Monomer, Dimer, and Tetramer States in Molybdenum Complexes of

Nov 19, 1996 - one hand and in slipper dimers on the other.28. In the latter case, the intermolecular overlap is much smaller. This might lead to a hi...
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J. Phys. Chem. B 1997, 101, 1561-1568

1561

Monomer, Dimer, and Tetramer States in Molybdenum Complexes of Tetracyanoquinodimethane E. Faulques,*,† A. Leblanc,‡ and P. Molinie´ ‡ Institut des Mate´ riaux de Nantes, 2 rue de la Houssinie` re, F-44072 Nantes Cedex 03, France

M. Decoster, F. Conan, and J. Sala-Pala Laboratoire de Chimie, Electrochimie Mole´ culaires et Chimie Analytique, URA 322 CNRS, UniVersite´ de Bretagne Occidentale, 6 AVenue Victor le Gorgeu, B.P. 809, F-29285 Brest Cedex, France ReceiVed: NoVember 19, 1996X

We report on the determination of monomers, dimers, and alternating states of tetracyanoquinodimethane (TCNQ) species in molybdenum complexes using optical absorption and Raman spectroscopy. The molecular crystals investigated are [MoVIO(dtc)3](TCNQ) 1a, [MoV(dtc)4](TCNQ) 2a and 2b, and [MoV(dtc)4](TCNQ)2 3a (a, dtc ) Et2NCS2; b, dtc ) Me2NCS2). Optical and micro-Raman experiments performed at 50, 78, and 293 K yield information about the electronic states and the mode of packing of TCNQ moieties in these crystals. The optical spectrum of 2a presents the characteristic bands of the isolated monomeric species TCNQ•- at 1.46, 1.63, and 3.02 eV. In complex 1a, the dimerized state is evidenced by the presence of three absorptions: two excitonic transitions LE1 at 1.9 eV and LE2 at 3.4 eV and an intermolecular charge transfer band (CT) near 1.38 eV. The bands LE1 and LE2 are blue-shifted by Davydov effect with respect to their position in the monomer states. Furthermore, it turns out that dimers are totally eclipsed in this system. Despite the fact that complex 2b differs only from 2a by a methyl substitution in the organometallic cation, we find that this salt presents dimer species which are well characterized by Raman spectroscopy. A coupling between the CT and LE1 bands establishes the presence of slipped dimers. At low temperature, however, Raman and optical spectra indicate a probable transition towards an eclipsed dimer state. Our optical spectra show that a perfect dimerization state is not possible in complex 3a since there is no excitonic band at 1.9 eV. However, the occurrence of a CT band at 1.23 eV corresponding to a charge transfer between TCNQ0 and TCNQ•- suggests the presence of TCNQ0/TCNQ•-/TCNQ•-/TCNQ0-stacked tetramers.

I. Introduction The 7,7,8,8-tetracyanoquinodimethane (TCNQ) is a fascinating compound because of its high electron affinity and its planar geometry. This highly versatile acceptor leads to three types of molecular complexes, namely, charge transfer complexes, simple salts Dn+((TCNQ)n)n-, and complex salts Dn+((TCNQ)m)nwith m * n. Many of these derivatives have interesting properties in the field of electrical conduction.1 However, although the number of TCNQ-containing compounds is increasing day after day, it is not yet well understood how the interactions between the organic radicals and between the organic and inorganic parts of the solids, and hence the physical properties, are affected by the nature of the cations. With the aim to further investigate this point, we have recently started a research program devoted to the synthesis, structural studies, and electronic characterization of TCNQ-containing solids with a variety of molybdenum cationic complexes. The main purpose for this program is to have simple systems in which the interactions could be modulated and investigated through the largest number of experimental techniques. We have previously reported on the syntheses and structural studies of the following dialkyl-dithio-carbamato-molybdenum complexes: [MoVIO(dtc)3](TCNQ) 1a, [MoV(dtc)4](TCNQ) 2a and 2b, and [MoV(dtc)4](TCNQ)2 3a (a, dtc ) Et2NCS2; b, dtc ) Me2NCS2). It is noteworthy that these complexes present a variety of TCNQ moieties, since X-ray studies showed the

presence of monomer TCNQ•- in 2a, slipped ((TCNQ)2)2dimers in 2b, and almost perfectly eclipsed ((TCNQ)2)2- dimers in 1a. Unfortunately, it was not possible to grow suitable crystals of the complex of TCNQ salt 3a for an X-ray study. In order to appreciate the possibility to distinguish monomer and dimer TCNQ anions by vibrational spectroscopy,2-4 a Raman study of complexes 1a, 2a, and 3a was performed. To some extent, a careful examination of specific Raman lines allowed such distinction.5 Continuing our efforts to compare these different anions by different experimental techniques, we studied the optical spectra of these molybdenum complexes. Since the pioneering work of Boyd6 on the electronic spectra of TCNQ anions in solutions, various electronic absorption spectra of TCNQ derivatives have been investigated. As an example, comparison of calculated values and observed values with single crystals has been made by Sakata7 for several TCNQ complexes while the optical properties of the two forms of [FeC*p2](TCNQ) (C*p ) C5Me5), one containing isolated paramagnetic TCNQ•- anions, the other containing isolated diamagnetic ((TCNQ)2)2- dimers, have been reported and discussed by Tanner.8 To illustrate the various situations encountered in dimeric compounds, Figures 1 and 2 summarize the different types of dimer states and the corresponding overlap of the π orbitals. II. Experimental Section

†

Laboratoire de Physique Cristalline. ‡ Laboratoire de Chimie des Solides. X Abstract published in AdVance ACS Abstracts, February 1, 1997.

S1089-5647(96)03865-5 CCC: $14.00

Optical spectra were recorded with a UV-vis Cary spectrophotometer in the 200-2000 nm range both at room and liquid © 1997 American Chemical Society

1562 J. Phys. Chem. B, Vol. 101, No. 9, 1997

Faulques et al.

Figure 3. Optical spectrum of complex 2a (T ) 293 K). Comparison with the spectrum of solid TCNQ is made in the inset.

Figure 1. Dimer types and intermolecular and intramolecular electronic transitions in and between TCNQ•- units (dark circles): (a) totally eclipsed dimer, (b-d) slipped dimers.

a sufficiently low laser power in order to avoid any thermal degradation. All compounds are dark green. Samples 1a, 2a,b are constituted of homogeneous parallepipedic crystallites, whereas sample 3a crystallizes as thin hairy needles. It is worth mentioning that the crystalline morphology of the samples is of particular importance. We note that the compounds 1a, 2a,b with one TCNQ unit per formula have the same rectangular crystalline shape, whereas the compound 3a with two TCNQ units per formula consists of long needles. Compound 3a is therefore susceptible to present columns with TCNQ stacks contrary to the case of the other compounds. III. Results and Discussion

Figure 2. Overlap of the π orbitals in the dimerized states with side and front views: (a) totally eclipsed dimer, (b-d) slipped dimers.

nitrogen temperature on powders ground in KBr and kept under dynamical vacuum. Subtraction of KBr and cryostat contributions was made with double-beam technique. For Raman spectroscopy, we used a Jobin-Yvon multichannel spectrometer equipped with a cooled charge device detector. The samples were analyzed under 50× and 100× microscope objectives with the λL ) 457.9 nm laser line at a spectral resolution of 5 cm-1. For low-temperature measurements, the samples were introduced in a special cryogenic stage adapted for the Raman microscope and were cooled with flowing He gas. The laser power density on the sample surface did not exceed 15 MW/m2; most of the heat was expected to be dissipated in the bulk since the samples were dark. However, one of the purposes of sample cooling, independent of the physical measurements, was to avoid any degradation. Additionally, the use of the microscope allowed us to check the stability of the samples under laser illumination. Accuracy of the temperature measurement was about 0.5 K in the nitrogen cryostat and less than 0.05 K in the Raman cooling stage. Other Raman experiments were recorded with a Fourier transform Raman spectrometer at ambient temperature using a 50× microscope objective and the 1064 nm excitation line. Under these operating conditions, it was not possible to design low-temperature experiments, however, care was taken to choose

A. Optical Absorption. 1. Monomer States. a. Comparison with TCNQ Charge Transfer Salts. Figure 3 presents the optical spectrum of the 2a complex in the 200-2000 nm range. We observe three main bands near 410, 760, and 850 nm which have been previously described for the monomeric state in LiTCNQ in very dilute aqueous or DMSO solutions.6,7,9-11 In DMSO solutions of [Rh(nbd)(N-N)]TCNQ, only two of these bands were mentioned at 394 and 842 nm.12 Haller et al.10 have examined a series of TCNQ-radical-ion solids structurally characterized by separate stacks of anions and cations (LiTCNQ, CKTCNQ, and TTF-TCNQ) (CKTCNQ ) dicyclohexyl-18-crown-6-complex of potassium TCNQ). To electronically isolate the radical ions from each other, they used dilutions of solids in a nematic solvent and attributed the three bands listed above to TCNQ•-. These experiments realized in a liquid-crystalline phase are in agreement with the results obtained from solutions and with those on the isolated monomeric state solid [FeC* p2](TCNQ) which crystallizes in two different structural forms, the first one with isolated TCNQ monomers, and the second one with isolated TCNQ dimers. X-ray structural data,13 magnetic measurements14 and optical spectra are thus strongly in favor of monomeric isolated species of TCNQ•- in the complex 2a. Our results are presented in Table 1 and compared to the literature. b. Nature of the Electronic Transitions. Lowitz found experimental transitions at approximately 832 nm (1.49 eV), 746 nm (1.66 eV), and 420 nm (2.95 eV) or 395 nm (3.14 eV). This author claimed that it is possible that high-energy theoretical transitions to states of A1u and B1u symmetries are experimentally evidenced in this group, whereas only one lowenergy transition (either at 1.49 or at 1.66 eV) is obtained theoretically. He did not know if both low-energy experimental bands (1.49 and 1.66 eV) stem from TCNQ•- π-π* transitions. In fact, only two transitions among the four have been calculated. Using another calculation method describing the excited states of TCNQ0 and TCNQ•-, Hiroma et al.15 attributed

Molybdenum Complexes of Tetracyanoquinodimethane

J. Phys. Chem. B, Vol. 101, No. 9, 1997 1563

TABLE 1: Optical Spectra of Monomer Species in Various TCNQ Complexes (nm). To Obtain the Values in eV, Divide 1239.8 by the Values in nm isolated stacked 2a isolated isolated LiTCNQc stacked stacked •- b (this work) [FeC* aq RbTCNQ(II)d,e RbTCNQ(II)f p2]- (TCNQ ) solid (TCNQ)a solution solution solid solid 850 760 410 400 320 270

918 765 420 394 331 279

886 775 413

825 737 410

1350 602

1538 620

353

380

a Ref 8, C* ) C Me . This compound presents both monomeric 5 5 p and dimeric structural forms. b Reference 11. c Reference 6. d Reference 7. e Reference 21. f Reference 16.

the first excited state (a doublet above the 2B2g ground state) to the 2B2g-2B3u transition (2B2g-2B1u for Lowitz). The two bands observed by Tanner at 918 nm (1.35 eV) and 765 nm (1.62 eV) and by us at 850 and 760 nm on isolated monomeric solids may be ascribed to transitions toward this excited state. The broad band we observed at 410 nm was already attributed by Hiroma to the superposition of two transitions 2B2g-2Au and 2B -2B at 425 and 394 nm. In addition, Sakata and Hiroma 2g 3u theoretically predicted7,15 another band near 250 nm assigned to the transition polarized along the shortest axis of the TCNQ anion radical. Hiroma describes it as a 2B2g-2Au transition. We experimentally observe this band at 270 nm as in Tanner et al., which means that it is likely not of cationic nature but characteristic of the monomeric state. The optical spectra of Sakata and Shirotani7,16 obtained from nonisolated columnar TCNQ monomers in alkali salts in the solid state, for example in RbTCNQ(II), present significant differences with those recorded in the present work on monomer species. In particular, they observe a low-energy band at 1350 nm (Sakata) and at 1538 nm (Shirotani) attributable to an intermolecular charge transfer (CT) transition. This band is not found in our spectrum. This confirms that our complex contains isolated and noninteracting monomers. Additionally, the two locally excited transitions (LE) observed by Sakata at 602 (LE1) and 353 nm (LE2) do not correspond to the bands observed in our spectrum at 850 and 400-410 nm. However, after theoretical calculations,9,15 these latter bands are also locally excited. In columnar compounds of Sakata and Shirotani there are couplings between CT and LE bands. Such couplings cannot appear in our isolated dimeric compounds since the CT band does not exist. It is therefore understandable that we obtain LE bands shifted in energy with respect to these workers. All these remarks are in agreement with results on (TTF)Cl1.0 of Torrance et al.11 These authors examined the case of solid state dimerized TTF and of solutions in which the TTF are isolated and noninteracting. Therefore, optical spectra point out two situations. In the first one, there is no CT band for the monomeric form. In the second one, there is a CT band for the isolated dimeric form. We deduce that the CT band appears in conjugated π systems as soon as two radical ions begin to stack.

Figure 4. Optical spectra of complex 2b for T ) 293 and 78 K.

c. Comparison of TCNQ0 and TCNQ•- Spectra. We have recorded the optical spectrum of the neutral TCNQ (Figure 3, inset) used to synthesize the molybdenum complex. Instead of the broad band previously observed by Tanner et al.8 at 443 nm (2.8 eV) corresponding to the lowest lying electronic transition, we now obtain two lines at 400 and 432 nm, exactly like those obtained by Pennelly.17 This broad band is assigned to the singlet transition 1A1g-1B3u (1B1u for Hiroma).9,17,18 In the case of a radical TCNQ anion, the single electron occupies an empty orbital of neutral TCNQ with B2g symmetry and participates in the absorption spectrum of the monomer moiety (Figure 3). In this spectrum there is a drastic change of the line shape of the TCNQ0 LE band (400-432 nm) which is now composed of features of 406, 427, and 444 nm. The latter one could be a cationic band. A vibrational structure of that type corresponds exactly to those previously described in Refs 9 and 10 and is in good agreement with the 726, 1191, and 2209 cm-1 TCNQ lines already determined by Raman spectroscopy.5 There is satisfactory correlation between the Raman spectrum (lines at 977, 1067, 1319, 1389, 1449, 1495, 1542, 1568, 1603, and 1613 cm-1, see Ref 5) and the optical transitions, since most of the energy differences measured from optical spectra do correspond with a good accuracy to these lines. In conclusion, when we analyze our spectra in terms of isolated anionic entities, we find strong conformity with previous workers. Some shoulders might be also attributed to the cationic spectrum given in Nieuwpoort’s thesis (709, 617, 532, 444, and 271 nm); in fact, we performed optical spectra on powders of [Mo(dtc)4]PF6 to identify the spectrum of cations and we concluded that only the bands at 532, 444, and 396 nm may come from cations.12,19 It is important to notice that our monomer species appear to be completely isolated as in [FeC*p2](TCNQ). This arrangement contrasts strongly with the situation found in other compounds like RbTCNQ(II).7,16 2. Slipped Dimers: Comparison with Charge Transfer Salts. The optical spectra of 2b were recorded first at room temperature (Figure 4). The differences between the monomer and the dimer spectra observed by Tanner et al.8 on the two phases of [FeC*p2](TCNQ) are completely similar to those gathered by us

TABLE 2: Optical Spectra of Dimer Species in Various TCNQ Complexes (nm) isolated slipped

isolated eclipsed

slipped and stacked

a (TCNQ2-)b LiTCNQc LiTCNQd 2b (this work) [FeC* p2](TCNQ) 2 solid solution solution aq solution solid

1a (this work) solid

KTCNQb KTCNQf RbTCNQ(I)d RbTCNQ(I)e LiTCNQf solution solid solid solid solid

866 690 480 390 a

1008 659 477 379

886 653

860 643

909 633

365

365

381

970-850 640 481 384 270

917 615

1176 610

1000 650

1163 621

1220 620

361

360

376

378

364

b c d e f Reference 8, C* p ) C5Me5. Reference 11. Reference 6. Reference 7. Reference 16. Reference 22.

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TABLE 3: Davydov Effect on C and D Bands of Optical Spectra in Various Isolated TCNQ Species (nm) C band (LE1) monomer slipped TCNQ solutionsa [FeC*p2](TCNQ) solidb this work (solid) 2a 2b eclipsed TTF saltsa this work (solid) 1a 2a a

886 918

dimer 653 659

TABLE 4: Energies of Charge Transfer (CT) and Locally Excited (LE) Bands for Simple Salts of TCNQ (nm)

D band (LE2) monomer

dimer

413 420

365 379

RbTCNQ(II) (monomer) RbTCNQ(I) (dimer)a RbTCNQ(II)b RbTCNQ(I)b a

850

B band CT1

C band LE1

D band LE2

1587 1163 1350 1000

615 621 602 650

380 379 353 376

Reference 16. b Reference 7.

410 690

579

a

508

390 435

640-670 850

394 384

408

Reference 11. b Reference 8, C* p ) C5Me5.

between the spectrum of 2b and that of 2a. In LiTCNQ solution where dimers are electronically isolated, a new band is observed for dimeric species at ca. 643 nm (LE1) and there is a downshift toward 365 nm of the LE2 band (or D band) initially at 410 nm in monomer solutions. Additionally, a CT1 band at 860 nm appears, whose value is close to the LE1 band (or C band) observed at 825 nm in the monomer state.6 In our case this band presents a maximum absorption at 866 nm. Our optical spectrum completely matches the ones described by these authors. For this reason, we ascribe it to a dimeric-type TCNQ arrangement. In our compounds and those of Tanner, the dimers are isolated as in the solution state.8,14,20 The spectra of alkali TCNQ salts either in solid state or in solution are less featured than those of the 2b compound. Between 600 and 400 nm we observe numerous lines likely corresponding to intermediate states TCNQd- with 0 < d < 1 whose existence would be corroborated by a background spectrum decreasing slowly beyond 1000 nm. Lines at 530, 470, and 390 nm have been identified as cationic lines from [Mo(dtc)4]PF6 spectrum. It has been shown in two cases8,11 that the low-energy bands at 1008 nm (1.23 eV) in the solid dimeric slipped state and that at 886 nm for TCNQ22 moieties isolated in solutions are CT bands, whereas the ones at 653 and 365 nm (1.9 and 3.4 eV) which appear at 690 and 390 nm in the 2b phase are the intramolecular LE transitions (bands C or LE1, D or LE2). According to these authors, the C-band (LE1) that they observe at 659 or 653 nm might evidence the dimerization state or a local exciton.6,8,11 A possible explanation of the occurrence of this C band and that at 390 nm (the D band of this paper) is given in Ref 11. These intramolecular transitions originate from the LE bands of the monomer state located after this work at 886 nm (1.4 eV) and 413 nm (3.0 eV) which are blue-shifted in the dimer state. This energy upshift results from the Davydov effect, due to a repulsive interaction between the two dipole moments of the dimer entity. To excite an intramolecular transition, it is necessary to give extra energy to the system which leads to a blue shift of about 0.3 eV in the absorption. Table 3 shows the relevance of the Davydov effect in various isolated TCNQ dimer salts and TTF salts with a systematic blue shift of the C and D intramolecular transition bands (LE1 and LE2). The band at about 270 nm could be due either to a LE transition polarized along the shortest axis of the TCNQ anion, as suggested by Sakata7 or a 2B2g-2Au transition.15 Recent susceptibility measurement using a SQUID magnetometer indicate a possible structural phase transition in 2b near 250 K.13,14 From the structural analysis we believe the stacking of TCNQ dimers modifies from slipped to eclipsed state. We

Figure 5. Optical spectrum of complex 1a (T ) 293 K).

then decided to perform optical measurements at low temperature to gain further information on these structural modifications. At 78 K, the C band of the dimer state (in this case at 690 nm) is enhanced, possibly indicating that the system goes from a slipped to a more eclipsed dimer state. In addition, a new band appears at 924 nm and broadens the CT1 band as in 1a, which is in a perfect eclipsed dimer state (see below). We note that, in the particular case of stacked columnar dimers, such as RbTCNQ(I), where TCNQ moieties are slipped and nonisolated in solid state, the transition from the monomer to the dimer state does not strictly obey to the blue shiftDavydov rule illustrated in the previous table. On the contrary, Table 4 shows that the LE bands are preferentially red-shifted in going from the monomer state to the dimer state in these salts. This red shift is attributed to a coupling between the CT band and the LE bands. This is a property of slipped TCNQ dimers11 stacked in columns. In the slipped case, the dipolar transition moment of the intermolecular charge transfer (CT) band is not strictly perpendicular to the plane of the molecule and has a component oriented along one axis of the TCNQ molecules. This parallel component can couple with an intramolecular transition dipole moment oriented along the molecular axis (LE band). The consequence for stacked columnar TCNQ is a strong mixing or coupling between the CT excitations and the intramolecular ones. The difference between the optical spectra of columnar stacked dimers and isolated dimers is therefore explained. This fact makes ascribing the band at 920-860 nm difficult. Furthermore, for the monomer-TCNQ state of our study (2a), the LE1 band (850 nm) is almost at the same energy as that of the CT1 band of the TCNQ dimer state (band at 866 nm). This ambiguity is the reason why some authors23,24 have assigned this band to an intramolecular excitation and not to an intermolecular one. 3. Eclipsed dimers. The optical spectrum at room temperature of the 1a complex is shown in Figure 5. The main features of the dimeric state are present in the spectrum of this compound. The C band (LE1) corresponding to the energy of the local exciton in 2b at 690 nm is present in 1a at 640 nm. In 2b the band at 866 nm is, however, not so broad as the corresponding one from 1a. The D band (LE2) appears at 384 nm. It is to be noticed that the band at 767 nm observed both

Molybdenum Complexes of Tetracyanoquinodimethane

J. Phys. Chem. B, Vol. 101, No. 9, 1997 1565 877 and 961 nm characterize stacked tetramers TCNQ0/ TCNQ•-/TCNQ•-/TCNQ0 with a wide gap between the TCNQ0’s.25 If these tetramers would be isolated in the 3a structure, we would notice the presence of an excitonic transition at 1.9 eV similar to that isolated for dimeric phases. This band is not found in our spectrum. We thus claim that 3a should belong to the stacked tetramer forms. 5. Intermolecular Charge Transfer Energy in Dimer and Tetramer States. According to Refs 1, 8, and 26-28, the charge transfer band energy of a dimer is given by

Figure 6. Optical spectra of complex 3a for T ) 293 and 78 K.

in the monomer 2a and in the 2b dimer spectra is also present in the 1a spectrum as a shoulder. However, it could be masked in compounds, such as 1a, where the optical response is dominated by the broad bands peaked at 640 and 860 nm. The main difference between 2b and 1a materials, which are both isolated dimeric phases in solid state, lies in the fact that in 2b the TCNQ anions are slipped as in RbTCNQ(I) and as in the Tanner complex, but are totally eclipsed in 1a. The excitonic LE1 band appears at 690 nm in 2b instead of 647 nm for the eclipsed dimers. This energy shift comes from a coupling between the CT band and the LE1 bands, which establishes the presence of slipped dimers in the 2b compound. 4. Tetramer State. a. Preliminary Remarks. The Raman spectrum of complex 3a characterizes the existence of TCNQ0 and TCNQ•- but does not correspond to a simple superimposition of the separate spectra of TCNQ0 and TCNQ•-, this spectrum is much more featured.5 Magnetic and electron spin resonance studies show paramagnetic behavior with a single electron per [Mo(dtc)4]TCNQ2 molecule. This electron occupies the d1 Mo level of [Mo(dtc)4]+.14 The optical spectra at room temperature and 78 K are plotted in Figure 6. These spectra do not correspond to either the 2a or the 2b spectrum. However, there are similarities with both of them. There is no excitoniclike band at 1.9 eV, which is characteristic of the isolated dimer state. As for the 2a and 2b compounds, some bands (532, 440 nm) could be also attributed to the cation. The peculiarities described above are found either at low or room temperature, which means that no dimer-monomer structural phase transition occurs at low temperature as often noticed in TCNQ salts. The band near 1010 nm is enhanced at 78 K. b. Charge Transfer Lines. The band in the 300-400 nm region is broad with a maximum at 350 nm. This feature is attributable to both the first excited 1B3u state of TCNQ0 and the 2B1u state of TCNQ•-.9,15 There is a noticeable shoulder at 410 nm in the spectrum taken at 293 K which is enhanced at 78 K. The band at 350 nm would be the signature of spatially stacked TCNQ0 and TCNQ•-.15,23 If we assume this stacking order, we should observe a CT band (labeled CT2 by Sakata and Hiroma) corresponding to a charge transfer between TCNQ0 and TCNQ•- (the notation CT1 is for the charge transfer between TCNQ•- and TCNQ•-). After Hiroma’s calculations,15 CT2 should occur at lower energy than CT1. The CT1 band of dimers in 1a and 2b is located at 860 and 866 nm. We may thus assume that the 1010 nm feature in 3a is a CT2 band. This corroborates the existence of a TCNQ0/TCNQ•- stacking order. In the above paragraph on slipped dimers, we pointed out the difficulty in assigning the 850 nm band either to a CT or a LE transition. This obstacle is really specific to slipped TCNQ dimers. We experience the same problem for the complex 3a. It turns out that the 850 nm band may stem from a CT1 band alone or a CT1 band coupled with a LE1 band. This conclusion is reinforced by the case of NPQ(TCNQ)2 where two bands at

E ) 1/2(U + (U2 + 16t2)1/2)

(1)

where t is the transfer integral between the two molecules of the dimer, and U is the on-site Coulomb repulsion energy. t is related to the bandwidth, which is mainly governed by the overlap between the two adjacent molecules. For [FeC*p2](TCNQ) (with slipped isolated dimers), Tanner et al.8 have measured U and t (1.0 and 0.27 eV, respectively). Many authors8,26 think that the approximation U ≈ 4t is valid in TCNQ dimers, in such a case

E ≈ 2t(1 + x2) ≈ 1.2U

(2)

In complex 1a, the charge transfer line is peaked at 860 nm, whereas in compound 2b (the methyl analog of 2a) we have identified this band at 866 nm. Using our optical data, we find that U ≈ 1.2 eV in 1a and 2b. In the case of 3a, the CT band is located at 1010 nm, which gives U ≈ 1.0 eV if U ≈ 4t is also valid in TCNQ tetramers. Extended Hu¨ckel calculations on TTF trimers yield bandwidths of 0.99 and 0.20 eV for the eclipsed and slipped geometries, respectively, 0.5 eV for D2h dimers, and 0.12 eV for C2h dimers.28 On the contrary, we find that the t values in 2b (isolated and slipped dimers) and in 1a (isolated and eclipsed dimers) are similar (≈ 0.3 eV). Stacked TTF moieties in salts and isolated dimers in 2b and 1a complexes account for these differences. B. Contribution of Raman Spectra to the Determination of TCNQ Electronic States. 1. Group Theory Analysis. Raman spectroscopy is one of the techniques which allows to directly evaluate the charge transfer in molecular salts of TTF, TCNQ, and BEDT-TTF. The charge transfer in the TCNQ system was found to be mostly determined by the frequency variation of the CdC ring bond at 1453 cm-1 in TCNQ0, which is downshifted to 1390 cm-1 for one electron transferred in TCNQ•-.5 If we restrict the discussion to the vibrations of the carboncarbon bonds of the TCNQ units (eight carbons per unit) we may write the vibrational representations of monomers, eclipsed and slipped dimers taken in their free states as

Γm ) 4Ag + B1g + 4B2g + 3B3g + Au + 3B1u + 2B2u + 3B3u Γe ) 7Ag + 5B1g + 5B2g + 7B3g + 5Au + 6B1u + 6B2u + 4B3u Γs ) 14Ag + 10Bg + 9Au + 12Bu Γv ) 24Ag + 21Au where m, e, s, and v refer, respectively, to the monomer, to the eclipsed dimer (D2h point group), to the slipped dimer along the long or short in-plane axis (z or x) of the TCNQ molecules (C2h), and to the slipped dimer along a direction parallel to the

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Figure 7. Fourier transform micro-Raman spectra of different TCNQ compounds: (a) TCNQ, (b) complex 2a, (c) complex 3a, (d) complex 1a, (e) complex 2b.

plane (Ci). The modes corresponding to the rotations along x, y, and z axes of the molecules have been included. In the solid, a factor-group analysis is necessary. Compounds 2a, 2b, and 1a crystallize in monoclinic and triclinic systems with respective symmetry C2/c, P21/c and P-1, corresponding to point groups C2h and Ci. In 2b, the distance between the TCNQ planes is 3.17 Å, similar to that between the TCNQ planes of the compound [FeC* p2](TCNQ), and the lateral gliding separation is 0.25 Å. In 1a, the intradimer separation is 3.28 Å less than the van der Waals spacing (3.5 Å). There is one dimer per unit cell, and the site group of the dimer is C1. For the complex 2a, the site group of cations and anions in the unit cell may be approximated to C2. From the correlation tables, it turns out that the internal modes belong to A or B symmetries since in the crystal the TCNQ molecules or dimers lie either at general sites or transform themselves through a binary axis. They may be activated both in IR (in-phase Au) and Raman spectra (outof-phase Ag). There are fewer Raman lines than expected from the symmetry analysis. This may be explained by the fact that van der Waals or ionic interactions have little influence on the vibrational modes of the isolated molecules. Covalent interactions and planar motions will contribute for the most part to the spectrum. 2. Raman Intensity. The Raman effect in organic molecules involves mainly the interaction of light with the π electrons which are strongly polarizable. Resonant Raman scattering (RRS) occurs when the energy of the exciting light falls at the edge or within an absorption band. In most cases of this study, one can consider that the 457.9 nm laser line fulfills this requirement, and again the 1064 nm line to a lesser extent, since the absorption of the TCNQ complexes is still important at this wavelength. The RRS intensity is proportional to

(RF,σ)ij|2 ∑ F,σ

(E - Eji)4|

(3)

where R is the polarisability tensor, (F,σ) stand for (x, y, and z), E is the energy of the exciting line, and Eji is the energy difference between initial and final vibronic states. In the Albrecht theory,29 R may be expressed as

R)A+B

(4)

Both A and B terms include the factors 〈e|Mσ|g〉, 〈g|M′F|e〉, 〈n|m〉, and 〈m|Q|m〉 which account for the transitions between the ground state g and an excited state e Via the electronic dipole moment operators Mσ and M′F, and for the contributions of the vibrational states n and m on the excited electronic states (Q ) bij + b†ij stands for the destruction-creation operator of the vibrational states). The transfer integral 〈n|m〉 expresses the overlap of the vibrational wavefunctions (Franck-Condon factor). The electronic-molecular-vibrational (EMV) coupling contributes to the B term in the factors

〈| |〉 e

∂He s ∂Qa

(5)

where He is the electronic Hamiltonian, Qa is a normal coordinate of the nucleus for the vibrational mode a taken at the equilibrium position e and s are excited electronic states. One could expect that the intermolecular overlap of the total electronic wavefunctions, for the Ag-bonding orbitals, follows the same behavior in eclipsed TCNQ and TTF dimers on the one hand and in slipper dimers on the other.28 In the latter case, the intermolecular overlap is much smaller. This might lead to a higher transfer integral in eclipsed dimers and to a more significant contribution of the Franck-Condon factor to the intensities. Additionally, the EMV coupling should be more important in an eclipsed dimer. Thus, one can assume that the intensity of the Ag Raman modes of the eclipsed dimer will be enhanced with respect to that of the slipped dimer. 3. Experimental Raman Spectra. In principle, there are 12 and 24 Raman active vibrations of CC bonds for monomers and dimers, respectively. This is more than observed in the experimental spectra of TCNQ, 1a, 2a, 2b, and 3a, which are presented in Figure 7, with a spectral resolution of 4 cm-1. The spectral range of interest, involving the C-C and CdC vibrations, lies between 1000 and 1700 cm-1. Their assignment has been described in Ref 5. The small number of observed modes may indicate that we only see the totally symmetric Ag modes. Table 5 lists the Raman frequencies found in Fourier transform (FT) Raman spectra of all compounds using the 1064 nm excitation line. The Raman bands appear to be close in

Molybdenum Complexes of Tetracyanoquinodimethane

J. Phys. Chem. B, Vol. 101, No. 9, 1997 1567

TABLE 5: Raman Frequencies (cm-1) of TCNQ and TCNQ Complexes of Molybdenum with λ ) 1.064 nm

a

TCNQ

2a

3a

1a

2b

2ba

2bb

102 mc 143 m 154 m 167 m 305 w 333 m 426 w 591 m 601 w 709 m 738 vw 751 m 946 w 1000 vw 1185 m 1205 s 1250 vw 1312 w 1320 w 1453 vs 1600 s 1654 w 1724 vw 2174 vw 2189 w 2224 vs 3027 vw 3044 w 3065 vw 3088 w

192 vwd 222 vw 335 m 410 vw 459 w 611 w 725 vw 806 vw 976 w 1052 w 1195 s 1276 vw 1317 vw 1335 vw 1389 vs 1416 vw 1531 w 1600 vs 1612 vs 1670 vw 1700 w 2155 w 2192 m 2583 w 2630 vw 2772 w 2802 w 2997 w 3059 vw 3212 w

225 w 334 m 610 w 976 w 1051 vw 1164 vw 1194 s 1343 vw 1387 vs 1418 vw 1568 vw 1602 vs 1610 vs 1625 w 1708 w 1920 w 2100 w 2188 m 2704 vw 2765 vw 2981 vw

554 vw 823 vw 1007 w 1176 w 1290 m 1336 s 1359 w 1440 m 1459 s 1490 vw 1562 vw 1593 m 1707 s 1778 w 2790 w 3156 m 3256 s 3365 w

332 w 464 w 616 w 1197 m 1203 vw 1387 vs 1607 vs 1620 1679 w 2186 m 2508 w 2758 w

41 se 142 s 222 w 277 w 349 w 367 w 476 w 500 w 536 m 687 m 724 m 857 m 923 w 977 s 1031 w 1182 w 1200 s 1910 w 1273 w 1340 m 1388 s 1450 w 1501 w 1538 w 1566 m 1606 s 1622 w 2082 w 2124 w 2182 w

720 m 727 m 923 w 960 m 980 m 1004 w 1053 w 1175 m 1200 m 1280 m 1327 m 1354 w 1388 m 1401 w 1453 w 1501 m 1528 w 1570 w 1608 vs 1624 vs 1705 w 1875 w 1928 w 1988 w 2100 w 2183 w 2210 s

T ) 293 K, λ ) 457.9 nm. b T ) 50 K, λ ) 457.9 nm. c m ) medium. d w ) weak. e s ) strong.

frequency and lineshape to those obtained using the 457.9 nm excitation line. Complex 1a was found to be fragile and exhibits a strong fluorescence background in the near infrared, removed in the plot, whereas the other compounds lead to intense spectra whose differences from the visible excitation spectra from Ref 5 are attributed to different resonance phenomena. Indeed, optical spectroscopy shows that in the near infrared region we expect a smaller Raman resonance with the electronic transitions than that in the blue region. A striking feature of these spectra is a strong energy shift of the line at 1453 cm-1 in pristine TCNQ toward 1389 cm-1 in the complexes. This line originates from pure CdC ring stretching. The shift may be explained by the ionization of the TCNQ species (TCNQ•-) due to the charge transfer in the salts. For all complexes, the line shape of the band close to 1600 cm-1 is broad, sometimes featured with two to three components close to 1600, 1612, and 1624 cm-1 (L1, L2, and L3) in contrast to the parent single line of TCNQ and LiTCNQ which appear at respectively 1600 (L1) and 1609 cm-1 (L2). It has been shown that this band stems from stretchings involving C-C ring and C-C(CN)2 motions. The FT-Raman spectrum of the 2a phase reveals lines at 1052, 1195, 1317, 1335, 1389, 1416, 1531, 1600, and 1612 cm-1. Since the 2a phase has only monomer moieties as shown by X-ray structural studies, one has to conclude that its Raman spectrum signalizes monomer species, in particular the salient features at 1195, 1600, and 1612 cm-1 are intrinsic to the monomer state. This FT-Raman spectrum resembles that of 3a; however, there are two additional weak components at 1569 (L0) and 1625 cm-1 (L4), and the intensity of the 1600 cm-1 line is stronger. The FT spectrum of the phase 1a exhibits lines at 1460 and 1594 cm-1 assigned to the TCNQ0 state, while the very weak lines at 1195, 1385, and 1609 cm-1, strongly resonant with the 514.5 nm excitation line, have been attributed to the presence of TCNQ•- moieties.

Figure 8. Raman spectra of complex 2b: (a) T ) 50 K, (b) T ) 293 K.

We now focus on the Raman and optical study of the methyl analog compound 2b, [MoV(dtc)4](TCNQ). Raman measurement on 2b for the 457.9 nm excitation at ambient and low temperature (50 K) (Figure 8) reveal the lines summarized in Table 5. At 293 K, the spectra taken for the visible and near infrared excitation lines are very similar. A very small shoulder (L3) occurs near 1624 cm-1 at the edge of the 1607 cm-1 line (L2); it is more prominent when using the 457.9 nm excitation. For T ) 50 K, the intensity of L3 is strongly enhanced as well as that of several other Raman bands at 960, 1004, 1175, 1280, 1501, and 1928 cm-1. These Raman intensity changes Vs temperature are quite strong and should be correlated to optical absorption changes between 293 and 78 K. At low temperature, the components of the L2 - L3 doublet have nearly the same intensity and are likely the signature of a fully dimerized state. As discussed above, the

1568 J. Phys. Chem. B, Vol. 101, No. 9, 1997 weak L3 component at room temperature may characterize a slipped dimer state because of the smaller overlap of the π orbitals (see Figure 2) which is likely to yield a smaller EMV coupling and, consequently, modes with reduced intensities. If a phase transition from slipped to eclipsed dimers would occur at temperatures down to 50 K, there would be a maximum π orbital overlap with an increase of the EMV coupling and enhancement of specific Raman modes. Furthermore, we note that at 78 K the optical spectrum shows an important vibronic progression between 400 and 600 nm. Some of the features correspond in the limits of the experimental peaking errors to the low temperature Raman lines at 1175, 1280, 1608, 1624, 1928, and 2210 cm-1. The Raman spectrum of the 3a compound, taken also for the Ar+ 457.9 nm line, has been described in Ref 5. It exhibits thin lines at 2209, 2217, 2232, and 2265 cm-1 for the CtN bands, as well as 1603, 1610, 1627, and 1632 cm-1 and 1150, 1172, 1194, and 1202 cm-1 for the CdC bands. This observation is in agreement with the paper of Yartsev on tetramers of (TEA(TCNQ)2),30 which pointed out the possibility of splitted experimental lines in reflectance spectra. These quadruplets may be ascribed to tetramers if we keep in mind that the vibrational representation of tetramer benzenoid rings leads to 48 Raman modes instead of 24 in the dimers:

Γt ) 14Ag + 10B1g + 10B2g + 14B3g + 10Au + 13B1u + 13B2u + 9B3u IV. Conclusion Solid salts of molybdenum-TCNQ complexes have proven to be interesting systems for the study of electronic properties in organic charge transfer salts. Optical and Raman spectra have provided valuable information on this class of compounds for studying intra- and intermolecular charge transfer. Their interesting properties lie in the fact that the series investigated in this paper presents various TCNQ ground states: monomers, slipped dimers, eclipsed dimers, and tetramers. Among these states, we note that the eclipsed dimers have never been found in TCNQ complexes, except in 1a. The materials of this study belong to a class of rare systems where monomer and dimer moieties can exist as isolated entities. For monomer states, we have found that the radical anion is the most stable form. The dimers are ionized and bear a 2- charge, i.e., we have

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