The rate constants of the nonradiative decay process of the excited singlet state of trans‐stilbene and trans‐1,1′‐biindanylidene (‘‘stiff ’’ stilbene) were measured in liquid n‐alkanes at room temperature These data were compared with the two models for isomerization in condensed phases proposed by Kramers and by Grote and Hynes For trans‐stilbene, the agreement with Kramers’ model was unsatisfactory, whereas better agreement was obtained with Grote and Hynes’ model The data for stiff stilbene agreed well with Kramers’ model in the Smoluchowski limit These results were explained in terms of the shape of the potential energy barrier which separates the trans from the twisted configuration of the excited singlet state of these molecules It is suggested that the deviation between theory and experiment is a result of a qualitative difference in the microscopic solvent–solute collisional interaction when the solvent molecules are longer than stilbene The best fit of theory to experiment for trans‐stilbe

Solvent influence on photoisomerization dynamicsa)

Femtosecond laser studies have been performed on the photoisomerization reactions of cis‐stilbene to obtain the most detailed understanding to date of a polyatomic isomerization reaction in a condensed phase environment. These experiments demonstrate that vibrationally hot product molecules are formed within a few hundred femtoseconds of the escape of the molecule from the cis* region of the potential energy surface. Although the cis to trans reaction may proceed via a twisted intermediate structure, this intermediate is not intercepted on the ∼150 fs time scale. The frictional effects on the cis to trans reaction coordinate are found to be important and account for the anisotropy of the trans product molecules. Specific experiments presented in detail are the absorption spectrum of electronically excited cis molecules (cis*); the anisotropy decays for cis* showing motion along the reaction coordinate; the detection of the trans‐stilbene product using transient fluorescence and transient absorption, confi...

Femtosecond laser studies of the cis‐stilbene photoisomerization reactions

Femtosecond spectroscopy of excited-state proton transfer in 2-(2′-hydroxyphenyl)benzothiazole

In this and the following paper, we present a full account of our earlier report [Syage et al., Chem. Phys. Lett. 88, 268 (1982)] on the spectra and picosecond dynamics of stilbene isomerization in supersonic jets. The jet-cooled excitation and dispersed fluorescence spectra of t-stilbene-h12 and -d12 are reported and assigned for the Bu 0 0 wavelengths for h12 and d12 (in excitation) are 3101.4 and 3092.5 A, respectively. Previously unidentified low frequency modes (as low as 20 cm^−1 in S0 for -h12) have been observed and tentatively assigned as out-of-plane modes of au symmetry in C2h. This indicates that t-stilbene has a propeller-like geometry involving phenyl rotation (i.e., C2 symmetry). A Franck–Condon analysis of the low frequency modes and particularly the ag, nu25 in-plane symmetric bend mode indicates that a large geometry change takes place upon electronic excitation possibly due to a delocalization of double bond character from the Ce–Ce bond to Ce–[cursive phi] bond. The geometry change of the in-plane Ce–Ce–[cursive phi] between S1 and S0 was determined from the Franck–Condon and a normal mode analysis to be 1.3°±0.3°. The rms amplitude of this bend motion for the symmetric nu25 bend mode (for one quanta in S0) is | ^2|^1/2=1.0±0.2°. Most ag modes involving benzene-type vibrations (other than C–H stretch modes) have been assigned. Dispersed fluorescence spectra exhibited a broad background indicative of IVR which increased rapidly with S1 vibrational energy. The spectra were completely diffuse above 1200 cm^−1 which is consistent with the barrier for isomerization being at about 1100–1200 cm^−1. The excitation spectra show a rapid decline in intensity at higher energies due to the process of isomerization which competes with radiative decay. However, sharp (albeit weak) structure could still be discerned at energies well in excess of 2000 cm^−1. In the accompanying paper, we present results on the dynamics of isomerization and its dependence on mode mixing and the nature of the reactive surface (adiabatic vs diabatic).

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Picosecond dynamics and photoisomerization of stilbene in supersonic beams. I. Spectra and mode assignments

▪ Abstract This review focuses on the study of the dynamics of isolated molecules and their control using coherent nonlinear spectroscopic methods. Emphasis is placed on topics such as bound-to-free excitation and the study of concerted elimination reactions, free-to-bound excitation and the study of bimolecular reactions, and bound-to-bound excitation and the study of intramolecular rovibrational dynamics and coherence relaxation. For each case the detailed time-resolved information reveals possible strategies to control the outcome. Experimental results are shown for each of the reactions discussed. The methods discussed include pump-probe and four-wave mixing processes such as transient grating and photon echo spectroscopy. Off-resonance transient-grating experiments are shown to be ideal for the study of ground state dynamics, molecular structure, and the molecular response to strong field excitation.

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Coherent nonlinear spectroscopy: from femtosecond dynamics to control.

Femtosecond-resolved ground-state recovery of cis-stilbene in solution

Excitation energy effects in the photophysics of trans-stilbene in solution

Picosecond and subpicosecond spectroscopic studies have revealed evidence for an isomerization intermediate between cis and trans in the photoinduced isomerism of both stilbene and biindanyledene ("stiff" stilbene). In stiff stilbene, a transient absorption at 351 nm displays time evolution and viscosity dependence consistent with absorption by a twisted intermediate ("phantom" state) with a lOps lifetime. An analagous bottleneck state with a life-time of 4ps is also consistent with the ground state recovery dynamics of t-stilbene following excitation of c-stilbene when monitored with 0.1ps resolution.

Isomerization Intermediates In Solution Phase Photochemistry Of Stilbenes

It is well known that substituted ethylene can undergo cis-trans isomerism following optical excitation. A large literature already exists on this topic and the particular case of stilbenes (1,2-diphenylethylenes) was recently reviewed by Saltiel and coworkers [1]. Phenyl substituted ethylenes have received considerable attention from photochemists because their optical spectra are in the near ultraviolet, readily accessible to traditional light sources. Although the ground work for understanding the photoisomerism process in solutions was laid on the basis of conventional (pre-laser) experiments [1,2] the Intrinsic molecular steps Involved are now known from vapor phase studies to be subnanosecond [3,4]. The basic model for the isomerism is that a twisted, less conjugated form of stilbene is formed following optical excitation [5]. This twisted form may then undergo relaxation processes that result in both cis and trans structures [6]. The effect of the solvent is to slow down the isomerization and at sufficiently low temperature or high viscosity the rate is found to be even slower than radiative processes [7]. The fluorescent properties of the system therefore can be used to monitor the isomerization. Saltiel and coworkers [8,9] have shown that the principal photochemistry of stilbene arises from the singlet excited surface although the triplet state can be involved when sensitizers are present [9,10]. There is also evidence that isomerization is not the exclusive photoprocess since small yields of rearranged species have been reported [11], but such processes will not be considered further in this article.

F. E. Doany

Papers

Femtosecond-resolved ground-state recovery of cis-stilbene in solution

Excitation energy effects in the photophysics of trans-stilbene in solution

Isomerization Intermediates In Solution Phase Photochemistry Of Stilbenes

Photoinduced Isomerism of Stilbenes: Picosecond Timescale Results