Triplet State Phosphorescence in Tris(8-hydroxyquinoline)Aluminum Light Emitting Diode MaterialsS. Perumal,* B. Minaev, and H. Ågren

Department of Theoretical Chemistry and Biology, School of Biotechnology, KTH Royal Institute of Technology, Roslagstullsbacken15, SE - 106 91, Stockholm, Sweden

ABSTRACT: Time-dependent density functional theory (TD-DFT) with thequadratic response technique is applied to the tris(8-hydroxyquinoline) aluminumcomplex (Alq3) to calculate spin−orbit coupling (SOC) effects and the mainmechanism of the Alq3 phosphorescence. This compound exhibits weakphosphorescence which provides additional emission in organic light emittingdiodes (OLEDs) besides the main fluorescence band. The phosphorescence affordsto overcome the efficiency limit imposed by the formation of triplet excitons in theemissive layer of OLEDs. The zero-field splitting (ZFS) parameters are alsocalculated taking into account the spin−spin coupling in the first-order perturbationtheory and the SOC in the second order. On the basis of the results obtained, wepropose that an efficient spin-polarized injection and transport in long channels ofAlq3, which have been recently achieved with new hybrid organic−inorganicinterfaces, can proceed with the triplet electronic state admixture to the chargecarriers. Our results also indicate that weak spin−orbit coupling may be responsible for the room-temperature magneto-resistance at the ferromagnetic−Alq3 interfaces and the spin-dependent charge transport.

■ INTRODUCTION

Although the discovery of organic electroluminescence (EL)and fabrication of the first EL devices of the monolayer type,based on anthracene crystal,1,2 came already in the 1960s, theorganic EL phenomenon remained unexploited for twodecades. This delay between discovery and exploitation oforganic EL crystals can be ascribed to the need of a monolayer,of high voltage (about 1 kV) and a large size of the singlecrystal. The first efficient organic EL diode was fabricated byEastman Kodak Company3 which consisted of a double-layeredorganic thin film sandwiched between two injecting electrodes.One layer (aromatic diamine) was capable of only charge(hole) transport; the second organic layer consisted of a tris(8-hydroxyquinoline) aluminum film (Figure 1)3 used as a lightemitter which was microcrystalline with a grain size of about 50nm. The ability to form smooth films in both layers was relatedto the large molecular geometry of the constituent molecules

and their low symmetry. The Alq3 organic chromophore waschosen since it belongs to a class of well-known fluorescentmetal chelate complexes.3,4 Such an emissive layer was provento be very useful (it also provides electron transport) and couldboost further progress in organic light emitting diode (OLED)technology.5

Multilayer devices with doping by cyclometalated complexesincluding heavy ions have attracted extensive attention in recentyears because of their potential to achieve a high externalquantum efficiency by utilizing both the singlet and tripletexcitons.5−11 Such OLEDs have an advantage that theiremission spectra can be tuned from blue to red by theperipheral substitution of the ligands with the electron-withdrawing and electron-donating substituents.9,12,13 Initially,the search for organic electroluminescent materials wasmotivated by the classical ruthenium(II) trisbipyridile complex,Ru(bpy)3

2+, which was widely employed as a photocatalyst insolar-driven photoconversion processes.4,14 The initial goal wasto modify the excited state redox potential of similar metalcomplexes by changing the central transition metal andreplacing the ligands to fit the Fermi levels of the electrodes.Along this way the Alq3 complex was chosen and became amilestone emitting electron-transport material in molecularelectronics and OLED techniques.Recently, this organic semiconductor was used in spintronics

devices, since an efficient spin-polarized injection and transportin long channels of Alq3 was achieved with new hybrid

Received: October 9, 2012Revised: December 21, 2012Published: January 18, 2013

Figure 1. Optimized structure of Alq3 and its complex with LiF. TheAl−F distance is 3.75 Å.

Article

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organic−inorganic interfaces.15,16 This material was employedin vertical spin valve devices with a direct interface between thebottom manganite electrode (La0.7Sr0.3MnO3 denoted LSMO)and an Alq3 layer, while the top layer was arranged as aninsulating tunnel barrier placed between the soft organicsemiconductor and the top cobalt electrode.16 Besides thestandard spintronic applications, the newly fabricated interfacescan be applied in special OLED devices and organic field-effecttransistors.15

Spin-dependent charge transport was first discovered inmetallic multilayers and magnetic tunnel junctions in the formof magnetoresistance effects. The field has now evolved tocommercial applications.16 Coherent spin transport in metalsand normal semiconductors suffers from dephasing induced byspin−orbit coupling (SOC). Organic materials have beensearched for alternative candidates because of their low SOCperturbations and ability to be integrated in hybrid organic−inorganic interfaces.15−17 Spin injection into organic semi-conductors was first demonstrated ten years ago in hybriddevices with highly spin-polarized manganite LSMO electrodesand sexithiophene as the organic channel material, in whichroom-temperature magnetoresistance has been detected.16

Xiong et al.17 chose the Alq3 complex, earlier used inOLEDs, to serve as an organic semiconductor spacer in thespintronic devices. The electrical resistance of such a spacerdepends on the spin of electrons passing through the deviceand can so be controlled by an external magnetic field. Thelow-temperature giant magneto-resistance effect has beenobtained with the Alq3 spacer. Recently the room-temperaturespin valve operation was realized through the improvement ofspin injection properties of both ferromagnetic−Alq3 inter-faces.15

The pioneering work carried out by Tang and Van Slykepaved the way for the highly efficient Alq3 OLED material.3

Since then many experimental studies have been devoted toexplore the fluorescence and phosphorescence for this material.In recent time Alq3 was used as a buffer to create polarizedspin-carriers. A LiF dipolar layer was sandwiched into the Alq3/metal interfaces. Several groups of researchers15,16 were able totune the HOMO−LUMO vacuum energy levels of Alq3, andhence the polarized charge carriers were transported throughthe thin films. With the advent of muon measurementsresearchers were able to detect polarized spin-carriers in organicsemiconductors, especially in Alq3.

15,16

There is clearly a lack of understanding of the phosphor-escence efficiency and its radiative lifetime of the Alq3 complex.The aim of the present paper is to calculate the opticalproperties of this complex as motivated by its fundamentalsignificance for OLED applications.5 At the same time, we wantto analyze the SOC effect on spin-polarized injection in theferromagnetic−Alq3 interfaces and on spin coherence duringthe spin-dependent charge transport in long channels of Alq3.We present connections between features of electronicstructures and spin-dependent properties including phosphor-escence efficiency and energy transfer mechanisms in the Alq3species. We consider the role of spin−orbit coupling effects indetermination of the EL properties as used in OLEDs and thespintronic parameters of the Alq3 spacer. We present acharacterization of zero-field splitting (ZFS) of the lowesttriplet state (T1) of the Alq3 complex with additional account ofspin−spin coupling and calculation of the phosphorescenceradiative lifetime.

Alq3 Thin Film as Electron Transport and EmittingLayer in OLEDs. Thin film Alq3 is among the most efficientorganic EL materials, which exhibit bright emission and highchemical stability upon electron−hole recombination processes.Alq3 thin films are also used as electron transport layers or holeblocking layers in OLEDs with light emission from conjugatedorganic polymers. Such polymers are useful for OLEDs as theyprovide the possibility to create electron−hole recombination(EHR) and formation of excitons with high efficiency of lightemission.5,18

Charge carriers (electrons and holes) are first injected fromthe electrodes into the organic semiconducting polymer layer.These oppositely charged polaronscharge carriersmigratethrough the organic layer by an applied voltage to capture eachother and form excitons. Since the charge pairs are injectedfrom different electrodes, they provide nongeminate ion-radicalpairs; thus, they have random spin orientation, and the singletand triplet colliding pairs are equally probable. Because ofstatistical arguments, the excitons are created in a 1:3 ratio ofsinglet to triplet since the triplet state has three spin projections(Ms = 0, ±1) and the singlet has only one (Ms = 0) microstate.The ground state of organic species is a closed-shell singletstate (S0). Intense fast fluorescence occurs from the singletexcitons (the S1 → S0 transition), whereas the triplet states arealmost nonemissive in typical pure organic polymers withoutcoordinating metal ions.11 The phosphorescence from tripletexcitons is characterized by a long radiative lifetime and smallEinstein coefficient for spontaneous emission. It is much lessintense since the triplet−singlet (T1 → S0) transitions inorganic polymers are 6 to 8 orders of magnitude weaker thanthe spin-allowed singlet−singlet (S1 → S0) fluorescence. Thephosphorescence can borrow dipole activity from spin-allowedtransitions through spin−orbit coupling (SOC), a perturbationthat is very weak in organic π-conjugated molecules.19,20 Thereason for the weak phosphorescence is 2-fold: (i) spin−orbitcoupling is rather weak in light elements like carbon and (ii)the orbital angular momentum matrix elements between theππ* states of conjugated chromophores are almost vanishing.11

SOC matrix elements between S and T states in moleculescan involve magnetic interactions which depend on effectivenuclear charges of atomic centers and on the change of theorbital angular momentum during the S−T transition at thesecenters. Since the T1 and S1 states in organic chromophores areof ππ* nature, there is no SOC-induced mixing between them,as well as between all other spectroscopically important ππ*states. Thus, in organic conjugated chromophores, one canexpect a very low rate constant for emission from the T1 tripletstate which can not compete with nonradiative quenching atroom temperature. Because of this circumstance, there is anadditional upper statistical limit of 1/4 for the quantum yield inOLEDs based on pure organic polymers.21 By doping theemitting layer of an OLED with heavy metal-ion-containingcomplexes, such as platinum porphyrins or iridium phenyl-pyridines6,18 with high spin−orbit coupling at the metal, it ispossible to arrange electron−hole recombination in the tripletstate on such centers and provide a competitively strong T1 →S0 transition. This chemical engineering with doping materialsaffords to compel the triplet excitons to do useful work inOLEDs, to give electrophosphorescence, and to increase theinternal quantum yield by a few times.6,18

Since the Al atom also belongs to the relatively heavy metals,one can anticipate some involvement of the triplet excitons forthe Alq3 light-emiting efficiency. Despite a great popularity of

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Alq3 in OLED technology, there is relatively little detailedinformation on its photophysics.22,23 Burrows et al.22 havereported on phosphorescence of Alq3 in the ethyl iodide glassmatrix at 77 K. Following pulse radiolysis of argon-saturatedsolution of Alq3 in benzene, they have observed the T−Tabsorption spectra of tris(8-hydroxyquinoline)aluminum(III).A lifetime of 25 ± 15 μs has been estimated for the Alq3 tripletstate in that solvent,22 which does not correspond to theradiative lifetime, as follows from the later measurements insolids.23,24

Colle and Garditz have detected phosphorescence of theAlq3-based OLEDs by measuring the delayed electrolumines-cence at different low temperatures after the end of the voltagepulse.23 Two distinct bands were clearly resolved: the typicalEL emission at 550 nm and a new band at about 700 nm, whichwas assigned as a phosphorescence of Alq3 from thetemperature dependence.23 Later, Colle et al.24 studied twocrystalline (α and δ) phases and amorphous Alq3 at 1.3 K anddetected their phosphorescence as well as a phosphorescence−microwave double resonance. We have undertaken TD-DFTcalculations to make a first-principle interpretation of such anelaborated experimental study.23,24

■ COMPUTATIONAL DETAILSSinglet−triplet transitions are strictly forbidden in the regime ofan unrelativistic treatment; however, spin−orbit interactionscouple these states, resulting in nonzero probability of spin-forbidden transitions. Although spin−orbit coupling is veryweak, it can by symmetry selection rules provide some nonzeroS−T mixing, which is very important for recombination ofcharge carriers and for phosphorescent emission. The electricdipole transition moments read

∑

∑

μ

μω ω

μω ω

= ⟨ Ψ | | Ψ ⟩

=⟨ Ψ | | Ψ ⟩⟨ Ψ | | Ψ ⟩

−

+⟨ Ψ | | Ψ ⟩⟨ Ψ | | Ψ ⟩

−

α α

α

α

Ψ Ψ

Ψ Ψ

M

H

H

k k

n

n nk

m

m mk

10 1

3

10

1 1so 1

3

10 so

3 31

3m m

m m

1 3

3 1 (1)

where 3Ψk is the k spin sublevel of the triplet 3Ψk state beingsubject to the ZFS induced by internal magnetic perturbations;k = x,y,z are the eigen axes of the ZFS tensor quantization; μα isan electric dipole moment operator projection on the α axis;Hso is the SOC operator, implemented here with the atomicmean-field approximations (AMFIs);25 and ω’s are eigenvaluesfor the triplet and singlet state energy. Such S−T electric dipoletransition moments are calculated in this work for a number ofthe excited triplet states (thus not only for T1) using quadraticresponse theory.25 The meridional form of Alq3, which is thelowest energy isomer, was optimized with the 6-31G* basis atthe B3LYP level of density functional theory (Figure 1). Wehave used singlet ground state geometry for the phosphor-escence calculations in meridional form of Alq3 and tripletgeometry for ZFS computation. No symmetry constraints wereimposed for the ground state singlet and other low lying singletand triplet states. Optimized geometries were obtained fromthe GAUSSIAN-09 suite of programs26 and utilized for thespin−orbit perturbation and phosphorescence calculations. Allthe fluorescence and phosphorescence calculations were donewith a local version of the DALTON code25 with the sameB3LYP/6-31G* level of theory. The spin−spin and spin−orbit

interactions for the open-shell triplet geometry were calculatedwith a Hamiltonian operating on zeroth-order triplet wavefunctions resulting in the unique ZFS spin parameters D and E.Many implementations of this kind have been reported earlier;here we employ the one implemented in the ORCA suite ofprograms for ZFS calculations.27 The spin−spin coupling isestimated as an expectation value of the spin dipole−dipoleinteraction operator, and the SOC contribution to ZFS isestimated in the second order of perturbation theory.27 Twodifferent methodsthe quasi-restricted orbitals (QROs) and thecoupled-perturbed (CP) methodfor the DFT calculations ofthe SOC contribution to ZFS were used.27,28 These twomethods of ZFS-SOC estimations are quite reasonable sincethe nonexact exchange approximation is used.29 Though thenonhybrid functionals (like BP86) are preferred in theseschemes for SOC account in ZFS, we have used both B3LYPand BP86 levels of theory and have found good ZFSpredictions with the QRO approximation.

■ RESULTS AND DISCUSSIONS

Molecular Structure and the Singlet State Spectra ofAlq3. The meridional Alq3 complex contains three quinolateligands labeled as A, B, and C from left to right clockwise asshown in Figure 1 (as in refs 30 and 31). The central Al(III)ion is surrounded by the ligands in such a way that the oxygenanions are covalently linked to the metal, and the weaklycoordinated nitrogen atoms produce a pseudo-octahedralconfiguration. The corresponding bond lengths are presentedin Table 1. Our results agree well with the X-ray measurementsin ref 32 and also with the calculated bond lengths presented inrefs 30−33.The studied meridional isomer (mer-Alq3) has no symmetry

(C1 point group) in contrast to the facial isomer ( fac-Alq3)which belongs to the symmetry C3 point group.31−33

Comparison of their structure−property relationship made in

Table 1. Geometries of Alq3 in the Ground State and in theFirst Excited Triplet State (Optimized at the UB3LYP/6-31G* Level)a

label bond length (Å) and angle exptl (ref 32)

SingletAl(1)−O(18) 1.855 1.850Al−O(35) 1.881 1.860Al−O(52) 1.884 1.857Al−N(2) 2.084 2.050Al−N(19) 2.126 2.087Al−N(36) 2.064 2.017N(2)−Al−N(36) 171.5 173.8N(19)−Al−O(18) 172.7 171.5O(52)−Al−O(35) 166.6 168.2TripletAl(1)−O(18) 1.851Al−O(35) 1.867Al−O(52) 1.963Al−N(2) 2.105Al−N(19) 2.123Al−N(36) 1.994N(2)−Al−N(36) 170.1N(19)−Al−O(18) 172.35O(52)−Al−O(35) 168.06

aNumeration of atoms is shown in the Figure 1.

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ref 33 is very instructive. Few crystalline phases of Alq3 areknown. The most stable α phase represents mer-Alq3, and theso-called δ phase is identified as consisting of the facialisomer.23 Since the phosphorescence data are available on bothcrystalline phases (α and δ)23,24 we should shortly mentionsome comparative spectroscopic and structural details of bothphases to discuss additional luminescence features forqualitative assignments.Since a huge amount of data are available on the more stable

mer-Alq3 isomer30−33 the rare facial species provides a goodchance to make a qualitative comparison because of its highersymmetry. Thus, the infrared and Raman spectra of thecrystalline δ phase exhibit a lower number of vibrational bandsthan the α phase33,34 because the C3 symmetry point groupprovides some forbidden vibrational transitions by more strictselection rules.Other manifestations of the different spectral properties in

the luminescence of α and δ crystalline phases we shall discusslater. Now we need to stress that in the studied meridionalisomer the longest Al−N bond distance corresponds to thenitrogen atom N(19) which is opposite to the oxygen (2.126Å). This oxygen atom O(18) corresponds to the shorter Al−Obond length (1.855 Å) being the sequence of the well-knowntrans-effect in the coordination sphere.33 In the facial isomer allAl−N bond distances are equal and are opposite to the oxygenatoms. Thus, they are relatively close to the longest Al−N bondin the mer-Alq3 isomer.33 This is the reason for the lowerstability of fac-Alq3.The calculated bond angles of the mer-Alq3 isomer ∠N(19)−

Al−O(18) = 172.7° and ∠O(52)−Al−O(35) = 166.6° stronglydeviate from linearity which is supported by a good agreementwith the experiment32 (171.5° and 168.2°, respectively) andalso by previous theoretical calculations.30,31,33 Thus thestructure of the meridional Alq3 isomer is well reproduced atthe B3LYP/6-31G* level. Structural changes upon the S−Ttransition, predicted at the UB3LYP level, show that excitationincludes mostly charge transfer from the A ligand (we refer to

Figure 1 for atom numeration, 36−52) to the C ligand (Figure1 atoms with 2−18 numbers). The bond Al−N (refer to Figure1, N labeled 36) is compressed by 0.07 Å, and the Al−O (referto Figure 1, O labeled 52) bond is elongated by 0.08 Å. Suchstrong distortion of the chelate cycle is determined by the moveof the excited electron from the A-quinolate ligand (mostlyfrom the π-O (refer to Figure 1, O labeled 52) orbital of thephenoxide side) and its occurrence at the C-ligand with largelocalization on the pyridil side (Figure 2) observed mainly fromthe electron distribution (Figure 2).In Tables 4−7 the calculated parameters for the S−S and S−

T transitions are presented. The calculated vertical excitationsin the gas phase provide a weak first absorption S0 → S1transition at 447 nm (2.77 eV) and a series of more intenseclose-lying transitions in the range 427−417 nm (2.9−2.97 eV).This is in a complete agreement with results of refs 30, 33, and35. The second and the fourth transitions are the most intenseand all together form the first broad absorption band. The firstvery weak S0 → S1 transition serves as an onset of the growingabsorption band. The broad experimental absorption band hasa maximum at 390 nm,36 being slightly shifted to the blue edgein comparison with our predication.Similar calculations by Zhang and Frenking37 have shown

that the red shift for the first S0 → S1 transition is predictedwith the basis set enlargement in the framework of the B3LYPfunctional. Thus, better agreement with experiment (415 nm)is obtained with the moderate 3-21+G* basis set.31 This is not afatal trend since the first vertical S0 → S1 transition has nothingin common with the maximum of the first absorption band(accounting for intensity of the first four vertical excitations,Table 4).Our TD-DFT calculations in benzene solvent by the PCM

model predict a correct blue shift for all absorption bands(Table 6). This provides more credit to the overall predictedabsorption picture since the experimental spectra are measuredin the condensed phase (solvents and polymers).36 The firstexcited singlet state can be responsible for fluorescence, which

Figure 2. SOMOs (triplet), HOMO, and LUMO (ground state) of Alq3.

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is detected experimentally at 515 nm.36 Geometry optimizationof the S1 state at the CIS/6-31G* level provides smallrelaxation and the further shift to longer wavelength foremission of Alq3 (523 nm).37 Our TD-DFT calculations of thesinglet states at the triplet-state optimized geometry also predicta large Stokes shift (155 nm) between fluorescence andabsorption spectra in a close agreement with observation.36

Before finishing the discussion of the singlet states of themer-Alq3 complex, we have to mention that according to TD-DFT calculations of Amati and Lelj33 the absorption andfluorescence spectra of the fac-Alq3 isomer are blue-shiftedrelative to the meridional counterpart. This is in goodagreement with experimental spectra of the α and δ crystallinephases.32,33 More important for our purpose is the fact that thephotophysical measurements indicate a lower triplet statepopulation for the irradiated δ crystalline phases relative to thetriplet quantum yield determined for the α phase.32 From thispoint we can switch to a more detailed analysis of the tripletstate properties.ZFS Parameters and Phosphorescence. Colle et al.23,24

detected the T1 → S0 transition over a broad temperature rangefrom 1.3 to 150 K including crystal and amorphous forms ofAlq3 and also studied the optical detection of magneticresonance (ODMR). By ODMR measurements they deter-mined the ZFS parameters (|D| = 0.0632 ± 0.0007 cm−1 and |E|= 0.0114 ± 0.0002 cm−1) in the same range for the α crystallinephases and for the evaporated amorphous films.24

Our ZFS calculations (Table 2) with account of both spin−spin and spin−orbit couplings in the framework of quasi-restricted orbitals (QROs) approximation support these ZFSvalues. Thus, there is no doubt about the nature of the observedphosphorescence of the meridional Alq3 molecule. The spin−spin coupling contribution alone can not explain theexperimental ODMR data; the SOC contribution to ZFS isvery essential in the QRO method (Table 2). Estimation of theZFS parameters of the meridional Alq3 molecule, starting fromthe D and E values of the isolated ligands, supports a mini-exciton-like behavior of the triplet state on the three ligands of

the Alq3 molecule.23,24 Our DFT calculation also indicates that

the three low-lying triplet states represent different combina-tions of the local ππ* excitations in the quinoxaline moietieswith simultaneous admixtures of electron transfer betweendifferent ligands.From the measured values of the ZFS, Colle et al.24

estimated the distance between the coupled nonpairedelectrons in the triplet state to be equal to 3.2 Å. This is anapproximate value since it considers only a crude form of spin−spin coupling operator and neglect SOC contribution.Nevertheless, this estimation indicates that the triplet excitonis spatially localized on one of the ligands of the Alq3 moleculewith some admixture of charge transfer (refs Tables 3, 5, and7).The ZFS parameters measured for the meridional isomer in

Alq3 are small compared to the respective values for the isolatedquinoline chromophores (|D| = 0.1028 cm−1 and |E| = 0.0167cm−1).24 That leads to the assumption that the triplet state isnot fixed on a single chromophore but is mobile, movingbetween the ligands24 in agreement with our DFT calculations(Table 2). Such a phenomenon is well-known for triplet stateEPR measurements in molecular crystals, like naphthalene oranthracene. When the triplet-excited molecule is found in twodifferent orientations in the unit cell and a mini-exciton isformed, then the triplet excitation has an equal probability ofpresence on both chromophores, which leads to a spatialaveraging of the spin Hamiltonian.24 In our case of the Alq3complex, the two quinoxalate ligands surrounding the Al centralion have to be considered as forming the triplet excitation(Figure 2).The low-temperature measurements indicate that the triplet

lifetime has a clear temperature dependence from 17 ms at 6 Kto 4 ms at 120 K.24 It is obvious that the lowest temperatureprovides the better estimation of the pure radiativephosphorescence lifetime since at 6 K the nonradiativequenching is often suppressed and does not compete withphosphorescent emission.38 Our DFT calculation of theradiative rate constants for the ten lowest triplet excited states

Table 2. Zero-Field Splitting Parameters (in cm−1) for the First Excited Triplet State of Alq3

exptl (ref 24)

functional method Dss Ess Dso+ss Eso+ss ⟨S2⟩ − S(S + 1) D E

UB3LYP CP 0.02252 0.00290 0.01734 0.00513 0.02535 0.0630 0.0114UB3LYP QRO 0.04539 0.00124 0.08489 0.00450 0.02535UBP86 CP −0.02337 0.01697 −0.01873 −0.00485 0.01088UBP86 QRO 0.03829 −0.00245 0.09034 0.00900 0.01088

Table 3. Radiative Lifetime of the Ten Lowest Triplet States of Alq3 and Rate Constant for the Tn−S0 Transitions at the SingletGround State Geometry

transition rate constant (in s−1) lifetime (tk) (s)

triplet state in gas phase in benzene near LiF in gas phase in benzene near LiF

1 27.721 × 10−3 40.413 × 10−3 54.507 × 10−3 36.073 24.745 18.3462 27.582 × 10−3 52.329 × 10−3 43.787 × 10−3 36.255 19.110 22.8383 27.833 × 10−3 31.818 × 10−3 0.110 35.929 31.429 9.1044 86.580 600.920 5.923 11.550 × 10−3 1.664 × 10−3 0.1695 386.330 438.410 3.795 2.588 × 10−3 2.281 × 10−3 0.2636 5.025 2.369 21.929 0.199 0.422 45.602 × 10−3

7 5.431 37.107 14781.000 0.184 26.949 × 10−3 67.656 × 10−6

8 5.283 8.288 23.077 0.190 0.121 43.334 × 10−3

9 8.290 62.012 0.235 0.121 16.126 × 10−3 4.25110 17.883 0.261 773.040 55.919 × 10−3 3.833 1.294 × 10−3

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of Alq3 indicates that the observed radiative lifetime (17 ms24)can not be assigned to the calculated T1 → S0 transition andeven to the three lowest quasidegenerate triplet states (Table3).All these states have a calculated radiative lifetime about 2000

times longer than experimentally measured. The calculatedlifetime (about 36 s) is typical for the oxyquinoline itself sinceits triplet state has ππ* character.38−40 The calculated radiativelifetimes for the upper triplet states, for the correspondingtransitions T4 → S0 and T5 → S0, are equal to 11.6 and 2.6 ms,respectively (Table 3), being occasionally close to the measuredT1 state lifetime. Account of vibronic mixing between the T1and the upper triplet states can provide a better agreement withthe measured lifetime of the Alq3 phosphorescence at lowtemperature in solid environments (one also needs to keep inmind that the measured lifetime of the Alq3 phosphorescenceprobably contains some contribution from nonradiativequenching). The main mechanism of the observed phosphor-escence includes vibronic contribution which explains the longprogression in the low-temperature EL spectrum of the OLED.Well-resolved vibrational progressions for all studied

samples24 allowed a precise determination of the triplet energy,namely, 2.11 ± 0.1 eV for the meridional isomer of the Alq3molecule.24 This is quite close to our prediction for the T1 ←S0 transition in mer-Alq3, 2.12 eV (Table 5). All delayedluminescence spectra detected at low temperature24 show twodistinct bands, one at about 520 nm, similar to the spontaneousphotoluminescence in OLEDs (this is delayed fluorescence,produced by the T−T annihilation process), and an additionalband with a maximum at about 700 nm.23 The maximum of thelow-energy band is located between 660 and 670 nm for thecrystalline α-Alq3 and for the yellowish-green Alq3 powder. It isshifted to about 690 nm for the amorphous film. Thisphosphorescence is comparable with that observed in ethyliodide glass at 660 nm.22 For the δ-Alq3 crystalline phase, thephosphorescence is slightly blue-shifted to 640−655 nm.24

On the high-energy side of the phosphorescence band, avibronic progression with an average distance of about 550cm−1 is clearly observed for all studied crystalline andamorphous phases.24 This is similar to the vibronic progressionobserved for the crystalline fluorescence34 and photolumines-cence in the OLED.23 Thus, this progression of the band atabout 690 nm is due to vibrational modes of the Alq3 complexin its ground state.According to the Raman spectra of the α - and δ-fractions of

polycrystalline Alq3, there are relatively intense lines at 525 and534 cm−1, respectively, which are attributed to Al−O stretchingand ring deformation vibrations.34 These are totally symmetricmodes which can promote the long fluorescence andphosphorescence progressions because of the large displace-ments in metal−ligand distances upon the S1 ← S0

37 and T1 ←S0 transitions. There are weak satellites (shoulders) of theselines in Raman spectra at 540 and 548 cm−1, respectively, whichhave similar vibrational nature with addition of Al−Nstretching. These modes seem to be the most active ones invibronic mixing between the T1 and T4 states and which couldbe responsible for the phosphorescence vibronic progressionwith an average interval of about 550 cm−1 and for a significantshortening of the observed radiative lifetime. One can stressthat for the δ-Alq3 (facial isomer) the average vibronic intervalis slightly larger (558 cm−1) than that for the α-Alq3 meridionalisomer (549.7 cm−1), taking into account the same fourvibronic bands in the phosphorescence progression for both

species. This correlates with the differences in the active Ramanvibrational frequencies of the two isomers. Thus, there is littledoubt that the vibronic progression of the phosphorescenceband at 690 nm is due to vibrational modes of the meridionalAlq3 complex determined by the metal−ligand stretchingmodes. Such modes are effective in the mixing of the π−π*electronic states of the quinolate ligands with the electroniccloud of the 3d shell of the Al atom.We should note that the absorption spectrum and

fluorescence emission of fac-Alq3 are blue-shifted relative tomer-Alq3. This is supported by TD-DFT calculations.33 Thecrystalline δ-phase ( fac-Alq3) shows a lower triplet statequantum yield relative to mer-Alq3 .

24 Only 2% of the excitedstates are triplets in the δ-phase, as compared with the 25% ofthe mer-Alq3 α-phase and amorphous films during lumines-cence measurements.24,33

The similarity of the T1 state population for amorphous andcrystalline α-phases (meridional isomer of Alq3) indicates thatmolecular packing or disorder does not influence theintersystem crossing (S1 ⇝ T1 relaxation) to any appreciableextent.24 Thus, this is an intrinsic property of the mer-Alq3molecule itself. The reduced T1 state population for thecrystalline δ-phase favors the explanation for the lowintersystem crossing as a pure intramolecular relaxation infac-Alq3.

24 The higher S1−S0 transition probability in the facialisomer can be responsible for the less competitive intersystemcrossing to the triplet state.33 The longer Al−N distances in fac-Alq3

33 can reduce the SOC integrals between the S1 and T1states; this and the higher vibrational frequency of thepromoting mode can lead to the lower S1 ⇝ T1 intersystemcrossing in the facial isomer. The above qualitative analysis, inaddition to our DFT calculations of SOC effects and thephosphorescence radiative lifetime in mer-Alq3, providesbackground for a complete explanation of all known featuresof the measured luminescence in crystalline phases and in EL ofOLEDs fabricated based on Alq3 emission layers. This directcharacterization of the triplet state in Alq3 is relevant for thepractical applications of Alq3-based OLEDs since most of theexcitons formed in OLEDs are triplet excitons.41

Alq3 in Spintronics. Spin transfer through interfaces is asignificant aspect for modeling spintronic devices. Successfulspin injections in one end and recovering it at the otherelectrode is not a trivial task to accomplish. Interfaces remain abottleneck, as it often meets difficulties with mismatchingFermi levels. Remarkable advances have been made in recentyears by introducing a polar material in between to raise thehighest occupied molecular orbitals in the organic semi-conductor. The underlying phenomenon is attributed to thespin−orbit coupling. Although this effect is meager, its selectionrules make it very important in recombination of electrons. Asin the case of a pair of electrons, the spin-polarized triplet statecan be quenched by SOC via intersystem crossing to the zero-spin ground singlet state; in a similar manner, a spin-dependentcurrent across an organic−inorganic interface can disappear.There are some similarities in the known organic materials

used in spintronic devices. Three organic molecules, out of fourknown, which show inverse spin-valve effect, possess the sameLUMO level (differing by less than 0.1 eV). These are Alq3,sexithiophene, and BNP (N′-bis-1-naphthyl-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine).16 In addition, Alq3, which is apure LUMO channel conductor, shows by far the bestspintronic performance. It has been demonstrated that thecobalt electrode injects spin-up carriers across a barrier and

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even across a hybrid Al2O3/Alq3 barrier.15,16 The sign of themagneto-resistance should then be explained considering bothelectron currents injected by the manganite electrode and bythe cobalt as spin-up currents. The authors of ref 16 achievedroom-temperature operation for organic spin injection devicesthrough engineering of the interfaces between the organic partand the spin-polarized electrodes. As shown here the organicsemiconductor does not represent any limitation in perform-ance at least up to room temperature, and the improvementachieved by the introduction of tunnel barriers in the organicspin valves may lead to improvement of such devices. This is ina good agreement with the recent results from Santos et al. whodemonstrated that in magnetic tunnel junctions the presence ofan Al2O3 barrier increases the spin injection effect at theinterface.42

A single LiF molecule being in a weak intermolecular contactwith Alq3 (Figure 2) may be a crude approximation to recreatethe LiF thin-film polar effect; however, the idea is here tocapture the polar effect at the molecular level. Such a procedureis also justified considering the results of ref 43, where a singleK atom is placed nearby the central aluminum of Alq3 tocalculate the electronic structure in pristine form. The additionof dipole moments results in an enhancement of the electricdipole moment of Alq3 + LiF, approximately becoming doubled(4.4 to 8.7 D; the electric dipole moment of LiF is about 5 D).The anisotropic electric field introduced by LiF indeed shiftsthe eigenvalues of the molecular orbitals of Alq3 (with amagnitude of the order 0.05 eV). In ref 15 the tuning of valenceMOs of the Alq3 molecule has been reported with the presenceof a polar LiF thin layer. This facilitates polarized spin carriersto pass through the organic layer. The energy shift in theHOMO of Alq3 is found in our calculations to be in the orderof ∼0.05 eV. This finding can be compared with the above-mentioned experiment of Schulz et al.15 in which they achievedspin injection operating at less than 100 mV.

As follows from the calculated singlet−triplet transitions(Table 5) in the gas phase, the first three excited states arequasidegenerate and are determined by HOMO-n → LUMOexcitations, where n = 1−3. At the same time the lowest singletexcited state includes the HOMO → LUMO (119−120)excitations with low oscillator strength (Table 4). This is alsoseen in Figure 2 where the HOMO in the ground statemolecule almost coincides with the SOMO-1 in the tripletexcited state; at the same time the LUMO of the ground statemolecule differs from the triplet state SOMO, being localizedon the different ligands. This leads to the fact that SOC matrixelements between the T1 state and the three singlet lowestexcited states are negligible,since they include SOC integralsbetween different hydroxyquinoline moieties.In spintronic devices based on the Alq3 material, the spin-

polarized current proceeds along the LUMO channel in theconducting band. In this case spin−orbit coupling can notremove spin polarization since the corresponding SOC matrixelements are almost negligible for the lowest triplet states(Table 5). They include only pure π-orbitals from differentorganic ligands without any contributions from the 3d-AO ofthe heavy metal ion. This is also the reason for the relativelyhigh phosphorescence intensity and quantum yield detected inexperiments.22−24 The nonradiative quenching T1 ⇝ S0 issuppressed by the weak direct SOC (Table 5), but the radiativerate is determined by a nondirect way through vibronic mixingwith the higher triplet states (Table 3).We have also studied integrals and S−T transitions for the

Alq3 complex with the LiF molecule in the nearest coordinationsphere (Figure 1). In this case a strong change of the S0−T7transition moment has been predicted, which indicates someparticular enhancement of SOC matrix elements betweenhighly excited states induced by weak intermolecular interactionwith the polar solvent. Besides this the phosphorescence S0 ←T1 transition rate constant is enhanced by an order of

Table 4. Singlet−Singlet Transition Electric Dipole Moments (in au), Transition Energies (in eV), and Their OscillatorStrength ( f) of Alq3 at the Optimized Singlet Ground State Geometry in the Gas Phase

state (n) ⟨S0|μx|Sn⟩ ⟨S0|μy|Sn⟩ ⟨S0|μz|Sn⟩ ESn−S0 f TD-DFT expansion coefficients

1 −0.103 −0.231 0.109 2.771 0.0051 0.135 (118 → 120) − 0.688 (119 → 120)2 0.777 0.494 0.309 2.902 0.0671 −0.154 (117 → 120) − 0.322 (118 → 120)

+0.081 (118 → 122) − 0.070 (119 → 120)−0.461 (119 → 121) + 0.320 (119 → 122)−0.065 (119 → 128)

3 −0.152 −0.069 −0.036 2.938 0.0021 −0.590 (118 → 120) − 0.111 (119 → 120)+0.310 (119 → 121) − 0.152 (119 → 122)

4 −0.015 −0.437 0.627 2.971 0.0425 −0.634 (117 → 120) + 0.111 (118 → 120)−0.085 (118 → 121) + 0.069 (119 → 120)+0.143 (119 → 121)

5 −0.023 −0.098 −0.037 3.025 0.0008 0.118 (117 → 120) − 0.134 (118 → 121)+0.355 (119 → 121) + 0.558 (119 → 122)

6 0.263 0.336 −0.055 3.149 0.0143 0.085 (118 → 120) + 0.629 (118 → 121)+0.176 (118 → 122) + 0.140 (119 → 121)+0.119 (119 → 122)

7 −0.203 −0.057 −0.099 3.269 0.0044 0.323 (117 → 121) + 0.171 (118 → 121)−0.593 (118 → 122)

8 −0.146 0.003 −0.207 3.275 0.0051 0.620 (117 → 121) - 0.105 (118 → 121)+0.302 (118 → 122)

9 0.151 −0.006 0.100 3.350 0.0027 0.701 (117 → 122)10 −0.065 0.092 −0.311 3.970 0.0107 −0.073 (116 → 121) + 0.111 (116 → 122)

−0.076 (118 → 123) + 0.672 (119 → 123)−0.121 (119 → 125)

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magnitude (Table 3). The LiF dipolar layer, being sandwichedinto the organic−metal interface in the spintronic devices,provides a tuning of the vacuum HOMO−LUMO levels andimplements transport of spin-polarized charge carriers throughthe thin films.16 Our calculations imply that not only theHOMO−LUMO levels are tuned but also some internalmagnetic perturbations occur that could be responsible for theLiF dipolar layer effect.Simulation of another organic layer in the form of a benzene

PCM solvent (Tables 6 and 7) indicates rather small differencesin the SOC integrals in comparison with the pure Alq3 speciesin vacuum (Tables 4 and 5). Our study of spin polarization inthe Alq3 conducting band and comparison with the SOC effectson the phosphorescent emission from different spin sublevels ofthe triplet excited states Mα

k (eq 1) will be continued in aforthcoming publication.

■ CONCLUSION

The tris-(8-hydroxyquinoline)aluminum complex (Alq3) is animportant emitting and electron-transporting material forOLEDs but has, even after two decades of intense research,still not been completely understood with respect to itsphotoluminescence. We have presented time-dependentdensity functional theory quadratic response calculations ofthe phosphorescence rate constant and the phosphorescencelifetimes of Alq3. The triplet states of Alq3 have been studied

theoretically for the first time. Our study is motivated by thefact that Alq3 phosphorescence has been recently observed

22−24

and that comparison with the triplet dopants in the OLED canbe made. We have shown that the pure electronic T1−S0transition is very weak and that the phosphorescence of Alq3borrows intensity from the upper T−S transitions by vibronicinteractions. The active mode (550 cm−1) well-seen in thevibronic progression of the phosphorescence band is connectedwith the metal−ligand vibrations. The ZFS parameters D and E

Table 5. Singlet−Triplet Transition Energy (in eV) and Spin−Orbit Coupling Matrix Elements (in cm−1) of Alq3 at the GroundState Geometry in the Gas Phase

state (n) ES0−Tn(in eV) ref 41 (in eV) ⟨S0|Hso

x |Tn⟩ ⟨S0|Hsoy |Tn⟩ ⟨S0|Hso

z |Tn⟩ TD-DFT expansion coefficients

1 2.123 2.13 0.150 0.057 −0.004 −0.082 (110 → 122) − 0.115(116 → 125)+0.107(118 → 121) − 0.191(118 → 122)+0.400(119 → 121) − 0.602 (119 → 122)−0.139 (119 → 128)

2 2.158 2.16 −0.112 −0.007 0.085 −0.097(108 → 120) + 0.119 (114 → 123)−0.705 (117 → 120) − 0.203 (117 → 121)−0.146 (117 → 122) + 0.149 (117 → 126)−0.107 (118 → 120)

3 2.183 2.19 0.008 0.259 0.029 0.075(109 → 121) − 0.123 (115 → 124)−0.066 (117 → 120) + 0.321 (118 → 120)−0.534(118 → 121) − 0.280 (118 → 122)+0.142 (118 → 127) − 0.162 (119 → 120)−0.227 (119 → 121) + 0.143 (119 → 122)

4 2.790 2.79 0.557 0.838 −0.758 0.151 (118 → 121) − 0.683 (119 → 120)5 2.952 2.95 −2.677 0.730 1.676 −0.072 (117 → 120) + 0.590 (118 → 120)

+0.141 (118 → 121) + 0.064 (118 → 122)+0.060 (119 → 120) − 0.298(119 → 121)−0.167 (119 → 122)

6 3.061 3.06 0.654 1.901 0.100 −0.047(117 → 120) + 0.218 (118 → 120)+0.324 (118 → 121) + 0.159 (118 → 122)+0.459 (119 → 121) + 0.324(119 → 122)

7 3.234 3.23 −2.614 0.582 2.527 −0.188(117 → 120) + 0.610 (117 → 121)+0.072 (117 → 122) − 0.105 (117 → 123)+0.161 (118 → 121) − 0.207 (118 → 122)

8 3.243 3.24 −1.709 −1.284 0.168 0.086 (117 → 120) − 0.243 (117 → 121)+0.262 (118 → 121) − 0.563 (118 → 122)−0.099 (119 → 121) + 0.166 (119 → 122)

9 3.314 3.31 −0.393 0.748 −2.993 −0.099 (117 → 120) − 0.116 (117 → 121)+0.658 (117 → 122) − 0.188(119 → 125)

10 3.346 3.35 −0.270 0.019 −0.889 0.068 (116 → 122) + 0.187 (117 → 122)+0.188 (118 → 125) − 0.199 (119 → 123)−0.101 (119 → 124) + 0.634 (119 → 125)

Table 6. Singlet−Singlet Transition Dipole Moments (inau), Transition Energy (in eV) of Alq3, and Their OscillatorStrength ( f) in Benzene Solvent Calculated by thePolarizable Continuum Model

state (n) ⟨S0|μx|Sn⟩ ⟨S0|μy|Sn⟩ ⟨S0|μz|Sn⟩ ESn−S0 (in eV) f

1 0.389 −0.334 −0.081 2.884 0.01902 1.093 0.181 −0.105 2.971 0.09023 0.157 −0.630 −0.234 3.021 0.03534 0.057 −0.496 −0.127 3.041 0.01985 0.245 −0.102 0.020 3.118 0.00546 −0.187 0.178 0.036 3.234 0.00547 0.393 0.117 −0.071 3.296 0.01408 −0.102 −0.222 −0.057 3.300 0.00519 −0.023 −0.021 0.056 3.360 0.000310 −0.115 −0.475 −0.170 4.064 0.0267

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being earlier determined by ODMR experiments23 are in areasonably good agreement with our calculation. Thus, we haveexplained a number of triplet state features connected with theZFS parameters in the ODMR spectra, the phosphorescencelifetime, and vibronic activity of the T1 → S0 transition of Alq3.Calculations of the spin−orbit coupling integrals provideexplanation of the observed relatively high phosphorescenceintensity.24 The weak direct SOC between the T1 and S0 states(Table 5) explains the slow nonradiative quenching T1 ⇝ S0; atthe same time the phosphorescence radiative rate is relativelylarge because it is determined by vibronic coupling with thehigher triplet states, where the charge transfer to the metal isinvolved and excited electrons penetrate into the Al(III)-ion-nuclear region. For similar reasons, Alq3 is a good material forspintronics devices: the SOC integrals, which could quenchspin-polarized charge carriers at the LUMO levels, arenegligible (Table 5). We predict shifts in the MO levels(∼0.05 eV) for the intermolecular contact with the dipolarspecies LiF which are consistent with recent observations forhybrid organic−inorganic spin-injection devices.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: sathya@theochem.kth.se.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe computations were performed on resources provided bythe Swedish National Infrastructure for Computing (SNIC) atthe Parallel Computer Center - PDC, through the project″Multiphysics Modeling of Molecular Materials″, SNIC 020/11-23

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Table 7. Spin−Orbit Coupling Matrix Elements (in cm−1)and Singlet−Triplet Transition Energy (in eV) of Alq3 inBenzene Solvents Calculated by the Polarizable ContinuumModel

state (n) ⟨S0|Hsox |Tn⟩ ⟨S0|Hso

y |Tn⟩ ⟨S0|Hsoz |Tn⟩ ES0−Tn

(in eV)

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