Evidence for the phantom state in photoinduced cis–trans isomerization of stilbene

doi:10.1016/j.cplett.2010.05.022

Chemical Physics Letters

Volume 493, Issues 4–6, 25 June 2010, Pages 255-258

https://doi.org/10.1016/j.cplett.2010.05.022 Get rights and content

Abstract

Transient absorption spectra of cis-stilbene in solution, recorded in the range 275–690 nm with 80 fs instrument response, reveal two successive evolving bands. One at 630 nm decays on a subpicosecond scale; it corresponds to vertically-excited nearly planar S₁ state. The other, growing and decaying band at 340 nm, is shown to be consistent with theoretically predicted phantom state p∗ of perpendicular molecular geometry. The lifetime of p∗ is determined to be $τ_{p^{*}} = 0.4 ps$ in acetonitrile and 0.3 ps in hexane.

Graphical abstract

Research highlights

► The intermediate phantom state is revealed in the course of cis-trans isomerization of stilbene, with its lifetime to be 0.4 ps in acetonitrile and 0.3 ps in hexane.

Introduction

Light-induced isomerization of stilbene [1], [2], [3], representing an important class of chemical reactions, has long been experimentally studied both with stationary [4] and ultrafast spectroscopies [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. A commonly accepted view [1], [2], [3], [4] of the excited-state evolution is sketched by Fig. 1. Upon optical excitation, trans (t∗) or cis (c∗) species isomerize via a common intermediate p∗ of perpendicular molecular geometry, when one of the phenyl rings is rotated by 90° about the central double bond. Subsequent internal conversion p∗ → p completes the reaction with one-to-one branching of ground-state t- and c-isomer. When starting from the trans side, the isomerization is relatively slow, $τ_{t^{*}} \sim 100 ps$ [5], [6], because of a barrier of 1200 cm⁻¹[4], [5], [6] between t∗ and p∗, while it is much faster for the cis species, $τ_{c^{*}} \sim 1 ps$ [5], [7], [8], [9], [10], [11], [12], [13], presumably due to a lower, <500 cm⁻¹[5], [12], [13], [14], [15], barrier between c∗ and p∗. The intermediate p∗ was called the phantom state [2] because of difficulties with its detection, as it is dark in emission and shortly lived. It was shown that in solution the lifetime $τ_{p^{*}}$ should be in the subpicosecond range [9]. Recently an indication of the p∗ state, with $τ_{p^{*}} = 0.16 ps$ , has been reported for gas-phase c∗ → p reaction [16]. The aim of this Letter is to present a detailed picture of isomerization of stilbene in solution and to provide convincing evidence for the p∗ state.

Section snippets

Experiment

Our broadband transient absorption setup has been described elsewhere [17], [18]. A Ti:Sa amplifier delivers 30 fs, 500 μJ, 920 Hz pulses at 800 nm. The beam is split in two parts. Four hundred and twenty microjoules are used to pump an optical parametric amplifier which provides 50 fs, 2 μJ excitation pulses at 318 nm. The remaining 80 μJ are converted into 400 nm, 15 μJ pulses to generate a probe supercontinuum in a 1 mm CaF₂ plate. The probe light is spectrally filtered and split for the signal and

trans-Stilbene

We start with t-stilbene evolution that interpretation is straightforward. Transient absorption spectra ΔA(λ, t) of t-stilbene in acetonitrile are shown in Fig. 2. Negative bands at 300 and 350 nm correspond to bleach and stimulated emission (SE), while a positive peak at 575 nm is due to excited-state absorption (ESA). It is clear that the SE and ESA bands belong to the t∗-minimum on S₁ (Fig. 1) as the both bands decay with the same time constant $τ_{t^{*}} = 36 ps$ [19]. This decay corresponds to the t∗ → p∗

Discussion

Previous ultrafast studies on c-stilbene reported a monoexponential decay of the c∗ band, with $τ_{c^{*}} = 0.38 ps$ in acetonitrile and 1.05 ps in hexane [5], [9], [10], [11], [12], [13]. Our recent broadband fluorescence measurements [24] show the monoexponential decay as well although with faster time constants, 0.21 and 0.75 ps, respectively. A monoexponential fit of c∗ corresponds to k₂ = 0, and such a fit is also possible with the present transient absorption data, resulting accordingly in 0.26 and 0.92

Acknowledgments

We thank the Deutsche Forschungsgemeinschaft and SFB-450 for financial support.

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Evidence for the phantom state in photoinduced cis–trans isomerization of stilbene

Abstract

Graphical abstract

Research highlights

Introduction

Section snippets

Experiment

trans-Stilbene

Discussion

Acknowledgments

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J. Photochem. Photobiol. A

Chem. Phys. Lett.

J. Photochem. Photobiol. A

Chem. Phys. Lett.

Chem. Phys. Lett.

Chem. Rev.

J. Am. Chem. Soc.

J. Chem. Phys.

J. Phys. Chem.

J. Chem. Phys.

J. Chem. Phys.

J. Chem. Phys.

J. Chem. Phys.

J. Phys. Chem.