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Excited singlet states of molybdenum chloride clusters
Volume 202, number 5
CHEMICAL PHYSICS LETTERS
29 January 1993
Excited singlet states of molybdenum chloride clusters U. Strauss, N. Perchenek, W.W. Riihle, H.J. Queisser and A. Simon Max-Planck-lnstitut fiir Festkijrperfrschung, Heisenbergstrasse 1, W-7000Stuttgart80, Germany Received 2 November 1992
Excited singlet states of [Mo&l,~]‘- cluster anions, as previously predicted but undiscovered, are detected in single crystals of [N(C,H,)&Mo&I,, by means of photoluminescence measurements with time resolution in the picosecond regime. The intensity of one of the transitions shows an unexpected dependence upon beam incidence angle relative to the crystallographic axes.
1. Introduction Compounds with octahedral transition-metal clusters capture much attention because of unusual structural and physical properties (for a detailed review, see ref. [ 1] ) [2-41. One particularly fascinating and fundamental example of this class of materials is the molybdenum cluster anion [ MosX,,] *[ 51. The octahedral metal-atom core is surrounded by halogen atoms, capping all faces and corners of the octahedron, as shown in fig. 1. Compounds with this anion are remarkably stable, they show fast light-
n
Fig. 1. Structure of the [ Mo&~,~] *- cluster anion.
induced electron transfer, which has made them candidates for specific photon-induced reactions utilizing long-lived metastable states possessing high electron affinity, and for investigating novel methods of energy storage [ 6-8 1. Remarkable quantum efficiencies for radiative emission have been reported [ 71. The physics of these anions and their solid state compounds are as yet only marginally understood. Theoretical calculations with a variety of computational methods predicted the existence of several closely spaced excited states [5,9-l 11. All previous spectroscopic investigations have, however, been unable to verify these calculations. No evidence was found for the existence of this manifold of excited states. Only long-lived luminescence from one of the excited triplet states has been experimentally confirmed; its lifetime was determined to lie in the range of a few tenths of a millisecond [6 1. No emission from the excited singlet states has thus far been observed, although these transitions ought to be spinallowed, as the cluster ion has a closed-shell ground state. Most of the available spectroscopic data were obtained on aqueous or acetonitrile solutions or on polycrystalline samples. Single-crystalinformation, such as orientation-dependent effects, have not yet been reported, Continuous-waveform spectroscopy was employed, which easily misses short-lived emission processes. This lack of qualitative and quantitative confirmation has seriously impeded the theoretical refinements necessary for understanding, improving, and utilizing these cluster compounds.
0009-2614/93/$06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
415
Volume 202, number 5
29 January 1993
CHEMICAL PHYSICS LETTERS
In this Letter, we present optical spectroscopy results for well-defined single crystal samples. Our time resolution extends into the picosecond regime, which enables us to ascertain the predicted excited states and to determine their energies and lifetimes.
2. Experimental Bis-tetrabutylammoniumhexachloro-octa-~S-chloro-hexa-octahedra-molybdate (2- ), [N ( C4H9) .II ZMo6Cl14,a compound containing individual cluster anions in a matrix of organic cations, is prepared according to established procedures [ 121. Single crystals up to 12 mm in diameter are grown from acetonitrile. The compound crystallizes in the monoclinic space group P2,/n (No. 14), with only a slight deviation (0.1’ ) from orthorhombic metrics [131* The samples are optically pumped by pulses with a width of 25 ps generated by a frequency-doubled, passively mode-locked Nd: YAG laser at wavelength A= 532 nm. Homogeneous excitation conditions are obtained by focusing the laser beam on an area of 800 pm in diameter. Only the central spot of the irradiated area is imaged onto the entrance slit of a 0.25 m grating spectrometer. The intensity of the scattered laser light in the detected spectrum is reduced by an optical filter. For lifetimes from picoseconds up to 2 ns, the spectrally dispersed luminescence is time-resolved by a streak camera, the output is recorded by a CCD (charge-coupled device) camera with a two-dimensional read-out system. For longer lifetimes, the time resolution is achieved by using a constant integration time of 200 ns and varying delay times. The effect of the optical filter, the wavelength dependence of the spectrometer and the inhomogeneity of the detector sensitivity are measured with a calibrated lamp; the spectra are corrected accordingly. Only the luminescence of the crystals is sufficiently intense for investigations on a picosecond time scale.
trum, and labeled A, B and C. (A: maximum at 600+ 10 nm; B: maximum at 6302 3 nm; C: maximum at 690 2 10 nm for T= 290 K, 760 + 10 nm for 5 < TG 100 K). Fig. 2 shows the luminescence spectra for several time intervals after excitation. Temperature dependences are systematically investigated; while the maxima of A and B do not change between 5 and 290 K, C shows a pronounced red-shift at low temperature. We study the spectra at various times (up to 2 ns) after the excitation and note that all emission maxima remain constant during the decay of the luminescence. The important new result is that emission bands A and B can only be detected with picosecond resolution. Band C is negligible on the picosecond time scale but dominates in spectra detected with integration times exceeding one microsecond. The measured time-dependences of emission bands A and B are shown in fig. 3. Emission band C also decays according to a simple exponential law. The evaluated decay times are listed in table 1. Variation of the pump excitation energy density between 0.5 and 500 mJ/cm’ yields no significant effect on the spectrum. emission intensities of the discovered bands A and B depend linearly on pump intensity. Our data obtained for C agree with earlier results reported by Maverick and Gray [ 61. Their energy transfer experiments have proven this luminescence to be an emission from the lowest triplet state of the cluster ion [ 81. This transition is spin-forbidden since IO00
800 600
550
600
650
700
750
800
WAVELENGTH (nm)
3. Results and discussion Three distinct emission bands are observed, identified by their maxima in the luminescence spec416
Fig. 2. Time-resolved luminescence spectra of [N(C4H9)4]2M06C114 at 290 K. (a) Integrated from 40 to 200 psafter laser pulse, (b) integrated from 360to 520ps after laser pulse, (c) time integrated, therefore not to the same scale as curves (a) and (b).
Volume 202, number 5
0
CHEMICAL PHYSICS LETTERS
500 TIME
1000
(ps)
Fig. 3. Time dependence of emissions for A, measured at 600 nm (solid lines) and B, measured at 630 nm (dotted;contributions from band A have been subtracted). Table 1 Decay times for the observed emissions for various sample temperatures Band
290 K
IOOK
5K
A B C
63Ok3Ops 40f20ps 130flOps
750+5ops 50+ 30 ps 225t20jls
1100+100ps 50f30ps 320f80ps
29 January 1993
spectra of compounds containing coordinated transition-metal ions. In contrast, B is much narrower, indicating that it does not originate from transitions in the individual cluster units. This conclusion is supported by our observation that B vanishes while excited states of higher energy, corresponding to A, are still populated; apparently there is no relaxation from A to B. Our experiments reveal another remarkable feature, previously undetectable in polycrystalline samples or solutions: the intensity of band B, relative to A, depends upon the orientation of the single crystal with respect to the incident pump beam. Maximal emission arises when the exciting beam is coincident with the crystallographic 6 axis, along which the cluster units align. The intensity falls to half-maximum at disorientation by about 15’. This intensity variation cannot be due to surface effects, as the penetration depth at our excitation wavelength is a few tenths of a millimeter, and no symmetric dependence on the angle between surface and incident beam is observed.
4. Conclusions the system has a closed-shell ground state ‘A,,. Consequently, this excited triplet state is characterized by a comparatively long lifetime. On the other hand, the newly discovered bands A and B can only be due to spin-allowed transitions, because they are intense on a picosecond time scale, which implies their large oscillator strengths. Band A can be identified as an emission from excited singlet states of the cluster ion: The observed energy of 2.1 eV agrees with the values calculated for the transition between the lowest excited singlet states and the ground state. Our semiempirical LCAO-MO calculations of the extendedHtickel type predict these energies to be 2.1 eV for the symmetry-allowed transition, and 1.9 and 1.5 eV, respectively, for the symmetry-forbidden transitions [ 131. Band A is no emission of a transition between the two lowest triplet states, since such a transition would occur at higher energies [ 131, and band A does not originate from a two-photon excitation of higher states of the cluster because of the observed linear dependence on the pump intensity. Both A and C are broadened by strong electronphonon coupling, a familiar feature in luminescence
Excited singlet states of [Mo,$&]~- cluster anions are verified for the first time by luminescence measurements (band A). Earlier discrepancies between experiments and theoretical calculations are now removed. Use of picosecond time scale spectroscopy and crystalline material are essential for the detection of emission from the singlet-state transitions. An unexpected, further emission on a picosecond time scale (band B) with emission strength depending upon beam incidence angle is observed. Our experiments inaugurate new topics for further investigations. Interpretation of the anisotropic emission strength of band B is still lacking. We hope to stimulate the development of models to resolve the open questions regarding these types of solid state materials.
Acknowledgement We gratefully acknowledge the advice and cooperation of M. Rinker regarding the spectroscopic sys417
Volume 202, number 5
CHEMICAL PHYSICS LETTERS
tern and thank K. Rother and H. Klann for exuert technical assistance.
References [ 1 ] A. Simon, Angew. Chem. Intern. Ed. Engl. 27 ( 1988) 159. (21 J.J. Finley, H. Nohl, E.E. Vogel, H. Imoto, R.E. Camley, V. Kevin, O.K. Andersen and A. Simon, Phys. Rev. Letters 46 (1981) 1472. [ 310. Fischer and M.B. Maple, eds., Topics in current physics, Superconductivity in ternary compounds, Vols. 32, 34 (Springer, Berlin, 1982). [4] L. Ouahab, P. Batail, C. P&n and C. Garrigou-Lagrange, Mater. Res. Bull. 21 (1986) 1223.
418
29 January I993
[S] O.K. Andersen, W. Klose and H. Nohl, Phys. Rev. B 17 (1978) 1209. [6] A.W. Maverick and H.B. Gray, J. Am. Chem. Sot. 103 (1981) 1298. [7] A.W. Maverick, J.S. Najdzionek, D. MacKenzie, D.G. NoceraandH.B.Gray, J.Am. Chem.Soc. 105 (1983) 1878. [ 8] T.C. Zietlow, M.D. Hopkins and H.B. Gray, J. Solid State Chem. 57 (1985) 112. [9] L.J. Guggenbergerand A.W. Sleight, Inorg. Chem. 8 (1969) 2041. [ 101R. Hoffmann and T. Hughbanks, J. Am. Chem. Sot. 105 (1983) 1150. [ 1I ] F.A. Cotton and G.C. Stanley, Chem. Phys. Letters 58 (1978) 450. [ 121J.C. Sheldon, J. Chem. Sot. 1960 ( 1960) 3 106. [ 131N. Perchenek, Ph.D. Thesis, University of Stuttgart ( 1992).
CHEMICAL PHYSICS LETTERS
29 January 1993
Excited singlet states of molybdenum chloride clusters U. Strauss, N. Perchenek, W.W. Riihle, H.J. Queisser and A. Simon Max-Planck-lnstitut fiir Festkijrperfrschung, Heisenbergstrasse 1, W-7000Stuttgart80, Germany Received 2 November 1992
Excited singlet states of [Mo&l,~]‘- cluster anions, as previously predicted but undiscovered, are detected in single crystals of [N(C,H,)&Mo&I,, by means of photoluminescence measurements with time resolution in the picosecond regime. The intensity of one of the transitions shows an unexpected dependence upon beam incidence angle relative to the crystallographic axes.
1. Introduction Compounds with octahedral transition-metal clusters capture much attention because of unusual structural and physical properties (for a detailed review, see ref. [ 1] ) [2-41. One particularly fascinating and fundamental example of this class of materials is the molybdenum cluster anion [ MosX,,] *[ 51. The octahedral metal-atom core is surrounded by halogen atoms, capping all faces and corners of the octahedron, as shown in fig. 1. Compounds with this anion are remarkably stable, they show fast light-
n
Fig. 1. Structure of the [ Mo&~,~] *- cluster anion.
induced electron transfer, which has made them candidates for specific photon-induced reactions utilizing long-lived metastable states possessing high electron affinity, and for investigating novel methods of energy storage [ 6-8 1. Remarkable quantum efficiencies for radiative emission have been reported [ 71. The physics of these anions and their solid state compounds are as yet only marginally understood. Theoretical calculations with a variety of computational methods predicted the existence of several closely spaced excited states [5,9-l 11. All previous spectroscopic investigations have, however, been unable to verify these calculations. No evidence was found for the existence of this manifold of excited states. Only long-lived luminescence from one of the excited triplet states has been experimentally confirmed; its lifetime was determined to lie in the range of a few tenths of a millisecond [6 1. No emission from the excited singlet states has thus far been observed, although these transitions ought to be spinallowed, as the cluster ion has a closed-shell ground state. Most of the available spectroscopic data were obtained on aqueous or acetonitrile solutions or on polycrystalline samples. Single-crystalinformation, such as orientation-dependent effects, have not yet been reported, Continuous-waveform spectroscopy was employed, which easily misses short-lived emission processes. This lack of qualitative and quantitative confirmation has seriously impeded the theoretical refinements necessary for understanding, improving, and utilizing these cluster compounds.
0009-2614/93/$06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
415
Volume 202, number 5
29 January 1993
CHEMICAL PHYSICS LETTERS
In this Letter, we present optical spectroscopy results for well-defined single crystal samples. Our time resolution extends into the picosecond regime, which enables us to ascertain the predicted excited states and to determine their energies and lifetimes.
2. Experimental Bis-tetrabutylammoniumhexachloro-octa-~S-chloro-hexa-octahedra-molybdate (2- ), [N ( C4H9) .II ZMo6Cl14,a compound containing individual cluster anions in a matrix of organic cations, is prepared according to established procedures [ 121. Single crystals up to 12 mm in diameter are grown from acetonitrile. The compound crystallizes in the monoclinic space group P2,/n (No. 14), with only a slight deviation (0.1’ ) from orthorhombic metrics [131* The samples are optically pumped by pulses with a width of 25 ps generated by a frequency-doubled, passively mode-locked Nd: YAG laser at wavelength A= 532 nm. Homogeneous excitation conditions are obtained by focusing the laser beam on an area of 800 pm in diameter. Only the central spot of the irradiated area is imaged onto the entrance slit of a 0.25 m grating spectrometer. The intensity of the scattered laser light in the detected spectrum is reduced by an optical filter. For lifetimes from picoseconds up to 2 ns, the spectrally dispersed luminescence is time-resolved by a streak camera, the output is recorded by a CCD (charge-coupled device) camera with a two-dimensional read-out system. For longer lifetimes, the time resolution is achieved by using a constant integration time of 200 ns and varying delay times. The effect of the optical filter, the wavelength dependence of the spectrometer and the inhomogeneity of the detector sensitivity are measured with a calibrated lamp; the spectra are corrected accordingly. Only the luminescence of the crystals is sufficiently intense for investigations on a picosecond time scale.
trum, and labeled A, B and C. (A: maximum at 600+ 10 nm; B: maximum at 6302 3 nm; C: maximum at 690 2 10 nm for T= 290 K, 760 + 10 nm for 5 < TG 100 K). Fig. 2 shows the luminescence spectra for several time intervals after excitation. Temperature dependences are systematically investigated; while the maxima of A and B do not change between 5 and 290 K, C shows a pronounced red-shift at low temperature. We study the spectra at various times (up to 2 ns) after the excitation and note that all emission maxima remain constant during the decay of the luminescence. The important new result is that emission bands A and B can only be detected with picosecond resolution. Band C is negligible on the picosecond time scale but dominates in spectra detected with integration times exceeding one microsecond. The measured time-dependences of emission bands A and B are shown in fig. 3. Emission band C also decays according to a simple exponential law. The evaluated decay times are listed in table 1. Variation of the pump excitation energy density between 0.5 and 500 mJ/cm’ yields no significant effect on the spectrum. emission intensities of the discovered bands A and B depend linearly on pump intensity. Our data obtained for C agree with earlier results reported by Maverick and Gray [ 61. Their energy transfer experiments have proven this luminescence to be an emission from the lowest triplet state of the cluster ion [ 81. This transition is spin-forbidden since IO00
800 600
550
600
650
700
750
800
WAVELENGTH (nm)
3. Results and discussion Three distinct emission bands are observed, identified by their maxima in the luminescence spec416
Fig. 2. Time-resolved luminescence spectra of [N(C4H9)4]2M06C114 at 290 K. (a) Integrated from 40 to 200 psafter laser pulse, (b) integrated from 360to 520ps after laser pulse, (c) time integrated, therefore not to the same scale as curves (a) and (b).
Volume 202, number 5
0
CHEMICAL PHYSICS LETTERS
500 TIME
1000
(ps)
Fig. 3. Time dependence of emissions for A, measured at 600 nm (solid lines) and B, measured at 630 nm (dotted;contributions from band A have been subtracted). Table 1 Decay times for the observed emissions for various sample temperatures Band
290 K
IOOK
5K
A B C
63Ok3Ops 40f20ps 130flOps
750+5ops 50+ 30 ps 225t20jls
1100+100ps 50f30ps 320f80ps
29 January 1993
spectra of compounds containing coordinated transition-metal ions. In contrast, B is much narrower, indicating that it does not originate from transitions in the individual cluster units. This conclusion is supported by our observation that B vanishes while excited states of higher energy, corresponding to A, are still populated; apparently there is no relaxation from A to B. Our experiments reveal another remarkable feature, previously undetectable in polycrystalline samples or solutions: the intensity of band B, relative to A, depends upon the orientation of the single crystal with respect to the incident pump beam. Maximal emission arises when the exciting beam is coincident with the crystallographic 6 axis, along which the cluster units align. The intensity falls to half-maximum at disorientation by about 15’. This intensity variation cannot be due to surface effects, as the penetration depth at our excitation wavelength is a few tenths of a millimeter, and no symmetric dependence on the angle between surface and incident beam is observed.
4. Conclusions the system has a closed-shell ground state ‘A,,. Consequently, this excited triplet state is characterized by a comparatively long lifetime. On the other hand, the newly discovered bands A and B can only be due to spin-allowed transitions, because they are intense on a picosecond time scale, which implies their large oscillator strengths. Band A can be identified as an emission from excited singlet states of the cluster ion: The observed energy of 2.1 eV agrees with the values calculated for the transition between the lowest excited singlet states and the ground state. Our semiempirical LCAO-MO calculations of the extendedHtickel type predict these energies to be 2.1 eV for the symmetry-allowed transition, and 1.9 and 1.5 eV, respectively, for the symmetry-forbidden transitions [ 131. Band A is no emission of a transition between the two lowest triplet states, since such a transition would occur at higher energies [ 131, and band A does not originate from a two-photon excitation of higher states of the cluster because of the observed linear dependence on the pump intensity. Both A and C are broadened by strong electronphonon coupling, a familiar feature in luminescence
Excited singlet states of [Mo,$&]~- cluster anions are verified for the first time by luminescence measurements (band A). Earlier discrepancies between experiments and theoretical calculations are now removed. Use of picosecond time scale spectroscopy and crystalline material are essential for the detection of emission from the singlet-state transitions. An unexpected, further emission on a picosecond time scale (band B) with emission strength depending upon beam incidence angle is observed. Our experiments inaugurate new topics for further investigations. Interpretation of the anisotropic emission strength of band B is still lacking. We hope to stimulate the development of models to resolve the open questions regarding these types of solid state materials.
Acknowledgement We gratefully acknowledge the advice and cooperation of M. Rinker regarding the spectroscopic sys417
Volume 202, number 5
CHEMICAL PHYSICS LETTERS
tern and thank K. Rother and H. Klann for exuert technical assistance.
References [ 1 ] A. Simon, Angew. Chem. Intern. Ed. Engl. 27 ( 1988) 159. (21 J.J. Finley, H. Nohl, E.E. Vogel, H. Imoto, R.E. Camley, V. Kevin, O.K. Andersen and A. Simon, Phys. Rev. Letters 46 (1981) 1472. [ 310. Fischer and M.B. Maple, eds., Topics in current physics, Superconductivity in ternary compounds, Vols. 32, 34 (Springer, Berlin, 1982). [4] L. Ouahab, P. Batail, C. P&n and C. Garrigou-Lagrange, Mater. Res. Bull. 21 (1986) 1223.
418
29 January I993
[S] O.K. Andersen, W. Klose and H. Nohl, Phys. Rev. B 17 (1978) 1209. [6] A.W. Maverick and H.B. Gray, J. Am. Chem. Sot. 103 (1981) 1298. [7] A.W. Maverick, J.S. Najdzionek, D. MacKenzie, D.G. NoceraandH.B.Gray, J.Am. Chem.Soc. 105 (1983) 1878. [ 8] T.C. Zietlow, M.D. Hopkins and H.B. Gray, J. Solid State Chem. 57 (1985) 112. [9] L.J. Guggenbergerand A.W. Sleight, Inorg. Chem. 8 (1969) 2041. [ 101R. Hoffmann and T. Hughbanks, J. Am. Chem. Sot. 105 (1983) 1150. [ 1I ] F.A. Cotton and G.C. Stanley, Chem. Phys. Letters 58 (1978) 450. [ 121J.C. Sheldon, J. Chem. Sot. 1960 ( 1960) 3 106. [ 131N. Perchenek, Ph.D. Thesis, University of Stuttgart ( 1992).