Photon upconversion in self-assembled materials

Abstract

Photon upconversion is the generation of high energy output photons from low energy input photons and has potential biomedical and solar energy applications. Recently, self-assembled materials have emerged as a platform to achieve upconversion via the triplet–triplet annihilation (TTA) mechanism. A prerequisite for TTA upconversion in a condensed phase is efficient triplet exciton migration through the material. In this regard, self-assembled materials (which include metal–organic frameworks (MOFs), covalent organic frameworks (COFs) and gels) contain non-photoactive structural elements which organise the spatial relationship between chromophores within the material, to allow systematic control over the triplet energy migration and upconversion efficiency to be made. In this review, we summarise recent results in this field, including the influence of chromophore spacing on upconversion vs. excimer formation, and the influence of particle morphology on threshold values. This is a promising area of research, but we conclude that a wider range of sensitiser-annihilator pairs are needed to expand the applicability of these materials more greatly. We hope that this review serves as a useful introductory text for those wishing to combine photon upconversion with the principles of self-assembly.