Combined Theoretical and Vibrational Study of Dihexylbithienoquinonoid Derivatives with Regioregular Head-to-Head, Head-to-Tail, and Tail-to-Tail Orientations

Benzo-thia-fused [n]thienoacenequinodimethanes with small to moderate diradical characters: the role of pro-aromaticity versus anti-aromaticity

Open-shell singlet diradicaloids have recently received much attention due to their unique optical, electronic and magnetic properties and promising applications in materials science. Among various diradicaloids, quinoidal π-conjugated molecules have become the prevailing design. However, the need for a fundamental understanding on how the fusion mode and pro-aromaticity/anti-aromaticity affect their diradical character and physical properties remains unaddressed. In this work, a series of pro-aromatic benzo-thia-fused [n]thienoacenequinodimethanes (Thn-TIPS (n = 1-3) and BDTh-TIPS) were synthesized and compared with the previously reported anti-aromatic bisindeno-[n]thienoacenes (Sn-TIPS, n = 1-4). The ground-state geometric and electronic structures of these new quinoidal molecules were systematically investigated by X-ray crystallographic analysis, variable temperature NMR, ESR, SQUID, Raman, and electronic absorption spectroscopy, assisted by DFT calculations. It was found that the diradical character index (y0) increased from nearly zero for Th1-TIPS to 2.4% for Th2-TIPS, 18.2% for Th3-TIPS, and 38.2% for BDTh-TIPS, due to the enhanced aromatic stabilization. Consequently, with the extension of molecular size, the one-photon absorption spectra are gradually red-shifted, the two-photon absorption (TPA) cross section values increase, and the singlet excited state lifetimes decrease. By comparison with the corresponding anti-aromatic analogues Sn-TIPS (n = 1-3), the pro-aromatic Thn-TIPS (n = 1-3) exhibit larger diradical character, longer singlet excited state lifetime and larger TPA cross section value. At the same time, they display distinctively different electronic absorption spectra and improved electrochemical amphotericity. Spectroelectrochemical studies revealed a good linear relationship between the optical energy gaps and the molecular length in the neutral, radical cationic and dicationic forms. Our research work discloses a significant difference between the pro-aromatic and anti-aromatic quinoidal compounds and provides guidance for the design of new diradicaloids with desirable properties.

Phenalenyl-fused porphyrins with different ground states

Materials based on biradicals/biradicaloids have potential applications for organic electronics, photonics and spintronics. In this work, we demonstrated that hybridization of porphyrin and polycyclic aromatic hydrocarbon could lead to a new type of stable biradicals/biradicaloids with tunable ground state and physical property. Mono- and bis-phenalenyl fused porphyrins 1 and 2 were synthesized via an intramolecular Friedel-Crafts alkylation-followed-by oxidative dehydrogenation strategy. Our detailed experimental and theoretical studies revealed that 1 has a closed-shell structure with a small biradical character (y = 0.06 by DFT calculation) in the ground state, while 2 exists as a persistent triplet biradical at room temperature under inert atmosphere. Compound 1 underwent hydrogen abstraction from solvent during the crystal growing process while compound 2 was easily oxidized in air to give two dioxo-porphyrin isomers 11a/11b, which can be correlated to their unique biradical character and spin distribution. The physical properties of 1 and 2, their dihydro/tetrahydro-precursors 7/10, and the dioxo-compounds 11a/11b were investigated and compared.

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

Antiaromatic bisindeno-[n]thienoacenes with small biradical character were synthesized and showed chain length dependent ground states and physical properties.

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

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

Push-pull bithienyl chromophore with an unusual transverse path of conjugation

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.

Synthesis and Characterization of a Novel Terthiophene-Based Quinodimethane Bearing a 3,4-Ethylenedioxythiophene Central Unit

The synthesis and a combined spectroscopic and density functional theoretical characterization of a 3',4'-ethylenedioxy-5,5' '-bis(dicyanomethylene)-5,5' '-dihydro-2,2':5',2' '-terthiophene analogue of 7,7,8,8-tetracyanoquinodimethane (TCNQ) are presented. Electrochemical data show that this novel trimer can be both reversibly reduced and oxidized at relatively low potentials. Quantum-chemical calculations show that the compound exhibits a quinoidal structure in its ground electronic state and that a certain degree of intramolecular charge transfer takes place from the central terthienyl moiety toward both =C(CN)2 end-caps. Therefore, the amphoteric redox behavior of this novel material can be related to the coexistence of an electron-impoverished terthienyl core endowed by two electron-enriched =C(CN)2 substituents. The UV-vis spectrum is dominated by the appearance of a strong absorption near 660 nm, attributable to the highest occupied molecular orbital (HOMO) --> lowest unoccupied molecular orbital (LUMO) pi-pi electronic transition of the terthienyl spine on the basis of time-dependent density functional theory (DFT) computations. The DFT calculations performed on the minimum-energy molecular geometry about the equilibrium atomic charge distribution, topologies, and energies of the frontier orbitals around the gap and about the Raman-active vibrations associated with the strongest Raman features are also consistent with a rather effective pi-electron conjugation and the partial degree of intramolecular charge transfer mentioned above. Our study reveals this novel heteroquinoid trimer could act as a promising candidate in organic field-effect transistor (OFET) applications.

Multidisciplinary Physicochemical Analysis of Oligothiophenes End-Capped by Nitriles: Electrochemistry, UV−Vis−Near-IR, IR, and Raman Spectroscopies and Quantum Chemistry

In this article, we investigate a series of alpha,omega-dicyano end-capped oligothiophenes NC(C(4)H(2)S)(n)()CN ranging in length from the dimer to the hexamer (n = 2-6), in the neutral state as pure solids, by means of Fourier transform IR and Fourier transform Raman (FT-Raman) spectroscopies. The cyclic voltammetry analysis of the compounds in dichloromethane reveals that most of them show two oxidation and two reduction waves (i.e., a dual or amphoteric electrochemical behavior), associated with the injection of either positive or negative charges into the pi-conjugated system. The doped species are characterized by in situ vis-near-IR and FT-Raman spectrochemistries. Density functional theory calculations have been also performed, at the B3LYP/6-31G level, to assess information about the molecular geometries and vibrational features of the neutral and doped species and about the topologies of the molecular orbitals involved in the main electronic transitions that appear for the neutral forms in the visible spectral region and for the doped species in the near-IR region.

Intermolecular interactions in pi-stacked conjugated molecules. Synthesis, structure, and spectral characterization of alkyl bithiazole oligomers

Syntheses are reported of new 4,4'-dialkyl-2,2'-bithiazole oligomers that have alkenoxy side chains that are capable of easy conversion to oligomers with functionalized side chains, e.g., terminally substituted hydroxy chains. The crystal structures of two representative oligomers (4,4',4' ',4' "-tetra-(2-propenoxymethyl)-2,2',5',5' ',2' ',2' "-quaterthiazole (3P2) and 4,4',4' ',4' "-tetra-(3-hydroxypropyloxymethyl)-2,2',5',5' ',2' ',2' "-quaterthiazole (3H2)) were determined; 3P2 crystallizes in a pi-stacked motif with two molecules per unit cell, whereas 3H2 forms pi-stacks that are linked with hydrogen bonds to form infinite two-dimensional sheets with one molecule per unit cell. A comparison of the UV-vis spectra of the compounds in solution and in the solid state provides unequivocal evidence for the presence of a Davydov splitting, W(D) approximately 0.2 eV, in solid 3P2. The spectra are interpreted in the framework of molecular exciton theory to extract a value of the intermolecular transfer integral, J approximately 0.2 eV, for a total exciton bandwidth of ca. 0.8 eV. Monte Carlo calculations were used to determine the density of states of the exciton band and the absorption and emission line shapes of the 0 <-- 0 transition. It is suggested that the "three-humped" absorption profile typical of partially crystalline, regioregular polymers is the "optical signature" of pi-stacking.

Quinonoid oligothiophenes as electron-donor and electron-acceptor materials. A spectroelectrochemical and theoretical study

Two quinonoid bis(dicyanomethylene) oligothiophenes, terthiophene and quaterthiophene analogues of TCNQ, have been investigated by spectroelectrochemical experiments and density functional theory calculations. Electrochemical data show that the molecules can be both reduced and oxidized at relatively low potentials, and that the quaterthiophene derivative forms four stable redox species, the dianion, neutral, cation radical, and dication. The neutral oligomers are characterized by a strong electronic absorption in the red or near-infrared region and can be viewed as structural and electronic analogues of aromatic oligothiophenes in the dication or bipolaron state. Upon reduction, dianions, not anion radicals, are formed which absorb in the visible region. The theoretical calculations show that the dianions have aromatic oligothiophene moieties with two anionic dicyanomethylene groups. The transition from a quinonoid to an aromatic structure is fully supported by UV-vis-near-IR and vibrational spectroscopic data. Oxidation, generating cation radicals and dications, occurs at rather low potentials similar to those reported for oligothiophenes. The electronic spectra of these cations are understood from the calculations, which suggest that the oxidized species are stabilized by the partial aromatization of the oligothiophene backbone. IR spectra of the species, especially the CN stretching frequencies, confirm the structural conclusions and allow comparison with TCNQ and the TCNQ dianion.

Synthesis and characterization of regiorandom and regioregular poly(3-octylfuran)

Poly(3-octylfuran) has been synthesized with three regioregularities: P3OF-95, P3OF-75, and P3OF-50, where the number signifies the percentage HT content. The 95% HT material is highly crystalline with a structure similar to that of HT-poly(3-octylthiophene), P3OT. The lamellar spacing is 22.1 A and the pi-stacking distance is 3.81 A. UV-vis spectroscopy reveals that P3OF-95 is aggregated in CHCl(3) solution, and solid films of P3OF-95, but not P3OF-75 or -50, show Davydov and exciton band splitting due to the interactions of the pi-systems in the stacked morphology. An estimate of the Davydov splitting is 0.15 eV (1200 cm(-1)). P3OF is reversibly oxidized at 0.32 V vs ferrocene/ferrocenium, but increasing the potential to 1.15 V leads to irreversible oxidation. Films of P3OF may be p-doped with iodine vapor. Doped P3OF-95 and -75 films have electrical conductivities of 10(-2) and 10(-7) S/cm, respectively. The UV-vis-NIR spectra of the iodine-doped films are interpreted in terms of molecular-like transitions involving the LUMO, HOMO, HOMO-1, and transitions across a Peierls distortion-induced gap in the intermolecular conduction band that is formed by the overlap of the pi-systems of the stacked partially oxidized chains. The conduction band gap estimated for P3OF-95 is 0.34 eV, and that for P3OF-75 is 0.9 eV. The P3OF samples are thermally stable in N(2) atmosphere to between 275 degrees C (P3OF-50) and 380 degrees C (P3OF-95), but suffer thermal oxidation above 150 degrees C or light-induced oxidation at room temperature.