Msa. Abdou et S. Holdcroft, SOLID-STATE PHOTOCHEMISTRY OF PI-CONJUGATED POLY(3-ALKYLTHIOPHENES), Canadian journal of chemistry, 73(11), 1995, pp. 1893-1901
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.