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2008, 112, 5699-5702 Published on Web 02/23/2008
Laser Flash-Photolysis Study of Organic-Inorganic Materials Derived from Zirconium Phosphates/Phosphonates of Ru(bpy)3 and C60 as Electron Donor-Acceptor Pairs Ernesto Brunet,*,† Marina Alonso,† M. Carmen Quintana,‡ Pedro Atienzar,£ Olga Juanes,† Juan Carlos Rodriguez-Ubis,† and Hermenegildo Garcı´a*,£ Departamento de Quı´mica Orga´ nica, and Departamento de Quı´mica Analı´tica, Facultad de Ciencias, UniVersidad Auto´ noma de Madrid, 28049-Madrid, Spain, and Instituto de Tecnologia Quimica, UPV-CSIC, UniVersidad Polite´ cnica de Valencia, AV. de los Naranjos s/n, Valencia, Spain ReceiVed: January 2, 2008; In Final Form: February 5, 2008
The laser flash photolysis of material 1 (layered γ-ZrP containing covalently attached Ru(bpy)3- and C60phosphonates), dyad 2, and material 3 (amorphous solid Zr(n-BuO) derivative of dyad 2) was accomplished. Zirconium salts, regardless of their amorphous or layered structures, appear to be excellent hosts for developing long-lived charge-separated states between C60 and Ru(bpy)3 derivatives. Moreover, the presented evidence points to the important fact that the inorganic scaffolding acted as a semiconductor, harvesting the electrons and/or holes produced at the active organic components. These findings suggest that the studied materials could be suitable for the building of practical solar cells.
Introduction There is a large interest in developing novel materials for photovoltaic applications.1-4 Upon light absorption, these materials should generate charge-separate states with high efficiency and a sufficiently long lifetime to permit charge migration.5 One strategy toward this objective is the incorporation of organic guests that can participate in photoinduced electron transfer in the intergallery space of suitable inorganic layered hosts.6,7 Ideally, the host should cooperate with the photochemistry of the guests, providing a highly polar environment, promoting charge separation, and favoring charge migration by accepting electron and/or holes from the photoactive guests. In addition to defining a confined 2D space of the appropriate polarity to promote electron transfer, the inorganic layered host must immobilize and protect the organic guests. In this context, we have previously reported8 on the preparation of organic-inorganic materials based on the γ laminar phase of zirconium phosphate (γ-ZrP), whose surface phosphates were sequentially replaced by phosphonates containing either the Ru(bpy)3 or C60 motifs. C60 is the preferred electron acceptor in organic and hybrid organic-inorganic solar cells, while polypyridyl ruthenium complexes are also widely used as photoexcitable electron donors, exhibiting a high absorption coefficient with visible light. The aim of this research was to build nanostructured solid materials where immobilization in close proximity by covalent attachment to the phosphate layers of photoexcitable electron donors and acceptors would lead to an efficiency enhancement of charge separation for light harvesting. Figure 1 sketches an idealized model of the previously reported laminar structure (material 1) of formula * To whom correspondence should be addressed. † Departamento de Quı´mica Orga ´ nica, Universidad Auto´noma de Madrid. ‡ Departamento de Quı´mica Analı´tica, Universidad Auto ´ noma de Madrid. £ Universidad Polite ´ cnica de Valencia.
10.1021/jp800026r CCC: $40.75
Zr(PO4)(H2PO4)0.83(C60-deriv.)0.07(C32H30P2O6N6Ru)0.05(decylNH2)0.17(H2O)1.2. The model of Figure 1 shows two consecutive layers of γ-ZrP at the experimental interlayer distance (2.15 nm, as measured by XRD) containing a grid of 6 × 5 surface phosphates per face. Therefore, following the molecular formula obtained by elemental analysis, TGA, and solid-state 31P-NMR, approximately 4 C60 derivatives, 3 Ru complexes, and 10 decylamine molecules had to be placed between the 30 + 30 exchangeable phosphates facing each other in the consecutive layers. The docking of theses molecules in the available space between the layers could be accomplished in the molecular model without any severe van der Waals contacts. On the other hand, we have also reported9 the synthesis of dyad species as that shown in Figure 2. This dyad 2 can be considered as the molecular analogue of the limit situation occurring in material 1 when the donor/acceptor units are in the closest proximity. Unfortunately, all attempts of slipping the dyad 2 within the layers of zirconium phosphate were unsuccessful using the typical exfoliating methods (1:1 water/acetone suspension at 80 °C and alkylamine intercalation). We thus used an alternative reaction10 by means of treating the dyad with Zr(n-BuO)4 in n-BuOH. In this way, we obtained an amorphous solid (it gave no discernible powder X-ray pattern) whose elemental analysis by combustion and ICP-MS was compatible with the approximate molecular formula Zr(dyad)0.2-0.4(n-BuO)3.6-3.8, suggesting that the dyad was, in fact, integrated in the inorganic matrix (material 2). Characterization details of this material were reported elsewhere.5 In this letter, we account for the laser flash photolysis of materials 1 (layered) and 3 (amorphous) and that of the dyad 2 in acetonitrile solution as a reference. The aim is to get evidence on the specific photochemistry and promotion of photoinduced charge separation when the donor-acceptor Ru complex and © 2008 American Chemical Society
5700 J. Phys. Chem. C, Vol. 112, No. 15, 2008
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Figure 1. Idealized model of material 1 (see text). The C60 moieties are in the overlapping spheres display mode. Ru complexes are in the balls and cylinders mode. The structures in green are n-decylamine, and the inorganic layers are represented as tubes (red, oxygen; yellow, phosphorus; white, zirconium).
Figure 2. Dyad 2, the main component of material 3 studied in this work.
C60 couple are sequentially incorporated in the intergallery space of the layered γ-ZrP or as a preorganized dyad within an amorphous Zr scaffold. Experimental Section Laser flash photolysis experiments were carried out using the second (532 nm) harmonic of a Q-switched Nd/YAG laser (Spectron Laser Systems, U.K.; pulse width ∼9 ns and 35 mJ × pulse-1). The signal from the monochromator/photomultiplier detection system was captured by a Tektronix TDS640A digitizer and transferred to a PC computer that controlled the experiment and provided suitable processing and data storage capabilities. The samples were contained in a septum-capped Suprasil quartz cells (1 × 0.5 for materials 1 and 3 and 1 × 1 cm2 for an acetonitrile solution of dyad 2). The samples were purged with nitrogen or oxygen at least 15 min before the laser experiments. Results and Discussion Laser flash photolysis studies of dyad 2 in acetonitrile were undertaken using the second harmonic of a Nd/YAG laser (532 nm). According to our previous study on the absorption spectra of dyad 2 and the individual ruthenium and fullerene components, 532 nm light should lead almost exclusively to excitation of the ruthenium moiety of the dyad. Laser excitation at 532 nm allowed recording of a transient spectrum decaying on the
submillisecond time scale characterized by two relative maxima at 300 and 700 nm (Figure 3). The temporal profiles of the signals at 300 and 700 nm were coincident, suggesting that these two peaks should correspond to a single transient (inset of Figure 3). On the basis of the reported optical spectrum of the triplet excited states of fullerene derivatives,11,12 we assigned this transient to a triplet excited state localized on the fullerene subunit of dyad 2. In agreement with this assignment to a triplet excited state, the transient was quenched by oxygen (inset of Figure 3). A detailed analysis of the transient signal corresponding to the localized triplet excited state reveals a growth of the signal occurring in the submicrosecond time scale, there being an increase in the ∆O.D. in the first hundreds of nanoseconds after the laser flash. This indicates that, in addition of an instantaneous generation of the triplet excited state localized on the fullerene moiety, there has to be a second mechanism giving rise to the generation of some more triplets well after the disappearance of the excitation laser pulse (7 ns). The proportions between these two mechanisms of triplet generation can be estimated from the relative intensity of the instantaneous ∆O.D. and the maximum ∆O.D. value reached at 500 ns after the laser pulse. It was determined that 18% of the triplets are formed through the non-instantaneous mechanism. Beyond the initial 500 ns, the decay part of the temporal signal profile could be fitted to a first-order kinetics with a half-life of 10 µs. On the basis of the previous knowledge on the photochemistry of fullerene dyads,13 we propose that the slow component in the fullerene triplet growth derives from the collapse of the charge-separated state Ru+-C60-, giving rise to the delayed formation of C60 triplets. According to this proposal, the lifetime of the charge-separated state can be estimated indirectly from this triplet growth as about 300 ns. As mentioned in the Introduction, the aim of this work is to produce layered materials containing a ruthenium complex and fullerene that could act as an electron donor and electron acceptor in photoinduced charge separation. Therefore, it is of interest to perform the transient spectra of materials 1 and 3 and compare these spectra with that of dyad 2. As it was explained above, material 3 contains dyad 2 covalently anchored to an amorphous Zr phosphonate/butoxide salt of approximate formula Zr(dyad)0.2-0.4(n-BuO)3.6-3.8. On the other hand, experimental evidence of material 1 showed that it contains almost equivalent amounts of the polypyridyl ruthenium complexes and fullerene derivative (vide supra molecular formula), similar to what happens in dyad 2. Yet, in contrast, the synthesis of
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Figure 3. Transient spectra recorded for dyad 2 in acetonitrile after 532 nm laser excitation of nitrogen purging (a, 0.5 and 1.5 µs) or oxygen purging (b, 1 µs). The inset shows the temporal decay of the signal monitored at 350 nm for the nitrogen (black) and oxygen (red) purged dyad 2.
Figure 4. Time-resolved diffuse reflectance spectra of nitrogen-purged material 3 200 and 500 ns after the 532 nm laser flash. The inset shows the signal monitored at 500 nm recorded for material 3 under nitrogen or oxygen purging.
material 1 (cf. Figure 1) was performed by the sequential introduction of the active species within the walls of true lamellar γ-ZrP. Therefore, the relative arrangement of C60 and the Ru complex cannot be exactly known in material 1. Despite this uncertainty, materials 1 and 3 displayed an almost complete quenching of the Ru complex luminescence in the solid state, thus suggesting that the sought charge transfer in fact occurred in both materials, despite their structural differences. In contrast to the photochemistry observed for dyad 2 in acetonitrile, time-resolved diffuse-reflectance laser flash photolysis of material 3 upon 532 nm excitation leads to a transient spectrum characterized by a continuous absorption expanding the entire wavelength range (Figure 4). This spectrum is completely different than that recorded for dyad 2 in solution. The temporal profile of the signals monitored at various wavelengths was coincident and could be fitted to a monoexponential decay with a half-life of 75 ns (inset of Figure 4).
The signal was not effected by oxygen purging. Although oxygen quenching in solids is not as effective as that in solution, the fact that the temporal profile of the signal is not altered by the presence of oxygen rules out triplets as the species responsible for the transient. Whatever the nature of the transients, the difference in the transient spectrum of dyad 2 and that of material 3 in which dyad 2 is anchored to the amorphous zirconium salt clearly shows the profound effect that incorporation of this dyad in the host scaffold has on the fate of the photogenerated transients. Importantly, laser flash photolysis of material 1 in which the ruthenium complex and fullerene are independently grafted to the layered zirconium phosphate structure gives rise to an identical spectrum to that recorded for material 3. Even more, the temporal profiles of the transients recorded for material 1 also coincide with those monitored for material 3, and the signal is again insensitive to oxygen. On the basis of the coincidence
5702 J. Phys. Chem. C, Vol. 112, No. 15, 2008 SCHEME 1: Mechanistic Proposal to Rationalize the Transients Observed by Laser Flash Photolysis from Materials 1 and 3
Letters charge separation in wide-band semiconductors such as zeolites has been attained by photoexcitation of incorporated anthracene guests.16 Conclusion Zirconium salts, regardless of their amorphous or layered structures, appear to be excellent hosts for developing longlived charge-separated states between C60 and Ru(bpy)3 derivatives. Moreover, the presented evidence points to the important fact that the inorganic scaffolding acts as a semiconductor, harvesting the electrons and/or holes produced at the active organic components. These findings suggest that the studied materials could be suitable for the building of practical solar cells. Our progress in this field will be published in due course.
of the spectra and the temporal profiles, we propose that the species being generated in materials 1 and 3 are the same, despite of the differences in the composition and structure of the two solids. By consecutively obtaining the transient spectra of materials 1 and 3 at the same laser power and experimental conditions, it was estimated that the efficiency of material 3 to generate the transient is 2.5 times that of material 1, probably due to the forced proximity of the active species in the dyad 2. A blank control in which zirconium phosphate devoid of ruthenium and fullerene guests was submitted to 532 nm laser flash photolysis did not give rise to the generation of any transient. Concerning the nature of the transient species observed in materials 1 and 3, we propose that this corresponds to electrons and holes in the conduction and valence bands of the Zr salts, thus acting as a semiconductor. In titanium dioxide, it has been observed that the state of charge separation gives rise to a transient spectra characterized by a continuous absorption expanding the entire wavelength range, as those observed here.14 Also, in other wide-band-gap semiconductors, such as zeolites, charge separation is also characterized by very broad absorptions, as those observed here.15 On the basis of the optical spectra, we propose that the species being monitored on materials 1 and 3 are delocalized electrons and holes. This charge-separated state had to be generated by photoexcitation of the ruthenium complex, which would be followed by electron transfer, the ruthenium complex acting as the electron donor. The ejected electron would be initially trapped by the fullerene moiety, giving rise to an early charge separation which subsequently collapses on the subnanosecond time scale into electrons and holes in the inorganic layered (material 1) or amorphous (material 3) scaffoldings. Scheme 1 summarizes our proposal. In support of it, there are recent precedents in which
Acknowledgment. Financial support by the Spanish Ministry of Science and Education (Grants MAT2003-03243, MAT200600570, CTQ06-0567) and indirect funding from ERCROSFarmacia S.A. (Aranjuez, Spain) are gratefully acknowledged. P.A. also thanks the Spanish Ministry of Education for a postgraduate scholarship. References and Notes (1) Granqvist, C.G. In Kirk-Othmer Encyclopedia of Chemical Technology, 5th ed.; Seidel, A. Ed., John Wiley & Sons, Inc, 2007; Vol. 23, pp 1-32; . (2) Bonifazi, D.; Enger, O.; Diederich, F. Chem. Soc. ReV. 2007, 36, 390-414. (3) Yagi, M.; Kaneko, M. AdV. Polym. Sci. 2006, 199, 143-188. (4) Sanchez, C.; Julian, B.; Belleville, P.; Popall, M. J. Mater. Chem. 2005, 15, 3559-3592. (5) Martin, N.; Sanchez, L.; Herranz, M. A.; Illescas, B.; Guldi, D. M. Acc. Chem. Res. 2007, 40, 1015-1024. (6) Cao, G.; Hong, H. G.; Mallouk, T. E. Acc. Chem. Res. 1992, 25, 420-427. (7) Brunet, E.; Alonso, M.; de la Mata, M. J.; Fernandez, S.; Juanes, O.; Chavanes, O.; Rodriguez-Ubis, J. C. Chem. Mater. 2003, 15, 12321234. (8) Brunet, E.; Alonso, M.; Cerro, C.; Juanes, O.; Rodriguez-Ubis, J. C.; Kaifer, A. E. AdV. Funct. Mater. 2007, 17, 1603-1610. (9) Brunet, E.; Alonso, M.; Quintana, M. C.; Juanes, O.; RodriguezUbis, J. C. Tetrahedron Lett. 2007, 48, 3739-3743. (10) Ngo, H. L.; Hu, A.; Lin, W. J. Mol. Catal. A: Chem. 2004, 215, 177-186. (11) Mohan, H.; Palit, D. K.; Chiang, L. Y.; Mittal, J. P. Fullerene Sci. Technol. 2001, 9, 37-53. (12) Asmus, K.-D.; Guldi, D. M. DeV. Fullerene Sci. 2000, 1, 87-106. (13) Thomas, K. G.; George, M. V.; Kamat, P. V. HelV. Chim. Acta 2005, 88, 1291-1308. (14) Yoshihara, T.; Tamaki, Y.; Furube, A.; Murai, M.; Hara, K.; Katoh, R. Chem. Phys. Lett. 2007, 438, 268-273. (15) Marquis, S.; Moissette, A.; Hureau, M.; Vezin, H.; Bremard, C. J. Phys. Chem. C 2007, 111, 17346-17356. (16) Marquis, S.; Moissette, A.; Bremard, C. ChemPhysChem. 2006, 7, 1525-1534.
