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
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