Thermally responsive AIE-active polyurethanes based on a tetraaniline derivative

Abstract

Polyurethanes with different soft-hard segment ratios were successfully synthesized, with an aggregation-induced-emission (AIE)-active tetraaniline derivative (NH₂-B₃-Ani₄-NH₂) as the hard segment. The resulting polyurethanes exhibited typical AIE features. The fluorescence intensities of polyurethane films changed with heat treatments. The fluorescence intensities of the polyurethane films decreased sharply after quenching treatment, yet their fluorescence intensities exceeded the original intensities of the films after thermal annealing at 80 °C for 24 h. Differential Scanning Calorimetry (DSC) results implied that the melting peaks in polyurethane films disappeared after quenching treatment, but the melting peaks appeared again after thermal annealing. These results proved that the arrangement of the structure had an important effect on the AIE properties of the polyurethane films. Meanwhile, the fluorescence intensities of these polyurethanes decreased with the increase of temperature, indicating that all three polyurethanes exhibited temperature-dependent fluorescent characteristics. Based on the above investigations, the AIE-active polyurethanes may provide a platform for the development of stimuli-responsive fluorescent materials.

Scheme 1. Synthesis rout of the AIE-active polyurethanes.

Fig. 1. Fluorescence intensity spectra of (a) PU-211, (b) PU-321, (c) PU-431 in DMF/H₂O mixtures with different water fractions; (d) plots of relative fluorescence intensity (I/I₀) of the polyurethanes versus the water fraction in DMF/H₂O solution, I₀: fluorescence intensity of the polyurethanes in pure DMF, mass concentration: 30 mg mL⁻¹, excitation wavelength: 350 nm.

Fig. 2. Fluorescence intensity spectra of (a) PU-211, (b) PU-321, (c) PU-431; (d) relative fluorescence intensity changes of the polyurethane films before and after heat treatment.

Fig. 3. DSC traces of (a) PU-211, (b) PU-321 and (c) PU-431 before and after the heat treatment.

Fig. 4. Temperature-dependent emission spectra of (a) PU-211, (b) PU-321 and (c) PU-431 films from 30 to 140 °C; (d) fluorescence intensities of PU-211, PU-321 and PU-431 versus the temperature.