Browsing by Author "N'Diaye, Alpha T."

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    Electronic structure of cobalt valence tautomeric molecules in different environments
    (RSC, 2023-02) Mishra, Ezra; Ekanayaka, Thilini K.; Panagiotakopoulos, Theodoros; Le, Duy; Rahman, Talat S.; Wang, Ping; McElveen, Kayleigh A.; Phillips, Jared P.; Zaz, M. Zaid; Yazdani, Saeed; N'Diaye, Alpha T.; Lai, Rebecca Y.; Streubel, Robert; Cheng, Ruihua; Shatruk, Michael; Dowben, Peter A.; Physics, School of Science
    Future molecular microelectronics require the electronic conductivity of the device to be tunable without impairing the voltage control of the molecular electronic properties. This work reports the influence of an interface between a semiconducting polyaniline polymer or a polar poly-D-lysine molecular film and one of two valence tautomeric complexes, i.e., [CoIII(SQ)(Cat)(4-CN-py)2] ↔ [CoII(SQ)2(4-CN-py)2] and [CoIII(SQ)(Cat)(3-tpp)2] ↔ [CoII(SQ)2(3-tpp)2]. The electronic transitions and orbitals are identified using X-ray photoemission, X-ray absorption, inverse photoemission, and optical absorption spectroscopy measurements that are guided by density functional theory. Except for slightly modified binding energies and shifted orbital levels, the choice of the underlying substrate layer has little effect on the electronic structure. A prominent unoccupied ligand-to-metal charge transfer state exists in [CoIII(SQ)(Cat)(3-tpp)2] ↔ [CoII(SQ)2(3-tpp)2] that is virtually insensitive to the interface between the polymer and tautomeric complexes in the CoII high-spin state.
  • Thumbnail Image
    Item
    Evidence of dynamical effects and critical field in a cobalt spin crossover complex
    (Royal Society of Chemistry, 2022-01) Ekanayaka, Thilini K.; Wang, Ping; Yazdani, Saeed; Phillips, Jared Paul; Mishra, Esha; Dale, Ashley S.; N'Diaye, Alpha T.; Klewe, Christoph; Shafer, Padraic; Freeland, John; Streubel, Robert; Wampler, James Paris; Zapf, Vivien; Cheng, Ruihua; Shatruk, Michael; Dowben, Peter A.; Physics, School of Science
    The [Co(SQ)2(4-CN-py)2] complex exhibits dynamical effects over a wide range of temperature. The orbital moment, determined by X-ray magnetic circular dichroism (XMCD) with decreasing applied magnetic field, indicates a nonzero critical field for net alignment of magnetic moments, an effect not seen with the spin moment of [Co(SQ)2(4-CN-py)2].
  • Thumbnail Image
    Item
    Probing the unpaired Fe spins across the spin crossover of a coordination polymer
    (RSC, 2021) Ekanayaka, Thilini K.; Kurz, Hannah; Dale, Ashley S.; Hao, Guanhua; Mosey, Aaron; Mishra, Esha; N'Diaye, Alpha T.; Cheng, Ruihua; Weber, Birgit; Dowben, Peter A.; Physics, School of Science
    For the spin crossover coordination polymer [Fe(L1)(bipy)]n (where L1 is a N2O22− coordinating Schiff base-like ligand bearing a phenazine fluorophore and bipy = 4,4′-bipyridine), there is compelling additional evidence of a spin state transition. Both Fe 2p X-ray absorption and X-ray core level photoemission spectroscopies confirm that a spin crossover takes place, as observed by magnetometry. Yet the details of the temperature dependent changes of the spin state inferred from both X-ray absorption and X-ray core level photoemission, differ from magnetometry, particularly with regard to the apparent critical transition temperatures and the cooperative nature of the curve progression in general. Comparing the experimental spin crossover data to Ising model simulations, a transition activation energy in the region of 160 to 175 meV is indicated, along with a nonzero exchange J. Overall, the implication is that there may be perturbations to the bistability of spin states, that are measurement dependent or that the surface differs from the bulk with regard to the cooperative effects observed upon spin transition.
  • Thumbnail Image
    Item
    Tunable spin-state bistability in a spin crossover molecular complex
    (IOP, 2019) Jiang, Xuanyuan; Hao, Guanhua; Wang, Xiao; Mosey, Aaron; Zhang, Xin; Yu, Le; Yost, Andrew J.; Zhang, Xin; DiChiara, Anthony D.; N'Diaye, Alpha T.; Cheng, Xuemei; Zhang, Jian; Cheng, Ruihua; Xu, Xiaoshan; Dowben, Peter A.; Physics, School of Sciences
    The spin crossover (SCO) transitions at both the surface and over the entire volume of the [Fe{H2B(pz)2}2(bipy)] polycrystalline films on Al2O3 substrates have been studied, where pz  =  pyrazol-1-yl and bipy  =  2,2'-bipyridine. For [Fe{H2B(pz)2}2(bipy)] films of hundreds of nm thick, magnetometry and x-ray absorption spectroscopy measurements show thermal hysteresis in the SCO transition with temperature, although the transition in bulk [Fe{H2B(pz)2}2(bipy)] occurs in a non-hysteretic fashion at 157 K. While the size of the crystallites in those films are similar, the hysteresis becomes more prominent in thinner films, indicating a significant effect of the [Fe{H2B(pz)2}2(bipy)]/Al2O3 interface. Bistability of spin states, which can be inferred from the thermal hysteresis, was directly observed using temperature-dependent x-ray diffraction; the crystallites behave as spin-state domains that coexist during the transition. The difference between the spin state of molecules at the surface of the [Fe{H2B(pz)2}2(bipy)] films and that of the molecules within the films, during the thermal cycle, indicates that both cooperative (intermolecular) effects and coordination are implicated in perturbations to the SCO transition.