Synthesis and characterization of photochromic triethylene glycol-containing spiropyrans and their assembly in solution

Abstract

A series of photochromic triethylene glycol (TEG)-containing spiropyrans (SPs) has been designed and synthesized. UV–Vis absorption and emission studies demonstrated that their photophysical and photochromic properties can be modulated by varying the linker group between the TEG and the SP components. TEG-containing SPs in THF–water mixtures were found to undergo solvent-induced ring-opening reactions without the need for photoirradiation, which could be attributed to the stabilization of the merocyanine (MC) form in polar solvents. Counterintuitively, a much slower thermal bleaching process of the open MC form has been observed in non-polar solvent media, possibly due to the stabilization of the zwitterionic open form through supramolecular self-assembly. Furthermore, the formation of spherical aggregates resulting from the planarized open MC form has been revealed by dynamic light scattering (DLS), transmission electron microscopy (TEM) and atomic force microscopy (AFM) measurements. Such rational molecular designs, together with the tuning of the solvent polarity and controlled UV light irradiation, have led to the self-assembly of this class of TEG-containing spiropyran derivatives, achieving the systematic and controlled formation of self-assembled functional materials.