Spectral and Photochemical Properties of Styrylquinoline–Carbazole Dyad in Neutral and Protonated Forms

The spectral–luminescent and photochemical properties of the neutral and protonated forms of the styrylquinoline–carbazole dyad SQ3Cbz, in which the carbazole unit (Cbz) is linked to the 2-styrylquinoline (SQ) residue via an oxytrimethylene bridge, have been studied. It has been shown that there is strong fluorescence quenching of the Cbz moiety in both forms of SQ3Cbz, which is due to energy transfer to the SQ moiety via the Förster mechanism. In the protonated form of the dyad, where the proton is localized on the SQ unit, a significant decrease in the quantum yields of trans–cis photoisomerization and fluorescence of the SQ moiety is observed, as well as a hypsofluoric shift of its emission band compared to the model SQ chromophore. It is assumed that the observed effects are due to the formation of folded dyad conformers, in which the SQ chromophore has a more rigid structure, and/or to the process of electron transfer from Cbz to the protonated SQ unit. DFT calculations predict the possibility of forming folded dyad conformers, which are stabilized by π-stacking interactions between Cbz and SQ units.