Electrogenerated Chemiluminescence from Heteroleptic Iridium(III) Complexes with Multicolor Emission

Abstract

Electrogenerated chemiluminescence (ECL) with different emission colors is important in the development of multichannel analytical techniques. In this report, five new heteroleptic iridium(III) complexes were synthesized, and their photophysical, electrochemical, and ECL properties were studied. Here, 2-(2,4-difluorophenyl)pyridine (dfppy, complex 1), 2-phenylbenzo[d]thiazole (bt, complex 2), and 2-phenylpyridine (ppy, complex 3) were used as the main ligands to tune the emission color, while avobenzone (avo) was used as the ancillary ligand. For comparison, complexes 4 and 5 with 2-phenylpyridine and 2-phenylbenzo[d]thiazole as the main ligand, respectively, and acetyl acetone (acac) as the ancillary ligand were also synthesized. All five iridium(III) complexes had strong intraligand absorption bands (π–π*) in the UV region (below 350 nm) and a featureless MLCT (d−π*) transition in the visible 400–500 nm range. Multicolored emissions were observed for these five iridium(III) complexes, including green, orange, and red for complexes 4, 5, 2, 1, 3, respectively. Density functional theory calculations indicate that the electronic density of the highest occupied molecular orbital is entirely located on the C^N ligands and the iridium atom, while the formation of the lowest unoccupied molecular orbital (LUMO) is complicated. The LUMO is mainly assigned to the ancillary ligand for complexes 1 and 3 but to the C^N ligand for complexes 2, 4, and 5. Cyclic voltammetry studies showed that all these complexes have a reversible oxidation wave, but no reduction waves were found in the electrochemical windows of CH2Cl2. The E1/2(ox) values of these complexes ranged from 0.642 to 0.978 V for complexes 3, 4, 2, 5, 1, (in increasing order) and are all lower than that of Ru(bpy)3(2+). Most importantly, when using tripropylamine as a coreactant, complexes 1–5 exhibited intense ECL signals with an emission wavelength centered at 616, 580, 663, 536, and 569 nm, respectively. In addition, complexes 1, 2, and 5 displayed approximately 2, 11, and 214 times higher ECL efficiencies than Ru(bpy)3(2+) under identical conditions.

Co‐reactant Electrogenerated Chemiluminescence of Iridium(III) Complexes Containing an Acetylacetonate Ligand

“…The UV‐visible absorption spectra of the Ir( ) 2 (acac) complexes (Figure a) were in good agreement with previous reports, in which the intense absorption bands between 250 nm and 350 nm were assigned to spin‐allowed singlet intra‐ligand ( 1 LC) transitions (π→π*, ppy/bt/pq) and the weaker bands above 400 nm to mixed singlet and triplet metal‐to‐ligand charge‐transfer (MLCT) transitions (dπ(Ir)→π*(ppy/bt/pq)) and intra‐ligand transitions …”

“…reported that the ECL signals for Ir(pq) 2 (acac) and Ir(pq) 2 (dm‐acac) with TPrA co‐reactant in acetonitrile were 10‐fold and 38‐fold greater than that of [Ru(bpy) 3 ] 2+ , under identical conditions. An even greater relative co‐reactant ECL intensity (214‐fold that of [Ru(bpy) 3 ] 2+ ) was observed for Ir(bt) 2 (acac) with TPrA in dichloromethane, but unlike the previous studies, their relative ECL intensities in acetonitrile were not reported.…”

“…With these considerations in mind, we have re‐examined several promising electrochemiluminophores (Ir(ppy) 2 (acac), Ir(bt) 2 (acac) and Ir(pq) 2 (acac)), in direct comparison with the archetypal [Ru(bpy) 3 ] 2+ and Ir(ppy) 3 complexes. We evaluate the relative ECL intensities of these complexes with TPrA co‐reactant in acetonitrile across a range of complex and co‐reactant concentrations, and instrument configurations.…”

Efficient green electrogenerated chemiluminescence from cyclometalated iridium(III) complex

“…Our research group has recently synthesized five iridium(III) complexes with green, orange and red efficient ECL emissions by changing the main and ancillary ligands [36]. Compared to the ECL efficiency of iridium(III) complexes with orange and red emissions, the ECL efficiency from iridium(III) complex with green emission is relatively low.…”

Electrogenerated chemiluminescence from newly synthesized α-diimine-ligated heteroleptic iridium(III) complexes

“…Lee and coworkers reported the synthesis and characterization of new iridium complexes of (pq) 2 Ir(acac) and (pq) 2 Ir(tmd) (pq = 2-phenylquinoline anion, acac = acetylacetonate anion, and tmd = 2,2′,6,6′-tetramethylhepta-3,5-dione anion) with much higher ECL efficiencies at 589 and 593 nm compared to that of Ru(bpy) 3 2+ [16]. Subsequently, a large number of novel iridium(III) complexes with various ligands have been prepared for ECL studies [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]. In particular, heteroleptic iridium(III) complexes such as Ir(dfppy) 2 (bpy) [27] and Ir(dfppy) 2 (PyBiz) [32] (dfppy = 2-(2,4-difluorophenyl) pyridine, PyBiz = 2-(2-pyridyl)benzimidazolato-N,N′) as the primary cyclometalating ligand have shown the highly enhanced ECL response compared to that of Ru(bpy) 3 2+ .…”