Photochromism of dihydroindolizines. Part XXIII: Sensitive tuning of the photophysical properties with region A and C substituents in the photochromic dihydroindolizines skeleton

Abstract

Twelve new photochromic materials based on dihydroindolizine (DHI) systems were synthesized using mechanochemical reaction strategy in one-pot solid state reaction with high yield and short reaction time. The DHI skeleton in these photochromic compounds is substituted in both the fluorene part (region A) or in the pyradizine (region B) with halogen and methyl groups, respectively, in order to predict the appropriate functionality for optimal tuning of the photochromic properties of the system. Upon Irradiation of the photochromic DHIs with polychromatic light, ring opened colored betaines are formed, which underwent ring-closure reactions via thermal1,5-electrocyclization processes. Because of their relatively slower 1,5-electrocyclization, the colored betaine forms are observable in dichloromethane solution with concentration of 1 × 10−4 mol/L at 15 °C. All the absorption maxima (λmax) of the colored betaines were detected using UV/VIS/NIR spectrophotometer which were found to be in the visible region and lie between 516 nm and 660 nm. The kinetics of the bleaching process of betaines to DHIs showed that the half-lives relative to betaines were found to take place in the second and third range and lie between 9 and 416 s and fit well with the first order thermal back reaction. It has been shown that, a highly pronounced decrease in the rate of electrocyclization of betaines bearing dimethyl substituted pyridazine was noted compared with non-substituted pyridazine betaines. It has been found that inversion of the trends in the rate of electrocyclization of the betaines takes place when passing from di-methyl to tetra-methyl substituted pyradizine. Interestingly, a remarkable photo-fatigue resistance of some studied betaines is monitored. A noticeable better photo-fatigue resistance was observed in blue-green colored betaines by comparison to the red-violet colored betaines. The observation deduced from the experimental part finds explanation through DFT calculations. The energy barriers, ΔE* and ΔE*rot(CC-TC), for the 12 photochromic compounds are calculated in the range from 95.5 to 127.7 kJ/mol and 17.1 to 84.6 kJ/mol, respectively. The effect of the substituent on the second step of the mechanism, the CC bond rotation, is much more important than that of the ring-opening step. The substitution in position 4 and 5 strongly increases the energy barrier for bond rotation: these positions are therefore interesting to modulate the reactivity of the open isomer. The substitution at the position 5 by a methyl group induces a strong bathochromic shift of the absorption.