Analysis of the Solvent- and Temperature-Dependent Raman Spectral Changes of S_1 trans-Stilbene and the Mechanism of the trans to cis Isomerization: Dynamic Polarization Model of Vibrational Dephasing and the C=C Double-Bond Rotation

摘要

Solvent- and temperature-dependent band shape changes of the olefinic C=C stretch Raman band of S_1 trans-stilbene have been analyzed on the basis of the dynamic polarization model. The analysis has shown that the solvent-induced dynamic polarization gives rise to the dephasing of the C=C stretch vibration and, concomitantly, triggers and facilitates the rotation around the C=C bond leading to the trans to perpendicular (and eventually to cis) isomerization. Picosecond time-resolved Raman spectra have been measured in the three alkane solvents, hexane, octane, and decane at a number of different temperatures ranging from 268 to 338 K and a total of 40 peak positions and the bandwidths have been determined for the C=C stretch band. The correlation plot of the bandwidth against peak position shows a clear linear relationship that is predicted by the dynamic polarization model. Picosecond time-resolved fluorescence measurements have been performed in the same three alkane solvents at five different temperatures from 283 to 313 K, and 15 rates of the trans to perpendicular isomerization have been determined. the plot of the peak position against the rate of isomerization indicates another linear relationship between these two quantities which have no obvious reason to be correlated with each other. The dynamic polarization model accounts very well for this linear relationship and yields a new formula that relates the rate of isomerization to the frequency of the solvent-induced dynamic polarization. This formula seems to possess certain generality because it shows an excellent numerical agreement with the Arrhenius formula. The new formula derived from the dynamic polarization model gives the molecular-level details of the reaction process as to how the reaction is triggered and in what time scale the reaction actually proceeds. A new view of the photoisomerization of trans-stilbene has thus been obtained.