Photofading of an Extended BOPHY Chromophore Dispersed in Poly(methyl methacrylate) as a Chemical Actinometer

The target compound is a member of the symmetric pyrrole-BF2 family, known as BOPHY, which has been equipped with 3,8-styryl groups that push the absorption maximum to lower energy. To assist solubility and facile dispersion in plastic films, poly(ethylene glycol) chains are added to the styryl groups. The photophysical properties are somewhat sensitive to changes in solvent polarity and temperature due to the imposition of weak, intramolecular charge-transfer character. The chromophore bleaches under white-light illumination to form an intermediate species that absorbs and emits at higher energy. This species is believed to be a BOPHY unit with one styryl arm removed from the conjugation path. The intermediate is quite resistant to photobleaching but does fade on prolonged illumination. The kinetics associated with photobleaching in poly(methyl methacrylate) films allow development of a simple model that can be used to establish a robust chemical actinometer for recording exposure to sunlight. Similar photofading behaviour is observed in polar solution, where self-catalysis is observed, with the overall kinetics depending on the nature of the solvent.