@article{HeRauchFriedrichetal.2019,
  author    = {He, Jiang and Rauch, Florian and Friedrich, Alexandra and Sieh, Daniel and Ribbeck, Tatjana and Krummenacher, Ivo and Braunschweig, Holger and Finze, Maik and Marder, Todd B.},
  title     = {N-Heterocyclic Olefins as Electron Donors in Combination with Triarylborane Acceptors: Synthesis, Optical and Electronic Properties of D-π-A Compounds},
  series = {Chemistry - A European Journal},
  volume    = {25},
  journal   = {Chemistry - A European Journal},
  doi       = {10.1002/chem.201903118},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-204690},
  pages     = {13777-13784},
  year      = {2019},
  abstract  = {N-heterocyclic olefins (NHOs), relatives of N-heterocyclic carbenes (NHCs), exhibit high nucleophilicity and soft Lewis basic character. To investigate their π-electron donating ability, NHOs were attached to triarylborane π-acceptors (A) giving donor (D)-π-A compounds 1-3. In addition, an enamine π-donor analogue (4) was synthesized for comparison. UV-visible absorption studies show a larger red shift for the NHO-containing boranes than for the enamine analogue, a relative of cyclic (alkyl)(amino) carbenes (CAACs). Solvent-dependent emission studies indicate that 1-4 have moderate intramolecular charge-transfer (ICT) behavior. Electrochemical investigations reveal that the NHO-containing boranes have extremely low reversible oxidation potentials (e.g., for 3, \(E^{ox}_{1/2}\) =-0.40 V vs. ferrocene/ferrocenium, Fc/Fc\(^+\), in THF). Time-dependent (TD) DFT calculations show that the HOMOs of 1-3 are much more destabilized than that of the enamine-containing 4, which confirms the stronger donating ability of NHOs.},
  language  = {en}
}
@article{HeRauchFriedrichetal.2021,
  author    = {He, Jiang and Rauch, Florian and Friedrich, Alexandra and Krebs, Johannes and Krummenacher, Ivo and Bertermann, R{\"u}diger and Nitsch, J{\"o}rn and Braunschweig, Holger and Finze, Maik and Marder, Todd B.},
  title     = {Phenylpyridyl-fused boroles: a unique coordination mode and weak B-N coordination-induced dual fluorescence},
  series = {Angewandte Chemie International Edition},
  volume    = {60},
  journal   = {Angewandte Chemie International Edition},
  number    = {9},
  doi       = {10.1002/anie.202013692},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-256423},
  pages     = {4833-4840},
  year      = {2021},
  abstract  = {Phenylpyridyl-fused boroles [TipPBB1]\(_4\) and TipPBB2 were synthesized and their properties investigated. [TipPBB1]\(_4\) forms a tetramer in both the solid state and solution. TipPBB2 contains a 4-coordinate boron atom in the solid state but dissociates to give a 3-coordinate boron species in solution. TipPBB2 shows interesting temperature-dependent dual fluorescence in solution because of the equilibrium between 3- and 4-coordinate boron species due to weak N⋅⋅⋅B intermolecular coordination.},
  language  = {en}
}
@phdthesis{He2020,
  author    = {He, Jiang},
  title     = {Studies of N-heterocyclic olefins as donors in triarylboranes and electron-poor phenylpyridyl-fused boroles},
  doi       = {10.25972/OPUS-21717},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-217175},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2020},
  abstract  = {Chapter 1 N-Heterocyclic olefins (NHOs), relatives of N-heterocyclic carbenes (NHCs), exhibit high nucleophilicity and soft Lewis basic character. To investigate their π-electron donating ability, NHOs were attached to triarylborane π-acceptors (A) giving donor(D)-π-A compounds 1-3. In addition, an enamine π-donor analogue (4) was synthesized for comparison. UV-visible absorption studies show a larger red shift for the NHO-containing boranes than for the enamine analogue, a relative of a CAAC. The red shifted absorption of NHO-containing boranes indicate smaller energy gaps of NHO-containing boranes than CAAC-containing boranes. Solvent-dependent emission studies indicate that 1-4 have moderate intramolecular charge transfer (ICT) behavior. Electrochemical investigations reveal that the NHO-containing boranes have extremely low reversible oxidation potentials (e.g., for 3, E1/2ox = -0.40 V vs. Fc/Fc+ in THF) which indicate the electron rich property of NHOs. Furthermore, TD-DFT calculations were carried out on these four D-π-A boranes. The results show that the LUMOs of 1-4 only show a small difference, but the HOMOs of 1-3 are much more destabilized than that of the enamine-containing 4, which is in agreement with the electrochemical investigations and confirms the stronger donating ability of NHOs. Chapter 2 Since the beginning of this century, the chemistry of (hetero)arene-fused boroles has attracted increasing interest. (Hetero)arene-fused boroles exhibit strong Lewis acidity, distinct fluorescence properties, strong electron accepting abilities, etc. However, their chemistry been only very briefly reviewed either as part of reviews on "free" boroles or on boron-doped polycyclic aromatic hydrocarbons (PAHs). In this chapter, we addressed the chemistry of (hetero)arene-fused boroles from fundamentals to their widely varying applications. It includes: 1) Synthetic methodology  Both historical and recently developed strategies for the synthesis of fused boroles. 2) Stabilities  A comparison of different kinetic protection strategies. 3) 9-Borafluorenes with a fluorinated backbone  Application as Lewis acids, forming ion pairs with Cp2Zr(CH3)2 and applied as activators for polymerization, activators of H2, and other related applications. 4) Donor-acceptor 9-borafluorenes  Applications as F- "turn on" sensors, potential applications as electron accepting units for organic (opto)electronics, bipolar transporting materials, TADF materials, and different functionalization strategies. 5) Heteroarene-fused boroles  Enhanced antiaromaticity, unique coordination mode and their interesting properties. 6) Intramolecular dative bonding in 9-borafluorenes  Bond-cleavage-induced intramolecular charge transfer (BICT), BICT-induced large Stoke shifts and dual emissions, application as a ratiometric sensor. 7) 9-Borafluorene-based main chain polymers  Application in polymer chemistry and their distinct properties, e.g., as a sensor for gaseous NH3. 8) Electrochemistry  A comparison of electron-accepting ability of different functionalized fused boroles through electrochemical studies. 9) Chemical reduction of fused boroles  Stable radical anions and dianions of fused boroles and their properties. 10) Three-coordinate borafluorenium cations  Cationic 9-borafluorenes and their interesting properties, e.g., in THF, reversible thermal colour switching properties. Finally, a conclusion and outlook regarding the chemistry, properties and applications, and suggestions for areas which require further study was provided.   Chapter 3 Interested in fusing electron-poor arene onto boroles, two electron-poor phenylpyridyl-fused boroles, [TipPBB1]4 and TipPBB2 were prepared. [TipPBB1]4 is a white solid adopting a unique coordination mode, which forming a tetramer with a cavity in both the solid state and solution (1H DOSY). The boron center of TipPBB2 is 4-coordinate in the solid state, evidenced by a solid-state 11B{1H} RSHE/MAS NMR study, but the system dissociates in solution, leading to 3-coordinate borole species. [TipPBB1]4 exhibits two reduction processes which are attributed to the phenylpyridyl cores. TipPBB2 also exhibits two reduction processes with the first half-reduction potential of E1/2red = -1.94 V. The electron accepting ability of TipPBB2 is largely enhanced and comparable to that of FMesBf. This enhanced electron accepting ability is attributed to the electron withdrawing property of the pyridyl group. TipPBB2 exhibits concentration- and temperature-dependent dual fluorescence in solution. With the temperature is lowered, the emission intensity decreases (Figure 6.4, left). We suggested that the dual fluorescence is caused by an equilibrium between 3-coordinate TipPBB2 and a weak intermolecular adduct of TipPBB2 via a B-N bond. This hypothesis was further supported by lifetime measurements at different concentrations, low temperature excitation spectra low temperature 1H NMR spectra and lifetime measurements upon addition of DMAP to a solution of TipPBB2 to simulate the 4-coordiante TipPBB2 species. Interestingly, the ratio of the relative percentages of the two lifetimes shows a linear relationship with temperature; thus, TipPBB2 could serve as a fluorescent thermometer. Furthermore, theoretical studies were carried out on TipPBB2, and two models, ((BMe3)TipPBB1(NMe3) and (BMe3)TipPBB2(NMe3)), which utilize a BMe3 group as the Lewis acid coordinated to pyridine and an NMe3 group as the Lewis base coordinated to the boron center of the borole, were used to simulate the [TipPBB1]4 and intermolecular 4-coordinate TipPBB2, respectively. Theoretical studies indicate that the HOMO of TipPBB2 is located at the Tip group, which is in contrast to its borafluorene derivatives for which the HOMOs are located on the borafluorene cores. Chapter 4 Two derivatives of phenylpyridyl-fused boroles were prepared via functionalization of the pyridyl groups in two different directions, namely an electron-rich dihydropyridine moiety (compound 10) and an electron-deficient N-methylpyridinium cation (compound 11). Both compounds were fully characterized. The 11B NMR signal of compound 10 was observed at 58.8 ppm in CDCl3, which suggests strong conjugation between the boron atom and dihydropyridine moiety. Compound 11 shows a reversible coordination to THF which was confirmed by NMR studies. Compared to other 2,4,6-triisopropylphenyl protected 9-borafluorenes which only coordinate to CH3CN or DMF, the coordination of the weaker and bulkier THF to compound 11 indicates an extremely electron-deficient boron center in compound 11. The electron-rich property of the dihydropyridine moiety of compound 10 was confirmed by its oxidation potential (Epc = +0.37 V). Due to the strong conjugation of the dihydropyridine moiety with the boron atom, the reduction potential of compound 10 shifts cathodically and is more negative than -2.5 V. Compound 11 exhibits three reduction processes with the first reversible reduction potential at Ered1/2 = -1.23 V, which is significantly anodically shifted compared to that of its precursor (TipPBB2) or its framework 1-methyl-2-phenylpyridin-1-ium triflate (12). This significantly anodically shifted reduction potential confirms an extremely electron-deficient property of compound 11. Photophysical studies indicate that the lowest energy transition of compound 10 is more likely a locally-excited (LE) transition and compound 11 exhibits a polarized ground state. Furthermore, we performed theoretical studies for both compounds. The electron cloud distribution of the HOMO of compound 10 supports the strong conjugation between the boron atom and the dihydropyridine moiety in the ground state. An extremely low LUMO energy was determined by theoretical studies which confirmed the extremely electron-deficient property of compound 11.   Chapter 5 Inspired by the enhancement of electron accepting ability with increasing numbers of electron withdrawing groups at boron, we tried to study the properties of a bis(pyridyl)arylboranes. In our attempt to synthesize a bis(pyridyl)arylborane, we obtained a bis(2-pyridyl)methoxyborate Li+ complex which is as a dimer both in solution and the solid state. In the solid state, compound [16]2 is a dimer containing two bis(2-pyridyl)methoxyborate which are linked by two lithium cations. Each lithium cation coordinates to one methoxy group and two pyridyl groups, one from each of the two bis(2-pyridyl)methoxyborate anions. The parameters of [16]2 were compared with other bis(2-pyridyl)methoxyborate stabilized Pt(IV) complex, bis(2-pyridyl)hydroxylborate stabilized Ru(II) complex and the dimer of EtAl(OMe)(2-pyridyl)2Li. To confirm the coordination mode in solution, 1H DOSY spectroscopy was carried out in CD2Cl2. The van der Waals radius obtained by 1H DOSY nicely matches with the result from the solid state and thus proves the dimer of 16 is persistent in solution. Finally, different Lewis acids (e.g., TMSCl, BF3•Et2O, AlCl3, HCl) were used to attempt to detach the methoxy group of [16]2. However, we observed either decomposition or selective cleavage of the Tip group, or no reaction at all, rather than cleavage of the methoxy group from boron.},
  subject      = {Triarylborane},
  language  = {en}
}
@article{ChenMengLiaoetal.2021,
  author    = {Chen, Xing and Meng, Guoyun and Liao, Guanming and Rauch, Florian and He, Jiang and Friedrich, Alexandra and Marder, Todd B. and Wang, Nan and Chen, Pangkuan and Wang, Suning and Yin, Xiaodong},
  title     = {Highly Emissive 9-Borafluorene Derivatives: Synthesis, Photophysical Properties and Device Fabrication},
  series = {Chemistry—A European Journal},
  volume    = {27},
  journal   = {Chemistry—A European Journal},
  number    = {20},
  doi       = {10.1002/chem.202005185},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-256738},
  pages     = {6274-6282},
  year      = {2021},
  abstract  = {A series of 9-borafluorene derivatives, functionalised with electron-donating groups, have been prepared. Some of these 9-borafluorene compounds exhibit strong yellowish emission in solution and in the solid state with relatively high quantum yields (up to 73.6 \% for FMesB-Cz as a neat film). The results suggest that the highly twisted donor groups suppress charge transfer, but the intrinsic photophysical properties of the 9-borafluorene systems remain. The new compounds showed enhanced stability towards the atmosphere, and exhibited excellent thermal stability, revealing their potential for application in materials science. Organic light-emitting diode (OLED) devices were fabricated with two of the highly emissive compounds, and they exhibited strong yellow-greenish electroluminescence, with a maximum luminance intensity of >22 000 cd m\(^{-2}\). These are the first two examples of 9-borafluorene derivatives being used as light-emitting materials in OLED devices, and they have enabled us to achieve a balance between maintaining their intrinsic properties while improving their stability.},
  language  = {en}
}