Orbital-Resolved Partial Charge Transfer from the Methoxy Groups of Substituted Pyrenes in Complexes with Tetracyanoquinodimethane鈥擜 NEXAFS Study

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• 作者：Katerina Medjanik ; Dennis Chercka ; Peter Nagel ; Michael Merz ; Stefan Schuppler ; Martin Baumgarten ; Klaus M眉llen ; Sergej A. Nepijko ; Hans-Joachim Elmers ; Gerd Sch枚nhense ; Harald O. Jeschke ; Roser Valenti
• 刊名：The Journal of the American Chemical Society
• 出版年：2012
• 出版时间：March 14, 2012
• 年：2012
• 卷：134
• 期：10
• 页码：4694-4699
• 全文大小：389K
• 年卷期：v.134,no.10(March 14, 2012)
• ISSN：1520-5126

文摘

It is demonstrated that the near-edge X-ray absorption fine structure (NEXAFS) provides a powerful local probe of functional groups in novel charge transfer (CT) compounds and their electronic properties. Microcrystals of tetra-/hexamethoxypyrene as donors with the strong acceptor tetracyano-p-quinodimethane (TMP/HMP-TCNQ) were grown by vapor diffusion. The oxygen and nitrogen K-edge spectra are spectroscopic fingerprints of the functional groups in the donor and acceptor moieties, respectively. The orbital selectivity of the NEXAFS pre-edge resonances allows us to precisely elucidate the participation of specific orbitals in the charge transfer process. Upon complex formation, the intensities of several resonances change substantially and a new resonance occurs in the oxygen K-edge spectrum. This gives evidence of a corresponding change of hybridization of specific orbitals in the functional groups of the donor (those derived from the frontier orbitals 2e and 6a₁ of the isolated methoxy group) and acceptor (orbitals b_3g, a_u, b_1g, and b_2u, all located at the cyano group) with 蟺*-orbitals of the ring systems. Along with this intensity effect, the resonance positions associated with the oxygen K-edge (donor) and nitrogen K-edge (acceptor) shift to higher and lower photon energies in the complex, respectively. A calculation based on density functional theory qualitatively explains the experimental results. NEXAFS measurements shine light on the action of the functional groups and elucidate charge transfer on a submolecular level.