Toward Functional Control of Molecular Nanomaterials
Based on Mechanistic Studies
We research various electronic functions of molecular nanomaterials and are aiming to clarify the mechanism and to manipulate those functionalities. Orbit, charge and spin of electron play essential roles for the optical, electrical and magnetic properties of materials. One of the fascinating natures of molecular materials is a hierarchic framework of interaction, which the size of interaction decreases in the order of element, molecule and nanostructured object. Based on the idea of interaction hierarchy, we can choose element, design molecular structure and integrate unit molecules. This build-up chemistry has a potential to realize the challenge to create and control the novel multifunction expanded from quantum effects in atoms and molecules. The strong point of our researches is on the elucidation of mechanism for the multifunction especially by means of the molecular spectroscopic measurements and the experiments of the illumination, electric and magnetic field effects on the properties of semiconductors formed by advanced molecular nanomaterials.
Using time-of-flight methods (i- and Q-TOF) with nanosecond time-resolution, we study the photocarrier generation of organized film systems such as nanotube or nanocrystal.
By means of the integral mode time-of-flight measurement of the drift motion of the photoinjected charge at room temperature, we observed magnetic field effect (MFE) on the photocharge in self-assembled nanotube of hexabenzocoronene (HBC) [Science, 314, 1761 (2006); J. Am. Chem. Soc., 129, 9276 (2007)] at the configurations where the magnetic field is parallel and perpendicular to the electric field. The detected MFEs are interpreted in terms of two mechanisms that are caused by the fast charge transportation (Hall effect) and the spin selective recombination (electron-hole pair mechanism). A high mobility in the nanotubes of π-stacked HBC was implied from the Hall data. The time dependence of the low field MFE due to the electron-hole pair mechanism clarified the recombination from both the singlet and triplet e-h pairs with the initial rate of ~108 s-1.
Y. Wakikawa, T. Ikoma, Y. Yamamoto, T. Fukushima and T. Aida, Syth. Met.,160, 285-290 (2010).
Single crystals of some diacetylene derivatives offer well-defined single crystals of polydiacetylene (PDA) through thermal solid state polymerization. The π-conjugated backbone structure of PDA indicates an attractive one-dimensional transportation with the high value of charge carrier mobility in PDA crystals. The mobility comparable or more than that of crystalline semiconductors is estimated in some papers. Thin film, in which oriented nanocrystals of PDA are assembled, can be obtained by a friction-transfer technique [Y. Ueda, et al., Jpn. J. Appl. Phys, Part1., 34(7B), 3876 (1995).].
Experiments of the magnetic field effect on the photoconductivity in this project provide more detail information about the photocarrier generation and transportation processes.
An amorphous molecular semiconductor, poly(N-vinylcarbazole) (PVCz), exhibits negative giant magnetoresistance (MR) under ambient conditions. The application of a weak magnetic field of 10 mT to PVCz films doped with lumichrome, in which light excitation immediately creates triplet electron–hole (e-h) pairs that are precursors of photocarriers, causes a steep decrease in resistivity by more than 20% at ambient temperature. Further, the resistivity of the doped film gradually reduces by approximately one-half under a field of 1 T, equivalent to an MR ratio of -55%. In addition, anomalous spikes are also detected at 0.07, 0.30, and 9.0 T, indicative of an exponential dependence with a decay distance of 0.1 nm in the exchange interaction of the e-h pair. A quantum mechanical calculation based on the density operator formalism clarifies that the observed MR effect can be comprehensively understood by the spin-selective charge dynamics and the coherent and incoherent spin dynamics of the e-h pair in a quasi-one-dimensional lattice for photocarrier generation. Model calculations also indicate the importance of the spin–lattice relaxation for the giant MR effect in organic molecular semiconductors.
T. Ikoma, T. Ogiwara, Y. Takahashi, K. Akiyama, S. Tero-Kubota, Y. Takahashi, T. Suzuki and Y. Wakikawa, Synth. Met., 160, 275-279 (2010).
Among the many organic photoconductive materials, aromatic vinyl polymers like poly(N-vinylcarbazole) (PVCz) have received much attention as one of the most durable compounds. Especially, PVCz films doped with a small amount of fullerenes (C60) have been reported to show excellent photoconductivity compared to the films without fullerenes [Nature 356, 585 (1992); J. Phys. Chem. B 101, 5627 (1997).], because the fullerene acts as an electron acceptor in the polymers due to its acceptor-ability. We clarified clarify the spin dynamic process during photoinduced carrier generation in a C60-doped PVCz film. The total number of carriers that escaped from the geminate pairs decreases in the presence of magnetic fields, which arises from a hyperfine mechanism of the distant ion pairs involving C60- and Cz+.
T. Ogiwara, T. Ikoma, K. Akiyama and S. Tero-Kubota, Chem. Phys. Lett. 411, 378-383 (2005).
A significant wavelength dependence of the excitation light was observed in the magnetic field effects (MFE) on the photocarrier generation of a benzene-1,2,4,5-tetracarbonitrile-doped PVCz film. The excitation of the carbazole induces positive MFEs, indicating an enhancement of the photoconductivity originating from the charge separation through the triplet exciton. In contrast, a singlet spin state governs the photocarrier generation in the case of the charge transfer band excitation, because negative MFEs are induced.
T. Ikoma, F. Ito, T. Ogiwara, K. Akiyama and S. Tero-Kubota, Chem. Lett. 34, 1424-1425 (2005).
The carrier injection yield depending on the spin state of the precursor indicates a possibility of photoinduced magnetoresistance based on the electron-hole pair mechanism in organic semiconductor. In our laboratory, we both theoretically and experimentally study the photoinduced magnetoresistance in nonmagnetic molecular solids.
Plastic photovoltaic device, which is one application of the fundamental science of organic semiconducting materials, offers the possibility of low-cost fabrication of large-area solar cells for harvesting energy from sunlight. Interest in the photovoltaic properties of conjugated polymers like polyacetylene, polythiophenes and poly(phenylenevinylenes) arises from the discovery of mobile photoinduced charged states in organic semiconductors. The energy conversion efficiencies for photovoltaic cells made with pure conjugated polymer above are typically less than 0.1%. However the best power conversion efficiency of 7.4 % is achieved in devices of a conjugated polymer and an electron accepting organic molecule, so called bulk heterojunction solar cells [Nature Photonics, 3, 649 (2009)]. The ultrafast and reversible photoinduced electron transfer from conjugated polymers onto the acceptor in the bulk heterojunction film generates a pair of negative polaron and positive polaron that is equal to an electron-hole pair. This point encourages us to study the effect of polaron spin on the properties of photovoltaic cell.
By means of the ESR technique, we investigate the electronic structures of the tridentate imino nitroxyl diradical complex with copper(II) (Cu-bisimpy), which has a square planar structure and a ground quartet state with an extremely strong ferromagnetic exchange interaction, and its related compounds (bisimpy = 2,6-bis(1'-oxyl-4',4',5',5'-tetramethyl-4',5'-dihydro-1'H-imidazol-2'-yl)pyridine). It was clarified that Cu-bisimpy had unique magnetic orbitals, compared with the biradical ligand (bisimpy), a zinc(II) biradical complex (Zn-bisimpy) and a copper(II) terpyridine complex (Cu-tpy) (tpy = 2,2';6',2''-terpyridine). Multifrequency ESR spectroscopy provided a reliable set of magnetic parameters of Cu-bisimpy, which has a small g anisotropy (gx = 2.02, gy = 2.01, gz = 2.08) and small hyperfine coupling with Cu (|Ax| = 42.0 MHz, |Ay| ≤ 14 MHz, |Az| = 153 MHz) but huge zero-field splitting (D = +17.4 GHz, E = -1.0 GHz). The maximum principle axis of zero-field interaction (zZF) is perpendicular to the z axis for the g and A-tensors, which is normal to the molecular plane. These characteristics of the magnetic properties prove that the substantial spin transfer from the dx2-y2 orbital of copper to the n-orbitals of the ligand is caused by a σ-type covalent bonding effect between the central metal and the ligand nitrogens. The covalent bonding effect produces carbene configurations on the nitrogen atoms of the imino nitroxyl radicals. The carbene configuration was concluded to be the main reason for the strong ferromagnetic coupling in Cu-bisimpy.
T. Ikoma, H. Oshio, M. Yamamoto, Y. Ohba and M. Nihei, J. Phys. Chem. A. 112(37), 8641-8648 (2008).
