Researchers have identified a fundamental 'switch' that controls the speed of a key quantum energy conversion process known as singlet fission (SF). Their work helps achieve a tenfold tuning of its rate by simply adjusting the molecular distance between two chemical units. The breakthrough by researchers from the Tata Institute of Fundamental Research, Jawaharlal Nehru University, and the Indian Association for the Cultivation of Science, offers a new path to dramatically increase the efficiency of future solar cells and advanced medical treatments.
When a single, very energetic packet of light or photon hits certain types of molecules, it can excite an electron from a lower-energy band to a higher one, leaving a vacancy or hole in the lower band. The excited electron and hole can then bind together due to their opposite charges and is together called a singlet exciton. Singlet fission (SF) is a molecular phenomenon where the high-energy singlet exciton instantly splits its energy to create two separate, lower-energy units, called triplet excitons.
The split occurs in two steps: first, the initial singlet exciton dissociates into a transient, intermediate state known as the correlated triplet-triplet pair. The speed of this initial step determines how efficiently energy can be harvested. Previous research has found it challenging to predict and control this rate because many SF systems use flexible molecules or solid materials where the geometry and electronic interactions are chaotic. This lack of control led to inconsistent SF rates for different distances and angles.
The new work improves on previous efforts by using highly rigid, ring-like molecules called Naphthalene-diimide (NDI) cyclophanes. This rigid chemical framework acts like a controlled laboratory, allowing the researchers to precisely tune the distance and angle between the two NDI units within the molecule. The team synthesized five new cyclophanes by changing the type of chemical linker connecting the two NDI units. These linkers systematically increased the center-to-center distance between the molecular units from approximately 3.3 angstroms to about 4.5 angstroms.
The fastest singlet fission rate clocked in this study was 400 femtoseconds, which is one quadrillionth of a second (0.000,000,000,000,4 seconds). A flash of lightning lasts about 30,000 times longer than one picosecond, which is already a thousand times slower than the fastest SF rate! |
To monitor the energy conversion, the researchers used femtosecond transient absorption (TA) spectroscopy, a technique that allows them to observe chemical reactions at unimaginably short timescales. The spectroscopy results showed a clear, strong inverse relationship between the physical distance and the speed of SF. As the inter-chromophoric distance increased, the electronic coupling (the degree to which the electronic clouds of the two NDI units interact) decreased. This, in turn, caused the SF state formation time to slow down dramatically, from an incredibly swift 400 femtoseconds to a more gradual 4.6 picoseconds. A femtosecond is a quadrillionth of a second, demonstrating the extraordinary speed and precision of this molecular mechanism.
The study's findings directly challenge a long-held rule in the field of singlet fission. For SF to occur, it was generally believed that the energy of the starting singlet exciton must be approximately double the energy of the resulting triplet exciton, known as the thermodynamic criterion. The NDI monomers used in this study did not meet this strict energetic requirement individually. The successful SF observed in the NDI dimers, therefore, provides strong evidence that the crucial factor is not the energy of the individual molecule, but the stabilisation of the multi-excitonic pair state within the structure.
By controlling the distance through a rigid structure, the researchers have shown that the formation rate of the energy-doubling state can be predictably and systematically tuned. SF is an important phenomenon because it can boost the power conversion efficiency of standard silicon solar cells beyond their current theoretical limit. Beyond energy, SF mechanisms are also being used to design highly efficient compounds for cancer photodynamic therapy.
By establishing a clear rule linking molecular structure to SF speed, this work could improve our control of the quantum states for several applications. Showing that a short-range interaction dictates SF rates is a significant step toward building a universal dimer system that can be designed to harness the energy-doubling process of singlet fission. The predictable structural control achieved in this research opens up a vast new chemical space for materials science, promising highly efficient energy harvesting for improved photovoltaics and medical advancements.
This article was written with the help of generative AI and edited by an editor at Research Matters.
