Strong enhancement of luminescence from an iridiumpolypyridyl complex via encapsulation in cucurbituril†
Lubna R. Alrawashdeh, Anthony I. Day and Lynne Wallace*
An exceptional, temperature-dependent enhancement of lumines-
cence is reported upon encapsulation of an iridium(III) polypyridyl
complex in cucurbit[10]uril (Q[10]). This is the first demonstrated
example of a luminescent transition metal complex occupying the
Q[10] cavity with this type of differential response.
Polypyridyl complexes of the transition metals are among themost extensively investigated luminescent systems, with thecomplexes of d6 metals (such as ruthenium(II), rhodium(III),rhenium(I) and iridium(III)) receiving particular attention formany years now due to their inert nature and attractive photo-physical properties. Extending from this, complexes of Ir(III)with C-metallating ligands such as 2-phenylpyridine (ppy), forexample [Ir(ppy)2(bpy)]
+ (bpy = 2,2′-bipyridine), were found toexhibit luminescence that is exceptionally sensitive to the localenvironment, prompting great interest in the application ofthese species in diverse areas from light-harvesting andelectro-optical devices1–4 to photocatalysis5,6 and as molecularprobes, particularly for biological systems.7–12 In most appli-cations the photophysical properties are critical to efficiency:for example the sensitivity of a luminescent sensor is directlycorrelated with the emission intensity; and for electrolumines-cent devices a long-lived excited state is also highly desirable.Approaches for enhancing luminescence of these transitionmetal complexes are therefore of continuing interest.2,13,14
Environmental factors such as solvent, ionic strength andpH can heavily influence the photophysical properties. Inmany cases, water is the preferred medium in which sensorsor photocatalysts will be applied, particularly when biologicalsystems are under study. Aqueous media may also be preferredfor economic and/or environmental reasons. However, forsome iridium phenylpyridine complexes, the luminescence
intensity can be much weaker in aqueous systems compared toorganic solvents.
Encapsulation of a luminescent species within a host mole-cule can sometimes enhance emission intensity.15–18 Thefamily of cucurbiturils (Q[n], n = 5, 6, 7, 8, 10) have by nowbeen used very extensively as host molecules, and significantenhancements in emission have been reported upon encapsu-lation of organic fluorophores in the smaller Q[n] (n =6–8).18–20 However encapsulation of luminescent metal tris-chelate complexes is less common, as these are too large toenter the commonly available hosts. Q[10], with the largestaccessible cavity, is a recently isolated member of the series andhas received modest attention as a molecular host, with guestphotophysical properties being unremarkable: to our knowl-edge there are only three examples of Q[10] complexation withluminescent transition metal complexes to date, and none haveshown any emission enhancement.21–23
We report here the first example of an exceptionally lumi-nescent host–guest system comprising an iridium polypyridylcomplex in the cavity of a cucurbituril (Q[10]) host. The photo-physical properties of the host–guest system were investigatedin aqueous buffer solution (pH 4.7). The iridium complex,[Ir(ppy-CHO)2(bpy)]
+ (where ppy-CHO = 4-(2-pyridyl)benz-aldehyde) is abbreviated here as IrCHO, and the structures ofQ[10] host and guest are shown in Fig. 1.
Fig. 1 (a) Structures of [Ir(ppy-CHO)2(bpy)]+ (IrCHO), with protons labeled, and
cucurbit[10]uril (Q[10]).†Electronic supplementary information (ESI) available: Experimental details,NMR data and additional figures. See DOI: 10.1039/c3dt51836a
School of Physical Environmental and Mathematical Sciences, The University of New
South Wales, UNSW Canberra, PO Box 7916, Canberra BC 2610, Australia.
Addition of Q[10] to IrCHO had negligible effect on theabsorption spectrum of the species (Fig. S1†), but stronglyaffected the emission spectrum, giving a very large enhance-ment in the luminescence intensity and a blue shift (from595 nm to 543 nm) in the emission maximum (Fig. 2). With a1 : 1 molar ratio of host to guest, the intensity from a 1 × 10−5 Msolution of IrCHO was enhanced by ≈40-fold, and this increasedto a maximum of ≈80-fold in the presence of excess Q[10] (>10×excess). The emission spectrum of Q[10]-encapsulated IrCHO inthe aqueous medium also became more structured. It is similarin profile to the spectrum of the free complex in media of lowerpolarity;9,24 however more structured spectra also occur forother iridium phenylpyridine complexes in low temperaturerigid media.7,25 This suggests the enhancement effect is relatedto the lower polarity of the Q[10] cavity relative to the aqueousmedium, or to the more rigid environment it provides.
The environment-sensitive nature of complexes based onthe [Ir(ppy)2(bpy)]
+ chromophore is often attributed to theclose proximity of two (or more) possible emitting states;3,9,25
these usually include LC and MLCT states.‡ In the aqueousmedium used here, free IrCHO exhibits photophysical pro-perties consistent with 3MLCT emission. However, in media oflower polarity the emitting state of free IrCHO has beenassigned as 3LC(ppy),9,24 with some 3MLCT(bpy) character alsoproposed based on experimental results and theoretical calcu-lations.9 As conditions of lower polarity or increased rigidity ofthe medium both tend to increase MLCT energies, encapsula-tion in Q[10] may have several possible effects: confinement ofthe molecule might produce effects similar to a rigid environ-ment (limited ability of the medium to undergo rearrange-ment to accommodate the charge distribution of the excited-state species and/or inhibition of nonradiative deactivationprocesses); while the non-polar environment of the cavitymight increase 3MLCT energy, possibly to the extent that 3LCemission is now favoured.
Consistent with the enhancement in intensity, the lumines-cence lifetime of IrCHO in aqueous buffer (36 ns) was alsoincreased, by almost an order of magnitude, in the presence of
excess Q[10]. A dual exponential was needed to fit the decayprofile. The lifetime of the long component for encapsulatedIrCHO (2.69 μs) begins to approach the lifetime reported forthe free complex in acetonitrile (4.71 μs);24 under these con-ditions the luminescence was assigned to a 3LC state. This pro-vides further evidence that the non-polar environment of theQ[10] cavity has a strong influence on the emitting state of theguest complex. However, the lifetime of the short component(160 ns) is longer than the lifetime of free IrCHO in the samemedium (36 ns), implying that a simple interpretation basedon free and encapsulated species is inadequate.
Further binding studies were carried out via steady-statefluorescence measurements. Titration of Q[10] into a solutionof IrCHO (Fig. 3) resulted in an increase in emission intensity.A Benesi–Hildebrand plot20 assuming 1 : 1 binding stoichio-metry (Fig. 3, inset) fitted very well to the data (R2 = 0.999)over the latter part of the curve and gave a binding constant of7.0 × 104 M−1. However, at lower Q[10] concentrations the rateof increase of intensity with added host was much lower, andthe Benesi–Hildebrand method could not be applied over thisrange.
At lower Q[10] : IrCHO ratio (≈0.3 : 1) the decay profilecould be fitted to a single exponential and the lifetime (140 ns)approached the value obtained for the short component in thepresence of excess Q[10]. These results suggest that when thehost availability is low, two IrCHO molecules may associatewith Q[10]; this would involve some binding at the portals,which generally gives a smaller enhancement effect than fullencapsulation.15,17 It is likely, therefore, that there is compe-tition between the two modes of binding to IrCHO, with 1 : 1stoichiometry being increasingly favoured as Q[10] availabilityincreases (Fig. 4).
The overall binding constant of 7.0 × 104 indicates that the1 : 1 association between IrCHO and Q[10] is reasonablystrong. However, the non-exponential decay of the lumines-cence observed for the Q[10]-IrCHO host–guest system impliesthat at least a small proportion of a shorter-lived species, prob-ably the 1 : 2 portal-bound species, is present even with excessQ[10].
Fig. 3 Fluorescence enhancement (F/F0) of IrCHO (1.2 × 10−5 mol L−1) at540 nm as a function of Q[10] : IrCHO ratio in aqueous buffer solution (pH =4.7) at 22 °C; inset: Benesi–Hildebrand plot over high Q[10] portion of the curve,assuming 1 : 1 binding stoichiometry.
Fig. 2 Left panel: emission spectra in aqueous buffer solution (pH 4.7) at22 °C: free IrCHO (dashed, red); with added Q[10] in 1 : 1 molar ratio (dotted,blue); and with excess Q[10] (solid, green). IrCHO concentration = 10−5 mol L−1.Right panel: solutions of free IrCHO (left) and Q[10]-encapsulated IrCHO (right),[IrCHO] = 4 × 10−5 M in both.
The measured binding constant is therefore likely to rep-resent some combination of the binding constants for 1 : 2(portal) binding and 1 : 1 (cavity) binding. Further studies areunderway to investigate this observation.
The emission of the host–guest system was also found to bevery strongly dependent on temperature, even in a range closeto ambient. For a solution of IrCHO in the presence of excessQ[10], increasing the temperature from 6 °C to 40 °C decreasedthe emission intensity very dramatically (Fig. 5), and the dualemission decay times also both decreased (from 230 and 3260 nsto 70 and 1150 ns for shorter- and longer-lived componentsrespectively); but the spectrum profile did not change mark-edly. While higher temperatures generally reduce excited statelifetimes, the effect in the solution regime is not normally thisstrong; indeed over the same temperature range, the lumines-cence intensity and lifetime of free IrCHO was little affected(<10% change in lifetime, and negligible change in intensity).These observations can be attributed at least partly to theeffect of temperature on the binding equilibrium: highertemperatures favour dissociation and thus a reduction in theamount of the strongly emissive 1 : 1 host–guest complex.However, as both decay components are temperature depen-dent this effect is not simply due to a change in the pro-portions of the different emitting species, and warrantsfurther investigation in a broader study.
The 1H NMR studies (see ESI†) of IrCHO in the absenceand presence of Q[10] showed that most of the resonances forthe iridium complex shifted upfield in the presence of Q[10],consistent with the shielding effect generally observed uponencapsulation of any guest within a cucurbituril host.17,26 Thestrongest shift was observed in the peaks for the ppy-CHOligands (maximum shift was 0.80 ppm for (a) resonance and0.79 ppm for (d)) (Fig. 1) while much weaker and mostly
downfield shifts were observed in peaks for the bpy ligand.This indicates that in the preferred orientation of the complex,the ppy-CHO ligands sit entirely within the cavity, while thebpy ligand mostly protrudes from the portal (Fig. 6). Thisarrangement may be convenient for applications requiring afunctionalised bpy ligand.
In conclusion, encapsulation of a large octahedral metalpolypyridyl complex within the cavity of a cucurbituril host isreported for the first time. The luminescence of the complex isdramatically enhanced upon encapsulation in Q[10], showingapproximately 40-fold increase in intensity at 1 : 1 molar ratio,a longer emission lifetime by an order of magnitude and ablue-shifted and more structured emission profile. Encapsula-tion within Q[10] may affect the nature of the emitting state ofIrCHO, increasing 3LC character. The results suggest that thecapacious host molecule Q[10] has potential for enhancingluminescence intensity of larger, transition metal-based mole-cules in various applications, particularly in cases where thesystems are only weakly emissive in water. We are currentlyextending this work to examine encapsulation of more highlyfunctionalised iridium systems.
Notes and references
‡CT (charge transfer) emission typically gives a broad unstructured spectrum,shorter lifetime and is more sensitive to environmental changes; LC (ligand-centred) emission is typically more structured, with longer excited state lifetime.
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