Photoinduced fluorescence changes on E–Z isomerisation in azobenzene derivatives

New fluorosensors based on azobenzene derivatives have been synthesised, characterised and their photophysical properties investigated. Compounds 2a and 2b undergo E→Z photoisomerisation on irradiation at ≈330 nm accompanied by fluorescence enhancement. Z→E isomerisation in the dark is accompanied by a decrease in the fluorescence intensity. These are the first reported examples of fluorescence enhancement on E→Z isomerisation across a NN bond. Protonation of the azo groups in 2a and 2b is also accompanied by fluorescence enhancement. The origin of the fluorescence enhancement in the cis isomer is explained on the basis of the inhibition of photoinduced electron transfer due to the nonplanar geometry of this isomer, which reduces the effective conjugation of the nitrogen lone pair electrons with the π electrons of the fluorophore.