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IV dimer chain complexes of platinum and palladium
Journal of Molecular Structure, 189 (1988) 173-185 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
173
RESONANCE RAMAN SPECTRA OF METAL II/IV DIMER CHAIN COMPLEXES OF PLATINUM AND PALLADIUM Analysis of the component structure to the band assigned to the symmetric XMX chain stretching mode (X=Cl or Br)*
ROBIN J.H. CLARK and DAVID J. MICHAEL Christopher Zngold Laboratories, University College London, 20 Gordon Street, London WClH OAJ (Gt. Britain) (Received 14 January 1988; in final form 15 February 1988)
ABSTRACT Resonance Raman spectra of the linear-chain, mixed-valence, halogen-bridged complexes [Pt(pn),] [Pt(pn),X,] (C104),,where X=Cl or Br, and [Pd(pn),] [Pd(pn),Br,] (ClO,), have been obtained over the range of excitation wavelengths 457.9 to 647.1 nm. Of particular interest is the symmetric metal-halogen stretch, vi, which has several components. The relative intensities of these components change with variation of the wavenumber of excitation within the intervalence electronic absorption. This effect and the origin of the different components are discussed.
INTRODUCTION
Linear-chain, mixed-valence, halogen-bridged complexes (LMHCs) of platinum have been synthesised and examined since the start of the twentieth red salt, the known being Wolffram’s century, best [Pt(etn),] [Pt(etn),Cl,]Cl,~nH,O, where etn=ethylamine. More recently substantial work has been done on palladium analogues and on their platinum/ palladium/nickel mixed-metal counterparts. The linear chains which these complexes form have the Mn/MIV dimer repeat unit shown below, where
.. . . . .‘,.. M%1‘. . . . . . . . . . . . L’
’L
MI1= Pt” Pd” or Ni”, M’V=Pt’V or Pdn’ (the latter occurring only when __ _I Mu= Pd”), X= Cl, Br or I, and L=equatorial ligand [l-4]. It is uncertain *Dedicated to Professor D.J. Millen on the occasion of his retirement.
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whether LMHCs with nickel in the M’” state can be prepared. The ligands L may be neutral or charged, uni-, bi-, ter-, or quadri-dentate. These complexes, which usually crystallise as long needles with the long axis coinciding with the linear chain axis (but occasionally as platelets), are intensely coloured and dichroic; the intense colour arises from the absorption of light with the electric vector parallel to the needle axis, while for light with the electric vector perpendicular to this axis, the crystal is colourless or yellow (like that of the constituent M” or M’” species). Analytical techniques commonly used to examine these complexes include electronic and resonance Raman spectroscopy. The electronic spectrum of an LMHC shows, in addition to features seen for its constituent M” and M’” species, a broad, intense peak in the visible to near infrared region, which is seen only with incoming light with its electric vector parallel to the chain axis. This absorption is responsible for the strong colour and dichroism observed and is understood to be due to excitation of M”/M’” pairs to an M”‘/M”’ state by the transfer of one electron between adjacent metal ions via the intervening halogen. Narrower, weaker peaks, likewise polarised, are observed just below the absorption edge of the intervalence transition (IT) and may be caused by exciton creation [ 51. Resonance Raman (RR) studies using exciting lines of wavenumbers corresponding to those of the IT bands show a very strong enhancement (an intensity increase of several orders of magnitude) of the band assigned to the XMI”-X symmetric stretch, vl, together with the appearance of many overtones U~Y, ( u1 up to 17 has been observed [ 61) and, in some cases, enhancement of combination bands involving ulv,. Significant RR scattering is observed primarily for light polarised parallel to the chain, the scattered light being mostly likewise polarised. These observations indicate a strong coupling between the intervalence electronic transition and the vi symmetric stretching vibration. By consideration of the energy bands of linear chain compounds [ 71, the spectra can be understood in terms of excitation from a mixed-valence metal ground state to a non-mixed-valence metal excited state. In addition to the v1 progressions a weak luminescence is observed underlying the U~Y,bands and a strong, broad luminescence band (several orders of magnitude more intense than the RR scattering and with a full width at half maximum (FWHM) of around 2000 cm-l) is observed at a wavenumber corresponding to roughly half the IT wavenumber [ 8,9]. Tanaka et al. [9] have observed that, for excitation of a single crystal of [Pt(en)z] [Pt(en),Cl,] (ClO,), within the peaks below the IT absorption edge, little RR scattering and no luminescence at half the IT wavenumber occur. They concluded that the maximum of the excitation profile of the RR scattering occurs at the absorption edge. Work on Wolffram’s red salt [8] indicates that, with increasing excitation wavenumber above the IT absorption edge, the intensity of the weak lumi-
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nescence increases at the expense of that of RR scattering, while the strong luminescence intensity is unchanged. It has been known since 1983 [lo] that, in the RR spectrum of some LMHCs, the wavenumber Y, of the symmetric X-MIV-X stretching fundamental appears to vary with excitation wavenumber, Y,, the extent of dispersion increasing with increasing mass of halogen. For vo
Ptbnhl Wbnhl
Pt(pn)&Ll (C104)4, F’d(pnMhl (C10d4,
WbnLl
Pt(pnMbl
(C1O4)4 and
henceforth abbreviated to [ Pt,pn,Cl] , [ Pt,pn,Br] and [ Pd,pn,Br], respectively, where pn = 1,2_diaminopropane, and the analysis of their RR spectra. The results are discussed with reference to the electronic theory of one-dimensional compounds. EXPERIMENTAL
[ Pt,pn,Br ] was made by cocrystallisation of equimolar amounts of the Pt” and PtiV monomeric species, following the method of Bekaroglu et al. [ 151. [Pt,pn,Cl] was made by an analogous method using chlorine gas instead of liquid bromine. [ Pd,pn,Br ] was made by partial oxidation of the Pd” species, following the method of Matsumoto et al. [ 161. The platinum complexes were recrystallised from aqueous solution to give good quality crystals. This was not possible with the palladium complex, crystals of which were not so fine or so large; they nevertheless gave good analyses and RR spectra. The observed and calculated analyses for C, H, N and Cl or Br of these complexes are satisfactory. RR spectra of the complexes as single crystals were obtained with a variety of exciting lines from Coherent Radiation Kr+ (models CR3000K and 52) and Ar+ (CR12 and Innova 70) ion lasers. A Spex 14018 (R6) double monochromator with Jobin-Yvon gratings (1800 lines/mm) was used. For each crystal high resolution spectra were obtained using different excitation lines ranging across the excitation profile (EP) associated with the IT. All spectra were
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obtained with the crystals at a temperature near to - 196” C and v,, incident on the crystals with a power of 5 mW or less, in order to improve the spectral resolution (by reducing hot band interferences) and to reduce the chance of decomposing the crystal either photolytically or thermally. Even under these conditions the crystals were occasionally burned, as was seen from the disappearance of the 1/l signal and the subsequent discolouration of the crystals. RESULTS
RR spectra of [ Pt,pn,Cl] , [ Pt,pn,Br ] and [ Pd,pn,Br ] are shown in Figs. l3, respectively. For each complex, the following features can be observed in the spectra.
330
320 Wavenumber
310 Icm-’
300
;
1
I90
180
Wavenumber
170
160
1 cm-’
Fig. 1. Resonance Raman spectra of [Pt,pn,Cl] showing the u, band at different excitation wavelengths. Fig. 2. Resonance Raman spectra of [Pt,pn,Br] showing the u1 band at different excitation wavelengths.
177
210
190
170
Vwrnunbrr
IS0
Fig. 3. Resonance wavelengths.
0
130
I cm-l
320 Wavenumber
Raman
310 I cm-’
spectra
of [Pd,pn,Br]
showing the V, band at different
20 0
190
180
Wawnumbcr
170
excitation
I60
I cm-’
Fig. 4. Synthesised v1 band profiles (full lines) which match the resonance [Pt,pn,Cl] shown in Fig. 1. Constituent peaks are shown as dashed lines.
Raman spectrum
of
Fig. 5. Synthesised v, band profiles (full lines) which match the resonance [Pt,pn,Br] shown in Fig. 2. Constituent peaks are shown as dashed lines.
Raman spectrum
of
178 TABLE 1 Deconvolution parameters of components of the V, band profiles for [Pt,pn,Cl] Stokes shift (cm-‘)
Height (%)
Gauss./ Lorentzmix (% Lorentz.)
FWHM (cm-‘)
457.9 457.9 457.9 457.9 457.9
308.7 311.5 313.4 315.5 316.8 318.5 319.9
20.5 73.0 42.5 47.0 25.5 17.0 7.0
30.0 30.0 30.0 30.0 30.0 30.0 30.0
3.15 3.15 3.15 3.15 3.15 3.15 3.15
482.5 482.5 482.5 482.5 482.5 482.5 482.5
308.5 311.3 313.3 315.4 316.8 318.5 319.8
21.0 75.5 42.0 48.5 21.5 15.0 5.0
30.0 30.0 30.0 30.0 30.0 30.0 30.0
3.10 3.10 3.10 3.10 3.10 3.10 3.10
530.9
530.9 530.9 530.9 530.9 530.9
308.3 311.4 313.2 315.5 317.4 318.6
22.0 80.0 33.0 51.0 14.0 11.0
30.0 30.0 30.0 30.0 30.0 30.0
2.95 2.95 2.95 2.95 2.95 2.95
568.2 568.2 568.2 568.2 568.2 568.2
308.0 311.1 312.9 315.3 317.1 318.3
22.0 83.0 30.0 51.0 10.6 8.9
30.0 30.0 30.0 30.0 30.0 30.0
2.95 2.95 2.95 2.95 2.95 2.95
647.1 647.1 647.1 647.1 647.1 647.1
308.4 311.4 313.1 315.5 317.4 318.7
21.0 80.0 33.0 53.0 15.0 10.0
30.0 30.0 30.0 30.0 30.0 30.0
2.95 2.95 2.95 2.95 2.95 2.95
10 (nm)
457.9 457.9
179 TABLE 2 Deconvolution parameters of components of the v1 band profiles for [Pt,pn,Br]
(nm)
Stokes shift (cm-‘)
Height (%)
Gauss./ Lorentzmix (% Lorentz.)
FWHM (cm-‘)
476.2 476.2 476.2 476.2 476.2
173.0 175.4 178.1 181.7 184.4
42.5 20.0 78.0 56.5 19.0
33.0 33.0 33.0 33.0 33.0
4.40 4.40 4.40 4.40 4.40
530.9
530.9 530.9 530.9 530.9
173.6 175.7 178.1 181.2 184.2
29.5 61.5 46.5 20.5 7.5
40.0 40.0 40.0 40.0 40.0
4.30 4.30 4.30 4.30 4.30
568.2 568.2 568.2 568.2 568.2
173.4 176.0 178.2 181.6 183.8
87.4 31.9 11.5 4.5 1.0
50.0 50.0 50.0 50.0 50.0
4.00 4.00 4.00 4.00 4.00
647.1 647.1 647.1 647.1 647.1 647.1 647.1
163.8 168.0 173.0 175.8 177.7 181.1 184.3
2.5 5.5 94.5 9.5 9.0 4.0 1.0
33.0 33.0 33.0 33.0 33.0 33.0 33.0
4.30 4.30 4.30 4.30 4.30 4.30 4.30
20
(a) With increasing wavenumber Y, within the IT absorption, the wavenumber of strongest scattering in the Y, profile shifts to a higher value. As a proportion of the vibrational wavenumber, this apparent dispersion is much greater for [ Pt,pn,Br ] than for [ Pt,pn,Cl ] over the same v,, range (530.9 to 476.2 nm) within the IT band of both complexes, although it should be noted that a different portion of the IT band was excited in each case. (b ) The v1 profile in all cases contains more than one component. (c) The shape of the V, profile changes with v. above the absorption edge. It would be of interest to know the origin of the structure in the v1 profiles and hence the spectra were analysed in order to try to determine the number of components and whether or not there is dispersion of the individual components with change of ZQ,.A computer program used to perform the deconvolutions of the vl band profiles had, as the variable parameters of the components,
180
IOOr
Excitation
Wavenumber / IO3 cm-’
Fig. 6. Variation of peak heights of the components of v1 for [Pt,pn,Br]. V , 0 , 0 , •i and A refer to components with average Stokes shifts of 184.2, 181.4, 178,0,175.7, and 173.3 cm-’ , respectively. % = percentage of maximum intensity.
the wavenumber, intensity, FWHM, and ratio of Lorentzian to Gaussian contribution to the bandshape. It was assumed that the bands are symmetric and that they are adequately fitted within a Gaussian/Lorentzian bandshape. Synthesised profiles intended to match the observed v1 profiles are shown in Figs. 4 and 5 for [ Pt,pn,Cl] and [ Pt,pn,Br 3, respectively. Constraints were put upon the parameters in the peak synthesis as follows. On the assumption that the components all arise from the same sort of vibration in chains identical or differing only in correlation length (the length over which a given chain is non-defective), the FWHM was constrained to have the same value for all components of any given profile. A similar constraint was put upon the Gaussian/Lorentzian ratio. It can be seen that the number of components necessary to synthesise accurately the V, profile varies from 5 to 7. The Stokes shifts of the components were determined by calibration with neon emission lines, the shifts being given in Tables 1 and 2, together with the relative intensity, FWHM, and Gaussian/Lorentzian bandshape ratio for each component. The Stokes shifts of V, components were determined to be invariant with excitation wavenumber, Y,. Figure 6 refers to [Pt,pn,Br] and shows how the height of each component in a profile varies with excitation energy. It demonstrates how the components at higher Stokes shift gain intensity relative to those at lower shift with increasing excitation wavenumber.
181 DISCUSSION
Since the IT is z-polarised, the most likely transition to be occurring is the transfer of one electron between adjacent dZ2orbitals via the halogenp, orbital, as indicated below. X...M”
. . . X_M’V_X
.
..
Iv-X.
. . M” . . . X_M’V_
However, it is useful to consider the cEzpelectrons to occupy bands of orbitals which span the whole chain of metal sites. A qualitative derivation [17,18] shows that, in the Peierls-distorted linear chain of metal sites with alternating oxidation states of 3+6 and 3-6, where O< 6< 1, two dL2bands exist. The lower (valence ) band is occupied by the c& electrons available from the metal atoms and has primarily M m--s( dz2) character, while the upper (conduction) band is empty and has primarily M”‘+’ (&) character. In this band orbital scheme the onset of electrical conduction occurs with excitation of electrons from the valence to the conduction band and this accounts for the observed semiconductivity in the chain direction. Optical excitation within the IT band of the electronic spectrum is believed to involve similar transitions. When electrons in the valence band are excited into the conduction band, the Peierls-distorted lattice may have no energetic advantage over the more symmetric lattice, in those parts of the one-dimensional chain with which the excitation is principally associated. This will depend on whether the electrons remain spin paired on a given metal site and on how the intermetal site electron density changes. It is possible, therefore, that the excited state of the linear chain would, given enough time, take up the more symmetric lattice geometry. The equilibrium position of X is well displaced by this excitation. It is therefore likely that, from a consideration of Franck-Condon factors, excitation will be to a high vibrational level of the excited state. Relaxation from this state to the ground electronic state could produce the long progression of vibrational overtones observed. With excitation within the IT, i.e. above the absorption edge, free electronhole pairs are created. The electrons and holes are not correlated spatially and permeate the chain independently. Although in this excited state the electrons and holes are never fully localised on specific (different) metal sites, it is useful to associate an electron-hole pair primarily with two M”’ sites in the chain at any one time. It is also reasonable, from the nature of the electron transfer occurring, to associate an electron-hole pair creation with the creation of two M”’ sites adjacent in the chain. Thus the creation of one electron-hole pair may be visualised as follows. Firstly, the M”’ pair creation MI1 M’v M” M’v M+Mrv MI’ M’v M” M’v followed by a “dissociation” of the M”’ pair, e.g.
182
M”
M’v
M”
M~MIII
McM”
M’v
M”
M’v
to give M” M’v M” Mm M’v M” Mm M’v M” M’v followed by movement of the independent Mm species throughout the chain. The dissociation stage occurs only for excitation beyond the absorption edge. It does not occur with excitation within any peak below the absorption edge. These peaks may correspond to excited states in which each electron-hole pair forms a spatially correlated pair (a Mott-Wannier exciton) . An electron and hole of this type are not energetic enough to be fully independent and therefore must move around together. In the band orbital scheme, the excitonic levels would occur just below the conduction band. In the above depiction, the Mm pair movement would differ as follows. Firstly the Mm creation: M” M’v M” M’v MpM’v Dissociation
M” M’v M” M’v
does not occur; the Mm must move around together,
M” M’v M” M’v MpM=Mn
e.g.
M’v Mn M’v
etc. The bound Mm pair may possibly be separated by several metal sites, depending on the energy of the exciton. It shouldbe emphasised that, in the above depictions, a majority of M” and MIv states is shown only to highlight the electron transfer occurring. In practice, many of the Mn/MIV pairs will be similarly excited in each chain. Onset of the RR effect in LMHCs occurs with excitation at or around the IT absorption edge. The scattering process is rapid, the lifetime of the excited state being of the order of lo-l3 s. However RR scattering is in competition with luminescence processes. After a free electron-hole pair has been created by irradiation at z+,the pair will have one of two possible fates. It may (rapidly) recombine and undergo a single transition to a vibrational level of the ground state. This would give rise to RR scattering. Alternatively, whilst in close proximity in the excited state, the electron and hole could, prior to recombination, interact with the Y, phonon mode in such a way as to be partially deactivated. It has been proposed [8] that this could occur via a phonon “cascade” down the k vector versus energy curve, all with the average position of the halogen unchanged from the ground state equilibrium position. This would then be followed by the favourable displacement of the halogen to a position central between the Mm sites. This Mm pair is now trapped, i.e. the electron and hole are bound, both to each other and primarily to one site in the chain. They will ultimately fluoresce to the ground state. Both previous work on analogous 1,2_diaminoethane (en) complexes (14,191
183
and the current work have demonstrated that the apparent shift in Y, with v,, within the IT absorption results from a change in the relative intensities of closely spaced constituent peaks, whose Stokes shifts are constant to within + 1 cm-‘. With increasing v, there is a progressive transfer of intensity towards components at greater Stokes shift. It is also observed that, when v, approaches the maximum in the RR excitation profile for a complex, one component in the v1 profile dominates over others. Various explanations have been proposed to explain the structure in I+. Tanaka and Kurita [ 121 have suggested that, by analogy with results for truns-polyacetylene, LMHC chains of different length will have both different IT absorption bands and different Y, wavenumbers. Others have reported that, for [ Pt,en,Cl] [ 201 and [ Pt,pn,Br ] [ 211, the peak of the IT absorption band shifts to higher energy with shorter chains. By varying v,, within the IT absorption band, different chain lengths are excited preferentially and scattering into the corresponding Y, bands is observed. This, however, implies that the v1 profile reflects the distribution of chain lengths. There are two ways in which this could be the case. The first possibility is that the LMHC chains could be very short, in which case adjacent components in the v1 profile correspond to chains which differ in length by only one repeat unit (given no unusual preference for particular chain lengths). This case seems unlikely, considering the highly anisotropic nature of the crystals, which is dependent on the integrity of the chains. The second possibility is that the distribution of chain lengths contains more than one maximum, as reflected in the v1 profile. The fact that z+ structures are approximately reproducible (individual components to within + 5% of maximum intensity) with different samples would then imply that the chain length distribution is nearly independent of sample. The probability of growth of LMHC chains to a small number of preferred chain lengths has not yet been investigated. Variation of z+ with the length of chain will depend upon the extent to which one unit in the chain interacts with the others. However great this is, there must also be a halogen isotope effect occurring, since it is the halogens which move in the vibration. Statistics suggest that, for chlorine-bridged LMHCs beyond chain lengths of about twenty units, chains with approximately equal numbers of 35C1and 37C1will strongly outweigh other distributions of halogen isotopes. The electrons in the dz2 valence band are unequally distributed between the M “i+’ and M”1-S sites in the chain. The electrons are still spin paired throughout the chain, and so there is no net spin associated with any metal site. The c&2electrons therefore form a charge density wave (CDW) . A similar phenomenon occurs in trans-polyacetylene, for which dispersion of phonon modes with v. is also observed in the RR spectra; this is attributed to photoselective enhancement of different lengths of conjugated C=C bonds. However, the dispersion differs from that reported here as follows. With truns-polyacetylene a RR-active phonon mode (e.g. the C=C stretch at approximately 1500 cm-‘)
184
shows a main peak and a satellite at higher wavenumber. It is the satellite which appears to disperse, its wavenumber increasing with increase in vo. Horovitz et al. [ 22 ] suggest that the CDW undergoes an amplitude oscillation because of interaction with phonons. The phonon wavenumbers are changed by this interaction, the extent of the change depending on the length of the conjugated chain. For long, undisturbed chains it is minimal, but for shorter segments of chain bounded by conjugation defects, the interaction is greater. Thus when v. excites ~+rr* transitions in the shorter chains, much of the scattering is observed at the satellite wavenumber. Since the tram-polyacetylene system is similar in many respects to LMHC systems, it may be feasible to explain the apparent dispersion in the RR spectra of an LMHC in terms of CDW interaction (it being noted that this is still founded on the basis of photoselective enhancement of Raman bands of chains of different lengths). It remains, however, to explain the significant difference between the truns-polyacetylene and the LMHC spectra, which is that the LMHC spectra do not show a main v, peak which remains unaffected by changes in vo. This may be related to a difference in chain length distribution. CONCLUSION
The symmetric metal-halogen stretch, v,, in the resonance Raman spectra of these linear chain complexes has a profile showing several components. They vary in relative intensity with the wavenumber of excitation for excitation within the intervalence transition band of the electronic spectrum. The origin of the different components and the reason for the changes in their relative intensities with excitation are not fully understood, but contributions from different chain lengths and different halogen isotopes would appear to provide a partial basis. ACKNOWLEDGMENT
The authors thank Dr. V.B. Croud for useful discussions.
REFERENCES 1 2 3 4 5 6
R.J.H. Clark, V.B. Croud and M. Kurmoo, Inorg. Chem., 23 (1984) 2499. R.J.H. Clark and V.B. Croud, Inorg. Chem., 24 (1985) 588. R.J.H. Clark, M. Kurmoo, D.N. Mountney and H. Toftlund, J. Chem. Sot., Dalton Trans., (1982) 1851. R.J.H. Clark, V.B. Croud and M. Kurmoo, J. Chem. Sot., Dalton Trans., (1985) 815. M. Tanaka, S. Kurita, T. Kojima and Y. Yamada, Chem. Phys., 91 (1984) 257. R.J.H. Clark, M. Kurmoo, H.J. Keller, B. Keppler and U. Traeger, J. Chem. Sot., Dalton Trans., (1980) 2498.
185 7 8 9 10 11 12 13 14 15 16 17 18 19
20 21 22
M.-H. Whangbo, Act. Chem. Res., 16 (1983) 95. H. Tanino and K. Kobayashi, J. Phys. Sot. Jpn., 52 (1983) 1446. M. Tanaka, S. Kurita, Y. Okada, T. Kojima and Y. Yamada, Chem. Phys., 96 (1985) 343. R.J.H. Clark and M. Kurmoo, J. Chem. Sot., Faraday Trans. 2,79 (1983) 519. R.J.H. Clark and V.B. Croud, J. Phys. C: Solid State Phys., 19 (1986) 3467. M. Tanaka and S. Kurita, J. Phys. C: Solid State Phys., 19 (1986) 3019. S.D. Allen, R.J.H. Clark, V.B. Croud and M. Kurmoo, Phil. Trans. R. Sot. London, Ser. A, 314 (1985) 131. SD. Conradson, R.F. Dallinger, B.I. Swanson, R.J.H. Clark and V.B. Croud, Chem. Phys. Lett., 135 (1987) 463. 0. Bekaroglu, H. Breer, H. Endres, H.J. Keller and H. Nam Gung, Inorg. Chim. Acta, 21 (1977) 183. N. Matsumoto, M. Yamashita and S. Kida, Bull. Chem. Sot. Jpn., 51 (1978) 2334. M.-H. Whangbo and M.J. Foshee, Inorg. Chem., 20 (1981) 113. J. Rouxel (Ed.), Crystal Chemistry and Properties of Materials with Quasi-One-Dimensional Structures, Reidel, Dordrecht, 1986, pp. 27-85. R.J.H. Clark and V.B. Croud, in P. Delhaes and M. Drillon (Eds.), Organic and Inorganic Low Dimensional Crystalline Materials, NATO Advanced Research Workshop, Plenum, New York, 1988, p. 341. G.C. Papavassiliou, R. Rapsomanikis, S. Mourikis and C.S. Jacobsen, J. Chem. Sot., Faraday Trans. 2,78 (1982) 17. R.J.H. Clark and M. Kurmoo, J. Chem. Sot., Dalton Trans., (1983) 761. B. Horovitz, Z. Vardeny, E. Ehrenfreund and 0. Brafmann, J. Phys. C: Solid State Phys., I9 (1986) 7291.
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RESONANCE RAMAN SPECTRA OF METAL II/IV DIMER CHAIN COMPLEXES OF PLATINUM AND PALLADIUM Analysis of the component structure to the band assigned to the symmetric XMX chain stretching mode (X=Cl or Br)*
ROBIN J.H. CLARK and DAVID J. MICHAEL Christopher Zngold Laboratories, University College London, 20 Gordon Street, London WClH OAJ (Gt. Britain) (Received 14 January 1988; in final form 15 February 1988)
ABSTRACT Resonance Raman spectra of the linear-chain, mixed-valence, halogen-bridged complexes [Pt(pn),] [Pt(pn),X,] (C104),,where X=Cl or Br, and [Pd(pn),] [Pd(pn),Br,] (ClO,), have been obtained over the range of excitation wavelengths 457.9 to 647.1 nm. Of particular interest is the symmetric metal-halogen stretch, vi, which has several components. The relative intensities of these components change with variation of the wavenumber of excitation within the intervalence electronic absorption. This effect and the origin of the different components are discussed.
INTRODUCTION
Linear-chain, mixed-valence, halogen-bridged complexes (LMHCs) of platinum have been synthesised and examined since the start of the twentieth red salt, the known being Wolffram’s century, best [Pt(etn),] [Pt(etn),Cl,]Cl,~nH,O, where etn=ethylamine. More recently substantial work has been done on palladium analogues and on their platinum/ palladium/nickel mixed-metal counterparts. The linear chains which these complexes form have the Mn/MIV dimer repeat unit shown below, where
.. . . . .‘,.. M%1‘. . . . . . . . . . . . L’
’L
MI1= Pt” Pd” or Ni”, M’V=Pt’V or Pdn’ (the latter occurring only when __ _I Mu= Pd”), X= Cl, Br or I, and L=equatorial ligand [l-4]. It is uncertain *Dedicated to Professor D.J. Millen on the occasion of his retirement.
0022-2860/88/$03.50
0 1988 Elsevier Science Publishers B.V.
174
whether LMHCs with nickel in the M’” state can be prepared. The ligands L may be neutral or charged, uni-, bi-, ter-, or quadri-dentate. These complexes, which usually crystallise as long needles with the long axis coinciding with the linear chain axis (but occasionally as platelets), are intensely coloured and dichroic; the intense colour arises from the absorption of light with the electric vector parallel to the needle axis, while for light with the electric vector perpendicular to this axis, the crystal is colourless or yellow (like that of the constituent M” or M’” species). Analytical techniques commonly used to examine these complexes include electronic and resonance Raman spectroscopy. The electronic spectrum of an LMHC shows, in addition to features seen for its constituent M” and M’” species, a broad, intense peak in the visible to near infrared region, which is seen only with incoming light with its electric vector parallel to the chain axis. This absorption is responsible for the strong colour and dichroism observed and is understood to be due to excitation of M”/M’” pairs to an M”‘/M”’ state by the transfer of one electron between adjacent metal ions via the intervening halogen. Narrower, weaker peaks, likewise polarised, are observed just below the absorption edge of the intervalence transition (IT) and may be caused by exciton creation [ 51. Resonance Raman (RR) studies using exciting lines of wavenumbers corresponding to those of the IT bands show a very strong enhancement (an intensity increase of several orders of magnitude) of the band assigned to the XMI”-X symmetric stretch, vl, together with the appearance of many overtones U~Y, ( u1 up to 17 has been observed [ 61) and, in some cases, enhancement of combination bands involving ulv,. Significant RR scattering is observed primarily for light polarised parallel to the chain, the scattered light being mostly likewise polarised. These observations indicate a strong coupling between the intervalence electronic transition and the vi symmetric stretching vibration. By consideration of the energy bands of linear chain compounds [ 71, the spectra can be understood in terms of excitation from a mixed-valence metal ground state to a non-mixed-valence metal excited state. In addition to the v1 progressions a weak luminescence is observed underlying the U~Y,bands and a strong, broad luminescence band (several orders of magnitude more intense than the RR scattering and with a full width at half maximum (FWHM) of around 2000 cm-l) is observed at a wavenumber corresponding to roughly half the IT wavenumber [ 8,9]. Tanaka et al. [9] have observed that, for excitation of a single crystal of [Pt(en)z] [Pt(en),Cl,] (ClO,), within the peaks below the IT absorption edge, little RR scattering and no luminescence at half the IT wavenumber occur. They concluded that the maximum of the excitation profile of the RR scattering occurs at the absorption edge. Work on Wolffram’s red salt [8] indicates that, with increasing excitation wavenumber above the IT absorption edge, the intensity of the weak lumi-
175
nescence increases at the expense of that of RR scattering, while the strong luminescence intensity is unchanged. It has been known since 1983 [lo] that, in the RR spectrum of some LMHCs, the wavenumber Y, of the symmetric X-MIV-X stretching fundamental appears to vary with excitation wavenumber, Y,, the extent of dispersion increasing with increasing mass of halogen. For vo
Ptbnhl Wbnhl
Pt(pn)&Ll (C104)4, F’d(pnMhl (C10d4,
WbnLl
Pt(pnMbl
(C1O4)4 and
henceforth abbreviated to [ Pt,pn,Cl] , [ Pt,pn,Br] and [ Pd,pn,Br], respectively, where pn = 1,2_diaminopropane, and the analysis of their RR spectra. The results are discussed with reference to the electronic theory of one-dimensional compounds. EXPERIMENTAL
[ Pt,pn,Br ] was made by cocrystallisation of equimolar amounts of the Pt” and PtiV monomeric species, following the method of Bekaroglu et al. [ 151. [Pt,pn,Cl] was made by an analogous method using chlorine gas instead of liquid bromine. [ Pd,pn,Br ] was made by partial oxidation of the Pd” species, following the method of Matsumoto et al. [ 161. The platinum complexes were recrystallised from aqueous solution to give good quality crystals. This was not possible with the palladium complex, crystals of which were not so fine or so large; they nevertheless gave good analyses and RR spectra. The observed and calculated analyses for C, H, N and Cl or Br of these complexes are satisfactory. RR spectra of the complexes as single crystals were obtained with a variety of exciting lines from Coherent Radiation Kr+ (models CR3000K and 52) and Ar+ (CR12 and Innova 70) ion lasers. A Spex 14018 (R6) double monochromator with Jobin-Yvon gratings (1800 lines/mm) was used. For each crystal high resolution spectra were obtained using different excitation lines ranging across the excitation profile (EP) associated with the IT. All spectra were
176
obtained with the crystals at a temperature near to - 196” C and v,, incident on the crystals with a power of 5 mW or less, in order to improve the spectral resolution (by reducing hot band interferences) and to reduce the chance of decomposing the crystal either photolytically or thermally. Even under these conditions the crystals were occasionally burned, as was seen from the disappearance of the 1/l signal and the subsequent discolouration of the crystals. RESULTS
RR spectra of [ Pt,pn,Cl] , [ Pt,pn,Br ] and [ Pd,pn,Br ] are shown in Figs. l3, respectively. For each complex, the following features can be observed in the spectra.
330
320 Wavenumber
310 Icm-’
300
;
1
I90
180
Wavenumber
170
160
1 cm-’
Fig. 1. Resonance Raman spectra of [Pt,pn,Cl] showing the u, band at different excitation wavelengths. Fig. 2. Resonance Raman spectra of [Pt,pn,Br] showing the u1 band at different excitation wavelengths.
177
210
190
170
Vwrnunbrr
IS0
Fig. 3. Resonance wavelengths.
0
130
I cm-l
320 Wavenumber
Raman
310 I cm-’
spectra
of [Pd,pn,Br]
showing the V, band at different
20 0
190
180
Wawnumbcr
170
excitation
I60
I cm-’
Fig. 4. Synthesised v1 band profiles (full lines) which match the resonance [Pt,pn,Cl] shown in Fig. 1. Constituent peaks are shown as dashed lines.
Raman spectrum
of
Fig. 5. Synthesised v, band profiles (full lines) which match the resonance [Pt,pn,Br] shown in Fig. 2. Constituent peaks are shown as dashed lines.
Raman spectrum
of
178 TABLE 1 Deconvolution parameters of components of the V, band profiles for [Pt,pn,Cl] Stokes shift (cm-‘)
Height (%)
Gauss./ Lorentzmix (% Lorentz.)
FWHM (cm-‘)
457.9 457.9 457.9 457.9 457.9
308.7 311.5 313.4 315.5 316.8 318.5 319.9
20.5 73.0 42.5 47.0 25.5 17.0 7.0
30.0 30.0 30.0 30.0 30.0 30.0 30.0
3.15 3.15 3.15 3.15 3.15 3.15 3.15
482.5 482.5 482.5 482.5 482.5 482.5 482.5
308.5 311.3 313.3 315.4 316.8 318.5 319.8
21.0 75.5 42.0 48.5 21.5 15.0 5.0
30.0 30.0 30.0 30.0 30.0 30.0 30.0
3.10 3.10 3.10 3.10 3.10 3.10 3.10
530.9
530.9 530.9 530.9 530.9 530.9
308.3 311.4 313.2 315.5 317.4 318.6
22.0 80.0 33.0 51.0 14.0 11.0
30.0 30.0 30.0 30.0 30.0 30.0
2.95 2.95 2.95 2.95 2.95 2.95
568.2 568.2 568.2 568.2 568.2 568.2
308.0 311.1 312.9 315.3 317.1 318.3
22.0 83.0 30.0 51.0 10.6 8.9
30.0 30.0 30.0 30.0 30.0 30.0
2.95 2.95 2.95 2.95 2.95 2.95
647.1 647.1 647.1 647.1 647.1 647.1
308.4 311.4 313.1 315.5 317.4 318.7
21.0 80.0 33.0 53.0 15.0 10.0
30.0 30.0 30.0 30.0 30.0 30.0
2.95 2.95 2.95 2.95 2.95 2.95
10 (nm)
457.9 457.9
179 TABLE 2 Deconvolution parameters of components of the v1 band profiles for [Pt,pn,Br]
(nm)
Stokes shift (cm-‘)
Height (%)
Gauss./ Lorentzmix (% Lorentz.)
FWHM (cm-‘)
476.2 476.2 476.2 476.2 476.2
173.0 175.4 178.1 181.7 184.4
42.5 20.0 78.0 56.5 19.0
33.0 33.0 33.0 33.0 33.0
4.40 4.40 4.40 4.40 4.40
530.9
530.9 530.9 530.9 530.9
173.6 175.7 178.1 181.2 184.2
29.5 61.5 46.5 20.5 7.5
40.0 40.0 40.0 40.0 40.0
4.30 4.30 4.30 4.30 4.30
568.2 568.2 568.2 568.2 568.2
173.4 176.0 178.2 181.6 183.8
87.4 31.9 11.5 4.5 1.0
50.0 50.0 50.0 50.0 50.0
4.00 4.00 4.00 4.00 4.00
647.1 647.1 647.1 647.1 647.1 647.1 647.1
163.8 168.0 173.0 175.8 177.7 181.1 184.3
2.5 5.5 94.5 9.5 9.0 4.0 1.0
33.0 33.0 33.0 33.0 33.0 33.0 33.0
4.30 4.30 4.30 4.30 4.30 4.30 4.30
20
(a) With increasing wavenumber Y, within the IT absorption, the wavenumber of strongest scattering in the Y, profile shifts to a higher value. As a proportion of the vibrational wavenumber, this apparent dispersion is much greater for [ Pt,pn,Br ] than for [ Pt,pn,Cl ] over the same v,, range (530.9 to 476.2 nm) within the IT band of both complexes, although it should be noted that a different portion of the IT band was excited in each case. (b ) The v1 profile in all cases contains more than one component. (c) The shape of the V, profile changes with v. above the absorption edge. It would be of interest to know the origin of the structure in the v1 profiles and hence the spectra were analysed in order to try to determine the number of components and whether or not there is dispersion of the individual components with change of ZQ,.A computer program used to perform the deconvolutions of the vl band profiles had, as the variable parameters of the components,
180
IOOr
Excitation
Wavenumber / IO3 cm-’
Fig. 6. Variation of peak heights of the components of v1 for [Pt,pn,Br]. V , 0 , 0 , •i and A refer to components with average Stokes shifts of 184.2, 181.4, 178,0,175.7, and 173.3 cm-’ , respectively. % = percentage of maximum intensity.
the wavenumber, intensity, FWHM, and ratio of Lorentzian to Gaussian contribution to the bandshape. It was assumed that the bands are symmetric and that they are adequately fitted within a Gaussian/Lorentzian bandshape. Synthesised profiles intended to match the observed v1 profiles are shown in Figs. 4 and 5 for [ Pt,pn,Cl] and [ Pt,pn,Br 3, respectively. Constraints were put upon the parameters in the peak synthesis as follows. On the assumption that the components all arise from the same sort of vibration in chains identical or differing only in correlation length (the length over which a given chain is non-defective), the FWHM was constrained to have the same value for all components of any given profile. A similar constraint was put upon the Gaussian/Lorentzian ratio. It can be seen that the number of components necessary to synthesise accurately the V, profile varies from 5 to 7. The Stokes shifts of the components were determined by calibration with neon emission lines, the shifts being given in Tables 1 and 2, together with the relative intensity, FWHM, and Gaussian/Lorentzian bandshape ratio for each component. The Stokes shifts of V, components were determined to be invariant with excitation wavenumber, Y,. Figure 6 refers to [Pt,pn,Br] and shows how the height of each component in a profile varies with excitation energy. It demonstrates how the components at higher Stokes shift gain intensity relative to those at lower shift with increasing excitation wavenumber.
181 DISCUSSION
Since the IT is z-polarised, the most likely transition to be occurring is the transfer of one electron between adjacent dZ2orbitals via the halogenp, orbital, as indicated below. X...M”
. . . X_M’V_X
.
..
Iv-X.
. . M” . . . X_M’V_
However, it is useful to consider the cEzpelectrons to occupy bands of orbitals which span the whole chain of metal sites. A qualitative derivation [17,18] shows that, in the Peierls-distorted linear chain of metal sites with alternating oxidation states of 3+6 and 3-6, where O< 6< 1, two dL2bands exist. The lower (valence ) band is occupied by the c& electrons available from the metal atoms and has primarily M m--s( dz2) character, while the upper (conduction) band is empty and has primarily M”‘+’ (&) character. In this band orbital scheme the onset of electrical conduction occurs with excitation of electrons from the valence to the conduction band and this accounts for the observed semiconductivity in the chain direction. Optical excitation within the IT band of the electronic spectrum is believed to involve similar transitions. When electrons in the valence band are excited into the conduction band, the Peierls-distorted lattice may have no energetic advantage over the more symmetric lattice, in those parts of the one-dimensional chain with which the excitation is principally associated. This will depend on whether the electrons remain spin paired on a given metal site and on how the intermetal site electron density changes. It is possible, therefore, that the excited state of the linear chain would, given enough time, take up the more symmetric lattice geometry. The equilibrium position of X is well displaced by this excitation. It is therefore likely that, from a consideration of Franck-Condon factors, excitation will be to a high vibrational level of the excited state. Relaxation from this state to the ground electronic state could produce the long progression of vibrational overtones observed. With excitation within the IT, i.e. above the absorption edge, free electronhole pairs are created. The electrons and holes are not correlated spatially and permeate the chain independently. Although in this excited state the electrons and holes are never fully localised on specific (different) metal sites, it is useful to associate an electron-hole pair primarily with two M”’ sites in the chain at any one time. It is also reasonable, from the nature of the electron transfer occurring, to associate an electron-hole pair creation with the creation of two M”’ sites adjacent in the chain. Thus the creation of one electron-hole pair may be visualised as follows. Firstly, the M”’ pair creation MI1 M’v M” M’v M+Mrv MI’ M’v M” M’v followed by a “dissociation” of the M”’ pair, e.g.
182
M”
M’v
M”
M~MIII
McM”
M’v
M”
M’v
to give M” M’v M” Mm M’v M” Mm M’v M” M’v followed by movement of the independent Mm species throughout the chain. The dissociation stage occurs only for excitation beyond the absorption edge. It does not occur with excitation within any peak below the absorption edge. These peaks may correspond to excited states in which each electron-hole pair forms a spatially correlated pair (a Mott-Wannier exciton) . An electron and hole of this type are not energetic enough to be fully independent and therefore must move around together. In the band orbital scheme, the excitonic levels would occur just below the conduction band. In the above depiction, the Mm pair movement would differ as follows. Firstly the Mm creation: M” M’v M” M’v MpM’v Dissociation
M” M’v M” M’v
does not occur; the Mm must move around together,
M” M’v M” M’v MpM=Mn
e.g.
M’v Mn M’v
etc. The bound Mm pair may possibly be separated by several metal sites, depending on the energy of the exciton. It shouldbe emphasised that, in the above depictions, a majority of M” and MIv states is shown only to highlight the electron transfer occurring. In practice, many of the Mn/MIV pairs will be similarly excited in each chain. Onset of the RR effect in LMHCs occurs with excitation at or around the IT absorption edge. The scattering process is rapid, the lifetime of the excited state being of the order of lo-l3 s. However RR scattering is in competition with luminescence processes. After a free electron-hole pair has been created by irradiation at z+,the pair will have one of two possible fates. It may (rapidly) recombine and undergo a single transition to a vibrational level of the ground state. This would give rise to RR scattering. Alternatively, whilst in close proximity in the excited state, the electron and hole could, prior to recombination, interact with the Y, phonon mode in such a way as to be partially deactivated. It has been proposed [8] that this could occur via a phonon “cascade” down the k vector versus energy curve, all with the average position of the halogen unchanged from the ground state equilibrium position. This would then be followed by the favourable displacement of the halogen to a position central between the Mm sites. This Mm pair is now trapped, i.e. the electron and hole are bound, both to each other and primarily to one site in the chain. They will ultimately fluoresce to the ground state. Both previous work on analogous 1,2_diaminoethane (en) complexes (14,191
183
and the current work have demonstrated that the apparent shift in Y, with v,, within the IT absorption results from a change in the relative intensities of closely spaced constituent peaks, whose Stokes shifts are constant to within + 1 cm-‘. With increasing v, there is a progressive transfer of intensity towards components at greater Stokes shift. It is also observed that, when v, approaches the maximum in the RR excitation profile for a complex, one component in the v1 profile dominates over others. Various explanations have been proposed to explain the structure in I+. Tanaka and Kurita [ 121 have suggested that, by analogy with results for truns-polyacetylene, LMHC chains of different length will have both different IT absorption bands and different Y, wavenumbers. Others have reported that, for [ Pt,en,Cl] [ 201 and [ Pt,pn,Br ] [ 211, the peak of the IT absorption band shifts to higher energy with shorter chains. By varying v,, within the IT absorption band, different chain lengths are excited preferentially and scattering into the corresponding Y, bands is observed. This, however, implies that the v1 profile reflects the distribution of chain lengths. There are two ways in which this could be the case. The first possibility is that the LMHC chains could be very short, in which case adjacent components in the v1 profile correspond to chains which differ in length by only one repeat unit (given no unusual preference for particular chain lengths). This case seems unlikely, considering the highly anisotropic nature of the crystals, which is dependent on the integrity of the chains. The second possibility is that the distribution of chain lengths contains more than one maximum, as reflected in the v1 profile. The fact that z+ structures are approximately reproducible (individual components to within + 5% of maximum intensity) with different samples would then imply that the chain length distribution is nearly independent of sample. The probability of growth of LMHC chains to a small number of preferred chain lengths has not yet been investigated. Variation of z+ with the length of chain will depend upon the extent to which one unit in the chain interacts with the others. However great this is, there must also be a halogen isotope effect occurring, since it is the halogens which move in the vibration. Statistics suggest that, for chlorine-bridged LMHCs beyond chain lengths of about twenty units, chains with approximately equal numbers of 35C1and 37C1will strongly outweigh other distributions of halogen isotopes. The electrons in the dz2 valence band are unequally distributed between the M “i+’ and M”1-S sites in the chain. The electrons are still spin paired throughout the chain, and so there is no net spin associated with any metal site. The c&2electrons therefore form a charge density wave (CDW) . A similar phenomenon occurs in trans-polyacetylene, for which dispersion of phonon modes with v. is also observed in the RR spectra; this is attributed to photoselective enhancement of different lengths of conjugated C=C bonds. However, the dispersion differs from that reported here as follows. With truns-polyacetylene a RR-active phonon mode (e.g. the C=C stretch at approximately 1500 cm-‘)
184
shows a main peak and a satellite at higher wavenumber. It is the satellite which appears to disperse, its wavenumber increasing with increase in vo. Horovitz et al. [ 22 ] suggest that the CDW undergoes an amplitude oscillation because of interaction with phonons. The phonon wavenumbers are changed by this interaction, the extent of the change depending on the length of the conjugated chain. For long, undisturbed chains it is minimal, but for shorter segments of chain bounded by conjugation defects, the interaction is greater. Thus when v. excites ~+rr* transitions in the shorter chains, much of the scattering is observed at the satellite wavenumber. Since the tram-polyacetylene system is similar in many respects to LMHC systems, it may be feasible to explain the apparent dispersion in the RR spectra of an LMHC in terms of CDW interaction (it being noted that this is still founded on the basis of photoselective enhancement of Raman bands of chains of different lengths). It remains, however, to explain the significant difference between the truns-polyacetylene and the LMHC spectra, which is that the LMHC spectra do not show a main v, peak which remains unaffected by changes in vo. This may be related to a difference in chain length distribution. CONCLUSION
The symmetric metal-halogen stretch, v,, in the resonance Raman spectra of these linear chain complexes has a profile showing several components. They vary in relative intensity with the wavenumber of excitation for excitation within the intervalence transition band of the electronic spectrum. The origin of the different components and the reason for the changes in their relative intensities with excitation are not fully understood, but contributions from different chain lengths and different halogen isotopes would appear to provide a partial basis. ACKNOWLEDGMENT
The authors thank Dr. V.B. Croud for useful discussions.
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