The replacement of C=C units by their isoelectronic and isosteric B=N units (BN/CC isosterism) in π‐conjugated organic compounds, as a strategy to produce novel organic–inorganic hybrid materials, ...has recently been successfully transferred to π‐conjugated polymers. This Concept provides an overview of the recent advances in this quickly evolving field, with a focus on synthesis, photophysical and electrochemical properties of the new polymers and related oligomers, as well as possible future applications in organic electronics and optoelectronics.
The strategy to create novel organic–inorganic hybrid materials by isoelectronic substitution of B=N for C=C units has recently been successfully transferred to π‐conjugated polymers. The advances in this quickly evolving field are discussed, with a focus on synthesis and electronic properties of the materials produced, as well as possible future applications in organic electronics.
Access to dithiophene‐fused oxadiborepins and the first azadiborepins attained via a modular synthesis route are presented. The new compounds emit intense blue light, some of which demonstrate ...fluorescence quantum yields close to unity. Cyclic voltammetry (CV) revealed electrochemically reversible one‐electron reduction processes. The weak aromatic character of the novel 1,2,7‐azadiborepin ring is demonstrated with in‐depth theoretical investigations using nucleus‐independent chemical shift (NICS) scans and anisotropy of the induced current density (ACID) calculations.
Dithienooxa‐ and ‐azadiborepins are accessible in a modular way from a common precursor. These BNB‐doped polycyclic aromatic hydrocarbons (PAHs) are highly efficient blue‐light emitters with quantum yields up to 96 %. In‐depth theoretical studies reveal that the previously unknown azadiborepin ring system is weakly aromatic.
The application of our newly developed B–C coupling method by catalytic Si/B exchange is demonstrated for the synthesis of a series of triarylboranes (1), monodisperse thienyl‐ and furylborane dimers ...(2) and trimers (9), extended oligomers (3) and polymers (3′), as well as mixed (oligo)thienyl‐/furylboranes. The structures of 1 aaTip, 1 bbTip, and 2 bbbMes*, determined by X‐ray crystallography, reveal largely coplanar hetarene rings and BR3 environments, which are most pronounced in the furylborane species. Photophysical investigations, supported by TD‐DFT calculations, revealed pronounced π‐electron delocalization over the hetarene backbones including the boron centers. With an extended series of derivatives of varying chain lengths available, we were able to determine the effective conjugation lengths (ECL) of poly(thienylborane)s and poly(furylborane)s, which have been reached with the highest‐molecular‐weight derivatives of our study. Through variation of the furan‐to‐thiophene ratio, the photophysical properties of these materials are effectively modulated. Significantly, higher furan contents lead to considerably increased fluorescence intensities. Compounds 1 aaTip, 1 bbTip, and 3 aTip showed the ability to bind fluoride anions. The binding process is signaled by a distinct change in their optical absorption characteristics, thus rendering these materials attractive targets for sensory applications.
Ob‐La‐B Ob‐La‐Si: Catalytic Si/B exchange polycondensation provided access to a series of thienyl‐ and furylboranes, monodisperse oligomers, and polymers. The photophysical properties of such species are effectively modulated through variation of the ratio of the heterocycles in the backbone (see scheme).
Fully aromatic, luminescent, and highly robust BNB-doped phenalenyls have been prepared, which are isoelectronic to the phenalenyl cation.
B
-Fluoromesityl-substitution leads to fluorescence in an ...extremely narrow range and significant increase in the reduction potential. Detailed theoretical investigations revealed an intramolecular aromaticity switch upon one-electron reduction.
Fully aromatic, fluorescent, and highly robust BNB-doped phenalenyls were prepared
via
ring expansion, which undergo an aromaticity switch upon reduction.
The kinetics of the metal-free hydrogen transfer from amine–borane Me2NH·BH3 to aminoborane iPr2NBH2, yielding iPr2NH·BH3 and cyclodiborazane Me2N-BH22 via transient Me2NBH2, have been investigated ...in detail, with further information derived from isotopic labeling and DFT computations. The approach of the system toward equilibrium was monitored in both directions by 11B{1H} NMR spectroscopy in a range of solvents and at variable temperatures in THF. Simulation of the resulting temporal–concentration data according to a simple two-stage hydrogen transfer/dimerization process yielded the rate constants and thermodynamic parameters attending both equilibria. At ambient temperature, the bimolecular hydrogen transfer is slightly endergonic in the forward direction (ΔG 1°(295) = 10 ± 7 kJ·mol–1; ΔG 1 ⧧ (295) = 91 ± 5 kJ·mol–1), with the overall equilibrium being driven forward by the subsequent exergonic dimerization of the aminoborane Me2NBH2 (ΔG 2°(295) = −28 ± 14 kJ·mol–1). Systematic deuterium labeling of the NH and BH moieties in Me2NH·BH3 and iPr2NBH2 allowed the kinetic isotope effects (KIEs) attending the hydrogen transfer to be determined. A small inverse KIE at boron (k H/k D = 0.9 ± 0.2) and a large normal KIE at nitrogen (k H/k D = 6.7 ± 0.9) are consistent with either a pre-equilibrium involving a B-to-B hydrogen transfer or a concerted but asynchronous hydrogen transfer via a cyclic six-membered transition state in which the B-to-B hydrogen transfer is highly advanced. DFT calculations are fully consistent with a concerted but asynchronous process.
Rainer Streubel's significant contribution to the field of low‐valent phosphorous chemistry has had a great impact on the field. This special collection was assembled to honor Prof. Streubel on the ...occasion of his retirement. The central theme is the beauty of inorganic chemistry.
This is a special collection celebrating Prof. Rainer Streubel on the occasion of his retirement and his fundamental contribution to the field of low‐valent phosphorous chemistry. Here, guest editors Holger Helten and Rory Waterman summarize Prof. Streubel's career and work.
The counteranion associated with the cationic initiator Cl3PNPCl3+ (4+) generated during the PCl5-initiated living, cationic chain growth polycondensation of the N-silylphosphoranimine Cl3PNSiMe3 ...(3) to give poly(dichlorophosphazene), NPCl2 n (2), has been found to have a dramatic effect on the polymerization. When the counteranion of 4+ was changed from PCl6 – or Cl– to the weakly coordinating anions BAr*F 4− and BArF 4− (Ar*F = 3,5-{CF3}2C6H3, ArF = C6F5) instead of the polymerization of 3 being complete in 4–6 h, no reaction was observed after 24 h. Remarkably, the polymerization of 3 may be initiated by Cl– anions even in the absence of an active cation such as 4+. However, in the presence of 4+, the reaction proceeded significantly faster and allowed for molecular weight control. These results reveal that the currently accepted mechanism for the PCl5-initiated living polymerization of 3 needs to be revised to reflect the key role of the counteranion present.
The replacement of C=C units by their isoelectronic and isosteric B=N units (BN/CC isosterism) in π-conjugated organic compounds, as a strategy to produce novel organic-inorganic hybrid materials, ...has recently been successfully transferred to π-conjugated polymers. This Concept provides an overview of the recent advances in this quickly evolving field, with a focus on synthesis, photophysical and electrochemical properties of the new polymers and related oligomers, as well as possible future applications in organic electronics and optoelectronics.
While certain inorganic main‐group polymers such as silicones (polysiloxanes) are ubiquitous in our everyday life, poly(iminoborane)s have been elusive for a long time. Incorporation of heterocyclic ...building blocks into their backbone has recently enabled access to the first derivatives of this inorganic‐polymer class, as this approach effectively prevents undesired side‐reactions to borazines. Information about the microstructure of these cyclolinear macromolecules, however, has been scarce. Herein, we present the synthesis of a series of monodisperse oligomers with up to 7 boron and 8 nitrogen atoms, representing the longest well‐defined molecular >B=N<x chain to date. We accomplished to characterize six of them structurally in the solid state by single‐crystal X‐ray diffraction, thus providing valuable insight into the microstructure of poly(iminoborane)s. In addition, we report on the synthesis of new poly(iminoborane) derivatives featuring aryl side groups, with an increased content of unsubstituted phenyl groups. The p‐(n‐butyl)phenyl‐substituted species 13 represents the highest‐molecular‐weight sample of this class of inorganic polymers to date.
Aryl‐substituted poly(iminoborane)s and a series of monodisperse oligo(iminoborane)s with increasing number of B=N units in a chain have been prepared – including the highest‐molecular‐weight poly(iminoborane) and the longest molecular >B=N<x chain, so far. X‐ray diffraction studies gave insights into the structures of the oligomers and, hence, into the microstructures of the corresponding polymers.
Substitution of selected CC units in π‐conjugated organic frameworks by their isoelectronic and isosteric BN units (BN/CC isosterism) has proven to be a successful concept for the development of ...BN‐doped polycyclic aromatic hydrocarbons (PAHs) with intriguing properties and functions. The first examples have just demonstrated the applicability of this approach to polymer chemistry. Herein, we present the synthesis and comprehensive characterization of the first poly(p‐phenylene iminoborane). This novel inorganic–organic hybrid polymer can be regarded as a BN analogue of the well‐known poly(p‐phenylene vinylene) (PPV). Photophysical investigations on the polymer and a series of model oligomers provide clear evidence of some π‐conjugation across the B=N bonds and extension of the conjugation path with increasing chain length. TD‐DFT calculations provide deeper insight into the electronic structure of the new materials.
Très BN: A BN analogue of the well‐known semiconducting organic polymer poly(p‐phenylene vinylene) (PPV) was synthesized through a facile Si/B exchange polycondensation. The novel inorganic–organic hybrid polymer and monodisperse oligomers of this type show some π‐conjugation across the B=N units along the backbone and extension of the conjugation path with increasing chain length.
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