Electrochemistry and photophysics of donor-substituted triarylboranes: symmetry breaking in ground and excited state

We synthesized a series of amino substituted triarylboranes (TABs) 1-3 by copper(I)-catalyzed cross-coupling reactions. The title compounds were investigated by means of cyclic voltammetry (CV) and UV-visible absorption and fluorescence spectroscopy. Electrochemical oxidation of tris(4-carbazolyl-2,6-dimethylphenyl)borane (3) leads to the formation of an electroactive polymer film on the electrode surface. The charge-transfer (CT) absorption band of all three TABs shows a pronounced negative solvatochromism, while the emission is positively solvatochromic. By combining Jortner's theory, AM1 computations, and electrooptical absorption measurements (EOAM), this unexpected behavior was shown to be due to a dipole inversion upon S0-->S1 excitation. Furthermore, polarized steady-state fluorescence spectroscopy and EOAM prove that the ground-state geometry of 3 is of lower symmetry than D3 and that the excitation energy can be transferred from one subchromophore to another within the lifetime of the excited state. Exciton-coupling theory was used to quantitatively analyze this excitation transfer.     

Ultrafast charge-transfer dynamics of donor-substituted truxenones

The photophysics of two donor-substituted truxenone derivatives has been studied by femtosecond time-resolved transient absorption spectroscopy. The systems consist of a central truxenone acceptor with three triarylamine (TARA) branches which act as electron donors. Upon excitation in the visible regime an electron is transferred from the donor to the acceptor, generating a charge-separated state. This state can be probed via the characteristic absorption of the TARA radical cation around 700 nm. A second absorption band around 420 nm exhibits the same kinetics and is assigned to an absorption of the radical anion of the truxenone moiety. The back electron transfer and the recovery of the ground state can be interpreted within the framework of Marcus theory. To study the dependence of the back electron transfer on the electronic coupling, the distance between the donor and the acceptor was adjusted. Two solvents were employed, dimethylsulfoxide and dichloroethane. A biexponential decay of the bands assigned to the charge-separated state was observed, with time constants in the picosecond range. Surprisingly, the rates for electron back transfer do not follow the simple picture of the donor-acceptor distance being the determining factor. The observations are explained within a model that additionally takes steric interactions between the donor and the acceptor into account.     

Reactions of donor-substituted furans with electrophilic ethylenes

2-Methoxyfuran and 2-p-tolyloxyfuran were attacked at the 5-position by tetracyanoethylene or the super-electrophilic 2,2-bis(trifluoromethyl)ethylene-1,1-dicarbonitrile affording (Z)-3-cyclopropylacrylic esters, substituted in the 3-membered ring. Initially formed zwitterions undergo simultaneous opening of the furan and closure of the cyclopropane ring. In the presence of pyridine, 1,3-prototropy converts the zwitterion to 5-substitution products of the furans.     

Synthesis and electrogenerated chemiluminescence of donor-substituted phenylethynylcoumarins

Two series of donor-bearing phenylethynylcoumarins have been synthesized, and their photophysical properties have been evaluated. Chemiluminescence was observed through the annihilation of their electrogenerated radical ions and was found to be only slightly affected by the presence of various donor groups on the phenyl moiety linked through the C-C triple bond. The overall properties of the two series of compounds are discussed with respect to their structures. The observed electronic absorption properties are explained with the help of computational studies.     

Synthesis, electronic properties, and electrochemiluminescence of donor-substituted phenylethynylanthronitriles

A new series of pi-conjugated donor-acceptor compounds (1-6) with inherent redox centers have been prepared and studied with respect to their electronic properties. The photophysical characteristics of these compounds have been studied in relation to their structures. Cyclic voltammetry and UV-vis spectroelectrochemistry were used to probe the ground-state electronic properties of the neutral and charged species. The observed electronic absorption properties of the neutral and charged molecules are explained with the help of frontier orbital structures and electrostatic potential maps obtained from density functional theory (DFT, B3LYP/6-31G) calculations. Electrochemiluminescence (ECL) of this series of donor-substituted phenylethynylanthronitriles with different donors was also studied. The structure-property relationship of all of the compounds is discussed.     

Pi-conjugation in donor-substituted cyanoethynylethenes: an EDA study

Pi-conjugation in several donor-substituted cyanoethynylethenes was estimated using energy decomposition analysis (EDA); very good linear correlations between the DeltaE(pi) values and experimental data are found.     

三芳基硼烷在有机电子学领域的研究与应用

近年来,三芳基硼烷在氟离子和氰根离子检测方面的应用受到了越来越广泛的关注.由于具有空的pπ轨道,三芳基硼烷可作为路易斯酸通过路易斯酸-碱相互作用实现对氟离子和氰根离子(路易斯碱)的高选择性检测.硼原子与氟离子或氰根离子的结合破坏了硼中心与芳香取代基的pπ-π共轭,引起三芳基硼烷光物理性质的变化.另外,三芳基硼烷具有优异的发光和载流子传输性质,已被广泛应用于有机电致发光领域,成为一类非常重要的有机光电材料.本文总结了近年来报道的三芳基硼烷的结构特点、构效关系以及在阴离子传感和有机电致发光显示器件中的应用,并在此基础上提出了三芳基硼烷在研究和应用中尚待解决的问题,展望了这类材料的应用前景.     

Planarized triarylboranes: stabilization by structural constraint and their plane-to-bowl conversion

Triphenylborane and 9,10-diphenyl-9,10-dihydro-9,10-diboraanthracene, constrained to a planar arrangement with methylene tethers, were synthesized by intramolecular multi-fold Friedel-Crafts cyclization. These compounds were stable toward air, water, and amines, despite the absence of steric protection in the vertical direction with respect to the B atoms, and showed characteristic structural, electronic, and photophysical properties. In addition, upon treatment with a fluoride ion, these compounds underwent a plane-to-bowl conversion in a controlled manner.     

Enhancement of the Lewis acidity by substitution of sulfur-containing hetero aromatics in triarylboranes

Triarylboranes bearing two thiophene, dimethylthiophene, or benzothiophene units were synthesized. X-ray crystallographic analysis revealed the planar structure around the boron atom and non-bonding S-S interaction of the thiophene. They exhibited several intense absorption bands in the UV region. They were stable in the air for months and showed extremely strong Lewis acidities.     

Synthesis and electrogenerated chemiluminescence of donor-substituted phenylquinolinylethynes and phenylisoquinolinylethynes: effect of positional isomerism

In furtherance of our research on the design, synthesis and study of electrogenerated chemiluminescence (ECL) of new donor substituted phenylquinolinylethynes, we report here more new series with the aim of studying the effect of positional isomerism on their overall photophysical properties with a special focus on ECL. For this study we have chosen 2-, 3-, and 4-(p-substituted phenyl)ethynylquinolines, and 1- and 4-(p-substituted phenyl)ethynylisoquinolines. These ethynes were synthesized in good yields by modified Sonogashira coupling of the corresponding terminal alkyne with the respective haloquinolines. The photophysical properties and ECL were studied in acetonitrile solvent and the various results are discussed.     

New strong organic acceptors by cycloaddition of TCNE and TCNQ to donor-substituted cyanoalkynes

Donor-substituted 1,1,2,4,4-pentacyanobuta-1,3-dienes and a cyclohexa-2,5-diene-1,4-diylidene-expanded derivative were prepared by a [2 + 2] cycloaddition of tetracyanoethene (TCNE) or 7,7,8,8-tetracyanoquinodimethane (TCNQ) to anilino-substituted cyanoalkynes, followed by retro-electrocyclisation; they feature intense bathochromically-shifted intramolecular charge-transfer bands and undergo their first one-electron reductions at potentials similar to those reported for TCNE and TCNQ.     

One-electron-reduced and -oxidized stages of donor-substituted 1,1,4,4-tetracyanobuta-1,3-dienes of different molecular architectures

A series of monomeric and oligomeric donor-substituted 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs) with various topologies have been synthesized by means of thermal [2+2] cycloaddition between tetracyanoethylene (TCNE) and donor-substituted alkynes, followed by retro-electrocyclization. One-electron-reduced and -oxidized stages of the donor-substituted TCBDs were generated by chemical methods. The obtained radical anions and radical cations were studied by using electron paramagnetic resonance/electron nuclear double resonance (EPR/ENDOR) spectroscopy, supported by density functional theory (DFT) calculations. The extent of pi-electron delocalization in the paramagnetic species was investigated in terms of the EPR parameters. Despite favorable molecular orbital (MO) coefficients, the EPR results suggest that in radical anions the spin and charge are confined to the electron-withdrawing TCBD moieties on the hyperfine EPR timescale. The observed spin localization is presumably caused by an interplay between the nonplanarity of the studied pi systems, limited pi-electron conjugation, and very likely counterion effects. In radical cations, an analogous spin and charge localization confined to the electron-donating N,N-dialkylaniline moieties was found. In this case, an efficient electron delocalization is disabled by small MO coefficients at the joints between the donor and acceptor portions of the studied TCBDs.     

Symmetry-dependent vibrational excitation in N 1s photoionization of N2: experiment and theory

We have measured the vibrational structures of the N 1s photoelectron mainline and satellites of the gaseous N2 molecule with the resolution better than 75 meV. The gerade and ungerade symmetries of the core-ionized (mainline) states are resolved energetically, and symmetry-dependent angular distributions for the satellite emission allow us to resolve the Sigma and Pi symmetries of the shake-up (satellite) states. Symmetry-adapted cluster-expansion configuration-interaction calculations of the potential energy curves for the mainline and satellite states along with a Franck-Condon analysis well reproduce the observed vibrational excitation of the bands, illustrating that the theoretical calculations well predict the symmetry-dependent geometry relaxation effects. The energies of both mainline states and satellite states, as well as the splitting between the mainline gerade and ungerade states, are also well reproduced by the calculation: the splitting between the satellite gerade and ungerade states is calculated to be smaller than the experimental detection limit.     

The solvation number of ions obtained from their entropies of solvation

The entropy of solvation of an ion contains contributions from i) the change of the volume at its disposal, ii) long-range electrostatic effects, iii) immobilization of solvent molecules in the first solvation shell, and iv) effects on the structure of the solvent. The last item is important in water, but can be ignored in less structured solvents. Standard ionic entropies of transfer from water to a dozen solvents are used for the estimation of the entropy of solvent immobilization, and the (extrapolated) entropy of freezing of the solvent is then used to estimate the number of solvent molecules immobilized.Presented in part at the IX ICNAS (International Conference on Non-Aqueous Solutions), Pittsburgh, PA, August 1984.     

Electron transfer in a donor-substituted acridinium dye: evidence for dynamical solvent control

Time-resolved emission is used to explore the intramolecular excited-state electron transfer in a donor-substituted acridinium dye in a variety of solvents at room temperature. Emission decays are predominantly bi-exponential, with the faster of the two time constants being a good measure of the charge transfer time. In 14 high-polarity solvents this time is correlated to the solvation times previously measured with the solvation probe Coumarin 153 via an apparent power law, τ_LE→CT≈8.4〈τ〉_solv^0.65. In solvents less polar than acetone the times observed deviate widely from this relationship, probably due to the effects of ion pairing.     

Efficient treatment of solvation shells in 3D molecular theory of solvation

We developed a technique to decrease memory requirements when solving the integral equations of three‐dimensional (3D) molecular theory of solvation, a.k.a. 3D reference interaction site model (3D‐RISM), using the modified direct inversion in the iterative subspace (MDIIS) numerical method of generalized minimal residual type. The latter provides robust convergence, in particular, for charged systems and electrolyte solutions with strong associative effects for which damped iterations do not converge. The MDIIS solver (typically, with 2 × 10 iterative vectors of argument and residual for fast convergence) treats the solute excluded volume (core), while handling the solvation shells in the 3D box with two vectors coupled with MDIIS iteratively and incorporating the electrostatic asymptotics outside the box analytically. For solvated systems from small to large macromolecules and solid–liquid interfaces, this results in 6‐ to 16‐fold memory reduction and corresponding CPU load decrease in MDIIS. We illustrated the new technique on solvated systems of chemical and biomolecular relevance with different dimensionality, both in ambient water and aqueous electrolyte solution, by solving the 3D‐RISM equations with the Kovalenko–Hirata (KH) closure, and the hypernetted chain (HNC) closure where convergent. This core–shell‐asymptotics technique coupling MDIIS for the excluded volume core with iteration of the solvation shells converges as efficiently as MDIIS for the whole 3D box and yields the solvation structure and thermodynamics without loss of accuracy. Although being of benefit for solutes of any size, this memory reduction becomes critical in 3D‐RISM calculations for large solvated systems, such as macromolecules in solution with ions, ligands, and other cofactors. © 2012 Wiley Periodicals, Inc.     

A laser desorption ionization mass spectrometry investigation of triarylboranes and tri-9-anthrylborane photolysis products

Laser desorption ionization (LDI) mass spectra have been obtained for the photolysis products of tri-9-anthrylborane (TAB) in the solvents toluene, cyclohexene, and tetrahydrofuran; and for the solid triarylboranes: TAB, trimesitylborane (TMB), and tri-(2,6-dimethylphenyl)borane (TXyB). The major single solution photolysis product of TAB is 9,9′-dianthryl. Other products are rationalized in terms of an anthrylborylene intermediate (AnthB:). LDI mass spectrometry of the triarylboranes yields molecular ions in good abundance, providing a simple and selective method of characterization. Fragmentation patterns in some cases are interpreted in terms of photochemical reactions prior to ionization.     

Linear solvation energy relations

Solvents have been parameterized by scales of dipolarity/polarizability ^*, hydrogen-bond donor (HBD) strength , and hydrogen-bond acceptor strength . Linear dependence (LSER's) on these solvent parameters are used to correlate and predict a wide variety of solvent effects, as well as to provide an analysis in terms of knowledge and theoretical concepts of molecular structural effects. Some recent applications utilizing this approach are presented. Included are analyses of solvent effects on (a) the free energies of transfer of tetraalkylammonium halide ion pairs and dissociated ions, (b) rates of nucleophilic substitution reactions, (c) the contrast in solvent effects of water (HBD) and dimethyl sulfoxide (non-HBD) on the acidities of m- and p-substituted phenols, (d) partition coefficients of non-HBD solutes between solvent bilayers, and (e) family relationships between proton transfer (and non-protonic Lewis acid) basicities and corresponding values for monomer HBA. A comprehensive summary of LSER with references is given.Session lecture, Ninth International Conference on Non-Aqueous Solutions, Pittsburgh, PA, August 1984.     

The statistical thermodynamics of solvation

Statistical mechanics provides the basis for a rigorous theory of molecules and molecular complexes in chemically inert media. It is found that the free energy of solvation may be calculated, provided the distribution function for the solvent molecules around the solute molecules is known. A relation is deduced between the volume change on dissolving, the free energy of solvation, and the isothermal compressibility of the pure solvent. The general equations are applied to a solution of solid spheres in a structureless medium. The theoretical deductions are found to be in qualitative agreement with the thermodynamic solvation parameters for oxygen and argon in inert solvents.