Quinoidal Thiophene Research Articles

Isothianaphthene quinoids: pyrazine-annelated structures for tuning electronic properties

Abstract The development of quinoidal systems with extended π-conjugation has elucidated the influence that diradical characteristics exert on structure–property relationships, which is significant because it broadens the possibilities for the use of organic semiconducting materials in organic electronics. However, the chain-length elongation of such quinoidal molecules has resulted in low chemical stability due to the large contribution of diradical characteristics and to the high level of the highest occupied molecular orbital (HOMO), both of which limit the synthesis of π-extended quinoidal molecules. Here, we describe solving this problem via aromatic stabilization. To accomplish this, we designed a system that utilizes electron-accepting pyrazine-fused thieno[3,4-b]pyrazine following the development of the isothianaphthene quinoids of thiophene 3-mer and 6-mer. Theoretical calculations indicate that the introduction of a pyrazine-annelated structure suppresses the diradical characteristics and stabilizes the HOMO energy level of quinoidal oligothiophenes. The thermal, photophysical, and physicochemical properties of newly synthesized quinoidal molecules with full annelation of the benzene and pyrazine rings were investigated. Quinoidal thiophene 3-mer functioned as an acceptor in organic solar cells with a power conversion efficiency of 1.04%. This study demonstrates that the introduction of pyrazine-fused rings is an effective molecular design to extend the chain length of quinoidal oligothiophenes.

(Invited) Development of Benzene-Annelated Quinoidal Systems for Acceptor Applications

Quinoidal oligothiophenes have attracted great attention as promising electron-transporting semiconductors. However, the use of quinoidal molecules as nonfullerene acceptors (NFAs) in organic solar cells has not been reported so far. We hypothesized that control of diradical characteristics to fine-tune the electronic properties of quinoidal oligothiophenes is important for the application to NFAs. Since the representative approach to increase the contribution of the quinoid character is the utilization of aromatic stabilization in a quinoidal form, we designed and developed new quinoidal oligothiophenes that contain benzene annelation. As a result, we revealed that quinoidal terthiophenes having fully benzene annelation functioned as NFAs. Furthermore, the quinoidal thiophene 5-mer showed acceptor characteristics in NIR organic phototransistors. In this presentation, we also discuss the influence of benzene annelation on the diradical character and physical properties.

Quinoidal Small Molecule Containing Ring-Extended Termini for Organic Field-Effect Transistors.

In this work, we synthesized and characterized two quinoidal small molecules based on benzothiophene modified and original isatin terminal units, benzothiophene quinoidal thiophene (BzTQuT) and quinoidal thiophene (QuT), respectively, to investigate the effect of introducing a fused ring into the termini of quinoidal molecules. Extending the terminal unit of the quinoidal molecule affected the extension of π-electron delocalization and decreased the bond length alternation, which led to the downshifting of the collective Raman band and dramatically lowering the band gap. Organic field-effect transistor (OFET) devices in neat BzTQuT films showed p-type transport behavior with low hole mobility, which was ascribed to the unsuitable film morphology for charge transport. By blending with an amorphous insulating polymer, polystyrene, and poly(2-vinylnaphthalene), an OFET based on a BzTQuT film annealed at 150 °C exhibited improved mobility up to 0.09 cm2 V–1 s–1. This work successfully demonstrated that the extension of terminal groups into the quinoidal structure should be an effective strategy for constructing narrow band gap and high charge transporting organic semiconductors.

Donor-Acceptor Conjugated Macrocycles with Polyradical Character and Global Aromaticity.

SummaryPolyradical character and global aromaticity are fundamental concepts that govern the rational design of cyclic conjugated macromolecules for optoelectronic applications. Here, we report donor-acceptor (D−A) conjugated macromolecules with and without π-spacer derivatives to tune the antiferromagnetic couplings between the unpaired electrons. The macromolecules without π-spacer have a closed-shell electronic configuration and show global nonaromatic character in the singlet and lowest triplet states. However, the derivatives with π-spacer develop a nearly pure open-shell diradical and a very high polyradical character, not reported for D−A type macromolecules. Furthermore, the π-spacer derivatives display global nonaromaticity in the singlet ground state, but global aromaticity in the lowest triplet state, according to Baird's rule. The absorption spectra of the open-shell macromolecules calculated with time-dependent density functional theory indicate intensive light absorption in the near-infrared region and broadening to 2,500 nm, making these materials suitable for numerous optoelectronic applications.

Theoretical study on the charge transport properties of three series of dicyanomethylene quinoidal thiophene derivatives.

It is very important to analyse the most advantageous connection style for quinoidal thiophene derivatives, which are used in n-type organic semiconductor transport materials. In the present work, the charge transport properties of three series of quinoidal thiophene derivatives, oligothiophene (series A), thienothiophene (series B) and benzothiophene (series C), are systematically investigated by employing full quantum charge transfer theory combined with kinetic Monte-Carlo simulation. The single crystal structures of the molecules we had constructed were predicted using the USPEX program combined with density functional theory (DFT) and considering the dispersion corrected. Our theoretical results expounded how the different connection styles, including oligo-, thieno-, and benzo-thiophene in the quinoidal thiophenes derivatives, effectively tune their electronic structures, and revealed how their intermolecular interactions affect the molecular packing patterns and hence their charge transport properties by symmetry-adapted perturbation theory (SAPT). In the meantime we also elucidated the role of end-cyano groups in noncovalent interactions. Furthermore, it is clarified that quinoidal thiophene derivatives show excellent carrier transport properties due to their optimal molecular stacking motifs and larger electronic couplings besides their low energy gap. In addition, our theoretical results demonstrate that quinoidal oligothiophene derivatives (n = 3-5) with more thiophene rings will have ambipolar transport properties, so quinoidal thienothiophene and benzothiophene derivatives should be promising alternatives as n-type OSCs. When we focused only on the electronic transport properties in the three series of molecules, quinoidal benzothiophene derivatives were slightly better than quinoidal oligothiophene or thienothiophene derivatives.

Fluorine-substituted quinoidal thiophene with a F-H hydrogen bond locked conformation for high-performance n-channel organic transistors.

The novel dicyanomethylene-substituted 1,2-bis(thieno[3,2-b]thiophene-2-yl)ethene based quinoidal compounds QBTTE and F2-QBTTE are reported. The FH (vinylene) hydrogen bonds lock the backbone conformation and greatly improve the transistor performance of F2-QBTTE.

π-Conjugated Polymers Incorporating a Novel Planar Quinoid Building Block with Extended Delocalization and High Charge Carrier Mobility.

Two novel conjugated polymers incorporating quinoidal thiophene are successfully synthesized. By combining 1D nuclear magnetic resonance (NMR) and 2D nuclear Overhauser effect spectroscopy analyses, the isomeric form of the major quinoid monomer is clearly identified as the asymmetric Z, E-configuration. The quinoidal polymers are synthesized via Stille polymerization with thiophene or bithiophene. Both quinoidal polymers exhibit the low band gap of 1.45 eV and amphoteric redox behavior, indicating extended conjugation owing to the quinoidal backbone. These quinoidal polymers show ambipolar behaviors with high charge carrier mobilities when applied in organic field-effect transistors. In addition, the radial alignment of polymer chains achieved by off-center spin-coating leads to further improvement of device performance, with poly(quinoidal thiophene-bithiophene) exhibiting a high hole mobility of 8.09 cm2 V-1 s-1 , which is the highest value among the quinoidal polymers up to now. Microstructural alteration via thermal annealing or off-center spin-coating is found to beneficially affect charge transport. The enhancement of crystallinity with strong π-π interactions and the nanofibrillar structure arising from planar well-delocalized quinoid units is considered to be responsible for the high charge carrier mobility.

A push-pull organic dye with a quinoidal thiophene linker: Photophysical properties and solvent effects

In the present work we perform a computational study of the properties of a push-pull organic dye with a quinoidal thiophene unit as the conjugated linker between the electron donor and acceptor groups. We investigate the photophysical properties of the dye related to its potential use as a molecular sensitizer in dye-sensitized solar cells. We rationalize the solvation effects on the absorption band of the dye in protic and aprotic solvents, identifying the interaction of alcohol solvents with the amine in the donor group as the source for the blue shift of the absorption band with respect to aprotic solvents.

High Yield Ultrafast Intramolecular Singlet Exciton Fission in a Quinoidal Bithiophene.

We report the process of singlet exciton fission with high-yield upon photoexcitation of a quinoidal thiophene molecule. Efficient ultrafast triplet photogeneration and its yield are determined by photoinduced triplet-triplet absorption, flash photolysis triplet lifetime measurements, as well as by femtosecond time-resolved transient absorption and fluorescence methods. These experiments show that optically excited quinoidal bithiophene molecule undergoes ultrafast formation of the triplet-like state with the lifetime ∼57 μs. CASPT2 and RAS-SF calculations have been performed to support the experimental findings. To date, high singlet fission rates have been reported for crystalline and polycrystalline materials, whereas for covalently linked dimers and small oligomers it was found to be relatively small. In this contribution, we show an unprecedented quantum yield of intramolecular singlet exciton fission of ∼180% for a quinoidal bithiophene system.

Small molecular thienoquinoidal dyes as electron donors for solution processable organic photovoltaic cells

Two small molecular quinoidal thiophene dyes, featuring low molecular weight, high molar extinction coefficient, and narrow band-gap, have been synthesized as donors for organic photovoltaic cells, giving a best power conversion efficiency of 5.12%.

Tetrathienodibenzocarbazole Based Donor–Acceptor Type Wide Band-Gap Copolymers for Polymer Solar Cell Applications

Two new donor–acceptor alternating wide band gap copolymers, PTTDBC-PhQC8 and PTTDBC-BTC12, based on an electron-rich tetrathienodibenzocarbazole (TTDBC) donor have been designed and synthesized, wherein two additional thienyl rings are fused into a 2,7-dithienylcarbazole skeleton to reinforce the structural coplanarity and rigidity of polymers. The quinoid thiophene in TTDBC can endow not only a wide optical band gap (∼1.9 eV) but also a deep HOMO (∼5.3 eV) for the resulting polymers. The conventional configuration solar cells based on PTTDBC-BTC12 exhibit a large Voc of 0.91 V and a power conversion efficiency of 4.30% (with a Jsc of 9.27 mA cm–2 and a FF of 0.51), while a slightly higher PCE of 4.50% can be achieved for the inverted structure devices. We believe that these wide band gap polymers have potential to be used for tandem cell applications.

Theoretical studies on the quinoidal thiophene based dyes for dye sensitized solar cell and NLO applications.

A series of quinoidal thiophene based dye molecules were designed and their optoelectronic properties were studied for dye sensitized solar cell (DSSC) applications. The efficiency of the designed dye molecules was analyzed using various parameters such as the HOMO-LUMO energy gap, absorption spectra, light harvesting efficiency (LHE), exciton biding energy (Eb) and free energy change for electron injection (ΔG(inject)). The simulated absorption spectra of the quinoidal thiophene molecules show that the electron withdrawing group substituted molecules exhibit dual band characteristics. We found that the cyano-[5'-(4″-amino benzylidene)-5H-thiophen-2'-ylidene] acetic acid based molecules, QT2B, QT4B, QT5 and QT6, are good candidates for DSSC applications. Furthermore, the study on the polarizability and hyperpolarizability of the designed molecules showed that the electron withdrawing group substituted QT2B-X molecules (X = Cl, Br, CF3, CN and NO2) are good candidates for NLO applications.

A donor–acceptor type organic dye connected with a quinoidal thiophene for dye-sensitized solar cells

A donor-acceptor type organic dye connected with a quinoidal thiophene as a π-conjugated chain, cyano-[5'-(4''-(N,N-dimethylamino)benzylidene)-5H-thiophen-2'-ylidene]acetic acid, was synthesized and applied to dye-sensitized solar cells (DSSCs). The absorption band of this quinoidal thiophene dye appeared at longer wavelengths than those of dyes with similar π-conjugation length, indicating the effective π-conjugation through the quinoidal structure. Although the excited state of the quinoidal thiophene dye is deactivated within several picoseconds even in solution, the DSSCs using the quinoidal thiophene dye showed incident photon to current conversion efficiency (IPCE) values of more than 90%, demonstrating the fast and efficient electron injection from the excited dye to TiO(2). By optimizing the fabrication conditions, the DSSC using this dye afforded a photoelectric conversion efficiency of 5.2%, without enlarging the molecular size.

Synthesis and Optical and Electronic Properties of Thiophene Derivatives

AbstractUsing the Hinsberg synthesis of thiophenes, a versatile method to prepare fully derivatized, π‐extended thiophenes is reported. Functionalized thiophenes were divergently synthesized to create three classes of compounds – electron‐deficient, extended conjugation and electron‐rich – to assess substituent effects on optical and electrochemical properties. Properties were assessed by solution absorption spectroscopy, solution‐ and solid‐state fluorescence spectroscopy, cyclic voltammetry and density functional theory calculations. Tetracyano derivatives, prepared through Knoevenagel condensations of malononitrile with thiophene‐2,5‐dicarbaldehydes, were used as electron‐poor analogs. These derivatives showed quasireversible reduction reactions and very low‐lying calculated LUMO energies (–0.55 V reduction potentials vs. Fc/Fc+). The effect of extending π conjugation on the optical and electrochemical properties was investigated by the installation of bis(2‐thienylacryonitrile) groups onto three thiophene cores. The extended conjugation led to compounds that demonstrated both solution‐ and solid‐state fluorescence and moderate, irreversible reduction potentials (–1.2 V versus Fc/Fc+). The Lewis‐acid‐catalyzed coupling of four indoles to thiophene‐2,5‐dicarbaldehydes was studied to assess the effects of both electron‐donating substituents and a quinoidal thiophene. The tetraindolic thiophenes possessed low HOMO–LUMO energy gaps of 1.91 eV, high‐lying HOMO energy levels and tunable LUMO energy levels, attributed to the thiophene quinoidal ground‐state structure. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

From linear quaterthiophene to sulflower: A comparative theoretical study

In this paper, we report a theoretical study of four types of thiophene-based oligomers showing the same number of C C double bonds and very different molecular structures. The comparative study has been performed on the basis of B3LYP/6-31G ∗∗ calculations. The way the thiophene rings are linked together has a remarkable influence on the molecular and electronic properties. Linear quaterthiophene and heptathienoacene show similar aromatic structures but a loss of π-conjugation is detected for the latter due to the condensation of thiophene rings. A blue shift of the most intense electronic transition is predicted for fused heptathienoacene compared with non-fused quaterthiophene. Cyclic quaterthiophene exhibits quinoid thiophene rings folded in an envelope shape and should be visualized as a sulphur-bridged, cis-transoid polyenic chain. Circularly-fused sulflower presents a loss of π-conjugation with respect to linear systems due to its highly-branched conjugated backbone. The loss of conjugation and the high symmetry of the molecule determine that sulflower shows no optical absorption in the visible or near-UV.

Electronic relaxation and vibrational dynamics in a thiophene oligomer studied under the same experimental condition with a sub-5 fs laser

Using a sub-5 fs pulse laser, the pump–probe experiment was performed on a quinoid thiophene derivative and both electronic relaxation and vibrational dynamics were clarified with the same experimental data. From the data, two population decay times of the lowest electronic excited state were determined to be 200 fs and 1.8 ps, which can be explained by the assumption that the 21Ag state lies below the exciton 11Bu state together with the very low fluorescence quantum efficiency. The relaxation process after excitation by the pump pulse was studied to be 11Bu→11B*u→21Ag→11Ag. According to the experimental data, the location of the energy level was also discussed. The electronic dephasing time was determined to be 64 ± 4 fs by utilizing the data in the ‘negative time’ range. The vibrational dephasing time constant of the most strongly coupled mode with frequency of 1466 cm-1 was determined to be 520 ± 20 fs from the widths of the corresponding Fourier power spectra.

Classification of Dynamic Vibronic Couplings in Vibrational Real-Time Spectra of a Thiophene Derivative by Few-Cycle Pulses

Pump-probe spectroscopy was performed with a few cycle pulses of 6.7 fs duration. The electronic transition intensity modulation was induced by molecular vibration in a quinoid thiophene molecule in solution. The real-time vibrational features were analyzed in terms of dependence of vibrational amplitude and phase on probe photon energy. The electronic transition probability is modulated by molecular vibration via vibronic coupling. Changes in the spectral shape and intensity of the time-resolved spectrum were studied by tracking characteristic spectral features including the peak frequency and intensity, spectral bandwidth, and band-integrated intensity. From the analysis the modulation mechanisms were classified into two groups: (1) Condon type and (2) non-Condon type. The features of the wave packet motions were also classified into zeroth-order derivatives due to quasi-pure non-Condon type and first- and second-order derivative types due to the displacement of the potential minimum and the potential curvature change associated with the relevant vibronic transition, respectively.

Real-time vibrational amplitude change in the ground and excited states of a quinoid thiophene induced by few-cycle pulses

Several vibrational modes impulsively excited by 6.7 fs pulses were observed in a quinoid thiophene. From the amplitude decay time and the phase of vibration, the modulation mechanism of the electronic transition by the wave packet motion is discussed and classified in terms of vibronic coupling. Modes with wavenumbers of 1343 and 1420 cm −1 are attributed to the wave packet motions on the potential surface of the excited and ground state, respectively, and a mode with 1539 cm −1 is due to those of both states. The apparent low-frequency modulation of these three modes is ascribed to the artificial interference effect in the calculation of the spectrogram.

Chirped modulation of molecular vibration in quinoidal thiophene after sub-5 fs excitation

Fourier transformation of the real-time trace of sub-5 fs pump-induced molecular vibration in quinoidal thiophene in solution revealed coupled modes to the electronic excitation at 206, 447, and 1466 cm −1. Spectrogram analysis of 1466 cm −1 (C C stretching) mode showed that the amplitude and frequency of the mode are modulated with another low-frequency mode at about 220 cm −1, which is down chirped being induced by the coupling with modes of even lower frequency, resulting in the irreversible relaxation due to high mode density in the low-frequency range. This Letter thus reports on observation of the modulation of vibrational frequency with a chirped frequency.

Confined breather-type excitation in a quinoidal thiophene after sub-5fs pulse excitation

A quinoidal thiophene molecule which is a prototype of the repeated unit of s-trans-cis-polyacetylene was studied by sub-5 fs spectroscopy. A breatherlike mode modulation of both the amplitude and frequency of the C[Double Bond]C stretching was observed for the first time. It generates two sidebands of 2238 and 700 cm(-1) with a separation of 769 cm(-1) corresponding to the modulation frequency. The latter mode is very close to the C-S-C ring deformation, which indicate that this mode mediates coupling between the breather mode and the C(beta)-C(beta) stretching mode. It was also found that the breather-type excitation has a longer lifetime (2-3 ps) than in all-trans-polyacetylene (50 fs) due to its confinement.