Intense annihilation electrogenerated chemiluminescence from tris(2-phenylpyridinato)iridium(III) with organic radical cations and anions

Abstract

The electrogenerated chemiluminescence (ECL) of 0.2 mM tris(2-phenylpyridinato)iridium(III) (Ir(ppy)3) in acetonitrile with a potential step method was dramatically enhanced by the electrochemically generated radical cation of 10-methylphenothiazine (MePT), thianthrene (TH), and dibenzo-1,4-dioxin (DD) or the radical anion of benzophenone (BP) and benzonitrile (BN). In the case of using the radical cation of phenothiazine (PT), no ECL was seen. Among the binary systems containing 0.2 mM Ir(ppy)3 and 10 mM parent molecule of the radical ions, intense annihilation ECL of Ir(ppy)3 was observed from a solution containing BN; the ECL intensity was about 250 times greater than that of the single system of 0.2 mM Ir(ppy)3. Among the cases using the radical cations, the ECL intensity with TH was the largest (about 90 times higher than that of the single system). The mechanism for enhancing the ECL is discussed based on the redox potential, energy level of the lowest triplet excited (T1) state, and electron-donating and −accepting characteristics. The electron transfer reaction between the reduced form of Ir(ppy)3 (Ir(ppy)3•−) and the radical cations can produce the T1 state of Ir(ppy)3 (3Ir(ppy)3*) and that of MePT, TH, and DD, whereas the electron transfer reaction between the oxidized form of Ir(ppy)3 (Ir(ppy)3•+) and the radical anions can produce 3Ir(ppy)3* but not the T1 state of BN or BP. The lower ECL intensity using the radical cations relative to that of using BN suggests that the triplet energy transfer from the parent molecule of the radical cations to Ir(ppy)3 is not efficient. It is also shown that the homogeneous electron transfer reaction between Ir(ppy)3 and the electrochemically generated radical cation of TH and radical anion of BN to produce Ir(ppy)3•+ and Ir(ppy)3•−, respectively, which facilitates an ion annihilation reaction between Ir(ppy)3•− and Ir(ppy)3•+, is also responsible for the intense ECL.