Π-stacked Polymers Research Articles

Surface hopping simulations on charge photogeneration in conjugated polymers.

The mechanism of charge photogeneration in neat conjugated polymers has long been controversial and a unified explanation has not been achieved so far. In this paper, we use a surface hopping method to simulate the excited-state dynamics of a system composed of five π-stacked polymer chains. In this system, polaron pairs (PPs) with large electron-hole distance can be seen as free charges. The surface hopping method is based on the Pariser-Parr-Pople (PPP) Hamiltonian and the excited states of the system are calculated with configuration interaction singles (CIS) formalism. During the simulations, the yields of PPs and free charges are calculated using a statistical method. By comparison, it is found that impurity and excess energy have significant effects on the yields of PPs and free charges. Free charges are difficult to be generated in neat systems with small excess energy. Free charges come from direct dissociation of high-energy excitons.

Fluorene pendant-functionalization of poly(N-vinylcarbazole) as deep-blue fluorescent and host materials for polymer light-emitting diodes

π-Electron coupling of pendant conjugated segment in π-stacked semiconducting polymers always causes the formation of defect trapped sites and further quenched high-band excitons, which is harmful to the performance and stability of deep-blue polymer light-emitting diodes (PLEDs). Herein, considerate of “defect” carbazole (Cz) electromers in poly(N-vinylcarbazole) (PVK), a series of fluorene units are introduced into pendant segments (PVCz-DMeF, PVCz-FMeNPh and PVCz-DFMeNPh) to suppress the strong π-electron coupling of pendant Cz units and enhance radiative transition toward fabricating sable PLEDs. Compared to PVCz-FMeNPh and PVCz-DFMeNPh, PVCz-DMeF spin-coated films show a relatively efficient deep-blue emission, completely similar to its single pendant chromophore, confirmed an extremely weak charge-transfer and electron coupling between adjacent pendant segments. Therefore, PLEDs based on PVCz-DMeF present stable and deep-blue emission with a high color purity (0.17, 0.08), associated with extremely weak defect emission at 600∼700 nm (induced by carbazole electromers). Finally, PLEDs based on PVCz-DMeF/F8BT blended films (1:1) also present the high maximum luminance (Lmax) of 6261 cd/m2 and current efficiency (CEmax) of 2.03 cd/A, confirmed slightly trapped sites formation. Therefore, precisely control the arrangement and packing model of pendant units in π-stacked polymer is an essential prerequisite for building efficient and stable emitter for optoelectronic devices.

Construction of Helically Stacked π-Electron Systems in Poly(quinolylene-2,3-methylene) Stabilized by Intramolecular Hydrogen Bonds.

π-Stacked polymers, which consist of layered π-electron systems in a polymer, can be expected to be used in molecular electronic devices. However, the construction of a stable π-stacked structure in a polymer is considerably challenging because it requires sophisticated designs and precise synthetic methods. Herein, we present a novel π-stacked architecture based on poly(quinolylene-2,3-methylene) bearing alanine derivatives as the side chain, obtained through the living cyclo-copolymerization of an o-allenylaryl isocyanide. In the resulting polymer, the neighboring quinoline rings of the main chain form a layered structure with π-π interactions, which is stabilized by intramolecular hydrogen bonds. The vicinal quinoline units form two independent helices and the whole molecule is a twisted-tape structure. This structure is established on the basis of UV/CD spectra, theoretical calculations, and atomic-force microscopy.

π-Stacked Polymer Consisting of a Pseudo⁻meta⁻[2.2]Paracyclophane Skeleton.

A novel π-stacked polymer based on a pseudo–meta–linked [2.2]paracyclophane moieties was synthesized by Sonogashira-Hagihara coupling. The UV-vis absorption spectra of the synthesized polymer and model compounds revealed an extension of the conjugation length owing to the through-space conjugation. The optical properties of the π-stacked dimer with the pseudo–meta–linked [2.2]paracyclophane unit were compared with those of the corresponding dimers with the pseudo–ortho– and pseudo–para–linked [2.2]paracyclophane units.

Cationic polymerization of dibenzofulvene leading to a π-stacked polymer

Cationic polymerization of dibenzofulvene is conducted using BF3-OEt2, CH3SO3H, CF3CO2H, and CH3CO2H as catalyst or initiator in CH2Cl2 under various conditions. The cationic polymerization leads to controlled stereochemistry and results in a π-stacked polymer through the polymerization. The polymers synthesized using BF3-OEt2 and CH3SO3H appear to possess different π-stacked structures. The polymer made using CH3SO3H has an all-trans main-chain C-C bond conformation while the one prepared using BF3-OEt2 may at least partially have a trans-gauche main-chain C-C bond conformation. π-Stacked conformations are confirmed on the basis of remarkable anisotropy effects on aromatic signals in 1H NMR spectra, hypochromic effects in electronic absorbance spectra, and dimer (excimer) emission in fluorescence spectra. π-Stacked conformation is found even for oligomer samples with Mn of as low as about 700, indicating that the conformation created by the cationic polymerization is rather stable.

Aggregation-Induced Förster Resonance Energy Transfer in Polybenzofulvene/Dye Nanoparticles

Polymeric nanoparticles are versatile scaffolds to develop smart multichromophoric systems whose collective optical properties can be tuned in view of a specific application. Addition of water (nonsolvent) to diluted THF solutions of a blue emitting π-stacked polymer and a yellow emitting dye induces nanoparticle polymer/dye coaggregation. At 70% water content, concomitantly to nanoparticle shrinking, the emission efficiency increases and the color switches from blue to yellow. Time-resolved fluorescence analysis demonstrates that the switch in the nanoparticle emission is governed by an aggregation-induced Forster resonance energy transfer (FRET) process from the polymer to the molecularly embedded dye activated at a local concentration of about 10–2 M within the nanoparticles.

Interchain Helically π-Stacked Assembly of Cationic Chiral Poly(para-phenylene) Derivatives Enforced by Anionic π-Conjugated Molecules through Both Electrostatic and π–π Interactions

Herein, a novel approach for preparing ionic, helically π-stacked polymer assemblies with high dissymmetry factors is presented. We synthesized cationic chiral poly(para-biphenylene) [PPP1] and poly(para-terphenylene) [PPP2] derivatives by introducing alkyl substituents containing both asymmetric centers and quaternary ammonium cations into the side chains of the polymers. We discovered that these polymers have an intrachain helical structure bearing one-handed twisted main chains in solution; however, there was only a slight preference for one twisted conformation over its opposite. Subsequently, by adding an anionic π-conjugated molecule to the cationic polymers, we prepared polymer assemblies bearing an interchain helically π-stacked structure between the polymers and the molecules. Additionally, we clarified that the anionic molecule functioned as the binding species that closed the cationic π-conjugated polymers through both electrostatic and π–π interactions. Although the polymer assembly of PPP1 wa...

π-Stacked polymers in drug delivery applications

Polybenzofulvenes are π-stacked polymers, which can be synthesized by spontaneous polymerization of the corresponding monomers without the use of catalysts or initiators. Therefore, they can be obtained completely free from byproducts, impurities, or harmful substances. The absence of any relevant toxic effects and cell viability impairments allows PEGylated polybenzofulvene brushes to be potentially functional in a wide range of biological, biomedical, and biotechnological applications. Moreover, the properties of these polymers, in terms of interaction with pharmacological active agents and the ability to self-assemble into nanoaggregates or a quite compact physical gel useful as drug delivery systems (DDSs), can be controlled by varying side chain moieties. Owing to the important role played by self-assembling DDS in the fields of the material and life sciences, the interaction and the delivery ability of polybenzofulvene polymers with model drug or protein molecules was definitively demonstrated. The present paper reviews the applications of polybenzofulvene derivatives in the drug delivery of a range of different drug molecules ranging from small molecules to peptides and proteins.

A photo-stable and electrochemically stable poly(dumbbell-shaped molecules) for blue electrophosphorescent host materials

Hindrance functionalization at the reactive sites makes π-stacked polymers highly stable with the potential applications related to organic electronic devices.

π-Stacked polybenzofulvene derivatives as hosts for yellow and red emitting OLEDs

A new class of π-stacked polymers possessing interesting charge transport properties is represented by polydibenzofulvene derivatives. We study the electroluminescence properties of three π-stacked polybenzofulvene derivatives and compare them to those of the standard π-stacked poly(N-vinylcarbazole). Solution processable devices using one of these polymers as host with benzothiadiazole based dyes display performances overcoming those of the standard poly(N-vinylcarbazole) host. From a photophysical study, the superior performances of the polybenzofulvene containing a fluorene unit at the position 6 of the indene nucleus are explained to arise from its ability to self-assemble with the dyes in supramolecular organizations that reduce their microaggregation.

Manipulating charge transport in a π-stacked polymer through silicon incorporation

The charge transport of a π-stacked polymer was manipulated by silicon incorporation to realize molecular nanofuse devices in a silicon-stimulated filament mechanism.

Hindrance-functionalized π-stacked polymer based on polystyrene with pendent cardo groups for organic electronics

A hindrance-functionalized π-stacked polymer, poly(4-(9-phenyl-fluoren-9-yl)styrene) (PPFS), was designed and synthesized by the introduction of bulky 9-phenylfluorenyl moieties (PFMs) with 3D cardo structure into all the phenyl units of polystyrene (PS) through the sp3 carbon linkage. The PFMs obviously alter π-stacked conformation and interchain interactions with respect to the precursor PS. The suitable HOMO–LUMO energy gap, high triplet energy, and excellent thermal and morphological stabilities enable PPFS to serve as not only electrically bistable materials but also phosphorescent hosts. Memory devices using PPFS as an active layer exhibited nonvolatile flash memory performance with ON/OFF current ratio up to 104, high stability in retention time up to 104 s and a number of read cycles up to 105 under a read voltage of 1.5 V in both ON and OFF states, while PS based devices exhibited no memory performance. Polymer light-emitting devices (PLEDs) using PPFS as the host and FIrpic as the guest exhibited blue emission with CIE chromaticity coordinates (x, y) of (0.146, 0.349). These results indicate that hindrance-functionalization opens a way to transform commercially available general purpose polystyrenes into functional π-stacked macromolecules.

Highly Photoconducting π-Stacked Polymer Accommodated in Coordination Nanochannels

We report on the formation of single poly(N-vinylcarbazole) (PVCz) chains in one-dimensional channels of [La(1,3,5-benzenetrisbenzoate)](n), where the side carbazolyl groups of the confined PVCz are effectively π-stacked. This ideal conformation of PVCz chains in the coordination nanochannels contributed to a drastic increase in hole mobility, which was 5 orders of magnitude higher than that in the bulk state. It is also noteworthy that PVCz isolated from the nanchannels still had a high hole mobility.

The steric effect of aromatic pendant groups and electrical bistability in π-stacked polymers for memory devices

In order to investigate the steric effect of aromatic pendant groups and the electrical bistability in nonconjugated polymers potentially for memory device applications, two π-stacked polymers with different steric structures are synthesized and characterized. They exhibit two conductivity states and can be switched from an initial low-conductivity (OFF) state to a high-conductivity (ON) state. Additionally, they demonstrate nonvolatile write-once-read-many-times (WORM) memory behavior with an ON/OFF current ratio up to 10(4), and flash memory behavior with an ON/OFF current ratio of approximately 10(5). Both steady-state and time-resolved fluorescence spectroscopies are used to examine the conformational change of the polymers responding to an applied external electrical voltage. The results provide useful information on different steric effects of pendant groups in polymer chains, resulting in various electrical behaviors. The possibility in realizing an "erasable" behavior through breaking π-stacked structures of pendant groups by a reversal of the electric field was also discussed on the basis of temperature-dependent fluorescence spectroscopy investigation. These results may thus offer a guideline for the design of practical polymer memory devices via tuning steric structure of π-stacked polymers.

A click chemistry-based “grafting through” approach to the synthesis of a biorelevant polymer brush

A new biorelevant polymer brush showing a polybenzofulvene backbone was synthesized by a “grafting through” approach based on click chemistry and spontaneous polymerization reactions. The easy polymerization of the relatively complex monomer (6-MOEG-9-TM-BF3k) suggests the existence of a particularly efficient recognition process capable of pre-organizing the monomer molecules for the spontaneous polymerization. 13C-NMR spectroscopy as well as UV-vis and fluorescence spectroscopy suggested for poly-6-MOEG-9-TM-BF3k the features of a vinyl (1,2) π-stacked polymer. The new polybenzofulvene derivative was found to interact with water at room temperature to give clear water solutions, but TEM analysis demonstrated the presence of macromolecular aggregates showing dimensions larger than those suggested by SEC-MALS analysis for the isolated macromolecules. DLS studies confirmed the presence of objects showing average dimensions in the range of 200–300 nm and suggested thermoresponsive colloidal properties for poly-6-MOEG-9-TM-BF3kmacromolecules. Finally, owing to its favourable absorption/emission properties and water solubility, the interaction of poly-6-MOEG-9-TM-BF3k with live cells was studied by fluorescence microscopy experiments, which revealed that the polymer brush was unable to enter live cells and alter cell morphology.

Naphthalene-based oligothiophene-stacked polymers

We report the synthesis and properties of π-stacked polymers consisting of oligothiophene and naphthalene as the stacked π-system and the scaffold, respectively. The titled polymers were obtained by the Suzuki–Miyaura coupling reaction. Oligothiophene units were layered in proximity, ∼3.0 Å from each other. Contribution of the quinoidal structure of the oligothiophene units involving the naphthalene scaffolds in the excited state resulted in relatively high photoluminescence quantum efficiencies. The polymers have potential application to optoelectronic devices such as hole-transporting materials. Oligothiophene-stacked polymers were synthesized by the Suzuki–Miyaura coupling reaction. Oligothiophene units were layered in proximity, ∼3.0 Å from each other. The polymers were characterized by various spectroscopic and electrochemical methods. The polymers have potential application to optoelectronic devices such as hole-transporting materials.

Photoinduced Charge Carrier Generation in Blends of Poly(Thienothiophene) Derivatives and [6,6]-Phenyl-C61-butyric Acid Methyl Ester: Phase Segregation versus Intercalation

The morphology, optical properties, and photoconductance of blends of the poly(thienothiophene) derivatives poly(3,6-dialkylthieno[3,2-b]thiophene-co-bithiophene) (pATBT), poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT), and poly(2,5-bis(3-dodecylthiophen-2-yl)thieno[2,3-b]thiophene) (pBTCT) with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were studied. After thermal annealing, the pATBT:PCBM blend exhibits formation of phase-segregated polymer and PCBM domains. Annealing of pBTTT:PCBM and pBTCT:PCBM yields a layered structure with PCBM molecules intercalated between layers of π-stacked polymers. In the intercalated systems the photoluminescence is almost completely quenched, in contrast to the phase-segregated pATBT:PCBM blend. The higher degree of exciton quenching in the intercalated systems likely results in a higher initial yield of charges. However, on longer time scales (>10 ns), the microwave photoconductance for the layered systems is lower than for pATBT:PCBM blend system...

Synthesis, structure and function of π-stacked polymers

Macromolecular conformation often has a significant effect on the properties of polymeric materials, and hence, conformational control in synthesizing a polymer is an important goal in polymer science. As a new conformational motif of vinyl polymers having side-chain π-electronic groups, we have introduced a ‘π-stacked structure’ in which side-chain chromophores are regularly stacked on top of each other and main-chain C–C bonds have a nearly all-trans, zigzag conformation. π-Stacked polymers can be prepared by vinyl polymerization of dibenzofulvene (DBF) and its derivatives. On the basis of the controlled structure, poly(DBF) and its derivatives exhibit unique photophysical and electronic properties as vinyl polymers. In addition, introduction of chirality to π-stacked polymers was also achieved. In this review, the synthesis of poly(DBF) and its derivatives and their structural analyses and properties are described.

Synthesis of π-Stacked Polymers on the Basis of [2.2]Paracyclophane

Abstract We synthesized π-stacked polymers by incorporating [2.2]paracyclophane units into conjugated polymer backbones and developed corresponding through-space-conjugated polymers and aromatic-ring-layered polymers. We also synthesized [2.2]paracyclophane-containing polyamidoamine (PAMAM) dendrimers and optically active polymethylenes. These polymers and dendrimers are the first well-defined and well-characterized conjugated polymers with cyclophane units in their main chain. We studied the properties of the [2.2]paracyclophane-containing through-space-conjugated polymers. We demonstrated the use of these through-space-conjugated polymers in optoelectronic applications such as an EL device. Further, we developed a synthetic strategy for new π-stacked polymers comprising layered aromatic rings on a xanthene skeleton as the scaffold. The occurrence of fluorescence resonance energy transfer from the layered cyclophanes to the end-capping aromatic moieties was verified.

Solution Behavior of Dendrimer-Coated Rodlike Coordination Polymers

We report here the synthesis and self-assembly behavior of a series of three coordination polymers (1−3) insulated with different generations and lengths of oligoether dendrons. Their self-assembly behavior in solutions was investigated according to the variation in generation of dendrons and solvent polarity by using spectroscopic measurements, molecular calculation, and transmission electron microscopy (TEM). In contrast, polymer 1 based on first-generation dendrons with diethylene oxide unit and polymers 2 and 3 based on higher volume fractions of dendrons are readily soluble in common organic solvents and H2O. Polymer 2 based on first-generation dendrons with triethylene oxide unit was observed to self-assemble into nanorods consisting of two π-stacked polymer backbones in both nonpolar CHCl3 solvent and film, whereas 2 exists as isolated single polymer chains in polar solvents. Polymer 3 based on second-generation dendrons with tetraethylene oxide units also exists as isolated single polymer chains in both solution and film states. Notably, the CHCl3 solution of 2 at concentrations above 2 wt % was observed to be a nematic fluid, resulting from the formation of rigid polymer backbone driven by π-stacking, whereas the THF solution of 2 undergoes gelation at concentrations above 2 wt %. The primary driving force for this unique self-assembling behavior is proposed to be the solvation ability of solvent molecules to silver salts and the coverage ability of dendrons to insulate the conjugated backbones.