Triplet excitation transfer in triphenylene columnar phases

Triplet excitation transport occurring in the columnar liquid crystalline phase of a triphenylene derivative at room temperature is studied by transient absorption spectroscopy with nanosecond resolution. The properties of the triplet excitons are evidenced by doping the mesophase with different concentrations of 2,4,6trinitrofluoren-9-one (TNF) which is inserted in the stacks of the triphenylene cores (T) and acts as energy trap. It is shown that triplet migration and trapping leads to the formation of the ion-pair 3(T⁺, TNF^-), whose recombination rate constant is 5.5 × 10⁵ s^-1. The comparison of the experimentally determined time dependence of the ion-pair concentration with numerically simulated curves on the basis of an one-dimensional random walk model allows the determination of the hopping time (2 ±;; 1 ps). The latter value is close to that found, in a previous study, for the singlet excitation transport (1.2 ±;; 0.5 ps) in the same mesophase. This is in agreement with the finding that interactions due to intermolecular orbital overlap, responsible for energy transport in the triplet state, are also the main driving force for singlet excitation transport. The migration length of the triplet exciton is limited by structural defects to a few hundreds of triphenylene cores.