FLUORESCENCE QUENCHING OF 5,10,15,20-TETRA(p-TOLYL)PORPHINE and ITS ZINC COMPLEX BY QUINONES. CHARGE-TRANSFER INTERACTION and TRANSIENT EFFECT

Molecular complexes of water-soluble porphyrins

Water-soluble porphyrins tend to form self-aggregates under certain conditions. The structure of the self-aggregate (H-type dimer, H-type higher aggregate, or J-aggregate) sensitively depends on the structure of the peripheral substituents at the meso-positions of the porphyrin. Water-soluble porphyrins also form relatively stable π-stacked complexes with various aromatics. Polar effects are important in complexation of anionic porphyrins with anthraquinonesulfonates. The results suggest that the main attractive interaction for formation of π-stacked complexes of water-soluble porphyrins is the σ-π interaction as claimed by Hunter and Sanders. Meanwhile, the aryl groups at the meso-positions of the water-soluble porphyrins are included by per-O-methylated β-cyclodextrin through van der Waals interactions to form extremely stable 1:2 complexes (porphyrin:cyclodextrin). Such a character of the water-soluble porphyrins can be applied to convenient preparation of supramolecular hetero-porphyrin arrays.

Membrane-anchored DNA assembly for energy and electron transfer

In this work we examine the trapping and conversion of visible light energy into chemical energy using a supramolecular assembly. The assembly consists of a light-absorbing antenna and a porphyrin redox center, which are covalently attached to two complementary 14-mer DNA strands, hybridized to form a double helix and anchored to a lipid membrane. The excitation energy is finally trapped in the lipid phase of the membrane as a benzoquinone radical anion that could potentially be used in subsequent chemical reactions. In addition, in this model complex, the hydrophobic porphyrin moiety acts as an anchor into the liposome positioning the DNA construct on the lipid membrane surface. The results show the suitability of our system as a prototype for DNA-based light-harvesting devices, in which energy transfer from the aqueous phase to the interior of the lipid membrane is followed by charge separation.

Light-induced electron transfer in porphyrin–calixarene conjugates

The fluorescence from a set of porphyrin-calixarene complexes is quenched upon addition of benzo-1,4-quinone (BQ) in fluid solution. In N,N-dimethylformamide solution, fluorescence quenching involves both static and dynamic interactions but there are no obvious differences between porphyrins with or without the appended calixarene. Under such conditions, the static quenching behaviour is attributed to pi-complexation between the reactants and it is concluded that the calixarene cavity does not bind BQ. An additional static component is apparent in dichloromethane solution. This latter effect involves partial fluorescence quenching, for which the intramolecular rate constant can be obtained by time-resolved fluorescence spectroscopy. The derived rate constants depend on molecular structure in a manner consistent with fluorescence quenching being due to electron transfer. In all cases, however, the dominant quenching step involves diffusional contact between the porphyrin nucleus and a non-bound molecule of BQ.

The analysis of the electron transfer rate from steady-state fluorescence spectroscopy

The feasibility of obtaining the electron transfer rate from steady-state fluorescence spectroscopy was investigated. Polymeric films with a fixed concentration of an electron donor molecule were studied as a function of an electron acceptor concentration. The transfer studied was from tetraphenylporphyrin to duroquinone in a polystyrene matrix. The data were analyzed according to Perrin model and the critical radius for electron transfer obtained was 7.1 A. There was an attempt to obtain the electron transfer rate from the nearest-neighbor and Inokuti-Hirayama models, but the parameters that define the rate could not be unequivocally determined.

Reversal of quinone-induced chlorophyll fluorescence quenching

We have used two methods to investigate the reversibility of the interaction of substituted quinones with the thylakoid membrane of plant chloroplasts. Treatment of chloroplasts with added quinones lowers the room-temperature Photosystem II chlorophyll fluorescence intensity by variable amounts depending on the identity and concentration of the quinone. The extent of restoration of the chlorophyll fluorescence level is used as a measure of the effectiveness of the reversal technique. One reversal method involves the addition of thiols to quinone-treated chloroplasts to alter the quinone in a chemical way via a nucleophilic 1,4-Michael addition. In general, the modified quinones exhibit a lower affinity for the thylakoid membrane, as evidenced by an accompanying increase in chlorophyll fluorescence. The thiol concentrations necessary for quenching reversal are found to be in the order [dithiothreitol] less than [2-mercaptoethanol] less than [glutathione]. The second reversal method examines the extent to which added quinones can be removed from thylakoid membranes using a concentration gradient established by resuspension of quinone-treated chloroplasts in quinone-free media. The results further support the reversible nature of the quinone inhibition and indicate that the extent of recovery is dependent upon the degree of fluorescence inhibition originally induced by the added quinone.