摘要:
1726 J.C.S. DaltonDIKETONATESA Study of Fluorescence in Some Tris- and Tetrakis-diketonates of Europ-ium(ii1) by Examination of Absorption and Excitation SpectraBy S. J. Lyle,' J. E. Newbery, and A. D. Witts, The Chemical Laboratory, University of Kent at Canterbury, KentFrom a spectroscopic study of tris- and tetrakis-diketonate complexes of europium(ii1) information on the mechan-ism by which radiation absorption in the ligand produces fluorescence characteristic of the metal ion has beenobtained. Excitation and absorption spectra of the complexes in the solid state and in solution are used to showthat energy transfer takes place wholly, or predominantly, from one level of the first excited singlet state to thetriplet manifold of the ligand from which excitation of the europium(li1) occurs.Problems in obtaining excitationspectra free from distortion are discussed.of europium(II1) are known to emitorange-red fluorescence- characteristic of the f-f tran-sitions in the europium when they are irradiated withlight in the near-u.v. region of the spectrum. Crosby l y 2has proposed a mechanism for the process wherebyenergy is transferred from the triplet state of the ligandto the 5D manifold of the europium(II1) with subsequentradiative decay to the 7F state. This outline mechanismhas been further supplemented by information from time-resolved spectroscopy 3~ and fine-structure in theemission spectra has been used to suggest stereochemicalarrangements for europium(II1) in the ~helates.~$*Published work of a mechanistic nature has beenbased on studies of the emission process or processes.However, although excitation spectra have beenexamined in photochemical studies of organic sub-stances9 their use in work on europium(II1) diketonatesystems appears to have been neglected.The onlyreport lo to our knowledge referring to europium excit-ation in such chelates deals but briefly with somestudies carried out on solutions.In the work to be described, fluorescence excitationspectra are examined for the europium(II1) diketonateswhich are listed in Table 1. While the emphasis hasbeen placed on measurements in the solid state, aspectsof several systems have also been studied in solution inorganic solvents. Reflectance spectra for the solidG. A.Crosby, R. E. Whan, and R. M. Alire, J . Chem. Phys.,1961, 34, 743.M. L. Bhaumik, G. L. Clark, J. Snell, and L. Ferder, Rev.M. L. Bhaumik and L. J. Nugent, J . Chem. Phys., 1965, 43,ti S. Sat0 and M. Wada, Bull. Chem. SOC. Japan, 1970, 43,M. Tanaka, G. Yamaguchi, J. Shioltawa, and C . Yamanaka,* G. A. Crosby, Mol. Crystals, 1966, 1, 37.Sci. Instr., 1965, 36, 37.1680.1955.Bull. Chem. SOC. Japan, 1970, 43, 549.substances and, where required, absorption spectra ofsolutions have been obtained in the wavelength range(240-500 nm) covered in the excitation measurements.TABLE 1Abbreviation EuropiumDiketone used 0 chelate1,3-Diphenylpropane-l,3-dione Hdbm [Eu(dbm),]PIP bl,l,l-Trifluor0-5,5-dimethyl- Hpta [Eu(pta),]NaI, l,l-Trifluor0-4-(2-thienyl)- Htta [Eu(tta),]PY 6l-Phenylbutane-l,3-dione Hba [Eu(ba),]PIPEu (ba) ,,2H20l,l,l-Trifluoro-4-phenylbutane- Hbta [Eu(bta),PIP]1 , l,l-Trifluoropentane-2,4-dione Htfa [Eu (tfa) ,] PIP1, 1 , 1 ,5,5,6-Hexafluoropentane- Hhfa [ Eu (hfa),]PIPEu(dbm),hexane-2,4-dionebutane-2,4-dione2,4-dione Eu (bta),,2H20Eu (tfa) ,,2H,O2,4-dione Eu (hfa) ,,2H20removal of a hydrogen ion.c PY Is the pyridinium ion.0 The ligand is considered to be derived from the diketone byb PIP Is the piperidinium ion.Problems relating to the elimination of spectral dis-tortion are considered.Practical implications of theresults and some aspects of energy transfer leading toeuropium(II1) excitation in the molecule are discussed.EXPERIMENTALThe europium(II1) diketonates (Table 1) were preparedK.Kreher. E. Butter, and W. Siefert. 2. Naturforsch.. 1967.and purified as previously described.ll22b, 242.J . Phys. Chenz., 1968, 72, 970.13 S. Bjorklund, N. Filipescu, N. McAvoy, and J. Degnan,a C. A. Parker, ' Photoluminescence of Solutions,' Elsevier.London, 1968.SOC. Japan, 1968, 41, 1513.10 Y. Matsukda, S. Makishima, and S. Shionoya, Bull. Chew.l1 S. J. Lyle and A. D. Witts, Inorg. Chim. Acta, 1971, 5, 4811972 1727Spectra.-Electronic reflectance spectra were obtainedusing a Pye-Unicam SP 500-2 spectrophotometer fittedwith a reflectance attachment having a magnesium oxidereference surface. The diketonates were diluted withpotassium bromide (5 VM to 1.00 g of potassium bromide).It was confirmed that fluorescence of the europium(II1)was not augmenting the reflected light by making a com-parison with the corresponding (non-fluorescent) lanthanumcomplex. Solution spectra were taken in hexane with cellsof 1 cm path-length using a Hitachi-Perkin-Elmer 139spectrophotometer.A standard Aminco-Bowman spectrophotofluorimeterwas employed to obtain the fluorescence excitation spectra.The emission monochromator was set ts pass light from the+ 'F2 transition (ca.613 nm) 2 in europium(xI1).Frontal illumination and detection were used in examin-ation of the solid samples (held in a de-mountable silica(b) Excitation spectra from solutions were corrected forinner filter effects 12 by assuming the validity of the Beer-Lambert law a t all wavelengths and using the measuredmolecular extinction coefficients.(c) A spike ' at 307 nm, due to second order diffractiona t the excitation monochromator, was eliminated graphicallyfrom each spectrum.RESULTSFigure 1 provides a range of uncorrected spectra takenusing solid samples.It is seen that there are considerabledifferences in shape between the absorption spectrum ob-tained by reflectance measurement and the excitationspectrum for the same chelate-potassium bromide mixture.Further, in most instances, excitation spectra appear tochange considerably with change in chelate ' concentration 'in the potassium bromide ' diluent.' A major problem in1, (iy , ... ...... :...FIGURE 1 Fluorescence excitation and U.V.reflectance spectra of solid samples of various europium(II1) diketonates ; (i)-(iv) thesolid line represents the U.V. reflectance spectrum of a KBr mixture and curve (A) represents the fluorescence excitation spectrum ofthe neat compound; (i) [Eu(bta),]PIP, curve (B) is the excitation spectrum of the solid diluted with KBr in the ratio 1 : 100(wtlwt); (ii) [Eu(ba) JPIP, curve (B) is the excitation spectrum of the solid diluted with KBr in the ratio 1 : 640; (iii) Eu(bta),,-2H,O, curve (B) is the excitation spectrum of the solid diluted with KBr in the ratio 1 : 100; (iv) Eu(tfa),,2H20, curves (B) and(C) are the excitation spectra of the solid diluted with KBr in the ratios 1 : 6 and 1 : 60 respectivelycell) and the thin films. The latter were prepared bycontrolled evaporation of ethanol or benzene solutions of adiketonate on a silica surface.On examination with amicroscope (450 x ) ' film ' produced from ethanolic solutionsturned out to consist of very small but discrete crystalscovering approximately one third of the surface area of thesilica. Benzene gave a continuous and therefore moreuniform deposit with evidence for crystallinity only at theedges of the preparation, Unfortunately this solvent onlyappears to be suitable for those chelates containing phenylgroups.Excitation spectra from hexane solution were examinedusing right angled illumination-detection geometry in acell of 8 mm X 8 mm cross-section.Corrections to Excitation Spectra.-(a) The light intensity-wavelength relation for the Xenon arc source (Mazda typeXE/D, ref. no.98-0352) was determined experimentallyby monitoring the output at 2 nm intervals between 240and 500 nm using a photomultiplier tube of known response(E.M.I. 9665B). A computer programme was written touse this data to correct excitation spectra to constant lightintensity.l2 Ref. 9, p. 220.obtaining excitation spectra in solution is known to arisefrom so-called inner filter effects.12 It is reasonable toassume that similar problems will be encountered inmeasurements on solids.The inner filter effect was therefore examined in solutionwhere it can be arranged to arise from attenuation of theincident light beam by the chelate substance. (Fluorescenceradiation is readily shown to be transmitted efficientlythrough the solution.) The results in Table 2 show howTABLE 2The apparent wavelength for maximum excitation, &,at various concentrations of [Eu(ba),]PIP in hexaneConcn.(106111) 230 60 30 15 8 4L m X . (4 368 357 350 340 332 330the apparent wavelength for maximum excitation canchange with solute concentration for a europium diketonate.If a correction is made for absorption of incident radiationthen for some but not all systems reasonable agreement isobtained between the solution absorption spectrum and theexcitation spectrum at the same concentration in thecommon solvent (Figure 2). There is aIso agreemen1728 J.C.S. Daltonbetween the corrected excitation spectra for the solutionsand the reflectance spectrum although one might anticipatesome minor discrepancies attributable to matrix differencesbetween the samples.Hence for solutions from which1 1 1 125 0 350Wavelength Inm)FIGURE 2 Fluorescence excitation and U.V. absorption spectraof [Eu(bta),]PIP in hexane solution; (A), excitation spectrum(concentration 15 x 1 0 - s ~ ) uncorrected for the internal filtereffect (i.f.e.) ; (B), excitation spectrum (concentration 6 x1 0 - 6 ~ ) uncorrected for the i.f.e.; (C), excitation spectrum(concentration 15 x l O - ' j ~ ) corrected for the i.f.e.; (D),absorption spectrum (concentration 6 x 1 0 - s ~ )excitation spectra are recorded by the method describedhere essentially undistorted spectra are obtained (see forexample Figure 2) by taking proper account of inner filtereffects.Exceptions can, however, arise if, as is not un-common for this class of substance, the chelate is partlydecomposed in solution. The example quoted in Table 2is a case in point. The corrected excitation spectra forsolutions of 15 x and 4 X 10-6~ were found to becoincident and to be in reasonable agreement with thereflectance spectrum but not with the absorption spectrumfrom the corresponding solution. Decomposition wasconfirmed by the eventual spontaneous deposition of awhite substance from the solution.It was concluded from the work on solutions that anexamination of solid samples under conditions whereby thisparticular filter effect could be accounted for or eliminatedwas required. This decision was reinforced by a com-parison of spectra in Figure 1.It will be seen that some ofthe broad excitation spectra have minima in the wave-length region of maximum light absorption. Spectra fromsolutions are, however, much more easily corrected thanthose from solids because the sample is homogeneous andthe incident light-sample-detector geometry better defined.Means were therefore sought whereby filter effects in thesolid samples could be eliminated. To this end methods forthe production of very thin layers of sample were in-vestigated.Silica plates were lightly dusted with finely dividedchelate in initial experiments. A typical spectrum isl3 A. P. B. Sinha in ' Spectroscopy in Inorganic Chemistry,'eds. C . AT. R. Rao and J. R. Ferraro, Academic Press, London,1971, vol.11, p. 255.recorded in Figure 3. Comparison of it with spectra inFigure 1 for the same substance would suggest that filtereffects are present in the former. The microcrystalline' film ' produced from ethanol solution gave much broaderexcitation spectra than the more uniform ' film derivedfrom benzene solution (see Figure 3). The latter gaveexcitation spectra coincident with the absorption spectrawithin the precision of the instruments employed (Figure 3).DISCUSSIONIt is now generally accepted that light absorption inthe europium diketonates results initially in a singlet --wsinglet (So + S,) transition in the ligand. As mightbe expected on general theoretical grounds directpopulation of triplet levels from So has been shown lo tohave a negligible probability.There is now, however,an appreciable body of evidenceI3 to show that energyin the S, manifold is transferred by radiationless pro-cesses (inter-systems crossing) to one or more tripletmanifolds, T , which in turn pass on all or part of theirenergy to the europium(II1). Thus there may be morethan one channel for energy transfer from the S, levelsto the 5D manifold of the europium(II1). For theefficiency of fluorescence to be independent of thewavelength of excitation all transfers from S, to Tshould occur from one particular vibrational level(probably the lowest) in the S, manifold.In the family of substances under consideration it isnot obvious that this condition will be fulfilled.SinceWavelength ( n m lFIGURE 3 Fluorescence excitation and U.V. absorption spectraof [Eu(bta),]PIP in the form of thin films. Curves (A)-(C)are fluorescence excitation spectra; (A), a thin sprinkling ofsolid complex; (B), an ' alcohol ' film; ( C ) , a ' benzene ' film;(D), absorption spectrum of a ' benzene ' film (for further detailssee text)triplet states are involved, the ligands might be expectedto have features characteristic of organic substancesexhibiting phosphorescence. From work on such sub1972 1729stances there is some evidence 14915 for direct inter-systems crossing from an upper, as well as from thelowest, vibrational level of the excited singlet stategiving rise to a wavelength dependent efficiency ofphosphorescence.Formerly it was estimatedl6 that energy transferfrom the triplet to the europium(Ii1) occurred in a timeof the order of s but more recent work has resultedin this time being revised to 10-8 s.It would thusappear that de-excitation to the lowest vibrational levelof the particular triplet state is probable beforeeuropium(II1) excitation occurs.From the experiments carried out on solid and liquidsamples of europium(II1) diketonates we consider thatthe excitation spectrum and corresponding absorptionspectrum are coincident for the tris- and tetrakis-l4 R. Bauer and A. Baczynski, Bull. Acud. polon. Sci., 1958,6, 113.l5 C . A. Parker and C. G. Hatchard, Trans. Faraduy SOL, 1961,57, 1894.complexes. The fluorescence efficiency for a giveneuropium(II1) diketonate is therefore essentially constantand independent of the wavelength of the excitingradiation. This constancy, in turn, may be taken asevidence for the predominance of energy transfer fromone level of the first excited singlet of the ligand to thetriplet manifold from which excitation of the europium-(111) occurs.Excitation at the wavelength of maximum absorptionof the chelate or, as is prevalent, by using a single linein the mercury emission spectrum need not result in theoptimum yield of fluorescence radiation from a givensample. This will be particularly true for solid prepar-ations as is readily seen from Figure 1.One of us (A. D. W.) thanks the S.R.C. for the award of aResearch Studentship during the tenure of which this workwas carried out.[2/104 Received, 18th Januavy, 19721l6 M. L. Bhaumik and M. A. El-Sayed, J. Cherut. Phys., 1965,42, 787
ISSN:1477-9226
DOI:10.1039/DT9720001726
出版商:RSC
年代:1972
数据来源: RSC