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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 &gt; 2 &gt; 3-exo ∼ 3-endo , which corroborates qualitatively the experimental non-radiative decay rate constants, k  nr : 1 ≫ 2 &gt; 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 &gt; 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.

chemical science

Theoretical studies on the photophysical properties of luminescent pincer gold(<scp>iii</scp>) arylacetylide complexes: the role of π-conjugation at the C-deprotonated [C^N^C] ligand

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