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)Au(III)C[triple bond, length as m-dash]CPh-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 C(H)^N^C ligand (1) by a naphthalenyl (2) or a fluorenyl moiety (3-exo and 3-endo; HC(H)^N^CH = 2,6-diphenylpyridine). Conforming to the conventional wisdom that extended π-conjugation imposes rigidity on the structure of the (3)IL(ππ*(C^N^C)) excited state (IL = intraligand), the calculated Huang–Rhys factors for the (3)IL → S(0) transition follow the order: 1 > 2 > 3-exo ∼ 3-endo, which corroborates qualitatively the experimental non-radiative decay rate constants, k(nr): 1 ≫ 2 > 3-exo, but not 3-endo. Density Functional Theory (DFT) calculations revealed that there is an additional triplet excited state minimum of (3)LLCT character (LLCT = ligand-to-ligand charge transfer; (3)[π(C[triple bond, length as m-dash]CPh-4-OMe) → π*(C^N^C)]) for complexes 1 and 3-endo. This (3)LLCT 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 (3)LLCT excited state and the ground state. Together with the larger spin–orbit coupling (SOC) and solvent reorganization energy for the (3)LLCT → S(0) transition compared with those for the (3)IL → S(0) transition, the calculated k(nr) values for the (3)LLCT → S(0) transition are more than 690- and 1500-fold greater than the corresponding (3)IL → S(0) transition for complexes 1 and 3-endo respectively. Importantly, when this (3)LLCT → S(0) decay channel is taken into consideration, the non-radiative decay rate constant k(nr) 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 (3)dd or (3)LMCT 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 (3)LLCT excited state while 3-exo does not.