J. Phys. Chem. B 1999, 103, 3545-3551
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XPS Study of Stilbazolium Chromophores and Their Intercalation Compounds in the MnPS3 Layered Phase Danielle Gonbeau* Laboratoire de Physicochimie Mole´ culaire, UMR 5624, UniVersite´ de Pau, 2 aVenue du Pre´ sident Angot, 64000 PAU, France
Thibaud Coradin and Rene Clement* Laboratoire de Chimie Inorganique, URA CNRS 420, UniVersite Paris-Sud, 91405 Orsay Cedex, France ReceiVed: NoVember 5, 1998; In Final Form: February 22, 1999
XPS spectra of a series of iodide salts of stilbazolium-type cations were recorded at the N1s core level. Analysis of the spectra, supported by ab initio calculations, revealed a great sensitivity of the charge distribution on the central part of the chromophores and a strong donor-acceptor intramolecular charge transfer leading to unusually intense shake-up satellite peaks. After intercalation in the MnPS3 layered phase, the XPS spectra of the chromophores showed a perturbation of their electronic distribution, this effect being especially significant for those containing a central imine double bond. These results are consistent with previous studies concerning the formation of stilbazolium J-aggregates in the interlamellar space of the host lattice.
1. Introduction In recent years, intercalation chemistry has seen a rapid development and found application in a wide variety of fields, from catalysis to electrochemical batteries.1-3 Of particular interest is the possibility of obtaining new materials exhibiting optical properties by insertion of photoactive species in transparent layered lattices;4 in this case, the latter not only protects the active species from photochemical or oxidation decomposition but may also modify its properties through host-guest interactions. In this context, the ability of the hexathiohypodiphosphate MPS35 layered compounds (where M is a metal in the +2 oxidation state) to insert reversibly cationic species,6 as well as their optical transparencies when M ) Cd, Mn, and Zn, makes them good candidates for the achievement of photoactive hybrid materials. Some of us have previously reported that the intercalation of a series of stilbazolium chromophores into MnPS3 could give rise to nonlinear optically (NLO) active materials.7 However, these properties were strongly dependent on the structure of the inserted dyes and, on the basis of UV-visible absorption studies,8 a model invoking the formation of J-aggregates9 in the interlamellar space of the host lattice has been proposed. As a matter of fact, stilbazolium derivatives have been extensively studied as typical one-dimensional charge transfer chromophores for second harmonic generation (SHG)10-12 and the effect of aggregation on their optical properties has been recently reported.13 Moreover, these chromophores have been intercalated in various other layered lattices,14,15 highlighting the importance of both guest-guest and host-guest interactions. To get a better understanding of the electronic characteristics of stilbazolium chromophores and to investigate the perturbation induced by the intercalation process, we have undertaken a XPS (X-ray photoelectron spectroscopy) study of both the chromophore salts and their MnPS3 intercalates. We report here the results of these investigations, and an interpretation, supported by ab initio calculations, is proposed that not only appears
consistent with previous studies on these materials but also brings new light on the aggregation phenomenon. 2. Materials and Methods Synthesis. The synthesis of the iodide salts of the stilbazolium used in this work (Scheme 1) and their intercalation in MnPS3 has already been reported elsewhere.8 It should be emphasized that the chromophore intercalates are synthesized via a cation exchange process between MnPS3 and the iodide salts in the absence of any redox process. The general formulation of the intercalates is therefore Mn1-xPS3(Chr)2x, where (Chr) stands for a monovalent chromophore cation.. XPS Measurements. The XPS analyses were performed with a Surface Science Instruments spectrometer (model 301), using focused monochromatized Al KR radiation (1486.6 eV). The diameter of the irradiated area was 600 µm, and the residual pressure inside the analysis chamber was in the range of 5 × 10-8 Pa. The spectrometer was calibrated using the photoemission lines of gold (Au 4f7/2, 83.9 eV with reference to the Fermi level) and copper (Cu 2p3/2, 932.5 eV). For the Au 4f7/2 line, the full width at half-maximum (fwhm) was 0.86 eV under the recording conditions; the peaks were recorded with a constant pass energy of 50 eV. The samples were examined as pressed powder pellets; they were fixed onto the sample holder in a glovebox (O2 and H2O levels below 2 and 7 ppm, respectively) attached directly to the introduction chamber of the spectrometer; thus, for all the compounds the oxygen (pollution) has been determined in the range 5-8 at. %. The charging effects were minimized with a low-energy flood gun. As the C 1s peak appears quite broad, the calibration was carried out with the I 3d5/2 line at 618.2 eV for all the stilbazolium chromophores studied; the detection of iodide in very low proportions (e0.5 at. %) in the case of MnPS3 intercalates allows us to use the same calibration for these compounds.
10.1021/jp9843196 CCC: $18.00 © 1999 American Chemical Society Published on Web 04/15/1999
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SCHEME 1: Formula and Abbreviated Names for the Series of Stilbazolium Chromophores Used in This Work
The XPS signals were analyzed using a peak synthesis program in which a nonlinear background16 is assumed and the peak fitting of the experimental curve is defined using a combination of Gaussian (80%) and Lorentzian (20%) distributions. Methods of Calculation. Ab initio calculations were performed with the Gaussian 94 package of programs.17 The optimization of the main geometrical parameters has been done at the 6-31G* level18 and the density matrix analyzed at the MP2/6-31G* level. 3. Results and Discussion XPS Study of the Stilbazolium Salts. The XPS spectra obtained for the stilbazolium chromophores at the N 1s core level are shown in Figure 1 and the results of the analysis are gathered in Table 1. DAMS+I-. The experimental spectrum of DAMS+I- (Figure 1a) has been analyzed as the sum of four peaks. On the basis of previously reported values,19,20 the two main 2p3/2 components at 399.4 and 401.1 eV could be readily attributed to the dimethylamino and pyridinium groups, respectively. Since the position and intensity of the two other peaks appeared to be insensitive to experimental conditions (sample, exposure time), they were identified as genuine satellite peaks rather than as parasite features due to impurity or radiation damage. Satellite peaks are usually expected at high energy but with low intensities as compared to main peaks. This is actually the case for the 404.5 eV weak peak, but the intensity of the 402.6 eV component is surprisingly large. Nevertheless, similar unusual features have been previously reported for a small number of highly polar donor-acceptor molecules.21 For instance, in the case of p-nitroaniline, an interpretation based on a “shake-up” process involving a (π f π*) charge transfer from the amino to the nitro group has been proposed21a and confirmed by ab initio calculations.22 In this frame, the two peaks at 401.1 and 402.6 eV can be both associated with the ionization of the pyridinium nitrogen of the DAMS+ chromophore; the occurrence of two energetically close final ionized state (ground and excited) would therefore be attributed to the stabilization of the excited ionized state by an effective charge transfer from the donor to the acceptor group. It is to be noted that the sum of the relative percentages for these two components (50.8%) is consistent with the expected intensity value for the pyridinium nitrogen (50%). It is also worth mentioning that the same results
Figure 1. XPS spectra obtained at the N 1s core level for (a) DAMS+I-, (b) RCl+I-, (c) DAZOP+I-, (d) IM1+I-, and (e) IM2+I-.
were obtained with the DAMS+p-Tos- salt (p-Tos- ) paratoluenesulfonate), thus underscoring the absence of any influence of the counteranion on the binding energies of the core levels of the DAMS+ chromophores.
XPS Study of Stilbazolium Chromophores
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TABLE 1: XPS Data for Stilbazolium Saltsa DAMS+I399.4 [47%] (1.4) 401.1 [29.5%] (1.4) 402.6 [21.3%] (1.5)
RCl+I-
DAZOP+I-
IM1+I-
IM2+I-
401.4 [85.4%] (1.5)
399.5 [60.1%] (1.6) 401.4 [24.7%] (1.5) 402.7 [12.7%] (1.7)
398 [21%] (1.4) 399.1 [35%] (1.4) 400.8 [36%] (1.5) 402.4 [4.2%] (1.6)
399.2 [67%] (1.5) 401.2 [21%] (1.5) 402.6 [12%] (1.7)
404.8 [2.5%] (1.6)
405.3 [3.8%] (1.9)
403.6 [14.6%] (1.8) 404.5 [2.2%] (1.8) a
Binding energies are given in electron volts; parentheses indicates fwhm values, and brackets show relative percentages.
TABLE 2: Geometrical Parameters Optimized at the 6-31G* Levela d12
d16
d47
d78
d89
d1215
2-1-6
4-7-8
7-8-9
11-12-13
1.35
1.38
1.43
1.35
1.44
1.34
120.5
125
127.9
118
1.35
1.38
1.37
1.26
1.34
1.33
120.7
113.2
118.
118
1.36
1.37
1.35
1.30
1.42
1.34
120.4
122.4
122.9
118.1
1.35
1.38
1.45
1.27
1.37
1.34
120.6
119.3
126.2
118.1
+
DAMS AdB:CdC DAZOP+ AdB:NdN IM1+ AdB:NdC IM2+ AdB:CdN
Bond lengths in angstroms, bond angles in degrees. d23 ) d56 ) 1.39 Å; d34 ) 1.43 Å; d45 ) 1.42 Å; d910 ) d914 ) 1.43 Å; d1011 ) d1314 ) 1.38 Å; d1112 ) 1.43 Å; d1213 ) 1.42 Å. a
RCl+I-. The two components observed in the spectrum of RCl+I- (Figure 1b), which possess only one type of (pyridinium) nitrogen atom, can be straightforwardly attributed to the main peak (401.4 eV) and the associated “shake-up” satellite (403.6 eV) of the pyridinium nitrogen. When compared to DAMS+, the lower intensity of the satellite band appears consistent with a less efficient charge transfer due to the weaker donor character of the chlorine substituant. DAZOP+I-. Although the DAZOP+ cation contains four nitrogen atoms, the spectrum of the DAZOP+I- salt (Figure 1c) could be fitted with four components at binding energies very close to those of DAMS+I-. However, although it seems reasonable to assign the 399.5 eV peak to the dimethylamino group and the 401.4 and 402.7 eV peaks to the pyridinium one, the relative intensities (60% and 37.4%) of these bands suggest that they also correspond to the ionization of the nitrogen atoms of the central NdN double bond. A more precise attribution is difficult. IM1+I-. In the case of IM1+I- (Figure 1d), the first component observed at 398.0 eV was attributed to the ionization of the nitrogen of the imine CdN central bond, a value consistent with data from the litterature19b and also with the absence of any peak in this energy domain for the other compounds studied in this work. The attribution of the 399.1 eV (dimethylamino group) and 400.8 and 402.4 eV (pyridinium group) peaks is similar to the DAMS+ and is supported by the value of the relative intensities (≈33%). Finally, the relative intensity and position of the 405.3 eV component suggests that this feature is associated with a “shake-up” satellite of the imine group. IM2+I-. In sharp contrast, the IM2+I- spectrum (Figure 1e) has been decomposed in only three bands. Because of its high relative intensity, the component at 399.2 eV probably corresponds to the unresolved dimethylamino group and the central imine bond, the two other peaks being once again attributed to the pyridinium group. This value suggests that the nitrogen atom of the CdN unit bears a lower negative charge in IM2+I- than in IM1+I-. Ab Initio Calculations on the Stilbazolium Chromphores. To gain further insight into the electronic structure of the stilbazolium salts, we have undertaken ab initio calculations with optimization of geometrical parameters. Taking into account the large size of the chromophores, calculations have been restricted
Figure 2. Visualization of the π frontier orbitals for the stilbazolium derivatives.
to the four cations DAMS+, DAZOP+, IM1+, and IM2+ using R1 ) H and R2 ) NH2 and keeping C-H and N-H bond lengths constant. The most significant geometrical parameters obtained after optimization (6-31G* level) are gathered in Table 2. The bond lengths around the pyridinium (d12, d16) and the amino (d1215) nitrogens appeared rather similar for the whole series of chromophores. In sharp contrast, the central parts of the molecules were found to be very sensitive to the nature of the double bond. A slight dissymmetry in the NdN bond was observed for the DAZOP+ system (4-7-8 ) 113.2°, 7-89)118°). As far as the two imines are concerned, they exhibited strong differences both in bond lengths and angles (IM1+: d78 ) 1.30 Å, 4-7-8 ) 122.4°. IM2+: d78 ) 1.27 Å, 4-7-8 ) 119.3°). Moreover, it is worth noticing that the central part of IM1+ is rather symmetrical (4-7-8 ) 122.4°, 7-8-9 ) 122.9°) in sharp contrast to IM2+ (4-7-8 ) 119.3°, 7-8-9 ) 126.2°). In a second step, a Mulliken population analysis was performed (MP2/6-31G* level) (Table 3). For the whole series, similar net charges on pyridinium and amine nitrogens were observed, the former exhibiting a less negative net total charge and a greater positive π net charge than the latter. Since the binding energies of the core electrons increase with the positive character of the probed atom, these results are consistent with our previous attributions for XPS peaks. If we now consider the charge distribution on the CdN central bond of IM1+ and IM2+, some differences were observed for net total charges (IM1+: qNtotal ) -0.50, qCtotal ) +0.15. IM2+: qNtotal ) -0.35,
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TABLE 3: Mulliken Population Analyses at the MP2/ 6-31G* Level (a) net total charges q1 DAMS+ AdB:CdC DAZOP+ AdB:NdN IM1+ AdB:NdC IM2+ AdB:CdN
q4
q7
q8
q9
q12
q15
-0.51 +0.04 -0.13 -0.08 -0.02 +0.27 -0.63 -0.50 +0.24 -0.25 -0.13 +0.10 +0.29 -0.62 -0.51 +0.26 -0.50 +0.15 -0.06 +0.27 -0.62 -0.49 -0.02 +0.08 -0.35 +0.10 +0.27 -0.63 (b) net Π charges q1
DAMS+ AdB:CdC DAZOP+ AdB:NdN IM1+ AdB:NdC IM2+ A)B: CdN
q4
q7
q8
q9
q12
q15
+0.52 +0.09 -0.06 +0.08 -0.05 +0.08 +0.28 +0.54 +0.08 -0.1
+0.19 -0.05 +0.10 +0.3
+0.53 +0.10 -0.21 +0.13 -0.04 +0.10 +0.31 +0.55 +0.08
0.01 +0.12 -0.02 +0.09 +0.29
qCtotal ) +0.08), but the values obtained for net π charges revealed much more significant changes (IM1+: qNπ ) -0.21, qCπ ) +0.13. IM2+: qNπ ) +0.12, qCπ ) -0.01). At this point, it is important to consider the following results. In a previous study, one of us has reported an XPS study of N,N′-diphenyl guanidine, a model molecule containing both amine and imine nitrogens.19b Its N 1s spectrum displayed two main peaks at 399.1 and 398.0 eV with a 2:1 intensity ratio which were therefore easily attributed to the amine and the imine nitrogen atoms, respectively. A Mulliken population analysis from ab initio calculations (MP2/6-31G*) showed us the following charges for the CdN bond (qNtotal ) -0.50, qCtotal ) +0.6, qNπ ) -0.28, qCπ ) +0.09) and amine nitrogen (qNtotal ) -0.67, qNπ ) +0.32). It must be pointed out that these results revealed the limits of the correlation between the binding energy and the net total charge; following this relationship, the higher binding energy of an amine nitrogen should correspond to a more positive nitrogen atom. In fact, several factors must be taken into account, especially the influence of the charge distribution on the atoms surrounding the ionized site. This effect appears better expressed by the variations of net π charges which reflect the electronic delocalization on the whole system, the positive and negative π net charges respectively obtained for amine and imine nitrogens being then consistent with the experimental results. In this context, the net π charge values obtained for the two imine chromophores, together with the calculated geometrical parameters reported above, reveal that the central double bond of IM1+ seems to retain the electronic characteristics of an imine group while IM2+ appears more perturbated by the charge transfer. This conclusion allows us to understand the difference observed in the N 1s core spectrum of those two molecules. Furthermore, the evolution of binding energies for the three nitrogens of IM1+I- (398, 399.1, 400.8 eV) is consistent with the changes observed in the π net charges (-0.21, +0.31, +0.53). The same trend exists for IM2+I- and DAZOP+I- and strengthens the XPS assignments. Finally, the study of the π frontier orbitals revealed a similar charge distribution for the four chromophores. As illustrated in Figure 2, the HOMO is mainly localized on the donor group and the LUMO on the acceptor part. It is therefore possible to understand why the multielectron process, involving the π f
Figure 3. XPS spectra for the MnPS3 host lattice at the Mn 2p, P 2p, and S 2p levels.
TABLE 4: XPS Results for the MnPS3 Host Lattice and MnPS3/Dams+ at the Mn2p, P2p, and S2p levelsa peak
Mn PS3
Mn PS3/DAMS+
Mn2p3/2-1/2
641.2 (2.8)-652.8 (2.7) 645.4 (3.9)-658 (4) 131.7 (1.2)-132.8 (1.2) 162.2 (1.2)-163.4 (1.2)
640.9 (2.8)-652.6 (2.8) 644.9 (3.9)-657.5 (4.1) 131.4 (1.1)-132.5 (1.1) 161.6 (1.2)-162.8 (1.2)
P2p3/2-1/2 S2p3/2-1/2 a
binding energies are given in eV; () indicates fwhm values
π* excitation, is more likely to happen when the core hole is located on the acceptor part of the system, i.e., the pyridinium nitrogen since, as for p-nitroaniline, this ionized state will be stabilized by the electron flow due to the charge transfer, leading to the observed shake-up peak at the N1s level. Moreover, each of the two atoms of the central double bond also appeared to contribute to a different orbital, a possible origin of the dissymmetry observed in the geometrical and electronic parameters for the four compounds and, especially, the difference between IM1+ and IM2+. XPS Study of the MnPS3 Intercalates. In a preliminary step, we have investigated the possible influence of the intercalation process on the host material; the results obtained at the Mn 2p, P 2p, and S 2p core levels are shown in Figure 3 and gathered in Table 4. In the case of the pure MnPS3 phase, the Mn 2p spectrum (Figure 3a) was decomposed as the sum of a main doublet (641.2-652.8 eV) and a satellite structure of high intensity
XPS Study of Stilbazolium Chromophores
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(645.8-658 eV). It is to be noted that for NiPS3 two satellite structures have been characterized, whereas no similar structure has been observed in the case of ZnPS3.23 The spectra obtained at the S 2p (Figure 3b) and P 2p (Figure 3c) levels were analyzed as doublets with binding energies close to those previously reported for NiPS3.23 The results obtained in the case of MnPS3 intercalates for Mn 2p, S 2p, and P 2p peaks did not seem to depend on the inserted species, the example of MnPS3/DAMS+ being given in Table 4, and were similar to those observed for MnPS3. The systematic shift of about 0.3 eV toward low binding energies is not significant and probably is due to a different calibration scale (C 1s at 284.6 eV for MnPS3 and I 3d5/2 at 618.2 eV for the intercalates). Since our main interest lies in the analysis of the effect of intercalation on the electronic structure of stilbazolium chromophores, we have focused our analysis on the N 1s core peak of the MnPS3 intercalates. Obtained spectra are shown in Figure 4, and the corresponding data are given in Table 5. When compared to DAMS+I-, the shape of the N 1s core peak of MnPS3/DAMS+ showed the same main features, the observed components exhibiting close binding energies and similar relative intensities; the only important difference was an increase in the intensity of the “shake-up” satellite at 404.8 eV. Similar results were obtained for MnPS3/RCl+ and MnPS3/ DAZOP+, the former presenting a more significant increase and a slight binding energy shift of the 403 eV shake up peak. In sharp contrast, IM1+ and IM2+ seemed to undergo more important changes upon intercalation. In the case of MnPS3/ IM1+, the component at 398 eV disappeared and the relative intensity of the 399.2 eV peak increased. For MnPS3/IM2+, a decrease of the component at 399.2 eV and an increase of the peak at 402.4 eV were observed; moreover, a satellite structure appeared at 404.2 eV. In connection with previous assignments for stilbazolium chromophores, these results can be explained in terms of a perturbation of the intramolecular charge transfer. Of particular interest are the striking changes observed for imine chromophores that can be explained by a modification of the π electronic charge on the nitrogen central atom. In the case of IM1+I-, the 398 eV line has been attributed to the ionization of the imine nitrogen; its disappearance after intercalation and the increase of the peak at 399.2 eV can be explained by a decrease of the π negative charge on the imine nitrogen. The same interpretation can be given for the decrease of the intensity of the line at 399.2 eV, attributed to the ionization of the amine and imine nitrogens in IM2+I-, in MnPS3/IM2+. However, the increase of the 402.4 eV line, a pyridinium satellite in IM2+I-, is difficult to explain unless this peak also contains a contribution from the central bond nitrogen. This hypothesis could be supported by the sum of relative intensities of 401.4 and 402.7 eV peaks in DAZOP+I- (37.4%) and its intercalates (38%) that cannot therefore be only attributed to the pyridinium nitrogen. Conclusion The aim of this work was to understand how intercalation in the MnPS3 layered phase could influence the electronic properties of stilbazolium chromophores, a problem related to the reported SHG properties of these materials. The main feature of the XPS study of the iodide salts of these chromophores is the effect of the strong donor-acceptor intramolecular charge transfer that stabilizes an excited state due to the ionization of the pyridinium nitrogen, leading to unusually intense shake-up peaks. However, the binding energies
Figure 4. XPS spectra obtained at the N 1s core level for (a) MnPS3/ DAMS+, (b) MnPS3/RCl+, (c) MnPS3/DAZOP+, (d) MnPS3/IM1+, and (e) MnPS3/IM2+.
of the terminal nitrogen atoms appear rather similar thoughout the series of chromophores, in sharp contrast to those of the central double bond. These results are supported by ab initio
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TABLE 5: XPS Results for Stilbazolium Intercalates at the N1s Levela peak
N1s a
Mn PS3/DAMS+
Mn PS3/RCl+
Mn PS3/DAZOP+
Mn PS3/IM1+
Mn PS3/IM2+
399.2 [47%] (1.5) 400.9 [28%] (1.6) 402.6 [20%] (1.7) 404.8 [5%] (1.8)
401.2 [77%] (1.4) 403.0 [23%] (1.7)
399.1 [57%] (1.7) 401.1 [28%] (1.5) 402.6 [10%] (1.6) 404.8 [5%] (1.7)
399.2 [51%] (1.5) 400.7 [40.5%] (1.6) 402.1 [5.5%] (1.6) 405.3 [3%] (1.6)
399.0 [53%] (1.6) 399.2 [22.3%] (1.6) 399.2 [20%] (1.6) 399.2 [4.7%] (1.8)
Binding energies are given in electron volts; parentheses indicate FWHM values, and barackets show relatives percentages.
calculations; the central region of these systems exhibits a great sensitivity of its π charge distribution, an illustration of this being the strong differences observed between the two imine chromophores. The XPS spectra obtained for the MnPS3/DAMS+ and MnPS3/DAZOP+ intercalates do not show any significant perturbation of the terminal nitrogen atoms, but changes in the intensity of shake-up peaks at high binding energies suggest a modification of the intramolecular charge transfer through intercalation. This effect is more clearly observed in the case of MnPS3/IM1+ and MnPS3/IM2+ where a significant shift in the binding energies of the imine nitrogen atoms accompanies intercalation. Such changes in the XPS spectra of the chromophores after intercalation reflect the effect of both host-guest and guestguest interactions on their electronic properties. It is therefore interesting to compare these results, especially the modification of the shake-up peaks, to our previous UV-visible studies which showed important energy shifts of the absorption band of the chromophores after their intercalation in MnPS3.8 In principle, it is possible to correlate XPS shake-up peaks and UV-visible transitions through MSXR (transition state formalism)24 or more specific calculations;25 for MPS3 intercalates, however, such a study would require further measurements, especially analyses of the C 1s core level, but this is out of reach of the present work. Both spectroscopies clearly evidence a strong perturbation of the donor-acceptor charge transfer due to the intercalation process. The observed bathochromic shifts of the UV-visible absorption band have been attributed to the formation of J-aggregates; in such systems, the electronic properties of a molecule are no longer those of a monomer but reflect the possibility of energy delocalization over two or more strongly interacting molecules.26 It is therefore tempting to suggest that the perturbation of the electronic structure of the chromophores revealed by XPS arises from their aggregation. The role of the MPS3 host lattice on aggregate formation has been discussed elsewhere,27 emphasizing the fact that the nonpolar nature of the sulfur layers leads to weak host-guest interactions, thus favoring guest-guest interactions. Furthermore, the proposed structure for stilbazolium aggregates,8 consistent with the basal spacing values of the intercalates as determined by powder XRD data, implies that the average molecular planes of the guest species stand essentially “edge on” with respect to the host lattice slabs, so that π-π interactions between the chromophore rings can develop. Additional calculations28 using the extended dipole model approximation29 have suggested a slipped rather than eclipsed cofacial configuration for the stilbazolium molecular dipoles (Scheme 2). This model therefore appears strengthened not only by the significant perturbations observed by XPS for the highly sensitive imine nitrogens in MnPS3/IM1+ and MnPS3/IM2+, but also by the picture of the frontier orbitals suggesting the possibility of more efficient stabilizing π-π* interactions (better HOMO-LUMO overlap) in the slipped configuration. In view of connecting the present study to the NLO properties of the intercalates, this last result is of particuler interest. The
SCHEME 2: Possible Conformations for Chromophores Molecular Dipoles: (a) Head-to-Tail, (b) Eclipsed Cofacial, (c) Slipped Cofacial
formation of molecular aggregates exhibiting the slipped cofacial configuration (c), in agreement with previous studies on organic salts,10,30 leads to a noncentrosymmetric packing of the chromophores in the van der Waals space thus offering a possible explanation to the SHG properties of these materials. Moreover, such molecular arrangement should lead to a more efficient charge transfer in the aggregates as compared to the monomer; this is ascertained by the increase of the shake-up peak intensities reported in this work. Since the value of the molecular quadratic hyperpolarizability β, and therefore of the second-order susceptibility χ(2) of the materials, strongly depends on the intensity of the charge transfer,31 this increase appears consistent with the significative SHG efficiencies reported for these compounds. Nevertheless, XPS appears more sensitive than UV-visible spectroscopy to perturbations in the electronic structure of the chromophores since RCl+ exhibits significative changes in its shake-up peak after intercalation even though no aggregation band could be identified on the absorption spectra. The Cl atom being a far poorer electron donor than the dimethylamino group, the donor-acceptor charge transfer is less efficient, thus reducing the intensity of the intermolecular interactions. In this case, it is therefore difficult to establish the relative influence of guest-guest and host-guest interactions on the XPS modifications observed and slight changes in the molecular geometry of the chromophores induced by intercalation cannot be ruled out. This result brings out the complexity of the intercalation process where the properties of the hybrid materials depend on the various possible interactions (steric constraints, polarization effects, redox charge transfers, ...) between its components. In this context, we hope to have demonstrated that XPS, which has been so far mainly applied to the study of the inorganic host phase, is also a useful method to probe the properties of the inserted species. References and Notes (1) Intercalation Chemistry; Whittingham, M. S., Jacobson, A. J., Eds.; Academic Press: London, 1982. (2) Intercalation in Layered Materials; Dresselhaus, M. S., Ed.; NATO ASI Series; Plenum Press: New York, 1986. (3) O’Hare, D. In Inorganic Materials; Bruce, D. W., O’Hare, D., Eds.; John Wiley & Sons: Chichester, 1993; Chapter 4. (4) Ogawa, M.; Kuroda, K. Chem. ReV. 1995, 95, 399. (5) Brec, R. Solid State Ionics 1986, 22, 3. (6) Clement, R.; Garnier, O.; Jegoudez, J. Inorg. Chem. 1986, 25, 1404. (7) Lacroix, P. G.; Clement, R.; Nakatani, K.; Zyss, J.; Ledoux, I. Science 1994, 263, 658. (8) Coradin, T.; Lacroix, P. G.; Nakatani, K.; Clement, R. Chem. Mater. 1996, 8, 2153.
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