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A dilute solution of benzene was photolyzed with a KrF excimer laser. The Sn←S1 and Tn←T1 absorption spectra are measured with time-resolved spectroscopy. The Sn←S1 absorptions in the energy range between 850 and 220 nm were analyzed. The transient absorption band at 500 nm, which was observed previously by several workers disappeared completely upon dilution of the sample, and thus was assigned to be the excimer band. We assigned the other states as follows: 620 nm (1E1u, f∼0.003), 540–325 nm (11E2g, f∼0.04), and 270 nm (21E2g, f∼0.12). The observed energies from the ground state to the 11E2g and 21E2g states 7.8 and 9.2 eV, respectively. The Tn←T1 spectrum showed a single peak at 235 nm (f∼0.35) and a shoulder around 310 nm (f∼0.12).

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Two components of fluorescence rise due to the photo tautomer generated in the excited-state proton transfer were observed in 3-methylpentane solutions of 3-hydroxyflavone (3-HF) and its derivatives, while only a fast rise was observed in 3hydroxychromone (3-HC) which lacks a 2-phenyl group. The shorter rise times are independent of temperature and substituent at the para position in the 2-phenyl group of 3-HF, whereas the longer rise times are dependent on each of them. As a result, the former and latter were ascribed to excited-state proton transfer in the Franck-Condon state (probably a twisted form of the phenyl group to the -pyrone ring) and to that in the vibrationally and/or torsionally relaxed excited state (a coplanar form of the phenyl group to the pyrone ring), respectively. The transient absorption of these compounds exhibits absorption bands at 430-530 nm with lifetimes of 7-30 ps at room temperature, which were attributable to the ground-state tautomers of these compounds. The two-step laser excitations (TSLE) of these transient absorption bands show TSLE fluorescence spectra being identical with the steady-state fluorescence spectra. Further, variable delay plots of TSLE fluorescence intensity afford lifetimes of the ground-state tautomers. From the temperature dependence of these lifetimes, the activation free-energy changes for the reverse proton transfer in the ground state were determined. Taking into account the reaction constant (p) of Hammett's plots of the activation free-energy change vs. substituent constants ( ), it was concluded that the electron-attracting group at the phenyl para position may assist the reverse proton transfer to the parent molecules and that the phenyl group in 3-HF may act as an electron-donating group to the y-pyrone ring to decrease the reaction rate of the reverse proton transfer in comparison with 3-HC.

The rates for direct cis→trans and trans→cis photoisomerizations of stilbene in solution were measured with a newly developed monitor light source for picosecond laser photolysis in the UV region. The rate constant of the trans→cis isomerization is 1.2×1010 s−1 in hexane at 295 K. The fluorescence decay rate constant of the S1 state of trans-stilbene has the same value. Thus, the decay of the S1 state is the rate limiting step and the lifetime of an intermediate perpendicular state (1p) has to be very short. The cis→trans isomerization is very fast and the half life of the excited cis-isomer is less than a few picosecond. These observations strongly indicate that the isomerization reaction proceeds through the potential surface of the singlet excited states.

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