Enhanced Quenching in an Azaphthalocyanine-Ferrocene System upon Charge-Transfer Complex Formation

Azaphthalocyanines are fluorescent dyes and photosensitizers with promising applications in photodynamic therapy and fluorescence sensing. However, achieving precise control over their photophysical behavior remains a major challenge. Here, we report a supramolecular approach to enhance photoinduced electron transfer quenching via charge-transfer complex formation. An electron-deficient azaphthalocyanine derivative incorporating a naphthalene 2,6-diol moiety as a charge-transfer donor was synthesized, and its fluorescence response toward a tailored quencher was evaluated. A ferrocene–methylviologen conjugate (the quencher) that simultaneously functions as a donor in photoinduced electron transfer and acceptor in charge-transfer complexes was designed and synthesized for this purpose. Compared to ferrocenemethanol and methylviologen alone, the new quencher exhibited enhanced quenching efficiency in acetonitrile. The quenching followed a non-linear Stern–Volmer dependence, indicating both static and dynamic quenching mechanisms with the former one being more efficient with K_S = 241 M⁻¹ due to effective charge-transfer complex formation between methylviologen and naphthalene-2,6-diol moiety that served as a staple. These findings demonstrate that the complexation can enhance fluorescence quenching of AzaPc derivatives and suggest a general approach for designing responsive photosensitizers in smart photodynamic therapy systems or molecular sensing.