Excitons in polymerized diacetylenes

A simple exciton theory for the excited electronic states of poly-2,4-hexadiyn-1,6-diol bis(p-toluene sulphonate) is described. Configuration interaction between bond excitons, charge transfer excitons and two-exciton states is included in the theory. The ? = 0 levels are described and their parentage traced to levels in two model compounds, vinyl acetylene and cyclododecatrienetriyne. The 2 eV transition of the polymer is predicted to be a collective state of the bond and charge transfer excitons with no two-exciton component. The lowest triplet state is predicted to lie below the first singlet and to also have extensive charge transfer character.     

Spiral Intramolecular Charge Transfer and Large First Hyperpolarizability in Möbius Cyclacenes: New Insight into the Localized π Electrons

Much effort has been devoted to investigating the unusual properties of the π electrons in Möbius cyclacenes, which are localized in a special region. However, the localized π electrons are a disadvantage for applications in optoelectronics, because intramolecular charge transfer is limited. This raises the question of how the intramolecular charge transfer of a Möbius cyclacene with clearly localized π electrons can be enhanced. To this end, [8]Möbius cyclacene ([8]MC) is used as a conjugated bridge in a donor–π‐conjugated bridge–acceptor (D–π–A) system, and NH₂‐6‐[8]MC‐10‐NO₂ exhibits a fascinating spiral charge‐transfer transition character that results in a significant difference in dipole moments Δμ between the ground state and the crucial excited state. The Δμ value of 6.832 D for NH₂‐6‐[8]MC‐10‐NO₂ is clearly larger than that of 0.209 D for [8]MC. Correspondingly, the first hyperpolarizability of NH₂‐6‐[8]MC‐10‐NO₂ of 12 467 a.u. is dramatically larger than that of 261 a.u. for [8]MC. Thus, constructing a D–π–A framework is an effective strategy to induce greater spiral intramolecular charge transfer in MC although the π electrons are localized in a special region. This new insight into the properties of π electrons in Möbius cyclacenes may provide valuable information for their applications in optoelectronics.     

Transient photoconductivity of a polydiacetylene single crystal

Pulsed photoconductivity results are presented for single crystals of a fully-conjugated polymer, poly-2,4-hexadiyne-1,6-diol bis(p-toluene sulfonate). Carriers of both sign are observed and are found to have essentially identical action spectra with a photoconduction onset around 3 eV. Upper and lower limits could be placed on the charge carrier mobilities normal to the (011) crystallographic plain, which is roughly the polymer chain direction: 3 ? μ_e,h ? 10^?3 cm²/V s. The mobilities normal to the chain are about a factor of 10 less. The carrier lifetimes are about 0.5 μs for both holes and electrons but strongly dependent on the method of polymerization of the precursor monomer crystals.     

Vibration and X-ray photoelectron spectroscopies of FeCl3-doped poly(p-diethynylbenzene)

The doping mechanism of poly(p-diethynylbenzene), chemically doped with FeCl₃, was investigated. Absorption, infrared, far infrared, Raman, X-ray photoelectron spectroscopies were used to determine the nature of the dopant in doped polymer. The experimental results suggest that the charge transfer reaction between the polymer chain and the dopant results in the formation of FeCl₄⁻ species, the π electron charge delocalization along the polymeric chain and the reduction of π-π^* transition energy.     

Electroreflectance spectra of polycrystalline gallium in alkali solutions

The electroreflectance spectra (ES) of polycrystalline gallium in alkali solutions have two bands of charge transfer (BCT) from adsorbed water molecules to metal. The energy of one band equals about 4.6 eV and is potential-independent, which points to the presence of water molecules with a zero charge transfer at the metal surface. The energy of the other is lower and drops with increasing potential, which points to the absence of decomposition of adsorbed water to OH_ groups. The plasma ES on gallium is determined largely by mechanism governing electromodulation of the concentration of the surface anion centers where the metal’s free electrons are scattered. The mechanism provides the ES intensity commensurate with that of the water BCT     

Negative polaron and triplet exciton diffusion in organometallic "molecular wires"

The dynamics of negative polaron and triplet exciton transport within a series of monodisperse platinum (Pt) acetylide oligomers is reported. The oligomers consist of Pt-acetylide repeats, [PtL(2)-C≡C-Ph-C≡C-](n) (where L = PBu(3) and Ph = 1,4-phenylene, n = 2, 3, 6, and 10), capped with naphthalene diimide (NDI) end groups. The Pt-acetylide segments are electro- and photoactive, and they serve as conduits for transport of electrons (negative polaron) and triplet excitons. The NDI end groups are relatively strong acceptors, serving as traps for the carriers. Negative polaron transport is studied by using pulse radiolysis/transient absorption at the Brookhaven National Laboratory Laser-Electron Accelerator Facility (LEAF). Electrons are rapidly attached to the oligomers, with some fraction initially residing upon the Pt-acetylide chains. The dynamics of transport are resolved by monitoring the spectral changes associated with transfer of electrons from the chain to the NDI end group. Triplet exciton transport is studied by femtosecond-picosecond transient absorption spectroscopy. Near-UV excitation leads to rapid production of triplet excitons localized on the Pt-acetylide chains. The excitons transport to the chain ends, where they are annihilated by charge separation with the NDI end group. The dynamics of triplet transport are resolved by transient absorption spectroscopy, taking advantage of the changes in spectra associated with decay of the triplet exciton and rise of the charge-separated state. The results indicate that negative polarons and excitons are transported rapidly, on average moving distances of ~3 nm in less than 200 ps. Analysis of the dynamics suggests diffusive transport by a site-to-site hopping mechanism with hopping times of ~27 ps for triplets and <10 ps for electrons.     

Near ultraviolet spectroscopic studies of 2,4-diamino-6-piperidinopyrimidine-3-oxide (minoxidil) and 2,4-diamino-6-piperidinopyrimidine (desoxyminoxidil)

The near u.v. spectra of 2,4-diamino-6-piperidinopyrimidine (desoxyminoxidil) and 2,4-diamino-6-piperidinopyrimidine-3-oxide (minoxidil) can be viewed as perturbed pyrimidine spectra. The u.v. properties of pyrimidine and a series of aminopyrimidines, specifically 2,4,6-triaminopyrimidine, are examined to obtain u.v. spectral assignments for desoxyminoxidil and minoxidil. Minoxidil and its desoxy counterpart have C_s symmetry, and all π → π* absorptions are allowed ¹A′ ← ¹A′ transitions. The two lowest energy π →- π* absorptions observed in minoxidil (262 nm, 292 nm) are tentatively assigned as very mild oxygen → pyrimidine ring charge-transfer transitions. Intensity decreases in protic solvents, and the results of simple Hückel molecular orbital calculations indicate that the 292 nm transition has more charge-transfer character than the 262 nm absorption. The protonated species of desoxyminoxidil and minoxidil have very similar u.v. spectra. This is due to the lack of oxygen-related charge transfer in protonated minoxidil, and the high probability that the positive charge resides in similar environments in the minoxidil and desoxyminoxidil molecular frameworks.     

Exciton transitions in quasi-one-dimensional crystals. 9,10-dichloroanthracene

Polarized reflection spectra of the first singlet transition of the α-crystalline form of 9,10-dichloroanthracene are reported. Crystal faces (001), (011) and (010) were examined in spectral range 450 to 350 nm at two temperatures, 5 K and 300 K. Two systems of transitions were observed. The first system is assigned to neutral excitons. Spectral similarities with unsubstituted anthracene and arguments based on the one-dimensional stacking of molecules are used to construct a model of the exciten band structures. The M-polarized ππ* molecular transition gives rise to a four branch band with two allowed transitions. The 0-0_b (A_g → A_u) transition lies 50–100 cm^?1 above the bottom of the exciton band and the 0-0_c′ (A_g → B_u) transition lies at the top of the band. In the reflection spectrum the Davydov splitting c′b for transverse excitons is 210 cm^?1. The exciton band of the 00 molecular transition is not isolated but overlaps the two-particle manifold of the 0–1 vibronic transition. As a result of the 0–1_c′ transition is unexpectedly strong in the spectra of the (010) face. The second system is polarized along the stack-axis a and starts 2500 cm^?1 above the first system. It is tentatively assigned as |a(A_g → B_u) charge transfer exciton transition in agreement with earlier observations.     

Absorption spectra

The first band in the absorption spectra of the Z isomers of aurones, thioindogenides, and selenoindogenides is due to an electronic transition that is associated primarily with charge redistribution on the heteroatom and the carbonyl group. The effect of a substituent in the 4′ position plays a secondary role. The low sensitivity of the spectra of selenoindogenides to the effect of substituents is due to the saturation of the acceptor center (carbonyl group) by electrons drawn off from the heteroatom during interaction of the 2π _z electrons of selenium with the π system of the molecule.     

Cis and trans effects and NMR correlations in organometallic cobaloximes

A study has been made of electronic cis and rans influences in cobaloximes of the type [Co(dmgH)₂X(L)], where dmgH is dimethylglyoximato, L is 2,6-dimethylpyrazine or 3,5-lutidine, and X is an alkyl or inorganic anionic ligand. The downfield shifts of the methylic protons of the equatorial (dmgH) ligands in the ¹H NMR spectra are linearly related to the upfield shifts of the α-protons of L, and the shifts are correlated with the wavelength of the Co » dmgH charge transfer band in the ultraviolet. Such correlations have been previously observed only for nonorganometallic cobaloximes, and explained in terms of the ring current produced by the delocalized π electrons of the metallacycles. the alkyl groups have the smallest effects on the chemical shifts of both the cis and trans ligand.     

Sensitized photoconduction in a polydiacetylene crystal (DCHD)

The photoconductivity of DCHD displays a maximum near 3.6 eV coinciding with the maximum of the S_o → S₁ absorption of the carbazole group. It is attributed to a sensitization involving charge transfer from the excited chromophore to the chain. The rate constant for non-radiative decay of the carbazole singlet due to energy transfer to the chain is 1.6 × 10¹³ s^?1, and for charge transfer ≈ 3 × 10¹¹ s^?1.     

Two‐Dimensional Acetylenic Scaffolding: Extended Donor‐Substituted Perethynylated Dehydroannulenes

Starting from (Z)‐bis(N,N‐diisopropylanilino)‐substituted tetraethynylethene (TEE), perethynylated octadehydro[12]‐ and dodecadehydro[18]annulenes were prepared by oxidative Hay coupling. The dodecadehydro[18]annulene with six peripheral N,N‐diisopropylanilino substituents was characterized by X‐ray crystallography. Elongation of the Z‐bisdeprotected TEE by Cadiot–Chodkiewicz coupling with 1‐bromo‐2‐(triisopropylsilyl)ethyne provided a Z‐configured bis(butadiyne), which after alkyne deprotection afforded under Hay coupling conditions N,N‐diisopropylanilino‐substituted perethynylated hexadecadehydro[20]‐ and tetracosadehydro[30]an‐nulenes. The diisopropylanilino substituents enhance the properties of these unprecedented all‐carbon perimeters in several distinct ways. They ensure their solubility, increase their stability, and importantly, engage in strong intramolecular charge‐transfer interactions with the electron‐accepting all‐carbon cores, resulting in intense, bathochromically shifted charge‐transfer bands in the UV/Vis spectra. The charge‐transfer character of these bands was confirmed by protonation‐neutralization experiments. The redox properties of the new carbon‐rich chromophores were investigated by cyclic voltammetry and rotating disk voltammetry, which indicated different redox behavior for aromatic (4n+2 π electrons) and antiaromatic (4n π electrons) dehydroannulenes.     

Charge transport in luminescent polymers studied by in situ fluorescence spectroscopy

Modulation of the photoluminescence of poly-[2,7-(fluorene)-1,4-(phenylene)] can be attained by reversible electrochemical modification of the conjugated chain (p- or n-doping). Controlled injection of charge quenches the fluorescent emission of the conjugated polymer. The injection of holes completely eliminates the emission, while the electrons only quench up to one-third of the initial fluorescence of the polymer. Analogous quenching effects have been previously reported for solid-state organoelectronic devices. Electrochemical Stern-Volmer plots permit the estimation of the relative mobility of charge carriers in the polymer layer. The mobility of holes is 1 order of magnitude higher that the mobility of electrons, as determined by this method.     

四齿配体[Ru(iph)(L)2]2+ (L=cpy,mpy, npy)配合物的结构和光谱特征

采用密度泛函的B3LYP和UB3LYP方法分别优化了一系列[Ru(iph)(L)₂]²⁺ (L=cpy (1), mpy (2), npy (3); 其中iph为2,9-双(1′-甲基-2′-咪唑)-1,10-邻二氮杂菲, cpy为4-氰基嘧啶, mpy为4-甲基嘧啶, npy为4-氮二甲基嘧啶)配合物的基态和激发态结构. 利用含时密度泛函理论(TD-DFT)方法, 结合极化连续介质(PCM)模型计算了它们在丙酮溶液中的吸收和发射光谱. 研究结果表明: 优化得到的几何结构参数和相应的实验值符合得非常好. 1和2的最高占据分子轨道主要由金属的d轨道和iph配体的π轨道构成, 但是3主要占据在npy配体上, 而它们的最低空轨道主要由iph配体的π反键轨道占据. 因此, 1和2的最低能吸收和发射属于金属到配体(MLCT)和配体内部(ILCT)的电荷转移跃迁, 而3属于两个配体之间的电荷转移(LLCT)跃迁. 三个配合物的最低能吸收分别在509 nm (1), 527 nm (2)和563 nm (3), 其磷光发射分别在683 nm (1), 852 nm (2)和757 nm (3). 这显示出通过调节L配体的π电子给予能力可以改变最低能吸收和发射的跃迁性质和发光颜色.     

Optical properties of a polydiacetylene crystal: poly-[1,6-di(N-carbazolyl)-2,4-hexadiyne]

Normal incidence reflection spectra and Kramers-Kronig transforms are reported for a polydiacetylene crystal, DCHD (substituent group: N-carbazolymethyl). The lowest energy optical transition for the polymer backbone is found at 15 300 cm^?1, the lowest value obtained thus far for a polydiacetylene. The polymer chain and the carbazolyl substituent group may be treated as isolated chromophores - the former dominating the visible portion of the spectra and the latter dominating the UV portion. Polarization studies for the (001) and (10$\text{1}\text{?}$) faces of DCHD permit unambiguous assignment of the two lowest energy transitions in the carbazolyl group at 28 750 cm^?1 and 33 250 cm^?1 as short- and long-axis polarized, respectively.     

YTao4：Nb，Eu的发光性能研究

用高温固相反应法合成了铌酸根NbO^3-4和Eu^3 共掺杂的正钽酸盐化合物Y1－xEuxTa1-yNbyO4,研究该体系中紫外光和X射线激发下的发光性能,研究表明,在紫外光激发下,YTaO4:Nb,Eu是一种比较有效的红色发光材料,激发能可以通过NbO^3 4离子传递给Eu^3 ,随钽酸盐中NbO^3-4基团浓度的增中,化合物的结构从M'型YTaO4变成褐钇铌型YNbO4结构,它的发光性质也随之改变。     

Electronic absorption spectra of some arylidene pyrazolone derivatives

The u.v. and visible spectra of some 1 - phenyl - 3 - methyl - 4 - arylidene - 2 - pyrazolin - 5 - one derivatives are investigated in pure and mixed organic solvents as well as in aqueous buffer solutions. Electronic transitions have been identified as either locally excited or predominantly charge transfer states. Moreover, the spectra of the hydroxy derivatives in proton acceptor solvents (DMF, DMSO, ethanol) are characterized by an extra band located at longer wavelengths, which is ascribed to an intermolecular CT transition. This involves an electron transfer from the lone pair of electrons of the oxygen atom of the solvent molecules (ψ_ol) to the antibonding orbital of the substituent OH group. The spectral shifts are discussed in terms of medium effects and in relation to molecular structure. The variation of absorbance with pH is utilized for the determination of pK_a for the dimethylamino and hydroxy derivatives.     

Nickel(II) Complex of N4 Schiff Base Ligand as a Building Block for a Conducting Metallopolymer with Multiple Redox States

Metal–ligand interactions in monomeric and polymeric transition metal complexes of Schiff base ligands largely define their functional properties and perspective applications. In this study, redox behavior of a nickel(II) N₄-anilinosalen complex, [NiAmben] (where H₂Amben = N,N′-bis(o-aminobenzylidene)ethylenediamine) was studied by cyclic voltammetry in solvents of different Lewis basicity. A poly-[NiAmben] film electrochemically synthesized from a 1,2-dichloroethane-based electrolyte was investigated by a combination of cyclic voltammetry, electrochemical quartz crystal microbalance, in situ UV-Vis spectroelectrochemistry, and in situ conductance measurements between −0.9 and 1.3 V vs. Ag/Ag⁺. The polymer displayed multistep redox processes involving reversible transfer of the total of ca. 1.6 electrons per repeat unit, electrical conductivity over a wide potential range, and multiple color changes in correlation with electrochemical processes. Performance advantages of poly-[NiAmben] over its nickel(II) N₂O₂ Schiff base analogue were identified and related to the increased number of accessible redox states in the polymer due to the higher extent of electronic communication between metal ions and ligand segments in the nickel(II) N₄-anilinosalen system. The obtained results suggest that electrosynthesized poly-[NiAmben] films may be viable candidates for energy storage and saving applications.     

3-Substituted 2,4-pentanedione complexes: electronic spectra

The electronic spectra of 85 γ-substituted acetylacetonates of the metal(III) ions, Sc, V, Cr, Mn, Fe, Co, Ga and Al are discussed. Previously unassigned bands with energies > 40 kK (1 kK = 1000 cm^?1) in the spectra of the V(III) and Cr(III) complexes are assigned to the component of highest energy (e → a₁) of the π₃ π ₄ transition. Shifts in the ligand π₃ → π ₄ and charge transfer t_2g → π ₄ transitions which are induced by varying either the metal ion or ligand substituent are interpreted in terms of the nature (M → L or L → M) and extent of metal-ligand π-bonding. Two types of substituent effect on the transition energies are distinguished: the gross effect of replacing the γ-hydrogen atom by any substituent shifts the ligand and charge transfer transitions to lower energies, while the effect of replacing an electron-releasing substituent (e.g. CH₃) by an electron-withdrawing substituent (e.g. NO₂) generally induces a high energy shift in both transitions. From the ⁴A_2g → ⁴T_2g transition energies in the Cr(III) complexes (which yield 10Dq directly) electron-withdrawing γ-substituents are shown to increase the crystal field strength of the β-ketoenolate ligands.     

Spectroscopic study of hydrogen bonding in aminoanthrapyridones

By an analysis of the IR spectra of solutions of aminoanthrapyridones in chloroform it was demonstrated that the primary and secondary amino groups in the 1 and 6 positions form an intermolecular hydrogen bond with the CO groups; the amino groups in the 6 position form a stronger hydrogen bond. An anomalous form of the absorption band of the stretching vibrations of the NH group was detected in the IR spectrum of 6-n-hexylamino-3-ethylanthrapyridone. The long-wave band in the electronic spectra of aminoanthrapyridone is related to the band of charge transfer of the unshared pair of electrons of the amino nitrogen atom to the π system of the rings.