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 Mes(F)BF (Mes(F) = 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 (Pr2NBF)-Pr-i ((Pr2N)-Pr-i = diisopropylamino) were synthesized and structurally characterized. The previously reported TipBF, ClBF (9-chloro-9-borafluorene) and (BuOBF)-Bu-t (9-(tert-butoxy)-9-borafluorene) were also included in this study. All of the aryl borafluorenes (TipBF, TipBF(OMe)(2), Mes(F)BF), and (BuOBF)-Bu-t are moderately air-stable. Both (Pr2NBF)-Pr-i 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 Mes(F) 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 Mes(F), Cl, and methoxy groups only cause subtle changes to the optical properties of the borafluorenes. The absorption spectra of both (Pr2NBF)-Pr-i and (BuOBF)-Bu-t are blue-shifted due to substituent pi-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.