J. Phys. Chem. C 2010, 114, 13967–13974
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Two-Dimensional Supramolecular Organization in Oligomers of DialkylterthiophenessEffect of the Alkyl Substituent Position Tomasz Jaroch,† Robert Nowakowski,*,† Małgorzata Zago´rska,‡ and Adam Pron´§ Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, Warszawa 01-224, Poland, Faculty of Chemistry, Warsaw UniVersity of Technology, Noakowskiego 3, Warszawa 00-664, Poland, and CEA-Grenoble, INAC/SPrAM (UMR 5891 CEA-CNRS-UniV. J. Fourier-Grenoble 1), Laboratoire d’Electronique Mole´culaire Organique et Hybride, 17 Rue des Martyrs, 38054 Grenoble Cedex 9, France ReceiVed: April 1, 2010; ReVised Manuscript ReceiVed: July 6, 2010
Strictly monodispersed fractions of two types of regioisomers of increasing molecular mass, namely, oligomers of 3,3′′-dioctyl-2,2′:5′,2′′-terthiophene (3,3′′DOTT) and 4,4′′-dioctyl-2,2′:5′,2′′-terthiophene (4,4′′DOTT) (nmers, n ) 2-5) were studied by STM with the goal to establish the effect of the type of regioregularity on the 2D supramolecular organization of these compounds on HOPG. Both regioisomers show strikingly different supramolecular organizations. In the 4,4′′DOTT series, a two-dimensional arrangement is found for short oligomers (2-mers and 3-mers). With increasing oligomer length (4-mers and 5-mers), this two-dimensional structure evolves toward a more one-dimensional one characterized by randomly oriented rows of parallelaligned molecules. This evolution arises from the interdigitation of the terminal and the side alkyl groups in two perpendicular directions. In the 3,3′′DOTT series, independent of the oligomer length, a two-dimensional organization is observed. Because of a significantly shorter distance between the two alkyl substituents in the mer, only one type of interdigitation takes place for this regioisomer in the direction perpendicular to the molecule’s long axis. 1. Introduction Oligo- and poly(alkylthiophene)s constitute an interesting class of solution-processable electroactive compounds suitable for various applications.1 One of the most obvious applications of these molecular semiconductors is organic electronics.2-4 However, making electronic devices based on these compounds is a delicate matter because the principal electrical parameters of solid oligo- and poly(alkylthiophene)s layers strongly depend on several molecular and supramolecular factors. Among them, the type of chain regioregularity plays a dominant role. It has been known for more than a decade that head-to-tail coupled (ht) regioregular poly(3-alkylthiophene)s and, in particular, poly(3-hexylthiophene) can serve as components of field effect transistors5,6 or organic photovoltaic cells,7,8 whereas their headto-head-tail-to-tail (hh-tt) analogues obtained by oxidative polymerization of 3,3′-dialkyl-2,2′-bithiophenes or 4,4′-dialkyl2,2′-bithiophenes9,10 show a several orders of magnitude lower charge carrier mobility, which eliminates them as prospective components of organic electronic devices. A similar effect is observed in the case of poly(alkylthiophene)s prepared from dialkylterthiophenes. Poly(3,3′′-dioctyl-2,2′:5′,2′′-terthiophene) is a well-known component of field effect transistors,11,12 but its poly(4,4′′-dioctyl-2,2′:5′,2′′-terthiophene) analogue shows no field effect. Another peculiar feature of poly(alkylthiophene)sbased semiconductors is the strong dependence of the charge carriers’ mobility on their molecular weight, which persists to high Mn values. This effect was observed not only for poly(3-hexylthiophene)13-15 but also for poly(3,3′′-dioctyl-2,2′: 5′,2′′-terthiophene).16 Therefore, several points need to be raised * To whom correspondence should be addressed. Tel: +48 22 (0) 343 32 26. Fax: +48 22 (0) 343 33 33. E-mail:
[email protected]. † Polish Academy of Sciences. ‡ Warsaw University of Technology. § CEA-Grenoble, INAC/SPrAM.
in elucidating these problems, including the following: (i) How does supramolecular organization in the solid state depend on the type of regioregularity? (ii) How does this organization evolve with increasing chain length? STM is especially well suited for this type of investigation. It has been used for more than a decade for precise characterization of thiophene-containing oligomer and polymer monomolecular layers at a molecular resolution.17-20 The molecular arrangement of various linear thiophene derivatives17-32 and, more recently, also derivatives more complex in shape33-38 has been investigated. In general, on substrates that interact weakly with thiophene units (for example, HOPG), highly ordered molecular adlayers were found.17,18,20-29,33-38 These supramolecular aggregations, formed spontaneously for alkylthiophene derivatives, are usually stabilized and controlled via multiple van der Waals interactions of alkyl side groups, separating planes of π-stacked molecular backbones. This is typical behavior also for alkyl derivatives of various organic molecules, for example, pyridine39 and annulene.40,41 In the case of substrates, such as gold, that interact strongly with these molecules, a pronounced influence of the substrate on their supramolecular organization is observed, as expected. As a result, the adlayers reveal a more disordered structure.19,30-32 STM investigations unequivocally indicate that the use of weakly interacting substrates together with appropriately designed molecules (macromolecules) can lead to the formation of a supramolecular aggregation with a controlled structure. Of course, this is of crucial importance for any application of these compounds in organic or molecular electronics. In the presented research, we have selected two types of conjugated regioisomers (Figure 1), namely, oligomers of 3,3′′dioctyl-2,2′:5′,2′′-terthiophene (abbreviated as 3,3′′DOTT) and oligomers of 4,4′′-dioctyl-2,2′:5′,2′′-terthiophene (4,4′′DOTT), which can be considered as model compounds for two isomeric
10.1021/jp102931b 2010 American Chemical Society Published on Web 07/29/2010
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Figure 1. Schematic representation of chemical structures of the oligomers studied (n ) 2-5): (a) 4,4′′DOTT [4,4′′-dioctyl-2,2′:5′,2′′terthiophene] and (b) 3,3′′DOTT [3,3′′-dioctyl-2,2′:5′,2′′-terthiophene].
polymers that significantly differ in electrical transport properties. The 2D supramolecular organizations of four 3,3′′DOTT and 4,4′′DOTT oligomers of increasing polymerization degree from 2 to 5 are studied comparatively. STM investigations of these compounds have enabled us to address the aforementioned two questions and to clearly demonstrate the effect of the regiochemistry and the oligomer chain length on the 2D supramolecular organization. 2. Experimental Section 2.1. Synthesis. 3,3′′DOTT was prepared by Suzuki-type coupling between a protected form of 2,5-thiophenediboronic acid and 2-bromo-3-octylthiophene. A detailed description of this procedure can be found in the electronic supporting information of ref 12. 4,4′′DOTT was also synthesized by Suzuki coupling using sodium 4-octylthiophene-2-thienoboronate and 2,5-dibromothiophene as substrates. The same procedure was applied as that developed for the preparation of an ethylacetate derivative of 4,4′′DOTT, which is described in detail in the electronic supporting information of ref 42. 3,3′′DOTT and 4,4′′DOTT were then oxidized with FeCl3, as described in refs 12 and 42. Oxidation with iron(III) chloride leads to a mixture of oligomeric and polymeric species. The separation of purely monodispersed oligomeric fractions from this mixture can be described as follows.29 The crude reaction product is first dissolved in chloroform. The resulting solution is then introduced into a chromatographic column filled with SilicaGel 60 (40-60 µm, Merck) using hexane as the eluent. In the next step, the first fraction consisting of a mixture of oligomers is collected and concentrated using a rotary evaporator. Purely monodispersed oligomers are obtained using thin layer chromatography (HPTLC RP2, Merck) in hexane. Chromatographically separated fractions are then removed from the substrate by scraping and finally redissolved. This very simple procedure yields four fractions of strictly monodispersed oligomers of polymerization degree n increasing from 2 to 5. The separated oligomers are termed n-mer (n ) 2-5), and the number of thiophene rings in each n-mer is equal to 3n. 2.2. STM Characterization. Monomolecular layers were prepared from each monodispersed fraction by drop-casting the solution (1.5 mg/L) in hexane (pure, POCh Gliwice (Poland)) on a freshly cleaved surface of highly oriented pyrolytic graphite (HOPG, SPI Supplies, U.S.A.). After drying under ambient conditions, the samples were then imaged in air by means of STM43 (University of Bonn, Germany). The mechanically cut STM tips were used for this investigation. The real-space models of 2D organization of the investigated oligomers on HOPG were postulated by the correlation of the layer structure observed by STM and the van der Waals size and geometry of the molecules (obtained using the HyperChem software package).
Jaroch et al. 3. Results and Discussion 3.1. Spectroscopic Studies. In their UV-vis spectra, conjugated molecules (macromolecules) exhibit a very characteristic absorption band originating from the π-π* transition in the conjugated backbone and corresponding to the optical gap. This band is extremely sensitive to the conjugation length. UV-vis investigations of high molecular weight analogues of the oligomers studied in this research, that is, poly(3,3′′-dioctyl2,2′:5′,2′′-terthiophene)12 and poly(4,4′′-dioctyl-2,2′:5′,2′′-terthiophene),42 have shown that the π-π* transition in the latter is significantly more energetic. This is caused by the fact that, in poly(3,3′′-dioctyl-2,2′:5′,2′′-terthiophene), the adjacent alkylthiophene rings are tail-to-tail (tt) coupled, whereas in poly(4,4′′dioctyl-2,2′:5′,2′′-terthiophene), this coupling is of the head-tohead (hh) type. The (hh) coupling causes a significant torsion between the adjacent terthiophene units. As a consequence, the overlap of the π orbitals is less efficient, which leads to an increase in the energy of the π-π* transition in the (hh) coupled polymers as compared with the (tt) coupled ones. The same applies to oligomers of 3,3′′DOTT and 4,4′′DOTT. Although the π-π* transition in 3,3′′DOTT is more energetic than in 4,4′′DOTT, for their oligomers, the same trend is observed as in the case of the polymers; that is, a bathochromic shift of ca. 27-31 nm is observed for the π-π* band in the case of 3,3′′DOTT oligomers, as compared with the same band in the corresponding 4,4′′DOTT oligomers (see Table 1 in the Supporting Information). These results unequivocally show that the optical gap widening in the oligomers of 4,4′′DOTT is caused by the (hh) coupling induced increase in the torsion angle between the adjacent terthiophene units. This finding is additionally corroborated by lower values of the oxidation potential of the 3,3′′DOTT oligomer series with respect to the 4,4′′DOTT oligomers one, showing a higher position of their HOMO level (corresponding to the π orbital). To summarize this part of the paper, the observed differences in the conjugation length are a consequence of a different regiochemical arrangement in both types of oligomers. They not only impose a more or less planar conformation of an isolated molecule but also strongly influence the supramolecular structure of these oligomers in the solid state, as demonstrated by STM investigations (vide infra). 3.2. STM Investigations. 3.2.1. Oligomers of 4,4′′DOTT. A comparison of monomolecular films of 4,4′′DOTT oligomers deposited on HOPG shows an interesting structural evolution of their packing with increasing chain length. Oligomers of lower molecular weight (2-mer and 3-mer) form well-defined 2D crystals, extending over several nanometers, which densely cover the graphite surface. Figure 2 summarizes the previously described results concerning self-organization of the lower molecular weight oligomers.29 The important difference between the organization of 2-mer and 3-mer monolayers, observed even on images at lower resolutions, is the shape of the adsorbate islands (compare panels a and b in Figure 2). Contrary to the case of 2-mer, the domains of 3-mer are not isotropic. Regardless of their orientation, the 3-mer domains show a characteristic elongation in the direction that is always parallel to the distinguished molecular rows (marked in Figure 2b by white lines). This anisotropy implies stronger interactions in the direction perpendicular to the longitudinal axis of molecules. Indeed, detailed inspection of highly resolved STM images combined with molecular modeling enabled us to propose realspace models of the adsorption geometry that fully support this conclusion. The lattice constants estimated for 2-mer (1.44 ( 0.04 nm/2.33 ( 0.05 nm/74 ( 2°) confirm that the alkyl
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Figure 2. STM images of monomolecular islands and corresponding models of adsorption geometry of 4,4′′DOTT oligomers: (a, c) 2-mer; (b, d) 3-mer; (a) 170 nm ×170 nm, Vbias ) 1 V, It ) 0.3 nA; (b) 123 nm ×123 nm, Vbias ) 1.2 V, It ) 0.3 nA.
substituents of the neighboring molecules are fully interdigitated (Figure 2c). In the most plausible molecular conformation, terminal and side alkyl chains of each 2-mer are differently oriented and interdigitated in two perpendicular directions. As a consequence, alkyl substituents of the oligomer molecules form a symmetrical two-dimensional lattice. This leads to uniform intermolecular interactions, extending in two directions, and confirms the strong tendency of 2-mer to self-organize in symmetrically shaped well-packed islands. As expected, the lattice constants estimated for 3-mer, which are longer than 2-mer, differ mainly by one dimension of the unit cellsalong the longitudinal axis of the oligomer (1.44 ( 0.05 nm/3.5 ( 0.05 nm/81 ( 2°). This confirms the same type of intermolecular interactions involving interdigitation of the alkyl substituents in two directions (Figure 2d). However, contrary to the shorter 2-mer, the numbers of side and terminal alkyl groups in 3-mer are not the same and correspond to four and two, respectively. The twice higher number of side alkyl groups as compared with the terminal ones leads to significantly stronger intermolecular interactions in the direction perpendicular to the longitudinal axis of the molecule. This anisotropy is manifested by the aforementioned and frequently observed, in the monolayers of 3-mer, elongation of the adsorbate islands in this direction. One can expect that further extension of the oligomer length, resulting in an increase of the side-to-terminal alkyl groups ratio, could amplify the intermolecular interactions only in one direction. Indeed, a typical layer of longer oligomers (4-mer, 5-mer) is evidently less ordered and consists of randomly oriented rows of parallel-aligned molecules (Figure 3). This observation confirms a strong tendency of these longer molecules to form one-dimensional structures. Careful inspection of layers enables us to distinguish two different internal structures of one-dimensional rows. In the majority of cases, longer oligomers are aligned parallel side-by-side and form rows that are oriented perpendicularly to the longitudinal axis of the molecule (for example, the rows marked as 1 in Figure 3a). This rowlike arrangement confirms that interdigitation is much
Figure 3. STM images of the 4,4′′DOTT oligomer monolayer on HOPG: (a) 5-mer, 65 nm × 65 nm; (b) 4-mer, 20 nm × 20 nm (Vbias ) 1 V, It ) 0.3 nA).
stronger in this direction. In the second structure, the molecules are aligned in a similar way but, additionally, they are mutually shifted along their longitudinal axis. If this shift exists only for a fraction of the parallel-aligned molecules, the row is locally bent. If it exists for all of the molecules in the row, a new structural arrangement is formed (see the chain marked as 2 in Figure 3b). It is important to note that, in both cases, the same new angle is imposed between the direction of the molecular row and the longitudinal axis of the molecules, equal to (50° (see the marked angle in structure 2 in Figure 3b). This indicates that the intermolecular shift of adjacent molecules in a particular molecular row is fixed. As appears from the molecular modeling, this shift is equal to the length of one mer. In our opinion, this conclusion is important because it confirms that, for the investigated structural unit (which is relatively long and gives various geometrical possibilities for packing of the alkyl group in two adjacent molecules), only one mode of the alkyl chain interdigitation is energetically favorable. At this point, an important question arises concerning the factors that determine the internal structure of one-dimensional molecular rows. Figure 4 shows two proposed molecule adsorption geometries forming a row (a) without and (b) with the molecular shift. Both geometries are characterized by different modes of the side alkyl chain interdigitation. This is a consequence of a possible rotation around the single C-C bond between the thiophene rings in the conjugated backbone of the molecule, which results in two different distribution patterns of the alkyl substituents. In the first case, (a), the conformation is characterized by two different distances between the alkyl
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Figure 5. STM images of the 3,3′′DOTT oligomer (4-mer) monolayer on HOPG: (a) 123 nm × 123 nm, (b) 30 nm × 30 nm (Vbias ) 1.1 V, It ) 0.2 nA).
Figure 4. Comparison of proposed adsorption geometries of the 4,4′′DOTT oligomer (4-mer) forming a rowlike arrangement (a) without and (b) with the molecular shift.
chains measured from the opposite sides of the molecule. This arrangement enables rows to form without any molecular shift. This happens when the two closer located alkyl groups fit the “gap” between the more distant alkyl groups of the neighboring molecule. However, molecules with this conformation are not able to form rows exhibiting the molecular shift and the characteristic (50°) angle between the axis of the molecule and the direction of the row. The second structure, (b), can be formed by molecules showing an alternative conformation in which the alkyl substituents are separated by the same distance on opposite sides of the molecule. In this case, the molecular shift along the longitudinal axis of the oligomer corresponds to the length of one mer. The presented analysis suggests that the two distinguished structures of molecular rows are directly related to the arrangement of the alkyl substituents in the oligomer molecule and result from two different modes of interdigitation. To summarize this part of the research, we can state that the self-organization of 4,4′′DOTT oligomers on HOPG is controlled by the interaction of the alkyl substituents that are interdigitated in two perpendicular directions. This conclusion is supported by the observed structural evolution with increasing oligomer length. It is a consequence of the fact that an increase
in the oligomer chain length leads to asymmetry in the intermolecular interactions due to an increase in the ratio of the side to terminal alkyl substituents. 3.2.2. Oligomers of 3,3′′DOTT. The main difference between 3,3′′DOTT oligomers and 4,4′′DOTT ones is the position of the alkyl substituents. This different regioregularity strongly influences the 2D supramolecular organization in the 3,3′′DOTT series. Significant differences are particularly evident for longer oligomers. Figure 5 shows a representative monomolecular island of 4-mers. Contrary to the case of the corresponding 4,4′′DOTT oligomer (vide supra), where one-dimensional molecular rows dominate in the monolayer, 4-mers of the 3,3′′DOTT series still exhibit a strong tendency to form well-packed, twodimensional crystals. This is confirmed by the observed ordered islands of mesoscopic size reaching several hundred nanometers. The above observation indicates that this type of regioregularity (tail-to-tail coupling between the terthiophene units) reduces the asymmetry in the intermolecular interactions characteristic of longer oligomers of the 4,4′′DOTT series. Careful inspection of the STM images provides more information. The image of this two-dimensional island is dominated by bright lines corresponding to the areas covered by conjugated backbones of the molecules. The longitudinal axis of the oligomer is always oriented perpendicularly to the direction of the observed lines. Individual 4-mers are seen as bright bars corresponding to the shape of the conjugated backbones. Randomly distributed point defects (marked in Figure 5a by black arrows) can frequently be found in the monolayer. There are black spots attributed to the local implementation of shorter contaminating oligomers (2- or 3-mer) into the ordered layer of 4-mers. This implementation occurs very locally and has no consequences for the supramolecular organization in larger areas. It can, therefore, be postulated that shorter oligomers can fit the bigger unit cell of ordered 4-mers. Evidently, the supramolecular interactions between molecules are strong enough to prevent reduction of the empty space by the movement of the surrounding 4-mer molecules. This phenomenon is also confirmed by the presence
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Figure 6. STM images of monomolecular islands and corresponding models of adsorption geometry of 3,3′′DOTT oligomers: (a, d) 2-mer; (b, e) 3-mer; (c, f) 4-mer; (a-c) 15 nm × 15 nm, Vbias ) 1.1 V, It ) 0.2 nA.
TABLE 1: Parameters of the Structure Unit Cell (Vectors a1 and a2, Angle r(a1, a2)), Estimated from STM Images, of Monomolecular Layers Formed from Monodispersed Fractions of 3,3′′DOTT Oligomers 2-mer
3-mer
4-mer
no of thiophene units fraction
6
9
12
a1 [nm] a2 [nm] R(a1, a2) [°]
1.60 ( 0.04 2.72 ( 0.05 94 ( 2
1.51 ( 0.05 3.83 ( 0.06 86 ( 2
1.51 ( 0.04 5.02 ( 0.05 92 ( 3
of a larger defect, in the shape of a black rectangle, marked in the image by a white arrow. This defect corresponds to a vacancy resulting from the absence of six 4-mers in the layer. As expected, a somewhat different situation occurs when a longer oligomer is implemented into the layer of 4-mers (see the bottom-right part of Figure 5b). This longer oligomer, whose longitudinal size extends the dimensions of the unit cell, creates a mismatch that can seriously disturb molecular ordering in larger dimensions. Using monodispersed fractions, we are able to investigate the evolution of the monolayer structure caused by the increasing length of the oligomer. Images obtained at the molecular resolution of the layers formed by 2-mers, 3-mers, and 4-mers are compared in Figure 6. A very similar structural organization is observed for these oligomers of different lengths. The images of the 2D layers are dominated by bright parallel rows of molecules aligned side-by-side. The molecules are always oriented perpendicularly to the direction of the rows. The parameters of their unit cells, estimated from the STM images, are compared in Table 1. The distance between two adjacent molecules in one row is essentially independent of the oligomer length (1.6 nm for 2-mer and 1.5 nm for 3-mer and 4-mer). Both values are significantly smaller than the calculated width of the oligomers (ca. 2.5 nm), confirming interdigitation of the alkyl chains of two adjacent molecules in the direction perpendicular to the longitudinal axis of the molecules. The slightly greater intermolecular distance observed for the shortest
molecule (2-mer) confirms that the interdigitation degree depends on the number of alkyl chains involved in this process. A higher number of interdigitated alkyl chains (in the cases of 3-mer and 4-mer) leads to stronger interactions, yielding a smaller intermolecular distance. The dimension of the unit cell in the direction perpendicular to the row axis is related to the length of the molecules forming the row. For all of the investigated oligomers (2-, 3-, and 4-mers), these values are only slightly larger (0.2-0.3 nm) than the length of the conjugated backbones of a given oligomer, estimated from the molecular modeling. This information, together with the analysis of the arrangements of conjugated backbones in the layer based on STM images, suggests that, for oligomers of the 3,3′′DOTT series, independent of the molecule’s length, interdigitation of the alkyl chains occurs only in one direction, that is, perpendicular to the longitudinal axis of the molecules. To verify this conclusion, we have decided to use the capability of STM to controllably manipulate a single molecule in the layer. This manipulation is made possible by decreasing the tunneling resistance during scanning, which is achieved by increasing the tunneling current at a constant bias voltage. Alternatively, the same effect can be reached by a decrease of the bias voltage at a constant tunneling current. In both cases, the closer proximity of the STM tip and the molecules is expected. The resulting stronger mutual interactions enable the tip-induced displacement of the molecules. An example of this process, performed inside an extended vacancy in a well-packed layer, is presented in Figure 7. Each scan performed in one direction (from left to right), in lower tunneling resistance conditions, leads to the movement of one molecule from the left to the right side of the vacancy. This displacement is realized only for molecules located at the vacancy edges. The process can be controlled by the resulting change of the observed vacancy shape (compare panels a-c in Figure 7). After each two-sequence scan, the shape of the vacancy is restored, as a result of the movement of two molecules from two adjacent rows. As a consequence, the large vacancy is displaced along the direction perpendicular to the longitudinal axis of molecules.
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Figure 7. Series of STM images of extended vacancy in the 3,3′′DOTT (4-mer) monolayer showing control manipulation of a single molecule. Each scan performed in one direction (from left to right), in lower tunneling resistance conditions, leads to the movement of one selected molecule from the left to the right side of the vacancy (43 nm × 27 nm, Vbias ) 1.1 V, It ) 0.2 nA (during scanning) and 0.8 nA (during manipulation)).
The displacement of the vacancy is confirmed by the change in its relative position with respect to the stable “witness” defect associated with the implementation of a shorter molecule in the well-packed layer (this defect is marked by a black circle in the bottom part of the image). The described manipulation inside the vacancy confirms the absence of alkyl chain interdigitation in the direction perpendicular to the direction of the movement, that is, along the longitudinal axis of the molecule. Otherwise, the movement of the molecule would be more complicated, and this process should force the displacement of surrounding molecules in the layer. A comparison of the results obtained for the two series of oligomers clearly indicates that the position of the alkyl substituents strongly influences the supramolecular organization of the molecules studied. An important observation concerns different structural evolutions caused by an increase in the oligomer length. For the 4,4′′DOTT series, an increase of the molecular length leads to an evolution from well-defined, twodimensional islands (as observed for 2- and 3-mers) to a strongly anisotropic one-dimensional organization characterized by more or less separate rows of molecules (for 4-mers and longer oligomers). This phenomenon indicates that, in the case of 4,4′′DOTT oligomers, due to the large distance between the alkyl substituents in the mer (ca. 0.86 nm), these groups are mutually independent and flexible. Therefore, they are able to independently interact in the layer in two perpendicular directions: along (terminal alkyl groups) and perpendicular (side alkyl groups) to the longitudinal axis of the molecules. As expected, the type of obtained supramolecular order is, therefore, the result of a balance between the strength of the intermolecular interactions in two different directions. The direct consequence of oligomer length augmentation is an increase in the number of side alkyl chains, which assures stronger interactions in the direction perpendicular to the longitudinal axis of the molecule. Therefore, a higher degree of anisotropy is expected for longer oligomers of this series, leading to the observed evolution of supramolecular ordering.
A significantly different behavior is observed for the series of 3,3′′DOTT oligomers. In this case, even longer oligomers exhibit a strong tendency to form well-packed and twodimensional islands. The observed dissimilarities result from different distributions of the alkyl groups along the molecule’s longitudinal axis, which leads to a different nature of the intermolecular interactions. In this series of oligomers, a much shorter distance (ca. 0.41 nm) between the two alkyl substituents in one mer is measured. As a consequence, the two alkyl chains of each mer mutually interact and form stiff segments always oriented perpendicularly to the longitudinal axis of the molecule. Therefore, the interactions of neighboring molecules in the layer involve interdigitation of whole segments (pairs of alkyl groups) only in this particular direction. Intermolecular interactions along the longitudinal axis of the molecules occur via direct interactions of the conjugated backbones of the neighboring oligomers. Here, an important practical question arises concerning the effect of the observed supramolecular organization on the electronic properties of monomolecular layers of the oligomers studied. This influence is evident in a comparison of the proposed real-space models of the adsorption geometry of 2-mers of both oligomer series studied (compare the structures presented in Figures 2c and 6d). It is well known that the electronic properties of an organic layer are determined by charge carrier movements in two different areas: along the conjugated backbones of the molecules and between the molecules. Because 4,4′′DOTT oligomers interact in the layer in two perpendicular directions in the same way, that is, via interdigitated alkyl groups, highly uniform electronic properties of the monolayer are expected. The different position of the alkyl substituents in 3,3′′DOTT oligomers leads to a different distribution of intermolecular interactions that become nonuniform. They are different in the directions along and perpendicular to the molecular axis. As a consequence, for this series, the electronic properties of the monolayer should depend on the direction. Therefore, a much higher mobility of the charge
Oligomers of Dialkylterthiophenes carriers, along the direction of directly interacting conjugated backbones, is expected. 4. Conclusions To summarize, STM investigations of a series of dialkylterthiophene oligomers, which can be considered as regioisomers, unequivocally showed that the type of their structural organization on HOPG is governed by the type of regioregularity and the molecule’s length. Structural differences were observed for oligomers of the same length but originating from the two different series studied. Moreover, the evolution of the supramolecular organization with increasing oligomer length was different in both series of compounds. The oligomers studied can be considered as model compounds of poly(3,3′′-dioctyl2,2′:5′,2′′-terthiophene) and poly(4,4′′-dioctyl-2,2′:5′,2′′-terthiophene)sthe former exhibiting much better electrical transport properties than the latter. The observed differences in the supramolecular organization of the oligomers clearly explain the superiority of (3,3′′-dioctyl-2,2′:5′,2′′-terthiophene) oligomers and polymers in organic electronics applications. On a broader perspective, the obtained results underline the importance of several molecular and structural factors in the future elaboration of new solution-processable semiconducting oligomers and polymers. First, the presence of solubilizing substituents should induce an anisotropic supramolecular organization, favoring intermolecular charge transport in one direction. This is determined by the regiochemistry. Second, using appropriate processing conditions, a high degree of the molecules’ orientation should be achieved with the parallel alignment direction depending on the organic electronic devices to be fabricated. For example, in the case of field effect transistors, this alignment direction should be parallel to the gate-semiconductor layer interface. Third, strict monodispersity of the oligomers (or significantly reduced polydispersity in polymers) should be achieved because the supramolecular organization depends not only on the type of regioisomerism but also on the molecular mass. Acknowledgment. This work was carried out in the framework of a PAN-CNRS cooperation project (No. 20135). M.Z. wishes to acknowledge financial support from the Polish Ministry of Science and Higher Education through a Research Project NN205105735 granted for the years 2008-2011. Supporting Information Available: Absorption band (λmax/ nm) originating from the π-π* transition in the conjugated backbone collected for all monodispersed fractions of the investigated oligomers. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Mishra, A.; Ma, C. Q.; Bauerle, P. Functional Oligothiophenes: Molecular Design for Multidimensional Nanostructures and their Applications. Chem. ReV. 2009, 109, 1141. (2) Murphy, A. R.; Frechet, J. M. J. Organic Semiconducting Oligomers for Use in Thin Film Transistors. Chem. ReV. 2007, 107, 1066. (3) Allard, S.; Forster, M.; Souharce, B.; Thiem, H.; Scherf, U. Organic Semiconductors for Solution-Processable Field-Effect Transistors (OFETs). Angew. Chem., Int. Ed. 2008, 47, 4070. (4) Rauch, T.; Boberl, M.; Tedde, S. F.; Furst, J.; Kovalenko, M. V.; Hesser, G.; Lemmer, U.; Heiss, W.; Hayden, O. Near-Infrared Imaging with Quantum-Dot-Sensitized Organic Photodiodes. Nat. Photonics 2009, 3, 332. (5) Bao, Z.; Dodabalapur, A.; Lovinger, A. J. Soluble and Processable Regioregular Poly(3-hexylthiophene) for Thin Film Field-Effect Transistor Applications with High Mobility. Appl. Phys. Lett. 1996, 69, 4108. (6) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Two-Dimensional
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