Synthesis and photochromic properties of 4,5-bisaryl-3(2H)-pyridazinones

• Six novel photochromic and fluorescent bisarylpyridazinones were synthesized.
• Their closed forms showed slight negative solvatochromism.
• Their open forms are fluorescent while the closed forms are not.
• N-methyl-(5-phenylthiazolyl)pyridazinone showed 94% conversion to the closed form.

Six novel photochromic bisarylpyridazinones containing 2,5-dimethylthiophene or 5-methyl-2-phenylthiazole unit were synthesized, and their photochromic and fluorescence properties were investigated. The bisarylpyridazinones underwent reversible color change upon irradiation with UV or visible light. The effect of solvents on the absorption spectra of the bisarylpyridazinones was investigated. The closed-ring forms of bisarylpyridazinones displayed negative solvatochromism which was attributed to the high dipolar characters of the molecule. The open-ring forms of bisarylpyridazinones showed fluorescence at 400–480 nm upon excitation at 302 nm, and the intensities of emission bands gradually decreased during the ring-closing photoreactions. Among the synthesized bisarylpyridazinones, 4,5-bis(5-methyl-2-phenylthiazol-4-yl)-2-methyl(or 2-phenyl) pyridazin-3(2H)-ones (5O and 6O) displayed rather large absorption and emission spectral change, higher quantum efficiency during the photoreaction compared to others. A high conversion ratio (94%) to the closed form was observed for 5O.

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