Photophysical and Electrical Properties of Highly Luminescent 2/6-Triazolyl-Substituted Push–Pull Purines

New push–pull N(9)-alkylated 6-piperidino-2-triazolylpurine and 2-piperidino-6-triazolylpurine derivatives are synthesized, and their optical and optoelectronic properties are comprehensively characterized with experimental and computational methods. The compounds possess intense violet or blue fluorescence with fluorescence quantum yields of up to 91% in solution and 40% in host-free films. Depending on their structural composition, the compounds have ionization energy in the range of 5.25–6.04 eV, electron affinity of 2.18–3.15 eV, and triplet energy of 2.52–2.95 eV. Due to the presence of hole-transporting purine and electron-transporting triazole fragments, compounds exhibit bipolar charge-transportation ability. Despite the favorable emissive properties of the studied push–pull purines, their electroluminescence in thin films is quenched owing to large current densities that are present even at a moderate driving voltage. This marks application directions related to a predominantly charge-transportation functionality as the most suitable for this compound class.


■ INTRODUCTION
DNA nucleobases have been successfully incorporated in electron-blocking or hole-transporting layers of organic lightemitting diodes (OLEDs) due to their large band gap (E g ) values and suitable energy levels. 1,2 Additionally, carbazolefunctionalized DNA has been used as a charge-transporting cohost material in the active emissive layer (EML), consequently showing performance improvements in comparison to some conventional host materials. 3 Nucleobases or their derivatives have also been used in organic field-effect transistors (OFETs), mainly as the dielectrics. 4,5 Recent studies have shown that chemical modifications of one of the nucleobase cores, the purine heterocycle, by an introduction of electron donor and acceptor fragments, provide highly emissive push−pull-type compounds, whose fluorescence (FL) quantum yield (Φ FL ) can reach unity. 6−11 While the fluorescent nature of these compounds limits the potentially attainable efficiency of the corresponding purine-based OLEDs, singlet harvesting compounds still find a practical use in blue light-emitting devices due to the poor chemical stability of the triplet harvesting alternatives. 12 Several early reports of OLEDs showing electroluminescence in the important deep-blue and violet parts of the spectrum have been demonstrated using purine-based emitters. 13,14 In addition, it was recently reported that 9-methylpurine-based derivatives containing phenoxazine groups in the para-position of the phenyl substituent at C6 or C2 and C6 positions of purine possess thermally activated delayed fluorescence, further underlining the potential prospects of the compound class for the application in organic electronic devices. 15 On the other hand, purine derivatives containing phenoxazine or phenothiazine groups in the meta-position of the phenyl substituent synthesized by us reached only up to 15% quantum yields in PMMA-based films. 16 In 2021, we designed TADF-active exciplexes based on 6cyano-9-phenylpurine-carbazole dyads, studied their photophysical properties in the solution, film, and crystalline phase and successfully used them as host materials for the emissive layer in OLEDs. 17 In this paper, we investigate the practical application feasibility and possible limitations of fluorescent push−pull purines containing triazolyl moieties, focusing on the compound integration in solution-processed films with electroluminescent or charge-transporting functionality and studying the optical and optoelectronic properties of synthesized purine derivatives. A series of N(9)-alkylated 6(or 2)-piperidino-2(or 6)-triazolylpurine push−pull-type molecules were acquired for the purpose of the study. The photophysical and thermal properties of the compounds and their energy level configuration were experimentally determined. The experimental data were supplemented with quantum-chemical calculations. The results indicate that the compounds exhibit excellent chargetransporting ability, which is detrimental to the electroluminescence process but can be potentially exploited in purely charge-transportation-related areas of organic electronics.

■ RESULTS AND DISCUSSION
Material Design and Synthesis. The newly synthesized materials were based on a known highly emissive molecular scaffold, 18−23 where 2-or 6-amino-substituted purines are functionalized with electron-deficient 1,2,3-triazole moieties at the corresponding unoccupied 2-or 6-position. It has been shown that natural 2-or 6-amino-substituted purine derivatives, guanine and adenine, possess good hole-transporting ability. 1 The presence of 1,2,3-triazole moieties, on the other hand, is shown to enhance the electron transport properties of the molecule. 24 It was predicted that the suggested bipolar composition of the target molecules would facilitate balanced charge-transporting properties. To better understand correlations between the structure and photophysical properties of the compounds, differently substituted phenyl groups were attached to the triazole cycle. The poor solubility and inherent crystallinity of the corresponding purine derivatives were overcome with the introduction of 5,5,5-triphenylpentyl groups. Triphenylmethane-based derivatives are known to promote amorphous phase formation for otherwise crystalline polar molecules. 25 For the synthesis of the desired compounds, we used two different approaches (Pathway A and B), yielding compound series 5a−d and 9a−d (Scheme 1). The commercially available 2,6-dichloropurine 1 was used as the starting material. For the synthesis of compound 5, first, we introduced a triphenylpentyl group at the N(9) position of purine using DIAD, Ph 3 P, and corresponding alcohol under the Mitsunobu conditions. 26,27 Then, we substituted chlorine atoms at C(2) and C(6) positions with azido groups and next introduced piperidine as a donating group. For N(9)-alkylated substrates, substitution with piperidine took place at the C(2) position and subsequently the triazolyl ring was introduced at the C(6) position via Cu(I)catalyzed cycloaddition reactions with different alkynes. Previously reported methods have been used for the copper-(I)-catalyzed alkyne-azide cycloaddition (CuAAC) reactions to produce triazole moieties. 19,28 Products 5a−d were obtained with 66−81% yields (Scheme 1).
On the other hand, series 9 were obtained using the same reaction conditions but in a reverse sequence (Scheme 1). First, unprotected 2,6-dichloropurine 1 was treated with NaN 3 followed by an S N Ar reaction with piperidine. In the case of N(9)-H 2,6-diazidopurine, the S N Ar reaction with piperidine took place regioselectively at the C(6) position, which is opposite to N(9)-alkylated substrates. Next, product 7 was alkylated at the N(9) position and products 8 were cyclized with different alkynes to target products 9a−d. These 1,3-dipolar cycloaddition reactions required longer times compared to previous ones but still produced products 9a−d with 73−88% yields. For example, a reaction with 4-dimethylaminophenylacetylene required 264 hours to be completed. An increase of the temperature to accelerate the cycloaddition was impossible due to the instability of azido starting materials. Moreover, all of the steps, where the azido group was involved, were performed while being covered from the day light. All azido compounds (4,7,8) should be stored at temperature ≤ −20°C to prevent degradation.
Phase Behavior and Thermal Characteristics. With the introduction of triphenylpentyl groups that enhance solubility and promote the formation of an amorphous phase, the synthesized compounds were primarily designed as being suitable for solution-based processing methods. The resulting purine derivatives are highly soluble in moderate polarity solvents (CHCl 3, DCM, THF) and form amorphous films upon spin-coating from the corresponding solutions. To closely investigate the solid-state phase behavior, the selected compounds 5a, 5d, 9a, 9b, and 9d were studied by employing differential scanning calorimetry (DSC). From this series, only compound 9d showed the presence of a crystalline state upon the first heating cycle (melting temperature 176.0°C). In the case of other compounds, glass transition temperature (T g ) was detected as the only phase transition. DSC plots of the second heating cycle for the purine derivatives are given in Figure 1. The obtained T g values vary in the range of 82−102°C. These parameters are sufficient to ensure the structural integrity and to prevent phase transition processes in optoelectronic devices under typical device operation conditions. For comparison, T g of a conventional charge-transporting host material as 4,4′-bis(Ncarbazolyl)-1,1′-biphenyl (CBP) is 62°C. 29 Structural correlations among the series of the synthesized compounds reveal that T g increase in the range of 13−20°C can be linked with the presence of the cyanophenyl group (compounds 5d, 9d). As it is indicated by DFT calculations, which evaluate the ground state dipole moment (μ) values of 5d and 9d at μ = 11.9 and 10.5 D (Table S1), these molecules are the most polar among the examined series. Further discussion concerning molecular orbital placement relates this to an increased spatial separation of electron-rich and poor regions of the molecules. Since μ can be associated with a strong amplification in the force of solidstate intermolecular interactions, 30 the consequential rigidification of the glassy lattice manifests in increased T g for 5d and 9d. The same explanation can be given for the outlier crystallization tendency for compound 9d, which is also caused by the high polarity of the compound.
The presence of N-heterocyclic molecular fragments has often been linked to a reduced thermal and chemical stability of the corresponding molecules. 31,32 Thermogravimetric analysis was performed to evaluate the thermal stability of the compounds 5d and 9d ( Figure S1). The results show that both the molecules possess good thermal stability with the corresponding decomposition temperatures of 298 and 258°C. A notable increase in stability can be observed for a purine derivative with a 2-amino-6-triazolyl substitution pattern in comparison to a 2triazolyl-6-amino counterpart.
Photophysical Characterization. The synthesized materials were initially characterized employing UV−vis absorption and photoluminescence (PL) spectroscopy. The obtained absorption and emission bands are given in Figure 2, and band maxima values are outlined in Table 1. Taking into account the structural impact on the photophysical properties, the placement of substituents at the purine ring in either 2-amino-6triazolyl (series 5) or 2-triazolyl-6-amino (series 9) arrangement has the most notable impact on electronic transitions. In the case of 5, UV−vis absorption spectra feature intense bands in the 250−300 nm range that can be attributed predominantly to the local excitation (LE) of the purine aromatic system. 33,34 The lowest energy bands with maxima at 360−367 nm, on the other hand, correspond to intramolecular charge-transfer (ICT) transitions. The FL spectra of 5 exhibit wide featureless emission bands in the blue spectral region (maxima at 439−448 nm). Considering the aromatic substituents at the 1,2,3-triazole ring, only the introduction of N,N-dimethylaniline (5c) causes a notable deviation in emissive properties, redshifting the FL band by approximately 60 nm. We relate this observation to a stronger solvatochromic response of the compound and the resulting stabilization of the excited state energy level. This is presumably caused by the presence of nitrogen lone pair, which may enhance specific solute−solvent interactions, such as hydrogen bonding. 35 Because of this, dialkyl substituted amines are common structural fragments met in strongly solvatochromic dyes. 36 Φ FL values for 5 vary in the range of 0.74−0.91 in solution. Generally, the introduction of electron-donating groups at 1,2,3-triazole can be associated with reduced Φ FL . In spin-coated amorphous films, the compounds retain fairly large emission efficiency (Φ FL = 0.40−0.28), indicating a moderate extent of solid-state quenching, 37,38 with the exception of 5c, which shows a dramatic emission efficiency drop to 0.03.
For the 2-triazolyl-6-amino purine series (9a−d), the lowest energy absorption and FL bands show a substantial hypsochromic shift in comparison to the structural analogues from the 6-triazolyl-2-amino purine series (5a−d). ICT absorption bands (λ abs max 300−305 nm) now partly overlap with LE transitions, while emission has shifted to the violet part of the spectrum (λ em max 388−455 nm). Analogous to the previously observed behavior, only the introduction of the N,Ndimethylaniline fragment to the 1,2,3-triazole moiety causes a notable redshift of absorption and FL bands. Φ FL values in solution for 9a−d (0.17−0.44) are notably lower than those measured for 5. On the other hand, in amorphous films, series 9 show a relatively lesser FL efficiency drop (0.15−0.22), with the exception of N,N-dimethylaniline-substituted 9c.
All compounds show monoexponential emission decay curves, associable with a fluorescence process ( Figure S2). Lifetime values vary in the range of 11−13 ns for series 5, while for compounds 9, the emissive relaxation to the ground state proceeds substantially faster, in the range of 2−8 ns ( Table 1). The corresponding radiative (k r ) and nonradiative (k nr ) decay rates (Table S2) indicate that k r is somewhat comparable among all compounds, but k nr is notably increased for series 9. It is worth noting that the FL lifetime of the compounds is longer than typical values for purine-based emitters. 6 As shown by Collier et al., the emission rate in such emitters decreases as the π-conjugation system of the emitter expands and the chargetransfer process is not confined to the purine moiety but assumes an intramolecular charge-transfer character. 9 A similar situation is observed for our compounds, where the emissive process involves charge transfer between different aromatic fragments.
All of the studied compounds show a positive solvatochromic response for FL bands, without notable band shape trans-  spin-coated amorphous films of the synthesized compounds were determined by photoemission yield spectroscopy, to find ionization energy (I E ), and photoconductivity measurements, to determine the photoconductivity threshold value (E th ). The electron affinity level (E A ) was then calculated as the difference between I E and E th . The obtained values are outlined in Table 1.
In agreement with the previously discussed results of photophysical characterization, the electronic level measurements confirm a clear distinction between compounds 5 and 9. E th of 5 (2.75−2.95 eV) are lower than for 9 (3.07−3.45 eV), indicating notably higher band gap values for the latter series of compounds. Several structure−property relationships can be observed. First, among the structurally comparable compound pairs that contain identical aryl substituents at the 1,2,3-triazole ring, the I E value is almost not influenced by the attachment pattern of amino and triazole substituents at the purine ring. This energy level is only affected by the substituents at the 1,2,3triazole-attached phenyl ring. The A E level placement, on the other hand, is affected by both these structural variations. In general, the introduction of the N,N-dimethylaminophenyl fragment at 1,2,3-triazole causes destabilization of I E and A E levels. This could be explained by the resulting donor− acceptor−donor (D−A−D) type configuration of the resulting conjugated molecules that reduce delocalization energy of the push−pull system. In contrast, the presence of the electrondeficient cyanophenyl group causes stabilization of I E and A E , as the molecules form an extended D−A system now. The compounds that contain an electronically neutral phenyl group show the largest E th values, as the lack of electronaccepting or -donating groups reduces their overall push−pull character.
The determination of triplet energy levels (E T ) is crucial for host materials that are being designed for triplet harvesting emitters. If E T of the charge-transporting material is higher than that for the emissive compound, exothermic excited triplet state migration proceeds to the emitter. If the situation is reversed, the triplet states largely stay on the host molecules, negatively affecting the corresponding OLED efficiency. 39 E T of the synthesized compounds was determined by employing lowtemperature phosphorescence measurements in a frozen 2methyltetrahydrofuran matrix at 77 K. Triplet energies were assigned at the highest energy phosphorescence band maxima (see Figures 3 and S3). For series 5, E T values vary in the range of 2.52−2.54 eV (Table 1), but for 9a, b, d, they are 2.87−2.95 eV, with 9c being an outlier at 2.75 eV. These values suggest that compounds 9 can be potentially used as host materials for bluephosphorescent compounds.
Quantum-Chemical Calculations. DFT calculations for the synthesized compounds were performed to better explain the correlations between the synthesized structures and their photophysical properties. The numerical values of the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energy levels, and singlet and triplet excitation energies are outlined in Table 1, but the visualizations  of the orbitals are given in Figure 4. The optimized structures of the compounds assume conformations, where all of the present aromatic ring systems are coplanar, indicating that π-electrons are delocalized in a conjugated system and no sterical overcrowding effects take place that would result in a twisted geometry. Generally, a good agreement between the experimental and DFT values can be found, as the calculated HOMO and LUMO energy level values correlate well with the measured ionization potentials and electron affinities. As it can be concluded from the previously discussed experimental results, regarding the substitution pattern at 2and 6-positions, the photophysical and electronic characteristics of the compounds change drastically. This discrepancy between 5 and 9 is partly explained by the molecular orbital configuration of the compounds. In the case of 5a, purine-centered HOMO and triazole-centered LUMO are much more spatially separated than for 9a, where these frontal orbitals are delocalized all over the conjugated ring system and largely overlap. This points to a more pronounced push−pull characteristic for 5a and its analogues. The assumption is additionally supported by the calculated band gap values that show a substantial drop in ΔE HOMO−LUMO for 5 in comparison to 9. The differing nature of the molecular orbital configuration may also explain the detected drastic variations in nonradiative excitation decay rates (k nr ) between the two compound subsets. The less contained molecular orbitals for series 9 compounds would result in the excited states that are delocalized across multiple aromatic ring systems, thus increasing the susceptibility to relaxation pathways through vibrational or conformational motion processes.
The introduction of electron-donating groups at the 1,2,3triazole fragment drastically changes the molecular orbital configuration for the compounds, as HOMO then localizes on the corresponding electron-rich aryltriazole moiety, while LUMO is mostly confined to the purine fragment. Despite this transformation of the electronic system, in terms of molecular orbital energy values, this causes no major deviations in regard to other synthesized compounds. For 5d and 9d, the presence of the electron-accepting cyanophenyl group enhances the spatial separation between the purine-centered HOMO and the triazole-based LUMO by shifting the LUMO to the corresponding electron-deficient benzene ring and the cyano group. Such increased push−pull characteristic is considered as the origin for the experimentally observed stabilization of I E and A E levels in comparison to the rest of the synthesized compounds. This also reflects in the increased calculated ground state dipole moment values of the compounds.
Optoelectronic Properties. The evaluation of the synthesized compounds for use in optoelectronic devices was initially carried out by the preparation of OLEDs, where solution-processed EMLs consisted only of the synthesized purine derivatives. Purines with the highest fluorescence quantum yield were chosen for OLED preparation, namely, 5a, 5d, 9a, and 9d. In all of the cases, no electroluminescence originating from purine derivatives was observed. At the same time, the prepared OLEDs showed relatively large current densities, as the values exceeding 100 mA/cm 2 were measured at a driving voltage of 5−7 V ( Figure S4). High current can be proposed as the main reason for the lack of detectable emission. Polaron concentration in EML at such current levels is high and, most likely, an extensive polaron-exciton quenching takes place. In addition, high current promotes a current leakage. The prevalence of nonradiative exciton recombination pathways under such conditions results in excessive heating that can further reduce the performance parameters of light-emitting diodes. 40 An unbalanced charge carrier injection in the devices was proposed as another possible cause for the lack of measurable emission.
After unsuccessful attempts to use the purines as the emitting compounds, these compounds were tested as potential chargetransporting host materials for phosphorescent triplet emitters. Based on its highest measured triplet energy value, compound 9b was chosen as the most suitable material for such a purpose. Tentative OLEDs with EML composed of conventional iridium(III) blue emitter FIrpic (10 wt %) and 9b were prepared. Unfortunately, an identical result as in purine-only samples was obtained since devices exhibited no electroluminescence. To better understand the possible cause for this poor OLED performance, a multi-host device with ITO/ PEDOT:PSS/EML/BPhen/LiF/Al architecture was prepared, where EML consisted of FIrpic/9b/CBP (10:30:60 wt %) ( Figure 5). In this case, a measurable amount of luminance was detected, originating from the phosphorescent host. In comparison with a reference device with FIrpic/PVK/CBP (10:30:60 wt %) EML, the purine bearing OLED showed a notable increase in current density, indicating improved charge transport properties. The device also showed improved efficiency parameters at the voltage range slightly above the luminance onset mark. The further driving voltage increase, however, resulted in a sharp decrease in luminance, while the reference device experienced a steady rise in current efficiency. Such behavior indicates a better charge carrier density and balance at turn-on voltage levels in a purine-containing device, but a further voltage increase causes a rapid deterioration in the operational stability. Two possible reasons could be mentioned for the steep OLED efficiency decrease. Although betterbalanced charge carries in the system, there is still high leaked current (around 10 mA/cm 2 at turn-on voltage) so that exciton quenching by polarons takes place at higher driving voltages. Second, the prepared emitter layer consists of only small molecules that could lead to higher FIrpic emitter aggregation compared to the system with an added polymer host (PVK). Consequently, exciton−exciton annihilation will be observed and will result in efficiency roll-off. The roll-off could be decreased by optimizing the ratio between 9b and CBP. At a voltage of 8 V, a partial current shortage was observed that destroyed the OLED and no electroluminescence was observed.
Finally, single charge carrier devices were prepared using architectures ITO/LiF/purine 5d or 9d/LiF/Al for electron-only or ITO/MoO 3 /purine 5d or 9d /MoO 3 /Cu for hole-only transport measurement purposes ( Figure 6). The obtained results indicate that the compounds possess bipolar chargetransporting properties, as the hole and electron-only devices have similar voltage−charge density dependencies. At the same time, both compounds show very high current levels. Even at voltages as low as 2 V, the current density exceeds 1 A/cm 2 and this parameter reaches the 3 A/cm 2 mark with a voltage increase to 7 V. These current density values are more than an order of magnitude greater than those measured for readily applied OLED charge-transporting host materials. 41,42 This result confirms that the large current density in EML is the main reason for the poor OLED performance of the studied push− pull purines. The good charge-transporting ability of the acquired triazolyl functionalized purines can be attributed to their planar geometry and dipolar nature. Our previous investigation of closely similar structures has revealed that in the crystalline state these compounds tend to form highly ordered vertical stacks that are determined by dipolar π−π interactions between the electron-rich purine and electrondeficient triazolyl ring systems. 43 Taking into account the good charge-transporting ability of the synthesized push−pull purines, their use as semiconductors in organic field-effect transistors (OFETs) or other devices with purely chargetransporting functionality can be proposed as the most promising application directions.
Interestingly, slight variations can be observed between charge transport properties for 2-amino-6-triazolyl (5d) and 2triazolyl-6-amino (9d) configured compounds. While both compounds show almost overlapping current density−voltage dependency in the hole-only mode, 5d exhibits vaguely better electron transportation ability. At the same time, 9d is more prone to hole transport. The calculated distribution of frontier molecular orbitals for the corresponding compounds (Figure 4) shows that the HOMO in both cases similarly lies on the purine ring system, explaining the almost identical hole-transporting properties. The LUMO, on the other hand, is located mainly on the cyanophenyl ring for 9d but is delocalized over the whole purine−triazole−phenyl system for 5d. Increased LUMO delocalization seems to be beneficial for the overall electrontransporting ability of the push−pull purines.

■ CONCLUSIONS
The optical and optoelectronic properties of push−pull N(9)alkylated 6-piperidino-2-triazolylpurine and 2-piperidino-6triazolylpurine derivatives have been investigated. The interchangeable 2/6-substitution pattern of the studied compounds provides a molecular platform in which frontier molecular orbital energy levels can be greatly modified. This allows obtaining molecules with ionization energy in the range of 5.25− 6.04 eV, electron affinity of 2.18−3.15 eV, and triplet energy of 2.52−2.95 eV. The compounds also exhibit strong fluorescence in violet and blue spectral regions, with the measured Φ FL in solutions reaching 0.91. The functionalization of purines with phase modifying groups prevents crystallization of compounds and reduces solid-state emission quenching, allowing the preparation of amorphous spin-coated host-free films with Φ FL of up to 0.40. While the emissive properties and high triplet energy levels could mark the synthesized compounds as promising candidates for either emitter or host materials in the emissive layers of OLEDs, no electroluminescence was observed in the corresponding devices. This can be attributed to an excessive charge-transporting ability of the compounds that leads to either current leakage or strong emission quenching due to the large current density in the emissive layer. On the other hand, the bipolar character and good charge-transporting ability of the investigated compounds can potentially be exploited to provide materials in other areas of organic electronics, such as organic field-effect transistors.
■ EXPERIMENTAL PART General Information. 1 H-and 13 C-NMR spectra were recorded on Bruker Avance at 300 and 75.5 MHz, respectively. The proton signals for residual nondeuterated solvents (δ 7.26 for CDCl 3 , δ 2.50 for DMSO-d 6 ) and carbon signals (δ 77.1 for CDCl 3 , δ 39.5 for DMSO-d 6 ) were used as an internal reference for 1 H-and 13 C-NMR spectra, respectively.
Optical measurements of solutions were carried out at a typical material concentration of 1 × 10 −5 mol L −1 . The UV−vis absorption spectra were recorded with a Perkin Elmer Lambda 35 spectrometer. Films for optical measurements were prepared using a spin-coating technique with a Laurell WS-400B-6NPP/ LITE spin-coater on glass slides, using CHCl 3 solutions with a material concentration of 30 mg/mL. After the preparation, all films were dried in an oven at 100°C for 2 h. Emission spectra, Φ FL , and lifetime values in solutions or thin films were measured using a QuantaMaster 40 steady-state spectrofluorometer (Photon Technology International, Inc.) equipped with a 6inch integrating sphere by LabSphere, using the software package provided by the manufacturer. For lifetime measure-ments, excitation at 365 nm was used, while emission was detected at the respective band maxima. Low-temperature emission measurements were carried out by cooling the cuvettes in a liquid nitrogen-filled quartz Dewar. Phosphorescence spectra were recorded using time-gated detection by collecting emission in a 50−5000 μs interval after the excitation. Differential scanning calorimetry (DSC) thermograms were acquired using a Mettler Toledo DSC-1/200W apparatus. The samples were heated from 50 to 190°C at a rate of 10°C min −1 and isothermally aged for 5 min, then cooled to 50°C at a rate of 10°C min −1 and isothermally aged for 5 min. The cycle was repeated 3 times. Decomposition temperatures were obtained using a Perkin Elmer STA 6000 thermal analyzer.
The molecular ionization energy in a thin film (I E ) and photoconductivity measurements (E th ) were carried out on a self-made experimental system using a method described in our previous work. 44 The values of electron affinity energies in thin film (A E ) were calculated using the difference of experimentally determined values of E th and I E .
Density functional theory (DFT) calculations were performed using the ORCA 45 program package. For auxiliary tasks, the Avogadro program 46 was used. The geometry optimizations for all compounds were obtained using nonlocal functional B3LYP with a 6-311G** 47 basis set. Ground state dipole moment (μ) values were calculated at the same theory level. Initial atomic coordinates were chosen based on X-ray analysis data of closely related structures. 43 For TDDFT calculations, def2-TZVP basis, def2/J auxiliary basis, and the RIJCOSX approximation were used. To reduce the computational cost, the structures were simplified by substituting triphenylpentyl moieties with ethyl groups.
Sandwich-type samples with the pixel size of 16 mm 2 were prepared for the OLED, electron-only, and hole-only devices. Indium tin oxide (ITO) glass (Prazisions Glas & Optik GmbH) with a sheet resistivity of 50 Ω 2 was used as a substrate in all cases. A 12 mm wide ITO strip line was made by wet etching at the middle of the substrate. ITO substrates were cleaned by the following method: sonicated in CHCl 3 ; sonicated in acetone; rinsed two times with deionized (DI) water; sonicated in water with 3 vol % of Hellmanex II detergent; rinsed with DI water; and sonicated in DI water and isopropyl alcohol.