OLEDs based on Ln(III) complexes for near-infrared emission

OLEDs based on Ln(III) complexes for near-infrared emission

4

A´ngel L. A´lvarez, Carmen Coya Rey Juan Carlos University, Madrid, Spain

4.1

Introduction

Since the discovery of efficient electroluminescence (EL) in small organic molecules [1] and conjugated polymers [2], there has been a considerable interest in the synthesis of new materials to increase the efficiency and stability of organic light-emitting diodes (OLEDs). In fact, because of the ability of organic semiconductors to retain their emission and conductive properties in thin large-area films, a wide range of new applications that are not accessible for the conventional electronics based on silicon—especially in relation to large surfaces, flexibility, and low-cost manufacturing—have emerged, most of which are oriented toward the lighting, photovoltaic energy, or wearable and flexible device fields. OLEDs operating in the near-infrared (NIR) spectral region have aroused a particular interest in view of the exciting potential of new and cost-effective applications for emerging photonic and optoelectronic technologies, such as lab-on-chip platforms for medical diagnostics, flexible self-medicated pads for photodynamic therapy, nightvision and plastic-based telecommunications, optical sensors, information processing, and special lighting (for instance, in greenhouses) [3]. This has promoted an intense activity in the research of novel materials, seeking for specific emission properties. OLED technology requires additional characteristics, such as the ability of the material to be processed into thin films. In fact, the attainment of electroluminescence in the infrared spectral range remains an open challenge due to the low photoluminescence efficiency of most narrow-gap organic emitters. Particularly challenging is the obtaining of solution-processed OLEDs. In this chapter, different approaches aimed at this goal, covering efficient NIR emitters that have the ability to be integrated into planar OLED devices, are reviewed. Among the diverse strategies that have been reported, emphasis will be placed on the principles of NIR emission for materials that have been demonstrated in OLEDs. The main emission mechanisms for the materials are phosphorescence emission mediated by heavy atoms, mainly using organometallic complexes; thermally activated delayed fluorescence (TADF) emission; aggregation-induced emission enhancement (AIEE) based on emitter molecules or oligomers; and, finally, the antenna effect in lanthanide complex-based OLEDs.

Lanthanide-Based Multifunctional Materials. https://doi.org/10.1016/B978-0-12-813840-3.00004-1 Copyright © 2018 Elsevier Inc. All rights reserved.

134

4.2

Lanthanide-Based Multifunctional Materials

Principle of operation and basic requirements of an organic light-emitting diode (OLED)

Light emission in OLEDs is caused by the radiative recombination of electrons and holes that are injected into layers widely separated from each other. The photon emission is then called electroluminescence (EL). In contrast to photoluminescence (PL), in which electron-hole pairs are generated within the same spatial region and in the form of singlet excitons, in EL a larger proportion of triplet states is expected, owing to the fact that charge injection is not spin-polarized. Concepts about singlet or triplet electronic states will be described in the next section. Moreover, recombination in EL occurs in the presence of an electric field that drives one type of carrier to meet the other. A scheme of an organic light-emitting diode OLED is shown in Fig. 4.1. It consists of a sequence of organic layers between the two electrodes; a low-work function metal for electron injection (cathode), typically Ca, Ba, Mg, Ag, or Al; and a high-work function material for hole injection (anode), typically Au, Co, Pt, or a conductive oxide, such as indium tin oxide (ITO). Among the most important ways to improve OLED performance, a good electronhole balance is really essential. This also contributes to the long-term stability of the device, since it minimizes the useless current that only contributes to heating and thus to the degradation of the device by Joule effect. In general, the highest-energy occupied molecular orbital (HOMO) and the lowest-energy unoccupied molecular orbital (LUMO), which are the transport bands of the emissive layer (EML), do not match the Fermi levels of the metal contacts to ensure the expected balance of the charge injection. Moreover, the mobilities of electrons and holes in conjugated organics are generally very different (that of holes being several orders of magnitude higher),

Fig. 4.1 (A) Multilayer structure (active layer thickness typically
20–200 nm) and (B) energy diagram of an OLED device.

NIR OLEDs based on Ln(III) complexes

135

which makes the problem of carrier balancing more difficult, while causing the recombination to take place in unsuitable sites, close to the electrodes. A carrier recombination excessively close to the electrode results in EL quenching by the metal. Thus, in order to obtain good emission efficiency, the active EML should be sandwiched between the so-called injection and transport layers, for both holes and electrons. These layers favor the required energy-level alignment between the electrodes and the organic emitter. Injector layers (HIL for holes and EIL for electrons) are designed to reduce the energetic barrier with the electrode Fermi levels by approaching as much as possible the EIL LUMO to the Fermi level of the cathode and the HIL HOMO to that of the anode. There are a variety of molecules widely used as HIL or EIL. Certain metal phthalocyanines (MPc, with M ¼ Cu or Zn) may be found among those good hole injectors, with HOMO levels close to 5 eV with respect to vacuum [4,5]. Molecules such as N,N0 -bis(3-methylphenyl)-N,N0 -diphenylbenzidine (TPD) and N,N0 -di (1-naphthyl)-N,N0 -diphenyl-(1,10 -biphenyl)-4,40 -diamine (NPD, also called NPB) are also widely used as hole injectors. Likewise, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is a very popular conductive polymer used as HIL and hole-transport layer in solution-based methods. Among the molecules that may play the role of EIL, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) and tris-(8-hydroxyquinoline)aluminum (Alq3) are commonly used. The injection barrier may alternatively be reduced by the general strategy of creating an interfacial dipole moment. Acceptor materials close to the anode or donors close to the cathode can play this role thanks to their ability to create a thin space-charge region. For example, derivatives of tetracyanoquinodimethane (TCNQ), with a very deep LUMO close to the Fermi level of ITO, are used as good hole injectors [6]. On the other hand, many inorganic materials with high permittivity (e.g., LiF or TiO2) or organic materials such as specific polar molecules or conjugated polyelectrolytes are good candidates for the electron-injection layer. Since these polar materials do not necessarily have to be conductive, this layer is usually very thin to minimize series resistance. Thereafter, the transport layers (HTL for holes and ETL for electrons) have the mission of softening the energy misalignment between the EML levels and those of the HIL or EIL. These layers should allow a high mobility of their respective carriers. The lower the mobility, the thinner the thickness of the transport layer should be in order to reduce its series resistance. In general, it is difficult to find amorphous organic materials with mobilities higher than 103 cm2 V1 s1 (except for certain materials with specific geometries and self-organizing ability, like acenes, which may attain mobilities of a few units of cm2 V1 s1), so the thicknesses of HTL and ETL are usually of a few tens of nanometers at most. More importantly, these transport layers must serve as a blocker to the carriers of the opposing sign. In the HTL, this condition is fulfilled by a LUMO level high enough to create an energy barrier for the passage of electrons toward the anode. In turn, the HOMO level of ETL must do the same for holes. Blocking carrier transport helps to confine and harvest excitons in the EML and thus greatly improves the efficiency. 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (bathocuproine, BCP), 4,7-diphenyl-1,10-phenanthroline (bathophenanthroline, Bphen), 1,3,5-benzinetriyl-tris(1-phenyl-1-benzimidazole) (TPBi), and 3-(4-biphenylyl)-4-

136

Lanthanide-Based Multifunctional Materials

phenyl-5-(4-tert-butyl-phenyl)-1,2,4-triazole (TAZ) are used as electron-conducting hole-blocking layers, whereas tris(4-carbazoyl-9-ylphenyl)amine (TCTA) is considered a hole-transporter material. In OLED technology, it is common that single layers of these materials adopt the role of both injector and transporter of charge carriers. In fact, those that exhibit luminescent properties can even be blended with the materials of the EML to improve their properties, as it will be shown in Section 4.4. Among the different techniques for the deposition of aforementioned layers, one can find plasma deposition, thermal evaporation, chemical vapor deposition, and solution-processed methods. Certain molecular materials can be deposited both by evaporation (due to their small weight and sufficiently high degradation temperature) and from solution, as they feature a certain degree of solubility in specific solvents. This is the case, for example, for TPBi, Alq3, PBD, or TCTA and for some Ln(III) complexes (to be discussed in Section 4.4.2.3). The main casting techniques compatible with cost-effective and large-area manufacturing methods include, but are not limited to, drop casting, spin coating, blading (knife-over-edge), gravure, slot die, screen printing, inkjet printing, and spray coating [7–9]. In general, fluorescent conjugated polymers have been the main field of work in solution-processed materials, although they present some drawbacks (viz., wide molecular-weight dispersion, inhomogeneity, uncontrolled intra- and interchain interactions, and the presence of impurities as a result of the polymerization routes). Soluble molecular compounds are not excluded, but are less used due to the general tendency of molecules to aggregate after liquid-phase deposition. Although solution processing is the most desirable procedure for massive and lowcost production, the lack of materials soluble in orthogonal solvents (to avoid redissolving the layer underneath) makes it really difficult to obtain multilayer stacking solely by this procedure. The combination of both good injection and transport properties in a single soluble material aids to overcome this difficulty, contributing to simplify the structures and reducing interface obstacles. Such is the case of PEDOT:PSS mentioned above, a material that acts as a good hole injector and transporter, which is supplied in an aqueous dispersion (orthogonal to most of the organic solvents). Unfortunately, PEDOT:PSS is not intrinsically an electron-blocking material, although it may occasionally play this role depending on the interface material [10]. With respect to electrons, in the last decade, a great effort has been made to synthesize water-soluble cationic polyelectrolytes to simultaneously inject and transport electrons within a single material [11], although most of these compounds are not yet commercial. Meanwhile, conjugated molecules such as PBD and Alq3, which allow deposition either by evaporation or from solution, have been used almost from the beginnings of OLED technology as reasonably well-suited electron-injector/ electron-transporter layers. In specific cases, advanced technology relies on hybrid approaches, in which some initial layers such as HIL and HTL and part of the emissive layers are inkjet printed

NIR OLEDs based on Ln(III) complexes

137

and patterned, whereas remaining emissive layers, together with ETL and EIL, are thermally evaporated on the previous patterns [12,13]. A general requirement is that optical transmission of all organic layers has to be high in the emission wavelength region. This also includes a significant Stokes shift (difference between the maxima of the absorption and emission spectral bands) in order to avoid self-absorption. In our case, at least one electrode must be transparent in the near-infrared range in order to achieve a high light outcoupling efficiency. One of the most frequently used transparent anodes is ITO. It consists of a nonstoichiometric alloy of In2O3 (80%–90%) and SnO2 (10%–20%) and is commercially available. Sn content in ITO modifies the conductivity and transmittance, in such a way that in the visible range, transmittance is more or less constant with increasing Sn content. However, Sn ions are very efficient scatterers in the infrared range, so the transmittance in the NIR region decreases from >85% for 100 Ω sq1. ITO to <35% for ITO with a reduced sheet resistance of 12 Ω sq1 (higher Sn content) [14]. With lower raw material costs, aluminum-doped zinc oxide (AZO) is a good candidate that meets the transmittance requirements better while keeping a reasonable resistance <50 Ω sq1 [15]. Nonetheless, both properties are strongly dependent on the preparation conditions, reducing its industrial impact. On the other hand, its lower work function (3.7–4.4 eV) as compared with ITO [16] implies a compromise when deciding its application in OLEDs. In an OLED device, the external quantum electroluminescence efficiency (ηex) is defined as the ratio of the number of emitted photons to the number of charge carriers injected into the device. The luminous efficacy of the source, expressed in lm W1, is the luminous flux of the emitting source as perceived by the human eye divided by the electric input power. The luminance or brightness (cd m2) gives the ratio of the light intensity from the source as perceived by the human eye in a certain direction to the area of the OLED in that direction. Using the brightness concept, the electroluminescence efficiency may be also defined as cd A1, relating the same light intensity to the current through the device. But the first requisite in order to achieve high external quantum efficiency is that the emitter material has a good emission performance itself, since ηex is directly proportional to internal emission efficiency of the material by a photon extraction efficiency factor.

4.3

Mechanisms to obtain near infrared emission

The development of emitting materials for OLEDs started by taking advantage of the emission properties of fluorescent materials and then moved to phosphorescent materials [17] and, recently, to materials involving new emission mechanisms such as thermally activated delayed fluorescence (TADF) [18] or aggregation-induced emission enhancement (AIEE) [19]. All these mechanisms are relevant for NIR-emitting devices. Fig. 4.2 schematically shows the three main mechanisms for light emission in organic semiconductors mentioned above.

138

Lanthanide-Based Multifunctional Materials

S1

Fluorescence

25%

75%

25%

75%

25% S1

S1 ISC T1

75% T1 RISC

T1

TADF

Phosphorescence

S0

S0

S0

(A)

(B)

(C)

Fig. 4.2 Schematic representation of the three main emission mechanisms: (A) fluorescence (internal quantum efficiency
25%), (B) phosphorescence (internal quantum efficiency
100%), and (C) recently reported thermally activated delayed fluorescence, TADF (internal quantum efficiency
100%). Solid and dashed yellow lines indicate radiative and nonradiative transitions, respectively.

A singlet is a molecular electronic state in which all electron spins are paired. Therefore, when an electron at the ground singlet state of a molecule (S0) is promoted to an excited singlet state (S1), the S1!S0 decay is an allowed transition and includes the emission of a photon. The radiative transition between singlet states is referred to as fluorescence. In a triplet state (T1), the excited electron is not paired with the ground-state electron because they possess the same spin. In consequence, the T1!S0 transition is not a spin-allowed transition. Since it is a forbidden spin transition, the radiative decay path happens to depend primarily on the spin-orbit interaction, which confers a certain mixed character to the state between triplet and singlet. However, in certain conditions, an excited singlet state can pass in a nonradiative manner to a triplet state, S1!T1, in a process known as intersystem crossing (ISC) relaxation. The radiative decay from the excited triplet state back to a singlet ground-state T1!S0 is called phosphorescence. Since a transition in spin multiplicity occurs, phosphorescence is always a manifestation of intersystem crossing. The timescale of intersystem crossing is in the order of 108–103 s [20]. So, the first step in the design of phosphorescent materials is to favor ISC as a relaxation path from S1 to T1. The main motivation is to take advantage of the longer lifetime of the nonradiative triplet-state decay, looking for 100% efficiency by simultaneously avoiding all possible paths for nonradiative deactivation of this state. ISC relaxation between S1 and T1 excited states may be largely favored by the influence of a heavy atom, which exhibits significant spin-orbit coupling [21]. S1 states use to have shorter lifetimes than T1 ones, so the probability for this process to take place is higher when the vibrational levels of the two excited states overlap, since little or no energy must be gained or lost in the transition. However, that state overlap would equally favor the reverse transition from T1 to S1, which would deactivate the process. Consequently, in general, a certain energetic separation S1>T1 is considered optimal to ensure efficient emission. Alternatively, thermally activated delayed fluorescence (TADF) process requires a careful design of the molecule due to the fact that it only occurs when ΔEST is small,

NIR OLEDs based on Ln(III) complexes

139

and thus, reverse intersystem crossing ISC can take place even in pure aromatic organic compounds containing no heavy metals [22].

4.3.1 Near infrared emission mediated by heavy atoms The phosphorescent approach to obtain NIR emission in OLEDs exploits the possibility of a radiative triplet- to singlet-state transition mainly by introducing heavy metal atoms, improving the overall electroluminescence (EL) internal quantum efficiency (IQE) of OLEDs up to 100% [17,23]. The introduction of phosphorescent emitters into fluorescent host materials can potentially harvest both singlet and triplet excitons upon electron-hole recombination, and hence, they may potentially improve the luminous efficiency. Because relaxation from T1 to S0 requires slower secondorder interactions, quantum relaxation is very slow and uncompetitive with vibrational decay. Complexes of third-row transition-metal ions are particularly suitable, due to their high spin-orbit coupling constant and associated high ISC, efficiently promoting the emission from the otherwise wasted triplet states, which represent up to 75% of the excited states formed upon charge recombination in an electroluminescent device [24–28]. This strategy has been mainly used by introducing iridium(III) or platinum(II) complexes [29–32]. Ir(III)- and Pt(III)-based phosphors can harvest both singlet (25%) and triplet (75%) excitons through heavy-atom-enhanced intersystem crossing (ISC) and can therefore reach a ηex that is four times higher than that of conventional fluorescent OLEDs [24,33,34]. Very recently, multilayer devices fabricated by thermal evaporation employing tetraphenyltetrabenzoporphyrin (PtTPTBP), aza-triphenyltetrabenzoporphyrin (PtNTBP), and Pt (II) cis-diazadiphenyltetrabenzoporphyrin (cis-PtN2TBP) have demonstrated EL emission at 770, 848, and 846 nm with external quantum efficiencies of 8.0%, 2.8%, and 1.5%, respectively [29]. OLEDs have also been prepared using Pt(II) complexes as phosphorescent emitters obtaining a maximum quantum efficiency of 1.2% at a current density of 10 mA cm2 (Fig. 4.2A) [31]. Kesarkar et al. [30] reported NIR-emitting, solutionprocessed phosphorescent organic light-emitting devices (PHOLEDs) based on novel Ir(III) complexes: [Ir(iqbt)2(dpm)], [Ir(iqbt)2(tta)], and [Ir(iqbt)2(dtdk)] based on 1-(benzo[b]thiophen-2-yl)-isoquinolinate (iqtb) (dpm ¼ 2,2,6,6-tetramethyl-3,5heptanedionate; tta ¼ 2-thenoyltrifluoroacetone; and dtdk ¼ 1,3-di(thiophen-2-yl)propane-1,3-dionate) emitting from 650 to 850 nm. The devices exhibited remarkable external quantum efficiencies (above 3%), exceeding the highest reported values for solution-processed emitters in this wavelength range. Recently, other metal complexes based on diketopyrrolopyrrole-boron have been proposed as deep-red or quasi-NIR emitters (760 nm) due to their high PL quantum yield and high chemical stability, demonstrating an all-solution-processed OLED [35] with an external quantum efficiency of 0.5%. It is worth noting that the efficiency of devices based on these complexes decreases as emission is shifted to longer wavelengths. Besides, their high cost prevents them from being used in many commercial applications such as mobile displays, largescreen displays, or general lighting.

140

Lanthanide-Based Multifunctional Materials

4.3.2 Near infrared emission by thermally-activated delayed fluorescence (TADF) Thermally activated delayed fluorescence (TADF) emission is an alternative approach that has been tested in order to obtain NIR emission in OLEDs without mediation of heavy atoms. Since statistically 25% of the excitons represent singlets and 75% triplets, an interesting strategy consists in trying to move electrons from the most populated triplet state to singlet states, where emission is more favored. In these materials, temperature accelerates the ISC from a triplet excited state (T1) to a singlet excited one (S1), that is, reverse intersystem crossing (RISC), thus leading to an increase in fluorescence intensity. TADF process requires a careful design of the molecule energetic levels due to the fact that it only occurs when the energy difference between the lowest singlet state and highest triplet state (ΔEST) is small. Thus, RISC can take place even in pure aromatic organic compounds containing no heavy metals [18,22,24,36]. The work of Endo et al. [18] with SnF2-Meso IX complex determined an activation energy of 0.24 eV for RISC from the analyses of T1 to S1 rate (kRISC) and temperature relationships, which precisely corresponded to the energy difference between T1 and S1 levels (Fig. 4.3A). Hence, in order to increase kRISC, it is necessary to have a smaller energy gap. Fig. 4.3B illustrates how in these compounds the total fluorescent emission, assisted by reverse ISC, is enhanced by temperature. The development of efficient long-wavelength (deep-red or NIR) TADF emitters is specially challenging as the limited orbital overlap associated to a small ΔEST generally leads to a low fluorescence rate (KF) and hence a low PL efficiency due to competition from radiationless transitions. Wang et al. [37] reported the first TADF molecule TPA-DCPP (TPA ¼ triphenylamine and DCPP ¼ 2,3-dicyanopyrazino phenanthrene) emitting in the deep-red spectral region (688 nm) with a singlet-triplet splitting of 0.13 eV. An OLED device exhibiting ηext of 2.1% was demonstrated, with Commission Internationale de l’Eclairage (CIE) coordinates of (0.70, 0.29), still insufficient to be considered a true NIR emitter.

4.3.3 Near infrared emission using a donor-acceptor design for oligomers and other molecular materials Attempts with other fluorescent materials, like novel oligomers, have been also tested for NIR emission in OLEDs [38,39]. A multilayer NIR OLED based on an evaporated conjugated imine oligomer, (E)-N-((E)-3-((E)-(4-iodophenyl-imino)methyl)benzyldine)-4-iodobenzenamine, as an active layer exhibited EL emission peaking at 800 nm (Fig. 4.4B) and a maximum external quantum efficiency, an electric-to-optical power efficiency, and a low turn-on voltage of 1.9%, 8.55 mW W1, and 2.3 V, respectively [40]. NIR OLEDs based on fluorescent donor-acceptor-donor (DAD) conjugated oligomer structures were also previously reported [41–43]. An electron-rich donor and electron-deficient acceptor are covalently bonded with one another in the DAD molecules. The energies of the HOMO-LUMO molecular orbitals of these oligomers are controlled by the donor and acceptor components, respectively. In these structures, the energy gap and therefore the emission wavelength can be tuned by changing the strengths of the donor and acceptor components. However, since many of these NIR organic fluorophores of the donor-acceptor type are flat π-conjugated

1010

Fluorescence S1

108

RISC

300 ΔE:S1

200

ΔE:T1

ΔE = 0.24 eV

30°C

100 0°C

kF

F PL

3.0

FF

2.5

F PHOS

2.0

F TADF

1.5

6

10

10 4

In (kRISC)

S0

Decay constant (s−1)

T1

PL quantum efficiency (%)

Emission intensity (au)

400 100°C

kISC

10

2

6

kRISC

4

10

2

100

0 0.0025 0.0030 1/T (K−1)

1.0 0.5

8

ΔE ST = 0.24 eV

kP

Phosphore-scence 0 500

0.0 600

0

700

100

150

200

250

300

350

400

10−2

Temperature (K)

Wavelength (nm)

(A)

50

(B)

0

50 100 150 200 250 300 350 400 Temperature (K)

(C)

Fig. 4.3 (A) Photoluminescence spectra for the SnF2-Meso IX complex as a function of temperature; (B) dependence of the total photoluminescence (ΦPL), fluorescence (ΦF), phosphorescence (ΦPHOS), and TADF (ΦTADF) quantum efficiencies on temperature; and (C) temperature dependence of the decay rates: kISC, intersystem crossing; kF, radiative decay from S1; kRISC, reverse intersystem crossing; and kP, radiative decay from T1. Modified from A. Endo, M. Ogasawara, A. Takahashi, D. Yokoyama, Y. Kato, C. Adachi, Thermally activated delayed fluorescence from Sn4+porphyrin complexes and their application to organic light emitting diodes—a novel mechanism for electroluminescence, Adv. Mater. 21 (2009) 4802–4906. Copyright 2017 Wiley InterScience.

142

Lanthanide-Based Multifunctional Materials

Normalized EL (au)

−5% in TCT −5% in CBP -neat film

(A)

(B)

−11 V −9 V −8 V −6 V −5 V

(C)

300

450

600

750

900

1050

Wavelength (nm)

Fig. 4.4 Examples of NIR EL emission from (A) OLEDs based on the phosphorescent approach, in particular OLEDs either incorporating PtL2Cl as a neat film or doped into TCTA or CBP as the emitting layer at 8 V; (B) OLEDs based on an imine oligomer at different voltages; and (C) spectra of nondoped devices based on fluorophores with aggregation-induced emission enhancement (AIEE). Black dashed arrows indicate the full width at half maximum (FHWM), ranging from 0.3 to 0.7 eV. Modified from (A) F. Nisic, A. Colombo, C. Dragonetti, D. Roberto, A. Valore, J. M. Malicka, M. Cocchi, G. R. Freeman, J. A. G. Williams, Platinum(II) complexes with cyclometallated 5-pi-delocalized-donor-1,3-di(2-pyridyl)benzene ligands as efficient phosphors for NIR-OLEDs, J. Mater. Chem. C 2(10) (2014) 1791–1800; (B), M.T. Sharbati, F. Panahi, A. Shourvarzi, S. Khademi, F. Emami, Near-infrared electroluminescence from organic light emitting diode based on imine oligomer with low turn on voltage, Optik–Int. J. Light Electron. Opt. 124(1) (2013) 52–54; (C), X.B. Du, J. Qi, Z.Q. Zhang, D.G. Ma, Z.Y. Wang, Efficient non-doped near infrared organic light-emitting devices based on fluorophores with aggregation-induced emission enhancement, Chem. Mater. 24(11) (2012) 2178–2185. Copyright 2017 American Chemical Society.

NIR OLEDs based on Ln(III) complexes

143

molecules, they tend to stack or eventually aggregate, showing the so-called aggregation-caused emission quenching (ACEQ) [44–46]. ACEQ is generally attributed to nonradiative deactivation processes, such as excitonic coupling, excimer formation, and excitation energy migration to the impurity traps [47,48]. Nonradiative deactivation pathways are also favored due to the higher coupling between ground and excited state associated with low bandgaps [49], so many NIR fluorophores can only be incorporated in small amounts as dopants in OLED devices [50,51]. An alternative is phosphorescent-sensitized fluorescence by codoping with a phosphorescent dye [41,52]. This has resulted in fluorescent NIR OLED device efficiencies comparable with those attained by phosphorescent NIR OLEDs [53–55]. Unfortunately, unless a precise control of the doping concentration is achieved, emission quenching by bimolecular interactions at high exciton densities will lead to a rapid decrease in quantum efficiency for high doping concentrations or at high currents [17]. Nondoped OLEDs with emission above 700 nm still show a low ηex and are not suitable for practical applications [56,57]. To overcome fluorescence quenching, Tang et al. [19] developed a molecular design that introduced a “propeller-like” group to reduce or eliminate the π-π stacking and molecular rotational motions in the solid state. The freely rotating groups are restricted in the solid state, and the PL efficiency (i.e., quantum yield) becomes higher than in the solution phase, exhibiting aggregation-induced emission enhancement (AIEE) [58,59]. However, there are still few works that demonstrate AIEE effect in materials for NIR OLEDS. Wang et al. [60] reported AIEE NIR fluorophores suitable for use in OLEDs by vapor-phase deposition using a DAD-type NIR emitter such as [1,2,5]thiadiazolo[3,4-g]quinoxaline (QTD), benzo[1,2-c;4,5-c0 ]bis[1,2,5]thiadiazole (BBTD) as an electron acceptor, and tetraphenylethene (TPE) and 2,2-bis(4-methoxyphenyl)-1-phenylethene (MTPE) as both electron donors and AIEE-enabling units. Nondoped OLEDs utilizing them as emitters were manufactured, and the NIR electroluminescence above 700 nm is shown in Fig. 4.4C. The ηext was 0.89% and remained fairly constant over the 100–300 mA cm2 current density range. These devices were stable and showed very little efficiency roll-off at high current densities. The device performance is considered as the best reported to date for nondoped NIR OLEDs at these wavelengths.

4.3.4 Near infrared emission by antenna-effect in Ln(III) complexes In general, with aforementioned approaches, it is difficult to reach emission wavelengths beyond 1 μm. Besides, they typically present broad emissions (i.e., large full width at half maximum, FHWM, values), as one can observe in Fig. 4.4. In contrast, traditional lanthanide ion transitions possess, among other unique properties, very narrow emission bands over a wide range of the near-infrared spectrum, large Stokes shifts, and long luminescence lifetimes. Those properties arise from their [Xe]4f n

144

Lanthanide-Based Multifunctional Materials

Fig. 4.5 Normalized emission spectra of some luminescent lanthanide complexes in solution, illustrating the sharp emission bands and minimal overlap of lanthanide luminescence. FHWM
0.08–0.2 eV.

electronic configurations. See, for instance, the work by B€unzli et al. on typical transitions of the most important NIR-emitting triply ionized lanthanide ions (Ln3+ or Ln(III) from now onward) [61]. The 4f orbitals of lanthanides are shielded by the 6s, 5p, and 5d orbitals; thus, the spectra arising from f-f transitions consist of narrow lines and are insensitive to their environment, resulting in aforementioned specific properties of lanthanide luminescence and in long lifetimes of the excited states. Fig. 4.5 shows the PL emissions of some representative lanthanide emitters in the 700–1600 nm region. Ln(III) in solid hosts typically exhibit emission line widths of
10–20 nm (FWHM), which are much narrower than those observed for organic dyes (
30–50 nm) or transition-metal ions (
100 nm) and even narrower than those observed for quantum dots (QDs,
25–40 nm) [62,63]. Among other advantages, this property allows more resolvable bands to be packed into the same spectral bandwidth, enabling, for instance, a larger number of distinct combinations. Further, lanthanide emissions involve only atomic transitions, and hence, they are extremely resistant to photobleaching (self-absorption). The energy-level structure in Ln3+ creates the possibility for large Stokes shifts of several hundred nanometers between the excitation and emission bands, even establishing discrete gaps between them with zero absorption. In contrast, the HOMO-LUMO transition in organic dyes typically results in overlapping excitation bands with Stokes shifts of only 10–30 nm between the absorption and emission maxima. The large variety of absorption and emission wavelengths, independence of host materials, and low vibration energy losses make lanthanides ideal candidates for spectral conversion. Ln3+ ions can be doped into a variety of solids such as crystals, fibers, or glass ceramics to give them the desired down and upconversion optical properties (discussed in Chapter 9).

NIR OLEDs based on Ln(III) complexes

145

Thus, their remarkable luminescence properties have been widely applied in lasers, solar cells, analytic sensors, photodynamic therapy, and optical imaging [64–66]. In view of these excellent luminescent properties, a number of synthetic routes have attempted to incorporate Ln3+ ions into organic compounds. A review that describes the process of the chelation of Ln3+ (mostly Eu3+ and Tb3+) with a large number of organic ligands and the emission properties of the resulting complexes was reported in 2007 [67]. A more recent extensive review on specific luminescent lanthanide β-diketonate complexes has been written by Binnemans [68]. For a comprehensive review on all sorts of Ln3+-based NIR-emitting molecular edifices (including macrocyclic ligands, such as porphyrinates, derivatized coronands and cryptands, derivatized cyclens, derivatized calixarenes, and resorcinarenes; acyclic ligands, including quinolinates, terphenyl-based ligands, polyaminocarboxylates, dyes and dendrimers; heterometallic functional assemblies; inorganic clusters; zeolites and composite mesoporous materials; and nanoparticles), the reader is referred to the review by Comby and B€ unzli [61]. Despite the expected emission in a narrow spectral range, some disadvantageous selection rules may obscure so many interesting transitions. For example, the f-f transitions of the lanthanide cations are forbidden according to Laporte selection rule and are therefore of negligible intensity. However, Weissman discovered in 1942 that when organic ligands were chelated to Eu3+ and then the complex was excited, the π!π* transition of the ligands sensitized the 4f!4f emission from Eu3+ [69]. This is the so-called antenna effect [70] or sensitization, by which energy is transferred from the surroundings of the metal ion (either an inorganic matrix or an organic ligand) to the metal ion. Furthermore, this sensitization process also has the advantage that the Stokes shift is very large, which allows an easy spectral separation of the remaining matrix luminescence from the metal ion emission. In lanthanide complexes, this antenna process includes a sequence of excitation, energy transfer, and emission steps, with the peculiarity that the absorbing (ligand, organic or inorganic) and emitting (lanthanide ion) components are distinct. This, in turn, it allows to achieve larger excited-state populations using four to five orders of magnitude lower light fluencies (J cm2) than those required for bare ions, overcoming their weak absorptivity. The antenna effect also results in an internal efficiency of the Ln3+ complex that is not limited to 25%, provided that the excitation energy can be transferred to the lanthanide ion both from excited singlet or triplet states, that is, the internal efficiency could theoretically approach 100%, as it happens in phosphorescent OLEDs. The energy transfer by antenna effect is illustrated in Fig. 4.6 for an Er3+ complex, using a simple Jablonski energy-level scheme [71]. The ligand singlet-state S1 is typically excited with UV-Vis radiation. Then, S1 can decay either to the ground-state S0 (molecular fluorescence) or to triplet-state Tn through an ISC relaxation mechanism, as described in Section 4.3. A careful selection of the ligand is essential in order to favor this last mechanism, which may be enhanced by heavy-atom effect. Subsequent sensitization of the rare-earth ion mainly depends on the matching between the triplet energy state (T1) and the Er3+ energy levels [72]. If the energy levels are adequate (energy levels below T1), excited triplets can then populate the upper levels of the

146

Lanthanide-Based Multifunctional Materials Ca/AI cathode 25

e−

S1

20

5 0

4

X

Phosphorescence

10

Fluorescence

15

2

RET

X T1 UV excitation

Energy (cm−1) × 10 −3

ISC

4

F9/2

4

I9/2 I11/2

4 4

4

(A)

h+ PEDOT:PSS

I13/2

ITO

1532 nm

S0 Ligand

H11/2 S3/2

Er(III)

I15/2

Glass substrate Erbium

NIR emission

(B)

Fig. 4.6 (A) Jablonski diagram illustrating the mechanism for the main intramolecular energy transfer between an ideal ligand and the 4f levels of Er3+ in order to favor antenna effect. Dashed lines correspond to nonradiative transitions. ISC, intersystem crossing; RET, resonant energy transfer. (B) Illustration of the emission process, assisted by antenna effect, in an OLED device.

Er3+ ion via resonant energy transfer (RET) [73]. The latter process can take place via either Dexter [74] or F€ orster [75] mechanisms, depending on the total angular momentum variation (ΔJ) undergone by the Er3+ ion [76]. After this indirect excitation by energy transfer, the excited lanthanide state may be deactivated by nonradiative processes, or it may undergo a radiative transition to a lower 4f state, resulting in the characteristic line-like emission. Thus, following a fast relaxation of the ion excited state, the radiative decay (2S+1)Γ J ! (2S+1)Γ J yields the emission from the Ln3+ [77]. According to the description discussed above, it is possible to formulate the overall efficiency or luminescence quantum yield for the sensitizing process of a lanthanide complex, ηS, as ηS ¼ ηISC  ηRET  ηLn where ηISC represents the efficacy of the intersystem crossing process, ηRET is the effectiveness of the ππ*-Ln transfer (ligand to metal), and ηLn is the intrinsic quantum yield for direct excitation of the lanthanide ion. These processes compete with other external relaxation mechanisms such as triplet oxygen quenching or internal intraligand charge transfer [78], which will eventually lower the overall quantum yield. Some of these processes are reviewed in the next subsection. NIR-based OLEDs based on Ln(III) complexes not only take advantage of the antenna effect but also can benefit from easier processability, enabling cost-effective manufacturing methods, thanks to the organic part of the compound. Very recently, Pushkarev and Bochkarev [79] published a thorough review on NIR-emitting pure organic or hybrid materials, which covers molecular materials, oligomers, polymers, and d-metal and Ln(III) complexes.

NIR OLEDs based on Ln(III) complexes

147

4.3.4.1 Radiation losses in Ln(III) complexes The ligands have to efficiently sensitize lanthanide ions and must provide coordination sites to shield them from impurities in the surrounding matrix, but at the same time, they may quench the luminescence by nonradiative mechanisms. Near-infrared PL from trivalent lanthanide ions with a relatively small energy gap between the excited and ground states, such as that from Er3+ or from Yb3+, is efficiently quenched by vibronic coupling with the ligand and, in the case of solutions, with the solvent molecules [80]. NIR PL of Er3+ ions is due to 4I13/2!4I15/2 transition, with an emission maximum located at around 1540 nm. For Yb3+ ions, which emit at 977 nm, emission is due to the 2F5/2!2F7/2 transition. As noted above, because of the small energy gap, these emissions can be quenched by the vibronic coupling with high-energy OdH stretching (from water and oxygenated anions) or CdH stretching vibrations (from alkyl chains of the ligands) in the neighborhood of the Ln3+ ion, which have been identified as the main causes for this effect. For example, it has been pointed out that the second harmonics of CdH and OdH bond vibrations (at 5900 and 6900 cml, respectively) of solvent molecules closely match the energy gap of the 4 F3/2!4I15/2 radiative transition in Nd+3 (5400 cml). This provokes the reduction of the NIR emission decay time from the value τ  8 ms for the isolated ion to values in the range of 1–2 μs for typical Nd3+ complexes. Consequently, the overall radiative efficiency is generally reduced to ηS  104 [81]. In general, coordination of OdH and CdH oscillators to the lanthanide ion will increase the nonradiative rates of the lanthanide excited states, in particular as the energy of the near-infrared transition decreases, as it can be observed in Fig. 4.7. 2000 5

0

4

F5/2

4

F3/2

I13/2

E1 − E0 (cm−1)

−2000 v=1

−4000 4

I15/2

4

I13/2

4

I11/2

−6000

4

−8000 2 −10000 F7/2

I15/2

v=2

v=3

4

I9/2

−12000 −14000 −16000

Yb

Nd

Er

O H C H C D C O C C

Fig. 4.7 Representation of the radiative transition energies of Yb3+, Nd3+, and Er3+ and the vibrational energies of common bonds found in organic systems. Modified from K. Binnemans, Rare-earth beta-diketonates, in: K.A. Gschneidner, J.C.G. B€ unzli, V.K. Pecharsky (Eds.), Handbook on the Physics and Chemistry of Rare Earths, vol. 35, Elsevier, Amsterdam, 2005, p. 107 (chapter 225). Copyright Elsevier, 2017.

148

Lanthanide-Based Multifunctional Materials

In 1996, Hasegawa et al. described the improved NIR luminescence in solution of a Nd3+ complex by using several β-diketonate complexes with deuterated and fluorinated ligands [82,83]. In this case, it was verified that fluorination in general did not significantly modify the triplet energy levels of the ligands, so the resonant transfer to the lanthanide ion was not compromised [84]. Recently, it has been shown that including fluorinated ligands and deuterated hydrogen also significantly improves the luminescence intensity of the Er3+ ions by reducing the fluorescence quenching caused by vibrational CdH bonds [85–87].

4.3.4.2 Ligand requirements As stated in the previous section, the exploitation of the lanthanide properties essentially depends on the ligands chosen to form complexes with the corresponding central Ln3+ ion. Generally speaking, the first role of organic ligands is to saturate the coordination sphere of the Ln3+ ion to facilitate its manipulation, to modulate its properties, and to provide a bonding strength that ensures good thermodynamic stability and high kinetic inertness. Focusing on emission properties, the ligands firstly ensure that the chelated ions remain apart to avoid quenching associated with Ln-Ln interactions. As it was discussed in the previous subsection, it is equally important to protect the Ln3+ ion from further coordination with solvent or moisture groups, already identified as the main infrared emission quenchers [88]. Molecular design and procedures during synthesis must prevent water molecules from penetrating into the inner coordination sphere. The nonradiative deactivation mechanisms of the excited Ln3+ are more likely to occur and therefore more critical for longer wavelength emitters such as Er3+. These processes are mainly attributed to high-energy vibrators such as OdH, NdH, or CdH, which are present in most of ligands, to the extent that any CdH bond within ˚ may act as an efficient quenching center [89,90]. Exclusion of these bonds via 20 A fluorination of the ligands is a known strategy to enhance the efficiency of the NIR emitters [91,92]. Synthesis of fully fluorinated complexes is chemically difficult, so several studies have used deuteration [93,94] or explored ligand structures with reduced quenching effects [95]. This strategy delivers encouraging results, and a lengthening of luminescence lifetimes of Yb3+ or Er3+ has been indeed reported, albeit—in general—by factors <10, so much work is still needed in this field. Secondly, as it has been previously stated, ligands should act as good sensitizers, that is, their energy-level scheme has to maximize the energy-transfer path to the corresponding Ln3+ excited state (ηISCηRET product). This means that, on the basis of similar functional groups, each Ln3+ ion requires particular ligands. Series of a single complex where the Ln3+ site is substituted by different rare-earth atoms (Pr, Nd, Ho, Er, Tm, or Yb) may exhibit very different emission efficiencies, being maximum only for one specific ion [96]. General trends are as follows: (i) The enhancement of spin-orbit coupling due to the heavy-atom effect associated with lanthanide ions should accelerate the ISC process (ηISC). (ii) Ligand-to-lanthanide energy transfer

NIR OLEDs based on Ln(III) complexes

149

(ηRET) depends on the distance and on the energy overlap. Proximity is required for the Dexter transfer mechanism to occur, since it involves charge exchange and, therefore, the best results should be expected when the antenna directly coordinates to the lanthanide center. (iii) Not only a short distance is needed but also an energy overlap between the triplet states of the sensitizer and the lanthanide to promote the F€orster energy-transfer mechanism. In this case, the ligand triplet energy must be closely matched and slightly above that of the Ln3+ triplet, but not so close that thermal back energy transfer can effectively compete with the Ln3+ emission [97,98]. Typically, an energy gap of about 1500–3000 cm1 has been recommended to prevent this detrimental phenomenon [72,99]. Thirdly, manufacturing of a device implies additional requirements, namely, that active layers have suitable properties to achieve uniform thickness upon deposition, either by thermal evaporation or by solution processing. Ln3+ are heavy ions and only form volatile complexes with certain ligand sets. Many types of lanthanide complexes cannot be evaporated without partial thermal decomposition and quality loss. It may occur, as in the case of some lanthanide βdiketonate complexes, that the most volatile ones are not among the best emitters [68]. In any case, thermal evaporation in vacuum has been widely used to deposit these materials, even though they exhibit a poor sublimation capacity as compared with most of the other molecules used in the layer stacking. Ligand design must pursue degradation temperatures as high as possible [100] and a decrease in intermolecular interactions by different strategies, such as fluorination of ligands [101]. In general, not only thermal evaporation performed in a vacuum chamber includes specific advantages, such as the accurate control of the layer thickness, but it also adds certain drawbacks, provided that it is an expensive technique that is not readily available for deposition on large-area substrates and that co-evaporation with multiple sources is a nontrivial process. Solution processing is the most desirable low-cost alternative for large-area manufacturing. This process removes previous restrictions on molecular weight and very high thermal stability and can be applied to complexes that are soluble or, if not, that may be embedded into a soluble matrix (host-guest strategy). However, this requirement implies that ligands must be specifically designed to be soluble in adequate solvents. Lately, great effort has been devoted to achieve good solubility of lanthanide complexes in water or in organic solvents, with moderate success in the final devices, presently constituting one of the frontiers in the state of the art [102]. Finally, the fourth requirement would be that the ligands should have good electron- and hole-transport properties to promote charge injection and exciton generation in the complex. To improve the carrier-transport properties of Ln(III) complexes, two main approaches have been explored: (i) introducing suitable chargetransport groups into the ligands and (ii) doping the lanthanide complexes into good transporter materials. In this latter case, the lanthanide complex (guest) is typically blended or dissolved into a polymer matrix (host), giving rise to a host-guest system. With regard to the transport properties of ligands, it is worth mentioning that regular β-diketones, which are the most widely used ligands, generally have a poor carrier-transport ability and cannot satisfy by themselves the conditions to fabricate

150

Lanthanide-Based Multifunctional Materials

OLEDs. This example demonstrates the great difference that can exist between the field of luminescent materials and that of electroluminescent devices. Starting from the base of tris-β-diketonate complexes, the addition of or substitution with different Lewis base adducts may improve transport properties and, eventually, enhance the final device performance. For example, it has been proved that 1,10-phenanthroline and some of its derivatives favor electron transport, whereas the addition of a carbazole fragment to the β-diketone ligands benefits hole transport [87,103]. The triphenylamine derivatives are also a well-known class of hole-transporting materials [104,105]. However, the strategy of introducing donor or acceptor substituents to confer transport properties unfortunately affects the triplet level energy of the ligands and therefore introduces a new factor that should be taken into account in the expected energy-transfer rate. The integration of the Ln(III) complex into a conductive polymer matrix (in the so-called host-guest strategy), apart from sorting out the problem of the transport for one of the electric species (e or h+) without modifying the structure of the Ln(III) complex, may also solve the solubility problems, enabling subsequent deposition of good-quality layers. However it introduces a new factor to consider in the efficiency: the charge transfer between the matrix and the Ln3+ ion via its ligands or, alternatively, in case of a fluorescent polymer, the tuning between the polymer emitting band and the ligand absorption one [106].

4.3.5 Redox properties to enhance resonance energy transfer Finally, it should be noted that the redox properties of some lanthanides may have an effect on the luminescent properties. In particular, Yb, Sm, Eu, Dy, and Tm have a metastable divalent state apart from their stable 3+ oxidation. Horrocks et al. [107] observed how Yb3+ PL could be directly sensitized by a distant chromophore via a long-range electron-transfer (ET) process. They used single tryptophan-containing calcium-binding protein parvalbumin where Ca2+ ions had been replaced by Yb3+ ions. In their work, they observed the 977 nm emission from the 2F5/2!2F7/2 transition of Yb3+ ions despite the fact that there was not an energy match with the ligand singlet or triplet levels. The spectral overlap integral of F€ orster theory approached zero, and indeed, they did not observe emission from the Eu3+ and Tb3+ ions, which did not present an energy-level match either. They proposed a redox mechanism that involved reversible ET to explain the observed 2F5/2!2F7/2 emission from Yb3+ ions in complexes with parvalbumin (a tryptophan-containing (Trp) protein, in particular tetra-psulfonatophenylporphyrin (TPPS)), in which there was not an energy match with the ligand singlet or triplet levels, neither within the porphyrin complexes (Fig. 4.8A). The redox excitation mechanism would explain the sensitization of the 2F5/2 level of Yb3+ emission, as shown in Fig. 4.8B. They stated that the redox potentials of the metal and ligand enabled complete and reversible electron transfer from the metal to the ligand, that is, reversible Ln3+/Ln2+ reduction, resulting into intermediate states formed by a tryptophan cation, Trp+, and Ln2+ ion, which eventually made the decay via Yb3+ system more energetically favorable. Fig. 4.8B shows the electron-transfer scheme proposed by Horrocks et al. [107] between Trp* and either Eu3+ or Yb3+. As the 2F5/2 state

NIR OLEDs based on Ln(III) complexes

151 4.27 eV

Indole

(A)

T1

Trp*, Yb3+ kYb f

T1

TPPS

2

F5/2

2

F7/2

Yb3+

D0

7

5

D4 Eu3+∗ (5D0)

2.14 eV

Trp+., Eu2+

1.66 eV kEu b

2.36 eV

F5 Trp, Eu3+ 0 eV

Trp+., Yb2+ kYb′ b

1.27 eV kYb b hn

7

F2

Eu3+

l = 290 nm

S1

5

Luminescence (545 nm)

S2

Luminescence (614 nm)

kEu f Luminescence (977 nm)

Excitation (600-475 nm)

10

Excitation (318-290 nm)

103 cm–1

20

Trp∗, Eu3+ 3.90 eV

S1

30

(l

=9

) m 7n

Trp, Yb3+∗(2F5/2)

7

Trp, Yb3+

Tb3+

(B)

Fig. 4.8 (A) Energy-level diagram for Yb3+, Eu3+, and Tb3+ and singlet and triplet levels of tetra-p-sulfonatophenylporphyrin (TPPS) and indole chromophores. (B) Proposed electronLn transfer scheme between Trp* and either Eu3+ or Yb3+ (kLn f and kb are the forward and 3+ 0 backward ET rate constants when the Ln ion is left in its ground state, respectively, and kYb b is 3+ 2 the backward ET rate constant when Yb is left in its excited emissive electronic state ( F5/2)). Reproduced with permission from W.D. Horrocks Jr., J.P. Bolender, W.D. Smith, R.M. Supkowski, Photosensitized near infrared luminescence of ytterbium(III) in proteins and complexes occurs via an internal redox process, J. Am. Chem. Soc. 119 (1997) 5972–5973. Copyright 2017 American Chemical Society.

energy (1.27 eV) is lower than the ET back-reaction energy (2.37 eV) from the intermediate Trp.+ and Yb2+ states, the Yb3+ formed may be either in the ground or in the excited state.

4.4

Electroluminescent devices in the near-infrared wavelength range based on lanthanides

4.4.1 Precedents and near infrared emission bands from lanthanides(III) in OLEDs Nd, Er, and Yb are the lanthanides with optical transitions in the NIR that have received more attention as emissive layers in OLEDs. Infrared band emissions from Pr, Tm, Sm, Ho, and Dy have also been studied for OLEDs but to a much lesser extent. In the review on electroluminescent devices based on organic materials with emission in the extreme spectral regions (UV and NIR) by Pushkarev and Bochkarev [79], Ln(III)-complex-based NIR emitters are thoroughly described and classified by their corresponding Ln3+ ions and their different organic complexes, paying attention to the different external efficiencies achieved in each case. In general, many of the observed emission bands are broadened by Stark splitting of the ground level, due to the differences in the local electric field perceived by the lanthanide ion. This splitting is dependent on the surrounding ligands. On the other hand, when transitions from several Ln3+ excited levels are simultaneously observed, their relative intensities vary from one complex to another depending on the ligands that

152

Lanthanide-Based Multifunctional Materials

sensitize the ion. In this case, regarding the relative intensity of the peaks, there may also be no coincidence between PL and EL spectra, reminding us of the different origins (viz., light excitation or carrier transport under an electric field) of the sensitizing process. Many of the lanthanides exhibit intense emission bands in the visible spectral region as described in Chapters 2 and 3, so in this section, emphasis will only be placed on the near-infrared range, from 800 nm onward. NIR emission from Nd3+-based OLEDs usually exhibits three bands, corresponding to 4F3/2!4I9/2, 4F3/2!4I11/2, and 4F3/2!4I13/2 transitions (at 890, 1070, and 1350 nm, respectively), the latter in the O band for optical communications (1260–1360 nm). As noted above, their intensity ratio may change depending on the ligands used. For example, when the emissive layer was formed by specific β-diketone ligands together with a phenantroline derivative, in tris(dibenzoylmethanato)mono (bathophenanthroline)neodymium(III), or [Nd(DBM)3(bath)], the peak at 890 nm was dominant, with almost no emission at 1350 nm (Fig. 4.9A) [108]. In contrast, by using tris(8-hydroxyquinoline)neodymium(III) as the emissive layer, the intensity of the peak at 1070 nm was 3.5 times higher than that of the 890 nm one, with a lower but significant 1350 nm band (Fig. 4.9B) [109]. More recently, EL emission has been

Fig. 4.9 Room-temperature electroluminescence (EL) spectra in the near-infrared region from emissive layers with three different Nd3+ complexes: (A) EL spectrum from an ITO/TPD/Nd (DBM)3bath/Alq3/Mg/Ag OLED at an applied voltage of 19 V, together with the PL spectrum (dotted line) of a deposited film of this complex upon excitation with 390 nm light; (B) EL from an ITO/TPD/NdQ/Al OLED; and (C) EL spectra of an OLED with multilayer structure ITO/ CuPc/[Nd(hfac)3(bipy)]/BCP/Alq3/LiFAl at different operating voltages. Modified from Y. Kawamura, Y. Wada, Y. Hasegawa, M. Iwamuro, T. Kitamura, S. Yanagida, Observation of neodymium electroluminescence, Appl. Phys. Lett. 74 (1999) 3245–3247; O.M. Khreis, R.J. Curry, M. Somerton, W.P. Gillin, Infrared organic light emitting diodes using neodymium tris-8-hydroxyquinoline, J. Appl. Phys. 88 (2000) 777–780; Z. Ahmed, K. Iftikhar, Variant coordination sphere, for efficient photo- and electroluminescence of 0.4–1.8 μm, of lanthanide(III) complexes containing a β-diketone ligand with low vibrational frequency C–F bonds and a flexible 2,20 -bipyridine ligand, Polyhedron 85 (2015) 570–592. Copyright American Institute of Physics (A and B) and Elsevier (C), 2017.

NIR OLEDs based on Ln(III) complexes

153

observed from a Nd3+ complex formed by another β-diketone ligand with low vibrational frequency CdF bonds, namely, tris(hexafluoroacetylacetone)(2,20 -bipyridine)neodymium(III), or [Nd(hfac)3(bipy)], where the intensity of the 1070 nm band was just 1.5-fold that of the band at 890 nm while the peak at 1390 nm narrowed and grew up to half the intensity of that at 1070 nm (Fig. 4.9C) [110]. NIR emission from Er3+-based OLEDs shows a single peak centered in the C-band for optical communications (1530–1565 nm), at around 1540 nm, corresponding to the 4 I13/2!4I15/2 transition. A significant Stark splitting from 1470 to 1570 nm, with a sharp peak at 1532 nm, can be observed [87,111]. The splitting is generally more broadened in case of optical excitation, and this was found to depend also on the excitation wavelength, as depicted in Fig. 4.10 from one of our previous works [87]. Yb3+ has a single resonance level lying at 10,200 cm1 (2F5/2) above the ground state, so NIR emission from Yb3+-based OLEDs shows a single sharp peak in the 975–983 nm range, corresponding to the 2F5/2!2F7/2 transition, with a significant Stark splitting that results in a broadening from 940 to 1050 nm [112,113]. Upon excitation of Sm3+, the radiative deactivation of the excited states finds plenty of possible energy levels that can act as the origin and target. These levels are roughly grouped in two sets separated by a gap. The lowest energy level of the upper group is 4G5/2, with a small energy difference to the closest levels, so a rapid relaxation to this one can be assumed. Thus, radiative recombination from 4G5/2 to energy levels of the lower group finds at least 10 possible emission bands, out of which 7 fall in the NIR spectral range and have been detected by photoexcitation. These correspond to the transitions to 6H13/2 (792 nm), 4F1/2 (877 nm), 4F3/2 or the nearby 6H15/2 (932–934 nm), 4F5/2 (947 nm), 4F7/2 (1030 nm), 4F9/2 (1167 nm), and 4F11/2 (1379 nm) levels [114]. NIR emission from OLEDs with emissive layers based on certain Sm3+ β-diketonate complexes (e.g., tris(dibenzoylmethanato)mono(bathophenanthroline)samarium(III), or [Sm(DBM)3(bath)]) showed a weak although dominant band formed by the four 4G5/2!4F1/2, 4F3/2, 6H15/2, and 4F5/2 transitions in the

4

Normalized emission (au)

I13/2

lexc = 337 nm

4

I15/2

lexc = 980 nm

EL

1400

1500

1600

Wavelength (nm)

1700

Fig. 4.10 PL emission at 1.5 μm (associated with Er3+: 4I13/2!4I15/2 transition) from [Er(tfnb)3(bipy)] complex in powder upon ligand excitation (λexc ¼ 337 nm, red dotted line) and upon Er3+: 4I11/2 level excitation (λexc ¼ 980 nm, black solid line) and EL emission (open blue squares) from the solution-processed OLED [87].

154

Lanthanide-Based Multifunctional Materials

887–953 nm range and emission vestiges from 4G5/2!4F7/2, 4F9/2, and 4F11/2 transitions [115]. Using a β-diketonate complex analogous to that mentioned above for Sm3+ but replacing the central lanthanide with Ho3+, namely, [Ho(DBM)3(bath)], the NIR emission from the OLED device revealed three weak bands: a sharp and dominant one at 980 nm and two at 1178 and 1489 nm, with less than half the intensity of the first one and broadened by Stark splitting, which were attributed to 5F5!5I7, 5I6!5I8, and 5 F5!5I6 transitions, respectively [116]. Other complexes, such as the corresponding tris(8-hydroxyquinolate)holmium(III) or more complex dimeric structures with N,Ochelated ligands, have been studied without detecting infrared bands [113,117]. Blending with a poly(p-phenylene) derivative was also attempted within a larger study involving several lanthanides, delivering a weak band from the 5I6!5I8 transition [118]. There is a need for further work to find appropriate ligands to sensitize Ho3+ NIR emission. Organic Tm3+-based complexes using several β-diketone ligands together with different phenantroline derivatives, namely [Tm(DBM)3(bath)] and [Tm(DBM)3(phen)], have been tested as emissive layers in OLEDs, revealing two weak bands: a sharp and dominant one at 802 nm, attributed to the 3F4!3H6 transition, and another at 1470 nm, attributed to 3F4!3H4 transition, with half the intensity of the former and broadened by Stark splitting (from 1380 to 1520 nm) [116]. It is difficult to sensitize this latter transition in a precise manner. Other complexes, such as the corresponding tris(8-hydroxyquinolate)thulium(III), β-diketonates based on acetylacetonates, or mercaptobenzothiazolates, have been studied detecting only the 800 nm band [117,119]. Dy3+ ions exhibit, as in the case of Sm3+, many energy levels with possible radiative deactivation. In fact, Dy3+-doped glasses are appreciated for their ability to generate white light, since they exhibit two intense emission bands at around 480–500 nm (blue spectral region) and at 580–600 nm (orange spectral region), corresponding to 4 F9/2!6H15/2 and 4F9/2!6H13/2 transitions, respectively. Dy3+ ions embedded in certain inorganic matrices exhibit three main fluorescence bands measured in the near- and mid-infrared spectral range, attributed to the following transitions: (6H9/2,6F11/2)!6H15/2 at 1.34 μm, 6H11/2!6H15/2 at 1.76 μm, and 6H13/2!6H15/2 at 2.86 μm [120]. Several acetylacetone-based β-diketonates with either 1,10phenanthroline or triphenyl phosphine oxide adducts, incorporated into sol-gel matrices, have shown sensitized emission in several infrared bands upon optical excitation, namely, at λ ¼ 836, 937, 1009, 1181, 1296, 1394, 1503, and 1680 nm, which are attributed to the 4F9/2!(6H7/2+6F9/2), 4F9/2!6H5/2, 4F9/2!6F7/2, 4F9/2!6F5/2, (6F11/2+6H9/2)!6H15/2, 4F9/2!6F1/2, 6F5/2!6H11/2, and 6H11/2!6H15/2 f-f transitions, respectively [121]. However, to the best of the authors’ knowledge, electroluminescence response from central Dy3+ ions has only been reported in the visible range [122,123], not in the infrared spectral region. Works reporting NIR emission from Pr3+-based OLEDs are scarce. In fact, to date, only the original work by Hong et al. has shown bands associated to the four expected NIR emissions at 890 nm (1D2!3F2), 1015 nm (1D2!3F3), 1065 nm (1D2!3F4), and 1550 nm (1D2!1G4) [124]. In this report, an emissive layer based on

NIR OLEDs based on Ln(III) complexes

155

[Pr(DBM)3(bath)], which—as mentioned above—has been demonstrated to be a successful sensitizer for other Ln3+ ions, was either used as a single component or coevaporated with TPD hole injector (which improved emission). The band at 1065 nm was dominant, with twice the intensity of the band at 1015 nm. The longest wavelength band (1550 nm) appeared to be broadened by a large splitting, and the 890 nm band exhibited residual intensity. Spectra recorded in the visible range revealed a band at 608 nm (1D2!1H6) and a broad band, from 400 to 700 nm, attributed to exciplex emission. Onset for EL was detected at 7 V, and a brightness of 59 cd m2 was delivered at 17 V. Other works have synthesized Ln(III) compound series exploiting other ligands, such as pentafluorophenolates [125], benzoxazolylphenolates, and benzothiazolylphenolates [113], which produced very efficient electroluminescent devices, but not in the case of Pr3+ that, at best, showed only the band in the visible spectral region.

4.4.2 Recent progress in solution processing and host-guest strategies In Section 4.3.4.2 it was stated that suitable ligands to optimize the emission of lanthanide complexes had to meet many conditions (appropriate sensitizing, carrier transport, thermal stability, and suitable characteristics for layer deposition), whose effects, in many cases, interfered with each other. This is why, despite the large number of ligands explored, the search for the optimal ones to coordinate around any Ln3+ ion is not an easy task, and much research is still necessary in this field. Some of the recent progresses and various strategies to improve the efficiency of OLEDs based on lanthanide complexes will now be reviewed.

4.4.2.1 Solubility and solution-processed OLEDs The activity in the field of new organic Ln(III) complex synthesis, with proposals of emissive layers for OLEDs as proof of concept, is currently very active. Most of the reported devices have been manufactured by vacuum evaporation [79,126,127]. In contrast to this trend, the possibility of handling organic complexes in solution, with the advantages of low-cost, fast thin-film deposition and access to large-area manufacturing, is currently a research issue within the state of the art. Not only NIR OLEDs but also biological analysis and imaging (i.e., bioprobes, covered in Chapter 11) or chemical handling (purification and extension of certain synthetic routes) need soluble complexes and have traditionally prompted research on this field. In this line, efforts have been made in two directions: (i) providing soluble character to the complex ligands in order to achieve direct deposition of thin films by spin coating, and (ii) doping of Ln(III) complexes into soluble matrices, releasing the ligand from the solubility requirement. Those blends used for photophysical studies or stimulated emission purposes may use inert polymers like poly(methyl methacrylate) or polystyrene, not paying attention to the polymer-conductive character, but those pursuing electronic device performance must focus on conducting conjugated materials.

156

Lanthanide-Based Multifunctional Materials

Some works have focused on endowing solubility to certain Ln(III) complexes to make their photophysical characterization easier, to enable bioimaging, or just to promote the formation of homogeneous thin films. For example, the synthesis of a ligand containing a single phenanthroline (phen) chromophore and a flexibly connected diethylenetriamine tetracarboxylic acid unit as a lanthanide coordination site has been reported to produce series of water-soluble Ln(III) complexes (Ln ¼ Sm, Dy, Pr, Ho, Yb, Nd, and Er) with remarkable NIR luminescence efficiencies [128]. Tetradentate and octadentate ligand design strategies for Yb3+ and Nd3+ by the incorporation of the 1-methyl-3-hydroxy-pyridin-2-one chelate group (Yb,Nd-3,2-HOPO) have delivered overall quantum yields of 0.22% in aqueous solution [129]. Recent works in the field of Er3+- and Yb3+-based porphyrinoids have made some porphyrazines soluble in organic solvents (DMF and chloroform) by the introduction of various aromatic phenyl rings into the periphery of the molecule, which was also accompanied by a bathochromic shift of a specific absorption band by 21 nm [130]. Likewise, a new porpholactone, which turned out to be a good sensitizer of Yb3+ and resulted in a good NIR-emitting complex, was made water-soluble by attaching a glucose moiety through the substitution of para-F atoms during its synthesis, to obtain a bioprobe for glucose oxidase detection [131]. In the 1990s, it was observed that a number of β-diketone ligands (e.g., hexafluoroacetylacetone, Hfac) could provide Ln(III) complexes with a sufficient degree of solubility in organic solvents (methanol, THF, and chloroform) to perform photophysical measurements and to demonstrate the absence of the expected quenching usually attributed to solvent molecules [82,83]. However, this characterization was carried out in highly diluted solutions (105 M) that were far from the high concentrations (e.g., 10 mg mL1) required for thin-layer deposition by spin coating. Some works can be found where photophysical characterization of the synthesized [Ln(hfac)3(bipy)] complexes was performed in chloroform solution, but NIR OLEDs were manufactured by vacuum evaporation [110]. It should be highlighted that NIR OLEDs with structure ITO/CuPc (15 nm)/[Ln(hfac)3(bipy)]/CBP (50 nm)/BCP (20 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm) (where Ln ¼ Yb, Nd, and Er) have been manufactured, showing maximum NIR irradiances and maximum external quantum efficiencies (ηex) of 28 μW cm2 and 0.022% for Nd3+, 0.50 μW cm2 and 0.011% for Er3+, and 93 μW cm2 and 0.18% for Yb3+, which indicate highly improved EL performance over previously reported devices. Just a few works have focused on solution-processed “complex-only” OLEDs (i.e., without resorting to a host-guest system). For example, Martı´n-Ramos et al., throughout different works, used β-diketones and N,N-donors as ancillary ligands to synthesize novel Er3+ complexes with structure [Er(β-diketonate)3(N,N-donor)], where β-diketones were 4,4,4-trifluoro-1-(2-naphthyl)-1,3-butanedione (Htfnb or Hntac, according to Binnemans’ notation [68]), 1,1,1-trifluoro-5,5-dimethyl-2,4hexanedione (Htmp), or 1,1,1-trifluoro-2,4-pentanedione (Htfac), whereas N,N-donor adducts were 2,20 -bipyridine, bathophenanthroline, or 5-nitro-1,10-phenanthroline. These Er3+ complexes exhibited good film-forming properties upon deposition from methanol solutions, although it should be noted that solute concentrations of about 4 wt% in this solvent were used to achieve layers as thick as 100 nm. This group

NIR OLEDs based on Ln(III) complexes

157

reported OLEDs with structure glass/ITO/PEDOT:PSS/Er3+ complex/Ca/Al with EL emission maxima at 1530–1540 nm and onset values for EL ranging from 4.5 V for a [Er(tfac)3(bath)] emissive layer, to 6.5 V for [Er(tfnb)3(bipy)], to 17.5 V for [Er (tfac)3(5NO2phen)] [85–87]. NIR EL from solution-processed, host-free OLEDs has also been demonstrated with emissive layers based on Ln(III) 9-9-anthracenates, [Ln(ant)3] (Ln ¼ Yb and Nd), recently reported by Utochnikova et al. [132]. Solubility of anthracenates was found to be almost negligible in water and common organic solvents, except for strong donors such as DMSO and DMF. Eventually, by spin coating of these [Ln(ant)3] complexes from a DMSO solution, OLEDs with structure ITO/PEDOT:PSS/[Yb(ant)3] (30 nm)/ HBL (10 nm)/Al were manufactured. In this case, the chosen HBL were well-known hole-blocking layers such as 1,3,5-benzinetriyl-tris(1-phenyl-1-benzimidazole) (TPBi) or 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butyl-phenyl)-1,2,4-triazole (TAZ) (mentioned in Section 4.2).

4.4.2.2 Host-guest strategies Doping organic conjugated compounds (either polymers or molecules) with Ln(III) complexes to form a blended layer is an easier approach than the direct deposition of complexes in solution, owing to the good film-forming properties of many of such conjugated compounds. Commercial fluorescent conjugated materials may also provide reasonable charge-transport properties and very good emission at short wavelengths suitable to excite the absorbance of many ligands. The price in terms of efficiency comes from adding a new energy-transfer step from the host to the dopant. Under electric excitation, electrons and holes may be injected into both the host and the Ln(III) complex. Those charges that combine to form singlet and triplet excitons in the host can then decay radiatively or undergo F€ orster or Dexter transfer to the triplet sensitizer, provided that its HOMO or LUMO is suitably tuned. This means that the guest energy gap should be similar or lower than that of the host. Further, some charges may be directly trapped in the complex ligand, as in “complex-only” devices, and eventually may combine with oppositely charged species to form singlets and triplets. Among all these possibilities, host-to-guest energy transfer by F€orster mechanism is considered dominant, since the host emission is usually dramatically quenched when it is suitably tuned with the absorption of the guest material. Moreover, in a polymeric environment, F€ orster transfer is expected to be three orders of magnitude or so faster than Dexter transfer [133]. A number of works have attempted to improve OLEDs with Ln(III)-based emissive layers in the visible spectral range with the host-guest strategy (focusing on Eu3+ or Tb3+), but just a few works have focused on the NIR spectral range. One of the first NIR OLEDs based on a host-guest strategy was reported by Slooff et al. The emissive layer consisted of a lissamine-functionalized terphenyl-based Nd3+ complex (LsNd3+) blended with the conjugated polymer poly(dioctylfluorene-co-benzothiadiazole), F8BT, in a dopant-to-polymer weight ratio of
1:10 [134]. The precursor solution for the blend was in chloroform at 0.5 wt%. The device structure was ITO/PEDOT: PSS (40 nm)/F8BT/LsNd3+ (80 nm)/Ca/Al, which showed an onset for EL of

158

Lanthanide-Based Multifunctional Materials

14 V. The F8BT fluorescence matched very closely the absorption band of LsNd3+ and was totally quenched, but the energy transfer between the lissamine ligand and Nd3+ was not very optimized, since the EL was dominated by the ligand emission (at λ ¼ 580 nm) with a 20 times higher intensity than that arising from Nd3+ (at λ ¼ 890 nm). Nonetheless, the authors noted that this intensity ratio was larger than that obtained by photoexcitation of the solid film. NIR EL from an emissive layer consisting of Nd(9-hydroxyphenalen-1-one)32H2O complex dispersed within a poly(N-vinylcarbazole) (PVK) matrix was reported by O’Riordan et al. [106,135]. PVK is a nonconjugated hole-conducting polymer, with a not very efficient fluorescence (peaking at 410 nm), although its emission matched well with the absorption band of the Nd3+ complex. Nd(9hydroxyphenalen-1-one)32H2O and PVK were mixed in a 1:1 ratio and codissolved in DMF at 2.8 wt%, in which the guest material had also shown good solubility. The ITO/PEDOT:PSS/PVK/Nd(9-hydroxyphenalen-1-one)3/Ca/Al devices exhibited a totally quenched luminescence from PVK and the characteristic Nd3+ emission centered at 1065 nm (the spectrum in the near-infrared region was recorded only in the 1000–1200 nm range), with a maximum irradiance of 8.5 nW mm2 and an external quantum efficiency of 0.007%. The onset for EL was very high (>30 V) revealing not only the poor conductivity of PVK but also a possible trap effect for the carriers from the Ln(III) complex. In a similar work, the authors dissolved an NIR-emitting neodymium tetrakis complex, [Nd(5,7-dichloro-8-hydroxyquinoline)4][NEt4] (where NEt4 is a tetraethylammonium counter ion), in a PVK matrix, with the same characteristics as the solution discussed above [135]. In this case, it was also expected that F€orster energy transfer from the host polymer to the quinoline ligands—and thus sensitization of the lanthanide ions—would be possible due to the spectral overlap between the emission from PVK and absorption by the complex. Nonetheless, Dexter transfer was also considered to be feasible since the T1 level of PVK is higher in energy than the T1 level of the quinoline ligands. The ITO/PEDOT:PSS/PVK/Ndquinoline/Ca/Al OLEDs showed a turn-on voltage of 12 V and delivered a NIR external quantum efficiency of 1103% with a NIR irradiance of 2.0 nW mm2 at 25 mA mm2 and 20 V. A commercial hole-transport molecule, 1,3-bis(9-carbazolyl)benzene (mCP), has been used as a host for the already known NIR emitter tris(thenoyltrifluoroacetone)mono(1,10-phenanthroline)neodymium(III), [Nd(TTA)3(phen)], to form the emissive layer of NIR-emitting OLEDs with structure ITO/PEDOT:PSS (30 nm)/mCP/[Nd (TTA)3(phen)] (60 nm)/TPBi (40 nm)/LiF/Al [136]. The concentration (x) of [Nd (TTA)3(phen)] in the mCP host was varied as x ¼ 7, 13, 16, and 20 wt%, and both compounds were codissolved in chloroform at a concentration of 10 mg mL1. As a result, the EL spectra showed the three Nd3+ infrared bands with an onset voltage of 12 V. The dominant band corresponded to 4F3/2!4I11/2 transition at 1060 nm, with the one at 890 nm (4F3/2!4I9/2) showing half the intensity and that at 1330 barely observable. The mCP host emission was not totally quenched until x ¼ 20 wt%. The external quantum efficiency value was noticeably higher than the best values reported in solution-processed Nd3+ host OLEDs (0.001 and 0.007%) [106] and was

NIR OLEDs based on Ln(III) complexes

159

comparable with the best values reported for thermally processed devices (0.02%– 0.04%) [137,138].

4.4.2.3 Challenging paths for NIR OLEDs based on Ln(III) complexes In this subsection, some of the most daring works that opened new paths to exploit lanthanide-based NIR OLEDs are covered. An interesting work by Zang et al. faced the co-emission of different infrared bands, in particular from Nd3+ and Er3+, by manufacturing a double emissive layer [139]. The complexes used exhibited the same ligand structure, the well-known [Nd(DBM)3(bath)] and [Er(DBM)3(bath)] (identified in Section 4.4.1), which has the experimentally observed ability of sensitizing transitions of several Ln3+ ions (viz., Nd3+, Er3+, Sm3+, and Ho3+). The authors tested other ligand combinations that were not so successful. OLEDs with structure ITO/TPD (60 nm)/[Nd(DBM)3(bath)] (x nm)/[Er(DBM)3(bath)] (60–x nm)/Mg/Ag were manufactured by thermal evaporation with different thicknesses of the emissive sublayers (x ¼ 1, 5, 15, and 20 nm), where the double emissive layer also acted as the electron-transport/electron-injection layer. In the recorded spectral range, emissions at 1064 nm and 1.3 μm from Nd3+ and at 1.54 μm from Er3+ were measured with an onset voltage of 6.5 V (Fig. 4.11A). The dramatic increase in the Nd3+ bands even for narrow thicknesses revealed that carrier recombination took place close to the TPD/Nd(III) complex interface. By selecting the structure with x ¼ 5 nm, the relative intensity of the bands was modulated from 0 to 1 between 6.5 and 18 V (Fig. 4.11B). One of the most efficient NIR OLEDs based on a Ln(III) complex included an emissive layer consisting of a coevaporated blend (with 1:1 M ratio) of an Ir(III)

0.8 0.6 0.4

(A)

5 10 15 20 0 Nd(DBM)3bath thickness (nm)

Nd3+ 4F3/2-4l13/2

1100

1200 1300 1400 Wavelength (nm)

1.0 0.8

1500

0.6

6.5 V 9V 12 V 18 V

0.4 0.2

1350 nm

0.2 0.0 1000

12 10 8 6 4 2 0

3+ 4I 4 13/2- l15/2 20 nm Er 15 nm 1540 nm 5 nm 1 nm

Intensity (a.u.)

1.0

1064 nm

1.5mm intensity (a.u.)

EL intensity (a.u.)

1.2 Nd3+ 4F -4l 3/2 11/2

1600

0.0 1000

(B)

1200 1400 Wavelength (nm)

1600

Fig. 4.11 Near-infrared electroluminescence from an OLED design to obtain simultaneous emission from Nd3+ and Er3+ complexes. (A) Spectra recorded at 12 V from structures with different emissive layer thicknesses of the Nd3+ complex (1, 5, 15, and 20 nm). Er3+ intensity ratio for each case is quantified and plotted in the inset. (B) Spectra recorded from that structure with Nd3+ complex thickness of 5 nm at different voltages. Modified from F. Zang, T.C. Sum, F. Zhu, Z.R. Hong, X. Sun, W.L. Li, C.H.A. Huan, Modulated infrared electroluminescence from organic light-emitting diodes, J. Lightwave Technol. 27 (2009) 1522–1526. Copyright Institute of Electrical and Electronics Engineers.

160

Lanthanide-Based Multifunctional Materials

complex and [Nd(pyrazolonate)3(bath)] or [Nd(pyrazolonate)3(TPPO)2] (where pyrazolonate is 1-phenyl-3-methyl-4-isobutyryl-5-pyrazolone and TPPO is the known electron-transporting group triphenylphosphine oxide) [138]. Two Ir(III) complexes were tested: Ir(III)bis(2-(40 ,60 -difluorophenyl)-pyridinato-N,C20 )(5-fluoro2-(pyrimidin-2-yl)-benzo[d]imidazole) and Ir(III)bis(2-phenyl-pyridinato-N,C20 ) (5-fluoro-2-(pyrimidin-2-yl)-benzo[d]imidazole). Both compounds were known to involve suitable triplet energy levels for sensitizing the Nd3+ ion. The OLED multilayer was deposited by thermal evaporation with the following structure: ITO/NPB (30 nm)/emitting layer (40 nm)/BCP (10 nm)/Alq3 (30 nm)/Mg0.9Ag0.1 (200 nm)/Ag (80 nm). A maximum irradiance of 6.1 μW cm2 at 24 V and 40 mA cm2 was obtained for the main Nd3+ emission wavelength (1060 nm), and an outstanding external quantum efficiency of 0.3% was claimed. The search for compatibility between silicon-based electronics and emitters in the C-band for optical communications (1530–1565 nm) has traditionally been a hot topic too. Qin et al. took advantage of p-doped silicon wafers to act as the anode in the fabrication of Er(III)-complex-based NIR OLEDs [140,141]. An attempt was made to remove as much as possible the thin native oxide from the top of the p-doped Si, and next, NPB was evaporated as the hole-injector/hole-transport layer. The emissive layers tested were coevaporated blends of commercial electron-transport molecules and known Er3+ β-diketonates: (i) Alq3/[Er(acac)3] [140] and (ii) Bphen/[Er (DBM)3(phen)] [141]. In the first case, OLEDs with structure Si/p+/SiO2 (1.5 nm)/ NPB (60 nm)/Alq3 (5 nm)/Alq3/[Er(acac)3] (10 nm)/Alq3 (45 nm)/Sm (15 nm)/Au (15 nm) were prepared. Different mass ratios of Alq3/[Er(acac)3] were tested, and the optimum results were recorded for 1:60, which delivered 0.1 cd A1 at 10 mA cm2. In the second case, OLEDs with structure Si/p+/SiO2 (1.5 nm)/NPB (60 nm)/Bphen/[Er(DBM)3(phen)] (20 nm)/ETL (40 nm)/Sm (15 nm)/Au (15 nm) were manufactured. Different ETLs were tested using Alq3, Bphen, and its blends with Cs2CO3. The results of type (ii) OLEDs were better than those from case (i), delivering a maximum irradiance of 0.93 μW cm2 at 12.5 V and 635 mA cm2, with a voltage onset for EL of 7.5 V. Owing to the capability of incorporating chiral ligands, some Ln(III) complexes may behave as excellent candidates for polarized light emitters (discussed in detail in Chapter 5). Recently, Zinna et al. have demonstrated the highest circularly polarized EL recorded from an OLED, based on a chiral Eu(III) complex and processed in solution [142]. Although the emission was in the visible range, the results obtained could be easily extended to NIR emission using suitable Ln3+ ions. Details on the multilayer architecture and strategies used in this device will be discussed in Chapter 5.

4.5

Conclusions

Since the late 1990s, the successful inclusion of molecular complexes of Ln3+ ions as emissive layers in OLEDs has allowed extending their emission to the infrared spectral range beyond 900 nm. Furthermore, the recorded peaks inherit the expected narrow bandwidths associated to lanthanide transitions between 4f orbitals. To date, most

NIR OLEDs based on Ln(III) complexes

161

of the predicted radiative transitions in the infrared region from Nd3+, Er3+, Yb3+, Pr3+, Tm3+, Sm3+, and Ho3+ have been detected using OLEDs. Among them, devices based on Nd3+, Er3+, and Yb3+ complexes have been the most studied due to their emission in bands of interest for telecommunications or, in the case of Yb3+, to have just a single band precisely located in this range. The radiative Ln3+ f-f transitions are actually characterized as being spinforbidden, so one has to resort to the strategy of indirect sensitization (via antenna effect) to get emission. Accordingly, a variety of different ligands, among which the β-diketonate family is the most popular, have been tested to sensitize the Ln3+ emission in complexes for OLEDs. This technology imposes additional requirements, particularly the inclusion of adducts with transport properties, which in turn influences the antenna effect in a way difficult to predict. Thus, the design and proposal of new suitable ligands is currently a very active field. Although lanthanides are heavy atoms, a significant part of the interesting molecules has a degradation temperature high enough to be deposited by thermal evaporation. Most of the Ln(III)-based OLEDs have been manufactured by this method. In fact, the ability of this technique to grow multilayer stacks with suitable carrier transport and injection layers has eventually delivered the highest external quantum efficiency achieved for an NIR OLED, around 0.3%, although most of the Ln(III)-based devices exhibit values <0.1% (the longer the wavelength, the lower the efficiency) in the infrared spectral range. Only a few works have focused on complexes processed in solution, not as much by designing new soluble ligands, as by taking advantage of the moderate solubility of certain complexes in specific solvents. An alternative that may partially overcome the solubility problem is to dope soluble matrices with Ln(III) complexes, forming the so-called host-guest system. These matrices must in turn provide good transport properties. This strategy generally implies an additional decrease in the efficiency owing to the necessary energy transfer from the host to the guest complex. In summary, combining good transport properties, solubility, and high sensitizing ability in a single Ln(III) complex is today one of the frontiers for the state of the art in NIR OLEDs. The efficiencies achieved (<0.3%) are still far from those recorded from visible-emitting devices, but there are still plenty of possible structures to be explored. New applications such as the emission of polarized infrared light or tuning the emitted wavelength between that of different lanthanides are among the exciting possibilities of this novel field.

References [1] J. Dresner, Double injection electroluminescence in anthracene, RCA Rev. 30 (1969) 322–334. [2] A. Kraft, A.C. Grimsdale, A.B. Holmes, Electroluminescent conjugated polymers— seeing polymers in a new light, Angew. Chem. Int. Ed. 37 (1998) 402–428. [3] H. Suzuki, Organic light-emitting materials and devices for optical communication technology, J. Photochem. Photobiol., A 166 (2004) 155–161.

162

Lanthanide-Based Multifunctional Materials

[4] M.-S. Liao, S. Scheiner, Electronic structure and bonding in metal phthalocyanines metal-Fe Co Ni Cu Zn Mg, J. Chem. Phys. 114 (22) (2001) 9780–9791. [5] I.G. Hill, A. Kahn, Combined photoemission/in vacuo transport study of the indium tin oxide/copper phthalocyanine/N,N0 -diphenyl-N,N0 -bis(l-naphthyl)1,10 biphenyl-4,4 diamine molecular organic semiconductor system, J. Appl. Phys. 86 (4) (1999) 2116–2122. [6] N. Koch, S. Duhm, J.P. Rabe, A. Vollmer, R.L. Johnson, Optimized hole injection with strong electron acceptors at organic-metal interfaces, Phys. Rev. Lett. 95 (2005) 237601. [7] (a) M. Eslamian, Spray-on thin film PV solar cells: advances, potentials and challenges, CoatingsTech 4 (1) (2014) 60–84; (b) S. Khan, L. Lorenzelli, R.S. Dahiya, Technologies for printing sensors and electronics over large flexible substrates: a review, IEEE Sensors J. 15 (6) (2015) 3164–3185. [8] Q. Wang, Y. Xie, F. Soltani-Kordshuli, M. Eslamian, Progress in emerging solutionprocessed thin film solar cells—part I: polymer solar cells, Renew. Sust. Energ. Rev. 56 (2016) 347–361. [9] F.C. Krebs, Polymer solar cell modules prepared using roll-to-roll methods: knife-overedge coating, slot-die coating and screen printing, Sol. Energy Mater. Sol. Cells 93 (4) (2009) 465–475. [10] S.A. Mauger, L. Chang, C.W. Rochester, A.J. Moule, Directional dependence of electron blocking in PEDOT:PSS, Org. Electron. 13 (2012) 2747–2756. [11] B. Liu, G.C. Bazan, Conjugated Polyelectrolytes: Fundamentals and Applications, Wiley-VCH Verlag & Co. KGaA, Weinheim, 2013. [12] H.Y. Shin, M.C. Suh, A study on full color organic light emitting diodes with blue common layer under the patterned emission layer, Org. Electron. 15 (2014) 2932–2941. [13] T. Matsumoto, T. Yoshinaga, T. Higo, T. Imai, T. Hirano, T. Sasaoka, High-performance solution-processed OLED enhanced by evaporated common layer, J. Soc. Inf. Disp. 42 (2011) 924–927. [14] I. Hamberg, G.C. Granqvist, Evaporated Sn-doped In2O3 films: basic optical properties and applications to energy-efficient windows, J. Appl. Phys. 60 (1986) R123–R159. [15] K. Ellmer, Past achievements and future challenges in the development of optically transparent electrodes, Nat. Photonics 6 (2012) 809–817. [16] X. Jiang, Aluminum-doped zinc oxide films as transparent conductive electrode for organic light-emitting devices, Appl. Phys. Lett. 83 (2003) 1875–1877. [17] M.A. Baldo, D. O’brien, Y. You, A. Shoustikov, S. Sibley, M. Thompson, S. Forrest, Highly efficient phosphorescent emission from organic electroluminescent devices, Nature 395 (1998) 151–154. [18] A. Endo, M. Ogasawara, A. Takahashi, D. Yokoyama, Y. Kato, C. Adachi, Thermally activated delayed fluorescence from Sn4+-porphyrin complexes and their application to organic light emitting diodes—a novel mechanism for electroluminescence, Adv. Mater. 21 (2009) 4802–4906. [19] Z. Zhao, S. Chen, J.W.Y. Lam, C.K.W. Jim, C.Y.K. Chan, Z. Wang, P. Lu, C. Deng, H. S. Kwok, Y. Ma, B.Z. Tang, Steric hindrance, electronic communication, and energy transfer in the photo- and electroluminescence processes of aggregation-induced emission luminogens, J. Phys. Chem. C 114 (2010) 7963–7972. [20] D.A. McQuarrie, J.D. Simon, Physical Chemistry, A Molecular Approach, University Science Books, Sausalito, CA, 1997. [21] N.J. Turro, Modern Molecular Photochemistry, University Science Book, Sausalito, CA, 1991. [22] B. Valeur, M.N. Berberan-Santos, Molecular Fluorescence: Principles and Applications, Wiley-VCH, Weinheim, 2002.

NIR OLEDs based on Ln(III) complexes

163

[23] Y. Ma, H. Zhang, J. Shen, C. Che, Electroluminescence from triplet metal—ligand chargetransfer excited state of transition metal complexes, Synth. Met. 94 (1998) 245–248. [24] H. Yersin (Ed.), Highly Efficient OLEDs With Phosphorescent Materials, Wiley-VCH, Weinheim, 2008. [25] A. Buckley (Ed.), Organic Light-Emitting Diodes: Materials, Devices and Applications, Woodhead, Cambridge, 2013. [26] Y. Chi, P.T. Chou, Transition-metal phosphors with cyclometalating ligands: fundamentals and applications, Chem. Soc. Rev. 39 (2010) 638–655. [27] P.T. Chou, Y. Chi, M.W. Chung, C.C. Lin, Harvesting luminescence via harnessing the photophysical properties of transition metal complexes, Coord. Chem. Rev. 255 (2011) 2653–2665. [28] C.L. Ho, W.-Y. Wong, Small-molecular blue phosphorescent dyes for organic lightemitting devices, New J. Chem. 37 (2013) 1665–1683. [29] L. Huang, C.D. Park, T. Fleetham, J. Li, Platinum (II) azatetrabenzoporphyrins for nearinfrared organic light emitting diodes, Appl. Phys. Lett. 109 (23) (2016) 233302. [30] S. Kesarkar, W. Mro´z, M. Penconi, M. Pasini, S. Destri, M. Cazzaniga, D. Ceresoli, P.R. Mussini, C. Baldoli, U. Giovanella, A. Bossi, Near-IR emitting iridium(III) complexes with heteroaromatic β-diketonate ancillary ligands for efficient solutionprocessed OLEDs: structure-property correlations, Angew. Chem. Int. Ed. Eng. 55 (8) (2016) 2714–2718. [31] F. Nisic, A. Colombo, C. Dragonetti, D. Roberto, A. Valore, J.M. Malicka, M. Cocchi, G.R. Freeman, J.A.G. Williams, Platinum(II) complexes with cyclometallated 5-pi-delocalized-donor-1,3-di(2-pyridyl)benzene ligands as efficient phosphors for NIR-OLEDs, J. Mater. Chem. C 2 (10) (2014) 1791–1800. [32] Z.Q. Chen, Z.Q. Bian, C.H. Huang, Functional Ir-III complexes and their applications, Adv. Mater. 22 (13) (2010) 1534–1539. [33] C. Adachi, M.A. Baldo, M.E. Thompson, S.R. Forrest, Nearly 100% internal phosphorescence efficiency in an organic light-emitting device, J. Appl. Phys. 90 (2001) 5048. [34] L. Xiao, Z. Chen, B. Qu, J. Luo, S. Kong, Q. Gong, J. Kido, Recent progresses on materials for electrophosphorescent organic light-emitting devices, J. Adv. Mater. 23 (2011) 926–952. [35] M. Sassi, N. Buccheri, M. Rooney, C. Botta, F. Bruni, U. Giovanella, S. Brovelli, L. Beverina, Near-infrared roll-off-free electroluminescence from highly stable diketopyrrolopyrrole light emitting diodes, Sci. Report. 6 (2016) 34096. [36] M. Klessinger, J. Michl, Excited States and Photochemistry of Organic Molecules, VCH, Weinheim, 1995. [37] S.P. Wang, X.J. Yan, Z. Cheng, H.Y. Zhang, Y. Liu, Y. Wang, Highly efficient nearinfrared delayed fluorescence organic light emitting diodes using a phenanthrene-based charge-transfer compound, Angew. Chem. Int. Ed. 54 (2015) 13068–13072. [38] M.T. Sharbati, F. Panahi, A. Gharavi, Near-infrared organic light-emitting diodes based on donor-pi-acceptor oligomers, IEEE Photon. Technol. Lett. 22 (22) (2010) 1695–1697. [39] M.T. Sharbati, F. Emami, Efficient NIR emission from organic light-emitting devices based on acceptor–donor–acceptor (A–D–A) and donor–acceptor–donor (D–A–D) oligomers, Opt. Express 19 (4) (2011) 3619–3626. [40] M.T. Sharbati, F. Panahi, A. Shourvarzi, S. Khademi, F. Emami, Near-infrared electroluminescence from organic light emitting diode based on imine oligomer with low turn on voltage, Optik–Int. J. Light Electron. Opt. 124 (1) (2013) 52–54. [41] Y.X. Yang, R.T. Farley, T.T. Steckler, S.H. Eom, J.R. Reynolds, K.S. Schanze, J.G. Xue, Efficient near-infrared organic light-emitting devices based on low-gap fluorescent oligomers, J. Appl. Physiol. 106 (4) (2009) 044509.

164

Lanthanide-Based Multifunctional Materials

[42] G. Qian, B. Dai, M. Luo, D. Yu, J. Zhan, Z. Zhang, D. Ma, Z.Y. Wang, Band gap tunable, donor-acceptor-donor charge-transfer heteroquinoid-based chromophores: near infrared photoluminescence and electroluminescence, Chem. Mater. 20 (2008) 6208–6216. [43] G. Qian, Z. Zhong, M. Luo, D. Yu, Z. Zhang, Z.Y. Wang, D. Ma, Simple and efficient near-infrared organic chromophores for light-emitting diodes with single electroluminescent emission above 1000 nm, Adv. Mater. 21 (2009) 111–116. [44] T. Mori, Organic conductors with unusual band fillings, Chem. Rev. 104 (2004) 4947–4970. [45] P. Bauer, H. Wietasch, S.M. Lindner, M. Thelakkat, Synthesis and characterization of donor-bridge-acceptor molecule containing tetraphenylbenzidine and perylene bisimide, Chem. Mater. 19 (1) (2007) 88–94. [46] G. Sonmez, H. Meng, F. Wudl, Very stable low band gap polymer for charge storage purposes and near-infrared applications, Chem. Mater. 15 (2003) 4923–4929. [47] S.A. Jenekhe, J.A. Osaheni, Excimers and exciplexes of conjugated polymers, Science 265 (1994) 765–768. [48] Z.R. Grabowski, K. Rotkiewicz, Structural changes accompanying intramolecular electron transfer: focus on twisted intramolecular charge-transfer states and structures, Chem. Rev. 103 (2003) 3899–4032. [49] J.V. Caspar, E.M. Kober, B.P. Sullivan, T.J. Meyer, Application of the energy gap law to the decay of charge-transfer excited states, J. Am. Chem. Soc. 104 (1982) 630–632. [50] O. Fenwick, J.K. Sprafke, J. Binas, D.V. Kondratuk, F.D. Stasio, H.L. Anderson, F. Cacialli, Linear and cyclic porphyrin hexamers as near-infrared emitters in organic light-emitting diodes, Nano Lett. 11 (2011) 2451–2456. [51] S. Ellinger, K.R. Graham, P. Shi, R.T. Farley, T.T. Steckler, R.N. Brookins, P. Taranekar, J. Mei, L.A. Padilha, T.R. Ensley, H. Hu, S. Webster, D.J. Hagan, E.W.V. Stryland, K.S. Schanze, J.R. Reynolds, Donor–acceptor–donor-based π-conjugated oligomers for nonlinear optics and near-IR emission, Chem. Mater. 23 (2011) 3805–3817. [52] C.C. Ho, H.F. Chen, Y.C. Ho, C.T. Liao, H.C. Su, K.T. Wang, Phosphorescent sensitized fluorescent solid-state near-infrared light-emitting electrochemical cells, Phys. Chem. Chem. Phys. 13 (2011) 17729–17736. [53] C. Borek, K. Hanson, P.I. Djurovich, M.E. Thompson, K. Aznavour, R. Bau, Y. Sun, S.R. Forrest, J. Brooks, L. Michalski, J. Brown Dr, Highly efficient, near-infrared electrophosphorescence from a Pt–metalloporphyrin complex, Angew. Chem. 119 (2007) 1127–1130. [54] J.R. Sommer, A.H. Shelton, A. Parthasarathy, I. Ghiviriga, J.R. Reynolds, K.S. Schanze, Photophysical properties of near-infrared phosphorescent π-extended platinum porphyrins, Chem. Mater. 23 (2011) 5296–5304. [55] K.R. Graham, Y. Yang, J.R. Sommer, A. Shelton, K.S. Schanze, J. Xue, J.R. Reynolds, Extended conjugation platinum(II) porphyrins for use in near-infrared emitting organic light emitting diodes, Chem. Mater. 23 (2011) 5305–5312. [56] K.Y. Cheng, R. Anthony, U.R. Kortshagen, R.J. Holmes, Hybrid silicon nanocrystalorganic light-emitting devices for infrared electroluminescence, Nano Lett. 10 (2010) 1154–1157. [57] (a) M.T. Sharbati, M.N.S. Rad, S. Behrouz, A. Gharavi, F. Emami, Near infrared organic light-emitting diodes based on acceptor–donor–acceptor (ADA) using novel conjugated isatin Schiff bases, J. Lumin. 131 (2011) 553–558; (b) G. Qian, Z. Zhong, M. Luo, D. Yu, Z. Zhang, Z.Y. Wang, D. Ma, Simple and efficient near-infrared organic chromophores for light-emitting diodes with single electroluminescent emission above 1000nm, Adv. Mater. 21 (2009) 111–116.

NIR OLEDs based on Ln(III) complexes

165

[58] Y. Hong, J.W.Y. Lam, B.Z. Tang, Aggregation-induced emission, Chem. Soc. Rev. 40 (11) (2011) 5361–5388. [59] J. Mei, Y. Hong, J.W. Lam, A. Qin, Y. Tang, B.Z. Tang, Aggregation-induced emission: the whole is more brilliant than the parts, Adv. Mater. 26 (31) (2014) 5429–5479. [60] X.B. Du, J. Qi, Z.Q. Zhang, D.G. Ma, Z.Y. Wang, Efficient non-doped near infrared organic light-emitting devices based on fluorophores with aggregation-induced emission enhancement, Chem. Mater. 24 (11) (2012) 2178–2185. [61] S. Comby, J.C.G. B€unzli, Lanthanide near-infrared luminescence in molecular probes and devices, in: K.A. Gschneidner, J.C.G. B€unzli, V.K. Pecharsky (Eds.), Handbook on the Physics and Chemistry of Rare Earths, vol. 37, (Chapter 235), Elsevier, Amsterdam, 2007, p. 217. [62] W.C. Chan, S. Nie, Quantum dot bioconjugates for ultrasensitive nonisotopic detection, Science 281 (1998) 2016–2018. [63] M. Han, X. Gao, J.Z. Su, S. Nie, Quantum-dot-tagged microbeads for multiplexed optical coding of biomolecules, Nat. Biotechnol. 19 (2001) 631–635. [64] H. Uh, S. Petoud, Novel antennae for the sensitization of near infrared luminescent lanthanide cations, C. R. Chim. 13 (2010) 668–680. [65] S. Petoud, S.M. Cohen, J.C.G. Bunzli, K.N. Raymond, Stable lanthanide luminescence agents highly emissive in aqueous solution: multidentate 2-hydroxyisophthalamide complexes of Sm3+, Eu3+, Tb3+, Dy3+, J. Am. Chem. Soc. 125 (2003) 13324–13325. [66] J. Zhang, P.D. Badger, S.J. Geib, S. Petoud, Sensitization of near-infrared-emitting lanthanide cations in solution by tropolonate ligands, Angew. Chem. Int. Ed. 44 (2005) 2508–2512. [67] E. Brunet, O. Juanes, J.C. Rodriguez-Ubis, Supramolecularly organized lanthanide complexes for efficient metal excitation and luminescence as sensors in organic and biological applications, Curr. Chem. Biol. 1 (1) (2007) 11–39. [68] K. Binnemans, Lanthanide-based luminescent hybrid materials, Chem. Rev. 109 (2009) 4283–4374; K. Binnemans, Rare-earth beta-diketonates. In Handbook on the Physics and Chemistry of Rare Earths; K. A. Gschneidner, J. C. G. B€ unzli, V. K. Pecharsky (Eds.); Elsevier: Amsterdam, 2005; vol. 35, (chapter 225), p. 107 [69] S.I. Weissman, Intramolecular energy transfer—the fluorescence of complexes of europium, J. Chem. Phys. 10 (1942) 214–217. [70] J.M. Lehn, Perspectives in supramolecular chemistry—from molecular recognition towards molecular information processing and self-organization, Angew. Chem. Int. Ed. Eng. 29 (1990) 1304–1319. [71] (a) J. Kido, Y. Okamoto, Organo lanthanide metal complexes for electroluminescent materials, Chem. Rev. 102 (2002) 2357–2368; (b) G.A. Crosby, R.E. Whan, R.M. Alire, Intramolecular energy transfer in rare earth chelates. role of the triplet state, J. Chem. Phys. 34 (1961) 743; (c) M.L. Bhaumik, M.A. El-Sayed, Studies on the triplettriplet energy transfer to rare earth chelates1a, J. Phys. Chem. 69 (1965) 275. [72] F.J. Steemers, W. Verboom, C.N. Reinhoudt, E.B. Vandertol, J.W. Verhoeven, New sensitizer-modified calix[4]arenes enabling near-UV excitation of complexed luminescent lanthanide ions, J. Am. Chem. Soc. 117 (1995) 9408–9414. [73] B.W. Van der Meer, G. Coker, S.Y.S. Chen, Resonance Energy Transfer Theory and Data, Wiley-VCH, New York, 1994. [74] D.L. Dexter, A theory of sensitized luminescence in solids, J. Chem. Phys. 21 (1953) 836–850. [75] (a) T. F€orster, 10th Spiers Memorial Lecture. Transfer mechanisms of electronic excitation, Discuss. Faraday Soc. 27 (1959) 7–17; (b) T. F€ orster, Zwischenmolekulare

166

[76]

[77]

[78]

[79] [80] [81]

[82]

[83]

[84]

[85]

[86]

[87]

[88] [89]

Lanthanide-Based Multifunctional Materials

Energiewanderung und Fluoreszenz, Ann. Phys. 437 (2) (1948) 55, (English translation, 1993). (a) M.P. Lowe, D. Parker, pH switched sensitisation of europium(III) by a dansyl group, Inorg. Chim. Acta 317 (2001) 163–173; (b) F. Vogtle, M. Gorka, V. Vicinelli, P. Ceroni, M. Maestri, V. Balzani, A dendritic antenna for near-infrared emission of Nd3+ ions, ChemPhysChem 2 (2001) 769–773. (a) H. Wang, G. Qian, M. Wang, J. Zhang, Y. Luo, Enhanced luminescence of an erbium (iii) ion-association ternary complex with a near-infrared dye, J. Phys. Chem. B 108 (2004) 8084–8088, and references therein; (b) P.A. Tanner, C.K. Duan, Luminescent lanthanide complexes: selection rules and design, Coord. Chem. Rev. 254 (2010) 3026–3029, and references therein; (c) S.V. Eliseeva, J.C.G. Bunzli, Lanthanide luminescence for functional materials and bio-sciences, Chem. Soc. Rev. 39 (2010) 189–227. J. Vuojola, M. Syrj€anp€a€a, U. Lamminm€aki, T. Soukka, Genetically encoded protease substrate based on lanthanide-binding peptide for time-gated fluorescence detection, Anal. Chem. 85 (2013) 1367–1373. A.P. Pushkarev, M.N. Bochkarev, Organic electroluminescent materials and devices emitting in UV and NIR regions, Russ. Chem. Rev. 85 (12) (2016) 1338–1368. R.E. Whan, G.A. Crosby, Luminescence studies of rare earth complexes: benzoylacetonate and dibenzoylmethide chelates, J. Mol. Spectrosc. 8 (1962) 315–327. R. Pizzoferrato, R. Francini, S. Pietrantoni, R. Paolesse, F. Mandoj, A. Monguzzi, F. Meinardi, Effects of progressive halogen substitution on the photoluminescence properties of an erbium-porphyrin complex, J. Phys. Chem. A 114 (12) (2010) 4163–4168. Y. Hasegawa, K. Murakoshi, Y. Wada, S. Yanagida, J.H. Kim, N. Nakashima, T. Yamanaka, Enhancement of luminescence of Nd3+ complexes with deuterated hexafluoroacetylacetonato ligands in organic solvent, Chem. Phys. Lett. 248 (1996) 8–12. Y. Hasegawa, Y. Kimura, K. Murakoshi, Y. Wada, J.H. Kim, N. Nakashima, T. Yamanaka, S. Yanagida, Enhanced emission of deuterated tris(hexafluoroacetylacetonato)neodymium (III) complex in solution by suppression of radiationless transition via vibrational excitation, J. Phys. Chem. 100 (1996) 10201–10205. S. Pietrantoni, R. Francini, R. Pizzoferrato, S. Penna, R. Paolesse, F. Mandoj, Energy transfer and excitation processes in thin films of rare-earth organic complexes for NIR emission, Phys. Status Solidi C 4 (2007) 1048–1051. ´ lvarez, M. Ramos Silva, C. Zaldo, J.A. Paixao, P. Martı´n-Ramos, C. Coya, A.L. A P. Chamorro, J. Martı´n-Gil, Charge transport and sensitized 1.5 μm electroluminescence properties of full solution processed NIR-OLED based on novel Er(III) fluorinated betadiketonate ternary complex, J. Phys. Chem. C 117 (19) (2013) 10020–10030. P. Martin-Ramos, C. Coya, V. Lavı´n, I.R. Martı´n, M. Ramos-Silva, P.S. Pereira Silva, M. Garcı´a-Velez, A.L. Alvarez, J. Martı´n-Gil, Active layer solution-processed NIROLEDs based on ternary erbium(III) complexes with 1,1,1-trifluoro-2,4-pentanedione and different N,N-donors, Dalton Trans. 43 (48) (2014) 18087–18096. ´ lvarez, S. A ´ lvarez-Garcı´ad, P. Martı´n-Ramos, M. Ramos Silva, C. Coya, C. Zaldo, A.L. A A.M. Matos-Beja, J. Martı´n-Gil, Novel erbium(III) fluorinated b-diketonate complexes with N,N-donors for optoelectronics: from synthesis to solution-processed devices, J. Mater. Chem. C 1 (2013) 2725–2734. F. Artizzu, M.L. Mercuri, A. Serpe, P. Deplano, NIR-emissive erbium–quinolinolate complexes, Coord. Chem. Rev. 255 (2011) 2514–2529. L. Winkless, R.H.C. Tan, Y. Zheng, M. Motevalli, P.B. Wyatt, W.P. Gillin, Quenching of Er(III) luminescence by ligand C–H vibrations: Implications for the use of erbium complexes in telecommunications, Appl. Phys. Lett. 89 (2006) 111115.

NIR OLEDs based on Ln(III) complexes

167

[90] J.C.G. Bunzli, S. Comby, A.-S. Chauvin, C.D.B. Vandevyver, New opportunities for lanthanide luminescence, J. Rare Earths 25 (2007) 257–274. [91] H.Q. Ye, Y. Peng, Z. Li, C.-C. Wang, Y.-X. Zheng, M. Motevalli, P.B. Wyatt, W.P. Gillin, I. Herna´ndez, Effect of fluorination on the radiative properties of Er3+ organic complexes: an opto-structural correlation study, J. Phys. Chem. C 117 (2013) 23970–23975. [92] P. Martı´n-Ramos, M. Ramos Silva, F. Lahoz, I.R. Martı´n, P. Chamorro-Posada, M.E. S. Eusebio, V. Lavı´n, J. Martı´n-Gil, Highly fluorinated erbium(III) complexes for emission in the C-band, J. Photochem. Photobiol. A Chem. 292 (2014) 16–25. [93] C. Bischof, J. Wahsner, J. Scholten, S. Trosien, M. Seitz, Quantification of C-H quenching in near-IR luminescent ytterbium and neodymium cryptates, J. Am. Chem. Soc. 132 (2010) 14334–14335. [94] C. Doffek, N. Alzakhem, M. Molon, M. Seitz, Rigid, perdeuterated lanthanoid cryptates: extraordinarily bright near-IR luminophores, Inorg. Chem. 51 (2012) 4539–4545. [95] A. Monguzzi, A. Milani, L. Lodi, M.I. Trioni, R. Tubino, C. Castiglioni, Vibrational overtones quenching of near infrared emission in Er3+ complexes, New J. Chem. 33 (2009) 1542–1548. [96] A.P. Pushkarev, V.A. Ilichev, T.V. Balashova, D.L. Vorozhtsov, M.E. Burin, D.M. Kuzyaev, G.K. Fukin, B.A. Andreev, D.I. Kryzhkov, A.N. Yablonskiy, M.N. Bochkarev, Lanthanide complexes with substituted naphtholate ligands: extraordinary bright near infrared luminescence of ytterbium, Russ. Chem. Bull. Int. Ed. 62 (2013) 392–397. [97] R.D. Archer, H.Y. Chen, L.C. Thompson, Synthesis, characterization, and luminescence of europium(III) schiff base complexes, Inorg. Chem. 37 (1998) 2089–2095. [98] F. Gutierrez, C. Tedeschi, L. Maron, J.P. Daudey, R. Poteau, J. Azema, P. Tisnes, C. Picard, Quantum chemistry-based interpretations on the lowest triplet state of luminescent lanthanides complexes. Part 1. Relation between the triplet state energy of hydroxamate complexes and their luminescence properties, Dalton Trans. (9) (2004) 1334–1347. [99] M. Latva, H. Takalo, V.-M. Mukkala, C. Matachescu, J.C. Rodrı´guez-Ubis, J. Kankare, Correlation between the lowest triplet state energy level of the ligand and lanthanide(III) luminescence quantum yield, J. Lumin. 75 (1997) 149–169. [100] H. Xu, K. Yin, L.H. Wang, W. Huang, Bright electroluminescence from a chelate phosphine oxide Eu(III) complex with high thermal performance, Thin Solid Films 516 (23) (2008) 8487–8492. [101] V.V. Grushin, N. Herron, D.D. LeCloux, W.J. Marshall, V.A. Petrov, Y. Wang, New, efficient electroluminescent materials based on organometallic Ir complexes, Chem. Commun. (16) (2001) 1494–1495. [102] J. Martins, P. Martı´n-Ramos, C. Coya, A.L. Alvarez, L.C. Pereira, R. Dı´az, J. Martı´n-Gil, M. Ramos-Silva, Lanthanide tetrakis-β-diketonate dimers for solution-processed OLEDs, Mater. Chem. Phys. 147 (3) (2014) 1157–1164. [103] M.R. Robinson, M.B. O’Regan, G.C. Bazan, Synthesis, morphology and optoelectronic properties of tris[(N-ethylcarbazolyl) (30 ,50 -hexyloxybenzoyl) methane] (phenanthroline) europium, Chem. Commun. (17) (2000) 1645–1646. [104] D. Zhang, W. Li, B. Chu, X. Li, L. Han, J. Zhu, T. Li, D. Bi, D. Yang, F. Yan, H. Liu, D. Wang, Sensitized photo- and electroluminescence from Er complexes mixed with Ir complex, Appl. Phys. Lett. 92 (2008) 093501. [105] P. Cias, C. Slugovc, G. Gescheidt, Hole transport in triphenylamine based OLED devices: from theoretical modeling to properties prediction, J. Phys. Chem. A 115 (50) (2011) 14519–14525.

168

Lanthanide-Based Multifunctional Materials

[106] A. O’Riordan, E. O’Connor, S. Moynihan, P. Nockemann, P. Fias, R. Van Deun, D. Cupertino, P. Mackie, G. Redmond, Near infrared electroluminescence from neodymium complex–doped polymer light emitting diodes, Thin Solid Films 497 (2006) 299–303. [107] W.D. Horrocks Jr., J.P. Bolender, W.D. Smith, R.M. Supkowski, Photosensitized near infrared luminescence of ytterbium(III) in proteins and complexes occurs via an internal redox process, J. Am. Chem. Soc. 119 (1997) 5972–5973. [108] Y. Kawamura, Y. Wada, Y. Hasegawa, M. Iwamuro, T. Kitamura, S. Yanagida, Observation of neodymium electroluminescence, Appl. Phys. Lett. 74 (1999) 3245–3247. [109] O.M. Khreis, R.J. Curry, M. Somerton, W.P. Gillin, Infrared organic light emitting diodes using neodymium tris-8-hydroxyquinoline, J. Appl. Phys. 88 (2000) 777–780. [110] Z. Ahmed, K. Iftikhar, Variant coordination sphere, for efficient photo- and electroluminescence of 0.4–1.8 μm, of lanthanide(III) complexes containing a β-diketone ligand with low vibrational frequency C–F bonds and a flexible 2,20 -bipyridine ligand, Polyhedron 85 (2015) 570–592. [111] R.J. Curry, W.P. Gillin, 1.54 μm electroluminescence from erbium (III) tris(8hydroxyquinoline) (ErQ)-based organic light-emitting diodes, Appl. Phys. Lett. 75 (1999) 1380–1382. [112] O.M. Khreis, W.P. Gillin, M. Somerton, R.J. Curry, 980 nm electroluminescence from ytterbium tris(8-hydroxyquinoline), Org. Electron. 2 (2001) 45–51. [113] M.A. Katkova, A.P. Pushkarev, T.V. Balashova, A.N. Konev, G.K. Fukin, S.Y. Ketkov, M.N. Bochkarev, Near-infrared electroluminescent lanthanide [Pr(III), Nd(III), Ho(III), Er(III), Tm(III), and Yb(III)] N,O-chelated complexes for organic light-emitting devices, J. Mater. Chem. 21 (2011) 16611–16620. [114] K. Lunstroot, P. Nockemann, K. van Hecke, L. van Meervelt, C. G€ orller-Walrand, K. Binnemans, K. Driesen, Visible and near-infrared emission by samarium(III)containing ionic liquid mixtures, Inorg. Chem. 48 (2009) 3018–3026. [115] B. Chu, W.L. Li, Z.R. Hong, F.X. Zang, H.Z. Wei, D.Y. Wang, M.T. Li, C.S. Lee, S.T. Lee, Observation of near infrared and enhanced visible emissions from electroluminescent devices with organo samarium(III) complex, J. Phys. D. Appl. Phys. 39 (2006) 4549–4552. [116] F.X. Zang, W.L. Li, Z.R. Hong, H.Z. Wei, M.T. Li, X.Y. Sun, C.S. Lee, Observation of 1.5 μm photoluminescence and electroluminescence from a holmium organic complex, Appl. Phys. Lett. 84 (2004) 5115–5117. [117] M.A. Katkova, V.A. Ilichev, A.N. Konev, M.N. Bochkarev, Rare-earth metal 8-hydroxyquinolinate complexes as materials for organic light-emitting diodes, Russ. Chem. Bull. 57 (2008) 2281–2284. [118] B.S. Harrison, T.J. Foley, A.S. Knefely, J.K. Mwaura, G.B. Cunningham, T.S. Kang, M. Bouguettaya, J.M. Boncella, J.R. Reynolds, K.S. Schanze, Near-infrared photoand electroluminescence of alkoxy-substituted poly(p-phenylene) and nonconjugated polymer/lanthanide tetraphenylporphyrin blends, Chem. Mater. 16 (2004) 2938–2947. [119] M.A. Katkova, V.A. Ilichev, A.N. Konev, I.I. Pestova, G.K. Fukin, M.N. Bochkarev, 2Mercaptobenzothiazolate complexes of rare earth metals and their electroluminescent properties, Org. Electron. 10 (2009) 623–630. [120] K. Wei, D.P. Machewirth, J. Wenzel, E. Snitzer, G.H. Sigel Jr., Spectroscopy of Dy3+ in Ge-Ga-S glass and its suitability for 1.3-μm fiber-optical amplifier applications, Opt. Lett. 19 (1994) 904–906. [121] J. Feng, L. Zhou, S.-Y. Song, Z.-F. Li, W.-Q. Fan, L.-N. Sun, Y.-N. Yua, H.-J. Zhang, A study on the near-infrared luminescent properties of xerogel materials doped with dysprosium complexes, Dalton Trans. (33) (2009) 6593–6598.

NIR OLEDs based on Ln(III) complexes

169

[122] Z.F. Li, L. Zhou, J. Bo Yu, H.J. Zhang, R.P. Deng, Z.P. Peng, Z.Y. Guo, Synthesis, structure, photoluminescence, and electroluminescence properties of a new dysprosium complex, J. Phys. Chem. C 111 (2007) 2295–2300. [123] Z. Ahmed, K. Iftikhar, et al., Efficient photo luminescent complexes of 400–1800 nm wavelength emitting lanthanides containing organic sensitizers for optoelectronic devices, RSC Adv. 4 (2014) 63696–63711. [124] Z. Hong, C. Liang, R. Li, F. Zang, D.I. Fan, W. Lia, L.S. Hung, S.T. Lee, Infrared and visible emission from organic electroluminescent devices based on praseodymium complex, Appl. Phys. Lett. 79 (2001) 1942–1944. [125] A.P. Pushkarev, V.A. Ilichev, A.A. Maleev, A.A. Fagin, A.N. Konev, A.F. Shestakov, R.V. Rumyantzev, G.K. Fukin, M.N. Bochkarev, Electroluminescent properties of lanthanide pentafluorophenolates, J. Mater. Chem. C 2 (2014) 1532–1538. [126] Z. Ahmed, R.E. Aderne, J. Kai, H.I.P. Chavarria, M. Cremona, Ytterbium β-diketonate complexes for near infra-red organic light-emitting devices, Thin Solid Films 620 (2016) 32–42. [127] Z. Ahmed, R.E. Aderne, J. Kai, J.A.L.C. Resende, H.I.P. Chavarria, M. Cremona, Near infrared organic light emitting devices based on a new erbium(III)beta-diketonate complex: synthesis and optoelectronic investigations, RSC Adv. 7 (2017) 18239–18251. [128] S. Quici, M. Cavazzini, G. Marzanni, G. Accorsi, N. Armaroli, B. Ventura, F. Barigelletti, Visible and near-infrared intense luminescence from water-soluble lanthanide [Tb(III), Eu(III), Sm(III), Dy(III), Pr(III), Ho(III), Yb(III), Nd(III), Er(III)] complexes, Inorg. Chem. 44 (2005) 529–537. [129] E.G. Moore, J. Xu, S.C. Dodani, C.J. Jocher, A. D’Aleo, M. Seitz, K.N. Raymond, 1Methyl-3-hydroxy-pyridin-2-one complexes of near infra-red emitting lanthanides: efficient sensitization of Yb(III) and Nd(III) in aqueous solution, Inorg. Chem. 49 (2010) 4156–4166. [130] G.R. Berezina, T.A. Rumyantseva, V.P. Kulinich, G.P. Shaposhnikov, Spectral properties and solubility of the lanthanide complexes with substituted and annelated porphyrazines, Russ. J. Coord. Chem. 37 (2011) 796–799. [131] X.S. Ke, B.Y. Yang, X. Cheng, S.L.F. Chan, J.L. Zhang, Ytterbium(III) porpholactones: b-Lactonization of porphyrin ligands enhances sensitization efficiency of lanthanide near-infrared luminescence, Chem. Eur. J. 20 (2014) 4324–4333. [132] V.V. Utochnikova, A.S. Kalyakina, I.S. Bushmarinov, A.A. Vashchenko, L. Marciniak, A.M. Kaczmarek, R. Van Deun, S. Br€ase, N.P. Kuzmina, Lanthanide 9-anthracenate: solution processable emitters for efficient purely NIR emitting host-free OLEDs, J. Mater. Chem. C 4 (2016) 9848–9855. [133] V. Cleave, G. Yahioglu, P. Le Barny, R.H. Friend, N. Tessler, Harvesting singlet and triplet energy in polymer LEDs, Adv. Mater. 11 (1999) 285–288. [134] L.H. Slooff, A. Polman, F. Cacialli, R.H. Friend, G.A. Hebbink, F.C.J.M. van Veggel, D.N. Reinhoudt, Near-infrared electroluminescence of polymer light-emitting diodes doped with a lissamine-sensitized Nd3+ complex, Appl. Phys. Lett. 78 (2001) 2122–2124. [135] A. O’Riordan, R. Van Deun, E. Mairiaux, S. Moynihan, P. Fias, P. Nockemann, K. Binnemans, G. Redmond, Synthesis of a neodymium-quinolate complex for nearinfrared electroluminescence applications, Thin Solid Films 516 (2008) 5098–5102. [136] A. Shahalizad, A. D’Aleo, C. Andraud, M.H. Sazzad, D.H. Kim, Y. Tsuchiya, J.C. Ribierre, J.M. Nunzi, C. Adachi, Near infrared electroluminescence from Nd(TTA)3phen in solution-processed small molecule organic light-emitting diodes, Org. Electron. 44 (2017) 50–58. [137] T.V. Balashova, N.A. Belova, M.E. Burin, D.M. Kuzyaev, R.V. Rumyantcev, G.K. Fukin, A.P. Pushkarev, V.A. Ilichev, A.F. Shestakov, I.D. Grishin,

170

[138]

[139]

[140]

[141]

[142]

Lanthanide-Based Multifunctional Materials

M.N. Bochkarev, Substituted naphtholates of rare earth metals as emissive materials, RSC Adv. 4 (2014) 35505–35510. Z.-Q. Chen, F. Ding, Z.-Q. Bian, C.-H. Huang, Efficient near-infrared organic lightemitting diodes based on multimetallic assemblies of lanthanides and iridium complexes, Org. Electron. 11 (2010) 369–376. F. Zang, T.C. Sum, F. Zhu, Z.R. Hong, X. Sun, W.L. Li, C.H.A. Huan, Modulated infrared electroluminescence from organic light-emitting diodes, J. Lightwave Technol. 27 (2009) 1522–1526. W.Q. Zhao, P.F. Wang, G.Z. Ran, G.L. Ma, B.R. Zhang, W.M. Liu, S.K. Wu, L. Dai, G.G. Qin, 1.54 μm Er3+ electroluminescence from an erbium-compound-doped organic light emitting diode with a p-type silicon anode, J. Phys. D. Appl. Phys. 39 (2006) 2711–2714. F. Wei, Y.Z. Li, G.Z. Ran, G.G. Qin, 1.54 μm electroluminescence from p-Si anode organic light emitting diode with Bphen: Er(DBM)3phen as emitter and Bphen as electron transport material, Opt. Express 18 (2010) 13542–13546. F. Zinna, U. Giovanella, L. di Bari, Highly circularly polarized electroluminescence from a chiral europium complex, Adv. Mater. 27 (2015) 1791–1795; F. Zinna, M. Pasini, F. Galeotti, C. Botta, L. di Bari, U. Giovanella, Design of lanthanide-based OLEDs with remarkable circularly polarized electroluminescence, Adv. Funct. Mater. 27 (2017) 1603719–1603726.

Biography ´ ngel L. A ´ lvarez received his M.Sc. degree in Physics from Universidad ComA plutense de Madrid in 1990, and his Ph.D. from E.T.S.I. Telecomunicacio´n, Universidad Politecnica de Madrid, in 1996. Since 2007 to date, he has been an Associate Professor in Electronics at Universidad Rey Juan Carlos. With more than 25 years of experience in solid-state electronics, he has been involved in OLED manufacturing in the last 15 years, and his current interests cover infrared light-emitting devices and graphene/graphene oxide optoelectronic properties. Carmen Coya was born in Madrid in 1968. She completed her Doctorate at the Institute of Materials Science of Madrid (CSIC) and works at Rey Juan Carlos University of Madrid, where she is Full Professor in Electronics. She is a founding member of the Research Group in Organic Optoelectronics (OOG) and directs the Laboratory of Characterization of Organic Devices (LABCADIO), belonging to the network of laboratories of excellence in Madrid (cod-351). Her lines of research are currently focused on the development of materials, manufacturing of devices, and optimization of profitable processes for organic electronics: hybrid materials based on graphene and organic-inorganic hybrids for optoelectronic applications.