Origin of Fluorescence from Boranils in the CrystallinePhase
2020
A small
series of boranil complexes has been studied by fluorescence
spectroscopy. Weakly fluorescent in most organic solvents at room
temperature, the target compounds display bright emission in the crystalline
phase. X-ray diffraction patterns obtained for single crystals indicate
a distorted tetrahedral geometry around the O–B–N center
with the boron atom being displaced from the plane of the heterobicyclic
ring. Consideration of the various bond lengths in comparison with
those of reference compounds indicates that the ancillary phenyl ring,
bearing different para-substituents, does not make a prominent contribution
to the molecular dipole moment in the solid state. Absorption and
fluorescence spectra recorded for the crystals remain remarkably similar
to those for liquid solutions and display large Stokes shifts. Proximity
broadening is observed in one case. The nitrophenyl derivative exhibits
additional absorption and emission bands unique to the solid state
and could be indicative of an intermolecular charge-transfer transition.
The optical properties are discussed in terms of the crystal packing
diagrams.
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