Journal of Biomedical OpticsPhotophysical properties of fluorescence probes I: dialkylamino stilbazolium dyes
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The photophysical and photochemical behavior of different isomers of dimethylamino-stilbazolium dyes was investigated in n-alcohols at different temperatures by stationary and time-resolved fluorescence measurements. At room temperature, an efficient nonradiative deactivation knr was observed, which increases in high-polarity and low-viscosity solvents. For viscosities < 10 cP, knr possesses a linear viscosity dependence. At low temperatures, the fluorescence spectra show considerable blue shifts, which are absent in solvent polarity changes relative to room temperature, and a spectral narrowing. From the combined quantum yield and lifetime measurements, knr is shown to decrease strongly with decreasing temperature, by more than two orders of magnitude, and kf is shown to increase. These facts and the observation of an initial lifetime lengthening by increasing the temperature from 77 K indicate several simultaneously emitting states with a temperature-dependent population efficiency. These states were tentatively assigned to several possible single-bond twisted excited conformers. This assignment is supported by the near absence of isomerization.