Carbazole-Linked Near-Infrared Aza-BODIPY Dyes as Triplet Sensitizers and Photoacoustic Contrast Agents for Deep-Tissue Imaging

Abstract

Four new N-ethylcarbazole-linked aza-boron-dipyrromethene (aza-BODIPY) dyes (8a,b and 9a,b) were synthesized and characterized. The presence of the N-ethylcarbazole moiety shifts their absorption and fluorescence spectra to the near-infrared region, l&650–730 nm, of the electromagnetic spectrum. These dyes possess strong molar absorptivity in the range of 3–4V104m@1cm@1 with low fluorescence quantum yields. The triplet excited state and singlet oxygen generation of these dyes were enhanced upon iodination at the core position. The core-iodinated dyes 9a,b showed excellent triplet quantum yields of about 90 and 75%, with singlet oxygen generation efficiency of about 70 and 60% relative to that of the parent dyes. Derivatives 8a,b showed dual absorption profiles, in contrast to dyes 9a,b, which had the characteristic absorption band of aza- BODIPY dyes. DFT calculations revealed that the electron density was spread over the iodine and dipyrromethene plane of 9a,b, whereas in 8a,b the electron density was distributed on the carbazole group and dipyrromethene plane of aza-BODIPY. The uniqueness of these aza-BODIPY systems is that they exhibit efficient triplet-state quantum yields, high singlet oxygen generation yields, and good photostability. Furthermore, the photoacoustic (PA) characteristics of these aza-BODIPY dyes was explored, and efficient PA signals for 8a were observed relative to blood serum with in vitro deep-tissue imaging, thereby confirming its use as a promising PA contrast agent.