Solid-state photochemistry of π-conjugated poly(3-alkylthiophenes)

Abstract

Films of poly(3-alkylthiophenes) undergo simultaneous photobleaching, cross-linking, and chain scission when irradiated in air with UV or visible light. The quantum yields of the latter two processes were determined using Chalesby–Pinner theory. Under the irradiation conditions employed the cross-linking density is ~ 4.5 times greater than the fracture density (chain scission). This corroborates the observation that poly(3-alkylthiophenes) are negative-type photoresists. Although the quantum yield of cross-linking is relatively low, the polymer maintains a relatively high degree of π-conjugation following irradiation. The mechanism of cross-linking and insolubilization proceeds via a classical photooxidation route that is initiated by photolysis of residual iron impurities. Photobleaching involves photosensitization, and reaction of singlet oxygen. 1O2 undergoes a 1,4 Diels–Alder addition to thienyl units which results in a loss of π-conjugation, a blue shift in the absorption spectrum of the polymer, and a decrease in the optical density. Photobleaching, cross-linking, and chain scission are considerably reduced in the absence of oxygen. Keywords: photochemistry, polymers, thin films, degradation, cross-linking.