Temperature dependent kinetics of the picosecond bimolecular reaction between S1 trans-stilbene and carbon tetrachloride: is the ultrafast photochemical reaction diffusion-controlled?

When trans-stilbene (tSB) is photoexcited to the S1 state in carbon tetrachloride (CCl4) solution, tSB reacts with CCl4 in 3 ps at room temperature. The rate constant measured with time-resolved fluorescence spectroscopy changed from 0.28 to 0.43 ps−1, as the temperature was changed from 274 to 314 K. This temperature dependence was explained quantitatively by the Smoluchowski model of diffusion-controlled reactions, with the known temperature dependence of the self-diffusion constant of CCl4. An encounter distance R of 0.46 nm was obtained, which agrees well with the radius of tSB estimated by its molecular volume.