CHARGE-CARRIER DYNAMICS IN MESOMORPHIC HEXAKIS(PENTYLOXY)TRIPHENYLENE

The minimum sum of the charge carrier mobilities, Sigma mu(min), within single domains of hexakis(pentyloxy)-triphenylene (HPT) has been determined, using the pulse-radiolysis time-resolved microwave conductivity technique. In the polycrystalline solid, Sigma mu(min) is 5 x 10(-7) m(2) V-1 s(-1) from -75 to +50 degrees C. In the mesophase, Sigma mu(min) decreases from ca. 2 x 10(-7) m(2) V-1 s(-1) at close to the transition temperature of 69 degrees C to 1.0 x 10(-7) m(2) V-1 s(-1) at 110 degrees C, just prior to the transition to the isotropic liquid for which Sigma mu(min) < 2 x 10(-8) m(2) V-1 s(-1). The maximum achievable, one-dimensional, intracolumnar mobility for a perfectly aligned single domain of HPT is estimated to be approximately 10 x Sigma mu(min) or close to 10(-6) m(2) V-1 s(-1) for the mesophase. The results are in reasonable agreement with recent time-of-flight measurements by Haarer et al. At the solid-to-mesophase transition, the half-life of the charge carriers suddenly;increases from 70 ns to 2 mu s. This is attributed to a decrease in the efficiency of intercolumnar charge recombination via through-bond electron tunneling caused by the melting of the saturated hydrocarbon barrier.