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

Abstract

CONSPECTUS: 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 rigi...

Ladder‐Type Heteroheptacenes with Different Heterocycles for Nonfullerene Acceptors

“…With the coplanar geometry and excellent p-electron delocalization feature,l adder-type heteroarenes have recently emerged as promising organic semiconductors with excellent device performance. [1,28,29] In this context, we design and synthesize two novel nonfullerene acceptors (M8 and M34 in Scheme 1) based on different heterocycles-based ladder-type heteroheptacenes both of which are free of sp 3 carbon atom. Thet wo pairs of neighboring side chains on the heteroheptacene are used to suppress the immoderate intermolecular aggregation thereby leading to suitable phase separation with optimal domain sizes.M8and M34 share the same A-D-A configuration with the identical side-chains as well as the same ending groups of 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile,b ut they differ in the aromaticity of the central electron-donating core.W ith the larger size of sulfur in the thiophene unit in relative to oxygen in the furan unit, the thiophene heterocycleh as an enhanced resonance energy of 121 kJ mol À1 compared to the furan heterocycle (67 kJ mol À1 ) [30] which may lead to al arger aromatic stabilization energy (increased aromaticity index) of the corresponding electron-donating core based on the thiophene heterocycles.Replacement of the furan heterocycles with the thiophene heterocycles in the heteroheptacene does not affect the molecular orientation and p-p-stacking distance of the resulting nonfullerene acceptors,b ut the replacement leads to hypsochromically shifted absorption, improved charge transport, and molecular orientation change in the blend film.…”

Extraordinary electrochemical stability and extended polaron delocalization of ladder-type polyaniline-analogous polymers

“…Ladder polymers can oen be synthesized by a step-wise approach: the rst strand of bonds is formed through a conventional linear polymerization, followed by a ladderization step in which the second strand of covalent bonds is formed along the backbone through annulation. 2 The laddertype PANI derivative (LPANI) in this study was synthesized (Fig. 1a) from a non-ladder polymer precursor P1 (M n ¼ 25.9 kg mol À1 , Đ ¼ 2.23), which could be synthesized using a scalable approach involving imine-condensation polymerization followed by in situ oxidation.…”

“…The covalently locked coplanar conformation of a conjugated ladder-type backbone, 1 in contrast with conventional non-ladder polymers with torsional rotation, allows for extensive intrachain delocalization of molecular orbitals and transport of quasi-particles such as charges, excitons, polarons, and spins. [2][3][4] The rigid coplanar conformation can also enhance interchain electronic coupling of conjugated ladder polymers due to the small reorganization energy of a rigid system upon electron transfer or photoexcitation, [5][6][7][8] which is important for solid-state materials properties. In addition, a signicantly higher activation energy is required to break the double-stranded backbones of a ladder polymer, translating to high stability that is important for applications under harsh conditions.…”

“…PANI derivatives feature reversible redox transformation, high conductivity, feasible synthesis, and good processability, [12][13][14][15][16][17][18] hence are widely employed in conductive protective coating, 19 electrochromism, 13 and energy storage. 20 However, PANI shares some of the common drawbacks with other organic electronic materials, such as (1) low stability of molecular constitutions and degraded performance in devices over time and (2) limited intrachain delocalization ranges of states or quasi-particles, such as polarons and excitons. One of the primary structural origins of these issues can be identied as the single-stranded and torsionally rotating bonds in the PANI polymer backbone, which gives rise to low barriers of bond scission and disordered chain conformation in the solidstate.…”

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

“…Fully conjugated ladder polymers (cLPs) have drawn extensive attention because of their rigid structure with π-conjugated systems [1,2,3,4,5,6]. The unique structure of cLPs confers favorable physical, optical, and chemical properties [7,8,9,10].…”