Theoretical studies on the photophysical properties of luminescent pincer gold(iii) arylacetylide complexes: the role of π-conjugation at the C-deprotonated [C^N^C] ligand†
We have performed theoretical analyses of the photophysical properties of a series of cyclometalated gold(III) arylacetylide complexes, [(C^N^C)AuIIICCPh-4-OMe], with different extents of π-conjugation at the doubly C-deprotonated [C^N^C] ligand via replacement of one of the phenyl moieties in the non-conjugated CH^N^C ligand (1) by a naphthalenyl (2) or a fluorenyl moiety (3-exo and 3-endo; HCH^N^CH = 2,6-diphenylpyridine). Conforming to the conventional wisdom that extended π-conjugation imposes rigidity on the structure of the 3IL(ππ*(C^N^C)) excited state (IL = intraligand), the calculated Huang–Rhys factors for the 3IL → S0 transition follow the order: 1 > 2 > 3-exo ∼ 3-endo, which corroborates qualitatively the experimental non-radiative decay rate constants, knr: 1 ≫ 2 > 3-exo, but not 3-endo. Density Functional Theory (DFT) calculations revealed that there is an additional triplet excited state minimum of 3LLCT character (LLCT = ligand-to-ligand charge transfer; 3[π(CCPh-4-OMe) → π*(C^N^C)]) for complexes 1 and 3-endo. This 3LLCT excited state, possessing a large out-of-plane torsional motion between the planes of the C^N^C and arylacetylide ligands, has a double minimum anharmonic potential energy surface along this torsional coordinate which leads to enhanced Franck–Condon overlap between the 3LLCT excited state and the ground state. Together with the larger spin–orbit coupling (SOC) and solvent reorganization energy for the 3LLCT → S0 transition compared with those for the 3IL → S0 transition, the calculated knr values for the 3LLCT → S0 transition are more than 690- and 1500-fold greater than the corresponding 3IL → S0 transition for complexes 1 and 3-endo respectively. Importantly, when this 3LLCT → S0 decay channel is taken into consideration, the non-radiative decay rate constant knr could be reproduced quantitatively and in the order of: 1 ≫ 3-endo, 2 > 3-exo. This challenges the common view that the facile non-radiative decay rate of transition metal complexes is due to the presence of a low-lying metal-centred 3dd or 3LMCT excited state (LMCT = ligand-to-metal charge transfer). By analysis of the relative order of MOs of the chromophoric [C^N^C] cyclometalated and arylacetylide ligands, one may discern why complexes 1 and 3-endo have a low-lying 3LLCT excited state while 3-exo does not.