J-dimers of phthalocyanine analogues: structural characterization and their use for determination of association constants between ligands and the central cation

Abstract

The characterization of the stability of self-assembled supramolecular systems is critical for numerous applications that rely on non-covalent interactions between the components. However, in phthalocyanines (Pcs), the coordination of ligands to the central metal is typically not accompanied by significant spectral changes, complicating the determination of association constants (K₁). In this study, we developed a reliable and widely applicable method based on fluorescence and absorption spectroscopy that allows straightforward determination of K₁ for a broad range of ligands, from heterocyclic to purely aliphatic, with weak (K₁ ∼ 10² M⁻¹) to strong (K₁ ∼ 10⁷ M⁻¹) binding affinities. The method benefits from the full characterization of unique J-dimers of Pcs, which are formed via coordination between the peripheral substituent of one molecule and the central metal of another Pc in a series of metal octa(dialkylamino)azaphthalocyanines. These J-dimers exhibit significantly red-shifted absorption bands (up to ∼710 nm) and retain red fluorescence with significant Stokes shifts (∼35–40 nm), making them ideal for spectroscopic analysis. The developed method allowed for the direct determination of dimerization constants (K_D) by monitoring temperature-induced J-dimer disassembly. The determined K_D values ranged from 10⁸ to 10¹⁵ M⁻¹, with the bulkiness of the coordinating substituents being the primary factor affecting the dimerization strength. The insights gained could be instrumental in the rational design of self-coordinating supramolecular systems that are important in, for example, energy and electron transfer processes.