I have written about the work of Mark Kuzyk on several occasions, most recently in relation to his discovery of the "self-healing" properties of a dye molecule known enigmatically as AF455. This compound excels at two-photon absorption, an important property in the future of optical data storage and in producing microelectronics for photolithography. I interviewed Kuzyk at the time, but several questions arose after publication of my original article, the answers to which I thought Sciencebase readers would be interested.
As such, I asked Professor Kuzyk about the complexities and applications of this compound, this is what he had to say:
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