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

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.