Understanding Electrogenerated Chemiluminescence Efficiency in Blue‐Shifted Iridium(III)‐Complexes: An Experimental and Theoretical Study

Abstract

Compared to tris(2-phenylpyridine)iridium(III) ([Ir(ppy)3 ]), iridium(III) complexes containing difluorophenylpyridine (df-ppy) and/or an ancillary triazolylpyridine ligand [3-phenyl-1,2,4-triazol-5-ylpyridinato (ptp) or 1-benzyl-1,2,3-triazol-4-ylpyridine (ptb)] exhibit considerable hypsochromic shifts (ca. 25-60 nm), due to the significant stabilising effect of these ligands on the HOMO energy, whilst having relatively little effect on the LUMO. Despite their lower photoluminescence quantum yields compared with [Ir(ppy)3 ] and [Ir(df-ppy)3 ], the iridium(III) complexes containing triazolylpyridine ligands gave greater electrogenerated chemiluminescence (ECL) intensities (using tri-n-propylamine (TPA) as a co-reactant), which can in part be ascribed to the more energetically favourable reactions of the oxidised complex (M(+) ) with both TPA and its neutral radical oxidation product. The calculated iridium(III) complex LUMO energies were shown to be a good predictor of the corresponding M(+) LUMO energies, and both HOMO and LUMO levels are related to ECL efficiency. The theoretical and experimental data together show that the best strategy for the design of efficient new blue-shifted electrochemiluminophores is to aim to stabilise the HOMO, while only moderately stabilising the LUMO, thereby increasing the energy gap but ensuring favourable thermodynamics and kinetics for the ECL reaction. Of the iridium(III) complexes examined, [Ir(df-ppy)2 (ptb)](+) was most attractive as a blue-emitter for ECL detection, featuring a large hypsochromic shift (λmax =454 and 484 nm), superior co-reactant ECL intensity than the archetypal homoleptic green and blue emitters: [Ir(ppy)3 ] and [Ir(df-ppy)3 ] (by over 16-fold and threefold, respectively), and greater solubility in polar solvents.

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

“…However, a stronger donor ligand in the iridium(III) complex rather increased the ECL response in the present work. It is reported that the lower the energy level of the LUMO (lowest unoccupied molecular orbital) of the iridium complex, the more efficient ECL process, and thus higher ECL response [28]. In this work, the energy level of the LUMO obtained from electrochemical studies are not well correlated with the observed ECL intensity.…”

“…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+ .…”

“…Up to now, The cyclometalated iridium(III) complexes have been applied for the ECL detection of the antibiotics [19], ammonium ion [20], cancer cell [21], ammonia [24], and cytosensing [26]. Since the ECL emission wavelengths in cyclometalated iridium(III) complexes can be tuned from the near-infrared to blue via changing the structure of the ligand, they are particularly useful in the development of multichannel ECL-based assays [15,28,32]. Many research groups have been working in designing or modifying the ligands of the iridium(III) complexes in order to tune the emission color as well as to enhance the emission efficiency.…”

Molecular-engineered [Ir(Fppy)2(Mepic)] towards efficient blue-emission

“…15,43,[52][53][54] Therefore, from the distinct emission properties, it is more appropriate to assume a mixed MLCT-LC character (Fppy-based) for the T 1 state of FIrMepic, harvested by singlet and triplet relaxations, similarly to FIrpic ( Figure S8 in supporting information). 29 . Data obtained using the three standards are in excellent agreement among them and also to the absolute value, measured using an integrating sphere.…”

Imaging Analysis Based on Electrogenerated Chemiluminescence

“…In comparison with the classical Ru(bpy) 3 2? , cyclometalated iridium(III) complexes generally possess much higher photoluminescence (PL) efficiency and easy tunability of the emission color. By modulating the ligand and changing the coordination mode, a series of cyclometalated iridium(III) complexes have been designed, synthesized, and thoroughly investigated through photophysics, electrochemistry, and ECL [23][24][25][26][27]. The development of new ECL emitters with different emission colors makes it possible to simultaneously determine multiplex analytes in clinical diagnostics.…”