Photoinduced intramolecular electron transfer in covalently linked porphyrin-triptycene-(bis)quinone diads and triads

Intramolecular photoinduced electron transfer (ET) in porphyrin-quinone diads and porphyrin-quinone(Q(A))-quinone(Q(B)) triads was investigated in different solvents by means of picosecond fluorescence and transient absorption spectroscopy. Charge separation rates (5 x 10(9) s(-1) to > 1 x 10(11) s(-1)) and charge recombination rates are presented and discussed in terms of the Marcus theory. The free enthalpy of the ET compounds varies from 0.52 to 1.2 eV, which led to ET in the normal and inverted region. It is shown that porphyrin-triptycene-bis-quinones have to be considered as an acceptor unit, rather than two separated quinone accepters, due to their electronic coupling. The data show that in the triads, intramolecular photoinduced ET to either the first or the second quinone occurs depending on the energetics of the compounds, which are influenced by the different bridges between donor and acceptor. There is therefore no indication of sequential ET from the porphyrin to one quinone and subsequently to the other. The results are discussed in comparison with previously described investigations of related porphyrin-acceptor-acceptor compounds in the literature. (C) 1999 Elsevier Science B.V. All rights reserved.