The molecules analyzed in this study, carboxylic acid-functionalized regioregular oligo(3-hexylthiophenes), H
TCOOH, are sketched in . The π-conjugated system, comprising the aromatic thiophene rings and being responsible for the electronic properties of the molecule, extends from 4 to 12 thiophene repeating units, and the molecular size stretches accordingly from 1.7 nm to 5.4 nm. The carboxylic acid group (COOH) at the terminal α-position of the oligothiophene backbone allows the formation of effective intermolecular hydrogen bonding through dimer formation. The regioregular hexyl-side-chain substitution pattern enhances the solubility of the compounds in organic solvents and their crystallinity on the substrate.
General formula of carboxylic acid functionalized oligothiophenes H
shows a large-scale STM image of a monolayer of tetramer H4TCOOH adsorbed at the solution–HOPG interface. Several terraces of the graphite substrate are visible in the image. On the terraces, a very well ordered monolayer of H4TCOOH can be seen extending over the micrometer range.
Figure 2 Left: STM image of H4TCOOH on HOPG (100 × 100 nm2, U = −120 mV, I = 50 pA). The letters label the different domains as explained in the text. Center: STM current image of H4TCOOH on HOPG (10 × 10 nm2, U = −100 mV, I = 50 (more ...)
In the first images after deposition, several domains with sizes ranging from tens to hundreds of nanometers can be recognized. In (left) four out of six possible domains (labeled “A” to “D”) can be seen. The six domains correspond to the two enantiomorphic molecular arrangements (with a relative angle of 10°) combined with the three crystallographic axes of the underlying HOPG substrate. In (left) “A” and “C” correspond to two orientations (at an angle of 120°) of the same enantiomorph, as do “B” and “D”.
The molecules are lying flat on the nonreactive HOPG surface. The oligothiophene backbone carrying the delocalized π-electron system is recognized in STM images as bright spots under the given scanning conditions, which correspond to a higher tunneling probability. On the contrary, the alkyl side chains, corresponding to the insulating part of the molecules, are extended perpendicular to the oligothiophene backbone and can be recognized as dark regions, reflecting the expected lower tunneling probability.
(center) shows a short range image of H4TCOOH
adsorbed on HOPG, exhibiting submolecular resolution. The unit cell contains two molecules and the cell parameters are a
= 3.6 ± 0.1 nm, b
= 2.1 ± 0.1 nm and α = 42 ± 2° (). The molecules are ordered in lamellae with an interlamellar distance of 1.4 nm. This value correlates very well with data published for ordered monolayers of hexyl-substituted oligo- and polythiophenes and can be explained by van der Waals interaction driven full interdigitation of the alkyl chains of two opposite molecules in their all-trans
conformation in different rows [9
STM and theoretical parameters of 2-D crystals of the investigated carboxylic acid functionalized oligothiophenes H
Theoretical calculations on a group of four molecules, excluding the substrate, support the dimer formation and are in very good agreement with the observed unit-cell parameters, with a
= 3.6 ± 0.1 nm, b
= 2.1 ± 0.1 nm and α = 39 ± 1° (, right, and ). In the lamellae the molecules form an angle of 8° between the molecular axis and the row direction. The two molecules building the pair are separated by small regions of low tunneling current (labeled with an arrow in , center), which we assign to the carboxylic acid ends of two H4TCOOH
molecules in a head-to-head arrangment. It is widely observed in STM images that carboxylic acid groups appear as dark spots under negative bias [19
]. Our theoretical calculations support the experimental findings (, right), showing no contribution of the end-function to the occupied frontier orbitals (HOMO and HOMO−1) close to the Fermi level. This arrangement stabilizes the monolayer by neutralizing the dipole moments of the molecules, which are oriented along the molecular axis (, right; plain arrows). The carboxylic acid groups of two head-to-head arranged molecules are able to undergo hydrogen-bond formation, additionally stabilizing the monolayer (, right).
In the current image (, center) submolecular resolution was obtained for the oligothiophene backbones. Due to the low negative bias applied and at the limit given by the tunneling barrier of this compound, we can assume that the observed eight lobes per molecule correspond to the local density of states (LDOS) of the molecule, which are very close to the Fermi level and are coupled, at this bias, to an almost anisotropic contribution from the LDOS of the substrate.
In , STM images of a 2-D crystal of the next higher homologue, hexamer H6TCOOH, are depicted. , left, shows a large-scale STM image of a densely packed monolayer. The molecules are perfectly arranged in lamellae and two domains (out of six possibilities, see above) can be observed. The lamellae are separated by about 1.3 nm, like in the case of H4TCOOH. In these regions the insulating hexyl chains interdigitate (, right). Despite a high degree of crystallinity, the monolayer reveals a few defects (, left), which are noticeable as faint depressions, probably related to molecules adsorbed at unstable sites. Adsorption and desorption processes equilibrating at the liquid–solid interface induce, in turn after 10–20 minutes, a self-healing process to form a perfectly ordered monolayer over several micrometers.
Figure 3 STM image of H6TCOOH. Left: 70 × 70 nm2, U = −360 mV, I = 50 pA. Center: 20 × 20 nm2, U = −361 mV, I = 50 pA. Right: Theoretical model. The small black arrows emphasize the H-bonding position. The big arrows indicate the (more ...)
(center) shows a small-scale image of a single domain. The molecular resolution allows the determination of the unit-cell parameters: a = 5.3 ± 0.1 nm, b = 3.0 ± 0.1 nm and α = 29 ± 2°. In the unit cell two molecules arrange as a dimer, with their carboxylic acid functional groups facing each other due to H-bonding, like in the case of H4TCOOH. With the help of theoretical calculations a model of the molecular packing can be obtained (, right). The unit-cell parameters are calculated to be a = 5.4 nm, b = 2.9 nm, and α = 27° in very good agreement with the experimental data. The plain arrows depicted in the model show that this molecular arrangement allows the full neutralization of the molecular dipoles within the plane.
In and , STM images of compounds H8TCOOH
adsorbed at the HOPG surface are shown. The large-scale images ( and , left) reveal monolayers with different orientations (see above). The STM image of octamer H8TCOOH
was taken immediately after adsorption and therefore reveals an increased number of vacancies relative to monolayers of the shorter oligomers. This phenomenon can be explained by a decrease in diffusivity as the size of the oligomers increase. However, the adsorption–desorption dynamics at the surface and the observed Ostwald ripening [21
] lead to an almost perfect monolayer with time. The arrangement of the molecules reveals a more columnar than lamellar ordering, although after careful analysis, the dimer formation can also be determined for these long oligomers ( and , center). The unit cell calculated for the crystalline regions of H8TCOOH
monolayers contains two molecules and the parameters correlate very well with the quantum-chemical model displayed in and , right, and in . The more extended π-system of H8TCOOH
induces a face-to-face molecular arrangement with a negligible offset and therefore a more columnar structure. The distance between the columns amounts to 1.3 nm and the space is filled by interdigitating alkyl chains ( and , right).
Figure 4 STM images of H8TCOOH. Left: 60 × 60 nm2, U = −640 mV, I = 44 pA. Center: 30 × 30 nm2, U = −725 mV, I = 35 pA. Right: Theoretical model. The small black arrows indicate the H-bonds. The big arrows show the orientation of (more ...)
Figure 5 STM images of H10TCOOH. Left: 50 × 50 nm2, U = −200 mV, I = 73 pA. Center: 24 × 19 nm2, U = −100 mV, I = 22 pA. Right: Theoretical model. The small black arrows point out the H-bonding position. The big arrows show the (more ...)
The 2-D crystal of the longest oligomer, H12TCOOH, is shown in , left. A densely packed monolayer can be seen. One can easily recognize the molecules, which are arranged in columns and domains related to the crystallographic axes of the HOPG substrate. The columns are not perfectly straight, in contrast to the stacks of derivatives H8TCOOH and H10TCOOH, due to several possibilities for an offset in the pseudo face-to-face arrangement of the long H12TCOOH molecules.
Figure 6 STM images of H12TCOOH. Left: 80 × 80 nm2, U = −200 mV, I = 73 pA. Center: 20 × 20 nm2, U = −750 mV, I = 68 pA. Details of the regions in the dashed rectangles are given in (i) and (ii). Right: Theoretical model. The small (more ...)
In the small-scale STM images of H12TCOOH
some very interesting effects can be seen (, center). At the domain boundaries, molecules in a nonlinear, bent shape can be distinguished (, center right (i) and (ii)). In order to fill the space between the domains, the adsorbed molecules partially change their conformation from the usual linear shape to a crescent shape. In , center right (i), the five terminal thiophenes are arranged in a syn
-conformation causing the observed hairpin bend. In , center right (ii), an oligomer conformation, in which several thiophene units are in syn-
arrangement, seems to be responsible for the observed crescent shape of the adsorbed H12TCOOH
molecules. The hairpin conformation was already observed for corresponding regioregular poly(3-alkylthiophenes) adsorbed on a HOPG surface [5
], but for an oligothiophene this behavior is shown here for the first time. More detailed analysis on the adsorption conformation at different solution concentrations of all the oligomers presented in this communication reveals that only the longest oligothiophene, H12TCOOH
, changes the typical all-anti
conformation to syn
-containing ones at the domain boundaries. X-ray structure analyses on different series of alkylated oligothiophenes [12
] have shown that, with a unique exception, smaller oligomers throughout prefer an all-anti
], whereas for longer oligomers syn-
conformations at the terminal thiophene units typically seem to be more favored.