Strongly Emissive and Photostable Four-Coordinate Organoboron N,C Chelates and Their Use in Fluorescence Microscopy

Six strongly fluorescent four-coordinate organoboron N,C chelates containing an aryl isoquinoline skeleton were prepared. Remarkably, the fluorescence quantum yields reach values of up to 0.74 in oxygen-free toluene. The strong B-N interaction was corroborated by the single-crystal X-ray analysis of two dyes. The intramolecular charge-transfer character of the fluorophores was evidenced by solvatochromism studies and time-dependent DFT calculations at the PCM(toluene)/CAM-B3LYP/6-311++G(2d,p)//PCM(toluene)/B3LYP/6-311G(2d,p) level of theory. The compounds combine high chemical stability with high photostability, especially when equipped with electron-donating substituents. The strong fluorescence and the large Stokes shifts predestine these compounds for use in confocal fluorescence microscopy. This was demonstrated for the imaging of the N13 mouse microglial cell line. Moreover, significant two-photon absorption cross sections (up to 61 GM) allow the use of excitation wavelengths in the near-infrared region (>800 nm).