Dimensionally oriented redox-active π-conjugated systems

Porphyrins bearing the redox-active phenylenediamine pendant groups are synthesized to afford dimensionally oriented π-conjugated systems. The structural and electronic characteristics depend on the atropisomers. In the fluorescence emission spectra, the emission from the porphyrin moiety is almost completely quenched. Zinc complexation of the αααα isomers gives the corresponding zinc porphyrins bearing four phenylenediamine strands. Treatment with a bidentate ligand, DABCO, leads to the sandwich dimer complex, in which the porphyrin moieties are surrounded by π-conjugated pendant groups. p-t-Butylcalix[4]arenes bearing four redox-active phenylenediamine pendant groups on the lower rim are synthesized and characterized both spectroscopically and electrochemically. The interconversion of the oxidation states of the pendant groups is demonstrated both chemically and electrochemically.     

Constructing a novel nonlinear optical materials: substituents and heteroatoms in π-π systems effect on the first hyperpolarizability

By doping π-π systems with Li atom, a series of Li@sandwich configuration and Li@T-shaped configuration compounds have been theoretically designed and investigated using density functional theory. It is revealed that energy gaps (E _gap) between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of all compounds are in a range of 0.4–0.9 ev. When Li atom is introduced into different sandwich configuration π-π systems (C₆₀-toluene, C₆₀-fluorobenzene, C₆₀-phenol, C₆₀-benzonitrile), Li@C₆₀-benzonitrile exhibits considerable first hyperpolarizability as large as 19,759 au, which is larger by about 18,372–18,664 au than those of other compounds. When Li atom is introduced into different T-shaped configuration π-π systems (C₆₀-pyridine, C₆₀-pyrazine, C₆₀-1, 3, 5-triazine, C₆₀-pyridazine), Li@C₆₀-pyridazine is found to present largest first hyperpolarizability up to 67,945 au in all compounds. All compounds are transparency in the deep ultraviolet spectrum range. We hope that this study could provide a new idea for designing nonlinear optical materials using π-π systems as building blocks.     

Further evidences of the quality of double-hybrid energy functionals for π-conjugated systems

Despite numerous interesting efforts along decades to improve the accuracy of density functionals with broad applicability, such as B3LYP, there are still large sets of molecular systems where improvements are badly needed. We select π-conjugated systems as an example of the latter due to the subtle interplay between some physical effects affecting possibly most of the calculations: self-interaction or delocalization error, medium-range correlation signatures, dispersive-like weak interactions, and static correlation effects. We further assess a recently proposed modification of the B2-PLYP double-hybrid functional, called B2π-PLYP, that is expected to yield substantial progress for this kind of systems. Generally speaking, when compared with other more popular and older density functionals, double hybrids behave particularly accurate for π-conjugated systems without suffering the large errors that are common in former yet conventional methods.     

DNA-templated assembly of dyes and extended π-conjugated systems

This feature article reports on the use of DNA as a template to assemble dyes and π-conjugated systems with the aim to construct functional multicomponent nanostructures that have a well-defined size, shape and sequence.     

4-substituted push-pull quinazoline chromophores with extended π-conjugated linker

A series of four push-pull compounds based on a 4-phenylquinazoline scaffold were designed. Three of these compounds exhibit dual state emission with intense fluorescence in solution and solid state. Their emission properties are sensible to environement stimuli with intense emission solvatochromism and halochromism. In case of methoxy derivative, both neutral and protonated form are luminescent and white light emission can be observed by controlled partial protonation.     

Spectroscopic probes with changeable π-conjugated systems

Spectroscopic probes have been extensively investigated and used widely in many fields because of their powerful ability to improve analytical sensitivity, and to offer greater temporal and spatial resolution (in some cases a molecule event may be visualized by the naked eye). So far, different photophysical mechanisms, such as charge transfer, photo-induced electron transfer and fluorescent resonance energy transfer, have been employed to develop various spectroscopic probes with superior properties. However, these photophysical mechanisms depend on the energy levels of molecular orbitals, which are usually difficult to accurately determine. This would lead to the poor prediction of analytical performance of the designed probe. Instead, the change of π-conjugated systems induced by chemical reactions is often accompanied by a distinct alteration in spectroscopic signal, which is more predictable and is of high signal/background ratio. This mechanism can serve as an effective measure for developing excellent spectroscopic probes, but to our knowledge, has not been systematically summarized. In this feature article, we review the development of spectroscopic probes with changeable π-conjugated systems, which is catalogued according to the fluorochromes: fluorescein, rhodamine, spiropyran, squaraine, coumarin, cyanine, etc. Two main strategies for constructing these spectroscopic probes, including ring-closing reaction and nucleophilic addition reaction, are summarized, and the merits and limitations of the probes are discussed.     

Recent advances with π-conjugated salen systems

Schiff base ligands have long been successfully employed as ligands in combination with various metals to give catalysts capable of realizing a variety of synthetic transformations. One of the most widely used Schiff base ligands, the "salen" ligand, has been extensively researched. Recently, there has been increased interest in π-conjugated salen systems, known as "salphen" ligands, as a result of the differences in reactivity of the complexes in catalytic applications compared with the salen analogues. Complexes of salphen ligands display interesting photophysical and supramolecular properties which are not always observed with salen systems as a result of their π-conjugation. This tutorial review therefore describes the most significant advances recently made with salphen and related π-conjugated ligand systems.     

Reviewing extrapolation procedures of the electronic properties on the π-conjugated polymer limit

In this article, the extrapolation procedures of π-π* electronic transition energy on π-conjugated oligomers are reexamined. Different models, including the simplest coupled oscillator, the free electron, the Hückel approach, the molecular exciton model, and some specific fitting-functions, are compared using the transition energies derived from theoretical calculations on three thiophene-based oligomer series. Specifically, oligomers of up to 30 repeating units have been considered to include the saturation effects as a function of chain length. The coupled oscillator model of W. Kuhn and the fitting-function of Hirayama are the models that present the better suit on the transition energy interpolation as a function of chain length. Using only the first four oligomers of the series (n = 2 up to 8) yields an estimation of the transition energy on the polymer limit with an average error of ～1.5%. The vertical and adiabatic ionization potential present a better fit with the Hückel model approach. Finally, implications of the environmental polarity on the electronic properties, molecular geometry, charge distribution, and aromaticity are shortly discussed.     

Long-range corrected hybrid functionals for π-conjugated systems: dependence of the range-separation parameter on conjugation length

Long-range corrected (range-separated hybrid) functionals represent a relatively new class of functionals for generalized Kohn-Sham theory that have proven to be very successful, for instance, when it comes to predicting ionization potentials and energy gaps for a wide range of molecules and solids. The results obtained from long-range corrected density functional theory approaches can be improved dramatically, if the range-separation parameter (ω) is optimized for each system separately. In this work, we have optimized ω for a series of π-conjugated molecular systems of increasing length by forcing the resulting functionals to obey the ionization potential-theorem, i.e., that their highest occupied eigenvalue be equal to the ΔSCF ionization potential. The optimized ω values are observed to vary substantially from their default values for the functionals. For highly conjugated chains such as oligoacenes and polyenes, we find that the characteristic length scale of the range-separation, i.e., 1/ω, grows almost linearly with the number of repeat units, for saturated alkane chains, however, 1/ω quickly saturates after 5-6 repeat units. For oligothiophenes, we find that 1/ω grows linearly for the shorter oligomers but then saturates at around 10 repeat units. Our results point to a close relation between the optimal range-separation parameter and the degree of conjugation in the system.     

On the effect of electron correlation on the static second hyperpolarizability of π conjugated oligomer chains

In this article, we report on the ab initio calculation of the static longitudinal second hyperpolarizability (γ) of π conjugated unsaturated oligomer chains using polyacetylene and polyyne as model compounds. The common observation is that the electron correlation enhances γ in these systems. The present study reveals that for extended chain lengths the opposite appears to be true: Electron correlation may have a damping effect on this property. For double-zeta basis sets, a negative contribution from electron correlation to γ is found within the range of chain lengths investigated. For triple-zeta basis sets, the same behavior must be anticipated at larger chain lengths based on extrapolation schemes. The analysis of the excitation energies and transition moments shows that transition moments between excited states as predicted by the Hartree-Fock and coupled cluster methods have a different response to chain length extension. There also are indications that higher order correlation effects will enhance γ.     

Topography of scalar fields: molecular clusters and π-conjugated systems

The pioneering works due to Bader and co-workers have generated widespread interest in the study of the topography of molecular scalar fields, the first step of which is the identification and characterization of the corresponding critical points (CPs). The topography of a molecular system becomes successively richer in going from the bare nuclear potential (BNP) to the molecular electrostatic potential (MESP) through the molecular electron density (MED). The present work clearly demonstrates, through the study of some π-conjugated test molecules as well as molecular clusters, that the CPs could be economically located by following this path within ab initio level theory. Further, the topography mapping of large molecules, especially at a higher level of theory, is known to be a demanding task. However, it is rendered possible by following the above sequential mapping assisted by a divide-and-conquer-type method termed as the molecular tailoring approach (MTA). This is demonstrated with the topography mapping of β-carotene and benzene nonamer at MP2 and a (H(2)O)(32) cluster at the HF level of theory, which are rather challenging problems with contemporary off-the-shelf computer hardware.     

N-heterocyclic carbene boranes as electron-donating and electron-accepting components of π-conjugated systems

Give and take: The introduction of NHC-borane moieties to thiophene-based π?skeletons endows a zwitterionic character, which makes the π?system electron-donating, while the NHC ring acts as an electron-accepting moiety. The NHC-borane-substituted thiophene underwent a clean photoisomerization with a drastic color change, however, the expanded bithiophene derivatives were inert to this photoreaction, showed low oxidation potentials, and formed a slipped π-stacked array in the crystal.     

Rhodium-catalyzed hydrothiolation of alkynes with thiols for construction of sulfur-containing π-conjugated systems

To synthesize sulfur-containing π-conjugated polymers, reaction conditions for rhodium-catalyzed hydrothiolation of terminal alkynes with arenethiols are optimized in detail. Under the optimized conditions, rhodium-catalyzed hydrothiolation of terminal alkynes proceeds regio- and stereoselectively to afford the corresponding vinyl sulfides via an anti-Markovnikov and syn-addition process. Then, the rhodium-catalyzed hydrothiolation is applied to polymerization of 2,5-diethynylthiophene with benzene-1,4-dithiol, which successfully provides sulfur-containing π-conjugated polymers with excellent regio- and stereoselectivities.     

First-principles approach to the understanding of π-conjugated organic semiconductors

We show how ab initio calculations based on density functional theory contribute to the understanding of the electronic and optical properties of organic semiconducting materials, which form the active layers in many opto-electronic applications. As a textbook example, we present the electronic structure and the optical properties of the oligo-phenylenes as evolving from their benzene-constituents. Thereby we discuss the dependence on the molecular length and introduce the modifications in the opto-electronic properties due to intermolecular interactions which are inherently present in the bulk phase.     

Quantum-coherence and correlations in π-conjugated molecules and multichromophoric systems

Chemistry is intrinsically founded on quantum mechanical principles and examples of quantum-mechanical phenomena abound on a range of energy and length scales. In this article some examples of quantum-mechanical phenomena that can be probed by optical spectroscopy are discussed. Recent experimental studies of quantumcoherence in electronic energy transfer in π-conjugated polymers are reported as examples of weak correlations. The nature of the electron-hole binding energy for excitons in organic systems is investigated as a case of intermediate correlations. Possible experimental probes of strong correlations involved in chemical reactions are critically examined in the final section of the paper.     

Synthesis and anticancer activity of new azo compounds containing extended π-conjugated systems

A series of novel azo compounds with extended π-conjugated systems were prepared by azo coupling reaction compounds trans-2-(4′-aminostyryl)-thiophene, 1-(4-aminophenyl)-4-phenyl-1,3-butadiene and 4-amino-4′-methoxystilbene with some phenols. The compounds were evaluated for their cytotoxicity against breast cancer adenocarcinoma (MCF-7), cervix adenocarcinoma (HeLa) and human embryonic kidney (HEK 293) cell lines using the MTT assay. The results showed all derivatives had more toxic effects than tamoxifen. Of all the compounds tested, the azo product obtained from coupling trans-2-(4′-Aminostyryl)-thiophene with 2-naphthol (compound 5b) exhibited the potent in vitro antiproliferative activity with IC₅₀ 27 ± 1 and 18 ± 0 µg/mL against MCF-7 and HeLa cell lines, respectively, while it was devoid of any cytotoxicity against normal HEK 293 cells even at 200 µg/mL.     

Substituent and counterion effects on the formation of π-dimer dications of end-capped heptathienoacenes

We have investigated the impact of the functionalization and the chemical nature of counterions on the π-dimer dications formation in two end-capped heptathienoacenes. Radical cations of an α-substituted heptathienoacene with triisopropylsilyl groups do not π-dimerize, while those of an α,β-substituted heptathienoacene with four n-decyl side chains show a high propensity toward π-dimerization, increased by PF(6)(-) counterions.