Liquid Crystals

We extended the use of Dan and Ndi derivatives to direct assembly of extended columns with alternating face-centered stacked structure in the solid state.  A variety of 1:1 Dan:Ndi mixtures produced mesophases that were found to be stable over temperature ranges extending up to 110˚C.  Analysis of these mesophases indicates mixtures with soft/plastic crystal phases and a few mixtures with the thermodynamic properties of true liquid crystals, all composed of alternating donor-acceptor columns within. Importantly, a correspondence was found between the clearing and crystallization points of the mesophase mixtures and the melting/clearing points of the component Ndi and Dan units, respectively. In particular, the clearing point transition is generally similar to the melting point of the Ndi component, while the crystallization temperature of the mixture is generally very similar to the Dan component melting points.A straightforward interpretation of these trends is that the 1:1 Dan:Ndi mixture clearing points track in a relative way with restriction of motion of the Ndi side chains, and the crystallization points track with restriction of motion of the Dan side chain component.  In other words, there is more order associated with Ndi long axes because the Ndi side chains have more restricted motion in the mesophase, and less order along the Dan long axes because the Dan residues maintain greater freedom of motion in the mesophase. 

Taking our previous research in liquid crystals in a new direction, we have designed novel conjugated donor/acceptor complexes from 1,4-naphthalenedicarboxylicimide and 1,3-dialkoxybenzene units linked by a triple bond (Figure1).  These complexes are similar in structure to the naphthaleneimide acceptors and phenolic donors that have been extensively studied by our group, but these new complexes are designed to have a large molecular dipole.  Crystal structures of some initial complexes show an unexpected and unusual stacking conformation: only half of the units stack donor to acceptor (Figure 1).  These complexes have interesting optical properties, which include fluorescence both in solution and in the solid state as well as solvatochromism.  These properties are due to aromatic stacking as well as conjugation.  However, more importantly, these complexes should have the ability to switch stacking configurations in the liquid crystalline state under an applied electric field.  In the presence of an electric field a heterojunction between donors and acceptors should develop (Figure 1).  This property combined with the optical characteristics of this material make this type of compound a possible candidate for use in organic photovoltaics.

Figure 1.  At top is a general structure of donor/acceptor complex where the donor is shown in red and the acceptor in blue.  A representation of the unusual stacking structure of the complex is shown on the left (the plane of the aromatics is perpendicular to the page).  In an electric field the stacking should change to that shown on the right, thus allowing for the possibility of photovoltaic behavior.

Polymers

Complementary Ndi and Dan repeat units were incorporated independently into long polymer chains.  These chains were dissolved together in basic water yielding a significant viscosity increase over independently dissolved chains.The significant increase in viscosity of the mixed polymers verifies a high degree of interchain interaction in the P1+P2 mixture. Films of the polymer solutions were analyzed using atomic force microscopy and scanning electron microscopy and showed unique macrostructure formation only from the polymer mixture.  Preliminary attempts at aligning the proposed P1 + P2 threads by forming fibers from the complementary polymer mixtures were made by precipitating them after quickly passing through a small aperture.  When an aqueous solution of P1 + P2 (0.2M NaOH, 3% total weight polymer) was injected via a 30 gauge needle into 1M HCl it yielded long, delicate fibers shown in the (a) side of the figure which were up to several centimeters in length.  Similar solutions of either P1 solubilized with SDS, or P2 formed no fiber, but rather very fine precipitates.The fiber appears to be made up of densely packed threads that are oriented in a uniform direction to form an elongated fiber shown in the (b) side of Figure. This is the first use of the directed non-covalent interactions between electron-deficient and electron-rich aromatics to affect polymer properties and structure in aqueous solution, films, and fibers.