New donor-acceptor conjugates based on a trifluorenylporphyrin linked to a redox-switchable ruthenium unit

Reactions of the 16-electron ruthenium complex [Ru(dppe) 2Cl][PF6] with metal-free and zinc ethynyl-phenyltrifluorenylporphyrins 1 and 2 respectively, gave the new dyads 3 and 4 with ethynylruthenium group as a potential electron donor and the porphyrin as a potential electron acceptor. The redox properties of the porphyrins 1-4 were investigated by cyclic voltammetry and UV spectroelectrochemistry (SEC), which reveal that the monocation and monoanion of metal-free porphyrin 1 are stable under these conditions whereas the formation of the corresponding radical cation or anion of the zinc porphyrin 2 was accompanied by partial decomplexation of the zinc ion. Oxidations of the dyads 3 and 4 gave stable radical cations as probed using IR, NIR and UV SEC methods. These cations show similar NIR and IR bands to those reported for the known 17-electron [Ru(dppe)(2)(Cu equivalent to CPh)Cl](+) radical cation. Remarkably, the dyad 3 has four stable redox states +2/+1/0/-1 where the second oxidation and first reduction processes take place at the porphyrin unit. Simulated absorption spectra on 1-4 at optimised geometries obtained by TD-DFT computations with the CAM-B3LYP functional are shown to be in very good agreement with the observed UV absorption spectra of 1-4. The spectra of 1-4 and their oxidised and reduced species were interpreted with the aid of the TD-DFT data. Fluorescence measurements reveal that the dyads 3 and 4 are only weakly emitting compared to 1 and 2, indicative of quenching of the porphyrinic singlet excited state by the ruthenium centre.