Photoinduced electron transfer in a directly linked meso-triphenylamine zinc porphyrin-quinone dyad

A multimodular donor-acceptor system composed of three triphenylamine entities at the meso-positions of a zinc porphyrin macrocycle and a quinone at the fourth meso-position was newly synthesized and characterized. The triphenylamine entities acted as energy transferring antenna units in addition of improving the electron donor ability of the zinc porphyrin. Appreciable electronic interactions of the triphenylamine and quinone entities with the porphyrin p-system were observed. In agreement with the spectral and electrochemical results, the computational studies performed by the DFT B3LYP/3-21G(*) method revealed delocalization of the frontier HOMO over the triphenylamine and the porphyrin macrocycle while the LUMO to be fully localized over the quinone entity. Free-energy calculations suggested photoinduced electron transfer from the singlet excited zinc porphyrin to the directly linked quinone to be exothermic and this was experimentally confirmed by the time-resolved pump probe and up-conversion techniques. In the investigated system, the ET reaction path was found to depend upon the excitation wavelength. That is, when Zn porphyrin was predominantly excited, a rapid charge separation followed by equally fast charge recombination was observed. However, excitation of the peripheral TPA substituents resulted in an extremely long-lived CS state with triplet spin character via the TPA triplet and Zn porphyrin triplet states.