Harnessing light energy: photoresponsive surfactants for enhanced micellar catalysis

In the context of green chemistry, minimizing waste and energy consumption is crucial in chemical processes. Organic solvents, known for their toxicity and hazards, pose a significant waste issue, prompting the search for non-toxic alternatives like water for organic reactions. Despite the hydrophobic nature of many organic compounds in aqueous environments, self-assembling surfactants enable "micellar catalysis”, overcoming this challenge by hosting reagents in their lipophilic pockets, offering conditions for high-yield, efficient, and sustainable reactions, if appropriately chosen.1 The chemical nature of surfactant-formed interphases significantly influences products distribution in competitive reactions. Within the micelle, an interfacial dipole induces selectivity between the hydrophilic and hydrophobic portions of the surfactant. Introducing selectivity in surfactant-enhanced reactions requires controlling the non-covalent interaction between surfactant and substrates, considering: 1) regioselectivity in competitive transformations induced by the preferential orientation of reaction intermediates due to interface dipoles at the hydrophilic/lipophilic interphase; 2) product selectivity induced by specific surfactant/substrate interactions; and 3) product selectivity due to compartmentalization effects between the water and oil phases. To enhance selectivity, we propose a novel surfactant design with a photochromic compound, specifically a spiropyran. The photochromic residue is incorporated at the boundary between the hydrophilic and the lipophilic portion of the amphiphilic molecule, which consists respectively of a PEG moiety and an alkyl chain. Responsive to UV light, it switches between a neutral-low polarity and a zwitterionic-high polarity state. This tuning ability, rooted in the concept of photostatic equilibrium, determines the relative population of open (polar) and closed (nonpolar) forms of the photochromic molecules.2 Adjusting irradiation conditions in terms of intensity, pulse duration and temperature, tunes the interfacial dipole and the Hydrophilic-Lipophilic Balance (HLB) of the surfactants used. This innovation holds promise for improving reaction selectivity in surfactant-enhanced processes. References: [1] B.H. Lipshutz, S. Ghorai, Green Chem 2014, 16, 3660 – 3679. [2] L. Kortekaas, W.R. Browne, Chem Soc Rev 2019, 48, 3406 – 3424.