Unusual conformational effects in proton transfer kinetics of an excited photochromic Schiff base

The compound 4-methoxy-2,5-bis(phenyliminomethyl)-phenol representing a group of recently synthesized new photochromic Schiff bases, has been studied by transient absorption and time-resolved fluorescence measurements in femto- and picosecond time domains. Two different conformers of the keto tautomer, decaying with the time constants 480 and 110 ps, have been detected. The first one is created within 50 fs, and the second one – exceptionally slowly – with the time constant of about 10 ps. The fast route occurs in the planar system, and the slow one involves the rotation around the CC bond between the central phenyl ring and the phenylimino group, leading to the metastable photochromic form. For the first keto conformer the evidence of fast vibrational relaxation taking place on the time scale of several hundreds of femtoseconds is observed.