Modulation of the electronic properties of conjugated polymers through design of polymer backbone

We present the results of valence effective hamiltonian (VEH) band structure calculations on stereoregular block copolymers of poly(thienylene vinylene) and poly(pyrrylene vinylene). We have examined the evolution of the electronic properties as a function of the extension and distribution of the block copolymer sequences. In both cases, the electronic-properties deviate from a linear evolution as a consequence of the localization of both HOCO and LUCO.     

Synthesis and properties of novel thiophene-based conjugated homologues: 9,9-diphenylfluorene-capped oligothiophenes

[reaction: see text] A series of novel 9,9-diarylfluorene-capped oligothiophenes were synthesized by Suzuki coupling reactions in good yields. The color of the emissions can be controlled by varying the conjugation length of the oligothiophene core. The bulky and rigid terminal groups of the resulting oligomers are significantly beneficial for their high morphological and thermal stability. These new oligothiophenes exhibit intriguing reversible oxidation and reduction redox behavior.     

Selective detection of polyaromatic hydrocarbons on diesel exhaust particles using sputtered neutral mass spectrometry

Aiming for the highly sensitive analysis of aromatic hydrocarbons in organic mixtures, selective ionization using an ultraviolet laser ionization technique was examined for the determination of sputtered neutral species on organic surfaces. The mass spectrum of a model mixture containing pyrene and n‐alkane showed that only pyrene was ionized and detected, whereas a mass spectrum without laser irradiation, i.e. in the case of secondary ion mass spectrometry, was dominated by many n‐alkane fragment peaks. This technique was applied for the detection of polyaromatic hydrocarbons adsorbed on diesel exhaust particles. Two kinds of polyaromatic hydrocarbons were clearly detected because many aliphatic fragment signals were suppressed by means of ultraviolet laser ionization. Copyright © 2013 John Wiley & Sons, Ltd.     

Structural control of the electronic properties of photodynamic azobenzene-derivatized pi-conjugated oligothiophenes

Quaterthiophenes containing a median 3,3'-dimethoxybithiophene (4MTZ) or bis(3,4-ethylenedioxythiophene) (4ETZ) block and an azobenzene group attached at two internal beta-positions of the end thiophene rings have been synthesized. The analysis of the crystal structure of the two compounds by X-ray diffraction shows that, for 4MTZ, the quaterthiophene chain adopts a syn-anti-syn conformation similar to the parent system based on unsubstituted quaterthiophene. In contrast, 4ETZ presents an all-anti conformation stabilized by noncovalent intramolecular interactions between oxygen atoms and thiophenic sulfur atoms. The effect of the photoisomerization of the azobenzene group on the electronic properties of the attached pi-conjugated quaterthiophene chain has been analyzed by UV-vis spectroscopy and cyclic voltammetry. The results obtained for 4MTZ suggest very limited geometrical changes due to the restricted rotation around the central interannular single bond caused by steric interactions between the methoxy groups. In contrast, upon trans to cis photoisomerization of the attached azobenzene group of 4ETZ, the quaterthiophene chain undergoes a reversible conformational switch to a final state with a lower HOMO level and a larger HOMO-LUMO gap than the initial state.     

Synthesis, optical, electrochemical, and thermal properties of conjugated α-fluorenyl oligothiophenes

A series of new α-fluorenyl oligothiophenes up to the pentamer have been synthesized using Suzuki cross-coupling and bromination reactions. The optical, electrochemical, and thermal properties of these materials can be tuned by varying the number of thiophene rings. The longer oligomers (n ? 4) were stable, crystalline, and unreactive to electrochemical oxidative dimerization.     

Synthesis and electronic properties of conjugated pentacene dimers

Conjugated pentacene dimers 1-3 were synthesized in two steps from readily available precursors. Noteworthy is the initial step, which assembles five independent fragments to form the carbon-rich molecular framework. Solution-cast films of these materials are air stable. Photocurrent measurements for solution-deposited thin films show that dimer 3 exhibits photoconductive gain >10.     

Modulation of optical and electronic properties of quinoxailine‐based conjugated polymers for organic photovoltaic cells

The synthesis of donor–acceptor type semiconducting copolymers is described. Quinoxaline (QX) or difluorinated quinoxaline (DFQX) derivatives serve as electron acceptors, while thiophene (T) or selenophene (Se) serve as electron donors. Alternating polymers are synthesized through Stille cross‐coupling, and their thermal stability, optical and electrochemical properties, field‐effect carrier mobilities, film crystallinities, and photovoltaic performances are investigated. The intramolecular charge transfer between the electron‐donating and electron‐accepting units in the backbone induces absorption from 450 to 750 nm. The optical band‐gap energies of the polymers are between 1.65 and 1.73 eV, and depend on the polymer structure. Organic photovoltaic cells fabricated using a polymer composed of DFQX and selenophene (PSe‐DFQX) exhibit a power conversion efficiency of 5.14% with an open‐circuit voltage of 0.78 V, a short‐circuit current density of 11.71 mA/cm², and fill factor of 0.57 under AM 1.5 G irradiation (100 mW cm^?2). © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1904–1914     

Density functional theoretical investigation of the aromatic nature of BN substituted benzene and four ring polyaromatic hydrocarbons

We have studied the topological and local aromaticity of BN-substituted benzene, pyrene, chrysene, triphenylene and tetracene molecules. The nucleus-independent chemical shielding (NICS), harmonic oscillator model of aromaticity (HOMA), para-delocalization index (PDI) and aromatic fluctuation index (FLU) have been calculated to quantify aromaticity in terms of magnetic and structural criteria. We find that charge separations due to the introduction of heteroatoms largely affect both the local and topological aromaticity of these molecules. Our studies show that the presence of any kind of heteroatom in the ring not only reduces the local delocalization in the six membered ring, but also affects strongly the topological aromaticity. In fact, the relative orders of the topological and local aromaticity depend strongly on the position of the heteroatoms in the structure. In general, more ring shared BN containing molecules are less aromatic than the less ring shared BN molecules. In addition our results provide evidence that the structural stability of the molecule is dominated by the σ bond rather than the π bond.     

Effect of ring fusion on the electronic absorption and emission properties of oligothiophenes

A series of fused-ring oligothiophenes were synthesized by a combination of Stille and oxidative coupling reactions. Compounds with the same number of double bonds, but varying in extent of planarization, display a similar longest wavelength absorption maximum in solution. However, the introduction of sulfur linkages into these oligothiophenes leads to a blue shift of the maximum emission wavelength and a correspondingly smaller Stokes shift.     

Determination of polyaromatic hydrocarbons in coal tar pitch

Different techniques for extracting polyaromatic hydrocarbons (PAHs) from coal tar pitch for their quantitative determination by gas chromatography-mass spectrometry are compared. It has been found that the ultrasonic treatment of a sample allowed the quantitative extraction of PAHs from coal tar pitch. The results of the quantitative determination of PAHs in samples of coal tar pitch obtained by Soxhlet extraction and liquid-liquid extraction with ultrasonic treatment agree within the experimental error range.     

Theoretical study of structural and electronic properties of oligo（thiophene-phenylene）s in comparison with oligothiophenes and oligophenylenes

In this work, a quantum-chemical investigation on the structural and opto-electronic properties of oligo（thiophene-phenylene） （4TP） is carried out. The results are discussed in comparison with the properties of corresponding oligothiophene （8T） and oligophenylene （8P）. As the opto-electronic properties of this type of conducting polymers are governed by their electronic band gap, we shall also present a comparison among HOMO, LUMO and band gap energies of these three materials.     

The electronic properties of conjugated ions. II

The calculations presented here demonstrate the relative importance of the excess charge effect for energy calculations, and its lack of importance for spin density calculations, in conjugated ions and radicals.Part 1 is considered to be Ref. [1].     

Molecular parameters and retention characteristics of unsubstituted polyaromatic hydrocarbons in HPLC

Summary The present research studies the possibility of using the correlation dependence between molecular parameters of unsubstituted polyaromatic hydrocarbons (PAH) and their retention in reversed-phase liquid chromatography to optimize the conditions for the separation and identification of unknown peaks on the chromatograms of multicomponent mixtures. A linear correlation equation, that takes the number and environment of the carbon atom in the PAH molecule into account as well as the differences in the specific interactions of isomeric molecules with polar eluent, has been proposed. The adequacy of the proposed PAH retention model was verified by comparing the calculated retention values with the experimental data. The possibility of identifying unsubstituted PAH according to the number of carbon atoms of various types and according to the values of the molecules lengths (calculated on the basis of the retention of these substances under different eluent compositions) was exemplified by various chromatographic systems (reversed phase-eluent-PAH molecules).     

Photophysics and electrochemistry of conjugated oligothiophenes prepared by using azomethine connections

Novel conjugated azomethines consisting of 1 to 5 thiophenes and up to 4 azomethine bonds prepared from a stable diaminothiophene are presented. The effect of the number of thiophene and azomethines bonds on the photophysics and electrochemistry was examined. A high degree of conjugation was confirmed by bathochromic shifts upward of 120 and 210 nm for the absorbance and fluorescence, respectively, relative to the diaminothiophene precursor. Acid doping with methanesulfonic acid resulted in further bathochromic shifts along with lowering of the HOMO-LUMO energy gaps to 1.3 eV. Moreover, the compounds are extremely stable as evidenced by the absence of decomposition products under acid conditions. The resulting heteroatomic covalent bonds are furthermore reductively and hydrolytically resistant. Increasing the degree of conjugation shifts the nonradiative mode of singlet excited state energy dissipation from internal conversion (IC) to intersystem crossing (ISC). The resulting triplet manifold produced by ISC was efficiently deactivated by intramolecular self-quenching from the azomethine bond leading to a nonemissive triplet. Cyclic voltammetry revealed unprecedented reversible radical cation formation of the azomethines. Both one-electron oxidations and reductions were found by electrochemical measurements demonstrating the azomethines' capacity to be mutually p- and n-doped. One of the azomethines exhibited reversible electrochromic behavior with the electrochemically generated radical cation absorbing in the NIR at 1630 and 792 nm. X-ray crystallography confirmed the thermodynamically stable E isomer was formed uniquely and that the thiophenes are coplanar adopting an antiparallel arrangement.     

Alkylenesulfanyl-bridged bithienyl cores for simultaneous tuning of electronic, filming, and thermal properties of oligothiophenes

DPY and DPE alkylenesulfanyl-bridged bithienyls were prepared by a highly effective ring-closing reaction via arylalkylsulfonium intermediate and used as inner cores in oligothiophenes. HOMO-LUMO energy levels, conformational flexibility, and intrinsic asymmetry of the cores are reflected in the electronic, film-forming, and thermal properties of the corresponding oligomers.     

Synthesis and structural, electronic, and optical properties of oligo(thienylfuran)s in comparison with oligothiophenes and oligofurans

[structure: see text] Alternate thiophene/furan oligomers having four and six heterocycles, i.e., oligo(thienylfuran) dimer and trimer 2 (n = 4 and 6), were newly synthesized by repetitive Stille coupling reactions. The structural, electronic, and optical properties of these oligomers were investigated by X-ray crystallography (for n = 4), cyclic voltammetry (CV), UV-vis and fluorescence spectroscopy, and DFT calculations, and the results were compared with those of corresponding oligothiophenes (1) and oligofurans (3). The inter-ring torsional energy profiles calculated for bithiophene 1 (n = 2), thienylfuran 2 (n = 2), and bifuran 3 (n = 2) at the B3LYP/6-31G(d) level indicated that the most stable conformers of 2 (n = 2) and 3 (n = 2) are fully coplanar with transoid structure while that of 1 (n = 2) is twisted with a dihedral angle of 158 degrees . In accord with this, X-ray crystallographic analysis of 2 (n = 4) revealed that the pi-conjugated system is nearly planar with the inter-ring C=C-C=C dihedral angles between the thiophene and furan rings of 173.6(7) degrees , -177.0(7) degrees , and 172.6(6) degrees . In the packing structure, these nearly planar molecules are arranged in a herringbone pattern. The CV on a series of oligo(thienylfuran)s 2 showed irreversible oxidation peaks at +0.90, +0.42, and +0.29 V vs Fc/Fc(+) for n = 2, 4, and 6, which were 0.15-0.18 V lower than those for corresponding oligothiophenes 1 and were closer to those for oligofurans 3. On the other hand, the UV-vis spectra of 2 showed the longest wavelength absorption to be almost identical with those of the corresponding 1, and more bathochromically shifted than those of the corresponding 3. The results of CV and UV-vis measurements were supported by DFT calculations (B3LYP/ 6-311+G(2d,p)//B3LYP/6-31G(d)). Thus, oligo(thienylfuran)s 2 have HOMOs which are higher than those of oligothiophenes 1 and close to those of 3, and HOMO-LUMO gaps which are close to those of 1 and smaller than those of 3. In fluorescence spectra, the quantum yield of 2 increased with elongation of the pi-system (n = 2 (3.5%), 4 (19%), 6 (24%)).     

On the electronic properties of dehydrogenated polycyclic aromatic hydrocarbons

The electronic excited-state properties of a series of dehydrogenated polycyclic aromatic hydrocarbons from phenyl through to decacyl are reported. The radicals were investigated by use of time-dependent density functional theory in conjunction with the B3LYP functional. The pi and n orbitals were seen to converge in energy as the system increased in size, yet all radicals were found to have A' ground states. In addition to the study of the electronic state symmetries, the excited-state transitions and oscillator strengths were investigated with the resulting transitions found within the visible region of the spectrum, placing these radicals in the large group of candidate carriers of the diffuse interstellar bands.