Light-Triggered Self-Assembly of a Spiropyran-Functionalized Dendron into Nano-/Micrometer-Sized Particles and Photoresponsive Organogel with Switchable Fluorescence

The synthesis, self-assembly, and spectroscopic investigation of spiropyran (SP)-functionalized dendron 1 are reported. Under UV light irradiation assembly of 1 into nano-/microparticles occurs due to the transformation of the closed form of SP into the open merocyanine (MC) form. The formation of these nano-/microparticles is confirmed by transmission electron microscopy (TEM) and dynamic light scattering (DLS) experiments in addition to the confocal laser scanning microscopy (CLSM) measurements. These nano-/microparticles exhibit relatively strong red emission. It is interesting to note that the direct cooling of the toluene/benzene solution of 1 to 0 degrees C lead to gel formation. Multivalent pi-pi interactions due to the dendron 1 may be the driving-force for the gelation. The UV light irradiation cannot destroy the gel phase, and in fact, the gel-gel transition is successfully realized. The purple blue gel exhibits relatively strong red fluorescence; moreover, the fluorescence can be reversibly switched by alternating UV and visible light irradiation. The results clearly indicate that the MC form after aggregation becomes more stable and fluorescent.