Visible-to-ultraviolet (<340 nm) photon upconversion by triplet-triplet annihilation in solvents

TL;DR

This study demonstrates visible-to-ultraviolet photon upconversion (<340 nm) via triplet-triplet annihilation in solvents, revealing how solvent properties influence efficiency, stability, and providing design guidelines for UV-UC systems.

Contribution

It explores UV-UC below 340 nm in solvents, elucidates solvent effects on efficiency and stability, and develops a theoretical model for reaction dynamics, offering new insights and design principles.

Findings

UV-UC efficiency depends on solvent polarity and triplet energy matching.

Sensitizer degradation correlates with frontier orbital energy differences.

Inclusion of emitter suppresses sensitizer degradation and enhances stability.

Abstract

In this article, visible-to-ultraviolet photon upconversion (UV-UC) by triplet-triplet annihilation in the emission range shorter than 340 nm, which is previously unexplored, is presented and the relevant physicochemical characteristics are elucidated. Investigations were carried out in several deaerated solvents using acridone and naphthalene derivatives as a sensitizer and emitter, respectively. Both upconversion quantum efficiency and sample photostability under continuous photoirradiation strongly depended on the solvent. The former dependence is governed by the solvent polarity, which affects the triplet energy level matching between the sensitizer and emitter because of the solvatochromism of the sensitizer. To elucidate the latter, first we investigated the photodegradation of samples without the emitter, which revealed that the sensitizer degradation rate is correlated with the difference between the frontier orbital energy levels of the sensitizer and solvent. Inclusion of the emitter effectively suppressed the degradation of the sensitizer, which is ascribed to fast quenching of the triplet sensitizer by the emitter and justifies the use of ketonic sensitizers for UV-UC in solvents. A theoretical model was developed to acquire insight into the observed temporal decays of the upconverted emission intensity under continuous photoirradiation. The theoretical curves generated by this model fitted the experimental decay curves well, which allowed the reaction rate between the emitter and solvent to be obtained. This rate was also correlated with difference between the frontier orbital energy levels of the emitter and solvent. Finally, based on the acquired findings, general design guidelines for developing UV-UC samples were proposed.