Charge Transport in Mesomorphic Phthalocyanines Studied by Pulse-Radiolysis Time-Resolved Microwave Conductivity

Abstract

The pulse‐radiolysis time‐resolved microwave conductivity (PR‐TRMC) technique has been used to obtain information on the transport of charge within columnarly stacked, peripherally octaalkoxy‐substituted phthalocyanines. Data are presented on the one‐dimensional, intracolumnar charge mobility and on the timescale of quasi‐two‐dimensional intercolumnar electron tunnelling. Particular attention is paid to materials that are liquid‐crystalline at room temperature, because of their potential technological importance in optoelectric charge transport layers and molecular semiconductor devices. The relevance of the data to channeled charge transport in aligned thin layers is discussed.

Charge Recombination via Intercolumnar Electron Tunneling through the Lipid-like Mantle of Discotic Hexa-alkyl-hexa-peri-hexabenzocoronenes

“…An analogous dramatic increase in the time scale for the decay of the radiation-induced conductivity was previously reported for a series of octa- n -alkoxy-subsituted phthalocyanines with chain lengths varying from 6 to 18 carbon atoms. , This was explained by a mechanism in which the “long-lived”, after-pulse conductivity was attributed to those electron hole pairs for which the electron and the hole become localized on separate columnar stacks. Their recombination is then retarded by the aliphatic hydrocarbon mantle between the stacks and can only occur via intercolumnar electron tunneling.…”

Mechanism of Charge Transport in Anisotropic Layers of a Phthalocyanine Polymer

“…In contrast to Σμ 1D , t 1/2 is seen to be very different for the two polymers with that for C1C18 approximately an order of magnitude larger than that for the shorter alkyl chain compound. This effect has previously been found for the monomeric compounds and is taken to indicate that the decay process involves charge recombination via intercolumnar electron tunneling through the intervening saturated hydrocarbon mantle …”

“…In previous TRMC measurements , on a C9C9 polysiloxane derivative the mobility was also found to display only a small temperature dependence. The absolute value, however, was a factor of 3 larger than the average value for the present compounds.…”

“…This effect has previously been found for the monomeric compounds and is taken to indicate that the decay process involves charge recombination via intercolumnar electron tunneling through the intervening saturated hydrocarbon mantle. 31 As for Σµ 1D, t 1/2 is seen to be almost independent of temperature with values of 30 and 380 ns given by the horizontal straight lines in Figure 9b for C1C8 and C1C18, respectively. This temperature independence of the rate of long-distance electron tunneling through saturated hydrocarbon barriers has also been found for charge recombination in model donorbridge-acceptor molecules.…”

“…The eventual after-pulse decay of the conductivity, an example of which is shown for C1C18 at room temperature in Figure 8, displays disperse kinetics. As in the past, 31 we take the first half-life time t 1/2 as indicated in Figure 8 to be a characteristic measure of the time scale for the decay. The values of t 1/2 determined are plotted against temperature in Figure 9b.…”