Photoinduced Charge and Energy Transfer in Phthalocyanine

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Photoinduced Charge and Energy Transfer in Phthalocyanine-Functionalized Gold Nanoparticles Anne Kotiaho,* Riikka Lahtinen, Alexander Efimov, Hanna-Kaisa Metsberg, Essi Sariola, Heli Lehtivuori, Nikolai V. Tkachenko, and Helge Lemmetyinen Department of Chemistry and Bioengineering, Tampere UniVersity of Technology, P.O. Box 541, 33101 Tampere, Finland ReceiVed: September 9, 2009; ReVised Manuscript ReceiVed: NoVember 2, 2009

Photoinduced processes in phthalocyanine-functionalized gold nanoparticles (Pc-AuNPs) have been investigated by spectroscopic measurements. The metal-free phthalocyanines used have two linkers with thioacetate groups for bonding to the gold nanoparticle surface, and the attachment was achieved using a ligand exchange reaction. The absorption spectrum of the Pc-AuNPs shows a broadening of the phthalocyanine Q-band absorption, probably due to a tight packing of the phthalocyanines on the gold nanoparticle surface. For the attached phthalocyanines, fluorescence is strongly quenched, and the fluorescence lifetimes determined by time-correlated single photon counting (TCSPC) are strongly reduced. The quenching mechanisms were studied in detail with time-resolved absorption (pump-probe) measurements. A selective excitation of the gold cores in the pump-probe experiment results in an energy transfer from the gold nanoparticles to the attached phthalocyanines in ∼2.4 ps. Photoexcitation of mainly the phthalocyanines in the functionalized nanoparticles leads to an electron transfer to the gold core in ∼3.0 ps. The recombination of charges in the Pc-AuNP takes place on a picosecond time scale. In addition, there is evidence of energy transfer from the photoexcited phthalocyanines to the gold nanoparticles. Introduction Gold nanoparticles are interesting building block candidates for several fields of applications1-3 because of their tunable optical properties, which depend on the size, shape, and surroundings of the particles.4-6 The functionality of the gold nanoparticles can be further enhanced by assembling photoactive molecules on their surfaces. Phthalocyanines are versatile molecules used as industrial colorants,7 and they have a number of applications: for example, photodynamic therapy8,9 and photovoltaic devices.10-13 Phthalocyanines have strong absorption at short and long wavelength ends of the visible spectrum and are thus well suited for optical applications. The optical and electrical properties of phthalocyanines can be tuned by changing the center metal and the peripheral substituents.14-16 For example, the fluorescence and phosphorescence quantum yields and the corresponding lifetimes of singlet and triplet states of phthalocyanines depend on their structure.16-19 Many different kinds of chromophores20-22 have been assembled on surfaces of gold nanoparticles, including pyrene,23 porphyrin,24-27 and fullerene.28-30 In most of the reported cases, the excitation of the chromophore is followed by energy transfer to the gold nanoparticle.20,31,32 Energy transfer is traditionally described by Fo¨rster theory,33 where the dipole-dipole interaction between the donor and the acceptor leads to the energy transfer. The rate of energy transfer is proportional to the overlap of the donor emission and the acceptor absorption spectra, and to the inverse sixth power of the distance between the donor and the acceptor.34,35 The limited distance range of Fo¨rster energy transfer (40 nm) of the gold nanoparticles, lead to fluorescence enhancement.40-42 There are several factors affecting which of the interaction types is seen for chromophore-gold nanoparticle systems: the size of the nanoparticle, the distance between the chromophore and the nanoparticle, and the orientation of the molecular dipole of the chromophore in relation to the nanoparticle surface. Apart from the interactions between the chromophore and the gold core, the packing of the molecules on the gold nanoparticle surface can be tight enough to allow interaction between the chromophore molecules.43,44 Phthalocyanines have been attached to semiconductor quantum dots (QDs), fullerenes (which can be considered as clusters of carbon) and gold nanoparticles. Energy transfer from photoexcited QDs to phthalocyanines has been observed in phthalocyanine-QD conjugates,45-47 and they are considered to be suitable for photodynamic therapy applications.48,49 Phthalocyanine-fullerene donor-acceptor molecules (dyads) undergo photoinduced electron transfer, where the photoexcited phthalocyanine acts as an electron donor.50-52 The efficient charge transfer in the phthalocyanine-fullerene dyads has been utilized in a solar cell application.53 Phthalocyanine-functionalized gold nanoparticles show promise for photodynamic therapy of cancer. Photoexcitation of the phthalocyanine results in a generation of cytotoxic singlet oxygen. Two different strategies have been utilized, where in the first one assembly of phthalocyanines on gold nanoparticles was reported to increase their activity in singlet oxygen generation, partly due to the phase transfer agent associated with the gold nanopar-

10.1021/jp9087173  2010 American Chemical Society Published on Web 12/03/2009

Photoinduced Processes in Pc-AuNPs ticles.54 In the second one, gold nanoparticles are used as delivery vehicles for loosely bound phthalocyanines, which are then released in the cancer cells.55 The phthalocyanine-gold nanoparticle systems clearly have potential applications, but their photoinduced processes have not yet been studied in detail. In the present report, phthalocyanines with two linkers have been assembled on 5 nm gold nanoparticles. Both the absorption and fluorescence of the phthalocyanines are affected by the attachment to the gold nanoparticles. Nearly selective excitations of both the phthalocyanines and the gold nanoparticles are possible because of their different absorption maxima wavelengths. Time-resolved absorption spectroscopy (pump-probe) on a picosecond time scale was used to monitor the relaxation processes of both the photoexcited phthalocyanines and the gold nanoparticles. The results suggest energy transfer from the photoexcited gold nanoparticles to the phthalocyanines and vice versa. In addition, electron transfer from the photoexcited phthalocyanines to the gold nanoparticles takes place. Experimental Methods Toluene was obtained from VWR International and used as received. Tetraoctylammonium bromide (TOABr) (98%) and sodium borohydride (NaBH4) (96%) were purchased from Fluka, and gold(III) chloride trihydrate (99.9%) was purchased from Aldrich. Gold Nanoparticles (TOABr-AuNPs). The method for preparing gold nanoparticles was similar to that described by Brust et al.,56 with a modification that the phase transfer agent TOABr serves also as the protecting ligand.57 Briefly, a gold(III) chloride trihydrate aqueous solution (25 mM, 2 mL) was vigorously stirred with a toluene solution of TOABr (85 mM, 3 mL) until all of the gold chloride was transferred to the organic phase. The reducing agent, NaBH4, in an aqueous solution (36 mM, 2 mL) was added dropwise for 10 min. After the addition of NaBH4, the mixture was stirred vigorously for 20 min. The organic phase was separated and washed with water. The core diameter of TOABr-AuNPs was estimated from transmission electron microscopy (TEM) images (figure not shown) to be approximately 5 nm. The absorption coefficient of the TOABrAuNPs was estimated to be 1.1 × 107 M-1 cm-1 at 522 nm, based on the dependence of the absorption coefficient on the core diameter.58,59 The TOABr-AuNPs were not dried at any time but kept as a toluene solution, for which the concentration was determined by an absorption measurement. Attachment of Phthalocyanines to Gold Nanoparticles. A phthalocyanine with thioacetate groups (Pc, Figure 1) was synthesized as described elsewhere.60 Mixture of Pc (3 mg, 2.8 µmol) and TOABr-AuNP (15 mg of gold, 0.02 µmol of 5 nm gold particles) in toluene (1 mL) was stirred for 24 h for the ligand exchange to take place. After this, the reaction mixture was diluted with toluene, and the unreacted Pc was separated in a size-exclusion column (Bio-Beads S-X1 from Bio-Rad) using toluene as an eluent. The TOABr-AuNPs are immobile on silica thin layer chromatography (TLC) plates, but the phthalocyanine-functionalized gold nanoparticles (Pc-AuNPs) move as a tailing spot, when a mixture of chloroform and ethanol (18:1) is used as an eluent. In addition, the TOABrAuNPs have a purple color, whereas the Pc-AuNPs are brownish. Steady-State Absorption and Emission Spectra. Absorption spectra were measured with a Shimadzu UV-3600 UV-vis-NIR spectrophotometer. Fluorescence spectra were recorded with a Fluorolog 3 Yobin Yvon-SPEX spectrofluorometer. The emis-

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Figure 1. Structure of phthalocyanine with two thioacetate linkers (Pc).

sion spectra were corrected to the instrument wavelength sensitivity using a correction spectrum supplied by the manufacturer. Time-Resolved Fluorescence. Time-correlated single photon counting (TCSPC) system consisting of a PicoHarp 300 controller and a PDL 800-B driver was used for the timeresolved fluorescence measurements on a nanosecond time scale. The excitation wavelength was 658 nm from a pulsed diode laser head LDH-P-650. The fluorescence signal was detected with a micro channel plate photomultiplier (Hamamatsu R2809U). The time resolution of the instrument was approximately 100 ps (fwhm). The fluorescence decays were fitted to a multiexponential model, I(t) ) a0 + Σai exp(-t/τi). Time-Resolved Absorption. The pump-probe measurement setup has been described elsewhere.61 Briefly, the samples were excited by ∼60 fs laser pulses at a wavelength of 500 or 680 nm. The pulse energies used for the sample excitation were 2.9 and 1.2 µJ for the wavelengths of 500 and 680 nm, respectively. A white continuum was used as the probe beam, and the spectra were recorded with a charge-coupled device (CCD) camera. Time-resolved absorption measurements of gold nanoparticles require that the dependence of the dynamics of the gold nanoparticles on the excitation power is taken into account. The strategy chosen was to measure the samples to be compared successively without any alteration of the instrumental setup, and with the absorbance of the gold nanoparticles at the surface plasmon band wavelength adjusted to be the same. This should guarantee the same excitation power for the samples and make the mutual comparison of the sample dynamics reliable. The overall time resolution of the pump-probe instrument was approximately 150 fs. The time range of the instrument extends to ∼1.2 ns. The primary data from the pump-probe measurements is in the form of differential transient absorption spectra at different delay times. From this data, transient absorption decay curves at different wavelengths are obtained. A global fit of the transient absorption decay curves with a multiexponential model gives the transient absorption decay component spectra. Results and Discussion Absorption and Fluorescence Spectra. All spectroscopic studies were carried out in toluene. A schematic presentation of Pc molecules attached to a gold nanoparticle is shown in Figure 2. The absorption maximum, that is, the surface plasmon band maximum of TOABr-AuNP, is at 522 nm (Figure 3A). Pc has a Soret band at 340 nm and a split Q-band with the

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Figure 2. Schematic illustration of a phthalocyanine-functionalized gold nanoparticle (Pc-AuNP). Size of the gold nanoparticle core is not in proportion to the size of the molecules, and the ratio and packing density of TOABr and phthalocyanine molecules are not accurately represented.

Figure 3. (A) Absorption and (B) fluorescence spectra of Pc (dotted line), TOABr-AuNP (dashed line) and Pc-AuNP (solid line) in toluene. Excitation wavelength for fluorescence measurements was 655 nm. The phthalocyanine absorbance at the excitation wavelength was adjusted to be the same for Pc and Pc-AuNP samples.

maxima at 685 and 715 nm. The splitting of the Q-band is typical for metal-free phthalocyanines, and it is caused by the

reduced symmetry of the molecule. The Q-band of Pc becomes broader after the attachment to the gold nanoparticles (Figure

Photoinduced Processes in Pc-AuNPs

Figure 4. Fluorescence decays of Pc (data: crosses, exponential fit: dotted line) and Pc-AuNP (data: open circles, exponential fit: solid line) on a logarithmic scale. The Pc-AuNP curve has been shifted for clarity. Excitation wavelength was 658 nm, and monitoring wavelength was 725 nm.

3A). The broadening of the Q-band can be attributed to aggregation of the phthalocyanine molecules62,63 in the PcAuNP. Phthalocyanines usually form face-to-face-oriented Haggregates, which are characterized by blueshifted absorption bands, but side-by-side-oriented J-aggregates are also possible.64 The peripheral substituents of phthalocyanines control their aggregation tendencies, and the properties of the two types of aggregates are different: for example, H-aggregates are considered to be nonfluorescent, whereas J-aggregates are fluorescent.64 The phthalocyanine molecules in the Pc-AuNP form most likely irregular aggregates as a result of their flexible linkers, the edge and terrace sites of the gold cores, and the accompanying TOABr molecules. The surface plasmon band of the gold nanoparticles in the Pc-AuNP has nearly the same position (519 nm) and bandwidth as before the attachment of the phthalocyanines, indicating that the core size of the particles is not affected by the ligand exchange. The molar absorption coefficient of Pc is 1.3 × 105 M-1 cm-1 at 715 nm.60 The molar absorption coefficient of TOABr-AuNP is estimated to be 1.1 × 107 M-1 cm-1 at 522 nm. Assuming that the absorption coefficient of Pc in the Q-band region does not change significantly upon assembling phthalocyanines on gold nanoparticles, the number of phthalocyanines per gold nanoparticle is estimated to be ∼60. This gives a mean molecular area of 130 Å2 for the phthalocyanine molecule, if the gold nanoparticle is approximated as a sphere with a diameter of 5 nm. This estimated mean molecular area is higher than that observed for a Langmuir film of a similar phthalocyanine molecule with shorter OH-terminated linkers (∼60 Å2).65 Both the Pc and the Pc-AuNP have a broad fluorescence band in toluene with the maximum at 726 nm. The fluorescence is quenched to 7% in the latter, but the position and shape of the spectrum remain the same (Figure 3B). Similar fluorescence quenching was observed at several excitation wavelengths. No fluorescence was observed from the TOABr-AuNP sample. PcAuNPs contain some TOABr, because not all TOABr is removed during the purification by the size exclusion column. An amount corresponding to the estimated maximum concentration of TOABr was added to a Pc solution, but no effect on the Pc fluorescence was observed. Fluorescence Lifetimes. The fluorescence decay curves of Pc and Pc-AuNP measured by the TCSPC instrument are shown in Figure 4. On the basis of the significant quenching of

J. Phys. Chem. C, Vol. 114, No. 1, 2010 165 the fluorescence intensity (to 7%, Figure 3B) of the Pc after the attachment to the gold nanoparticles, a drastic change in the fluorescence lifetime was expected. Instead, the lifetimes of the Pc-AuNP and the Pc appear to be the same, except for a slightly faster decay for the Pc-AuNP compared to that of the Pc. It is thus assumed that 93% of the Pc molecules are assembled in such a way, that they are completely quenched due to the interactions with the gold nanoparticles or between the phthalocyanines. The lifetime of the completely quenched phthalocyanines is so short that it cannot be resolved by the TCSPC instrument. The remaining 7% fluorescence is somehow ambiguous. A simple explanation for the 7% fluorescent phthalocyanines would be the presence of free Pc molecules among the Pc-AuNP, but because of the careful purification process, this should not be the case. Thus, it is assumed that the remaining 7% fluorescence of the Pc-AuNP comes from the attached, but unquenched, phthalocyanines packed differently on the gold nanoparticle surface compared to the main 93% portion. Pump-Probe. Pump-probe measurements for the Pc-AuNP were done by exciting the gold nanoparticles at 500 nm. Phthalocyanine absorption at this wavelength is negligible. The behavior of gold nanoparticles excited with a short laser pulse is explained by three relaxation processes.66,67 First, when the electronic temperature of the gold nanoparticles is increased as a result of the excitation pulse, the excited electrons thermalize via electron-electron scattering. Second, the hot electrons lose their energy to the lattice by electron-phonon scattering. Finally, the lattice transfers energy to the surroundings by phonon-phonon scattering. For gold nanoparticles with size around ∼5 nm, the two latter processes may have overlapping time scales.68 In the time-resolved differential absorption spectra of the two samples, TOABr-AuNP and Pc-AuNP, there are two wavelength regions where the transient absorption represents mainly the phthalocyanine or the gold nanoparticles: 690 and 520 nm, respectively. The global fitting of the TOABr-AuNP transient absorption decays gave best results when a biexponential model was used (Figure 5A). The first component with a lifetime of 3.4 ps has the typical shape of a gold nanoparticle transient absorption with bleaching around 530 nm and photoinduced absorption at wavelengths longer than 590 nm, and can be attributed to the electron-phonon scattering. The second component with a lifetime of ∼15 ps arises most likely from the phonon-phonon scattering. The relaxation times for the electron-phonon and the phonon-phonon scattering are dependent on excitation power.67,69 A decay component spectrum of the Pc-AuNP was obtained from a monoexponential fit, and has a lifetime of 2.4 ps (Figure 5B). The recovery of the plasmon band bleaching at ∼520 nm is faster in the Pc-AuNP (2.4 ps) compared to that of the TOABr-AuNP (3.4 ps), and could be ascribed to a different capping of the gold core, which has been observed to affect the relaxation.70 The different behavior of the Pc-AuNP sample compared to the TOABr-AuNP is more striking at wavelengths around 690 nm, where a bleaching of the phthalocyanine Q-band was observed for Pc-AuNP already at early stages of the relaxation, although the excitation at 500 nm does not affect phthalocyanines directly. The bleaching of the phthalocyanine Q-band in Pc-AuNP together with the faster recovery of the gold nanoparticle surface plasmon band bleach indicates energy transfer from the photoexcited gold nanoparticles to the attached phthalocyanine. Later it will be shown that a direct excitation of the phthalocyanine in the Pc-AuNP will result in a shorter lifetime than 2.4 ps.

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Figure 5. Decay component spectra obtained from global fitting of the pump-probe decay curves for (A) TOABr-AuNP and (B) Pc-AuNP. Excitation wavelength was 500 nm.

Figure 6. Decay component spectra obtained from global fitting of the pump-probe decay curves for (A) Pc and TOABr-AuNP in toluene and (B) Pc-AuNP in toluene. Excitation wavelength was 680 nm.

The excitation of the gold nanoparticles in the Pc-AuNP seems to lead to energy transfer from the photoexcited gold nanoparticles to the phthalocyanines. Excitation of mainly phthalocyanines in the Pc-AuNP is possible when an excitation wavelength of 680 nm is used. Absorbances of the Pc and the TOABr-AuNP in toluene were adjusted to correspond to the absorbances of the phthalocyanine and the gold nanoparticles in the Pc-AuNP sample. The absorbance of the TOABr-AuNP sample is relatively low at 680 nm, but the transient absorbance is still quite high (Figure 6A). The transient absorption decay curves could be fitted with a monoexponential decay yielding a lifetime of 1.8 ps (Figure 6A). The excitation energy at 680 nm is lower compared to 500 nm, and therefore the phonon-phonon scattering process is not seen. The decay component spectrum shows bleaching of the plasmon band around 530 nm and a positive wing in the wavelength range of 570-750 nm. The excitation wavelength of 680 nm is in the range of the Pc Q-band and the Pc is excited to the first excited singlet state. The decay component spectrum of Pc shows bleaching of the Q-band together with positive absorptions on both sides of the bleaching (Figure 6A). The lifetime of this component (>2 ns) is longer than the upper time-limit of the pump-probe instrument. This lifetime of the bleached signal corresponds to the fluorescence lifetime of Pc measured by TCSPC method (5.0 ns) and can thus be attributed to the first excited singlet state. It is important to notice that the transient absorption of the phthalocyanine singlet state in the near-infrared part of the spectrum has a lower intensity at 850-900 nm compared to that around 1000 nm. The pump-probe decay curves of Pc-AuNP, after excitation at 680 nm, were fitted with a three-exponential model (Figure 6B). The 0.3 ps component was mainly observed in the wavelength range from 520 to 800 nm. The overall shape of this component is similar to that of Pc, except for a positive band at ∼530 nm, which probably belongs to the gold nanoparticles. The second component with a lifetime of 3.0 ps

resembles the first one, but the features from the gold nanoparticles are more pronounced with bleaching at 530 nm. The band around 600 nm and enhanced absorption around 850-900 nm, as compared to the phthalocyanine singlet excited state, could have some contribution from radical ions of phthalocyanines.71 It is clear that the lifetimes of the phthalocyanines and gold nanoparticles in the Pc-AuNP are too close in time to be properly separated. The longest lifetime of >2 ns for Pc-AuNP corresponds to the differently packed phthalocyanine molecules, which have intact fluorescence properties. The two important conclusions from the lifetimes of the PcAuNP are (1) the relaxation of the gold nanoparticle excited state is slower compared to the TOABr-AuNP, and (2) the lifetime of the first excited singlet state of the phthalocyanines is shorter compared to the free Pc. The slower relaxation of gold nanoparticles observed for the Pc-AuNP compared to the TOABr-AuNP can be explained by energy transfer from the photoexcited phthalocyanines to the gold nanoparticles. Slower relaxation of the gold nanoparticles due to energy transfer from attached molecules has been observed.72 Energy transfer from photoexcited, attached chromophores results in a similar effect as an increased excitation power: relaxation of the gold nanoparticles becomes slower due to an increased electronic temperature. On the other hand, the lifetimes of the gold nanoparticles and phthalocyanines are not well separated in the analysis, and therefore the conclusion on energy transfer from the phthalocyanines to the gold cores is, to some extent, tentative. The reduction of the phthalocyanine lifetime can arise both from the close packing of phthalocyanines on the gold nanoparticle surface, or from the interaction between phthalocyanines and gold nanoparticles. The lifetime of the singlet excited state of a phthalocyanine similar to Pc was reduced ∼12 ps in thin films as a result of a close packing of the molecules.65 It is likely that both intermolecular interaction and photoinduced

Photoinduced Processes in Pc-AuNPs processes between the phthalocyanine and the gold nanoparticles affect the phthalocyanine lifetime in the Pc-AuNP. The important feature still to be considered in more detail in the decay component spectra of the Pc-AuNP is the absorption at 840-1000 nm for both the 0.3 and 3.0 ps components (Figure 6B). Phthalocyanine radical cations have an absorption band at ∼830-940 nm51,71,73-76 and radical anions at ∼990 nm.71,77 The intermolecular interaction between the Pc molecules attached to the gold nanoparticle surface could lead to charge transfer between phthalocyanines, generating both radical cations and anions. Energies for creating electron-hole pairs in phthalocyanines are close to the optical gap,78,79 and photoexcitation can thus easily create mobile charges in films or solids of phthalocyanines. The band at 840-1000 nm observed for PcAuNP covers only the absorption range of the phthalocyanine radical cation,80 and no phthalocyanine radical anion is observed. It can therefore be concluded that the charge transfer between phthalocyanine molecules in Pc-AuNPs does not take place. If charge transfer between the phthalocyanines in the PcAuNP can be excluded, the reason for the appearance of the phthalocyanine radical cation absorption in the decay component spectra (Figure 6B) at 840-1000 nm must be electron transfer from the photoexcited phthalocyanines to the gold cores. The distance between the phthalocyanine and the surface of the gold nanoparticles in the Pc-AuNP is at the most ∼1 nm, when the linkers are fully extended. In pyrene-functionalized gold nanoparticles (∼3 nm core), the electron transfer from the photoexcited pyrenes to the gold nanoparticles is dependent on the linker: eight-atom-linkers will yield electron transfer but 11 will not.43 The lifetime of the pyrene radical cation in this kind of systems is several microseconds.23 Also, bigger gold nanoparticles (8 nm) can accept electrons from photoexcited chlorophyll molecules assembled with electrostatic interactions.81 The charge transfer process in the Pc-AuNPs thus seems to be possible from the point of view of the distance between the phthalocyanine and the gold core and the gold nanoparticle core size. In addition, the appearance of phthalocyanine radical cation absorption in the near-infrared (NIR) region is a good indication of charge transfer. The lifetime of phthalocyanine radical cation in Pc-AuNP is 3 ps, which means a quick recombination of the photoinduced charges. A short distance between the separated charges causes a fast, direct recombination. The actual distance between phthalocyanines and the gold nanoparticle surface in Pc-AuNPs can be shorter than 1 nm, because the linkers are flexible. In summary, the photoexcitation of the phthalocyanines in the Pc-AuNP leads to energy and electron transfers from the phthalocyanine molecules to the gold nanoparticles. In principle, it should be possible to create charge separation in the Pc-AuNP also by exciting the phthalocyanines via energy transfer from the excited gold nanoparticles. Conclusions Phthalocyanine-functionalized 5 nm gold nanoparticles (PcAuNPs) can be easily prepared with a partial exchange of TOABr-ligands to phthalocyanines with two thioacetate groups. Phthalocyanines are tightly packed on the surface of the gold nanoparticles in the Pc-AuNPs, and intermolecular interactions between the phthalocyanines were observed as well as interactions between phthalocyanines and gold nanoparticles. Selective excitation of the gold nanoparticles in the Pc-AuNPs leads to energy transfer from the gold nanoparticles to the attached phthalocyanines. Excitation of the phthalocyanines in the PcAuNPs, on the other hand, results in energy transfer to the gold

J. Phys. Chem. C, Vol. 114, No. 1, 2010 167 nanoparticles, but also electron transfer from the attached phthalocyanines to the gold nanoparticles. The energy-donating ability of the gold nanoparticles can be used to extend the absorption range of phthalocyanine, and, at the same time, charge separation between phthalocyanines and gold nanoparticles is achieved. Acknowledgment. A.K. and R.L. acknowledge the Academy of Finland (No. 107182) for the financial support. References and Notes (1) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293. (2) Pelton, M.; Aizpurua, J.; Bryant, G. Laser Photon. ReV. 2008, 2, 136. (3) Biju, V.; Itoh, T.; Anas, A.; Sujith, A.; Ishikawa, M. Anal. Bioanal. Chem. 2008, 391, 2469. (4) Doremus, R. H. J. Chem. Phys. 1964, 40, 2389. (5) Alvarez, M. M.; Khoury, J. T.; Schaaf, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706. (6) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668. (7) Gregory, P. J. Porphyrins Phthalocyanines 2000, 4, 432. (8) Fabris, C.; Ometto, C.; Milanesi, C.; Jori, G.; Cook, M. J.; Russell, D. A. J. Photochem. Photobiol. B: Biol. 1997, 39, 279. (9) Miller, J. D.; Baron, E. D.; Scull, H.; Hsia, A.; Berlin, J. C.; McCormick, T.; Colussi, V.; Kenney, M. E.; Cooper, K. D.; Oleinick, N. L. Toxicol. Appl. Pharmacol. 2007, 224, 290. (10) Eichhorn, H. J. Porphyrins Phthalocyanines 2000, 4, 88. (11) Hohnholz, D.; Steinbrecher, S.; Hanack, M. J. Mol. Struct. 2000, 521, 231. (12) Loi, M. A.; Denk, P.; Hoppe, H.; Neugebauer, H.; Winder, C.; Meissner, D.; Brabec, C.; Sariciftci, N. S.; Gouloumis, A.; Vazquez, P.; Torres, T. J. Mater. Chem. 2003, 13, 700. (13) Xue, J.; Uchida, S.; Rand, B. P.; Forrest, S. R. Appl. Phys. Lett. 2004, 84, 3013. (14) de la Torre, G.; Claessens, C. G.; Torres, T. Eur. J. Org. Chem. 2000, 2821. (15) Kobayashi, N.; Fukuda, T. Recent progress in phthalocyanine chemistry: Synthesis and characterization; In Functional Dyes; Kim, S.H., Ed.; Elsevier, B.V.: Amsterdam, 2006. (16) Nyokong, T. Coord. Chem. ReV. 2007, 251, 1707. (17) Howe, L.; Zhang, J. Z. J. Phys. Chem. A 1997, 101, 3207. (18) Freyer, W.; Mueller, S.; Teuchner, K. J. Photochem. Photobiol. A 2004, 163, 231. (19) Savolainen, J.; van der Linden, D.; Dijkhuizen, N.; Herek, J. L. J. Photochem. Photobiol. A 2008, 196, 99. (20) Aguila, A.; Murray, R. W. Langmuir 2000, 16, 5949. (21) Huang, T.; Murray, R. W. Langmuir 2002, 18, 7077. (22) Gu, T.; Whitesell, J. K.; Fox, M. A. Chem. Mater. 2003, 15, 1358. (23) Ipe, B. I.; Thomas, K. G.; Barazzouk, S.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. B 2002, 106, 18. (24) Imahori, H.; Arimura, M.; Hanada, T.; Nishimura, Y.; Yamazaki, I.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 335. (25) Beer, P. D.; Cormode, D. P.; Davis, J. J. Chem. Commun. 2004, 414. (26) Kanehara, M.; Takanishi, H.; Teranishi, T. Angew. Chem., Int. Ed. 2008, 47, 307. (27) Ohyama, J.; Hitomi, Y.; Higuchi, Y.; Shinagawa, M.; Mukai, H.; Kodera, M.; Teramura, K.; Shishido, T.; Tanaka, T. Chem. Commun. 2008, 6300. (28) Sudeep, P. K.; Ipe, B. I.; Thomas, K. G.; George, M. V.; Barazzouk, S.; Hotchandani, S.; Kamat, P. V. Nano Lett. 2002, 2, 29. (29) Deng, F.; Yang, Y.; Hwang, S.; Shon, Y.-S.; Chen, S. Anal. Chem. 2004, 76, 6102. (30) Amendola, V.; Mattei, G.; Cusan, C.; Prato, M.; Meneghetti, M. Synth. Met. 2005, 155, 283. (31) Jennings, T. L.; Singh, M. P.; Strouse, G. F. J. Am. Chem. Soc. 2006, 128, 5462. (32) Sen, T.; Patra, A. J. Phys. Chem. C 2008, 112, 3216. (33) Valeur, B. Molecular Fluorescence Principles and Applications; Wiley-VCH: Weinheim, Germany, 2002. (34) Saini, S.; Srinivas, G.; Bagchi, B. J. Phys. Chem. B 2009, 113, 1817. (35) Clapp, A. R.; Medintz, I. L.; Mauro, J. M.; Fisher, B. R.; Bawendi, M. G.; Mattoussi, H. J. Am. Chem. Soc. 2004, 126, 301. (36) Yun, C. S.; Javier, A.; Jennings, T.; Fisher, M.; Hira, S.; Peterson, S.; Hopkins, B.; Reich, N. O.; Strouse, G. F. J. Am. Chem. Soc. 2005, 127, 3115. (37) Pons, T.; Medintz, I. L.; Sapsfors, K. E.; Higashiya, S.; Grimes, A. F.; English, D. S.; Mattoussi, H. Nano Lett. 2007, 7, 3157.

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