1. Could you explain the significance of your article to the non-specialist? 

We have prepared a new class of oxonol dyes (salts of oxonol anions and 4,4'-bispyridinium cations) that are highly photo-resistant due to a mechanism in which the excited state of oxonol anion was rapidly quenched by transferring one electron to the counter cation followed by a reverse electron transfer leading to recovery of the original ground state dye. It is noteworthy that we have enhanced photo-resistance without using any heavy metal ion, which is often used in light-fast dyes. In addition, the new dyes have large extinction coefficients, narrow bandwidth and a wide variation of absorption-wavelengths from ultra violet to near infrared. These characteristics will satisfy the requirements of many new applications in organic-glass based photonic devices.

 

2. What has motivated you to conduct this work? 

While we were engaged in the development of new dyes for DVD-R (recordable digital versatile disks), we encountered a problem with the poor light-resistance of oxonol dyes. Addition of and complexation with heavy-metal containing quenchers are the usual means for improvement. For fear of harmful effects due to heavy metal ions, and in expectation of a wider scope of applications, we sought a heavy-metal free technology. 

Inspired by knowledge of excitation behavior of photographic sensitizing dyes, we pictured a quenching mechanism in which a strong electron acceptor, e.g., bipyridinium, mediates the process of putting the excited electron back.

 

3. Where do you see this work developing in the future? 

We have actually introduced new dyes based on the present design principle into a DVD-R that has recently been put on the market. Besides optical disks there are many potential applications. In view of the usefulness of organic-glass forming dyes in such devices as photovoltaic cells and organic light emitting diodes, the present finding of heavy-metal free and organic-glass forming oxonol dyes with enhanced light-resistance will find new applications. Large extinction coefficients enable the dye to absorb light effectively even in a very thin film. This implies its usefulness for making very small and thin devices. The very rapid electron exchange that took place in the amorphous solid films of the present oxonol dyes is worth detailed investigation in relation to its potential application in devices based on electron transport.

 

4. Are there any particular challenges facing future research in this area? 

To meet various requirements of potential future applications, substantial effort will have to be made to fine-tune various properties of the new dyes. We are quite optimistic about this because the present design principle applies to any oxonol dye, and properties of the oxonol dyes can be easily and widely changed by modifying their molecular structure.