Photochromic dibenzobarrlenes: Long–lived triplet biradical intermediates

Abstract

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 molecular oxygen; the Stern-Volmer quenching constant is 6.9 ± 0.2x10 rise to 8 M rise to -1 S rise to -1. Similarly, the disubstituted dibensobarrelenes 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 single state of 1a undergoes rapid intersystem crossing to its triplet state, followed by intramolecular delta-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 delta-H abstraction on the triplet-state potential energy surface, calculated at the B3LYP/6-31G* level to density functional theory (DFT), has an activation energy of 18.5 kcal/mol. Further, the absorption spectrum of the triplet biradical 32, obtained from the time-dependent DFT calculations, displays an intense absorption maximum at 670 nm, which is in good agreement with the observed absorption peak at 700nm. 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 supports the intervention of a long-lived triplet diradical 32 in the photochromism of appropriately substituted dibenzobarrelenes.