A donor–acceptor cross-conjugated phenazine macrocycle with a large Stokes shift for sensing transition metal ions with “turn-on” fluorescence

Abstract

Organic conjugated macrocycles have been widely studied in the fields of photoelectric devices, supramolecular recognition and topological self-assembly because of their unique photoelectronic properties and terminal-less symmetrical structures. In this study, an alternating donor–acceptor conjugated macrocycle (MC-1) with phenazine moieties as electron acceptors (A) and aniline moieties as electron donors (D) was designed and synthesized. Interestingly, MC-1 exhibited simultaneously ultraviolet (UV) absorption and near-infrared (NIR) emission in dichloromethane, resulting in a large Stokes shift (12 422 cm⁻¹, 361 nm). Furthermore, nonpolar MC-1 exhibited a strong positive solvatochromic effect with a significant red shift (256 nm) in the emission from cyclohexane to dichloromethane. Comparative investigations of the linear fragment molecules through experiments and theoretical calculations have shown that the large Stokes shift of MC-1 originated mainly from the strong local intramolecular charge transfer (ICT), probably caused by localized π-conjugation through the cross-conjugated structure. Additionally, MC-1, as a dual-channel fluorescence sensor, exhibited reasonably high selectivity upon the recognition of Fe³⁺ along with a blue-shifted turn-on emission, which can be ascribed to the blocked ICT triggered by Fe³⁺. Similarly, all fragment molecules were able to recognize transition metal ions with relatively low selectivity. This study not only expands the types of alternating D–A macrocycles and applications in ion recognition but also provides a new design strategy for accessing fluorescent molecules with NIR emissions and large Stokes shifts.