Contribution of long-range Coulomb interactions to bimolecular luminescence quenching reactions

Abstract

9660J . Phys. Chem. 1991. 95, 9660-9666 flexible molecules. However, the resolution obtained from a single measurement can be modest (see xR2 surfaces for single files) and estimated values of the recovered parameters can be questionable. Beechem and H a d have recently discussed this problem for time-domain measurements and suggested simultaneous analysis of the donor and acceptor decay kinetics. This type of global analysis is potentially valuable and intuitively obvious, but is very difficult to realize in practice. Usually, acceptors are excited simultaneously during the excitation of the donor, and the amount of excited acceptor molecules due to energy transfer can be small compared with the whole population of excited-state acceptors. That is, the population or^ excited acceptors can be predominately the result of acceptors which are directly excited. The time-dependent decays of these acceptors do not contain any information on the distance distributions or dynamics, and this component can only result in decreased information content and/or resolution from the data. One must also separate donor and acceptor absorption and emission spectra and introduce additional parameters into the analysis to account for direct acceptor excitation and/or spectral overlap. I n contrast, the global analysis obtained by varying the donor lifetime does not introduce any additional parameters, gives an acceptable increase of the resolution for each fitted parameter, and can be used for any donor-acceptor system including nonfluorescent acceptors.The recovered initial distance distribution (Figure 9, insert, solid line) is in very good agreement with that found previously for TU2D in viscous solution32 (Figure 9, insert, dashed line). The diffusion coefficient, D = I .26 X cm2/s (I26 A2/ns), is in the range of expected values for small molecules in low-viscosity solutions. I t is interesting to compare the end-to-end diffusion coefficient with that obtained for a nonlinked system. Indoleto-dansyl intermolecular energy transfer has been investigated previously in propylene glycol and methanol.33 The data were fitted well to a Gosele et al. model" yielding a diffusion coefficient in methanol at 20 "C, D = 2.64 X cm2/s. The end-to-end diffusion coefficient obtained for TU2D in methanol is only twice smaller than that recovered from the mixture of indole and dansyl, where the polyethylene linker was not present. The data obtained for the TU2D donor-acceptor system and indole-dansyl mixture show that the polyethylene linker containing 22 carbons is highly flexible and has a moderate influence on end-to-end diffusion.The luminescence quenching of the hexanuclear cluster ions Mo,CI,,~-and W6II4'-by pyridinium and bipyridinium ions in nonaqueous solution has been investigated. Stern-Volmer (SV) plots of the emission intensity and lifetime quenching data for the reaction of M6X142-* with pyridinium acceptors are linear, and slopes of -0.48 for plots of R T In kobs vs the free energy driving force for electron transfer are in accordance...

Photonic interfacial supramolecular assemblies incorporating transition metals

“…Taken in conjunction with the Stern-Volmer constant, the electrontransfer rate constant is between 8 × 10 6 and 3 × 10 8 M −1 s −1 . This rate constant compares favorably with those reported elsewhere for ruthenium polypyridyl complexes in solution, solids and thin films [56][57][58][59][60]. However, when describing lateral quenching it is more appropriate to consider the process as occurring in twodimensional space.…”

Bimolecular Electron Transfer in the Marcus Inverted Region

“…As shown in Figure , the parabolic dependence predicted by eq 1 is truncated by the diffusion limit. Consequently, the observed rate constants of most bimolecular reactions display an increase with increasing free energy followed by a leveling at the diffusional limit. − Typically, the driving forces of bimolecular ET reactions have not been sufficiently energetic to permit the ET rates to lie outside the diffusion limit. Pioneering studies by Sutin and Creutz reported “vestiges” of the inverted region for the bimolecular reactions of metal polypyridyl complexes, and the inverted region has been indirectly inferred from chemiluminescent bimolecular reactions. , Direct observation of inverted region behavior for a bimolecular reaction has only recently been achieved by Gray and co-workers in their studies of the back reaction between the products resulting from the quenching of an electronically excited binuclear iridium complex by pyridinium acceptors …”

Divergent Electronic Structures of Isoelectronic Metalloclusters: Tungsten(II) Halides and Rhenium(III) Chalcogenide Halides

“…It is apparent that the face-bridging ligands decisively influence the emission maxima; luminescence parameters of clusters having the same bridging ligand gravitate together in Figure 2; the E em values of the [W 6 I 8 X' 6 ] 2À clusters are identical within experimental error at 21 AE 2 8C in acetonitrile.Alongside the luminescence lifetimes, other experimental evidence suggests emission from a spin-triplet excited state. Tungsten(II) halide cluster emission undergoes bimolecular quenching both oxidatively and by energy-transfer mechanisms; [37,[48][49][50] molybdenum analogues are quenched both oxidatively and reductively, and by energy transfer. 57] The luminescence quenching property of [Mo 6 Cl 14 ] 2À has been applied in the building of a fiber-optic oxygen sensor active at partial pressures encountered in automotive and biological applications, and having a theoretical response time of 1 s. [58] Quantum-chemical characterization specifically of the tungsten(II) halide clusters is sparse.…”