Fluorescent phenanthroimidazoles functionalized with heterocyclic spacers: synthesis, optical chemosensory ability and two-photon absorption (TPA) properties

Abstract

Three series of fluorescent phenanthroimidazoles bearing heterocyclic spacers were synthesized in moderate to excellent yields and evaluated as optical chemosensors for ions as well as two-photon absorbing chromophores. Interaction of compounds 5–7 with anions and cations in acetonitrile and acetonitrile/H₂O (95 : 5) showed them to be selective receptors for several anions (AcO⁻, CN⁻ and F⁻) and cations (Fe³⁺, Cu²⁺ and Pd²⁺), with compound 7a being the most sensitive receptor for Fe³⁺ and Cu²⁺. On the other hand, compounds 5a, 7b and 7c were the most sensitive receptors for AcO⁻, CN⁻ and F⁻. The binding stoichiometry between the receptors and the anions and cations was found to be 1 : 2 (ligand to anion/metal cation). The binding process was also followed by ¹H NMR titrations. The evaluation of the TPA properties of chosen phenanthroimidazoles 7a–c by the two-photon induced fluorescence method revealed that compound 7c, which contains a bithienyl spacer, exhibited the highest TPA cross-section (σ₂) value.