On the insensitivity of the non-adiabatic relaxation of solvated electrons to the details of their local solvent environment

Utilizing the charge-transfer-to-solvent transition of Na , we measure the excited-state lifetime of the solvated electron (e s ) in tetrahydrofuran (THF) in each of three different well-defined local solvent environments: an isolated THF cavity, a THF cavity with a sodium atom located one solvent shell away, and a THF cavity with a sodium atom in the first solvent shell. We find that the excited-state lifetimes of the e s in each environment are the same, suggesting that the solvent-induced non-adiabatic coupling that determines the radiationless transition rate is insensitive to the details of the specific atoms or molecules in the first solvent shell. 2002 Elsevier Science B.V. All rights reserved. The dynamics of chemical reactions in solution are strongly affected by solvent molecules whose motions are coupled to the electronic states of the reactant. One of the most important effects is nonadiabatic coupling, in which the adiabatic potential surfaces of the reacting species are mixed together due to rapid nuclear motions of the solvent, a manifestation of breakdown of the Born–Oppenheimer approximation [1]. Thus, non-adiabatic coupling allows for solvent-induced transitions between reactant energy levels. These transitions can provide access for a reacting system to enter new regions of phase space, permitting the creation of new products or significantly altering reaction rates. Non-adiabatic coupling is particularly important in electron transfer reactions, in which the reaction coordinate can consist mostly or entirely of the nuclear motions of the solvent [2]. One of the prototypical systems for studying solvent-induced non-adiabatic coupling has been the solvated electron (e s ). Solvated electrons are confined within a solvent cavity; the eigenstates of such excess electrons are similar to those of a particle in a finite spherical box. Solvent molecule motions are strongly coupled to the energy levels of this solute: even subtle changes in the size and shape of the solvent cavity lead to large modulation of the electronic energy levels [3]. The importance of this coupling is reflected in the short excited-state lifetime of the e s : the solvent-induced 3 July 2002 Chemical Physics Letters 360 (2002) 22–30 www.elsevier.com/locate/cplett * Corresponding author. Fax: +1-310-206-4038. E-mail address: [email protected] (B.J. Schwartz). 0009-2614/02/$ see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0009 -2614 (02 )00771 -6 mixing of the ground and excited states is so strong that photo-excited solvated electrons return to their ground state in under a picosecond [4–7], despite expected radiative lifetimes in the nanosecond regime [8]. The excited-state lifetime of solvated electrons has been investigated theoretically by several groups [9–13], including calculations by Prezhdo and Rossky [14–16] that directly addressed the specific solvent motions that nonadiabatically couple the ground and excited electronic states. The theoretical lifetime predictions are in good general agreement with experiments performed on solvated electrons in water and alcohols by several groups [17–19], including a series of detailed studies by Barbara and co-workers [4–7]. While it is clear that changing solvents affects the excited-state lifetime of the e s , there has been no experimental work addressing how the details of the local solvent structure alters the non-adiabatic coupling that controls the electron’s lifetime. This is because it is extremely difficult to prepare solvated electrons in different well-defined local solvent environments. In this Letter, we show how we have overcome this difficulty, allowing us to measure the relaxation dynamics of the solvated electron in tetrahydrofuran (THF) in three different well-defined local solvent environments. We find that whether the e s is in an isolated THF cavity, a THF cavity with a sodium atom located one solvent shell away, or a THF cavity with a sodium atom in the first solvent shell, the excitedstate lifetime is the same. This suggests that it is not just the motions of the nearest molecules that control the electron’s lifetime, but that the solvent motions that control non-adiabatic coupling are spread throughout the solvent, a result with important implications for condensed-phase chemical reaction dynamics. The usual method for creating solvated electrons is by multiphoton ionization of the solvent (or of an easily ionizable solute, such as I or 1⁄2FeðCNÞ6 4 ). The initial ionization produces conduction band electrons with a great deal of kinetic energy. These hot, delocalized conduction band electrons can travel through the solvent before they ultimately localize into cavities, leading to a continuous distribution of distances between the localized electrons and the parent species from which they were generated [20]. Because of this, e s excited-state dynamics are typically studied in experiments that use three ultrafast laser pulses. The first pulse is used to synthesize the electrons via multiphoton ionization. The second and third pulses, which are used to perform pump–probe spectroscopy on the freshly created solvated electrons, are not applied until after the localization is completed. In this way, the experiments are able to measure the excited-state lifetimes starting from fully relaxed solvated electrons [4–8,17–19]. For the experiments discussed here, we generated solvated electrons via a different route, making use of the charge-transfer-to-solvent (CTTS) [21] transition of the sodium anion (Na ) in THF (Fig. 1a, solid curve) [22,23]. We have extensively studied this reaction and elucidated the molecular details of the CTTS electron detachment process [24–26], which are illustrated schematically in Fig. 1b. We found that upon photoexcitation, the solvent reorganizes around the newly created CTTS excited state, causing the electron to detach and localize in a nearby solvent cavity. Excitation into the low energy side of the CTTS band (near 800 nm) predominantly creates electrons that reside in the same solvent cavity as their sodium atom (Na) partners; we refer to these species as ‘immediate’ contact pairs (Fig. 1b, left). The wavefunction of an immediate-pair e s has significant overlap with the Na in the first shell, leading to rapid back electron transfer (geminate recombination) to reform Na within a couple of picoseconds. Excitation on the high-energy side of the CTTS band (near 400 nm), on the other hand, produces solvated electrons that are located one solvent shell away from their sodium atom partners (Fig. 1b, right). The wavefunctions of these ‘solvent-separated’ contact pair electrons do not overlap with their geminate partners and recombine on the hundreds-of-picoseconds time scale. High excitation energies also produce a few ‘free’ solvated electrons that are ejected to locations in the solvent further away than the second solvent shell (Fig. 1b, right). These electrons do not recombine with their Na partners on sub-ns time scales. The initial distribution of electrons localized at the three different distances from their Na partners changes I.B. Martini, B.J. Schwartz / Chemical Physics Letters 360 (2002) 22–30 23