Soret bands in the Fig. 3 film spectra reflects a small proportion of
monomeric ZnCA(PE)xBPP adsorbed on TiO2 films. This is
consistent with the fact that the TiO2 surfaces are partially covered
by the porphyrins. For the blue-shifted absorptions, the quality of
the TiO2 films (e.g., thickness) seems to have a small effect on the
intensities of these blue shoulders. The appearance of these blue
shoulders is thus not due to the effects of reflection.14 The film
spectra reveal little interaction between ZnCA(PE)xBPP and TiO2
nanoparticles, and the appearance of these blue shoulders is
attributed to aggregation of ZnCA(PE)xBPP assemblies on the
surfaces of the TiO2 films (Figs. 3a and b, right). The stacked, face-
to-face porphyrin p-aggregation (or H-aggregation) is known to
produce a blue shift of the absorption, whereas the side-by-side
porphyrin p-aggregation (or J-aggregation) results in a red shift.15
We hence suggest that the Soret blue shoulders (Figs. 3a and b,
bold curves) are caused by H-aggregation of porphyrins on the
TiO2 surfaces. In contrast to the ZnCA(PE)1BPP/TiO2 system,
red-shifted and much broadened Soret bands are observed for
ZnCA(PE)1BPP spin-coated on a glass substrate, indicating the
formation of J-type porphyrin aggregates on such a flat glass
surface. These phenomena are consistent with literature
reports.16,17
Notes and references
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Soc. Rev., 2000, 29, 87–96; (d) A. Hagfeldt and M. Gra¨tzel, Chem. Rev.,
1995, 95, 49–68.
3 M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Muller,
P. Liska, N. Vlachopoulos and M. Gra¨tzel, J. Am. Chem. Soc., 1993,
115, 6382–6390.
4 M. K. Nazeeruddin, P. Pechy, T. Renouard, S. M. Zakeeruddin,
R. Humphry-Baker, P. Comte, P. Liska, L. Cevey, E. Costa,
V. Shklover, L. Spiccia, G. B. Deacon, C. A. Bignozzi and
M. Gra¨tzel, J. Am. Chem. Soc., 2001, 123, 1613–1624.
5 Q. Wang, W. M. Campbell, E. E. Bonfantani, K. W. Jolley,
D. L. Officer, P. J. Walsh, K. Gordon, R. Humphry-Baker,
M. K. Nazeeruddin and M. Gra¨tzel, J. Phys. Chem. B, 2005, 109,
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6 (a) S. Cherian and C. C. Wamser, J. Phys. Chem. B, 2000, 104,
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W. M. Campbell, A. K. Burrell and M. Gra¨tzel, Langmuir, 2004, 20,
6514–6517 and the references therein.
7 F. Odobel, E. Blart, M. Lagree, M. Villieras, H. Boujtita, N. E. Murr,
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9 (a) P. Piotrowiak, E. Galoppini, Q. Wei, G. J. Meyer and P. Wiewior,
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To extend these spectral observations for ZnCA(PE)xBPP
molecules H-aggregated on TiO2 surfaces and to demonstrate that
this aggregation can be well controlled by ligating solvents, we
conducted further UV-visible spectral measurements for
ZnCA(PE)xBPP/TiO2 films in air, THF and pyridine. As
compared in Figs. 3a–c (left, bold solid curves), the Soret blue
shoulders of ZnCA(PE)xBPP/TiO2/air samples are more intense
than those of a 1 : 1 mixture of the [ZnCA(PE)1BPP +
ZnCA(PE)4BPP]/TiO2/air system, indicating that the difference
in the length of bridge hinders H-aggregation (Fig. 3c, right).
Furthermore, the blue shoulders of ZnCA(PE)xBPP/TiO2/air
samples become less intense when the films are immersed in
THF (thin solid curves) or pyridine (dotted curves). This
phenomenon implies ligation of THF or pyridine molecules to
the central metal zinc(II) ion of these complexes,2a,18 which
effectively hampers H-type aggregation of ZnCA(PE)xBPP
molecues on the TiO2 surface, and decreases the absorbance in
the blue shoulders.19
10 (a) K. Sonogashira, Y. Tohda and N. Hagihara, Tetrahedron Lett.,
1975, 4467–4470; (b) S. Takahashi, Y. Kuroyama and K. Sonogashira,
Synthesis, 1980, 627–630.
11 C. J. Barbe, F. Arendse, P. Comte, M. Jirousek, F. Lenzmann,
V. Shklover and M. Gra¨tzel, J. Am. Ceram. Soc., 1997, 80, 3157.
12 L. Luo, C.-F. Lo, C.-Y. Lin, I.-J. Chang and E. W.-G. Diau, J. Phys.
Chem. B, 2006, 110, 410–419.
13 M. Gouterman, J. Mol. Spectrosc., 1961, 6, 138–163.
14 H. Donker, R. B. M. Koehorst and T. J. Schaafsma, J. Phys. Chem. B,
2005, 109, 17031–17037.
15 M. Kasha, Radiat. Res., 1963, 20, 55–71.
16 (a) A. Osuka and K. Maruyama, J. Am. Chem. Soc., 1988, 110,
4454–4456; (b) N. C. Maiti, S. Mazumdar and N. Periasamy, J. Phys.
Chem. B, 1998, 102, 1528–1538.
17 (a) S. Jiang and M. Liu, J. Phys. Chem. B, 2004, 108, 2880–2884; (b)
H. Imahori, H. Norieda, Y. Nishimura, I. Yamazaki, K. Higuchi,
N. Kato, T. Motohiro, H. Yamada, K. Tamaki, M. Arimura and
Y. Sakata, J. Phys. Chem. B, 2000, 104, 1253–1260.
18 M. Nappa and J. S. Valentine, J. Am. Chem. Soc., 1978, 100,
5075–5080.
In summary, analysis of UV-visible spectra of ZnCA(PE)xBPP
indicates strongly that ZnCA(PE)xBPP are assembled on
surfaces of nanocrystalline TiO2 films in an H-type manner.
This finding is consistent with the AFM observation that
shows little change in the TiO2 surface mean roughness upon
porphyrin adsorption. Detailed results regarding the synthesis of
ZnCA(PE)xBPP (x = 2–3), UV-visible absorption, steady-state
and time-resolved fluorescence, and electrochemical properties of
ZnCA(PE)xBPP (x = 1–4) will be reported elsewhere.
This work is supported by the National Science Council of
Republic of China (project contracts NSC 94-2113-M-260-005 for
CYL and 94-2113-M-009-016, 94-2120-M-009-014 for EWGD).
19 Only a negligible proportion of ZnCA(PE)xBPP was dissolved back into
THF or pyridine during these experiments. The red shifts of the
ZnCA(PE)xBPP/TiO2 absorptions in pyridine are consistent with ref. 15.
1432 | Chem. Commun., 2006, 1430–1432
This journal is ß The Royal Society of Chemistry 2006