Consequences of Hydrophobic Association in Photoreactions: Photodimerization of Stilbenes in Water

Hydrophobic interactions are of considerable importance in maintaining the structure of biological membranes, proteins, and nucleic acids. The same interaction is also responsible for the association of organic solutes in water, a well-substantiated phenomenon.¹ Such an association could play a significant role during cycloaddition reactions of organic molecules in aqueous medium. Indeed, Diels-Alder reactions are strikingly accelerated when water is the solvent.² Furthermore, significant selectivity in the product distribution (endo, exo) also resulted.³ Although the potential role of “hydrophobic effect” in thermal reactions has attracted considerable attention in the last few years, curiously, however, the use of such an effect has not been reported for any typical photodimerization reactions of small molecules in aqueous solution except for cases which are fairly well soluble in water. The role of aggregates in concentrated aqueous solution during the photodimerization of thymine, uracil, and their derivatives has been demonstrated.⁴ Our attention was drawn to the fact that many organic molecules tend to aggregate at concentrations even lower than 10^-5 M. Clearly, the application of such a phenomenon could be of considerable interest to photochemists. Although the dimerization of stilbene has been known⁵ since the beginning of this century it has eluded extensive investigation probably due to its poor efficiency. Typically, frans-stilbene in benzene (0.75 M) yields only 27% of dimer after two months of irradiation.⁶ We report below that efficient dimerization can be obtained within 24 h of irradiation of stilbene at concentrations as low as ~10-6 M in water. This remarkable acceleration of the efficiency of dimerization is demonstrated to be due to the association of stilbene molecules in water.