Self-assembled via metal-ligand coordination AzaBODIPY-zinc phthalocyanine and AzaBODIPY-zinc naphthalocyanine conjugates: Synthesis, structure, and photoinduced electron transfer

Using near-IR emitting photosensitizers, viz., zinc phthalocyanine (ZnPc), zinc naphthalocyanine (ZnNc), and BF₂-chelated azadipyrromethene (azaBODIPY), donor-acceptor conjugates have been newly formed using the well-known metal-ligand axial coordination approach. To accomplish this task, the electron-deficient azaBODIPY was functionalized to possess either two pyridine or imidazole ligating entities. The X-ray crystal structure of one such derivative revealed the presence of axial binding capable pyridine entities on the azaBODIPY macrocycle. The structural integrity of the newly formed conjugates was established from optical absorption, emission, ¹H NMR, electrochemical, and computational methods. The experimentally determined binding constants suggested moderately stable conjugates. The absence of excitation energy transfer from singlet excited azaBODIPY to either ZnPc or ZnNc in the conjugates was confirmed from the steady-state emission measurements. However, free-energy calculations suggested photoinduced electron transfer to be exothermic from singlet excited ZnPc or ZnNc to azaBODIPY. Femtosecond transient studies confirmed such predictions from where occurrence of ultrafast photoinduced electron transfer was observed. The measured rates of charge separation and charge recombination (k_CS and k_CR) revealed charge stabilization to some extent in these conjugates.