# Aminal‐linked Covalent Organic Frameworks for Light Energy Upconversion

## TL;DR

Researchers created solid-state materials that efficiently convert low-energy light into higher-energy light, which could improve solar energy and bioimaging.

## Contribution

The first covalent organic frameworks (COFs) capable of sensitized triplet–triplet annihilation upconversion are reported.

## Key findings

- Aminal-linked COFs with anthracene chromophores showed strong photoluminescence with quantum yields up to 40%.
- Upconversion quantum yields of 1.8% were achieved, outperforming conventional solution-based systems.
- Energy transfer within the frameworks occurs via intra-framework triplet migration rather than diffusion.

## Abstract

The conversion of two low‐energy photons into a single higher‐energy photon at low irradiance is highly desirable for bioimaging and solar energy harvesting. Yet translating established solution‐phase triplet–triplet annihilation upconversion (TTA‐UC) systems into robust solid‐state platforms remains a challenge. Here, we report the first covalent organic frameworks (COFs) capable of sensitized TTA‐UC. Two aminal‐linked frameworks integrating anthracene chromophores, Ant‐COF‐H and Ant‐COF‐OH, were synthesized and structurally characterized, revealing high crystallinity and strong photoluminescence (ΦF
≈ 40%). When sensitized with a palladium porphyrin complex, both COFs display upconverted emission with quantum yields up to 1.8%, surpassing the performance of the conventional all‐in‐solution reference system. Notably, the onset of saturation occurs at excitation power densities as low as 100 mW cm−2. Time‐resolved emission spectroscopy reveals fast energy‐transfer consistent with intra‐framework triplet migration rather than diffusion. Finally, we correlate framework structural features with energy‐loss pathways, providing design guidelines for further improvement. This work establishes a foundation for practical, low‐power light management in crystalline polymers by demonstrating that aminal‐linked COFs can be engineered to support efficient energy transfer and function as solid‐state upconverters.

Aminal‐linked covalent organic frameworks incorporating anthracene units are introduced as solid‐state platforms for triplet–triplet annihilation upconversion. The crystalline materials retain chromophore electronic properties and exhibit fluorescence quantum yields of up to 40%. Internal energy transfer processes, governed by the alignment of anthracene units within the framework, enable upconversion with quantum yields of 1.8% under low‐intensity excitation. This work demostrates, for the first time, TTA upconversion in COFs, opening new avenues for energy conversion and optoelectronic applications.