00:00:30 I feel very honored by this award, but I would like to add that I think the many

00:00:40 students, the graduate students, the postdoctorals, and the visiting faculty

00:00:45 that have come through our laboratory are also recognized by this award. It was

00:00:51 they who did the work and from whom many of the ideas came. Beyond this, I think

00:00:58 also this award is a recognition of a field, a field of inorganic photochemistry.

00:01:05 This could be regarded as a part of spectroscopy and some papers in this

00:01:10 session will be in that collection. It could be regarded as an aspect of

00:01:15 physical chemistry and some papers at this meeting will be in such sessions. I

00:01:20 think with this award, the Division of Inorganic Chemistry has in effect

00:01:26 welcomed the group of inorganic photochemists to a home and not only in

00:01:34 the United States, but in Europe and other countries where this type of work

00:01:39 is practiced. I believe this award is a recognition that will give status to all

00:01:45 of us and therefore I think it is a landmark event in the history of this

00:01:53 field. Now I want to dwell on primarily two themes. The first, a very central one,

00:02:03 is the theme that excited states are good chemical species. When a molecule

00:02:13 absorbs a quantum of light, sure, it is first born in an excited state with

00:02:19 excess vibrational energy, but most of our studies are in solution. The molecule

00:02:26 is solvated, it is in good contact with the solvent, and this excess energy is

00:02:32 lost, probably within less than a picosecond. The molecule becomes

00:02:36 thermally equilibrated in the excited state. Now there is an unfortunate

00:02:42 ambiguity in the word state. To a spectroscopist, a molecule has a state. It

00:02:50 has a set of quantum numbers, it may have a symmetry designation, and so forth. But

00:02:56 there is also the idea of a thermodynamic state. Now this is a

00:03:01 condition of a collection, of an ensemble of molecules, and this is the sense in

00:03:07 which I mean that excited states are good chemical species. An ensemble of

00:03:13 thermally equilibrated excited states has not only an average energy, it has an

00:03:20 entropy, which means a free energy, which means a standard redox potential. It has

00:03:28 an absorption spectrum, which is independent of how that collection of

00:03:32 states is prepared. It has reaction chemistry, it has reaction kinetics, it

00:03:40 has structure. We have felt that this was an important enough point that one

00:03:47 should have some term, other than things borrowed from spectroscopists, and have

00:03:54 used the term sexy state, either to mean thermally equilibrated excited state, or

00:04:00 if you wish, a thermodynamic excited state, to emphasize this other aspect. I

00:04:06 want, in the first part of this lecture, to give some illustrations of these

00:04:12 properties, and especially with respect to reactions. But let's start with the

00:04:17 first slide, the very simple crystal or ligand field diagram for a d3 system.

00:04:27 I'll talk primarily about d3 and d6 octahedral systems for lack of time, to

00:04:34 do more than that. And you'll see there are two kinds of excited states shown. A

00:04:39 quartet with an electron promoted to an anti-bonding, sigma anti-bonding orbital,

00:04:44 and a doublet, with the pairing keeping the electrons in the non-bonding t2g set.

00:04:50 And on the right, the conventional ligand field term symbols. This is how

00:04:56 the spectroscopist would look at these states. It will not be exactly how we'll

00:05:01 be looking at them. For the d6 case, cobalt 3, same sort of thing, except now

00:05:10 all excited states have one or more electrons in a sigma anti-bonding orbital.

00:05:16 There's no room to hide them by spin pairing in the non-bonding set. Now the

00:05:24 ligand field theoretician can do much better than the preceding slide. In fact,

00:05:30 this is the bottom half of what gets incredibly soupy toward the top. This is

00:05:36 taken from the compilation by Koenig. However, other than to impress you with

00:05:41 complexity, the reason for showing this is that for a tetragonally distorted state

00:05:47 in d6, under suitable conditions of parameters, the ground state is singlet.

00:05:54 There will be now a singlet e state somewhere up here. Let me try to focus

00:06:01 right here. But now you see triplet states and a relatively low-lying quintet state.

00:06:11 So we have not one, not two, not three, but three low-lying states. Well now,

00:06:21 keeping with the spirit of ligand field theory, one can ask about structure. But

00:06:29 first let me point out that excited states generally are changed in structure.

00:06:36 The simplest case maybe is the one of the oxygen. Singlet oxygen can only change

00:06:41 its bond length, which it does. But it's now well established that the chemistry

00:06:46 of singlet oxygen is different and characteristically different from that

00:06:52 of ground state oxygen. Chris Foote and that other institution in my town has

00:06:57 done a great deal of work in this area as just one example. The simple tri and

00:07:05 tetraatomic molecules. Excited states are thoroughly considered to be altered

00:07:11 in bond angle and not merely in bond length. There are at least two cases

00:07:17 involving square planar complexes, where low-lying excited states are considered

00:07:24 to be tetrahedral or distorted tetrahedral in structure. We don't have

00:07:31 very much information as yet on the structure of firmly equilibrated states

00:07:37 of octahedral complexes. This is a field of enormous interest and enormous

00:07:42 importance in my opinion, and is yet to be, yet to be really developed. On the

00:07:49 bottom line, this is a bit of a hooker, but I wanted to illustrate that for the

00:07:55 this famous ruthenium-tris-bipyridine complex, there's quite a range of thought

00:08:01 that has been exhibited as to the structure of the first and emitting

00:08:06 excited state, thought to be a charge transfer state. The electron that has

00:08:11 moved from the central metal to the ligand could be completely delocalized

00:08:17 as one possibility. It could be semi-delocalized, meaning that it is in

00:08:22 the area of one of the bipyridines, rather than all three. It could be fully

00:08:28 localized and create an active site, which Gillard would then proceed to

00:08:34 hydrate. There's quite a range here. We have come gradually to believe that

00:08:40 there is a considerable localization, and this partly through the very interesting

00:08:44 results of Woodruff and co-workers on excited state resonance Raman. And I

00:08:50 think this technique will, for the next few years, be one of the important

00:08:54 techniques for getting at excited state structure. Well, one way of trying to

00:09:01 argue on structural possibilities in an excited state is going back to ligand

00:09:08 field theory, talking about crystal field stabilization, which we've all had in our

00:09:13 undergraduate inorganic course. For a d3 system, it's 12 dq. In the first excited

00:09:20 state, it would be 2 dq. But now if we allow a distortion to a pentagonal

00:09:26 pyramidal geometry, and keeping the same ligand field strength, which is the usual

00:09:31 drastic approximation that is made, the stabilization is now almost 8 dq, or

00:09:39 almost 6 greater. What this is saying, or suggesting, is that the first excited

00:09:46 state, the first quartet excited state in a d3 system, is stabilized by distorting

00:09:52 to a pentagonal pyramidal shape. A dq is worth about 5 kilocalories, so we're

00:09:58 talking about 25 or 30 kilocalories. In the case of d6, stabilization is 24 in

00:10:06 the ground state, 14 in the first excited state, and only about 15 and a half in

00:10:13 the distorted. Not much gain. And just qualitatively, one at least could

00:10:19 conclude that there's a good chance that d3 systems, in their excited quartet

00:10:25 states, will be considerably more altered in geometry, and have a different

00:10:29 reactivity pattern than d6 systems. We have liked to use excited state diagrams

00:10:39 of this kind, rather than Jablonski or other related types, because of the

00:10:47 importance we attach to distortion, excited state distortion. And this is a

00:10:53 d3 case, a quartet ground state, quartet excited state with a electron and a

00:10:59 sigma antibonding orbital, and the first doublet state. Solid lines are processes

00:11:06 involving a photon, wavy lines are non-radiated. These light lines have a

00:11:12 somewhat complicated meaning, which I'll try to explain in just a second. The

00:11:17 heavy bars locate the thermally equilibrated or sexy states. Now, what's

00:11:26 involved with those light lines is approximately the following idea. In

00:11:31 terms of a square planar complex, in solution, there will be solvent molecules

00:11:37 all around the shaded circles here. In other words, the complex is in a dense

00:11:42 crowd of solvent molecules. And if you've ever been in a dense crowd of people,

00:11:48 you'll agree that it's very different, very difficult to change your shape

00:11:52 quickly. You have to push somebody aside. And if this molecule wants to change its

00:11:58 shape to a, say, tetrahedral shape, solvent molecules are going to have to

00:12:04 move. Now, they don't move that easily because they in turn are crowded. They

00:12:11 diffuse. And roughly speaking, every few picoseconds or after every few hundred

00:12:17 vibrations, a solvent molecule is able to make a jump. It can make the jump. The

00:12:23 coordination compound can begin to relax towards the geometry that it wants. This

00:12:30 is an artistic slide. It's also the first needlepoint that my wife ever prepared.

00:12:35 She was learning the art, asked me for a suggestion, and that's what she got.

00:12:44 A succession of hot molecules that are quivering and changing in geometry, but

00:12:53 in particular, the red bars are intended to represent successive solvent cage

00:13:00 configurations in the solvent, so that really one has a transitory existence

00:13:10 around, say, this energy level with vibrational states moderately well

00:13:15 defined, then a jump to another cage configuration, and then another jump.

00:13:20 That's the sort of thing that these red lines are intended to convey. Well now, on

00:13:27 the matter of distortion, I'll repeat Kutal's slide. I didn't know he was going to

00:13:34 show it, but since you read his slide, of course, I don't have to repeat it. What I

00:13:40 do want to do is to give a couple of examples of this rule, too, which was the

00:13:46 more interesting because of its predictions, which were at the time

00:13:51 generally unexpected. Namely, that if you consider an octahedron as three

00:13:58 mutually perpendicular axes and assigned to each a ligand or average ligand field

00:14:04 of strength, that the labialization effect will locate on the weaker field

00:14:10 axis, and if that axis contains two different ligands, it'll be the stronger

00:14:15 field ligand of the two that will be made labial, and this will be labiality

00:14:21 towards substitution. The two or three examples I want to talk about, and I've

00:14:28 named the people involved in the publication, the trans-di-chlorobisethylenediamine

00:14:34 at the top, the chloride-chloride axis is the weak field axis, and indeed chloride

00:14:40 is labialized photochemically and is replaced by water in aqueous solution. In

00:14:46 the second one, the chloropentamine, by isotopic labeling of the ammonias, indeed

00:14:53 it is the trans-ammonia, which is the strong field ligand on the weak field

00:14:58 axis that is labialized, and this is what is observed. And this is now not the

00:15:03 thermal reaction. It's an antithermal reaction, and it's predictions of this

00:15:08 kind that made, I think, these rules very useful. This was work by Ed Zinato on a

00:15:14 visit to our laboratory, and then the third case work done by Charles Putau in

00:15:21 our laboratory. The interesting situation with the third, which is the cyclam

00:15:27 complex, is that, well, from the ligand field point of view, it's not very

00:15:32 different from the trans-di-chlorobisethylenediamine. The spectrum is very

00:15:37 similar. Spectroscopically, one would say there's not much difference, but

00:15:42 photochemically there is. The cyclam ligand is not one that can permit

00:15:47 isomerization to cis very easily. It'd be highly activated, and correspondingly

00:15:53 this does not occur, and photochemically the product is the trans-aquachloro, same

00:15:59 as the thermal reaction now. But now the quantum yield has dropped a thousand

00:16:04 fold, almost, over what it is for the dichlorobisethylenediamine. There's

00:16:11 stereochemistry evidently involved here. You notice that in the first case the

00:16:16 product is a cis-aquachloro, although it's a trans-di-chloro axis that's been

00:16:22 labelized. And in the second case, although it's a trans-ammonia that's

00:16:26 been labelized, the product is a cis-aquachloro. Something is going on, and

00:16:34 with the cyclam complex, if isomerization is prevented, the efficiency of quantum

00:16:41 chemical reaction of excited state has dropped enormously. So there's something

00:16:46 inherently stereochemically directed towards cis that's involved here. As

00:16:53 another kind of example, I might say there's been a great deal of work done

00:16:58 on the photochemistry of cobalt amines. The work with the analogous V6 system of

00:17:05 rhodium is a little more recent by and large, and I've picked this. Again, I'll

00:17:10 just talk about the first two or three cases. For the chloropentamine, contrary

00:17:17 to the rules, and the D6 cases don't obey them quite as well, chloride

00:17:22 aquation is what's been observed by Endicott, also by Chakutau. With the

00:17:29 bromopentamine, we now get photochemically aquation of the ammonia,

00:17:35 which is what would be predicted. So just between chloride and bromine, there's a

00:17:40 switch in the principal mode of photochemical reaction. Notice that we

00:17:46 don't have the trans to cis isomerization. It stays trans in the D6

00:17:52 case, so there's a difference in the stereochemistry. And when one puts on the

00:17:57 cyclam ligand, there is some but not much reduction in quantum yield. So with these

00:18:04 D6 complexes, the stereochemistry is different, the rules are not obeyed quite

00:18:11 as well, and there is certainly an indication of differences in mechanism.

00:18:16 Well now, mechanistic excited-state chemistry is relatively recent. Perhaps

00:18:23 one could take around 1967 as a starting point. So we needn't be too ashamed that

00:18:31 we still have quite a ways to go. After all, the ground-state reaction

00:18:36 kineticists have been at it quite a while. I think the next slide reminds you

00:18:42 of a rather classic paper by Brown, Ingold, and Nyholm back in 1953 in

00:18:49 proposing a then very new controversial mechanistic idea of edge displacement.

00:18:58 Today in JACS, or inorganic chemistry, you will find papers on mechanism of

00:19:04 substitution in cobalt complexes. The subject is still alive after 30 years

00:19:10 and is still not completely settled. Now I put in the next slide. I couldn't

00:19:17 resist it. There are some of you for whom this is an in-house slide. You know some

00:19:23 of the story. The face is the face of Sir Ronald Nyholm. The body is the body of

00:19:28 Jeremy Bentram, the founder of University College of the University of London.

00:19:35 Ronald Nyholm saw this slide. He has been joshed by it in his lifetime, so I felt no

00:19:42 hesitation in showing this slide to show what what some inorganic chemists do to

00:19:49 each other by way of jokes. But coming back to the matter of excited state

00:19:57 mechanism, I'll indicate possibilities. These are in no way established in my

00:20:03 opinion, not yet, and this is one of the current frontiers of work in this

00:20:09 field. For the D3 case, one could talk about something akin to an edge or

00:20:15 facial approach, which could lead to a pentagonal type of intermediate,

00:20:21 pentagonal pyramidal type of intermediate, and produce the observed

00:20:26 geometry. This is a rationalization. It will be in agreement with the data. It is

00:20:33 not established by the data. For the D6 case, on the bottom line, one could talk

00:20:40 about a frontside displacement reaction or a solvent-assisted reaction. This

00:20:48 would be consistent with the observation. It is not proven by the observations.

00:20:53 Other people, and most recently Van Quickenborn and co-workers, have looked

00:20:59 at the sequence shown in the middle, in which a ligand promptly is lost, a square

00:21:05 pyramidal intermediate, which rearranges to a trigonal bipyramidal intermediate,

00:21:10 is formed. And then by arguments that are reminiscent of use of Woodward-Hoffman

00:21:16 rules, namely of looking where the lobes are, one can explain the stereochemistry

00:21:24 of substitution reactions. Again, it's a rationalization. It is not yet proven. And

00:21:31 I could go into this in a little more detail, but I think for lack of time, I

00:21:35 want to move on. The next part of this talk, and I think I've come to the next

00:21:42 section, has to do more closely with the title. The title had to do with In Search

00:21:49 of the Reactive Excited State. The phrasing is in no way original with me.

00:21:55 I borrowed it from a series of travelogue books that I once ran into. There would

00:22:01 be a book on In Search of Brazil, In Search of London, In Search of England, In Search

00:22:09 of This or That Place. And I remember seeing the book In Search of London, and having lived

00:22:15 a year in London, I decided he may have been in search of it, but I didn't think he had

00:22:19 found it. And maybe it had been a very foggy day that he had passed through London. But

00:22:26 this is a contemporary, very considerable activity among the inorganic photochemists

00:22:34 to be in search of the reactive excited state. And we're still somewhat foggy on this, but

00:22:41 maybe the fog is clearing at least a little bit.

00:22:45 One of the very useful, very important handles that we have is that gained when the compound

00:22:55 shows emission. Many complexes were known and worked by Forster and many others over

00:23:03 the years to show emission at low temperatures. It's been relatively recently that people,

00:23:11 and I think with better equipment, have been able to observe emission in room temperature

00:23:17 fluid solution. If an excited state emits, you have a handle on it. And you must remember

00:23:24 that we are shorter of handles than is the ground state chemists. You people who work

00:23:29 in the ground state, after all, generally know what your starting molecule is, and you

00:23:35 generally know its structure. As you'll see, there can be uncertainty for us what our reactant

00:23:42 is, and there's a double uncertainty as to what its structure. We need all the help we

00:23:47 can get. If there's emission, that is a big help, as you'll see.

00:23:52 What you observe here is an emission spectrum. Just look at the one on the right, obtained,

00:23:58 I believe, by Tom Walters when he was with our group, for the hexamine of chromium. It

00:24:04 shows a lot of vibrational structure. It's emitting in the red, and the dashed line shows

00:24:09 what happens on further duration. There is a major shift, in this case and other shifts,

00:24:16 which strongly corroborate that this is a vibrationally-assisted transition. Probably

00:24:24 an ammonia rocking frequency is involved in that particular feature.

00:24:31 Now, one of the classic pieces of work, and this is low-temperature studies, was done

00:24:39 some 20 years ago by Gerald Porter on a visit to Schlaefer's laboratory in Concord, which

00:24:46 was at that time an important laboratory in inorganic photochemistry. He found with one

00:24:53 of the very few complexes, the hexaurea complex of chromium, that shows fluorescence as well

00:25:01 as phosphorescence. To take you through this, on the right is the absorption spectrum, the

00:25:07 first quartet-to-quartet transition. These little rumps are the doublet absorption features,

00:25:13 which you see not as pronounced as others you'll come to in the next slide or two.

00:25:19 Then on the left side, the emission. There is emission from the doublet state, which is on

00:25:26 this scale very sharp and occurring at almost the same wave number as the absorption, a pretty

00:25:34 convincing indication that the doublet excited state is not very distorted from the ground state

00:25:40 if emission comes back down at about the same energy as absorption. But look at the fluorescence.

00:25:46 This is emission from the first excited quartet state. It's shifted by some 4,000 wave numbers,

00:25:53 12 kilocalories in energy from the absorption maximum. It's broad, it's mirror image-like.

00:26:02 The crossing you notice, which is a way of estimating where the fecsi or zero-zero transition

00:26:09 would be, the energy of the fecsi state, puts the value at not very different from that of the doublet.

00:26:17 And this is one reason to go back again to one of these distortion diagrams. Why

00:26:24 this is located outward indicating distortion. This is above indicating very little geometry change.

00:26:33 You can see quite a few rate constants in a slide like this. Rates of inter-system crossing prior

00:26:41 to thermal equilibration, back inter-system crossing, chemical reaction from either state,

00:26:49 non-radiative and radiative processes. We get into a very complex situation

00:26:56 of rate constants as soon as we begin to inquire about the details of kinetics of excited state

00:27:03 reactions. But the question that I'm addressing myself to here is, what is the reactive excited

00:27:12 state? Is it the doublet state? Is it the quartet state? Historically, a strong case could be made

00:27:20 and was made by Schaefer's group for the doublet state. An analogy to organic triplet excited states

00:27:28 is the long-lived state. It also has all the electrons bunched up in the non-bonding set,

00:27:36 well prepared for a substitution reaction. When the photolysis rules came along and proved to be

00:27:45 useful and successful, the explanation that made sense was that the reaction was from the quartet

00:27:52 state. And a number of people added at Lickenfield confirmation to these rules.

00:27:59 Gray and Wright and especially Zink and his co-workers more recently than Quickenborn.

00:28:08 And the reactivity of quartet excited states became almost the entire picture for a while,

00:28:16 although in our group we retained the idea that, hey, the doublet state still is a long-lived state.

00:28:24 It's set up to react. Let's not forget about it. One way of trying to get at reactivity

00:28:31 of which state is reactive is to vary the wavelength of a radiation. This is some

00:28:38 relatively early work from our laboratory. For the chromium thiol cyanatopentamine,

00:28:45 it shows the absorption spectrum and on the expanded scale at the right, the doublet feature.

00:28:52 The bars show the quantum yields. The principal mode of photoreaction is ammonia equation as

00:28:57 these rules would predict. The minor mode is thiocyanate and the ratio is 15 to 20 to 1

00:29:05 depending on wavelength in this region. I'm going to the wavelength region of the doublet state.

00:29:12 We found the ratio to drop to 8, not because the quantum yield for thiocyanate equation changed

00:29:18 much, but because that for ammonia equation dropped. And a reasonable conclusion from this

00:29:25 kind of result is that the quartet excited states led to ammonia substitution, and the doublet

00:29:33 excited state led to thiocyanate substitution. Another kind of approach, and this is one of the

00:29:44 examples of the importance of emission studies, is in studies of quenching of emission.

00:29:52 The emission lifetimes of chromium amines at low temperature are around milliseconds. In

00:29:59 room temperature solution, they drop to microseconds or nanoseconds in lifetime.

00:30:06 We don't believe that the radiative or spontaneous emission time is altered with temperature

00:30:15 particularly, so the shortening in lifetime of emission at room temperature we attribute to

00:30:23 the intervention of some other process to remove excited states, either non-radiative or chemical

00:30:32 reaction processes. Now, because of the still relatively long lifetime, microseconds or long

00:30:44 nanoseconds, another way of removing excited states is to intercept them with a quencher

00:30:50 by a bimolecular process that may involve energy transfer, may involve an electron transfer,

00:30:57 or proton transfer. There are various possibilities, but the point of interest is that if a given

00:31:04 emitting state is involved or implicated as a chemically on the reaction path towards

00:31:12 excited state chemistry, then if you kill that state, if you quench it out, you should also kill

00:31:19 the photochemistry. So one has a diagnostic tool. If you look at the sequence here, it ranges the

00:31:28 complete range. The hexacyanide quenching the emission does nothing to the photochemistry.

00:31:35 One concludes it must all be through the quartet state. With the tris-bipyridine complex, quenching

00:31:44 the emission completely quenches the photochemistry, and one could say the photochemistry is from the

00:31:52 doublet state, or one could say the doublet state can back inter-system cross to the quartet,

00:32:00 and that is a reaction via the doublet state. A number of people, for various reasons,

00:32:07 somewhat coincidental, have looked at the tris-ethylenediamine complex of chromium.

00:32:14 With quenching studies, with somewhat variable results, I've listed some of the names.

00:32:19 We found, and this is work of Fukuda and Mackey and others in our laboratory,

00:32:24 a higher figure of about 70 percent. The method we used was not a method of quenching, however.

00:32:34 There was a pulse laser catalysis technique, and I'll show a slide that was probably originally

00:32:40 prepared by Tom Walters. I'm not sure, but it's a block diagram of the equipment.

00:32:47 In the upper left, you see the typical oscilloscope trace. This is the intensity of a monitoring beam

00:32:54 at right angles to the laser pulse. Here is the laser pulse. The time scales are not synchronized,

00:33:02 so this shows a 20 nanosecond laser pulse. Here is the, on this trace, the start of the laser

00:33:08 pulse, and this is on a scale of some microsecond per division.

00:33:18 Immediately following the laser pulse, there was a prompt change in absorbance of the monitoring

00:33:24 beam, meaning a prompt formation of primary photoproduct. Then a further change in absorbance,

00:33:33 which decreased with time, indicating a slow formation of primary photoproduct,

00:33:41 and the rate of this slower formation was exactly the rate of decay of the emission.

00:33:48 So the slow formation was connected with the emitting excited state, the doublet state.

00:33:55 By using a very intense laser pulse, we could partially bleach or significantly convert ground

00:34:02 state to excited state, and this, without going into the algebra, allowed us to reach a conclusion

00:34:10 from our data that the doublet state was actually the reactive state. It was not just on the path

00:34:17 to reaction. I might say there's been controversy on this, as much controversy perhaps as some of

00:34:24 you ground state people who deal in kinetics have gotten into on kinetic questions. I think the next

00:34:31 slide moves on, but I'll return to this point in just a minute. There is one other possible

00:34:39 approach, and Tom Waters and others in our laboratory, Al Gutierrez, have looked at emission

00:34:47 lifetimes in various solvents and with various complexes, and it gradually appeared that there

00:34:55 was a consistency that again led to some rules. Like the perhaps frontal load idea, these rules

00:35:06 are not absolute by any means, but I think they're very, very stimulating. They have been to us.

00:35:13 Again, the idea of ligand-filled axes, again the idea of focusing on the weak-filled axis,

00:35:20 but now the statement is that in room temperature fluid solution, if the weak-filled axis contains

00:35:28 two different kinds of ligands, and if one of those ligands is the same that is thermally

00:35:34 labeled, the lifetime will be short. Otherwise, it'll be relatively long, and I'll give a couple

00:35:41 of examples. The trans-thiocyanato-bisethylenediamine, the emission lifetime in

00:35:50 six microseconds is relatively long, and it turns out from work by Link's group in San Diego that

00:35:57 the thermal reaction in this case involves primarily a substitution at one end of an

00:36:03 ethylenediamine, so the thermally labeled ligand is not on the weak-filled axis. In the

00:36:10 trans-dithiocyanatotetramine, the thermal reaction is substitution of thiocyanate,

00:36:19 it is on the ligand on the weak-filled axis, and now the lifetime is relatively short. Again,

00:36:26 we have two complexes that are spectroscopically almost identical. A ligand-filled theoretician

00:36:32 would find little leverage to pick out any basis of difference in behavior, but there's a big

00:36:39 difference when you come to the emission properties. The rules have predictive value. We

00:36:45 were able then to predict for the cyanopentamine an interesting case where now the ammonia axis

00:36:54 is the weak field, because cyanide is a strong field over ammonia, that now we have a case where

00:37:01 the reactive ligand, which is in the thermal reaction with cyanide, does not lie on the

00:37:07 weak-filled axis. We predict a long emission time, and it is a relatively long 24 microseconds.

00:37:17 More recently, Mack and Jean-Pierre Pouault, working in our laboratories, have looked at

00:37:23 cyclam complexes. In both cases, the thermally labeled ligand lies on the weak-filled axis,

00:37:32 and one would say the lifetime should be long in both cases for emission. It's long here and

00:37:39 short here. Now, there is a remarkable difference between these two compounds as to the thermal

00:37:45 reactivity. The rate of thermal substitution of the acetyl group, in this case the pi-cyanate group,

00:37:54 is about a thousand times or more slower than for chloride. It's a remarkable difference,

00:38:01 considering that otherwise the nature of as a ligand of chloride and pi-cyanate is not all

00:38:08 that different. And we are seeing, in another work of Mack's, we're seeing correlations between

00:38:16 excited state emission lifetimes and rate constants for thermal or ground state reactivity.

00:38:24 All of this has added up to us to a suggestion that the doublet state lifetime, the lifetime of

00:38:31 existence of the doublet state in fluid room temperature solution, is strongly affected,

00:38:38 if not entirely controlled, by rate of chemical reaction. And that the rate of chemical reaction

00:38:44 is the rate of a reaction which parallels and may have the same mechanism as the ground state

00:38:50 reaction. And if the ground state reaction is unusually labile, so will be the excited state

00:38:55 reaction. So here I've put together several indications of doublet reactivity. I'll tease

00:39:06 you with this one. This is a picture of the doublet absorption band for the trisethylenediamine

00:39:11 complex of chromium. We have a very sharp feature here, and it can be somewhat diagnostic

00:39:21 to irradiate exactly that feature and see what happens to the quantum yield. I won't go further

00:39:27 than this. Dr. Kutow will be talking more about results of some very careful experiments in this

00:39:35 area later in this week. You'll see for the final time this distortion diagram. And what I've been

00:39:44 saying now is that it has seemed to us, for certain complexes at least, that lifetime and some of the

00:39:52 chemical reactivity, photochemical reactivity, comes from the doublet, some of it from the quartet.

00:40:00 And as a general rule, I would say that whenever the photochemical reaction differs from the

00:40:07 thermal reaction, it is probably occurring from the quartet state. Where the thermal, where the

00:40:13 photochemical and thermal reactions are the same, be careful. It may be coming from the doublet.

00:40:20 Now this is not a purely trivial matter or purely a chess game of interest only to

00:40:26 those people who do the kinetics. Theoreticians, ligand field people who want to rationalize,

00:40:32 to explain, to predict reactivity by ligand field arguments, are going to have to be cautious until

00:40:41 the evidence settles in on what is the reactive excited state. I'm sure there's nothing quite so

00:40:47 embarrassing to a theoretician to give a full-fledged theoretical explanation for what turns

00:40:53 out to be wrong experimental data. This happened in the case of anomalous water.

00:41:00 On the next slide, I'm going to move a little more rapidly through some additional examples.

00:41:07 A d6 system, rhodium, this shows three kinds of spectra on one slide. This is work of Marina

00:41:14 Larson in our laboratory. The ordinary absorption spectrum on the left, on the far right, the

00:41:20 emission spectrum, again in the red, and in the middle the excited state absorption spectrum.

00:41:26 As I say, these excited states in general have a characteristic absorption spectrum.

00:41:32 This is for the chloropentamine. For the bromopentamine, the emission spectrum moves

00:41:36 significantly. The emission spectrum does not change. The absorption spectrum of the excited

00:41:42 state moves significantly. I don't have time to, or don't want to take time to go into much detail,

00:41:48 but hydroxide ion is a quencher in these systems. We think probably by proton transfer from excited

00:41:57 state to hydroxide. Be that as it may, the emission from the chloropentamine is beautifully and

00:42:04 fairly simply kinetically quenched by hydroxide. In the case of the bromopentamine, the quenching

00:42:12 of emission lifetime and the quenching of quantum yield for ammonia equation, remember with the

00:42:18 bromopentamine it was ammonia equation that was the principal reaction, are essentially in parallel.

00:42:26 And we can therefore say that the emitting state is implicated, that the chemical reaction of ammonia

00:42:35 substitution is either from or goes through the emitting excited state. There's about 10 percent

00:42:43 of the photochemistry that results in bromide substitution, and the interesting thing here

00:42:50 is that the quantum yield of bromide substitution was unaffected, was unaffected by quenching the

00:42:56 emission from the excited state. This is a clear indication that that aspect of photochemistry,

00:43:04 that mode, does not implicate the emitting state. On the next slide I indicate a couple of

00:43:11 possibilities for excited state scheme. For reasons which I won't try to explain in full,

00:43:20 we tend to prefer the lower scheme, but it's a three-state scheme that involves a quintet

00:43:26 and a triplet, and in fact two triplets, in order to account for similarity in emission properties

00:43:34 difference in excited state absorption properties and difference in photochemistry, plus the

00:43:40 difference in quenching pattern, in that with the chloropentamine the photochemistry is chloride

00:43:47 equation and is fully quenched on quenching emission. With the bromopentamine the principal

00:43:53 mode is ammonia equation and the bromide component is not quenched. Putting all these together led us

00:43:59 to this pattern of excited state relationships. Again, any theoretician wanting to deal with

00:44:08 Lickenfield arguments is going to have to think rather carefully about how much is known as to

00:44:16 what is the reactive excited state. One more brief set of examples. Mark Wrighton and his

00:44:25 collaborators did some beautiful work with a series of pentacarbonyls involving substituted

00:44:31 pyridine, and we have built on this work. Alastair Lees in particular has done this to

00:44:39 study room temperature photochemistry. The first absorption band is very solvent sensitive. It

00:44:48 shifts quite dramatically if you change solvent polarity. The next feature is essentially

00:44:56 solvent independent as well as independent of the four position substituent. For these reasons and

00:45:05 others we believe, and Mark Wrighton and his group believe, that the first feature is a charge

00:45:12 transfer type of transition. The second is a DD or Lickenfield type of transition. Alastair found

00:45:22 room temperature emission and fluid solution. Not just at low temperature, but fluid solution.

00:45:29 This is the emission spectrum. The lifetimes are readily measurable with the type of laser

00:45:36 equipment I described, 20 nanosecond type pulses. The emission is quenchable very cleanly by

00:45:45 anthracene and by a number of other quenchers, and Dr. Lees later this week I think will presenting

00:45:52 some of this kind of work in much more detail. Here I'll point out that the quantum yield or

00:45:59 photochemistry is quenched in exact parallel to lifetime quenching, so the emitting state is

00:46:05 implicated in the photochemistry. The temperature dependence of the emission lifetime corresponds

00:46:13 to about two kilocalories of apparent activation energy. The temperature dependence of the quantum

00:46:20 yield has about eight kilocalories apparent activation energy. They aren't the same. For this

00:46:27 and for some other reasons involving the studies with quenchers, we have suggested as a preference

00:46:35 case B rather than case A for the excited state scheme. An emitting state which is in thermal

00:46:43 equilibrium with another perhaps triplet charge transfer state of a different symmetry species,

00:46:50 but also back intersystem crossing to the ligand field state, which is the chemically reactive

00:46:55 state, and there's where the eight kilocalories we suppose comes from. But it certainly makes a

00:47:02 difference as to whether in case A the charge transfer state is the reactive state or in case B

00:47:08 the ligand field state is the reactive state. This is really a very exciting area because

00:47:15 every day people are discovering more and new systems that show room temperature emission,

00:47:21 and the emission gives us this handle. We can look at emission quenching, emission lifetime,

00:47:27 all the properties and relate this to the photochemistry. Well, I have a few more minutes,

00:47:35 and I propose to use these to say something that no general conversation on inorganic

00:47:42 photochemistry could be without, namely the YIT complex, the ruthenium-tryspite-pyridine,

00:47:49 and here is a slide from some of the early data of Harry Gaffney,

00:47:57 the observation that excited state ruthenium-2-tryspite-pyridine

00:48:02 would reduce various cobalt amines, the product being the ruthenium plus three

00:48:09 excited state. This was controversial at the time. It seems amazing now that only 10 years ago

00:48:18 that it would be considered so unlikely a suggestion as excited state electron transfer.

00:48:25 Work by Sutin and his collaborators, work by Meyer and Witten and other laboratories,

00:48:33 added beautiful confirmation to the supposition of excited state electron transfer.

00:48:40 This is now widely accepted. Apart from the scientific interest, there was a practical

00:48:46 interest, the water split. We have a certain collegiality amongst inorganic photochemists,

00:48:55 and we have something called the Ligand Field Songs. There are many stanzas to this,

00:49:01 and we have sung them on different occasions, and there's one that deals with the water split.

00:49:06 Well, I won't repeat it here, but here's the pipe drain. The excited state of ruthenium-2-tryspite-pyridine

00:49:13 has the energy to reduce water to hydrogen. The product, the ruthenium-3-tryspite-pyridine,

00:49:20 has the energy to oxidize water to oxygen. You put these together and you have the catalyzed

00:49:27 decomposition of water, and catalyzed by visible light. As this complex absorbs in the visible

00:49:33 region, it's an orangey color. There's no problem decomposing water with ultraviolet light.

00:49:40 The interesting and valuable thing is to do it with light in the visible region,

00:49:46 if one's talking about solar energy. Well, these reactions are fine. On a molecule scale, it's very

00:49:54 difficult to produce a half molecule of hydrogen. It's even more difficult to produce a quarter

00:50:00 molecule of oxygen on a single molecule scale. This does not work. There's been an intense effort

00:50:09 in numerous laboratories, as many of you know, to find ways of substituting reaction paths that will

00:50:17 allow the scheme, or a similar scheme, to work, as well as making various modifications of the basic

00:50:24 complex. Most recently, Gretzel in Lausanne has announced a considerable degree of success

00:50:32 at the catalyzed splitting of water. Well now, I've reached my conclusion, I think,

00:50:40 except for one thing. This is just a beautiful field of work to work in. It's colorful.

00:50:48 A lecture of this kind absolutely should have lecture demonstrations, and yet the Las Vegas

00:50:54 Convention Center didn't seem to be the right venue and facilities for a lecture demonstration.

00:51:02 I've tried to do the next best thing, and now, Herb, if you could get the lights off, it will help.

00:51:07 Namely, to have slides of a lecture demonstration. What you see here is the following.

00:51:16 The ruthenium-2-trispipyridine complex has been adsorbed on silica gel.

00:51:22 This is a dry, free-flowing orange powder now. The powder is clinging

00:51:28 to a sticky surface and on the cardboard, and is being illuminated with a UV lamp.

00:51:36 So what you see is the very beautiful fluorescence, very beautiful, forget it, very beautiful

00:51:46 fluorescence of this complex. The cards are sitting in a beaker, and there's a

00:51:53 lamp. The cards are sitting in a beaker, and the lamp is on top of the pair of beakers.

00:52:02 What's been done in this second sequence, the identical cards, identical lamp, identical

00:52:07 position, has been that a piece of dry ice has been dropped in the left-hand beaker.

00:52:15 Now, why should dry ice turn on the emission? As you see, it's considerably brighter.

00:52:20 Well, the answer is the dry ice, the dry ice does not turn on the emission.

00:52:27 It drives away something that's been killing the emission. In the right-hand beaker, ah, great!

00:52:35 In the right-hand beaker, the oxygen of air, which is about 100th molar, is highly quenching.

00:52:44 And in fact, as Jim Demas has shown and other people, the quenching is strongly by energy

00:52:50 transfer, and one is producing singlet excited state oxygen. So in the right-hand beaker,

00:52:56 you can see the singlet oxygen oozing away there, a lot of quenching. The carbon dioxide displaces

00:53:03 the air, and now the emission is turned on. I have a slightly shorter exposure

00:53:11 photo slide. I thought it might contrast better. I wasn't sure of the lighting conditions,

00:53:18 but that is then the demonstration. And one very active interest amongst inorganic

00:53:26 photochemists today is to find means of making this energy that's present in the emission

00:53:34 available, to transfer it to other molecules, to transfer it to organic molecules, to produce

00:53:41 fuels of one kind or another. Well, this brings me to the end of my talk.

00:53:47 I'll repeat the two themes. First, that excited states are good chemical species. Their study is

00:53:55 a study in a second world of chemistry. It's a real world of chemistry with all the attributes

00:54:02 of ground state chemistry, just as interesting and just as complicated and just as exciting.

00:54:11 And secondly, that we are at two kinds of frontiers amongst others, but one has to do very

00:54:20 much with what is the structure of excited states, a very important question that is just going to

00:54:27 be resolved one way or another in the next few years in important cases. And the second,

00:54:33 in terms of photochemistry, just what is the reactive excited state?

00:54:38 Well, thank you very much for your attention.