Dynamics of photochromism in salicylideneaniline: A femtosecond spectroscopic study

Excited state intramolecular proton transfer (ESIPT) of salicylideneaniline enol and subsequent isomerization of the proton transferred keto structure to the final long-lived photochromic product has been studied in different solvents using femtosecond transient absorption and picosecond fluorescence spectroscopy. Three different types of transient absorption have been identified, viz. (a) from initially excited enol form, (b) proton transferred keto form and (c) corresponding to the final photochromic product. The ESIPT rate was determined from the rise time of the characteristic transient absorption originated from the proton transferred keto form and was found to be in the order of (200-300 fs)(-1) in most of the solvents. Photochromic product formation is completed within a few tens of picoseconds. The time dependent spectral change observed at the early time delay after excitation was considered to be due to cooling of the vibrationally "hot" proton transferred keto form towards the formation of the final photochromic product. The cooling process leads to the formation of some "metastable" state within a few hundred of femtoseconds in the ground state potential energy surface of the final trans product.