Singlet fission is the process in which certain molecules can generate two excited states following the absorption of a single photon. It has received increasing interest in recent years for its potential to boost solar cell efficiency by expanding the types of incident light to which they respond and reducing heat loss. One new paper explores how to better understand the intermolecular forces at play behind singlet fission.

Grieco et al. report using native vibrational modes to probe causes of singlet fission in a solution. Examining the carbon-hydrogen stretch in side groups of a model organic singlet fission chromophore called TIPS-pentacene, the group found the molecules aggregated in solution through their side groups rather than by encountering each other through diffusion.

Such work marks one of the first uses of vibrational modes to understand the structural information of molecules involved in singlet fission.

The experiments were conducted in solution, rather than in crystalline form, because this provided a way to control the amount of fission that took place through dilution or concentration.

“The solution study allowed us to tune the intermolecular interactions,” said John Asbury, an author on the paper. “Vibrations are a unique probe for those interactions, which helps us understand singlet fission in amorphous systems.”

The group found TIPS-pentacene’s alkyne stretch modes were sensitive to the electronic excited states present during singlet fission reactions. Such modes can provide unique signatures for the formation of triplet excited states.

The C-H vibrational modes in the side groups allowed for detection of TIPS-pentacene aggregation in solution.

The next steps are to use the vibrational technique to examine other materials, including derivatives of anthradithiophene, rylene and polymeric systems.

Source: “Vibrational probe of the origin of singlet exciton fission in TIPS-pentacene solutions,” by Christopher Grieco, Grayson S. Doucette, Kyle T. Munson, John R. Swartzfager, Jason M. Munro, John E. Anthony, Ismaila Dabo, and John B. Asbury, Journal of Chemical Physics (2019). The article can be accessed at https://doi.org/10.1063/1.5116586.