Thermoresponsive Fluorescent Microgels through In Situ [2 + 2] Photocycloaddition Cross-Linking of Thermally Induced Aggregates of Dendrimers

Abstract

An efficient strategy is reported for preparing smart microgels through in situ cross-linking thermally induced aggregates from thermoresponsive dendrimers in aqueous media via photoinduced [2 + 2] cycloaddition. These dendrimers are composed of a fluorescent 4,4′,4″-nitrilotribenzamide core and decorated with three dendritic oligoethylene glycol-carrying cinnamate moieties. The photocycloaddition approach is fast and straight without the use of any additional additives such as initiators, cross-linking agents, or surfactants, which affords narrowly dispersed microgels with controllable sizes. These microgels inherit characteristic thermoresponsiveness from the parent dendrimer and undergo thermally induced reversible volume phase transitions in aqueous solutions with tailorable deswelling ratios. In contrast to the parent dendrimers which are fluorescence silent due to the quenching of cinnamate moieties to the 4,4′,4″-nitrilotribenzamide core, the microgels show strong blue fluorescence since this quenching effect was vanished with the photocycloaddition of cinnamate moieties into nonconjugated cyclobutanes. By further loading with BODIPY dyes, fluorescence of the microgels can be modulated ratiometrically from blue to green through thermally induced volume phase transitions. These new dendrimer-based microgels with densely packed architectures provided supplemental entities to the existing microgels fabricated from linear polymers, and their characteristic thermoresponsiveness and tunable fluorescence would promote their fascinating applications in cargo delivery and nanosensing.

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

“…Dendritic macromolecules carrying dendritic oligoethylene glycol (OEG) pendants are a class of fascinating water-soluble macromolecules with characteristic thermoresponsiveness and excellent biocompatibility. , A variety of OEG-based dendritic macromolecules with different architectures, including dendrons, dendrimers, and dendronized polymers, have been developed in the past decades and shown broad applications in drug delivery, sensors, and stimuli-responsive hydrogels. − These dendritic OEG-based macromolecules exhibited fast and reversible phase transitions as well as tunable phase transition temperatures in aqueous solutions, which are different from those of linear OEG counterparts. Importantly, the dense packing of OEG segments afforded thermally switchable microenvironments which can reversibly encapsulate and release various guest molecules such as dyes, proteins, and nucleic acids in aqueous media. − Recently, we further exploited [2 + 2] photocycloadditions to mediate intra-and intermolecular cross-linking of OEG-based dendronized polymers or dendrimers into single-chain nanoparticles and microgels, respectively. − However, the cycloadditions of cinnamates used in these systems were activated by high-energy 365 nm UV light, which is harmful to environments and may also cause degradation of polymer backbones. , Herein, we report visible light-induced [2 + 2] cycloadditions of dendronized styryltriazines that can be controlled by dendritic architectures and solvent environments. Dendronization of styryltriazines with dendritic OEGs can also afford them characteristic thermoresponsiveness in water, and simultaneously provide tunable microenvironments to encapsulate and release guest dye through [2 + 2] photocycloadditions (Figure ).…”