The Ultrafast Charge-Transfer-to-Solvent Dynamics of Iodide in Tetrahydrofuran. 1. Exploring the Roles of Solvent and Solute Electronic Structure in Condensed-Phase Charge-Transfer Reactions

Abstract

Although they represent the simplest possible charge-transfer reactions, the charge-transfer-to-solvent (CTTS) dynamics of atomic anions exhibit considerable complexity. For example, the CTTS dynamics of iodide in water are very different from those of sodide (Na -) in tetrahydrofuran (THF), leading to the question of the relative importance of the solvent and solute electronic structures in controlling charge-transfer dynamics. In this work, we address this issue by investigating the CTTS spectroscopy and dynamics of I -in THF, allowing us to make detailed comparisons to the previously studied I -/H 2 O and Na -/THF CTTS systems. Since THF is weakly polar, ion pairing with the counterion can have a substantial impact on the CTTS spectroscopy and dynamics of I -in this solvent. In this study, we have isolated "counterion-free" I -in THF by complexing the Na + counterion with 18-crown-6 ether. Ultrafast pump-probe experiments reveal that THF-solvated electrons (e THF -) appear 380 ( 60 fs following the CTTS excitation of "free" I -in THF. The absorption kinetics are identical at all probe wavelengths, indicating that the ejected electrons appear with no significant dynamic solvation but rather with their equilibrium absorption spectrum. After their initial appearance, ejected electrons do not exhibit any additional dynamics on time scales up to ∼1 ns, indicating that geminate recombination of e THF -with its iodine atom partner does not occur. Competitive electron scavenging measurements demonstrate that the CTTS excited state of I -in THF is quite large and has contact with scavengers that are several nanometers away from the iodide ion. The ejection time and lack of electron solvation observed for I -in THF are similar to what is observed following CTTS excitation of Na -in THF. However, the relatively slow ejection time, the complete lack of dynamic solvation, and the large ejection distance/lack of recombination dynamics are in marked contrast to the CTTS dynamics observed for I -in water, in which fast electron ejection, substantial solvation, and appreciable recombination have been observed. These differences in dynamical behavior can be understood in terms of the presence of preexisting, electropositive cavities in liquid THF that are a natural part of its liquid structure; these cavities provide a mechanism for excited electrons to relocate to places in the liquid that can be nanometers away, explaining the large ejection distance and lack of recombination following the CTTS excitation of I -in THF. We argue that the lack of dynamic solvation observed following CTTS excitation of both I -and Na -in THF is a direct consequence of the fact that little additional relaxation is required once an excited electron nonadiabatically relaxes into one of the preexisting cavities. In contrast, liquid water contains no such cavities, and CTTS excitation of I -in water leads to local electron ejection that involves substantial solvent reorganization.

Photoinduced Electron Transfer and Solvation in Iodide-doped Acetonitrile Clusters

“…The eKE then decreases over the next few picoseconds, reaching a minimum smaller than the initial eKE. For the n ) 5 cluster in Figure 1, the eKE distribution shifts to slightly higher values over several hundred picoseconds; no such shift is seen for I -(CH 3 CN) 10 . The PE yield for excited-state detachment of the I -(CH 3 CN) 5 cluster is highest just after absorption of the pump photon, then drops and remains essentially constant throughout the examined time range.…”

“…Photoelectron yields are plotted as false color images as a function of both electron kinetic energy (eKE) and pump/probe delay t. Although eKE is shown on a linear scale between zero and 2.0 eV, a nonlinear scale was chosen for the delay time axis to allow for better discrimination of early dynamics. The pump energies for I -(CH 3 CN) 5 in Figure 1 and I -(CH 3 CN) 10 ( Figure 2) were 4.68 and 5.06 eV, respectively, and the same probe energy, 1.56 eV, was used in all spectra. The higher pump energy was needed for the n ) 9 and 10 clusters since they did not absorb enough light at 4.68 eV to produce sufficient signal.…”

“…Values for the excited-state feature in clusters I -(CH 3 CN) 5 and I -(CH 3 CN) 10 are plotted as filled red circles in Figures 3 and 4, respectively, along with the detachment yield.…”

“…3,4 The development of ultrafast lasers has led to new insights into the formation and reaction dynamics of solvated electrons. 5 Although most of these studies have been carried out in water, experiments have been carried out in other dipolar solvents including alcohols, [6][7][8] ethers, [9][10][11] and other polar molecules. [12][13][14] In several of these experiments, 6,8,11,15,16 solvated electrons were generated by photodetachment of dissolved negative ions, most commonly halide ions, via excitation of charge-transfer-tosolvent (CTTS) bands in which the electron is ejected from the halide into the surrounding solvent.…”

Real-time observation of the charge transfer to solvent dynamics

“…From the experimental point of view, there has been a huge effort aimed at observing the early-time dynamics of aqueous CTTS states using ultrafast transient absorption (TA) spectroscopy [1][2][3][4]15,22 and, more recently, ultrafast photoemission from liquid microjets 5,6 . However, these methods turned out to be exclusively sensitive to the electron signal, that is, the end product of the photodetachment process, because of its very large oscillator strength over a broad spectral range and the fact that the excited CTTS absorption is close to that of the solvated electron 5,6 .…”