Helical Polyisocyanides Research Articles

Cover Picture: Unexpected Thermally Stable, Cholesteric Liquid‐Crystalline Helical Polyisocyanides with Memory of Macromolecular Helicity (Chem. Asian J. 6/2007)

A good memory for macromolecular helicity is displayed by the one‐handed helical sodium salt of poly(4‐carboxyphenyl isocyanide) after the complete removal of the optically active amines that induced the helicity. This memory persists even with further modification to the side groups of the amide residues, thus giving designer helical polyisocyanides with outstanding thermal stability in solution, some of which also exhibit liquid crystallinity. To find out more, turn to the Full Paper by E. Yashima et al. on page 755 ff.

Synthesis and Characterization of Long Perylenediimide Polymer Fibers: From Bulk to the Single-Molecule Level

The synthesis and characterization of perylenediimide polyisocyanides is reported. In addition to short oligomers, our synthetic approach results in the formation of extremely long, well-defined, and rigid perylenediimide polymers. Ordering and close-packing of the chromophores in these long polymers is guaranteed by attachment to a polyisocyanide backbone with amino acid side chains. Hydrogen bonding interactions between those groups stabilize and rigidify the helical polymer structure. The rodlike nature of the synthesized long perylenediimide pendant polyisocyanides as well as the helical arrangement of the chromophores is demonstrated by means of atomic force microscopy. Remarkably, polymer fibers up to 1 mum in length have been visualized, containing several thousands of perylenediimide molecules. Circular dichroism spectroscopy reveals the chiral organization of the chromophore units in the polymer, whereas absorption and emission measurements prove the occurrence of excited-state interactions between those moieties due to the close packing of the chromophore groups. However, an intricate optical behavior is encountered in bulk as a result of the coexistence of short oligomers and long polymers of perylenediimide, a situation subsequently uncovered by means of single-molecule experiments. Individual long helical perylenediimide polyisocyanides exhibit a typical red-shifted fluorescence spectrum, which, together with depolarized emission continuously decreasing in time, demonstrate that fluorescence arises from multiple excimer-like species in the polymer. Upon continuous irradiation of these long polymers, a fast decay in fluorescence lifetime is observed, a situation explained by photoinduced creation of quenching sites. Radical/ion formation by intramolecular electron transfer between close-by perylenediimide moieties is the most probable mechanism for this process. Appropriate control of the electron-transfer process might open the possibility of applying these polymers as perylenediimide-based supramolecular nanowires.

Helix-Sense Controlled Polymerization of a Single Phenyl Isocyanide Enantiomer Leading to Diastereomeric Helical Polyisocyanides with Opposite Helix-Sense and Cholesteric Liquid Crystals with Opposite Twist-Sense

We report the unprecedented helix-sense controlled polymerization of enantiomerically pure phenyl isocyanides bearing an l- or d-alanine pendant with a long alkyl chain. The polymerization with an achiral nickel catalyst diastereoselectively proceeds, resulting in either a right- or left-handed helical polymer, whose helix-sense can be controlled by the polymerization solvent and temperature. Both the diastereomeric right- and left-handed helical polymers further self-assemble into lyotropic cholesteric liquid crystals with opposite twist-senses. Consequently, the macromolecular helicity and mesoscopic, supramolecular cholesteric twist can be controlled by the molecular chirality of the pendant of a single enantiomeric phenyl isocyanide through the polymerization under either kinetic or thermodynamic control assisted by hydrogen bonds. High-resolution atomic force microscopy revealed their helical conformations and enabled the determination of the helical sense.

Di- and Trifunctional Initiators Containing Pt−Pd μ-Ethynediyl Units for Living Polymerization of Aryl Isocyanides

Multinuclear acetylide complexes containing two or three Pd−Pt μ-ethynediyl units were prepared as multifunctional initiators for the living polymerization of aryl isocyanides. The polymerization smoothly proceeded to give two- or three-armed polymers with narrow polydispersity indexes in quantitative yields. Trace experiments using gel permeation chromatography and 31P NMR spectroscopy as well as kinetics revealed that all the Pd−Pt μ-ethynediyl units acted as initiators for the living polymerization. The polyisocyanide chains with narrow polydispersity indexes were cleaved off from the multiarmed polymers. The reaction of chiral monomer produced polymers having two and three helical chiral arms. Kinetic study of the polymerization of the chiral monomer and comparison of the chiroptical properties of resulting polymers with those of polymers prepared with Pd−Pt μ-ethynediyl complex suggested that the helical chiral polyisocyanide arms did not exhibit intramolecular interaction.

Synthesis, characterization and aggregation behavior of block copolymers containing a polyisocyanopeptide segment

Following up on previous preliminary communications the synthesis of a series of block copolymers by applying amine end-capped polymers as initiators for the nickel(II) catalyzed polymerization of isocyanides is reported. Using a polystyrene derivative as the initiator, superamphiphiles containing a hydrophobic polystyrene tail and a charged helical polyisocyanide headgroup were prepared. Under proper conditions these superamphiphiles self-assembled in water to give a variety of aggregate morphologies, among which are superhelical architectures. Initiators derived from carbosilane dendritic wedges gave block copolymers with a unique combination of structural elements, i.e. a flexible dendritic block and a rigid polyisocyanide block. Block copolymers derived from the 3rd generation dendrimers form well-defined micellar aggregates in the presence of Ag+ ions. These aggregates have been used to construct nanoarrays of metallic silver.

Rapid Exciton Migration and Fluorescent Energy Transfer in Helical Polyisocyanides with Regularly Arranged Porphyrin Pendants

Photophysical properties of a series of polyisocyanides possessing porphyrin pendants (porphyrin polymers) were investigated by steady state and transient spectroscopic methods. The B (Soret)-band of the free base porphyrin polymers showed splitting indicating substantial exciton coupling in the porphyrin polymers due to a face-to-face stacking conformation. On the other hand, the B-band of the zinc porphyrin polymers split into four peaks indicating deviation from the strict face-to-face stacking conformation. The fluorescence quantum yields of the porphyrin polymers were small compared with those of the corresponding monomers and dimers. Furthermore, the fluorescence decay profiles of the porphyrin polymers deviated from single-exponential decay due to the structural heterogeneity of the polymers. It was revealed that the porphyrin polymers showed transient absorption changes attributable to the exciton−exciton annihilation accompanying structural change with the rate constant of (2−4) × 1010 s-1 regardless of the length of the polymer chain. The quantum yields of the intersystem crossing of the polymers were smaller than those of monomers due to the efficient exciton−exciton annihilation process in the singlet excited states. In the block copolymers of zinc and free base porphyrins, energy transfer processes from zinc to free base porphyrin moieties were confirmed by time-resolved fluorescence spectral and transient absorption spectral measurements. It became clear that the energy transfer rates were almost independent of the sequence of the block copolymers, suggesting that the rate-determining step of the energy transfer in the present block copolymers is the process at the interface of zinc and free base porphyrins because of the sufficiently fast exciton migration in each block.

Hierarchical transfer of stereochemical information in synthetic macromolecules

Abstract In Nature, a wide variety of complex tasks (e.g., catalysis, transport, and information storage) is performed by biomacromolecules with precise composition, dimensions, and architecture. To achieve the precise organization required for efficient function, structural information is built into the smallest building blocks of biomacromolecules (i.e., amino and nucleic acids) and subsequently transferred in a hierarchical fashion to form larger quaternary structures. These construction principles have been an inspiration for synthetic and supramolecular chemists alike, and a number of synthetic biomimetic helical macromolecules have been reported in the recent literature. Here, some recent developments in the field of helical polyisocyanides will be reviewed in combination with new prospects on the self-organization of protein/polymer hybrid architectures.

Helical Chiral Polyisocyanides Possessing Porphyrin Pendants: Determination of Helicity by Exciton-Coupled Circular Dichroism

Der Exciton-gekoppelte Circulardichroismus zur Aufnahme der Porphyrin-Soret-Bande von Triblockcopolymeren, die aus chiralen Isocyanidmonomeren und einem achiralen Tetraphenylporphyrin(TPP)-Derivat aufgebaut sind, bietet sich als neue Methode zur Bestimmung der Helicität von Poly(arylisocyaniden) an (siehe schematische Darstellung der Struktur).

Helical Chiral Polyisocyanides Possessing Porphyrin Pendants: Determination of Helicity by Exciton-Coupled Circular Dichroism.

Exciton-coupled circular dichroism of the porphyrin Soret band of triblock copolymers prepared from chiral isocyanide monomers and an achiral tetraphenylporphyrin derivative (TPP) provides a novel method for determining the helical sense of poly(aryl isocyanide)s (see the schematic representation of the structure).

Helical Chiral Polyisocyanides Possessing Porphyrin Pendants: Determination of Helicity by Exciton-Coupled Circular Dichroism This work was supported by a Grant-in-Aid for COE Research and Scientific Research from the Ministry of Education, Science, Sports, and Culture. F.T. gratefully acknowledges partial support of this work by the Hayashi Memorial Foundation for Female Natural Scientists.

Der Exciton-gekoppelte Circulardichroismus zur Aufnahme der Porphyrin-Soret-Bande von Triblockcopolymeren, die aus chiralen Isocyanidmonomeren und einem achiralen Tetraphenylporphyrin(TPP)-Derivat aufgebaut sind, bietet sich als neue Methode zur Bestimmung der Helicität von Poly(arylisocyaniden) an (siehe schematische Darstellung der Struktur).

Formation of an Optically Active Helical Polyisocyanide Langmuir–Blodgett Film

Abstract An optically active helical polyisocyanide synthesized with a screw-sense-selective living polymerization formed a condensed monolayer on the water surface and was transferred onto a solid support as Y-type LB film with an ordered orientation.

Synthesis, characterization, and application of helical polymers

AbstractThis review mainly describes the asymmetric synthesis of optically active polymers with helical conformation. Bulky methacrylates such as triphenylmethyl methacrylate and 1‐phenyldibenzosuberyl methacrylate give one‐handed helical and optically active polymers with almost perfectly isotactic main chain conformation by polymerization with chiral anionic initiators. The radical polymerization and copolymerization of these monomers under chiral conditions also afford optically active polymers with prevailing one‐handed helicity. N, N‐Disubstituted acrylamides also give optically active, helical polymers in the asymmetric anionic polymerization. Optically active polyisocyanates with a prevailing one‐handed helical structure have been prepared in the copolymerization of an achiral isocyanate with a small amount of an optically active isocyanate and also in the polymerization of alkyl and aromatic isocyanates with optically active lithium alkoxide or amide compounds. The existence of a stable helical structure for polychloral has been successfully proved with the helical oligomers of chloral. One‐handed helical polyisocyanides have been prepared by helix‐sense‐selective polymerization of bulky isocyanides and also by the cyclopolymerization of a 1, 2‐diisocyanobenzene derivative with the Pd complex of a one‐handed helical oligomer.

"Change of mechanism" block copolymerizations: formation of block copolymers containing helical polyisocyanide and elastomeric polybutadiene segments

"Change of mechanism" block copolymerizations: formation of block copolymers containing helical polyisocyanide and elastomeric polybutadiene segments

Main‐Chain Chirality and Optical Activity in Polymers Consisting of CC Chains

AbstractMain‐chain chirality is the optical activity resulting from the configurational or conformational arrangement in the main chain of a polymer. The chirality of the most important types of structures has been investigated on the basis of systematic considerations of symmetry. This has led to the surprising result that even in polymers derived from 1‐substituted or nonsymmetric 1,1‐disubstituted olefins (the technologically most important polymers) several types of chiral structures exist, which are expected to result in optical activity if a particular enantiomer is favorably formed. By carrying out an asymmetric cyclopolymerization, it has been possible to obtain certain structural types in the form of optically active copolymers or homopolymers (e.g., copolymers of styrene with methyl methacrylate, or even the homopolymer of styrene). Another new group of optically active polymers consists of the atropisomeric helical polyisocyanides, poly(trityl methacrylates), and polychlorals. Optically active polymers are already used as adsorbents for the chromatographic separation of racemic mixtures. Further applications are likely to emerge.