Donor-Acceptor Biradicals as Ground State Analogues of Photoinduced Charge Separated States
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- 作者：Martin L. Kirk ; David A. Shultz ; Ezra C. Depperman ; Candice L. Brannen
- 刊名：Journal of the American Chemical Society
- 出版年：2007
- 出版时间：February 21, 2007
- 年：2007
- 卷：129
- 期：7
- 页码：1937 - 1943
- 全文大小：282K
- 年卷期：v.129,no.7(February 21, 2007)
- ISSN：1520-5126
文摘
A Valence Bond Configuration Interaction (VBCI) model is used to relate the intraligand magneticexchange interaction (J) to the electronic coupling matrix element (HAB) in TpCum,MeZn(SQNN), a compoundthat possesses a Donor-Acceptor (D-A) SemiQuinone-NitronylNitroxide (SQNN) biradical ligand. Withinthis framework, an SQ 
NN charge transfer state mixes with the ground state and stabilizes the spintriplet (S = 1). This charge-transfer transition is observed spectroscopically and probed using resonanceRaman spectroscopy. In addition, the temperature-dependent electronic absorption spectrum of the Ni(II)complex, TpCum,MeNi(SQNN), has been studied. Exchange coupling between the S = 1 Ni(II) ion and S =1 SQNN provides a mechanism for observing the formally spin-forbidden, ligand-based 3GC 1CTCtransition. This provides a means of determining U, the mean GC CTC energy, and a one-center exchangeintegral, K0. The experimental determination of J, U, and K0 permits facile calculation of HAB, and we showthat this methodology can be extended to determine the electronic coupling matrix element in related SQ-Bridge-NN molecules. As magnetic susceptibility measurements are easily acquired in the solid state,HAB may be effectively determined for single molecules in a known geometry, provided a crystal structureexists for the biradical complex. Thus, SQ-Bridge-NN molecules possess considerable potential for probingboth geometric and electronic structure contributions to the magnitude of the electronic coupling matrixelement associated with a given bridge fragment.
NN charge transfer state mixes with the ground state and stabilizes the spintriplet (S = 1). This charge-transfer transition is observed spectroscopically and probed using resonanceRaman spectroscopy. In addition, the temperature-dependent electronic absorption spectrum of the Ni(II)complex, TpCum,MeNi(SQNN), has been studied. Exchange coupling between the S = 1 Ni(II) ion and S =1 SQNN provides a mechanism for observing the formally spin-forbidden, ligand-based 3GC 1CTCtransition. This provides a means of determining U, the mean GC CTC energy, and a one-center exchangeintegral, K0. The experimental determination of J, U, and K0 permits facile calculation of HAB, and we showthat this methodology can be extended to determine the electronic coupling matrix element in related SQ-Bridge-NN molecules. As magnetic susceptibility measurements are easily acquired in the solid state,HAB may be effectively determined for single molecules in a known geometry, provided a crystal structureexists for the biradical complex. Thus, SQ-Bridge-NN molecules possess considerable potential for probingboth geometric and electronic structure contributions to the magnitude of the electronic coupling matrixelement associated with a given bridge fragment.