An Experimental Look into Subelectron Charge Flow Roie Yerushalmi, ² Kim K. Baldridge, ‡ and Avigdor Scherz* ,² Department of Plant Sciences, The Weizmann Institute of Science, 76100 RehoVot, Israel, and Department of Chemistry, UniVersity of California at San Diego, La Jolla, California 92093 Received May 5, 2003; E-mail: avigdor.scherz@weizmann.ac.il The prediction and measurement of charge distribution and fragmental charge flow between interacting chemical entities in complex environments is a major challenge and an urgent need for modern chemistry, biology, material sciences, and other rapidly developing molecular disciplines. 1 It encompasses information related to fundamental quantities such as the electronic chemical potential (µ e ) and hardness (η) of molecular fragments as well as their interactions with the surroundings. Advances in quantum mechanical (QM) methodologies, particularly the density functional theory (DFT), 2 and computational capabilities have enabled the detailed calculation of electronic structures and properties of large molecular systems and provide a rigorous counterpart to the more “intuitive” concepts, such as electronegativity, that have served chemists in the design of such systems for decades. However, at the very fundamental level, the concept of atomic or group charges in a molecule has not been uniquely formulated, because it is not rigorously defined within the QM postulates. 3-5 As a result, the judgment of the quality of computational predictions relies on the availability of high-precision experimental data and the interpreta- tion of related experimental observables. Furthermore, the use of computational techniques as an aid in designing large and complex molecules is practically limited. These shortcomings underscore the importance of developing experimental tools for reliable monitoring and prediction of charge flow between molecular fragments. Here, we demonstrate a novel experimental approach capable of monitoring charge distribution and fragmental charge flow between a chelated metal center and reversibly bound molecules. The experimental approach shown here utilizes the recently described “molecular potentiometer”. 6 In the demonstrated setting, the metal probe is a Ni(II) atom, and the interacting ligand molecules are changed in a modular manner (Figure 1). This includes ligands that have been systematically modified in a specific position with different functional groups while the rest of the molecular structure remains unchanged, for example, in the series: 4-picoline (5), pyridine (6), 4-bromopyridine (8), and 4-cyanopy- ridine (9). The choice of a Ni(II) metal center allows the study of all possible coordination geometries (tetra-, penta-, and hexacoor- dinated) in a systematic manner, in contrast to [Pd(II)]- or [Co- (II)]BChls, for example, where only the tetra- or pentacoordinated complexes, respectively, are observed in solution. [Ni]BChl was titrated with different ligand molecules in anhydrous acetonitrile. The resolved spectroscopic (UV-vis-NIR) band shifts of 16 [Ni]- BChl complexes (ΔEQ y , ΔEQ x , ΔEB x , and ΔEB y ) with one and two axial ligands are listed in Table 1. The charge flow (ΔQ M o ) between each ligand and the [Ni]BChl molecule was derived from the absorption spectra in solution as previously described. 6 The spectroscopic data reported here suggest that when using a particular metal center, for example, Ni(II), changes in ΔQ M o , because of different ligand molecules, can be accurately determined by measuring the energetic band shift of a single electronic transition (ΔΕQ x , Figure 1C). This result is expressed through the linear correlation shown in Figure 1C, ΔQ M o ) a*ΔEQ x + b, for the 16 complexes studied here. Thus, additional spectroscopic contributions to ΔQ M o values that originate from changes in core size are constant (mainly reflected in the Q y position 8 ). This observation agrees with our computational data for the optimized structures of the nonligated low-spin [Ni]- BChl and the high-spin (S ) 1) [Ni]BChl‚L n complexes. Following geometry optimization, charge analysis was performed for the set of 16 complexes to provide an independent computational deter- mination of fragmental charge flow (ΔN Lig , NPA). 9,10 Comparison of ΔQ M o and ΔN Lig shows excellent correlation (Figure 2, 0, R 2 ) 0.99) for the entire data. Therefore, the Q x band shift can be used for directly measuring the amount of charge transfer upon bond formation using the simple equation: ΔEQ x )R*ΔN lig (NPA) +  (Figure 2, O, R 2 ) 0.99). The need for a scaling factor when comparing the experimental (ΔQ M o ) and computational (ΔN Lig , NPA) charge flow values reflects the experimental parameters and ² Weizmann Institute of Science. ‡ University of California. Figure 1. Binding of ligand molecules (gray and red molecules) to [Ni]- BChl changes the effective charge at the nickel metal center (violet). 7 This change consequently affects the orbital energies, via electrostatic interactions with the π electrons. 8 The orbital shifts are observed in the optical band transition energy shifts. The [Ni]BChl Qx band shifts as a result of (A) one axial ligand, and (B) two axial ligands. The noncoordinated [Ni]BChl Qx band (not shown) is located at 532 ( 1 nm. (C) The amount of charge flow (ΔQ M o ) correlates linearly with the Qx energetic band shift (ΔEQx); R 2 > 0.99. Published on Web 09/26/2003 12706 9 J. AM. CHEM. SOC. 2003, 125, 12706-12707 10.1021/ja035946y CCC: $25.00 © 2003 American Chemical Society