Molecular rotor dye thioflavin T (ThT) is almost nonfluorescent
in low-viscosity solvents but highly fluorescent when bound to amyloid
fibrils. This unique property arises from the rotation of the dimethylaniline
moiety relative to the benzothiazole moiety in the excited state,
which drives the dye from an emissive locally excited state to a twisted
intramolecular charge-transfer state. This process is viscosity-controlled,
and therefore, we can use the quantum yield of ThT to assess the viscosity
of the environment. In this study, we have investigated the quantum
yield of ThT (φThT) in various compositions of six
alcoholic solvent mixtures of glycerol with methanol, ethanol, n-propanol, iso-propanol, n-butanol, and tert-butanol. We have proposed an
empirical model using φThT as a function of the mole
fraction of glycerol to estimate the interaction parameters between
the components of the solvent mixtures. This analysis allowed us to
predict the extent of nonideality of the solvent mixtures. The Förster–Hoffmann-
and Loutfy–Arnold-type power law relationship was established
between the quantum yield of ThT and bulk viscosity for solvent mixtures
of methanol, ethanol, n-butanol, and tert-butanol with glycerol, and it was found to be similar in nature
in all the four mixtures. Applying this knowledge, we proposed a methodology
to quantify and predict the bulk viscosity coefficient values of several
compositions of n-propanol–glycerol and iso-propanol–glycerol mixtures which have not been
previously documented.