Regulation of Singlet and Triplet Excitons in a Single Emission Layer: Efficient Fluorescent/Phosphorescent Hybrid White Organic Light-Emitting Diodes

Two efficient fluorescent molecules, viz., (E)-2-(2-4-(1-2,3-dihydrobenzo[b][1,4]dioxin-5-yl)-4,5-diphenyl-H-imidazole-2-yl)-[1,1-biphenyl]-4-yl)vinyl-1-yl(naphthalene-1-yl)-1H-phenanthro[9,10-d]imidazole (DDIBNPPI) and (E)-4-(2-(2-(-4′-1(2,3-dihyderobenzo[b][1,4]dioxin-5-yl)-4,5-diphenyl-1H-imidazole-2-yl)-[1,1′-biphenyl]-4-yl)vinyl)-1H-phenanthr[9,10-d]imidazole-1-yl)-1-napthronitrile (DDIBPPIN), were designed and synthesized. DDIBNPPI and DDIBPPIN were obtained by rupturing the covalent bond of phenanthrimidazole core to prevent aggregation-induced quenching. In DDIBPPIN, naphthonitrile group was incorporated into azomethine nitrogen of phenanthrimidazole to enhance charge-transfer ability. The DDIBPPIN/CBP:DDIBPPIN-based device shows blue emission with ηc (current efficiency) of 4.91/4.10 cd/A, ηp (power efficiency) of 4.56/3.84 lm/W, and ηex (external quantum efficiency) of 5.11/5.96%. The ηs (exciton utilization efficiency) values of DDIPNPPI and DDIBPPIN are of 27.0 and 30.3%, respectively. The DDIPNPPI and DDIBPPIN materials employed as a host to fabricate green and red phosphorescent organic light-emitting diodes. The red/white devices (with 0.4% dopant concentration) with DDIBPPIN:Ir(MDQ)2(acac) exhibit maximum L of 69889/26319 cd/m2, ηex of 19.6/16.6%, ηc of 34.6/35.6 cd/A, and ηp of 35.8/36.6 lm/W. The device with DDIBPPIN:Ir(ppy)3/DDIPNPPI:Ir(ppy)3 exhibits green emission [Commission Internationale de l’Eclairage (0.30,0.60)/(0.30,0.60)] with maximum L of 69906/69482 cd/m2, ηex of 17.9/17.0%, ηc of 59.8/58.6 cd/A, and ηp of 63.6/61.3 lm/W. The white device using DDIBPPI:Ir(ppy)3 (with 0.4% dopant concentration) exhibits maximum L of 22152 cd/m2, ηex of 15.8%, ηc of 31.4 cd/A, and ηp of 36.1 lm/W.


S3
Figure S1 Solvatochromic a) absorption and b) emission spectra of DDIBNPPI and DDIBPPIN.

SI-II: Potential energy scan (PES)
The ground (S 0 ) and excited (S 1 ) state geometries of DDIBNPPI and DDIBPPIN were optimized with DFT/B3LYP/6-31G (d, p) and TD-DFT/B3LYP/6-31G (d, p) methods, respectively, using Gaussian-09 ( Figure S2). The three key twist angles (θ°): PPI-θ° (between phenanthrimidazole core and styryl spacer, LIN-θ° (between styryl linker and phenyl linker) and DPI-θ° (between diphenyl imidazole and phenyl linker) play a key role in electron overlap of frontier molecular orbitals in ground and excited states. The ground and excited state energy of DDIBNPPI and DDIBPPIN reveal that S4 PPI-θ° was limited at room temperature (13.3 eV and 14.5 eV) ( Figure S2).The larger twist angle (DPI-θ°) of DDIBPPIN compared to DDIBNPPI is due to the stronger repulsion between the bulky substituent at azomethine nitrogen of diphenyl imidazole fragment with linker fragment. The excited state (S 1 ) twist angles, PPI-θ°, LIN-θ° and DPI-θ° of DDIBNPPI and DDIBPPIN are increased compared to ground state (S 0 ) twist angles. Only small change in geometry of DDIBPPIN from S 0 →S 1 was observed compared to DDIBNPPI, which facilitate the suppression of non-radiation for the enhancement of η PL . 1 From the potential energy surface of the twisted geometry of DDIBPPIN at ground and excimer at excited states ( Figure   S3) reveal that the DDIBPPIN needs very small relaxation energy to form excimer of excited state corresponding to slightly increased interplanar separation of PPI from linker fragment

SI-III: Charge-Transfer indexes
The excited state properties of DDIBNPPI and DDIPPIN were discussed briefly in SI-III: charge-transfer indexes ( Figure S4-S7).
The hole-particle pair interactions have been related to the distance covered during the excitations one possible descriptor Δr intex could be used to calculate the average distance which is weighted in function of the excitation coefficients.
is the norm of the orbital centroid. Δr-index will be expressed in Å.
The density variation associated to the electronic transition is given by where and are the electronic densities of to the ground and excited states, ( ) ( ) respectively. Two functions, and , corresponds to the points in space where an increment or a depletion of the density upon absorption is produced and they can be defined as follows: .
The barycenters of the spatial regions R + and Rare related with and and are The centroid along X axis is expected. The t intex represents the difference between D CT and H: ............... (S16) = -S9 Figure S4 Natural transition orbital pairs with (HONTOs and LUNTOs) transition character analysis for singlet states (S 1 -S 10 ) and triplet states (T 1 -T 10 ) of DDIBNPPI [f-oscillator strength and % weights of hole-particle]. S10 Figure S5 Natural transition orbital pairs with (HONTOs and LUNTOs) transition character analysis for singlet states (S 1 -S 10 ) and triplet states (T 1 -T 10 ) of DDIBPPIN [f-oscillator strength and % weights of hole-particle].