Coherent Electronic and Nuclear Dynamics for Charge Transfer in 1-Ethyl-4-(carbomethoxy)pyridinium Iodide

Abstract

Although polaronic interactions and states abound in charge transfer processes and reactions, quantitative and separable determination of electronic and nuclear relaxation is still challenging. The present paper employs the amplitudes, polarizations, and phases of four-wave mixing signals to obtain unique dynamical information on relaxation processes following photoinduced charge transfer between iodide and 1-ethyl-4-(carbomethoxy)-pyridinium ions. Pump-probe signal amplitudes reveal the coherent coupling of an underdamped 115 cm -1 nuclear mode to the charge transfer excitation. Assignments of this recurrence to intramolecular vibrational modes of the acceptor and to modulation of the intermolecular donor-acceptor distance are discussed on the basis of a high-level density functional theory normal-mode analysis and previously observed wave packet dynamics of solvated molecular iodine. Nuclear relaxation of the acceptor induces sub-picosecond decay of the pump-probe polarization anisotropy from an initial value of 0.4 to an asymptotic value of -0.05. Electronic structure calculations suggest that relaxation along the torsional coordinate of the ethyl group is the origin of the anisotropy decay. Electric-field-resolved transient grating (EFR-TG) signal fields are obtained by spectral interferometry with a diffractive optic based interferometer. These measurements show that the signal phase and amplitude possess similar dynamics. Model calculations are used to demonstrate how the EFR-TG signal phase yields unique information on transient material resonances located outside the laser pulse spectrum. This effect can be rationalized in that the real and imaginary parts of the nonlinear polarization are related by the Kramers-Kronig transformation, which allows the dispersive component of the polarization response to exhibit spectral sensitivity over a larger frequency range than that defined by the absorption bandwidth.

Ultrafast dynamics of solvation: the story so far

“…Solute dynamics of ECMPI in methanol and acetonitrile have been previously characterized, where the electron-transfer rates were found to correlate with the acceptor-donor distance, and the charge transfer reaction was found to be nearly a factor of ten times faster in acetonitrile than in methanol. 55 Interestingly, PORS spectra for ECMPI in acetonitrile and methanol revealed that the polarizability spectral density was time-dependent ( Figure 6). The general trend observed in both solvents was that of an initial broadband inertial response followed by a decrease in the spectral density linewidth.…”

“…50 Despite its success, variants of the formalism that consider anharmonicity of the liquid state potential [51][52][53] and additional electronic resonances in-/outside the laser bandwidth have been discussed. 54,55 Figure 2 shows 3-pulse PE peak shift (3 PEPS) vs. population time T data from IR144 for several different polar solvents: methanol, ethanol, butanol, and ethylene glycol. The 3PEPS data from IR144 in different solvents have common features.…”

“…From that perspective, the timedependence of PORS spectra indicated that non-equilibrium solvent fluctuations were coupled to the reaction coordinate, a sign of breakdown of linear response theory that has also been reported in other systems. 55 …”

How a solute-pump/solvent-probe spectroscopy can reveal structural dynamics: Polarizability response spectra as a two-dimensional solvation spectroscopy

“…The observable, however, is not the solute fluorescence, but a time-delayed 4-wave-mixing (effectively, an intermolecular vibrational) spectrum of the surrounding solvent. [11][12][13][14][15][16][17][18][19][20][21] In the resonantpump polarizability response spectra (RP-PORS) carried out by the Scherer group, for example ( Fig. 1), [15][16][17] a transientgrating spectrum is taken by crossing two non-resonant optical pulses in the sample a time T after the initial resonant solute excitation, and then watching the light scattering from another non-resonant optical pulse a time t after that.…”

Field-resolved measurement of reaction-induced spectral densities by polarizability response spectroscopy

“…24,25,35 Our previous design employed heterodyne detection to separate real and imaginary signal components but did not fully resolve the signal phase. 36,37 Therefore, the PORS signals are dominated by motions of the solvent that are coupled to/driven by the charge transfer reaction; these are the dynamics PORS is designed to measure. In addition, the Raman probe measurement is performed in the frequency domain with fixed pulse delays, as in the FSRS, [30][31][32][33][34] to speed up data acquisition by about 300-fold.…”