J . Phys. Chem. 1984, 88, 3007-3014
3007
Infrared, Raman, and Resonance Raman Studies of the Ru(2,2'-bpy),*+ Cation in Its Chloride Crystal and as an Intercalate in the Layered MnPS, Compound 0. Poizat*+ and C. Sourissead LASIR, CNRS, 94320 Thiais, France, and Laboratoire de Spectroscopie Infrarouge, LA 124, UniversitP de Bordeaux I , 33405 Talence, France (Received: August 29, 1983)
Infrared, Raman, and resonance Raman spectra (1700-180 cm-l) of Ru(bpy),C12, h24 and d24 compounds, and of (Ru(bpy),)o,2,Mn,,5PS3Nao.s(H20)2, in the solid state, have been investigated. A complete vibrational assignment of the R~(bpy),~+ cation modes is proposed. The analysis of excitation profiles for many Raman bands definitely demonstrates that the unresolved structure of the main visible absorption band of Ru(bpy),C12 is due to two distinct metal to ligand charge-transfer transitions. These profiles have been satisfactorilyfitted by using a theoretical model based on two interfering Franck-Condon processes. Finally, similar spectroscopicstudies of the Ru(bpy),'+ species intercalated in MnPS, have shown that this cation is weakly interacting with the host lattice and that its electronic structure is practically not perturbed by this intercalation.
Introduction The electronic structure and properties of trisubstituted metal complexes of the 2,2'-bipyridine ligand have been recently the subject of several investigations in relation to their potential activities in photochemistry and photolysis. Most of the spectroscopic studies were focused on the UV-vis transmission and emission spectral-'" of the Fe2+, Ru2+, and Osz+cation complexes; some works were carried out on the resonance Raman spectra of these compounds in their ground states and in their first excited triplet states."-I6 It is well-known that visible spectra of M(bpy):+ cations exhibit an intense and broad band at about 21 000 cm-I with an additional unresolved structure on its high-energy side; the assignment of these signals to metal (t2J to ligand (K') charge-transfer transitions is well established.17J8 From recent polarized absorption and circular dichroism (CD) measurements, Fergusson et a1.?*l0Elfring and Crosby,6 and Ceulemans et al.' agreed to assign these visible bands to two distinct spin-allowed, 'A, 'E, charge-transfer (CT) transitions. However, from previous Raman studies and resonance ~ ~aqueous + Raman excitation profile investigations of F e ( b p ~ ) in solutions, Clark et a1.16 and Miller et al.I4 have concluded that the splitting of the intense visible band comes from vibrational progressions. Then, the main purpose of this work is to solve this open question in the particular case of the Ru(bpy),2+ complex by a careful analysis of the vibrational spectra, which should lead to a better insight into the energy and charge-transfer processes. In fact, to the best of our knowledge, complete vibrational results have not been published yet and, in particular, low-frequency data are missing. One thus expects to get a coherent interpretation of both the resonance Raman excitation profiles and the electronic spectra. In the first part of this work, we report a precise analysis of the infrared and Raman spectra (1800-120 cm-') for solid Ru(bpy),C12.5H20 samples (using hZ4 and d24 derivatives) and propose new vibrational assignments. Then, the resonance Raman profiles of several modes have been obtained by excitation within the contour of the first CT transitions; these experimental profiles have been satisfactorily fitted by using a theoretical approach described by Siebrand and ZgierskilF2' and by Clark and Dines.22 Finally, we present the analysis of the infrared, Raman, and resonance Raman spectra of the R ~ ( b p y ) , ~cation + intercalated in the layered MnPS, compound.23 This study is developed in the context of a general investigation of cationic entities intercalated in lamellar inert materials; in fact, it has turned out from recent works24-26that, for such systems, vibrational spectroscopy is able to provide structural and electronic information concerning the guest species. We have thus performed the intercalation of
-
LASIR.
* Laboratoire de Spectroscopie Infrarouge. 0022-365418412088-3007$01.50/0
new reactive molecules with interesting photochemical properties in order to find out by which amount these properties are modified upon intercalation. In the present case, the Raman excitation profiles of the intercalated R ~ ( b p y ) , ~modes + have been investigated and fitted, and the results have been compared to those obtained with the "free" cation. Interactions between the host lattice and the guest species and electronic perturbations of the L C T excited-state properties have been evaluated. M
-
Experimental Section Synthesis. The solid complexes, h24 and d24, of Ru(bpy),C12.5H20 were prepared from 2,2'-bipyridine, h8 and d8, respectively, according to literature method^.^'^^^ The totally
Fergusson, J.; Herren, F. Chem. Phys. 1983, 76, 45. (2) Fergusson, J.; Kransz, E. Chem. Phys. Lett. 1982, 93, 21. (3) Fergusson, J.; Herren, F. Chem. Phys. Lett. 1982, 89, 371. (4) Fergusson, J.; Herren, F.; Laughlin, G. M. Chem. Phys. Lett. 1982, 89, 376. (5) Kobar, E. M.; Meyer, T. J. Inorg. Chem. 1982, 21, 3967. (6) Elfring, W. H.; Crosby, G. A. J . Am. Chem. SOC.1981, 103, 2683. (7) Ceulemans,A.; Vanquickenborne,L. G. J. Am. Chem. SOC.1981,103, 2238. (8) Decurtins, S.; Felix, F.; Fergusson, J.; Giidel, H. U.; Ludi, A. J. Am. Chem. SOC.1980, 102,4102. (9) Felix, F.; Fergusson, J.; Giidel, H. U.; Ludi, A. J. Am. Chem. SOC. 1980, 102, 4096. (10) Felix, F.; Fergusson, J.; Giidel, H. U.; Ludi, A. Chem. Phys. Lett. 1979, 62, 153. (1 1) Basu, A,; Gafney, H. D.; Strekas, T. C. Inorg. Chem. 1982,21,2231. (12) Forster, M.; Hester, R. E., Chem. Phys. Lett. 1981, 81, 42. (13) Bradley, G.;Kress, N.; Hornberger, B. A.; Dallinger,R. F.; Woodruff, W. H. J . Am. Chem. SOC.1981, 103, 7441. (14) Miller, P. J.; Chao, R. S. L. J . Raman Spectrosc. 1979, 8, 17. (15) Dallinger, R. F.; Woodruff, W. H. J . Am. Chem. SOC.1979, 101, 4392. (16) Clark, R. J. H.; Turtle, P. C.; Strommen, D. P.; Streusand, B.; Kincaid, J.; Nakamoto, R. Inorg. Chem. 1977, 16, 84. (17) Williams, R. J. P. J . Chem. SOC.1955, 137. (18) Lytle, F. E.; Hercules, D. H. J . Am. Chem. SOC.1969, 91, 253. (19) Siebrand, W.; Zgierski, M. Z. In "Excited States"; Academic Press: New York, 1979; Vol. 4. (20) Siebrand, W.; Zgierski, M. Z. J . Chem. Phys. 1979, 71, 3561. (21) Siebrand, W.; Zgierski, M. Z . J . Raman Spectrosc. 1982, 13, 78. (22) Clark, R. J. H.; Dines, T. J. Mol. Phys. 1981, 42, 193. (23) Clement, R. J . Am. Chem. SOC.1981, 103, 6998. (24) Mathey, Y.; Clement, R.; Sourisseau, C.; Lucazeau, G. Inorg. Chem. 1980, 19, 2773. (25) Sourisseau, C.; Forgerit, J. P.; Mathey, Y. J . Phys. Chem. Solids 1983, 44, 119. (26) Sourisseau, C.; Forgerit, J. P.; Mathey, Y . J . Solid State Chem. 1983, 49, 134. (1)
0 1984 American Chemical Society
3008 The Journal of Physical Chemistry, Vol. 88, No. 14, 1984
Poizat and Sourisseau frequencies can be easily compared. Therefore, we have used not only the notation already known for benzene34and pyridine3, to describe these normal modes but also the correlation table between the C2, symmetry group of pyridine and cis-2,2’-bipyridine and the D, group of Ru(bpy):+: thus, each pyridyl ring vibration splits B, or A2 + B2) in the cis-2,2’-biinto two components (A, pyridine molecule and into four components (A, A2 + 2 E) in the D3 complex; the latter splitting effect is expected to be very weak as all the vibrational results yet reported for tris(bipyridine) metal are quite similar to those known for the corresponding monopyridine corn pound^.^^ Finally, site and correlation crystal effects are also expected to be negligible in the solid compound Ru(bpy),C12. Inter-ring vibrations include one stretch (a,), two in-plane bends (02 and w3), and three out-of-plane deformations (I’,,r2,and I’,) which give rise to 18 modes in the D, complex (4A1 + 2A2 + 6E). Finally, the metal-ring modes can be classified into six Ru-N stretches (A, A2 2E) and nine bending vibrations (2A1 A2 3E); these deformations must give rise to very low frequency bands and will not be investigated therein. ( b ) Methods of Ring Mode Assignments. First of all, the recorded spectra (Figure 1) are quite similar to those of monobipyridyl c o m p l e x e ~and ~ ~ ,exhibit ~ ~ nearly the same number of infrared and Raman bands. Then, band assignments were essentially established using the following criteria: (i) We have extensively used the vibrational analogies among benzene,29 pyridine,,, toluene,37 picoline,38 biphenyl,31 2,2’-bipyridine,,, and R ~ ( b p y ) , ~species, + and we have analyzed the group frequency sequences as commonly encountered in the sixmembered aromatic molecules. In particular, the strongly coupled symmetric ring modes called hereafter as “6a”, ”l”, and “a,”are generally characterized by the Raman bands labeled as “c”, “d”, and “e” and localized at -1300, 760, and 400 cm-’. We thus assigned with confidence the Raman bands observed at 1317,765, and 464 cm-’ to these modes (Table I). (ii) Many band assignments have been confirmed by the vibrational data obtained for the d24 derivative and the comparison of the isotopic ratios to those observed in related corn pound^.^^,^^ (iii) Finally, the values of the resonance Raman enhancements have been compared in the h24 and d24 complexes (Table I). In a first approximation, one can expect similar enhancement factors for the corresponding bands in the spectra of both derivatives; however, such a comparison must be handled with care since some factors may be sensitive to vibrational coupling effects. (c) Metal-Ligand Stretching Vibrations. The assignments of Ru-N stretches were based on the results previously reported by Saito et al.39 for a set of 2,2’-bipyridyl complexes of first-row transition metals. Using the metal isotopic t e c h n i q ~ e ,these ~~,~~ authors have characterized two v(Ru-N) modes, the frequencies of which are sensitive to the electronic configuration of the metal: these vibrations are expected in the 39C-300-cm-’ range for metals with empty eg orbitals and in the 290-180-cm-’ region for metals with partially filled or completely filled eg orbitals. In Ru(bpy)?+, the ruthenium ion has a low-spin (t”,, e:) configuration; consequently, the two Raman bands observed at 372 and 337 cm-’ for the h24complex and at 360 and 327 cm-’ for the d24compound
+
I
+
1800
1400
1000
600
ZOO cm-1
Figure 1. Infrared and Raman spectra (A, = 6471 A), in the 1700-
180-cm-’ region, of the h24 (upper trace) and dZ4(lower trace) derivatives of Ru(bpy),C12.5H20 in the solid state. deuterated sample (d,) of 2,2’-bipyridine was synthesized as follows: CloH8N2(2.5 X lo-, mol) was added to a nickel-Raney catalytic mixture (50 mL of D 2 0 + 1.5 g of N a + 2.0 g of Ni-Al-Raney) and heated at 85 OC under stirring for 6 h. The filtered solution was evaporated to dryness, and the partially deuterated bipyridine was sublimated. Such a procedure was repeated four times. Finally, one gets 1.5 g of 2,2’-bipyridine (d,) with a deuterium content of 95% as checked by mass spectroscopy. The intercalation reaction of R ~ ( b p y ) in ~ ~the + MnPS, layered system has already been described.23 Spectra. Absorption spectra in the visible region were recorded with a Cary 17 spectrometer in the transmission mode with potassium halide pressed disks (with ca. l %O in KCl) at 300 and 100 K. Infrared spectra were recorded from Nujol or fluorolube mulls at 300 K and at ca. 100 K by use of Perkin-Elmer 225 and 180 spectrometers. Raman spectra were obtained either with a Dilor R T I triple monochromator equipped with Spectra Physics 164 krypton (647.1 nm) and argon (514.5-454.5 nm) lasers or with a Jobin Yvon (Ramanor HG2S) double-monochromator instrument using the 363.8- and 351.1-nm exciting lines from a Spectral Physics 171 argon laser and other lines in the range 430.0-450.0 nm from the stilbene3 dye pumped by the previous UV excitations. The polycrystalline samples were dispersed in KC1 or KC104 (internal standard) and pressed on a sample holder rotating at ca. 1600 rpm in order to minimize any local heating effects. Integrated band intensities were measured with respect to the 943- or 465cm-’ bands of KC104 and corrected from the spectral responses of the instruments.
Results and Discussion 1. Vibrational Spectra of Ru(bpy),C12. The infrared and Raman spectra (A, = 647.1 nm) of R ~ ( b p y ) , ~at+ 300 K, in the 1700-180-~m-~range, are shown in Figure 1 for the h24 and d24 derivatives. The corresponding band wavenumbers and proposed assignments are reported in Table I. (a) Description of the Normal Modes and Structural Considerations. The vibrations of R ~ ( b p y ) cations ~ ~ + can be classified into three groups: ring modes, inter-ring vibrations (within a bipyridine ligand), and metal-ligand modes. Most of the ring vibrations in R ~ ( b p y ) , ~can + be referred to their analogues in six-membered aromatic cycles such as benzene:9 pyridine,,, bipheny1,31-32 and 2,2’-bipyridine alone,33 and their (27) Palmer, R. A,; Piper, T. S. Inorg. Chem. 1966, 5, 864. (28) Burstall, F. H. J. Chem. SOC.1936, 173. (29) Herzfeld, N.; Ingold, C. K.; Poole, H . G. J . Chem. SOC.1946, 316.
+
+
+
+
(30) Long, D. A,; Murfin, F. S.; Thomas, F. L. Trans. Faraday SOC.1963, 12, 59. (31) Zerbi, G.; Sandroni, S . Spectrochim. Acta, Part A 1968, 24A, 483. (32) Pasquier, B.; Lebas, J. M., J . Chim. Phys. Phys.-Chirn. Biol. 1967, 64, 765. (33) Neto, N.; Muniz-Miranda, H.; Angeloni, L.; Castellucci, E. Spectrochim. Acta. Part A 1983, 39A, 97. (34) Wilson, E. B.; Decius, J. C.; Cross, P. C. “Molecular Vibrations”; McGraw-Hill: New York, 1955. (35) Strukl, J. S.; Walter, J. L. Spectrochim. Acta, Part A 1971,27A, 228. (36) McConnell, A. A,; Brown, D. H.; Smith, W. E. Spectrochim. Acta. Part A 1982, 38A, 265. (37) Fuson, N.;Garrigou-Lagrange,C.; Josien, M. L. Spectrochim. Acta 1960, 16, 106. (38) Long, D. A.; George, W. 0. Spectrochim. Acta 1963, 19, 1777. (39) Saito, Y.; Takemoto, J.; Hutchinson, B.; Nakamoto, K. Inorg. Chem. 1972, 1 1 , 2003. (40) Hutchinson, B.; Takemoto, J.; Nakamoto, K. J . Am. Chem. SOC. 1970, 92, 3335.
The Journal of Physical Chemistry, Vol. 88, No. 14, 1984 3009
>
-m
c
z Y
I-
z H
z
a
a K w
->
+ 4 A iy
K
Figure 2. Raman spectra (1650-180 cm-I) of the hydrogenated (h2J Ru(bpy),Cl, complex in the solid state, obtained with the exciting lines 6471, 4658, and 3638 A. The dashed lines correspond to the 943-cm-' reference band of the C104- ion. I
I
!
2'
Ru(bipy)g
ii
I
(0")
!
Figure 3. Raman spectra (1600-180 cm-I) of the perdeuterated (dZ4)Ru(bpy),Clz complex in the solid state, obtained with the exciting lines 6471, 4658, and 3638 A. The dashed lines refer to the 943-cm-' reference band of the C104- ion.
-
are assigned to these ~(Ru-N)vibrations. It is worthwhile to point out that the corresponding isotopic ratios p ( v H / Y ~ ) 1.03 are the lowest ones observed in the 500-200-~m-~ range; the remaining bands observed below 300 cm-' arise probably from inter-ring ligand deformations. 2. Resonance Raman Spectra and Excitation Profiles in Ru(bpy)C12. Some Raman spectra (1700-1200 cm-l) recorded with different exciting radiations within the contour of the first electronic C T transitions are shown in Figures 2 and 3 for the
h24 and dZ4Ru(bpy),CI2 complexes. The electronic spectrum together with excitation profiles of many Raman modes for the hydrogenated Ru(bpy),2+ cation is shown in Figure 4. The relative maximum-enhancement factors (F)for all the bands (normalized with respect to the intensities measured by using the 647.1-nm laser line) are reported in Table I. It is well-known that Raman bands are enhanced by a resonance process when the exciting laser line falls within the contour of a dipole-allowed electronic transition that presents a vibrational
The Journal of Physical Chemistry, Vol. 88, No. 14, 1984
3010
Poizat and Sourisseau
TABLE I: Infrared and Raman Band Wavenumbers (cm-’) and Assignments for the h,, and d,, Ru(bpy),CI, Complexes h,, V
int
1600 1565
m
V
1607
1503 1495 1485 1462 1441 1420
d24
Raman
IR
inta S
Fb
1558
m
58
1490
vs
25
m m m 1317
1311
m
1267 1243 1225
m m m
1159 1121
m
1066
W
1040 1024 1007
vw
S
int
1566 1534
m
1360 1335 1299
sh
1232
m
ms
6
1173 1160
m
20 1
732
468 421 371 337 332
195
24
1523
s
3o
1423
vs
1302
w
1200
s
1110 1069
W
2 2
S
1
S W
8 4
W
W W
1040 1025 1012
} 8a, u(ring) } 8b, u(ring)
1 774 + 732 l5
} }
6
1005 992 982 934
} }
vw vw
vw vw
1017 1003 988
s
s w
sh
866
m
845 836 792
sh
vw
736
vw
882 864 850 844
w w
w sh
730
vw
m
vw
vw
668 659 643
vw
1i
W
W W
>5
495 464
vw
372
W
4
337 295 281 25 2 236 198
m
8 3
vw
6 20
W
vw
720 680
m vw
2
640 631 621 600
vw w m vw
100 -4 2
vw vw
441
w
m vw
360
w
327 271 262 242 227 187
m w
W
632 621
W
592 45 8 440 378 35 9
S
vw
m
W
324
m
vw
vw vw vw
6 4 4
184
m
vw vw w
vw
} }
12, ring breathing
8
; I
6 (CD)
+ v(ring)
17% Y(CD) 11, dCH) “d”, A(ring) + v(ring) + v(ring-ring) 1% r(CD) 4, r(CH) + r (ring) “d”, A(ring) + v(ring) + u(ring-ring) 10a,r(cD)
;;;;E)
1 Il,r(CD)
>15
W
sh
1,ring breathing
1 18a,
2
150 4 1
6 (CH)
} } 9b, 6 (CD) + v(ring)
vs S
3, 6 (CH) + u(ring)
17a, r(CH) 17b, 18b, 6(CD) + v(ring)
m
m
19b, v(ring) “c”, u(ring-ring) + v(ring) + A(ring) 14, u(ring) + 6 (CH/CD) “ ,, c ,v(ring-ring) + v(ring) + A(ring)
1 3,6(CD) + u(ring)
vw
vw
765
19a, u(ring)
+ u(ring) } 18b, 6 (CH) + v(ring) 1Sa, 6 (CH) + v(ring)
W
661 658 642
w
1 9b,
888
774
1567
S S
assigntC
Fb
} 1030
968 897
m
inta
U
12
1278
W
V
14
vw
W
Raman
IR
}
16a, r(ring) “e”, A(ring) + v(ring) + u(ring-ring) 16b, r(ring) v(Ru-N)
I?,
A(rhg-ring)
8
Measured by using 6471 A excitation. Highest enhancement factors calculated as the ratio Imax/1664,1. u = stretch; 6 , A = in plane deformation; 7, r = out-of-planedeformation. a
activity. Such a vibrational activity can arise either from the Franck-Condon principle or/and from vibronic couplings?1 The first phenomenon affects only the totally symmetric modes whose potential minima are shifted between the ground and the resonant excited state (r,); according to Albrecht’s formalism, an A term mechanism is then effective.“2 Vibronic coupling effects take place (41) Herzberg, G. “Electronic Spectra and Electronic Structure of Polyatomic Molecules”; Van Nostrand: Princeton, NJ, 1967. (42) Albrecht, A. C. J . Chem. Phys. 1961, 34, 1476.
when a second allowed electronic transition (rz) is coupled with the resonant transition (rl) via a vibrational mode; the scattering mechanism is governed by Albrecht’s B term,42 and any mode whose symmetry is contained in the direct product of the representations, r r , @ r r z , will be Raman enhanced. ( a ) Experimental Results. As shown in Figure 4, many ring modes of the 2,2’-bipyridine ligand are enhanced at resonance by a factor larger than 10; moreover, the Raman band at 668 cm-’ which corresponds to the “6a” ring deformation, is by far the most enhanced and exhibits the largest factor (F L 150) at resonance.
The Journal of Physical Chemistry, Vol. 88, No. 14, 1984 3011
50 z
0 II
E D
1 00
N
g
II r m
1317
D
2
1278 e
2 5 0 II E D z
00
-z E E < 700
600 c m - ’
700
600 cni-1
Figure 5. Detailed Raman spectra of the hZ4(700-580 cm-I) and d24 io
0
(740-560 cm-I) Ru(bpy),C12 derivatives from the excitation lines 4765 and 6471 A. (A spectral resolution of 4 cm-I has always been used.)
the energy difference, equal to 1800 cm-I, between the two electronic absorption bands cannot be explained by the vibrational data, and we conclude the absence of a normal mode corresponding to this vibronic spacing. Therefore, in agreement with Fergusson et al.,9 we assign with confidence these strong visible absorption bands to two distinct overlapping MLCT transitions, ‘A, If the resonance Raman mechanism were due to a vibronic coupling between these transitions, there would he no symmetry selection, as the direct product E @ E contains A, Az E in the D3 group, and all Raman modes should be enhanced. However, it has been shown from previous depolarization ratio mea~ u r e m e n t s ’ ~that J ~ only the totally symmetric modes were active. We thus conclude that Franck-Condon effects (A term) are mainly responsible for the observed resonance Raman results while any contribution of a vibronic coupling ( B term) must be negligible. ( b ) Theoretical Treatment. According to Siebrand and Zgierski2’ and to Clark and Dines,22when two electronic transitions are overlapping, the Raman spectrum of a mode active in both transitions shows the effect of interference between the two resonant states. In the absence of coupling, this two-state interference is a direct consequence of the Franck-Condon principle. In fact, each state leads, for totally symmetric modes, to a scattering contribution whose magnitude is very sensitive to the value and the sign of the displacement parameter of the equilibrium position, B,, in this excited state. Raman intensities being proportional to the squared sum of these contributions, negative or positive cross terms can appear, and the interference effects can then be constructive or destructive.21 This phenomenon can account for differences noted between absorption band shapes and resonance Raman excitation profiles. We have thus used a mathematical model, firstly developed by Siebrand and Zgierski,zo in order to reproduce the experimental Raman profiles of the symmetric vibrations of Ru(bpy),2+. In a Franck-Condon process, the general expression of the scattering tensor element, namely ai:, corresponding to a Ogs lgs fundamental Raman transition of the sth vibration is
-
+ +
Figure 4. The absorption spectrum (300-650 nm) and Raman excitation profiles for several modes of the h24Ru(bpy)&12 complex in the solid state. All intensities are normalized with respect to those obtained at 647.1 nm, and for clarity the profiles are shifted along the axis of relative Raman intensity. Arrows on the absorption spectrum indicate the position of the laser exciting lines.
Surprisingly, this effect has never been pointed out in previous Raman studies.I4J6 As a matter of fact, this may be explained by the presence of two other weakly enhanced bands in the vicinity (at 658 and 643 cm-l) and by the very weak intensity of this mode when far from resonance. These phenomena are illustrated in Figure 5 where we have included the similar experimental features observed for the band at 640 cm-l (F I100) in the d24complex. We thus conclude that many ring modes are perturbed and involved in the C T transitions but that the totally symmetric 6a ring deformation is probably the more responsible one for transforming the equilibrium geometry of the bipyridine ligand from that of the ground state to that corresponding to the resonant first excited state.43 Under these conditions, any vibrational structure expected on the MLCT absorption band could only be the result of a complex superposition of progressions arising from many modes; moreover, if a particular spacing is observed on the absorption spectrum, it will not be obvious to connect it with an excited-state vibrational mode. The detailed observations and theoretical explanations for such a phenomenon called the “missing mode effect” (MIME) have been recently reported by Tutt et al.44 Finally, (43) Nishimura, Y . :Hirakawa, A. Y . ;Tsuboi, M. ‘Advances in Infrared and Raman Spectroscopy”: Heyden: London, 1978;Vol. 5. (44) Tutt, L.; Tadnor, D.; Heller, E. J.: Zink, J. I. Inorg. Chern. 1982, 21,
3858.
-
a;: =
CCP
,, gm
( Ag 1(s) I
L v )
(Amvlrgo )
E,, - Ego- hvo - ir,,
where hvo is the energy of the exciting radiation, E,,,, - E,, is the energy difference between the ground state and the vibronic level mv of the excited state m, r,, is the damping factor, and pgm is the electronic transition moment g m. Here, the first sum runs over the two resonant electronic states (m = rl, rz) and the second sum is carried out over all the intermediate vibrational levels. The
-
3012 The Journal of Physical Chemistry, Vol. 88, No. 14, 1984
Poizat and Sourisseau TABLE 111: Infrared and Raman Band Wavenumbers (cm-’) and Assignments for the MnPS, Compound Intercalated with the Ru(bpy),’+ Cation IR
int
1630
br
1600 1562 1485 1460 1442 1420 1309 1267 1241 1225 1163 1122 1105 1064 1043 1024 1007 966 897 878 773 Figure 6. Comparison of the calculated (dashed lines) and observed (crosses) excitation profiles for the Raman bands at 668, 1173, 1490, and 1558 cm-I, respectively. Raman intensities are normaIized with respect to those at 6471 A, and the profiles are shifted for clarity. The absorption spectrum (full line) and the calculated origins of both electronic transitions (vertical arrows) are also shown. TABLE II: Parameter Values Used in the Calculation of the Resonance Raman Excitation Profiles of the Totally Symmetric Modes at 668. 1173, 1490. and 1558 cm-I of the Ru(bovL*+Cation B,,, cm-I MLCT EmO,*O, rm, transition cm-I P~~ 668 1173 1490 1558 cm-’ rl 20500 3 0.55 0.2 0.2 0.3 600 r2 21800 2 -0.8 -0.5 -0.5 -0.5 600 vibrational wave function (AI in the multimode approach20q22 must be written as products of normal-mode functions, so that (Ago1
(AmvI
=
v(OgiI
( IgsIVms) (VrnsIOgs)
= ?pgm2$
sh
n(OgrIVmi)(VmrIOgi)
f#S
+
EmO- Ego cvm,w,- hvo - ir,,
1558 1488
W
/8b, u(ring)
} 19a, u(ring)
1319
S
1275
m
119b, v(ring) c , u(ring-ring) t u(ring) 14, u(ring) + 6 (CH)
W
m
\8a, u(ring)
vs
W
1175 1160
W
1112
W
1066 1043 1026 1012
vw
sh
W
L’
I\
W W W W W
S S
vw
m sh
564
vs
W W
515
vw
555 495 470
vw vw
9b, 6 (CH) + u(ring)
1
VS
65 8 645 608 591
3,6 (CH) t u(ring)
2 X 564 18a, 6 (CH) + u(ring) 1 12, u(ring) 17a 17b, r(CH) 564 + 322 11,4(CH) “d”, u(ring) + A(ring) 4, r(ring) + r(CH)
vw 766 732 66 8 65 8 640
51
1 18b, 6(CH) + u(ring)
vw vw
S
25 3
m
m m m W W
assigntb water
1604 m
132
470 45 5 420 389 372 335 322 23 8
Raman intu
u
vw
W
vw sh
6a
16b, A(ring)
vs S
vw
16a, r(ring) “e”, A(ring) + u(ring) u (P-P) 16b, r(ring)
vw
S
W
m sh
sh m sh
386 372 339 295 274
vs W
w
vw m
vs
23 7 220 200
= n(vmiI
One thus obtains CY;,”
-
u
vw
vw
vw
\:E;) 6 ,(PS,)
ring-ring def 6d(ps3) T’,,(PS3) ring-ring def R‘xy
ring-ring def
a Measured by using 6471 A excitation. u = stretch; 6 , A = in-plane deformation; 7,r = out-of-planedeformation.
The first contribution involves up to four vibronic levels vms, so that
I
where wi is the excited-state frequency of the ith normal mode. In the first step, we have considered only the intermediate states v, involving the mode s (v, # 0; all u,, 0) in order to estimate mO. In the second stage, all the the electronic origins, go intermediate vibronic levels whose go mv transition energies are in the contours of the MLCT absorption bands were included; thus, only vibronic levels which give rise to nonvanishing Franck-Condon integrals have been kept in the calculation. Therefore, we have calculated three main contributions to the scattering tensor which can be readily expressed from the basic equations proposed by Siebrand and Z g i e r ~ k i ’ and ~ , ~ ~using Manneback‘s recurrence formulas for Franck-Condon i11tegrals.4~
-
-
(45) Manneback, C, Physica (Amsterdam) 1951, 17, 1001.
IC
c
+ 2ws
c+ws
(46) Clark, R.J. H.; Stewart, B. Struct. Bonding (Berlin) 1979, 36, 54. (47) Abdo, S.; Canesson, P.; Cruz, M.; Fripiat, J. J.; Van Damme, H. J . Phys. Chem. 1981.85, 797.
The Journal of Physical Chemistry, Vol. 88, No. 14, 1984 3013 Finally, the third contribution comes from three combination levels between s and t modes, i.e. -Bmr2 1 (B;.' -- 1
) ;
-Bm; ( B;? - 1
+
c
+ w, + 2wr
)
t
The resulting complete expression of the scattering tensor element is therefore
I
1
22000
18000
and the Raman intensity is proportional to the square of this tensor element. (c) Best Fit Parameters and Comparison of Experimental and Theoretical Profiles. The experimental and calculated resonance Raman excitation profiles correspondingto the four more enhanced modes at 668, 1173, 1490, an d 1558 cm-' in the hZ4Ru(bpy),*+ cation are compared in Figure 6. Only these modes were taken into account to calculate each fundamental profile by the multimode approach, since we had checked that contributions from other modes do not significantly perturb nor improve the theoretical results. The best fit parameters obtained after further refinements are listed in Table 11. It must be pointed out that all Bmidisplacement parameters are small and exhibit values lower than 1.O; this is consistent with the experimental spectra where no overtone has been observed in resonance Raman. Consequently, in agreement with Clark and Dines,22these results show once more that in polyatomic species of low symmetry several normal modes may be enhanced at resonance, each one involving small displacements of a large number of atoms; as a result, the displacement of the excited-state potential minimum along any particular normal coordinate will
1
J/cm-'
Figure 7. Calculated Raman excitation profiles of the bands at 668,
1490, and 1558 cm-' for the Ru(bpy),C12 compound. Peak maxima are normalized to the same intensity. where k, = -(1/21/2)p,m2B, ~ X P ( - ' / ~ C B ~ and ; ) c = EmO - Ego - hvo - irmv. B , corresponds to the displacement parameter of equilibrium position for the mode s in the state m; B, = (ims)'"(Qms
'
Qgs)'
We have assumed no frequency changes (w,) within the different ~electronic states: these w, frequencies a r e unknown from experiment, but they are expected to be equal to the wgsvalues; that is usually considered as a good a p p r o ~ i m a t i o n . ~ ~ The second term represents the contributions of other normal modes t of frequency ut within the vibronic levels vmI:
x 24
1
I
I
I
Figure 8. Infrared and Raman spectra (A, = 4658 and 6471 A) in the 1650-150-cm-' range of the Ru(bpy),*+ cation intercalated in MnPS,. The stars indicate bands assigned to vibrations of the MnPSl host lattice. The dashed lines correspond to the 943-cm-' reference band of the Clod-ion.
3014
The Journal of Physical Chemistry, Vol. 88, No. 14, 1984 A
-150
$ 3 D
1 m N
0
II
-100
7 D
! m
a D D
-50 H 2
+
m
z E 4
