Abstract
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•An efficient mechanochemical synthesis of photochromic tetrahydroindolizines has been established.•Pronounced tuning of the photochromic properties with different substitutions in THI regions have recorded.•Collective substituents effect in THI skeleton represents a noticeable decreasing the rotational freedom upon cyclization.•The computational results fit-well with the experimental data and clarified the ring opening/closure mechanism.
A new mechanochemical synthetic strategy for the synthesis of novel tetrahydroindolizines (THIs) photochromes incorporating different substituents in both regions B (ester part), and region C (base part) has been developed. The UV-irradiation of the colored betaine forms of the novel THIs and their absorption maxima (λmax) were determined in dichloromethane solution. The betaines exhibited one absorption maxima around 440–450 nm, and the second one in the white region at 680–740 nm. The thermodynamic activation parameters for the ring closure of colored betaines were calculated and compared with similar compounds. The kinetics of the 1,5-electrocyclization back reaction the designed photochromic by incorporating substitutions in all the three THI regions was also determined. Interestingly, the substitutions resulted in pronounced tunable of the kinetic behavior. Moreover, the mechanism and optical properties of the thermal fading process of betaine were investigated based on the theoretical simulations (density functional theory, DFT) providing insights into ring opening and closing process. The geometries and the relative energies of the THI, betaine isomers and transition states along reaction coordinates were optimized and evaluated by density functional theory (DFT). The theoretical calculation studies gave more insight into the ring opening and ring closure processes in addition to of distinct THI-substituents effect. The obtained results offer opportunity of further development in the quest of high-performance photochromic materials.