Charge Localization in Isolated Mixed-Valence Complexes: An STM and Theoretical Study Yuhui Lu, † Rebecca Quardokus, ‡ Craig S. Lent, † Frederic Justaud, § Claude Lapinte, § and S. Alex Kandel* ,‡ Department of Electrical Engineering and Department of Chemistry and Biochemistry, UniVersity of Notre Dame, Notre Dame, Indiana 46556, and Sciences Chimiques de Rennes, UniVersite ´ de Rennes 1, UMR CNRS 6226, Campus de Beaulieu, F-35042 Rennes, France Received July 6, 2010; E-mail: skandel@nd.edu Abstract: {Cp*(dppe)Fe(CtC-)} 2 (1,3-C 6 H 4 ) is studied both as a neutral molecule, Fe(II)-Fe(II), and as a mixed-valence complex, Fe(II)-Fe(III). Scanning tunneling microscopy (STM) is used to image these species at 77 K under ultrahigh-vacuum conditions. The neutral molecule Fe(II)-Fe(II) has a symmetric, “dumbbell” appearance in STM images, while the mixed-valence complex Fe(II)-Fe(III) demonstrates an asymmetric, bright-dim double-dot structure. This asymmetry results from localization of the electron to one of the iron- ligand centers, a result which is confirmed through comparison to theoretical STM images calculated using constrained density-functional theory (CDFT). The observation of charge localization in mixed-valence complexes outside of the solution environment opens up new avenues for the control and patterning of charge on surfaces, with potential applications in smart materials and molecular electronic devices. I. Introduction Beginning with the work of Creutz and Taube 1,2 in 1969, there has been extensive research into the chemistry and physics of mixed-valence compounds. These molecular species have multiple oxidation-reduction (redox) centers, and electron transfer between these centers results in more than one energeti- cally accessible electronic state for the molecule. These compounds are ideal candidates for the study of intramolecular electron and energy transfer. 3 Furthermore, mixed-valence complexes can be conductive and can exhibit significant nonlinear optical and magnetic properties, resulting in potential applications in molecular electronics. 4-6 The electronic proper- ties of mixed-valence compounds strongly depend on the extent of the electronic interaction between the redox centers and range, in accordance with Robin-Day classification, 7 from small (class I), to slight (class II), to strong (class III) interactions. Various experimental techniques including observation of the interval- ence transfer (IT) band, 8 electron spin resonance, 9 and ultraviolet photoelectron spectroscopy have been used to probe such interactions; there is also an extensive theoretical literature. 10,11 In recent years, the investigation of the properties of isolated, individual molecules has been greatly facilitated by scanning tunneling microscopy (STM). STM has been used to study the adsorption site and geometry of molecules on surfaces, 12,13 to obtain high-resolution images of intramolecular structure, 14,15 to investigate the local electronic properties of individual molecules, 16-22 and to detect different spin states of transition- metal complexes. 23 In this work, we use STM to observe charge localization within a mixed-valence complex. Each mixed-valence molecule has two electronic states which, in the isolated molecule, are degenerate: they have the same energy but are spatially distinct. Small perturbations in the environment mix these states to produce a highly asymmetric electron distribution, which can then be probed using STM. 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