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 H2O2‐labile group. The photoacoustic signals of these hydroxy‐masked BPcs were increased upon NIR excitation at 880 nm in the presence of esterase or H2O2, respectively. These are rare examples of activatable probes utilizing NIR light at around 900 nm.

We achieved the first synthesis of N-alkynylpyridinium salts, by reacting pyridines with alkynyl-λ-iodanes. The N-alkynylpyridiniums exhibit highly electron-accepting character with extended π-conjugation. The electrophilic alkynyl groups were readily susceptible to Michael addition and 1,3-dipolar cycloaddition to afford various N-alkenylpyridiniums. Ring-fused pyridiniums were synthesized through intramolecular cyclization, demonstrating the utility of N-alkynylpyridiniums for the design of various electron-deficient cationic nitrogen-embedded polycyclic aromatic hydrocarbons with unique optical and electrochemical properties.

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.H owever,t here are very few stable,a ctivatable molecular probes that can utilize NIR light in the wavelength range beyond 800 nm. Herein, we report an ew activatable NIR system for photoacoustic imaging based on tautomeric benziphthalocyanines (BPcs). We found that the existence of afree hydroxyl group is crucial for NIR absorption of BPcs. Synthesized water-soluble hydroxy BPcs exhibited high photostability and no fluorescence,w hicha re desirable features for photoacoustic imaging. We synthesized BPcs in which the free hydroxyl group was masked by an esterase-labile or an H 2 O 2labile group.T he photoacoustic signals of these hydroxymasked BPcs were increased upon NIR excitation at 880 nm in the presence of esterase or H 2 O 2 ,r espectively.T hese are rare examples of activatable probes utilizing NIR light at around 900 nm.