Pi-conjugated Oligomers Research Articles

Suppression of local electron-phonon interaction in pi-conjugated oligomers and its monitoring using Raman scattering

For efficient operation of many organic electronic devices, organic semiconductors with high charge carrier mobility are required. However, in most of the known organic semiconductors, the charge mobility is low, since it is limited by the strong local electron-phonon interaction. In the present work, using the example of thiophene-phenylene co-oligomers, a class of organic semiconductors that combine a sufficiently high charge mobility with light emission and therefore promising for light-emitting transistors and electrically pumped lasers, the mechanism of suppression of the electron-phonon interaction by introducing electronegative atoms or an additional thiophene ring is studied. It was found that such structural changes alter the contribution of various vibrational modes to the local electron-phonon interaction, in particular, to the suppression of the contribution of the low-frequency torsion mode. In addition, it is shown that for the two modes that make the largest contribution to the local electron-phonon interaction in an unsubstituted oligomer, this change correlates with their intensity of Raman scattering, and this confirms the promise of studying the electron-phonon interaction using Raman spectroscopy. The results obtained improve the understanding of the relationship between the local electron-phonon interaction and the molecular structure of organic semiconductors, which is extremely important for the directed design of such materials with high charge mobility.

High-Throughput Preparation of Antibacterial Polymers from Natural Product Derivatives via the Hantzsch Reaction.

SummaryThe Hantzsch and free-radical polymerization reactions were combined in a one-pot high-throughput (HTP) system to simultaneously prepare 30 unique polymers in parallel. Six aldehydes derived from natural products were used as the starting materials to rapidly prepare the library of 30 poly(1,4-dihydropyridines). From this library, HTP evaluation methods led to the identification of an antibacterial polymer. Mechanistic studies revealed that the dihydropyridine group in the polymer side-chain structure plays an important role in resisting bacterial attachment to the polymer surface, thus leading to the antibacterial function of this polymer. This research demonstrates the value of multicomponent reactions (MCRs) in interdisciplinary fields by discovering functional polymers for possible practical applications. It also provides insights to further developing new functional polymers using MCRs and HTP methods with important implications in organic chemistry, polymer chemistry, and materials science.

Electro-active pi-Conjugated Oligomers and Polymers A Molecular Picture of Charge-Transfer Processes

<span>Inorganic semiconductor devices such as transistors have been instrumental in shaping the development of our society of information and communication. Recently, the electronics and photonics technologies have opened their materials base to organics, in particular p-conjugated oligomers and polymers. The goal with organics-based devices is not necessarily to attain or exceed the level of performance of inorganic semiconductor technologies...</span>

Local contact potential difference of molecular self-assemblies investigated by Kelvin probe force microscopy

Self-assembled pi-conjugated oligomer nanowires have been investigated by frequency modulation atomic force microscopy and amplitude modulation Kelvin probe force microscopy under ultra high vacuum. The distance dependence of the contact potential difference (CPD) has been analyzed by combining high resolution imaging with distance-spectroscopy measurements. It is shown that the apparition of a damping contrast characterizes the onset of short range electrostatic (SRE) forces, which are responsible for the occurrence of local CPD (LCPD) modulations correlated with the molecular lattice. By working at the onset of the damping contrast, the tip-surface separation can be adjusted to minimize the contribution of SRE forces to the measured CPD.

Modulation of Phase Separation Between Spherical and Rodlike Molecules Using Geometric Surfactancy

A Dyad consisting of C(60) linked to a pi-conjugated oligomer by a propylene spacer was synthesized to explore its ability to modulate phase separation between OFTB and PCBM using differential scanning calorimetry, hot-stage polarizing optical microscopy, X-ray diffraction, and atomic force microscopy techniques. Upon thermal annealing at 10 degrees C above its T(g) for 12-48 h, the 1:1 blend of OFTB and PCBM resulted in a eutectic mixture. Thermal annealing of a OFTB:Dyad:PCBM film with a 9:2:9 mass ratio at 10 degrees C above its T(g) for 12 h resulted in an amorphous film. Its AFM phase image indicated phase separation into two interspersed 30 nm amorphous domains at approximately equal fractions. Geometric surfactancy is inferred from the formation of microemulsions in analogy to widely reported traditional oil-surfactant-water systems and ternary polymer blends. In contrast, thermal annealing of a 7:6:7 film under a similar condition resulted in an amorphous film with compositional uniformity.

Non-Traditional Aromatic Topologies and Biomimetic Assembly Motifs as Components of Functional Pi-Conjugated Oligomers

This article will highlight our recent work using conjugated oligomers as precursors to electroactive polymer films and self-assembling nanomaterials. One area of investigation has focused on nonbenzenoid aromaticity in the context of charge delocalization in conjugated polymers. In these studies, polymerizable pi-conjugated units were coupled onto unusual aromatic cores such as methano[10]annulene. This article will also show how biologically-inspired assembly of molecularly well-defined oligopeptides that flank pi-conjugated oligomers has resulted in the aqueous construction of 1-dimensional nanomaterials that encourage electronic delocalization among the pi-electron systems.

Conjugated Oligoyne-Bridged [60]Fullerene Molecular Dumbbells: Syntheses and Thermal and Morphological Properties

A series of linear and star-shaped pi-conjugated oligomer hybrids (2-5) composed of an oligoyne core, ranging from 1,3-butadiyne to 1,3,5,7,9,11-dodecahexayne, and fullerenyl end-capping groups has been synthesized and studied. The molecular structures of these fullerene-oligoyne hybrids were assembled through three key reactions: Pd-catalyzed cross-coupling, Cu-catalyzed oxidative homocoupling, and an in situ alkynylation reaction on [60]fullerene. The properties of these compounds were investigated by UV-vis spectroscopy, differential scanning calorimetry (DSC), and atomic force microscopy (AFM) with the purpose of understanding the thermal reactivity arising from the oligoyne moieties as well as the morphological properties on surface. Our study shows that these fullerene-oligoyne hybrids tend to aggregate in different morphologies, including nanospheres, nanoflakes, and continuous thin films, while the morphological properties appear to be subject to the influence of molecular factors such as oligomer chain length, solubilizing alkylphenyl groups, and the thermal reactivity of the oligoyne unit. The correlation between molecular property and interfacial aggregation behavior evinced by these fullerene-oligoyne hybrids suggests a viable approach to exert bottom-up control over the structures and properties of fullerene based nanomaterials.

Persistent, Well-Defined, Monodisperse, π-Conjugated Organic Nanoparticles via G-Quadruplex Self-Assembly

Several oligo(p-phenylene-vinylene) oligomers capped with a guanosine or a guanine moiety have been prepared via a palladium-catalyzed cross-coupling reaction. Their self-assembly, in both the absence and presence of alkaline salts, has been studied by means of different techniques in solution (NMR, MS, UV-vis, CD, fluorescence), solid state (X-ray diffraction), and on surfaces (STM, AFM). When no salt is added, these pi-conjugated molecules self-associate in a mixture of hydrogen-bonded oligomers, among which the G-quartet structure may be predominant if the steric hindrance around the guanine base becomes important. In contrast, in the presence of sodium or potassium salts, well-defined assemblies of eight functional molecules (8mers) can be formed selectively and quantitatively. In these assemblies, the pi-conjugated oligomers are maintained in a chirally tilted (J-type) stacking arrangement, which is manifested by negative Cotton effects, small bathochromic absorption and emission shifts, and fluorescence enhancements. Furthermore, these self-assembled organic nanostructures, approximately 1.5-2.0 nm high and 8.5 nm wide, exhibit an extraordinary stability to temperature or concentration changes in apolar media, and they can be transferred and imaged over solid substrates as individual nanoparticles, showing no significant dissociation or further aggregation.

Chemistry as a Prism: A Review of Light‐Emitting Materials Having Tunable Emission Wavelengths

Photoluminescent materials have been extensively applied in various fields of science because of their numerous advantages, such as excellent sensitivity, good specificity, a large linear range of analysis, ease of handling, and so on. Many strategies have been used to understand and manipulate the photophysical properties of photoluminescent materials. This Focus Review describes recent progress focused on tuning the photophysical properties, especially the emission wavelengths of pi-conjugated oligomers, photoluminescent organometallic complexes, and fluorescent organic dyes by chemical modification.

Detection and Identification of Alkylating Agents by Using a Bioinspired “Chemical Nose”

Alkylating agents are simple and reactive molecules that are commonly used in many and diverse fields such as organic synthesis, medicine, and agriculture. Some highly reactive alkylating species are also being used as blister chemical-warfare agents. The detection and identification of alkylating agents is not a trivial issue because of their high reactivity and simple structure. Herein, we report on a new multispot luminescence-based approach to the detection and identification of alkylating agents. In order to demonstrate the potential of the approach, seven pi-conjugated oligomers and polymers bearing nucleophilic pyridine groups, 1-7, were adsorbed onto a solid support and exposed to vapors of alkylators 8-15. The alkylation-induced color-shift patterns of the seven-spot array allow clear discrimination of the different alkylators. The spots are sensitive to minute concentrations of alkylators and, because the detection is based on the formation of new covalent bonds, these spots saturate at about 50 ppb.

Fluorene-Based π-Conjugated Oligomers for Efficient Three-Photon Excited Photoluminescence and Lasing

A novel series of diphenylamino- and 1,2,4-triazole-end-capped, fluorene-based, pi-conjugated oligomers that includes extended oligofluorenes and oligothienylfluorenes has been synthesized by means of the palladium-catalyzed Suzuki cross-coupling of 9,9-dibutyl-7-(diphenylamino)-2-fluorenylboronic acid and the corresponding 1,2,4,-triazole-based aryl halide as a key step. It was demonstrated that efficient two- and three-photon excited photoluminescence and lasing in the blue region are obtained by pumping near-infrared femtosecond lasers on these materials. Although the absorption and emission maxima of the highly fluorescent and extended oligofluorenes reach a saturation limit, there exists an effective conjugation length for an optimum three-photon absorption cross section in the homologous oligofluorene series. On the other hand, the multiphoton excited emission spectrum and lasing wavelength can easily be modified or tuned by an incorporation of thienyl unit(s) into the fluorene-based pi-conjugated core with which exceptionally large three-photon absorption cross sections up to 3.59 x 10(-77) cm6 s2 in the femtosecond regime have been obtained, thereby highlighting the potential of this series of photonic materials. The optimized full width at half-maximum of the cavityless three-photon upconverted blue lasing spectra are sharply narrowed to approximately 6 nm with an efficiency of up to 0.013%.

Fullerene and pi-Conjugated Oligomer Hybrids as Advanced Functional Nanomaterials

Two classes of pi-conjugated oligomer linked bis-fullerene molecules with a triad motif of C60-pi-C60 were synthesized and investigated. In the first series of C60-pi-C60 hybrids, electron-rich OPE/OPV conjugated oligomers were employed to function as the central pi-bridges and as the electron donors. Electrochemical and electronic absorption/emission spectral analyses indicate that the electronic interactions between the fullerene cage (acceptor) and the oligomer unit (donor) varied depending on the nature of the bridging unit. Complex excited state decay dynamics on the nanosecond timesccale were observed with time-resolved transient absorption spectroscopy. The second series of fullerene-oligomer hybrids use conjugated polyynes as the pi-bridges. The unique structural rigidity and solid-state reactivity of the polyyne units impart these bisfullerene molecular dumbbells interesting interfacial aggregation behavior by forming spherical nanostructures on s urfaces. This property could be useful for solid-state synthesis of fullerene based nanomaterials.

Fullerene and pi-Conjugated Oligomer Hybrids as Advanced Functional Nanomaterials

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Fine Tuning of the Electronic Properties of Linear π‐Conjugated Oligomers by Covalent Bridging

A series of oligothienylenevinylenes, pi-conjugated oligomers rigidified by ethylene bridges attached at different sites of the conjugated backbone, have been constructed by multistep synthetic methodologies. Electronic absorption spectra show that the rigidification of the conjugated system produces a bathochromic shift of the absorption maximum and a narrowing of the HOMO-LUMO energy gap, as compared to the spectra of an open-chain reference compound. The cyclic voltammograms of all oligomers show that these compounds can be reversibly oxidized into their cation radicals and dications and that rigidification produces a large negative shift of the first oxidation potential, which is indicative of a considerable increase of the HOMO level. Electrochemical data confirm that covalent bridging strongly affects the HOMO and LUMO levels and these data demonstrate that the sites of fixation of the bridges on the pi-conjugated backbone exert a determining effect on the relative stability of the cation radical and dication. Examination of these various results in the light of theoretical calculations shows that in addition to a local control of bond length alternation, and hence of the HOMO-LUMO gap, the fixation of covalent bridges at selected positions of the pi-conjugated system limits the deformation of the pi-conjugated structure upon oxidation to the charged states.

Control of Film Morphology by Folding Hydrogen-Bonded Oligo(p-phenylenevinylene) Polymers in Solution

The film morphology of pi-conjugated oligomers has been controlled by self-assembly in solution. To this end supramolecular hydrogen-bonded systems of oligo(p-phenylenevinylene) (OPV) carrying urei ...

Reverse polarization in charged π-conjugated oligomers

Single-photon excitation in a charged pi-conjugated oligomer is studied theoretically. An apparent reverse polarization is obtained through single-photon excitation, which is different from that obtained through a double-photon excitation. The polarizability is calculated and it is found that a maximum reverse polarization will appear at a suitable conjugation length. In addition, we indicate that the reverse polarization is a nonlinear behavior with the induced electric field. Effects of nondegenerate confinement and interchain interactions on the reverse polarization are also discussed.

Density matrix renormalization group study of conjugated polymers with transverseπ-conjugation

We report accurate numerical studies of excited state orderings in long hypothetical pi-conjugated oligomers in which the hydrogen atoms of trans-polyacetylene are replaced with conjugated sidegroups, within modified Hubbard models. There exists a range of the bare Coulomb repulsion for which the excited state ordering is conducive to photoluminescence in the substituted systems, even as this ordering is opposite in the unsubstituted polyenes of the same lengths. Our work provides motivation to study real pi-conjugated polymers with transverse conjugation and small optical gaps.

Probing Molecular Wires: Synthesis, Structural, and Electronic Study of Donor–Acceptor Assemblies Exhibiting Long‐Range Electron Transfer

A series of donor-acceptor arrays (C60-oligo-PPV-exTTF; 16-20) incorporating pi-conjugated oligo(phenylenevinylene) wires (oligo-PPV) of different length between pi-extended tetrathiafulvalene (exTTF) as electron donor and C60 as electron acceptor has been prepared by multistep convergent synthetic approaches. The electronic interactions between the three electroactive species present in 16-20 were investigated by UV-visible spectroscopy and cyclic voltammetry (CV). Our studies clearly show that, although the C60 units are connected to the exTTF donors through a pi-conjugated oligo-PPV framework, no significant electronic interactions are observed in the ground state. Interestingly, photoinduced electron-transfer processes over distances of up to 50 Angstroms afford highly stabilized radical ion pairs. The measured lifetimes for the photogenerated charge-separated states are in the range of hundreds of nanoseconds (approximately 500 ns) in benzonitrile, regardless of the oligomer length (i.e., from the monomer to the pentamer). A different lifetime (4.35 micros) is observed for the heptamer-containing array. This difference in lifetime has been accounted for by the loss of planarity of the oPPV moiety that increases with the wire length, as established by semi-empirical (PM3) theoretical calculations carried out with 19 and 20. The charge recombination dynamics reveal a very low attenuation factor (beta = 0.01 +/- 0.005 Angstroms(-1)). This beta value, as well as the strong electron coupling (V approximately 5.5 cm(-1)) between the donor and the acceptor units, clearly reveals a nanowire behavior for the pi-conjugated oligomer, which paves the way for applications in nanotechnology.

Structure−Property Relationships for Electron−Vibrational Coupling in Conjugated Organic Oligomeric Systems

A series of pi-conjugated oligomers containing one to six monomer units were studied by absorption and photoluminescence spectroscopy. As is common for these systems, a linear relationship between the positioning of the lowest-energy absorption and the highest-energy photoluminescence maxima plotted versus inverse conjugation length is observed, in good agreement with a simple nearly free electron model, one of the earliest descriptions of the properties of one-dimensional organic molecules. It was observed that the Stokes shift and therefore Huang-Rhys factor also exhibit a well-defined relationship with increasing conjugation length, implying a correlation between the electron-vibrational coupling and chain length. This correlation is further examined using Raman spectroscopy, whereby the integrated relative Raman scattering is seen to behave superlinearly with chain length. The Stokes shift and the Raman activity are also well-correlated in these systems. There is a clear indication that the vibrational activity and thus nonradiative decay processes are controllable through molecular structure.

Materials for organic solar cells: the C60/π-conjugated oligomer approach

This tutorial review surveys recent advances in the field of C60/pi-conjugated oligomer donor-acceptor ensembles. In particular, different synthetic strategies are discussed that were developed to link pi-conjugated oligomers, as versatile photoexcited state electron donors, to C60. We highlight relationships between the nature/structural aspects of pi-conjugated donor systems and a variety of physico-chemical features. Modifications of the oligomeric components are discussed under aspects of tailoring (i) the absorption cross-section of the chromophore in the visible region, (ii) the oxidation potential of the oligomeric donor moiety, (iii) the size, shape, or chemical makeup of the oligomer, and (iv) the stabilization of the charge-separated radical ion pairs. In the final section, the applicability of selected materials for the fabrication of photovoltaic devices is analyzed.