Impact of Donor Substitution Pattern on the TADF ... - ACS Publications

Article Cite This: J. Phys. Chem. C 2017, 121, 23618-23625

pubs.acs.org/JPCC

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 Bučiunas, ̅ ‡ ‡ ,† ‡ Dovydas Banevičius, Karolis Kazlauskas, Juozas V. Gražulevičius,* and Saulius Juršeṅ as †

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 S Supporting Information *

ABSTRACT: In this work, we report on the synthesis and photophysical investigation of a new star-shaped triazinecarbazole derivative 2,4,6-tris(3-(3,6-di-tert-butyl-9H-carbazol-9yl)phenyl)-1,3,5-triazine. Comparative study of the photophysical properties of the newly synthesized emitter along with its para-substituted isomer 2,4,6-tris(4-(3,6-di-tert-butyl-9Hcarbazol-9-yl)phenyl)-1,3,5-triazine was performed. While paralinkage caused higher oscillator strength of the lowest energy absorption band and high fluorescence quantum yield, the metalinkage resulted in stronger charge transfer character as well as higher triplet energy. Delayed emission of meta-isomer was found to be 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.

■

3

INTRODUCTION

CT, as well as n−π* 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, and 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 ultralow 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

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 TADF.2,3 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 nonemissive triplet excitons back to emissive singlet state,1 thus internal quantum efficiency of 100% can be achieved. Gibson et al.4 recently 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 systems.5,6 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 3 LE state of the donor facilitates the efficient rISC. These observations clearly indicate the equal importance of 3LE and © 2017 American Chemical Society

Received: August 11, 2017 Revised: October 5, 2017 Published: October 10, 2017 23618

DOI: 10.1021/acs.jpcc.7b08034 J. Phys. Chem. C 2017, 121, 23618−23625

Article

The Journal of Physical Chemistry C Scheme 1. Synthetic Pathway toward TmCZ

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 the zoomed lowest energy band of TmCZ. The lowest energy transitions are depicted, and (b) corresponding NTOs are presented.

modify the existing TCPZ structure by adding branched tertbutyl 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 2,4,6-tris(3-(3,6-di-tert-butyl-9H-carbazol9-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 para-linkage possessing isomeric derivative TpCZ31 allows to highlight the isomeric differences, which influence TADF properties of these emitters.

successfully fabricated PhOLEDs in panchromatic range using TCPZ as host. In former studies, TCPZ was only 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 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 a 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

■

SYNTHESIS Synthesis of TmCZ is presented in Scheme 1. Compounds 1, 2, and 3 were synthesized according the literature procedures in refs 32, 33, and 34, respectively. 2,4,6-Tris(3-(3,6-di-tert-butyl9H-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 (3) (0.80 g, 1.16 mmol), 3,6-di-tert-buthylcarbazole (1) (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 (thin 23619


Article

The Journal of Physical Chemistry C

meta-conjugation (0.008). A similar tendency can be observed in the experimentally obtained spectra: the LEB of TpCZ (360−440 nm) is of higher intensity, as compared to that of TmCZ (Figure 1a). Note that both experimental absorption spectra are normalized according local excitation peak of carbazole moiety (296−297 nm). 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 (Figure 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 a 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 the 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, Figure S3) also points at the CT nature, and the larger excited state charge separation in TmCZ.

layer chromatography (TLC) control), the reaction mixture was cooled down to 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). mp = 385−387 °C (DSC 388 °C). 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.

■

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 (Figure S3). Strong electron affinity of 1,3,5-triazine affects the adjacent phenyl rings at C-2, C-4, and C-6 positions of the 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. Figure 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 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 exchange-correlation (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 the Supporting Information for details). Consequently, vertical transitions, as well as natural transition orbitals (NTOs)39 were simulated with MPW1B9540 (31% HF) functional. It should be noted that standard longrange-corrected functionals tend to overestimate the excitation energy of TADF compounds,38,41 which is also true in our case (Figure S5). Theoretical and experimental (1 wt % solid solutions in cyclo-olefin polymer Zeonex) absorption spectra of the compounds are presented Figure 1a. Evidently, careful selection of the appropriate functional provided eligible results. The first transitions of both compounds coincide with the maxima of the lowest energy bands (LEBs) 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

■

THERMAL AND ELECTROCHEMICAL PROPERTIES 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 (Figure S1). However, para-isomer 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 (Figure 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 and 2.83 eV for TpCZ and 5.65 and 2.78 eV for TmCZ, respectively.

■

STEADY-STATE PHOTOPHYSICS

One of the key parameters for TADF compounds is the singlet−triplet energy splitting (ΔEST). Small ΔEST generally facilitates reverse intersystem crossing (rISC), thus enhancing the efficiency of TADF.4,22,42−44 However, estimation of ΔEST 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 The 1CT singlet state is known to be sensitive to the polarity of the surrounding media,48 and it is also the state involved in the TADF process,49 while 3LE is usually nonemissive at room temperature, not affected by the 23620


Article


Figure 2. Steady-state photoluminescence spectra of TpCZ and TmCZ: (a) in the solvents of different polarity and (b) dispersed in different hosts in ambient atmosphere. Phosphorescence spectra from solid solutions (1 wt %) in Zeonex at 77 K is also depicted.

CT nature of the compound.43,44,49 In the 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 the 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 investigation.53 Upon changing the polarity of the surrounding media (Figure S6), the LEB of TpCZ shifts to lower energies, indicating the solely π−π* origin of this band. On the other hand, the position of the LEB of TmCZ remains the same for the solutions in all of the tested solvents. Such polarity insensitive behavior of the LEB can be accounted for by the mixed n−π* and π−π* origin, which was confirmed by theoretical calculations. A blue-shift of the LEB upon increase of polarity, which is a characteristic of n−π* transitions,53 was unobserved, which is evidence of a mixed n−π* and π−π* origin of the 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. Figure 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 (Figure 2b), while in the case of TpCZ, due to its weaker CT character and lower ET, DPEPO polarity seems to be insufficient to minimize ΔEST, and 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 the PLQY for TmCZ (from 0.05 to 0.25) was found to be 5 times higher than that of

polarity of the surrounding media, and is also involved in TADF. Determination of the T1 energy level is possible by measuring the emission spectrum at low temperatures. Then, by choosing a host of certain polarity, it is possible to tune the position of the 1CT level, shifting it as close to T1 as possible, thus minimizing ΔEST.44 Interesting observation can be made by analyzing PL spectra of both isomers in the most nonpolar media: hexane and Zeonex (Figure 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 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 bonds 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 the 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 nonpolar medium (hexane, dielectric constant ε = 1.88) possess wellresolved vibronic structure, indicating the 1LE state to be the origin of emission (Figure 2a). A small increase of solvent polarity (toluene, ε = 2.38) facilitates formation of the 1CT state, since a 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 a broad structureless CT emission band even in nonpolar media, indicating a pure 23621


Article


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 (ΔESTpeak), from the onsets of the spectra (ΔESTonset), as well as average values (ΔESTav.).

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.

samples show that delayed fluorescence is more intense for TmCZ. 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. Figure 5

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. 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 possessing emission spectra (Figure 3). The recorded phosphorescence spectra suggest a stronger electronic coupling of the 3LE and 1CT states for TmCZ.49 The recorded spectra also allow estimation of the energy gap ΔEST, which can be evaluated from the PL spectra. The evaluation yields ΔEST values of 0.209 and 0.125 eV for TpCZ and TmCZ, respectively.

■

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 and 80 μs for TpCZ and TmCZ, respectively (Figure 4a). To confirm that the delayed fluorescence is due to rISC, luminescence intensities of prompt and delayed fluorescence were measured at room temperature in a deoxygenated environment as a function of excitation energy (Figure 4b, c). The same power law dependence at low energy excitations for prompt and delayed fluorescence suggests 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

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 gray.

shows photoluminescence decay dynamics at 440 nm of the solid solutions of TpCZ and TmCZ in PMMA recorded at different temperatures. A 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. 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/ 23622


Article


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 Figure 7a. Due to 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 Figure 7b, c. 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 nonradiative exciton relaxation pathway, degrading the device performance. Nevertheless, taking into account that EQE = γφS/TφPLφOUT , 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 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 2-fold-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.,49 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.

PF versus temperature is plotted for both compounds (Figure 6a,b). At 330 K, this ratio reaches the values of 0.06 and 27 for

Figure 6. (a,b) 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 four data points.

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 the Arrhenius plot

(

(k rISC = A exp −

ΔEA kT

)),

55

as shown in Figure 6c,d. The

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

■

OLED CHARACTERISTICS 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)/5 wt %-TmCZ:DPEPO (20 nm)/TPBi (45 nm)/LiF (0.8 nm)/Al (100 nm)], where m-MTDATA (4,4′,4″tris[phenyl(m-tolyl)amino]triphenylamine), NPB (N,N′-di(1naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine), and mCP (1,3-di(9H-carbazol-9-yl)benzene) were used as hole

■

CONCLUSIONS We synthesized and investigated the properties of new triazinecarbazole derivative 2,4,6-tris(3-(3,6-di-tert-butyl-9H-carbazol9-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; that is, para-linkage was found to be responsible for higher

Figure 7. Energy level diagram (a) and properties (b,c) of OLED. 23623


Article


Organic Emitting Diodes. Phys. Chem. Chem. Phys. 2015, 17 (20), 13553−13558. (10) Wang, C.; Liu, G.; Liu, S.; Zhang, X.; Ou, C.; Wang, L.; Sun, W.; Mi, B.; Song, J.; Gao, Z. A Bipolar and Small Singlet-Triplet Splitting Energy Host with Triplet Energy Lower Than a Blue Phosphor for Phosphorescent OLEDs in Panchromatic Range. Chin. J. Chem. 2016, 34 (8), 763−770. (11) Su, S.-J.; Cai, C.; Takamatsu, J.; Kido, J. A Host Material with a Small Singlet−triplet Exchange Energy for Phosphorescent Organic Light-Emitting Diodes: Guest, Host, and Exciplex Emission. Org. Electron. 2012, 13 (10), 1937−1947. (12) Inomata, H.; Goushi, K.; Masuko, T.; Konno, T.; Imai, T.; Sasabe, H.; Brown, J. J.; Adachi, C. High-Efficiency Organic Electrophosphorescent Diodes Using 1,3,5-Triazine Electron Transport Materials. Chem. Mater. 2004, 16 (7), 1285−1291. (13) Wang, Q.; Wallace, J. U.; Lee, T. Y.-H.; Ou, J. J.; Tsai, Y.-T.; Huang, Y.-H.; Wu, C.-C.; Rothberg, L. J.; Chen, S. H. Evaluation of Propylene-, Meta-, and Para-Linked Triazine and Tert-Butyltriphenylamine as Bipolar Hosts for Phosphorescent Organic Light-Emitting Diodes. J. Mater. Chem. C 2013, 1 (11), 2224. (14) Cha, J.-R.; Lee, C. W.; Lee, J. Y.; Gong, M.-S. Design of OrthoLinkage Carbazole-Triazine Structure for High-Efficiency Blue Thermally Activated Delayed Fluorescent Emitters. Dyes Pigm. 2016, 134, 562−568. (15) Serevičius, T.; Nakagawa, T.; Kuo, M.-C.; Cheng, S.-H.; Wong, K.-T.; Chang, C.-H.; Kwong, R. C.; Xia, S.; Adachi, C. Enhanced Electroluminescence Based on Thermally Activated Delayed Fluorescence from a Carbazole−triazine Derivative. Phys. Chem. Chem. Phys. 2013, 15 (38), 15850. (16) Endo, A.; Sato, K.; Yoshimura, K.; Kai, T.; Kawada, A.; Miyazaki, H.; Adachi, C. Efficient up-Conversion of Triplet Excitons into a Singlet State and Its Application for Organic Light Emitting Diodes. Appl. Phys. Lett. 2011, 98 (8), 083302. (17) Wang, Y.; Liu, W.; Deng, J.; Xie, G.; Liao, Y.; Qu, Z.; Tan, H.; Liu, Y.; Zhu, W. Engineering the Interconnecting Position of StarShaped Donor-π-Acceptor Molecules Based on Triazine, Spirofluorene, and Triphenylamine Moieties for Color Tuning from Deep Blue to Green. Chem. - Asian J. 2016, 11 (18), 2555−2563. (18) Huang, B.; Yin, Z.; Ban, X.; Jiang, W.; Dai, Y.; Zhang, J.; Liu, Y.; Yang, Y.; Sun, Y. Thermally Activated Delayed Fluorescence of NPhenylcarbazole and Triphenylamine Functionalised Tris(aryl)triazines. Dyes Pigm. 2015, 117, 141−148. (19) Ren, Z.; Nobuyasu, R. S.; Dias, F. B.; Monkman, A. P.; Yan, S.; Bryce, M. R. Pendant Homopolymer and Copolymers as SolutionProcessable Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes. Macromolecules 2016, 49 (15), 5452− 5460. (20) Tanaka, H.; Shizu, K.; Nakanotani, H.; Adachi, C. Dual Intramolecular Charge-Transfer Fluorescence Derived from a Phenothiazine-Triphenyltriazine Derivative. J. Phys. Chem. C 2014, 118 (29), 15985−15994. (21) Kim, M.; Choi, J. M.; Lee, J. Y. Simultaneous Improvement of Emission Color, Singlet−triplet Energy Gap, and Quantum Efficiency of Blue Thermally Activated Delayed Fluorescent Emitters Using a 1Carbazolylcarbazole Based Donor. Chem. Commun. 2016, 52 (65), 10032−10035. (22) Cui, L.-S.; Nomura, H.; Geng, Y.; Kim, J. U.; Nakanotani, H.; Adachi, C. Controlling Singlet-Triplet Energy Splitting for Deep-Blue Thermally Activated Delayed Fluorescence Emitters. Angew. Chem., Int. Ed. 2017, 56 (6), 1571−1575. (23) Ban, X.; Jiang, W.; Lu, T.; Jing, X.; Tang, Q.; Huang, S.; Sun, K.; Huang, B.; Lin, B.; Sun, Y. Self-Host Thermally Activated Delayed Fluorescent Dendrimers with Flexible Chains: An Effective Strategy for Non-Doped Electroluminescent Devices Based on Solution Processing. J. Mater. Chem. C 2016, 4 (37), 8810−8816. (24) Im, Y.; Kim, M.; Cho, Y. J.; Seo, J.-A.; Yook, K. S.; Lee, J. Y. Molecular Design Strategy of Organic Thermally Activated Delayed Fluorescence Emitters. Chem. Mater. 2017, 29 (5), 1946−1963.

oscillator strength of the lowest energy absorption band, as well as fluorescence quantum yield, as compared to those of metalinked 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.

■

ASSOCIATED CONTENT

S 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 (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +37037 300193. E-mail: [email protected]. ORCID

Tomas Matulaitis: 0000-0003-0470-7356 Nadzeya A. Kukhta: 0000-0001-7311-228X Present Address §

N.A.K.: Department of Chemistry, Durham University, South Road, DH1 3LE, United Kingdom. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was supported by the Horizon 2020 ICT29-2014 project PHEBE (grant No 641725). REFERENCES

(1) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly Efficient Organic Light-Emitting Diodes from Delayed Fluorescence. Nature 2012, 492 (7428), 234−238. (2) Wong, M. Y.; Zysman-Colman, E. Purely Organic Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes. Adv. Mater. 2017, 29 (22), 1605444. (3) Bergmann, L.; Zink, D. M.; Bräse, S.; Baumann, T.; Volz, D. Metal−Organic and Organic TADF-Materials: Status, Challenges and Characterization. Top. Curr. Chem. 2016, 374 (3), 22. (4) Gibson, J.; Monkman, A. P.; Penfold, T. J. The Importance of Vibronic Coupling for Efficient Reverse Intersystem Crossing in Thermally Activated Delayed Fluorescence Molecules. ChemPhysChem 2016, 17 (19), 2956−2961. (5) Etherington, M. K.; Gibson, J.; Higginbotham, H. F.; Penfold, T. J.; Monkman, A. P. Revealing the Spin−vibronic Coupling Mechanism of Thermally Activated Delayed Fluorescence. Nat. Commun. 2016, 7, 13680. (6) Marian, C. M. Mechanism of the Triplet-to-Singlet Upconversion in the Assistant Dopant ACRXTN. J. Phys. Chem. C 2016, 120 (7), 3715−3721. (7) Liu, X.-K.; Zheng, C.-J.; Xiao, J.; Ye, J.; Liu, C.-L.; Wang, S.-D.; Zhao, W.-M.; Zhang, X.-H. Novel Bipolar Host Materials Based on 1,3,5-Triazine Derivatives for Highly Efficient Phosphorescent OLEDs with Extremely Low Efficiency Roll-Off. Phys. Chem. Chem. Phys. 2012, 14 (41), 14255. (8) Wagner, D.; Hoffmann, S. T.; Heinemeyer, U.; Münster, I.; Köhler, A.; Strohriegl, P. Triazine Based Bipolar Host Materials for Blue Phosphorescent OLEDs. Chem. Mater. 2013, 25 (18), 3758− 3765. (9) Kim, M.; Jeon, S. K.; Hwang, S.-H.; Lee, J. Y. Molecular Design of Triazine and Carbazole Based Host Materials for Blue Phosphorescent 23624


Article


Splitting in a TADF Molecule. J. Mater. Chem. C 2016, 4 (17), 3815− 3824. (44) Dos Santos, P. L.; Ward, J. S.; Bryce, M. R.; Monkman, A. P. Using Guest−Host Interactions To Optimize the Efficiency of TADF OLEDs. J. Phys. Chem. Lett. 2016, 7 (17), 3341−3346. (45) Sun, H.; Zhong, C.; Brédas, J.-L. Reliable Prediction with Tuned Range-Separated Functionals of the Singlet−Triplet Gap in Organic Emitters for Thermally Activated Delayed Fluorescence. J. Chem. Theory Comput. 2015, 11 (8), 3851−3858. (46) Sun, H.; Hu, Z.; Zhong, C.; Chen, X.; Sun, Z.; Brédas, J.-L. Impact of Dielectric Constant on the Singlet−Triplet Gap in Thermally Activated Delayed Fluorescence Materials. J. Phys. Chem. Lett. 2017, 8, 2393−2398. (47) Samanta, P. K.; Kim, D.; Coropceanu, V.; Brédas, J.-L. UpConversion Intersystem Crossing Rates in Organic Emitters for Thermally Activated Delayed Fluorescence: Impact of the Nature of Singlet vs Triplet Excited States. J. Am. Chem. Soc. 2017, 139 (11), 4042−4051. (48) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Lakowicz, J. R., Ed.; Springer US: Boston, MA, 2006. (49) Dias, F. B.; Santos, J.; Graves, D. R.; Data, P.; Nobuyasu, R. S.; Fox, M. A.; Batsanov, A. S.; Palmeira, T.; Berberan-Santos, M. N.; Bryce, M. R.; et al. The Role of Local Triplet Excited States and D-A Relative Orientation in Thermally Activated Delayed Fluorescence: Photophysics and Devices. Adv. Sci. 2016, 3 (12), 1600080. (50) Bagnich, S. A.; Rudnick, A.; Schroegel, P.; Strohriegl, P.; Köhler, A. Triplet Energies and Excimer Formation in Meta- and Para-Linked Carbazolebiphenyl Matrix Materials. Philos. Trans. R. Soc., A 2015, 373 (2044), 20140446−20140446. (51) Guiglion, P.; Zwijnenburg, M. A. Contrasting the Optical Properties of the Different Isomers of Oligophenylene. Phys. Chem. Chem. Phys. 2015, 17 (27), 17854−17863. (52) Romain, M.; Thiery, S.; Shirinskaya, A.; Declairieux, C.; Tondelier, D.; Geffroy, B.; Jeannin, O.; Rault-Berthelot, J.; Métivier, R.; Poriel, C. Ortho-, Meta-, and Para -Dihydroindenofluorene Derivatives as Host Materials for Phosphorescent OLEDs. Angew. Chem., Int. Ed. 2015, 54 (4), 1176−1180. (53) Etherington, M. K.; Franchello, F.; Gibson, J.; Northey, T.; Santos, J.; Ward, J. S.; Higginbotham, H. F.; Data, P.; Kurowska, A.; Dos Santos, P. L.; et al. Regio- and Conformational Isomerization Critical to Design of Efficient Thermally-Activated Delayed Fluorescence Emitters. Nat. Commun. 2017, 8, 14987. (54) Valeur, B. Molecular Fluorescence; Wiley-VCH Verlag GmbH: Weinheim, FRG, 2001. (55) Dias, F. B.; Penfold, T. J.; Monkman, A. P. Photophysics of Thermally Activated Delayed Fluorescence Molecules. Methods Appl. Fluoresc. 2017, 5 (1), 012001.

(25) Williams, R. T.; Bridges, J. W. Fluorescence of Solutions: A Review. J. Clin. Pathol. 1964, 17 (4), 371−394. (26) Li, W.; Li, J.; Liu, D.; Wang, F.; Zhang, S. Bipolar Host Materials for High-Efficiency Blue Phosphorescent and Delayed-Fluorescence OLEDs. J. Mater. Chem. C 2015, 3 (48), 12529−12538. (27) Shizu, K.; Sakai, Y.; Tanaka, H.; Hirata, S.; Adachi, C.; Kaji, H. Meta-Linking Strategy for Thermally Activated Delayed Fluorescence Emitters with a Small Singlet-Triplet Energy Gap. ITE Trans. Media Technol. Appl. 2015, 3 (2), 108−113. (28) Matsuoka, K.; Albrecht, K.; Yamamoto, K.; Fujita, K. Mulifunctional Dendritic Emitter: Aggregation-Induced Emission Enhanced, Thermally Activated Delayed Fluorescent Material for Solution-Processed Multilayered Organic Light-Emitting Diodes. Sci. Rep. 2017, 7, 41780. (29) Li, Y.; Xie, G.; Gong, S.; Wu, K.; Yang, C. Dendronized Delayed Fluorescence Emitters for Non-Doped, Solution-Processed Organic Light-Emitting Diodes with High Efficiency and Low Efficiency Rolloff Simultaneously: Two Parallel Emissive Channels. Chem. Sci. 2016, 7 (8), 5441−5447. (30) Albrecht, K.; Matsuoka, K.; Fujita, K.; Yamamoto, K. Carbazole Dendrimers as Solution-Processable Thermally Activated DelayedFluorescence Materials. Angew. Chem., Int. Ed. 2015, 54 (19), 5677− 5682. (31) Matulaitis, T.; Kostiv, N.; Grazulevicius, J. V.; Peciulyte, L.; Simokaitiene, J.; Jankauskas, V.; Luszczynska, B.; Ulanski, J. Synthesis and Properties of Bipolar Derivatives of 1,3,5-Triazine and Carbazole. Dyes Pigm. 2016, 127, 45−58. (32) Neugebauer, F. A.; Fischer, H. Tert.-Butyl-Substituierte Carbazole. Chem. Ber. 1972, 105 (8), 2686−2693. (33) Wang, L.; Wang, G. T.; Wang, X.; Tong, Y.; Sullivan, G.; Park, D.; Leonard, N. M.; Li, Q.; Cohen, J.; Gu, W.-Z.; et al. Design, Synthesis, and Biological Activity of 4-[(4-Cyano-2-Arylbenzyloxy)-(3Methyl-3 H -Imidazol-4-Yl)methyl]benzonitriles as Potent and Selective Farnesyltransferase Inhibitors. J. Med. Chem. 2004, 47 (3), 612−626. (34) Hayami, S.; Inoue, K. Structure and Magnetic Property of the Organic Triradical with Triazine Skeleton; 2,4,6-Tris{p-(N-Oxy-NTert-Butylamino)phenyl}triazine. Chem. Lett. 1999, 28 (7), 545−546. (35) Weingarten, H. Ullmann Condensation. J. Org. Chem. 1964, 29 (4), 977−978. (36) Brédas, J.-L. Organic Electronics: Does a Plot of the HOMO− LUMO Wave Functions Provide Useful Information? Chem. Mater. 2017, 29 (2), 477−478. (37) Kukhta, N. A.; da Silva Filho, D. A.; Volyniuk, D.; Grazulevicius, J. V.; Sini, G. Can Fluorenone-Based Compounds Emit in the Blue Region? Impact of the Conjugation Length and the Ground-State Aggregation. Chem. Mater. 2017, 29 (4), 1695−1707. (38) Huang, S.; Zhang, Q.; Shiota, Y.; Nakagawa, T.; Kuwabara, K.; Yoshizawa, K.; Adachi, C. Computational Prediction for Singlet- and Triplet-Transition Energies of Charge-Transfer Compounds. J. Chem. Theory Comput. 2013, 9 (9), 3872−3877. (39) Martin, R. L. Natural Transition Orbitals. J. Chem. Phys. 2003, 118 (11), 4775−4777. (40) Zhao, Y.; Truhlar, D. G. Hybrid Meta Density Functional Theory Methods for Thermochemistry, Thermochemical Kinetics, and Noncovalent Interactions: The MPW1B95 and MPWB1K Models and Comparative Assessments for Hydrogen Bonding and van Der Waals Interactions. J. Phys. Chem. A 2004, 108 (33), 6908−6918. (41) Liang, K.; Zheng, C.; Wang, K.; Liu, W.; Guo, Z.; Li, Y.; Zhang, X. Theoretical Investigation of the Singlet−triplet Splittings for Carbazole-Based Thermally Activated Delayed Fluorescence Emitters. Phys. Chem. Chem. Phys. 2016, 18 (38), 26623−26629. (42) Hirata, S.; Sakai, Y.; Masui, K.; Tanaka, H.; Lee, S. Y.; Nomura, H.; Nakamura, N.; Yasumatsu, M.; Nakanotani, H.; Zhang, Q.; et al. Highly Efficient Blue Electroluminescence Based on Thermally Activated Delayed Fluorescence. Nat. Mater. 2014, 14 (3), 330−336. (43) Santos, P. L.; Ward, J. S.; Data, P.; Batsanov, A. S.; Bryce, M. R.; Dias, F. B.; Monkman, A. P. Engineering the Singlet−triplet Energy 23625


Impact of Donor Substitution Pattern on the TADF ... - ACS Publications

Recommend Documents