The photoinduced isomerization and its implication in the photo-dynamical processes in two simple Schiff bases isolated in solid argon

Abstract

Two Schiff bases: 2-(1-(methylimino)methyl)-phenol (SMA) and its chlorosubstituted derivative 2-(1-(methylimino)methyl)-6-chlorophenol (SMAC), and SMA complexes with water were studied by infrared matrix isolation spectroscopy and DFT/B3LYP/6-311G++(2d,2p) quantum chemical calculations. SMA and SMAC bases trapped in an argon matrix from the vapor above the liquid and solid samples have the most stable enol conformation with intramolecular O–H⋯N bonding. Irradiation (λ > 320 nm) leads in both bases to a rotational isomerization reaction in which the scission of the O–H⋯N bond occurs and the C(H)NCH₃ and OH groups are turned by 180° around the C–C and C–O bonds, respectively. In SMAC a competitive photoreaction channel yields the trans-keto tautomer. The identification of the two SMAC photoproducts evidences that in the excited enol form of this compound two processes compete with each other: the rotational isomerization and intramolecular proton transfer (ESIPT). In the argon matrices doped with SMA and H₂O the SMA–water complexes were identified and characterized spectroscopically. Interaction of SMA with one or two water molecules does not affect the photochemistry of SMA.