Direct Observation of Long-Lived Upper Excited Triplet States and Intersystem Crossing in Anthracene-Containing PtII Complexes

Exceptionally long-lived T-2 states (7 ns) were observed with the N<^>N Pt-II bisacetylide complex (Pt-1) and trans-bis(phosphine) Pt-II bisacetylide complexes (Pt-2, Pt-3) containing anthryl acetylide ligands. For Pt-1, fluorescence of the anthryl moiety (An) was quenched and phosphorescence was observed. Under 350 nm excitation, the upper long-lived triplet state T-2 ((3)An) was populated via ultrafast intersystem crossing (ISC) of S-1 ((1)An) -> T-2 ((3)An) (within 0.2 ps). Interestingly, Pt-3, after population of the S-1 state, emits strong fluorescence (Phi(F) = 89%); the poor ISC is due to the high-lying T-2 ((3)An, 3.36 eV) versus S-1 ((1)An, 2.55 eV) state and the large energy gap between S-1 ((1)An, 2.55 eV) and T-1 ((3)An, 1.32 eV) states. The population of the upper excited state S-2 ((LLCT)-L-1, 3.49 eV) turns to an efficient S-2 -> T-2 -> T-1, and ISC yield increases by 55% compared with S-0 -> S-1 excitation. These results present new in-depth insights into fundamental photochemistry of upper excited states.