 Hi everyone. Hi everyone. I'm Giovanni and I'm going to talk to you about femtosecond clearance spectroscopy of large porphyrinan rings today. So I'm going to start from the basics. So I'm going to start by describing a simplest, let's say time result, experimenting the femtosecond picosecond time domain, time scale can be described. So in the simplest experiment that we can conceive is called, let's say in the visible range is transduction. So it's a two poles experiment. We have a first intense and short femtosecond light poles, which induces a perturbation in the sample. And after some time, we look at this perturbation by a second. The weaker poles, which is usually Broadman is typically a white light in the modern days. And these second poles we look at in the frequency domain on a detector. What we do here is to that the pump pulse is usually modulated at a frequency so that we can record sort of sequential spectra in of the probe in which we have the sample which has been perturbed by the pump or not perturbed by the pump. And then we can perform a subtraction of the, let's say pump or minus pump of spectra so that we record differential spectra between the two different conditions. And if we do so for a series of time delays, different time delays between these two laser pulses, of course we can recover dynamical evolution of the system in real time, even on really fast and short time scales. And on top of that, if the pump and probe actually as well pulses are short enough in the time domain and broad enough in the spectral domain. On top of that we can also access to coherent dynamics of our molecular system or let's say solid state system if you're interested in that's what we're interested in. How do we, what we can learn from these differential spectra so basically a different spectrum you can have either positive or negative signals. If we basically have like more light that reaches the detector when the sample has been perturbed by a pump or if you have less light. So here in this figure I'm showing of molecule at increasing time duration in different colors and so we can see that we have positive and negative regions. So the, let's say the first negative region wants to look at is the one enclosed by orange square. And that's what we call ground state bleach so it's a negative signal. And it's called by the fact that the pump excites the sample so the probe sees a sample which is more transparent let's say that when it's not perturbed. And that's a negative signal as we can see. And of course usually matches in the spectral domain the steady state absorption of our molecule. And so to the right side in the wavelength domain, we have another region where have we usually have negative signals which are called stimulated emission. So this is when the basically the pump excited sample the probe sees an excited state and it stimulates that excited state to decay back to the ground state emitting an identical photon. So we have more light reaching the sample in this case and so this is negative again, and if you move to the blue instead in the green rectangular thing. We can see that we have some positive signals and this is called excited state absorption. These doesn't have a steady state correspondence and it's basically the excited state that is prepared by the pump, which is totally excited to, let's say, higher energy excited states by the pro pulses, and of course if the first pulse generates chemical reaction triggers a chemical reaction and we produce a photo product that for the product is a new thing that's not there in the ground in the amplitude sample so will that will produce a positive signal as well. We said that we posted which are short enough on top of population dynamics with these techniques we can look at coherent dynamics in the time domain indeed so what happens is that when we have a short palm polls, namely shorter than for example the vibrational period of molecular vibration, we can impulsively excited these mode, creating a way packet that can be either in the excited state which is more intuitive, but also in the ground state. I'm talking about electronic states now, and that is due to a sort of ramen kind of like process. So those way packets of course will evolve over time they will move around in the ground or excited state electronic potential energy surface and as a consequence of that. We will modulate the energy gap between these two electronic states as a function of time and that is seen in the spectra as a sort of oscillation in the time domain. In order to isolate these oscillatory behavior which what we're interested in when we're doing femtosecond coherence spectroscopy is that we take the pump roll the trans absorption data set and we get rid of the underlying slow population decay which is shown here in the bottom as a sort of global solid line fit to the experimental data which are in teal and what we have left is basically those oscillation around like zero amplitudes which are the what the features were interested in. So we said that we have both ground and excited state contributions so the problem is now how do we assign that how do you like separate that between ground and excited state. So the first thing one can do is think and say okay if I see a beating and oscillation in the time domain on top of a ground state bleach signal. That's probably a ground state wave packet so vibrational coherence in the electronic ground state if these beatings are seen on top of stimulated emission or excited state absorption it has to be a vibronic coherence in the excited state electronic excited state. But another thing that can facilitate the assignment is that those oscillation carry phase, which is depending on the probe wavelength, and they tend to have a sort of destructive interference for a frequency that matches the steady state absorption molecule for the ground state coherences or the, or the other match these know these amplitude not caused by destructive interference matches the maximum of the fluorescence emission for each other state with packets. And so, this is seen of course in the amplitude isn't as a deep, and it's seen as a jump of the face by pie in terms of like when we're looking at the phase of those oscillation. When we have these things, we can look at them in the frequency domain also basically performing a photo transform over the time pro delay time to yield sort of frequent pro frequency result Raman spectrum so Raman spectrum for each of the wavelengths contained in the probe bandwidth. Okay, this is the technique for we use it to look at those large zinc porphyrin amaryllis. We look at those molecules called they're interesting because in a way they resemble some kind of pigment protein complexes that we are found in bacteria. In bacteria, we normally have a number of chlorophylls which are sort of put on a ring shape. And here it's porphyrin so it's still tetrapyrus they're bound in a covalent fashion by those sort of diacetylene linkers. And what's interesting is that we observe, we can tell from the steady state absorption spectrum and from also time result measurements that the exit on structure of these molecules completely different, namely the 10 member nano rings are fully sized the 20 sort of transition regime and 30 and 40 despite the ring size, increasing the accident does not span more than 20 promo for units as what could tell from the steady state absorption which are the solid line in here, which are red shifting as a function of the ring size up to 20 and then the state constant. And that's quite interesting because despite like looking molecules are the same family actually the electronic excited states are quite different in character. And there's no indication also by chronic coupling so any vibration which is evident from these steady state absorption. And here as shaded blue and red areas I've reported that pump spectra that we used to study this morning so we basically change the wavelength of the pump to see the dynamic for any difference. And we found out that indeed they are so these are the femtosecond clearance spectra here on the left. This is what we get when we excite with these blue poles. And this is what we get when we excite with these like red poles. So we can clearly see that in the time domain. And also in the frequency domain, basically these two graphs are the full transform of these ones. So here we have the delay between pump and probe and here we have the wave number which is the full transform of these. The nodes of these oscillation moves. And when we excite here on the blue side of the steady state absorption spectrum of the molecule we get a node which is more or less here. When we excite on the red side of the molecule we get a node which matches really well the maximum of the fluorescence emission. So what we see that pump wavelength tuning allows us to observe better wave packets in the brown state, according to the theory explained earlier on, or wave packets residing in the excited electronic state of this molecule. So a further confirmation of these is given if we take a slice of these sort of probe resolved Raman spectra at a frequency that we care about which is these one quite intense 370 wave number mode. And we can clearly see that the amplitude as a deep, which is very close to the maximum of the absorption when we excite on this side, and the phase which is in blue jumps by pi and then jumps back. While when we excite in resonance with the fluorescence we get the nodes in the amplitude and sorry the deep in the amplitude and the phase jump which are matching quite closely the emission, the maximum the emission of this model. So another interesting thing is that the phase is jumping really abruptly here while here it has a sort of wavelength dependence, and these could tell something about the how an harmonic. The excited state potential energy surface is compared to the ground state for example. So what we've done here was basically here what we learn is that exciting with different pump wavelength we can look at ground or excited state with packets so if we now integrate over all the frequencies in the probe. We get basically an impulsive Raman spectrum of the ground or the excited state of these molecules. So here we have the color code for 1020 30 and 40 member ring, and the ground and excited state Raman spectra. So we can see that in the ground state, what we have is this mode at 370 for the smallest ring 375 for the smallest ring that shifts to the red as the ring grows and it also narrows down that has to do with the strain which is induced by those ring structure which is way for smaller rings. And this mode is stretching of the zinc nitrogen bonds so that it's centered in the porphyrin core. Well here we have these lesson 10s mode which is still a ground state mode of course when we look in here, which is not a combination but it's a complete motion that involves both methenic and pyrolic carbon carbon bending and in plain bending and stretching. On top of that we also get strong solvent contribution we are in toluene here because the ground state, because toluene is not resonance so the signal should be weaker but there's also a lot of it because it's the solvent so we can still see all these kinds of contribution that are coming from solvents. If we move to the excited state we can see a few interesting things. So we can see that the for example these zinc nitrogen Raman mode is basically insensitive to the displacement of electron density from the ground to the excited state. And that's because it's applied by star transition which is weakly affecting the molecular orbital involved in forming the zinc nitrogen bonds. And these modes that come from methenic and pyrolic shift to the red quite a lot and it appears indeed as a shoulder here. So it's shifted by 26 wave numbers more or less it's 430 roughly here and here is like 405. And this is due to the fact that due to the symmetry of the molecule that like the porphyrin monomer that belongs to the H2 point group. Basically the those molecular orbitals are more antibonding in the excited state so the frequency shifts to the red. And on top of that we also have this signal which is not there at all in the ground state which is a mode that comes from a sort of distortion of the porphyrin ring as well but it's Raman cross section gets announced in excited state due to hyper conjugation with the substituents on the porphyrin side. So the conclusions are that we were able in this way to observe vibrational coherences both in the ground and excited state that we're assigned looking at the frequency dependence of these node in the oscillations. And by moving these information to the frequency domain we were able to obtain impulsive resonant Raman spectral of the ground or the excited state of these molecule, and surprising somehow conclusion is that here the vibration are quite relative to the degree of the external localization because we had substantially identical spectra for molecules that display quite different excitonic behavior. So the main vibrational modes are indeed coming from the, let's say the monomeric units which are constituting those large porphyrin nano ring and as similar effects as being observed previously in pigment brolin structures I feel like all the beatings are given by internal motor the problem for rather than to the overall structure. So, that's it, I want to thank my PI, Steve Mitch I did my part of my PhD with him as well, and I was an imperial for a while and I came back at you a these are the most iconic building at you a in Norwich in the UK, or these are like student residents are called the students. I want to thank Professor Harry Anderson and Michael, who actually synthesized those nice molecules in Oxford is my that was my former PhD supervisor which is now back in Brazil my colleagues palace and they'll we work on theory and simulation of ultrafast spectroscopy and DPSC for funding our lab. So, thank everyone for having listened to me. And it was interesting. Thank you. It was an excellent talk. Thank you.