is expected to facilitate column formation in the bulk due to its partial disc-like design () already bestowed upon synthesis. It is expected to self-assemble into a complete disc and thereupon to a stacked arrangement of discs. It has been reported that symmetrically substituted methyloxy-3CHd-1 and ethyloxy-3CHd-2 form crystalline compounds that melt above 177 °C, while 3CHd with longer chains as well as 2CHd-10
form a columnar, hexagonal, disordered (C
) mesophase in the bulk [6
]. AFM images in show the morphology of 2CHd-10
on HOPG after deposition from high-concentration solutions: around 2.4 wt % for 2CHd-10
and 2.8 wt % for 1CHn-10
. Albeit 2CHd-10
possessing a particularly favourable geometry for column formation in the bulk, its morphology on the graphite surface is that of fibrillar crystallites of varied lengths rather than pure fibrils, as shown in .
AFM images of randomly oriented (a) 2CHd-10 crystallites and (b) 1CHn-10 fibril bundles on HOPG obtained in high-concentration solutions.
Intrigued by the crystallite morphology of 2CHd-10, we investigated its chain length variants, namely 2CHd-6 and 2CHd-14, by AFM imaging, which vindicates their fibrillar-crystallite nature, with 2CHd-14 seeming to produce the longest fibres amongst the three, as demonstrated in . On the other hand, 1CHn-10 deviates from the wedge-shape due to having only one alkoxy chain, but optical microscopy and AFM images reveal that 1CHn-10 is capable of forming fibril assemblies extending up to several tens of micrometres, as evident from . Note that 1CHn-10, however, offers an additional hydrogen-bonding site (believed to enhance column ordering in the bulk). An explanation for this seemingly contrasting behaviour of 1CHn-10 and 2CHd-10 critically depends on the knowledge of the respective elementary fibril structures.
AFM images of (a) 2CHd-14 and (b) 2CHd-6 on HOPG taken to demonstrate the capability of the 2CHd-n series to produce fibrillar crystallites.
To investigate the structure formation of 2CHd-10 on HOPG at a molecular scale, a drop of a dilute solution of 2CHd-10 (≈0.24 wt %) dissolved in 1,2,4-trichlorobenzene (C6H3Cl3) was deposited on HOPG and the liquid/solid interface searched for the thinnest fibrils by STM. The use of dilute solutions for STM studies is prompted by the requisite of locating isolated elementary strands on the substrate. Fibre bundles are oriented mostly randomly but isolated elementary fibrils follow low-index graphite surface directions. Generally, the length of isolated fibrils exceeds the scan range of the STM (≈800 nm). is a typical STM image showing a bundle consisting of two elementary fibrils, while is a close-up of the left fibril taken to reveal the internal structure and its dimensions.
Figure 4 (a) STM image of two adjacent 2CHd-10 elementary fibrils on HOPG. Imaging parameters are V
t = 1.3 V and I
t = 0.6 nA. (b) Structure of a 2CHd-10 dimer. (c) Dimer arrangement in fibril. (d) Magnified view of the left fibril from (a) revealing details (more ...)
Having a width of 5.6 nm, the fibril consists of bright blobs arranged side-by-side in a zigzag pattern that slightly varies along the fibril. Bright blobs can sometimes be resolved into two elliptical features that are about 1.4 nm long (arrows in ). Assuming that electron-rich delocalized π clouds of the aromatic rings dominate the image contrast [37
], the bright blobs are interpreted as hydrogen-bonded dimers, as shown in . Note that the distance between adjacent bright blobs is 1.5 and 2.5 nm in directions perpendicular and parallel to the fibril axis, which is much larger than the interstack distance of 0.35 nm observed in mesophase columns in the bulk [10
]. This means that the fibrils cannot be explained by a stacked structure stabilized by π* orbital overlap of adjacent aromatic rings. The measured heights (brightness) of individual bright blobs in a zigzag vary slightly, which could be a convolution of electronic and topographic effects implying the three-dimensional nature of the structure hidden in the topographic image.
A structural model is proposed for the 2CHd-10 fibril based on dimer precursors involving mainly hydrogen bonding along the circumference and van der Waals bonding between interdigitated dangling alkyl chains along the fibril axis, as shown in as a “net” of the fibril. Note that the periodicity of the structure along the fibril axis, predicted by this model and the molecular dimensions given in , is in reasonable agreement with the periodicity of 2.5 nm observed in experiments (see ). is a 3-D model in which individual ellipsoids represent 2CHd-10 molecules. The ellipse representing the bright STM contrast feature defines the symmetry axis of the molecule while the fibril axis is defined by the direction of the alkyl chains, as illustrated in . The tilt of α = 9° between the molecular axis and the fibril axis is determined by aligning the alkyl chains while reducing their bending with respect to the aromatic rings to a minimum.
(a) Planar-sheet model (net) of a 2CHd-10 fibril section. The dashed line is drawn parallel to the fibrillar axis. (b) 3-D representation of a fibrillar fragment.
One could construct a perfectly planar molecular layer of surface-filling molecules based on the described construction principles. However, zigzag structures result from defects introduced by dimers flipped by 180° around the fibril axis (shaded blue and green in ). As evident from and , such a flipped molecule can form only one hydrogen bond with the neighbouring dimer and is tilted in the opposite direction yielding a step in the molecular contour of the hydrogen-bonded units. The loss of one hydrogen bond at defect sites is partially compensated by additional interdimer hydrogen bonds (see ). Due to a perfect interdigitation of alkyl chains, the fibril has only a weak interaction with the substrate. We speculate that defects introduce internal stress resulting in a small bending of the initially planar sheet. A fibril fragment may result if the specific zigzag structure facilitates a hydrogen-bond closure from open hydrogen bonds, as indicated (unbonded H atoms at the top of the net and O atoms at the bottom) in .
Fibril fragments can grow with different diameters depending on the number of molecules in the sheet, while the detailed zigzag structure determines whether a closure is possible or not. Once a closed fragment is formed, the fibril can easily grow along its axis by the attachment of more dimer units. Planar fragments that are unable to close may still grow axially leading to fibrillar crystallites, as the axial growth mechanism is basically the same as that for a closed net, i.e., through van der Waals interactions. It can also be conjectured that a closure is most plausible for nets with a small diameter, whereas large nets may lie flat on the surface and grow as crystallites. It is worth noting that the model described here displays striking similarities with the bulk mesophase fibrils in its basic constitution. First, from X-ray data for the bulk mesophase fibrils, the number of molecules per column (a disc) cross section is also found to be two, i.e., a dimer [6
]. Second, the periphery of the fibril cross section in the columnar hexagonal disordered mesophase consists of six dimer units just as for the six-membered dimer fibril cross section shown in .
Next, we investigated the structure formation of 1CHn-10 on HOPG at the molecular scale. shows an STM image taken after a drop of a dilute solution of 1CHn-10 in 1,2,4-trichlorobenzene (≈0.29 wt %) had been deposited on HOPG. Again fibrils with a length covering the entire STM scan range are observed. Unlike for 2CHd-10, we observe single-strand and three-strand fibrils (top view) as shown in . As revealed by , the periodicity of bright blobs along the fibril bundle as well as the lateral distance between strands is 2.1 nm. The individual bright blobs of about 1.5 nm appear as rather elusive features in single-strand fibrils. We assume hydrogen-bonded tetramers, as shown in , to be the building blocks for the strand structure. The tiny single-strand fibrils yielding only unstable STM contrast may simply be linear arrangements of tetramers between overlapping alkyl chains.
Figure 6 (a) STM image showing a single-strand and a three-strand fibril of 1CHn-10 on HOPG. Imaging parameters are V
t = 1.29 V and I
t = 0.69 nA. (b) Structure of a 1CHn-10 tetramer. (c) Model of the repeating unit of a three-strand fibril formed by four tetramers. (more ...)
The three-strand geometry appears as a much more rigid structure, and a plausible repeating unit for it is illustrated in in which four tetramer building blocks form a ring stabilized by van der Waals interactions between dangling alkyl chains. This ring is a highly symmetric unit in which the tetramer aryl cores (assuming the tetramers to be planar) appear pairwise parallel (tetramers 1║3 and 2║4), with 2 and 4 displaced from 1 and 3 by half the periodicity along the fibril axis. In a projection perpendicular to tetramer 2, tetramer 4 appears precisely below tetramer 2, and 1 and 3 appear symmetrically at the sides of 2 where the connecting lines 1–2 and 2–3 enclose an angle of 120°. In such a ring unit, 8 of the 16 available alkyl chains are van der Waals bonded to each other while four are dangling at one side of the ring and four at the other side (two alkyl chains each from 2 and 4 are not shown in ).
Unlike 2CHd-10, here no condition is to be met for the closure of 1CHn-10 tetramers to form a ring. Due to the symmetry of the ring unit, the dangling alkyl chains are at the right positions to connect to alkyl chains of a following ring in the very same manner the tetramers in a ring unit are bonded internally, i.e., through van der Waals interactions. This interaction between subsequent tetramer rings leads to their growth into a linear chain. Hence, a string of ring units yields a fibril with a well-defined diameter and a saturation of all possible van der Waals bonds between alkyl chains. Assuming the strand of tetramers numbered 4 is lying flat on the HOPG surface, the fibril structure appears in the above-mentioned projection, and tetramers 1, 2 and 3 appear as bright blobs in the STM image of ordered in a linear herringbone arrangement exhibiting the 120° angle. A 3-D space-filling model for the fibril is shown in , which is a periodic structure of the ring unit of at a periodicity of 2.1 nm.
To visualise the structure of the 1CHn-10 fibril more clearly, a simplified model is shown in . The “net” in (fibril dimensions are not drawn to scale for the 2-D representation) shows the interdigitation of alkyl chains between neighbouring tetramers in which the tetramer building blocks are represented by squares. The structure constituted by blocks 1, 2, 3 and 4 represents a repeating unit of the fibril. The aliphatic chains of the subsequent tetramers interact through van der Waals forces between the interdigitating chains. Thus, the capability of 1CHn-10 to achieve a fibril structure is based on its tendency to form tetramers, which is a crucial step in the process. The closure of the “net” is facilitated by the coupling between tetramers 3 and 3′ and tetramers 4 and 4′ and similarly all equivalent tetramers along the fibril, naturally defining the unique diameter of the fibril.
(a) Planar sheet (net) model (for representational purpose only) of a 1CHn-10 fibril section. The dashed line is drawn parallel to the fibril axis. (b) 3-D configuration for a three-strand fibril.