Sadamu TAKEDA (Gunma Univ. and IMS), Hiroki FUKUMOTO (Osaka Univ.), Kazushi MASHIMA (Osaka Univ.), Goro MARUTA (Osaka Univ.), Kizashi YAMAGUCHI (Osaka Univ.) and Akira NAKAMURA (Osaka Univ.)
[J. Phys. Chem. 101, 278 (1997)]
Conjugation in organic compounds is represented by extended p(pi)-p(pi) interactions. Trans-polyacetylene is typical and fascinating one-dimensional conjugated system. Many interesting physical and chemical properties have been found for this p(pi) conjugated system upon doping. A modified conjugation and/or doping category can be seen in organometallic systems, in which extended (pi)-interaction involving metal-carbon bonding is realized.1) The uniqueness of the organometallic (pi)-systems containing various transition metals exists because of their large variety of the electronic and molecular structures. An example is metal (pi)-complexes of organic conjugated systems. Particularly polyene complexes of early transition metals such as zirconium are interesting, since these electron-deficient metal ions are highly reactive and thus the polyene system may be largely perturbed by the bonding with zirconium which is both electron-donating ((pi)-type) and -attracting ((sigma)-type) in varying degrees. The situation is basically different from the simple doping of conventional conjugated systems.
We observed the thermochromism of the deeply colored zirconocene complex of 1,4-diphenyl-1,3-butadiene (DPBD) in the solid state. The color of the polycrystals of this complex is in between deep red and deep purple and changes continuously as the temperature is decreased from room temperature to liquid nitrogen temperature, while the precursors of the complex, ZrCp2Cl2 and 1,4-diphenyl-1,3-butadiene, are almost colorless. Variable temperature13C-CP/MAS NMR spectroscopy revealed that a remarkable change of the electronic structure of the DPBD molecule by constructing the complex propagates all over the carbon sites and that the distortion of the electronic structure of the complex becomes continuously large as temperature is decreased. This phenomenon is closely related to the thermochromism. The mechanism of the temperature variation of the electronic structure of the complex was attributed to the dynamics of phenyl ring and cycropentadienyl ring of the complex. Fast rotation of the cyclopentadienyl ring around its five fold axis with the extremely small activation energy of 28 meV (= 2.6 kJ/mol) and 180º-flip rotation of the phenyl ring about the C-C bond with the activation energy of 84 meV (= 7.7 kJ/mol) were recognized by the measurement of the spin-lattice relaxation rate of proton NMR. Quasielastic neutron scattering experiment suggests a small angle flipping of the phenyl ring, which is much faster than the two motional modes mentioned above. Semiempirical ZINDO molecular orbital calculation revealed that the deep color of the complex originates in the HOMO-LUMO and HOMO-LUMO+1 transitions. These molecular orbitals consists of d-orbitals of zirconium and (pi)-orbital of organic components and are sensitive to the orientation of the (pi)-orbital of cycropentadienyl rings, suggesting that the dynamic perturbation is operative for the transition energies.