Proof of the Absence of trans–cis Photoisomerization of Methyl Orange Rydimers in an Aqueous Medium and Inclusion Complexes

Data are presented on the ultrafast laser probing of photoinduced states of methyl orange in an aqueous medium and in complexes with cyclodextrins, along with trans -aminoazobenzene in ethanol, obtained using an ultrafast transient lens (UTL) and transient absorption spectroscopy (TAS). The analysis is based on the concept of the rydimeric structure of azo dyes in the ground state. It is shown that the discrepancy between the UTL and TAS signals of methyl orange rydimers, and the UTL and TAS signals of aminoazobenzene rydimers, is due to the inability of methyl orange rydimers to undergo trans – cis photoisomerization in an aqueous medium and in inclusion complexes, due to the dissociation of aminoazobenzene rydimers into monomers. In ethanol, an aminoazobenzene monomer with two phenylaminyl cations and Vis absorption in the wavelength range of 500–750 nm is converted to a cis- isomer. In contrast, the photodissociation of methyl orange rydimers into monomers does not result in the formation of cis -isomers. The reasons for this difference are the immobilization and limited mobility of methyl orange rydimers in aqua capsules and cyclodextrin nanocavities.