Why Molecules and Light Don’t Mix
December 13, 2015
Why do chemical molecules disintegrate rapidly when they strongly interact with light? Researchers from the Polish Academy of Sciences’ Institute of Physical Chemistry in Warsaw know the answer. Their insight promises to substantially enhance the stability of chemical molecules in the lab as well as has practical significance for manufacturers of common objects, especially those made from dyed polymers.
Why do newly painted walls fade in a flash? Why does a plastic grass sprinkler become useless after just a few months of operation? Why do laboratory researchers have problems with quickly disintegrating chemical molecules when they use laser light? In all these cases, light is a factor responsible for the disintegration of molecules. The scientists from the Institute of Physical Chemistry, led by Prof. Jacek Waluk, have discovered how to significantly slow down this process. The researchers managed to identify the main mechanism accelerating the photodestruction of chemical molecules.
“In our lab we observe single chemical molecules,” says Waluk, one of the authors of a research paper published in the Journal of Physical Chemistry Letters. “We use a fluorescence microscope for this, examining the light they emit.”
He says the main problem is the life span of the molecules: they disintegrate after just a few seconds. “So we decided to extend their life span,” Waluk says. “The first step toward this goal was understanding the main phenomenon responsible for the destruction of molecules.”
For the measurements to be reliable, it is necessary to observe thousands of photons emitted by a molecule in the process of fluorescence. Molecules emit photons in all directions, but only those traveling toward the detectors are recorded. In the case of the terrylene diimide (TDI) molecules studied at the institute, the fluorescence quantum yield—the ratio of the number of photons emitted to the number of photons absorbed—was around 70, says Waluk. This means that during the measurement each molecule had to absorb nearly 2 million photons.
“Each instance of photon absorption causes the molecule to enter an excited state that generally increases its reactivity, which means its ability to enter into chemical reactions,” says Waluk. “Thus, each absorbed photon brings the particle closer to its own death.”
It is generally accepted that a molecule is photostable—able to retain its integrity upon exposure to light—if it has a 50-percent chance of survival after absorbing a million photons. “In our conditions, this means that we can observe it for a few seconds,” Waluk says.
The key to enhancing the photostability of the molecules was the analysis of the way in which they were made fit for observation under a fluorescence microscope. The procedure starts with the preparation of a heavily diluted solution of the molecules in a dissolved polymer. A droplet of this solution is then placed on a microscope slide located on a spinning disc. Spinning spreads the droplet over the surface, and the solvent evaporates. A thin layer of polymer—with a thickness of about 30 nanometers—remains on the slide, together with trapped single molecules of the studied substance, which are about 1 nanometer in size. If the concentration of the molecules in the initial solution is selected in the right way, the single studied molecules will be positioned in the polymer film at relatively large distances from each other—on the order of microns. A slide prepared in this way is then placed under the microscope, where the polymer layer is swept by a narrow laser beam with a light wave energy selected so as to excite the studied molecules. Any fluorescence appearing in the area will most probably come from a single molecule of the studied compound.
What can the excited molecule react with? From the beginning, the primary “suspect” was oxygen, which can be dissolved in polymer solutions.
The researchers therefore examined the impact of seven polymers on the life span of the TDI molecules, but they did not detect any link between the capacity for improved oxygen dissolution and the accelerated photodestruction of TDI. The correlation appeared only when they examined the impact that the rate of oxygen permeation through the polymer layer had on the studied molecules. The differences were significant: polymers through which oxygen permeated slowly significantly enhanced the photostability of the molecules. One of the polymers, polyvinyl chloride, was capable of increasing the life span of the molecules up to 100-fold. This figure increased with the age of the polymer. This was another argument in favor of the crucial role of oxygen because it is known that oxygen permeates more slowly through older polymers.
“We are convinced that it is not a molecule’s interaction with light that is responsible for the accelerated photodestruction of molecules, but its reaction with oxygen,” says Waluk.
He explains that the excited molecule passes into what is technically known as a triplet state, when it can combine with triplet oxygen in the basic state. The oxygen is activated and moves to a singlet state—a molecular electronic state in which all electron spins are paired. “Singlet-state oxygen is extremely voracious and instantly enters into reactions with anything that is within its reach,” Waluk says.
The research results, obtained with funding from Poland’s National Science Center, are of substantial practical significance. Applied in laboratories, they will create new opportunities in the study of single chemical molecules using fluorescent methods, a kind of research that has been a major challenge for the last two decades. Understanding the mechanism of photodestruction of molecules will also prove useful wherever everyday objects are produced with the use of polymers and dyes. The selection of suitable polymers hindering the migration of oxygen may significantly extend the durability of dyes and the life spans of various objects of everyday use.