Langmuir 2008, 24, 5366-5374
5366
Zinc(II) Tetraarylporphyrins Anchored to TiO2, ZnO, and ZrO2
Nanoparticle Films through Rigid-Rod Linkers
Jonathan Rochford and Elena Galoppini*
Chemistry Department, Rutgers UniVersity, 73 Warren Street, Newark, New Jersey 07102
ReceiVed December 10, 2007. ReVised Manuscript ReceiVed February 14, 2008
A series of six Zn(II) tetraphenylporphyrins (ZnTPP), with a phenyl (P) or oligophenyleneethynylene (OPE ) (PE)n)
rigid-rod bridge varying in length (9-30 Å) and terminated with an isophthalic acid (Ipa) anchoring unit, were prepared
as model dyes for the study of sensitization processes on metal oxide semiconductor nanoparticle surfaces (MOn )
TiO2, ZnO, and insulating ZrO2). The dyes were designed such that the electronic properties of the central porphyrin
chromophore remained consistent throughout the series, with the rigid-rod anchoring unit allowing each porphyrin
unit to be located at a fixed distance from the metal oxide nanoparticle surface. Electronic communication between
the porphyrin and the rigid-rod unit was not desired. Rigid-rod porphyrins ZnTPP-Ipa, ZnTPP-P-Ipa, ZnTPP-PE-Ipa,
ZnTPP-(PE)2-Ipa, ZnTPP-(PE)3-Ipa, and ZnTMP-Ipa (with mesityl substituents on the porphyrin ring) were
synthesized using combinations of mixed aldehyde condensations and Pd-catalyzed cross-coupling reactions. Their
properties, in solution and bound, were compared with that of Zn(II) 5,10,15,20-tetra(4-carboxyphenyl)porphyrin
(p-ZnTCPP) as the reference compound. Solution UV–vis and steady-state fluorescence spectra for all six rigidrod-Ipa porphyrins were almost identical to each other and to that of p-ZnTCPP. Cyclic voltammetry and differential
pulse voltammetry scans of the methyl ester derivatives of the six rigid-rod-Ipa porphyrins, recorded in dichloromethane/
electrolyte, exhibited redox behavior typical of ZnTPP porphyrins, with the first oxidation in the range +0.99 to 1.09
V vs NHE. All six rigid-rod-Ipa porphyrins and p-ZnTCPP were bound to metal oxide (MOn ) TiO2, ZnO, and
insulating ZrO2) nanoparticle films. The Fourier transform infrared attenuated total reflectance spectra of all compounds
bound to MOn films showed a broad band at 1553-1560 cm-1 assigned to the V(CO2-) asymmetric stretching mode.
Splitting of the Soret band into two bands at 411 and 423 nm in the UV–vis spectra of the bound compounds, and
broadening and convergence of both fluorescence emission bands in the fluorescence spectra of the porphyrins bound
to insulating ZrO2 were also observed. Such changes were less evident for ZnTMP-Ipa, which has mesityl substituents
on the porphyrin ring to prevent aggregation. Steady-state fluorescence emission of rigid-rod-Ipa porphyrins bound
to TiO2 and ZnO through the longest bridges (>14 Å) showed residual fluorescence emission, while fluorescence
quenching was observed for the shortest compounds.
1. Introduction
The study of the molecule/MOn semiconductor boundary and
the characterization of heterogeneous electron transfer processes
at this interface are of great importance for solar energy
conversion.1,2 For instance, many groups worldwide are developing dye-sensitized solar cells (DSSCs) that utilize photoanodes
consisting of chromophores covalently anchored to TiO2 nanoparticle films.3 Other applications of similar materials include
heterogeneous photocatalytic systems4 and electrochromic devices.5 More recently, interest in the molecule/MO interface has
led to the development of model dyes attached onto the surface
* Corresponding author, [email protected].
(1) (a) Basic Research Needs for Solar Energy Utilization. The Department
of Energy Technical Reports, April 18–21, 2005. http://www.sc.doe.gov/bes/
reports/list.html. (b) Adams, D. M.; Brus, L.; Chidsey, C. E. D.; Creager, S.;
Creutz, C.; Kagan, C. R.; Kamat, P. V.; Lieberman, M.; Lindsay, S.; Marcus,
R. A.; Metzger, R. M.; Michel-Beyerle, M. E.; Miller, J. R.; Newton, M. D.;
Rolison, D. R.; Sankey, O.; Schanze, K. S.; Yardley, J.; Zhu, X. J. Phys. Chem.
B 2003, 107, 6668. (c) Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834. (d)
Armaroli, V.; Balzani, V. Angew. Chem., Int. Ed. 2007, 46, 52. (e) Koval, C. A.;
Howard, J. N. Chem. ReV. 1992, 92, 411. (f) Lewis, N. S. Inorg. Chem. 2005,
44, 6900.
(2) (a) Duncan, W. R.; Prezhdo, O. V. Annu. ReV. Phys. Chem. 2007, 58, 143.
(b) Persson, P.; Lundqvist, M. J.; Ernstorfer, R.; Goddard, W. A., III; Willig, F
J. Chem. Theory Comput. 2006, 2, 441. (c) Lundqvist, M. J.; Nilsing, M.; Lunell,
S.; Åkermark, B.; Persson, P. J. Phys. Chem. B 2006, 110, 20513.
(3) (a) Grätzel, M. Inorg. Chem. 2005, 44, 6841. (b) Kalyanasundaram, K.;
Grätzel, M. Coord. Chem. ReV. 1998, 177, 347. (c) Nozik, A. J. Annu. ReV. Phys.
Chem. 2001, 52, 193. (d) Meyer, G. J. Inorg. Chem. 2006, 45, 6852. (e) O’Reagan,
B.; Grätzel, M. Nature 1991, 353, 737.
(4) (a) Tachikawa, T.; Fujitsuka, M.; Majima, T. J. Phys. Chem. C 2007, 111,
5259. (b) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341. (c) Fujishima, A.;
Honda, K. Nature 1972, 238, 37.
of a semiconductor through a rigid linker.6,7 This design allows
control of properties such as distance and orientation of the dye
unit with respect to the metal oxide surface and is inspired to
determine the parameters that influence heterogeneous electron
transfer processes and solar cell efficiencies. Although Ru
polypyridine complexes and their analogues remain the most
frequently used dyes due to their chemical stability and broad,
tunable, metal-to-ligand charge-transfer absorption in the UV–vis
region, there is increasing interest in the development of organic
dyes, in particular porphyrins.8,9
Our interest in developing anchor-bridge-porphyrin model
compounds started with the electron transport study of DSSCs
prepared from MOCVD-grown ZnO “nanotip” films sensitized
with Zn(II) 5,10,15,20-tetra(3-carboxyphenyl)porphyrin (mZnTCPP).10 This study indicated that zinc tetraphenylporphyrins
are excellent dyes to study interfacial processes in ZnO nanotips
(5) (a) Cummins, D.; Boschloo, G.; Ryan, M.; Corr, D.; Rao, S. N.; Fitzmaurice,
D. J. Phys. Chem. B 2000, 104, 11449. (b) Biancardo, M.; Schwab, P. F. H.;
Argazzi, R.; Bignozzi, C. A. Inorg. Chem. 2003, 42, 3966. (c) Choi, S. Y.; Mamak,
M.; Coombs, N.; Chopra, N.; Ozin, G. A. Nano Lett. 2004, 4, 1231. (d)
Marguerettaz, X.; Fitzmaurice, D. J. Am. Chem. Soc. 1994, 116, 5017.
(6) Galoppini, E. Coord. Chem. ReV. 2004, 248, 1283.
(7) (a) Ernstorfer, R.; Gundlach, L.; Felber, S.; Storck, W.; Eichberger, R.;
Willig, F. J. Phys. Chem. B 2006, 110, 25383. (b) Gundlach, L.; Ernstorfer, R.;
Willig, F. J. Phys. Chem. C 2007, 111, 13586. (c) Myahkostupov, M.; Piotrowiak,
P.; Wang, D.; Galoppini, E J. Phys. Chem. C 2007, 111, 2827. (d) Taratula, O.;
Rochford, J.; Piotrowiak, P.; Galoppini, E.; Carlisle, R. A.; Meyer, G. J. J. Phys.
Chem. B 2006, 110, 15734. (e) Galoppini, E.; Guo, W.; Zhang, W.; Hoertz, P. G.;
Qu, P.; Meyer, G. J. J. Am. Chem. Soc. 2002, 124, 7801. (f) Wang, D.; Mendelsohn,
R.; Galoppini, E.; Hoertz, P. G.; Carlisle, R. A.; Meyer, G. J. J. Phys. Chem. B
2004, 108, 16642. (g) Galoppini, E.; Guo, W.; Qu, P.; Meyer, G. J. J. Am. Chem.
Soc. 2001, 123, 4342. (h) Kilså, K.; Mayo, E. I.; Kuciauskas, D.; Villahermosa,
R.; Lewis, N. S.; Winkler, J. R.; Gray, H. B. J. Phys. Chem. A 2003, 107, 3379.
10.1021/la703845u CCC: $40.75  2008 American Chemical Society
Published on Web 04/15/2008
Dyes Anchored to Nanoparticle Films
Langmuir, Vol. 24, No. 10, 2008 5367
Figure 1. Structure of the of rigid-rod-Ipa porphyrins 1–6 and reference compound p-ZnTCPP. Also shown are the anticipated binding geometries
on metal oxide surfaces. The cartoons show ideal behavior of the sensitizers on the metal oxide surface, but we must assume that other binding modes
are also possible.13
films. More recently, we developed a series of porphyrins designed
to bind “flat” and at an increasing distance from the surface of
(8) (a) Clifford, J. N.; Palomares, E.; Nazeeruddin, Md. K.; Grätzel, M.; Nelson,
J.; Li, X.; Long, N. J.; Durrant, J. R. J. Am. Chem. Soc. 2004, 126, 5225. (b)
Durrant, J. R.; Haque, S. A.; Palomares, E. Coord. Chem. ReV. 2004, 248, 1247.
(c) Campbell, W. M.; Burrell, A. K.; Officer, D. L.; Jolley, K. W. Coord. Chem.
ReV. 2004, 248, 1363. (d) Argazzi, R.; Murakami Iha, N. Y.; Zabri, H.; Odobel,
F.; Bignozzi, C. A Coord. Chem. ReV. 2004, 248, 1299. (e) Nazeeruddin, M. K.;
Humphry-Baker, R.; Officer, D. L.; Campbell, W. M.; Burrell, A. K.; Grätzel,
M. Langmuir 2004, 20, 6514. (f) Watson, D. F.; Marton, A.; Stux, A. M.; Meyer,
G. J. J. Phys. Chem. B 2004, 108, 11680. (g) Brune, A.; Jeong, G.; Liddell, P. A.;
Sotomura, T.; Moore, T. A.; Moore, A. L.; Gust, D. Langmuir 2004, 20, 8366.
(h) Jasieniak, J.; Johnston, M.; Waclawik, E. R. J. Phys. Chem. B 2004, 108,
12962. (i) Michaelis, E.; Nonomura, K.; Schlettwein, D.; Yoshida, T.; Minoura,
H.; Wöhrle, D. J. Porphyrins Phtalocyanines 2004, 12, 1366. (j) Schmidt-Mende,
L.; Campbell, W. M.; Wang, Q.; Jolley, K. W.; Officer, D. L.; Nazeeruddin,
M. K.; Grätzel, M. ChemPhysChem 2005, 6, 1253. (k) Hasobe, T.; Hattori, S.;
Kamat, P. V.; Urano, Y.; Umezawa, N.; Nagano, T.; Fukuzumi, S. Chem. Phys.
2005, 319, 243. (l) Wang, Q.; Campbell, W. M.; Bonfantani, E. E.; Jolley, K. W.;
Officer, D. L.; Walsh, P. J.; Gordon, K.; Humphry-Baker, R.; Nazeeruddin, M. K.;
Grätzel, M. J. Phys. Chem. B 2005, 109, 15397. (m) Hasobe, T.; Hattori, S.;
Kamat, P. V.; Fukuzumi, S. Tetrahedron 2006, 62, 1937. (n) Lo, C.-F.; Luo, L.;
Wei-Guang Diau, E.; Chang, I.-J.; Lin, C.-Y. Chem. Commun. 2006, 1430. (o)
Luo, L.; Lo, C.-F.; Lin, C.-Y.; Chang, I.-J.; Wei-Guang Diau, E. J. Phys. Chem.
B 2006, 110, 410. (p) Eu, S.; Hayashi, S.; Umeyama, T.; Oguro, A.; Kawasaki,
M.; Kadota, N.; Matano, Y.; Imahori, H. J. Phys. Chem. C 2007, 111, 3528. (q)
Huijser, A.; Marek, P. L.; Savenije, T. J.; Siebbeles, L. D. A.; Scherer, T.; Hauschild,
R.; Szmytkowski, J.; Kalt, H.; Hahn, H.; Balaban, T. S. J. Phys. Chem. C 2007,
111, 11726. (r) Campbell, W. M.; Jolley, K. W.; Wagner, P.; Wagner, K.; Walsh,
P. J.; Gordon, K. C.; Schmidt-Mende, L.; Nazeeruddin, M. K.; Wang, Q.; Grätzel,
M.; Officer, D. L. J. Phys. Chem. C 2007, 111, 11760. (s) Kira, A.; Tanaka, M.;
Umeyama, T.; Matano, Y.; Yoshimoto, N.; Zhang, Y.; Ye, S.; Lehtivuori, H.;
Tkachenko, N. V.; Lemmetyinen, H.; Imahori, H. J. Phys. Chem. C 2007, 111,
13618. (t) Balany, M. P.; Dipaling, C. V. P.; Lee, S. H.; Kim, D. H.; Lee, K. H.
Sol. Energy Mater. Sol. Cells 2007, 91, 1775. (u) Tanaka, M.; Hayashi, S.; Eu,
S.; Umeyama, T.; Matano, Y.; Imahori, H. Chem. Commun. 2007, 2069. (v)
Fungo, F.; Milanesio, M. E.; Durantino, E. N.; Otero, L.; Dittrich, T. J. Mater.
Chem. 2007, 17, 2107.
the metal oxide semiconductor.11 This study suggested that the
binding orientation as well as the distance from the semiconductor
surface influence the photophysical properties of the bound dyes
and their solar cell efficiencies.11 In an effort to expand and
complete the study of distance and orientation effects, we report
in this paper the synthesis and the photophysical and electrochemical properties of a second series of anchor-bridge-porphyrin model compounds, named here rigid-rod-Ipa porphyrins.
(9) (a) For the synthesis of “tripodal” porphyrins see: Loewe, R. S.; Ambroise,
A.; Muthukumaran, K.; Padmaja, K.; Lysenko, A. B.; Mathur, G.; Li, Q.; Bocian,
D. F.; Misra, V.; Lindsey, J. S. J. Org. Chem. 2004, 69, 1453. (b) Wei, L.;
Padmaja, K.; Youngblood, W. J.; Lysenko, A. B.; Lindsey, J. S.; Bocian, D. F.
J. Org. Chem. 2004, 69, 1461.
(10) Galoppini, E.; Rochford, J.; Chen, H.; Saraf, G.; Lu, Y.; Hagfeldt, A.;
Boschloo, G. J. Phys. Chem. B 2006, 110, 16159.
(11) Rochford, J.; Chu, D.; Hagfeldt, A.; Galoppini, E. J. Am. Chem. Soc.
2007, 129, 4655.
(12) While this paper was in preparation, the development of a tethered porphyrin
and chlorin that bind through a Ipa unit has been reported: Stromberg, J. R.;
Marton, A.; Kee, H. L.; Kirmaier, C.; Diers, J. R.; Muthiah, C.; Taniguchi, M.;
Lindsey, J. S.; Bocian, D. F.; Meyer, G. J.; Holten, D. J. Phys. Chem. C 2007,
111, 15464. (b) Muthiah, C.; Taniguchi, M.; Kim, H.-J.; Schmidt, I.; Kee, H. L.;
Holten, D.; Bocian, D. F.; Lindsey, J. S. Photochem. Photobiol. 2007, 83, 1513.
(13) The length of the sensitizer-anchoring unit is here defined as the distance
between the Zn(II) metal center and the midpoint between the two oxygen atoms
of the C-OH bond on the isophthalic acid unit, as shown in Figure 2. Due to
the unknown binding nature of each Ipa-rod linkers, i.e., at what angle(s) the
sensitizers bind with respect to the plane of the metal oxide surface, it is not
possible to ascertain their orientation at the semiconductor surface. Therefore the
length of the rigid-rod anchoring unit, which is here reported, may not correspond
to the effective sensitizer-semiconductor distance.
(14) The experimental procedure employed is described in the Supporting
Information of ref 11 and was based on the method reported by: Chemseddine,
A.; Moritz, T. Eur. J. Inorg. Chem. 1999, 235.
(15) Taratula, O.; Galoppini, E.; Wang, D.; Chu, D.; Zhang, Z.; Chen, H.;
Saraf, G.; Lu, Y. J. Phys. Chem. B 2006, 110, 6506.
5368 Langmuir, Vol. 24, No. 10, 2008
Rochford and Galoppini
Figure 2. Lengths of 1-6 and reference compound p-ZnTCPP. The length of the sensitizer-anchoring unit is here defined as the distance between
the Zn(II) metal center and the midpoint between the two oxygen atoms of the C-OH bond on the isophthalic acid unit for the Ipa rods, and the
distance between the Zn(II) metal center and the oxygens of the COOH group for p-ZnTCPP, as shown.13
These are rigid-rod porphyrins that bind through an isophthalic
acid anchoring group (Ipa) and have phenyl or oligophenyleneethynylene (OPE) bridges varying in length, Figure 1.12,13
The compounds were studied in solution and bound to MOn
films.
2. Experimental Section
Metal Oxide Film Preparation. Colloidal TiO2 and ZrO2 films
were prepared by a sol–gel technique that produces mesoporous
films approximately 10 µm thick and that consist of nanoparticles
with an average diameter of ∼20 nm.14 ZnO films were prepared
by a previously published method that produces mesoporous films
approximately 2 µm thick with an average particle size of 15 nm.15
Briefly, the TiO2 films were prepared by casting the colloidal paste
by the doctor blade technique onto the substrate over an area of 1
× 2 cm2, followed by sintering at ∼450 °C for 30 min. Lower
temperatures were used for sintering the ZnO films (∼300 to 350
°C). The colloidal paste was employed within 1 or 2 days and then
discarded. The films were stored in the air, but reheated (see below)
prior to binding. For absorption and fluorescence studies the films
were cast on cover glass slides (VWR). For electrochemical studies
the films were cast on transparent, conductive glass with a tin-doped
indium oxide layer (8 Ω/square inch ITO, Pilkington). We refer to
films cast on cover glass or ITO conductive glass as MOn/G or MOn/
ITO, respectively, where MOn is the metal oxide (TiO2, ZnO, or ZrO2).
Binding. Binding of the dye molecules to the TiO2, ZnO, and
ZrO2 films was accomplished by immersing the films in a 0.4 mM
MeOH solution of the carboxylic acid derivatives of the porphyrin
dyes (Ipa derivatives) at room temperature for 1 h. Basic or acidic
pretreatment of the metal oxide films was tested, but binding was
found to occur best on untreated films. All films, which were stored
in the air, were dried by heating to 150 °C for 30 min followed by
cooling to 80 °C prior to immersion in the dye solution. The sensitized
films were rinsed, immersed in MeOH to remove any physisorbed
dye, and then used for the measurements.
Spectroscopy. The Fourier transform infrared attenuated total
reflectance (FT-IR-ATR) spectra of all carboxylic acid derivatives
of the neat porphyrins (as powders) were compared to the spectra
of the porphyrins bound to TiO2 or ZnO. FT-ATR-IR spectra were
acquired on a Thermo Electron Corp. Nicolet 6700 FT-IR. UV–visible
absorption spectra were acquired at ambient temperature in MeOH
(Acros, spectroscopic grade) using a Varian Cary-500 spectrometer.
The sensitized MOn/G films were placed diagonally in a 1 cm2 quartz
cuvette in air. Steady-state fluorescence spectra were acquired on
a Spex Fluorolog that had been calibrated with a standard NIST
tungsten-halogen lamp. Sensitized MOn/G films were placed
diagonally in a 1 cm2 quartz cuvette. The excitation beam was directed
45° to the film surface, and the emitted light was monitored from
the front face of the sample and from a 90° angle in the case of fluid
solutions. Fluorescence quantum yields for the samples (ΦFl) were
calculated in MeOH solutions deareated by freeze–pump–thaw
technique at λexc ) 565 nm, using the optically dilute technique with
Zn(II) 5,10,15,20-tetraphenylporphyrin (ZnTPP) as the actinometer
(Φref ) 0.033),16 according to
ΦFl ) (Aref ⁄ As)(Is ⁄ Iref)(ηs ⁄ ηref)2Φref
(1)
The subscript “s” refers to the sample, the subscript “ref” to the
reference sample (in this case ZnTPP), A is the absorbance at the
excitation wavelength, I is the integrated emission area, and η is
the solvent refraction index.
Electrochemistry. Cyclic voltammetry was performed with a
BAS potentiostat under nitrogen at room temperature with a
conventional three-electrode configuration at a scan rate of 100 mV
(16) Strachan, J. P.; Gentemann, S.; Seth, J.; Kalsbeck, W. A.; Lindsey, J. S.;
Holten, D.; Bocian, D. F. J. Am. Chem. Soc. 1997, 119, 11191.
Langmuir, Vol. 24, No. 10, 2008 5369
Dyes Anchored to Nanoparticle Films
Scheme 1. General Scheme for the Synthesis of 3–6a
a
Reagents and conditions: (i) Pd(dba)2/K2CO3/THF:H2O (2:1)/70 °C/12 h; 42% yield. (ii) Pd2(dba)3/AsPh3/NEt3/THF/ 60 °C/12 h; 28–40%.
s-1. The electrochemical properties of the methyl ester derivatives
of the dyes (Ipe derivatives) were studied in dichloromethane with
0.1 M Bu4NBF4 supporting electrolyte. A glassy-carbon working
electrode (2 mm diameter) was used along with a platinum wire
auxiliary electrode and a Ag/AgCl reference electrode. When using
a porphyrin/MO/ITO film as the working electrode, acetonitrile was
used with a 0.1 M Bu4NClO4 supporting electrolyte. The potential
of the Ag/AgCl electrode was calibrated externally versus the
ferrocene/ferrocenium (Fc+/Fc) redox couple. The half-wave redox
potentials (E1/2) were determined as (Epa + Epc)/2, where Epa and
Epc are the anodic and cathodic peak potentials, respectively. It was
assumed that E1/2 = E0′for all measurements. All potentials were
referenced to the normal hydrogen electrode (NHE) to make a
comparison with the conduction bands of TiO2 and ZnO. Where E1/2
could not be calculated, due to irreversible oxidation or reduction
processes, Epa and Epc were reported, respectively. All redox processes
were also studied by differential pulse voltammetry (DPV), using
the relationship
E1⁄2 ) Emax + ∆E ⁄ 2
(2)
where Emax is the peak maxima in the DPV scan and ∆E is the pulse
amplitude.17 DPV experiments were carried out at a scan rate of 20
mV s-1, pulse amplitude 50 mV, pulse width 50 mV, and pulse
period 0.2 s. In some cases, namely, the third reduction process of
some of the porphyrin dyes, the differential pulse voltammetry data
are reported, due to poor quality cyclic voltammetry scans at such
negative potentials.
The excited-state oxidation potentials E1/2(P+/P*) of all porphyrins
in MeOH and bound to TiO2 and ZnO surfaces were calculated from
the relationship
E1⁄2(P+ ⁄ P*) ) E1⁄2(P+ ⁄ P) - E0-0
(3)
(P+/P)
where E1/2
is equivalent to the first oxidation potential of the
ground-state porphyrin chromophore and E0–0 is the zero-zero
(17) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and
Applications, 2nd ed.; Wiley: New York, 2001.
(18) Kuciauskas, D.; Monat, J. E.; Villahermosa, R.; Gray, H. B.; Lewis, N. S.;
McCusker, J. K. J. Phys. Chem. B 2002, 106, 9347.
excitation energy.18 E0–0 was calculated from the intersection of the
porphyrin absorption spectrum with the fluorescence emission
spectrum in MeOH at normalized absorption/emission intensity.
For the solid-state measurements E0–0 was similarly calculated from
the UV–vis and fluorescence measurements made on the derivatized
MOn/G and ZrO2/G films, respectively. E1/2(P+/P*) values were
calculated from the solid-state cyclic voltammetry, UV–vis, and
fluorescence data to give a closer approximation of E1/2(P+/P*) in
a solid-state device as possible.
3. Results and Discussion
Synthesis. The synthesis of the rigid-rod-Ipa porphyrins is
described in the Supporting Information. Briefly, the dimethylisophthalate esters (Ipe) of porphyrins ZnTMP-Ipe (1) and
ZnTPP-Ipe (2) where both prepared by a mixed aldehyde
condensation of dimethyl-5-formylisophthalate with mesitaldehyde and benzaldehyde, respectively, following reported methods
(Scheme S3).19,20 To prepare all of the extended rigid-rod-Ipa
porphyrin systems, we employed a Pd-catalyzed cross-coupling
reaction21 between Zn(II) 5-(4-iodophenyl)-10,15,20-triphenylporphyrin and the rigid-rod-Ipa component, as illustrated in
the general scheme below (for details see Supporting Information).
FT-IR-ATR. Neat Rigid-Rod-Ipa Porphyrins. All solid rigidrod-Ipa porphyrin samples (Figure 3 and Figures S1, S2, S3,
S4, S5 in Supporting Information) showed spectra typical of the
meso-tetraphenylporphyrin macrocycle, with symmetric and
asymmetric stretching bands of the pyrrole ring (ν(C-H),
ν(CdC), and ν(CdN)) in the range 700–1500 cm-1.22 All rigidrod-Ipa porphyrin acids here studied showed a strong band in
(19) Lindsey, J. S.; Schreiman, I. C.; Hsu, H. C.; Kearney, P.; Marguerettaz,
A. M. J. Org. Chem. 1987, 52, 827.
(20) Lindsey, J. S. In The Porphyrin Handbook; Kadish K. M., Smith K. M.,
Guilard R., Eds.; Academic Press: Boston, 2000; Vol. 1, Chapter 2.
(21) Wagner, R. W.; Ciringh, Y.; Clausen, C.; Lindsey, J. S. Chem. Mater.
1999, 11, 2974.
(22) (a) Thomas, D. W.; Martell, A. E. J. Am. Chem. Soc. 1959, 81, 5111. (b)
Datta-Gupta, N.; Bardos, T. J. J. Heterocycl. Chem. 1966, 3, 495. (c) Longo, F. R.;
Finarelli, M. G.; Kim, J. B. J. Heterocycl. Chem 1969, 6, 927.
5370 Langmuir, Vol. 24, No. 10, 2008
Rochford and Galoppini
Figure 4. Solution UV–vis absorption spectra of p-ZnTCPP and the
rigid-rod-Ipa porphyrins recorded in MeOH.
Figure 3. FT-IR-ATR spectra of ZnTPP-(PE)3-Ipa (6) as the neat solid
(bottom) and adsorbed on TiO2 and ZnO (middle and top, respectively).
The broad bands observed at ∼3000 cm-1 for the TiO2 and ZnO samples
are assigned to adsorbed moisture on the films. Also shown are the
possible binding modes of the carboxylate group to TiO2 and ZnO.
the region of 1687–1700 cm-1 due to the ν(CdO) stretch and
a broad band at 1262–1286 cm-1 due to the ν(C-O) stretch of
the carboxylic acid groups. Weak acetylenic stretching bands
were observed in the region of 2210–2213 cm-1 in the spectra
of 4, 5, and 6. The spectrum of porphyrin 1 exhibited additional
bands in the region 2850–2970 cm-1, which were assigned to
the C-H stretches of the peripheral mesityl substituents.
Bound to TiO2and ZnO. All rigid-rod-Ipa porphyrins here
studied did bind strongly to TiO2, ZnO, and ZrO2 nanoparticle
films. The nature of the binding interaction between the rigidrod-Ipa porphyrins and the metal oxide surfaces was characterized by comparing changes in the region of the carbonyl group
in the FT-IR-ATR spectra, Figure 3. Upon binding to TiO2 the
ν(CdO) and ν(C-O) stretching modes of the isophthalic acid
group disappeared, with the appearance of a broad shoulder on
the high-frequency side of the phenyl breathing mode (1600–1750
cm-1). New bands were observed in the regions 1553-1560 cm
and 1402-1406 cm-1. These bands are assigned to the ν(CO2-)
asymmetric and symmetric stretching modes respectively corresponding to the TiO2-bound isophthalic moiety of the rigidrod-Ipa porphyrins in accordance with previously reported TiO2bound Ipa systems.7d,f Similar changes were observed upon
binding to ZnO nanoparticle films (Figure 3, top). Although
these spectral changes were in good agreement with previously
reported ZnO-bound Ipa,15 the poor quality of the spectra makes
the interpretation and assignments of the ZnO-bound compounds
difficult. Overall, the spectral changes, as well as the differences
between the symmetric and asymmetric bands of the COOH
group upon binding to TiO2 (∆ν ∼ 240 cm-1), suggests a
prevalence of chelating and/or bidentate binding modes23
compared to an ester-type binding for all rigid-rod-Ipa porphyrins/MOn (Figure 3). The possibility that only one of the
COOH groups is binding to the metal oxide and the other remains
unbound cannot be ruled out. Computational studies aimed to
determine whether an isophthalic acid binding group can bind
both acid functionalities are in progress. The weak symmetric
stretch of the acetylenic units for porphyrins 4and 5 was still
evident on TiO2 and ZnO but was not observable in the spectra
of 6 on TiO2 and ZnO.
UV–vis Absorption and Fluorescence Emission Spectra.
Solution. Table 1 summarizes the solution absorption and emission
data for the rigid-rod-Ipa porphyrins in MeOH solutions. The
UV–vis ground-state absorption and fluorescence emission
spectra in fluid solutions of all rigid-rod-Ipa porphyrins were almost identical to that of the reference compound
p-ZnTCPP. All solution UV–vis absorption spectra exhibited
features typical of the p-ZnTCPP chromophore with a Soret
absorption band between 422 and 424 nm and Q bands in the
556-558 nm and 596-597 nm regions, respectively, Figure 4.24
Porphyrins 3,4, 5, and 6 each showed an additional high-energy
absorption in the 300–350 nm region, consistent with the π-π*
absorptions typical of the phenyl or OPE rigid-rod linkers.25 The
slightly lower extinction coefficient for p-ZnTCPP compared to
the Ipa-derivatized porphyrins (Table 1) had been reported
before.26 The fluorescence spectra were also typical of the
p-ZnTCPP chromophoric group, with two emission maxima
observed at 602-604 nm and 654-656 nm, from the Q(0,0)*
and Q(1,0)* excited states, respectively. Quantum yields ranged
from 0.027 to 0.038 for the rigid-rod-Ipa porphyrins (Table 1),
with p-ZnTCPP having a slightly lower quantum yield (0.026).
It can be concluded from the convergence of the λmax observed
for all porphyrins in the UV and fluorescence spectra that the
phenyl or OPE rigid-rod substituents have little or no influence
on the electronic and spectroscopic properties of the porphyrin
ring. Neither the number of carboxy groups (p-ZnTCPP vs rigidrod-Ipa porphyrins) nor the length of the phenyl or OPE spacer
(23) (a) Deacon, G. B.; Phillips, R. J. Coord. Chem. ReV. 1980, 33, 227. (b)
Finnie, K. S.; Bartlett, J. R.; Woolfrey, J. L. Langmuir 1998, 14, 2744. (c) Vittadini,
A.; Selloni, A.; Rotzinger, F. P.; Grätzel, M. J. Phys. Chem. B 2000, 104, 1300.
(d) Nazeeruddin, M. K.; Humphrey-Baker, R.; Liska, P.; Grätzel, M. J. Phys.
Chem. B 2003, 107, 8981.
(24) In the classical nomenclature of porphyrin absorption spectra the “Soret”
band corresponds to the S0 f S2 transition, whereas the Q bands, Q(1,0) and
Q(0,0), correspond to the first and lowest vibrational modes of the S0 f S1
transition, respectively.
(25) Sudeep, P. K.; James, P. V.; Thomas, K. G.; Kamat, P. V. J. Phys. Chem.
A 2006, 110, 5642.
(26) Finikova, O. S.; Chernov, S. Y.; Cheprakov, A. V.; Filatov, M. A.;
Vinogradov, S. A.; Beletskaya, I. P. Dokl. Chem. 2003, 391, 222.
Langmuir, Vol. 24, No. 10, 2008 5371
Dyes Anchored to Nanoparticle Films
Figure 5. UV–vis absorption spectra of reference compound p-ZnTCPP
(solid red line) and the rigid-rod-Ipa porphyrins bound to ZnO films.
between the porphyrin ring and the Ipa groups caused any
significant shift in their absorption or emission maxima. These
similarities may be attributed to the lack of electronic communication between the aryl substituents and the central porphyrin
ring itself and are consistent with what we had observed in our
previous studies of tetrachelate p-ZnTPP systems.11
In this respect, the rigid-rod porphyrins here studied represent
ideal models to determine the effect of the sensitizer distance13 on
excited-state electron transfer processes from thep-ZnTPP porphyrin
chromophore to nanocrystalline TiO2 and ZnO semiconductors.
Figure 4 shows an overlay of the UV–vis spectra of all rigidrod-Ipa porphyrins in MeOH. An overlay of the fluorescence
spectra, also recorded in MeOH, is shown in Figure S-6.
Bound to TiO2, ZnO and ZrO2 Films. The UV–vis absorption
spectra of p-ZnTCPP and the rigid-rod-Ipa porphyrins bound
to ZnO are shown in Figure 5 (for TiO2 see Figure S-7). One
notable feature in the spectrum of p-ZnTCPP is the splitting of
the Soret band into two bands at 411 and 423 nm. This
phenomenon is due to exciton coupling between adjacent
porphyrin chromophores at the ZnO surface as a result of
H-aggregate formation (face-to-face stacking). Such behavior
has been observed previously and was described in our recent
study11 of p-ZnTCPP in comparison to a number of tetrachelate
systems.27 All Soret maxima of the rigid-rod-Ipa porphyrins
were not observable on the 10 µm thick TiO2 film due to a high
optical density of sensitizer (Figure S-7). H-aggregate formation
was evident, however, for all sensitizers (p-ZnTCPP and rigidrod-Ipa porphyrins) on the thinner ∼2 µm thick ZnO films
(Figure 5). As expected, aggregation was largely decreased for
the mesityl-substituted porphyrin ZnTMP-Ipa, which only showed
a slight shoulder on the high energy side of its Soret absorption.
Almost identical UV–vis spectra were observed for all
porphyrins on ZrO2 and TiO2 (Figure S-7 and S-8 in Supporting
Information). Because of the wider bandgap, ZrO2 behaves as
an insulator precluding electron injection from the lowest excited
state of all porphyrins studied here (Ebg ∼ 5 eV for ZrO2 compared
Figure 6. Fluorescence emission spectra of p-ZnTCPP and the rigidrod-Ipa porphyrins bound to ZrO2 films.
Figure 7. Residual fluorescence emission observed for porphyrins
ZnTPP-P-Ipa, ZnTPP-PE-Ipa, ZnTPP-(PE)2-Ipa, and ZnTPP(PE)3-Ipa bound toZnO films. Spectra of p-ZnTCPP, ZnTMP-Ipa and
ZnTPP-Ipa are also included for comparison.
to ∼3 eV for TiO2 and ZnO).3b Since the fluorescence emission
is not quenched, ZrO2 films are an excellent substrate to study
the fluorescence emission of dyes bound to a surface that closely
resembles the morphology of TiO2 films. The fluorescence spectra
of p-ZnTCPP and the rigid-rod-Ipa porphyrins bound to ZrO2,
all showed broadening and convergence of both their fluorescence
bands (Figure 6).
This behavior is consistent with H-aggregate formation of the
porphyrin chromophores on the ZrO2 surface (as observed in the
TiO2 and ZnO UV–vis spectra) and once more correlate well
with our previous study comparing p-ZnTCPP to a number of
tetrachelate systems.11 Aggregation effects were again less
pronounced for the mesityl-substituted porphyrin ZnTMP-Ipa
which showed the highest resolution between its Q(0,0)* and
Q(1,0)* emission bands. Interestingly, residual fluorescence was
observed for some of the rigid-rod-Ipa porphyrins on the TiO2
and ZnO semiconductors (Figure S-9 and Figure 7, respectively).
It should be emphasized here that although strong quenching of
Table 1. Solution UV-vis Absorption and Fluorescence Emission Data of Rigid-Rod-Ipa Porphyrins in MeOH
UV–vis absorption
porphyrin
p-ZnTCPP
ZnTMP-Ipa (1)
ZnTPP-Ipa (2)
ZnTPP-P-Ipa (3)
ZnTPP-PE-Ipa (4)
ZnTPP-(PE)2-Ipa (5)
ZnTPP-(PE)3-Ipa (6)
Soret λmax, nm
(! × 105, M-1 L-1)
424 (2.78)
424 (5.49)
422 (5.66)
422 (5.27)
422 (5.34)
423 (5.36)
423 (5.38)
Q(1,0)λmax, nm
(! × 104, M-1 L-1)
557 (1.39)
558 (1.72)
556 (1.64)
556 (1.47)
557 (1.55)
557 (1.82)
557 (1.79)
Q(0,0)λmax, nm
(! × 103, M-1 L-1)
597 (4.59)
597 (5.05)
596 (6.12)
596 (6.33)
596 (6.79)
597 (8.39)
597 (8.50)
fluorescence Q(0,0)*,
Q(1,0)*λmax, nm (Φ)
602, 654 (0.023)
602, 654 (0.032)
602, 656 (0.034)
604, 656 (0.035)
604, 656 (0.038)
604, 656 (0.031)
605, 654 (0.027)
5372 Langmuir, Vol. 24, No. 10, 2008
Figure 8. Plot of “emission quantum efficiency” vs porphyrin rod lengths
for both the Q(0,0)* and Q(1,0)* emission bands in the fluorescence
spectra of ZnTPP-P-Ipa, ZnTPP-PE-Ipa, ZnTPP-(PE)2-Ipa, and
ZnTPP-(PE)3-Ipa bound to ZnO. Higher emission quantum efficiencies
correspond to lesseffective fluorescence quenching of the porphyrin
excited state by the semiconductor.
the static fluorescence of these porphyrin systems bound to TiO2
and ZnO is indicative of electron injection into the conduction
band of the semiconductors, it is not direct evidence. Ultrafast
time-resolved fluorescence and IPCE measurements are currently
in progress. It is assumed from this point on, however, in
accordance with previous results from this laboratory10,11 and
other literature3b,8 that the observed quenching of the porphyrin
static fluorescence on TiO2 and ZnO is due to electron injection
into the conduction band of the semiconductors. The fact that
quenching was only observed on TiO2 and ZnO semiconductors
and not on the ZrO2 insulator substantiates this assumption.
It appears that, as the porphyrin chromophore becomes more
distant from the semiconductor surface, due to the increasing
length of the rigid-rod linker,12 charge injection from its
photoexcited state to the metal oxide conductance band
becomes less efficient. Such behavior was not observed
previously for the planar tetrachelate systems, 11 which showed
no residual fluorescence emission when bound to either TiO2or
ZnO nanoparticle films. In fact, the planar orientation of
these tetrachelate systems showed highly improved chargeinjection characteristics in comparison to the vertically
assembled p-ZnTCPP, even for the most distant sensitizer
(ca. 9 Å).11 Residual fluorescence emission on the semiconductor TiO2 and ZnO surfaces was here observed only from
the rigid-rod-Ipa porphyrins with the longer bridges, i.e.,
ZnTPP-P-Ipa, ZnTPP-PE-Ipa, ZnTPP-(PE)2-Ipa, and
ZnTPP-(PE)3-Ipa, having estimated distances of 14, 16, 23,
and 30 Å, respectively (Figure 2).12
Also, the growth of the Q(0,0)* emission band with the linker’s
length (and, therefore, distance from the surface) was less efficient
than that for the Q(1,0)* emission band of these rigid-rod-Ipa
porphyrins. A possible explanation for this observation is that a
more efficient charge injection occurs from the Q(0,0)* excited
state than for the Q(1,0)* excited state for the Zn(II) porphyrin
chromophores 10–30 Å long. This is more evident in Figures 8-10
and Supplementary Figure S-10, where is given a plot of the
‘emission quantum efficiency’ of both the Q(0,0)* and Q(1,0)*
emission bands vs the length of porphyrins ZnTPP-P-Ipa,
ZnTPP-PE-Ipa, ZnTPP-(PE)2-Ipa, and ZnTPP-(PE)3-Ipa
bound to ZnO (TiO2 in Supplementary Figure S-10).28 These
results provide evidence the complexity of heterogeneous electron
(27) (a) Aratani, N.; Osuka, N.; Cho, H. S.; Kim, D. J. Photochem. Photobiol.,
C 2002, 3, 25. (b) Lo, C.-F.; Luo, L.; Diau, E. W.-G.; Chang, I.-J.; Lin, C. Y.
Chem. Commun. 2006, 1430.
Rochford and Galoppini
Figure 9. Solution cyclic voltammogram (top, 0.1 M Bu4NBF4, CH2Cl2,
scan rate 100 mV s-1) and differential pulse voltammogram (bottom,
0.1 M Bu4NBF4, CH2Cl2, scan rate 20 mV s-1; pulse amplitude, 50 mV;
pulse width, 50 mV; pulse period, 0.2 s) scans of ZnTPP-(PE)3-Ipa.
Figure 10. Cyclic voltammogram scans of ZnTPP-(PE)3-Ipa bound
to TiO2/ITO (top) and ZnO/ITO electrodes (bottom) (0.1 M Bu4NClO4,
CH3CN, scan rate 100 mV s-1).
transfer at the dye-semiconductor interface, i.e., varying chargeinjection efficiency from the same electronic excited state of
the porphyrin chromophore was observed depending on (i) the
vibrational states involved, i.e., Q(0,0) or Q(1,0), and (ii) the
porphyrin rigid-rod’s length.
Interestingly, the “shorter” (<10 Å) porphyrins p-ZnTCPP,
ZnTMP-Ipa, and ZnTPP-Ipa showed no residual emission both
on TiO2 and on ZnO films, indicating efficient charge injection
from their excited states into the conduction bands of TiO2 or
ZnO over these distances.12 Although we cannot be certain
whether the linkers bind perpendicularly or at an angle(s) with
respect to the semiconductor surface, prior injection kinetics
studies of Ru-polypyridyl-Ipa rods bound to TiO2 suggest that
the longer the bridge, the longer the distance from the metal
oxide surface.7c
Electrochemical Properties. Solution. Redox potentials vs
NHE of the methylester derivatives (Ipe) of the rigid-rod
porphyrins recorded in CH2Cl2 are listed in Table 2. The methyl
ester derivatives were employed for all compounds, since a
common organic solvent in which all the isophthalic acid
derivatives were soluble, and which was suitable for electrochemical measurements, could not be found.
All rigid-rod-Ipe porphyrins exhibited redox behavior typical
of ZnIITPP porphyrins. Formation of the porphyrin radical cation
was observed over the range +0.99 to 1.10 V vs NHE. The dication
was observed over the range +1.38 to +1.47 V vs NHE. In the
negative potential region three reversible/quasi-reversible reduction
(28) The Q(0,0)* and Q(1,0)* emission peaks were resolved using OriginLab
and integrated to give the “emission quantum efficiency”. The peak of greatest
area, i.e., the Q(1,0)* emission peak of ZnTPP-(PE)2-Ipa, was taken as the
reference and used to calculate the relative efficiency for all other emission peaks.
A similar plot was observed for TiO2 (Supporting Information, Figure S-10).
Langmuir, Vol. 24, No. 10, 2008 5373
Dyes Anchored to Nanoparticle Films
Table 2. Solution Redox Potentials of Porphyrin Methyl Esters in Dichloromethane Reported vs NHE (V)a
oxidation (V)
reduction (V)
porphyrin
1st
2nd
1st
2ndb
3rdb
E0–0 (eV)
E1/2(P+/P*) (eV)
p-ZnTCPP ester
ZnTMP-Ipe (1 ester)
ZnTPP-Ipe (2 ester)
ZnTPP-P-Ipe (3 ester)
ZnTPP-PE-Ipe (4 ester)
ZnTPP-(PE)2-Ipe (5 ester)
ZnTPP-(PE)-Ipe (6 ester)
1.10
0.99
1.08
1.09
1.08
1.09
1.09
1.47
1.41
1.39
1.40
1.39
1.38
1.38
-1.13
-1.44
-1.10
-1.10
-1.11
-1.12
-1.10
-1.46
-1.65
-1.47
-1.42
-1.36
-1.36
-1.30
-1.65
2.06
2.07
2.07
2.07
2.07
2.07
2.07
-0.96
-1.08
-1.01
-1.02
-1.01
-1.02
-1.02
-1.50
-1.51
-1.51
a
All cyclic voltammograms were measured at a scan rate of 100 mV s-1 using a glassy carbon working electrode. b Determined from differential pulse
voltammetry experiments.
Table 3. Redox Potentials of p-ZnTCPP and Rigid-Rod-Ipa Porphyrins Bound to TiO2/ITO and ZnO/ITO Films vs NHE (V)a
TiO2/ITO (V)
porphyrin
p-ZnTCPP
ZnTMP-Ipa (1)
ZnTPP-Ipa (2)
ZnTPP-P-Ipa (3)
ZnTPP-PE-Ipa (4)
ZnTPP-(PE)2-Ipa (5)
ZnTPP-(PE)3-Ipa (6)
a
1st
1.09
1.04
1.07
1.05
1.04
1.07
1.06
2nd
b
1.38
1.36
1.37
1.36
1.38
1.38
1.37
ZnO/ITO (V)
E0–0 (eV)
E1/2(P+/P*) (eV)
1.99
2.00
2.00
2.00
2.00
2.00
2.00
-0.90
-0.96
-0.93
-0.95
-0.96
-0.93
-0.94
1st
1.07
1.04
1.06
1.04
1.05
1.06
1.06
2nd
E0–0 (eV)
E1/2(P+/P*) (eV)
2.00
1.99
2.00
2.00
2.00
2.00
2.00
-0.93
-0.96
-0.94
-0.96
-0.95
-0.94
-0.94
b
1.43
1.36
1.35
1.37
1.36
1.36
1.37
All samples were run at a scan rate of 100 mV s-1 using the sensitized MOn/ITO films as the working electrode.
processes were observed over the range-1.10 to -1.65 V vs NHE.29
Cyclic voltammetry and differential pulse voltammetry scans for
ZnTPP-(PE)3-Ipa (6) are shown in Figure 9.
Bound to TiO2 and ZnO. A summary of the electrochemical
behavior for the rigid-rod-Ipa porphyrins bound to TiO2/ITO
and ZnO/ITO mesoporous thin films is shown in Table 3.
Analogous to the solution studies, a reversible first oxidation
was observed in the range +1.04 to +1.07 V vs NHE for the
rigid-rod-Ipa porphyrins on the TiO2and ZnO surfaces, due to
the formation of the porphyrin radical cation species. Formation
of the dication species was observed in the range +1.36 to +
1.43 V vs NHE and is quasi-reversible.
Cyclic voltammetry scans for ZnTPP-(PE)3-Ipa bound to
TiO2/ITO and ZnO/ITO working electrodes are shown in Figure
10. The oxidation of sensitizers bound to wide band gap
semiconductors, such as TiO2, has been explained in terms of
an electron–hole transfer process which permeates through the
adsorbed layer of dye lateral to the surface of the nanocrystalline
semiconductor, the latter acting as an inert support at potentials
positive of its conduction band.30 The decreased current observed
for the second oxidation process relative to the first redox couple
could possibly be explained by a reduced efficiency of charge
transport through the already oxidized layer of porphyrin on the
nanocrystalline surface. Desorption of the dye following formation
of the radical cation may also be involved, in fact slight
discoloration of the electrolyte solution was observed during
each of these experiments. The excited-state oxidation potentials,
E1/2(P+/P*), of the rigid-rod-Ipa porphyrins bound to TiO2 and
ZnO lie in the range -0.93 to -0.96 V vs NHE. This value is
quite negative with respect to the conduction bands positions of
both the TiO231 and ZnO32 semiconductors, therefore facilitating
a thermodynamically favored electron transfer to the lower-lying
conduction band of the semiconductors. Such consistency in the
electronic properties of this series of rigid-rod sensitizers again
(29) Wolberg, A.; Manassen, J. J. Am. Chem. Soc. 1970, 92, 2982.
(30) Bonhôte, P.; Gogniat, E.; Tingry, S.; Barbé, C.; Vlachopoulos, N.;
Lenzmann, F.; Comte, P.; Grätzel, M. J. Phys. Chem. B 1998, 102, 1498.
(31) Liu, G.; Jaegermann, W.; He, J.; Sundström, V.; Sun, L. J. Phys. Chem.
B 2002, 106, 5814.
(32) Redmond, R.; O’Keeffe, A.; Burgess, C.; MacHale, C.; Fitzmaurice, D.
J. Phys. Chem. 1997, 97, 11081.
b
Anodic peak maxima (Epa).
suggests that the rigid-rod-Ipa porphyrins could be useful models
for studying the effect of the linker length (and therefore the
chromophore’s distance) on charge transfer at the dyesemiconductor interface.12
4. Conclusions
A series of rigid-rod-Ipa porphyrins, made of Zn(II) tetraphenylporphyrin (ZnTPP) chromophores, with a phenyl or
oligophenyleneethynylene rigid-rod bridge varying in length,
and terminated with an Ipa anchoring unit, were studied in solution
and bound to metal oxide (TiO2, ZnO, and ZrO2) nanoparticle
films. Zn(II) porphyrin p-ZnTCPP was used as the reference
compound. The series was designed to determine the effect of
the linkers’ length (9-30 Å)12 on their photoelectrochemical
and photophysical properties and to make comparisons with a
series of previously studied porphyrins11 that bind flat on the
surface of the semiconductor. Solution UV–vis and fluorescence
spectra, and redox properties for all model compounds suggested
that the electronic structure of the porphyrin chromophoric unit
is mostly unchanged by the linkers’ structure and length. In this
respect, the rigid-rod porphyrins here studied are good models
to determine the effect of the sensitizer distance13 on excitedstate electron transfer processes. Electrochemical studies were
performed in solution and bound to MOn/ITO films. The methyl
ester (Ipe) derivatives, which were soluble in dichloromethane/
electrolyte, were employed for solution electrochemical studies.
The excited-state oxidation potentials, E1/2(P+/P*), of the rigidrod-Ipa porphyrins bound to TiO2 and ZnO lie in the range
-0.93 to -0.96 V vs NHE. This value is quite negative with
respect to the conduction band positions of both the TiO231 and
ZnO32 semiconductors. The surface bonds of the Ipa unit,
characterized by FT-IR-ATR, occurred through bidentate binding
modes of the carboxylic groups. The rigid-rod-Ipa porphyrin
molecules formed H-aggregates on the nanoparticle surfaces upon
binding, as suggested in changes in the UV–vis spectra of the
compounds bound to TiO2, ZnO, and ZrO2 and by the merging
and broadening of bands in the fluorescence emission spectra on
insulating ZrO2. Aggregation effects were less evident for
ZnTMP-Ipa that contained methyl groups designed to prevent
face-to-face stacking. Residual fluorescence emission was
5374 Langmuir, Vol. 24, No. 10, 2008
observed for the longest model compounds (10–30 Å) on TiO2
and ZnO films, suggesting a reduced charge injection efficiency
with increasing distance from the semiconductor surface. Timeresolved injection and recombination studies for the rigid-rod-Ipa
porphyrins studied here as well as of the previously reported
porphyrins that bind “flat” to the surface11 are in progress.
Acknowledgment. E.G. is grateful to DOE (DE-FG0201ER15256) for funding this research and to Professor R.
Mendelsohn for generously making available IR instrumentation,
Rochford and Galoppini
E.G. and J.R. thank the Rutgers University Research Council for
travel support.
Supporting Information Available: Synthesis of all porphyrins
here studied, FT-IR-ATR spectra of 1, 2, 3, 4, and 5, neat, bound to
TiO2, and bound to ZnO, UV–vis absorption spectra on TiO2 and ZrO2,
fluorescence emission spectra in solution and bound to TiO2, plot of
“emission quantum efficiency” vs porphyrin rods length of 3, 4, 5, and
6 bound to TiO2, cyclic voltammetry of 1, 2, 3, 4, and 5 bound to TiO2
and bound to ZnO, and complete ref . This information is available free
of charge via the Internet at http://pubs.acs.org.
LA703845U