trans-Stilbene Photochemistry Beyond 500 nm

trans-Stilbene and its derivatives have been widely investigated over the past decades in terms of both photophysics and photochemistry. This study presents an intramolecular energy transfer within a triple chromophore from an optically excited fluorescein onto an extremely low-lying trans-stilbene T₁ state. As in many stilbenes an isomerization along the triplet pathway was sufficiently effective to be monitored by a decrease in the trans- and a concomitant increase in the cis-stilbene absorption bands. Semiempirical calculations and sensitizing experiments were performed to obtain a good estimate of the S₀–T₁ energy difference, which was found to be about 142 kJ/mol. This uncommonly low value enables a long-wavelength photochemistry of trans-stilbene at exciting light wavelengths ranging from 500 nm (fluorescein) up to at least 680 nm (applying an external triplet sensitizer such as methylene blue).