Vibronic coupling to simulate the phosphorescence spectra of Ir(III)-based OLED systems: TD-DFT results meet experimental data

The electronic and optical properties of six iridium imidazolylidene complexes (1a, 1b, 2, 2b, 3, 3b) that are strong candidates for use in OLED systems were investigated theoretically. Computations using DFT and TD-DFT methods were performed to explain the observed optical properties of these complexes. Observed absorption bands were assigned and the lowest triplet excited states were computed. Whereas complexes 1a and 1b are nonemissive in solution, the simulated phosphorescence spectra of complexes 2, 2b, 3, and 3b were in good agreement with the observed spectra when the vibrational contributions to the electronic transitions were taken into account. The use of vibronic coupling allowed us to reproduce and explain the structured phosphorescence spectra of complexes 2 and 2b, as well as the absence of such structure from the spectra of complexes 3 and 3b.