Jeffrey K. Nagle

Redox properties of metalloporphyrin excited states, lifetimes, and related properties of a series of para-substituted tetraphenylporphine carbonyl complexes of ruthenium(II)

Excited-state and redox properties of Ru(p-XTPP)(CO), X = MeO, Me, H, F, Cl, H and Br, have been defined. Emission bands were centered at 730 ± 3 nm and excited-state lifetimes were in the range of 30 ± 10 μ

Successful Bifunctional Photoswitching and Electronic Communication of Two Platinum(II) Acetylide Bridged Dithienylethenes

. Two one-electron oxidations in CH2C12 ranged from 0.74 to 0.86 V for the first step and from 1.18 to 1.27 V for the second one. A one-electron reduction process in (CH3)2SO ranged from -1.35 to -1.24 V. Excited-state lifetimes and redox potentials exhibit a weak dependence on the Hammett function. In general, redox potentials increase as the electron-withdrawing power of the substituents increases, whereas excited-state lifetimes decrease. The first oxidation step (0.74-0.86 V) and the reduction step are, respectively, assigned to -electron removal or acceptance by the porphyrin ring. The second oxidation is assigned to removal of an electron from the ruthenium(II) center. The excited state is shown to be the ( *) state of the porphyrin ring and to exhibit photoredox behavior involving both oxidative and reductive quenching. Redox product separation occurred in flash photolysis quenching experiments and back-reactions took place at near-diffusion-controlled rates. The redox potential of the Ru(TPP)(CO)+/* couple was estimated from emission and redox data to be -0.57 ± 0.03 V; it was determined from oxidative quenching studies to be -0.56 ± 0.10 V. Comparison to the excited-state properties of Ru(bpy)32+ is made, and the utility of porphyrin complexes as potential solar energy storage catalysts is examined.

Luminescent excimers and exciplexes of PtII compounds

Coordinating two dithienylethenes to a platinum center results in the reversible ring closure of both photochromic units in a model for a photoresponsive pi-conjugated polymer. This system demonstrates how metal-sensitized photochemistry, from a triplet excited state, circumvents the problems associated with other multicomponent photochromic systems, where significant electronic interactions in the ground state and singlet excited state prevent full photoswitching. Changes in charge-transfer behavior based upon conversion of both dithienylethenes to their ring-closed forms illustrate how photomodulation of conductivity through a conjugated polymer might be achieved using Pt-bis(acetylide)s.

Linker-Dependent Metal-Sensitized Photoswitching of Dithienylethenes

Abstract Spectroscopic and lifetime studies of luminescent excimer formation involving the lowest triplet excited state of Pt(4,7-diphenyl-1,10-phenanthroline)(CN)2 in dichloromethane solutions at room temperature are reported. Such experiments yield information related to the rate constants characterizing excimeric formation and decay. The PtII dimer Pt2(P2O5H2)44− together with Au(CN)2 in deoxygenated aqueous solution exhibits a low energy luminescence band. An exciplex resulting from direct PtII AuI interactions between these ions is postulated to be responsible for the observed luminescence. This appears to be the first exciplex reported between square planar d8 and linear d10 ions, as well as the first between like-charged ions.

Multiple emissions from charge transfer excited states of ruthenium(II)—polypyridine complexes

Intramolecular energy-transfer results in sensitized ring closing of a pendant dithienylethene from a platinum terpyridyl complex only when the two components are connected with a short pi-conjugated linker.

Application of electron-transfer theory to excited-state redox processes

Abstract The complex cis-[(bpy)2Ru 2]4+ (bpy is 2,2′-bipyridine) has been prepared by methylation of (bpy)2Ru 2]2+. Electrochemical studies show that introduction of the bound pyridinium group creates a chemically attached electron acceptor site (E1/2 = −0.76 V in 0.1 M [N(n-C4H9)4]PF6-acetonitrile versus the SSCE). Evidence for a low-lying dπ — π* charge transfer (CT) state has been obtained by the appearance of a low energy emission at λmax 680 nm in ecetonitrile (τ0 = 104 ns) and for an upper dπ — π* (bpy) state by a higher energy emission at 580 nm in a methanol glass at 77 K (τ0 = 7.59 μs). Both emissions appear in a water—ethylene glycol solution containing 5% by weight polyvinyl alcohol at room temperature.

Enhanced Photoluminescence of Sulfur-Bridged Organic Chromophores

Abstract Quenching rate constants have been obtained for oxidative electron transfer quenching of Ru(bpy) 2++ 3 by a series of nitroaromatic and bipyridinium quenchers and reductive quenching by a series of aromatic amines. The results are consistent with electron-transfer theory when competing processes which occur following the electron-transfer quenching step are taken into account. The competing processes are back-electron-transfer to give the excited state Ru(bpy) 2++ 3 and net quenching by the sum of redox-product-separation and back-electron-transfer to give Ru(bpy) 2++ 3

Charge transfer and intraligand excited state interactions in platinum-sensitized dithienylethenes.

A general approach to enhancing the emission quantum yield of several widely studied organic chromophores is presented. The luminescence properties of a series of symmetrical sulfur-bridged chromophores are reported as a function of the oxidation state of the bridging sulfur atom. The photoluminescence quantum yield is significantly enhanced by successively oxidizing the sulfur bridge from sulfide (S), to sulfoxide (SO), to sulfone (SO2).

Phosphorescent lifetime and quantum yield measurements of K4Pt2(H2P2O5)4 from 1.6 to 300 K

The photophysical behavior for two photochromic Pt-terpyridine acetylide complexes containing pendant dithienylethenes (DTEs) bound to the metal through the alkynyl linkage is presented. Selective excitation of the Pt complex with visible light resulted in the metal-sensitized ring closing of the DTE unit. The central purpose of this study was to understand how excited state interactions govern the photophysics by correlating differences in the linkage of the two components with differences in the intramolecular energy transfer processes that occur between the Pt complex and the DTE. A series of model complexes without photochromic ligands were prepared and studied to elucidate the contributions of the triplet metal-to-ligand charge transfer and triplet intraligand states. It is demonstrated that reducing the orbital overlap of the metal-based and intraligand states by lengthening the linkage and eliminating a conjugated pathway is effective at dramatically decreasing the efficiency of intramolecular energy transfer. This is evidenced by the appearance of Pt-terpyridine based phosphorescence and a significant decrease in the observed rate of metal-sensitized ring closing of the DTE.

Tunable Luminescence of Bithiophene-Based Flexible Lewis Pairs

Abstract Phosphorescent lifetime measurements were performed on K 4 Pt 2 (H 2 P 2 O 5 ) 4 in 2/1 ethylene glycol/water solution, as well as on green and brown crystalline forms, front 1.6 to 300 K. Quantum yield measurements on K 4 Pt 2 (H 2 P 2 O 5 ) 4 in solution were performed over the same temperature range. Data for the dissolved sample were fit to yield a 41.8 cm −1 energy splitting of the assumed 3 A 2u excited term into E u (τ = 4.2 μs) and A 1u (τ = 6.06 ms) components. The Quantum yield for this sample varied from 0.13 at 4.2 K to 0.43 at 77 K. The green and brown crystalline form showed similar behavior. with somewhat shorter lifetimes. A purple crystalline form showed no detectable luminescence from 1.6 to 300 K.