Dynamical Solvent Control of Electron Transfer in a Flexible, Tethered Donor−Acceptor Pair

Abstract

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.

Ultrafast Dynamics of Nonequilibrium Electron Transfer in Photoinduced Redox Cycle: Solvent Mediation and Conformation Flexibility

“…Over the past decades, a large number of chemical systems, consisting of covalently linked electron donor (D) and acceptor (A), have been synthesized and investigated to examine electron transfer (ET) reactions − and theories − with controlled parameters of free energy gap (Δ G ), reorganization energy (λ), and donor–acceptor coupling ( J ) or distance (r). These reactions were usually studied by changing one parameter while the other two parameters were fixed.…”

The Key Role of Solvation Dynamics in Intramolecular Electron Transfer: Time-Resolved Photophysics of Crystal Violet Lactone

“…Generally, a two-dimensional reaction coordinate is used in the description to include both the vibronic couplings and the solvent degrees of freedom . The kinetics of solvent-controlled ET processes has been extensively studied theoretically, , but the existing body of time-dependent studies demonstrating the dynamic solvent control of ET − still leaves important fundamental questions open. In this paper, we want to experimentally clarify the impact of solvation on the ET dynamics and how the progress of the solvation influences the ET.…”

“…Up to now, experimental investigations of the dynamical solvent effect on the reaction dynamics have focused on small organic molecules, ,,,,, metal complexes, ,, and CT to solvent states . To clearly separate the influence of the solvation from the intramolecular properties that determine the speed of ET, molecules are needed which allow ET under favorable conditions faster than the solvation time scale.…”

Charge Separation Distance for Flexible Donor−Bridge−Acceptor Systems after Electron-Transfer Quenching

“…Figure exhibits the steady-state emission spectra of both DBA3 and DBA4 in tetrahydrofuran (THF) after excitation of the anthracene moiety at 355 nm. The emission at wavelengths from 375 to 450 nm is from the unquenched, locally excited anthracene moiety . The broad emission at the wavelengths longer than 450 nm is from exciplexes formed after intramolecular electron-transfer quenching of the excited anthracene moiety by the N , N -dimethylaniline moiety.…”

“…It has been shown − that, in nonpolar solvents like cyclohexane and n-hexane, the intramolecular electron transfer quenching of DBA3 takes 2−4 ns and can only happen at contact configuration. Wang et al and Kauffman et al have shown that in nonipolar solvents the electron-transfer rate in DBA3 exhibits a power law dependence on viscosity η.…”