Truxenes columnar phases

In discotic systems, the roles of ATu and K22 are reversed, because in such phases the bend deformations require the lowest energy [181]. Measurements of splay and bend constants in a homologuous series of discotic n-hexa(alkanoylox)truxenes [76] revealed that K22 is always smaller than A, . The splay/bend ratio approached unity at the high temperature transition to the columnar phase. Qualitatively different results have been obtained, however, by Raghunathan et al. [75], who found K22>Kn in a disco-tic nematic phase enclosed between two columnar phases. The authors interpreted this unexpected result as being a consequence of short-range columnar order. 

The structure of the columnar phase of the truxene compound is essentially the same as that proposed originally by Chandrasekhar et al. [1]. The possibility of such a structure i.e. a three-dimensional body in which the density is a periodic function of only two dimensions, was in fact envisaged many years ago by Landau and Lifshitz [20] (see Sec. 10.3). 

Even though no tabular data are available for truxene derivatives, inclusion in this Datareview is important because these materials exhibit discotic nematic phases. The truxene phase data presented by Busine and co-workers [11] are for the transition of the discotic nematic phase to the disordered rectangular columnar phase (TABLE 18). Over the phase range studied which was up to 900 bar, the relationship between T and p is most conveniently represented as linear. Once again the magnitudes... 

This Datareview has concentrated on the nematic-isotropic transition except for the case of the truxenes where the discotic nematic to a columnar phase was considered. No smectic or crystal phase data have been considered here even though the literature reviewed often contained pressure-temperature data involving these phases. Data for the crystal-nematic transition are almost always included in the studies, but only sometimes even when the compound exhibits a smectic phase are such data included. No surprises exist in the body of pressure-temperature data for the nematic-isotropic transition. This transition for any nematogen could be described by the polynomial fitting equation with surprisingly small uncertainties in the average fitting parameters for example... 

Optically active esters of benzene, triphenylene and truxene have been studied [40], The triphenylene and truxene derivatives (structures 2 and 4, respectively, with R = (-h)- or (-)-CH3-(CH2)5-CH(CH3)-CH2-CO O) do show mesophases, but of the non-chiral columnar type. In other words, this chiral substituent does not change the nature of the phases. The benzene derivative is non-mesomorphic. However, when these compounds are mixed with... 

Kato and coworkers also synthesized Cj symmetric, truxene-based TTF and TCNQ derivatives 244 and 245 (Scheme 6.65). These compounds exhibit columnar hquid crystalline (LC) phases, and serve as n- and p-type semiconducting materials with the carrier mobilities on the order of 10 cm V" s", ... 

TABLE 18 Fitting equati(Hi parameto-s fw discotic nematic-disra-dered rectangular columnar [vessure-temperature phase transition lines for hmnologous truxenes. Fitting equation is T/K = a + b(p/bar). 

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