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Advances in Photochemistry, Vol. 16
J. Photochem. PhotobioL A: Chem.,
63 (1992) 127-130
127
Book Review
Advances
in Photochemistry, Vol.
16
Edited by David H. Volman, George S. Hammond and Douglas C. Neckers, Wiley, New York, 1991, X +372 pp., US$95_00, TSBN O-471-81526-8.
I am happy to see Volume 16 of this series appear only just over a year after its predecessor. There was a period when there were quite long gaps between volumes, so that the current publication schedule represents a significant improvement_ It is also satisfying to see a smattering of references to papers from 1990 in a review volume published early in 1991. Authoritative reviews of the kind that are presented here are just what the photochemist needs to keep abreast of developments in experiment and theory, so that all moves to bring the material into print rapidly are most welcome. There has been one change to the group of editors since the last volume was prepared. Klaus Gollnick, who worked on Volumes 9-15 is now replaced by Douglas Neckers. Of the team that was responsible for Volume 1 in 1963, only George Hammond remains, although I note that David Volman was an author of a paper in that volume (readers of the present Journal might also like to remember that Volman was an author of the very first paper published in it). The current editors remind us that the fundamental objective of phorochemistry is to elucidate the detailed history of a molecule that absorbs radiation, and they point out how the emergence of the laser and the evolution of computers have together influenced research on the dynamics of excited states and other transient species. The reviews in this volume chart some of the progress that has been made in several important areas. There are five contributions in the volume, the lengths ranging from less than 40 pages to more than 100. Photosynthesis is obviously one of the key photochemical processes in nature. Over the years, a considerable depth of understanding of the details of photosynthesis has been achieved, and this understanding of the biological systems has conversely provided great insight into the fundamentals of photochemistry and photophysics. The progress in interpretation has also stimulated a search for man-made photosynthetic systems that mimic some aspects of the natural systems. In Mimicking photosynthetic electron and energy transfer, Gust and Moore focus their research on one facet of the artificial systems, that of the synthesis and spectroscopy of multicomponent molecular devices. These devices are made up of pigments, and electron donors and acceptors, whose structures resemble those found in the natural photosynthetic system, but in which the organizational constraints are provided by covalent linkages rather than by the protein environment. The complexity ranges from simple two-component systems to pentad molecules, and several aspects of natural photosynthesis can be mimicked. Various antenna effects, protection from singlet oxygen damage, and multistep electron transfer to generate charge-separated states have a11 been investigated with the model systems. Quantum yields can, in some cases, approach unity so that there is an obvious
Elsevier Sequoia
128 potential application of the results to energy conversion using man-made molecular devices. Photosynthetic organisms may provide paradigms for much basic photochemistry, but perhaps the best known photochemistry is that of ketones. Processes include bond cleavage in the Type I reaction, intramolecular abstraction of a hydrogen atom by the carbonyl oxygen (the Type II reaction), intermolecular hydrogen abstraction, and various cycloadditions. There remain, however, several paradoxes in ketone photochemistry, and a unifying interpretation is clearly needed. Formosinho and Arnaut present such Their rationalization treats an interpretation in A unified view of ketone photochemistry. the reactions of ketones as radiationless transitions from reactant to product potential energy surfaces. It may appear bizarre to invoke nuclear tunnelling where heavy atoms are involved, but there is increasing evidence for tunnelling in photoreactions and Formosinho and Arnaut develop a unified tunnel effect theory. This theory uses both the concept of a tunnelling mechanism for the radiationless transition and of normal vibrational modes to couple the initial and final states vibronically. Only one partially adjustable parameter, connected with the geometry of the transition state, is used in calculating reactivity parameters. The authors show how an impressive number of aspects of ketone photochemistry can be explained with the theory. The tunnelling theory is not universally accepted, but the debate between its proponents and those of thermal activation models have been most fruitful. The next review in the volume is entitled Molecular distortions in excited electronic states determined from electronic and resonance Raman spectioscopy, and is by Zink and Shin. According to the Franck-Condon principle, the geometry of a molecule hardly alters in the initial absorption of a photon. Subsequent relaxation can well lead to a change in shape because of changes in bonding properties that may follow excitation. Organic molecules generally undergo rather small distortions in their excited states, but much larger effects may be observed with transition metal complexes and organometallic compounds. Lengthening of metal-ligand bonds by as much as 10% along two or more bonds sometimes occurs. These classes of compound are thus amongst the most interesting in which to study excited state distortions, especially since the electronic transitions are often in spectroscopically convenient regions of the spectrum. The high symmetry of many metal compounds offers a further advantage of studying these compounds in terms of the theoretical interpretation of the effects. Zink and Shin accordingly emphasize metal-containing systems in their chapter, although the methods and principles that they set out apply to all molecules. Their review examines the origins, magnitudes and consequences of excited state distortions, and the experimental and theoretical techniques used for their measurement and calculation. The time-dependent theory of electronic and Raman spectroscopy is developed in order to interpret the observations, which include unusual features such as the missing mode effect, energy gaps, and beats. As the authors demonstrate, working in the time domain offers new insights into the common source of the different spectroscopic effects. The patterns of distortions in a molecule are related to the orbital states involved in a transition and thus to the photochemical rectivity of a molecule, so that the spectroscopic measurements and their subsequent interpretation constitute yet another tool in the photochemist’s armoury. The theme of transition metal photochemistry is continued in Forster’s Primly photoprocesses in hansition metal complwes. Photoprocesses in metal complexes have been studied for many decades, and they show some distinctive features_ A profusion of complexes is available for a given metal ion. The excited states can be metal localized, ligand localized, or charge transfer; both electron-transfer and ligand-
129
substitution photochemistry exist. There is often long-lived emission in fluid solution. The emphasis of Forster’s review is on the use of directly measured kinetic data, including lifetimes and quantum yields, to interpret the photochemical and photophysical steps. Effects of solvent motion are treated explicitly. Forster illustrates his treatment with examples drawn from the photochemistry of a few hexacoordinated complexes. He shows that, if the photoactive state can be identified, then photophysical and photochemical results can be combined to evaluate many of the primary rate constants for the photoprocesses. Mechanistic assignments are, however, mainly based on stereochemical and theoreticai, rather than kinetic, evidence. The intermediates involved have not been unequivocally identified, and in this context the studies of the photoprocesses in solvents of different mobility may prove useful in elucidating the nature of the primary steps. Sophisticated investigations of ever more intimate details of chemical reactions and their dynamics have extended to the study of aligned and oriented gas-phase reactants. In general, the geometric relation of the reactants to the laboratory system has been achieved with fields derived from multipoles or from laser radiation itself. However, one of the types of process discussed in the last review of this volume provides another way of defining the geometric starting conditions for bimolecular reactions. When a weakly bonded complex is used as a photochemical precursor of the reactants, the fragment that is liberated is thrust towards the other constituent within the geometrical framework of the complex. Within the limitations of knowledge of the structure of the complex, the initial geometry of any subsequent reaction is thus defined. Shin, Chen, Nickolaisen, Sharpe, Beaudet and Wittig are concerned more generally with photochemistry in weakly bound systems, and their review Photoinitiated reactions in weakly bonded complexes covers three specific topics. Statespecific and time-resolved interrogation of the systems in each case reveals detailed information about the processes occurring. One important aspect of the studies surveyed is, indeed, the initiation of reactions in binary complexes in the gas phase. Many of the results are from the authors’ own research, and involve formation of the binary complexes in supersonic nozzle expansions. Reaction is initiated photochemically in the complex and nascent product distributions, the variation of product yield with photolytic wavelength, and the build-up (on the picosecond time-scale) of products are all measured to infer the dynamical nature of the interaction_ However, the authors preface this aspect of the subject by a description of studies performed on binary pairs isolated in inert cryogenic matrices. Products and even reaction intermediates can be investigated quantitatively, in spite of the fairly complicated overall photochemistry that occurs. Indeed, the ability of the matrix to quench excitation can serve as a valuable diagnostic tool. Evolution of the excited state towards products is a concerted process in the matrix system, and it cannot be separated into sequential photodissociation and reaction steps. Yet another kind of experiment discussed in this review relates to the photochemistry of complexes between metal atoms and small molecules. The basic idea is to impart electronic orbital specificity to the complexes through excitation of an atomic chromophore. Orbitals are selectively oriented in the body frame of the complex, and the specificity may be carried through in subsequent reaction to the products as chemical or energy selectivity. This last review discusses rather unusual photochemistry, but it is evident that the basic ideas can be extended to cover a broad range of issues. Much may be expected from studies of this kind in the future. The five articles in this volume make it abundantly clear that photochemistry is a vital, energetic, and expanding field. The range encompassed by the reviews illustrates
130 at once the diversity of interests of photochemists, and it underlines the value of the series. Photochemists of all persuasions have good reason to be thankful that some of their distinguished colleagues have been prepared to share their knowledge and enthusiasm, and that the editors of this series were able to encourage the authors to produce their reviews. R. P. Wayne Physical Chemisny Laboratory, University of Oxford, South Parks Road, Oxford, Oxon OX1 3QZ (UK)
63 (1992) 127-130
127
Book Review
Advances
in Photochemistry, Vol.
16
Edited by David H. Volman, George S. Hammond and Douglas C. Neckers, Wiley, New York, 1991, X +372 pp., US$95_00, TSBN O-471-81526-8.
I am happy to see Volume 16 of this series appear only just over a year after its predecessor. There was a period when there were quite long gaps between volumes, so that the current publication schedule represents a significant improvement_ It is also satisfying to see a smattering of references to papers from 1990 in a review volume published early in 1991. Authoritative reviews of the kind that are presented here are just what the photochemist needs to keep abreast of developments in experiment and theory, so that all moves to bring the material into print rapidly are most welcome. There has been one change to the group of editors since the last volume was prepared. Klaus Gollnick, who worked on Volumes 9-15 is now replaced by Douglas Neckers. Of the team that was responsible for Volume 1 in 1963, only George Hammond remains, although I note that David Volman was an author of a paper in that volume (readers of the present Journal might also like to remember that Volman was an author of the very first paper published in it). The current editors remind us that the fundamental objective of phorochemistry is to elucidate the detailed history of a molecule that absorbs radiation, and they point out how the emergence of the laser and the evolution of computers have together influenced research on the dynamics of excited states and other transient species. The reviews in this volume chart some of the progress that has been made in several important areas. There are five contributions in the volume, the lengths ranging from less than 40 pages to more than 100. Photosynthesis is obviously one of the key photochemical processes in nature. Over the years, a considerable depth of understanding of the details of photosynthesis has been achieved, and this understanding of the biological systems has conversely provided great insight into the fundamentals of photochemistry and photophysics. The progress in interpretation has also stimulated a search for man-made photosynthetic systems that mimic some aspects of the natural systems. In Mimicking photosynthetic electron and energy transfer, Gust and Moore focus their research on one facet of the artificial systems, that of the synthesis and spectroscopy of multicomponent molecular devices. These devices are made up of pigments, and electron donors and acceptors, whose structures resemble those found in the natural photosynthetic system, but in which the organizational constraints are provided by covalent linkages rather than by the protein environment. The complexity ranges from simple two-component systems to pentad molecules, and several aspects of natural photosynthesis can be mimicked. Various antenna effects, protection from singlet oxygen damage, and multistep electron transfer to generate charge-separated states have a11 been investigated with the model systems. Quantum yields can, in some cases, approach unity so that there is an obvious
Elsevier Sequoia
128 potential application of the results to energy conversion using man-made molecular devices. Photosynthetic organisms may provide paradigms for much basic photochemistry, but perhaps the best known photochemistry is that of ketones. Processes include bond cleavage in the Type I reaction, intramolecular abstraction of a hydrogen atom by the carbonyl oxygen (the Type II reaction), intermolecular hydrogen abstraction, and various cycloadditions. There remain, however, several paradoxes in ketone photochemistry, and a unifying interpretation is clearly needed. Formosinho and Arnaut present such Their rationalization treats an interpretation in A unified view of ketone photochemistry. the reactions of ketones as radiationless transitions from reactant to product potential energy surfaces. It may appear bizarre to invoke nuclear tunnelling where heavy atoms are involved, but there is increasing evidence for tunnelling in photoreactions and Formosinho and Arnaut develop a unified tunnel effect theory. This theory uses both the concept of a tunnelling mechanism for the radiationless transition and of normal vibrational modes to couple the initial and final states vibronically. Only one partially adjustable parameter, connected with the geometry of the transition state, is used in calculating reactivity parameters. The authors show how an impressive number of aspects of ketone photochemistry can be explained with the theory. The tunnelling theory is not universally accepted, but the debate between its proponents and those of thermal activation models have been most fruitful. The next review in the volume is entitled Molecular distortions in excited electronic states determined from electronic and resonance Raman spectioscopy, and is by Zink and Shin. According to the Franck-Condon principle, the geometry of a molecule hardly alters in the initial absorption of a photon. Subsequent relaxation can well lead to a change in shape because of changes in bonding properties that may follow excitation. Organic molecules generally undergo rather small distortions in their excited states, but much larger effects may be observed with transition metal complexes and organometallic compounds. Lengthening of metal-ligand bonds by as much as 10% along two or more bonds sometimes occurs. These classes of compound are thus amongst the most interesting in which to study excited state distortions, especially since the electronic transitions are often in spectroscopically convenient regions of the spectrum. The high symmetry of many metal compounds offers a further advantage of studying these compounds in terms of the theoretical interpretation of the effects. Zink and Shin accordingly emphasize metal-containing systems in their chapter, although the methods and principles that they set out apply to all molecules. Their review examines the origins, magnitudes and consequences of excited state distortions, and the experimental and theoretical techniques used for their measurement and calculation. The time-dependent theory of electronic and Raman spectroscopy is developed in order to interpret the observations, which include unusual features such as the missing mode effect, energy gaps, and beats. As the authors demonstrate, working in the time domain offers new insights into the common source of the different spectroscopic effects. The patterns of distortions in a molecule are related to the orbital states involved in a transition and thus to the photochemical rectivity of a molecule, so that the spectroscopic measurements and their subsequent interpretation constitute yet another tool in the photochemist’s armoury. The theme of transition metal photochemistry is continued in Forster’s Primly photoprocesses in hansition metal complwes. Photoprocesses in metal complexes have been studied for many decades, and they show some distinctive features_ A profusion of complexes is available for a given metal ion. The excited states can be metal localized, ligand localized, or charge transfer; both electron-transfer and ligand-
129
substitution photochemistry exist. There is often long-lived emission in fluid solution. The emphasis of Forster’s review is on the use of directly measured kinetic data, including lifetimes and quantum yields, to interpret the photochemical and photophysical steps. Effects of solvent motion are treated explicitly. Forster illustrates his treatment with examples drawn from the photochemistry of a few hexacoordinated complexes. He shows that, if the photoactive state can be identified, then photophysical and photochemical results can be combined to evaluate many of the primary rate constants for the photoprocesses. Mechanistic assignments are, however, mainly based on stereochemical and theoreticai, rather than kinetic, evidence. The intermediates involved have not been unequivocally identified, and in this context the studies of the photoprocesses in solvents of different mobility may prove useful in elucidating the nature of the primary steps. Sophisticated investigations of ever more intimate details of chemical reactions and their dynamics have extended to the study of aligned and oriented gas-phase reactants. In general, the geometric relation of the reactants to the laboratory system has been achieved with fields derived from multipoles or from laser radiation itself. However, one of the types of process discussed in the last review of this volume provides another way of defining the geometric starting conditions for bimolecular reactions. When a weakly bonded complex is used as a photochemical precursor of the reactants, the fragment that is liberated is thrust towards the other constituent within the geometrical framework of the complex. Within the limitations of knowledge of the structure of the complex, the initial geometry of any subsequent reaction is thus defined. Shin, Chen, Nickolaisen, Sharpe, Beaudet and Wittig are concerned more generally with photochemistry in weakly bound systems, and their review Photoinitiated reactions in weakly bonded complexes covers three specific topics. Statespecific and time-resolved interrogation of the systems in each case reveals detailed information about the processes occurring. One important aspect of the studies surveyed is, indeed, the initiation of reactions in binary complexes in the gas phase. Many of the results are from the authors’ own research, and involve formation of the binary complexes in supersonic nozzle expansions. Reaction is initiated photochemically in the complex and nascent product distributions, the variation of product yield with photolytic wavelength, and the build-up (on the picosecond time-scale) of products are all measured to infer the dynamical nature of the interaction_ However, the authors preface this aspect of the subject by a description of studies performed on binary pairs isolated in inert cryogenic matrices. Products and even reaction intermediates can be investigated quantitatively, in spite of the fairly complicated overall photochemistry that occurs. Indeed, the ability of the matrix to quench excitation can serve as a valuable diagnostic tool. Evolution of the excited state towards products is a concerted process in the matrix system, and it cannot be separated into sequential photodissociation and reaction steps. Yet another kind of experiment discussed in this review relates to the photochemistry of complexes between metal atoms and small molecules. The basic idea is to impart electronic orbital specificity to the complexes through excitation of an atomic chromophore. Orbitals are selectively oriented in the body frame of the complex, and the specificity may be carried through in subsequent reaction to the products as chemical or energy selectivity. This last review discusses rather unusual photochemistry, but it is evident that the basic ideas can be extended to cover a broad range of issues. Much may be expected from studies of this kind in the future. The five articles in this volume make it abundantly clear that photochemistry is a vital, energetic, and expanding field. The range encompassed by the reviews illustrates
130 at once the diversity of interests of photochemists, and it underlines the value of the series. Photochemists of all persuasions have good reason to be thankful that some of their distinguished colleagues have been prepared to share their knowledge and enthusiasm, and that the editors of this series were able to encourage the authors to produce their reviews. R. P. Wayne Physical Chemisny Laboratory, University of Oxford, South Parks Road, Oxford, Oxon OX1 3QZ (UK)