Fiscal 1997

(1) "Picosecond energy dissipation of photoexcited molecules in solution : how a solute interacts with solvents"

Energy dissipation from excited states plays a crucial role in chemical reactions in solution. With the help of energy dissipation, the newly generated "hot" product molecule can release excess energy to reach the stable final state; otherwise it might go back to the energetically equivalent reaction complex or even to the reactant. We studied the process of excess-energy dissipation from photoexcited trans-stilbene in a number of different solvents by using picosecond time-resolved Raman spectroscopy. The 1570 cm^-1 band of S₁ trans-stilbene (C=C stretch) was found to be useful as an accurate temperature marker. The cooling-down kinetics in the time range of 0 to 70 ps was analyzed in terms of a diffusion equation of heat. The analysis led to a molecular model of the solute-solvent energy dissipation: the excess energy given to the solute by photoexcitation is shared with solvent molecules in the first solvation sphere in less than 3 ps. It is then dissipated to the bulk solvent in time-scales of 10-15 ps depending on the thermal diffusivity of the solvent (Fig. 1). The number of solvent molecules in the first solvation sphere is estimated to be five (V-V, V-R, V-T processes considered) to twelve (only V-R and V-T processes considered), from the temperature rise just after the photoexcitation.

Fig. 1. Solvent-solute interaction and picosecond excess-energy dissipation.

(2) "New photoisomerization of diphenylacetylene as revealed by picosecond 2-D multiplex CARS spectroscopy"

Interaction among the closely-lying excited singlet states of diphenylacetylene has been a subject of considerable interest both experimentally and theoretically, as a model for strong electronic interactions. We used our picosecond 2-D CARS method to obtain structural information on these excited states. CARS spectra of both the S₂ and S₁ states were obtained with high S/N ratios. The central carbon-carbon stretch frequency of the S₂ state was found to be 2009 cm^-1. This frequency is indicative of a linear structure having a C≡C triple bond. No band was observed in the C≡C frequency region of the S₁ CARS spectra. Isotopic substitution experiments and a two-mode normal coordinate analysis finally showed that the central carbon-carbon stretch frequency of the S₁ state is ∼1570 cm^-1. This value means that the central carbon-carbon bond of diphenylacetylene is a double bond (most probably in a bent form) rather than a triple bond. Diphenyl acetylene in the S₁ state is not an acetylene. The S₂ to S₁ internal conversion of diphenylacetylene is known to have an activation energy of 1200 cm^-1, which is unusual for a purely intramolecular relaxation process. It is most likely that the S₂ to S₁ internal conversion is a linear (S₂) to bent (S₁) isomerization that is accompanied by a large structural change (Fig. 2). This proposition is consistent with an empirical MO calculation and explains many curious photophysics of diphenyl acetylene reported previously.

Fig. 2. Photoinduced dynamics of diphenylacetylene and new isomerization reaction