Imidazoporphyrins with appended polycyclic aromatic hydrocarbons: to conjugate or not to conjugate?

Abstract Herein we report the synthesis and characterization of a series of new dyads – imidazo-fused Ni(II) porphyrinates, containing polycyclic aromatic hydrocarbons (PAH) at the C-2 position of the imidazole ring, including pyrene – 1Pyr, anthracene – 1An, and diphenylanthracene – 1Ph2An. Electronic communication between the tetrapyrrolic macrocycles and PAH moieties was comprehensively studied by UV-Vis and fluorescence spectra together with quantum-chemical modelling. DFT calculations, including geometry optimization and analysis of the frontier orbitals, has revealed the efficient conjugation of pyrene and anthracene with porphyrin π-system, while 1Ph2An dyad possessed two isolated non-conjugated π-systems. Interestingly, these conclusions could not be unambiguously ruled out of UV-Vis spectroscopy data alone. Finally, the presence or absence of conjugation was confirmed using the analysis of ellipticities of bond critical points, thus proving this method to be a convenient tool to study the conjugation in multichromophoric porphyrin-based systems.