Photobleaching and spectral diffusion in disordered media

The photochemical bleaching of an energetically disordered molecular system is studied theoretically. Spectral diffusion is taken into account, which is marked by downhill energy transfer of excitons between the molecular sites. The resulting redshift in the excited-state occupation gives rise to a relatively stronger bleaching of the lower-lying states, which is observed in the blueshift of both the absorption spectrum and the photoluminescence spectrum of the bleached system. During the photochemical bleaching reaction, quenching sites are formed, at which the excitons decay nonradiatively. These quenching sites modify the excited-state dynamics and thus the bleaching process. The relevance of our model to the photobleaching of poly$(p$-phenylene-vinylene) is pointed out.