Spectroscopic Studies of Bridge Contributions to Electronic Coupling in a Donor-Bridge-Acceptor Biradical System

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• 作者：Martin L. Kirk ; David A. Shultz ; Ezra C. Depperman ; Diana Habel-Rodriguez ; Robert D. Schmidt
• 刊名：The Journal of the American Chemical Society
• 出版年：2012
• 出版时间：May 9, 2012
• 年：2012
• 卷：134
• 期：18
• 页码：7812-7819
• 全文大小：424K
• 年卷期：v.134,no.18(May 9, 2012)
• ISSN：1520-5126

文摘

Variable-temperature electronic absorption and resonance Raman spectroscopies are used to probe the excited state electronic structure of Tp^Cum,MeZn(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(蟺)_SOMO 鈫?NN-Ph (蟺*)_LUMO D 鈫?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 鈫?A interaction given by the magnitude of the electronic coupling matrix element, H_ab.