Near-infrared reversible photoswitching of an isolated azobenzene-stilbene dye

Photoswitching of a charged azobenzene-stilbene dye is investigated through laser excitation in a tandem ion mobility mass spectrometer. Action spectra associated with E→Z and Z→E photoisomerisation of the stilbene group exhibit bands at 685 and 440 nm, corresponding to S1←S0 and S3←S0 transitions, respectively. The data suggest that isomers possessing a Z configuration of the azobenzene unit rapidly convert to E isomers and are not discernible using ion mobility spectrometry, and that photoisomerisation occurs through excited state dynamics rather than statistical isomerisation on the ground state potential energy surface.