Chiral BINOL-Based Covalent Organic Frameworks for Enantioselective Sensing

Covalent
organic frameworks (COFs) have emerged as a novel platform
for material design and functional explorations, but it remains a
challenge to synthetically functionalize targeted structures for task-specific
applications. Optically pure 1,1′-bi-2-naphthol (BINOL) is
one of the most important sources of chirality for organic synthesis
and materials science, but it has not yet been used in construction
of COFs for enantioselective processes. Here, by elaborately designing
and choosing an enantiopure BINOL-based linear dialdehyde and a tris­(4-aminophenyl)­benzene
derivative or tetrakis­(4-aminophenyl)­ethene as building blocks, two
imine-linked chiral fluorescent COFs with a 2D layered hexagonal or
tetragonal structure are prepared. The COF containing flexible tetraphenylethylene
units can be readily exfoliated into ultrathin 2D nanosheets and electrospun
to make free-standing nanofiber membrane. In both the solution and
membrane systems, the fluorescence of COF nanosheets can be effectively
quenched by chiral odor vapors via supramolecular interactions with
the immobilized BINOL moieties, leading to remarkable chiral vapor
sensors. Compared to the BINOL-based homogeneous and membrane systems,
the COF nanosheets exhibited greatly enhanced sensitivity and enantioselectivity
owing to the confinement effect and the conformational rigidity of
the sensing BINOL groups in the framework. The ability to place such
a useful BINOL chiral auxiliary inside open channels of COFs capable
of amplifying chiral discrimination of the analytes represents a major
step toward the rational synthesis of porous molecular materials for
more chirality applications.

共价有机框架（Covalent Organic Frameworks, COFs）已成为材料设计与功能探索的新型平台，但针对特定任务应用合成功能化目标结构仍是一项挑战性难题。光学纯1,1'-联-2-萘酚（1,1′-bi-2-naphthol, BINOL）是有机合成与材料科学领域最重要的手性源之一，但目前尚未被用于构建可用于对映选择性过程的COFs。本研究通过精心设计并选择基于光学纯BINOL的线性二醛，以及三(4-氨基苯基)苯衍生物或四(4-氨基苯基)乙烯作为构筑单元，成功制备了两种亚胺连接的手性荧光COFs，二者分别具有二维层状六边形或四方晶体结构。含柔性四苯乙烯单元的COF可轻松剥离为超薄二维纳米片，并可通过静电纺丝工艺制备自支撑纳米纤维膜。在溶液与膜两种体系中，COF纳米片的荧光均可通过固定化BINOL基团与手性气味蒸气之间的超分子相互作用被有效猝灭，从而实现性能优异的手性蒸气传感。相较于基于BINOL的均相体系与膜体系，由于框架内传感BINOL基团的限域效应与构象刚性，COF纳米片展现出大幅提升的灵敏度与对映选择性。将这类实用的BINOL手性助剂置入能够放大分析物手性识别能力的COFs孔道内，这一策略向着合理合成用于更多手性应用场景的多孔分子材料迈出了关键一步。