 Okay, now I like the terrible, the... Henrik, your voice is breaking quite a bit now. Oh, thank you. Thank you for the kitchen. Okay. Henrik, your voice is breaking quite a bit. Ken, is it better? I think it was better before. But do anything. I don't do anything. Okay, fine. Just share your screen. I mean, you will see if we are able to see your screen. Yeah. Is it... You're not seeing... We are not able to see your screen yet. It shows it has started sharing, but I'm seeing a black screen as of now. Yeah, we can see your screen now, Henrik. Yeah. Thank you. Your voice is breaking quite a bit. Can you go back to your previous internet connection that you had? No, no, no. Please bear with us. I think he's having some internet connectivity issues and he has left the meeting probably joining again. He's not in. He's not in, yes. He has left the meeting now, I think. He's trying to reconnect possibly. As soon as we see him, we promote the co-host. Yeah. Are the other contributed talk speakers here? Yeah. I mean, we have probably done a test for Stefan, but we have not done a test for Thomas Lam and Ramai. They want to... I'm here. Hello. Okay. Can you hear me? Yeah, we can hear you and we can see you fine. So that is good. Hello. Do you want me to try to share screen? Henrik is here. Henrik is back. I think he's back, yes. Yeah, but I don't know if I will be able to do... Henrik, we are able to hear you perfectly fine now. We are hearing you much better. Okay. Thank you. We are here. It's perfectly fine. But it seems I had problems printing before also. So it seems to be something with the network today here at... But we can hear you perfectly fine and we can see your slides. So I think we can start now. Thank you. Thank you. Great. So, yeah. Thank you very much for the introduction. And I would also like to be in Kerala. It would have been... We would have not had these problems then. So what I'll first talk about is the history of the excited state of authenticity topic. The theory qualitative theory in particular and also experimental manifestations. Then what I see as the scopes, how it could be used in future, particularly for understanding photochemistry and also designing molecules for different applications. But I also want to stress that the concepts have limitations and that it's complications and also pitfalls, particularly these two molecules which are polycyclic or have other functionalities, heterocycles and so on. And then in my second part, I will go in a little bit more looking at signet fission, chromophore design, and also looking on photoreactivities, particularly excited state proton transfer reactions. So the aromaticity concept, it's been around now for more than 150 years. It is rather fussy concepts, but there's very many molecules that are aromatic. About two thirds of all known molecules are aromatic or contain aromatic parts. And we can also see this importance by looking at the number of papers using the concept. So one sees that it's nearly as often used as concepts such as ionicity and acidity. But it's essentially only used in the electronic ground state. Someone can, I think there's a kind of a big possibility to develop new research science when going to the excited states. So my aim broadly is to put the concepts in the broader use. And I think it can be used, everything from design of new photo reaction for synthetic chemistry to solar energy research, exemplifying here by this drawing. So everything from astrochemistry to environmental chemistry. But when we want to take it into applications, these concepts, we must also know a little bit better when it cannot be applied. What are the complications? There's of course kind of complications in assessing aromaticity in excited states. We must rely on computational methods and so on. So going into the history, the concept was kind of excited state aromaticity was first used in the mid-60s by Michael Jewer and Howard Zimmerman when they were looking at so-called pericyclic reactions. They were looking at both thermal and photochemical pericyclic reactions and concluded that those that kind of are found, that occur or allowed, they proceed via aromatic transition states. And those pericyclic reactions that we do not observe, they proceed via anti-aromatic transition states. And this can then be exemplified in these kind of fundamental cycloaddition reactions. They deal solder reaction. We know that it's a thermal pericyclic reaction. It goes via an allowed aromatic transition state. Well then, the 2 plus 2 reaction is not occurring in the ground state. And it's a forbidden reaction in the ground state and goes via an anti-aromatic transition state. And now there's a reversal when going to the lowest excited state. So there, the deal solder reaction is forbidden. It goes via an anti-aromatic transition state. While then, according to this theory, the 2 plus 2 cycloaddition goes via an aromatic transition state and is allowed. So this was kind of developed in the mid-60s. And then Colin Baird, who was a postdoc in 6-State 69 by Michael Juer, he developed this further and used a perturbation molecule orbital theory to look at triplet states of different annulings. And he could then conclude that arguments based on simple perturbation theory indicate that the rules for ground state aromaticity are reversed in the Pi-Pi-Star triplet state, which then means that foreign rings are aromatic and foreign plus 2 rings are anti-aromatic. And we can understand this if you look qualitatively on triplet state benzene and triplet state cycloctatetraene. And if we construct these two annulings, triplet state annulings, from two polynomial monoradicals. So for benzene, we then create triplet state benzene from two annul radicals. And we first look at the type 1 orbital interaction, which is interaction between the singly-occupied MOs. And there this leads, this is kind of a destabilizing interaction because the anti-bonding combination between these two is more destabilizing than the bonding is stabilizing according to PMO theory. But for the triplet state cycloctatetraene, when we constructed from an annul and a pentadienyl fragment, we see that there's no interaction because they have the opposite symmetries, these orbitals. And yeah, it becomes zero, the interaction. But there are also other orbital interactions between this somo and the doubly-occupied and the empty orbitals on the opposite radical fragment. But for benzene, this type of type 2 interaction is nil because we have the opposite symmetries between the doubly-occupied and the empty here. While for triplet state CUT, we have an interaction here because they have the same symmetry like this. And this then means that when summing up the type 1 and the type 2 interactions, we get an energy loss in the case of benzene, triplet state benzene, we get an interaction from 2 allyl while for cycloctatetraene, we get an energy gain upon the cyclization to the triplet state. And this can summarize, as I said, that the Bayard's rule is opposite to Hückel's rule for the singlet ground state. Colin Bayard, actually, he didn't stay on in this field. He went on to atmospheric chemistry, then to environmental chemistry and became one of the textbook author of one book here in environmental chemistry that has sold the most worldwide. But he's still around. And two years ago, we organized the first conference on excited state aromaticity in Sigtuna, here in Sweden. And my hash is, I think, here. So hopefully it will be a more conference. So one can also look on other topologies, orbital topologies, Möbius topology. And this is what Jo-Nitja Ajhara did a little bit after Colin Bayard. And he could then see that when going from the ground state, using the Hückel resonance energies, going to the excited state, in all cases we see a reversal in the side, which indicates then that we have reversal in the aromatic and anti-aromatic characters upon excitation. And it can be summarized very kind of simply as this cube here. We change from an aromatic to an anti-aromatic character when we either change the topology, when we change the number of electrons, or when we change the electronic state from a pi, close to pi, to a pi, pi star state. So understanding the qualitatively we can look also using Weyler-Spomb theory. And this is what Schmold-Zielberg and Jo-Wood Haas did. And then understanding the triplet state, a triplet state, cycle butadiene here, as it has 2n plus 2 pi electron, pi alpha electrons, and then 2n minus 1 pi beta electrons. And this can only be used to 2n minus 1 pi bonds, which means that we describe it with these resonance structures. And it means that we actually have a 4n minus 2 pi electron cycle and two non-bonding same-spin pi electrons. And a 4n minus 2 pi electron cycle is also Hückel aromatic. So they concluded that triplet state benzene, triplet state cycle butadiene based on Weyler-Spomb theory, should be aromatic, or have an aromatic character. And one can then also simplify it like this. Cycloctatetraindicatine, two non-bonding same-spin pi electrons, gives us a triplet COT, triplet state benzene. So we're generating from benzene diketion, which is Hückel anti-aromatic, and we're adding two non-bonding same-spin pi electrons, and we get the anti-aromatic triplet state benzene. So then, I'm going down, and I also can use an approach developed by Marcos Mandardo. If one looks on Hückel's rule as in separate spins, then one has the Hückel's 2n plus 1 rule. And if n is the same as m, then we get the regular Hückel aromaticity. If n equals m plus 1, then we get to bad aromaticity, which means then that a bad foreign rule is just simply twice Hückel's rule when we have n equals m plus 1. And the benzene anti-aromaticity in the triplet state can be understood from the fact that we have 4 pi alpha electrons and 2 pi beta electrons, meaning that it's both pi alpha and pi beta anti-aromatic. While Cycloctatetrain in the T1 state, here we have 5 pi alpha and 3 pi beta, which means that both the pi alpha and the pi beta components are aromatic, according to Hückel's rule for separate spins. So computationally going over from qualitative theory to computations now, there's a number of different tools for assessing aromaticity, and normally one should use as many of these as possible of different aspects. We have the geometry aspect, where aromatic molecules should have a great bond equalization and planarity. And this is often measured by the so-called HOMA index, a harmonic oscillate model of aromaticity, where values between 1 half and 1 1 corresponds to aromatic. HOMA runs here on non-aromatic and negative HOMAS are anti-aromatic. There's also magnetic properties, and here I can look on the induced current, ring currents, if they are paratropic or diatropic. One can also use the Nucleus independent chemical shift method developed by Schleyer and now further developed by Amnon Stangler at Technion and Renana Poran also. And here a negative Nix value corresponds to an aromatic situation, a Nix value around zero, non-aromatic, and a positive Nix value, an anti-aromatic cycle. We also have the energetic aspects of aromaticity, and here one compares a potentially aromatic cycle with a non-aromatic isomer. Among them kind of gets the isomerization, stabilization energy. One can also look on the electron density and see kind of how well distributed are the electrons within a certain cycle. So the first to kind of restart or open the field again was Paul Schleyer in the late 90s. He was applying then the Nix method and looked at a simple annulins, foreign annulins. And if we here look on the cyclobutadiene and cyclopentadienyl cation, we can see that in the singlet state we have for this reaction, we have positive aromatic stabilization energies, which actually indicates a destabilization when we go to these foreign and the cyclic species. And he also observed positive Nix values indicating anti-aromaticity. But then going to the triplet state, we had the reversal both in the energetic and the magnetic aromaticity indicators. So both cyclobutadiene and cyclopentadienyl cation in the triplet state are aromatic. And there's also been earlier, kind of in the 70s, it was EPR studies on cyclopentadienyl cation, which was kind of indicated that it has a pentagonal structure indicating aromaticity and also that it has a triplet ground state. This would support that the Cp plus is an aromatic ion. Okay, so the first explicit application of excited state aromaticity outside the field of pericyclic reactions was posted by Peter Wan in the mid-80s. He was looking at photosolvolysis reactions and he could conclude then that fluorinol is easily photosolvolyzed in the excited state. And he concluded that it's because formation of a 4-pi cationic system aromatic in the excited state. There's been kind of also studies now in the singlet excited states and the most kind of fundamental is one by Peter Karadakov where he looked on the three simplest aniline cyclopentadienyl benzene and cyclopentadienyl in the ground state and the lowest pi-pi star excited states. And here you can see benzene having a negative value in the ground state, of course aromatic and then in the T1 and S1 state anti-aromatic positive NICS values. While we have the opposite trend for CBD and cyclopentadienyl strongly anti-aromatic according to NICS in the ground state and aromatic in the T1 and S1 states and nearly cyclopentadienyl is nearly as aromatic as benzene is in the ground state. So this is just exemplifying the paratropic and diatropic ring currents. So here diatropic ring current indicating aromaticity is clockwise in the way that we have applied the magnetic field out of the plane while the paratropicity is counterclockwise. And this is then for the, you see the reversal when going from the ground state to the T1 state. So the spectroscopic evidence for triplet state damage or excited state dermaticity was also pushed forward by Peter Wan. He looked on dibenzo oxopins and saw then very large molecules in this and analogous molecules and he attributed this the driving force for this geometry change to the attainment of an 8 pi electron system like this which gave the molecular planner structure in contrast to the ground state where it's strongly parked. We could also more recently calculate the aromaticity of dibenzoxopins and asopins and similar compound and could confirm here his postulation. So this is for the triplet state but one can also see the same for the singlet excited state which large negative NICS values for the particular the central rings in these compounds. Another kind of possibility is to show how a substituent patterns can be used to tune the excited state properties and energies. Full veins which are molecules like this they have if we in the ground state they can be influenced by Huckel aromaticity and particularly if we have electron donating groups while in the excited state they can instead be influenced by bear aromaticity cp plus ring and that is favored if we have electron withdrawing substituent at the exocyclic position. We can just simply look at these the colors of these compounds and we can see that they have very different photophysical properties with the dision of full vein having the lowest excitation energy in agreement with this qualitative scheme here. So what we did some years ago was to summarize the theory that had been published up until then 2014-15 and then use this to reinterpret earlier experimental observations and I would if you're interested in excited state aromaticity I would propose to read either of these reviews. There's also been further experimental spectroscopic studies by the group of Professor Dong Ho Kim and Atsuiro Osoka looked on the excited state absorption spectra, triplet state absorption spectra of these two expanded porphyrins one aromatic and the other anti-aromatic and they could see that the spectral features swapped when going to the triplet state and then by linking this to aromaticity indicators they could conclude that there was also a switch in the aromatic anti-aromatic character upon this excitation. One can also assess the energetic stabilization and this is work that I've been involved in in this cyclic tetrothiophene which is chiral and can be separated in its two enantiomeric forms and then one can record the resumization rate and the resumization enthalpy in the ground state and in the excited states and since the central COT ring should become aromatic in the excited state we postulated that there should be a reduced barrier in the excited states compared to the ground state and this is indeed what one observes looking at the CD spectroscopy time-dependent CD spectral one can see that there's a faster resumization when irradiated under sensitized conditions which of course then is connected with the lifetime of this species and from this the iron plots we could kind of determine the resumization enthalpies and there was a significant lowering in the excited state both in the T1 and in the S1 state and with this we could assess the aromatic stabilization energy to be around 21 to 22 k-gel per mole and one can also kind of look into this computationally this being the ring current which actually at the minimum we have a diatropic ring current but it's enhanced when we're going to the transition state but this actually Hendrik you have around 5 minutes for your tutorial talk yeah thank you so yeah one can conclude that excited state aromaticity is actually growing in activity and I think I stress that the scopes I will be talking more about this and the second part but then a little bit about the limitations or how it can be used Klar's rule for the ground state tells that we in polybenzene hydrocarbons we have the most stable isomer is the one where we can form the number of benzene sexteds and one can do the same kind of conclusion combining Klar's rule and bad's rule one can see that this can be used to conclude that this one is the most stable in the triplet state where we have a central bad octet on the pentalin unit and this can I will show it in the second part can be used to design singlet fission chromophores but a little bit about the complications limitations of the pitfalls complications is what computation and method should one use limitations when does the bad aromaticity vanish one gets two large molecules and pitfalls as we see there can also be Huckel aromaticity in excited states negative nix value is not always corresponding to bad aromaticity and we explored this on micro cycles because they are computationally quite complex or intricate one can have several different conformers of these one can have conformers where one has bad aromaticity the complete cycle and one can also have the conformers where one has a local aromaticity on a few of the mono cycles and we looked on different mono cycles benzene rings, furan rings and so on so of course yeah I can varies with computational methods and it also varies particularly difficult for the singlet excited states because we have a limited number of aromaticity indices is primarily just nix calculated with the causes here and here you can see a little bit of a variation how the different functionals be three lip can be three lip and m o six twigs performs these different micro cycles and in all cases here we found that the be three lip when with the loop geometry when assessed by a couple cluster calculations at this level front nears is approximative couple cluster we find that the be three gives the best geometries which is actually in contrast to what it was what it is in the in the ground state and then one can also see that one is different aromaticity different aromaticity indices gives different answers we can see for this eight CFU that with m o six twigs we have essentially non aromaticity but with nix but with homa we get an aromaticity with that method we have also different different conformers in the excited state can be here with m o six twigs we have two conformers and I'll actually skip this one can also have in the excited states and as revealed I'll show this better with this compound here a TMTQ which is a quinoidal compound that can have both game influence both bad aromaticity and Huckel aromaticity we can have central 10 pi electron aromatic cycle and an 8 pi electron bad aromatic cycle and this being a sweetrion is kind of a charge has been charged transfer character we could assess that the bad aromatic structure contributes about 10 to 15 percent and primarily Huckel aromaticity is what these compounds are one can also do a similar analysis for the S1 state which recently was done by Michael Solan David Kasanova and myself one can actually what one must do in order to understand if the compound is Huckel or bad aromatic is to make use of so-called spin separate aromaticity indices and I can go to this one here from this the flow index here the delta flow we can realize that the TMTQ in the triplet state is very far from the correct 100 percent bad aromatic structure so there I would like to and I think it has potential but one needs to be a little bit careful when using these concepts thank you okay thank you we already have a question in the chat box and Alessio is asking could it be mixed calculated outside of the ring plane mode reliable since there will be the lockout of sigma electrons effects in the TMTQ yeah yes let's see where did yeah we did here and Nick's scan so it is actually scanned out down from and up until 5 oz from below the this unit now on the out of plane component and we can see that it's strongly negative so in molecules that are difficult one should not calculate the nicks in the center but one should calculate one or a little bit further out from the ring and we are either doing nicks z scan as here or nicks x y scan so yes it's also what we have done here okay are there any more questions for Hendrik in the meantime okay Swati you can unmute yourself and ask a question now we are not able to hear you do you want to type your question Swati you might be having a problem with your audio do you want to go ahead and type your question in the chat yes please go ahead so while Swati is here her question is here suppose we are calculating Nick's value of a benzene dimer which functional will be more accurate yes yes I would you need to have some well dispersion there but that's general I would well generally we are I think this has to be assessed more kind of in detail it's a little bit unclear but it might not be that what we know from the ground state that applies in the excited state here based on what we found from these couple of cluster calculations on the macro cycles the general conclusion has been that can be trillip should be used for large pie conjugated systems that have kind of aromatic systems in general but this sets I cannot give a definite answer I think it should be investigated further with functional should be used if that this was actually a question this was actually a question from Keith he has now typed her question how can we figure out the pitfall of getting false negative Nick's value false negative Nick's values or yes this is a pitfall that I didn't bring up in some molecules if one takes for example by fennel where one has two triplets or two anti-aromatic benzene rings then in that void or that saturated cycle outside those two two paratropic current would in their outside give rise to a false paratropic ring current and negative Nick's values when running Nick's one should always try to plot the current density either with acid from Reinhard Herges or the gimmick method from Dago Sundholm or some other current density program because the Nick's values can be a little bit shady sometimes so there's a question from Fabio and do you want to ask him you are so sorry very interesting topic and I would like to know whether you can quantify this excited state of aromaticity experimentally because obviously in order to confirm the concept it is necessary to measure it this is a little bit what Dong-Ho Kim has been working on we are time-resolved vibrational spectroscopy but I don't know if it's to quantify is really experimental is really difficult I rather the quantify let's say confirm the concept confirm I mean for example if we measure the it's possible to measure the effect of the ring current in the benzene experimentally I think or at least the effect of the ring current in the ground state of benzene so is it possible to do such a measurement for the excited state I don't know I'm half theoretician but I would not really I would think that the best would be some really ultrafast where you can see a change in the electron density kind of upon excitation that you would for example cycloctatetrain when it's excited if you would see that it becomes more electron delocalized but this would be really ultrafast would I suppose it would be a two second spectroscopy that is what I think is needed so it's unfortunately the the field so far is very heavily relying on computational computation but it would be great to develop experimental means to assess excited state armatism thanks a lot very interesting thank you thank you for your nice questions Fabio are there any other questions for Henry before we move over to his research talk I don't see any so Henry you can now stop sharing this one or you have the research talk in the same screen yeah go ahead with it I need to change but here see their part two and now it's not at the start I need to stop sharing again because it's there it's on the wrong screen there now so do you see it yes it is here now yeah so now I'm going over a little bit more towards applications using it first for signature chromophore design and then for understanding photoreactivity especially excited state proton transfer reactions and as I showed very quickly before one can use the the concept to kind of tune exemplified on the full and then we can change the excitation energies by substituents and this should be a useful way to design using the chromophores and also I showed very quickly that we consider benzene to be a molecular doctor Mr. Hyde the ugly personality came out under darkness and darkness the ugly benzene Mr. Hyde comes out when we are irradiating benzene so continuing on this on the single fission topic first it begins starting on this combination of Clares and Baird's rule where we saw that we can these isomers which can have localized bed aromatic units often have very low excited states low line excited states in the triplet state where we have large localization of the spin density to the central pentalin unit and we can also see that we have some diatropic current in the central pentalin and then in the four outer benzene hexagons so this can be used then we can see that we have around this four pi electron unit kind of influences the energy so if we have the linear we have high triplet energies but if we go to this where we can have localized the doubly bent we can have a localized triplet state character in the electron unit and then what do we need for singlet fission we need to have chromophores which are such that the singlet excited state must be at least twice above the triplet state because the first triplet state we can use one singlet exciton to produce two triplet excitons into neighboring molecules or into chromophores within the same molecule and there's a lot of ongoing theory on how this how it goes from this excited from the first step from the singlet excited singlet excitation to the triplet excitons but we have these requirements that the S1 state should be more than twice above the triplets first triplet state in energy we should also have the T2 state ideally above the S1 state so that we don't have any kind of the T2 state and then further decay there so how can we make use of bed's rule to identify compounds that fulfill these criteria well if I look on the two most prototypical anulines benzene and cycbutadiene one can see that benzene being an anti-aromatic in the ground state to stabilize the molecule and it's anti-aromatic in its first triplet state which means that it's destabilized relatively and that means that we have for benzene a large energy gap between the ground state and the T1 state the opposite is the case for cycbutadiene here we have an anti-aromatic ground state in the T1 and S1 state which means that we have a small gap but cycbutadiene is not the ideal molecule for a singlet fission chromophore it's very reactive and it has a too low T1 energy ideally triplet energies should be around one electron volt so we should be searching in a region and then utilizing this concept that I looking at the different connectives of the topologists of benzene-anulated foreign pi-electron anulines we could identify a series of pentalines that could be interested for singlet fission chromophores the problem with these ones being though that they have very low oscillators strength so very low absorption but one can see also kind of one can identify one can see that when we change the connectivity in certain patterns we have a rather constant energy difference between the S1 and the T1 state because this would then be kind of valuable when identifying new chromophores and yes it actually builds on this model here so that we have compound classes where we have a rather constant energy difference between the S1 and the T1 state then where we have constant size of the exchange interaction then we would be able to just by tuning the aromaticity we should be able to identify singlet fission chromophores and here we looked on substituted full veins for this purpose because one can kind of conclude that the excitation is fairly localized to the central five-member drain and the exocyclic double bond and we can by then enhancing the triplet state aromaticity by either proper substitution at the endocyclic position or at the exocyclic positions we can stabilize the triplet state and according to computations one should even be able to get the triplet ground state which it's not surprising because this species in the triplet state will be a cyclopentadienyl cation and as you saw on one of the slides in the previous presentation cyclopentadienyl cation has a triplet ground state and we can look on these compounds here and just take the ratio between these excitation energies and we can see that for this one we are approaching a value of two which indicates that we are then close to what is required for a singlet fission chromophore and what we did was now to investigate a large series of differently substituted full veins and we could see that the assumption that we have a rather kind of constant energy gap between the two the S1 and the T1 state that assumption holds to some degree there is of course a large spread in the values here but we can see both the S1 and the T1 state goes down in energy when we increase the ground state anti-armaticity here so in analogy to the conclusion based on comparing benzene and cyclopentadienyl and interestingly also the T2 state as you see here does not really vary with the extent of ground state aromaticity so at this position here we will get gradually more full veins which have the T2 state above the S1 state so with this tool a very qualitative tool we should be able to identify using the fission chromophores and presently we are investigating experimentally a series of various substituted pentapole veins but the dilemma with the simple ones is the fact that they also can rotate around the exocyclic double bond and in that way non-radiatively decay to the ground state it seems like that if we bends and elate further we will get compounds that potentially are suitable for signal fission photovoltaics but this is what we are presently ongoing and part here is that I will be talking on photoreactivity and here building on a study we published some years ago on the photochemistry of cyclopropyl substituted anolines we considered that the cyclopropyl group could be used as an indicator for excited state aromaticity but the cyclopropyl group sits on a cyclopropyl group ring opens when it sits next to a radical center and also to a bi-radical but if it would sit next to a cyclic aromatic unit it should remain intact and this is what one can observe while cyclopropyl benzene polymerizes quite rapidly within an hour or something like that while cyclopropyl cycle octatetrain under both direct and sensitized irradiation it only remains as unreacted and qualitatively one can understand this from the fact that benzene cyclopropyl benzene of course will alleviate its anti-aromaticity getting back to something which is partially hygleromatic here while cyclopropyl COT which is aromatic in this when the cyclopropyl group is ring closed and the triplet state aromaticity when the cyclopropyl group is ring opened and this should be an uphill reaction and looking on the triplet state this is actually what is seen very low activation energy for the cyclopropyl ring opening in the triplet state and it was also it was a little bit higher in the I think it was 10 kekel per mole here according to cos pt2 and one can look on aromaticity indices here the geometric index HOMA where the negative HOMA value indicates anti-aromaticity which we have in the ring closed cyclopropyl benzene but as soon as the cyclopropyl group opens we get back a highly positive value on the benzene ring indicating that we have regained hygel aromaticity and that this is then more like a benzene radical for the cyclopropyl COT we have the opposite relationship now the ring closed form has a high positive HOMA HOMA value indicating aromaticity and this aromaticity is lost when we open the cyclopropyl group but is this just due to the spin density at the group at the carbon to which the cyclopropyl group is attached or is it really does it depend on the extent of excited state aromaticity anti-aromaticity we looked on the cyclopropyl ring opening for a set of non-aromatic compounds and the triplet state activation barriers and one could see then rather kind of that the activation energy for these varied with the spin density at the carbon to which the cyclopropyl group was attached so yeah but then what is the how does it differ when we have potentially excited state aromatic and anti-aromatic annulins we looked on this for these species cycle rings with 4n pylacton heterocycles or cycles and rings with 4n plus 2 cycles and then plotting the activation energies calculated for these compounds triplet state activation energies we could see that the activation energies for the 4n species were higher than what would expected if they had been non-aromatic and the species with 4n plus 2 pylacton cycles with the excited state anti-aromatic or R they had lower activation barriers than had there been non-aromatic so this clearly seems to be an effect on the by excited state aromaticity anti-aromaticity on the height of the activation barrier and this fact that we have higher activation barriers for the for the chemical activation barriers for the 4n pylacton cycles is also an agreement with Peter Van's observations in the early 90s because when looking on dibenzooxapine you found that it had a much higher photo-stability than this compounds the hydrobenz benzooxapine and this compound rearranged to some spiral adducts so going over then into the to the excited state proton transfer this is work that we've done together with Professor Judy Vuette Houston and Lukas Karras who is a postdoc in Judith's group and Lukas is very great cartoonist also so we get very interesting nice talk images the starting with salicylic acid here we of course have we all know that one can observe a very large stokes shift which indicates that one has a large structural rearrangement in the excited state and we consider this to be due to a kind of a desire for the for the benzene ring to relieve its excited state and the aromaticity and looking then to Tautomers the in all form and the keto form the in all form in the ground state is of course aromatic with the benzene ring and this one is not or only very weakly and it's not favourable but in the excited state this will be bad and what it can do then by alleviating pushing the proton over to the other side we alleviate relief the anti-aromaticity and this is also in agreement with the calculated mixed values the ground state we have a negative mixed value corresponding to aromaticity but we're going to the S1 state we have strongly positive corresponding situation but when the proton is shuffled over to the acetyl group we have a significant reduction in the anti-aromaticity it's still anti-aromatic but much less one can look on other molecules such as this benzo oxazoles and found here that we have again in the ground state we have negative mixed values for this Tautomer but upon excitation it becomes more strongly positive or becomes positive indicating anti-aromaticity but then when the upon excited state proton transfer we get a relief of that anti-aromaticity and in the T1 state even some I would say some aromaticity and this also kind of goes along the energy changes for this process the Tautomerization is an endothermic reaction in the ground state but then an exothermic in both the S1 and the T1 state indicating that we have a loss or reduction of excited state anti-aromaticity finally we also have been investigated and coupled electron transfer reactions I was participating on one looking on the DNA base base pairs I will not be able to talk on that but kind of I would like to direct you to either of these two papers here and I would like to conclude and say that time is ready to make use of Bert's rule but when using the rules and the concept of excited state we must also be aware of the limitations the complications and the pitfalls so there I would like to thank you now Uppsala looks a little bit like this I prefer much more when it looks like this Uppsala is a very nice student town and good summer town thank you thank you Henry for a very nice talk and now the talk is open for questions so I don't see any hands raised but maybe I can start so relating to a question that Fabio asked in the previous tutorial section when I look at this Keto-Inult-Automerism would you be able to do like transient absorption spectroscopy and that would give you changes in the absorption spectrum when these species can then be studied experimentally hmm yeah that should be yeah I think it's a very good suggestion to I mean I'm not in the field so I don't know I've been just learning from the talk that I've been hearing here so it would be nice if other people yeah yes yes I'm not an experimental physical chemist so I would not but I think it would be very interesting try to kind of match with the computations and of yes with the vibrational time-resolved vibrational spectroscopy yeah okay I don't see any questions but I just wanted to then ask you another quick question this negative nucleus independent chemical shift what does it actually so this is a calculation as far as I understand right so this is basically what we relate this to in terms like in terms of physical term the physical justification I mean in terms of you know when I think about chemical shift I'm thinking in PPM I'm thinking of NMR but the value is in PPM so I'm just trying to understand you know what it's the one place as a so-called BQ atom or a ghost atom in empty space outside and calculates the chemical shielding at that tensor and it would not really correspond to anything which is experimentally measurable so well one can there were Schleyer I think did some correlations to or investigating connection to lithium NMR spectroscopy lithium-6-lithium-7 which also could be used as a probe because it was but I see so basically you're placing a ghost atom and you're trying to see what kind of electric field that is seen on the aromatic yeah yes well the induced recording then the induced ring current so that it becomes the chemical shielding tensor in this point but it's completely atom there's no shielding to it because there is a shielding but it's not a chemical well yeah it's a chemical shielding but it's further it's difficult to explain but it's okay no I understand that now but so then mostly the aromatic systems are showing a negative value and the anti-aromatic systems are showing a positive value so that is how you're distinguishing between the two right yes but I would say that it has there are some caveats some pitfalls so one should always also calculate the current density to try to see what type of ring current do I have it could be that one has diatropic ring current localized at certain atoms and they can then give rise to negative mixed values but in order to classify as aromatic you must have a ring current that goes through the complete cycle okay Fabio do you have a question thank you well I think you comment about the nicks the advantage of nicks is that it is a single number right so with one number it's possible to characterize aromaticity but on the other hand it doesn't have an experimental counterpart so as Eric said a ring current is actually something that is more realistic but it's more difficult to calculate and I would like to ask to Eric about the method of calculation of the ring current because there are several different methods for example besides the acid also there are methods for example used by for example in the group of Dages Sundol you may know that he's also working on this type of research the most realistic and why do you use specifically the acid method we now use acid because it's simple but it's not it's not really the ring or the current density it's to calculate it more directly would be to calculate it or use a gimmick or something the Guillermo Monaco for example or have but we are using here acid more because of facility there but yes there are several different packages I hear you very weakly sorry I'm not sure I should maybe increase the sound just a moment please just a moment I will perhaps in the interest of time Fabio you can type a question in the chat and we can go to the break and Henry can answer your question because we are already going a little bit ahead of time you can just keep on chatting with Henry but in the meantime let's thank Henry for your very nice talk Henry