DOI: 10.1002/chem.201000017

A Scanning Probe Microscopy Study of Annulated Redox-Active Moleculesat a Liquid/Solid Interface: The Overruling of the Alkyl Chain Paradigm

Bo Liu, Ying-Fen Ran, Zhihai Li, Shi-Xia Liu,* Chunyang Jia, Silvio Decurtins, andThomas Wandlowski*[a]

The self-assembly of organic molecules at liquid/solid in-terfaces has attracted particular attention in recent years forthe rational design and fabrication of functional surfaceswith unique electrical, optical and catalytic properties.[1–3]

Surface deposition at solid/liquid interfaces is driven by thedelicate interplay of the adsorbate with itself, the substrate,as well as interactions of both with the solvent. This inter-play involves basic concepts of 2D crystal engineering, suchas hydrogen bonding,[4] p–p stacking,[5] metal–ligand coordi-nation[6] and van der Waals interactions.[1,2, 7] Scanning probemicroscopy (SPM) is a unique nanoscience tool to addressstructures and electronic properties of molecular ensemblesand single molecules with unprecedented resolution in realspace and time. Investigations at (electrified) liquid/solid in-terfaces enable the construction of equilibrated self-assem-bled monolayers or more complex hierarchical architectures,and to address their functionalities in a well-controlled envi-ronment. Scanning tunnelling spectroscopy (STS) as appliedto the liquid/solid interphases is capable of detecting in situlocal changes in the electronic properties of molecules,[1c] re-sulting, for example, from intermolecular donor–acceptor(D–A) or charge-transfer interactions.[8a–d]

Herein, we report the first in situ scanning tunnelling mi-croscopy (STM) and spectroscopy (STS) study on the self-assembly of a new class of p-conjugated bifunctional tri-starmolecules (Scheme 1),[9] physisorbed from solution onhighly oriented pyrolitic graphite (HOPG) under ambientconditions. We also demonstrate, based on STS measure-ments, that the molecular system exhibits a pronounced rec-tification response.

The redox-active and chromophoric D–A target mole-cules 1 a–d (Scheme 1 and the Supporting Information) rep-resent planar, fused p-conjugated systems of high symmetry,which were obtained by annulation of three functionalisedtetrathiafulvalene (TTF) subunits with a hexaazatripheny-lene (HAT) core. The TTF building block is a strong p-elec-tron donor (D)[10] and the annulated electron-deficient HATsystem acts as an acceptor (A) unit.[11] The extended p-con-jugated molecules (TTF–HAT) are terminally substitutedwith six symmetrically arranged and rather flexible thioalkylgroups of variable length. Specifically, compounds 1 a–dwere obtained by a direct condensation reaction of hexake-tocyclohexane with 5,6-diamino-2-(4,5-bis(alkylthio)-1,3-dithio-2-ylidene)benzo[d]-1,3-dithiole. The latter was pre-pared by a phosphite-mediated cross-coupling reaction of4,5-bis(alkylthio)-1,3-dithiole-2-one with 5,6-diaminoben-zene-1,3-dithiole-2-thione.[9] All compounds were purifiedby chromatographic separation and have been fully charac-terised (see the Supporting Information). The covalent link-age between the molecular electron-donor and electron-ac-

[a] Dr. B. Liu, Y.-F. Ran, Dr. Z. Li, Dr. S.-X. Liu, Prof. C. Jia,Prof. S. Decurtins, Prof. T. WandlowskiDepartement f�r Chemie und Biochemie, Universit�t BernFreiestrasse 3, 3012 Bern (Switzerland)Fax: (+41) 301-631-5384E-mail : thomas.wandlowski@dcb.unibe.ch

liu@iac.unibe.ch

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.201000017.

Scheme 1. Structure of the fused TTF–HAT molecules 1 with R=

CnH2n +1, n=6 (1a), 10 (1b), 14 (1 c), 18 (1 d).

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ceptor fragments provides high geometric control over therelative positions and angles of the subunits. A particularlyunique feature of this class of target molecules, as revealedin a recent DFT calculation,[9] comprises the distinct locali-sation and spatial separation of the highest-occupied molec-ular orbital (HOMO) and the lowest-unoccupied molecularorbital (LUMO) within the extended p-conjugated mole-cules, despite the annulation of the D and A moieties intoone framework. Thin layer cyclic voltammetry revealed aHOMO–LUMO gap of approximately (1.6�0.1) eV as at-tributed to the radical cation state of the TTF subunits andthe first reduction state of the pyrazine HAT core.[9] We em-phasise that, despite the rich chemistry of TTF, only a fewannulated TTF-fused D–A systems have been reported sofar.[12]

The self-assembling properties of compounds 1 a–d havebeen studied at the liquid/solid interface by STM upon de-ACHTUNGTRENNUNGposition of a 1 to 10 mm solution of 1 a–d in 1-phenyloctaneon a HOPG surface (for details, see the Supporting Infor-mation). After equilibration, all TTF–HAT molecules formlong-range ordered monolayers with few defects coveringentire terraces of the HOPG substrate with a uniform hon-eycomb motif, as illustrated for molecule 1 b in Figure 1 A.The rarely observed domain boundaries appear ratherbroad, indicating an in-plane structure mismatch betweenadjacent domains and the formation kinetics controlled by2D nucleation and growth of adlayer islands. We also noticethat fuzzy domain boundary regions alternate with sharpedge lines following the symmetry directions of the honey-comb network and that neighbouring domains are rotatedby b= (7�2)8 with respect to each other. High-resolution

images at positive bias voltages, Ebias =Etip�EHOPG withEHOPG<Etip, reveal that the honeycomb pattern is composedof six main bright features and a dark depression in thecentre (Figure 1 B). Cross-section profiles of type I (Fig-ure 1 C) demonstrate an apparent corrugation height of thebright spots, ranging from 0.10 to 0.18 nm, and a value of0.02 nm for the circular dark region (diameter (1.4�0.2) nm). Both features alternate with a repeat distance of(3.2�0.2) nm. Cross-sections of type II (Figure 1 C) revealthat the dominant bright features, which are separated by(1.5�0.2) nm, are interspaced by less bright protrusions ex-hibiting a corrugation amplitude of approximately 0.10 nm.Similar data were obtained along the other symmetry direc-tions of the honeycomb lattice.

The highly symmetric motif leads to an experimentally de-termined rhombohedral unit cell with a= b= (3.2�0.2) nm,both vectors are separated by an angle a= (60�5)8 for 1 b(Figure 1 B). Alteration of tunnelling current and/or biasvoltage allowed the inspection of the underlying substratelattice revealing an angle of (4�2)8 between the main direc-tion of the unit cell and the closest symmetry axis of HOPG.In an attempt to identify the STM contrast pattern of theTTF–HAT compound 1 b in more detail, we also studied aseries of structural analogues with the length of the six pe-ripheral alkyl chains systematically varying from C6 to C18(1 a–d). Except that the increasing length of the alkyl chainsreduced the tendency of the TTF–HAT molecules to aggre-gate with time in small nanometre-sized clusters, STM con-trast pattern and unit cell dimensions of 1 a and 1 c werefound to be nearly identical to the results obtained with 1 b(Table 1 in the Supporting Information). No long-range or-dered 2D adlayer structure could be resolved with 1 d (Fig-ure S2 in the Supporting Information). This observationdemonstrates convincingly that the alkyl chains are notaligned with their backbone parallel to the HOPG substratesurface, for example, they do not act as “molecular glue” asoften observed as a structure-determining element at liquid/HOPG interfaces,[1] but are rather exposed to the liquidphase exhibiting a high conformational mobility. The latteroccurs at a timescale much faster than the scanning frequen-cy, and as a consequence, their structures could not be re-solved. However, we assigned the circular dark depressionsin the centre of the honeycomb network, for example, areasof lower tunnelling current, to the positions of the alkylchains. No resonance contribution to the tunnelling currentis expected due to the large energy gap of this subunit be-tween the HOMO, the LUMO and the Fermi levels of tipand substrate.[13] The spatial assignment of the thioalkylchains is also supported by the bulk crystallographic struc-ture of an alkylated TTF-phenazine derivative.[14] The lift-ing-off of the alkyl chains in self-assembled monolayers onHOPG was previously hypothesised for functionalised hexa-perihexabenzocoronenes (f-HBC)[15] and alkylated deriva-tives of dehydrobenzo[12]annulenes.[7] The comparison ofthe STM contrast pattern and apparent corrugation heightwith those of alkylated TTF derivatives[16] (Figure S3 in theSupporting Information) suggests that the circular protru-

Figure 1. A) Large-scale STM image of 1b (60 � 60 nm, Ebias =0.8 V, Iset =

20 pA). B) High-resolution STM image of 1 b (15 � 15 nm, Ebias =0.8 V,Iset =20 pA) with the unit cell. C) Cross-section profiles along the direc-tions I and II indicated in (B). D) Proposed packing model and rhombicunit cell.

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sions of the highest apparent corrugations represent the spa-tial positions of the TTF moieties, which alternate with theinterstitial positions of the p-electron-deficient HAT groups.The flat adsorption geometry maximises the favourable in-teraction sites of the p-electronic states with those of thegraphite substrate. Based on these boundary conditions, andassuming that the planar and C3 symmetric TTF–HAT mole-cule extends the graphite lattice into the adlayer, we pro-pose the structure model illustrated in Figure 1 D. The di-mensions of the isolated molecular tri-star system were cal-culated at the MM2 level. The aromatic rings inclusive ofthe TTF units were positioned in or close to three-foldhollow sites of the substrate lattice, allowing a maximumoverlap with the p states of the substrate. The TTF–HATmolecules are mutually rotated by 608 and the molecules arestabilised by an antiparallel arrangement of all TTF sidebranches with those of their neighbouring molecules. Thecalculated spacing between the TTF–HAT side branches isobtained as 0.6 nm. The honeycomb pattern is then repre-sented by a clockwise or an anti-clockwise arrangement ofsix molecules enclosing a well-defined circular centre capa-ble of accommodating the solution-directed alkyl chains.The repeat motif is described by a rhombohedral unit cellwith aM = bM = 3.08 nm and a=608 composed of two planaradsorbed TTF–HAT molecules. The area per molecule isobtained to AM = 4.5 nm2 leading to a coverage GM =0.38 �10�10 mol cm�2. The unit cell of the model is rotated by 3.658with respect to the main symmetry direction of the sub-strate. These characteristics and the densely packed arrange-ment of the fused TTF–HAT molecules on HOPG (Fig-ure S4 in the Supporting Information) provide an excellentrepresentation of the experimentally observed dimensionsof the TTF–HAT network. The model also predicts the exis-tence of chiral adlayer domains, mutually rotated by anangle bM =7.98, which were indeed observed experimentally(Figure 2).

The fused and largely extended p-conjugated TTF–HATsystem represents a unique geometrical building block forsurface functionalisation, and for which the core–substrateinteractions completely overrule the directional force of theterminal alkyl chains. Only for thioalkyl chains longer thanC14, the latter contribution appears to distort the formationof the long-range ordered honeycomb network.

The resolved structure model of 1 a–c on HOPG motivat-ed exploring the electronic properties of the chiral molecu-lar monolayers in more detail. Firstly, we aimed to addressthe bias-dependent visualisation of the donor and acceptormoieties. Scanning the bias voltage Ebias from �0.80 to0.80 V did not modify the corrugation patterns (Figure 1 Aand B) of the adlayers at the 1-phenyloctane/HOPG inter-face qualitatively. On the other hand, closer inspection ofthe high-resolution images revealed that the six prominentbright features, assigned to the region of the TTF moieties,appear less bright, while the contrast of the p-acceptor HATunit and the central cavity do not change.

Local STS measurements were carried out in the absenceand in the presence of the TTF–HAT monolayer. Figure 3

displays tunnelling current (iT) versus bias voltage (Ebias)curves recorded on top of the prominent bright features at-tributed to the TTF sites as well as in the dark central cavityregions. The curves shown in Figure 3 A represent the aver-age of 15 individual traces. The slightly asymmetric iT versusEbias curves at the later positions (alkyl chains) reflect the in-trinsic asymmetry of the tunnelling junction enclosed by thetwo electrodes without contributions of energy states of mo-lecular levels. The response coincides with data acquired forthe adsorbate-free 1-phenyloctan/HOPG system. In con-trast, the iT versus Ebias characteristics through the TTFdonor units exhibit a rectifying behaviour with distinctlylarger currents at positive bias voltages. Diode-like or recti-fying behaviour in the tip–substrate gap at the liquid/solidinterface has been observed before for conjugated deriva-

Figure 2. Chiral domains of 1 b (85 � 40 nm, Ebias =0.8 V, Iset =12 pA) withthe main symmetry directions indicated (A) and the corresponding pack-ing model of molecules arranged clockwise and anti-clockwise (B). Theexperimental rotation angle between the two chiral domains is estimatedto be b= (7�2)8.

Figure 3. A) STS curves of 1b with the tip located above the centralcavity region (black line) or on top of a TTF donor unit (blue line); set-point iT =20 pA, Ebias =0.30 V, scan rate 0.5 V s�1. B) Energy diagram il-lustrating the origin of the asymmetry in the iT–Ebias characteristics withthe HOMO of the molecule placed closer to the substrate Fermi levelthan the LUMO.

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S.-X. Liu, T. Wandlowski et al.

tives of hexabenzocoronene,[8c,17] alkylated TTF molecu-les[16a–c] and substituted thiophenes.[18]

Assuming that the fused TTF–HAT molecules are closerto the substrate than to the tip, and the Fermi levels of theadjacent electrodes lying energetically between the HOMOand the LUMO of the molecule, we attributed the increasedtunnelling probability at positive bias to a resonance contri-bution caused by the molecular HOMO. Ab initio calcula-tions revealed that the HOMO is spatially localised at theTTF subunits with an estimated vertical ionisation energy of7.0 eV below the vacuum level, which is close to the valueof the free TTF molecule.[9]With the Fermi level of theHOPG and the Pt/Ir STM tip being 4.7 eV[19] and approxi-mately 5.3–5.7 eV,[20] the conditions for TTF HOMO-medi-ated tunnelling are well established.

Quantum chemical calculations also identified a LUMOorbital centred at the p-deficient HAT moiety and charac-terised by an electron affinity of approximately 0.5 eV.[9]

The above energetics, as well as related electrochemicalmeasurements,[9] predict at negative bias potentials aLUMO-mediated resonance in the iT versus Ebias spectra atthe HAT sites. However, this feature was not observed inour experiments at the liquid/solid interface. We speculatethat possible reasons are related to solvent effects, such assolvent-induced surface work function changes, solvent-mediated coupling between substrate and tip orbital ormixing of the surface states with molecular energy levels.[21]

In conclusion, we have studied, for the first time, the self-assembly of a heteroaromatic fused D–A molecule compris-ing hexaazatriphenylene and tetrathiafulvalene branches atthe liquid/solid interface. The structure of the observed 2Dchiral porous network, which exhibits C3 symmetry, is de-scribed in detail. The analysis reveals convincingly that thestrong interactions of the extended p-conjugated cores ofthe fused units with the HOPG substrate overrule the direc-tional forces of alkyl chains commonly used as “molecularglue” to facilitate physisorption of functional systems at or-ganic liquid/solid interfaces. Current–voltage spectroscopydata revealed a distinct rectification on top of the TTF sites.Current efforts are geared towards the design, immobilisa-tion and local addressing of TTF-based fused systems withlow band gaps, but energetically and spatially well-separatedelectrical donor and acceptor moieties.

Experimental Section

Experimental procedures, characterisation data of the new compounds,and complete details about molecule assembling and STM experimentalresult are available in the Supporting Information.

Acknowledgements

This work was supported by the Swiss National Science Foundation(grant no. 200020-116003 and 200021-124643), the Volkswagen Founda-tion and by EC FP7 ITN “FUNMOLS”, project 212942.

Keywords: donor–acceptor systems · scanning probemicroscopy · self-assembly · tetrathiafulvalenes

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Received: January 7, 2010Published online: March 31, 2010

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