Isomerisation of an intramolecular hydrogen-bonded photoswitch: Protonated azobis(2-imidazole)

Photoisomerisation of protonated azobis(2-imidazole), an intramolecular hydrogen-bonded azoheteroarene photoswitch molecule, is investigated in the gas phase using tandem ion mobility mass spectrometry. The E and Z isomers exhibit distinct spectral responses, with E-Z photoisomerisation occurring over the 360-520 nm range (peak at 460 nm), and Z-E photoisomerisation taking place over the 320-420 nm range (peak at 390 nm). A minor photodissociation channel involving loss of N₂ is observed for the E-isomer with a maximum efficiency at 390 nm, blue-shifted by ≈70 nm relative to the wavelength for maximum photoisomerisation response. Loss of N₂ is also the predominant collision-induced dissociation channel. Electronic structure calculations suggest that E-isomer photoisomerisation involves S₁(ππ∗) excitation, whereas the Z-isomer photoisomerisation involves S₂(ππ∗) excitation. Conversion between the E and Z isomers through collisional excitation, which is calculated to occur through both inversion and torsion pathways, is investigated experimentally by colliding the molecular ions with nitrogen buffer gas over a range of electric fields. This study demonstrates the versatility of tandem ion mobility mass spectrometry for exploring the isomerisation of molecular photoswitches initiated by either light or collisions.