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Spectroscopy and non-radiative processes in Gd3+, Eu3+ and Tb3+ tropolonates
Spectrochimica Acta Part A 54 (1998) 2237 – 2245
Spectroscopy and non-radiative processes in Gd3 + , Eu3 + and Tb3 + tropolonates B.S. Santos a, C. de Mello Donega´ a, G.F. de Sa´ a,*, L.F.C. de Oliveira b, P.S. Santos c b
a Depto. de Quı´mica Fundamental, UFPE, Cid. Uni6ersita´ria, CEP 50670 -901, Recife PE, Brazil Depto. de Quı´mica, Instituto de Cieˆncias Exatas, Uni6ersidade Federal de Juiz de Fora, CEP 36036 -330, Juiz de Fora MG, Brazil c Depto. de Quı´mica Fundamental, Instituto de Quı´mica, USP, C.P.26077, CEP 05599 -970, Sa˜o Paulo SP, Brazil
Received 10 November 1997; accepted 17 February 1998
Abstract Complexes of Eu3 + , Gd3 + , and Tb3 + with tropolone (tropolone =2-hydroxy-2,4,6-cycloheptatrien-1-one) were synthesized and characterized by C and H elemental analysis, infrared vibrational spectroscopy, X-ray powder diffraction analysis and Raman vibrational spectroscopy. The results show that the complexes are isostructural and have the formula [Ln(trop)3], where Ln=Eu3 + , Gd3 + or Tb3 + and trop= tropolone. The characteristic luminescence of the Eu3 + and Tb3 + ions was not observed either upon ligand excitation (300 – 380nm) or upon direct f–f excitation, not even at 4.2 K. In order to understand this phenomenon the spectroscopic properties of the Gd(trop)3 complex (absorption spectra at 298 K, excitation and emission spectra from 4.2 to 298 K and decay time measurements from 20 to 298 K) were investigated. The emission spectra show three distinct bands, which are ascribed to ligand electronic states: a singlet state, S1 (p– p*), at 24270 cm − 1, and two triplet states, T1 and T2, at 15630cm − 1 and 16610cm − 1, respectively. Both triplet states are situated below the emitting states of the Eu3 + and Tb3 + ions (5Do and 5D4, respectively), thus suppressing the luminescence from these states by back-transfer. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Lanthanide complexes; Raman spectroscopy; Luminescence; Non-radiative relaxation
1. Introduction The design and investigation of lanthanide complexes with organic ligands as efficient lightconversion molecular devices (LCMD) has become an important theme in coordination * Corresponding author. Tel.: + 55 81 27418441; fax: + 55 81 2710359; e-mail: [email protected]
chemistry, being pursued by several groups [1– 13]. Efficient LCMD may find many applications, such as luminescent labels in fluoroimmunoassays [2], lasers [14], and thin film electroluminescent devices [15]. In order to optimize the quantum yield and the light output of a LCMD several processes must be controlled: (i) the ligand absorption and internal decay processes; (ii) the efficiency of the ligand–to–metal energy transfer;
1386-1425/98/$ - see front matter © 1998 Elsevier Science B.V. All rights reserved. PII S1386-1425(98)00143-7
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and (iii) the luminescence efficiency of the metal ion. To achieve such a goal our group has adopted a strategy based upon both theoretical and experimental approaches [11 – 13,16 –28], investigating qualitative and quantitative aspects. Therefore, in order to gain insight into the factors that determine the quantum yield of lanthanide complexes we have synthesized and investigated the photophysical properties of a number of new lanthanide(III) complexes [11– 13,20,25–28]. Furthermore, our group has been successfully developing theoretical models to determine the geometry of a lanthanide complex [16,17], the position and nature of the ligand excited states in the complex [18,19], the efficiency of the ligand – to – metal energy transfer [21–23], and the luminescence quantum yields [24]. The results have shown that the quantum yields can be enhanced by an appropriate choice of ligands (e.g. – CF3 containing ligands [13,19,27], N-oxide ligands [11,13,26 – 28]) and are lowered not only by inefficient energy transfer ligand Eu3 + , but also by several 5Do nonradiative relaxation processes (e.g. multiphonon relaxation [11,22,24], phonon-assisted crossover to ligand-to-Eu3 + charge transfer state [25], etc.). Also, we have been able to develop complexes which are very promising LCMD, with room temperature quantum yields as high as 66% [13] and light outputs comparable to those of commercial phosphors. In this paper we will present new results on another quenching mechanism, viz. backtransfer to ligand states. Preliminary results on the spectroscopic properties of the Eu3 + , Gd3 + , and Tb3 + complexes with tropolone, (Ln(trop)3), have been previously reported [29]. The lack of the characteristic luminescence of Eu3 + and Tb3 + ions in these complexes was ascribed to non-radiative decay processes via excited ligand states. In order to better understand the quenching processes operative in lanthanide tropolonates we have carried out further spectroscopic studies on these complexes, including excitation and emission spectra from 298 to 4.2 K. As a part of the structural characterization of the complexes, special attention has been given to the analysis of their Raman spectra.
2. Experimental The complexes were prepared following the procedure reported by Muetterties and Wright [30], which consists of mixing an ethanolic solution of tropolone with an aqueous solution of lanthanide nitrate in the molar proportion of 3:1. The resultant solution is stirred for about 1 h at 60°C, yielding a pale yellow solid, which is filtered, washed with hot ethanol and dried under reduced pressure (B 1 mmHg) over P2O5. The solution of the lanthanide nitrate is obtained by dissolving the sesquioxide in an aqueous solution of nitric acid (molar ratio 1:3), and adjusting the pH to 6.0. The lanthanide oxides Eu2O3 (Aldrich, 99.99%), Gd2O3 (Molycorp, 99.99%) and Tb4O7 (Aldrich, 99.99%) were used without further purification, while tropolone (Aldrich, 98%) was recrystallized from ethanol and sublimed. The complexes were characterized by elemental analysis (C and H), infrared (JR) and Raman vibrational spectroscopies, UV-Visible electronic absorption spectroscopy, X-ray powder diffractometry and thermogravimetry. The UV-visible absorption spectra were obtained at room temperature with a Perkin-Elmer Lambda 6 spectrophotometer (spectral resolution of 0.5 nm). The spectrum of tropolone was obtained from aqueous and cyclohexane solutions, whereas the spectra of the complexes were obtained in the solid state with the samples dispersed in KBr pellets. Thermogravimetric analyses were carried out on a Shimadzu TGA-50, under a N2 flow, at 10 K min − 1, up to 900°C. The IR vibrational spectra were measured on a Bruker IF566 FTIR spectrophotometer (resolution, 4 cm − 1) using KBr pellet and nujol mull techniques. The Raman spectra were obtained with a Jobin-Yvon U- 1000 Raman spectrometer using a photomultiplier tube and photon counting system for detection. The spectral resolution was 5 cm − 1, the acquisition time 0.5 s at steps of 1 cm − 1. The spectra of the solid samples were obtained using a rotating disk in the 30–1800 cm − 1 range. The spectrum of sodium tropolonate in alkaline aqueous solution, in the 100– 1800 cm − 1 region, was obtained with a spinning cell for liquids. The spectra were excited using the
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514.5 nm line of an argon laser (Coherent Innova 90) and the power level was kept to a minimum to avoid photodecomposition of the sample. The luminescence and excitation spectra of the solid complexes were obtained by using a SPEX Fluorolog DM3000F Spectrofluorometer with double grating 0.22m SPEX monochromators (spectral resolution: 10 cm − 1) and a 450 UV Xe lamp as the excitation source. This set-up is equipped with an Oxford LF205 liquid helium flow cryostat, allowing measurements down to 4.2 K. The spectra are corrected for the instrumental response. Selective excitation of the 5Do energy level of Eu3 + in the complex (580nm), was performed at room temperature with a Nd:YAG pumped dye laser (Rhodamine 6G). The photomultiplier used in all the measurements was an RCA C31034-02. Decay time measurements for the emitting states of the Gd(trop)3 complex were performed between 20 and 293 K, using a closed cycle helium cryostat. The sample was excited with the third harmonic of a Nd:YAG pulsed laser (lexc = 355 nm) and the emission was monitored at three wavelengths: 440, 602 and 640 nm. The photocurrent was acquired and analyzed by a digital oscilloscope (HP 54501A 100 MHz). The temporal resolution of this set-up is limited to ca. 200 ns.
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Fig. 1. Structural formula of tropolone
tropolone are displayed in Fig. 2. The band positions are presented in Table 1 together with a tentative assignment based on previous work by Ikegami [31] and Thornton et al [32,38]. By analyzing the spectra of the lanthanide complexes and comparing them to that of the free ligand, some important features are observed. First, the Raman spectra of the complexes all have the same pattern, suggesting the same coordination geometry. This is consistent with the absence of any significant difference between the vibrational IR spectra of the three complexes. Considering that the Xray powder diffraction patterns of the complexes are also very similar one can conclude that the complexes are isostructural. The second important feature is a clear increase in the number of bands in the region characteris-
3. Results and discussion
3.1. Structural aspects: Raman spectroscopy The chemical analytical data (C and H) of the complexes indicate the formula [Ln(trop)3], where Ln=Eu3 + , Gd3 + or Tb3 + and trop =tropolonate ion (C7H5O2) [Eu(trop)3 =found: C, 48.75; H, 3.02; calc.: C, 48.96; H, 2.93; Gd(trop)3 = found: C, 48.70; H, 2.72; calc.: C, 48.40; H, 2.88; Tb(trop)3 =found: C, 48.40; H, 2.93; calc.: C, 48.31; H, 2.89%]. The proposed formula is also consistent with the thermogravimetric analytical data and the vibrational IR spectra of the complexes. The structural formula of tropolone is presented in Fig. 1. The Raman spectra of the tropolonate lanthanide complexes as well as the spectrum of
Fig. 2. Raman spectra of (a) solid Gd(trop)3, (b) solid Tb(trop)3 (c) solid Eu(trop)3 and (d) solid tropolone. For details see Table 1 and text.
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Table 1 Observed Raman lines (in cm−1) of solid tropolone and of the Eu3+, Gd3+ and Tb3+ tropolonate complexes Solid tropolone
Eu(trop)3
Gd(trop)3
Tb(trop)3
Assignment
1606
1597 1570 1533 1469 1448 1420 1404 1385
1598 1571 1533 1469 1449 1421 1406 1385 1359
1599 1572 1534 1470 1451 1423
1246 1223
1244 1221 1195 1071 976
1246 1224
877 739 707 564 502 491 421 410 383
876 742 707 566 503 490 420 410
877 738 709 569 506 496 425 412 380
n(C =O)+n(C= C) n(C =O)+n(C =C) n(C =O)+n(C =C) n(C =C)+n(C =O) C–H bending C–H bending C–H bending n(C–C) C–H bending+n(C–C) C–H bending C–O–H group C–O–H group C–H bending C–H bending+n(C–C) n(C–C) n(C–C) C–H bending Ring breathing nLn–O nLn–O
236
240
242
213 169 108 48
170 110 50
220 173 113 50
1540 1460 1423 1403
1271 1237 1190 982 961 875 743 688
451 370 342
1071 977
1386
1072 977
204
66
tic of the carbon – carbon and carbonyl stretching modes ( 1400 – 1600 cm − 1) after complexation. These are coordination-sensitive modes and an increase of the number of bands in this region could be regarded as a consequence of a decrease of the ligand symmetry. The ring breathing mode of solid tropolone in the Raman spectrum was assigned by Ikegami [31] to a band at 744 cm − 1 and is observed in this work as a sharp peak in 742 cm − 1. This same frequency mode is found as a rather weak and poorly defined band with highest intensity at 739, 742 and 738 cm − 1 respectively for the Eu3 + , Tb3 + and Gd3 + tropolonate complexes. This lowering of the ring breathing mode could be interpreted as a deviation from
nLn–O Skeletal deformation nLn–O Skeletal deformation Skeletal deformation nLn–O Skeletal deformation nLn–O dO–Ln–O dO–Ln–O Lattice mode
planarity (the seven-membered ring is essentially planar in solid tropolone due to intramolecular hydrogen bonding [33]). Different degrees of ringdistortion have been observed in other tropolonate complexes [34–37]. The metal–ligand stretching modes are tentatively assigned (Table 1) based on the previous assignments made by Hullet and Thornton [37] and Burden et al. [32] in the infrared vibrational spectra of tropolonate transition metal and lanthanide complexes. This assumption indicates a rather strong metal–ligand interaction in the complexes, that is corroborated by the observed change in the pattern of the ligand spectrum. Such an observation is in marked contrast with the
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Raman spectra of squarate lanthanide complexes [39], in which the pattern in the Raman spectrum of the ligand was almost unaffected by complexation with the Eu3 + and Tb3 + ions. It is also interesting to note that the analysis of the emission spectra of the Gd(trop)3 (Fig. 4, see below) reveals a vibronic progression based on the 1470 and 1595cm − 1 modes, assigned to C – C and C–O stretching. This is a clear indication of a significant change in the geometry of the molecule in the excited state along the corresponding normal coordinates. Consistent with this, the analysis of the Raman spectra shows that the same modes are very intense, which is in accord with the vibronic theory of the Raman effect, in which it is expected that the more intense modes are the ones showing greater geometric changes of the corresponding normal coordinates in the excited state. On the same basis, one should expect that a substantial variation in the geometry of the ligand in the excited state should also reflect the nature of the metal–ligand bonding. This is in line with the remarkable intensity of the metal – oxygen stretching mode, at ca. 700 cm − 1, in the Raman spectra.
3.2. Electronic spectra The absorption spectra of the complexes (Fig. 3) are very similar to each other, and correspond well to the tropolone absorption spectrum, apart
Fig. 3. Room temperature absorption spectra of (a) solid Gd(trop)3, (b) solid Eu(trop)3, (c) solid Tb(trop)3 and (d) tropolone in cyclohexane solution. The intensities have been normalized to the most intense band of each spectrum.
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Fig. 4. Emission (lexc =360 nm, solid line) and excitation (lem =597 nm, dotted line) spectra of Gd(trop)3 at 4.2 K. qr is the relative quantum output for the excitation spectrum and f gives the photon flux per constant wavelength interval in the emission spectrum (arbitrary units).
from the red shift due to the complexation to the metal ion. The tropolone absorption spectrum shows two separate bands in the 210–270 nm and 270–400 nm wavelength ranges. From previous studies on tropolone solutions [40,41] it has been established that each of the two absorption bands results from the superposition of two (p, p*) electronic transitions. Correspondingly, the absorption spectra of the complexes also show two broad bands in the UV region: B 200–270 nm (with a maximum at 237 nm) and 270–420 nm (with maxima at around 334, 387, 397 and 410 nm). Despite the poorer definition of this band one may still observe some structure, which resembles that observed in the tropolone absorption spectrum. The characteristic f–f emission lines of the Eu3 + and Tb3 + ions were not observed, either upon ligand excitation (300–380 nm) or upon direct f–f excitation, even at 4.2 K. The Gd(trop)3 complex shows ligand related luminescence. This luminescence is also observed in the Tb(trop)3 complex, although at low temperatures only, and even then it is much weaker than that observed in the gadolinium complex. The complex Eu(trop)3 does not show any ligand luminescence at all, even at 4.2 K. The emission spectrum of solid Gd(trop)3 at 4.2 K (Fig. 4) consists of two distinct
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bands: 400– 525 nm and 597 – 740 nm. The zerophonon line of the higher energy emission band is observed at 412 nm (24270 cm − 1), resonant with the zero-phonon line of the excitation band. The lower energy emission band is actually composed of two overlapping emission bands: one consisting of a sharp zero-phonon line at 602 nm (16610 cm − 1) with a vibrational progression based on two frequencies: 1470 and 1595 cm − 1 (C =C and C–O stretching modes, respectively, both observed in the Raman and IR spectra), and the other characterized by a vibrational progression based on a 1450 cm − 1 frequency mode, with the zero-phonon line at 640 nm (15630 cm − 1). The excitation spectrum of Gd(trop)3 at 4.2 K (Fig. 4) bears a very close resemblance with the absorption spectrum of tropolone, apart from the expected shift to lower energies. From a consideration of the vibronic frequencies one may conclude that the electronic states involved in the absorption-luminescence processes belong to molecular orbitals localized in the region defined by the carbonyl group and the C=C bond closest to it, and that they are not strongly disturbed by complexation to the metal ion, i.e. they remain localized in the same region of the tropolone molecule. The excited state decay times reveal the nature of the emitting states: one singlet, S1, at 24270 cm − 1 (412 nm), and two triplets, T2 at 16610 cm − 1 and T1 at 15630 cm − 1. The decay curves for the state at 24270 cm − 1 are strongly non-exponential, with a temperature independent decay time of 3009 100 ns. We note that, even though this decay time appears at first sight to be too long for a singlet – singlet transition, one should bear in mind that this is simply the limit of the instrumental set-up, and that the zero-phonon line of this emission is actually resonant with the lowest energy excitation line, thus being most likely due to a singlet state. The decay curves observed for the states at 16610 and 15630 cm − 1 are single exponential, with temperature-dependent decay times, which are summarized in Table 2. Luminescence from any triplet state of tropolone has never been observed before. Nevertheless, an indirect estimate of the position of a triplet state (16800 cm − 1) was given by
Croteau and Leblanc [41], based on the quenching ability of tropolone towards the phosphorescence of several molecules. This value is consistent with our observation, taking into consideration the lowering of the energy states due to the complexation of tropolone to the lanthanide ion. The emission spectrum of Gd(trop)3 changes dramatically with increasing temperature, as shown in Fig. 5. Consistently with the decay time measurements, the emission of T2 is observed to be completely quenched above 60 K, while the emission of T1, is strongly quenched above 120 K, remaining up to 160 K. The fluorescence of S1, is essentially independent of temperature. The radiative and non-radiative processes occurring in the Gd(trop)3 complex are interpreted as follows. Under UV excitation the lowest excited singlet state, S1, is populated by internal conversion, decaying to the ground state nonradiatively and radiatively (fluorescence) and to both triplets, T1 and T2, by non-radiative intersystem crossing. The triplets in turn decay to the ground state radiatively (phosphorescence) and non-radiatively by thermally activated crossover. Therefore, the phosphorescence intensities are highest at 4.2 K, and decrease drastically with increasing temperature. The emission of T2 is totally quenched above 60 K, whereas the emission of T1, is no longer observed above 160 K, only the fluorescence remaining. Table 2 Decay times for two of the Gd(trop)3 emitting states, at different temperatures T (K)
17 20 22 25 30 40 50 56 70 100 114 150
Decay time, t (ms) 602 nm
640 nm
300 240 160 90 60 30 15 15
130 130 140 140 140 140 130 130 90 50 0,6
The monitored wavelengths were 602 and 640 nm.
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Fig. 5. Emission spectra of Gd(trop)3 upon 360 nm excitation at several temperatures: (a) 4.2 K, (b) 30 K, (c) 77 K, (d) 120 K, (e) 150 K. f gives the photon flux per constant wavelength interval (arbitrary units). The instrumental conditions were kept unchanged.
We will now consider the europium and terbium tropolonates. Although the energy levels of the Eu3 + and Tb3 + ions in tropolonate complexes cannot be directly observed, their position can be easily estimated from that in other compounds (e.g. LaF3 [42]), due to the weak interaction between the 4f electrons and their surroundings [43]. The absence of the ligand fluorescence (S1 S0) in the Eu(trop)3 complex can be ascribed to an efficient ligand–to–metal energy transfer. The 5D3 level of Eu3 + (ca. 24355 cm − 1 [42]) is nearly resonant with the singlet S1, of tropolone (24270 cm − 1), therefore allowing an efficient energytransfer process to take place. Considering the decay time of the fluorescence, the energy transfer rate S1(trop) 5D3(Eu3 + ) is estimated to be ] 109 s − 1. In the case of the Tb(trop)3 complex the ligand fluorescence is observed at low temperatures, but is no longer observed at room temperature, thus indicating that energytransfer ligandTb3 + also occurs, but is
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phonon assisted. This is not surprising considering the large energy mismatch between the S1 ligand state and the closest Tb3 + excited state (viz. the 5D3 level at ca. 26360 cm − 1 [42]), thus requiring phonon assistance to achieve the resonance condition. We note that for Gd(trop)3 these energy-transfer processes are not possible, since the lowest energy excited state of Gd3 + lies at ca. 32177 cm − 1 [42]. As shown above, after excitation into the singlet states of tropolone, energy-transfer S1 Ln3 + (Ln= Eu or Tb) takes place, thus populating the excited states of these ions. Since both triplet states (T2 at 16610 cm − 1 and T1 at 15630 cm − 1) lie below the emitting levels of the Eu3 + and Tb3 + ions (viz. 5D0 at ca. 17293 cm − 1 [42], and 5D4 at ca. 20568 cm − 1 [42]), the characteristic luminescence of these ions is totally quenched, even at 4.2 K, by back-transfer to the triplet states of tropolone. Considering that the radiative decay rates for Eu3 + and Tb3 + are typically 103 s − 1 [44], the back-transfer rates must be ] 105 s − 1. To explain that the phosphorescence of both triplets is also suppressed in Eu(trop)3 and Tb(trop)3 even at 4.2 K, one has to consider yet another quenching mechanism: a phonon-assisted Stokes cross-relaxation process, where the ligands relax down to an intermediate excited state and simultaneously the lanthanide ion is raised to an intermediate excited state, in such a way that energy is conserved and partially dissipated as heat. The initial states should be the ligand triplet states and the lanthanide ground level (7F0 for Eu3 + and 7F6 for Tb3 + [42]) and the final levels should be the 7th vibrational level of the ligand ground level (at 10290 cm − 1, considering coupling to the 1470 cm − 1 mode) and the 7F6 level of Eu3 + (at ca. 4907 cm − 1 [42]) or the 7F0 level of Tb3 + (at ca. 5784 cm − 1 [42]). Considering the decay times of the phosphorescence of T1, and T2, the cross-relaxation rates should be ] 105 s − 1. Although such a process has never been observed before for lanthanide complexes, it is rather common for lanthanide ions doped in inorganic hosts, such as oxide and fluoride crystals and glasses [44].
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4. Conclusions The complexes Gd(trop)3, Eu(trop)3, and Tb(trop)3 (trop =tropolone) are isostructural. Three tropolone excited states have been observed in these complexes: a singlet state S1, at 24270 cm − 1, and two triplet states, T2 and T1, at 16610 cm − 1 and 15630 cm − 1 respectively. In the Gd(trop)3 complex no energy-transfer ligand Gd3 + takes place, so that only ligand fluorescence and phosphorescence are observed, each with characteristic temperature dependencies. In contrast, in the Eu(trop)3 and Tb(trop)3 complexes no ligand luminescence is observed: the S1, fluorescence is quenched by energy-transfer to excited levels of Eu3 + or Tb3 + ions, and the T1 and T2 phosphorescence are quenched by a cross-relaxation process involving the vibrational levels of the tropolone ground level and the 7F6 level of Eu3 + or the 7F0 level of Tb3 + . The characteristic luminescence of the Eu3 + and Tb3 + ions is not observed due to back-transfer to the triplet states of tropolone.
Acknowledgements The authors are grateful to CAPES, CNPq, and FINEP (Brazilian agencies) for financial support and to Professor Cid B. de Aranjo and Mr Leonardo Meneses (Department of Physics-UFPE, Brazil) for help with the decay-time measurements. We are indebted to Professor Andries Meijerink (Universiteit Utrecht-The Netherlands) for kindly allowing the use of his laboratory facilities and for insightful discussions.
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[40] E.F. Breheret, M.M. Martin, J. Lumin. 17 (1978) 49. [41] R. Croteau, R. Leblanc, Photochem. and Photobiol. 28 (1978) 33. [42] W.T. Carnall, H. Crosswhite, H.M. Crosswhite, Energy Level Structure and Transition Probabilities of the Trivalent Lanthanides in LaF3, Argonne Natl. Laboratory report, 1977. [43] B. Henderson, G.F. Imbusch, Optical Spectroscopy of Inorganic Solids, Ch. 8, Clarendon Press, Oxford, 1989. [44] B. Henderson, G.F. Imbusch, Optical Spectroscopy of Inorganic Solids, Ch. 10, Clarendon Press, Oxford, 1989.
Spectroscopy and non-radiative processes in Gd3 + , Eu3 + and Tb3 + tropolonates B.S. Santos a, C. de Mello Donega´ a, G.F. de Sa´ a,*, L.F.C. de Oliveira b, P.S. Santos c b
a Depto. de Quı´mica Fundamental, UFPE, Cid. Uni6ersita´ria, CEP 50670 -901, Recife PE, Brazil Depto. de Quı´mica, Instituto de Cieˆncias Exatas, Uni6ersidade Federal de Juiz de Fora, CEP 36036 -330, Juiz de Fora MG, Brazil c Depto. de Quı´mica Fundamental, Instituto de Quı´mica, USP, C.P.26077, CEP 05599 -970, Sa˜o Paulo SP, Brazil
Received 10 November 1997; accepted 17 February 1998
Abstract Complexes of Eu3 + , Gd3 + , and Tb3 + with tropolone (tropolone =2-hydroxy-2,4,6-cycloheptatrien-1-one) were synthesized and characterized by C and H elemental analysis, infrared vibrational spectroscopy, X-ray powder diffraction analysis and Raman vibrational spectroscopy. The results show that the complexes are isostructural and have the formula [Ln(trop)3], where Ln=Eu3 + , Gd3 + or Tb3 + and trop= tropolone. The characteristic luminescence of the Eu3 + and Tb3 + ions was not observed either upon ligand excitation (300 – 380nm) or upon direct f–f excitation, not even at 4.2 K. In order to understand this phenomenon the spectroscopic properties of the Gd(trop)3 complex (absorption spectra at 298 K, excitation and emission spectra from 4.2 to 298 K and decay time measurements from 20 to 298 K) were investigated. The emission spectra show three distinct bands, which are ascribed to ligand electronic states: a singlet state, S1 (p– p*), at 24270 cm − 1, and two triplet states, T1 and T2, at 15630cm − 1 and 16610cm − 1, respectively. Both triplet states are situated below the emitting states of the Eu3 + and Tb3 + ions (5Do and 5D4, respectively), thus suppressing the luminescence from these states by back-transfer. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Lanthanide complexes; Raman spectroscopy; Luminescence; Non-radiative relaxation
1. Introduction The design and investigation of lanthanide complexes with organic ligands as efficient lightconversion molecular devices (LCMD) has become an important theme in coordination * Corresponding author. Tel.: + 55 81 27418441; fax: + 55 81 2710359; e-mail: [email protected]
chemistry, being pursued by several groups [1– 13]. Efficient LCMD may find many applications, such as luminescent labels in fluoroimmunoassays [2], lasers [14], and thin film electroluminescent devices [15]. In order to optimize the quantum yield and the light output of a LCMD several processes must be controlled: (i) the ligand absorption and internal decay processes; (ii) the efficiency of the ligand–to–metal energy transfer;
1386-1425/98/$ - see front matter © 1998 Elsevier Science B.V. All rights reserved. PII S1386-1425(98)00143-7
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and (iii) the luminescence efficiency of the metal ion. To achieve such a goal our group has adopted a strategy based upon both theoretical and experimental approaches [11 – 13,16 –28], investigating qualitative and quantitative aspects. Therefore, in order to gain insight into the factors that determine the quantum yield of lanthanide complexes we have synthesized and investigated the photophysical properties of a number of new lanthanide(III) complexes [11– 13,20,25–28]. Furthermore, our group has been successfully developing theoretical models to determine the geometry of a lanthanide complex [16,17], the position and nature of the ligand excited states in the complex [18,19], the efficiency of the ligand – to – metal energy transfer [21–23], and the luminescence quantum yields [24]. The results have shown that the quantum yields can be enhanced by an appropriate choice of ligands (e.g. – CF3 containing ligands [13,19,27], N-oxide ligands [11,13,26 – 28]) and are lowered not only by inefficient energy transfer ligand Eu3 + , but also by several 5Do nonradiative relaxation processes (e.g. multiphonon relaxation [11,22,24], phonon-assisted crossover to ligand-to-Eu3 + charge transfer state [25], etc.). Also, we have been able to develop complexes which are very promising LCMD, with room temperature quantum yields as high as 66% [13] and light outputs comparable to those of commercial phosphors. In this paper we will present new results on another quenching mechanism, viz. backtransfer to ligand states. Preliminary results on the spectroscopic properties of the Eu3 + , Gd3 + , and Tb3 + complexes with tropolone, (Ln(trop)3), have been previously reported [29]. The lack of the characteristic luminescence of Eu3 + and Tb3 + ions in these complexes was ascribed to non-radiative decay processes via excited ligand states. In order to better understand the quenching processes operative in lanthanide tropolonates we have carried out further spectroscopic studies on these complexes, including excitation and emission spectra from 298 to 4.2 K. As a part of the structural characterization of the complexes, special attention has been given to the analysis of their Raman spectra.
2. Experimental The complexes were prepared following the procedure reported by Muetterties and Wright [30], which consists of mixing an ethanolic solution of tropolone with an aqueous solution of lanthanide nitrate in the molar proportion of 3:1. The resultant solution is stirred for about 1 h at 60°C, yielding a pale yellow solid, which is filtered, washed with hot ethanol and dried under reduced pressure (B 1 mmHg) over P2O5. The solution of the lanthanide nitrate is obtained by dissolving the sesquioxide in an aqueous solution of nitric acid (molar ratio 1:3), and adjusting the pH to 6.0. The lanthanide oxides Eu2O3 (Aldrich, 99.99%), Gd2O3 (Molycorp, 99.99%) and Tb4O7 (Aldrich, 99.99%) were used without further purification, while tropolone (Aldrich, 98%) was recrystallized from ethanol and sublimed. The complexes were characterized by elemental analysis (C and H), infrared (JR) and Raman vibrational spectroscopies, UV-Visible electronic absorption spectroscopy, X-ray powder diffractometry and thermogravimetry. The UV-visible absorption spectra were obtained at room temperature with a Perkin-Elmer Lambda 6 spectrophotometer (spectral resolution of 0.5 nm). The spectrum of tropolone was obtained from aqueous and cyclohexane solutions, whereas the spectra of the complexes were obtained in the solid state with the samples dispersed in KBr pellets. Thermogravimetric analyses were carried out on a Shimadzu TGA-50, under a N2 flow, at 10 K min − 1, up to 900°C. The IR vibrational spectra were measured on a Bruker IF566 FTIR spectrophotometer (resolution, 4 cm − 1) using KBr pellet and nujol mull techniques. The Raman spectra were obtained with a Jobin-Yvon U- 1000 Raman spectrometer using a photomultiplier tube and photon counting system for detection. The spectral resolution was 5 cm − 1, the acquisition time 0.5 s at steps of 1 cm − 1. The spectra of the solid samples were obtained using a rotating disk in the 30–1800 cm − 1 range. The spectrum of sodium tropolonate in alkaline aqueous solution, in the 100– 1800 cm − 1 region, was obtained with a spinning cell for liquids. The spectra were excited using the
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514.5 nm line of an argon laser (Coherent Innova 90) and the power level was kept to a minimum to avoid photodecomposition of the sample. The luminescence and excitation spectra of the solid complexes were obtained by using a SPEX Fluorolog DM3000F Spectrofluorometer with double grating 0.22m SPEX monochromators (spectral resolution: 10 cm − 1) and a 450 UV Xe lamp as the excitation source. This set-up is equipped with an Oxford LF205 liquid helium flow cryostat, allowing measurements down to 4.2 K. The spectra are corrected for the instrumental response. Selective excitation of the 5Do energy level of Eu3 + in the complex (580nm), was performed at room temperature with a Nd:YAG pumped dye laser (Rhodamine 6G). The photomultiplier used in all the measurements was an RCA C31034-02. Decay time measurements for the emitting states of the Gd(trop)3 complex were performed between 20 and 293 K, using a closed cycle helium cryostat. The sample was excited with the third harmonic of a Nd:YAG pulsed laser (lexc = 355 nm) and the emission was monitored at three wavelengths: 440, 602 and 640 nm. The photocurrent was acquired and analyzed by a digital oscilloscope (HP 54501A 100 MHz). The temporal resolution of this set-up is limited to ca. 200 ns.
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Fig. 1. Structural formula of tropolone
tropolone are displayed in Fig. 2. The band positions are presented in Table 1 together with a tentative assignment based on previous work by Ikegami [31] and Thornton et al [32,38]. By analyzing the spectra of the lanthanide complexes and comparing them to that of the free ligand, some important features are observed. First, the Raman spectra of the complexes all have the same pattern, suggesting the same coordination geometry. This is consistent with the absence of any significant difference between the vibrational IR spectra of the three complexes. Considering that the Xray powder diffraction patterns of the complexes are also very similar one can conclude that the complexes are isostructural. The second important feature is a clear increase in the number of bands in the region characteris-
3. Results and discussion
3.1. Structural aspects: Raman spectroscopy The chemical analytical data (C and H) of the complexes indicate the formula [Ln(trop)3], where Ln=Eu3 + , Gd3 + or Tb3 + and trop =tropolonate ion (C7H5O2) [Eu(trop)3 =found: C, 48.75; H, 3.02; calc.: C, 48.96; H, 2.93; Gd(trop)3 = found: C, 48.70; H, 2.72; calc.: C, 48.40; H, 2.88; Tb(trop)3 =found: C, 48.40; H, 2.93; calc.: C, 48.31; H, 2.89%]. The proposed formula is also consistent with the thermogravimetric analytical data and the vibrational IR spectra of the complexes. The structural formula of tropolone is presented in Fig. 1. The Raman spectra of the tropolonate lanthanide complexes as well as the spectrum of
Fig. 2. Raman spectra of (a) solid Gd(trop)3, (b) solid Tb(trop)3 (c) solid Eu(trop)3 and (d) solid tropolone. For details see Table 1 and text.
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Table 1 Observed Raman lines (in cm−1) of solid tropolone and of the Eu3+, Gd3+ and Tb3+ tropolonate complexes Solid tropolone
Eu(trop)3
Gd(trop)3
Tb(trop)3
Assignment
1606
1597 1570 1533 1469 1448 1420 1404 1385
1598 1571 1533 1469 1449 1421 1406 1385 1359
1599 1572 1534 1470 1451 1423
1246 1223
1244 1221 1195 1071 976
1246 1224
877 739 707 564 502 491 421 410 383
876 742 707 566 503 490 420 410
877 738 709 569 506 496 425 412 380
n(C =O)+n(C= C) n(C =O)+n(C =C) n(C =O)+n(C =C) n(C =C)+n(C =O) C–H bending C–H bending C–H bending n(C–C) C–H bending+n(C–C) C–H bending C–O–H group C–O–H group C–H bending C–H bending+n(C–C) n(C–C) n(C–C) C–H bending Ring breathing nLn–O nLn–O
236
240
242
213 169 108 48
170 110 50
220 173 113 50
1540 1460 1423 1403
1271 1237 1190 982 961 875 743 688
451 370 342
1071 977
1386
1072 977
204
66
tic of the carbon – carbon and carbonyl stretching modes ( 1400 – 1600 cm − 1) after complexation. These are coordination-sensitive modes and an increase of the number of bands in this region could be regarded as a consequence of a decrease of the ligand symmetry. The ring breathing mode of solid tropolone in the Raman spectrum was assigned by Ikegami [31] to a band at 744 cm − 1 and is observed in this work as a sharp peak in 742 cm − 1. This same frequency mode is found as a rather weak and poorly defined band with highest intensity at 739, 742 and 738 cm − 1 respectively for the Eu3 + , Tb3 + and Gd3 + tropolonate complexes. This lowering of the ring breathing mode could be interpreted as a deviation from
nLn–O Skeletal deformation nLn–O Skeletal deformation Skeletal deformation nLn–O Skeletal deformation nLn–O dO–Ln–O dO–Ln–O Lattice mode
planarity (the seven-membered ring is essentially planar in solid tropolone due to intramolecular hydrogen bonding [33]). Different degrees of ringdistortion have been observed in other tropolonate complexes [34–37]. The metal–ligand stretching modes are tentatively assigned (Table 1) based on the previous assignments made by Hullet and Thornton [37] and Burden et al. [32] in the infrared vibrational spectra of tropolonate transition metal and lanthanide complexes. This assumption indicates a rather strong metal–ligand interaction in the complexes, that is corroborated by the observed change in the pattern of the ligand spectrum. Such an observation is in marked contrast with the
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Raman spectra of squarate lanthanide complexes [39], in which the pattern in the Raman spectrum of the ligand was almost unaffected by complexation with the Eu3 + and Tb3 + ions. It is also interesting to note that the analysis of the emission spectra of the Gd(trop)3 (Fig. 4, see below) reveals a vibronic progression based on the 1470 and 1595cm − 1 modes, assigned to C – C and C–O stretching. This is a clear indication of a significant change in the geometry of the molecule in the excited state along the corresponding normal coordinates. Consistent with this, the analysis of the Raman spectra shows that the same modes are very intense, which is in accord with the vibronic theory of the Raman effect, in which it is expected that the more intense modes are the ones showing greater geometric changes of the corresponding normal coordinates in the excited state. On the same basis, one should expect that a substantial variation in the geometry of the ligand in the excited state should also reflect the nature of the metal–ligand bonding. This is in line with the remarkable intensity of the metal – oxygen stretching mode, at ca. 700 cm − 1, in the Raman spectra.
3.2. Electronic spectra The absorption spectra of the complexes (Fig. 3) are very similar to each other, and correspond well to the tropolone absorption spectrum, apart
Fig. 3. Room temperature absorption spectra of (a) solid Gd(trop)3, (b) solid Eu(trop)3, (c) solid Tb(trop)3 and (d) tropolone in cyclohexane solution. The intensities have been normalized to the most intense band of each spectrum.
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Fig. 4. Emission (lexc =360 nm, solid line) and excitation (lem =597 nm, dotted line) spectra of Gd(trop)3 at 4.2 K. qr is the relative quantum output for the excitation spectrum and f gives the photon flux per constant wavelength interval in the emission spectrum (arbitrary units).
from the red shift due to the complexation to the metal ion. The tropolone absorption spectrum shows two separate bands in the 210–270 nm and 270–400 nm wavelength ranges. From previous studies on tropolone solutions [40,41] it has been established that each of the two absorption bands results from the superposition of two (p, p*) electronic transitions. Correspondingly, the absorption spectra of the complexes also show two broad bands in the UV region: B 200–270 nm (with a maximum at 237 nm) and 270–420 nm (with maxima at around 334, 387, 397 and 410 nm). Despite the poorer definition of this band one may still observe some structure, which resembles that observed in the tropolone absorption spectrum. The characteristic f–f emission lines of the Eu3 + and Tb3 + ions were not observed, either upon ligand excitation (300–380 nm) or upon direct f–f excitation, even at 4.2 K. The Gd(trop)3 complex shows ligand related luminescence. This luminescence is also observed in the Tb(trop)3 complex, although at low temperatures only, and even then it is much weaker than that observed in the gadolinium complex. The complex Eu(trop)3 does not show any ligand luminescence at all, even at 4.2 K. The emission spectrum of solid Gd(trop)3 at 4.2 K (Fig. 4) consists of two distinct
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bands: 400– 525 nm and 597 – 740 nm. The zerophonon line of the higher energy emission band is observed at 412 nm (24270 cm − 1), resonant with the zero-phonon line of the excitation band. The lower energy emission band is actually composed of two overlapping emission bands: one consisting of a sharp zero-phonon line at 602 nm (16610 cm − 1) with a vibrational progression based on two frequencies: 1470 and 1595 cm − 1 (C =C and C–O stretching modes, respectively, both observed in the Raman and IR spectra), and the other characterized by a vibrational progression based on a 1450 cm − 1 frequency mode, with the zero-phonon line at 640 nm (15630 cm − 1). The excitation spectrum of Gd(trop)3 at 4.2 K (Fig. 4) bears a very close resemblance with the absorption spectrum of tropolone, apart from the expected shift to lower energies. From a consideration of the vibronic frequencies one may conclude that the electronic states involved in the absorption-luminescence processes belong to molecular orbitals localized in the region defined by the carbonyl group and the C=C bond closest to it, and that they are not strongly disturbed by complexation to the metal ion, i.e. they remain localized in the same region of the tropolone molecule. The excited state decay times reveal the nature of the emitting states: one singlet, S1, at 24270 cm − 1 (412 nm), and two triplets, T2 at 16610 cm − 1 and T1 at 15630 cm − 1. The decay curves for the state at 24270 cm − 1 are strongly non-exponential, with a temperature independent decay time of 3009 100 ns. We note that, even though this decay time appears at first sight to be too long for a singlet – singlet transition, one should bear in mind that this is simply the limit of the instrumental set-up, and that the zero-phonon line of this emission is actually resonant with the lowest energy excitation line, thus being most likely due to a singlet state. The decay curves observed for the states at 16610 and 15630 cm − 1 are single exponential, with temperature-dependent decay times, which are summarized in Table 2. Luminescence from any triplet state of tropolone has never been observed before. Nevertheless, an indirect estimate of the position of a triplet state (16800 cm − 1) was given by
Croteau and Leblanc [41], based on the quenching ability of tropolone towards the phosphorescence of several molecules. This value is consistent with our observation, taking into consideration the lowering of the energy states due to the complexation of tropolone to the lanthanide ion. The emission spectrum of Gd(trop)3 changes dramatically with increasing temperature, as shown in Fig. 5. Consistently with the decay time measurements, the emission of T2 is observed to be completely quenched above 60 K, while the emission of T1, is strongly quenched above 120 K, remaining up to 160 K. The fluorescence of S1, is essentially independent of temperature. The radiative and non-radiative processes occurring in the Gd(trop)3 complex are interpreted as follows. Under UV excitation the lowest excited singlet state, S1, is populated by internal conversion, decaying to the ground state nonradiatively and radiatively (fluorescence) and to both triplets, T1 and T2, by non-radiative intersystem crossing. The triplets in turn decay to the ground state radiatively (phosphorescence) and non-radiatively by thermally activated crossover. Therefore, the phosphorescence intensities are highest at 4.2 K, and decrease drastically with increasing temperature. The emission of T2 is totally quenched above 60 K, whereas the emission of T1, is no longer observed above 160 K, only the fluorescence remaining. Table 2 Decay times for two of the Gd(trop)3 emitting states, at different temperatures T (K)
17 20 22 25 30 40 50 56 70 100 114 150
Decay time, t (ms) 602 nm
640 nm
300 240 160 90 60 30 15 15
130 130 140 140 140 140 130 130 90 50 0,6
The monitored wavelengths were 602 and 640 nm.
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Fig. 5. Emission spectra of Gd(trop)3 upon 360 nm excitation at several temperatures: (a) 4.2 K, (b) 30 K, (c) 77 K, (d) 120 K, (e) 150 K. f gives the photon flux per constant wavelength interval (arbitrary units). The instrumental conditions were kept unchanged.
We will now consider the europium and terbium tropolonates. Although the energy levels of the Eu3 + and Tb3 + ions in tropolonate complexes cannot be directly observed, their position can be easily estimated from that in other compounds (e.g. LaF3 [42]), due to the weak interaction between the 4f electrons and their surroundings [43]. The absence of the ligand fluorescence (S1 S0) in the Eu(trop)3 complex can be ascribed to an efficient ligand–to–metal energy transfer. The 5D3 level of Eu3 + (ca. 24355 cm − 1 [42]) is nearly resonant with the singlet S1, of tropolone (24270 cm − 1), therefore allowing an efficient energytransfer process to take place. Considering the decay time of the fluorescence, the energy transfer rate S1(trop) 5D3(Eu3 + ) is estimated to be ] 109 s − 1. In the case of the Tb(trop)3 complex the ligand fluorescence is observed at low temperatures, but is no longer observed at room temperature, thus indicating that energytransfer ligandTb3 + also occurs, but is
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phonon assisted. This is not surprising considering the large energy mismatch between the S1 ligand state and the closest Tb3 + excited state (viz. the 5D3 level at ca. 26360 cm − 1 [42]), thus requiring phonon assistance to achieve the resonance condition. We note that for Gd(trop)3 these energy-transfer processes are not possible, since the lowest energy excited state of Gd3 + lies at ca. 32177 cm − 1 [42]. As shown above, after excitation into the singlet states of tropolone, energy-transfer S1 Ln3 + (Ln= Eu or Tb) takes place, thus populating the excited states of these ions. Since both triplet states (T2 at 16610 cm − 1 and T1 at 15630 cm − 1) lie below the emitting levels of the Eu3 + and Tb3 + ions (viz. 5D0 at ca. 17293 cm − 1 [42], and 5D4 at ca. 20568 cm − 1 [42]), the characteristic luminescence of these ions is totally quenched, even at 4.2 K, by back-transfer to the triplet states of tropolone. Considering that the radiative decay rates for Eu3 + and Tb3 + are typically 103 s − 1 [44], the back-transfer rates must be ] 105 s − 1. To explain that the phosphorescence of both triplets is also suppressed in Eu(trop)3 and Tb(trop)3 even at 4.2 K, one has to consider yet another quenching mechanism: a phonon-assisted Stokes cross-relaxation process, where the ligands relax down to an intermediate excited state and simultaneously the lanthanide ion is raised to an intermediate excited state, in such a way that energy is conserved and partially dissipated as heat. The initial states should be the ligand triplet states and the lanthanide ground level (7F0 for Eu3 + and 7F6 for Tb3 + [42]) and the final levels should be the 7th vibrational level of the ligand ground level (at 10290 cm − 1, considering coupling to the 1470 cm − 1 mode) and the 7F6 level of Eu3 + (at ca. 4907 cm − 1 [42]) or the 7F0 level of Tb3 + (at ca. 5784 cm − 1 [42]). Considering the decay times of the phosphorescence of T1, and T2, the cross-relaxation rates should be ] 105 s − 1. Although such a process has never been observed before for lanthanide complexes, it is rather common for lanthanide ions doped in inorganic hosts, such as oxide and fluoride crystals and glasses [44].
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4. Conclusions The complexes Gd(trop)3, Eu(trop)3, and Tb(trop)3 (trop =tropolone) are isostructural. Three tropolone excited states have been observed in these complexes: a singlet state S1, at 24270 cm − 1, and two triplet states, T2 and T1, at 16610 cm − 1 and 15630 cm − 1 respectively. In the Gd(trop)3 complex no energy-transfer ligand Gd3 + takes place, so that only ligand fluorescence and phosphorescence are observed, each with characteristic temperature dependencies. In contrast, in the Eu(trop)3 and Tb(trop)3 complexes no ligand luminescence is observed: the S1, fluorescence is quenched by energy-transfer to excited levels of Eu3 + or Tb3 + ions, and the T1 and T2 phosphorescence are quenched by a cross-relaxation process involving the vibrational levels of the tropolone ground level and the 7F6 level of Eu3 + or the 7F0 level of Tb3 + . The characteristic luminescence of the Eu3 + and Tb3 + ions is not observed due to back-transfer to the triplet states of tropolone.
Acknowledgements The authors are grateful to CAPES, CNPq, and FINEP (Brazilian agencies) for financial support and to Professor Cid B. de Aranjo and Mr Leonardo Meneses (Department of Physics-UFPE, Brazil) for help with the decay-time measurements. We are indebted to Professor Andries Meijerink (Universiteit Utrecht-The Netherlands) for kindly allowing the use of his laboratory facilities and for insightful discussions.
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