Two-photon absorption in quadrupolar pi-conjugated molecules: influence of the nature of the conjugated bridge and the donor-acceptor separation

Quadrupolar-type substitution of pi-conjugated chromophores with donor and acceptor groups has been shown to increase their two-photon absorption (TPA) response by up to two orders of magnitude. Here, we apply highly correlated quantum-chemical calculations to evaluate the impact of the nature of conjugated bridge and the charge-transfer distance on that enhancement. We compare chromophores with phenylenevinylene-, thienylenevinylene-, polyene-, and indenofluorene-type backbones substituted by dimethylamino and cyano groups. In all compounds, we find a strongly TPA-active A(g) state (either 2A(g) or 3A(g)) in the low-energy region, as well as a higher lying TPA-active state (mA(g)) at close to twice the energy of the lowest lying one-photon allowed state; the smaller energy detuning in the mA(g) states results in very large TPA cross sections delta. We also investigate the influence of the degree of ground-state polarization on TPA. Independent of the nature of the backbone and the donor-acceptor separation, delta displays the same qualitative evolution with a maximum before the cyanine-like limit; the highest TPA cross sections are calculated for distirylbenzene- and polyene-based systems.     

Origin of high second- and third-order nonlinear optical response in ammonio/borato diphenylpolyene zwitterions: the remarkable role of polarized aromatic groups

We present a quantum-chemical analysis of the molecular structure and second- and third-order polarizabilities in a series of promising nonlinear optical (NLO) chromophores, the zwitterionic ammonio/borato diphenylpolyenes, R3N+Ph(C=C)nPhB-R3, whose synthesis has been reported recently. The molecular geometries are obtained via MP2/6-31G optimization, while the NLO properties are calculated with the INDO Hamiltonian using the sum-over-states and finite-field real-space methods. The real-space approach allows the direct evaluation of the NLO-active segments of the molecules, while the sum-over-states results illustrate the virtual excitations and charge-transfer pathways that are essential in the NLO response. Both methods highlight the remarkable and unexpected result that it is the strongly polarized phenylene groups that play the key role in generating a high NLO response.     

Negative differential resistance in phenylene ethynylene oligomers

The origin of the sharp peak profile (i.e., negative differential resistance, NDR) observed in the I/V curves of three-ring phenylene ethynylene oligomers is a topic of major current interest. Here, quantum-chemical calculations are performed to analyze the evolution of the one-electron structure of an unsubstituted three-ring oligomer under the influence of a static electric field (which models the driving voltage applied in the experiments). The results indicate that the rotation of the central ring of the oligomer induces resonant tunneling processes over a limited voltage range. This can thus be responsible for the NDR signature observed experimentally.     

Electronic structure of highly conducting conjugated polymers: evolution upon doping of polyacetylene, polythiophene, and polyemeraldine

The electronic properties of three types of conducting polymers: trans-polyacetylene (proto-typical of systems with a degenerate ground state), polythiophene (as an example of compounds with a nondegenerate ground state), and polyemeraldine (which can be doped via protonation) are reviewed. The structural and electronic band structure properties of these systems are studied at various defect concentrations corresponding to undoped, lightly doped, and highly doped polymers. Geometry optimizations of oligomeric equivalents to the undoped and doped polymers are performed using the semi-empirical MNDO and AM1 methods. The electronic band structures are calculated using the VEH method. The interpretation of the optical absorption data is discussed in terms of interband transitions; for doped trans-polyacetylene including soliton defects and for doped polythiophene including bipolaron defects. For highly doped trans-polyacetylene and polythiophene as well as for protonated polyemeraldine, the electronic structure of a polaron lattice conformation is discussed and shown to be in agreement with existing optical and magnetic data on these polymers.     

Indium(I) bromide-mediated coupling of dibromoacetonitrile with aldehydes followed by Boord elimination of bromine and oxygen of β-bromo alkoxides for preparation of 3-organyl-2-alkenenitriles

The organoindium compound derived from indium monobromide and dibromoacetonitrile reacts with carbonyl compounds to afford the corresponding 2-bromo-2-cyano-indium(III) alkoxide. The action of a second equivalent of indium monobromide onto the alkoxides derived from aldehydes promotes the Boord elimination of the β-related oxygen and bromine atoms leading to 2-alkenenitriles.     

HRGC of wine free amino acids as a tool for elementary wine characterization

In this paper we show that wine free amino acids derivatized as isopropyl N-heptafluorobutyryl esters can be used as a tool for wine characterization. Elementary wines of eight Vitis vinifera varieties were studied during a seven year period. The characteristic wine amino acids for each variety were assessed by means of pattern recognition techniques. A “star symbol plot” was used for graphic representation.     

Impact of the computational method on the geometric and electronic properties of oligo(phenylene vinylene) radical cations

We report on a quantum-chemical study of the electronic and optical properties of unsubstituted oligo(phenylene vinylene) (OPV) radical cations. Our goal is to distinguish the impact of the choice of molecular geometry from the impact of the choice of quantum-chemical method, on the calculated optical transition energies. The geometry modifications upon ionization of the OPV chains are found to depend critically on the theoretical formalism: Hartree-Fock (HF) geometry optimizations lead to self-localization of the charged defects while pure density functional theory (DFT) results in a complete delocalization of the geometric modifications over the whole conjugated backbone. The electronic structure and vertical transition energy associated with the lowest excited state of the radical cations have been calculated at the post-Hartree-Fock level within a configuration interaction (HF-CI) scheme and using the time-dependent DFT (TD-DFT) formalism for different radical cation geometries. Interestingly, the changes in the calculated optical properties obtained when using different geometric structures are less important within a given method than the differences between methods for a given structure. The optical excitation is localized with HF-CI and delocalized with TD-DFT, almost irrespective of the molecular geometry; as a result, HF-CI excitation energies tend to saturate as the chain length increases, in contrast to the results from TD-DFT.     

Determination of lead in seawater by inductively coupled plasma optical emission spectrometry after separation and pre-concentration with cocrystallized naphthalene alizarin

An analytical method for separation and pre-concentration of lead in seawater for determination by inductively coupled plasma optical emission spectrometry has been investigated. Lead was retained in the solid phase (0.5 g) composed of co-precipitated naphthalene and alizarin red. The solid phase quantitatively sorbs Pb(II) at pH 8–9, and the metal was eluted using 5.0 ml of 2 mol l⁻¹ nitric acid. The effect of NaCl, KCl, BaCl₂, CaCl₂, Na₂SO₄, MgCl₂ and Na₃PO₄ on the sorption of Pb(II) in the solid phase was studied. A set of solutions containing varying amounts of electrolytes (0.5; 1.0; 3.0 and 5.0% m/v) with Pb (50 μg) was prepared and the recommended procedure applied. The Na₃PO₄ was found to interfere; the other electrolytes did not interfere up to 5% m/v. A pre-concentration factor of 40 was obtained in this analytical procedure. The limit of detection and limit of quantification for Pb(II) were 53 and 176 μg l⁻¹, respectively. Lead was determined in seawater samples collected in Salvador city, Bahia, Brazil. The precision, expressed as R.S.D., was 1.8–4.6%, and the recovery of lead added to seawater samples was 95–97%.     

The conformation of amine- and amide-terminated poly(propylene imine) dendrimers as investigated by molecular simulation methods

The parameters that influence the conformation of poly(propylene imine) dendrimers were investigated by molecular simulations using molecular mechanics and simulated annealing methods. Dendrimers with two types of peripheral units able to communicate via hydrogen bonding-amine and amide moieties-were considered in order to study the role that secondary interactions among the end groups have in the spatial organization of the dendritic branches. Radial atomic density profiles and radial atomic probability distributions were used to extract global properties, such as the degree of packing of the branches, the distribution of the monomers throughout the molecular volume, and the extent and characteristics of the surface region. Information was also obtained about the nature, location, and extent of formation of the hydrogen bonds, as well as their evolution with dendrimer generation and their assembly into networks. The analyses were supported by a detailed investigation of the first two generations, with an emphasis on the relationship between hydrogen bonding and the compactness and stability of the molecules; this allowed us to account for the generational evolution of hydrogen bonding that is experimentally observed in several poly(propylene imine) dendrimers.