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Reversible photoredox switching of porphyrin-pridged Bis-2,6-di-tert-butylphenols
journal contributionposted on 15.08.2014, 11:27 by Shinsuke Ishihara, Jonathan P. Hill, Atsuomi Shundo, Gary J. Richards, Jan Labuta, Kei Ohkubo, Shunichi Fukuzumi, Akira Sato, Mark ElsegoodMark Elsegood, Simon J. Teat, Katsuhiko Ariga
Porphyrin derivatives bearing 2,6-di-tert-butylphenol substituents at their 5,15-positions undergo reversible photoredox switching between porphyrin and porphodimethene states as revealed by UV vis spectroscopy, fluorescence spectroscopy, and X-ray singlecrystal analyses. Photoredox interconversion is accompanied by substantial variations in electronic absorption and fluorescence emission spectra and a change of conformation of the tetrapyrrole macrocycle from planar to roof-shaped. Oxidation proceeds only under photoillumination of a dianionic state prepared through deprotonation using fluoride anions. Conversely, photoreduction occurs in the presence of a sacrificial electron donor. Transient absorption spectroscopy and electron spin resonance spectroscopy were applied to investigate the processes in photochemical reaction, and radical intermediates were characterized. That is, photooxidation initially results in a phenol-substituent-centered radical, while the reduction process occurs via a delocalized radical state involving both the macrocycle and 5,15-substituents. Forward and reverse photochemical processes are governed by different chemical mechanisms, giving the important benefit that conversion reactions are completely isolated, leading to better separation of the end states. Furthermore, energy diagrams based on electrochemical analyses (cyclic voltammetry) were used to account for the processes occurring during the photochemical reactions. Our molecular design indicates a simple and versatile method for producing photoredox macrocyclic compounds, which should lead to a new class of advanced functional materials suitable for application in molecular devices and machines.
This research was partially supported by the World Premier International (WPI) Research Center Initiative on Materials Nanoarchitectonics and Grant-in-Aid (No. 20108010) from MEXT (Japan), Core Research for Evolutional Science and Technology (CREST) from JST (Japan), and KOSEF/MEST through the WCU project (R31-2008-000-10010-0). A.S. and J.L. are grateful to Japan Society for Promotion of Science (JSPS) for Fellowships. Advanced Light Source (ALS) is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02- 05CH11231.