New diarylaminophenyl derivatives of carbazole: Effect of substituent position on their redox, spectroscopic and electroluminescent properties

Synthetic Metals 228 (2017) 1–8

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New diarylaminophenyl derivatives of carbazole: Effect of substituent position on their redox, spectroscopic and electroluminescent properties

MARK

Łukasz Skórkaa, Piotr Kurzepa, Gabriela Wiosna-Sałygab, Beata Łuszczyńskab, Ireneusz Wielgusa, ⁎ Zbigniew Wróbelc, Jacek Ulańskib, Irena Kulszewicz-Bajera, a b c

Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland Department of Molecular Physics, Technical University of Lodz, Zeromskiego 116, 90-924 Lodz, Poland Institute of Organic Chemistry, Polish Academy of Science, Kasprzaka 44/52, 01-224 Warsaw, Poland

A R T I C L E I N F O

A B S T R A C T

Keywords: Aminocarbazoles Luminescence properties Electrochemical oxidation OLED

Three new derivatives of carbazole were synthesized, namely two isomers disubstituted with diarylaminophenyl groups in 2,7- and 3,6- positions (C1 and C2) and diarylaminophenyl symmetrically disubstituted with carbazole C3. All three compounds showed similar electrochemical behavior with one reversible redox couple, originating from the oxidation of the diarylaminophenyl units to radical cations, followed by an irreversible anodic peak associated with the oxidation of the carbazole unit. The electrochemical results were supported by DFT calculations which predicted the same order of ionization potential values as experimentally found. Absorption and emission bands in the spectra of C1 and C3 were bathochromically shifted as compared to the corresponding bands in C2, again consistent with DFT calculations. C1 showed a very high value of the photoluminescence quantum yield in THF solution, i.e. 90%. Moreover, the position of the emission band turned out to be solvent dependent and underwent a hypsochromic shift in toluene with simultaneous decrease of the photoluminescence quantum yield value to 68%. Blue light emitting C1 was used as an electroluminophore in a simple, guest/hosttype single-layer light emitting diodes exhibiting luminance exciding 1000 cd/m2 and luminous efficiency of 0.7 cd/A.

1. Introduction Carbazole derivatives constitute popular building blocks of several low and high molecular weight compounds exhibiting interesting electrochemical [1–4] and electrical transport [5–7] properties. For this reason they have been successfully used as components of electrochromic devices [8–11], active layers of field effect transistors [12–14], hole transporting materials in various electronic devices [15–18] etc. Photo- and electroluminescence of carbazole derivatives are other features of this family of compounds which are technologically promising. Therefore, carbazole unit-containing compounds have been successfully used as organic emitters [19–22], matrix materials for blue OLEDs [23–25] or electro donating units in molecules exhibiting the thermally activated delayed fluorescence (TADF) effect [26–29]. Each of the above listed applications requires the synthesis of compounds exhibiting different physical properties. Thus, low and high molecular weight carbazole derivatives used as hole transporting layer should exhibit high glass transition temperatures and good hole mobility. For derivatives considered as potential organic emitters high

⁎

Corresponding author. E-mail address: [email protected] (I. Kulszewicz-Bajer).

http://dx.doi.org/10.1016/j.synthmet.2017.04.004 Received 5 December 2016; Received in revised form 9 March 2017; Accepted 1 April 2017 0379-6779/ © 2017 Elsevier B.V. All rights reserved.

values of the electroluminescence quantum yield are required. Matrix materials for blue OLEDs should, in turn, show high triplet energy as well as balanced charge injection. For TADF emitters small ΔEST values must be achieved due to well separated HOMO and LUMO levels. Tuning of all these properties is possible through appropriate functionalization of the carbazole unit either in its 2,7- or 3,6- positions. In this paper we describe the studies on the effect of the type of substitution on redox photo- and electroluminescent properties of three new carbazole derivatives containing diarylaminophenyl substituents. In two of them the central carbazole unit is differently disubstituted with diarylaminophenyl (2,7- or 3,6- substitution). The third derivative studied consists of the central diarylaminophenyl unit symmetrically disubstituted with carbazole moieties. 2. Results and discussion 2.1. Synthesis The new synthesized carbazole derivatives are depicted in Scheme

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The detailed description of the synthesized target compounds as well as all their precursors together with their spectroscopic characterization and elemental analyses can be found in Supplementary Materials. 3. Electrochemical studies Representative cyclic voltammograms of C1–C3 are shown in Fig. 1. All three voltammograms reveal some common features, namely a reversible redox couple at lower potentials and an irreversible oxidation at higher potentials. The first one is associated with the diarylaminephenyl oxidation to a radical cation. In the case of C1 and C2 both diarylaminephenyl substituents are oxidized at very similar potentials and these redox processes are manifested by some broadening of the anodic and cathodic peaks without the possibility of their resolution. In the case of C3, which contains only one triphenylamine unit undergoing oxidation, the peak of the first redox couple is narrower and separated by 70 mV, consistent with a one-electron redox process. The second redox process of predominantly irreversible (C1) or irreversible (C2 and C3) nature has to be attributed to the oxidation of the carbazole moiety [30]. Electrochemical data derived from the chronovoltamperometric investigations are collected in Table 1. As expected the redox properties of the investigated derivatives are dependent on the effective conjugation between both moieties constituting the molecule. Better conjugated 2,7- substituted derivative (C1) is easier to oxidize than its less conjugated isomer (C2). Diarylaminophenyl group in C3 undergoes oxidation at higher potentials than in the case of C1 and C2 which probably is associated with a weak electron withdrawing effect of two aromatic rings of two carbazole moieties. In C1 and C2 diarylaminophenyl groups are in terminal positions and this effect must be less pronounced. In general, the oxidation of substituted diarylaminophenyl is very sensitive to the presence of electrodonating (accepting) groups in its closest vicinity. For example, in arylene bisimides disubstituted with diarylaminophenyl in their core, the potential of the substituent oxidation is shifted to higher potentials by ca. 350–400 mV as compared to C1–C3 [31]. From the formal first oxidation potential it is possible to calculate the ionization potentials, IP, of C1–C3 using the absolute potentials scale, i.e. expressing their values with respect to a vacuum level [32]. The obtained IP values, listed in Table 1, are relatively low as expected for conjugated molecules containing easily oxidizable arylamino moieties [33]. We were tempted to verify whether DFT calculations support these conclusions. At the early stage, the calculations were aimed at the equilibrium geometry elaboration since all three compounds could accommodate various possible conformations. At that point in all cases the geometry was constrained to C2 symmetry point group and the calculations were performed without the alkoxy substituents. Then for the least energetic conformations –OMe groups were attached to simplify the calculations, which also removed the symmetry constrains and introduced C1 symmetry. Then the geometries were re-optimized for neutral and charged species applying also the solvation model for IP and EA determination. The resulting HOMO/LUMO and IP/EA values along with respective Eg are collected in Table 2. The data collected in Table 2 clearly demonstrate that the solvation changes the HOMO/LUMO levels, dropping down both values by ca. 0.2–0.3 eV. Since the changes are almost the same for HOMO and LUMO, the band gap, Eg, values determined in vacuum and in solution remain essentially the same. The calculated IP values are ca. 0.4 eV smaller than those derived from the cyclic voltammetry investigations, however, the trend remains the same (compare data in Tables 1 and 2). The calculated EA values are very low, consistent with the fact that all three compounds are easy to oxidize but very difficult to reduce. The calculations also clearly indicate that the 3,6-substituted derivative (C2) is less conjugated that the 2,7-substituted isomer (C1). In Fig. 2 the frontier orbitals contours are presented. As it can be

Scheme 1. Chemical structures of the studied aminophenyl carbazoles C1–C3. C1: 4,4′(9-(4-hexyloxyphenyl)-9H-carbazole-2,7-diyl)bis(N,N-bis(4-methoxyphenyl)aniline), C2: 3,3′-(9-(4-methoxyphenyl)-9H-carbazole-3,6-diyl)bis(N,N-bis(4-methoxyphenyl)aniline), C3: (4-methoxy-N,N-bis(4-(9-(4-(octyloxyphenyl)-9H-carbazol-2-yl)phenyl)-aniline).

1. The specific features of the applied synthetic strategy can be outlined as follows. Aminophenyl disubstituted carbazoles i.e. C1 and C2 were synthesized using Suzuki cross coupling (see Scheme 2). In the case of C1 commercially available 2,7-dibromo derivative was used as a substrate, which was substituted with 4-hexyloxyphenyl at 9-carbazole position in a standard procedure. Thus, the reaction of 2,7-dibromocarbazole with 4-hexyloxy-1-iodobenzene in the presence of CuI and 1,2-diaminocyclohexane gave 1 with 62% yield. In the procedure of the preparation of C2, carbazole was first N-substituted with a 4-methoxyphenyl group using the same protocol as that applied in the case of the synthesis of C1 (58% yield). The resulting product (2) was brominated with NBS to afford the 3,6-dibromo derivative 3 with 97% yield. Independently, 4-amino- and 3-aminophenyl boronic acid pinacol esters S1 and S2 were prepared (see Supplementary Material) with 69% and 59% yields, respectively. Palladium-catalyzed Suzuki coupling reactions between dibromocarbazole derivatives 1 and 3 and the corresponding borolanes S1 and S2 afforded the target products C1 and C2 with 55% and 64% yields, respectively. C3 was synthesized in a four-step procedure (Supplementary Materials, Scheme S2). Anizidinediphenyl amine 4 was obtained with 92% yield in a Buchwald–Hartwig–type reaction using Pd(OAc)2/tBu3P as a catalyst. 4 was then brominated with NBS to give its dibromoderivative 5 with 91% yield. Suzuki-type coupling of 5 with commercially available carbazole boronate led to the dicarbazolamine derivative 6 (46% yield), which was finally N-substituted with 4octyloxyphenyl in the presence of CuI/1,2-diaminocyclohexane to afford C3 with 62% yield. 2

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Scheme 2. Synthesis of the carbazole derivatives C1 and C2: (a) 4-hexyloxy-1-iodobenzene, CuI, 1,2-cyclohexyldiamine, K3PO4, dioxane, 100 °C, (b) 4-iodoanisole, CuI, 1,2cyclohexyldiamine, K3PO4, dioxane, 110 °C, (c) NBS, DMF, 0 °C, (d) compound S1, Pd(OAc)2, SPhos, K3PO4, dioxane, 100 °C.

degenerate HOMO orbitals (see Fig. 3) [34].

immediately noticed, in each case HOMO and LUMO have preferential localization sites and small differences, in this respect, occur between all three compounds. For C1 the HOMO is located mainly on the triarylamine moieties, but also has a substantial contribution from the central carbazole unit. On the other hand the LUMO is mainly located on the central carbazole unit with contribution from the adjacent phenylene ring. These shapes of the frontier orbitals reflect the best conjugation between C(2) and C(7) of carbazole and p-phenylene ring of the triarylamine substituents. In the case of C2 the situation is even more complex since HOMO becomes degenerate and the two corresponding orbitals occupy the two lateral triarylamine moieties. This type of HOMO degeneracy where it occurs as two separate orbitals rather than a linear combination of them is rarely spotted [34]. On the other hand LUMO of C2 is on exclusively limited to the central carbazole unit. Among the three derivatives studied this is the only case, where the frontier orbitals are located exclusively on different sites with no overlap. For C3, similarly as in the case of C1, HOMO is localized on the central triarylamine moiety with some extension to the phenyl rings of the two carbazole substituents, whereas LUMO is spread over the whole molecule, with main focus on the lateral carbazoles. Spin density distributions of the radical cation forms of C1–C3 also correspond to their HOMO localizations, and especially in the case of C2 it is a direct superposition of the two,

4. Optical properties In Fig. 4 the UV–vis absorption spectra of C1–C3 are presented. They show a strong band in the spectral range 300–380 nm, corresponding to the π–π* transition in the conjugated backbone. Its position strongly depends on the type of substitution (2,7- vs. 3,6-) (see Table 3). It is instructive to compare spectral features of C1 and C2 with that of N-phenyl carbazole, i.e. the compound, which corresponds to the central part of both molecules. Its π–π* band is located at 340 nm [18]. A bathochromic shift of this band in the spectrum of C1 by nearly 40 nm is a consequence of the electrodonating effect of amine and extension of the conjugation to the substituent. To the contrary, in the case of C2 this band is hypsochromically shifted by 30 nm indicating less planar geometry which pushes the amine substituents out of conjugation. A bathochromic shift of nearly 30 nm is also observed for the π–π* band in the spectrum of C3. Triphenyl amine substituents in C1 and C2 are linked with the carbazole central unit via phenyl ring, however if the amine atom is directly attached to the carbazole ring larger bathochromic shifts are observed due to stronger electrodonating effect, as reported by Tomkeviciene et al. [35]. All studied compounds are luminescent. The photoluminescence 3

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Table 1 Oxidation potentials and ionization potentials of C1–C3. Comp.

Eox1a (V)

Ered1a (V)

(Eox1 + Ered1)/2 (V)

Eox2a (V)

Eox3a (V)

IPb (eV)

C1 C2 C3

0.28 0.34 0.38

0.16 0.21 0.31

0.220 0.275 0.345

0.87 0.91 0.94

0.95 – –

5.020 5.075 5.125

a

Potential values vs. Fc/Fc+. IP values were calculated according to half-wave potentials using the following equation: IP = {4.8 + [(Eox1 + Ered1)/2]} eV. b

Table 2 HOMO/LUMO and IP/EA levels with corresponding Eg values for compounds C1, C2 and C3 calculated at B3LYP/6-31G(d,p) level of theory in vacuum and dichloromethane solution. Comp.

Solvation

HOMO

LUMO

Eg

IP

EA

C1

Vacuum Solvation

−4.43 −4.66

−0.91 −1.20

3.52 3.45

5.17 4.59

0.19 1.41

C2

Vacuum Solvation

−4.53 −4.77

−0.61 −0.89

3.92 3.88

5.30 4.66

0.32 1.00

C3

Vacuum Solvation

−4.59 −4.80

−0.95 −1.20

3.64 3.60

5.51 4.71

0.20 1.34

Fig. 2. Graphical representation of HOMOs and LUMOs of compounds C1–C3 calculated at B3LYP/6-31G(d,p) level of theory (isosurface value = 0.03 green for negative and red for positive wavefunction sign).

spectra of C1 recorded in three different solutions, namely toluene, THF and dichlorobenzene (DCB), are shown in Fig. 5. Contrary to the absorption spectra of this compound, which are solvent independent, its photoluminescence spectra are strongly influenced by the type of the solvent (compare Figs. 4 and 5 and the data collected in Table 3). The emission band registered for the solution of C1 in THF and DCB shows loss of vibrational structure and bathochromic shift with respect to the

Fig. 1. Cyclic voltammograms of C1 (a), C2 (b) and C3 (c) in CH2Cl2 solution (the concentration of the compounds was c = 10−3 M) containing an electrolyte – 0.1 M Bu4NBF4, (reference electrode – Ag/0.1 M AgNO3 in acetonitrile, scan rate – 100 mV/s).

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higher in the THF solution than in the toluene one (90% vs. 68%). Moreover, the non-radiative processes are slowed down upon passing from nonpolar toluene to polar THF (Table 3). The small Stokes shift and the most pronounced vibrational structure of the emission spectrum of C1 in toluene reflect a high rigidity of this molecule in nonpolar solvent. On the other hand, low photoluminescence QY (6%) was measured for C2. The value of radiative decay rate constant for C2 is evidently lower than that for C1 and C3. Such behavior may be interpreted in terms of limited conjugation between triarylamine and carbazole moieties in the excited state. It is generally known that the radiative decay constant kr is related to the transition dipole moment, thus to the electronic wave functions of the ground and excited states. In simple approximation it depends on the overlap between HOMO and LUMO [36]. The DFT calculations presented above showed that there is no spacial overlap between HOMO and LUMO in C2 and the frontier orbitals are located exclusively on different sites. The lack of the overlap decreases significantly the luminescent properties of C2. Similar effect was observed by Volyniuk et al. for carbazole derivatives substituted at 2,7- and 3,6-positions [37]. A comparison of the photoluminescence data of C1 and C2 with the corresponding data obtained for carbazole derivatives in which arylamine nitrogen is directly linked to the central unit [35] shows that 2,7- substitution via phenyl ring leads to improved photoluminescence properties whereas 3,6- substitution results in a significant lowering of QY value to 6%. Further elucidation of the optical properties of C1–C3 was possible through a series of TD-DFT calculations carried out in various solvents (namely: THF, toluene and dichloromethane). This was done with the aim to verify whether the calculations can predict the experimentally observed dependence of the emission band position on the type of solvent. All data are collected in Supplementary Material (Tables S3–S5). In the case of C1 and C3 the first transition (HOMO→LUMO) is privileged, what is manifested by a large value of the oscillator strength. However, for C2 this transition is forbidden and only the 8th vertical excitation gives raise to the electronic spectrum. This is a direct consequence of the aforementioned localization/delocalization of the frontier molecular orbitals and their overlap, which in the case of C2 is substantially diminished. These theoretical predictions correspond well to the experimental results and elucidate the presence of very weak shoulder ca. 360 nm observed in the absorption spectrum of C2. Emission wavelengths were calculated using both linear- and statespecific-approach, with the results being presented in Table S6. The aim of that study was to verify whether it is possible to reproduce the experimental trends in the emission wavelengths and to establish the impact of solvation of the excited state or, in other words, to predict the experimentally observed dependence of the emission band position on the type of solvent. According to the data collected in Table S6 among two ways of approximation of the absorption and emission wavelength, namely linear response and state-specific solvation, the latter are greatly exaggerated, while the traditional linear response correctly reproduces the experimental trends in the absorption bands positions with deviations of 29 nm and 37 nm for C1 and C3 respectively and 56 nm for C2. Also when the emission wavelengths are calculated with linear-response solvation they very well correspond to the experimental ones, again with small deviations (45 nm for C1, 41 nm for C2 and 33 nm for C3 in THF solutions). In the calculations of emission spectra effects associated with the used solvents become non-negligible indicating, for C1, a bathochromic shift of the emission band by 11 nm with changing the solvent from toluene to THF. Similar problems with this type of approach already occurred in the studies of various types of emissive materials [38], but in general it provides good results [39]. In this particular case it may suggest, that upon excitation the adjustment of the geometry is slower than the emission, what is always a big question for determining the mechanism of the emission process [40].

Fig. 3. Spin density distribution of the radical cation forms of compounds C1 (a), C2 (b) and C3 (c) calculated at B3LYP/6-31G(d,p) level of theory (isosurface value = 0.001, blue for negative and green for positive sign).

Fig. 4. Absorption spectra of C1–C3 in THF solutions.

corresponding band recorded in toluene. The location of the emission maxima is very sensitive to the solvent polarity. An increase of the solvent polarity causes the shift of the emission band from 429 nm in toluene to 452 in THF and 462 nm in DCB. Similar solvent sensitivity, indicating the dipolar nature of excited states, was observed in the spectra of C2 and C3, however, the effect was not so distinct than that for C1 (all emission spectra are presented in Supplementary Material). The mechanism of the spectral shift may be due to the changes in fluorophores dipole moment during excitation. The measured photoluminescence quantum yield for C1 is significantly 5

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Table 3 Photophysical parameters derived from stationary and time-resolved spectroscopic measurements of the compounds C1–C3. Compound

Absorption band [nm]

Emission band [nm]

Stokes shift [nm]

Photo. QYa [%]

Emission lifetime [ns]

Radiative rate constant [108 s−1]

Non-radiative rate const. [108 s−1]

C1 in toluene C1 in THF

380 380

429 452

49 72

68 90

1.1 1.8

6.9 5

2.2 0.6

C2 in toluene C2 in THF

300 300

413 425

53 65

6 5

3 4

0.2 0.13

3.1 2.4

C3 in toluene C3 in THF

360 360

422 435

62 75

66 58

1.2 1.6

5.5 3.6

2.8 2.6

a

Photoluminescence quantum yield.

Fig. 6. Photoluminescence spectra obtained for thin films of PVK:PBD containing 1 wt% and 3 wt% of molecularly dispersed C1 using two excitation lines: 340 nm and 390 nm. Inset: Normalized emission spectra for thin films of PVK:PBD containing 1 wt% of C1 after excitation at 340 nm and 390 nm.

Fig. 5. Photoluminescence spectra of C1 in toluene, THF and DCB.

5. Electroluminescent properties The very high value of photoluminescence QY determined for C1 prompted us to investigate the electroluminescent properties of this blue emitter. We fabricated guest/host-type simple test light emitting diodes of the following configuration:

system with 3% of C1 and only barely noticeable trace of it can be found for the layer with 1 wt% of C1 (see the Inset in Fig. 6). This means that the excitation energy of the matrix was efficiently transmitted to the dispersed C1 molecules. Usually low contents of guest molecules are used in the fabrication of matrices, in order to avoid aggregation, which quenches the electroluminescence. A slight red shift of emission band was observed for the layer with higher luminophore content, which may be due to stronger intermolecular interaction. In Fig. 7 photo- and electroluminescence spectra of the matrix layer

ITO/PEDOT:PSS/PVK: PBD + 1% (3%) C1/Ca/Ag and ITO/PEDOT:PSS/PVK: PBD + 1% (3%) C1/LiF/Al. In the active layer of this type of diodes luminescent “guest molecules” are molecularly dispersed in a host matrix which usually consists of a blend of two wide-gap organic semiconductors capable of transporting electrons and holes. We used poly(vinylcarbazole) (PVK) as a hole conducting phase and 2-tert-butylphenyl-5-biphenyl-1,3,4oxadiazole (PBD) as an electron conductor [23]. In this configuration singlet excitons, formed in the host phase are transferred to the guest molecules through Förster energy transfer [41]. Alternative mechanism involves guest molecules acting as traps for electron and/or holes with the formation and anihilation of excitons in situ within the molecule [42]. In studied layer, because of relative position of HOMO and LUMO energy levels of the host matrix components and the guest molecules, C1 should constitute traps for holes. In order to verify whether the energy is efficiently transferred from the host matrix to the dispersed C1 luminophore, the photoluminescence spectra for two different C1 contents in the matrix (1 wt% and 3 wt%) were recorded using two excitation lines (λex = 340 nm and 390 nm), corresponding to the maxima of the absorption bands of the host matrix and C1, respectively (see Fig. 6). The normalized emission spectra obtained for these two excitation lines were almost identical, indicating that the photoluminescence was derived almost exclusively from the C1 molecules without the emission from the matrix for the

Fig. 7. Comparison of normalized photoluminescence and electroluminescence spectra obtained for a thin film of PVK:PBD containing 1 wt% of molecularly dispersed C1.

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voltage curve mimicking the current density vs. voltage one. The diode turns on at voltage 8 V and reaches the luminance of ca. 800 cd/m2 at about 20 V. Diodes with the LiF/Al cathode showed better luminance, exceeding 1000 cd/m2. The current efficiency measured at ca. 9 V was ca. 0.55 cd/A for the device with the Ca/Ag cathode and ca. 0.70 cd/A for that with the LiF/Al cathode. It is known, that LiF interfacial layer improves device performance by protecting the emissive layer during the cathode deposition and by preventing the diffusion of metal atoms of the cathode into the organic layer. Moreover, the presence of LiF can also reduce the number of quenching sites at the interface [45]. As one can see in Fig. 8a, the current efficiency dropped significantly with increasing voltage, possibly due to some degradation of the emitter molecules, because the diodes were not encapsulated and measured in air. In the molecule of C1 the 3,6- positions of the carbazole moiety remain active, thus irreversible oxidation/degradation processes can take place at high voltage. These preliminary results, obtained for simple and non-optimized diodes clearly demonstrates promising optoelectronic properties of the diarylaminephenyl-substituted carbazoles. 6. Conclusions To summarize, we have demonstrated that carbazole moiety can be effectively modified by diarylaminophenyl substituents at 2,7- or 3,6positions. In the 2,7- substituted derivatives conjugation extends from the carbazole unit to diarylaminephenyl substituents which is manifested by a significantly lower oxidation potential and bathochromic shift of their absorption and photoluminescence bands as compared to the 3,6- substituted derivative. Both 2,7- and 3,6- substituted derivatives show interesting electrochemical properties involving reversible oxidation of the diarylaminophenyl group to its radical cation form. The 2,7- substituted derivatives show in addition very high photoluminescence QY values and promising electroluminescent properties yielding blue light emitting diodes. Supplementary material Synthesis, characterization of all compounds, optical absorption and fluorescence spectra, OLED preparation, DFT calculations.

Fig. 8. Characteristics of a diode with an active layer consisting of PVK:PBD containing 1 wt% of molecularly dispersed C1: (a) luminance vs. voltage and current density vs. voltage; (b) external quantum efficiency vs. current density and luminance vs. current density.

Acknowledgements L.S., P.K., I.W. and I.K-B. wish to acknowledge the financial support from National Science Centre in Poland, of NCN, Grant No. 2015/17/B/ ST5/00179 whereas Z.W. acknowledges the support of Grant No. 2015/ 17/B/ST4/03837. G.W-S., B.L. and J.U. acknowledge the support of Grant No. 8862/E370/S/2016 from the Polish Ministry of Science and Higher Education. The DFT calculations were carried out in the Wroclaw Centre for Networking and Supercomputing, WCSS, Wroclaw, Poland under Grant No. 283. We thank professor Adam Proń for critical review of the manuscript.

containing 1 wt% of C1 are compared. It can be noted that the electroluminescence band is broader and bathochromically shifted by ca. 30 nm with respect to the photoluminescence one. The difference in the photoluminescence and electroluminescence spectra may confirm important role of a trapping mechanism in the electroluminescence, via an electroplex formation. The term “electroplex” describes the excited states that can only be seen in EL rather than PL and is used to distinguish from exciplex emission which is visible in both EL and PL. The origin of this unique phenomenon is explained in terms of varying distance between two molecules involved in a charge transfer complex. When molecules are close to each other, it is easy to observe optical transition (exciplex case), but when the distance is large this transition is possible only under applied electric field. It is well known that oxadiazole and carbazole derivatives show the tendency to form excited state complexes such as excimers, exciplexes, electroplexes [43,44]. The presented electroluminescence spectrum seems to be superposition of the emission being very similar to that observed upon photoexcitation and the emission originating from electroplex, probably created between PVK and PBD [44]. However, to confirm this hypothesis and to clarify the nature of excited state complexes further studies are needed. Fig. 8 shows representative characteristics of the fabricated diodes. A typical diode-like behavior is observed with the luminance vs. applied

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.synthmet.2017.04.004. References [1] G. Zotti, G. Schiavon, S. Zecchin, J.F. Morin, M. Leclerc, Macromolecules 35 (2002) 2122–2128. [2] K. Karon, M. Lapkowski, A. Dabuliene, A. Tomkeviciene, N. Kostiv, J.V. Grazulevicius, Electrochem. Acta 154 (2015) 119–127. [3] Z. Wei, J. Xu, G. Nie, Y. Du, S. Pu, J. Electroanal. Chem. 589 (2006) 112–119. [4] N. Blouin, A. Michaud, M. Leclerc, Adv. Mater. 19 (2007) 2295–2300. [5] R.B. Aich, N. Bloin, A. Bouchard, M. Leclerc, Chem. Mater. 21 (2009) 751–757.

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