Combined effects of electron correlations and lattice distortions are investigated on the charge ordering in θ-(BEDT-TTF)2RbZn(SCN)4 theoretically in a two-dimensional 3/4-filled extended Hubbard model with electron-lattice couplings. It is known that this material undergoes a phase transition from a high-symmetry metallic state to a low-symmetry insulating state with a horizontalstripe charge order (CO) by lowering temperature. By means of the exact-diagonalization method, we show that electron-phonon interactions are crucial to stabilize the horizontal-stripe CO and to realize the low-symmetry crystal structure.

To elucidate different photoinduced melting dynamics of charge orders observed in quasi-twodimensional organic conductors θ-(BEDT-TTF)2RbZn(SCN)4 and α-(BEDT-TTF)2I3 [BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene], we theoretically study photoinduced time evolution of charge and spin correlation functions on the basis of exact many-electron wave functions coupled with classical phonons in extended Peierls-Hubbard models on anisotropic triangular lattices. In both salts, the so-called horizontal-stripe charge order is stabilized by nearest-neighbor repulsive interactions and by electron-lattice interactions. In θ-(BEDT-TTF)2RbZn(SCN)4 (abbreviated as θ-RbZn), the stabilization energy due to lattice distortion is larger, so that larger quantity of energy needs to be absorbed for the melting of the charge and lattice orders. The photoinduced charge dynamics shows a complex behavior owing to a substantial number of nearly degenerate eigenstates involved. This is related to the high structural symmetry when the lattice is undistorted. In α-(BEDT-TTF)2I3 (abbreviated as α-I3), the lattice stabilization energy is smaller, and smaller quantity of energy is sufficient to melt the charge and lattice orders leading to a metallic phase. The photoinduced charge dynamics shows a sinusoidal oscillation. In α-I3, the low structural symmetry ensures nearly spin-singlet bonds between hole-rich sites, where the spin correlation survives even after photoexcitation.

The formation of the C-U base pair in a duplex was observed in solution by means of the temperature profile of (15)N chemical shifts, and the precise geometry of the C-U base pair was also determined by NOE-based structure calculation. From the solution structure of the RNA oligomer, r[CGACUCAGG].r[CCUGCGUCG], it was found that a single C-U mismatch preferred being stacked in the duplex rather than being flipped-out even in solution. Moreover, it adopts an irregular geometry, where the amino nitrogen (N4) of the cytidine and keto-oxygen (O4) of the uridine are within hydrogen-bonding distance, as seen in crystals. To further prove the presence of a hydrogen bond in the C-U pair, we employed a point-labeled cytidine at the exocyclic amino nitrogen of the cytidine in the C-U pair. The temperature profile of its (15)N chemical shift showed a sigmoidal transition curve, indicating the presence of a hydrogen bond in the C-U pair in the duplex.

The effects of spin and charge fluctuations and electron-phonon couplings on charge ordering in α-(BEDT-TTF) 2 I 3 (BEDT-TTF=bis(ethylenedithio)-tetrathiafulvalence) are investigated theoretically for an anisotropic triangular lattice at 3/4 filling. By the exactdiagonalization method, we have calculated the hole density distributions and the modulations of transfer integrals from high-temperature values as a function of electron-phonon coupling strength. The results clearly show that the lattice effect on α-(BEDT-TTF) 2 I 3 is weak compared with that on θ-(BEDT-TTF) 2 RbZn(SCN) 4 , as previously found by experiments. This finding, which is also consistent with recent mean-field results, is systematically explained by strong-coupling perturbation theory; the effects of spin fluctuations are partially canceled by charge fluctuations in α-(BEDT-TTF) 2 I 3 .