Change in the site of electron-transfer reduction of a zinc- quinoxalinoporphyrin/gold-quinoxalinoporphyrin dyad by binding of scandium ions and the resulting remarkable elongation of the chargeshifted-state lifetime

The site of electron-transfer reduction of AuPQ⁺ (PQ = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b']porphyrin) and AuQPQ⁺ (QPQ = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl) bisquinoxalino[2,3-b':12, 13-b'']porphyrin) is changed from the Au^III center to the quinoxaline part of the PQ macrocycle in the presence of Sc ³⁺ in benzonitrile because of strong binding of Sc³⁺ to the two nitrogen atoms of the quinoxaline moiety. Strong binding of Sc ³⁺ to the corresponding nitrogen atoms on the quinoxaline unit of ZnPQ also occurs for the neutral form. The effects of Sc³⁺ on the photodynamics of an electron donor-acceptor compound containing a linked Zn ^II and Au^III porphyrin ([ZnPQ-AuPQ]PF⁶) have been examined by femto- and nanosecond laser flash photolysis measurements. The observed transient absorption bands at 630 and 670 nm after laser pulse irradiation in the absence of Sc³⁺ in benzonitrile are assigned to the charge-shifted (CS) state (ZnPQ^.+-AuPQ). The CS state decays through back electron transfer (BET) to the ground state rather than to the triplet excited state. The BET rate was determined from the disappearance of the absorption band due to the CS state. The decay of the CS state obeys first-order kinetics. The CS lifetime was determined to be 250 ps in benzonitrile. Addition of Sc³⁺ to a solution of ZnPQ-AuPQ⁺ in benzonitrile caused a drastic lengthening of the CS lifetime that was determined to be 430 ns, a value 1700 times longer than the 250 ps lifetime measured in the absence of Sc³⁺. Such remarkable prolongation of the CS lifetime in the presence of Sc³⁺ results from a change in the site of electron transfer from the Au^III center to the quinoxaline part of the PQ macrocycle when Sc³⁺ binds to the quinoxaline moiety, which decelerate BET due to a large reorganization energy of electron transfer. The change in the site of electron transfer was confirmed by ESR measurements, redox potentials, and UV/Vis spectra of the singly reduced products.