Calculation of Electron Transfer Rate Constants from Emission Spectra in Re(I) Chromophore−Quencher Complexes

Rigid medium effects on photophysical properties of MLCT excited states of polypyridyl Os(II) complexes in polymerized poly(ethylene glycol)dimethacrylate monoliths

Higher-energy emissions from the metal-to-ligand charge-transfer (MLCT) excited states of a series of polypyridyl Os(II) complexes were observed at the fluid-to-film transition in PEG-DMA550. The higher-energy excited states, caused by a "rigid medium effect" in the film, led to enhanced emission quantum yields and longer excited-state lifetimes. Detailed analyses of spectra and excited-state dynamics by Franck-Condon emission spectral analysis and application of the energy gap law for nonradiative excited-state decay reveal that the rigid medium effect arises from the inability of part of the local medium dielectric environment to respond to the change in charge distribution in the excited state during its lifetime. Enhanced excited-state lifetimes are consistent with qualitative and quantitative predictions of the energy gap law.

Influence of the fluid-to-film transition on photophysical properties of MLCT excited states in a polymerizable dimethacrylate fluid

Photophysical properties of the salts [Ru(bpy)(3)](p-Tos)(2), [Ru(dmb)(3)](PF(6))(2), [Ru(vbpy)(3)](PF(6))(2), and [Ru(phen)(3)](p-Tos)(2) (bpy = 2,2'-bipyridine, dmb = 4,4'-dimethyl-2,2'-bipyridine, vbpy = 4-methyl-4'-vinyl-2,2'-bipyridine, phen = 1,10-phenanthroline, and p-Tos = p-toluene sulfonate) in fluid and film polyethylene glycol dimethacrylate containing nine ethylene glycol spacers (PEG-DMA550) are reported. MLCT absorption energies and bandshapes are similar in fluid and film PEG-DMA550 pointing to similar local dielectric environments, presumably dominated by the polar acrylate groups. Emission energies and excited-to-ground state 0-0 energy gaps (E(0)), determined by emission spectral fitting, are blue-shifted, and band-widths-at-half height (Δv(0,1/2)) decreased, due to an expected "rigid medium effect" in PEG-DMA550 film. The extent of loss of medium dipole reorientation in the rigid environment, and the increased emission energies in the film, resulted in enhanced emission quantum yields and excited state lifetimes in accordance with the energy gap law. The "rigid medium effect" in PEG-DMA550 is less pronounced than in films of poly(methyl methacrylate) (PMMA) pointing to a more fluid-like local environment presumably arising from the ethylene glycol linker spacers in PEG-DMA550. Comparison of the absorption, emission, emission spectral fitting, and emission lifetime results for [Ru(dmb)(3)](PF(6))(2) and [Ru(vbpy)(3)](PF(6))(2) shows that the vinyl groups of vbpy copolymerize with PEG-DMA550 covalently incorporating Ru(vbpy)(3)(2+) as a cross-linker into the polymer network. The most dramatic effect of the fluid-to-film transition is seen in the emission lifetime data for [Ru(phen)(3)](p-Tos)(2), with an increase of ~3 in the PEG-DMA550 film. Ru(phen)(3)(2+) cations appear to occupy a low symmetry site in the films probably close to the polar acrylate groups in a structurally confined environment.

Chemical approaches to artificial photosynthesis. 2

The goal of artificial photosynthesis is to use the energy of the sun to make high-energy chemicals for energy production. One approach, described here, is to use light absorption and excited-state electron transfer to create oxidative and reductive equivalents for driving relevant fuel-forming half-reactions such as the oxidation of water to O2 and its reduction to H2. In this "integrated modular assembly" approach, separate components for light absorption, energy transfer, and long-range electron transfer by use of free-energy gradients are integrated with oxidative and reductive catalysts into single molecular assemblies or on separate electrodes in photelectrochemical cells. Derivatized porphyrins and metalloporphyrins and metal polypyridyl complexes have been most commonly used in these assemblies, with the latter the focus of the current account. The underlying physical principles--light absorption, energy transfer, radiative and nonradiative excited-state decay, electron transfer, proton-coupled electron transfer, and catalysis--are outlined with an eye toward their roles in molecular assemblies for energy conversion. Synthetic approaches based on sequential covalent bond formation, derivatization of preformed polymers, and stepwise polypeptide synthesis have been used to prepare molecular assemblies. A higher level hierarchial "assembly of assemblies" strategy is required for a working device, and progress has been made for metal polypyridyl complex assemblies based on sol-gels, electropolymerized thin films, and chemical adsorption to thin films of metal oxide nanoparticles.

Utilization of the Highly Environment-Sensitive Emission Properties of Rhenium(I) Amidodipyridoquinoxaline Biotin Complexes in the Development of Biological Probes

We report the synthesis and characterization of luminescent rhenium(I) amidodipyridoquinoxaline biotin complexes [Re(CO)3(dpqa)(L)](PF6) (dpqa = 2-(n-butylamido)dipyrido[3,2-f:2',3'-h]quinoxaline; L = 4-(biotinamidomethyl)pyridine (py-4-CH2-NH-biotin) (1), 3-(N-((2-biotinamido)ethyl)amido)pyridine (py-3-CO-NH-en-NH-biotin) (2), 4-(N-((6-biotinamido)hexanoyl)aminomethyl)pyridine (py-4-CH2-NH-cap-NH-biotin) (3)), and their biotin-free counterpart [Re(CO)3(dpqa)(py)](PF6) (py = pyridine (4)). Upon irradiation, these complexes exhibited intense triplet metal-to-ligand charge-transfer (3MLCT) (dpi(Re) --> pi(dpqa)) emission in fluid solutions at 298 K and in alcohol glass at 77 K. However, the emission became much weaker in aqueous buffer, probably due to the interactions of water molecules with the amide substituent of the dpqa ligand. These properties render the complexes good candidates as luminescent probes for hydrophobic media, such as the substrate-binding sites of proteins. The avidin-binding properties of the new biotin complexes have been studied by 4'-hydroxyazobenzene-2-carboxylic acid (HABA) assays, emission titrations, and competitive association and dissociation assays. Most importantly, the complexes showed a profound increase in emission intensities upon binding to avidin. Additionally, we found that the fluorescence of anthracene was quenched by these rhenium(I) complexes, and the 3MLCT emission of the complexes was also quenched by anthracene. On the basis of these findings, new homogeneous assays for biotin using these complexes, avidin, and anthracene-labeled avidin have been designed.

Synthesis, Characterization, Crystal Structure, and Electrochemical, Photophysical, and Protein-Binding Properties of Luminescent Rhenium(I) Diimine Indole Complexes

We report the synthesis, characterization, and photophysical and electrochemical properties of a series of luminescent rhenium(I) diimine indole complexes, [Re(N-N)(CO)3(L)](CF3SO3) (N-N = 3,4,7,8-tetramethyl-1,10-phenanthroline (Me4-phen), L = N-(3-pyridoyl)tryptamine (py-3-CONHC2H4-indole) (1a), N-[N-(3-pyridoyl)-6-aminohexanoyl]tryptamine, (py-3-CONHC5H10CONHC2H4-indole) (1b); N-N = 1,10-phenanthroline (phen), L = py-3-CONHC2H4-indole (2a), py-3-CONHC5H10CONHC2H4-indole (2b); N-N = 2,9-dimethyl-1,10-phenanthroline (Me2-phen), L = py-3-CONHC2H4-indole (3a), py-3-CONHC5H10CONHC2H4-indole (3b); N-N = 4,7-diphenyl-1,10-phenanthroline (Ph2-phen), L = py-3-CONHC2H4-indole (4a), py-3-CONHC5H10CONHC2H4-indole (4b)), and their indole-free counterparts, [Re(N-N)(CO)3(py-3-CONH-Et)](CF3SO3) (py-3-CONH-Et = N-ethyl-(3-pyridyl)formamide; N-N = Me4-phen (1c), phen (2c), Me2-phen (3c), Ph2-phen (4c)). The X-ray crystal structure of complex 3a has also been investigated. Upon irradiation, most of the complexes exhibited triplet metal-to-ligand charge-transfer (3MLCT) (d pi(Re) --> pi*(diimine)) emission in fluid solutions at 298 K and in low-temperature glass. However, the structural features and long emission lifetimes of the Me4-phen complexes in solutions at room temperature suggest that the excited state of these complexes exhibited substantial triplet intraligand (3IL) (pi --> pi*) (Me4-phen) character. The binding interactions of these complexes to indole-binding proteins including bovine serum albumin and tryptophanase have been examined.

Probing the Excited States of Ru(II) Complexes with Dipyrido[2,3-a:3‘,2‘-c]phenazine: A Transient Resonance Raman Spectroscopy and Computational Study

The lifetimes and transient resonance Raman spectra for Ru(II) complexes with the dipyrido[2,3-a:3',2'-c]phenazine (ppb) ligand and substituted analogues have been measured. The effect of altering the Ru(II) center ([Ru(CN)4]2- versus [Ru(bpy)2]2+), of the complex, on the excited-state lifetimes and spectra has been considered. For [Ru(bpy)2L]2+ complexes the excited-state lifetimes range from 124 to 600 ns in MeCN depending on the substituents on the ppb ligand. For the [Ru(CN)4L]2- complexes the lifetimes in H2O are approximately 5 ns. The transient resonance Raman spectra for the MLCT excited states of these complexes have been measured. The data are analyzed by comparison with the resonance Raman spectra of the electrochemically reduced [(PPh3)2Cu(mu-L*-)Cu(PPh3)2]+ complexes. The vibrational spectra of the complexes have been modeled using DFT methods. For experimental ground-state vibrational spectra of the complexes the data may be compared to calculated spectra of the ligand or metal complex. It is found that the mean absolute deviation between experimental and calculated frequencies is less for the calculation on the respective metal complexes than for the ligand. For the transient resonance Raman spectra of the complexes the observed vibrational bands may be compared with those of the calculated ligand radical anion, the reduced complex [Ru(CN)4L*-]3-, or the triplet state of the complex. In terms of a correlation with the observed transient RR spectra, calculations on the metal complex models offered no significant improvement compared to those based on the ligand radical anion alone. In all cases small structural changes are predicted on going from the ground to excited state.

Evidence for Through-Space Electron Transfer in the Distance Dependence of Normal and Inverted Electron Transfer in Oligoproline Arrays

Four new helical oligoproline assemblies containing 16, 17, 18, and 19 proline residues and ordered arrays of a Ru(II)-bipyridyl chromophore and a phenothiazine electron-transfer donor have been synthesized in a modular fashion by solid-phase peptide synthesis. These arrays are illustrated and abbreviated as CH(3)CO-Pro(6)-Pra(PTZ)-Pro(n)()-Pra(Ru(II)b(2)m)(2+)-Pro(6)-NH(2), where PTZ is 3-(10H-phenothiazine-10)propanoyl and (Ru(II)b'(2)m)(2+) is bis(4,4'-diethylamide-2,2'-bipyridine)(4-methyl,4'-carboxylate,2,2'-bipyridine)ruthenium(II) dication with n = 2 (2), 3 (3), 4 (4), and 5 (5). They contain PTZ as an electron-transfer donor and (Ru(II)b'(2)m)(2+) as a metal-to-ligand charge transfer (MLCT) light absorber and are separated by proline-to-proline through-space distances ranging from 0 (n = 2) to 12.9 A (n = 5) relative to the n = 2 case. They exist in the proline-II helix form in water, as shown by circular dichroism measurements. Following laser flash Ru(II) --> b'(2)m MLCT excitation at 460 nm in water, excited-state PTZ --> Ru(2+) quenching (k(2)) occurs by reductive electron transfer, followed by Ru(+) --> PTZ(+) back electron transfer (k(3)), as shown by transient absorption and emission measurements in water at 25 degrees C. Quenching with DeltaG degrees = -0.1 eV is an activated process, while back electron transfer occurs in the inverted region, DeltaG degrees = -1.8 eV, and is activationless, as shown by temperature dependence measurements. Coincidentally, both reactions have comparable distance dependences, with k(2)( )()varying from = 1.9 x 10(9) (n = 2) to 2.2 x 10(6) s(-)(1) (n = 4) and k(3) from approximately 2.0 x 10(9) (n = 2) to 2.2 x 10(6) s(-)(1) (n = 4). For both series there is a rate constant enhancement of approximately 10 for n = 5 compared to n = 4 and a linear decrease in ln k with the through-space separation distance, pointing to a significant and probably dominant through-space component to intrahelical electron transfer.

Solvent-Dependent Dynamics of the MQ•→ReII Excited-State Electron Transfer in [Re(MQ+)(CO)3(dmb)]2+

The Re-->MQ(+) MLCT excited state of [Re(MQ(+))(CO)(3)(dmb)](2+) (MQ(+) = N-methyl-4,4'-bipyridinium, dmb = 4,4'-dimethyl-2,2'-bipyridine), which is populated upon 400-nm irradiation, was characterized by picosecond time-resolved IR and resonance Raman spectroscopy, which indicate large structural differences relative to the ground state. The Re-->MQ(+) MLCT excited state can be formulated as [Re(II)(MQ*)(CO)(3)(dmb)](2+). It decays to the ground state by a MQ*-->Re(II) back-electron transfer, whose time constant is moderately dependent on the molecular nature of the solvent, instead of its bulk parameters: formamides approximately DMSO approximately MeOH (1.2-2.2 ns) < THF, aliphatic nitriles (3.2-3.9 ns) << ethylene-glycol approximately 2-ethoxyethanol (4.2-4.8 ns) < pyridine (5.7 ns) < MeOCH(2)CH(2)OMe (6.9 ns) < PhCN (7.5 ns) < MeNO(2) (8.6 ns) <<< CH(2)Cl(2), ClCH(2)CH(2)Cl (25.9-28.9 ns). An approximate correlation was found between the back-reaction rate constant and the Gutmann donor number. Temperature dependence of the decay rate measured in CH(2)Cl(2), MeOH, and BuCN indicates that the inverted MQ*-->Re(II) back-electron transfer populates a manifold of higher vibrational levels of the ground state. The solvent dependence of the electron transfer rate is explained by solvent effects on inner reorganization energy and on frequencies of electron-accepting vibrations, by interactions between the positively charged MQ(+) pyridinium ring and solvent molecules in the electron-transfer product, that is the [Re(MQ(+))(CO)(3)(dmb)](2+) ground state.

Calculation of triplet-triplet energy transfer rates from emission and absorption spectra. The quenching of hemicarcerated triplet biacetyl by aromatic hydrocarbons

The kinetics of triplet-triplet (T-T) energy transfer have been analysed with a view to linking theories of chemical reactions (involving the rupture and formation of bonds) with theories of processes, such as electron transfer or energy transfer, which preserve chemical bonding. As for the latter, our analysis does not support the claim that, of the two rival expressions for T-T energy transfer, both rooted in the golden rule, only one is applicable to electron transfer or T-T transfer. Though the two expressions do reflect different standpoints, the distinction is eroded by the assumption of a delta-function distribution for the vibrational spectrum. It is shown that theories of chemical reactions also furnish estimates of Franck-Condon factors; rates of chemical reactions and chemical processes are both related to the properties (strengths and lengths) of the reactive bonds, but differ in the mode of energy dissipation. The relationship between the rates of reactions and processes presents new possibilities for a unified view of chemical reactivity.