Photoelectronic Utility of Photoinduced Electron Transfer by Phosphorescent Ir(III) Complexes
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Photoelectronic Utility of Photoinduced Electron Transfer by Phosphorescent Ir(III) Complexes Youngmin You1 1 Department of Advanced Materials Engineering for Information and Electronics, Kyung Hee University, Yongin, Gyeonggi-do 446-701, Korea ABSTRACT Intermolecular photoinduced electron transfer (PeT) has found a wide range of photoelectronic utility. One of the most notable examples includes the natural photosynthesis, where PeT between chlorophyll and quinone triggers photon-to-chemical energy conversion. We observed that phosphorescent Ir(III) complexes exhibited efficient PeT to trigger a cascade of catalytic intermolecular electron transfer among electrochemically active molecules. To establish the photoelectronic utility of PeT, a series of cyclometalated Ir(III) complexes were prepared and evaluated for photoelectrocatalytic conversion of dithienylethene (DTE) compounds. Selective photoexcitation of the Ir(III) complexes facilitated ultrafast PeT from DTE. The oxidative PeT initiated electrocatalytic cycloreversion of DTE, yielding one order of magnitude enhancement in quantum yields relative to direct photochromic conversion. INTRODUCTION Photochromism refers to reversible interconversion between two isomeric species under alternating photoirradiation.[1] Among the various classes of photochromic molecules, cis-1,2dithienylethene (DTE) compounds are most promising because their photochromic properties exhibit several advantages, including ultrafast conversion, high fatigue resistance, and thermal stability of two isomers, open and closed forms.[2] The open form of the DTE compound undergoes an intramolecular [2+4] cycloaddition reaction under UV irradiation to produce the closed form, while photoirradiation at visible regions restores the open form. The two isomeric species differ significantly by optical, electrical, and magnetic properties due to changes in the πconjugation along the molecular frameworks. A variety of molecular switches have thus been developed to take advantage of the photo-modulated physical properties of DTE compounds.[3] Despite the advances, the use of DTE molecules for creation of truly reversible molecular switches has been retarded by low quantum yields for the ring-opening reaction. In fact, the quantum yields for the photochromic ring-opening process is one order of magnitude smaller than that for the photochromic ring-closing reaction.[4−6] The asymmetry in the photochromic quantum yields originates from the excited-state potential energy surface that favors ring-closing reaction. To circumvent this challenge, several approaches utilizing multi-photon excitation[7] and triplet sensitization[8] were pursued. We also developed a photoelectrocatalytic strategy which takes advantage of barrierless electrochromic ring opening of DTE compounds.[9] Specifically, photoexcitation of a photoredox catalyst, 9-mesityl-10-methylacridinium ion promotes one-electron oxidation of the closed form of DTE compounds which undergo spontaneous ring opening. The radical cation of the open form is n
