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 III C≡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 蠑 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≡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 蠑 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.