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.

Carbonyl-functionalized quaterthiophenes: a study of the vibrational Raman and electronic absorption/emission properties guided by theoretical calculations.

This work investigates the evolution of the molecular, vibrational, and optical properties within a family of carbonyl-functionalized quaterthiophenes: 5,5'''-diheptanoyl-2,2':5',2'':5'',2'''-quaterthiophene (1), 5,5'''-diperfluorohexylcarbonyl-2,2':5',2'':5'',2'''-quaterthiophene (2), and 2,7-[bis(5-perfluorohexylcarbonylthien-2-yl)]-4H-cyclopenta[2,1-b:3,4-b']-dithiophene-4-one (3). The analysis is performed by Raman and UV/Vis absorption/excitation/fluorescence spectroscopy in combination with density functional calculations. Theoretical calculations show that substitution with carbonyl groups and perfluorohexyl chains induces progressive quinoidization of the π-conjugated backbone in comparison to the carbonyl-free compound 5,5'''-dimethyl-2,2':5',2'':5'',2'''-quaterthiophene (DM-4T) used as reference. Raman spectra are dominated by a strong Raman line which mainly corresponds to a combination of C-C/C=C stretching vibrations spreading over the whole thiophene core. This band undergoes a remarkable downshift as a consequence of the structural changes induced by the electron-withdrawing groups on the π-conjugated backbone. The band splitting on incorporation of a central carbonyl bridge evidences the formation of two structural domains in the molecule. The excitation and fluorescence spectra recorded at low temperature show well-resolved vibronic structures associated with the most intense collective C-C/C=C stretching mode. Optical absorption and fluorescence bands exhibit remarkable bathochromic dispersion on carbonyl functionalization, indicative of extension of π conjugation. TDDFT calculations enable a detailed description of the trends observed in the absorption spectra. Resonance Raman spectra reflect the structural changes predicted for the S(0)→S(1) electronic transition and evidence the cross-conjugated character that the central carbonyl group confers on 3.

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.

Oligothienoacenes versus oligothiophenes: impact of ring fusion on the optical properties.

The impact of backbone rigidity on the optical properties of thiophene-based compounds is studied by analyzing in detail the geometrical, electronic, optical and vibronic features of a family of oligothienoacenes (nTAs) in comparison to non-fused α-oligothiophenes (nTs) by means of quantum-chemical calculations. Ring fusion in nTAs provokes a greater conjugation in the ground state. However, the change in the bond length alternation upon electronic excitation is very similar in both systems, which is also reflected in a similar evolution of the first optical transition energy with increasing oligomer size. Larger transition energies in nTAsvs.nTs arise from an electronic effect rather than from a structural one. nTAs present a normal mode predicted at ca. 500 cm(-1) which displays significantly higher Franck-Condon activity compared to nTs and which leads to pronounced differences in the optical spectra. Due to the rigid structure of nTAs, persistent mirror symmetry of absorption and emission is observed, very different to nTs.

Diferrocenyl oligothiophene wires: Raman and quantum chemical study of valence-trapped cations.

A combination of Raman spectroscopy and density functional theory calculations is used to describe the structural and spectroscopic properties of the different isomeric cations of diferrocenyl quaterthiophenes. Isomerisation of the thienyl ß-positions provides site selective oxidation, which gives rise to species that can interconvert by moving the charge over the bridge. The spectroscopic study allows us to describe a sequence of stationary trapped cationic, either ferrocenyl or thienyl, states which constitutes an energy cascade of accessible sites through which the charge transfer can proceed.

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.

Neutral and oxidized triisopropylsilyl end-capped oligothienoacenes: a combined electrochemical, spectroscopic, and theoretical study.

This work presents an analysis of the structural, electrochemical, and optical properties of a family of triisopropylsilyl end-capped oligothienoacenes (TIPS-Tn-TIPS, n=4-8) by combining cyclic voltammetry, spectroscopic techniques, and quantum-chemical calculations. TIPS-Tn-TIPS compounds form stable radical cations, and dications are only obtained for the longest oligomers (n=7 and 8). Oxidation leads to the quinoidization of the conjugated backbone, from which electrons are mainly extracted. The absorption and fluorescence spectra show partially resolved vibronic structures even at room temperature, due to the rigid molecular geometry. Two well-resolved vibronic progressions are observed at low temperatures due to the vibronic coupling, with normal modes showing wavenumbers of approximately 1525 and approximately 480 cm(-1). Optical absorption bands display remarkable bathochromic dispersion with the oligomer length, indicative of the extent of pi conjugation. The optical properties of the oxidized compounds are characterized by in situ UV/Vis/NIR spectroelectrochemistry. The radical cation species show two intense absorption bands emerging at energies lower than in the neutral compounds. The formation of the dication is only detected for the heptamer and the octamer, and shows a new band at intermediate energies. Optical data are interpreted with the help of density functional theory calculations performed at the B3LYP/6-31G** level, both for the neutral and the oxidized compounds.

FT Raman and DFT study on a series of all-anti oligothienoacenes end-capped with triisopropylsilyl groups.

Herein, we study the pi-conjugational properties of a homologous series of all-anti oligothienoacenes containing four to eight fused thiophene rings by means of FT Raman spectroscopy and DFT calculations. The theoretical analysis of the spectroscopic data provides evidence that selective enhancement of a very limited number of Raman scatterings is related to the occurrence in these oligothienoacenes of strong vibronic coupling between collective nu(C=C) stretching modes in the 1600-1300 cm(-1) region and the HOMO/LUMO frontier orbitals (HOMO=highest occupied molecular orbital; LUMO=lowest unoccupied molecular orbital). The correlation of the Raman spectroscopic data and theoretical results for these all-anti oligothienoacenes with those previously collected for a number of all-syn oligothienohelicenes gives further support to the expectation that cross-conjugation is dominant in heterohelicenes. Fully planar all-anti oligothienoacenes display linear pi conjugation which seemingly does not reach saturation with increasing number of annulated thiophene rings in the oligomeric chain at least up to the octamer.

Weighting non-covalent forces in the molecular recognition of C(60). 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.

Helical supramolecular polymerization of C3-symmetric amides and retroamides: on the origin of cooperativity and handedness.

The cooperative supramolecular polymerization of 1 and 2 yields P- or M-type helical aggregates depending on the absolute configuration (S or R) of the stereogenic centres attached to the side chains. The connectivity of the amide group does not affect the handedness of the helical aggregates, but determines a larger cooperativity for retroamides 1.

Synthesis, Characterization, and Theoretical Study of Sulfur-Containing Donor-Acceptor DCNQI Derivatives with Photoinduced Intramolecular Electron Transfer.

The donor-acceptor compounds N,N'-dicyanobenzo[b]naphtho[2,3-e][1,4]dithiin-6,11-quinonediimine (9a) and N,N'-dicyanobenzo[b]naphtho[2,3-e][1,4]oxathiin-6,11-quinonediimine (10a) and their methyl-substituted derivatives (9b and 10b-d, respectively) have been prepared, and their structural and electronic properties have been characterized by both experimental techniques and quantum-chemical calculations. The (1)H-NMR spectra evidence the existence of a syn/anti isomerism in solution. Both experimental and theoretical data suggest that the preferred configuration of the N-CN groups corresponds to a syn isomer for 9 and to an anti isomer for 10. The X-ray analysis performed for 9b reveals that molecules are not planar and pack in vertical stacks showing an overlap between donor and acceptor moieties of adjacent molecules. In agreement with X-ray data, theoretical calculations predict that both for 9 and 10 the acceptor DCNQI moiety is folded and adopts a butterfly-type structure and the donor moiety is bent along the line passing through the heteroatoms. The energy difference between planar and butterfly structures is calculated to be < 3 kcal/mol at the ab initio 6-31G level. The UV-vis spectra present a broad absorption in the visible which corresponds to a photoinduced intramolecular electron transfer from the high-energy HOMO furnished by the donor moiety to the low-energy LUMO located on the DCNQI fragment. Cyclic voltammetry displays one oxidation peak to the cation and two one-electron reduction waves to the anion and dianion. Theoretical calculations show the planarization of the acceptor/donor moiety induced by reduction/oxidation. The formation of stable radical anions is corroborated by the intense EPR signals recorded for reduced 9. The assignment of the hyperfine coupling constants of the EPR spectra is consistent with the existence of a preferred syn configuration.

Theoretical insight on novel donor-acceptor exTTF-based dyes for dye-sensitized solar cells.

A thorough density functional theory study is performed for the three carboxyl-based derivatives of the exTTF-TCF chromophore, where the π-extended tetrathiafulvalene (exTTF) electron-donor is linked to the tricyanofuran (TCF) electron-acceptor through an ethylene bridge, as dyes for dye-sensitized solar cells. Calculations predict that the carboxyl group in the acceptor moiety adopts an adequate orientation for an efficient anchoring on the semiconductor TiO2 surface. The carboxylic acid group holds a negative charge twice larger than the cyano moiety that favors the electron injection to the semiconductor. Time-dependent calculations allow for the assignment of the absorption bands in the UV-vis spectrum of exTTF-TCF and confirm the presence of two low-lying charge-transfer electronic transitions that account for the moderately-intense absorption in the 450-800 nm range. The striking optical absorption properties of exTTF-TCF are preserved for the carboxylic analogues. Finally, periodic calculations show relevant topological differences between the carboxylic derivatives anchored on the TiO2 surface, which would notably influence in the power conversion efficiency of a dye-sensitized solar cell.

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.

Controlled self-assembly of electron donor nanotubes.

We employ a combination of urea-urea hydrogen bonds and pi-pi stacking interactions to obtain soluble self-assembled nanotubes decorated with electron-donor TTF derivatives on the periphery. We have investigated the structure and stability of the nanotubes with a combination of experiments and high-level DFT calculations. We also demonstrate that the association process can be controlled by changes in the hydrogen-bonding ability of the solvent and electrochemically.

Predicting the activity of single isolated Lewis acid sites in solid catalysts.

An experimental study of the activity of Ti-, Zr- and Sn-beta catalysts in different types of oxidation reactions is combined with a quantum-chemical analysis of the electronic properties of the active sites and the adsorbed reactants. The differences observed in the catalytic behaviour of the three materials are explained in terms of the molecular orbital distribution of each system. The intrinsic Lewis acid strength of the isolated active site, the degree of back-donation from the catalyst to the empty orbitals of the organic reactant and the net atomic charges on selected atoms are proposed as predictors of reactivity.

Stable single-layer light-emitting electrochemical cell using 4,7-diphenyl-1,10-phenanthroline-bis(2-phenylpyridine)iridium(III) hexafluorophosphate.

A significant improvement in the stability of a light emitting electrochemical cell was achieved by utilizing a novel iridium(III) complex: 4,7-diphenyl-1,10-phenanthroline-bis(2-phenylpyridine)iridium(III) hexafluorophosphate. The enhanced device stability is correlated by means of DFT studies to be related to a more efficient shielding of the reactive LUMO of the complex.

Raman and theoretical study of the solvent effects on the sizable intramolecular charge transfer in the push-pull 5-(dimethylamino)-5'-nitro-2,2'-bithiophene.

In this paper, we analyze the degree of intramolecular charge transfer in a push-pull pi-conjugated system, 5-(dimethylamino)-5'-nitro-2,2'-bithiophene, from changes in frequencies and relative intensities of its strongest Raman scatterings in a bunch of solvents with different polarities. Density functional theory (DFT) was used as a support of the experimental study. Solvent effects on the molecular and electronic structures and on the vibrational properties were estimated by performing B3LYP/6-31G calculations within the framework of the polarized continuum model (PCM) developed by Tomasi. Calculations reveal that the molecule is highly polarized in the ground state and behaves as a very efficient photoinduced push-pull system. The polarization of the molecule strongly increases with solvent polarity and determines that the profile of the Raman spectra greatly changes from one solvent to another and in going to the solid. The strongest Raman scattering associated with the nu(sym)(NO(2)) stretching undergoes a downshift of 48 cm(-1) in passing from CCl(4) to the solid. DFT calculations provide a comprehensive interpretation of the evolution of the Raman spectra with solvent polarity.