Force-Induced Spiropyran–Merocyanine Interconversion: An Enforced Geometry Optimization Study

Mechanically induced spiropyran (SP)–merocyanine (MC) interconversion was examined using the enforced geometry optimization (EGO) method at the B3LYP/6-311G(d,p) level. The calculations followed a complete mechanical cycle for the SP–CH₃ model: tensile opening of the spiro ring, force removal, and subsequent compression. This allowed both SP → MC conversion and recovery of the closed SP structure to be considered within the same computational framework. Across eleven spiropyran derivatives, the first observed opening forces show a clear overall Hammett-like trend, with strongly electron-withdrawing substituents opening earlier and strongly electron-donating substituents opening later. A closer analysis of the H-, NH₂-, and Cl-substituted systems showed that C–O cleavage is accompanied by loss of the localized spiro bond, Wiberg bond indices falling to values near zero, charge redistribution, and narrowing of the HOMO–LUMO gap. Calculations in the gas phase and in thirteen COSMO environments revealed a further feature: a low-force geometric reorganization precedes bond cleavage. The solvent also influences which relaxed MC conformer is reached, giving TTC, CTC, CCC, or CCT structures. Neither the calculated ring-opening point nor the final MC conformation is determined solely by the dielectric constant.