Carbon Nanotubes

Jul 31, 2008 - Atula S. D. Sandanayaka , Raghu Chitta , Navaneetha K. Subbaiyan , Lawrence D'Souza , Osamu Ito and Francis D'Souza. The Journal of ...
1 downloads 0 Views 155KB Size
Download PDF
13000

J. Phys. Chem. C 2008, 112, 13000–13003

Photoinduced Charge Separation in Riboflavin/Carbon Nanotubes Superstructures Dewu Long,†,§ Guozhong Wu,*,† and Aibin Wu‡,⊥ Department of Nuclear Analysis, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, and Shanghai Key Laboratory of Chemical Biology, School of Pharmacy, East China UniVersity of Science and Technology, Shanghai 200237 ReceiVed: March 2, 2008; ReVised Manuscript ReceiVed: June 11, 2008

A natural pigment, riboflavin (RF), was successful in immobilizing to the multiwalled carbon nanotubes (MWNTs) as antennae by a noncovalent approach. 1H NMR analysis indicated their π-π stacking interaction by the proton shift at the isoalloxazine ring of RF. Subsequently, time-resolved spectroscopic experiments were carried out to monitor the photoinduced charge separation states in the riboflavin/carbon nanotubes (RF/MWNTs) superstructures. It was found that charge separation was accessible in the superstructures with 355 nm laser excitation. The recombination of charge separation states was observed to be an obvious twostep process. A fast decay with a lifetime of ∼ 40 ns was attributed to the recombination of surface-immobilized RF radicals and localized electrons in MWNTs. Another relative longer process with a lifetime of ∼ 3.0 µs was assigned to the storage electron in MWNTs and thus retarded the recombination of charge separation states. Introduction Immobilization of photosensitive molecules to carbon nanotubes (CNTs) has attracted a lot of interest in recent years due to their potential application in optoelectronic devices and solar cells.1–3 Special attention was devoted to the CNT-based superstructures modified with natural pigments (e.g., chlorophyll,4,5 peptides, and porphyrin6–8). Such superstructures possess both high light conversion ability and biocompatible surfaces, extending the use of CNT-based nanohybrids from donoracceptor models to medicinal materials. Covalent and noncovalent approaches are two general strategies to immobilize molecules to CNTs,9–13 of which the noncovalent functionalization approach is regarded as a nondestructive method achieved by the wrapping and/or stacking of molecules on the surface of CNTs.14,15 The one-dimensional electric structure and property of nanotubes can be better preserved in the modified procedures, which is vitally important to further application. As reported previously,16–18 molecules with conjugated flat aromatic rings are ideal to attach to the surface of CNTs. The interaction between the flat aromatic rings and the graphitic surface of CNTs is strong due to their π-π stacking interaction. Upon light excitation, the role of CNTs in the donor-acceptor superstructures is usually that of the capable acceptor, while the surface-immobilized molecules act as antennae.19,20 As a result, charge separation takes place in the functional composites to form transient species (radical cations and anions). Monitoring of the kinetics of these transient species in superstructures provides the information of the charge separation process.15–20 Herein, we report the immobilization of riboflavin onto multiwalled carbon nanotubes. 1H NMR analysis indicated that the interaction between RF and MWNTs is π-π stacking via the isoalloxazine ring of RF and the graphite surface of MWNTs. * To whom correspondence should be addressed. Tel/Fax: 86-21-5955 8905. E-mail: [email protected]. † Chinese Academy of Sciences. ‡ East China University of Science and Technology. § E-mail: [email protected]. ⊥ E-mail: [email protected].

SCHEME 1: Illustration of Noncovalently Modified Multiwalled Carbon Nanotubes with RF via a π-π Stacking Interactiona

a The isoalloxazine ring of RF is proposed to be tethered at the surface of the MWNTs while its tailor is protended to the surface. The interpretation of this is discussed in the text.

We subsequently used the time-resolved spectroscopy technique (laser photolysis) to monitor the transient species after photoinduced charge separation. The results indicated that the photoinduced charge separation process is achieved and the recombination process is between tens of nanoseconds and several microseconds, demonstrating that this superstructure is an efficient donor-acceptor nanohybrid. Experimental Section Materials and Reagents. MWNTs (10-30 nm in diameter and 1-2 µm in length) and water-soluble MWNTs were purchased from Shenzhen Nanotechnologies. Co. Ltd. (China) and used without further purification. Spectral-grade RF was purchased from Sigma-Aldrich. Millipore water was used throughout all experiments in this study. Analytical-grade HClO4 and NaOH were purchased from Chinese Chemical Reagents Co. Ltd. Preparation of RF/MWNTs Composites. As illustrated in Scheme 1, the noncovalent approach to immobilize RF on MWNTs is readily accessible only by agate mortar and pestle mulling. Typical modification procedures are mentioned as

10.1021/jp801842d CCC: $40.75  2008 American Chemical Society Published on Web 07/31/2008

Photoinduced Charge Separation in RF/MWNTs

J. Phys. Chem. C, Vol. 112, No. 33, 2008 13001

Figure 1. 1H NMR spectra of RF (below) and RF/MWNTs (upper) in D2O. The spectral shifts at δ 7.880 (aromatic ring) and 2.552 (methyl group) ppm of RF/MWNTs superstructures clearly indicate a π-π stacking interaction between the tethered isoalloxazine ring of RF and the graphitic surface of MWNTs. Inset: Structure of RF (upper left) and UV-vis spectra of RF and RF/MWNTs obtained in aqueous solutions (upper right).

follows. First, 2.5 mg of MWNTs and 0.7 mg of RF were mixed with several drops of water in agate mortar and then vigorously mulled for 10 min to get exfoliative black solid. The solid was then transferred into a cuvette and dispersed in water by shaking. The dispersion was centrifuged at 12 000 rpm for 20 min, discarding the supernatant to remove the nonimmobilized RF. The washing and centrifugation were repeated several times until the supernatant was colorless. The composite obtained after the last washing and centrifugation procedure was then directly dispersed in water and stored in the dark for the following experiments. Characterization of RF/MWNTs. UV-visible absorption spectra were obtained with a Hitachi-3010 spectrophotometer. The TEM was taken with a JEOL JEM-1230 at an accelerating voltage of 80 kV. The samples for TEM observations were prepared by dropping the modified MWNT suspension on a carbon-coated copper grid for 30 s and then blotting off. NMR results, RF: 1H NMR (400 MHz, D2O): δ 7.810 (s, 1H), 7.769 (s, 1H), 5.031-4.970 (m, 1H), 4.839-4.772 (m, 2H), 4.320-4.306 (m, 1H), 3.903-3.789 (m, 3H), 3.684-3.638 (m, 1H), 2.492 (s, 3H), 2.381 (s, 3H). RF/MWNTs: 1H NMR (400 MHz, D2O): δ 7.880 (s, 1H), 7.856 (s, 1H), 5.450-5.420 (m, 1H), 5.100-5.040 (m, 2H), 4.390-4.352 (m, 1H), 3.950-3.853 (m, 3H), 3.744-3.697 (m, 1H), 3.339 (s, 2H), 2.552 (s, 3H), 2.441 (s, 3H). A detailed description of the laser photolysis apparatus can be found elsewhere.28 In this work, the third harmonic incident laser (355 nm) with an energy of ∼30 mJ/pulse or the forth harmonic incident laser (266 nm) with an energy of ∼20 mJ/ pulse (fwhm 3-6 ns and spot size 0.5 mm in diameter) was used as the excitation source. The RF/MWNTs solution was obtained from the directly dispersed functionalized superstruc-

ture after washing by water, as formerly mentioned. For all RF/ MWNTs superstructures, their concentrations used in the laser photolysis experiments were controlled by measuring the steady absorption at 440 nm (Abs440 ) 0.8). Results and Discussion Riboflavin is widely distributed in organisms and plants, usually acting as a photosensitizer in many metabolism processes and/or photosynthetic processes.21–24 The isoalloxazine ring of riboflavin is the main chromophore group and provides a large π electronic cloud (Figure 1). Therefore, riboflavin can be readily tethered to the graphite surface of MWNTs via π-π stacking. In our strategies, RF and MWNT mixtures are mulled in agate mortar to produce an exfoliative solid. This solid is followed by washing and centrifugation procedures to remove the nonimmobilized RF (Scheme 1). Unlike the insoluble pristine MWNTs, the RF/MWNTs superstructures exhibit excellent water solubility and can be stored in the dark for weeks without precipitation. The excellent dispersion property was also confirmed by the TEM observation (Figure S1). The presence of RF in RF/MWNTs superstructures and the strong π-π stacking interaction between RF and MWNTs are confirmed by UV-vis absorption spectra and 1H NMR (Figure 1). The UV-vis absorption spectrum of RF/MWNTs shows the characteristic absorption peaks of RF at 260, 370, and 450 nm 21–24,26 and the monotonical absorption of MWNTs at 230-800 nm.25 The chemical shifts of the protons in aromatic rings (δ: 7.880) and the methyl group (δ: 2.552) at the isoalloxazine ring of RF are clearly observed for the suspension of RF/MWNTs, indicating the presence of RF in the composites. The strong π-π stacking interaction between the surface-

13002 J. Phys. Chem. C, Vol. 112, No. 33, 2008

Figure 2. Transient absorption spectra of RF•+/MWNTs•- recorded at 100 ns, 1.0 µs, and 5.0 µs after 355 nm laser excitation. Inset: Characteristic absorption spectrum of 3RF* recorded at 1.0 µs in aqueous solution.

immobilized RF molecules and MWNTs is proven by the polarization of the isoalloxazine ring of RF after immobilization to MWNTs. As compared to free RF, a significant shift to the low field was observed for the proton of the isoalloxazine ring in the bound RF (Figure 1). The largest shifts were observed for the protons at the aromatic ring and methyl group (about 0.1 ppm) due to the tethering of the isoalloxazine ring at the surface of MWNTs. This notable NMR spectral shift of the bound molecules in the RF/MWNTs superstructures implies a strong interaction between the bound RF and MWNTs. Such an interaction is attributed to the strong polarization of bound RF molecules at the graphite surface of MWNTs by their π-π stacking interaction. This polarization is also confirmed by the deshielding effect on the proton of the chain since the tail of the molecule is proposed to extend far from the tethered rings at the MWNTs (Scheme 1). It is noted that two sharp peaks, δ 3.339 (s, 2H) and δ 5.450-5.420 (m, 1H), are also observed in RF/MWNTs superstructures. The former peak is attributed to the active hydrogen of the hydroxyl group in the chain of the immobilized molecule in superstructures, owing to a strong π-π interaction between RF and MWNTs. To the contrary, the signal of the same hydrogen atom in free RF in solution is a bump. The peak at 5.450-5.420 (m, 1H) is due to a proton, corresponding to the upshield of the proton at 5.031-4.970 (m, 1H) of free RF in the solution. Photoinduced charge separation in RF/MWNTs superstructures is observed by nanosecond laser photolysis experiments. Figure 2 shows the transient absorption spectra of RF•+/MWNTs•-, the radical counters produced by charge separation in RF/MWNTs superstructures after 355 nm laser excitation. The transient spectroscopic studies of RF were widely documented.22–24,26,27 Upon UV/visible laser excitation, riboflavin excited states (3RF*) are the predominant transient species (λmax: 680, 310 nm) due to their high intersystem crossing efficiency, also accompanied by a negative bleaching at 440 nm due to photodepletion of groundstate molecules (Figure 2).22–24,26,27 The recovery of bleaching occurs via the deactivation of 3RF*, which is confirmed by the corresponding decay profiles at 680 and 440 nm (Figure S2). The lifetime of 3RF* in our experiments was determined to be 8.5 µs, consistent with the literature value.22–24,26 Compared to the free 3RF*, the transient absorption of RF•+/ MWNTs•- shows a distinct difference; it is composed of three

Long et al. discernible absorption regions, namely, two intensive absorption peaks at 320 and 500 nm accompanied by a hump at 370 nm, and a featureless broad absorption region in the range of 530-800 nm, as well as the negative bleaching of RF at 440 nm (Figure 2). The disappearance of the intensive absorption of 3RF* at 680 nm in the transient spectra of the RF/MWNTs composite implies the scavenging of triplet states by the MWNTs. The intermediate with absorption peaks at 320 and 500 nm and the hump at 370 nm was assigned to the oxidation species of RF (RF•+/RF•(-H)), in accordance with previous reports.22 The suggestion was further confirmed by the comparison of transient absorption spectra with that obtained in RF aqueous solution, in which the oxidation species of RF (RF•+/ RF•(-H)) was directly formed upon 266 nm laser photoionization (Figures S3 and S4). Since the reported pKa of RF•+ was 6.1,22 one can infer that the surface-bound RF cation radical (RF•+) in RF/MWNTs superstructures may undergo deprotonation to form neutral RF radical (RF•(-H)) in this study. In the case of the featureless broad absorption in the longwavelength region, it is assigned to the reduction species of MWNTs (MWNTs•-). Though it was reported previously that the oxidization RF species has transient absorption in this region, its very low molar absorption coefficient (ε ) 200 dm-3 · cm-1 · cm-1)22 cannot have such a strong absorption (near 0.5 in Figure 2). The charge separation in RF/MWNTs superstructures is likely achieved by thermodynamic electron transfer from 3RF* to carbon nanotubes since the redox potential of 3RF* is reported to be 1.7 V versus NHE24 while the redox potential of CNTs is determined to be 1.0 V versus NHE.13,29 It is noted that the absorption of RF•+ radicals at the surface of MWNTs has a slight blue shift (20 nm) as compared to the reported free radical cation (λmax ) 520 nm).22 This shift may be interpreted by the strong π-π interaction between the bound RF molecules and MWNTs and the subsequent polarization of RF molecules. The charge separation occurs in the RF/MWNTs superstructures between surface-immobilized RF and MWNTs instead of the interaction between free RF molecules in solution and MWNTs. Such a conclusion is confirmed by the following reasons. First, the RF/MWNTs superstructures used in laser photolysis experiments were submitted to a thorough washing procedure to remove the nonimmobilized RF. Second, a comparison of the transient absorption spectra of RF/MWNTs superstructure suspensions and the mixture of aqueous the RF and MWNT solution was made (Figure S5). Typically, the transient absorption of RF radical cations at 320 and 500 nm and the hump at 370 nm was observed for the RF/MWNTs solution, but no obvious absorption was observed for the mixed solution of RF and MWNTs. The results demonstrated that the photoinduced electron transfer readily took place between surface-immobilized RF molecules and MWNTs in the RF/ MWNTs superstructures, rather than occurring via an intermolecular interaction between free RF molecules and MWNTs in solutions. Time profiles of two transient species (RF•+/MWNTs•-) monitored at 500 (RF•+) and 650 nm (MWNTs•-) imply the charge recombination in the superstructures. The charge recombination of RF•+ was monitored at 500 nm but not at 320 nm because the transient absorption peak at 320 nm was contributed to by various transient species22 and also by carbon nanotubes. Both of the species follow an obvious two-step decay process (Figure 3). Lifetimes of the two steps are fitted to be 0.04 and 2.2 µs for RF•+ and 0.05 and 3.0 µs for MWNTs•-. As discussed in our previous report,28 the fast decay immediately after laser excitation can be attributed to the fast recombination

Photoinduced Charge Separation in RF/MWNTs

J. Phys. Chem. C, Vol. 112, No. 33, 2008 13003 Supporting Information Available: TEM observation, time profiles of 3RF* at 680 and 440 nm, transient absorption spectra of RF•+/RF•(-H) after 266 nm laser photoionization at various pH values, and the comparison transient spectra of free RF and MWNTs and of the dispersion of RF/MWNTs superstructures. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes

Figure 3. Time profiles of RF•+ at 500 nm (9) and MWNTs•- at 650 nm (O) in RF/MWNTs superstructures after 355 nm pulse laser excitation. The time axis is treated logarithmic to clarity the weak but long-lived absorbance.

of surface-bound radical cations and the injected electrons in MWNTs. Such recombination in the superstructures is analogous to the intramolecular backward electron transfer. Thus, a fast decay process was observed for the radicals and for the reduced MWNTs species immediately after pulse laser excitation. The slow decay process, however, is contributed to by the electron storage ability of carbon nanotubes.30,31 Previous literature has reported the efficient storage of injected electrons from excited surface-immobilized molecules by nanotubes in similar superstructures (e.g., the storage ability of the SWNTs is determined to be up to 1 electron per 32 carbon atoms,31 and the lifetime of charge recombination is observed up to hundreds of microseconds8). Therefore, the storage of electrons in SWNTs can significantly increase the lifetime of charge separation species (MWNTs•- and RF•+). Similar long-lived charge separation states were also observed in porphyrin-modified carbon nanotube superstructures30,31 and oligmer peptide functional ones.8 Conclusion In conclusion, a photosensitized molecule, RF, was noncovalently immobilized onto MWNTs as antennae via their π-π stacking interactions. 1H NMR spectra directly confirmed this π-π stacking interaction by monitoring the resonance peaks’ shifts. Using this composite as a donor-acceptor model, we investigated the photoinduced charge separation by laser photolysis. The lifetimes of the separated transient species lie in the range of tens of nanoseconds to several microseconds. The results demonstrated that such RF/MWNTs superstructures are suitable building blocks for novel photoinduced charge separation materials. Acknowledgment. This work was jointly supported by the Frontier Science Foundation of Chinese Academy of Sciences (Grant No. 55120703) and the Natural National Sciences Foundation (Grant No. 20673137).

(1) Strano, M. S.; Dyke, C. A.; Usrey, M. L.; Barone, P. W.; Allen, M. J.; Shan, H. W.; Kittrell, C.; Hauge, R. H.; Tour, J. M.; Smalley, R. E. Science 2003, 31, 1519–1521. (2) Guldi, D. M.; Marcaccio, M.; Paolucci, D.; Paolucci, F.; Tagmatarchis, N.; Tasis, D.; Va´zquez, E.; Prato, M. Angew. Chem., Int. Ed. 2003, 42, 4206–4209. ` .; Martin, N.; Campidelli, S.; Prato, M.; Brehm, G.; (3) Herranz, M. A Guldi, D. M. Angew. Chem., Int. Ed. 2006, 45, 4478–4482. (4) Kavakka, J. S.; Heikkinen, S.; Kilpela¨inen, I.; Mattila, M.; Lipsanenb, H.; Helaja, J. Chem. Commun. 2007, 519–521. (5) Carmeli, I.; Mangold, M.; Frolov, L.; Zebli, B.; Carmeli, C.; Richter, S.; Holleitner, A. W. AdV. Mater. 2007, 19, 3901–3905. (6) Baskaran, D.; Mays, J. W.; Zhang, X. P.; Bratcher, M. S. J. Am. Chem. Soc. 2005, 127, 6916–6917. (7) Boul, P. J.; Cho, D. G.; Aminur Rahman, G. M.; Marquez, M.; Ou, Z. P.; Kadish, K. M.; Guldi, D. M.; Sessler, J. L. J. Am. Chem. Soc. 2007, 129, 5683–5687. (8) Saito, K.; Troiani, V.; Qiu, H.; Solladie, N.; Sakata, T.; Mori, H.; Ohama, M.; Fukuzumi, S. J. Phys. Chem. C 2007, 111, 1194–1199. (9) Hirsch, A. Angew. Chem., Int. Ed. 2002, 41, 1853–1859. (10) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. ReV. 2006, 106, 1105–1136. (11) Holzinger, M.; Vostrowsky, O.; Hirsch, A.; Hennrich, F.; Kappes, M.; Weiss, R.; Jellen, F. Angew. Chem., Int. Ed. 2001, 40, 4002–4005. (12) Baskaran, D.; Mays, J. W.; Bratcher, M. S. Angew. Chem., Int. Ed. 2004, 43, 2138–2142. (13) Chen, S. M.; Wu, G. Z.; Liu, Y. D.; Long, D. W. Macromolecules 2006, 39, 330–334. (14) Star, A.; Stoddart, J. F.; Steuerman, D.; Diehl, M.; Boukai, A.; Wong, E. W.; Yang, X.; Chung, S. W.; Choi, H.; Heath, J. R. Angew. Chem., Int. Ed. 2001, 40, 1721–1725. (15) Hasobe, T.; Fukuzumi, S.; Kamat, P. V. J. Phys. Chem. B 2006, 110, 25477–25484. (16) Martin, R. B.; Qu, L. W.; Lin, Y.; Harruff, B. A.; Bunker, C. E.; Gord, J. R.; Allard, L. F.; Sun, Y. P. J. Phys. Chem. B 2004, 108, 11447– 11453. (17) Ge, J. J.; Zhang, D.; Li, Q.; Hou, H. Q.; Graham, M. J.; Dai, L. M.; Harris, F. W.; Cheng, S. Z. D. J. Am. Chem. Soc. 2005, 127, 9984–9985. (18) Nakashima, N. Sci. Technol. AdV. Mater. 2006, 7, 609–616. (19) Guldi, D. M.; Rahman, G. M. A.; Zerbetto, F.; Prato, M. Acc. Chem. Res. 2005, 38, 871–878. (20) Ehli, C.; Rahman, G. M. A.; Jux, N.; Balbinot, D.; Guldi, D. M.; Paolucci, F.; Marcaccio, M.; Paolucci, D.; Melle-Franco, M.; Zerbetto, F.; Campidelli, S.; Prato, M. J. Am. Chem. Soc. 2006, 128, 11222–11231. (21) Alva, S.; Phadke, R. S. Biosystems 1995, 35, 153–156. (22) Lu, C. Y.; Wang, W. F.; Lin, W. Z.; Han, Z. H.; Yao, S. D.; Lin, N. Y. J. Photochem. Photobiol. B 1999, 52, 111–116. (23) Lu, C. Y.; Lin, W. Z.; Wang, W. F.; Han, Z. H.; Yao, S. D.; Lin, N. Y. Phys. Chem. Chem. Phys. 2000, 2, 329–334. (24) Lu, C. Y.; Yao, S. D.; Lin, N. Y. Chem. Phys. Lett. 2000, 330, 389–396. (25) Zhang, Q.; Zhang, L.; Li, J. H. J. Phys. Chem. C 2007, 111, 8655– 8660. (26) Cardoso, D. R.; Franco, D. W.; Olesen, K.; Andersen, M. L.; Skibsted, L. H. J. Agric. Food Chem. 2004, 52, 6602–6606. (27) Stanley, R. J.; MacFarlane, A. W., IV. J. Phys. Chem. A 2000, 104, 6899–6906. (28) Long, D.; Wu, G.; Zhu, G. Int. J. Mol. Sci. 2008, 9, 120–130. (29) Zheng, M.; Diner, B. A. J. Am. Chem. Soc. 2004, 126, 15490– 15494. (30) Cognet, L.; Tsyboulski, D. A.; Rocha, J. R.; Doyle, C. D.; Tour, J. M.; Weisman, R. B. Science 2007, 316, 1465–1468. (31) Kongkanand, A.; Kamat, P. V. ACS Nano 2007, 1, 13–21.

JP801842D