A pentaporphyrin as a switching device activated by proton and redox stimuli

A new pentaporphyrin array, constituted by a peptidic backbone and lateral chains with two free-base, one Mg(II), and two Zn(II) porphyrins, has been synthesized. The electrochemical and photophysical properties are not the mere superposition of those of its model compounds: slight shifts of the E(1/2) values and strong perturbation of both the Soret and Q-band absorption show substantial ground-state interactions among the component units, which take advantage of the rather flexible nature of the peptidic links. This multiporphyrin array, despite the flexible and nonconjugated nature of the peptidic spacers, plays the role of an antenna for visible light: an efficient photoinduced energy transfer takes place from the metalated porphyrin units to the free-base ones. Furthermore, the light emitted by the antenna can be: 1) tuned upon protonation of the free-base units, or 2) turned off by a redox input, since the formation of the Mg porphyrin radical cation, by either electrochemical or chemical methods, quenches the free-base porphyrin emission. Both quenching and tuning of the emission from the light-collecting center can be fully reverted by redox or chemical stimuli.