 So, in the previous module, we have seen how the dynamics of the fastest possible process breaking of a bond was determined in Amit Sewell's group and we have also seen how they could show you not only give you a time constant but actually tell you how the process is taking place making and breaking, making and breaking on and on until it breaks and goes away. Now what we will do is we will jump cut to a couple of decades later and present to you the glimpses of a couple of pieces of work from Eric Nebering's group in Max Born Institute and this is my favorite I like it very much one reason is that we read it in real time the moment they were published and also Nebering had come and I heard him present this work. So this is elegant so what has been done is this okay all of us have studied acid based reactions in school and college right and what is written invariably in chemistry textbooks is that acid based reaction is very fast and the mechanism is unknown, mechanism cannot be established because it is so fast right. Now what we are going to discuss in the next 10-15 minutes is how the mechanism of this supposedly impossible to establish mechanism kind of reaction how then that mechanism was elucidated by pump pro spectroscopy once again I will not discuss the whole story I will just present to you the logic and then you should read these two science papers once you know a little bit about pump pro spectroscopy those papers are not very difficult to understand alright. So what it did is again in the beginning of the course we have presented the concept of photo acidity we have said that organic acids become stronger acids when they are excited organic bases become stronger base and we had explained why in the light of molecular orbital theory. So what Nebering's group did is that they used a strong photo acid pyranine or HPTS as it is called so here you see and they used OD, OD signal is easier to see in this experiment and they used a regular base that is acetate ion and the idea is that they would excite pyranine it would lose D plus ion to form the what they call the photo base and this acetate ion would get deuterated to give you deuterated acetic acid okay this is the reaction and how do you follow it by using pump pro spectroscopy but here the pump is visible 400 nanometer or something because that is where pyranine absorbs anyway the probe is IR probe is not your visible because the good thing is if you use IR there is a lot of data do not look at everything just show at wherever the arrow points just see wherever the arrow points this here is the IR spectrum of acetic acid and it is a little different from the IR spectrum we are used to see because y axis is absorbance and not transmittance. So the idea is this this is your acetic acid absorption in IR of course that OD will come deuterated acetic acid and this is the absorption of absorptions of the photo base and photo acid remember what photo base and photo acid is? Photo acid is pyranine deuterated pyranine photo base is the deuterium removed right the anion corresponding anion of pyranine these are the IR spectra of those. Now the idea is to start with the ground state what do you have you have the photo acid and you have no acetic acid so when you excite by an ultrafast pulse if you use the frequency of characteristic absorption frequency of deuterated acetic acid then you should see a transient absorption signal right point to note is that this transient absorption signal arises due to absorption of a ground state not an excited state okay and techniques like this are often used in other experiments like recombination of axial ligands of heme proteins. So transient absorption does not necessarily have to be of an excited state okay it is forming as a result of excitation of the photo acid that is right but what you are looking at is the ground state absorption spectrum of acetic acid and this is actually the most useful range of frequencies as you see that the transient absorption spectrum change significantly you can also work with this range and neighboring soup has actually done that but there is significant overlap between the absorptions of photo acid and photo base. So you might understand that analysis of this part of the transient absorption signal is a little more complicated read the paper they have actually done it but this is very easy to follow. So what you should see in the second panel is how the absorption of acetic acid changes as a function of time post excitation of bidenate photo acid okay is the problem understood what do you expect to see you expect to see a rise time yeah. So they looked at the rise time and what they saw is that first of all rise time is there secondly your you see a smaller rise time if you increase the concentration of acetate ion why because if you have a higher concentration of acetate ion then the moment this deuterium ion is liberated I do not know whether it is a good idea to use words like moment when you are talking about ultrafast phenomena but I think you understand what I am trying to say when the D plus ion is liberated as a result of photo excitation of pyronein it finds an acetate ion faster that is what I mean if the concentration is more that is why the rise time is shorter as you increase the CSTCO- concentration and also they compared the evolution time evolution of the signals for photo base and CSTCOH. So at high CSTCO- concentration you can see the dynamics are more or less the same see photo base is also produced as a result of excitation right because you start with the photo acid when H the D plus goes out from pyronein two things are produced first is photo base is produced right away and then that liberated D plus is taken up by acetate ion and then acetic acid is produced the question is are the dynamics same for these two processes the rise in the population of photo base and rise in the population of acetic acid are the same or different what do you expect if it is a simple mechanism involving something like this where you have contact ion pairs HOD and B minus then when D plus is liberated what do you get well ROD. So you get RO minus and you get DB let us say so dynamics of formation of RO minus and dynamics of formation of DB should be the same understand what I am saying so rise in population of the anion of pyronein and rise in population of acetic acid should be the same if they are just two ions in contact and it happens instantly that is what you get for high concentration of acetate ion however when you work at low concentration of acetate ion this is what happens this is the dynamics of photo base and this is the dynamics of your acetic acid actually I should have written CHT COD here because it is D plus so which one is first of all they are not the same which one is faster which one is slower formation of photo base is faster right and formation of acetic acid is slower what does that mean it takes some time for D plus to get liberated right and time taken for that is reflected in the time taken for population of the photo base to build okay after that if the proton is doing something else gets engaged somewhere else then the time of formation of acetic acid will be longer understand what I am saying yeah so I pass on a message to you if I speak to you the time taken for me to get rid of the message and the time taken for you to get the message is the same but suppose I do not tell you I tell somebody else that person tells somebody else that person tells somebody else then I have delivered the message but sometime would go before you get it that is what is reflected in this kind of a different difference in population dynamics of the photo base and CHT COD okay and then of course this is a very qualitative way of putting it but it is very important because it gives you the idea the idea you get is that at high CHT CO minus concentration this is a situation at low CHT CO minus concentration when you have a chain of solvents in between the proton donor and proton acceptor then it takes a little longer okay this is the qualitative picture to get the quantitative picture what they had to and that is why they got not one but two science papers out of this they did a thorough analysis of the data and established quantitatively how much time it takes for the proton to get lost from the photo acid how much time it takes for it to get associated with water and then after a thorough analysis they came up with this picture looks very simple when you look at it right and in fact looks almost intuitive so what they said is ROD plus V minus first there is a diffusion stage in diffusion stage you have a lot of water molecules they are separated so they come a little closer and they have this you can call it water polymer between the proton donor and the proton acceptor this is called the encounter stage where this polymer is eliminated and you form the contact time pair and then you have the reaction stage where the proton actually gets transferred so what you see in front of your eyes is the reaction mechanism of acid base reaction something that has been believed for a long long time to be beyond our capabilities so for the last 13 14 years this has not been beyond our capabilities we know exactly what the mechanism is and what is not written here is that you see all these rate constants all the values of these rate constants are exactly known that is the power of ultrafast spectroscopy not only do you get to know how a bond is broken you also get to unravel mechanisms of reactions that no other technique can so this is a fantastic piece of work from relatively recent times after all we have been showing you papers from 1972 1965 and so on and so forth which is still recent compared to what we usually do in classrooms classrooms we often talk about things that were already known by 1900 but what you see now is the paper was published in 2005 science I hope to get back to the work of neighboring once again when we talk about energy transfer between vibrational modes of water but before that let me see if you can introduce another excited state process which is perhaps the most widely debated excited state process of all times and that goes by the name of twisted intermolecular charge transfer and there have been beta fights over decades over whether that first T in TICT should be there or not be there okay so to cut a long story short and I show you the example suppose you have a coplanar molecule in which you have this donor moiety and the acceptor moiety right they are bonded to each other and the coplanar coplanarity is essential at or at least so it was thought because when they are coplanar two rings then the pi electrons talk to each other right if they are perpendicular then that communication is lost so the idea was that you have this donor and acceptor coplanar excite by light then the electron transfer takes place and then that is associated with the twist then what will happen you have delta minus on one side delta plus on one side and since this delta plus and delta minus fragments are perpendicular to each other that electronic communication is lost so you cannot have back electron transfer the reason why interest grew in this kind of systems was because people wanted to develop things like dye sensorized solar cell what is the meaning of a battery or a cell if you have a plus end and a minus end right essentially that is what makes a cell so here the idea was that light energy would be converted potentially into electrical energy because you are producing a molecule one end of which is plus one end of which is minus now how you are going to connect the two ends of the molecule that is a different issue altogether in any case we are not going there but then what it as it turned out that more than making a solar cell out of this a very rich field of photo physics was unraveled by this kind of molecules the most celebrated perhaps molecule of this class goes by the name DMABN dimethyl amino benzo nitrile even now people work on it I do not know how many thousand papers have been published on the photo physics of DMABN and its brothers and sisters and cousins but even now people find use for it so DMABN you can see what the structure is and for a chemist if I say dimethyl amino benzo nitrile that should be enough you can see that you have this donor moiety and acceptor moiety and the idea was that after charge transfer they become perpendicular to each other the fact that charge transfer takes place is not very difficult to understand because if you look at the fluorescence spectra and here when you look at the fluorescence spectra be careful this paper is not very recent well I am showing you a 1992 paper but that is sort of a review this is after two decades of work okay so this data is not really from this paper data is from some 1970s paper so they used to draw things differently at that time as you can see x axis is new bar in centimeter inverse I do not know if you can read and this side is the high energy spectrum high energy side this is the low energy side is it right yes it is this is 16 on the left side this is 24,000 or something 16,000 24,000 so what you see is you have two kinds of bands spectra reported in several solvents NXN is nonpolar and there there is only one band the B band so called B band which is higher energy and if you go all the way to acetone nitrile then you get only a band a stroke shifted band and remember what we said the stroke shift is one of the signatures of excited state process happening so something is happening post excitation in this molecule and it is charge transfer because we will see why we think it is charge transfer right what you see here is not exactly leopard plot but it is the maximum of spectrum fluorescent spectrum centimeter inverse plotted against a polarity parameter this polarity parameter is obtained from leopard Mataga equation which we will talk about very briefly in the next module so what you see is this look at absorption this is the graph for change of absorption maximum and centimeter inverse with polarity not much this is the second one is the variation of the emission maximum of the B band the higher energy band the change in polarity well you do not see it in very highly polar solvents or use it is very feeble but as long as you can see it there is not much of a variation but when you look at a emission there is a distinct dependence on delta f more delta f is delta f is a polarity parameter greater value of delta f means the solvent is more polar we will see what it is little later. So for a more polar molecule we see a greater rate shift of the a band what does it mean that means the a band the stroke shifted band is associated with a polar emissive state right and that corroborates nicely with this kind of a picture that the higher energy state corresponds to this where charge transfer is not taken place lower energy excited state corresponds to this where charge transfer has taken place but why do you think there is a twist in the data that we have shown so far we might have been able to establish that the stroke shifted band is associated with the polar excited state who has said that there is a twist lot of calculations said first of all we are not going to show you all that but we will present another piece of work extremely arrogant piece of work this time from Isenthal's group Columbia University Department of Chemistry Kenneth Isenthal. So what he said is this that we think that it is associated with the twist from theory calculations and some other preliminary steady state fluorescence spectra if there is a twist actually then the photo physics would depend not only on the polarity but also on the viscosity because whenever there is segmental motion that has to take place overcoming the viscosity of the medium. So for more viscous mediums the process would get slowed down. So before this work in 1987 there was plenty of work of usual leap armataga plot and all that of DMABN. So the reason why I am very fond of this work and actually everybody should read this paper chemical physics later. So the paper is not very big and not very difficult to understand either. This is a must read paper for anyone who wants to work with photo physics of molecules especially because it is extremely instructive it teaches you tools in one paper that can be very very useful if you are going to work with your molecules. So beauty of this work is that Isenthal and suddenly cannot remember the name of the then student who worked on this they said that well what about viscosity dependence. If you want to talk about polarity dependence of the rate constant associated with this then we should also take into account the role of viscosity that might be there. So what they did is they did two things first experiment result of which that you see here is due to mixture of isoviscus polar and non-polar solvents at room temperature. So what happens when you take mixture of isoviscus polar and non-polar solvent two pairs are shown here one is C3H7CN nitrile and they actually did a series of nitrile and octane and the reason why they chose these two as you can see none of these are commonly used solvents right whoever works with octane whoever works with C3H7CN of all things but they chose this pair because this pair has the same viscosity more or less same but octane of course is non-polar nitrile is polar. And another representative mixture is C4H9OH butane and hexadecane again you might think that sometimes we might have done experiments with butane butanol sorry not butane butanol but hexadecane of all things the reason why they were chosen once again is that they have same viscosity and then the first solvent mixture is the mixture of a non-polar solvent and a polar a protic solvent. Second one is a mixture of a non-polar solvent and a polar protic solvent. So not only do you get to understand the role of polarity and viscosity but also the role of a protocity. So and everything is done in room temperature X-axis ET30 ET30 is an empirical micro polarity parameter introduced by Richard as a book on properties of solvents where ET30 is discussed very nicely essentially what Richard's group did was they looked at Reichardt is what we often pronounce his name but I think the correct pronunciation is Richard but I am not good at German pronunciation anyway. So they worked with a library of molecules which exhibit solvatochromism and they found out that the absorption maximum of this between di number 30 30 is basically a roll number right. When you work with a large library of molecules Faria here works with molecule 72 all the time who is going to mention the IUPSE name all of us spent so much of time in school learning IUPSE name we hardly ever use them because you work with molecules that have IUPSE name if you try to write it is not a name it is like a name address a lot of things okay. So absorption maximum of between di number 30 in their inventory okay if somebody else might have called it di number 82 I do not know but they called this di number 30 and they worked with a series of beta indies they found that their di number 30 has this absorption maximum which is nicely dependent on polarity. So what they said is this why do they call it micro polarity they say that see organization of a solvent around a solute is very different from the organization of a bulk solvent especially for polar solvents and polar solutes. So the polarity that we measure by measuring dielectric constant need not necessarily be the polarity experience by the solute. In fact sometimes the polarity is much lower right around a solute polarity of water epsilon of water is about 5 whereas bulk polarity everybody knows very high value. So they said that when you talk about photo physics when you talk about a property of a solvent a solute you should talk about not epsilon not dielectric constant but some kind of a polarity parameter that is seen by the solute. See when you do spectroscopy you use solus and probes or something they are very myopic they do not see very far away they see the first solution shell maybe second solution shell beyond that nothing. So they introduced this and they said that this is a better polarity parameter when you are talking about photo physics of solutes. So that is what they used and they found that when they plotted against 8030 you get this two different lines one for the nitrile non polar solvent mixture the other for your alcohol hexadecane mixture okay good thing about these plots is that there is no effect of viscosity in any of the plots okay the plots are not the same right that could be because this mixture has a different viscosity than this mixture it would also be because here we have proteic solvent here we do not that will elucidate later on but then what is done here is that if you forget about viscosity polarity dependence is clearly there. So this is a superior piece of result then what I showed you a little earlier simple leopard plot because viscosity is taken care of another way of handling situations like this is to do multi parametric analysis what is called cum leg tuft analysis that is also very useful in understanding complex situations.