Synthetic tuning of the quantum properties of open-shell radicaloids
We unravel the structure-property relations for the quantum behavior in a whole family of open-shell radicaloids that vary in size and conjugation. The effect of conjugation and edge functionalization are clarified, as well as the role of the molecular environment, for the quantum performance. Record quantum performances are obtained at high temperatures, and, by individuating the decoherence channels, synthetic strategies are proposed in order to further improve coherence in molecular conjugated materials.
INTRODUCTION<!>The bigger picture<!>Synthesis<!>Magnetic properties<!>Pulsed electron paramagnetic resonance<!>Spin reversal times<!>Spin coherence times<!>Theoretical modeling<!>Conclusions<!>EXPERIMENTAL PROCEDURES<!>Materials availability<!>Data and code availability
<p>Spin-based technologies have fueled the race toward quantum information processing. 1 Coherent two-level systems offer a splendid platform where microwave pulses and a static magnetic field allow creating quantum superpositions of states and quantum gates. Numerous approaches use solid-state systems, such as donors in silicon, 2 nitrogen vacancies in diamond, 3 or defects in silicon carbide. 4 Molecular systems have also embarked on such a route, 5 offering the perspective of chemical tuning of the quantum functionalities. Initial efforts have focused on inorganic complexes [6][7][8] that, despite issues with robustness, 9 have allowed reaching considerable spin coherence times via the design of ligands, 10 and also allowed exploiting clock transitions. 11,12 Polycyclic hydrocarbons (PHs), with a p-conjugated backbone that makes them akin to small pieces of graphene, are now emerging as a promising platform for quantum molecular materials. 13 The magnetic properties are introduced synthetically by using open-shell radicaloids, which produce rather stable spin states that are unaffected by spin-orbit coupling. 14,15 Magnetic edge states have been recently demonstrated in synthetic graphene nanoribbons, 16 and topological effects play a key role in radicaloids, 17 with coherence times that already outperform most solid-state systems at room temperature. 18 In order to optimize their quantum coherence behavior, it is now necessary to understand the relationship between the molecular structures and quantum properties. 18 For example, it is unclear how the size of the extended p-systems affects the coherence properties, or how the molecular geometry and electron-nuclear interactions determine the coherence time.</p><!><p>Molecular compounds are emerging as appealing units for quantum computation and quantum technologies. For these purposes, open-shell molecular radicaloids hold an exceptional interest: they offer p-conjugated units with an unpaired electron ground state, and they can be considered as atomically precise pieces of graphene. The next frontier to improve the quantum performance is to determine structure-quantum properties relations, so as to channel the synthetic efforts toward blocking decoherence pathways. At present, we have little clue as to how the shaping of radicaloids can affect their quantum behavior, and what their performance limit could be. In this work, we synthesize and characterize a family of radicaloids and explore how molecular design leads to quantum memory loss. These results are crucial for the synthesis of efficient quantum units and contribute to the fundamental understanding of spin decoherence in graphene-related materials, opening the path to multifunctional quantum materials. Here, we investigate these issues, unraveling the structure-property relations of three related open-shell radicaloids 1, 2, and 3, developed from a common meta-quino-di-methane unit (Chart 1). The three molecular systems are tailored so as to investigate the effect of a different number of pentagonal defects, different degrees of conjugation, and different areas of the graphenoid plane over which the p orbitals extend. Compound 1 is a highly stable, curved diradicaloid derived from the indeno[2,1-b]fluorene scaffold, 19 2 and 3 are related to it because they contain two units of 1 linked by dimerization and fusion, respectively. Compound 2 is the dimer in which two units of 1 are connected by a single C-C bond, which results into a highly twisting non-planar structure with a bi-chrysene subunit. In contrast to 2, compound 3 is a p-extended derivative with a larger degree of conjugation in which the two chrysene units are fused, possessing a stable singlet tetraradical character. 20 Using pulsed electron paramagnetic resonance (EPR), we identify different quantum regimes at different temperatures, T, and the mechanisms of decoherence that are dominant in each of them. 18 Comparison of the quantum properties in powders and deuterated and non-deuterated solvents reveals the influence of the environment and shows the effects of aggregation and the molecule-solvent decoherence pathways. Rotationally free side groups are found to be extremely important in limiting coherence, while a degree of synthetic freedom is established in extending the conjugated backbone. With these results, we reach record-high coherence times at room temperature and layout the synthetic pathway to even higher values.</p><!><p>The synthetic routes of the two novel singlet radicaloids, 1 and 2, are shown in Scheme 1, while radicaloid 3 was prepared according to our previous report. 20 First, 5-bromochrysene (7) was obtained in 52% yield from ((2-iodophenyl)ethynyl)triisopropylsilane (4) in three steps. Then, a Suzuki coupling between 7 and the reported 4 20 gave the key dialdehyde intermediate 10 in a 53% yield. Subsequently, treatment of 10 with 2-mesitylmagnesium bromide formed the diol compound, which was further subjected to a BF 3 $OEt 2 -mediated Friedel-Crafts cyclization to afford the precursor 11 in a 48% yield over two steps. Finally, the oxidative dehydrogenation of 11 with 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) provided the target benzo [6,7]-s-indaceno[1,2-g]chrysene derivative (1) as a dark-cyan solid in a 76% yield. The radicaloid dimer 2, a deep-green solid, was obtained by a similar synthetic path, starting from the 11,11 0 -dibromo-5,5 0 -bichrysene (8) and compound 9. Remarkably, both molecules 1 and 2 were highly stable under ambient conditions (Figure S4), and hence, they could be purified by column chromatography without Ar protection. Radicaloids 1 and 2 were fully characterized by high-resolution MALDI-TOF mass spectrometry, NMR Article spectroscopy, UV-vis-near infrared absorption, and cyclic voltammetry measurements (Supplemental information).</p><!><p>Variable-temperature 1 H-NMR measurements on 1 and 2 showed increasing linewidths with increasing temperatures, in agreement with a thermally populated paramagnetic state (Figures S24A and S32A). Within the studied temperature range, the line broadening by paramagnetic effects was selective, and the linewidth of protons with a negligible triplet spin density was not affected. Superconducting quantum interference device (SQUID) magnetometry was carried out on powders of 1 and 2. In both cases, the susceptibility increased with an increasing T (Figures S40 and  S41), in agreement with the EPR signal (Figure S42) and the radicaloid character, in the presence of a singlet ground state with a thermally excited paramagnetic state. Fitting with the Bleaney-Bowers 21 equation yielded a singlet-to-triplet gap DE SÀT = À 2:40 kcal/mol for 1, and DE SÀT = À 2:06 kcal/mol for 2, in overall agreement with the DE SÀT = À 2:08 kcal/mol value of the previous reports for 3. 20 In addition, as shown in the literature on poly-radicaloids, SQUID measurements cannot clarify if the first excited state is a triplet or a quintet. 20,[22][23][24] The EPR spectra of compounds 1, 2, and 3 show one single peak, indicating that the spin-spin exchange is much larger than the microwave frequency, and display an increasing level of structuring on passing from 3 to 2 to 1 (Figure 1). The choice of solvent does not modify the CW EPR spectrum, and thus, only toluene is shown,  Article for simplicity. All spectra can be simulated with an excellent agreement to the data 25 using the following Hamiltonian equation:</p><p>where, b S i are the electron spin operators, J is the electron-electron exchange coupling, m B the Bohr magneton, g the isotropic (scalar) Lande ´factor for the electron spin, A i the hyperfine matrix for the ith nucleus, b I i the nuclear operators, and D the dipolar coupling. For all three compounds, we obtained g = 2.0027, which is comparable to other open-shell radicaloids [26][27][28][29][30] and other carbon nanostructures with weak spin-orbit coupling. 31 In liquids, the averaging produced by molecular tumbling hides trends in the anisotropy and dipolar coupling, but it is possible to distinguish isotropic hyperfine contributions for both 1 and 2. In Figure 1B, the solution spectra of 1-3 are directly compared. The width of the CW spectra is only determined by the strength of the isotropic hyperfine coupling, which can be used as a fingerprint of the delocalization of the electron spin over the whole molecule. 32 Different from other radical systems, such as nitroxide or nitronyl-nitroxides, the radicaloids have a singlet-triplet splitting and behave diamagnetically at low T, so that even NMR spectra could be measured around 300 K. This is phenomenon in good agreement with the modest radical character of the system at room temperature, and is generally observed in open-shell molecules. 33,34 Because most molecules are in their singlet state, no appreciable SQUID signal is observed at low T, while a very weak signal is still detectable with EPR. As reported in the literature, 18,[35][36][37] this low-T signal is produced by partially hydrogenated species with half-integer spin configurations: the final step of the synthesis involves dehydrogenation of the pentagonal sites, and there is a small ($1%) probability that the reaction will not remove the hydrogen on one pentagonal site. Bi-hydrogenated species are not necessarily magnetic, while tri-hydrogenated ones occur with negligible probability ($1 every 10 6 molecules) and can be neglected. As a result, we can investigate not only the behavior of the radicaloid species but also that of the mono-hydrogenated radicals, which are also interesting and representative of the quantum properties obtainable with molecularly made graphenoid structures. The structure and spin density of these hydrogenated species are simulated using density functional theory (DFT) in the same way as for 1-3 (vide infra). The signal from hydrogenated species is dominant between 10-100 K, where 1-3 are in their singlet state, and is negligible above 150 K, where the thermally populated paramagnetic states of 1-3 dominate the behavior. The 100-150 K range sees a co-existence of the two signals, where the broadening of the linewidth by unresolved hyperfine couplings (vide infra) does not allow spectral resolution of the two species. Therefore, the discussion will be carried out cohesively but highlights that any measurement below 100 K solely refers to such additional species.</p><!><p>Pulsed microwave excitation allows probing the quantum properties of the molecules. In order to distinguish and quantify environmental effects on the quantum properties, we examined Article of progressively weaker hyperfine interactions; the d 14 OTP allowed probing a solid environment with suppressed molecule-molecule and matrix hyperfine interactions. All solutions (2G0.5 mM) were degassed with 5 cycles of pump-freeze thaw before flame sealing quartz tubes. Since just a small fraction of the molecule was in the thermally populated paramagnetic state, the spin concentration was much lower than 2 mM. Different concentrations were tested, and further lowering the concentration did not affect the results, except for the reduction in signal-tonoise ratio (Figure S44).</p><p>Two characteristic times, T 1 and T m , describe the spin dynamics. 38 The spin-lattice relaxation time, T 1 , describes the recovery time of magnetization from out of equilibrium, by spin reversal (i.e., the up-down state in the classical picture) following exchange of angular momentum. We measured T 1 through Inversion or Picket Fence recovery, which is fitted with a bi-exponential function that takes into consideration the effect of instantaneous diffusion that usually appears as a faster recovery (Figure 2A). De-oxygenation of the solution has a large effect on T 1 (Figure S51) because oxygen introduces an additional way to flip electron spins, as also reported for other systems. 39 The coherence time T m , as considered in this work, takes into account spin diffusion, nuclear diffusion, and partial-dilution effects, which all affect the coherent evolution of the quantum states in a non-ideal ensemble on non-isolated molecules. Therefore, our measured T m represents a lower boundary of the true quantum coherence of the system. The measurement of T m was performed via the Hahn echo decay. Hyperfine effects immediately appeared in the Hahn echo experiments as a modulation superimposed onto the signal decay, with a frequency corresponding to the Larmor frequency of the relevant nuclei (Figure 2B). Because of molecular tumbling, no hyperfine modulation was observed in the liquid phases. In the solid state, we observed 1 H modulation in the powders and in toluene (14.8 MHz at 330 mT), deuterium modulation in d 8 Tol and d 14 OTP (2.27 MHz at 330 mT), and no modulation in CS 2 . This modulated echo decay was fitted with the following equation:</p><p>where, Y 0 is the normalized amplitude, T m is the coherence time, k is the modulation depth, u is the Larmor frequency, 4 a phase adjustment, and t is the time coordinate. 6,40 Excellent agreement is always obtained, at every T and in all solvents (Figure 2B).</p><!><p>The spin reversal time is an important upper limit for the quantum performance of the systems; the quantum coherence usually cannot survive a spin-flip, and it is thus important to maximize T 1 as well as T m . The T 1 À1 increases monotonically with T, with minimal variations on changing solvent size, viscosity, or mass variation (as produced by deuteration).</p><p>This indicates that spin-lattice relaxation is primarily due to the exchange of angular momentum with intramolecular modes but via different mechanisms when at different temperatures (Figure 3). A model that includes direct and Raman processes for half-integer spins 34,39,41 provides an excellent agreement for the d 8 Tol data in the frozen region 10-125 K:</p><p>x 8 e x ðe x À 1Þ 2 dx; (Equation 3)</p><p>where A dir and A Ram are rate constants and q D is the Debye temperature. The low-T regime is dominated by the direct process, while Raman processes become dominant above 20 K, where more phonon modes are available.</p><p>The solid d 14 OTP matrix shows that T 1 is only weakly T-dependent above 150 K, and a good fit is obtained,</p><p>, where C Ram is the fit parameter for a Raman process above the Debye temperature. 41,42 This indicates that the main process for T 1 remains a Raman one up to room T and, different from graphene nanoribbons, 16 radicaloids display no contribution from local modes. Without any particular molecular optimization, this leads to a T 1 $1 ms at room T. This value is comparable with those of the best solid-state quantum units-nitrogen-vacancy centers in diamond (6 ms) 43 and endohedral fullerenes N@C 60 . 44 These values are more than two orders of magnitude higher than molecular qubits based on metal-centers. 45,46 These very long relaxation times become partially masked, in liquid solvents, by the molecular tumbling. In solution, three main processes contribute to the spin-lattice relaxation-the modulation of the g-factor due to the rotation of the molecule (SR); the modulation of the coupling between electron and hydrogen nuclei by electronnuclear dipolar interactions (END); and solvent-probe translational diffusion (SD). 42,47,48 All three processes happen in parallel, so that</p><p>, and the timescale associated with each process can be evaluated using the following equation:</p><p>c + 1 2u e t c 1=4 (Equation 4)</p><p>where Dg = ðg x À g e Þ 2 + ðg y À g e Þ 2 + ðg z À g e Þ 2 is the g-anisotropy modulation, DA = 2 9 IðI + 1Þ</p><p>is the hyperfine coupling anisotropy modulation, u e the microwave frequency, R represents the inverse of the spin diffusion time, andt c is the tumbling correlation time. Following the Stoke-Einstein equation, t c = hV ðk B TÞ À1 , where V is the hydrodynamic volume of the molecule, h the viscosity of the solvent, and k B Boltzmann's constant. Dg and DA can be obtained by DFT calculations (vide infra), and an excellent agreement is obtained with the data above the melting point with just V and R as free parameters (Supplemental information for details). The T END 1 term provides the smallest contribution because the nuclear coupling is largely isotropic, while rotation and diffusion terms contribute almost equally. For all three graphenoids, the spin-lattice relaxation times assume very similar values, indicating that, for these three systems, the size and shape of the molecule only mildly affect T 1 , although they might influence the correlation time in the viscous regime. These observations indicate that radicaloids with very longs backbones 49,50 or wheels 22 could provide excellent platforms for quantum units in solution.</p><!><p>The T dependence of the coherence time T m was measured for 1-3, in all solvents, using the aforementioned fitting of the Hahn echo decay (Figure 4). Environmental parameters affect all compounds in a similar manner. T m in the powders is always at least one order of magnitude lower than the solutions, with a trend that is largely independent of T. Coherence in the powders is likely suppressed by the presence of columnar stacks of molecules connected by p-stacking interactions, 20 as for other Article graphenoids. 18 In CS 2, Tol and d 8 Tol, the rather high room-temperature T m ( $10 ms) gradually decreases on lowering T, until the melting point of the solvent, where it increases abruptly because of the freezing of the matrix. It then decreases again, reaching a minimum between 120-130 K in all compounds, to then increase steadily again until a maximum at 80 K that roughly at the same value of the room-temperature T m . In d 14 OTP, on the contrary, T m increases steeply on lowering T and reaches a broad maximum immediately below room temperature, at $200 K.</p><p>Data in d 14 OTP offer proof of the impressive performance of the radicaloids in solid state at room T, with T m values up to 12 ms, and peak values 38 ms for 1 at 200 K, an unprecedented result for molecular systems that can only be equaled by the high-T coherence of N@C 60 . 44 The highest value is obtained for 1, which is the system that sees the smallest delocalization. This is understandable-delocalization over a larger surface leads to intramolecular interactions with more nuclei, and the edge hydrogens in particular. The fact that 3 has a faster decoherence than 2 (8 against 22 ms) is more surprising: 3 has fewer edge hydrogens, and the single s bond between the two moieties of 2 provides an internal motional degree of freedom that should enhance decoherence. On the contrary, the effect is more than counterbalanced by the fact that the delocalization of 2 is spread over a smaller area, with respect to 3. Synthetically, this indicates that it may not be necessary to create vast delocalized structures, so that a number of s bonds can be maintained in the compounds.</p><p>It is then useful to understand what structural groups contribute most to decoherence. Different from T 1 , T m does not need angular momentum exchange, and is mainly limited by fluctuating magnetic fields. Electron-electron dipolar coupling is vanishingly small, at the concentrations used, as we do not observe variations with concentration (Figure S44). Instantaneous diffusion is also negligible for the observed spectral linewidths. Nuclear spin diffusion depends on energy-conserving spin-flip-flop processes and produces a temperature-independent contribution. Hydrogens constitute one major pathway-nuclear-nuclear dipolar couplings are strong between hydrogens because of the large gyromagnetic ratios; additionally, hydrogens are weakly coupled, so that their flip-flops are not suppressed, and they can act as an efficient random modulation. With 1 having 46 hydrogens, 2 having 90, and 3 having 86, this is a point with ample room for optimization. We calculate the nuclear-nuclear dipolar couplings that drive the flip-flops by point-dipole approximations, and we find them to be all in the 0.1-1.5 KHz for 1-3 (Supplemental information). In Figure 5A, we highlight the groups of hydrogens that contribute differently to decoherence, and we then discuss their separate roles.</p><p>Hydrogens on the edges of the aromatic structure play different roles depending on their hyperfine couplings. Hydrogens in mobile groups plays a major role via molecular motion-slow rotations of nuclei-bearing molecular groups produce varying local magnetic fields owing to the reorientation of the nuclear spins. To unravel the two contributions, and which groups contribute to decoherence, we highlight that nuclear spin diffusion is T independent, while molecular motions are thermally activated processes. The T m peak at 200 K cannot be connected to methyl rotations, which happen with a frequency of 250 MHz (see below), and are already too fast and averaged out at room T, but bulkier mesityl groups can rotate around the C-C bond at <0.1 MHz frequency. 51,52 Such degrees of motional freedom are known to be present because of the softness of the matrix around the glass temperature. 53,54 In-solution trends are important for the biological use of quantum probes. 55 In this regime, decoherence is dominated by tumbling, and we can use t c , as previously Article extracted from the T 1 fits, to understand the T m trends. Just above the melting point, the solvent is viscous and the spin Larmor frequency matches t c , so that tumbling resonates with the spin precession, with maximal decoherence effects. At a higher T, tumbling becomes faster and off-resonance and T m increases monotonically. Three observations support this argument-Tol and d 8 Tol have an analogous viscosity and produce identical T m times; CS 2 is less viscous so that the molecule enters the fast tumbling regime at lower T and T m values are higher; 1 has a smaller hydrodynamic volume, yielding a smaller t c and T m values larger than 2 and 3, just because of its dimensions.</p><p>In the frozen matrices of Tol, d 8 Tol and CS 2 , 56 T m displays a minimum of around 130 K common to all three solvents. Here, decoherence should be attributed to intramolecular motions facilitated by the softness of the matrix, which will affect 1-3 and their hydrogenated counterparts in equal measure. The steep decay in T m around 130 K is linked to a resonance between the methyl rotational rate and the hyperfine energies of the methyl hydrogens in the different positions. Cr-based 57 and Cu(II) complexes, 58 and organic radicals 40,48 all see a minimum of T m in the 100-140 K range, owing to such motional degrees of freedom become available. 59 Below 100 K the signal is dominated by the mono-hydrogenated species, and the peak observed around 80 K is found in all compounds. Once again, this decoherence pathway is connected to a structural feature, i.e., the hindered rotation of methyl groups in the frozen matrices: nuclear spin tunneling of the hydrogens in the methyl groups enhances the decohering effects of nuclei, and the same effect is observed in methyl malonic acid [60][61][62] and nitroxyl radicals 63 in the same T range.</p><!><p>In order to gain an insight into the properties of 1-3, we used spin-unrestricted DFT with the software Orca 64,65 to simulate the structure, the spin delocalization (Figures  Article 5B-5D), and the EPR parameters, g-factor and hyperfine couplings, at the B3LYP/6-31G** level of theory. We obtained an excellent agreement between the simulated g-factor and the experimental one. DFT indicates that the several hydrogen nuclei on the compound edges are only rather weakly coupled to the electron spin, so that these hydrogens merely increase the signal linewidth, leading to the unresolved line shape of Figure 1A. While this dipolar contribution remained completely unresolved in 2 and 3, a small splitting could be observed in 1, as its smaller size concentrated the spin density on fewer hydrogens. The main contributions arose, in all compounds, from the hydrogens close to the mesithyl groups. This indicates that this site should be a prime target for substitution with bromine or chlorine atoms: substitution would decrease the hyperfine contribution and hinder the mesithyl rotation. This result also confirms the previous assignments on different p-delocalized paramagnetic structures with unresolved electron spin dipolar interactions, [26][27][28][29][30]34,[66][67][68][69] and is consistent with the previous observation of dipolar coupling solely in small radicaloids. 19,35 Trends reported for linear acenes show that the value of the dipolar coupling strongly decreases when increasing the length of the molecule, 70,71 and this agrees perfectly with our observations and the lack of a half-field line. DFT theory was further used to simulate hyperfine couplings and calculate the average difference in anisotropic hyperfine couplings of the different structural groups. For example, 1 has 9 methyl groups, calculations of the average difference between the 3 protons of each methyl group yielded DA = 0:4G0:2 MHz. If we consider an Arrhenius behavior as t c = t 0 expðE a =k B TÞ, assuming t À1 c z 2pDAz2:5 MHz at the minimum of T m at 130 K, and t 0 z10 À12 s, we obtain an average activation barrier for methyl rotation of E a z3:3 kcal/mol. This value is comparable with literature data 57,[72][73][74] and strongly supports the assignment of the decoherence channels by methyl group rotations, as discussed earlier.</p><p>If we compare T m for the three compounds, 1 has a better performance along with all matrices, followed by 2 of a small margin that falls within experimental errors, and 3 performs slightly worse. While the size effect is not dramatic, we interpret it in terms of a nuclear spin diffusion barrier, as already introduced in the literature of molecular spin systems is the presence of nuclei increasingly farther away from a spin center. 75,76 Intuitively, the nuclei closer to the sites of high spin density are so strongly coupled to the spin that they are less likely to produce flip-flops leading to decoherence, which is what might happen in 2 and 3 with peripheral nuclei being farther from the central spin density than in 1. This, anyway, does not indicate a trade-off between long T m times and availability of spin sites: once the nuclear spin diffusion threshold is overcome, the addition of repeating units will lower T m only very gradually. Larger molecules can thus still support long coherence times, while at the same time hosting multiple pentagonal defects.</p><!><p>The results show that carbon-based radicaloids present an optimal structure to achieve extremely appealing quantum features even at high temperatures. They offer remarkably large spin-flip times, with Raman and direct processes influencing T 1 at low T, but no localized mode limiting T 1 at room temperature. Bulkier radicaloids 27,49,50 and wheels 22 might provide even higher T 1 times liquid environments, e.g., as necessary for in-vivo quantum probes, and should be interesting targets for future investigations, as their quantum properties are still unexplored.</p><p>The coherence times are found to be long and appealing for quantum technologies applications. A record T m = 38 ms is achieved in 1, comparable only to the performance of N@C 60 . 44 Moreover, our analysis shows that these values are still nowhere Article close to the maximum attainable ones, and the data offer indications for further synthetic tuning and optimization. Fusion or dimerization of the individual units is much less significant than expected, and its effects are modulated by a nuclear spin diffusion barrier effect as well as the different delocalization of the spin density. Counterintuitively, residual s-bonds are not found to hamper the coherence, so that synthetic approaches that allow a certain extent of incomplete conjugation, but still preserve the desired quantum properties, appear accessible. The investigation of the different T regions reveals that intramolecular motions of the side appendages are most detrimental to spin coherence, which is never found to be limited by T 1 , in contrast to most previous literature. 6,7,38,39 These observations thus indicate that it is possible to boost the quantum coherence using just synthesis-substitution of hydrogens on the methyl-and mesithyl groups with chlorine or bromine would have a dramatic effects, much higher than substitution along the edges. Such substituted groups would also be heavier and would therefore shift the inertia of rotating groups to higher temperatures, possibly solving the problem of molecular motions entirely. Our theoretical modeling also indicates that substitution of the specific hydrogens close to the mesityl groups would improve the performance by contemporarily blocking the group rotation and substituting the edge positions with the highest spin densities. We thus expect such variations to be most desirable to increase the performances. 70 While the coherence time is still below that of the optimized solid-state systems, such strategies may lead to coherence times comparable to those of NV vacancy centers in diamonds, which have seen 15 years of optimization. 77 Moreover, these results highlight the importance not only of the radicaloid itself but also of its chemical matrix, for quantum applications. Scaffolding into metal-organic frameworks, imprisonment into matrices, or onto surfaces will be tested as the next steps. A synthetic pathway that allows crystal engineering by altering the bulky separating appendages is also highlighted by these results: bulky immobile groups could simultaneously interrupt the stacking channel that limits coherence in powders, tune T 1 for faster initialization and limit the main room-temperature decoherence channel produced by group rotations. The observed structure-property relations thus now open up the use of radicaloids for multifunctional quantum materials, e.g., exploiting their non-linear optical 16 and photovoltaics properties. 17</p><!><p>Resource availability Lead contact Further information and requests for resources should be directed to and will be fulfilled by the Lead Contact, Lapo Bogani (lapo.bogani@materials.ox.ac.uk)</p><!><p>The materials generated in this study will be made available on request.</p><!><p>The datasets supporting the current study are available from the Lead Contact on request.</p><p>Full experimental procedures are available in the Supplemental information.</p>