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

Exceptionally long-lived T2 states (7 ns) were observed with the N^N PtII bisacetylide complex (Pt-1) and trans-bis(phosphine) PtII 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 T2 (3An) was populated via ultrafast intersystem crossing (ISC) of S1 (1An) → T2 (3An) (within 0.2 ps). Interestingly, Pt-3, after population of the S1 state, emits strong fluorescence (ΦF = 89%); the poor ISC is due to the high-lying T2 (3An, 3.36 eV) versus S1 (1An, 2.55 eV) state and the large energy gap between S1 (1An, 2.55 eV) and T1 (3An, 1.32 eV) states. The population of the upper excited state S2 (1LLCT, 3.49 eV) turns to an efficient S2 → T2 → T1, and ISC yield increases by 55% compared with S0 → S1 excitation. These results present new in-depth insights into fundamental photochemistry of upper excited states.