Twisted Benzothieno-Fused BOPHY Dyes as Triplet Photosensitizers with Long-Lived Triplet Excited States.

The synthesis of a series of photostable [b]-benzothieno-fused BOPHY derivatives is reported via one-pot condensation of formylated isoindoles or formylindoles with hydrazine and subsequent boron complexation. These dyes show strong absorption in the deep red region and acceptable fluorescence quantum yields (∼30%). The two fused benzothiophene moieties are slightly deviated from the BOPHY core (with dihedral angles of 6.1 and 10.2°). This slightly twisted conformation brings an enhanced spin-orbit coupling and a reduced energy band gap between singlet and triplet states. The enhanced intersystem cross process endows these series of dyes with a good singlet oxygen quantum yield (up to 63%), a high triplet-state quantum yield (up to 78%), and a long lifetime value (up to 127 μs). Density functional theory calculations indicate that the transition from S1 to T2 states is crucial for triplet-state formation, highlighting their high efficiency in intersystem crossing. The calculated triplet electron spin surface reveals a widespread distribution of triplet states across the conjugated molecular structure, which enhances the Dexter mechanism for triplet energy transfer in these BOPHY photosensitizers. These findings are helpful for thorough understanding of the fundamental ISC process and developing triplet photosensitizers.