Paula Mayorga Burrezo

Push-Pull Type Oligo(N-annulated perylene)quinodimethanes: Chain Length and Solvent-Dependent Ground States and Physical Properties.

Research on stable open-shell singlet diradicaloids recently became a hot topic because of their unique optical, electronic, and magnetic properties and promising applications in materials science. So far, most reported singlet diradicaloid molecules have a symmetric structure, while asymmetric diradicaloids with an additional contribution of a dipolar zwitterionic form to the ground state were rarely studied. In this Article, a series of new push-pull type oligo(N-annulated perylene)quinodimethanes were synthesized. Their chain length and solvent-dependent ground states and physical properties were systematically investigated by various experimental methods such as steady-state and transient absorption, two-photon absorption, X-ray crystallographic analysis, electron spin resonance, superconducting quantum interference device, Raman spectroscopy, and electrochemistry. It was found that with extension of the chain length, the diradical character increases while the contribution of the zwitterionic form to the ground state becomes smaller. Because of the intramolecular charge transfer character, the physical properties of this push-pull system showed solvent dependence. In addition, density functional theory calculations on the diradical character and Hirshfeld charge were conducted to understand the chain length and solvent dependence of both symmetric and asymmetric systems. Our studies provided a comprehensive understanding on the fundamental structure- and environment-property relationships in the new asymmetric diradicaloid systems.

Antiaromatic bisindeno-[n]thienoacenes with small singlet biradical characters: syntheses, structures and chain length dependent physical properties

Recent studies demonstrated that aromaticity and biradical character play important roles in determining the ground-state structures and physical properties of quinoidal polycyclic hydrocarbons and oligothiophenes, a kind of molecular materials showing promising applications for organic electronics, photonics and spintronics. In this work, we designed and synthesized a new type of hybrid system, the so-called bisindeno-[n]thienoacenes (n = 1–4), by annulation of quinoidal fused α-oligothiophenes with two indene units. The obtained molecules can be regarded as antiaromatic systems containing 4n π electrons with small singlet biradical character (y0). Their ground-state geometry and electronic structures were studied by X-ray crystallographic analysis, NMR, ESR and Raman spectroscopy, assisted by density functional theory calculations. With extension of the chain length, the molecules showed a gradual increase of the singlet biradical character accompanied by decreased antiaromaticity, finally leading to a highly reactive bisindeno[4]thienoacene (S4-TIPS) which has a singlet biradical ground state (y0 = 0.202). Their optical and electronic properties in the neutral and charged states were systematically investigated by one-photon absorption, two-photon absorption, transient absorption spectroscopy, cyclic voltammetry and spectroelectrochemistry, which could be correlated to the chain length dependent antiaromaticity and biradical character. Our detailed studies revealed a clear structure–aromaticity–biradical character–physical properties–reactivity relationship, which is of importance for tailored material design in the future.

Fully Fused Quinoidal/Aromatic Carbazole Macrocycles with Poly-radical Characters.

While the chemistry of open-shell singlet diradicaloids has been successfully developed in recent years, the synthesis of π-conjugated systems with poly-radical characters (i.e., beyond diradical) in the singlet ground state has been mostly unsuccessful. In this study, we report the synthesis and isolation of two fully fused macrocycles containing four (4MC) and six (6MC) alternatingly arranged quinoidal/aromatic carbazole units. Ab initio electronic structure calculations and various experimental measurements indicate that both 4MC and 6MC have an open-shell singlet ground state with moderate tetraradical and hexaradical characters, respectively. Both compounds can be thermally populated to high-spin excited states, resulting in weak magnetization at room temperature. Our study represents the first demonstration of singlet π-conjugated molecules with poly-radical characters and also gives some insights into molecular magnetism in neutral π-conjugated polycyclic heteroarenes.

Molecular and electronic-structure basis of the ambipolar behavior of naphthalimide-terthiophene derivatives: Implementation in organic field-effect transistors

A new family of naphthalimide-fused thienopyrazine derivatives for ambipolar charge transport in organic field-effect transistors is presented. Their electronic and molecular structures were elucidated through optical and vibrational spectroscopy aided by DFT calculations. The results indicate that these compounds have completely planar molecular skeletons which promote good film crystallinity and low reorganization energies for both electron and hole transport. Their performance in organic field-effect transistors is compared with twisted and planar naphthaleneamidine monoimide-fused terthiophenes in order to understand the origin of ambipolarity in this new series of molecular semiconductors.

Evidence for Multicenter Bonding in Dianionic Tetracyanoethylene Dimers by Raman Spectroscopy

Raman spectroscopic studies have the potential to pro-vide a deeper insight into the properties of long bonds byexploring the shape of the intradimer C···C stretching vibra-tionalmode.ThismodeisexpectedtobeRaman-activebasedon symmetry considerations, and its observation unequivo-cally characterizes the presence of the long, multicenter bondin p-[TCNE]

Planarization, Fusion, and Strain of Carbon-Bridged Phenylenevinylene Oligomers Enhance π-Electron and Charge Conjugation: A Dissectional Vibrational Raman Study

We have used Raman spectroscopy to study the molecular and electronic structures of the radical cations and dications of carbon-bridged oligo(para-phenylenevinylene)s (COPVn, n = 1-6) possessing consecutive fused pentagons and hexagons, up to 19, along with COPV derivatives having electron-donating and -withdrawing groups. This study was made possible by the outstanding stability of the charged states of COPVs. We could untangle the effects of π-conjugation in the planar structure on the Raman frequency by distinguishing it from other structural effects, such as strain in the vinylene groups shared by the two pentagons. The analyses showed that the radical cations have benzo-quinoidal structures confined in the center of the molecule, as well as benzo-aromatic rings at the terminal sites. In contrast, dications of COPVn longer than n = 3 exhibit a biradicaloid character because of the recovery of aromaticity in the central rings and quinoidal rings at the terminal positions. These biradicaloids favor a singlet nature in their ground electronic states because of the double spin polarization. The introduction of electron-donating and -withdrawing groups on the termini of a COPV core affords, upon oxidation or reduction, a fully delocalized class III mixed valence system because of the high degree of conjugation of the COPV platform, which favors extensive charge delocalization.

Polarization, second-order nonlinear optical properties and electrochromism in 4H-pyranylidene chromophores with a quinoid/aromatic thiophene ring bridge

Push-pull systems, in which the proaromatic 4H-pyranylidene electron donor is conjugated with a dicyanomethylene acceptor through a quinoid thiophene as (part of) the electron relay, have been prepared, and their properties have been compared to those of a parent compound featuring an aromatic thiophene moiety. Different experimental techniques (X-ray diffraction, 1H NMR, IR, Raman, UV-vis, cyclic voltammetry, spectroelectrochemistry, and NLO measurements) combined with theoretical calculations have been used for the study of the chromophores. Quinoidal derivatives, although neutral, show strongly polarized structures, with positive μβ values and a progressive increase of the intramolecular charge transfer (ICT) on lengthening the π-spacer. Comparison between compounds that only differ in the character (quinoid or aromatic) of the thiophene unit shows a more efficient ICT for the quinoid thiophene-containing chromophore, which influences the second-order NLO response. Furthermore, the thienyl ring has been also found to play a significant role in the ICT process for the analogous aromatic derivative.

Quinoidal/Aromatic Transformations in π-Conjugated Oligomers: Vibrational Raman studies on the Limits of Rupture for π-Bonds

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.

Bis(aminoaryl) Carbon-Bridged Oligo(phenylenevinylene)s Expand the Limits of Electronic Couplings

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.

Robust Ethylenedioxythiophene–Vinylene Oligomers from Fragile Thiophene–Vinylene Cores: Synthesis and Optical, Chemical and Electrochemical Properties of Multicharged Shapes

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.