Intramolecular Electron Transfer of Diporphyrins Comprised of Electron-Deficient Porphyrin and Electron-Rich Porphyrin with Photocontrolled Isomerization

Abstract

New azobenzene-linked diporphyrin Zn complexes and free base analogues were synthesized in an attempt to develop new materials for molecular electronics. These diporphyrin Zn complexes and the corresponding diporphyrins consist of electron-deficient porphyrin Zn complex and electron-rich porphyrin Zn complex or electron-deficient porphyrin and electron-rich porphyrin. These compounds, which are a new family of diporphyrins, exhibited the photoresponsive property with a structual change in the antenna; the trans−cis photoisomerization and cis−trans thermal recovery occurred and this process was reversible. A particularly noteworthy characteristic of the diporphyrin Zn complex is that its fluorescence spectrum changes with isomerization; the fluorescence intensity of the cis-isomer produced by photoirradiation is smaller than that of the trans-isomer. This fluorescence quenching of the cis-isomer arises from the intramolecular electron transfer from electron-rich porphyrin Zn complex to electron-deficient porphyrin Zn complex. Similar phenomena were observed for azobenzene-linked diporphyrins. This discovery that intramolecular electron transfer between the porphyrin rings is caused by photocontrolled isomerization is worthy of notice. Additionally, these compounds bearing eight fluorine atoms at the β-position of one porphyrin ring are the ideal materials for the practical application, because of their robust properties. This finding clearly suggests that new molecules are particularly useful in the development of photocontrolled molecular electronics such as molecular switches.