Density Functional Theory Study Of Ionic Iridium(?)Complexes For Light-emitting Electrochemical Cells

Cationic iridium complexes have been extensively used in electroluminescence applications like organic light-emitting diodes (OLEDs) and light-emitting electrochemical cells (LECs). Compared with the conventional multi-layer OLEDs, LECs have a much simpler architecture and do not rely on air-sensitive charge-injection layers or metals for electron injection. This greatly simplifies their preparation and makes them more cost efficient. In the simplest form, it consists of a single active layer composed of an ionic transition-metal complex (iTMC). The presence of mobile ions facilitates the formation of ionic junctions that lowers the barrier for electron arid hole injection and makes these devices independent of the work function of the electrode material. Furthermore, higher electroluminescence efficiencies are expected owing to the phosphorescent nature of the iTMCs. Significant advances have been obtained in iTMCs-based LECs during the last years. This was possible not in the least due to a more thorough understanding of the operation mechanism. Despite these advances, there is one noteworthy deficiency with this type of device, the lack of wide bandgap iTMCs that leads to efficient blue and green iTMC-LECs. Although several blue-green and blue light-emitting iTMCs-based LECs have been reported, their efficiencies and stability are rather poor. The above urgency prompted us to search for better and new blue or blue-green iTMCs. In this paper, the geometrical, electronic structures, absorption, emission spectra and phosphorescence efficiency as well as the photophysical properties of several iridium(III) complexes are investigated by density functional theory (DFT) and time-dependent density function theory (TDDFT).(1) We investigated a series of iridium(III) complexes [Ir(ppyn)2(P?Pn)]+with P?P ancillary ligands to shed light on the effects of the trans-cis stereoisomerism and substituents on the photophysical properties. According to the results, we found that the same substituent has different influences on the electronic structures and spectra properties of N,N-trans and-cis counterparts. In the N,N-trans complexes, owing to the C2symmetry, the geometrical structure and electronic structure are symmetrical, whereas N,N-cis complexes are not. In addition, the N,N-trans complexes have more intense absorption than N,N-cis ones. The trans-cis stereoisomerism may be an efficient way to tune the emitting colors of these iridium(III) complexes. The complex N,N-trans[Ir(ppy2)2(P?P2)]+(3b) show high quantum phosphorescence efficiency (?PL) of0.91, while an extremely low ?PL (0.05) was observed for N,N-trans[Ir(ppy4)2(P?P)1]+(2d). The detailed analyses on quantum efficiency showed that the different ?PL are closely linked to kr and knr constants, which are elucidated with the aid of the calculated the S1-Tn splitting energy (?Es1-Tn), the transition dipole moment (?S1) upon the So?S1transition and energy gap between3MLCT/?-?*and3MC/d-d states (?EMC-MLCT), respectively. On the basis of these parameters, the higher ?PL of3b with respect to that for2d was explained, and lc,1d,2b and2e were considered to have relatively better physical properties with respect to the experimentally synthesized complexes2,2d and3b. The newly designed molecular lc,1d,2b and2e are expected to be highly emissive in blue-green region for LECs application.(2) In this study, we have employed DFT and TDDFT calculations on a series of cationic iridium(III) complexes with2-phenylpyridine derivatives (ppyn) as major ligand and2-pyridyl azolate (N?Nn) as ancillary ligand. The calculated results reveal that both the difluoro-substituent and the different2-pyridyl azolate ancillary ligand have a large influence on tuning the emission energies and quantum yields of the studied complexes. The difluoro-substituent shortened the Ir-N?Nn bond length, leading to more involvement of the N?Nn ligand in both the FMOs and the excited states. Besides, the difluoro-substituent rendered an increase of the HOMO-LUMO energy gap in comparison with that of4a-10a. The emission wavelength of4b-10b with difluoro-substituent shows a blueshift of18-38nm compared with that of4a-10a. On the basis of the results reported herein, we attempt to explain the experimental observation that the complexes4a,10a and10b show different ?PL. The detailed analysis on quantum efficiency showed that the different ?PL for4a,10a and10b are closely linked to the metal-to-ligand charge transfer contributions (MLCT%), the S1-T1splitting energy (?ES1-T1) and the transition dipole moment (?S1) upon the S0?S1transition, respectively. Drastically small ?ES1-T1and large MLCT%for10a (0.06eV and28.2%, respectively) and10b (0.08eV and24.6%, respectively) compared to those for4a (0.60eV and19.5%, respectively) account for their relatively high ?PL observed experimentally. Besides, the remarkably small ?ES1-T1large MLCT%and similar ?S1for5a (0.04eV,29.1%and0.026D, respectively) compared with those for5b (0.56eV,2.3%and0.022D, respectively) could be interpreted by the difluoro-substituent effects. Furthermore,5a-7a and7b-8b are considered to have relatively better physical properties with respect to the experimentally synthesized complexes10a,10b and4a. Therefore, the newly designed complexes5a-7a and7b-8b are expected to be highly efficient sky-blue and blue-green emitters for LECs application.