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8 May 1995 Cyanine dyes with branched (trinuclear) polymethine chains: long-wavelength biomembrane probes
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We suggest a new photophysical principle of response of dye fluorescence spectra to the dynamics of its molecular environment. The branched symmetric dye molecule is excited to the degenerate level, which, being unstable, is relaxing to a low-symmetry isomer with substantial Stokes shift. Thus, solvent-viscosity-dependent ratio of fluorescence intensities of unrelaxed and relaxed forms may be observed. The principle was applied to the dyes with branched three-chain polymethine chromophore. In the ground state they are in propeller-like conformation of symmetry C3. In the excited-state one of the degenerate lower vacant molecular orbitals becomes occupied, which makes the dye conformation unstable. This results in out-of-plane rotation of the end groups with isomerization in the chain and temporal loss of symmetry. This effect should be observed in fluorescence spectra and result in dual emission from unrelaxed and relaxed states, which we observed in glycerol solutions, but do not see in low-viscous solvents. In biomembranes this effect allows to suggest a simple and convenient method of observation of dynamics in lipid bilayer. In phospholipid liposomes however, only the long-wavelength band is seen, which is probably due to the probe location on the surface, in highly mobile environment.
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Alexander P. Demchenko, Zoya N. Volovik, Yulia L. Briks, and Alexey D. Kachkovsky "Cyanine dyes with branched (trinuclear) polymethine chains: long-wavelength biomembrane probes", Proc. SPIE 2388, Advances in Fluorescence Sensing Technology II, (8 May 1995);

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