Lanthanide-Based Luminescent Hybrid Materials

92 Pages·2009·2.66 MB·English

by  Koen Binnemans*

Chem.Rev.2009,109,4283–4374 4283
Lanthanide-Based Luminescent Hybrid Materials
Koen Binnemans*
KatholiekeUniversiteitLeuven,DepartmentofChemistry,Celestijnenlaan200F,bus2404,B-3001Leuven,Belgium
Received February 5, 2007
Contents
1. Introduction 4283
2. Luminescence of Lanthanide Complexes 4284
2.1. Principles of Lanthanide Luminescence 4284
2.2. Lanthanide (cid:1)-Diketonates 4287
3. Sol-Gel Hybrid Materials 4290
3.1. Inorganic Sol-Gel Systems 4290
3.2. Confinement of Liquids in Silica Matrices 4298
3.3. Organically Modified Xerogels 4300
org 3.4. Bridged Polysilsesquioxanes 4305
bs.acs.3983 33..56.. SCiolivcaal/ePnotllyymBeornNdeadnoCcoommppolesxiteess 44330077
u0
27, 2009 | http://pdoi: 10.1021/cr80 456...44..PIP12n..ooterlyorZMocuexaesoolsalmHoittiepyoesobntarroilCdautoseMmsSapti(loePicurOianatMdlessss) 44444333331112255954 KMUISnocn.Sietievhcnnee.trBidsfipeiicetnygrnriRoeeoedemfsLa(1e1en9a9us9r9vc92ewh-)naF,a2snl0uadb0nno5ddPr,eenhrhr.sDeitnh.(ewBdGaeesesgulegrplaei,ueemrBpv(oe)1is.sl9gitoH9diun6oem)cotd,ifoniidrPnCarlhpo1feof9e.ms7llCto0dis.w.otrGcHyotoeo¨afrrtlatolhetlbhertew-aWFoiCnuraaeknltrddhawonhfilotdiihscr.
mber 009 | 7. Polymer Materials 4328 PUr.oKf..).JIancq2u0e0s0,Lhuecarsec(Reievnendetsh,eFfirrasntcEeR)aEnSdJPurnoifo.rDAuwnacardn(WER.BErSu,cEeu(rEoxpeetearn,
e2 7.1. Complexes Blended with Polymers 4328
on Septgust 4, 7.2. MCoamtripxlexes Covalently Bonded to the Polymer 4332 RtCimaartehe)o-EalicsasrUtohcniiaavnteedrspiAtrycotfoienfsidsLeoeru.SvPoecrnei.estHeyn)is.tlyFc,ruohrmreeni2st0rp0er2soefueansrcstiholr2in0ot0fe5cre,hsehtmesiaswtrareysma(pteatthartel--
ns Au 7.3. Complexes of Dendrimeric Ligands 4334 containingliquidcrystals(metallomesogens),lanthanide-mediatedorganic
oen BinnemaDate (Web): 89..7.HN4.yadnCroocogooemrldspinoaasntiditoenOMrPgaoatlenyrmoiagelesrlss 444333344830 crioeonaliccotiroliqnpusui,drlsait.nythoafnitdheespeemctirtotescdopliyg,hlutm.Tinheseceenmtimsosileocnulcaromloartedreiaplse,nadnds
Kn 9.1. Surface-Modified Nanoparticles 4343
ded by blicatio 99..23.. NNaannooppaarrttiiccllee-s iPnoSlyoml-erGCeolmGplaosssiteess 44334456 oennvitrhoenmlaennthtaonfidaegiivoennblauntthisanliadregeiolyn.iMndoespteonfdtehnet sotufdtihees
oaPu on these compounds have been limited to either inorganic
wnl 10. Applications 4347 compounds(lanthanidephosphors)ormolecularlanthanide
o 10.1. Luminescent Thin Films 4347
D compounds (for instance, the (cid:1)-diketonate complexes). For
10.2. Polymeric Optical Amplifiers 4348 about one decade, there has been a strong interest in
10.3. Lasers 4349 lanthanide-based organic-inorganic hybrid materials. In
10.4. OLEDs 4350 thesematerials,amolecularlanthanidecomplexisembedded
10.5. Luminescent Chemical Sensors 4355 inaninorganichostmatrix(likethesol-gel-derivedmateri-
10.6. Luminescent Molecular Thermometers 4356 als),oralternatively,aninorganiclanthanidecompound(like
11. Conclusions and Outlook 4358 apolyoxometalatecomplexoralanthanide-dopednanopar-
12. List of Abbreviations and Symbols 4358 ticle)isembeddedinanorganicpolymermatrix.Ofcourse,
13. Acknowledgments 4359 thedistinctionbetweentheseclassesisnotwell-defined,as
14. References 4359 illustrated by the organically modified xerogels. The study
ofluminescentlanthanidecompoundsinhybridmaterialsis
1. Introduction notonlyoffundamentalinterest,becausethesematerialshave
also a high potential for different applications (optical
The photoluminescence properties of rare-earth (lan-
amplifiers, optical waveguides, OLEDs, etc.). In general,
thanide) compounds have been fascinating researchers for
these hybrid materials have superior mechanical properties
decades.1-14Anattractivefeatureofluminescentlanthanide
and have a better processability than the pure molecular
compoundsistheirline-likeemission,whichresultsinahigh
lanthanide complexes. Moreover, embedding a lanthanide
complexinahybridmatrixisalsobeneficialforitsthermal
*Fax:[phone].E-mail:[email protected]. stability and luminescence output.
10.1021/cr8003983CCC:$71.50 2009AmericanChemicalSociety
PublishedonWeb08/04/2009
4284 ChemicalReviews,2009,Vol.109,No.9 Binnemans
Table1. ElectronicStructureoftheTrivalentLanthanideIons emission is due to transitions inside the 4f shell, thus
atomic configuration ground intraconfigurational f-f transitions. Because the partially
element symbol number(Z) Ln3+ stateLn3+ filled 4f shell is well shielded from its environment by the
lanthanum La 57 [Xe] 1S closed5s2and5p6shells,theligandsinthefirstandsecond
0
cerium Ce 58 [Xe]4f1 2F coordinationsphereperturbtheelectronicconfigurationsof
5/2
praseodymium Pr 59 [Xe]4f2 3H4 the trivalent lanthanide ions only to a very limited extent.
neodymium Nd 60 [Xe]4f3 4I
9/2 This shielding is responsible for the specific properties of
promethium Pm 61 [Xe]4f4 5I
samarium Sm 62 [Xe]4f5 6H4 lanthanide luminescence, more particularly for the narrow-
5/2
europium Eu 63 [Xe]4f6 7F bandemissionandforthelonglifetimesoftheexcitedstates.
0
gadolinium Gd 64 [Xe]4f7 8S7/2 Ce3+ is a special case because this ion emits intense
terbium Tb 65 [Xe]4f8 7F6 broadband emission due to allowed f-d transitions. The
dysprosium Dy 66 [Xe]4f9 6H
15/2 positionoftheemissionmaximumstronglydependsonthe
holmium Ho 67 [Xe]4f10 5I
erbium Er 68 [Xe]4f11 4I8 ligand environment of the Ce3+ ion. Depending on the
15/2
thulium Tm 69 [Xe]4f12 3H method of excitation, different types of luminescence are
6
ytterbium Yb 70 [Xe]4f13 2F7/2 defined, for example, photoluminescence (emission after
lutetium Lu 71 [Xe]4f14 1S
0 excitation by irradiation with electromagnetic radiation),
electroluminescence(emissionbyrecombinationofelectrons
The aim of this review is to give an overview of the andholes undertheinfluence of anelectricfield),chemilu-
different types of lanthanide-based hybrid materials and to minescence (nonthermal production of light by a chemical
comparetheirrespectiveadvantagesanddisadvantages.The reaction), or triboluminescence (emission observed by ap-
g literature has been covered until March 2009. Both the plyingmechanicalstresstocrystalsorbyfractureofcrystals).
s.or preparationofthesematerialsandtheirluminescenceproper- Althoughphotoluminescenceoflanthanideionscanbean
s.ac983 ties will be discussed. The device construction and the efficient process, all lanthanide ions suffer from weak light
ub03 physics behind these devices will not be covered in detail. absorption. Because the molar absorption coefficients ε of
9 | http://p1021/cr80 Tignlhaetshsdiesesfiranenivtdiioepwno,olfysmohyelbarrsnidtdhomapnaeitddeer-iwaiolitnihs-dionortpgeeardpnriecinteloadrngrtaahntahinceirdsbeorloc-aodgmley-l mtorniovlysatlaeonvfterltayhneltihmtarinateinddseiatiimoonnossunaitrneofsthmreaadlialaebtrsiootnhrpaitnsioa1nb0sosLprbemecdotrlab-y1ocdfmirte-hc1et,
27, 200doi: 10. pthleaxneidse-adlsoopeadrenadnioscpuasrstiecdle.sBiescaeuxspelosthiveellyitegrraotuwriengo,nalasne-- eisxcniotattoionnlyinprtohpeo4rtfiolenvaelltso.Sthinecleumthienelusmceinnceescqeunacnetuinmteyniseiltdy
ember 2009 | lbeecetniomnoadfeth.eworkmostrelevanttomaterialschemistryhas babustoraplstioontoretshueltsaminouwnetakoflulimghintesacbesnocreb.edH,owweeavker,ligthhet
n Septust 4, hyFbroirdsmpaetceirailailzseadndintfhoerimraatpipolnicoatniotnhse,tdhieffreeraednetrcislarsesfeesrreodf psoro-cballelmedoafnwteenankaliegfhfetcatb(soorrpsteinosnitcizaantiboen)o.vWereciosmsmeabnydtihse-
ns oAug to the available books15,16 and reviews.17-35 A special covered that intense metal-centered luminescence can be
nnemaWeb): rheyfberriedncmeaitsemriaaldse.36to-3r8ecentreviewsonlanthanide-containing oexbcseitravteiodnfoirnlaannthaabnisdoerpctoiomnplbeaxnedswoifththoergaonrgicanliigcanlidgsaunpdo.3n9
oen BiDate ( 2. Luminescence of Lanthanide Complexes Bmeocpahuosreeso,fmtuhcehinmteonreseliagbhstocrapntiobneabbasnodrsbeodfboyrgthaneiocrgchanroic-
Kn
d by catio 2.1. Principles of Lanthanide Luminescence leixgcaintadtsiotnhaennebrygythiesltarnanthsafenrirdeediofrnomitstehlfe.oSrugbasneiqculeignatlnyd,sthtoe
debli
au the lanthanide ion by intramolecular energy transfer. In his
oP Thetrivalentionsofthelanthanideseriesarecharacterized
wnl byagradualfillingofthe4forbitals,from4f0(forLa3+)to seminalpaper,Weissmandescribedthisphenomenonforthe
Do 4f14(forLu3+)(Table1).Oneofthemostinterestingfeatures europium(III)complexesofsalicylaldehyde,benzoylacetone,
dibenzoylmethane, and meta-nitrobenzoylacetone. It took
oftheseionsistheirphotoluminescence.Severallanthanide
about 20 years before the importance of Weissman’s work
ions show luminescence in the visible or near-infrared
was fully appreciated, although Sevchenko and Trofimov
spectral regions upon irradiation with ultraviolet radiation.
showedthathisexperimentscouldbereproduced.40However,
Thecoloroftheemittedlightdependsonthelanthanideion.
For instance, Eu3+ emits red light, Tb3+ green light, Sm3+ sincethemechanismsoftheenergytransferfromtheorganic
orangelight,andTm3+bluelight.Yb3+,Nd3+,andEr3+are ligand to the lanthanide ion were investigated in the early
1960s and since it was realized that the lanthanide (cid:1)-dike-
well-known for their near-infrared luminescence, but other
lanthanide ions (Pr3+, Sm3+, Dy3+, Ho3+, and Tm3+) also tonatecomplexeshavepotentialastheactivecomponentin
show transitions in the near-infrared region. Gd3+ emits in chelate lasers, intense research activity has been going on
in the field of luminescent materials based on molecular
the ultraviolet region, but its luminescence can only be
lanthanide complexes.
observed in the absence of organic ligands with low-lying
singlet and triplet levels. When the light emission by The commonly accepted mechanism of energy transfer
lanthanide ions is discussed, one often uses the term fromtheorganicligandstothelanthanideionisthatproposed
“luminescence”, rather than the terms “fluorescence” or byCrosbyandWhan(Figure1).41-43Uponirradiationwith
“phosphorescence”.Thereasonisthatthetermsfluorescence ultraviolet radiation, the organic ligands of the lanthanide
andphosphorescenceareusedtodescribelightemissionby complexareexcitedtoavibrationallevelofthefirstexcited
organicmoleculesandthatthesetermsincorporateinforma- singletstate(S rS ).Themoleculeundergoesfastinternal
1 0
tionontheemissionmechanism:fluorescenceissinglet-to- conVersion to lower vibrational levels of the S state, for
1
singlet emission (i.e., a spin-allowed transition) and phos- instance, through interaction with solvent molecules. The
phorescence is triplet-to-singlet emission (i.e., a spin- excited singlet state can be deactived radiatively to the
forbidden transition). In the case of the lanthanides, the ground state (molecular fluorescence, S f S ) or can
1 0
Lanthanide-BasedLuminescentHybridMaterials ChemicalReviews,2009,Vol.109,No.9 4285
are below that of the resonance level of the lanthanide ion,
molecular fluorescence or phosphorescence of the ligand is
observed, or no light emission at all is observed. The
luminescenceobservedforaspecificlanthanidecomplexis
therefore a sensitive function of the lowest triplet level of
the complex relative to a resonance level of the lanthanide
ion.Becausethepositionofthetripletleveldependsonthe
type of ligand, it is therefore possible to control the
luminescence intensity observed for a given lanthanide ion
byvariationoftheligand.49Thepositionofthetripletlevel
is also temperature dependent, so the luminescence caused
byindirectexcitationthroughorganicligandsismuchmore
temperature sensitive than luminescence caused by direct
excitation of the 4f levels. Sato and Wada investigated the
relationship between the efficiency of the intermolecular
energy transfer from the triplet state to the lanthanide ion
Figure1. Schematicrepresentationofphotophysicalprocessesin and the energy difference between the triplet state and
lanthanide(III) complexes (antenna effect). Abbreviations: A ) resonance levels of the lanthanide ions.50 The authors
absorption; F ) fluorescence; P ) phosphorescence; L ) lan-
thanide-centeredluminescence;ISC)intersystemcrossing;ET) determinedtheenergyofthetripletstatesbymeasuringthe
energytransfer;S)singlet;T)triplet.Fullverticallinesindicate phosphorescence spectra of gadolinium(III) (cid:1)-diketonate
g radiative transitions; dotted vertical lines indicate nonradiative complexesat77Kinaglass-formingEPAsolution(5parts
or transitions. diethyl ether, 5 parts 3-methylpentane, and 5 parts ethanol
s.
ubs.ac03983 undergo nonradiative intersystem crossing from the singlet tbhyevtroilpulmetel)e.vBelesc,anuosemtehteal4-cfelnetveerlesdoefmGisds3i+onarcealnocbaeteodbsaebrovvede
p://pcr80 sdteaatcetiSv1atteodtrhaeditartiipvleeltysttoatethTe1g.rTouhnedtrsitpalteet, Ssta,tebyTt1hecasnpibne- for Gd3+. Moreover, the presence of a heavy paramagnetic
9 | htt1021/ forbidden transition T1 f S0. This results0in molecular itohne terniphlaentcsetastethbeecinatuesresyosftemmixcinrogssoifngthefrotrmipltehteansdingsilnegtlteot
7, 200oi: 10. pahnoosnprhaodrieasticveenctrea.nAsilttieornnaftriovmelyth,ethteripcloemtpstlaetxemtoaaynuenxdceirtgeod isntatetersac(tpioanra,mtahgenetrtiipcleeftfesctta)t.5e1,5a2cBqyuitrheesspainp-arotribaliltycosuipnlgilnegt
mber 2009 | d setnaetregoyftrthaneslfaenrt,htahneidlaenitohnan.iAdefteiornthmisaiynduinrdeecrtgeoxcaitraatdioiantibvye cohfarthaceteGradn3+dtihoens,eltehcetiodnecrauylestiamreereolfaxtehde.Btryipthleetpsrteasteencies
n Septeust 4, 2 tprhaontsoitliuomnintoescaenlcoewoerr m4faystbaetedebayctcivhaatreadctberyisntiocnrlaindeia-tliivkee orebdsuecrveed.p53hoCsrpyhoogreenscicentceem,bpeecraatuusreesotahreerwoifsteenthenetcriepslseatrsytattoe
og processes. According to Whan and Crosby the main cause
oen Binnemans Date (Web): Au KeocnofleuenripognlnyeinrramgfdrowaiamnittihvtpherteohdpeeeoxalsiccgetiiatdvendadatisaoimnnndgeolcsefhottalhsnvetieaslntmaetnSmtho1oaftlnoedicdtiuhreeleeicoset.nn4te1irrsaAgnvylsitbhfleeroorvunegoilhcsf icqaAsorulemetdhnaepolcaweuhctgitaenihysvgseewdolnofeicbtrthahygteyenpdttohrrianoaptrnslapeasdtfhleisooartrwtaeittvoeseecrtieehspnnernceoeelrca.ggenlysiAtgshteithabsanl.7neid7A.teThlsKehiooe,snifltntrtuhigapoelklereetestsslolecpelvevlvneaeelccnlseest.
Kn ofthelanthanideion,thismechanismisnowconsiderednot
d by catio tobeofgreatimportance.44Indeedthisprocessisoftennot forbosmervtheeinlowtheestpthroipslpehtolerevsecleTnc1,eithiisghseormleytiinmgestrippolsestibstlaetetos
oadePubli eEfxficciiteantitodnuoefttohethleansthhoarntidliefeitoimnevioafththeessiningglelettsteaxtceiitsedknsotawten. such as T2 as well. The efficiency of the energy transfer is
wnl for Tb3+45 and for Eu3+.46 Direct excitation from a singlet proportional to the overlap between the phosphorescence
Do statetothe4flevelsofaNd3+ionwasdemonstratedbyvan spectrum of the ligand and the absorption spectrum of the
lanthanide ion. The overlap decreases as the triplet state
Veggel and co-workers for dansyl- and lissamine-function-
energy increases. A close match between the energy of the
alizedneodymium(III)complexes.47Directlightinthetriplet
triplet state and the energy of the receiving 4f level of the
level followed by energy transfer to the lanthanide ion is a
lanthanide ion is not desirable either, because energy back
lessobservedandlessstudiedphenomenon.48Luminescence
transfer of the lanthanide ion to the triplet state can occur.
by the lanthanide ion is only possible from certain levels,
which are termed resonance leVels. The main resonance Forinstance,Latvaetal.observedthatenergybacktransfer
levelsare4G forSm3+(17800cm-1),5D forEu3+(17250 from the excited Tb3+ ion to the ligand occurs when the
cm-1),5D fo5r/2Tb3+(20430cm-1),and4F 0forDy3+(20960 energy difference between the 5D4 level of Tb3+ and the
cm-1).Ift4helanthanideionisexcitedtoa9/n2onemittinglevel, lowest triplet state of the ligand is less than 1850 cm-1.54
Becauseoftheenergytransferoftheorganicligandstothe
eitherdirectlybyexcitationinthe4flevelsorindirectlyby
lanthanideion,thefluorescenceandphosphorescenceofthe
energy transfer, the excitation energy is dissipated via
ligandsisquenched.55,56Whentheenergytransferisnotvery
nonradiative processes until a resonance level is reached.
efficient, it is possible to observe some remaining ligand
Radiative transitions become then competitive with the
emissionincombinationwiththelanthanide-centeredemission.
nonradiativeprocesses,andmetal-centeredemissioncanbe
observed.Lineemissionbyalanthanideionisonlypossible Another possibility to sensitize lanthanide luminescence
if nonradiative deactivation, molecular fluorescence, and isviacharge-transferstates.57-60Thisisespeciallythecase
phosphorescence can be minimized. In order to populate a for trivalent lanthanide ions that can easily be reduced to
resonance level of a lanthanide ion, it is necessary that the thedivalentstate(redox-sensitivelanthanideions)likeSm3+,
lowest triplet state of the complex is located at an energy Eu3+, and Yb3+, where light can be absorbed by an intense
nearly equal or above the resonance level of the lanthanide ligand-to-metal charge transfer state (LMCT state) from
ion,notbelow.Whentheenergylevelsoftheorganicligands whichtheexcitationenergycanbetransferredtothe4flevels
4286 ChemicalReviews,2009,Vol.109,No.9 Binnemans
of the lanthanide ion. This process only works well if the τ
LMCT state lies at high-enough energy. For instance, for ΦLn ) τobs (4)
Eu3+, sensitization through a LMCT state is efficient if the rad
LMCTliesabove40000cm-1.Low-lyingLMCTstateswill
The observed lifetime, τ , can be derived from intensity
partially or totally quench the luminescence.61 In the case obs
decay curves. However, it is not easy to experimentally
ofEu3+,metal-centeredluminescenceistotallyquenchedif
the energy of the LMCT is less than 25000 cm-1. Also determine the radiative lifetime, τR. Many authors consider
this quantity to be a constant value for a given lanthanide
stronglyabsorbingchromophorescontainingd-blockmetals
ion.However,thisisanincorrectassumption.Equallywrong
can be used for sensitizing lanthanide luminescence.62-68
is the assumption that τ can be obtained by measurement
Because these chromophores absorb in general at longer R
of τ after cooling the sample to a sufficiently low
wavelengthsthanthemostoftenusedorganicchromophores obs
temperature (77 K or lower). The best approach to obtain
(typically in the visible spectral region), d-block chro-
τ is by calculation of this value with the aid of the
mophores are especially useful for sensitizing the near- R
infrared luminescence of lanthanide ions like Nd3+, Er3+, experimentallyderivedJudd-Ofeltintensityparameters,Ωλ
and Yb3+. (λ)2,4,6).Theseparameterscanbederivedfromoptical
absorption spectra of the lanthanide complex.74 The Judd-
The luminescence quantum yield Φ is an important
Ofeltparametrizationschemeworksremarkablywellforions
parameter for evaluation of the efficiency of the emission
like Nd3+, Er3+, and Ho3+ but is more difficult to apply for
process in luminescent materials. The quantum yield is
Pr3+.ThefactthattheJudd-Ofelttheorydoesnotproduce
definedastheratioofthenumberofemittedphotonstothe
well the intensities of Pr3+ spectra has been the subject of
number of absorbed photons per time unit:69
much debate.75-78 A special case is Eu3+, where the τ can
R
bs.acs.org 3983 Φ ) nnuummbbeerrooffaebmsoitrtbeeddpphhoototonnss (1) tbuheseindmegtaetghrmneeEitniicendsdtiwepiiontlhecooturetafnkfisncioiteiwonnltefd5oDgre0sfopfot7nhFtea1,nintehtoeeunrsseieftrmyacpistaisrviaoemnineAdteeorxsf
pu00 Forluminescentlanthanidecomplexes,theoVeralllumines- n, and the ratio of the total integrated intensity to the
27, 2009 | http://doi: 10.1021/cr8 ldtcureeamntnecsirenfmeeqrisnuc(eaeηdnnsectnuebsm)ysatyentiphdeel(dbΦ,yefΦLfitnh)tco:eit7,e0qnuucpayonntouefmxcsiyetnaietsiliodtinzoaoftfitohtnheeloalringtaehnnadensrigdiyes mecinraietsTteayigchsaruetaalrtseoaekdnvd.,deTirbrnauehtlqteleuntqhmisrueieteaysdanssetouputferemrectmhmiayeielnine5eaDltqtdiu0oo,ifnfpΦmaotbo7fetsF,nro1tecllbuatirenataecbnablqseueiustvaieeoanxinlttupu.i7eems0srnimiyesiceenenlosdtstasalarliynys
ember 2009 | Φtot ) ηsensΦLn (2) tTohkenseowmethaseuarmemoeunnttsoafreexdcointeedblyigthhteruesceeiovfesdcbatytetrhiengsaamgepnlets.
on Septgust 4, ΦitLisnitshecaqllueadntthueminytireinldsicdeltuemrminiensecdenbcyedqiuraecnttuemxcyiitealtdio,nanind awnodrka,noinneteigsroaftitnengssapthisefireedtowciathlibthraetedettheermsyinstaetmio.nFoofrrreoluattiinVee
ns Au the 4f levels of the Ln3+ ion. The intrinsic quantum yield, quantum yields. In this case, the quantum yield of the
nnemaWeb): ΦdeLanc,tiivsadtiiornec(tkly)raenldatendontroadtihaetivraetedecaocntisvtaantitosnf(okr r)a,dbiyatitvhee unknown is compared w(ith)t(hat o)(f a r)eference sample:
aded by Koen Biublication Date ( relationship r Φtot ) (kr +krknr) nr (3) where Φ is theΦlXum)ineAAscXRencEEeXRqunnaXRntu2ΦmRyield, A is (th5e)
oP absorbanceattheexcitationwavenumber,Eistheareaunder
nl
w Φ expresseshowwelltheradiativeprocessescompetewith the corrected emission curve (expressed in number of
o Ln
D nonradiative processes. The factor k is temperature- photons),andnistherefractiveindexofthesolventsused.
r
independent.Processesthatcontributetok areenergyback The subscripts R and X refer to the reference and the
nr
transfertotheligands,energytransferquenching(important unknown, respectively. The ideal absorbance values for
for Eu3+), and matrix vibrations. Especially, OH and NH luminescencemeasurementsliebetween0.05and0.04.When
vibrations are effective in quenching lanthanide lumines- theabsorbanceisabove0.05,theemissionintensitycanno
cence. Haas and Stein investigated the different pathways longer be assumed proportional to the concentration of the
forradiativeandradiationlessdeactivationofexcitedstates analyte(nolinearrelationshipbetweentheemissionintensity
of lanthanide ions, and they point especially to the role of and the concentration). Only when the sample and the
high-energy vibrations in the radiationless deactivation reference have the same absorbance at the excitation
processes.71,72 The nonradiative rate constant contains con- wavelength, absorbance up to 0.5 can be tolerated. When
tributions from a temperature-independent term, which the absorbance is too low, the impurities from the medium
accounts for the deactivation to the ground state, and a maybecomeimportantwithrespecttotheamountofanalyte.
temperature-dependent term, which can play a role when Moreover, at low concentrations the dissociation of the
upper-lying short-lived excited states are thermally acces- complex in solution can be a problem, especially when the
sible.73 The nonradiative processes influence the obserVed formation constants are not very high. It is advisible to use
luminescence lifetime, τ , whereas they do not influence the same excitation wavelength for measuring the lumines-
obs
the radiatiVe lifetime, τ () 1/k). The radiatiVe lifetime is cence of the standard and the unknown. One should not
R r
thelifetimeofanexcitedstateintheabsenceofnonradiative choosetheexcitationwavelengthontheedgeofanexcitation
transitions.Althoughsometimestheterm“naturallifetime” band, because upon excitation on the edge, a slight change
isusedinsteadof“radiativelifetime”,theformertermshould in wavelength will induce a large change in the amount of
beabandoned.Theintrinsicquantumyieldcanbedetermined light absorbed. When the same solvent is used for both the
using the equation referenceandtheunknown,thefactor(n /n )2willbeequal
X R
Lanthanide-BasedLuminescentHybridMaterials ChemicalReviews,2009,Vol.109,No.9 4287
to unity. For lanthanide complexes, the quantum yield luminescence decay time indicates the extent of quenching
depends on the excited state of the ligand or metal ion, attheemittingionsiteonly.Bhaumikstudiedthetemperature
because sensitization of the lanthanide ion can go through variation of the luminescence quantum yield and the decay
several energy migration paths, the efficiency of which times of various europium(III) (cid:1)-diketonate complexes in
dependsontheparticularlevelsinvolved.Forintegrationof various solvents.89 Although the luminescence decay times
the emission spectra, the spectra have to be expressed as a of the complexes were quite different from one another at
functionofthewavenumber(cm-1)andnotasafunctionof room temperature, the values for the different complexes
thewavelength.Ofcourse,theluminescencequantumyield wereverymuchthesameat77K(ca.450µs).Thisindicates
hastobedeterminedbytheuseofcorrectedemissionspectra. that the rate of quenching at the Eu3+ site approached a
Finding a suitable reference (standard) is often a serious constant value at this temperature. Fluorine substitution in
problem, especially when one wants to perform measure- the ligand resulted in a decrease of the quenching and thus
mentsonluminescentmaterialsthatemitinthenear-infrared inanincreaseofthedecaytimeatroomtemperature(upto
region. The reference compound has to emit in the same a factor 2). Although the decay times of the europium(III)
region as the lanthanide ion of interest does. Most of the complexes were similar at 77 K, this was not the case for
fluorescence standards are organic compounds that show thecorrespondingluminescencequantumyields.Thisgives
broadbandemission,whereasthelanthanideionsexhibitline- an indication of the fact that quenching occurs not only at
likeemission.Fordeterminationoftheluminescencequan- the lanthanide ion site but in the ligand as well. The
tum yield of europium(III) complexes, cresyl violet (Φ ) luminescence quantum yields of the fluorinated complexes
54%inmethanol)orrhodamine101(Φ)100%inethanol) were found to be higher than those of the nonfluorinated
can be used as standards.79 For terbium(III) complexes, complexes.Duringatime-resolvedstudyofthespectroscopic
quinine sulfate (Φ ) 54.6% in 0.5 M aqueous H SO ) and properties of tris and tetrakis complexes of europium(III)
g 2 4
or 9,10-diphenylanthracene(Φ)90%incyclohexane)canbe with dibenzoylmethane ligands in glass-forming solvents,
ubs.acs.03983 uinsetdh.e79vAisnibolteherergsitoanndiasr[dRfuo(rbliapnyt)h3a]Cnild2e(λceoxm)pl4e0x0esnemm,iΦttin)g eWxpatosnoennteitalalyl.forobmseravneadptphaartenthtein5iDtia0leinmteisnssiiotyntiontaemnsaixtyimrousme
9 | http://p1021/cr80 2oasf.8e%suercoionpniuwdmaart(yeIIrI)s).t8aa0nnBddua¨trendrzbsliiufamonr(dIIlcuI)omt-rwiinsoe(drskcipeeirncscoeplirnoqaputeoa)snectudommthpeyleiuxeseldes twahiemthu5iDnch1selloevvneegralelrwm(amiscirlfoloissueenccdoonntdods)dateinmcdaeydseoccnaalyeae.9d0meTixchpreoosneeemcnoitsnisadilolytnimoonef
27, 200doi: 10. detFeormr sinoalitdiosna.8m1ples, standard phosphors can be used.82-85 isdceanlet,icaanldwtihthetrhiseedteicmaeyotifmteheof5Dth0ee5mDi1sseiomniswsiaosn.vIitrtuisalalyn
ember 2009 | The relevant expression(i1s - R )(φ ) iinsteshreosrttienrgtphhaennothmaetnoofntthheattethtrealkifiseticmomeopfletxheestraistc7o7mKpl,exbeust
n Septust 4, ΦX ) 1 - RXR φXR ΦR (6) theTrheeverersaedeirswtruheoaist rionotemrestetemdpienramtuorree. theoretical aspects
og
d by Koen Binnemans cation Date (Web): Au T)wφpquhbhiao3es+n9srp,t9teCuhh%mReeo3)ri+isynsf(itteeoCthhlgrBadertsaTeatca-uemUardrenoo7uppY3bnih4ue2toOmPtouo3h(fs/nIi3elIrid%eIflp)flusae,ExsecΦumt(se3pti+d)ahssno(ei9Ydtxoo5acnOn%ridtsXa)ats-fnfiU-oood1rrn7)t.1lreGu9aCrmddboPMiiimuahntmgieimlosBi(pcneI5IesrOIac,n)ni1c.Φa80de6/l olDfbwbioofetiireoetarhlkktaadhoestnmeu9u,t95trh-1iaetaas9tW.7nessidTiaygtdnrhbnteodheomnarusgcteeprlvnnteabhitecsaee,sotwc9yrif2kocsagJst1hw4rucr,eoe7odo4updn,er9ny,8kon9,d9st3e9ireaisiagnannrysdermytelhaHfeatvievotsu¨arherfiuefilelnsamndeebdlorald,etef9tor4i.tacsThartstaeelh.htnheeSttdroheciusevlvfapogaesberhlrsecoairniicloetatakowlmiidstozoheuenorersdkssrft
debli Asolidstandardthatiseasilyobtainableissodiumsalicylate, isthe“bluebook”ofCarnallandco-workers,butthisinternal
au
nlo P which has a broadband emission with a maximum at 450 reportoftheArgonneNationalLaboratoryhadunfortunately
ow nm and a luminescence quantum yield of 60% at room onlyalimitedcirculation.100Therefore,theseriesofpapers
D temperature.83 One of the few examples of direct determi- ofCarnallontheenergylevelstructureoftrivalentlanthanide
nation of absolute quantum yield of lanthanide complexes ions in aqueous solution is suggested as an alternative for
istheworkofGudmundsenetal.87Theseauthorsdetermined the“bluebook”.101-104ThetwopapersofJuddandOfelton
theabsolutequantumefficiencyoftheeuropium(III)2-thenoyl- the theory of intensities of lanthanide spectra are citation
trifluoroacetonate complex [Eu(tta)3] in acetone by a calo- classics (Judd-Ofelt theory),105,106 but the well-known Ω
λ
rimetric method. By this technique, the temperature rise of intensity parameters were introduced by Axe.107 Carnall
the samples due to nonradiative deactivation is measured. appliedJudd-Ofelttheorytothelanthanideionsinaqueous
Thequantumefficiencyinacetoneat25°Cwasdetermined solution.108,109 Several other reviews on intensities of f-f
as0.56(0.08.Onlythe5D0f7F2transitionwasconsidered, transitions have been published.74,110-113
because the authors argue that this transition accounts for
more than 95% of the total emission of the complex. For
2.2. Lanthanide (cid:1)-Diketonates
thedeterminationoftheluminescencequantumyields,itis
notnecessarytorecordemissionspectraathighresolution.72 Lanthanide (cid:1)-diketonates are complexes of (cid:1)-diketone
The errors on the experimentally determined luminescence ligands (1,3-diketones) with lanthanide ions. These com-
quantumyieldscanbequitehigh(upto30%),soonehasto plexes are the most popular and the most intensively
be careful with drawing conclusions when quantum yields investigatedluminescentlanthanidecoordinationcompounds.
ofdifferentsystemsarecompared.Theluminescencequan- Theirpopularityispartiallybecausemanydifferent(cid:1)-dike-
tum efficiency of the europium(III) (cid:1)-diketonate complex tones are commercially available and the synthesis of the
[Eu(nta) (dmso) )](0.75)isoneofthehighestobservedfor corresponding lanthanide complexes is relatively easy but
3 2
solid europium(III) complexes.88 also because of their excellent luminescence properties.
Whereas the luminescence quantum yield gives an idea Unfortunately, they suffer from a poor photostability upon
of the luminescence quenching in the whole system, the UV irradiation. An extensive review on lanthanide (cid:1)-dike-
4288 ChemicalReviews,2009,Vol.109,No.9 Binnemans
Table2. Overviewof(cid:1)-diketonesusedasligandsinlanthanidecomplexes
abbreviation name synonym
Hacac acetylacetone 2,4-pentanedione
Hbtfac benzoyltrifluoroacetone
Hbzac benzoylacetone 1-phenyl-1,3-butanedione
Hdnm dinaphthoylmethane
Hfacam 3-(trifluoroacetyl)-d-camphor
Hfod 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione
Hhfac hexafluoroacetylacetone 1,1,1,5,5,5-hexafluoro-2,4-pentanedione
Hntac 2-naphthoyltrifluoroacetone 4,4,4-trifluoro-1-(2-naphtyl)-1,3-butanedione
Hptp 1-phenyl-3-(2-thienyl)-1,3-propanedione
Htfac trifluoroacetylacetone 1,1,1-trifluoro-2,4-pentanedione
Hthd(Hdpm) 2,2,6,6-tetramethyl-3,5-heptanedione dipivaloylmethane
Htta 2-thenoyltrifluoroacetone 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione
tonate complexes has been written by Binnemans.114 In expected for the (cid:1)-diketonate complexes of samarium(III)
several reviews on luminescent lanthanide compounds, the anddysprosium(III).121Weakvisibleornear-infraredemis-
(cid:1)-diketonatecomplexesaredescribed.115-118Becauseoftheir sion is possible for the complexes of praseodymium(III),
frequentuseinlanthanide-basedluminescenthybridmateri- neodymium(III),holmium(III),erbium(III),thulium(III),and
als, some of the general properties of the lanthanide ytterbium(III).Ithasalreadybeenmentionedthatnometal-
(cid:1)-diketonate complexes will be discussed in this section. centered photoluminescence can be observed for gadolin-
Three main types of lanthanide(III) (cid:1)-diketonate com- ium(III) complexes because of the high energy of the 4f
org plexes have to be considered: tris complexes, Lewis base levels.BecauseLa3+hasanempty4fshellandLu3+afilled
bs.acs.3983 andatdeusc),tsaonfdttheetrtarkisisccoommpplelexxeess(.teTrhnearnyeluatnrathlatnriisdeco(cid:1)m-dpilkeexteos- 4fofrshtheell,cnoompmleextaels-coenftethreedseluiomnisn.esSceernacfiencaanndbceoo-wbsoerrkveerds
u0
p0 ortris((cid:1)-diketonates)havethree(cid:1)-diketonateligandsforeach investigated the influence of different substituents on the
p://cr8 lanthanide(III)ion,andtheycanberepresentedbythegeneral (cid:1)-diketonateligandsontheintramolecularenergytransfer.122,49
9 | htt1021/ formula[Ln((cid:1)-diketonate)3].Becausethecoordinationsphere They determined the position of the first excited singlet S1
7, 200oi: 10. ocofmthpelelxaensth,athneidelaniothnainsiduensiaotnurcaatendeixnpathnedseitssicxo-coordoirndaintiaotne bauythmoerassfuoruinngdtthheatatbhseoprpotsiiotinonspoefctSr1adoofetshneoctoamffpelcetxdeisr.eTcthlye
mber 2009 | d slipghaenrdes)bbyuotlailgsoombeyrafdodrumcattfioornm(awtioitnhwbirtihdgLienwgi(cid:1)s-bdaiskeest,osnuacthe tlhanetheanneirdgeyiotnra.nOsfnerthferoomthetrhhean(cid:1)d-,dtihkeetoinntaetresylsitgeamndcrotossitnhge
n Septeust 4, 2 aosrwtrai-tne-ro,c1t,y1l0p-hpohsepnhainntehrooxliindee((pthoepno)),.2I,t2′i-sbiaplysoridpionses(ibbliepyt)o, (Wsihnegnletth-teo-etnrieprlgeyt ttrraannssfietirofnr)omdetpheendlisgaonnd ttohethseulbasnttihtuaennidtse.
og arrange four (cid:1)-diketonate ligands around a single lanthani- ion is inefficient, the metal-centered luminescence is weak
Koen Binnemans n Date (Web): Au anaddnreeiek(uaIetIlrtfkIoao)alnrilimatioytnmee,isdse)a.tnaawcdTlhiihtioihneensvtteh(heLidesic+bgow,yemaNnyapea,lrc+eatoe,xlutKerfnsao+tkre,miarCsrcuecasl+otai,amon[RnLpi.bolneT+n(x)(cid:1)ih,ece-s,bdcuoiakatrntemitdtoeootnnrreaeaclketaoeicnsf)t(t4rbe(cid:1)]i-nec- aptioohnhfnedo.tsthWarpitehphtolheeprentehsssotchatsaemeptnehlecaoeintsritiemhsosancoenelbyiansdnecepreeavlmuretbmdiiaas.inlsnOlidyeonsnqeccueabcnenaacnnnecdashipasdepyduarbetohbsatayeoctnhtitthnh,etetthhhlmeaaetnsionetlhoteceafcnanusstielihdatsyeer,
ded by blicatio iutmisH+a,pisrootqouniantoeldinoiurmgaHni+c, ebtacs.e) o(pryariqduinatieurmnaHr+y,apmipmeorindiiunmi- cthoerreenseprognydtirnagnsgfaedrofrlionmiumth(eIIoIr)gacnoimcplilgeaxn.dTthoetheeffilacnietnhcaynidoef
oaPu ion(Et N+,But N+,Hex N+,etc.).Alistofthemostoften ion is proportional to the overlap between the ligand
nl 4 4 4
w used (cid:1)-diketones is given in Table 2 and in Chart 1. A phosphorescence spectrum and the absorption spectrum of
o
D difference is made between the (cid:1)-diketone and the corre- the lanthanide(III) ion.123
sponding (cid:1)-diketonate ligand that is obtained by deproto- Althoughtheeuropium(III)(cid:1)-diketonatecomplexesoften
nation of the (cid:1)-diketone (i.e., the conjugate base of the showanintenseluminescence,theluminescenceintensities
(cid:1)-diketone). For instance, Hacac stands for acetylacetone, arestronglydependentonthetypeof(cid:1)-diketoneandonthe
andacacistheacetylacetonateligand.Lewisbasesfoundin type of complex. Moreover, it is very often difficult to
lanthanide(cid:1)-diketonatecomplexesareshowninChart2.In comparetheluminescenceoutputofdifferentsamples.The
twooften-citedpapersthatwerepublishedinthesameissue luminescence intensity is related not only to the quantum
oftheJournaloftheAmericanChemicalSociety,Melbyet yield of luminescence but also to the amount of absorbed
al.119 and Bauer et al.120 give experimental procedures for radiation.Forthisreason,theluminescenceofsolidsamples
thesynthesisoftheadductsofthetrisandtetrakiscomplexes. will also depend on the position of the sample in the light
These procedures are still the standard procedures for the beamthatisusedforexcitation.Theluminescenceintensity
synthesis of the lanthanide (cid:1)-diketonates. oflanthanidechelatesthatareexcitedintheligandbandsis
Most europium(III) (cid:1)-diketonate complexes show an muchmoredependentonthetemperaturethanthelumines-
intenseluminescence,butmany(cid:1)-diketonatesarenotgood cencethatisobservedupondirectexcitationinthef-flevels.
ligands to sensitize the luminescence of terbium(III) ions. It is possible to find some regularities in the luminescence
The reason is that the triplet level of many (cid:1)-diketonate output of europium(III) (cid:1)-diketonates. The weakest lumi-
ligandswitharomaticsubstituentsisbelowthatofthereso- nescence is observed for the tris complexes. Lewis base
nancelevel5D ofTb3+.Terbium(III)(cid:1)-diketonatecomplexes adductsgivehigherintensities,andthetetrakis(cid:1)-diketonate
4
often show weak or no luminescence at room temperature, complexes gives the highest luminescence intensity. The
although sometimes stronger luminescence is observed at complexes of aliphatic (cid:1)-diketones (acetylacetone, trifluo-
liquid nitrogen temperature. Besides europium(III) and roacetylacetone, or hexafluoroacetylacetone) give weakly
terbium(III) complexes, visible photoluminescence can be luminescent europium(III) complexes because of the large
Lanthanide-BasedLuminescentHybridMaterials ChemicalReviews,2009,Vol.109,No.9 4289
Chart1. Structuresof(cid:1)-DiketoneLigandsforLanthanideComplexesa
g
or
s.
s.ac983
b3
u0
p0
p://cr8
9 | htt1021/
7, 200oi: 10.
2d
mber 009 |
e2
n Septust 4,
og
ns Au
nnemaWeb): ChaaTrhte2m.olSectruulecstaurreeisnothfeLkeewtoisfoBrma.seTsheUasbebdreivniaLtioannstahraeneixdpelainedinTable1.
oen BiDate ( (cid:1)-DiketonateComplexes
Kn
d by catio
debli
au
oP
nl
w
o Figure2. Structureoftheluminescenteuropium(III)(cid:1)-diketonate
D
complex[Eu(tta)(phen)].
3
known for its good luminescence properties is [Eu(tta) -
3
(phen)], where tta is the conjugated base of 2-thenoyltrif-
luoroacetone(Htta)andphenrepresents1,10-phenanthroline
(Figures 2 and 3). Among the terbium(III) (cid:1)-diketonate
complexes, strong luminescence is observed for the tris
complexes of acetylacetone, di-p-fluorodibenzoylmethane,
and trifluoroacetylacetone.49 The highest luminescence in-
tensity is observed for terbium(III) complexes of acetyl-
acetone.124 In order to obtain luminescent terbium(III)
energy gap between the resonance levels of the Eu3+ ion (cid:1)-diketonate complexes with aromatic substituents, com-
and the triplet state of the ligand, which makes energy plexes of 1-indoleacetylacetone and 3-indoleacetylacetone
transfertotheeuropium(III)ionlessefficient.49Combinations wereprepared.125,126Thereasonforthischoicewasthatthe
of aromatic and aliphatic substituents on the (cid:1)-diketones triplet level of the indole group is at a higher energy than
(benzoylacetone, benzoyltrifluoroacetone, 2-thenoyltrifluo- theenergyofthetripletlevelofaphenylgroup.Yangetal.
roacetone)giveeuropium(III)complexeswithamoreintense stated that the presence of a rigid planar structure in the
luminescence.Inthesesystems,theenergytransferfromthe complex causes a higher intensity of the sensitized lumi-
ligand to the lanthanide ion is more efficient. The increase nescence because such structure allows a better energy
in luminescence intensity in such systems is also attributed transfer.124Thefactthatastrongerluminescenceisobserved
to the increased anisotropy around the europium(III) ion.49 for[Eu(tta) (phen)]thanfor[Eu(tta) (bipy)]isinagreement
3 3
Among the Lewis base adducts, a complex that is well- withthisrule.Theluminescenceofeuropium(III)complexes
4290 ChemicalReviews,2009,Vol.109,No.9 Binnemans
condensation reactions take place in solution. These con-
densation reactions result in the formation of Si-O-Si
siloxanebondsandleadtotheproductionofalcoholorwater.
A simplified reaction scheme for the hydrolysis and con-
densation of a tetraalkoxysilane Si(OR) is
4
Hydrolysis:
Si(OR) + nH O f Si(OR) (OH) + nROH (7)
4 2 4-n n
Condensation:
(RO) Si-OR + HO-Si(OR) f
3 3
(RO) Si-O-Si(OR) + ROH (8)
3 3
or
Figure3. Luminescencespectrumat77Kof[Eu(tta)(phen)]in
3 (RO) Si-OH + HO-Si(OR) f
a KBr pellet. The excitation wavelength was 396 nm. All the 3 3
transitionsstartfromthe5D0state. (RO)3Si-O-Si(OR)3 + H2O (9)
can be quenched by a low-lying ligand-to-metal charge-
transfer state (LMCT state).127,128 Such a LMCT state can Besides the type of alkoxide used for the hydrolysis,
important reaction parameters are r ) H O(mol)/Si(mol),
g deactivate the excited singlet or triplet states of the ligand. 2
or solvent,catalyst,pH,temperature,andpressure.Waterisused
bs.acs.3983 3. Sol-Gel Hybrid Materials absyparordeuacgteonftthinecthoendheyndsarotiloynsirsearcetaioctni.oAn,mboultaritraitsioarls)o 2a
u0
p://pcr80 3.1. Inorganic Sol-Gel Systems ihsoswuefvfiecri,enthtefohryadcroolmyspilsetoefrtehaectaiolkno.xUidnedeirsminocsotmcpolnedteitiaonnds
7, 2009 | httoi: 10.1021/ sairleiLcapanrteothmgalniasiisdnsege-sdmohpaaetvederiobaxlesiednfeoigrnlthaesingssehis-vpleiolkyweesprthulodasisepedhrsab1t3ee,,c12ab9u-osr1ea31tetah,neodyr tcAhaleskeoc,xocynosdmielanpnsleaesttieoannhdyrdewaraoctlteyirosnaissreipsirmoamccehiesidecvibseildme.uoTlnthlaeynreewofouhrseelnya.rsIon>lvt1ehn0ist.
mber 2009 | d ogplatsicsaels awmitphlihfiiegrhs.1l3a2ntHhaonwideevecro,ntcheentprraetpioanrastiiosnproofblseimlicaattiec iins uwsheidc.hTbhoethhycodmroplyosniesnctsanarbeesoacluidb-leo,rsubcahsea-scaatnalyalzceodh.oAl,
n Septeust 4, 2 dmuaetrtiocetsheanlodwtosotlhuebilrietsyuoltfinlagnpthhaansiedeseopxairdaetsioinn.1t3h3esSeilgiclaastes tcyaptaiclyalstasciadrieccsaotdaliyusmtishhyyddroroxcidhleo,ripcoatcaisds,iuwmhilheytdyrpoicxaidleb,asoicr
og glasses with doping concentrations higher than 0.5 mol % ammonium hydroxide. Also a nucleophile like the fluoride
Koen Binnemans n Date (Web): Au oaihtholfitganhlnhsaoienudirtsgeh-thandhnooaoitnpbdeteaa2dnii0noseinonmaslgs-oyclaagnte%ualsnbgkiehfl.o1aap3srvr4ms-eeep1s3dba.71rie3seeL8tdnraivnbritueahtpaitoonhnrietdeoesdfocltf-hooenrgceltealhnnepttsrrheoaatcnileoaisdnnses-, ihTtwohyhanedetercaoraallkncymiosdbxoi-esicldeuaoecstfaueglldayerzoatetsueodpttrh,afheoafyolrckdmlalortooaxwlalyyysesspiditles.anTbnitnyehavecanooaruloecvcriedldsesih-onoatpahwntheedindlpbicirainonsatetStoe-tcrcnamhacatekteaimodlbynieayzteo1edaf,.
d by catio Thesol-gelprocessisachemicalsynthesistechniquethat followed by the elimination of an alcohol molecule. The
debli is used for the preparation of gels, glasses, and ceramic base-catalyzed hydrolysis involves the nucleophilic attack
oaPu powders.139-143Anattractivefeatureoftheprocessisthatit on the silicon atom by the hydroxide anion to form a
nl
w enablespreparationofglassesatfarlowertemperaturesthan negativelychargedpentacoordinatedintermediate,followed
o
D thoseoftheconventionalmeltprocesses.Notonlydoesthis by the elimination of an alkoxide anion.
allow avoidance of the problems of phase separation and Oligomersandpolymersareformedbythecondensation
crystallization that are often observed for high-temperature reaction,andthisleadstotheformationofcolloidalparticles
meltprocesses,butitisalsopossibletoencapsulateorganic in the solution. The term sol is used for the suspension of
compounds or metal complexes in the sol-gel glass.144-148 colloidal particles that is formed by the hydrolysis of the
It is often observed that encapsulation in sol-gel glasses tetraorthosilicate precursor and the initial phases of the
increases the photostability of organic luminophores.149,150 condensationreaction.Whenthecolloidalparticlesundergo
The sol-gel process can be used to prepare bulk samples additionalpolymerizationreactions,aninterconnectingnet-
(monoliths) as well as thin films and fibers. The gels can workisformed.Theviscosityofthesolutionincreasesduring
easily be formed in different shapes. The starting products thisgelationprocess,andagelisobtained.Whereasthesol
forthepreparationofsilicatesol-gelglassesaretetraortho- isafluid,thegelhasarigidthree-dimensionalstructurewith
silicatesortetraalkoxysilanes,Si(OR) ,liketetraethylortho- an interstitial liquid phase. At the gel point, when the last
4
silicate(TEOS,Si(OC H ) ),whichismixedwithwaterand bondthatcompletesthethree-dimensionalnetworkisformed,
2 5 4
a mutual solvent to form a homogeneous solution. This asuddenincreaseintheviscosityofthesolutionisobserved.
solvent is typically the same alcohol as generated by the The sol-gel transformation can be monitored by different
hydrolysis reaction of tetrahydrofuran. Another often used spectroscopicmethods.Aftergelation,thegelsarekeptfor
alkoxide precursor is tetramethylorthosilicate (TMOS, sometimeinasealedvesselsothatnoevaporationofsolvent
Si(OCH ) ). Hydrolysis of TEOS is slower than that of can take place. This is the aging process, during which the
3 4
TMOSbecauseoftheretardingeffectofthebulkierethoxy condensation reactions continue and further cross-links in
groups. Partial hydrolysis of the alkoxide leads to the the network are formed. The aging process is followed by
formation of silanol groups, Si-OH, with the release of the drying process, which involves removal of the liquid
alcohol. Instead of complete hydrolysis to silicic acid, phasefromtheinterconnectedporousnetwork.Iftheliquid
Lanthanide-BasedLuminescentHybridMaterials ChemicalReviews,2009,Vol.109,No.9 4291
Scheme1. Acid-andBase-CatalyzedHydrolysisofaTetraalkoxysilane,Si(OR)
4
phase is removed by conventional drying, a xerogel is Three general methods can be used to incorporate or
obtained. The drying step can be performed at room immobilizeluminescentcomplexesintosol-gelglasses:(1)
temperature or at elevated temperatures (<200 °C). During impregnation;(2)doping;(3)chemicalimmobilization.156To
the drying process, the gel shrinks. Additional cross-links impregnateacomplexintothesol-gelglass,thesilicamatrix
org can be formed when the network collapses and unreacted isimmersedforsometimeinasolutionthatcontainsafairly
s.acs.983 -liqOuHidpanhdase-wOiRllbgeroruepmsovcoedmdeuirnintogcthoentdarcyti.nNgoptroaclelsosf,atnhde hpiegrihodconocfenimtrmatieornsioonf,thtehelumcoimnepslceexntwciollmpdlieffxu.sDeuirnitnog tthhee
b3
u0
p0 aconsiderableamountofliquidisretainedinthepores.The channels or pores of the silica glass. To dope the complex
p://cr8 finalvolumeofthexerogelisonlyabout1/8thoftheoriginal in the sol-gel glass, the complex is added to the silica sol
9 | htt1021/ volume.Itshouldbenotedthatitisoftendifficulttoobtain priortogelation.Duringgelation,thecomplexistrappedin
7, 200oi: 10. htoigchr-aqcukaldituyrimngontohleithdircyixnegropgreolcse,sbse.1c5a1uTsehethseuxrfearcoegetlentesniodns tthoedpoopreesthaencdocmhpalnenxelisnothfethseiliscilaicmaahtorisxt.bIyt iasdadlisnogpaosmsiebtalel
mber 2009 | d onfetwthoerkr,eamnadinthinegselsiqtrueisds fionrctehsecapnorceasussetrseesvseesretchreacskiilnicga. sisalftoarmndedliginansditutointhtehesilgieclaogreiln,wthheerxeebryogtehle(cooftmenpldeuxriintsgelaf
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Koen Binnemans n Date (Web): Au s(maxDodeardrCtoeseCgodrielAaulplt)icrtoloaoinpkne.ye1nb5lNe2senuT,Nerthre-aodocnmixrsmaifadcoseekrtemh-tftyreoaledlfe.oraterdompdrpuyaalcimineecgdiodtmoahefepp(tDaahgcrMetetilcgsaFuiet)lillioa.tc1tona5e3ttesthTiiemlghiclpeeaayr.sefisacTllulbhseroyer- otmohbreegTttaalhhuinnoemiedcidfia.nl.relsyTstchelmeannotmtdchooiafismnteipdodlfeets-xedineloiscupiasnetedteodsstmho(eole-rtshmigloieocdlsaiflsmosi,lraictisrnaeicxeogirlsbaptesohslreoeawstdioo)wpnieanorrgeef
ded by blicatio wheealtlinbgelotowhtihgehmteemltpinegraptuorienst oufpstiolic1a1.10504 S°Cev,ewrahlicphroicsessstielsl sliakmephlyedsraptreedpaeruerdopbiuymd(iIsIsIo)lcvhinlogriidneoorgraenuircoplaiunmth(aInIIid)enistraalttes
oaPu take place as the temperature increases: elimination of inthesol.Inaseminalpaper,Levyetal.usedtheEu3+ion
nl
w residualwaterandorganiccomponents,relaxationofthegel asasensitiveluminescentprobetomonitorthetransforma-
o
D structure,andfinallydensificationbyviscoussintering.When tion of a gel into a glass.157 The authors noticed a gradual
the liquid phase is removed from the gel by supercritical increaseofthetotalintensityoftheluminescencespectrum
drying in an autoclave, an aerogel is formed. During andarelativeincreaseintheintensityofthehypersensitive
supercritical drying, the gel is heated above the critical transition5D f7F asafunctionoftimeandasafunction
0 2
temperatureandpressureoftheliquidphase.Aerogelshave of increase in temperature of dehydration of the gel. The
verylowdensitiesandaregoodthermalinsulators,butthey authors concluded that the environment of the Eu3+ ion in
arenotoftenusedashostforspectroscopicallyactivemetal the final sol-gel glass resembled that of Eu3+ in oxide
ionsororganicmolecules.155Theaerogelcanalsobefurther glasses. Reisfeld and co-workers compared the intensity
densifiedtoasilicaglass.Togetthinfilms,thesilicagelis ratios I(5D f 7F )/I(5D f 7F ) for many different glass
0 2 0 1
spin-coated or dip-coated prior to gelation on a suitable systems,includingsol-gelglasses.158Adistinctincreasein
substrate(e.g.,amicroscopeglassslideoraquartzdisk).In theratioasafunctionofincreasingtemperatureisobserved.
thespincoatingprocess,anexcessamountofasolsolution Campostrinietal.performedadetailedspectroscopicstudy
isplacedonthesubstrate,whichisthenrotatedathighspeed of Eu3+-doped silica sol-gel glasses.159 By comparing the
(typicallybetween1000and10000rpm)tospreadthefluid luminescencespectrarecordedatdifferenttemperatures,they
by centrifugal force. Rotation is continued while the fluid were able to follow the changes in the matrix from a wet
spinsofftheedgesofthesubstrate,untilthedesiredthickness geltoacompactsilicaglass.Parametersthatwereconsidered
ofthefilmisachieved.Becausethesolventisusuallyvolatile, are the luminescence decay of the 5D state, the line width
0
it simultaneously evaporates. The thickness of the film of the emission bands, and the crystal-field fine structure.
dependsonthespinningrate,theconcentrationandviscosity These authors considered the changes in the luminescence
of the solution, and the solvent. In the dip coating process, spectra at lower temperatures (25-250 °C) to investigate
asubstrateisimmersedinthesolandthenslowlywithdrawn thegel-to-xerogeltransitionintheseEu3+-dopedsystems.160
at a constant rate. Luminescence studies revealed that the xerogel-to-glass
4292 ChemicalReviews,2009,Vol.109,No.9 Binnemans
transition is a continuous process.161-165 During the initial
dehydration steps, two different environments could be
observed for the Eu3+ ion, a liquid-like and a dry environ-
ment.161 In samples heated to temperatures below 250 °C,
the hydroxyl groups present in the matrix can efficiently
quench the luminescence by nonradiative relaxation. In
samplesheatedtotemperaturesabove250°C,clusteringof
Eu3+ionsandquenchingbyenergytransferisobserved.This
clusteringisalsoevidentfrommoreefficientenergymigra-
tion between Eu3+ ions after the gel-to-glass transition.166
Theclusteringoflanthanideionsduringheattreatmentwas
observed by Jia and co-workers for silica glasses codoped
with yttrium(III) oxide.167 The problems associated with
hydroxyl quenching and clustering of lanthanide ions will
bediscussedindetailbelow.High-resolutionspectroscopic
studies at low temperatures were used to get an idea about Figure4. In-situmonitoringofcerium(III)luminescenceduring
the energy distribution of the different Eu3+ sites.168,169 thecondensationofaTEOS-derivedgelcontaining2.4%Ce/silica.
Spectrawereobtainedattimesaftermixingof(a)0,(b)3,(c)5,
StudiesonthedensificationprocessofsilicaglassesbyEu3+
(d) 7, and (e) 8.5 h. The gel time was 3 h. Reproduced with
luminescence measurements have been complemented by permissionfromref181.Copyright1995Elsevier.
Raman studies.170 Lochhead and Bray investigated the
g influence of the counteranion (nitrate, chloride, or perchlo- of sol-gel glasses is Ce3+, because of the broadband
s.or rate)onthespectroscopicbehaviorofeuropium(III)saltsin emission of this ion (Figure 4).181 It should be noted that
s.ac983 sol-gelglasses.171Theymeasuredtheenergyofthe5D0r Ce3+ is partially oxidized to Ce4+ in the drying gels at 100
pub003 7F0transitionintheexcitationspectratodeterminewhether °C.182 Above 200 °C, total oxidation of Ce3+ to Ce4+ takes
p://cr8 thecounterionwasinthefirstorinthesecondcoordination place.Valencechangesoflanthanideionsinsol-gelglasses
9 | htt1021/ sopbhseerrev.edInfnoerrtshpehneirteractoemanpdlexcehslowriidtehstahletsc(osutrnotenrgieosntsewffeercet ddoupriendgghlaesastest.reHaetmatetnretahtmaveentaolsfoEub3e+e-ndoopbesdesrvoel-dgfeolrgElauss3+es-
27, 200doi: 10. fuopronnitthraetremiaolntrse)abtmutenntootfftohretxheeropgeerlcsh,ltohreatceousnatlet.riHonowefefvecetrs, cthaen lsetraodngtoesptaretfifaelcrtesduwcetrioenoobfseEruv3e+dtofoErus2o+l.-173geHlogwlaesvseers,
ember 2009 | wsoelr-egreelmgolavsesde.sTchoentfaoirnminagtiohnigohfcaoncrcyesnttarlaltiinoenpshoafsCeain(NsiOli3c)a2 caoddvoanpteadgew,bitehcaaulsuemthineal.i1g83h-t1e85neTrghyisabrseodrubcetdiobnycthaneinbteenasne
on Septgust 4, capopueldarabnecemoofncitroyrsetadl-bfiyeldEfiu3n+eslutrmucitnuerseciennctheetlhuamniknsestcoenthcee UleVadianbgstoorpatihoinghbearnldumoifneEsuce2+ncceaenffibceietnrcayn.s1f7e3,r1r8e4dEuto3+Eioun3+s
ns Au spectraduringcrystallization.172Theluminescenceproperties boundto15-crown-5insilicaglasscanbephotoreducedby
nnemaWeb): othfoEseu3o+fiEnua3+soiln-ageml-edleterdivgedlasgsla.1s7s3hTahveesboel-engceolmglpaasrsegdawveitha imrreacdhiaatniiosnmwfiothrUthVerpahdoiatotiroendu(λct)ion32i0snthma)t.1t8h6eThEeup3+roipoonseids
oaded by Koen BiPublication Date ( ilmadnureoomrturihvienneedeedsfs2ofics5lec-i0nileicgcnneeamtlsl.gutguTlmladahsisyinsssee.osbsfTcahthenienatdasceniesnibadvmeuie(roeTnosinttOosmlti2auk)edenstilthteyriodondnfugfibEeylamutbo3Ess+oEXpirunrApe2t+FpsiooaSpnlr.r-e1eb7dsg4aeebnnAlyd-t oEfiEeufruusrt33oth++pepiihoiousnoxmtryroeegedicesxeuncicsviaetteeatdsodbmaitltoniozoeEefaldeutcch2ht+beray.onrngeTtiefhhgr-eeohtrmba1don5itrvsh-ifanceelrgreuonnswsitstlanhaot-ane5xroeialddlnigagdetrialooetnuhcndpter.ssoetIsnaxtotcpewitathieoaardssft
wnl the sol-gel process revealed that the luminescence decay observed that the reduction continues after the irradiation
Do timeofthe5D0stateofEu3+increaseduponheattreatment with UV radiation had stopped. The explanation for this
to 400 °C, but samples heated to 500 °C showed a shorter phenomenon is that the oxygen-associated hole centers
luminescence decay time.175 This difference was attributed formedbyUVradiationslowlyreleasetheirelectronssothat
to differences in the crystalline structure of TiO2. The Eu3+ can be reduced in the absence of any UV light. The
transformations sol f gel f glass have been studied for rapid photoreduction of Eu3+ to Eu2+ gives excellent per-
Eu3+-doped SnO2 gels.176 It is also possible to monitor the sistentspectralholeburning(PSHB)propertiestothisglass,
changesinthesol-gelmatrixduringthedehydrationofthe andtheseholesarestableatroomtemperature.187Additional
geluponformationofthexerogelbyrecordingtheabsorption studies confirmed the stabilizing effect of 15-crown-5 and
spectra rather than the luminescence spectra. The intensity related compounds on the stability of Eu2+ in sol-gel
andfinestructureofthehypersensitivetransitionswerefound glasses.188 Zaitoun et al. observed that during the sol-gel
to change during dehydration and further heat treatment of process,alargeconcentrationofstructuraldefectsareformed
the samples.177-179 A useful ion for studies by UV-vis in the polymeric oxo-bridged silica network and that these
absorptionspectroscopyisNd3+,becauseofthepresenceof defects generate a large amount of electron-hole (e--h+)
thehypersensitivetransition5G5/2r4I9/2around585nmin carriers.189,190 Eu3+ ion present in the gel can harvest these
the absorption spectrum. e--h+ pairs, resulting in a reduction of Eu3+ to Eu2+. The
Although most studies on structural changes in sol-gel Eu2+ ions are stabilized at the cation vacancies by forming
glasses have used Eu3+ as a luminescent structural probe, Eu2+-hole complexes. In Figure 5, the emission spectrum
some authors have considered other lanthanide ions as of an aqueous solution of Eu3+ is compared with that of a
luminescent probe for the glass structure. Pucker et al. Eu3+-doped sol at the gelation point. In the latter case,
reportedonacombinedRamanandluminescencespectros- simultaneous emission by Eu2+ and Eu3+ is clearly visible.
copy study with Tb3+ as luminescent probe.180 A less ThereductionisinhibitedinthepresenceofEDTA,because
conventionalluminescentprobeforthegelationanddrying ofthestrongstabilizingeffectofEDTAonEu3+.Sm3+can