Rotation of aromatic hydrocarbons in viscous alkanes. 1. Methylcyclohexane

Time-resolved single-photon counting has been used to measure the decay of fluorescence anisotropy of a number of aromatic hydrocarbons in methylcyclo hexane excited by synchrotron radiation. Changing the temperature (-60 to - 140 degrees C) produces large changes in the viscosity, eta. Most solutes w ere excited both parallel and perpendicular to the emission axis with the a im of determining all three principal rotational diffusion coefficients, D. The results are compared to predictions from hydrodynamic theory made usin g the "slip" and "stick" approximations and literature data for the n-alkan es and other solvents. Rotation times should be proportional to eta/T, i.e. , D eta/T should be constant. This is found to be approximately true for 9, 10-disubstituted anthracenes; it does not hold for anthracene, perylene, an d triphenylene for which D eta/T increases markedly on cooling. Also, again st prediction, the ratios between D values for different molecular axes are found to change with temperature.