Spectroscopic studies of bridge contributions to electronic coupling in a donor-bridge-acceptor biradical system

J Am Chem Soc. 2012 May 9;134(18):7812-9. doi: 10.1021/ja300233a. Epub 2012 May 1.

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).

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Benzene / chemistry*
  • Benzoquinones / chemistry*
  • Electron Transport
  • Electrons
  • Models, Molecular
  • Nitrogen Oxides / chemistry*
  • Spectrum Analysis, Raman
  • Zinc / chemistry*

Substances

  • Benzoquinones
  • Nitrogen Oxides
  • semiquinone radicals
  • nitroxyl
  • Zinc
  • Benzene