Photophysical Characterization of a Bis-Acridinium Zn(II) Porphyrin Conjugate

Acridinium moieties are emerging building blocks in supramolecular chemistry since they are single components that can modify their chemical and electronic structures upon chemical or redox stimuli. In addition, thanks to their behaviour as ?-acceptors they have been used as guest molecules in molecular recognitions systems based on porphyrins, where they are stabilized due to charge transfer interactions. In some of these host-guest systems, a fast photoinduced electron transfer process between the porphyrin host and the acridinium guest has been demonstrated. However, the association of porphyrins and acridinium units in the same structure has remained almost unexplored. One example was reported where a free-base porphyrin, connected to two acridiniums, was working as a fluorescent detector for superoxide anion. The fluorescence enhancement of the triad, following a two-electron reduction of the acridinium units by the superoxide anion, was exploited as a detection signalling probe. Here, we report on the absorption and emission properties of a bis-acridinium-Zn(II) porphyrin conjugate (Figure 1), as well as on the characterization of its photophysics by means of ultrafast absorption and emission techniques. The conjugate was synthesized in four synthetic steps starting from commercially available 4-bromobenzaldehyde. The work is supported by the H2020-MSCA-ITN-2017-765297 project "NOAH".