Molecular oxygen forms a reversible charge transfer complex (CTC) with poly(3-alkylthiophenes). The complex is weakly bound but possesses a distinct absorption band in the visible region. The electronic properties of the neutral polymer are modulated by the CTC. The extent of modulation of these properties is proportional to oxygen pressure and is fully reversible. Analysis of poly(3-hexylthiophene) field effect transistors under increasing pressures of oxygen shows that the carrier concentration increases, conductivity increases, and the charge carrier mobility is lowered by the formation of the charge transfer complex. The CTC manifests itself as a fluorescence quencher of mobile polaronic excitons, and it explains why oxygen quenches luminescence with much greater efficiency than predicted by a collisional quenching model. Implications of the charge transfer complex on the photochemistry of poly(3-alkylthiophenes) and photosensitization of singlet oxygen are discussed.

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Communications which is significantly lower than the expected M,,, ( = gS) of 2.87 for isotropic S = 1 (assuming g = 2.87). This could be the result of spin canting as is found in manganese(@ phthalocyanine (MnPc), a canted ferromagnet with

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 IT-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. '0, undergoes a 1,4 Diels-Alder addition to thienyl units which results in a loss of IT-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.Key words: photochemistry, polymers, thin films, degradation, cross-linking.RCsumC : Lorsqu'on les soumet B une irradiation UV ou visible dans l'air, les films de poly(3-alkylthiophbnes) subissent des photoblanchiments, des rtticulations et des scissions de chaines simultants. Utilisant la thCorie de Chalesby-Pinner, on a determink les rendements quantiques des deux derniers processus. Dans les conditions d'irradiation employtes, la densitk de la rkticulation est environ 4,5 fois plus grande que la densitk de fracture (scission des chaines). Ces rksultats corroborent l'observation que les poly (3-alkylthiophbnes) sont des photorksistances de type nkgatif. M&me si le rendement quantique de la rkticulation est relativement bas, le polymbre maintient un degrk relativernent klevk de conjugaison IT aprbs l'irradiation. Le mkcanisme de rkticulation et d'insolubilisation procbde par le biais d'une photooxydation classique qui est initike par la photolyse d'impuretks ferriques rksiduelles. Le photoblanchiment irnplique une photosensibilitation et la reaction d'un oxygbne singulet. Le '0, subit une addition-1,4 de Diels-Alder sur les unites thiknyles qui provoquent une perte de conjugaison IT, un dkplacement vers le bleu dans le spectre d'absorption du polymbre et une diminution de la densitk optique. Les rkactions de photoblanchiment, rkticulation et scission de chaine sont forternent rtduites en l'absence d'oxygbne.

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