Phenylene Thiophene Research Articles

Tuning Molecular Packing, Charge Transport, and Luminescence in Thiophene–Phenylene Co-Oligomer Crystals via Terminal Substituents

Tuning Molecular Packing, Charge Transport, and Luminescence in Thiophene–Phenylene Co-Oligomer Crystals via Terminal Substituents

Effect of Fluorination on Properties of Thiophene–Phenylene Co-Oligomers with Annulated Core: A DFT Study

Effect of Fluorination on Properties of Thiophene–Phenylene Co-Oligomers with Annulated Core: A DFT Study

Polarity Switching in Organic Electronic Devices via Terminal Substitution of Active-Layer Molecules

Organic electronic devices often suffer from poor charge injection limiting their performance. Specifically, high-performance electronic devices usually need Ohmic contacts, but it is not easy to realize them in junctions of the organic semiconductor with the electrodes because of the contact problems. In this work, polarity switching in organic field-effect transistors (OFETs) that is independent of the electrode work function is demonstrated─the switching of charge injection from the p-type to ambipolar and to the n-type─via modification of the donor/acceptor character of the molecular terminal substituents. By using three thiophene–phenylene co-oligomers with the same conjugated core, similar crystal packings, but different terminal substituents (methyl, trimethylsilyl, and trifluoromethyl), the polarity switching in both thin-film and single-crystal OFETs is demonstrated. The ultraviolet photoelectron spectroscopy studies and electronic structure calculations justify a definitive role of the interface dipole stemming from the terminal groups in controlling the heights of charge injection barriers and hence in charge injection into the OFET active layer. The results obtained are expected to facilitate rational design of organic semiconductors for high-performance electronic devices.

Luminescent 2D single crystals of thiophene–phenylene co-oligomers for field-effect devices

Photo/electroluminescent and semiconductor properties of macroscopic 2D single crystals are presented, and the impact of phenylene insertion is investigated.

Impact of chirality on the aggregation modes of l-phenylalanine- and d-glucose-decorated phenylene–thiophene oligomers

Three l-phenylalanine- or d-glucose-decorated phenylene–thiophene oligomers have been studied using UV-Vis and ECD spectroscopies in different conditions of solution aggregation and thin films, investigating the impact of chirality on their aggregation modes.

Unoccupied Electron States of Ultrathin Films of Thiophene–Phenylene Cooligomers on the Surface of Polycrystalline Gold

Unoccupied electronic states in the energy range from 5 to 20 eV above the Fermi level have been studied in ultrathin films of dimethyl-substituted thiophene–phenylene cooligomers CH3-phenylene–thiophene–thiophene–phenylene–CH3 (CH3–PTTP–CH3) on polycrystalline gold surfaces of two types: the ex situ Au layer thermally deposited in a special chamber and the in situ Au surface prepared inside an analytical chamber. The film structure is studied by the X-ray diffraction (XRD) method. The formation of a superposition of the amorphous phase and the crystalline phase with period 3.8 nm is discussed. The energy positions of the maxima of the unoccupied electronic states and the character of formation of the boundary potential barrier have been studied by the total current spectroscopy (TCS). The structures of the FSTCS maxima of the 5–7-nm-thick CH3–PTTP–CH3 films are not different when using various types of Au substrates and the ZnO semiconductor surface prepared by atomic layer deposition (ALD). As a CH3–PTTP–CH3 layer is deposited on the ex situ Au and in situ Au surfaces, the electron work function increases insignificantly (by ~0.1 eV) as the coating thickness increases to 5–7 nm. At such thicknesses of the CH3–PTTP–CH3 films, the electron work function is 4.7 ± 0.1 eV in the case of the ex situ Au substrate and 4.9 ± 0.1 eV in the case of the in situ Au substrate. A possible influence of the processes of physicochemical interaction at the boundary between the film and the substrate on the formation of the boundary potential barrier in the structures under study is discussed.

Influences of the terminal groups on the performances of asymmetric small molecule acceptors-based polymer solar cells

By altering the end-capping groups from 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile to 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile and 2-(5,6-dichloro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile, three asymmetric non-fullerene electron-acceptors (T-TT, T-TT-4F, and T-TT-4Cl) containing thiophene–phenylene–thieno[3,2-b]thiophene fused-ring as the electron-donating core were synthesized. Although the three asymmetric molecules all have bent geometries, the di-fluorinated and di-chlorinated acceptors show bathochromically-shifted spectra, enhanced molar extinct coefficients, deepened molecular energy levels, and reduced dipole moments. When blending with a wide-bandgap polymer PM6 in inverted solar cells, the T-TT-based device exhibits the highest open-circuit voltage (VOC) of 0.966 V, while the T-TT-4Cl-based device has the biggest short-circuit current density (JSC) of 19.00 mA cm−2, and the T-TT-4F-based device possesses the best fill factor (FF) of 66.1%, a moderate VOC (0.859 V), and a medium JSC (18.48 mA cm−2). As a result of the above properties and parameters, the T-TT-4F- and T-TT-4Cl-based solar cells successfully achieved high power conversion efficiencies of 10.49% and 10.16%, in comparison with that of 9.70% for the T-TT-based device, illustrating the importance of the influences of the terminal groups on the performances of asymmetric small molecular acceptors-based polymer solar cells.

Impact of N-substitution on structural, electronic, optical, and vibrational properties of a thiophene-phenylene co-oligomer.

Properties of the organic semiconductors can be finely tuned via changes in their molecular structure. However, the relationship between the molecular structure, molecular packing, and (opto)electronic properties of the organic semiconductors to guide their smart design remains elusive. In this study, we address computationally and experimentally the impact of subtle modification of a thiophene–phenylene co-oligomer CF3-PTTP-CF3 on the molecular properties, crystal structure, charge transport, and optoelectronic properties. This modification consists in the substitution of two C–H atom pairs by N atoms in the thiophene units and hence converting them to thiazole units. A dramatic effect of the N-substitution on the crystal structure—the crossover from the herringbone packing motif to π-stacking—is attributed to significant changes in the molecular electrostatic potential. The changes in the molecular and crystal structure resulting from the N-substitution clearly reveal themselves in the Raman spectra. The increase of the calculated electron mobility in the corresponding crystals as a result of the N-substitution is rationalized in terms of the changes in the molecular and crystal structure. The charge transport, electroluminescence, and photoelectric properties are compared in thin-film organic field-effect transistors based on CF3-PTTP-CF3 and its N-substituted counterpart. An intriguing similarity between the effects of N-substitution in the thiophene rings and fluorination of the thiophene–phenylene oligomer is revealed, which is probably associated with a more general effect of electronegative substitution. The obtained results are anticipated to facilitate the rational design of organic semiconductors.

The Electron-Vibrational Interaction in a Thiophene—Phenylene Cooligomer and Its Relationship to the Raman Spectrum

Electron-vibrational interactions play a key role in limiting charge mobility in organic semi-conductors. This paper reports a theoretical study of the electron—phonon interaction in the 5,5′-diphenyl-2,2′-bitiophene (PTTP) molecule, which belongs to the class of thiophene-phenylene cooligomers, which are of great interest for organic optoelectronics due to their electron-transport and luminescent properties; these results are compared with anthracene, which is a model organic semiconductor. The contributions of various vibrational modes to the reorganization energy of PTTP and the anthracene molecules are revealed and it is shown that these contributions correlate with the intensities of the corresponding bands in the Raman spectrum. In particular, it is found that for the PTTP molecule the so-called I-mode with a frequency of ∼1460 cm−1, which corresponds to collective vibration of atoms of all oligomer units, has the highest intensity in both spectra. These results indicate the promise of Raman spectroscopy for studying electron-vibrational interactions in organic semiconductors. Finally, the mobility of holes in PTTP and anthracene is estimated in the framework of the jump model and the reasons for their difference are analyzed. Based on these results, we propose some ways to reduce the electron-vibrational interaction in thiophene—phenylene cooligomers, which is important for the directional molecular design of organic semiconductors.

Noncovalent Close Contacts in Fluorinated Thiophene–Phenylene–Thiophene Conjugated Units: Understanding the Nature and Dominance of O···H versus S···F and O···F Interactions with Respect to the Control of Polymer Conformation

Using a simple π-conjugated trimer, EDOT–phenylene–EDOT (EDOT = 3,4-ethylenedioxythiophene), we evaluate the effect that fluorine substituents have upon changes in conformation, conjugation, and ox...

Impact of terminal substituents on the electronic, vibrational and optical properties of thiophene-phenylene co-oligomers.

Owing to the combination of efficient charge transport and bright luminescence, thiophene-phenylene co-oligomers (TPCOs) are promising materials for organic light-emitting devices such as diodes, transistors and lasers. The synthetic flexibility of TPCOs enables facile tuning of their properties. In this study, we address the effect of various electron-donating and electron-withdrawing symmetric terminal substituents (fluorine, methyl, trifluoromethyl, methoxy, tert-butyl, and trimethylsilyl) on frontier orbitals, charge distribution, static polarizabilities, molecular vibrations, bandgaps and photoluminescence quantum yields of 5,5'-diphenyl-2,2'-bithiophene (PTTP). By combining DFT calculations with cyclic voltammetry and absorption, photoluminescence, and Raman spectroscopies, we show that symmetric terminal substitution tunes the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energies of TPCOs within a range of ∼0.7 eV, shifts the frequencies of the vibrational modes associated with the phenyl rings, changes the photoluminescence quantum yield by about two-fold and slightly changes the bandgap by ∼0.1 eV. We demonstrate that these effects are governed by two factors: the Hammet constant of the substituents and their involvement in the π-conjugation/hyperconjugation described by the effective conjugation length of the substituted oligomer. A detailed picture underlying the effect of the terminal substituents on the electronic, vibrational and optical properties of TPCOs is presented. Overall, the unraveled relationships between the structure and the properties of the substituted PTTPs should facilitate a rational design of π-conjugated (co-)oligomers for efficient organic optoelectronic devices.

Long side-chain grafting imparts intrinsic adhesiveness to poly(thiophene phenylene) conjugated polymer

Herein, poly(n-butyl acrylate) (PBA)-grafted poly(thiophene phenylene) (PThP) (PThP-g-PBA) conducting polymers have been synthesised to introduce the softness and adhesiveness of PBA to a rigid conjugated PThP backbone. We investigate the effects of PBA chain lengths on the morphological, optical, electrical and mechanical properties of the graft copolymers, using atomic force microscopy (AFM), electrochemical and in situ spectroelectrochemical analysis and stress-strain measurements. Despite the insulating nature of the grafted PBA side chains, the graft copolymers with short to medium graft lengths (17 and 30 units) showed comparable electroactivity to the ungrafted PThP and appreciable electrical conductivity, while graft length of 60 units significantly diminished electrical properties of the PThP. All of the graft copolymers displayed significant adhesive properties, characterized both macro- and microscopically, showing enhancement in the adhesion force with increasing PBA side chain length. In addition, the adhesion as well as solution processability of the graft copolymers were utilised in fabrication of a stretchable strain sensor based on these copolymers. This work presents a simple approach to producing solution-processable adhesive conducting polymers for flexible and stretchable electronics.

Unoccupied Electron States and the Formation of Interface between Films of Dimethyl-Substituted Thiophene–Phenylene Coolygomers and Oxidized Silicon Surface

The unoccupied electron states and the boundary potential barrier during deposition of ultrathin films of dimethyl-substituted thiophene–phenylene coolygomers of the type of CH3–phenylene–thiophene–thiophene–phenylene–CH3 (CH3–PTTP–CH3) on an oxidized silicon surface have been studied. The electronic characteristics have been measured in the energy range from 5 to 20 eV above the Fermi level using total current spectroscopy (TCS). The structure of the CH3–PTTP–CH3 film surfaces has been studied by atomic force microscopy (AFM), and the atomic compositions of the films have been studied by X-ray photoelectron spectroscopy (XPS). The changes in the maximum intensities measured by the TCS method obtained from the deposited CH3–PTTP–CH3 film and from the substrate during increasing in the organic coating thickness to 6 nm is discussed. The formation of the boundary potential barrier in the n-Si/SiO2/CH3–PTTP–CH3 is accompanied by the decrease in the surface work function from 4.2 ± 0.1 to 4.0 ± 0.1 eV as the organic coating thickness increases to 3 nm. The ratio of atomic concentrations C: S in the CH3–PTTP–CH3 films well corresponds to the chemical formula of CH3–PTTP–CH3 molecules. The roughness of the CH3–PTTP–CH3 coating surface was not higher than 10 nm on the ~10 × 10 μm areas as the total CH3–PTTP–CH3-layer thickness was about 100 nm.

Molecular Self‐Doping Controls Luminescence of Pure Organic Single Crystals

AbstractOrganic optoelectronics calls for materials combining bright luminescence and efficient charge transport. The former is readily achieved in isolated molecules, while the latter requires strong molecular aggregation, which usually quenches luminescence. This hurdle is generally resolved by doping the host material with highly luminescent molecules collecting the excitation energy from the host. Here, a novel concept of molecular self‐doping is introduced in which a higher luminescent dopant emerges as a minute‐amount byproduct during the host material synthesis. As a one‐stage process, self‐doping is more advantageous than widely used external doping. The concept is proved on thiophene–phenylene cooligomers (TPCO) consisting of four (host) and six (dopant) conjugated rings. It is shown that <1% self‐doping doubles the photoluminescence in the TPCO single crystals, while not affecting much their charge transport properties. The Monte‐Carlo modeling of photoluminescence dynamics reveals that host–dopant energy transfer is controlled by both excitonic transport in the host and host–dopant Förster resonant energy transfer. The self‐doping concept is further broadened to a variety of conjugated oligomers synthesized via Suzuki, Kumada, and Stille crosscoupling reactions. It is concluded that self‐doping combined with improved excitonic transport and host–dopant energy transfer is a promising route to highly luminescent semiconducting organic single crystals for optoelectronics.

Незаполненные электронные состояния и формирование интерфейса между пленками диметил замещенных тиофен- фенилен соолигомеров и поверхностью окисленного кремния

AbstractThe unoccupied electron states and the boundary potential barrier during deposition of ultrathin films of dimethyl-substituted thiophene–phenylene coolygomers of the type of CH_3–phenylene–thiophene–thiophene–phenylene–CH_3 (CH_3–PTTP–CH_3) on an oxidized silicon surface have been studied. The electronic characteristics have been measured in the energy range from 5 to 20 eV above the Fermi level using total current spectroscopy (TCS). The structure of the CH_3–PTTP–CH_3 film surfaces has been studied by atomic force microscopy (AFM), and the atomic compositions of the films have been studied by X-ray photoelectron spectroscopy (XPS). The changes in the maximum intensities measured by the TCS method obtained from the deposited CH_3–PTTP–CH_3 film and from the substrate during increasing in the organic coating thickness to 6 nm is discussed. The formation of the boundary potential barrier in the n -Si/SiO_2/CH_3–PTTP–CH_3 is accompanied by the decrease in the surface work function from 4.2 ± 0.1 to 4.0 ± 0.1 eV as the organic coating thickness increases to 3 nm. The ratio of atomic concentrations C: S in the CH_3–PTTP–CH_3 films well corresponds to the chemical formula of CH_3–PTTP–CH_3 molecules. The roughness of the CH_3–PTTP–CH_3 coating surface was not higher than 10 nm on the ~10 × 10 μm areas as the total CH_3–PTTP–CH_3-layer thickness was about 100 nm.

Effect of chemical structure and deposition method on optical properties of polyazomethines with alkyloxy side groups

Three (two novel) conjugated, thiophene phenylene based polyazomethines have been obtained parallel, by chemical vapour deposition (CVD) and by chemical synthesis. They were subsequently identified and characterised by an X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry and detailed UV–Vis-NIR spectroscopy. Absorption spectra of the polymers' solutions and thin films have been examined. The spectra of polyazomethines without, with methoxy and with octyloxy side chains attached to the polymer backbone showed a conjugation-improving effect of alkyloxy side chain introduction. A comparison of spectra of the same polyazomethine's thin films deposited separately by spin-on and CVD methods revealed decrease in conjugation of compounds obtained during CVD. Additionally, amorphous character of the investigated thin films allowed to calculate the absorption edge parameters, in a way typical for amorphous semiconductor and to discuss their values in the context of chemical structure and deposition method. Surface imaging with atomic force microscope revealed different character of thin films deposited by spin-on and CVD methods.

Coumarin-based donor–π–acceptor organic dyes for a dye-sensitized solar cell: photophysical properties and electron injection mechanism

The electronic structure and photophysical properties of five coumarin-based donor–π–acceptor (D–π–A)-type organic dyes for a dye-sensitized solar cell (DSSC) which were recently developed have been investigated using the time-dependent density functional theory and the symmetry-adapted cluster configuration interaction method. Theoretical calculations including the solvent effect in state-specific and linear-response scheme reproduced the experimental UV–Vis absorption spectra of these dyes satisfactorily. The π-spacers, thiophene and thiophene–phenylene mixed units, affect the planarity of the molecular structures which is relevant to the photophysical properties and charge polarization. Energy levels of the frontier orbitals and charge separation were analyzed, and the thiophene linker was found to be effective for the electron injection in DSSC. The adsorption of these dyes on the TiO2 anatase (101) surface and the electron injection mechanism were also investigated using a dye–(TiO2)38 cluster model employing PBE and TD-CAM-B3LYP calculations, respectively. The adsorption energies of these dyes were estimated to be ~14 kcal/mol, indicating strong adsorption of dye to a TiO2 surface by carboxylate group. The possible direct electron injection mechanism was suggested in the present coumarin-based D–π–A dyes in a dye–TiO2 interacting system.

Synthesis and optical properties of poly(phenylenethiophene)s bearing conjugated side chains

A series of π-conjugated polymers bearing conjugated side chain were prepared by Migita–Kosugi–Stille coupling polycondensation reaction. The polymers consisting of phenylene–thiophene (CCPTO), phenylene–isothianaphthene (CCPIO), and phenylene-3,4-ethylenedioxythiophene (CCPEO) conjugated backbones in the main chains showed optical absorption peaks in the same region, while fluorescence peak of CCPTO was at larger wavelengths compared with those of CCPIO and CCPEO. The polymers showed optical rotary dispersion, indicating the optically active substituents induced one-handed helicity to the polymers.

Molecularly Smooth Single-Crystalline Films of Thiophene–Phenylene Co-Oligomers Grown at the Gas–Liquid Interface

Single crystals of thiophene–phenelyne co-oligomers (TPCOs) have previously shown their potential for organic optoelectronics. Here we report on solution growth of large-area thin single-crystalline films of TPCOs at the gas–liquid interface by using solvent–antisolvent crystallization, isothermal slow solvent evaporation, and isochoric cooling. The studied co-oligomers contain identical conjugated core (5,5′-diphyenyl-2,2′-bithiophene) and different terminal substituents, fluorine, trimethylsilyl, or trifluoromethyl. The fabricated films are molecularly smooth over areas larger than 10 × 10 μm2, which is of high importance for organic field-effect devices. The low-defect structure of the TPCO crystals is suggested from the monoexponential kinetics of the PL decay measured in a wide dynamic range (up to four decades) and from low crystal mosaicity assessed by microfocus X-ray diffraction. The TPCO crystal structure is solved using a combination of X-ray and electron diffraction. The terminal substituents affect the crystal structure of TPCOs, bringing about the formation of a noncentrosymmetric crystal lattice with a crystal symmetry Cc for the bulkiest trimethylsilyl terminal groups, which is unusual for linear conjugated oligomers. Comparing the different crystal growth techniques, it is concluded that the solvent–antisolvent crystallization is the most robust for fabrication of single-crystalline TPCOs films. The possible nucleation and crystallization mechanisms operating at the gas–solution interface are discussed.

Novel Thiophene–Phenylene–Thiophene Fused Bislactam-Based Donor–Acceptor Type Conjugate Polymers: Synthesis by Direct Arylation and Properties

Three new donor–acceptor copolymers based on thiophene–phenylene–thiophene fused bislactam and various donors (3,4-dodecylthiophene, 4,4′-didodecyl-2,2′-bithiophene, and ethylenedioxythiophene) were synthesized, characterized, and used in field-effect transistors. Polycondensation was performed using nonactivated thiophene derivatives by employing palladium-catalyzed direct arylation under phosphine-free conditions. This method is superior to traditional cross-coupling polymerization because it requires fewer synthetic operations and does not employ toxic organometallic intermediates. Regioselective polymers can also be generated by using β-substituted thiophene derivatives. The studied polymers were tested in a bottom gate top contact thin film transistor (OTFT) architecture. The best electronic performance was shown by polymer P3, with enhanced π-conjugation due to the appearance of intramolecular attractive interactions.