Abstract
Donor-acceptor Stenhouse adducts (DASAs) are a novel class of solvatochromic photoswitches with increasing importance in photochemistry. Known for their reversibility between open triene and closed cyclized states, these push-pull molecules are applicable in a suite of light-controlled applications. Recent works have sought to understand the DASA photoswitching mechanism and reactive state, as DASAs are vulnerable to irreversible “dark switching” in polar protic solvents. Despite the utility of fluorescence spectroscopy for providing information regarding the electronic structure of organic compounds and gaining mechanistic insight, there have been few studies of DASA fluorescence. Herein, we characterize various photophysical properties of two common DASAs based on Meldrum’s acid and dimethylbarbituric acid by fluorescence spectroscopy. This approach is applied in tandem with complexation by cyclodextrins and cucurbiturils to reveal the zwitterionic charge separation of these photoswitches in aqueous solution and the protective nature of supramolecular complexation against degradative dark switching. DASA-M, for example, was found to form a weak host-guest inclusion complex with (2-hydroxypropyl)-γ-cyclodextrin, with a binding constant K = 60 M−1, but a very strong inclusion complex with cucurbit[7]uril, with K = 27,000 M−1. This complexation within the host cavity was found to increase the half-life of both DASAs in aqueous solution, indicating the significant and potentially useful stabilization of these DASAs by host encapsulation.
Highlights
The term “photoswitch” refers to a broad palette of compounds capable of undergoing some reversible change in structure, conformation, charge, etc. in response to the absorption of light.This reversibility allows for the application of photoswitches as binary state switches, analogous to turning a light switch on or off [1]
A three-step “fast dilution” method was developed: (i) an aliquot of an aqueous Donor-acceptor Stenhouse adducts (DASAs) stock solution was used to dissolve a certain mass of the host molecule, giving a stock containing the DASA and a known concentration of host; (ii) various host concentrations were quickly achieved by mixing precise volumes of the host-absent and host-containing DASA stocks; and (iii) the fluorescence emission of each sample was measured over a 20 nm range centered about λF,max
This work successfully lays a foundation for using fluorescence spectroscopy in the study of these interesting molecules
Summary
The term “photoswitch” refers to a broad palette of compounds capable of undergoing some reversible change in structure, conformation, charge, etc. in response to the absorption of light. The ability to convert molecules between forms using light presents a wide range of applications ranging from photocontrol of enzyme activity [3], to tissue-specific drug release in vivo [4], to super-resolution fluorescence microscopy techniques [5,6]. Donor-acceptor Stenhouse adducts (DASAs) are molecular photoswitches that can be unidirectionally controlled using visible light This class of push-pull olefins consists of a triene skeleton with opposing. This class of push-pull olefins consists of a triene electron-donating and electron-withdrawing groups (EDG and EWG, respectively).
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