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 (E_a 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 on the triplet-state potential energy surface, calculated at the B3LYP/6-31G* level of density functional theory (DFT), has an activation energy of 18.5 kcal/mol. Further, the absorption spectrum of the triplet biradical ³2, obtained from time-dependent DFT calculations, displays an intense absorption maximum at 670 nm, which is in good agreement with the observed absorption peak at 700 nm. The molecular-orbital analysis of the triplet diradical ³2 suggests that its long-wavelength absorption involves the transition of the unpaired electron from the comparatively localized benzyl-type HOMO to the extensively conjugated benzoyl-type LUMO. The present experimental and theoretical results strongly support the intervention of a long-lived triplet biradical ³2 in the photochromism of appropriately substituted dibenzobarrelenes.