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Involvement of C60 fullerene monomers and aggregates in the photoconductivity of ultrathin bilayer lipid membranes
ELSEVIER
SyntheticMetals77 (1996) 103-106
Involvement of CGOfullerene monomers and aggregates in the photoconductivity of ultrathin bilayer lipid membranes J.M. Janot a, P. Seta al*, R.V. Bensasson b, S. Leach ‘yd a Laboratoire b Laboratoire
des Membranes et Procedes Membranaires, CNRS UMR 9987, 1919 route de Mende, BP 5051,34033 Montpellier Cedex 01, France de Biophysique, Muse’um National d’tiistoire Naturelle, CNRS URA 481, INSERM U 201, 43 rue Cuvier, 75231 Paris Cedex 05, France ’ Laboratoire de Photophysique Mol&laire du CNRS, Universite’de Paris-&d, 91405 Orsay, France ’ DAMAP, CNRS URA 812, Observatoire de Paris-Meudon, 92195 Meudon, France
Abstract
Due to its hydrophobic character,the hydrocarboncore of ultrathin phospholipid bilayer membraneslends itself to the insertion of fullerenes. When doped with CeOthesemembranes arephotoconducting, the photoconductivity being affectedby the presenceof quenchersof the excited triplet state such as oxygen. The photoconductivity involves trans-membrane electron transfer; its dependenceon CeOconcentration and on the excitation wavelength suggeststhe participation of small aggregatesin the membrane. As derived from time-resolved triplet absorption and fluorescencequenching measurements, the suggestedmechanism makes more precise (i) the role of the aggregateswhich might act as a collecting antenna for the benefit of the monomers, and (ii) the role of the monomers which have a much higher yield of photoinduced triplet stateformation than the aggregates. Keywords:
Fullerene;Photoconductivity;Membranes;Lipids
1. Introduction Since their discovery, fullerenes have continuously attracted the interest of the scientific community, due to the diversity of their physicochemical properties and of their structural arrangements. They not only appear as individual molecules in organic solutions or in various matrices, polymers [ 1,2], cyclodextrins [ 31, but also as diverse forms of molecular solids and condensed species (microcrystals, tubules, onion-like structures [ 41) . The photoluminescence of fullerenes has been shown to be tightly linked to their physical state of organization [5], and the electric conductivity and photoconductivity depend on factors such as the presence of oxygen [ 61 or the state of polymerization [ 71. Thus, it appeared interesting to know how the fullerenes behave in host matrices, as isolated molecules without cooperativity, or as molecules associated within molecular aggregates. Ultrathin membranes are a convenient system in order to address this issue. This paper builds on previous results which showed that CGoincorporated in bilayer lipid membranes (BLMs) is a better electrical conductor under photoexcitation than in the ground state [ 81. On the basis of new time-resolved spectro* Correspondingauthor.TeI.: +33 67 613389; fax: + 33 67 042820. 0379-6779/96/$15.00
0 1996ElsevierScienceS.A.All rights reserved
scopic results (triplet absorption and fluorescence lifetimes), we make more precise the photoconductivity mechanism which implies the formation of Cho- anion radicals thanks to the high electron acceptor affinity of the singlet and triplet excited states of Cco. The possible role of small fullerene aggregates which act as an antenna for collecting photons, in the presence of monomers, will be discussed.
2. Experimental 2.1. Time-resolvedtriplet-triplet absorption spectroscopy
The spectrometer built at the Laboratoire des Membranes et Pro&d& Membranaires (LMPM), Montpellier, has for pump Iight source a frequency doubled 532 nm, 6 ns pulsed YAG Quantel model YG 585 10G at the repetition rate of 10 Hz. The probe light source is a 75 W OSRAM xenon lamp. The absorption spectra are processed by Princeton Applied Research instruments, an OMA 3 optical multichannel analyser connected to a model 1455 BHQ diode array detector and gated by means of a model 1304 pulse amplifier. The system is linked to a Hewlett-Packard HP 9000 computer for spectra collection. Each spectrum corresponds to an averaging of 100 shots which are stored for 100 ns and are initiated
104
J.M. Janot et al. /Synthetic
less than 1 ns after the excitation pulse. The samples are degassed with argon bubbling prior to the experiments. 2.2. Time-resolvedfluorescence
The fluorescence lifetimes have been measured using the single photon counting technique. The excitation pulse is created by a Spectra Physics model 2030, 82 MHz mode locked argon laser, synchronously pumping a Spectra Physics model 375 rhodamine 6G dye laser. The time between successive excitation pulses is controlled by means of a Spectra Physics model 344 cavity dumper, the frequency being reduced to 4 MHz. The fluorescence detector is a Hamamatsu model 1564 U multichannel plate photomultiplier. The response function of the apparatus (ORTEC model 457 time amplitude converter, TRACOR NORTHERN model 1750 multichannel analyser) is in the range of 100 ps (FWHM) , measured by scattering the excitation light in a dilute solution of Ludox. The pump profile is accumulated using the same count rate and the same number of counts at the maximum of the decay (about 30 000 counts at the peak of the decay curve or 2X lo6 counts for the integrated decay). The lifetimes are obtained from the experimental decays with a program using the Marquardt algorithm. The decay curves are fitted by convolution of a calculated fluorescence decay, calculated as i(t) =Ca,exp(
-t/rJ
with the excitation profile obtained from the scattering sample (ai and ri are the amplitude and lifetime of the different components of the total fluorescence emission i(t) at time t, respectively). The adjustment of the parameters of the fit is stopped when the minimum of the statistical x2 criterion is reached. 2.3. BLM experiments
The set-up is described in Ref. [ 81. The membrane-forming solution is 4X 10v4 M Cc0 and lo-’ M L-a-diphytanoyllecithin in n-decane; the buffer is 10-i M NaCl and lo-’ M HEPES in water, The oxidant compartment is 3 X 10m4 M anthraquinone-2-sulfonic acid sodium salt; the reductant one 5 X lo-’ M L-ascorbic acid sodium salt.
Metals
77 (1996)
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as the result of an increase by ten of the fullerene concentration (from 4 X low5 to 4 X 10m4 M) in the membrane-forming solution. This observation has been tentatively interpreted as the result of the formation of fullerenes associated in small molecular aggregates. The aggregates could either be less efficient in the electron-hopping mechanism, or have a lower yield of photoinduced triplet state [ 91. In the solid the fullerene triplet state was not detectable by absorption spectroscopy [lo], but it has been observed by optically detected magnetic resonance [ 111. The dominant role of the fullerenephotoexcited triplet state (ET = 1.60 eV [ 81) in the photoconduction process has been shown by the quenching of the photocurrents by oxygen or tetracene (ET = 1.27 eV [ 121). However, even at quencher concentrations much higher than the C6e concentration in the membrane, the photocurrents are not totally suppressed. In order to address the issue of the respective roles of the singlet and the triplet states on the one hand, and of monomers and aggregates on the other hand, we have designed spectroscopic measurements (singlet absorption, triplet-triplet absorption and fluorescence lifetime) on three systems, the membrane-forming solution, Triton X-100 micelles and phospholipid vesicles; the aims are the characterization of the formation of the triplet state in these media, which are the precursors or the models of the BLMs, and the elucidation of the role of the singlet excited state in the photoconductivity. 3.1. Influence of the mediumon the appearanceof aggregates
The spectra of Cc0in various media are collected in Fig. 1. The absorbance of a membrane-forming solution containing &/lipid (phosphatidylcholine) /n-decane 2/100/1460 wt./wt. (Fig. l(a)) shows the spike at 410 nm, but not the valley at 4.50 nm observed with a solution of C6a in n-hexane (Fig. 1(b) ) and which are characteristic of the monomers [ 13,141. The spectra of a micellar Triton X-100 solution and
3. Results Our previous results concerned the stationary photocurrents measured under steady illumination of a BLM doped with C6,, separating two aqueous phases, a reductant phase and an oxidant one [ 81. The results were in accordance with the formation of a fullerene anion at the reductant interface, followed by electron transport across the membrane, mediated by the fullerene molecules. The dependence of the photocurrents with fullerene concentration was not linear. A change in photocurrent of only a factor of two was observed
400
600 600 700 800 Wavelength (nm) Fig, 1. Absorption spectra of C& in various media: (a) C,/phosphatidylcholine 2% wt./wt. n-decane solution; (b) Cm in n-hexane; (c) C,,/Triton X-100 aqueous micelles; (d) Cm incorporated in phosphatidylcholine vesicles; (e) Cm film deposited on a quartz substrateby the Langmuir-Blodgett technique from a layer of 50 ,&*/molecule molecular area.
J.M. Janot et al. /Synthetic
Metals
77 (1996)
103-106
105
7-
I
65-
I Jz 4p” 32 :/ wavelength
(nm)
Fig. 2. Transient triplet-triplet absorption spectrum of a CJphosphatidylcholine 2% wt./wt. n-decane solution (ultrathin membrane-forming solution).
of vesicles of C,,/lipid (phosphatidylcholine) 2/ 100 wt./ wt. in water (Fig. 1(c) , (d) ) still less resemble the spectrum of the solution. They show a broad band peaking at approximately 450 nm characteristic of the solid state (C6,, layers on a glass substrate, Fig. 1(e) ) . Moreover, the fluorescence emission of CeOin the membrane-forming solution resembles that of the toluene solution, confirming the presence of monomers in the membrane-forming solution. Thus, the membrane-forming solution contains both monomers and aggregates, whereas the micelles and vesicles contain aggregates. We are not equipped to study the absorption of the BLMs directly. It should be in between that of the membrane-forming solution and that of the vesicles. The BLM contains much less decane than its precursor, but some decane is still present.
/
3
lfl 4 0.0
L
,
0.2
I
0.4
I
0.6
I
0.8
' 0
IQl/M Fig. 3. Variation of the Cm fluorescence lifetime ratio TJ~ vs. tripropylamine quencher concentration Q.
incorporated in the lipid vesicles (2%), fluorescence lifetimes could not be measured in the vesicle system which closely resembles the BLM. The lifetimes were measured in toluene solution in which C,, is sufficiently soluble. The electron donor ascorbic acid, used in the BLM system, is not soluble in toluene and tripropylamine was chosen instead; amines are known to give electrons to photoexcitedfullerenes [ 171. A linear relation (Fig. 3) was observed between r,,/~ (7 fluorescence lifetime with amine, r. without amine) and the amine concentration Q (0 to 1 M) . This linearity is in accordance with a competition between electron transfer from the amine to the singlet state and intersystem crossing from the singlet state to the triplet state. The equation: T~/T= 1 +knr[ Q] r0
3.2. Aggregates and triplet state
The transient absorption spectrum of the membrane-forming solution measured for 100 ns after the excitation at 532 nm (Fig. 2) shows an absorption maximum at 750 nm characteristic of the triplet-triplet absorption measured in C,, benzene solutions [ 151. The observed AA at the 750 nm peak is identical to the one measured for a reference CeObenzene solution having the same ground state absorbance at 532 nm. Thus, the yield of formation of the CeOtriplet state in this environment can be estimated to be near unity, as for the benzene solution [ 161. Neither in the micellar solution, nor in the vesicle suspension did we observe any triplet-triplet absorption. However, in the vesicle case, the non-existence of triplet states could not be ascertained, because from the first laser shots the suspension turned brownish and black solid particles deposited on the cell walls, probably due to a photodegradation of the sample. 3.3. Electron transfer rate constant to C,, singlet excited state
Due to the very low fluorescence quantum yield of the fullerenes, and the low maximum percentage of CGObeing
where Q is the amine concentration, gives a 5 X lo9 M- ’ s- ’ value for the electron transfer rate constant kET. 3.4. Wavelength dependenceof the photocurrent
The photocurrents measured at a 457.9 nm excitation wavelength are more than twice the one measured at a 514.5 nm excitation wavelength.
4. Discussion
The quenching of the photocurrents measured in C,,-doped BLMs by oxygen and other quenchers of triplet excited states (added in the membrane-forming solution) proves the contribution of this state to the photoconduction process, However, the quenching is not complete; electron transfer is also possible in the singlet excited state, in spite of the very rapid intersystem crossing process which has a lo9 s-l estimated rate constant [ 181. In order to ensure 5 to 10% of the photocurrent value in the presence of a quencher [ 81, the electron transfer rate from ascorbic acid to the C6e singlet excited state has to be of the order of 5X107 to ~O*M-‘S-~. With an experimental 5 X lo-’ M donor concentration, the rate con-
106
J.M. Janot et al. /Synthetic
stant for the electron transfer is estimated as 10’ to 2X 10’ S - *. This value is of the order of magnitude of the electron transfer rate constant between tripropylamine and photoexcited Cc0 in toluene (see Section 3), consistent with the singlet excited state participation. The importance of the triplet state participation on the one hand and of the aggregates in dispersed phases on the other hand is at first sight in contradiction. No CeOtriplet states were detected by laser flash photolysis for fullerenes dispersed in liposomes, except when decane was added to create the membrane-forming solution. In the latter case a triplet yield similar to that observed in aromatic solvents was measured [ 151. This demonstrates that aggregation, more marked in the absence of decane, results in a drastic decrease in the yield of observed triplet state. The predominating importance of the photocurrent linked to the triplet state participation argues in favour of the greater resemblance of the BLM to the membrane-forming solution. The hydrocarbon which stays in the BLM must favour the solubilization of monomers and, consequently, the formation of triplet states. However, the photoconductivity cannot be uniquely ascribed to monomers, because the photocurrents measured at a 457.9 nm excitation wavelength (where the aggregates have stronger absorption than the monomers) are much higher than the ones measured at a 514.5 nm excitation wavelength. In order to reconcile all the results, we hypothesize the coexistence in the membrane of Cc0monomers and small size microaggregates (a few molecules taking account of the approximately 35 p\ width of the hydrophobic core of the BLMs [ 81). The extinction coefficient of the aggregated form at the excitation wavelength being larger than that of the monomers, the absorption spectrum is dominated by the contribution of the aggregates. The energy absorbed by the aggregates is then transferred to the monomers by a singletsinglet excitation energy transfer, in the same way as the energy is collected by the chlorophyll antenna in reaction centres. The excited monomers are then reduced by the donor species at the BLM interface, either in the singlet excited state, or predominantly in the triplet excited state classically derived by intersystem crossing. An alternative mechanism involves exciton-induced charge injection at the interface of
Metals
77 (1996)
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the Cc0 aggregates with the BLM, without transfer to molecularly dispersed fullerene molecules. Acknowledgements We thank J.L. Garaud and G. Miquel of the LMPM for their contribution to the membrane experiments, E. Bienvenue for helpful discussions and C. Bertrand and B. d’Epenoux for technical assistance. We are grateful for support from the GDR ‘ComposBs de 1aFamille de CGO’of the CNRS. References [l] K. Lee, R.A.J. Janssen,N.S. Sariciftci and A.J. Heegcr, Phys. Rev. B, 49 (1994) 5781. [2] A. Watanabe and 0. Ito, J. Phys. Chem., 98 (1994) 7736. [3] T. Anderson, G. Westman, 0. WennerstrSm and M. Sundahl, J. Chel?l. SOL, Perkin Trans. 2, (1994) 1097. [4] MS. Dresselhaus, G. Dresselhaus and PC. Eklund, J. Muter. Res., 8 (1993) 2054. [5] W. Guss, J. Feldmann, E.O. Gabel, C. Taliani, H. Mohn, W. Miiller, P. HLussler and H.U. ter Meer, Phyys. Rev. Lett., 72 (1994) 2644. [6] S. Kazaoui, R. Ross and N. Minami, SolidState Comrmm., 90 (1994) 623. [7] N. Takahashi, H. Dock, N. Matsuzawa and M. Ata, J. Appl. Phyys., 74 (1993) 5790. [8] R.V. Bensasson,J.L. Garaud, S. Leach, G. Miquel and P. Seta, Cl~ern. Phys. Lett., 210 (1993) 141. [9] F. Wilkinson, in J.B. Birks (ed.), Organic Molecular Phofophyysics, Wiley, London, 1975, p. 95. [lo] K.L. Akers, C. Douketis, T.L. Haslett and M. Moskovits, J. Phyys. Chem., 98 (1994) 10 824. [ll] P.A. Lane, L.S. Swanson, Q.X. Ni, J. Shiner, J.P. Engel, T.J. Barton, J. Wheelock and L. Jones,Phys. Rev. Mt., 68 (1992) 887. [12] A.A. Lamola, W.G. Herkstoer, J.C. Dalton and G&Hammond, J. Chem. Phys., 42 (1965) 1715. [ 131 S. Leach, M. Vervloet, A. Despres, E. Breheret, J.P. Hare, T.J. Dennis, H.W. Kroto, R. Taylor and D.R.M. Walton, Chern. Phys., 160 (1992) 451. [ 141 R.V. Bensasson,E. Bienveniie, M. Dellinger, S. Leach and P. Seta, J. Phys. Chern., 98 (1994)
3492.
[15] R.V. Bensasson, T. Hill, C. Lambert, E.J. Land, S. Leach and T.G. Truscott, Chem. Phys. Lett., 701 (1993) 326. [16] J.W. Arbogast, A.P. Darmangan, C.S. Foote, Y. Rubin, F. Diederich, M.M. Alvarez, S.J.Anz and R.L. Whetten,J. Phys. Chem., 95 (1991). [ 171 D. Kim and M. Lee, .I. Am. Chem. Sot., 114 (1992) 4429. [18] R.J. Sension, CM. Phillips, A.Z. Szarka, W.J. Romanow, A.R. McGhie, J.P. McCauley, Jr., A.B. Smith III and R.M. Hochstreasser, J. Phys. Chem., 95 (1991) 6075.
SyntheticMetals77 (1996) 103-106
Involvement of CGOfullerene monomers and aggregates in the photoconductivity of ultrathin bilayer lipid membranes J.M. Janot a, P. Seta al*, R.V. Bensasson b, S. Leach ‘yd a Laboratoire b Laboratoire
des Membranes et Procedes Membranaires, CNRS UMR 9987, 1919 route de Mende, BP 5051,34033 Montpellier Cedex 01, France de Biophysique, Muse’um National d’tiistoire Naturelle, CNRS URA 481, INSERM U 201, 43 rue Cuvier, 75231 Paris Cedex 05, France ’ Laboratoire de Photophysique Mol&laire du CNRS, Universite’de Paris-&d, 91405 Orsay, France ’ DAMAP, CNRS URA 812, Observatoire de Paris-Meudon, 92195 Meudon, France
Abstract
Due to its hydrophobic character,the hydrocarboncore of ultrathin phospholipid bilayer membraneslends itself to the insertion of fullerenes. When doped with CeOthesemembranes arephotoconducting, the photoconductivity being affectedby the presenceof quenchersof the excited triplet state such as oxygen. The photoconductivity involves trans-membrane electron transfer; its dependenceon CeOconcentration and on the excitation wavelength suggeststhe participation of small aggregatesin the membrane. As derived from time-resolved triplet absorption and fluorescencequenching measurements, the suggestedmechanism makes more precise (i) the role of the aggregateswhich might act as a collecting antenna for the benefit of the monomers, and (ii) the role of the monomers which have a much higher yield of photoinduced triplet stateformation than the aggregates. Keywords:
Fullerene;Photoconductivity;Membranes;Lipids
1. Introduction Since their discovery, fullerenes have continuously attracted the interest of the scientific community, due to the diversity of their physicochemical properties and of their structural arrangements. They not only appear as individual molecules in organic solutions or in various matrices, polymers [ 1,2], cyclodextrins [ 31, but also as diverse forms of molecular solids and condensed species (microcrystals, tubules, onion-like structures [ 41) . The photoluminescence of fullerenes has been shown to be tightly linked to their physical state of organization [5], and the electric conductivity and photoconductivity depend on factors such as the presence of oxygen [ 61 or the state of polymerization [ 71. Thus, it appeared interesting to know how the fullerenes behave in host matrices, as isolated molecules without cooperativity, or as molecules associated within molecular aggregates. Ultrathin membranes are a convenient system in order to address this issue. This paper builds on previous results which showed that CGoincorporated in bilayer lipid membranes (BLMs) is a better electrical conductor under photoexcitation than in the ground state [ 81. On the basis of new time-resolved spectro* Correspondingauthor.TeI.: +33 67 613389; fax: + 33 67 042820. 0379-6779/96/$15.00
0 1996ElsevierScienceS.A.All rights reserved
scopic results (triplet absorption and fluorescence lifetimes), we make more precise the photoconductivity mechanism which implies the formation of Cho- anion radicals thanks to the high electron acceptor affinity of the singlet and triplet excited states of Cco. The possible role of small fullerene aggregates which act as an antenna for collecting photons, in the presence of monomers, will be discussed.
2. Experimental 2.1. Time-resolvedtriplet-triplet absorption spectroscopy
The spectrometer built at the Laboratoire des Membranes et Pro&d& Membranaires (LMPM), Montpellier, has for pump Iight source a frequency doubled 532 nm, 6 ns pulsed YAG Quantel model YG 585 10G at the repetition rate of 10 Hz. The probe light source is a 75 W OSRAM xenon lamp. The absorption spectra are processed by Princeton Applied Research instruments, an OMA 3 optical multichannel analyser connected to a model 1455 BHQ diode array detector and gated by means of a model 1304 pulse amplifier. The system is linked to a Hewlett-Packard HP 9000 computer for spectra collection. Each spectrum corresponds to an averaging of 100 shots which are stored for 100 ns and are initiated
104
J.M. Janot et al. /Synthetic
less than 1 ns after the excitation pulse. The samples are degassed with argon bubbling prior to the experiments. 2.2. Time-resolvedfluorescence
The fluorescence lifetimes have been measured using the single photon counting technique. The excitation pulse is created by a Spectra Physics model 2030, 82 MHz mode locked argon laser, synchronously pumping a Spectra Physics model 375 rhodamine 6G dye laser. The time between successive excitation pulses is controlled by means of a Spectra Physics model 344 cavity dumper, the frequency being reduced to 4 MHz. The fluorescence detector is a Hamamatsu model 1564 U multichannel plate photomultiplier. The response function of the apparatus (ORTEC model 457 time amplitude converter, TRACOR NORTHERN model 1750 multichannel analyser) is in the range of 100 ps (FWHM) , measured by scattering the excitation light in a dilute solution of Ludox. The pump profile is accumulated using the same count rate and the same number of counts at the maximum of the decay (about 30 000 counts at the peak of the decay curve or 2X lo6 counts for the integrated decay). The lifetimes are obtained from the experimental decays with a program using the Marquardt algorithm. The decay curves are fitted by convolution of a calculated fluorescence decay, calculated as i(t) =Ca,exp(
-t/rJ
with the excitation profile obtained from the scattering sample (ai and ri are the amplitude and lifetime of the different components of the total fluorescence emission i(t) at time t, respectively). The adjustment of the parameters of the fit is stopped when the minimum of the statistical x2 criterion is reached. 2.3. BLM experiments
The set-up is described in Ref. [ 81. The membrane-forming solution is 4X 10v4 M Cc0 and lo-’ M L-a-diphytanoyllecithin in n-decane; the buffer is 10-i M NaCl and lo-’ M HEPES in water, The oxidant compartment is 3 X 10m4 M anthraquinone-2-sulfonic acid sodium salt; the reductant one 5 X lo-’ M L-ascorbic acid sodium salt.
Metals
77 (1996)
103406
as the result of an increase by ten of the fullerene concentration (from 4 X low5 to 4 X 10m4 M) in the membrane-forming solution. This observation has been tentatively interpreted as the result of the formation of fullerenes associated in small molecular aggregates. The aggregates could either be less efficient in the electron-hopping mechanism, or have a lower yield of photoinduced triplet state [ 91. In the solid the fullerene triplet state was not detectable by absorption spectroscopy [lo], but it has been observed by optically detected magnetic resonance [ 111. The dominant role of the fullerenephotoexcited triplet state (ET = 1.60 eV [ 81) in the photoconduction process has been shown by the quenching of the photocurrents by oxygen or tetracene (ET = 1.27 eV [ 121). However, even at quencher concentrations much higher than the C6e concentration in the membrane, the photocurrents are not totally suppressed. In order to address the issue of the respective roles of the singlet and the triplet states on the one hand, and of monomers and aggregates on the other hand, we have designed spectroscopic measurements (singlet absorption, triplet-triplet absorption and fluorescence lifetime) on three systems, the membrane-forming solution, Triton X-100 micelles and phospholipid vesicles; the aims are the characterization of the formation of the triplet state in these media, which are the precursors or the models of the BLMs, and the elucidation of the role of the singlet excited state in the photoconductivity. 3.1. Influence of the mediumon the appearanceof aggregates
The spectra of Cc0in various media are collected in Fig. 1. The absorbance of a membrane-forming solution containing &/lipid (phosphatidylcholine) /n-decane 2/100/1460 wt./wt. (Fig. l(a)) shows the spike at 410 nm, but not the valley at 4.50 nm observed with a solution of C6a in n-hexane (Fig. 1(b) ) and which are characteristic of the monomers [ 13,141. The spectra of a micellar Triton X-100 solution and
3. Results Our previous results concerned the stationary photocurrents measured under steady illumination of a BLM doped with C6,, separating two aqueous phases, a reductant phase and an oxidant one [ 81. The results were in accordance with the formation of a fullerene anion at the reductant interface, followed by electron transport across the membrane, mediated by the fullerene molecules. The dependence of the photocurrents with fullerene concentration was not linear. A change in photocurrent of only a factor of two was observed
400
600 600 700 800 Wavelength (nm) Fig, 1. Absorption spectra of C& in various media: (a) C,/phosphatidylcholine 2% wt./wt. n-decane solution; (b) Cm in n-hexane; (c) C,,/Triton X-100 aqueous micelles; (d) Cm incorporated in phosphatidylcholine vesicles; (e) Cm film deposited on a quartz substrateby the Langmuir-Blodgett technique from a layer of 50 ,&*/molecule molecular area.
J.M. Janot et al. /Synthetic
Metals
77 (1996)
103-106
105
7-
I
65-
I Jz 4p” 32 :/ wavelength
(nm)
Fig. 2. Transient triplet-triplet absorption spectrum of a CJphosphatidylcholine 2% wt./wt. n-decane solution (ultrathin membrane-forming solution).
of vesicles of C,,/lipid (phosphatidylcholine) 2/ 100 wt./ wt. in water (Fig. 1(c) , (d) ) still less resemble the spectrum of the solution. They show a broad band peaking at approximately 450 nm characteristic of the solid state (C6,, layers on a glass substrate, Fig. 1(e) ) . Moreover, the fluorescence emission of CeOin the membrane-forming solution resembles that of the toluene solution, confirming the presence of monomers in the membrane-forming solution. Thus, the membrane-forming solution contains both monomers and aggregates, whereas the micelles and vesicles contain aggregates. We are not equipped to study the absorption of the BLMs directly. It should be in between that of the membrane-forming solution and that of the vesicles. The BLM contains much less decane than its precursor, but some decane is still present.
/
3
lfl 4 0.0
L
,
0.2
I
0.4
I
0.6
I
0.8
' 0
IQl/M Fig. 3. Variation of the Cm fluorescence lifetime ratio TJ~ vs. tripropylamine quencher concentration Q.
incorporated in the lipid vesicles (2%), fluorescence lifetimes could not be measured in the vesicle system which closely resembles the BLM. The lifetimes were measured in toluene solution in which C,, is sufficiently soluble. The electron donor ascorbic acid, used in the BLM system, is not soluble in toluene and tripropylamine was chosen instead; amines are known to give electrons to photoexcitedfullerenes [ 171. A linear relation (Fig. 3) was observed between r,,/~ (7 fluorescence lifetime with amine, r. without amine) and the amine concentration Q (0 to 1 M) . This linearity is in accordance with a competition between electron transfer from the amine to the singlet state and intersystem crossing from the singlet state to the triplet state. The equation: T~/T= 1 +knr[ Q] r0
3.2. Aggregates and triplet state
The transient absorption spectrum of the membrane-forming solution measured for 100 ns after the excitation at 532 nm (Fig. 2) shows an absorption maximum at 750 nm characteristic of the triplet-triplet absorption measured in C,, benzene solutions [ 151. The observed AA at the 750 nm peak is identical to the one measured for a reference CeObenzene solution having the same ground state absorbance at 532 nm. Thus, the yield of formation of the CeOtriplet state in this environment can be estimated to be near unity, as for the benzene solution [ 161. Neither in the micellar solution, nor in the vesicle suspension did we observe any triplet-triplet absorption. However, in the vesicle case, the non-existence of triplet states could not be ascertained, because from the first laser shots the suspension turned brownish and black solid particles deposited on the cell walls, probably due to a photodegradation of the sample. 3.3. Electron transfer rate constant to C,, singlet excited state
Due to the very low fluorescence quantum yield of the fullerenes, and the low maximum percentage of CGObeing
where Q is the amine concentration, gives a 5 X lo9 M- ’ s- ’ value for the electron transfer rate constant kET. 3.4. Wavelength dependenceof the photocurrent
The photocurrents measured at a 457.9 nm excitation wavelength are more than twice the one measured at a 514.5 nm excitation wavelength.
4. Discussion
The quenching of the photocurrents measured in C,,-doped BLMs by oxygen and other quenchers of triplet excited states (added in the membrane-forming solution) proves the contribution of this state to the photoconduction process, However, the quenching is not complete; electron transfer is also possible in the singlet excited state, in spite of the very rapid intersystem crossing process which has a lo9 s-l estimated rate constant [ 181. In order to ensure 5 to 10% of the photocurrent value in the presence of a quencher [ 81, the electron transfer rate from ascorbic acid to the C6e singlet excited state has to be of the order of 5X107 to ~O*M-‘S-~. With an experimental 5 X lo-’ M donor concentration, the rate con-
106
J.M. Janot et al. /Synthetic
stant for the electron transfer is estimated as 10’ to 2X 10’ S - *. This value is of the order of magnitude of the electron transfer rate constant between tripropylamine and photoexcited Cc0 in toluene (see Section 3), consistent with the singlet excited state participation. The importance of the triplet state participation on the one hand and of the aggregates in dispersed phases on the other hand is at first sight in contradiction. No CeOtriplet states were detected by laser flash photolysis for fullerenes dispersed in liposomes, except when decane was added to create the membrane-forming solution. In the latter case a triplet yield similar to that observed in aromatic solvents was measured [ 151. This demonstrates that aggregation, more marked in the absence of decane, results in a drastic decrease in the yield of observed triplet state. The predominating importance of the photocurrent linked to the triplet state participation argues in favour of the greater resemblance of the BLM to the membrane-forming solution. The hydrocarbon which stays in the BLM must favour the solubilization of monomers and, consequently, the formation of triplet states. However, the photoconductivity cannot be uniquely ascribed to monomers, because the photocurrents measured at a 457.9 nm excitation wavelength (where the aggregates have stronger absorption than the monomers) are much higher than the ones measured at a 514.5 nm excitation wavelength. In order to reconcile all the results, we hypothesize the coexistence in the membrane of Cc0monomers and small size microaggregates (a few molecules taking account of the approximately 35 p\ width of the hydrophobic core of the BLMs [ 81). The extinction coefficient of the aggregated form at the excitation wavelength being larger than that of the monomers, the absorption spectrum is dominated by the contribution of the aggregates. The energy absorbed by the aggregates is then transferred to the monomers by a singletsinglet excitation energy transfer, in the same way as the energy is collected by the chlorophyll antenna in reaction centres. The excited monomers are then reduced by the donor species at the BLM interface, either in the singlet excited state, or predominantly in the triplet excited state classically derived by intersystem crossing. An alternative mechanism involves exciton-induced charge injection at the interface of
Metals
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the Cc0 aggregates with the BLM, without transfer to molecularly dispersed fullerene molecules. Acknowledgements We thank J.L. Garaud and G. Miquel of the LMPM for their contribution to the membrane experiments, E. Bienvenue for helpful discussions and C. Bertrand and B. d’Epenoux for technical assistance. We are grateful for support from the GDR ‘ComposBs de 1aFamille de CGO’of the CNRS. References [l] K. Lee, R.A.J. Janssen,N.S. Sariciftci and A.J. Heegcr, Phys. Rev. B, 49 (1994) 5781. [2] A. Watanabe and 0. Ito, J. Phys. Chem., 98 (1994) 7736. [3] T. Anderson, G. Westman, 0. WennerstrSm and M. Sundahl, J. Chel?l. SOL, Perkin Trans. 2, (1994) 1097. [4] MS. Dresselhaus, G. Dresselhaus and PC. Eklund, J. Muter. Res., 8 (1993) 2054. [5] W. Guss, J. Feldmann, E.O. Gabel, C. Taliani, H. Mohn, W. Miiller, P. HLussler and H.U. ter Meer, Phyys. Rev. Lett., 72 (1994) 2644. [6] S. Kazaoui, R. Ross and N. Minami, SolidState Comrmm., 90 (1994) 623. [7] N. Takahashi, H. Dock, N. Matsuzawa and M. Ata, J. Appl. Phyys., 74 (1993) 5790. [8] R.V. Bensasson,J.L. Garaud, S. Leach, G. Miquel and P. Seta, Cl~ern. Phys. Lett., 210 (1993) 141. [9] F. Wilkinson, in J.B. Birks (ed.), Organic Molecular Phofophyysics, Wiley, London, 1975, p. 95. [lo] K.L. Akers, C. Douketis, T.L. Haslett and M. Moskovits, J. Phyys. Chem., 98 (1994) 10 824. [ll] P.A. Lane, L.S. Swanson, Q.X. Ni, J. Shiner, J.P. Engel, T.J. Barton, J. Wheelock and L. Jones,Phys. Rev. Mt., 68 (1992) 887. [12] A.A. Lamola, W.G. Herkstoer, J.C. Dalton and G&Hammond, J. Chem. Phys., 42 (1965) 1715. [ 131 S. Leach, M. Vervloet, A. Despres, E. Breheret, J.P. Hare, T.J. Dennis, H.W. Kroto, R. Taylor and D.R.M. Walton, Chern. Phys., 160 (1992) 451. [ 141 R.V. Bensasson,E. Bienveniie, M. Dellinger, S. Leach and P. Seta, J. Phys. Chern., 98 (1994)
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