Solvent effects on phototriggered conformational changes in an azobenzene-functionalized poly(p-phenylene vinylene)

Poly(2-hexyloxy-5-((10-(4-(phenylazo)phenoxy)decyl)oxy)-1,4-phenylene vinylene) (HPA-10-PPV) is an azobenzene-functionalized poly(p-phenylene vinylene) derivative that previously has been shown to undergo changes in the position and shape of its fluorescence spectrum in response to trans → cis-azobenzene photoisomerization. This effect was initially observed in a binary dichloromethane-methanol solution and was attributed to a phototriggered expansion of polymer coil dimensions. In this report we explore the mechanism by which azobenzene isomerization induces this conformational change and the solvent properties that govern the magnitude of the change. Phototriggered fluorescence shifts are observed in mixtures of dichloromethane and each of 12 cosolvents, 7 protic and 5 aprotic. The shifts are largest in protic solvents but do not correlate with any single solvent parameter or combination of parameters. Smaller shifts are observed in mixtures with aprotic cosolvents, and the magnitude of these shifts correlates with solvent polarity/polarizability. The conformational changes responsible for the fluorescence shifts are attributed to a change in polymer–solvent interactions induced by the difference between properties of the trans- and cis-azobenzene isomers. Isomerization to the more polar cis-azobenzene promotes an increase in polymer–solvent interactions. These results suggest a means by which the magnitude of phototriggered fluorescence changes could be increased in future derivatives.