Editorial pubs.acs.org/JPCA
Virtual Issue in Memory of Ahmed Zewail
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e were sad to hear that our good friend Ahmed Zewail passed away August 2, 2016. Zewail was a giant in the field of physical chemistry, having led the world starting in the 1980s in the application of femtosecond lasers to chemical problems, and more recently in time-resolved electron imaging. He won the Nobel Prize in 1999, as well as many other awards, including the ACS Priestley Medal in 2011. It is noteworthy that Zewail’s contributions to education and policy were equally important to his science, including the founding of a university in Egypt, and his role in building scientific research in the Middle East. The Zewail City for Science and Technology in Cairo, Egypt, was instituted by Zewail and paid for by the people of Egypt to help him in his efforts to return the importance of science and technology to that which was present during the ancient Egyptian civilization. Zewail was a Senior Editor of the Journal of Physical Chemistry in the 1980s. This was a time of significant growth of the Journal, as JPC evolved to become the dominant physical chemistry journal, and Zewail’s leadership was important to this process. Later he played an even more important role as Editor of Chemical Physics Letters, and we are delighted to see that CPL is doing a special issue for him. For our part, we have compiled this Virtual Issue in his honor, including 25 of his most important papers published in the JPC. A brief commentary on each of these papers is given below. We hope that this compendium will help illustrate the exciting work that Zewail contributed, including both the breadth and depth of his accomplishments. Zewail published a total of 83 papers in the JPC, spanning the time from the early 1980s to very recently. It is especially noteworthy that during his period of peak creativity in femtochemistry, from the late 1980s through the early 2000s he published 5−10 papers each year in the journal, including many papers that can be considered classics in the femtochemistry field. The papers we have chosen span the time 1984−2007, during which time Zewail transitioned from doing picosecond to femtosecond experiments, and the experiments evolved from gas phase isolated molecule studies, to dynamics in clusters, and then to ultrafast processes in liquids and in biological systems. Also, starting in the 1990s, Zewail initiated experiments in ultrafast electron diffraction, and after 2006 this topic, including crystallography and imaging, dominated in his publications. 1. Bloembergen, N.; Zewail, A. H. Energy Redistribution in Isolated Molecules and the Question of Mode-Selective Laser Chemistry Revisited. J. Phys. Chem. 1984, 88 (23), 5459−5465. DOI: 10.1021/j150667a004. This was a Feature Article in which the opportunities for laser selective chemistry using picosecond pulses were explored. This was the era when there was interest in using multiphoton excitation to dissociate small molecules such as CF3Br in the gas phase, and there were serious questions about the importance of intramolecular vibrational redistribution (IVR). Several experiments were described, and it was concluded that IVR was generally fast enough that much shorter pulses were desirable. © 2016 American Chemical Society
2. Felker, P. M.; Zewail, A. H. Rates of Photoisomerization of trans-Stilbene in Isolated and Solvated Molecules: Experiments on the Deuterium-Isotope Effect and RRKM Behavior. J. Phys. Chem. 1985, 89 (25), 5402− 5411. DOI: 10.1021/j100271a018. Here the photoisomerization of trans-stilbene after photoexcitation with picosecond excitation was studied by probing fluorescence. The results, including isotope effects, were modeled using RRKM theory. This work identified disagreement between experiment and RRKM that was the subject of later studies by many groups. 3. Leland, B. A.; Joran, A. D.; Felker, P. M.; Hopfield, J. J.; Zewail, A. H.; Dervan, P. B. Picosecond Fluorescence Studies on Intramolecular Photochemical Electron Transfer in Porphyrins Linked to Quinones at Two Different Fixed Distances. J. Phys. Chem. 1985, 89 (26), 5571−5573. DOI: 10.1021/j100272a002. This picosecond study examined fluorescence lifetimes associated with porphyrin−spacer−quinone structures both in solution and at 77 K, revealing electron transfer rates as a function of solvent and number of bicyclo[2.2.2]octyl spacers. 4. Scherer, N. F.; Knee, J. L.; Smith, D. D.; Zewail, A. H. Femtosecond Photofragment Spectroscopy: The Reaction ICN → CN + I. J. Phys. Chem. 1985, 89 (24), 5141− 5143. DOI: 10.1021/j100270a001. This Letter was the beginning of femtosecond photofragment spectroscopy, here involving the use of 400 fs pulses to initiate ICN photodissociation, and then using fluorescence to probe the dynamics. A 600 fs time for formation of the products was observed. 5. Felker, P. M.; Baskin, J. S.; Zewail, A. H. Rephasing of Collisionless Molecular Rotational Coherence in Large Molecules. J. Phys. Chem. 1986, 90 (5), 724−728. DOI: 10.1021/j100277a006. This Letter reported the first observation of rotational quantum coherence transients, here for the trans-stilbene molecule. The experiment used 20 ps photoexcitation followed by fluorescence detection of rotational dephasing that could be interpreted using a rotational wavepacket analysis. 6. Peng, L. W.; Dantus, M.; Zewail, A. H.; Kemnitz, K.; Hicks, J. M.; Eisenthal, K. B. Stepwise Solvation of the Intramolecular-Charge-Transfer Molecule p-(dimethylamino)benzonitrile. J. Phys. Chem. 1987, 91 (24), 6162− 6167. DOI: 10.1021/j100308a021. This collaboration of Zewail with the Eisenthal group compared supersonic jet expansion and thermalized vapor spectroscopy of the p-(N,N-dimethylamino)benzonitrile molecule. The jet studies include both the isolated molecule and complexes of the molecule with water and other solvents. This work demonstrated that Published: September 29, 2016 7405
DOI: 10.1021/acs.jpca.6b08645 J. Phys. Chem. A 2016, 120, 7405−7407
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1:1 complexes are not large enough to enable charge transfer. Baskin, J. S.; Zewail, A. H. Determination of ExcitedState Rotational-Constants and Structures by DopplerFree Picosecond Spectroscopy. J. Phys. Chem. 1989, 93 (15), 5701−5717. DOI: 10.1021/j100352a014. Here the use of picosecond time-resolved fluorescence measurements of rotational coherence (paper 5 above) is developed as a method for determing rotational constants for the electronic states of trans-stilbene, anthracene and related van der Waals complexes. Zewail, A. H. Femtochemistry. J. Phys. Chem. 1993, 97 (48), 12427−12446. DOI: 10.1021/j100150a001. This was a Feature Article that reviewed Zewail’s work in the femtochemistry field as it existed in 1993. Included were sections concerned with a number of gas phase elementary reactions as studied by pump−probe methods, sections concerned with reactions in solvent cages and clusters, and sections concerned with wavepacket phasing and coherence. This was part of the proceedings of the International Meeting on Femtosecond Chemistry - The Berlin Conference, March 1−4, 1993. This conference was the first of a series that continues to this day, and the JPC has published special issues for three of them: Femto1 in 1993, Femto3 in 1997, and Femto10 in 2011 (published in 2012). Dantus, M.; Kim, S. B.; Williamson, J. C.; Zewail, A. H. Ultrafast Electron-Diffraction. 5. Experimental Time Resolution and Applications. J. Phys. Chem. 1994, 98 (11), 2782−2796. DOI: 10.1021/j100062a011. This paper presented a detailed description of the ultrafast electron diffraction apparatus that was built at Caltech, and included diffraction images of several molecules, along with a study of the photodissociation of CF3I. This was an early version of a more sophisticated apparatus that is described in paper 25. Kim, S. K.; Breen, J. J.; Willberg, D. M.; Peng, L. W.; Heikal, A.; Syage, J. A.; Zewail, A. H. Solvation Ultrafast Dynamics of Reactions. 8. Acid−Base Reactions in Finite-Sized Clusters of Naphthol in Ammonia, Water, and Piperidine. J. Phys. Chem. 1995, 99 (19), 7421−7435. DOI: 10.1021/j100019a027. This presented time-resolved (sub-picosecond) studies of 1-naphthol in solvent cages composed of ammonia, piperidine, or water (in a molecular beam). The rate of proton transfer was determined as a function of cage size, and from this it was possible to determine the importance of caging on the photodissociation dynamics. Baskin, J. S.; Banares, L.; Pedersen, S.; Zewail, A. H. Femtosecond Real-Time Probing of Reactions. 20. Dynamics of Twisting, Alignment, and IVR in the trans-Stilbene Isomerization Reaction. J. Phys. Chem. 1996, 100 (29), 11920−11933. DOI: 10.1021/jp960909x. Here the isomerization of trans-stilbene under collisionless conditions was studied using REMPI and femtosecond depletion spectroscopy, revealing ethylene twisting in combination with IVR as a function of excess vibrational energy in the S1 state. Cong, P.; Roberts, G.; Herek, J. L.; Mohktari, A.; Zewail, A. H. Femtosecond Real-Time Probing of Reactions. 18. Experimental and Theoretical Mapping of Trajectories and Potentials in the NaI Dissociation Reaction. J. Phys.
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Chem. 1996, 100 (19), 7832−7848. DOI: 10.1021/ jp9534038. This paper was part of the classic studies of the NaI dissociation dynamics using a combination of femtosecond pump−probe methods and wavepacket calculations. The calculations demonstrated how the experiment is sensitive to extension and contraction of the Na−I bond. Lienau, C.; Zewail, A. H. Solvation Ultrafast Dynamics of Reactions. 11. Dissociation and Caging Dynamics in the Gas-to-Liquid Transition Region. J. Phys. Chem. 1996, 100 (48), 18629−18649. DOI: 10.1021/jp962430a. This was another classic femtosecond paper concerned with I2 photodissociation in supercritical rare-gas solvents, in which changes in the gas density were used to study wavepacket coherence, collision-induced predissociation, and geminate recombination. Molecular dynamics calculations were used to model the results. Motzkus, M.; Pedersen, S.; Zewail, A. H. Femtosecond Real-Time Probing of Reactions. 19. Nonlinear (DFWM) Techniques for Probing Transition States of Uni- and Bimolecular Reactions. J. Phys. Chem. 1996, 100 (14), 5620−5633. DOI: 10.1021/jp960265t. Here a degenerate four-wave mixing technology was presented for describing the dynamics of chemical processes, including the Na + H2 reaction. Zewail, A. H. Femtochemistry: Recent Progress in Studies of Dynamics and Control of Reactions and Their Transition States. J. Phys. Chem. 1996, 100 (31), 12701−12724. DOI: 10.1021/jp960658s. This review article appeared in the Centennial Issue of the Journal of Physical Chemistry in 1996. It summarizes Zewail’s femtochemistry work to that point, including his studies of transition state dynamics. Chachisvilis, M.; Fiebig, T.; Douhal, A.; Zewail, A. H. Femtosecond Dynamics of a Hydrogen-Bonded Model Base Pair in the Condensed Phase: Double Proton Transfer in 7-Azaindole. J. Phys. Chem. A 1998, 102 (4), 669−673. DOI: 10.1021/jp980337b. Here the femtosecond dynamics of double proton transfer in the 7-azaindole dimer was studied in both the gas phase and condensed phase, and the relative importance of direct and indirect reaction pathways was determined. Douhal, A.; Fiebig, T.; Chachisvilis, M.; Zewail, A. H. Femtochemistry in Nanocavities: Reactions in Cyclodextrins. J. Phys. Chem. A 1998, 102 (10), 1657−1660. DOI: 10.1021/jp980518d. The intramolecular proton transfer and isomerization dynamics of the HPMO molecule was studied in the β-cyclodextrin cavity using fluorescence up-conversion, showing that encapsulation significantly slows reaction. Zhong, D. P.; Zewail, A. H. Femtosecond Real-Time Probing of Reactions. 23. Studies of Temporal, Velocity, Angular, and State Dynamics from Transition States to Final Products by Femtosecond-Resolved Mass Spectrometry. J. Phys. Chem. A 1998, 102 (23), 4031−4058. DOI: 10.1021/jp9805196. This presented a femtosecond, time-resolved mass spectrometry method for studying complex reactions, with applications to a number of molecular dimers such as methyl iodide, including the characterization of charge DOI: 10.1021/acs.jpca.6b08645 J. Phys. Chem. A 2016, 120, 7405−7407
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Nanometer-Scale Structures. J. Phys. Chem. C 2007, 111 (13), 4889−4919. DOI: 10.1021/jp067466+. This was the first of a series of several papers in which Zewail described the development of ultrafast electron crystallography (UEC), including applications to nanometer scale semiconductors and quantum well structures.
transfer, dissociation, elimination, and other reactive processes. Chachisvilis, M.; Zewail, A. H. Femtosecond Dynamics of Pyridine in the Condensed Phase: Valence Isomerization by Conical Intersections. J. Phys. Chem. A 1999, 103 (37), 7408−7418. DOI: 10.1021/jp991821x. Femtosecond transient absorption in combination with theory was used to study pyridine photochemistry, including excited state deactivation and dissociation pathways. Fiebig, T.; Chachisvilis, M.; Manger, M.; Zewail, A. H.; Douhal, A.; Garcia-Ochoa, I.; Ayuso, A. D. H. Femtosecond Dynamics of Double Proton Transfer in a Model DNA Base Pair: 7-Azaindole Dimers in the Condensed Phase. J. Phys. Chem. A 1999, 103 (37), 7419−7431. DOI: 10.1021/jp991822p. This work extended earlier studies of double proton transfer in 7-azaindole dimers, including femtosecond transient absorption of fluorescence upconversion, with consideration of tunneling and concertedness of the process. Zewail, A. H. Femtochemistry: Atomic-Scale Dynamics of the Chemical Bond. J. Phys. Chem. A 2000, 104 (24), 5660−5694. DOI: 10.1021/jp001460h. This was adapted from Zewail’s Nobel Lecture, and summarizes his work on Femtochemistry. This is Zewail’s highest cited paper in any journal. This provides a historical perspective on the development of femtochemistry in the Zewail group and other groups, including detailed studies of experiments on systems such as ICN and NaI photodissociation in which it was demonstrated that the femtosecond experiments could be understood in terms of wavepacket dynamics on Born−Oppenheimer potential surfaces, thus linking with concepts that were already understood from spectroscopy and molecular beam experiments. Baskin, J. S.; Yu, H. Z.; Zewail, A. H. Ultrafast Dynamics of Porphyrins in the Condensed Phase: I. Free Base Tetraphenylporphyrin. J. Phys. Chem. A 2002, 106 (42), 9837−9844. DOI: 10.1021/jp020398g. Fluorescence up-conversion and transient absorption is used to study ultrafast relaxation of the tetraphenylporphyrin molecule in benzene solution, determining time scales of IVR, for vibrational relaxtion caused by solvent interactions, and for thermal equilibration. Pal, S. K.; Peon, J.; Bagchi, B.; Zewail, A. H. Biological Water: Femtosecond Dynamics of Macromolecular Hydration. J. Phys. Chem. B 2002, 106 (48), 12376− 12395. DOI: 10.1021/jp0213506. This Feature Article reviewed femtosecond studies of the residence times associated with hydration of proteins. Qiu, W. H.; Zhang, L. Y.; Okobiah, O.; Yang, Y.; Wang, L. J.; Zhong, D. P.; Zewail, A. H. Ultrafast Solvation Dynamics of Human Serum Albumin: Correlations with Conformational Transitions and Site-Selected Recognition. J. Phys. Chem. B 2006, 110 (21), 10540−10549. DOI: 10.1021/jp055989w. This was a study of the solvation dynamics and local rigidity of human serum albumin based on the photophysics of a tryptophan residue, including studies of the effects of pH on the protein structure. Yang, D.-S.; Gedik, N.; Zewail, A. H. Ultrafast Electron Crystallography. 1. Nonequilibrium Dynamics of
Mostafa A. El-Sayed, Editor-in-Chief, 1980−2004 Georgia Institute of Technology
George C. Schatz, Editor-in-Chief
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Northwestern University
AUTHOR INFORMATION
Notes
Views expressed in this editorial are those of the authors and not necessarily the views of the ACS.
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DOI: 10.1021/acs.jpca.6b08645 J. Phys. Chem. A 2016, 120, 7405−7407
