Photoinduced Processes in Molecular and Supramolecular Arrays Based on Porphyrins and Acridinium Units

Multichromophoric arrays based on porphyrins are of high interest for their mimicry of naturally occurring photoinduced energy and electron transfer processes leading to energy collection and conversion. [1] Moreover, the rich photophysics and electrochemistry of porphyrins make them suitable components of molecular wires, switches or chemical sensors. [2] In this perspective, it is interesting to pursue the combination of porphyrins with units that can act as multiresponsive components by modifying their chemical and electronic structure upon chemical or redox stimuli. N-acridinium ions have been recently explored as a new class of switches that respond to various orthogonal stimulations (pH or electrochemical/photochemical inputs) that trigger their reversible conversion into the non-aromatic acridane form. [3] Few examples reporting the association of acridinium units and porphyrins indicate that these systems are characterized by fast photoinduced electron transfer reactions between the porphyrin donor and the acridinium acceptor. [4,5] We recently investigated the photophysical properties of molecular and supramolecular arrays based on porphyrins and acridinium units, where the type of porphyrin (free-base or metallated) and/or the form of the switchable component (acridinium or acridane) has been varied (Figure 1). We found that the association of porphyrins and acridinium ions always leads to ultrafast electron transfer processes, and that these reactions can be altered by changing the temperature or by converting the acridinium electron acceptor into an acridane energy donor. Moreover, the combination of two Zn-porphyrins with two acridinium/acridane units results in switchable molecular receptors able to coordinate suitable hosts. The photophysics of the arrays and of the complexes, analyzed by means of steady-state and time-resolved luminescence techniques and ultrafast transient absorption spectroscopy, will be herein presented and discussed. The work is supported by the H2020-MSCA-ITN-2017-765297 project "NOAH" and by the H2020-LC-SC3-2020-RES-RIA-101006839 project "CONDOR".