Molecules That Have Memory

molecules, phthalocyanine, solid state memory, molecular memory, tautomers, tautomerization

The future of incredibly fast solid state computers could be built on phthalocyanine—a type of molecule recently reported to be capable of possessing a kind of molecular memory. This molecular memory is composed of a simple binary system, much like the 1s and 0s in computer code. The “on” and “off” statuses of these molecular switches are the product of the unique configurations created when the atoms in the molecule bind to each other in different ways.

Phthalocyanines are a particular type of synthetic organic molecule that have much in common with the porphyrins—a class of molecules that make up the chlorophyll in plants and the hemoglobin in blood. Like these porphyrins, the atoms in a phthalocyanine form a ring (specifically, an aromatic macrocycle) about a nanometer across. The center of this ring contains a metal ion. These structural attributes give phthalocyanines a wide range of potential applications, particularly with regards to light absorption and emission. To date, they have been used as photo-conducting materials in laser printers, industrial catalysts, components of cancer therapy, and an important element in creating the light absorbing layer on CDs.

One particular kind of phthalocyanine, metal-free phthalocyanine (H2Pc) recently attracted the research community’s attention when it was featured in the cover story for the May 20th issue of Chemical Communications. Specifically, H2Pc can be used to imbue molecules with a kind of “molecular memory” depending on the particular configuration of the bonds. At specific sites within an H2Pc molecule, protons can migrate from one bound atom to another, shifting which atoms are held together by double and single bonds. Such a process, called “tautomerization,” generally establishes a dynamic equilibrium in which variations of the H2Pc molecule form and then conform back to their original state at a constant rate.

The rate at which tautomerization occurs is extremely fast. In the case of H2Pc, experimental evidence shows that at room temperature variants of the molecule (tautomers) form and revert over 100,000 times per second. Such a rapid, uncontrolled rate of tautomerization would make industrial applications of H2Pc very difficult to actualize.

To see if they could slow down the hectic speed of tautomerization, Professor Hiroyuki Noji and Dr. Tomohiro Ikeda from the University of the Tokyo Graduate School of Engineering teamed up with Professor Ryota Iino from the National Institute of Natural Sciences and Okazaki Instute for Integrative Bioscience. The researchers knew that at least theoretically it would be possible to induce H2Pc to tautomerize at a much slower rate. Using wide-field fluorescence microscopy, the researcher observed that in carefully controlled conditions, single molecules of H2Pc at an air/glass interface exhibited the predicted slower rate.

So what’s the big deal? The slower, controlled rate of H2Pc tautomerization means that the molecule could be manipulated by chemical or electrical means to form one tautomer or another. Having two such recognizable states of a single molecule can potentially be applied to creating a new form of solid-state memory. Just as a series of 1s and 0s in a computer code can together form complex programs, so too might the “on” and “off” switches of H2Pc molecules be used as a new kind of memory—molecular memory. Using H2Pc to store data has the advantage of beint extremely compact. Because the molecule is flat, an estimated 13 terabytes of data could be stored in a single square centimeter.


By Sarah Takushi


Asian Scientist 
Chemical Communications
Encyclopedia Britannica  

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