Spectroscopic and Electrochemical Studies of Photochemical Electron Transfer in Linked Donor-Acceptor Molecules

Some covalently-linked porphyrin-quinone (PQ) molecules of restricted molecular geometry are described. The excited state P*Q, Where P* is the porphyrin S1 state, probably undergoes intramolecular electron transfer to produce the charge-separated state P†Q. The forward rate constant et f at 295 K for this electron transfer, measured from fluorescence quenching in PAQ (Scheme 1) compared with its hydroquinone analogue PAQH2, lies in tne range (1 − 230) × 107 s− and is strongly solvent dependent. The electrochemically measured energy U± of P†Q with respect to the ground state PQ is 1.35 – 1.41 eV and is also somewhat solvent dependent. Rationalization of the solvent dependence of k et f and of U± is attempted both in terms of the Marcus theory of electron transfer and in terms of the Onsager reaction field theory; the latter appears to be more successful.