Consequences of Heterogeneity of Organic Molecules in Water: Enhanced Photodimerization of Olefins

Photodimerization of organic molecules such as indene and coumarin is dramatically enhanced in water compared to that in an organic solvent. In this study, we have probed the origin of this phenomenon through NMR spectroscopy, dynamic light scattering (DLS) experiments, quantum chemical calculations, and molecular dynamic simulations. Indene molecules are inferred to exist as an equilibrium mixture of monomers, noncovalent dimers, and small (NMR-detectable) and large (DLS-detectable) aggregates in water. This behavior is distinctly different from that observed in an organic solvent, where indene molecules remain homogeneously distributed as monomers. The enhancement of thermal bimolecular reactions such as the Diels-Alder reaction is analyzed in terms of "in-water", "on-water", and "on the surface of microdroplets". The inhomogeneous distribution of small organic molecules identified in this study could be a reason for their enhanced photodimerization in water. The presence of small aggregates, detectable by their unusually sharp 1H NMR signals, rules out the need for diffusion, which is often slower than the decay rates of excited molecules. The results presented here demonstrate that the knowledge of reactions in organic solvents cannot be directly extended to those occurring in water.