Experimental evidence for water mediated electron transfer through bis-amino acid donor-bridge-acceptor oligomers.

This work compares the photoinduced unimolecular electron transfer rate constants for two different solute molecules (D-SSS-A and D-SRR-A) in water and DMSO solvents. The D-SSS-A solute has a cleft between the electron donor and acceptor units, which is able to contain a water molecule but is too small for DMSO. The rate constant for D-SSS-A in water is significantly higher than that for D-SRR-A, which lacks a cleft, and significantly higher for either solute in DMSO. The enhancement of the rate constant is explained by an electron tunneling pathway that involves water molecule(s).

Solvent dynamical effects on electron transfer in U-shaped donor-bridge-acceptor molecules.

This study explores how the electron transfer in a class of donor-bridge-acceptor (DBA) supermolecules is affected by the dynamical response of the solvent. These DBA molecules have a pendant group juxtaposed between the donor and acceptor groups (Figure 1). The pendant provides intermediate electronic coupling strengths of a few hundred wavenumbers by way of its nonbonded contacts with the donor and acceptor and it can be tuned by substituents added to the pendant. This design allows the measurement of electron transfer rates from a regime in which the mechanism is nonadiabatic to a regime in which the solvent friction modifies the rate substantially. The rate constants and mechanistic parameters are compared with the expectations of models for solvent dynamical effects on the reaction rate.

Competing electron-transfer pathways in hydrocarbon frameworks: short-circuiting through-bond coupling by nonbonded contacts in rigid U-shaped norbornylogous systems containing a cavity-bound aromatic pendant group.

This work explores electron transfer through nonbonded contacts in two U-shaped DBA molecules 1DBA and 2DBA by measuring electron-transfer rates in organic solvents of different polarities. These molecules have identical U-shaped norbornylogous frameworks, 12 bonds in length and with diphenyldimethoxynaphthalene (DPMN) donor and dicyanovinyl (DCV) acceptor groups fused at the ends. The U-shaped cavity of each molecule contains an aromatic pendant group of different electronic character, namely p-ethylphenyl, in 1DBA, and p-methoxyphenyl, in 2DBA. Electronic coupling matrix elements, Gibbs free energy, and reorganization energy were calculated from experimental photophysical data for these compounds, and the experimental results were compared with computational values. The magnitude of the electronic coupling for photoinduced charge separation, /V(CS)/, in 1DBA and 2DBA were found to be 147 and 274 cm(-1), respectively, and suggests that the origin of this difference lies in the electronic nature of the pendant aromatic group and charge separation occurs by tunneling through the pendant group, rather than through the bridge. 2DBA, but not 1DBA, displayed charge transfer (CT) fluorescence in nonpolar and weakly polar solvents, and this observation enabled the electronic coupling for charge recombination, /V(CR)/, in 2DBA to be made, the magnitude of which is approximately 500 cm(-1), significantly larger than that for charge separation. This difference is explained by changes in the geometry of the molecule in the relevant states; because of electrostatic effects, the donor and acceptor chromophores are about 1 A closer to the pendant group in the charge-separated state than in the locally excited state. Consequently the through-pendant-group electronic coupling is stronger in the charge-separated state--which controls the CT fluorescence process--than in the locally excited state--which controls the charge separation process. The magnitude of /V(CR)/ for 2DBA is almost 2 orders of magnitude greater than that in DMN-12-DCV, having the same length bridge as for the former molecule, but lacking a pendant group. This result unequivocally demonstrates the operation of the through-pendant-group mechanism of electron transfer in the pendant-containing U-shaped systems of the type 1DBA and 2DBA.

Axial resolution limit of a fiber-optic fluorescence probe.

We examine the limit of spatial resolution achievable when a sine optical fiber is used for excitation and collection of fluorescence from a bulk specimen. We calculate the probability of detecting a fluorescent particle as a function of its position relative to the fiber face, using excitation wavelength lambda, radius a, numerical aperture N.A., and the particle's fluorescence and absorbance spectra. Treating Rhodamine B as a model fluorescent analyte and using appropriate fiber parameters, we show that the maximum axial resolution (defined as the axial distance in a homogenous solution within which 50% of the detected signal originates) achievable is approximately 10 microm. We experimentally measured the axial resolution for a 500-microM aqueous solution of Rhodamine B with lambda = 543 nm, a = 1.31 microm, and a N.A. of 0.16 and found good qualitative agreement with the calculation.