A STUDY OF THE FLUORESCENCE AND REORIENTATION DYNAMICS OF CARMINIC ACID IN PRIMARY ALCOHOLS

We have studied the fluorescence lifetime and reorientation dynamics o f carminic acid in primary aliphatic alcohols and water to develop an understanding of its dynamic properties. The fluorescence response exh ibits a biexponential decay, which is attributed to emission from two stable conformers of carminic acid. The reorientation behavior of carm inic acid depends sensitively on the chemical identity of the solvent. The dynamical response of carminic acid is predicted by the Debye-Sto kes-Einstein (DSE) equation with a sticking boundary condition for wat er, methanol, ethanol, and 1-propanol. In higher alcohols, carminic ac id reorientation follows DSE behavior in the slip limit. These data, t aken collectively, indicate that carminic acid interacts strongly with polar solvents and that the structural degrees of freedom available t o this probe molecule do not affect its dynamical responses over a tim e scale of several nanoseconds.