Effect of different solvent condition on cis-trans isomerization of two benzodiazopyrrole derivatives

The trans-to-cis photo-isomerization of azobenzene (AzB) and its derivatives (AzDs) is a well known process (1), that, in the course of the last two decades has been studied extensively for its high potential in a wide range of technological applications, extending from the photo-control of biological events (2) to the production of engineered materials (3). In this frame, we synthesized and studied the chemical-physical properties of two benzodiazopyrrole derivatives, named 1RS and 1RR (Figure 1). The photo-chemical properties of both molecules were studied, by 1H NMR, UV, and fast UV spectroscopy, in dark and under irradiation by LED light at 435 nm, using different solvent conditions. In methanol, UV spectra registered on samples of 1RS or 1RR kept in dark show the highest band centered at 400 nm. After irradiation, the shape and the intensity of this band strongly changed, thus evidencing that trans-cis conversion should occur in the solution. 1H NMR spectra registered before and after irradiation also show significant differences, attributable to light induced trans-cis conversion. Fast UV experiments were used to measure the time required to convert trans 1RS and 1RR to the corresponding metastable cis isomers and the half lives of the latter species. Using aqueous solutions, the half lives of both metastable cis form strongly depend on the pH of the solution. In particular the half lives of both cis 1RR and 1RS increase from ~1s at pH=5.7 to > 90s at pH=8. Notably, increasing the LED light exposition of the samples, the UV profile of 1RR in buffered solution at pH=5.7, but not that of 1RS, undergoes to irreversible change in the intensity and the position of bands. We are now studying the origin of this phenomenon.