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Impact of Donor Substitution Pattern on the TADF Properties in the Carbazolyl-Substituted Triazine Derivatives Tomas Matulaitis, Paulius Imbrasas, Nadzeya A. Kukhta, Paulius Baronas, Tadas Buciunas, Dovydas Banevicius, Karolis Kazlauskas, Juozas Vidas Grazulevicius, and Saulius Jursenas J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08034 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 12, 2017

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The Journal of Physical Chemistry

Impact of Donor Substitution Pattern on the TADF Properties in the Carbazolyl-Substituted Triazine Derivatives Tomas Matulaitis,† Paulius Imbrasas,‡ Nadzeya A. Kukhta,†# Paulius Baronas, ‡ Tadas Bučiūnas, ‡

†

Dovydas Banevičius, ‡ Karolis Kazlauskas, ‡ Juozas V. Gražulevičius,*,† and Saulius Juršėnas‡ Department of Polymer Chemistry and Technology, Kaunas University of Technology,

Radvilenu pl. 19, LT-50254, Kaunas, Lithuania. ‡

Institute of Applied Research, Vilnius University, Sauletekio 9-III, LT-10222 Vilnius,

Lithuania. #

Current address: Department of Chemistry, Durham University, South Road, DH1 3LE, United

Kingdom

ABSTRACT

In this work we report on the synthesis and photophysical investigation of new star-shaped triazine-carbazole

derivative

2,4,6-tris(3-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)-1,3,5-

triazine. Comparative study of the photophysical properties of the newly synthesized emitter

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along with its para-substituted isomer 2,4,6-tris(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)1,3,5-triazine was performed. While para-linkage caused higher oscillator strength of the lowest energy absorption band and high fluorescence quantum yield, the meta-linkage resulted in stronger charge transfer character as well as higher triplet energy. Delayed emission of metaisomer was found to be by 3 orders of magnitude more intense than that of para-isomer. Temperature dependent measurements of meta-isomer confirmed the thermally activated delayed fluorescence origin of its delayed fluorescence with the activation energy of 0.07 eV. Organic light emitting diode containing this emitter dispersed in bis[2-(diphenylphosphino)phenyl] ether oxide with emission spectrum peak at 475 nm was fabricated. Commission Internationale de l'Éclairage color coordinates corresponded to a sky-blue emission color (0.16, 0.23). The turn-on voltage of the electroluminescent device was found to be in the range of 5-6 V with a maximum external quantum efficiency of 9.5%. These results confirm the importance of the linking pattern between donor and acceptor moieties in the molecular design of thermally activated delayed fluorescence emitters.

INTRODUCTION After the first application of thermally activated delayed fluorescence (TADF) in organic light emitting diodes (OLEDs)1, a great deal of interest has been paid to the design, synthesis and application of new, purely organic emitters exhibiting TADF2,3. The basic principle of TADF is the equalization of the lowest singlet and triplet energy levels, which enables thermally activated reverse intersystem crossing (rISC) of non-emissive triplet excitons back to emissive singlet state1, thus internal quantum efficiency of 100% can be achieved. Gibson et. al.4 recently


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reported that the main driving force of TADF is the ISC, induced by spin-orbit coupling (SOC) between local triplet (3LE) and charge-transfer singlet (1CT) states. This process by itself is a second order process, since before SOC can appear, 3LE must vibronically couple with the close in energy lying charge-transfer triplet (3CT) state. This theoretical model was further experimentally confirmed in different molecular systems5,6. It was discovered that direct SOC between 1CT and 3CT is too small to mediate the rISC; instead, the mixing of 3CT with close in energy lying 3LE state of the donor facilitates the efficient rISC. These observations clearly indicate the equal importance of 3LE and 3CT, as well as nπ* states in efficient TADF molecular systems. Derivatives of triazine and carbazole are versatile bipolar materials which have found applications as high triplet energy hosts for blue/green phosphorescent OLEDs7–13 as well as TADF emitters.14–23 Thermal, electrochemical, environmental stability, ease of functionalization of both carbazole and triazine derivatives make these units attractive for the design of stable devices24 including the most stable blue-emitting TADF OLEDs.14,23,24 In 2012, Su et al.11 reported a C-3 symmetry possessing host material with meta- linked triazine and carbazole substituents (TCPZ) with a small singlet-triplet exchange energy and have successfully applied it in RGB and white phosphorescent OLEDs (PhOLEDs) with ultra-low driving voltage as well as sufficient triplet exciton confinement. However, excimer emission in blue and green devices was observed. This phenomenon was further investigated by Wang et al.10, who successfully fabricated PhOLEDs in panchromatic range using TCPZ as host. In former studies TCPZ was examined as a host, however, its properties as a TADF emitter have not been reported yet. It is known that meta-conjugated molecular systems tend to exhibit


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reduced values of PLQY as compared to those of para-conjugated analogous systems.25 However, for electron donor-acceptor (D-A) systems it is not always the case.26 Shizu et. al.27 investigated D-A-D molecular systems, consisting of triazine core and two carbazolyl arms, structurally similar to that of previously reported TCPZ; however, TCPZ is a C-3 symmetry possessing compound with three carbazoles connected to the central triazine core. It was shown by Shizu et. al. that by changing the linking pattern from para- to meta-phenylene in these isomeric triazine-carbazole systems it is possible to enable the TADF phenomenon. However, to our knowledge those emitters were not tested in OLEDs. Dendrimeric conjugated systems with higher degree of branching usually possess enhanced photoluminescence quantum yield (PLQY), higher thermal stability, better film forming ability and even self-hosting.23,28–30 Due to these fundamental advantages of dendritic structures, we decided to modify the existing TCPZ structure by adding branched tert-butyl alkyls, expecting to create thermally, electrochemically stable compound, which could be applied as a potential TADF emitter. In this work we present the synthesis and properties of the new compound 2,4,6tris(3-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)-1,3,5-triazine (TmCZ). Since in TmCZ metalinking pattern is used to connect donors to the core, comparison of its properties to its paralinkage possessing isomeric derivative TpCZ31 allows to highlight the isomeric differences, which influence TADF properties of the emitters.

SYNTHESIS Synthesis of TmCZ is presented in Scheme 1. Compounds 1, 2 and 3 were synthesized according the literature procedures 32, 33 and 34, respectively. 2,4,6-tris(3-(3,6-di-tert-butyl-9H-


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carbazol-9-yl)phenyl)-1,3,5-triazine (TmCZ) was prepared by Ullmann’s coupling reaction.35 2,4,6-tris(m-iodophenyl)1,3,5-triazine (2) (0.80 g, 1.16 mmol), 3,6-di-tert-buthylcarbazole (3) (1.56 g, 7.00 mmol) and 18-crown-6 (0.08 g, 0.12 mmol) were dissolved in o-dichlorobenzene (20 mL) and the resulting solution was heated to reflux under nitrogen atmosphere. Then, K2CO3 (1.64 g, 3.96 mmol) and Cu (0.44 g, 2.32 mmol) were added. After being stirred for 24 h (TLC control), the reaction mixture was cooled down to the room temperature and filtered. The residue was carefully washed with chloroform and the solvent was evaporated under vacuum. The product was purified by silica gel column chromatography, using the mixture of hexane and dichloromethane in the volume ratio of 4:1 as an eluent, and recrystallized from the eluent mixture of solvents to afford the off-white crystals (0.95 g, 72% yield). m.p. = 385-387 oC (DSC 388 oC). 1H NMR (400 MHz, CDCl3, δ ppm): 8.89 (d, J = 8.7 Hz, 3H), 8.72 (d, J = 8.0 Hz, 3H), 8.24 (d, J＝7.3 Hz, 6H), 7.78 (d, J = 7.7 Hz, 3H), 7.70 (t, J = 8.0 Hz, 3H), 7.40 (d, J = 8.0 Hz, 6H), 7.33 (t, J = 7.0 Hz, 6H), 1.50 (s, 54H, CH3-). 13C NMR (75.5 MHz, CDCl3, δ ppm): 171.4, 140.8, 138.3, 137.8, 131.3, 130.3, 128.1, 127.6, 126.1, 123.4, 120.3, 120.1, 109.6, 35.1, 32.3. Elemental analysis found: C, 85.23; H, 7.41; N, 7.36%; molecular formula C81H84N6 requires: C, 85.22; H, 7.42; N, 7.36%. MALDI-TOF MS (m/z) calculated for C81H84N6 1141.61 (M+ + H), found: 1140.


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Scheme 1. Synthetic pathway towards TmCZ.

GEOMETRIES AND ELECTRONIC PROPERTIES The molecular geometries of the compounds in ground state were optimized using density functional theory (DFT) calculations with B3LYP/6-31G (vacuum) level of theory (Fig. S3). Strong electron affinity of 1,3,5-triazine affects the adjacent phenyl rings at C-2, C-4 and C-6 positions of triazine unit. The lack of steric hindrance predetermines planar conformation of 2,4,6-triphenyl-1,3,5-triazine moiety. Carbazole units linked to phenyl rings via C-N bonds, are twisted by ca. 48° in TpCZ and by 51° in TmCZ. Fig. 1 represents the summary of theoretical calculations results along with the experimental UV/Vis absorption spectra for the compounds under consideration. Understanding the origin of the first excited states in D-A compounds is important, since these states determine the overall photophysical behavior of compounds. Interpretation of only HOMO and LUMO plots is not informative enough, since these orbitals represent one-electron wave function. Moreover, in the case of D-A compounds, the frequently used B3LYP functional underestimates the energy of


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electronic states.36,37 To assess these issues and correctly interpret the experimental results, we have employed the Optimal Hartree-Fock (OHF) method, developed by Huang et. al.38 The authors revealed a strong relationship between the amount of charge transferred in the excitation process and the optimal Hartree-Fock exchange energy percentage (HF%) in exchangecorrelation (XC) functional. According to the OHF method, the optimal HF for TpCZ was found to be 31%, and that for TmCZ was estimated to be 35% (see S.I. for details). Consequently, vertical transitions, as well as natural transition orbitals (NTOs)39 were simulated with MPW1B9540 (31%) functional. It should be noted that standard long-range-corrected functionals tend to overestimate the excitation energy of TADF compounds38,41, which is also true in our case (Fig. S5). Theoretical and experimental (1 wt. % solid solutions in cyclo-olefin polymer Zeonex) absorption spectra of the compounds are presented Fig. 1a.

Figure 1. (a) Experimental absorption spectra (green curves, solid solution in Zeonex) and theoretical transitions (blue columns, MPW1B95/6-31G in vacuum) of TpCZ and TmCZ. The inset represents zoomed lowest energy band of TmCZ. The lowest energy transitions are depicted and (b) corresponding NTOs are presented.


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Evidently, the careful selection of the appropriate functional provided eligible results. The first transitions of both compounds coincide with the maxima of the lowest energy bands of the experimental spectra of the solid solutions in Zeonex. Para-conjugation in TpCZ is responsible for substantially higher oscillator strength (0.73) of the first electronic transition, as compared to that of TmCZ with meta-conjugation (0.008). The similar tendency can be observed in the experimentally obtained spectra – the lowest energy band (LEB) of TpCZ (360-440 nm) is of higher intensity, as compared to that of TmCZ (Fig. 1a). In order to identify the origin of the first (S0→S1,2 for TpCZ and S0→S1-3 for TmCZ) electronic transitions, one may find it difficult to analyze the overall impact of major excitations, especially when a singular transition consists of multiple excitations.36 Therefore, we have assessed the NTOs (Fig. 1b) of these degenerate transitions of the compounds under investigation. Apparently, para-conjugation results in higher degree of π-π* and n-π* mixing, since the orbital delocalization area in TpCZ spreads through N-9 carbazole atom, 2,4,6-phenyl substituents of triazine moiety and 1,3,5-N atoms of triazine. Therefore, these transitions are highly allowed, since there is significant amount of π-π* character in them. On the other hand, meta-linkage is responsible for the larger excited state charge separation (q): in this case, N-9 carbazole atom and 1,3,5-N atoms of triazine moiety omitted the orbital delocalization area, indicating smaller degree of π-π* and n-π* mixing in TmCZ. The analysis of the frontier orbital contributions corresponding to these transitions (Table S1, Fig. S3) also points at the CT nature, and the larger excited state charge separation in TmCZ.

THERMAL AND ELECTROCHEMICAL PROPERTIES


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Investigation of thermal characteristics by the combination of differential scanning calorimetry and thermogravimetric analysis revealed high thermal stability of both compounds. For TmCZ 5% mass loss temperature (TID) was detected to be as high as 447 °C (Fig. S1). However, paraisomer demonstrated even higher TID, which exceeded 514 °C. Glass transition temperatures of both isomers are similar (226 and 222 °C for TpCZ and TmCZ, respectively), while TmCZ showed a lower melting point (388 °C) than para-isomer by 60 °C. Estimation of the energy levels of the compounds is required in order to implement them as emitters in OLED stacks, for this reason cyclic voltammetry (CV) method was invoked. During CV experiments both compounds demonstrated bipolar behavior. Application of positive or negative voltage resulted in reversible oxidation and reduction of the compounds (Fig. S2), which means that they are capable of forming stable cations and anions within the applied potential range. The calculated values of ionization potential (IPCV) and electron affinity (EACV) were found to be 5.68 eV and 2.83 eV for TpCZ and 5.65 eV and 2.78 eV for TmCZ, respectively.

STEADY-STATE PHOTOPHYSICS One of the key parameters for TADF compounds is the singlet-triplet energy splitting (∆EST). Small ∆EST generally facilitates reverse intersystem crossing (rISC), thus enhancing the efficiency of TADF.4,22,42–44 However, estimation of ∆EST is complicated and the results obtained must be considered with caution.45–47 In D-A systems there are two competing singlet states (local excitation 1LE and charge-transfer 1CT) and two types of triplet states (3LE and 3CT) involved in the emission mechanisms.48 1CT singlet state is known to be sensitive to the polarity


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of the surrounding media48 and it is also the state involved in the TADF process49, while 3LE is usually non-emissive at room temperature, not affected by the polarity of the surrounding media, and is also involved in TADF. Determination of the T1 energy level is possible by measuring emission spectrum at low temperatures. Then, by choosing a host of certain polarity it is possible to tune the position of 1CT level, shifting it as close to T1 as possible, thus minimizing ∆EST.44

Figure 2. Steady-state photoluminescence spectra of TpCZ and TmCZ: (a) of solvents of different polarity and (b) of the compounds dispersed in different hosts in ambient and degassed atmosphere. Phosphorescence spectra from solid solutions (1 wt. %) in Zeonex at 77 K is also depicted. Interesting observation can be made by analyzing PL spectra of both isomers in the most nonpolar media: hexane and Zeonex (Fig. 2, violet curves). Since polarity is almost the same, the main difference between these two types of media is the degree of molecular motion. In solution the molecules are free to move, rotate and adjust to various factors that may impact the molecular geometry. However, in solid state the freedom for molecules to change their conformation is heavily suppressed. Therefore, the identical spectra of the solution of TmCZ in


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hexane and Zeonex indicate that molecular conformations remain basically the same, which suggests that molecules of TmCZ are more rigid than those of TpCZ. In the meantime, more of rotational freedom around D-A bond in the molecules of TpCZ is possible and, therefore, in solution more planarized conformation is inherent, which results in PL band mirroring the absorption band. However, in solid state TpCZ molecules are locked in different conformation, resulting different electron-vibronic interaction, what is witnessed by the change in PL spectra. PL spectra of both compounds in the most non-polar medium (hexane, dielectric constant ε = 1.88) possess well-resolved vibronic structure, indicating 1LE state to be the origin of emission (Fig. 2a). A small increase of solvent polarity (toluene, ε = 2.38) facilitates formation of 1CT state, since broad and structureless red-shifted PL band appears. Further increase in polarity (tetrahydrofuran (THF), ε = 7.58) results in even more significant red-shifting of PL spectra. It should be noted that PL spectra of the solutions of TmCZ were found to be batochromically shifted as compared to those of the solutions of TpCZ. This is unexpected, as generally PL spectra of meta-isomers, due to reduced conjugation, appear to be blue-shifted as compared to those of their para-analogues.25,48,50–52 However, some reported efficient TADF emitters readily possess broad structureless CT emission band even in nonpolar media, indicating pure CT nature of compound.43,44,49 In case of TmCZ, the vibronic structure can still be observed in nonpolar hexane; however, it is smeared out, as compared to that of TpCZ, indicating that even in nonpolar medium a competition between 1LE and 1CT states occurs in TmCZ. This correlates with theoretically calculated higher values of excited state charge separation factor q for TmCZ. It was shown that apparent Stokes shift defines the strength of CT character in the compound under investigation53. Upon changing polarity of surrounding media (Fig.S6), LEB of TpCZ shifts to lower energies, indicating the solely π-π* origin of this band. On the other hand, the


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position of LEB of TmCZ remains the same for the solutions in all of the tested solvents. Such polarity insensitive behavior of LEB can be accounted for by the mixed n-π* and π-π* origin, which was confirmed by theoretical calculations. A blue-shift of LEB upon increase of polarity, which is a characteristic of n-π* transitions53, was unobserved, which is evidence of a mixed nπ* and π-π* origin of LEB in TmCZ. However, the estimation of Stokes shifts yields higher values for TmCZ (Table S2), as compared to those of TpCZ, indicating that the former possesses stronger CT character. Fig. 2b presents fluorescence spectra of TpCZ and TmCZ dispersed in polymers of different polarity. It should be noted that TpCZ demonstrated higher values of PLQY, which is consistent with higher oscillator strength of the first excitations, constituting the lowest energy band of TpCZ, as compared to that of TmCZ. Furthermore, the PL spectra of thermally evaporated samples of the emitters doped into DPEPO, which corresponds to the more polar medium (DPEPO polarity is somewhere between that of toluene and THF), were assessed. Interestingly, the steady-state PL spectrum of the dispersion of TmCZ in DPEPO almost entirely overlaps with the phosphorescence spectrum (Fig. 2b), while in the case of TpCZ, due to its weaker CT character and lower ET, DPEPO polarity seems to be insufficient to minimize ∆EST, thus reduced TADF efficiency is expected. To evaluate the contribution of delayed fluorescence, the PLQYs of the evaporated layers were measured in ambient and degassed atmospheres. The increase of PLQY for TmCZ (from 0.05 to 0.25) was found to be five times higher than that of TpCZ (from 0.33 to 0.35). The above results indicate that the contribution of delayed component to total emission is more prominent in TmCZ.


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For temperature resolved measurements, we chose poly(methyl methacrylate) (PMMA) as a host for TpCZ and TmCZ. At lower temperatures phosphorescence is observed for both compounds, noted by a shifted and vibronic structure possesing emission spectra (Fig. 3). The recorded phosphorescence spectra suggest a stronger electronic coupling of the 3LE and 1CT states for TmCZ49. The recorded spectra also allow estimation of the energy gap ∆EST, which can be evaluated from the PL spectra. The evaluation yields ∆EST values of 0.209 eV and 0.125 eV for TpCZ and TmCZ, respectively.

Figure 3. Fluorescence (black) and phosphorescence (blue) spectra of: (a) TpCZ and (b) TmCZ in PMMA. Singlet-triplet energy gap calculated from the position of 0-0 vibronic peak (∆EST peak), from the onsets of the spectra (∆EST onset), as well as average values (∆EST av.).

TIME-RESOLVED MEASUREMENTS The decay transients of both compounds molecularly dispersed in PMMA exhibit a fast prompt and a slower delayed fluorescence component (τDF) with lifetime values for delayed fluorescence of 500 µs and 80 µs for TpCZ and TmCZ, respectively (Fig. 4a). To confirm that the delayed


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fluorescence is due to rISC, luminescence intensities of prompt and delayed fluorescence were measured at room temperature in deoxygenated environment as a function of excitation energy (Fig. 4b, c). The same power law dependence at low energy excitations for prompt and delayed fluorescence suggest that delayed fluorescence is not a result of triplet-triplet annihilation, but instead is a thermally activated process.54 The power law of ca. 0.5 at higher excitation energies is probably due to singlet-singlet annihilation, as the delayed fluorescence transients are insensitive to excitation energies (not illustrated here). Emission decay kinetics of both samples show that delayed fluorescence is more intense for TmCZ.

Figure 4. (a) Decay kinetics of prompt and delayed fluorescence and (b,c) prompt and delayed fluorescence intensity versus excitation energy for the solid solutions of TpCZ and TmCZ in PMMA (detection wavelength was 440 nm). Data (points) were fitted with power law function (lines), with the exponent n values displayed. To evaluate the efficiency and the activation energy of thermally activated process the decay transients of studied compounds were investigated in solid PMMA solutions in deoxygenated environment at different temperatures. Fig. 5 shows photoluminescence decay dynamics at 440 nm of the solid solutions of TpCZ and TmCZ in PMMA recorded at different temperatures. A


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significant decrease of delayed fluorescence lifetime with increasing temperature is observed, which is due to increasing rate of rISC, a thermally activated process. The prompt fluorescence of both compounds was found to be temperature independent. The phosphorescence decay transient appears as a line due to its lifetime being much longer than the maximum available delay time, which is 10 ms.

Figure 5. Photoluminescence decay dynamics recorded at different temperatures of the solid solutions of: (a) TpCZ and (b) TmCZ in PMMA. The prompt component (PF) is marked grey. While the prompt fluorescence is more intense for TpCZ, TADF is more prominent and observed at lower temperatures for TmCZ. To illustrate this, the ratio of time-integrated DF/PF versus temperature is plotted for both compounds (Fig.6a,b). At 330K this ratio reaches the values of 0.06 and 27 for TpCZ and TmCZ, respectively, which indicates that TADF is the dominating PL decay process for the meta-linked TmCZ. Activation energies were calculated by fitting Arrhenius plot (

∆

)55, as shown in Fig. 6c,d. The resulting activation

energies (EA) of 0.199 eV and 0.070 eV for TpCZ and TmCZ, respectively, are in good agreement with ∆EST values estimated from low-temperature fluorescence and phosphorescence spectra.


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Figure 6. (a,b) The ratio of time-integrated intensities of delayed and prompt fluorescence and (c,d) Arrhenius type plot of temperature dependence of the integrated intensity of delayed fluorescence for the solid solutions of TpCZ and TmCZ in PMMA (1 %). Activation energy EA was calculated from the exponential fit of the first 4 data points. OLED CARACTERISTICS Since TmCZ showed more efficient rISC and thus more prominent delayed fluorescence, it was chosen as a viable emitter for the fabrication of TADF OLED. The device structure was [ITO/m-MTDATA (10 nm)/NPB (15 nm)/mCP (10 nm)/5wt%-TmCZ:DPEPO (20 nm)/TPBi (45 nm)/LiF (0.8 nm)/Al (100 nm)], where m-MTDATA (4,4′,4′′-tris[phenyl(mtolyl)amino]triphenylamine), NPB (N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′diamine), mCP (1,3-di(9H-carbazol-9-yl)benzene) were used as hole injection and transport, hole transport, hole transport and exciton blocking layers, respectively. TPBi (2,2',2"-(1,3,5benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)) and LiF were used as electron transport and injection layers, respectively. The energy level diagram of the OLED is shown in Fig. 7a. Due to


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electron transporting properties of the host DPEPO (bis(2-(diphenylphosphino)phenyl) ether oxide), the recombination zone is located at the interface of mCP and DPEPO. Thus, mCP plays a role of exciton blocking, as its triplet energy (ET = 2.9 eV) is higher than that of TmCZ (ET = 2.78 eV). The current density, luminance versus voltage as well as EQE and luminous efficacy versus current density characteristics are shown in Fig. 7b, c.

Figure 7. The energy level diagram (a) and properties (b,c) of OLED. The turn-on voltage of the OLED was found to be in the range of 5-6 V. The EL emission spectrum peak was found to be at 475 nm and Commission Internationale de l'Éclairage (CIE) color coordinates were calculated to be (0.16, 0.23), corresponding to a sky-blue emission color. The device displayed EL spectra similar to the corresponding PL spectra of TmCZ, confirming that EL emission was generated from TmCZ via the same radiative decay process as in PL. The maximal obtained EQE for the device was 9.5%. However, the device showed severe roll-off at high current densities, which is likely due to annihilation processes in the emitter. TmCZ PL efficiency in DPEPO was found to be 25%, which implies a prominent non-radiative exciton relaxation pathway, degrading the device performance. Nevertheless, taking into account that / , where γ is the charge balance factor, φS/T is the emitting exciton fraction (which is 1 for TADF materials), φOUT is the light outcoupling factor and φPL is the PLQY of the


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material and assuming γ = 1, φS/T = 1, φOUT = 0.22 and using the φPL = 0.25, one can expect an EQE of 5.5%. The experimental value of 9.5% shows nearly a twofold-higher value, which implies an underestimation of either φPL or φOUT. The former may be due to direct CT exciton generation in the OLED stack, which was observed in a similar situation by Dias et al. [44], or due to preferable in-plane alignment of emitter dipole moments within the emission layer, which could lead to enhanced light outcoupling from the device.

CONCLUSIONS We synthesized and investigated the properties of new triazine-carbazole derivative 2,4,6-tris(3(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)-1,3,5-triazine (TmCZ). The comparative study of the photophysical properties of TmCZ and the previously reported para-substituted analogue TpCZ revealed considerable differences of the properties of these isomeric derivatives, i.e., para-linkage was found to be responsible for higher oscillator strength of the lowest energy absorption band, as well as fluorescence quantum yield, as compared to those of meta-linked compound. However, meta-substituted isomer was found to possess higher triplet energy, as well as stronger charge transfer character, which resulted in smaller values of singlet-triplet energy splitting and more intense TADF. A blue OLED with EQE reaching 9.5% was fabricated using TmCZ, with its emission originating mostly from TADF.

AUTHOR INFORMATION Corresponding Author


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*prof. Juozas V. Grazulevicius, Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilenu pl. 19, LT-50254, Kaunas, Lithuania, Tel. No. +37037 300193, e-mail: [email protected]. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b08034. Instrumentation, computational details, thermal, electrochemical data are available (PDF). ACKNOWLEDGMENT This work was supported by the Horizon 2020 ICT29-2014 project PHEBE (grant No 641725). ABBREVIATIONS TLC, thin layer chromatography. REFERENCES (1)

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