Intramolecular Photochemical Electron Transfer. 4. Singlet and Triplet Mechanisms of Electron Transfer in a Covalently Linked Porphyrin-Amide-Quinone Molecule

We have carried out an extensive photophysical analysis of a tetraarylporphine linked through a single amide bridge to either methyl-p-benzoquinone (PAQ) or the corresponding hydroquinone (PAQH₂) in benzonitrile as the solvent. The photophysical properties of PAQH₂ are closely similar to those of nonlinked tetraarylporphine species, while for PAQ significant lifetime quenching of both the lowest excited singlet and triplet states is observed. Picosecond transient absorption spectroscopy and fluorescence lifetime measurements were used to show that quenching of the excited singlet state of PAQ is due to intramolecular electron transfer to the singlet radical ion pair ¹(P^·+'+AQ^·--) with a rate constant of 4.1 (±0.3) × 10⁸ s^{-1 1}(P^·+AQ^·- ) subsequently decays to the ground state by reverse electron transfer with a rate constant of 1.6 (±0.2) × 10⁸ s^-1. This reaction has ΔG° ≅-1.4 eV and is predicted to be in the Marcus inverted region. The experimental ratio of the forward to reverse rate constants is very similar to that predicted by Marcus theory. Nanosecond flash photolysis studies show that the lowest triplet state of PAQ is also quenched, most likely by electron transfer to the triplet radical ion pair ³(P^·+AQ^·-), with a rate constant of 4.6 (±0.2) × 10⁴ s^-1. We suggest that ³(P^·+AQ^·-) interconverts rapidly with ¹(P^·+AQ^·-), which then decays rapidly to the ground state.