Zhao, L., et al., Multi-responsive cellulose nanocrystal-rhodamine conjugates – An advanced structure study by solid-state dynamic nuclear polarization (DNP) NMR. Phys. Chem. Chem. Phys., 2014.
Multi-stimuli responsive materials based on cellulose nanocrystals (CNC), especially using non-conventional stimuli including light, still need more explorations, to fulfill the requirement of complicated application environments. The structure determination of functional groups on CNC surface constitutes a significant challenge, partially due to their low amounts. In this study, rhodamine spiroamide groups are immobilized onto the surface of CNC leading to a hybrid compound being responsive to pH-value, heating and UV light. After the treatment with external stimuli, the fluorescent and correlated optical color change can be induced, which refers to a ring opening and closing process. Amine and amide groups in rhodamine spiroamide play the critical role during this switching process. Solid-state NMR spectroscopy coupled with sensitivity-enhanced dynamic nuclear polarization (DNP) was used to measure 13C and 15N in natural abundance, allowing the determination of structural changes during the switching process. It is shown that a temporary bond through an electrostatic interaction could be formed within the confined environment on the CNC surface during the heating treatment. The carboxyl groups on CNC surface plays a pivotal role in stabilizing the open status of rhodamine spiroamide groups.