Pedro M. Viruela

Weighting non-covalent forces in the molecular recognition of C60. Relevance of concave–convex complementarity

The relative contributions of several weak intermolecular forces to the overall stability of the complexes formed between structurally related receptors and [60]fullerene are compared, revealing a discernible contribution from concave-convex complementarity.

Quinoidal Oligothiophenes: Towards Biradical Ground‐State Species

A family of quinoidal oligothiophenes, from the dimer to the hexamer, with fused bis(butoxymethyl)cyclopentane groups has been extensively investigated by means of electronic and vibrational spectroscopy, electrochemical measurements, and density functional calculations. The latter predict that the electronic ground state always corresponds to a singlet state and that, for the longest oligomers, this state has biradical character that increases with increasing oligomer length. The shortest oligomers display closed-shell quinoidal structures. Calculations also predict the existence of very low energy excited triplet states that can be populated at room temperature. Aromatization of the conjugated carbon backbone is the driving force that determines the increasing biradical character of the ground state and the appearance of low-lying triplet states. UV/Vis, Raman, IR, and electrochemical experiments support the aromatic biradical structures predicted for the ground state of the longest oligomers and reveal that population of the low-lying triplet state accounts for the magnetic activity displayed by these compounds.

Cooperative Supramolecular Polymerization and Amplification of Chirality in C3‐Symmetrical OPE‐Based Trisamides

The folding of biopolymers, the binding of oxygen by hemoglobin, and some selected processes of supramolecular polymerization are model examples of allosteric cooperativity. The cooperative supramolecular polymerization is especially relevant since the hierarchical organization of relatively small and simple molecules produces organized structures with applicability in chemistry, biology, and materials science. The morphology and the properties of these structures can be directly related with the mechanism—isodesmic or nucleation–elongation, also named cooperative—that operates in the self-assembly process. The isodesmic mechanism is determined by only one binding constant (K), whereas the cooperative mechanism is characterized by an activation step defined by Ka and an elongation process governed by K. A number of organic compounds has been reported to self-assemble by following a cooperative mechanism. The supramolecular polymerization of these molecules forms columnar aggregates in which it is possible to observe an amplification of chirality that transforms racemic mixtures into enantiomerically enriched helical structures. Herein, we report on the cooperative supramolecular polymerization of C3-symmetrical oligo(phenylene ethynylene)s (OPEs) endowed with three amide functional groups (1, (S)-2, and (R)-3 in Figure 1). The reported OPE-based trisamides self-assemble by following a nucleation–elongation mechanism that contrasts with the isodesmic aggregation experienced by referable OPEs. The helical organization of the chiral trisamides (S)-2 and (R)-3 as well as achiral 1 has been investigated by variable temperature (VT), circular dichroism (CD), and “sergeants-and-soldiers” experiments. The latter demonstrate that the homochirality of 1 is strongly biased upon addition and subsequent intercalation of small amounts of chiral (S)-2 or (R)-3. X-ray diffraction and scanning electron microscopy (SEM) imaging confirm the columnar organization of these trisamides in the bulk. SEM images of 1, (S)-2, and (R)-3 reveal the formation of star-like, globular, and flower-like supramolecular structures, respectively. The preparation of target trisamides 1, (S)-2, and (R)-3 requires only three synthetic steps from commercial reagents or previously reported compounds and their chemical structures have been fully characterized by a number of techniques (Scheme S1 in the Supporting Information). The self-association of achiral 1 was first evaluated by concentration-dependent H NMR experiments (Figure S4 in the Supporting Information). These studies demonstrate that all the aromatic protons that shield upon increasing concentration, and also the amide protons that deshield upon increasing concentration are involved in the self-association of 1 due to the synergy of p–p interactions and H-bonding. The structure and stability of the aggregates formed by 1 was studied by performing density functional theory (DFT) calculations at the MPWB1 K/6-31G** level (see the Supporting Information for full computational details). The isolated monomer shows a minimum-energy C3 geometry in which the OPE moiety remains almost planar and the amide groups are twisted by 21.58 (Figure 2 a). This molecular structure favors the coexistence of p–p and H-bonding intermolecular interactions, and compound 1 aggregates into helical columnar stacks in which adjacent molecules are sepa[a] F. Garc a, Prof. Dr. L. S nchez Departamento de Qu mica Org nica Facultad de Ciencias Qu micas Ciudad Universitaria s/n. 28040 Madrid (Spain) Fax: (+34) 913944103 E-mail : lusamar@quim.ucm.es [b] Dr. P. M. Viruela, Prof. Dr. E. Ort Instituto de Ciencia Molecular Universidad de Valencia, 46980 Paterna (Spain) Fax: (+34) 963543274 E-mail : enrique.orti@uv.es [c] Dr. E. Matesanz C.A.I. Difraccion de Rayos X Facultad de Ciencias Qu micas Ciudad Universitaria s/n. 28040 Madrid (Spain) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201100898. Figure 1. Chemical structure of trisamides 1, (S)-2, and (R)-3.

Donor–π‐Acceptor Species Derived from Functionalised 1,3‐Dithiol‐2‐ylidene Anthracene Donor Units Exhibiting Photoinduced Electron Transfer Properties: Spectroscopic, Electrochemical, X‐Ray Crystallographic and Theoretical Studies

Steric interactions between the anthraquinoid core and the 1,3-dithiole and dicyanomethylene groups play a key role in determining the physical properties of system 1. The intramolecular charge transfer properties of this donor–π-acceptor species have been explored and cyclic voltammetric data, X-ray crystal structures and ab initio calculations are also reported.

Electronic spectra of tetrathiafulvalene and its radical cation: analysis of the performance of the time-dependent DFT approach

Abstract The electronic spectra of tetrathiafulvalene and its radical cation have been studied within the framework of the time-dependent density functional theory by using a conventional hybrid functional. The behaviour of the method has been analyzed through the computed vertical excitation energies for the low-lying electronic excited states. Although the procedure provides a correct description of many of the features of the spectra, deviations in the range 0.4–0.7 eV have been obtained for several transitions, from which one can conclude misleading assignments.

New functionalized tetrathiafulvalenes: X-ray crystal structures and physico-chemical properties of TTF–C(O)NMe2 and TTF–C(O)–O–C4H9: a joint experimental and theoretical study

The synthesis of the new mono- and di-functionalized tetrathiafulvalene (TTF) derivatives 4, 5 and 12–15 is reported. Compounds 4 and 5 have been studied in detail: their single crystal X-ray structures are reported and theoretical calculations at both semiempirical and ab initio levels have been performed. The TTF moiety in the crystal of 4 is folded, whereas in the crystal of 5 it is essentially planar. In the crystal structure of 4 the molecules related via an inversion centre form close dimers which pack in a severely distorted kappa-arrangement. Molecules of 5 assemble into uniform stacks with TTF moieties overlapping the side chains of adjacent molecules. Calculated geometries are in good agreement with the X-ray data, and the distortion from planarity in the crystal of 4 can be explained by nonbonding interactions between the amide substituent and the TTF rings of adjacent molecules. Evidence for intramolecular charge-transfer from the TTF ring to the electron-withdrawing amide and ester substituents of 4 and 5, respectively, is provided by UV-VIS spectroscopy in solution, analysis of the bond lengths in the crystal structure, and by theoretical calculations.

Non-Centrosymmetric Homochiral Supramolecular Polymers of Tetrahedral Subphthalocyanine Molecules**

A combination of spectroscopy (UV/Vis absorption, emission, and circular dichroism), microscopy (AFM and TEM), and computational studies reveal the formation of non-centrosymmetric homochiral columnar subphthalocyanine assemblies. These assemblies form through a cooperative supramolecular polymerization process driven by hydrogen-bonding between amide groups, π-π stacking, and dipolar interactions between axial B-F bonds.

Tuning the electronic properties of nonplanar exTTF-based push-pull chromophores by aryl substitution.

A new family of π-extended tetrathiafulvalene (exTTF) donor-acceptor chromophores has been synthesized by [2 + 2] cycloaddition of TCNE with exTTF-substituted alkynes and subsequent cycloreversion. X-ray data and theoretical calculations, performed at the B3LYP/6-31G** level, show that the new chromophores exhibit highly distorted nonplanar molecular structures with largely twisted 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) units. The electronic and optical properties, investigated by UV/vis spectroscopy and electrochemical measurements, are significantly modified when the TCBD acceptor unit is substituted with a donor phenyl group, which increases the twisting of the TCBD units and reduces the conjugation between the two dicyanovinyl subunits. The introduction of phenyl substituents hampers the oxidation and reduction processes and, at the same time, largely increases the optical band gap. An effective electronic communication between the donor and acceptor units, although limited by the distorted molecular geometry, is evidenced both in the ground and in the excited electronic states. The electronic absorption spectra are characterized by low- to medium-intense charge-transfer bands that extend to the near-infrared.

ExTTF-based dyes absorbing over the whole visible spectrum.

New push-pull dyes featuring π-extended tetrathiafulvalene (exTTF) as the donor group and tricyanofuran (TCF) as the acceptor group were synthesized and characterized. Their broad absorption covers the entire visible spectral range and enters the near-infrared region. Electrochemistry and theoretical calculations provided an understanding of these singular electronic properties. The new dyes are appealing candidates as light harvesters in photovoltaic devices.

Flexibility of TTF. a theoretical study

Abstract The folding of tetrathiafulvalene along the S···S axes has been investigated using highly-accurate quantum-chemical calculations. A very flat, two-fold potential is found for neutral TTF where minima correspond to boat conformations (θ ≈ 15–20 °). The flatness of the folding potential indicates that TTF is conformationally more flexible than usually thought.