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.

Variable temperature electronic absorption and resonance Raman spectroscopies are used to probe the excited state electronic structure of TpCum,MeZn(SQ-Ph-NN) (1) – a donor-bridge-acceptor (D-B-A) biradical complex and a ground state analog of the charge-separated excited state formed in photoinduced electron transfer reactions. Strong ferromagnetic exchange coupling that is mediated by the para-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 localized phenylene bridge LUMO wavefunction 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, Hab.

The preparation and characterization of new model complexes for the molybdenum cofactor are reported. The new models are distinctive for the inclusion of pterin-substituted dithiolene chelates and have the formulation Tp*MoX(pterin-R-dithiolene) (Tp* = tris(3,5,-dimethylpyrazolyl)borate), X= O, S, R= aryl or −C(OH)(CH 3 ) 2 ). Syntheses of Mo(4+) and (5+) complexes of two pterin-dithiolene derivatives as both oxo and sulfido compounds, and improved syntheses for pterinyl alkynes and [Et 4 N][Tp*Mo IV (S)S 4 ] reagents are described. Characterization methods include electrospray ionization mass spectrometry, electrochemistry, infrared spectroscopy, electron paramagnetic resonance and magnetic circular dichroism. Cyclic voltammetry reveals that the Mo(5+/4+) reduction potential is intermediate between that for dithiolene with electron-withdrawing substituents and simple dithiolate chelates. Electron paramagnetic resonance and magnetic circular dichroism of Mo(5+) complexes where X = O, R = aryl indicates that the molybdenum environment in the new models is electronically similar to that in Tp*MoO(benzenedithiolate).

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.