The near-infrared optical properties of an Nd (III) complex and its potential application in electroluminescence

Inorganic Chemistry Communications 12 (2009) 151–153

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The near-infrared optical properties of an Nd (III) complex and its potential application in electroluminescence Zhefeng Li a,b, Jiangbo Yu a, Liang Zhou a,b, Ruiping Deng a, Hongjie Zhang a,* a b

State Key Laboratory of Rare Earth Resource Utilizations, Changchun Institute of Applied Chemistry, 5625 Renmin Street, Chinese Academy of Sciences, Changchun 130022, PR China Graduate School of the Chinese Academy of Sciences, Beijing 100064, PR China

a r t i c l e

i n f o

Article history: Received 7 October 2008 Accepted 2 December 2008 Available online 10 December 2008 Keywords: Near-infrared Nd (III) complex Electroluminescence

a b s t r a c t A trivalent neodymium ion (Nd3+) complex Nd(PM)3(TP)2 was synthesized, and its optical properties was studied by introducing Judd–Ofelt theory to calculate the radiative transition rate and the radiative decay time of the 4F3/2 ? 4IJ0 transitions in this Nd(III) complex. The strong emissions of this complex at nearinfrared region were owing to the efficient energy transfer from ligands to center metal ion. The potential application of this complex in NIR electroluminescence was studied by fabricating several devices. The maximum NIR irradiance was obtained as 2.1 mW/m2 at 16.5 V. Ó 2008 Elsevier B.V. All rights reserved.

Near infrared luminescent lanthanide complexes have attracted much attention because of their academic interest and potential utility in a variety of photonic applications, such as planar waveguide amplifiers, plastic lasers, light-emitting diodes, and luminescent probes [1–3]. It is well known that lanthanide (Ln3+) ions have an incompletely filled f-subshell, which is shielded by the outer filled 5s and 5p electrons shells. The 4f–4f electronic transitions are in principle spin-forbidden because the energy levels of 4f electron shells have an equal parity. Therefore, luminescent ligands are introduced to coordinate with Ln3+ ions and act as sensitizers or antenna chromophores, absorbing and transferring the energy to excite the Ln3+ ions via an energy-transfer process [4,5]. The organic ligands can be selected effectively to sensitize Nd3+ ion and provide enough coordination sites to shield it from impurities in the surrounding matrix that may quench the luminescence. For instance, the luminescence is usually quenched by hydroxyl groups in the system due to coupling of the excited state of lanthanide ion with high-energy vibrational modes of the O–H bond [6]. The ligand 1-phenyl-3-methyl-4-isobutyryl-5-pyrazolone (PM) is easy to coordinate with Nd3+ ion and form a stable complex with triphenyl phosphine oxide (TP) as the neutral ligand. These ligands have large conjugated structure and the isopropyl in PM ligand is the electron donor, all of which can improve the luminescent property of the complex [7,8]. Nd (III) complex-based OLEDs have reported several times [9], but performance of devices in near infrared region was not studied in detail such as the intensity of NIR irradiance. In this work, the optical properties of the complex Nd(PM)3(TP)2 are investigated * Corresponding author. Tel.: +86 431 85262127; fax: +86 431 85698041. E-mail address: [email protected] (H. Zhang). 1387-7003/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2008.12.001

by absorption, excited, and emission spectra. Judd–Ofelt theory is introduced to study the radiative rates of different transitions in this complex and obtain the natural lifetime of 4F3/2 excited state of Nd(III). The potential application of this complex in NIR electroluminescence was studied. The room-temperature absorption spectrum for Nd(PM)3(TP)2 (in CHCl3) is shown in Fig. 1. In the 500–1000 nm region, various f–f absorption bands of Nd3+ are observed (originate from the 4 I9/2 ground state of Nd3+), they correspond to the energy states: 4 G5/2 (584 nm), 4F9/2 (686 nm), 4F7/2 + 4S3/2 (749 nm), 4F5/2 (801 nm), 4F3/2 (872 nm) [10]. All the observed absorption bands are numerically integrated to obtain the experimental line strength, and the values are summarized in Table 1. The excitation spectrum (monitored at 1060 nm) and the emission spectrum of Nd(PM)3(TP)2 are shown in Fig. 2. With direct excitation (kex = 350 nm), we obtain three emission bands in the near-infrared region, which are originated from the characteristic f–f transition of Nd3+ ion at 893 nm (4F3/2 ? 4I9/2), 1060 nm (4F3/2 ? 4I11/2), and 1336 nm (4F3/2 ? 4I13/2). The energy transfer process of ligand PM to the Nd3+ ion can be described as follows: firstly, the electrons of the ligand PM are excited from the singlet ground state (S0) to the singlet excited state (S1) by absorbing the energy. Then the singlet excited state (1pp*) is transferred into triplet state (3pp*) via intersystem crossing, because the intersystem crossing yield of the ligand is increased greatly by a heavy atom effect. The excited state 4G7/2 of Nd3+ ion is at the level of 19,103 cm1, which is the mainly energy accepter level by an electron exchange mechanism [11]. Finally, the energy transfer to 4F3/2 level of Nd3+ ion by vibrational relaxation and radiate to 4IJ levels. The lifetime of Nd(PM)3(TP)2 was obtained at the emission wavelength 1060 nm. It is fit to a single exponential with a decay

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in the absorption spectrum, are shown in Table 1. The measured absorption line strengths, Smea, from the ground 4I9/2 manifold (J = 9/2) to the excited J’ manifold can be obtained using the following Eq. (1) [14]:

Z

" # 1 8p3 e2 k ðn2 þ 2Þ2 Smea ; ODðkÞ dk ¼ NNd L Ln10 3chð2J þ 1Þn 9

ð1Þ

where OD(k) is the absorption coefficient at wavelength k; k is the mean wavelength of the specific absorption band; NNd is the con3þ centration of Nd ðNNd ¼ 3:01  1017 cm3 Þ, L is the optical length (L = 1 cm); e, h, and c are the electron charge, Planck’s constant, and velocity of light, respectively; and n is the refractive index of the chloroform solution (n = 1.446).According to Judd–Ofelt theory, the line strength for electric-dipole (ED) transition between the ini tial J manifold Fig. 3 jðS; LÞJi and terminal J’ manifold jðS0 ; L0 ÞJ can be expressed by following Eq. (2) [15]:

S¼ Fig. 1. The absorption spectrum of Nd(PM)3(TP)2 in CHCl3 solvent.

Table 1 Measured and calculated line strengths of Nd3+ ion in Nd(PM)3(TP)2 complex (all transitions are from 4I9/2). S0 L0 J0

k (nm)

Smea (1020 cm2)

Scal (1020 cm2)

4

584 686 749 801 872

28.305 2.3843 15.232 18.142 8.2645

28.305 1.5881 15.087 18.438 7.8745

G5/2 F9/2 F7/2+4S3/2 4 F5/2 4 F3/2 rms DS = 0.668  1020 cm2 4 4

X



2

Xt hðS; LÞJkU t kðS’; L’ÞJ’i ;

ð2Þ

t¼2;4;6

where hkU ðtÞ ki are the doubly reduced matrix elements corresponding to the J–J’ transition. The oscillator strength parameters Xt (t = 2, 4, 6) are independent of electronic quantum numbers for the ground configuration of the Nd3+ ion. Using Eq. (1), the experimental oscillator strength Smea for the electric-dipole transition can be obtained. Then, the parameters Xt can be derived by least-squares fitting of Eq. (2): X2 = 1.848  1020 cm2, X4 = 2.581  1020 cm2, X6 = 3.199  1020 cm2. Using these parameters, we recalculate the transition line strengths Scal of the absorption bands using Eq. (2). The calculated line strengths are listed in Table 1. The rms (root–mean–square) deviation between the experimental and calculated line strengths is defined by Eq. (3)

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u N uX rmsDS ¼ t ðSmea  Scal Þ2 =ðN  3Þ;

ð3Þ

i¼1

where N is the number of absorption bands analyzed. A measurement of the relative error is givenffi by the rms error = rms DS/ qffiffiffiffiffiffiffiffiffiffiffiffi P S2mea rmsS  100%, where rms S ¼ .The rms error of the fitting is N 2.9%, which indicates that the fitting results are in good agreement with the experimental data. The parameters Xt are used to determine the radiative transition rate Aed from the initial state J (4F3/2) to the terminal state 4IJ’ (J’ = 13/2, 11/2, 9/2), and Aed can be calculated using Eq. (4) [16]

Fig. 2. The excitation and emission spectra of Nd(PM)3(TP)2 crystal. Inset is the chemical structure of Nd(PM)3(TP)2.

time of 2.2 ls. The nature lifetime of 4F3/2 excited state in the Nd(PM)3(TP)2 complex is calculated by Judd–Ofelt theory as follows: Judd–Ofelt theory [12,13] is one of the successful theories for estimating the magnitude of the forced electric-dipole transition of rare-earth ions. From the RT absorption spectrum of Nd(PM)3(TP)2, we can find the optical absorption corresponds to electron transitions from the ground 4I9/2 multiplet to the upper energy states of Nd3+. Five Nd3+ ion absorption bands in the spectrum were selected to determine the phenomenological oscillator strength parameters. The band positions, along with assignments

Fig. 3. Current density-NIR Irradiance-Voltage curves of device B.

Z. Li et al. / Inorganic Chemistry Communications 12 (2009) 151–153

Aed ¼

 2 64p4 e2 nðn2 þ 2Þ2 X Xt  ðS; LÞJkU t kðS0 ; L0 ÞJ0  ; 9 3hð2J þ 1Þk3 t¼2;4;6

153

ð4Þ

where n is the refractive index, and the value of jh½4 F 3=2  kU t k½4 IJ0 ij2 are calculated based on the intermediate-coupling wave functions obtain from the energy-level fitting. The radiative lifetime for the excited 4F3/2 state of Nd3+ ion is determined from the radiative transition rate A using Eq. (5)

sR ¼ P

1 : AðJ ! J0 Þ

ð5Þ

The radiative quantum efficiency is defined as the following

ge = s/sR, where s is the fluorescence lifetime. The calculated radiative lifetime and quantum efficiency are listed in Table 2. The calculated radiative decay time of 208.5 ls for the 4F3/2 ? 4IJ0 transition of Nd3+ ion in this complex is excellent agreement with the reported value of 260 ls for 4F3/2 excited state of Nd3+ [17]. To fabricate the devices, all the organic layers were evaporated onto a precleaned ITO (with a sheet resistance of 12 X/sq) glass substrate with the speed of 0.05 nm/s under high vacuum (63.0  105 Pa). LiF and Al were evaporated in another vacuum chamber with different speed of 0.01 nm/s and 0.5 nm/s without being exposed to the atmosphere. The thicknesses of the deposited layers and the evaporation speed of individual materials were monitored in vacuum with quartz crystal monitors. To investigate EL properties of Nd(PM)3(TP)2, several devices based on this Nd complex as emitter layers were fabricated. Firstly, we fabricated device A, which comprised 20 nm NPB for holetransport layer and 50 nm Nd(PM)3(TP)2 for emission layers; 30 nm AlQ was then deposited for a electron-transport layer; finally, a 150 nm LiF/Al bilayer cathode was made for efficient electron injection. Current-irradiance–voltage properties were measured by using a computer-controlled Keithley source measurement unit (Keithley 2400 and Keithley 2000) with a calibrated UDT Model 280 InGaAs Detector. The maximum NIR irradiance of this device was obtained as 0.98 mW/m2 at applied voltage of 19.2 V (341 mA/cm2). To improve performance of the device, a hole-block material BCP was introduced in device to make device B as the structure of ITO/NPB 15/Nd 55/BCP 15/AlQ 35/LiF/Al 150 (nm). BCP is helpful to confine electrons and excitons within the heterostructure emissive region and then improving characteristics of devices. Compared with device A, the NIR irradiance of device B got two times increase and turn-on voltage decreased to 8.1 V. The maximum near-infrared irradiance was obtained as 2.1 mW/m2 at 16.5 V. The better performance of this device owes to: firstly, thickening emission layer makes the recombination zone broader; secondly, carriers-transport is balance and excitons are formed mainly in emission layer. Fig. 4 displays the EL spectrum of device B at different voltages. The emissions at 890 nm and 1060 nm were clearly detected, which was arisen from the transitions between the 4F3/2 ? 4I9/2, 4 I11/2 levels of Nd3+. The emission at 1330 nm (4F3/2 ? 4I13/2) was a weak band, only can be found at applied voltage of 16 V. In summary We have conducted detail analyses of optical properties of the Nd(PM)3(TP)2 complex and found the energy-transfer from ligands to center Nd3+ ion is possible and the characteristic

Table 2 Calculated radiative lifetime, the radiative transition rate, and the radiative quantum efficiency for the emission from 4F3/2 to the lower manifolds. Manifold

k (nm)

Aed (s1)

sR (ls)

ge (%)

4

1333 1053 886

546.4 2674 1578

208.5

1.06

I13/2 4 I11/2 4 I9/2

Fig. 4. EL spectra of device B at operating voltages of 12 V and 16 V.

emission from Nd3+ ion is detected at 890 nm 1060 nm and 1336 nm. Judd–Ofelt theory calculation has been introduced successfully to obtain the radiative lifetime from the 4F9/2 manifold of 208.5 ls. The time-resolved luminescence spectrum shows monoexponential decay with a lifetime of 2.2 ls for Nd3+ ion. By using the Nd complex as the emitting layer, the NIR emission device with the structure of ITO/NPB 20 nm/Nd(PM)3(TP)2 50 nm/ BCP 20/AlQ 40/LiF 1 nm/Al 120 nm shows a strong NIR irradiance of 2.1 mW/m2 at 16.5 V. To the best of our knowledge, this is one of the best values reported of NIR OLEDs based on Nd (III) complexes as emitters. Acknowledgement The authors are grateful to the financial aid from the National Natural Science Foundation of China (Grant Nos. 20631040, and 20771099) and the MOST of China (Grant Nos. 2006CB601103, 2006DFA42610). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.inoche.2008.12.001. References [1] K. Kuriki, Y. Koike, Y. Okamoto, Chem. Rev. 102 (2002) 2347. [2] L.H. Slooff, A. Polman, F. Cacialli, R.H. Friend, G.A. Hebbink, F.C.J.M. van Veggel, D.N. Reinhoud, Appl. Phys. Lett. 78 (2001) 2122. [3] S.I. Klink, L. Grave, D.N. Reinhoudt, Frank C.J.M. van Veggel, J. Phys. Chem. A 104 (2000) 5457. [4] S.I. Klink, G.A. Hebbink, L. Grave, F.C.J.M. van Veggel, D.N. Reinhoudt, L.H. Slooff, A. Polman, J.W. Hofstraat, J. Appl. Phys. 86 (1999) 1181. [5] G. Mancino, A.J. Ferguson, A. Beeby, N.J. Long, T.S. Jones, J. Am. Chem. Soc. 127 (2005) 524. [6] V.P. Gapontsev, A.A. Izyneev, Sov. J. Quantum Electron. 11 (1981) 1101. [7] Zhefeng Li, Liang Zhou, Jiangbo Yu, Hongjie Zhang, Ruiping Deng, Zeping Peng, Zhiyong Guo, J. Phys. Chem. C 111 (2007) 2295–2300. [8] Zhefeng Li, Jiangbo Yu, Liang Zhou, Hongjie Zhang, Ruiping Deng, Zhiyong Guo, Org. Electron. 9 (2008) 487–497. [9] (a) Joo Han Kim, Paul H. Holloway, Appl. Phys. Lett. 85 (2004) 1689; (b) O.M. Khreis, R.J. Curry, M. Somerton, W.P. Gillin, J. Appl. Phys. 88 (2000) 777. [10] W.T. Carnall, P.R. Fields, K. Rajnal, J. Chem. Phys. 49 (1968) 4424. [11] D.L. Dexter, J. Chem. Phys. 21 (1953) 836. [12] B.R. Judd, Phys. Rev. 127 (1962) 750. [13] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. [14] Guohua Jia, Chaoyang Tu, Jianfu Li, Zhenyu You, Inorg. Chem. 45 (2006) 9326– 9331. [15] G.F. Wang, W.Z. Chen, Z.B. Li, Z.S. Hu, Phys. Rev. B 60 (1999) 15469. [16] X.Y. Chen, M.P. Jensen, G.K. Liu, J. Phys. Chem. B 109 (2005) 13991–13999. [17] M.J. Weber, Phys. Rev. 171 (1968) 283.