Photoreduction of some dyes by styrene

Abstract

Some heteroatom-containing anthracene dyes are bleached out in a reductive pathway with quantum yields, Φ_b, between 10^–2 and 10^–4 by excitation at their long-wavelength maximum in methanol solutions containing styrene (5 mol dm^–3). The photoreduction of the dye safranine T by styrene has been investigated. The excited safranine T singlet undergoes and electron-transfer reaction with a rate constant of k^s_q= 5 × 10⁷ dm³ mol^–1 s^–1 and an overall efficiency of η_el= 0.019. Three different experimental procedures (fluorescence quenching, triplet-state yield measurement, Φ_b measurement) give the same value within the experimental limits. As determined by means of short-time techniques, the excited triplet state does not interact with the alkene. In the presence of oxygen, there is an additional oxidative pathway, involving the primary products of the photoreduction process. The Φ_b values are dependent on the solvent used. A correlation with Gutmann's donor numbers, DN, has been obtained (–log Φ_b= 0.030DN + 1.9). The semireduced safranine T species are able to add onto the styrene double bond with a rate constant of k_rm= 6 × 10³ dm³ mol^–1 s^–1 giving rise to initiation of free radical polymerization of this alkene.