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

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