The OLED emitter Ir(btp)_1tn2(acac) [Elektronische Ressource] : photophysical properties of the triplet state studied by highly-resolving spectroscopy / vorgelegt von Walter J. Finkenzeller
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The OLED emitter Ir(btp)_1tn2(acac) [Elektronische Ressource] : photophysical properties of the triplet state studied by highly-resolving spectroscopy / vorgelegt von Walter J. Finkenzeller

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The OLED Emitter Ir(btp) (acac) –2Photophysical Properties of the Triplet StateStudied byHighly Resolving SpectroscopyDissertationzurErlangungdesDoktorgradesderNaturwissenschaften(Dr. rer. nat.)anderNaturwissenschaftlichenFakultat¨ IV–ChemieundPharmazie–derUniversitat¨ RegensburgvorgelegtvonWalterJ.FinkenzellerausIngolstadtRegensburg,2008Promotionsgesucheingereichtam26.03.2008DieArbeitwurdeangeleitetvonProf. Dr. H.YersinamInstitutfur¨ PhysikalischeundTheoretischeChemiederUniversitat¨ Regensburg.Prufungsausschuss:¨ Prof. Dr. R.Winter,VorsitzenderProf. Dr. H.Yersin,1. GutachterProf. Dr. B.Dick,2.Prof. Dr. A.PenzkoferPartsofthisworkarealreadypublished:Bauer,R;Finkenzeller,W.J.;Bogner,U.;Thompson,M.E.;Yersin,H.Matrix Influence on the OLED Emitter Ir(btp) (acac) in Polymeric Host Materials2–StudiesbyPersistentSpectralHoleBurningOrganicElectronics2008,inpress.Yersin,H;Finkenzeller,W.J.In Highly Efficient OLEDs with Phosphorescent Materials; Yersin, H., Ed.; Wiley VCH:Weinheim,2007,p.1.Finkenzeller,W.J.;Thompson,M.E.,Yersin,H.PhosphorescenceDynamicsandSpin LatticeRelaxationoftheOLEDEmitterIr(btp) (acac)2ChemicalPhysicsLetters2007,444,273.Finkenzeller,W.J.;Hofbeck,T.;Thompson,M.E.,Yersin,H.Triplet State Properties of the OLED Emitter Ir(btp) (acac) – Characterization by2Site SelectiveSpectroscopyandApplicationofHighMagneticFieldsInorganicChemistry2007,46,5076.ContentsIntroduction 51 OLEDs–AnIntroduction 91.1 Basicworkingprinciple . . . . . . . . .

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The OLED Emitter Ir(btp) (acac) –2
Photophysical Properties of the Triplet State
Studied by
Highly Resolving Spectroscopy
Dissertation
zurErlangungdesDoktorgradesderNaturwissenschaften(Dr. rer. nat.)
anderNaturwissenschaftlichenFakultat¨ IV–ChemieundPharmazie–
derUniversitat¨ Regensburg
vorgelegtvon
WalterJ.Finkenzeller
ausIngolstadt
Regensburg,2008Promotionsgesucheingereichtam26.03.2008
DieArbeitwurdeangeleitetvonProf. Dr. H.YersinamInstitutfur¨ Physikalische
undTheoretischeChemiederUniversitat¨ Regensburg.
Prufungsausschuss:¨ Prof. Dr. R.Winter,Vorsitzender
Prof. Dr. H.Yersin,1. Gutachter
Prof. Dr. B.Dick,2.
Prof. Dr. A.PenzkoferPartsofthisworkarealreadypublished:
Bauer,R;Finkenzeller,W.J.;Bogner,U.;Thompson,M.E.;Yersin,H.
Matrix Influence on the OLED Emitter Ir(btp) (acac) in Polymeric Host Materials2
–StudiesbyPersistentSpectralHoleBurning
OrganicElectronics2008,inpress.
Yersin,H;Finkenzeller,W.J.
In Highly Efficient OLEDs with Phosphorescent Materials; Yersin, H., Ed.; Wiley
VCH:Weinheim,2007,p.1.
Finkenzeller,W.J.;Thompson,M.E.,Yersin,H.
PhosphorescenceDynamicsandSpin LatticeRelaxationoftheOLEDEmitter
Ir(btp) (acac)2
ChemicalPhysicsLetters2007,444,273.
Finkenzeller,W.J.;Hofbeck,T.;Thompson,M.E.,Yersin,H.
Triplet State Properties of the OLED Emitter Ir(btp) (acac) – Characterization by2
Site SelectiveSpectroscopyandApplicationofHighMagneticFields
InorganicChemistry2007,46,5076.Contents
Introduction 5
1 OLEDs–AnIntroduction 9
1.1 Basicworkingprinciple . . . . . . . . . . . . . . . . . . . . . . . . 9
1.2 Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.3 Deviceoptimization . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.4 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
1.5 Materialsandfabrication . . . . . . . . . . . . . . . . . . . . . . . 20
1.6 Stateoftheart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2 OrganometallicTripletEmitters 27
2.1 Spin e ffectsandtripletharvesting . . . . . . . . . . . . . . . . . . 27
2.2 EnergystatesoftypicalOLEDemitters . . . . . . . . . . . . . . . 31
2.3 Originofphosphorescence–Spin orbitcoupling . . . . . . . . . . 34
2.4 Zero fieldsplittingandMLCTperturbation . . . . . . . . . . . . . 37
2.5 Spin orbitcouplingroutes–Whyoctahedralcomplexesmaybebetter 41
2.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3 Ir(btp) (acac)–ARedOLEDEmitter 452
3.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.2 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.3 Spectroscopicintroduction . . . . . . . . . . . . . . . . . . . . . . 48
4 EmissionSpectraofIr(btp) (acac)–ElectronicOrigins 522
4.1 Low temperaturespectraandsitedistribution . . . . . . . . . . . . 52
4.2 Electronicoriginsandenergyleveldiagram . . . . . . . . . . . . . 54
4.3 Magneticfieldeffects . . . . . . . . . . . . . . . . . . . . . . . . . 58
4.4 Variationofzero fieldsplitting–matrixinfluence . . . . . . . . . . 63
4.5 Assignmentoftheemittingstate–Conclusions . . . . . . . . . . . 652 Contents
5 EmissionDecayBehaviorofIr(btp) (acac) 692
5.1 IndividualemissiondecaytimesoftheT substates . . . . . . . . . 691
5.2 Processesofspin latticerelaxation . . . . . . . . . . . . . . . . . . 72
5.3 EffectsofinIr(btp) (acac) . . . . . . . . . . 752
5.4 Emissiondecaybehaviorandmatrixinfluence–Conclusions . . . . 81
6 Emission Spectra of Ir(btp) (acac) in CH Cl – Vibrational Satellite2 2 2
Structures 87
6.1 Emissionspectrumundersite selectiveexcitation . . . . . . . . . . 87
6.2 Franck CondonandHerzberg Telleractivity . . . . . . . . . . . . . 89
6.3 Temperaturedependence . . . . . . . . . . . . . . . . . . . . . . . 94
6.4 IndividualemissionspectrafromthetripletsubstatesI,II,andIII . . 97
6.5 Assignmentofvibrationalsatellites. . . . . . . . . . . . . . . . . . 100
6.6 ConsiderationsontheelectronicallowednessofthetransitionI→ 0 105
6.7 Magneticfieldeffect . . . . . . . . . . . . . . . . . . . . . . . . . 106
6.8 Time resolvedemission . . . . . . . . . . . . . . . . . . . . . . . . 109
6.9 Satellitestructureinothersites . . . . . . . . . . . . . . . . . . . . 111
6.10 Vibrationalsatellitestructures–Conclusions . . . . . . . . . . . . . 116
7 SpectralHoleBurningofIr(btp) (acac) 1192
7.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
7.2 PhosphorescencelinenarrowingofIr(btp) (acac) . . . . . . . . . . 1202
7.3 Persistentspectralholeburning . . . . . . . . . . . . . . . . . . . . 122
7.4 Detection of spectral holes by a synchronous excitation detection
scantechnique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
7.5 TripletsubstatesofIr(btp) (acac)–Holeburningresults . . . . . . 1252
7.6 Evaluationofthesynchronousscantechniqueofholedetection . . . 132
7.7 Persistentspectralholeburning–Outcomes . . . . . . . . . . . . . 134
8 Experimental 137
8.1 Samplepreparationandcooling . . . . . . . . . . . . . . . . . . . 137
8.2 Standardopticalequipment . . . . . . . . . . . . . . . . . . . . . . 138
8.3 Setting upofanewspectrometer . . . . . . . . . . . . . . . . . . . 139
Summary 147
Appendix 155
A Intensityratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155Contents 3
B Vibrationalenergies . . . . . . . . . . . . . . . . . . . . . . . . . . 157
C Emission spectra of Ir(btp) (acac) in CH Cl (site I) – Vibrational2 2 2
satellitestructure . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
References 163
Acknowledgment 179Introduction
Since many years, organo transition metal complexes are known to show great po
tentialforavarietyofapplicationsinchemistry,physicsandengineering. Examples
arechemo andbiosensors[1–6],photo catalyzersinpreparativechemistry[7–10],
and photo sensitizers for singlet oxygen [11] or dye sensitized solar cells [12–17].
Because of these fascinating possibilities, organo transition metal complexes have
been under scientific research for a fairly long time and complexes such as, for ex
2+ 1ample, Ru(bpy) [18–23] have been under investigation in applied as well as in
3
fundamentalresearchfields.
It is not so long, since a new application for organo transition metal complexes
has attracted attention. Organic light emitting devices (OLEDs) (see, e.g. [24–
29] utilize the light emission that can, under certain conditions, occur in organic
materials upon application of an electric field. This so called (organic) electrolu
minescence was discovered by Pope et al. already in 1963 in a crystalline layer of
anthracene.[30] However, the onset of electroluminescence was observed at com
paratively high voltages and it took more than 20 years, until Tang and van Slyke
coulddemonstratethatorganicelectroluminescenceisalsopossibletobeobserved
atlowervoltages(below10V).[31]Thiswasthebeginningofarapiddevelopment
oftheOLEDtechnology.
Organic light emitting devices are attractive for display technology and lighting
andopenupnewpossibilitiesforboth. Thusitwillnotonlybepossibletofabricate
flat panel displays with a maximum in viewing quality in scalable size and at low
cost, but even flexible and transparent displays will become realizable. The appli
cations in focus range from large television screens and displays for advertising to
2mobileapplicationssuchassmalldisplaysforcellularphones,PDAs, digitalcam
eras and camcorders, and portable media players. For some applications, OLED
technology has already entered the commercial market. Especially in portable au
dio players and cellular phones, OLED displays already replace the conventional
liquid crystal display (LCD) technology to a growing extent. More exotic applica
1bpy=2,2’ bipyridine
2PDA=PersonalDigitalAssistant6 Introduction
tionsliketransparentdisplaysapplicableascarhead updisplaysorflexibledisplays
tobeused,forexample,asrolloutdisplays,areunderdevelopment.
For many of these applications, the key requirement is a minimized power con
sumption. In this regard, OLEDs offer certain advantages compared to other tech
nologies (see Sects. 1.2 and 2.1) Among other reasons, this renders the OLED
technology also extremely attractive in solid state lighting, where it is ascribed a
great potential to deliver highest power efficiencies at very low production costs.
By replacing conventional lighting systems such as incandescent light bulbs, the
United States alone speculate to accumulate energy savings until 2025 of more
than $100 billion and therefore could defer the construction of forty 1 GW power
plants.[32,33]
IthasalreadybeendemonstratedthatOLEDscanreachanenergyefficacyof100
lm/Wormore,whichiscomparabletothebestinorganicLEDsorevenbetter.[34–
36]Interestingly,thesehighefficienciescanonlybeobtainedbyusingphosphores
cent emitter materials.[34–39] Application of these materials allows the utilization
of both singlet and triplet excited states of the emitter, which are usually involved
in the operation of an OLED due to spin statistics (Sect. 2.1).This so called trip
letharvestingcanprovideuptofourfoldelectroluminescencequantumefficiencies
of phosphorescent emitters compared to fluorescent ones. Therefore it is not sur-
prising that a great deal of interest has been dedicated to organo transition metal
complexes to be employed as emitter materials in OLEDs. These complexes can
exhibit a phosphorescence of very high efficiency up to nearly 100% and more
over, can be adapted to meet further requirements due to their chemical versatil
ity concerning the central metal ion and the choice of ligands (see Sects. 2.3 and
2.4). Especially iridium complexes seem to be exceedingly suited for OLEDs and,
therefore, are usually the first choice for fabrication or engineering of high effi-
ciency devices.[34–44] Up to the present, substantial progress has been made in
the development of organo transition metal emitters for OLED application, espe
cially in the green spectral region.[34, 35, 37, 39] In contrast, blue and red emitter
complexesstillhavetobeenhancedwithrespecttostabilityandluminescenceeffi-
ciency – especially in view of lighting applications. This can possibly be achieved
by a controlled chemical tuning of organo transition metal complexes on the basis
of a detailed understanding of emission properties and their relation to chemical
characteristics. Usually, the electroluminescence in an OLED originates from the
very same excited state which also governs the emission after optical excitation
(photoluminescence). Therefore, optical spectroscopy can be a powerful means to
study a complex also with respect to its suitability for application in OLEDs. The