MECHANISMS OF PHOTODEGRADATION OF POLY(3-ALKYLTHIOPHENES) IN SOLUTION

Poly(3-hexylthiophene) degrades in nonaqueous solvents containing diss olved molecular oxygen when irradiated with UV or visible light. Degra dation takes the form of both reduced pi-conjugation and chain scissio n. The former manifests itself as photobleaching and is largely the pr oduct of photosensitization and reaction of singlet oxygen. Chain scis sion occurs via the classical route of photooxidation where scission i s initiated by photolysis of trace amounts of transition-metal salts. In the case of poly(3-hexylthiophene), prepared by chemical oxidation using FeCl3, the photoactive impurities are iron(III) salts. Free-radi cal attack on thienyl rings provides an additional, but minor, route t o photobleaching. Similar polymers prepared electrochemically and by G rignard polycondensation reactions also undergo photochain scission bu t to a lesser degree than polymers prepared by FeCl3. The variation in rates of scission is attributed to differences in the nature and conc entration of the impurities.