Novel azaphalocyanines containing thymol and carvacrol: Investigation of fluorescence properties

Aryloxyazaphthalocyanines (AzaPcs) with different phenol derivatives have been extensively studied in the literature due to their photophysical properties. However, incorporating natural bulky phenols such as Thymol and Carvacrol into AzaPcs remains underexplored. In this study, novel Thymol- and Carvacrol-substituted AzaPcs were synthesized via a solvent-free method, and their structural and fluorescence properties were systematically investigated. The molecular structures of the synthesized compounds were characterized using Fourier-transform infrared (FT-IR) spectroscopy, ultraviolet-visible (UV–Vis) spectroscopy, proton and carbon nuclear magnetic resonance (¹H and ¹³C NMR) spectroscopy. Fluorescence emission studies in dimethyl sulfoxide (DMSO) revealed that all synthesized Zn-azaPcs exhibited red fluorescence. The fluorescence behavior was further analyzed regarding Stokes shifts, fluorescence quantum yields (Φ_f), and aggregation tendencies. Notably, octa-substituted Zn-azaPcs exhibited superior fluorescence quantum yields compared to their tetra-substituted counterparts, with Φ_f values reaching 0.22 for 3a and 0.19 for 4a. The presence of Zn²⁺ in the macrocyclic core was found to enhance fluorescence efficiency by stabilizing the electronic structure and minimizing non-radiative decay pathways. Aggregation studies revealed that tetra-substituted Zn-azaPcs (1a and 2a) exhibited a higher tendency to aggregate, as indicated by broadened Q-bands in UV–Vis spectra and increased Stokes shifts. In contrast, octa-substituted derivatives (3a and 4a) demonstrated lower aggregation tendencies due to steric hindrance from bulky Thymol and Carvacrol groups, which limited π-π stacking interactions. This reduction in aggregation contributed to higher fluorescence efficiencies and reduced Stokes shifts. These findings provide insights into the structure-property relationships of AzaPcs and suggest their potential applications in optoelectronics and bioimaging, particularly in environments where aggregation-induced fluorescence quenching is a concern.