Hot exciton transition for organic light-emitting diodes: tailoring excited-state properties and electroluminescence performances of donor–spacer–acceptor molecules

The photophysical, electrochemical and electroluminescent properties of newly synthesized blue emitters with donor–π–acceptor geometry, namely, 4′-(1-(naphthalen-1-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenyl-(2-[1,1′-biphenyl]vinyl)-4-amine (NSPI-TPA), 4′-(1-(2-methylnaphthalen-1-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenyl-(2-[1,1′-biphenyl]vinyl)-4-amine (MNSPI-TPA), 4-(2-(4′-(diphenylamino)-(2-[1,1′-biphenyl]vinyl)-4-yl)-1H-phenanthro[9,10-d]imidazol-1-yl)-1-naphthalene-1-carbonitrile (SPNCN-TPA) and 4-(2-(4-(9H-carbazol-9-yl)styryl)-1H-phenanthro[9,10-d]imidazol-1-yl)naphthalene-1-carbonitrile (SPNCN-Cz) were analyzed. The conjugation length in the emitters is not conducive to pure emission and hence, a molecular twisting strategy was adopted in NSPI-TPA, MNSPI-TPA, SPNCN-TPA and SPNCN-Cz to enhance pure emission. The emissive state (HLCT) of twisted D–π–A molecules containing both LE and CT (HLCT) states was tuned for high PL (ηPL) (LE) and high exciton utilization (ηs) (CT) efficiencies by replacing triphenylamine (strong donor) with carbazole (weak donor). Among strong donor compounds, namely, NSPI-TPA, MNSPI-TPA and SPNCN-TPA, the SPNCN-TPA-based device exhibited blue emission (451 nm) with CIE coordinates (0.15, 0.08), maximum current efficiency (ηc) of 2.32 cd A−1, power efficiency (ηp) of 2.01 lm W−1 and external quantum efficiency (ηex) of 3.02%. The device with SPNCN-Cz emitter exhibited higher electroluminescence efficiencies than the SPNCN-TPA-based device, with pure blue emission (443 nm, CIE: 0.15,0.07), ηex of 3.15%, ηc of 2.56 cd A−1 and ηp of 2.45 lm W−1.


Introduction
Organic light-emitting diodes (OLEDs) have been widely investigated and tested commercially in recent decades and utilized in at-panel displays. 1 However, their commercialization is still restricted because of the scarcity of blue emitters. [2][3][4] Efficient green and red emissive materials are highly exploited. However, fabrication of blue OLEDs is a major problem due to wide band gap and unbalanced carrier injection. [5][6][7][8][9][10] Therefore, the design of efficient pure blue emitters with narrow full width at half maximum (FWHM) is still an important task. 11,12 The aggregated arrangement of the emitter-induced molecular interaction results in bathochromic shi with low quantum efficiency. 13 However, non-doped blue emitters with restricted intermolecular interaction exhibit expected quantum yields. 14 Their potential carrier injection and transporting abilities provide balanced charge recombination, which results in enhanced efficiency. [15][16][17] For TV displays, blue OLEDs with CIE (0.14, 0.08) and (0. 15, 0.06) are required by the National Television System Committee (NTSC) and high-denition television (HDTV), respectively. Pyrene 18 and anthracene 19 derivative-based blue OLEDs exhibit high efficiency with poor color purity. Therefore, to achieve pure blue emission, emitters having D-p-A geometry with twisted conguration are employed as they reduce the p-conjugation and consequently exhibit blue emission. 20-26 Furthermore, the emissive state of twisted D-p-A molecules [27][28][29][30] possesses both LE and CT states (hybridized local and charge transfer state/HLCT) and shows high PL efficiency (LE) and high exciton utilization (h s ) (CT), which are attributed to hot exciton mechanism. [31][32][33] The LEdominated (low lying) HLCT state provided high radiative rate (k r ), resulting in high photoluminescence efficiency (h PL ) of the lm, whereas CT-dominated HLCT state is responsible for high h S through RISC via the hot exciton principle. 34 The larger energy gap between T 2 and T 1 states greatly reduces the internal conversion (IC) ðT 2 ! kIC T 1 Þ, resulting in hot RISC ðT 2 ! kRISC S 1 =S 2 Þ rather than cold RISC (T 1 / S 1 ) TADF mechanism. 35,36 Hot exciton process with HLCT increases external quantum efficiency (h ex ) because of high h PL and high h S . h ex can be calculated as follows: where h IQE is the internal quantum efficiency, h out is the light out coupling efficiency (20%), h rec is the efficiency for electron-hole recombination (100%), h PL is the photoluminescence efficiency of the lm and h S is the exciton utilization efficiency [h S ¼ h rec Â h PL Â h out O h EL ]. 37 The ambipolar phenanthrimidazole derivatives have been shown as potential blue emitters. The increase in conjugation in phenanthrimidazoles inuenced the blue emission, [38][39][40][41][42] and conjugation length was restricted by incorporating a bulky fragment in the core molecule, which resulted in twisted conformation. [43][44][45][46] In line with this discussion and our own research interest, we report donor-spacer-acceptor derivatives, namely, 4 0 -(1-(naphthalen-1-yl)-1H-phenanthro [9,10- [9,10-d]imidazol-1-yl)-1-naphthalene-1-carbonitrile (SPNCN-TPA) and 4-(2-(4-(9H-carbazol-9-yl)styryl)-1H-phenanthro [9,10-d] imidazol-1-yl)naphthalene-1-carbonitrile (SPNCN-Cz) using triphenylamine as a strong donor and carbazole as weak donor (Scheme 1). The H-H repulsion of bulky styryl fragment with the phenyl moiety of TPA and the carbazole moieties leads to twisted conguration, which enhances the twist angle, thus shortening the conjugation length. The solvatochromic effect of NPSI-TPA, MNSPI-TPA, SPNCN-TPA and SPNCN-Cz was examined to understand the excited state characteristics and interstate coupling strength of LE and CT components. Combining theoretical (TD-DFT) and experimental data, the LE and CT compositions were discussed using natural transition orbital (NTO), centroids of charges and transition density matrix (TDM) analysis. Hybridization of LE and CT energy states was used for molecular design, and their composition in HLCT was tuned, resulting in high EL efficiency. Triphenylamine (TPA) of SPNCN-TPA was replaced by carbazole (Cz) in SPNCN-Cz, both of which differ in electron donating ability. As a result, the CT composition decreases with an increase in LE composition in the S 1 HLCT state. Thus, the SPNCN-Cz-based device exhibited maximum electroluminescent efficiency with h ex of 3.15%, h c of 2.56 cd A À1 and h p of 2.45 lm W À1 and CIE of (0.15, 0.07), which were higher than those of the SPNCN-TPA-based device. Higher h PL of SPNCN-Cz lm was observed compared with that of SPNCN-TPA lm; thus, high h PL and high h S enhanced the h ex of the SPNCN-Cz-based device. These results can be used to design low cost uorescent materials via subtle molecular modications using the HLCT emissive state principle.

Measurements and general methods
Reagents and solvents were purchased from commercial sources. 1 H and 13 C NMR spectra were recorded on a Bruker (400 MHz) spectrometer and an Agilent instrument (LCMS VL SD) was employed to record the mass spectra. UV-vis absorption was measured using a Perkin-Elmer Lambda 35 (solution) and Lambda 35 spectrophotometer with an integrated sphere (RSA-PE-20) (lm). Photoluminescence (PL) spectra were recorded on a PerkinElmer LS55 uorescence spectrometer. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed using a PerkinElmer thermal analysis system and NETZSCH-DSC-204, respectively. Decay analysis was conducted using a nanosecond time correlated single photon counting (TCSPC) spectrometer (Horiba Fluorocube-01-NL lifetime system) with nanoLED excitation source and TBX-PS detector; DAS6 soware was used to t the decay curve and c 2 values lie between 0.8 and 1.2. The absolute photoluminescence quantum yield (PLQY) was determined using a uorescence spectrometer (Model-F7100). Cyclic voltammetry was performed using Potentiostat CHI 630A electrochemical analyzer (platinum electrode and platinum wire as the working electrode and counter electrode, respectively; Ag/Ag + as the reference electrode, with 100 mV s À1 scan). Ferrocene was used as the internal standard, with the highest occupied molecular orbital energy of À4.80 eV, and 0.1 M tetrabutylammoniumperchlorate in CH 2 Cl 2 was used as the supporting electrolyte. The HOMO energies (E HOMO ) were calculated using oxidation potential [E HOMO ¼ À(E ox + 4.8 eV)] and the LUMO energies (E LUMO ) were calculated by subtracting the optical band gap from the HOMO energy [E LUMO ¼ E HOMO À 1239/l onset ]. For theoretical calculations, ground state (DFT)/excited state (TD-DFT) geometrical properties were optimized by employing Gaussian 09 program. 47 Multifunctional wavefunction analyzer (Multiwfn) 29 was used to determine the nature of electronic transitions of excited states and natural transition orbitals (NTOs).

Results and discussion
Since the LUMO energies (E LUMO ) of À2.04 eV (NSPI-TPA), À2.06 eV (MNSPI-TPA), À2.02 eV (SPNCN-TPA) and 2.15 eV (SPNCN-Cz) are close to that of 1,3,5-tris(N-phenylimidazol-2-yl) benzene (À2.40 eV), these materials also possess electroninjection abilities. 51 Furthermore, the frontier molecular orbital (HOMO-LUMO) analysis conrms the carrier injection abilities and hence, these materials can be employed as potential emitters in OLEDs. 52,53 The calculated high uorescence quantum yields, which are 0.58/0.46 (NPS-TPA), 0.69/0.56 (MNPS-TPA), 0.48/0.42 (SPNCN-TPA) and 0.52/0.49 (SPNCN-Cz), in both solution and lm support the co-emission phenomenon from LE and CT of emissive materials. The radiative (k r ) and non-radiative (k nr ) transition rates of SPNCN-TPA and SPNCN-  Cz were calculated from lifetime (s) and quantum yield (f) data. Compared with SPNCN-TPA, the increase in radiative rate (k r ) and decrease in non-radiative rate (k nr ) of SPNCN-Cz are also in agreement with the aim of our molecular design.  (Fig. 3). The intramolecular charge transfer (ICT) from donor triphenylamine/carbazole to acceptor (naphthonitrilephenanthrimidazole) is likely to be the cause for strong absorption, and the absorption at around 248 nm is attributed to p-p* transition. 54 The intramolecular charge transfer (ICT) was also conrmed by the MEP diagram (Fig. 4). Compared with solution of the materials, negligible absorption shis in their corresponding lms reveal that suppressed p-p* stacking exists in the solid state. 55 The observed larger red shi further supports the charge-transfer (CT) in the twisted geometry of NSPI-TPA, MNSPI-TPA, SPNCN-TPA and SPNCN-Cz. Compared with SPNCN-TPA, SPNCN-Cz exhibits higher blue shi for both absorption and emission, which is attributed to the poor electron donor ability of Cz relative to TPA. The increase in LE composition with decrease in CT in the S 1 -emissive HLCT state is likely to be the reason for this blue shi. The full width at half maximum in the absorption spectrum of SPNCN-Cz (40 nm) is narrowed relative to that of SPNCN-TPA (58 nm, Fig. 3). This observation indicated that the decrease in CT component of SPNCN-Cz in the S 1 state is in good agreement with the NTO description for S 0 / S 1 transition. However, the absorption peaks of both SPNCN-TPA and SPNCN-Cz are narrower than those of the cyano-free parent compounds, indicating more LE character. The emission peaks of SPNCN-TPA and SPNCN-Cz show blue shi relative to those of their parent compounds, which is in contrast to the general observation, i.e., extension of p-conjugation leads to a red-shied emission. 56 The enhanced LE component is equivalent to the suppressed CT component in the emissive states of SPNCN-TPA and SPNCN-Cz, resulting in blue shi. The increase in LE composition is expected to result in a red-shied PL spectrum, whereas the suppressed CT results in a blue-shied PL spectrum. From the experimental observation, it is known that the latter factor is more dominant than the former. In addition, there is an overlap between UV and PL spectra of both SPNCN-TPA and SPNCN-Cz because of the enhanced LE character in SPNCN-TPA and SPNCN-Cz than that in their respective parent compounds. SPNCN-Cz exhibits solvatochromic red shi (45 nm), which is smaller than that of SPNCN-TPA (75 nm) (Fig. S10, Table S1 and S2 †). Similarly, small absorption shis of 22 nm and 32 nm were observed for SPNCN-Cz and SPNCN-TPA, respectively (Fig. S11, Tables S1 and S2 †). These solvatochromic shis conrmed that the low-lying S 1 -excited states of SPNCN-Cz and SPNCN-TPA must possess CT character. [57][58][59] The % CT character of the S 1 state of SPNCN-Cz is lower than that of the S 1 state of SPNCN-TPA, whereas the This journal is © The Royal Society of Chemistry 2018 % LE character of the S 1 state of SPNCN-Cz is higher than that of the S 1 state of SPNCN-TPA (Table 2). In hexane, both SPNCN-Cz and SPNCN-TPA show LE-like character because of vibronic PL spectrum. Compared with the broad and smooth PL spectrum of cyano-free compounds, there is higher % LE in the S 1 state of both SPNCN-Cz and SPNCN-TPA. The full width at half maximum (FWHM) of SPNCN-Cz (40 nm) and SPNCN-TPA (58 nm) indicates that these compounds possess higher PL efficiency due to higher % LE in the S 1 emissive state. The solvatochromic effect using Lippert-Mataga plot is displayed in Fig. 3 (Tables S1 and S2 †). When the solvent polarity increased, the blue emitters exhibited larger red shi, which supports charge transfer (CT) in these molecules. 57 From the plot of Stokes shi against solvent polarity function, ground state dipole moment (m g ) can be calculated: where m g and m e are the ground state and excited state dipole moments,ỹ abs andỹ nac abs are the solvent-equilibrated absorption maximum and that extrapolated to gas phase, respectively,ỹ u andỹ nac u are the solvent equilibrated uorescence maximum and that extrapolated to gas-phase, respectively, a o is the Ons-    Table 4) is higher than that of SPNCN-TPA (0.1261, Table 4), which results in the higher PL efficiency (h PL ) of SPNCN-Cz. Molecular modication from TPA to Cz caused an increase in % LE in the S 1 emissive state and enhanced the h PL of SPNCN-Cz. In order to supplement experimental results, theoretical calculations (natural transition orbital analysis) were performed to describe the excited state characteristics of SPNCN-TPA and SPNCN-Cz materials (Fig. 4). In SPNCN-TPA and SPNCN-Cz, holes are located over the horizontal backbone, whereas particles are located on vertical naphthonitrile in the S 1 state. CT transition is maintained from the horizontal backbone to vertical naphthonitrile in the S 1 states of SPNCN-TPA and SPNCN-Cz. The overlap density between a hole and a particle is expanded due to spacer styryl moiety, indicating an enhanced % LE in the S 1 state ( Fig. S12 and S13 †). The NTOs of S 1 and S 2 excited states of SPNCN-TPA and SPNCN-Cz exhibit a hybrid splitting state character that derives from the interstate coupling of LE and CT levels (Tables 3, 4, Fig. S14 and S15 †), i.e., formation of HLCT state. The wave function symmetry of a particle on naphthonitrile is opposite between S 1 and S 2 states, indicating interstate hybridization coupling J S 1 /S 2 ¼ c LE J LE AE c CT J CT . Similar holeelectron wave functions between S 1 and S 2 are observed in both SPNCN-TPA and SPNCN-Cz, indicating a quasi-equivalent hybridization between LE and CT states as a result of the almost isoenergies of initial LE and CT states (Fig. 5). Therefore, the degree of hybridization between LE and CT states is dependent not only on the initial E LE -E CT energy gap but also on their interstate coupling strength. 60 Compared with nonequivalent hybridization, quasi-equivalent hybridization is expected to achieve the combination of high h PL and high h s to maximize the EL efficiency of uorescent OLED materials due to the more balanced LE and CT components in HLCT states of SPNCN-TPA and SPNCN-Cz. The formation of the HLCT state  Paper can be analysed through the excitation energies of LE and CT states (Tables 3 and 4). In SPNCN-Cz and SPNCN-TPA, the LE state is stabilized as compared with the CT state and energy gap (E S 2 -E S 1 ) is small when compared with their cyano-free parent compounds, resulting in quasi-hybridization. In the case of SPNCN-Cz, the energy gap (E S 2 -E S 1 ) is reduced, resulting in effective hybridization and improved OLED efficiency. The overlap between the hole and the particle is also displayed in Fig. S12 † (SPNCN-TPA) and Fig. S13 † (SPNCN-Cz). Composition of HLCT can be determined from the wave function of electronhole pair transition density matrix (TDM) plotted in a twodimensional color-lled map (Fig. S16 and S17 †). The axes represent the atom in a molecule, which is proportional to the probability of nding an electron and a hole in an atomic orbital located on a non-hydrogen atom. The diagonal represents the LE component localized on the main backbone, while the off-diagonal region shows the CT component. The qualitatively calculated percentages of LE and CT in S 1 -S 10 and T 1 -T 10 states are displayed in Table 2. This nding also supports that HLCT state contributes to hybridization, in addition to the LE and CT states. Upon excitation, an electron is transferred from a donor and localized on an acceptor. Depending upon intramolecular geometrical and electronic coupling, the transferred electron is delocalized from the region of nearby the donor molecule to the vicinity of the acceptor. This effect can be qualitatively studied by analyzing the electron density distribution at the ground and excited states. Computed electron-hole properties, distance between hole and electron, transition density, H and t indexes and RMSD of electrons and holes of SPNCN-Cz and SPNCN-TPA are displayed in Table S3-S8. † The integral value of a hole and an electron of SPICN-Cz is less than that in SPICN-TPA with transition density. The integral overlap of hole-electron distribution (S) is a measure of spatial separation of holes and electrons. The integral overlap (S) of  (Table S9 †) and the eigenvalue is greater than 0.96, supporting the hybridization and described in terms of dominant excitation pair in terms of 94% of transition. This is further evidenced by Dr index (Tables S3 and S4 †). The Dr index (eqn (S1) †) is the average hole (h + )-electron (e À ) distance (d h + -e À ) upon excitation, which reveals the nature of the excitation, viz., LE or CT. Valence excitation (LE) is related to short distances (d h + -e À ), while larger distances (d h + -e À) are related to CT excitation. The triplet exciton is transformed to the singlet excitons in SPNCN-TPA and SPNCN-Cz via RISC process with high energy excited state (hot CT channel), 61,62 which is benecial for triplet exciton conversion in electroluminescence processes without any delayed uorescence. CT excitons are formed with weak binding energy (E b ) in higher excited states. 63 As a result, the exciton utilization can be harvested in SPNCN-TPA and SPNCN-Cz, similar to that observed in phosphorescent materials. The quasi-equivalent hybridized materials SPNCN-TPA and SPNCN-Cz exhibit excellent device performances due to ne modulation in the excited states. The enhanced LE component and hybridization between LE and CT components results in high h PL and high h s . The coexisting LE/CT composition in SPNCN-TPA and SPNCN-Cz harvested high h PL and high h s and enhanced the OLEDs' performances (Table 1) is elongated S 1 / S 0 by 0.05, 0.07, 0.14 and 0.16Å (Fig. 6). The smaller change in geometry (S 0 to S 1 ) decreases the nonradiative emission (k nr ), which results in enhanced photoluminescence efficiency (h PL ). The twisted naphthonitrilephenanthrimidazoles NSPI-TPA, MNSPI-TPA, SPNCN-TPA and SPNCN-Cz can effectively suppress molecular aggregation and the almost orthogonal dihedral angles ($89.0 ) between styryl and phenyl of TPA/Cz core can effectively minimize the intermolecular packing (Fig. 7) and can be used as holetrapping sites, whereas the peripheral phenanthrimidazole core blocks electron-trapping sites. Thus, both carrier injection and transport ability are expected from these reported emitters. Furthermore, the relative carrier transport of the title materials was investigated by fabricating hole-only devices as well as electron-only devices (Fig. 8)  from triplet to singlet. SPNCN-TPA and SPNCN-Cz decay sharply (Fig. 2). Hence, radiative excitons in SPNCN-TPA and SPNCN-Cz are short-lived without TADF contribution. Exciton utilization efficiency (h s ) in SPNCN-TPA and SPNCN-Cz follows neither TTA nor TADF mechanism. 65 The non-doped EL devices were fabricated to investigate the relationship between excited state properties and EL performances of NSPI-TPA, MNSPI-TPA, SPNCN-TPA and SPNCN-Cz (Fig. 9, Table 1  The h s of SPNCN-Cz and SPNCN-TPA was calculated to be 32.14 and 30.8%, respectively, which was superior to the 25% spin statistics limit. The increased h s and h IQE (15.75 for SPNCN-Cz and 15.10 for SPNCN-TPA) was due to the CT state contributed from the cyano group. The twisted geometry of emissive materials through introduction of a sterically hindered group enhanced the color purity. The fabricated devices also show excellent efficiency with no roll-off external quantum efficiency. Efficiencies of the as-fabricated current devices were compared with literature reported efficiencies [66][67][68][69][70][71][72][73][74][75][76][77][78][79] and displayed in Table  S10, † which shows that newly synthesized twisted D-p-A blue emitters are the best in view of efficiency (Table 5).

Conclusions
We have reported new deep-blue emitters using twisted donorp-acceptor molecular design strategy. The photophysical and thermal stabilities and electrochemical properties of cyanosubstituted blue uorescent materials SPNCN-TPA and SPNCN-Cz can be modulated by the chemical modication of TPA moiety by a Cz fragment, which results in an HLCT emissive state with the increase in LE, decrease in CT and an increase in quantum efficiency. A ne modulation of the emissive state was performed between LE and CT composition to form a quasi-equivalent hybridized HLCT state in SPNCN-Cz and SPNCN-TPA, in which the LE component contributes high h PL , whereas the CT component generates high h s . SPNCN-Czbased device shows external quantum efficiency of 3.28%, current efficiency of 2.90 cd A À1 and power efficiency of 2.26 lm W À1 . The molecular design of the twisted conformation can be used to fabricate low cost uorescent OLED materials using HLCT state principle.

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