Micellar Effects and their Relevance in Photochemistry and Photocatalysis

Abstract

The encapsulation of substrates within micellar structures gives rise to a multitude of effects beyond simple solubilization in water. When incorporated in photochemical reaction design micellar media may offer possibilities to control both reactivity and selectivity. Herein, we describe various important micellar phenomena such as encapsulation, polarity differences, electro-static interactions and hydrogen bonding, and discuss the various resulting mechanisms by which reaction outcomes can be greatly influenced. In this concept article, we showcase selected photochemical reactions to demonstrate how micellar effects can be selectively applied in reaction design.

Facilitating [2+2] Photocycloadditions by Promoting Oxygen Tolerance and Substrate Activation in Water

“…In micellar systems, a wide range of phenomena can influence excited state properties, for example, electrostatic interactions, hydrogen bonding, a more rigid environment, etc. These micellar phenomena sometimes result in longer excited state lifetimes, [26] which may also partially explain the increased product formation in micellar media compared to MeOH. Our results showcase the micellar system as a more versatile system that might help substrates towards more efficient energy transfer catalysis.…”

“…We propose that the reaction takes place in the charged interphase as opposed to the core of the micelle. Furthermore, hydrogen bonding has been proposed to be present in the micellar media [26] and an activation of the substrate to lower the triplet energy and allow for the use of lower triplet energy catalysts could be present.…”

“…However, it has been shown, especially in triplet‐triplet annihilation‐upconversion (TTA‐UC) that oxygen sensitive processes, like TTA‐UC [25] can occur efficiently in aqueous media with supramolecular assemblies in the presence of oxygen (Figure 1). [26] This was proposed to be the result of two possible effects; the low solubility of oxygen in water in comparison to organic solvents [27] and a localization of oxygen in the micellar core [28] depending on the system. Noteworthy, the sensitized [2+2] photocycloaddition of cinnamate esters in water was shown to be highly dependent on the removal of oxygen [29] .…”

A survey of the iron ligand-to-metal charge transfer chemistry in water

“…[95][96][97] Efforts were made to overcome the solubility problems of organic starting materials by adding phase transfer catalysts or performing reactions in micellar systems. 98,99 For the research field of ligand-to-metal charge transfer catalysis the hydrolysis of the metal is the main challenge as hydroxy radicals that result from the competitive bond homolysis of a metal-OH 2 bond can hardly be controlled in their reactivity and therefore stable iron complexes are promising candidates for selective LMCT reactions in water. 21,37,100 Additionally, performing photoreactions in water reduces the use of harmful chemicals as many organic solvents show medium to high toxicity.…”

A micelle-mediated approach enables facile access to bridged oxabicyclo[n.3.1]alkene scaffolds

“…Over the years, micellar catalysis has witnessed a range of chemical transformations, particularly the metal-catalyzed and now the bio-catalyzed reactions. 12 Though the area still needs to be fully explored, some research groups, 13 especially the Lipshutz group, 14 have made significant contributions in this direction. In our group, we also explore novel micelle-guided strategies that steer towards functionalized natural product scaffolds.…”