Spiropyran-based sensors have garnered significant attention in recent years due to their unique photochromic properties and their potential applications in various fields. This collaborative study focuses on the molecular and photophysical properties of an amphiphilic molecule that contains spiropyran. The research specifically investigating these properties in both organic and aqueous solutions. In organic solvents, the spiropyran-containing system exhibits positive photochromism, meaning that upon exposure to UV light, the initially colorless spiropyran form undergoes a transformation into a colorful merocyanine isomer. However, when the amphiphile is dissolved in an aqueous solution, it displays negative photochromism. In water, the orange-red merocyanine form becomes thermodynamically more stable, and both UV and visible light stimuli cause partial or complete photobleaching of the solution.
The explanation for this phenomenon is elucidated through the use of density functional theory calculations and classical modeling, which includes thermodynamic integration. The simulations uncover that the stabilization of the merocyanine form in water occurs with an energy of approximately 70 kJ mol−1. Furthermore, the Helmholtz free energy of hydration for the merocyanine form is 100 kJ mol−1 lower than that of the spiropyran isomer in water. This disparity arises from the molecular properties of merocyanine: after undergoing a ring-opening reaction, the molecule transforms into a zwitterionic form, as confirmed by the electrostatic potential plotted around the opened structure. The presence of three charged groups on the periphery of the planar conjugated backbone facilitates the self-assembly of merocyanine molecules in water. This self-assembly leads to the formation of elongated associates with stack-like building blocks, as observed in molecular dynamics simulations of the aqueous solution at concentrations surpassing the critical micelle concentration.
By quantitatively evaluating the hydrophilicity switching in surfactants containing spiropyran/merocyanine, this study provides insights that could drive the exploration of new sersoric systems, including colloidal and polymeric ones. The aim is to enable the remote tuning of their morphology, thereby generating promising shapes and patterns relevant to the demands of modern nanotechnology.
