Photoinduced Energy and Charge Transfer in Layered Porphyrin-Gold

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J. Phys. Chem. C 2008, 112, 10316–10322

Photoinduced Energy and Charge Transfer in Layered Porphyrin-Gold Nanoparticle Thin Films Anne Kotiaho,* Riikka Lahtinen, 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: March 7, 2008; ReVised Manuscript ReceiVed: April 18, 2008

In thin films of porphyrin (H2P) and gold nanoparticles (AuNPs), photoexcitation of porphyrins leads to energy and charge transfer to the gold nanoparticles. Alternating layers of porphyrins and octanethiol protected gold nanoparticles (dcore ∼3 nm) were deposited on solid substrates via the Langmuir-Scha¨fer method, forming bilayer films denoted as H2P/AuNP. Photoinduced electron transfer from the gold nanoparticle layer to the porphyrin layer was observed as a distinct photovoltage response of the H2P/AuNP film when studied via the time-resolved Maxwell displacement charge (TRMDC) method. Time-resolved fluorescence and absorption measurements of the H2P/AuNP film demonstrated a significant reduction of the lifetime of the excited singlet state of porphyrin caused by the gold nanoparticles. Transients of the charge transfer reaction were not observed in the time-resolved absorption measurements, which indicates that the quantum yield of the charge transfer is low in the H2P/AuNP film. Energy transfer from the excited singlet state of porphyrin to the gold nanoparticles is the main deactivation path of excited porphyrins in the H2P/AuNP film. The critical distance of the energy transfer was estimated to be 6.4 nm, based on the dependence of fluorescence quenching on the distance between the porphyrin and gold nanoparticle layers. Introduction Metal nanoparticles and chromophores are possible components of thin film hybrid materials for photovoltaic, optoelectronic, and bioanalytical applications. Different chromophores, for example, pyrenes and porphyrins, have been assembled on the surfaces of gold nanoparticles.1,2 The process following the excitation of the chromophore can be energy3 or charge4 transfer to the gold nanoparticles. Besides changing the nonradiative decay rate, the proximity of the nanoparticles can also change the radiative decay rate of the chromophore.5 The relaxation path of the excited chromophore depends on the distance between the chromophore and the gold nanoparticle,2,4,6 on the orientation of the molecular transition dipole moment of the chromophore relative to the gold nanoparticle surface,5 and on the gold nanoparticle size.7 The excited gold nanoparticles can also act as energy donors to chromophores.8 Gold nanoparticles have special size-dependent optical and electrical properties. Metal clusters with a core diameter larger than 1 nm are considered to be metallic.9 Gold nanoparticles have two main electronical transitions: the plasmon band caused by oscillation of the d-electrons and the interband transition between the 5d and 6sp energy levels. Photoexcited gold nanoparticles relax either nonradiatively10 or by luminescence.11,12 In addition, gold nanoparticles can participate in charge transfer reactions.13,14 Preparation of chromophore-functionalized gold nanoparticles requires modification of the chromophore with a linker for covalent attachment to the gold nanoparticle surface. Alternative ways to assemble chromophores in close contact with gold nanoparticle surfaces are, for example, spin coating and Langmuir-Blodgett methods. Langmuir films of gold nano* To whom correspondence should be addressed. [email protected].

particles have been transferred to solid substrates, and they form ordered structures.15 Solid devices combining gold nanoparticles and chromophores have already been built, for example, photovoltaic devices16,17 and light-emitting diodes (LEDs).18,19 Study of the interaction mechanisms of chromophores and gold nanoparticles in solid assemblies aids to optimize these solid devices. Photoinduced charge transfer between porphyrin and gold nanoparticle films was observed in a previous study on the assembly and photoelectrical properties of films of gold nanoparticles and porphyrin-fullerene dyads.20 In the present study, the charge and energy transfer processes between porphyrin and gold nanoparticle films are studied in detail by optical measurements, that is, steady-state fluorescence, time-resolved fluorescence (nanosecond time scale), and transient absorption (picosecond and microsecond time scales). The charge movement is measured photoelectrically via the time-resolved Maxwell displacement charge (TRMDC) method. Porphyrin and gold nanoparticle films are prepared by horizontal lifting of Langmuir films to a substrate (Langmuir-Scha¨fer method). The film composition of porphyrins and gold nanoparticles has been optimized for successful film deposition and molecular density to obtain strong interaction. The usual mechanism of fluorescence quenching of chromophores by gold nanoparticles is energy transfer, and thus, it can be expected that energy transfer plays a role also in the present porphyrin and gold nanoparticle films. The relative efficiencies of charge and energy transfer processes are discussed. Experimental Methods Materials. Solvents of analytical grade were obtained from commercial sources and used as received. MilliQ water was derived from a Millipore system. Gold(III) chloride trihydrate (99%),

10.1021/jp802026w CCC: $40.75  2008 American Chemical Society Published on Web 06/14/2008

Energy and Charge Transfer in H2P/AuNP Films SCHEME 1: Schematic Illustration of a H2P/AuNP Bilayer

tetraoctylammonium bromide (TOABr, 98%), 1-octanethiol (98.5%), sodium borohydride (NaBH4, 96%), and octadecylamine (ODA, 99%) were purchased from Sigma-Aldrich. 5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)porphyrin (denoted in further text as H2P) was synthesized as described previously.21 Octanethiol protected gold nanoparticles (AuNPs) were prepared according to the Brust method.22 In brief, TOABr in toluene (3.2 mmol, 25 mL) was mixed with aqueous gold chloride solution (1.3 mmol, 50 mL) and stirred for 20 min to transfer all the gold to the organic phase. Octanethiol (0.5 mmol) was added to the separated organic phase, and stirring was continued for 20 min to form a complex between the gold and the thiol. Aqueous NaBH4 solution (15.1 mmol, 10 mL) was added rapidly, and the mixture was stirred for 3.5 h. The dark brown product was separated and washed with water. The product was precipitated from excess ethanol, and a fraction of the particles was precipitated from an ethanol-toluene mixture (2:1). The core diameter of the gold nanoparticles is estimated from transmission electron microscopy (TEM) images to be approximately 3 nm. Film Preparation. Surface pressure-mean molecular area isotherm measurements and film deposition were done using LB 5000, LB Minitrough, and Minialternate systems from KSV Instruments. The subphase temperature was set to 18 ( 1 °C by using a thermostat. The concentration of H2P in the film was 30 mol % in an ODA matrix. H2P stock solution (0.6 mM) was prepared in hexane and diluted in chloroform with ODA to provide a total concentration of 1 mM for the spreading solution. The subphase was a phosphate buffer containing 0.5 mM Na2HPO4 and 0.1 mM NaH2PO4 in MilliQ water. The deposition pressure was 26 mN m-1, and the deposition was done by horizontal lifting, that is, via the Langmuir-Scha¨fer method. Gold nanoparticles were spread from 0.4 mg mL-1 chloroform solution on a MilliQ water subphase. The AuNP films were deposited via the Langmuir-Scha¨fer method at surface pressure of 10 mN m-1. Samples for optical measurements were prepared on quartz or glass plates cleaned in an ultrasonic bath in chloroform and chrome sulfuric acid and treated with sodium hydroxide to improve the hydrophilicity of the surface. Samples for photoelectrical measurements were prepared on indium tin oxide (ITO) coated glass plates with a sheet resistance of approximately 10 Ω/square. The plates were cleaned in an ultrasonic bath in acetone and in chloroform. Before film deposition, both glass and ITO plates were plasma etched in a low-pressure nitrogen atmosphere plasma cleaner PDC23G from Harrick. Three ODA layers were used to make the surfaces of the glass plates hydrophobic before horizontal deposition of the photoactive layers. ODA layers were used as an insulator between the electrodes and the photoactive layers in samples prepared for photoelectrical measurements. Typically, 11 layers of ODA were deposited on an ITO plate before the photoactive layers, and finally, the samples were covered with 20 ODA layers. H2P/AuNP bilayer samples (Scheme 1; the organization of the mixed porphyrin layer is discussed in the Results and

J. Phys. Chem. C, Vol. 112, No. 27, 2008 10317 Discussion section) were used for photoelectrical measurements and for steady-state and time-resolved fluorescence measurements. Pump-probe and flash photolysis experiments require an absorbance higher than 0.4, and therefore, the photoactive layers were repeated 23 times and separated from each other by 3 ODA layers, for example, quartz/(3ODA/H2P/AuNP) × 23. The separating ODA layers prevent interaction between the repeating H2P/AuNP bilayers. It is important that the properties of the films stay the same regardless of the number of the layers, and therefore, the properties of the multilayer samples are compared to the mono- and bilayer samples with respect to the absorption and fluorescence spectra and fluorescence lifetimes. Steady-State Absorption and Emission Spectra. Absorption spectra of the films were measured with a Shimadzu UV-3600 UV-vis-NIR spectrophotometer. Steady-state fluorescence spectra were recorded with a Fluorolog 3 Yobin Yvon-SPEX spectrofluorometer. The emission spectra were corrected to instrument wavelength sensitivity using a correction spectrum supplied by the manufacturer. Measurements were carried out in room temperature and ambient air. Photoelectrical Measurement. Photoinduced charge movement was studied with the time-resolved Maxwell displacement charge (TRMDC) method.23,24 Samples were excited by 10 ns pulses at a wavelength of 422 nm from a tunable Ti:sapphire laser (pumped by the second harmonic of a Nd:YAG laser). Briefly, the photoactive layers are insulated from the ITO and InGa electrodes, and no current passes through the sample. Photoinduced electron movement perpendicular to the sample plane in the photoactive layers induces a potential difference between the electrodes, which is then measured as a photovoltage signal. Time-Resolved Emission. A time-correlated single photon counting (TCSPC) system consisting of a PicoHarp 300 controller and PDL 800-B driver was used for time-resolved fluorescence measurements. The excitation wavelength was 404 nm from a pulsed diode laser head LDH-P-C-405B. The fluorescence signal was detected with a microchannel plate photomultiplier tube (Hamamatsu R2809U). The time resolution was approximately 100 ps. Time-Resolved Absorption. The pump-probe measurement setup has been described elsewhere.25 Briefly, the films were excited by 50 fs laser pulses at a wavelength of 420 nm with an excitation energy density of 0.2 mJ cm-2. A white continuum was used as the probe beam, and spectra were recorded with a charge-coupled device (CCD) camera. The time resolution was approximately 150 fs. Measurements were carried out in ambient air. The flash photolysis experiments on the microsecond time scale were performed with a modified Luzchem laser flash system (mLFP111 prototype from Luzchem Co.) controlled with a computer. The excitation source was the same as that in the TRMDC experiment. The excitation power density was 1.4 mJ cm-2. A continuous Xe lamp (Oriel Simplesity Arc Source) provided the monitoring light, and the signal was recorded with a digitizing oscilloscope (Tektronix, TDS3032B, 300 MHz). Measurements were carried out under nitrogen atmosphere or in air. Results and Discussion Absorption and Emission Spectra. A monolayer of H2P has a B-band (Soret-band) at 422 nm (Figure 1) with a bandwidth of 12 nm (fwhm). The B-band position is red-shifted by 5 nm and broadened by 2 nm in the H2P monolayer compared to that of H2P in hexane solution (417 and 10 nm),26 indicating some aggregation of porphyrins in the film. Qy bands of the H2P

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Figure 1. Absorption spectra of the H2P monolayer (thin line), AuNP monolayer (dashed line), and H2P/AuNP bilayer (bold line).

Figure 2. Fluorescence spectra of the H2P monolayer (thin line) and H2P/AuNP bilayer (bold line). The excitation wavelength was 420 nm.

monolayer are located at 515 and 549 nm, and Qx bands are located at 592 and 650 nm (Figure 1). The film formation and structure of H2P in an ODA matrix have been studied earlier.26 The limiting area is approximately 100 Å2 for the 30 mol % H2P monolayer. Such a small limiting area indicates at least dimer formation. Probably some of the porphyrins are squeezed out of the film plane formed by ODA molecules. The relative coverage of H2P molecules in the mixed film, calculated from the limiting area, is 70%. In a H2P multilayer film, where the H2P layers are separated from each other by the layers of ODA, absorption at 422 nm increases linearly by 0.038 as a function of the number of H2P layers. During the multilayer deposition of H2P, the Soret-band broadens from 12 nm (fwhm) for a monolayer to 15 nm for 23 layers. This broadening can be explained by the different organization, and thus slightly different absorption, of porphyrins in each layer. The AuNP plasmon band positions were estimated from differentiated absorption spectra. The AuNP monolayer shows plasmon band absorption at ∼520 nm (Figure 1). The plasmon band is red-shifted ∼30 nm compared to that for particle absorption in toluene (∼490 nm). The AuNP plasmon band absorption in the multilayer film is slightly blue-shifted, to 510 nm, compared to that of the monolayer. The plasmon band position depends on the dielectric constant of the surroundings,15 and therefore, the peak position of the AuNP monolayer is red-shifted compared to the solution. The ODA molecules incorporated between the AuNP layers during multilayer film deposition have again different dielectric constants compared to the nanoparticles, and thus, the peak position of the multilayer film is in between the values for the toluene solution and the monolayer of AuNP. During the deposition of AuNP multilayer films, absorption at 470 nm increases linearly by 0.025 after each layer. The interband absorption threshold of gold is at 2.4 eV (∼520 nm),10 which is very close to the plasmon band absorption maximum. The absorption spectrum of a H2P/AuNP bilayer is the sum of the corresponding monolayers (Figure 1). The ratio between the number of porphyrin moieties and gold nanoparticles is difficult to determine accurately. On the basis of molecular dimensions, one can estimate that the ratio is more than 1. The growth of AuNP absorption can be monitored at 470 nm in the H2P/AuNP multilayer film, because H2P absorbs weakly at this wavelength. The absorbance increases by 0.024 at 470 nm during each deposition, which is similar to the value of the AuNP multilayer film. At 422 nm, the absorption increases by a factor of 0.062, which corresponds to the sum of H2P and AuNP absorptions. The absorption spectra of the multilayer films and absorbances of the multilayer films as a function of deposited layers are presented in the Supporting Information Figures S1 and S2. In all the multilayer films, the absorption increase during the deposition of

the two first layers is higher compared to that of the subsequent layers. This is probably due to the increasing roughness of the sample and partly to the wearing of the Langmuir films after multiple depositions. The final absorbances of the multilayer films at 420 nm are 0.81, 0.61, and 1.40 for H2P, AuNP, and H2P/AuNP, respectively. The absorption measurements show that films of H2P and AuNP can be deposited in a reproducible manner. Multilayer deposition of H2P/AuNP films was done successfully. The absorption spectra of H2P/AuNP films indicate no interaction between the two layers. The typical free-base porphyrin fluorescence at 600-800 nm originates from the Qx state. The fluorescence bands are located at ∼650 and ∼720 nm for the 30 mol % H2P monolayer (Figure 2). The excitation wavelength was 420 nm. The ratio of fluorescence quenching is defined as FH2P/FH2P/AuNP, where FH2P and FH2P/AuNP are the fluorescence intensities of the H2P and H2P/AuNP films, respectively, at 654 nm. In the H2P/AuNP bilayer structure, the quenching ratio is 0.02 (Figure 2). The gold nanoparticle film is nonfluorescent under these experimental conditions. In the multilayer structure of H2P, the fluorescence bands are slightly red-shifted and broadened compared to those of the H2P monolayer. The difference between the spectral shapes of the monolayer and multilayer films is quite small, though. The fluorescence quenching ratio in the H2P/AuNP multilayer film is 0.007, but the absorbance of the AuNP layers reduces the excitation power at 422 nm. The fluorescence spectra of the multilayer films are shown in the Supporting Information Figure S3. The actual factor for fluorescence quenching due to energy or electron transfer in the H2P/AuNP multilayer film is obtained, when the fluorescence quenching ratio is divided by the transmittance of the AuNP layers at the excitation wavelength: 0.007/0.25 ) 0.03. This value is similar to the fluorescence quenching ratio observed for the H2P/AuNP bilayer. As a conclusion for the fluorescence spectra, the fluorescence of the H2P film is significantly quenched by the AuNP film. This fluorescence quenching could be due to electron or energy transfer, enhanced intersystem crossing, or enhanced internal conversion. Charge Transfer Studied by Photoelectrical Measurements. The photoactive layers are isolated from the ITO and InGa electrodes by insulating layers in the photoelectrical measurements. The time-resolved Maxwell displacement charge (TRMDC) method gives a photovoltage signal, if charges are moving perpendicular to the sample plane in the photoactive film. The amplitude of photovoltage is proportional to the number of shifted charges and to the charge separation distance. The sign of the photovoltage gives the direction of electron

Energy and Charge Transfer in H2P/AuNP Films

Figure 3. Photovoltage decays of (1) ITO/ODAs/H2P/AuNP, (2) ITO/ ODAs/AuNP, (3) ITO/ODAs/H2P, and (4) ITO/ODAs/AuNP/H2P. The excitation wavelength was 422 nm, and the excitation energy density was 0.01 mJ cm-2.

Figure 4. Photovoltage dependence on excitation energy density for ITO/ODAs/H2P (×), ITO/ODAs/H2P/AuNP (b, the saturation fit is drawn as a thin line), and ITO/ODAs/AuNP/H2P (4, the saturation fit is drawn as a dashed line).

transfer: positive photovoltage indicates that electrons shift toward the ITO electrode and vice versa. Electron transfer between the H2P and AuNP layers can be confirmed by the TRMDC measurements. The photovoltage for the ITO/ODAs/H2P/AuNP film is positive (Figure 3), indicating electron transfer from the gold nanoparticle layer to the porphyrin layer. The direction of the electron transfer is further confirmed by the change of the photovoltage sign, when the deposition order of the H2P and AuNP layers is changed to ITO/ODAs/AuNP/H2P (Figure 3). The photovoltage response from the H2P monolayer is negative and weak (Figure 3) and can be attributed to the charge shift caused by the electrical field between the electrodes.23 For the AuNP film, the photovoltage is positive and very weak. The photovoltage amplitude saturates with increasing excitation energy, and the species creating the photovoltage can be identified based on the dependence of the photovoltage on the excitation energy. The saturation of the photovoltage is fitted with the following equation:27

(

( ))

Uout ) U0 1 - exp -

Iexc I0

where I0 ) hν/σ, U0 is the saturation photovoltage, Iexc is the excitation energy density, I0 is the saturation energy density, h is the Planck constant, ν is the frequency, and σ is the absorption cross section. The photovoltages of ITO/ODAs/H2P/AuNP and ITO/ODAs/AuNP/H2P films clearly saturate with increasing excitation energy (Figure 4). The values obtained from the saturation fits are shown in Table 1. The saturation voltage values are similar, but they have opposite signs for ITO/ODA/ H2P/AuNP and ITO/ODA/AuNP/H2P films. The absorption cross section values determined from the photovoltage saturation

J. Phys. Chem. C, Vol. 112, No. 27, 2008 10319 of the H2P/AuNP films correspond well to that of H2P estimated from the absorption spectrum and the Langmuir isotherm (Table 1). The absorption cross section value of AuNP is significantly higher than that obtained from the photovoltage saturation of the H2P/AuNP films. This indicates that excitation of H2P causes the photovoltage signal of the H2P/AuNP films. The H2P monolayer shows a linear dependence of the photovoltage on the excitation energy density at the used energy density range (Figure 4), which indicates a field-induced charge movement. Photoinduced electron transfer from the AuNP layer to the H2P layer was observed via the photovoltage measurements. The charge separated state has a lifetime of over hundreds of microseconds. The electron transfer process requires excitation of the porphyrin layer to take place. A possible mechanism of the charge transfer is a hole transfer from the excited porphyrin to the gold nanoparticle,20 favored by alignment of the gold work function relative to the highest occupied molecular orbital (HOMO) level of porphyrin. This process is equivalent to electron transfer from the gold nanoparticle to porphyrin. Determination of the quantum efficiency of the charge transfer requires a known value for the charge separation distance, and in the case of bilayer films reliable estimation is difficult. The quantum efficiency estimation is also complicated by the time resolution of the instrument, which is slow compared to the time scale of the initial recombination, making the maximum of the photovoltage amplitude impossible to measure. The probability for electron transfer is estimated using the following equation:24

φ)

nH2Pnexc

)

U0CD gSelednH2P

where nH2P- is the surface density of porphyrin anions formed, nexc is the surface density of excited porphyrins, C is the capacitance of the sample, D is the total distance between the ITO and InGa electrodes, g is the amplifying factor of the instrument, Sel is the area of the electrode, e is the elementary charge, d is the distance of charge separation, and nH2P is the surface density of porphyrin molecules. The saturation photovoltage U0 is used in the above equation, and thus, all of the porphyrin molecules are assumed to be excited. It should be emphasized that this calculation yields the low limit of the charge transfer probability and the actual value is expected to be higher. A rough estimation of the lower limit of the charge transfer probability is obtained as 0.3%,28 when the distance of charge transfer is estimated to be 1 nm, the same as the thickness of the octanethiol layer protecting the gold nanoparticles. Fluorescence Lifetimes. Photovoltage measurements indicate that charge transfer to gold nanoparticles is one pathway for relaxation of the excited H2P film. Time-resolved optical measurements are necessary in order to determine which of the porphyrin excited states is the one interacting with the gold nanoparticles and what contribution energy transfer to gold nanoparticles has in the relaxation of the excited H2P. The fluorescence decay of the H2P monolayer at a monitoring wavelength of 660 nm is shown in Figure 5. Porphyrin emits from the Qx state at this wavelength. Fluorescence decays of H2P films are best described by a two-exponential function.26 The two lifetimes are 2.6 ns (47%) and 5.8 ns (53%) for the 30 mol % H2P monolayer. Fluorescence decay is clearly faster for the H2P/AuNP bilayer than for the H2P monolayer (Figure 5). The major lifetimes in the H2P/AuNP bilayer are 0.1 ns (76%) and 0.4 ns (23%), and only a very small portion (1%) of porphyrins have a lifetime of 2.0 ns. There is some variation of the average lifetime (4-7 ns) of H2P between repetitive film

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TABLE 1: Saturation Photovoltages, Saturation Energy Densities, and Absorption Cross Sections for H2P/AuNP Films Film structure

U0 (V)

I0 (mJ cm-2)

σ (×10-16 cm2) from photovoltage

σH2P (×10-16 cm2) from absorption

σAuNP (×10-16 cm2) from absorption

ITO/ODAs/H2P/AuNP ITO/ODAs/AuNP/H2P

1.5 -1.4

0.31 0.19

15 25

17 17

120 120

depositions, which is probably due to the structural sensitivity of mixed films and Langmuir-Scha¨fer deposition. The average fluorescence lifetimes are 4.3 and 0.3 ns for H2P and H2P/AuNP multilayer films, respectively. The decrease of the H2P fluorescence lifetime to less than one tenth of the original in the H2P/AuNP film is in agreement with the ratio of the fluorescence quenching observed in the steady-state measurement. In earlier measurements, the fluorescence of a 10 mol % H2P film was quenched to half by a 2 mol % gold nanoparticle film.20 The two longer fluorescence lifetimes for H2P (10%)/AuNP (2%) were the same as those for 10 mol % H2P, but there was a third, shorter lifetime (∼200 ps, pre-exponential factor 24%) that was attributed to a charge or energy transfer process.20 The low portion of the short lifetime was explained by the organization of the mixed layers: only one-quarter of porphyrins was close enough to the gold nanoparticles for the fluorescence quenching to occur.20 Almost all porphyrins interact with the gold nanoparticles in the 30 mol % H2P and 100 mol % AuNP films used in the present study, showing clear improvement of the film geometry in terms of contact area. Time-Resolved Absorption: Pump-Probe. Formation and decay of the porphyrin Q states can be observed by the pump-probe measurements on the picosecond time scale. Cooling of the laser pulse heated gold nanoparticles can be seen on this time scale. The excitation wavelength was 420 nm, which corresponds to the excitation of the porphyrin B state and to interband excitation of the gold nanoparticles. The H2P film degraded during the measurements when the excitation energy was higher than 0.2 mJ cm-2. Thus, the comparison of the films had to be done at this excitation energy density, though the AuNP and H2P/AuNP films endured higher excitation energies. Due to the low excitation energy density, the pump-probe signal in the IR region was too weak for obtaining reliable results. In gold nanoparticles, the laser pulse excitation increases the energy of the electrons, which then thermalize by electronelectron scattering and cool further to the lattice temperature by electron-phonon scattering and finally release heat to the surrounding media by phonon-phonon scattering.29 A typical transient absorption spectrum of gold nanoparticles features a plasmon band bleach, with positive wings on both sides of the bleach signal.30 These features are observed for particles with core diameters greater than 2 nm.31 The bleach recovery has a

time constant of 1.0 ps in a drop-cast film of dodecanethiol protected gold nanoparticles (core diameter 6 nm).32 For the AuNP multilayer film, the bleaching of the plasmon band and the positive wings are observed with the decay component lifetime of ∼1.4 ps (Figure 6A), corresponding to electron-phonon scattering. The phonon-phonon scattering is not seen for the AuNP film on a time scale of ∼100 ps, because the phonon-phonon scattering strongly depends on the excitation energy and disappears at low excitation.29,33 The transient absorption features with the surface plasmon bleach at 500-630 nm and the positive wings extending in the wavelength ranges 450-490 and 640-1100 nm are similar for AuNPs both in film and in toluene solution (Supporting Information, Figure S4). For the H2P multilayer film, two decay components with lifetimes of ∼23 and ∼690 ps are observed (Figure 6B). The longer lifetime is attributed to the Q states of porphyrin. The measured lifetime is shorter than that obtained from the fluorescence lifetime measurement (4.3 ns), but it is within the experimental error limit

Figure 5. Fluorescence decays at 660 nm for a H2P monolayer (×, two-exponential fit shown as a bold line) and H2P/AuNP bilayer (O, three-exponential fit shown as a thin line).

Figure 6. Decay component spectra obtained from global fitting of the pump-probe decay curves: (A) AuNP, (B) H2P, and (C) H2P/AuNP multilayer films.

Energy and Charge Transfer in H2P/AuNP Films due to the inaccuracy of the pump-probe system in measuring these long lifetimes. Also, the shape of the spectrum of the ∼700 ps component is typical for the transient spectrum of porphyrin Q states. The shape of the 23 ps component spectrum also indicates that this component belongs to the Q states of H2P, with a short lifetime. This component is probably a result of energy transfer within the H2P film. The two-exponential model was sufficient for fitting the H2P/ AuNP multilayer film decays, because addition of a third exponent did not result in significant improvement of the fit. The H2P/AuNP film has two decay components with lifetimes of ∼1.3 and ∼20 ps (Figure 6C). The shorter component is the same as that for the AuNP film. This can be expected, because the electron-phonon scattering is controlled by the pump power rather than by the environment of the particle.29,34 The second component has a lifetime of 20 ps and a shape corresponding to the porphyrin Q states. The AuNP film thus reduces the lifetime of the Q states of the H2P film. This is in agreement with the reduction of the H2P fluorescence lifetime in the H2P/ AuNP film. No transient absorption with a long lifetime, which could be attributed to charge transfer products, was observed in the pump-probe measurements. The pump-probe measurements indicate that energy transfer from the Q states of porphyrin to the gold nanoparticles is the main relaxation path of excited porphyrins. Time-Resolved Absorption: Flash Photolysis. The photovoltage signal of a H2P/AuNP film decays in hundreds of microseconds, indicating existence of some long living charge separated state. Radical ions formed after the charge separation should be visible in transient absorption measurements done on the microsecond time scale. The porphyrin triplet state is also expected to be visible, because its lifetime is ∼1 ms in deaerated solutions.35 The porphyrin triplet absorption can be monitored around 450 nm.36 The porphyrin radical anion has absorption bands around 450 nm (sharp) and 700-900 nm (broad).37 The transient absorption decays for the H2P and H2P/AuNP multilayer films at a monitoring wavelength of 450 nm are shown in Figure 7. The transient absorption spectrum is not shown, because the signals were weak at other wavelengths compared to the signal at 450 nm. The signal for the H2P film is due to the triplet state and has an average lifetime of ∼200 µs under nitrogen atmosphere. For the H2P/AuNP film, the transient absorption at 450 nm is 10 times lower than that of H2P under the same conditions. This signal originates most likely from the triplet state. In air atmosphere, no transient absorption was observed at 450 nm for the H2P/AuNP film. If there is some contribution from the radical ion at the wavelength of 450 nm, it is too low to be resolved. No porphyrin radical ion formation was observed in the wavelength range 600-900 nm. If the quantum efficiency of the charge transfer from the excited H2P to AuNPs is less than roughly 10%, the radical anion absorption is not clearly distinguishable from the noise level at wavelengths of 700-900 nm.38 Distance Dependence of Energy Transfer Studied by Fluorescence Spectroscopy. An increasing number of ODA layers was added between H2P and AuNP monolayers to determine the efficiency of the energy transfer from the H2P layer to the AuNP layer. The dependence of the fluorescence intensities ratio Fseparated layers/FH2P on the distance between the H2P and AuNP monolayers is shown in Figure 8. The H2P fluorescence is strongly quenched when there are less than three ODA layers to separate the H2P and AuNP monolayers. When the number of ODA layers is more than three, the fluorescence gradually increases and reaches the FH2P value at a separation distance of eight layers.

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Figure 7. Transient absorption decays from flash photolysis for H2P (upper curve) and H2P/AuNP (lower curve) multilayer films at a wavelength of 450 nm. The excitation wavelength was 422 nm.

Figure 8. Fluorescence intensity ratio Fseparated layers/FH2P at a wavelength of 654 nm as a function of monolayer distance for H2P/mODA/AuNP films, m ) 0-10. The excitation wavelength was 419 nm.

Fluorescence quenching by energy transfer is described by the following equation:39

Fseparated layers ) FH2P

1 d0 1+ d

()

n

where d0 is the critical distance, d is the distance between the donor and the acceptor, and n is the parameter determined by the molecular organization. At the critical distance, the relaxation by energy transfer is as efficient as it is through other relaxation routes. In case of the Fo¨rster mechanism, energy is transferred between two isolated dipoles and n ) 6.40 In energy transfer from a point dipole to a layer, n ) 4, and in layer-to-layer energy transfer n ) 2.40 Energy transfer between two layers is chosen here, because porphyrins are probably organized close to each other and behave more as a film than as single dipoles. Fo¨rster type energy transfer from the Qx state of porphyrin to the gold nanoparticles can be expected based on the overlap of the H2P fluorescence and AuNP absorption. The thickness of an ODA layer, estimated from the number of bonds, is 2.5 nm. The minimum distance between the adjacent layers of H2P and AuNP is estimated to be the thickness of the protecting octanethiol monolayer of the gold nanoparticles, which is approximately 1 nm. When n is set to 2 and the data are fitted (Figure 8) with the equation above, a value of d0 ) 6.4 ( 0.6 nm is obtained. In dyes attached via a rigid linker to a 1.5 nm core diameter gold nanoparticle, the energy transfer efficiency has a d-4 dependence on the distance and the critical distance is 7-8 nm.41 The rate of energy transfer (kEnT) between two layers at a certain distance can be calculated when the fluorescence lifetime of the energy donor (τ) and the critical distance for energy transfer are known: kEnT ) τ-1(d02d-2).42 For the energy transfer rate in the H2P/AuNP film, where d ) 1 nm, a value of kEnT ≈ 9.5 × 109 s-1

10322 J. Phys. Chem. C, Vol. 112, No. 27, 2008 can be calculated (the lifetime is thus τEnT ∼ 100 ps). The average distance between the adjacent gold nanoparticles and porphyrin films was roughly estimated to be 1 nm. The actual distance probably has a distribution from less than one to a few nanometers. From the pump-probe measurements, a lifetime of 20 ps was obtained for the energy transfer between the porphyrin and gold nanoparticle layers. These lifetime values obtained by the two methods are of the same order of magnitude. Conclusions The interaction between porphyrins and gold nanoparticle films was studied by photoelectrical and time-resolved optical measurements. Absorption spectra confirm that H2P/AuNP film preparation is reproducible both as bilayers and multilayers. Significant quenching of the H2P film fluorescence by the AuNP film was observed. Photovoltage measurements indicate photoinduced electron transfer from the AuNP film to the H2P film. The excited species generating the photovoltage is H2P. Time-resolved fluorescence and absorption, on nanosecond and picosecond time scales, both show a reduction of the lifetime of the excited singlet state of H2P by AuNP. Time-resolved absorption measurements on the microsecond time scale indicate that less than 10% of the excited porphyrins in the H2P/AuNP film relax through the triplet state. No charge transfer products were observed in the timeresolved absorption measurements. Even though the photovoltage measurements indicate electron transfer from the gold nanoparticles to porphyrins, the efficiency of the charge transfer is expected to be low (less than 10%). The charge transfer is probably restricted to porphyrin molecules closest to the gold nanoparticles. The energy transfer from the Qx state of the porphyrins to the gold nanoparticles is thus attributed to be the main relaxation process of the excited porphyrin in the H2P/AuNP structure. More than 80% of the excited porphyrins relax by energy transfer to gold nanoparticles. This is in agreement with studies of porphyrin-functionalized gold nanoparticles in solution, where energy transfer from the porphyrin excited singlet state to the gold nanoparticle was observed to be the main photoinduced process.2 Acknowledgment. A.K. and R.L. acknowledge the Academy of Finland (No. 107182) for financial support. The authors are very grateful to Dr. Christoffer Johans (Helsinki University of Technology, Finland) for TEM imaging. Supporting Information Available: Absorption spectra of multilayer films (Figure S1); multilayer film absorbances as a function of deposited layers (Figure S2 and Table S1); fluorescence spectra of multilayer films (Figure S3); and pump-probe decay component spectra of AuNP multilayer film and AuNP in toluene (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Thomas, K. G.; Kamat, P. V. Acc. Chem. Res. 2003, 36, 888. (2) Imahori, H.; Kashiwagi, Y.; Endo, Y.; Hanada, T.; Nishimura, Y.; Yamazaki, I.; Araki, Y.; Ito, O.; Fukuzumi, S. Langmuir 2004, 20, 73. (3) Templeton, A. C.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 7081. (4) Ipe, B. I.; Thomas, K. G. J. Phys. Chem. B 2004, 108, 13265. (5) Hernandez, F. E.; Yu, S.; Garcı´a, M.; Campiglia, A. D. J. Phys. Chem. B 2005, 109, 9499. (6) Aguila, A.; Murray, R. W. Langmuir 2000, 16, 5949.

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