Substituent Effects on the Properties of Borafluorenes

A series
of substituted 9-borafluorenes were studied both experimentally
and computationally in order to assess substituent effects on the
optical and electronic properties and the stability of 9-borafluorenes.
The previously unknown 9-substituted-9-borafluorenes MesFBF (MesF = 2,4,6-tris­(trifluoromethyl)­phenyl), TipBF­(OMe)2 (Tip = 2,4,6-tris­(triisopropyl)­phenyl,
(OMe)2= methoxy at the borafluorene 3 and 6 positions),
and iPr2NBF (iPr2N = diisopropylamino)
were synthesized and structurally characterized. The previously reported TipBF, ClBF (9-chloro-9-borafluorene) and tBuOBF (9-(tert-butoxy)-9-borafluorene) were also included in this study. All of
the aryl borafluorenes (TipBF, TipBF­(OMe)2, MesFBF), and tBuOBF are moderately air-stable. Both iPr2NBF and ClBF degrade
rapidly in air. Cyclic voltammogram measurements and density functional
theory (DFT) calculations reveal that (a) borafluorenes have higher
electron affinities relative to comparable boranes and (b) substituents
have a strong influence on the lowest unoccupied molecular orbital
(LUMO) levels of borafluorenes but less influence over the highest
occupied molecular orbital (HOMO) levels. The DFT calculations show
that, in general, borafluorenes exhibit low electron reorganization
energies, a predictor of good electron mobility. However, the MesF group, which is finding popularity as a stabilizing group
in borane chemistry, significantly increases the electron reorganization
energy of MesFBF compared to the other borafluorenes. The Lewis acidities of the
borafluorenes were probed using Et3PO as a Lewis
base (the Gutmann–Beckett method) and found to be dictated
primarily by steric considerations. Calculated fluoride affinities
(Lewis acidities) correlate with the LUMO energies of the borafluorenes.
UV–visible and fluorescence spectroscopic measurements showed
that compared to the Tip substituent, the MesF, Cl, and
methoxy groups only cause subtle changes to the optical properties
of the borafluorenes. The absorption spectra of both iPr2NBF and tBuOBF are blue-shifted due to substituent
π-backbonding with the p-orbital on boron. The results of this
study provide insights into substituent effects on conjugated boron
systems and will help in the design of future boron containing materials.