Correlated Polaron Transport in a Quasi-One-Dimensional Liquid Crystal

Correlated polaron transport in a quasi-one-dimensional liquid crystal

V. Duzhko,1 A. Semyonov,2 R. J. Twieg,2 and K. D. Singer1

1Department of Physics, Case Western Reserve University, Ohio 44106

2Department of Chemistry, Kent State University, Kent, Ohio 44242

2,3,6,7,10,11-Hexakis(pentyloxy)triphenylene HAT-5

OC5H11

H C O OC5H11 11 5 a

H C O OC5H11 11 5

OC5H11

a) VOCl3, CH2Cl2, room temperature, 10 min.

A two-neck 250 ml round bottom flask with stirbar was charged with 1,2- bis(pentyloxy)benzene [1] (17.0 g, 67.9 mmol) and anhydrous dichloromethane (150 ml). The flask was fitted with a thermometer/nitrogen inlet and vanadium oxychloride (27.1 g, 156 mmol) was added dropwise from an addition funnel at such a rate that the temperature of the reaction mixture does not rise above 30-34°C (addition takes approximately 20 min, the color changes to bluish- green). The reaction mixture was stirred for an additional 20 min and then anhydrous methanol

(100 ml) was added at once to destroy the complex with vanadium. After about five minutes water

(250 ml) was added all at once. The mixture was extracted with dichloromethane (4×200 ml) and after removal of the solvent the residue was subjected to two consecutive chromatographic separations; first with hexane:dichloromethane 1:1 as eluent and second, hexane:dichloromethane

9:1 to afford, after solvent evaporation, 14.4 g (85.4%) of grayish crystals.

At this stage the crude triphenylene product (14.4 g, ~19.3 mmol) may contain some phenolic groups and so it was realkylated with 1-bromopentane (5.8 g, 38 mmol), potassium carbonate (6.0 g, 43 mmol), potassium iodide (0.1 g, 0.6 mmol), and 2-butanone (150 ml) at 90°C for 12 hrs. The reaction mixture was cooled to room temperature, filtered, solvent evaporated on a rotavap and the residue dissolved in boiling cyclohexane. The warm solution was treated with charcoal (2×15 g), filtered, and applied onto silica gel (50 g). The impregnated material was placed on top of a 1500 ml flash chromatography column, filled with silica gel (25 cm of column height), was eluted with hexane (3×500 ml) to remove excess of 1-bromopentane and then the column was left under suction for 12 hrs (air is pulled through the column through the bed of silica gel). Then elution was resumed with dichloromethane (1 to 10% gradient in hexane). The first ~10% of the HAT-5 off the column was discarded, the rest of the fraction was collected, solvents evaporated on a rotavap to provide 8.3 g (49%) of white crystals. This purified material was recrystallized from 1- propanol/chloroform (residue-free), dissolved in dichloromethane (residue-free), the solution was filtered through 0.45 syringe filter, solvent evaporated and the residue was recrystallized from cyclohexane to provide 6.04 g (35%) of snow-white crystals.

Crystals of HAT-5 prepared and purified in this fashion do not change color when exposed to ambient lab conditions (air and light) for two weeks. The spectroscopic and thermal data were in accordance with the literature. [2] FIG.1. Temperature dependence of the hole mobility within the liquid crystal mesophase of HAT5 for two sample thicknesses of 5 m and 12.5 m measured at the electric field F= 2×104 V/cm . FIG.2 Differential scanning calorimetry of HAT5 with heating and cooling rates of 10 oC/min. The peak transition temperatures and the associated enthalpies are shown. FIG.3 Photocurrent transients measured with ac or dc applied biases of 50 V in a 12.5 m-thick HAT5. Parameters of the ac bias are: repetition rate of 10 Hz, pulse duration of 10 ms, and delay between the onsets of the bias and laser pulses of 200 s. [1] CAS Registry Number 142947-21-5

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