Application of time-resolved linear dichroism spectroscopy: Relaxation of excited hexamethylbenzene/1,2,4,5-tetracyanobenzene charge-transfer complexes

The charge-transfer complex formed between hexamethylbenzene and 1,2,4,5-tetracyanobenzene has been studied using time-resolved linear dichroism spectroscopy. These studies allowed the angle between the transition moment vectors for the absorptions of the ground-state charge-transfer complex and the product contact-radical ion pair to be measured. The results were compared with predictions based on the Mulliken two-state model acid an assumed structure of the ground-state complex. The comparison between the observed and predicted angles supports the notion that relaxation of the excited charge-transfer complex leads to a change in the complex geometry. The time dependence of the dichroism allows the average rotational diffusion time constant to be determined. For this complex in 1,2-dichloroethane at room temperature, tau (OR) = 75 ps. This value is comparable to what would be expected based on Debye-Stokes-Einstein diffusion theory at the stick boundary limit.