Preferential solvation studied by the fluorescence lifetime of acridine in water-alcohol mixtures

Fluorescence lifetimes (tau(F)) of acridine have been measured in water-alcohol solvent mixtures at room temperature. The tau(F) of acridine in pure water (11.5 +/- 0.1 ns) was drastically decreased upon the addition of small amounts of alcohol; i.e. tau(F) 1.2 +/- 0.01 in 0.5 mole fraction of ethanol in water and tau(F) = 0.4 +/- 0.01 ns in pure ethanol. The extent of the decrease in tau(F) becomes more evident in the following order: methanol < ethanol <propan-1-ol. This is the order of increasing hydrophobic interaction between alcohols and acridine, suggesting that alcohol molecules preferentially solvate acridine in water. Such an observation has been discussed in terms of the degree of preferential solvation.