 So, in today's lecture I am going to continue on optoelectronic materials. In the last lecture we saw about the genie in organic molecules. The organic molecules have special potential to find applications in organic LEDs and this is what we have seen in the last class and I am going to continue giving some examples of how these organic molecules can be used to fine tune organic LEDs and also how this can be used to modulate the display device properties. So, in the next few slides I just want to recap on whatever I have told in the first lecture just to keep in perspective what we are discussing. As I told you this is a simple demonstration of a diode where holes come from here and electrons come from the cathode and when they combine together they form exciton pair and they release a photon and that photon when it is in the middle layer which is emissive layer then the photon can excite the organic molecule as a result we can get the desired light. And also in the last class in the last lecture I touched on the basics of organic LED where I told that this is a transparent anode which is called indium tin oxide which is a transparent electrode and then you have a buffer layer to help grow a hole transport layer and this is the emissive layer and this is the electron transport layer. Now hole transport layer and electron transport layers can be polymeric in type or they can also be small organic molecules, but we are actually talking about the organic molecules here which are called popularly as emissive layers. So at the recombination zone which is the emissive layer if you can bring the number of holes and number of electrons together to recombine at this interface then you get the desired light which is due to the emissive layer. So this is a basic principle of the organic LED there are different issues that we can understand in the last lecture I told you how to get white light emission and also I told you how these interfaces are important because of these interfaces I also showed you another example of how X-applex formation can lead to broad emission that is white light I showed you an example which we will further see today. So what is important here is how these holes can easily come to the emissive layer and how the electrons can come to the emissive layer and thereby they form the excitonic pan which can release a photon here. So the number of holes that you generate the speed with which the holes travel the number of electrons you generate and then speed with which you moderate electrons all are important in order to harvest the desired light at the emissive layer which forms the fundamental principle for organic light emitting diode. I also told you about the mechanism what exactly happens to electro luminescent process because you have a hole and electron combining to form a singlet exciton it can also form triplet exciton but singlet excitons are the ones which ultimately are going to give the external emission which we see in terms of light output in the OLED device therefore we are more concerned about this protocol how we can harvest the light from a singlet exciton. And also I told you in the last class there are two basic organic class of compounds namely molecular materials which are based on ALQ3 or polymers which hold this display technology. By and large it is been observed that this molecular materials are more rugged and it can withstand thermal issues therefore from thermal stability point of view small molecules have been used till now for commercial OLED devices whereas polymers are slightly degradable they are sensitive to both light and air and other issues nevertheless there is a great potential with polymer because you can go for large area display materials so much of work is actually going into polymeric LED which call it as POLED polymer LED so we will come to this issue later but we saw in the last class that there are two class of compounds and in the last lecture we saw two case studies of organic molecule and a metal organic complex they show white light emission and I also discussed with you about why this white light emission is coming in the case of organic molecule I showed you that it is due to confirmation of the molecule if it is in cis form then it shows and I also showed from metal organic complex that is zinc BZT complex I showed how X-Aplex mediated white light emission can come in today's lecture I am going to present to you three case studies again showing the potential of what all is buried inside organic molecule which can show fundamental properties and it is very easy for us to moderate the light emission in OLED devices if we understand the solid state properties of this organic molecules because the organic molecules in solution gives a entire different picture when you consider against the solid state properties so once we study the PL emission in solid state we can get to understand what sort of device that we can anticipate from this group of molecules first I am going to talk to you about selectivity of PL emission with different chromophoric groups in the organic molecule so this is one example and as a passing thought I am also going to give a brief example on hydrogen bonding on the PL emission in bensate resolves how hydrogen bonding can play in tuning the PL properties and on tuning the color PL efficiency mobility how we can use metal organic complexes to vary the PL emission so these three will be the basis for today's lecture as I told you in the last class if I have the confirmation dependent white light emission either if it is a trans DBE or cis DBE molecule how we can tune the white light emission and in the last lecture I also showed to you about zinc bensate thiasol how when you make a device out of it how the interface is actually helping in white light emission now in today's class I am going to talk to you about specificity and selectivity on the photo luminescent properties of pi conjugated bensate resolves molecule why we are laying more emphasis on bensate resolves because this is a class of compound which we all see in everyday life if you remember this is nothing but the cartoon of firefly which we all have seen and specially during winter days or in dark environment in dark environment you will see this firefly glowing and it gives light and then it puts off it gives light and puts off this is nothing but a classic example of chemiluminescence when there is chemiluminescence there is a oxidation reduction process which is going on which actually is responsible for this light emission and this is a enzymatic catalyzed enzyme catalyzed reactions therefore the enzyme is called luciferase because it helps in helping the molecule to glow so what is the molecule that is responsible it is actually called luminal or luciferase molecule because in this molecule the functional groups or the chromophoric groups that are responsible for strong emission comes from this Benz thysol moiety so this is important in fact it is very difficult to isolate this molecule therefore one can actually try to prepare several of this Benz thysol base molecules to see and to understand what is exactly responsible for the fine tuning of colors so with this in perspective we have actually chosen a range of Benz thysol molecules and try to understand what is the role of this functional groups and the chromophoric groups that are present in the molecules which will help in fine tuning the light emission so to this end I am going to pick out some molecules and then try to show you by playing around with different combinations how we can fine tune the color for example this is example of two Benz heterosol molecules this can be nitrogen and sulphur or nitrogen oxygen or nitrogen nitrogen so you can call this as a Benz heterosol in general or Benz thysol if it is sulphur Benz oxysol if it is oxygen Benz imidazole if it is nitrogen now when you have two of this these are planar molecules and you can try to have a extended pi conjugation by introducing a linker molecule such as ethylene or you can have a linker molecule such as phenyl group so either ethylene or phenyl group can be used to provide a extended pi conjugation so in this case how does the electron density really moderate and how we can try to control the PL emission it is in such pi conjugated systems another way of looking at it keep both the Benz thysol moieties away from each other not in closer proximity therefore you can have a linker like this and substitute the Benz thysol in one four position so electron pathway will be in this direction so if it is either this way or this way or this way how does the electron delocalization in this molecules which is going to affect the PL emission is what to be studied now as a general protocol synthesis of this is very important and if you are really interested to know how we make this sort of molecules it is very easy we can try to make malic acid or we can try to take carboxylic acid with the carbonyl carboxyl groups in adjacent position or you can actually have the carboxylic group in the para position now a or you can either add this with a or with a and b or with a now in this case if it is a for example you can actually generate three series of compounds based on x x can be either sulphur or oxygen or nitrogen therefore you can call this either as thiozol or oxazol or imidozol similarly we can actually try to add here we can try to put either one thiozol and one oxazol or one thiozol and one imidozol therefore you can get a mixed ligand complexes with benzene as phenyl moiety as your linker or we can actually go for other combinations therefore we are going to generate say four sets of molecules and we will try to see generally what is the what is the nature of the PLM mission to understand how this chromophoric groups are going to alter the issue for example let us take the case of the absorption this is the absorption data of one a molecule and this is the emission spectra of the one series now if you see here you can very well understand that if it is a thiozol based molecule in thiozol you see the bandages shifted in the same bandage at by over 70 nanometers it is more towards the visible region in case of thiozol species and imidozol is blue shifted here you can see that imidozol is having a bandage at 400 nanometer now if you go for oxazol interestingly oxazol has a sharp bandage somewhere around 400 but then you can see that it is actually tailing and extending up to 550 so oxazol seemingly have a extended optical bandage compared to thiozol and imidozol but distinctly you can see there is a large shift between the thiozol and oxazol when compared to thiozol compounds so what is this implication on the emission data if you look at the emission you can very clearly see that this one a molecule has a fine structure emission structure which is more resolved and you see several satellite features coming along with a peak maximum which is coming at 550 now compared to that if you look at one b molecule which is nothing but your imidozol imidozol gives a broad emission peak and one c compound which is oxazol is clearly red shifted so we can see from here one which is absorbing at the visible region or which has a bandage more towards visible is actually showing a featured emission which in which if you actually look at the full width at half maxima the full width of half maxima is around 165 so 165 nanometer which means this is more like a white light emitting molecule whereas if you look at the imidozol although it is it is showing a one peak the full width of half maxima is considerably less and oxazol component shows a very narrow emission and it is more preferentially in the red region so we can clearly understand that the thiozol might is seemingly have a white spread in the emission feature and it is also showing fine structures in terms of emission now we will look at another group of compound the two series where you are actually instead of having alkene linker you are going to have a phenyl linker if you have a phenyl linker then the situation now remains the same as far as oxazol is concerned you see here oxazol after a sharp bandage here around 350 nanometers you see that there is a tailing and this extends up to the entire visible region oxazol seemingly is having it is having a band gap which is much much lower as a result you can again see in the emission feature that the 2 c molecule which is nothing but your oxazol derivative oxazol derivative again showing a red emission compared to the thiozol and imidozol case when you look at the bandage of imidozol when you have pi electron you can clearly see that the imidozol is extending pi bandage up to 450 or so and this is while the thiozol is actually showing a blue shifted emission which is reverse of the one series compound nevertheless you can see here the emission spectra the emission spectra of 2 a molecule again shows a full width at half maxima which is 153 nanometer which means even the 2 a molecule that is thiozol molecule shows a white light emission full with that half maxima which is greater than 150 nanometer. So, what is peculiar in both the cases one series and two series thiozol are showing a featured emission and it is also showing a near white light emission whereas, if you look at oxazol whether it is with the phenyl linker or with the alkene linker you would see that it is showing a preferred red emission imidozol on the other hand always shows a green emission you can see here imidozol always shows a green emission. So, this seems to be characteristic of all the bends heterosols now if you go to three series where you are going to actually play around with your x in other words this is a mixed bends heterosols you would see the same feature coming for 3 c molecule whereas, where 3 c is involving a oxazol and imidozol ring you can see that the bend a bandage is extending up to 500 nanometer whereas, 3 a and 3 b molecules are predominantly those which have thiozol and which shows a strong blue shifted emission now if you look at the 3 c molecule again wherever oxazol is involved you see these are red shifted therefore, it is showing orange color whereas, if you have the 3 b molecule which involves imidozol again you see a green emission and in the case of sulphur and oxygen combination oxazol and thiozol you see near u v emission coming somewhere around 4 10 nanometers or so. So, what is specular in this case is when oxazol is present oxazol predominantly contributes to red shifted emission whereas, sulphur shows more preference to blue and imidozols usually show preference to green color and the same is true for the 4 series also if you are going to keep the bends heterosol rings away from each other not in closer proximity then you can see that there is no tailing of this bandage and there is no tailing here and you can also see that these have sharp bandages below 400 nanometer and you can also see that the emission spectras are more concerted towards blue region. So, this is quite true of both the thiozol as well as imidozol moieties if you look at the absorption spectra you can actually do a density functional calculation DFT calculation and if you try to generate the u v visible spectra to see whether this is matching with the with the experimental data one would find that there is a excellent agreement of the theoretical prediction of this u v visible spectra with the experimental value this is the experimental curve and what you see here is a one line spectra and here you see a two line spectra exactly resembling that of the experimental data why we do not see all this featured emissions because in when we are calculating the u v visible spectra you are actually considering only one molecule whereas in the experiments you actually consider hundreds of molecules together. So, when you have hundreds of molecules in solid state when they are packed together then you would see a featured emission whereas when you do a calculation for a single molecule you see the most predominant emission which is mentioned. So, we can say that the theoretical analysis goes well with the experimental data therefore whatever structure that we are predicting due to optimization of this geometry is nearly the same as that of what we observe in the experiments and these are some of the energies and the oscillatory strength of this molecules which are calculated specifically we have studied one a and two a compounds because we would like to see whether we can make some correlation and try to understand why one a and two a molecules among the series of compounds that we are studying are standing out and why they have a specifically a white light emitting properties that is the whole notion and these are some of the data that one can arrive at by doing the calculations. Now just to give a visual picture of what this calculations mean if you look at the frontier molecular orbitals and if you use this B 3 L Y P program which with this subset if you have this basis set then this is the sort of charge resonance picture that evolves when you consider one a molecule. Remember one a molecule is nothing but two benz thaisol groups coupled by ethylene linker and two a molecule is nothing but two benz thaisol groups coupled by a phenyl linker. So, this is what we mean and if you look at the frontier molecular orbitals you can see in the case of Homo that is highest occupied molecular orbital the charge resonance is spread around the ring and this also involves the alkene moiety ethylene moiety and if you look at the lowest unoccupied molecular orbital again you see in the excited state the charge resonance is spread along the ring and even so in the case of Lomo and if you look at one of the higher excited states Lomo plus 2 you would see the charge resonance is now confined more to the benz thaisol rings and also you would see the charge resonance is actually mediating across the sulfur bonds or sulfur atoms why because sulfur atoms are much bigger and they carry lone pair of electrons therefore the charge resonance is not only through the bond but also through the space. So, you have a through space resonance in the case of sulfur atoms which is peculiarly absent in the case of imidazole or oxazole. You would not see the same picture if you look at oxazole or imidazole compounds now this is not only true in the case where you have sulfur sulfur in adjacent positions in cis positions with a ethylene linker. Suppose you try to push it away by introducing a phenyl linker then even in terms of phenyl linker you would see here the charge resonance is through the bond across the linker molecule and it is the same as in the case of 1A compound and when you come here to lumo plus 2 again you see that in this case the charge is confined more to the benz thaisol rings and also you would see that the charge resonance is across the sulfur atoms. Therefore, the bigger the atom and more the charge density in other words because of the availability of the lone pair of electrons now you have the charge resonance is spread through the space. So, when you have a through space and through bond resonance then you would see a very different emission phenomena happening because there is more of charge delocalization as a result you would have a broad emission coming into picture. So, the broad emission that we talk about broad emission is mainly due to the through space and through bond resonance that is typical of only sulfur compounds compared to imidazole and oxazole. Now, if you take 3A compound as I told you 3A compound is not having thaisol ring in one case it is actually imidazole and in one case it is actually thaisol ring. If you have such a combination you would see that there is through bond resonance here and also here, but this resonance is actually delocalized sorry it is localized and it is not spread throughout the ring and same as the case for 4A molecule also you can see that charge resonance is across the ring and it is across the ring here and there is no through space resonance. Therefore, through space resonance is absent in the case of 3A and 4A molecule and as a result we can sort of make a general conclusion that the featured emission for thaisoles is mainly because of a extended charge resonance which is spread throughout the ring which brings down the homo lumo gap and because the homo lumo gap is smaller the emission is now involving the blue, green and red region as a result you are full with that half maxima is always bow 150 nanometer. So, these sulfur based molecules or thaisole based molecules can act as potential ligands for a for extended charge localization as a result they are good candidates for white lead emission compared to imidazole and oxazole compounds. We can get some more clues when you look at the 1A and 2A compound what is different here is only the linker that is the ethylene linker or the phenyl linker you would see the time decay curves if you look at the time decay curves and you fit it to a double exponential decay model then you would see both has a same component that is 0.7 nanosecond time decay which mainly comes from the thaisole molecules which we can conclude because both have a common common tau 1 value and the tau 2 seems to be coming from the linker in this case it is 1.42 seconds it is comparatively faster for aromatic linker compared to ethylene linker. So, we can have a guess as to what is the molecule or what is the functional group or the chromophory group that is responsible for this time decay the fast decay component seems to be coming from the benz thaisole as compared to the linker groups. And we can also extend this case now to make devices for example, we have made 2 devices using 1A compound and 2A compound because of all the compounds I have told you that only these 2 compounds seemingly give white light emission as a result we can try to make emissive layer either depositing exclusive 2A compound layer or we can mix it with PFO as a matrix if you are unable to effectively deposit this in a thin film situation. Now, whatever be the case whether it is spin coated with a PFO 1A composite or whether it is vacuum sublimated thermally evaporated layer one would see whatever be the device configuration you can see almost a similar feature is seen in the electro luminescence. This is the electro luminescence data electro luminescence data and one could see that the full width at half maxima is nearly 150 nanometers as a result in both cases you would see there is a white light emission coming out of this devices. So, conceptually we can make some logical conclusion or summary to say that benz thaisole molecules preferentially show white light emission and the devices that we have made has this sort of energy level picture which really falls well because your homo lumo level has to be compatible to I T O and to the cathode and which exactly happens therefore, the holes are injected this way and electrons are injected this way therefore, the combination exactly appears at this interface and similarly in the case of 2A compound you can see that the holes can come easily and then electrons can go this way therefore, the electron hole pair can actually have the fusion occurring here and therefore, you can get the white light emission. So, as you are choosing a molecule one should also know that the homo level and the lumo level should be comparable to the adjacent layers otherwise you would not get the right set of devices and incidentally in the case of 1A and 2A compound we have a excellent match where the device falls in picture and as a result we can harvest a white light emission. Now as a passing thought I would like to sort of give another small idea about what really happens if hydrogen bond exists in this sort of molecules take the case of same phenyl linker, but in this case not Benzthaisol we are actually having imidazole moieties. So, this is imidazole moiety now this imidazole can actually have the NH groups facing outward or the NH group 1NH group facing inward. So, depending on that I can either talk about the possibility of a intermolecular hydrogen bonding or I can talk about intermolecular hydrogen bonding both are possible in such cases you can actually expect different sort of PL properties and that is exactly what happens there is a blue two green shifted fluorescence when you have a inter or a intermolecular hydrogen bonding and I will show you as a quick representation how this happens. Now if you take the synthesis part as you synthesize this molecule one would actually come end up with D I B that is dibenz imidazole benzene and this compound we mark it B because it gives blue color. Now if this blue colored compound if you try to vacuum sublimate what happens on vacuum sublimation you get a compound which is actually showing green in color. So, this is a simple sublimation process, but during sublimation a blue color compound goes to a green color compound which is of interest to us and we can try to see what really happens. Now you can take this blue color compounds which we call it as D I B B and you can take this green color compound if you look at the thermal analysis you can clearly see the one which is having intermolecular hydrogen bonding suddenly collapses and then the molecule undergoes a 100 percent weight loss and this happens somewhere around 500 degree C when you have intermolecular hydrogen bonding. Whereas in the intramolecular hydrogen bonding which is the case of the green emitting molecule you can see that there is a sharp fall around 400, but then it extends down up to 1000 degree which means the intramolecular hydrogen bonded compound is more thermally stable compared to intermolecular hydrogen bonded compound and because of this hydrogen bonding network the solid state property of these two compounds will vary. As a result you would see what happens to the absorption spectra the intramolecular hydrogen bonded one absorbs up to 450 or so whereas the regular hydrogen bonded shows a very sharp optical bandage and that is narrow at 350 nanometers and because of this reason you would also see that the emission spectra is very clearly different and what is the difference here you can see here the blue emitting molecule is showing your emission spectra somewhere around 380 nanometers whereas in the case of green emitting molecule the emission spectra is somewhere around 520 nanometers and if you compare the solid and the solution spectra emission spectra of the intramolecular hydrogen bonded molecule you can see that there is a collapse of this intramolecular hydrogen bonding once you put it in solution as a result there is a red shifted emission in solution compared to blue emission in the solid state whereas there is no such feature present in the case of intramolecular the salvation does not seem to affect. So this is a classic example to show that hydrogen bonding controls the PL emission even if it is a small molecule you can have a precise fine tuning of color if you if you can hold the hydrogen bonding which will decide the PL emission. So we have seen two case studies in one case I have shown you a spectrum of molecules and I have tried to impress upon you how the sulfur based compounds lead preferentially to white light emission and I have shown you some examples including devices as to why they show white light emission and also I have shown you in the second case how hydrogen bonding can affect the PL property. In the third case that I want to present today before I close is the photo and electro luminescent properties of a quinoline complex and this is called as a solid solution clathrate. I want to share with you here the excitement because all of us know that this is the most famous molecule called Al Q 3 molecule which has three quinoline units bonded to this it forms octahedral complex and this Al Q 3 molecule is one of the well known emissive layer in organic LEDs because it is also used in today's display devices including those display in digital cameras. Now Al Q 3 cannot just be the only molecule we need to look for newer molecules for new properties therefore what will happen suppose I take aluminum indium gallium these are in the same group therefore if I form Al Q 3 indium Q 3 gallium Q 3 is it possible for me to make one complex having three metal ions in the same complex and this is possible only if there are some number of molecules in a single unit cell then you can distribute atomically all these three atoms and in that case you can get all three atoms in one molecule which we call it as solid solution and this is possible because there is one polymorph which is called as a clathrate it is a caged structure or it is a larger molecule with pi stackings and I will show you some of the features of it this is a typical crystal structure of a alpha isomer of Al Q 3 and even in Al Q 3 there are different polymorphs and this is typically the x-ray crystal structure of a Al Q 3 alpha isomer and the Al Q 3 can either be in meridional form or facial form meridional isomer has a structure a core structure which has two oxygens in close proximity against one away from these two oxygens or these three oxygens can be equidistant so based on the way these oxygens are arranged either you can categorize this as facial isomer or meridional isomer but there is also one other phase which is called as clathrate and this clathrate phase is a low temperature phase if you try to heat this clathrate actually if you sub limit then you come across the alpha form or these are all categorized as meridional forms and there is another form called facial form so there can be inter conversion from meridional to facial or you can sub limit or you can suspend it in acetone and you can suspend it in acetone and you can realize either alpha gamma or delta form and you also have this crucial polymorph which is called clathrate which is actually made out of Al Q 3 complex stacked with several of the solvent molecules therefore this has a bigger crystal structure compared to the alpha isomers or the mer or facial isomer so we can actually try to look at the clathrate form and see if you can make all three atoms in a single molecule so if you look at the alpha form here z is equal to 2 so suppose you have gallium Q 3 you can see two gallium atoms sitting in this picture and this is the situation for a clathrate you can see there are four molecules of clathrate sitting in a cage structure and this is by and large a bigger one therefore there is a possibility in a single unit cell it is possible to put aluminum gallium and indium together in atomic level precision and how can we diagnose that if you take the x-ray diffraction peak the alpha isomer typically looks like this the delta isomer and the beta isomer they all have a characteristic pattern but essentially they all have only three quinoline moieties but the way they solid state packing goes they all differ from each other therefore they are called as different polymorphs but if you look at the clathrate this is called methanol clathrate the methanol clathrates have rather a very simple x-ray pattern which has a 100 percent peak somewhere around 10 now if you have a 10 degree peak then you can carefully try to look at al Q 3 clathrate gallium Q 3 clathrate indium Q 3 clathrate and try to see whether they all have the same x-ray peak and therefore we can try to look at what is the order of this solid solutions I will show you the x-ray peak the x-ray diffraction data of al Q 3 gallium Q 3 indium Q 3 and the solid solution if you see here all these peaks have a common 100 percent peak at 10 degree which means principally they are all x-ray isomorphous and because they are x-ray isomorphous it is possible to have all the three atoms together in same molecule why we are looking at it if you look at the aluminum Q 3 this has more p l efficiency compared to indium Q 3 because as you go down the series the p l efficiency goes down whereas if you look at the mobility as you go up the series the mobility decreases in other words mobility of the charge carrier is much better in indium Q 3 whereas the p l efficiency is low whereas in this case p l efficiency is much better in aluminum whereas the charge mobility is comparatively less because of the ionic size and because of the electro positive character so as a result now we need to look at two things one p l efficiency goes down as you go down the group and the ionic mobility of the charge carriers goes down as you go up the series so you need a compromise as a result one can go for a solid solution which will have both p l and mobility values in a compromising ratio and for this reason we can try to prepare this complexes how do we prepare such complex individual complexes you can prepare by taking the nitrate and this is hydroxy quinoline so if you reflect this at a optimum condition then you get aluminum Q 3 or indium Q 3 or gallium Q 3 now if you want to prepare a solid solution what you do you take this in stoichiometric proportion if you reflex it then you get this compound as you see although you take point three point three point three in net you seem to get only this sort of compound as a solid solution which means this is the most preferred stoichiometry at which the solid solution can be stable so you can see the x-ray pattern is exactly matching with the individual compounds. How do we know that this is a solid solution you can take the secondary electron image and the backscattered image secondary electron when backscattered image will give you clue whether it is a simple mixture of all the three complex together image you will see they are all in different colors in black and gray resolution they will all be in different shades but what you see here from this picture to this picture almost the color contrast is exactly the same therefore this is not a mixture of three complex but this is a solid solution and because it is a solid solution the stoichiometry at this point is going to be the same at this point the stoichiometry in any given place will be the same and that is the simple definition of a solid solution precursor now we have tried to refine we can try to refine this data as you see here al q 3 and a g they show the similar x-ray pattern and we can do the rate well analysis and we can see that both are crystallizing in monoclinic form and these are the structural datas but what is interesting that I would like to point out to you is the absorption spectra you can see that the absorption spectra of the al q 3 indium q 3 gallium q 3 and a g they all show similar feature which means the pi to pi star transition and the n to pi star transition you can see is happening at around 260 nanometer and somewhere around 380 nanometer this is due to n to pi star transition and this is due to pi to pi star transition and it is all the same for both the solid solutions and individual molecules but interesting point here is the emission data if you see the emission data we exactly have got what we anticipated because al q 3 and indium q 3 they have a very sharp change in the p l intensity al q 3 is highly luminescing compared to indium q 3 whereas if you prepare a solid solution which involves all 3 molecules atoms together then you can see that it is exactly place in the middle so this is your a g which means I am able to now make a compromise between the mobility of charge carriers and the p l intensity by bringing all 3 metal ions together in a single compound and this is possible only because it has a clathrate feature. So by knowing the crystal structure by knowing the x-ray pattern it is possible to find you to get the right composition which will have dual property of both the mobility as well as the p l emission in one single form and you can clearly see that if it is al q 3, gallium or indium they have preferred emission feature and if it is d i g in solid it exactly comes between these solids indicating that these are really the solid solution really the solid solution and the solid solution seems to be very typical of its structure and therefore you do not see much of a change whether you record it in solid or in liquid form and you can also try to fit this to a double exponential d k you can see 2 components tau 1 and tau 2 and mostly coming from the quinoline moieties and these are all some of the data's that we can agree upon and try to see how the solid solution is performing in comparison to the individual molecules you can see that the absorption is placed somewhere closer to indium and gallium and then the p l is exactly mid place between aluminum and gallium complex and so is all the homo luma values everything is recorded which clearly shows that such a solid solution can be achieved now this is the electro luminescence data e l spectra of a device which you can make with i t o as your anode then p dot p s s as the buffer layer t p d as your whole transport layer and this is the emissive layer and then you have the electron transport layer and this is the barrier and this is your cathode so in a device configuration like this you can see that a g is giving a nearly good green emission and one can see that the current and the light intensity curves are going hand in hand with a turn on voltage somewhere around 6 volts fairly good one and one can achieve a compromise between p l and mobility if one can actually play around with the amount of aluminum indium and gallium that you can take in a single solid solution and this is the e l spectra which shows that this is nearly a greenish orange emission greenish orange emission that is coming out of this e l spectra so what do we see here the clathrate architecture with z is equal to 4 formula unit in a unit cell facilitates doping of different metal ions in a single unit cell the essential feature that helps in realizing such a solid solution is the x ray isomorphous nature the e l device suggest that a g can be used as a emissive layer in the device structure with a low turn on voltage having said this I just want to make some quick summary in the first place we looked at the specificity of these aromatic compounds Benz heterosols and I have shown you that one a and two a preferentially shows white light the imidazoles show green and the oxazoles show red emission and a combination of thiasol and oxazole or thiasol and imidazole leads to either orange or to blue so with this notion one can say that there is a very definite way more like a thumb rule one can try to exercise in rationally preparing the decide organic compounds that will give you a preferred emission and of all the library of compounds studied thiasol insist configuration show selective white light emission extended pi conjugated structures not only alter the band gap but they give more thermal stability useful for device performance and lastly I have shown you an example where x ray isomorphous clathrates can help us engineer solid solution clathrates to get a compromise between p l emission and mobility of the charge carriers so I stop here with this examples and I will try to discuss in the next lecture how inorganic first can be used for display technology.