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

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 similar to 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 pi-conjugation following irradiation. The mechanism of crosslinking 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. O-1(2) undergoes a 1,4 Diels-Alder addition to thienyl units which results in a loss of pi-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.