Pulsed laser spectroscopic study of the photoisomerization of azo labels at three different locations on a polystyrene chain

Abstract

Azobenzene chromophores have been incorporated as molecular labels at three specific sites on a polystyrene chain: the chain end, the chain center, or as the side group. Trans * cis photoisomerization kinetic behavior of azo labela has been studied both in dilute solution and in the glassy state at 20 C, using nanosecond pulsed laser spectroscopy, which reduced the measurement time to only 2 s. In dilute solution, a small difference in the label's photoisomerization behavior has been observed at different sites of the chain; the end label can photoisomerize a little faster than the center label or the side label. A much greater difference is observed in the glassy state, especially between the end label and the center label, mainly due to the free volume differences in the vicinity of each label. Assuming that in the glassy state, only a certain fraction of the label can isomerize with the same rate as in dilute solution, only 8% of the center label is found to photoisomerize while about 45% of the chain end or the side group photoisomerizes at 20 C. The experimental results are discussed in terms of the theoretical predictions based on the free volume size distribution theory of Robertson, which was applied to meet the photoisomerization requirement of the azobenzene label. This theory predicts that 23% of the azo label can isomerize at 20 OC. In view of the packing differences and the resulting free volume differences in the chain end vs. the chain center, the relationships between the theoretical value and the experimental values seem reasonable. We recently utilized azobenzene chromophores as, mo- lecular labels in the main chains of amorphous poly- urethanes and found that the photoisomerization of such azo labels is very sensitive to the volume changes taking place within the solid film.' While photoisomerization in dilute solution occurs by a single rate process, its initial portion in the solid films may be fitted by two separate rate processes. The first is as fast as in dilute solution and is followed by a slower one. The fractional amount of the fast process decreases with physical aging but increases with temperature, plasticization, or glassy deformation.lb We suggested that this fraction may be proportional to the number of regions where local free volumes are greater than a critical size necessary for the photoisomerization of the azobenzene group. The azo chromophores in our previous study were attached in the main chain of the amorphous polymers but their location was not specific. In this study, we have incorporated azo chromophores at