Stimuli-responsive liquid crystal host materials for energy upconversion systems

University of Cambridge
Materials Science & Metallurgy

Photon upconversion (UC) refers to a mechanism in which two low energy photons (e.g. infrared) are absorbed by a material; and converted into a single higher energy photon (e.g. visible). UC has emerged as a key energy efficiency conversion mechanism widely used in technological applications such as solar energy conversion, optical communication, and various biological functions, e.g. bioimaging and cancer therapy. In particular, UC can be achieved through triplet–triplet annihilation (TTA), which features beneficial characteristics, such as easy tunability of excitation and emission wavelengths and low excitation power density requirements.

The TTA-UC process involves the collision of two triplet excitons (T1), which fuse to give one singlet exciton (S1). Efficient TTA-UC has only been achieved in solution, where it has limited practical use. Solid-state TTA- UC systems present a potential for real-world application as they can be used in form of films and layers. Moreover, using a host material to control the TTA-UC efficiency or switch the process “on/off” remains largely unexplored.

In this project, I will design and study liquid crystal (LC) host matrices capable of facilitating TTA-UC. This project is built on my proof-of-concept study that demonstrated for the first time that LC molecules can undergo TTA-UC. LC molecules share the anisotropic order of a crystalline solid state and the fluidity of a liquid. Owing to the “liquid-like” molecular mobility, LCs can promote the triplet diffusion and molecular mobility needed to facilitate efficient TTA-UC, which is easy to achieve in a liquid but much harder in a solid material. Moreover, molecular and macroscopic alignment of LCs is controlled by external stimuli, such as temperature or electric field. Hence, using an LC host will allow me to design smart layers in which the TTA-UC response is switched on or off by applying external stimuli. Due to their high responsivity, using LCs as host matrices for TTA-UC systems will expand their application as specific properties can be activated or deactivated by controlling their alignment.