Dynamics of photochromism in salicylideneaniline: A femtosecond spectroscopic study

Abstract

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) À1 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.

Deconvolution of femtosecond time‐resolved spectroscopy data in multivariate curve resolution. Application to the characterization of ultrafast photo‐induced intramolecular proton transfer

“…It was also checked that no improvement can be obtained when including different number of components in the resolution. The profiles Ĉ and Ŝ extracted from MCR analysis are also found in quite good agreement with results obtained previously for this class of molecules in different solvents [7,31,32].…”

“…The rate constant k 1 typically would describe an ultrafast transition ($ few femtoseconds) between the ground state and the excited state, which is usually not accessible from classical experimental data. The rate constants k 3 would correspond to the formation of a cis-keto intermediate ($200 fs) [7,32]. Finally, k 4 would correspond to the relaxation of this intermediate to the trans-keto photochromic form ($2 ps).…”

“…The photo-reaction is based on an excitedstate intramolecular proton transfer (ESIPT) between an enol and a keto form of the molecule (Figure 1), which then creates the photoproduct. The detailed mechanism is still controversial but the ESIPT usually occurs in less than 50 fs in solution [29][30][31][32][33][34]. In the previous analysis [7], new insights into the photo-reaction were obtained but as deconvolution was not considered, the kinetic analysis had to be considered very carefully at ultrashort time scale.…”

Hybrid hard- and soft-modelling applied to analyze ultrafast processes by femtosecond transient absorption spectroscopy: Study of the photochromism of salicylidene anilines

“…This mechanism involves in the first step, an excited state intramolecular proton transfer (ESIPT) [15] which is of potential interest for fast optical switching applications and from fundamental point of view [16,17]. The generally accepted photoreaction is (1) an ESIPT between an enol form and a cis-keto form of the molecule and (2) a cis-trans isomerization to the trans-keto photoproduct [15,18,19]. Back reaction from the trans-keto state to the enol form can occur both by irradiation with visible light or by thermal conversion with a kinetics which ranges from a few ms in solution to several months in solid state (see Fig.…”

Chemometric analysis of femtosecond transient absorption spectroscopy data: Study of the photochromism of anils

“…This is an important result with regards to applications where switching speed should be considered. Such hypothetical schemes for the photochromism of anils were made previously in several studies [27,28,31,42,43] but have never been shown directly, due to the presence of strong overlapping species with almost similar spectra.…”