Ultrafast excited-state dynamics in photochromic N-salicylideneaniline studied by femtosecond time-resolved REMPI spectroscopy

TLDR: Investigation of ultrafast processes in photoexcited N-salicylideneaniline with femtosecond time-resolved resonance-enhanced multiphoton ionization spectroscopy reveals that an internal conversion to the S(1)(n,pi( *)) state of the enol form may provide opposite effect on the quantum yield of photochromic products.

Abstract: Ultrafast processes in photoexcited N-salicylideneaniline have been investigated with femtosecond time-resolved resonance-enhanced multiphoton ionization spectroscopy. The ion signals via the S1(n,π*) state of the enol form as well as the proton-transferred cis-keto form emerge within a few hundred femtoseconds after photoexcitation to the first S1(π,π*) state of the enol form. This reveals that two ultrafast processes, excited-state intramolecular proton transfer (ESIPT) reaction and an internal conversion (IC) to the S1(n,π*) state, occur on a time scale less than a few hundred femtoseconds from the S1(π,π*) state of the enol form. The rise time of the transient corresponding to the production of the proton-transferred cis-keto form is within 750 fs when near the red edge of the absorption is excited, indicating that the ESIPT reaction occurs within 750 fs. The decay time of the S1(π,π*) state of the cis-keto form is 8.9 ps by exciting the enol form at 370 nm, but it dramatically decreases to be 1.5–1.6...

Figures

FIG. 3. Energy diagram and ionization scheme for the enol andcis-keto forms of SA. The probe wavelength of;400 nm is resonant with theSn-Si transition of thecis-keto form. Resonant wavelength for theSn-S1 transition of the enol form may be longer than 700 nm.

FIG. 4. Decay profiles of SA excited at 320 nm and probed at~a! 790 nm and ~b! 395 nm. The solid circles are experimental data, while the so curves are the best-fitted curves obtained by two or three decay compon The dotted curves represent the components used to decompose the profiles.

FIG. 7. Photoexcited processes of S drawn on the basis of the femtosecon time-resolved REMPI spectroscopy i gas phase. Two ultrafast processes E IPT and IC occur by photoexcitation of a high vibronic state of the eno form. When the pump wavelength i shrorter than;365 nm, an IC occurs from the1(p,p* ) to the1(n,p* ) state of the proton-transferredcis-keto form in addition to the radiative (hnF) and nonradiative decay to theS0 state. The IC efficiently produces thetrans-keto form as the final photochromic prod ucts. Thetrans-keto form may be produced as a minor channel when th enol form is excited at wavelength longer than 370 nm. TS indicates transition state.

FIG. 1. Stable structures and photochromic reaction of SA.

TABLE I. Best-fit time constants of SA in units of ps obtained from fem tosecond time-resolved measurement.t1 andt2 are time constants obtaine from the decay profiles ofS1(p,p* ) andS1(n,p* ) of the enol form, while t3 andt4 are time constants obtained from the decay profiles ofS1(p,p* ) of the keto form.

FIG. 6. ~a! Electronic absorption spectrum of SA inn-hexane solution.~b! Decay profiles of SA measured by varying the pump wavelength from to 373 nm, while probing at 395 nm. The pump wavelength is given on right-hand side of each time profile. The solid circles are experimental d while the solid curves are the best-fitted curves obtained by biexpone decay components.