PHOTOBLEACHING AND SPECTRAL DIFFUSION IN DISORDERED MEDIA

The photochemical bleaching of an energetically disordered molecular s ystem is studied theoretically. Spectral diffusion is taken into accou nt, which is marked by downhill energy transfer of excitons between th e molecular sites. The resulting redshift in the excited-state occupat ion gives rise to a relatively stronger bleaching of the lower-lying s tates, which is observed in the blueshift of both the absorption spect rum and the photoluminescence spectrum of the bleached system. During the photochemical bleaching reaction, quenching sites are formed, at w hich the excitons decay nonradiatively. These quenching sites modify t he excited-state dynamics and thus the bleaching process. The relevanc e of our model to the photobleaching of poly(p-phenylene-vinylene) is pointed out.