Reversible spin-optical interface in luminescent organic radicals

Abstract

Molecules present a versatile platform for quantum information science1,2 and are candidates for sensing and computation applications3,4. Robust spin-optical interfaces are key to harnessing the quantum resources of materials5. To date, carbon-based candidates have been non-luminescent6,7, which prevents optical readout via emission. Here we report organic molecules showing both efficient luminescence and near-unity generation yield of excited states with spin multiplicity S > 1. This was achieved by designing an energy resonance between emissive doublet and triplet levels, here on covalently coupled tris(2,4,6-trichlorophenyl) methyl-carbazole radicals and anthracene. We observed that the doublet photoexcitation delocalized onto the linked acene within a few picoseconds and subsequently evolved to a pure high-spin state (quartet for monoradical, quintet for biradical) of mixed radical–triplet character near 1.8 eV. These high-spin states are coherently addressable with microwaves even at 295 K, with optical readout enabled by reverse intersystem crossing to emissive states. Furthermore, for the biradical, on return to the ground state the previously uncorrelated radical spins either side of the anthracene shows strong spin correlation. Our approach simultaneously supports a high efficiency of initialization, spin manipulations and light-based readout at room temperature. The integration of luminescence and high-spin states creates an organic materials platform for emerging quantum technologies.

Luminescent Radicals

“…Notably, the radicals have been elucidated to exhibit emission characteristics and excited-state dynamics unique to the doublet or higher-multiplet spin states, such as highly efficient electron–photon conversion, absence of heavy atom effect, and magnetic field-controlled luminescence, which are unexpected for conventional closed-shell luminophores. Emerging properties and phenomena are now being discovered one after another, such as an OLED with the EQE of 28%, optical readout of high spin states with S > 1 at room temperature, and single-molecule magnetoluminescence. − Overall, organic radicals have great potential to expand further the chemical and spin spaces of luminescent molecular systems, thus broadening their applicability to photofunctional materials and devices.…”

Designing thermally activated delayed fluorescence emitters with through-space charge transfer: a theoretical study

“…1–9 The photophysical performance of organic molecules depends highly on their excited state properties. 10–12 By designing TADF molecules with twisted donor (D) and acceptor (A) configurations, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) can be effectively separated, leading to a reduction in the singlet–triplet energy gap (Δ E ST ) and an increase in the reverse intersystem crossing rate ( K RISC ). 13–16 For instance, a group of D–A–D type TADF emitters employing 10-(pyridin-2-yl)-acridin-9(10 H )-one and substituted carbazoles can achieve a high K RISC of 10 5 s −1 .…”

Oriented Triplet Excitons as Long-Lived Electron Spin Qutrits in a Molecular Donor–Acceptor Single Cocrystal

“…11,13,28 This is primarily due to the fact that most studies of triplet states are performed on randomly oriented ensemble samples with a large distribution of zero-field splitting tensors relative to the applied magnetic field, leading to significant line width broadening and overlap of the EPR transitions. 11,14,29,30 These spin sublevel transitions are also usually spread over a wide spectral range due to the large zero-field splittings that are typical of molecular triplet states (>1 GHz). 11,12,30 The combination of these effects makes it difficult to generate microwave pulses with sufficiently broad bandwidths to address the triplet spin transitions either simultaneously or selectively, as is essential, to perform nontrivial quantum operations using coupled electron spins.…”

“…11,14,29,30 These spin sublevel transitions are also usually spread over a wide spectral range due to the large zero-field splittings that are typical of molecular triplet states (>1 GHz). 11,12,30 The combination of these effects makes it difficult to generate microwave pulses with sufficiently broad bandwidths to address the triplet spin transitions either simultaneously or selectively, as is essential, to perform nontrivial quantum operations using coupled electron spins. 13,21,26,27,31−34 Additionally, the DiVincenzo requirements assert that the coherence time of these spin states must be sufficiently long to allow the execution of quantum operations, 1 a task that has remained challenging for molecular electron spins.…”

“…Despite the advantages outlined above, demonstrations of complex quantum operations using molecular electronic triplet states as qubits or qutrits have been limited to a few cases. ,, This is primarily due to the fact that most studies of triplet states are performed on randomly oriented ensemble samples with a large distribution of zero-field splitting tensors relative to the applied magnetic field, leading to significant line width broadening and overlap of the EPR transitions. ,,, These spin sublevel transitions are also usually spread over a wide spectral range due to the large zero-field splittings that are typical of molecular triplet states (>1 GHz). ,, The combination of these effects makes it difficult to generate microwave pulses with sufficiently broad bandwidths to address the triplet spin transitions either simultaneously or selectively, as is essential, to perform nontrivial quantum operations using coupled electron spins. ,,,,− Additionally, the DiVincenzo requirements assert that the coherence time of these spin states must be sufficiently long to allow the execution of quantum operations, a task that has remained challenging for molecular electron spins. ,,− As a result, strategies to design high-spin-state molecular systems with selectively addressable spin transitions and long spin coherence times are highly sought after …”