Influence of the Medium Polarity on the Mechanism of Thermal Transformation of Cis-4-Aminoazobenzene Molecules Into Dye Rydimers

For the first time, an analysis was made of the kinetic features of the complex process of thermal regeneration of trans-4-aminoazobenzene dye rydimers (t-AAB2 is the ground state) after their photodissociation with the formation of cis-4-aminoazobenzene (c-AAB) molecules in solvents that differ greatly in empirical polarity according to the Reichardt scale. Previously unknown facts have been established: 1) a combination of the second stoichiometric order of regeneration (2c-AAB → 2t-AAB → t-AAB 2 ) with the kinetic first order of assembly t-AAB2; 2) implementation of different ways of t-AAB2assembly in weakly and strongly polar media. Both these facts were shown to be related to intermediate complexes (rydiplexes) between the amino groups of c-AAB and t-AAB molecules and the HO-, >O, C=O groups of solvent molecules. In highly polar alcohols (methanol, ethylene glycol), the isomerization events c-AAB → t-AAB and subsequent assembly 2t-AAB → t-AAB2occur inside relatively strong micellar cocoons formed around the primary molecular pairs of dissociated t-AAB2 rydimers. The formation of such cocoons is induced by amino-alcohol rydiplexes and is additionally fixed by hydrogen (H-) bonds of the alcohol molecules included in the cocoons. Such cocoons do not form in weakly polar diethyl ether and acetone, which do not form H-bonds between their molecules, but the presence of oxygen atoms in these solvents ensures the solvation of c-AAB, t-AAB due to the formation of rydiplexes. The latter receive relative freedom of self-diffusion with concomitant c-AAB → t-AAB isomerization. In both isomerization pathways, energy barriers are overcome in acts of thermal excitation of c-AAB molecules, followed by a change in the sp2-hybridization of nitrogen atoms of the azo group to sp-hybridization. In both cases, the reason for the decrease in the energy barrier of isomerization at the c-AAB → t-AAB stage, which was not previously understood in the literature, is explained by the formation of aminorydiplexes of c-AAB with oxygen-containing solvents, and these rydiplexes accelerate isomerization steps which do more effective inside micellar cocoons. The formation of rydiplexes with amino groups of t-AAB molecules is also responsible for the first-order kinetics in both pathways of assembly of t-AAB2 rydimers. Specific schemes for the regeneration of t-AAB2 dye are presented.