# Organic Molecular Cages

Organic molecular cages represent a unique class of porous materials characterized by their discrete molecular nature and well-defined internal cavities, formed through covalent bonds between precisely designed organic building blocks. These molecular structures contain organized frameworks surrounding a central cavity, where organic components are precisely arranged to create functional internal spaces. Unlike extended networks such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), these cage compounds exist as distinct molecular entities, offering advantages in solution processability and structural precision.
The field of organic molecular cages emerged in the early 2000s, pioneered by the work of Cram, Lehn, and Pedersen, whose foundational research on host-guest chemistry and molecular recognition earned them the 1987 Nobel Prize. The first discrete organic cages were reported by Tozawa and Cooper in 2009, introducing permanently porous organic cages with intrinsic cavities. Since then, the field has grown significantly, driven by advances in synthetic chemistry and characterization techniques. Early examples demonstrated basic molecular containment, but modern designs achieve sophisticated functions, including selective molecular recognition, catalysis, and stimuli-responsive behavior. The ability to control cavity size and chemical environment at the molecular level distinguishes these materials from traditional porous systems.

## Structure and Design
### Basic Structural Components

The architecture of organic molecular cages follows precise geometric principles that dictate their three-dimensional structure and functional properties. Understanding these structural components is crucial for the design of cages with desired characteristics. The assembly process relies on the careful selection and combination of two primary components: nodes and linkers, whose geometric relationship determines the final cage structure.
Nodes are the cornerstones of cage architecture and are typically composed of rigid organic molecules with specific geometric arrangements. The geometry of these nodes directly influences the final cage structure and its internal cavity dimensions, making their selection crucial for targeted applications. Common node geometries include trigonal (three-directional), tetrahedral (four-directional), and octahedral (six-directional).
Complementing the nodes, linkers act as edges that connect these vertices to form the complete cage structure. These linkers are typically linear or slightly bent organic molecules that contain reactive end groups essential for cage formation. The careful selection of linker length and flexibility helps control the size of the internal cavity and the overall cage stability. The most employed linkers are dialdehyde units, diamine compounds, and diboronic acids.
This modular approach of combining specific nodes and linkers enables the rational design of cages with predetermined geometries and functionalities. The interplay between node geometry and linker characteristics determines not only the structural features but also the chemical environment of the resulting cavity, which is crucial for applications in molecular recognition and catalysis.

## Classification
### Chemical Composition-Based Classification

The most extensively studied category of organic molecular cages are imine-based cages, formed through Schiff base condensation reactions. This reaction involves the condensation of aldehyde and amine groups to form imine bonds (C=N). The reversible nature of imine bond formation enables error correction during synthesis, leading to highly ordered structures, where multiple imine bonds connect the organic linkers and nodes to form a well-defined cage structure. This self-correcting mechanism makes imine-based cages particularly attractive for developing new cage architectures and has contributed to their widespread study in the field.
Boronic ester cages are another important class, characterized by their reversible boronic ester bonds and remarkable stability in non-aqueous conditions. Their unique chemical nature allows for post-synthetic modification, enabling the fine-tuning of cage properties after initial synthesis. This adaptability makes them valuable for applications requiring specific chemical functionalities, particularly in conditions where imine bonds are unstable.
A third major category includes alkyne-based cages, which feature irreversible acetylene linkages that provide enhanced structural rigidity. The strong covalent bonds in these structures result in high thermal stability, making them suitable for applications under demanding conditions. Their rigid framework ensures consistent cavity size and shape, making them particularly valuable for selective molecular recognition applications where structural integrity is crucial.

### Structural Classification

Beyond chemical composition, cages are also classified based on their structural characteristics. Shape-persistent cages maintain fixed conformations due to their rigid building blocks and strong covalent bonds, providing stable and predictable cavity environments. In contrast, flexible cages exhibit dynamic structures that can adapt to guest molecules through conformational changes, allowing for responsive host-guest interactions. This flexibility can be advantageous in applications requiring adaptive binding, such as selective molecular capture under varying conditions. Some systems even form hierarchical assemblies, creating cage-of-cage structures with complex internal architectures that can provide multiple distinct environments for guest molecules or cascade reactions.

### Size-Based Classification

The classification of cages by cavity size provides practical guidance for applications and directly relates to their synthetic components and geometry. Small cages (< 1 nm internal diameter) are typically constructed from compact building blocks and feature tight binding pockets suitable for gas molecule separation and storage, particularly for gases like CO$_2$ and CH$_4$. Medium cages (1-2 nm) represent the most versatile category, finding applications in selective molecular recognition and catalysis due to their ability to accommodate a wide range of organic molecules and maintain specific chemical environments. Large cages (> 2 nm), often synthesized using extended linear components or through hierarchical assembly, can accommodate bigger guest molecules such as proteins or large organic compounds, making them valuable for applications in drug delivery and enzyme encapsulation. The relationship between cage size and function has been extensively studied, revealing optimal size ranges for specific applications and guiding the design of new cage systems.