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 th e liquid crystalline dimer 1,10-di-[3',6',7',10',11'-pentabutyloxytriphenyl enyl-2-oxy]decane using the pulse-radiolysis time-resolved microwave conduc tivity (PR-TRMC) and the time-of-flight (TOF) techniques. The high frequenc y (30 GHz) and TOF intracolumnar mobilities approach the same value (ca. 0. 01 cm(2) V-1 s(-1)) at the highest temperatures studied (about 400 K). When the temperature is lowered, the high-frequency mobility remains almost con stant within the range (1.5 +/- 0.5) x 10(-2) cm(2) V-1 s(-1) down to 170 K . The value of mu(TOF) in contrast decreases dramatically at lower temperat ures, reaching a value as low as 2 x 10(-6) cm(2) V-1 s(-1) at 130 K. The d ifferent temperature dependences found are attributed to structural disorde r within the columnar stacks. The experimental data are compared with predi ctions of the influence of static disorder on charge transport using differ ent transport models. Our analytical and computer simulation studies show t hat the only transport mechanism consistent with both sets of-experimental data is one involving thermally activated jumps over barriers with an expon ential distribution of barrier heights. The experimental mobility values co uld be reproduced with a mean barrier height of 0.024 eV and an attempt fre quency for jumping equal to 1 ps(-1). Our experimental and theoretical find ings illustrate the added insights into the underlying mechanism of charge transport in complex molecular materials that can be gained from the combin ed results of TOF and high-frequency mobility measurements.