A research group consisting of Prof. Yoshio Teki of the Osaka Metropolitan University Graduate School of Engineering, Prof. Kohei Yoshida of the Kwansei Gakuin University Graduate School of Science, and others has achieved photostability more than 100 times higher than that of TIPS-pentacene, a famous commercially-available pentacene derivative, by increasing the planarity of the molecule and strengthening the conjugation of π electrons between a radical substituent and pentacene moiety. At the same time, to elucidate the mechanism of the remarkable photostability, ultrafast transient absorption measurements using a femtosecond pulsed laser were performed to clarify the peculiar excited-state dynamics of this system. Focusing on the pentacene moiety of the system, they found that intersystem crossing occurs at an ultrafast rate (10^-13 seconds), which has never been achieved before in purely organic materials containing no heavy atoms. Furthermore, the subsequent ultrafast deactivation to the ground state was observed to occur within a time of about 10^-10 seconds.

Due to their high hole mobility, pentacene and its derivatives have been the representative organic semiconductors and have been the subject of much research, both basic and applied. In particular, they are expected to be applied to semiconductor devices such as field-effect transistors. In addition, organic semiconductors have the advantage of being inexpensive to produce by inkjet printing and having low environmental impacts because they do not use metals. However, the backbone of organic semiconductors such as pentacene easily reacts with oxygen molecules under visible light, resulting in the loss of useful properties.

This article was published online on April 11, 2022, and featured on the back cover of the physical edition of Physical Chemistry Chemical Physics.