Blue organic light-emitting diodes with hybridized local and charge-transfer excited state realizing high external quantum efficiency

Donor–spacer–acceptor (D–π–A) materials CAPI and CCAPI, with hybridized local and charge transfer (HLCT) emissive states, have been synthesized. The twisting D–π–A architecture promotes the partial separation of HOMO and LUMO, leading to an enhanced % CT component, and the anthracene moiety in CAPI and CCAPI increases the conjugation length, leading to an enhanced % LE component. The non-doped device with CCAPIb shows the blue emission (450 nm) with maximum current efficiency (ηc), power efficiency (ηp), and external quantum efficiency (ηex) of 16.83 cd A−1, 15.32 lm W−1, and 12.0%, respectively, as well as exciton utilization efficiency (EUE) of 95% with a luminance of 32 546 cd m−2 and a roll-off efficiency of 0.53%. The new design strategy has great potential for developing high-performance blue electroluminescent materials.


Thermal and electrochemical studies
The twisted molecular architecture of CAPI and CCAPI, having a twist angle of $52 between the styryl substituent (C2) and the phenanthrimidazole core, effectively suppressed the conjugation and intermolecular p-p stacking (Fig. 1). The twisted molecular architecture increased the thermal stability of CAPI (T g /T d : 218/500 C) and CCAPI (T g /T d : 228/528 C) (Fig. 2). Because of the C/N-side coupling with bulky substituents, the rigid phenanthroimidazole showed high glass transition temperature and interaction of substituents at C2 with N1 of the phenanthroimidazole core induced more condensed molecular packing. The higher T g and T d are essential for applications in devices. The morphological stability of these compounds was examined by atomic force microscopy at 30 C and 90 C for 12 h (Fig. 2). The root-mean-square [CAPI -0.28 nm; CCAPI -0. 19 nm] analysis showed the absence of remarkable surface modi-cation before and aer annealing, which further supports the suitability of these materials for fabrication. CAPI and CCAPI showed a redox process with an oxidation onset potential (E onset ) of 0.63 V (CAPI) and 0.61 V (CCAPI), which revealed that these bipolar carrier transporting materials are electrochemically stable (Fig. 2). The HOMO ¼ À(E ox vs. Ag/Ag + À E 1/2 + vs. Ag/ Ag + + 4.8) eV/LUMO ¼ À(E red vs. Ag/Ag + À E 1/2 À vs. Ag/Ag + + 4.8) eV have been calculated as CAPI (À5.20/À2.58 eV) and CCAPI (À5.18/À2.54 eV).

Photophysical studies
The absorption of emissive materials has been studied with various solvents (Fig. S1, Tables S1 and S2 †) and in the lm state ( Fig. 3 and Table 1). The absorption in the region of 320-332 nm is attributed to the p-p* (LE) transition of phenanthroimidazole and carbazole moieties, 41,42 whereas the absorption at around 370 nm is ascribed to the p-p* transition of the anthracene moiety. 43 44,45 The small shi in the lm state shows the existence of weak p-p* intermolecular stacking. 48 The UV absorption spectra of CAPI and CCAPI changed shape and position with increasing solvent polarity due to small dipole  Table 1). The solvatochromic emission spectra of CAPI and CCAPI from higher polarity solvents were remarkably broad and red-shied from n-hexane to acetonitrile due to twisting by 9,10substituted anthracene, and were further supported by natural transition orbitals ( [(3 À 1/2 3 + 1) À 1/2 (n 2 À 1/2n 2 + 1)]; [m g (ground state dipole moment), m e (excited state dipole moment), f (orientation polarizability),3 abs (absorption maximum),3 vac abs (absorption maximum extrapolated to gas phase),3 u (uorescence maximum),3 vac u (uorescence maximum extrapolated to gasphase), a o (Onsager cavity), 3 (solvent dielectric constant) and n (solvent refractive index)] (Fig. 3). The emission spectra gradually broadened and showed less structure with larger red-shis, which supports that the excited state has a strong CT component. The red-shied emission is due to the twisted conformation of these emitters, which leads to easier charge transfer from donor to acceptor via anthracene linker.
The two-section linear relation observed from the Lippert-Mataga plot (Fig. 3b) revealed that a line with a small slope in solvents with f # 0.1 is due to the LE-like excited state component with a lower dipole moment (CAPI -7.32 D and CCAPI -  51 The higher dipole moment corresponding to the CT-region is greater than that of the CT molecule 4-(N,N-dimethylamino) benzonitrile (DMABN, 23 D), whereas the lower dipole moment in the LE-region is a little higher than those of conventional LE molecules such as anthracene and PI, indicating that CAPI and CCAPI are HLCT materials.
The solvatochromic data of these compounds were tted with two straight lines corresponding to two different dipole moments because of the coexistence and hybridization of the LE and CT excited state components. 50 The transformation of the slope between ether (f ¼ 0. 10 3 -372 (f À 1.8932)/450 (f À 2.4531)] implies that higher luminescence will be obtained from the intercrossed excited state. These materials demonstrate one nanosecond lifetime without any delayed components (Fig. 3) and the energy gap of the singlet and triplet energy levels of CAPI and CCAPI is estimated to be less than 0.1 eV, which is also a feature of typical HLCT materials.
In low-polarity solvents, the PLQYs of CAPI/CCAPI remain unchanged (0.60/0.66 À n-hexane, 0.65/0.68isopropyl ether), which implies that the LE dominated emission from the low lying S 1 state. In a medium-polarity solvent (THF), a high PLQY of 0.80/0.88 for CAPI/CCAPI was obtained due to the hybridization of LE with the CT excited state. In a high-polarity solvent      Table 2). The CAPI/ CCAPI oscillator strength (0.6089/0.6636) in the S 1 excited state is consistent with PLQY in low polarity solutions. The excited state (S 1 ) dipole moments of CAPI and CCAPI of 7.32 D and 5.00 D are also in accordance with the experimental results. Therefore, the emissive state of CAPI/CCAPI belongs to the S 1 excited state and serves as exciton utilization channel. The S 0 excited state exhibited obvious CT character according to NTO analysis. The NTO for S 0 / S 3 (CAPI) and S 0 / S 4 (CCAPI) transition displays a total CT transition character with hole distribution on the carbazole moiety and particle distribution on anthracene with minor overlap on the adjacent moiety, and T 13 and T 4 are the corresponding triplet CT excited states for CAPI (E S 3 -T 13 -0.00 eV) and CCAPI (E S 4 -T 8 -0.00 eV), respectively. Therefore, this could increase the RISC between S 3 and T 13 (CAPI) and S 3 and T 4 (CCAPI) excited states and enhance the exciton utilisation efficiency [% EUE: CAPI/CCAPI -64/83: b 70/95].

Theoretical calculation
To gain more insight into the analysis of the HLCT emissive state, molecular optimization and frontier molecular orbital (FMO) analysis for CAPI and CCAPI were carried out (DFT/ B3LYP/6-31G (d, p)) ( Fig. 1). The blue emitters CAPI and CCAPI consist of the 9,10-substituted anthracene moiety involving the twisted D-p-A molecular conguration with torsional angles of 52 and 51 for N-side coupling, and 66 and 63 for C-side coupling, respectively. These carbazole derivatives show higher C-coupling torsional angles but smaller Ncoupling angles as compared with triphenylamine derivatives TPAAnPI (torsional angles of 53.7 -C-side) and 69.0 -N-side). 51 Therefore, the calculated FMO exhibit partially separated characteristics. The highest occupied molecular orbital (HOMO) of CAPI is mainly localized on the phenanthrimidazole core and the LUMO is localized on the phenyl, anthracene and partially on phenanthrimidazole. In CCAPI, the HOMO is distributed on anthracene and the LUMO is fully distributed on the anthracene moiety, suggesting that the HOMO / LUMO transition involves an intercrossed CT and p-p* transition character, reecting HLCT character. Furthermore, NTO analyses were performed for the singlet and triplet excited states based on the S 0 state geometry using the time-dependent DFT (TD-DFT) method at the same level as S 0 . The S 0 / S 1 and S 0 / S 2 transitions of CAPI and CCAPI are the radiative p-p* and non-radiative n-p* transitions of anthracene, respectively, which efficiently increased the RISC between the S 3 and T 13 excited states (CAPI) and the S 4 and T 8 (CCAPI) excited states, and enhanced the exciton utilization efficiency.
The HONTOs and LUNTOs of CAPI and CCAPI (Fig. S2, S3, Tables S3 and S4 †) exhibited hybrid splitting state character from the interstate coupling of LE with CT levels. The interstate hybridization coupling of LE with the CT state wave function is given by J S 1 /S 2 ¼ c LE J LE AE c CT J CT . The % CT of these emitters increased with steric hindrance with increasing aromatic substituent size and the increase in % LE in the S 1 state resulted in higher photoluminescence efficiency (h PL ). The single emissive state of CAPI and CCAPI has been investigated by the excitation energies of the LE and CT states. A large energy gap (DE ST ) between T 13 and T 1 of CAPI (1.89 eV) and T 8 and T 1 of CCAPI (1.08 eV) arose from phenanthroimidazole acceptor group. 50,51 A very small DE ST between S 3 and T 13 (0.00 eV -CAPI) and S 4 and T 8 (0.00 eV -CCAPI) states facilitated the RISC (T n / S 1 ) process due to HLCT character (the S 3 -T 12 energy gap of the triphenylamine derivative TPAAnPPI is only 0.0013 eV). 51 CAPI and CCAPI show high photoluminescence efficiency (h PL ), high exciton utilisation efficiency (EUE) and high external quantum efficiency (EQE) because of the increased LE component in the S 1 state. The small DE ST of these materials arises from spatially separated HOMO and LUMO. 46 Similar hole-electron wave functions between the singlet excited states in CAPI and CCAPI indicate the non-equivalent hybridization of the initial LE with the CT state (CAPI and CCAPI). The degree of hybridization in CAPI and CCAPI depends on both the initial DE LE,CT and the coupling strength. 54 In the transition density matrix (TDM) plot (Fig. S4 -CAPI and Fig. S5 -CCAPI †), the diagonal parts represent the LE component and the off-diagonal zone represents the CT component. Depending on electronic coupling, electrons are transferred from donor to acceptor on excitation, which was studied via the electron density distribution in both the ground and excited states. The computed distance between holes and electrons, H as well as t indexes and the RMSD of electrons and holes of these emitters are shown in Tables S5-S12. † The formation of HLCT is supported by the Dr index (average of the hole-electron distance (d h + -e À ): r < 2.0Å LE; Dr > 2.0Å CT) and indicates the nature of excitation (LE or CT) (Tables S13 and S14 †). The LE (valence excitation) is associated with short distances (d h + -e À), whereas CT excitation is related to larger distances (d h + -e À ). The dark triplet exciton is harvested through the RISC process by a hot CT mechanism in the electroluminescence process without delayed emission and leads to high exciton utilization (h s ) in CAPI and CCAPI like phosphorescent materials. 44,45 The increasing % LE component and hybridization of LE with CT components result in high h PL , and high h s leads to enhanced device performances ( Table 3). The computed overlap of the condensed function (r + and r À ) in CAPI and CCAPI is 0.99 ( Fig. 4 and Table S15 †), and the H/t index for CAPI and CCAPI are 5.91/5.61 and 5.91/5.85Å, respectively. The CT index (D CT À H index) is another measure of the hole-electron separation (eqn S15 and S16 †), and the calculated D CT /m CT of CAPI (0.33/37.28) and CCAPI (0.13/15.06) further conrmed the HLCT formation. A non-zero t index implies the severe overlap of holes with electrons and the Eigenvalue (>0.98) conrmed the hybridization with predominant excitation pairs (94% of transition). Fig. S6 † shows the potential energy surface (PES) scan of CAPI and CCAPI in the gas phase and different polarity solvents. In the gas phase, the S 3 (CAPI) and S 4 (CCAPI) states are not mixed with the S 1 state due to a large DE S 3 -S 1 (CAPI) and DE S 4 -S 1 (CCAPI). In low polarity solvents, the S 3 (CAPI) and S 4 (CCAPI) states crossed the S1 state, whereas in high polarity solvents, E S 3 (CAPI) and E S 4 (CCAPI) decreased sharply and became the lowest excited state. The energetic closeness in moderate-polarity solvents leads to the enhanced mixing of S 3 (CAPI) and S 4 (CCAPI) with S 1 (the larger dipole moments of the S 3 (CAPI) and S 4 (CCAPI) states lead to stabilization in high polarity solvents). Therefore, the S 1 state is dominated by LE character in low polarity medium; the S 1 state is dominated by mixing the LE and CT character in moderate polarity medium and the S 1 state is dominated by CT character in high polarity medium.

Electroluminescent properties
To understand the carrier transport abilities of CAPI and CCAPI  52 The current densities versus voltage characteristics of hole-only and electron-only devices revealed that these compounds are bipolar materials (Fig. 5). The higher electron current density of CAPI and CCAPI based devices relative to the CBP device revealed that these bipolar materials transport electrons as well as holes effectively than CBP. [55][56][57] Table 2). The external quantum efficiency [h EQE ¼ h out Â h rc Â h g Â F PL , 53 F PL -quantum yield of lm, h outout-coupling efficiency (20%), h rcproduct of charge recombination efficiency (100%), h gradiative exciton-production (25%)] and EUE can be estimated [h s ¼ 5 Â h ex /F PL Â 100]: maximum exciton utilizing the efficiency of the devices based on CAPI and CCAPI have been calculated as 64 and 83% (Fig. 8 -CAPI and S 8 -CCAPI †), respectively and exceed the 25% theoretical limit of spin statistics for conventional uorescent OLEDs. The efficiency roll-off (h roll-off ) is 0.92% only at a luminance of 1000 cd m 2 ; however, the EQE is still not satisfactory for display applications. Therefore, the non-doped device with a conguration of b ITO/PEDOT:PSS (40 nm)/TCTA (30 nm)/CAPI or CCAPI (20 nm)/TPBi(30 nm)/LiF (1 nm)/Al (100 nm) has been fabricated to enhance the efficiencies (Fig. S7 †). The non-doped device with CAPI/CCAPI showed blue emission (467/450 nm) maximum current efficiency, power efficiency and EQE of 14.06/16.83; 14.81/15.32; 8.89/12.0, respectively, with a luminance of 28 801/32 546 cd m À2 and the efficiency roll-off (h roll-off ) was 0.53% only at a luminance of 1000 cd m À2 . The high EQE and low roll-off efficiency further emphasized the great potential of new CAPI and CCAPI materials for industrial applications. The EL spectra are stable with a driving voltage range of 3 V to 10 V (Fig. 7). The transient EL decay curves of devices with the conguration of b CAPI/ b CCAPI at different voltages correspond to two components: rapid EL decay originating from the uorescence of S 1 and delayed decay. The ratio of the delayed component decreased from 3 V to 10 V because of the quenching of dark triplet excitons and the amplied delayed component obtained from the plot of log EL intensity and log time [slope: À0.21 (initial time); À1.46 (log time)] (Fig. 3); 58 however, the slope should be À2 instead of À1.46 for the TTA mechanism. Therefore, the TTA conguration is insignicant in the EL process. The TPAANPPI exhibited h c , h p and EQE of 12.51 cd A À1 , 11.47 lm W À1 and 8.00%, respectively. 51 The small energy splitting (DE ST z 0) between S 3 and T 13 (0.00 eV -CAPI) (Fig. 9) and S 4 and T 8 (0.00 eV -CCAPI) (Fig. S9 †) promotes potential RISC processes and the dark triplet excitons are converted into singlet excitons in the EL process [ a CAPI/ b CAPI -39/45%; a CCAPI/ b CCAPI -58/70%]. According to the energy-gap law, 59 the larger energy-gap between T 13 and T 1 of CAPI (1.89 eV) and T 8 and T 1 of CCAPI (1.08 eV) inhibits the internal conversion (IC) process and the more competitive RISC process between T 13 and S 3 of CAPI (0.00 eV) and T 8 and S 4 of CCAPI (0.00 eV) is predominant because of a narrower energy gap. Therefore, a large fraction of electrogenerated dark triplet excitons were transferred into singlet excitons via T 13 / S 3 (CAPI) and T 8 / S 4 (CCAPI) channels. Moreover, the larger energy gap between S 1 and T 1 (DE S 1 T 1 ¼ 1.62 eV -CAPI and 0.51 eV -CCAPI) prevents the intersystem  crossing process from low-lying S 1 to T 1 and excludes the existence of the TADF mechanism. To the best of our knowledge, the efficiencies of CAPI and CCAPI are comparable with those of blue-emitting devices reported recently (Table 4). 1-4,60-75 Therefore, the HLCT mechanism is likely responsible for the radiative exciton ratio above 25% in the non-doped OLED based on CAPI and CCAPI.

Conclusion
We have synthesized novel D-p-A materials with HLCT character enhancing the exciton utilization efficiency. The small energy splitting (DE ST z 0) between S 3 and T 13 (0.00 eV -CAPI) and S 4 and T 8 (0.00 eV -CCAPI) promotes a potential RISC process and the dark triplet excitons are effectively converted to singlet excitons in the EL process. The non-doped device with CCAPI shows blue emission (450 nm) with maximum current efficiency (h c ), power efficiency (h p ), external quantum efficiency (h ex ) of 16.83 cd A À1 , 15.32 lm W À1 , 12.0%, respectively, and exciton utilization efficiency (EUE) of 95% with a luminance of 32 546 cd m À2 and a small roll-off efficiency of 0.53%. The amplied delayed component with obtained slope ruled out the contribution of the TTA mechanism and conrmed that the HLCT mechanism is responsible for the radiative exciton ratio above 25% in the non-doped OLED based on CAPI and CCAPI. The high EQE and small roll-off efficiency of devices emphasize the great potential of CAPI and CCAPI for industrial applications.

Conflicts of interest
There are no conicts to declare.