Effects of Viscosity on the Isomerization of Calix[4]arene Capped Azobenzene

Student: John Nugent
Major: Chemistry
Advisor: Dr. Paul Bonvallet

Calix[4]arene capped azobenzene (CCA) takes advantage of the isomerization of azobenzene and the host-guest interactions of calixarenes to act as a molecular capsule that can be opened or closed. The supramolecular capsule can contain positively charged species while in the cis (closed) isomer and release them in the trans (open) isomer. The cis to trans (opening) reaction was the subject of study because it is slow and can be driven by thermal energy. The main goal of this project was to see if increasing the viscosity of the solution would cause the reaction to go slower. The experiments were initially done with azobenzene, then with CCA-1. It was found that there is no relationship between the viscosity of the solution and the rate constant of the cis to trans isomerization in both azobenzene and CCA-1. When the solvent viscosity is increased, the rate of the isomerization remained constant for both.

John will be online to field comments on May 8:
10am-noon EDT (Asia: late evening, PST 6am-8am, Africa/Europe: late afternoon)

41 thoughts on “Effects of Viscosity on the Isomerization of Calix[4]arene Capped Azobenzene”

  1. Thank you for sharing the research, John! It is an interesting investigation. What do you think is the insensitivity to viscosity indicates about the isomerization mechanism? Is CCA thought to follow the same isomerization mechanism as uncapped azobenzene?

    1. Hi Dr. Sobeck,
      The mechanism for CCA is one of the most interesting parts! Unfortunately, I had to cut it out of the presentation due to the restrictions of this mode of communication. With CCA there is still the question of whether it follows the inversion or rotation mechanism. While my experiments don’t provide clear evidence for either one, it seems that the insensitivity to the change in viscosity may indicate that it follows an inversion mechanism. There is some literature based on computational evidence that the rotation mechanism requires more space (free volume) to isomerize, while the inversion mechanism takes up less space when transitioning between isomers (1). To me it seems that if the viscosity of the solvent is not impacting the rate of isomerization, then the reaction is not taking up very much space, thus it is potentially following an inversion mechanism. On the other hand, the results could have simply been limited by the viscosities that I tested. I was limited in that regard by the materials and methods I was using and performing.
      -John

      1. Here is the article I referenced, if you are interested. I forgot to attach it to that comment:

        Dokić, J.; Gothe, M.; Wirth, J.; Peters, M. V.; Schwarz, J.; Hecht, S.; Saalfrank, P. Quantum Chemical Investigation of Thermal Cis-to-Trans Isomerization of Azobenzene Derivatives: Substituent Effects, Solvent Effects, and Comparison to Experimental Data. J. Phys. Chem. A 2009, 113, 6763–6773

        1. Thanks John! This is a wonderful physical organic project and I look forward to seeing how it continues. There could be some interesting kinetic and solvent studies to be had. Also your thesis will be a nice thing to note for the azobenzene experiment in our 318 lab.

          I truly enjoyed getting to know you during your time at Wooster. It was a joy to have you in class from FYS to PChem. Best wishes to you and please don’t be a stranger to Wooster. Cheers to you!

      2. Great job John so proud of you! Is there a specific type of guest molecule you are interested in?

        1. Dr. Baker,
          Thank you, I really appreciate it! For this project I didn’t do any investigations into potential guests, but in the past Dr. Bonvallet and previous IS students have looked into guests with aromatic rings and positively charge nitrogen groups. This site doesn’t allow images to be attached, so here is a direct link to structures of guests that have been used in the past.

          https://acswebcontent.acs.org/prfar/2014/abimages/Paper_12796_abstract_24494_0.png

          -John

  2. Super cool research topic here, John! Did you have a chance to do the isomerization mechanism of CCA-1 with any type of guest species present? What do you imagine future work to be here? Would you want to try the different generations of CCA?
    Thank you for sharing, and CONGRATULATIONS!!

    1. Hi Leah!
      I looked into the mechanism a lot, but it just didn’t make the cut for the presentation because I was trying to aim at a more general audience with this presentation and I don’t know how long it would take to begin introducing inversion and rotation mechanisms. If you look at my response to Dr. Sobeck, I included my thoughts and findings about the mechanism in that reply. When doing the isomerizations I did them without a guest, but it would be interesting to do them with a guest present. As far as the future work is concerned, how much do you know about Chloe’s project? Because if her work is continued and her findings about the isomers are correct, then it may change everything we think about CCA and adjust the future work.
      -John

      1. Super fascinating stuff here! Thanks, John — I hope you enjoyed the experience and best of luck to you in the future!

  3. Congratulations John Nugent! I enjoyed learning more about your research. Great job.

  4. How did you vary the viscosity of the solution? I assume it was by adding another solute rather than changing the solvent.
    A very good presentation! Congratulations and best of luck in the future.

    1. Hi David!
      I varied the viscosity of the solution by adding polystyrene, which is a polymer that can dissolve in solution with CCA. It acted as a thickening agent without changing the polarity of the solution.
      -John

      1. Thanks for the reply. Another thought if this work is to be continued by others: It would likely be a more difficult synthesis, but if the propoxy groups were on the inside rather than the outside of the calixarene caps, they might be better able to complex with any added positively charged guest.
        David

        1. That is an interesting thought. Like you said, it would be a difficult synthesis, but in that structure they could complex better with positive guests. It could also have some interesting effects on the three-dimensional structure of the calixarenes, which might change the interactions in an unexpected way.
          -John

    1. Dr. Pasteur,
      Thank you! I’m currently applying to medical schools to hopefully begin in Fall 2021. I am taking a gap year where I will be volunteering/working with City Year in Kansas City (it is a subprogram within AmeriCorps).
      -John

  5. Excellent work, John! What was the range of viscosities that you worked with? (I am not envious of whoever has to clean the buret!)

    1. Dr. Faust,
      Thank you! I ended up working with pure toluene, then 0.01 g/mL to 0.10 g/mL of polystyrene in toluene. Anything more concentrated clogged the buret (I tried up to 0.30 g/mL, it was VERY viscous)! Yes, cleaning up my materials will be a bit of a nightmare. I had a couple days set aside to take care of them after spring break, but with everything that has happened, I guess I won’t be doing it!
      -John

  6. Hi John,
    Nice presentation and very interesting concept!
    I was wondering if this isomerization could be considered a prototype chemical nano machine for performing environmental or other desired functions?

    1. Yes! Azobenzene has been an area of increasing research in the nanotechnology field. Because of its very consistent isomerization reaction and its response to light it has been investigated for use in a number of fields. Including; as a supra molecule capsule, adjusting flow over surfaces, etc.

      In the natural world, an isomerization reaction (not azobenzene) is present in the rod and cone cells in our eyes, and plays a major role in our ability to see.
      -John

  7. Thanks for being a volunteer for this initial venture!

    What sort of guest is imagined to be “captured” between the calix crowns? Does Pr stand for propyl? The innermost surfaces or the crowns might then have a slight negative charge due to inductive effects, but the distance between the crowns is large. Free rotation about the biphenyl rings seems not to favor any encapsulation.

    1. Hi David,
      While I didn’t focus much on the host-guest interactions of calixarenes, the literature around it is pretty well established. Previous IS students and Dr. Bonvallet’s research has focused on those interactions with CCA, proving that the guest do form that interaction with at least one calixarene based on hydrogen shifts in the NMR spectra they have taken. Yes, the OPr is a propoxy group which allows for that interaction to take place. You are spot on with your assessment that the guest may not interact with both calixarene rings. It is something research is currently being done on. It is possible (maybe likely) that the guest is only interacting with one of the calixarenes. Because of this, the group as also synthesize CCA-3, which has removed the additional aromatic ring between azobenzene and the calixarenes on either side to bring the calixarenes closer and to see if that compound does a better job of forming interactions with guest molecules. Because CCA-3 is fairly new and we don’t have a lot of it, I didn’t use it for this project. It is definitely a point of interest and is currently being investigated.
      All the best,
      John

  8. I really enjoyed your presentation, John! I’m impressed by your creative method to quantify viscosity, since that can be really tricky to measure without specialized instrumentation. What was your upper bound in terms of the most viscous solution you tested – was it something with high viscosity, comparable to honey? Also, do you know if there are any applications for the host-guest interactions of CCA that would occur in a highly viscous environment?

    1. Dr. Craig,
      Thank you! The upper bounds of what I was able to test using my methods was 0.10 g/mL of polystyrene in toluene. It was fairly high in viscosity, maybe a little more viscous than honey. The problem I ran into was that when I tried to run it through the buret’s small tip, it would congeal and become a nightmare! What comes to mind is potentially an industrial process that require the release of a particular compound at a specific time, which a molecule like CCA could help with. The main reason why I looked at viscosity was to try to find a way to slow down the “opening” reaction. In the past, Dr. Bonvallet identified that adjusting the solution polarity and acidity can speed up the opening reaction, so this was an attempt to find a process that could match these adjustments, but slow down the reaction.
      -John

      1. Thanks John! I do not envy your job cleaning those burets! Congratulations on a great project and best of luck as you move on from Wooster!

  9. Hi, John:
    Excellent presentation. Congratulations on your project and upcoming graduation. It was a pleasure to have you in class!

  10. Excellent presentation. Congratulations!
    I hope you will be considering University of Pittsburgh, School of Medicine.

    1. Thank you and yes it is! I love Pittsburgh and have a lot of family from and living there.
      -John

  11. Great presentation, John! Thanks for stepping us so clearly and logically through all aspects of your project.

  12. This is a beautiful presentation, John. It’s clear that you have thought carefully about the key details and how to explain them to a non-Chemistry audience, while including those extra details in your slides that the Chemistry folks would appreciate. Congratulations on a job well done!

    1. Thank you, I really appreciate it! It was a little difficult to make it understandable to a larger audience, but I’m happy I could engage more people.
      All the best,
      -John

  13. Great presentation! I have a brand new Ostwald viscometer on my shelf in the stockroom. I wish I knew earlier, I would have lent it to you… The buret idea is a clever work-around. Nice thinking.

    Really interesting project and good results. Best wishes in your future endeavors!

  14. Hi John, nice presentation! Perhaps I missed this at the beginning, but why were you interested in viscosity as a variable? Is this just to round out the characterization of CCA in various solutions, or are you trying to slow the reaction to make it more useful somehow? It would make sense that you would want to be able to exert some control over the rate but I’m just making sure I’m understanding properly.
    (Also glad to see Dr. Baker in the comments here too, I miss him!)

    1. Thank you! Yeah a lot of research on CCA has been focused on how to control the isomerization of CCA. If it was to be used as a tool, then it would be helpful to be able to control the rate at which the capsule opens and closes. The closing reaction (trans to cis) happens very quickly when exposed to light, so that can’t be controlled very well. On the other hand, the thermal isomerization of the opening reaction (cis to trans) happens slowly and can potentially be influenced. In previous research, Dr. B and other IS students were able to identify that changing the polarity and acidity of the solution will increase the rate of opening reaction. So with the ability to speed up the reaction, it would be helpful to have a way to slow down the reaction, just to be able to influence the reaction in both direction. We thought that increasing the viscosity could slow down the reaction because the calixarene groups are fairly large, so our hypothesis was that the increased resistance to movement in a highly viscous solution would slow down the isomerization reaction. Ultimately, that turned out not to be the case, but it was worth the effort.

      On a side note, the lack of a result may give a clue into the mechanism of the isomerization reaction. For azobenzene (the center of CCA), there is a lot of debate whether it follows a rotational or inversion mechanism. The lack of an influence of viscosity on the rate may indicate that the reaction could be following an inversion mechanism, which would be really interesting. Here is what I responded to Dr. Sobeck:
      “While my experiments don’t provide clear evidence for either one, it seems that the insensitivity to the change in viscosity may indicate that it follows an inversion mechanism. There is some literature based on computational evidence that the rotation mechanism requires more space (free volume) to isomerize, while the inversion mechanism takes up less space when transitioning between isomers. To me it seems that if the viscosity of the solvent is not impacting the rate of isomerization, then the reaction is not taking up very much space, thus it is potentially following an inversion mechanism. On the other hand, the results could have simply been limited by the viscosities that I tested. I was limited in that regard by the materials and methods I was using and performing.”

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