A novel library of tunable negative photochromic compounds, donor-acceptor Stenhouse adducts (DASAs), is reported. Tailoring the electron deficient "acceptor" moiety yielded DASAs that can be activated with mild visible and far red light. The effect of acceptor composition on reactivity, absorption, equilibrium, and cyclability is exploited for the design of high performance photoswitches. The structural changes to the carbon acid acceptor also provide access to new, more structurally diverse DASA derivatives by facilitating the ring-opening reaction with electron deficient amine donors.

The effects of solution-state dielectric and intermolecular interactions on the degree of charges eparation provide ar oute to understanding the switchingp roperties and concentration dependence of donor-acceptorS tenhouse adducts (DASAs). Through solvatochromic analysis of the open-form DASA in conjunction with X-ray diffraction and computational theory,w eh ave analyzed the ionic character of as eries of DASAs. First-and third-generation architectures lead to ah igher zwitterionic resonance contribution of the open form and az witterionic closed form, whereas the second-generation architecture possesses al ess charge-separatedo penf orm and neutral closed form. This can be correlated with equilibrium controla nd photoswitching solvent compatibility. As ar esult of the high contribution of the zwitterionic resonance forms of first-and third-generation DASAs, we were ablet oc ontrol their switching kinetics by meanso fi on concentration,w hereas second-generation DASAs were less affected. Importantly,t hese results show how the previously reported concentration dependenceo f DASAs is not universal,a nd that DASAs with am ore hybrid structure in the open form can achievep hotoswitching at high concentrations.

We investigate the influence of the host matrix on the photothermally driven actuation performance of negatively photochromic, donor–acceptor Stenhouse adduct (DASA)-based polymers. Using a modular Diels–Alder “click” platform, we designed polymeric materials with varying DASA incorporation and investigated the relationships between the material composition and the resulting physical, mechanical, and photoswitching properties. We demonstrate that increasing the DASA concentration in polymer conjugates has a dramatic effect on the material’s physical and mechanical properties, such as the glass transition temperature (T g) and elastic modulus, as well as the photoswitching properties, which are found to be highly dependent on T g. We establish using a simple photoresponsive bilayer that actuation performance is controlled by the bilayer stiffness rather than the photochrome incorporation of DASA. Finally, we report and compare the light-induced property changes in T g and the elastic modulus between the materials comprising the open or closed forms of DASAs. Our results demonstrate the importance of designing a material that is stiff enough to provide the mechanical strength required for actuation under load, but soft enough to reversibly switch at the operational temperature and provide key considerations for the development of application-geared photoswitchable materials.

Photoisomerization of molecules dissolved in a polymer film can modulate its properties. In a previous paper (Mostafavi, S. H.;et al. Macromolecules 2018et al. Macromolecules , 51, 2388et al. Macromolecules −2394, it was found that the ultraviolet light-induced photoisomerization of spiropyran dopants could substantially increase adhesion to a glass surface. In this work, a different photochromic reaction, the visible-light-induced cyclization of a donor−acceptor Stenhouse adduct (DASA), leads to the opposite effect: the deadhesion of a polystyrene film from a clean glass surface. Measurements of the shear and pull-off adhesion strengths before and after visible irradiation show a light-induced decrease of 20−30%. The time required for delamination in water shows an even more dramatic decrease of 90%. Changes in the water contact angle and other measurements suggest that molecular-level noncovalent interactions between the polymer and glass are weakened after photoisomerization, possibly due to the molecular contraction of the DASA that disrupts the interaction between its amine groups and the surface silanols. The ability to reduce polymer adhesion using visible light enables the controlled release of dye molecules from a glass container, where these have been stored as a dry powder, into an aqueous solution. Embedding photochromic molecules in a polymer can lead to new effects that may have practical applications in stimuli-responsive materials.

Donor-acceptor Stenhouse adducts (DASAs) are visible-light-responsive photoswitches with avariety of emerging applications in photoresponsive materials.T heir 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,a llowing greatly reduced reaction times and increased yields for knownderivatives.Inaddition, it provides access to previously unattainable DASA-based photoswitches and DASA-polymer conjugates.T he role of HFIP and the photochromic properties of as et of new DASAs is probed using ac ombination of 1 HNMR and UV/Vis spectroscopy. The use of sterically hindered, electron-poor amines enabled the dark equilibrium to be decoupled from closed-isomer halflives for the first time.