Photoinduced bimolecular electron transfer kinetics in small unilamellar vesicles

Journal Article · Wed Nov 21 00:00:00 EST 2007 · Journal of Chemical Physics

DOI:https://doi.org/10.1063/1.2794765· OSTI ID:21024504

Choudhury, Sharmistha Dutta; Kumbhakar, Manoj; Nath, Sukhendu; Pal, Haridas ^[1]

---

1. Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085 (India)

Photoinduced electron transfer (ET) from N,N-dimethylaniline to some coumarin derivatives has been studied in small unilamellar vesicles (SUVs) of the phospholipid, DL-{alpha}-dimyristoyl-phosphatidylcholine, using steady-state and time-resolved fluorescence quenching, both below and above the phase transition temperature of the vesicles. The primary interest was to examine whether Marcus inversion [H. Sumi and R. A. Marcus, J. Chem. Phys. 84, 4894 (1986)] could be observed for the present ET systems in these organized assemblies. The influence of the topology of SUVs on the photophysical properties of the reactants and consequently on their ET kinetics has also been investigated. Absorption and fluorescence spectral data of the coumarins in SUVs and the variation of their fluorescence decays with temperature indicate that the dyes are localized in the bilayer of the SUVs. Time-resolved area normalized emission spectra analysis, however, reveals that the dyes are distributed in two different microenvironments in the SUVs, which we attribute to the two leaflets of the bilayer, one toward bulk water and the other toward the inner water pool. The microenvironments in the two leaflets are, however, not indicated to be that significantly different. Time-resolved anisotropy decays were biexponential for all the dyes in SUVs, and this has been interpreted in terms of the compound motion model according to which the dye molecules can experience a fast wobbling-in-cone type of motion as well as a slow overall rotating motion of the cone containing the molecule. The expected bimolecular diffusion-controlled rates in SUVs, as estimated by comparing the microviscosities in SUVs (determined from rotational correlation times) and that in acetonitrile solution, are much slower than the observed fluorescence quenching rates, suggesting that reactant diffusion (translational) does not play any role in the quenching kinetics in the present systems. Accordingly, clear inversions are observed in the correlation of the fluorescence quenching rate constants k{sub q} with the free energy change, {delta}G{sup 0} of the reactions. However, the coumarin dyes, C152 and C481 (cf. Scheme 1), show unusually high k{sub q} values and high activation barriers, which is not expected from Marcus ET theory. This unusual behavior is explained on the basis of participation of the twisted intramolecular charge transfer states of these two dyes in the ET kinetics.

• OSTI ID: 21024504
• Journal Information: Journal of Chemical Physics, Vol. 127, Issue 19; Other Information: DOI: 10.1063/1.2794765; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); ISSN 0021-9606
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
ACETONITRILE
ANISOTROPY
COUMARIN
DYES
ELECTRON TRANSFER
EMISSION SPECTRA
FLUORESCENCE
LECITHINS
PHASE TRANSFORMATIONS
QUENCHING
REACTION KINETICS
SPECTRAL SHIFT
STEADY-STATE CONDITIONS
TIME RESOLUTION
TRANSITION TEMPERATURE
ULTRAVIOLET SPECTRA
VISCOSITY
VISIBLE SPECTRA