Designing luminescent organic diradicals with optical-spin interfaces

In the fields of quantum information science and sensing, electron spins are often purified into a specific polarisation through an optical-spin interface, a process known as optically-detected magnetic resonance (ODMR). Diamond-NV centres and transition metals are both excellent platforms for these so-called colour centres, while metal-free molecular analogues are also gaining popularity for their extended polarisation lifetimes, milder environmental impacts, and reduced costs. In this session, we will first discuss our theoretical proposal for alternant organic high-spin π-diradicals with ground-state spins that can be polarised by shelving triplet M_S = ±1 populations as singlets. This was recently verified by experiments albeit with low ODMR contrasts of < 1% at temperatures above 5 K. Following that, we will present how we can improve the ODMR signal by moving singlet populations back into the triplet M_S = 0 sublevel, thereby designing a true carbon-based molecular analogue to the NV centre. To achieve both spin channels in π-diradicals, we leverage weaker spin-orbit couplings beyond the nearest-neighbour approximation, made possible by careful control of orbital nodal structures and group-theoretical considerations. These analyses are further confirmed by ab initio calculations of a realistic trityl-based radical dimer. Microkinetic analyses point towards high ODMR contrasts of around 30% under experimentally-feasible conditions, a stark improvement from previous works. Finally, in our quest towards ground-state optically-addressable molecular spin qubits, we exemplify how our symmetry-based design avoids Zeeman-induced singlet-triplet mixings during electron paramagnetic resonance (EPR) experiments, setting the scene for realising electron spin qubit gates.