A biological photosynthetic reaction center contains a spatially ordered array of chromophores, electron donors, and electron acceptors that efficiently separate reductive and oxidative equivalents upon irradiation with visible light. This fundamental process of redox separation has been modeled in redox assemblies based on metal-bipyridyl complexes (1) or porphyrin-quinone systems (2-5). We have described the synthesis and photophysical characterization of lysine-based donorchromophore-acceptor triads that undergo light-induced redox separation (6-9). We describe here a general method for designing and constructing a helical oligoproline assembly having a spatially ordered array of redox or other functional groups protruding from a proline-II helical rod. Solid-phase peptide synthesis (10) is an efficient method for assembling a modular chain containing functional sites at specific positions. An a-helical peptide chain can serve as a molecular framework to display functional sites in a spatially ordered array (9). Alternatively, a chain of nine or more proline residues folds into a stable proline-II helix even when several of the proline residues have large functional sites on their side chains (11). The distance between two functional sites is determined by their helical location and orientation. The position of a modified proline residue in the proline chain determines its location on the helical surface. The configuration and conformation of the modified proline residue determine its orientation on the helical surface. To illustrate this general method, we have assembled an oligoproline assembly (triad 1) bearing three different redox sites (Fig. 1). Folding of the peptide triad 1 into a proline-II helix places the donor, chromophore, and acceptor sites in a linear array on one side of the helical rod.

A series of chromophore−quencher complexes, trans-[Ru(bpy)2(pya)(pyb)] n + (bpy = 2,2‘-bipyridyl; pya, pyb = substituted pyridyl ligands; n = 2−4) have been prepared and characterized. These contain combinations of the electron donor PTZ in py-PTZ (py-PTZ = 10-(4-picolyl)phenothiazine) and the electron acceptors MQ+ (1-methyl-4,4‘-bipyridinium cation), py-AQ (4-((((9,10-anthraquinon-2-yl)carbonyl)amino)methyl)pyridine), and py-MPAA (N-(4-pyridyl)-β-(1-methylpyridinium-3-yl)acrylamide cation), and 4-ethylpyridine (4-Etpy). The asymmetrical complexes (pya ≠ pyb) are synthesized by using the precursor trans-[Ru(bpy)2(DMSO)2](CF3SO3)2·0.5H2O, which is converted into trans-[Ru(bpy)2(py-PTZ)(DMSO)](CF3SO3)2·0.5H2O (1) by reaction with py-PTZ in DMSO/acetone at room temperature. 1 is treated with LiCl in aqueous DMF at 105 °C to prepare trans-[RuCl(bpy)2(py-PTZ)]CF3SO3·H2O (2). 2 is converted into the derivatives trans-[Ru(bpy)2(py-PTZ)(4-Etpy)](PF6)2·0.5H2O (5), trans-[Ru(bpy)2(py-PTZ)(MQ+)](PF6)3 (7), trans-[Ru(bpy)2(py-PTZ)(py-AQ)](PF6)2·H2O (9), and trans-[Ru(bpy)2(py-PTZ)(py-MPAA)](PF6)3·H2O (11) by chloride abstraction at room temperature in the presence of 4-Etpy, [MQ](PF6), py-AQ, and [py-MPAA](PF6), respectively. The salts trans-[Ru(bpy)2(4-Etpy)(MQ+)](PF6)3·H2O (6), trans-[Ru(bpy)2(4-Etpy)(py-AQ)](PF6)2 (8), and trans-[Ru(bpy)2(4-Etpy)(py-MPAA)](PF6)3 (10) are synthesized by chloride abstraction from trans-[RuCl(bpy)2(4-Etpy)]PF6·H2O in the presence of [MQ](PF6), py-AQ, or [py-MPAA](PF6). The symmetrical complexes trans-[Ru(bpy)2(MQ+)2](PF6)4·3H2O (3) and trans-[Ru(bpy)2(py-PTZ)2](PF6)2·H2O (4) are synthesized by reaction of trans-[Ru(bpy)2(H2O)2](CF3SO3)2 with an excess of the appropriate ligand in DMF at 100 °C. Visible light irradiation of these complexes at room temperature leads to photochemical decomposition. At lower temperatures (<200 K), emission typical of RuIII−bpy•- (and RuIII−MQ•) MLCT states is observed. The appearance of a thermally activated decay pathway above 150 K signals the presence of a crossing to a photochemically reactive ligand-field state. As a result, significant formation of redox-separated states by electron transfer is not observed for the redox triads trans-[Ru(bpy)2(py-PTZ)(X)] n + (X = py-AQ, n = 2; X = py-MPAA or MQ+, n = 3). Nonetheless, the results obtained have important implications for the stereochemical design of Ru(II)-based chromophore−quencher complexes.

The polymer poly(4{2-[N,N-bis(trimethylsilyl)amino]ethyl}styrene), prepared by anionic polymerization and of low polydispersity (M(w)/M(n) = 1.10-1.18), has been derivatized by amide linkage to [Ru(II)(bpy)(2)(4-(CO-)-4'-CH(3)-bpy)-](2+) (bpy is 2,2'-bipyridine; 4-(CO-)-4'-CH(3)-bpy is 4-carbonyl-4'-methyl-2,2'-bipyridine). Unreacted amine sites were converted into acetamides by treatment with acetic anhydride to give derivatized polymers of general formula [PS-CH(2)CH(2)NHCO(Ru(II)(n)()Me(m)())](PF(6))(2)(n)(), where m + n = 11, 18, or 25, PS represents the polystyrene backbone, and Ru(II) and Me represent the attached complex and acetamide, respectively. Spectral and electrochemical properties of the derivatized polymers are similar to those of the model [Ru(bpy)(2)(4-CONHCH(2)CH(2)C(6)H(5)-4'-CH(3)-bpy)](2+) (4-CONHCH(2)CH(2)C(6)H(5)-4'-CH(3)-bpy is 4'-methyl-2,2'-bipyridinyl-4-(2-phenylethylamide)), but emission quantum yields (phi(em)) and time-resolved emission decays are slightly dependent on the level of Ru(II) loading, with nonexponential, irradiation-dependent decays appearing at high loadings. The decays could be fitted satisfactorily to the first derivative of the Williams-Watts distribution function. These results are discussed with reference to possible structural and multichromophoric effects on excited-state decay.

The helical oligoproline assembly CH3-CO-Pro-Pro-Pro-Pra(Ptzpn)-Pro-Pro-Pra(RuIIb2m2+ -Pro-Pro-Pra(Anq)-Pro-Pro-Pro-NH2, having a spatially ordered array of functional sites protruding from the proline backbone, has been prepared. The 13-residue assembly formed a linear array containing a phenothiazine electron donor, a tris(bipyridine)ruthenium(II) chromophore, and an anthraquinone electron acceptor with the proline II secondary structure as shown by circular dichroism measurements. Following RuII --> b2m metal-to-ligand charge-transfer (MLCT) excitation at 457 nm, electron-transfer quenching occurs, ultimately to give a redox-separated (RS) state containing a phenothiazine (PTZ) radical cation at the Pra(Ptzpn) site and an anthraquinone (ANQ) radical anion at the Pra(Anq) site. The redox-separated state was formed with 33-96% efficiency depending on the solvent, and the transient stored energy varied from -1.46 to -1.71 eV at 22 +/- 2 degrees C. The dominant quenching mechanism is PTZ reductive quenching of the initial RuIII(b2m*-) MLCT excited state which is followed by m*- --> ANQ electron transfer to give the RS state. Back electron transfer is highly exergonic and occurs in the inverted region. The rate constant for back electron transfer is solvent dependent and varies from 5.2 x 10(6) to 7.7 x 10(6) s-1 at 22 +/- 2 degrees C. It is concluded that back electron transfer occurs by direct ANQ*- --> PTZ*+ electron transfer. Based on independently evaluated kinetic parameters, the electron-transfer matrix element is HDA approximately 0.13 cm-1.