7846 | Chem. Commun., 2016, 52, 7846--7849 This journal is©The Royal Society of Chemistry 2016

Cite this:Chem. Commun., 2016,

52, 7846

Enhanced photophysics from self-assembledcyclometalated Ir(III) complexes in water†

Michelle M. McGoorty,a Rony S. Khnayzerb and Felix N. Castellano*a

Two water-soluble anionic cyclometalated Ir(III) complexes, Ir(ppy)2BPS

[1] and Ir(F-mppy)2BPS [2] have been synthesized and display clear

evidence of self-assembly in water. Concentration-induced aggre-

gation enhances the excited-state properties of both complexes,

blue-shifting the photoluminescence emission energies as well as

increasing the corresponding excited state lifetimes and quantum

yields up to a factor of 5.

Over the last fifteen years, the library of luminescent Ir(III)complexes has expanded significantly.1 Early synthetic effortstowards Ir(III) polypyridine complexes revealed the propensity of theiridium centre to spontaneously cyclometalate,2 yielding moleculeswith large ligand-field stabilization energies and thereby enhancedthermal/photochemical stability as well as impressive emissiontunability across the visible region of the spectrum.3–6 Consequently,such Ir(III) complexes have been studied for a variety of applicationssuch as emitters in OLEDs,7–9 sensitizers in photochemicalproton reduction reactions,10 and as chromophores for bio-logical labelling4,11 and analyte sensing.12,13 However, the limitedaqueous solubility of these complexes imposes severe restrictionson their utility in water-based applications, particularly solar fuelsphotochemistry.

A common approach for promoting the water solubility oftransition metal complexes is the incorporation of appropriatesolubilizing groups on the ligand framework.14–20 Here, twoanionic bis-cyclometalated Ir(III) chromophores have been studied,Ir(ppy)2BPS [1] and Ir(F-mppy)2BPS [2] (Fig. 1), which utilize thecommercially available bathophenanthroline disulfonate disodiumsalt (BPS) as the ancillary ligand. The peripheral sulfonate groupspromote the solubility of the resulting complex, and its excited-state properties can be adjusted through modification of the

cyclometalating ligand. Chromophore 2 incorporates the 2-(4-fluoro-phenyl)-5-methylpyridine (F-mppy) cyclometalating ligand, whichhas been shown to markedly improve the excited state lifetimesand quantum yields of the non-sulfonated derivative.10

To our surprise, upon nitrogen sparging aqueous solutions of1 and 2, the immediate formation of photoluminescent bubblesvisualized using 365 nm blacklight excitation was observed(Fig. 1). In fact, each dissolved chromophore can be completelyremoved from water as a soapy material using slow bubbledegassing (Fig. S1, ESI†). Further investigation into the aqueousbehaviour of these complexes suggested a self-assembly process,occurring in water and scaling with concentration, which resultsin significant enhancements of the photophysical properties of 1and 2. This effect is reminiscent of the aggregation-inducedemission (AIE) phenomenon first described by the Tang groupin 2001,21 which has since been reported in several organicmolecules as well as transition metal complexes.22,23

It is particularly uncommon for aggregation to occur inoctahedral transition metal complexes, as the molecular shapeis not conducive to the formation of higher-ordered structures.Work by the DeCola group has shown that such complexes canform metallosurfactants through the incorporation of longalkyl chains, and has characterized the photophysical changesobserved upon aggregation of these molecules.24–27 The Bruce

Fig. 1 Structures of 1 and 2 and photoluminescent bubbles formedduring nitrogen sparging.

a Department of Chemistry, North Carolina State University, Raleigh,

NC 27695-8204, USA. E-mail: fncastel@ncsu.edub Department of Natural Sciences, Lebanese American University, Chouran,

Beirut 1102-2801, Lebanon

† Electronic supplementary information (ESI) available: Syntheses, structuralcharacterization, photoluminescence intensity decay kinetics and fitting, andadditional experiments. See DOI: 10.1039/c6cc03932d

Received 11th May 2016,Accepted 24th May 2016

DOI: 10.1039/c6cc03932d

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group has shown that through bulky, pro-mesogenic ligandsubstitutions, octahedral complexes can be made to form liquidcrystalline phases.28–31 However, analogous molecular features arenoticeably lacking in 1 and 2. Here we describe the unexpectedself-assembly phenomenon that is observed in aqueous solutionsof 1 and 2 and characterize the resultant effects on their photo-physical properties.

The UV-vis absorption spectra of 1 and 2 (Fig. S2, ESI†) arerepresentative of bis-cyclometalated diimine Ir(III) complexes.Based on comparisons with the literature,32–35 the high-energytransitions below 320 nm are assigned to spin allowed p–p*transitions localized on the cyclometalating and diimineligands. Due to the anionic charge of the cyclometalatingcarbon, these ligand-centred (LC) transitions are expected tooccur at higher energy than those of the ancillary ligand.36 Atlower energy, we observe less intense overlapping bands attrib-uted to spin allowed metal-to-ligand charge transfer (MLCT)and ligand-to-ligand charge transfer (LLCT) transitions. Thelowest intensity bands above 430 nm have been assigned tospin-forbidden charge transfer (CT) transitions, which gainintensity through the large spin–orbit coupling constant ofthe iridium centre. No changes in the absorption profile wereobserved with increasing concentration, and no Mie scatteringwas detected at any concentrations investigated. The aqueoussolubility of these complexes was confirmed by identical UV-Visabsorption spectra before and after solution filtration with a200 nm syringe filter (Fig. S3, ESI†).

Both molecules display broad, unstructured photolumines-cence profiles in solution, consistent with the dominant CTcharacter of their emissive excited states. The HOMO for thesecomplexes contains contributions from the Ir(III) metal orbitalsas well as from the cyclometalating ligand, whereas the LUMO

is centred on the ancillary BPS ligand, making the lowest energytransition a mixture of MLCT/LLCT.14

The emission of CT type complexes is particularly sensitiveto solvent/environment polarity,37 which is observed in thesolvatochromic shifting of 1 and 2 in organic solvents ofdiffering polarity (Fig. S4, ESI†). In water, the emission energyof both complexes displayed strong concentration dependence(Fig. 2). A discrete shifting of photoluminescence profiles towardshigher energy was observed as the aqueous concentration wasincreased in the range of 20–400 mm for 1 (629 to 610 nm,495 cm�1) and 2–100 mm for 2 (585 to 578 nm, 207 cm�1).These blue shifts indicate a self-assembly process that renders theimmediate chromophore domain less polar than the solvated waterenvironment; similar shifts in Ir(III) emission energies have beenreported in the formation of aggregates by metallosurfactants25 andmetallomesogens.38 The emission spectrum of the neat powderwas red-shifted relative to the solution spectrum, indicatingthat these effects are not simply due to the precipitation of thecomplex in water (Fig. S5, ESI†). In organic solvents, the emis-sion energy does not change with concentration, consistent withmolecularly derived photophysics.

Further evidence of aqueous self-assembly was observed inthe time-resolved photoluminescence intensity decays of 1 and2 (Fig. 2). In dilute solution (r20 mm for 1 and r10 mm for 2),the decays were concentration-independent and were adequatelyfit with single exponential functions. Above these concentra-tions, the decay profiles systematically lengthen with increasingconcentration. From 50–100 mm for 1 and 20–40 mm for 2 theresultant fits become biexponential, suggesting the presenceof two (or a narrow distribution of two) distinct chromophoreenvironments. Further increases in the concentration beyondthis range produce excited state decays that are best fit using a

Fig. 2 Concentration-dependent (a) emission of 1, lex = 443 nm (b) excited-state decays of 1, lex = 405 nm (c) quantum yields of 1 (d) emission of 2,lex = 443 nm (e) excited-state decays of 2, lex = 405 nm (f) quantum yields of 2. All samples are contained in aerated water at the concentrationsspecified in the legends.

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7848 | Chem. Commun., 2016, 52, 7846--7849 This journal is©The Royal Society of Chemistry 2016

stretched exponential function I(t) = I0 exp(�kt)b,39 suggesting aheterogeneous chromophore environment modelled by a distri-bution of excited state lifetimes. The width of the distribution(effectively the heterogeneity of the environment) is given by thefitting parameter b. For both complexes this value is smaller forthe lower concentrations in this fitting regime and approachesunity (less heterogeneity) as the concentration is increased to1 mM, where the observed lifetime enhancements level off. Inorganic solvents the intensity decays remain constant and singleexponential over the same concentration range in both 1 and 2.

Similar to the trend observed in excited-state lifetimes, thequantum yields calculated for both complexes show significantenhancement with increasing concentration in water (Fig. 2).The quantum yield of 1 increased by a factor of 5, from 3 to 15%,while that of 2 increased from 7 to 21% on going from 10 mM to1 mM. Likely, a combination of factors contributes to thesequantum efficiency enhancements, which are summarized inTable 1. As the photoluminescence energy increases with concen-tration, the lifetimes and quantum yields would also be expectedto increase as a consequence of the energy gap law. It has alsobeen suggested that self-assembled structures can protect theexcited chromophore from quenching by diffusing oxygen in air-equilibrated samples.26 At a concentration of 1 mM, a minimalimprovement in lifetime is observed between air-equilibrated andair-free samples of 1 despite an average lifetime of 353 ns,suggesting that the aggregate is quite effective at protecting thelong-lived triplet excited state from oxygen quenching (Fig. S6,ESI†). Additionally, the more rigid aggregate environment canpotentially suppress non-radiative decay pathways, further con-tributing to improvements in lifetime and quantum yield.26

Table 1 indicates that the radiative rate constants for 1 and 2are essentially constant over the concentration range measured,whereas the non-radiative decay rates show a significant decreaseas the concentration increases. In air-equilibrated solutions,this rate constant would include contributions from oxygenquenching as well as a more rigid environment.

Similar photophysical enhancements are observed in aqueoussolutions of 1 and 2 upon addition of salt. A 20 mm solution of 1

shows a lengthening of excited-state decay profiles as well asenhancement in quantum yield from 3 to 12% in solutions ofsodium chloride up to 1.0 M. A 10 mm solution of 2 displayssimilar behaviour, with quantum yields increasing from 7 to16% (Fig. S7, ESI†). This effect was also observed by theadditions of either potassium chloride or potassium nitrate(Fig. S8, ESI†). Salt effects are well known in ionic surfactants,where the addition of salt lowers the CMC by shielding thecharged molecules from repulsive interactions, facilitatingaggregate formation at lower concentrations.40 While we believe asimilar mechanism is responsible for the photophysical enhance-ments observed in 1 and 2 upon addition of salt, no changeswere observed in the surface tension of aqueous solutions withincreasing Ir(III) concentration, indicating that these complexesare not surfactant-like.

Dynamic light scattering (DLS) measurements were used toestimate the size of aggregates in aqueous solution (Fig. S9,ESI†). At a concentration of 1 mM, solutions of 1 indicateparticles with a hydrodynamic radius of 2 nm, and 2 indicatea radius of 5 nm. Both complexes also show a polydisperse peakin the data around a radius of 100 nm, however, as larger particlesscatter more light, these peaks are overrepresented in the presentscattering data. It is likely that the aqueous compositions containmostly smaller particles in solution, consistent with the lack ofMie scattering observed by UV-Vis experiments.

Cryo-SEM images confirm the formation of smaller aggregatesin 1 mM aqueous solutions of 1 (Fig. 3). While these imagesrepresent aggregates in frozen solution, the size distribution agreesreasonably well with that obtained by DLS for 1 in liquid water,indicating an average aggregate radius of approximately 10 nm.

In conclusion, Ir(III) chromophores 1 and 2 were renderedwater-soluble by the incorporation of peripheral sulfonategroups on the diimine ancillary ligand. Both octahedral mole-cules were shown to form concentration-dependent aggregatesin aqueous solution, without the need for structurally complexligands to induce self-assembly. Work is currently underway tofurther understand the driving forces behind the aggregationbehaviour observed in 1 and 2. In the future, this unusualself-assembly phenomenon can potentially be exploited in solar

Table 1 Concentration-dependent photophysical properties of 1 and 2 inair-equilibrated water

ComplexConcentration(mM)

fEma

(%)Averagelifetimeb (ns)

kr

(�105 s�1)knr

(�105 s�1)

1 10 2.9 69c 4.19 140.450 3.2 106d 3.00 91.4

100 5.5 234d 2.37 40.3400 13.0 306e 4.26 28.5

1000 14.9 353e 4.23 24.12 10 6.7 211c 3.18 44.2

50 14.0 338e 4.17 25.4100 18.3 432e 4.24 18.9400 21.6 654e 3.30 12.0

1000 21.0 665e 3.16 11.9

a Average of 2 relative quantum yield measurements accurate to �1%.b Average of two time-resolved emission experiments, all lifetimeswithin �13 ns. c Monoexponential decay. d Biexponential decay, averagelifetime, hti =

Paiti

2/P

aiti.e Stretched exponential decay, average lifetime,

hti = (t/b)G(1/b). Fig. 3 CryoSEM images of 1 (1 mM) in frozen aqueous solution.

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fuels photochemistry, where increased photosensitizer concen-tration renders the composition more reactive than its mole-cular counterpart.

This material is based upon work supported by the U.S. Depart-ment of Energy, Office of Science, Office of Basic Energy Sciences,under Award Number DE-SC0011979, and the National ScienceFoundation (CHE-1465068). We thank the Voronov Group at NorthDakota State University for surface tension measurements.

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