Ultrafast dynamics and photochemistry of π–π* excited trans‐azobenzene—a comprehensive analysis of resonance Raman intensities

Abstract

Azobenzene and its derivatives have widely been recognized as systems with potential applications such as optical switching elements, and the mechanism of trans-cis photoisomerization, which is the basis for all these applications, has long been sought. The long-standing controversy on the relaxation mechanism of p-p * (S 2 ) excited trans-azobenzene (TAB) is addressed in this paper. The role of S 1 -S 2 intersection and the possible involvement of a third state in the S 2 photochemistry are investigated. The excited state structure and dynamics of TAB are extracted by modeling the Raman excitation profiles (REPs) of the S 1 and S 2 states and the absorption spectrum of TAB by Heller's time-dependent formalism. The REPs of S 2 are modeled in the single resonant state approximation by explicitly including the eight most Franck-Condon (FC) active modes. The obtained NN and CN bond lengths of S 2 and the intial dynamics in these coordinates following photoexcitation to S 2 are in agreement with recent theoretical calculations. The structural parameters of S 1 are obtained by exploiting the nature of interference in the REPs of S 1 . From these parameters a three-mode-two-state vibronic coupling model, based on harmonic diabatic potentials and linear coupling, is formulated to study the S 2 -S 1 internal conversion dynamics. The resultsindicate that the S 1 -S 2 intersection is not directly accessible within electronic dephasing time of S 2 to cause significant population decay from S 2 to S 1 . The current conception of p-p * relaxation mechanisms in TAB is reviewed and revised on the basis of our observations and also from recently reported results of time-resolved and theoretical studies. The role of a third state (S 3 ), which appears to be an integral part of in the relaxation mechanism of S 2 , is pointed out and the feasibility of two competing relaxation pathways for S 2 state is identified. One is the direct rotationless S 2 -S 1 (C 2h ) internal conversion and the other is S 2 -S 3 internal conversion along rotational pathway followed by a branching of the S 3 population to S 0 and S 1 .

Manifestation of conical intersections in resonance Raman intensities

“…Multimode damped correlation functions for the FC-active modes of S 2 in TAB are seen to survive for about 30 fs. [53,60] Thus if, indeed, the S 2 -S 1 CI is active within 15 fs, the resulting nonadiabatic dynamics should reflect in the REPs. The absorption spectrum and REPs of S 2 state of TAB are simulated by using a two-statethree-mode model to model the S 2 -S 1 intersection.…”

“…These conclusions are crucial for ascertaining the ππ * state relaxation mechanism, which has been extensively investigated and is yet to be established. [60] It is worth mentioning that the S 2 REPs in TAB could actually distinguish between an intersection of the sort in model I and a remote S 2 -S 1 intersection based on the energy-dependent response of REPs, specific for model I. [53] The details of the modeling of S 1 -S 2 CI in TAB is the subject of our recent work.…”

Editorial

“…These include: a study on polarization-sensitive CARS spectroscopy of free-base porphyrins; [1] a study on the ultrafast bimolecular radical reaction between S 1 p-terphenyl and carbon tetrachloride; [2] a time-resolved resonance Raman and density functional theory study of the deprotonation reaction of the triplet state of para-hydroxybenzophenone in mixed acetonitrile/water solutions; [3] an investigation of femtosecond Raman-induced Kerr effect spectroscopy; [4] a paper on ultrafast vibrational dynamics and solvation complexes of methyl acetate in methanol studied by sub-picosecond infrared spectroscopy; [5] an ultrafast dynamics and photochemistry study of π − π * excited trans-azobenzene; [6] a paper on ultrafast dynamics in Cu(I)bisdiimine complexes from resonance Raman intensities; [7] a three-state model for femtosecond broadband stimulated Raman scattering; [8] a picosecond time-resolved ultraviolet resonance Raman study on photoinduced electron transfer in glucose oxidase; [9] a study on molecular vibrational imaging of a human cell by multiplex coherent anti-Stokes Raman scattering microspectroscopy using a supercontinuum light source; [10] a paper on characteristic wavenumber shifts of the stimulated Raman scattering from interfacial water molecules induced by laser-induced plasma generation at air-water and water-silver interfaces; [11] a study on the identification of chemical species of fluorescein isothiocyanate isomer-I (FITC) monolayers on platinum by doubly resonant sum-frequency generation spectroscopy; [12] a work on coherent acoustic phonons in a thin gold film probed by femtosecond surface plasmon resonance; [13] a review of recent advances in linear and nonlinear Raman spectroscopy; [14] an investigation of the halide-ion-assisted increase of surfaceenhanced hyper-Raman scattering; [15] and finally a study on ultrafast Raman loss spectroscopy [16] . …”

Recent advances in linear and non‐linear Raman spectroscopy. Part III

“…For the poly(aniline) emeraldine base, only a small deprotonation was observed. Chowdary and Umapathy [218] made a comprehensive analysis of Raman intensities from resonance scattering in trans-azobenzene via the π -π * excited electronic state. In particular, the longstanding controversy on the relaxation mechanism of the π -π * (S 2 ) excited trans-azobenzene was addressed in this paper, and the excited state structure and dynamics of trans-azobenzene were extracted by modelling the Raman excitation profiles of the S 1 and S 2 states and the absorption spectrum by a time-dependent formalism.…”