Theoretical investigation on radical anion promoted electrocyclization in photochromes

Rapid electrocyclization is proposed under radical anionic conditions in organic photochromes. DFT calculations have been performed to investigate the radical anion mediated electrocyclization in different organic photochromes. Furthermore, the activation barriers under radical anionic conditions are compared with those in neutral and radical cationic conditions. The nuclear independent chemical shift (NICS(0)) and synchronicity calculations have been performed for the confirmation of concerted nature and aromatic character of transition states, respectively. The activation barrier for thermal return of cyclophanediene (CPD) to dihydropyrene (DHP) under radical anionic conditions is very lower (ΔH = 5.92 kcal/mol, ΔG = 6.97 kcal/mol) than under neutral conditions, but higher than that in radical cationic conditions (ΔH = 3.13 kcal/mol, ΔG = 4.0 kcal/mol). Similarly, the other prominent classes of photochromes; dithienylethene (ΔH = 20.12 kcal/mol, ΔG = 21.55 kcal/mol) and vinylheptafulvene (ΔH = 23.72 kcal/mol, ΔG = 24.82 kcal/mol) have shown decreased activation barrier under radical anionic condition. However, activation barrier of fulgide under radical anionic conditions is not different than those under neutral and radical cationic conditions. Synchronicity and NICS(0) values for organic photochromes also show significant changes under radical anionic conditions.