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

Acridinium moieties as building blocks in the design of supramolecular entities are of great interest thanks to their ability of modifying their chemical and electronic structure upon chemical or redox stimuli. Besides, their ?-acceptor behavior allows them to be used as guest molecules in molecular recognition systems constituted by porphyrins, where they are stabilized due to charge transfer interactions. It has been reported that in some host-guest systems where the acridiniums units and the porphyrins are involved, a fast photoinduced electron transfer process between the porphyrin host and the acridinium guest takes place. Even though these interesting results, the conjugation of porphyrin and acridinium units in the same supramolecular structure has been rarely explored. One example is reported where a free-base porphyrin, connected to two acridiniums, is 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 the detection signal. Herein, we report on the absorption and emission properties of a bis-acridinium-Zn(II) porphyrin conjugate (Figure 1) as well as on the study of its photophysics by means of ultrafast transient absorption. 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".