Photochromic Dibenzobarrlenes: Long-Lived Triplet Biradical Intermediates

Upon exposure to UV light, the disubstituted dibenzobarrelene derivative 1a turns green in the solid phase and reverts back to its original pale-yellow color within several hours in the dark. The lifetime of the colored species in degassed benzene at room temperature is 37 ± 2 s (Ea for decoloration is 14.5 ± 0.7 kcal mol-1 and log A is 8.92 ± 0.5 s-1) and highly sensitive to molecular oxygen; the Stern−Volmer quenching constant is 6.9 ± 0.2 × 108 M-1 s-1. Similarly, the disubstituted dibenzobarrelenes 1b and 1c exhibited pink coloration when exposed to UV light in the solid phase. On the basis of combined experimental and theoretical evidence, it is proposed that upon photoexcitation the excited singlet state of 1a undergoes rapid intersystem crossing to its triplet state, followed by intramolecular δ-H abstraction, to yield the triplet biradical intermediate 32. Upon prolonged irradiation, 2 undergoes cyclization to the alcohol 3, which affords the enone 4 as the final photoproduct. The δ-H abstraction ...