Singlet fission is a process in which one absorbed photon can generate two triplet excitations, offering a potential route toward solar energy materials that make more efficient use of incoming light. Xin Chang, a postdoctoral researcher, and Wennie Wang, an assistant professor in the McKetta Department of Chemical Engineering and an affiliate of TMI, have developed a theoretical framework that explains when these normally difficult-to-observe triplet-pair states can leave a detectable fingerprint in a material's optical spectrum.
Understanding how electronic excitations move and interact between neighboring molecules is challenging because triplet-pair states are typically optically dark or only weakly visible. Chang and Wang's model shows that molecular arrangement, charge transfer, and the symmetry of the electronic wave functions work together to determine whether a triplet pair can acquire an observable spectral signature.
In their model, charge-transfer states act as a bridge between the optically excited singlet state and the triplet pair. Importantly, this pathway depends on the parity symmetry of the interacting states. When the symmetries and electronic couplings are favorable, the triplet pair can mix with optically accessible states and become visible in the absorption spectrum. When the couplings interfere or the symmetries do not match, the triplet pair's spectral signature can remain strongly suppressed, making it difficult to detect.
The work provides a molecular-level picture of how crystal packing controls the relationship between charge transfer, triplet-pair formation, and optical spectroscopy. These insights can help researchers interpret spectral signatures of singlet-fission materials and guide the design of molecular structures that make better use of absorbed light.
To learn more, read "The Role of Charge Transfer and Parity Symmetry of the Wave Function in N,N′-Bis(2-phenylethyl)-3,4,9,10-perylenedicarboximide Crystals: A Frenkel-Holstein Approach," in The Journal of Physical Chemistry Letters.
