We present a study of the torsional potential of biisothianaphthene and compare it to that of bithiophene. The calculations are performed at the ab initio and semiempirical Hartree−Fock (HF), ab initio post-Hartree−Fock, and density functional theory (DFT) levels. Our study has two major aims:  (i) on the physico-chemical side, to asses the optimal conformation of biisothianaphthene and evaluate the rotational barriers toward coplanar structures and (ii) on the methodological side, to asses the usefulness of DFT approaches. In contrast to previous estimates, the torsional potential of biisothianaphthene is found to differ markedly from that of bithiophene. For biisothianaphthene, strongly rotated s-cis- and s-trans-gauche minima are predicted as the most stable structures. The structural analysis fully justifies the greater stability of the s-cis-gauche conformer, thus explaining the “unexpected” s-cis-like structure observed experimentally in the crystal. The attainment of planar conformations is prevented by the high rotational barriers:  ∼22 kJ/mol (s-trans) and ∼63 kJ/mol (s-cis) at the MP2 level. Aromatic polyisothianaphthene chains are therefore predicted to be highly distorted from planarity even in the solid state, which is of importance with regard to their electronic and optical properties. DFT calculations are shown to provide geometries very close to those obtained at the MP2 level, but fail in describing the energetics of the torsional potentials because they overstabilize planar conformers. The results allow us to propose a very efficient computational approach for reliable determinations of conformational potentials in conjugated compounds. The poor quality of the potentials provided by semiempirical HF methods is emphasized.

A series of highly conjugated tetrathiafulvalene (TTF) analogues with a quinonoid structure has been synthesized, and their structural and electronic properties have been characterized by both experimental techniques and quantum-chemical calculations. Cyclic voltammetry measurements show a two-electron oxidation wave to form the dication, which is mainly located on the dithiole rings. The second irreversible oxidation wave to form the trication-radical corresponds to the oxidation of the polyacenic backbone. The temperature dependence of the reduction peak corresponding to the donor2+ → donor0 process is explained in terms of the low stability of the cation and the high aromaticity of the dication. Charge-transfer complexes are formed with the strong acceptor TCNQF4 showing a 1:2 (D:A) stoichiometry and a semiconducting behavior. The molecular structures of neutral and oxidized compounds are investigated by performing theoretical calculations at the semiempirical, ab initio, and density functional theory levels. The steric hindrance introduced by lateral benzoannulation determines the loss of planarity of the neutral molecules, which adopt butterfly shaped structures. The folded structures are retained in the cations, reducing the gain of aromaticity in the first oxidation step. The dications are by contrast predicted to be fully aromatic and are formed by a planar polyacenic moiety and two orthogonal, singly charged dithiole rings. The destabilization of the cations and the high aromaticity of the dications explain the redox properties observed experimentally. Theoretical calculations also help to rationalize the UV−vis data since they predict the appearance of a low-energy charge-transfer absorption band for the neutral compounds where the laterally fused polyacenic units act as acceptors.

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An exTTF-based macrocyclic receptor that associates C(60) with a binding constant >10(6) M(-1) in chlorobenzene at room temperature is described. This represents an improvement of 3 orders of magnitude with respect to the previous examples of exTTF-based receptors and one of the highest binding constants toward C(60) reported to date.

On the ball: Charge transfer occurs readily in tightly interacting complexes formed from π‐extended tetrathiafulvalenes, which act as pincerlike receptors, and C60 in a variety of solvents upon photoexcitation (see picture; PET=photoelectron transfer). It should be feasible to construct simple photovoltaic devices from systems based on similar recognition motifs.

Spiroannelated methanofullerenes bearing quinone-type addends including TCNQ and DCNQI analogues (3a − c, 6a,b, 8, 10, and 11) have been prepared, and their structural and electronic properties have been characterized by both experimental techniques and quantum-chemical calculations. The spiro[2,5-cyclohexadienone-4,61‘-methanofullerene] derivatives (3a − c), the spiro[10-anthrone-9,61‘-methanofullerene] (8), and the TCNQ- and DCNQI-type derivatives (10 and 11) were isolated as [6,6] adducts. The spiro[cyclohexanone-4,61‘-methanofullerene] (6) was however obtained as a mixture of [5,6] and [6,6] isomers. The novel methanofullerenes, with the only exception of 6, show irreversible cyclic voltammograms with additional reduction peaks. The conjugated cyclohexadienone derivatives 3 exhibit better acceptor abilities than the parent C60. Semiempirical PM3 calculations show that the addend lies perpendicular to the transanular bond in 3, while it folds down and adopts a butterfly shaped structure for compounds 8, 10, and 11. For compounds 3, periconjugative interactions transmit the inductive effect of the addend and produce a small stabilization of the orbitals of C60, resulting in a less negative first-reduction potentials compared to C60. For compounds 8, 10, and 11, the folding of the addend prevents periconjugative effects. Theoretical calculations performed on 3a • - and 3a 2- at the semiempirical (PM3), density functional (B3P86/3-21G), and ab initio (HF/6-31G*) levels indicate that the attachment of the first electron causes the homolytic cleavage of one of the bonds connecting the addend to C60. The resulting open-cyclopropane structure is stabilized by the aromaticity of the phenoxyl radical structure presented by the addend. The second electron enters in the addend forming the phenoxyl anion. This ring opening is supported by ESR measurements and explains the irreversible electrochemical behavior of compounds 3. The nonconjugated nature of the cyclohexanone ring in 6 determines that reduction takes place via a closed-cyclopropane structure with an electrochemical behavior similar to that observed for C60. Compounds 8, 10, and 11 are proposed to undergo reduction via an open-cyclopropane structure now obtained after the attachment of the second electron which produces the heterolytic opening of the cyclopropane ring. The lack of planarity shifts the reduction of the addend to more negative potentials.

The [4 + 2] cycloaddition reaction of o-quinodimethanes, generated in situ from 4,5-benzo-3,6-dihydro-1,2-oxathiin 2-oxides (10a,b, 13, and 19) (sultines), to [60]fullerene is described. Sultines are readily accesible from the commercially available rongalite and smoothly generate o-quinodimethanes, by extrusion of sulfur dioxide, which are efficiently trapped by the active dienophile C60. The cycloadducts formed (21a−d) were further oxidized to the respective p-benzoquinone-containing fullerenes 23a−c. The temperature dependent 1H NMR spectra show a dynamic process of the methylene protons. The activation free energy determined for the boat-to-boat inversion (11.3−11.6 kcal/mol) is remarkably lower than that obtained for other related carbocyclic or heterocyclic analogues. Semiempirical PM3 calculations show that the geometrical features and not the electronic properties of the organic addend in 23 are responsible for the low activation energy barriers. A linear correlation is found between the activation energy barriers and the length of the C62−C63 bond. The electrochemical properties of 23a−c have been rationalized on the basis of DFT-B3P86/3-21G calculations. The attachment of the first electron in the reduction process takes place in either the C60 cage or the organic addend depending upon the nature of the substituents on the p-benzoquinone ring, which controls the relative energy of the LUMO of the p-benzoquinone moiety. A full agreement between the theoretical predictions and the electrochemical measurements is found.

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.The construction of molecular receptors for fullerenes continues to be a very active area of research, with their purification from fullerite and the construction of self-organized electroactive nanostructures as main driving forces.1-11 To achieve these objectives, the formation of stable associates with fullerenes is a prerequisite. In this regard, the group of Kawase has recently coined the term ''concave-convex interactions '' 5-8 to denote the increase in non-covalent interactions between curved aromatic hosts and guests, and suggested these might play a distinct role in the stabilization of the complexes. A fair and quantitative comparison of the stability of the complexes formed between fullerenes and receptors based on flat or concave recognizing fragments would require a collection of receptors with enough structural similarity-that is, dissimilar only with regard to the recognizing units-to be studied under experimentally identical conditions. Herein, we investigate the relative contributions of p-p, van der Waals, electrostatic, and concave-convex interactions to the molecular recognition of C 60 by a series of related receptors.We have reported receptors that exploit the concave, electron-rich, aromatic surface of p-extended tetrathiafulvalene derivatives to associate [60]fullerene.9,10 Receptor 1 features 2-[9-(1,3-dithiol-2-ylidene)anthracen-10(9H)-ylidene]-1,3-dithiole (exTTF) as the recognizing element (Chart 1).9 Despite the lack of preorganization in its design, 1 forms stable associates with C 60 (see Table 1). Since a charge-transfer band is experimentally observed in the UV-Vis titrations (l max E 482 nm) of 1 against C 60 , up to four ''separate'' contributions to the stability of the complex can be envisaged: p-p aromatic interactions, van der Waals forces, electrostatic interactions, and concave-convex complementarity. With the aim of weighting those contributions separately, we designed and synthesized a collection of tweezer-like receptors, 1-4, in which the size, shape and electronic character of the recognizing motifs are selectively tuned. The solubility of receptors 1-4 at the concentrations employed in titration experiments (r1 mM) is sufficient to rule out solvophobic effects as a major factor in the stability of the complexes.As shown in Chart 1, receptors 1-4 consist of an isophthalic diester spacer to which two units of the corresponding recognizing moieties are attached. All the receptors were synthesized from the commercially available or previously reported methylene alcohols and isophthaloyl dichloride by standard condensation reactions in good to excellent yields, and unambiguously characterized.w Table 1 and are the average of at least two titration experiments (for details, see the Supplementary Informationw). Unfo...