The people involved: Katrin Adamczyk, Mirabelle Prémont-Schwarz, Jens Dreyer, Erik T. J. Nibbering
Former team members: Christian Chudoba, Frank Tschirschwitz, Matteo Rini, Omar F. Mohammed, Andreas Kummrow, Michael Pfeiffer, Anwar Usman, Hirendra N. Ghosh, Thomas Elsaesser

International collaboration: Ben-Zion Magnes,^† Dina Pines,^† Ehud Pines ,^† Omar F. Mohammed,^# Nathalie Banerji,^# Bernhard Lang,^# Eric Vauthey^,# Hirendra N. Ghoshº
^†: Department of Chemistry, Ben Gurion University of the Negev, Beer-Sheva, 84105 Israel
^#: Department of Physical Chemistry, University of Geneva, CH-1211 Geneva 4, Switzerland
º: Bhabha Atomic Research Centre, Radiation and Photochemistry Division, Mumbai, India

Transfer of an elementary particle, i.e. an electron, a proton, or a hydrogen, is a fundamental chemical reaction, which are at the core of many energy conversion and associated charge transport processes in chemistry and biology. Examples include light harvesting in plants, enzymatic conversion of carbon dioxide into (bi)carbonate, proton pumping through biological membranes, excitation conversion mechanisms in photostabilisers , acid-base neutralization reactions and the von Grotthuss mechanism of proton transport in bulk water. Techniques to study the transfer of elctrons, protons or hydrogen atoms range from nuclear magnetic resonance, inelastic neutron scattering, infrared and Raman spectroscopy, and electronic spectroscopy. In all these cases the transfer reaction is described as motion in a double well potential energy surface along the reaction coordinate, either over the barrier by solvent- or temperature induced effects or by tunnelling through the barrier. In the aforementioned techniques the observed relative populations are always ensemble-averaged over the possible configurations and dynamical aspects can only be deduced in an indirect way. Ultrafast spectroscopy with an optical pump trigger pulse allows for a direct monitoring of the transfer reaction in real time. Until now these time-resolved studies have been performed with optical probing techniques, with which only limited structural information can be obatined. We pursue the implementation of femtosecond mid-infrared spectroscopy to study the structural dynamics of charge transfer processes.

Excited state intramolecular hydrogen transfer

Initial studies focussed on intramolecular hydrogen and proton transfer dynamics, where the molecular configurations are well-defined. For further reading follow the link.

Bimolecular reaction dynamics in solution

We have explored bimolecular electron transfer and bimolecular proton transfer reactions in liquid solution. For either case the interplay between molecular diffusion and reaction dynamics of on-contact complexes has to be taken into account.

Bimolecular electron transfer

Bimolecular electron transfer has often been assumed to be the most simple type of reaction. We explore photoinduced bimolecular electron transfer of donor-acceptor complexes in polar and nonpolar solution.

5. We have recently used 3-methylperylene (3-MePe) as donor and tetracyanoethene (TCNE) as acceptor in acetonitrile and dichloromethane solution. Local excitation (LE) into the S1-state of 3-MePe is followed by the transfer to TCNE (after mutual diffusion forming the encounter pair) with a large driving force, forming a 3-MePe.+ radical ion and TCNE.- radical anion which eventually can dissociate or recombine. A direct charge transfer (CT) excitation of 3-MePE - TCNE tight pairs is also possible. In the case of LE one expects that loose pairs pay a key role, in contrast for CT excitation only tight pairs are expected to occur.

6. We use polarization-sensitive UV-pump IR-probe spectroscopy with which we can learn orientational preferences of the reactant pairs.

7. Monitoring the CN-stretching modes of TCNE.- radical anion allows for a spectral distinction of tight ion pairs (TIPs) and loose ion pairs (LIPs). TIPs are observed for both LE and CT cases in acetonitrile, LIPs are only detected in the LE case. The early time component in the dynamics is caused by TIPs, which have a finite value for the anisotropy, decaying with the rotational diffusion time constant of the tight complex.

8. Based on the observed initial value of the anisotropy (r = 0.1), several possible cofigurations of 3-MePe and TCNE can be considered. All these configurations are in accordance with the sandwich-type of complexes calculated for 3-MePe-TCNE in the electronic ground state, and for the charge transfer complex.

Bimolecular proton transfer in aqueous acid-base neutralization reactions

Bimolecular acid-base neutralization in aqueous solution involves proton transfer reactions between acids and bases with water playing an active role in mediating the exchange of the protons..

9. Intermolecular proton transfer in condensed phase acid-base neutralization reactions proceed by mutual diffusion of acid and base to each other and subsequent reaction when the acid and base are "on-contact". Since the pioneering work of Weller and Eigen it is known that typical "on-contact" reaction rates typically lie in the picosecond range.

10. Photoacids are ideal systems to study the proton transfer dynamics in real-time. When in the electronic ground state proton transfer does not take place when the difference between the pKa-values of the conjugate acid HB and the photoacid R-OH is smaller than 2. When photo-excited, the pKa -value of the photoacid drops with values between 5-10. Proton transfer is then made possible by the optical trigger pulse.

11. We have used 8-hydroxy-pyrene-1,3,6-trisulfonate (HPTS) as photoacid and acetate as base., dissolved in deuterated water. We excite the photoacid with a 400 nm pump pulse, and follow the dynamics with an IR-pulse tuned to marker modes of the photoacid, of the conjugate photobase and of acetic acid, that is formed when acetate accepts a deuteron. We thus follow the reaction two sides: the photoacid/photobase (telling us when the deuteron leaves the photoacid) and the product acetic acid (telling us when the deuteron arrives at the base acetate).

12. At high concentrations of acetate we observe a fast component, that is due to pre-formed photoacid-base complexes. In these complexes the photoacid and base are in direct contact through a hydrogen-bond, along which the reaction can proceed rapidly. The deuteron transfer is found to be faster than our time resolution of 150 fs.

13. For the fraction of photoacid molecules that are initially uncomplexed when they are excited by the optical pump pulse, the reaction dynamics with the base acetate is diffusion-controlled. Modelling this diffusion-controlled reaction (solid lines) tells us that the "on-contact" reaction dynamics occurs on picosecond time scales.

14. In the case of the deuteron transfer reaction of HPTS in 1 M potassium chloroacetate (KOAc-Cl) 20% of the photoacid releases the deuteron within time resolution (< 150 fs), as evidenced by the initial rise of the 1435 cm-1 photobase band. Comparison with the case of HPTS in D2O (no base added) reveals that in this case the signal is generated in the "loose" HPTS--D2O--OAcCl complexes when the deuteron is released to the water bridge.

15. Comparison of the dynamics of the hydrated deuteron and hydrated proton bands measured for HPTS in 1 M KOAc-Cl in D2O and H2O, respectively. At longer pulse delays the deuteron/proton is transferred to the base, as indicated by the decay of the hydrated deuteron/hydrated proton bands and the rise of the C=O stretching band of DOAc-Cl.

16. Schematic comparison of the von Grotthuss hopping mechanism for proton conductivity in water (top) and the proposed proton hopping mechanism in the neutralization reaction between the photoacid HPTS and the chloroacetate base, separated by one water molecule.

17. Several proton transfer pathways between acids and bases having different number of water molecules as bridge can be considered. Because proton transfer reactions are typically reversible, many peripatetic pathways may play a key role in the solution phase dynamics. The phrase "peripatetic" in this context has been used first by Casey Hynes, see his News & Views commentary published in 2007 on this.

18. An example from the analysis of the broad data set obtained on the photoinduced reaction between HPTS and different carboxylate bases is shown here. For the loose complex pathway (only a single water molecule bridging photoacid and base) it follows that for the second proton transfer step the forward rate diminishes as function of basicity of the base (and the backward rate increases). As such the equilibrium shifts from the right hand side when using acetate to the centre when using trichloroacetate. The proton transfer reaction rate observed in the current experiment (red dots) follow the general trend found for proton dissociation of a large class of photoacid molecules (open blue circles), obeying a Marcus-type free energy correlation for proton transfer in aqueous conditions. This points to an active role the solvent water plays in such reactions.

19. Aqueous carbon dioxide chemistry is usually described using an equilibrium between CO2/H2O and HCO3-/H3O+, with the effective acid dissociation constant Ka(CO2) for which pKa = 6.35. In reality the hydration/dehydration and deprotonation/protonation steps have to be treated separately, with carbonic acid as intermediate. Carbonic acid has not been directly observed in aqueous solution, and as a result the real acidity of carbonic acid has only been roughly estimated to be pKa ~ 3.6 .

20. Ultrafast protonation of bicarbonate can be achieved using 2-naphthol-6,8-disulfonate (2N-6,8S). Experimental results have been obtained in D2O for two isotopomers of bicarbonate. The results show that the disappearance of bicarbonate by is correlated with the formation of carbonic acid.

21. Formation dynamics of carbonic acid as function of base concentration: 0.25 M (red), 0.5 M (green) and 0.8 M (blue). At the highest base concentration the early time dynamics is dominated by encounter pair reactions, at longer pulse delay the dynamics is governed by diffusional motions of photoacid and base. The experimentally found reaction rate for the protonation of bicarbonate (red dot) is only consistent with previously obtained protonation rates of carboxylate bases CH3-xClxCOO- (blue triangles) and HCOO-(black square) when the real acidity for carbonic acid is used: pKa = 3.45 ± 0.15. In contrast the effective value for CO2/H2O cannot be used for the results obtained with the ultrafast protonation experiment.

22. The facts that carbonic acid has an acidity between that of chloroacetic acid and formic acid, and the carbonic acid is relatively long lived suggests strongly the possibility that carbonic acid plays a key role as an intact molecule in the CO2 household in living animals, in chemical weathering of rock formations, in the CO2 household of Earth and in CO2 sequestration schemes where water is present.