The change taking place between singlet and triplet states of electron pairs in charge-separated states plays a crucial role in nature. The compass of migratory birds could even potentially be explained by the impact of the geomagnetic field on the magnetic interplay among these two spin states.
Basic diagram of the experiment. The charge-separated radical pair (CSRP, black curve) decays within about 1000 ns by recombination of the electrons to singlet or triplet product. Here, the dynamic alternation of the CSRP between singlet (S) and triplet (T) state is recorded only averaged over the total reaction time. Using the pump-push-pulse technique, singlet and triplet character of the CSRP can be read out at any time. Image Credit: Christoph Lambert/Universität Würzburg.
So far, this quantum process could not be tracked directly optically and only be assessed summarily in the final result. In the present issue of the journal Science, research collaboration, with Professor Ulrich Steiner from the University of Konstanz and scientists from the Universities of Würzburg and Novosibirsk (RUS), presents the pump-push-pulse method. This enables the researchers to optically identify the time course of the singlet or triplet ratio for the first time. This reveals new avenues, for instance in the field of organic solar cells, as well as for qubits in quantum computers.
Generally, electrons present in a molecule occupy the quantum theoretically allowed orbits pair-wise. The electrons’ intrinsic angular momentum property, their spin, is of key significance in this context. As per the Pauli Exclusion Principle of quantum mechanics, two electrons have the ability to travel along the same orbit only if their spins are antiparallel. If one electron tends to rotate clockwise, the other must rotate counter-clockwise.
In the molecular ground state, generally, all electron spins are paired. Through light excitation, a single electron gets separated from the paired constellation and lifted to a greater level of energy, where it holds a free orbit singlehandedly.
From here, it could further jump to a free orbit in an appropriate neighboring molecule. This leads to photo-induced electron transfer. The two isolated electrons can currently alter their spin settings independently via magnetic interaction with their corresponding surroundings, as they are no longer forced by the Pauli principle.
The Two Lone Electrons Form a Radical Pair
Furthermore, such charge separation by photo-induced electron transfer occurs, as in photosynthesis. There is only a slight reduction in the energy of the transferred electron during this step, so that majority of the electronic energy initially absorbed via the light excitation is still maintained. Therefore, this original excitation energy is stored in chemical form.
In chemistry, the charge-separated state along with the two lone electrons is also called a radical pair. If the spins of the two isolated electrons are arranged in parallel, one tends to speak of a triplet state and if their alignment seems to be antiparallel, this is a singlet state of the radical pair.
As a result of the free individual evolutions of the two spins, the spin state of the radical pair alternates between the singlet and triplet state. As there is not much variation between these spin alignments concerning energy, so far they have not been directly optically distinguishable.
Achieving energy stabilization of the radical pair could be done by the radical electron jumping back from the acceptor molecule to the donor molecule, whereby the original singlet state has been restored, thereby discharging energy in the form of heat.
But to be able to pair again with the original partner electron, its spin should have remained opposite to that of the latter, which is not essential, as spin reorientation might have taken place in the meantime.
If its present alignment seems to be different, it cannot get back to its original orbit, but alternatively, it can discharge energy by transitioning into another, lower orbit that is still free. This leads to a triplet product that can be optically differentiated from the singlet product.
Radical Pair as Model for Qubits and the Magnetic Field Sensor of Migratory Birds
As far as several aspects are concerned, the phase in which the radical pairs tend to oscillate between singlet and triplet states is of specific interest. As it is a coherent motion controlled by quantum mechanics, it can be regulated, for instance by an external magnetic field. These motions are used in physics to implement in quantum computers.
Our radical pair can serve as a model for qubits, as they exist as elements in quantum computers, or for understanding the function of radical pairs in the biological compass model of migratory birds mentioned above. For such reasons, it is of interest to know how the spin is currently positioned in this process.
Ulrich Steiner, Photokinetics and Spin Chemistry, University of Konstanz
“Pump-Push-Pulse” Technique to Read Out Singlet/Triplet Ratio
With the “pump-push-pulse” technique, the team has developed a procedure that achieves the possibility for the first time to read out the singlet/triplet ratio at specific points in time. Firstly, the electron transfer from the donor to the acceptor molecule is started with a pump laser pulse.
This, in turn, gives rise to the charge-separated state with singlet spin. Now, the uncoupled electron spins can evolve. After some time, a second laser pulse is followed.
This push laser pulse in turn transfers an electron from the acceptor back to the donor, whereby the second laser pulse forces the system to immediately make the decision between triplet or singlet product formation, for which the radical pair would normally take several spin oscillation periods.
Ulrich Steiner, Photokinetics and Spin Chemistry, University of Konstanz
Ulrich Steiner together with his Russian collaborator has verified the interpretation of the experiments by model calculations depending on quantum theory. By adopting this method, it is possible to take the so-called snapshots of the spin state of the radical pair at different times.
Journal Reference:
Mims, D., et al. (2021) Readout of spin quantum beats in a charge-separated radical pair by pump-push spectroscopy. Science. doi.org/10.1126/science.abl4254.