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 vi sible light. The quantum yields of the latter two processes were deter mined using Chalesby-Pinner theory. Under the irradiation conditions e mployed the cross-linking density is similar to 4.5 times greater than the fracture density (chain scission). This corroborates the observat ion that poly(3-alkylthiophenes) are negative-type photoresists. Altho ugh the quantum yield of cross-linking is relatively low the polymer m aintains a relatively high degree of pi-conjugation following irradiat ion. The mechanism of crosslinking and insolubilization proceeds via a classical photooxidation route that is initiated by photolysis of res idual iron impurities. Photobleaching involves photosensitization, and reaction of singlet oxygen. O-1(2) undergoes a 1,4 Diels-Alder additi on 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 ar e considerably reduced in the absence of oxygen.