A new donor–acceptor biradical complex, TpCum,MeZn(SQ-VD) (TpCum,MeZn+ = zinc(II) hydro-tris(3-cumenyl-5-methylpyrazolyl)borate complex cation; SQ = orthosemiquinone; VD = oxoverdazyl), which is ...a ground-state analogue of a charge-separated excited state, has been synthesized and structurally characterized. The magnetic exchange interaction between the S = 1/2 SQ and the S = 1/2 VD within the SQ-VD biradical ligand is observed to be ferromagnetic, with J SQ‑VD = +77 cm–1 (H = −2J SQ‑VD ŜSQ·ŜVD) determined from an analysis of the variable-temperature magnetic susceptibility data. The pairwise biradical exchange interaction in TpCum,MeZn(SQ-VD) can be compared with that of the related donor–acceptor biradical complex TpCum,MeZn(SQ-NN) (NN = nitronyl nitroxide, S = 1/2), where J SQ‑NN ≅ +550 cm–1. This represents a dramatic reduction in the biradical exchange by a factor of ∼7, despite the isolobal nature of the VD and NN acceptor radical SOMOs. Computations assessing the magnitude of the exchange were performed using a broken-symmetry density functional theory (DFT) approach. These computations are in good agreement with those computed at the CASSCF NEVPT2 level, which also reveals an S = 1 triplet ground state as observed in the magnetic susceptibility measurements. A combination of electronic absorption spectroscopy and CASSCF computations has been used to elucidate the electronic origin of the large difference in the magnitude of the biradical exchange coupling between TpCum,MeZn(SQ-VD) and TpCum,MeZn(SQ-NN). A Valence Bond Configuration Interaction (VBCI) model was previously employed to highlight the importance of mixing an SQSOMO → NNLUMO charge transfer configuration into the electronic ground state to facilitate the stabilization of the high-spin triplet (S = 1) ground state in TpCum,MeZn(SQ-NN). Here, CASSCF computations confirm the importance of mixing the pendant radical (e.g., VD, NN) LUMO (VDLUMO and NNLUMO) with the SOMO of the SQ radical (SQSOMO) for stabilizing the triplet, in addition to spin polarization and charge transfer contributions to the exchange. An important electronic structure difference between TpCum,MeZn(SQ-VD) and TpCum,MeZn(SQ-NN), which leads to their different exchange couplings, is the reduced admixture of excited states that promote ferromagnetic exchange into the TpCum,MeZn(SQ-VD) ground state, and the intrinsically weaker mixing between the VDLUMO and the SQSOMO compared to that observed for TpCum,MeZn(SQ-NN), where this orbital mixing is significant. The results of this comparative study contribute to a greater understanding of biradical exchange interactions, which are important to our understanding of excited-state singlet–triplet energy gaps, electron delocalization, and the generation of electron spin polarization in both the ground and excited states of (bpy)Pt(CAT-radical) complexes.
Excited state processes involving multiple electron spin centers are crucial elements for both spintronics and quantum information processing. Herein, we describe an addressable excited state ...mechanism for precise control of electron spin polarization. This mechanism derives from excited state magnetic exchange couplings that occur between the electron spins of a photogenerated electron–hole pair and that of an organic radical. The process is initiated by absorption of a photon followed by ultrafast relaxation within the excited state spin manifold. This leads to dramatic changes in spin polarization between excited states of the same multiplicity. Moreover, this photoinitiated spin polarization process can be “read” spectroscopically using a magnetooptical technique that is sensitive to the excited state electron spin polarizations and allows for the evaluation of wave functions that give rise to these polarizations. This system is unique in that it requires neither intersystem crossing nor magnetic resonance techniques to create dynamic spin-polarization effects in molecules.
Abstract
The ejecta velocities of Type Ia supernovae (SNe Ia), as measured by the Si
ii
λ
6355 line, have been shown to correlate with other supernova properties, including color and standardized ...luminosity. We investigate these results using the Foundation Supernova Survey, with a spectroscopic data release presented here, and photometry analyzed with the SALT2 light-curve fitter. We find that the Foundation data do not show significant evidence for an offset in color between SNe Ia with high and normal photospheric velocities, with Δ
c
= 0.004 ± 0.015. Our SALT2 analysis does show evidence for redder high-velocity SNe Ia in other samples, including objects from the Carnegie Supernova Project, with a combined sample yielding Δ
c
= 0.018 ± 0.008. When split on velocity, the Foundation SNe Ia also do not show a significant difference in Hubble diagram residual, Δ
HR
= 0.015 ± 0.049 mag. Intriguingly, we find that SN Ia ejecta velocity information may be gleaned from photometry, particularly in redder optical bands. For high-redshift SNe Ia, these rest-frame red wavelengths will be observed by the Nancy Grace Roman Space Telescope. Our results are in line with previous work that suggests SN Ia host-galaxy stellar mass is correlated with ejecta velocity: high-velocity SNe Ia are found nearly exclusively in high-stellar-mass hosts. However, host-galaxy properties alone do not explain velocity-dependent differences in supernova colors and luminosities across samples. Measuring and understanding the connection between intrinsic explosion properties and supernova environments, across cosmic time, will be important for precision cosmology with SNe Ia.
The Askaryan Radio Array (ARA) experiment at the South Pole is designed to detect high-energy neutrinos which, via in-ice interactions, produce coherent radiation at frequencies up to 1000 MHz. ...Characterization of ice birefringence, and its effect upon wave polarization, is proposed to enable range estimation to a neutrino interaction and hence aid in neutrino energy reconstruction. Using radio transmitter calibration sources, the ARA collaboration recently measured polarization-dependent time delay variations and reported significant time delays for trajectories perpendicular to ice flow, but not parallel. To explain these observations, and assess the capability for range estimation, we use fabric data from the SPICE ice core to model ice birefringence and construct a bounding radio propagation model that predicts polarization time delays. We compare the model with new data from December 2018 and demonstrate that the measurements are consistent with the prevailing horizontal crystallographic axis aligned near-perpendicular to ice flow. The study supports the notion that range estimation can be performed for near flow-perpendicular trajectories, although tighter constraints on fabric orientation are desirable for improving the accuracy of estimates.
This study examines English speakers' relative weighting of two voicing cues in production and perception. Participants repeated words differing in initial consonant voicing (b or p) and labeled ...synthesized tokens ranging between ba and pa orthogonally according to voice onset time (VOT) and onset f0. Discriminant function analysis and logistic regression were used to calculate individuals' relative weighting of each cue. Production results showed a significant negative correlation of VOT and onset f0, while perception results showed a trend toward a positive correlation. No significant correlations were found across perception and production, suggesting a complex relationship between the two domains.
Calculated conductance through Au
n
-S-Bridge-S-Au
n
(Bridge = organic σ/π-system) constructs are compared to experimentally-determined magnetic exchange coupling parameters in a series of Tp
Cum,Me
...ZnSQ-Bridge-NN complexes, where Tp
Cum,Me
= hydro-tris(3-cumenyl-1-methylpyrazolyl)borate ancillary ligand, Zn = diamagnetic zinc(
ii
), SQ = semiquinone (
S
= 1/2), and NN = nitronylnitroxide radical (
S
= 1/2). We find that there is a nonlinear functional relationship between the biradical magnetic exchange coupling,
J
D→A
, and the computed conductance,
g
mb
. Although different bridge types (monomer
vs.
dimer) do not lie on the same
J
D→A
vs. g
mb
, curve, there is a scale invariance between the monomeric and dimeric bridges which shows that the two data sets are related by a proportionate scaling of
J
D→A
. For exchange and conductance mediated by a given bridge fragment, we find that the ratio of distance dependent decay constants for conductance (
β
g
) and magnetic exchange coupling (
β
J
) does not equal unity, indicating that inherent differences in the tunneling energy gaps, Δ
, and the bridge-bridge electronic coupling,
H
BB
, are not directly transferrable properties as they relate to exchange and conductance. The results of these observations are described in valence bond terms, with resonance structure contributions to the ground state bridge wavefunction being different for SQ-Bridge-NN and Au
n
-S-Bridge-S-Au
n
systems.
Calculated conductance through Au
n
-S-Bridge-S-Au
n
constructs are compared to experimental magnetic exchange coupling parameters in Tp
Cum,Me
Zn(SQ-Bridge-NN) complexes, where SQ = semiquinone radical and NN = nitronylnitroxide radical.
Scanning tunneling microscopy and local conductance mapping show spin-state coexistence in bilayer films of Fe(H2Bpz2)2bpy on Au(111) that is independent of temperature between 131 and 300 K. This ...modification of bulk behavior is attributed in part to the unique packing constraints of the bilayer film that promote deviations from bulk behavior. The local density of states measured for different spin states shows that high-spin molecules have a smaller transport gap than low-spin molecules and are in agreement with density functional theory calculations.
The thermal-induced and photoinduced valence tautomerism of a series of Co(dioxolene)2(4-X-py)2 complexes (dioxolene = 3,5-di-tert-butylcatecholate or 3,5-di-tert-butylsemiquinonate; 4-X-py = ...4-(X)pyridine, X = H (1), OMe (2), Me (3), CN (4), Br (5), NO2 (6)) is described. The thermal valence tautomerism (ls-CoIII(SQ)(Cat)(4-X-py)2 ↔ hs-CoII(SQ)(SQ)(4-X-py)2) is only observed for complexes 4, 5, and 6 where each is accompanied by a hysteresis loop of ca. 5 K. When a crystalline sample of 4−6 is held at 10 K in a SQUID magnetometer and irradiated with white light (λ = 400−850 nm), the hs-CoII tautomer is formed. When the light source is removed, and the sample is slowly heated, the hs-CoII tautomer persists until ca. 90 K, approximately 40 K higher than the thermal stability of previously reported complexes. Heating and cooling the sample while maintaining irradiation results in the appearance of a new light-induced thermal hysteresis loop below 90 K (ΔT = ca. 25 K). Below 50 K, the hs-CoII tautomer displays temperature-independent relaxation to the ls-CoIII form, and above 50 K, the relaxation is thermally activated with an activation energy E a > ca. 1500 cm−1. The coordination geometry (trans-pyridines), pyridine substitution, and crystal packing forces conspire to create the comparatively thermally stable photogenerated hs-CoII tautomer, thus providing an excellent handle for molecular and crystal engineering studies.
The synthesis and characterization of an FeIII catecholate–nitronylnitroxide (CAT-NN) complex (1-NN) that undergoes FeIII spin-crossover is described. Our aim is to determine whether the intraligand ...exchange coupling of the semiquinone–nitronylnitroxide FeII(SQ-NN) excited state resulting from irradiation of the CAT → FeIII LMCT band would affect either the intrinsic photophysics or the iron spin-crossover event when compared to the complex lacking the nitronylnitroxide radical (1). X-ray crystallographic analysis provides bond lengths consistent with a ferric catecholate charge distribution. Mössbauer spectroscopy clearly demonstrates FeIII spin-crossover, hyperfine couplings, and a weak ferromagnetic FeIII–CAT-NN exchange, and spin-crossover is corroborated by variable-temperature magnetic susceptibility and electronic absorption studies. To explore the effect of the NN radical on photophysical processes, we conducted room-temperature transient absorption experiments. Upon excitation of the ligand-to-metal charge transfer band, an FeIISQ state is populated and most likely undergoes fast intersystem crossing to the ligand field manifold, where it rapidly decays into a metastable low-spin FeIIICAT state, followed by repopulation of the high-spin FeIIICAT ground state. The decay components of 1-NN are slightly faster than those obtained for 1, perhaps due to the higher number of microstates present within the LMCT and LF manifolds for 1-NN. Although the effects of the NN radical are manifest in neither the spin-crossover nor the photophysics, our results lay the groundwork for future studies.
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