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

Abstract

We have performed theoretical analyses of the photophysical properties of a series of cyclometalated gold(III) arylacetylide complexes, [(C^N^C)Au^IIICCPh-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 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, 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 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 k_nr 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 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 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.