Molecular cubes constructed from catechol-functionalized tribenzotriquinacenes and 1,4-phenylene diboronic acids were synthesized in a one-pot procedure by crosslinking 20 individual components through a dynamic covalent approach. Structural identity of the nanocubes was confirmed by mass spectrometry and (1)H-NMR spectroscopy.
Phosphines are important ligands in homogenous catalysis and have been crucial for many advances, such as in cross‐coupling, hydrofunctionalization, or hydrogenation reactions. Herein we report the synthesis and application of a novel class of phosphines bearing ylide substituents. These phosphines are easily accessible via different synthetic routes from commercially available starting materials. Owing to the extra donation from the ylide group to the phosphorus center the ligands are unusually electron‐rich and can thus function as strong electron donors. The donor capacity surpasses that of commonly used phosphines and carbenes and can easily be tuned by changing the substitution pattern at the ylidic carbon atom. The huge potential of ylide‐functionalized phosphines in catalysis is demonstrated by their use in gold catalysis. Excellent performance at low catalyst loadings under mild reaction conditions is thus seen in different types of transformations.
Ylide‐functionalized phosphine ligands (YPhos) were rationally designed to fit the requirements of Buchwald–Hartwig aminations at room temperature. This ligand class combines a strong electron‐donating ability comparable to NHC ligands with high steric demand similar to biaryl phosphines. The active Pd species are stabilized by agostic C−H⋅⋅⋅Pd rather than by Pd–arene interactions. The practical advantage of YPhos ligands arises from their easy and scalable synthesis from widely available, inexpensive starting materials. Benchmark studies showed that YPhos‐Pd complexes are superior to the best‐known phosphine ligands in room‐temperature aminations of aryl chlorides. The utility of the catalysts was demonstrated by the synthesis of various arylamines in high yields within short reaction times.
Combining a squaraine (S) and a BODIPY (B) chromophore in a heterodimer (SB) and two heterotrimers (BSB and SBS) by alkyne bridges leads to the formation of coupled oscillators whose fluorescence properties are superior compared to the parent squaraine chromophore. The lowest energy absorption and emission properties of these superchromophores are mainly governed by the squaraine part and are shifted by more than 1000 cm(-1) to the red by excitonic interaction between the squaraine and the BODIPY dye. Employing polarization-dependent transient absorption and fluorescence upconversion measurements, we could prove that the lowest energy absorption in SB and BSB is caused by a single excitonic state but by two for SBS. Despite the spectral red-shift of their lowest absorption band, the fluorescence quantum yields increase for SB and BSB compared to the parent squaraine chromophore SQA. This is caused by intensity borrowing from the BODIPY states, which increases the squared transition moments of the lowest energy band dramatically by 29% for SB and 63% for BSB compared to SQA. Thereby, exciton coupling leads to a substantial enhancement of fluorescence quantum yield by 26% for SB and by 46% for BSB and shifts the emission from the red into the near-infrared. In this way, the BODIPY-squaraine conjugates combine the best properties of each class of dye. Thus, exciton coupling in heterodimers and -trimers is a valuable alternative to tuning fluorescence properties by, e.g., attaching substituents to chromophores.
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