Pi-electron Conjugation Research Articles

Valence Topological Charge-Transfer Indices for Reflecting Polarity: Correction for Heteromolecules

Valence topological charge-transfer (CT) indices are applied to the calculation of dipole moments μ. The μ calculated by algebraic and vector semisums of the CT indices are defined. The model is generalized for molecules with heteroatoms and corrected for sp3-heteromolecules. The ability of the indices for the description of the molecular charge distribution is established by comparing them with μ of the valence-isoelectronic series of cyclopentadiene, benzene and styrene. Two CT indices, μvec (vector semisum of vertex-pair μ) and μvecV (valence μvec) are proposed. The μvecV behaviour is intermediate between μvec and μexperiment. The correction is produced in the correct direction. The best results are obtained for the greatest group. Inclusion of the heteroatom in the π-electron system is beneficial for the description of μ, owing to either the role of additional p and/or d orbitals provided by the heteroatom or the role of steric factors in the π-electron conjugation. The steric effect is almost constant along the series and the dominating effect is electronic. Inclusion of the heteroatom enhances μ, which can improve the solubility of the molecule. For heteroatoms in the same group, the ring size and the degree of ring flattering are inversely proportional to their electronegativity.

Oligomeric Ligands Incorporating Multiple 5,5‘-Diethynyl-2,2‘-bipyridine Moieties Bridged and End-Capped by 3,4-Dibutylthiophene Units

In this work, we detail the synthesis and photophysical properties of a series of soluble polybipyridine ligands comprising one to five bipyridine units sandwiched between rigid carbon-carbon triple bonds substituted by 3,4-dibutylthiophene repeating units. The dual Sonogashira coupling reaction of 5,5'-dibromo-2,2'-bipyridine with TMS- and (CH3)2C(OH)-protected acetylene allows the synthesis of a dissymmetrically functionalized building block which was selectively deprotected at either the TMS or 2-hydroxyprop-2-yl site. Various combinations allow the interconnection of the terminal alkyne to 3,4-dibutyl-2,5-diiodothiophene or 3,4-dibutyl-2-iodothiophene leading to bipyridine frameworks bearing two acetylene-protected groups or one acetylenethiophene/one acetylene-protected function. It is possible therefore to construct dimeric to pentameric bipyridine ligands where the chelating subunit is bridged by a 3,4-dibutyl-2,5-diethynylthiophene spacer and end-capped by a 3,4-dibutyl-2-ethynylthiophene stopper. All cross-coupling reactions are promoted with palladium(0) tetrakistriphenylphosphine under mild conditions. Spectroscopic data for the new oligomers are discussed in terms of the extent of pi-electron conjugation. Upon increasing the number of pi-electrons from 24 to 104, there is a progressive lowering in the energy of absorption and fluorescence transitions, while the emission quantum yields remain essentially constant. The LUMO levels of these large molecules, estimated by cyclic voltammetry, lie in the range -3.06 to -3.18 eV.

Hydrophilic Labeling Reagents of Dipyrrylmethene-BF2Dyes for Two-Photon Excited Fluorometry: Syntheses and Photophysical Characterization

Recently introduced bioaffinity assay technology, ArcDia TPX, is based on two-photon excited fluorescence (TPE) and it enables separation-free ultra-sensitive immunoassays from microvolumes. Here we present syntheses of novel two-photon excitable fluorescent labeling reagents which have been specially designed to be used as label molecules in the ArcDia TPX assay technique. The labeling reagents are based on dipyrrylmetheneboron difluoride (dipyrrylmethene-BF2) chromophore, which have been substituted with aryl, heteroaryl or arylalkenyl chemical groups to extend the pi-electron conjugation. These substitutions results in a series of dipyrrylmethene-BF2 fluorophores with different photophysical properties. Dipyrrylmethene-BF2 fluorophores have been further substituted with a dipeptide linker unit and finally activated as succinimidyl esters to enable specific coupling with primary amino groups. The dipeptide linker serves as a spacer arm between the label and a target, and enhances the solubility of the label in aqueous solutions. Study of the chemical and photophysical performance of the new labeling reagents is described. The new labeling reagents exhibit high fluorescence quantum yields, and molar absorption coefficients. The results show that the new labels with the hydrophilic dipeptide linker unit provide large two-photon excitation cross-sections, high fluorescence quantum efficiency and good solubility in aqueous solutions. The results suggest that the novel dipyrrylmethene-BF2 labels are highly applicable to bioaffinity assays based on two-photon excitation of fluorescence.

Steady-State and Time-Resolved Spectroscopic Characteristics of Mesoionic Oxazolones Solutions

Three new mesoionic oxazolo[3,2-b]pyridazin-2-one derivatives, in different solutions have been investigated by UV-Vis absorption, steady-state and time-resolved fluorescence methods. The effect of substituents on the extension of conjugation of the pi-electrons from mesoionic oxazolone has been evidenced by bathochromic shifts of the absorption and fluorescence maxima positions. The fluorescence decay data could be fitted to single-exponential or double-exponential function. The lifetime values are much higher in aprotic polar solvents and in the case of the derivatives that present an extension of the conjugation of pi-electrons. The properties of the compounds present a solvent dependence, being tested in micellar solutions as potential molecular probe "sensitive" to the environment polarity.

A chemical index inspired by biological plastic evolution: valence-isoelectronic series of aromatics.

Plastic evolution is a new perspective of the evolutionary process conjugating the effect, on one hand, of the acquired characters and, on the other, of the relationships that emerge among the principle of evolutionary indeterminacy, the principle of morphologic determination, and natural selection. Plastic evolution is applied to design the coordination index Ic. Ic is used to characterize the valence-isoelectronic series of cyclopentadiene, benzene, toluene, and styrene and compared to charge indices for dipole moment. The parameters needed to calculate Ic are minus the standard enthalpy of formation T, molecular surface area S, and molecular weight W. With the exception of the O heteromolecule, Ic allows differentiating quantitatively the styrene series as well as the cyclopentadiene from the toluene series. Ic improves the multivariable regression equations for dipole moment and is a predictive index when it is used together with the topological charge indices. The correction introduced in the dipole moment is produced in the correct direction. Inclusion of the heteroatom in the pi-electron system is beneficial for the description of the dipole moment, owing to either the role of additional p and/or d orbitals provided by the heteroatom or the role of steric factors in the pi-electron conjugation. Principal component analysis groups the chemical indices in three classes, which are in agreement with cluster analysis.

Electrical conduction through linear porphyrin arrays.

Electrical conduction measurements were made on two extreme types of directly linked porphyrin arrays by using nanoelectrodes. One type is the directly linked Zn(II)porphyrin arrays, consisting of 48 Zn(II)porphyrin moieties (Z48), and the other type is the completely flat, tape-shaped Zn(II)porphyrin arrays, consisting of eight Zn(II)porphyrin units (T8). The I-V curve for Z48 exhibits the diode-like behavior and the hysteresis depending on the voltage sweep direction presumably due to the conformational heterogeneity arising from the dihedral angle distribution in Z48. On the other hand, the I-V curve for T8 is nearly symmetric without any hysteresis, leading to the higher conductivity and the smaller band gap. These results illustrate that the stronger pi-electron conjugation in T8, as compared with that of Z48, results in better electrical conduction.

A concise modular synthesis of 2,5-diethynyl-3,4-dibutyl-thiophene-bridged back-to-back terpyridine ligands.

The efficient synthesis of soluble and rigid terpyridine-based ditopic ligands bearing an increasing number of 2,5-diethynyl-3,4-dibutylthiophene (DEDBT) modules has demonstrated the advantages of a single convergent strategy based on a double coupling in a final step of monoterpyridine building blocks carrying the adequate number of thiophene modules with a diiodo-substituted thiophene subunit. This protocol enjoys the advantages of both efficiency and versatility and requires pivotal intermediates, which were produced by a step-by-step implementation of monoterpyridine fragments with a key thiophene intermediate carrying an iodo function, a propargylic-protecting group, and two butyl-solubilizing fragments. One set of experimental conditions is required to produce all the intermediates and the final ligands. Oxidative dimerization of monosubstituted terpyridine skeletons bearing one or two thiophene substituents and a terminal alkyne function, in the presence of cupric salts and oxygen, afforded the homotopic ligands with a central dithienylbutadiyne spacer. Optical properties for the new oligomers have been investigated and are discussed in terms of effective conjugation length and pi-electron conjugation. Upon increasing the number of interspersed DEDBT units, a significant lowering in energy of absorption and fluorescence transitions as well as of the quantum yields is observed.

Attachment of styrene and phenylacetylene on Si(111)-7 x 7: the influence of substitution groups on the reaction mechanism and formation of pi-conjugated skeletons.

The interactions of styrene and phenylacetylene and their isotope substitutions with a Si(111)-7 x 7 surface have been studied as model systems to mechanistically understand the chemical binding of conjugated pi-electron systems to di-radical-like silicon dangling bonds of the adjacent adatom-rest atom pair. Vibrational studies show that styrene mainly binds to the surface through a diradical reaction involving both the external C=C and its conjugated internal C=C of the phenyl ring with an adjacent adatom-rest atom pair, forming a 5-ethylidene-1,3-cyclohexadiene-like skeleton. On the other hand, phenylacetylene was shown to be covalently attached to Si(111)-7 x 7 through the external C[triple bond]C, forming a styrene-like conjugation system. These experimental results are consistent with density functional theory calculations. The different binding mechanisms for styrene and phenylacetylene clearly demonstrate that reaction channels for multifunctional organic molecules are strongly dependent on the chemical and physical properties of the functional groups. The resulting pi-electron conjugation structures may possibly be employed as intermediates for further organic syntheses and fabrication of multilayer organic films on semiconductor surfaces.

Synthesis of soluble bis-terpyridine ligands bearing ethynylene-phenylene spacers

Soluble and rigid terpyridine-based ditopic ligands bearing one to five phenylene/ethynylene modules have been synthesized by way of a stepwise procedure. Each module is attached to the terpyridine unit via an ethynylene fragment and functionalized at the 4-position with an additional ethynylene connector and in the 2,5-positions with two flexible dodecyloxy chains. The synthetic protocol is based on sequential Pd(0)-catalyzed cross-coupling reactions between a terpyridine subunit grafted with the necessary diethynyl/phenyl or ethynylphenyl/bromide appendage. For ditopic ligands displaying an even number of phenyl/ethynylene modules, the final step involves a single cross-coupling reaction between 4'-ethynylene-2,2':6',6' '-terpyridine and the appropriate bromo derivative. In the case of the ligands having an odd number of phenylene/ethynylene fragments, a double cross-coupling reaction between an extended dibromopolyphenylene intermediate and 4'-ethynylene-2,2':6',6' '-terpyridine or 1-(4'-ethynylene-2,2':6',2' '-terpyridine)-4-ethynylene-2,5-didodecyloxy-benzene is required. For ligands I-V, optimal preparative conditions were found with [Pd(0)(PPh(3))(4)] (6 mol %) in n-propylamine at 70 degrees C. Oxidative dimerization of the 1-(4'-ethynylene-2,2':6',2' '-terpyridine)-4-ethynylene-2,5-didodecyloxybenzene derivative in the presence of cupric salts and oxygen gives the corresponding homoditopic ligand II(2)() bearing a central diphenyldiacetylene spacer. Spectroscopic data for the new oligomers are discussed in terms of the extent of pi-electron conjugation. Upon increasing the number of phenylene/ethynylene modules, there is a progressive lowering in energy of absorption and fluorescence transitions.

Electric fields in active sites: substrate switching from null to strong fields in thiol- and selenol-subtilisins.

Although known to be important factors in promoting catalysis, electric field effects in enzyme active sites are difficult to characterize from an experimental standpoint. Among optical probes of electric fields, Raman spectroscopy has the advantage of being able to distinguish electronic ground-state and excited-state effects. Earlier Raman studies on acyl derivatives of cysteine proteases [Doran, J. D., and Carey, P. R. (1996) Biochemistry 35, 12495-502], where the acyl group has extensive pi-electron conjugation, showed that electric field effects in the active site manifest themselves by polarizing the pi-electrons of the acyl group. Polarization gives rise to large shifts in certain Raman bands, e.g. , the C=C stretching band of the alpha,beta-unsaturated acyl group, and a large red shift in the absorption maximum. It was postulated that a major source of polarization is the alpha-helix dipole that originates from the alpha-helix terminating at the active-site cysteine of the cysteine protease family. In contrast, using the acyl group 5-methylthiophene acryloyl (5-MTA) as an active-site Raman probe, acyl enzymes of thiol- or selenol-subtilisin exhibit no polarization even though the acylating amino acid is at the terminus of an alpha-helix. Quantum mechanical calculations on 5-MTA ethyl thiol and selenol ethyl esters allowed us to identify the conformational states of these molecules along with their corresponding vibrational signatures. The Raman spectra of 5-MTA thiol and selenol subtilisins both showed that the acyl group binds in a single conformation in the active site that is s-trans about the =C-C=O single bond. Moreover, the positions of the C=C stretching bands show that the acyl group is not experiencing polarization. However, the release of steric constraints in the active site by mutagenesis, by creating the N155G form of selenol-subtilisin and the P225A form of thiol-subtilisin, results in the appearance of a second conformer in the active sites that is s-cis about the =C-C=O bond. The Raman signature of this second conformer indicates that it is strongly polarized with a permanent dipole being set up through the acyl group's pi-electron chain. Molecular modeling for 5-MTA in the active sites of selenol-subtilisin and N155G selenol-subtilisin confirms the findings from Raman spectroscopic studies and identifies the active-site features that give rise to polarization. The determinants of polarization appear to be strong electron pull at the acyl carbonyl group by a combination of hydrogen bonds and the field at the N-terminus of the alpha-helix and electron push from a negatively charged group placed at the opposite end of the chromophore.

Proton affinity of some substituted 2,6-dimethylbenzamides and their N,N-dimethyl derivatives: the susceptibility of the amide group to polar substituent effects

The proton affinity (PA) of 2,6-dimethylbenzamide and its derivatives substituted by an amino or a nitro group at the para- or meta-position, as well as of the corresponding N,N-dimethylbenzamides, have been determined by the kinetic method using bracketing by a pair of reference bases B-i which give a slightly more or less intense signal of the protonated base [BHi](+) compared with that of the protonated amide [AH](+), The amide group of the 2,6-dimethylbenzamides is twisted out of the plane of the aromatic ring by the methyl substituents at both ortho-positions which interrupts the pi-electron conjugation between amide group and benzene ring. The amino group and the nitro group have been chosen as prototypical polar substituents with electron donating and with electron withdrawing properties to reveal the effect of steric hindrance of pi-conjugation on the PA of benzamides, A comparison with the PA of the corresponding sterically unhindered benzamides shows that this effect is generally small and manifests itself clearly only in the case of para-amino substituents. This result is discussed considering different conformations of the benzamides using semi-empirical PM3 calculation. The absolute PA values calculated by PM3 agree only modestly with the experimental values, but the calculated conformation and the relative PA values corroborate a weak interaction between amide group and benzene ring as suggested by experiments. Finally, the modest susceptibility of the amide group and protonated amide group to polar substituent effects in the gas phase agrees with comparable small substituent effects observed on the pK(a) in solution and shows that the amide group is singular with respect to a resonance saturation.

Synthesis of conjugated polymers with alternating aromatic and quinonoid sequences via elimination on precursors

Abstract : An elimination reaction for the formation of conjugated polymers from polymer precursors containing alternating sp superscript 3 carbon atom and conjugatged segments in the main chain is described. The resulting conjugated polymers with alternating aromatic and quinonoid sequences are inaccessible by conventional polymerization processes. The Pi-electron conjugation extension process is exemplified by reactions performed on poly(5,5',alpha-bithiophenediyl p acetoxybenzylidene) at 23 C and followed by electronic and infrared spectra. The semiconductor band gap narrowed from an initial value of 1.53 ev for the precursor polymer to about 0.83 ev for the conjugated derivative. Further evidence for the sp superscript 3 to sp superscript 2 carbon atom conversion by the elimination process is provided by infrared data.

Pi-electron conjugation in porphyrins

The pi-electron conjugation path in the porphyrin ring has been examined by Hueckel-type molecular orbital calculations. The assumption that the path is along the sixteen-member inner ring together with the consideration of different carbon-carbon resonance energies for different bond lengths has been found to account for the observed electronic spectra and to reproduce the ground-state geometry.