Studies on porphyrin-quinhydrone complexes: molecular recognition of quinone and hydroquinone in solution

Free-base and zinc(II) porphyrins bearing either one, two, or four hydroquinone entities at the meso positions are shown to bind quinones in solutions via a quinhydrone pairing mechanism. Electrochemical studies reveal that the quinhydrone complexes are stabilized by charge-transfer interactions between the donor (hydroquinone) and the acceptor (quinone). The redox potentials of the quinhydrone complexes are governed by the potentials of the quinones utilized to form quinhydrone. The (1)H NMR studies reveal that the quinhydrone complexes are stabilized by H-bonding in addition to the charge-transfer interactions. Singlet emission studies have shown that the fluorescence quenching of the porphyrin increases with an increase in the number of receptors, i.e., hydroquinone entities on the porphyrin macrocycle. Control experiments performed by using zinc porphyrin bearing a dimethoxyphenyl group, i.e., a receptor entity with no H-bonding ability, indicate that the H-bonding plays an important role in quinhydrone formation. Porphyrin-quinhydrone formed by using covalently linked porphyrin-quinone and hydroquinone present in solution shows fluorescence enhancement. The measured fluorescence quantum yields, phi(f), are found to depend on the metal ion in the porphyrin cavity and the oxidation potential of the employed hydroquinones. The present studies also reveal that the measured phi(f) values depend on how the quinhydrone is linked to the porphyrin macrocycle, i.e., either through quinone or hydroquinone. Generally, porphyrin-quinhydrone formed by hydroquinone-appended porphyrins shows decreased phi(f) values as compared to porphyrin-quinhydrone formed by quinone-appended porphyrins.