J . Phys. Chem. 1984, 88, 2870-2874
2870
Electron Spin Resonance Study of Nitroxide Spin Probes Dissolved in the Discotic Mesophase of Hexakis(n-hexy1oxy)triphenylene E. Meirovitch,* Z. Luz, Isotope Department, The Weizmann Institute of Science, 76100 Rehovot, Israel
and H. Zimmermann Max- Planck-Institiit fur Medizinische Forschung, 0 - 6 9 0 0 Heidelberg, West Germany (Receiued: August 2, 1983; In Final Form: November 8, 1983) ESR spectra of five nitroxide spin probes dissolved in the discotic mesophase of hexakis(hexy1oxy)triphenylene (THE6) were measured as function of temperature in the mesophase region. The different spin probes exhibit a variety of line shapes and temperature dependencies. The results could be explained in terms of the existence of two solvation sites for the probe molecules. These sites are assumed to correspond to (I) probe molecules intercalated within the columnar structures of the discotic mesophase and (11) probe molecules dissolved in the aliphatic region between the columns. In one case (CSL) the experimental results actually exhibited distinct spectra due to two solvation sites. The other spin probes gave simpler spectra but their temperature dependence was unusual and could in general be interpreted in terms of fast dynamic equilibrium between the two solvation sites. Introduction In recent years a new class of liquid crystalline mesophases was discovered, Le., the discotic mesophase which appears in compounds consisting of disklike molecules.’ The most common discogenic compounds are hexasubstituted derivatives of benzene, triphenylene, and truxene. It was originally anticipated that the mesophases of these compounds might serve as ideal liquid crystalline solvents with high ordering potential, in particular for aromatic solutes. However, 2DN M R measurements on a series of deuterated probe molecules dissolved in the discotic mesophase of hexakis(hexy1oxy)triphenylene (THE6) indicated that the solute ordering is unusually low and highly temperature dependent.2 These results are surprising, in particular in view of the fact that the aromatic core of the discotic phase is known to be highly ordered and very weakly temperature dependenta3 The discotic mesophase of THE6 was shown by X-ray diffractometry to belong to the Dhoclass4 This class is characterized by a structure in which the mesogen molecules are stacked into columnar units which in turn are arranged in a regular hexagonal array (Figure 1). On the basis of this structure, the N M R results of the deuterated probe molecules .were interpreted2 in terms of a two-solvation-site model. The solvation sites were assumed to correspond to (I) solute molecules intercalated between the mesogen molecules within the columns and (11) solute molecules dissolved within the aliphatic chains between the columns. Each site is characterized by its own order parameter and, since there is fast (on the N M R time scale) exchange of solute molecules between the sites, the observed spectrum reflects the overall weighted average order parameter. The small degree of ordering and its strong temperature dependence result, according to this model, from the fact that the order parameters in the two sites are of opposite signs and the partitioning between the sites is temperature dependent. The purpose of the present study was to try to substantiate this model by using ESR spectroscopy of paramagnetic spin probes dissolved in the discotic mesophase. The magnetic interactions (Zeeman and hyperfine) in ESR spectroscopy and 3-4 orders of magnitude larger than the quadrupole interactions which give rise to the splitting in the deuterium NMR spectrum. It was therefore hoped that, by use of ESR of spin probes, separate signals for the various solvation sites might perhaps be observed, and more detailed information on the various sites, as well as on the exchange dynamics between them, be obtained. For this study we have (1) For a recent review see C. Destrade, N. H. Tinh, Gasparoux, J. Maltheste, and A. M. Levelut, Mol. Cryst. Liq. Cryst. 71, 111 (1981). (2) D. Goldfarb, Z. Luz, and H. Zimmermann, J . Phys. (Orsay, Fr.), 43, 421, 1255 (1982). (3) D. Goldfarb, 2.Luz, and H. Zimmermann, J . Phys. (Orsay. Fr.), 42, 1303 (1981). (4) A. M. Levelut, J . Phys. Lett., 40, L-81 (1979).
chosen the spin probes shown in Figure 2 and we have used the same discotic mesogen as in ref 3, Le., THE6 (see Figure 1). For one spin probe (CSL) the experimental spectra could be interpreted in terms of two superimposed components, which we attribute to two distinct solvation sites. The other spin probes gave simpler spectra, but for most of them the temperature dependence was quite unusual and could also be interpreted in terms of the two-site model. Experimental Section Material. Hexakis(hexy1oxy)triphenylene was prepared as described in ref 3 and was purified twice by column chromatography. The phase transition temperatures of the neat compound were 68 OC for solid to mesophase and 97 OC from mesophase to isotropic liquid. The reported values in ref 3 are 68 and 99 OC, respectively. The ESR dopants lowered the transition temperatures by at most 1 O C . Solutions of probe molecules were prepared by adding weighed amounts of the solute into known quantities of the mesogen to form solutions in the concentration range 5 X 10-4-10-3 M. The spin probes CSL (spiro[5a-cholestane-3,2’-N-oxy-4’,4’-dimethyloxazolidine]) and ADL (3-spiro[5a-androstan-l7~-ol3,2’-N-oxy-4’,4’-dimethyloxazolidine]) were purchased from Syva Associates, and perdeuterated Tempone (PD-Tempone, 2,2,6,6tetramethyl-4-oxopiperidinyl-l-oxy-d16) was purchased from Merck Sharp and Dohme, Canada. TBBP (2,2,6,6-tetramethyl-4- [ [ [4-(butyloxy)phenyl]carbonyl]amino]piperidinyl- 1oxy) was obtained from Professor J. H. Freed and POATP (2,2,6,6-tetramethyl-4-(propionylamino)piperidinyl1-oxy) was prepared in this laboratory as described before.$ ESR Measurements. The ESR measurements were performed on a Varian E-12 spectrometer at X-band frequency (9.3 MHz) using a Varian 257 temperature control unit. The ESR experiments were done on “planar” multidomain samples in which the directors were distributed uniformly in a plane perpendicular to the magnetic field of the ESR spectrometer. These samples were obtained by slow cooling from the isotropic phase into the mesophase region in a sufficiently strong field. In practice, we have stepped the field up to 20 kG during the cooling process and then reduced it to 3.5 kG for the ESR measurements. Results and Discussion In this section we present the results of the ESR measurements on the various probes and discuss them in terms of the two-site (5) D. Destrade, M. C. Mondon, and J. Malthete, J . Phys., Colloq. (Orsay, I+.), 40, C3-17 (1979). ( 6 ) K. V. S. Rao, C. F. Polnaszek, and J. H. Freed, J . Phys. Chem., 81, 449 (1977). (7) W. J. Lin and J. H. Freed, J . Phys. Chem., 83, 379 (1979).
0022-3654/84/2088-2870$01.50/00 1984 American Chemical Society
ESR of Spin Probes in a Discotic Mesophase
The Journal of Physical Chemistry, Vol. 88, No. 13, 1984 2871 n Y
0
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I Y
Figure 1. Molecular structure and a schematic diagram of the discotic mesophase of hexakis(hexy1oxy)triphenylene (THE6). The rings in the diagram represent the molecules of the mesogen. They are stacked together into columnar structures which in turn are arranged in an ordered hexagonal (DhO) array. X
X
t,
POATP
ry
Lh-N3J
TEMPONE
d
Figure 2. Structural formula and symbols of the spin probes used in the present study. The coordinate systems centered at the nitroxide oxygen represent the approximate direction of the principal components of the hyperfine and g tensors. z is the direction of the major component of the hyperfine interaction.
model described in the Introduction. Since considerably more information was obtained from the spectra of CSL, we discuss this case first and then briefly describe the other probes. CSL. Experimental ESR spectra of a "planar" sample of CSL-doped THE6 at various temperatures in the mesophase region are shown in Figure 3a. A qualitative analysis of these spectra shows that at lower temperatures they cannot be interpreted in terms of a single paramagnetic species: for a rapidly
reorienting nitroxide probe there are just too many features, while attempts to simulate the spectra and their temperature dependence within the slow-motion regime were unsuccessful. (Temperature-induced spectral modifications in anisotropic solutions are observed frequently with spin probe ESR. These are assigned primarily to changes in the rate of rotational reorientation and in the local ordering, Le., in the details of the averaging process, with the straightforward consequence of altering the shape of the ESR spectrum. Yet, the Figure 3a spectra basically preserve their shape and differ mainly in the relative intensities of particular spectral features while their position in the field is being preserved.) On the other hand, the temperature dependence of the spectrum (and as will be shown below the effect of addition of small amounts of organic solvents) can be explained in terms of a two-component system: one component gives rise to the central three-peak spectrum while the other exhibits broad signals of which only the outer components can be discerned. The spectra shown in Figure 3a indicate that, as the temperature is increased, the component giving rise to a broad peaks loses intensity relative to the other component. We interpret these observations in terms of the two-solvation-site model described in the Introduction and designate the site giving rise to the central triplet as site I1 and that responsible for the outer components as site 1. In earlier measurements done on impure THE6 which contained residual organic solvents similar spectra were obtained with, however, different intensity ratios between the two components. It appeared that the abundance of site I increased in the contaminated mesogen. To demonstrate this point we added to the solution giving rise to the spectra in Figure 3a some benzene ( 2 7 wt %) and recorded the ESR spectra. The results are shown in Figure 3b. They clearly demonstrate the increase in intensity of the site I spectrum on account of site 11. In order to obtain quantitative information on the two sites it is necessary to deconvolute the spectra into their components and analyze each one separately. The degree of overlap between the spectra is however too large for such an analysis to be made accurately. Nevertheless, some qualitative conclusions can be obtained from the general behavior of the spectra. In particular we note that the spectrum of site I1 corresponds to the relaxation limit, indicating fast reorientation of the probe molecules in this site.* The observed hyperfine splitting which corresponds to all1 = 19.4 G at 84 OC (remember that the mesophases of THE6 orient (8) C. F. Polnaszek and J. H. Freed, J . Phys. Chem., 79, 2283 (1975).
2872 The Journal of Physical Chemistry, Vol. 88, No. 13, 1984
Meirovitch et al.
b n n \\Ill
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Figure 3. (a) Experimental ESR spectra of CSL in THE6 at different temperatures in the mesophase region. The spectra correspond to a planar sample with the magnetic field perpendicular to the director. (b) Same as part a, except that =7 wt % benzene was added to the sample. perpendicular to the field3) is larger than aiso(14.7 G). This indicates that the molecules in this site prefer an orientation in which the long molecular axis 0,in Figure 2) is aligned along the director, and from the known values of the nitroxide hyperfine parameters an orientational order parameter of 0.1-0.2 for this site can be estimated. Much less can be said about the probe molecules at site I. The wide spread in field of this component indicates that it corresponds to the slow-motion regime. It is tempting to identify this site with probe molecules intercalated within the discotic columns, apparently with their long axes perpendicular to the columnar axes. By analogy with the discussion in ref 2, site I1 will then correspond to probe molecules dissolved in the aliphatic region in between the rigid columnar structures. In this site the molecules are relatively free to move and, as indicated above, slightly oriented along the director. The results of Figure 3 also indicate that, with increasing temperature, probe molecules transfer from site I to site 11. ADL. The molecular structure of this spin probe is very similar to that of CSL, the only difference being the lack of an alkyl chain at carbon 17 of the cholesterol moiety. Its ESR spectrum in THE6 (see Figure 4) is however considerably simpler than that of CSL. It exhibits a single-component spectrum very similar to that of site I1 of CSL and is essentially isotropic. It is.possible that the observed spectrum indeed corresponds to ADL in site I1 and that the fractional population of site I is too small to observe. TBBP. Several spectra of this spin probe at different temperatures are depicted in Figure 5 . The spectra are typical of the relaxation limit and exhibit a small but measurable anisotropy in the hyperfine splitting. The temperature dependence of the hyperfine splitting is anomalous in that it first drops discontinuously at the isotropic-to-mesophase transition, but on further cooling it increases again and approaches the isotropic value. A plot of the hyperfine splitting vs. temperature is shown in the insert of Figure 5. (In an earlier experiment using impure THE6, the splitting actually increased to values above ais,,.) This behavior can again be explained in terms of the two-site model: we assume that the probe molecules in the two sites have different order parameters, most probably of opposite signs. Fast exchange of probe molecules between the two sites results in a single spectrum whose parameters correspond to weighted average interactions. Thus ( a , ) = p'a,'
+ p"a,"
where pi and ai are the fractional populations of the (average) hyperfine interactions of the ith site. The decrease in splitting
X
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Figure 4. Experimental ESR spectra of an ADL solution in THE6 at different temperatures. The upper spectrum corresonds to the isotropic liquid, and the others ta the discotic mesophase region. The spectra correspond to a planar sample with the magnetic field perpendicular to the director. with increasing temperature results, according to this model, from the shift in the population ratio of the two sites. From these results alone it is not possible to identify sites I and 11. If, however, we assume that, similar to CSL, increase in temperature shifts the equilibrium towards site 11, we must conclude that all' < aI1 (and most probably ull > also). These qualitative estimates imply that the N-0 bond (or the piperidine ring, to which the radical moiety is rigidly attached) aligns preferentially along the director, for molecules intercalated within the rigid THE6 cores (site I) and perpendicular to it, in the hydrocarbon-chain region (site 11). Yet, for an extended molecular conformation (see Figure 2), the opposite would be expected from purely geometrical considerations. It is possible that the average configuration of TBBP in the discotic phase of
ESR of Spin Probes in a Discotic Mesophase
The Journal of Physical Chemistry, Vol. 88, No. 13, 1984 2873
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Figure 5. ESR spectra of TBBP-doped T H E 6 at various temperatures in the isotropic and mesophase regions. The measurements were taken with the director perpendicular to the magnetic field. The insert gives a plot of the experimental 14Nhyperfine splitting as a function of temperature within the isotrapic and mesophase regions.
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Figure 6. ESR spectra of POATP-doped T H E 6 in the isotropic and discotic phases. The insert gives a plot of the observed hyperfine splitting as a function of temperature in both phases.
THE6 is folded rather than extended. An alternative interpretation, which however we consider less likely, would be to assume that, when the temperature is increased, probe molecules shift from
the aliphatic site (11) to the intercalation site (I). POATP. Examples of spectra of POATP-doped THE6 in the isotropic and mesophase regions are shown in Figure 6. In both
Meirovitch et al.
2874 The Journal of Physical Chemistry, Vol. 88, No. 13, 1984
dis
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they exhibit extremely narrow lines in both the isotropic and mesophase regions and that the hyperfine splitting is temperature independent and essentially the same in the isotropic and mesophase solutions. (Note that in all spectra the weak I3C and 15N t (“C) satellites are readily observed.) The lack of sensitivity of this probe to ordering in the mesophase region is no doubt related to the small size of its molecules which apparently reorient very fast not only I IO in the isotropic phase but also in the discotic phase. It is felt that 7the Tempone probe occupies predominantly the aliphatic site which is more mobile and liquidlike. In this connection it is interesting to note that in experiments performed with impure THE6 (Le., 87.5 containing organic solvent) some line broadening of the ESR signal was observed within the mesophase region upon increasing the temperature. It is possible that, as in the case of CSL, the presence of organic solvents allows more probe molecules to occupy the intercalation site where the lines are expected to broaden due to 67.0 hindrance of molecular reorientation. The invariance of the line width upon decreasing the temperature (Figure 6) may thus be interpreted in terms of two opposing H ’ IOG effects: line narrowing due to motional averaging and line * broadening due to shift in the equilibrium population.
-
TEMPONE
Figure 7. ESR spectra of PD-Tempone-doped THE6 at various temperatures in the isotropic and mesophase regions.
phases the spectra correspond to the relaxation limit, yielding a well-resolved triplet structure. Note that lull in the mesophase is considerably smaller than in the isotropic liquid. In fact, we found that the experimental hyperfine splitting drops discontinuously upon cooling the sample from the isotropic to the mesophase region, whereas within each phase it is fairly constant, as indicated by the insert of Figure 6, where the a l splittings are plotted as a function of temperature in both the isotropic and discotic phases. The general behavior of the spectrum is similar to that of the TBBP probe, except for the fact that, for POATP, a is independent of temperature in the mesophase region. These results are consistent with an extended POATP molecule residing predominantly in the columnar region. PD-Tempone. Examples of ESR spectra of PD-Tempone-doped THE6 in the isotropic and mesophase region are shown in Figure 7. These spectra differ from those of the other probes in that
Conclusions The main results of the present work are the observations which support the two-solvation-site model for the columnar mesophses of discotic liquid crystals.* The results are also consistent with the previous identification of these sites as the aromatic core intercalation site (I) and aliphatic intercolumnar site (11). With one probe (CSL), separate signals from the two sites could be observed, indicating that on the ESR time scale this probe exchanges slowly between the two environments. The other spin probes occupy only one site or are in fast dynamic equilibrium between the two. Observations on mesogen samples contaminated with organic solvents showed that the distribution of probe molecules between the two sites is affected by the concentration of the foreign compounds. In the examples studied in the present work, organic solvents appeared to shift the equilibrium toward the aromatic intercalation site. Acknowledgment. This research was partly supported by the National Research Council of Israel, by the United States-Israel Binational Science Foundation, and partly by the funds granted by the Charles H. Revson Foundation (to E.M.). We thank D. Goldfarb and R. Poupko for their assistance and many helpful discussions. Registry No. THE6, 70351-86-9; CSL, 18353-76-9; ADL, 2552133-9; TBBP, 64120-21-4; POATP, 21270-94-0; PD-Tempone, 3676353-8.
