The Journal of
Physical Chemistry
0 Copyright, 1985, by the American Chemical Society
VOLUME 89, NUMBER 21 OCTOBER 10, 1985
LETTERS Temperature Dependence of the Luminescence Lifetime of Hexanuclear Molybdenum(II)Chlorlde Cluster. Identification of Lower Excited Triplet Sublevels Yuria Saito, Hideaki K. Tanaka, Yoichi Sasaki, and Tohru Azumi* Department of Chemistry, Faculty of Science, Tohoku University. Sendai 980, Japan (Received: April 16, 1985; In Final Form: June 18, 1985)
The temperature dependence of the luminescence lifetime has been investigated for the tetraethylammonium salt of the octakis(p3-chloro)hexachlorohexamolybdate(2-) ion, [ M o ~ C ~in~the ~ ]1.4-300 ~ - ,K range. The lifetime tends to decrease as the temperature increases. The observed temperature dependence has been analyzed in terms of the emissions from several Boltzmann populated triplet sublevels. The lowest triplet state has been identified as 3Tluwhich is due to the t,, to t,, orbital excitation. The lifetimes of the individual spin sublevels of the lowest triplet state and the energy gaps among them were determined.
Introduction The hexanuclear molybdenum( 11) cluster ion, [Mo6ClI4] is known1V2to emit red luminescence both in solid salts and in solutions. The nature of the luminescence process, however, is not at all understood. Specifically, even the assignment of the emitting state has not been given. In view of the current interest in the photophysics and photochemistry of metal clusters, a clear understanding of the electronic excited states of the cluster is indispensable. One of the keys to understanding the excited-state properties appears to exist in the temperature dependence of the luminescence lifetime. According to Maverick and Gray,' the lifetime of 120 I.CS observed for the tetrabutylammonium salt a t 300 K is lengthened to 210 ps at 80 K. No interpretation of this temperature dependence, however, has been given. In view of the long lifetime, the observed luminescence is undoubtedly the emission from the triplet state; that is, the lu-
,-,
(1) Maverick, A. W.; Gray, H. B. J. Am. Chem. SOC.1981, 103, 1298. (2) Maverick, A. W.; Najdzionek, J. S.; MacKenzie, D.; Nocera, D. G.; Gray, H. B. J. Am. Chem. SOC.1983, 105, 1878.
minescence should be termed as the phosphorescence. Because of the high symmetry (i.e., Oh)of the cluster, the lowest triplet state is likely to be both spatially and spin degenerate. This multifold degeneracy may be partly lifted by the spin-orbit coupling. Once the degeneracy is lifted, the populations of the sublevels change with temperature. The observed temperature dependence of the lifetime is most likely attributed to the temperature dependence of the populations of the triplet sub level^.^ Detailed analysis of the temperature dependence may, therefore, lead to a better understanding of the triplet sublevels. This is what we aimed at in this paper. In addition to the experiments discussed above, theoretical considerations are also required. Previous semiempirical (orbital-overlap type c a l ~ u l a t i o nand ) ~ ~a~b initio (SCF X a SW)596 theories were mainly concerned with the ground-state properties ~
~
~
~
~
~~~
(3) Crosby, G. A. Acc. Chem. Res. 1975, 8, 231. (4) Cotton, F. A.; Haas, T. E. Inorg. Chem. 1964, 3, 10 (5) Burstein, B. E.; Cotton, F. A.; Stanley, G . G . Isr. J . Chem 1980, 19, 132. (6) Cotton, F. A,; Stanley, G G. Chem. Phys. Left 1978, 58, 450.
0022-3654/85/2089-4413$01.50/00 1985 American Chemical Society
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The Journal of Physical Chemistry, Vol. 89, No. 21, 1985
and are not particularly helpful in understanding the excited states, especially the triplet sublevels. We have, therefore, made our own theoretical analysis, in which the electronic repulsion and the spin-orbit coupling are taken into account. This paper, however, mainly deals with the experimental parts, and only the essential results of the theoretical analysis will be mentioned. Details of the theories will be described elsewhere. Experimental Section Crystals of the tetraethylammonium salt, [(CzH5)4N]z[Mo6Cl14],were used in the experiments. The cluster was synthesized as follows. First, Mo6Cll2was prepared from powdered Mo metal and MoCl, by the known method.' This was converted into (H,0)z[Mo6C1,4]~6Hz0 on cooling in 6 M HCI solution. The potassium salt, K , [ M o ~ C ~ , was ~ ] , obtained by adding 10 cm3 of 3 M KCl to the solution of (H30)z[Mo6Cl14].6H20 in 3 M HCI (ca. 2 g in 20 cm3) at ca. 50 "C. Finally the tetraethylammonium salt was precipitated from a 3 M HCI solution of the potassium salt (ca. 1 g in 20 cm3) by adding 10 cm3 of 2 M (C2H5)4NC1 solution, collected by filtration, washed with 6 M HCl, and dried over solid NaOH in vacuo. This was purified by recrystallization from acetonitrile. Anal. Calcd for C16H40NzC114M06: c , 14.42; H, 3.03; N, 2.10; C1, 37.25. Found: C, 14.66; H, 2.96; N, 2.00; C1, 36.58. The luminescence was observed with a Spex 1702 monochromator equipped with a Hamamatsu R928 photomultiplier tube. In the decay measurements, the excitation was carried out with a Molectron UV24 nitrogen laser. The decay signals were digitized with a Kawasaki TMR-10 transient memory, and the digital data were accumulated on a microcomputer for 21° or 211 times. Special caution was exercised to minimize the intensity of the exciting laser light. Since the emitting state undergoes very efficient bimolecular quenching, the intensity of the laser light was reduced until strict exponential decay was obtained. For the measurements at 4.2 K and below, the crystals were directly immersed into liquid helium. The measurements a t temperatures higher than 10 K were made with a Displex CSW-202 closed-cycle cryogenic system.
Outlines of the Theory In a similar manner as Cotton and Haas: the molecular orbitals are constructed from the 4d atomic orbitals of the six molybdenum atoms. The highest occupied molecular orbital (HOMO) is composed of d,, and d,, atomic orbitals and belongs to the t2, representation. The lowest unoccupied molecular orbital (LUMO), on the other hand, is composed of dz2 atomic orbitals and belongs to the tl, representation. (The coordinates to describe atomic orbitals are chosen as Cotton and H a a ~ . ~From ) the tzg to tl, orbital excitation, the four triplet states are produced: 3Tlu, 3Tzu,3E,, and 3Azu. Calculation of the excited-state energies by incorporating the electronic repulsion terms has shown that 3Tlu is the lowest triplet state. Introduction of the spin-orbital coupling lets this state further split into four spin sublevels, their double group representation being Tzu,E,, Tlu,and AI, in the order of increasing energy. The lowest two sublevels, Tzuand E,, remain degenerate if the spin-orbit coupling is limited to the first order. The splitting between these sublevels is introduced only by the second-order spin-orbit coupling, and is therefore expected to be of small magnitude. The next higher sublevel, TI,,is theoretically expected to be apart from the lowest sublevel by the amount equal to the atomic spin-orbit coupling parameter (. It is noted that only the emission from the TI, sublevel is electronically allowed. Emission from the other sublevels should accompany the vibronic coupling, and hence the lifetimes associated with these "forbidden" states are expected to be longer than the lifetime of the T,, sublevel. Results and Discussion The temperature dependence of the observed lifetime is shown in Figure 1. As the temperature increases, the lifetime tends to (7) Nannelli, P.; Block, B. P. Inorg. Synth. 1970, I t , 170.
Letters T/P*
400
loo
'37-Au 560d
IT
I
0;
200
100
J
300
T / K
Figure 1. Temperature dependence of the phosphorescence lifetime of The black circles represent the excrystalline [(C2H5)4N]2[Mo6C114]. perimental points, and the solid curve represents the best fit based on eq 1 of the text. The lifetimes associated with the individual sublevels and the energy gaps among them were determined as is shown schematically in the insert.
decrease in agreement with the previous notion.'s2 The solid curve in Figure 1 represents the best fit based on the three-level scheme 7
= [Cgi ex~(-Ei/kr)I/[Cgi ex~(-Ei/kr)/riI
(1)
where gi, Ei, and riare respectively the multiplicity, energy, and lifetime associated with the ith sublevel. The finally obtained energy gap and lifetimes are schematically shown in the insert of Figure 1. The location and lifetime of the next higher sublevel, AI,, cannot be determined, since the existence of a forbidden sublevel above the strongly allowed one hardly affects the observed lifetime. As is mentioned above, the energy gap between the TI, and E, sublevels is expected to be equal to the atomic spin-orbit coupling parameter (. Thus, the experiments yield (of 540 cm-', which roughly agrees with the value determined from atomic spectros~opy.~ That ~ ~ the lifetime of TI, is significantly shorter than those of the other sublevels is consistent with the notion that only the TI, emission is electronically allowed. Whether the allowed character of the E, and Tzuemissions comes from the vibronic coupling or other sources (for example, slight geometry deformation) is not at the moment clear; this subject needs to be elucidated in the future. In a very recent article, Zietlow et a1.I0 also reported the temperature dependence of the lifetime for a similar cluster (tetrabutylammonium salt) in the temperature range higher than -80 K. They observed a sharp change of lifetime at around 300 K, from which the activation energy of -3000 cm-' was obtained. It would be of interest to examine the source of this activation energy, because it might give information on much higher triplet states. Unfortunately, however, we were unable to reproduce their data both in general behavior and in absolute magnitude. For example, their lifetime of -130 ~s at -80 K (estimated from the figure) is much shorter than -210 ps of ours and of Marverick andGray.' As is discussed above, our calculation identifies the t,, orbital as HOMO. If one adopts a larger orbital exponent for the atomic d orbital, the tzuorbital becomes HOMO., In view of the difficulty of properly choosing the orbital exponent, we have also examined what sorts of excited states are available from the tzu to tl, excitation. If the electronic repulsion is taken into account, 3A2, becomes the lowest. The spin-orbit coupling, in this case, does not lift the spin degeneracy. Hence, no temperature dependence of the lifetime is expected, in conflict with the experiment. (8) Fiffith, J. S. 'The Theory of Transition-Metal Ions"; Cambridge University Press: New York, 1964. (9) McClure, D. S . Solid State Phys. 1959, 9, 428. (10) Zietlow, T. C.; Hopkins, M. D.; Gray, H. B. J. Solid State Chem. 1985, 57, 112.
J. Phys. Chem. 1985, 89, 4415-4418
The molecular orbital t h e o r i e ~ ~ incorporating v~.~~ the atomic orbitals of halogens give the eg HOMO. In this case, the lowest triplet state becomes 3T2u.When the spin-orbit coupling is introduced, the TI, sublevel is located slightly above the E, sublevel, the splitting between these two levels being due to the second-order spin-orbit coupling only. In this case, the lifetime should have to exhibit a very sharp temperature dependence at a very low (11) Guggenberger, L. J.; Sleight, A. W. Inorg. Chem. 1969, 8, 2041.
4415
temperature range, which again conflicts with the experimental finding. In this way, among the three candidates of HOMO, t2g, tzu, and eg, only the tza H O M O is consistent with the observed temperature dependence. Acknowfedgment. We thank Professor T. Nakajima of this Department and Professor K. Saito of the Institute for Molecular Science for discussions. We also thank Professors T. Mukai and T. Miyashi for the opportunities to use their displex cryogenic system.
Gas-Phase Radical Formation during the Reactions of Methane, Ethane, Ethylene, and Propylene over Selected Oxide Catalysts Daniel J. Driscoll and Jack H. Lunsford* Department of Chemistry, Texas A&M University, College Station, Texas 77843 (Received: June 6, 1984)
The formation of surface-generated gas-phase radicals during the reactions of methane, ethane, ethylene, and propylene over Bi203,yBi203-Mo03,PbO, MgO, and Li/MgO was examined by EPR matrix isolation spectroscopy. Allyl radicals were detected during the reaction of propylene over all of the oxides examined. Gas-phase methyl and ethyl radicals were detected during the reactions of methane and ethane, respectively, over MgO and Li/MgO while BizO3, y-Bi20,.Mo03, and PbO were essentially inactive. Gas-phase vinyl radicals were not detected during the reaction of ethylene over any of the oxides. These differences in reactivity are attributed to differences in reactivity of the oxide ions on these surfaces toward hydrogen atom abstraction.
Introduction Recently there has been a renewed interest in the formation and detection of surface-generated gas-phase radicals during heterogeneous catalytic reactions. Early work by Hart and Friedli’ and Dolejsek and Novakova2 employed conventional mass spectrometry for detection of these species. With current advances in instrumentation, a number of new spectroscopic techniques have been developed which allow detection of surface-generated gasphase radicals. These include matrix isolation infrared spect r o ~ c o p y laser-induced ,~ fluore~cence,’’~~ modulated beam mass spectrometry: photoelectron spectroscopy,’ resonance-enhanced multiphoton ionization (REMPI),* and matrix isolation EPR.+12 In most cases, investigations with these techniques have been limited to the detection of surface-generated radicals during several specific reactions; thus, at present, no general comparisons of catalysts and reactants have been conducted. Previous reports, from this laboratory, have shown that EPR matrix isolation is a very sensitive and versatile technique for detecting the formation of surface-generated gas-phase radicals.9JoJ2 With this system it was possible not only to detect the formation of gas-phase radicals, but also to determine the amounts (1) Hart, P. J.; Friedli, H. R. J. Chem. SOC.,Chem. Commun. 1970, 1 1 , 621. (2) Dolejsek, Z.; Novakova, J. J. Catal. 1975, 37, 540.
(3) Tevault, D. E.; Lin, M. C.; Umstead, M. E.; Smardewski, R. R. Int. J . Chem. Kinet. 1979, 1 1 , 445. (4) Talky, L. D.; Sanders, W. A.; Bogan, D. J.; Lin, M. C. Chem. Phys. Lett. 1981, 78, 500. ( 5 ) Dulcey, C . S.;Lin, M. C.; Hsu, C. C. Chem. Phys. Lett. 1985, 115, 481. (6) Amorebieta, V. T.; Colussi, A. J. J . Phys. Chem. 1982, 86, 2760. (7) Schultz, J. C.; Beauchamp, J. L. J. Phys. Chem. 1983, 87, 3587. (8) Squire, D. W.; Dulcey, C. S.;Lin, M. C., to be submitted for publi-
cation. (9) Martir, W.; Lunsford, J. H. J. Am. Chem. SOC.1981, 103, 3728. (10) Driscoll, D. J.; Lunsford, J. H. J . Phys. Chem. 1983, 87, 301. (11) Berlowitz, P.; Driscoll, D. J.; Lunsford, J. H.; Butt, J. B.; Kung, H. H.Combust. Sei. Technol. 1984, 40, 317. (12) Driscoll, D. J.; Martir, W.; Wang, J.-X.; Lunsford, J. H. J. Am. Chem. SOC.1985, 107, 58.
of the radicals produced. Furthermore, the versatility of the technique allows a variety of different catalysts and gas reactants to be examined without modification of the system. Therefore, this technique is suitable for conducting a general survey on the formation of gas-phase radicals over solid surfaces. In the present study, five oxides (BizO3, y-Bi2O3-MoO3,PbO, MgO, Li/MgO), known to be active for the formation of gas-phase radicals, were compared in their ability to promote the formation of different gas-phase radical species. The hydrocarbons examined were methane, ethane, ethylene, and propylene. In addition to detecting and quantifying the radicals produced, the results obtained also provide insight into the relative reactivity of surface oxygen ions for hydrogen atom abstraction.
Experimental Section The system used to carry out the matrix isolation experiments has been described in considerable detail e l s e ~ h e r e ; ~therefore, J~J~ only the more important aspects of the system will be described. The same conditions and experimental system were used for all the reactions presented here. The reactor was constructed of fused quartz (2.5-cm i.d., 35.8-cm length) and had a thermocouple well centered along its axis which allowed measurement of the temperature along the entire reactor length. A perforated quartz plate was positioned 9 cm from the exit end of the reactor to support the catalyst bed. The reactor was resistively heated over a 23.5 cm length and consisted of a 9.0-cm-long reaction zone and a 14.5-cm-long preheater zone. A temperature profile of the reaction zone showed a maximum temperature variation of f 2 OC. A gas leak was positioned between the exit of the reactor and the collection system creating a pressure gradient. Pressure in the reaction zone was approximately 0.9 torr while in the collection zone the pressure was typically 2 X torr. The reaction temperature in all experiments was 475 OC. The MgO and Li,CO, (99.0%) were obtained from Fisher Scientific (Certified ACS grade). Bismuth oxide (Bi2O3, 99.8%) and lead monoxide (PbO, 99.9%) were obtained from Ventron, Alpha Division and Aldrich Chemical, respectively. The y-bis-
0022-3654/85/2089-4415fi01.50/0 0 1985 American Chemical Society
