# All-visible-light-responsive porous aromatic frameworks manipulate CO2 uptake by reversible bulk isomerization of azobenzene pendants

## TL;DR

This paper introduces a new class of porous materials that respond to visible light by changing their CO2 uptake properties through reversible isomerization.

## Contribution

The novel contribution is the development of visible-light-responsive porous aromatic frameworks with reversible CO2 uptake modulation via bulk isomerization.

## Key findings

- The porous aromatic frameworks undergo reversible isomerization under visible light, confirmed by solid-state NMR.
- The materials show significant changes in CO2 uptake depending on the photoswitch state.
- The system is robust, with no photo fatigue or decomposition after multiple cycles.

## Abstract

Despite the clear advantages of low-energy photon activation, the development of porous materials that respond to visible light remains a significant challenge. Here, we report switchable porous aromatic frameworks (PAFs) appended with o-fluoroazobenzene that undergo reversible isomerization under alternating irradiation with distinct wavelengths of visible light. Solid-state NMR confirms that isomerization occurs throughout the bulk of the framework. This light-driven switching leads to marked changes in gas adsorption properties, including CO2 uptake. Notably, the relative modulation of CO2 uptake correlates with the content of the photoswitch. By demonstrating the example of visible-light-driven bulk isomerization in a porous solid, this work sets a benchmark for designing robust, light-responsive materials offering a pathway toward sustainable, visible-light-controlled gas storage and separation technologies.

Embedding light-responsive small molecules in a porous solid is a promising strategy to achieve dynamic control over material properties. Powering these systems with low-energy photons is essential for their future applications, since visible light, compared to UV light, is less damaging and offers more selective isomerization with higher penetration depth. However, the construction of visible light-responsive porous materials remains a significant challenge. Here, we report the construction of a series of visible-light-responsive porous aromatic switchable framework materials grafted with o-fluoroazobenzene pendants (Azo-PSFs). The materials exhibit high microporosity and reversible photoswitching upon irradiation with visible light. The highly robust materials can be cycled between two distinct states multiple times without showing any photo fatigue or decomposition. Remarkably, solid-state NMR revealed that the azobenzene moiety undergoes reversible bulk isomerization in the framework. The isomerization of azobenzene within the framework is associated with substantial changes in adsorption capacity and CO2 uptake-release by the material. This work presents the example of visible-light-triggered bulk isomerization in an azobenzene-based porous material, providing a benchmark characterization of photoresponsive systems and paving the way for the future advancements in light-driven materials.