Microscopic details of rotational diffusion of perylene in organic solvents: Molecular dynamics simulation and experiment vs Debye-Stokes-Einstein theory

Molecular dynamics simulations and time-resolved linear dichroism measurements have been employed to investigate rotational diffusion of perylene in two organic solvents, cyclohexane, a nonpolar solvent, and 2-propanol, a polar solvent. Both experiments and simulations yield a biexponential rotational anisotropy decay for the long in-plane axis. The calculated time constants were 9 and 44 ps in cyclohexane at 300 K, 13 and 75 ps in 2-propanol at 300 K, and 25 and 126 ps in 2-propanol at 263 K, in excellent agreement with corresponding time-resolved linear dichroism measurements of 14 and 52 ps, 10 and 51 ps, and 22 and 240 ps respectively. Although the viscosity of 2-propanol is more than two times that of cyclohexane at room temperature, the measured rotational reorientation times and the calculated average rotational diffusion coefficients of perylene are similar in the two solvents, demonstrating a breakdown of simple hydrodynamic theory. Analysis of the calculated rotational diffusion coefficients for the individual molecular axes showed that diffusion was highly anisotropic, with the fastest rotation around the out-of-plane axis z. This dominant motion occurred at comparable rates for perylene in cyclohexane and 2-propanol, leading to similar values of average rotational diffusion coefficients in the two solvents. The hindered spinning of perylene in cyclohexane relative to 2-propanol could be rationalized in terms of tighter packing of the former solvent around the solute in the molecular plane.