Tuning Photophysical Properties in Closo-Decaborane Cluster Derivatives

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Abdulmojeed, Mustapha Babatunde
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Middle Tennessee State University
ABSTRACT Closo-decaborane ([closo-B10H10]2-) is a three-dimensional, electron rich, sigma aromatic system with orbital symmetry that allows for efficient electronic interaction with π-substituents at the apical positions. This thesis project deals with the design, synthesis, and characterization of a class of compounds with tunable photophysical properties. The prototypical system is the pyridinium [closo-B10H9-NC5H5]- anion (1a), which exhibits an intense band in the visible region that results from intramolecular electron transfer from the HOMO, predominantly localized on the cage, to the LUMO, localized on the pyridine. The level of the HOMO is controlled by substituents at the antipodal B1 boron atom (series 1), while the energy of the LUMO is changed by variation of the azine at the B10 positions (series 7). The control of the HOMO-LUMO gap leads to the control of the absorption energy. Two synthetic methods were developed for installation of nucleophiles (CN, N3, OAc, halogen, and SCN, pyridine, morpholine; series 1) at the B1 and azines (pyridine, pyridazine, pyrimidine, pyrazine; series 7) at the B10 positions, relying on selective activation of the apical B-H bond towards nucleophilic substitution via oxidation with PhI(OAc)2. Trends in substituent effects on spectroscopic properties (NMR and UV) were analyzed and compared to other [closo-B10H10]2- derivatives. The HOMO-LUMO gap as measured by the max varied from 329.5 to 368.5 nm in series 1 (0.398 eV range) and from 339.5 to 409 nm (0.621 eV range) in series 7, indicating that the system is more sensitive to perturbations of the LUMO energy.
Organic chemistry