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 S1states of 1a-3a. Upon increasing π-conjugation as from 1a to 3a and to 2a, the kISCdecreases consecutively by ∼2 orders of magnitude from ∼1.2 × 1012s-1in 1a to ∼2.6 × 1010and ∼1.6 × 108s-1in 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 kISCand 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.
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