Ligand π-conjugation dictated intersystem crossing in phenyleneethynylene gold(i) complexes

Abstract

The excited state dynamics of gold(I) complexes containing monomeric phenyleneethynylene (PE) or oligo (p- or m-PE) ligand, namely PhCCAu(PCy3) (1a) and PhCC(C6H4-1,m-CC)2Au(PCy3) (m = 4, 2a; m = 3, 3a) were investigated by combined methods of femtosecond transient absorption (fs-TA), fs time-resolved fluorescence (fs-TRF) and nanosecond time-resolved emission (ns-TRE). Comparative study was also performed on the corresponding metal free p- and m-PE oligomers (2b and 3b) to allow for evaluating the effect of ligand π-conjugation and the effect of gold(I) on the overall dynamics and emission pathways of the gold(I)-PE complexes. The results reveal that, although the incorporation of heavy gold(I) into PE ligand is effective in promoting the spin-forbidden ligand centered S11ππ* to T13ππ* intersystem crossing (ISC), the exact ISC rate constant (kISC) is largely controlled by π-conjugation of the PE ligand in the S1 states of 1a–3a. Upon increasing π-conjugation as from 1a to 3a and to 2a, the kISC decreases consecutively by ∼2 orders of magnitude from ∼1.2 × 1012 s−1 in 1a to ∼2.6 × 1010 and ∼1.6 × 108 s−1 in 3a and 2a, respectively. As a result, the prompt fluorescence (PF) from the initial S11ππ* state is quenched to markedly different extent, with the lifetime (τPF) ranging from ∼0.7 ps in 1a to ∼450 ps in 2a. Aside from PF, long-lived delayed fluorescence (DF, lifetime on microsecond timescale) formed due to triplet–triplet annihilation (TTA) has also been identified to contribute to the fluorescence of 2a and 3a but not 1a. The ligand dependent behavior of both the kISC and DF provides a rationale to the varied efficiency and unusual composition of the emissions of 1a–3a. The findings present a compelling case that impacts the conventional scenario of ISC due to the heavy-atom effect in the excited state decay of transition-metal complexes; the photophysical parameters and the deactivation pattern derived in this work are useful information for understanding the structure–property relationship of transition-metal-PE complexes.