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