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

Azaphthalocyanines are fluorescent dyes and photosensitizers with promising applications in photodynamic therapy (PDT) and fluorescence sensing. However, achieving precise control over their photophysical behavior remains a major challenge. Here, we report a supramolecular approach to enhance fluorescence 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 that simultaneously functions as a quencher and an acceptor in charge-transfer complexes was designed and synthesized for this purpose. Compared to ferrocenemethanol and methylviologen alone, the conjugate quencher exhibited an enhanced quenching efficiency in acetonitrile. The quenching followed a nonlinear 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 directed charge-transfer complex formation between methylviologen and the naphthalene-2,6-diol moiety serving as a staple. These findings demonstrate that complexation can enhance the fluorescence quenching of AzaPc derivatives and suggest a general approach for designing responsive photosensitizers in smart PDT systems or molecular sensing.