Synthesis and Optical Studies of Organoboron Compounds
Four-coordinate aryl borafluorenes with boron-oxygen and boron-nitrogen dative bondswere recently found to exhibit very large fluorescence Stokes shifts. The large Stokes shifts arehypothesized to arise from cleavage of the boron-donor dative bond in the excited state, via amechanism called bond-cleavage-induced intramolecular charge transfer (BICT). The primarygoal of this current investigation is to directly observe the BICT state by performing transientabsorption spectroscopy on four-coordinate borafluorenes which have improved optical stability.A family of four-coordinate borafluorenes was synthesized, each containing a dative B-O bondand two tert-butyl moieties on the aryl ring. Transient absorption spectroscopy (TAS) of the newborafluorenes showed existence of a single species in the excited state that is consistent with theBICT state. The TAS data is supported by fluorescence lifetime measurements and DFTcalculations.In 2016, Higuchi reported photochromic boronium-bipyridine complexes. It washypothesized that the photochromism arose from radical species that form via the photooxidationof iodide and other anions by the boronium cations. Although EPR data was provided to supportthis hypothesis, the EPR data was inconsistent with that expected for the proposed iodineradicals and boron center radicals. In the present work, boronium-bipyridine complexes withdifferent counter anions were synthesized to gain an understanding of how the anion affects thephotochromic properties of boronium-bipyridine complexes. EPR spectroscopy was employedto observe radicals generated on the complexes and further clarify the mechanism behind thephotochromic properties. The boronium-bipyridine complexes exhibited nearly identical radicalsignals to one another by EPR spectroscopy, however there was no evidence of radical formationon the anion, suggesting the reduction of the complex is occurring through a differentmechanism.The first polymer with pendant boronium-bipyridine complexes was designed andsynthesized to investigate the generation of photochromic polymer films. Electron paramagneticresonance experiments performed on the monomer complex suggested there is formation of aradical species in the solid-state following irradiation with UV light, coinciding withphotochromism. The resulting polymer lacked any photochromic characteristics, however thisallowed for better understanding of the photochromic mechanism of boronium-bipyridinecomplexes, paving a path forward to determine better designs to incorporate these complexesinto photochromic polymers.
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