Electric field effects on emission line shapes when electron transfer competes with emission: An example from photosynthetic reaction centers

Electron-transfer reactions are expected to be particularly sensitive to externally applied electric fields because of the presence of charge-separated, dipolar states, and the effects of the field on photoinduced reactions can be determined by monitoring the competing emission. The rates of photoinduced electron-transfer reactions between a donor and acceptor are often measured by comparing the quantum yield of the competing emission from the state undergoing the reaction in the presence and absence of electron transfer. Calculations are presented for the fluorescence line shape and amplitude changes expected for isotropic and oriented samples of molecules undergoing electron transfer in an applied electric field. The calculations are based on a model that includes the expected effects of the field on both the fluorescence transition energy and the rate of the competing electron-transfer reaction. These model calculations indicate that a surprisingly wide range of electric-field-modulated line shapes are possible and that the line shape is very sensitive to key parameters that characterize the electron-transfer reaction. An example of these effects has been obtained in a structurally characterized donor/acceptor system, the bacterial photosynthetic reaction center, whose electric-field-modulated fluorescence changes have been characterized in detail. Information on the magnitude and direction of the dipole moment of the emitting state and on the mechanism and rate versus free energy curve for this picosecond long-distance electron-transfer reaction are obtained by comparing the experimental data with the results of the model calculations.