Spectral properties (anisotropy coefficients calculated for absorption, emission and fluorescence decay time) of two stilbazolium merocyanine dyes have been determined to evaluate the applicability of these dyes as sensitizers in photodynamic therapy. The dyes were embedded in an anisotropic polymer matrix. Analysis of the emission decay components measured in polarized light provides information on the interactions of the dye molecules with the polymer matrix being a model of an anisotropic biological system. Different values of the emission anisotropies obtained from various polarized components of fluorescence decays have shown that the orientations of the dye molecules influence their interactions with the polymer. This means that differently oriented dye molecules located in biological systems should exhibit different interactions with membranes. The chain length and type of side groups attached as well as the salt form of the dye molecule were shown to influence the dye-polymer interactions and should be taken into account before the application of merocyanine dyes in medicine. These dyes seem to be promising optical sensors with spectral properties, including the calculated anisotropy coefficients, sensitive to the molecular environment, useful to study orientation and interaction with neighbouring molecules in biological membranes.
Time-resolved photothermal responses of merocyanine dyes were used to estimate such parameters as the quenching rate constant of dye triplet states and dye capability of singlet oxygen generation, which are helpful in explanation of the dye photochemical activity. The results have shown that the high photostability of the salt form of merocyanine dyes and their resistance against reactive oxygen species as well as dye-oxygen interaction by triplet-triplet energy transfer make merocyanines efficient singlet oxygen generators in simple and complex systems.
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