Photoreduction of some dyes by styrene

Some heteroatom-containing anthracene dyes are bleached out in a reductive pathway with quantum yields, Φb, 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 ksq= 5 × 107 dm3 mol–1 s–1 and an overall efficiency of ηel= 0.019. Three different experimental procedures (fluorescence quenching, triplet-state yield measurement, Φb 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 Φb values are dependent on the solvent used. A correlation with Gutmann's donor numbers, DN, has been obtained (–log Φb= 0.030DN + 1.9). The semireduced safranine T species are able to add onto the styrene double bond with a rate constant of krm= 6 × 103 dm3 mol–1 s–1 giving rise to initiation of free radical polymerization of this alkene.