Interplay of solvent polarity and hydrogen bonding in thermal isomerization of spiropyran

Spiropyran is a versatile photochromic molecule in the area of photo-responsive materials. Along with photochromism, the isomerization of spiropyran can be controlled by solvents, acid-base, metal ions etc. Often photo-induced generation of ‘merocyanine (MC)’ isomer leads to thermal relaxation to thermodynamically more stable isomer ‘spiropyran (SP)’. The current study aims to decipher the critical role of solvent polarity and hydrogen bonding ability on the isomerization of spiropyran systems. UV–visible spectroscopic study, Solvation energy calculations and kinetic measurements unequivocally indicated that presence of hydrogen bond donors play a key role in stabilizing ‘MC’ isomer of spiropyrans. This finding was leveraged through demonstrating a general strategy to decelerate the thermal isomerization of ‘MC’ to ‘SP’ in the presence of a series of hydrogen bond donors.