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A phenalenyl‐based Kekulé hydrocarbon with singlet biradical character has been isolated and characterized. Strong intra‐ and intermolecular interactions between the unpaired electrons lead to short π–π contacts and formation of one‐dimensional chains (see picture). Thus, wide valence and conduction bands are established, and the compound shows semiconductive behavior.

We reinvestigated the two C=C stretching modes of the five-membered rings of ET (ET = bis(ethylenedithio)tetrathiafulvalene), namely, nu(2) (in-phase mode) and nu(27) (out-of-phase mode). The frequency of the nu(27) mode of ET(+) was corrected to be approximately 1400 cm(-1), which was identified from the polarized infrared reflectance spectra of (ET)(ClO(4)), (ET)(AuBr(2)Cl(2)), and the deuterium- or (13)C-substituted compounds of (ET)(AuBr(2)Cl(2)). It was clarified from DFT calculations that the frequency of the nu(27) mode of the flat ET(0) molecule was significantly different from that of the boat-shaped ET(0) molecule. We obtained the linear relationship between the frequency and the charge on the molecule, rho, for the flat ET molecule, which was shown to be nu(27)(rho) = 1398 + 140(1 - rho) cm(-1). The frequency shift due to oxidation is remarkably larger than that reported in previous studies. The fractional charges of several ET salts in a charge-ordered state can be successfully estimated by applying this relationship. Therefore, the nu(27) mode is an efficient probe to detect rho in the charge-transfer salts of ET. Similarly, a linear relationship for the nu(2) mode was obtained as nu(2)(rho) = 1447 + 120(1 - rho). This relationship was successfully applied to the charge-poor molecule of theta-type ET salts in the charge-ordered state but could not be applied to the charge-rich molecule. This discrepancy was semiquantitatively explained by the hybridization between the nu(2) and nu(3) modes.

We report experimental evidence for the generation of ferroelectric polarization in an organic conductor -[bis(ethylenedithio)tetrathiafulvalene] 2 I 3 obtained by optical second-harmonic generation. The spontaneous polarization emerges along with a metal-to-insulator transition that is driven by the Wigner crystallization of electrons. The strong optical nonlinearity and its ultrafast photoresponse demonstrated by this study exemplify the nature of the ferroelectric polarization that originates from the electron ordering.

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Charge distribution in the insulating phase of -(BEDT-TTF) 2 RbZn(SCN) 4 ͓BEDT-TTF denotes bis͑ethyl-enedithiolo͒tetrathiafulvalene͔ is investigated using the polarized Raman and infrared ͑IR͒ spectroscopy. Both of the Raman and IR spectra showed a multiple-peaks pattern below metal-insulator transition temperature. We assigned the carbon-double-bond (CvC) stretching-modes based on the isotope shift in the 13 C-substituted sample, and performed a factor-group analysis for the polarized IR and Raman spectra. These results proved that charge disproportionation ͑CD͒ takes place in the insulating phase, and that the spatial pattern of the charge ordering agrees with the symmetry determined by the x-ray diffraction. The CD ratio was estimated from the frequency of a ring CvC stretching mode that is weakly coupled with electronic excited states. A systematic way to investigate the CD phenomenon based on vibrational spectroscopy is discussed.

Photoinduced melting of charge-order (CO) in [bis(ethylenedithiolo)]-tetrathiafulvalene (BEDT-TTF) salts was investigated by femotosecond spectroscopy. Comparative studies on two polytypes exhibiting large [theta-(BEDT-TTF)2RbZn(SCN)_{4}] and small [alpha-(BEDT-TTF)2I3] molecular rearrangements through the CO transition were performed. Ultrafast melting of CO for both compounds demonstrates the major contribution of the electronic instability which is due to Coulomb interaction. The roles of the molecular rearrangements on the formation of the CO and the metallic domain are discussed on the basis of low-frequency lattice dynamics.

Present status of experiments on charge ordering (CO) phenomena observed in organic molecular conductors is reviewed. Experimental techniques to detect and analyze the CO, i.e., NMR, X-ray and Infrared/Raman spectroscopy, are described and the experimental results in typical materials exhibiting CO are summarized. (DI-DCNQI) 2 Ag, (TMTTF) 2 X, -(BEDT-TTF) 2 I 3 , -(BEDT-TTF) 2 RbZn(SCN) 4 and related materials are discussed. Charge disproportionation without long-range order observed in numbers of materials are also included.