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PREFACE
M ^ O L E C U L E S IN THEIR EXCITED STATES and molecules of transient existence generated by photochemical stimulation or by other processes, such as electrochemistry, are rapidly drawing considerable interest and gaining importance. The excited state of a molecule is, in many ways, a new species different chemically from the ground state molecule and endowed with additional energy; it is often capable of chemical processes that are not possible in the ground state. The ability to do "test tube" experiments with such short-lived species is currently under intensive development. The conference from which this book was developed addressed many of the techniques that may be used to probe these systems and dealt with the new chemistry that is being learned. Approximately 160 participants took part in the Biennial Inorganic Chemical Symposium 1985. The participants came primarily from Canada and the United States; however, some came from as far as Japan, England, Belgium, Italy, East and West Germany, and The Netherlands, thus generating an international atmosphere. They heard the latest ideas in excited state photochemistry and photophysics, species at electrode surfaces, resonance Raman spectroscopy, electrochemiluminescence, photochemistry of organometallic and cluster species, and gas phase organometallic chemistry to name a few topics. Some fifty posters were also presented and the Chemical Abstracts Services displayed the latest in on-line searching. Funding for the conference came from the divisions of inorganic chemistry of the American Chemical Society (ACS) and the Chemical Institute of Canada, the Petroleum Research Fund (ACS), the Natural Sciences and Engineering Research Council, York University, and eight industrial companies: Strem Chemicals Inc.; Merck Frosst Canada Inc.; Union Carbide Canada Limited; EG&G Canada Limited; Xerox Research Centre of Canada; Tasman Scientific Inc.; Guided Wave, Inc.; and Lumonics Inc. I hope this volume will stimulate the readers to consider how they might also contribute to this rapidly growing area. A. B. P. LEVER
York University Toronto Ontario Canada M3J 1P3 January 1986 vii Lever; Excited States and Reactive Intermediates ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
Lever; Excited States and Reactive Intermediates ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
INTRODUCTION
CHEMISTS HAVE BEEN CONCERNED
predominantly with the chemistry o f species that exist i n their m o l e c u l a r g r o u n d state, often stable for an indefinite p e r i o d . Structures c a n , i n p r i n c i p l e , be obtained by X - r a y crystallographic methods, and physical data such as N M R , 1R, and U V / V I S spectra can be obtained with conventional spectrometers. T h e advent o f lasers a n d electronic devices that can record extremely fast events has led to g r o w i n g interest in the study o f molecules in excited states, usually, though not exclusively, the lowest excited state. T h e detailed study o f excited states is a field that is now g r o w i n g rapidly and promises to deliver a fascinating new view o f chemistry in the future.
Excited States: Characteristics A molecule i n its first excited state is, i n a very real sense, a different molecule from the g r o u n d state o f the species. It possesses a d d i t i o n a l energy and p r o b a b l y has a different structure, at least in respect to small changes in b o n d lengths and angles, and indeed may have a totally different stereochemistry. It has different electronic a n d v i b r a t i o n a l spectra a n d clearly has a different chemistry. S u c h chemistry is referred to as photochemistry because it is accessed by a light a b s o r p t i o n event. These excited state molecules c o m m o n l y exist for time intervals ranging f r o m picoseconds to m i c r o seconds, rarely longer, except when solids at cryogenic temperatures are being studied. Nevertheless, methods o f analysis are available to probe the photophysics and photochemistry o f these species even o n such a short time frame. Light absorption w i l l not generally occur to the lowest excited state, but rather a series o f excited states may become populated. These will generally decay rapidly (in picoseconds) to the lowest excited state. O n e can anticipate the even richer chemistry o f these higher excited states, but their lifetime w i l l usually—though not exclusively—be so short that this chemistry has no time to be expressed. Nevertheless, light emission and photochemistry may sometimes be observed from these higher energy levels. Special techniques such as ultra-short laser pulses (measured i n femtoseconds) are b e c o m i n g available to probe this chemistry (for e x a m p l e , o n the time scale o f b o n d breaking). In general the lowest excited state will decay back to the ground state by IX
Lever; Excited States and Reactive Intermediates ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
one or more pathways, i n c l u d i n g radiationless deactivation (loss o f excited state energy as heat to the surroundings), one or more p h o t o c h e m i c a l reactions, or by luminescence (fluorescence or phosphorescence). T h e study of these processes leads to a better understanding o f the electronic structure of the excited state. In the short term, the value o f such studies must lie i n what we can learn about h o w chemistry changes when the q u a n t u m mechanical state of a molecule changes, a n d h o w the a d d i t i o n a l energy, distributed over the m o l e c u l e , modifies its chemistry. In the l o n g t e r m , new i n d u s t r i a l l y important processes may depend u p o n the use o f excited state molecules. W i t h the exception o f a few highly studied states, such as, for example, the redox active lowest metal-to-ligand charge transfer excited state ( M L C T ) in the [ R u ( b i p y r i d i n e ) ] cation, or the ligand field active E state o f C r ( l l l ) , we k n o w relatively little about these excited states. A n enormous b o d y o f knowledge is waiting to be explored. 2+
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Excited State Quenching Reactions A n i m p o r t a n t facet o f the chemistry o f excited states is that a d d i t i o n a l energy confers u p o n the state both greater o x i d i z i n g power a n d greater reducing power, relative to the g r o u n d state. T h e greater reducing power originates i n the higher energy electron that has been excited, while the greater o x i d i z i n g power resides i n the hole created by the excitation of an electron. E l e c t r o n transfer reactions may be observed by reaction o f the excited state with an electron d o n o r or acceptor. Where the excited state luminesces, redox reactions with various species can be m o n i t o r e d by observing the quenching o f excited state luminescence, or reduction i n excited state lifetime, as a function o f the concentration o f quenching species ( S t e r n - V o l m e r plot). In this fashion one can determine the rates o f c h e m i c a l reaction between excited state a n d quencher, a n d , using models such as those developed by M a r c u s or A n g m o n and Levine, determine various parameters such as free energies o f a c t i v a t i o n and reorganization energies. Because o f the m u c h greater d r i v i n g forces potentially available in reactions between substrates and excited state molecules, difficult—but valuable—electron transfer reactions, such as the o x i d a t i o n o f water or c h l o r i d e i o n , may be accessed t h r o u g h excited state photochemistry. T h e question o f h o w to separate hole-electron pairs generated in a quenching reaction, h o w to provide kinetic pathways to lead these two highly reactive species far apart f r o m each another, a n d h o w to couple in some useful chemistry are currently o f interest. O f related interest is the problem o f how far apart, in a fixed sense, an χ
Lever; Excited States and Reactive Intermediates ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
excited state a n d a quencher can be, and yet have electron transfer take place. T h u s one can have quenching via a collision process, or by overlap o f d o n o r a n d acceptor orbitals at l o n g distances. If the excited state and quencher are linked by a l o n g conjugated pathway, quenching might be expected to take place. However, what about nonconjugated and throughspace pathways, both o f w h i c h can also lead to quenching? S u c h studies can lead to geometric information about proteins and impurity sites in crystal. S o m e e x t r e m e l y f a s c i n a t i n g o x i d a t i v e a d d i t i o n - t y p e c h e m i s t r y is possible at cryogenic temperatures when metal atoms are irradiated in the presence o f C - H and C - O bonds. M e t a l atoms are inserted, presumably via an excited state. T h i s may well have significance for the activation o f alkanes. S o m e o f the more interesting and valuable redox processes are m u l t i electron i n nature, suggesting the utility o f c o u p l i n g a t w o - or many-electron event i n t o a n e x c i t e d state process. T h e study o f the excited state photochemistry a n d photophysics o f binuclear and polynuclear (cluster) molecules is thus b e c o m i n g o f importance, a n d two-electron reactions are being identified. T h e sensitization o f semiconductors is a special example o f electron transfer q u e n c h i n g a n d may prove to be very important. A photoexcited electron may, for example, be injected w i t h high q u a n t u m yield into the semiconductor c o n d u c t i o n b a n d , to produce a p h o t o v o l t a i c device. T h e " h o l e " that is "left b e h i n d " may then perform some useful o x i d a t i o n process. Excited state quenching is not restricted to electron transfer processes, but may also occur by a t o m abstraction (for example, hydrogen a t o m abstraction), or by energy transfer to another species. In a d d i t i o n , the excited state energy may be used a l o n g a reaction coordinate leading ultimately to ligand loss or ligand exchange. N e w molecules may be formed by s h i n i n g light u p o n the o l d . O r g a n o m e t a l l i c photochemistry is particularly rich in p r o v i d i n g unusual molecules after stimulation by light. T h e mechanisms and dynamics o f such reactions are areas o f serious study. Indeed e l u c i d a t i o n a n d understanding o f the m a n y processes that c a n o c c u r u p o n light s t i m u l a t i o n , a n d the c h e m i c a l d y n a m i c s associated therewith, are major goals o f current excited state chemistry. P h o t o e x c i t a t i o n o f biological molecules, proteins, and enzymes also has interest (such as watching a c a r b o n y l group photodissociate from c a r b o n y l heme and studying the chemistry o f the resulting products).
Spectroscopy A s one might expect, various spectroscopies, especially electronic spectroscopy a n d resonance R a m a n spectroscopy, can provide detailed information about the electronic and v i b r a t i o n a l nature o f an excited state. C o n v e n t i o n a l XI
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electronic spectroscopy, a b s o r p t i o n a n d emission, can provide i n f o r m a t i o n about the geometry a n d b o n d distances i n excited states, while resonance R a m a n reveals the nature o f the c o u p l i n g between a n electronic state a n d v i b r a t i o n a l modes o f the molecule. Transient absorption spectroscopy, wherein one measures the electronic absorption spectrum o f a molecule i n a n excited state, is still in its infancy, but the g r o w i n g a v a i l a b i l i t y o f ultra-high-speed, rapid-scan spectrometers augurs well for this area o f spectroscopy. T h u s one may, i n the future, routinely probe excited state a b s o r p t i o n spectra as well as g r o u n d state a b s o r p t i o n spectra. T h e former can be expected to be as valuable in o b t a i n i n g i n f o r m a t i o n about the excited state as is the latter for the g r o u n d state. Time-resolved spectroscopies o f various kinds have proven useful in p r o b i n g the life o f a n excited state. A s a n excited state decays, perhaps t h r o u g h a c h a i n o f species, time-resolved spectroscopy (e.g., luminescence, excitation, resonance R a m a n ) can provide data for these various steps. S u c h studies have led, for example, to the view that the first M L C T excited state in [ R u ( b i p y r i d i n e ) ] , is localized i n one b i p y r i d i n e ring rather than delocalized over a l l three rings. 2+
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Electrochemistry Electrochemically generated chemiluminescence provides a n unusual method for studying excited state energies. T h u s , for example, a n o x i d a n t a n d a reductant c a n be generated at the same electrode ( w i t h a l t e r n a t i n g p o l a r i z a t i o n ) , o r at t w o closely spaced electrodes. G i v e n a p p r o p r i a t e energetics, the o x i d a n t a n d reductant quench one another to generate a n excited state rather than a g r o u n d state product, and luminescence may be observed.
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Lever; Excited States and Reactive Intermediates ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
