Photochromism of dihydroindolizines. Part XXII: Significant effect of region B substituents on tuning the photophysical properties of photochromic dihydroindolizines: Absorption, kinetic and computational studies.

Novel photochromic dihydroindolizines (DHIs) bearing different alkyl and cyclic alkyl ester group (region B) were prepared via electrophilic addition of fluorene spirocyclopropenes substituted with different alkyl ester groups in region B of the DHI skeleton to N-heterocyclic pyridazine in dry ether solution. The maxima of absorption (λ max ) of the colored betaines (formed after UV-irradiation) were detected by UV/Vis absorption measurement. The studied betaines showed one visible region absorption maxima between 517 and 548 nm. The kinetics of the 1,5-electrocyclization process of the colored betaines to closed DHIs system was studied spectrophotometrically in dichloromethane solution. Interestingly, a significant decrease in the half-life time by a factor of ∼8 was observed when alkyl of the ester group changed from methyl to pentyl alkyl groups. The activation parameters indicated that the changes of ΔH ≠ are counterbalanced by changes in ΔS ≠ ʺ compensation effectʺ. The enthalpy and entropy of the activation energies for the thermal back-reaction were compared to investigate the generalization and the limitation of the molecular intention of the photochromic DHIs lengthways with the correlation between unwieldiness of the substituents (region B) and the rate constant of the thermal back-reaction. DFT calculations are performed to explain the 1,5-electrocyclization back-reaction mechanism, including a partial rotation prior to the final ring closure. [ABSTRACT FROM AUTHOR]