 Today, we are going to see how the EPR spectra recorded when the radicals are just created by a pulse laser light and how they differ from the steady-state EPR spectra and what can you learn about the mechanism of the radical production. So, here broadly the theme will be electron spin spolarization in photochemical reaction. Before you understand what is the meaning of this term is, let us see some example. So, here the steady-state EPR spectrum of this particular system is shown here. They have acetone mixed with isopropanol as solvent and we sign UV light and then light is on all the time. We record this EPR spectrum the way normally we do. This is the steady-state EPR spectra recorded under continuous photolysis. The spectra looks like this and then from the intensity of this various hyperfine lines, we can definitely attribute it to this radical. So, the chemical reaction is this absorbed light. So, this must be going to the excited state and in the excited state some reaction is taking place. This is the isopropanol in the radical that we see here this radical. This protons give the major 7 lines EPR spectrum and this little spreading comes from this proton here. So, here the reaction is that this hydrogen atom gets abstracted by this to produce this two radicals of this kind. Now this is the overall reaction. Acetone is excited by the light and it abstracts a hydrogen atom from the alcohol. Now do get the EPR spectrum of this radical just at the time they are created by a laser light. We have a pulse laser which is repeated again and again. So, we look at the microbe signal and try to get a snapshot of the voltage at a given time after the laser pulse. And then average this voltage after every laser pulse to improve the signal to noise ratio using a boxcar average. So, we get a spectrum that will be characteristic of the whatever it is at a given time after the laser pulse. We call this the time resolved EPR spectrum of the transient species. Now that is the way the comparison between the steady-steady EPR spectrum under continuous photolysis and this is the time resolved EPR spectrum under a pulse laser light. The second one was recorded at a time 0.5 microsecond after the laser pulse. And one can see that the line positions are matching perfectly well that is we are getting the same radical here even in the and the nothing new is happening so far the chemistry is concerned. Photochemistry is same, but the EPR spectra are quite different when it do the experiment in the time resolved fashion. Different in what sense? Spectrum is actually the coming from the same radical. It is the same thing happening there. So, the line positions are the same only the intensities are very different. That is important that it does not produce any different radical. Radical is the same we only get different intensities and that intensities are very difficult to understand from the way we have understood the EPR spectrum that they must follow the usual Pascal triangle type of intensity ratios among the various hyperfine lines or similar type of relative intensity ratios. So this spectrum shows that some levels are getting selectively populated some levels of the Zeeman's split line. So, this particular transition involves pair of Zeeman line this involves some other pair of Zeeman line that depends on the nuclear spin state. So, some are populated more than the other sometimes when it is other around. So, if I have set up the spectrum properly to get the absorption spectrum in this fashion the way signal goes up that is then these lines are showing absorptive signal and these lines are showing opposite of absorption and opposite of absorption means what? It has to be only emissive in nature and that is very unusual because the normal distribution of spins in a pair of Zeeman level will be this is more and less here. So, you get a net absorptive signal in this kind. So, when you see emissive signal which is going downwards means these levels are more populated than this one that is the only way one can explain this observation. So, we call this type of radicals which are produced in a selectively populated spin states or we call it electron spin polarized state that is some levels are selectively populated electron spin polarization is the phenomenon we are observing here and therefore the spin populations at different Zeeman level do not follow the Boltzmann distribution. So, then we can define a quantity called polarization which is defined as the number of spins in the beta level and number of spin alpha level N alpha and N beta then this though you can define the polarization N beta minus N alpha by the total number of spin. The advantage of this is that the EPR signal intensity depends on see among other things the population difference if these two are equal we will not get any signal. So, intensity is proportional to the polarization. So, when polarization is positive I get a net absorptive signal and polarization is negative I can get emissive signal. Now the first time such signal was seen was way back in 1963 when this two scientist Fressenden and Schuler were actually irradiating liquid methane by an electron pulse I suppose that this electron beam was breaking this molecule and producing hydrogen atom and what they saw here is the two lines of hydrogen atom one is here other is there they have exactly opposite phase please note that these are not the first derivative EPR signal they are second derivative EPR signal. So, when you going down and here high field line going up as exactly opposite phase normally hydrogen atom should give two lines at a distance of let us say 509 gauss or so. So, here the intensity is of this kind that means the population is exactly opposite for this pair of transition they even did experiment with deteriorated methane. So, that they could produce deteriorated atom and deteriorated atom has got nuclear spin of 1. So, if i equal to 1 this should give three intense line of this kind equal intensity with a relative intensity of 1 is to 1 is to 1 instead what they found was that this is negative this intense is not very big this is the opposite one here. So, this is negative intensity this is positive, but small this is the more positive one this is something else. So, this is the first observation something unusual was happening and they wrote in their paper that for the moment the cause of the inversion remains unknown people probably did not pay much attention, but soon similar observation was seen in NMR experiment. The only different condition was that when some NMR signal of a reacting system was recorded one could see such unusual signal there is some observed to MEC signal appears there will see some example soon before that I like to point out that one does not necessarily have to go to time reserve repair experiment even in steady state also one can see similar behavior here this is the xanthone molecule and continuous photolysis was done in isopropanol and this is the steady state repair spectrum as you can see the derivative lines here. Now, but the way they did intensities are you can compare with the expected spectrum the similar to spectrum that see this bunch of line is sort of matching with this one, but corresponding bunch of lines absolutely not seen and they are buried in the noise level. In other words the left to right symmetry is lost here. So, intensity that goes down on this side from right to left which is not consistent with or if you have spectrum that we discussed as a first order spectrum where it will be perfect symmetric from middle to outside. So, here the intensities are more on the high field region than low field region therefore, it also means that some lines are not following Boltzmann distribution. So, here is another comparison between steady state repair and time reserve repair para benzo quinone photolyzed in isopropanol in a steady state you will light. So, one can get the repair spectrum of this kind and this is the radical that you have attributed to this radical and the reaction on this is para benzo quinone plus isopropanol gives this para benzo quinone plus isopropanol radical that is what is seen here let us say that is all you see we do not know what else is forming there there is no evidence of any other signal in the spectrum. So, here again this para benzo quinone structure is this and if this is the radical is formed there then this is a abstracting a hydrogen from the solvent and producing this radical alright let us you know time reserve spectrum of this one here it is the same experiment is done in the time reserve fashion and the spectrum that is seen here is recorded after 0.5 micro second after the laser pulse. So, here this lines which are marked with the symbol here they actually match with the spectrum of this radical. CS 3 whole 2 COH they here little here that this is not seen, but one could presumably believe that partner of this must be here. So, that means this time reserve spectrum shows that the other radical is this one a steady state spectrum did not show this and this doublet this doublet and this doublet they are the three doublet corresponding to here this radical you notice the scale here this much is 5 gauss and here this much 20 gauss naturally this compressed here. So, first thing is that time reserve spectrum gives evidence of presence of both the radicals, but that is not so interesting interesting part is that the appearance of the signal all the lines are in the downward phase that means all the lines are appearing in the emissive fashion that is where the main difference between this system and this system. So, something unusual different in the two cases even though the chemical reaction is the same this hydrogen goes from here to there here also I could write that this hydrogen goes from here to this to produce this radical. So, chemically there is no difference some difference in there way the spins are populated in the two cases just to show that indeed this lines are coming from the semi quinone radical this region expanded you can see clearly that that matches exactly well with the spectrum due to this. Now, I said earlier that when this first 1960's observation of fission and solar on hydrogen atom giving this two lines in the this opposite fashion many people may not have paid any attention, but now later early 60's NMR experiment showed lots of such signal here let us see one of them. This is the NMR spectrum of this chemical reacting system where acetyl peroxide is reacting with this solvent here peroxides are known to breakdown when they are heated and then undergo some chemical reactions and I see here this is ethane this is CH3Cl the LS come here. So, as a spectrum here is all going up these are going down. So, that these are absorptive signal this is absorptive signal, but this signal from this molecule and from this signal from this molecule they are coming in the opposite sense they are emissive in nature. Another example here thermal decommodation of propionyl peroxide in exocloroacetone when it is heated propionyl peroxide breaks down and it does some chemical reaction to produce ultimately this molecule here CH3, CH2Cl. CH3, CH2Cl the NMR spectrum of this should be very simple to predict this there are 3 protons here 2 protons there. So, this will be split into quarter due to this and this will be split into triplet due to this one. So, this type of thing other one will be this type of NMR spectrum is expected there. Now what is seen here is quite different. So, this is the expected NMR signal from the arising from this CH2 protons for what is his what appears here is the first 2 lines are absorptive and other 2 lines are emissive. And here for the triplet this line is absorptive this line is emissive and middle line is almost 0 intensity. So, very peculiar in that indeed. So, then lots of such experiments started appearing and then people remember the yes in 1963 Faisal Nandu Sular indeed saw the hydrogen spectrum appearing in the opposite phase then there are lot of activities to understand why such things are happening what is so special going on in such cases and one gets unusual NMR signal as well as EPR signal. So, here selective population of various nucleus pin levels are taking place here that is why some levels give positive signal and some other levels give emissive signal. This new phenomenon was termed as chemically induced dynamic electron polarization for polarization of electron spin. And for polarization of nuclear spin this used to be called chemically induced dynamic nuclear polarization even now these terms are used. By and large they mean that electron spins are polarized and they are in a dynamic state in a sense that they do not stay there forever. The electron spin lattice relaxation process always tries to restore the Boltzmann distribution. Similarly, in case of NMR spin also the NMR spin lattice relaxation mechanism tries to restore the spin population to the thermal distribution. Now, since the early days almost all the examples were seen to arise from chemical processes that is the NMR signal was recorded for chemical reacting system or EPR signal was recorded at the time of creation of the radical. So, this used to be called chemically induced dynamic electron polarization or chemically induced dynamic nuclear polarization. And the general term was given as chemically induced magnetic polarization to which will be a inclusive of both of these terms. Then it turned out that any interaction which are spin selective can give rise to a spin polarized radicals. And it was found out that involvement of chemical or photo chemical processes is not always necessary. Some photo physical processes can be also electron spin selective and they can give rise to spin polarization. So, then the new term was coined and it is called electron spin polarization or ESP. So, today we are going to learn how this sort of electron spin polarization arises ESP in photochemical generated transient radicals. So, radicals can be generated either by light that will bring the molecule excited state or it can be electron beam the way FSN and in solar or the hydrogen atom this involves radiolysis or one can heats the sample which will be thermolysis. Now, in these two cases reaction usually takes place in the ground state excited state will be of course, excited state if the light is involved. Now, these are general types of EPR signal that can appear. Let us take an example of methyl radical methyl radical EPR spectrum will be 3 protons. So, it will be looking like this 1, 3, 3, 1. Notice that I will be always drawing the EPR spectrum in this fashion either absorptive or emissive will not use derivative representation because this experiment involves recording the spectrum using the direct detection method and that does not use any magnetic field modulation. So, I can get directly the appears of the spectrum and from there I can decide whether it is absorptive or emissive. Now, in the steady state condition whatever the intensity of these lines are this will follow 1 is to 3 is to 3 is to 1, but the actual intensity will depend on the Boltzmann distribution among the various hyperfine levels. So, this is the absorptive signal at thermal equilibrium. So, these are the possibilities that we can encounter. One is that all the lines get inverted and intensity is much more than the steady state intensity. So, we say that P the polarization is negative and P is negative and value of this one magnitude wise much, much bigger than the equilibrium value of the polarization and this otherwise this shape is exactly same as what is seen in the steady state mode. So, relative intensity is still 1 is to 3 is to 3 is to 1. So, we say polarization is hyperfine independent this is the emissive nature. Other possibility is that I get absorptive polarization where P is greater than 0 at the same time P is much bigger than the equilibrium value. So, I get a huge signal, but the relative intensities are such that they are same as the thermal population. So, again this intensity distribution is hyperfine independent. Now, third possibility is what I showed you for acetonoyster overall system the some lines are emissive other lines are absorptive. So, it could be emissive absorptive and emissive on the lowest field region absorptive on the high field region and hyperfine dependent now see different hyperfine line have different densities or to other end also that low field lines are absorptive and high field lines are emissive. So, these are the various possibilities in general one can expect. What are the general things we have seen so far is that 2 types of spectra can be seen. One is that hyperfine independent polarization here either emissive or absorptive one type of situation and second situation is one of emission other half enhanced absorption a e or e a depending upon the hyperfine line. And also thing which I may not have a detention earlier is that any of this reaction what we have here or here the starting compounds are always in singular state diamagnetic state there is no unpaired electron. So, when the reaction takes place there cannot be one radical there has to be two radicals produced at the same time otherwise you cannot get this sort of radical formation from nowhere since everything was singular state when there some either bond breaks or electron goes from one to the other there will be always pair of radicals. So, what was seen here see that solvent radical aceton ketone radical and the semi-quinone radical both have the same sense of polarization here both are emissive. This is the summary is that in photoelectric generation there will always be a pair of radicals one may or may not be able to date both of them for whatever reason one radical may be more reactive than the other, but there will always be two radicals produced at the same time. And the two type of polarization that we have seen here both radicals in the same phase either both are absorptive or emissive independent of hyperfine line or one of emission other half enhanced absorption either a e or e a this is the background of observation now let us can we understand why these signals are appearing in this fashion. So, what are the mechanisms of electron spin polarization? So, we make a few key I would not say assumption, but accept the laws of nature without questioning because they are so fundamental that they are always supposed to be true one of them is this in an isolated system the total angular momentum of a system is conserved in all processes. So, that there cannot be any creation or destruction of angular momentum out of no here there has to be somebody to do that. So, all photo physical and photo chemical processes will conserved the total angular momentum. So, we take it to be a fundamental law of nature and see how much we can progress based on this. Let us first consider this mechanism single phase hyperfine independent polarization where the example was the paramanja quinone reacting in isopropanol giving rise to this two radical here this emissive spectrum this also give emissive both are in emissive phase this is single phase hyperfine independent polarization. So, we can just symbolically write that instead of this particular reaction let us say I have got a and b molecule reacting in the presence of light producing a radical number 1 and radical number 2 and both are in emissive polarization. So, radical 1 therefore, this is got the 2 g 1 level upper level is mostly populated. So, that it gives emissive signal radical 2 again upper level is mostly populated compared to lower level. So, this also gives emission. So, then all the electrons are paired when a and b were in the ground state and the radicals are formed as one you see suddenly that radical 1 has alpha spin state now radical 2 has also alpha spin state. So, here all electrons are paired and suddenly these 2 radicals have 2 electron unpaired. So, everything was paired in the beginning and then these 2 are unpaired also parallel to each other both are alpha spin how is it possible apparently there is this law of conundrum angle momentum is breaking down. So, there is something interesting going on there. So, if you go back now from this radical to one step backward that molecule a let us say absorbs light goes to excited state s 1 in the scheme we are starting with from the observation or observation is that this has got alpha spin state is also alpha spin state. So, if the reaction a only a absorbs light and it goes excited state and reaction takes place there. So, if the reaction is in the single state of this one while it is all spins are paired and here of course, all spins are paired then it just cannot produce a pair of radical with parallel spin. So, this is therefore, single state cannot react to produce this one that important conclusion one can draw immediately. How about triplet state now suppose a is in the triplet state triplet state what shall I write here I write in a moment. So, triplet state this is alpha and alpha it is not difficult visualize that this could also be alpha alpha that is one of the triplet then the angle of momentum in terms of the number of spins that are unpaired is perfectly satisfied. So, we can therefore, develop our reaction scheme in this fashion that it is not the single state of a that is reacting with the triplet state of a that is reacting and triplet particularly not all that three triplet level, but only triplet a with alpha alpha spin state. So, then it reacts with b and produce two radicals a and b everything is conserved here and both of them come in the emissive polarized state. So, similarly if the triplet beta-beta reacts excited state this triplet also I will get a radical one which is beta spin state radical two will also beta spin state. So, this will give a polarization which will be seen in the form of enhanced absorptive signal for radical a and radical b. So, this is the mechanism which must be taking place to produce a polarized signal in this case. So, you see that how the involvement of various photophysical processes are necessary to have this sort of polarized spectrum here turning it other around how much you can learn about the involvement of here photophysical processes when you see if your spectrum of this kind. Now, the triplet level as the name suggests there are three levels present there. Now, why does only one level react for example, to give emissive signal only one of them is reacting here not all why not all three of them why does the process choose only one level also what happens with the t zero level here the two extreme cases I said here triplet of alpha alpha will produce two radicals in the emissive phase if the beta-beta is reacting then this will produce two radicals in the absorptive phase enhanced absorptive phase. So, this is the question still remains to be answered here I have shown the three triplet level since the experiments are carried out in the spectrometer in the presence of magnetic field one could write that the triplet state also will experience the one interaction and t one t zero t minus n will be three the one split line of the triplet level. So, what is inter system cross thing process do is to bring this singlet excited state to this three triplet sub levels, but the experiments shows that only one of them is reacting the t plus one state is reacting here the spin states alpha alpha alpha beta plus beta alpha and beta beta the way the polarization of this two radicals are there this is the one which reacts. So, is it likely that reactivity of the three triplet level different that is not very likely why should the reactivity depend on whether it is t one or t zero t minus one. So, that is not a very likely reason then how about this inter system crossing rate the rates are different that could explain this behavior there may be all three levels are not equally populated. So, we hypothesize that may be the IAC rates are different for different triplet level see with that assumption one can justify the complete mechanism in this fashion that molecule a observe slide go to singlet state inter system crossing brings this molecule to triplet level, but not equally it brings selectively to one of the triplet level may be dominantly to one of the triplet level and reaction takes place in all the three triplets level, but the one which has got dominant population produces the radical. So, if s one goes to t plus one state selectively then it gives emissive signal s one goes to t minus one of state selectively it gives observed signal, but it goes to t zero then what happens since we are going to understand the whole mechanism in a quality fashion we just state here that s one to t zero just does not take place which is not rigorously correct and a much better model to which looks into the detail inter system crossing and the polarization in the two molecules will involve much more refined treatment. So, then this ad hoc approximation is not necessary will be satisfied with this one right now that we get two types of spectrum either totally emissive or totally absorptive and there is because of this. So, the conversion of a singulate to triplet seems to create a unit of spin angle momentum again we are back to some other problem now we have been talking about the conservation of angle momentum, but when this process takes place s one to t one s one has no angle momentum spin angle momentum that is, but t one triplet has a one unit of spin angle momentum how is it created now what happened to the conservation of angle momentum rule. So, to understand that let us figure out what is causing the inter system crossing process the perturbation is the spin of it interaction I call h s o. So, in the presence of spin of it interaction the spin and orbital angle momentum separately are not good quantum numbers the total angle momentum is conserved. So, you have let us say l and s this is the orbital and this is the spin angle momentum and this totally can give rise to j in the presence of h s o. So, this is conserved not this and this individually. So, there is a clue to this now that when the inter system crossing takes place in the presence of this perturbation then this singlet to psi triplet this may not be same for all three triplet level whether I have got t plus one t zero t minus one this might be dependent on that that might explain why we get this selective population among the three triplet level well just keep in mind that this triplets are the triplet of the first molecule which get excited for example, here that is the molecule which goes excited not the radical pair. So, little more understanding will come if we try to see why this thing is possible that different level triplet level may not be equally efficient in this inter system crossing to take place. So, for that we have got triplet molecule as two unparallel electron. So, I could write the spin of interaction in this fashion l one dot s one l two dot s two now this could be simply little bit manipulated to write in this fashion. Now, here you see the first term is symmetric l one plus l two s one plus s two. So, that total spin and orbital angle momentum remains constant, but second term is the difference of them l one minus l two s one minus s two that is the this is supposed to be conserved j has to be conserved not necessarily this and this. So, the difference shows that if one is changing other must change the opposite sense. So, singular to triplet inter system crossing means I am creating one unit of spin angle momentum. So, that can be compensated by having one unit of orbital angle momentum changing in the opposite sense. So, that is the key to this conserving conserving the total angle momentum as well as try justify why this could be different for different triplet level. So, most molecule are not spherical and hence orbital motions along certain directions will be favorable to changes in the orbital angle momentum. So, the molecules are not spherical let us say a quinoa on itself. So, it is the carbonyl group which is the reactive center. So, now electrons can move about in this direction electron can move much more freely let us say than movement of electron in this direction. So, this movement that I am showing by hand is sort of nothing but the orbital angle momentum. So, different direction therefore molecule can have different type of orbital angle momentum and some direction it may more efficient than the other direction. So, coming back to this some direction this L can change more easily than some other direction. So, these three triplet level are molecular triplet level based on the coordinate of the molecule one can think of that along that direction. Then this triplet level can be connected to molecular triplet and then this orbital angle momentum change in the molecular coordinate system can be dependent on the which direction we choose. So, by arguing in this fashion we can therefore see that the inter system crossing rate may not be same for all three triplet level and therefore that is the key to the appearance of the selective population of the triplet state and then finally the reaction taking place there to produce the polarized radical. So, let us summarize this single phase hyperfine independent polarization what is happening there key point is that all three triplet levels are not equally populated by inter system crossing. So, an acetylpic inter system crossing is taking place. So, the triplet that itself is produced in the beginning it is polarized triplet because this also does not follow the Boltzmann distribution. So, one might call this as the photo physical origin of electron spin polarization and then electron spin lattice reaction will try to equalize the population here, but then if the reaction is competing with this spin lattice reaction process then the radicals can be produced with the spin selective manner. So, here again figuratively this thing is described here the let us say T1 state is populated first selectively then this relaxation processes are that spin lattice relaxation within the triplet manifold. So, that this follows the Boltzmann distribution or Boltzmann distribution is restored within the triplet manifold. Now, the B molecule comes and reacts with this if the reaction is faster or comparable to this rate then this extra population of spins can be transferred to the radical. Therefore, the most important requirement is that chemical reaction must compete with the spin lattice relaxation of the triplet for generating radicals with electron spin polarization. The summary of the single phase hyperfine independent polarization is that reaction takes place in the triplet excited state first requirement. Then anisotropic ISC selectively populates either T plus 1 or T minus 1 state and then first chemical reaction competing with the triplet spin lattice relaxation transfers the spin polarization of the triplet to the radical. The reaction rate must be rate constant must be comparable to the spin lattice relaxation rate constant of the triplet. Now, this is the condition that need to be satisfied but to keep in mind that this is not easy to satisfy because this triplet spin lattice relaxation rate is of the order of 10 nanosecond at room temperature. So, the reaction has to be also comparable must be comparable to this time. So, not all reactions are such fast reaction usually electron transfer reaction or hydrogen on transfer reactions are fast compared to that the spin lattice relaxation of radicals mostly are in the microsecond range. So, that whatever you see the radicals are formed at least they will maintain their spin polarization for several microsecond for the time to appear spectrometer to record that this mechanism which primarily centered around the triplet state involvement of the photochemical reaction is called the triplet mechanism of electron spin polarization. The characteristic feature of this is that both the radicals will give same type of spin polarization either totally absorptive or totally emissive this does not depend on the nuclear spin state the spectrum will be very much like the steady state spectrum but either will be going down or going up relative index it will be same as what one sees in the steady state nuclear spectrum. So, with this we will stop and then in the next class we will take up the other spin polarization mechanism where one sees the appearance signal which is of this kind this or that the nucleus spin dependent electron spin polarization what we saw in case of this rubbed it out acetone isopropanol system.