Design of Photostable, Activatable Near-Infrared Photoacoustic Probes Using Tautomeric Benziphthalocyanine as a Platform.

Near-infrared (NIR) imaging techniques have attracted significant attention for biological and medicinal applications due to the ability of NIR to penetrate deeply into tissues. However, there are very few stable, activatable molecular probes that can utilize NIR light in the wavelength range beyond 800 nm. Herein, we report a new activatable NIR system for photoacoustic imaging based on tautomeric benziphthalocyanines (BPcs). We found that the existence of a free hydroxyl group is crucial for NIR absorption of BPcs. Synthesized water-soluble hydroxy BPcs exhibited high photostability and no fluorescence, which are desirable features for photoacoustic imaging. We synthesized BPcs in which the free hydroxyl group was masked by an esterase-labile or an H2 O2 -labile group. The photoacoustic signals of these hydroxy-masked BPcs were increased upon NIR excitation at 880 nm in the presence of esterase or H2 O2 , respectively. These are rare examples of activatable probes utilizing NIR light at around 900 nm.