@article{kirk_shultz_hewitt_marri_est_2023, title={Competitive reversed quartet mechanisms for photogenerated ground state electron spin polarization}, volume={8}, ISSN={["2041-6539"]}, DOI={10.1039/d3sc03049}, journal={CHEMICAL SCIENCE}, author={Kirk, Martin L. and Shultz, David A. and Hewitt, Patrick and Marri, Anil Reddy and Est, Art}, year={2023}, month={Aug} }
 @article{kirk_shultz_hewitt_marri_est_2023, title={Competitive reversed quartet mechanisms for photogenerated ground state electron spin polarization}, volume={14}, ISSN={["2041-6539"]}, url={https://doi.org/10.1039/D3SC03049K}, DOI={10.1039/d3sc03049k}, abstractNote={Photoinduced electron spin polarization (ESP) of a spin-½ organic radical (nitronyl nitroxide, NN) in a series of Pt(ii) complexes comprised of 4,4′-di-tert-butyl-2,2′-bipyridine (bpy) and 3-tert-butylcatecholate (CAT) ligands, where the CAT ligand is substituted with (CH3)n-meta-phenyl-NN (bridge-NN) groups, is presented and discussed. We show the importance of attenuating the energy gap between localized NN radical and chromophoric excited states to control both the magnitude and sign of the optically-generated ESP, and to provide deeper insight into the details of the ESP mechanism. Understanding electronic structure contributions to optically generated ESP will enhance our ability to control the nature of prepared states for a variety of quantum information science applications, where strong ESP facilitates enhanced sensitivity and readout capabilities at low applied magnetic fields and higher temperatures.}, number={36}, journal={CHEMICAL SCIENCE}, author={Kirk, Martin L. and Shultz, David A. and Hewitt, Patrick and Marri, Anil Reddy and Est, Art}, year={2023}, month={Sep}, pages={9689–9695} }
 @article{kirk_shultz_hewitt_est_2022, title={Excited State Exchange Control of Photoinduced Electron Spin Polarization in Electronic Ground States}, volume={1}, ISSN={["1948-7185"]}, url={https://doi.org/10.1021/acs.jpclett.1c03491}, DOI={10.1021/acs.jpclett.1c03491}, abstractNote={Ground-state electron spin polarization (ESP) is generated in radical elaborated (bpy)Pt(CAT-NN) and (bpy)Pt(CAT-p-Me2PhMe2-NN) (bpy = 5,5'-di-tert-butyl-2,2'-bipyridine, CAT = 3-tert-butylcatecholate, p-Ph = para-phenylene, NN = nitronylnitroxide). Photoexcitation produces an exchange-coupled, three-spin, charge-separated doublet 2S1 (S = chromophore excited spin singlet configuration) excited state that rapidly decays to a 2T1 (T = chromophore excited spin triplet configuration) excited state. The SQ-bridge-NN bond torsions affect the magnitude of the excited state exchange interaction (JSQ-NN), which determines the 2T1-4T1 energy gap. Ground state ESP is dependent on the magnitude of JSQ-NN, and we postulate that this results from differences in 2T1 and 4T1 state mixing. Mechanisms that lead to the rapid transfer of the excited state ESP to the ground state are discussed. Although subnanosecond 2T1 state lifetimes are measured optically in solution, the ground state ESP decays very slowly at 20 K and is observable for more than a millisecond.}, journal={JOURNAL OF PHYSICAL CHEMISTRY LETTERS}, author={Kirk, Martin L. and Shultz, David A. and Hewitt, Patrick and Est, Art}, year={2022}, month={Jan} }
 @article{kirk_shultz_hewitt_chen_est_2022, title={Excited State Magneto-Structural Correlations Related to Photoinduced Electron Spin Polarization}, volume={7}, ISSN={["1520-5126"]}, url={https://doi.org/10.1021/jacs.2c03490}, DOI={10.1021/jacs.2c03490}, abstractNote={Photoinduced electron spin polarization (ESP) is reported in the ground state of a series of complexes consisting of an organic radical (nitronylnitroxide, NN) covalently attached to a donor-acceptor chromophore either directly or via para-phenylene bridges substituted with 0-4 methyl groups. These molecules represent a class of chromophores that undergo visible light excitation to produce an initial exchange-coupled, three-spin [bpy•-, CAT•+ (= semiquinone, SQ) and NN•], charge-separated doublet 2S1 (S = chromophore spin singlet configuration) excited state that rapidly decays by magnetic exchange-enhanced internal conversion to a 2T1 (T = chromophore excited spin triplet configuration) state. The 2T1 state equilibrates with chromophoric and NN radical-derived excited states, resulting in absorptive ESP of the recovered ground state, which persists for greater than a millisecond and can be measured by low-temperature time-resolved electron paramagnetic resonance spectroscopy. The magnitude of the ground state ESP is found to correlate with the excited state magnetic exchange interaction between the CAT+• and NN• radicals, which in turn is controlled by the structure of the bridge fragment.}, journal={JOURNAL OF THE AMERICAN CHEMICAL SOCIETY}, publisher={American Chemical Society (ACS)}, author={Kirk, Martin L. and Shultz, David A. and Hewitt, Patrick and Chen, Ju and Est, Art van der}, year={2022}, month={Jul} }
 @article{kirk_shultz_marri_hewitt_van der est_2022, title={Single-Photon-Induced Electron Spin Polarization of Two Exchange-Coupled Stable Radicals}, volume={144}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/jacs.2c09680}, DOI={10.1021/jacs.2c09680}, abstractNote={Transient electron paramagnetic resonance spectroscopy has been used to probe photoinduced electron spin polarization of a stable exchange-coupled organic biradical in a Pt(II) complex comprising 4,4'-di-tert-butyl-2,2'-bipyridine (bpy) and 3,6-bis(ethynyl-para-phenyl-nitronyl nitroxide)-o-catecholate (CAT(o-C≡C-Ph-NN)2). Photoexcitation results in four unpaired spins in excited states of this complex, with spins being localized on each of the two radicals, CAT•+ and bpy•-. The four spins are all exchange-coupled in these excited states, and an off-diagonal matrix element in the CAT•+-NN exchange allows for exchange-enhanced intersystem crossing to the 3T1a state, which possesses (bpy•-)Pt(CAT•+) chromophoric triplet character. Fast mixing between this 3T1a state and thermally accessible excited LL'CT state(s) followed by fast relaxation provides spin polarization of the exchange-coupled NN radicals in the 3S0 ground state of the complex. Our results demonstrate that well-defined quantum states of a ground-state biradical can be initialized with single-photon excitation and have the potential for further spin manipulation directed toward quantum information science applications.}, number={46}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Kirk, Martin L. and Shultz, David A. and Marri, Anil Reddy and Hewitt, Patrick and van der Est, Art}, year={2022}, month={Nov}, pages={21005–21009} }
 @article{kirk_shultz_hewitt_stasiw_chen_est_2021, title={Chromophore-radical excited state antiferromagnetic exchange controls the sign of photoinduced ground state spin polarization}, volume={9}, ISSN={["2041-6539"]}, DOI={10.1039/d1sc02965g}, abstractNote={A change in the sign of the ground-state electron spin polarization (ESP) is reported in complexes where an organic radical (nitronylnitroxide, NN) is covalently attached to a donor–acceptor chromophore via two different meta-phenylene bridges in (bpy)Pt(CAT-m-Ph-NN) (mPh-Pt) and (bpy)Pt(CAT-6-Me-m-Ph-NN) (6-Me-mPh-Pt) (bpy = 5,5′-di-tert-butyl-2,2′-bipyridine, CAT = 3-tert-butylcatecholate, m-Ph = meta-phenylene). These molecules represent a new class of chromophores that can be photoexcited with visible light to produce an initial exchange-coupled, 3-spin (bpy˙−, CAT+˙ = semiquinone (SQ), and NN), charge-separated doublet 2S1 (S = chromophore excited spin singlet configuration) excited state. Following excitation, the 2S1 state rapidly decays to the ground state by magnetic exchange-mediated enhanced internal conversion via the 2T1 (T = chromophore excited spin triplet configuration) state. This process generates emissive ground state ESP in 6-Me-mPh-Pt while for mPh-Pt the ESP is absorptive. It is proposed that the emissive polarization in 6-Me-mPh-Pt results from zero-field splitting induced transitions between the chromophoric 2T1 and 4T1 states, whereas predominant spin–orbit induced transitions between 2T1 and low-energy NN-based states give rise to the absorptive polarization observed for mPh-Pt. The difference in the sign of the ESP for these molecules is consistent with a smaller excited state 2T1 – 4T1 gap for 6-Me-mPh-Pt that derives from steric interactions with the 6-methyl group. These steric interactions reduce the excited state pairwise SQ-NN exchange coupling compared to that in mPh-Pt.}, journal={CHEMICAL SCIENCE}, author={Kirk, Martin L. and Shultz, David A. and Hewitt, Patrick and Stasiw, Daniel E. and Chen, Ju and Est, Art}, year={2021}, month={Sep} }
 @article{hewitt_shultz_kirk_2021, title={Magnetic Exchange Coupling through the Nonalternant Cyclopentadienyl pi-System of Ferrocene}, volume={23}, ISSN={["1523-7052"]}, url={https://doi.org/10.1021/acs.orglett.1c02982}, DOI={10.1021/acs.orglett.1c02982}, abstractNote={Electronic and magnetic coupling through nonalternant π-systems is an area of intense interest in photonics and molecular electronics research, yet relatively little is known regarding coupling through nonalternant π-systems. Herein we present magnetic exchange coupling in two semiquinone-based biradicals: 1,3-SQ2Fc has two semiquinone radicals attached to the one- and three-positions of the same cyclopentadienyl ligand (a nonalternant π-system) of ferrocene, whereas 1,1'-SQ2Fc has one semiquinone radical attached to each of the two cyclopentadienyl ligands of ferrocene.}, number={21}, journal={ORGANIC LETTERS}, publisher={American Chemical Society (ACS)}, author={Hewitt, Patrick and Shultz, David A. and Kirk, Martin L.}, year={2021}, month={Nov}, pages={8235–8239} }
 @article{myers_ranieri_smirnova_hewitt_peterson_quesada_lenker_stapelmann_2021, title={Measuring plasma-generated center dot OH and O atoms in liquid using EPR spectroscopy and the non-selectivity of the HTA assay}, volume={54}, ISSN={["1361-6463"]}, url={https://doi.org/10.1088/1361-6463/abd9a6}, DOI={10.1088/1361-6463/abd9a6}, abstractNote={Plasma-generated hydroxyl radicals (·OH) and oxygen atoms (O) produced by the COST reference plasma jet, a micro-scaled atmospheric pressure plasma jet, were investigated using a variety of experimental techniques. Several gas admixtures were studied to distinguish the contributions of the two reactive oxygen species. Large discrepancies between inferred aqueous ·OH densities were noted when using a 2-hydroxyterephthalic acid (HTA) fluorescence assay and electron paramagnetic resonance (EPR) measurements with the spin trap 5,5-dimethyl-1-pyrroline N-oxide—especially when oxygen was present in the feed gas. A series of follow-up experiments including optical emission spectroscopy, H2O2 quantification, and EPR measurements of atomic oxygen using the spin trap 2,2,6,6-tetramethylpiperidine, revealed that the inconsistencies between the measured aqueous ·OH were likely due to the propensity of atomic oxygen to hydroxylate TA in a manner indistinguishable from ·OH. This renders the HTA assay non-selective when both ·OH radicals and atomic oxygen are present, which we report for all three gas admixtures in our experiments. Additionally, considerable degradation of both HTA and the spin adducts measured using EPR spectroscopy was apparent, meaning actual radical densities in the plasma-treated liquid may be considerably higher than implied. Degradation rates compared favorably to previously measured gas phase densities of atomic oxygen in the predecessor of the COST jet and reported degradation of other chemical probes. These results show the prolific role of atomic oxygen in plasma-induced liquid chemistry and caution against diagnostic techniques that are unable to account for it.}, number={14}, journal={JOURNAL OF PHYSICS D-APPLIED PHYSICS}, publisher={IOP Publishing}, author={Myers, B. and Ranieri, P. and Smirnova, T. and Hewitt, P. and Peterson, D. and Quesada, M. Herrera and Lenker, E. and Stapelmann, K.}, year={2021}, month={Apr} }
 @article{kirk_shultz_chen_hewitt_daley_paudel_der est_2021, title={Metal Ion Control of Photoinduced Electron Spin Polarization in Electronic Ground States}, volume={143}, ISSN={["1520-5126"]}, url={https://doi.org/10.1021/jacs.1c04149}, DOI={10.1021/jacs.1c04149}, abstractNote={Both the sign and intensity of photoinduced electron spin polarization (ESP) in the electronic ground state doublet (2S0/D0) of chromophore-radical complexes can be controlled by changing the nature of the metal ion. The complexes consist of an organic radical (nitronyl nitroxide, NN) covalently attached to a donor-acceptor chromophore via a m-phenylene bridge, (bpy)M(CAT-m-Ph-NN) (1) (bpy = 4,4'-di-tert-butyl-2,2'-bipyridine, M = PdII (1-Pd) or PtII (1-Pt), CAT = 3-tert-butylcatecholate, m-Ph = meta-phenylene). In both complexes, photoexcitation with visible light produces an initial exchange-coupled, three-spin (bpy•-, CAT•+ = semiquinone (SQ), and NN•), charge-separated doublet 2S1 (S = chromophore excited spin singlet configuration) excited state that rapidly decays to the ground state via a 2T1 (T = chromophore excited spin triplet configuration) state. This process is not expected to be spin selective, and only very weak emissive ESP is found for 1-Pd. In contrast, strong absorptive ESP is generated in 1-Pt. It is postulated that zero-field-splitting-induced transitions between the chromophoric 2T1 and 4T1 states (1-Pd and 1-Pt) and spin-orbit-induced transitions between 2T1 and NN-based quartet states (1-Pt) account for the differences in polarization.}, number={28}, journal={JOURNAL OF THE AMERICAN CHEMICAL SOCIETY}, publisher={American Chemical Society (ACS)}, author={Kirk, Martin L. and Shultz, David A. and Chen, Ju and Hewitt, Patrick and Daley, David and Paudel, Sangita and Der Est, Art}, year={2021}, month={Jul}, pages={10519–10523} }
 @article{hewitt_shultz_kirk_2021, title={Rules for Magnetic Exchange in Azulene-Bridged Biradicals: Quo Vadis?}, volume={86}, ISSN={["1520-6904"]}, url={https://doi.org/10.1021/acs.joc.1c02085}, DOI={10.1021/acs.joc.1c02085}, abstractNote={Electronic coupling through organic bridges facilitates magnetic exchange interactions and controls electron transfer and single-molecule device electron transport. Electronic coupling through alternant π-systems (e.g., benzene) is better understood than the corresponding coupling through nonalternant π-systems (e.g., azulene). Herein, we examine the structure, spectroscopy, and magnetic exchange coupling in two biradicals (1,3-SQ2Az and 1,3-SQ-Az-NN; SQ = the zinc(II) complex of spin-1/2 semiquinone radical anion, NN = spin-1/2 nitronylnitroxide; Az = azulene) that possess nonalternant azulene π-system bridges. The SQ radical spin density in both molecules is delocalized into the Az π-system, while the NN spin is effectively localized onto the five-atom ONCNO π-system of NN radical. The spin distributions and interactions are probed by EPR spectroscopy and magnetic susceptibility measurements. We find that J = +38 cm-1 for 1,3-SQ2Az and J = +9 cm-1 for 1,3-SQ-Az-NN (H=-2JS^SQ·S^SQorNN). Our results highlight the differences in exchange coupling mediated by azulene compared to exchange coupling mediated by alternant π-systems.}, number={21}, journal={JOURNAL OF ORGANIC CHEMISTRY}, publisher={American Chemical Society (ACS)}, author={Hewitt, Patrick and Shultz, David A. and Kirk, Martin L.}, year={2021}, month={Nov}, pages={15577–15587} }
 @article{hewitt_shultz_2020, title={In Search of Stable, High-Spin Polymers}, volume={51}, ISSN={["1613-7507"]}, url={https://doi.org/10.1007/s00723-020-01293-z}, DOI={10.1007/s00723-020-01293-z}, number={11}, journal={APPLIED MAGNETIC RESONANCE}, author={Hewitt, Patrick and Shultz, David A.}, year={2020}, month={Nov}, pages={1331–1341} }