Entropy-controlled thermal back-reaction of photochromic [2.2]paracyclophane-bridged imidazole dimer

A [2.2]paracyclophane-bridged imidazole dimer having bulky 3′,4′,5′-triphenyl-1,1′:2′,1″-terphenyl substituents was synthesized and the photochromic properties were investigated. The half-life of the colored species generated by the UV irradiation of the parent imidazole dimer was 1.0 ms at 298 K in benzene. Although such bulky substituents are introduced on the imidazole dimer, the fast thermal back-reaction can be observed as found in other [2.2]paracyclophane-bridged imidazole dimers. Herein, enthalpy and entropy of the activation energies (∆H‡ and ∆S‡, respectively) for the thermal back-reaction of the present molecule were compared with other [2.2]paracyclophane-bridged imidazole dimers in order to investigate the generality and the limitation of the molecular design of the photochromic imidazole dimer along with the correlation between bulkiness of the substituents and the rate constant of the thermal back-reaction. Finally, we referred to a new method for controlling the rate constant of the thermal back-reaction of [2.2]paracyclophane-bridged imidazole dimers.

► We designed thermally reversible photochromic molecules bearing bulky substituents.
► The photochromic molecules showed efficient photo-reaction even in the solid state.
► We investigated the correlation between the bulkiness and the thermal back-reaction.
► The bulky substituents decreased the rate of the thermal back-reaction entropically.
► We showed that the thermal back-reaction could be controlled entropically.