A pendent necklace and a new insight about self-healing molecules

In a recent post on our research on self healing, I discussed our new theory, which is posted in the Physics Archives (see it here). The paper has been accepted for publication in the Journal of Chemical Physics and will appear soon.

We used lots of data as input to construct the model, which took years to complete. Data that seemed to support one model initially would later be contradicted by additional data. Over time, the model evolved into a coherent picture as more hypotheses were eliminated by experiments. Finally, we had a model that fit the data AND had as its cornerstone the formation of domains of molecules that together, would help a damaged molecule heal.

There is no direct evidence for domain formation, though the behavior of all the experiments to date are consistent with this model, and only this model. Remove the domains and the predicitve power of the theory is lost. The burning question pertains to the nature of the domains. What are they? Are they clumps of molecules or molecules that are somehow stuck to the same polymer chain? What is the nature of the force that keeps the domains together, and how is it that a domain of healthy molecules acts to promote healing in a damaged one?

We may be closer to an answer.

The lab is in a wonderful buzz of activity with lots of new measurements -- always an exciting time. There are bold new hypotheses based on initial data that generalize our model, followed by letdowns after new data or a more detailed calculation proves us wrong. The process is highly stimulating. I can just smell it; something new and wonderful is brewing.

In the midst of all this activity, I found myself sitting at my computer writing my conference paper for SPIE, where I will give a couple of papers in August. I completed writing the introduction and then explained our new model. What next? I needed something new that did not detract from the presentations of my students. So, I drew the molecular structures of the polymer and the molecules, and started to play with them, rotating this one this way and that one here, etc.

In less than a few minutes, I realized again that a molecule could stick to a polymer through what is called hydrogen bond -- an attractive force between a hydrogen molecule and in this case, an oxygen, very much like the forces found between water molecules. This thought had crossed my mind in the past, and is indeed a motivation for a subset of projects. However, having all this jumbled data running around my head made me realize that Shiva, my coauthor on the theory paper, had already determined the three parameters of our model, one of which is the force that binds the molecule to a domain. If the molecules are sticking to the polymer chain through a hydrogen bond, the hydrogen bond energy should have the same value as the corresponding parameter in the model.

This is an excellent example of a model that we built to explain the data is now guiding us in figuring out what is going on.

I got on the internet and searched for hydrogen boding and found a table of numbers. The energy between a hydrogen and oxygen was one of the first values listed, at 0.3 eV. Then I nervously clicked through the directory tree on my computer to find its measured value. As I scrolled to the table with the results, my eyes focused on the value of the lambda parameter -- 0.29 eV with an uncertainty of 0.01. The two matched!

It is not often that things work out this easily, so I considered the next question, and that was how self-healing is mediated by molecules attached to a chain. A polymer with molecules connected by hydrogen bonding looks a lot like a necklace (polymer) with pendents (molecules) thrown on the night dresser as shown in the figure below.The hypothesis that I proposed is as follows. (a) When a molecule absorbs a photon, (b) it breaks into two fragments that are charged. There is evidence from earlier work that charged species are involved. One of the fragments is fixed in place by the polymer and (c-e) the other hops from molecule to molecule along the chain (f) until it finds its mate and recombines.

An alternative explanation is that the attached fragment attracts a small fragment from a neighboring molecule. The neighboring molecule then attracts a fragment from its neighbor, and so on, which propogates down chain like a wave of fans at a stadium until the original damaged piece combines with an adjacent fragment. The more molecules in the domain (i.e number of molecules attached to a polymer chain), the bigger the chance that there is a contiguous path for the fragment to find a mate.

This is indeed an exciting time. In addition to this work, there are other very exciting developments that I will post in the near future. Breakthroughs can be addictive. I can't wait for the next one!

Zeroing in on the cause of self healing

As I have mentioned in the past, one of our biggest projects seeks to develop an understanding of the mysterious self healing process following damage to a molecule by a zap of light. Recently, a former graduate and I developed a model of the healing process that hinges on the formation of domains of molecules. Members of these domains are highly cooperative: they accelerate the healing of a damaged molecule in proportion to the size of the group and they prevent their comrades from being damaged. This behavior is as strange from the sociological perspective as it is from the underlying physics. Why do the molecules aggregate and how does their community enhance healing and prevent physical damage?

We have gone out on a limb and made what I believe is a bold assertion; that there are forces between the molecules that cause them to aggregate, and that these same forces are responsible for healing. Such an assertion would be just a wild guess if it were not for lots of data that we find to be consistent with our model. With only three parameters, our data fits the model as a function of temperature, concentration, time, and intensity. The model also makes predictions beyond our present experimental capabilities, so it will gain acceptance only if it holds up to future scrutiny.

When submitting something this interesting (at least to us) that may go past the present paradigms (Shiva got some lifted eyebrows and jaw dropping during an interview talk, which turned to nods of approval after he presented supporting evidence), one always worries that the work will not be understood. There are many examples of Nobel-prizewinning work being rejected by a journal. In our case, the first journal did not even send the paper out to review, claiming that our work was not appropriate. How can a physics paper not be appropriate to a physics journal?

Of course, I have no illusions that this is a Nobel-prizewinning paper, but if the underlying mechanism is found to be new, it could very well end up being a significant achievement for whoever makes this discovery.

Rather than fight the editor, back in mid May, we sent the paper to a second journal of equal quality. Then we waited. I was still concerned that the reviewers may not see the importance of the work. But alas, they accepted it on the first pass, suggesting only minor revisions. And it was also incredibly fast given the nature of our paper. The first reviewer summarizes the paper as follows,

"This interesting manuscript continues the authors' work aimed at discovering the mechanism behind the observation of self-healing of photoluminescence in chromophore doped polymers. The authors have proposed a phenomenological model for their observations that is able to predict aspects of the time, temperature, concentration and intensity dependence. The model focuses on the formation of dye domains in the polymer and studies the dynamics of these..."

Then (s)he goes on,

"While these are interesting results, the manuscript could be more satisfying if the authors did more to understand the physical mechanisms behind the model. Some well-considered speculation on the materials physics in the conclusions would suffice. "

We tried to hold back on speculation, but this review gives us an opportunity to present what we think is happening. Incidentally, the reviewer is right that we need to work more on the mechanisms, which is exactly what we are doing now. We are already getting data that is pointing at the mechanism, but its still too premature to mention.

The second reviewer made no suggestions for revisions and believes that the paper is in good shape in its present form. (S)he writes,

"In this paper authors present a model on photodegradation/self-healing kinetics of dye molecules doped in a polymer matrix. This investigation is an extension of their previous work. Using phenomenological arguments the authors generalize their model. They allow (implicitly) for association of dye molecules which form correlated domains interacting with the polymer matrix. A healing rate is assumed to be proportional to the number of undamaged molecules in a correlated region and a decay rate is proportional to the intensity normalized to the correlation volume. The model proposed by the authors predicts decay and recovery of the population of doped molecules. The results of the theory are successfully tested with experimental data.

"The paper is generally well written and contains several interesting results. I recommend it to be published as it stands..."

The next step will be to determine the physical significance of these parameters. I am excited by the prospects that we may be looking at some very new physics because this process is like no other that I have ever seen. As I sit at my computer bogged down with lots of administrative tasks, new physics is in the air. I hope to be able to get back with pencil and paper to work on the next set of ideas. But first I need to work on some proposals so that we have the resources to do lots of wonderful work in the future. And as penance for writing proposals, I also have some that I need to review. Similarly, I have a pileup of papers to review.

Hopefully in my next post I will report on even more interesting physics. On another project, something very exciting is brewing. Again, new physics! Until then, ...

My voice from the past

A while ago, David Bradly, a reporter from ScienceBase had contacted me about a paper from my group on self healing in a molecule called AF455. He wrote a short news piece on our work. After the piece was posted, he contacted me with additional questions. In response, I shot him an email, which he posted in its entirety. This was back in April of 2007, more than 4 years into my past.

Just 5 minutes ago, I was searching for articles related to our research and ran across my email. I tend to write emails from the top of my head, without much editing, so it was eerie to see myself in an unguarded moment. In effect, it was my own voice form the past, real and uncensored. When writing for the public, as I do in papers and proposals - and even in this blog, I choose my wording carefully, though often not with good results. While you may not notice the tone, reading this email rekindles in me the excitement of discovery that I was feeling at that time. It is better than any diary entry.

I am glad that David Bradley posted this email, which is truly a window into my past. It is reproduced below. As you may have guessed, he asked me about applications of our work.

Dear David,

The molecule AF455 is indeed complex, and that is what makes its irreversibility so puzzling. The DO11 dye, which we previously studied for reversibility is a relatively small molecule; and, the mechanisms for the recovery is the breaking up of dimers that form in the degradation process. This requires the molecules to be able to move around a bit. AF455 clearly can not move around easily, so another mechanism must be responsible.

Any device that operates at high intensity, such as lasers, displays, and all-optical switches and logic, suffer from photodegradation. Solid state lasers, for example, live longer than ion lasers and dye lasers; but, dye lasers have much more flexibility is the range of colors that are available. Polymer displays, on the other hand can be mechanically flexible and can be used to host all sorts of organic molecules. The general theme is that organic molecules have a much broader pallet of what they can do, but, they are not as stable.

So in our work, we are not so much interested in targeting specific applications. Rather, we want to understand the mechanisms for recovery since most materials degrade irreversibly. And here we have two very different molecules that behave the same way. There is one similarity. We discovered this property by accident!

If a material absorbs light strongly, it will damage when the absorbed optical power reaches the material's damage threshold. In applications where the material is transparent, light can be absorbed through a two-photon absorption process. Not as much light is absorbed in the process, but, over long-enough periods of time, cumulative effects cause the material to degrade.

Bright light can cause all sorts of things to happen in a material. If it induces a chemical reaction that causes a molecule to break apart into pieces, that process is irreversible. On the other hand, if the light causes the molecules to change shape into a form that no longer absorbs light or perhaps causes some charge to jump from one side of the molecule to the other, this change is reversible. The trick is to find materials that are not killed by the zap of laser, but that prefer to take a nap.

Another intriguing observation is that when such molecules wear out, rest, then recover many times, they seem to degrade more slowly and recover to a higher level of efficiency upon further cycling. It's like a weight lifter that gets stronger after each workout. So, it may be possible to make our molecules more buff by giving them a good workout. We observed this kind of response in the DO11 dye, but have not seen it in the AF455 dye.

So, while we see two-photon absorption (TPA) as a universal nuisance that destroys materials, and that's the motivation for our studies, there are many important applications. Two-photon absorption is strongest where the light intensity is the highest, and is ideal in applications where a chemical reaction in a material operates above a certain threshold power. The important consideration is that for absorption to occur, two photons must participate.

Cancer therapies are one such application. The patient drinks a cocktail of molecules that like to stick to a particular type of tumor cell. Also, these molecules are tailored to be strong two-photon absorbers to a color of light to which cells and flesh are transparent. Then, just aim a laser beam at the tumor right through the skin. In this way, only the tumor cells are zapped. Since the skin is not perfectly transparent, it will also absorb some of this light, causing a bit of damage. Ideally, you want to make the strength of two-photon absorption as high as possible so that the amount of damage to the tumor is as big as possible relative to the damage to healthy cells. You want the special molecules to live as long as possible so that they can be repeatedly zapped without the patient having to ingest more of the cocktail, which could have side effects.

Since TPA is a process where two photons are simultaneously absorbed, it can be used to drive chemical reactions at the intersection point of two beams of light. As an example, a liquid can be made to turn solid (i.e. polymerize) at the crossing points. In this way, a three-dimensional object can be made piece by piece inside the liquid, such as gears, shafts, and other nano-scale parts. So, it's like having the ultimate nanolab.

So, TPA is something that is simultaneously very useful in important applications; but, can be a nuisance in all applications that require the use of light. We are thinking more about ways to make a molecule snooze to help it recover rather than find more ways to put it to work. Happy dreams!

Mark