Bis(phenylethynyl)arene Linkers in Tetracationic Bis-triarylborane Chromophores Control Fluorimetric and Raman Sensing of Various DNAs and RNAs

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 analogue 6 employs 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 than 3 – 5 . Pronounced aggregation–deaggregation processes are observed in fluorimetric titration experiments with DNA for compounds 3 and 5 . Molecular modelling of complexes of 5 with AT-DNA, suggest the minor groove as the dominant binding site for monomeric 5 , but demonstrate that dimers of 5 can also be accommodated. Strong SERS responses for 3 – 5 versus a very weak response for 6 , particularly the strong signals from anthracene itself observed for 5 but not for 6 , 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.