Electronic structure changes in cobalt phthalocyanine due to nanotube

encapsulation probed using resonant inelastic X-ray scattering

Janine C. Swarbrick,*a Tsu-Chien Weng,a Karina Schulte,b Andrei N. Khlobystovc

and Pieter Glatzela

Received 10th February 2010, Accepted 15th April 2010

DOI: 10.1039/c002501a

The electronic structure of cobalt phthalocyanine (CoPc) changes upon encapsulation inside

multi-walled carbon nanotubes (CoPc@MWNT), as detected in this research using Co-K-edge

X-ray absorption near-edge structure spectroscopy (XANES) and Co-Ka1 resonant inelasticX-ray scattering (RIXS). The CoPc molecules are no longer planar once inside the nanotubes,

and the molecular symmetry is found to change upon encapsulation from D4h to C4v symmetry.

This change of symmetry increases the amount of p–d orbital mixing, which is seen in the spectra

as a change in peak intensity. Energy shifts are also seen between CoPc and CoPc@MWNT,

showing that Co in the encapsulated species is more oxidized due to electron donation from the

phthalocyanine molecule to the surrounding nanotube. Trends seen in the spectra between CoPc

and CoPc@MWNT can be calculated using density functional theory (DFT), which shows the

molecular orbitals involved in different spectral features.

1. Introduction

Phthalocyanines (Pcs) are metalloorganic macrocyclic molecules

with a readily tunable central metal atom. They have received

much attention due to their wide-ranging applicability in

cancer treatments,1 sensors2 and organic electronics3,4 amongst

other areas of research. Cobalt phthalocyanine (CoPc) (Fig. 1a)

is currently being studied in such fields as biosensors5 and

organic photovoltaics.6 The central Co2+ ion has an unpaired

3d electron, which also makes CoPc an interesting candidate

for magnetic applications.

The ability to encapsulate molecules, such as fullerenes,

phthalocyanines and rare earth compounds inside carbon

nanotubes has been achieved recently,7–9 producing a new

class of nanomaterials. The encapsulation of CoPc inside

multi-walled carbon nanotubes (CoPc@MWNT, Fig. 1) has

been performed,7 limiting the molecules to one-dimensional

arrangements and raising interesting possibilities for controlling

molecular arrays with magnetic character10,11 and optical

properties.12 Understanding the electronic properties of such

systems is vital for potential applications. This article describes

a hard X-ray absorption near-edge structure spectroscopy

(XANES) and resonant inelastic X-ray scattering (RIXS)

study of CoPc and CoPc@MWNT. Due to their size, CoPc

molecules can only be encapsulated in larger inner diameter

nanotubes, which are multi-walled (usually between two and

five carbon layers). Hard X-rays are a bulk sensitive probe and

as such the ‘buried’ Co atoms can be accessed without

significant signal attenuation.

XANES spectroscopy at the Co-K edge can be used to

determine the local coordination and oxidation state of the Co

atoms. The post edge peaks arise from 1s–4p and 1s–continuum

transitions.13,14 In systems with inversion symmetry, the pre-edge

arises from 1s–3d quadrupolar transitions; lower symmetry

causes p–d orbital mixing to occur.

To further investigate the electronic structure of the CoPc

systems, an X-ray emission spectrometer was used to record

RIXS planes. In this study the Co-K edge absorption features

were recorded over the Co-Ka1 emission line to give 1s–2p3/2RIXS planes. The incident energy can be tuned, as can the

detected emitted energy, to plot a 2-D RIXS plane. Here the

RIXS data is plotted against incident energy and energy

transfer (incident minus emission energy).

From the RIXS map, a line scan may be extracted along the

diagonal as shown in Fig. 3 which is the so-called high energy

resolution fluorescence detected (HERFD) absorption spectrum.

This HERFD spectrum shows essentially the same features as

the total fluorescence yield (TFY) XANES spectrum, but with

higher resolution due to the apparent reduction in the core-hole

lifetime broadening.15–17 The increase in the spectral resolution

obtained over TFY XANES, combined with a zero back-

ground count level due to detection using an avalanche

photodiode (APD) allow the pre-edge peaks to be more clearly

recorded and the spectral features to appear sharper.

2. Method

2.1 Experimental

The experiments were performed on the high brilliance X-ray

beamline ID26 at the European Synchrotron Radiation Facility

(ESRF). The electron energy was 6.0 GeV with a ring current

of 170–200 mA. A monochromator employing a pair of

Sih311i single crystals was used to choose the incident energy

for access to the Co edge at 7709 eV. Higher harmonics were

suppressed using two Si mirrors at 2.5 mrad. The beam size

was approximately 0.3 mm vertically � 0.9 mm horizontally.

a European Synchrotron Radiation Facility (ESRF), BP 220,F-38043, Grenoble Cedex 9, France. E-mail: janine.grattage@esrf.fr

b Lund University, MAX-lab, P.O. Box 188, SE-221 00 Lund, Swedenc School of Chemistry, University of Nottingham, University Park,Nottingham, NG7 2RD, UK

This journal is �c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 9693–9699 | 9693

PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics

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The Co-Ka1 emission line at 6931 eV was measured using a

vertical Rowland circle spectrometer using the (531) reflection

of a spherically bent Si Bragg crystal analyzer of 1 m bending

radius crystal. The Bragg angle was 771. Co-Ka1 RIXS

spectra were measured by recording the Co-K absorption edge

between 7707 and 7720 eV over the Co-Ka1 emission line. The

standard TFY XANES was also recorded with a photodiode.

Emitted photons were detected in the emission spectrometer

using a 100 mm APD. The emission spectrometer energy

bandwidth was found to be 0.6 eV taken from the full width

at half maximum (FHWM) of the elastic peak taken at

7700 eV.

Radiation damage studies were performed at room

temperature by measuring rapid (5 s) XANES spectra at the

pre-edge region and monitoring any shift in the pre-edge peak

shape and intensity, and any shift of the edge. For all samples,

no change was observed over 30 min and all measurements

were taken on sample spots exposed for less than 30 min.

Energy calibration of the monochromator was performed by

recording the XANES spectrum of a Co metal foil.

2.2 Synthesis

CoPc (97% purity) was purchased from Sigma Aldrich and

used without further purification for the CoPc samples. Thin

multi-walled carbon nanotubes (MWNT) were obtained from

Nanocyl, having an average outer diameter of 7–10 nm.

Purified carbon nanotubes were first oxidized in air at

400 1C for 40 min, resulting in a weight loss of around 40%.

This thermal treatment opens the nanotube caps and also

partly removes amorphous carbon from the surface of the

tubes. They were then mixed with an excess of sublimation

purified CoPc in a quartz tube, sealed in a vacuum of 10�6 Torr

and heated at 375 1C for three days. CoPc that had settled on

the outside of the nanotubes was successfully removed

by repeated rinsing with a mixture of chloroform and 1%

trifluoroacetic acid. A control sample was also prepared by

mixing the nanotubes and CoPc only with no encapsulation

treatment. The resulting powder samples were prepared for

study using X-rays by being pressed into pellets.

2.3 Calculations

Density functional theory (DFT) calculations were performed

in the ORCA code18 to model the transitions seen in the

XANES spectra. The input structure is that proposed in

ref. 19. The BP86 method was employed with a TZVP basis

set. A finer grid was used over the Co atom, as was a CP(PPP)

basis set. Quadrupolar transitions were included. A broadening

of 1 eV was applied to the calculated transitions.

3. Results and discussion

3.1 TFY XANES

Co-K edge TFY XANES for CoPc, a CoPc/MWNT mixture

and CoPc@MWNT are shown in Fig. 2. The Co-K edge

shows the unoccupied states close to the Fermi level. The

spectra have been normalised using the edge jump of the

EXAFS region. The count rates obtained were similar for all

samples, thus the signal from Co in the CoPc@MWNT and

CoPc/MWNT mixture samples was not significantly attenuated

due to the surrounding C atoms, which can be problematic in

surface-sensitive soft X-ray studies. The spectra are very

similar for all compounds, demonstrating that the molecular

structure has not been changed significantly upon mixing or

encapsulation (to the extent that the molecule has broken up

or become destroyed). Here the pre-edge peaks (between

7706–7712 eV) and the edge peak, which is characteristic of

Pc molecules (between 7714–7717 eV),14 are discussed. The

largest edge peak intensity at 7715.5 eV is observed for the

CoPc reference sample, with the CoPc@MWNT having the

lowest intensity edge peak. It is difficult to see changes in the

pre-edge region as these peaks are very low intensity and are

not well resolved. Co-Ka1 RIXS plots (vide infra) are used to

show spectral details in the pre-edge region.

The differences in relative peak intensity should be discussed

in relation to self-absorption effects. The samples are concen-

trated bulk powders and as such the spectra may appear

artificially flattened at higher energy due to self-absorption.

However self-absorption would affect the spectral intensity

above the edge more than the pre-edge, which has a lower

absorption cross-section. Self-absorption effects would also be

expected to affect the spectrum of CoPc the most as it has the

highest proportion of Co, which is the strongest absorber in

these samples. However, the CoPc sample shows the strongest

edge peak, so these relative intensity differences do not originate

from self-absorption, but are real spectroscopic changes.

Fig. 1 Schematic diagram of (a) the CoPc molecule and (b) schematic structure of CoPc@MWNT (only one NT layer is shown).

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3.2 Co-Ka1 RIXS planes

To obtain further information on the transitions being probed,

the Co-Ka1 RIXS planes were recorded for the three samples.

Fig. 3(a) shows a schematic with the electronic transitions

involved in recording a Ka RIXS plane. In the Ka RIXS

process a 1s core electron is excited into the localized unoccupied

states just above the Fermi level. A 2p electron relaxes to fill

the core-hole (the relaxation of the 2p3/2(2p1/2) electron gives

the Ka1(Ka2) line). The overall energy transferred to the

system is the incident energy O minus the emitted energy o.In this study we only measure transitions over the stronger

Ka1 emission line.

The Co-Ka1 RIXS plane for CoPc is shown in Fig. 3(b). The

pre-edge peaks are seen in the 7706–7712 eV incident energy

range and 779–784 eV energy transfer range. The edge peak is

seen between 7714–7717 eV incident energy and 785–789 eV

energy transfer. Above 7718 eV incident energy and 789 eV

energy transfer the intensity arises from transitions to the

continuum (post-edge). The diagonal line along the maximum

intensity of the edge peak and continuum features represents

the cut along which a HERFD spectrum can be extracted.

In the RIXS plane (and the extracted HERFD line scan), the

pre-edge transitions are clearer due to the higher resolution

compared with TFY XANES. Thus we will focus on this

region of the RIXS plane to obtain information on the

pre-edge, and hence on the unoccupied electronic levels just

above the Fermi level.

3.3 Co-Ka1 RIXS over the Co-K pre-edge

The Co-Ka1 RIXS planes over the pre-edge peak region are

shown for the three samples in Fig. 4. It can be seen that the

pre-edge peaks are similar for CoPc and a CoPc/MWNT

mixture, which is expected as most of the CoPc in the mixture

sample is still in the bulk phase since no special treatment was

used. The two main pre-edge peaks are at the same energy for

both samples, and have similar intensities. The main difference

between the two can be seen as a broadening of the second

pre-edge peak in a CoPc/MWNT mixture around 7710.5 eV

incident energy, but the main features are the same. The pre-edge

for CoPc@MWNT, however, is very different with two

pre-edge peaks rather than three. The CoPc@MWNT Ka1RIXS shows a strong pre-edge peak at 7709.8 eV incident

energy. The lower energy pre-edge peak around 7708 eV for

CoPc and a CoPc/MWNT mixture is shifted to higher energy

in CoPc@MWNT to 7708.4 eV, a shift of 0.4 � 0.1 eV. This

shift is revisited in the HERFD spectral analysis later.

If HERFD spectra are to be used for detailed analysis it is

important to ensure the equivalence of the features measured

in the HERFD spectra compared with those measured in TFY

XANES. The full RIXS plane should be recorded to understand

the energy position and shape of the peaks. In these RIXS

planes it can be seen that the pre-edge peaks are not aligned

along the HERFD diagonal. The diagonal is positioned along

the maxima of the post-edge and edge peak as shown in

Fig. 3(b). The HERFD diagonal passes through the center

Fig. 2 TFY XANES spectra for the three CoPc samples.

Fig. 3 (a) Schematic of the electronic excitations involved in measuring

Co-Ka RIXS planes. O in the incident energy and o is the emitted

energy. (b) Co-Ka1 RIXS plane for CoPc. The diagonal dashed line

shows the cut which gives a HERFD spectrum. The dotted line box

indicates the pre-edge region shown in detail in Fig. 4.

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of the first pre-edge peak but is below the second peak in the

energy transfer direction. Firstly, this shows that the HERFD

spectra are not simply a higher resolution version of the TFY

XANES scan since the full intensity of the second pre-edge

peak is not recorded in the HERFD line scans. The second

peak is at higher energy transfer, which relates to final state

interactions, electron-electron interactions and core-hole effects.

3.4 HERFD line scans

The HERFD line scans over the pre-edge and edge peak,

extracted from the Co-Ka1 RIXS planes, are shown in Fig. 5

alongside the equivalent TFY XANES spectra. The increase in

resolution is apparent in the HERFD spectrum. Also shown is

the first pre-edge peak extracted from the RIXS planes by

integrating over the final states in the range 778.8–780.7 eV

(see Fig. 4). This peak can be used as an indicator of the shift

in incident energy between the samples.15 Using the method in

ref. 20 for estimating the overall spectral broadening from

core-hole lifetimes and experimental considerations (using a

monochromator with resolving power 35 000 and an emission

spectrometer energy bandwidth of 0.6 eV), the overall

HERFD peak broadening using the Co-Ka1 line was calculatedto be 0.42 eV.

Differences in resolution in the pre-edge region between the

TFY XANES and HERFD spectra are clear. The main edge

and edge peak at 7715.5 eV follow the same trend in both TFY

XANES and HERFD. Using the HERFD technique the

pre-edge peak is better separated from the main edge and

facilitates peak identification.13 As seen in the Co-Ka1 RIXS,

the CoPc/MWNT mixture sample has a more intense second

pre-edge peak which starts to merge with the third peak at

around 7709.6 eV, but the shape is still similar to that of CoPc.

The CoPc@MWNT sample has a differently shaped pre-edge

with two main peaks, of which the second one at 7709.8 eV is

Fig. 4 Co-Ka1 RIXS planes over the pre-edge for (a) CoPc (b) a

CoPc/MWNT mixture and (c) CoPc@MWNT. HERFD diagonal

cuts are shown as in Fig. 3, and the pre-edge peaks are clearly seen.

The horizontal dashed lines mark the upper bound of the region of

integration (778.8–780.7 eV) used to show the energy position of the

first pre-edge peak (see Fig. 5).

Fig. 5 TFY XANES pre-edge and edge region compared to HERFD

spectra taken using the Co-Ka1 emission line for the three CoPc

samples. The pre-edge is magnified �5 to show the peaks more clearly.

Also shown is the integrated intensity over the first pre-edge peak from

the RIXS planes to demonstrate the shift in incident energy between

the samples.

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around twice as intense that the corresponding peak(s) from

CoPc or a CoPc/MWNT mixture.

Considering the edge peak at 7715.5 eV, we see the reverse

trend in intensity to that seen in the pre-edge. The highest peak

intensity is seen for CoPc. A slightly lower peak intensity is

observed for a CoPc/MWNT mixture, and the same peak

measured for CoPc@MWNT has much lower intensity, is

broader, and appears at slightly higher incident energy. The

combination of an increased pre-edge intensity, and a decrease

in the edge peak intensity for CoPc@MWNT shows a change

in the p–d orbital mixing due to encapsulation.

There is a small shift to higher incident energy of the first

pre-edge peak in CoPc@MWNT compared to CoPc or

a CoPc/MWNT mixture, as seen most clearly using the

integrated intensity of the first pre-edge peak taken from

the RIXS planes. This peak is a reliable indicator of the

energy position as it represents the localized Co unoccupied

electronic levels and thus the Co electronic structure. A

clear shift to higher incident energy is seen in this first

pre-edge for CoPc@MWNT compared with the non-encapsulated

samples, which is consistent with a more oxidized Co species

in CoPc@MWNT. Encapsulated molecular species are

found to be more oxidized in previous soft X-ray studies

of CoPc,21 cobaltocene22 and fullerenes.23 Electron density is

donated from the encapsulated species to the surrounding

nanotube.

The peak shifts seen in the CoPc@MWNT data are subtle,

which can be understood in terms of the arrangement of

CoPc inside the nanotubes. From previous studies8 it is

understood that the CoPc molecules arrange inside the

nanotube so as to stack similarly (at least over a short range)

to the packing in the bulk a-phase, shown schematically in

Fig. 1. Thus the Co atom is not located close to the surrounding

nanotube. While the Co atom ‘feels’ the effect of electron

donation to the nanotube, making it more oxidized, the

electron donation is likely to be greatest from the outer

atoms (C and H) of the CoPc, which are closer to the

nanotube. The electron donation affects the central Co atom,

not because the Co atom is close to the nanotube, but because

of the delocalisation of electronic density over the whole

phthalocyanine.

3.5 Symmetry changes upon encapsulation

Previous studies state that the edge peak, characteristic of Pc

molecules, arises from the Co 4pz orbitals.14,24,25 CoPc

molecules in the bulk phase have D4h symmetry, as they are

flat square-planar molecules. To understand the changes in the

pre-edge peak and edge peak intensities between CoPc, a

CoPc/MWNT mixture and CoPc@MWNT, seen in the

Co-Ka1 RIXS and highlighted in HERFD line scans, changes

in the molecular symmetry will be considered. A change in

symmetry would change the p–d hybridisation and therefore

the spectral features.

STM studies of CoPc and modified CoPc monolayers

adsorbed on a gold surface show the molecular plane is

distorted to give C4v symmetry.26 Perfluorinated CuPc on

the Cu(111) and Ag(111) surfaces was found via X-ray standing

wave studies to distort, and the center of the molecule

moved closer to the substrate giving the Pc molecule an

‘‘umbrella-shaped’’ structure.27 Fe, Co and Cu phthalocyanine

molecules absorbed on the Cu (100) surface also show

molecular distortion from four-fold to two-fold symmetry.28

In Fe K-edge XANES studies of FePc absorbed on carbon

electrodes,29 the small pre-edge peak increases and the edge

peak decreases after absorption, and these changes are attributed

to a distortion of the molecular shape, giving rise to lower

symmetry and a higher pre-edge peak.

Firstly the small differences between CoPc and a CoPc/

MWNT mixture are considered. For a CoPc/MWNT mixture,

it is expected that a small fraction of the CoPc molecules are

adsorbed onto the outer surface of the nanotubes, but that

most of the CoPc remains in the bulk crystalline phase. This

adsorption can be thought to change the symmetry of the

molecules by moving the Co atom out of the plane of the

molecule, resulting in a molecule with C4v symmetry rather

than D4h symmetry. If the peak changes seen in the Co K-edge

XANES are due to a small proportion of molecules (since only

a small fraction of the CoPc present in the mixture could be

absorbed on the outer nanotube layer) having lower symmetry,

the changes are expected to be very small.

The trend in changing peak intensity and increased peak

broadening continues from the CoPc/MWNT mixture to

CoPc@MNWT, which agrees with the hypothesis that the

molecular symmetry has changed, since more CoPc molecules

are affected on encapsulation than in the mixture. All the CoPc

present in the CoPc@MWNT sample is encapsulated inside

MWNTs rather than existing in bulk crystals as in CoPc and

the CoPc/MWNT mixture. A lower molecular symmetry

arises due to distortion of the CoPc molecules because of

geometry constraints within the nanotubes. It was previously

shown8 that the CoPc molecules stack inside nanotubes in a

similar way to that seen in the bulk phase; the molecular

geometry may relax by distorting the molecular plane to

ensure the molecules can pack inside the nanotubes in such

a way. The effect of changing the molecular geometry on

the orbitals involved in a XANES/HERFD spectrum was

investigated using DFT.

3.6 DFT modelling

To test the proposal of symmetry breaking to give the spectral

changes seen, DFT calculations were performed to obtain the

transitions contributing to the XANES/HERFD spectrum.

A lower symmetry molecule was modelled by using the atomic

coordinates of a CoPc molecule and moving the Co atom out

of plane by 0.1 and 0.2 A. This is still much less than the

out-of-plane distance found in naturally non-planar phthalo-

cyanines such as SnPc30 or PbPc.31

The calculated spectra for planar and distorted CoPc are

shown in Fig. 6. They have been shifted in incident energy so

that the Fermi level for both calculated and measured spectra

are the same. The calculated spectra are normalised to the

transitions far above the edge where there are fewer differences

between the two molecular conformations. The pre-edge

transitions are not well reproduced in the calculations. The

calculation of the pre-edge peaks and the 3d electronic structure

in CoPc and other Co systems depends on several factors,

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including multiplet effects due to interatomic electron–electron

interactions, and changes in the local environment,13,32 which

at present cannot all be considered in a single theoretical code.

Current theoretical approaches also show that the calculated

3d electronic structures depend on the particular theoretical

model chosen.33 Spectral structures higher than the pre-edge in

molecular systems reflect more delocalized orbitals, thus they

do not strongly depend on multiplet effects and can be more

readily modelled using e.g. DFT, as done here.

The DFT calculations give the transitions present in the

XANES beyond the pre-edge, shown in Fig. 6, and for each

transition the molecular orbital (MO) can be plotted. The edge

peak is formed from two main contributions, labelled I and II,

with the corresponding MOs shown in Fig. 7. It can be seen

that as the Co atom is moved out of the molecular plane, the

overall contribution to the edge peak becomes less intense, and

the two orbitals involved split in energy, consistent with an

increase in orbital hybridisation. The decrease in intensity is

seen in the measured spectra as a smaller edge peak in

CoPc@MWNT compared to CoPc. The increase in splitting

is seen as a broadening of the edge peak.

For planar CoPc, orbital I has Co 4pz character only, and

orbital II has Co 4s hybridized with Co 3dz2 orbitals. The dz2

and s orbitals can mix as they both have A1 symmetry for a

molecule with D4h symmetry, but the pz orbitals have A2u

symmetry and therefore cannot mix. The calculated atomic

orbital compositions are in agreement with previous studies

assigning this peak to an orbital with pz, or more generally, out

of plane orbital character. For the two non-planar CoPc

representations, the molecular symmetry is reduced to C4v,

and the orbitals can mix in a different way than for the planar

molecule. Both orbitals I and II have Co s, Co 4pz and Co 3dz2

character, which all have A1 symmetry in the reduced

symmetry and can thus mix. This mixing of p and d type

orbitals has an effect in the pre-edge also, leading to a broader,

higher intensity second pre-edge peak in CoPc@MWNT. The

fractions of each atomic orbital contributing to the MOs are

shown in Fig. 7. The hybridisation is stronger for the more

distorted molecule. The calculations follow the same trend as

that seen in the experimental spectra.

It may be thought that peak intensity changes are stronger

for CoPc@MWNT than for CoPc mixed with MWNT, due to

the Co atom being positioned further out of the molecular

plane rather than a higher proportion of CoPc atoms being

distorted. However, the proportion of CoPc not in the bulk

structure is known to be much greater for CoPc@MWNT

than for CoPc mixed with MWNT, therefore this is more

likely the reason for a further peak intensity change. A higher

proportion of the molecules are affected in the encapsulated

sample, rather than the effect being greater per molecule.

Fig. 6 (a) TFY XANES spectrum of CoPc compared with DFT

calculations for (b) a planar CoPc molecule, (c) a molecule where the

Co is out of plane by 0.1 A and (d) by 0.2 A. The symmetry change

induces a change in p–d orbital mixing giving a lower intensity edge

peak as seen for CoPc@MWNT compared with CoPc. The two main

MOs contributing to the edge peak (labelled I and II) are shown for

each molecular configuration in Fig. 7.

Fig. 7 Representations of the two MOs involved in the edge peak for

(b) planar CoPc, (c) CoPc with Co out of plane by 0.1 A and (d) by

0.2 A. Orbital mixing changes upon symmetry breaking, and percentage

components of the MOs due to Co are shown. For (c) and (d) the Co is

above the plane.

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4. Conclusions

The general structure of CoPc does not change significantly

upon encapsulation, as the XANES/HERFD spectra do not

change dramatically before and after encapsulation. However,

changes are seen in the pre-edge and edge peaks in the XANES

and Co-Ka1 RIXS spectra for CoPc@MWNT compared with

CoPc and a CoPc/MWNT mixture which indicate a change in

molecular symmetry upon encapsulation.

Co-Ka1 RIXS measurements and the first pre-edge peak

energy position extracted from this RIXS data show, from a

small shift to higher energy of the first pre-edge peak for

CoPc@MWNT compared with CoPc or a CoPc/MWNT

mixture, that electron density is donated from the encapsulated

CoPc to the surrounding nanotube. This is in agreement with

previous photoemission studies. The Co atoms become more

oxidized upon encapsulation, not just the outer ligands which

are in closer proximity to the nanotube, because extensive

molecular orbital delocalisation is present across the whole

CoPc molecule.

Intensity changes in the HERFD pre-edge and edge peaks

between CoPc and CoPc@MWNT have been modelled using

DFT calculations. These results show that the geometry of the

CoPc molecules change upon encapsulation due to the Co

atom moving out of the molecular plane by a fraction of an

Angstrom, and this changes the molecular symmetry from D4h

to C4v. The MOs involved in the edge peak are calculated, and

relate to 4s, 4pz and 3dz2 orbitals, which mix differently

depending on the molecular geometry. A very small change

is seen for CoPc mixed with MWNTs, which is most likely due

to a small proportion of CoPc molecules changing geometry

after adsorption on the outer NT surface. Changes due to

encapsulation relate to conformational changes, so that the

molecules fit inside the nanotubes.

This study shows how the electronic structure of molecules

and nanotube-encapsulated molecules can may be determined

using XANES and Co-Ka1 RIXS spectroscopy. Subtle

changes in electronic and geometric structure can be extracted

and confirmed using DFT modelling to understand the

changes in molecular orbitals. Understanding the properties

of versatile and magnetically interesting nanomaterials such as

CoPc@MWNT are important to realize the potential uses of

these materials.

Acknowledgements

The authors acknowledge C. Lapras of the ESRF for technical

support.

References

1 M. Ochsner, J. Photochem. Photobiol., B, 1997, 39, 1–18.2 R. Zhou, F. Josse, W. Gopel, Z. Z. Ozturk and O. Bekaroglu,Appl. Organomet. Chem., 1996, 10, 557–577.

3 J. Zhang, J. Wang, H. B. Wang and D. H. Yan, Appl. Phys. Lett.,2004, 84, 142–144.

4 G. Witte and C. Woll, J. Mater. Res., 2004, 19, 1889–1916.5 A. B. P. Lever, J. Porphyrins Phthalocyanines, 1999, 3, 488–499.6 P. Peumans and S. R. Forrest, Appl. Phys. Lett., 2001, 79, 126–128.7 A. N. Khlobystov, D. A. Britz and G. A. D. Briggs, Acc. Chem.Res., 2005, 38, 901–909.

8 K. Schulte, J. C. Swarbrick, N. A. Smith, F. Bondino, E. Magnanoand A. N. Khlobystov, Adv. Mater., 2007, 19, 3312–3316.

9 H. Shiozawa, T. Pichler, C. Kramberger, M. Rummeli,D. Batchelor, Z. Liu, K. Suenaga, H. Kataura and S. R. P. Silva,Phys. Rev. Lett., 2009, 102, 046804.

10 S. C. Tsang, Y. K. Chen, P. J. F. Harris and M. L. H. Green,Nature, 1994, 372, 159–162.

11 J. V. Barth, G. Costantini and K. Kern, Nature, 2005, 437,671–679.

12 M. M. El-Nahass, Z. El-Gohary and H. Soliman, Opt. LaserTechnol., 2003, 35, 523–531.

13 G. Vanko, F. M. F. de Groot, S. Huotari, R. J. Cava, T. Lorenzand M. Reuther, arXiv:cond-mat.str-el 0802.2744v1 20 Feb 2008.

14 M. C. Martins Alves, J. P. Dodelet, D. Guay, M. Ladouceur andG. Tourillon, J. Phys. Chem., 1992, 96, 10898–10905.

15 P. Glatzel and U. Bergmann, Coord. Chem. Rev., 2005, 249, 65–95.16 H. Hayashi, R. Takeda, Y. Udagawa, T. Nakamura,

H. Miyagawa, H. Shoji, S. Nanao and N. Kawamura, Phys.Scr., 2005, T115, 1094–1096.

17 K. Hamalainen, D. P. Siddons, J. B. Hastings and L. E. Berman,Phys. Rev. Lett., 1991, 67, 2850–2853.

18 F. Neese, ORCA: An ab initio, DFT and semiempirical SCF-MOpackage, Universitat Bonn, http://www.thch.uni-bonn.de/tc/orca/,2009.

19 P. Ballirano, R. Caminiti, C. Ercolani, A. Maras and M. A. Orru,J. Am. Chem. Soc., 1998, 120, 12798–12807.

20 J. C. Swarbrick, U. Skyllberg, T. Karlsson and P. Glatzel, Inorg.Chem., 2009, 48, 10748–10756.

21 K. Schulte, C. Yan, M. Ahola-Tuomi, A. Strozecka, P. J. Moriartyand A. N. Khlobystov, J. Phys. Conf. Ser., 2008, 100, 012017.

22 L. J. Li, A. N. Khlobystov, J. G. Wiltshire, G. A. D. Briggs andR. J. Nicholas, Nat. Mater., 2005, 4, 481–485.

23 H. Kataura, Y. Maniwa, M. Abe, A. Fujiwara, T. Kodama,K. Kikuchi, H. Imahori, Y. Misaki, S. Suzuki and Y. Achiba,Appl. Phys. A: Mater. Sci. Process., 2002, 74, 349–354.

24 A. Rosa and E. J. Baerends, Inorg. Chem., 1994, 33, 584–595.25 B. Y. Baik, G. Kwag and S. Kim, Bull. Korean Chem. Soc., 2006,

27, 329–332.26 A. D. Zhao, Q. X. Li, L. Chen, H. J. Xiang, W. H. Wang, S. Pan,

B. Wang, X. D. Xiao, J. L. Yang, J. G. Hou and Q. S. Zhu,Science, 2005, 309, 1542–1544.

27 A. Gerlach, F. Schreiber, S. Sellner, H. Dosch, I. A. Vartanyants,B. C. C. Cowie, T.-L. Lee and J. Zegenhagen, Phys. Rev. B:Condens. Matter Mater. Phys., 2005, 71, 205425.

28 S. H. Chang, S. Kuck, J. Brede, L. Lichtenstein, G. Hoffmann andR. Wiesendanger, Phys. Rev. B: Condens. Matter Mater. Phys.,2008, 78, 233409.

29 S. Kim, T. Ohta and G. Kwag, Bull. Korean Chem. Soc., 2000, 21,588–594.

30 R. A. J. Woolley, C. P. Martin, G. Miller, V. R. Dhanak andP. J. Moriarty, Surf. Sci., 2007, 601, 1231–1238.

31 N. Papageorgiou, Y. Ferro, E. Salomon, A. Allouche, J. M. Layet,L. Giovanelli and G. L. Lay, Phys. Rev. B: Condens. Matter Mater.Phys., 2003, 68, 235105.

32 T. E. Westre, P. Kennepohl, J. G. DeWitt, B. Hedman,K. O. Hodgson and E. I. Solomon, J. Am. Chem. Soc., 1997,119, 6297–6314.

33 T. Kroll, V. Y. Aristov, O. V. Molodtsova, Y. A. Ossipyan,D. V. Vyalikh, B. Buchner and M. Knupfer, J. Phys. Chem. A,2009, 113, 8917–8922.

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