Redox Control of Stilbylvinylpyridine Chormophore Pairwise Orientations: Towards Solid State Materials for Molecular Ele
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Steffen Zahn and James W. Canary Department of Chemistry, New York University New York, NY 10003
ABSTRACT Tripodal ligand complexes of copper salts are described that display redoxdependent exciton coupled circular dichroism spectra. Significant spectral amplitude is observed at 400 nm in both solution and microcrystalline films. The synthesis and characterization of two new ligands and the preliminary characterization of the circular dichroism properties of their copper complexes is reported. The results constitute a significant step toward the development of molecules that can be used as on/off molecular switches. INTRODUCTION The development of electrochemical switches has recently attracted much attention due to possible applications in molecular electronics applications. [1, 2, 3, 4, 5, 6, 7, 8] Redox switches require (a) components whose structures and physical properties can be turned on or off electrochemically, [3, 6, 7, 8] and (b) bistable complexes with sufficiently different optical spectra that the individual states can be addressed. [2, 4] In this paper we describe coordination complexes that show dramatically different circular dichroic spectra as a function of redox control of ligand conformation. We have studied a series of compounds as potential redox switches with the following design elements: [9] (a) coordination complexes that can accommodate single metal centers in two different oxidation states, and (b) chiral ligands bearing easily distinguished optical properties. Copper(I/II) complexes are particularly suitable due to fast rates of ligand exchange which potentiates fast signal interchange. In a previous study, we showed that the ligand (S)-N,N-bis[(2-quinolyl)methyl]-1-((2quinolyl)ethyl)amine, (S)-α-MeTQA, provided a prototype of a redox-mediated, chiroptical molecular switch.[10] This ligand was selected because of the steric hindrance presented by the 2-quinolyl groups, their strong chromophoric properties and the expected optimal chromophoric orientation provided by the propeller-like geometry of the complexes. Reduction of sterically hindered CuII complexes resulted in a change in the orientation of the arms with respect to central axis. Additionally, in the presence of SCN- as counterion, one of the quinoline arms decoordinated from the metal ion. These two factors contributed to a remarkable difference in the amplitude of the excitoncoupled circular dichroism (ECCD) signal generated from these complexes. As such spectra are critically dependent upon the distance and dihedral angle between the planar chromophores, [11] the magnitude of the ECCD spectra was expected to vary with the oxidation state of the copper ion. The obvious next step is to attach the molecule to an
electrically conductive and optically transparent surface so that the switching may be done electrochemically rather than chemically in solution. Unfortunately, the UV wavelength of the quinoline chromophore is problematic. [12] Indeed, it would be worthwhile to try to modify the chromophore so that cheaper, less en
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