Diffusion process of methyl red in organic solvents studied by the transient grating method

Time dependence of the transient grating (TG) signal after the photoexcitation of methyl red (MR) is investigated in organic solvents, especially in benzene, ethanol, and 2-propanol. The origin of the TG signal, which represents the mass diffusion of MR, is attributed to the phase grating. The cis form of MR, which has a slightly smaller molecular size than the trans form, is found to diffuse more slowly than the trans form in alcoholic and aldehyde solutions. The measured diffusion coefficients (D) in the three solvents are compared with theoretically calculated values. It is shown that the measured D are in good agreement with the values calculated by using the modified Stokes-Einstein equation. Diffusion coefficients in 2-propanol at low temperatures are also measured and compared with the calculated values.