Theoretical Studies on Photophysical Properties of Isomeric Iridium(III) Complexes Ir(ppy)2(acac) Containing Dimesitylboron Moiety

The phosphorescent photophysical properties for three Ir(III) complexes 1 similar to 3 containing dimesitylboryl moiety were investigated by DFT. The electronic structure of the ground and excited state, absorption and emission spectra, the spin-orbital coupling matrix < T-1(alpha)vertical bar H-SOC vertical bar S-n > radiative and non-radiative transition process for complexes 1 similar to 3 were calculated by DFT/TD-DFT approach. The effect of dimesitylboryl substitution at different site of Ir(III) complex with phenylpyridine and acetylacetone ligand on the phosphorescent radiative and non-radiative process was discussed. The results reveal that the introduction of B(Mes)(2) group to the pyridine ring of the phenylpyridine (ppy) ligand can strengthen the interactions between the metal and the acetylacetone (acac) ligand, reduce the structure relaxation of the molecule from the ground state to the excited triplet state, and maintain the structures of octahedral field, which is conducive to restricted non-radiative transition. Moreover the singlet-triplet energy splitting Delta E(S-1-T-1) is decreased, the intersystem crossing rate and radiative transition rate are increased. In addition, compared with the substitution at the pyridinyl in complex 1, modifying phenyl group with B(Mes)(2) group in complex 2 and 3 could induce larger structural changes from So to T-1 state and enhance the < S-0 vertical bar H-SOC vertical bar T-1 > value, the spin orbit coupling matrix element between S-0 and T-1 state of 2 and 3 are greater than that of 1, which will induce a larger non-radiative transition rate for 2 and 3. The variety of substitution position of B(Mes)(2) group leads to different d-splitting, different spin-orbital coupling effect in the x, y or z direction, induces the changes of zero field splitting energy and the inequality of radiative transition rates in the three substates (namely, k(r)(x), k(r)(y), and k(r)(z)), and the largest radiative rates of 1 similar to 3 are all located in z substates with values of 2.32 X 10(5), 1.20 X 10(5), and 5.50 X 10(5) s(-1), respectively. Therefore, we explained the reason that complex 1 has higher phosphorescence quantum efficiency through modifying the pyridine ring of the ppy ligand rather than the benzene ring.