Ultrafast Spectroscopy of Hydroxy-Substituted Azobenzenes in Water.

Ultrafast UV/Vis pump/probe experiments on ortho-, meta- and para-hydroxy-substituted azobenzenes (HO-ABs), as well as for sulfasalazine, an AB-based drug, were performed in aqueous solution. For meta-HO-AB, AB-like isomerisation behaviour can be observed, whereas, for ortho-HO-AB, fast proton transfer occurs, resulting in an excited keto species. For para-HO-AB, considerable keto/enol tautomerism proceeds in the ground state, so after excitation the trans-keto species isomerises into the cis form. Aided by TD-DFT calculations, insight is provided into different deactivation pathways for HO-AB, and reveals the role of hydroxy groups in the photochemistry of ABs, as well as their acetylation regarding sulfasalazine. Hydroxy groups are position-specific substituents for AB, which allow tuning of the timescale of thermal relaxation, as well as the amount and contribution of the keto species to photochemical processes.