Visible-to-ultraviolet (

Yoichi Murakami,Ayumu Motooka,Atsushi Kaiho,Riku Enomoto,Noriko Kiyoyanagi,Kazuki Niimi

doi:10.1039/d0cp04923a

Abstract

In this article, visible-to-ultraviolet photon upconversion (UV-UC) by triplet-triplet annihilation in the emission range shorter than 340 nm, which has not been explored well, 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 the 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.

Highlights

Photon upconversion (UC) is a technology to convert presently wasted sub-bandgap photons into those with higher energies, which are useful in many fields including photovoltaics and photocatalysis
If triplet annihilation (TTA)-UC technology can be reliably extended to the ultraviolet (UV) region (o400 nm), it will become suitable for a broader range of applications, such as for hydrogen generation by water splitting using anatase titanium dioxide (a-TiO2), which has a band gap of 3.2 eV.[41]
From the relation between ksen,degr and the energetic separations of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) between 1 and the solvents, we found a clear correlation of ksen,degr with D|HOMO|, whereas no obvious correlation was found between ksen,degr and D|LUMO| (Fig. 3d)

Summary

Introduction

Photon upconversion (UC) is a technology to convert presently wasted sub-bandgap photons into those with higher energies (i.e., light of shorter wavelength), which are useful in many fields including photovoltaics and photocatalysis. UC using triplet–triplet annihilation (TTA) between organic molecules has been widely explored because of its applicability to low-intensity and non-coherent light.[1,2,3,4,5] Most of the previous studies focused on visible-to-visible UC.[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40] If TTA-UC technology can be reliably extended to the ultraviolet (UV) region (o400 nm), it will become suitable for a broader range of applications, such as for hydrogen generation by water splitting using anatase titanium dioxide (a-TiO2), which has a band gap of 3.2 eV (lgap B 385 nm).[41].

Objectives

Results

Conclusion