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arXiv:2609.30899

Room-temperature quantum-sensing molecular crystals grown in minutes

Madhur Parashar, Guangzhao Chen, Emanuel Druga, Liang Z. Tan, Jeffrey Reimer, Ashok Ajoy

quant-phSubmitted Sep 25, 2026• Announced Sep 28, 2026
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Abstract

Quantum sensing increasingly demands materials combining room-temperature optical spin addressability and coherent control with rapid, accessible fabrication. Yet semiconductor platforms require costly materials and specialized processing, while molecular systems often depend on bespoke synthesis or involved crystal growth. Here we apply microspacing in-air sublimation (MAS) to rapidly fabricate quantum-sensing materials within minutes. Microgram quantities of commercially available precursors crystallize on glass, in air and without vacuum or inert-gas handling. Monitored in situ, growth yields optically clear, stoichiometric single crystals of excellent quality, including plates and needles with intrinsic optical waveguiding. Donor-acceptor charge-transfer co-crystals pairing anthracene or biphenyl with tetracyanobenzene, together with localized-exciton pentacene:pentacenequinone (P:PQ), exhibit room-temperature optically detected magnetic resonance (ODMR). Charge-transfer ODMR resonances reach linewidths below $4\mathrm{MHz}$, consistent with motional narrowing of mobile triplet excitons, while P:PQ supports coherent ensemble spin control with $T_2=0.56μ\mathrm{s}$. Functionality across chemically and electronically distinct pairs establishes MAS as an accessible screen for molecular quantum-sensing materials, connecting molecular crystal engineering with low-cost, chemically tunable thin triplet-spin layers and waveguiding sensor geometries for quantum sensing and microscopy.

Comments
34 Pages, 6 Main Figures, 18 Supplementary figures

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Quantum sensing increasingly demands materials combining room-temperature optical spin addressability and coherent control with rapid, accessible fabrication. Yet semiconductor platforms require costly materials and specialized processing, while molecular systems often depend on bespoke synthesis or involved crystal growth. Here we apply microspacing in-air sublimation (MAS) to rapidly fabricate quantum-sensing materials within minutes. Microgram quantities of commercially available precursors crystallize on glass, in air and without vacuum or inert-gas handling. Monitored in situ, growth yields optically clear, stoichiometric single crystals of excellent quality, including plates and needles with intrinsic optical waveguiding. Donor-acceptor charge-transfer co-crystals pairing anthracene or biphenyl with tetracyanobenzene, together with localized-exciton pentacene:pentacenequinone (P:PQ), exhibit room-temperature optically detected magnetic resonance (ODMR). Charge-transfer ODMR resonances reach linewidths below 4MHz4\mathrm{MHz}4MHz, consistent with motional narrowing of mobile triplet excitons, while P:PQ supports coherent ensemble spin control with T2=0.56μsT_2=0.56μ\mathrm{s}T2​=0.56μs. Functionality across chemically and electronically distinct pairs establishes MAS as an accessible screen for molecular quantum-sensing materials, connecting molecular crystal engineering with low-cost, chemically tunable thin triplet-spin layers and waveguiding sensor geometries for quantum sensing and microscopy.
Molecular materials that enable coherent control over an electron's spin state at room temperature are promising candidates for quantum technologies, including quantum sensors and ultra-low noise microwave amplifiers, known as masers. Host-guest molecular crystals enable independent control of spin-active guests and their local environments to enhance molecular spin properties and so improve device performance. Using electron paramagnetic resonance and optically-detected magnetic resonance, we demonstrate the ability to tune triplet population, depopulation, and spin-lattice relaxation by modulating host-dependent lattice rigidity and vibrational coupling to significantly reduce the operating requirements for building useful masers. Importantly, the most rigid host, picene, reveals the ability to slow spin-lattice relaxation without lengthening triplet lifetime, though at the cost of strain-induced line width broadening and reduced triplet spin polarisation. We also find that deuteration reduces the triplet resonance line width and vibrationally-mediated triplet depopulation. Consequently, we find that perdeuterated pentacene in perdeuterated p-terphenyl is the most viable candidate for building a continuous wave maser. This work demonstrates host-guest engineering as an important and practical method for tuning the spin-dependent performance of room-temperature molecular quantum technologies.
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Quantum sensors based on electronic spins have emerged as powerful probes of microwave-frequency fields. Among other solid-state platforms, spins in molecular crystals offer a range of advantages, from high spin density to functionalization via chemical tunability. Here, we demonstrate microwave vector magnetometry using the photoexcited spin triplet of deuterated pentacene molecules, operating at zero external magnetic field and room temperature. We achieve full three-dimensional microwave field reconstruction by detecting the Rabi frequencies of anisotropic spin-triplet transitions associated with two crystallographic orientations of pentacene in naphthalene crystals. We further introduce a phase alternated protocol that extends the rotating-frame coherence time by an order of magnitude and enables sensitivities of 1 μT/Hz1~μ\mathrm{T}/\sqrt{\mathrm{Hz}}1 μT/Hz​ with sub-micrometer spatial resolution. These results establish pentacene-based molecular spins as a practical and high-performance platform for microwave quantum sensing, and the control techniques are broadly applicable to other molecular and solid-state spin systems.
Optically addressable spins at material surfaces have represented a long-standing ambition in quantum sensing, providing atomic resolution and quantum-limited sensitivity. However, they are constrained by a finite depth at which the quantum spins can be stabilized. Here, we demonstrate a hybrid molecular-2D architecture that realizes quantum spin sensors directly on top of the surface. By anchoring spin-active molecules onto hexagonal boron nitride (hBN), we eliminate the depth of the quantum sensor while also exhibiting robust spin properties from 4~K to room temperature (RT). The Hahn-echo spin coherence time exceeds \(T_2 = 3.4~\upmu\text{s}\) at 4~K, outperforming values in bulk organic crystals and overturning the prevailing expectation that spin inevitably deteriorates upon approaching the surface. By chemically tuning the molecule through deuteration, T2T_2T2​ improves by more than 10-fold, and under dynamic decoupling, coherence is prolonged to the intrinsic lifetime limit, exceeding 300~\(\upmu\text{s}\). Proximal proton spins and the magnetic response of two-dimensional magnets beneath the hBN layer have been detected at RT. These molecular spins form surface quantum sensors with long coherence, optical addressability, and interfacial versatility, enabling a scalable, adaptable architecture beyond what conventional solid-state platforms offer.
Maximizing the number of addressable spins within a fixed volume can improve ensemble quantum sensor sensitivity, but dense packing usually increases dipolar interactions and excited-state transport, shortening coherence and suppressing optical readout. Here we report a 2:1 cocrystal of 6,13-dihydropentacene and pentacene (DHP/Pc) containing 33.3 mol% pentacene (3.3×1053.3\times10^{5}3.3×105 ppm; 9.51×10209.51\times10^{20}9.51×1020 cm−3^{-3}−3), a volumetric spin-site density more than two orders of magnitude above previous benchmarks, NV-diamond and pentacene-doped p-terphenyl (PDP). Despite this density, DHP/Pc exhibits microsecond spin coherence, room-temperature optically detected magnetic resonance and coherent control. Time-resolved measurements indicate that the long-lived triplet population is generated predominantly by intersystem crossing and exhibits strong, non-thermal sublevel polarization. Density-functional theory calculations further suggest that the native cocrystal geometry weakens electronic coupling between neighboring pentacenes, while the higher DHP triplet energy creates a barrier to triplet migration. These results identify molecular packing and coformer triplet energetics as complementary design parameters for preserving coherence in high spin-site density systems, demonstrating cocrystallization as a promising route to dense, optically addressable spin materials for room-temperature ensemble quantum sensing.
Molecular quantum sensors represent a promising frontier for the detection of nuclear magnetic resonance signals and alternating current magnetic fields at the nanoscale, potentially reaching single-proton sensitivity. Although the triplet states of molecular pentacene provide a viable sensing architecture, the triplet pair states produced by singlet fission of pentacene dimers could enable more flexible quantum manipulations through entanglement. In this work, we model the quantum sensing efficacy of a spin-polarized quintet manifold in a photoexcited pentacene dimer generated via intramolecular singlet fission. Using a Lindblad master equation approach, we simulate the evolution of the triplet pair state under standard dynamical decoupling sequences, including spin echo, XY4, and XY8 and provide a direct performance comparison to the traditional pentacene monomer benchmark. While both architectures exhibit comparable sensitivity for isolated single-spin detection, our findings indicate that the dimer architecture provides a superior interaction cross-section for detecting small ensembles of nuclear spins. Analytical expressions derived for fluorescence modulation demonstrate that sensitivity is optimized in the low-magnetic field regime and scales with the number of pulses in the sensing protocol. This study establishes a theoretical baseline for utilizing high-spin multi-excitonic states as chemically tunable, high-sensitivity quantum probes.