Complexation of Selected Polycyclic Aromatic Hydrocarbons with Hexaprotonated Hexaazacyclophane Macrocycles

Abstract

Abstract: This research focused on the host-guest complexation-driven detection of three selected polycyclic aromatic hydrocarbons (PAHs), naphthalene, anthracene and phenanthrene, in solution using two protonated hexaazacyclophane macrocycles (1⁶⁺), specifically 1a⁶⁺ and 1b⁶⁺. The geometry optimization of PAHs and 1⁶⁺ performed using quantum chemical density functional theory calculations demonstrated their electronic compatibility to undergo host-guest complexation. The macrocycles 1a⁶⁺ and 1b⁶⁺, which possess different cavity geometries, were synthesized using [2+2] Schiff-base cyclization of diethylenetriamine separately with terephthalaldehyde and isophthalaldehyde, respec­tively, followed by reduction and protonation. After spectroscopic characterization of the macrocycles, the fluorescence spectroscopic determination of stoichiometry, binding constant and quenching constant between 1a⁶⁺/1b⁶⁺ and PAH was performed using the Job’s plot method, Benesi-Hildebrand method and Stern-Volmer plot, respectively. The stoichiometry of each combination of 1⁶⁺ with PAH exhibits a 1:1 complexation. Among the PAHs studied, phenanthrene demonstrated the highest binding and quenching constants with both 1⁶⁺, while anthracene and naphthalene showed the lowest binding and quenching constants, respectively, with both 1⁶⁺. The cavity geometry of 1⁶⁺ and the shape and number of fused rings of PAHs contributed to the binding constant.