Ruling Out E/Z Isomerization in the Inverted Solvatochromism of Brooker’s Merocyanine

Brooker’s merocyanine (BM) is the prototypical dye exhibiting inverted solvatochromism; however, the possible contribution of E/Z isomerization to this phenomenon has remained unresolved. Here, we combine selective photoexcitation in steady-state fluorescence spectroscopy, 1H NMR experiments, and quantum-mechanics calculations to evaluate the configurational behavior of BM and its protonated precursor BMH. While BMH underwent efficient E/Z photoisomerization and reached photostationary equilibrium, no detectable formation of the Z-isomer of BM was observed. Consistently, SCS-MP2/def2-QZVPP calculations combined with COSMO-RS solvation models showed that the Z-isomer of BM is thermodynamically disfavored by approximately 6 kcal mol–1 across solvents ranging from benzene to water. Even without any change in configuration, BM displayed its remarkable inverted solvatochromism, clearly revealing that this behavior arises exclusively from the intrinsic electronic properties of the E-isomer.