Transient Absorption Spectroscopy of the Stilbenes in Methanol─Adiabatic cis→ trans Photoisomerization by Sequential Two-Photon Excitation.

Femtosecond pulse excitation gave the time evolution of the transient absorption spectra (TAS) of trans- and cis-stilbene (t- and c-St) in methanol. Global analysis of t-St TAS gives the spectrum of the singlet excited state of t-St, 1t*, with a lifetime, 46 ps, consistent with previous measurements. The spectrum of the singlet excited state of c-St, 1c*, has bands at 360 and 630 nm in agreement with early literature spectra in other solvents. However, the 1c* spectrum obtained on multiexponential global analysis of c-St TAS is much more intense in the UV relative to the visible in disagreement with the observed TAS. Although the lifetime of the 630 nm band, 0.47 ps, agrees with the reported lifetimes, monoexponential fits in the UV region yield lifetimes that are significantly longer. We attribute this discrepancy to interference by the twisted phantom singlet, 1p*, whose growth and decay occur in the same UV region. The 360 nm band rises together with a weak 405 nm band that decays faster and may be the Franck-Condon 1c* transient or a 1c* transient on the way to dihydrophenanthrene formation. Also observed on exciting c-St is the TAS of 1t*. A mechanism involving two-photon sequential excitation of c-St accounts for this result.