Specific orientation of dipole moments in azocrown cetocyanine dyes determined by electrooptical absorption measurements (EOAM)

Abstract From electrooptical absorption measurements (EOAM) follows that the dipole moments of azacrown ketocyanine dyes 2,5-di{(E)-1-[4-(4,7,10,13-tetraoxa-1-azacyclopentadecyl)phenyl]methylidene}-1-cyclopentanone (Compound 1) and (E)-1-(2-hydroxy-4methoxyphenyl)-3-[4-(4,7,10,13-tetraoxa-1-azacyclopentadecyl)phenyl]-2-propen-1-one (Compound 2) in the equilibrium ground state μ g and the change of dipole moments upon transition to the excited Franck–Condon state Δ a μ are large. This alteration causes a significant long-wavelength shift of the absorption and emission spectra, as well as large fluorescence Stokes shift with increasing polarity of the solvent. From the DFT calculations follows that the terminal groups of the dyes are located outside the plane of the central part of the molecules by the torsion angles that allow good conjugation of the nitrogen atoms lone pairs with π-system of the molecule. The transition dipole moment of Compound 1 m a is perpendicular to the dipole moment μ g in the equilibrium ground state and for Compound 2 m a is parallel to μ g . This is due to the differences in the geometric structure of the dyes. The distribution of fluorescence lifetime of Compound 1 significantly depends on the polarity of the solvent. Thus, ketocyanine dyes with azacrown cycles can serve not only as ion indicators but also as fluorescent probes for studying the polarity of the environment.