J . Phys. Chem. 1989, 93, 4487-4489
Luminescence of Single Crystal [Rh,(bridge),](BPh,), Magnetic Fields
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and Its Behavior under External
Stefan Oberneder and Giinter Gliemann* Institut fur Physikalische und Theoretische Chemie, Universitat Regensburg, 0-8400 Regensburg, Federal Republic of Germany (Received: November 30, 1988)
The polarized phosphorescence of single crystals of [Rh,(bridge),](BPh,), as a function of temperature (2 5 T 5 150 K) and of applied homogeneous magnetic fields (0 I H I 6 T) is reported. The decay of the phosphorescence is monoexponential, and the decay rate at T = 2 K can be described by an expression of the form l/su = l/r,+,, + eH + r H 2 with e # 0. The phosphorescence is assigned to the emission from the spin-orbit components E', and Allu of the state 3A2u,which originates from the excited configuration a2,(du*)alB(pu) of the system Rh(1)-Rh(1). E', is about 11 cm-I above A'lu.
Introduction The compounds M,[Pt(CN)4],.nH20 (M = alkali-, alkalineearth-, rare-earth-metal cation) are spectroscopically well-studied Pt(1I) salts.' They show several conspicuous optical properties, which are based mainly on the quasi-one-dimensional crystal structure due to parallel columns formed by the square-planar [ Pt(CN),]*- ions. Variations of temperature, high pressure, and applied magnetic fields influence considerably the optical properties such as the absorption and the emission. Similar sensitivity has been observed for a series of other Pt(I1) compounds such as [Pt(bpy)(CN),] (bpy = bipyridine)? [Pt(o-phen)(CN),] (o-phen = o-phenanthr~line),~ the red modification of [Pt(bpy)CI,]: and [Pt(bpm)(CN);?](bpm = bi~yrimidine).~These salts crystallize also in a columnar structure. Recently d8 systems of smaller size, e.g., binuclear d8-d8 complexes, have found increasing interest. Examples are [Pt2(P205H2),]' (ref 6) and [Rh2(bridge)4]2+ (bridge = 1,3-diisocyanopropane).' They exhibit unusual optical properties and can be used as model systems for further study of metal-metal interactions. As has been described by Gray et a1.,8 the title compound shows at low temperature a highly structured absorption spectrum, which can be assigned to electronic transitions in the Rh(1)-Rh(1) system. The purpose of this paper is to report the polarized optical emission spectrum of single crystals of [Rh2(bridge),](BPh4), and the temperature dependence of the emission and its behavior in strong magnetic fields. The experimental data will be used to describe the involved deactivation processes and to establish the energy level diagram of the lowest excited electronic states. Experimental Section [Rhl(bridge)J(BPh4), was prepared by the reported proced~re.~ The elemental analysis yielded the following: C, 66.41; H, 5.37; N, 10.81 (calcd: C, 66.37; H, 5.41; N , 10.75). Single crystals of 1.5 X 0.5 X 0.2 mm3 were grown from saturated solutions
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( I ) Gliemann, G.; Yersin, H. Struct. Bonding 1985,62,87, and references therein. (2) Biedermann, J.; Wallfahrer, M.; Gliemann, G. J . Luminesc. 1987, 37, 323. ( 3 ) Schwarz, R.; Lindner, M.; Gliemann, G. Ber. Bunsen-Ges. Phys. Chem. 1987, 91, 1233. (4) Wallfahrer, M.; Gliemann, G., unpublished results. (5) Biedermann, J.; Gliemann, G., unpublished results. (6) (a) BPr, L.; Gliemann, G. Chem. Phys. Lett. 1984, 108, 14. (b) Zipp, A. P. Coord. Chem. Rev. 1988, 84, 47, and references therein. (7) (a) Mann, K. R.; Lewis, N. S.; Miskowski, V. M.; Erwin, D. K.; Hammond, G. S.;Gray, H. B. J . Am. Chem. SOC.1977, 99, 5525. (b) Miskowski, V. M.; Nobinger, G. L.; Kliger, D. S.;Hammond, G. S.; Lewis, N. S.; Mann, K. R.; Gray, H. B. J . Am. Chem. Soc. 1978, 100, 485. (c) Yaneff, P. V.; Powell, J. J . Organomet. Chem. 1979, 179, 101. (d) Milder, S.J.; Goldbeck, R. A,; Kliger, D. S.; Gray, H . B. J . Am. Chem. SOC.1980, 102,6761. (e) Dallinger, R. F.; Miskowski, V. M.; Gray, H. B.; Woodruff, W. H . J . Am. Chem. SOC.1981, 103, 1595. ( f ) Milder, S.J.; Kliger, D. S. J . Phys. Chem. 1985, 89, 4170. (g) Milder, S. J.; Kliger, D. S.; Butler, L. G.; Gray, H. B. J . Phys. Chem. 1986, 90, 5567. (8) Rice, S. F.; Gray, H. B. J . Am. Chem. SOC.1981, 103, 1593. (9) Lewis, N. S.;Mann, K. R.; Gordon 11, J. G.; Gray, H. B. J . Am. Chem. SOC.1976, 98, 7461.
of [Rh2(bridge)4](BPh4)2in acetonitrile by slow evaporation (6-10 weeks). The brick-shaped crystals are highly dichroic. Light polarized parallel to the long edge of the crystals ( b axis) is absorbed many times stronger than light with perpendicular polarization. For comparison, we had several well-shaped single crystals that were provided by H. B. Gray and V. M. Miskowski. The polarized emission of single crystals was measured with a liquid helium bath cryostat of a superconducting magnet system (Oxford Co., SM4) yielding magnetic field strengths up to H = 6 T. As the excitation source for the continuous-wave emission spectra an argon-ion laser (A,, = 364 nm, Coherent Innova 90) was used. The lifetime was measured by a cavity-dumped dye laser system (A = 435 nm; dye = stilbene 3).1° The emitted light was analyzed by a double-grating monochromator (Spex 1404, and detected by an EM1 S 20 photomultiplier. 1200 lines/") The polarization of the exciting laser beam was parallel to the b axis of the crystals. Results As mentioned by Gray et al., [Rh,(bridge),](BPh,), exhibits in addition to a phosphorescence at -12000 cm-I a broad fluorescence with a maximum at about 15000 cm-I.l1 Our experiments have shown that a variation of the excitation wavelength does not affect the spectral positions of the phosphorescence and the fluorescence, but it changes the ratio of their intensities. With increasing excitation wavelength A,, the intensity of the = 331 nm the fluorescence fluorescence decreases. Whereas at kXc and the phosphorescence have nearly equal intensities, at A,, = 364 nm the fluorescence is less intense by a factor of about 5 than < 514 nm only a very weak the phosphorescence, and at 458 < hC fluorescence could be detected. Figure 1 shows the polarized phosphorescence spectra of single-crystal [Rh2(bridge),] (BPh,), at different temperatures. For both polarizations the phosphorescence band has its maximum at I = 11 990 cm-I and exhibits no fine structure. Within the temperature range 1.7 I T 5 100 K the E l b phosphorescence is more intense by a factor of about 10 than the Eiib phosphorescence. With increasing temperature no energy shift of the phosphorescence band was observed; the total phosphorescence intensity, however, decreases, and at T k 150 K it is below the limit of detection. The influence of a homogeneous magnetic field H / / bon the 2 K is presented in polarized phosphorescence spectra at T Figure 2. The intensity of the Eilb band increases by a factor of -3 if the field strength is raised to H = 6 T; the E l b band, however, is unchanged within the limits of experimental detection. Magnetic fields with H l b and 0 IH I6 T yield no effect on the phosphorescence spectra (Ellb and E l b ) . Because of the very low intensity of the Ellb phosphorescence, only for the E l b polarized band the decay behavior of the
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(IO) v. Ammon, W.; Gliemann, G. J . Chem. Phys. 1982, 77, 2266. ( 1 1 ) Rice, S.F.; Milder, S. J.; Gray, H. B.; Goldbeck, R. A,; Kliger, D. S.Coord. Chem. Rev. 1982, 43, 349.
0022-3654/89/2093-4487$01.50/00 1989 American Chemical Society
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The Journal of Physical Chemistry, Vol. 93, No. 11, I989
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Oberneder and Gliemann
