Encapsulation of a tetra-pyridyl porphyrin guest in bis(acridinium-porphyrin) tweezers: a photophysical study

Metalloporphyrins are of great interest as coordination compounds thanks to their photophysical and electrochemical properties. Interestingly, their redox potentials can be tuned by modifying the lateral substituents or by changing the central metal ion; this makes them widely used constituents in multicomponent systems undergoing photoinduced electron transfer.[1] Recently, porphyrins associated to N-acridinium chromophores in supramolecular conjugates that could act as sensors or receptors have been investigated.[2] The covalent coupling of acridinium units to porphyrin moieties has resulted in an ultrafast photoinduced electron transfer from the porphyrin donor to the acridinium acceptor, which quenches the fluorescence of both units.[3] Novel bis(acridinium-porphyrin) tweezers (1, as an example, Figure 1) were designed as multi-responsive arrays containing two Zn(II) porphyrins, as coordinating sites, and two acridinium ions as switching units. In the present work we report on the spectroscopic characterization of the complexation process among the bis(acridinium-porphyrin) conjugates and a meso-(5,10,15,20-tetra(4?-pyridyl)porphyrin, TPyP, (Figure 1) and on the photophysical characterization of the related host-guest complexes, in order to explore the use of the tweezers as receptors for photoactive hosts.