ISSN 10637826, Semiconductors, 2010, Vol. 44, No. 8, pp. 1070–1073. © Pleiades Publishing, Ltd., 2010. Original Russian Text © A.V. Ziminov, Yu.A. Polevaya, T.A. Jourre, S.M. Ramsh, M.M. Mezdrogina, N.K. Poletaev, 2010, published in Fizika i Tekhnika Poluprovodnikov, 2010, Vol. 44, No. 8, pp. 1104–1107.
AMORPHOUS, VITREOUS, POROUS, ORGANIC, AND MICROCRYSTALLINE SEMICONDUCTORS; SEMICONDUCTOR COMPOSITES
Photoluminescence of NitroSubstituted Europium (III) Phthalocyanines A. V. Ziminova^, Yu. A. Polevayaa, T. A. Jourrea, S. M. Ramsha, M. M. Mezdroginab, and N. K. Poletaeva a
St. Petersburg State Institute of Technology (Technical University), St. Petersburg, 190013 Russia ^email:
[email protected] b Ioffe Physical Technical Institute, Russian Academy of Sciences, St. Petersburg, 194021 Russia Submitted December 28, 2009; accepted for publication January 11, 2010
Abstract—Europium monophthalocyanine Eu(acac)Pc, europium monotetranitrophthalocyanine Eu(acac)Pc(NO2)4, and heteroleptic europium tetranitrobisphthalocyanine Eu(Pc)(Pc(NO2)4) are synthe sized. The spectral characteristics of the phthalocyanine complexes in the visible and nearinfrared regions are studied. The photoluminescence spectra are recorded. The luminescence bands are detected in the S0) and 670–730 nm (S1 S0). The peaks are attributed to electronic tran regions 450–500 nm (S2 sitions in the organic ligands. DOI: 10.1134/S1063782610080208
1. INTRODUCTION Metal phthalocyanines are extensively studied organic semiconductors that are promising for mod ern fields of science and technology, e.g., optoelec tronics [1], organic nanophotonics, etc. [2–7]. Special attention is given to complexes based on phthalocya nine ligands (Pc) in combination with lanthanides (Ln). Lanthanides can form various complex com pounds with phthalocyanine ligands: monophthalocy anines with axial ligands, doubledeck homoleptic and heteroleptic complexes, and tripledeck supercom plexes containing two lanthanide atoms arranged between three phthalocyanine ligands [8]. Lanthanide complexes are extensively explored as lightemitting layers in organic lightemitting diodes [9]. The best understood compounds are βdiketonate complexes, especially lanthanide acetylacetonates (acac) [10, 11]. At the same time, phthalocyanine complexes, mainly copper phthalocyanine, are used as ptype layers [12]. It is known that the photochemical activity of chromophores of organic compounds is enhanced upon the introduction of nitro groups into the com pounds. Nitro groups introduced into aromatic and heteroaromatic compounds noticeably modify the spectral characteristics of the compounds. In nitro substituted phthalocyanines, the nitro group is outside the plane of the macroheterocyclic ring [13]. This yields splitting of the Q band in the absorption spectra [14, 15], a decrease in the luminescence intensity, and some shift of the peaks in the absorption spectra [16].
In this context, the purpose of this study was to analyze the role of the molecular structure of nitro substituted europium phthalocyanines in the forma tion of emission spectra of the molecules in solutions. This problem is of considerable interest, since it is known that a nitro group introduced into an aromatic ring enhances the photosensitivity. 2. EXPERIMENTAL The spectral characteristics of a number of europium phthalocyanines synthesized by procedures described elsewhere [17, 18] have been studied. The subjects of inquiry were samples of (1) europium monophthalocya nine Eu(acac)Pc (Fig. 1A); (2) europium monotetrani trophthalocyanine Eu(acac)Pc(NO2)4, in which the role of the axial ligand is played by acetylacetonate (Fig. 1B); and (3) heteroleptic tetranitrodiphthalocy anine Eu(Pc)(Pc(NO2)4) (Fig. 1C). Complexes A and B were produced on interaction of the corresponding ligands with europium acetylacetonates in boiling ortodichlorobenzene in the presence of a strong organic base, DBU (1.8diazabicyclo[5.4.0]undec7ene). Complex C was produced on interaction of complex B with an unsubstituted phthalocyanine ligand in nitrobenzene with a catalytic admixture of DBU. The absorption and photoluminescence (PL) spec tra were recorded for complexes dissolved in DMFA (dimethylformamide), CHCl3, and CH3CN. The absorption spectra were recorded with the use of an SF2000 spectrophotometer in the wavelength range from 200 to 800 nm. The PL measurements were con
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PHOTOLUMINESCENCE (A) O O Eu N
N
N N
N N
N
N O2N
N
N N
O2N
N
N N
N N Eu N N
1 0.5
NO2
2 0
N
300
400 500 600 Wavelength, nm
NO2 NO2
N N
(C)
N
700
0
Fig. 2. Absorption (1) and PL (2) spectra of the solution of Eu(Pc)(Pc(NO2)4) (complex C) in DMFA. The PL signal is excited at λex = 325 nm.
NO2 N N
N
Fig. 1. Molecular complexes under study: (A) europium monophthalocyanine Eu(acac)Pc, (B) europium monotetra nitrophthalocyanine Eu(acac)Pc(NO2)4, and (C) europium tetranitrodiphthalocyanine Eu(Pc)(Pc(NO2)4).
ducted with the use of an experimental system assem bled on the basis of an SDL2 grating spectrometer with the reciprocal linear dispersion 1.3 nm mm–1. For the radiation source to excite the PL signal, we used (1) a cw He–Cd laser emitting at the wavelength λ = 3250 Å with a emission power of 15 mW (interband excitation of phthalocyanines) and (2) an LGI21 pulsed nitrogen laser with pulse duration τ = 7 ns (measured at the pulse halfmaximum), repetition fre quency 100 Hz, average power 3 mW, and emission wavelength λ = 3371 Å; the pulse energy density pro vided by the defocused laser beam was ~10 kW cm–2. In order to correlate the emission spectra of different samples correctly, we kept the controllable parameters, specifically, the angle of incidence of the excitation beam and the temperature (300 and 77 K), constant. 3. DISCUSSION Figure 2 shows the absorption (curve 1) and PL (curve 2) spectra of the solutions of Eu(Pc)(Pc(NO2)4) in DMFA. In the electron absorp tion spectra of the solution of complex C in DMFA, we observe a number of bands: a shortwavelength N band (287 nm), a B band or Soret band (360 nm), and a Q band (692 and 700 nm). The Q band is broadened and split, which is attributed to the presence of nitro groups [13, 14]. From comparison of the absorption SEMICONDUCTORS
10000
5000
O O Eu N N N N N N
O2N
PL intensity, arb. units
Absorbance, arb. units 1.5
1.0
N (B)
O2N
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S0 spectra with the PL spectra, we can see that the S1 PL band is a direct reflection of the longwavelength Q band, whereas the more broadened and less resolved S0) in the range 400–500 nm emission band (S2 corresponds to the B absorption band. Figure 3 shows the PL spectra of the solutions of Eu(Pc)(Pc(NO2)4) (complex B) in different solvents. The experimentally observed PL spectra are typical of phthalocyanine complexes containing metals of dif ferent natures [19–24]. The absorption spectrum of europium monotetranitrophthalocyanine (complex B) heavily depends on the nature of solvents. Complex B is hardly dissolved in chloroform, which has an impact on the relative intensity of the Q band. The ratio between the band intensities in the absorption and PL spectra of complex B in different solvents is the same. In the absorption spectra, as well as in the PL spectra, we observe a gypsochromic shift of the bands if one solvent is exchanged for another in the series PL intensity, arb. units 10000 1 2
5000 3
0
400
500
600 700 Wavelength, nm
Fig. 3. PL spectra of Eu(acac)Pc(NO2)4 (complex B) in the (1) CH3CN, (2) CHCl3, and (3) DMFA solvents.
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PL intensity, arb. units 8000 6000 4000
2
2000 0
1 3 400
500
600 700 Wavelength, nm
Fig. 4. PL spectra of the solutions of (1) Eu(acac)Pc (com plex A), (2) Eu(acac)Pc(NO2)4 (complex B), and (3) Eu(Pc)(Pc(NO2)4) (complex C) in DMFA.
DMFA CHCl3 CH3CN. In analyzing the PL spectra, we clearly observe the solvatochromic effect: λmax shifts from 711 nm (in DMFA) to 684 nm (in CH3CN). The S1 S0 band is most sensitive to changes of solvents, whereas the S2 S0 band changes only its intensity under changes of solvents. In analyzing the PL spectra of unsubstituted europium phthalocyanine and europium tetranitro phthalocyanine A and B, we must note that, on intro duction of the nitro group, we observe splitting of the longwavelength band (a similar effect is observed in the absorption spectra as well) and a bathochromic shift of the entire PL spectrum. In turn, when we con sider heteroleptic europium bisphthalocyanine C instead of monophthalocyanines A and B, we observe a gypsochromic shift of the longwavelength band (with respect to its position in complexes A and B), whereas the very broad band in the range 450–600 nm exhibits a bathochromic shift in parallel with a decrease in the intensity (Fig. 4). In the table, the spectral characteristics of the solu tions under study are summarized for comparison. In [20], heteroleptic double and tripledeck com plexes of europium and terbium with phthalocyanine and porphyrin ligands were studied. The authors of [20] note that, between tetrapyrrole ligands, there is a strong π–π interaction dependent on the radius of the lanthanide atom. (The principle role of the central rareearth (RE) atom is that, the smaller the atomic Spectral characteristics of complexes A and B in the solu tion in DMFA Compound
Absorption bands, nm
PL bands, nm
Eu(acac)Pc (A) Eu(acac)Pc(NO2)4 (B)
338, 607, 670 336, 612, 679
457, 697 450, 618, 696
radius, the stronger the interaction.) The RE atoms only slightly influence the absorption spectra of the complexes, but these atoms have an effect on the dynamics of excited states. The luminescence spectra were studied in toluene at an excitation wavelength corresponding to the Soret band for porphyrin com plexes (420 nm). The luminescence spectra were reflectionsymmetric to the absorption spectra. For the tripledeck complexes (with the phthalocyanine toprophyrin ratio Pc : Por = 1 : 2), the most intense luminescence band is the emission bands at 650 and S0). When one considers doubledeck 720 nm (S1 complexes (with the ratio Pc : Por = 1 : 1) instead of tripledeck ones, the most intense luminescence band S0). This sug is the emission band at 447 nm (S2 gests that, for the doubledeck compounds, the 447nm emission becomes the basic channel of conversion of the excitation energy. In the nearIR range (1300–1450 nm) of the spec trum of terbium complexes, two bands correspond to phosphorescence at transitions from triplet states of the porphyrin ligand in the molecule. In contrast to terbium complexes, europium complex compounds exhibit neither such phosphorescence nor lumines cence at interatomic transitions in RE atoms. These differences in the phosphorescence spectra are attrib uted to differences in the energy position of low energy ground states of the RE atoms. For terbium, the levels of its ground state are higher than the triplet level of the ligand; therefore, energy transfer from the ligand to the metal is impossible and the energy is emitted as a phosphorescence photon from the triplet level. For europium, the situation is opposite: the trip let level of the ligand is higher than the levels of the europium ground state and, therefore, the energy is transferred to the europium ground state and then converted into heat or given up to the molecules of the solvent. Figure 5 shows a proposed diagram of electronic transitions on excitation of europium complexes in the region of the Soret band. The primary process is opti cal excitation at the wavelength λ = 325 nm corre sponding to high excited states (process (1) in Fig. 5). Furthermore, on internal conversion, the system tran sits from the corresponding vibration states to the quasistable excited level PcS2. Then the molecule in the excited state PcS2 can experience different photo physical processes. Among these is the allowed fast nonradiative internal conversion into the stable state PcS (process (3) in Fig. 5), with subsequent radiative 1 (process 4) or nonradiative transition to the ground state PcS0. Since the energy separation between the first and second excited states is rather large (about 300– 400 nm), some of the excited molecules in the state PcS can fluoresce (i.e., immediately transit to the 2 ground state with emission of a photon) [24]. The characteristic emission bands are lacking in the PL spectra of the complexes, since the first excited singlet state of the axial ligand (acetylacetone) is higher in SEMICONDUCTORS
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PHOTOLUMINESCENCE L
S1 L
T1
Pc
S2
(1)
(5)
Pc
S1
5 D1 5D 0
(2) (3) PcT 1
(4)
(6) 7
Fj j = 6. .. . 0
(7)
Pc
S0
L
S0
Pc ligand
Eu3+
Axial ligand
Fig. 5. The proposed diagram of optical processes on laser excitation of europium complexes in the region of the Soret band (λex = 325 nm).
energy than the second excited state of the phthalocy anine ligand and, consequently, no energy transfer occurs on laser excitation of the molecules in the region of the Soret band (PcS2). 4. CONCLUSIONS Europium (III) phthalocyanine complexes con taining nitro groups as peripheral substitutes are syn thesized. The effect of the nitro group on the elec tronic absorption and PL spectra of the complexes dis solved in DMFA is shown. In analysis of the spectral curves, the splitting of the Q band directly reproduc ible in the PL spectra is found. A diagram of electronic transitions in europium nitrophthalocyanine com plexes with axial acetylacetonate ligands is suggested. REFERENCES 1. D. Hohmholz, S. Steinbrecher, and M. Hanack, J. Mol. Struct. 5231, 231 (2000). 2. Ch. H. Cheng, Z. Q. Fan, S. K. Yu, et al., Appl. Phys. Lett. 88, 213 (505) (2006).
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Translated by É. Smorgonskaya
