A New Photochromic System Based on a Pyridazinopyrrolo[1,2-b]pyridazine with Ultrafast Thermal Decoloration
The nucleophilic addition of the substituted pyridazines 2a−e to diacetylspirocyclopropene (1) in dry ether solution afforded the dihydroindolizines 3a−e, which undergo ring opening to the betaine 3′a−e after irradation with UV light. Condensation of the diacetyl groups of the dihydroindolizines (DHIs) 3a−e with hydrazine hydrate in a diethyl ether/ethanol mixture gave the new pyridazinopyrrolo[1,2-b]pyridazines 4a−e. Compounds 4a−e showed no photochromism at room temperature or after cooling with liquid nitrogen. However laser flash spectroscopy was successfully used for the determination of both the half-life and absorption maxima of the betaines 4′a−e.