Triplet-pair spin signatures from macroscopically aligned heteroacenes in an oriented single crystal

Abstract

The photo-driven process of singlet fission generates coupled triplet pairs (TT) with fundamentally intriguing and potentially useful properties. The quintet 5 TT 0 sublevel is particularly interesting for quantum information because it is highly entangled, is addressable with microwave pulses, and could be detected using optical techniques. Previous theoretical work on a model Hamiltonian and nonadiabatic transition theory, called the JDE model, has determined that this sublevel can be selectively populated if certain conditions are met. Among the most challenging, the molecules within the dimer undergoing singlet fission must have their principal magnetic axes parallel to one another and to an applied Zeeman field. Here, we present time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy of a single crystal sample of a tetracenethiophene compound featuring arrays of dimers aligned in this manner, which were mounted so that the orientation of the field relative to the molecular axes could be controlled. The observed spin sublevel populations in the paired TT and unpaired (T+T) triplets are consistent with predictions from the JDE model, including preferential 5 TT 0 formation at z ‖ B 0 , with one caveat—two 5 TT spin sublevels have little to no population. This may be due to crossings between the 5 TT and 3 TT manifolds in the field range investigated by TR-EPR, consistent with the intertriplet exchange energy determined by monitoring photoluminescence at varying magnetic fields.

Singlet fission as a polarized spin generator for dynamic nuclear polarization

“…According to the JDE model, the 5 (TT) 0 quintet state of chromophore dimers can be generated as a nearly pure quantum state by making the exchange interaction between two chromophores sufficiently large and by making the principal axes of the two chromophores parallel to each other and to the Zeeman field 19 . This model has explained well the experimental results of oriented crystalline samples 20 , 21 . To date, the spin aspect of SF has only been used to explain the microscopic mechanisms of SF.…”

“…There was an approximate correlation between ESR and DNP profiles, confirming that DNP was performed using triplets and quintets, respectively. Although triplet gave the larger DNP effect than quintet in the present system, it is possible to suppress the formation of triplet by selectively synthesizing dimers, and it would be possible to improve the quintet DNP performance more by selectively generating 5 (TT) 0 among the quintet sublevels by controlling the orientation of the dimers relative to the magnetic field 21 . These results indicate that nuclear hyperpolarization of water molecules was successfully achieved using SF-derived quintet electron polarization.…”

Entangled spin-polarized excitons from singlet fission in a rigid dimer

“…Future work will compare the relative merits between the two types of experiments. Notably, there are two recent works in the literature that use singlet fission in the pursuit of quantum information applications, but with chromophores in crystals that are oriented in the field, not dimers 8 , 32 . The value of T 2 for TIPS-BP1' dimers reported here at 75 K is similar to that reported in ref.…”

“…For example, in solid-state SF systems, individual chromophores are aligned through crystallization. Triplet pairs may form at sites where J is large, but in crystals the excitons are mobile and hopping unpairs the biexciton state, which can then decohere 8 – 12 . There have been no reports of an aligned system of molecular SF dimers, where chromophores are covalently coupled through synthetic design.…”

Near-Infrared Absorption Features of Triplet-Pair States Assigned by Photoinduced-Absorption-Detected Magnetic Resonance

“…Singlet fission (SF) is a multiexciton generation process wherein a singlet excited state on one chromophore, S 1 , partitions its energy with a nearby chromophore in its ground state, S 0 , to generate two triplet excited states, T 1 + T 1 . As a downconversion process, SF can be used in photovoltaics to reduce thermalization losses, − but recent work has also suggested that intermediate triplet-pair states, particularly 5 TT, may have quantum information (QI) applications, − since these may form with high initial spin polarization. Distinct pair states, n TT, form when two adjacent triplet states experience a strong interchromophore exchange interaction, such that pairs with different spin multiplicities, n = 1, 3, or 5, possess different energies.…”