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Electronic absorption and emission spectra of Alq3 in solution with special attention to a delayed fluorescence
ARTICLE IN PRESS
Journal of Luminescence 128 (2008) 1353–1358 www.elsevier.com/locate/jlumin
Electronic absorption and emission spectra of Alq3 in solution with special attention to a delayed fluorescence Toshihiko Hoshi, Ken-ichi Kumagai, Keita Inoue, Shigendo Enomoto, Yoko Nobe, Michio Kobayashi Department of Chemistry and Biological Science, College of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara-shi, Kanagawa-ken 229-8558, Japan Received 24 August 2007; received in revised form 6 December 2007; accepted 4 January 2008 Available online 10 January 2008
Abstract Tris(8-quinolinolato)aluminum(III) (Alq3) shows electronic absorption bands at 378, 360 (in a 1:1 mixed solvent of methanol and ethanol (ME) at 77 K), 334, 316, 300, 263, 255.8, and 233 nm in ethanol at room temperature. According to the polarized fluorescence excitation spectrum together with MO calculations, for instance, the 360 nm band is assigned to an LL CT transition (an intramolecular charge transfer transition between two ligands), and the 378 nm band to an LM/ML CT one (an intramolecular charge transfer transition between ligand and metal). Alq3 shows a broad fluorescence band peaking at around 478 nm in the ME matrix at 77 K. The emission spectrum measured with a phosphoroscope has two emission bands at 567 and 478 nm. The 567 nm band accompanies vibronic bands at 578 and 605 nm, being safely assigned to a phosphorescence of Alq3. The lifetimes of the 478 and 567 nm bands are both 5.4 ms. The lifetime of the 478 nm band together with the band position and its band shape indicate that this band can be assigned to a delayed fluorescence. r 2008 Elsevier B.V. All rights reserved. PACS: 31.15; 33.20; 33.50; 78.55 Keywords: Metal complex; Electronic absorption spectra; Delayed fluorescence; Phosphorescence; MO calculation
1. Introduction It is well known that tris(8-quinolinolato)aluminum(III) (abbreviated to Alq3) shows electroluminescence (EL) with a relatively high quantum yield [1,2]. From this viewpoint, Alq3 has received much attention by many investigators, and many papers have been published on its EL emission [3–7]. It is believed or often said that an EL emission is caused by the encounter of positive holes (cation radicals M+d) and negatively charged species (anion radicals Md) in the emission layer, producing electronically exited states of emitting molecules. It seems, however, that this interpretation is not a unique one. That is, the electronically excited molecule might be produced via other mechanisms, e.g., there are possibilities that an encounter Corresponding author. Tel.: +81 42 759 6415; fax: +81 42 759 6493.
E-mail address: [email protected] (T. Hoshi). 0022-2313/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2008.01.003
of Md (or M+d) and a neutral molecule M also gives rise to a light-emitting excited molecule M*. In the latter interpretation, the interaction between Md (or M+d) and nearest neighbor M enables an electron transfer from the highest doubly occupied molecular orbital (HOMO) of Md (or M) to LUMO of M (or M+d) to give M*+Md (or M*+M+d) getting the excitation energy from electric current. In the former interpretation, though encounter probabilities of M+d and Md are considered to be very small, an electron transfer from singly occupied MO (SOMO) of Md to LUMO (SOMO+1) of M+d is considered to occur without an additional energy if the SOMO level of Md is higher than the LUMO level of M+d. To know a more precise mechanism of EL emission, it is very important to investigate the electronic properties of the molecules showing EL. Tang and Vanslyke [1] constructed an EL diode using Alq3 as an emitting layer. This EL diode emits a green light
ARTICLE IN PRESS T. Hoshi et al. / Journal of Luminescence 128 (2008) 1353–1358
(550 nm). Recently, Co¨lle and Ga¨rditz [8] measured an emission spectrum of Alq3 based on organic light-emitting diode, and they found two kinds of emission bands at around 550 and 700 nm. Furthermore, they measured lifetimes for the EL emissions at 520 and 730 nm at various temperatures. The lifetimes measured at 520 and 730 nm are 2.8 and 5.6 ms at 80 K, respectively. From these lifetime data, Co¨lle et al. assigned the 520 and 730 nm EL bands to a delayed fluorescence and phosphorescence, respectively. It is found that the delayed fluorescence is caused by a triplet–triplet annihilation, since the lifetime of the delayed fluorescence is shorter by a factor of two than that of the phosphorescence. The emission properties of Alq3 based on optical excitations have also been investigated by Co¨lle and Ga¨rditz [9] using evaporated polycrystalline and amorphous films. They have found two kinds of emission bands at about 500 and 700 nm corresponding to the 550 and 700 nm EL bands, respectively. The 700 nm phosphorescence band is accompanied by a vibrational structure from which the 0–0 transitions of the phosphorescence emissions for mer- and fac-Alq3 have been determined to be at 588 and 574 nm, respectively. As described above, many papers have been published concerning EL, fluorescence, and phosphorescence spectra of the evaporated films of Alq3. However, there are few papers on Alq3 studied in solutions. It is very interesting to know the electronic properties of Alq3 in monomolecularly dispersed systems such as in solutions or matrices. Thus in this investigation, electronic absorption, emission, and polarized excitation spectra have been measured for Alq3 in solutions and matrices at low temperatures. Moreover, measurements of emission lifetimes and MO calculations have been performed. 2. Experimental Commercially available Alq3 (Wako, S grade) was recrystallized by a solvent diffusion method in a mixed solvent of ethanol and chloroform. The sample Alq3 thus obtained has been confirmed to be a meridional isomer from the single-crystal X-ray analysis [10]. Ethanol (Wako, S grade), acetonitrile (Wako, Sp grade), and N, Ndimethylformamide (abbreviated to DMF, Wako, Sp grade) were used as received. Methanol (Wako, S grade) was used after distillation. Electronic absorption spectra were measured with a Shimadzu UV-3100PC spectrophotometer. Fluorescence spectra were recorded on a Hitachi F-3010 fluorophotometer, with which a phosphoroscope attachment was equipped for the measurements of phosphorescence and delayed fluorescence spectra together with emission lifetimes. 3. MO calculation To date, many kinds of MO methods have been contrived. However, it seems that there are no methods,
exceeding the PPP method, to interpret the nature of electronic transitions of organic molecules and metal complexes with organic ligands. Thus, in the present MO calculation, an extended PPP method has been adopted [11]. Two center electron repulsion and resonance integrals were computed by the Nishimoto-Mataga [12] and Nishimoto-Forster [13,14] equations, respectively. The valence state ionization energies (Ip(r)) and electron affinities (Ea(r)) for carbon, aza-nitrogen and oxido ion used are Ip(C)=11.22 eV, Ip(–N=)=14.16 eV, Ip(O)=25.6 eV, Ea(C)=0.62 eV, Ea(–N=)=1.67 eV, and Ea(O)=10.5 eV [11,15,16]. In CI calculations, 64 singly electronic excited configurations among the highesteight-occupied and the lowest eight unoccupied orbitals were taken into account. The effects of the central Al3+ ion on the p electronic systems of the ligands are treated as a point charge (0.4 e) [11]. 4. Results and discussion Many papers have been published on the electronic absorption, optically excited emission, and electroluminescence properties of Alq3. It seems, however, that few papers have been found on the complete assignment of electronic transitions for Alq3. Thus, electronic absorption spectra of Alq3 have been measured in methanol, ethanol, acetonitrile, and DMF (Fig. 1). Alq3, for instance, shows strong electronic absorption bands at 378 and 255.8 nm in ethanol, the former band accompanying weak shoulders at 334, 316, and 300 nm and the latter at 233 nm. It is interesting to note that the weak bands at 334, 316, and 300 nm remain unshifted irrespective of solvent polarities, but strong first band is largely shifted to the lower energy side roughly depending on the solvent polarities. This means that the former three weak bands can be assigned to the different electronic transitions from that of the main 1.0 methanol ethanol 0.8
acetonitrile DMF
Absorbance
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0.6
0.4 X 10 0.2
0.0 200
250
300
350 400 Wavelength / nm
450
500
550
Fig. 1. The electronic absorption spectra of Alq3 in methanol, ethanol, acetonitrile, and DMF.
ARTICLE IN PRESS T. Hoshi et al. / Journal of Luminescence 128 (2008) 1353–1358
strong band. Fig. 2 shows the fluorescence spectrum of Alq3 measured in ME (a 1:1 mixed solvent of methanol and ethanol) along with the first electronic absorption band. As seen from this figure, the fluorescence spectrum peaks at 478 nm and the mirror image between absorption and fluorescence bands is only held for the 378 nm broad absorption band. This means that the weak shoulder bands at 334, 316, and 300 nm are due to different electronic transitions from the main broad absorption band in accordance with assignment obtained from the solvent effect experiments described above. In order to understand the nature of the electronic transitions, we measured a polarized fluorescence excitation spectrum of Alq3 in ME at 77 K by a photo-selection method (Fig. 3) [17,18]. In this figure, IJ and I? are fluorescence intensities measured with incident polarized light beams, whose polarizations are 1.6 Fluorescence Band Fluorescence Relative Intensity
Absorption Band
Absorbance
1.2
0.8
0.4
0.0 300
350
400 450 500 Wavelength / nm
550
600
Fig. 2. The electronic absorption and fluorescence spectra (excited at 388 nm) of Alq3 in ME at room temperature.
Monitored at 483 nm
I//
Polarization Degree (P)
Relative Intensity
P
I⊥
250
300
350 Wavelength / nm
400
450
Fig. 3. The polarized fluorescence excitation spectrum of Alq3 in ME at 77 K by the method of photo-selection monitored at 483 nm.
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parallel to and perpendicular to the polarization axis of the analyzer, respectively. The polarization degree P is defined as P=(IJI?)/(IJ+I?). The P values for the weak shoulder bands corresponding to the 334, 316, and 300 nm absorption bands in ME are obviously lower than those of the main band, further indicating that the weak three bands are due to different electronic transitions from that of the main band. In the polarization spectrum, an additional shoulder band (360 nm) is seen at the higher energy side of the main 385 nm band (corresponding to the 378 nm band in ethanol). The additional 360 nm band is considered to be a different electronic transition from that of the main band, since the P values for the shoulder bands are obviously different from those of the main 385 nm band. The P curve shows a minimum at the peak of the 266 nm band (corresponding to the 255.8 nm band in ethanol) and steeply heightens on both sides of this band peak. This indicates that at least two weak bands, having almost the same polarization nature with that of the 385 nm band, are hidden on both sides of the 266 nm band. In fact, a weak shoulder (or peak) band is found at 233 nm (Fig. 1). Moreover, enlarging the spectrum, a weak shoulder is obviously recognized in each solvent, e.g., at 263 nm in ethanol. The first electronic absorption band (378 nm) of Alq3 was interpreted as corresponding to the first pp transition of the qH+ 2 (protonated molecular species of 8-quinolinol) from the following two reasons: (1) the lower energy band contour of Alq3 resembles well that of qH+ 2 and (2) each electronic band position of Alq3 is unchanged if the central metal ion of the complex is changed from Al3+ to Bi3+ [19]. However, there remain ambiguities in this assignment because the first bands of Ptq2 (bis(8-quinolinolato)platinum(II)) and Irq3 (tris(8-quinolinolato)iridium(III)) are largely red-shifted compared with those of Alq3 and Biq3 ((8-quinolinolato)bismuth(III)) [19]. As for the first electronic absorption band of Ptq2, Bartocci et al. [20] reported that this band should be assigned to dp or ML CT transition. According to the present polarization spectrum (Fig. 3), an additional electronic band is newly found in the higherenergy side (at 360 nm) of the intense first band (at 385 nm corresponding to the 378 nm band in ethanol). The above assignment [19] that the first band of Alq3 is due to a localized electronic transition on the 8-quinolinolato moiety cannot explain the presence of the newly found 360 nm band. To know the detailed information on the electronic transitions of Alq3, an extended PPP calculation was performed, being extended to be suitable for metal complexes. The calculated transition energies, intensities, and polarizations are shown in Table 1 in comparison with observations. The observed four weak bands at 360 (a new band found from the polarization spectrum), 334, 316, and 300 nm are assigned to the calculated pp1 , pp2 , pp3 , and pp6 transitions, respectively, pp4 and pp5 transitions being too weak to be assigned to the observed bands. The 316 nm band was assigned to a vibronic band originating at the
ARTICLE IN PRESS T. Hoshi et al. / Journal of Luminescence 128 (2008) 1353–1358
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Table 1 Comparison of the calculated- and observed-transition energies, intensities and polarizations for AlQ3 pp Transition Transitions energy (nm) Calcd.
Intensity
Obsd.a Calcd.b
e
378 337.8 360f 335.1 334 324.2 316 322.7 319.9 319.1 300 285.0 280.8 273.5 272.3 265.8 256.5 251.8 o 263 247.5 255.8 241.6 232.1 233 231.9
pp1 pp2 pp3 pp4 pp5 pp6 pp7 pp8 pp9 pp10 pp11 pp12 pp13 pp14 pp15 pp16 pp17
Polarization Obsd.c Calcd. (degree)
Obsd.d
y
j
4 70 66 66 6 75 55 69 34 33 51 52 3 75 65 30 55
105 71 103 93 93 108 99 100 98 98 169 144 107 134 144 88 66
e
0.10 0.347 — 0.235 0.057 0.040 0.052 0.001 0.001 0.015 0.042 0.000 0.001 0.001 0.000 0.001 0.000 0.169 o 0.5 1.281 1.0 2.467 0.100 0.3 0.050
0.01 0.07 0.15 0.20
0.21
O-
0.23 0.35 0.20
N
y
N Al 3+ N
C3 ¼ 0:5322w18;21 0:5067w16;19 þ 0:3925w17;20 þ C6 ¼ 0:6009w18;24 þ 0:5870w15;21 þ
z ϕ
C1 ¼ 0:6630w17;20 þ 0:4463w17;19 0:4216w16;20 0:2913w16;19 þ C2 ¼ 0:6919w18;21 0:4817w16;19 þ 0:3055w17;20 þ
In this calculation, the effect of Al3+ on the p electronic systems of the ligands is regarded as a point charge (0.4e). a Observed in ethanol. b Oscillator strength. c Observed relative intensity with respect to the 255.8 nm band. d Apparent P values. e ML/LM CT transition. f Observed in ME at 77 K z
O-
respectively. No calculated transition exists corresponding to the observed 378 nm band, suggesting that the 378 nm band may be assigned to an LM/ML CT transition (an intramolecular charge transfer transition between the ligand and the metal) mixed with pp transitions localized on the 8-quinolinolato skeletons. The total wavefunctions corresponding to the lowest four allowed singlet pp transitions, except almost accidentally forbidden ones, are shown as
y
O-
Here, wi,j means a configuration wavefunction for one electron excitation from the ith occupied MO to the jth unoccupied MO. From the above total wavefunction together with MO’s represented in Fig. 4, it is found that w17,19 and w16,20 are the configuration wavefunctions corresponding to the intramolecular charge transfer excitations between two ligands. These configuration wavefunctions contribute to C1 (the first pp transition) corresponding to the observed 360 nm band by 40%. Therefore, the 360 nm band is considered to be based on LL CT transitions (intramolecular charge transfer transitions between two ligands) mixed with a localized transition on the 8-quinolinolato skeleton. This means that the p electronic systems localized on 8-quinolinolato skeletons are considerably interacting. The observed 334, 316, and 300 nm bands can be interpreted as being due to localized transitions on 8-quinolinolato skeletons. As for the measurements of the phosphorescence spectrum of Alq3 in solution or low-temperature matrix, a few attempts have been performed, but the emission was not recognized except for the case due to Burrows et al. [22]. That is, they succeeded to measure the phosphorescence spectrum of Alq3 using an external heavy atom effect
θ x
Occupied MO
x
φ19
φ15
334 nm band [21]. However, according to the present polarization spectrum, the P values for the 334 nm band are clearly different from those of the 316 nm band, indicating that the 316 nm band is not vibronic but pure electronic transition band. This assignment is in agreement with the facts that vibronic bands are difficult to appear in metal complexes. Since the calculated pp14 and pp15 transitions are polarized along nearly the same directions and have almost same transition energies, the 255.8 nm band may be assigned to the overlap of the two transitions pp14 and pp15 . The observed 263 and 233 nm bands are safely assigned to the calculated pp13 and pp16 transitions,
Unoccupied MO
φ16
φ17
φ21 φ18
φ20
φ24
Fig. 4. Diagrammatic representation of MOs of Alq3.
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Relative Intensity
T. Hoshi et al. / Journal of Luminescence 128 (2008) 1353–1358
400
450
500
550 600 Wavelength / nm
650
700
Fig. 5. The emission spectrum (excited at 390 nm) of Alq3 measured with a phosphoroscope in ME at 77 K.
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the 478 nm emission band is due to a delayed fluorescence. Inspecting in detail, the phosphorescence spectrum measured by Burrows et al., an additional weak emission band is recognized at around 500 nm. This weak emission band may correspond to the delayed fluorescence [22]. If the delayed fluorescence occurs based on a triplet–triplet annihilation, the lifetime of the delayed fluorescence should take a half value of the phosphorescence lifetime [8]. It is, therefore, considered that the present delayed fluorescence is not derived from a triplet–triplet annihilation process but from another one such as the fluorescence from thermally populated S1 state. Since the molecules of Alq3 are considered to be dispersed monomolecularly in ME at 77 K, triplet–triplet annihilation hardly occurs in the present condition. As seen from Fig. 5, the fluorescence and phosphorescence bands are overlapped significantly. This suggests the possibility of a thermal process, i.e., the separation between S1 and T1 levels may be small enough to enable the thermal process. 5. Concluding remarks
(in an ethyl iodide glass). However, the intensity of the phosphorescence is too weak to resolve the vibrational structure. An emission spectrum of Alq3 has been measured in ME at 77 K using a phosphoroscope (Fig. 5). As seen from this figure, using ME as a matrix, the phosphorescence of Alq3 has been easily measured without an external heavy atom effect. The present phosphorescence spectrum is composed of two bands originating and peaking at 567 and 478 nm, respectively. The 567 nm 0–0 band is accompanied by two vibronic bands at 605 and 578 nm. Generally, S0’T1 transitions are forbidden, and these transitions are considered to occur through spin-orbit coupling along with vibronic mixings. Thus, most phosphorescence bands are accompanied by vibrational structures. The appearance of the band structure together with the band position suggests that the 567 nm emission band is safely assigned to a phosphorescence of Alq3. The 478 nm emission band is located at the same position as that of the prompt fluorescence band and is very similar to the fluorescence band in shape. This suggests that the 478 nm band is due to a delayed fluorescence. To confirm this assignment, the emission lifetimes for both bands have been measured in ME at 77 K, and the lifetime for the phosphorescence band (measured at 610 nm) has been determined to be 5.4 ms. This lifetime has almost the same value as that (5.6 ms) observed in an evaporated film at 80 K [8]. The lifetime measured at 478 nm is in accordance with that of the phosphorescence lifetime within experimental error, confirming that the 478 nm emission band is assigned to a delayed fluorescence. Furthermore, the excitation spectra monitored at the delayed fluorescence and phosphorescence band positions have reproduced well the absorption spectrum and excitation spectrum monitored at the prompt fluorescence band position. These experimental results are further evidence for the fact that
Fluorescence excitation spectrum of Alq3 measured at 77 K in ME shows an extra shoulder band at the higher energy side (360 nm) of the intense 378 nm LM/ML CT band. Since the extra 360 nm band is differently polarized from the main band at 378 nm, this band has been assigned to a different electronic transition from the 378 nm band. The MO method used in this investigation can explain the nature of pp* transitions localized on the ligand together with the LL CT transitions, but LM/ML CT transitions cannot be computed explicitly. Though many kinds of MO methods have been proposed, few methods exist to reproduce well the electronic spectra of metal complexes. It is, therefore, necessary to develop a new method, which can explain the electronic properties of metal complexes. A computer programming for the new MO method is now in progress, which can be reproducible electronic transitions of metal complexes including LM/ML CT transitions. In this method, interactions between d orbitals of the central metal ion and p orbitals of the ligand will be taken into account explicitly along with configuration interactions. References [1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] C.W. Tang, S.A. VanSlyke, C.H. Chen, J. Appl. Phys. 65 (1989) 3610. [3] S.A. Van Slyke, C.H. Chen, C.W. Tang, Appl. Phys. Lett. 69 (1996) 2160. [4] E.-M. Han, L.-M. Do, N. Yamamoto, M. Fujihira, Thin Solid Films 273 (1996) 202. [5] F. Papadimitrakopoulos, X.-M. Zhang, Synth. Meth. 85 (1997) 1221. [6] M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature 395 (1998) 151. [7] S.C. Kim, S.N. Kwon, M.-W. Choi, C.N. Whang, K. Jeong, S.H. Lee, J.-G. Lee, S. Kim, Appl. Phys. Lett. 79 (2001) 3726. [8] M. Co¨lle, C. Ga¨rditz, Appl. Phys. Lett. 84 (2004) 3160. [9] M. Co¨lle, C. Ga¨rditz, M. Braun, J. Appl. Phys. 96 (2004) 6133.
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[10] A. Nakao, T. Hoshi, M. Hasegawa, K. Kumagai, K. Inoue, unpublished data. [11] T. Hoshi, Y. Inomaki, M. Wada, Y. Yamada, J. Okubo, M. Kobayashi, H. Inoue, Ber. Bunsenges. Phys. Chem. 98 (1994) 585. [12] K. Nishimoto, N. Mataga, Z. Phys. Chem. (Neue Folge) 12 (1957) 335. [13] K. Nishimoto, L.S. Forster, Theor. Chim. Acta 3 (1965) 407. [14] K. Nishimoto, L.S. Forster, Theor. Chim. Acta 4 (1966) 155. [15] H.A. Skinner, H.O. Pritchard, Trans. Fraday Soc. 49 (1953) 1254. [16] T. Hoshi, J. Yoshino, K. Hayashi, Z. Phys. Chem. (Neue Folge) 83 (1973) 31.
[17] F. Do¨rr, M. Held, Angew. Chem. 72 (1960) 287. [18] H. Zimmermann, M. Joop, Z. Elektrochem. 64 (1960) 1215. [19] R. Ballardini, G. Varani, M.T. Indelli, F. Scandola, Inorg. Chem. 25 (1986) 3858. [20] C. Bartocci, S. Sostero, O. Traverso, A. Cox, T.J. Keme, W.J. Reed, Faraday Tras. 1 (76) (1980) 797. [21] V.V.N. Ravi Kishore, A. Aziz, K.L. Narasimhan, N. Periasamy, P.S. Meenakshi, S. Wategaonkar, Synth. Meth. 126 (2002) 199. [22] H.D. Burrows, M. Fernandes, J. Seixas de Melo, A.P. Monkman, S. Navaratnam, J. Am. Chem. Soc. 125 (2003) 15310.
Journal of Luminescence 128 (2008) 1353–1358 www.elsevier.com/locate/jlumin
Electronic absorption and emission spectra of Alq3 in solution with special attention to a delayed fluorescence Toshihiko Hoshi, Ken-ichi Kumagai, Keita Inoue, Shigendo Enomoto, Yoko Nobe, Michio Kobayashi Department of Chemistry and Biological Science, College of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara-shi, Kanagawa-ken 229-8558, Japan Received 24 August 2007; received in revised form 6 December 2007; accepted 4 January 2008 Available online 10 January 2008
Abstract Tris(8-quinolinolato)aluminum(III) (Alq3) shows electronic absorption bands at 378, 360 (in a 1:1 mixed solvent of methanol and ethanol (ME) at 77 K), 334, 316, 300, 263, 255.8, and 233 nm in ethanol at room temperature. According to the polarized fluorescence excitation spectrum together with MO calculations, for instance, the 360 nm band is assigned to an LL CT transition (an intramolecular charge transfer transition between two ligands), and the 378 nm band to an LM/ML CT one (an intramolecular charge transfer transition between ligand and metal). Alq3 shows a broad fluorescence band peaking at around 478 nm in the ME matrix at 77 K. The emission spectrum measured with a phosphoroscope has two emission bands at 567 and 478 nm. The 567 nm band accompanies vibronic bands at 578 and 605 nm, being safely assigned to a phosphorescence of Alq3. The lifetimes of the 478 and 567 nm bands are both 5.4 ms. The lifetime of the 478 nm band together with the band position and its band shape indicate that this band can be assigned to a delayed fluorescence. r 2008 Elsevier B.V. All rights reserved. PACS: 31.15; 33.20; 33.50; 78.55 Keywords: Metal complex; Electronic absorption spectra; Delayed fluorescence; Phosphorescence; MO calculation
1. Introduction It is well known that tris(8-quinolinolato)aluminum(III) (abbreviated to Alq3) shows electroluminescence (EL) with a relatively high quantum yield [1,2]. From this viewpoint, Alq3 has received much attention by many investigators, and many papers have been published on its EL emission [3–7]. It is believed or often said that an EL emission is caused by the encounter of positive holes (cation radicals M+d) and negatively charged species (anion radicals Md) in the emission layer, producing electronically exited states of emitting molecules. It seems, however, that this interpretation is not a unique one. That is, the electronically excited molecule might be produced via other mechanisms, e.g., there are possibilities that an encounter Corresponding author. Tel.: +81 42 759 6415; fax: +81 42 759 6493.
E-mail address: [email protected] (T. Hoshi). 0022-2313/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2008.01.003
of Md (or M+d) and a neutral molecule M also gives rise to a light-emitting excited molecule M*. In the latter interpretation, the interaction between Md (or M+d) and nearest neighbor M enables an electron transfer from the highest doubly occupied molecular orbital (HOMO) of Md (or M) to LUMO of M (or M+d) to give M*+Md (or M*+M+d) getting the excitation energy from electric current. In the former interpretation, though encounter probabilities of M+d and Md are considered to be very small, an electron transfer from singly occupied MO (SOMO) of Md to LUMO (SOMO+1) of M+d is considered to occur without an additional energy if the SOMO level of Md is higher than the LUMO level of M+d. To know a more precise mechanism of EL emission, it is very important to investigate the electronic properties of the molecules showing EL. Tang and Vanslyke [1] constructed an EL diode using Alq3 as an emitting layer. This EL diode emits a green light
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(550 nm). Recently, Co¨lle and Ga¨rditz [8] measured an emission spectrum of Alq3 based on organic light-emitting diode, and they found two kinds of emission bands at around 550 and 700 nm. Furthermore, they measured lifetimes for the EL emissions at 520 and 730 nm at various temperatures. The lifetimes measured at 520 and 730 nm are 2.8 and 5.6 ms at 80 K, respectively. From these lifetime data, Co¨lle et al. assigned the 520 and 730 nm EL bands to a delayed fluorescence and phosphorescence, respectively. It is found that the delayed fluorescence is caused by a triplet–triplet annihilation, since the lifetime of the delayed fluorescence is shorter by a factor of two than that of the phosphorescence. The emission properties of Alq3 based on optical excitations have also been investigated by Co¨lle and Ga¨rditz [9] using evaporated polycrystalline and amorphous films. They have found two kinds of emission bands at about 500 and 700 nm corresponding to the 550 and 700 nm EL bands, respectively. The 700 nm phosphorescence band is accompanied by a vibrational structure from which the 0–0 transitions of the phosphorescence emissions for mer- and fac-Alq3 have been determined to be at 588 and 574 nm, respectively. As described above, many papers have been published concerning EL, fluorescence, and phosphorescence spectra of the evaporated films of Alq3. However, there are few papers on Alq3 studied in solutions. It is very interesting to know the electronic properties of Alq3 in monomolecularly dispersed systems such as in solutions or matrices. Thus in this investigation, electronic absorption, emission, and polarized excitation spectra have been measured for Alq3 in solutions and matrices at low temperatures. Moreover, measurements of emission lifetimes and MO calculations have been performed. 2. Experimental Commercially available Alq3 (Wako, S grade) was recrystallized by a solvent diffusion method in a mixed solvent of ethanol and chloroform. The sample Alq3 thus obtained has been confirmed to be a meridional isomer from the single-crystal X-ray analysis [10]. Ethanol (Wako, S grade), acetonitrile (Wako, Sp grade), and N, Ndimethylformamide (abbreviated to DMF, Wako, Sp grade) were used as received. Methanol (Wako, S grade) was used after distillation. Electronic absorption spectra were measured with a Shimadzu UV-3100PC spectrophotometer. Fluorescence spectra were recorded on a Hitachi F-3010 fluorophotometer, with which a phosphoroscope attachment was equipped for the measurements of phosphorescence and delayed fluorescence spectra together with emission lifetimes. 3. MO calculation To date, many kinds of MO methods have been contrived. However, it seems that there are no methods,
exceeding the PPP method, to interpret the nature of electronic transitions of organic molecules and metal complexes with organic ligands. Thus, in the present MO calculation, an extended PPP method has been adopted [11]. Two center electron repulsion and resonance integrals were computed by the Nishimoto-Mataga [12] and Nishimoto-Forster [13,14] equations, respectively. The valence state ionization energies (Ip(r)) and electron affinities (Ea(r)) for carbon, aza-nitrogen and oxido ion used are Ip(C)=11.22 eV, Ip(–N=)=14.16 eV, Ip(O)=25.6 eV, Ea(C)=0.62 eV, Ea(–N=)=1.67 eV, and Ea(O)=10.5 eV [11,15,16]. In CI calculations, 64 singly electronic excited configurations among the highesteight-occupied and the lowest eight unoccupied orbitals were taken into account. The effects of the central Al3+ ion on the p electronic systems of the ligands are treated as a point charge (0.4 e) [11]. 4. Results and discussion Many papers have been published on the electronic absorption, optically excited emission, and electroluminescence properties of Alq3. It seems, however, that few papers have been found on the complete assignment of electronic transitions for Alq3. Thus, electronic absorption spectra of Alq3 have been measured in methanol, ethanol, acetonitrile, and DMF (Fig. 1). Alq3, for instance, shows strong electronic absorption bands at 378 and 255.8 nm in ethanol, the former band accompanying weak shoulders at 334, 316, and 300 nm and the latter at 233 nm. It is interesting to note that the weak bands at 334, 316, and 300 nm remain unshifted irrespective of solvent polarities, but strong first band is largely shifted to the lower energy side roughly depending on the solvent polarities. This means that the former three weak bands can be assigned to the different electronic transitions from that of the main 1.0 methanol ethanol 0.8
acetonitrile DMF
Absorbance
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0.6
0.4 X 10 0.2
0.0 200
250
300
350 400 Wavelength / nm
450
500
550
Fig. 1. The electronic absorption spectra of Alq3 in methanol, ethanol, acetonitrile, and DMF.
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strong band. Fig. 2 shows the fluorescence spectrum of Alq3 measured in ME (a 1:1 mixed solvent of methanol and ethanol) along with the first electronic absorption band. As seen from this figure, the fluorescence spectrum peaks at 478 nm and the mirror image between absorption and fluorescence bands is only held for the 378 nm broad absorption band. This means that the weak shoulder bands at 334, 316, and 300 nm are due to different electronic transitions from the main broad absorption band in accordance with assignment obtained from the solvent effect experiments described above. In order to understand the nature of the electronic transitions, we measured a polarized fluorescence excitation spectrum of Alq3 in ME at 77 K by a photo-selection method (Fig. 3) [17,18]. In this figure, IJ and I? are fluorescence intensities measured with incident polarized light beams, whose polarizations are 1.6 Fluorescence Band Fluorescence Relative Intensity
Absorption Band
Absorbance
1.2
0.8
0.4
0.0 300
350
400 450 500 Wavelength / nm
550
600
Fig. 2. The electronic absorption and fluorescence spectra (excited at 388 nm) of Alq3 in ME at room temperature.
Monitored at 483 nm
I//
Polarization Degree (P)
Relative Intensity
P
I⊥
250
300
350 Wavelength / nm
400
450
Fig. 3. The polarized fluorescence excitation spectrum of Alq3 in ME at 77 K by the method of photo-selection monitored at 483 nm.
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parallel to and perpendicular to the polarization axis of the analyzer, respectively. The polarization degree P is defined as P=(IJI?)/(IJ+I?). The P values for the weak shoulder bands corresponding to the 334, 316, and 300 nm absorption bands in ME are obviously lower than those of the main band, further indicating that the weak three bands are due to different electronic transitions from that of the main band. In the polarization spectrum, an additional shoulder band (360 nm) is seen at the higher energy side of the main 385 nm band (corresponding to the 378 nm band in ethanol). The additional 360 nm band is considered to be a different electronic transition from that of the main band, since the P values for the shoulder bands are obviously different from those of the main 385 nm band. The P curve shows a minimum at the peak of the 266 nm band (corresponding to the 255.8 nm band in ethanol) and steeply heightens on both sides of this band peak. This indicates that at least two weak bands, having almost the same polarization nature with that of the 385 nm band, are hidden on both sides of the 266 nm band. In fact, a weak shoulder (or peak) band is found at 233 nm (Fig. 1). Moreover, enlarging the spectrum, a weak shoulder is obviously recognized in each solvent, e.g., at 263 nm in ethanol. The first electronic absorption band (378 nm) of Alq3 was interpreted as corresponding to the first pp transition of the qH+ 2 (protonated molecular species of 8-quinolinol) from the following two reasons: (1) the lower energy band contour of Alq3 resembles well that of qH+ 2 and (2) each electronic band position of Alq3 is unchanged if the central metal ion of the complex is changed from Al3+ to Bi3+ [19]. However, there remain ambiguities in this assignment because the first bands of Ptq2 (bis(8-quinolinolato)platinum(II)) and Irq3 (tris(8-quinolinolato)iridium(III)) are largely red-shifted compared with those of Alq3 and Biq3 ((8-quinolinolato)bismuth(III)) [19]. As for the first electronic absorption band of Ptq2, Bartocci et al. [20] reported that this band should be assigned to dp or ML CT transition. According to the present polarization spectrum (Fig. 3), an additional electronic band is newly found in the higherenergy side (at 360 nm) of the intense first band (at 385 nm corresponding to the 378 nm band in ethanol). The above assignment [19] that the first band of Alq3 is due to a localized electronic transition on the 8-quinolinolato moiety cannot explain the presence of the newly found 360 nm band. To know the detailed information on the electronic transitions of Alq3, an extended PPP calculation was performed, being extended to be suitable for metal complexes. The calculated transition energies, intensities, and polarizations are shown in Table 1 in comparison with observations. The observed four weak bands at 360 (a new band found from the polarization spectrum), 334, 316, and 300 nm are assigned to the calculated pp1 , pp2 , pp3 , and pp6 transitions, respectively, pp4 and pp5 transitions being too weak to be assigned to the observed bands. The 316 nm band was assigned to a vibronic band originating at the
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Table 1 Comparison of the calculated- and observed-transition energies, intensities and polarizations for AlQ3 pp Transition Transitions energy (nm) Calcd.
Intensity
Obsd.a Calcd.b
e
378 337.8 360f 335.1 334 324.2 316 322.7 319.9 319.1 300 285.0 280.8 273.5 272.3 265.8 256.5 251.8 o 263 247.5 255.8 241.6 232.1 233 231.9
pp1 pp2 pp3 pp4 pp5 pp6 pp7 pp8 pp9 pp10 pp11 pp12 pp13 pp14 pp15 pp16 pp17
Polarization Obsd.c Calcd. (degree)
Obsd.d
y
j
4 70 66 66 6 75 55 69 34 33 51 52 3 75 65 30 55
105 71 103 93 93 108 99 100 98 98 169 144 107 134 144 88 66
e
0.10 0.347 — 0.235 0.057 0.040 0.052 0.001 0.001 0.015 0.042 0.000 0.001 0.001 0.000 0.001 0.000 0.169 o 0.5 1.281 1.0 2.467 0.100 0.3 0.050
0.01 0.07 0.15 0.20
0.21
O-
0.23 0.35 0.20
N
y
N Al 3+ N
C3 ¼ 0:5322w18;21 0:5067w16;19 þ 0:3925w17;20 þ C6 ¼ 0:6009w18;24 þ 0:5870w15;21 þ
z ϕ
C1 ¼ 0:6630w17;20 þ 0:4463w17;19 0:4216w16;20 0:2913w16;19 þ C2 ¼ 0:6919w18;21 0:4817w16;19 þ 0:3055w17;20 þ
In this calculation, the effect of Al3+ on the p electronic systems of the ligands is regarded as a point charge (0.4e). a Observed in ethanol. b Oscillator strength. c Observed relative intensity with respect to the 255.8 nm band. d Apparent P values. e ML/LM CT transition. f Observed in ME at 77 K z
O-
respectively. No calculated transition exists corresponding to the observed 378 nm band, suggesting that the 378 nm band may be assigned to an LM/ML CT transition (an intramolecular charge transfer transition between the ligand and the metal) mixed with pp transitions localized on the 8-quinolinolato skeletons. The total wavefunctions corresponding to the lowest four allowed singlet pp transitions, except almost accidentally forbidden ones, are shown as
y
O-
Here, wi,j means a configuration wavefunction for one electron excitation from the ith occupied MO to the jth unoccupied MO. From the above total wavefunction together with MO’s represented in Fig. 4, it is found that w17,19 and w16,20 are the configuration wavefunctions corresponding to the intramolecular charge transfer excitations between two ligands. These configuration wavefunctions contribute to C1 (the first pp transition) corresponding to the observed 360 nm band by 40%. Therefore, the 360 nm band is considered to be based on LL CT transitions (intramolecular charge transfer transitions between two ligands) mixed with a localized transition on the 8-quinolinolato skeleton. This means that the p electronic systems localized on 8-quinolinolato skeletons are considerably interacting. The observed 334, 316, and 300 nm bands can be interpreted as being due to localized transitions on 8-quinolinolato skeletons. As for the measurements of the phosphorescence spectrum of Alq3 in solution or low-temperature matrix, a few attempts have been performed, but the emission was not recognized except for the case due to Burrows et al. [22]. That is, they succeeded to measure the phosphorescence spectrum of Alq3 using an external heavy atom effect
θ x
Occupied MO
x
φ19
φ15
334 nm band [21]. However, according to the present polarization spectrum, the P values for the 334 nm band are clearly different from those of the 316 nm band, indicating that the 316 nm band is not vibronic but pure electronic transition band. This assignment is in agreement with the facts that vibronic bands are difficult to appear in metal complexes. Since the calculated pp14 and pp15 transitions are polarized along nearly the same directions and have almost same transition energies, the 255.8 nm band may be assigned to the overlap of the two transitions pp14 and pp15 . The observed 263 and 233 nm bands are safely assigned to the calculated pp13 and pp16 transitions,
Unoccupied MO
φ16
φ17
φ21 φ18
φ20
φ24
Fig. 4. Diagrammatic representation of MOs of Alq3.
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T. Hoshi et al. / Journal of Luminescence 128 (2008) 1353–1358
400
450
500
550 600 Wavelength / nm
650
700
Fig. 5. The emission spectrum (excited at 390 nm) of Alq3 measured with a phosphoroscope in ME at 77 K.
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the 478 nm emission band is due to a delayed fluorescence. Inspecting in detail, the phosphorescence spectrum measured by Burrows et al., an additional weak emission band is recognized at around 500 nm. This weak emission band may correspond to the delayed fluorescence [22]. If the delayed fluorescence occurs based on a triplet–triplet annihilation, the lifetime of the delayed fluorescence should take a half value of the phosphorescence lifetime [8]. It is, therefore, considered that the present delayed fluorescence is not derived from a triplet–triplet annihilation process but from another one such as the fluorescence from thermally populated S1 state. Since the molecules of Alq3 are considered to be dispersed monomolecularly in ME at 77 K, triplet–triplet annihilation hardly occurs in the present condition. As seen from Fig. 5, the fluorescence and phosphorescence bands are overlapped significantly. This suggests the possibility of a thermal process, i.e., the separation between S1 and T1 levels may be small enough to enable the thermal process. 5. Concluding remarks
(in an ethyl iodide glass). However, the intensity of the phosphorescence is too weak to resolve the vibrational structure. An emission spectrum of Alq3 has been measured in ME at 77 K using a phosphoroscope (Fig. 5). As seen from this figure, using ME as a matrix, the phosphorescence of Alq3 has been easily measured without an external heavy atom effect. The present phosphorescence spectrum is composed of two bands originating and peaking at 567 and 478 nm, respectively. The 567 nm 0–0 band is accompanied by two vibronic bands at 605 and 578 nm. Generally, S0’T1 transitions are forbidden, and these transitions are considered to occur through spin-orbit coupling along with vibronic mixings. Thus, most phosphorescence bands are accompanied by vibrational structures. The appearance of the band structure together with the band position suggests that the 567 nm emission band is safely assigned to a phosphorescence of Alq3. The 478 nm emission band is located at the same position as that of the prompt fluorescence band and is very similar to the fluorescence band in shape. This suggests that the 478 nm band is due to a delayed fluorescence. To confirm this assignment, the emission lifetimes for both bands have been measured in ME at 77 K, and the lifetime for the phosphorescence band (measured at 610 nm) has been determined to be 5.4 ms. This lifetime has almost the same value as that (5.6 ms) observed in an evaporated film at 80 K [8]. The lifetime measured at 478 nm is in accordance with that of the phosphorescence lifetime within experimental error, confirming that the 478 nm emission band is assigned to a delayed fluorescence. Furthermore, the excitation spectra monitored at the delayed fluorescence and phosphorescence band positions have reproduced well the absorption spectrum and excitation spectrum monitored at the prompt fluorescence band position. These experimental results are further evidence for the fact that
Fluorescence excitation spectrum of Alq3 measured at 77 K in ME shows an extra shoulder band at the higher energy side (360 nm) of the intense 378 nm LM/ML CT band. Since the extra 360 nm band is differently polarized from the main band at 378 nm, this band has been assigned to a different electronic transition from the 378 nm band. The MO method used in this investigation can explain the nature of pp* transitions localized on the ligand together with the LL CT transitions, but LM/ML CT transitions cannot be computed explicitly. Though many kinds of MO methods have been proposed, few methods exist to reproduce well the electronic spectra of metal complexes. It is, therefore, necessary to develop a new method, which can explain the electronic properties of metal complexes. A computer programming for the new MO method is now in progress, which can be reproducible electronic transitions of metal complexes including LM/ML CT transitions. In this method, interactions between d orbitals of the central metal ion and p orbitals of the ligand will be taken into account explicitly along with configuration interactions. References [1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] C.W. Tang, S.A. VanSlyke, C.H. Chen, J. Appl. Phys. 65 (1989) 3610. [3] S.A. Van Slyke, C.H. Chen, C.W. Tang, Appl. Phys. Lett. 69 (1996) 2160. [4] E.-M. Han, L.-M. Do, N. Yamamoto, M. Fujihira, Thin Solid Films 273 (1996) 202. [5] F. Papadimitrakopoulos, X.-M. Zhang, Synth. Meth. 85 (1997) 1221. [6] M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature 395 (1998) 151. [7] S.C. Kim, S.N. Kwon, M.-W. Choi, C.N. Whang, K. Jeong, S.H. Lee, J.-G. Lee, S. Kim, Appl. Phys. Lett. 79 (2001) 3726. [8] M. Co¨lle, C. Ga¨rditz, Appl. Phys. Lett. 84 (2004) 3160. [9] M. Co¨lle, C. Ga¨rditz, M. Braun, J. Appl. Phys. 96 (2004) 6133.
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[10] A. Nakao, T. Hoshi, M. Hasegawa, K. Kumagai, K. Inoue, unpublished data. [11] T. Hoshi, Y. Inomaki, M. Wada, Y. Yamada, J. Okubo, M. Kobayashi, H. Inoue, Ber. Bunsenges. Phys. Chem. 98 (1994) 585. [12] K. Nishimoto, N. Mataga, Z. Phys. Chem. (Neue Folge) 12 (1957) 335. [13] K. Nishimoto, L.S. Forster, Theor. Chim. Acta 3 (1965) 407. [14] K. Nishimoto, L.S. Forster, Theor. Chim. Acta 4 (1966) 155. [15] H.A. Skinner, H.O. Pritchard, Trans. Fraday Soc. 49 (1953) 1254. [16] T. Hoshi, J. Yoshino, K. Hayashi, Z. Phys. Chem. (Neue Folge) 83 (1973) 31.
[17] F. Do¨rr, M. Held, Angew. Chem. 72 (1960) 287. [18] H. Zimmermann, M. Joop, Z. Elektrochem. 64 (1960) 1215. [19] R. Ballardini, G. Varani, M.T. Indelli, F. Scandola, Inorg. Chem. 25 (1986) 3858. [20] C. Bartocci, S. Sostero, O. Traverso, A. Cox, T.J. Keme, W.J. Reed, Faraday Tras. 1 (76) (1980) 797. [21] V.V.N. Ravi Kishore, A. Aziz, K.L. Narasimhan, N. Periasamy, P.S. Meenakshi, S. Wategaonkar, Synth. Meth. 126 (2002) 199. [22] H.D. Burrows, M. Fernandes, J. Seixas de Melo, A.P. Monkman, S. Navaratnam, J. Am. Chem. Soc. 125 (2003) 15310.