Vectorization and Intracellular Distribution of a Two‐Photon‐Absorbing, Near‐Infrared‐Emitting π‐Extended Boranil Dye

A π‐extended boranil (PEB) dye has been synthesized and its photophysical properties have been recorded in a range of solvents. The dye exhibits an intense solvatochromic fluorescence emission covering the entire visible spectrum and the near infra‐red (NIR) region (from λ =534 to 764 nm) with quantum yields up to 69 %. Embedment of the PEB dye in the amphiphilic block copolymer Cremophor EL ® leads to a strong emission ( λ em =617 nm, Φ =0.33) in PBS buffer. Moreover, two‐photon absorption (TPA) spectroscopy evidenced a large two‐photon (2P) cross section (420 GM), enabling the use of the PEB dye in 2P‐excitation fluorescence microscopy. The cellular uptake and distribution of PEB in HeLa cells was monitored using fluorescence lifetime imaging microscopy (FLIM) and real‐time widefield imaging. It was found that the emission of the dye is strongly influenced by the local environment and that the dye is preferentially internalized in acidic vesicles.