The diketone 1a exhibited strong yellow emission in solution (Figure ). The steady-state photoluminescence spectrum of 1a in cyclohexane had a single, sharp emission peak at ~560 nm accompanied by weak vibrational bands. The total photoluminescence quantum yield PL was 38.2% under Ar and a lifetime was 72.7 s (Figure ). Time-resolved photoluminescence spectra of 1a revealed that the emission emerged just after photoexcitation (within the <100 ps instrumental response function), and exhibited a long lifetime (Figure ). Hence, the emission was phosphorescence, as reported previously for thienyl diketone analogs, 15 and ISC occurred over a timescale less than the instrumental response.