@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{ribeiro_stasiw_pattison_raithby_shultz_pinheiro_2016, title={Toward Controlling the Solid State Valence Tautomeric Interconversion Character by Solvation}, volume={16}, ISSN={["1528-7505"]}, DOI={10.1021/acs.cgd.6b00159}, abstractNote={Crystals of [Co(diox)2(4-NO2-py)2] (1) and [Co(diox)2(4-CN-py)2] (2) where diox are the o-dioxolene 3,5-di-t-butylsemiquinonate (SQ•–) and/or 3,5-di-t-butylcatecholate (Cat2–) ions, 4-NO2-py is 4-nitro-pyridine, 4-CN-py is 4-cyano-pyridine, are among the few known crystals presenting both thermally induced and photoinduced ls-[M+3(SQ•–)(Cat2–)] ↔ hs-[M2+(SQ•–)2] valence tautomeric interconversion (VTI). In 2, the thermal-induced VTI is cooperative, characterizing an abrupt conversion, and in 1 it is noncooperative. In this work, crystals of [Co(diox)2(4-NO2-py)2]·benzene (1BZ), [Co(diox)2(4-NO2-py)2]·toluene (1TL), [Co(diox)2(4-CN-py)2]·benzene (2BZ), and [Co(diox)2(4-CN-py)2]·toluene (2TL) have been prepared and analyzed by single crystal X-ray diffraction in order to investigate how solvation modulates thermally induced VTI. Crystallographic data were also successfully used together with the two-state equilibrium equation to estimate ΔH° and ΔS° VTI thermodynamic parameters. The solvate crystals, like t...}, number={4}, journal={CRYSTAL GROWTH & DESIGN}, author={Ribeiro, Marcos A. and Stasiw, Daniel E. and Pattison, Philip and Raithby, Paul R. and Shultz, David A. and Pinheiro, Carlos B.}, year={2016}, month={Apr}, pages={2385–2393} }
 @article{stasiw_zhang_wang_dangi_stein_shultz_kirk_wojtas_sommer_2015, title={Determining the Conformational Landscape of a and pi Coupling Using para-Phenylene and "Aviram-Ratner" Bridges}, volume={137}, ISSN={["0002-7863"]}, DOI={10.1021/jacs.5b04629}, abstractNote={The torsional dependence of donor-bridge-acceptor (D-B-A) electronic coupling matrix elements (H(DA), determined from the magnetic exchange coupling, J) involving a spin SD = 1/2 metal semiquinone (Zn-SQ) donor and a spin S(A) = 1/2 nitronylnitroxide (NN) acceptor mediated by the σ/π-systems of para-phenylene and methyl-substituted para-phenylene bridges and by the σ-system of a bicyclo[2.2.2]octane (BCO) bridge are presented and discussed. The positions of methyl group(s) on the phenylene bridge allow for an experimentally determined evaluation of conformationally dependent (π) and conformationally independent (σ) contributions to the electronic and magnetic exchange couplings in these D-B-A biradicals at parity of D and A. The trend in the experimental magnetic exchange couplings are well described by CASSCF calculations. The torsional dependence of the pairwise exchange interactions are further illuminated in three-dimensional, "Ramachandran-type" plots that relate D-B and B-A torsions to both electronic and exchange couplings. Analysis of the magnetic data shows large variations in magnetic exchange (J ≈ 1-175 cm(-1)) and electronic coupling (H(DA) ≈ 450-6000 cm(-1)) as a function of bridge conformation relative to the donor and acceptor. This has allowed for an experimental determination of both the σ- and π-orbital contributions to the exchange and electronic couplings.}, number={29}, journal={JOURNAL OF THE AMERICAN CHEMICAL SOCIETY}, author={Stasiw, Daniel E. and Zhang, Jinyuan and Wang, Guangbin and Dangi, Ranjana and Stein, Benjamin W. and Shultz, David A. and Kirk, Martin L. and Wojtas, Lukasz and Sommer, Roger D.}, year={2015}, month={Jul}, pages={9222–9225} }
 @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} }