Ultrafast Charge-Transfer-to-Solvent Dynamics of Iodide in Tetrahydrofuran. 2. Photoinduced Electron Transfer to Counterions in Solution

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.

Real-time observation of the charge transfer to solvent dynamics

“…As the CTTS state and the ground state of the solvated electron have a similar, s-like symmetry, each CTTS state likely evolves on a singlepotential energy surface all the way to the aqueous electron. This contrasts to the behaviour of, for example, Na À , whose CTTS state has a reversed, p-like symmetry such that a non-adiabatic transition is required to go from the reactant to the product state 13 . However, our findings show that the characteristic time scale of the dynamics (ultimately, the fluorescence lifetime) changes from excited ionic centre to the next because it strongly depends on the structure of the surrounding solvation cage at the time of photo-excitation.…”

“…The valence electrons of ground-state anions are bound by the nucleus, but the excited states are bound by solvent polarization 1 . The CTTS states of halides rapidly decay by ejection of an electron, which is then stabilized by solvation, leaving a neutral halogen atom behind [2][3][4][5][6][7][8][9][10][11][12][13][14] . Atomic anions are ideal for studies of the electron-transfer dynamics to the solvent, because the solute lacks internal (nuclear) degrees of freedom, such that the process of CTTSmediated electron ejection is entirely governed by the structure and motions of the solvation shell.…”

“…Femtosecond X-ray absorption spectroscopy at the I L-edges detected the birth of the neutral halogen, but the time resolution was not sufficient to capture the early dynamics 7 . Of particular relevance here are the studies by Ruhman et al 11,12 and Schwartz et al 13,14,[22][23][24][25] on the CTTS states of sodide (Na À ) in tetrahydrofuran (THF) and ether solvents, which enabled them to probe at high-time resolution both the signal of the ejected electron and the nascent neutral sodium atom. They found that electrons photodetached from the parent ion appear with their equilibrium spectrum after 500 fs with no further spectral evolution, nor a pump wavelength dependence.…”

Nature of Sodium Atoms/(Na+, e−) Contact Pairs in Liquid Tetrahydrofuran

“…Our preliminary simulation studies of this system found that Na 0 clearly exists as a TCP in liquid THF, 18 in agreement with experimental interpretations. [1][2][3][4][13][14][15]19 In this work, we explore the molecular nature of the sodium TCP in significantly more detail. We find that the driving force for TCP formation lies in the creation of a tight solvation shell of ∼4 THF oxygens around the partially exposed Na + end of the TCP.…”

Searching for solvent cavities via electron photodetachment: The ultrafast charge-transfer-to-solvent dynamics of sodide in a series of ether solvents

“…[14][15][16][17] For example, in the red-edge excitation of anions such as I − in THF, coupling of the local charge-transfer-to-solvent ͑CTTS͒ excited state to solvent-supported disjoint states with similar energies leads to photoejection of electrons to significant distances ͑ϳ6 nm͒ from the parent anion. 16 Another experimental signature of cavities is that the detached electrons produced after exciting nearly any anion in liquid THF-I − , 17 Na − , 14 K − , 18 or even the solvated electron itself 19 -appear after a ϳ0.5 ps time delay with their equilibrium spectrum. 20 This indicates that excited electrons relax directly into pre-existing solvent traps from the initially prepared solvent-supported excited state.…”