DFT study of acceleration of electrocyclization in photochromes under radical cationic conditions: Comparison with recent experimental data

Radical cation formation is proposed for the rapid cyclization of 1, 2-bis[5-phenyl-2-methylthien-3-yl]cyclopentene and oligothiophene functionalized dimethyldihydropyrenes (DMDHP). Density functional theory calculations have been performed to rationalize the effect of a radical cation on the activation barrier of different classes of electrocyclic photochromes (DHP, dithienylethene, dihydroazulene and fulgide). For exact comparative analysis, the activation barrier of neutral (singlet) analogues at the same level of theory are also calculated. In addition, the concerted nature and aromaticity of transition states were investigated with the help of synchronicity (Sy.) and nuclear independent chemical shift values NICS(0) calculations, respectively, for both the radical cation and neutral systems. In case of the radical cation, thermal return of CPD to DHP, the activation barrier is very low (ΔH = 3.13 kcal mol⁻¹, ΔG = 4.01 kcal mol⁻¹) as compared to the neutral analogue (ΔH = 20.6 kcal mol⁻¹, ΔG = 20.98 kcal mol⁻¹), which is consistent with experimental observations. Similarly for dithenylethenes, radical cation formation has a large impact on the activation barrier (ΔH = 19.44 kcal mol⁻¹, ΔG = 22.29 kcal mol⁻¹). However, radical cation formation has almost negligible impact on the activation barrier of VHF-DHA and fulgide isomerization. The significant difference has been observed for synchronicity and NICS(0) values of all types of photochromes under radical cation conditions as compared to the neutral system.