This article reviews the static and dynamic properties of spontaneous superstructures formed by electrons. Representations of such electronic crystals are charge density waves (CDW) and spin density waves in inorganic as well as organic low-dimensional materials. A special attention is paid to the collective effects in pinning and sliding of these superstructures, and the glassy properties at low temperature. Charge order and charge disproportionation which occur in organic materials resulting from correlation effects are analysed. Experiments under magnetic field, and more specifically field-induced CDWs are discussed. Properties of meso-and nanostructures of CDWs are also reviewed.

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Electronic crystals: an experimental overview

Optical properties of small inorganic and organic metal particles

Ground-state integer charge transfer is commonly regarded as the basic mechanism of molecular electrical doping in both, conjugated polymers and oligomers. Here, we demonstrate that fundamentally different processes can occur in the two types of organic semiconductors instead. Using complementary experimental techniques supported by theory, we contrast a polythiophene, where molecular p-doping leads to integer charge transfer reportedly localized to one quaterthiophene backbone segment, to the quaterthiophene oligomer itself. Despite a comparable relative increase in conductivity, we observe only partial charge transfer for the latter. In contrast to the parent polymer, pronounced intermolecular frontier-orbital hybridization of oligomer and dopant in 1:1 mixed-stack co-crystallites leads to the emergence of empty electronic states within the energy gap of the surrounding quaterthiophene matrix. It is their Fermi-Dirac occupation that yields mobile charge carriers and, therefore, the co-crystallites-rather than individual acceptor molecules-should be regarded as the dopants in such systems.

/pdf/charge-transfer-crystallites-as-molecular-electrical-dopants-3j3cqrt9qu.pdf

Charge-transfer crystallites as molecular electrical dopants

Despite the high charge-carrier mobility in covalent organic frameworks (COFs), the low intrinsic conductivity and poor solution processability still impose a great challenge for their applications in flexible electronics. We report the growth of oriented thin films of a tetrathiafulvalene-based COF (TTF-COF) and its tunable doping. The porous structure of the crystalline TTF-COF thin film allows the diffusion of dopants such as I2 and tetracyanoquinodimethane (TCNQ) for redox reactions, while the closely packed 2D grid sheets facilitate the cross-layer delocalization of thus-formed TTF radical cations to generate more conductive mixed-valence TTF species, as is verified by UV-vis-NIR and electron paramagnetic resonance spectra. Conductivity as high as 0.28 S m−1 is observed for the doped COF thin films, which is three orders of magnitude higher than that of the pristine film and is among the highest for COF materials.

http://yaghi.berkeley.edu/pdfPublications/14-cofconductivity.pdf

Tunable electrical conductivity in oriented thin films of tetrathiafulvalene-based covalent organic framework

Supramolecular conducting nanowires from organogels.

An extensive vibrational assignment of TTF and TTF‐d4 is achieved, improving the previously reported one through the use of polarized infrared spectra of single crystals of the monoclinic form. Infrared spectra of the monoclinic and triclinic forms are compared and the different crystal field effects discussed. Powder Raman and infrared spectra of (TTF)Br1.0 and (TTF‐d4)Br1.0, Raman depolarization ratios and infrared spectra of (TTF)ClO4 and (TTF‐d4)ClO4 solutions are reported. The assignment of the ag, b1u, b2u, and b3u fundamental modes of (TTF)+ and (TTF‐d4)+ radicals allows the identification of most of the relevant frequency shifts following the ionization of the TTF structure. The possible use of the ionization shifts for the study of the electronic charge distribution in the conducting TTF systems is considered. The parallel investigation of the concentration effects on the visible and infrared absorption spectra of TTF+ in solution let us to identify anomalous infrared absorptions associated with ...

Vibrational spectroscopy of molecular constituents of one‐dimensional organic conductors. Tetrathiofulvalene (TTF), TTF+, and (TTF+)2 dimer

The results of a study of the effect of concentration on the electronic and infrared absorption spectra of LiTCNQ and Li[2H4]TCNQ in dimethylsulphoxide, as well as the Raman and infrared spectra of aqueous solutions of the same salts, are reported. The formation of the self-dimer of the TCNQ radical anion is accompanied by the appearance of some peculiar absorptions in the infrared. They are attributed to a vibronic intensity enhancement of some vibrational modes of the dimer, corresponding to the out-of-phase coupling of totally symmetric modes of the isolated radicals. The phenomenon is interpreted in terms of the Ferguson–Person vibronic model, which embodies the basic concepts of the charge transfer (CT) interaction.The comparison of the vibronic effects discovered for the TCNQ radical dimer in solution with those reported in the literature for the infrared spectrum of crystalline KTCNQ shows that the dimeric unit essentially contains the same vibronic features induced by the CT interaction in the solid state. The infrared spectra of the two polymorphic forms RbTCNQ(I) and RbTCNQ(II) are also reported. Their comparison shows that the intensity of the vibronic features is sensitive to different types of anion stacking in the two structures.An overall analysis of the reported data allows us to give the complete list of the infrared absorptions of vibronic origin and to reassign some infrared active fundamentals of the monomeric TCNQ radical anion.

Influence of the intermolecular charge transfer interaction on the solution and solid state infrared spectra of 7,7,8,8-tetracyanoquinodimethane (TCNQ) alkaline salts

The results of an extensive vibrational analysis of tetramethyltetrathiafulvalene (TMTTF) and tetramethyltetraselenafulvalene (TMTSF) and of their radical cations are presented. The polarized infrared absorption spectra of oriented crystalline samples of neutral TMTTF and TMTSF (4000–80 cm−1) are reported and compared with powder and solution spectra. The polarization data are used as a basis for the symmetry assignment of the infrared active fundamental modes. Powder and solution Raman spectra are presented and discussed considering the values of the depolarization ratios measured for some bands. Raman and infrared spectra of powder and solution samples of the (1:1) bromide and perchlorate salts of TMTTF and TMTSF are reported. Vibronic infrared absorptions originated by the coupling of the unpaired electrons to totally symmetric modes are identified and compared with those previously reported for unsubstituted tetrathiafulvalene (TTF) radical systems. The assignment of the normal modes of the radical ca...

Vibrational behavior of molecular constituents of organic superconductors: TMTSF, its radical cation and the sulphur analogs TMTTF and TMTTF +

The vibrational assignment of the fundamental modes of 2,3,5,6‐tetrachloro‐ and 2,3,5,6‐tetrabromo‐p‐benzoquinone (chloranil and bromanil, respectively) radical anions is reported and compared with that of the neutral molecules. For this purpose the single crystal Raman and infrared spectra of bromanil are also given and interpreted. The absence of the vibronic effects present in the infrared spectra of some free radical salts is demonstrated, and the vibrational spectra of the potassium salts of both radical ions are shown to be interpretable in terms of substantially unperturbed molecular structures even in the case of crystalline samples. The vibrational assignments and their normal coordinate analysis allow one to obtain most of the relevant information on the frequency shifts proceeding from the addition of an extra electron to the perhalo‐p‐benzoquinone structure.

Raman and infrared frequency shifts proceeding from ionization of perhalo-p-benzoquinones to radical anions

Analyse vibrationnelle des transitions vibroniques du TCNQ dans un solvant rigide, a basse temperature. Calcul des constantes de couplage vibroniques par la methode CNDO/S en utilisant les vecteurs propres cartesiens des modes de vibration obtenus a partir d'une analyse en coordonnees normales

Ines Zanon

Papers

Vibrational spectroscopy of molecular constituents of one‐dimensional organic conductors. Tetrathiofulvalene (TTF), TTF+, and (TTF+)2 dimer

Influence of the intermolecular charge transfer interaction on the solution and solid state infrared spectra of 7,7,8,8-tetracyanoquinodimethane (TCNQ) alkaline salts

Vibrational behavior of molecular constituents of organic superconductors: TMTSF, its radical cation and the sulphur analogs TMTTF and TMTTF +

Raman and infrared frequency shifts proceeding from ionization of perhalo-p-benzoquinones to radical anions

Vibronic structure of the near-infrared and visible electronic transitions of 7,7,8,8-tetracyanoquinodimethane radical anion