Photochromism of tetrahydroindolizines. Part XXVI: Mechanochemical synthesis, tunable photophysical properties and combined experimental and theoretical studies of novel photochromic tetrahydroindolizines

Herein, eighteen novel photochromic tetrahydroindolizines (THIs) have been successfully prepared using a well-established mechanochemical synthetic approach. These products were obtained in good to excellent yields and the purification process was relatively straightforward devoid of any isolation challenges. The new THIs incorporate various substituents in region C (base part) which include both electron donating and withdrawing groups in the ortho, meta and para positions of the phenyl ring. The absorption maxima (λ^max) of both closed form (THI) and the colored form (betaine, after UV-irradiation) were spectrophotometrically determined in dichloromethane solution. The colored betaines displayed two absorption maxima, one between 472 and 485 nm, and the second one appearing in the white region between 658 and 724 nm with tailing extending to the near infra-red (NIR) region. The impact of the nature and position of substituent in the 10′b-phenyl ring on the absorption of the betaines indicated that substitution in the ortho-position with absorption maxima between 658 and 690 nm leads to a hypsochromic shift largely caused by the steric effect of the substituents in the o-position of the THI molecule 10′b-phenyl ring compared to the meta- and para-substituted species with absorption maxima between 700 and 724 nm. The calculated ring closure thermodynamic activation parameters of colored betaines were found to be in good agreement with previous reported results. The 1,5-electrocyclization reaction kinetics of the investigated photochromic THIs were monitored and calculated. Intriguingly, substitution with electron donating and withdrawing groups in the o-, m-, and p-positions of the phenyl ring enabled distinctive tunability of the kinetic behavior. Furthermore, possible mechanism of the thermal 1,5-electrocyclization back reaction of betaines was explored based on density functional theory calculations (DFT), delivering more understanding into ring opening and ring closure processes in addition to the distinct THI-substituents effect (region C). The geometries and the relative energies of the THI, betaine isomers, and transition states along reaction coordinates were optimized and assessed by DFT. Based on the obtained synthetic methodology results, unprecedent tunability of the photochromic properties and the combined experimental and theoretical studies, the current work launches a new dimension and contributes further developments to the field of high-performance photochromic materials.