Theoretical study of spectroscopic properties of dimethoxy-p-phenylene-ethynylene oligomers: planarization of the conjugated backbone

The optical spectra of the dimethoxy-p-phenylene-ethynylene oligomers (up to n = 10) are calculated by DFT and TD-DFT methods. It is found that the conformational rotations around the cylindrical triple-bonded carbon links impact significantly the optical spectrum. The effective conjugation length (ECL) of the oligomer is obtained by extrapolating the HOMO-LUMO gap to infinite chain length with an alternative exponential function. The spectral shift is mainly dependent on the high pi-conjugation segment of oligomers, resulting from the planarization of the backbone. Although the rotational barrier is very low, the calculated results further indicate that rotation about the cylindrical triple bond still interrupts the conjugation of rod-like oligomers to some extent, and leads to an angle-dependent HOMO-LUMO gap. The results are helpful to interpret the conformational-dependent spectroscopic phenomena of p-phenyleneethynylene oligomers and polymers (PPEs) observed in ensemble and single molecule spectroscopy.     

用耦合簇运动方程计算分子的多光子吸收截面

简短评述了多光子吸收过程的原理与应用．由于该过程的高度非线性，对于分子的高激发态性质的描述要求很高，因为从本质上讲，这涉及到从低激发态到高激发态的跃迁过程．我们提出了用耦合簇运动方程方法并结合半经验哈密顿参数来计算大分子体系的多光子吸收截面．以蒽一卟啉一蒽分子为例，从激发态之间的跃迁密度分析发现，分子内电荷转移过程可以极大地增强三光子吸收．     

Stimuli-responsive azulene-based conjugated oligomers with polyaniline-like properties

Novel azulene building blocks, prepared via the cycloaddition of thiophene-S,S-dioxides and fulvenes, allow for incorporation of the seven-membered ring of the azulene nucleus directly into the backbone of conjugated materials. This unique mode of incorporation gives remarkably stable, stimuli-responsive materials upon exposure to acid. This simple doping/dedoping strategy provides for effective optical band gap control and on/off fluorescence switching, reminiscent of polyaniline.     

Stability and spectroscopic properties of singly and doubly charged anions

Using density functional theory and hybrid B3LYP exchange-correlation energy functional we have studied the structure, stability, and spectroscopic properties of singly and doubly charged anions composed of simple metal atoms (Na, Mg, Al) decorated with halogens such as Cl and pseudohalogens such as CN. Since pseudohalogens mimic the chemistry of halogen atoms, our objective is to see if pseudohalogens can also form superhalogens much as halogens do and if the critical size for a doubly charged anion depends upon the ligand. The electron affinities of MCl(n) (M = Na, Mg, Al) exceed the value of Cl for n ≥ (k + 1), where k is the normal valence of the metal atom. However, for M(CN)(n) complexes this is only true when n = k + 1. In addition, while the electron affinities and vertical detachment energies of MCl(n) complexes are close to each other, they are markedly different when Cl is replaced by pseudohalogen, CN. The origin of these anomalous results is found to be due to the large binding energy of cyanogen, (NCCN) molecule. Because of the tendency of CN molecules to dimerize, the ground state geometries of the neutral and anionic M(CN)(n) complexes are very different when their number exceed the normal valence of the metal atom. While our calculations support the conclusion of Skurski and co-workers that pseudohalogens can form the building blocks of superhalogens, we show that there is a limitation on the number of CN moieties where this is true. Equally important, we find large differences between the ground state geometries of the neutral and anionic M(CN)(n) complexes for n ≥ (k + 2) which could play an important role in interpreting future experimental data on M(CN)(n) complexes. This is because the electron affinity defined as the energy difference between the ground states of the anion and neutral can be very different from the adiabatic detachment energy defined as the energy difference between the ground state of the anion and its structurally similar neutral isomer.     

The surprising nonlinear optical properties of conjugated polyyne oligomers

Polyynes represent a unique class of conjugated organic compounds. The third-order nonlinear optical response of polyynes has been extensively modeled theoretically, and it is generally believed that the increase in molecular second hyperpolarizability (gamma) as a function of length for polyynes should be lower than that for polyenes. Experimental evidence to test this prediction, however, has been absent. We have synthesized conjugated polyynes that contain up to 20 consecutive sp-hybridized carbons, and we have determined their nonresonant gamma-values as a function of the number of acetylene repeat units (n). These gamma-values demonstrate a power-law behavior versus n(gamma approximately n(4.28+/-0.13)), with an exponent that is both larger than theoretically predicted for polyynes and substantially higher than that observed for polyenes or polyenynes. Furthermore, no saturation of the linear or nonlinear optical properties is observed.     

Chemical properties of positive singly charged astatine ion in aqueous solution

The mobility of oxidized astatine in solutions H(Na)ClO₄ (=0.4 M) – 1·10^–4M K₂Cr₂O₇ has been measured at 25°C in the interval 0.63pH1.68. Under these conditions astatine migrates to the cathode only. The speed of the migration depends upon the concentration of hydrogen ions in solution: pH 1.68 U_c = 1.17 · 10^-4 cm² V^-1 s^-1 pH 0.63 U_c = 2.67 · 10^-4 cm² V^-1 s^-1 The effect agrees with the opinion that the singly charged cation of astatine formed in acidic solutions is a strong aquacomplex [(H₂O)_xAt]⁺ (x=1–2) (protonated hypoastatic acid). Deprotonation constant of this cation is K_dp=0.032±0.005. Specific properties of the astatine cation are given. They can be explained, probably, through the peculiarities of its structure.     

Theoretical study on electronic structure and optical properties of phenothiazine-containing conjugated oligomers and polymers

The application of polyfluorenes in polymeric light-emitting diodes has been hampered because of the charge injection difficulties and the troublesome formation of a tailed emission band at long wavelengths (>500 nm) during device fabrication and operation, leading to both a color instability and reduced efficiency. The incorporation of the phenothiazine units has been proven to significantly enhance the hole injection and charge carrier balance and at the same time efficiently suppress the keto defect emission. In this contribution, we apply quantum-chemical techniques to investigate poly[10-(N-(2'-methyl)phenothiazine-3,7-diyl) and its fluorene copolymer poly[10-(N-(2'-methyl)phenothiazine-3,7-diyl)-co-alt-2,7-(9,9-dimethylfluorene)] (PFPTZ) and gain a detailed understanding the influence of phenothiazine units on the electronic and optical properties of fluorene derivatives. Density functional theory (DFT) and time-dependent DFT approaches are employed to study the neutral molecules, HOMO-LUMO gaps (Delta(H-L)), the lowest excitation energies (E(g)'s), positive and negative ions, as well as the IPs and EAs, focusing on the superiority of the electronic and optical properties attributed to the introduction of electron-donating moiety phenothiazine (PTZ) through comparing with pristine polyfluorene. The outcomes show that the highly nonplanar conformation of phenothiazine ring in the ground state preclude sufficiently close intermolecular interactions essential to forming aggregates or excimers. Furthermore, the HOMO energies lift about 0.4 eV, and thus, the IPs decrease about 0.3 eV in PFPTZ, suggesting the significant improved hole-accepting and transporting abilities, due to the electron-donating properties of phenothiazine ring by the presence of electron-rich sulfur and nitrogen heteroatoms and highly nonplanar characters, resulting in the enhanced performances in both efficiency and brightness compared with pristine polyfluorene. In addition, even though the introduction of electron-donating moiety PTZ onto fluorene leads to a slight bathochromic shift in absorption and emission spectra, the copolymer still exhibited strong blue emission.     

Electron-pair densities of singly charged atoms

For the singly charged 53 cations from Li⁺ to Cs⁺ and 43 anions from H⁻ to I⁻ in their ground states, spherically averaged electron-pair intracule (relative-motion) density h(u), extracule (center-of-mass-motion) density d(R), and their moments uⁿ and Rⁿ are examined, where u and R are the interelectronic distance and the center-of-mass radius of a pair of electrons, respectively. The intracule and extracule densities of all the 96 ions are found to be monotonically decreasing functions, as for neutral atoms. Approximate relations d(R)8h(2R) and uⁿ/Rⁿ2ⁿ are confirmed to be valid for the charged atoms as well.     

Optical and redox properties of a series of 3,4-ethylenedioxythiophene oligomers

The optical and redox properties of a series of 3,4-ethylenedioxythiophene oligomers (EDOTn, n=1-4) and their beta,beta'-unsubstituted analogues (Tn, n=1-4) are described. Both series are end capped with phenyl groups to prevent irreversible alpha-coupling reactions during oxidative doping. Absorption and fluorescence spectra of both series reveal a significantly higher degree of intrachain conformational order in the EDOTn oligomers. Oxidation potentials (E(PA1) and E(PA2)) determined by cyclic voltammetry reveal that those of EDOTn are significantly lower than the corresponding Tn oligomers as a consequence of the electron-donating 3,4-ethylenedioxy substitution. Linear fits of E(PA1) and E(PA2) versus the reciprocal number of double bonds reveal significantly steeper slopes for the EDOTn than for the Tn oligomers. This could indicate a more effective conjugation for the EDOTn series, confirmed by the fact that coalescence of E(PA1) and E(PA2) is reached already at relatively short chain lengths ( approximately 5 EDOT units) in contrast to the Tn series (>10 thiophene units). The stepwise chemical oxidation of the EDOTn and Tn oligomers in solution was carried out to obtain radical cations and dications. The energies of the optical transitions of the radical cations and dications as determined by UV/Vis/NIR spectroscopy were similar for the two series. These spectroscopic observations are consistent with quantum-chemical calculations performed on the singly charged molecules. Cooling solutions containing T2.+, T3.+, EDOT2.+, and EDOT3.+ revealed the reversible formation of dimers, albeit with a somewhat different tendency, expressed in the values for the dimerization enthalpy.     

Structural properties of reciprocal form factor in neutral atoms and singly charged ions

Structural characteristics of the spherically averaged internally folded density or reciprocal form factor Br are studied within the Hartree-Fock framework for 103 neutral atoms, 54 singly charged cations, and 43 anions in their ground state. The function Br is classified throughout the Periodic Table into three types: (i) monotonic decrease from the origin, (ii) maximum at r=0 and a negative minimum at r>0, and (iii) a local maximum at r=0 and a pair maximum-minimum out of the origin. A detailed study of the corresponding properties for individual subshells as well as their relative weight for the total Br is also carried out. For completeness, the analytical Br for hydrogenlike atoms in both ground and excited states is also analyzed.     

Twisting of conjugated oligomers and polymers: case study of oligo- and polythiophene

Interring twisting (change in the dihedral angle between conjugated rings) of polythiophene was studied theoretically using periodic boundary conditions (PBC) at the B3LYP/6-31G(d) level. We find that the band gap of polymers is strongly dependent on the interring twist angle; yet twisting requires very little energy. A twist of 30 degrees increases the band gap by 0.75 eV in polythiophene, while requiring only 0.41 kcal mol(-1) per monomer unit. Such a small energetic value is of the order of crystal packing or van der Waals forces. These results are compared with calculations performed on model oligomers. Sexithiophene, its radical cations, and its dication are optimized at 0-180 degrees end-to-end twist angles (which correspond to 0-36 degrees interring dihedral angles) using the B3LYP/6-31G(d) method. The theoretical results suggest that the HOMO-LUMO gap, ionization potential, and charge distribution of oligomers are strongly dependent on twisting, whereas, similar to the case of polythiophene, twisting of neutral oligothiophenes costs very little energy. In the case of the radical cation, the lowest energy transition is shifted to a longer wavelength region on twisting, while the second-lowest energy transition is shifted to a shorter wavelength region. This implies that twisted, doped conducting polymers (modeled here by an oligomer radical cation), in contrast to planar, doped polymers, should be transparent within a certain optical window (in the far-visible region, at approximately 1.5 eV). This observation is explained on the basis of changes in the shape and overlap of the frontier molecular orbitals.     

From oligomers to polymer: convergence in the HOMO-LUMO gaps of conjugated oligomers

[Structure: see text] Extrapolation of HOMO-LUMO gaps for pi-conjugated oligomers at the B3LYP/6-31G(d) level of theory predict accurately (within 0.1-0.2 eV) the band gaps of conjugated polymers only when long (at least 20-mer) pi-conjugated oligomers are used for the extrapolation.     

基于含硫芳杂稠环化合物的可溶性共轭齐聚物的合成、表征及其在有机薄膜晶体管中的应用

设计并合成了4个基于含硫芳杂稠环化合物的可溶性共轭齐聚物,即以3-十一烷基苯并[d,d’]噻吩并[3,2-b;4,5-b ’]并二噻吩(BTTT)为末端芳香单元,噻吩(T)、二噻吩(bT)、N-十二烷基-二噻吩并[3,2-b]吡咯(TP)或2,5-双(3-十二烷基噻吩)[3,2-b]并二噻吩 (qT)为中间芳香单元的...     

Fourier transform ion cyclotron resonance study of multiply charged aggregates of small singly charged peptides formed by electrospray ionization

Aggregates of singly protonated peptides formed with a nanoelectrospray ion source have been observed in the gas phase using Fourier transform ion cyclotron resonance (FT-ICR). Employment of “soft” ion sampling conditions in the source, which were developed previously to generate water clusters of biomolecules, provides significant yields of aggregates of singly protonated GGDPG ([2GGDPG + 2H]²⁺), GGEPG ([2GGEPG + 2H]²⁺), and VEPIPY (2VEPIPY + 2H]²⁺). With peptide mixtures, heteroaggregates, e.g., [GGDPG + GGEPG + 2H]²⁺ have also been observed along with the homoaggregates. These weakly bound noncovalent complexes undergo facile exothermic dissociation into the corresponding singly protonated monomer species with normal operation of the electrospray ion source. For example, the aggregates were not observed in FT-ICR experiments utilizing a conventional electrospray ionization (ESI) or fast atom bombardment source or with a quadrupolar ion trap mass spectrometer equipped with a conventional ESI source. The formation and metastability of these aggregates are dependent on highly specific intermolecular hydrogen bonding between the monomers. The amino acid sequence (DPG) of GGDPG mimics the well-known β reverse turn of proteins and semiempirical calculations show that it provides excellent hydrogen bonding sites for a protonated N-terminus amino group. Support for this conjecture is provided by the failure to observe aggregate formation of singly protonated peptides with several larger peptides, including hexaglycine and hexaalanine.     

Effect of chromophore-charge distance on the energy transfer properties of water-soluble conjugated oligomers

The synthesis of 1,4-bis(9,9'-bis(3"-(N,N,N-trimethylammonium)-propyl)-2'-fluorenyl)benzene tetrabromide (C3), 1,4-bis(9,9'-bis(4"-(N,N,N-trimethylammonium)-butyl)-2'-fluorenyl)benzene tetrabromide (C4), 1,4-bis(9,9'-bis(6"-(N,N,N-trimethylammonium)-hexyl)-2'-fluorenyl)benzene tetrabromide (C6), and 1,4-bis(9,9'-bis(8"-(N,N,N-trimethylammonium)-octyl)-2'-fluorenyl)benzene tetrabromide (C8) is reported. Fluorescence energy transfer experiments between C3-C8 and the acceptors pentasodium 1,4-bis(4'(2",4"-bis(butoxysulfonate)-styryl)styryl)-2-(butoxysulfonate)-5-methoxybenzene (3), fluorescein labeled single-stranded DNA and fluorescein labeled double-stranded DNA in water, buffer, and methanol reveal the importance of hydrophobic and electrostatic forces in determining chromophore-chromophore close proximity. In water, the oligomers with longer side chain length show better energy transfer, as well as higher Stern-Volmer quenching constants (K(sv)), largely due to a stronger hydrophobic attraction between the optically active components. In methanol, the differences in energy transfer are leveled, and the oligomers with shorter side chain lengths show higher K(sv) values. Compounds C3, C4, C6, and C8 were also used to dissect the different contributors to DNA hybridization assays based on cationic conjugated polymers.     

Constructing hybrid films of conjugated oligomers and gold nanoparticles for efficient photoelectronic properties

Hybrid films of conjugated oligomers and gold nanoparticles have been fabricated by a coordination approach on the surface of gold nanoparticles. This method facilitates mild linkage of the oligomer/nanoparticle layers in ambient conditions, which provides a general route for preparation of organic/inorganic interlayer superstructures. Characterization of the as-obtained hybrid film has been carried out by UV-vis absorption, fluorescence spectroscopy, and atomic force microscopy (AFM). The hybrid film exhibits dramatic changes in both optical and photovoltaic properties upon encapsulation of fullerene. A photoelectrical application is presented by taking electrochemical measurements of the self-assembled film. The results reveal potential technological uses in photovoltaic devices.     

Structure and electric properties of the hydration shell of a singly charged chloride ion in a nanopore with hydrophilic walls

The effect of hydrophilic walls on the structure of the hydration shell of a Cl^? ion is studied in terms of the model flat nanopore in contact with water vapors at room temperature by the Monte Carlo computerassisted simulations. In the field of hydrophilic walls, the hydration shell falls into two parts: the ion-enveloping part and the molecular-film spots spread over the wall surface above and under the ion. Both parts have the pronounced radial-layered structure. The three-dimensional scheme of distribution of the averaged local shell density represents a system of conical coaxial layers expanding in the direction from wall to ion. The effect of forcing out the ion from its own hydration shell is also observed for hydrophilic walls. The specific electric polarizability of the shell is strongly anisotropic. Its longitudinal component is several times larger than the transversal component and behaves nonmonotonically as the hydration shell grows, passing through the maximum. The molecular order near the walls is characterized by the preferential orientation of the molecule plane in parallel to the wall plane and the turn of symmetry axes of molecules in the direction parallel to the normal to the pore plane in the vicinity of the ion.     

Structural properties of polystyrene oligomers in different environments: a molecular dynamics study

We have performed detailed molecular dynamics simulations to investigate the effects of solvation and confinement on the structure of polystyrene (PS) oligomers in four different environments, melt, concentrated solution, dilute solution and confined concentrated solution at 450 K and 1 bar, respectively. Local packing of the monomers and the solvent (toluene, good solvent) molecules were monitored by means of radial distribution functions (RDFs). We have also investigated bond, angle, and dihedral angle distributions of the monomers. End-to-end distances, radii of gyration and persistence lengths were calculated to characterize the static properties. The chain in the dilute solution was found to exhibit more stretched conformations. Dilution effect of the solvent was observed in the RDFs between the monomer centers. Only slight conformational changes in the polymers were observed by solvation. The effect of confinement was mainly seen in the density profiles, which showed an oscillatory behavior in the confined system.     

Computational study of the structure, dynamics, and photophysical properties of conjugated polymers and oligomers under nanoscale confinement

Computational simulations were used to investigate the dynamics and resulting structures of several para-phenylenevinylene (PPV) based polymers and oligomers (PPV, 2-methoxy-5-(2'-ethyl-hexyloxy)-p-phenylenevinylene --> MEH-PPV and 2,5,2',5'-tetrahexyloxy-7,8'-dicyano-p-phenylenevinylene --> CN-PPV). The results show how the morphology and structure are controlled to a large extent by the nature of the solute-solvent interactions in the initial solution-phase preparation. Secondary structural organization is induced by using the solution-phase structures to generate solvent-free single molecule nanoparticles. Isolation of these single molecule nanostructures from microdroplets of dilute solution results in the formation of electrostatically oriented nanostructures at a glass surface. Our structural modeling suggests that these oriented nanostructures consist of folded PPV conjugated segments with folds occurring at tetrahedral defects (sp3 C-C bonds) within the polymer chain. This picture is supported by detailed experimental fluorescence and scanning probe microscopy studies. We also present results from a fully quantum theoretical treatment of these systems which support the general conclusion of structure-mediated photophysical properties.     

单分散共轭齐聚物的合成方法

简要介绍了线性、星形和环状单分散共轭齐聚物的主要合成方法,并对不同方法的优缺点做了简要评述.其中,线性单分散共轭齐聚物的合成方法主要有4种,即控制聚合方法,发散合成方法,发散/收敛合成方法和指数增长合成方法.在星状单分散共轭齐聚物方面,主要介绍了两种苯环成环反应的应用.环状共轭齐聚物的合成主要介绍了聚合过程中的成环反应、线性共轭齐聚物的分子内环化反应和模板导向环化反应.