Josephine
Truong
a,
Kara B.
Spilstead
a,
Gregory J.
Barbante
a,
Egan H.
Doeven
a,
David J. D.
Wilson
b,
Neil W.
Barnett
a,
Luke C.
Henderson
ac,
Jarrad M.
Altimari
a,
Samantha C.
Hockey
a,
Ming
Zhou
d and
Paul S.
Francis
*a
aCentre for Chemistry and Biotechnology, School of Life and Environmental Sciences, Faculty of Science, Engineering and Built Environment, Deakin University, 75 Pigdons Road, Waurn Ponds, Victoria 3216, Australia. E-mail: paul.francis@deakin.edu.au
bDepartment of Chemistry, La Trobe Institute for Molecular Sciences, La Trobe University, Victoria 3086, Australia
cInstitute for Frontier Materials, Deakin University, 75 Pigdons Road, Waurn Ponds, Victoria 3216, Australia
dSunaTech Inc., BioBAY, Suzhou Industrial Park, Suzhou, Jiangsu 215123, P. R. China
First published on 12th September 2014
The chemiluminescence from four cyclometalated iridium(III) complexes containing an ancillary bathophenanthroline-disulfonate ligand exhibited a wide range of emission colours (green to red), and in some cases intensities that are far greater than the commonly employed benchmark reagent, [Ru(bpy)3]2+. A similar complex incorporating a sulfonated triazolylpyridine-based ligand enabled the emission to be shifted into the blue region of the spectrum, but the responses with this complex were relatively poor. DFT calculations of electronic structure and emission spectra support the experimental findings.
We have previously demonstrated the use of the highly polar bathophenanthroline-disulfonate (BPS) as an ancillary ligand in a cyclometalated iridium(III) complex to increase its solubility in aqueous solution.15 Moreover, we showed that bis(2-phenylpyridine-C2,N)(bathophenanthroline-disulfonate)iridium(III) ([Ir(ppy)2(BPS)]−, Fig. 1a: 1) provided greater sensitivity (in terms of the calibration gradient) than [Ru(bpy)3]2+ for the determination of oxalate, but the ruthenium-based reagent still provided better limits of detection due to its lower blank responses.15 Investigations with the difluoro-phenylpyridine analogue, [Ir(df-ppy)2(BPS)]− (2), revealed significant differences in the selectivity of light producing reactions of 1 and 2.17 Moreover, complex 2 provided a superior limit of detection for the pharmaceutical furosemide than that obtained using [Ru(bpy)3]2+. Following the publication of this work,15,17 these and other bis-cyclometalated iridium complexes containing an ancillary BPS ligand (Fig. 1a; 1–4) have been utilised for a variety of luminescence-based applications5,12,22–24 and they are now commercially available.
Fig. 1 Iridium(III) complexes containing sulfonate-functionalised ligands. (a) Complexes containing the BPS ligand. The position of the sulfonate groups on BPS varies depending on the source of the ligand.22,25–27 In this study, the p–m′-regioisomer23 was used. (b) A complex containing the 1-phenylsulfonate-1,2,3-triazol-4-ylpyridine (STP) ligand. |
Herein we explore, for the first time, the chemiluminescence of recently commercialised complexes [Ir(bt)2(BPS)]− (3) and [Ir(piq)2(BPS)]− (4), in addition to a novel complex containing a sulfonated triazolylpyridine ancillary ligand (Fig. 1b; 5), in direct comparison with the previously studied [Ir(df-ppy)2(BPS)]− (2) and [Ru(bpy)3]2+ reagents.3,17
Photoluminescence | Oxidation | ||
---|---|---|---|
λ em (nm) | ϕ PL 23 | E ox (V vs. Ag/AgCl)b | |
a 10 μM in 50:50 acetonitrile–water at room temperature. Spectra were corrected for the wavelength dependence of the detector response and monochromator transmission.29 b 0.1 mM in acetonitrile–water containing 0.1 M TBAPF6 at room temperature, measured using a potentiostat with three-electrode configuration. | |||
[Ir(df-ppy)2(BPS)]− | 549 | 0.29 | 1.54 |
[Ir(bt)2(BPS)]− | 573 | 0.40 | 1.36 |
[Ru(bpy)3]2+ | 624 | 0.06 | 1.23 |
[Ir(piq)2(BPS)]− | 632, 595 | 0.10 | 1.19 |
For quantitative comparison, the reagents (1 × 10−5 M) were injected into the analyte solution (1 × 10−6 M), which merged with the oxidant (1 × 10−3 M in 0.05 M H2SO4) in a T-piece just prior to entering a transparent PTFE coil flow-cell mounted in front of a photomultiplier tube. As observed in our previous work under these conditions,17 the green-light emitting [Ir(df-ppy)2(BPS)]− complex gave the greatest chemiluminescence signals (and signal-to-blank (S/B) ratios) with furosemide (Table 2). However, the current work revealed that [Ir(bt)2(BPS)]− provided greater chemiluminescence signals (and S/B ratios) with ofloxacin and codeine, in addition to the second greatest values with furosemide. This is considerably different to the reported order of ‘oxidative–reduction’ ECL intensities ([Ru(bipy)3]2+ (1) > [Ir(piq)2(BPS)]− (0.079) > [Ir(bt)2(BPS)]− (0.016) > [Ir(df-ppy)2(BPS)]− (0.001)) at relatively low metal-complex concentration with tri-n-propylamine co-reactant in buffered aqueous solution, despite the similarities in their light-producing reaction pathways.30,31 The complex exhibiting the highest photoluminescence quantum yield ([Ir(bt)2(BPS)]−, see Table 1) generally gave the largest chemiluminescence intensities, but overall, the correlation between photoluminescence quantum yields and chemiluminescence intensity was poor.
Ofloxacin | Furosemide | Codeine | |
---|---|---|---|
a n = 3; the relative standard deviation (RSD) of replicate injections was generally below 2.5%. | |||
[Ir(df-ppy)2(BPS)]− | 3.7 (28) | 3.3 (25) | 0.13 (1.0) |
[Ir(bt)2(BPS)]− | 29 (51) | 3.1 (4.6) | 2.4 (2.7) |
[Ru(bpy)3]2+ | 4.6 (15) | 0.7 (2.5) | 0.45 (1.6) |
[Ir(piq)2(BPS)]− | 6.6 (12) | 1.1 (1.2) | 0.97 (1.6) |
Ofloxacin calibrations prepared under these conditions exhibited a 7-fold steeper gradient, 2-fold higher intercept, and an order of magnitude superior limit of detection (3σ) of 3 × 10−9 M ofloxacin using [Ir(bt)2(BPS)]−, compared to that obtained using [Ru(bpy)3]2+.
When we increased the chemiluminescence reagent concentrations by orders of magnitude (to 1 mM, for example), the advantage of the iridium(III) complexes was diminished or even overcome. We have observed similar effects in the case of the [Ir(df-ppy)2(BPS)]− complex,17 and also for some related ruthenium(II) complexes,26 such as [Ru(BPS)3]4−. There is evidence to suggest that this (at least in part) arises from differences in the kinetics of the competing light-producing reactions of the oxidised reagent with the analyte and with the solvent.17 This effect explains the apparent discrepancy between the data in Table 2 and the relative intensities of light seen emanating from the flow-cells in Fig. 2, as the photographs were obtained using much higher concentrations of the reactants.
The presence of the BPS ligands in complexes 1–4 elicits a considerable bathochromic shift in the emission (compared to their homoleptic tris-cyclometalated analogues), towards a less sensitive region of the photodetector. To the naked eye, [Ir(ppy)3] emits green light (λmax = 530 nm),9 whereas the luminescence of [Ir(ppy)2(BPS)]− is orange (λmax = 628 nm).17 Similarly, the light from [Ir(df-ppy)3] is blue (λmax = 492 nm),9 but that from [Ir(df-ppy)2(BPS)]− is shifted into the green (λmax = 547 nm;17Fig. 2). To avoid this effect while maintaining reasonable solubility in water, we sought to prepare a sulfonated derivative of 1-phenyl-1,2,3-triazol-4-ylpyridine to use as an alternative ancillary ligand to BPS. In recent investigations of electrochemiluminescence detection with iridium(III) complexes,6,9 triazolylpyridine ligands have been shown to exert hypsochromic effects by stabilising the HOMO energy (compared to their homoleptic cyclometalated counterparts). In initial attempts to synthesise the sulfonated ligand and then form the iridium(III) complex, purification of the product from precursors was problematic. However, the target was readily obtained by first preparing the analogous thiol ligand (7) via an efficient two step synthetic sequence (Fig. 3a), then forming the iridium(III) complex (9; Fig. 3b), before oxidising the thiol with potassium peroxymonosulfate to form the desired sulfonate (5). The reagent was then directly prepared, without further isolation of the product, by appropriate dilution with an aqueous sulfuric acid solution.
Fig. 3 Synthesis of (a) the thiol ligand, and (b) the iridium(III) complex with an ancillary sulfonate-functionalised ligand. |
As expected, the emission maxima of [Ir(df-ppy)2(STP)] (λmax = 453 and 482 nm; Fig. 4a) occurred at much shorter wavelengths than those of [Ir(df-ppy)2(BPS)]− (λmax = 498 and 526 nm; Fig. 4b). For both complexes, the presence of vibronic fine structure is indicative of ligand-centred (LC) character.32
Fig. 4 Photoluminescence emission spectrum of (a) [Ir(df-ppy)2(STP)], (b) [Ir(df-ppy)2(BPS)]−, (c) [Ir(bt)2(BPS)]−, and (d) [Ru(bpy)3]2+, at 10 μM in 0.05 M H2SO4. |
Comparison of the novel complex against [Ir(df-ppy)2(BPS)]−, [Ir(bt)2(BPS)]−, and [Ru(bpy)3]2+ in aqueous solution (without acetonitrile) was conducted using flow injection analysis, after optimisation of conditions (manifold configuration, flow rate, cerium(IV) concentration). The presence of only one sulfonate group on the ancillary ligand (and the fluorine groups on the phenylpyridine ligands) of [Ir(df-ppy)2(STP)] lowered its solubility in aqueous solution compared to the other complexes. We therefore limited the reagent concentration in these comparisons to 1 × 10−5 M. Greater signals and S/B ratios were obtained by injecting the reagent into a cerium(IV) stream which then merged with the analyte in the detector (which in this case was a GloCel™ with dual-inlet serpentine flow cell).28
As shown in Table 3, [Ir(df-ppy)2(STP)] generally gave the lowest chemiluminescence intensities, but its S/B ratios for furosemide were superior to those of other iridium complexes. Nevertheless, the emission from [Ir(df-ppy)2(STP)] is the shortest wavelength chemically-induced luminescence from a metal complex in aqueous solution reported to date. Similar to the above findings in mixed solvents, [Ir(bt)2(BPS)]− and [Ir(df-ppy)2(BPS)]− exhibited much higher chemiluminescence intensities than [Ru(bpy)3]2+ (Table 3). Furthermore, the considerable differences in their intensities were observed over a wide range of analyte concentrations (e.g.Fig. 5). Under these conditions, however, the conventional ruthenium(II) chelate gave superior S/B ratios, due to its much lower blank responses. Consequently, the limit of detection (3σ) of ofloxacin using [Ru(bpy)3]2+ (3 × 10−9 M) was better than that using [Ir(bt)2(BPS)]− (6 × 10−9 M).
Ofloxacin | Furosemide | Codeine | |
---|---|---|---|
a n = 3; the RSD of replicate injections was generally below 2.5%. | |||
[Ir(df-ppy)2(STP)] | 0.6 (1.3) | 3.2 (6) | 0.2 (0.5) |
[Ir(df-ppy)2(BPS)]− | 91.4 (16) | 8.4 (1.4) | 17.9 (3) |
[Ir(bt)2(BPS)]− | 536 (31) | 79.9 (4) | 16.6 (1.1) |
[Ru(bpy)3]2+ | 24.9 (196) | 2.6 (20) | 0.8 (8) |
The differences in the blank signals of the reagents arise from: (i) the rate that each metal complex is oxidised by cerium(IV) (i.e. the proportion of the metal complex that is in its oxidised form within the detection zone); (ii) the relative rates of the chemiluminescent (blank) reaction of the oxidised metal-complexes with the acidic aqueous solvent;33 (iii) the efficiency that the corresponding excited state is generated;34 and (iv) the efficiency of the emission (i.e. photons emitted per excited luminophore molecules). The same or analogous considerations must be made when comparing the different responses of each reagent with the same analyte.
As the emissions corresponding to the solvent (blank) and analyte are both transient and may occur at different rates, the greatest S/B ratio observed in this flow-analytical system was dependent on the solution flow rate, which was optimised for each combination of analyte and reagent.
Fig. 6 Ground-state singlet molecular energy surfaces of (a) [Ru(bpy)3]2+, (b) [Ir(df-ppy)2(BPS)]− and (c) [Ir(df-ppy)2(STP)]. |
Analysis of the MOs of [Ir(df-ppy)2(BPS)]− (Fig. 6b and 7b) is illustrative of the theoretical results for each of the [Ir(C^N)2(BPS)]− series (n.b.: frontier MO and triplet spin density surfaces for all complexes are included in Table S1 in ESI†). The singlet HOMO of [Ir(df-ppy)2(BPS)]− is principally composed of a mixture of the iridium d and the phenyl π orbitals, distributed equally across the two C^N-type df-ppy ligands. The LUMO is localised on the BPS ligand, predominantly on the phenanthroline moiety. The triplet spin density surface (Fig. 7b) shares the same spatial extent as the singlet LUMO and HOMO,37 which in this case leads to a description of the lowest energy excited state as having metal–ligand-to-ligand charge-transfer (MLLCT) character.
The results for the novel [Ir(df-ppy)2(STP)] complex (Fig. 6c) are analogous to those of the [Ir(C^N)2(BPS)]− series, where the singlet HOMO is composed of a mixture of the Ir d and the phenyl π orbitals of the df-ppy ligands, and the LUMO is localised on the sulfonate ligand. Mulliken population analysis of fragment contributions to the HOMO and LUMO (Fig. 8) highlighted the similarities of these iridium compounds. In each case, Ir contributes 35–37% of the HOMO while the phenyl (π) ring of the C^N ligand contributes ∼60%. The LUMO is almost exclusively composed of the sulfonate ligand (94–97%). The triplet spin density surface of [Ir(df-ppy)2(STP)] shares the same spatial extent as the singlet LUMO and HOMO.
For each of the iridium complexes under investigation, there is very little overlap between the singlet-state HOMO and LUMO (i.e. they are largely orthogonal), which indicates that the HOMO and LUMO energies can be independently ‘tuned’ by appropriate substitution of donor/acceptor groups on the C^N or sulfonate ligands, respectively. For example, the HOMO energies for [Ir(df-ppy)2(BPS)]− and [Ir(df-ppy)2(STP)]− are very similar (−6.07 and −6.11 eV), but the LUMO energies are −2.70 and −2.26 eV (Fig. S1; ESI†). That is, the common df-ppy ligand ensures little change to the HOMO properties, but modification of the sulfonate ligand significantly influences the LUMO.
The singlet–triplet transition energy of the complexes can be estimated from the difference between the triplet state highest singly occupied MO (HSOMO) and the singlet HOMO. For example, the B3LYP/def2-TZVP calculated energy difference for [Ir(bt)2(BPS)]− is 2.32 eV (534 nm). The experimental measurement is 576 nm, but this is only formally comparable to the theoretical results in the limit of low-temperature measurements (in this work all measurements were recorded at room temperature). Nevertheless, a general trend emerges that the rank order of calculated singlet–triplet energy gaps of the complexes matches that of the experimental results (Fig. S2; ESI†).
To probe the nature of the luminescence emission bands, TD-DFT calculations of the lowest-energy vertical emissions were carried out (estimated by singlet state TD-DFT calculations at the triplet-state optimised geometries; Table S2†). As discussed above, for the iridium compounds (Fig. 6 and 8), a HOMO–LUMO transition would be attributed to a mixture of MLCT and LLCT (HOMO is Ir d orbital and π orbital of the C^N ligand, LUMO is on the sulfonate ligand). However, the emission spectrum arises from a range of electronic transitions, and moreover, the HOMO–LUMO transition is not necessarily the dominant transition.
TD-DFT results indicate that the complexes that contain the df-ppy ligand exhibit emission bands with significant ligand-centred (LC) character. For [Ir(df-ppy)2(BPS)]−, the LC transition occurs within the BPS ligand (HOMO−1 to LUMO), while for [Ir(df-ppy)2(STP)]−, the LC transition occurs within the df-ppy ligand (HOMO to LUMO+1). Similarly, both the [Ir(bt)2(BPS)]− and [Ir(piq)2(BPS)]− complexes are predicted to exhibit some LC character. The [Ir(ppy)2(BPS)]− and [Ir(ppy)2(STP)]− complexes are not expected to show LC character, but are dominated by MLCT and LLCT transitions.
In general, emission bands from charge-transfer (CT) states are broad and featureless, while ligand-centred (LC) states typically give emissions with vibronic structure.32 The presence of vibronic structure in the emission spectra of [Ir(df-ppy)2(BPS)]− and [Ir(df-ppy)2(STP)]− (Fig. 4) is indicative of contributions from ligand-centred (LC) transitions,38,39 and is consistent with the theoretical results. The [Ir(bt)2(BPS)]− (Fig. 4) and [Ir(piq)2(BPS)]− (not shown) complexes also exhibit minor shoulder bands that are attributed to LC transitions. The emission bands of [Ir(ppy)2(BPS)]− and [Ir(ppy)2(STP)]− (not shown) are broad and featureless, consistent with MLCT and LLCT (MLLCT) character.
Fig. 9 (a) Generalised mechanism of the chemiluminescence reactions of transition metal complexes with tertiary amines,30,31,40 and (b) an alternative light producing pathway involving energy transfer from an excited intermediate derived from the tertiary amine. |
We also observed a weak emission of light from the reaction of cerium(IV) with ofloxacin, in the absence of the metal complex, indicating that another electronically excited species (derived from the tertiary amine compound) can be generated in this reaction. In the presence of the metal complex, the contribution of the direct emission from the alternative excited state species to the overall chemiluminescence would not be significant. However, it is possible that the alternative excited species is capable of transferring energy to the more efficient metal complex luminophore (Fig. 9b), and this alternative light producing pathway could make a significant contribution to the overall emission.
To test the contribution of this alternative light-producing pathway, we replaced the metal complexes with two efficient luminophores, rhodamine B and quinine. These compounds have commonly been used to sensitise the weak chemiluminescence oxidation of various organic compounds with strong inorganic oxidants such as cerium(IV), bromate and permanganate.41–45 However, in this case, no significant emission from either luminophore was observed, indicating that the postulated energy transfer pathway (Fig. 9b) does not make a significant contribution to the chemiluminescence reaction of metal complexes.
The increased sensitivity of certain iridium complex reagents compared to [Ru(bpy)3]2+ was therefore largely attributed to a combination of their greater reactivity with the analytes after oxidation, in addition to superior excitation and/or luminescence efficiencies. Moreover, the observed analyte dependence of these effects highlights the relative importance of the reactivity of the respective oxidised complex towards the analyte to generate the radical intermediates, and towards the α-amino alkyl radical as the source of chemi-excitation.
Footnote |
† Electronic supplementary information (ESI) available: Additional ground state singlet HOMO and LUMO and excited state triplet spin density surfaces; HOMO, LUMO and singlet–triplet transition energies; and Cartesian coordinates of optimized geometries. See DOI: 10.1039/c4an01366b |
This journal is © The Royal Society of Chemistry 2014 |