16222
J. Phys. Chem. 1996, 100, 16222-16231
Theoretical Vibrational Spectra of Neutral and Doped Poly(p-phenylene sulfide) Oligomers† G. F. Musso,* R. Narizzano, P. Piaggio, and G. Dellepiane Dipartimento di Chimica e Chimica Industriale, UniVersita` , Via Dodecaneso 31, I-16146 GenoVa, Italy
A. Borghesi Dipartimento di Fisica, UniVersita` , Via Campi 213/a, I-41100 Modena, Italy ReceiVed: February 9, 1996; In Final Form: June 19, 1996X
The vibrational properties of the first oligomers of poly(p-phenylene sulfide) (PPS) in their neutral, singly and doubly ionized states have been computed at the ab-initio SCF level followed by scaling of the ab-initio force constants with appropriate scale factors. The effect of the basis set on geometries and vibrational spectra has been studied, and the results have been compared with the available experimental data relative to the oligomers and to the polymer. The 3-21G* basis set has been found to be a satisfactory compromise between accuracy and computional effort. In the trimer, the smallest oligomer whose spectra contain all the essential features pertinent to the polymer, ionization has been found to give rise to a strong intensity enhancement of a small number of selected vibrational bands in the infrared and in the Raman spectra, in good agreement with the experimental findings. Combining the experimental and theoretical Raman spectra of the doped trimer with the experimental and theoretical electronic spectra of the same system suggests that dicationssin addition to radical cationssare formed upon doping.
Introduction Vibrational spectroscopic methods are a powerful tool for studying conjugated polymers in the pristine or in the doped state, and much experimental as well as theoretical work has been devoted to this subject.1,2 Theoretical methods are of much interest to achieve structural information which is not easily obtained experimentally. As a matter of fact, today quantum chemical calculations on the appropriate oligomers can in principle provide reliable results regarding the structure and force constants of a polymer with an acceptable computational effort. Ab-initio calculations of the vibrational properties of oligomers related to conjugated polymers in their pristine states have in fact appeared in the last years, yielding results in good general agreement with available experimental data.3-10 Only recently, however, was this kind of approach extended to predict the vibrational properties of the doped systems,11-14 a comparison with experimental results being often proposed. Poly(p-phenylene sulfide) (PPS) is an interesting conducting polymer for both fundamental research and applications.15 Although it is not strictly a conjugated polymer, it has been recognized that in conditions of a controlled low degree of doping a film of this polymer shows both reversible dopinginduced electronic and infrared bands16-18 and photoinduced electronic and infrared absorptions,19 similar to what is observed in conjugated materials. This behavior may be attributed to the sulfur atoms, which are believed to allow a certain degree of conjugation between the phenylenic rings.20,21 The doping process of PPS and of some of its oligomers has been extensively studied in our laboratory by using different solvents and various doping agents, e.g. TaF5, 98% H2SO4, oleum, and FeCl3. The doping-induced electronic spectra in 98% H2SO422 clearly show that the trimer is the smallest oligomer whose spectrum is qualitatively similar to the spectrum † Paper presented in part at the EMRS 1995 Spring Meeting, Strasbourg, France, May 22-26, 1995. X Abstract published in AdVance ACS Abstracts, September 1, 1996.
S0022-3654(96)00414-5 CCC: $12.00
of the polymer. The vibrational spectra of the polymer and of its first few oligomers in the pristine state were reported in an early paper,23 while the effect of FeCl3 doping on the infrared and Raman spectra of a film of the polymer has been studied by us in detail recently.24 The most striking feature of the doping-induced spectra is the strong enhancement of the intensity in very few selected spectral regions. In particular, two strong Raman bands (at 1043 and 1553 cm-1, respectively) have been observed and believed to correspond to the two strong IRAV modes at 1033 and 1538 cm-1 in the infrared spectrum. In this work we present an ab-initio quantum chemical study of the vibrational properties of the trimer molecule both in the pristine and in the doped states. This system was chosen because it contains all the essential features pertinent to the polymer, still remaining computationally tractable at a reasonable level. The results will be compared with the available vibrational data relative both to the trimer itself and to the polymer. In the meantime, the main features of the electronic spectrum of the doped trimer, which had been already studied by us theoretically,25 will be revisited. The scope of the work is to provide an answer to the following questions: (i) Can the vibrational properties of the pristine and doped trimer (and hopefully of the polymer) be predicted and interpreted by ab-initio quantum chemical calculations of a reasonable quality? (ii) Can the results of the theoretical vibrational analysis be of help in assessing the nature of the doping-induced defects in these systems? Method of Calculation The ab-initio part of the calculations was done with the GAUSSIAN 92 system of programs.26 As usual, the effect of doping was mimicked by considering the radical cation and dication species of the actual oligomer. We remark, however, that the assumption of an ideal charge transfer between the dopant molecule and the oligomer could affect in some way the comparison with experimental results, given that in PPS the frequencies of the already-mentioned IRAV bands show appreciable variations with the nature of the doping agent.24 The © 1996 American Chemical Society
Spectra of Poly(p-phenylene sulfide)
J. Phys. Chem., Vol. 100, No. 40, 1996 16223
TABLE 1: Scaling Factors for the 3-21G* Basis Set C-S str. C-C str. C-H str. CSC bend. CCS bend. CCC bend. CCH bend. SCCC tors. CSCC tors. CCCC tors. CCH oop CC/CC, orthob CC/CC, meta, parab
monomer
trimer
0.789 0.844 0.823 0.514 0.908 0.771 0.762 0.842 0.904 0.829 0.708 0.911 0.874
0.884 0.831 0.823 0.494 0.824 0.767 0.760 0.777 0.999a 0.898 0.671 0.875 0.883
a Held fixed (see also ref 29). b “Extra” scale factors which scale the interaction force constants between aromatic CC bonds.
calculations were performed in the framework of the independent particle model, and the neutral and doubly ionized forms were treated as closed shell systems (RHF procedure), while for the radical cations the restricted open (ROHF) procedure was adopted in order to obtain pure spin states. Full geometry optimizations were done under the constraint of C2 symmetry. The trimer (T) calculations were preceeded by a complete set of test calculations on the monomer (M), which were aimed to assess the overall quality of the results, to compare them with the available experimental data, and to study the dependance of the results on the basis set. In the M calculations two basis sets were used, namely 3-21G* and 6-31G*, while in the T case we have been forced to use the 3-21G* basis for computational reasons. The results obtained in this way suffer from various approximations, namely the neglect of electron correlation effects, the limitations in the basis set, and also the isolated molecule approximation. A well-established procedure to take into account these effects is to scale the ab-initio force constants with a set of scale factors which are mostly numbers less than 1.27-29 We have used a total of 13 scale factors, 11 of which are “normal” scale factors7,27 referring to all different kinds of internal coordinates, the last two being “extra” scale factors,7,28 which scale only the interaction force constants between the aromatic CC bonds. The latter scale factors must be included to account for the correlation effects inherent to the aromatic phenyl rings.7 The optimum values of the scale factors were determined by requiring the mean deviation between the computed and experimental vibration frequencies of the neutral M or T species to be a minimum (SCALE3 program30) and are reported in Table 1. A few T scale factors, involving in particular some sulfur atom motions, are seen to differ appreciably from the corresponding M values. We have however checked that using for the T calculation the optimized scale factors of the monomer instead of the trimer’s ones raises the frequency mean deviation (see below) by only 2 cm-1. Although the reasons for the discrepancy are not fully clear, it may be noted that the -SPh-S- moiety is present in T, but not in M. Moreover, a lesser number of experimental data in the low-frequency region (where the sulfur atom motions preferentially absorb) have been included in the fitting for T than for M, due to their experimental uncertainties. Finally we remark that the vibrational spectra of M and of T have been recorded in different phases, namely in the liquid phase for M and in the solid state for T. This fact could be a further origin of possible differences in the values of the optimized scale factors between the two oligomers. Transferability of the scale factors between different molecular systems is well-established,3,27,28 although some limitations to its validity have been discussed recently.29,31 In this respect
Figure 1. 3-21G* molecular geometries (Å and deg) for the neutral (a) and radical cation (b) monomers.
we may compare our results for the phenyl ring scale factors with those reported in refs 7, 32, and 33 for benzene. Our results for the C-C and C-H stretchings, the in-plane CCC and CCH bendings, and the CCH out-of-plane vibrations are seen to be in essential agreement with the reference values (notice that the scale factor for the CC in-plane deformation in ref 7 is taken from biphenyl), while our values for the CCCC torsion and the CC/CC couplings are markedly higher (i.e., nearer to 1). We have found it difficult to give an explanation for this behavior, although the effect of the sulfur atom(s) on the substituentsensitive phenyl vibrations cannot be discarded. The resulting values of the scale factors were used as such to scale the ab-initio force constants relative to the ionized species. At present it is not clear if this transfer of scale factors between the neutral species and its positive ions can give force fields of similar accuracy. We remark, however, that in their calculations on diphenylacetylene Shimojima et al.32 obtained good agreement between observed and calculated normal frequencies for the neutral species, the radical cation, and the radical anion by using the same scale factors throughout. Similar results have been obtained by Pauzat et al.34 for neutral naphtalene and its radical cation. All the theoretical vibrational frequencies reported in this paper are scaled values, while IR intensities and Raman activities have been computed at the ab-initio level. In fact, scaling has only a small effect on intensities (see also ref 3). Results and Discussion Monomer Results. The 3-21G* fully optimized ab-initio geometries of the neutral and radical cation forms of M are displayed in Figure 1 (only the most significant geometry parameters are reported). The change in the basis would be seen to affect the bond lengths for less than 8 × 10-3 Å and the bond angles and dihedral angles for about 1°. The results for the neutral species are in good agreement with the experimental electron-diffraction data [rCS ) 1.771 Å, rCC ) 1.399 Å, CSC ) 103.7°, angle of rotation of phenyl rings ) 45° (assumed)].35 In going from M to M•+, we observe a shortening of the CS bond of about 0.05 Å, accompanied by an enlargement of the CSC bond angle of more than 6° and by the rise of some quinoid character of the phenyl rings (maximum difference in length between longitudinal and transverse CC bonds ) 0.024 Å). Also, the phenyl rotation angle is seen to be more or less halved, revealing some tendency of the rings to become coplanar. All these effects
16224 J. Phys. Chem., Vol. 100, No. 40, 1996
Figure 2. Computed IR spectra of the neutral monomer: (a) 3-21G* basis set; (b) 6-31G* basis set.
Musso et al.
Figure 4. Computed IR spectra of the monomer radical cation: (a) 3-21G* basis set; (b) 6-31G* basis set.
Figure 5. Comparison between the theoretical 6-31G* IR spectrum (a) and the experimental spectrum (b) for the neutral monomer.
Figure 3. Computed Raman spectra of the neutral monomer: (a) 3-21G* basis set; (b) 6-31G* basis set.
are qualitatively similar to those already found in truly conjugated oligomers, although they are less pronunced here due to the nature of the present system. We remark that the present geometries are somewhat different from those previously obtained by us25 through a partial optimization with a STO3G* basis set. In Figures 2-6 the theoretical vibrational spectra of M and M•+ obtained with the two basis sets are compared between themselves and with the experimental data. We recall that all of the 63 vibrational frequencies are both IR and Raman active, but GAUSSIAN 92 does not give the Raman activities for ROHF wave functions. The basis set comparisons are displayed in Figures 2-4. The agreement between the two sets of results is seen to be very satisfactory both for the frequencies (mean frequency deviation for M and M•+ ) 11 and 12 cm-1, respectively) and for the IR intensities and Raman activities. These findings indicate that the 3-21G* basis set can be safely used in the T calculations. This is confirmed by the comparison of the vibrational spectra of M with the experimental results,23 which is presented in Figures 5 and 6 (no comparison is possible for M•+ due to experimental difficulties).
Figure 6. Comparison between the theoretical 6-31G* Raman spectrum (a) and the experimental spectrum (b) for the neutral monomer.
Again the comparison is seen to be very favorable, the mean frequency deviation being 14 and 19 cm-1 for the 6-31G* and 3-21G* basis sets, respectively, while the intensities too show a satisfactory behavior. We report in Table 2 the assignments
Spectra of Poly(p-phenylene sulfide)
J. Phys. Chem., Vol. 100, No. 40, 1996 16225
TABLE 2: Observed and Calculateda Normal Frequencies and Intensities for the Neutral Monomer calcd
IR intensityb
183 212 273 399 413 438 478 519 613 615 696 689 691 703 748 757 843 925 980 981 1017 1017 1052 1076 1082 1095 1161 1162 1183 1186 1310 1313 1432 1437 1487 1488 1582 1596 1599
0.5 1.1 0.6 0 1.4 0.7 8.3 20.5 0 0.1 3.6 31.3 14 19 57.6 23.8 0.2 2.5 0.3 4.5 11.2 1.6 2.6 3.3 0.6 5.6 1.8 0.1 0.6 0 2.9 1 15.5 6.5 35 10.8 4.1 14.8 0.9
a
Raman activityc 0.9 5.8 3.6 3.5 0.4 3.5 0.1 2.3 7.6 2.5 7.6 3.3 0.4 1.6 0.5 2.3 1.8 0.1 64.9 9.8 3.6 19.5 0.2 0 22.2 7.2 2.8 6.3 1 0.4 0.5 1.6 0.5 2 0.5 0.5 4.1 31.7 63.5
]
obsd 185 215 269 411 412 443 464 516 615
IR intensity
Raman intensity
vw vw vw
w w mw w
vw vw mw mw vw
696
]
] ] ] ] ] ]
689 737 746 837 916 999 1000 1024 1025 1068 1080 1093
vw w mw
vs vs sh vw b, w vs w m m m ms m
1157
w
w
1178
vw
w
1301 1326 1439
w vw s
vw
1476
vs
vw
1580
s
1582
ms
PEDd 0.79SCCC + 0.16CCCC 0.43SCCC + 0.27CCCC + 0.16CSC 0.65CCS + 0.15CS 0.38CCC + 0.31CCCC + 0.28CS 0.77CCCC + 0.18OOP 0.49CCC + 0.36CS + 0.10CCS 1.58CCCC + 0.19OOP + 0.15SCCC 1.01CCCC + 0.18OOP + 0.17SCCC + 0.13CSC 0.92CCC + 0.13CC 0.98CCC + 0.13CC 0.34CCCC + 0.31OOP + 0.16CC + 0.14CS 0.89CCCC + 0.32OOP + 0.31CCC + 0.10CC 1.32CCCC + 0.55OOP 0.69CCC + 0.29CS + 0.27CC 0.73OOP + 0.43CCCC 0.66OOP + 0.41CCCC 1.00OOP 0.83OOP + 0.22CCCC 1.11CCC + 0.44CC + 0.23OOP + 0.17CCCC 0.89CCC + 0.36OOP + 0.33CC + 0.26CCCC 1.84CCC + 0.22CC + 0.17CCH 1.77CCC + 0.22CC + 0.18CCH 0.62CC + 0.28CCH + 0.18CCC 0.75CCC + 0.72CC + 0.23CCH 0.75CCC + 0.73CC + 0.25CCH 0.83CCC + 0.28CC + 0.20CCH + 0.18CS 0.74CCH + 0.28CC 0.73CCH + 0.28CC 0.73CCH + 0.39CC 0.72CCH + 0.39CC 0.87CCH + 0.11CC 0.88CCH + 0.10CC 0.62CCH + 0.33CC 0.61CCH + 0.33CC 0.65CCH + 0.37CCC 0.64CCH + 0.38CCC 0.81CCC + 0.62CC + 0.22CCH 0.65CC + 0.43CCC + 0.24CCH 0.65CC + 0.42CCC + 0.24CCH
6-31G* basis set. b km/mol. c Å4/amu. d Values less than 10% are neglected.
of the vibrational frequencies together with the computed and experimental intensities and the potential energy distributions (PED).36 All the frequencies are reported, except the CH stretchings and the frequencies to which a zero weight has been given in the scale factor optimization due to their experimental uncertainties. No bands have been left unassigned, and all of them have been assigned satisfactorily. The calculations are seen to reproduce quantitatively the known features which are characteristic of the monosubstituted phenyl rings both for the substituent-sensitive23 and the substituent-insensitive bands,37 as shown also by the PED’s. The theoretical analysis allows for good assignments even in the low-frequency region, where group correspondences are less well-known. A comparison with the assignments of Zeraoui et al.,38 which are based on a valence-force field, is hardly possible in view of the fact that they assume a planar geometry for the monomer and the polymer and consider only in-plane modes. As a consequence, for instance, they assign as a ring deformation the 690 cm-1 frequency, which on the contrary is known to be an out-ofplane vibration, as correctly predicted by us. The most interesting considerations are suggested from the computed IR spectrum of M•+ as compared with that of M (Figure 7, 6-31G* basis set). The frequency variations in corresponding bands are limited, while four to five bands show a strong intensity increase upon ionization. This fact becomes still more interesting as the two
Figure 7. Comparison between the theoretical 6-31G* spectra of the radical cation (a) and the neutral (b) monomers.
16226 J. Phys. Chem., Vol. 100, No. 40, 1996
Musso et al.
Figure 8. 3-21G* molecular geometries (Å and deg) for the neutral (a), radical cation (b), and dication (c) trimers.
most intense IR bands of M•+ (I ) 600/700 km/mol) are predicted to occur around 1000 and 1550 cm-1, respectively, i.e. in the vicinity of the two strong IRAV bands observed in PPS.24 If, however, we note that many IR bands of M show intensities of ≈50-60 km/mol and that the degree of doping does not in general exceed 2-3%, we can anticipate that no appreciable changes in the spectrum of M would be observed upon doping. The same is true for the 3-21G* spectra. Trimer Results. 3-21G* calculations have been performed on T, T•+, and T2+, whose fully optimized geometries are shown in Figure 8 (only the most significant parameters reported). The geometry of the central part of the molecule closely parallels that of the monomer both in T and in T•+, while the outer portion remains essentially unchanged in all species, thus indicating that the charged defects should not include more than two rings and three sulfur atoms. An equally extended defect has been also found in the 3-21G* optimization of the geometry of the pentamer dication (results not reported here), which shows the outer -Ph-S-Ph moieties to be negligibly perturbed by the double ionization. The outer S-C bonds are appreciably rotated with respect to the plane of the central CSC moiety in T but become nearly coplanar in T•+ and T2+, while the terminal phenyl rings tend to become perpendicular to that plane in all forms. The already noted effects of ionization in T•+ result to be more and more enhanced in T2+. As a check of the basis set effects, the geometry of T2+ has been reoptimized at the 6-31G* level, and the results obtained deviate from those in Figure 8c by less than 1 × 10-2 Å for bond lengths and 1.5° for bond angles and dihedral angles. Similar to what happens in the monomer, the present geometries differ appreciably, if not very much, from the earlier ones which were possibly less reliable, having been obtained by us through only partial optimizations carried out with the smaller STO-3G* basis set.25 This has, however, appreciable effects on the low-frequency side of the electronic spectra of T•+ and T2+, as computed at the VEH39 level. The VEH (Valence Electron Hamiltonian) nonempirical method is known to be able to reproduce very
Figure 9. VEH relevant electronic transitions for the trimer radical cation and dication: (a) calculations based on the earlier STO-3G* geometries (ref 25); (b) calculations based on the present geometries. The oscillator strengths reported are relative values.
well the frequencies of optical transitions (e.g., band gaps) in many conducting polymers.40,41 The earlier and the present data are reported in Figure 9. The experimental spectrum of the doped trimer25 shows in the low-frequency side an intense peak at 1.05 eV followed by a shoulder at 1.60 eV. The earlier theoretical spectrum of T•+, which was based on a low-quality geometry (see above), contains one main peak at 0.71 eV and two very weak bands at 1.43 and 2.14 eV, while T2+ gives a peak at 1.07 eV. It is seen that the present results are in much better agreement with the experimental data, showing the main T•+ and T2+ absorptions at 0.72 and 0.96 eV, respectively, but only one less intense
Spectra of Poly(p-phenylene sulfide)
Figure 10. Comparison between the theoretical IR spectrum (a) and the experimental spectrum (b) for the neutral trimer.
Figure 11. Comparison between the theoretical Raman spectrum (a) and the experimental spectrum (b) for the neutral trimer.
band of T•+ at 1.61 eV. Of course the agreement would be better if T2+, in addition to T•+, was present. The vibrational frequencies of the trimer are 129, all of which are both Raman and IR active. Figures 10 and 11 show the theoretical IR and Raman spectra of T together with their experimental counterparts.23 Table 3 reports the assignments of the vibrational frequencies. CH stretchings and zero-weight experimental frequencies are omitted, as well as frequencies whose intensities are both observed and predicted to be weak. Regarding the assignments, essentially similar considerations hold as those already given for the monomer. The comparison between computed and observed frequencies is satisfactory (mean deviation ) 12 cm-1), but the overall quality of the calculated spectra is somewhat worse than that found in the monomer for what concerns the IR intensities and Raman activities. As a consequence, we believe that some care must be exercised in analyzing the results obtained even for the ionized species, in particular by taking for granted only the most notable variations in intensities. The theoretical IR spectra of T and T•+ are compared in Figure 12. The most relevant feature in Figure 12 is that the spectrum of T•+ is considerably simpler than that of T. In fact, only the T•+ vibrational modes of B symmetry at 935 and 1500 cm-1 are seen to experience intensity enhancements of roughly 3 orders of magnitude, their absolute intensities being 8600 and 4600 km/mol, respectively. If we consider that the vast majority of the T frequencies is predicted to have I e 60-70 km/mol, we can conclude that the above two intensity maxima should now appear as new peaks in the IR spectrum of the doped trimer.
J. Phys. Chem., Vol. 100, No. 40, 1996 16227 Unfortunately, we have not been able to obtain a good infrared spectrum neither in transmission nor in reflection, and for a comparison with experiment, we can only refer to the two already-mentioned IRAV bands in PPS.24 Taking into account that the trimer is the smallest oligomer whose spectroscopical behavior is qualitatively similar to that of the polymer, the agreement can be considered satisfactory. PED is of help in assessing the nature of the vibrational modes in these systems. Such an analysis shows for instance that by no means are out-of-plane CH vibrations nor torsional carbon or sulfur motions involved in intensity enhancements. On the contrary, in T•+ (and in M•+ alike), ionization is seen to invariably affect the intensities of the phenyl ring breathing modes, eventually somewhat mixed with CS stretchings and CCH bendings. In fact, the 935 and 1500 cm-1 modes of T•+ have PED’s 1.61CCC + 0.62CC + 0.10CS and 0.62CC + 0.34CCH, respectively, and similar results have been found for the corresponding maxima of M•+. No theoretical Raman spectrum is available for T•+ because GAUSSIAN 92 does not provide the Raman activities for ROHF wave functions. On the contrary, T2+ is a closed-shell system, and its theoretical IR and Raman spectra are displayed in Figures 13-14, together with those of T. By comparing Figure 13 with Figure 12 it is seen that, despite the two spectra being different, there is a strict similarity between their main features, Figure 13 too showing two very intense peaks of B symmetry at 902 and 1513 cm-1, whose PED’s are 1.16CCC + 0.35CS + 0.22CC and 0.72CC + 0.24CCH + 0.11CCC, respectively. The same similarity is found between the main peaks of the IR spectra of M•+ vs M2+. Continuum model calculations on polythiophene42,43 suggested that one cannot distinguish between polarons and bipolarons from the IRAV alone, and not very different results have been obtained from recent calculations on PPV oligomers44 based on a two-dimensional SSH-like model Hamiltonian. In the latter case, however, a few significant differences between polaron and bipolaron IR spectra have been underlined. The present calculations give the first ab-initio quantum chemical confirmation of the above suggestions. The Raman spectrum of T2+ in Figure 14 exhibits three high activity maxima of A symmetry at 1082 (PED ) 0.52CC + 0.35CCC + 0.26CS + 0.12CCH), 1156 (PED ) 0.30CC + 0.27CCC + 0.26CS + 0.18CCH), and 1579 cm-1 (PED ) 0.72CC + 0.23CCC + 0.22CCH) together with other less important features. The comparison with the experimental Raman spectrum (excitation wavelength ) 1064 nm ≡ 1.16 eV) of the doped trimer (Figure 15) is very interesting. The trimer has been efficiently doped with FeCl3 in CH2Cl2 solution, and its spectrum has been recorded using the same experimental procedures described in ref 24. The lower curve in Figure 15 refers to the neutral trimer in CH2Cl2 solution. Note that practically only solvent lines are observed and that to obtain the spectrum of the neutral trimer reported in Figure 11 microcrystalline pellets have been used. A nearly identical spectrum is given by the system FeCl3/CH2Cl2. The upper curve is relative to the trimer/FeCl3/CH2Cl2 system and shows that the background of the spectrum continuously increases toward higher wavenumbers, probably due to a thermoluminescence effect. On top of this background, four new strong peaks are observed at 1067, 1116, 1559, and 1581 cm-1, respectively. Among these bands, however, the 1559 cm-1 one shows a different time evolution during doping with respect to the others, casting some doubt on whether or not it is originated by the same molecular species. In any case, the agreement between the computed spectrum of T2+ and the experimental spectrum of the doped trimer is impressive, and
16228 J. Phys. Chem., Vol. 100, No. 40, 1996
Musso et al.
TABLE 3: Observed and Calculated Normal Frequencies and Intensities for the Neutral Trimer calcd
IR intensitya
111 163 185 224 310 429 441 509 510 519 552 625 642 683 683 712 740 746 749 761 811 817 840 845 939 981 986 1002 1007 1031 1068 1070 1081 1087 1081 1093 1104 1106 1110 1123 1174 1378 1381 1435 1458 1462 1464 1565 1565 1573 1575 1577 1578
0 0.3 1.3 1.1 1.2 5.1 6.8 10.1 45.7 8.4 50.5 0.1 0.3 82.2 60.9 2.3 0 0.7 2.7 70 15 7.7 52.3 19.4 4.8 0.1 0.2 19.7 16.5 0 20.8 6.5 5.7 3.9 0.1 0 23.5 9.4 4.6 26.4 0.6 21.6 14.4 28 53 97.7 21.4 2.5 1 30.3 3.5 2.8 2.7
a
km/mole.
b
Raman activityb 0.8 6.7 0.5 7.7 9.1 0.1 3 0.5 0 2.1 0.4 10.6 13.3 0.6 2.3 12.5 35.2 5.9 7.2 4.7 3.8 6.6 0.2 3.3 3 108 3.7 5.5 0 21 20.1 105 8 6.1 7 10.3 0.7 11.5 25.2 0.5 40.7 0.9 2 2 0.8 0.6 0.5 6.5 10.9 105 81.5 21.3 380
Å4/amu. c
] ] ] ] ] ] ] ]
] ] ]
obsd 108 147 171 228 315 430 475 496 507 520 550 616 632 690 701 704 728 743 738 750 815 825 811
IR intensity
w sh mw m s s w w vs w ms vs mw vs s s sh
1115 1116 1183 1386
ms
1439
m
1472
vs
w mw m
1570 1574 1580
0.45SCCC + 0.27CCCC + 0.20CSC + 0.13CSCC + 0.12CCS 0.44CCS + 0.37SCCC 0.56CCS + 0.29SCCC + 0.10CCC 0.26CCC + 0.24SCCC + 0.24CSC + 0.17CCCC + 0.13CS + 0.10CCS 0.43CCS + 0.42SCCC + 0.24CCC 0.75CCCC + 0.24OOP 0.50CCC + 0.29CCCC + 0.26CS + 0.14CCS 0.71CCCC + 0.32CCC + 0.29CS + 0.17OOP 1.17CCCC + 0.30OOP + 0.10SCCC 0.98CCCC + 0.28OOP + 0.10SCCC 0.44CCCC + 0.33CS + 0.23OOP + 0.17CCC 0.97CCC + 0.10CC 1.33CCC + 0.12CC 0.86OOP + 0.45CCCC 0.86OOP + 0.45CCCC 0.73CCC + 0.28CS + 0.24CC 0.92CCC + 0.29CCCC + 0.19CS + 0.18OOP + 0.16CC 1.11CCC + 0.27CS + 0.22CC 0.68CCCC + 0.48OOP + 0.31CCC 0.68OOP + 0.42CCCC 0.99OOP 0.99OOP 0.64OOP + 0.39CCCC 0.61OOP + 0.42CCCC 0.73OOP + 0.34CCCC 1.08CCC + 0.30CC + 0.10CCH 0.79CCCC + 0.73OOP 2.04CCC + 0.11CCH 1.54CCC + 0.11CCH 1.74CCCC + 0.35OOP 1.03CCC + 0.70CC + 0.18CS 1.02CCC + 0.59CC + 0.18CS 0.84CC + 0.30CCC 0.87CC 0.85CC + 0.32CCC 0.90CC 0.37CC + 0.33CCH + 0.25CCC + 0.15CS 0.37CC + 0.35CCH + 0.28CCC + 0.14CS 0.58CC + 0.33CCC + 0.20CS + 0.17CCH 0.51CC + 0.35CCH + 0.11CCC 0.79CCH + 0.23CC 0.47CCH + 0.34CC + 0.20CCC 0.46CCH + 0.34CC + 0.20CCC 0.61CCH + 0.36CC 0.67CCH + 0.29CCC 0.66CCH + 0.34CCC + 0.12CC 0.65CCH + 0.34CCC + 0.12CC 0.73CC + 0.36CCC + 0.21CCH 0.73CC + 0.36CCC + 0.21CCH 0.73CC + 0.56CCC + 0.21CCH 0.74CC + 0.62CCC + 0.20CCH 0.74CC + 0.70CCC + 0.20CCH 0.74CC + 0.65CCC + 0.20CCH
s
1088 1098
m s m m mw
w mw
1025 1075 1085
PEDc
w s sh mw w m
942 998 1007
Raman intensity
s b, ms s
Values less than 10% are neglected.
we believe it is of much relevance in connection with the problem of which species are generated upon doping. That T•+ is present in the doped material is supported by the fact that it would otherwise be impossible to explain the observed electronic spectrum of the doped trimer (see above). Polaron formation is also suggested by the in-situ spectroelectrochemical study of PPS45 in which spins have been monitored through ESR during the whole process. On the other hand, divalent ion formation has been experimentally detected46,47 in PPV oligomers of equivalentsor even lowerssize with respect to the trimer of PPS. In the latter papers, as well as in the already-cited calculations based on a two-dimensional SSH-like Hamiltonian,44 it has been shown that the Raman spectra of radical ions differ markedly from the spectra of divalent ions. The spectra of the different ions referring to the same doped oligomer are
there selectively recorded as Raman resonance spectra by using different excitation wavelengths, each of them corresponding to a maximum in the electronic absorption spectrum of the indicated ionized species. We can apply the same kind of reasoning to our case as follows. If dications were not present, the 1.05 eV absorption in the electronic spectrum would be due to T•+ only and the exciting wavelength at 1.16 eV would produce the Raman spectrum of this species. In view of the above theoretical and experimental findings,44,46-47 however, the experimental Raman spectrum of T•+ could not be expected to match the computed spectrum of T2+ so well as it happens. On the contrary, in the presence of both T•+ and T2+ the electronic absorption of T2+ is predicted to occur at a frequency appreciably higher than that of T•+ (as confirmed by Figure 9) and consequently the experimental peak at 1.05 eV to be due
Spectra of Poly(p-phenylene sulfide)
J. Phys. Chem., Vol. 100, No. 40, 1996 16229
Figure 14. Comparison between the theoretical Raman spectra of the dication (a) and neutral (b) trimers. Figure 12. Comparison between the theoretical IR spectra of the radical cation (a) and neutral (b) trimers.
Figure 13. Comparison between the theoretical IR spectra of the dication (a) and neutral (b) trimers.
Raman bands of the trimer and the corresponding experimental data for PPS (see ref 24). We are not in the position of performing a detailed theoretical analysis of the doping-induced bands in PPS, which would require a different approach, for instance along the lines proposed in ref 12. Such a study is beyond the scope of this paper but is presently under consideration in our laboratory. We believe, however, that the results reported in Table 4 give some preliminary indications. Only in the case of the Raman spectra can we compare the experimental results for the trimer and PPS. We observe a strict correspondence of their dopinginduced peaks, which is in agreement with our theoretical findings about the limited extension of the defects in these systems (see above) and in turn suggests that the defects themselves are similar in the two cases. The same is suggested by the comparison between the IR doping-induced peaks in PPS and the theoretical results for T•+ and T2+, although here a somewhat lesser agreement is found between theory and experiment. A possible explanation could be related to the slower convergence found by Karpfen and co-workers in their theoretical studies of the bipolaronic defects in oligothiophenes.14 We finally remark that the question remains open of the physical origin of the intensity enhancement of selected vibrations following ionization. An interesting approach to this problem, which takes into account the electron-molecular vibration coupling, was applied long time ago48 to TTF-TCNQ systems and is now under investigation in our laboratory. Summary and Conclusions
to T2+. This implies the Raman spectrum of the doped trimer to be dominated by that of T2+, as is in fact the case. Finally, a good correspondence is also observed between the computed peaks of the Raman spectrum of T2+ and the strongest bands in the Raman spectrum of the FeCl3-doped polymer (excitation wavelength again )1064 nm) at 1043, 1109, and 1553 cm-1.24 We have collected in Table 4 our theoretical and experimental results referring to the doping-induced IR and
The results reported in this paper suggest the following considerations: (i) The vibrational propertiessincluding intensitiessof pristine and doped PPS oligomers are well reproduced by the abinitio SCF approach followed by a proper scaling of the force constants. Using the moderate-sized 3-21G* basis set instead of the more elaborate 6-31G* one does not appreciably affect the computed geometries nor the vibrational properties. The assumption of a complete charge transfer during the doping
16230 J. Phys. Chem., Vol. 100, No. 40, 1996
Musso et al.
Figure 15. Raman spectrum of the trimer doped with FeCl3 in CH2Cl2 solution (upper curve). Lower curve: trimer in CH2Cl2 solution before adding FeCl3.
TABLE 4: Experimental and Theoretical Doping-Induced Vibrational Bands in PPS and Its Trimer (cm-1) PPS exptla 1043 1109 1553 1033 1538 a
trimer exptl 1067 1116 1559b 1581 .... ....
trimer theor T•+ T2+
approximate description
.... ....
(i) Raman 1082 substituent-sensitive ring stretching 1156 substituent-sensitive ring stretching
....
1579
935 1500
(ii) IR 902 substituent-sensitive ring stretching 1513 ring stretching
ring stretching
Reference 24. b See text.
process seems to play a relatively minor role in nearly all cases, with the possible exception of the experimental IRAV band of PPS at 1033 cm-1. (ii) The trimer is the first oligomer whose spectra contain all the essential features pertinent to the polymer. Its vibrational spectra in the doped state appear to be dominated by a few very strong intensity maxima. Even if one takes into account the low doping levels achieved both in the doped trimer and in the doped polymer, these are predicted to give rise to new dopinginduced peaks, in agreement with the experimental findings. (iii) As also pointed out by other authors,42-44 the analysis of the theoretical IR spectra of the singly and doubly ionized trimer shows that IR spectroscopy is of little help in assessing the nature of the species which are formed upon doping. On the contrary, if we combine the theoretical and experimental Raman spectra relative to the doped trimer with the theoretical and experimental electronic spectra of the same system, a clear suggestion emerges for the dicationsbesides radical cations formation upon doping. This in turn is a demonstration of how the resonance Raman approach introduced in refs 46 and 47 could take advantage of theoretical calculations in cases where it is hardly possible to separately prepare oligomer radical ions and divalent ions. Of course this would be even more true if
the theoretical Raman spectra of radical ions at the ROHF level would be available. (iv) Five intense peaks are predicted as a consequence of ionization of the trimer molecule, two of which exhibiting strong IR intensity (at 935 and 1500 cm-1) and three strong Raman activity (at 1082, 1156, and 1579 cm-1). Their PED's reveal substantial contributions of different internal coordinates relative to phenyl ring motions, namely CC stretching and CCC and CCH bending, with appreciable mixing of CS stretching for the lower frequencies. A deeper analysis is greatly complicated by the absence of any symmetry in the phenyl rings, but we can observe that only the internal coordinates of the inner rings and the inner CS stretchings are implied in the above normal coordinates. In fact, ionization is seen to affect only the geometry of the central -S-Ph-S-Ph-S- section of the oligomer, as also confirmed by the geometry optimization on the pentamer dication. The small defect length inferred from the above calculations is due to the limited degree of conjugation inherent to the present systems. (v) Although a detailed analysis of PPS spectra is beyond the scope of the present work, experimental and theoretical results suggest that the picture given by us of the trimer dopinginduced defects holds almost equally well for the polymer. Acknowledgment. We acknowledge support by the Italian Ministry of University and Scientific and Technological Research (MURST) and by the National Research Council (CNR). We thank Dr. Gabor Pongo´r for supplying us with a copy of the SCALE3 program. References and Notes (1) Zerbi, G.; Gussoni, M.; Castiglioni, C. In Conjugated Polymers; Bre´das, J. L., Silbey, R., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1991; p 435. (2) Gussoni, M.; Castiglioni, C.; Zerbi, G. In Spectroscopy of AdVanced Materials; Clark, R. J. H., Hester, R. E., Eds.; John Wiley and Sons: Chichester, England, 1991; p 251.
Spectra of Poly(p-phenylene sulfide) (3) Cui, C. X.; Kertesz, M. J. Chem. Phys. 1990, 93, 5257. (4) Tian, B.; Zerbi, G.; Schenck, R.; Mu¨llen, K. J. Chem. Phys. 1991, 95, 3191. (5) Tian, B.; Zerbi, G.; Mu¨llen K. J. Chem. Phys. 1991, 95, 3198. (6) Cui, C. X.; Kertesz, M. Macromolecules 1992, 25, 1103. (7) Cuff, L.; Kertesz, M. Macromolecules 1994, 27, 762. (8) Cuff, L.; Kertesz, M. J. Phys. Chem. 1994, 98, 12223. (9) Hernandez, V.; Ramirez, F. J.; Otero, T. F.; Lopez Navarrete, J. T. J. Chem. Phys. 1994, 100, 114. (10) Ku¨rti, J.; Magyar, C.; Bala´sz, A.; Rajczy, P. Synth. Met. 1995, 71, 1865. (11) Cuff, L.; Kertesz, M.; Geisselbrecht, J.; Ku¨rti, J.; Kuzmany, H. Synth. Met. 1993, 55-57, 564. (12) Cuff, L.; Cui, C.; Kertesz, M. J. Am. Chem. Soc. 1994, 116, 9269. (13) Kastner, J.; Kuzmany, H.; Vegh, D.; Landl, M.; Cuff, L.; Kertesz, M. Macromolecules 1995, 28, 2922. (14) Ehrendorfer, C.; Karpfen, A. J. Phys. Chem. 1995, 99, 5341. (15) Reisch, M. S. Chem. Eng. News 1989, 4, 21. (16) Friend, R. H.; Giles, J. R. M. Synth. Met. 1985, 10, 377. (17) Shimizu, H.; Kanetsuna, H.; Tanabe, Y. Polym. J. (Tokio) 1987, 8, 915. (18) Piaggio, P.; Giribone, D.; Musso, G. F.; Cuniberti, C.; Dellepiane, G.; Borghesi, A. Synth. Met. 1991, 41, 1359. (19) Ginder, J. M.; Epstein, A. J.; McDiarmid, A. G. Synth. Met. 1991, 43, 3431. (20) Bre´das, J. L.; Chance, R. R.; Silbey, R.; Nicolas, G.; Durand, P. J. Chem. Phys. 1982, 77, 371. (21) Riga, J.; Snauwaert, P.; De Pryck, A.; Lazzaroni, R.; Boutique, J. P.; Verbist, J. J.; Bre´das, J. L.; Andre´, J. M.; Taliani, C. Synth. Met. 1987, 21, 223. (22) Musso, G. F.; Piaggio, P.; Dellepiane, G.; Cuniberti, C.; Borghesi, A. Synth. Met. 1992, 48, 283. (23) Piaggio, P.; Cuniberti, C.; Dellepiane, G.; Campani, E.; Gorini, G.; Masetti, G., Novi, M.; Petrillo, G. Spectrochim. Acta 1989, 45A, 347. (24) Piaggio, P.; Musso, G. F.; Dellepiane, G. J. Phys. Chem. 1995, 99, 4187. (25) Musso, G. F.; Piaggio, P.; Dellepiane, G.; Cuniberti, C.; Comoretto, D.; Borghesi, A. Synth. Met. 1993, 55-57, 4807. (26) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzales, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian Inc., Pittsburgh, PA, 1992.
J. Phys. Chem., Vol. 100, No. 40, 1996 16231 (27) Pulay, P.; Fogarasi, G.; Boggs, J. E.; Vargha, A. J. Am. Chem. Soc. 1983, 105, 7037. (28) Pulay, P.; Fogarasi, G.; Boggs, J. E. J. Chem. Phys. 1981, 74, 3999. (29) Pupyshev, V. I.; Panchenko, Y. N.; Bock, C. W.; Pongor, G. J. Chem. Phys. 1991, 94, 1247. (30) Pongor, G. Department of Theoretical Chemistry, Eo¨tvo¨s L. University, Budapest, Hungary. (31) Panchenko,Y. N.; De Mare´, G. R.; Pupyshev, V. I. J. Phys. Chem. 1995, 99, 17544. (32) Shimojima, A.; Takahashi, H. J. Phys. Chem. 1993, 97, 9103. (33) Arenas, J. F.; Lo`pez Toco`n, I.; Otero, J. C.; Marcos, J. I. J. Phys. Chem. 1995, 99, 11392. (34) Pauzat, F.; Talbi, D.; Miller, M. D.; Defrees, D. J.; Ellinger, Y. J. Phys. Chem. 1992, 96, 7882. (35) Rozsondai, B.; Schultz, G.; Hargittai, I. J. Mol. Struct. 1981, 70, 309. (36) Califano, S. Vibrational States; John Wiley and Sons: London, 1976; p 235. (37) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy; Academic Press: New York, 1990; Chapter 8. (38) Zeraoui, S.; Buisson, J. P.; Mevellec, Y. J.; Lefrant, S. Synth. Met. 1993, 55-57, 487. (39) Andre´, J. M.; Bre´das, J. L.; Delhalle, J.; Vanderverken, D. J.; Vercauteren, D. P.; Fripiat, J. G. In Modern Techniques in Computational Chemistry: MOTECC90; Clementi, E., Ed.; ESCOM Science Publisher: Leiden, The Netherlands, 1990; p 745. MOTECC90 is a trademark of IBM. (40) Andre´, J. M.; Delhalle, J.; Bre´das, J. L. Quantum Chemistry Aided Design of Organic Polymers; World Scientific: Singapore, 1991. (41) Stafstro¨m, J. L.; Bre´das, J. L. Synth. Met. 1989, 28, D477. (42) Hicks, J. C.; Mele, E. J. Phys. ReV. B 1986, 34, 1091. (43) Schaffer, H. E.; Heeger, A. J. Solid State Commun. 1986, 59, 415. (44) Wong, W. Z.; Saxena, A.; Bishop, A. R. Phys. ReV. B 1994, 50, 6068. (45) Arbizzani, C.; Mastragostino, M.; Dellepiane, G.; Piaggio, P.; Zotti, G. Synth. Met. 1993, 55-57, 1354. (46) Sakamoto, A.; Furukawa, Y.; Tasumi, M. J. Phys. Chem. 1992, 96, 3870. (47) Sakamoto, A.; Furukawa, Y.; Tasumi, M. J. Phys. Chem. 1994, 98, 4635. (48) Lipari, N. O.; Rice, M. J.; Duke, C. B.; Bozio, R.; Girlando, A.; Pecile, C. Int. J. Quantum Chem. 1977, S11, 583.
JP9604148
