The fluorescence of a polyanionic conjugated polymer can be quenched by extremely low concentrations of cationic electron acceptors in aqueous solutions. We report a greater than millionfold amplification of the sensitivity to fluorescence quenching compared with corresponding ''molecular excited states.'' Using a combination of steady-state and ultrafast spectroscopy, we have established that the dramatic quenching results from weak complex formation [polymer (؊) ͞quencher (؉) ], followed by ultrafast electron transfer from excitations on the entire polymer chain to the quencher, with a time constant of 650 fs. Because of the weak complex formation, the quenching can be selectively reversed by using a quencher-recognition diad. We have constructed such a diad and demonstrate that the fluorescence is fully recovered on binding between the recognition site and a specific analyte protein. In both solutions and thin films, this reversible fluorescence quenching provides the basis for a new class of highly sensitive biological and chemical sensors. With the rising awareness of the public vulnerability to chemical and biological terrorism, there is a heightened need for detection techniques that show both high sensitivity and selectivity. Such techniques also would find wide use in medical diagnostics and biomedical research applications. Methods of identifying biological molecules such as the enzyme-linked immunosorbant assay (ELISA) achieve selectivity by using specific antibody͞antigen interactions to anchor the antigen to a substrate, with a subsequent colorimetric change or fluorescence signal on addition of secondary reagents; these techniques can be time-consuming and require multistep procedures. Other approaches have used molecular recognition ligands to link to specific receptor sites on a biological species, usually as a means also of fixing the biomolecule to a substrate or membrane (1-6) It has remained a challenge to incorporate the selectivity offered by ligand͞receptor interactions into a sensor that can be extremely sensitive, robust, and versatile.We have recently explored the photophysical properties of a fluorescent, water-soluble polyanionic conjugated polymer [poly (2-methoxy-5-propyloxy sulfonate phenylene vinylene (MPS-PPV)] (Fig. 1B), one of a larger class of related molecules [poly phenylene vinylene (PPV)] (Fig. 1 A and derivatives) that has been the subject of almost explosive recent interest (7-13). Although much attention has focused on the well known potential for use of PPV derivatives as electronic materials [e.g., electrochemical sensors (14-16) light-emitting diodes (17, 18), and integrated circuits (19,20)], the highly charged backbone of MPS-PPV (with charge density approximating that of polynucleic acids such as DNA and RNA), also makes it a model polymer for understanding the interactions and self-assembly properties of charged biopolymers. In this paper, we report a striking discovery: the use of this fluorescent anionic polymer leads to a greater than million-fold amplificatio...

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A number of phosphatidyl choline derivatives containing trans-azobenzene units in the fatty ester backbone have been synthesized and studied in aqueous dispersions both pure and in the presence of saturated and unsaturated phospholipids. The structures of the assemblies formed have been investigated by microcalorimetry, dynamic light scattering, cryo-transmission electron microscopy, and reagent entrapment. While many of the mixed phospholipid dispersions give evidence for the formation of small unilamellar vesicles, the aqueous dispersions of pure azobenzene phospholipids (APL's) give evidence for several different structures, including relatively large plates in at least one case. The azobenzenes show strong evidence of “H” aggregate formation both in the pure and mixed dispersions. The aggregation number has been estimated for several of the APL's and found to be typically 3 or a multiple thereof. On the basis of simulations and studies with similar stilbene phospholipids as well as on the strong induced circular dichroism signals observed for the aggregate, we infer a chiral “pinwheel” unit aggregate structure similar to that found for several aromatics. The azobenzenes in the aqueous dispersions have been found to photoisomerize to give cis-rich photostationary states; the cis-azobenzenes show no evidence for aggregation and no induced circular dichroism. The cis-azobenzenes can be isomerized back to the trans either by irradiation or by thermal paths. Mixed aqueous dispersions of trans-APL's with saturated or unsaturated phospholipids can be prepared which entrap the fluorescent dye carboxyfluorescein (CF) under conditions where the CF fluorescence is very low due to self-quenching. By varying the APL/host phospholipid ratio the azobenzene can be aggregate, monomer, or dimer. In cases where the azobenzene is monomer or dimer, irradiation produces complete isomerization but little “leakage” of CF from the vesicle interior. In contrast, where the azobenzene is predominantly aggregate, irradiation results in both photoisomerization and reagent release. That photoisomerization in the latter case can result in “catastrophic” destruction of the vesicle can also be shown by cryo-transmission electron microscopy.

The syntheses of several surfactant analogues of tris(2,2'-bipyridine)rutheni~m(iI)~+ is described together with a study of their photochemistry in spread films, monolayer assemblies, and at a monolayer assembly-water interface. It has been found that highly purified samples of the dioctadecyl ester of (4,4'-dicarboxy-2,2'-bipyridine)bis(2,2'-bipyridine)ruthenium( 11)2+ are inactive as catalysts for the photocleavage of water in monolayer assemblies. This is contrary to our previous findings, which employed a sample of the surfactant complex which has now been shown to contain several impurities, including other surfactant ruthenium(i1) complexes. The properties of films and monolayer assemblies of these complexes are found to be quite sample sensitive even when different samples of high indicated purity are employed. The failure of the highly purified complex to serve as a catalyst appears partially due to a light-induced destruction of assemblies irradiated in contact with water, which is due at least in part to a photohydrolysis of the ester group.The photochemistry of the tris(2,2'-bipyridine)ruthenium(I1) dication (Ru(bpy)32+) and related metal complexes has been the subject of considerable recent i n v e~t i g a t i o n .~-~~ It has been found that the excited states of Ru(bpy)32+ and other complexes having relatively long excited state lifetimes can be quenched by energy transfer, complex formation, and electron transfer processes. The latter have been observed both for cases where the excited state of Ru(bpy)32+ serves as an electron d~n o r~-~ as well as for instances where it acts as an electron acceptor.8-'0 There has been intense interest in the possibility that light driven electron transfer reactions can serve as useful energy conversion and storage processes,20-2' since the initial products of electron transfer are usually highly energetic, but stable, when isolated, molecules. In solution the high-energy products are rapidly degraded through reverse electron transfer with each other to form ground-state starting material^.^.^ Recently, interest has developed in the possibility of inhibiting these back reactions by various techniques, including the initiation of competitive processes or the carrying out of the reactions in other media.11,21 I n recent investigations, we have examined the photochemical reactivity of a number of chromophores in organized monolayer a~s e m b l i e s . l .~,~~-*~ In several cases it has been found that the controlled and highly condensed environment provided by the assemblies can strongly modify photoreactivity and luminescence behavior from that observed in solution. Our interest in light driven electron transfer processes involving Ru(bpy)32+ led us to prepare surfactant complexes of ruthenium( 11) for incorporation into monolayer assemblies. In a recent communication2 we reported that the strong luminescence of a surfactant ruthenium complex could be quenched by immersion of assemblies containing the complex into water. Concomitantly, the light-induced cleavage of w...