Computational Insights into the Open–Closed Equilibrium of Substituted Spiropyrans: Stability and Optical Properties

Spiropyran (SP)–merocyanine (MC) systems are dynamic molecular switches that have been widely studied for their tunable optical properties. The equilibrium between the closed (SP) and open (MC) forms can be modulated by structural features, enabling their application in sensing, photoresponsive materials, and fluorescence-based devices. In a recent experimental study by Alonso et al., two SP–MC pairs were synthesized: one bearing a single hydroxyl substituent on the aromatic ring (SP1/MC1), and another featuring a tri-hydroxylated aromatic moiety, also known as gallol (SP3/MC3). While SP1 predominantly adopts the closed configuration, the second system exists exclusively in the open MC3 form and exhibits strong fluorescence.

Herein, we present a computational study based on density functional theory (DFT) to rationalize these differences. Conformational searches using xTB/CREST were followed by geometry optimizations (B97-3c) and single-point energy calculations (B3LYP-D4/def2-TZVP). Relevant properties, including relative free energies, dipole moments, charge distribution, and orbital energies, were evaluated. TDDFT calculations (CAM-B3LYP/def2-TZVP) were employed to simulate UV–Vis absorption and explore fluorescence behavior. Solvent effects were modeled using the SMD model to assess the influence of polarity on the SP–MC equilibrium as well as on the optical properties.

This computational analysis aims to test the hypothesis that gallol substitution (SP3/MC3) stabilizes the MC configuration. The results are expected to clarify the impact of substitution and solvation on structural and optical properties, providing insights for the rational design of fluorescent photoactive molecules with tunable SP–MC interconversion.