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Characterization of functionalised porphyrin films using synchrotron radiation
Applied Surface Science 248 (2005) 40–44 www.elsevier.com/locate/apsusc
Characterization of functionalised porphyrin films using synchrotron radiation V. Arima a,*, F. Matino a, J. Thompson a, R. Del Sole b, G. Mele b, G. Vasapollo b, R. Cingolani a, R. Rinaldi a, R.I.R. Blyth a a
National Nanotechnology Laboratory of INFM, Distretto Tecnologico ISUFI, Universita’ di Lecce Via Arnesano, 73100 Lecce, Italy b Dipartimento di Ingegneria dell’Innovazione, Universita` di Lecce, via per Arnesano, 73100 Lecce, Italy Available online 20 March 2005
Abstract Porphyrins and C60 are strategic materials for the fabrication of nanoscale molecular devices by virtue of their optical, photo– electro-chemical and chemical properties. We have developed procedures to immobilise cobalt tetra-butyl-phenyl porphyrins (CoTBPPs) on gold surfaces via ligation to self-assembled monolayers of aromatic aminothiophenols (4-ATP). We have used synchrotron radiation photoemission and near-edge X-ray absorption, NEXAFS, to characterise such films, both in their native state, and with ligated fulleropyrrolidines N-methyl-2-( p-pyridyl)-3,4-fulleropyrrolidine (Py-C60), forming charge-separation complexes which may have applications in solar cells. While photoemission spectra appear dominated by the individual CoTBPP and Py-C60 components, we observe an apparent signature of charge separation in fulleropyrrolidine NEXAFS spectra. # 2005 Elsevier B.V. All rights reserved. Keywords: Porphyrins; Synchrotron radiation; Fulleropyrrolidines; Photoemission; NEXAFS
1. Introduction In the field of nanotechnology, a promising route towards cheap, mass-produced solar cells are organic charge-separation complexes. One approach to mimic photosynthetic energy conversion is the construction of synthetic supramolecular systems containing chromophores, electron donors and electron acceptors linked by covalent bonds [1,2]. Many artificial * Corresponding author. Tel.: +39 0832 298383; fax: +39 0832 298238. E-mail address: [email protected] (V. Arima).
assemblies incorporate porphyrin derivatives as light receptors [3–5] and C60 derivatives as electron acceptors [6–8]. Control of the morphology of, and thus the donor–acceptor separation in, such assemblies has been shown to be crucial for device performance [9]. In addition, these complexes can sometimes be difficult to synthesize and to purify, therefore we have developed a simple procedure to sequentially deposit such molecules, and control the porphyrin-C60 separation, by axially connecting the metal core of the porphyrin to a C60 functionalized by an N-donor group. A fulleropyrrolidine, the N-methyl-2-( p-pyr-
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.03.029
V. Arima et al. / Applied Surface Science 248 (2005) 40–44
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Fig. 1. (a) Molecular formula of N-methyl-2-(p-pyridyl)-3,4-fulleropyrrolidine (Py-C60). (b) 4-ATP-CoTBPP-Py-C60 system.
idyl)-3,4-fulleropyrrolidine (Py-C60), see Fig. 1a, is axially ligated to cobalt tetra-butyl-phenyl porphyrin (CoTBPP). The complex is anchored to the Au(1 1 1) surface via an aminothiophenol (4-ATP) as shown in Fig. 1b. Synchrotron radiation-based spectroscopies are ideally suited to the study of such assemblies, not only because they provide access to their electronic structure, but also because effectively probe the system in a light-induced excited state. In this work we discuss the electronic structure modifications occurring step by step during the deposition process, by employing photoemission and near-edge X-ray absorption fine structure (NEXAFS) spectroscopies. Synchrotron-radiation photoemission allows the investigation of chemical states and electronic structure, with higher resolution than conventional XPS, as well as atomic concentration ratios with a semi-quantitative estimate. NEXAFS probes the local unoccupied electronic states of the atoms absorbing the X-rays, providing information on the chemical states. NEXAFS is also capable of giving insight about the spatial orientation of the probed states (e.g. for molecules, the molecular orientations). These techniques permit to identify and characterize all the molecules adsorbed on the surface, made possible by the properties of tun-
ability, high intensity and high resolution of the employed light source.
2. Experimental details To allow wet processing and axial ligation, we have synthesised a soluble porphyrin cobalt(II) 5,10,15,20-tetrakis(4-tert-butylphenyl)-porphyrin (CoTBPP), and N-methyl-2-( p-pyridyl)-3,4-fulleropyrrolidine (Py-C60), by following standard procedures [10,11]. 4-Aminothiophenol (4-ATP), 90%, was purchased from Aldrich and used as received. Ethanol and chloroform used were analytical grade. Chloroform was purified by filtration through a column of basic alumina. We anchored Py-C60 to the Au(1 1 1) surface via axial ligation to a self-assembled monolayer of CoTBPP, by immersing an Au/4-ATP/CoTBPP SAM into a chloroform solution of Py-C60 (10 5 M) for 24 h. Further details of the immobilization procedure are described in Ref. [12]. Photoemission and NEXAFS spectra, at room temperature, were taken using the BEAR beamline [13,14] at the ELETTRA synchrotron, Trieste, Italy. Samples were introduced into the vacuum system via a load lock. The sample mounting geometry used is
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V. Arima et al. / Applied Surface Science 248 (2005) 40–44
reported in Ref. [15]. Carbon 1s and nitrogen 1s photoemission spectra were recorded using photon energies of 400 and 510 eV, respectively. A cylindrical mirror analyser was used to acquire the spectra, which had an overall resolution (beamline plus analyser) of 0.8 eV (C and N 1s). Carbon and nitrogen K-edge NEXAFS spectra were recorded using Auger yield, with resolutions of 0.3 and 0.4 eV, respectively.
3. Results and discussion Core level photoemission was used to investigate the electronic modifications in the self-assembled monolayers (SAMs) at different immobilization steps, up to the final step where charge transfer phenomena can occur. The C 1s and N 1s spectra are shown in Fig. 2. Concerning the C 1s spectra, the broad peak shape indicates the presence of several contributions from chemically inequivalent carbon atoms, including some contributions from adventitious contaminants. When the 4-ATP monolayer is formed, we observe a peak centred at 285.2 eV, characteristic of aromatic carbon. After axial ligation of CoTBPP there is a shift of the C 1s peak toward higher energies around +0.55 eV. Although CoTBPP consists largely of aromatic carbons, the reduced screening of the photoemission final state in a molecule distant from the metallic surface will result in a higher apparent binding energy. The third step of immobilization induces a strong shift towards the energy position of the reference PyC60 sample (Py-C60 aggregates deposited directly on bare gold). This suggests that the final Py-C60 monolayer chemisorbed on the 4-ATP-CoTBPP
complex entirely covers the gold surface, since a peak at the CoTBPP binding energy is no longer observed, and that the C 1s peak position is not strongly affected by the underlying monolayers. Flux normalised N 1s photoemission spectra are shown in Fig. 2b. The presence of the CoTBPP is clear from these spectra, as there is considerably more nitrogen present on the surface than for 4-ATP alone – comparing the areas of the peaks suggests a ratio of about 4:1. The presence of only one N 1s peak in the 4ATP-CoTBPP data of Fig. 2b confirms the presence of a metalloporphyrin, since metal-free porphyrin shows two peaks, due to protonated and unprotonated nitrogen. [16] The binding energy of the N 1s peak for 4-ATP is consistent with that of Rosink et al. [17] The value for CoTBPP is slightly higher than that of a neat CoTPP film [18] again probably due to the reduced screening of the hole. After adding the Py-C60 monolayer, the position of the N 1s peak of the CoTBPP is not modified, however its intensity is reduced due to the presence of the C60. No N 1s signal was detectable for the reference sample of Py-C60, most likely because the probable higher packing density, compared to the assumed one Py-C60 per porphyrin, leads to essentially total suppression of the N 1s signal. From the C 1s and N 1s data alone it seems that the electronic features of the complex 4-ATP-CoTBPPPy-C60 are similar to those of the CoTBPP and Py-C60 single components. This conclusion appears premature on examining the N and C-edge NEXAFS spectra shown in Fig. 3. In our previous work [15,19] we have determined an angle between the 4-ATP and the surface normal of 42 88 and an angle of 70 58 relative to the surface for the CoTBPP ring. In Fig. 3a
Fig. 2. (a) Flux normalised C 1s photoemission and (b) N 1s spectra of 4-ATP, 4-ATP-CoTBPP, 4-ATP-CoTBPP-Py-C60 and Py-C60.
V. Arima et al. / Applied Surface Science 248 (2005) 40–44
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Fig. 3. (a) Nitrogen K-edge NEXAFS spectra of 4-ATP (incidence angle of 508), 4-ATP-CoTBPP (incidence angle of 808) and 4-ATP-CoTBPPPy-C60 (incidence angle of 08). (b) Carbon K-edge NEXAFS spectra of 4-ATP-CoTBPP-Py-C60 and Py-C60, both taken at 08.
the NEXAFS spectra taken at the N K-edge are reported for 4-ATP (incident angle of 508), 4-ATPCoTBPP (incident angle of 808) and the 4-ATPCoTBPP-Py-C60 complex (incident angle of 08). The reported data correspond to incident angles at which the peaks are most prominent, such that their energies can be compared. Comparing data from the same angles presupposes that the molecular orientation does not change during the deposition process, which cannot be assumed. The 4-ATP and 4-ATP-CoTBPP N K-edge spectra are described in detail in Refs. [15,19]. If we compare the 4-ATP-CoTBPP and the 4-ATPCoTBPP-Py-C60 spectra we observe that the three main peaks, corresponding to the p* (the two structures at lower energies) and s* resonances (the broad peaks at higher energies) of the metalloporphyrin, are significantly shifted, by more than 1 eV, to lower values. Given that the energies of the N 1s initial states are unchanged, this effect suggests that the Py-C60 ligation strongly modifies the energies of the excited states of the CoTBPP, due to the inherent dipole in the excited state of the donor–acceptor complex. We cannot compare relative intensities if the angle has changed, so the following is not certain. Moreover, it seems that, after the Py-C60 immobilization, the p* resonances decrease in intensity (particularly the peak at lower energies) and change their shape as well, the s* resonance undergoes a rise in intensity. This behaviour could be considered such as a consequence of changes in the molecular orientation of the porphyrin ring occurring when Py-C60 is axially connected to the 4-ATP-CoTBPP system. However, this is not certain as we are not comparing like angles.
Since the C 1s spectrum of the complex is similar to that recorded on the reference Py-C60 sample, we have performed a direct comparison between the C K-edge NEXAFS spectrum of the physisorbed Py-C60 and that of the porphyrin-ligated C60. For the latter case, the two main peaks are both shifted toward higher energies, but the shifts are inequivalent, being for the low energy peak about 0.28 eV, and 1.2 eV for the other. This shows that the observed energy shifts are not due simply to an electrostatic effect, which would shift all peaks equally, but reflects the different interactions in the possible donor–acceptor excited state complexes. The observed spectral changes suggest that the charge redistribution essentially involves the empty electronic states and does not influence core levels because the C 1s spectra are similar to those of the PyC60 aggregates and the N 1s spectra to those of the 4ATP-CoTBPP SAMs. Given that the photoemission process final state leaves only a core hole, rather than the resonantly populated excited states of the NEXAFS process, this shows the utility of X-ray absorption for probing solar cell materials.
4. Conclusions In summary, we have developed a simple procedure to sequentially deposit charge-separation complexes, which may have applications in solar cells, by axially connecting the metal core of a metallo-porphyrin to a C60 functionalized by an N-donor group. We have discussed the electronic structure modifications occurring step by step, by employing photoemission
44
V. Arima et al. / Applied Surface Science 248 (2005) 40–44
and near-edge X-ray absorption fine structure (NEXAFS) spectroscopies. The recorded spectra suggest that the charge redistribution essentially involves the empty electronic states and does not influence the initial state because the C 1s spectra are similar to those of the Py-C60 aggregates and the N 1s spectra to those of the 4-ATP-CoTBPP SAMs.
Acknowledgments This work was funded by the MIUR FIRB project ‘‘Molecular Devices’’. The authors are grateful to the BEAR staff, particularly Bryan Doyle, for assistance at ELETTRA.
References [1] H. Yamada, H. Imahori, Y. Nishimura, I. Yamazaki, T.K. Ahn, S.K. Kim, D. Kim, S. Fukuzumi, J. Am. Chem. Soc. 125 (2003) 9129. [2] F. D’Souza, G.R. Deviprasad, M.E. El-Khouly, M. Fujitsuka, O. Ito, J. Am. Chem. Soc. 123 (2001) 5277. [3] H. Yamada, H. Imahori, Y. Nishimura, I. Yamazaki, T.K. Ahn, S.K. Kim, D. Kim, S. Fukuzumi, J. Am. Chem. Soc. 125 (2003) 9129. [4] F. Fungo, L. Otero, E.N. Durantini, J.J. Silber, L. Sereno, J. Phys. Chem. B 104 (2000) 7644. [5] R.B.M. Koehorst, G.K. Boschloo, T.J. Savenije, A. Goossens, T.J. Schaafsma, J. Phys. Chem. 104 (2000) 2371.
[6] N.V. Tkachenko, H. Lemmetyinen, J. Sonoda, K. Ohkubo, T. Sato, H. Imahori, S. Fukuzumi, J. Phys. Chem. A. 107 (2003) 8834. [7] P.A. Liddel, J.P. Sumida, A.N. Macpherson, A.L. Moore, T.A. Moore, D. Gust, Photochem. Photobiol. 60 (1994) 537. [8] H. Imahori, Y. Sakata, Eur. J. Org. Chem. 10 (1999) 2445. [9] J.K.J. Van Duren, X. Yang, J. Loos, C.W.T. Bulle-Lieuwma, A.B. Sieval, J.C. Hummelen, R.A.J. Janssen, Adv. Mater. 14 (2004) 425. [10] A.D. Adler, F.R. Longo, W. Shergalis, J. Am. Chem. Soc. 86 (1964) 3145. [11] M. Maggini, G. Scorrano, M. Prato, J. Am. Chem. Soc. 115 (1993) 9798. [12] V. Arima, R.I.R. Blyth, F. Della Sala, R. Del Sole, F. Matino, G. Mele, G. Vasapollo, R. Cingolani, R. Rinaldi, Mater. Sci. Eng. C 24 (2004) 569. [13] S. Nannarone, F. Borgatti, A. DeLuisa, B.P. Doyle, G.C. Gazzadi, A. Giglia, P. Finetti, N. Mahne, L. Pasquali, M. Pedio, G. Selvaggi, G. Naletto, M.G. Pelizzo, G. Tondello, AIP Conf. Proc. 705 (2004) 450. [14] F. Borgatti, A. De Luisa, B. Doyle, A. Giglia, N. Mahne, I. Pasquali, M. Pedio, G. Selvaggi, S. Nannarone, G. Naletto, M.G. Pelizzo, G. Tondello, ELETTRA News 47 (2003). [15] V. Arima, E. Fabiano, R.I.R. Blyth, F. Della Sala, F. Matino, J. Thompson, R. Cingolani, R. Rinaldi, J. Am. Chem. Soc. 126 (2004) 16951. [16] G. Polzonetti, A. Ferri, M.V. Russo, G. Iucci, S. Licoccia, R. Paolesse, J. Vac. Sci. Technol. A 17 (1999) 832. [17] J.J.W.M. Rosink, M.A. Blauw, L.J. Geerligs, E. Van der Drift, B.A.C. Rousseeuw, S. Radelaar, W.G. Sloof, E.J.M. Fakkeldij, Langmuir 16 (2000) 4547. [18] F. Da Cruz, K. Driaf, C. Berthier, J.-M. Lamelle, F. Armand, Thin Solid Films 349 (1999) 155. [19] V. Arima, F. Matino, J. Thompson, R. Cingolani, R. Rinaldi, R.I.R. Blyth, Surf. Sci., in press.
Characterization of functionalised porphyrin films using synchrotron radiation V. Arima a,*, F. Matino a, J. Thompson a, R. Del Sole b, G. Mele b, G. Vasapollo b, R. Cingolani a, R. Rinaldi a, R.I.R. Blyth a a
National Nanotechnology Laboratory of INFM, Distretto Tecnologico ISUFI, Universita’ di Lecce Via Arnesano, 73100 Lecce, Italy b Dipartimento di Ingegneria dell’Innovazione, Universita` di Lecce, via per Arnesano, 73100 Lecce, Italy Available online 20 March 2005
Abstract Porphyrins and C60 are strategic materials for the fabrication of nanoscale molecular devices by virtue of their optical, photo– electro-chemical and chemical properties. We have developed procedures to immobilise cobalt tetra-butyl-phenyl porphyrins (CoTBPPs) on gold surfaces via ligation to self-assembled monolayers of aromatic aminothiophenols (4-ATP). We have used synchrotron radiation photoemission and near-edge X-ray absorption, NEXAFS, to characterise such films, both in their native state, and with ligated fulleropyrrolidines N-methyl-2-( p-pyridyl)-3,4-fulleropyrrolidine (Py-C60), forming charge-separation complexes which may have applications in solar cells. While photoemission spectra appear dominated by the individual CoTBPP and Py-C60 components, we observe an apparent signature of charge separation in fulleropyrrolidine NEXAFS spectra. # 2005 Elsevier B.V. All rights reserved. Keywords: Porphyrins; Synchrotron radiation; Fulleropyrrolidines; Photoemission; NEXAFS
1. Introduction In the field of nanotechnology, a promising route towards cheap, mass-produced solar cells are organic charge-separation complexes. One approach to mimic photosynthetic energy conversion is the construction of synthetic supramolecular systems containing chromophores, electron donors and electron acceptors linked by covalent bonds [1,2]. Many artificial * Corresponding author. Tel.: +39 0832 298383; fax: +39 0832 298238. E-mail address: [email protected] (V. Arima).
assemblies incorporate porphyrin derivatives as light receptors [3–5] and C60 derivatives as electron acceptors [6–8]. Control of the morphology of, and thus the donor–acceptor separation in, such assemblies has been shown to be crucial for device performance [9]. In addition, these complexes can sometimes be difficult to synthesize and to purify, therefore we have developed a simple procedure to sequentially deposit such molecules, and control the porphyrin-C60 separation, by axially connecting the metal core of the porphyrin to a C60 functionalized by an N-donor group. A fulleropyrrolidine, the N-methyl-2-( p-pyr-
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.03.029
V. Arima et al. / Applied Surface Science 248 (2005) 40–44
41
Fig. 1. (a) Molecular formula of N-methyl-2-(p-pyridyl)-3,4-fulleropyrrolidine (Py-C60). (b) 4-ATP-CoTBPP-Py-C60 system.
idyl)-3,4-fulleropyrrolidine (Py-C60), see Fig. 1a, is axially ligated to cobalt tetra-butyl-phenyl porphyrin (CoTBPP). The complex is anchored to the Au(1 1 1) surface via an aminothiophenol (4-ATP) as shown in Fig. 1b. Synchrotron radiation-based spectroscopies are ideally suited to the study of such assemblies, not only because they provide access to their electronic structure, but also because effectively probe the system in a light-induced excited state. In this work we discuss the electronic structure modifications occurring step by step during the deposition process, by employing photoemission and near-edge X-ray absorption fine structure (NEXAFS) spectroscopies. Synchrotron-radiation photoemission allows the investigation of chemical states and electronic structure, with higher resolution than conventional XPS, as well as atomic concentration ratios with a semi-quantitative estimate. NEXAFS probes the local unoccupied electronic states of the atoms absorbing the X-rays, providing information on the chemical states. NEXAFS is also capable of giving insight about the spatial orientation of the probed states (e.g. for molecules, the molecular orientations). These techniques permit to identify and characterize all the molecules adsorbed on the surface, made possible by the properties of tun-
ability, high intensity and high resolution of the employed light source.
2. Experimental details To allow wet processing and axial ligation, we have synthesised a soluble porphyrin cobalt(II) 5,10,15,20-tetrakis(4-tert-butylphenyl)-porphyrin (CoTBPP), and N-methyl-2-( p-pyridyl)-3,4-fulleropyrrolidine (Py-C60), by following standard procedures [10,11]. 4-Aminothiophenol (4-ATP), 90%, was purchased from Aldrich and used as received. Ethanol and chloroform used were analytical grade. Chloroform was purified by filtration through a column of basic alumina. We anchored Py-C60 to the Au(1 1 1) surface via axial ligation to a self-assembled monolayer of CoTBPP, by immersing an Au/4-ATP/CoTBPP SAM into a chloroform solution of Py-C60 (10 5 M) for 24 h. Further details of the immobilization procedure are described in Ref. [12]. Photoemission and NEXAFS spectra, at room temperature, were taken using the BEAR beamline [13,14] at the ELETTRA synchrotron, Trieste, Italy. Samples were introduced into the vacuum system via a load lock. The sample mounting geometry used is
42
V. Arima et al. / Applied Surface Science 248 (2005) 40–44
reported in Ref. [15]. Carbon 1s and nitrogen 1s photoemission spectra were recorded using photon energies of 400 and 510 eV, respectively. A cylindrical mirror analyser was used to acquire the spectra, which had an overall resolution (beamline plus analyser) of 0.8 eV (C and N 1s). Carbon and nitrogen K-edge NEXAFS spectra were recorded using Auger yield, with resolutions of 0.3 and 0.4 eV, respectively.
3. Results and discussion Core level photoemission was used to investigate the electronic modifications in the self-assembled monolayers (SAMs) at different immobilization steps, up to the final step where charge transfer phenomena can occur. The C 1s and N 1s spectra are shown in Fig. 2. Concerning the C 1s spectra, the broad peak shape indicates the presence of several contributions from chemically inequivalent carbon atoms, including some contributions from adventitious contaminants. When the 4-ATP monolayer is formed, we observe a peak centred at 285.2 eV, characteristic of aromatic carbon. After axial ligation of CoTBPP there is a shift of the C 1s peak toward higher energies around +0.55 eV. Although CoTBPP consists largely of aromatic carbons, the reduced screening of the photoemission final state in a molecule distant from the metallic surface will result in a higher apparent binding energy. The third step of immobilization induces a strong shift towards the energy position of the reference PyC60 sample (Py-C60 aggregates deposited directly on bare gold). This suggests that the final Py-C60 monolayer chemisorbed on the 4-ATP-CoTBPP
complex entirely covers the gold surface, since a peak at the CoTBPP binding energy is no longer observed, and that the C 1s peak position is not strongly affected by the underlying monolayers. Flux normalised N 1s photoemission spectra are shown in Fig. 2b. The presence of the CoTBPP is clear from these spectra, as there is considerably more nitrogen present on the surface than for 4-ATP alone – comparing the areas of the peaks suggests a ratio of about 4:1. The presence of only one N 1s peak in the 4ATP-CoTBPP data of Fig. 2b confirms the presence of a metalloporphyrin, since metal-free porphyrin shows two peaks, due to protonated and unprotonated nitrogen. [16] The binding energy of the N 1s peak for 4-ATP is consistent with that of Rosink et al. [17] The value for CoTBPP is slightly higher than that of a neat CoTPP film [18] again probably due to the reduced screening of the hole. After adding the Py-C60 monolayer, the position of the N 1s peak of the CoTBPP is not modified, however its intensity is reduced due to the presence of the C60. No N 1s signal was detectable for the reference sample of Py-C60, most likely because the probable higher packing density, compared to the assumed one Py-C60 per porphyrin, leads to essentially total suppression of the N 1s signal. From the C 1s and N 1s data alone it seems that the electronic features of the complex 4-ATP-CoTBPPPy-C60 are similar to those of the CoTBPP and Py-C60 single components. This conclusion appears premature on examining the N and C-edge NEXAFS spectra shown in Fig. 3. In our previous work [15,19] we have determined an angle between the 4-ATP and the surface normal of 42 88 and an angle of 70 58 relative to the surface for the CoTBPP ring. In Fig. 3a
Fig. 2. (a) Flux normalised C 1s photoemission and (b) N 1s spectra of 4-ATP, 4-ATP-CoTBPP, 4-ATP-CoTBPP-Py-C60 and Py-C60.
V. Arima et al. / Applied Surface Science 248 (2005) 40–44
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Fig. 3. (a) Nitrogen K-edge NEXAFS spectra of 4-ATP (incidence angle of 508), 4-ATP-CoTBPP (incidence angle of 808) and 4-ATP-CoTBPPPy-C60 (incidence angle of 08). (b) Carbon K-edge NEXAFS spectra of 4-ATP-CoTBPP-Py-C60 and Py-C60, both taken at 08.
the NEXAFS spectra taken at the N K-edge are reported for 4-ATP (incident angle of 508), 4-ATPCoTBPP (incident angle of 808) and the 4-ATPCoTBPP-Py-C60 complex (incident angle of 08). The reported data correspond to incident angles at which the peaks are most prominent, such that their energies can be compared. Comparing data from the same angles presupposes that the molecular orientation does not change during the deposition process, which cannot be assumed. The 4-ATP and 4-ATP-CoTBPP N K-edge spectra are described in detail in Refs. [15,19]. If we compare the 4-ATP-CoTBPP and the 4-ATPCoTBPP-Py-C60 spectra we observe that the three main peaks, corresponding to the p* (the two structures at lower energies) and s* resonances (the broad peaks at higher energies) of the metalloporphyrin, are significantly shifted, by more than 1 eV, to lower values. Given that the energies of the N 1s initial states are unchanged, this effect suggests that the Py-C60 ligation strongly modifies the energies of the excited states of the CoTBPP, due to the inherent dipole in the excited state of the donor–acceptor complex. We cannot compare relative intensities if the angle has changed, so the following is not certain. Moreover, it seems that, after the Py-C60 immobilization, the p* resonances decrease in intensity (particularly the peak at lower energies) and change their shape as well, the s* resonance undergoes a rise in intensity. This behaviour could be considered such as a consequence of changes in the molecular orientation of the porphyrin ring occurring when Py-C60 is axially connected to the 4-ATP-CoTBPP system. However, this is not certain as we are not comparing like angles.
Since the C 1s spectrum of the complex is similar to that recorded on the reference Py-C60 sample, we have performed a direct comparison between the C K-edge NEXAFS spectrum of the physisorbed Py-C60 and that of the porphyrin-ligated C60. For the latter case, the two main peaks are both shifted toward higher energies, but the shifts are inequivalent, being for the low energy peak about 0.28 eV, and 1.2 eV for the other. This shows that the observed energy shifts are not due simply to an electrostatic effect, which would shift all peaks equally, but reflects the different interactions in the possible donor–acceptor excited state complexes. The observed spectral changes suggest that the charge redistribution essentially involves the empty electronic states and does not influence core levels because the C 1s spectra are similar to those of the PyC60 aggregates and the N 1s spectra to those of the 4ATP-CoTBPP SAMs. Given that the photoemission process final state leaves only a core hole, rather than the resonantly populated excited states of the NEXAFS process, this shows the utility of X-ray absorption for probing solar cell materials.
4. Conclusions In summary, we have developed a simple procedure to sequentially deposit charge-separation complexes, which may have applications in solar cells, by axially connecting the metal core of a metallo-porphyrin to a C60 functionalized by an N-donor group. We have discussed the electronic structure modifications occurring step by step, by employing photoemission
44
V. Arima et al. / Applied Surface Science 248 (2005) 40–44
and near-edge X-ray absorption fine structure (NEXAFS) spectroscopies. The recorded spectra suggest that the charge redistribution essentially involves the empty electronic states and does not influence the initial state because the C 1s spectra are similar to those of the Py-C60 aggregates and the N 1s spectra to those of the 4-ATP-CoTBPP SAMs.
Acknowledgments This work was funded by the MIUR FIRB project ‘‘Molecular Devices’’. The authors are grateful to the BEAR staff, particularly Bryan Doyle, for assistance at ELETTRA.
References [1] H. Yamada, H. Imahori, Y. Nishimura, I. Yamazaki, T.K. Ahn, S.K. Kim, D. Kim, S. Fukuzumi, J. Am. Chem. Soc. 125 (2003) 9129. [2] F. D’Souza, G.R. Deviprasad, M.E. El-Khouly, M. Fujitsuka, O. Ito, J. Am. Chem. Soc. 123 (2001) 5277. [3] H. Yamada, H. Imahori, Y. Nishimura, I. Yamazaki, T.K. Ahn, S.K. Kim, D. Kim, S. Fukuzumi, J. Am. Chem. Soc. 125 (2003) 9129. [4] F. Fungo, L. Otero, E.N. Durantini, J.J. Silber, L. Sereno, J. Phys. Chem. B 104 (2000) 7644. [5] R.B.M. Koehorst, G.K. Boschloo, T.J. Savenije, A. Goossens, T.J. Schaafsma, J. Phys. Chem. 104 (2000) 2371.
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