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.