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In this article, we report the characterization of novel oligothienoacenes with five and seven fused thiophene rings, materials with potential applications in organic electronics. In contrast to usual alpha-linked oligothiophenes, these fused oligothiophenes have a larger band gap than most semiconductors currently used in the fabrication of organic field-effect transistors (OFETs) and therefore they are expected to be more stable in air. The synthesis of these fused-ring oligomers was motivated by the notion that a more rigid and planar structure should reduce defects (such as torsion about single bonds between alpha-linked units or S-syn defects) and thus improve conjugation for better charge-carrier mobility. The conjugational properties of these two molecular materials have been investigated by means of FT-Raman spectroscopy, revealing that conjugation still increases in passing from the five-ring oligomer to that with seven-rings. DFT and TDDFT quantum chemical calculations have been performed, at the B3LYP/6-31G level, to assess information regarding the minimum-energy molecular structure, topologies, and absolute energies of the frontier molecular orbitals (MOs.) around the gap, vibrational normal modes related to the main Raman features, and vertical one-electron excitations giving rise to the main optical absorptions.

Substituted phenols having pendant isoquinoline N-oxide were synthesized and their photochemical and luminiscent properties studied. Photolysis in an acid medium was found to yield the related photohydroxylation products, in a regioselective process, in addition to the isoquinoline deoxygenated precursor. Photoinduced electron transfer from the donor phenols to the protonated form of the first excited singlet state (S1) of the pendant isoquinoline N-oxide acting as acceptor leads to a red-shifted emissive charge transfer (CT) state that is in fact a radical/cation-radical pair. Homolysis of the N–OH bond restores the aromatic isoquinoline nucleus and produces a hydroxyl radical that can couple to the required ring carbon in the phenol cation-radical to give the photohydroxylation products in a regioselective process controlled by the spin density of the phenol cation-radical. These photohydroxylation processes efficiently compete with the reported tendency to deprotonation in phenol cation-radicals. The photohydroxylation process by itself, and its regioselectivity, exclude a proton-coupled electron transfer mechanism or a consecutive electron transfer/deprotonation reaction. By contrast, the phenol cation-radical exists long enough to undergo the hydroxyl radical coupling reaction that leads to the photohydroxylation products.

The electrocopolymerization of 3,4-ethylenedioxythiophene (EDOT) with the branched thiophene building block 2,2′:3′,2″-terthiophene (3T) is presented as a versatile route to functional polymer films. Comparisons to blend systems of the respective homopolymers PEDOT and P3T by in situ spectroelectrochemistry and Raman spectroscopy prove the successful copolymer formation and the access to tailored redox properties and energy levels. The use of EDOT-N3 as co-monomer furthermore allows modifications of the films by polymer analogous reactions. Here, we exemplarily describe the post-functionalization with ionic moieties by 1,3-dipolar cycloaddition (“click”-chemistry) which allows to tune the surface polarity of the copolymer films from water contact angles of 140° down to 40°.

A series of four 1 : 1 host–guest supramolecular complexes of [n]CPPs and C70 have been analyzed by Raman spectroscopy in the solid state and complemented with the analysis of their spectroscopic responses under mechanical and thermal stresses. By following the frequency behaviour of the G and RBM modes we have found that [10]CPP in the [10]CPP@C70 complex displays a more “ordered” structure. However, in [11]CPP@C70, the nanoring becomes oval-shaped with closer contacts with the C70 poles and less conformational restriction in the flattened region. By mechanical and thermal stresses we are able to modify the lying conformation of [10]CPP@C70 towards a standing orientation. [11]CPP@C70 resists pressure changes, although it tends to shift from the standing to the lying orientation by heating. As for the crystal cell, the [n]CPPs occupy the residual empty spaces while the main crystallographic positions are reserved for C70. These are new examples of the impressive adaptability of the [n]CPP molecules to different physico-chemical environments, a chameleon-like property which reveals the delicate equilibrium provided by cyclic conjugation and ring strain.

A series of alpha,omega-bis(mesitylthio)oligothiophenes of various chain lengths and with different side substitution patterns have been studied in their oxidized states by means of electron absorption and Raman spectroscopies in combination with theory in the framework of the density functional theory. Upon chemical oxidation, stable radical cations, dications, and even radical trications are generated. Longer chain lengths better stabilize higher oxidation states. The tetramer can be easily converted to the dication, and a trication can be obtained for the ethylenedioxy derivative. The alpha,omega-sulfur atoms are actively involved in the formation of the charged species and exert a favorable tuning of their electronic structure. Raman spectra provide experimental evidence of the attainment of quinoidal structures within the conjugated path, initially heteroaromatic, with different extension as a function of the p-doping level.

This paper reports on the analysis of the Raman spectra, supported by DFT quantum chemical calculations, of a series of terthiophenes which present the novelty of being selectively functionalized with sulfone groups to increase their photoluminescence properties. The spectra have been carefully assigned on the basis of the theoretical vibrational eigenvectors. They suggest the occurrence of a segmentation of the electronic structure which can become a crucial feature for optimizing linear and non linear optical properties. Sulfonation fixes the two lone electron pairs of the sulfur atom, in particular that involved in the aromatization of the thiophene ring, thus coexisting two different kind of five member (aromatic and non aromatic) units in the same chain. This duality of the structure gives rise to the splitting of the characteristic Raman lines associated to the C C/C–C stretching modes into components mainly located either on the external rings or on the internal one. The study also focuses on the electrosynthesized polymers obtained upon electropolymerization of the corresponding terthiophenes.

A series of branched poly(6-azulenylethenyl)benzene derivatives consisting of a central benzene ring surrounded by acetylene-bridged 1,3-bis(n-hexyloxycarbonyl) azulene arms has been investigated b...

In this paper, we study three symmetrical D-pi-D chromophores containing dithieno[3,2-b:2',3'-d]thiophene (DTT) as the pi-center and various donor end moieties, by means of UV-vis-NIR and FT-Raman spectroscopy and in situ spectroelectrochemistry. The compounds display dual redox properties: all exhibited two oxidations and single stable reduction peaks contrarily to the one or two oxidations and none reduction which could be anticipated in view of their chemical structures. We analyze the possible electronic modulation by the pi-conjugated DTT relay in the redox process and electronic coupling between the two electron donor (D) units attached through conjugation to opposite sides of the pi-linker. To this end, the UV-vis-NIR and FT-Raman spectra of the neutral compounds and of the charged species generated upon in situ p- or n-doping have been recorded and interpreted with the help of Density Functional Theory (DFT) calculations. The analysis of the peculiar Raman features of these pi-conjugated chromophores is guided by the formalism of the Effective Conjugation Coordinate (ECC) theory. This research shows that the Raman spectroscopic characterization of this type of D-pi-D structures is a powerful tool to derive information about their pi-conjugational properties in the pristine and doped states.

A series of radial thiophene-based structures consisting of a central benzene or thiophene ring surrounded by acetylene-bridged terthienyl arms has been investigated by physical and theoretical methods. Fourier transform Raman spectroscopy of the neutral solids shows that the terthiophene arms are weakly coupled across the core (benzene plus acetylene groups) likely due to cross-conjugation or meta-conjugation effects that may prevent full delocalization. By increasing the number of arms around the central ring, the electronic structure of the molecules seems to be affected only at the core, whereas the outer terthiophene arms remain almost unaltered. Raman spectroelectrochemistry and quantum chemical calculations provide further insight into the charge delocalization of the oxidized species. There is no evidence to suggest that these oxidized forms, obtained upon electrochemical doping of the molecules, show charge delocalization across the core.

Two isomeric dithienyl naphthodithiophenes and their corresponding polymers have been thoroughly studied by using a combination of spectroscopic, electrochemical, and quantum chemistry techniques. The change between the syn and anti configuration of the sulfur atoms of the core translates into important differences in their optical and electrochemical properties. This study reveals the existence in the syn isomer of a bonding interaction, while in the anti isomer there exists a more delocalized π-electron distribution. The full assignment of the Raman spectra of the monomers has allowed to interpret, assign, and correlate the spectra of the polymers. Upon polymerization the Raman data indicate an increment of the bond length relaxation pattern for the electronic structure of the polymer regarding the monomers. The two polymers are surprisingly strong amphoters, showing very stable oxidation and reduction waves at relatively low potentials at the same time that they behave as low band gap polymers. From a ...

In this paper we analyze, with the help of density-functional theory calculations, the relationship between the molecular structure and the optical and vibrational properties of two narrow band gap ﷿-conjugated cooligomers containing an alternating sequence of aromatic donor and o-quinoid acceptor units. The optimized molecular geometries of these co-oligomers reveal that short inter-ring S‚‚‚N contacts occur in their minimumenergy structure between the two types of constituiting units and that the resulting rigid coplanar arrangement of the rings enhances the degree of ﷿ conjugation and lowers the band gap.

Two types of push ± pull chro- mophores built around thiophene-based -conjugating spacers rigidified by ei- ther covalent bonds or noncovalent intramolecular interactions have been analysed by means of IR and Raman spectroscopical measurements in the solid state as well as in a variety of solvents. Comparison of the Raman features of NLO-phores based on a covalently rigidified dithienylene (DTE) spacer with those of their open chain DTE analogues shows that the bridging of the central double bond of DTE with the nearest -positions of the thienyl units through two ethylene bridges significantly improves the intra- molecular charge transfer. This also occurs for NLO-phores based on a 2,2- bi(3,4-ethylenedioxythiophene) (BED- OT) spacer as compared with their corresponding parent compounds based on an unsubstituted bithiophene (BT) spacer. For NLO-phores based on a BEDOT spacer, noncovalent intramo- lecular interactions between sulfur and oxygen atoms are responsible for the rigidification of the spacer. The Raman spectra of these NLO-phores obtained in the form of solutes in dilute solutions reveal two different behaviours: i) chro- mophores based on covalently bridged or open chain DTE spacers display Raman spectral profiles in solution quite similar to those of the corresponding solids, with a very little dependence on the polarity of the solvent, while ii) - larger spectral changes are noticed for NLO-phores built around BEDOT or BT spacers on going from solids to solutions. In the second case, spectral changes must be ascribed not solely to conformational distortions of the donor and acceptor end groups with respect to the -conjugated backbone mean- square-plane (as for the DTE-based NLO-phores) but also to distortions of the thienyl units of the -conjugating spacer from coplanarity. The insertion of vinylenic bridges between the thienyl units of the -conjugating spacer and between the spacer and the donor and acceptor end groups is a suitable strat- egy to reach a fairly large intramolecular charge transfer both in polar and non- polar solvents. Density functional theory (DFT) calculations have been carried out to assign the relevant electronic and vibrational features and to derive useful information about the molecular struc- ture of these NLO-phores.

The Raman spectrum of a quaterthiophene oligomer incorporating a central vinylene spacer has been investigated using density functional theory B3LYP/6-31G** calculations. The spectrum has been fully assigned with the aid of the calculations and in comparison with unsubstituted quaterthiophene. The spectrum preserves most of the vibrational features of linear oligothiophenes. The vibrations of the vinylene spacer are clearly differentiated from those of the rest of the chain. The vinylene spacer increases the conjugation length of the molecule and induces a frequency downshift of < 20 cm 21 for the normal modes associated with the nas(CyC) and ns(CyC) vibrations of the thiophene rings. The molecule is predicted to have a structure in which both the thiophene rings and the vinylene spacer are in an all-anti conformation. q 2003 Elsevier Science B.V. All rights reserved.

Surprisingly, thecarbon backbones of 3 and 5adeviate by 4.4(3)8 and 4.6(2)8from the linearity that would be expected based on their sp-hybridized central carbon atoms (Figure 2). Furthermore, thetwo terminal F-C-H planes in5a have a torsion angle of91.9(2)8. The previously reported structures derived frommicrowave spectroscopy do not show the same distortions(Supporting Information, Table S2), because erroneously, therefinement was based on a constrained linear geometry.

A analysis of the electronic and molecular structures of new molecular materials based on zethrene is presented with particular attention to those systems having a central benzo-quinoidal core able to generate Kekule biradicals whose stability is provided by the aromaticity recovery in this central unit. These Kekule biradicals display singlet ground electronic states thanks to double spin polarization and have low-energy lying triplet excited states also featured by the aromaticity gain. Pro-aromatization is also the driving force for the stabilization of the ionized species. Moreover, the low energy lying singlet excited states also display a profound biradical fingerprint allowing to singlet exciton fission. These properties are discussed in the context of the size of the zethrene core and of its substitution. The work encompasses all known long zethrenes and makes use of a variety of experimental techniques, such as Raman, UV-Vis-NIR absorption, transient absorption, in situ spectroelectrochemistry and quantum chemical calculations. This study reveals how the insertion of suitable molecular modules (i.e., quinoidal) opens the door to new intriguing molecular properties exploitable in organic electronics.

Chromophores bearing a 2-dicyanomethylenethieno[3,2-b]thiophene moiety in their quinoidal form have been synthesized, exploring for the first time the reactivity of this system towards aldehydes. Their ground state polarization and linear and second-order nonlinear optical (NLO) properties have been determined by a combined experimental and theoretical study, and compared to those of analogous compounds featuring an aromatic thienothiophene unit. Due to the gaining of aromaticity, quinoid systems have been found to display more polarized electronic ground states and higher NLO responses with respect to their aromatic counterparts.

By reformulating Raman and ROA invariants we provide ground for the definition of robust modes in ROA spectroscopy. Introduction of two parameters defining robustness helps characterization and assignment of ROA bands. Application and use of robustness parameters to [n]helicenes and oxirane/thiirane derivatives are presented.

The first synthesis of a conjugated hybrid of C60–fullerene and a single-wall carbon nanotube (SWCNT) is reported. The new nanohybrid material has been prepared by amidation of acid functionalized SWCNTs by an amine functionalized-fullerene derivative. The formation of this new nanomaterial was demonstrated by high resolution TEM, attenuated total reflectance/FTIR and Raman spectroscopy. Acid hydrolysis of the new material produced carboxylic-SWCNTs and the recovery of the aminofullerene derivative as complementary evidence of their covalent link. This new SWCNT–fullerene material may be of interest in optoelectronic applications.

In 1990 the article on the first Raman spectrum of a protein using a circularly polarized light as excitation radiation, authored by the group of Prof. L. Barron, was published in ChemComm. In this viewpoint we analyze this pioneering work and we used it as a starting point to highlight, after 20 years since its publication, the most outstanding contributions in the application of the Raman Optical Activity to proteins and biological systems.

Efficient charge transport is key to the operation of the various devices based on organic semiconductors, such as OLEDs, OPVs and OFETs. Both academia and industry are investing heavily in the development of new organic materials and processing techniques to improve device performance. An important parameter to tweak is the mobility of charge carriers. Triggered by an experimental result, here we investigate whether this parameter can be probed indirectly using UV-VIS spectroscopy. This would simplify the process of characterization and optimization of the mobility in amorphous molecular films, for example.

A chiral bisurea-based superhydrogelator that is capable of forming supramolecular hydrogels at concentrations as low as 0.2 mM is reported. This soft material has been characterized by thermal studies, rheology, X-ray diffraction analysis, transmission electron microscopy (TEM), and by various spectroscopic techniques (electronic and vibrational circular dichroism and by FTIR and Raman spectroscopy). The expression of chirality on the molecular and supramolecular levels has been studied and a clear amplification of its chirality into the achiral analogue has been observed. Furthermore, thermal analysis showed that the hydrogelation of compound 1 has a high response to temperature, which corresponds to an enthalpy-driven self-assembly process. These particular thermal characteristics make these materials easy to handle for soft-application technologies.

The high responsiveness of π-conjugated materials to external stimuli, such as electrons and photons, accounts for both their utility in optoelectronic applications and their chemical instability. Extensive studies on heteroatom-stabilized π-conjugated systems notwithstanding, it is still difficult to combine high performance and stability. We report here that carbon-bridged oligo(p-phenylenevinylene)s (COPV-n) are not only more responsive to doping and photoexcitation but also more stable than the conventional p-phenylenevinylenes and poly(3-hexylthiophene), surviving photolysis very well in air, suggesting that they could serve as building blocks for optoelectronic applications. Activation of the ground state by installation of bond angle strain toward the doped or photoexcited state and the flat, rigid, and hindered structure endows COPVs with stimuli-responsiveness and stability without recourse to heteroatoms. For example, COPV-6 can be doped with an extremely small reorganization energy and form a bipolaron delocalized over the entire π-conjugated system. Applications to bulk and molecular optoelectronic devices are foreseen.

The main chemical, spectroscopic and material research done in tetracyano quinoidal oligothiophenes in the last 30 years has been reviewed. Their use as semiconducting substrates in organic electronic and their versatility to act as multifunctional materials have been highlighted. This tutorial review has been paralleled by the description of the main findings provided by Raman spectroscopy, in particular, associated with the discovery of the intrinsic diradical properties inherent to the pro-aromatic character of the quinoidal arrangement. It turns out that the fascinating properties of these materials are the manifestation of the diradical fingerprint behind a rather unconventional electronic structure.

Rational creation of polymeric semiconductors from novel building blocks is critical to polymer solar cell (PSC) development. We report a new series of bithiopheneimide-based donor-acceptor copolymers for bulk-heterojunction (BHJ) PSCs. The bithiopheneimide electron-deficiency compresses polymer bandgaps and lowers the HOMOs--essential to maximize power conversion efficiency (PCE). While the dithiophene bridge progression R(2)Si→R(2)Ge minimally impacts bandgaps, it substantially alters the HOMO energies. Furthermore, imide N-substituent variation has negligible impact on polymer opto-electrical properties, but greatly affects solubility and microstructure. Grazing incidence wide-angle X-ray scattering (GIWAXS) indicates that branched N-alkyl substituents increased polymer π-π spacings vs linear N-alkyl substituents, and the dithienosilole-based PBTISi series exhibits more ordered packing than the dithienogermole-based PBTIGe analogues. Further insights into structure-property-device performance correlations are provided by a thieno[3,4-c]pyrrole-4,6-dione (TPD)-dithienosilole copolymer PTPDSi. DFT computation and optical spectroscopy show that the TPD-based polymers achieve greater subunit-subunit coplanarity via intramolecular (thienyl)S···O(carbonyl) interactions, and GIWAXS indicates that PBTISi-C8 has lower lamellar ordering, but closer π-π spacing than does the TPD-based analogue. Inverted BHJ solar cells using bithiopheneimide-based polymer as donor and PC(71)BM as acceptor exhibit promising device performance with PCEs up to 6.41% and V(oc)0.80 V. In analogous cells, the TPD analogue exhibits 0.08 V higher V(oc) with an enhanced PCE of 6.83%, mainly attributable to the lower-lying HOMO induced by the higher imide group density. These results demonstrate the potential of BTI-based polymers for high-performance solar cells, and provide generalizable insights into structure-property relationships in TPD, BTI, and related polymer semiconductors.

A Raman spectroscopic analysis revealed that π-conjugation does not reach saturation at least up to the octamer in long α-oligofurans and spreads over 14–15 furan units in the polyfuran. Comparing DFT calculations with experimental results suggests that a considerable amount of HF exchange is required to reproduce computationally the observed conjugation.

Polycyclic aromatic hydrocarbons with an open-shell singlet biradical ground state are of fundamental interest and have potential applications in materials science. However, the inherent high reactivity makes their synthesis and characterization very challenging. In this work, a convenient synthetic route was developed to synthesize two kinetically blocked heptazethrene (HZ-TIPS) and octazethrene (OZ-TIPS) compounds with good stability. Their ground-state electronic structures were systematically investigated by a combination of different experimental methods, including steady-state and transient absorption spectroscopy, variable temperature NMR, electron spin resonance (ESR), superconducting quantum interfering device (SQUID), FT Raman, and X-ray crystallographic analysis, assisted by unrestricted symmetry-broken density functional theory (DFT) calculations. All these demonstrated that the heptazethrene derivative HZ-TIPS has a closed-shell ground state while its octazethrene analogue OZ-TIPS with a smaller energy gap exists as an open-shell singlet biradical with a large measured biradical character (y = 0.56). Large two-photon absorption (TPA) cross sections (σ((2))) were determined for HZ-TIPS (σ((2))(max) = 920 GM at 1250 nm) and OZ-TIPS (σ((2))(max) = 1200 GM at 1250 nm). In addition, HZ-TIPS and OZ-TIPS show a closely stacked 1D polymer chain in single crystals.

We have investigated the impact of the functionalization and the chemical nature of counterions on the π-dimer dications formation in two end-capped heptathienoacenes. Radical cations of an α-substituted heptathienoacene with triisopropylsilyl groups do not π-dimerize, while those of an α,β-substituted heptathienoacene with four n-decyl side chains show a high propensity toward π-dimerization, increased by PF(6)(-) counterions.

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.

13 páginas, 13 figuras, 3 tablas, 2 esquemas.-- et al., In this paper we have prepared a new series of oligothiophenes capped with hexyl groups and a variety of strong acceptors, mainly cyanovinyl moieties. An exhaustive analysis of the absorption, photophysical, electrochemical, solid state, nonlinear optical and vibrational properties has been presented guided by theoretical calculations. The investigation is centered on the efficiency of the intramolecular charge transfer (i.e., chain length and acceptor dependence) and its impact on all the relevant electronic, structural, optical, and vibrational properties. The most significant features imparted by the acceptors through the π–conjugated oligothiophene path are (i) intense visible electronic absorptions, (ii) tuned fluorescence wavelength emissions, (iii) solid state π–stacking, (iv) ambipolar redox behavior, (v) S1 -- S0 internal conversion as being the major route for the deactivation of the excited state, and (vi) large electronic and vibrational contributions to their nonlinear optical response (hyperpolarizability). The analysis establishes connections between the different properties of the materials and structure–function relationships useful in organic electronics., We are indebted to the Ministerio de Ciencia e Innovaci on through the project references CTQ2009-10098, CTQ2008- 02942, and MAT2008-06522-C02-02. S.R.G. and R.A. acknowledge the Junta de Andalucía and the BES 2006-12104 grant, respectively. The Gobierno de Arag on through the project reference E39 and the Junta de Andalucía by means of the research project PO9-4708 are also acknowledged.

The main research on the family of aromatic and quinoidal oligothiophenes is described, with a focus on their longest members. We have described how the comprehensive understanding of the vibrational Raman spectra of oligothiophenes in a variety of situations permitted the elucidation of properties of fundamental importance, not only for basic research in molecular physics and in physical chemistry, but also for applications. We have written the Raman story that brought us to the elucidation of the biradicaloid nature of the longest quinoidal oligothiophene. DOI 10.1002/tcr.201000022

We present the determination of the conformational properties of aeroplysinin-1 in aqueous solution by means of a combined experimental and theoretical Raman optical activity (ROA) and vibrational circular dichroism (VCD) study. Aeroplysinin-1 is an antiangiogenic drug extracted from the sponge Aplysina cavernicola which has been proved to be a valuable candidate for the treatment of cancer and other antiangiogenic diseases. Our study shows that this molecule possesses the 1S,6R absolute configuration in aqueous solution, where only two conformers are present to a significant level. We discuss in detail the relationships between the chiro-optical ROA and VCD features, and the structural properties of various energy accessible conformers are described. The present work is one of the first studies in which both ROA and VCD have been used as complementary tools for the determination of absolute configuration and dominant solution-state conformations of an unknown therapeutically significant molecule.

The Raman spectra of a series of fluorine (F)/dibenzothiophene-S,S-dioxide co-oligomers (S) of different length and alternation sequences in their backbones (FSF, FFSFF, FSFSF, and FASAF; A is the –C≡C– bridge) have been recorded and simulated theoretically. It is shown that Raman spectroscopy is useful to probe π conjugation and ground state electron polarization in these molecules, phenomena directly related with the existence of intramolecular charge-transfer processes owing to the combination of electron donor (fluorene) and acceptor (dibenzothiophene-S,S-dioxide) groups. Their geometric, electronic, and vibrational properties have been studied by density functional theory B3LYP/6-311G(2d,p) quantum chemical calculations and compared with those for fluorene homo-oligomers. Comparative studies in solution/solid/melted phase led to the conclusion that the Raman wavenumbers are also sensitive to intermolecular interactions.

Oligothiophene-substituted 1,1,4,4-tetracyanobutadienes (TCBDs) have been synthesized by [2þ 2] cycloaddition reactions between tetracyanoethylene and oligothiophene alkynes. The TCBD moiety is compared to other electron acceptors attached to dibutylterthiophene including dicyanovinyl (DCV) and tricyanovinyl (TCV).These donor-acceptor molecules (TCBD-3T, DCV-3T, andTCV-3T) show red-shifted absorption spectra relative to the unsubstituted oligothiophene as a result of intramolecular charge-transfer (ICT). Monosubstituted terthiophenes bearing the electron acceptors show both oxidation and reduction processes as characterized by cyclic voltammetry. Density functional theory (DFT) calculations are used to explain the electronic and redox properties of the materials. Electrochemical oxidation of a bis(terthienyl)-substitutedTCBDmolecule (3T-TCBD-3T) yields a conducting polymer exhibiting balanced ambipolar redox conduction with similar values for the oxidized and reduced states of the polymer (1 _ 10-3 S cm-1). Raman spectra of the asymmetric donor-acceptor materials are characterized by two intense bands characteristic of the aromatic and quinoidal regions in the conjugated À-system of the oligothiophene.

Push-pull compounds, in which a proaromatic electron donor is conjugated to a 2-dicyanomethylenethiazole acceptor, have been prepared, and their properties compared to those of model compounds featuring an aromatic donor. A combined experimental (X-ray diffraction, 1HNMR, IR, Raman, UV/Vis, nonlinear optical (NLO) measurements) and theoretical study reveals that structural and solvent effects determine the ground-state polarisation of these merocyanines: whereas 4H-pyran-4-ylidene- and 4-pyridylidene-containing compounds are zwitterionic and 1,3-dithiol-2-ylidene derivatives are close to the cyanine limit, anilino-derived merocyanines are essentially neutral. This very large range of intramolecular charge transfer (ICT) gives rise to efficient second-order NLO chromophores with μβ values ranging from strongly negative to strongly positive. In particular, pyranylidene derivatives are unusual in that they show an increase in the degree of ICT on lengthening the π-spacer, a feature that lies behind the very large negative μβ values they display. The linking of the formally quinoidal 2- dicyanomethylenethiazole moiety to proaromatic donors seems a promising approach towards the optimisation of zwitterionic NLO chromophores., Financial support from MICINN-FEDER (CTQ2008-02942, MAT2008-06522C02-02 and CTQ2009-10098), Gobierno de Aragón- Fondo Social Europeo (E39) and Junta de Andalucía (Proyecto de Excelencia P09-4708) is gratefully acknowledged. The predoctoral fellowships for E. Galán (CSIC, JAE 2008) and R. Alicante (FPI BES2006-12104) are also acknowledged.

We have prepared a new series of mixed thiophene-pyrrole oligomers to investigate the electronic benefits arising from the combination of these two heterocycles. The oligomers are functionalized with several hexyl and aryl groups to improve both processability and chemical robustness. An analysis of their spectroscopic (absorption and emission), photophysical, electrochemical, solid state, and vibrational properties is performed in combination with quantum-chemical calculations. This analysis provides relevant information regarding the use of these materials as organic semiconductors. The balance between the high aromatic character of pyrrole and the moderate aromaticity of thiophene allows us to address the impact of the coupling of these heterocycles in conjugated systems. The data are interpreted on the basis of the aromaticity, molecular conformations, ground and excited electronic state structures, frontier orbital topologies and energies, oxidative states, and quinoidal versus aromatic competition.

A comprehensive photophysical and spectroscopic (electronic and Raman) study guided by density functional theory, DFT, CIS, and correlated ab initio calculations has been performed on a series of fully oxidized oligothiophenes with variable chain length, and up to four rings. A comparison with the properties of oligoenes and oligothiophenes is proposed. Absorption, fluorescence, lifetimes, flash-photolysis, phosphorescence, two-photon absorption, Raman, resonance Raman, and thermospectroscopy data are collected and interpreted according to the obtained theoretical results. The interest is focused on the ground electronic state and in the low-lying excited electronic states. Full oxygenation of the sulfur atoms of oligothiophenes results in: (i) restricted inter-ring isomerization such as observed from the absorption spectra; (ii) an effective quenching of fluorescence, and (iii) the appearance of dual emission. The emission features are explained by the interference of a dipole-allowed and a dipole-forbidden singlet excited states leading to simultaneous lighting from a local Frenkel and an intramolecular charge transfer photon -releasing configurations. These two excited states contribute to the broadening of the light emission spectrum. These properties highlight the similarity of these samples to that of oligoenes with comparable number of π-electrons.

This work presents an analy- sis of the structural, electrochemical, and optical properties of a family of triisopropylsilyl end-capped oligothie- noacenes (TIPS-Tn-TIPS, n = 4-8) by combining cyclic voltammetry, spectro- scopic techniques, and quantum-chemi- cal calculations. TIPS-Tn-TIPS com- pounds form stable radical cations, and dications are only obtained for the lon- gest oligomers (n = 7 and 8). Oxidation leads to the quinoidization of the con- jugated backbone, from which elec- trons are mainly extracted. The absorp- tion and fluorescence spectra show par- tially 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 � 1525 and � 480 cm � 1 . Optical absorption bands display remarkable bathochromic dis- persion with the oligomer length, indi- cative of the extent of p conjugation. The optical properties of the oxidized compounds are characterized by in situ UV/Vis/NIR spectroelectrochemistry. The radical cation species show two in- tense absorption bands emerging at en- ergies lower than in the neutral com- pounds. 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 per- formed at the B3LYP/6-31G** level, both for the neutral and the oxidized compounds.

Calculations of Fluorene-Dibenzothiophene-S,Sdioxide Oligomers Jose L. Zafra, Irene I. Perepichka, Igor F. Perepichka, Martin R. Bryce, Francisco J. Ramirez, Juan Casado, Juan T. Lopez Navarrete a Departamento de Quimica Fisica, Universidad de Malaga, Malaga 29071, Spain b Departement de Chimie, Universite de Montreal, Montreal, Quebec H3C 3J7, Canada c Centre for Materials Science, University of Central Lancashire, Preston PR1 2HE, United Kingdom d Department of Chemistry, Durham University, Durham DH1 3LE, United Kingdom

We have synthesized three new molecules that have three thienylethynyl arms substituting a central benzene core and different electron donor/acceptor groups in the three remaining phenyl positions. The absorption, fluorescence, phosphorescence, and transient triplet–triplet spectra are analyzed in the light of the electronic structure of the ground and excited states obtained from quantum-chemical calculations. From the above, the relevant photophysical data (including quantum yields, lifetimes, and rate constants) could be derived. It was found that the major deactivation pathway is internal conversion, which competes with the fluorescence and intersystem crossing processes. For the three investigated compounds, we provide convincing theoretical support corroborating these findings and further conclusions based on the theoretical information obtained. These molecules are one of the very few cases in which the depolarization ratios, obtained from the NLO optical measurements, clearly reflect the octopolar configuration. Molecular hyperpolarizabilities have been measured and display a typical dependence on the donor–acceptor substitution pattern.

The electronic and molecular structures of a family of oligothienylenevinylenes for organic solar cells are studied by means of UV/Vis, fluorescence and Raman spectroscopy, aided by quantum chemical calculations. By using different anchoring groups, the alteration of the electronic properties upon inserting electron-withdrawing groups into different positions on the oligothienylenevinylene backbone is determined. In addition, a thorough study of the photophysical properties is carried out to understand their potential use in optoelectronic devices. The charge defect of one of these systems is analysed to elucidate the possible charge carriers photogenerated during device operation.

A comprehensive photophysical and spectroscopic Raman study guided by Density Functional Theory and Hartree-Fock calculations has been done in a series of fully oxidized oligothiophenes with variable chain length, up to four rings. A comparison with the properties of oligoenes and oligothiophenes is proposed.

We present a new terthiophene derivative substituted at the thienyl beta positions with a naphthalenediimide functionalization. The UV-vis absorption and emission spectroscopic properties as well as the electrochemical properties have been discussed to describe its electronic structure. The vibrational Raman data are used to inspect the molecular architecture. All of the experimental data are supported by quantum chemical calculations with different approaches. A close comparison with other terthiophenes available in the literature is conducted, always stressing the effect of the relative orientation of the donor and acceptor groups either with long-axis or short-axis polarizations. The electronic structure of the molecule has been understood in terms of the HOMO-LUMO absolute energy values. A considerable reduction of the band gap from the constituting units to the studied molecule is detected, although electronic interaction is not optimal for this configuration. Fluorescence quenching is interpreted by the possibility of intersystem crossing and internal conversion. Finally, Raman spectroscopy gives additional information of the distribution of conjugation along the molecular domain, of subtle conformational effects, and of the intermolecular interaction by means of temperature-dependent Raman data. This study provides guidelines for controlling the electronic structure and for exploring new strategies pursuing improved dyes.

6 pages, 6 tables, 4 figures., In this article, we present a computational study of the nonlinear optical properties of pyridine-based octupolar molecules in their neutral and fully triprotonated states. The effect of the core substitution and the degree of conjugation with the periphery has been also established and confirms the possibility of fine-tuning the nonlinear optical response. Computations involving the time-dependent density-functional theory approach serve to further explore the existence of excited states with nonzero dipole moment. From these results, the origin of the high second-order nonlinear optical activity upon protonation is addressed., The present work was supported in part by the Dirección General de Enseñanza Superior DGES, MEC, Spain through research Project Nos. CTQ2006-14987-C02-01 and CTQ2005-01368. The authors are also indebted to Junta de Andalucía and Gobierno de Aragón Spain for funding their research groups FQM-0159 and E39. M.C.R.D. is grateful to the MEC/Fulbright for her Postdoctoral Fellowship at the Georgia Institute of Techology. J.C. is grateful to the MEC of Spain for an I3 professorship position of Chemistry at the University of Málaga and M.M.O. acknowledges the MEC for a personal grant.

In this work, we present a comparative study of the second order nonlinear optical properties of a series of chromophores containing a fused terthiophene, namely dithienothiophene (DTT), as electron relay with either D-π-A or D-π-D substitution patterns. The effect of the acceptor/donor strength and the solvent polarity confirm the possibility of fine-tuning optical non-linearities in the asymmetric samples. The geometrical and electronic properties calculated in solution reveal that push-pull chromophores become highly polarized as the dielectric constant of the solvent increases. Theoretical NLO calculations furthermore reveal a moderate nonlinear optical activity for the symmetric samples.

An analysis is performed on the molecular and electronic features in a series of trigonal molecules constituted by a central truxene core which is ramified with three oligofluorene moieties of different lengths. Arms and core are studied independently and upon threefold unification. Special emphasis is paid to the modulation of the conjugational properties in relation to substitution, molecular dimension, ring aromaticity, intermolecular forces, oxidation state, etc. Raman and optical absorption/emission spectroscopies in conjunction with computational theoretical results are combined for this purpose. The evolution of some key intensity ratios in the Raman spectra (i.e., I(1300)/I(1235)) is followed as an indication of electronic interaction between the core and the branches. The changes of the electronic delocalization upon solvation, with varying temperature in the solid state, with the nature of the aromatic unit (bithiophene/fluorene) or after electrochemical oxidation are interpreted. The modulation of the optical properties on the basis of the structure and energetics of the orbital around the gap is also addressed. Density functional theory was used to assign the vibrational and electronic spectra.

The synthesis, structure, and electronic properties of a novel cross-conjugated 10H-bisthienodithiocin-10-dicyanoethylene are reported. The X-ray single-crystal structure of the compound reveals a nonplanar conformation. The FT-IR and FT-Raman spectra of the compound show a great resemblance, which is a spectroscopic observation common to many push-pull systems. The UV-vis spectrum in CHCl3 displays a strong absorption at 370 nm accompanied by a shoulder at 430 nm so that the optical gap is 2.88 eV. On the other hand, the electrochemical gap amounts to 2.38 V. DFT and TDDFT quantum chemical calculations, at the B3LYP/6-31G** level, have been also performed to (i) determine the minimum-energy molecular structure, (ii) gain knowledge about the equilibrium atomic charges distribution, the topologies, and absolute energies of the frontier molecular orbitals around the gap and about the molecular vibrations which give rise to the most outstanding Raman bands experimentally evidenced, and (iii) to analyze the nature of the vertical one-electron excitations associated to the strongest UV-vis absorptions.