A family of bi-heterocyclic dyes have been first synthesized by using heterocyclic pyridone and benzothiazole components. UV−Vis spectral analyses reveal distinct spectral behaviors depending on the introduced electron-withdrawing and electron-donating groups in the benzothiazol ring as well as the polarity of solvents. Namely, nitrobenzothiazol based bi-heterocyclic dyes display more obvious solvatochromism compared with both methylbenzothiazol based ones and previously reported azobenzene based half-heterocyclic pyridone dyes. More interestingly, 1 H NMR and UV−Vis spectral results as well as single-crystal structures manifest that these dyes are in the hydrazone form in the solid state and in the non-polar solvents. However, they adopt dominating azo form in polar solvents and it can be transformed to the hydrazone form in the acidic condition, which are both different from all known half-heterocyclic pyridone dyes. In addition, transformation from hydrazone to deprotonated azo form can be achieved via metal-ion complexation, and four Ni II and Cu II complexes with different pyridone N -tails have been structurally characterized. It is noted that the ligands adopt different tridentate coordination fashions with Ni II and Cu II ions and a rare tetranuclear Cu II complex with equal Cu⋯Cu separation is obtained.
In this study, three typical D–π–A type heteroaromatic organic dyes containing carbazole (CBZ), phenothiazine (PTZ), and phenoxazine (POZ) moiety as electron donor cores and linked to g-C 3 N 4 with amide covalent bonds were synthesized. Thereafter, we studied the long-term photochemical stability and degradation mechanism of the associated dye-sensitised g-C 3 N 4 with covalent linkage or weak interaction. The results revealed that most of the dyes degraded and became unstable after absorption on g-C 3 N 4 under light irradiation. Interestingly, after the target dyes were linked to g-C 3 N 4 with amide covalent bonds, the photostability could be effectively promoted and improved to high levels of stability. For POZ-S dye covalent bond-linked sample g-C 3 N 4 /POZ-S, ∼90.5% percent of the dyes were reserved, whereas only ∼25.3% remained for the physically absorbed dye sample g-C 3 N 4 +POZ-S after 30 min of irradiation aging test. Moreover, the obtained g-C 3 N 4 /POZ-S sample exhibited enhanced H 2 evolution activity with photocatalytic H 2 production rates that were approximately 96- fold of that of pure g-C 3 N 4 . Then, we applied various instrumental analysis and DFT calculation to clarify the mechanism involved with dye photostability and degradation pathway. The typical decarboxylation process was confirmed, and such an unfavourable reaction process could be effectively inhibited if the target dye is linked to g-C 3 N 4 with amide covalent bond instead of the weak interaction. Overall, the existed covalent bond is the key factor for enhancing both the photochemical stability and photocatalytic activity of organic dye-sensitised g-C 3 N 4 for hydrogen evolution .
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