PHOTOELECTRON TRANSFER BETWEEN A MAGNESIUM-FREE-BASE PORPHYRIN HETERODIMER AND DUROQUINONE - SELECTIVE EXCITATION AND TIME-RESOLVED EPR STUDIES

The photoexcited triplet state of a cofacial heterodimer, [Mg-H-2], comprised of Mg and H-2 porphyrins, and photoinduced electron transfer (ET) between the heterodimer and duroquinone in 1:1 mixture of CH2Cl2/ethanol were studied by selective laser excitation combined with time-resolved CW or with pulsed EPR spectroscopies. ET originates from the photoexcited triplet or triplet radical pair states. Upon photoexcitation of the Mg subunit (580 nm), a noticeable delay time of approximately 20 ns in ET is whereas with photoexcitation of the H-2 part (620 nm), no delay time in ET is noticed. The dependence of ET upon excitation wavelength is interpreted in terms of the formation and participation of a charge-transfer state that is operative at 205 K. The delay time is attributed to an intradimer ET that produces the triplet mdical pair-state 3[Mg.+-H-2.-]. Photoexcitation at 620 nm results in ET via the lower-lying triplet of H-2 without involvement of the charge-transfer state.