Photoactive Arrays Based on Porphyrins and Acridinium Units

Porphyrins are attractive photoactive molecules, widely used in energy conversion systems and materials for optoelectronics and biomedical research. Thanks to their interesting photophysical and electrochemical properties, they are suitable components of molecular wires, switches and chemical sensors. On this basis, the design of multichromophoric arrays based on porphyrins is of high interest for the mimic of naturally occurring photoinduced energy and electron transfer processes, leading to energy collection and conversion. In this view, the combination of porphyrins with other units that can act as multiresponsive components, reacting to chemical or redox inputs, can lead to fascinating stimuli-responsive architectures. N-substituted acridinium units are a newly explored class of switches; their reversible conversion into the non-aromatic acridane form is triggered by various orthogonal stimulations (pH or electrochemical/photochemical inputs). The association of acridinium components with porphyrins is thus a promising strategy to develop photoactive molecular and supramolecular arrays with useful functions. In this scenario, our recent work focused on the investigation of the photophysical properties of 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) can be varied (Figure 1). In particular, a bisacridinium Zn(II)-porphyrin conjugate, a bisacridinium diphenylporphyrin conjugate and a multi-responsive bis(acridinium-Zn(II) porphyrin) receptor (Figure 1) have been studied.1-3 Interestingly, the studies revealed that the association of porphyrins with acridinium ions leads to ultrafast electron transfer processes, controlled by the temperature or by the conversion of the acridinium unit, acting as an electron acceptor, into the acridane form, behaving as an energy donor. Moreover, the combination of two Zn-porphyrins with two acridinium/acridane units results in a multi-responsive eight-state molecular receptor, able to coordinate suitable hosts. The photophysics of the arrays and of their complexes, analyzed by means of absorption and emission spectroscopies, including ultrafast pump and probe, will be presented and discussed.