Chapter 9
Near-Threshold Photoionization Dynamics of Indole in Water
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Jorge Peon, J. David Hoerner, and Bern
*
Kohler
th
Department of Chemistry, Ohio State University, 100 West 18 Avenue, Columbus, OH 43210
Abstract The photoionization of indole in water was studied at 262 nm by the femtosecond transient absorption technique. The initial excited state generated by one-photon absorption is about 0.4 eV above the photoionization threshold of indole in water. Under these near-threshold conditions ionization most likely occurs by an electron-transfer mechanism in which an electron is directly transferred to a trapping site in the solvent without the intermediate generation of a delocalized electron in the conduction band of water. Solvated electrons were formed within our time resolution (= 200 fs). No recombination between the solvated electron and the indole radical cation could be observed up to 600 ps after the pump pulse, despite the very high electron affinity of the latter species. A similar result was obtained for aqueous tryptophan. The absence of diffusion -limitedcharge recombination for this highly exergonic reaction is consistent with Marcus-inverted behavior.
Introduction Photoionization is a fondamental photochemical decay channel of an electronically excited molecule. When a neutral molecule is photoionized, an
122
© 2002 American Chemical Society
In Liquid Dynamics; Fourkas, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
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123 electron and a positive ion are produced, and these charged products interact strongly with the molecules in the liquid. Consequently, less energy is needed to photoionize a molecule in a polar solvent compared to vacuum conditions. The liquid environment also profoundly alters the dynamics of photoionization, just as it does for other photochemical processes of excited molecules. These effects are still poorly understood. We have been studying the near-threshold photoionization of aromatic solutes in polar solvents in order to learn more about the microscopic mechanisms underlying charge separation and recombination in the liquid phase. For an isolated, uncharged molecule, the first ionization potential is the minimum energy required to produce a free electron with zero kinetic energy, which is infinitely far away from its parent ion. This definition is problematic for a molecule in a liquid because of the difficulty of detecting electrons with high efficiency and in an energy-resolved manner in a condensed phase. Electrons can take on a wide range of energies in liquids. In a polar solvent the solvated electron, e~, is the most stable form of the excess electron. The solvated electron in water, e ", lies at least 1.7 eV lower in energy than a delocalized ("quasifree") electron in the lowest energy state in the conduction band. This fact suggests that it might be possible to photoionize a molecule in a liquid by direct formation of e " without ever forming a conduction-band electron, e \ Such a mechanism requires less energy, and would therefore be particularly important near threshold. Sander, Luther, Troe discussed this issue in an elegant paper that divided liquid-phase photoionization mechanisms into two classes, depending on whether ionization and electron localization are distinct mechanistic steps (7). When these steps are distinct, e is an intermediate in photoionization, and the mechanism, which we refer to as conduction-band photoionization (CB-PI), can be represented as follows, s
aq
s
cb
cb
ionization
M*
-»
localization
M^ +e" cb
-»
solvation
ΜΓ + eT - »
NT* + e " s
(CB-PI)
In this mechanism, an electron is "ejected" from an electronically excited state, M * , of molecule M into the conduction band of the liquid. Ionization is thus the initial act of charge separation, which creates the molecular ion, NT*. Since charge separation depends primarily on M * , ionization occurs in essentially the same manner as for an isolated molecule in vacuum. The conduction band electron, e ~, may travel a considerable distance before becoming localized at a suitable trapping site in the liquid. The trapped electron, et", then undergoes further solvation to yield a fully solvated electron, e ". Solvation of also occurs during this time period, but has not been explicitly included in the above scheme. cb
s
In Liquid Dynamics; Fourkas, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
124 When ionization and localization are part of the same elementary step, ionization may occur without formation of e \ Instead, an electron is transferred directly to a trapping site. We refer to this type of mechanism as electron-transfer photoionization (ET-PI), cb
ionization, localization
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*
.
ι
-
> M +e
t
solvation
.
-
/ i - - r τιτ\
> M +e
»
(ET-PI)
s
In this case, ionization occurs due to electronic coupling between M * and the localization site. Because charge separation in ET-PI occurs by transferring an electronfromthe excited state of the ionizing molecule to an acceptor site, the rate of ionization is likely to depend sensitively on factors that control conventional electron transfer (ET) reactions such as free energies, donoracceptor geometries, and the rate of solvent fluctuations. Here, the acceptor site could be a single solvent molecule, but is more likely a suitable cluster, which provides a high electron affinity site for an incipient cavity electron. This mechanism, which is impossible for an isolated molecule, can occur for excitation energies below those required for the production of e " and it is therefore also called sub-conduction-band photoionization. Crowell and Bartels have termed this "indirect photoionization" (2). ET-PI has been discussed extensively to explain the photoionization of neat water at low excitation energies (3-5). It has also been discussed in connection with solute photoionization (6). M* is the immediate precursor to ionization. It is most likely a continuum state in the case of CB-PI, although an autoionizing bound state is also conceivable (7). For ET-PI, M* is always an electronic bound state, although it may not be the same excited state prepared by the optical transition. Recent calculations suggest that the ionizing state of indole is optically dark, but can be accessed by vibronic coupling to optically allowed states (7,8). The rate of ETPI will then depend on both excited state dynamics of the ionizing molecule and on fluctuations in the environment that promote electron transfer. Afiniterate of photoionization has now been observed in ultrafast photodetachment experiments on negative ions in solution (9,10). While it has been proposed that tetramethyl-p-phenylenediamine can be photoionized from its lowest excited singlet (Si) state (7/), photoionization appears to only occurfromhigher-lying electronic states for most molecules. Higher singlet states generally relax in < 100 fs by internal conversion to St. We therefore believe that the very rapid decay of upper singlet states will limit ET-PI to timescales of a few tens to hundreds of femtoseconds for most molecules. Photoionization in liquids gives rise to a second phenomenon that has no counterpart in isolated molecules: charge recombination. This process, the analog of back electron transfer, refers to the reaction of e~ with M** to reform cb
s
In Liquid Dynamics; Fourkas, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
125 the parent molecule, M** + e" -» M. In CB-PI, e^" is quite mobile, but still traps within several multiples of the Onsager distance from the initial ionization site. The total distance traveled is thought to depend on the excess energy. In ET-PI, the short timescale for charge separation rules out long-range electron transfer, suggesting that the M**, e" pairs will always be separated by small distances. For both mechanisms, photoionization generates a pair of geminate ions that are separated by some distance. Aided by their mutual Coulombic attraction, they may undergo recombination or diffuse away from each other. The geminate recombination dynamics provide information about the initial ion pair separation. Thus, energy-independent geminate recombination dynamics for water atfinalstate energies below 9 eV provided evidence for a sub-conductionband photoionization mechanism (2). s
Downloaded by COLUMBIA UNIV on September 14, 2012 | http://pubs.acs.org Publication Date: June 3, 2002 | doi: 10.1021/bk-2002-0820.ch009
s
Our interest in these issues has led us to study the photoionization of neutral aromatic molecules in polar solvents using femtosecond pump-probe methods. Charge ejection following absorption of a UV or near UV photon is a common deactivation channel for many aromatic compounds in solution (12). This allows these solutes to be monophotonicaily ionized under carefully controlled excitation conditions, circumventing some of the difficulties in multiphoton ionization experiments on molecules such as water. We are concentrating on the near-threshold regime where ET-PI is expected to dominate. In addition to looking for 'delayed photoionization' due to this mechanism, an additional motivation was to quantify the actual photoionization yield at short times before geminate recombination has reduced the yield of ions. The geminate recombination dynamics provide insight into reactions of a simple radical ion pair consisting of an organic radical cation and the solvated electron. Unlike neat water, geminate recombination involves reaction between two partners instead of three. The rates of reaction between solvated electrons and a large number of substrates vary over a wide range, but are often masked by diffusion. Tight ion pairs created by ET-PI make it possible to investigate true reaction rates in the absence of transport. We chose indole for thesefirststudies of ultrafast solute photoionization for several reasons. Indole photoprocesses have been extensively studied for decades due to the importance of this molecule as the chromophore of the aromatic amino acid, tryptophan. The exquisite sensitivity of the indole chromophore to its local environment has been used extensively to study proteins (73). This important application has been the stimulus for hundreds of studies of indole photoprocesses. This past work has identified photoionization as an important deactivation channel for electronically excited indole, but the mechanism is still unclear. Indole is also one of the only polyatomic molecules for which condensed-phase photoionization thresholds have been measured in a range of solvents. The datafromBernas and co-workers (14) is summarized in Table 1.
In Liquid Dynamics; Fourkas, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
126 Table I. Photoionization thresholds for indole in various solvents/ Solvent
Threshold Energy / e V h
l-butanol ~5M 1-propanol 5.15* tetramethylsilane 4.95 ethanol 4.85 methanol 4.60 H 0 4.35 "Datafromref14. ^Extrapolatedfromvalues for shorter alcohols.
Results and Discussion In our experiments (15), the third harmonic pulse (τ ~ 180 fs) from a femtosecond titaniumrsapphire laser was used to excite a 6 mM aqueous solution of indole at λ = 262 nm (see Figure 1). A continuum probe pulse was then used to measure transient absorption at wavelengths throughout the visible and nearIR. Figure 2 compares the transient signals for aqueous indole (top panel) with ones obtained from the two-photon ionization of neat water using the same pump and probe wavelengths (lower panel). The signals for aqueous indole were recorded at low pump intensities. They result solely from monophotonic
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