Comparative analysis of the absorption and fluorescence spectra and fluorescence excitation spectra of thioflavin T (ThT) in various solvents and in the composition of amyloid fibrils has shown that ThT, when excited in the region of the long-wavelength absorption band, fluoresces in the spectral region with a maximum at 478-484 nm. The appearance in aqueous and alcohol solutions of a fluorescence band with a maximum near 440 nm has been attributed to the presence in the composition of the ThT preparations of an impurity with an absorption band in the 340-350-nm range. The literature data showing that in glycerol ThT has a wide fluorescence spectrum with two maxima are due to the artifact connected with the use of a high concentration of the dye. It has been suggested that the cause of the low quantum yield of ThT aqueous and alcohol solutions is the breakage of the system of conjugated bonds due to the reorientation of the benzothiozole and benzaminic rings of ThT in the excited state with respect to one another. The main factor determining the high quantum yield of fluorescence of ThT incorporated in fibrils is the steric restriction of the rotation of the rings about one another under these conditions. The suggestions made have been verified by the quantumchemical calculation of the ThT molecule geometry in the ground and excited states.

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Spectral and luminescent properties of an indotricarbocyanine dye are studied in solutions and after deposition on quartz or silicon substrates. It is found that the dye molecules self-assemble in aqueous EtOH solutions to form H * -aggregates. The absorption band of the H * -aggregates shows a hypsochromic shift of 192 nm (5291 cm -1 ) relative to the absorption maximum of dye monomers (706 nm) and has a full width at half maximum of 21 nm (797 cm -1 ). The morphology of the H * -aggregates of the indotricarbocyanine dye is studied for the fi rst time. It is found that the aggregates are rod-like species ~10 nm high, 100 nm wide, and several micrometers long. H-aggregates with a fl uorescence maximum at 560 nm and Stokes shift of 325 cm -1 in addition to non-fl uorescent H * -aggregates form in aqueous EtOH solutions and are nanoparticles with a height of 1-3 nm and lateral dimensions of ~100 nm.

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