Mechanism of charge transport along columnar stacks of a triphenylene dimer

We have measured the mobility of charge carriers along the one-dimensional conducting pathways provided by columnar stacks of triphenylene units in the liquid crystalline dimer 1,10-di-[3′,6′,7′,10′,11′-pentabutyl- oxytriphenylenyl-2-oxy]decane using the pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) and the time-of-flight (TOF) techniques. The high frequency (30 GHz) and TOF intracolumnar mobilities approach the same value (ca. 0.01 cm² V^-1 s^-1) at the highest temperatures studied (about 400 K). When the temperature is lowered, the high-frequency mobility remains almost constant within the range (1.5 ± 0.5) × 10^-2 cm^-2 V^-1 s^-1 down to 170 K. The value of μ_TOF in contrast decreases dramatically at lower temperatures, reaching a value as low as 2 × 10^-6 cm² V^-1 s^-1 at 130 K. The different temperature dependences found are attributed to structural disorder within the columnar stacks. The experimental data are compared with predictions of the influence of static disorder on charge transport using different transport models. Our analytical and computer simulation studies show that the only transport mechanism consistent with both sets of experimental data is one involving thermally activated jumps over barriers with an exponential distribution of barrier heights. The experimental mobility values could be reproduced with a mean barrier height of 0.024 eV and an attempt frequency for jumping equal to 1 ps^-1. Our experimental and theoretical findings illustrate the added insights into the underlying mechanism of charge transport in complex molecular materials that can be gained from the combined results of TOF and high-frequency mobility measurements.