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

## TL;DR

Researchers developed a supramolecular system that enhances fluorescence quenching in azaphthalocyanines through charge-transfer complex formation, offering potential for smart photodynamic therapy and sensing.

## Contribution

A novel supramolecular dyad combining azaphthalocyanine and ferrocene improves quenching efficiency via charge-transfer complex formation.

## Key findings

- The ferrocene–methylviologen conjugate showed higher quenching efficiency than individual components.
- Nonlinear Stern–Volmer behavior indicated both static and dynamic quenching mechanisms.
- Directed charge-transfer complex formation increased quenching with a binding constant of K_S = 241 M–1.

## Abstract

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.