Our group is interested in "Organic Photochromism." We are mainly dealing with thermally irreversible fulgides and diarylethenes, and thermally reversible spiropyrans and spirooxazines. However, we are also interested in developing new photochromic compounds other than listed above.

What are the major potential applications of photochromic compounds?

(1) High-density rewritable photon-mode optical memory media

     The possible application of thermally irreversible photochromic compounds may be optical recording media. In order to be so, the following conditions should be achieved.

    1. Thermal stability of the records

    2. Durability towards iterative write/erase cycles

    3. High-speed writing and erasing

    4. High sensitivity in writing and erasing

    5. Responsive to diode laser light

    6. Active in polymer matrix

    7. Records are non-destructively readable

     Among these, we are most interested in the last "Non-destructive readout". We have so far demonstrated three of non-destructive readout methods. As we believe one of them, using the change of optical rotation values, is the most realistic, we are continuing the research from various aspects.

     Reading of the records must be done, like writing and erasing, by optical method in order to attain high-speed processing. The easiest way is to read the presence or absence of absorption on the recording point. However, this always induces photoreaction when absorption is present, which results in gradual eraseing of the record. Reading should be done by the light outside the absorption. Fortunately, as the optical rotation has its value outside the absorption band, when a large change of optical rotation may be induced by photoirradiation, it can be used for readout of the records.

     With this method, we require three-head optic system because writing, erasing, and reading require independent light sources. Secondary harmonic generation of lights of higher energy may reduce the number of the light sources. In our laboratory, however, will not set-up the real devices. We are mostly concerning to the development of new methods and realization of the prototype systems.

(2) Photofunctional materials switchable by light

    The structures and properties of the thermally irreversible photochromic compounds can be switched by a one-time photoirradiation. Therefore, when a light-controllable certain function is introduced to a photochromic compound, you can switch-on the function by irradiating the material by light for a short time, and switch-off the function by irradiating with another light. In this context, we are doing research shown below to control the mutual interaction between the photochromic compound and the second chemical species by photoirradiation.

1 Photochemical control of interaction between the photochromic compounds possessig crown-ether moiety and cations such as metal ions or ammonium ions.

2 Photochemical control of hydrogen-bonding ability of photochromic compounds which can form multiplex hydrogen bonding

3 Photochemical control of properties of liquid crystals by the change of structures of photochromic compounds doped in liquid crystals

4 Development/discovery of chiral organid photochromic system

5 Photochemical control of optical/chiroptical properties of materials such as refractive index and optical rotation

    These projects are now under active investigation. In the "Era of Photonics" which is coming close, we believe that these molecules are governing the on-off switching of functions. There will be, in practice, a number of application ways that we can never imagine (but you can).

If you have any of that kind of ideas, or you are interested in our research projects, feel free to contact the following address. Thank you.

    Professor Yasushi Yokoyama, Ph. D.
    Tel/Fax: +81-45-339-3934
     e-mail: yyokoyam\8653\ynu.ac.jp

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