Stefanie Griesbeck

The synthesis, photophysical, and electrochemical properties of selectivelymono‐,bis‐ andtris‐dimethylamino‐ and trimethylammonium‐substitutedbis‐triarylborane bithiophene chromophores are presented along with the water solubility and singlet oxygen sensitizing efficiency of the cationic compoundsCat¹⁺,Cat²⁺,Cat(i)²⁺, andCat³⁺. Comparison with themono‐triarylboranes reveals the large influence of the bridging unit on the properties of thebis‐triarylboranes, especially those of the cationic compounds. Based on these preliminary investigations, the interactions ofCat¹⁺,Cat²⁺,Cat(i)²⁺, andCat³⁺with DNA, RNA, and DNApore were investigated in buffered solutions. The same compounds were investigated for their ability to enter and localize within organelles of human lung carcinoma (A549) and normal lung (WI38) cells showing that not only the number of charges but also their distribution over the chromophore influences interactions and staining properties.

We report four new luminescent tetracationic bis‐triarylborane DNA and RNA sensors that show high binding affinities, in several cases even in the nanomolar range. Three of the compounds contain substituted, highly emissive and structurally flexible bis(2,6‐dimethylphenyl‐4‐ethynyl)arene linkers (3: arene=5,5′‐2,2′‐bithiophene;4: arene=1,4‐benzene;5: arene=9,10‐anthracene) between the two boryl moieties and serve as efficient dual Raman and fluorescence chromophores. The shorter analogue6employs 9,10‐anthracene as the linker and demonstrates the importance of an adequate linker length with a certain level of flexibility by exhibiting generally lower binding affinities than3–5. Pronounced aggregation–deaggregation processes are observed in fluorimetric titration experiments with DNA for compounds3and5. Molecular modelling of complexes of5with AT‐DNA, suggest the minor groove as the dominant binding site for monomeric5, but demonstrate that dimers of5can also be accommodated. Strong SERS responses for3–5versus a very weak response for6, particularly the strong signals from anthracene itself observed for5but not for6, demonstrate the importance of triple bonds for strong Raman activity in molecules of this compound class. The energy of the characteristic stretching vibration of the C≡C bonds is significantly dependent on the aromatic moiety between the triple bonds. The insertion of aromatic moieties between two C≡C bonds thus offers an alternative design for dual Raman and fluorescence chromophores, applicable in multiplex biological Raman imaging.

Invited for the cover of this issue are the groups of Todd B. Marder at the Julius‐Maximilians‐Universität Würzburg and Ivo Piantanida at the Ruder Boškovic Institute. The image depicts the molecular structure of a bis‐triarylborane‐based chromophore that is simultaneously detecting ds‐DNA and proteins. Read the full text of the article at 10.1002/chem.201903936.

A water‐soluble tetracationic quadrupolar bis‐triarylborane chromophore showed strong binding to ds‐DNA, ds‐RNA, ss‐RNA, as well as to the naturally most abundant protein, BSA. The novel dye can distinguish between DNA/RNA and BSA by fluorescence emission separated by Δ=3600 cm⁻¹, allowing for the simultaneous quantification of DNA/RNA and protein (BSA) in a mixture. The applicability of such fluorimetric differentiation in vitro was demonstrated, strongly supporting a protein‐like target as a dominant binding site of1in cells. Moreover, our dye also bound strongly to ss‐RNA, with the unusual rod‐like structure of the dye, decorated by four positive charges at its termini and having a hydrophobic core, acting as a spindle for wrapping A, C and U ss‐RNAs, but not poly G, the latter preserving its secondary structure. To the best of our knowledge, such unmatched, multifaceted binding activity of a small molecule toward DNA, RNA, and proteins and the selectivity of its fluorimetric and chirooptic response makes the quadrupolar bis‐triarylborane a novel chromophore/fluorophore moiety for biochemical applications.

Two different chromophores, namely a dipolar and an octupolar system, were prepared and their linear and nonlinear optical properties as well as their bioimaging capabilities were compared. Both contain triphenylamine as the donor and a triarylborane as the acceptor, the latter modified with cationic trimethylammonio groups to provide solubility in aqueous media. The octupolar system exhibits a much higher two‐photon brightness, and also better cell viability and enhanced selectivity for lysosomes compared with the dipolar chromophore. Furthermore, both dyes were applied in two‐photon excited fluorescence (TPEF) live‐cell imaging.

The tetracationic diketopyrrolopyrrole compound 5M exhibits a σ₂ value of 4560 GM at 740 nm.

The stability of tetracationic triarylboranes in dilute aqueous solution was investigated by tuning the steric demand of the linker in a (para‐(N,N,N‐trimethylammonio)xylyl)₂B–(linker)–B(para‐(N,N,N‐trimethylammonio)xylyl)₂ structure. With increasing steric bulk of the linker, namely 1,4‐phenylene, 2,2′′′‐(3,3′′′‐dimethyl)‐5,2′:5′,2′′:5′′,5′′′‐quaterthiophene, 9,10‐anthracenylene, and 4,4′′′‐(5′‐(3,5‐dimethylphenyl))(5′′‐(3′′′,5′′′‐dimethylphenyl))‐2′,2′′‐bithiophene, the stability of the compounds increased. The anthracene‐based chromophore, compound 3M is water‐stable for at least 48 h, is nontoxic to cells and exhibits an exceedingly high fluorescence quantum yield of 0.86 in water making it an ideal candidate for confocal live‐cell imaging of lysosomes.