 So, we have polarity dependent rate constants that is for sure and there is some effect of hydrogen bonding. So the second piece of experiment that they did earlier one remember what it was? It was mixtures isoviscus mixtures at room temperature. Second experiment is in isoviscus liquids at different temperatures it is important to understand what we are talking about here. As you change the temperature I think it is very well understood that viscosity of a liquid is going to change. Does it increase or decrease usually when I increase temperature and what about a gas why? For a gas viscosity increases with increase in temperature that is right for a liquid it decreases the molecule collision that is liquid you understand nicely right. What about gases? So you think of laminar flow there is exchange between different lamellas that is why okay but right now we are talking about liquids anyway. So what it did was they painstakingly determined the viscosities of neat liquids at different temperatures and then of course you have to eliminate the you cannot compare an alcohol with a non-polar solvent okay and then at different temperatures they took different solvents keeping in mind that viscosity has to be same for all these points. So solvent A at 25 degree centigrade, solvent B at 35 degree centigrade, solvent C at 45 degree centigrade so on and so forth the common factor is that at 25 degree centigrade whatever viscosity solvent A has is the viscosity of solvent C at 45 degree centigrade okay. Isoviscus neat liquids in different temperatures and then they observed the surprising result that the rate of TICT increases at lower temperature that is not a very usual phenomenon right. It is not as if it is unknown again if you go back to Atkins physical chemistry book or any standard physical chemistry book you will see that if there is a pre equilibrium then you are going to have certain conditions are met you can have a decrease in the rate constant with increase in temperature okay. But here the explanation is this this here let us say is your energy surface for the locally excited state this is the energy surface for the charge transferred state. So what they said is that the charge transfer state is going to be stabilized further if polarity is more yeah and polarity is more at low temperature yes so what will happen you can see there are two surfaces here for CT the upper one and the lower one if suppose there are two temperatures which one will correspond to the lower temperature which one will correspond to the higher temperature yeah lower one corresponds to higher temperature or lower temperature lower temperature actually because at lower temperature your polarity is more or less oh I did not even discuss that we only talked about viscosity sorry at lower temperature actually polarity is more because at higher temperature what would happen is your solvent dipoles to rotate and all that would cause a decrease in dielectric constant. So at lower temperatures due to greater polarity this would be stabilized you might think that that should have no effect on the on the activation energy anyway because you are stabilizing the product what does that have to do with your activation energy actually something is there what is there look at this diagram carefully. If it comes down what is the activated complex it is a point at which the two potential energy surfaces for LE and CT cut right. So if the LE surface is where it is and the CT surface is lowered then what will happen is that the two surfaces will intersect at a lower energy point okay that is what will least lead to a decrease in activation energy at low temperature okay. So Arrhenius equation is not violated Arrhenius equation gives you the expectation that if you increase the temperature rate should increase but that is true only for constant activation energy scenario here the activation energy itself becomes smaller because it is not only temperature that you are changing and this is instructive because you have to remember when you try to understand the photo physics of any molecule that you change some parameter it is not the only thing that is changing it can have a domino effect. So to be very careful about what you are actually looking at you think you are only changing temperature are you affecting viscosity are you affecting polarity are you affecting molecular aggregation or de-aggregation one needs to think of that. So now to conclude the story they observed this empirical expression but EA equal to EA0 minus A into ET30 minus 30 now it is accidental degeneracy the second 30 is nothing to do with the first 30 the first 30 is a roll number second 30 is a number that arises out of experiment it is fortuous that they are the same or maybe they made sure that maybe they changed the roll number so that this becomes roll number 30 so that both are 30 but these two 30s are actually not related okay. So from here what they established is that the activation energy of alcohols is greater than the activation energy of nitrile and more interestingly the difference between the activation energies of alcohols and nitrile is 6 kilo calorie per mole does this number ring a bell 6 kilo calorie per mole what is it the 6 kilo calorie per mole it is the energy of a hydrogen bond so this difference in activation energies of alcohols and nitriles is due to hydrogen bond so hydrogen bond also has a role to play and this one later contains so much of information and it is so instructive it is imperative that we all read this paper and understand it there is more to it than what I have presented of course it seems important that we understand everything please read this paper great to conclude this story even though we have overshot the time a little bit we will finish this and then maybe today we will stop. So TICT in DMABN DMABN is the champion molecule of TICT this what champion is used very commonly especially now in the head is of material science people make devices they make 200 devices and the one that is best performing is called the champion device. So there is no harm in learning from there and calling DMABN the champion molecule of TICT but then that is the not the only prominent TICT molecule there are plenty two other these are also champion molecules classes of molecules are Nile rate and TNS or ANS for that matter T for toluidino A for anilino so these are all TICT molecules and Nile rate and TNS ANS these the claim to fame of these molecules is that they are used extensively by people who try to understand proteins using fluorescence these are very well established protein stains extrinsic protein stains and the idea is and you can see so this is the data that we recorded I think when I was a PhD student or something I think this is ANS or TNS I forgot maybe ANS. So this is the spectrum in water but multiplied by 66 so if I do not multiply by 66 it will be a baseline right and you see when we put it in my cell there is a huge increase and there is a blue shift why this is the TICT emission that you see in water and since TICT state is energetically closer to the ground state non-radiative deactivation of that state is much more that is called the energy gap law if the energy gap between 2 states is small then the rate constant of non-radiative deactivation between these 2 states is an exponential function of that energy gap widely used in things like metal ion complexes and when it is put in my cell then the micropolarity is much less that is why the locally excited state is selectively populated that is much more energy and more energy okay. So this is these are 2 examples and there are many applications that are worked out this is not such a great example I do not know why you have put this one there are if you read the work of say P. K. Bharadwaj of IIT Kanpur they have made numerous devices and all not devices sensors and all and their strategy is always you have an ICT molecule initially ICT is there since ICT is there fluorescence is quenched then you bring in a metal ion or whatever you want to sense and that engages the lone pair so ICT is stopped and fluorescence shoots up that is what makes turn on sensors. So you can read this it is not so bad also you can read this paper for description of this sensors that they made of different things okay. Now we go back once again to 1993 and show you some data where people have tried to do structural modification of DMMBN I am going very fast on this part because I want you to actually read this papers this is homework I will I do not expect you to understand everything I am saying right now it is just an introduction but the crux of the matter is people tried to make DMMBN derivatives of several different structures one in which this coplanarity will be maintained one in which coplanarity is forcibly destroyed and all these studies what they inferred was that the twist takes place if you do not allow the molecule to twist you do not see that TICT band okay if you force it to be at 90 degrees you only see that TICT band but then there are problems also because if you make the linkers a little too long then all results go awry. Now before leaving this subject I should at least show you some real ultrafast dynamics data so let me do that and this is where we will show the data we promised okay. In this 2009 paper what you see is this is what data is this can somebody guess up conversion right what up conversion with the difference as we will see. So this is the dynamics in the blue end of the spectrum okay one is not one day I think it means monitored at 350 nanometer emission emission wavelength you see there is a decay and the decay constant is 3.07 picosecond ignore that 2 in bracket and when this trace is recorded in the red end of the spectrum you get a rise associated with 3.07 picosecond. So this tells that 3.07 picosecond is the time associated with the charge transfer locally excited state to charge transfer state transformation okay and this is a kind of a nice way of putting it and it gets nicer when I show you the time resolved emission spectrum here they are something strike you here these are not fitting curves these are actual fluorescence spectrum recorded at different time delays the entire fluorescence spectrum is recorded together okay and now again I want to give another homework the homework is this we are actually going to our lab and looked at the FOX setup and saw how it worked femtosecond optical gating depends on generating a sum frequency and as we have discussed earlier for different wavelengths well the gate light is the same wavelength for different emission wavelengths some frequency generation would take place at different tilts of the non-linear crystal and over the last 10 12 modules we have learnt why tilt is important also okay. So what common sense would tell us is that the tilt at which we get some frequency for a particular emission wavelength lambda 1 should not be appropriate to generate some frequency using another wavelength lambda 2 is that right this is the tilt it is optimum for lambda 1 which means some frequency for lambda 2 is not going to take place then how do I generate the entire spectrum the entire spectrum is recorded at one go like our transient type of spectrum how has that been done it can be done in two ways first the hard way or the more time consuming way use a stepper motor on the sum frequency generation crystal itself right and keep tilting so this tilt may be the angle for say emission wavelength of what is given here 350 nanometer this may be the actual the appropriate tilt for 355 nanometer so keep on tilting the crystal okay and as you know we use uniaxial crystals so tilt is the only parameter and keep on recording data okay hence construct the spectrum but if you want to do that do not forget how it is done fix the tilt then run the scan so essentially you generate the decay at that wavelength then change the tilt again record another scan that is a decay so in this method if you want to do it in this method you actually generate the decays first and from there you construct the time-resolved emission spectra okay the other way of doing it and that is something that we have not really discussed is to use a very thin crystal in which that tilt is not important you can get some frequency for all the wavelengths what I want you to do is I want you to read this paper and tell me next day which method has been used in this paper is it a variable tilt or is it a crystal at a particular angle but thin enough so that you can get all the wavelengths together okay and in the second case how does it work next day we start with this question right so we have talked about TICT we have shown plenty of evidences for it and we have discussed the ultrafast dynamics of it we have discussed your applications of it as well the only thing that remains to be discussed now is the opposition to TICT because remember I started off saying that this is perhaps the most controversial excited state process ever discussed may be not if you get time we will talk about the proton transfer debate between Tahara and Zoel 7 as a Indole dimers and I say Tahara and Zoel because Tahara turned out to be right Zoel turned out to be wrong in that debate that lasted a decade but why do I say it is controversial TICT that is because there has been a school of thought a minority school of thought which has said from day one very vigorously that this twist is not important and that school has been led by professor Zacharias K. Zacharias and in areas of TICT whenever I read their papers I felt that this guy is making too much of sound for no reason but then in 2004 or so there was this paper from Zacharias group and this is a pump probe experiment but it is a UV pump X-ray probe experiment and it is easier said than done it is not so easy to do an X-ray probe experiment it is quite in why if you read this paper you will see that there are things that we need to do beyond what we have discussed what is a good thing about having an X-ray probe who are the people who love to do X-rays in chemistry department of course inorganic chemist why because they have such strange complicated complexes right so X-ray crystallography is what tells them what the structure is and it is not only inorganic chemist even protein crystallographers love to do it okay so X-rays give you the structure so what this paper did is that it showed now you are doing pump probe and probe is X-ray so it basically gives you the structure of the molecule in the excited state and when once you know the structure of the molecule in the excited state the question of whether it is tilted or whether it is straight in the excited state is late to rest forever because you can see the structure analysis is not trivial and also the molecule they used is a little different but what they saw there is they said that there is an a greater amount of linearization in the excited state compared to the ground state they are completely opposite view TICD says the ground state is planar coplanar excited state is twisted what these guys say is that the ground state is not planar it is something like this excited state is more like this linearization so all this discussion falls apart what does not fall apart is the fact that the charge transfer state is highly polar and there is a viscosity dependence okay after this paper not many papers have been written on the TICD but it is not as if the debate is over I leave you I leave it to you to read this paper by Catalan I will only highlight the acknowledgement what you wrote there is that I am indebted to professor Zacharias whose apt criticism of my recent paper on the photo physics of DMABN encouraged me to devote the past 3 months to carefully revisiting existing evidence for this compound so even though the intensity of debate has gone down frequency of papers has gone down it does not look like anybody is giving up so over the last 30 35 years maybe there is intense discussion about what exactly goes on in so called TICD molecules one thing is for sure that charged so ICT part of TICD there is no problem whether it is T or whether it is P or whether it is nothing that is where the debate lies and the useful byproduct if you want to call it that of this debate is that we have first of all got a lot of useful information on these kind of molecules based on which some applications have been designed to me what is more important is that in the attempt to understand this the intricacies of this process we have actually learned a lot about how to perform experiments and how to interpret data if you are going to attempt to answer a question like this to me that is the biggest outcome of this entire debate stop here today we talk about something that is another fundamental that is about another fundamental question how much time does it take for a solvent shell to form around a polar solute or in other words solvation dynamics.