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 (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 i Pr 2 NBF (iPr2N = diisopropylamino) were synthesized and structurally characterized. The previously reported TipBF, ClBF (9-chloro-9-borafluorene) and t BuOBF (9-(tert-butoxy)-9-borafluorene) were also included in this study. All of the aryl borafluorenes (TipBF, TipBF(OMe) 2 , Mes F BF), and t BuOBF are moderately air-stable. Both i Pr 2 NBF 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 Mes F BF compared to the other borafluorenes. The Lewis acidities of the borafluorenes were probed using Et3PO 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 i Pr 2 NBF and t BuOBF 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.
The anionic ring-opening copolymerization of 1-(methylsulfonyl)aziridine (MsAz) and 1-(sec-butylsulfonyl)aziridine ( s BsAz) produces a soluble random copolymer P(MsAz-r-s BsAz), which can subsequently be converted to linear poly(ethylenimine) (lPEI). The copolymerization of MsAz and s BsAz is living and allows for the synthesis of copolymers with low molecular weight distributions. Sequential anionic polymerization of MsAz and s BsAz with 2-methyl-1-(methylsulfonyl)aziridine (MsMAz) creates P(MsAz-r-s BsAz)-b-P(MeMsAz). Removal of the sulfonyl groups from P(MsAz-r-s BsAz)-b-P(MsMAz) gives lPEI-b-poly(propylenimine). For the first time, lPEI can be synthesized by a controlled anionic polymerization.
The synthesis and optical characterization of 9-(2,6bis(methoxymethyl)phenyl) borafluorene (BMMP-BF) are reported. NMR spectroscopic data and single-crystal X-ray diffraction data of BMMP-BF show intramolecular chelation by the 2,6-bis(methoxymethyl)phenyl moiety via a boron−oxygen dative bond. The optical spectra of BMMP-BF are unusual in that absorption is entirely in the UV region (λ max = 284 nm), yet fluorescence occurs at 536 nm. This equates to a Stokes shift of 2.05 eV (16 500 cm −1 ) and is among the highest Stokes shifts ever reported for a small molecule. Density functional theory (DFT) calculations show that the boron−oxygen dative bond in BMMP-BF is ruptured in the excited state and that emission occurs from a trigonal planar boron geometry. This bond cleavage and the concurrent planarization of the boron center are responsible for the high Stokes shift. Two borafluorenes related to BMMP-BF were also examined: 9-(2,6-bis((methylthio)methyl)phenyl) borafluorene (BMTMP-BF) and 9-(2,6-bis(tert-butoxymethyl)phenyl) borafluorene (B t BuMP-BF). Both BMTMP-BF and B t BuMP-BF have optical properties similar to those of BMMP-BF with remarkably large Stokes shifts. Finally, BMMP-BF-(2T) 2 , which possesses bithiophene moieties on the 2 and 7 positions of a BMMP-BF core, was also synthesized and studied. The absorption spectrum of BMMP-BF-(2T) 2 is red-shifted compared to BMMP-BF. BMMP-BF-(2T) 2 was found to exhibit dual emissions rather than the single, high Stokes shift emission of BMMP-BF. DFT calculations suggest that the dual emissions of BMMP-BF-(2T) 2 arise due to radiative relaxation from two different structures in the excited state.
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