J. Phys. Chem. 1987, 91, 2242-2243
2242
flips about the C-C vector. In their single-crystal N M R study, Meier et a1.4 concluded that the hydrogen bond protons undergo correlated jumps. Such correlated motions should be readily observable in the zero field N M R spectrum even for a polycrystalline sample, and we have performed a study to observe such motions.
D = D ,X
- 3 sin2 a) + 2 9 p 3
0
Dl
0
40 P 2 P l ( l - 3 cos2 a) - 2
0
0
D,
2P,
Results and Discussion Details of the zero field N M R experimentla and the spectrometer6 have been presented elsewhere. The ptoluic acid sample used was -60% deuteriated on the acid sites and 98% deuteriated elsewhere. By diluting the acid site protons, coupling between dimers is reduced and the splitting of the lines is better resolved. The room temperature proton zero field N M R spectrum of ptoluic acid appears in Figure 2. Simulated zero field spectra for the case of static protons as well as correlated and uncorrelated jumps were produced for comparison to the experimental spectrum as follows. In the most general case, the average dipole coupling tensor is written
D = PiDI
+ P2D2 + P3D3 + P4D4
(1)
where P,, is the fractional population of orientation n. The tensors, D,,, describe the dipole coupling for the two protons in a particular orientation, as explained in the caption to Figure 1. A few simplifications to this expression can be made immediately. First, when the tensor is viewed in the molecular based frame of Figure 1, the interproton vectors 3 and 4 are equivalent and thus D3 = D4,unlike the interproton vectors 1 and 2 which have a different dependence on the jump angle a. Additionally, at room tem,~ perature, sites 1 and 2 are assumed to be equally p o p ~ l a t e d as are 3 and 4. Writing the average dipole coupling tensor in the molecule based frame (x,y,z) of Figure 1 yields eq 2, where a is the jump angle shown in Figure 1 and D, = hy2/r,‘. The value of D3 is based (6) Bielecki, A.; Zax, D. B.; Zilm, K. W.; Pines, A. Reu. Sei. Instrum. 1986, 57, 393.
-1
+ 2D3P3 Dl
on the internuclear distances taken from X-ray crystallographic data for pentafluorobenzoic acid.’ The normalized zero field signal is calculatedla to be S(t1) =
COS o12t1 + COS ~ 1 3 t +l COS ~ 2 3 t l ]
(3)
Thus one expects three lines of equal intensity at the frequencies corresponding to the differences in eigenvalues derived from eq 2. Simulated spectra were produced for the static (P1 = l.O), correlated (PI = P2 = 0.5, P3 = P4 = 0 ) , and uncorrelated ( P I = P2 = P3 = P4 = 0.25) cases and appear in Figure 3. Comparison to the experimental spectra indicates that correlated jumps between two orientations is the most plausible situation. The values of rl and a derived from the experimental spectrum are rl = 2.33 f 0.05 A and a = 16.0 f 2O which are in good agreement with results previously reported by Meier et aL4 from single crystal experiments in high field. More complex dynamics are also accessible with zero field N M R and have been treated theoretically.2b,d
Acknowledgmenr. We thank H . Zimmermann for the synthesis of the deuteriated p-toluic acid. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Science Division of the US.Department of Energy under Contract DE-AC03-76SF00098. (7) Benghait, V.; Leiserowitz, L. J. Chem. SOC.,Perkin Trans. 2 1972, 1779. Pentafluorobenzoic acid is a molecule of related structure whose carboxyl hydrogens show no disorder at room temperature and is thus a suitable model compound.
Antiferroelectric Stacking in Lead Phthalocyanlne Columnar Mesophases P.Weber, D. Guillon, and A. Skoulios* Institut Charles Sadron (CRM-EAHP), ULP-CNRS, 67083 Strasbourg Cedex, France (Received: December 23, 1986)
Experimental evidence is presented for the first example of columnar mesophase (1) observed far below room temperature, (2) obtained with flat discotic molecules possessing a strong dipole moment normal to the molecular plane, and (3) showing an intramolecular antiferroelectric coupling of the molecules along the columns.
Introduction In a series of recent octakis(alkoxymethy1) derivatives of phthalocyanine were shown to produce columnar liquid-crystalline structures upon heating. The crystal/mesophase transition occurred in a range from 40 to about 80 OC. The mesophase/ isotropic liquid transition was generally difficult to detect: most of the products studied remained in the mesomorphic state up to temperatures of the order of 300 OC, where they started to de(1) Piechocki, C.; Simon, J.; Skoulios, A,; Guillon, D.; Weber, P. J . Am. Chem. SOC.1982, 104, 5245. ( 2 ) Guillon, D.; Skoulios, A.; Piechocki, C.; Simon, J.; Weber, P. Mol. Cryst. Liq. Cryst. 1983, 100, 275. (3) Guillon, D.; Weber, P.; Skoulios, A.; Piechocki, C.; Simon, J. Mol. Cryst. Liq. Cryst. 1985, 130, 223
compose. The structure of the mesophases was studied by optical microscopy and X-ray diffraction. It was described as corresponding to the stacking of the flat aromatic cores of the phthalocyanine molecules in columns, surrounded by the aliphatic chains in disordered conformations, and laterally assembled in a two-dimensional hexagonal lattice. The majority of the phthalocyanine derivatives were studied with a metal ion (Cu, Zn,Mn) included in the empty central cavity of the charged aromatic core. Because they are smaller size than the cavity, and because of their electrostatic interactions with the phthalocyanine moiety, these metal ions lie in the average plane of the molecule. Among the metal ions which are known to form complexes with phthalocyanine (Pc), lead (Pb2+)is of particular interest, because its diameter (2.4 A) is larger than that (1.6 A) of the Pc cavity to which it tends to be attracted. In both monoclinic and triclinic
0022-36S4/87/2091-2242$01.50/00 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 9, 1987 2243
Letters crystalline forms of nonsubstituted lead phthal~cyanine,~ X-ray diffraction shows that the Pc moieties are not strictly planar: they are slightly curved to adopt the shape of a flattened dome. The Pb atoms protrude from the central hole on the convex side of the molecules, deviating by about 1 A from the plane defined by the four isoindole N atoms. This geometry results in a strong electrical dipole moment oriented normally to the plane of the molecule. Our interest in the columnar mesophases of octakis(a1koxy)-substituted phthalocyanine derivatives, with metal ions located along the stacking axis of the molecules, prompted us to investigate the special case of lead. Indeed, the electric interactions among the dipoles which are oriented parallel to the axis of the columns should produce special ordering of the superposed molecules. This paper describes the mesomorphic behavior and the structural characterization of C8PcPb: (CH~(CH~),OCH~)~PCP~
Results and Discussion Differential scanning calorimetry shows5that CsPcPb undergoes one thermal transition at -45 OC, with a latent heat of 0.5 kcal/mol. To give exact details for this transition, and eventually to analyze completely the polymorphic scheme involved, polarizing optical microscopy and X-ray diffraction have been used in the present work. The following scheme has thus been established: K-D-I
-45 O C
158 OC
0.5 kcal/mol
The optical textures of phase D closely resemble those already observed for the columnar mesophases produced by hexakis(a1koxybenzo) derivatives of tris(benzo)furan.6 Powder X-ray diffraction patterns have been recorded in the temperature range from 25 to about 110 "C with a Searle camera equipped with Elliott toroidal mirror optics (Ni-filtered copper radiation). They fully confirm the columnar structure of the mesophase; indeed, they contain a series of three sharp small-angle reflections with reciprocal spacings in the ratio 1:31/2:41/2,typical of hexagonal packing of columns. Independent of the temperature, the distance between the axes of neighboring cylinders is 26.9 A, in perfect agreement with the molecular dimensions. It is'of interest to note that, apart from the special case of the phthalocyanine derivatives with eight tri(ethy1ene oxide) peripheral chains, where the crystal to mesophase transition temperature lies just below 25 "C3this is the first time a columnar mesophase has been found to be perfectly stable at room temperature and over a wide temperature range from -45 to about 158 O C . X-ray patterns also contain one broad peak located at 7.4 8, in the wide-angle region. This spacing corresponds to twice the thickness of one lead phthalocyanine molecule and was therefore interpreted as being related to the repeat distance along the columns of pairs of superposed molecules. The same peak (at 7.3 A) had also been observed for the columnar mesophase of the octakis(octadecyloxymethy1) derivative of lutetium phthalocyanine,' the molecules of which are known to contain one metal ion sandwiched between two phthalocyanine m o i e t i e ~ . ~ . ~ (4) (a) Przyborowski, F.; Hamann,C. Cryst. Res. Technol. 1982, 17, 1041. (b) Iyechika, Y.; Yakushi, K.; Ikemoto, I.; Kuroda, H. Acta Crystallogr.,Sect. B 1982, 38, 166. (5) Piechocki, C. Ph.D. Dissertation, University of Paris VI, 1985. (6) Mamlok, L.; Malthste, J.; Nguyen Huu Tinh; Destrade, C.; Levelut, A. M. J . Phys. Lett. 1982, 43, 641. (7) Piechocki, C.; Simon, J.; Andre, J. J.; Guillon, D.; Petit, P.; Skoulios, A.; Weber, P. Chem. Phys. Lett. 1985, 122, 124. (B).Kasuga, K.; Tsutsui, M.; Petterson, R. C.; Tatsumi, K.; van Optenbnsh, N.; P e p , G.; Meyer, E. F. J . Am. Chem. SOC.1980, 102, 4836. (9) de Cian, A.; Moussavi, M.; Fischer, J.; Weiss, R. Inorg. Chem. 1985, 24, 3162.
a
b
Figure 1. Schematic view of the (a) nontilted and (b) tilted stacking of the PcPb dimers within the columns.
The existence of dimers clearly suggests an antiferroelectric coupling of axial dipoles of lead phthalocyanine molecules, similar to that mentioned for the monoclinic crystalline modification of lead p h t h a l ~ y a n i n e .However, ~ an antiferroelectric coupling such as that illustrated in Figure l a , with the opposed dipoles on the same axis, is unlikely due to the repulsive interactions between dipoles. An energetically favorable configuration can be obtained by merely tilting the molecules within the columns, as tentatively sketched in Figure 1b. A further argument supporting an antiferroelectric dipole coupling may be found in the unusually weak enthalpy (0.5 kcal/mol, to be compared with the value of 10 kcal/mol generally measured for other octakis(octyloxymethy1) phthalocyanine derivatives5), and correspondingly in the unusually low temperature of the crystal to mesophase transition. It is worth noting that the enthalpy drop (of about 10 kcal/mol, that is about 1.2 eV per dimer) is of the order of what would be expected for an antiferroelectric coupling of dipoles with a magnitude of a few Debye units. Systematic X-ray diffraction analyses are currently in progress to investigate the mesomorphic and structural behavior of lead phthalocyanine derivatives carrying alkoxymethyl peripheral chains of various lengths.
Acknowledgment. This work has been carried out in the framework of the G R I M M (Groupe de Recherches Interdisciplinaires sur les MatEriaux MolEculaires) cooperative research program. We thank Prof. J. Simon and Drs. J. C. Boulou and C. Piechocki (Laboratoire de Chimie Inorganique et d'Electrochimie des MatEriaux MoMculaires, ESPCI, Paris) for having provided us with the octakis(octy1oxy) derivatives of lead phthalocyanine.
