J. Phys. Chem. B 2001, 105, 1547-1553
1547
Photoluminescence Properties of Magnesium, Chloroaluminum, Bromoaluminum, and Metal-Free Phthalocyanine Solid Films Youichi Sakakibara,* Raghu N. Bera, Toshiyuki Mizutani, Kohtaro Ishida, Madoka Tokumoto, and Toshiro Tani Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305-8568, Japan, and Faculty of Science and Technology, Science UniVersity of Tokyo, 2641 Yamazaki, Noda-shi, Chiba 278-8510, Japan, and Department of Applied Physics, Tokyo UniVersity of Agriculture and Technology, 2-24-16 Naka-machi, Koganei, Tokyo 184-8588, Japan ReceiVed: August 15, 2000; In Final Form: NoVember 21, 2000
Photoluminescence properties were measured for vacuum-deposited thin films of magnesium, chloroaluminum, bromoaluminum, and metal-free phthalocyanines (MgPc, AlClPc, AlBrPc, and H2Pc). For MgPc, AlClPc, and AlBrPc, besides the as-deposited films, phase-transformed films having red-shifted absorption bands at approximately 800 nm were also prepared by a vapor treatment of acetone, dichloromethane, and ethanol. Fluorescence quantum yields of the films at room temperature were 10-5-10-4. These values were much smaller than those of the corresponding monomers (>0.5), indicating that nonradiative relaxation is dominant in the solid films. Increase of the fluorescence intensity with decreasing temperatures was observed in all the samples, but the extent of the increase was at most as large as 10 times, even at the liquid helium temperature, indicating that nonradiative relaxation is still dominant. The spectral features were very different depending on the crystal phases and the materials. All the red-shifted films showed distinct emission bands located at 850-950 nm overlapped with the absorption edge. This feature was interpreted by the emission from the allowed lowest-lying exciton states. The as-deposited AlBrPc and H2Pc films showed broad emission bands at approximately 1000 and 900 nm, respectively, located away from the absorption edge. This feature was interpreted by the very weakly allowed transition from the forbidden lowest-lying exciton state to the vibronic sublevel of the ground state. In the as-deposited AlClPc film new emission bands appeared with decreasing temperature. From the corresponding change in the absorption spectrum, the appearance of the new bands was ascribed to the change of crystal packing.
1. Introduction
TABLE 1: Fluorescence Quantum Yields of Metal Phthalocyanines in Dilute Solution Reported So Far
Phthalocyanine (Pc) is a prominent class of organic materials with many optical and optoelectronic applications. For instance, the blue-coloring property of copper phthalocyanine (CuPc) has been widely used as excellent pigment.1 In this decade, because of the excellent photoconductive response at the near-infrared GaAsAl laser wavelength, titanyl phthalocyanine (TiOPc) in the Y-form crystal has acquired a leading status as a photoreceptor in laser printing systems.2-5 Moreover, some substituted phthalocyanine compounds are used as CD-R dyes.6 Besides these applications now in use, many optical functions have been investigated so far for potential applications such as photovoltaic cells,7 nonlinear optical devices,8 and photochemical holeburning memory.9 In addition, some recent reports10-12 on the electroluminescence (EL) of Pc compounds offer a new potential application as light-emitting materials. Most of the functions corresponding to the above-mentioned applications are generated through the excited states of solidphase phthalocyanines. Therefore, knowledge of their excited electronic states and relaxation processes is significant for the understanding of their functions. Photoluminescence (PL) is a powerful probe for the excited states, because we can obtain fruitful information by measuring its properties such as quantum yields, spectra, time decay, and temperature dependence. For the EL application PL properties provide important reference data, because we can estimate the EL spectra and the EL
central atom
quantum yield
reference number
H2 Mg AlCl Zn GaCl Cd InCl Pd Cu Ni
0.7 0.6 0.58 0.3 0.31 3-8 × 10-2 0.031 5 × 10-4 undetectable undetectable
16 16 17 16 17 16 17 16 16 16
emission efficiencies of the materials through the corresponding PL spectra and the quantum yields. Therefore, it is important to investigate PL phenomena of Pc compounds in the solid phases. Photoluminescence of Pc molecules in solution (and in vapor) has been investigated extensively and, as a result, many fluorescence spectra and quantum yields are known.13-17 (Table 1 summarizes fluorescence quantum yields.) Convincing interpretation of the emission process has been also given.14 However, PL properties in the solid (or crystalline) phases have been investigated for only a few Pc materials such as H2Pc,18,19 ZnPc,19 AlClPc,20 VOPc,21 and TiOPc.4,5 In particular, quantum yields, although a very fundamental property, have been estimated roughly only for H2Pc (∼10-4)18 and VOPc (∼10-5-
10.1021/jp002943o CCC: $20.00 © 2001 American Chemical Society Published on Web 02/01/2001
1548 J. Phys. Chem. B, Vol. 105, No. 8, 2001 10-7).21 Therefore, it is valuable to extend a basic study on the PL properties for various phthalocyanines in solid phases including quantum yields determined by a precise method. It also seems valuable to interpret the emission process of the solid materials in conjunction with the excitonic states that are characteristic in the solid phases. In this article we describe fluorescence properties of H2Pc, MgPc, AlClPc, and AlBrPc thin solid films. The centers of the Pc ligands of these materials are not coordinated with transition metal ions; therefore, their molecular electronic structures are basically of Pc ligand origin.22 As a result, as shown in Table 1, these materials have high quantum yields in the monomer states. This means that they have relatively small internal nonradiative relaxation within the molecules which occurs by the cascade relaxation between the internal energy levels of the metal ions located between the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbital (LUMOs) of the Pc ligands,14,23 or through the intersystem crossing to the triplet states of the Pc ligands, which is enhanced by the heavy atom effect.14,16 Therefore, these molecular materials seem favorable for the study of the emission process in the solid phases in which intermolecular interaction induces excitonic states. In addition, these materials have several crystal phases (polymorphism) that have different excitonic states. 24-27 This can be used to elucidate the influence of the intermolecular interactions on the emission process. Therefore, in this work we prepare two crystal phases for each molecular material and compare their fluorescence quantum yields at room temperature and their spectral changes with decreasing temperature. We discuss the emission process in the solid materials by considering the exciton migration and the following relaxation to the radiative and the nonradiative sites with a particular interest in the allowed or forbidden nature of the lower-lying exciton states. 2. Experimental Section 2.1. Samples. Thin films of H2Pc, MgPc, AlClPc, and AlBrPc were deposited on quartz glass slides (10 mm × 10 mm × 0.5 mm) in a vacuum of 10-6 Torr with a conventional vacuum deposition apparatus using an oil diffusion pump (ULVAC EBH-6). Each Pc material was put into an alumina crucible (diameter, 10 mm; depth, 20 mm) and heated with a tungsten heater. The quartz glass substrates were set at distances of 10, 20, and 30 cm from the evaporation crucible and, therefore, three types of thin films with different thickness (0.5) are high as shown in Table 1, whereas the quantum yields of the solid films obtained (10-5-10-4) are very low. This indicates that nonradiative relaxation is dominant in the solid films. The relaxation process of the excited states in the solid Pc materials has been discussed by the exciton migration and the following capture of the exciton at the radiative and the nonradiative sites (traps).38,39 As the radiative sites we may list lower-lying localized excitons, surface excitons, deformed molecules (including excimers), and impurities. As for the nonradiative process, although many seem to remain uncertain, we may list candidates such as internal conversion within the molecules, intersystem crossing to triplet excitons, photocarrier generation, impurity traps, surface traps, lattice defect and deformation, and exciton scattering by lattice phonons. Among these processes the last one seems most sensitive to temperature because lattice phonons are greatly thermally activated. There-
fore, we infer that the increase of the fluorescence intensity with decreasing temperature observed in all the samples (Figure 5) is mainly due to the depression of the exciton scattering by the lattice phonons. However, the increase is at most as large as 10 times even at the liquid helium temperature, indicating that quantum yields are still very low (10-4-10-3). This suggests that the other nonradiative processes are dominant in the films. In contrast to the very low quantum yields of these films, Blasse et al.38 have observed a high quantum yield of 5-10% at room temperature in a solid octa(n-dodecoxy)H2Pc. We have also observed a quantum yield of ∼10-2 in a solid PcSi[OSi(CH3)3]2 film.40 With decreasing temperature to liquid helium temperature, fluorescence quantum yields of these materials increase to an extent comparable with the monomer quantum yields.38,40 In these materials intermolecular distances are enlarged because of the large outer or out-of-plane substituents.38,41 From this we surmise that the large molecular contact between neighboring molecules may induce the dominant nonradiative pathways in the nonsubstituted Pc materials. The spectral features of the fluorescences were very different depending on the crystal phases and the materials. All the vaportreated films showed distinct emission bands located at 850950 nm overlapped with the absorption edge. In these films the absorption edges are largely red-shifted from the monomer bands, suggesting that the lowest-lying exciton is allowed. At low temperatures the emission bands moved slightly to red corresponding to the red movement of the absorption edge due to the thermochromism. These features seem to support that the luminescence is emitted from the lowest-lying allowed exciton state. The higher quantum yields of the red-shifted phases than those of the as-deposited phases in the same molecular materials may be related to this allowed nature of the lowest-lying exciton states, because the red-side oscillator strengths of the red-shifted phases are stronger than those of the as-deposited phases and, therefore, natural radiation rates become larger in the former phases. The as-deposited AlBrPc film emitted a broad luminescence, the peak of which seems to be located at about 1000 nm, far away from the absorption edge. As mentioned previously, in this film the absorption band is blue-shifted, suggesting that
1552 J. Phys. Chem. B, Vol. 105, No. 8, 2001 the lowest-lying exciton is forbidden. In such a case the explanation on the weak fluorescence of a silicon Pc dimer presented by Oddos-Marcel et al.37 seems applicable. They have argued that, even though the lower-lying exciton is forbidden, the transition to the vibronic sublevels of the ground state can be very weakly allowed. In this way the spectral separation of the emission band from the absorption band edge and the very low quantum yield might be understood. The as-deposited H2Pc film also emitted a broad luminescence band located away from the absorption edge. This feature is also understandable with an explanation similar to that for the as-deposited AlBrPc film if the lowest-lying exciton state is forbidden. However, in this case whether the lowest-lying exciton state is allowed or forbidden is a subtle problem. The simplest judgment is to check if the absorption edge is located at lower energy than the absorption band of the monomer. However, this judgment is not valid in practice because molecular distortion occurs in the crystal that induces a splitting of the monomer band3,30,42 and because a shift of the center of gravity of the two exciton states arises from electrostatic interactions between molecules.37 Therefore, other judgment is necessary. Here we note that in the β-form H2Pc crystal the low-energy edge of the absorption band and the high-energy edge of the emission band overlap at about 750 nm.19,30 This strongly suggests that the lowest-lying exciton state is allowed in the β-form H2Pc crystal. In contrast, in the R-form crystal the absorption band and the emission band are separated,18,19,30 suggesting that the lowest-lying exciton state is forbidden. The spectral feature of the as-deposited AlClPc films is characteristic in that new emission bands appear with decreasing temperature. In this film, as obvious from the change of the absorption spectrum depending on the temperatures, the change of crystal structure occurs at low temperatures, producing some different exciton states. At low temperatures, because the nonradiative scatterings by lattice phonons are depressed, several excitonic emission bands become more distinct. The spectral feature of the as-deposited MgPc film is characterized by the coexistence of a distinct emission band at the absorption edge (at about 700 nm) and a broad emission band located at longer wavelengths. We avoid giving a conclusive interpretation at this stage, however, because it has been pointed out recently that MgPc is sensitive to moisture,35 but we have not taken a special care about it in the course of the experiments. A detailed study on MgPc films is now in progress. In brief, we also comment on the potential of the materials studied here as light-emitting materials. The invisible nearinfrared emission range (>700 nm) is a great disadvantage for EL display devices, although it might be useful for a lightemitting diode (LED) used for a printer head in combination with near-infrared sensitive organic photoconductors. The very low quantum yields (10-4-10-5) are also a great disadvantage for EL devices. From these we estimate the low potential of the materials for EL application. 4. Conclusion Photoluminescence properties were measured for the vacuumdeposited thin films of H2Pc, MgPc, AlClPc, and AlBrPc in two different crystal phases, that is, as-deposited phases and red-shifted phases prepared by a vapor treatment of organic solvents. Fluorescence quantum yields of the films at room temperature were 10-5-10-4. These values were much smaller than those of the corresponding monomers (>0.5), indicating that nonradiative relaxation is dominant in the solid films. Increase of the fluorescence intensity was observed with
Sakakibara et al. decreasing temperature in all the samples, but the extent of the increase was at most as large as 10 times even at the liquid helium temperature, indicating that nonradiative relaxation is still dominant. The spectral features were very different depending on the crystal phases and the materials. The distinct emission bands of the vapor-treated MgPc, AlClPc, and AlBrPc films overlapped with the red-shifted absorption edge and were interpreted by the emission from the allowed lowest-lying exciton states. The broad emission bands of the as-deposited AlBrPc and H2Pc films located away from the absorption edge were interpreted by the very weakly allowed transition from the forbidden lowest-lying exciton state to the vibronic sublevel of the ground state. The appearance of new emission bands in the as-deposited AlClPc film with decreasing temperature was ascribed to the change of crystal packing from the corresponding change in the absorption spectrum. References and Notes (1) Moser, F. H.; Thomas, A. L. The Phthalocyanines; CRC Press: Boca Raton, 1983; vols. I and II. (2) Saito, T.; Iwakabe, Y.; Kobayashi, T.; Suzuki, S.; Iwayanagi, T. J. Phys. Chem. 1994, 98, 2726. (3) Mizuguchi, J.; Rihs, G.; Karfunkel, H. R. J. Phys. Chem. 1995, 99, 16217. (4) Popovic, Z. D.; Khan, M. I.; Atherton, S. J.; Hor, A. M.; Goodman, J. L. J. Phys. Chem. B 1998, 102, 657. (5) Yamaguchi, S.; Sasaki, Y. J. Phys. Chem. B 1999, 103, 6835. (6) Namba, N. Phthalocyanine - Chemistry and Functions; Shirai, Y., Kobayashi, N., Eds.; IPC: Tokyo, 1997; p 247 (in Japanese). (7) Wo¨hrle, D.; Kreienhoop, L.; Schlettwein, D. Phthalocyanines Properties and Applications; Leznoff, C. C., Lever, A. B. P., Eds.; VCH: New York, 1996; vol. 4, p 219. (8) Nalwa, H. S.; Shirk, J. S. Phthalocyanines - Properties and Applications; Leznoff, C. C., Lever, A. B. P., Eds.; VCH: New York, 1996; vol. 4, p 79. (9) Machida, S.; Horie, K. Phthalocyanine - Chemistry and Functions; Shirai, Y., Kobayashi, N., Eds.; IPC: Tokyo, 1997; p 260 (in Japanese). (10) Fujii, A.; Yoshida, M.; Ohmori, Y.; Yoshino, K. Jpn. J. Appl. Phys. 1996, 35, L37. (11) Ottmar M.; Hohnholz, D.; Wedel, A.; Hanack, M. Synth. Met. 1999, 105, 145. (12) Tang, Ch.; Weidner, Ch.; Comfort, D. U.S. Patent 5409783 A 950425, 1994. (13) Eastwood, D.; Edwards, L.; Gouterman, M.; Steinfeld, J. L. Mol. Spectrosc. 1966, 20, 381. (14) Gouterman, M. The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; vol. 3, chapter 1. (15) Spaeth, M. L.; Sooy, W. R. J. Chem. Phys. 1968, 48, 2315. (16) Vincett, P. S.; Voigt, E. M.; Rieckhoff, K. E. J. Chem. Phys. 1971, 55, 4131. (17) Brannon, J. H.; Magde, D. J. Am. Chem. Soc. 1980, 102, 62. (18) Menzel, E. R.; Popovic, Z. D. Chem. Phys. Lett. 1978, 55, 177. (19) Yoshino, K.; Hikida, M.; Tatsuno, K.; Kaneto, K.; Inuishi, Y. J. Phys. Soc. Jpn. 1973, 34, 441. (20) Aroca, R.; Jennings, C.; Loutfy, R. O.; Hor, A. M. Spectrochim. Acta 1987, 43A, 725. (21) Huang, T. H.; Sharp, J. H. Chem. Phys. 1982, 65, 205. (22) Lee, L. K.; Sabelli, N. H.; LeBreton, P. R. J. Phys. Chem. 1982, 86, 3926. (23) Greene, B. I.; Millard, R. R. J. Phys. Chem. 1985, 89, 2976. (24) Loutfy, R. O.; Hor, A. M.; DiPaola-Baranyi, G.; Hsiao, K. J. Imag. Sci. 1985, 29, 116. (25) Hor, A. M.; Loutfy, R. O. Thin Solid Films 1983, 106, 291. (26) Khe, N. C.; Aizawa, M. Nippon Kagaku Kaishi 1986, 393 (in Japanese). (27) Daidoh, T.; Matsunaga, H.; Iwata, K. Nippon Kagaku Kaishi 1988, 1090 (in Japanese). (28) Djurisˇicˇ, A. B.; Fritz, T.; Leo, K.; Li, E. H. Appl. Opt. 2000, 39, 1174. (29) Williams, V. S.; Mazumdar, S.; Armstrong, N. R.; Ho, Z. Z.; Peyghambarian, N. J. Phys. Chem. 1992, 96, 4500. (30) Lucia, E. A.; Verderame, F. D. J. Chem. Phys. 1968, 48, 2674. (31) Hoshi, H.; Dann, A. J.; Maruyama, Y. J. Appl. Phys. 1990, 67, 1845. (32) Fujiki, M.; Tabei, H.; Kurihara, T. J. Phys. Chem. 1988, 92, 1281. (33) Hush, N. S.; Woolsey, I. S. Mol. Phys. 1971, 21, 465.
Photoluminescence Properties of Solid Films (34) Sakakibara, Y.; Saito, K.; Tani, T. Jpn. J. Appl. Phys. 1998, 37, 695. (35) Endo, A.; Matsumoto, S.; Mizuguchi, J. J. Phys. Chem. 1999, 103, 8193. (36) Galanin, M. D. Luminescence of Molecules and Crystals; Cambridge International Science Publishing: Cambridge, 1996; p 8. (37) Oddos-Marcel, L.; Madeore, F.; Bock, A.; Neher, D.; Ferencz, A.; Rengel, H.; Wegner, G.; Kryschi, C.; Trommsdorff, H. P. J. Phys. Chem. 1996, 100, 11850.
J. Phys. Chem. B, Vol. 105, No. 8, 2001 1553 (38) Blasse, G.; Dirksen, G. J.; Meijerink, A.; van der Pol, J. F.; Neeleman, E.; Drenth, W. Chem. Phys. Lett. 1989, 154, 420. (39) Popovic, Z. D.; Menzel, E. R. J. Chem. Phys. 1979, 71, 5090. (40) Sakakibara, Y.; Vacha, M.; Tani, T. Mol. Cryst. Liq. Cryst. 1998, 314, 71. (41) Mooney, J. R.; Choy, C. K.; Knox, K.; Kenney, M. E. J. Am. Chem. Soc. 1975, 97, 3033. (42) Mizuguchi, J.; Matsumoto, S. J. Phys. Chem. A 1999, 103, 614.
