Structure of new carbonaceous materials: The role of vibrational spectroscopy

Carbon 43 (2005) 1593–1609 www.elsevier.com/locate/carbon

Structure of new carbonaceous materials: The role of vibrational spectroscopy Andrea Centrone a, Luigi Brambilla a, Thierry Renouard b, Lileta Gherghel c, Claude Mathis b, Klaus Mu¨llen c, Giuseppe Zerbi a,* a

Politecnico di Milano, Dipartimento di chimica, Materiali e Ingegneria Chimica ‘‘Giulio Natta’’, Piazza Leonardo da Vinci 32, 20133 Milano, Italy b CNRS Institut Charles Sadron, 6 Rue Boussingault, 67083 Strasbourg, France c Max-Planck-Institut f €ur Polymerforschung, Ackermannweg 10, Mainz, Germany Received 15 October 2004; accepted 7 January 2005 Available online 8 March 2005

Abstract The search for materials aimed at energy storage has prompted the synthesis of new materials which were obtained by pyrolysis of aromatic precursors under controlled conditions for the production of carbon based structurally disordered networks aimed at hydrogen or lithium storage. Obviously these materials consist of fully insoluble mixtures of different molecular species which escape the traditional physico-chemical techniques for structure determination. With the purpose of overcoming this difficulty we have developed and report here a systematic vibrational spectroscopic work which lays the basic concepts to be considered in the structural understanding of the molecules studied and can be extended to similar classes of complex carbonaceous materials. The (partial) structure of these systems and some of the reaction pathways at the molecular level can be inferred from the spectroscopic signals presented and discussed to be taken as key features for structural analysis. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Pyrolytic carbon, Carbon precursor; Infrared spectroscopy; Microstructure

1. Introduction In recent years Material Science and Soft Matter Physics have shifted their attention on carbonaceous materials ranging from amorphous carbon to fullerenes and nanotubes [1–7]. Each of this class of materials seems to become essential for the development of new and revolutionary technologies. Numerous are the ways adopted by various groups for the preparation of carbon based materials and great difficulties have been met in finding suitable methods for their characterisation. More than once the properties of these substances have been strongly affected and altered *

Corresponding author. Tel.: +39 02 2399 3235; fax: +39 02 2399 3231. E-mail address: [email protected] (G. Zerbi). 0008-6223/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.01.040

by the large amount of impurities which were not properly removed [8]. Necessarily the preparation and purification processes need to be followed and supported by whichever technique of molecular and bulk characterisation is found suitable for the particular system considered. Obviously carbonaceous materials are black, amorphous and insoluble and any attempt of characterisation has to face such unpleasant reality. We adopt the molecular approach to obtain a new class of carbon based materials, exploiting the pyrolysis of chosen precursor molecules, in order to obtain suitable and peculiar properties and features. We had found earlier that poly phenyl precursors, when having the appropriate topology, can smoothly be transformed into flat polycyclic aromatic hydrocarbons by an intramolecular cyclodehydrogeneration. The simplest case is that of the propeller-shaped

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cited states; these studies have been carried out with quantum chemical calculations which are reported independently elsewhere [6,7]; analogously the precise dynamics of the simplest systems has been considered also from the quantum mechanical viewpoint [6,7]. We discuss in this paper the dynamical problem in terms of group theory and of dynamics and propose spectroscopic correlations useful for the understanding of the chemical nature specifically of the disordered carbonaceous materials obtained from the pyrolysis of suitable precursors. We report here first the results obtained from the infrared spectra which may be of more immediate use for the characterisation of unknown materials. The measured decrease of hydrogen content after pyrolysis is necessarily accounted for with the formation of intermolecular (a sort of polymerisation) or intramolecular (ring condensation) reactions that lead to the formation of CC bonds, respectively between or within, the precursor molecules; it follows that the extention of p electrons delocalisation in these materials is unknown and may be variable from structure to structure throughout the sample. One can envisage: (i) the formation of mostly twodimensional disordered (probably rippled) sheets of graphitic systems, (ii) the formation of a crosslinked dendrimeric three-dimensional polyphenylenic molecules, (iii) the formation of a three-dimensional disordered networks of bonded benzene rings connected in a complex structure where both dendrimeric and graphitic structures coexist. We first identified the most meaningful spectroscopic signals of the precursor molecules, then, keeping in mind the three structural scenarios presented above we proceeded in a comparison of the spectra of many pyrolytic products obtained under different reaction conditions (time and temperature) (see Fig. 1).

hexa(phenyl)benzene (I) which upon treatment with, e.g., iron trichloride produces hexa-peri-hexabenzocoronene (II) C42H18 (HBC) [9,10]. HBC is a remarkable disc-type p-system because it has a D6h symmetry and can be regarded as a super benzene (C42) with three times the size of triphenylene (III). Hexa alkyl derivatives of HBC are soluble and meltable and due to their phase-forming properties play an important role as semiconductors compounds of electronics devices [10–12]. What prompted the present study was the expectation that pyrolysis of (I) might also lead to intramolecular dehydrogenation processes under formation of larger conjugated discs. Pyrolysis in the lamella, however, could also lead to intermolecular dehydrogenation giving rise to aryl–aryl-coupling processes. Inclusion of hexa(p-bromophenyl)benzene (IV) and its iodo analogue (V) seemed to have appropriate aryl-aryl bound cleavage, and thus rearrangements of phenyl groups might also occur at elevated temperatures. While this scenario might then appear quite complex it has the potential of forming a three-dimensional network containing more or less extended graphitic areas. Such a network appears particularly useful for the uptake of guest atoms or molecules such as occurring in the lithium storage of secondary battery elements. In principle many are the reactions that might occur during the pyrolytic process and consequently many could be the synthesised structures; in order to investigate the main processes involved and the structures obtained we developed a method of spectroscopic characterisation aimed at discovering spectroscopic signals characteristic of specific structural features which may help in the characterisation of the material at the ‘‘molecular’’ scale. The interpretation of the Raman spectra required considerations of the electronic properties in the ground and ex-

Br Br

Br

Br

Br Br

(II)

(I)

(IV)

(III)

I I

I

I

I I

(V)

(VI)

(VII)

Fig. 1. (I) Hexa(p-phenyl)benzene, (II) Hexabenzocoronene, (III) triphenylene (IV) hexa(p-bromo-phenyl)benzene, (V) hexa(p-iodo-phenyl)benzene, (VI) penta-phenyl-cyclopentadiene, (VII) and 1,2,3,4 tetra-phenyl-naphthalene.

A. Centrone et al. / Carbon 43 (2005) 1593–1609

(a)

(b)

(c)

(d)

1595

Fig. 2. Examples of regular structure: (a) C126H42 (TRIO/MONO/DUO = 5/2/2), (b) C252H54 (TRIO/MONO/DUO = 2/1/1) and irregular structure: (c) C126H42 (TRIO/MONO/DUO = 5/2/2), (d) C252H60 (TRIO/MONO/DUO = 2/2/1).

In this paper we show that the products obtained upon pyrolysis consist of mostly two-dimensional polycyclic aromatic hydrocarbon (PAH) sheets with peculiar features (‘‘holes’’) strictly related to the ‘‘molecular approach’’ of the synthetic method. With respect to an ideal graphite plane those ‘‘holes’’ can be described as six missing carbon ‘‘replaced’’ by six hydrogens (see Fig. 2). Necessarily the concept of ‘‘molecular’’ purity in these systems becomes very vague or even meaningless since they have to be considered complex mixtures of structures where carbon atoms are likely to be in sp2 and sp3 hybridisation states linked to each other in a disordered network of C=C and C–C bonds where (full or partial) p electron delocalisation can also occur over topologically small or large domains. There is no doubt that these systems can be considered as molecularly heterogeneous structures and any description originated from any physical or chemical test must be taken as ‘‘average’’ over several structural situations. Moreover, each physical analytical technique focuses mostly on specific structural or chemical features. XPS may indicate the average amount of carbon atoms in sp, sp2 and sp3 hybridisation, STM and AFM can provide an overall picture at the nanometer level, infrared or Raman may provide informations on more or less localised structures through a sort of group frequency correlations.

2. Materials and experiments In this paper we focus our attention on two new classes of carbon containing materials namely (i) polyaromatic hydrocarbons (PAH) which have been recently prepared as strictly monodisperse systems with preassigned shapes consisting of a larger and larger number

of condensed rings [9] and (ii) materials obtained from the pyrolysis of simple polyphenylene precursors hexa(phenyl)benzene (I), hexa(p-bromo-phenyl)benzene (IV) and hexa(p-iodo-phenyl)benzene (V) which by heating form inter and intramolecular C–C bonds. Under formation of PAH sheets materials as in (i) are indeed precise models of graphite and they may provide data useful for the understanding of the structure of more complex and less ordered systems [6,7], materials as in (ii) are of recent production which is part of a more extended project on carbonaceous materials whose target is the storage of energy through lithium storage and hydrogen storage [13]. The spectra of the following simple molecular models have also been examined: penta-phenyl-cyclopentadiene (VI) and 1,2,3,4 tetra-phenyl-naphtalene (VII), which have been purchased from Aldrich and used without further purification. We report the spectra obtained from hexa(phenyl) benzene (I) precursor that has been pyrolysed for one day at 450 °C (HPB-450-1d), 500 °C (HPB-500-1d), 600 (HPB-600-1d), 800 °C (HPB-800-1d) and for five days at 600 °C (HPB-600-5d); hexa(p-bromo-phenyl)benzene (IV) precursor that has been pyrolysed at 450 °C for six days (HPB-Br-450-6d), at 550 °C for one day (HPBBr-550-1d) and five days (HPB-Br-550-5d); hexa (p-iodo-phenyl)benzene (V) precursor has been pyrolysed at 450 °C for six days (HPB-I-450-6d). The pyrolytic process is described in reference [14]. Infrared Spectra have been recorded with FTIR spectrometer Nicolet Nexus, for most of the materials in the solid state as KBr pellets. When during the analysis it was felt that we were dealing with mixtures of several chemical species extractions with suitable solvents have been made and the extracted materials were studied as solids after solvent evaporation.

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Raman spectra were recorded with Dilor XY spectrometer; micro-Raman spectra were also recorded with the same spectrometer in the visible range. If a sample showed a non homogeneous microscopic morphology Raman spectra aiming at different zones were recorded with the aid of a microscope. All solid samples as powders have been examined with the optical microscope; when necessary Electron microscopy as well as AFM techniques were used for the study of their morphology.

Obviously ki  k0i if the various elements DFml and/or DGml are negligibly small and/or if the elements of the matrix L are negligibly small i.e. the motion Qi does not involve atoms at large distances within the molecule. A collective motion takes place when all L elements and possibly also DFml and/or DGml are large. In order to answer this question several dynamical and structural factors need to be considered and are going to be discussed below.

3. The dynamical problem

4. Results and discussion

A rigorous approach to the understanding of the vibrational spectra of the carbonaceous materials (either considered in this paper or for the whole class of these types of substances) necessarily has to answer the first relevant question whether, and to what extent, the ‘‘molecular’’ vibrations of these systems are topologically localised within restricted molecular domains or are able to extend over larger molecular domains (generating ‘‘collective’’ motions), thus probing either the structure and dynamics of small or large portions of the material. The phenomenon of localisation and delocalisation of normal vibrations in finite molecules can be better understood with a few conceptual definitions. In molecular dynamics it is usual to calculate normal frequencies (ki) from the following eigenvalue equation (in matrix notation): GFL = LK, where G takes care of the atomic masses and their geometrical arrangement within the molecule and the matrix F describes the force field acting during the vibrational motions. If these dynamical matrices are known the eigenvalues K (i.e. the diagonal matrix with vibrational frequencies) can be calculated as well as the eigenvectors L which describe the vibrational displacements for each ki, thus providing the required information whether during a given normal mode Qi with frequency ki a few adjacent atoms (localised mode) or many atoms throughout the whole molecule (collective mode) are moving. Let G0, F0, K0 and L0 be the above dynamical quantities referred to a given molecule taken as reference (i.e. unperturbed); then we consider the introduction of some perturbation on the frequencies (by chemical modification) of the reference molecule (e.g., substitution, C–C link etc. in our case). The corresponding matrices of the molecule so modified can be written as: G = G0 + DG, F = F0 + DF, L = L0 + DL. Let us focus on a specific normal mode Qi with reference frequency k0i ; expanding the eigenvalue equation and retaining the first order changes DG and DF and the zeroth order eigenvectors one obtains s X  0 ki ¼ ðk0i Þ þ ðL Þim  ðL0 Þli  DF ml þ ðk0i Þ

4.1. The precursor hexa(phenyl)benzene (HPB)

l;m¼1

ðL1 Þim  ðL1 Þli  DGml



Hexa(phenyl)benzene (HPB) is the simplest precursor molecule which, subjected to pyrolysis at various temperatures, provides the pyrolytic materials studied in this paper. Three geometries can be considered for HPB namely (i) A molecule where all torsional angles h take the value of h = 90° (i.e. when the six benzene rings are orthogonal to the plane of the central ring) with symmetry point group D6h. (ii) A molecule in which all benzene rings attached to the central ring are equally tilted with a certain torsional angle h (0° < h < 90°) with symmetry point group D6. (iii) A fully planar molecule (just as limiting geometry, not physically conceivable because of the repulsive interactions between hydrogens); the symmetry point group becomes again D6h. The structures of the irreducible representations of the above three models are listed below, while Table 1 summarises the theoretical predictions as of their infrared and Raman activity. Model (i) D6hðh¼90 Þ

Cvibr:

¼ 12A1g þ 7A2g þ 4B1g þ 12B2g þ 15E1g þ 20E2g þ 4A1u þ 11A2u þ 12B1u þ 8B2u þ 19E1u þ 16E2u

According to Table 1 one expects 12 medium/strong totally symmetric (TS) modes in the Raman spectrum accompanied by at most 35 much weaker ones. Polarisation measurements of a sample in solution should also identify the fully polarised TS modes, but these are overwhelmed by the scattering of THF, into which HPB is slightly soluble. Because of the existence of the centre of symmetry strict mutual exclusion between g (Raman active) and u (infrared active) modes should be observed.

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Table 1 Distribution of vibrational modes, IR and Raman activity as predicted from group theory for the three geometries considered Raman active

IR active

Model (i) D6h (h = 90°)

12A1g 15E1g 20E2g 11A2u 19E1u

Yes Yes Yes No No

(Polarised totally symmetric) predicted strong (Depolarised) predicted medium-weak (Depolarised) predicted weak

No No No Yes Yes

Model (ii) D6 (0° < h < 90°)

16A1 34E1 36E2 18A2

Yes Yes Yes No

(Polarised totally symmetric) predicted strong (Depolarised) predicted medium-weak (Depolarised) predicted weak

No Yes No Yes

Model (iii) D6h (h = 0°)

12A1g 11E1g 24E2g 7A2u 23E1u

Yes Yes Yes No No

(Polarised totally symmetric) predicted strong (Depolarised) predicted medium-weak (Depolarised) predicted weak

No No No Yes Yes

Model (ii) D6ð0
Cvibr:

¼ 16A1 þ 18A2 þ 16B1 þ 20B2 þ 34E1 þ 36E2

For this structure we expect 16 strong polarised TS Raman lines, 34 transitions coinciding in frequencies both in IR and Raman and 18 modes only IR active. 36 E2 modes are generally extremely weak. Model (iii) D6hðh¼0 Þ

Cvibr:

¼ 12A1g þ 11A2g þ 4B1g þ 8B2g þ 11E1g þ 24E2g þ 4A1u þ 7A2u þ 12B1u þ 12B2u þ þ23E1u þ 12E2u

As for model (i) g and u modes should not coincide; the distribution of the normal modes in the various symmetry species is different from that predicted for model (iii) because the geometries in the two models are different, even if both molecules belongs to the same symmetry point group. At the first look the spectra of HPB in Fig. 3 show a striking evidence of non coincidence between infrared and Raman spectra which could lead to the immediate conclusion that the molecule has a centre of symmetry, thus suggesting that model (i) or (iii) should be considered. Indeed this observation turns out to be deceiving as discussed below. Let us consider the vibrations of the molecule of biphenyl which is the simplest model of the molecules we are examining in this paper. Biphenyl consists of two benzene rings which we label as (ring 1) and (ring 2) linked to each other by a single C–C bond (inter-ring delocalisation of p electrons is purposely overlooked for the time being). Each of the normal modes of (ring 1) may be coupled in-phase or out-of-phase with the corresponding mode of (ring 2). If the coupling is sizeable two vibrational levels, well spaced in energy, are produced

and two bands, one IR and one Raman active, in principle should be observed (Fig. 4). If the coupling is negligibly small the two levels are accidentally degenerate and the TS in-phase mode (gerade, Raman active) coincides in frequency with the out-of-phase infrared active ungerade mode. Localisation (or coupling) plays a relevant role. This has been precisely what has been calculated and experimentally observed for biphenyl [15,16]. Similar situations can be predicted for HPB when each normal mode of one ring has to couple with the other identical modes of the five neighbours. If no coupling occurs sixfold accidental degeneracy takes place and the selection rules are only those which dominate within one phenyl ring. It has to be pointed out that coupling may be simply through kinetic energy (i.e. due to atomic masses and geometry; G matrix, see Section 3) and/or through potential energy (i.e. force constants due to electronic interactions such as p conjugation; F matrix). Let i and j be two internal coordinates of the molecule. Dynamics tells us that the coupling terms gij in the kinetic energy matrix certainly are = 0 when i and j have no atom in common [17]. Thus the extent of kinetic coupling is very restricted. As to the quadratic potential energy terms fij the coupling may derive from the strength of the diagonal term fii in the potential energy matrix of the bond directly connecting the two benzene rings or through the interaction force constants fij which depend on the distance and strength of interaction caused mostly by p electron delocalisation. This problem is common in the large class of polyconjugated materials and has been extensively discussed in specialised papers and books [18–20]. We checked the above statements with two model compounds (VI, VII) with a few benzene rings attached to a central ring which probably, because of symmetry and different electronic delocalisation, must introduce

A. Centrone et al. / Carbon 43 (2005) 1593–1609

224 201

1496

Wavenumbers (cm-1) 3054

1000

4052

a.u.

3080

3056

IR Raman

3023

1577

1599

555

a.u.

730

1598

257

995

1344

IR Raman

696

1598

4000

3500

3000

Wavenumbers (cm-1)

Fig. 3. IR spectrum (continuous line) and Raman spectrum (dotted line) of Hexa(Phenyl)Benzene (HPB). The Raman spectrum is recorded with 632.8 nm exciting line. The ordinate scale are in arbitrary units.

L1-L2

IR active IR, Raman active

L1, L2

DECOUPLED MODES

L1+L2

Raman active

COUPLED MODES f (coupling)=0

ACCIDENTAL DEGENERATE MODES f (coupling)=0

Fig. 4. Splitting of vibrational energy levels of symmetrically equivalent oscillators (L1 and L2) as function of their coupling and consequent appearance of the infrared or Raman spectra.

some kind of perturbation. We have first analysed the infrared and Raman spectra of penta-phenyl cyclopentadiene (PPCPD) where the central ring may be taken (in a first approximation) as planar; of the five phenyl rings one lies certainly out of the plane of the central ring while the other four, in pairs, take up a different location in space. With such geometry the five phenyl rings can be grouped into three symmetrically inequivalent sets where their differences are either geometrical and/or

chemical. If this is true and the criteria presented above for HPB are true each of the vibrations of the monosubstituted benzene rings has to generate a triplet of bands in the vibrational spectra. This is exactly what happens as shown in Fig. 5 where the spectra of PPCPD are compared with those of HPB. However, two of the components of each triplet (no effort is made here for their identification) are still accidentally doubly degenerate since in and out-of-phase motions within the pairs of equivalent rings are still practically de-coupled. A further check has been attempted with another model molecule, namely 1,2,3,4 tetra-phenyl-naphtalene (TPN), in which two pairs of symmetrically equivalent phenyl rings occur. Doublets are expected and indeed observed (Fig. 5). As a conclusion we must state that the observed IR/ Raman non coincidence observed in the vibrational spectra of HPB cannot be taken as evidence of a centrosymmetric structure. Moreover if the pyrolytic process produces a dendritic-like asymmetric structure multiplets of bands are expected in correspondence of a single

A. Centrone et al. / Carbon 43 (2005) 1593–1609

1599

Fig. 5. IR spectra penta-phenyl-cyclopentadiene, PPCPD, (dotted line), 1,2,3,4 tetra-phenyl-naphtalene, TPN, (continuous line) and hexa(phenyl)benzene, HPB, (sketched line).

band for HPB, but this not the case of the spectra of the pyrolitic products, as it will be shown in the text. We can then proceed to the qualitative vibrational assignment of HPB. While the detailed dynamical vibrational analysis will be reported in a specialised spectroscopic journal, in this paper we extract a few information which are relevant to the target of this work, namely we wish to extract from the spectra structurally relevant spectroscopic features to be used in a general structural analysis of carbonaceous materials. Bands relevant to our purpose are the following (see Fig. 3); in infrared: (a) 555 cm1 associated to a sort of collective out of plane deformation mode (a complex butterfly-like motion). (b) 696 and 730 cm1 out-of-plane C–H deformation modes characteristic of monosubstituted benzene ring [21] (i.e. five adjacent C–H groups are vibrating out of the plane (opla modes)). The opla modes of benzene rings in general enjoy great popularity in spectroscopic chemical diagnosis since they are characteristic group frequency bands directly related to the location and number of substitution within the benzene ring. From the work reported here it turns out that these correlations must be limited to isolated benzene rings with some kind of chemical substitution which does not introduce too large kinetic or electronic perturbations. Moreover they cannot be used in the case of condensed polyaromatic structures. (c) 3023 cm1 mainly associated to the single C–H stretch in para position [22]. (d) 4052 cm1. Particular attention should be paid to this band which has never been reported as struc-

turally meaningful in the previous literature. It derives from a two quantum transition (combination tone) between the IR active C–H stretching at 3056 cm1 and the ‘‘ring breathing mode’’ of the phenyl ring, Raman active at 995 cm1 (995 + 3056 = 4051 cm1). This band is observed also in the infrared spectrum of benzene, thus it must be the result of a combination of (g)x(u) = (u) modes. This band can be taken as characteristic of benzene rings which can develop an unperturbed breathing mode, hence it can used in the diagnosis of the existence of non condensed benzene rings. Indeed the IR spectrum of naphthalene does not show this absorption. The band is observed also in substituted benzenes as long as several C–H groups are still attached to the ring. This band can be taken as a marker of the existence in the sample of ‘‘free’’ (i.e. not condensed) substituted benzene rings as long as aromatic C–H groups exist in the rings. The appearance of such overtone implies the existence of a strong anharmonic coupling (either mechanical and/or electrical) between the C–H oscillators and the ‘‘ring breathing’’ oscillation. Such preferential anharmonic coupling (yet to be accounted for theoretically) occurs (and spectral features are observed) in many other molecules where the C–H group is attached to a C=C bond; p-bonding does play a dominant role [23]. (e) The traditional group frequencies used in spectroscopic chemical analysis [21,24,25] are obviously observed namely: (i) the triplet of band at 1599 (m), 1577 (w) and 1496 (m) cm1 associated to the ring stretching motions (their intensity is strongly dependent on p-electron delocalisation) and (ii) the complex structure above 3000 cm1

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A. Centrone et al. / Carbon 43 (2005) 1593–1609

associated to the C–H stretchings of C–H groups with the C atoms in sp2 hybridisation; we notice the band at 3056 cm1 which generates the combination transition infrared active at 4053 cm1 as discussed above at (d). The Raman spectrum (free of fluorescence with excitation at 632.8 nm) of HPB shows the following lines of diagnostic use: (f) 201, 224 and 257 cm1 due to collective skeletal deformations. (g) 995 cm1 (very strong) due to the ring breathing mode which generates the combination transition infrared active at 4053 cm1 as discussed above at (d). (h) 1344 cm1 originating from the breathing of the central ring. A similar mode in the same frequency range has a great relevance in the interpretation of the whole class of carbonaceous materials, graphite, PAH etc which are the subject of interest in our research group [6,7]. (i) 1598 cm1 C–C ring stretching mode. (j) 3054 cm1 C–H stretching mode. It has to be kept in mind that Fermi resonances are likely to occur in this frequency range.

450 °C is the lowest temperature used in the pyrolysis of HPB and its Raman and infrared spectra provide the first description of the molecular processes which take place during the reaction. Even if preliminary tests on the sample indicate that HPB-450-1d (pyrolysed at 450 °C for one day) consists of a mixture of various chemically different fractions new spectroscopic data are revealed which will be of great use for the further development of these kinds of studies as shown later in this paper. The main fractions are labelled as follows: (i) pristine material (P), (ii) material quickly soluble in THF (QS) and (iii) material slowly soluble in THF for approximately 24 h (SS). The first look at the IR spectrum of HPB-450-1d if compared with that of the precursor HPB (Fig. 6) seems to suggest that no reaction took place. At a closer look of the spectrum of QS one notices that a new absorption starts arising near 759 cm1 which requires some detailed interpretation. The infrared spectra of SS after evaporation of the solvent is not very significant (it shows only a weaker band, if compared with the spectrum of QS, at 759 cm1) but the Raman spectra provide further information if compared with the Raman spectrum of HPB. Referring to Figs. 7 and 8 we notice:

4.2. Pyrolysis of HPB at 450 °C (HPB-450-1d) The materials whose vibrational spectra are the subjects of this paper derive from the pyrolysis at increasing temperatures of HPB. The aim of such pyrolytic reactions is to favour the formation of inter-ring CC (a sort of polymerisation) and intra-ring CC bonds (ring condensation).

(a) The relative intensities of the Raman lines within the spectrum are significantly different from those of the corresponding ratios for the original HPB. (b) A few lines are not shifted (e.g., 201, 224, 661, 995, 1598 cm1). (c) A few lines are shifted (e.g., 264 (+7), 767 (+8), 1341 (3), 1492 (3) and 1538 (3) cm1). (d) Lines at 693 and 700 cm1 gain intensity.

759

Absorbance a.u.

HPB-450-1d P HPB-450-1d QS HPB

800

750

700

Wavenumbers (cm-1) Fig. 6. details of out-of-plane vibration region in the IR spectra of precursor HPB (sketched line), pristine HPB-450-1d (dotted line) and quickly soluble fraction of HPB-450-1d (continuous line).

995

A. Centrone et al. / Carbon 43 (2005) 1593–1609

1601

201 224

238 407

399

693 767

1492

700

661

264

1598 1538

Raman intensity a.u.

1341

HPB HPB-450-1d SS

1000

Wavenumbers (cm-1)

IR HPB Raman HPB-450-1d SS Raman HPB

555

730

Absorbance a.u.

696

Fig. 7. Raman spectra (exciting line at 632.8 nm) of precursor HPB (dotted line) and SS fraction of HPB-450-1d (continuous line).

600

700

Wavenumbers

(cm-1)

Fig. 8. Detail of the low frequency region of the Raman spectra (exciting line at 632.8 nm) of precursor HPB (sketched line), and SS fraction of HPB450-1d (continuous line) which shows the activation of some IR active line (dotted spectrum).

(e) The new lines appearing at 556, 696 (structured) and 730 cm1 coincide with strong infrared bands (see Fig. 8). (f) The skeletal modes at 264 and 402 cm1 appear to be non negligibly perturbed and weak new signals are observed at 238 cm1. (g) New scattering appears in the C–H stretching range on the high frequency side of the corresponding scattering of HPB above 3000 cm1 (not shown). The structural analysis which derives from the above spectroscopic observations is the following: certainly the material is a mixture of the pristine HPB and at least of another fraction consisting of HPB molecules linked together (weak band at 759 cm1) as it will be discussed later in this paper. Only a few molecules, may be linked

together because of the solubility of the material and because the IR spectra are very similar to the IR spectrum of HPB. Such ‘‘oligomeric HPBÕs’’ may also possess very few intramolecular CC links (intramolecular condensation). The existence of a dominant population of phenyl rings is documented by the observation of the overtone at 4053 cm1 assessing that many phenyl rings can freely ‘‘breathe’’ without any intermolecular covalent bonding. The Raman spectrum of the SS sample clearly indicates that the pyrolysis has produced more than one species, in particular species which activate in the Raman infrared active bands, thus showing that the symmetry of the system has been lowered from that of HPB. From Figs. 7 and 8 it is apparent that the changes of the Raman spectrum are quite noticeable if compared with those observed in the IR. It is known that the Raman spectrum is mostly determined by the large changes in

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polarisability originating from collective modes, thus showing that the overall shape(s) of the system(s) has (have) changed from that of HPB even if, chemically, HPB and the new ‘‘oligomers’’ are not much different. On the other hand the local symmetry probed by the IR does not change much. 4.3. New spectroscopic and dynamical aspects In the pyrolytic process inter and intramolecular reactions lead to the formation of a network of CC bonds and the elimination of H2 generating various possible carbonaceous structures. A nice system which all attempts of characterisation refer to is graphite [2–7]. Single crystals can be prepared and their structure has been clearly determined from diffraction studies, intra and intermolecular C–C bond distances have been measured, infrared (very week and hard to be seen) and Raman spectra (with strong characteristic scattering, G line) have been studied with the help of detailed dynamical treatments as well as quantum mechanical calculations [6,7]. But even for graphite as soon as a small amount of chemical or structural impurities are left in the sample new signals are clearly seen, for instance in the Raman spectrum with the ubiquitous D line. When more common samples of graphite or other carbonaceous materials are studied the D peak may strongly dominate in intensity and width. The interpretation of these features in terms of the structure of these materials is a challenge by itself which is presently debated by many research groups [1–7]. It is well known that the infrared spectrum of pure graphite is almost featureless and flat because of the optical selection rules: the structure of the irreducible representation and the derived selection rules are the following: for graphite reduced to a two-dimensional infinite network of condensed benzene rings the space group is isomorphous with D6h point group CD6h vibr: ¼ B1g ðinactiveÞ þ E 2g ðRamanÞ For a tridimensional ordered crystalline arrangement where sheets are suitably stacked one on top of the others CD6h vibr: ¼ A2u ðIRÞ þ 2B1g ðinactiveÞ þ E 1u ðIRÞ þ 2E2g ðRamanÞ Selection rules tell us that for tridimensional graphite the infrared active modes correspond to the out-of-plane modes of the graphite sheets. Due to lack of sizeable equilibrium atomic charges and/or charge fluxes [26–28] during the motions the absorptions in infrared turn out to be extremely week and very hard to be observed. Practically the infrared spectrum of graphite is expected to be flat and if features are observed they must arise from chemical impurities or from particular structural features not related to the pure and simple graphitic material.

If large PAH type platelets are formed (with more or less rippled structures) the vibrational selection rules will first approach those of two-dimensional graphite (i.e. practically no IR spectrum). If flat or disordered carbon networks originate weak or practically uninteresting IR spectra, the weak absorptions which are generally observed for such systems (including monodisperse PAH) floating on top of a very broad background should derive from the vibrations of C–H bonds still left in the systems because of an incomplete process of condensation during pyrolysis. Generally the C–H bonds to be considered are peripheral bonds, i.e. bonds which are saturating carbon atoms in sp2 hybridisation located at the edge of whatever kind of cluster, or sheets are formed in the sample. A word of caution needs to be said regarding the ‘‘chemistÕs approach’’ to the diagnostic spectroscopy of aromatic systems which is generally restricted to the universally used correlations originally proposed by Jones [21] more than fifty years ago based on the use of the infrared absorptions associated to the out-of-plane deformation modes of the aromatic C–H groups. These correlations work beautifully for ‘‘isolated’’ benzene rings variously substituted with aliphatic groups, but they may fail when molecules contain condensed benzene rings, as it will shown below. We find that the analysis of the infrared spectra of many molecules consisting of several condensed rings proposed by Zander [29] provides a very useful way for the understanding of the lower frequency range of the infrared spectra of carbonaceous pyrolytic materials. Zander proposes a new nomenclature: solo, duo, trio, quatro (Fig. 9) referring to the number of adjacent aromatic C–H groups which are vibrating most likely ‘‘outof-plane’’ in the fused rings. The frequencies which we will use in this work are reported in Table 2. Our analysis suggests that these correlations work better with large aromatic systems. The meaning of such correlations by theoretical dynamics is not yet clearly understood and work is in progress in our group. 4.4. Boundaries and holes in graphitic systems The diagnostic capabilities of the C–H out-of-plane bands are related to the kinds of ‘‘borders’’ (solo, duo, trio, quatro) of the large carbonaceous objects obtained

H

H

H H

H H

H

H

H H

SOLO

DUO

TRIO

QUATRO

Fig. 9. Description of solo, duo, trio and quatro structures.

A. Centrone et al. / Carbon 43 (2005) 1593–1609

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Table 2 Patterns of absorption bands in the infrared to be associated to specific sets of adjacent aromatic C–H groups belonging to ‘‘fused’’ benzene rings Structure

Characteristic C–H OPLA of C–H bands of C–H group belonging to condensed rings (cm1)

QUATRO TRIO DUO SOLO

730–750 750–770 800–810 860–910

750–770 770–800 810–860 –

– 800–810 – –

from the pyrolysis. We have reconstructed the kinds of borders from the observed spectra proposing classes of structures which must show bands associated to solo, duo and trio groups (no bands associated to quatro group are found in the IR spectra). Let us assume that during the pyrolysis HPB molecules are not broken up but are only forced to ‘‘condense’’ and/or link to each other. We have constructed two main repeating ‘‘motives’’ presented in Fig. 2 which show borders we label as regular (reg) and irregular (irreg) respectively. The two classes of molecules are very similar, but when the sizes of the clusters increase only reg can pretend to reach the graphitic limit; irreg clusters necessarily contain ‘‘holes’’ where six atoms are missing and are ‘‘replaced’’ by six hydrogens (Fig. 2). In absence of any regio-selective reactions simple statistical reasoning shows that the growing of reg structures becomes less and less probable favouring the formation of irreg structures with plenty of holes. Whenever the size of the clusters increases the relative population of trio structures decreases in favour of solo and duo. In principle we can distinguish reg or irreg clusters by the relative population of solo and duo, namely for reg structures the ratio (no. of duo/no. of solo) is always = 1; for irreg structures with holes the population of mono is larger than the population of duo. As an example we refer to Fig. 10. In going from HPB-500-1d to HPB-600-1d the infrared spectra show that the intensities of solo and duo bands increase relative to trio band, thus indicating that the sizes of the clusters have increased. Moreover, since the solo band has increased remarkably more than duo this is an evidence of the formation of irregular clusters, i.e. with the formation of holes. Quantitative studies are in progress.

The Raman spectrum unfortunately shows only a broad and strong background due to fluorescence even if excited at different wavelengths and even with excitation in the near IR. It can be concluded that the sample of HPB-500-1d does not contain any more hexa(phenyl)benzene (a); however a sizeable fraction of non condensed phenyl groups survive the pyrolysis but most likely they are not any more attached to a framework of the type of HPB (b, c). Ring condensation has certainly taken place (d–f) with the formation of inter and intramolecularly linked benzene rings.

4.5. HPB-500-1d

4.6. HPB-600-1d

In our spectroscopic experiments we have not identified a soluble fraction in HPB-500-1d. The IR spectrum of HPB-500-1d is very different from that of HPB-450-1d (QS fraction), see Fig. 10. In particular:

The sample of HPB pyrolysed at 600 °C appears as heterogeneous, mostly translucent and black mixed with a coloured fraction (yellow-greenish, orange-reddish). Approximately 20% of the material is soluble in tetrahydrofurane. We have recorded the spectra both of the soluble (HPB-600-1d-S) and of the insoluble (HPB600-1d-Ins) fractions. The infrared spectrum of HPB-600-1d-S indicates that during the reaction triphenylene (III) or triphenylene-like

(a) The band at 555 cm1 (typical of HPB originating from the ‘‘butterfly-like motion’’ has fully disappeared.

(b) The strongest band in the low frequency range is that at 756 cm1 characteristic of trio structures: the bands at 698 and 730 cm1 characteristic of monosubstituted non condensed benzene rings weaken, but do not disappear. (c) The combination band near 4050 cm1 weakens. (d) The bands associated to C–H stretchings weaken, become broader and less structured (not shown). (e) Bands appear near 791 cm1 (trio), 816 cm1 (trio + duo) and 875 cm1 (solo). (f) In the CC stretching range strong differences are found between IR spectra of HPB-450-1d and HPB-500-1d: 1401 ! 1384 cm1; 1442 cm1 weakens remarkably, 1495 ! 1481 cm1, 1599 ! 1596 cm1. (g) The whole spectrum shows bands remarkably broader in shape, thus indicating that the sample is not ‘‘monomolecular’’ and that strong disorder exists in the molecule(s) which, however, have reached already a sizeable dimension and complexity. In particular in the CC stretching range the observed spectral pattern approaches already that observed for PAH systems.

696

730 759

791

756

791

867 809 875

816

HPB-450-1d QS HPB-500-1d HPB-600-1d-ins

1384

1442

1481

1596

Absorbance a.u.

754

A. Centrone et al. / Carbon 43 (2005) 1593–1609 1104 SiO2

1604

1500

1000

Wavenumbers (cm-1)

H

HPB-450-1d QS HPB-500-1d HPB-600-1d-ins

730

759 Orto-Di-subs.

791

696

736

698

756

809 791 816

875

Absorbance a.u.

H

867

H

698

H

754

H

H

600

800

Wavenumbers (cm-1) Fig. 10. IR spectra of HPB-450-1d QS (continuous line), HPB-500-1d (dotted line) and HPB-600-1d-ins (sketched line).

molecules have been formed. Moreover, larger (but still relatively small because of their solubility in THF) polycondensed systems start being formed. The spectra of HPB-600-1d-Ins (Fig. 10) is obviously free from the contribution of triphenylene and triphenylene-like molecules which are observed in the spectrum of the soluble fraction. The original sample was carefully washed with THF. The relevant observations are the following: (a) The broad band near 1104 cm1 is due to impurities of SiO2 originating from the walls of the reactor. (b) The absorption at 698 cm1 due to monosubstituted benzene rings has almost vanished. (c) The combination band at 4053 cm1 is decreased in intensity even if compared to the analogous band of HPB-500-1d. (d) The strongest band occurs at 754 cm1 (trio).

(e) The band at 791 cm1 (trio), already observed in the spectrum of HPB-500-1d, becomes stronger. (f) The bands at 809 cm1 (duo) and 867 cm1 (solo) are slightly shifted with respect to HPB-500-1d. (g) The whole spectrum floats on top of a strong and broad background with and overall weakening of the absorption spectrum. (h) Bands flatten from 1000 cm1 towards higher frequencies. (i) The CC stretching band near 1600 cm1 broadens, weakens and melts in the broad background. (j) The CH stretching region is weak and featureless with a very broad maximum near 3024 cm1. The Raman HPB-600-Ins spectrum (with excitation at 632.8 nm) escapes fluorescence and shows the two typical Raman G and D lines common to carbonaceous materials and also to the class of large PAH molecules. A thorough discussion of the Raman spectra of carbo-

A. Centrone et al. / Carbon 43 (2005) 1593–1609

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Raman intensity a.u.

HPB-800-1d wall’s reactor side HPB-800-1d spheres side HPB-600-Ins

1800

1700

1600

1500

1400

1300

1200

1100

Wavenumbers (cm-1)

Fig. 11. Raman spectra (exciting line 632.8 nm) of HPB-600-Ins (thin dotted line) and HPB-800-1d on the spheres side (continuous line) and on the wall of the reactor side (thick dotted line).

naceous materials and PAH is reported elsewhere [6,7] and a full analysis of the complex theory of the Raman scattering of graphitic systems [6,7] is beyond the scope of this paper which is mostly based on the understanding of the infrared spectra aimed at the elucidation of the reaction mechanism during pyrolysis. In Fig. 11 10 we notice the typical strong Raman doublet near 1600 and 1330 cm1 commonly labelled and G and D lines respectively. This means that at 600 °C almost complete ‘‘graphitisation’’ has taken place. In conclusion the pyrolysis of HPB carried out at 600 °C produces a fraction of materials still soluble in THF mostly consisting of few condensed benzene rings with a dominant concentration of triphenylene and/or triphenylene substituted by non condensed phenyl rings. The insoluble fraction, in agreement with Raman spectrum, indicates that graphitisation has taken place almost completely (g)–(j). Phenyl groups have almost vanished (b), (c), but structures containing aromatic C–H groups still exist in the system (d)–(f). 4.7. HPB-800-1d The sample pyrolysed from HPB at 800 for one day consists of chips of metallic brightness, that when analysed by SEM, reveal an interesting morphology (Fig. 12). The sample appears to consist of spheres which tend to form chains like pearls in a necklace. In particular, every chip of material presents two different sides namely one flat (the one in contact with the wall of the reactor) and the other with the spheres. In the effort for a spectroscopic characterisation of this material practically no help is obtained from the infrared spectrum (not showed here) since, as expected and as discussed earlier, we are approaching the limit of the infrared spectrum of graphite. Only some indication of a small residue of solo structures can be

revealed and can be taken as indications of the existence of holes. The Raman spectrum, recorded under a microscope with 632.8 nm exciting line, by focussing at the two different sides of the sample is reported in Fig. 11. The spectra show dispersion of both lines G and D (spheres side 1596; 1329 ! reactorÕs wall side: 1582, 1320 cm1) as if larger graphitic clusters, approaching the graphite limit (1580 cm1) were formed in contact with the wall of the reactor. Line G arising from the small spherical objects occurs at 1600 cm1 suggests the existence of more confined delocalisation. 4.8. Pyrolysis of hexa(p-bromo-phenyl)benzene The rationale behind the pyrolysis of bromo-derivatives of HPB (IV) is that Br may favour the linking of molecules, due to the energy of cleavage of the bond C–Br (smaller than C–H), either with the elimination of HBr or Br2. If Br2 is preferentially eliminated ‘‘head-to-head’’ linking of the phenyl rings in para position should be the preferential process during the pyrolysis. The infrared spectra of HPB-Br 550-1d and HPB-Br 550-5d (Fig. 13) show few weak bands in the C–H outof-plane motions, thus suggesting that dehydrogenation has not been completed. In Fig. 13 we compare the IR spectra of HPB-600-1dIns with the IR spectrum of HPB-Br-550-1d. It turns out that in spite of the fact that the temperature of pyrolysis of HBP-600 is higher than that of HPB-Br 550-1d, the latter has lost more hydrogen (see the weak intensities of the out of plane deformation modes of C–H). However while the intensities are different the frequencies of the bands are the same, thus indicating that the kind of chemical species formed upon pyrolysis are practically

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867 809 791

1500

Wavenumbers (cm-1)

742 700 755

808 758

878

867 866 811 810 789 752

698

Absorbance a.u.

HPB-600-1d-Ins HPB-600-5d HPB-Br-550-1d HPB-Br-550-5d

754

Fig. 12. SEM images of HPB-800-1d at different magnification.

1000

Fig. 13. IR spectra of HPB-600-1d-Ins (thin continuous line), HPB 600-5d (sketched line), HPB-Br 550-1d (dotted line) and HPB-Br 550-5d (thick continuous line). In the box the OPLA region for HPB-Br 550-1d and HPB-Br 550-5d is reported for the evaluation of the intensity ratios (base line correction applied).

the same in the two samples, only their relative concentrations in the two samples turn out to be different. The pyrolysis of the bromo-derivative makes dehydrogenation easier, but seems to produce the same final materials, or better, materials with similar periphery. This structural scenario is compatible with a reaction mechanism upon heating which seems to favour the elimination of HBr instead of Br2. If the preferred reaction would cause the elimination of Br2 the material should be preferentially composed either by polyphenylenic networks or connected hexa-benzo-coronene unit. This is not the case as indicated both by IR and Raman spectra (the latter not showed here). In order to obtain a complete picture of the situation we look at the infrared spectra of materials (HPB-6001d, HPB-600-5d, HPB-Br-1d, HPB-Br-5d) which differ only by the time of the pyrolitic process (Fig. 13). The spectra of the materials differ mainly for the intensities of the bands associated to the out-of-plane motion; the intensities are different, but the frequencies are practically very similar. As it becomes obvious by considering the reaction times, we learn that the two samples pyrolysed for one

day contain more C–H groups (larger fraction of boundaries) with respect to samples subjected to five days of treatment which show that the solo intensity is increased remarkably with respect to duo and trio, thus indicating that the relative holes population has increased with increasing PAH dimension, as our model suggests. 4.9. Pyrolysis of hexa(p-Iodo-phenyl)benzene Analogous reasoning can be applied to the iodinederivative material (HPB-I 450-6d) that makes pyrolysis easier with respect to the brome-derivative; but, again, the C–H out of plane frequencies are the same (Fig. 14). The halogenated precursors certainly make the first step of reaction easier, but it is also plausible that the gas formed (that remains in the batch) could favour ‘‘parasitic’’ reactions. One effect is the broad (and weak) absorption band that is clearly seen in the halogenderivative materials in the region of 1100–1300 cm1. In the literature [30] attempts are made to ascribe such broad absorption to normal modes of sp2 carbon activated in the IR by the confinement and symmetry breaking by carbon in sp3 hybridisation.

1168

HPB-I-450-6d HPB-Br-450-6d

789

787 752

867

807 803

864 1500

754

1216

1607

1166

1282

Absorbance a.u.

1294

A. Centrone et al. / Carbon 43 (2005) 1593–1609

1000 Wavenumbers (cm-1)

Fig. 14. IR spectra of HPB-I 450-6d (continuous line), HPB-Br 450-6d (dotted line).

5. Conclusions (1) As usual, the normal modes localised at specific chemical groups offer an easier way to extract from the infrared spectra information to be used in chemical diagnosis. This is precisely the case encountered in this work. Again as usual, the C– H out-of-plane vibrations in aromatic rings generally show relatively stronger intensities because, for locally planar systems, during the out-of-plane motion of the C–H charge fluxes do not play any role and the intensity is only associated to the fixed atomic charge of the hydrogen atoms [26–28]. Because of the complexity of the systems studied any concept of symmetry cannot be invoked; symmetry seemingly may be required for mechanically localised modes in solo, duo, trio and quatro structure. (2) Attention should be paid that in the infrared spectra of many samples the absorptions due to C–H are very weak and need to be blown up for a careful (even quantitative) analysis. Their weakness is only due to the small relative concentration of H left in the sample after pyrolysis. Great attention should also be paid to the fact that, on the whole, the infrared spectra of these carbonaceous materials are intrinsically extremely weak as discussed in this paper. Caution must be paid not to take unavoidable weak optical artefacts as molecular signals. (3) While localised modes are easily observed, following the concepts of solid state dynamics ‘‘resonance modes’’ may certainly occur, but they are necessarily strongly coupled [31] with the collective modes of the carbonaceous C–C skeleton and their intensity becomes smeared out over the many coupled

modes. Indeed the density of vibrational state due to CC modes is large in this frequency range and increases with the size of the systems. For this reason no clear specific absorptions should be expected in the higher frequency (900–1300 cm1) region where skeletal motions occur. This must be the case not only for the motions of C–H but also for any kind of ‘‘defect modes’’ associated to geometrical distorsions of the CC skeleton unless strong specific localisation takes place for the charges + charge fluxes [26–28,30,32–34] such as to show a medium-weak absorption. The identification of the normal modes behind the possibly observed resonance modes is neither straightforward nor unambiguous. It follows that the judgment on the ‘‘corrugation’’ or geometrical disorder in such mostly graphitic structures is not easy to be made from infrared. As previously done for the case of disordered one-dimensional polymer systems, the calculation of the density of vibrational states may make this task easier [32–34]. This approach is being followed in our group. (4) Signals arising from phenyl rings attached to some kind of carbonaceous framework have been identified and used. The identification has implied a rigorous analysis of the dynamical coupling of the modes of the phenyl rings within the framework. The dynamical concepts used in this paper to identify localised and de-coupled modes are very similar to the concepts used in the literature for the definition of the ‘‘local modes’’ [31] and the development of Local Mode Spectroscopy of diagnostic and analytic use. (5) It becomes apparent from the spectroscopy discussed in this paper that graphitisation of HPB takes place mostly around 500–550 °C.

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(6) Pyrolysis of haxa(p-bromo-phenyl)benzene suggests that inter-ring CC bonds may be easier to be formed by elimination of HBr than by elimination of Br2. Pyrolysis of haxa(p-Iodo-phenyl) benzene makes the reaction easier also with respect to the pyrolysis of the bromo derivative of HPB. (7) With increasing temperature or reaction times the size of PAHs formed increases, but also the population of holes increases as witnessed by the relative intensity of the solo band (with respect a to an ideal graphite sheet the ‘‘holes’’ can be described as six missing adjacent carbons ‘‘replaced’’ by six hydrogens). These holes are strictly related to the synthetic process of the materials analysed in this paper (pyrolisis from precursor molecules) and may be the imprint of the precursor molecule on the macroscopic material. In the theoretical and experimental studies of the capability of the materials studied in this work to store lithium for the development of new batteries ab initio calculations [35] have suggested that lithium could be preferentially bound at the periphery of PAHÕs. In this sense these new materials, with a large population of holes can offer large peripheral areas as smaller PAHÕs do. However while small PAHÕs molecules are not suitable for battery devices these new materials with holes could be better suited for the increasing amount of lithium stored [14]. (8) Certainly additional information may be derived from a parallel study of the Raman spectra which, however, requires consideration of quantum chemical problems related to the electronic states of the systems. These studies have been made and are reported elsewhere [6,7].

[4] [5]

[6]

[7]

[8]

[9]

[10]

[11]

[12] [13]

[14]

[15]

[16]

Acknowledgements [17]

This work has been supported by the European Commission, Fifth Framework Programme, Growth Programme (Research Project ‘‘MAC-MES; Molecular Approach to Carbon Based Materials for energy Storage’’, G5RD-CT2001-00571). We thank Prof. C. Castiglioni and Dr. M. Tommasini for very helpful discussions.

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