Triplet-triplet annihilation: An efficient photon upconversion route

Researchers at Fysikum recently investigated the triplet-triplet annihilation (TTA) dynamics of naphthalene. The annihilation process involves the conversion of two spin triplet states into one singlet excited state. The triplet state contains two unpaired electrons with same spin whereas the singlet state electrons are all paired and have opposite spins.

This TTA process provides an efficient route to generate a photon of almost twice the frequency than the incident light. Their latest paper involves exploring the fundamental mechanistic details of the TTA reaction and understand the nuclear dynamics by employing quantum wave packet dynamics. Additionally, the study also demonstrates that the strong light-matter coupling assists in increasing the upconversion efficiency of TTA process in naphthalene.

Cavity-molecule coupled system
The front part of the figure displays the cavity-molecule coupled system along with the conversion oflow-energy (red) light into high-energy (green) light. The surface crossings between the potential energysurfaces involved in the TTA process are also shown.

The TTA-based photon upconversion has witnessed significant growth

Photon upconversion, generating high-energy photons from low-energy
photons, is an outstanding approach for overcoming the efficiency limitations of light-driven processes such as photocatalysis, photodynamic therapy, optogenetics, and bioimaging. Within this area, the TTA-based photon upconversion has witnessed a significant growth due to its unique advantage of photon conversion using incoherent and low-power light sources.

This phenomenon was firstly reported by Hatchard and Parker over 60 years ago and rapidly developed in the 21st century. However, the upconversion efficiency is still far away from the maximum quantum yield. The results of our study should provide fundamental insights on the upconversion process and assist in developing the well-functioning TTA molecules.

The process is mediated by the seams of degenerate points

TTA is a bimolecular process in which multiple energy transfer steps are involved in converting the two low-energy triplet electronic states into one singlet excited state. Our results suggest that the annihilation process is mediated by the seams of degenerate points, the so-called conical intersections of two potential energy surfaces. In the presence of an optical cavity, the molecular electronic states are strongly coupled to the tightly confined electromagnetic mode and from hybrid field-matter states called polaritons.

Strong light-matter interaction increases the upconversion to more than 50%

These hybrid states manipulate the potential energy surfaces by creating additional conical intersections. The strong light-matter interaction has been found to be increasing the upconversion efficiency of naphthalene from
35% to more than 50%.

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