Janice M. DeGraziano

Efficient multistep photoinitiated electron transfer in a molecular pentad.

A synthetic five-part molecular device has been prepared that uses a multistep electron transfer strategy similar to that of photosynthetic organisms to capture light energy and convert it to chemical potential in the form of long-lived charge separation. It consists of two covalently linked porphyrin moieties, one containing a zinc ion (PZn) and the other present as the free base (P). The metailated porphyrin bears a carotenoid polyene (C) and the other a diquinone species (QA-QB). Excitation of the free-base porphyrin in a chloroform solution of the pentad yields an initial charge-separated state, C-PZn-P.+.-QA--QB, with a quantum yield of 0.85. Subsequent electron transfer steps lead to a final charge-separated state, C.+-PZn-P-QA-QB.-, which is formed with an overall quantum yield of 0.83 and has a lifetime of 55 microseconds. Irradiation of the free-base form of the pentad, C-P-P-QA-QB, gives a similar charge-separated state with a lower quantum yield (0.15 in dichloromethane), although the lifetime is increased to ∼340 microseconds. The artificial photosynthetic system preserves a significant fraction (∼1.0 electron volt) of the initial excitation energy (1.9 electron volts) in the long-lived, charge-separated state.

Kinetics of multistep photoinitiated electron transfer reactions in a molecular triad

Abstract Time-resolved fluorescence and subpicosecond transient absorption experiments have been carried out on a triad molecular device (CPQ) comprising a porphyrin (P) covalently linked to an electron-donating carotenoid pigment (C) and a quinone (Q), which acts as an electron acceptor. Rate constants governing the intramolecular electron transfer processes originating from the excited singlet state of the porphyrin have been determined. Excitation of the porphyrin moiety of the triad in benzonitrile solution results in photoinduced electron transfer to give CP .+ Q .− ( k 1 =2.5 × 10 9 s −1 ) followed by rapid non-photochemical electron transfer to yield C .+  PQ .− ( k 3 ≈ 1 × 10 11 s −1 ). The intermediate CP .+ Q .− , also decays by charge recombination to the ground state with a rate constant k 2 of about 6 × 10 11 s −1 . Nanosecond laser flash experiments were used to measure a quantum yield of 0.13 for the long-lived (370 ns) charge-separated species C .+ PQ .− . Excitation of the carotenoid moiety of the triad results in singlet energy transfer to the attached porphyrin with a quantum yield of about 0.07.

Photoinduced charge separation in a carotenoid-porphyrin-diquinone tetrad: enhancement of quantum yields via control of electronic coupling

Abstract A molecular tetrad consisting of a free-base porphyrin (P) linked to a carotenoid polyene (C) and a diquinone moiety (Q A –Q B ) has been synthesized, and its photochemistry has been investigated using time-resolved techniques. Excitation of the porphyrin moiety of the tetrad in dichloromethane solution is followed by photoinduced electron transfer to yield an initial C-P + -Q − A –Q B state, which is formed with arate constant of 2.3×10 9 s −1 and a quantum yield of 0.87. In chloroform, the rate is 4.1×10 9 s −1 and the quantum yield is 0.94. Transient absorption studies show that this state evolves by subsequent electron transfer pathways to a final C + -P-Q A –Q − B charge-separated state whose lifetime is 7.4 μs in dichloromethane and 740 ns in chloroform. The quantum yield of the final state is 0.49 in dichloromethane and 0.57 in chloroform. The yield of the final state is substantially higher than that in a related, previously-reported tetrad in spite of the fact that the quantum yield of the initial C-P + -Q − A –Q B species is lower. This fact is interpreted in terms of the rates of charge-separation reactions relative to those of charge recombination. It is shown that yields of charge separation in multicomponent molecules may be altered in a predictable fashion using the basic tenets of electron transfer theory.

Multistep Photoinitiated Charge Separation In A Molecularpentad

A number of multicomponent molecular devices which mimic the multistep electron transfer strategy of natural photosynthesis by which light energy is captured and converted to chemical potential energy have been prepared. The most complex of these are pentads consisting of a diporphyrin moiety bearing both a carotenoid polyene and a diquinone. Excitation of the porphyrin leads to the formation of a charge separated state with a quantum yield as high as 0.83 and a lifetime up to 340 CIS, depending upon the structure of the pentad, The final charge separated state preserves over one-half of the initial excitation energy as chemical potential.