Wavelength-Dependent Pathways of Poly-3-hexylthiophene Photo-Oxidation

The photodegradation kinetics of regioregular poly-3-hexylthiophene (P3HT) thin films (∼150 nm) upon illumination with both visible light of 525 nm and UV light of 365 nm has been studied using UV/vis and FTIR spectroscopy in order to probe both the disruption of the π-conjugated system and the chemical product evolution. Reaction effectiveness and product evolution turn out to depend strongly on the wavelength of the incident light. Under UV light, the π-conjugated system and the hexyl side chain are degraded almost simultaneously, involving Norrish-type reactions that lead to complete disintegration of the alkyl chains. Under visible light only the π-conjugated system is destroyed, whereas the aliphatic side chains are hardly affected. While the reaction under UV light is consistent with a radical reaction starting at the α-carbon of the alkyl side chain, the reaction under visible illumination is more likely to involve a photosensitized species that primarily attacks the π-conjugated system of the polymer.