Visible Light [2 + 2] Cycloadditions of Thermoresponsive Dendronized Styryltriazines To Exhibit Tunable Microconfinement.

Visible light-triggered photochemical reactions in aqueous media are highly valuable to tailor molecular structures and properties in an ecofriendly manner. Here we report visible light-induced catalyst-free [2 + 2] cycloadditions of thermoresponsive dendronized styryltriazines, which show tunable microconfinement to guest dyes in aqueous media. These dendronized styryltriazines are constituted of conjugated mono- or tristyryltriazines, which carry hydrophilic dendritic oligoethylene glycol (OEG) pendants. They underwent efficient [2 + 2] cycloadditions to form dendronized cyclobutane dimers or oligomers in water through irradiation with visible light of 400 nm, and their cycloaddition behavior was dominated by dendritic architectures and solvent conditions. Dendronization with dendritic OEGs also afforded them characteristic thermoresponsive properties with tunable phase transition temperatures in the range 36-65 °C, which can be further modulated through photocycloaddition of styryltriazine chromophores. Importantly, dendronized styryltriazines can form tunable microenvironments in aqueous media, which encapsulate hydrophobic solvatochromic Nile red to exhibit variable photophysical properties. The encapsulated guest dye can be simultaneously released through noninvasive visible light-induced [2 + 2] cycloaddition reactions.

Thermoresponsive Helical Dendronized Poly(phenylacetylene)s: Remarkable Stabilization of Their Helicity via Photo-Dimerization of the Dendritic Pendants.

Dynamic helical polymers can change their helicity according to external stimuli due to the low helix-inversion barriers, while helicity stabilization for polymers is important for applications in chiral recognition or chiral separations. Here, we present a convenient methodology to stabilize dynamic helical conformations of polymers through intramolecular cross-linking. Thermoresponsive dendronized poly(phenylacetylene)s (PPAs) carrying 3-fold dendritic oligoethylene glycol pendants containing cinnamate moieties were synthesized. These polymers exhibit typical features of dynamic helical structures in different solvents, that is, racemic contracted conformations in less polar organic solvents and predominantly one-handed stretched helical conformations in highly polar solvents. This dynamic helicity can be enhanced through selective solvation by increasing the polarity of the organic solvents or simply via their thermally mediated dehydration in water. However, through photocycloaddition of the cinnamate moieties between the neighboring pendants via UV irradiation, these dendronized PPAs adopt stable helical conformations either below or above their phase transition temperatures in water, and their helical conformations can even be retained in less polar organic solvents. Spectroscopic and atomic force microscopy measurements demonstrate that photocycloaddition between the cinnamate moieties occurs on the individual molecular level, and this is found to be helpful in restraining the photodegradation of the PPA backbones. Molecular dynamics simulations reveal that the spatial orientation of the pendants along the rigid polyene backbone is crucial for the photodimerization of cinnamates within one helix pitch.