Molecular Photoswitching in Confined Spaces

Abstract

Conspectus In nature, light is harvested by photoactive proteins to drive a range of biological processes, including photosynthesis, phototaxis, vision, and ultimately life. Bacteriorhodopsin, for example, is a protein embedded within archaeal cell membranes that binds the chromophore retinal within its hydrophobic pocket. Exposure to light triggers regioselective photoisomerization of the confined retinal, which in turn initiates a cascade of conformational changes within the protein, triggering proton flux against the concentration gradient, providing the microorganisms with the energy to live. We are inspired by these functions in nature to harness light energy using synthetic photoswitches under confinement. Like retinal, synthetic photoswitches require some degree of conformational flexibility to isomerize. In nature, the conformational change associated with retinal isomerization is accommodated by the structural flexibility of the opsin host, yet it results in steric communication between the chromophore and the protein. Similarly, we strive to design systems wherein isomerization of confined photoswitches results in steric communication between a photoswitch and its confining environment. To achieve this aim, a balance must be struck between molecular crowding and conformational freedom under confinement: too much crowding prevents switching, whereas too much freedom resembles switching of isolated molecules in solution, preventing communication. In this Account, we discuss five classes of synthetic light-switchable compoundsdiarylethenes, anthracenes, azobenzenes, spiropyrans, and donor–acceptor Stenhouse adductscomparing their behaviors under confinement and in solution. The environments employed to confine these photoswitches are diverse, ranging from planar surfaces to nanosized cavities within coordination cages, nanoporous frameworks, and nanoparticle aggregates. The trends that emerge are primarily dependent on the nature of the photoswitch and not on the material used for confinement. In general, we find that photoswitches requiring less conformational freedom for switching are, as expected, more straightforward to isomerize reversibly under confinement. Because these compounds undergo only small structural changes upon isomerization, however, switching does not propagate into communication with their environment. Conversely, photoswitches that require more conformational freedom are more challenging to switch under confinement but also can influence system-wide behavior. Although we are primarily interested in the effects of geometric constraints on photoswitching under confinement, additional effects inevitably emerge when a compound is removed from solution and placed within a new, more crowded environment. For instance, we have found that compounds that convert to zwitterionic isomers upon light irradiation often experience stabilization of these forms under confinement. This effect results from the mutual stabilization of zwitterions that are brought into close proximity on surfaces or with...

Construction and Properties of Donor–Acceptor Stenhouse Adducts on Gold Surfaces

“…The curved Au particles were expected to facilitate both the interfacial reaction and the switching process. 43 − 45 …”

Confined space design by nanoparticle self-assembly

“…To date, the most extensive study in this field is the one performed by Klajn and coworkers. 90 First, they focused on surface-confined species and demonstrated that UV irradiation ( λ ∼ 365 nm) of ethynylanthracenyl-thiol immobilized on spherical Au NPs gave [4 + 4] dimerization, instead of the well-known selective [4 + 2] Diels–Alder reaction. The [4 + 2] vs. [4 + 4] selectivity could be tuned tailoring the curvature of the underlying NP surface, as well as the flexibility of the linker chains between gold and ethynylanthracene moieties.…”

聚合物中螺吡喃原位“自笼”及光响应调控

“…酰胺键之间的氢键增加了聚合物链的刚性 [30][31][32][33] , MC 之间的有序堆叠 [33] , 以及 MC 和酯−酰胺结构片段之 间的偶极-偶极相互作用 [15,17,[33][34] 进一步加强了抑制 MC 运动的效应. 我们把这种 MC 催化交联锁定自身 结构的策略称为 MC 的"自笼"化.…”