The influence of the nitro group on the Cspiro-O bond dissociation and recovery in spiropyrans: A comparative theoretical analysis of 6-nitro-BIPS and BIPS

Abstract

In this work, we elucidated the role of the nitro group in the greater propensity of substituted spiropyran (6-nitro-BIPS) to open form compared to unsubstituted (BIPS) by theoretical analysis using DFT/TD-DFT. Methanol was used as a solvent, which made it possible to take into account only the singlet mechanism of phototransformations. Testing of hybrid functionals showed that CAM-B3LYP, M052X, M062X, and ωB97XD are suitable for comparative analysis of 6-nitro-BIPS and BIPS. M08HX functional can only be applied to 6-nitro-BIPS. The solvent was modeled using the SMD model as well as four solvating methanol molecules. Calculations of purely electronic excitation of spiropyrans showed that the NO2 group, when the solvent is implicitly specified, contributes to the weakening of the Cspiro-O bond, and, on the contrary, strengthens this bond when explicitly specified. However, analysis of the vibronic excitation of 6-nitro-BIPS showed that it activates the contraction-stretching vibration of the Cspiro-O bond in the excited state. Thus, the vibronic transition upon UV absorption for 6-nitro-BIPS contributes to the opening of this spiropyran. While BIPS restores its closed form in the ground state, in the case of 6-nitro-BIPS this is prevented by a potential barrier, the magnitude of which is determined both by the polarity of the solvent and its ability to form hydrogen bonds.