[Show abstract][Hide abstract] ABSTRACT: 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 BF bonds.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: A new family of pi-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 non-planar 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.
The Journal of Organic Chemistry 11/2012; 77(23). DOI:10.1021/jo302047m · 4.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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 β-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.
The Journal of Chemical Physics 12/2011; 135(23):234705. DOI:10.1063/1.3669653 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Sergeants-and-soldiers experiments demonstrate the amplification of chirality that transforms the racemic mixture of helical columns formed from OPE-based trisamides into enantiomerically enriched helical structures (see scheme).The cooperative supramolecular polymerization of the trisamides is investigated by theoretical calculations and temperature-dependent UV/Vis and CD experiments.
Chemistry - A European Journal 07/2011; 17(28):7755-9. DOI:10.1002/chem.201100898 · 5.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Two charged organometallic complexes containing bulky hydrophobic ligands based on ruthenium (II) and iridium (III) were synthesized and their performance in solid state light emitting electrochemical cells is described. The complexes were chosen as due to their large ligands a diminished susceptibility towards the formation of destructive complexes during device operation is expected. The LEC device performances reveal the longest living devices reported so far under dc bias. Quantum chemical calculations confirm that the major effect of the bulky diphenylphenanthroline ligands is of steric origin and not related with changes in the molecular electronic structure of the complexes.
[Show abstract][Hide abstract] ABSTRACT: Owing to the practical interest of the acid-catalyzed isomerization of n-butane and 1-butene, the mechanism of the isomerization and scrambling reactions of the n-butyl cation has been studied theoretically using ab initio methods which include electron correlation and extended basis sets. It has been found that the protonated cyclopropane ring does not appear as a common intermediate for carbon scrambling and branching isomerization of the n-butyl cation, since it is a transition state and not a minimum on the potential energy surface. The transition states for both reactions have been determined and the activation energies calculated. These values are in very good agreement with those obtained experimentally.
[Show abstract][Hide abstract] ABSTRACT: 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 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 C-60. The cycloadducts formed (21a-d) were further oxidized to the respective p-benzoquinone-containing fullerenes 23a-c. The temperature dependent H-1 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 C-60 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.
[Show abstract][Hide abstract] ABSTRACT: 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.
Chemistry - A European Journal 03/2010; 16(18):5481-91. DOI:10.1002/chem.200903343 · 5.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This work presents an analysis of the evolution of the molecular, vibrational and optical properties of a family of functionalized
pentacenes. The analysis is performed on the basis of DFT quantum-chemical calculations in combination with spectroscopic
techniques (Raman and UV–Vis). Theoretical calculations show that the bond length C–C/C=C alternation along the peripheral
oligoenic ribbons increases with electron-releasing dioxolane substituents and diminishes with electron-withdrawing chlorine
atoms and cyano groups. The attachment of triisopropylsilylethynyl groups increases the complexity of the Raman spectra. The
spectra present many intense features of similar intensities in the 1,200–1,600cm−1 range which are described by a combination of C–C/C=C stretching vibrations and in-plane C–H deformations spreading over
the whole pentacene backbone. The absorption spectra display absorption bands in three different energy regions of the UV–Vis
electromagnetic range. The spectra are dominated by a strong absorption band measured in the 300–350nm region, which undergoes
a sizeable red-shift with the substitution pattern, and a low-intensity, three-peak band in the 500–700nm region, which undergoes
a blue- or a red-shift depending on the electronic nature of the substituents. TDDFT calculations enable a detailed description
of the trends observed in the absorption spectra.
KeywordsRaman spectra–Electronic transitions–Pentacenes–Density functional calculations
[Show abstract][Hide abstract] ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
[Show abstract][Hide abstract] ABSTRACT: 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.
Chemistry - A European Journal 11/2009; 16(2):470-84. DOI:10.1002/chem.200902037 · 5.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The [Ir(ppy-F2)2Me4phen]+1 complex, where ppy-F2 is 2-(2′,4′-fluorophenyl)pyridine and Me4phen is 3,4,7,8-tetramethyl-1,10-phenanthroline, has been theoretically investigated by means of DFT calculations. The molecular and electronic properties calculated for [Ir(ppy-F2)2Me4phen]+1 are compared with those obtained for the simpler [Ir(ppy)(bpy)]+1 complex. The introduction of fluorine substituents in the ppy ligands and the use of phenanthroline instead of 2,2′-bipyridine as the diimine ligand increase the HOMO–LUMO energy gap and blue-shift the emission colour. The phenanthroline ligand causes the appearance of two nearly-degenerate LUMO orbitals of different symmetry in [Ir(ppy-F2)2Me4phen]+1 and determines that two almost isoenergetic doublet states are obtained for the reduced complex. Calculations predict three lowest-energy triplet excited states of different nature in an energy difference of only 0.10eV. Emission from the 3LC π–π* state takes place at higher energies and accounts for the structured blue emission observed in solution. Emission from the lowest-energy 3MLCT state occurs in the green region and explains the structureless broad band observed in solid films.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: In this paper, we report a theoretical study of four types of thiophene-based oligomers showing the same number of CC double bonds and very different molecular structures. The comparative study has been performed on the basis of B3LYP/6-31G∗∗ calculations. The way the thiophene rings are linked together has a remarkable influence on the molecular and electronic properties. Linear quaterthiophene and heptathienoacene show similar aromatic structures but a loss of π-conjugation is detected for the latter due to the condensation of thiophene rings. A blue shift of the most intense electronic transition is predicted for fused heptathienoacene compared with non-fused quaterthiophene. Cyclic quaterthiophene exhibits quinoid thiophene rings folded in an envelope shape and should be visualized as a sulphur-bridged, cis-transoid polyenic chain. Circularly-fused sulflower presents a loss of π-conjugation with respect to linear systems due to its highly-branched conjugated backbone. The loss of conjugation and the high symmetry of the molecule determine that sulflower shows no optical absorption in the visible or near-UV.
[Show abstract][Hide abstract] ABSTRACT: 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.