Add time:07/17/2019         Source:sciencedirect.com

The ground state electronic structure of boron, zinc and aluminum complexes of 3,5-di-tert-butylsalicylic acid (t-BSA) was studied by fluorescence spectroscopy. These compounds are negative charge control additives (CCAs) commercially known as LR120, Bontron E84 and Bontron E88 respectively. They are synthesized by condensing the corresponding metal salts with t-BSA and are primarily characterized by elemental analysis. Spectroscopic analysis, by IR and 13C nuclear magnetic resonance (NMR), fails to provide adequate structural information on these materials. Are they salts or chelate complexes? Previous studies in our laboratory have shown that lithium 3,5-di-tert-butylsalicylate (Lit-BSA) exhibits a characteristic, long-wavelength fluorescence. The Stokes shift (from peak to peak) is approximately 9300 cm−1. In this work, we use the fluorescence of Lit-BSA as a probe of the ionicity of the dibutylsalicylato groups in LR120, Bontron E84 and Bontron E88 (in the ground state). Comparison of the fluorescence spectra reveals that the dibutylsalicylato groups in LR120 are not ionic. In fact, they resemble an ester. We conclude that LR120 is a cyclic boron ester and that the BO bond is covalent. The comparison also reveals that the dibutylsalicylato groups in Bontron E84 and Bontron E88 are ionic in both polar and non-polar solvents. The strong ionic character in non-polar solvents, which differs from that of Lit-BSA, suggests that Bontron E84 and Bontron E88 are chelate complexes and that the di-tert-butylsalicylato groups are bidentate ligands for the Zn2+ and Al3+ ions respectively. The technological implications of the results obtained on the toner charging mechanism are discussed.

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