Donor-acceptor Compounds Research Articles

Benzo[1,2,5]selenadiazole bridged amines: electro-optical properties

New diamines and tetramines bridged by a low band gap rendering chromophore benzo[1,2,5]selenadiazole were synthesized by following palladium-catalyzed methodologies developed by Hartwig, Stille, or Suzuki. These donor–acceptor compounds display unique absorption and emission characteristics, which are dependent on the strength of the donors.

TTF derivatives containing fused-pyrazine rings for novel donor- acceptor systems

In this work, novel TTF derivatives fused with electron-accepting pyrazine rings are described. We have succeeded in preparing two kinds of TTF derivatives containing two pyrazine rings. Furthermore, we have prepared asymmetrical TTF derivatives containing one pyrazine ring to increase the electron-donating properties. In some cases, hydroxyl groups were introduced to induce intcrmolecular hydrogen bonds. Oxidation of hydroquinone derivatives afforded a new type of donor-acceptor compounds with quinone units, which showed both oxidation and reduction potentials. The electrical conductivity of the D-A compound was about 10 −8 S/cm as single component. We have also prepared several CT complexes based on the new TTF derivatives and investigated the structures and properties.

Ferrocene triphenylmethyl radical donor-acceptor compounds. Towards development of multifunctional molecular switches

Ferrocene-substituted polychlorinated triphenylmethyl radical (1) is a neutral paramagnetic donor-acceptor dyad that exhibits an intramolecular charge-transfer band in the near-infrared (NIR) region. It can be converted reversibly into its diamagnetic precursors 1 - and 1H by chemical reduction and subsequent acid/base reaction or oxidized to the ferrocenium diradical 1 + ; each interconversion being accompanied with drastic changes of the optical properties. The new synthetic route described here allows the isolation of K + (18-Crown-6) (1) - salt in crystalline form, making possible the X-ray crystal structure determination. Because all derivatives exhibit extraordinary chemical and thermal stability, both in solution and in the solid state, radical 1 may be used as a model compound for the development of new multifunctional molecular switches.

Electrogenerated Chemiluminescence. 81. Influence of Donor and Acceptor Substituents on the ECL of a Spirobifluorene-Bridged Bipolar System

The electrochemistry and radical ion annihilation electrogenerated chemiluminescence (ECL) of 9,9'-spirobifluorene-bridged bipolar systems containing 1,3,4-oxadiazole-conjugated oligoaryl and triarylamine substituents were investigated. The stability of the oxidized spirobifluorenes was improved by functionalization with triarylamine centers. These donor-acceptor (DA) compounds exhibited a good fluorescence efficiency with an emission maximum that correlated with the potential difference between radical anion and cation formation, suggesting a charge transfer (CT) emission band. An ECL mechanism based on the formation of the CT excited state by radical ion annihilation or production of the triplet state followed by triplet-triplet annihilation, with perhaps some excimer contribution, is proposed.

Computation of quantum mechanical hybridization and dipole correlation of the electronic structure of the F3B–NH3supermolecule

AbstractA valence bond description of the electronic structure and the process of the formation of the adduct supermolecule, F3B–NH3by charge transfer interactions between a Lewis acid, BF3, and a Lewis base, NH3, is computed in terms of the quantum mechanical localized molecular orbital (LMO) method. A dipole correlation of electronic structure is also reported. The valence bond description in terms of bond pair and lone pair is transparent in the generated LMOs of the donor, acceptor, and adduct supermolecule. It is transparent in the computed LMOs that the lone pair of N in NH3is actually involved in forming a new dative bond between B and N atoms. The present work reports the evaluated hybridization status of the boron, nitrogen, and fluorine atoms in the interesting donor–acceptor compound, F3B–NH3. It is transparent in the computed results that the quantum mechanical hybridization automatically incorporates the effect of change of environment in the pattern of hybridization perfectly in accordance with Bent's rule. The calculated dipole moment of the supermolecule is high; it is partitioned into contributions resulting from bond moments and atomic dipoles, as contributions from hybrid orbitals accommodating lone pairs of electrons. The magnitude of atomic dipole is small compared with the bond dipole component. A rationale of the bond moment of the molecule is attempted in terms of the charge density distribution on atomic sites and the atomic dipole is correlated in terms of the pattern of hybridization of lone pairs on the F atoms. The generated LMOs reveal that only one lone pair on each F atom is in a hybrid orbital, and the remaining two lone pairs are in pureporbitals. The small lone pair contribution to molecular dipole is thus transparent in computed quantum mechanical hybridization that shows that, out of three, only one lone pair on the F atoms can set in atomic dipole. This study demonstrates that atomic dipole can be used as a descriptor of the hybridization status of lone pairs on atoms in the molecules. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Versatile Self‐Complexing Compounds Based on Covalently Linked Donor–Acceptor Cyclophanes

A range of covalently linked donor-acceptor compounds which contain 1) a hydroquinone (HQ) unit, 2) a 1,5-dioxynaphthalene (DNP) ring system, or 3) a tetrathiafulvalene (TTF) unit as the pi-donor, and 4) cyclobis(paraquat-p-phenylene) (CBPQT(4+)) as the pi-accepting tetracationic cyclophane were prepared and shown to operate as simple molecular machines. The pi-donating arms can be included inside the cyclophane in an intramolecular fashion by virtue of stabilizing noncovalent bonding interactions. What amounts to self-complexing/decomplexing equilibria were shown to be highly temperature dependent when the pi-donating arm contains either an HQ or DNP moiety. The thermodynamic parameters associated with the equilibria have been unraveled by using variable-temperature (1)H NMR spectroscopy. The negative DeltaH degrees and DeltaS degrees values account for the fact that the "uncomplexed" conformation becomes the dominant species, since the entropy gain associated with the decomplexation process overcomes the enthalpy loss resulting from the breaking of the donor-acceptor interactions. The arm's in-and-out movements with respect to the linked cyclophanes can be arrested by installing a bulky substituent at the end of the arm. In the case of compounds carrying a DNP ring system in their side arm, two diastereoisomeric, self-complexing conformations are observed below 272 K in hexadeuterioacetone. By contrast, control over the TTF-containing arm's movement is more or less ineffective through the thermally sensitive equilibrium although it can be realized by chemical and electrochemical ways as a result of TTF's excellent redox properties. Such self-complexing compounds could find applications as thermo- and electroswitches. In addition, the thermochromism associated with the arm's movement could lead to some of the compounds finding uses as imaging and sensing materials.

Studies on TMPD:TCNB; a donor-acceptor with room temperature paramagnetism and n-pi interaction.

A donor-acceptor compound based on N,N,N',N'-tetramethyl-p-phenylene-diamine and 1,2,4,5-tetracyanobenzene (TMPD:TCNB) has been synthesized. The crystal structure of the black 1:1 complex formed between TMPD and TCNB has been determined by single crystal X-ray diffraction at room temperature. The compound crystallizes in the triclinic space group P-1 with cell dimensions: a = 7.4986(15) å, b =7.6772(11) å, c = 8.0764(15) å, alpha = 78.822(12) degrees, beta = 83.3779(19) degrees, gamma = 86.289(17) degrees .TMPD and TCNB molecules are stacked alternately in infinite columns along the a-axis. The structure does not seem to show the usual pi-pi interaction between the two aromatic rings, but indicates an n-pi interaction localized between the nitrogen atoms of the donor and the cyano groups of the acceptor.

The meso–β-linkage as structural motif in porphyrin-based donor–acceptor compounds

Synthetic strategies for using the meso–β-linkage as a structural motif in electron transfer mimics have been tested. Exploratory syntheses of directly meso–β-linked bis- and trisporphyrins and the first representative X-ray structure of a meso–β-linked bisporphyrins are reported. The structure reveals a unique form of intramolecular π–π stabilization between one porphyrin and a meso-aryl substituent in a second porphyrin unit that accounts for the stability of different atropisomers in trimers. Using β-formyl porphyrins, dipyrromethanes, and suitable quinone precursor aldehydes, mixed condensations gave convenient access to porphyrin–porphyrin–quinone (P–P–Q) donor–acceptor systems consisting of a meso–β-linked bisporphyrin, a spacer, and a quinone acceptor.

Novel Fluorophores: Efficient Synthesis and Photophysical Properties

The synthesis of new donor-acceptor acetylene-bridged compounds was realized via organophosphorus reactions and Sonogashira couplings. These systems exhibit high fluorescence quantum yields, and excitation induces very efficient charge redistribution in the molecules. [structure: see text]

Time-resolved measurement system of electrofluorescence spectra

A time-resolved measurement system for electrofluorescence spectra (plots of electric-field-induced change in fluorescence intensity as a function of wavelength) has been constructed by combining the time-correlated single-photon-counting system for emission decay measurement with modulated electric field as a sample bias. Four fluorescence decays, which correspond to positive, zero, negative, and zero sample bias, respectively, can be measured simultaneously, and a small field-induced change in decay profile can be detected precisely by using the constructed system. Measurements of time-resolved electrofluorescence spectra have been performed in electron donor–acceptor compounds which show a photoinduced electron transfer.

Decay pathways of charge-separated states in strongly fluorescent electron donor-acceptor compounds.

The nonradiative decay pathways are studied of the 1-cyano-4-cyclohexylidenenaphthalene chromophore and electron donor acceptor compounds containing the same chromophore. It is argued that the rapid decay at room temperature of the locally excited singlet state of the electron acceptor chromophore involves twisting of the double bond via a low barrier, and a polar transition state. In non-polar solvents the charge transfer excited states also decay mostly via a thermally activated decay process, possibly via a similar pathway as the locally excited state of the acceptor unit. In more polar solvents, decay via direct charge recombination to the ground state obeys the energy gap law, but in contrast to the prediction of semiclassical Marcus theory an activation energy of 1-2 kcal mol(-1) is found.

Chiral Reagents for the Determination of Enantiomeric Excess and Absolute Configuration Using NMR Spectroscopy

AbstractFor Abstract see ChemInform Abstract in Full Text.

Structural, magnetic, and optoelectronic properties of (diimine)(dithiolato)platinum(II) and -palladium(II) complexes and their charge-transfer adducts with nitrile acceptors.

Two new diimine dithiolato complexes, (dbbpy)Pt(dmid), 1, and (dbbpy)Pd(dmid), 2, were prepared and characterized (dbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine; dmid = 2-oxo-1,3-dithiole-4,5-dithiolate). Both complexes interact with the nitrile acceptor TCNQ, and 1 also interacts with TCNQF(4) and TCNE (TCNQ = 7,7,8,8-tetracyanoquinodimethane; TCNQF(4) = 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane; TCNE = tetracyanoethylene) to form supramolecular 2:1 charge-transfer solids that stack in the manner -DDADDADDA- (D = electron donor; A = electron acceptor). All compounds have been fully characterized by magnetic, spectroscopic, electrochemical, and single-crystal X-ray crystallographic analyses. Magnetic susceptibility studies of the charge-transfer compounds revealed that the platinum-based complexes exhibit temperature-independent paramagnetism of approximately 10(-3) emu/mol. The donor complexes exhibit continuous absorption bands across the UV/visible and into the NIR region. Upon interaction with the nitrile acceptors, the extinction coefficients of the absorption bands increase and the energies of some d-d transitions in the NIR region change. The donor-acceptor compounds possess desired spectral features for solar cell dyes, but low conversion efficiencies resulted when a representative compound was tested in a TiO(2) solar cell. The results, however, serve to illustrate that the donor-acceptor interactions persist in solution and the adsorption of the dye molecules to the semiconductor surface occurs in the absence of typical anchoring groups. Evaluation of the spectral and electrochemical data for the title compounds and the results of the preliminary solar cell study serve as guides for future research in choosing promising candidates for efficient solar cell dyes.

Chiral reagents for the determination of enantiomeric excess and absolute configuration using NMR spectroscopy.

Recent advances in the development of chiral derivatizing and solvating agents that facilitate the determination of enantiomeric excess and absolute configuration are reviewed. These include metal-containing species, host-guest systems, donor-acceptor compounds, and liquid crystal discriminating agents. In the aggregate, these reagents can be used to analyze a wide range of compound classes.

Orientational effect on the photophysical properties of quaterthiophene-C60 dyads.

Two quaterthiophene-[60]fullerene dyads in which C60 is singly (4TsC) or doubly (4TdC) connected to the inner beta-position of the terminal thiophene rings have been synthesized. The electronic properties of these donor-acceptor compounds were analyzed by UV/Vis spectroscopy and cyclic voltammetry, and their photophysical properties in solution and in the solid state by (time-resolved) photoluminescence (PL) and photoinduced absorption (PIA) spectroscopy. Both the flexible and geometrically constrained 4TsC and 4TdC dyads exhibit photoinduced charge transfer from the quaterthiophene to the fullerene in toluene and o-dichlorobenzene (ODCB). In toluene, charge transfer occurs in both dyads by an indirect mechanism, the first step of which is a singlet-energy transfer from the 4T(S1) state to the C60(S1) state. In the more polar ODCB, direct electron transfer from 4T(S1) competes with energy transfer, and both direct and indirect charge transfers are observed. The geometrical fixation of the donor and acceptor chromophores in 4TdC results in rate constants for energy and electron transfer that are more than an order of magnitude larger than those of the flexible 4TsC system. For both dyads, charge recombination is extremely fast, as inferred from picosecond-resolved temporal evolution of the excited state absorption of the 4T.+ radical cation both in toluene and ODCB.

Electronic Tunneling through Self-Assembled Monolayers of Phenylene Oligomers on Au(111)

We investigated the electrical properties of self-assembled monolayers (SAMs), which are composed of a single component of benzylic derivatives of oligophenylene thiols, HSCH2(C6H4)m-1C6H5 (m=1, 2, 3) on an Au(111) substrate, using conducting atomic force microscopy (AFM). The use of an Au-coated AFM tip with a decanethiol SAM resulted in a bilayer junction at the contact area between the Au-coated tip with the decanethiol SAM and the Au(111) substrate covered with one of the phenylene oligomer SAMs, i.e., Au-decanethiol SAM/oligophenylene SAM-Au(111). The electrical properties of the phenylene oligomers were studied in terms of the dependence of the observed tunneling current on the length of the oligomers at the same constant sample bias voltage. An exponential decrease in the tunneling current was observed with increasing length of the oligomers. The decay constant, β, of the tunneling current through the SAMs is related to the electronic structure of the molecules. The present study gave a β value of 5.5 nm-1 for the phenylene oligomers by measuring the current–voltage (I–V) curves of the bilayer junctions and plotting the resulting current (at +1.0 V) as a function of the length of the oligomers. The I–V curves were symmetrical around V=0. The value of β did not depend significantly on the applied voltage and was close to reported ones of 6.6 nm-1 for electron transfer through thin organic films of a series of corresponding Hg-hexadecanethiol SAM/oligophenylene SAM-Ag junctions and of 4.0 nm-1 for photoinduced electron transfer in bis-porphyrin donor-acceptor compounds with polyphenylene spacers. The slight difference among these three values was attributed to the difference in barrier height not for electron transfer but for hole transfer through the phenylene oligomers.

Photophysics of novel donor–{saturated rigid hydrocarbon bridge}–acceptor systems exhibiting efficient excitation energy transfer

Intramolecular singlet–singlet energy transfer is reported in a series of novel donor–{saturated rigid hydrocarbon bridge}–acceptor (D–B–A) molecules containing dimethoxynaphthalene (DMN) or dimethoxybenzene (DMB) as the energy donor chromophore, and a dione group as the acceptor. In the case of the DMN donor, the efficiency of energy transfer has been studied as a function of the bridge length with the number of sigma bonds linking the donor and acceptor groups ranging from 6 to 12. Highly efficient energy transfer is observed in all the D–B–A molecules studied, and the results are discussed with reference to the role of dipole–dipole and through-bond donor–acceptor coupling mechanisms. The energy transfer process facilitates efficient photosensitized decomposition of the dione moiety. These results are compared to data published on related donor–acceptor compounds. The chemical structure of the backbone linking the donor and acceptor is shown to have a marked effect on the photophysics of the molecules.

Electron donor–acceptor compounds: exploiting the triptycene geometry for the synthesis of porphyrin quinone diads, triads, and a tetrad

Rigidly and covalently linked porphyrin quinones are well established as model compounds for studying photo-induced electron transfer (PET) reactions like those occurring during the primary processes of photosynthesis. In this context, the synthesis of a number of porphyrin quinones is reported in which one or two porphyrin electron donors are connected to either one or two quinone electron acceptors, resulting in diad, triad, and tetrad model systems, respectively. The porphyrin(s) and the quinone(s) are linked by triptycene, 1,4-phenylene, and cis- or trans-1,4-cyclohexylene bridges. The use of the 1,4-phenylene, and cis- or trans-1,4-cyclohexylene bridges results in donor–acceptor compounds with the same number of bonds between donor and acceptor(s), but differing in distance and orientation. Analysis of the 1H NMR spectra confirmed the chair conformation for the cis- and trans-cyclohexylene-linked diads and triads. NOE experiments gave information about the spatial arrangement of the target compounds. The key compounds in the syntheses of all these new PET model systems are the triptycene quinones, which are formed via a [πs4+πs2]-cycloaddition between an anthracene derivative and a suitable quinone. The triptycene system enforces a rigid orientation on the quinone acceptor(s) in the final model system. Evidence is given that the triptycene system has further potential for constructing tailor-made donor–acceptor compounds.

A donor-acceptor compound composed of a dinuclear zinc(II) complex of a robson macrocycle with 8-methylquinoline

A donor-acceptor compound, diaqua-1kappaO,2kappaO-[&mgr;-11, 23-dimethyl-3,7,15,19-tetraazatricyclo[19.3.1.1(9,13)]hexacosa-1(25), 2,7,9,11,13(26),14,19,21,23-decaene-25,26-diolato-1kappa(4)N(3),N(7), O(25),O(26):2kappa(4)N(15),N(19),O(25),O(26)]dizinc(II) diperchlorate bis(8-methylquinoline) ethanol disolvate, [Zn(2)(C(24)H(26)N(4)O(2))(H(2)O)(2)](ClO(4))(2).2C(10)H(9)N. 2C(2)H(6)O, obtained by the reaction of a dinuclear zinc(II) complex of a Robson macrocycle (acceptor) and 8-methylquinoline (donor), lies about an inversion centre and the coordination about the unique Zn atom is a distorted square pyramid. The fifth coordination site is occupied by the water molecule, Zn-O = 2.016 (2) A, and the average macrocyclic Zn-O and Zn-N distances are 2.059 (6) and 2.059 (3) A, respectively.

Nonlinear optics of molecular donor–acceptor compounds

Molecular donor–acceptor–donor trimer is considered as a model system to study the effects of electron-intramolecular vibration coupling on the nonlinear optical properties. The static polarisability α and the static second hyperpolarisability γ are calculated and their dependencies on the model parameters are analysed. The compounds with molecules of low symmetry are predicted to have a potential for a high nonlinearity. Sharp changes of hyperpolarisabilities are expected upon neutral-to-ionic transition in the bis-tetrathiafulvalene-iodanil compound.