The pyren-1'-yl-functionalized dithiadiazolyl (DTDA) radical, CHCNSSN (1), is monomeric in solution and exhibits fluorescence in the deep-blue region of the visible spectrum (440 nm) upon excitation at 241 nm. The salt [1][GaCl] exhibits similar emission, reflecting the largely spectator nature of the radical in the fluorescence process, although the presence of the radical leads to a modest quenching of emission (Φ = 98% for 1 and 50% for 1) through enhancement of non-radiative decay processes. Time-dependent density functional theory studies on 1 coupled with the similar emission profiles of both 1 and 1 are consistent with the initial excitation being of predominantly pyrene π-π* character. Spectroscopic studies indicate stabilization of the excited state in polar media, with the fluorescence lifetime for 1 (τ = 5 ns) indicative of a short-lived excited state. Comparative studies between the energies of the frontier orbitals of pyren-1'-yl nitronyl nitroxide (2, which is not fluorescent) and 1 reveal that the energy mismatch and poor spatial overlap between the DTDA radical SOMO and the pyrene π manifold in 1 efficiently inhibit the non-radiative electron-electron exchange relaxation pathway previously described for 2. Solid-state films of both 1 and [1][GaCl] exhibit broad emission bands at 509 and 545 nm, respectively. Incorporation of 1 within a host matrix for OLED fabrication revealed electroluminescence, with CIE coordinates of (0.205, 0.280) corresponding to a sky-blue emission. The brightness of the device reached 1934 cd/m at an applied voltage of 16 V. The crystal structure of 1 reveals a distorted π-stacked motif with almost regular distances between the pyrene rings but alternating long-short contacts between DTDA radicals. Solid state measurements on a thin film of 1 reveal emission occurs at shorter wavelengths (375 nm) whereas conductivity measurements on a single crystal of 1 show a photoconducting response at longer wavelength excitation (455 nm).

The fluorescent 9′-anthracenyl-functionalized dithiadiazolyl radical (3) exhibits four structurally determined crystalline phases, all of which are monomeric in the solid-state. Polymorph 3α (monoclinic P21/c, Z' = 2) is isolated when the radical is condensed onto a cold substrate (enthalpically favored polymorph) whereas 3β (orthorhombic P212121, Z' = 3) is collected on a warm substrate (entropically favored polymorph). The α and β polymorphs exhibit chemically distinct structures with 3α exhibiting face-to-face π−π interactions between anthracenyl groups while 3β exhibits edge-to-face π−π interactions. 3α undergoes an irreversible conversion to 3β on warming to 120 o C (393 K). The β-phase undergoes a series of reversible solid-state transformations on cooling; below 300 K a phase transition occurs to form 3γ (monoclinic P21/c, Z' = 1) and on further cooling below 165 K a further transition is observed to 3δ (monoclinic P21/n, Z' = 2). Both 3β  3γ and 3γ  3δ transitions are reversible (single-crystal X-ray diffraction) and the 3γ  3δ process exhibits thermal hysteresis with a clear feature observed by heat capacity measurements. Heating 3β above 160 o C generates a fifth polymorph (3ε) which is distinct from 3α -3δ based on PXRD data. The magnetic behavior of both 3α and the 3β/3γ/3δ system reflect an S = ½ paramagnet with weak antiferromagnetic coupling. The reversible 3δ ↔ 3γ phase transition exhibits thermal hysteresis of 20 K. Below 50 K the value of χmT for 3δ approaches 0 emu•K•mol -1 consistent with formation of a gapped state with an S = 0 ground state configuration. In solution both paramagnetic 3 and diamagnetic [3][GaCl4] exhibit similar absorption and emission profiles reflecting similar absorption and emission mechanisms for paramagnetic and diamagnetic forms. Both emit in the deep-blue region of the visible spectrum (λem ~ 440 nm) upon excitation at 255 nm with quantum yields of 4% (3) and 30% ([3][GaCl4]) affording a switching ratio [ΦF(3 + )/ΦF(3)] of 7.5 in quantum efficiency with oxidation state. Solid-state films of both 3 and [3][GaCl4] exhibit emission bands at longer wavelength (490 nm) attributed to excimer emission.

The phenanthrene-functionalised dithiadiazolyl radical 2 provides a rare example of a fluorescent radical, where the unpaired electron does not efficiently quench fluorophore emission (MeCN: ΦF = 0.11).

Formation of radical–radical cocrystals is an important step towards the design of organic ferrimagnets. We describe a simple approach to generate radical–radical cocrystals through the identification and implementation of well‐defined supramolecular synthons which favor cocrystallization over phase separation. In the current paper we implement the structure‐directing interactions of the E−E bond (E=S, Se) of dithiadiazolyl (DTDA) and diselenadiazolyl (DSDA) radicals to form close contacts to electronegative groups. This is exemplified through the preparation and structural characterization of three sets of radical cocrystals; the 2:2 cocrystal [PhCNSSN]2[MBDTA]2 (4) [MBDTA=methyl benzodithiazolyl] and the 2:1 cocrystals [C6F5CNEEN]2[TEMPO] (E=S, 5; E=Se, 6). In 4 the two types of radical are linked via bifurcated inter‐dimer δ+S⋅⋅⋅Nδ− interactions whereas 5 and 6 exhibit a set of five‐centre δ+E⋅⋅⋅Oδ− contacts (E=S, Se).

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The preparation and characterization of the halo-functionalized dithiadiazolyl radicals p-XC 6 F 4 CNSSN (X = Br (1) or I (2)) are described. Compound 1 is trimorphic. The previously reported phase 1α (Z′ = 1) comprises monomeric radicals, whereas 1β comprises a mixture of one cis-oid π*−π* dimer and one monomer (Z′ = 3), and 1γ exhibits a single cis-oid dimer (Z′ = 2) in the asymmetric unit. We have only been able to isolate a single polymorph of 2, isomorphous with 1α. Both the bromo and iodo groups in 1 and 2 promote sigma-hole type interactions of the type C−X•••N (X = Br, I), reflecting the increasing strength of this interaction for the heavier halo-derivatives. An analysis of the intermolecular forces is made using dispersion corrected density functional theory (DFT) (UM06-2X-D3/LACV3P*) and compared to a unified pair potential model (UNI) embodied in the crystallographic software Mercury. While there is a correlation between DFT and UNI force-field models, there are some discrepancies, although both reveal that a number of intermolecular contacts beyond the sum of the van der Waals radii are significant (>5 kJ mol −1 ). A natural bond order analysis of the intermolecular interactions reveals lone pair donation from the heterocyclic N atom to C−X or S−S σ* orbitals contributes to these intermolecular interactions with relative energies in the order C−I > SN-II > C−Br > SN-III. The magnetism of 2 reveals a broad maximum in χ around 20 K indicative of short-range antiferromagnetic interactions. These are supported by DFT calculations that reveal a set of three significant exchange interactions which propagate in two dimensions.

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Cocrystallization of the dithiadiazolyl (DTDA) radicals p-XC 6 F 4 CNSSN (X=F, Cl, Br, I, CN) with TEMPO afforded the 2 : 1 cocrystals [p-XC 6 F 4 CNSSN] 2 [TEMPO] (1-5) whose structures all reflect a common S 4 •••O supramolecular motif.The nature of this interaction was probed by DFT calculations (M06/aug-cc-pVDZ) on 1 which revealed that the enthalpy of formation of the [C 6 F 5 CNSSN] 2 [TEMPO] supramolecular motif from [C 6 F 5 CNSSN] 2 and TEMPO is substantial (À 54.0 kJ mol À 1 ). Electronic structure calculations revealed a TEMPO-based doublet S = 1 = 2 configuration as the ground state with limited spin density on the DTDA rings (2.4 %). The corresponding spin quartet state is + 78.9 kJ mol À 1 higher in energy. An atoms-in-molecules analysis reveals four bond critical points (BCPs) between the TEMPO O and the DTDA S atoms as well as additional BCPs between selected DTDA S atoms and methyl H atoms of the TEMPO molecule. Herein, the structures of 2-5 are considered within the context of a hierarchical view of competing and complementary intermolecular interactions; in particular, the established supramolecular CN•••SÀ S synthon is sacrificed in order to form the new S 4 •••O interaction.