Photophysical behaviours of some 2-styrylindolium dyes in aqueous solutions and in the presence of cyclodextrins

Host–guest inclusion type association between native β-cyclodextrin and randomly substituted methyl-β-CD and two 2-styrylindolium cationic dyes, e.g. 1,3,3-trimethyl-2-(4-diethylaminostyryl)-3H-indolium iodide (D1) and 1,3,3-trimethyl-2-4-(N-2-cyanoethyl,N-methyl)-aminostyryl]-3H-indolium iodide (D2), are reported. The described indolium derivatives belong to the rarely investigated class of unsymmetrical polymethines. The complex formation was studied in aqueous buffer solutions with two pH values (7.2 and 3) by means of absorption and steady-state fluorescence spectroscopy. The association equilibrium constant (K), the molar absorptivity and the stoichiometry of the complexes were evaluated using the modified Benesi-Hildebrand approach. The complex stability was affected by the pH of the solution and by the type of CD. The results obtained indicate that D1 forms 1:1 complexes with both β-CD and Me-O-β-CD, whereas D2 does not form stable complexes with Me-O-β-CD and in acidic medium. The fluorescent intensity of D1 in the presence of CDs increases over four times relative to the intensity of the pure dye solutions, presumably via inclusion of the dye into the cyclodextrin cavity due to rigidity of the structure.