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Electronic energy levels in lyotropic chromonic liquid crystals formed by ionic perylene diimide derivatives
Synthetic Metals 257 (2019) 116147
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Electronic energy levels in lyotropic chromonic liquid crystals formed by ionic perylene diimide derivatives
T
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Oleksandr P. Boikoa, , Bohdan Ya. Lenykb, Oleg Yu. Posudievskiyc, Yurii. L. Slominskiyd, Sergii A. Tsybuliae, Yuriy A. Nastishine, Vassili G. Nazarenkoa a
Institute of Physics of the NAS of Ukraine, prospect Nauky 46, Kyiv, 03028, Ukraine Taras Shevchenko National University of Kyiv, prospect Glushkov 4g, Kyiv, 03680, Ukraine c L.V. Pisarzhevsky Institute of Physical Chemistry of the NAS of Ukraine, prospect Nauky 31, Kyiv 03028, Ukraine d Institute of Organic Chemistry of the NAS of Ukraine, Murmanska Str. 5, Kyiv, 02660, Ukraine e Hetman Petro Sahaidachnyi National Army Academy, 32, Heroes of Maidan street., Lviv, 79012, Ukraine b
A R T I C LE I N FO
A B S T R A C T
Keywords: Lyotropic chromonic liquid crystals Polyaromatic dyes Cyclic voltammetry Light absorption spectra
Lyotropic Chromonic Liquid Crystals (LCLCs) represent dispersions of disk-like organic molecules in water that form columnar aggregates bound together by weak hydrophobic interactions and aligned parallel to each other. Dry films of LCLCs preserve the aggregated molecular packing and orientational order. Our idea is that this class of materials is ideally suited for flexible electronics applications since their structure is shaped by relatively weak non-covalent molecular interactions. We report an experimental study that aims to elucidate the influence of ionic substitutes on the energy band of LCLCs based on perylene-diimide derivatives. Using cyclic voltammetry and light absorption techniques, we explore the role of functionalization of several materials of this class with ionic end groups (positive and negative) on the molecular LUMO/HOMO energy level positions. We find that both positively and negatively charged LCLC molecules show decrease in energy positions in comparison with chemically-similar non-mesogenic materials. The experimentally estimated parameters show the deep-lying LUMO energy levels (below −4.0 eV) for all studied LCLC materials with ionic substitutes.
1. Introduction Perylene-diimide derivatives are known as a promising class of organic semiconductors for electronic applications [1,2]. The most studied representatives of this class are 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) and N,N′-Diphenyl-3,4,9,10-perylene-dicarboximide (PDI-Ph), which are commonly used as electron-transporting layers for fabrication of n-channel organic field-effect transistors (OFETs) thanks to their high electron affinity and relatively large electron mobility in the solid state [3–5]. PTCDA and PDI-Ph molecules typically form polycrystalline films with two nearly coplanar (stacked) molecules within the unit cell. The distance between consecutive molecular planes along the a-direction is relatively short, resulting in a large overlap of molecular π orbitals and formation of strongly coupled systems. Further development of organic electronic devices requires macroscopically highly ordered semiconducting films with well-defined molecular alignment [6–11], which still remains to challenge for traditional methods of organic film deposition. To improve the charge-transporting properties of perylene
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derivatives films an alternative approach to use lyotropic chromonic liquid crystals (LCLCs) [12] was recently proposed. LCLCs are formed by flat polyaromatic molecules with ionic groups at the periphery that make them solvable in water. In water (or in a similar polar solvent such as glycerol), the molecules self-assemble into aggregates, which then self-organize into structures with long range orientational order, i.e. a liquid crystal (see [13–15] for review on LCLCs). In water solutions perylene cores of the molecules tend to stack face-to-face due to the strong π-π interaction, thus forming elongated columnar aggregates. The distance between stacked molecular planes is ˜3.4 Å [16,17]. The LCLCs are unique liquid crystals, structurally composed of building units in the form of rod-like aggregates, which are polydisperse in length from monomers to thousands of the molecules in the stack. In this sense they are akin to polymers. However in the LCLC aggregates the molecules are not fixed by the covalent bonds; they can be added to the stack or removed from it. As a result the average length of the aggregates, is a function of such external actions as temperature, concentration [13], UV irradiation [18]. For this reason the LCLCs are called “physical polymers” to distinguish them from conventional
Corresponding author. E-mail address: [email protected] (O.P. Boiko).
https://doi.org/10.1016/j.synthmet.2019.116147 Received 7 February 2019; Received in revised form 30 July 2019; Accepted 13 August 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.
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“chemical polymers. It is important to note that the orientational order with dense packing is preserved when the aggregates are transferred from the nematic water solutions into thin solid films by shear deposition and drying [19,20]. Experiments on the perylene-based ionic LCLCs provide a direct evidence for the improved energetic ordering (smaller effective energetic disorder) in aggregated LCLC films as compared to conventional PDI-C5 films [21]. Although, the charge transport properties of organic semiconductors are to great extent governed by their molecular packing and macroscopic orientational ordering, the impact of functionalization (end and core substitutions) of PDI molecules is important as well [22–24]. Typically, perylene diimide derivatives demonstrate the nchannel behavior [25,26]. However, the p-channel [27–29] and ambipolar behavior have also been observed [28–31,4]. It was shown [22] that the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy levels can be varied considerably by using different substituents in these materials, such that the charge transport can be altered from the n- to p-type. Detailed mechanism of charge transfers and positions of molecular orbitals in LCLC films is still less studied and requires additional experiments especially for molecules with charged groups. 2. Experimental and discussion In this Letter we present the experimentally determined energy band structure for the PDIs with different peripheral groups containing positive and negative ions. In contrast to conventional perylene systems, wherein molecule ordering is limited, their charged analogues generate the structure with the long orientation order. Using cyclic voltammetry (CV) and light absorption techniques, we perform comparative study of electronic properties for several ionic LCLC perylene derivatives against PDI-Ph, that we consider as a relevant reference system which, alternatively, features no liquid crystal properties. The studied LCLC materials have the same core structure based on perylene diimide with different periphery groups containing ions, whose molecular structures are shown in Fig. 1. For comparison purposes, the studies have also been performed for a neutral N,N′-diphenyl3,4,9,10-perylenedicarboximide (PDI-Ph), molecules, shown in Fig. 1a which are known as a benchmark organic semiconductors having a similar molecular structure, however, have no charged species. The negatively Perylene-3,4,9,10-bis(dicarboximide)- N,N’-bis(3-benzene sulfonic acid) bis-ammonium salt (Red 2334), Fig. 1b, and positively Perylene-3,4,9,10-bis(dicarboximide)-N,N’-bis(1-methyl-3-pyridinium) bis-n toluenesulfonate (Red 2416), Fig. 1c and, Perylene-3,4,9,10-bis (dicarboximide)-N,N’-bis(propyl-trimethyl-ammonium) bis-n toluenesulfonate) (Red 2582), Fig. 1d, charged dyes used in experiments were synthesized at the Institute of Organic Chemistry of the NAS of Ukraine. The dyes were prepared using the classic method by reaction of Perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and a corresponding substituents in quinoline as solvent [32,33]. The PDI-Ph and PTCDA were obtained from Sigma-Aldrich. The dyes have a structure, which is common for many other molecules of the chromonic class: a rigid planar poly-aromatic core with zwitterionic groups at the periphery [13]. The Red 2334 (Fig. 1b) and Red 2416 (Fig. 1c) possess the same core structure but oppositely charged peripheries. The materials with charged substitutes demonstrate the nematic (N) phase when dissolved in water with concentration above 16 wt.%. The concentration phase diagrams of aqueous solutions of the synthesized LCLC materials are shown in Fig. 2. At room temperature, the aqueous solutions are in the isotropic phase for the weight concentration of the materials up to 10 wt %. In the range 10–16 wt %, the solutions of Red 2334 and Red 2416 demonstrate a wide biphasic region of coexisting nematic and isotropic phases. The N phase is observed for the range of concentration of about 16–20 wt %. For higher concentrations, another biphasic region with coexisting nematic and solid crystalline phases is observed. The dye Red 2582 demonstrates similar
Fig. 1. Chemical structures of the studied perylene based dyes. a) - PDI-Ph, b) Red 2334, c) - Red 2416 and d) - Red 2582 (for ionic dyes (b, c, d) counterion are presented separately).
behavior but with rather short range of concentration 16–17 wt %. The all phase diagrams feature relatively wide biphasic regions, which is common for LCLC. CV measurements were performed using the electrochemical analyzer μAUTOLAB III/FRA2 (ECO CHEMIE) in the potential range −2.0–2.0 V vs. an Ag/Ag+ standard electrode. Electrochemical measurements were performed in methyl chloride solution (in the case of PDI-Ph) and in DMF solutions of ionic LCLC dyes with TBAPF6 (TBA – tetrabutyl ammonium) as an electrolyte using Pt working electrode. Estimation of HOMO/LUMO energy levels was carried out using spectroscopic and CV data [34–37]. UV–VIS spectra were measured on the double beam spectrophotometer Shimadzu 2450. Determination of the optical gap was performed using linearization of the absorption spectra by Tauc equation [38] assuming indirect allowed electron transitions: AE = const (E − Eg . opt .) , where A , E and Eg . opt . is absorbance, photon energy and the value of the optical gap, respectively. The results obtained for PDI-Ph are shown in Fig. 3. The optical gap for this compound equals to 2.26 eV. The LUMO energy level calculated Ered.1/2 = −1.00 eV taking in account the value is ELUMO = −(Ered.1/2 + 4.44 = 3.44 eV . The HOMO energy level calculated using data from absorption spectra is Eg . opt . = 2.26eV EHOMO = Eg . opt . + ELUMO = 5.70 eV . The same method was used to study ionic LCLC dyes. For the negatively charged Red 2334, and positively charged Red 2416, Red 2582, CVs (Fig. 3) show only peaks of electrochemical reduction. Due to the complicate nature of the CV traces we also performed differential pulse voltammograms (DPVs) measurements of charged dies to better explore reduction potentials. The results are presented in Fig. 4. DPVs was performed with the following parameters: modulation time 5 ms, interval time 200 ms, step potential 2 mV, modulation amplitude 100 mV. The surface of a glassy carbon electrode was carefully polished with a superfine Al2O3 powder before measurements. Concentration of the dyes was 2 mM. Before each measurement, an oxygen
2
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Fig. 2. Concentration dependencies of phase transition of the studied perylene based dyes.
corresponding molecule containing a single center. Only the magnitude of the current is enhanced by the presence of additional electroactive centers [39]. We do observe that the neutral PDI-Ph in the cathode region is characterized by the presence of only one transition, whereas for all derivatives having charged functional groups (Red 2416, Red 2334 and Red 2582), there are several reversible redox transitions. This effect may have complex explanation which is based on important factor of the molecular aggregation. While the substituents have little influence on the electrochemical behavior, a large effect is noted with extension of the aromatic system between the redox groups [40]. Being in aggregate the molecules are no more non-interacting. Within the aggregates, the molecules are associated through non-covalent interactions such as π-π attraction. They are closely packed at the distance of approximately 3.4A leading to a strong coupling between the aromatic perylene cores [41]. As the aromatic system increases, oxidation becomes easier and the potentials between the reduction steps become closer that may result in two, as for Red 2334, or even in three, as for Red 2582 and Red 2416, reversible redox transitions. Moreover, the
was removed from the electrolyte by 30 min purging of nitrogen. CVs and DPVs data presented in Figs. 3 and 4a shows the presence of two reduction peaks for Red 2334 and three reduction peaks for Red 2416 and Red 2582. The observed three reduction transitions for positively charged molecules (Red 2582 and Red 2416) as compared to Red 2334, may originate from hoping electron on positively charged imide position. In comparison with the neutral dye PDI-Ph, the potential of their first reduction peak, which determines the position of their LUMO, is substantially shifted to less negative potentials as E1/ 2416 < E1/22582 < E1/22334. Interestingly, the effect does not depend on 2 the electric charge of the substituent as it is observed for negatively charged Red 2334 dye and positively charged Red 2416 and Red 2582 dyes. This striking aspect in the behavior of charged perylene derivatives indicates the governing role of aggregation in the properties of mesogenic materials. Indeed, molecules containing a number of identical, noninteracting centers that accept or give up electrons exhibit currentpotential responses having the same shape as that obtained with the
Fig. 3. Cyclic voltammograms and UV–vis spectra (insets) for different materials indicated at the bottom of the corresponding graph. Bold lines on voltammogram curves demonstrate linear fit for calculation of reduction potential (Ered.1/2 ). Bold lines on UV–vis spectra demonstrate linear fit for calculation of optical gap (Eg . opt . ). 3
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Fig. 4. a) Differential pulse voltammograms for Red 2416, Red 2334, Red 2582 dies. b–d) Possible structure of aggregates, calculated for Red 2334 (AM1 approximation, HyperChem package): b) parallel dimer, c) rotated dimer, d) rotated dimer, top view.
with different background electrolytes, without taking into account the effects of the medium [48]. However, it has been shown that the replacement of water with organic solvents substantially affects this value [49,1–51]. The CV measurements were performed at the following conditions: glassy carbon (GC) work electrode, Pt wire counter-electrode, Ag/Ag+ (= +0.68eV vs. NHE) reference electrode, 0.1 M TBAPDMF electrolyte with v = 100mV / s . Therefore, empirically adjusted formula is of the form ELUMO = −(Ered.1/2 + 4.87eV ) , where 4.87eV = 4.19eV (ENHE in DMF ) + 0.68eV (EAg / Ag + vs NHE ) [50]. The positions of LUMO/HOMO energy levels of perylene derivatives measured in the present study are summarized in Table. 1. The data demonstrate that both positively and negatively charged mesogenic materials showed decrease in energy positions as compared to classical PDI-Ph. The data presented in Table 1 are schematically depicted in Fig. 5. The information on HOMO and LUMO energy levels can be used to estimate of the charge-carrier type of the studied material. Although no well-defined criteria certifying the charge-carrier type of a material are available in the literature, some reference HOMO/LUMO values, which might be used for this purpose, have been suggested. Using the data for HOMO/LUMO energy levels experimentally measured with the CV and light absorption techniques for a series of 19 functionalized acenes, Tang et. al. [30] have concluded that electron injection prevails if the LUMO energy level is below 3.15 eV and the hole injection is no longer favorable if the HOMO energy level is lower than 5.6 eV. It was also suggested that the materials showing the HOMO/LUMO energy levels within the range of [5.6, −3.15] eV, display ambipolar behavior. Below and above this ambipolar range, one expects purely electron- and purely hole-charge-carrier type, respectively. However, there is no evidence for the universality of this rule and its applicability to other materials, since these arguments may be altered by the existence of traps with different energy levels. The remarkable result of our study is that the deep-lying LUMO energy levels are found for all studied dyes with ionic substitutes, namely: -4.08 eV for Red2334, −4.12 eV for Red 2582 and −4.24 eV for Red 2416. The reports on organic molecules with the LUMO energy level below −4.0 eV are rare [52]. Importantly, the low-lying LUMO level is associated with relatively high efficiency of acceptor properties
aggregation has another merit to influence CV-traces. The commonly accepted idealized model according to which the chromonic aggregates are simple rods dispersed in the solvent is only a convenient simplification [42]. The attempt to explore the real structure of the aggregates built from perylene derivatives was performed in [43]. The authors suggested two types of molecular assemblies: the molecules are positioned exactly parallel with the polar groups localized at the different side of molecule (Fig. 4b parallel dimer); the molecules are rotated relative to each other (Fig. 4c and d rotated dimer). The growth in concentration accompanies a growth of the dimers number as well as the creation of aggregates with a larger number of molecules. For parallel dimers the spatial obstacles (large charged species) limit the creation of trimers, tetramers and so on with equivalent distance between the molecules of 3.4A. The only tetramers or higher associates with 2 N molecules and with alternative distances 3.4A and 5.5A are possible. In this case the translation unit is the parallel dimer. The rotated aggregates can be formed from the separate molecules. Theoretically, there are two possible aggregates: a) helix, when each molecules turns by some angle forming a helix; b) alternative, when an angle of rotation alternates as θ− and θ+ from one molecules to another. The all these species demonstrate different overlap of aromatic systems of molecules that influences observed CV traces. Also, the elegant mechanism of generating radical anions and dianions was described in [44,45]. Analyzing absorption spectra, the authors report that perylene diimide formed dimers may be transformed into a π-stacked aggregated form upon one-electron reduction. Addition of a second electron, however, resulted in dissociation of the aggregates, giving the dianion in the monomeric form. The experimental spectra however do not permit to make a clear choice between possible structure of aggregates [43,46]. The charged dyes exhibit no light emission in polar solvents. The only very dilute water solution (the concentration below 10−5 wt. %) demonstrates molecular emission for the dyes [43,47]. The maximum emission is slightly red-shifted from the absorption band. With increase in concentration the absorption band form does not noticeable change while the emission intensity dramatic diminishes. The correct interpretation of this phenomena is possible assuming that even at the lowest concentration LC molecules associate into H-type of dimers and aggregates with larger number of molecules (isodesmic aggregation) [13]. Further detailed studies would be desirable to explore the exact mechanism behind the electrochemical behavior of charged perylenes. For determination of EHOMO and ELUMO from CV data for charged molecules one switches from the electrochemical scale to the absolute potential scale. In the commonly employed empirical formulas one uses ELUMO = −(Ered.1/2 + 4.44) , EHOMO = −(Eox.1/2 + 4.44eV ; the value 4.44eV implies a normal hydrogen electrode (NHE) in aqueous solutions. This value is also used for electrochemical studies in non-aqueous solutions
Table 1 LUMO/HOMO energy levels for studied materials.
4
Sample
CV Ered .1/2
DPV Ered .1/2
ELUMO
Eg . opt .
EHOMO
PDI-Ph 2334 2416 2582
−1.00 −0.79 −0.63 −0.75
– −0.80 −0.62 −0.75
−3.44* −4.08 −4.24 −4.12
2.26 2.24 2.23 2.25
−5.70 −6.32 −6.47 −6.37
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Fig. 5. Diagram depicting data from Table 1 for LUMO and HOMO energy levels of the studied materials compared to the literature data for SdiCNPBI [52], PDI-4 [4] and the work function for Au.
of the materials and demonstrate a potential as alternatives to fullerene derivatives in bulk heterojunction organic solar cells [2,53,54]. Although perylene diimides molecules with amino or amino N-oxide as the terminal substituent were successfully implemented as high-performance organic interlayer materials [55,56], the liquid crystal approach provides further benefit achieved also via the enhanced orientational ordering of charged perylene diimides molecules and its aggregates over the large area.
3. Conclusions In summary, we investigated Lyotropic Chromonic Liquid Crystals that are formed by flat polyaromatic molecules with ionic groups at the periphery. Using cyclic voltammetry (CV) and light absorption techniques, we explore the role of functionalization of perylene-diimide derivatives with ionic end groups on the molecular LUMO/HOMO energy level positions. We found that both the positively and negatively charged mesogenic materials showed a decrease in energy positions as compared to the classical chemically-similar non-mesogenic PDI-Ph material. The ionic substitutes lowered both HOMO and LUMO energy levels by 0.6 eV without changing the band gap (˜2.24 eV) resulting in a LUMO level of less than −4 eV. Our results suggest that the noncovalent interaction between charged molecules act to decrease the lowest unoccupied molecular orbital. The findings point an effective way to develop promising nonfullerene acceptors for organic solar cells.
Author information Author Contributions: OB, BL and OP performed the experiments. YS synthesized the material. ST, YN and VG envisioned the experiments. The manuscript was written through contributions of all authors.
Funding The research was supported by the National Academy of Sciences of Ukraine within the projects 1.4.BC#202, 1.4.BC#188 and 1.4.В#186. The authors declare no competing financial interests.
Acknowledgments The authors are also acknowledged Prof. O. Lavrentovich and Dr. A. Kadashchuk for helpful discussion and critical reading of the manuscript. 5
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Electronic energy levels in lyotropic chromonic liquid crystals formed by ionic perylene diimide derivatives
T
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Oleksandr P. Boikoa, , Bohdan Ya. Lenykb, Oleg Yu. Posudievskiyc, Yurii. L. Slominskiyd, Sergii A. Tsybuliae, Yuriy A. Nastishine, Vassili G. Nazarenkoa a
Institute of Physics of the NAS of Ukraine, prospect Nauky 46, Kyiv, 03028, Ukraine Taras Shevchenko National University of Kyiv, prospect Glushkov 4g, Kyiv, 03680, Ukraine c L.V. Pisarzhevsky Institute of Physical Chemistry of the NAS of Ukraine, prospect Nauky 31, Kyiv 03028, Ukraine d Institute of Organic Chemistry of the NAS of Ukraine, Murmanska Str. 5, Kyiv, 02660, Ukraine e Hetman Petro Sahaidachnyi National Army Academy, 32, Heroes of Maidan street., Lviv, 79012, Ukraine b
A R T I C LE I N FO
A B S T R A C T
Keywords: Lyotropic chromonic liquid crystals Polyaromatic dyes Cyclic voltammetry Light absorption spectra
Lyotropic Chromonic Liquid Crystals (LCLCs) represent dispersions of disk-like organic molecules in water that form columnar aggregates bound together by weak hydrophobic interactions and aligned parallel to each other. Dry films of LCLCs preserve the aggregated molecular packing and orientational order. Our idea is that this class of materials is ideally suited for flexible electronics applications since their structure is shaped by relatively weak non-covalent molecular interactions. We report an experimental study that aims to elucidate the influence of ionic substitutes on the energy band of LCLCs based on perylene-diimide derivatives. Using cyclic voltammetry and light absorption techniques, we explore the role of functionalization of several materials of this class with ionic end groups (positive and negative) on the molecular LUMO/HOMO energy level positions. We find that both positively and negatively charged LCLC molecules show decrease in energy positions in comparison with chemically-similar non-mesogenic materials. The experimentally estimated parameters show the deep-lying LUMO energy levels (below −4.0 eV) for all studied LCLC materials with ionic substitutes.
1. Introduction Perylene-diimide derivatives are known as a promising class of organic semiconductors for electronic applications [1,2]. The most studied representatives of this class are 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) and N,N′-Diphenyl-3,4,9,10-perylene-dicarboximide (PDI-Ph), which are commonly used as electron-transporting layers for fabrication of n-channel organic field-effect transistors (OFETs) thanks to their high electron affinity and relatively large electron mobility in the solid state [3–5]. PTCDA and PDI-Ph molecules typically form polycrystalline films with two nearly coplanar (stacked) molecules within the unit cell. The distance between consecutive molecular planes along the a-direction is relatively short, resulting in a large overlap of molecular π orbitals and formation of strongly coupled systems. Further development of organic electronic devices requires macroscopically highly ordered semiconducting films with well-defined molecular alignment [6–11], which still remains to challenge for traditional methods of organic film deposition. To improve the charge-transporting properties of perylene
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derivatives films an alternative approach to use lyotropic chromonic liquid crystals (LCLCs) [12] was recently proposed. LCLCs are formed by flat polyaromatic molecules with ionic groups at the periphery that make them solvable in water. In water (or in a similar polar solvent such as glycerol), the molecules self-assemble into aggregates, which then self-organize into structures with long range orientational order, i.e. a liquid crystal (see [13–15] for review on LCLCs). In water solutions perylene cores of the molecules tend to stack face-to-face due to the strong π-π interaction, thus forming elongated columnar aggregates. The distance between stacked molecular planes is ˜3.4 Å [16,17]. The LCLCs are unique liquid crystals, structurally composed of building units in the form of rod-like aggregates, which are polydisperse in length from monomers to thousands of the molecules in the stack. In this sense they are akin to polymers. However in the LCLC aggregates the molecules are not fixed by the covalent bonds; they can be added to the stack or removed from it. As a result the average length of the aggregates, is a function of such external actions as temperature, concentration [13], UV irradiation [18]. For this reason the LCLCs are called “physical polymers” to distinguish them from conventional
Corresponding author. E-mail address: [email protected] (O.P. Boiko).
https://doi.org/10.1016/j.synthmet.2019.116147 Received 7 February 2019; Received in revised form 30 July 2019; Accepted 13 August 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.
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“chemical polymers. It is important to note that the orientational order with dense packing is preserved when the aggregates are transferred from the nematic water solutions into thin solid films by shear deposition and drying [19,20]. Experiments on the perylene-based ionic LCLCs provide a direct evidence for the improved energetic ordering (smaller effective energetic disorder) in aggregated LCLC films as compared to conventional PDI-C5 films [21]. Although, the charge transport properties of organic semiconductors are to great extent governed by their molecular packing and macroscopic orientational ordering, the impact of functionalization (end and core substitutions) of PDI molecules is important as well [22–24]. Typically, perylene diimide derivatives demonstrate the nchannel behavior [25,26]. However, the p-channel [27–29] and ambipolar behavior have also been observed [28–31,4]. It was shown [22] that the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy levels can be varied considerably by using different substituents in these materials, such that the charge transport can be altered from the n- to p-type. Detailed mechanism of charge transfers and positions of molecular orbitals in LCLC films is still less studied and requires additional experiments especially for molecules with charged groups. 2. Experimental and discussion In this Letter we present the experimentally determined energy band structure for the PDIs with different peripheral groups containing positive and negative ions. In contrast to conventional perylene systems, wherein molecule ordering is limited, their charged analogues generate the structure with the long orientation order. Using cyclic voltammetry (CV) and light absorption techniques, we perform comparative study of electronic properties for several ionic LCLC perylene derivatives against PDI-Ph, that we consider as a relevant reference system which, alternatively, features no liquid crystal properties. The studied LCLC materials have the same core structure based on perylene diimide with different periphery groups containing ions, whose molecular structures are shown in Fig. 1. For comparison purposes, the studies have also been performed for a neutral N,N′-diphenyl3,4,9,10-perylenedicarboximide (PDI-Ph), molecules, shown in Fig. 1a which are known as a benchmark organic semiconductors having a similar molecular structure, however, have no charged species. The negatively Perylene-3,4,9,10-bis(dicarboximide)- N,N’-bis(3-benzene sulfonic acid) bis-ammonium salt (Red 2334), Fig. 1b, and positively Perylene-3,4,9,10-bis(dicarboximide)-N,N’-bis(1-methyl-3-pyridinium) bis-n toluenesulfonate (Red 2416), Fig. 1c and, Perylene-3,4,9,10-bis (dicarboximide)-N,N’-bis(propyl-trimethyl-ammonium) bis-n toluenesulfonate) (Red 2582), Fig. 1d, charged dyes used in experiments were synthesized at the Institute of Organic Chemistry of the NAS of Ukraine. The dyes were prepared using the classic method by reaction of Perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and a corresponding substituents in quinoline as solvent [32,33]. The PDI-Ph and PTCDA were obtained from Sigma-Aldrich. The dyes have a structure, which is common for many other molecules of the chromonic class: a rigid planar poly-aromatic core with zwitterionic groups at the periphery [13]. The Red 2334 (Fig. 1b) and Red 2416 (Fig. 1c) possess the same core structure but oppositely charged peripheries. The materials with charged substitutes demonstrate the nematic (N) phase when dissolved in water with concentration above 16 wt.%. The concentration phase diagrams of aqueous solutions of the synthesized LCLC materials are shown in Fig. 2. At room temperature, the aqueous solutions are in the isotropic phase for the weight concentration of the materials up to 10 wt %. In the range 10–16 wt %, the solutions of Red 2334 and Red 2416 demonstrate a wide biphasic region of coexisting nematic and isotropic phases. The N phase is observed for the range of concentration of about 16–20 wt %. For higher concentrations, another biphasic region with coexisting nematic and solid crystalline phases is observed. The dye Red 2582 demonstrates similar
Fig. 1. Chemical structures of the studied perylene based dyes. a) - PDI-Ph, b) Red 2334, c) - Red 2416 and d) - Red 2582 (for ionic dyes (b, c, d) counterion are presented separately).
behavior but with rather short range of concentration 16–17 wt %. The all phase diagrams feature relatively wide biphasic regions, which is common for LCLC. CV measurements were performed using the electrochemical analyzer μAUTOLAB III/FRA2 (ECO CHEMIE) in the potential range −2.0–2.0 V vs. an Ag/Ag+ standard electrode. Electrochemical measurements were performed in methyl chloride solution (in the case of PDI-Ph) and in DMF solutions of ionic LCLC dyes with TBAPF6 (TBA – tetrabutyl ammonium) as an electrolyte using Pt working electrode. Estimation of HOMO/LUMO energy levels was carried out using spectroscopic and CV data [34–37]. UV–VIS spectra were measured on the double beam spectrophotometer Shimadzu 2450. Determination of the optical gap was performed using linearization of the absorption spectra by Tauc equation [38] assuming indirect allowed electron transitions: AE = const (E − Eg . opt .) , where A , E and Eg . opt . is absorbance, photon energy and the value of the optical gap, respectively. The results obtained for PDI-Ph are shown in Fig. 3. The optical gap for this compound equals to 2.26 eV. The LUMO energy level calculated Ered.1/2 = −1.00 eV taking in account the value is ELUMO = −(Ered.1/2 + 4.44 = 3.44 eV . The HOMO energy level calculated using data from absorption spectra is Eg . opt . = 2.26eV EHOMO = Eg . opt . + ELUMO = 5.70 eV . The same method was used to study ionic LCLC dyes. For the negatively charged Red 2334, and positively charged Red 2416, Red 2582, CVs (Fig. 3) show only peaks of electrochemical reduction. Due to the complicate nature of the CV traces we also performed differential pulse voltammograms (DPVs) measurements of charged dies to better explore reduction potentials. The results are presented in Fig. 4. DPVs was performed with the following parameters: modulation time 5 ms, interval time 200 ms, step potential 2 mV, modulation amplitude 100 mV. The surface of a glassy carbon electrode was carefully polished with a superfine Al2O3 powder before measurements. Concentration of the dyes was 2 mM. Before each measurement, an oxygen
2
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Fig. 2. Concentration dependencies of phase transition of the studied perylene based dyes.
corresponding molecule containing a single center. Only the magnitude of the current is enhanced by the presence of additional electroactive centers [39]. We do observe that the neutral PDI-Ph in the cathode region is characterized by the presence of only one transition, whereas for all derivatives having charged functional groups (Red 2416, Red 2334 and Red 2582), there are several reversible redox transitions. This effect may have complex explanation which is based on important factor of the molecular aggregation. While the substituents have little influence on the electrochemical behavior, a large effect is noted with extension of the aromatic system between the redox groups [40]. Being in aggregate the molecules are no more non-interacting. Within the aggregates, the molecules are associated through non-covalent interactions such as π-π attraction. They are closely packed at the distance of approximately 3.4A leading to a strong coupling between the aromatic perylene cores [41]. As the aromatic system increases, oxidation becomes easier and the potentials between the reduction steps become closer that may result in two, as for Red 2334, or even in three, as for Red 2582 and Red 2416, reversible redox transitions. Moreover, the
was removed from the electrolyte by 30 min purging of nitrogen. CVs and DPVs data presented in Figs. 3 and 4a shows the presence of two reduction peaks for Red 2334 and three reduction peaks for Red 2416 and Red 2582. The observed three reduction transitions for positively charged molecules (Red 2582 and Red 2416) as compared to Red 2334, may originate from hoping electron on positively charged imide position. In comparison with the neutral dye PDI-Ph, the potential of their first reduction peak, which determines the position of their LUMO, is substantially shifted to less negative potentials as E1/ 2416 < E1/22582 < E1/22334. Interestingly, the effect does not depend on 2 the electric charge of the substituent as it is observed for negatively charged Red 2334 dye and positively charged Red 2416 and Red 2582 dyes. This striking aspect in the behavior of charged perylene derivatives indicates the governing role of aggregation in the properties of mesogenic materials. Indeed, molecules containing a number of identical, noninteracting centers that accept or give up electrons exhibit currentpotential responses having the same shape as that obtained with the
Fig. 3. Cyclic voltammograms and UV–vis spectra (insets) for different materials indicated at the bottom of the corresponding graph. Bold lines on voltammogram curves demonstrate linear fit for calculation of reduction potential (Ered.1/2 ). Bold lines on UV–vis spectra demonstrate linear fit for calculation of optical gap (Eg . opt . ). 3
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Fig. 4. a) Differential pulse voltammograms for Red 2416, Red 2334, Red 2582 dies. b–d) Possible structure of aggregates, calculated for Red 2334 (AM1 approximation, HyperChem package): b) parallel dimer, c) rotated dimer, d) rotated dimer, top view.
with different background electrolytes, without taking into account the effects of the medium [48]. However, it has been shown that the replacement of water with organic solvents substantially affects this value [49,1–51]. The CV measurements were performed at the following conditions: glassy carbon (GC) work electrode, Pt wire counter-electrode, Ag/Ag+ (= +0.68eV vs. NHE) reference electrode, 0.1 M TBAPDMF electrolyte with v = 100mV / s . Therefore, empirically adjusted formula is of the form ELUMO = −(Ered.1/2 + 4.87eV ) , where 4.87eV = 4.19eV (ENHE in DMF ) + 0.68eV (EAg / Ag + vs NHE ) [50]. The positions of LUMO/HOMO energy levels of perylene derivatives measured in the present study are summarized in Table. 1. The data demonstrate that both positively and negatively charged mesogenic materials showed decrease in energy positions as compared to classical PDI-Ph. The data presented in Table 1 are schematically depicted in Fig. 5. The information on HOMO and LUMO energy levels can be used to estimate of the charge-carrier type of the studied material. Although no well-defined criteria certifying the charge-carrier type of a material are available in the literature, some reference HOMO/LUMO values, which might be used for this purpose, have been suggested. Using the data for HOMO/LUMO energy levels experimentally measured with the CV and light absorption techniques for a series of 19 functionalized acenes, Tang et. al. [30] have concluded that electron injection prevails if the LUMO energy level is below 3.15 eV and the hole injection is no longer favorable if the HOMO energy level is lower than 5.6 eV. It was also suggested that the materials showing the HOMO/LUMO energy levels within the range of [5.6, −3.15] eV, display ambipolar behavior. Below and above this ambipolar range, one expects purely electron- and purely hole-charge-carrier type, respectively. However, there is no evidence for the universality of this rule and its applicability to other materials, since these arguments may be altered by the existence of traps with different energy levels. The remarkable result of our study is that the deep-lying LUMO energy levels are found for all studied dyes with ionic substitutes, namely: -4.08 eV for Red2334, −4.12 eV for Red 2582 and −4.24 eV for Red 2416. The reports on organic molecules with the LUMO energy level below −4.0 eV are rare [52]. Importantly, the low-lying LUMO level is associated with relatively high efficiency of acceptor properties
aggregation has another merit to influence CV-traces. The commonly accepted idealized model according to which the chromonic aggregates are simple rods dispersed in the solvent is only a convenient simplification [42]. The attempt to explore the real structure of the aggregates built from perylene derivatives was performed in [43]. The authors suggested two types of molecular assemblies: the molecules are positioned exactly parallel with the polar groups localized at the different side of molecule (Fig. 4b parallel dimer); the molecules are rotated relative to each other (Fig. 4c and d rotated dimer). The growth in concentration accompanies a growth of the dimers number as well as the creation of aggregates with a larger number of molecules. For parallel dimers the spatial obstacles (large charged species) limit the creation of trimers, tetramers and so on with equivalent distance between the molecules of 3.4A. The only tetramers or higher associates with 2 N molecules and with alternative distances 3.4A and 5.5A are possible. In this case the translation unit is the parallel dimer. The rotated aggregates can be formed from the separate molecules. Theoretically, there are two possible aggregates: a) helix, when each molecules turns by some angle forming a helix; b) alternative, when an angle of rotation alternates as θ− and θ+ from one molecules to another. The all these species demonstrate different overlap of aromatic systems of molecules that influences observed CV traces. Also, the elegant mechanism of generating radical anions and dianions was described in [44,45]. Analyzing absorption spectra, the authors report that perylene diimide formed dimers may be transformed into a π-stacked aggregated form upon one-electron reduction. Addition of a second electron, however, resulted in dissociation of the aggregates, giving the dianion in the monomeric form. The experimental spectra however do not permit to make a clear choice between possible structure of aggregates [43,46]. The charged dyes exhibit no light emission in polar solvents. The only very dilute water solution (the concentration below 10−5 wt. %) demonstrates molecular emission for the dyes [43,47]. The maximum emission is slightly red-shifted from the absorption band. With increase in concentration the absorption band form does not noticeable change while the emission intensity dramatic diminishes. The correct interpretation of this phenomena is possible assuming that even at the lowest concentration LC molecules associate into H-type of dimers and aggregates with larger number of molecules (isodesmic aggregation) [13]. Further detailed studies would be desirable to explore the exact mechanism behind the electrochemical behavior of charged perylenes. For determination of EHOMO and ELUMO from CV data for charged molecules one switches from the electrochemical scale to the absolute potential scale. In the commonly employed empirical formulas one uses ELUMO = −(Ered.1/2 + 4.44) , EHOMO = −(Eox.1/2 + 4.44eV ; the value 4.44eV implies a normal hydrogen electrode (NHE) in aqueous solutions. This value is also used for electrochemical studies in non-aqueous solutions
Table 1 LUMO/HOMO energy levels for studied materials.
4
Sample
CV Ered .1/2
DPV Ered .1/2
ELUMO
Eg . opt .
EHOMO
PDI-Ph 2334 2416 2582
−1.00 −0.79 −0.63 −0.75
– −0.80 −0.62 −0.75
−3.44* −4.08 −4.24 −4.12
2.26 2.24 2.23 2.25
−5.70 −6.32 −6.47 −6.37
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Fig. 5. Diagram depicting data from Table 1 for LUMO and HOMO energy levels of the studied materials compared to the literature data for SdiCNPBI [52], PDI-4 [4] and the work function for Au.
of the materials and demonstrate a potential as alternatives to fullerene derivatives in bulk heterojunction organic solar cells [2,53,54]. Although perylene diimides molecules with amino or amino N-oxide as the terminal substituent were successfully implemented as high-performance organic interlayer materials [55,56], the liquid crystal approach provides further benefit achieved also via the enhanced orientational ordering of charged perylene diimides molecules and its aggregates over the large area.
3. Conclusions In summary, we investigated Lyotropic Chromonic Liquid Crystals that are formed by flat polyaromatic molecules with ionic groups at the periphery. Using cyclic voltammetry (CV) and light absorption techniques, we explore the role of functionalization of perylene-diimide derivatives with ionic end groups on the molecular LUMO/HOMO energy level positions. We found that both the positively and negatively charged mesogenic materials showed a decrease in energy positions as compared to the classical chemically-similar non-mesogenic PDI-Ph material. The ionic substitutes lowered both HOMO and LUMO energy levels by 0.6 eV without changing the band gap (˜2.24 eV) resulting in a LUMO level of less than −4 eV. Our results suggest that the noncovalent interaction between charged molecules act to decrease the lowest unoccupied molecular orbital. The findings point an effective way to develop promising nonfullerene acceptors for organic solar cells.
Author information Author Contributions: OB, BL and OP performed the experiments. YS synthesized the material. ST, YN and VG envisioned the experiments. The manuscript was written through contributions of all authors.
Funding The research was supported by the National Academy of Sciences of Ukraine within the projects 1.4.BC#202, 1.4.BC#188 and 1.4.В#186. The authors declare no competing financial interests.
Acknowledgments The authors are also acknowledged Prof. O. Lavrentovich and Dr. A. Kadashchuk for helpful discussion and critical reading of the manuscript. 5
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