Combined experimental and theoretical insights into the solvent-dependent photochromic properties of nitrospiropyrans

Spiropyrans exhibit reversible photochromism that strongly depends on solvent polarity; yet a quantitative understanding of this relationship remains incomplete. Here we combine detailed spectroscopic and kinetic experiments with ab initio calculations to investigate 5'-methoxy-1',3',3'-trimethyl-6-nitrospiro[chromene-2,2'-indoline] (SP). Ultraviolet irradiation at 368 nm populates the S₃ (¹ππ*) state, initiating the ring-opening reaction to the merocyanine (MC) form, which is subsequently followed by thermal ring-closing. Theoretical calculations indicate that photoexcited SP undergoes nonradiative relaxation to the S₁ charge-transfer state, from which the system reaches the ground state via a ring-opened conical intersection. Experimental kinetics studies have revealed that increasing solvent polarity slows relaxation and raises the activation energy from 89.7 kJ mol⁻¹ in dioxane to 104.6 kJ mol⁻¹ in acetone. Calculations have reproduced this trend, showing that polar solvents preferentially stabilize MC over the transition state. The observed blue shift of the MC absorption band with increasing polarity confirms the presence of negative solvatochromism. This combined experimental-theoretical analysis provides a quantitative framework linking solvent polarity, photochromic efficiency, and the activation energy in a model nitrospiropyran system.