# Improving Triplet–Triplet Annihilation Upconversion Output by a Triplet Mediator Approach: Mechanistic Insights on Homo and Hetero-Annihilation in Three-Component Systems

## TL;DR

This paper introduces a new method to improve photon upconversion by using a neutral mediator molecule, enhancing efficiency in solar and imaging applications.

## Contribution

The study introduces a mediator-assisted TTA-UC approach and reports the first estimation of a hetero-TTA rate constant.

## Key findings

- A mediator molecule enables efficient upconversion in UV and visible regions.
- Hetero-TTA rate constant is twice as high as homo-TTA rate.
- Neutral mediators expand design possibilities for TTA-UC systems.

## Abstract

Triplet–triplet
annihilation photon upconversion (TTA-UC)
is a promising strategy for converting low-energy photons into higher-energy
emission, with potential applications in solar energy harvesting,
bioimaging, and photocatalysis. A challenge in TTA-UC systems is minimizing
the reabsorption of upconverted photons by the annihilator molecules.
To address this, we present a mediator-assisted TTA-UC approach utilizing
a neutral mediator molecule to facilitate upconversion in the ultraviolet
(UV) and visible regions. Our study introduces a general protocol,
and through detailed kinetic modeling, we elucidate the underlying
mechanism, highlighting the role of hetero-TTA (triplet–triplet
annihilation between the mediator and annihilator). Notably, we report
the first estimation of a hetero-TTA rate constant, which exceeds
the homo-TTA rate by a factor of 2. This work broadens the design
space for TTA-UC systems by enabling the use of neutral, noncovalently
linked mediators, expanding beyond the conventional reliance on charged
or covalently tethered species.