Solid-State E-to-Z Photoisomerization of 4-Dimethylaminoazobenzene Examined by Higher-Order Derivative Spectra

E-to-Z photoisomerization of 4-dimethylaminoazo-benzene (DMAz) in solution and in aqueous dispersion of fine crystals was analyzed by means of higher-order derivatives. UV-induced changes in derivative spectra of DMAz in hexane led to the identification of vibration level transitions (VLTs) of both E- and Z-isomers and to the estimation of conversions of photoisomerization by tracing extrema of VLT sub-peaks of E-isomer. An aqueous dispersion of DMAz crystals prepared by bead-milling exhibits an extraordinarily broad n,pi*-band, which is quite different from that in solution. The irradiation of the dispersion with 365 nm light resulted in the decrement of the absorption band due to the solid-state photoisomerization. The corresponding eighth-derivatives showed the distinctive decline of VLT bands assignable to E-isomer to confirm the photoisomerization. The UV-induced decrement of VLT bands was followed by plotting extremum values of the eighth-order derivatives as a function of exposure time. It was proposed that the solid-state photoisomerization of DMAz is comprised of the fast process at the surface layers and the subsequent slow one in the bulk, taking the observation into account that the melting depression of melting transition occurs by the nanodownsizing of the crystals.