Thermodynamic synthesis of solution processable ladder polymers

Tether-entangled conjugated helices

A new design concept, tether-entangled conjugated helices (TECHs), is introduced for helical polyaromatic molecules. TECHs consist of a linear polyaromatic ladder backbone and periodically entangling tethers with the same planar chirality. By limiting the length of tether, all tethers synchronously bend and twist the backbone with the same manner, and change it into a helical ribbon with a determinate helical chirality. The 3D helical features are customizable via modular synthesis by using two types of synthons, the planar chiral tethering unit (C 2 symmetry) and the docking unit (C 2h symmetry), and no post chiral resolution is needed. Moreover, TECHs possess persistent chiral properties due to the covalent locking of helical configuration by tethers. Concave-type and convex-type oligomeric TECHs are prepared as a proof-of-concept. Unconventional double-helix π-dimers are observed in the single crystals of concave-type TECHs. Theoretical studies indicate the smaller binding energies in double-helix π-dimers than conventional planar π-dimers. A concentration-depend emission is found for concave-type TECHs, probably due to the formation of double-helix π-dimers in the excited state. All TECHs show strong circularly polarized luminescence (CPL) with dissymmetric factors (|g lum|) generally over 10-3. Among them, the (P)-T4-tBu shows the highest |g lum| of 1.0 × 10-2 and a high CPL brightness of 316 M-1 cm-1.

Ambipolar conjugated ladder polymers by room-temperature Knoevenagel polymerization

Two soluble conjugated ladder polymers (cLPs), decorated with multiple electron-poor species (i.e., cyano groups, fused pentagons, and N-heterocyclic rings), have been synthesized from the newly developed tetraketo-functionalized double aza[5]helicene building blocks using a single-step Knoevenagel polycondensation strategy. This facile approach features mild conditions (e.g., room temperature) and high efficiency, allowing us to quickly access a nonalternant ladder-like conjugated system with the in situ formation of multicyano substituents in the backbone. Analysis by 1H NMR, FT-Raman, and FT-IR spectra confirms the successful synthesis of the resulting cLPs. The combination of theoretical calculations and experimental characterizations reveals that the slightly contorted geometry coupled with a random assignment of trans- and cis-isomeric repeating units in each main chain contributes to improving the solubility of such rigid, multicyano nanoribbon systems. Apart from outstanding thermal stability, the resulting cLPs exhibit attractive red fluorescence, excellent redox properties, and strong π-π interactions coupled with orderly face-on packing in their thin-film states. They are proven to be the first example of ambipolar cLPs that show satisfactory hole and electron mobilities of up to 0.01 and 0.01 cm2 V-1 s-1, respectively. As we demonstrate, the Knoevenagel polycondensation chemistries open a new window to create complex and unique ladder-like nanoribbon systems under mild reaction conditions that are otherwise challenging to achieve.

Coplanar Conformational Structure of π-Conjugated Polymers for Optoelectronic Applications

Hierarchical structure of conjugated polymers is critical to dominating their optoelectronic properties and applications. Compared to nonplanar conformational segments, coplanar conformational segments of conjugated polymers (CPs) demonstrate favorable properties for applications as a semiconductor. Herein, recent developments in the coplanar conformational structure of CPs for optoelectronic devices are summarized. First, this review comprehensively summarizes the unique properties of planar conformational structures. Second, the characteristics of the coplanar conformation in terms of optoelectrical properties and other polymer physics characteristics are emphasized. Five primary characterization methods for investigating the complanate backbone structures are illustrated, providing a systematical toolbox for studying this specific conformation. Third, internal and external conditions for inducing the coplanar conformational structure are presented, offering guidelines for designing this conformation. Fourth, the optoelectronic applications of this segment, such as light-emitting diodes, solar cells, and field-effect transistors, are briefly summarized. Finally, a conclusion and outlook for the coplanar conformational segment regarding molecular design and applications are provided.

Periphery Fusion Strategy of a Carbazole-Based Macrocycle toward Coplanar N-Heterocycloarene for High-Mobility Single-Crystal Transistors

Designing (hetero)cycloarenes through the modifications of the π-topology and molecular packing of organic semiconductors has recently garnered considerable attention. However, their applications as an organic active layer in field-effect transistors are very limited, and the obtained hole carrier mobilities are less than 1 cm² V^-1 s^-1 . In this work, a novel alkyl-substituted coplanar N-heterocycloarene (FM-C4) containing four carbazole units is successfully synthesized in crystalline form. As compared to the corresponding single-bond-linked carbazole-based macrocycle M-C4, it is found that the periphery fusion strategy greatly changes the electronic structures, energy levels, photophysical properties, host-guest interactions with fullerenes, and molecular crystal stacking motifs. In particular, the fully fused N-heterocycloarene FM-C4 exhibits a herringbone packing structure with an unusual long-range π-π overlap distance as low as 3.19 Å, whereas the single crystal of M-C4 demonstrates no π-π interactions. As a consequence, FM-C4 in single-crystal transistors displays the highest hole mobility of 2.06 cm² V^-1 s^-1 , significantly outperforming M-C4 and all the reported (hetero)cycloarenes and suggesting the high potential of (hetero)cycloarenes for organic electronic applications.

Precise Synthesis of Concentric Ring, Helicoid, and Ladder Metallo-Polymers with Chevron-Shaped Monomers

Molecular geometry represents one of the most important structural features and governs physical properties and functions of materials. Nature creates a wide array of substances with distinct geometries but similar chemical composition with superior efficiency and precision. However, it remains a formidable challenge to construct abiological macromolecules with various geometries based on identical repeating units, owing to the lack of corresponding synthetic approaches for precisely manipulating the connectivity between monomers and feasible techniques for characterizing macromolecules at the single-molecule level. Herein, we design and synthesize a series of tetratopic monomers with chevron stripe shape which serve as the key precursors to produce four distinct types of metallo-macromolecules with well-defined geometries, viz., the concentric hexagon, helicoid polymer, ladder polymer, and cross-linked polymer, via platinum-acetylide couplings. Concentric hexagon, helicoid, and ladder metallo-polymers are directly visualized by transmission electron microscopy, atomic force microscopy, and ultra-high-vacuum low-temperature scanning tunneling microscopy at the single-molecule level. Finally, single-walled carbon nanotubes (SWCNTs) are selected as the guest to investigate the structure-property relationship based on such macromolecules, among which the helicoid metallo-polymer shows high efficiency in wrapping SWCNTs with geometry-dependent selectivity.

Synthesis of Ladder-Type 9,9'-Bifluorenylidene-Based Conjugated Oligomers via a Pd-Catalyzed Tandem Suzuki Coupling/Heck Cyclization Approach

For new ladder-type oligomers and polymers with versatile and robust synthetic strategies, in this study, four fully conjugated ladder-type overcrowded 9,9'-bifluorenylidene-based compounds and oligomers (BFY1, BFY2, BFY3, and BFY4) were synthesized via a Pd-catalyzed tandem Suzuki coupling/Heck cyclization reaction. By monomer screening and route optimization, the target products were obtained in high yields and characterized by ¹H and ¹³C NMR spectroscopy and high resolution mass spectroscopy.

Multifunctional Heteropentalenes: From Synthesis to Optoelectronic Applications

In the broad family of heteropentalenes, the combination of two five-membered heterocyclic rings fused in the [3,2-b] mode has attracted the most significant attention. The relatively straightforward access to these structures, being a consequence of the advances in the last two decades, combined with their physicochemical properties which match the requirements associated with many applications has led to an explosion of applied research. In this Perspective, we will discuss the recent progress of heteropentalenes' usefulness as an active element of organic light-emitting diodes and organic field-effect transistors. Among the myriad of possible combinations for the different heteroatoms, thieno[3,2-b]thiophenes and 1,4-dihydropyrrolo[3,2-b]pyrroles are subject to the most intense studies. Together they comprise a potent optoelectronics tool resulting from the combination of appreciable photophysical properties, chemical reactivity, and straightforward synthesis.

Nanoconfined synthesis of conjugated ladder polymers

This review highlights recent advances in controlled synthesis of conjugated ladder polymers using templates.

Spiers Memorial Lecture. Carbon nanostructures by macromolecular design - from branched polyphenylenes to nanographenes and graphene nanoribbons

Nanographenes (NGs) and graphene nanoribbons (GNRs) are unique connectors between the domains of 1D-conjugated polymers and 2D-graphenes. They can be synthesized with high precision by oxidative flattening processes from dendritic or branched 3D-polyphenylene precursors. Their size, shape and edge type enable not only accurate control of classical (opto)electronic properties, but also access to unprecedented high-spin structures and exotic quantum states. NGs and GNRs serve as active components of devices such as field-effect transistors and as ideal objects for nanoscience. This field of research includes their synthesis after the deposition of suitable monomers on surfaces. An additional advantage of this novel concept is in situ monitoring of the reactions by scanning tunnelling microscopy and electronic characterization of the products by scanning tunnelling spectroscopy.

Effective Synthesis of Ladder-type Oligo(p-aniline)s and Poly(p-aniline)s via Intramolecular SNAr Reaction

Symmetric ladder-type oligo(p-aniline)s and poly(p-aniline)s were successfully synthesized by an intramolecular ring closure in a highly efficient S_NAr reaction from oligo(p-phenylene)s and poly(p-phenylene)s with fluorine (F) and secondary amine (NH) groups. Unlike Cadogan ring closure, the newly designed cyclization reaction will not produce a mixture of symmetric and nonsymmetric structures. Moreover, the introduction of the F atom does not hinder Suzuki polymerization. The result indicates that preparing regular oligomers and polymers with a nitrogen bridge is possible.

Simple Route to Synthesize Fully Conjugated Ladder Isomer Copolymers with Carbazole Units

Two isomer polymers, P3 and P6, with fully conjugated ladder structures are presented by simple synthetic routes. The well-defined structures of fully conjugated ladder polymers P3 and P6 were ensured by the high yields of every reaction step. The fully rigid ladder structures were confirmed by nuclear magnetic resonance (NMR), fourier transform infrared spectroscopy (FTIR), and photophysical test. Polymers P3 and P6 with bulky alkyl side chains exhibit good solution processability and desirable thermostable properties. After the intramolecular cyclization reaction, the band gaps of polymers P3 and P6 become lower (2.86 eV and 2.66 eV, respectively) compared with polymers P1 and P4. This initial study provides insight for the rational design of fully ladder-conjugated isomeric polymers with well-defined structures.

Covalent and Noncovalent Approaches to Rigid Coplanar π-Conjugated Molecules and Macromolecules

Molecular conformation and rigidity are essential factors in determining the properties of individual molecules, the associated supramolecular assemblies, and bulk materials. This correlation is particularly important for π-conjugated molecular and macromolecular systems. Within such an individual molecule, a coplanar conformation facilitates the delocalization of not only molecular orbitals but also charges, excitons, and spins, leading to synergistically ensembled properties of the entire conjugated system. A rigid backbone, meanwhile, imposes a high energy cost to disrupt such a favorable conformation, ensuring the robustness and persistence of coplanarity. From a supramolecular and material point of view, coplanarity and rigidity often promote strong intermolecular electronic coupling and reduce the energy barrier for the intermolecular transport of charges, excitons, and phonons, affording advanced materials properties in bulk. In this context, pursuing a rigid and coplanar molecular conformation often represents one of the primary objectives when designing and synthesizing conjugated molecules for electronic and optical applications. Two general bottom-up strategies-covalent annulation and noncovalent conformational control-are often employed to construct rigid coplanar π systems. These strategies have afforded various classes of such molecules and macromolecules, including so-called conjugated ladder polymers, graphene nanoribbons, polyacenes, and conformationally locked organic semiconductors. While pursuing these targets, however, one often confronts challenges associated with precise synthesis and limited solubility of the rigid coplanar systems, which could further impede their large-scale preparation, characterization, processing, and application. To address these issues, we developed and utilized a number of synthetic methods and molecular engineering approaches to construct and to process rigid coplanar conjugated molecules and macromolecules. Structure-property correlations of this unique class of organic materials were established, providing important chemical principles for molecular design and materials applications. In this Account, we first describe our efforts to synthesize rigid coplanar π systems fused by various types of bonds, including kinetically formed covalent bonds, thermodynamically formed covalent bonds, N→B coordinate bonds, and hydrogen bonds, in order of increasing dynamic character. The subsequent section discusses the characteristic properties of selected examples of these rigid coplanar π systems in comparison with control compounds that are not rigid and coplanar, particularly focusing on the optical, electronic, and electrochemical properties. For systems bridged with noncovalent interactions, active manipulation of the dynamic bonds can tune variable properties at the molecular or collective level. Intermolecular interactions, solid-state packing, and processing of several cases are then discussed to lay the foundation for future materials applications of rigid coplanar π conjugated compounds.

New Triphenylamine-Based Oligomeric Schiff Bases Containing Tetraphenylsilane Moieties in the Backbone

Three new triphenylamine-based oligomeric Schiff bases (polySB1, polySB2 and polySB3) containing tetraphenylsilane core (TPS-core) in the main chain were obtained from TPS-core-based diamines and bis(4-formylphenyl)phenylamine by a high-temperature polycondensation reaction. These new oligomers were structurally characterized by FT-IR, NMR and elemental analysis. All polySBs were highly soluble in common organic solvents, such as chloroform, tetrahydrofuran and chlorobenzene. Samples showed moderate molecular average molecular weight (Mw) and a high thermal stability above 410 °C. Likewise, polySBs showed absorption near 400 nm in the UV-vis range and photoluminescence. The HOMO levels and band-gap values were found in the ranges of -6.06 to -6.18 eV and 2.65⁻2.72 eV, respectively. The lowest band-gap value was observed for polySB2, which could be attributed to a more effective π-conjugation across the main chain. The results suggest that silicon-containing polySBs are promising wide-band-gap semiconductors materials for optoelectronic applications.

Ladderization of polystyrene derivatives by palladium-catalyzed polymer direct arylation

Pseudo-ladderized polystyrenes could be obtained in high yield by the palladium-catalyzed polymer direct arylation using poly(2-bromostyrene) as a prepolymer.

Synthesis of an indacenodithiophene-based fully conjugated ladder polymer and its optical and electronic properties

A fully conjugated ladder polymer (PFIDT) based on the indacenodithiophene unit was synthesized via a simple strategy.

Donor–acceptor conjugated ladder polymer via aromatization-driven thermodynamic annulation

The construction of coplanar conjugated ladder polymers featuring alternating donor–acceptor units has been achieved in high efficiency using ring-closing olefin metathesis.

Extended Ladder-Type Benzo[k]tetraphene-Derived Oligomers

Well-defined, fused-ring aromatic oligomers represent promising candidates for the fundamental understanding and application of advanced carbon-rich materials, though bottom-up synthesis and structure-property correlation of these compounds remain challenging. In this work, an efficient synthetic route was employed to construct extended benzo[k]tetraphene-derived oligomers with up to 13 fused rings. The molecular and electronic structures of these compounds were clearly elucidated. Precise correlation of molecular sizes and crystallization dynamics was established, thus demonstrating the pivotal balance between intermolecular interaction and molecular mobility for optimized processing of highly ordered solids of these extended conjugated molecules.

Rigid-to-Flexible Conformational Transformation: An Efficient Route to Ring-Opening of a Tröger's Base-Containing Ladder Polymer

The synthesis of ladder polymers is still a big challenge in polymer chemistry, and in particular, there are few examples of conformationally flexible well-defined ladder polymers. Here we report an efficient and convenient route to conformationally flexible ladder polymers, which is based on a postpolymerization reaction of a rigid ladder polymer containing Tröger's base in its main chain. The postpolymerization reaction involves sequential N-methylation and hydrolysis for the Tröger's base unit, resulting in a diazacyclooctane skeleton that can exhibit a ring-flipping motion. Molecular dynamics simulations predicted that this motion provides conformational flexibility with the resultant ladder polymer, which was demonstrated by ¹H NMR spectroscopy in solution. The presence of the diazacyclooctane units in the flexible ladder polymer allowed further functionalization through reactions involving its secondary amine moiety. The present synthetic method may lead to the development of a new class of ladder polymers that exhibit both conformational and design flexibility.

Fully conjugated ladder polymers

Fully conjugated ladder polymers (cLPs), in which all the backbone units on the polymer main-chain are π-conjugated and fused, have attracted great interest owing to their intriguing properties, remarkable chemical and thermal stability, and potential suitability as functional organic materials. The synthesis of cLPs can be, in general, achieved by two main strategies: single-step ladderization and post-polymerization ladderization. Although a variety of synthetic methods have been developed, the chemistry of cLPs must contend with structural defects and low solubility that prevents complete control over synthesis and structural characterization. Despite these challenges, cLPs have been used for a wide range of applications such as organic light emitting diodes (OLEDs) and organic field effect transistors (OFETs), paralleling developments in processing methods. In this perspective, we discuss the background of historical syntheses including the most recent synthetic approaches, challenges related to the synthesis and structural characterization of well-defined cLPs with minimum levels of structural defects, cLPs' unique properties, and wide range of applications. In addition, we propose outlooks to overcome the challenges limiting the synthesis, analysis, and processing of cLPs in order to fully unlock the potential of this intriguing class of organic materials.

Low Band Gap Coplanar Conjugated Molecules Featuring Dynamic Intramolecular Lewis Acid-Base Coordination

Ladder-type conjugated molecules with a low band gap and low LUMO level were synthesized through an N-directed borylation reaction of pyrazine-derived donor-acceptor-donor precursors. The intramolecular boron-nitrogen coordination bonds played a key role in rendering the rigid and coplanar conformation of these molecules and their corresponding electronic structures. Experimental investigation and theoretical simulation revealed the dynamic nature of such coordination, which allowed for active manipulation of the optical properties of these molecules by using competing Lewis basic solvents.