Photochromism of quinolylhydrazones. III. The mechanism of isomerization of the photocolored α-quinolylimino-(Z)-hydrazone to the α-quinolylamino-(E)-hydrazone

The kinetics and mechanism of the thermal decay of the photocolored form 1a of salicylaldehyde 2-quinolylhydrazone are reported. Two isomerization reactions, viz., α-imino- to α-aminoquinoline and Z → E hydrazone, are involved. The first conversion occurs via an intramolecular transfer of the phenolic hydrogen to the α-imino group. This is deduced on the basis of medium effect, concentration effect, and base inhibition studies. The decay of the 8-nitro colored form confirms this and implicates the quinoline NH as the source of the phenolic hydrogen of the uncolored form. The overall deuterium isotope effect of 1.84 denotes the non-rate-determining nature of the participation of the OH and NH. Since plots of the logarithm of the decay rate constant k vs. solvent Z or E/sub T/ values show linear relationship and large negative ΔS values accompany the decay process, a rotation mechanism is postulated for the Z→ E hydrazone isomerization. Also, acid catalysis substantiates the mechanistic scheme proposed for the decay process. (U.S.)