Structural criteria for hydrazone photochromism in solution

Abstract

In view of the unique photochromic property of salicylaldehyde 2-quinolylhydrazone (1) in solution, a systematic study of the effects of hydrazone substituents on photochromism has been made. We have synthesized nine derivatives of (1) which are photochromic, eleven non-photochromic derivatives, and three non-quinolylhydrazones for comparison. On this basis, the following structural criteria for a hydrazone R¹R²N·N:CR³R⁴ to exhibit photochromism in solution are derived: R¹ and R² should be H and an unsaturated system containing an α-imino-group; R³ and R⁴ should be H and an α-enol, the latter being capable of internal chelation.