Effects of Halo-substitution on 2'-Chloro-5'-halo-phenyl-1,2,3,5-dithiadiazolyl Radicals: A Crystallographic, Magnetic and EPR Case Study

The syntheses and characterization of the aryl-substituted dithiadiazolyls, 2ʹ-Cl-5ʹ- X-C6H3CNSSN• [1 (X = F), 2 (X = Cl), 3 (X = Br), 4 (X = I)] are described. In all four cases the radicals adopt distorted stacks of π*-π* dimers with inter-stack S···X contacts. In 1 (monoclinic P2/c) S···Cl contacts are manifested through a non-crystallographic 3-fold axis forming supramolecular trimers whereas the inter-stack S…X contacts in 2 (triclinic P-1), 3 and 4 (which form an isostructural pair, orthorhombic Pna21) form supramolecular chains. While all the structures adopt π*-π* cis-oid dimer motifs, tuning the halogen modifies the intra-dimer S···S distance. Variable temperature SQUID magnetometry and X-band CW-EPR studies revealed the presence of a thermally accessible triplet state in all cases, with the singlet-triplet separation appearing in the order 1 > 2 > 3 > 4, consistent with a reduction in the overlap integral with increasing intra-dimer S···S separation. Variable temperature structural studies on both 2 and 4 reveal a structural evolution from a distorted π-stack motif towards a regular π-stacked array on warming. This is particularly pronounced in 2 where the intermolecular S…S separations along the stacking direction converge on a regular 3.6 Å spacing at ambient temperature

Studies on a “Disappearing Polymorph”: Thermal and Magnetic Characterization of α‑<i>p</i>‑NCC₆F₄CNSSN^•

The α-and β-phases of the thiazyl radical <i>p</i>-NCC₆F₄­CNSSN^• (<b>1</b>) can be selectively prepared by careful control of the sublimation conditions, with the α-phase crystallizing preferentially when the substrate temperature is maintained below −10 °C, whereas the β-phase is isolated when the substrate temperature is maintained at or above ambient temperature. Differential scanning calorimatry studies reveal that the α-phase converts to the β-phase upon warming over the range 111–117 °C (Δ<i>H</i> = +4 kJ·mol^–1) via a melt–recrystallization process, with the β-phase itself melting at 167–170 °C (Δ<i>H</i>_fus = 27 kJ·mol^–1). IR and Raman spectroscopy can be used to clearly discriminate between <b>1α</b> and <b>1β</b>. The α-phase shows a broad maximum in the magnetic susceptibility around 8 K that, coupled with a broad maximum in the heat capacity, is indicative of short-range order. Some field dependence of the susceptibility below 3 K is observed, but the lack of features in the ac susceptibility, <i>M</i> vs <i>H</i> plots, or heat capacity mitigates against long-range order in <b>1α</b>

Studies on a “Disappearing Polymorph”: Thermal and Magnetic Characterization of α‑<i>p</i>‑NCC₆F₄CNSSN^•

The α-and β-phases of the thiazyl radical <i>p</i>-NCC₆F₄­CNSSN^• (<b>1</b>) can be selectively prepared by careful control of the sublimation conditions, with the α-phase crystallizing preferentially when the substrate temperature is maintained below −10 °C, whereas the β-phase is isolated when the substrate temperature is maintained at or above ambient temperature. Differential scanning calorimatry studies reveal that the α-phase converts to the β-phase upon warming over the range 111–117 °C (Δ<i>H</i> = +4 kJ·mol^–1) via a melt–recrystallization process, with the β-phase itself melting at 167–170 °C (Δ<i>H</i>_fus = 27 kJ·mol^–1). IR and Raman spectroscopy can be used to clearly discriminate between <b>1α</b> and <b>1β</b>. The α-phase shows a broad maximum in the magnetic susceptibility around 8 K that, coupled with a broad maximum in the heat capacity, is indicative of short-range order. Some field dependence of the susceptibility below 3 K is observed, but the lack of features in the ac susceptibility, <i>M</i> vs <i>H</i> plots, or heat capacity mitigates against long-range order in <b>1α</b>

Structural, magnetic, and optical studies of the polymorphic 9′-anthracenyl dithiadiazolyl radical

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 °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 °C generates a fifth polymorph (3ε) which is distinct from 3α–3δ based on powder X-ray diffraction data. The magnetic behavior of both 3α and the 3β/3γ/3δ system reflect an S = 1/2 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 a longer wavelength (490 nm) attributed to excimer emission.We would like to thank the Canada Research Chairs Program for financial support (J.M.R.) and the University of Windsor for a scholarship (Y.B.). E.G. and M.P. would like to thank NSERC. A.A. and J.C. acknowledge support from grant MAT2015-68200-C2−2-P from the Ministerio de Economía y Competividad of Spain and the European Regional Development Fund. Additional support from Diputacion General de Aragon (DGA-M4) is also acknowledged.Peer reviewe

Polysilicon Microchips Functionalized with Bipyridinium-Based Cyclophanes for a Highly Efficient Cytotoxicity in Cancerous Cells

The use of micrometric-sized vehicles could greatly improve selectivity of cytotoxic compounds as their lack of self-diffusion could maximize their retention in tissues. We have used polysilicon microparticles (SiμP) to conjugate bipyridinium-based compounds, able to induce cytotoxicity under regular intracellular conditions. Homogeneous functionalization in suspension was achieved, where the open-chain structure exhibits a more dense packing than cyclic analogues. The microparticles internalized induce high cytotoxicity per particle in cancerous HeLa cells, and the less densely packed functionalization using cyclophanes promotes higher cytotoxicity per bipy than with open-chain analogues. The self-renewing ability of the particles and their proximity to cell membranes may account for increased lipid peroxidation, achieving toxicity at much lower concentrations than that in solution and in less time, inducing highly efficient cytotoxicity in cancerous cells. Keywords: HeLa cells; bipyridinium; cancer; cytotoxicity; lipid peroxidation; polysilicon microparticles

Structural Variations in the Dithiadiazolyl Radicals <i>p</i>‑ROC₆F₄CNSSN (R = Me, Et, ^<i>n</i>Pr, ^<i>n</i>Bu): A Case Study of Reversible and Irreversible Phase Transitions in <i>p</i>‑EtOC₆F₄CNSSN

The 4′-alkoxy-tetrafluorophenyl dithiadiazolyls, ROC₆F₄CNSSN [R = Me (<b>1</b>), Et (<b>2</b>), ^<i>n</i>Pr (<b>3</b>), ^<i>n</i>Bu­(<b>4</b>)] all adopt <i>cis-oid</i> dimers in the solid state. The methoxy derivative <b>1</b> adopts a π-stacked AA’AA’ motif, whereas propoxy (<b>3</b>) and butoxy (<b>4</b>) derivatives exhibit an AA’BB’ stacking. The ethoxy derivative (<b>2</b>) is polymorphic. The α-phase (<b>2α</b>) adopts an AA’BB’ motif comparable with <b>3</b> and <b>4</b>, whereas <b>2β</b> and <b>2γ</b> are reminiscent of <b>1</b> but combine a mixture of both monomers and dimers in the solid state. The structure of <b>2β</b> exhibits <i>Z</i>’ = 6 with two dimers and two monomers in the asymmetric unit but undergoes a thermally induced phase transition upon cooling below −25 °C to form <b>2γ</b> (<i>Z</i>’ = 14) with six dimers and two monomers in the asymmetric unit. The transition is associated with both rotation and translation of the dithiadiazolyl ring. Detailed differential scanning calorimetry and variable temperature powder X-ray diffraction studies coupled with SQUID magnetometry have been used to show that <b>2α</b> converts irreversibly to <b>2β</b> upon heating and that <b>2β</b> and <b>2γ</b> interconvert through a reversible phase transition with a small thermal hysteresis in its magnetic response

Multifunctional Dithiadiazolyl Radicals: Fluorescence, Electroluminescence, and Photoconducting Behavior in Pyren-1′-yl-dithiadiazolyl

The pyren-1′-yl-functionalized dithiadiazolyl (DTDA) radical, C₁₆H₉CNSSN (<b>1</b>), 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 [<b>1</b>]­[GaCl₄] 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_F = 98% for <b>1</b>⁺ and 50% for <b>1</b>) through enhancement of non-radiative decay processes. Time-dependent density functional theory studies on <b>1</b> coupled with the similar emission profiles of both <b>1</b>⁺ and <b>1</b> 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 <b>1</b> (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 (<b>2</b>, which is not fluorescent) and <b>1</b> reveal that the energy mismatch and poor spatial overlap between the DTDA radical SOMO and the pyrene π manifold in <b>1</b> efficiently inhibit the non-radiative electron–electron exchange relaxation pathway previously described for <b>2</b>. Solid-state films of both <b>1</b> and [<b>1</b>]­[GaCl₄] exhibit broad emission bands at 509 and 545 nm, respectively. Incorporation of <b>1</b> 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/m² at an applied voltage of 16 V. The crystal structure of <b>1</b> 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 <b>1</b> reveal emission occurs at shorter wavelengths (375 nm) whereas conductivity measurements on a single crystal of <b>1</b> show a photoconducting response at longer wavelength excitation (455 nm)

Route to benzo- and pyrido-fused 1,2,4-triazinyl radicals via N′-(het)aryl-N′-[2-nitro(het)aryl]hydrazides

A two-step route to 1,3-disubstituted benzo- and pyrido-fused 1,2,4-triazinyl radicals is presented. The route involves the N′-(2-nitroarylation) of easily prepared N′-(het)arylhydrazides via nucleophilic aromatic substitution of 1-halo-2-nitroarenes, which in most cases gives N′-(het)aryl-N′-[2-nitro(het)aryl]hydrazides in good yields. Mild reduction of the nitro group followed by an acid-mediated cyclodehydration gives the fused triazines, which upon alkali treatment afford the desired radicals. Fifteen examples of radicals are presented bearing a range of substituents at N-1, C-3, and C-7, including the pyrid-2-yl and 8-aza analogues. This route to the N′-(het)aryl-N′-[2-nitro(het)aryl]hydrazides, which works well with benzo- and picolinohydrazides, required a modification for aceto- and trifluoroacetohydrazides that involved a multistep synthesis of asymmetrically 1,1-diaryl-substituted hydrazines