luminescent lanthanide complexes for advanced photonics ...luminescent lanthanide complexes for...

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Rapid Communication in Photoscience(2012), Volume. 1, pp. 1-9

Luminescent Lanthanide Complexes for Advanced Photonics Applications

Yu Kyung Eom, Jung Ho Ryu, Hwan Kyu Kim*

Department of Advanced Materials Chemistry and WCU Center for Next Generation Photovoltaic Systems, Korea University, Jochiwon, Chungnam 339-700, Republic of Korea.

ABSTRACT: Luminescent lanthanide complexes havebeen overviewed for advanced photonics applications.Lanthanide(III) ions (Ln3+) were encapsulated by theluminescent ligands such as metalloporphyrins,naphthalenes, anthracene, push-pull diketone derivativesand boron dipyrromethene(bodipy). The energy levels of theluminescent ligands were tailored to maintain the effectiveenergy transfer process from luminescent ligands to Ln3+ ionsfor getting a higher optical amplification gain. Also, keyparameters for emission enhancement and efficient energytransfer pathways for the sensitization of Ln3+ ions byluminescent ligands were investigated. Furthermore, toenhance the optophysical properties of novel luminescentLn3+ complexes, aryl ether-functionalized dendrons asphoton antennas have been incorporated into luminescentLn3+ complexes, yielding novel Ln3+-cored dendrimercomplex such as metalloporphyrins, naphthalenes, and

anthracenes bearing the Fr chet aryl-ether dendrons,namely, (Er3+-[Gn-Pt-Por]3(terpy), Er3+-[Gn-Naph]3(terpy) and Er3+-[Gn-An]3(terpy)). These complexsshowed much stronger near-IR emission bands at 1530nm, originated from the 4f-4f electronic transition of the firstexcited state (4I13/2) to the ground state (4I15/2) of the partiallyfilled 4f shell. A significant decrease in the fluorescence ofmetalloporphyrins, naphthalenes and anthracene ligand wereaccompanied by a strong increase in the near IR emission ofthe Ln3+ ions. The near IR emission intensities of Ln3+ ions inthe lanthanide(III)-encapsulated dendrimer complexes weredramatically enhanced with increasing the generationnumber (n) of dendrons, due to the site-isolation and thelight-harvesting(LH) effects. Furthermore, it was firstattempted to distinguish between the site-isolation and thelight-harvesting effects in the present complexes. In thisreview, synthesis and photophysical studies of inert andstable luminescent Ln3+ complexes will be dealt for theadvanced photonics applications. Also, the review willinclude the exploratory investigation of the key parametersfor emission enhancement and the effective energy transferpathways from luminescent ligands to Ln3+ ions with Ln3+-chelated prototype complexes.

Advanced photonic devices based on semiconductors andinorganic materials have continuously been pursued for therealization of superhigh speed communication systems oflarge capacity optical communication, information storage,and processing. However, the currently existing limitationsof these materials on the performance, extensive researchon the development of new advanced materials is essentialto overcome such limitations and to achieve photonic devicesfor the next generation superhigh speed communications.1

With regard to this, the organic polymeric materials havebeen considered as such future photonic materials and theirmain advantages, compared with semiconductors andinorganic materials, are as follows: ease of property controland synthetic feasibility through molecular chemistry, fastresponse time and large broad bandwidth, low cost, goodprocessibility, and simple device manufacturing process atlow temperatures, and ease of integrated device fabrication.However, their thermal instability in the optical property andhigh optical transmission loss in the practical use atwavelength of 1.30 or 1.53 μm are still limited in real deviceapplications. High transmission loss has been attributed tothe optical absorption by the inherent presence of C-Hovertone band in organic polymers. To compensate for theabove mentioned shortcomings, the development ofintegrated planar waveguide optical amplifiers utilizing thebasic principle of optical amplification in optical fibers fortelecommunication is extremely high in demand. Inparticular, integrated optical amplifier wavelength divisionmultiplexing(WDM) devices are recognized to be essentialfor successfully realizing photonic devices in WDMtechnology, which has attracted much attention because ofits paramount importance in superhigh speed communicationsystems. Despite such needs, the problem still remains in thematerials for the use in the real optical amplifiers.1

At present, erbium(Er3+)-doped silica amplifiers are widelyused for the optical telecommunication can maintain very lowloss (below 0.18 dB/km) when operated at wavelength of1.53 μm. But, the poor solubility of lanthanide cations (Ln3+)in conventional inorganic/organic media leads to lowamplification property. In addition, Er3+-doped silicaamplifiers is limited to 100-1000 ppm. If higher than thelimiting concentration is to be employed, Er3+ ion interactionbetween themselves would cause non-radiative process tooccur, hence rapidly reducing the amplification gain. Forthese reasons, it is not possible to obtain the amplificationgain of higher than 30 dB with Er3+-doped silica optical fibersfor developing integrated planar waveguide amplifiers.2

In order to enhance the luminescence intensity, luminescent

*To whom correspondence should be addressed.E-mail: [email protected] 9, 2012; accepted February 20, 2012

Introduction

This Journal is ⓒ Korean Society of Photoscience 2012

2This Journal is ⓒ Korean Society of Photoscience 2012

H .K. Kim

ligands are being used to excite the Ln3+ ions via an energytransfer from the luminescent ligands to the Ln3+ ions.The development of luminescent Ln3+ complexes based onthe energy transfer phenomena by using the supramolecularcomplexes as organic ligands has been extensively studiedand attracted considerable attention because of theiracademic interests and potential utility in a wide variety ofapplications, such as planar waveguide amplifiers, plasticlasers, and light-emitting diodes.3-15 In most cases,luminescent Ln3+ complexes consist of Ln3+ ion and chelatingluminescent ligand which acts as a sensitizer that transfersexcitation energy to the encapsulated Ln3+ ion; the presenceof this kind of ligand overcomes the lanthanide ion’sintrinsically low luminescence intensity by direct excitationof the Ln3+ ion having low absorption and emission cross-sections. Luminescent ligands efficiently absorb photons andtransfer the excited energy to the central Ln3+ ions. Thecentral Ln3+ions accumulate energy from the antennachromophores, producing a strong and narrow bandwidthemission. This sensitization process is much more effectivethan the direct excitation of Ln3+ions, since the absorptioncoefficients of organic chromophores are many orders ofmagnitude larger than the intrinsically low molar absorptioncoefficients (typically 1 - 10 M-1cm-1) of Ln3+ ions.

Lanthanide-containing near-infrared(NIR)-emittingcompounds16 are classified according to the syntheticstrategy used to encapsulate the metal ion starting withmacrocyclic compounds,17 then moving to acyclic receptorsbefore describing sensitization with transition metal ions and,finally, various other approaches leading to new materials.Numerous NIR-emitting lanthanide-porphyrin complexeshave been reported, particularly with Yb3+ ion in view of thefavorable energy of the porphyrin triplet state.17 Porphyrinsdisplay rich photochemical properties and various applicationfor sensitizing Ln3+ luminescence.18,19

A coronandis a cyclic molecule containing several donoratoms in its ring such as cyclic ethers termed crown ethers.For example, bimetallic complexes with coronand are muchmore stable and they provide a tighter environment for thefirst coordination sphere of the metal ion. Ln2(coronand)complexes display the highest quantum yields reported forYb3+ complex in water (0.53%).

In the case of acyclic ligands, lanthanide β-diketonates in1897 by Urbain until now, hundreds of different complexesformed by reaction between Ln3+ ions and β-diketonederivatives have been described in the literature.20 Thesecomplexes have their potential application in diverse domain.In presence of lanthanide ions, β-diketonates generally formneutral tris complexes, [Ln(β-diketonate)3], thedelocalization of the negative charge of the ligand leading tothe formation of stable six-membered chlate rings. Crosbyand Kaska reported the NIR luminescence of trivalent Yb3+

ion between the organic liand, in this casedibenzoylmethanate(dbm), and the lanthanide ion.20

8-Hydroxyquinoline (8-HQ) is a bidentate ligandpossessing good coordination properties towards a widerange of metal ions, including lanthanide ion. After thediscovery of the 1.53 μm Er3+ luminescence in [Er(8-HQ)3],where 8-HQ is 8-hydroxyquinolinate, in 1999 by Gillin etal.9 and subsequently metal-centred luminescence in thecorresponding [Nd(8-HQ)3]21 and [Yb(8-HQ)3]22complexes, several derivatives of thischromophore23,24, including podands25,26 and macrocycles27

have been proposed with success for the sensitization ofNd3+, Er3+, and Yb3+.

The acyclic m-terphenylligand derivatized withcarboxylic groups provides suitable coordination ability toefficiently encapsulate lanthanide ions. It has been fitted withchromophoric dyes such as triphenylene, dansyl, coumarin,lissamine, Texas red, fluorescein, eosin, erythrosine(seeterphenyl ligand in Scheme 1) which have low-energytriplet states allowing sizeable sensitization of Ln3+ NIRluminescence.28

In order to overcome the problem of very low Er3+ and Nd3+

quantum yields, these ions have been inserted into dendriticligands. For Nd3+ ion, the light-harvesting chromophores chosenwerethe dansyl group29,30, derivatized benzenetricarboxylate31,32,naphtyl moieties33, or a more intricate edifice in which lightis transferred first onto a Ru2+ complex which sensitizes Nd3+

luminescence by means of the 3MLCT state.34 For Er3+ ion,dansyl units are also adequate chromophores30, as wellas metalloporphyrins.35-37

Very recently, to investigate the longer excitationwavelength of Er3+ ion in Er3+-doped fiberamplifiers(EDFA)s, Er3+-chelated complexes have beenfocused on efficient energy transfer process between theemission of ligand and absorption of Er3+ ion (such as 600-750 nm region). Therefore, Ln3+-cored complexes based onboron dipyrromethene (bodipy) ligands have beenintroduced for NIR emission.38

Two bodipy-based chromophores have been synthesized,which bear two methoxyphenyl substituents in the 1,7positions and either hydrogen (L1) or bromine (L2) atoms inthe 2,6 positions, with the aim of testing their ability tosensitize the luminescence of NIR emitting Ln3+ ions (seeFigure1). Despite the presence of the two halogen

Scheme 1. Examples of ligands for the sensitization ofNIR-emitting Ln3+ ions.

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substituents in L2, the two chromophores display verysimilar absorption and emission properties, the onlydifference being a drop in the quantum yield from 31% forL1 to 21% for L2. In addition, no triplet state emission couldbe evidenced for both chromophores as well as for their triscomplexes with Gd3+ ions. The experimental evidences reported here point to the

bodipy framework being adequate for transferring energyonto NIR-emitting Ln3+ ions, with excitation in the orangeregion of the visible spectrum; in addition the complexeshave high molar absorption coefficients (log ε ≈ 5.0-5.2).While remaining modest, the amount of energy transferredfrom the ligands (20-40%) is sizeable; the weak metal-centered luminescence observed at room temperature ismainly due to deactivation through high-energy vibrations,which explains why Er3+ complexes are less luminescentthan their Yb3+ counterparts, as observed for complexes witha 8-hydroxyquinoline-derivatized bodipy ligand.

Up to date, luminescent Ln3+complexes containingsupramolecular ligands as antenna chromophores were notdeveloped for the real use in various photonics applications,such as erbium-doped fiber amplifiers (EDFA), erbium-doped waveguide amplifiers (EDWA) and light-emittingdiodes. They are simply supramolecular complexescontaining well-known antenna chromophores tophotoexcite the Ln3+ ions via the energy transfer process.The quantum yield of energy transfer and the luminescenceefficiency were not satisfied yet. Also, such efforts are justin the early stage and not only the basic concept notestablished, but also the structure-property relationship isnot yet clearly understood. Therefore, the dependence ofLn3+ ion and ligand structure on amplification principles (suchas the optical amplification lifetime, excited state dynamics,etc) need to be systematically established. Based on therelationship established, the design and synthesis ofluminescent Ln3+ complexes using molecular engineeringapproach need to be investigated.

In this review, Ln3+ -cored complexes based on variousorganic ligands were overviewed for advanced photonicsapplications. Ln3+ ions were encapsulated by the luminescentligands. Also, key parameters for emission enhancement and

efficient energy transfer pathways for the sensitization ofLn3+ ions by luminescent ligands were investigated.

The electronic configurations of Ln3+ ions, referred to as thelanthanide ions or rare earth ions, have 4fn (n = 1 ~ 14)structures, which fill the 4f orbital according to the atomicnumbers starting from Ce3+ to Lu3+ ions. These ions all havean incompletely filled f subshell in which the f-electrons areshielded by the outer 5s and 5p electrons, which are lower inenergy, but spatially located outside the 4f orbitals. Theseelectrons are slightly perturbed by the effects of latticephonons and static strain fields in the coordinationenvironment of ions, lead to the sharp spectral lines. Sincethe spin-orbit interaction is larger than the effect of thecrystal field, the luminescence spectra consist of groups oflines which arise from crystal field splittings of J multiplets ofthe free ion in general.39

Judd40 and Ofelt41 independently derived expressions for theoscillator strength of induced dipole transitions within the 4fn

— 4fn configuration. Since their results are similar and werepublished simultaneously, their results are known as theJudd-Ofelt theory. They summed over the intensities of theindividual crystal field components of a given state. The fundamental mechanism of the Judd-Ofelt theory is the

perturbation caused by mixing of the crystal field potentialsbetween the orbitals with different parities and 4fn orbitals.The 4fn — 4fn transitions are forbidden because 4∫n of rareearth ions and energy levels of electron shells have equalparity. However, absorption spectra between 4fn — 4fn transition areexperimentally observed. This is explained by assuming thata higher-lying opposite parity configuration is mixed into the4fn states via the potential due to the ligand field.42 Incontrary, 4f n— 4f n-15d1 transitions or charge transfertransitions (4fn — 4fn-15d1, L = ligand) are partly allowed bymixing with the odd-parity wave functions via the potentialdue to the crystal field. The absorption and emission cross-sections are very small in solid phase and the correspondingfluorescence decay times are sufficiently long (as of theorder of ms), compared with the rate at which it is populatedin the excitation process. Also there is a Stark splittingbetween the degenerated 4f energy levels by the effect ofelectric field near Ln3+ ions removing the degeneracy of the4f level.

In analyzing the mechanisms of excited state relaxation oflanthanides in crystal hosts, two modes of relaxation arerecognized: radiative and radiationless (or non-radiative)processes. Axe43 represented the radiative processes inquantitative terms using the Judd-Ofelt theory.Radiationless relaxation was formulated in terms ofmultiphonon relaxation processes.44,45 Such processesbecome less probable as the energy gap between an excitedstate and the next lower energy state increases. Sinceexcited state relaxation is generally achieved prior totransitions to several lower-lying states, a total radiativerelaxation rate can be defined as a summation over all stateslower in energy than the fluorescing state. Scheme 2 showsthe principal fluorescence states of the Ln3+ ions in crystalhosts. Fluorescence from many of these levels is observed

Figure 1. Chemical structures of lanthanide(III)-coredcomplexes based on bodipy ligands.

Lanthanide Ions and Basic Optical AmplificationPrinciple of Lanthanide Ions



H .K. Kim

only at low temperatures since rapid relaxation of an excitedstate by radiationless processes compete strongly with theradiative mode unless the energy gap to the next lower levelis large. As the gap increases, the process rapidly decreasesin probability such that radiative decay can efficientlycompete with a relaxation mechanism. Usually both radiativeand radiationless processes operate to relax an excited state.The total fluorescence lifetime of the state is a sum of theradiative rate and the rates of the various radiationlessprocesses (see Scheme 2).

Several lanthanide ions, such as Pr3+, Nd3+, Dy3+, Er3+, andYb3+ ions, show luminescence in the near infrared region.The luminescence of wavelengths 1.53 ㎛ from Er3+ ion and1.06 ㎛ from Nd3+ ion is used as standard wavelengths inoptical communications, in which light is used to transportinformation between different users. Some of lanthanideions, such as Sm3+, Tb3+ and Eu3+ ions, exhibit the emissionband in the visible region. Their luminescent complexes havebeen considered the potential candidates for light-emittingdiodes.Among them, erbium-based materials have attracted much

attention because they have the authentic opticalamplification (OA) property with the strong stimulatedemission band. This specific property renders erbium-based materials to use in several photonics applications, suchas erbium-doped fiber amplifiers (EDFA), erbium-dopedwaveguide amplifiers (EDWA) and light-emitting diodes.The basic optical amplification principle of Er3+ ion isdescribed in Scheme 3. The Er3+ion is excited to the upperenergy level than the lower excited energy level (Er3+, 4I13/2)of the lanthanide ions, by absorbing photons of pumping lasersuch as 488 or 980 nm, etc. And then, the excited state ofthe upper energy level is transited to the lower excited

energy level (Er3+, 4I13/2) of the Ln3+ ions through the latticerelaxation process (or the vibrational relaxation process).The population inversion inherently takes place at theenergy level of the lanthanide ions (Er3+, 4I13/2). Finally, itloses the photon energy in a radiative decay to fall back tothe ground state (Er3+, 4I15/2) of the Ln3+ ions, emitting thestimulated light at 1.53 ㎛. When the same input signal withthe metastable excited state is introduced at the same time,the input signal and the stimulated emission beam aresummed to generate the amplified output signal, due to thepower interchange between the input signal and thestimulated emission beam. The stimulated emission lifetimeof the 4I13/2→4I15/2 radiative transition at 1.53 ㎛ of Er3+ ion ina solid, which depends on the presence of impurities anddefects in the host, is of the order of 10 ms. In luminescentLn3+ complexes, instead of direct excitation of Er3+ ion in asolid, the luminescent ligands, as photon sensitizers,efficiently absorb and transfer the light to excite the Ln3+

ions via an energy transfer between the luminescent ligandand the Ln3+ ion. It is very important to note that theaccumulating photon energy from the excited state of theluminescent ligands can excite the Ln3+ ions as much aspossible, thus enhancing the optical amplification gain (seeScheme 3).

Efficient Energy Transfer Pathways for theSensitization of Ln3+ Ions by Luminescent Ligands inLuminescent Ln3+ ComplexesTo date, it is well-believed that only energy transfer from

the triplet state of luminescent ligands to Ln3+ ion is the mostdominant mechanism.46 Therefore, most researches towardlanthanide ion sensitizers have been focused on developingthe luminescent ligands with a triplet state matching thereceiving lanthanide ion energy levels. Interestingly,however, several reports mentioned the energy transferfrom the excited singlet state to Ln3+ ion.47 We also reportedall possible ET pathways using Pt-porphyrin (ETt)35,37 andanthracene ligand (ETs)48, and additionally, demonstrated theunusual ET pathway through the intramolecular chargetransfer (ICT) state of the naphthalene ligand to Ln3+ ion(ETc) for the first time.49

To investigate all three possible ET pathways and compare

Scheme 2. Energy levels of Ln3+ ions.

Scheme 3. Basic optical amplification principle of Er3+ ionsfor advanced photonics applications.

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the lanthanide emission efficiency for the various ETpathways in Er3+-cored complexes, naphthalene derivatives(NA-0~NA-3)50 have been rationally designed andsynthesized the structurally and UV-absorbinglysimilarligands and their stable 9-coordinated Ln3+-complexes.51 The energy transfer of the Er3+- complexbased on NA-2 takes places from the ICT state to Ln3+ ion.The Er3+complexes based on NA-0 and NA-1 ligands havethe ET mechanism via the excited triplet state. Bromine-substituted NA-1 ligand, as compared with NA-0 ligand,increases the rate of intersystem crossing (ISC) anddecreases the fluorescence quantum yield, due to theinternal heavy atom effect. In the case of Er3+-[NA-3]3(tpy), the energy transfer takes place from the singletstate of NA-3 ligand to central Er3+ ion (see Figure 2).Although ET process via the triplet state to Ln3+ ion isconsidered as main efficient ET pathway in Ln3+ complex,this sensitization process is not strongly dominated on Ln3+

emission, compared with other ET pathways via singlet orcharge transfer state.

Very recently, to enhance the luminescence properties ofLn3+ complexes, lanthanide complexes with two push-pulldiketone derivatives as sensitizers have been developed fornear-infrared emitting materials52(see Figure 3-(a)).

The ligand substituents consist of a carbazole moiety withhole-transport properties and an aromatic or heteroaromaticunit. According to quantitative NMR analysis andcomplementary HPLC experiments, the diketones essentiallyappear under the thermodynamically stable enol form insolution displaying extended resonance electronic structure:the cis-enol tautomer amounts to 95-98% of the speciationin THF at room temperature. These ligands possess an intracharge-transfer(ICT) electronic state, the energy of whichis sensitive to the polarity of the solvent and which allowsconvenient sensitization of the luminescence of NIR emittingions such as Nd3+ and Er3+ in the visible spectrum. Theresulting tris(diketonate)ternary complexes with terpyridine(Ln = Nd, Er) display sizeable near-IR emission withlong luminescence lifetimes. Upon excitation into the ICT bandof the ligands, the spectra of the Er(III) compounds displaythe characteristic 4I13/2 →4I15/2 transition of Er3+ ion at 1530nm(see Figure 3-(b)). It was also confirmed that the rate

constant from singlet excited state to ICT state (kICT)islarger than the rate constant of intersystem-crossingprocess(kISC). As a result, the new design criteria oflanthanide materials has been established to improve theluminescence intensity of Ln3+ ion by the ICT process,35, 36, 48,

49as compared with the existing theory of ET process via thetriplet state to Ln3+ ion(see Figure 3-(c)).Surprisingly, the ISC process caused by the heavy Ln3+ ion

effect in Ln3+ complexes is not dominant process over theICT or the radiative relaxation process and the ET processvia the triplet state to Ln3+ ion is not strongly dominated onLn3+ emission, compared with other ET pathways via singletor charge transfer state. As concluded, no matter which ETpathway takes place, the key factor for strong lanthanideemission is the ET efficiency between the organic ligand andLn3+ ion, associated with the quantum yield of a ligand and aproper J value. This study can provide the criteria fordesigning the ligand to achieve efficient Ln3+ emission.50

Er3+-chelated Prototype Complexes: Key Parametersfor Near IR Emission Enhancement

To enhance the NIR emission intensity and maintain theeffective energy transfer process, our research efforts havebeen focused on developing stable and inertEr3+encapsulated complexes with artificial light-harvestingsystems using dendritic luminescent ligands based onmetalloporphyrins, naphthalenes, and anthracenes bearingthe Fr chet aryl-ether dendrons, namely, (Er3+-[Gn-Pt-Por]3(terpy)35, Er3+-[Gn-Naph]3(terpy)36 and Er3+-[Gn-An]3(terpy)48 (see Scheme 4).

Figure 2. All possible ET pathways and chemical structuresof lanthanide(III)-cored complexes based on naphthaleneacid ligands.

Figure3. (a) Scheme of [Ln(diketonate)3(tpy)] complexes(Ln= Er3+ and Nd3+, R= CTPD and CNPD) (b) NIR emissionspectra of CTPDand [Er(CTPD)3(tpy)] 2.0×10-5M inMeCN (λex = 410 nm). (c) Energy transfer diagram ofCTPD complex by ICT process.



H .K. Kim

A series of inert and photostable encapsulated lanthanide(III)complexes based on dendritic anthracene ligands was shownfor the first time to exhibit strong near-IR emission bandsvia efficient energy transfer from the excited states of theperipheral antenna to the Ln3+ ions (Er3+, Yb3+, and Nd3+).37 Asignificant decrease in the fluorescence of the anthraceneligand was accompanied by a strong increase in the near-IRemission of the Ln3+ ions. The near-IR emission intensitiesof Ln3+ ions in the encapsulated Ln3+-dendrimer complexesare dramatically enhanced on increasing the generationnumber (n) of Fr chet aryl-ether dendrons, owing to site-isolation and light-harvesting effects48(see Figure 4).

Furthermore, a first attempt was made to distinguishbetween the site-isolation and light-harvesting effects inthe present complexes. Photophysical studies indicated thesensitization of Ln3+ luminescence by energy transferthrough the excited singlet state of the anthracene ligands,

and the energy-transfer efficiency between the dendriticanthracene ligands and the Ln3+ ion was evaluated to be inthe range of 90 to 97 %. As a result, we observed that theNIR emission intensity of the lanthanide complexes wasdramatically enhanced with increasing the generationnumber (n) of the Fr chet aryl-ether dendrons, due to thesite-isolation and light-harvesting effects.

It was also demonstrated that a series of stable and inertEr3+-cored complexes with dendritic Pt2+-porphyrin ligandsexhibit NIR emission bands via efficient energy transfer fromthe excited triplet state of the Pt2+-porphyrin ligands to theEr3+ ions.35 The Pt2+-porphyrin derivatives exerted aheavy- Pt2+-atom effect, leading to a higher intersystemcrossing (ISC) efficiency for the conversion of a singlet stateinto a triplet state, as compared to the lighter Zn2+-porphyrinderivatives. The NIR emission intensity of the lanthanidecomplexes was dramatically enhanced with increasinggeneration number (n) of the Fr chet aryl-ether-typedendrons, largely due to site-isolation and LH effects.Surprisingly, the site-isolation effect was dominant over theLH effect in the Er3+-[Gn-PtP]3(terpy) complexes due tothe low quantum yield of the arylether dendrons (Φf =0.02) for exciting Pt2+-porphyrin units. The NIR emissionintensity of Er3+-[G3- PtP]3(terpy) was seen to be 20-times stronger than that of Er3+-[G1-PtP]3(terpy), whencomparing thin films of these complexes35(see Figure 5).

Very recently, also, the photophysical criteriafor lanthanideemission enhancement for three Er(III)-cored dendriticcomplexes , such as Er3+-[G2-An]3(terpy),(Er3+-[G2-Pt-Por]3(terpy), Er3+-[G2-Naph]3(terpy) wereinvestigated, as summarized in Table I.36The Er(III)-coreddendritic complex of Er3+-[G2-An]3(terpy)exhibits thestronger PL intensity than the Er(III)-coreddendriticcomplex of Er3+-[G2-Naph]3(terpy) or theEr(III)-coreddendritic complex of Er3+-[G2-Pt-Por]3(terpy) by 1.6 or 8.3 times, respectively, uponphotoexcitation of the ligand at the absorption maximumwavelength. It may be due to the fact that the anthraceneligand in Er3+-[G2-An]3(terpy) has a higher spectraloverlap integral (J)value and a higher luminescence quantumefficiency than naphthalene moiety or Pt(II)-porphyrin ligandin Er(III)-encapsulated dendrimer complexes.

Scheme 4. The chemical structures of Er3+-coredcomplexes based on (a) dendritic anthracene, (b)naphthalene and (c) Pt-porphyrin ligands.

Figure 4. (a) Schematics of the light-harvesting and site-isolation effects in a series of Er3+-[Gn-An]3(terpy)complexesenergy transfer. (b) Near-IR emission spectra ofa series of Er3+-[Gn-An]3(terpy) inthin film uponphotoexcitation at 290 nm.

Figure 5. (a) Scheme of [Er(PtP)3(tpy)] complexe (b) NIRemission spectra of Pt-porphyrin ligand and [Er(PtP)3(tpy)]in THF (λex = 405 nm). (c) Energy transfer diagram of[Er(PtP)3(tpy)] complex. (d) NIR emission spectra of Er3+-[Gn-Ptp]3(terpy) complexes in thin film uponphotoexcitation at 410 nm.

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Therefore, to enhance of the NIR emission in luminescentlanthanide complexes, no matterwhich ET pathway takesplace, the key parameters for NIR emission enhancement isas follows (1) The direct luminescent Ln3+ complexeshave much higher PL efficiency than the indirectluminescent Ln3+complexes. (2) Highly coordinatedlanthanide-coredcomplexes (at least 8 to 9 coordinationnumber) have muchhigher PL efficiency than unsaturated6-coordinated complex. (3) The key factor for stronglanthanide emission is the ET efficiency between the organicligand and Ln3+ ion, associated with the quantum yield of aligand and a proper J value.

Summary and Outlook

New synthetic methodology of the saturated and unsaturatedEr3+-chelated prototype complexes based onmetalloporphyrins, naphthalenes, anthracenes, push-pulldiketone derivatives and bodipy ligands were successfullydeveloped through the ligand-exchange reaction. The stable and inert Er3+-encapsulated complexes based onanthracene, naphthalene, and Pt2+-porphyrin ligands bearinga second generation Fr chet aryl-ether dendron (G2) exhibitmuch stronger near-IR emission bands at 1530 nm,originated from the 4fn-4fn electronic transition of the firstexcited state (4I13/2) to the ground state (4I15/2) of thepartially filled 4f shell. A strong decrease of fluorescenceintensity of luminescent ligands, such as anthracene,naphthalene, and Pt2+-porphyrin units, is accompanied bystrongly increasing the near-IR emission of the Er3+ ions inEr3+-encapsulated dendrimer complexes, uponphotoexcitation of luminescent ligands at the correspondingabsorption maximum wavelength. It could be attributed tothe efficient energy transfer process occurring betweendendritic luminescent ligand and Er3+ ion. The emissionintensity of the lanthanide complexes, upon photoexcitationof luminescent ligand at the corresponding absorptionmaximum wavelength, was dramatically enhanced byintroducing a third generation Frechet aryl-ether dendron

(G3), as compared with those of their corresponding Er3+-encapsulated complexes, mainly due to the site-isolationand LH effects. Surprisingly, the site-isolation effect wasdominant over the LH effect in the Er3+-encapsulatedcomplexes due to the low quantum yield of the aryl etherdendrons (Φf = 0.02).From various researches, it can suggest that energy transferpathway does not influence the lanthanide emissionenhancement: Although ET process via the triplet state toLn3+ ion is considered as main efficient ET pathway inLn3+ complex, this sensitization process is not stronglydominated on Ln3+ emission, compared with other ETpathwaysvia singlet or charge transfer state. In other words,the ISC process caused by the heavy Ln3+ ion effect inLn3+complexes is not dominant process over the ICT or theradiative relaxation process. Therefore, to enhance of the near-IR emission inluminescent lanthanide complexes, no matter which ETpathway takes place, the key parameters for NIR emissionenhancement are the ET efficiency between the organicligand and Ln3+ ion, associated with the quantum yield of aligand and a proper J value.To determine the structure-property relationships,ultrahighly efficient light-harvesting supramolecularstructures will be optimized and their application as thefuture optical amplification devices will be examined. Utilizingthe developed materials, photonics community can have theleverage to lead the world in tomorrow’s communicationtechnology and the technological impact will be similar to theintegrated circuit development in semiconductor industries.This will not only be considered to be the material revolutionin the heart of exploratory materials in optical informationand communication, but also will open up many new areas ofapplications of polymeric materials for use in ultrahigh speedcommunication and information systems for the futuregeneration.

KEYWORDS Luminescent lanthanide complexes, Nearinfrared emission, Energy transfer pathways, Moleculardesign criteria, Advanced photonics applications

ACKNOWLEDGEMENTThis research was supported by a grant from theFundamental R&D Program for Core Technology ofMaterials funded by the Ministry of Knowledge Economyand World Class University program funded by the Ministryof Education, Science and Technology through the NationalResearch Foundation of Korea(R31-2008-10035-0).

Table 1. Summary on photophysical results for lanthanideemission enhancement in luminescent lanthanide complexes

aSpectral overlap integral J value between ligand and erbium ion.

G2-An-Er(Ⅲ) G2-PtPor-Er(Ⅲ) G2-NA-Er(Ⅲ)

ET Pathway

RelativeJ

ET Efficiency

Фlig

Relative Intensityof Er(Ⅲ)

τobs

ФLn

Singlet state

3.4

77%

0.77(solution)

8.3

1.8㎲

2.3×10-4

Triplet state

1.0

<77%

0.45(solution at 77K)

1.0

1.65㎲

2.1×10-4

ICT

2.3

100%

0.57(solution)

1.7

1.3㎲

1.6×10-4



H .K. Kim

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9


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