J. Phys. Chem. 1991, 95, 1067-1073
1067
Vibronic and Magnetic Coupling in the Radiative Deactivation of the Lowest Excited State of [Os(bpy),12+ Doped into [Ru(bpy),](PF,), D. Braun, C. Hensler,+ E. Callhuber,* and H. Yersin* Institut fur Physikalische und Theoretische Chemie, Universitat Regensburg, 0-8400 Regensburg, FRG (Received: June 27, 1990)
Small amounts of [ O ~ ( b p y ) ~doped ] ~ + into single-crystal [h~(bpy),](PF~)~ exhibit highly resolved MLCT spectra corresponding to the transitions between the ground state and the lowest excited states. The eledtronic origins as well as the vibronic satellites appear as sharp lines with half-widths of 2 2 cm-I. Three distinct spectroscopic sites are identified. For the lowest energy site the lowest excited state 11) is located at 14423 cm-' and the second excited state 111) lies 72 cm-I above 11). Due to the polarization properties of the origins both states are assigned to be doubly degenerate (E representations in the D3 double group). The emission from 11) shows a very weak origin line compared to the intense vibronic satellites which mostly correspond to IR-active vibrations. It is proposed that the vibronic intensity is induced by spin-vibronic and/or spin-orbit-vibronic coupling. The electronic state(s) supplying allowedness to the radiative decay from 11) are assigned to doubly degenerate E state(s) of singlet parentage. Further, magnetic fields induce a mixing of the wave functions of 11) and Ill), which results in an intensity increase of the electronic origin of the perturbed state lI')B by a factor of about 1000. This is accompanied by the appearance of resonance-Raman vibrations, which display the properties of the (unperturbed) state 111). Thus, the vibronic coupling properties of the lowest excited state are tunable by magnetic fields. The intensities of the magnetic field induced lines increase with the square of the magnetic field strength, as anticipated by first-order perturbation theory. The highly resolved emission spectra did not allow to detect any progression. Thus, the nuclear equilibrium positions of the lowest excited states and the ground state should be very similar. This, as well as the occurrence of doubly degenerate states, is not compatibel with the model of localization of the excited electron on one particular bpy ligand.
1. Introduction The [0s(bpy)J2+ complex (bpy = 2,2'-bipyridine) is similar to [Ru(bpy)J2+, in its chemical as well as its physical behavior. Though [Ru(bpy)J2+ has attracted many research groups (see re~iewsl-~), the [0s(bpy)J2+ complex has been somewhat neglected. Early e ~ p e r i m e n t a land ~~ work was stimulated by the close analogy of [0s(bpy)J2+ to [Ru(bpy),12+. The lowest excited electronic states in both complexes are commonly assigned to be of metal to ligand charge transfer (MLCT) character of the type Os(5d) bpy(n*) and Ru(4d) b p y ( ~ * ) , respectively. In spite of the large (but different) spin-orbit couplings in both compounds the lowest excited states possess mostly triplet character. (The ground state is a singlet.) Until recently, only badly resolved absorption and emission spectra could be obtained for [0s(bpy)J2+. But since it was found that [O~(bpy)~]*+ doped into certain [Ru(bpy),]X2 matrices (X= PF6-, AsF6-, SbF6-, c10,) exhibits highly resolved emission and absorption spectra,'2-16 it seems to be very promising to investigate these systems in more detail. In a recent publicationr2we thoroughly described the spectroscopic properties of [ O ~ ( b p y ) ~ doped ] ~ + into single-crystal [Ru(bpy),](PF,), at zero magnetic field. The spectra corresponding to the lowest excited states are relatively complicated due to the Occurrence of three distinct sites (called A, B, and C) and we were able to identify the electronic origins (zero-phonon lines) of the lowest excited states for every site. Since it is possible for the title compound to obtain one-site spectra of the site of lowest energy (denoted A), the analysis of the spectra is facilitated and the vibronic satellites can be identified and assigned. It could be shown that the radiative decay from the lowest excited state 11) to the ground state 10) is vibronically induced and dominated by 1R-active vibrations.'* Thus, the lowest excited state of the title compound appears to be a good model system for investigations on the magnetic field behavior of highly forbidden transitions. Drastic changes in the spectra are expected because any additional perturbation might overrule a vibronic intensity mechanism. The purpose of this paper is to study these effects with site-selective spectroscopic techniques
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Author for correspondence. 'Present address: Oitzing 6, D-8351 Grattersdorf, FRG. IPresent address: BASF AG, D-6700 Ludwigshafen, FRG.
0022-3654/91/2095- 1067$02.50/0
and to discuss the magnetically induced changes in the radiative pathways. 2. Experimental Section
[R~(bpy)~l(PF,), and [o~(bpy)~](PF,), were prepared as described Mixed single crystals of [Ru,-,Os,( b ~ y ) ~ ] ( p with F ~ )x~ = 0.01 were grown from sohtions of 1:I acetonitrile/ethanol by slow evaporation of the solvents at room ] ~ +determined from temperature. The content x of [ O ~ ( b p y ) ~was the absorption of redissolved crystals. The details concerning the emission spectrophotometer'* and the superconducting magnet (Oxford Instruments S M 4)19 are
( I ) Meyer, T. J. Pure Appl. Chem. 1986, 58, 1193. (2) Krause, R. A. Struct. Bonding (Berlin) 1987, 67, 1. (3) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.;Belser, P.; von Zelewsky, A. Coord. Chem. Rev. 1988,84, 85. (4)(a) Krausz, E.Comments Inorg. Chem. 1988, 7 , 139. (b) Krausz. E.; Ferguson, J. frog. Inorg. Chem. 1989, 37, 293. ( 5 ) Yersin, H.; Braun, D.;Hensler, G.;Gallhuber, E. In Vibronic Processes in Inorganic Chemistry; Flint, C. D., Ed.; Kluwer Academic: Dordrecht, 1989; p 195. (6) Felix, F.; Ferguson, J.; Giidel, H. U.; Ludi, A. Chem. Phys. Left. 1979, 62, 153. (7) Decurtins, S.;Felix, F.; Ferguson, J.; Giidel, H. U.; Ludi, A. J. Am. Chem. Soc. 1980, /02,4102. (8) Pankuch, B. J.; Lacky, D. E.; Crosby, G. A. J. Phys. Chem. 1980,84, 2061. (9) Lacky, D.E.;Pankuch, B. J.; Crosby, G. A. J. Phys. Chem. 1980,84, 2068. (IO) Kober, E. M.; Meyer, T. J. Inorg. Chem. 1982, 21, 3967. (11) Ferguson, J.; Herren, F. Chem. Phys. 1983, 76, 45. (12)Braun, D.; Gallhuber, E.; Hensler, G.;Yersin, H. Mol. Phys. 1989, 67,417. (13) Yersin, H.;Gallhuber, E.; Hensler, G. Chem. Phys. Lett. 1987, 140. 157. (14) Hensler, G.;Gallhuber, E.; Yersin, H. In Phorochemistry and Photophysics of Coordination Compounds; Yersin, H., Vogler, A.. Eds.; Springer-Verlag: Berlin, 1987;p 107. ( 1 5 ) Krausz, E.; Moran, G. Adu. Magneto-Opt.. Proc. Int. Symp. Magnefo-Opt.,J . Magn. Sor. Jpn. 1987, / / , Suppl. SI, 23. (16)Yersin, H.; Huber, P.; Braun, D. J . Phys. Chem. 1990, 94, 3560. (I 7) Yersin, H.; Gallhuber, E.; Vogler, A.; Kunkely, H. J . Am. Chem. Soc. 1983, 105, 4155. (18)Yersin, H.; Gliemann, G.Messterhnik (Braumchweig) 1972,80,99.
0 199 1 American Chemical Society
1068 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 ;; ~ 5 0 0cm"
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Braun et al. 3
1L300
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Figure 1. E
I c polarized absorption and emission spectra of singlecrystal [ R u , , O S , ( ~ ~ ~ ) ~ ] (at P FT~=) ~2 K and B = 0 T in the region ( x = 0.01, bXc = 363.8 nm). For of the electronic origins of [O~(bpy),]~+ E 11 c the emission is weaker by a factor of ,IO2 and no absorption for E 11 c could be detected with X 3 685 nm. The crystal thickness for the absorption measurements was (100 f 5 ) pm.
I c polarized absorption and emission spectra of singlecrystal [R~~,os,(bpy)~](PF~), at T = 2 K and B = 6 T (B Ic, E 11 B) in the region of the electronic origins of [ O ~ ( b p y ) ~( ]x ~=+0.01, A,, = 363.8 nm). The emission E 11 c is weaker by a factor of 3 X lo3. The absorption line A-IIM does not show any magnetic field dependence, within limits of experimental error.
described in the literature. The orientation of the magnetic field B was B I c. B I 0 (c, needle axis of the crystal; 0, direction of detection). UV excitation was achieved with the 363.8-nm line of an argon ion laser (Coherent Innova 90). The excitation source for the site-selective experiments was a nitrogen laser (Lambda Physik M 1000) pumped dye laser (Lambda Physik FL 2000) with DCM or pyridine 1 as laser dyes. The half-width of the laser line was =0.7 cm-". In all experiments the incident laser power was held low enough to avoid sample heating. Emission and excitation spectra were corrected neither for the spectral dependence of the response of the detection system nor for the power of the exciting dye laser, respectively. The monochromator read-out was calibrated with a low-pressure mercury lamp (Oriel Penray), which led to an experimental error of fl cm-l. For further details about measurements of polarized emission from single crystals see ref 20. Absorption measurements were carried out with the same type of apparatus,l* using a halogen quartz lamp (Osram Xenophot HLX bulb). After shielding the sample against infrared radiation (to avoid sample heating), the light was focused onto the crystal (typical thickness of -100 bm). The imaging system for the transmitted light was the same as for the emitted light. The spectra were plotted in a logarithmic scale. The concentration of [0s(bpy)J2+ was determined by using the content of [Os(bpy)J2+ in the mixed single crystals and the known crystal structure2' of the [R~(bpy),](pF,)~ host. From that it was possible to estimate the extinction coefficients. The infrared spectra of [ O ~ ( b p y ) , ] ( P F ~ ) ~ - 4were H ~ 0recorded at room temperature with a Nicolet 60 SX IT-IR spectrometer with a resolution of =4 cm-'. The spectra from 120 to 500 cm-I were measured in polyethylene pellets and those from 350 to 4000 cm-' in KBr pellets. For comparison with literature values,22we also measured the resonance-Raman spectra in aqueous solution at room temperature and expanded the measuring range to lower Raman frequencies. Several UV and visible lines of an argon ion laser (Spectra Physics 171) were used for Raman excitation. The
detection system was the same as for the emission measurements.
(19) v. Ammon, W.; Hidvegi. 1.; Gliemann, G.J . Chem. Phys. 1984, 80, 2837. (20) Yersin, H.: Hensler. G.;Gallhuber, E. Inorg. Chim. Acto 1987, 132, 187. (21) Rillema, D. P.; Jones, D. S.; Levy, H. A. J . Chem. Soc., Chem. Commun. 1979, 849. (22) Caspar, J. V.; Westmoreland, T. D.; Allen, G. H.; Bradley, P. G.; Meyer, T. J.; W d r u f f , W. H. J . Am. Chem. SOC.1984, 106. 3492.
Figure 2. E
3. Results The emission of [ R ~ ~ - , 0 s , ( b p y ) ~ ] ( P Fafter ~ ) ~UV excitation consists of two distinct components resulting from the [Ru(bpy),](PF,), host matrix ( 4 5 0 to ~ 6 8 nm) 0 and the [os( b p ~ ) ~guest ] ~ + molecules ( ~ 6 8 5to =410 nm). Thus, the luminescence of [0s(bpy),l2+ is clearly separated from that of the host. With increasing [0s(bpy)J2' concentration the [Ru( b p ~ ) , ] ~emission ' is quenched and its lifetime is shortened. The ] ~ +not changed if the complex emission spectra of [ O ~ ( b p y ) ~are is excited with A,, = 632.8 nm light. The spectra of [ O s ( b p ~ ) ~ ]doped ~ + into single-crystal [Ru(bpy),](PF,), exhibit highly resolved fine structures. The halfwidths of the lines amount to =2 cm-I in all electronic spectra discussed in this contribution. Figure 1 shows the E I c polarized emission (A,, = 363.8 nm, all sites A, B, and C) and absorption of [ R U ~ , O S , ( ~ ~ ~ ) , ] ( P Fat, ) ~T = 2 K and B = 0 T in the region of the electronic origins of [ O ~ ( b p y ) ~ (E, ] ~ +electric field vector). The absorption line of lowest energy (designated A-llM)23 lies nearly at the same energy as the emission line at highest energy (designated C-IM). However, these two lines belong to different sites. No absorption could be detected for energies lower than line A-HM at zero magnetic field. The line A-IIM has an extinction coefficient t = 5 X IO3 L/(mol.cm) and is accompanied by satellites and shoulders in distances of 15,21, 23, and 40 cm-' to higher energy. For the polarization E 11 c the emission is weaker by a factor of 2102and no absorption could be detected for A 2 685 nm. The spectra in the wavelength range of Figure I change drastically after application of a magnetic field (Figure 2). At B = 6 T (B I c, B 11 E) three dominating lines (A-lM, B-lM, and C-IM) are grown in by a factor of about lo3 in the E I c polarized emission. Within limits of experimental error the emission polarized E 11 c (E I B) is not influenced by a magnetic field B I c. The intensity of the line emission (A-IM, B-lM. C-iM) in E I c is by a factor of =3 X IO3 stronger than in E (23) For the designation of the different lines we chose the following nomenclature: The capital letters (A, B, and C ) denote the corresponding sites of [Os(bpy),]*+ in (Ru(bpy),](PF,),. They are followed by roman numbers ( I and 11) which label the excited states 11) and 111). The subscript (M indicates that the lines correspond to the electronic origins (zero-phonon tfansitions) whereas a number as subscript designates the energy separation (in cm-I) of a vibronic line to the corresponding origin.
The Journal of Physical Chemistry, Vol. 95, No. 3, I991 1069
[ O ~ @ P Y ) ~ IDoped * + into [ R ~ ( ~ P Y ) ~ I ( P F ~ ) ~
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700 720 7L0 mo 780 nm h. 800 Figure 3. Unpolarized emission of site A of [O~(bpy)~]~' in single-crystal [ R ~ ( b p y ) ~ ] ( p(A,F, ~ ) = ~ 689.9 nm, position of line A-IIW) at. T = 2 K and (a) B = 0 T (b) E = 6 T (B I c). The electronic origin of the lowest excited state 11) is marked A-I,,+ The numbers give the energy differences of vibronic satellites relative to the origin line A-I, in cm-l. The emission can be regarded to be E I c polarized due to the extremely weak (factor 3 IO2) E 11 c polarized component for-the electr&rc origin as well as for the vibron; satellites.
11 c and the vibronic satellites (not reproduced) are stronger by a factor of =IO2 in E Ic compared to E 11 c at B = 6 T. Thus, unpolarized emission spectra at all applied magnetic field strengths B Ic may be considered to be E Ic polarized. With application of a magnetic field also E I c polarized absorption lines grow in at exactly the same energy positions as the lines A-IM and B-IM in emission. The extinction coefficients of both lines are estimated to be in the order of c = 4 X 'Of L/(mol.cm) at B = 6 T. (The absorption peak of line G I M is hidden under the strong absorption line A-lIw) Within experimental error the absorption line A-IIw is not influenced by magnetic fields. Figure 3 shows the site-selective emission of site A of [Os( b ~ y ) ~ doped ] ~ + into [ R ~ ( b p y ) ~ ] ( p F , )(AFxc , = 689.9 nm) for B = 0 T (Figure 3a) and B = 6 T (B I c, Figure 3b) at T = 2 K . The given numbers mark energy differences between emission lines (vibronic satellites) and the line of highest energy (electronic origin) denoted as A-lw The energies of these vibrational modes are listed in Table I for all lines detected. At B = 0 T the line marked 479 cm-l is by a factor of about 50 larger than the origin line A-lM (Figure 3a). At B = 6 T several new satellites appear in the emission spectrum (Figure 3b). These satellites cannot be detected at B = 0 T. The intensity of the line A-I, is increased by a factor of = I O 3 with magnetic field increase from B = 0 T to B = 6 T in accordance with the result obtained with A,, = 363.8 nm. The magnetic field induced growing-in of the line A-I, can also be observed in the excitation spectra. Figure 4 shows this for [0s(bpy)J2+ in [ R ~ ( b p y ) ~ l ( Pdetecting F ~ ) ~ the emission at Ad,, = 71 7.2 nm (479 cm-' vibronic satellite of site A). At B = 0 T no excitation can be achieved with A,, = 693.3 nm, being the position of the electronic origin A-lw. However, on applying a magnetic field B Ic exactly at this position the excitation line A-IH grows in. The intensity of the line A-11, stays (within experimental error of 20%) constant at all magnetic field strengths. This is also found for a further line called A-I'. At B = 6 T the excitation line A-I, is more intense than line A - r by a factor of about 20. Together with line A-lM some additional peaks grow in with an energy separation of 18, 39, and 48 cm-' to higher energy relative to the position of A-lw. (Equivalent results as presented in Figures 3 and 4 for line A-lw are also found for the lines B-lWo and C-IH. These results are not reproduced.) At T = 2 K and E = 6 T the emission spectrum excited at A,, = 693.3 nm (line. A-lWo)is equivalent to the one which is excited at A,, = 689.9 nm (line A-llm, shown in Figure 3b). N o energy shifts and no splittings of the lines, in emission, absorption, or excitation, could be observed within the limits of the experimcntal uncertainty of f 1 cm-I. The vibrational frequencies obtained from 1R and resonanceRaman measurements are also included in Table I (spectra not
7 1L500 cm-' 1LL60 9
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690 692 nm X 69L Figure 4. Unpolarized excitation spectra of site A of [0s(bpy),l2' (Ad,, = 717.2 nm, corresponding to the 479-cm-' vibronic satellite) in [Ru(bpy),](PF& at T = 2 K and (a) B = 0 T; (b) B = 6 T (B Ic). Within limits of experimental error, the intensity and the energy position of line A-IIw do not depend on the magnetic field strength.
reproduced). For the resonance-Raman frequencies in the region of the ligand modes (=lo00 to =I600 cm-I) our results are in agreement with the literature valuesz2and therefore the latter are listed for convenience. 4. Discussion [ O ~ ( b p y ) ~guests ] ~ + doped into the [ R ~ ( b p y ) , l ( P F ~host ) ~ are mainly excited by radiationless energy transfer via the host when the doped crystals are irradiated with &, = 363.8 nm light. The process of energy transfer does not influence the emission properties of [ O ~ ( b p y ) ~ ]since ~ + , the spectra do not change when the complex is excited directly, e.g., with A,, = 632.8 nm. The properties of energy transfer have been discussed p r e v i o u ~ l y ~ ~ ~ ~ ~ as well as the behavior of the host m a t r i ~ . Therefore, ~ these subjects will not be, discussed further in this contribution. The emission spectra of [Os(bpy),lZ+ in [Ru(bpy),](PF& are governed by triple structures resulting from a superposition of spectra of three different sites A, B, and C of [ O ~ ( b p y ) J ~ + . ' ~ . ' ~ (24) Yersin, H.; Braun, D.;Gallhuber, E.; Hensler, G.Ber. Bunsen-Ges. Phys. Chem. 1987. 91. 1228. (25) Yersin, H.; Hensler, G.; Gallhuber, E.J. Lumin. 1988, 40/41, 676.
1070 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 TABLE I: Vibronic Satellites in the Emission (7' = 2 K) from the Lowest ~ ) ~Also Excited State 11) of [Os(bpy)#+ Doped into [ R ~ ( b p y ) ~ l ( P F(See Figure 3) Compared to IR and Resonance-Enhanced Raman Vibrations' vibronic magn field reson satellitesr IR-active induced vibrnl Raman atE=OT vibrnsb satellites' vibrns' 16 I5 31 21 40 38 45 52 68 71 ai 75 115 104 155 I34 196 I75 191 210 242 258 238 296 278 2aa 312 309 327 383 372 369 377 386 405 417 417 424 442 479 482 551 56 I 558 596 653 646, 659 667 675 696 730 729 738 766 77 1 772 791 849 862 868 a80 878 a9 I aa5 993 I009 I024 1026 1028 1 029' I 048' 1062 I 048 1069 1067 1084 1 IO6 1124 I125 I163 1154 1 I72 1 175' 1222 1241 I242 I263 1270 I260 1 26ad I267 1320 131 1 1314 1 322d I420 I446 I446 1461 1482 1491 1491' I 528 1559 1560 I 55ad 1552 I604 1610 1610' 1650 1699 1764 I aoi I937 1935 1969 "The vibronic satellites at B = 0 T are assigned to be nontotally symmetric (a2 and/or e symmetry) whereas the magnetic field induced ones are assigned to be totally symmetric (a, symmetry). The magnetic field induced satellites cannot be observed at B = 0 T. (For a comparison with the vibrational freuqencies of perdeuterated [Os(bpy-d,)J2+ see ref 16.) All numbers are given in cm-'. The experimental uncertainty is -I cm-I. Measured with [O~(bpy)~l(PF,),from 120 to 500 cm-' in polyethylene and from 350 to 4000 cm-' in KBr ( T = 300 K). Peaks above 2000 cm-l are not listed here, but see ref 12. CMeasuredin H20at 300 K. dFrom ref 22.
