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

Abstract

Exceptionally long-lived T₂ states (7 ns) were observed with the N^N Pt^II bisacetylide complex (<b>Pt-1</b>) and <i>trans</i>-bis(phosphine) Pt^II bisacetylide complexes (<b>Pt-2</b>, <b>Pt-3</b>) containing anthryl acetylide ligands. For <b>Pt-1</b>, 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, <b>Pt-3</b>, after population of the S₁ state, emits strong fluorescence (Φ_F = 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.