Excited-State Structures DOI: 10.1002/anie.200900741 Structural Tracking of a Bimolecular Reaction in Solution by Time- Resolved X-Ray Scattering** Kristoffer Haldrup, Morten Christensen, Marco Cammarata, Qingyu Kong, Michael Wulff, Simon O. Mariager, Klaus Bechgaard, Robert Feidenhansl, Niels Harrit,* and Martin M. Nielsen* Every photochemical reaction starts with an electronically excited state and ends with a ground-state molecule. This is also true in bimolecular photoreactions, in which the excited molecule collides with a ground-state reactant. The collision complex may follow a potential energy surface directly to the primary ground-state product. The reaction may also proceed on an excited-state surface and reach a stable minimum configuration. This excited complex—an exciplex—is antici- pated to precede the primary product of many types of bimolecular photoreactions, most prominently in photoin- duced electron-transfer reactions. [1] The presence of an exciplex is easily demonstrated if it emits a characteristic emission. However, this situation is the exception rather than the rule, and exciplexes are generally not easily detected. An alternative to emission spectroscopy is time-resolved absorption spectroscopy based on laser meth- ods. Unfortunately, neither type of spectroscopy provides direct structural information. The present study presents time-resolved X-ray scattering in aqueous solution as a new means of directly obtaining a model-independent structure of this kind of elusive intermediate. The advent of intense and pulsed X-ray beams from synchrotron insertion devices has opened up unprecedented opportunities to obtain structural information on transient molecules. [2, 3] In the crystalline state, diffraction studies have allowed structure determination of photoexcited mole- cules [4–7] including the 3 A 2u state of salts of octahydrogen[te- trakis-m-diphosphito-1kP :2kP’-diplatinate](4-) (PtPOP*) [5, 6] and excimers. [8] Only recently have time-resolved scattering studies of reactions in solutions become possible despite the large background scattering from the solvent [9] and the absence of signal amplification from long-range order. The theoretical foundation of the laser/X-ray pump–probe method has been established, [10, 11] and the solvent response to impulsive heating has been described. [12, 13] Thus, solution- state pump–probe X-ray scattering studies on time scales of picoseconds to nanoseconds have been reported [14] for light- induced reactions of small molecules such as I 2 , HgI 2 , CH 2 I 2 , and C 2 H 4 I 2 [15, 16] dissolved in CCl 4 and methanol, and for gold nanoparticles in water. [17] The excited 3 A 2u state of PtPOP* represents the most complex structure hitherto investigated in solution by time-resolved X-ray scattering. [18] PtPOP* displays characteristic photophysical (phosphor- escence quantum yield, F p = 0.5; lifetime, t p = 10 ms [19, 20] ) and photochemical properties. Hydrogen/halogen atom abstrac- tion and photocatalytical generation of H 2 by C H bond cleavage are among the reactions reported. [21] PtPOP* also forms excited-state complexes (exciplexes) with, for example, Tl + ions. This bimolecular reaction is of particular interest as it was the first metal–metal bonded exciplex formation to be reported. [22–24] The present study deals with the exciplex TlPtPOP*, which forms between PtPOP* and a single Tl + ion, in the time frame from 100 ps to 100 ns after excitation. The processes occurring are illustrated in Figure 1. On longer time scales, or at high concentrations of Tl + ions, an additional Tl + ion can combine with TlPtPOP* to form Tl 2 PtPOP*. [24] The liquid-scattering experiments were carried out on beamline ID09B at the ESRF facility following a well- established laser/X-ray pump–probe protocol. [9, 18, 25] The con- centrations in a typical experiment were [PtPOP] = 12 mm and [Tl + ] = 7.2 mm. The aqueous solution was circulated under oxygen-free conditions through a sapphire nozzle producing a 0.3 mm thick sheet of liquid. This was crossed by the laser and X-ray beams. The optical pump pulse at 390 nm was generated by frequency doubling the 780 nm output of a Ti:sapphire laser. To reduce multiphoton excita- tion of the solvent as well as other higher-order effects, the peak intensity of the pump pulse was reduced by stretching the 150 fs pulse to 2 ps. The X-ray probe pulse was created by an undulator insertion device producing a “pink-beam” energy profile with a maximum at 18.2 keV and 3 % bandwidth after passing the X-ray optics. The X-rays from a single bunch in the storage [*] Dr. K. Haldrup, M. Christensen, S. O. Mariager, Prof. K. Bechgaard, Prof. R. Feidenhans’l, Prof. N. Harrit, Prof. M. M. Nielsen Centre for Molecular Movies, Department of Chemistry and Niels Bohr Institute, University of Copenhagen Universitetsparken 5, 2100 Copenhagen (Denmark) Fax: (+ 45) 3532-0460 E-mail: harrit@nano.ku.dk meedom@nbi.dk Dr. M. Cammarata, Dr. Q. Kong, [+] Prof. M. Wulff European Synchrotron Radiation Facility, Grenoble Cedex 38043 (France) [ + ] Current address: SociØtØ Civile Synchrotron SOLEIL, Gif-sur-Yvette Cedex 9112 (France) [**] The expert assistance provided by E. Pontecorvo and F. Ewald at the ID09B Beamline at ESRF, Grenoble, is very much appreciated. This work was supported by the Danish National Research Foundation’s Centre for Molecular Movies and DANSCATT. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200900741. Communications 4180  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2009, 48, 4180 –4184