Photoisomerization of Bis(9-anthryl)methane and Other Linked Anthracenes. The Role of Excimers and Biradicals in Photodimerization

Fluorescence characteristics, including quantum yields and lifetimes, and quantum efficiencies for photoisomerization (1 → 2) for a series of anthracenes linked at the 9 position have been measured. Emission yields and lifetimes (ns) decrease for the series dianthryl carbonate (τ = 5), dianthrylethanes (τ ~ 2), dianthrylmethanes (τ ≤ 1). Reaction quantum yields are not so much a function of chromophore linkage but are responsive to the substitution pattern at 10,10′ positions. For dianthrylethanes but not for dianthrylmethanes, excimer emission is observed for sandwich dimers generated in glasses at low temperatures. High photoreactivity for proximal anthracenes is identified with intersecting plane as opposed to parallel plane (sandwich excimer) geometries. In accord with recent theory, biradicals are proposed as intermediates in photodimerization. Formation and partitioning of these species (14) respectively control excited singlet lifetimes and photodimerization quantum yields. Thermochemical and kinetics data for the thermal back reaction (2 → 1) are reported. The measurement and significance of latent heats of isomerization (photon energy storage) are discussed. © 1978, American Chemical Society. All rights reserved