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NMR, DFTand luminescence studies of the complexation of Al(III) with8-hydroxyquinoline-5-sulfonate†

M. Luísa Ramos,*a,b Licínia L. G. Justino,a,b Andreia I. N. Salvador,a,b Andreia R. E. de Sousa,a,b

Paulo E. Abreu,a Sofia M. Fonsecaa and Hugh D. Burrowsa

Received 27th June 2012, Accepted 16th August 2012DOI: 10.1039/c2dt31381b

Multinuclear (1H, 13C and 27Al) magnetic resonance spectroscopy (1D and 2D), DFT calculations andfluorescence have been used to study the complexation of 8-hydroxyquinoline-5-sulfonate (8-HQS)with Al(III). The study combines the high sensitivity of luminescence techniques, the selectivity ofmultinuclear NMR spectroscopy with the structural details accessible through DFT calculations, andaims to provide a detailed understanding of the complexation between the Al3+ ion and 8-HQS. A fullspeciation study has been performed and over the concentration region studied, the Al3+ ion formscomplexes with 8-HQS in an aqueous solution in the pH range 2–6. At higher pH, the extensivehydrolysis of the metal limits complexation. Using Job’s method, three complexes were detected, with1 : 1, 1 : 2 and 1 : 3 (metal : ligand) stoichiometries. These results are in agreement with those previouslyreported using potentiometric and electrochemical techniques. The geometries of the complexes areproposed based on the combination of NMR results with optimized DFT calculations. All the complexesin aqueous solutions at 25 °C are mononuclear species, and have an approximately octahedral geometrywith the metal coordinated to one molecule of 8-HQS and four molecules of water (1 : 1 complex), twomolecules of 8-HQS and two molecules of water mutually cis (1 : 2 complex), and to three molecules of8-HQS in non-symmetrical arrangement (mer-isomer), for the 1 : 3 (metal : ligand) complex. On bindingto Al(III), 8-HQS shows a more marked fluorescence than the weakly fluorescent free ligand. In addition,as previously noted, there are marked changes in the absorption spectra, which support the use of 8-HQSas a sensitive optical sensor to detect Al3+ metal ions in surface waters and biological fluids. Thesecomplexes also show potential for applications in organic light emitting diodes (OLEDs).

1. Introduction

Aluminium, the most abundant metal on earth, consists as thenatural element entirely of the 27Al isotope, and is normallypresent as its trivalent ion. It is a non-essential element, but hasbeen implicated in various neurodegenerative and neurologicaldisorders, such as Alzheimer’s disease,1–5 dialysis encephalo-pathy, and problems in bone, the haematopoietic system, musclesand joints.5–7 It may be relevant that aluminium(III) can induceformation of amyloid spherulites.8 The general population isexposed to Al compounds through dissolution of aluminiumcontaining minerals, their widespread use in water treatment, asfood additives, in various Al-based pharmaceuticals, health careproducts, and from Al containers and cooking utensils. Thisincrease in exposure to aluminium is of concern following

epidemiological studies that show an increased risk of variousdisorders, including Alzheimer’s disease, associated with Allevels in drinking water.7 This has led to a growing interest inthe development of novel materials and methods for the detec-tion and quantification of low concentrations of environmentallyhazardous metals, such as Al(III).

Luminescent chemical sensors based on increases in fluor-escence in the presence of metals are particularly attractivebecause of their ease of use, and high sensitivity, even at lowconcentrations of metals. They can be used over a wide range ofanalyte concentrations, and applied to real time and space moni-toring of these metals.9,10 On binding to a metal, they may emitlight of a different wavelength or intensity from the metal freesystem, which can be indicative of the type of metal that isbound.11,12 Free 8-hydroxyquinoline (8-HQ) is only very weaklyluminescent. This is probably because of fast excited state intra-molecular proton transfer leading to a nonfluorescent photo-tautomer.13 This pathway is inhibited on complexation, and thedramatic intensity changes seen in the presence of suitable metalions can provide the basis for sensitive and selective metal ionsensors.14 These systems have been extended through rationaldesign to the development of 8-hydroxyquinolates linked to

†Electronic supplementary information (ESI) available. See DOI:10.1039/c2dt31381b

aDepartamento de Química, Universidade de Coimbra, 3004-535Coimbra, PortugalbCentro de Neurociências e Biologia Celular, Universidade de Coimbra,Portugal. E-mail: mlramos@ci.uc.pt; Fax: +351-239-827703;Tel: +351-239-854453

12478 | Dalton Trans., 2012, 41, 12478–12489 This journal is © The Royal Society of Chemistry 2012

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fluorene-based conjugated polymers to enhance sensitivity andselectivity.9,10 Water soluble 8-hydroxyquinoline derivatives,such as 8-HQS, are also good candidates for these applications,and provide a sensitive analytical technique for selective detec-tion and quantification of metal ions through steady state15 ortime resolved measurements.16 Recently, we have studied theinteraction of 8-HQS with the Zn2+ ion and characterized asingle 1 : 2 (metal : ligand) complex, which has potential for itsdetection in aqueous medium, such as surface waters and bio-logical fluids.17 This shows formation of a complex with octa-hedral geometry having two bidentate 8-HQS ligands and twocoordinated water molecules mutually trans. With the corres-ponding 8-HQS complexes of the other group 12 metals Cd2+

and Hg2+, again there is formation of a dominant 1 : 2 complexwith octahedral coordination. However, in this case the twowater molecules show a cis geometry.18

We are interested in extending these studies to the trivalentAl3+ metal ion. Although the complexation of Al(III) with 8-HQand derivatives, such as 8-HQS, has been studied for a longtime, our knowledge of these systems is far from complete. Mostof the initial studies on 8-HQ concerned its use as a precipitantfor metal ions, such as aluminium(III), in gravimetric analysis.19

However, with the demise of standard gravimetric analysis foranalytical applications, the excellent chemical and optoelectronicproperties of these complexes have resulted in their developmentfor other applications. In particular, the observation at Kodak ofelectroluminescence from tris(8-hydroxyquinoline) aluminium(III)(Alq3)

20 has led to its use in commercial organic light emittingdiodes (OLEDs), including high efficiency electrophosphorescentdevices.21 Alq3 is a cost-efficient green light-emitting materialand an excellent electron transport layer. However, physicochem-ical properties of Alq3 and related complexes are still underinvestigation to optimize their applications, in particular in thedevelopment of blue light emitters.22 This includes the control ofgeometry of 1 : 3 complexes of Alq3 and derivatives with facialor meridional ligand configuration around the metal centre (fac-or mer-isomers), in addition to device degradation mechanismsin order to increase the lifetime of OLEDs.23 We note thatdevice degradation with Alq3 may involve changes in the coordi-nation number.23

Metal ion complexes of 8-hydroxyquinoline (8-HQ) and itsderivatives find applications in other areas, including imagingwith radionuclide complexes,24 in pharmaceutical formulations,as potential drugs in cancer treatment, particularly as tumorgrowth inhibitors25,26 and also show potential as chelatingagents for treating neurodegenerative disorders. Development ofremediation strategies for limiting the amount of metal ions inthe body is becoming increasingly important and chelationtherapy shows considerable potential for delaying the onset ofdegenerative conditions.27 Complexes must have appropriatethermodynamic and kinetic stability to compete with biologicalchelators, such as transferrin, for metal ion coordination. Watersoluble 8-hydroxyquinoline derivatives, such as 8-HQS, arelikely to be excellent candidates for this area.28,29 The 8-hydro-xyquinolate ligand also finds use as a selective sorbent of toxicmetals,30 and it is of interest to combine the sensing and adsorp-tion applications in areas such as surface water remediation.

For all these applications, characterisation of the complexspecies is important. We report a detailed structural study of the

complexation of the Al3+ ion with 8-hydroxyquinoline-5-sulfo-nate (8-hydroxy-5-sulfoquinoline, 8-HQS) using multinuclearNMR spectroscopy (1H, 13C and 27Al), DFT calculations inparallel with luminescence techniques. A full speciation studyis reported, and results will be compared with literaturereports using potentiometric and electrochemical techniques.31

Additionally, we have studied the possibility of mer–fac isomeri-zation of the complex with a 1 : 3 (metal : ligand) stoichiometryin an aqueous solution, using 1H and 27Al NMR spectroscopyand luminescence techniques as a function of temperature overthe range 25 to 80 °C. On binding to Al(III), 8-HQS shows amuch stronger fluorescence than the free ligand; these changesare accompanied by modifications in the absorption spectra, inline with previous studies.32 These results allow a full structuralcharacterization of the systems, and can be the basis for thedevelopment of luminescence sensors for Al(III) and may be rele-vant for applications in organic light emitting devices.

2. Experimental

(a) NMR experiments

Analytical grade aluminium nitrate and commercially avail-able 8-hydroxyquinoline-5-sulfonic acid hydrate were used asreceived.

The pH was adjusted (cautiously, to reduce the possibility ofdrastic local disturbances of equilibria that may be slow to dis-appear) by addition of DCl and NaOD; the pH* values quotedare the direct pH-meter readings (room temperature) after stan-dardization with aqueous (H2O) buffers.

The 1H, 13C and 27Al spectra were obtained on a VarianUnity-500 NMR spectrometer (at 499.843, 125.695 and130.285 MHz, respectively). The 13C spectra were recordedusing proton decoupling techniques (Waltz-16) taking advantageof the nuclear Overhauser effect. As a consequence, signal inten-sities for 13C spectra are not quantitative. The methyl signal oftert-butyl alcohol was used as an internal reference for1H (δ 1.3) and 13C (δ 31.2) relative to TMS. The 2D NMRspectra, DQF-COSY33 and HETCOR34–36 were recorded onthe same spectrometer. 27Al NMR spectra were obtained usingAl(NO3)3 acidified at pH 2.0, [Al(H2O)6]

3+, (δ = 0) as an exter-nal reference and typically, spectral widths of 8000 Hz, acqui-sition times of 0.5 s, pulse delays of 0.5 s and about 5000pulses.

(b) Computational details

All the molecular structures were optimized at the DFT levelusing the GAMESS code37 employing the B3LYP (Becke three-parameter Lee–Yang–Parr)38,39 exchange correlation functional.The all-electron 6-31G* double-ζ plus polarization basis set wasused for the expansion of the Kohn–Sham orbitals of sulfur,aluminum, oxygen, carbon and hydrogen. The structures wereoptimized both in the gas phase and under the polarizable conti-nuum model (PCM)40,41 to simulate bulk solvent (water) effects.In PCM, default van der Waals radii were used for all atoms.In the optimization of the fac-[Al(HQS)3]

3−, cis-[Al(HQS)2-(H2O)2]

− and trans-[Al(HQS)2(H2O)2]− structures in the gas

phase, C3, C2 and Ci symmetries were imposed, respectively.

This journal is © The Royal Society of Chemistry 2012 Dalton Trans., 2012, 41, 12478–12489 | 12479

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In the optimization in PCM the structures were built imposingthose symmetries, however, during PCM computations pointgroup symmetry is switched off internally in GAMESS. At thefinal equilibrium geometry with minimum energy the maximumgradient was 10−5 hartree bohr−1. The Hessian was calculatedfor the resultant stationary points and all were characterized astrue minima (i.e., no imaginary frequencies). The relative ener-gies of isomers presented in the discussion include zero-pointvibrational energy corrections. The 27Al nuclear shieldings werecomputed for the mer-[Al(HQS)3]

3− and fac-[Al(HQS)3]3− struc-

tures using the NWCHEM42 program at the B3LYP/GIAO(gauge-including atomic orbital method) level. The 6-31G(d,p)basis sets and a fine integration grid (FINE option) were used.27Al relative chemical shifts (δ) are given with respect to theabsolute shielding value (σ) of the hexaaquoaluminum(III)([Al(H2O)6]

3+) complex obtained at the same computationallevel (δ = σref − σ).

(c) Photophysical studies

Cetyltrimethylammonium bromide (CTAB) and poly(allylaminehydrochloride) were obtained from Sigma and were used asreceived. UV/visible absorption and fluorescence spectra wererecorded on Shimadzu UV-2100 and Jobin-Ivon SPEX Fluoro-log 3-22 spectrometers, respectively. Fluorescence spectra wereregistered with an excitation at 330 nm and were corrected forthe wavelength response of the system. When not being used formeasurements, all samples were kept in the absence of light.Fluorescence quantum yields were measured using quininesulfate in 0.5 M sulfuric acid (ΦF = 0.55)43 as standard. Flashphotolysis experiments were performed with an Applied Photo-physics laser flash photolysis equipment pumped by the thirdharmonic (355 nm) of a Nd:YAG laser (Spectra Physics).Signals were detected with Hamamatsu IP28 and R928 photo-multipliers and transient spectra were obtained by monitoring theoptical density change at 10 nm intervals over the 300–800 nmrange, averaging at least 10 decays at each wavelength. Furtherdetails have been given elsewhere.44

3. Results and discussion

Schemes 1 and 2 show the structures of 8-hydroxyquinoline(8-HQ) and the water soluble derivative 8-hydroxyquinoline-5-sulfonic acid (8-HQS).

The proton and carbon chemical shifts, as well as the proton–proton coupling constants of 8-HQS, have been measured, in anaqueous solution over the whole pH range, and completelyassigned in a previous study.17 The 1H and 13C NMR chemicalshifts were found to change with pH, indicating differences inthe degree of protonation/deprotonation of the various acidicfunctions present in the molecule, whilst the proton–protoncoupling constants show no significant changes. This indicatesthat there are no significant conformational changes, such aswould be expected if any keto–enol tautomerism was present.

Proton and carbon chemical shifts provide a good indicationof the chelation sites of the ligand upon complexation. In par-ticular, broadening or coordination induced shifts of the 1H and13C signals of the ligand in the presence of the metal ions, when

compared with those of the free ligand, can give clear indicationsof the ligand-to-metal coordination sites. In addition, with aro-matic ligands, inductive effects at specific positions in thearomatic rings allow the identification of the electronic environ-ment of the coordination site.45 For 8-HQS, changes in thechemical shifts of the ring carbons and protons upon chelationprovide insights into the nature of the metal–ligand bondsand the structure of the chelate species, such as the symmetry ofthe coordinated ligands. The importance of proton and carbonchemical shifts in providing indications of the metal chelationwas recently illustrated in the study of the complexation of8-HQS and Zn2+.17 This, together with the metal ion NMR,provides valuable structural information, including the type ofmetal centre present in the complexes, as has been widely exem-plified in our previous work on the complexation of metal ions,such as vanadate, molybdate and tungstate with relevantligands.46–52

In this study, in addition to the information from 1H and 13Cchemical shifts, the 27Al NMR spectra provide valuable struc-tural details, including the type of metal centre present in thesecomplexes, from the characteristic chemical shift and the sym-metry around the metal centre from the spectral linewidth.

Aluminium-27 (100%, I = 5/2) has a moderately large quad-rupole moment (Q = 0.149 × 10−24 m2).53 Although this nor-mally gives rise to broad signals, since the quadrupolarrelaxation mechanism is dominant, it is characterised by a highsensitivity to detection by NMR and large chemical shift ranges,two factors that make its study relatively easy. In fact, rather thanquadrupolar nuclei (spin I ≥ 1) being undesirable, they can giveunique structural information. The relaxation of such nuclei ismainly due to the interaction of the quadrupole moment withelectrical field gradients present at the position of the nucleus.This interaction allows an efficient transfer of energy from thenucleus to the molecular rotational motions. Asymmetry in theligand field produces an increase in the NMR linewidth, which

Scheme 2

Scheme 1

12480 | Dalton Trans., 2012, 41, 12478–12489 This journal is © The Royal Society of Chemistry 2012

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can provide information on the coordination environment.54–5627Al has been shown to be more amenable to NMR studies thanother quadrupolar nuclei due to its large receptivity and the exist-ence of only one isotope. Another attractive feature of thisnucleus is the relatively small quadrupolar moment associatedwith its high nuclear spin (I = 5/2). These two favourable factorsresult in a high relative peak height, which means that sufficientsignal/noise ratios can be obtained even with dilute (≈0.01 M)solutions of aluminum compounds.57 27Al linewidths may varyfrom 3 Hz to several kHz, while the signal may even completelyvanish into the baseline noise in some instances. This negativeaspect of quadrupolar relaxation is counterbalanced by theadditional information obtained concerning molecular sym-metries from the magnitude of quadrupolar line broadenings andthe favorable large range of 27Al NMR chemical shifts, which isabout 450 ppm.

In aqueous solution, at low pH, Al(III) is predominantlypresent as the octahedral coordinated hexaaquoaluminium(III)ion, [Al(H2O)6]

3+, giving rise to a sharp 27Al NMR signal (Δν1/2≈ 25 Hz), which is usually taken as the reference (δ = 0 ppm).Upon increase of the pH to 3.5, additional signals are observed,which are dependent on the concentration. Signals at around1–4 ppm, which become broader with increasing pH, can beattributed to octahedral Al(III) present in hydrolytic oligomericspecies. With the increase of the pH value, precipitation canoccur and with an excess of base, the re-dissolution of the pre-cipitate is observed and a sharp single signal is detected in the27Al NMR spectra (δ = 80 ppm, Δν1/2 ≈ 70 Hz), coming from

the complex [Al(OH)4]−. The change of the coordination

number of Al(III) is accompanied by a large change in the 27AlNMR chemical shift.56 Generally, in aqueous solution, octa-hedral coordinated Al(III) is observed from −10 to around50 ppm and tetrahedral coordinated Al(III) from around 60 to80 ppm. Intermediate chemical shifts have been assigned to five-coordinated Al(III) species.58,59

For a complete structural characterization of the interaction ofAl(III) metal ions with 8-HQS, we have obtained 1H, 13C NMRand 27Al NMR spectra for total concentrations of metal andligand ranging from 10 mM to 20 mM, metal : ligand molarratios from 2 : 1 to 1 : 4, and pH* values ranging from 2 to 6.5.Above pH 6, hydrolysis of Al(III) begins to take place, becomingdominant at higher pH values. This competes with complexation,giving rise to hydrolytic species of Al(III).

Mixtures of aluminium nitrate and 8-HQS, in aqueous solu-tion, show additional 1H, 13C due to complexed forms, depend-ing on pH, concentration, and molar ratios, in addition to thoseof any free ligand when present. Fig. 1 and 2 show a series of 1Hand 13C NMR spectra, respectively, of 8-HQS alone and in thepresence of aluminium nitrate, where three complexes, a, b andc are detected. The observation of distinct signals for free andcomplexed ligands in 1H and 13C NMR spectra suggests thatfree and complexed ligands are in slow exchange on the NMRtime scale. Tables 1 and 2 show the 1H and 13C NMR para-meters, respectively, for this system. The 1H and 13C NMRassignments of the signals were obtained by using DQF-COSYand HETCOR sequences, respectively.

Fig. 1 1H NMR spectra (499.824 MHz) of D2O solutions of (i) Al(III)/8-HQS 10 : 10 mmol dm−3, pH* 4, (ii) Al(III)/8-HQS 3.3 : 10 mmol dm−3,pH* 4.0 and (iii) Al(III)/8-HQS 2.5 : 7.5 mmol dm−3 pH* 6.0, temp 298 K. *1H free ligand signals.

This journal is © The Royal Society of Chemistry 2012 Dalton Trans., 2012, 41, 12478–12489 | 12481

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Whereas species a is most notable at pH values ranging from2 to 4, especially in solutions with the molar ratio 1 : 1 andspecies b in 1 : 2 solutions at slightly higher pH values, 2.5 to5.0, species c is detected particularly in 1 : 3 (metal : ligand)solutions at pH values ranging from 3.5 to 6.

Each of the species a and b gives rise to one set of 1H and13C signals. For species a, the carbon nuclei C-2, C-4, C-5, C-7,C-8, C-9 and C-10 give rise to 13C NMR signals, which undergosignificant changes in their chemical shifts on complexation,

whereas the other carbon atoms, C-3 and C-6, show muchsmaller shifts relative to the free ligand under the same condi-tions. C-2, C-3, C-5, C-6 and C-8 carbon signals move to highfrequencies. In contrast, C-4, C-7, C-9 and C-10 move to lowfrequency on complexation, as do all the proton signals (Tables1 and 2). The coordination induced shifts observed for the C-2and C-8 nuclei support the involvement of deprotonated quino-line (N) and hydroxyl (O) groups in the complexation.17,44–50

The shielding or deshielding observed for the other carbon

Fig. 2 13C NMR spectra (125.695 MHz) of D2O solutions of (i) Al(III)/8-HQS 10 : 10 mmol dm−3, pH* 2.2, (ii) Al(III)/8-HQS 5 : 10 mmol dm−3,pH* 4 and (iii) Al(III)/8-HQS 2.5 : 7.5 mmol dm−3, pH* 6, temp 298 K; *13C free ligand signals.

Table 1 1H NMR parametersa for 8-HQS and its complexes with Al(III) (298 K)

H-2 H-3 H-4 H-6 H-7 J2,3 J3,4 J2,4 J6,7

8-HQSb

pH* = 2.1 9.17 8.28 9.74 8.36 7.53 5.0 8.6 1.2 8.3pH* = 3.0 9.14 8.21 9.65 8.33 7.49 5.0 8.6 1.2 8.3pH* = 4.0 9.00 7.98 9.32 8.21 7.33 5.4 8.8 1.2 8.2pH* = 6.5 8.95 7.81 9.09 8.14 7.25 4.2 8.7 1.0 8.2

Al(III)/8-HQSComplex ac (pH* 2.7)δ 8.94 8.00 9.29 8.23 7.10 4.9 8.5 1.2 8.2Δδ −0.20 −0.21 −0.36 −0.10 −0.39Complex bd (pH* 4.0)δ 8.62 7.65 9.06 8.27 7.31 4.4 8.6 1.2 8.2Δδ −0.38 −0.33 −0.26 0.06 −0.02Complex ce (pH* 6.0)δI 7.69 7.69 9.22 8.24 7.06 4.9 8.7 1.2 8.3Δδ −1.26 −0.12 0.13 0.10 −0.19δII 8.97 7.87 9.21 8.20 7.12 4.8 8.7 1.2 8.3Δδ 0.02 0.06 0.12 0.06 −0.13δIII 8.89 7.80 9.18 8.22 7.13 4.8 8.8 1.2 8.3Δδ −0.06 −0.01 0.09 0.08 −0.12a δ values, in ppm, relative to Me4Si, using tert-butyl alcohol (δH = 1.3) as an internal reference; J values in Hz. b 10 mmol dm−3 8-HQSsolution.c 10 : 10 mmol dm−3 Al(III)–8-HQS solution. d 5.0 : 10 mmol dm−3 Al(III)–8-HQS solution. e 2.5 : 7.5 mmol dm−3 Al(III)–8-HQS solution.

12482 | Dalton Trans., 2012, 41, 12478–12489 This journal is © The Royal Society of Chemistry 2012

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nuclei is in agreement with the effects of deprotonation of theligand upon complex formation.16 C-2, C-3, C-5, C-6 and C-8nuclei are observed to be deshielded whereas C-4, C-7, C-9 andC-10 are shielded. The large shielding and deshielding observedfor C-4 and C-5, respectively, are in agreement with what occursin acidic solutions of the free ligand upon the NH+ deprotonation(pKa 4.09).

17,60,61 This observation for C-4 with the NH+ depro-tonation agrees with the results of previous calculations for pyri-dine.62,63 C-9 and C-10 nuclei also show a slight shielding uponcomplexation, in parallel to what occurs with the OH deprotona-tion (pKa 8.66)

60,61 perhaps due to combined effects involvingthe pyridine and the phenol-sulfonate rings.17 In addition, all theproton signals are observed to be deshielded, as a consequenceof the deprotonation which occurs upon complexation, as pre-viously discussed for 8-HQ and calculated for pyridine.45,64

Also, with respect to complex a, one molecule of 8-HQS coordi-nates one octahedral aluminium centre, with the remaining posi-tions bound to four molecules of H2O, as suggested by theobserved 27Al NMR chemical shift 58 (δ = 9.6 ppm; Δν1/2 =830 Hz), indicating that species a has a 1 : 1 (metal : ligand) sto-ichiometry, in agreement with the Job’s plot based on 1H NMRstudies (Fig. 3). This 1 : 1 stoichiometry is in accordance withprevious studies using other techniques.31,65

Complex b also gives rise to one set of 1H and 13C signals. 1Hand 13C NMR signals due to this complex undergoing significantchanges in the chemical shift upon complexation relative to thefree ligand under the same conditions, although the induced shiftpatterns show some slight differences from those observed forcomplex a, which is probably a result of the higher pH range forwhich complex b appears. For species b, all of carbon nucleishow much smaller shifts relative to the free ligand under thesame conditions. C-3, C-5, C-6, C-8 and C-10 carbon signalsmove to high frequencies while, in contrast, C-2, C-4, C-7 andC-9 move to low frequency on complexation, as do all the

proton signals, except H-6 that suffers a small positive shift(Tables 1 and 2). The coordination induced shifts observedfor the C-2 and C-8 nuclei also support the involvement ofdeprotonated quinoline (N) and hydroxyl (O) groups in thecomplexation,17,45–51 despite the lower value of the C-2 coordi-nation induced shift observed in complex b in comparison tocomplex a. The coordination induced shift for carbon nucleiwhich bears a coordinated function results from several contri-butions to σp

66,67 including changes in the electronic densityrelated with the coordination and with the ionization whichoccurs with complexation. C-2 nuclei are mainly affected by theionization of the quinolinium N–H+ group (pKa = 4.09). It isinstructive to compare the C-2 chemical shift changes as a func-tion of pH observed for free 8-HQS with the shifts induced uponmetal ion complexation. Chemical shifts of C-2 in free 8-HQSincrease with pH until a pH value around 7, and then decreasewith the increasing negative charge of quinoline (N) for highervalues of pH, which parallels the shielding observed upon com-plexation. In addition, the other carbon shifts observed are inagreement with the effects of deprotonation of the ligand uponcomplex formation17,60–63 and the 1H NMR signals were foundto change accordingly with the complexation and the consequentdeprotonation of NH+ and OH groups.45,64 A Job’s plot based on1H NMR studies confirms a 1 : 2 (metal : ligand) species forcomplex b (Fig. 3), and is in agreement with previousstudies,31,65 while the 27Al NMR signal (δ = 11.9 ppm; Δν1/2 =660 Hz) is compatible with a mononuclear hexacoordinatedcomplex having an Al3+ centre coordinated by two molecules of8-HQS, through deprotonated quinoline (N) and hydroxyl (O)groups, with the remaining two positions occupied by coordi-nated water molecules.58 This is supported by the fact that, fromthe spectra, corresponding nuclei in the two complexed ligandmoieties are magnetically equivalent (i.e. they have the same 1Hand 13C chemical shift), indicating a symmetrical 1 : 2 complex.

Table 2 13C NMR parametersa for 8-HQS and its complexes with Al(III) (298 K)

C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-10

8-HQSb

pH* = 2.1 146.17 124.87 144.77 131.58 132.34 115.74 151.56 131.39 127.66pH* = 3.1 146.23 124.82 143.63 133.59 131.68 114.64 153.26 131.29 127.48pH* = 4.0 148.09 124.75 140.34 136.33 130.81 113.22 154.91 130.91 127.07pH* = 6.5 150.19 124.71 136.79 139.54 129.91 111.73 156.91 130.35 126.78

Al(III) + 8-HQSComplex ac (pH* 2.2)δ 147.10 125.34 140.65 139.69 132.56 112.19 160.30 129.43 126.77Δδ 0.93 0.41 −4.12 8.11 0.22 −3.54 8.74 −1.96 −0.89Complex bd (pH* 4.0)δ 147.70 125.97 140.65 140.13 133.21 112.82 161.53 127.45 127.85Δδ −0.39 1.22 −0.31 3.80 2.40 −0.40 6.62 −3.36 0.78Complex ce (pH* 6.0)δI 146.71 125.22 140.29 140.15f 131.83 112.26 160.95f 132.41f 127.01f

Δδ −3.48 0.51 3.50 0.61 1.82 0.53 4.04 2.06 0.23δII 148.58 125.40 140.73 140.06f 132.85 112.44 160.79f 132.44f 126.80f

Δδ −1.61 0.69 3.94 0.52 2.94 0.71 3.88 1.89 0.02δIII 148.05 125.57 140.42 139.91f 132.58 112.18 160.37f 132.58f 126.80f

Δδ −2.14 0.86 3.63 0.37 2.67 0.45 3.46 2.23 0.02

a δ Values, in ppm, relative to Me4Si, using tert-butyl alcohol (δC 31.2) as an internal reference. b 10 mmol dm−3 8-HQS solution. c 10 : 10 mmoldm−3 Al(III)–8-HQS solution. d 5 : 10 mmol dm−3 Al(III)–8-HQS solution. e 2.5 : 7.5 mmol dm−3 Al(III)–8-HQS solution. f The signals arising fromquaternary carbons can be interchanged in sets I, II and III.

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For complex c, detected at higher pH values, three sets ofsignals of equal intensities are detected in the 1H NMR spectra.The 13C NMR spectra also show three different sets of signalsfor complexed ligands (labelled in Tables 1 and 2 by I, II andIII). One mononuclear centre of Al3+ coordinates three molecules

of 8-HQS in an asymmetrical manner (mer-isomer). The corres-ponding fac-isomer is ruled out since this would have threeequivalent ligands. Job’s plot confirms the 1 : 3 (metal : ligand)stoichiometry (Fig. 3), which is in agreement with previousstudies.31,65

The three sets of 13C signals detected for complex c havesimilar patterns of coordination induced shifts. C-3, C-4, C-5,C-6, C-7, C-8, C-9 and C-10 carbon signals move to high fre-quencies and C-2 moves to low frequencies upon complexation,though C-2 in set I is more shielded than C-2 in either of theother two sets, II and III. In agreement with this observation,H-2 also shows a significant shielding in set I, compared withthe sets II and III. The coordination induced shifts observed forthe C-2, C-8 and C-9 nuclei for all the sets of signals I, II and IIIsupport the involvement of deprotonated quinoline (N) andhydroxyl (O) groups in the complexation, in all the three inequi-valent molecules of 8-HQS coordinated to the metalcentre.17,45–51 The three inequivalent molecules of 8-HQS arecoordinated to the metal centre in a mer-arrangement. One of thecoordinated molecules in the mer isomer has the configurationO trans to O and N trans to N (set of signals I), whereas theother two coordinated molecules have, respectively, O trans to Nand N trans to N, and N trans to O and O trans to O (set ofsignals II and III). 27Al NMR spectra show a signal around50 ppm, very near the broad resonance around 60 ppm associ-ated with the aluminium of the probe signal,56 but is compatiblewith a mononuclear octahedral complex having an Al3+ centrecoordinated by three 8-HQS ligands, through deprotonated quinoline(N) and hydroxyl (O) groups in a meridional arrangment.55,56

From the NMR data, conditional formation constants forcomplexes a, b and c can be estimated. The values obtained arelog K′(a), log K′(b) 2.67 and 2.71 (pH 3.0) and log K′(b), log K′(c) 5.85 and 10.47 (for pH 4.2). Quantitative comparison withthe thermodynamic values obtained electrochemically31 is notpossible because the NMR values are not thermodynamic (activitybased) values. However, the order of the conditional formationconstants obtained from NMR is in qualitative agreement withthe equilibrium constants determined potentiometrically.

To help elucidate the coordination mode of Al3+ with 8-HQSin solution, the structures of the complexes have been optimizedat the DFT level and the relative energies of isomers determined.For complex b, we have considered two possible geometries: asquare bipyramidal geometry for a trans-[Al(HQS)2(H2O)2]

−

complex and the octahedral cis-[Al(HQS)2(H2O)2]−. These two

geometries were optimized at the DFT level and the trans struc-ture was found to be more stable than the cis by 2.5 kcal mol−1

under PCM conditions and by 9.1 kcal mol−1 in the gas phase.For complex c, the meridional and facial isomers were con-sidered and the mer isomer was found to be more stable thanthe fac isomer by 2.7 kcal mol−1 under PCM conditions andby 3.1 kcal mol−1 in the gas phase. This energetic differencebetween the mer and the fac isomers is smaller than the onecalculated for the Al(III)/8-HQ system, which is 4.9 kcal mol−1

(gas phase calculation).68 In an aqueous solution, the establish-ment of hydrogen bonding involving the complexes is expectedto play an important role in the stabilization of the species. Inview of the presence of water molecules in the coordinationsphere of the cis and trans structures, hydrogen bridging is likelyto have a larger effect in this case than for the mer and fac

Fig. 3 Job’s plots based on 1H NMR spectral intensities for the systemAl(III)/8-HQS; total concentration of complexing species 10 mmol dm−3

(D2O solutions, temp. 298 K); (a) pH* 2.4; (b) pH* 3.0; (c) pH* 4.2.

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isomers. To simulate approximately the effect of hydrogen brid-ging in the energies of the structures, in addition to PCM, wehave included for both the cis and trans structures of complex bfour explicit solvent water molecules bound through hydrogenbridges to the coordinated water ligands and we have optimizedthese structures. It is known that continuum solvation methodsare unable to treat specific solute–solvent interactions such ashydrogen bonding41,69 due to the lack of explicit solvent mo-lecules. For both the cis and trans arrangements, in the resultingoptimized geometry, one of the explicit water molecules estab-lishes a second hydrogen bridge with the oxygen atom of one ofthe 8-HQS ligands and, interestingly, the energy gap no longerfavours the trans structure, but instead is favourable to the cisstructure by 0.80 kcal mol−1. Although this value is within thecalculation error, it unquestionably points to a stabilization of thecis configuration in an aqueous solution. This result was alreadyanticipated when comparing the energy difference between thecis and trans structures in the gas phase (9.1 kcal mol−1) and inPCM (2.5 kcal mol−1). Thus, although in the gas phase a transarrangement is more stable for the 1 : 2 complex, the calculationspoint to a cis arrangement in an aqueous solution. Previously wehave found that the inclusion of water molecules hydrogenbonded to a system allows a better reproduction of the solutionNMR chemical shifts,17,70 and this is, at least in part, accom-plished because of the improvement in the geometries. Opti-mized structures for complexes a, b and c are shown in Fig. 4.

Table 3 presents selected geometrical parameters of the com-plexes optimized under PCM conditions and in the gas phase.The 1 : 1 and the meridional 1 : 3 complexes have C1 pointgroup symmetry, while the cis 1 : 2 complex has C2 symmetryand the facial 1 : 3 complex has C3 point group symmetry. Theshortest Al–N and Al–O bonds are found for the 1 : 1 complex,followed by the 1 : 2 complex, as expected in view of the pres-ence in the two complexes of the less bulky water ligand. In themeridional 1 : 3 complex the longest Al–N bond is the one oppo-site to the Al–O bond, reflecting the trans effect of the oxygen.As expected, the shortest Al–O bond in the meridional complexis that opposite to the Al–N bond.

Taking into account the small energetic difference between themer- and fac-isomers indicated by the calculations (2.7 kcal mol−1

in PCM), the possibility of mer–fac isomerisation (Fig. 5), inthe accessible range of temperatures, was also studied using1H and 27Al NMR spectra as a function of temperature. Variabletemperature 1H and 27Al (not shown) NMR spectra wereobtained for a solution of Al3+/8-HQS, 3.3 : 10 mmol dm−3, pH6.0, where complex c (mer-isomer) is dominant, and the1H NMR spectra are shown in Fig. 6. For the Al(III)/8-HQsystem in DMSO, when the aluminium chelate is heated insolution, the three sets of signals in the 1H NMR spectrum col-lapse to a single set of signals, which had been attributed to theconversion of mer- to fac-isomer,45 consistent with studies inCDCl3.

71

Table 3 Selected bond lengths (Å) calculated at the B3LYP/6-31G(d) level in PCM and in the gas phase (values in parentheses refer to the gasphase)

mer-[Al(HQS)3]3− fac-[Al(HQS)3]

3− [Al(HQS)2(H2O)2]− [Al(HQS)(H2O)4]

+

Al–OI 1.886 (1.883) 1.868a (1.860) 1.845a (1.825) 1.813 (1.761)Al–OII 1.885 (1.885)Al–OIII 1.870 (1.862)Al–NI 2.047 (2.073) 2.101a (2.138) 2.031a (2.017) 1.988 (1.930)Al–NII 2.113 (2.143)Al–NIII 2.074 (2.094)

aAs a consequence of point group symmetry being switched off internally in GAMESS during PCM calculation, very minor differences in the bondlengths were found for symmetry-equivalent ligands in the PCM optimized geometries (in the fac-[Al(HQS)3]

3− complex, PCM calculated values areAl–O: 1.868, 1.868, 1.867 Å and Al–N: 2.097, 2.093, 2.112 Å; for the [Al(HQS)2(H2O)2]

− complex, PCM calculated values are Al–O: 1.845,1.844 Å and Al–N: 2.031, 2.029 Å.

Fig. 4 B3LYP/6-31G(d) optimized geometries in PCM (water) of complexes [Al(HQS)(H2O)4]+, cis-[Al(HQS)2(H2O)2]

− and mer-[Al(HQS)3]3−.

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At 25 °C, the 1H NMR spectra show sharp and distinct signalsfor the free ligand and each of the sets I, II and III of complexedmolecules of 8-HQS in the mer-isomer, which means that atroom temperature, the mer-isomer is the stable form in solution(Fig. 6). With increasing temperature, free ligand signals are notaffected and signals of the complex become broader, suggesting

fast exchange on the NMR time scale. Small changes areobserved on the broad peak in the 27Al NMR spectra, which areconsistent with the formation of a small amount of the fac-isomer, as suggested by the shoulder observed in the spectra athigh temperatures (δ = 27 ppm). We have calculated the 27Alchemical shifts for the mer- and fac-isomers of Al(8-HQS)3 andthe chemical shifts of 33.8 and 32.9 ppm, respectively, wereobtained. The corresponding experimental shifts are around 50and 27 ppm. Although a larger error exists on the calculation ofthe mer-chemical shift, the calculation correctly predicts a largerchemical shift for the mer-isomer. Previous studies on experi-mental and calculated values for the mer- and fac-isomers ofAl(8-HQ)3 show equivalent results.72

Additionally, the highest occupied (HOMO) and the lowestunoccupied (LUMO) molecular orbitals of the mer isomer arepresented in Fig. 7 and the energies of the molecular orbitalsclose to the frontier orbitals are indicated for the mer and facisomers in Table 4.

The complexation between 8-HQS and Al(III) was also investi-gated by UV-Vis absorption and fluorescence spectroscopy forthe pH range 2–6. Fig. 8 shows the absorption and fluorescencespectra of 8-HQS alone and with increasing concentrations ofAl(III) at pH 5. As the Al(III) concentration increases and com-plexation between 8-HQS and the metal is occurring, there is adecrease in the absorption band with a maximum around 310 nmand the formation of a new band with a maximum around365 nm, corresponding to an allowed transition. An isosbesticpoint can clearly be seen showing that there is a single equili-brium between two species under these conditions of excessligand:

½AlðH2OÞ6�3þ þ 3ð8� HQSÞ2� ! ½Alð8� HQSÞ3�3� þ 6H2O

This contrasts with the behaviour in the NMR studies wherewe are approaching the equilibria from an excess of Al(III), andsee formation of 1 : 1, 1 : 2 and 1 : 3 (metal : ligand) complexes.As previously reported for Zn(II), upon addition of Al(III), thefluorescence intensity of 8-HQS increases as a result of the com-plexation between the metal and 8-HQS.17 Fluorescencequantum yields for the 1 : 1, 1 : 2 and 1 : 3 (metal : ligand) com-plexes were determined with quinine sulfate in 0.5 mol dm−3

sulfuric acid (ΦF = 0.55) as standard.43 Values of 0.045, 0.030

Fig. 5 Isomerization between the meridional and facial isomers of[Al(HQS)3]

3−. Geometries were optimized at the B3LYP/6-31G(d) levelin PCM (water).

Fig. 6 1H NMR spectra (499.824 MHz) of a D2O solution of Al3+/8-HQS 3.3 : 10 mmol dm−3, pH 6.0, as a function of temperature: (i)25 °C, (ii) 30 °C, (iii) 40 °C, (iv) 50 °C, (v) 60 °C, (vi) 70 °C, (vii)80 °C.

Fig. 7 HOMO and LUMO orbitals of mer-[Al(HQS)3]3− calculated at

the B3LYP/6-31G(d) level in PCM (water).

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and 0.020 were obtained, respectively. For the 1 : 3 complex theeffect of varying the pH on the fluorescence quantum yields wasalso investigated and is shown in Fig. 9. In an acidic medium, onincreasing the pH there is an increase in the fluorescencequantum yields of the 1 : 3 complexes. The addition of the cat-ionic surfactant cetyltrimethylammonium bromide (CTAB) tothe 1 : 3 complexes was also studied. A sharp increase in fluor-escence intensity is observed until the CTAB critical micelleconcentration (CMC, 8 × 10−4 mol dm−3 at 25 °C73) is reached.With further addition of CTAB the fluorescence remains almostunchanged. Since with the formation of CTAB micelles there is

release of bromide anions to the solution, the effect of addingKBr to the 1 : 3 complex was investigated. No significant changein the absorption and fluorescence spectra was observed,suggesting that there is no interaction between the bromideanions and the Al(III)/8-HQS complexes. The effect of CTAB(9 × 10−4 mol dm−3) on the fluorescence quantum yields wasinvestigated and the results as a function of pH are shown inFig. 9. In the presence of CTAB, with a concentration above itsCMC, a significant increase in the fluorescence quantum yieldsof the 1 : 3 complexes is observed due to the electrostatic inter-action between the cationic CTAB micelles and the anionic com-plexes, [Al(8-HQS)3]

3−. In accordance with the results in theabsence of CTAB, in an acidic medium, on increasing the pHthere is an increase in the fluorescence quantum yields of the1 : 3 complexes. That increase is more pronounced in the pHrange 4–4.5, suggesting that the electrostatic interaction betweenthe cationic CTAB micelles and the anionic Al(III)/8-HQS com-plexes is optimized in this pH range.

The interaction between the cationic polyelectrolyte poly(allyl-amine) and the 1 : 3 Al(III)/8-HQS complexes was also studied,but the results were not reproducible. We note that significantenhancement of tris(8-hydroxyquinolate-5-sulfonate)aluminiumhas been observed with polyanions in layered double hydrox-ides.74 It is possible that such rigid environments are necessaryto achieve fluorescence enhancements with polyelectrolytes.

Fluorescence spectra were obtained, as a function of tempera-ture, for the 1 : 3 complexes at pH 4. The temperature was variedfrom 25 to 80 °C and a 5 nm shift of the emission maximum tolower wavelengths accompanied by a decrease in intensity wasobserved. In agreement with the variable temperature NMRresults, this is possibly due to mer-to-fac isomerization, since ithas been reported that the fac isomer of tris(8-hydroxyquinoline)aluminium(III) (Alq3) presents a blue-shifted fluorescence.22

Attempts were made to study the triplet state of the 1 : 3 com-plexes by flash photolysis. The signals were very weak, buta broad band with a maximum in the same region of thetriplet spectra of tris(8-hydroxyquinoline)aluminium(III) (Alq3)(∼520 nm) could be observed.75 The triplet-singlet differenceabsorption spectra observed with different delays following laser

Table 4 Energy (eV) of molecular orbitals close to HOMO andLUMO calculated for mer-[Al(8-HQS)3]

3− and fac-[Al(8-HQS)3]3− at

the B3LYP/6-31Gd level in PCM (water)

Molecular orbital mer-Al(8-HQS)3 (eV) fac-Al(8-HQS)3 (eV)

HOMO − 8 −7.00 −7.04 aHOMO − 7 −6.95 −6.98 eHOMO − 6 −6.93 −6.97 eHOMO − 5 −6.19 −6.21 aHOMO − 4 −6.17 −6.17 eHOMO − 3 −6.15 −6.16 eHOMO − 2 −5.44 −5.43 eHOMO − 1 −5.40 −5.42 eHOMO −5.31 −5.39 aLUMO −1.75 −1.73 eLUMO + 1 −1.62 −1.71 eLUMO + 2 −1.52 −1.61 aLUMO + 3 −0.59 −0.68 aLUMO + 4 −0.49 −0.49 eLUMO + 5 −0.42 −0.47 e

Fig. 8 Absorption (a) and fluorescence (b) spectra of 8-HQS (1.0 ×10−4 mol dm−3) alone and in the presence of increasing concentrationsof Al(III) (2.0 × 10−6–1.1 × 10−4 mol dm−3) for pH 5.

Fig. 9 Plot of fluorescence quantum yields (ΦF) as a function of pHfor a 1 : 3 Al(III)/8-HQS solution in the absence (squares) and in the pres-ence (circles) of the cationic surfactant CTAB.

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excitation at 355 nm of a solution of the 1 : 3 Al(III)/8-HQScomplex at pH 4 can be found in the ESI.†

4. Conclusion

This paper addresses the detailed structural characterisation ofspecies formed in aqueous solutions from the interaction of Al3+

with the ligand 8-HQS. By using one- and two-dimensionalnuclear magnetic resonance spectroscopy (1H, 13C and 27Al), itwas possible to carry out the complete speciation studies of thesystem. In addition, the use of theoretical techniques, in particu-lar DFT, was important in the structural characterization. In par-allel, UV/visible and luminescence spectra have also beenmeasured.

We identify three species, 1 : 1, 1 : 2 and 1 : 3 (metal : ligand)complexes, using 1H, 13C and 27Al NMR, UV/visible absorptionand fluorescence spectroscopy. The detection of three complexeswith metal : ligand 1 : 1, 1 : 2 and 1 : 3 is in complete agreementwith previous work, using potentiometric and electrochemicaltechniques.31 One, two and three molecules of 8-HQS coordinatea single octahedral aluminium centre through the deprotonatedquinoline (N) and the hydroxyl (O−) groups in these complexes,as shown by the 1H, 13C and 27Al NMR spectra. In the 1 : 1complex, a single bidentate ligand binds the metal, with theremaining positions occupied by four water molecules, whereastwo 8-HQS and two H2O molecules bind the metal in the 1 : 2(metal : ligand) complex. The fact that corresponding nuclei inthe two complexed ligand moieties are chemically equivalent(i.e. they have the same chemical shift) points to a symmetricalcomplex. We have used DFT calculations to evaluate the natureof the metal centre, and this indicates that the complex has anoctahedral geometry, with the two coordinated water moleculesmutually cis, the remaining positions being occupied by thedonor groups of the two coordinated 8-HQS molecules. For the1 : 3 (metal : ligand) species, three molecules of 8-HQS bindthe metal in an asymmetrical fashion (mer-isomer). We have alsoused DFT calculations to evaluate the relative stability of the twopossible 1 : 3 (metal : ligand) isomers and this indicates thatthe mer-isomer is more stable than the fac-isomer by about3.1 kcal mol−1. We have investigated the possibility of mer–facisomerization, in aqueous solutions using 1H and 27Al NMR as afunction of the temperature over the range 25–80 °C, in additionto variable temperature fluorescence measurements. All resultsconfirm the possibility of this isomerization.

The complexation of Al3+ metal ions with 8-HQS isaccompanied by marked changes in the UV/visible absorptionand fluorescence spectra of 8-HQS, in line with previous studies.These observed changes are likely to be particularly importantfor the potential application of 8-HQS as a luminescent sensorfor the detection and remediation of Al(III) in surface waters andbiological fluids. It is shown that sensitivity in the fluorescencemeasurements can be markedly increased in the presence of thecationic surfactant cetyltrimethylammonium bromide. Thesecomplexes may also have potential in organic light emittingdevices. A logical extension will involve the development ofself-assembled structures in rigid matrices. Recent work hasdemonstrated the feasibility of such systems.74

Acknowledgements

The authors thank FCT, the Portuguese agency for scientificresearch, which has supported this work and Rede Nacional deRMN (REDE/1517/RMN/2005), the Portuguese NMR Network,for spectrometer facilities. L.L.G.J. also thanks FCT, of the Por-tuguese Ministry for Science, Technology and Higher Education,for the postdoctoral grant SFRH/BPD/26415/2006 and the“Laboratório de Computação Avançada”, of the Department ofPhysics of the University of Coimbra, for the computing facili-ties (Milipeia Cluster). S.M.F. also thanks FCT, of the Portu-guese Ministry for Science, Technology and Higher Education,for the postdoctoral grant SFRH/BPD/34703/2007.

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