Ligand π-conjunction dictated intersystem crossing in phenyleneethynylene gold(I) complexes

The excited state dynamics of gold(i) complexes containing monomeric phenyleneethynylene (PE) or oligo (p- or m-PE) ligand, namely PhCCAu(PCy 3) (1a) and PhCC(C 6H 4-1,m-CC) 2Au(PCy 3) (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 S 1 1ππ* to T 1 3ππ* intersystem crossing (ISC), the exact ISC rate constant (k ISC) is largely controlled by π-conjugation of the PE ligand in the S 1 states of 1a-3a. Upon increasing π-conjugation as from 1a to 3a and to 2a, the k ISC decreases consecutively by ∼2 orders of magnitude from ∼1.2 × 10 12 s -1 in 1a to ∼2.6 × 10 10 and ∼1.6 × 10 8 s -1 in 3a and 2a, respectively. As a result, the prompt fluorescence (PF) from the initial S 1 1ππ* state is quenched to markedly different extent, with the lifetime (τ PF) 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 k ISC 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. © 2012 The Royal Society of Chemistry.link_to_subscribed_fulltex