Photophysical Properties of Thermally Activated Delayed Fluorescent

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C: Energy Conversion and Storage; Energy and Charge Transport

Photo-Physical Properties of Thermally Activated Delayed Fluorescent Materials upon Distortion of Central Axis of Donor Moiety Hyung Suk Kim, Hansol Park, So-Ra Park, Sang Hoon Lee, Yunho Ahn, Yong Sup Lee, and Min Chul Suh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11144 • Publication Date (Web): 21 Nov 2018 Downloaded from http://pubs.acs.org on November 27, 2018

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The Journal of Physical Chemistry

Photo-physical Properties of Thermally Activated Delayed Fluorescent Materials upon Distortion of Central Axis of Donor Moiety Hyung Suk Kim a, Hansol Park a, So-Ra Park a, Sang Hoon Lee a, Yunho Ahn a, Yong Sup Lee b, and Min Chul Suh a,* aDepartment

of Information Display, Kyung Hee University, Seoul 02447, Republic of Korea of Life and Nanopharmaceutical Sciences, College of Pharmacy, Kyung Hee University, Seoul 02447, Republic of Korea

bDepartment

Abstract In this study, we showed the distortion of central axis of donor moiety can switch critically the rate of reverse intersystem crossing (kRISC) process, which is the trigger point to modulate the lifetime of delayed fluorescence. To achieve kRISC what we desired (105 to 106 s-1) in a series of donor-acceptor-donor (D-A-D) type thermally activated delayed fluorescence (TADF) materials, the donor groups (phenoxazine and phenothiazine) was selectively introduced. Maintaining the near orthogonality between donor and acceptor (benzonitrile) moiety, the occurrence of the distortion of central axis of donor moiety could make the effect of locally excited triplet state (3LE). In other words, the interaction between 3LE and the charge transfer counterparts (i.e., 1CT and 3CT) contributes an opposite propensity of kRISC for each target TADF materials when those are dissolved in solution and condensed in solid-state. Herein, we have theoretically and experimentally shown the photo-physical behavior of common D-A-D type TADF upon the different system.

I. Introduction In the field of organic light emitting diodes (OLEDs), there have been lots of interests on the achievement of ~100 % internal quantum efficiency (IQE) to overcome the exciton ratio generated from the electron-hole recombination dependent on spin-statistics.1-4 The spins of injected carriers are uncorrelated, which roughly says that three of four pumped excitons are triplet states under the spin multiplicity.5-6 Thus, the harnessing triplet excitons (so-called ‘dark states’) was paramount issue to make the charge-to-photon IQE reach the achievement as high as 100 %. Unfortunately, the current OLED devices still select the fluorescent blue emitters which can only realize 25% IQE.7 The main reason that we cannot make the stable blue emitters as high as 100 % is due to the difficulty of facilitating the host materials which withstand the excitation energy greater than 3.5 eV. Thus, there has been lots of trial to reduce the singlet energy of the organic molecules by using the insight realizing small energy splitting (EST) between the lowest excited singlet state (S1) and triplet state (T1). From this approach, lots of groups are developing the materials with very small EST. Interestingly, those materials show delayed fluorescence at the ambient condition. From this reason, Adachi group named those phenomenon as a thermally activated delayed fluorescence (TADF). They have embodied their ideas with 1,2,3,5-tetrakis(carbazol9-yl)-4,6-dicyanobenzene (4CzIPN), a highly efficient TADF molecule.8 Hereafter, numerous

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highly efficient TADF molecule, showing IQE as high as approximately 100 %, has been reported9-11. The following formula illustrates this phenomenon well. 𝑒2 ∆𝐸ST = 2∬HOMO(𝑟1)LUMO(𝑟2)(𝑟1 ― 𝑟2)HOMO(𝑟2)LUMO(𝑟1)𝑑𝑟1𝑑𝑟2 = 2J (1) wherein HOMO and LUMO are the highest occupied molecular orbital and the lowest unoccupied molecular orbital wavefunctions. r1 and r2 are the electron coordinates, r1-

r2 is the separation of two electrons (i.e., the average distance between two electrons). The Coulomb-exchange correlation kernel follows the Hartree kernel in this equation.12-14 J is the electron exchange energy. Eq. 1 simply delivers the meaning that the large spatial overlap between HOMO and LUMO can make the larger EST.14 Thus, to minimize the EST, the spatial separation between the ground state and excited one was essential for being utilized to synthesize most TADF materials with an intramolecular charge transfer (ICT). Very recently, it has been known that the 3LE state (locally excited triplet state) of TADF molecule could also influence the delayed fluorescence. Rather, T1 can be more stabilized if a part of the electrons inside certain acceptor (A) and donor (D) moieties. These 3LE state15-17 induces the significant stabilization of estimated T118-19. However, in general, J should equate 1/2 EST as described in Eq. 1 if there is no previous case which unconventionally stabilizes only the T1. Then, we can express the energy for S1 (ES) and T1 (ET) as follows ES = ELUMO - EHOMO + J1 ET = ELUMO - EHOMO – J2 (2) Thus, we commonly say this fundamental approach ‘a universal design rule (EST = 2J)’.20 However, the presence of 3LE state makes different J values (J1 and J2) for singlet charge transfer (1CT, S1 in general) state, and their actual triplet (T1) state, respectively. In other words, the value corresponding J1 and J2 can be different. Thus, we can understand the case of J2 > J1 when 3LE character dominates over 3CT one for reverse intersystem crossing (RSIC) by unconventional stabilization. In this condition, we could observe that 3LE→1CT transfer is the major RISC process because the interaction between 1CT and 3CT state would not be efficient due to the large stabilization of 3CT state (J2) rather than 1CT (J1). Interestingly, this is quite different from that through a universal design rule (i.e., EST ≠ 2J). Driven by such interesting systematic process, we scrutinized the photo-physical behavior of two target D-A-D type TADF materials. In particular, 3,5-di(10H-phenothiazin-10yl)benzonitrile (DPtBn) and 3,5-di(10H-phenoxazin-10-yl)benzonitrile (DPxBn). In this study, those materials showed the rate of reverse intersystem crossing (kRISC) can effectively be realized by differentiating the distortion of central axis of donor moiety from ~ 105 to 106 s-1,


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maintaining near orthogonality between donor and acceptor moiety. Further, those benzonitrile based TADF groups showed an opposite behavior for kRISC in the different systems (i.e., solution and solid-state).

II. Experimental Section 2.1. Synthesis of TADF dopant materials 2.1.2. Synthesis of 3,5-di(10H-phenothiazin-10-yl)benzonitrile (DPtBn) 3,5-Dichlorobenzonitirle (0.86 g, 5 mmol) was dissolved in anhydrous toluene (30 ml) after bubbling with N2. 10H-phenothiazine (1.99 g, 10 mmol) and t-BuONa (1.92 g, 20 mmol) were added and stirred for 10 min at room temperature. Then, tris(dibenzylideneacetone)dipalladium(0) (137 mg, 0.15 mmol) and tri-tert-butylphosphonium tertafluoroborate (87 mg, 0.3 mmol) were added after stirring the solution. The mixture was stirred at 100 ℃ for 2 h and filtered using silica gel and Celite to remove palladium. The mixture was evaporated under reduced pressure, then recrystallized from dichloromethane/methanol. The product was isolated as yellow powder, yield 50 %, 1H NMR (DMSO-d6, 400MHz) 7.49 (d, J = 2.0 Hz, 2 H), 7.33 (dd, J = 1.5 Hz, 4 H), 7.2 (m, 5H), 7.19 (td, J = 1.0 Hz, 4 H), 6.89 (dd, J = 1.0 Hz, 4 H) ppm. HRMS [M + H]+ : m/z calcd. 498.10932; found 498.10523. 2.1.2. Synthesis of 3,5-di(10H-phenoxazin-10-yl)benzonitrile (DPxBn) 3,5-Dichlorobenzonitirle (0.69 g, 4 mmol) was dissolved in anhydrous toluene (20 ml) after bubbling with N2. 10H-phenoxazine (1.47 g, 8 mmol) and t-BuONa (1.54 g, 16 mmol) were added and stirred for 10 min at room temperature. Then, tris(dibenzylideneacetone)dipalladium(0) (92 mg, 0.1 mmol) and tri-tert-butylphosphonium tertafluoroborate (58 mg, 0.2 mmol) were added after stirring the solution. The mixture was stirred at 60 ℃ for 1.5 h and filtered using silica gel and Celite to remove palladium. The mixture was evaporated under reduced pressure, then recrystallized from dichloromethane-methanol. The product was isolated as yellow crystal, yield 52 %, 1H NMR (CDCl3, 400MHz) 7.8 (d, J = 2.0 Hz, 2 H), 7.73 (t, J = 2.0 Hz, 1 H), 6.72 (m, 12H), 5.99 (d, J = 7.5 Hz, 4 H) ppm. HRMS [M + H]+ : m/z calcd. 466.14773; found 466.14152. We synthesized TADF materials (DPtBn, DPxBn) with one step reaction using BuchwaldHartwig amination of 3,5-dichlorobenzonitrile with PTZ and PXZ in the presence of Pd2(dba)3/(t-Bu)3PBF4H/t-BuONa as summarized in Scheme 1. 2.2. Instruments 1H NMR spectra were recorded on a Bruker Avance 400 MHz NMR spectrometer and chemical shifts were referenced to chloroform (7.26 ppm). HR-MS spectra were recorded using AccuTOF-TLC singlet-reflection time-of-flight (TOF) mass spectrometer (JEOL Ltd., Japan). Thermogravimetric analysis (TGA) were measured by TGA 50 (Shimadazu). HOMO was measured with a cyclic voltammetry (CV) using BASi (Bioanalytical Systems, Inc.) analysis equipment (C-3 standard). The absorption spectra of solid-state film and solution were obtained by UV-vis spectrophotometer (S-4100, SINCO). The low temperature photoluminescence (PL) spectra of organic molecules dissolve in the solution were collected by a PerkinElmer PL spectrophotometer (LS 55 model) and by cross-check with a spectrofluorometer (JACSO, FP8500). Furthermore, low (at 77K) and room temperature PL spectra of solid film deposited on a quartz substrate were collected by the time-resolved PL (Tr-PL) spectroscopy equipped with a cryostat. Tr-PL decay characteristics were recorded with a time-correlated single photon counting (TCSPC) method using an LED of wavelength at 340 nm and a Quantaurus-Tau


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fluorescence lifetime measurement system (C11367-03, Hamamatsu Photonics). The prompt and delayed photoluminescence quantum yield (PLQY) were extracted by separation from the total PLQY measured in an integrating sphere at N2 atmosphere. Those are also referenced from PLQY of 4,4 ′ -bis(N-carbazolyl)biphenyl (CBP) film doped 5 wt. % 1,2,3,5tetrakis(carbazol-9-yl)-4,6-dicyanobenzene(4CzIPN). 2.3. Device fabrication and measurements Indium-tin oxide (ITO) patterned with 2 x 2 mm2 of active area was formed on the glass substrate by the photolithography process. The substrate was cleaned in acetone and isopropyl alcohol by sonication, rinsed in deionized water, and finally irradiated in a UV-ozone chamber to eliminate all of the remaining organic impurities during the previous process. Organic materials were deposited on ITO substrates by the thermal vacuum evaporation technique under a pressure of ~ 5 x 10-7 Torr without breaking the vacuum. The deposition rate of organic layers was ~ 0.5 Å/s. Lithium fluoride (LiF) and aluminum (Al) were deposited with rates of 0.1 Å/s and 3 Å/s, respectively. The current density–voltage (J–V) and luminance–voltage (L–V) data of OLEDs were measured by Keithley 2635A and Konica Minolta CS-100A, respectively. The EL spectra and the Commission Internationale de’Eclairage (CIE) coordinates were obtained using a Konica Minolta CS-2000 spectroradiometer. 2.4. Materials We purchased or synthesized the following materials. The donor moieties, phenoxazine (PXZ) and phenothiazine (PTZ), for target TADF materials, were purchased from Alfa Aesar and Aldrich Corp. respectively. N,N’-bis(naphthalen-1-yl)-N,N’-bis(phenyl)benzidine (NPB) was used for hole injection layer (HIL). 4,4',4''-tris(carbazol-9-yl)-triphenylamine (TCTA) and N,N ′ -dicarbazolyl-3,5-benzene (mCP) were utilized as a hole transport layer (HTL). DPtBn and DPxBn were synthesized and used as a TADF dopant for emitting layer (EML). We selected 3,3-di(9H-carbazol-9-yl)biphenyl (m-CBP) as a host material for EML. Besides, 1,3,5tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB), LiF and Al were used as an electron transport layer (ETL), an electron injection layer (EIL), and a cathode, respectively. Other organic materials for the OLED fabrication except of DPtBn and DPxBn were purchased from Jilin OLED Material Tech. Corp.

III. Results and discussion 3.1. The molecular geometry and their actual triplet excited state for TADF We know that S1 and T1 of the TADF dopant could have pure CT character, but most of them show mixed behavior of CT character as well as LE character. Hence, the electronic wavefunction of ICT complex can be expressed by following eqation21: ΨICT = 𝐶1|D ∗ A > LE + 𝐶2|DA ∗ > LE + 𝐶3|D + A ― > CT (3) where Cn (n = 1,2,3) is a factor for each electronic wavefunction. Eq. 3 connotes the idea that the state may have CT and LE characters where the extent of photo-physical property change for TADF molecule may differ, which is swayed by the polarity of surrounding molecules (solvatochromic) or TADF molecular itself (self-quenching) with strong CT.22-23 Notably, the lowest excited triplet state (T1) has the propensity that shows considerable LE characteristic


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rather than CT. 24 In other words, the lowest excited triplet energy, ET, would be stabilized according to Eq. 2; 𝐸T = ELUMO - EHOMO – J2. Thus, we can increase the value of J2 if the T1 has more LE character. To understand more clearly, we calculated the hole/particle distribution in molecule geometries at excited states on the basis of the natural transition orbital (NTO) theory25 and investigated the phosphorescence spectra of each donor moiety. To understand the role of 3LE state affecting RISC behavior, we prepared two different donor groups (PTZ15-17, 26-31 and PXZ21,30-44) and strong electron acceptor group (benznonitrile, BN) as shown in Figure 1a. Especially, both donor moieties have similar chemical structure, but their molecular geometry is somehow different with the respect to the planarity of the central axis. To examine their molecular geometry, we proceeded geometrical optimization in their ground-state (S0) by using DFT (density functional theory) calculation using the B3LYP39-45 hybrid functional and 6-311G(d) basis set supported by Gaussian 09W46. Meanwhile, the geometries in excited state were optimized 6-311G+(d)/B3LYP basis set and hybrid functional (20% HF) at time-dependent DFT (TD-DFT) level. As we expected, phenazine (PZ, 9,10diazzaanthracene)47 has a planar molecular conformation but PXZ and PTZ moieties becomes non planar due to the presence of substituted with O and S atoms (in the central six-membered ring). Notably, PTZ has curved bow-like conformation while PXZ has moderately planar structure (see Figure 1b). The result of DFT calculation were well consistent with those of single-crystal X-ray crystallographic data reported before15-16,27-28,30-32. The distortion of PTZ moiety (a bent conformation) may be originated from the large discrepancy in bond length of C-S bonds (1.76-1.77 Å) and C-N bonds (1.44-1.45 Å)28. Thus, we can also observe the conformational distortion at PXZ moiety but less than that from PTZ moiety. To demonstrate TADF characteristic, two or more donor units are required to have a stable triplet state (delocalized CT). Intrinsically, the electron transition from pure 3CT to 1CT is impossible since the spin-orbit coupling (SOC) matrix element (SOCME) for ⟨1CT│ℋ𝑆𝑂𝐶│3CT⟩ is zero.16 In that regard, PTZ and PXZ were introduced into the m-positions of one BN moiety 48-49 (i.e., D-A-D skeleton with meta-linking effect). From this approach, the steric hindrance between the H atoms of PTZ and PXZ (at 1,9 positions) and neighboring phenyl ring on BN could arise. This can be supported by DFT calculation of S0 geometries optimized at the functional (B3LYP, PBE0, LC-wPBE) and 6-311G(d) basis set in the gas phase (refer Table S1 to 5). The torsion angle between donor and acceptor moieties ( calculated to be 106.2o, 109.0 o, 109.9o and 96.9o, 97.4o, 109.4o for DPtBn and DPxBn, respectively. And, their HOMO and LUMOs are spatially separated on which HOMO delocalized on PTZ and PXZ moieties and LUMO located on BN unit. Given that the frontier orbitals at S0 could be understood as an orbital showing any one electron character directly related to the electronic transition50, those can have a TADF character. As an interesting result from DFT based on three functional, some of the electron density remains at PTZ in case of DPtBn while LUMO is vacant in the case of DPxBn (Figure 2). As a deep understanding of their behaviors, NTO analysis was performed to show the nature of the excited states in each TADF material. From Figure 3, the NTO pairs, for the highest occupied NTOs (HONTOs, hole) and the lowest unoccupied NTOs (LUNTOs, particle) at T1 and S1 of TADF molecules (DPtBn and DPxBn), can support the behavior that S1 has a dominant CT character, and T1 consists of the combination of LE and CT states or only CT character. As discussed, the presence of intermediate state, such as LE state, can affect the RISC behavior in each TADF molecule due to the vanished SOCME value between pure CT molecular orbitals.16 Thus, this can be the contribution of the components, such as C1|D ∗ A > LE and C3|D + A ― > CT, resulting in two different J values (i.e., J1 and J2) at each excited states (S1 and T1). Thus, the mixing of the pure CT state with LE state could increase the value of EST. Notably, the different propensity


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occurs at T1 for DPtBn and DPxBn. As expected, DPtBn has the partial LE character, localized electron density on the donor subunit, while DPxBn does not have such behavior as shown in Figure 3. For the rigorous calculation due to the limitation of B3LYP hybrid functional, we also proceeded TD-DFT calculation for excited states based on the long-range electron exchange correlation functional set (LC-wPBE) where we changed the parameter w from 0.05 to 0.40 and 6-311+G(d) basis set from the optimized S0 geometry in the gas phase. Meanwhile, the mixing LE state in T1 induces the more stabilization of excited state energy due to larger J value, corresponding to the case of J1 < J2. This leads to the deviation of EST from HOMO/LUMO spatial separation. The value of calculated EST and f were 5 meV, 0.0004 and 4 meV, 0.0001 for DPtBn and DPxBn, respectively). HOMO and LUMO energy levels for electron donor (PTZ, PXZ), acceptor (BN) moiety and TADF molecules (DPtBn, DPxBn) in the gas phase were depicted in Figure S1.

3.2. Photo-physical properties of DPtBn and DPxBn at solution state Thermal properties of the TADF molecules are described in Figure S2. The decomposition temperature (Td) measured from TGA was 314 oC for both DPtBn and DPxBn (see Figure S2a). Those showed relatively low glass transition temperature (Tg) of 82 oC and 74 oC for DPtBn and DPxBn, respectively. This stems from the increased free volume of such materials with highly twisted molecular skeleton to facilitate ICT. The melting point (Tm) for target molecules were observed in DSC 2nd scans (see Figure S2b and Table 1). Meanwhile, both DPtBn and DPxBn molecules showed weak absorption at around 350-450 nm. Those are originated from the ICT transition from donor to acceptor (refer to UV-vis absorption spectra in the Figure 4a). The HOMO-LUMO energy gaps (Eg) of target TADF molecules were determined from band-edge to be 3.03 and 2.84 eV, respectively. The PL spectra for a series of TADF molecules and their donor components, dissolved at toluene (conc. 1.0 x 10-4 M), were recorded. A small bathochromic shift is observed for DPtBn relative to DPxBn in Figure 4b. Given that the orthogonality () of respective TADF molecule, this could be consistent with the assertion27 that PXZ is a better electron-donating group rather than PTZ. The peak wavelengths (PL), for PTZ and PTX, were estimated as 445, 410 nm, respectively. The locally excited singlet state (1LE) corresponds to 2.79 and 3.02 eV at the excitation wavelength (ex) of 340 nm. Further, apparent ICT peaks (i.e., broad and structureless) of the DPtBn and DPxBn were collected after N2 purge (PL paired with 536 and 520 nm, respectively). In the Marcus framework51, the electronic transition from S1 with dominant CT character results in the broad Gaussian-shaped PL spectra, originated from the large vibrational relaxation in the ICT transition, which can be fitted by the following formula52;

(𝐸CT ― 𝜆S ― 𝐸)2 𝐸 𝐼PL(𝐸) ∝ 𝑒𝑥𝑝 ― 4𝜆S𝑘B𝑇 4𝜋𝜆S𝑘B𝑇

[

]

(4) Where the PL intensity of TADF material, IPL, is the function of the photon energy (E). ECT corresponds to the energy difference between S1 and S0. The reorganization energy, S, relevant to the surrounding window determines the emission wavelength (PL). kB is the Boltzmann constant and T is the temperature. The schematic diagram of potential energy surfaces (PES) for electronic transition is illustrated as Figure 4c. The fitted PL spectra for DPtBn and DPxBn


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using Eq. 4 well matched with the experimentally observed PL at solution state from Figure 4d. The values for S were tabulated as 0.59 eV for those cases. This large S stems from the solvation effect even at non-polar toluene (dielectric constant,  = 2.38)16. The prompt lifetime (PF), for DPtBn and DPxBn, dissolved in toluene (conc. 1.0 x 10-4 M), were 22.6 ns and 26.1 ns, respectively, which shows the similar time-scale while the delayed lifetime (DF) for target TADF molecules were 20.8 s and 2.9 s, respectively (see Figure 5a and b). In other words, the extremely different RISC behavior was observed although the similar molecular D-A-D backbone was utilized. And, this clearly supports the main mechanism stems from the delayed portion. Thus, we note the T1 and LE state to explain this behavior. It is well known that 3LE state is impervious to the presence of solvent in principle.15-17,53 3LE state contributes as the mediator to reach fast equilibrium state between 3CT and 3LE via second order spin-vibronic 2 coupling, which can be written as |⟨1CT│ℋ𝑆𝑂𝐶│3LE⟩⟨3LE│ℋ𝑣𝑖𝑏│3CT⟩| since the electronic SOCME between 1CT and 3CT is very small or zero as discussed previously.16,54 The exact mechanism of RISC has been actively debated and suggested so far. To investigate the contribution of this intermediate state, we estimated energy level of PTZ15-17,55 and PXZ56 moiety at toluene (conc. 1 x 10-4 M) paired with 2.45 eV [3LEs(PTZ)] and 2.68 eV [3LEs(PXZ)], respectively. Further, our reproduced estimation was commensurate with precedent report1517,55-60 (see Figure S3). The energy difference between 3LE and 1CT (i.e., dissolved in toluene) s s [define, ∆E3LE ― 1CT] was tabulated as 0.14 (0.30) eV for DPtBn (DPxBn). Indeed, the value of ∆E3LE ― 1CT for DPxBn is relatively large energy barrier to interact between 3CT and 3LE. However, the value of DF for DPxBn is smaller than one for DPtBn. Notably, when we collected low temperature PL (LTPL) spectra at 77K for a series of TADF materials, we cannot observe any behavior of T1 in both cases (refer to Figure 5c and d). Instead, we can assign the blue-shifted LTPL even at the delay with 74 ms. Strikingly, this estimation values were 2.43 eV in the DPtBn molecule and 2.60 eV in the DPxBn molecule, which strongly supports the actual 3LE (little bit stabilized) in each TADF molecule. In other words, the observation of 3LE in TADF materials might imply that this state can effectively work as intermediate state under the excited condition. According to Kobayashi et al.,57 the contribution of a higher triplet excited states (Tn) can be observable at low temperature and this can be explicable by an introduction of the four-level model. Thus, we can understand the behavior of Tn as an intermediate state in accordance with the temperature, which can support the electronic transition between T1 to S1.

3.3. The different propensity of RISC behavior in the solid-state. The ICT character also associated with the solvent polarity within the framework of LippertMataga equation58-59. Further, concentration quenching (CQ) occurs between TADF molecules itself22-23. Thus, this strategy can be exploited as a key to modulate CT counterparts in line with solid-state solvation (SSS)23,60-63. A selection of m-CBP, with the non-polar character as a host, can confine the TADF materials. This efficiently transfers optically pumped excitons to DPtBn or DPxBn molecule in each host-guest system. The absorption spectra of doped films (film thickness 50 nm, Figure 6a) corresponds to m-CBP absorption due to the dominant presence as a host in the solid film. The doping concentration condition was 10 wt. %. The value of PF, for DPtBn and DPxBn, diluted in m-CBP host film, were 18.2 ns and 22.5 ns, respectively. This indicates the enhancement of the value of kPF, but, interestingly, the value of DF showed different trend at solution and solid-state (15.6 and 5.3 s paired with DPtBn and DPxBn, respectively) in Figure 6b. Using m-CBP host induces hypsochromic shift of 1CTf at each


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TADF compared to those dissolved in toluene [2.31 eV (1CTs) → 2.44 eV (1CTf) for DPtBn and 2.38 (1CTs) → 2.45 eV (1CTf) for DPxBn, refer to Table 2 and Figure 7). The subscript s and f are denoted as solution and solid-state. At a conception of TADF molecule design, NTO analysis supported our assertion that the mismatching of J (J1 ≠J2) could induce a large EST as we estimated. Further, the theoretical approach that meta-position with respect to the closest cyano substituent is more prone to secure a larger LE character rather than ortho- position.54,64 From the Figure 6c–d, the values of EST for DPtBn and DPxBn are 0.14 eV and 0.05 eV, respectively. To investigate the actual LE behavior, we also collected LTPL spectra of m-CBP:PTZ (30 nm, 5 wt. %) and m-CBP:PXZ (30nm, 5 wt.%) as shown in Figure S4. Strikingly, the estimated 3LE of PXZ and PTZ in solid-state are different from those of solution, wherein 3LE < 3LE f f s relevant to the electronic stabilization [2.45 eV (3LEs) → 2.30 eV (3LEf) for DPtBn and 2.68 (3LEs) → 2.34 eV (3LEf) for DPxBn]. Then, we can summarize the energy diagram of BN derivatives relevant to the 3LE as depicted in Figure 7. In the case of DPtBn, 3LEf (2.30 eV) is enough to interact with 3CTf (2.30 eV) in the solid film (i.e., energetically degenerated). That is the reason why the value of kDF could be enhanced (4.8 x 104 to 6.4 x 104 s-1) even at relatively large EST ( eV) in the solid-state. Notably, the value of kRISC for DPtBn in the solid film was 3.3 x 105 s-1 where the rate constants are estimated on the basis of the three-level model2223,57 while DPxBn with strong ICT gives rise to nearly degenerated E ST (0.05 eV) where this photo-physical result could match with the similar nature of the S1 and T1 at those geometries. The common TADF material, 4CzIPN, with moderate SOCME (0.27 cm-1), for which the emission process based complex transitions. In the case of 4CzIPN, EST could be nearly reduced degenerated upon the solvent polarity (140 ± 19 at toluene to 92 ± 20 meV at acetonitrile)65. 3LEf state, typically, are more stable than its CT state counterparts with regard to energy61. From the solution case, we observed reversed trend in both cases (3LEs > 1CTs and 3CT ), whereas the condensed phase, such as solid-state, showed the general case in TADF s molecules (3LEf ≤ 3CTf

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