Gold(0) porphyrins on gold nanoparticles

Article abstract of DOI:10.1002/anie.200703943

Bound through S-Au bonds, multidentate macrocyclic porphyrin thioester derivatives densely protect Au nanoparticles in a face-coordination fashion (see picture) to form quite stable Au⁰ porphyrins. The spectroscopic Soret-band intensity can be tuned by the distance between the porphyrin ring and the Au surface. (Figure Presented).

Full text of DOI:10.1002/anie.200703943

Angewandte
Chemie
DOI: 10.1002/anie.200703943
Gold(0) Porphyrins
Gold(0) Porphyrins on Gold Nanoparticles**
Masayuki Kanehara, Hirokazu Takahashi, and Toshiharu Teranishi*
Gold nanoparticles play important roles in different branches
of science, such as in nanoelectronics, nonlinear optics,
biological labeling, and oxidation catalysis, among others.[1–5]
Many ligand-functionalized metal nanoparticles have been
reported based on ligation using the chemical affinity of
organic functional groups toward the nanoparticle surface to
stabilize the nanoparticles. Polymers,[6] linear molecules with
long alkyl chains,[7] and dendrimers[8] have all been effectively
used for this purpose, relying on the s electrons of the
functional groups. For the application of nanoparticles in
nanoelectronic devices, exploiting the organoelectronic p-
orbital interactions, which are generally used in electron-
conductive polymers and organic transistors, is quite impor-
tant for the reduction of the tunneling resistance of the
surrounding ligands. Porphyrins are one of the most impor-
tant p-conjugated compounds, and a recent study of porphy-
rins on Au(111)[9] encouraged us to investigate the interaction
between Au nanoparticles and p-conjugated porphyrin sys-
tems. Herein, we report the preparation, structural analysis,
and unique optical properties of novel porphyrin speices on
Au nanoparticles. The nitrogen atoms of the porphyrin rings
were found to coordinate to the Au nanoparticle surface, and
the Soret-band intensity could be tuned by changing the
distance between the porphyrin rings and the Au nanoparticle
surface.
To obtain stable Au nanoparticles surrounded by p orbi-
tals perpendicular to the Au surface, we focused on Au
nanoparticles formed through strong multidentate ligation
using thiol derivatives. As shown in Figure 1, the multidentate
macrocyclic porphyrin thioester derivatives tetrakis-
5,10,15,20-(2-acetylthiophenyl)porphyrin (SC0P) and tetra-
kis-5,10,15,20-(2-acetylthiomethylphenyl)porphyrin (SC1P)
were synthesized. The SC1P ligand was designed with
methylene groups inserted between the benzene rings and
the acetylthio groups in order to increase the distance
between the porphyrin ring and the Au surface. The SCnP
(n = 0, 1) ligands were synthesized from the corresponding
aldehyde and pyrrole using Lindseyꢀs method[10] in 15% and
40% yields, respectively.[11] Since the acetylthio groups easily
dissociate to form S–Au bonds on bare Au surfaces in a
slightly alkaline condition,[12] these groups are considered an
excellent thiol source to protect the Au surface. The SCnP-
protected Au (SCnP-Au) nanoparticles were prepared by
ligand-exchange reactions from citrate-protected Au (CA-
Au) nanoparticles. After ligand exchange, the nanoparticles
became insoluble in water but soluble in N,N-dimethylaceta-
mide (DMAc), indicating that ligand exchange was accom-
plished. Further evidence was confirmed from X-ray photo-
electron spectroscopy (XPS) measurements. The C1s peaks
assigned to the carbonyl carbon atoms of both citrate and
acetylthio groups of the SCnP ligands disappeared (Figure S1
in the Supporting Information), indicating that the citrate
ligands were completely exchanged with the SCnP ligands and
that the acetylthio groups dissociated to protect the Au
surface. To confirm the stability of SCnP, these ligands were
annealed with citrate and tannic acid in the absence of Au
nanoparticles at 1208C in a DMAc/water mixture. No UV/Vis
spectral change was observed, thus demonstrating that the
SCnP ligands are stable under the ligand-exchange conditions.
Figure 1 shows TEM images of the CA-Au and SCnP-Au
nanoparticles. The sizes of the SCnP-Au nanoparticles
remained unchanged after the ligand exchange. The DMAc
solutions of SCnP-Au nanoparticles are quite stable under
ambient conditions, and no size change was observed over at
least one year because of the tetradentate nature of the SCnP
ligands. To confirm the existence of SCnP on the Au surface
and to reveal the coordination geometry, laser Raman
measurements, thermogravimetric analysis (TGA), and XPS
analyses were conducted. The laser Raman spectra of the
SCnP-Au nanoparticles were similar to those of the SCnP
ligands, indicating the existence of porphyrin rings on the Au
nanoparticles (Figure S2 in the Supporting Information).
Further evidence was obtained by cyanide decomposition of
the SCnP-Au nanoparticles.[13] When the SCnP-Au nanopar-
ticles were treated with an excess amount of sodium cyanide,
the intensities of both the Soret band and Q band were
[*] Dr. M. Kanehara, H. Takahashi, Prof. T. Teranishi
Graduate School ofPure and Applied Sciences
University ofTsukuba
1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571 (Japan)
Fax: (+81)29-853-6503
E-mail: teranisi@chem.tsukuba.ac.jp
[**] We thank Dr. Nakayama and Dr. Shingaya for laser Raman
measurements, and Prof. Sugimura for XPS measurements. This
work was supported by a Grant-in-Aid for Scientific Research (A)
(No. 19205016) and Scientific Research on Priority Area“Chemistry
ofCoordination Space” (No. 18033007) rfom MEXT (Japan) and
Industrial Technology Research Grant Program in 2004 from NEDO
ofJapan (T.T.).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Figure 1. TEM images ofa) citrate- b) SC P-, and c) SC1P-protected Au
0
nanoparticles. Particle diameters are 10.5Æ1.0 nm (a), 10.5Æ1.0 nm
(b), and 10.0Æ0.9 nm (c). Scale bars: 50 nm. d) Schematic illustra-
tions ofthe coordination ofSC nP ligands on the Au surface.
Figure 2. N1s core-level XPS spectra for a) SC0P, b) SC1P, c) SC0P-Au,
recovered regardless of the decomposition of the SCnP
ligands (Figure S3 in the Supporting Information). TGA
analysis revealed weight losses of 3.1% and 3.0% for SC0P-
Au and SC1P-Au, corresponding to 246 and 226 ligands on a
single Au nanoparticle for SC0P-Au and SC1P-Au, respec-
tively. Considering that the length of the neighboring meso-
substituted phenyl groups is about 1.25 nm, approximately
222 SCnP ligands can cover a single Au nanoparticle surface,
thus suggesting that these porphyrin ligands densely protect
the Au nanoparticles in a face-coordination fashion, as shown
in Figure 1d.[11]
and d) SC1P-Au
nate to a zero-valent metal surface has never been reported,
not only for porphyrins on bulk Au(111) or Ag(111) surfaces
but also for previously reported Ru0 and Fe0 porphyrins.
Furthermore, the Ru0 [14] and Fe0 [15] porphyrins have a quite
high reactivity toward both organic and inorganic electro-
philes, while our SCnP-Au nanoparticles are exceedingly
stable against various electrophiles including water and
DMAc, and no decomposition of the SCnP ligands on the
Au nanoparticles was observed by UV/Vis spectra under
ambient conditions. The ligand-exchange conditions were
almost pH-neutral, and UV/Vis spectral changes are not
observed for the SCnP-Au nanoparticles after adding acetic
acid to the SCnP-Au nanoparticle solution. Thus, it was
concluded that the deprotonation of the porphyrins was not
induced by some bases. Presumably, the strong tetradentate
coordination of the SCnP ligands fixes the porphyrin rings
onto the Au nanoparticle surfaces to stabilize the deproton-
ated porphyrin rings. In addition, refluxing a toluene solution
containing Au nanoparticles, tetraphenylporphyrin, and
dodecanethiol did not result in Au0 porphyrin formation.
Interestingly, these SCnP-Au nanoparticles exhibit unique
optical properties. Figure 3 shows the UV/Vis spectra of the
DMAc solutions containing SCnP ligands or SCnP-Au nano-
particles. The DMAc solutions of SC0P and SC1P gave Soret
bands at 421 and 423 nm, respectively, and four distinct
Q bands in the range 515–646 nm. Upon coordination of the
SCnP ligands onto the Au nanoparticles, the Q bands for the
SCnP-Au nanoparticles completely disappeared, and the
Soret bands for the SC0P-Au and SC1P-Au nanoparticles
were broadened and red-shifted to 432 and 427 nm, respec-
tively. The molar absorption coefficients of the Soret bands
for the SC0P-Au and SC1P-Au nanoparticles were calculated
to be 2.5 ” 104 and 7.0 ” 104 mÀ1 cmÀ1, respectively, using TGA
results and subtraction of the Au nanoparticle background.
These values are smaller by one order of magnitude than
those for the SCnP ligands themselves (ca. 3.9 ” 105 mÀ1 cmÀ1).
Further evidence for the face coordination of the SCnP
ligands on the Au nanoparticles could be confirmed by XPS
measurements. The SCnP ligands exhibit two distinct chemi-
cally inequivalent N1s core-level spectra corresponding to
=
free nitrogen atoms of the imine (-C N-) and pyrrole (-NH-)
groups, from lower to higher binding energy, respectively
(Figure 2). However, both the SC0P-Au and SC1P-Au nano-
particles exhibit a single N1s peak with a binding energy of
399.9 eV, which is similar to that of the imine nitrogen atoms
coordinating to the Au(111) surface, as reported by Feringa
and co-workers.[9] They observed three distinct N1s peaks
(free imine, coordinating imine, and free pyrrole) on a
Au(111) substrate, whereas SCnP-Au nanoparticles exhibit a
single N1s peak, indicating that the four nitrogen atoms of the
SCnP ligands deprotonate to coordinate to the Au surface to
form Au(0) porphyrins. Although the position of the peak
with a binding energy of about 399.9 eV corresponds to the
binding energy of nitrogen atoms in metalloporphyrins,[9] the
possibility of the formation of AuIII porphyrins was ruled out,
because no Au 4f7/2 peak assigned to oxidized gold was
observed in SCnP-Au (Figure S4 in the Supporting
Information). We conclude that this unique coordination
fashion is a nanoparticle effect. The porphyrin derivatives are
expected to coordinate mainly to the step edges and plane
boundaries that the nanoparticles possess on their surfaces,
centering at the porphyrin nitrogen atoms. To the best of our
knowledge, this coordination fashion resulting from the
porphyrin molecules spontaneously deprotonating to coordi-
308
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coordination fashion is completely different from that dis-
played by conventional metalloporphyrins, in which the
porphyrin coordinates to an isolated metal ion. The SCnP-
Au nanoparticles show a ligand-dependent decrease of the
Soret-band intensity, whereby the shift and decrease of the
Soret band for SC0P-Au are larger than those for SC1P-Au.
The diminishing of the Soret band, which is tuned by the
coordination distance between the porphyrin ring and the Au
surface, implies an electronic interaction between the por-
phyrin and Au nanoparticle. A first-principles calculation
approach to confirm our claim is currently in progress.
Figure 3. UV/Vis spectra ofa) SC nP ligands and b) SCnP-Au nano-
particles. The inset in (a) shows the Q bands expanded by 10. Each set
ofspectra was normalized by ligand (a) and nanoparticle (b) concen-
trations.
Experimental Section
Preparation of SCnP-Au nanoparticles: Citrate-capped Au (CA-Au)
nanoparticles with a size of 10.5 Æ 1.0 nm were prepared according to
the literature.[23] The CA-Au aqueous solution was poured into the
same volume of a N,N-dimethylacetamide (DMAc) solution of SCnP,
where a half equivalent of ligand to surface Au atoms was used based
on the spherical model.[24] Because the SCnP ligands have four
atropisomers at room temperature,[25] the DMAc solution of the
nanoparticles was annealed at 1208C to promote the coordination of
four thiolate groups onto the same nanoparticle surface. After
removing the solvent under reduced pressure, water was added and
the resulting precipitate was filtered and washed with water,
methanol, and toluene to obtain pure SCnP-Au nanoparticles.
Purification was checked by analytical gel permeation chromatog-
raphy.
These results are completely different from those of the
Au nanoparticles protected by meso-3-acetylthiophenyl-sub-
stituted porphyrin[16] and other porphyrin derivatives,[17,18] in
which the porphyrin rings are tilted at the Au surface and the
extensive diminishing of both the Soret band and Q band has
not been observed. The drastic changes in the absorbance of
the Soret band and Q band for porphyrin molecules on Au
nanoparticles have to result from the change in electronic
state of the porphyrins. Generally, a red-shift of the Soret
band results from the formation of a side-by-side partially p–p
stacked structure of the J aggregate type,[19] although the
formation of J aggregates cannot explain the decrease in
intensity of the Soret band and Q band of the SCnP-Au
nanoparticles. Distortion of the porphyrin ring may lead to a
decrease in the Soret-band intensity, although a large
distortion of the porphyrin ring must accompany the consid-
erable shift of the Soret band.[20] Thus, this distortion model is
not applicable to our case, because red-shifts of only a few
nanometers are observed for the SCnP-Au nanoparticles.
Though the details are not understood yet, the unique optical
change exhibited by the porphyrin rings may be derived from
the direct interaction between the porphyrin p and Au
orbitals, in other words, from the partial charge transfer
between the porphyrin and Au surface, which is often
observed between porphyrin rings and fullerenes.[21] Another
possibility includes the hybridization of the porphyrin p and
Au orbitals to form new orbitals between the porphyrin rings
and Au surfaces, as calculated for a conductive carbon
nanotube on an aluminum surface.[22] Considering the
longer porphyrin–Au distance for SC1P-Au, as illustrated in
Figure 1d, the Au–porphyrin interaction for SC0P-Au is
expected to be much larger than that for SC1P-Au, resulting
in the larger red-shift and the broadening of the Soret band
for SC0P-Au.
Received: August 28, 2007
Revised: September 26, 2007
Published online: November 19, 2007
Keywords: electronic interactions · gold · metalloporphyrins ·
.
nanostructures · porphyrinoids
[1] M.-C. Daniel, D. Astruc, Chem. Rev. 2004, 104, 293.
[2] M. Kanehara, Y. Oumi, T. Sano, T. Teranishi, J. Am. Chem. Soc.
2003, 125, 8708.
[3] Y. Azuma, M. Kanehara, T. Teranishi, Y. Majima, Phys. Rev.
Lett. 2006, 96, 016108.
[4] M. Kanehara, E. Kodzuka, T. Teranishi, J. Am. Chem. Soc. 2006,
128, 13084.
[5] D. Astruc, F. Lu, J. R. Aranzaes, Angew. Chem. 2005, 117, 8062;
Angew. Chem. Int. Ed. 2005, 44, 7852.
[6] T. Teranishi, I. Kiyokawa, M. Miyake, Adv. Mater. 1998, 10, 596.
[7] M. Brust, M. Walker, D. Bethell, D. J. Schiffrin, R. Whyman, J.
Chem. Soc. Chem. Commun. 1994, 801.
[8] J. Zheng, J. T. Petty, R. M. Dickson, J. Am. Chem. Soc. 2003, 125,
7780.
[9] N. Katsonis, J. Vicario, T. Kudernac, J. Visser, M. M. Pollard,
B. L. Feringa, J. Am. Chem. Soc. 2006, 128, 15537.
[10] D. T. Gryko, C. Clausen, K. M. Roth, N. Dontha, D. F. Bocian,
W. G. Kuhr, J. S. Lindsey, J. Org. Chem. 2000, 65, 7345.
[11] See the Supporting Information.
[12] A. A. Yasseri, D. Syomin, V. L. Malinovskii, R. S. Loewe, J. S.
Lindsey, F. Zaera, D. F. Bocian, J. Am. Chem. Soc. 2004, 126,
11944.
In conclusion, we have synthesized macrocyclic porphyrin
ligands which protect Au nanoparticles with the porphyrin
rings parallel to the Au surface. The porphyrin rings on the Au
nanoparticles have a stronger coordination ability to the Au
surface than to a Au(111) substrate, since all imine nitrogen
atoms of the SCnP ligands participate in the ligation to the Au
nanoparticle surface to form quite stable Au0 porphyrins. The
Au0 porphyrin has a novel structure in that the porphyrin
molecule on the Au nanoparticle surface has been deproton-
ated to coordinate to a surface Au atom, and such a
[13] K. R. Gopidas, J. K. Whitesell, M. A. Fox, J. Am. Chem. Soc.
2003, 125, 6491.
[14] J. P. Collman, P. J. Brothers, L. McElwee-White, E. Rose, J. Am.
Chem. Soc. 1985, 107, 6110.
Angew. Chem. Int. Ed. 2008, 47, 307 –310
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
309

Communications
[15] T. Mashiko, C. A. Reed, K. J. Haller, W. R. Scheidt, Inorg.
[21] A. Hosseini, S. Taylor, G. Accorsi, N. Armaroli, C. A. Reed,
P. D. W. Boyd, J. Am. Chem. Soc. 2006, 128, 15903.
Chem. 1984, 23, 3192.
[16] M. Kanehara, T. Teranishi, e-J. Surf. Sci. Nanotechnol. 2005, 3,
30.
[22] S. Okada, A. Oshiyama, Phys. Rev. Lett. 2005, 95, 206804.
[23] J. W. Slot, H. J. Geuze, Eur. J. Cell Biol. 1985, 38, 87.
[24] R. H. Terrill, T. A. Postlethwaite, C.-h. Chen, C.-D. Poon, A.
Terzis, A. Chen, J. E. Hutchison, M. R. Clark, G. Wignall, J. D.
Londono, R. Superfine, M. Falvo, C. S. Johnson, Jr., E. T.
Samulski, R. W. Murray, J. Am. Chem. Soc. 1995, 117, 12537.
[25] Y. Kuroda, T. Hiroshige, T. Sera, Y. Shiroiwa, H. Tanaka, H.
Ogoshi, J. Am. Chem. Soc. 1989, 111, 1912.
[17] I. Sloufova-Srnova, B. Vlkova, Nano Lett. 2002, 2, 121.
[18] H. Imahori, Y. Kashiwagi, Y. Endo, T. Hanada, Y. Nishimura, I.
Yamazaki, Y. Araki, O. Ito, S. Fukuzumi, Langmuir 2004, 20, 73.
[19] H. Imahori, H. Norieda, Y. Nishimura, I. Yamazaki, K. Higuchi,
N. Kato, T. Motohiro, H. Yamada, K. Tamaki, M. Arimura, Y.
Sakata, J. Phys. Chem. B 2000, 104, 1253.
[20] J. P. Hill, I. J. Hewitt, C. E. Anson, A. K. Powell, A. L. McCarty,
P. A. Karr, M. E. Zandler, F. DꢀSouza, J. Org. Chem. 2004, 69,
5861.
310
www.angewandte.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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