‘Quantum magic’ reveals absence of emissive excimers in singlet fission

A little touch of ‘quantum magic’ has clarified the role of excimers and molecular aggregation during singlet fission in TIPS-pentacene solutions.

Singlet fission is a process in which one singlet exciton is converted into two triplet excitons following the absorption of a photon of light. Excitons are quasiparticles useful for carrying energy, and singlet fission is a pathway to increasing the efficiency of solar cells beyond conventional theoretical limits.

The world’s leading physicists and chemists are still seeking to increase our collective, fundamental understanding of singlet fission, with a particular focus on the role of excimers.

Excimers can form when an excited molecule meets a molecule in a ground state, and the excitation spreads over both chromophores in an attractive interaction that leaves them stuck together.

Some researchers have suggested excimers are a necessary step in the process of singlet fission.

A competing view, held by Exciton Science’s Professor Tim Schmidt and his collaborators at UNSW Sydney, is that excimers represent a trap in which energy is lost. Only excited molecules which avoid falling into this trap are able to successfully undergo singlet fission.

Some previous attempts to clarify singlet fission mechanisms and the role of excimers suffered from a key weakness: When performed with high concentrations of solution, an inner filter effect will lead to the reabsorption of high energy light, creating the illusion of a spectra shift and causing excimers to be erroneously credited with a role in emission.

To solve this problem, Tim and his collaborators employed some literal tricks of the light – an evanescent wave and total internal reflection, a phenomenon which enables us to use optical fibres in information technology.

“When light bounces off an interface, it has a little feel out the other side,” Tim said.

“It’s just checking to see if there’s something there. The light wave decays exponentially over a very short scale. But because it’s there it can excite molecules.

“So what we were able to do is use this effect to excite only a very shallow region of the solution. And therefore, even at very high concentrations we don’t get the reabsorption effect when we see the light coming back up the other side.”

By modifying the experiment in this way, the researchers were able to conclusively demonstrate the absence of emissive excimers during the process of singlet fission, and also found evidence for small amounts of molecular aggregation in the highest concentration of the pentacene solution.

It had been thought that aggregation, which is when molecules become transiently bound in an unexcited state, and can help to achieve singlet fission, was not occurring in this context.

Having found evidence to the contrary, the researchers will now seek to investigate this part of the process in greater detail to enhance of our understanding of its role and impact.

The results of this study have been published in the Journal of the American Chemical Society and are available here.