Switchable singlet fission: pH triggers molecules to split or emit light for sensors

An international team of researchers has successfully controlled the flow of energy in a molecule with the help of its pH value. The results of the study, led by Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), could contribute to the development of new sensors for medical diagnostics, for example.

The findings are also of interest for building more efficient solar cells and for use in quantum computing. The results have been published in the journal Nature Communications.

A process called singlet fission is at the center of the study. In future generations of solar cells, it should improve the utilization of light and thus increase efficiency. Until now, a large proportion of the energy that shines onto solar cells is lost and released as heat.

This is due to the way solar cells work and the principle is similar to that of a car horn. It doesn’t matter whether you press down gently on the center of the horn or hit it with force with your fist—the result is the same, namely one single sound of the horn. This is the same with conventional solar cells. Each photon, regardless of how energy-rich it is, excites a single electron that is then available as a charge carrier.

However, some light particles could have enough power for two electrons. “This is where singlet fission comes in,” explains Prof. Dr. Dirk Guldi, Chair of Physical Chemistry at FAU. “This process ensures that the energy of the photon is split, so to speak, to enable the excitation of two electrons. In conjunction with a team at the University of Alberta in Canada, we have now succeeded in making this process switchable.”

Hitting the steering wheel produces two sounds of the horn

To do so, the researchers used a molecule from a group known as tetracenes. The compound is put into something known as the singlet excitation state by energy-rich photons. This then splits into two lower-energy triplet states within a short period of time—so a sufficiently strong tap on the steering wheel now produces two sounds of the horn.

“We chemically modified our molecule in such a way that it binds protons in an acidic environment,” explains Guldi. “This changes its properties so that singlet fission can no longer take place.”

If no fission occurs, the singlet state disintegrates within a short period of time while emitting light. This means that the compound lights up in an acidic environment. It stays dark in an alkaline environment. “This mechanism could be used for a new type of sensor for medical diagnostics,” explains Guldi.

In addition, this success provides new insights into how singlet fission in tetracenes works. The researcher hopes that this knowledge will enable the process to potentially be optimized further in future.

“However, there’s a great deal of work to be done before we can use it to design new and significantly more efficient solar cells,” he says. “But our results could still be an important step in the right direction.” They may also open up new perspectives for the development of quantum computers that can solve certain problems incredibly quickly.