Making a Molecule Functional
Project Leader: Hiroaki Isago (D. Sc.)
(Correspondence should be addressed to ISAGO.Hiroaki@nims.go.jp)
For What?
In these days, as a larger amount of data needs to be more rapidly processed, more highly integrated electric devices are to be required. On the other hand, so far as the conventional processes to fabricate electric devices are adopted, in which narrow electric circuits are drawn on silicon wafers, principle limits of integration can not be avoided and are coming in the near future. That is why an innovative breakthrough, such as fabrication of device on atomic or molecular scales, will be essential.
The term “a molecule” may remind you of small simple molecules, such as H2O (water) or CO2 (carbon dioxide), and you might doubt if such an idea could make sense. However, this is not a fairy tale: We can see a lot of excellent models in biological systems, such as many kinds of enzymes in our bodies, hemoglobin in our blood, and chlorophylls in plants that play important roles in photosynthesis.
In our laboratory, we are studying compounds
of phthalocyanine (figure 1), which is a
well-known organic dye, and metals (figure
2). Because the molecular structure of phthalocyanine
is similar to that of porphyrin (figure 3)
that is a core skeleton in the above mentioned
hemoglobin and chlorophyll, compounds of
phthalocyanine and metals have attracted
much attention as models of such important
biologically systems.
From the left; Fig.1 Molecular structure of metal phthalocyanine compound (M: metal ion), Fig.2 Crystals of copper-phthalocyanine, Fig.3 Molecular structure of porphyrin.
Not only academically interesting, but also
phthalocyanine compounds are very important
in industry as blue or green pigments (famous
as those for the Shinkansen super-express),
as absorbents in optical disks (for CD-R),
as charge generators for photocopiers, etc.
Therefore, phthalocyanine is very familiar
to our daily life although it may sound unfamiliar.
Furthermore, phthalocyanine compounds are
expected to be applied in many fields, such
as medicine (for example, as photosensitizers
in photodynamic therapy of cancer), optical
communication (in nonlinear optics), and
displays (as liquid crystals) (figure 4).
Yes, phthalocyanine is the Superman in chemical
compounds, isn’t it?
Fig. 4 Actual and potential applications of phthalocyanines in industry.
Antimony Compounds: Extraordinary Guys in Phthalocyanine Compounds
Because phthalocyanine is very versatile in industry as mentioned above and forms compounds with most of metal elements, numbers of compounds of metal elements are known. However, those of group-15 elements (family of nitrogen) had been unknown until we first reported compounds of bismuth (Bi; atomic number = 83) and antimony (Sb; 51) in 1994. Particularly, compounds of antimony (figure 5) have been found very unusual in some respects when compared to the known phthalocyanine compounds. For example, “normal” phthalocyanine compounds significantly absorb visible light and are intensely colored blue or green depending on the wavelength of the absorbed light (that is why these compounds are being used as blue or green pigments). On the other hand, the antimony compounds intensely absorb invisible light that has longer wavelength (near-infrared light) (figure 6). So far as the absorption maximum wavelength is concerned, the antimony compounds are the champions among the phthalocyanine compounds. This property of near-infrared absorption is very important for industrial technologies, particularly for those utilizing semiconductor laser beam (for example, as absorbents for CD-R) and will be more and more important in the field of optical communication and therapy of cancer. As normal phthalocyanine compounds do not absorb near-infrared light, hard tasks, such as chemical modification of molecular structure of phthalocyanine itself or rearrangement of conformation between phthalocyanine molecules, had to be done to use phthalocyanine compounds as near-infrared absorbents. However, since such efforts are not necessary for the antimony compounds, these can be applied in various fields more easily.
From the left, Fig. 5 Molecular structure of antimony compound, Fig. 6 Optical absorption spectrum of antimony compound (solid line); broken line denotes that of "normal" phthalocyanine compound. Light with a wavelength in 400～700 nm is visible and that with longer wavelength is near-infrared light.
Another “extraordinary” characteristics found for these guys is their facile reduction (that is, they accept electrons much more easily than “normal” guys). With respect to ease of reduction (that can be expressed by using the term “reduction potential”), the antimony compounds are the champions also in this field (Yes, the antimony compounds have won the double crown!). In general, normal phthalocyanine compounds are prone to release electrons but not easily to accept additional electrons while characteristics of the antimony compounds are just the opposite. We might be able to find something interesting by combining normal and extraordinary phthalocyanine compounds. Today, transistors and diodes that are used in electric circuits are made of silicon, but in the future, such devices could be made of phthalocyanines.
What is going on?
It is still unknown at present point why only the antimony compounds are so “extraordinary”. If we are able to find a clear-cut reason, we may be able to find “more extraordinary guys”. Also we could be able to reform the known “normal guys” extraordinary. Thus, it is very important to figure out the mechanism of their anomalies. Recently, we have found that the antimony compounds turned unstable upon reduction although they themselves are very stable. It is also an interesting subject to reform them with the combination of phthalocyanine and antimony unchanged so that these attractive “extraordinary guys” can be stable under any conditions. In addition, we are trying to make more compounds to find new attractive “extraordinary guys” and trying to figure out their various spectroscopic properties (optical absorption, emission, etc.,) and electrochemical properties (acceptance or release of electrons).
List of publications (selected).