Chiral N,C-Chelate Organoboron Compounds: Photoreactivity and Optoelectronic Applications
The works described herein are broadly concerned with exploring the interactions between light and boron-containing π-conjugated systems in order to build a complete understanding of the relationship between molecular structure and reactivity. Despite rapid progress in the field of N,C- and C,C-chelate organoborate photochemistry over the past ten years, only the effects of different π-conjugated backbones have been well documented, with the impact of the aryl substituents being virtually unknown. To remedy this deficiency and harness the full potential of this class of photochromic materials, methodologies for obtaining prochiral organoboranes and their respective chiral N,C- or C,C-chelate organoborates have been developed in order to investigate the influence of boron substitution on their excited-state reactivity. Chiral N,C-chelate organoboron compounds bearing two different aryl groups at the boron center have been found to undergo regioselective photoisomerization involving the less bulky substituent exclusively, generating various highly colored base-stabilized boriranes with an H-atom on the three-membered ring. These species thermally isomerize to 4bH-azaborepins via direct H-atom transfer from boracycle to pyridine with concomitant ring expansion. Furthermore, appropriate functionalization with mesityl/heterocycle substituents (thienyl, furyl and derivatives) enables quantitative phototransformations yielding rare, chiral N,B,X-embedded heterocycles (e.g. base-stabilized 1,2-thiaborinines and 1,2-oxaborinines), which display strong blue-green to orange-red emission in the solid state. Mechanistic insights on these highly regioselective transformations were obtained via kinetic data and computational investigations on their excited-states. The effect of charge-transfer character on the photoreactivity of this class of photochromic molecules was also investigated by substituting the aryl groups of N,C-chelate organoborates with varying amine-donors. These compounds possess bright and tunable charge-transfer luminescence depending on the donor strength of the amine functionality, as well as donor-dependent photochromic switching. These new findings help elucidate the influence of electronic structure on the photoreactivity of N,C-chelate organoborates. Lastly, combining a three-coordinated boron (BMes2) moiety with a four-coordinated photochromic organoboron unit leads to a series of new diboron compounds that undergo four-state reversible color switching in response to stimuli of light, heat, and fluoride ions.
URI for this recordhttp://hdl.handle.net/1974/24477
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