750 � WILEY-VCH-Verlag GmbH, D-69451 Weinheim, 2001 1439-4235/01/02/12 $ 17.50+.50/0 CHEMPHYSCHEM 2001, No. 12

Molecular Superlattices Induced byAlkyl Substitutions in Self-AssembledTriphenylene Monolayers

Peng Wu, Qingdao Zeng, Shandong Xu, Chen Wang,Shuxia Yin, and Chun-Li Bai*[a]

KEYWORDS:

monolayers ´ scanning tunneling microscopy ´ self-assembly ´steric interaction ´ superlattices

Alkyl substitution is commonly practiced in preparing discoticliquid crystal species. It is one of the more effective moleculardesign strategies used to generate a rich variety of physical andchemical properties, such as solubility, phase transitions, andoptoelectric behaviors. Planar molecules, such as phthalocya-nines, porphyrins, and polyphenylenes, are an importantcategory of molecules associated with alkyl substitution. It haslong been known that the alkyl-substituted molecules couldform columnar structures; some even display discerniblechirality.[1±4] The substitution by alkyl chains is essential indetermining such properties as the intercolumnar spacing andtransition temperatures.

Scanning tunneling microscopy (STM) studies provide ahelpful addition to the available knowledge on molecularassemblies. STM observations reveal many details of theadsorption and assembling behavior of a variety of molecules.Recent studies clarified the interdigitate behavior of the alkylmoieties, and packing symmetries on graphite, of alkyl-substi-tuted phthalocynines and porphyrins.[5]

stant and the frequency factor were clearly larger for the atomichydrogen than for the atomic deuterium. According to theabsolute rate theory in chemical kinetics proposed by Eyringet al. ,[7] the frequency factor A in the Arrhenius equationcorresponds to (1/pb�h) sinh(0.5b�hw) for the system investigatedand may be approximated with w/2p (�n), where b� 1/kT, �h�h/2p, and w� 2pn. The isotope effect should be representedtheoretically by Equation (2).

nH

nD

��������mD

mH

r� ���

2p

(2)

However, the experimental value of nH/nD, obtained from theratio of frequency factors AH/AD, was 1.14. A small deviation fromthe theoretical value is probably explained by anharmonicity. Atthis stage, a comparison between experimental and theoreticalresults is in good agreement.

This kinetic study on the isotope effect is an unprecedentedexperimental example in the system that consists of themovement of a single particle. Further close experimental andtheoretical investigations are under way.

Experimental Section

Q8M8 (400 mg) in powder form was placed in a 20 mL vial, and[D12]cyclohexane (1 g, ISOTEC, 99.6 atom %) was added. The vial waskept covered with a polypropylene cap, and was irradiated with301.2 kGy of 60Co g-ray at room temperature. All irradiated specimenswere collected together and then recrystallized with hexane. Thepolycrystalline sample obtained (100 mg) was transferred into an EPRtube with an outside diameter of 5 mm. EPR measurements werecarried out using a JEOL JES RE-1X X-band spectrometer, equippedwith a variable temperature apparatus JEOL ES-DVT2, in the temper-ature range 353 ± 383 K. All EPR measurements were carried out at amicrowave power of 10 mW (below microwave saturation, max.0.2 mW) and a field modulation width of 0.1 mT. Peak-to-peakderivative amplitudes were measured to determine the signalintensities. In the measurement using EPR, the highest field signal(352� 0.75 mT) of hydrogen atoms and the highest field signal(336� 0.75 mT) of deuterium atoms were recorded alternatively inthe sweep time of about 1 min. After a constant temperature was set,the time point corresponding to the center of the EPR signal wasdefined as the decay time t. The intensities of the EPR signal for thehydrogen and the deuterium atoms, which remained in the D4Rcages, were determined and their thermal decay curves wereobtained.

We would like to thank Dr. T. Okai and Mr. K. Matsumoto at theInstitute for Irradiation and Analysis of Quantum Radiation, KyushuUniversity, for their valuable technical assistance in performing g-ray irradiation.

[1] R. Sasamori, Y. Okaue, T. Isobe, Y. Matsuda, Science 1994, 265, 1691 ± 1693.[2] M. Päch, R. Stöûer, J. Phys. Chem. A 1997, 101, 8360 ± 8365.[3] M. Mattori, K. Mogi, Y. Sakai, T. Isobe, J. Phys. Chem. A 2000, 104, 10 868 ±

10 872.[4] Y. Hayashino, T. Isobe, Y. Matsuda, Inorg. Chem. 2001, 40, 2218 ± 2219.[5] H. Dilger, E. Roduner, R. Scheuermann, J. Major, M. Schefzik, R. Stöûer, M.

Päch, D. G. Fleming, Physica B (Amsterdam) 2000, 289/290, 482 ± 486.[6] G. Scholz, R. Stöûer, J. A. Momand, A. Zehl, J. Klein, Angew. Chem. 2000, 112,

2570 ± 2573; Angew. Chem. Int. Ed. 2000, 39, 2516 ± 2519.[7] S. Glasstone, K. J. Laidler, H. Eyring, The Theory of Rate Processes, McGraw-

Hill, New York, NY, 1941.

Received: June 25, 2001 [Z 252]

[a] Prof. C.-L. Bai, P. Wu, Dr. Q. Zeng, S. Xu, Prof. C. Wang, Dr. S. YinCenter for Molecular SciencesInstitute of ChemistryChinese Academy of SciencesBeijing 100080 (China)Fax: (�86) 10-62557908E-mail : clbai@infoc3.icas.ac.cn

Table 2. Arrhenius parameters of H and D atom release

A [sÿ1] Ea [kJ molÿ1] R

H@Q8M8!H � Q8M8 1.27� 1013 116.8 0.9997D@Q8M8!D � Q8M8 1.11� 1013 117.1 0.9998

CHEMPHYSCHEM 2001, No. 12 � WILEY-VCH-Verlag GmbH, D-69451 Weinheim, 2001 1439-4235/01/02/12 $ 17.50+.50/0 751

In 1988, Foster and Frommer first studied liquid crystals suchas 4-n-octyl-4'-cyanobiphenyl (8CB) on highly orientated pyro-lytic graphite (HOPG).[6] Subsequently, the assemblies of nCB(n� 6 ± 12) were intensively investigated by varying the numberof carbon atoms n of the alkyl chains.[7±9] For different alkyl chainsthe strength of intermolecular forces changes, and the packingbehavior can vary appreciably, as compared to the adhesionstrength and the registry of molecular systems at the substrate/film interface. For 8CB, each row is shown to interact in anamphiphilic fashion with neighboring rows: head (cyanobiphen-yl group)-to-head and tail (alkane chain)-to-tail. The phenylgroups are considered to be lying flat on the substrate. Withincreasing alkyl-chain length, the row spacing for cyanobiphenylderivatives of different alkyl-chain lengths increases proportion-ally. In addition, for 6CB, every second biphenyl unit appears tobe canted, wedged between its two adjacent biphenyl neigh-bors, which are lying flat. With 7CB, there is an alternativepacking,[10] which could arise from its odd number of carbons inthe alkyl moiety.

2,3,6,7,10,11-Hexakisalkoxy-substituted triphenylenes with n-carbon side chains (Tn), another typical kind of discotic liquidcrystal, have been studied in recent years. In 1992, Rabe firststudied T7 and T16,[11] which present different packing modes.When n equals seven, the molecules inthe assembly keep three-fold symmetry,while in T16 all alkyl chains are orientedpreferentially parallel to each other.Subsequently T5, T7, T9, and T11 werestudied by Charra and Cousty.[12] Theyreported that a chiral ordering transitionappears when increasing the triangularaspect ratio of the molecules, throughthe tuning of alkoxy side-chain length.There is a threshold when n is equal tonine. When the number of the methyleneunits is above nine, a chiral configurationis presented. The structure could beinterpreted as being a frustrated antifer-romagnetic-like triangular Ising net.

In the current study using a directsolution-deposition method, we inves-tigated 2,3,6,7,10,11-hexakisalkoxy-sub-stituted triphenylenes with n-carbonside chains (n� 12, 14, 16, 18, or 20;see Scheme 1), and found that therewere several distinctly different arrange-ments presented with different alkylsubstituents.

Using the described sample prepara-tion procedure, we can readily obtain alarge and uniformly distributed molec-ular monolayer, as observed by STM.Figure 1 a presents an example of asubmonolayer assembly of hexakis(do-decyloxy)triphenylene (T12).

The high-resolution image is shown inFigure 1 b. It can be seen that the

Scheme 1. Synthesis of triphenylenes. a) RBr, K2CO3 , cyclohexanone ; b) MoCl5 ,CH2Cl2 .

conjugated cores consist of three adjacent rings as subunitscorresponding to the aromatic moiety, in agreement with thecalculated HOMO (Figure 1 c). The surrounding zigzag linesinterconnecting the bright ring correspond to the linear alkylsubstituents. As illustrated in the unit cell, every seven moleculesform an approximate hexagon (with the seventh molecule in thecenter). There are two kinds of molecules: The central one hastypical six-fold symmetry, while the molecules located at thecorners of the hexagon have three-fold symmetry. The alkylchains are separated into three groups and the angle betweeneach two-chain group is about 1208 (Figure 1 d). This arrange-

Figure 1. a) STM image (82.0� 82.0 nm2) of a self-assembled monolayer of T12 on a graphite surface. Theimaging conditions are I� 1.30 nA and V�ÿ854.0 mV. The z-axis is 0.8 nm. b) A higher-resolution STM image(10.0� 10.0 nm2) of T12. The imaging conditions are I� 1.08 nA and V�ÿ689.4 mV. The z-axis is 0.8 nm. c) TheHOMO electron density of triphenylenes. d) The packing model of T12.

752 � WILEY-VCH-Verlag GmbH, D-69451 Weinheim, 2001 1439-4235/01/02/12 $ 17.50+.50/0 CHEMPHYSCHEM 2001, No. 12

ment was first observed in an earlier study on T11 in which achirality model was proposed to describe the assembly and afrustrated triangular Ising net was introduced to describe thesteric hindrance interaction of the core molecules.[12] The high-resolution image is in good agreement with the latter report. Inaddition, with the improved resolution of both core moleculeand substituents, we can obtain detailed information of theassembly configuration. It is discernible that the center moleculehas a fixed orientation with six alkyl chains extended as shown inFigure 1 b. We measured the angle of the chains relative to themolecular cores as illustrated in Figure 1 b. It is worthpointing out that, when only the molecule ± substrateinteraction is concerned, both the aromatic core andthe alkyl chains would choose a preferred orientationcommensurate with the underlying graphite lattice.Therefore, the measured relative angles could provide ameasure of the interaction strength of the adsorbedmolecules with the substrate. It can be seen fromTable 1 that the measured angles are far from those atideal orientation (explained in Figure 6 later). In theimage (Figure 1 b) the angle between the alkyl groupsis 1208, indicating that alkyl chains are positionedcommensurate with the graphite lattice. The locallyinterdigitated alkyl chains are supportive to the viewthat the 2 D crystallization of alkyl chains is important indetermining the assembly structure. The deviation

from the ideal value is attributed to the steric hinderince of thetriphenyl cores.[12]

Further support of the above observation is provided by thefollowing data. With the two additional methylene units on thealkyl substituents of hexakis(tetradecyloxy)triphenylene (T14),the whole arrangement of the molecule changes drastically. Acharacteristic dimer structure is persistent over a large area(Figure 2 a). Figure 2 b is a high-resolution image. The carbonchains of the molecule are divided into two oriented groups,which leads to the loss of the original molecular symmetry. The

triphenylene cores are anti-parallel to each other within thedimer, as seen from the experimental image, similar to theantiferromagnetic arrangement in the 1 D Ising model. This isagain an indication of steric hindrance of the aromatic moieties.The relative angles again do not agree with the values of theideal adsorption configuration (Table 1).

When the length of the alkyl substituent increases by twomore methylene units to form hexakis(hexadecyloxy)tripheny-lene (T16), the molecular arrangement changes again. Themolecules form a different kind of two-dimensional superlattice,every two neighboring molecules orient in an antiparallel way toform dimers, as directly observable from the STM image

Table 1. The angles between the midline of the triangle of the triphenylenecores and the alkyl chains.[a]

Measured relativeangles [8]

a b g

T12[b] 46� 6 (60) 186� 1 (180) 315� 10 (300)T14 68� 1 (60) 130� 2 (120) 230� 4 (240)T16[c] 0 (0) 180 (180)

[a] The solid ± line arrowhead rotates clockwise along the alkyl chains tomeasure the angle values. The values of the ideal orientation are inparentheses, with the configuration illustrated in Figure 6. [b] Values for themolecule with three-fold symmetry of T12. [c] There are only twoorientations for T16.

Figure 3. a) STM image (48.2� 48.2 nm2) of T16. The imaging conditions are I� 1.12 nA and V�ÿ994.8 mV. The z-axis is 0.8 nm. b) A higher-resolution image (18.7�18.7 nm2) of T16. The imaging conditions are I� 1.05 nA and V�ÿ900.0 mV. The z-axis is 1.0 nm. c) The simplified model of T16.

Figure 2. a) STM image (75.6� 75.6 nm2) of T14. The imaging conditions are I� 1.12 nA andV�ÿ704.5 mV. The z-axis is 0.8 nm. b) A higher resolution image (12.9� 12.9 nm2) of T14.The imaging conditions are I� 1.29 nA and V�ÿ 741.3 mV. The z-axis is 1.0 nm.

CHEMPHYSCHEM 2001, No. 12 � WILEY-VCH-Verlag GmbH, D-69451 Weinheim, 2001 1439-4235/01/02/12 $ 17.50+.50/0 753

(Figure 3 a). It can be seen that the moleculesform a well ordered arrangement and that all thealkyl groups are parallel to each other, in order toform a tightly packed structure (Figure 3 b). In theimage the benzene moiety can be recognizedclearly. The two molecules in one dimer have theopposite polarity (Figure 3 b). The distance be-tween the two molecules of the dimer in T16 is2.01�0.04 nm, compared to 1.57� 0.05 nm inT14. The packing model is presented in Figure 3 c.Earlier studies revealed a uniform arrangementwhere the molecules were proposed to orientatein a parallel manner.[11] We believe that theimproved resolution of the assembly helps toclarify the configuration.

When the number of the methylene unitsreaches eighteen (T18) and twenty (T20), theintermolecular spacing within the triphenylene arrays becomesless uniform than that in T16. Figure 4 is the large-scale image ofT18; Figure 5 a is the high-resolution image of T20. The packingmodel is presented in Figure 5 b. It can be seen that the aromaticcores are no longer uniformly spaced along the array, but ratherdisplay a sporadic spacing distribution. This behavior can be

accounted for by the consideration that as the 2 D crystallizationforce becomes dominant for longer alkyl chains, the sterichindrance of aromatic cores is less important. As a result, thearomatic cores could be either in a parallel or antiparallelmanner, which leads to the observed random spacing betweenaromatic cores.

Figure 6 is an idealized model. Both triphenyl cores and alkylbackbones are commensurate with the graphite lattice. Labels a,b, and g represent the angles between the orientation of themidline of the triangle of the triphenylene cores and theorientation of the alkyl chains. The values in the ideal orientationand in the images are presented in Table 1.

Alkyl substitution is beneficial in stabilizing molecules on solidsurfaces through van der Waals interactions, which could in-crease the desorption barrier of the combined molecules.[14, 15]

Furthermore, it was shown that the 2 D crystallization of alkylmoieties is also important to immobilize molecules within theassembly.[16] The local crystallization of the alkyl moieties resultsin highly parallel ordering of linear alkanes regardless of coremolecule symmetries.[17, 18] An ideal match would require that thedimension and symmetry of the molecules meet the periodicityof alkane lamellae. The steric hindrance of core molecules wouldfavor antiparallel molecular orientations. The outcome of the

Figure 4. STM image (59.8� 59.8 nm2) of a self-assembled monolayer of T18.The imaging conditions are I� 1.12 nA and V�ÿ825.0 mV. The z-axis is 1.0 nm.

Figure 5. a) A high-resolution image (29.8� 29.8 nm2) of T20. The imaging conditions are I�1.13 nA and V�ÿ799.0 mV. The z-axis is 1.0 nm. b) The simplified model of T20.

Figure 6. Four different ideal molecular models with different alkyl lengths on HOPG (I, II : T12 ; III : T14 ; IV: T16). Both triphenyl cores and alkyl backbones arecommensurate with the graphite lattice. The broken line represents the orientation of the midline of the triangle of the triphenylene cores, the solid line the orientation ofthe alkyl chains.

754 � WILEY-VCH-Verlag GmbH, D-69451 Weinheim, 2001 1439-4235/01/02/12 $ 17.50+.50/0 CHEMPHYSCHEM 2001, No. 12

Evidence for a Transition BetweenSinglet and Triplet States in theElectrochemical Reduction of2,2'-4,4'-Tetranitrobiphenyl

Iluminada Gallardo,* Gonzalo Guirado, Jordi Marquet,and Miquel Moreno[a]

KEYWORDS:

anions ´ electrochemistry ´ structure elucidation ´transition states

Radical centres, separated by suitable spacers, constitute one ofthe basic principles in the design of organic molecular ferro-magnets.[1] However, the use of extended quinones as electronacceptors for the production of organic conducting materials[2±4]

has been increasing. Thus, 2,6-di-tert-butylphenoxyl radicalsystems[5, 6] connected by m-phenylene units in the 4-positions,have been largely used to synthesise oligoradicals with triplet,quartet and quintet ground states.[7±10] In some cases, thebiradical systems, in which two phenoxyl radicals are coupled byvarious p-arylene spacers, quantitatively exist in an equilibriumwith extended quinones.[11±13] In this case, the biradical iselectrochemically generated (two successive one-electron oxi-dations of the correspondent bisphenol) and characterised bycyclic voltammetry and EPR spectroscopy. The variation of thespacers in the extended quinones influence both the electro-chemical behaviour and the spectroscopic properties of theseextended quinones and their biradicals.[14]

It is well known that, for a given molecular system, differentelectronic states have a very different chemical behaviour. Wellknown examples are the singlet and triplet states of oxygen and

molecular assembly is determined by the joint effect of stericinteraction and frustrated 2 D alkane lamellae.

To summarize, we studied the arrangement of Tn on HOPGusing STM. The results show that alkyl components can lead tovarious packing patterns, forming a variety of superlattices in theself-assembled monolayer on the graphite surface. When n isequal to twelve, the molecular interaction dominates thecrystallization, and the whole arrangement contains the three-fold symmetry. A transition occurs when n is equal to fourteen,and part of the alkyl chains form a tightly packed structure.When n reaches sixteen, the alkyl moiety directs the intermo-lecular ordering and the molecules lose their symmetry.

Experimental Section

Synthetic methods: 2,3,6,7,10,11-Hexakisalkoxy-substituted tripheny-lenes with n-carbon side chains (n� 12, 14, 16, 18, or 20) wereprepared using a two-step synthesis, as described in an earlierpaper.[13]

Sample preparation for STM: The samples were dissolved in tolueneor chloroform with a concentration <1 %. A droplet of the solutionwas deposited onto a freshly cleaved surface of HOPG (quality ZYB,Digital Instruments). The solvent was evaporated to dryness andthen the image was taken.

The experiment was performed on a Nanoscope IIIa SPM (DigitalInstruments) at ambient conditions. STM tips were mechanicallyformed Pt/Ir wire (90/10). All the STM images were recorded usingthe constant current mode of operation, and only the flattenprocessing was performed. The specific tunneling conditions aregiven in the figure captions.

The preliminary calculation of the electron densities of the molecularorbitals was calculated with the PM3 method using the Hyperchemprogram.

The authors thank the National Natural Science Foundation andthe Foundation of the Chinese Academy of Sciences for financialsupport. Support from the National Key Project on Basic Research(grant G2000077501) is also acknowledged.

[1] D. K. Schwartz, R. Viswanathan, J. A. N. Zasadzinski, J. Am. Chem. Soc.1993, 115, 7374.

[2] D. K. Schwartz, R. Viswanathan, J. A. N. Zasadzinski, Phys. Rev. Lett. 1993,70, 1267.

[3] J. V. Selinger, Z.-G. Wang, R. F. Bruinsma, C. M. Knobler, Phys. Rev. Lett.1993, 70, 1139.

[4] R. Viswanathan, J. A. Zasadzinski, D. K. Schwartz, Nature 1994, 368, 440.[5] X. H. Qiu, C. Wang, Q. D. Zeng, B. Xu, S. X. Yin, H. N. Wang, S. D. Xu, C. L. Bai,

J. Am. Chem. Soc. 2000, 122, 5550.[6] J. S. Foster, J. Frommer, Nature 1988, 333, 542.[7] D. P. E. Smith, H. Hörber, Ch. Gerber, G. Binnig, Science 1989, 245, 43;

D. P. E. Smith, J. K. H. Hörber, G. Binning, H. Nejoh, Nature 1990, 344, 641.[8] M. Hara, Y. Iwakabe, K. Tochigi, H. Sasabe, A. Garito, A. Yamada, Nature

1990, 344, 228.[9] D. P. E. Smith, W. Heckl, Nature 1990, 346, 616; D. Smith, J. Vac. Sci. Technol.

B 1991, 9, 19.[10] W. Mizutani, M. Shigeno, Y. Sakakibara, K. Kajimura, M. Ono, S. Tanishima,

K. Ohno, N. Toshima, J. Vac. Sci. Technol. A 1990, 8, 675.[11] L. Askadskaya, C. Boeffel , J. P. Rabe, Ber. Bunsen-Ges. Phys. Chem. 1993, 97,

3, 517.[12] F. Charra, J. Cousty, Phys. Rev. Lett. 1998, 80, 1682.

[13] S. Kumar, M. Manickam, Chem. Commun. 1997, 1615.[14] K. Eichhorst-Gerner, A. Stabel, G. Moessner, D. Declerq, S. Valigaveettil, V.

Enkelmann, K. Müllen, J. P. Rabe, Angew. Chem. 1996, 108, 1599; Angew.Chem. Int. Ed. Engl. 1996, 35, 1492.

[15] X. H. Qiu, C. Wang, S. X. Yin, Q. D. Zeng, B. Xu, C. L. Bai, J. Phys. Chem. B2000, 104, 3570.

[16] B. Xu, C. Wang, S. X. Yin, X. H. Qiu, Q. D. Zeng, C. L. Bai, J. Phys. Chem. B2000, 104, 10 502.

[17] V. S. Iyer, K. Yoshimura, V. Enkelmann, R. Epsch, J. P. Rabe, K. Müllen,Angew. Chem. 1998, 110, 2843; Angew. Chem. Int. Ed. 1998, 37, 2696.

[18] A. Stabel, P. Herwig, K Müllen, J. P. Rabe, Angew. Chem. 1995, 107, 1768;Angew. Chem. Int. Ed. Engl. 1995, 34, 1609.

Received: June 11, 2001 [Z 241]

[a] Dr. I. Gallardo, Dipl.-Chem. G. Guirado, Prof. J. Marquet, Dr. M. MorenoDepartament de QuímicaUniversitat AutoÁnoma de Barcelona08193 Bellaterra, Barcelona (Spain)Fax: (�34)93-581-2920E-mail : Iluminada.Gallardo@uab.es