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each other is reduced. Indeed, a reduced sliding ability of the molecules, dependent on T,,, and thus an increase in intermolecular cross-linkage is demonstrated by the delayed increase in [?/Ii shown in Figure 2.

We showed for the first time that discotic liquid crystals are well suited for a new class of fast photoconducting materials. Due to their spontaneous orientation and their dynamical fluctuations in the mesophase, they show exceptionally high mobilities of 1 * cm2/Vs, about two to three orders of magnitude higher than those obtained for conventional amorphous polymers. Further on, the Gaussian transport (for holes in the mesophase) is remarkable, which is characterized by the existence of a conduction band and the absence of trapping states. In contrast, the charge carrier transport in amorphous photoconductors is generally trap-dominated which limits technical properties, leading to low carrier mobilities and undesirable memory effects. Measurements of the transient photocurrents, using the time-of-flight technique, have been carried out in the discotic liquid crystalline photoconductor hexapentyloxytriphenylene. The carrier mobilities and the underlying transport mechanisms both in the crystalline phase and in the mesophase are discussed in the framework of ideal intrinsic (Gaussian) transport, multiple shallow trapping, and deep trapping transport, respectively.

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As we have shown through several examples, combinations of donor and acceptor substituents stabilize cyclopolyenes containing (4n) 7c electrons. They may even take on the properties of arenes (e.g., positive resonance energies)."] The question of double-bond delocalization in such systems could only be approached so far for diethyl 2,4-bis(diethylamino)cyclobutadiene-1 , 3 -d i~a r b o x y l a t e ;~~-~~ in contrast to tetra-tert-butylcyclobutadiene,'5,61 it exhibits a "benzene-like" double-bond delocalization ( " .2, , , " = 1.24; see below). In continuation of our studies on 2,5-diazapentalene~,'~] we posed the question whether it might also be possible to achieve a delocalization of the double bonds in pentalene by the incorporation of suitable donoracceptor substituents. By way of 1,3,5-tri-tert-butylpentalene as example it has been that alkylated pentalenes display bond alternation and that a rapid valence isomerization occurs.These findings are in agreement with the double-bond localization parameter " . 2, , , " (see Table 1) calculated according to Binsch and Heilbronner et a1.;[".'21 its value is significantly higher than the critical value. The "A, , , " values of the compounds 1,3,4,6-tetraamino-, 2,5-diaza-, and, above all, 1,3,4,6-tetraamino-2,5-diazapentalene are less than the critical value. This finding and the larger HOMO-LUMO gap lead to the expectation that 1,3,4,6-tetradonor-2,5-diacceptor-substituted pentalenes should exhibit aromatic stabilization and a delocalized n-bonding system (cf. also Ref. 1131). Table I. The effect of substituents on the double-bond localization in pentalenes (A,,,,== 1.7) [ax=ac+hxflcc; hNnz= 1.5, h,=O.S]). The ring nitrogen atoms are also regarded here as substituents.