Photochromism of dihydroindolizines. Part 16: Tuning of the photophysical behavior of photochromic dihydroindolizines in solution and in polymeric thin film

In this work, photochromic materials based on the dihydroindolizine (DHI) system were synthesized in multistep reactions using chemical and photochemical methods. Some of the synthesized photochromic dihydroindolizine derivatives were substituted on the fluorene (region A) and pyridazine (region C) moieties in order to provide the appropriate functionality for optimal tuning of the photochromic properties of the system. Irradiation of the photochromic DHIs with polychromatic light led to ring opened colored betaines, which underwent thermal 1,5-electrocyclization. The red to green colored betaines produced after UV irradiation returned back through 1,5-electrocyclization to the corresponding DHIs with different rate constants depending on the substituents in both fluorene and pyridazine regions. The kinetic measurements of the thermal 1,5-electrocyclization under different temperatures that ranged from -10 to 25 °C showed that the half-lives of the colored betaines fall in the second to hours domain. Interestingly, these materials showed a very good photochromic behavior not only in solution but also in the PMMA matrix. Irradiation of a slide prepared by the deep-coating method led to the formation of the colored betaine and the kinetics of the thermally reversible 1,5-electrocyclization and the AFM image of the film has been recorded. Indeed, the chemical and thermal stability of the investigated betaines in polymer (PMMA) will render such species useful for a plethora of new of applications.