Sushanta K. Pal

Hysteretic spin and charge delocalization in a phenalenyl-based molecular conductor.

We have investigated the solid-state electronic structure and properties of a phenalenyl-based butyl-substituted neutral radical, 3, that shows a hysteretic phase transition just above room temperature. We quantitatively analyzed the electron density distribution of this radical throughout both branches of the hysteretic phase transition using solid-state X-ray structures and found two distinct electronic states in the hysteresis loop that accompanies the phase transition. The bistability of the two electronic states was observed through a number of measurements, including IR transmittance spectra of single crystals in the vicinity of the phase transition. By comparing the changes in the crystal structures of 3 and the related ethyl-substituted radical 1 (which exhibits no hysteresis) at various temperatures, we show that the change in the interplanar π-π distance within dimers is the most important structural parameter in determining the physical properties of the radicals. The large change in the C-H···π interaction in 3 occurs in concert with the spin redistribution during the phase transition, but these factors are not responsible for the hysteresis effect. We suggest that the presence of a high-temperature state inside the hysteretic loop during the cooling cycle is due to thermodynamic stability, while the existence of the low-temperature state during the heating cycle is due to the presence of a large energy barrier between the two states (estimated to be greater than 100 kJ/mol) that results from the large-amplitude motion of the phenalenyl rings and the associated lattice reorganization energy that is required at the phase transition.

Trisphenalenyl-Based Neutral Radical Molecular Conductor

We report the preparation, crystallization, and solid-state characterization of the first member of a new family of tris(1,9-disubstituted phenalenyl)silicon neutral radicals. In the solid state, the radical packs as weak partial pi-dimers with intermolecular carbon...carbon contacts that fall at the van der Waals atomic separation. Magnetic susceptibility measurements indicate approximately 0.7 Curie spins per molecule from room temperature down to 50 K, below which antiferromagnetic coupling becomes apparent; the compound has a room-temperature single-crystal conductivity of sigmaRT = 2.4 x 10(-6) S cm(-1).

Localization of Spin and Charge in Phenalenyl-Based Neutral Radical Conductors

We report the development of an experimentally based structural analysis to examine the degree of localization of the spin and charge in the phenalenyl-based neutral radical molecular conductors--the results motivate a reinterpretation of the electronic structure of a number of the radicals that we have reported over the past 10 years. The analysis is based on the well-known relationship between bond order and bond length and makes use of the experimental bond distance deviations between the molecular structure of the radical and its corresponding cation. We determined the single crystal X-ray structure of the ethyl radical (1) at 11 temperatures between 90 K and room temperature so that we could follow the evolution of the structure and the electron density distribution through the magnetic phase transition that occurs in the vicinity of 140 K. We show that the enhanced conductivity in the dimeric ethyl (1) and butyl (3) radicals at the magnetic phase transition results from the development of a complex, but highly delocalized electronic structure and not to the formation of a diamagnetic pi-dimer. We find that the monomeric radicals 4, 12, and 13 have an asymmetric electron density distribution in the crystal lattice whereas radical 11 is the only monomeric radical which remains fully delocalized. The pi-chain radicals (7, 8, 14, and 15) retain the strongly delocalized electronic structures expected for a resonating valence bond ground-state structure.

Resonating valence bond and {sigma}-charge density wave phases in a benzannulated phenalenyl radical.

We report the preparation of the first benzannulated phenalenyl neutral radical conductor (18), and we show that the compound displays unprecedented solid state behavior: the structure is dominated by two sets of intermolecular interactions: (1) a pi-chain structure with superimposed pi-overlap of the benzannulated phenalenyls along [0 0 1], and (2) an interchain overlap involving a pair of carbon atoms (C4) along [0 1 0]. The pi-chain-type stacking motif is reminiscent of previously reported phenalenyl radicals and the room temperature structure (space group P2/c) together with the conductivity of sigma(RT) = 0.03 S/cm and the Pauli-like magnetic susceptibility are best described by the resonating valence bond (RVB) model. The interchain interaction is unstable with respect to the formation of a sigma-charge density wave (sigma-CDW) involving pairs of C4 carbon atoms between adjacent radicals and this phase is characterized by the P2(1)/c space group which involves a doubling of the unit cell along the [0 1 0] direction. The RVB and CDW phases compete for structural occupancy throughout the whole temperature range (15-293 K) with the RVB phase predominating at 15 and 293 K and the sigma-CDW phase achieving a maximum structural occupancy of about 60% at 150 K where it produces clearly discernible effects on the magnetism and conductivity.

Enhanced Electrical Conductivity in a Substitutionally Doped Spiro-bis(phenalenyl)boron Radical Molecular Solid

We report the crystallization of a subsitutionally doped organic conductor based on a host lattice composed of spiro-bis(phenalenyl)boron radicals. Co-crystallization of solutions of spiro-bis(9-oxidophenalenone)boron radical [PLY(O,O)]2B mixed with selected amounts of spiro-bis(9-oxidophenalenone)beryllium [PLY(O,O)]2Be leads to the formation of a series of solid-state solutions of composition [PLY(O,O)]2B(1-x)Be(x). The dopant molecules [PLY(O,O)]2Be serve to introduce holes into the lattice of spins provided by the [PLY(O,O)]2B radicals and lead to a systematic increase in the conductivity while decreasing the activation energy of the conduction process and leaving the solid-state structure relatively unperturbed. While the energies of the hole sites are expected to be high, the results are consistent with the interpretation of the electronic structure of [PLY(O,O)]2B in terms of the resonating valence bond model.

Sulfur and selenium substituted spiro-biphenalenyl-boron neutral radicals

We report a synthetic scheme for the preparation of alkylthio, dithio-bridged and diseleno-bridged 9-hydroxyphenalenones and associated spiro-biphenalenyl boron neutral radicals. We show that the strategy of sulfur substitution at the active positions of the phenalenyl units reduces the electrochemical disproportionation potential (ΔE2−1 = E2½ − E1½, where E1½ and E2½ are the first and second reduction potentials of corresponding cations) of the alkylthio-radicals [3,7-SR-PLY(O,O)]2B, (R = Me, 9), (R = Et, 10) and (R = Pr, 11), but brings about a significant reduction of the ΔE2−1 value in the case of disulfide and diselenide substitution, [3,4-S,S-PLY(O,O)]2B (12) and [3,4-Se,Se-PLY(O,O)]2B (13). The crystal structures of 10 and 11 show that the radicals exist as one dimensional (1-D) π-chains of superimposed phenalenyl units, and the molecular units pack more efficiently than the oxygen-substituted analog [3,7-OMe-PLY(O,O)]2B (8). Magnetic susceptibility measurements indicate that in the solid state the radicals remain paramagnetic while there is spin–spin interaction between the molecules along the π-chains. Band structure calculations are in accordance with the magnetic measurement data and indicate the presence of interactions between the molecules. The room temperature electrical conductivities of both compounds are found to be σRT = 1.5 × 10−2 S cm−1.

Synthesis of Tetrachalcogenide-Substituted Phenalenyl Derivatives: Preparation and Solid-State Characterization of Bis(3,4,6,7-tetrathioalkyl-phenalenyl)boron Radicals

We report the synthesis and properties of a series of spiro-bis(3,4,6,7-tetrachalcogenide-substituted-phenalenyl)boron salts and two of the corresponding tetrathioalkyl-substituted spiro-bis(phenalenyl)boron radicals [tetrathiomethyl (10) and tetrathioethyl (11)] in which all of the active positions of the phenalenyl (PLY) nucleus are functionalized. In the solid state, radicals 10 and 11 exist as a weak π-dimers due to the steric congestion of the thioalkyl groups in the superimposed PLY units. As a result, the spins are localized in the isolated (nonsuperimposed) PLY rings, and the structure, magnetic susceptibility measurements, and band structure calculations confirm that these PLY units are unable to undergo strong intermolecular interaction as a result of the orientation of the thioalkyl groups.

Band Structure Engineering by Substitutional Doping in Solid-State Solutions of [5-Me-PLY(O,O)]2B(1–x)Bex Radical Crystals

We report the substitutional doping of solid-state spiro-bis(5-methyl-1,9-oxido-phenalenyl)boron radical ([2]2B) by co-crystallization of this radical with the corresponding spiro-bis(5-methyl-1,9-oxido-phenalenyl)beryllium compound ([2]2Be). The pure compounds crystallize in different space groups ([2]2B, P1̅, Z = 2; [2]2Be, P2₁/c, Z = 4) with distinct packing arrangements, yet we are able to isolate crystals of composition [2]2B(1-x)Be(x), where x = 0-0.59. The phase transition from the P1̅ to the P2₁/c space group occurs at x = 0.1, but the conductivities of the solid solutions are enhanced and the activation energies reduced for values of x = 0-0.25. The molecular packing is driven by the relative concentration of the spin-bearing ([2]2B) and spin-free ([2]2Be) molecules in the crystals, and the extended Hückel theory band structures show that the progressive incorporation of spin-free [2]2Be in the lattice of the [2]2B radical (overall bandwidth, W = 1.4 eV, in the pure compound) leads to very strong narrowing of the bandwidth, which reaches a minimum at [2]2Be (W = 0.3 eV). The results provide a graphic picture of the structural transformations undergone by the lattice, and at certain compositions we are able to identify distinct structures for the [2]2B and [2]2Be molecules in a single crystalline phase.