Multiynes

J. Reiche, R. Dietel, D. Janietz, H. Lemmetyinen, L. Brehmer, Edge-on Langmuir-Blodget multilayers derived from disc-shaped multiyne mesogens. Thin Solid Films 226, 265-269... 

The Ncoi phase is a nematic phase were the structural units are columns of mesogens—in this case columns containing both the multiynes and TNF. See Refs 38-40 for more details. 

As an aside, it may be of interest to the reader that although selective reflection is usually a phenonemon associated with cal-amitic liquid crystalline systems, various multiyne materials have recently been demonstrated to show selective reflection as well as a helix inversion in chiral discotic nematic phases [15, 16]. 

A widely studied class of discotic benzene derivatives are the multiynes prepared by Praefcke and coworkers [24], Like the structurally similar naphthalenes, these materials are prepared by palladium-catalysed coupling of polybromobenzenes with acetylene derivatives (Scheme 8). Their mesophase behaviour is given in Table 8. The hexakisalkynylbenzenes themselves exhib-... 

Table 8. Transition temperatures for benzene multiynes (see Scheme 8 for structures).

Scheme 11. Synthesis of naphthalene multiynes. Reagents (a) RCbH4C=CH/Pd[0].

Two general series of naphthalene derivatives are known to give discotic liquid crystal phases, the multiynes [27, 33] and the p-alkoxyphenoxymethyl derivatives [28]. The multiynes are synthesized through... 

Scheme 17. Synthesis of multiyne derivatives of triphenylene [24], Reagents (a)RCfiH4CsCH/Pd(PPh3)2Cl2/PPh3/Cu(l)/ 100°C/38h, 52-51%.

Likewise, as for instance analogously substituted members of the radial multiyne families 3a and 3C compiled in TABLE 3, demonstrate, it seems friat the wider their molecular core, the more stable and broader the No phase formed on heating [12]. Re-entrant occiurences of an No phase are also known,... 

TABLE 3 Transition temperatures ( C, on heating) of forty-six derivatives of six different radial multiynes (type 3a to 3f) and of the cyclic multiyne type 3g with siq>er-disc cores of different diameters each exhibiting mostly an No phase wliich in four cases of group 3e are inverse. 

Five of the radial multiynes of the types 3b, 3c, and 3e exhibit die chiral nematic discotic (N d) phase. 

Multiyne structure R R Phase transition tempo atures Ref... 

Obviously, the structurally biaxial members of the naphthalene family 3e do not behave in line with the otha, benzene or triphenylene centred, multiynes and in four cases there is an inversion of the phase sequence (see TABLE 3 and compare the examples in TABLE 2). 

The syntheses of the multiynes are straightforward and are described in the references quoted in TABLE 3. 

However, in contrast to their fascinating flat molecular structures voy little attention has y been paid to possible applications. Probabfy, more discotic materials with still lower phase transition temperatures must be designed fmr such purposes. An into esting ( >tically controlled electro-optic effect in discotic nematic radial multiynes has been observed very recently [57], and hopefully, this encouraging experience will induce more and systematic work on these materials. 

---