@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{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_habel-rodriguez_schmidt_sullivan_2010, title={Hyperfine Interaction, Spin Polarization, and Spin Delocalization as Probes of Donor-Bridge-Acceptor Interactions in Exchange-Coupled Biradicals}, volume={114}, ISSN={["1520-5207"]}, DOI={10.1021/jp102955j}, abstractNote={Computations and EPR spectroscopy are used to probe the spin distribution of donor-bridge-acceptor (D-B-A) biradical complexes: Tp(Cum,Me)Zn(SQ-NN) (1), Tp(Cum,Me)Zn(SQ-1,4-Ph-NN) (2), Tp(Cum,Me)Zn(SQ-2,5-TP-NN) (3), and Tp(Cum,Me)Zn(SQ-2,5-Xyl-NN) (4) (SQ = orthosemiquinone and NN = nitronylnitroxide). These complexes are ground-state analogs of the charge-separated excited states formed in photoinduced electron transfer reactions. The intraligand magnetic exchange interaction (J) in these complexes is mediated by the bridges and has been found to stabilize the triplet ground states of 1 and 2. Detailed spectroscopic and bonding calculations have been used to elucidate the role of the bridge fragment (B) and its conformation relative to donor (SQ) and acceptor (NN) on spin density distributions. The computed results correlate well with experimental nitrogen hyperfine coupling constants.}, number={45}, journal={JOURNAL OF PHYSICAL CHEMISTRY B}, author={Kirk, Martin L. and Shultz, David A. and Habel-Rodriguez, Diana and Schmidt, Robert D. and Sullivan, Ubie}, year={2010}, month={Nov}, pages={14712–14716} }
 @article{schmidt_shultz_martin_2010, title={Magnetic Bistability in a Cobalt Bis(dioxolene) Complex: Long-Lived Photoinduced Valence Tautomerism}, volume={49}, ISSN={["1520-510X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77950273381&partnerID=MN8TOARS}, DOI={10.1021/ic901998p}, abstractNote={The thermal- and photoinduced valence tautomerism of a cobalt bis(dioxolene) complex is described. The thermal conversion is precipitous, complete within 10 K, and is accompanied by a 5 K hysteresis loop (107 K < T(1/2) < 112 K). Rapid thermal quenching (300 K --> 10 K in ca. 5 s) and photoinduced valence tautomerism result in trapping of the metastable Co(II)-state at low temperatures through intermolecular hydrogen bonding. This lattice stabilization results in unmatched kinetic and thermal stability for a valence tautomer from 10-50 K, with residual hs-Co(II) persisting until about 90 K.}, number={7}, journal={INORGANIC CHEMISTRY}, author={Schmidt, Robert D. and Shultz, David A. and Martin, James D.}, year={2010}, month={Apr}, pages={3162–3168} }
 @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{sloop_jackson_schmidt_2009, title={Microwave-Mediated Pyrazole Fluorinations Using Selectfluor (R)}, volume={20}, ISSN={["1098-1071"]}, DOI={10.1002/hc.20556}, abstractNote={Abstract}, number={6}, journal={HETEROATOM CHEMISTRY}, author={Sloop, Joseph C. and Jackson, James L. and Schmidt, Robert D.}, year={2009}, pages={341–345} }