@article{francisco_gee_shepherd_warren_shultz_raithby_pinheiro_2017, title={Hard x-ray-induced valence tautomeric interconversion in cobalt-o-dioxolene complexes}, volume={8}, number={19}, journal={Journal of Physical Chemistry Letters}, author={Francisco, T. M. and Gee, W. J. and Shepherd, H. J. and Warren, M. R. and Shultz, D. A. and Raithby, P. R. and Pinheiro, C. B.}, year={2017}, pages={4774–4778} }
 @article{kirk_shultz_zhang_dangi_ingersol_yang_finney_sommer_wojtas_2017, title={Heterospin biradicals provide insight into molecular conductance and rectification}, volume={8}, ISSN={2041-6520 2041-6539}, url={http://dx.doi.org/10.1039/C7SC00073A}, DOI={10.1039/c7sc00073a}, abstractNote={Properties of two isomeric triplet ground state biradical molecules, which serve as constant-bias analogs of single-molecule electronic devices, are described and experimental results are compared with transport calculations in a device geometry.}, number={8}, journal={Chemical Science}, publisher={Royal Society of Chemistry (RSC)}, author={Kirk, Martin L. and Shultz, David A. and Zhang, Jinyuan and Dangi, Ranjana and Ingersol, Laura and Yang, Jing and Finney, Nathaniel S. and Sommer, Roger D. and Wojtas, Lukasz}, year={2017}, pages={5408–5415} }
 @article{kirk_shultz_stasiw_habel-rodriguez_stein_boyle_2013, title={Electronic and Exchange Coupling in a Cross-Conjugated D–B–A Biradical: Mechanistic Implications for Quantum Interference Effects}, volume={135}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/JA405354X}, DOI={10.1021/ja405354x}, abstractNote={A combination of variable-temperature EPR spectroscopy, electronic absorption spectroscopy, and magnetic susceptibility measurements have been performed on Tp(Cum,Me)Zn(SQ-m-Ph-NN) (1-meta) a donor-bridge-acceptor (D-B-A) biradical that possesses a cross-conjugated meta-phenylene (m-Ph) bridge and a spin singlet ground state. The experimental results have been interpreted in the context of detailed bonding and excited-state computations in order to understand the excited-state electronic structure of 1-meta. The results reveal important excited-state contributions to the ground-state singlet-triplet splitting in this cross-conjugated D-B-A biradical that contribute to our understanding of electronic coupling in cross-conjugated molecules and specifically to quantum interference effects. In contrast to the conjugated isomer, which is a D-B-A biradical possessing a para-phenylene bridge, admixture of a single low-lying singly excited D → A type configuration into the cross-conjugated D-B-A biradical ground state makes a negligible contribution to the ground-state magnetic exchange interaction. Instead, an excited state formed by a Ph-NN (HOMO) → Ph-NN (LUMO) one-electron promotion configurationally mixes into the ground state of the m-Ph bridged D-A biradical. This results in a double (dynamic) spin polarization mechanism as the dominant contributor to ground-state antiferromagnetic exchange coupling between the SQ and NN spins. Thus, the dominant exchange mechanism is one that activates the bridge moiety via the spin polarization of a doubly occupied orbital with phenylene bridge character. This mechanism is important, as it enhances the electronic and magnetic communication in cross-conjugated D-B-A molecules where, in the case of 1-meta, the magnetic exchange in the active electron approximation is expected to be J ~ 0 cm(-1). We hypothesize that similar superexchange mechanisms are common to all cross-conjugated D-B-A triads. Our results are compared to quantum interference effects on electron transfer/transport when cross-conjugated molecules are employed as the bridge or molecular wire component and suggest a mechanism by which electronic coupling (and therefore electron transfer/transport) can be modulated.}, number={39}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Kirk, Martin L. and Shultz, David A. and Stasiw, Daniel E. and Habel-Rodriguez, Diana and Stein, Benjamin and Boyle, Paul D.}, year={2013}, month={Sep}, pages={14713–14725} }
 @article{kirk_shultz_stasiw_lewis_wang_brannen_sommer_boyle_2013, title={Superexchange Contributions to Distance Dependence of Electron Transfer/Transport: Exchange and Electronic Coupling in Oligo(para-Phenylene)- and Oligo(2,5-Thiophene)-Bridged Donor–Bridge–Acceptor Biradical Complexes}, volume={135}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/JA4081887}, DOI={10.1021/ja4081887}, abstractNote={The preparation and characterization of three new donor-bridge-acceptor biradical complexes are described. Using variable-temperature magnetic susceptibility, EPR hyperfine coupling constants, and the results of X-ray crystal structures, we evaluate both exchange and electronic couplings as a function of bridge length for two quintessential molecular bridges: oligo(para-phenylene), β = 0.39 Å(-1) and oligo(2,5-thiophene), β = 0.22 Å(-1). This report represents the first direct comparison of exchange/electronic couplings and distance attenuation parameters (β) for these bridges. The work provides a direct measurement of superexchange contributions to β, with no contribution from incoherent hopping. The different β values determined for oligo(para-phenylene) and oligo(2,5-thiophene) are due primarily to the D-B energy gap, Δ, rather than bridge-bridge electronic couplings, H(BB). This is supported by the fact that the H(BB) values extracted from the experimental data for oligo(para-phenylene) (H(BB) = 11,400 cm(-1)) and oligo(2,5-thiophene) (12,300 cm(-1)) differ by <10%. The results presented here offer unique insight into the intrinsic molecular factors that govern H(DA) and β, which are important for understanding the electronic origin of electron transfer and electron transport mediated by molecular bridges.}, number={45}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Kirk, Martin L. and Shultz, David A. and Stasiw, Daniel E. and Lewis, Geoffrey F. and Wang, Guangbin and Brannen, Candice L. and Sommer, Roger D. and Boyle, Paul D.}, year={2013}, month={Nov}, pages={17144–17154} }
 @misc{kirk_shultz_2013, title={Transition metal complexes of donor-acceptor biradicals}, volume={257}, number={1}, journal={Coordination Chemistry Reviews}, author={Kirk, M. L. and Shultz, D. A.}, year={2013}, pages={218–233} }
 @article{sloop_shultz_marcus_shepler_2012, title={Linear free-energy relationships in semiquinone species and their Mn(II) and Cu(II) complexes}, volume={25}, number={2}, journal={Journal of Physical Organic Chemistry}, author={Sloop, J. C. and Shultz, D. A. and Marcus, M. B. and Shepler, B.}, year={2012}, pages={101–109} }
 @article{kirk_shultz_depperman_habel-rodriguez_schmidt_2012, title={Spectroscopic Studies of Bridge Contributions to Electronic Coupling in a Donor-Bridge-Acceptor Biradical System}, volume={134}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/ja300233a}, DOI={10.1021/ja300233a}, abstractNote={Variable-temperature electronic absorption and resonance Raman spectroscopies are used to probe the excited state electronic structure of Tp(Cum,Me)Zn(SQ-Ph-NN) (1), a donor-bridge-acceptor (D-B-A) biradical complex and a ground state analogue of the charge-separated excited state formed in photoinduced electron transfer reactions. Strong electronic coupling mediated by the p-phenylene bridge stabilizes the triplet ground state of this molecule. Detailed spectroscopic and bonding calculations elucidate key bridge distortions that are involved in the SQ(π)(SOMO) → NN-Ph (π*)(LUMO) D → A charge transfer (CT) transition. We show that the primary excited state distortion that accompanies this CT is along a vibrational coordinate best described as a symmetric Ph(8a) + SQ(in-plane) linear combination and underscores the dominant role of the phenylene bridge fragment acting as an electron acceptor in the D-B-A charge transfer state. Our results show the importance of the phenylene bridge in promoting (1) electron transfer in D-Ph-A systems and (2) electron transport in biased electrode devices that employ a 1,4-phenylene linkage. We have also developed a relationship between the spin density on the acceptor, as measured via the isotropic NN nitrogen hyperfine interaction, and the strength of the D → A interaction given by the magnitude of the electronic coupling matrix element, H(ab).}, number={18}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Kirk, Martin L. and Shultz, David A. and Depperman, Ezra C. and Habel-Rodriguez, Diana and Schmidt, Robert D.}, year={2012}, month={May}, pages={7812–7819} }
 @article{lee_lee_kim_bang_shultz_schmidt_forbes_lee_2011, title={Nitronyl Nitroxide Radicals as Organic Memory Elements with Both n- and p-Type Properties}, volume={50}, ISSN={1433-7851}, url={http://dx.doi.org/10.1002/anie.201004899}, DOI={10.1002/anie.201004899}, abstractNote={Organic molecules are being actively explored for use in logical devices, either as individual memory elements or as components embedded in small organic and polymeric materials. Conventional inorganic semiconductor devices are limited in terms of performance improvement owing to increased costs for device fabrication as well as physical limitations on minimum feature dimensions. Organic memory, however, is a possible substitute for both volatile and non-volatile memory devices. It has the advantages of facile tailoring through organic synthesis, simple device fabrication (even upon flexible substrates), and very low power consumption. Volatile organic memory is expected to be applied towards dynamic random access memory (DRAM), which typically requires a data refresh every few milliseconds, while non-volatile organic memory can be applied to read-only memory (ROM) and flash-type memory. Several types of organic and polymeric materials have been reported for this purpose, such as organic semiconductors, charge-transfer complexes (including redoxactive compounds), and metal-nanoparticle-dispersed thin films. Recently, a new type of organic memory has been added to this list, namely organic radical molecules (nitroxide radicals, NOC) that contain an unpaired electron that is capable of undergoing oxidation or reduction by applied bias voltages. In 1901, Piloty and Schwerin succeeded in the synthesis and isolation of porphyrexide, the first organic nitroxide. The most prominent member of this class of compounds is the 2,2,6,6-tetramethylpiperidine-N-oxyl radical (TEMPO). TEMPO and many other NO radicals belong to the category of persistent radicals. Since this pioneering work, the Nakahara group has reported the synthesis of a polymeric TEMPO radical derivative, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), for an organic radical battery. The research group of Nishide extended this work toward applications, such as radical batteries as cathode active materials, organic light-emitting diodes as holeinjection layers, and memory as p-type redox active materials. The TEMPO radical is easily oxidized to yield the corresponding oxoammonium salt, returning to the TEMPO radical by a p-type one-electron reduction. However, for the complete circuit of the organic semiconducting device using PTMA, an n-type redox active material as a partner to the p-type material is required. Some previously reported polymer-based organic radical memory devices required additional organic layers, such as an electron-accepting layer for n-type and even a metalparticle-dispersed dielectric layer for actuation of the organic memory device. While previous research into polymerbased organic radical memory has led to significant advances, for a complete organic radical memory circuit it is crucial to find new organic radical molecules that demonstrate switchability and present both pand n-type properties within the molecule. Also, new molecules can facilitate understanding of the origin of memory effects and whether that effect is induced by the organic radical alone or whether other environmental or chemical factors must be considered. Herein we report novel molecular radical memory behavior using a stable organic radical molecule. We have synthesized and characterized the nitronyl nitroxide (NN) radical molecule 2-(3’-tert-butyl-4’,5’-dimethoxymethoxybiphenyl-4-yl)-4,4,5,5-tetramethylimidazolidine-1-oxyl-3-oxide (NN-Ph-CatMOM2) [14] (see also the Supporting Information). The NN radical possesses one unpaired electron that is delocalized across the two equivalent N O groups (Scheme 1). Owing to delocalization, the oxidized and reduced states of the NN radicals were expected to be stabilized over a wide window of applied voltages, leading to a high switchability for the NN radical memory. The ability of the NN-Ph-CatMOM2 to act as both electron donor and acceptor was investigated by cyclic voltammetry (CV) and simultaneous electrochemical electron paramagnetic resonance (SEEPR) spectroscopy under an applied voltage. Cyclic voltammograms were [*] Dr. J. Lee, E. Lee, S. Kim, Dr. G. S. Bang, Prof. H. Lee NCRI, Center for Smart Molecular Memory Department of Chemistry, Sungkyunkwan University Suwon 440-746 (Republic of Korea) Fax: (+ 82)31-299-5934 E-mail: hyoyoung@skku.edu Prof. D. A. Shultz, Dr. R. D. Schmidt Department of Chemistry, North Carolina State University Raleigh, NC 27695-8204 (USA) Fax: (+ 1)919-515-8920 E-mail: david_shultz@ncsu.edu}, number={19}, journal={Angewandte Chemie International Edition}, publisher={Wiley}, author={Lee, Junghyun and Lee, Eunkyo and Kim, Sangkwan and Bang, Gyeong Sook and Shultz, David A. and Schmidt, Robert D. and Forbes, Malcolm D. E. and Lee, Hyoyoung}, year={2011}, month={Apr}, pages={4414–4418} }
 @article{sloop_shultz_coote_shepler_sullivan_kampf_boyle_2011, title={Synthesis of and structure-property relationships in zinc complexes of bis-metaphenylene semiquinone biradical species}, volume={25}, ISSN={0894-3230}, url={http://dx.doi.org/10.1002/poc.1917}, DOI={10.1002/poc.1917}, abstractNote={The design of novel, functionalized bis‐metaphenylene semiquinone (SQ) ligands and their corresponding metal complexes which combine conformational flexibility and electron‐withdrawing, electron‐donating, and conjugating substituents enable investigation of multiple structure–property relationships. Along these lines, we report the synthesis of three new bis(ZnII(SQ)TpCum,Me) complexes containing the bis‐metaphenylene coupling fragment. Using electron paramagnetic resonance spectroscopy, ab initio computations and superconducting quantum interference device magnetometry, we show how spin‐density is affected by the bis‐metaphenylene system substituents. Copyright © 2011 John Wiley & Sons, Ltd.}, number={4}, journal={Journal of Physical Organic Chemistry}, publisher={Wiley}, author={Sloop, Joseph C. and Shultz, David A. and Coote, Tashni and Shepler, Benjamin and Sullivan, Ubie and Kampf, Jeff W. and Boyle, Paul D.}, year={2011}, month={Aug}, pages={314–321} }
 @article{schmidt_shultz_martin_boyle_2010, title={Goldilocks Effect in Magnetic Bistability: Remote Substituent Modulation and Lattice Control of Photoinduced Valence Tautomerism and Light-Induced Thermal Hysteresis}, volume={132}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/ja101957c}, DOI={10.1021/ja101957c}, abstractNote={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), NO(2) (6)) is described. The thermal valence tautomerism (ls-Co(III)(SQ)(Cat)(4-X-py)(2) <--> hs-Co(II)(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 (lambda = 400-850 nm), the hs-Co(II) tautomer is formed. When the light source is removed, and the sample is slowly heated, the hs-Co(II) 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 (DeltaT = ca. 25 K). Below 50 K, the hs-Co(II) tautomer displays temperature-independent relaxation to the ls-Co(III) 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-Co(II) tautomer, thus providing an excellent handle for molecular and crystal engineering studies.}, number={17}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Schmidt, R. D. and Shultz, D. A. and Martin, J. D. and Boyle, P. D.}, year={2010}, month={May}, pages={6261–6273} }
 @article{kirk_shultz_schmidt_habel-rodriguez_lee_lee_2009, title={Ferromagnetic Nanoscale Electron Correlation Promoted by Organic Spin-Dependent Delocalization}, volume={131}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/ja904648r}, DOI={10.1021/ja904648r}, abstractNote={We describe the electronic structure and the origin of ferromagnetic exchange coupling in two new metal complexes, NN-SQ-Co(III)(py)(2)Cat-NN (1) and NN-Ph-SQ-Co(III)(py)(2)Cat-Ph-NN (2) (NN = nitronylnitroxide radical, Ph = 1,4-phenylene, SQ = S = (1)/(2) semiquinone radical, Cat = S = 0 catecholate, and py = pyridine). Near-IR electronic absorption spectroscopy for 1 and 2 reveals a low-energy optical band that has been assigned as a Psi(u) --> Psi(g) transition involving bonding and antibonding linear combinations of delocalized dioxolene (SQ/Cat) valence frontier molecular orbitals. The ferromagnetic exchange interaction in 1 is so strong that only the high-spin quartet state (S(T) = (3)/(2)) is thermally populated at temperatures up to 300 K. The temperature-dependent magnetic susceptibility data for 2 reveals that an excited state spin doublet (S(T) = (1)/(2)) is populated at higher temperatures, indicating that the phenylene spacer modulates the magnitude of the magnetic exchange. The valence delocalization within the dioxolene dyad of 2 results in ferromagnetic alignment of two localized NN radicals separated by over 22 A. The ferromagnetic exchange in 1 and 2 results from a spin-dependent delocalization (double exchange type) process and the origin of this strong electron correlation has been understood in terms of a valence bond configuration interaction (VBCI) model. We show that ferromagnetic coupling promoted by organic mixed-valency provides keen insight into the ability of single molecules to communicate spin information over nanoscale distances. Furthermore, the strong interaction between the itinerant dioxolene electron and localized NN electron spins impacts our ability to understand the exchange interaction between delocalized electrons and pinned magnetic impurities in technologically important dilute magnetic semiconductor materials. The long correlation length (22 A) of the itinerant electron that mediates this coupling indicates that high-spin pi-delocalized organic molecules could find applications as nanoscale spin-polarized electron injectors and molecular wires.}, number={51}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Kirk, Martin L. and Shultz, David A. and Schmidt, Robert D. and Habel-Rodriguez, Diana and Lee, Hyoyoung and Lee, Junghyun}, year={2009}, month={Dec}, pages={18304–18313} }
 @article{kirk_shultz_depperman_brannen_2007, title={Donor−Acceptor Biradicals as Ground State Analogues of Photoinduced Charge Separated States}, volume={129}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/ja065384t}, DOI={10.1021/ja065384t}, abstractNote={A Valence Bond Configuration Interaction (VBCI) model is used to relate the intraligand magnetic exchange interaction (J) to the electronic coupling matrix element (HAB) in Tp(Cum,MeZn)(SQNN), a compound that possesses a Donor-Acceptor (D-A) SemiQuinone-NitronylNitroxide (SQNN) biradical ligand. Within this framework, an SQ --> NN charge transfer state mixes with the ground state and stabilizes the spin triplet (S = 1). This charge-transfer transition is observed spectroscopically and probed using resonance Raman spectroscopy. In addition, the temperature-dependent electronic absorption spectrum of the Ni(II) complex, Tp(Cum,MeNi)(SQNN), has been studied. Exchange coupling between the S = 1 Ni(II) ion and S = 1 SQNN provides a mechanism for observing the formally spin-forbidden, ligand-based 3GC --> 1CTC transition. This provides a means of determining U, the mean GC --> CTC energy, and a one-center exchange integral, K(0). The experimental determination of J, U, and K(0) permits facile calculation of HAB, and we show that this methodology can be extended to determine the electronic coupling matrix element in related SQ-Bridge-NN molecules. As magnetic susceptibility measurements are easily acquired in the solid state, H(AB) may be effectively determined for single molecules in a known geometry, provided a crystal structure exists for the biradical complex. Thus, SQ-Bridge-NN molecules possess considerable potential for probing both geometric and electronic structure contributions to the magnitude of the electronic coupling matrix element associated with a given bridge fragment.}, number={7}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Kirk, Martin L. and Shultz, David A. and Depperman, Ezra C. and Brannen, Candice L.}, year={2007}, month={Feb}, pages={1937–1943} }
 @article{carlson_kuehl_ogallo_shultz_thompson_kirk_martin_john_morris_2007, title={Influence of Ligand Geometry in Bimetallic Ytterbocene Complexes of Bridging Bis(bipyridyl) Ligands}, volume={26}, ISSN={0276-7333 1520-6041}, url={http://dx.doi.org/10.1021/om061157o}, DOI={10.1021/om061157o}, abstractNote={Two new bimetallic complexes, [Cp*2Yb]2(μ-1,3-(2,2‘-bipyridyl)-5-tBu-C6H3) (1) and [Cp*2Yb]2(μ-1,4-(2,2‘-bipyridyl)-C6H4) (2), and their corresponding two-electron oxidation products [1]2+ and [2]2+ have been synthesized with the aim of determining the impact of the bridging ligand geometry on the electronic and magnetic properties of these materials. Electrochemistry, optical spectroscopy, and bulk susceptibility measurements all support a ground-state electronic configuration of the type [(f)13-(πa*)1-(πb*)1-(f)13]. Density functional theory calculations on the uncomplexed bridging ligands as doubly reduced species also indicate that the diradical electronic configuration is the lowest lying for both meta- and para-bis(bipyridyl) systems. The electrochemical and optical spectroscopic data indicate that the electronic coupling between the metal centers mediated by the diradical bridges is weak, as evidenced by the small separation of the metal-based redox couples and the similarity of the f−f transitions...}, number={17}, journal={Organometallics}, publisher={American Chemical Society (ACS)}, author={Carlson, Christin N. and Kuehl, Christopher J. and Ogallo, Linda and Shultz, David A. and Thompson, J. D. and Kirk, Martin L. and Martin, Richard L. and John, Kevin D. and Morris, David E.}, year={2007}, month={Aug}, pages={4234–4242} }
 @article{shultz_sloop_coote_beikmohammadi_kampf_boyle_2007, title={Substituent Effects on Exchange Coupling:  5-Aryl-Substituted Semiquinones and Their Complexes with MnIIand CuII}, volume={46}, ISSN={0020-1669 1520-510X}, url={http://dx.doi.org/10.1021/ic061807g}, DOI={10.1021/ic061807g}, abstractNote={A series of functionalized radical anion semiquinone (SQ-Ar) ligands and their MnII- and CuIIhydro-tris(3-cumenyl-5-methylpyrazolyl)borate (TpCum,MeMII) complexes were prepared and characterized. The semiquinone ligands have substituted phenyl rings (Ar = -C6H5NO2, -C6H5OMe, -C6H5-tert-Bu, etc.) attached to the SQ 5-position. Despite the "remoteness" of the phenyl ring substituents, the MII-SQ exchange parameters, J, were found to vary nearly 3-fold. Attempts to quantify the substituent effects on J are complicated by the fact that not all complexes could be structurally characterized. As such, substituent effects and phenyl-ring torsion angles could conspire to produce the observed variation in J values. Although there is no clear trend in the J values as a function of SQ substituent for the MnII complexes, for the CuII complexes, electron-withdrawing substituents on the phenyl ring have greater ferromagnetic J values than the CuII complexes of SQ ligands with electron-donating substituents. This trend suggests a FM contribution from MLCT excited states in the copper complexes.}, number={1}, journal={Inorganic Chemistry}, publisher={American Chemical Society (ACS)}, author={Shultz, David A. and Sloop, Joseph C. and Coote, Tashni-Ann and Beikmohammadi, Mithra and Kampf, Jeff and Boyle, Paul D.}, year={2007}, month={Jan}, pages={273–277} }
 @article{shultz_sloop_washington_2006, title={Design, Synthesis, and Properties of Conformationally Fixed Semiquinone Monoradical Species}, volume={71}, ISSN={0022-3263 1520-6904}, url={http://dx.doi.org/10.1021/jo061502j}, DOI={10.1021/jo061502j}, abstractNote={The design of novel, functionalized semiquinone (SQ) ligands which combine structural rigidity and electron-withdrawing, electron-donating, and electroneutral substituents enables investigation of multiple structure-property relationships and building blocks for new materials, including components of sensors, switches, and molecular spintronics. Along these lines, we report the synthesis of several new SQ ligands containing fused heterocyclic ring systems. Using both electron paramagnetic resonance spectroscopy and quantum chemical calculations, we show how spin density is affected by the fused ring system substituents.}, number={24}, journal={The Journal of Organic Chemistry}, publisher={American Chemical Society (ACS)}, author={Shultz, David A. and Sloop, Joseph C. and Washington, Gary}, year={2006}, month={Nov}, pages={9104–9113} }
 @article{shultz_mussari_ramanathan_kampf_2006, title={Electron Spin−Spin Exchange Coupling Mediated by the Porphyrin π System}, volume={45}, ISSN={0020-1669 1520-510X}, url={http://dx.doi.org/10.1021/ic060209z}, DOI={10.1021/ic060209z}, abstractNote={The syntheses and electron paramagnetic resonance (EPR) spectral characterizations of porphyrins (1-3) substituted with two radical groups bound to trans-meso positions are described. One of these compounds, 3, has been studied by variable-temperature magnetic susceptibility and has been structurally characterized. Biradical porphyrin 3 is monoclinic, space group P2(1)/n, with a = 12.239(2) A, b = 17.819(3) A, c = 34.445(7) A, alpha = 90 degrees , beta = 97.466(3) degrees , gamma = 90 degrees , and Z = 2. The bis(nitroxide) porphyrins 1 and 2 exhibit fluid solution EPR spectra consistent with |J| >> |a|. No evidence was observed for conformational modulation of J by rotation about single bonds as shown by the lack of change of the EPR spectra as a function of temperature. The bis(semiquinone) porphyrin 3 exhibits frozen-solution EPR spectra with zero-field splitting and a Deltam(s) = 2 transition characteristic of a triplet state. The intensity of the Deltam(s) = 2 transition of 3 was measured as a function of temperature, and the data fit according to a singlet-triplet model to yield J(3,solution) = -75 cm(-1) (H = - 2Jŝ1.ŝ2). Polycrystalline samples of porphryin 3 were examined by variable-temperature magnetometry. The paramagnetic susceptibility data were fit using a modified Bleaney-Bowers equation to give J(3,solid) = -29 cm(-1) (H = - 2Jŝ(1).ŝ(2)). The antiferromagnetic J values are consistent with the pi topology of the porphyrin ring.}, number={15}, journal={Inorganic Chemistry}, publisher={American Chemical Society (ACS)}, author={Shultz, David A. and Mussari, Christopher P. and Ramanathan, Krishna Kumar and Kampf, Jeff W.}, year={2006}, month={Jul}, pages={5752–5759} }
 @article{bin-salamon_brewer_depperman_franzen_kampf_kirk_kumar_lappi_peariso_preuss_et al._2006, title={Testing Bridge-Mediated Differences in Dinuclear Valence Tautomeric Behavior}, volume={45}, ISSN={0020-1669 1520-510X}, url={http://dx.doi.org/10.1021/ic060170y}, DOI={10.1021/ic060170y}, abstractNote={Two structurally characterized dinuclear valence tautomers are described. Cobalt ions are bridged by p- and m-phenylene units connected to 2,2'-bipyridines. X-ray crystal structures show that the molecules are in the [(Co(III))(Co(III))] forms at ca. 125 K, while spectroscopic studies show that both molecules can achieve the [(Co(II))(Co(II))] form above 400 K and confirm the [(Co(III))(Co(III))] form below 10 K. Magnetic susceptibility studies are also included. Our results highlight the necessity of studying both crystalline and amorphous samples to distinguish the effects of intrinsic electronic structure and intermolecular forces on valence tautomeric behavior.}, number={11}, journal={Inorganic Chemistry}, publisher={American Chemical Society (ACS)}, author={Bin-Salamon, Sofi and Brewer, Scott H. and Depperman, Ezra C. and Franzen, Stefan and Kampf, Jeff W. and Kirk, Martin L. and Kumar, R. Krishna and Lappi, Simon and Peariso, Katrina and Preuss, Kathryn E. and et al.}, year={2006}, month={May}, pages={4461–4467} }