The Optical and Charge Transport Properties of Discotic Materials with Large Aromatic Hydrocarbon Cores

The optical absorption and charge transport properties of a series of discotic molecules consisting of peripherally alkyl-substituted polycyclic aromatic cores have been investigated for core sizes, n, of 24, 42, 60, 78, 96, and 132 carbon atoms. In dilute solution, the wavelength maximum of the first absorption band increases linearly with n according to γ_max = 280 + 2n and the spectral features become increasingly broadened. The two smallest core compounds display a slight red-shift and increased spectral broadening in spin-coated films. For derivatives with n = 24, 42, 60, and 96, the one-dimensional, intracolumnar charge mobility, ∑μ_1D, was determined using the pulse-radiolysis time-resolved microwave conductivity technique. For the compounds which were crystalline solids at room temperature, ∑μ_1D lay within the range 0.4-1.0 cm²/Vs. In the discotic mesophases at ca. 100 °C, ∑μ_1D was somewhat lower and varied from 0.08 to 0.38 cm²/Vs. The mobility values in both phases are considerably larger than the maximum values found previously for discotic triphenylene derivatives. However, the recently proposed trend toward increasing mobility with increasing core size is not substantiated by the results on the present series of increasingly large aromatic core compounds.