Effect of gold nanoparticles on intramolecular exciplex emission in organized porphyrin–fullerene dyad films

Chemical Physics Letters 471 (2009) 269–275

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Effect of gold nanoparticles on intramolecular exciplex emission in organized porphyrin–fullerene dyad films Anne Kotiaho *, Riikka Lahtinen, Hanna-Kaisa Latvala, Alexander Efimov, Nikolai V. Tkachenko, Helge Lemmetyinen Department of Chemistry and Bioengineering, Tampere University of Technology, P.O. Box 541, 33101 Tampere, Finland

a r t i c l e

i n f o

Article history: Received 22 January 2009 In final form 15 February 2009 Available online 20 February 2009

a b s t r a c t Interlayer processes of porphyrin–fullerene dyad (PF) and octanethiol-protected 3 nm-gold nanoparticles (AuNP), assembled as films by Langmuir–Schäfer method, were studied by fluorescence and photoelectrical measurements. The PF and AuNP films appear homogeneous on lm-scale and have full coverage on each other. Both the exciplex emission and the photoelectrical response of the PF dyad are altered by the AuNP film. The results show that the AuNP film has an interaction with the intramolecular exciplex of the PF dyad. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Porphyrin–fullerene dyads undergo a fast and efficient intramolecular charge transfer in polar solvents [1]. Charge transfer state is reached via an exciplex, i.e. partial charge transfer intermediate state both in solutions [2,3] and in solid films [4,5]. Dyads containing hydrophilic groups either on the porphyrin or the fullerene end of the compound can be deposited as oriented Langmuir–Blodgett (LB) films. The photoinduced electron transfer between the donor and acceptor planes is therefore vectorial [4,5]. Intensive research efforts have been focused on the interaction of gold nanoparticles with chromophores [6]. Especially chromophores attached covalently on the surface of gold nanoparticles [7,8], and measured in solutions, have gained lot of interest. Possible processes following the excitation of the chromophore are e.g. energy [9] or charge transfer [10] to the gold nanoparticle. Gold nanoparticles have strong interaction with singlet excited states of molecules [6] and this phenomenon has been studied widely. There are only few reports on metal clusters and excimer-forming molecules. For example, coupling of pyrene excimer fluorescence to surface plasmons of 100 nm silver islands [11] and formation of excimer between pyrenes attached on surface of gold nanoparticles [10] have been demonstrated. Studies on donor–acceptor dyads and gold nanoparticles are quite rare: for example bis-pyridinium–protoporhyrin dyads have been assembled with citrate-protected gold nanoparticles into conductive films [12]. Our previous study [13] showed that photoelectrical signal, i.e. transient photovoltage response of the porphyrin–fullerene dyad layer was significantly altered by an adjacent gold nanoparticle * Corresponding author. E-mail address: anne.kotiaho@tut.fi (A. Kotiaho). 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.02.042

layer. In addition, the change in the dyad photovoltage depended on the location of the gold nanoparticles relative to the dyad, i.e. gold nanoparticles approaching dyad from the porphyrin or fullerene side [13]. In the present Letter the effect of the gold nanoparticles on the intramolecular exciplex of the porphyrin–fullerene dyad is studied in detail. The porphyrin–fullerene dyads (Scheme 1A) and the gold nanoparticles are arranged in alternating films (Scheme 1B and C). Structures of the dyad and gold nanoparticle films, on a lm-scale, were studied using Brewster angle microscopy (BAM) and fluorescence lifetime microscopy (FLM). Fluorescence spectra and decay measurements on ns-scale are used for monitoring the exciplex state in the films. These results are combined with the photoelectrical studies to investigate the relaxation processes of the intramolecular exciplex in the proximity of the gold nanoparticles. Knowledge of the interaction of the gold nanoparticles with excited states is crucial in order to utilize these particles in photochemical systems.

2. Experimental 2.1. Materials Solvents of analytical grade were obtained from commercial sources. MilliQ water was derived from a Millipore system. Octadecylamine (ODA, 99 %) was purchased from Sigma–Aldrich. Porphyrin–fullerene dyad (PF), Scheme 1A, was synthesized as described previously [14]. Octanethiol-protected gold nanoparticles (AuNP) were prepared according to Brust method [15] and described in detail elsewhere [16]. The core diameter of the gold nanoparticles, estimated from transmission electron microscopy (TEM) images, is approximately 3 nm.

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B

A

Au

Au

F

F

F

P

P

P

HO

OH O

O

ODA

O

O

substrate

O

O O

Ph(tBu)2 NH N

O

N HN

C

F

F

F

P

P

P

Au

time. Fluorescence decays were measured with a time-correlated single photon counting (TCSPC) system from PicoHarp as described elsewhere [16]. Excitation wavelength was 405 nm. Time resolution was approximately 100 ps. The decays were fitted globally to P a multiexponential model, I(t) = a0 + aiexp( t/si). The pre-exponential factors ai were corrected to instrument wavelength sensitivity using a correction spectrum supplied by the manufacturer. 2.5.2. Fluorescence lifetime microscopy (FLM) A time-resolved fluorescence microscope MicroTime 200 from PicoQuant was used for recording the fluorescence lifetime images of the films. Excitation wavelength was 405 nm from a pulsed diode laser head LDH-P-C-405B. The emission was detected on a broad wavelength range, approximately 410–1000 nm.

Au ODA

substrate Scheme 1. (A) Chemical structure of the PF dyad, and schematic illustrations of (B) PF|AuNP and (C) AuNP|PF film structures.

2.2. Film preparation Surface pressure–mean molecular area isotherm measurements and film depositions were carried out using LB 5000, LB Minitrough and Minialternate systems from KSV Instruments. Subphase temperature was set to 18 ± 1 °C. Chloroform solution of PF dyad (<0.5 mM) was spread on a subphase containing 0.5 mM Na2HPO4 and 0.1 mM NaH2PO4 in MilliQ water. The PF deposition was done by horizontal lifting, i.e. Langmuir–Schäfer (LS) method at surface pressure of 17 mN m 1. Gold nanoparticles were spread using a 0.5 mg mL 1 chloroform solution on a MilliQ water subphase. The AuNP films were deposited by LS method at surface pressure of 10 mN m 1. Samples for optical and photoelectrical measurements were deposited on cleaned [5,13] glass and indium tin oxide (ITO) coated (sheet resistance approximately 10 X/h) glass plates, respectively. Three ODA layers were deposited on the glass plates to make the surface hydrophobic before horizontal deposition of photoactive layers. For photoelectrical measurements, the photoactive layers were insulated from the ITO and InGa electrodes by ODA layers [5]. 2.3. Brewster angle microscopy (BAM) BAM images were recorded by a BAM 300 setup combined with a KSV 10001 minitrough system, both from KSV Instruments. 2.4. 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. 2.5. Time-resolved fluorescence 2.5.1. Decay associated spectra (DAS) The decay associated spectra were determined by measuring fluorescence decays at each monitoring wavelength for a constant

Fig. 1. BAM images (600  400 lm) of PF Langmuir film at different surface pressures (A) 0 mN/m, (B) 1 mN/m, and (C) 18 mN/m.

A. Kotiaho et al. / Chemical Physics Letters 471 (2009) 269–275

2.6. Photovoltage Photoinduced charge transfer was studied with the time-resolved Maxwell displacement charge (TRMDC) method [5,17]. Samples were excited by 10 ns pulses at a wavelength of 430 nm from a tunable Ti:sapphire laser. Briefly, the photoactive layers are insulated from ITO and InGa electrodes and thus no current passes through the sample. Photoinduced charge movement perpendicular to film plane in the photoactive layers induces a potential difference between the electrodes, i.e. photovoltage, and decay of this signal is recorded. The sign of the photovoltage is determined by the direction of the charge movement in such a way that a positive photovoltage indicates electron transfer in direction from InGa electrode to ITO electrode and vice versa. Photovoltage amplitude is proportional to the number of charges moving, and to the charge separation distance. 3. Results and discussion 3.1. Brewster angle microscopy (BAM) PF dyads are deposited as 100% LS films to obtain full surface coverage of PF dyads and higher monolayer absorbance compared to amphiphile-mixed PF monolayers. Film organization on molecular scale cannot be resolved by BAM, but an overall impression of the quality of 100% PF Langmuir film on lm-scale is obtained. Langmuir films of porphyrin–fullerene pairs have been prepared earlier by using an anionic, water-soluble porphyrin combined with a cationic fullerene [18], ionic porphyrin–fullerene dyads [19], or by mixing relatively hydrophobic dyads with amphiphilic matrix molecules [5]. Recently, phthalocyanine-fullerene dyads with relatively high hydrophobicity have been deposited as 100% LS films, for which AFM images indicate formation of aggregated structures [20]. The PF dyads form clusters already before compression of the Langmuir film (Fig. 1A). The island-like clusters are then compressed together (Fig. 1B), and at the deposition pressure the film is free of voids and appears homogeneous (Fig. 1C). The limiting area of PF dyad corresponds to monomolecular layer rather than a bilayer [5]. AuNPs form strongly colored islands during spreading. These islands are interconnected upon compression, but some small voids maintain even at the deposition pressure (Supporting information

Fig. 2. Absorption spectra of PF monolayer (dashed line), AuNP monolayer (dotted line), and PF|AuNP bilayer (solid line). Absorption spectrum of AuNP|PF is similar to that of PF|AuNP bilayer and is left out for clarity. Inset: increase of the absorbances as a function of layer number for glass|(3ODA|PF)  5 (squares), glass|(3ODA|AuNP)  5 (solid circles), glass|(3ODA|PF|AuNP)  5 (solid triangles), and glass|(3ODA|AuNP|PF)  5 (open triangles).

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Fig. 1). The initial domain formation is due to attraction of the particles induced by solvent evaporation, and these domains have persistent voids between them until overcompressed [21]. 3.2. Absorption spectra The PF dyad monolayer has an absorption maximum around 430 nm as shown in Fig. 2. The AuNP monolayer shows a plasmon band at 530 nm (Fig. 2A). The PF and AuNP layers were deposited on top of each other to prepare film structures shown in Scheme 1B and C. Absorption spectra of PF|AuNP and AuNP|PF bilayers are the sums of the corresponding monolayer absorbances (Fig. 2). Multilayer films for DAS measurements were prepared by depositing 3 layers of ODA between the photoactive layers. Absorbances of the multilayer films increase linearly as a function of the number of deposited PF and AuNP layers, as shown in Fig. 2 inset. The increase is linear with slopes of 0.022 for PF, of 0.027 for AuNP, of 0.047 for PF|AuNP, and of 0.046 for AuNP|PF multilayers, respectively. Based on the absorbance spectra, both bi- and multilayer depositions of PF dyad and AuNP films are reproducible. 3.3. Steady-state fluorescence spectra For the porphyrin–fullerene dyads in non-polar solvents, a clear exciplex emission is observed at 700–900 nm and porphyrin monomer emission is very low [22]. The exciplex emission band in LB films is weak compared to solution and less reproducible [5]. For 100% PF monolayer, porphyrin emission around 660 and 720 nm is accompanied with weak and broad exciplex emission at 700–850 nm (Fig. 3A). At the excitation wavelength of 426 nm, the porphyrin absorption is much stronger than that of fullerene. Even though the exciplex emission is weak, it is well reproducible in 100% LS films, on the contrary to ODA-mixed films. The AuNP layer quenches the emission of the PF layers by 30–50% at different wavelengths and independently of the orientation of the dyads relative to the AuNP film, as can be seen from the emission spectra of PF|AuNP and AuNP|PF films shown in Fig. 3A. Transmittance of the AuNP layer is >90% at the excitation wavelength and thus AuNP absorption can be excluded as a reason for the observed fluorescence quenching. It was observed that the intensity of porphyrin emission increases for PF films during aging, but this is not the case for PF|AuNP or AuNP|PF films. The effect of aging on these films is however beyond the scope of this Letter. Contributions of porphyrin and exciplex to the fluorescence spectra are obtained by using appropriate fitting functions for the different emission bands. Porphyrin emission is presented by two Gaussian bands, for which fit parameters are: wavelength of the maximum absorption, ki, and bandwidth, bwi. For the exciplex emission band the fit parameters are: transition free energy, DG0, external reorganization energy, Eext, vibrational energy of the molecule, Evibr, and electron-vibrational constant of the molecule, S (see Ref. [23] for the detailed model). The fit parameters (see Supporting information Table 1) are relatively similar for all of the films: PF, PF|AuNP and AuNP|PF. The exciplex emission fit parameters are in qualitative agreement with those observed for PF dyad in non-polar solvents [22]. The most important conclusion from the results of fittings is the quenching of the PF exciplex emission by one third by the AuNP layer, as seen by comparing the exciplex bands for the different films in Fig. 3B. After the exciplex has formed, it can in principle transfer energy by Förster mechanism to the adjacent AuNP layer, because of some overlap of the PF exciplex emission and AuNP absorption. This energy transfer might play some role in the quenching of the PF exciplex emission by AuNP layer. In any case, quenching of the exciplex emission indicates that this state is relaxing into some other transient state, e.g. charge separated

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Fig. 3. (A) Fluorescence spectra of PF monolayer (solid line), PF|AuNP bilayer (dotted line) and AuNP|PF bilayer (dashed line). Excitation wavelength was 426 nm. (B) Fits of fluorescence spectra by two Gaussian bands and an exciplex emission band for PF (solid line), PF|AuNP (dotted line) and AuNP|PF (dashed line).

state, faster in films containing both PF and AuNP compared to PF film. The quenching of the porphyrin emission by AuNP could lead to quenching of the exciplex emission, but this is not the case: intramolecular exciplex formation is faster than interlayer processes as will be discussed later in the text in more detail. The porphyrin emission of the PF layer, presented by the Gaussian bands, is quenched approximately to half by the AuNP layer. This can be explained by energy transfer between porphyrin and gold nanoparticles, taking place also at distances of few nanometers [16].

Fluorescence decays of the films, containing PF dyad layer together with AuNP layer, were fitted with 4-exponential model, even though it is not exactly known how the gold nanoparticles

3.4. Fluorescence lifetime microscopy (FLM) Fluorescence lifetime imaging is useful in both determining the film structure on the lm-scale and measuring fluorescence lifetimes. The PF film is quite homogeneous (Fig. 4A), though some aggregates or holes could be found by changing the imaged spot. The bilayer films PF|AuNP and AuNP|PF (Fig. 4B and C, respectively) are fairly smooth as well, and the AuNP and PF layers seem to have good coverage on each other. Illustrated colors of the films in Fig. 4A–C are related to fluorescence lifetimes, and change of green color of PF film to blue in PF|AuNP and AuNP|PF films indicates decrease of fluorescence lifetime. In FLM measurements the emitted photons are collected from a wide wavelength range, 600–900 nm. Fluorescence decay curves were obtained from the fluorescence lifetime images of the PF, PF|AuNP and AuNP|PF films presented in Fig. 4. The number of exponential components was chosen so that the fit quality (v2 value) improved more than 10% after addition of one more component. For PF film, a 4-exponential fit was necessary; see Table 1 for lifetimes and pre-exponential factors. The lifetimes are attributed, based on literature values of lifetimes [5,24], from the shortest to the longest, to porphyrin (0.1 ns), fullerene (0.4 ns) and exciplex (1.2 ns) emission of the dyad and to emission from unbound residual porphyrin (5.4 ns), respectively. Decay of the fastest component happens in a timescale shorter than the resolution of FLM instrument (100 ps) and the shortest lifetime is therefore considered to be inaccurate. Addition of the fourth exponent improves the fit quality, but the amplitude of this component is very small, as can be expected for a porphyrin impurity. The three shortest lifetimes are in qualitative agreement with those observed for PF dyad in ODA-mixed films after excitation at 405 nm: porphyrin (s  70 ps), fullerene (s  0.6 ns) and exciplex (s  2 ns) [5]. The longest lifetime attributed to unbound porphyrin in 100% PF film is comparable to fluorescence lifetime of non-aggregated porphyrin in dilute ODA-mixed film (8 ns) [24].

Fig. 4. Fluorescence lifetime microscope images of (A) PF monolayer, (B) PF|AuNP and (C) AuNP|PF bilayers. Size of the images is 15.5 lm  15.5 lm. Excitation wavelength was 405 nm.

A. Kotiaho et al. / Chemical Physics Letters 471 (2009) 269–275 Table 1 Fluorescence lifetimes (si) and pre-exponential factors (ai) obtained from 4-exponential fits of FLM decays. savg is amplitude-weighted lifetime and v2 is weighted mean square deviation.

s1 (ns) PF PF|AuNP AuNP|PF

s2 (ns) a2 (%)

s3 (ns) a3 (%)

s4 (ns)

a1 (%) 0.11 ± 0.01 46.6 0.030 ± 0.001 55.4 0.020 ± 0.001 59.8

0.39 ± 0.01 48.7 0.16 ± 0.01 32.3 0.11 ± 0.01 26.7

1.2 ± 0.1 4.5 0.49 ± 0.02 11.8 0.35 ± 0.01 13.0

5.4 ± 0.7 0.2 2.4 ± 0.2 0.5 1.4 ± 0.2 0.4

savg (ns)

v2

0.31

1.067

0.14

1.046

0.094

1.086

a4 (%)

affect the fluorescence decays of the different emitting species. Decrease in lifetimes of all four components compared to PF is seen for both PF|AuNP and AuNP|PF films (Table 1). 3.5. Decay associated spectra The emitting species of the PF dyad film, i.e. porphyrin, fullerene and exciplex can be separated by the different spectral shapes in addition to the different fluorescence lifetimes [5]. A global fit using 4-exponential model was applied to the decay curves collected with the TCSPC instrument in emission wavelength range 620–800 nm for the PF, PF|AuNP and AuNP|PF films. The number of exponents used in the fit was based on knowledge of four emitting species, supported by the FLM decays. This fit resulted in components that have different lifetimes and partly different spectral shapes, and which should be attributable to specific emitting species.

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In the PF film, the 0.14 ns component has the narrowest band, corresponding in shape to the porphyrin emission (Fig. 5A). The component with lifetime 0.46 ns in PF film is extended to the red wavelengths compared to that of 0.14 ns component. Fullerene has a emission band around 720 nm in non-polar solvents [25]. The broadening of the peak and a fluorescence lifetime corresponding to that of fullerene indicate that 0.46 ns component is composed of both porphyrin and fullerene emissions. The broadest band of PF film belongs to the 1.5 ns component, with a lifetime corresponding to that of the exciplex. The 1.5 ns component contains porphyrin emission at the shorter wavelengths and exciplex emission at the longer wavelengths. The longest component has small amplitude, and its shape and lifetime indicate unbound residual porphyrin, and this spectrum was left out from the figure for clarity. This was also done in the case of PF|AuNP and AuNP|PF decay associated spectra. When the component spectra with different lifetimes are summed, the resulting spectra for PF, PF|AuNP and AuNP|PF are in a qualitative agreement with the steady-state spectra of the corresponding film structures. The similarities of the different components (i.e. 0.14; 0.46 and 1.5 ns) in the PF decay associated spectra, arise from the porphyrin emission of the dyad, which is strong and distributed in a wide lifetime range due to the variation of environment and organization of the PF dyads in film. Relaxation of exciplex in porphyrin–fullerene dyad film is non-exponential [26]. Exponential fit may thus lead to similarity of the time-resolved components in the case of films. In PF|AuNP film, the shape of the spectrum of the shortest component, 0.06 ns is quite similar to porphyrin emission (Fig. 5B). The spectra of the two longer components, 0.25 and 1.2 ns, are very similar to each other and correspond in shape to combination of

Fig. 5. Normalized decay associated spectra of (A) PF, (B) PF|AuNP and (C) AuNP|PF multilayer films. Excitation wavelength was 405 nm.

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porphyrin and fullerene emissions. The major change in PF|AuNP film decay associated spectra compared to PF film is the less significant presence of the spectral feature attributable to the exciplex. This indicates that there might be some specific interaction between the AuNP film and the fullerene moieties of the dyad, thus causing a change in the relaxation path of the excited PF dyad film. The lifetimes for the porphyrin and fullerene emissions are also shorter than those of the PF film. Spectra of the AuNP|PF film (Fig. 5C) has three components, and they have similar shapes as for PF film (Fig. 5A) but shorter lifetimes. This indicates that even though all the emissive species of the PF dyad decay faster in AuNP|PF film compared to the PF film, the relative amounts of these species actually stay the same. The AuNP film thus has no effect on the relaxation path of the excited PF dyad in AuNP|PF film, except for the shorter fluorescence lifetimes. 3.6. Photovoltage The TRMDC photovoltage method is a valuable tool for studying charge transfer in films. With this method, it has been shown that PF dyads incorporated in an amphiphilic matrix orient in LB films with the fullerene moiety towards the water and the porphyrin moiety towards the air phase [5]. The observed photovoltage signal of the 100% PF dyad monolayer (Fig. 6) confirms that PF dyads are oriented also in 100% films. Some of the film forming PF dyads may however differ from the ideal organization on the air–water interface. The photovoltage signals of PF|AuNP films are in agreement with the previous results obtained with amphiphile-mixed films of porphyrin–fullerene dyad, with hydrophilic groups on the porphyrin moiety, deposited with octanethiol-protected gold nanoparticle films [13]. The electrons move from the AuNP layer to the porphyrin moiety in AuNP|PF film, thus enhancing the electrical signal of the dyad (Fig. 6). Also in agreement with the previous results [13], the photovoltage signal of the PF|AuNP film is opposite to that of the dyad reference, indicating electron transfer from the AuNP layer to the PF layer (Fig. 6). The AuNP film shows a photovoltage close to zero (not shown in the figure). Relaxation of the porphyrin from the singlet excited state to exciplex in PF dyad is very fast (<100 fs) in films [26] and any interlayer processes between AuNP and PF layers are assumed to be slower than this, for example energy transfer between porphyrin film and adjacent gold nanoparticle film was observed to take place in 20 ps [16].

Charge transfer in porphyrin–fullerene dyad films has been demonstrated by photoelectrical [5] and optical [27] measurements, and efficiency for charge transfer in drop casted films of a porphyrin–fullerene dyad is estimated to be <25% [26]. The AuNP film facing porphyrin moiety of the dyad, i.e. in AuNP|PF film, evidently enhances the photovoltage response of the dyad. Two parallel mechanisms for this enhancement can be presented: (1) after the charge transfer in the PF dyad, a hole is transferred to AuNP layer thus increasing the charge separation distance, and (2) the excitation of porphyrin moiety of the PF dyad leads to exciplex formation, and AuNP layer enhances charge transfer in the PF dyad, thus increasing the efficiency and distance of the charge transfer. Both of these mechanisms are in agreement with the fluorescence studies, where the exciplex character of the PF dyad is preserved in AuNP|PF film, and the lifetime of the exciplex is decreased. Explanations for the reversed photovoltage signal of PF|AuNP compared to PF film are (1) competing electron transfer from AuNP to fullerene (2) competing electron transfer from AuNP to porphyrin and (3) reduction of the charge transfer efficiency of the PF dyad by AuNP layer. Photoinduced electron transfer from AuNP film to fullerene film has been observed [13]. Diminishing exciplex character of PF|AuNP film observed in DAS measurement indicates some more complex mechanism than electron transfer from AuNP to porphyrin moiety of the dyad. Energy transfer processes are not observable by photovoltage method, but would be seen in fluorescence measurements. Energy transfer from gold nanoparticles, excited at the surface plasmon absorbance, to fullerene attached on the nanoparticle surface has been observed in solution [28]. The energy and electron transfers from AuNP to fullerene moiety of the dyad might thus oppose the exciplex changing in to a charge separated state in PF|AuNP film. 4. Conclusions Porphyrin–fullerene dyads can be deposited as 100% LS monolayers. Alternating films of PF dyads and AuNPs can be deposited with good relative coverage between the adjacent layers. Reduction of fluorescence intensity and lifetime of the PF exciplex by the adjacent gold nanoparticle film is observed. Also the porphyrin monomer emission of the PF dyad is quenched by the gold nanoparticle film. Based on photoelectrical response, gold nanoparticle film transfers electrons to the PF dyad layer after photoexcitation. Energy transfer reactions may play some role in quenching of the exciplex as well. These studies indicate interlayer interaction of gold nanoparticles with singlet excited state and intramolecular exciplex state of a chromophore, i.e. porphyrin–fullerene dyad. A gold nanoparticle monolayer thus has relatively long range interaction with adjacent chromophore films. Furthermore, the gold nanoparticle thin film has the ability to affect even intramolecular processes, i.e. exciplex relaxation in porphyrin–fullerene dyad film. Acknowledgement A.K. and R.L. acknowledge the Academy of Finland (No. 107182) for financial support. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2009.02.042. References

Fig. 6. Photovoltage decays of PF monolayer (solid line), PF|AuNP bilayer (dotted line) and AuNP|PF bilayer (dashed line). Excitation wavelength was 430 nm and excitation energy density 0.06 mJ/cm2. Inset shows the decay curve on a shorter timescale.

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