TY - JOUR
T1 - Photoinitiated multistep charge separation in ferrocene-zinc porphyrin-diiron hydrogenase model complex triads
AU - Poddutoori, Premaladha
AU - Co, Dick T.
AU - Samuel, Amanda P S
AU - Kim, Chul Hoon
AU - Vagnini, Michael T.
AU - Wasielewski, Michael R.
PY - 2011/7
Y1 - 2011/7
N2 - Two covalently linked linear electron donor-acceptor triads Fc-ZnTPP-[NMI-FeI-FeI-S2(CO)6] (1) and Fc-Ph-ZnTPP-[NMI-FeI-FeI-S2(CO) 6] (2) consisting of a zinc meso-tetraphenylporphyrin (ZnTPP) chromophore, a naphthalene monoimide diiron hydrogenase active site model [NMI-FeI-FeI-S2(CO)6], and a ferrocene (Fc) secondary electron donor have been synthesized along with their corresponding dyad reference molecules ZnTPP-[NMI-FeI-Fe I-S2(CO)6] (3), Fc-ZnTPP (4), and Fc-Ph-ZnTPP (5). Time-resolved transient absorption and emission studies in CH 2Cl2 show that selective photoexcitation of ZnTPP in triads 1 and 2 results in two competing quenching pathways for 1*ZnTPP: electron transfer from 1*ZnTPP to [NMI-FeI-FeI-S2(CO)6] and energy transfer from 1*ZnTPP to low-lying Fc excited states. Our studies on reference dyads 4 and 5 show that the majority of 1*ZnTPP produced by the laser pulse decays rapidly by energy transfer to Fc in triad 1 (τ < 10 ps), while electron transfer to [NMI-FeI-FeI-S2(CO)6] dominates in triad 2, allowing the second rapid electron transfer step from Fc to ZnTPP + to proceed. Quantum yields of the fully charge separated states Fc+-ZnTPP-[NMI-Fe0-FeI-S2(CO) 6] and Fc+-Ph-ZnTPP-[NMI-Fe0-Fe I-S2(CO)6] are 0.13 and 0.71, respectively. Charge recombination in Fc+-ZnTPP-[NMI-Fe0-Fe I-S2(CO)6] occurs with τCR = 9 ± 1 ns and τCR = 67 ± 2 ns for Fc +-Ph-ZnTPP-[NMI-Fe0-FeI-S2(CO) 6]. By incorporating a secondary electron donor, the lifetime of the reduced diironhydrogenase mimic was extended by a factor of >450. Studies of photochemical hydrogen evolution using 1 and 2 reveal that the hydrogen generation efficiency depends on the lifetime of the final charge separated state. The ability to execute a multi-electron proton-coupled electron transfer mechanism in a stepwise manner will allow us to investigate the structural and electronic requirements for each step aiding in overall system optimization. Thus, it is possible to use the same multi-step electron transfer strategy that has been employed to extend the lifetime of charge-separated states in photodriven donor-acceptor systems to extend the lifetime of the reduced states of metal complexes of potential use in catalytic proton reduction.
AB - Two covalently linked linear electron donor-acceptor triads Fc-ZnTPP-[NMI-FeI-FeI-S2(CO)6] (1) and Fc-Ph-ZnTPP-[NMI-FeI-FeI-S2(CO) 6] (2) consisting of a zinc meso-tetraphenylporphyrin (ZnTPP) chromophore, a naphthalene monoimide diiron hydrogenase active site model [NMI-FeI-FeI-S2(CO)6], and a ferrocene (Fc) secondary electron donor have been synthesized along with their corresponding dyad reference molecules ZnTPP-[NMI-FeI-Fe I-S2(CO)6] (3), Fc-ZnTPP (4), and Fc-Ph-ZnTPP (5). Time-resolved transient absorption and emission studies in CH 2Cl2 show that selective photoexcitation of ZnTPP in triads 1 and 2 results in two competing quenching pathways for 1*ZnTPP: electron transfer from 1*ZnTPP to [NMI-FeI-FeI-S2(CO)6] and energy transfer from 1*ZnTPP to low-lying Fc excited states. Our studies on reference dyads 4 and 5 show that the majority of 1*ZnTPP produced by the laser pulse decays rapidly by energy transfer to Fc in triad 1 (τ < 10 ps), while electron transfer to [NMI-FeI-FeI-S2(CO)6] dominates in triad 2, allowing the second rapid electron transfer step from Fc to ZnTPP + to proceed. Quantum yields of the fully charge separated states Fc+-ZnTPP-[NMI-Fe0-FeI-S2(CO) 6] and Fc+-Ph-ZnTPP-[NMI-Fe0-Fe I-S2(CO)6] are 0.13 and 0.71, respectively. Charge recombination in Fc+-ZnTPP-[NMI-Fe0-Fe I-S2(CO)6] occurs with τCR = 9 ± 1 ns and τCR = 67 ± 2 ns for Fc +-Ph-ZnTPP-[NMI-Fe0-FeI-S2(CO) 6]. By incorporating a secondary electron donor, the lifetime of the reduced diironhydrogenase mimic was extended by a factor of >450. Studies of photochemical hydrogen evolution using 1 and 2 reveal that the hydrogen generation efficiency depends on the lifetime of the final charge separated state. The ability to execute a multi-electron proton-coupled electron transfer mechanism in a stepwise manner will allow us to investigate the structural and electronic requirements for each step aiding in overall system optimization. Thus, it is possible to use the same multi-step electron transfer strategy that has been employed to extend the lifetime of charge-separated states in photodriven donor-acceptor systems to extend the lifetime of the reduced states of metal complexes of potential use in catalytic proton reduction.
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U2 - 10.1039/c1ee01334c
DO - 10.1039/c1ee01334c
M3 - Article
AN - SCOPUS:79959834389
SN - 1754-5692
VL - 4
SP - 2441
EP - 2450
JO - Energy and Environmental Science
JF - Energy and Environmental Science
IS - 7
ER -
