Phonon bands in a π-electron charge-transfer complex

Solid State Communications, Vol. 21, pp. 871—873, 1977.

Pergamon Press.

PHONON BANDS IN A u-ELECTRON CHARGE-TRANSFER COMPLEX

Printed in Great Britain

*

Freeman P. Chen and Paras N. Prasad Department of Chemistry, State University of New York at Buffalo Buffalo, New York 14214 (Received 8 October 1976; in revised form 23 December by R. H. Silsbee) Phonon bands in mixed-stack ui-electron charge-transfer complexes. are investigated using Raman spectroscopy. Specifically, the Raman phonon spectra of naphthalene: antimony trichioride system is discussed in detail. The effect of the substitution of the donor and the acceptor is used to examine relative amplitudes of various motions on the donor and the acceptor. Phonon delocalization is investigated using various mixed crystals containing donor and acceptor impurities. The result obtained is discussed in terms of two models: (i) the giant molecule model, and (ii) the sublattice model. Conducting properties of organic chargetransfer complexes have received2. a great dealtheof attention during recentcomplexes years’’ haveHowever, phonon bands of these not drawn much attention even when they play an important role in determining their conducting properties. In a recent paper3 we proposed that phonon motions of a charge transfer complex can be described by two models which represent the two limiting cases: (i) the giant molecule model, and (ii) the sublattice model. In a giant molecule model the phonon bands correspond to

benzene (TNB) complex show isotope shift both with respect to the donor acceptor isotope substitution. Thisandwastheshown to fit a giant molecule model. Here we report the phonon spectrum in another mixed—stack charge-transfer complex, naphthalene:2SbCl,, which fits the sublattice model. The phonons of the naphthalene donor sublattice and the antimony trichloride acceptor sublattice are found to be decoupled. This was established by isotopic substitution (naphthalened 8) of the donor and chemical substitution (SbBr3) of the acceptor. The entire series of the donor and acceptor mixed crystals was studied by Raman spectroscopy. Single crystals were grown either from the melt or from a methylene chloride solution. The spectra were obtained at l30°Kusing Spex double monochromator (model 14018) and the 5145A°line of a Coherent Radiation argon ion laser. The Raman spectra of naphthalene—h8: 2SbC13 are shown in Figure 1. The results of the mixed crystal series are shown in Figure 2, where the frequencies of the phonons are plotted as a function of the composition of the charge-transfer complex. The solid curve was obtained by the substitution of the donor in which the X-coordinate corresponds to the mole fraction of the naphthalene-d8. It shows that only two phonon bands (109 cm 1 and 127 cm ‘) which will also be referred as high frequency phonons, shift with respect to donor 5 (also amalgamation6’7) substitution. These phononcalled motions show the one and delocalized with a lower limit for mode hence type are behavior the bandwidth8 ~k-dispersion) as 6 cni’. The bands at 140 cm ‘, 146 cm ‘ and 166 cm 1 are the internal motions of SbCl 3. These were identified from the solution spectra of SbCl, and from the melt of the complex. All the low frequency phonons show practically no shift with the donor substitution. The broken curve is obtained by the substitution of the acceptor, and for this, the X coordinate corresponds to the mole fraction of SbBr3. It can be seen that cm do not shift atphonons all, whereas the 1 high frequency at 109 the cm 1 low and fre127 quenc~ phonons (at 32 cm ‘, 34.5 cm’, 45 cm’, 55 cm 1) shift to the lower frequencies in a one

the motion of the donor-acceptor (D-A) unit as a whole. In a sublattice model, on the other hand, the phonons could be classified as belonging either to the donor sublattice or to the acceptor sublattice with a relatively weak coupling between the two sublattice motions. The condition for the applicability of these models can be derived by visualizing the charge-transfer crystal as an ordered solution of two components (the donor and the acceptor) in a definite stoichiometry. In such solid solutions, the defect perturbation can be defined as the difference between various mass coefficients (molecular mass for tanslations, moments of inertia for librations), and the difference between the D-D and A-A force constants. If the defect perturbation is small compared to the 0-A force constant, the resulting motions will be described by the giant molecule model. On the other hand, a large defect perturbation can give rise to lattice which model. are representative of the submotions In a segregated stack charge-transfer complex~ one might expect the sublattice model to be more appropriate. In a mixed-stack complex~, the closer proximity of the donor and the acceptor would make one expect the applicability of a giant molecule model for the external lattice phonons. However, we find examples of both the giant molecule model and the sublattice model in the experimental investigation of lattice phonon bands in a mixed-stack charge3 that the phonon Inbands in the durene-trinitrotransfer complex. an earlier study we report— ed * Supported by NSF Grant # DMR75-02628 871

872

PHONON BANDS IN A ui—ELECTRON CHARGE-TRANSFER COMPLEX

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Vol. 21, No. 9

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Low frequency Raman spectra (at l30~K)

of naphthalene-h8: 2SbC13 and naphthalene—dn: 2SbC13 complexes. Spectral reso ution is 0.5 cm-’. mode type (amalgamation) manner with respect to the acceptor substitution. Again the one mode behavior of the acceptor substitution is indicative of delocalized phonons with relatively wide bandwidths (k-dispersion at least 10 cm’). The bands at 59 cm~,90 ciii’, and 98 ciii’ are assigned as the internal vibrations of SbBr3 which were also identified from the solution spectra of t~pebehavior SbBr3 and from the melt of the5 complex. These internal (also called motions separated show two bandmode limit ) with respect to the acceptor substitution. The band at 157 ciii’ is believed to be a defect induced band which appears only in the mixed crystal of naphthalene: 2 (SbC1 3),_c (SbBrs)c complex, where c is the mole fraction. The 87 cm (weak) and 37 ciii’ (shoulder) bands of the naphthalene: 2 SbC13 system were not observed in the naphthalene: 2 (SbCl,),c(SbBr,)c mixed crystals. Thus, we were unable to make assignments for these bands, The isotopic substitution experiments reveal that a large amplitdue exists on the naphth— alene molecule for the high frequency phonons. One might suspect that this large amplitude is, perhaps, related to an effect due to the 1:2 stoichiometry. In this 1 (donor): 2 (acceptor) stoichiometric complex, naphthalene occupies’ a special position, the center of inversion. The topology of the arrangement of molecules in the complex is similar to that of a linear triatomic molecule in which an internal mode exists with a large amplitude on the central atom. In order to consider effects of stoichiometry on the isotopic shift, we examine the complexes of

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Fig. 2. Graphical representation of the variation in phonon frequencies (at 13O°K) with the donor and acceptor substitu— tiorus. The curve represented by the broken line is obtained by the acceptor substitution. In this case the data point is represented by hollow diamonds (o-) and the X—coordinate-refers to the mole fraction of doped SbBr3. para—xylene with SbC1, which form’° in the ratio shift substitution of of bothupon 1:1 donor and 1:2. We found was thatindependent the isotopic the stoichiometry of the complex. We conclude that the stoichionietry does not play an important role in these complexes. Furthermore, the observed isotopic shift of phonons at 109 cm’ and 127 ciii’ could not be ex— plained by a theoretically expected shift calculated using a giant molecule model. We have calculated the mass and various moments of inertia of the giant molecule on the assumption that the principal axes of the complex coincides with the principal axes of the center species (naphthalene). The calculated isotopic shift for naphthalene-d8 substitution is very small. In view of the above analysis, our result suggests that the 109 ciii’ and 127 cm’ bands can be associated with the delocalized librational optical phonons of the donor naphthalene (centrosymmetric’’) sublattice. On the other hand, the low frequency optical phonons (up to 55 cm’) appear to be predominantly the acceptor sublattice motions which may have mixed translational and librational characters. Our experimental observation on phonons in naphthalene:2SbCl3 crystals can be explained by considering the complex as an ordered mixed crys—

Vol. 21, No. 9

PHONON BANDS IN A ui-ELECTRON CHARGE—TRANSFER COMPLEX

tal with naphthalenes as impurities embedded in the SbC1, host lattice. In these systems the impurity perturbation consists of both diagonal (mass coefficients differences) and off-diagonal (force constants differences) terms. While the diagonal term arising from mass coefficient differences is readily estimated, the off-diagonal perturbation term can not easily be obtained, However, a qualitative understanding of the cornbined mass and force constant perturbation can be achieved by comparing the phonon frequencies observed in pure one component crystals of naphthalene and SbCl,. This comparison gives us a measure of the nature of the dynamical perturbation because the geometrical effects are similar (intermolecular distances in both crystals are comparable). As the naphthalene is centrosynunetnc, only librational motions are Raman active. Frequencies (the average of the two factor group components) for Raman active phonons observed cm ‘, in 100 the crystals are’2 130 cm 1 pure and naphthalene 55 cm They correspond, respectively, to the librations of naphthalene along the long and short in-plane molecular axes’2 and the axis normal to the molecular plane. On the other hand, the pure SbC1, crystal phonon frequency’’ is at most 80 cm ‘. In other words, at least for ~.

long and short molecular axes librations of naphthalene, the frequencies of naphthalene: the highest phonon 2SbC1, crystal may frequency be expectedof tothebeSbCl higher than 3 sub1attice~ This can give rise to naphthalene phonons which are localized with respect to the SbCl, sublattice. However, the phonon band corresponding to the libration of naphthalene along the normal molecular axis will, most probab— ly, be within the SbCl3 phonon density of states and be in resonance with them. Additional support to this interpretation is derived from our calculation of librational phonon frequencies from the thermal parameters of the published room ternperature X-ray data’ for the complex. The root mean square amplitudes of librational phonons were calculated for both SbC13 and naphthalene. The librational phonon frequencies calculated from these root mean square amplitudes by using

873

Cruickshank’s formula’~ are 90 cm’, 89 ciii’, and 28 cm ‘ for naphthalene. The corres~onding-‘ values for SbC1, are 66 cm ‘,5~cm 50 cm This again reveals that the high freguency naphthalene phonons at 90 cm ‘ and 89 cm ‘ fall outside the SbC1, phonon density of states and are thus localized on naphthalenes with respect to the SbCl, sublattice. These values compare qualitatively with our observed values in the room temperature spectra. On the basis of the above discussion the 109 cm ‘ and 127 cm ‘ phonon bands are assigned as librational motions of the naphthalene donor along the long and short inplane molecular axes. Furthermore, as we inferred above from our isotopic mixed crystal study, these phonon bands are delocalized i.e. the amplitudes are distributed over naphthalene molecules. This is not surprising, because the crystal structure’ of this complex shows that SbCl, (acceptor) in the bc(donor). plane alternating with layers ofstack naphthalene Important naphthalene-naphthalene interactions can give rise to delocalization of the phonons in naphthalene planes. Although the low frequency region consists of both a naphthalene librational phonon and SbCl, phonons, it represents predominantly the motions of the SbCl 3 sublattice. The observation of decoupled sublattice phonons is not limited to inorganic acceptors5 in (SbCl,). We have observed similar behavior’ the naphthalene:octafluoronaphthalene (C,0H,:C,,F,) system. Here, again we find a set of high frequency phonons (77.5, 81, 98, 101 and 107 cii’) which belong to the naphthalene sublattice, while the octafluoronaphthalene phonons comprise the low frequency region at 37, 43, and 50, 61 and 68 ciii’. This system, again, meets the theoretical condition for the applicability of the sublattice model where the defect perturbation arises mainly from differences in moments of inertia. We are currently investigating phonon bands of various organic charge-transfer complexes in order to establish the general nature and pro— perties of molecular motions in these systems. ,

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