Artificial Light-Harvesting System Based on Multifunctional Surface-Cross-Linked Micelles
Hui-Qing Peng, Yu-Zhe Chen, Yan Zhao, Qing-Zheng Yang, Li-Zhu Wu, Chen-Ho Tung, Li- Ping Zhang, & Qing-Xiao Tong
Angew. Chem., Int. Ed., 2012, 51, 1-6. DOI: 10.1002/anie.201107723
By Luis R. Rivera-Ríos (Berti)
Energetic crisis has made scientists examine the efficient chemical processes used by plants for doing work. In particular, into the details of how chlorophyll aggregates harvest Sunlight. In 1990, A. Scherz and colleagues reported (Scherz et al. PNAS, 1990, 5430) the use of the Triton X-100 detergent to encapsulate photosynthetic pigments making in the process the first model system using self-assembled micelles. The approached used by A. Scherz to construct the light-harvesting system (LHS) was from a biological perspective. In contrast, the article I’m summarizing this week, although also using micelles, reports the construction of an antenna from a more “chemical” perspective. Here Hui-Qing and coworkers used a cross-linked micelle (SCMs) as a template to first covalently modify the exterior with a donor (DPA) chromophore and, second, to attach an anionic acceptor (EY) molecule using electrostatic interactions. This model system used the “bullet proof” copper catalyzed dipolar cyclo-addition to first make the cross-link micelles and second to modify them with one equivalent of DPA per micelle monomer. The synthesis of the antenna is a one-pot reaction at room temperature and the purification is a simple product filtration after it precipitates out from the reaction mixture. 1H NMR was used for monomer characterization; IR spectroscopy (alkyne stretch bands) was used to monitor the attachment of the DPA; and dynamic light scattering was used for the characterization of the micelles before and after donor attachment. The resulting active micelles range from 15 nm to 25 nm in size. If higher efficiencies are desired for the energy transfer process, the composition of the system could be easily modified by attaching a different alkynyl donor and by adding a different anionic acceptor. In this example the resulted LHS presented no self-quenching with a donor quantum yield of 80% and with no excimer formation. DPA–SCMs have a λext at 330–420 nm and a λem band at 390–520 nm, this emission is complementary to EY λext at 530 nm. By using this strategy the dipole–dipole coupling-mediated energy transfer was not lost and self-quenching was prevented. All of the luminescence experiments (and the corresponding controls) show that the donor contributes directly to the acceptor emission. The Förster radius was estimated to be 3.6 nm for donor-acceptor and 2.5 nm for donor-donnor. DPA and EY effective concentrations were of [DPA-SCMs] = 23 mm and [EY] = 1.34 mm, this gives us an idea of the low concentrations needed for making an efficient LHS. The authors decided to investigate how the chromophores communicate between them after realizing that one acceptor quenched multiple donors. They concluded that there were two mechanisms of energy transfer: 1) direct quenching, and 2) the “energy-migration pathway”, which is an effect of the antenna architecture. Finally, a model of the LHS was built in which each micelle was divided into harvesting units composed of ~48 DPAs surrounding each acceptor. This work can’t be compared to that of Scherz in 1990, but I still think that the authors should’ve cited that article. This work represents a very promising strategy for the building of harvesting systems due to the simplicity of its chemistry and the efficiency of the product. Although, a better system will consist of an assembly that can self-correct errors within the Förster radius, prior to any needed covalent fixation step.
Commented previously in: http://www.chemistryviews.org/details/ezine/1482681/Efficient_Artificial_Light-harvesting_System.html
Supramolecular Space 
A nice article to enjoy, I like the “light harvesting” field because when you finish to design/construct your system, you can tested and modify according with the requirements that have to be addressed. In this article (collaboration between Yan Zhao and Tong??), an artificial light harvesting system is constructed by multifunctionalization of a cross-linked micelle. Practically, with a click reaction they attach the donors (DPA) that will perform an energy transfer to an acceptor (EY). When they add EY, you can see that the acceptor absorption increase. They study the Micelle structure by DLS and the quenching by increasing the concentration of EY.
Some attributes of this system are not only the antenna effect or light harvesting properties but also the method of elaboration is attractive for their production and applications. In addition, the high quantum yield and the avoiding of self-quenching make this system very promising for further applications. They provide different possible explanations for that behavior, but we have to realize that concentration is the key for that inhibition in self-quenching. Also, micelles are attractive systems for construction of nanomaterials, but how they assign these units remain a little confusing for me. Let’s said that all the micellar components are functionalized, how we can control or known the exact amounts of subunits for this non-discrete system? The micelle will continue accepting or incorporating subunits in these dynamic arrangements or assemblies.
I like the article in terms of the design and applications of the system; I really like the system a lot, because in previous articles this approach was used also to encapsulate pyrene. But I have some concerns about their preparation. First, for the cross-linking with 1,4-diazidobu- tane-2,3-diol (2), how we know the degree of crosslinking here? I think that there are unreacted functional groups and for that reason after the next click, the donor composition in the micelles will vary. For that reason, if we have variation in that composition, how this will affect the energetic efficiency of this light harvesting system? How many times this absorption/fluorescence/DLS experiments were performed? Just to see, how variation in that surface composition affect these properties and the quenching in a certain concentration.
About Bertí’s synopsis, I think that follow the general guidelines, I would suggest to separate the synopsis or ideas in different paragraphs to help the reader. Also, I would suggest a better organization or explanation of the founded results to understand the exposed conclusions. About the picture, it helps a lot to understand this article, it would help if he use the same color of the non polar region in the molecules and micelle components.
In this article is presented the synthesis of an acceptor-donor FRET pair and the self-assembly of the system. They state that one of the main problems in the area of light-harvesting is the quenching of the system resulting in low efficiency. They propose a micelle like self-assembled system with the donor exposed to the environment. They develop a theory that this system would not self-quench due to steric interactions, the linkers of the aromatics are short so for them to stack would be almost impossible and because their donor is positively charge, so the repulsion would decrease the possibility for intermicellar interactions. So the acceptor would simply interact in the surface with the donors or between donors as presented in one of their figures. With proper controlled experiments they demonstrate an efficient donor-acceptor energy transfer, also noticing that the acceptor ratio is much less than the donor. They hold responsible this behavior to possible different paths that the energy transfer, other than the direct energy transfers a donor->donor->acceptor energy transfer which we has seen before. Their results may look as any other kind of FRET paper that we have read before but what is amusing about this system is the structure of the self-assembled pair and the donor:acceptor ratio is replaced by a miscelle:acceptor ratio (1:1). Yet there is no estimate of monomers of micelle or a critical micelle concentration. Not much characterization was presented other than a I.R. with purification/reaction purposes and 1H-NMR for the synthesis. Gotta say that in general the paper was easy to read and very consistent with the presentation of data and discussion, telling you the rational of why doing each experiment.
In regards of Berti’s synopsis I have to say that is was ok and follows the guidelines. I had to read the paper first to later understand the synopsis. Maybe he tried to say as much detail as he could in a small space. The picture was ok, but as the synopsis, had to read the paper to later get the picture.
This paper I really enjoyed: the chemistry is simple and practical, something I really like, and the results are very promising. I guess the authors were lucky: there system doesn’t quench (something I’d rather expect), and the transfer occurs efficiently. I don’t understand these types of measurements all the time, but if I get it correctly, the evidence lies on the emission quenching of the donor dye (the emission of the acceptor would increase anyway, because you increasing the concentration of the acceptor dye). I’d like to know a bit about the authors’ background: what have they previously done that led them to these results. Was the choice of system, dye, etc. simply the result of good luck?
The blogger calls the click-reaction bullet proof, and indeed it seems like the most practical and fail-proof reaction ever: simple to perform, with overall very convenient orthogonality (can be performed in many cases in the presence of a variety of other groups), mild reaction conditions, and a stable, sturdy product: it really has enabled a lot of good chemistry to be made.
Regarding the paper’s narrative: it’s a good, simple-to-read paper. The intro was engaging; actually it sounded like it could’ve been the intro to Berti’s paper. They didn’t say anything about the next step to make this actually work?? I don’t know what more to look out for in the literature for LHS’s. I would’ve like the paper to include a bit more of insight on this: how will they optimize, or is this ready for real world application? What are the drawbacks? The synopsis doesn’t add much to this, but I really liked how it orderly mentioned the characterization methods, which helps get a better view on the paper. I’d recommend dividing in paragraphs to make it lighter to read. The picture was really nice, and I really liked the 3D effect managed with the different sizes and transparencies of the dyes. Very well illustrated.
The construction of an artificial light harvesting system is described in this article. The approach consisted of two self-assembling strategies and covalent fixation to prepare the antenna system. DPA was the antenna chromophore used and Eosin Y was used as the energy acceptor. Then, the authors explain that light harvesting systems have presented problems when there are several donors, these problems include quenching and or the formation of excimers because the chromophores are pretty close. These problems interfere with the energy transfer and lowers the efficiency of the system. This is why most of these systems have a low energy transfer efficiency and when researchers improve it by just a little they become very excited. However, for DPA the quantum yield was 0.80 and 0.90 for the free chrompphore, indicating that this system is very efficient and none of the above factors are interfering. Steric interactions and limited freedom explain this. When EY was added to the DPA-SCM the DPA absorption stayed constant, but that of EY increased. This also lowered the donor emission and increased the acceptor emission to 550 nm, which indicates a Forster energy transfer. A reverse titration also supported this finding. A control experiment with Bodipy as the acceptor showed no energy transfer, indicating that an electrostatic assembly with EY is occurring. Micellization and electrostatic attractions were used for the self-assembly to prepare an efficient light harvesting system, as well as covalent capture. The antenna created showed no self-quenching nor excimer formation, therefore the energy transfer was highly efficient. This system has a potential for photocatalytic devices. This article was really nice in my opinion and it could be understood. The results are really good and, but unfortunately this cannot be achieved with most light harvesting systems because of the physical properties of the compounds involved. Nevertheless, I really liked the article.
The synopsis presented many details of the results, it could have been summarized more. I liked the fact that Berti presented some background information because this helps understand the importance of the light harvesting systems and their purpose maybe. However, my suggestion for the next time is to use paragraphs in the synopsis. The use of paragraphs helps in the organization of the ideas and to divide different topics. A person with limited knowledge of chemistry will obviously not understand this synopsis because of the complexity of the material (they might understand the part about chlorophyll, but maybe by using paragraphs it can be more attractive or something.
Tong, Yang, Zhao and colleagues reported on the design of an artificial light-harvesting system based on a micelle with its surface covalently linked (SCMs) via a simple and convenient click reaction. The resulting surface of the SCMs also have alkyne groups to similarly incorporate antenna chromophores like DPA (DPA- SCMs) and it is slightly cationic to allows its interaction with anionic species like the acceptor EY. DPA- SCMs have very good quantum yields with no detectable quench of the chromophores but still through one pathway apparently the energy can be transfer from donor to donor until it gets to the acceptor (antenna effect). Regarding the latter, I appreciated that the authors made a really good rationalization of the possible light-harvesting pathways complimented with helpful figures. Considering that there are, already reported, so many other light-harvesting systems it seems that novelty relies in the combination of having various nature inspired features in a very efficient light-harvesting unit that is relatively much simpler/cheap to be prepared. For example it was remarkable that in the one pot synthesis, after the first click to have the SCMs, addition of the DPA-N3 in a THF solution generates the appropriate difference in solubility between the SCMs and the DPA-functionalized SCMs resulting in the selective precipitation of the desired product. I guess that the scope of this strategy is useful for N3-donors soluble in THF and some other organic solvents but for those seeking the synthesis of water soluble functionalized-SCMs additional challenges awaits them.
The light-harvesting properties of the system develop by Tong, Yang, Zhao and colleagues were well characterized using various techniques with the appropriate control experiments to validate their conclusions. As for what could be the next step… I have two ideas; the easiest one to implement is to evaluate what happens when increasing amounts of plain DPA-SCMs are added to a solution of the DPA-SCMs loaded with EY. If the emission of EY increase, could this suggest that their system is potentially capable of doing long-range energy migrations between the DPA’ on plain DPA-SCMs and the DPA’s on the DPA-SCMs loaded with EY??? Interesting… My second idea is that the authors could evaluate the scope of their system by evaluating the light-harvesting properties of a family of SCMs having different antenna chromophores/acceptors.
In general the authors made good use of the first two paragraphs highlighting the main challenges behind the design of efficient light harvesting systems. It was recognized that there is already many impressive results but its complexity limits how practical they could be.
I found a very similar example of an artificial light-harvesting system in which the center core instead of being a micelle is a C60 fullerene which can be decorated with yellow or blue boron dipyrromethene (Bodipy) dyes “in such a way that the dyes retain their individuality and assist solubility of the fullerene” (Ref. http://pubs.acs.org/doi/abs/10.1021/ja206894z). The rationale behind this other article by Nierengarten , Harriman, Ziessel and colleagues is very similar to that of Tong, Yang, Zhao and colleagues since both articles try to resemble the natural photosynthetic light-harvesting complexes using structurally similar systems. Unfortunately as many other systems the reference I am mentioned have some limitations and both dyes are covalently linked. This article should have been incorporated as a reference but I guess that this was somewhat difficult to do because it came out available on the web while the article we are discussing was probably under revisions.
There is an interesting webpage in which the “hot topic” is click chemistry therefore various articles are listed to put in perspective the wide scope of click chemistry and they mentioned the article by Tong and colleagues as an example (http://www.wiley-vch.de/util/hottopics/clickchem/).
Berti prepare a very good figure since in an engaging way it includes many details of the light-harvesting unit and highlights one its main energy-transfer pathways. His synopsis was also good; it includes the main findings in a relatively simple way and in its introduction he include additional information to highlight an interesting preliminary work on the field. My recommendation is that he should have included a more critical evaluation of the system described in the article. It would have been a much longer yet interesting synopsis, otherwise might be he could have also sacrifice some details from the narrative in order to make space for his interesting perspectives. Even though he knows a lot about light-harvesting system so for sure he will bring those criticisms in his presentation.
This is a very excited and useful work in the artificial light harvesting system (LHS). The collaborators Yang and Tong groups developed amazing and highly efficient antenna system based on non-covalent and covalent bonds. In this great thing is ‘one-pot synthesis’ of such big molecule. In my opinion this synthesis looks like simple but we have to appreciate a lot for their thought.
The synthesis starts by the covalent bonds formations by mixing in 1:1 ratio of 1,4-diazidobutane-2,3-diol and 4-(dodecyloxy)benzyltripropargylammonium bromide to form link micelles, so here still one more alkyn is ready to use another ‘click’, which up on treating with chromophore 9,10-bis(4-methylphenyl)anthracene (DPA) forms the micelle monomer, then the monomer was treated with an energy acceptor Eosin Y disodium salt (EY) to form LHS by ionic interactions, the electrostatic assembly of the EY on the DPA–SCM surface was confirmed by control experiments. They used IR and dynamic light scattering to determine the formation products and also they calculated DPA concentration in DPA-SCMs system by using the absorption spectrum.
Here they compared the fluorescence quantum yields of individual DPA and DPA-SCMs from this they got very good results (0.9 and 0.8 respectively). The quantum yield result shows formation of DPA-SCMs prevents the self-quenching of energy and from all other experimental results they said that energy transfer takes place from donor-donor-acceptor.
About Berti synopsis, for me it is good and also the figure is very appropriate to illustrate the theme of the whole article. Simply I can say ‘I like it’.
Hui-Qing and coworkers report how they constructed a synthetic light-harvesting system using cross-linked micelle’s (SCM’s). By combining two self-assembling strategies and covalent fixation they prepared their antenna system; using DPA as a model antenna cromophore and Eosin Y as the energy acceptor. The entire synthesis reaction was done in a single pot and precipitated spontaneously. One problem with this light harvesting systems is the proposed self-quenching of the system due to the proximity of the cromophore. However their systems presented no self-quenching with a donor quantum yield of 80% and no excimer formation. Also the effective concentrations were very low [DPA-SCMs}=23 nm and [EY]= 1.34 nm resulting in no self-quenching, which gives us a clear idea of the low concentrations needed to achieve this… Finally they found energy transfer takes place from donor-donor-acceptor
I thought the article was a very interesting and with good-engaging narrative. The way they present their findings is very clear and attractive.
About Berti’s synopsis it was ok and it followed the general guidelines. But, something about its organization was kinda off for me (maybe using some paragraphs to divide the synopsis and his thoughts…I don’t know). His picture displayed the essence of the paper; I got the message
In this article, Tong and Yan present a light harvesting system with the use of self-assembly strategies and electrostatic interactions. They star out by using 9,10-bis(4-methylphenyl)anthracene (DPA) as the donor molecule and Eosyn Y as the acceptor. With the use of cationic surfactants, they were able to construct micelles, which were covered with alkynyl groups. They modify the exterior of the micelles with the DPA molecules, and they then add the EY which interacts with the DPA molecules by electrostatic forces. To test this artificial light harvesting system, they did titration experiments and monitored the absorbance of the donor and the acceptor groups. They observed that the absorption for the donor was nearly the same, while the absorption of the acceptor groups gradually increased. They also observed a decrease in the emission of the donor groups, while the emission of the acceptor groups increased. As control experiments, they titrated free DPA and added EY, but no energy transfer was observed.
I enjoyed reading this paper; it was simple and easy to follow. I think that they did all the proper control experiments by making sure that the increased in emission of the acceptor was due to the energy transfer of the light harvesting system and not by aggregation of the molecules. Overall it had a good narrative, and the authors explain very well the data they obtained. As for the synopsis, the blogger did a good job summarizing the main points in the article. However, I don’t think that a person with general knowledge of the chemistry would have understood the whole thing. The use of supplementary information was very helpful. As for the picture, it was really helpful and creative.
This article presents interesting work by the Tong group: the synthesis of an artificial light harvesting system. The system employs a convenient micelle that has alkynes in the periphery. These alkynes can be readily modified via Click-Chemistry. In this case they modified the positively charged surface with DPA, a highly fluorescent compound that does not form excimers or shows any self-quenching. The addition of the negatively charged acceptor, Eosin Yellow (EY) finishes the synthesis of these micelles. A series of experiments confirm the quenching of multiple DPA molecules by a single EY molecule, suggesting efficient DPA to DPA and DPA to EY energy transfer. These results are important because they are relatively similar to other light harvesting arrays found in nature and could be a big step in the development and implementation of more efficient light harvesting devices. I find the length of the article to be appropriate since the experiments they performed were all necessary.
The authors perform appropriate control experiments to support their claims, which include a titration of the micelles with a neutral dye. The control experiments and their experimental results definitely support their claims.
The narrative of the article was engaging and the figures helped me imagine the micelle. The problem was appropriately introduced and the reference to the photosynthetic apparatus was well made.
Bertis synopsis was well written. I would have written some things differently but, ours is more of a difference of style. I think a person in the field can understand the synopsis but reading the article would be better. Berti followed some of the guidelines but ignored others. He used some links in his synopsis and I find them adequate. In the future he should separate things into different paragraphs in order to make the synopsis easier to read and also to help him organize his ideas.
Tong and coworkers report a biomimetic approach to constructing artificial light harvesting systems. Through the combination of self assembly and covalent fixation, they prepare the system with commercially available compounds. They used DPA as their chromophore, which they chose due to its high fluorescence quantum yield. They used it to modify the surfactant micelles via click chemistry. It starts getting interesting with the fact that even after this, the DPA still showed a pretty high fluorescence quantum yield, because neither self-quenching nor excimer formation was observed. They then hypothesized that the high density of the DPA on the surface of the DPA-SCM would make it a good energy donor for EY. They followed by testing this by doing titration experiments on DPA-SCM with EY, and they observed Förster energy transfer from DPA-SCM to EY was occurring. This was further proven by the fact that the escitation spectrum was nearly identical to the absorption spectrum of DPA-SCM, indicating that the donor was directly contributing to acceptor emission. This same experiment was conducted on plain DPA and they didn’t observe the same effects.
I have to say that I liked the narrative very much, the way they segmented everything was really nice. As far as I know, they conducted all the experiments they needed to. I think the most interesting part of this is that they used such a basic example of supramolecular chemistry to create their system, which is actually a nice light-harvesting antenna. I found that the images they used were very helpful and complementary to the narrative, and that their results unambiguously support their claims.
Regarding Berti’s synopsis, I don’t think it’s bad, it does summarize the key aspects of the article, but I think it could’ve been done with a little bit more elegance. It’s those simple things like paragraph breaks that help you attend to each aspect that you’re talking about in a more organized manner. It is also just easier to read when you don’t have just a big wall of text. That aside, I thought the picture was lovely and that it is a good visual summary of what the paper is about.