Antiaromatic Covalent Organic Frameworks Based on Dibenzopentalenes

Abstract

Despite their inherent instability, 4n π systems have recently received significant attention due to their unique optical and electronic properties. In dibenzopentalene (DBP), benzanellation stabilizes the highly antiaromatic pentalene core, without compromising its amphoteric redox behavior or small HOMO− LUMO energy gap. However, incorporating such molecules in organic devices as discrete small molecules or amorphous polymers can limit the performance (e.g., due to solubility in the battery electrolyte solution or low internal surface area). Covalent organic frameworks (COFs), on the contrary, are highly ordered, porous, and crystalline materials that can provide a platform to align molecules with specific properties in a well-defined, ordered environment. We synthesized the first antiaromatic framework materials and obtained a series of three highly crystalline and porous COFs based on DBP. Potential applications of such antiaromatic bulk materials were explored: COF films show a conductivity of 4 × 10 −8 S cm −1 upon doping and exhibit photoconductivity upon irradiation with visible light. Application as positive electrode materials in Li-organic batteries demonstrates a significant enhancement of performance when the antiaromaticity of the DBP unit in the COF is exploited in its redox activity with a discharge capacity of 26 mA h g −1 at a potential of 3.9 V vs. Li/Li + . This work showcases antiaromaticity as a new design principle for functional framework materials.

Organic electrodes based on redox-active covalent organic frameworks for lithium batteries

“…However, the practical capacities can be influenced by several factors such as the electrode composition, electronic conductivity, ion diffusion, binder availability, particle size of the active material, or the electrolyte used. The maximum practical specific capacities and average discharge voltages reported for some representative COF-based cathodes 35,36,[50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65] for lithium batteries are summarised in Fig. 2.…”

“…9,73 The redox potentials of representative redox-active groups used in COFs have recently been summarised. 33 In the particular case of COF cathodes for LIBs, the highest voltages have been achieved with the TEMPO radical (B3.6 vs. Li/Li + ), 36 phenoxazine-(B3.6 vs. Li/Li + ), 64 and dibenzopentalene-based COFs (B3.9 vs. Li/Li + ) 54 (Fig. 2).…”

Synthesis and Characterization of Dibenzothieno[a,f]pentalenes Enabling Large Antiaromaticity and Moderate Open-Shell Character through a Small Energy Barrier for Bond-Shift Valence Tautomerization

“…Since the first description by Breslow in 1967, antiaromaticity has been one of the most attractive concepts to describe the electronic features of cyclic π-conjugated molecules. − Unlike aromaticity, antiaromaticity gives rise to destabilization, π-bond localization, paratropic ring current, and a decrease in the highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) energy gap in the π-conjugated system . These electronic properties have attracted interest for their utility in advanced organic materials. − One interesting topic of study in antiaromatic molecules involves the interconversion of single and double bonds through a process called bond-shift valence tautomerization. An antiaromatic molecule distorts toward a lower-symmetric structure with pronounced bond length alternation (BLA) to avoid the electronic instability caused by π electron delocalization.…”

Substituted benzophenone imines for COF synthesis via formal transimination

“…One example has been shown recently where a COF building block equipped with benzophenone imines with –OiPr groups for solubility was successfully used. 20 This is the first demonstration of how difficulties in the synthesis of complex COF building blocks can be tackled by functionalization of benzophenone imines. Another example could be conceived where electron-withdrawing groups could help further stabilize oxidatively unstable electron-rich COF building blocks.…”