Bistability and the Phase Transition in 1,3,2-Dithiazolo[4,5-b]pyrazin-2-yl
23 Oct 2004-Journal of the American Chemical Society (American Chemical Society)-Vol. 126, Iss: 45, pp 14692-14693
TL;DR: The structural interconversion is suggested to proceed via the cooperative breaking and making of intermolecular S- - -N interactions and an inversion symmetry-preserving "domino cascade" of the pi-stacked rings.
Abstract: The molecular radical 1,3,2-dithiazolo[4,5-b]pyrazin-2-yl (PDTA) exhibits magnetic bistability just above room temperature, undergoing a well-defined hysteretic phase change with TC↓ = 297(1) K and TC↑ = 343(1) K. The crystal structures of the two phases of PDTA have been determined by single-crystal X-ray diffraction at 323(2) K. LT-PDTA consists of diamagnetic (S = 0) nearly superimposed π-dimer stacks, while that of HT-PDTA comprises slipped stacks of π-radicals (S = 1/2). The structural interconversion is suggested to proceed via the cooperative breaking and making of intermolecular S- - -N interactions and an inversion symmetry-preserving “domino cascade” of the π-stacked rings.
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References
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TL;DR: A new organic molecular conductor, based on a spiro-biphenalenyl neutral radical, simultaneously exhibits bistability in three physical channels: electrical, optical, and magnetic.
Abstract: A new organic molecular conductor, based on a spiro-biphenalenyl neutral radical, simultaneously exhibits bistability in three physical channels: electrical, optical, and magnetic. In the paramagnetic state, the unpaired electrons are located in the exterior phenalenyl units of the dimer, whereas in the diamagnetic state the electrons migrate to the interior phenalenyl units and spin pair as a π-dimer. Against all expectations, the conductivity increases by two orders of magnitude in the diamagnetic state, and the band gap decreases. This type of multifunctional material has the potential to be used as the basis for new types of electronic devices, where multiple physical channels are used for writing, reading, and transferring information.
500 citations
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TL;DR: A large first-order magnetic phase transition in an organic radical, 1,3,5-trithia-2,4,6-triazapentalenyl, is described and may have applications in thermal sensors, switching units, and information storage media based on organic radical crystals.
Abstract: A large first-order magnetic phase transition in an organic radical, 1,3,5-trithia-2,4,6-triazapentalenyl, is described. The transition occurs with a wide thermal hysteresis loop over the temperature range 230 to 305 kelvin. The high-temperature phase is paramagnetic, and its structure consists of a uniform one-dimensional stacking of the radical. The low-temperature phase is diamagnetic because of strong dimerization along the stacking direction. The results may have applications in thermal sensors, switching units, and information storage media based on organic radical crystals.
439 citations
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TL;DR: A mechanism for the interconversion of the two phases of PDTA and related structures is proposed in which hysteretic behavior arises from cooperative effects associated with the breaking and making of a lattice-wide network of intermolecular S - - -N' and/or S- - -S' interactions.
Abstract: New synthetic methods for heterocyclic 1,3,2-dithiazolyl (DTA) radicals have been developed, and trends in the molecular spin distributions and electrochemical properties of a series of DTA radicals are reported. The crystal structures of [1,2,5]thiadiazolo[3,4-f][1,3,2]benzodithiazol-2-yl (TBDTA) and [1,3,2]pyrazinodithiazol-2-yl (PDTA) have been determined. The structure of TBDTA (at 293 and 95 K) contains two molecules in the asymmetric unit, each of which generates π-stacked arrays, one consisting of antiparallel chains of centrosymmetrically associated dimers, the other comprising parallel chains of unassociated radicals. The structure of PDTA (at 293 and 95 K) is simpler, consisting of slipped stacks of π-dimers. Variable-temperature magnetic susceptibility (χP) measurements on TBDTA indicate essentially paramagnetic behavior for the unassociated radical π-stacks over the range 5−400 K. By contrast PDTA is diamagnetic at all temperatures below 300 K, but between 300 and 350 K the value of χP follows...
176 citations
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TL;DR: In this article, the characterization of the heterocyclic radical 1,2,5-thiadiazolo [3,4-b]-1,3,2-dithiazolo[3, 4-b]pyrazin-2-yl (TDP-DTA) is described.
Abstract: The characterization of the heterocyclic radical 1,2,5-thiadiazolo[3,4-b]-1,3,2-dithiazolo[3,4-b]pyrazin-2-yl (TDP-DTA) is described. The compound is prepared by treatment of 5,6-dithiolo-1,2,5-thiadiazolo[3,4-b]pyrazine with S3N3Cl3 and purified by fractional sublimation in vacuo. The results of cyclic voltammetry and ESR analysis of TDP-DTA and related heterocyclic dithiazolyls indicate that the spin distributions and donor/acceptor properties of these radicals are extremely sensitive to the nature of the 4,5-substituents. The crystal structure of TDP-DTA has been determined at two temperatures. At 293 K, the crystals are triclinic, space group P1, with a = 4.4456(8), b = 8.407(2), c = 9.671(3) A, α = 71.34(2), β = 89.28(2), γ = 87.80(2)°, Z = 2 (for C4N5S3); at 150 K the crystals are triclinic, space group P1, with a = 7.489(7), b = 9.593(4), c = 10.759(6) A, α = 65.77(4), β = 74.10(6), γ = 74.64(6)°, Z = 2 (for C8N10S6). At 293 K the structure consists of ribbons of TDP-DTA radicals packed in a slip...
118 citations
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TL;DR: In this article, a novel synthesis of the title compound, C2S3N3======677(1) is reported, which is based on DFT calculations and X-ray diffraction in the region 300-225k and reveals a regular π-stacked structure.
Abstract: A novel synthesis of the title compound, C2S3N3
(1) is reported. X- and K-band EPR spectra on dilute solutions of 1 indicate delocalisation of the unpaired spin density over both heterocyclic rings in agreement with DFT calculations. An XRPD study indicates that it crystallises in two morphologies with both phases formed during vacuum sublimation. The XRPD studies indicate that on cooling below 230 K, only the triclinic phase (P) becomes detectable, whereas on warming above 320 K, just the monoclinic phase (P21/c) becomes observed. The crystal structure of the monoclinic phase has been examined by variable temperature single crystal X-ray diffraction in the region 300–225 K and reveals a regular π-stacked structure. A crystal structure of the triclinic
phase is reported at 150 K and exhibits a dimeric π-stacked motif. Susceptibility measurements show that the monoclinic phase is paramagnetic whereas the triclinic phase is diamagnetic. This radical exhibits thermal hysteresis with a wide range of bistability; EPR and magnetic susceptibility measurements indicate Tc↓ = 234 K, and Tc↑ = 317 K. The magnetic behaviour of the monoclinic phase is consistent with strong antiferromagnetic exchange interactions between open shell doublet states (J = −320 K) along the π-stacking direction, although significant inter-stack interactions are required to model the data adequately. In contrast the dimeric phase is essentially diamagnetic, with the residual paramagnetism indicating a very large singlet–triplet separation (|2J| > 2000 K).
The magnetic exchange interactions in both phases are probed through a series of DFT calculations using the broken-symmetry approach. These confirm the presence of strong magnetic exchange interactions along the π-stacking direction in the high temperature phase (2J = −182 K), but with additional interstack interactions which are an order of magnitude smaller. Calculations on the triclinic phase indicate that it is best considered as a dimer with an open-shell singlet state with a very large singlet–triplet separation (2J = −2657 K). The magnitude of J for both phases from theory and experiment are in good agreement. The origin of the thermal hysteresis is attributed to the presence of two energetically similar structures which have a low energy barrier to interconversion. The thermodynamic parameters associated with the interconversion process
have been probed by DSC studies. It confirms the first order nature of the transition with Tc↓ = 232.3 K (ΔH↓ = 1.41 kJ mol−1, ΔS↓ = 6.0 J mol−1 K−1) and Tc↑ = 320.5 K (ΔH↑ = 1.86 kJ mol−1, ΔS↑ = 5.8 J mol−1 K−1).
117 citations