Unraveling the Marked Differences of the Excited-State Properties of Arylgold(III) Complexes with C⁽sic)N⁽sic)C Tridentate Ligands

Three structurally similar gold(III) complexes with C(sic)N(sic)C tridentate ligands, [1; C(sic)N(sic)C = 2,6-diphenylpyridine], [2; C(sic)N(sic)C = 2,6-diphenylpyrazine], and [3; C(sic)N(sic)C = 2,6-diphenyltriazine], have been investigated theoretically to rationalize the marked difference in emission behaviors. The geometrical and electronic structures, spectra properties, radiative and nonradiative decay processes, as well as reverse intersystem crossing and reverse internal conversion (RIC) processes were thoroughly analyzed using density functional theory (DFT) and time-dependent DFT calculations. The computed results indicate that there is a small energy difference Delta ET1 = T-1(') between the lowest-energy triplet state (T-1) and the second lowest-energy triplet state (T-1') of complexes 2 and 3, suggesting that the excitons in the T-1 state can reach the emissive higher-energy T-1' through the RIC process. In addition, the non-emissive T-1 states of gold(III) complexes in solution can be ascribed to the easily accessible metal-centered (3MC) state or possibly tunneling into high-energy vibrationally excited singlet states for nonradiative decay. The low efficiency of 3 is attributed to the deactivation pathway via the 3MC state. The present study elucidates the relationship between structure and property of gold(III) complexes featuring C(sic)N(sic)C ligands and providing a comprehensive understanding of the significant differences in their luminescence behaviors.