Π-conjugated Macromolecules Research Articles

Excitation energy transfer and vibronic relaxation through light-harvesting dendrimer building blocks: A nonadiabatic perspective.

The light-harvesting excitonic properties of poly(phenylene ethynylene) (PPE) extended dendrimers (tree-like π-conjugated macromolecules) involve a directional cascade of local excitation energy transfer (EET) processes occurring from the "leaves" (shortest branches) to the "trunk" (longest branch), which can be viewed from a vibronic perspective as a sequence of internal conversions occurring among a connected graph of nonadiabatically coupled locally excited electronic states via conical intersections. The smallest PPE building block that is able to exhibit EET, the asymmetrically meta-substituted PPE oligomer with one acetylenic bond on one side and two parallel ones on the other side (hence, 2-ring and 3-ring para-substituted pseudo-fragments), is a prototype and the focus of the present work. From linear-response time-dependent density functional theory electronic-structure calculations of the molecule as regards its first two nonadiabatically coupled, optically active, singlet excited states, we built a (1 + 2)-state-8-dimensional vibronic-coupling Hamiltonian model for running subsequent multiconfiguration time-dependent Hartree wavepacket relaxations and propagations, yielding both steady-state absorption and emission spectra as well as real-time dynamics. The EET process from the shortest branch to the longest one occurs quite efficiently (about 80% quantum yield) within the first 25fs after light excitation and is mediated vibrationally through acetylenic and quinoidal bond-stretching modes together with a particular role given to the central-ring anti-quinoidal rock-bending mode. Electronic and vibrational energy relaxations, together with redistributions of quantum populations and coherences, are interpreted herein through the lens of a nonadiabatic perspective, showing some interesting segregation among the foremost photoactive degrees of freedom as regards spectroscopy and reactivity.

Donor-Acceptor Conjugated Macrocycles with Polyradical Character and Global Aromaticity.

SummaryPolyradical character and global aromaticity are fundamental concepts that govern the rational design of cyclic conjugated macromolecules for optoelectronic applications. Here, we report donor-acceptor (D−A) conjugated macromolecules with and without π-spacer derivatives to tune the antiferromagnetic couplings between the unpaired electrons. The macromolecules without π-spacer have a closed-shell electronic configuration and show global nonaromatic character in the singlet and lowest triplet states. However, the derivatives with π-spacer develop a nearly pure open-shell diradical and a very high polyradical character, not reported for D−A type macromolecules. Furthermore, the π-spacer derivatives display global nonaromaticity in the singlet ground state, but global aromaticity in the lowest triplet state, according to Baird's rule. The absorption spectra of the open-shell macromolecules calculated with time-dependent density functional theory indicate intensive light absorption in the near-infrared region and broadening to 2,500 nm, making these materials suitable for numerous optoelectronic applications.

Precise molecular shape control of linear and branched strips with chirality-assisted synthesis

ABSTRACT This account describes the latest developments in the field of chirality-assisted synthesis (CAS). CAS has emerged as a versatile method to control the shapes of π-conjugated macromolecules and supramolecular assemblies. With CAS, the sequence and chirality of the monomeric building blocks dictates the shape and functions of the resulting macromolecules in a programmable fashion. Supramolecular materials with special functions, e.g. the ability to encapsulate macrocycles, can be created with CAS. CAS has been utilized to synthesize a variety of shape-defined nanoscale structures, including polyaromatic strips, helices, molecular claws, and very large hydrogen-bonded closed-shell capsules. Applications, challenges, and future directions of the growing CAS field are discussed in light of the unique shape-control abilities CAS has to offer.

Interaction of anionic dyes with polyaniline implanted cellulose: Organic π-conjugated macromolecules in environmental applications

The selective response of most of the adsorbents towards a particular dye creates problem in launching them as adsorbent in real field applications. Owing to that we have synthesized organic macromolecule of polyaniline implanted cellulose through vapor phase polymerization method. Polyaniline implanted cellulose was found as a cationic charge transport medium and thus enthusiastically chosen to observe its interaction with anionic dyes as adsorbent media. The prepared material was found crystalline in X-ray diffraction (XRD) measurement. Field emission scanning electron microscopy (FESEM), Raman spectroscopy and optical absorbance, measurements were carried out to explore its physical properties. The polyaniline implanted cellulose substrate has shown excellent adsorptive response with anionic dyes of methyl orange (MO) and eosin yellow (EY). More than 97% removal efficiency was observed at optimum experimental conditions for both the dyes and the MO dye adsorption process followed Langmuir isotherm strictly. However, Freundlich isotherm model has shown better fit for equilibrium adsorption data of EY dye onto polyaniline implanted paper. Kinetic analysis indicated the applicability of second order kinetic model with involvement of intra-particle diffusion. The polaron and bipolaron charged defect states in the polyaniline chain played the key role in adsorption of anionic dyes through chemical interaction. Thus it appeared as an efficient adsorbent for anionic dyes with wide range of selectivity.

Direct Arylation Strategies in the Synthesis of π-Extended Monomers for Organic Polymeric Solar Cells.

π-conjugated macromolecules for organic polymeric solar cells can be rationally engineered at the molecular level in order to tune the optical, electrochemical and solid-state morphology characteristics, and thus to address requirements for the efficient solid state device implementation. The synthetic accessibility of monomers and polymers required for the device is getting increasing attention. Direct arylation reactions for the production of the π-extended scaffolds are gaining importance, bearing clear advantages over traditional carbon-carbon forming methodologies. Although their use in the final polymerization step is already established, there is a need for improving synthetic accessibility to implement them also in the monomer synthesis. In this review, we discuss recent examples highlighting this useful strategy.

Synthesis and thin-film self-assembly of radical-containing diblock copolymers

Abstract

Porphyrin–Bodipy combination: synthesis, characterization and antenna effect

This paper described the synthesis of multichromophoric and π-conjugated macromolecules consisting of two equal Bodipy units and one porphyrin unit (free base). Then, a zinc(II) complex of this macromolecule based on Bodipy and porphyrin was prepared by a classical complex reaction. The formation of nitroporphyrin, aminoporphyrin, Bodipy–porphyrin (PB) and Bodipy–metalloporphyrin (Zn–PB) was confirmed by 1H-NMR, 13C-NMR, elemental analysis, UV-vis/fluorescence spectra, melting point and ESI-TOF-MS techniques. Moreover, the photophysical properties of chromophoric groups such as absorption, emission and quantum yields were investigated for the photoinduced energy transfer in three different organic solvents. The result indicated that the emission of Bodipy in the Bodipy–porphyrin combination was significantly quenched with the formation of the free base and zinc(II) complex of porphyrin. Efficient energy transfer from the Bodipy unit to the porphyrins (free base or Zn–chelate) was observed between 88% and 96% due to high π-conjugation. The energy transfer also occurred from Bodipy to porphyrin though the high polarity of acetonitrile.

Phenylene−Ethynylene Macrocycles as Model Systems of Interchromophoric Interactions in π-Conjugated Macromolecules

Unraveling the complex photophysics of macromolecular pi-conjugated systems requires both the development of suitable model systems to access a particular subset of a material's parameter space and the choice of matching spectroscopic techniques. We address the question of the strength of interchromophoric interactions in macromolecular systems by studying the fluorescence depolarization kinetics of a family of prototypical conjugated macrocycles. Shrinking the size of the molecular system decelerates fluorescence depolarization even though the radius of gyration decreases. Although the smaller macrocycles show faster rotational diffusion, the larger compounds exhibit an additional initial depolarization mechanism, attributed to intramolecular interchromophoric energy transfer. Comparison with fragments of the molecule illustrates that the larger macrocycles can be interpreted as bichromophoric systems, whereas the effectively parallel chromophoric elements of the smaller ring are indistinguishable in terms of polarization. The potential role of strong interchromophoric interaction is discussed. The results illustrate a subtle link between interchromophoric arrangement and ultrafast fluorescence depolarization, phenomena, which are often considered in the context of conjugated polymers: chromophoric alignment can potentially counteract the effect of polarization memory loss through energy transfer.

Functional polymers: scanning force microscopy insights

Scanning force microscopy (SFM) and related techniques make it possible to visualize polymer systems with a molecular resolution. Beyond imaging, they also enable the unveiling of a variety of (dynamic) physico-chemical properties of both isolated polymer chains and their supramolecular architectures, including structural, mechanical and electronic properties. This article reviews recent progress in the use of SFM on polymers, with a particular emphasis on the mechanical properties of copolymers and single polymer chains, as well as on the bottom-up fabrication of supramolecular polymeric (helical) nanostructures in particular based upon pi-conjugated macromolecules as building blocks for nanoelectronics. Through a detailed understanding of the polymer behavior, we propose solutions for the generation of organic functional (nano)systems.

Electro- and Magneto-Responsible Chiral Polymers

Helical polymers and macromolecules have precisely ordered stereostructures, and their potential applications include chiral separation and sensing based on molecular recognition and liquid crystalline formation by molecular ordering. Among them, πconjugated polymers with a helical structure have often exhibited unique electro- and magnetoresponsibilies. In this review, we describe the helical πconjugated macromolecules as electro- and magneto-responsible materials including our recent studies. In Chapter 2, the recent studies of helical polymeric molecules with πconjugated structures are reviewed. We focus on "helicene" derivatives, which consist of fused-benzene and/or thiophene rings, in Chapter 3 and summarize their syntheses, characteristics, and optical properties based on the stiff structures and molecular ordering. Helical ladder polymers, "poly(thiaheterohelicene)s" comprised of fused-benzothiophene rings, are discussed in Chapter 4 among their stiff helical structure and πconjugation. The combined functions of electro- and magnetoproperties of the chiral polymer are described using the example of poly(thiaheterohelicene)s, such as electron transmission along the helical main-chain and a molecule model of a molecular solenoid, in the final section. Keywords: conductive polymers, Polyacetylenes, MACROMOLECULES, Polythiophenes, Helicenes, Magnetic circular dichroism

Poly(p-phenylenephosphaalkene): A π-Conjugated Macromolecule Containing PC Bonds in the Main Chain

Niedervalenter Phosphor im Polymerrückgrat zeichnet ein neues, gelbes Polymer aus (siehe im Bild rechts), das in polaren organischen Solventien löslich ist. Durch 31P-NMR-spektroskopische Endgruppenanalyse wurden Molekulargewichte zwischen 2900 und 10 500 g mol−1 ermittelt. Die mögliche π-Konjugation wurde durch UV/Vis-Spektroskopie untersucht, wobei beim Vergleich mit farblosen Modellsystemen (links) eine Rotverschiebung von λmax beobachtet wurde.

Poly(p-phenylenephosphaalkene): a pi-conjugated macromolecule containing P=C bonds in the main chain.

Poly(p-phenylenephosphaalkene): a pi-conjugated macromolecule containing P=C bonds in the main chain.

Excitation energy migration in highly emissive semiconducting polymers

The process of excitation energy migration (EEM) in conjugated polymers, which is relevant both for light emitting diodes and laser applications, is probed by doping the blue light emitting methyl-substituted ladder-type poly( para-phenylene) with small concentrations of a highly fluorescent π-conjugated macromolecule. The experimentally attained temperature and concentration dependence of the steady state photoluminescence is modeled and discussed by means of a two-step EEM process: (1) a thermally activated migration within the host and (2) transfer from the host to the guest. In particular we show that a Förster-type mechanism alone cannot account for the experimental facts in such a guest host system.

Nanoribbons from conjugated macromolecules on amorphous substrates observed by SFM and TEM

Micrometre long nanoribbons have been grown from solutions of functionalized poly( para-phenyleneethynylene)s (PPE)s on noncrystalline insulating substrates including glass and carbon coated copper grids. Tapping mode scanning force microscopy (SFM) and transmission electron microscopy (TEM) revealed that these nanostructures possess a molecular cross section with a typical thickness of 2-3 molecular layers and a width which reflects the distribution of macromolecular lengths. The ribbons are therefore quite similar to the ones found on the crystalline mica substrate except that they are not oriented within the surface plane. This indicates that the growth of these architectures from solution is mainly governed by intermolecular interactions between the π-conjugated macromolecules. The possibility to self-assemble these nanoribbons also on amorphous silica opens a prospect for their application as molecular nanowires bridging gold nanoelectrodes grown on oxidized silicon wafers.