Spiropyran-induced one-dimensional cyclodextrin microcrystals with light-driven fluorescence change

Systems that exhibit light-driven fluorescence change have received much attention due to their applications in luminescent switches or as sensors. However, such solid-state systems are still limited relative to those in solution form, and how to obtain photochromic fluorescent film materials for use in device fabrication is a long-standing problem. In this work, one-dimensional (1D) solid-state microcrystals combining a host molecule cyclodextrin (CD) and UV-responsive guest molecule spiropyran (SP) have been developed, which present a light-driven fluorescence change based on the isomerization of the SP molecule in the CD matrix. Firstly, the supramolecular forces including inherent hydrogen bonding and hydrophobic interactions in this self-assembled system were studied using electrospray ionization mass spectrometry (ESI-MS), isothermal titration calorimetry (ITC), and Fourier transform infrared spectra (FTIR). The scanning electron microscopy (SEM) comparison between the SP@CD system and the pristine CD or SP confirms that the SP guest plays a crucial role in the formation of 1D microcrystals. Then, through a spin-casting process, a thin film of SP@CD microcrystals was further fabricated, which shows a reversible photochromic fluorescence between fluorescence-free and high red-emission states. Moreover, the open-ring form of SP@CD microcrystals also presents two-photon emission and polarized fluorescence. Therefore, this work has offered a facile way to obtain a reversible photochromic fluorescent film, which may be applied in dynamic light-controlled fluorescent sensors and two-photon imaging.