 So, this is where we are the we are actually discussing only one paper so far in the last couple of modules that 1998 Jeff is came a paper of Takeuchi and Tahara. So we are doing that because we want to introduce a problem slowly and then later on we will just flash the papers and you will see how the quality of papers and number of papers change over time and then it is up to you to read them. So, the crux of the matter is that excitation is taking place to not one but to two states and again when you said that this is not talking in the air there is some existing literature about some very well known so therefore okay and the additional evidence that came here came in the form of fluorescence anisotropy we all know that we excite by polarized light and then look at parallel ad perpendicularly polarized emission light you can get fluorescence anisotropy I parallel minus I perpendicular divided by I parallel plus I to I perpendicular and this fluorescence anisotropy where is it used most of the time what does it stand for rotational relaxation right that is I mean if there are at least in last 30 years if there are n number of papers in fluorescence anisotropy perhaps 0.9 n or more would be of about rotational depolarization but one of the very if you go back to early times look at very early papers on fluorescence spectroscopy you will see fluorescence anisotropy has been put to use to address a more fundamental issue rotation is not the only thing that causes depolarization why does this question of polarization arise why does the question of polarization arise in spectroscopy because as we know the most important quantity in spectroscopy is this what is this quantity transition moment integral I like to call it TMI different people call it different things mu and all what is psi I psi I is a wave function of this state from which the transition originates initial I for initial what is psi f final wave function of the state to which the transition takes place we are not even saying absorption or emission and this mu is basically the dipole moment so dipole moment is a vector and can be resolved into x y and z components so a transition as we know takes place only when this transition moment integral is nonzero and when you say this transition moment integral is nonzero what we mean is one of these three integrals psi f x psi I or psi f y psi I or psi f z psi I of course I should not say psi f x psi I it is integral psi f star x psi I d tau one of these has to be nonzero essentially and many transitions for many transitions only this transition is nonzero while this may be zero or the other way round okay in emission spectroscopy the idea is this you excite vice a z polarized vertically polarized light and well excitation by vertically polarized light is better than horizontally polarized light if you are going to detect in 90 degrees so then the issue is the when emission takes place if emission takes place from the same state since you have already done photo selection you have already excited by z polarized light the emitted light should also have z polarization vertical polarization right the thing gets messed up apparently if you use horizontally polarized light because if this is the polarization of excitation light this is the direction of propagation and when emission takes place from here the problem is generally you detect at 90 degrees if you detect in the same direction there is no problem if you detect at 90 degrees then this direction is the polarization is a direction of polarization of light as well and that messes up things completely both this as well as this are actually perpendicular but coming back to the question in hand you expect the emission to be completely z polarized vertically polarized if the emission is from the excited state so one thing that can happen is that the molecule can tumble before can rotate before emission takes place and then starting from this polarization you can get emission in different directions and that is where fluorescence anisotropy comes in is of a great help to tell you whether rotational depolarization is taking place or not however it takes some time for the molecules to rotate if you are talking about time scales of less than picosecond the molecule will move this much and not enough fluorescence depolarization will take place in that ultra short time due to rotation however suppose this is the situation where you excite yeah and you excite to a state which has some kind of a polarization and then there is an a rapid excited state process ultrafast excited state process that takes it to another state which has some polarization in transition dipole moment in some other direction so this is the direction of transition dipole moment of the locally excited state this is the direction of the transition dipole moment of a state that is produced by some rapid excited state process then what will happen then also you will have some depolarization and this depolarization will take place in ultrafast time scale which is not accessible to rotation so if you have a little slow depolarization then you can think that it is due to rotation something like 10s of picosecond and longer if you are talking about proteins and all it becomes nanosecond however if you have depolarization which is ultrafast 0.1 picosecond 1 picosecond 2 picosecond then it is not enough time for rotation that means there is some rapid transformation from one state to the other okay right and it is actually known that this phenomenon takes place in some kinds of molecules we will come back to that in a moment but now let me show you the data of Takeuchi and Tahara fluorescence anisotropy data this is the fluorescence decay excited at 380 nanometer this is the rise followed by a long decay at lambda m 380 nanometer sorry sorry this is lambda fluorescence 380 nanometer excitation is at 270 nanometer excitation is at 270 nanometer benzenoid band and this is the first decay you see at 380 nanometer this is the rise followed by a very slow decay that you see at 500 nanometer emission now here this is the time evolution of anisotropy you see the first component and here the time evolution of anisotropy is slower it is associated with 12 picosecond right so when you look at 500 nanometer Tautomer band the fluorescence depolarization that takes place is mainly due to rotation however when you look at the locally excited state there is a very very first component ultra first component will tell you how much it is that cannot be due to rotation and the reason came from classical knowledge of this kind of molecules it is known that Sebenazza indole has two very closely lying excited states singlet LA and singlet LB and for once again as a result of experiments as well as calculations it was well established by that time that singlet LA has a transition moment transition dipole moment in this direction singlet LB has transition dipole moment in this direction here we are talking about monomers do not forget okay excitation essentially is absorption does not change much does this remind you of something you have two transition moments one almost perpendicular to the other not exactly perpendicular and one we have why have you drawn a longer arrow in one case and shorter arrow in the other case double-headed arrow because this one passes through more atoms this one passes through fewer atoms so this is called the long axis for the singlet LB state is known to have transition moment in transition dipole moment along the long axis and singlet LA is known to have transition dipole moment along short axis does that ring a bell long axis short axis those of us who have studied the spectroscopy of naphthalene would perhaps remember that for naphthalene there are very well defined long and short axis transitions at 90 degrees to each other perfect 90 degrees now for seven as I indulge and for trip to fan it is well known that you have this again this short axis long axis business it is just that the axis are tilted they are not exactly a right angles because the two rings are not the same so this idea of long axis and short axis and this closed line excited state all this stems really from the symmetry discussion of naphthalene energetics of naphthalene okay that is the starting point so those who might be interested we have discussed this naphthalene case in detail in our symmetry NPTEL course symmetry in chemistry once again the lectures are freely available on YouTube alright so this is this was known so what Takeuchi and Tahara said was a simple explanation Tahara's group specializes in providing simple explanation in fact Tahara once told me that you should not think matters in a very complicated way and it is very typical manner he said I do not understand things that are complicated there has to be simple explanations for everything of course if you say that quantum chemist they might not agree but that is not too bad an approach so what they said is this we know very well that there is singlet LB state which is a little higher line state than singlet LA so singlet LB would be S2 singlet LA would be S1 yeah and from the time resolved spectroscopic experiment that is there their assignment was when you excite you excite to this singlet LB that gets transformed to singlet LA without the proton being transferred so it is basically S2 to S1 vibrational cooling in 0.2 picosecond and then the double proton transferred takes place from this electronically relaxed S1 state singlet LA state of the dimer or rather I should say dimer of singlet LA states because after all the singlet and singlet LA and singlet LB states are defined for the monomer not the dimer and herein started the trouble because their interpretation was that the ultrafast component 0.2 picosecond is associated with sort of vibrational cooling say S2 to S1 non radiative transformation and the proton transfer is concerted both the proton gets transferred at the same time proton transfer is not sequential but concerted 1.1 picosecond is the time associated with transfer of both the protons together right note the time line is important here 1998 also note the page number 7740 Jeff is came a okay and the reason why that was the beginning of some trouble was that already about 3 years ago and that is not the only paper by the way there was this 1997 CPL paper and well as I said one reason I am very excited about it is that I did all these papers in real time as they were published because that is when I was a student the reason why this part of this debate is that 2 years prior to this CPL paper 1995 there was a paper published by Duhal came and Zuhal in nature where they had studied the same 7 as a indole dimer in gas phase using mass spectroscopy you can see the setup here it is very similar to the setup we have next to our lab in towards the nourish patwari's lab the only difference is that this is a time resolved mass spectrometer okay same time of light mass spectrometer but time resolved with femtosecond resolution this is where a lot of that noble prize winning work has been done. So there 1995 is 4 years before Zuhal's noble prize when everybody knew that noble prize was coming to this gentleman that time this nature paper came where they said based on this mass spectroscopy data that they also got by the way the interesting thing is they got very similar time constant 0.2 picosecond and 1.1 picosecond almost close to what Tahrar group had got but based on the mass data they said that the 0.2 picosecond component is associated with transfer of one proton 1.1 picosecond is associated with transfer of the second proton it is pK1 pK2 pK2 kind of thing and then you get that automer. So now the question was that Zuhal's group said the proton transfer is sequential 2 step Tahrar's group said it is concerted and preceded by S2 to S1 cooling which one is correct more or less around the same time 1998 again Kasselman is very well known name in this field Kasselman's group in Penn state did some experiment once again on mass spectroscopy and it is very easy to understand what they did and perhaps it is not very difficult to understand where the problem lies here. So what they said is this you take a monomer the mass is 118 AMU if a proton gets transferred then the protonated species has a mass of 119 AMU the deprotonated species has a mass of 117 AMU it is as simple as that when both the proton gets transferred what is the mass again both have 118 so what they their argument was based on the fact that if I look at that 119 AMU fragment how much of it is there in relation to 118 fragment they said this is the evidence of single proton transfer species okay and this is what they got can you read the y axis can you read the x axis time delay in femtoseconds very easy to understand can you read the y axis ratio of masses 119 to 118 and there they see a rise and that is associated with 660 femtosecond so they said yes it is sequential first one proton gets transferred otherwise why are we getting 119 AMU so the problem of course with this experiment is that you are differentiating 118 and 119 with femtosecond resolution so is the data really that good right that skepticism is going to come but this is what Castleman's group said anyway but then do not think that these are only gas phase experiments that said that it is sequential Zuhl also did experiments in solution Zuhl's group and they did in different solvent so this is these are the papers that I am not going to discuss in detail but I strongly advise that you did this then only you will understand the intricacies of this discussion several studies mostly by in one case when dual was in Zuhl's lab in another case when dual went back and their groups collaborated every time yeah same dual every time of the result dual every time they said it is sequential so this is the debate this is what it is all about is it concerted Taha model is it step wise Zuhl model and then well to somebody who of course you know in any field understand the importance of anything if you are interested in so to an outsider this might seem to be frivolous who cares who cares if it is one at a time or both at a time but it is not frivolous then as promised we will discuss what all we have learned and one evidence that this was not frivolous but rather any very interesting thing in this in this community was that father figure one of the father figures of fluorescence when you see fluorescence this is the name that comes to your mind Michael Kasha actually got into this 1999 Kasha's group published a paper and it is not only Kasha Catalan Catalan is also very major player Catalan and Kasha and all they published a paper and this was published in PNS as you can see this PNS paper was essentially a discussion of what these two groups had seen Zuhl group and the Hara group please read this paper first of all it is the work of a master well commentary of a master Kasha and what he said I will just show you one part and I will read it what it said is that Zuhl group observed for 7 as a indole dimers in solution a similar range of ultrafast and fast components for deuterated dimers so Zuhl group also did the deuteration work. Takeuchi and Tahara assigned this rate to vibrational cooling by analogy to establish rates for IVR for analogous hydrocarbon molecules so as you know every research is really linear combination of previous knowledge and something more so you cannot really do anything in the air so that is what Tahara had done the Zuhl group adopted the deficient RHF calculation results so one of the main strengths of Zuhl's work was whenever you do any gas phase work to understand it you have to do calculations quantum chemical calculations and quantum chemical calculations are very helpful but if you are not careful or if you are if you are biased towards a particular result you want to see you actually get to see the result you want to see so for an RHF calculation results for an intermediate reaction potential minimum exaggerated in their figure in their figure one and interpreted all of the dynamics data accordingly. Takeuchi and Tahara took full account of the exact 7 as a indole lowest molecular energy states involved analogous to the S1, LB and S2 LA states of isoelectronic naphthalene Takeuchi and Tahara omit the lower AG electronic state preserving the upper BU split components of S1U and S1B because the S0 singlet AG to S1A doublet AG transition is electric dipole forbidden and is observed as a biphotonic transition as we have discussed above as a dipole moments are rotated in plane in the PT Tautomer excited state the AG and BU components reverse order there is a reversing of orders that takes place the S1U state metastability contributes to the tautomerization dynamics and is the state from which tautomerization occurs in the dimer. So Kasha actually said at that point itself that for solution it did not want to comment on the gas phase results but for solution the conclusion of Takeuchi and Tahara is much more reliable that is what Kasha had to say at that point but this was immediately followed up by paper on Kasselman where he stood his ground and sort of defended their stand. So this is like a war of giants Michael Kasha was a member fellow of the National Academy of Science USA since 1971 Kasselman had just become at that time a fellow of the academy. And this really got eyeballs and this is something that I usually do not show in any presentation but in this case just to bring out how the kind of interest generated I show you how drastic was the increase in the number of papers in related fields until 90 to nothing the slowly it goes up and from here it just takes off and most of this work was about this debate is it concerted is it stepwise. Then Tahara's group did some experiment what they said is fine we are saying that when we excite we excite to two different states. So now if I change the excitation wavelength then what happens if you have something like this where the absorption spectrum is made up of absorption spectra for S0 to S2 and S0 to S1 transitions and this is of course a cartoon representation their contention was that if I excite here then I will excite primarily to S2 if I excite here I will excite primarily to S1. So when I excite here that 0.2 because again should be seen if it is S2 to S1 transition if I excite in the red end of absorption spectrum it should not be seen yeah. However if it is only one state that is excited to and if the proton transfer is actually sequential then no matter where you excite you should still see two time constant 0.2 picosecond and 1.1 picosecond of course this is easier said than done this is what you want to do this here is the absorption you want to excite at different places and that is what they did they started with 270 nanometer then 280, 287, 293, 300, 307, 313. Now do not forget that the problem here is that again same thing again and again you excite in with an ultra-fast pulse it is not monochromatic okay so it is important to do a lot of experiments and when you excite here 270 nanometer you cannot go here you cannot go to 260 because there is hardly any absorption. So this was a difficult experiment not at all an easy experiment and data analysis was also not at all easy but I will show you what they get this is what they got what is compared here is the decays of up conversion 50 second up conversion upon excitation at 287 nanometer 300 nanometer 313 nanometer and look at the decays these are not normalized these are not peak normalized if anything they are tail matched. So see the tails are matched very nicely what is this tail 1.1 picosecond component so if the second case is correct that 0.2 as well as 1.1 picosecond both are associated with proton transfer the first one and the second one then these decays should have become superimposable a tail matched decay would also get peak normalized a peak normalized decay would also get tail matched that is obviously not the case and when you excite at 287 nanometer and when you excite at 313 nanometer what is the difference contribution from the ultrafast component is more when you excite at 287 nanometer and next step is how you represent your data it is not enough to have data what you say is important how you say it also important right. So they did it beautifully by making log normal plots and log normal plot this is the this is data right actual data yet to get this kind of data in up conversion third harmonic excitation is very very difficult you can understand that so this required very careful experiment and then in semi log plots what they did is these lines that you see these lines come when you have this 1.1 picosecond time constant right so in semi log plot 1.1 picosecond time constant decay would look like a line with the corresponding slope. So for 313 nanometer excitation you see the decay is coincident with this line as you go towards blue I mean higher energy excitation there is a deviation in the smaller time scale that is because when you excite at 280 nanometer the decay is by exponential you get 0.2 picosecond as well as 1.1 picosecond when you excite at 313 nanometer there excitation wavelength remember the decay is single exponential only 1.1 picosecond component is there right so this is the thing moreover they also looked at anisotropy when you excite at 313 nanometer anisotropy decay also is almost single exponential and long lived associated with that 12 picosecond component which is for rotation when they excite it at 270 nanometer you see there is an initial ultrafast component 0.2 picosecond so that 0.2 picosecond turns out to be the time constant as well as the time constant associated with anisotropy decay that is what is telling you that one state gives mix way for the other with this 0.2 picosecond time constant and that is associated not with not only with decay of population but also change in direction of transition moment integral well transition rifle moment that is why your you see it here. So the see in long time these two are almost parallel right that is due to rotation anyway but in short time you get something like that that tells you that in that time scale some excited state process is happening and that is seen only for well not only for that is seen prominently for excitation at higher energy not seen for excitation at lower energy which means that when you excite at 330 nanometer you are not exciting the S2 state you only excite the S1 state when you excite at 270 nanometer you do excite the S2 state right. So that was explained knowing the directions of all these transition moment integrals from calculations by Nakajima et al. and other people like Walook and all and finally this was the PNAS paper published in 2007 10 years after the debate started and there the title of the paper I think is something like the answer to concerted versus stepwise controversy for the double proton transfer mechanism of 7 as a endowed dimer in solution and the answer is so simple this is the answer to a 10 year long debate yeah and so from here they said that yes it is definite that S2 involvement is there and see the change in quality of the paper also starting from campus later 1997 here we have this last paper in PNAS it was not the end of the debate because alright this is 2007 PNAS page number 5285 2007 PNAS page number 8703 where Zuel's group published one more paper defending their stand I am not going into what they said because by the time actually this is not really the debate raging on it is just well the Tahara paper is actually conclusive still some more papers are published I have already referred to the Sakeuchi and Tahara 2007 PNAS and Kohn and Zuel 2007 PNAS then Catalan remember Catalan Catalan and Kasha had written a paper remember so Catalan wrote a comment in response to Zuel's paper 2008 and here since the slides are from Tahara San he has actually written the exact timeline this paper was accepted in 2007 January 12 Zuel's paper in April on April 3rd same year so Catalan wrote this comment encourage you to read it and Zuel wrote a response to the comment and that was the end of the debate. So well this is very small print you can read it here it is blown up for our benefit so what Zuel said here is that as we pointed out in our recent publication another group of researchers stated the concerted mechanism by its definition does not require such a strict simultaneity so what it had boiled down to is it clearly concerted at the both going out exactly at time t equal to 0 with attosecond resolution so Tahara had said no who is saying that it might be that there is a little bit of difference but you cannot make out the fact remains at that initial 0.2 because second component is not the first proto transfer so they discussed a little bit about symmetry breaking and then this is where this is a statement of yielding at the end it seems not profitable to have in the scientific literature the same claims in different colors and we hope that this letter will be the epilogue and it was that brings us to the end of this decade long discussion on what seem to be what might seem to be an outsider a debate of no consequence but actually that is very wrong approach this is an engrossing debate because it teaches you several things some scientific and at least one philosophical the scientific thing that we learn is we learn how to handle situations like this where you have closely lying states what are the experiments you need to do how do you analyze your data to come up with an explanation that is that differentiates states the difference of which is really really subtle a less careful experiment would completely miss this so this body of literature that has been produced teaches us specifically those in the ultrafast dynamics field but also generally in other fields of how experiment should be done how data should be analyzed and how questions should be asked to understand the problem that is not easy to solve and the philosophical outcome of this is published paper is not gospel just because somebody has said something in a paper we cannot say what that is published so it is right that is completely wrong science progresses as a result of series of mistakes people do something they interpreted in such a way and more often than not it is wrong rather for theory was wrong was it completely wrong it was not completely wrong after all there is a nucleus and there is an electron outside the nucleus but this planetary motion and all that is wrong both theory was wrong but we are not done we would not have reached the present state of the art so somebody who has one Nobel Prize or is about to win Nobel Prize it is not necessary that whatever he says is the absolute truth and even if you are Goliath or less than Goliath in front of that David if you are convinced about what we are saying we should have the metal of taking it forward and following through until the epilogue comes right so that is why I am really very fond of this debate and I thought we will discuss it as a part of this course that brings us to a to an end of whatever we wanted to say about molecules of course what we have presented on molecules again is the tip of the iceberg we have not discussed the fast body of literature that exists on say things like photoisomerization there is another story in itself you can teach half a course on it so all that and I leave to you to read but since in any case we are coming to the end of the course you know few more modules maybe 15 more modules what we will do is we will move on to metal nanoclusters after this and then to metal nanoparticles and semiconductor nanoparticles and then we will discuss 2D electronics 2D electronic and IR one of those spectroscopy and perhaps we will come back to Tahara's work as well and we will discuss what is called some frequency generation at the surface that is again a classical problem that has been reinvented and new sites have been achieved in the last 15 years that will bring us to the end of the course.