Electrogenerated\nChemiluminescence from Heteroleptic\nIridium(III) Complexes with Multicolor Emission

Abstract

Electrogenerated chemiluminescence\n(ECL) with different emission\ncolors is important in the development of multichannel analytical\ntechniques. In this report, five new heteroleptic iridium­(III) complexes\nwere synthesized, and their photophysical, electrochemical, and ECL\nproperties were studied. Here, 2-(2,4-difluorophenyl)­pyridine (dfppy,\ncomplex <b>1</b>), 2-phenylbenzo­[d]­thiazole (bt, complex <b>2</b>), and 2-phenylpyridine (ppy, complex <b>3</b>) were\nused as the main ligands to tune the emission color, while avobenzone\n(avo) was used as the ancillary ligand. For comparison, complexes <b>4</b> and <b>5</b> with 2-phenylpyridine and 2-phenylbenzo­[d]­thiazole\nas the main ligand, respectively, and acetyl acetone (acac) as the\nancillary ligand were also synthesized. All five iridium­(III) complexes\nhad strong intraligand absorption bands (π–π*)\nin the UV region (below 350 nm) and a featureless MLCT (d−π*)\ntransition in the visible 400–500 nm range. Multicolored emissions\nwere observed for these five iridium­(III) complexes, including green,\norange, and red for complexes <b>4</b>, <b>5</b>, <b>2</b>, <b>1</b>, <b>3</b>, respectively. Density functional\ntheory calculations indicate that the electronic density of the highest\noccupied molecular orbital is entirely located on the C^N ligands\nand the iridium atom, while the formation of the lowest unoccupied\nmolecular orbital (LUMO) is complicated. The LUMO is mainly assigned\nto the ancillary ligand for complexes <b>1</b> and <b>3</b> but to the C^N ligand for complexes <b>2</b>, <b>4</b>, and <b>5</b>. Cyclic voltammetry studies showed that all\nthese complexes have a reversible oxidation wave, but no reduction\nwaves were found in the electrochemical windows of CH₂Cl₂. The <i>E</i>_1/2^ox values of these complexes ranged from 0.642\nto 0.978 V for complexes <b>3</b>, <b>4</b>, <b>2</b>, <b>5</b>, <b>1</b>, (in increasing order) and are all\nlower than that of Ru­(bpy)₃²⁺. Most importantly, when using tripropylamine\nas a coreactant, complexes <b>1</b>–<b>5</b> exhibited\nintense ECL signals with an emission wavelength centered at 616, 580,\n663, 536, and 569 nm, respectively. In addition, complexes <b>1</b>, <b>2</b>, and <b>5</b> displayed approximately 2, 11,\nand 214 times higher ECL efficiencies than Ru­(bpy)₃²⁺ under identical conditions.