Excited state proton transfer and photochromism of an aromatic Schiff base. Pico- and femtosecond kinetics of the N,N′-bis(salicylidene)-p-phenylenediamine (BSP)

The results of time-resolved pico- and femtosecond absorption and emission study performed for the title photochromic Schiff base, (BSP) are presented. Transient absorption spectra of intermediates, appearing in the excited state intramolecular proton transfer (ESIPT) were identified. A full scheme of deactivation of the excited BSP molecule, including the enol-, keto- and photochromic tautomers, was proposed. In particular, the characteristic time of the ESIPT process was determined as <50 fs, and the time of the back proton transfer, as 0.5–1.5 ps. The photochromic transient in its ground state is created from the S1 excited keto-tautomer with the rate constant kK−PC=2.8×1010 s−1.