Editorial pubs.acs.org/JPCL
Photoinduced Dynamics in Membranes and at Interfaces
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ne of the areas of physical chemistry that continues to evolve apace is the field of chemical dynamics, a subdiscipline once a bastion of the pristine environment of the gas phase or homogeneous solution. Much current work is extending studies of chemical dynamics to challenging domains where our intuition is not well-developed, and therefore, the results are often unanticipated. In this issue, two Perspectives provide examples that illustrate the point. Vauthey and coworkers1 describe the various experimental methods and key results aimed at probing photoinduced dynamics at air/liquid and liquid/liquid interfaces. Here, the challenge is to selectively detect the few molecules at the interface relative to the bulk and do so in a pump−probe arrangement that catches the photoinduced processes “in the act” in real time. The authors walk the reader through the various experimental options, pointing out the strengths and weaknesses of each. They focus particular attention on a couple of methods, time-resolved surface second-harmonic generation (TR-SSHG) and its sumfrequency generation analogue (TR-SSFG), which show particular promise. Both are nonlinear probe schemes that only detect interfacial molecules where the symmetry of the environment is broken by the interface. The field is clearly poised for further advancement as these methods are perfected and applied to a wider range of chemical systems. In the other Perspective, Jankowiak2 describes studies of the dynamics of electron transfer in photosynthetic reaction centers. There, the challenge is to sort through the complexity inherent to the biological structure under investigation, with multiple chromophores assembled in a membrane environment serving as antennae for photoabsorption leading to charge separation in the reaction center. While revolutionary studies from two-dimensional electronic spectroscopy (2D-ES) are providing femtosecond time-domain probes of the excitonic coupling and electronic energy transfer at room temperature,3 Jankowiak describes experiments taking a complementary track. With the goal of sorting through the local chromophore site and interchromophore couplings, the photosystems are cooled to within a few degrees of absolute zero, and hole-burning methods are used to study the homogeneous and inhomogeneous contributions to the absorption profiles. Both photochemical and photophysical processes can be probed. The results of such studies serve as input to models of the entire process and feed back into the 2D-ES studies. This close coupling between experiment and theory is a hallmark of current research in this area. As good Perspectives do, these provide a spotlight on an emerging field also represented in the Letters themselves. A notable example is the Letter by Hauer and co-workers, who discuss the role of vibrational and electronic coherences in 2DES and present criteria that distinguish the two.4 We hope that the pages of The Journal of Physical Chemistry Letters continue to be a prime venue for advancements in the increasingly complex field of light-induced chemical dynamics.
REFERENCES
(1) Richert, S.; Fedoseeva, M.; Vauthey, E. J. Ultrafast Photoinduced Dynamics at Air/Liquid and Liquid/Liquid Interfaces. J. Phys. Chem. Lett. 2012, 3, 1635−1642. (2) Jankowiak, R. Probing Electron-Transfer Times in Photosynthetic Reaction Centers by Hole-Burning Spectroscopy. J. Phys. Chem. Lett. 2012, 3, 1684−1694. (3) Ishizaki, A.; Fleming, G. R. Quantum Coherence in Photosynthetic Harvesting. Ann. Rev. Condens. Matter Phys. 2012, 3, 333− 361. (4) Mancal, T.; Christensson, N.; Lukes, V.; Milota, F.; Bixner, O.; Kauffmann, H. F.; Hauer, J. System-Dependent Signatures of Electronic and Vibrational Coherences in Electronic Two-Dimensional Spectra. J. Phys. Chem. Lett. 2012, 3, 1497−1502.
Timothy S. Zwier, Senior Editor
Purdue University, West Lafayette, Indiana, United States © 2012 American Chemical Society
Published: June 21, 2012 1721
dx.doi.org/10.1021/jz300645m | J. Phys. Chem. Lett. 2012, 3, 1721−1721
