Comparative Investigation of Ultrafast Photoinduced Processes in Salicylidene-Aminopyridine in Solution and Solid State

Abstract

Photodynamics and excited-state intramolecular proton transfer (ESIPT) of N-(3,5-ditert-butylsalicylidene)-4-aminopyridine (4P) and N-(3,5-ditert-butylsalicylidene)-2-aminopyridine (2P) are studied by steady-state and transient optical spectroscopy in solution and solid state (microcrystalline powder). These two compounds are representative of anils Classes A and B, respectively. The results confirm that in solution both compounds have a similar dynamic behavior: upon excitation at 390 nm, ESIPT leads to different cis-keto* fluorescent species which relax to the photochromic trans-keto photoproduct within a few tens of picoseconds. Furthermore, for the first time, picosecond data are recorded for the two classes of anils in the solid state: in 2P, two cis-keto* fluorescent excited state species (23 and 250 ps lifetime) are relaxing exclusively to the cis-keto ground state, whereas in 4P, a single cis-keto* fluorescent species is observed which leads to the trans-keto photoproduct in 250 ps via an intermediate metastable species. † Part of the "Hiroshi Masuhara Festschrift".

Initial Processes of Proton Transfer in Salicylideneaniline Studied by Time-Resolved Photoelectron Spectroscopy

“…9,[14][15][16][17]22,24,28,29 Theory predicts a ground-state torsion angle of 36°about the anil twist angle α (C 7 NC 6 C 5 dihedral angle; see Figure 1c for a definition of the critical angles considered in this study) at the B3LYP/6-31G* level of theory 28 and 44°at the HF/6-31G* level. 29 An angle of 49°was found by X-ray diffraction of crystalline SA.…”

“…28,29 Most experimental results can be understood in terms of this picture. 9,[15][16][17]24,32 For example, REMPI experiments showed a relaxation of the S 1 state of the enol form within less than a few hundred femtoseconds, 24 which was explained by the two processes taking place in parallel. Similar studies with a higher time resolution but excitation to a higher-lying state claimed the dynamics to occur within 100 fs.…”

“…Similar studies with a higher time resolution but excitation to a higher-lying state claimed the dynamics to occur within 100 fs. 16,17 As both relaxation pathways involve twisting motions of anilino or phenyl rings, it is difficult to experimentally differentiate between the two reaction channels.…”

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

“…The spectrokinetic data sets D 1 -D 5 were artificially constructed in general agreement with the previously published results on SA photodynamics (transient absorption spectra and kinetic profiles) [29,30,33]. They contain 400 spectra of 1340 difference optical density (DOD) values each.…”

“…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.…”