Thermally Activated Delayed Fluorescence (Green) in Undoped Film

Article Cite This: J. Phys. Chem. C 2019, 123, 1003−1014

pubs.acs.org/JPCC

Thermally Activated Delayed Fluorescence (Green) in Undoped Film and Exciplex Emission (Blue) in Acridone−Carbazole Derivatives for OLEDs Qamar T. Siddiqui,†,§ Ankur A. Awasthi,† Prabhjyot Bhui,‡ Mohammad Muneer,§ Kuttay R. S. Chandrakumar,∥ Sangita Bose,*,‡ and Neeraj Agarwal*,†

J. Phys. Chem. C 2019.123:1003-1014. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/20/19. For personal use only.

†

School of Chemical Sciences and ‡School of Physical Sciences, UM-DAE Centre for Excellence in Basic Sciences, University of Mumbai, Kalina, Santacruz (E), Mumbai 400098, India § Department of Chemistry, Aligarh Muslim University, Aligarh 202001, India ∥ Theoretical Chemistry Section, Bhabha Atomic Research Centre, Mumbai 400085, India S Supporting Information *

ABSTRACT: Donor−acceptor−donor materials (1,2) having acridone as acceptor unit and carbazole as donor were synthesized for optoelectronic applications. Carbazole was substituted on 2,7 positions of acridone in 1, while 3,6-trifluoromethylphenyl carbazole was substituted in 2. Steady-state and time-dependent emission properties of these compounds were studied in detail to get insight into their possible thermally activated delayed fluorescence (TADF) behavior. The singlet−triplet energy gap (ΔEST) was found to be as low as 0.17 eV (1) and 0.15 eV (2), favorable for TADF materials. Both these materials were found to be efficient green TADF emitters in organic light-emitting diode (OLED) devices. Interestingly, the TADF properties were observed for the first time in undoped 1,2-based devices, i.e., without host matrix, unlike the most commonly reported TADF emitters. Furthermore, an exciplex emission at 465 nm was observed in the blends of 1,2 with poly(vinylcarbazole) (PVK) in 1:7 (w/w) ratio. OLEDs with the blend of 1,2 with PVK as the active layer showed an intense electroluminescence at 465 nm matching well with the exciplex photoluminescence. Thus, we show that acridone−carbazole derivatives (1,2) offer variable electroluminescence as undoped TADF green emitter and blue exciplex emitter when doped in PVK.

■

Delayed fluorescence (DF), namely, thermally activated delayed fluorescence (TADF) and triplet fusion delayed fluorescence (TFDF) in fluorescent molecules, have been identified as other ways to improve the OLED efficiency.18−25 DF significantly improves electroluminescence as it allows us to generate more singlet excitons. For TFDF, internal efficiency can reach ∼62.5% as additional singlet excitons are generated via triplet−triplet annihilation (TTA).19−22 On the other hand, TADF materials possess a very small energy gap between the singlet and triplet energy levels (ΔEST), which enable the conversion of triplet excitons into singlet excitons via reverse intersystem crossing (RISC), using thermal energy. Thus, theoretically, TADF materials offer 100% internal quantum efficiency.23−26 Reports by Adachi et al. opened up the OLED research field for detailed investigation of TADF emitters.25,27 Design and engineering of materials possessing TADF properties is a major challenge, as the molecule should possess

INTRODUCTION Organic light-emitting diodes (OLEDs) have garnered a significant attention due to their eco-friendly nature, low cost of production, and superior performance.1−11 A simple architecture of OLED consists of two working electrodes sandwiching the emissive material (active layer). In OLEDs, it is important to generate the exciton (electron−hole pair) in the emissive layer. To control the hole and electron mobilities for optimal operation, other organic materials such as hole and electron transporters have been used.12−14 At room temperature, under electrical excitation, the fluorescent molecule typically generates 75% triplet and 25% singlet excitons, resulting in maximum 25% internal quantum efficiency (IQE).13 Usually, triplet excitons are nonemissive at room temperature as they are deactivated by heat, prompting widescale research into how to improve efficiency of devices. Phosphorescent materials have been used to harvest singlet and triplet excitons to achieve 100% internal efficiency in OLED. However, such emissive, phosphorescent materials are synthesized using nonrenewable, expensive, and rare heavymetal atoms (e.g., Pt, Ir, etc.), hampering their prospect for large-scale application.1,3,15−17 © 2018 American Chemical Society

Received: August 28, 2018 Revised: November 7, 2018 Published: December 13, 2018 1003

DOI: 10.1021/acs.jpcc.8b08357 J. Phys. Chem. C 2019, 123, 1003−1014

Article

The Journal of Physical Chemistry C Scheme 1. Synthesis of Acridone−Carbazole Derivatives, 1−2

small ΔEST to promote RISC from lowest triplet state to lowest singlet state. Simultaneously, for high-fluorescence quantum yield, rate of internal conversion (kIC) should be considerably less than radiative decay constant (kr). Also, it is important to have short TADF lifetime for improved device efficiency as the chances of TTA gets reduced.28−30 It has been reported that low ΔEST can be achieved in push−pull molecules comprising electron donor and acceptor units, yielding singlet and triplet states with strong charge-transfer (CT) character.31,32 Although charge transfer is not a prerequisite for TADF, it lowers the ΔEST. Charge-transfer state results in small highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) overlap and leads to decrease in electronic exchange energy, thus small ΔEST can be achieved. Dielectric nature of medium and molecular geometry also influence the photophysical properties of these molecules and, in turn, TADF efficiency, making it a challenge to design good TADF materials. Recently, push−pull molecules based on acridone, quinacridone, and acridine have been reported for their applications in organic electronics.33−37 Acridone core acts as electron acceptor and can be substituted with various electron-donating units easily, while maintaining its emissive nature. Substitution of secondary acyclic aromatic amines has been previously reported; however, substitution with cyclic amine has been rarely investigated. In this article, we designed and synthesized two acridone derivatives substituted with carbazole (1) or 3,6trifluoromethylphenyl carbazole at 2,7 positions (2). The photophysical properties of these compounds were studied in detail to get insight into their TADF properties. Several studies on the TADF properties in doped system have been reported recently;36−39 however, to the best of our knowledge, TADF properties in undoped system are rare. Host matrix influences the several photophysical properties of emitters; thus, we report TADF properties of undoped 1,2 (without host matrix). Furthermore, the effect of host poly(vinylcarbazole) (PVK) on emission of 1,2 was also studied in solution as well as thin film. Interestingly, both 1 and 2 showed blue-shifted emission with very high fluorescence intensity (∼10-fold) compared to neat emitters, attributed to exciplex emission. Applications of 1,2 as emitter and their exciplex in OLEDs are also discussed.

dibromoacridone with secondary noncyclic aryl amines.33 B− H reaction with cyclic amine such as carbazole did not work in our hands. Besides B−H amination, several other methods have been reported in the literature.41−53 The Ullmann coupling, Cu-catalyzed Chan−Lam coupling, Pd-catalyzed C−N cross-coupling reaction of sulfonamides and aryl halides, and cross-coupling reaction of aryl halides (iodide or bromide) with aromatic/heteroaromatic amines employing lanthanum(III) oxide are few of the many reports.54−58 For the amination of acridone with cyclic aryl amine, here carbazole, we adopted palladium-free reaction using 1,10-phenanthroline and copper iodide as catalyzing agents. Compounds 1,2 were synthesized as shown in Scheme 1. Aryl-substituted carbazole (3) was synthesized by the Suzuki−Miyaura coupling of 3,6-dibromocarbazole and p-trifluoromethylphenyl boronic acid. For amination reaction, 2,7-dibromoacridone was reacted with excess of carbazole in the presence of copper iodide, and 1,10phenathroline at 140 °C for 24 h. Mono- and diamination was observed, products were extracted and purified by column chromatography, followed by recrystallization with ethyl acetate/hexane. Compounds 1,2 were characterized by multiple spectroscopic techniques like 1H NMR spectroscopy, 13C NMR spectroscopy, Fourier transform infrared spectroscopy, matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry, etc. (see Supporting Information). Solubilities of these compounds were found to be good in organic polar solvents, e.g. 15 mg/mL in dichloromethane. 1 H NMR spectra of 2,7-bis(carbazolyl)-10-methylacridone (1) and 2,7-bis((3,6-p-trifloromethyl-phenyl)carbazolyl)-10methylacridone in CDCl3 are shown in the Supporting Information (Figure S32). Singlets of two hydrogens at 1and 8-positions of acridone appeared at 8.83 and 8.85 ppm, respectively, in 1,2. The singlet nature of this signal shows the substitution of amine at 2- and 7-positions of acridone. A doublet at 8.2 ppm for four protons in 1 is assigned to hydrogen at 4- and 5-positions of carbazole moiety. Similarly, a signal at 8.46 ppm in 2 appears as singlet and is assigned for 4,5-positions of aryl-substituted carbazole. A signal at ∼7.8 ppm is assigned to 4,5-positions of acridone. N-Methyl protons appeared at about 4.1 ppm in 1,2. A molecular ion peak was observed for compounds 1,2 in MALDI-TOF. Photophysical Studies. Ground- and excited-state properties of acridone−carbazole derivatives 1,2 were studied using absorption and emission spectroscopy in solution and thin films. Absorption and emission spectra of 1,2 were recorded in

■

RESULTS AND DISCUSSION Synthesis. Generally, amination is carried out with the Buchwald−Hartwig coupling under palladium-catalyzed reaction.40 We have recently reported B−H coupling on 2,71004


Article

The Journal of Physical Chemistry C

Figure 1. Normalized absorption, fluorescence, and phosphorescence spectra of (A) 1 and (B) 2 in Me-THF. Phosphorescence peak is shown in circle.

Figure 2. Transient Lifetime measurements (λex = 406 nm) of (A) 1 and (B) 2 in tetrahydrofuran (THF) under air and nitrogen atmosphere.

emission from triplet state.16,17,60−62 Therefore, sensitization was carried out using ethyl iodide. On addition of ethyl iodide (1:10 v/v in N2 purged Me-THF), the intensity of both the low-energy peaks at ∼520 and ∼550 nm increases many folds and hence is attributed to phosphorescence. The singlet and triplet energy levels were estimated from the onset of fluorescence and phosphorescence peaks. The singlet−triplet energy gap (ΔEST) was calculated for both locally excited state (LEΔEST) and charge-transfer state (CTΔEST). The LEΔEST value was found to be 0.4 eV for 1 and 0.34 eV for 2, whereas CT ΔEST was found to be 0.17 eV (1) and 0.15 eV (2). It is worth mentioning that ΔEST of 1 has been previously reported to be 0.42 eV, which we believe is the difference between the singlet and triplet of the locally excited states.36 It is realized to have low ΔEST (0.1−0.3 eV) for thermally activated delayed fluorescence and the charge-transfer states of the compounds 1,2 satisfy this condition. TADF was further studied using fluorescence transient measurements. Fluorescence lifetimes of 1,2 were determined in various solvents and traces are shown in Figure 2. It represents the prompt fluorescence lifetimes of 1 and 2, in solution, obtained on time-correlated single-photon counting (TCSPC) setup, which is described in the Methods section. The lifetimes were obtained in a 0−200 ns time window, to obtain only the prompt fluorescence lifetime. Fluorescence decay was expected to be multiexponential due to prompt and delayed fluorescence components. In our case, transient measurements at room temperature yielded single exponential decay for 1,2 arising from prompt fluorescence only. The absence of any other component could be due to triplet state quenching by dissolved molecular oxygen. Thus, the lifetimes were recorded again on purging the solution with N2 gas, to enhance triplet

2-methyl tetrahydrofuran (Me-THF), and are shown in Figure 1. A low-intensity broad peak at 410 nm and emission were observed at about 490 nm. The absorption at 410 nm is assigned to charge-transfer band investigated through solvatochromic studies (see Supporting Information). Absorption studies revealed a negligible change in peak position (410 nm) on varying solvent polarity; however, steady-state peak maxima altered with solvent polarity. Emission maxima displayed significant red shift of up to 55 nm (∼460 nm in hexane and ∼515 nm in dimethyl sulfoxide) and broadening in emission when going to higher polarity of solvents (see Supporting Information). In nonpolar solvents, the spectrum is well resolved showing two peaks. The first peak belongs to the locally excited (LE) state, whereas the second peak arises from the charge-transfer (CT) state. It is well reported that locally excited (LE) S1−S0 transition is independent of solvent polarity; therefore, emission maxima remain unaltered in solvents of varying polarity.59 The CT transitions are broad, and their emission maxima depend on polarity of the medium. Our results indicate that the emission maxima arise from 1CT transition. Furthermore, steady-state emission studies of both the molecules in Me-THF were studied at room temperature as well as at low temperature (77 K). At room temperature, emission maxima were observed at 480 nm for 1, which is assigned to relaxation from charge-transfer state. However, 2 displays emission maxima at (450 nm) arising from locally excited singlet state (1LE) and charge-transfer emission at 480 nm. At 77 K, two new peaks in lower-energy region at ∼520 and ∼550 nm are observed for both 1 and 2, which are expected to arise from LE and CT triplet states. It is well known that sensitization by heavy atom (e.g., Br, I, Pt, Ir, etc.) promotes spin−orbit coupling and thus enhancement in 1005


Article

1006

0.60 0.48 a

In dichloromethane.

bI

n thin film. cIn acetonitrile. dIn poly(methyl methacrylate) (PMMA)-doped film. eIn nitrogen-purged dichloromethane. fIn Me-THF.

362 16 305.8 (O2), 446.1 (N2) 12.1 (O2), 27.1 (N2) 11.5 (O2), 17.6 (N2) 12.1 (O2), 17.2 (N2) 501 502 425 424 471 472 242 (4.83), 292 (4.48), 321 (4.44), 416 (3.60) 260 (4.85), 299 (4.87), 324 (4.80), 415 (3.60) 1 2

0.44 0.40

λema (nm) λabs,a nm (log ε) comp

Table 1. Photophysical Data of 1,2

ϕFa

λabsbI (nm)

λembI (nm)

τPFc (ns)

τDFc (μs)

τDFd (μs)

ϕFe

ΔESTf

utilization for TADF. Interestingly, we observed the prompt fluorescence lifetime increase by ∼6 ns for 1 and ∼5 ns for 2 (Table 1) in various solvents (Supporting Information). Such an increase in lifetime on nitrogen purging is possible when triplet state is in close proximity to singlet. This result correlates well with the reported results of Adachi et al.26,63 TADF lifetime is dependent on triplet state of molecule, and short TADF lifetime (below milliseconds) indicates chance of shorter triplet lifetime. It has been proposed that a short TADF lifetime is important for prospective TADF materials, which is beneficial to suppress triplet quenching processes like triplet− triplet annihilation, triplet−polaron annihilation, etc., thereby improving singlet fluorescence efficiency via TADF.30 The delayed fluorescence lifetimes in TADF molecules are known to be in microseconds−milliseconds scale.4,32 Generally, TADF lifetime has been obtained by doping the emitter in a solid host matrix, such as mCBP, Zeonex, etc.29,36 We believe that the presence of dopant may slightly alter the photophysical behavior of the emitter. Here, we have attempted to evaluate the TADF lifetime of undoped pure 1,2 in acetonitrile. Efficiency of thermally activated delayed fluorescence depends on various factors associated with singlet and triplet states, such as the presence of molecular oxygen (air) will affect TADF emission and lifetime at room temperature (see Scheme S2). Thus, we carried out extensive fluorimetric analysis to find out the TADF lifetime in air-saturated and nitrogen-purged atmosphere at room temperature. Figure 3 shows the TADF lifetimes of 1,2 (in solution) obtained from emission spectra taken at various delay times and then peak intensity was plotted against delay time and fit to the equation I = Io e−t/τ. The lifetimes of all of the samples were obtained in air- and nitrogen-saturated acetonitrile solutions. Here, delay time is much longer than 100 ns; therefore, prompt fluorescence can be ruled out. Fluorescence spectra of 1,2 were collected at various delay times (100 μs to 1 ms), in airand nitrogen-saturated acetonitrile. Associated emission spectra of 1,2 corresponding to various delay times are shown in the Supporting Information (Figures S20−S23). Interestingly, emission peak maxima obtained with delay time match well with prompt fluorescence for both 1 and 2. It is worth mentioning that the lifetime of prompt fluorescence is 0.5 eV) between LUMO of acceptor and donor is favorable.72 The charge transfer depends on the energy levels of HOMO and LUMO of the donor and acceptor molecules, respectively. Also, it is required that both EHOMO and ELUMO of donor should be greater than those of the acceptor. If the above-mentioned conditions are not met, energy transfer may take place rather than CT. In the present study, we find that in PVK:1/2, this condition is satisfied, where we see emission by exciplex only. According to Weller, energy of true exciplex emission (Eex) can be expressed by eq 173,74

general analysis of these frontier orbitals indicates that the electron density is transferred predominantly from the donor carbozole unit to the acceptor acridone central unit. Interestingly, it can also be observed that the peak located at 353 nm (HOMO to LUMO) is mostly associated with the dominant charge transfer from nitrogen atom of the carbozole group to the CO group of the acridone central acceptor unit. This is illustrated in terms of the isodensity molecular orbital plots, as presented in Figure 4. Exciplex Formation. As envisaged earlier, fluorescent emitters doped in host matrix may display altered photophysical properties by host−guest aggregation or exciplex formation. Exciplex offers tuning of emission maxima and peak width. The formation of an exciplex is a heterobimolecular process, where the ground-state species undergoes complex formation with an excited species via Coulombic forces (Figure 5).69 During exciplex formation, partial or complete charge transfer occurs from donor to acceptor species. Exciplex formation depends mainly on the orbitals from the donor and acceptor. The photophysical properties of exciplex has been explained by local excited states of donor and acceptor molecules and by considering intermolecular charge transfer (CT) state. This explanation of exciplex is quite identical to TADF emitter, which comprises broken or ineffective conjugation between donor and acceptor units. Therefore, exciplex emitters are expected to have small ΔEST, making them capable of undergoing TADF.25,70,71 We studied the photophysical properties of 1,2 in poly(vinylcarbazole) (PVK) matrix as well as N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′biphenyl)-4,4′-diamine (NPD). Carbazole moiety is known to form aggregates through π−π interaction in PVK host. Thus, thin films of the PVK and compounds 1,2 in varying ratios were prepared by spin-casting method and their photophysical properties were studied. Neat thin film of PVK shows emission maxima at 400 nm. Interestingly, we found emission maxima for PVK:1/2 blend at 470 nm, which does not belong to either PVK (400 nm) or 1,2 (510 nm). This new peak at 470 nm was

Eex = E HOMO(donor) − E LUMO(acceptor) + Udest − Ustab − ΔHe sol + 0.32 eV

(1)

where Udest and Ustab are, respectively, the destabilization and stabilization effects during exciplex formation, which cancel out each other. ΔHesol is the energy gain due to solubilization of electron transfer and can be considered nearly zero in solid state. Considering this, the energy level of exciplex (Eex) for PVK:1/2 films would be 2.72 eV, which matches well with the experimental value, i.e. 2.79 eV. This new emission was assigned to an exciplex formation between PVK and active emitters (1,2). Fluorescence lifetime measurement of PVK:1/2 blend, monitored at 470 nm, was found to be multiexponential, and we attribute it to varying degrees of aggregates and exciplex. Decay lifetime for PVK (0.6 and 8.0 ns) has been reported as biexponential.75 Figure 5b shows the clear difference between the multiexponential decay profile for 1 and its blend with PVK in thin film, hence confirming that this new emission arises from exciplex. The transient decay of 1 decreases multiexponentially at 510 nm, with three time constants of 0.3, 1.3, and 7.8 ns due to a varying degree of 1008


Article


Figure 6. Device characteristics of 1 and 2 as an emitter. (a) Three-dimensional (3D) schematic of the device geometry: ITO/(PVK + NPD) blend/1 or 2/Bphen/LiF/Al; (b) EL spectra for 1 and 2 at 20 and 5 mA/cm2, respectively; (c, d) device characteristics of 1 and 2.

Figure 7. Device characteristics of exciplex systems formed using blends of PVK with 1 and 2 (7:1 by weight). (a) Three-dimensional (3D) schematic of the device geometry: ITO/PEDOT:PSS/(PVK + 1 or 2)blend/Bphen/LiF/Al. (b) Electroluminance spectra for the two devices (with 1 or 2) at J values of 10 and 5 mA/cm2, respectively. The inset shows the blue emission from the devices. (c−e) Device characteristics of 1 and 2.

1009


Article

The Journal of Physical Chemistry C aggregation in solid film (see Table 2). On analyzing the blend of PVK and 1, we again find the decay to be triexponential but with an increase in the lifetimes. Of the three time constants obtained, the longest of 18 ns is due to the formation of an exciplex between PVK and 1 in thin films. We find that the emission from the exciplex is maximum at a ratio of 7:1 w/w (PVK:1/2). It is also of interest to note that the emission from the exciplex is much more intense, roughly 20-fold, compared to pure films of PVK or 1,2 having same concentration as in the blend (see Supporting Information). A possible reason for this intense emission is that, with PVK being a polymer, excitonic migration takes place across the polymeric backbone, which is prevented in exciplex formation. Since all excitons are utilized in the formation of an exciplex and no loss of excitons happens due to migration,76−79 the intensity of the exciplex emission is found to be much greater compared to emission of pure films of compound and PVK having same concentration. Theoretically, TADF emitters can achieve 100% singlet exciton harvestation, making them prospective candidates for OLED fabrication. Exciplex formation in OLED is well reported.74,80−82 This phenomenon has been extensively utilized to attain color tunability ranging from pure blue (Jankus et al.) to green to white OLED devices as well.83−87 Such wide range of color tunability could be achieved in exciplex systems as the emission wavelength is independent of optical gap of emitters, but depends on the HOMO−LUMO gap between the donor and acceptor species. To study the prospects of these materials for OLEDs, devices were fabricated and studied. Initially, devices were fabricated with 1 or 2 as green emitter. Poly(vinylcarbazole) (PVK), which is a well-known hole transport layer (HTL), is used in these studies. For devices where the organic compound (1 and 2) was used as an emitter, the best results were obtained by taking a blend of PVK and N,N′-di(1-naphthyl)N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPD) in 1:1 (w/ w). The most optimized device structure used was indium tin oxide (ITO)/(PVK + NPD) blend/1 or 2/4,7-diphenyl-1,10phenanthroline (Bphen)/LiF/Al (see schematic of the device geometry in Figure 6a). Typically, hole-blocking layers (HBLs) are most commonly placed after the emissive layer closer to cathode to confine the carriers and excitons. 4,7-Diphenyl1,10-phenanthroline (Bphen) was used as a hole-blocking layer (HBL). Electroluminance (EL) spectra of the devices showed the peak emission at 490 nm for 1 and 505 for 2 (see Figure 6b). Peak maxima of EL and photoluminescence in thin films match quite well. The turn-on voltage, VON, was lower for the OLED composed of 1, and also the current efficiency at high luminance (∼1000 Cd/m2) was almost twice as that of 2 (see Figure 6c,d), indicating that the device geometry is more suited for 1 for exciton harvesting. Thus, 1 behaves as a good nondopant TADF emitter for OLEDs possibly because of better donor properties of carbazole than 3,6-trifluoromethylphenyl carbazole. Also, it correlates well with the lower ΔEST of 1 than 2. As observed from the photophysical characterizations, the emission wavelength and efficiency of the devices are expected to be tuned by the formation of an exciplex with PVK. Thus, devices with the following geometry, ITO/poly(2,3-dihydrothieno-1,4-dioxin):poly(styrenesulfonate) (PEDOT:PSS)/ (PVK + 1 or 2)blend/Bphen/LiF/Al, were fabricated (see Figure 7a). In these devices, blends of PVK with 1 or 2 were taken as active emitting materials. The ratio of PVK:1 or 2 was

kept at 7:1 (w/w), which had shown the maximum quantum efficiency in thin films as found in the photophysical studies. For these devices, an additional layer of poly(2,3-dihydrothieno-1,4-dioxin):poly(styrenesulfonate) (PEDOT:PSS) was used as an HTL. Both these devices emitted blue light with the maximum emission obtained at 464 nm as seen from the EL spectrum in Figure 7b. This EL spectrum matches well with the exciplex emission observed in the thin film. VON for 2 was found to be 5.3 V, which is lower than that of the device based on 1 (Figure 7c). Bright light emission was obtained from both devices (∼1000 Cd/m2 @ 10 mA/cm2) with a current efficiency of ∼65 Cd/A at high luminance of 9800 Cd/m2 for the device based on 2 (Figure 7d,e). Thus, the overall performance of 1,2 as exciplex emitters was better than their use as neat emitters (see Table 2 for comparison of the devices).

■

CONCLUSIONS Carbazole and 3,6-trifluoromethylphenyl carbazole were substituted on acridone to get donor−acceptor−donor materials for organic light-emitting diodes. The photophysical (steady-state and time-dependent emission) properties of these compounds were studied, which revealed thermally activated delayed fluorescence (TADF) in them. The ΔEST values for both these materials were found to be as low as 0.15 eV, favorable for TADF properties. Both these materials were employed as green emitters in OLED devices. Unlike the commonly reported doped TADF emitters, we showed the TADF properties of emitters in undoped devices, i.e., without host matrix. OLEDs of 1 showed a low turn-on voltage and high luminance (∼1000 Cd/m2), which is roughly 2 times than that of 2. Thus, 1 behaves as a good nondopant TADF emitter for OLEDs, which we attribute to the lower ΔEST and better donor properties of carbazole than 3,6-trifluoromethylphenyl carbazole. Furthermore, an exciplex emission (roughly 10 times more than neat emitter) was observed in blends of 1 and 2 with PVK. Exciplex emission (at 465 nm) was also seen in the OLEDs having blend of 1 or 2 with PVK as the active layer. Bright blue light emission was obtained from both devices (∼1000 Cd/m2 @ 10 mA/cm2) with a current efficiency of ∼65 Cd/A at a high luminance of 9800 Cd/m2 for the device based on 2. We found that performance of 1,2 as exciplex emitters were better than their use as neat emitters. Thus, we show that acridone−carbazole derivatives (1,2) offer variable electroluminescence as undoped TADF green emitter and blue exciplex emitter when doped in PVK.

■

METHODS All chemicals were purchased from Sigma-Aldrich and used as received. Organic solvents were dried using standard procedures, wherever anhydrous solvents were required. 1H and 13C NMR spectra were recorded using a Varian 600 MHz spectrometer. Tetramethylsilane in CDCl3 was used as an internal reference (residual proton; δ = 7.26 ppm), for recording of 1H NMR spectra. Mass spectra were recorded using Bruker MALDI-TOF. Cyclic voltammetry was performed on CH Instruments 620D electrochemical analyzer. Typically, a three-electrode cell was employed with glassy carbon working electrode, Ag/AgCl (nonaqueous) reference electrode, and Pt wire counter electrode. The measurements were performed at room temperature in anhydrous acetonitrile with tetrabutylammonium hexafluorophosphate solution (0.1 1010


Article


For blue-emitting devices, exciplex formation was initiated by making a blend of PVK with the organic compounds 1 and 2. A 7:1 weight ratio of PVK and organic compound was taken, which was dissolved in chloroform. This was spin-coated at 5000 rpm for 30 s, which resulted in ∼100 nm thick films. In these devices, best results were obtained by using a 45 nm layer of PEDOT:PSS as an HTL. This was grown by spin coating at 8000 rpm for 40 s. The geometries of the most optimized devices were ITO/PEDOT:PSS/(PVK + 1 or 2)/Bphen/LiF/ Al. The active areas of the devices were around 9−12 mm2. J− V measurements of the devices were carried out using a 2400 Keithley sourcemeter at the emitting wavelength, and the luminance−voltage (L−V) spectra were simultaneously recorded using lock-in detection. The lock-in signal was converted to light units using the conversion factors, which were estimated using a calibrated light source Electroluminance spectroscopy of the devices were measured using a setup consisting of a Bausch and Lomb 350−750 nm monochromator and a Hamamatsu R212 photomultiplier tube as the detector.

M) as supporting electrolyte at a scan rate of 100 mV/s. The oxidation potential of Fc/Fc+ was used as internal reference. The absorption and fluorescence data were acquired with ∼6 × 10−6 M solution. UV−visible spectra were recorded using Shimadzu 1800, and steady-state fluorescence spectra were recorded on a Horiba Fluoromax-4 spectrofluorometer. A diode laser-based time-correlated single-photon counting (TCSPC) spectrometer (IBH, U.K.) was used to obtain the time-resolved fluorescence measurements and has been described in detail elsewhere.88−90 Excitation of 1,2 was done using a 374 nm diode laser (1 MHz repetition rate). The fluorescence transients were collected at magic angle (54.7°) configuration, which ensures that the observed transient decays are not influenced by the rotational relaxation of the probe molecule. To obtain instrument response function (IRF), scattered excitation light from the suspended TiO2 particles in water was monitored. The IRF obtained was ∼160 ps. TADF lifetime was obtained on a Agilent spectrofluorometer, with emission spectra taken at varying delay times (100 μs to 1 ms), and then peak intensity was plotted against delay time and fitted to the equation I = Io e−t/τ. Doped films of 1/2:PMMA (1:9 w/w in chloroform) were spin-coated on quartz at 4000 rpm for 30 s. Steady-state fluorescence and its lifetime studies of PMMA-doped films of 1,2 were carried out on a Fluorolog-3 spectrofluorometer. Delays of 100 and 5 μs for 1 and 2, respectively, were given and decay data were collected. For OLED fabrication, solid films of the compounds were prepared with a spin coater (Holmarc HO-TH-05). Spin coating was done at 2000 rpm for 40 s. Vacuum deposition was done at a base vacuum of 2 × 10−6 mbar. ITO-coated substrate (15−25 Ω/sq, Sigma-Aldrich) was cut into 22 × 12 mm2 dimension. It was etched into the desired pattern to incorporate four active devices (see device geometry in Figures 6a and 7a) with the help of Zn powder and 10% HCl. Substrate cleaning was done in three simple steps. It was first cleaned with soap solution and rinsed with distilled water. Further, it was sonicated with distilled water and propanol. The sonication time was 10 min in each step. The last step included cleaning with trichloroethylene vapors. After UV treatment for 1 h, the substrates were ready for device fabrication. Organic layer of 1 and 2 was thermally evaporated in vacuum (base vacuum ∼10−6 mbar). The thickness was ∼20 nm. Bathophenanthroline (Bphen), a well-known hole-blocking layer (HBL), was also thermally evaporated over the active layer, followed by 1 nm LiF and 160 nm Al. The geometry of the most optimized device was ITO/(PVK + NPD)/1 or 2/ Bphen/LiF/Al. All of the theoretical calculations have been performed using the ORCA electronic structure-based programs.91 RHF types of Kohn−Sham-based density functional theory (DFT) methods have been applied and the def2TZVP basis set has been employed for all atoms to obtain the optimized geometry as well as their energies using the B3LYP exchange-correlation functional.92,93 The geometry optimization and time-dependent DFT (TDDFT)-based calculations for the evaluation of electronic excitation spectrum have been performed using the Becke three-parameter exchange functional or Lee−Yang−Parr correlation functional (B3LYP) and BHHLYP exchange-correlation functional, respectively. The grid-based DFT has been used, which employs a typical grid quadrature for the computation of the integrals. Typically, the grid consists of 96 radial shells with 36 and 72 angular points during the self-consistent field procedure.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b08357.

■

NMR spectra; mass, absorption, emission, and transient lifetime studies; cyclic voltammograms; and figures related to DFT studies (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.B.). *E-mail: [email protected] (N.A.). ORCID

Kuttay R. S. Chandrakumar: 0000-0002-0121-3556 Neeraj Agarwal: 0000-0003-2853-2730 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank Swati Dixit for assistance in cyclic voltammetric studies. They acknowledge Tata Institute of Fundamental Research, Mumbai, for NMR and MALDI-TOF. They also acknowledge Radiation and Photochemistry Division, Bhabha Atomic Research Centre for TCSPC. N.A. and S.B. acknowledge Department of Science and Technology for partial financial support (EMR/2017/000805).

■

REFERENCES

(1) Cao, X.; Zhang, D.; Zhang, S.; Tao, Y.; Huanga, W. CNContaining Donor−acceptor-type Small-molecule Materials for Thermally Activated Delayed Fluorescence OLEDs. J. Mater. Chem. C 2017, 5, 7699−7714. (2) Chiang, C. J.; Kimyonok, A.; Etherington, M. K.; Griffiths, G. C.; Jankus, V.; Turksoy, F.; Monkman, A. P. Ultrahigh Efficiency Fluorescent Single and Bi-Layer Organic Light Emitting Diodes: The Key Role of Triplet Fusion. Adv. Funct. Mater. 2013, 23, 739− 746.

1011


Article


Efficient Fluorescent Organic light-Emitting Diodes. J. Appl. Phys. 2009, 106, No. 124510. (21) King, S. M.; Cass, M.; Pintani, M.; Coward, C.; Dias, F. B.; Monkman, A. P.; Roberts, M. The Contribution of Triplet−Triplet Annihilation to the Lifetime and Efficiency of Fluorescent Polymer Organic Light Emitting Diodes. J. Appl. Phys. 2011, 109, No. 074502. (22) Fukagawa, H.; Shimizu, T.; Ohbe, N.; Tokito, S.; Tokumaru, K.; Fujikake, H. Anthracene Derivatives as Efficient Emitting Hosts for Blue Organic Light-Emitting Diodes Utilizing Triplet-Triplet Annihilation. Org. Electron. 2012, 13, 1197−1203. (23) Kaji, H.; Suzuki, H.; Fukushima, T.; Shizu, K.; Suzuki, K.; Kubo, S.; Komino, T.; Oiwa, H.; Suzuki, F.; Wakamiya, A.; Murata, Y.; Adachi, C. Purely Organic Electroluminescent Material Realizing 100% Conversion from Electricity to Light. Nat. Commun. 2015, 6, No. 8476. (24) Wong, M. Y.; Zysman-Colman, E. Purely Organic Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes. Adv. Mater. 2017, 29, No. 1605444. (25) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly Efficient Organic Light-Emitting Diodes from Delayed Fluorescence. Nature 2012, 492, 234−238. (26) Zhang, Q.; Li, J.; Shizu, K.; Huang, S.; Hirata, S.; Miyazaki, H.; Adachi, C. Design of Efficient Thermally Activated Delayed Fluorescence Materials for Pure Blue Organic Light Emitting Diodes. J. Am. Chem. Soc. 2012, 134, 14706−14709. (27) 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, No. 083302. (28) Yersin, H.; Czerwieniec, R.; Shafikov, M. Z.; Suleymanova, A. F. TADF Material Design: Photophysical Background and Case Studies Focusing on CuI and AgI Complexes. ChemPhysChem 2017, 18, 3508−3535. (29) Godumala, M.; Choi, S.; Cho, M. J.; Cho, D. H. Thermally Activated Delayed Fluorescence Blue Dopants and Hosts: From the Design Strategy to Organic Light-Emitting Diode Applications. J. Mater. Chem. C 2016, 4, 11355−11381. (30) Masui, K.; Nakanotani, H.; Adachi, C. Analysis of Exciton Annihilation in High-efficiency Sky-Blue Organic Light-Emitting Diodes with Thermally Activated Delayed Fluorescence. Org. Electron. 2013, 14, 2721−2726. (31) 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 Splitting in a TADF Molecule. J. Mater. Chem. C 2016, 4, 3815−3824. (32) 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, 15850. (33) Sharma, B. K.; Shaikh, A. M.; Agarwal, N.; Kamble, R. M. Synthesis, Photophysical and Electrochemical Studies of AcridoneAmine Based Donor−Acceptors for Hole Transport Materials. RSC Adv. 2016, 6, 17129−17137. (34) Wang, C.; Zhang, Z.; Wang, Y. Quinacridone-Based πConjugated Electronic Materials. J. Mater. Chem. C 2016, 4, 9918− 9936. (35) Wang, Y.; Zhu, Y.; Lin, X.; Yang, Y.; Zhang, B.; Zhan, H.; Xie, Z.; Cheng, Y. Efficient Non-doped Yellow OLEDs Based on Thermally Activated Delayed Fluorescence Conjugated Polymers with an Acridine/Carbazole Donor Backbone and Triphenyltriazine Acceptor Pendant. J. Mater. Chem. C 2018, 6, 568−574. (36) Pander, P.; Swist, A.; Motyka, R.; Soloducho, J.; Dias, F. B.; Data, P. Thermally Activated Delayed Fluorescence with a Narrow Emission Spectrum and Organic Room Temperature Phosphorescence by Controlling Spin-Orbit Coupling and Phosphorescence Lifetime of Metal-free Organic Molecules. J. Mater. Chem. C 2018, 6, 5434−5443.

(3) Cui, L. S.; Kim, J. U.; Nomura, H.; Nakanotani, H.; Adachi, C. Benzimidazobenzothiazole-Based Bipolar Hosts to Harvest Nearly All of the Excitons from Blue Delayed Fluorescence and Phosphorescent Organic Light-Emitting Diodes. Angew. Chem. 2016, 128, 6978−6982. (4) Méhes, G.; Nomura, H.; Zhang, Q.; Nakagawa, T.; Adachi, C. Enhanced Electroluminescence Efficiency in a Spiro-acridine Derivative through Thermally Activated Delayed Fluorescence. Angew. Chem., Int. Ed. 2012, 51, 11311−11315. (5) Nasu, K.; Nakagawa, T.; Nomura, H.; Lin, C.-J.; Cheng, C.-H.; Tseng, M.-R.; Yasuda, T.; Adachi, C. A Highly Luminescent SpiroAnthracenone-Based Organic Light-Emitting Diode Exhibiting Thermally Activated Delayed Fluorescence. Chem. Commun. 2013, 49, 10385−10387. (6) Reineke, S. Organic Light-Emitting Diodes: Phosphorescence meets its Match. Nat. Photonics 2014, 8, 269−270. (7) Zhang, D.; Duan, L.; Li, C.; Li, Y.; Li, H.; Zhang, D.; Qiu, Y. High-efficiency Fluorescent Organic Light-Emitting Devices using Sensitizing Hosts with a Small Singlet−Triplet Exchange Energy. Adv. Mater. 2014, 26, 5050−5055. (8) Agarwal, N.; Nayak, P. K.; Ali, F.; Patankar, M. P.; Narasimhan, K. L.; Periasamy, N. Tuning of HOMO levels of Carbazole Derivatives: New Molecules for Blue OLED. Synth. Met. 2011, 161, 466−473. (9) Agarwal, N.; Patil, M.; Patil, M. Synthesis of Highly Emissive 1,8Diaryl Anthracene Derivatives and Fabrication of their Micro/ Nanostructures. RSC Adv. 2015, 5, 98447−98455. (10) Montes, V. A.; Pérez, C. B.; Agarwal, N.; Shinar, J.; Anzenbacher, J. P. Molecular-Wire Behavior of OLED Materials: Exciton Dynamics in Multichromophoric Alq3-OligoFluorene-Pt(II)Porphyrin Triads. J. Am. Chem. Soc. 2006, 128, 12436−12438. (11) Montes, V. A.; Zyryanov, G. V.; Danilov, E.; Agarwal, N.; Palacios, M. A.; Anzenbacher, J. P. Ultrafast Energy Transfer in Oligofluorene−Aluminum Bis(8-hydroxyquinoline)acetylacetone Coordination Polymers. J. Am. Chem. Soc. 2009, 131, 1787−1795. (12) Kafafi, Z. H. Organic Electroluminescence; Taylor & Francis Group: Abingdon, U.K., 2005. (13) Forrest, S. R.; Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E. Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices. Nature 1998, 395, 151−154. (14) Ilic, S.; Brown, E. S.; Xie, Y.; Maldonado, S.; Glusac, K. D. Sensitization of p-GaP with Monocationic Dyes: The Effect of Dye Excited-State Lifetime on Hole Injection Efficiencies. J. Phys. Chem. C 2016, 120, 3145−3155. (15) Lee, S. Y.; Yasuda, T.; Yang, Y. S.; Zhang, Q.; Adachi, C. Luminous Butterflies: Efficient Exciton Harvesting by Benzophenone Derivatives for Full-Color Delayed Fluorescence OLEDs. Angew. Chem., Int. Ed. 2014, 53, 6402−6406. (16) Obara, S.; Itabashi, M.; Okuda, F.; Tamaki, S.; Tanabe, Y.; Ishii, Y.; Nozaki, K.; Haga, M.-A. Highly Phosphorescent Iridium Complexes Containing Both Tridentate Bis(benzimidazolyl)-benzene or -Pyridine and Bidentate Phenylpyridine: Synthesis, Photophysical Properties, and Theoretical Study of Ir-Bis(benzimidazolyl)benzene Complex. Inorg. Chem. 2006, 45, 8907−8921. (17) Rausch, A. F.; Homeier, H. H. H.; Yersin, H. Organometallic Pt(II) and Ir(III) Triplet Emitters for OLED Applications and the Role of Spin-Orbit Coupling - A Study Based on High-Resolution Optical Spectroscopy. Top. Organomet. Chem. 2010, 29, 193−235. (18) Kim, S.-K.; Yang, B.; Ma, Y.; Lee, J.-H.; Park, J.-W. Exceedingly Efficient Deep-blue Electroluminescence from New Anthracenes obtained using Rational Molecular Design. J. Mater. Chem. 2008, 18, 3376−3384. (19) Zhou, J.; Chen, P.; Wang, X.; Wang, Y.; Wang, Y.; Li, F.; Yang, M.; Huang, Y.; Yu, J.; Lu, Z. Charge-Transfer-Featured Materials Promising Hosts for Fabrication of Efficient OLEDs through Triplet Harvesting via Triplet Fusion. Chem. Commun. 2014, 50, 7586−7589. (20) Kondakov, D. Y.; Pawlik, T. D.; Hatwar, T. K.; Spindler, J. P. Triplet Annihilation Exceeding Spin Statistical Limit in Highly 1012


Article


(57) Sun, X.; Tu, X.; Dai, C.; Zhang, X.; Zhang, B.; Zeng, Q. Palladium-Catalyzed C-N Cross Coupling of Sulfinamides and Aryl Halides. J. Org. Chem. 2012, 77, 4454−4459. (58) Murthy, S. N.; Madhav, B.; Reddy, V. P.; Nageswar, Y. V. D. A New, Efficient and Recyclable Lanthanum(III) Oxide-Catalyzed C-N Cross-Coupling. Adv. Synth. Catal. 2010, 352, 3241−3245. (59) Noda, H.; Nakanotani, H.; Adachi, C. Excited State Engineering for Efficient Reverse Intersystem Crossing. Sci. Adv. 2018, 4, No. eaao6910. (60) Abedin-Siddique, Z.; Ohno, T.; Nozaki, K.; Tsubomura, T. Intense Fluorescence of Metal-to-Ligand Charge Transfer in [Pt(0)(binap)2] [binap = 2,2′-Bis(diphenylphosphino)-1,1′-binaphthyl]. Inorg. Chem. 2004, 43, 663−673. (61) Yersin, H.; Finkenzeller, W. J. Highly Efficient OLEDs with Phosphorescent Materials; Wiley-VCH: Weinheim, 2008; pp 1−97. (62) Yersin, H.; Rausch, A. F.; Czerwieniec, R.; Hofbeck, T.; Fischer, T. The Triplet State of Organo-Transition Metal Compounds. Triplet Harvesting and Singlet Harvesting for Efficient OLEDs. Coord. Chem. Rev. 2011, 255, 2622−2652. (63) Li, J.; Zhang, Q.; Nomura, H.; Miyazaki, H.; Adachi, C. Thermally Activated Delayed Fluorescence from 3n* to 1n* UpConversion and its Application to Organic Light-Emitting Diodes. Appl. Phys. Lett. 2014, 105, No. 013301. (64) Gritzner, G.; Kuta, J. Recommendations on Reporting Electrode Potentials in Nonaqueous Solvents. J. Pure Appl. Chem. 1984, 56, 461−466. (65) Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures. Chem. Rev. 2003, 103, 3899−4032. (66) Herbich, J.; Kapturkiewicz, A.; Nowacki, J. Phosphorescent Intramolecular Charge Transfer Triplet States. Chem. Phys. Lett. 1996, 262, 633−642. (67) Kapturkiewicz, A.; Herbich, J.; Nowacki, J. Highly Efficient Electrochemical Generation of Fluorescent Intramolecular ChargeTransfer States. Chem. Phys. Lett. 1997, 275, 355−362. (68) Zhang, Q.; Kuwabara, H.; Potscavage, W. J., Jr.; Huang, S.; Hatae, Y.; Shibata, T.; Adachi, C. Anthraquinone-Based Intramolecular Charge-Transfer Compounds: Computational Molecular Design, Thermally Activated Delayed Fluorescence, and Highly Efficient Red Electroluminescence. J. Am. Chem. Soc. 2014, 136, 18070−18081. (69) Valeur, B.; Berberan-Santos, M. N. Molecular Fluorescence: Principles and Applications, 2nd ed.; Wiley-VCH: Weinheim, Germany, 2012. (70) 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. 2015, 14, 330−336. (71) dos Santos, P. L.; Dias, F. B.; Monkman, A. P. Investigation of the Mechanisms Giving Rise to TADF in Exciplex States. J. Phys. Chem. C 2016, 120, 18259−18267. (72) Pander, P.; Kudelko, A.; Brzeczek, A.; Wroblowska, M.; Walczak, K.; Data, P. Analysis of Exciplex Emitters. Disp. Imaging 2016, 2, 265−277. (73) Weller, A. The Exciplex; Academic Press Inc.: London, 1975. (74) Jankus, V.; Data, P.; Graves, D.; McGuinness, C.; Santos, J.; Bryce, M. R.; Dias, F. B.; Monkman, A. P. Highly Efficient TADF OLEDs: How the Emitter−Host Interaction Controls Both the Excited State Species and Electrical Properties of the Devices to Achieve Near 100% Triplet Harvesting and High Efficiency. Adv. Funct. Mater. 2014, 24, 6178−6186. (75) Gong, X.; Lim, S.-H.; Ostrowski, J. C.; Moses, D.; Bardeen, C. J.; Bazan, G. C. Phosphorescence from Iridium Complexes Doped into Polymer Blends. J. Appl. Phys. 2004, 95, 948−953. (76) Keller, J. M.; Glusac, K. D.; Danilov, E. O.; McIlroy, S.; Sreearuothai, P.; Cook, A. R.; Jiang, H.; Miller, J. R.; Schanze, K. S. Negative Polaron and Triplet Exciton Diffusion in Organometallic “Molecular Wires”. J. Am. Chem. Soc. 2011, 133, 11289−11298.

(37) Mamada, M.; Inada, K.; Komino, T.; Potscavage, W. J., Jr.; Nakanotani, H.; Adachi, C. Highly Efficient Thermally Activated Delayed Fluorescence from an Excited-State Intramolecular Proton Transfer System. ACS Cent. Sci. 2017, 3, 769−777. (38) Nakanotani, H.; Higuchi, T.; Furukawa, T.; Masui, K.; Morimoto, K.; Numata, M.; Tanaka, H.; Sagara, Y.; Yasuda, T.; Adachi, C. High-Efficiency Organic Light-Emitting Diodes with Fluorescent Emitters. Nat. Commun. 2014, 5, No. 4016. (39) Zhang, D.; Cao, X.; Wu, Q.; Zhang, M.; Sun, N.; Zhang, X.; Tao, Y. Purely Organic Materials for Extremely Simple All-TADF White OLEDs: A New Carbazole/oxadiazole Hybrid Material as a Dual-Role Non-Doped Light Blue Emitter and Highly Efficient Orange Host. J. Mater. Chem. C 2018, 6, 3675−3682. (40) Hartwig, J. F. Palladium-Catalyzed amination of Aryl Aalides: Mechanism and Rational Catalyst Design. Synlett 1997, 1997, 329− 340. (41) Ley, S. V.; Thomas, A. W. Modern Synthetic Methods for Copper-Mediated C(aryl)[bond]O, C(aryl)[bond]N, and C(aryl)[bond]S Bond Formation. Angew. Chem., Int. Ed. 2003, 42, 5400− 5449. (42) Maejima, T.; Shimoda, Y.; Nozaki, K.; Mori, S.; Sawama, Y.; Monguchi, Y.; Sajiki, H. One-pot Aromatic Amination Based on Carbon−Nitrogen Coupling Reaction Between Aryl Halides and Azido Compounds. Tetrahedron 2012, 68, 1712−1722. (43) Paul, F.; Patt, J.; Hartwig, J. F. Palladium-Catalyzed Formation of Carbon-Nitrogen Bonds. Reaction Intermediates and Catalyst Improvements in the Hetero Cross-Coupling of Aryl Halides and Tin Amides. J. Am. Chem. Soc. 1994, 116, 5969−5970. (44) Sang, P.; Xie, Y.; Zou, J.; Zhang, Y. Copper-Catalyzed Sequential Ullmann N-Arylation and Aerobic Oxidative C−H Amination: A Convenient Route to Indolo[1,2-c]quinazoline Derivatives. Org. Lett. 2012, 14, 3894−3897. (45) Wei, W.; Shao, Y.; Hu, H.; Zhang, F.; Zhang, C.; Xu, Y.; Wan, X. Coupling of Methyl Ketones and Primary or Secondary Amines Leading to α-Ketoamides. J. Org. Chem. 2012, 77, 7157−7165. (46) Xia, Q.; Wanzhi, C.; Qiu, H. Direct C−N Coupling of Imidazoles and Benzylic Compounds via Iron-Catalyzed Oxidative Activation of C−H Bonds. J. Org. Chem. 2011, 76, 7577−7582. (47) Surry, D. S.; Buchwald, S. L. Dialkylbiaryl Phosphines in Pdcatalyzed Amination: A User’s Guide. Chem. Sci. 2011, 2, 27−50. (48) Beletskaya, I. P.; Cheprakov, A. V. Copper in Cross-Coupling Reactions The Post-Ullmann Chemistry. Coord. Chem. Rev. 2004, 248, 2337−2364. (49) Hie, L.; Ramgren, S. D.; Mesganaw, T.; Garg, N. K. NickelCatalyzed Amination of Aryl Sulfamates and Carbamates Using an Air-Stable Precatalyst. Org. Lett. 2012, 14, 4182−4185. (50) Bariwal, J.; Eycken, E. V. d C-N Bond Forming Cross-Coupling Reaction: An Overview. Chem. Soc. Rev. 2013, 42, 9283−9303. (51) Ma, D.; Cai, Q. Copper/Amino Acid Catalyzed CrossCouplings of Aryl and Vinyl Halides with Nucleophiles. Acc. Chem. Res. 2008, 41, 1450−1460. (52) Guram, A. S.; Buchwald, S. L. Palladium-Catalyzed Aromatic Aminations with in situ Generated Aminostannanes. J. Am. Chem. Soc. 1994, 116, 7901−7902. (53) Berrino, R.; Cacchi, S.; Fabrizi, G.; Goggiamani, A. 4-Aryl-2quinolones from 3,3-Diarylacrylamides through Intramolecular Copper-Catalyzed C−H Functionalization/C−N Bond Formation. J. Org. Chem. 2012, 77, 2537−2542. (54) Ullmann, F. Ueber eine neue Bildungsweise von Diphenylaminderivaten. Ber. Dtsch. Chem. Ges. 1903, 36, 2382−2384. (55) Chan, D. M. T.; Monaco, K. L.; Wang, R.; Winters, M. P. New N- and O-arylations with Phenylboronic Acids and Cupric Acetate. Tetrahedron Lett. 1998, 39, 2933−2966. (56) Lam, P. Y. S.; Clark, C. G.; Saubern, S.; Adams, J.; Winters, M. P.; Chan, D. M. T.; Combs, A. New Aryl/Heteroaryl C-N Bond Cross-Coupling Reactions via Arylboronic Acid/Cupric Acetate Arylation. Tetrahedron Lett. 1998, 39, 2941−2944. 1013


Article

The Journal of Physical Chemistry C (77) Dykstra, T. E. Photophysics of Conjugated Polymers. PhD thesis. University of Toronto, 2008. (78) Hennebicq, E.; Pourtois, G.; Scholes, G. D.; Herz, L. M.; Russell, D. M.; Silva, C.; Setayesh, S.; Grimsdale, A. C.; Müllen, K.; Brédas, J.-L.; et al. Exciton Migration in Rigid-Rod Conjugated Polymers: An Improved Forster Model. J. Am. Chem. Soc. 2005, 127, 4744−4762. (79) Schwartz, B. J.; Nguyen, T.-Q.; Wu, J.; Tolbert, S. H. Interchain and Intrachain Exciton Transport in Conjugated Polymers: Ultrafast Studies of Energy Migration in Aligned MEH-PPV/Mesoporous Silica Composites. Synth. Met. 2001, 116, 35−40. (80) Sarma, M.; Wong, K.-T. Exciplex: An Intermolecular ChargeTransfer Approach for TADF. ACS Appl. Mater. Interfaces 2018, 10, 19279−19304. (81) Jeon, S. K.; Yook, K. S.; Lee, J. Y. Highly Efficient Exciplex Organic Light-Emitting Diodes Using Thermally Activated Delayed Fluorescent Emitters as Donor and Acceptor Materials. Nanotechnology 2016, 27, No. 224001. (82) Nayak, P. K.; Agarwal, N.; Periasamy, N.; Patankar, M. P.; Narasimhan, K. L. Pure Exciplex Electroluminescence in Blended Film of Small Organic Molecules. Synth. Met. 2010, 160, 722−727. (83) Grybauskaite-Kaminskiene, G.; Ivaniuk, K.; Bagdziunas, G.; Turyk, P.; Stakhira, P.; Baryshnikov, G.; Volyniuk, D.; Cherpak, V.; Minaev, B.; Hotra, Z.; Agren, H.; Grazuleviciusa, J. V. Contribution of TADF and Exciplex Emission for Efficient “Warm-White” OLEDs. J. Mater. Chem. C 2018, 6, 1543−1550. (84) Jankus, V.; Monkman, A. P. Is Poly(vinylcarbazole) a Good Host for Blue Phosphorescent Dopants in PLEDs? Dimer Formation and Their Effects on the Triplet Energy Level of Poly(Nvinylcarbazole) and Poly(N-Ethyl-2-Vinylcarbazole). Adv. Funct. Mater. 2011, 21, 3350−3356. (85) Deksnys, T.; Simokaitiene, J.; Keruckas, J.; Volyniuk, D.; Bezvikonnyi, O.; Cherpak, V.; Stakhira, P.; Ivaniuk, K.; Helzhynskyy, I.; Baryshnikov, G.; Minaev, B.; Grazulevicius, J. V. Synthesis and Characterisation of a Carbazole-Based Bipolar Exciplex-Forming Compound for Efficient and Color-Tunable OLEDs. New J. Chem. 2017, 41, 559−568. (86) Hung, W.-Y.; Fang, G.-C.; Chang, Y.-C.; Kuo, T.-Y.; Chou, P.T.; Lin, S.-W.; Wong, K.-T. Highly Efficient Bilayer Interface Exciplex For Yellow Organic Light-Emitting Diode. ACS Appl. Mater. Interfaces 2013, 5, 6826−6831. (87) Hung, W.-Y.; Fang, G.-C.; Lin, S.-W.; Cheng, S.-H.; Wong, K.T.; Kuo, T.-Y.; Chou, P.-T. The First Tandem, All-Exciplex-Based WOLED. Sci. Rep. 2014, 4, No. 5161. (88) Singh, P. K.; Kumbhakar, M.; Pal, H.; Nath, S. Modulation in the Solute Location in Block Copolymer−Surfactant Supramolecular Assembly: A Time-Resolved Fluorescence Study. J. Phys. Chem. B 2009, 113, 1353−1359. (89) Singh, P. K.; Mora, A. K.; Nath, S. Ultrafast Torsional Relaxation of Thioflavin-T in Tris(pentafluoroethyl)trifluorophosphate (FAP) Anion-Based Ionic Liquids. J. Phys. Chem. B 2015, 119, 14252−14260. (90) Singh, P. K.; Murudkar, S.; Mora, A. K.; Nath, S. Ultrafast Torsional Dynamics of Thioflavin-T in an Anionic Cyclodextrin Cavity. J. Photochem. Photobiol., A 2015, 298, 40−48. (91) Nesse, F. The ORCA program system. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2012, 2, 73−78. (92) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648. (93) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields. J. Phys. Chem. 1994, 98, 11623−11627.

1014


Thermally Activated Delayed Fluorescence (Green) in Undoped Film

Recommend Documents