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This work describes the synthesis and properties of a dicyanomethylene‐substituted indolo[3,2‐b]carbazole diradical ICz‐CN. This quinoidal system dimerises almost completely to (ICz‐CN)2, which contains two long C(sp3)−C(sp3) σ‐bonds between the dicyanomethylene units. The minor open‐shell ICz‐CN component in the solid‐state mixture was identified by EPR spectroscopy. Cyclic voltammetry and UV–visible spectroelectrochemical data, as well as comparison with reference monomer ICz‐Br reveal that the nature of the one‐electron oxidation of (ICz‐CN)2 at ambient temperature and ICz‐CN at elevated temperature is very similar in all these compounds due to the prevailing localization of their HOMO on the ICz backbone. The peculiar cathodic behaviour reflects the co‐existence of (ICz‐CN)2 and ICz‐CN. The involvement of the dicyanomethylene groups stabilizes the close‐lying LUMO and LUMO+1 of (ICz‐CN)2 and especially ICz‐CN compared to ICz‐Br, resulting in a distinctive cathodic response at low overpotentials. Differently from neutral ICz‐CN, its radical anion and dianion are remarkably stable under ambient conditions. The UV/Vis(–NIR) electronic transitions in parent (ICz‐CN)2 and ICz‐CN and their different redox forms have been assigned convincingly with the aid of TD‐DFT calculations. The σ‐bond in neutral (ICz‐CN)2 is cleaved in solution and in the solid‐state upon soft external stimuli (temperature, pressure), showing a strong chromism from light yellow to blue–green. Notably, in the solid state, the monomeric diradical species is predominantly formed under high hydrostatic pressure (>1 GPa). [ABSTRACT FROM AUTHOR]

The synthesis of a tetra‐functionalized pentaerythritol core decorated with N‐methyl‐N,N‐diphenylamine‐based push‐pull chromophores and its electropolymerization to 3D push‐pull networks are described. The electrochemical and absorption behaviors of the tetramer are compared with the one of two reference linear push‐pull compounds, carrying triphenylamine (TPA) or methyldiphenylamine (MeDPA) donor groups, a thienyl linker and a dicyanovinyl acceptor group (DCV). We found that substituting the outer phenyl with a methyl group causes important differences in the radical cation stability, such that MeDPA chromophore generates stable dimers and TPA is reversibly oxidized. Interestingly, DFT calculations suggest that steric hindrance and electrostatic interactions dominate the radical cation reactivity. [ABSTRACT FROM AUTHOR]

Abstract: A detailed analysis is undertaken of positively charged species generated on a series of thienylenevinylene (nTV) wires terminally substituted with two perchlorotriphenylmethyl (.PTM) radical acceptor groups, .PTM‐nTV‐PTM. (n=2–7). Motivated by the counterintuitive key role played by holes in the nTV bridges on the operating mechanism of electron transfer in their radical anion mixed‐valence derivatives, a wide combination of experimental and theoretical techniques is used, with the aim of gaining further insights into their structural location. Consequently, contributions of the .PTM units for the stabilization of the radical cations and hole localization, particularly in the case of the shortest molecular wire, are probed. In this sense, the formation of quinoidal ring segments, resulting from the coupling of the unpaired electron of the .PTM radical site with those generated along the nTV chains is found. Additionally, open‐shell dications, described by the recovery of the central aromaticity and two terminal quinoidal segments, assisted by the .PTM units, are detected. [ABSTRACT FROM AUTHOR]

Evidence for the surprising formation of polymeric phases under high pressure for conjugated nanohoop molecules was found. This paper represents one of the unique cases, in which the molecular-level effects of pressure in crystalline organic solids is addressed, and provides a general approach based on vibrational Raman spectroscopy combining experiments and computations. In particular, we studied the structural and supramolecular chemistry of the cyclic conjugated nanohoop molecule [5]cyclo- para-phenylene ([5]CPP) under high pressures up to 10 GPa experimentally and up to 20 GPa computationally. The theoretical modeling for periodic crystals predicts good agreements with the experimentally obtained Raman spectra in the molecular phase. In addition, we have discovered two stable polymeric phases that arise in the simulation. The critical pressures in the simulation are too high, but the formation of polymeric phases at high pressures provides a natural explanation for the observed irreversibility of the Raman spectra upon pressure release between 6 and 7 GPa. The geometric parameters show a deformation toward quinonoid structures at high pressures accompanied by other deformations of the [5]CPP nanohoops. The quinonoidization of the benzene rings is linked to the systematic change of the bond length alternation as a function of the pressure, providing a qualitative interpretation of the observed spectral shifts of the molecular phase. [ABSTRACT FROM AUTHOR]

This work reports on a quinodimethane-type molecule, 2,7-dicyanomethylene-9-(2-ethylhexyl)carbazole ( 1), one of the shortest π-conjugated biradicaloids reported to be stable in solution under ambient conditions. This carbazole-based quinoidal precursor is able to form a macrocyclic σ-bonded tetramer ( 2). The resolved single-crystal X-ray structure of tetramer 2 shows that four molecules of 1 are linked together through four long (CN)2C−C(CN)2 bonds (1.631 Å) resulting from coupling of the unpaired electrons in biradicaloid 1. Dynamic interconversion between monomer 1 and cyclophane tetramer 2 is achieved by reversible cleavage and recovery of the four (CN)2C−C(CN)2 bonds upon soft external stimuli (light absorption, temperature and pressure), which is accompanied by significant color changes. These novel photo-, thermo-, and mechanochromic properties expand the versatility of π-conjugated biradicaloid compounds as novel functional materials that, in combination with spin chemistry and dynamic covalent chemistry, can be relevant in molecular machines, sensors, and switches. [ABSTRACT FROM AUTHOR]

Carbon-bridged bis(aminoaryl) oligo( para-phenylenevinylene)s have been prepared and their optical, electrochemical, and structural properties analyzed. Their radical cations are class III and class II mixed-valence systems, depending on the molecular size, and they show electronic couplings which are among the largest for the self-exchange reaction of purely organic molecules. In their dication states, the antiferromagnetic coupling is progressively tuned with size from quinoidal closed-shell to open-shell biradicals. The data prove that the electronic coupling in the radical cations and the singlet-triplet gap in the dications show similar small attenuation factors, thus allowing charge/spin transfer over rather large distances. [ABSTRACT FROM AUTHOR]

The vibrational Raman spectra of several series of aromatic and quinoidal compounds have been analyzed considering the downshifts and upshifts of the frequencies of the relevant Raman bands as a function of the number of repeating units. Oligothiophenes, oligophenylene-vinylenes, and oligoperylenes (oligophenyls) derivatives are studied in a common context. These shifts are taken as spectroscopic fingerprints of the changes in π-conjugation. For a given family, aromatic and quinoidal oligomers have been studied together, and according to their Raman frequency shifts located in the two-well BLA-energy curve of their ground electronic state as a function of the bond-length-alternation pattern (BLA). The connection among BLA values, π-conjugation, and Raman frequencies is taken here as the basis of the study. These Raman shifts/BLA changes have been related to important electronic properties of these one-dimensional linear π-electron delocalized systems such as quinoidal (polyene) and aromatic characters. [ABSTRACT FROM AUTHOR]

The combination of a bis-alkynyl-helicene moiety with two iron centers leads to novel electroactive species displaying unprecedented redox-triggered chiroptical switching. Upon oxidation, strong changes of vibrational modes (either local or extended coupled modes) are detected by vibrational circular dichroism and Raman optical activity. Remarkably, the sign of the optical rotation at 1.54 mm (that is, at wavelengths typically used for telecommunications) changes upon oxidation while the topology and stereochemistry of the helicene remain unchanged. [ABSTRACT FROM AUTHOR]

The combination of a bis-alkynyl-helicene moiety with two iron centers leads to novel electroactive species displaying unprecedented redox-triggered chiroptical switching. Upon oxidation, strong changes of vibrational modes (either local or extended coupled modes) are detected by vibrational circular dichroism and Raman optical activity. Remarkably, the sign of the optical rotation at 1.54 μm (that is, at wavelengths typically used for telecommunications) changes upon oxidation while the topology and stereochemistry of the helicene remain unchanged. [ABSTRACT FROM AUTHOR]

A general description of the use of the Raman spectra of polyconjugated molecules to infer their electronic and molecular properties and how these change in a number of situations is illustrated with several examples, with particular emphasis on oligothiophene molecules. The most noticeable features of the Raman spectra of these molecules and the main theoretical models developed for their understanding have been revised and explained in terms related to π-electron delocalization intimately connected with the aromatic and quinoidal characters. [ABSTRACT FROM AUTHOR]

Herein we present novel Raman spectroscopic studies carried out for polyselenophene (PSe), poly(3,4-ethylenedioxyselenophene) (PEDOS), and alkyl-substituted poly(alkyl-3,4-ethylenedioxyselenophene) (PEDOS-Cn: n = 2, 8, and 12) homologues. These spectroscopic data are combined with morphological and structural analysis techniques, such as AFM, and DFT simulations. An investigation into the extent of p-conjugation in PSe is presented by comparing its spectral signatures with those of a homologous series of oligoselenophenes. Our experimental data suggest that p-conjugation in the PSe backbone extends to more than six selenophene units. The incorporation of ethylenedioxy (EDO) groups in PEDOS results in a more complex spectrum and a downshift of the C=C/C--C stretching vibration (or effective conjugation coordinate (ECC) mode); this provides evidence for an improvement in the π-conjugational properties along the PSe backbone upon EDO substitution. The dispersity of p-conjugation lengths in different PEDOS samples obtained through different synthetic routes is evaluated as a function of different structural factors. Finally, we present experimental evidence that the insertion of alkyl chains into the EDO units slightly impacts on the pconjugational properties of the PEDOS backbone in the non-annealed samples (with an improved p-electron delocalization found in PEDOS-C8), but moderately affects the degree of molecular order of polymer films upon thermal annealing. [ABSTRACT FROM AUTHOR]

Oligomers of ethylendioxythiophene-vinylene have been prepared. Their optical, electrochemical and chemical properties have been studied in detail by absorption and emission spectroscopy as well as cyclic voltammetry, Raman techniques and spectroelectrochemistry complemented with quantum chemical calculations. A comparison with their non-ethylendioxy and non-vinylene parents has been done. The inclusion of the EDO plus the vinylene function generates more robust electronic ground states regarding the largely flexible thiophene-vinylene ( n TV) oligomeric homologues. The redox features of the new compounds are also rich of oxidative processes arising as an interesting stabilising balance effect between the oxygen fragment in the EDO groups (mesomeric effect) and the linear π-conjugated structure. The oxidised species have been characterised, which show the ability for the formation of mixed valence charge-transfer complexes and π dimers of different oxidation states, in particular, in the electrochemical medium, resulting that the electrochemical response is accounted for a succession of aggregation and electron-transfer steps. With this work, a full understanding of the optical and electronic properties of these new oligomers in the context of the oligomer approach has been proposed. [ABSTRACT FROM AUTHOR]

Radical cations of a heptathienoacene α,β-substituted with four n-decyl side groups (D4T7 .+) form exceptionally stable π-dimer dications already at ambient temperature ( Chem. Comm. 2011, 47, 12622). This extraordinary π-dimerization process is investigated here with a focus on the ultimate [D4T7 .+]2 π-dimer dication and yet-unreported transitory species formed during and after the oxidation. To this end, we use a joint experimental and theoretical approach that combines cyclic voltammetry, in situ spectrochemistry and spectroelectrochemistry, EPR spectroscopy, and DFT calculations. The impact of temperature, thienoacene concentration, and the nature and concentration of counteranions on the π-dimerization process is also investigated in detail. Two different transitory species were detected in the course of the one-electron oxidation: 1) a different transient conformation of the ultimate [D4T7 .+]2 π-dimer dications, the stability of which is strongly affected by the applied experimental conditions, and 2) intermediate [D4T7]2 .+ π-dimer radical cations formed prior to the fully oxidized [D4T7]2 .+ π-dimer dications. Thus, this comprehensive work demonstrates the formation of peculiar supramolecular species of heptathienoacene radical cations, the stability, nature, and structure of which have been successfully analyzed. We therefore believe that this study leads to a deeper fundamental understanding of the mechanism of dimer formation between conjugated aromatic systems. [ABSTRACT FROM AUTHOR]

Herein we present novel Raman spectroscopic studies carried out for polyselenophene (PSe), poly(3,4-ethylenedioxyselenophene) (PEDOS), and alkyl-substituted poly(alkyl-3,4-ethylenedioxyselenophene) (PEDOS-Cn: n=2, 8, and 12) homologues. These spectroscopic data are combined with morphological and structural analysis techniques, such as AFM, and DFT simulations. An investigation into the extent of π-conjugation in PSe is presented by comparing its spectral signatures with those of a homologous series of oligoselenophenes. Our experimental data suggest that π-conjugation in the PSe backbone extends to more than six selenophene units. The incorporation of ethylenedioxy (EDO) groups in PEDOS results in a more complex spectrum and a downshift of the C[DOUBLE BOND]C/C[BOND]C stretching vibration (or effective conjugation coordinate (ECC) mode); this provides evidence for an improvement in the π-conjugational properties along the PSe backbone upon EDO substitution. The dispersity of π-conjugation lengths in different PEDOS samples obtained through different synthetic routes is evaluated as a function of different structural factors. Finally, we present experimental evidence that the insertion of alkyl chains into the EDO units slightly impacts on the π-conjugational properties of the PEDOS backbone in the non-annealed samples (with an improved π-electron delocalization found in PEDOS-C8), but moderately affects the degree of molecular order of polymer films upon thermal annealing. [ABSTRACT FROM AUTHOR]

The helical organization of oligo- p-phenylene-based organogelators has been investigated by atomic force microscopy, circular and vibrational circular dichroism, and Raman techniques. Whilst OPPs with more than two phenyl rings in the core self-assemble into left-handed helices, that with a biphenyl core shows an inversion of the supramolecular helicity depending on the formation conditions through the atropisomerism of the biphenyl central unit. The results presented herein outline a new example of kinetically controlled modulation of supramolecular helicity. [ABSTRACT FROM AUTHOR]

This article investigates the excited and charged states of three branched oligothiophenes with methyl-thienyl side groups as models to promote 3D arrangements. A comparison with the properties of the parent systems, linear all-α,α-oligothiophenes, is proposed. A wide variety of spectroscopic methods (i.e., absorption, emission, triplet-triplet transient absorption, and spectroelectrochemistry) in combination with DFT calculations have been used for this purpose. Whereas the absorption spectra are slightly blueshifted upon branching, both the emission spectra and triplet-triplet absorption spectra are moderately redshifted; this indicates a larger contribution of the β-linked thienyl groups in the delocalization of the S1 and T1 states rather than into the S0 state. The delocalization through the α,β-conjugated path was found to be crucial for the stabilization of the trication species in the larger branched systems, whereas the linear sexithiophene homologue can only be stabilized up to the dication species. [ABSTRACT FROM AUTHOR]

Biradicaloids based on quinoidal rylenes! Soluble and stable tetracyanoquaterrylenequinodimethane (QR‐CN) and tetracyanohexarylenequinodimethane (HR‐CN) were synthesized. QR‐CN has a closed‐shell quinoidal structure in the ground state, whereas HR‐CN has a singlet biradical ground state. Both compounds showed very strong one‐photon and two‐photon absorption in the NIR range. [ABSTRACT FROM AUTHOR]

Amplified spontaneous emission (ASE) is observed, under optical pump, in polystyrene films doped with two pentathienoacene derivatives functionalised with thienyl‐S,S‐dioxide groups (compounds 2 and 3). The effect of the dioxide groups on the ASE properties is analysed by comparing the performance with that of its corresponding non‐oxidized analogue (1). Films containing either 2 or 3 show ASE at 511 and 574 nm, respectively, when excited directly (at 435 nm) on their absorption bands, showing thresholds and linewidths larger than those obtained from films doped with 1, pumped at 355 nm. ASE is also observed under excitation at 355 nm, in samples containing 1 (host) and either 2 or 3 (guests), due to energy transfer from host to guest. For the blends with 3, the ASE threshold is lower than that obtained when the films are excited directly. Results are interpreted in terms of the photophysical parameters such as absorption capacity, fluorescence efficiency, singlet‐to‐triplet intersystem crossing leading to triplet‐triplet re‐absorptions, bimolecular energy‐transfer efficiency, efficiency of internal conversion process, etc. State‐of‐the‐art quantum chemical calculations are used in the interpretation of the experimental results. [ABSTRACT FROM AUTHOR]

The linear and non-linear optical properties of a family of dumbbell-shaped dinuclear complexes, in which an oligothiophene chain with various numbers of rings (1, 3, and 6) acts as a bridge between two homoleptic tris(2,2′-bipyridine)ruthenium(II) complexes, have been fully investigated by using a range of spectroscopic techniques (absorption and luminescence, transient absorption, Raman, and non-linear absorption), together with density functional theory calculations. Our results shed light on the impact of the synergistic collaboration between the electronic structures of the two chemical moieties on the optical properties of these materials. Experiments on the linear optical properties of these compounds indicated that the length of the oligothiophene bridge was critical for luminescent behavior. Indeed, no emission was detected for compounds with long oligothiophene bridges (compounds 3 and 4, with 3 and 6 thiophene rings, respectively), owing to the presence of the 3ππ* state of the conjugated bridge below the 3MLCT-emitting states of the end-capping RuII complexes. In contrast, the compound with the shortest bridge ( 2, one thiophene ring) shows excellent photophysical features. Non-linear optical experiments showed that the investigated compounds were strong non-linear absorbers in wide energy ranges. Indeed, their non-linear absorption was augmented upon increasing the length of the oligothiophene bridge. In particular, the compound with the longest oligothiophene bridge not only showed strong two-photon absorption (TPA) but also noteworthy three-photon-absorption behavior, with a cross-section value of 4×10−78 cm6 s2 at 1450 nm. This characteristic was complemented by the strong excited-state absorption (ESA) that was observed for compounds 3 and 4. As a matter of fact, the overlap between the non-linear absorption and ESA establishes compounds 3 and 4 as good candidates for optical-power-limiting applications. [ABSTRACT FROM AUTHOR]