Thermoresponsive Luminescent Nanobowls with Controlled Fluorescence Properties: The Role of a Flexible Spacer Length

The controlled preparation of stimulus-responsive soft nanomaterials, especially those with anisotropic nanostructures, has attracted wide attention. Herein, a series of N-(2-(6-(4-(diphenylamino)phenyl)-1,3-dioxo-1H-benzo[de]isoquinolin-2(3H)-yl)ethyl)acrylamide (TNAA, n = 0, 1, 2, 3, or 4)-containing luminescent monomers with an adjustable flexible spacer length are randomly copolymerized with N-isopropylacrylamide (NIPAM) to afford thermoresponsive polymers with a tunable cloud point temperature (CP) and fluorescence emission. By adjusting the molar ratio of TNAA to NIPAM, the CP of the polymers can be regulated from 49.1 ± 0.7 to 26.0 ± 0.8 °C, while the emission wavelength is controlled from 598 to 633 nm as the length of the flexible spacer shortens. Density functional theory calculation results verify that as the length of flexible spacers increases, the path for intramolecular electron transfer becomes longer. It induces an elevation in the energy required for electrons to transfer from the highest occupied molecular orbital to the lowest unoccupied molecular orbital, leading to the increase in the energy gap. Importantly, nanobowls with controlled diameter, opening size, and inherited thermoresponsive and tunable fluorescence properties are formed by self-assembly. As the temperature increases from below the CP of the polymer to 60 °C, the hydrophilic-to-hydrophobic transition of PNIPAM segments occurs, leading to the enhancement of the hydrophobic interactions and a more compact aggregation of the polymer chains. Consequently, the nanobowls also change from their original loose and porous structure to a relatively dense state. Overall, thermoresponsive luminescent nanobowls with controlled dimensions and fluorescence properties are achieved by manipulating the spacer length between fluorophores and the polymer backbone.