Photophysical, photochemical and electrochemical properties of a series of aromatic electron acceptors based on N-heterocycles

The electronic absorption and luminescence spectra, photoreactivity, and the electrochemical properties of a series of aromatic electron acceptors based on the 4,4′-bipyridinium, 1,2-bis(4-pyridinium)ethylene, and 2,7-diazapyrenium cations have been investigated. All these species exhibit distinctive absorption spectra and some of them show fluorescence and phosphorescence bands. The compounds based on the 1,2-bis(4-pyridinium)ethylene unit provide the interesting possibility of studying the E–Z photoisomerization of the vinylic double bond. The photophysical and photochemical properties have been also interpreted on the basis of quantum chemical calculations. All the examined compounds exhibit reduction processes at mild negative potentials that reveal their electron accepting character. We found that the photophysical, photochemical and electrochemical properties of such compounds are not only determined by the structure of the N-heterocyclic central moiety, but are also remarkably affected by the peripheral substituents linked to the quaternarized nitrogen atoms.