Thiaheptapyrrin and Thiatetrapyrrin Armed p-Phenylene-Bridged Norrole as NIR-II Dyes: Photothermal Behavior Effectively Enhanced by Protonation and Deprotonation

To extend the absorption spectra of p-phenylene-bridged norroles with the absorption peaks below 700 nm (1 and 2) and probe the photothermal applications in the near-infrared region, cyclization of thiaheptapyrrin (3) afforded a p-phenylene-bridged norrole armed with a thiatetrapyrrin side chain (4) in a 52% yield. Compared to 1 and 2, 4 demonstrates a much narrower HOMO–LUMO gap and considerably red-shifted absorption spectrum tailing to ca. 1150 nm, which may be related to its extended molecular structure. Interestingly, the absorption spectra of both 3 and 4 can be considerably intensified and red-shifted into the near-infrared II region upon protonation and deprotonation. On this basis, photothermal conversion behavior was investigated by irradiating 20 µM solutions with a 1064 nm laser for 10 min. Thus, 3 and 4 exhibited high photothermal conversion efficiencies (PCEs) of 64.5% and 58.9% with temperatures rising to 48.5°C and 36.0°C, respectively. Notably, under identical illumination conditions, protonated and deprotonated forms of 3 demonstrated higher temperature rise to 73.8°C and 77.7°C, respectively. Meanwhile, 4 also shows similar behavior upon protonation and deprotonation. This work provides insights into designing NIR-II photothermal agents by synthesizing long chain oligopyrrins and p-phenylene-bridged norroles in combination with supramolecular means like protonation and deprotonation.