Donor-Acceptor Stenhouse Adducts: Exploring the Effects of Ionic Character.

The effects of solution-state dielectric and intermolecular interactions on the degree of charge separation provide a route to understanding the switching properties and concentration dependence of donor-acceptor Stenhouse adducts (DASAs). Through solvatochromic analysis of the open-form DASA in conjunction with X-ray diffraction and computational theory, we have analyzed the ionic character of a series of DASAs. First- and third-generation architectures lead to a higher zwitterionic resonance contribution of the open form and a zwitterionic closed form, whereas the second-generation architecture possesses a less charge-separated open form and neutral closed form. This can be correlated with equilibrium control and photoswitching solvent compatibility. As a result of the high contribution of the zwitterionic resonance forms of first- and third-generation DASAs, we were able to control their switching kinetics by means of ion concentration, whereas second-generation DASAs were less affected. Importantly, these results show how the previously reported concentration dependence of DASAs is not universal, and that DASAs with a more hybrid structure in the open form can achieve photoswitching at high concentrations.

Influence of Polarity Change and Photophysical Effects on Photosurfactant-Driven Wetting.

Photosurfactants have shown considerable promise for enabling stimuli-responsive control of the properties and motion of fluid interfaces. Recently, a number of photoswitch chemistries have emerged to tailor the photoresponsive properties of photosurfactants. However, systematic studies investigating how photoresponsive surfactant behavior depends on the photochemical and photophysical properties of the switch remain scarce. In this work, we develop synthetic schemes and surfactant designs to produce a well-controlled library of photosurfactants to comparatively assess the behavior of photoswitch chemistry on interfacial behavior. We employ photoinduced spreading of droplets at fluid interfaces as a model for such studies. We show that although photosurfactant response is largely guided by expected trends with changes in polarity of the photoswitch, interfacial behavior also depends nontrivially and sometimes counter-intuitively on the kinetics and mechanisms of photoswitching, particularly at the interface of two solvents, as well as on complex interactions with other surfactants. Understanding these complexities enables the design of new photosurfactant systems and their optimization toward responsive functions including triggered spreading, dewetting, and destabilization of droplets on solid and fluid surfaces.

Promoting the Furan Ring-Opening Reaction to Access New Donor-Acceptor Stenhouse Adducts with Hexafluoroisopropanol.

Donor-acceptor Stenhouse adducts (DASAs) are visible-light-responsive photoswitches with a variety of emerging applications in photoresponsive materials. Their two-step modular synthesis, centered on the nucleophilic ring opening of an activated furan, makes DASAs readily accessible. However, the use of less reactive donors or acceptors renders the process slow and low yielding, which has limited their development. We demonstrate here that 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) promotes the ring-opening reaction and stabilizes the open isomer, allowing greatly reduced reaction times and increased yields for known derivatives. In addition, it provides access to previously unattainable DASA-based photoswitches and DASA-polymer conjugates. The role of HFIP and the photochromic properties of a set of new DASAs is probed using a combination of 1 H NMR and UV/Vis spectroscopy. The use of sterically hindered, electron-poor amines enabled the dark equilibrium to be decoupled from closed-isomer half-lives for the first time.

Unusual concentration dependence of the photoisomerization reaction in donor-acceptor Stenhouse adducts.

Donor-acceptor Stenhouse adducts comprise a new class of reversible photochromic molecules that absorb in the visible and near-infrared spectral regions. Unimolecular photoisomerization reactions are usually assumed to be insensitive to photochrome density, at least up to millimolar concentrations. In this paper, the photoisomerization kinetics of a third-generation donor-acceptor Stenhouse adduct molecule (denoted DASA) are examined over a range of concentrations. DASA switches efficiently at micromolar concentrations in both liquid solution and in polymers, but as the photochrome concentration is increased there is a dramatic inhibition of the photoisomerization. A kinetic study of both the reactant and photoproduct decays at varying concentrations and in different hosts indicates that the forward photoisomerization and the thermal backward reaction can change by factors of 20 or more depending on DASA concentration. Femtosecond transient absorption experiments show that the initial cis → trans step of the isomerization is not affected by concentration. It is hypothesized that long-range coulombic interactions interfere with the ground state electrocyclization stage of the isomerization, which is unique to the DASA family of photochromes. The physical origin of the inhibition of photoswitching at high photochrome concentrations must be understood if the DASA class of molecules is to be used for applications that require high photochrome concentrations, including photomechanical actuation.

Self-regulating photochemical Rayleigh-Bénard convection using a highly-absorbing organic photoswitch.

We identify unique features of a highly-absorbing negatively photochromic molecular switch, donor acceptor Stenhouse adduct (DASA), that enable its use for self-regulating light-activated control of fluid flow. Leveraging features of DASA's chemical properties and solvent-dependent reaction kinetics, we demonstrate its use for photo-controlled Rayleigh-Bénard convection to generate dynamic, self-regulating flows with unparalleled fluid velocities (~mm s-1) simply by illuminating the fluid with visible light. The exceptional absorbance of DASAs in solution, uniquely controllable reaction kinetics and resulting spatially-confined photothermal flows demonstrate the ways in which photoswitches present exciting opportunities for their use in optofluidics applications requiring tunable flow behavior.

Tunable Photothermal Actuation Enabled by Photoswitching of Donor-Acceptor Stenhouse Adducts.

We report a visible light-responsive bilayer actuator driven by the photothermal properties of a unique molecular photoswitch: donor-acceptor Stenhouse adduct (DASA). We demonstrate a synthetic platform to chemically conjugate DASA to a load-bearing poly(hexyl methacrylate) (PHMA) matrix via Diels-Alder click chemistry that enables access to stimuli-responsive materials on scale. By taking advantage of the negative photochromism and switching kinetics of DASA, we can tune the thermal expansion and actuation performance of DASA-PHMA under constant light intensity. This extends the capabilities of currently available responsive soft actuators for which mechanical response is determined exclusively by light intensity and enables the use of abundant broadband light sources to trigger tunable responses. We demonstrate actuation performance using a visible light-powered cantilever capable of lifting weight against gravity as well as a simple crawler. These results add a new strategy to the toolbox of tunable photothermal actuation by using the molecular photoswitch DASA.

Direct introduction of nitrogen and oxygen functionality with spatial control using copper catalysis.

Synthetic chemists have spent considerable effort optimizing the synthesis of nitrogen and oxygen containing compounds through a number of methods; however, direct introduction of N- and O-functionality remains challenging. Presented herein is a general method to allow for the simultaneous installation of N- and O-functionality to construct unexplored N-O heterocyclic and amino-alcohol scaffolds. This transformation uses earth abundant copper salts to facilitate the formation of a carbon-centered radical and subsequent carbon-nitrogen bond formation. The intermediate aminoxyl radical is terminated by an intramolecularly appended carbon-centered radical. We have exploited this methodology to also access amino-alcohols with a range of aliphatic and aromatic linkers.