Photophysical Properties of Donor-Acceptor Stenhouse Adducts and Their Inclusion Complexes with Cyclodextrins and Cucurbit[7]uril

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.

Donor–Acceptor Stenhouse Adduct Displaying Reversible Photoswitching in Water and Neuronal Activity

“…To find out whether HP-β-CD stabilizes linear DASAbarbital in an aqueous environment following a similar mechanism to that observed with supramolecular vessels, 65,66 we examined the absorption spectra of the different formulations. The solvatochromism observed in the normalized UV−vis spectra of the molecule confirms that HP-β-CD immerses DASA-barbital in a hydrophobic environment (Figure 2g).…”

Promoting the Furan Ring‐Opening Reaction to Access New Donor–Acceptor Stenhouse Adducts with Hexafluoroisopropanol

“…However, we cannot completely exclude that trace amounts of highly fluorescent, free 2,2'dinaphthylamine is present, which could complicate the fluorescence measurements.D ASAs in general are weakly fluorescent when excited at their maximum absorbance wavelength (p-p*t ransition of open form), so that prior examples of strongly fluorescent DASAs have been limited to systems that are based on the attachment of separate fluorophores. [47][48][49][50] Introducing easily accessible fluorescent donors could thus enable further development of multifunctional materials and easily identifiable DASA-functionalized materials. a) The use of time-dependentUV/Vis spectroscopy to observe the photochromism of DASA-5 and DASA-12 in chloroform at 10 mm (initial absorbance: 0.9 and 0.1) followed at l max (645 nm and 646 nm).…”

Donor–Acceptor Stenhouse Adducts: Exploring the Effects of Ionic Character

“…In this study we investigated the medium effects on the charge separation of DASAs through as eries of X-ray diffraction (XRD) analyses, solution-state studies, and time-dependent pump-probe UV/Vis spectroscopica nalyses. This work builds on four important independentp reliminarys tudies by Beves, [15,21] Jacquemin, [23] Feringa, [24] and Wagner [25] that showedt he zwitterionic character of the openf orm of DASAs. Using XRD data, Beves and co-workers reported open linear zwitterionic forms for first-and second-generation DASAs, concluding that the polyene systemsi ns econd-generation DASAs are significantly delocalized.…”