An infrared study of azobenzene photoisomerization in a polymer matrix

A technique involving in situ Fourier transform infrared spectroscopy in the reflection-absorbance mode (FTIR-RA) was used to study photoisomerization and the thermal back reaction of a solid solution of trans-azobenzene in a poly(methyl methacrylate) (PMMA) matrix. The FTIR-RA method, although inherently less sensitive than UV spectrophotometry, affords better resolution of specific isomer absorbances and permits simultaneous observation of changes in the matrix and chromophore vibrational bands. A change in the orientation of the acrylate side groups in isotactic sequences of the atactic PMMA matrix was observed to coincide with photoisomerization of the dissolved azobenzene. This orientational change is a direct confirmation of the matrix effect on isomerization reactions postulated by other researchers. The frequencies of the most intense vibrational absorption bands in the thermally unstable cis isomer were determined. With this information, the relative quantum yields $\text{φ}{}_{\mathrm{<mtext>t\to c</mtext>}}\text{φ}{}_{\mathrm{<mtext>c\to t</mtext>}}$ for photoisomerization of azobenzene and the half-life for thermal cis→trans isomerization were obtained and compared with literature values.