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 diacetylsp irocyclopropene (1) in dry ether solution afforded the dihydroindolizines 3 a-e, which undergo ring opening to the betaine 3'a-a after irradation with UV light. Condensation of the diacetyl groups of the dihydroindolizines (DH Is) 3a-a with hydrazine hydrate in a diethyl ether/ethanol mixture gave the new pyridazinopyrrolo[1,2-b]pyridazines 4a-e. Compounds 4a-a showed no pho tochromism at room temperature or after cooling with liquid nitrogen. Howev er laser flash spectroscopy was successfully used for the determination of both the half-life and absorption maxima of the betaines 4'a-e.