title: Spectroscopic studies of bridge contributions to electronic coupling in a donor-bridge-acceptor biradical system.
abstract: 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).
authors.affiliation: Department of Chemistry and Chemical Biology, The University of New Mexico, Albuquerque, New Mexico 87131-0001, USA. mkirk@unm.edu
authors.forename: Martin L
authors.initials: ML
authors.lastname: Kirk
authors.forename: David A
authors.initials: DA
authors.lastname: Shultz
authors.forename: Ezra C
authors.initials: EC
authors.lastname: Depperman
authors.forename: Diana
authors.initials: D
authors.lastname: Habel-Rodriguez
authors.forename: Robert D
authors.initials: RD
authors.lastname: Schmidt
mesh_terms: D001554:Benzene / Q000737:chemistry*; D016227:Benzoquinones / Q000737:chemistry*; D004579:Electron Transport; D004583:Electrons; D008958:Models, Molecular; D009589:Nitrogen Oxides / Q000737:chemistry*; D013059:Spectrum Analysis, Raman; D015032:Zinc / Q000737:chemistry*
chemical_list: D016227:Benzoquinones; D009589:Nitrogen Oxides; C025232:semiquinone radicals; C039900:nitroxyl; D015032:Zinc; D001554:Benzene