The solvent dependence of the photoinduced intramolecular charge transfer rate constants of 1-(9-anthryl)-3-(4-dimethylaniline)propane (ADMA) is investigated by picosecond time-resolved fluorescence spectroscopy in polar solvents. ADMA undergoes electron transfer by two distinct mechanisms, depending on the solvent polarity. In nonpolar solvents the excited ADMA molecule must fold prior to electron transfer, whereas in polar solvents electron transfer can occur in an extended conformation. In polar solvents. ADMA exhibits biexponential fluorescence decay. This is consistent with the proposed polar mechanism of electron transfer, in which electron transfer occurs in an extended conformation, and is followed by conformational interconversion to an emissive, folded conformation. Examination of the kinetic expressions for the time-dependent population of the locally excited state of ADMA indicates that the fast decay time of the fluorescence decay is approximately equal to the inverse of the forward electron-transfer rate constant. A linear correlation is observed between the fast decay time and the amplitude weighted average solvation time determined from time-dependent Stokes shift measurements. This indicates that solvent dynamics influences the electron-transfer rate. Analysis of the results on the basis of an expression for the rate constant of adiabatic electron transfer demonstrates that the results are consistent with previous estimates of the reorganization energy and electron-transfer barrier using a dielectric continuum model. Comparison of ADMA electron-transfer rate constants in ethers with electron-transfer rate constants in polar solvents shows that ADMA undergoes electron transfer by a mechanism that is intermediate between the polar and nonpolar mechanisms in solvents of intermediate polarity.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry