Charge localization in 1D tetramerized organic conductors: the special case of (tTTF)2ClO4

We report a detailed structural and spectroscopic study of the 1D 2:1 cation radical salt (tTTF)₂ClO₄, where tTTF  =  trimethylenetetrathiafulvalene, which exhibits a semiconductor-semiconductor phase transition at ca. T  =  137 K. Crystal structures are determined above and below the transition; the tTTF molecules in stacks are grouped into weakly interacting tetramers. The reorganization of tTTF stacks is accompanied with an order-disorder transition in anion sublattice. Polarized infrared and Raman spectra of (tTTF)₂ClO₄ are measured in the broad frequency range as a function of the temperature (10-293 K). The structural and vibrational features are investigated to elucidate the origin of the semiconductor-semiconductor phase transition. We discuss the electron-intramolecular vibration coupling effects in the vibrational spectra of (tTTF)₂ClO₄ and identify signatures of high- and low-temperature states of charge localization in the tetramerized system. Both the C=C and C-S stretching modes of tTTF give evidence of strong charge distribution fluctuations in conducting stacks for T  >  137 K, which are responsible for the appearance of molecules with charge  +1e, and charge localization in tTTF tetramers for T  <  137 K. The uniqueness of the salt (tTTF)₂ClO₄ in comparison with other tetramerized 1D systems is discussed.

Electronic criterion for categorizing the chalcogen and halogen bonds: sulfur–iodine interactions in crystals

Diversity of mutual orientations of Y–S and I–X and covalent bonds in molecular crystals complicate categorizing noncovalent chalcogen and halogen bonds. Here, the different types of S...I interactions with short interatomic distances are analysed. The selection of S...I interactions for the categorization of the chalcogen and halogen bonds has been made using angles that determine the mutual orientation of electron lone pairs and σ-holes interacted S and I atoms. In complicated cases of noncovalent interactions with `hole-to-hole' of S and I orientations, distinguishing the chalcogen and halogen bonds is only possible if the atom is uniquely determined, which also provides the electrophilic site. The electronic criterion for chalcogen/halogen bonds categorizing that is based on analysis of dispositions of electron density and electrostatic potential minima along the interatomic lines has been suggested and its effectiveness has been demonstrated.

The Halogen Bond

The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design.

Correlating conduction properties with the molecular symmetry: segregation of Z and E isomers in the charge-assisted, halogen-bonded cocrystal [(Z,E)-Me2I2TTF]2Br

Co-crystallization of theZandEisomers of Me₂I₂TTF in a mixed-valence bromide salt leads to segregated stacks with two different charge order patterns and associated charge-assisted halogen bonding.

Bilayer Molecular Metals Based on Dissymmetrical Electron Donors

The electrocrystallization from solutions of cyanobenzene-ethylenedithio-tetrathiafulvalene (CNB-EDT-TTF) in the presence of different anions X = ClO4(-), PF6(-), and I3(-), affords a new type of 2D molecular metals with composition (CNB-EDT-TTF)4X based on an unprecedented bilayer structure of the donors induced by effective head to head interdonor interactions through the nitrile groups, which is responsible for 2D metallic systems with unusual properties such as the higher band filling, larger effective mass of carriers, and almost degenerated double Fermi surfaces.

Cocrystals of 6-chlorouracil and 6-chloro-3-methyluracil: exploring their hydrogen-bond-based synthon motifs with several triazine and pyrimidine derivatives

In order to obtain complexes held together by hydrogen as well as halogen bonds, 6-chlorouracil [6-chloropyrimidin-2,4(1H,3H)-dione; 6CU] and its 3-methyl derivative [6-chloro-3-methylpyrimidin-2,4(1H,3H)-dione; M6CU] were cocrystallized with 2,4,6-triaminopyrimidine and the three triazine derivatives 2,4,6-triamino-1,3,5-triazine (melamine), 2,4-diamino-6-methyl-1,3,5-triazine and 2-chloro-4,6-diamino-1,3,5-triazine, which all offer complementary hydrogen-bonding sites. Three of these compounds form cocrystals with 6CU; however, melamine yielded only a new pseudopolymorph with 6CU, but formed a cocrystal with M6CU. All six cocrystals contain solvent molecules (N,N-dimethylformamide,N,N-dimethylacetamide orN-methylpyrrolidin-2-one), whose intermolecular interactions contribute significantly to the stabilization of the crystal packing. Each of these structures comprises chains, which are primarily formed by strong hydrogen bonds with a basic framework built byR22(8) hydrogen bonds of either pure N—H...N or mixed patterns. Solvent molecules are aligned to the border of these chainsviaN—H...O hydrogen bonds. Two of the reported crystal structures containing 6CU show additional Cl...O halogen bonds, which connect the chains to two-dimensional layers, while one weak and one strong Cl...Cl interaction are observed in the two structures in which molecules of M6CU are present.

Halogen-bonded halide networks from chiral neutral spacers

Chiral, ditopic, bis-iodinated molecules can form helical networks due to halogen bonding interactions when co-crystallised with halide tetraalkylammonium salts.