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J. Phys. Chem. C 2007, 111, 1146-1149
Electronic Interactions between Inorganic Nanowires and Organic Electron Acceptors: Drastic Changes in Optical Response and Molecular Vibration Jun-ichi Fujisawa,*,† Naoya Tajima,‡ Koichi Tamaki,† Masatsugu Shimomura,†,§ and Teruya Ishihara†,| Frontier Research System, The Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako 351-0198, Japan, DiscoVery Research Institute, The Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako 351-0198, Japan, Nanotechnology Research Center, Research Institute for Electronic Science (RIES), Hokkaido UniVersity, N21W10, Kita-ku, Sapporo 001-0021, Japan, and Department of Physics, Graduate School of Science, Tohoku UniVersity, Aramaki-Aoba, Sendai 980-8578, Japan ReceiVed: May 24, 2006; In Final Form: September 30, 2006
We have studied electronic interactions between inorganic nanowires and organic molecules and the effects on optical response and molecular vibration, using quasi-one-dimensional organic-inorganic hybrid compounds, methylviologen lead iodide (MVPb2I6) and piperidinium lead iodide (PDPbI3). The materials consist of lead iodide nanowires with a diameter of 0.5 nm and intervening organic molecules, methylviologen (MV2+) and piperidinium ion (PD+), respectively. MV2+ has a very high electron affinity, in contrast to PD+. By comparison of the optical absorption spectra between the compounds, it is shown that a broad absorption band appears in the visible region by varying organic molecules from PD+ to MV2+. The absorption band in MVPb2I6 is assigned to charge-transfer transitions from the lead iodide nanowire to MV2+. In the infrared absorption spectrum of MVPb2I6, a large amount of low-energy shift and a striking increase in oscillator strength were observed for the molecular vibration around the nitrogen atoms in MV2+. These features in MVPb2I6 are owing to electronic interactions between the nanowire and MV2+. Electronic interactions in MVPb2I6 are attributed to charge-transfer interactions, which result from electronic couplings between the valence band (VB) of the nanowire and the lowest unoccupied molecular orbital (LUMO) of MV2+.
1. Introduction Inorganic nanowires have attracted considerable attention as primary components in nanoscale electronic devices. Recently, functions obtained by hybridization of nanowires with organic molecules, which exhibit a wide variety of electronic properties and structures, have received much interest.1,2 For chemical functionalization of nanowires, electronic interactions with organic molecules are of fundamental importance. However, electronic interactions between inorganic nanowires and organic molecules and the effects on physical properties have not been clarified yet. Here, we have studied electronic interactions between inorganic nanowires and organic electron acceptors and the effects on optical response and molecular vibration, using organicinorganic hybrid quasi-one-dimensional compounds, methylviologen lead iodide (MVPb2I6) and piperidinium lead iodide (PDPbI3). These materials consist of lead iodide nanowires of face-sharing lead iodide octahedra ([PbI6]4-) with a diameter of ca. 0.5 nm and intervening organic molecules, methylviologen (MV2+) and piperidinium ions (PD+), respectively, as shown in Figure 1a,b.3,4 The length of the nanowire is as long as that (∼1 mm) of the crystal along the c-axis. MV2+ is well-known * Address correspondence to this author. E-mail; ufujisw@ mail.ecc.u-tokyo.ac.jp. † Frontier Research System, The Institute of Physical and Chemical Research. ‡ Discovery Research Institute, The Institute of Physical and Chemical Research. § Nanotechnology Research Center, Research Institute for Electronic Science, Hokkaido University. | Department of Physics, Graduate School of Science, Tohoku University.
Figure 1. (a) Crystal structure of MVPb2I6 viewed from the perpendicular direction to the c-axis (left) and along the c-axis (right). (b) Molecular structures of piperidinium ions (PD+) and methylviologen (MV2+). (c) Photographs of PDPbI3 and MVPb2I6 single crystals measured with transmission configurations. Scale bar ) 300 µm.
as a strong electron acceptor with a high electron affinity, in contrast to PD+.5 The compounds have two merits for the fundamental research. One is to be able to obtain single crystals by a chemical synthesis, which are stable under air and light illumination. By using such single crystals, structural information including the distance and configuration between nanowires and molecules cannot only be obtained by X-ray crystal structure analysis, but also various spectroscopies can be applied. The
10.1021/jp063171k CCC: $37.00 © 2007 American Chemical Society Published on Web 01/03/2007
Interactions between Inorganic Nanowires and Organic Molecules second is to be capable of varying the organic molecules, retaining the basic crystal structure mentioned above. For these reasons, the compounds can be a model system suitable for studies of nanowire-molecule interactions. Interestingly, we found that the crystal color changes drastically from pale yellow to dark red by varying organic molecules from PD+ to MV2+, as shown in Figure 1c, although both the MV2+ and PD+ molecules and the lead iodide nanowire exhibit no significant optical absorption in the visible region. In our previous report, optical absorption in the visible region in MVPb2I6 was assigned to charge-transfer transitions from the lead iodide nanowire to MV2+.6,7 In the present paper, we examine the difference in crystal color between PDPbI3 and MVPb2I6 spectroscopically. Furthermore, the vibrational spectrum of MV2+ in MVPb2I6 is examined by comparing to those of charge-transfer complexes, methylviologen halides (MVCl2 and MVI2) and methylviologen cation radical chloride (MVCl). The features observed in optical absorption and vibrational spectra of MVPb2I6 are attributed to electronic interactions between the nanowire and MV2+. The electronic interaction in MVPb2I6 is identified to be chargetransfer interactions, which are defined as electronic couplings between the D-A and D+-A- states in electron donor (D) and acceptor (A) pairs. We discuss charge-transfer interactions between the inorganic nanowire and MV2+. 2. Materials and Methods MVPb2I6 was synthesized by mixing dimethylsulfoxide (DMSO) solutions of lead iodide (PbI2) and methylviologen iodide (MVI2) in an equimolar ratio.6 PDPbI3 was prepared by reacting PbI2 and piperidinium iodide (PDI) in an equimolar ratio in N,N-dimethylformamide (DMF).8 Single crystals of MVPb2I6 and PDPbI3 were obtained by slow evaporation of the solvents after a few weeks. The chloride salt (MVCl) of methylviologen cation radical (MV+•) was synthesized under argon atmosphere by reacting MVCl2 with active Zn power in distilled water, which was degassed by freeze-pump-thaw cycles beforehand. MV+• is rather stable as a solid in air, in contrast to in solution. Optical absorption spectra in the near-ultraviolet and visible region were measured by using a halogen lamp and a chargecoupled device (CCD) camera cooled by liquid nitrogen and equipped with a polychromator. Thin cleaved crystals for optical absorption measurements were obtained by using adhesive tapes. IR spectra were measured by means of a Fourier transform (FT)IR spectrometer. Photoinduced absorption spectra of MVPb2I6 were measured with a CW He-Cd laser (441.6 nm) and a halogen lamp as pump and probe light sources, respectively. The pump and probe lights were focused on a pinhole (about 100 µm) in aluminum foil that covered a single crystal. All the experiments were carried out at room temperature. 3. Results 3.1. Optical Absorption Spectra. Figure 2 shows the optical absorption spectra of the thin cleaved crystals of PDPbI3 and MVPb2I6. In the absorption spectrum of PDPbI3, a strong absorption peak is observed at 3.1 eV. This absorption band is attributed to the lowest energy one-dimensional exciton, which is a bound state of a photogenerated electron and a hole confined in the lead iodide nanowire, as reported by Fukumoto et al.8 Since optical absorption in the visible region in PDPbI3 is just owing to the absorption tail of the excitonic band, the crystal looks pale yellow, as shown in Figure 1c. On the other hand, in the spectrum of MVPb2I6, a broad absorption band appears between 2.0 and 2.7 eV, in addition to a similar excitonic
J. Phys. Chem. C, Vol. 111, No. 3, 2007 1147
Figure 2. Optical absorption spectra of the cleaved thin crystals of PDPbI3 and MVPb2I6. The spectrum for PDPbI3 is offset for clarity. (Inset) Photoinduced absorption spectrum of MVPb2I6 measured under illumination by CW He-Cd laser (441.6 nm).
absorption band at 3.1 eV to that in PDPbI3. This absorption band is responsible for the dark red color of the crystal shown in Figure 1c. Since intramolecular electronic transitions in MV2+ take place in the ultraviolet region above 4 eV and the excitonic absorption at 3.1 eV is the lowest lying electronic transition in the nanowire, the absorption cannot be explained by electronic transitions in the organic and inorganic components.9 The absorption is attributed to charge-transfer transitions from the lead iodide nanowire to MV2+, as reported in the previous paper.6 The inset in Figure 2 shows a photoinduced absorption spectrum in MVPb2I6 by irradiation of a CW He-Cd laser (441.6 nm). A photoinduced absorption band is observed at 2.0 eV. This absorption is assigned to a methylviologen cation radical (MV+• ).10 The detection of MV+• manifests charge separation between the nanowire and MV2+ via the chargetransfer transition, supporting the above assignment. The appearance of charge-transfer transitions in MVPb2I6 indicates electronic state mixing between the lead iodide nanowire and MV2+ due to electronic interactions. Next, we examine the effects of electronic interactions on molecular vibrations of MV2+ in MVPb2I6. 3.2. Vibrational Spectra. The IR spectrum of MVPb2I6 is shown in Figure 3, together with those of MVCl2, MVI2, and MVCl for comparison.11 The samples used for the former three compounds are single crystals and that for MVCl is a powder. MVCl2 and MVI2 are known to have charge-transfer interactions between MV2+ and the halide ions in the solids, which exhibit optical absorption bands due to charge-transfer transitions from the halide ions to MV2+.12 For MVPb2I6, since phonon energies in the lead-iodide nanowire are less than 200 cm-1,13 the IR absorption peaks observed originate from molecular vibrations of MV2+ as well as in the spectra of MVCl2, MVI2, and MVCl. At first, we examine the dependence of the IR spectrum of MV2+ on counterions. The vibrational peaks around 800 cm-1 in MVI2 and MVPb2I6 are observed to shift remarkably to the low-energy side by 40 and 62 cm-1, respectively, as compared to that (854 cm-1) of MVCl2. The deviation amount (62 cm-1) in MVPb2I6 reaches more than 7% of the vibrational energy in MVCl2. For MVPb2I6, furthermore, the oscillator strength is enhanced approximately twice compared with the other compounds. This behavior is contrastive to molecular vibrations between 1200 and 1400 cm-1. Although the molecular vibrations between 1200 and 1400 cm-1 hardly change among the three compounds, the molecular vibration around 800 cm-1 is very sensitive to the kind of counterions. The vibrational mode around
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Figure 4. Energy diagram of the electronic structures of MVPb2I6 and PDPbI3. VB, CB, and Ex denote valence and conduction bands and one-dimensional excitons, respectively. HOMO and LUMO stand for the highest occupied and lowest unoccupied molecular orbitals, respectively.
Figure 3. IR spectra of (a) MVCl, (b) MVCl2, (c) MVI2, and (d) MVPb2I6. The numbers denote the peak energies (cm-1). The spectra of MVCl, MVCl2, and MVI2 are offset for clarity.
800 cm-1 is assigned to the vibration about the nitrogen atoms.14,15 The quite large low-energy shift and the accompanying intensity change in MVPb2I6 indicate this vibrational mode can be changed by electronic interactions with the inorganic nanowire.16 The IR spectrum of MVCl was measured for specifying the cause of the low-energy shift in MVPb2I6, which will be described later. Here, we examine effects of one-electron reduction of MV2+, by comparing the IR spectra of MVCl2 and MVCl. In the IR spectrum of MVCl, three representative peaks are observed at 827, 1195, and 1330 cm-1 for molecular vibrations of MV+•. IR spectra of MV+• above 1100 cm-1 were reported by Ito et al.17 and Brienne et al.18 to have strong peaks at around 1196 and 1338 cm-1, which are in good agreement with our data. From the spectra, it is seen that one-electron reduction of MV2+ changes the vibrational spectrum entirely. Furthermore, the vibrational peak at 854 cm-1 in MVCl2 shifts to the low-energy side by 27 cm-1 and the peak intensity decreases remarkably with one-electron reduction. It is noted that the amount of low-energy shift in MVPb2I6 is obviously larger than that due to one-electron reduction of MV2+. 4. Discussion Here, we discuss electronic interactions between the lead iodide nanowire and MV2+ in MVPb2I6. Figure 4 illustrates a schematic energy diagram of the electronic structures of MVPb2I6 and PDPbI3.6,19 In MVPb2I6, the lowest unoccupied molecular orbital (LUMO) of MV2+ is much lower in energy than that of PD+ owing to the high electron affinity, as shown in the figure. On the basis of this fact, charge-transfer interactions are the most probable as electronic interactions in MVPb2I6. Charge-transfer interactions in MVPb2I6 result from electronic couplings between the valence band (VB) of the inorganic nanowire and LUMO of MV2+ by regarding the lead iodide nanowire and MV2+ as an electron donor and an acceptor, respectively. With this interaction, the electronic states of the inorganic nanowire and methylviologen are hybridized with each
other. As a result, the charges of MV2+ and the nanowire are deviated as follows, MV2+ + Pb2I62- f MV2-δ + Pb2I6-2+δ, 0 < δ , 1. To discuss charge-transfer interactions between nanowires and organic electron acceptors, suppose a system that consists of a nanowire and a molecule for clarity. In this case, δ is expressed by the following equation in the first order approximation with the condition of δ , Hint, which is the case:
δ≈L
( ) H ()
int ∫D() ∆E()
2
d
(1)
where ∆E is the energy gap between VB and LUMO without charge-transfer interactions. Hint is the matrix element of electron-nuclear Coulombic interactions between VB and LUMO, which depends on electronic overlaps between the two states and effective charges of interacting nuclei. Note that these two terms are functions of the state energy () of VB. D() is the density-of-state of VB, and L is the length of the inorganic nanowire. The equation shows δ to be proportional to the square of the matrix element and the square inverse of the unperturbed energy gap. Since the LUMO of MV2+ is much lower in energy than that of PD+, as mentioned above, the energy gap between VB and LUMO is much smaller in MVPb2I6 than in PDPbI3. Therefore, stronger charge-transfer interactions arise in MVPb2I6. Finally, we discuss the low-energy shift observed in the IR spectrum of MVPb2I6. Charge-transfer interactions affect molecular vibrations of MV2+ via two effects. One is due to partial charge deviation of MV2+. The other is owing to the formation of a weak covalent bond with the nanowire. In the former case, charge deviation changes directly force constants of molecular vibrations, as observed in Figure 3. In this case, the vibrational energy should be between those of MV1+ (827 cm-1) and MV2+ (854 cm-1). However, this is not the case, as mentioned above. Moreover, the charge deviation changes other vibration modes, as seen in Figure 3a, which is not observed in MVPb2I6. For these reasons, the contribution of the first mechanism is not predominant. From this result, the low-energy shift is considered to be due to modulation of the molecular vibration by weak bond formation between MV2+ and the lead iodide nanowire. A similar low-energy shift of molecular vibrations, which cannot be explained by partial charge deviation, was reported for the charge-transfer complexes of transition metal ions and tetra-
Interactions between Inorganic Nanowires and Organic Molecules cyanoethylene (TCNE).20 It was attributed to charge-transfer type coordinate bonds between them. To clarify the low-energy shift in more detail, further investigation is required. 5. Conclusion It was found that charge-transfer interactions between lead iodide nanowires and MV2+ in MVPb2I6 provide drastic effects on optical response and molecular vibration. In the absorption spectrum, charge-transfer transitions from the nanowire to MV2+ appear in the visible region. In the IR spectrum, a large lowenergy shift and the enhancement in oscillator strength are observed for the molecular vibration about the nitrogen atoms. From analysis of the low-energy shift, it is indicated that the vibrational mode is changed by formation of a weak covalent bond between the nanowire and MV2+ via charge-transfer interactions. Acknowledgment. J.F. is grateful to Dr. D. Hashizume (RIKEN) for fruitful discussion. The research was partly supported by the Grant-in-Aid for Young Scientists (B) (No. 14740223, 15750129, and 1775014) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) in Japan. References and Notes (1) Collier, C. P.; Wong, E. W.; Belohradsky, M.; Raymo, F. M.; Stoddart, J. F.; Kuekes, P. J.; Williams, R. S.; Heath, J. R. Science 1999, 285, 391.
J. Phys. Chem. C, Vol. 111, No. 3, 2007 1149 (2) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Science 2001, 293, 1289. (3) Tang, Z.; Guloy, A. M. J. Am. Chem. Soc. 1999, 121, 452. (4) Gridunova, G. B.; Ziger, E. A.; Koshkin, V. M.; Lindeman, S. V.; Struchkov, I. T.; Shklover, V. E. Dokl. Akad. Nuak SSSR 1984, 278, 656. (5) Vermeulen L. A.; Thompson M. E. Nature 1992, 358, 656 and references therein. (6) Fujisawa, J.; Ishihara, T. Phys. ReV. B 2004, 70, 113203. (7) Fujisawa, J.; Tajima, N. Phys. ReV. B 2005, 72, 125201. (8) Fukumoto T.; Hirasawa M.; Ishihara T. J. Lumin. 2000, 497, 8789. (9) Watanabe, T.; Honda, K. J. Phys. Chem. 1982, 86, 2617. (10) The intensity decays abruptly above 2.1 eV owing to the saturation effect. The sample used was not a cleaved thin crystal, but a relatively thick single crystal. (11) A broad peak at 1019 cm-1 in MVPb2I6 is probably due to dimethylsulfoxide contaminated in the crystal, which was used as a solvent in the synthesis. (12) Russel, J. H.; Wallwork, S. C. Acta Crysallogr. 1972, B28, 1527. (13) Fukumoto T. MS Thesis at Hiroshima University, 1999. (14) Poizat, O.; Sourisseau, C.; Mathey, Y. J. Chem. Soc., Faraday Trans. 1 1984, 80, 3257. (15) Forster, M.; Girling, R. B.; Hester, R. E. J. Raman Spectrrosc. 1982, 12, 36. (16) Tang et al. reported in ref 3 down-shifts of molecular vibrations in MVPb2I6 by 8-15 cm-1 in comparison to methylviologen dichloride hydrate and methylviologen gold(I) iodide, pointing out charge-transfer interactions in MVPb2I6. (17) Ito, M.; Sasaki, H.; Takahashi, M. J. Phys. Chem. 1987, 91, 3932. (18) Brienne, S. H. R.; Cooney, R. P.; Bowmaker, G. A. J. Chem. Soc., Farady Trans. 1991, 87, 1355. (19) Azuma, J.; Tanaka, K.; Kamada, M.; Kan’no K. J. Phys. Soc. Jpn. 2002, 71, 2730. (20) Manriquez, J. M.; Yee, G. T.; McLean, R. S.; Epstein, A. J.; Miller, J. S. Science 1991, 252, 1415.
