Synthesis and photochromism of E,E-3,4-(3,5-dimethoxybenzylidene)succinic anhydride and its infra red active 2-dicyanomethylene derivative

The pale yellow fulgide 7 was prepared via two successive Stobbe condensations of one molar equivalent of diethylsuccinate and two molar equivalent of 3,5-dimethoxybenzaldehyde. The pale yellow fulgide 7, in both solid and solution, turned pink when irradiated with 366 nm light. The fulgide 7 was found to undergo a Knovenagel type reaction when condensed with malononitrile under basic condition to give, after ring closure, the dicyanomethylene derivative 14. The later on irradiation with 366 nm gave the infrared active 1,8a-DHN derivative 16. The 1,8a-DHN derivative of fulgide 7 (e.g. 15) showed remarkable positive solvatochromism as the solvent polarity is increased.