Rotation diffusion of aromatic dianions

Rotational diffusion of the major axis of anthracene, tetracene, and pentacene dianions in dimethyl ether at room temperature is measured by picosecond spectroscopy. Data is compared with predictions of three types of models; two based upon the hydrodynamics of a fluid about spheroidal particles, with sticking and slipping boundary conditions, and a third based upon hard-sphere, hard cylinder collisions. The experimental rotational correlation times are proportional to the third power of molecular length. The hydrodynamic model with slipping boundary conditions mimics this observation but predicts magnitudes that are too small. Sticking boundary conditions give the correct magnitude for anthracene but incorrect axis length dependence. The collision model provides roughly correct magnitude and length dependence.