1876
The Journal of Physical Chemistry, Vol. 83, No. 14, 1979
(5) P. P. Infelta and M. Gratzel, J. Chem. Phys., 70, 179 (1979). (6) R. C.Dorrance and T. F. Hunter, J . Chem. Soc., Faraday Trans. 1 , 68, 1312 (1972);70, 1572 (1974);73, 1891 (1977). (7) Y. Moroi, A. Braun, and M. Gratzel, J . Am. Chem. Soc., 101, 567 (1979). (8) P. Ekwall, L. Mandell, and P. Solyom, J. Colloid. Inferface Sci., 35, 519 (1971). (9) Using the partition coefficient of naphthalene between the micelle and aqueous phases (93% in the micelles) given in N. J. Turro et
T. J. Klingen and L.
R. Sherman
al., J . Am. Chem. Soc., 95,5508 (1973),one obtains K, = 0.91 and v = 90. (10) (a) K. Sandros and H. L. J. Backstrom, Acta Chem. Scand., 14, 48 (1960);16, 958 (1962);K. Sandros, lbid., 18, 2355 (1964); (b) K. Kikuchi, H. Kokubuzz, and M. Koizumi, Bull. Chem. SOC.Jpn., 43, 2732 (1970);A. Klra and J. K. Thomas, J . Phys. Chem., 78, 196
(1974). (11) S. L. Murov, “Handbook of Photochemistry”, Marcel Dekker, New York, 1973,p 30.
Effects of Temperature and Electric Field on the Visible Polarized Transmission Spectra of Several Orientationally Disordered Crystals T. J. Klingen” and L. R. Shermant Department of Chemistry, The Unlversity of Mississippi, University, Mlssissippi 38677 (Received February 8, 1979) Publication costs assisted by the U.S. Department of Energy
Investigation of the polarized visible light transmission spectra of orientationally disordered (plastic) crystals as a function of applied electric field and temperature has shown a relationship between the transmitted light intensity and the rotational freedom of the mesophase. In the a mesophase, an Arrhenius type relationship was observed for 1-vinyl-o-carboranebetween 298 and 348 K and for 1-bromoadamantane between 308 and 348 K. The calculated rotational energies of these compounds from the Arrhenius relationships were determined to be 17.2 and 26.4 kJ/mol, respectively. These values are in reasonably good agreement with the rotational energy barriers reported in the literature for these spherical compounds from NMR studies.
Introduction In our previous investigations of orientationally disordered (plastic) the primary concern was to evaluate whether or not these materials belong to this type of mesomorphic state of matter. In addition to determining the phase transition entropies and X-ray diffraction of these systems, we also studied their electro-optical
proper tie^.^,^ Orientationally disordered crystals are well known to involve varying degrees of rotational freedom of the molecules within their crystal lattice The amount of freedom of rotation, from highly restricted to almost free rotors, is also known to depend on the nature of the material and on the entropy release in forming the given mesophase,1° with the a phase having the largest amount of freedom of rotation followed by the fl phase. In the present study, the polarized visible light transmission spectra of two orientationally disordered crystals, 1-vinyl-o-carborane and 1-bromoadamantane, were examined as a function of temperature in the CY mesophase. Also, the effect of an electric field applied to these two compounds was determined in a Kerr-type cell for the a phase of both compounds and the phase for l-bromoadamantane.
Experimental Section Materials. Both of the compounds used in this study were commercially available from Aldrich Chemical Co. and from Dexsil Chemical Co. The 1-bromoadamantane and 1-vinyl-o-carborane were purified by vacuum sublimation one to three times or until a distinct melting point to within &1“C of the published value was obtained. The freshly sublimed material was packed into a 1 X 1 cm quartz cuvet and heated above its melting point until it ‘Department of Chemistry, University of Akron, Akron, Ohio 44325.
had compacted into the cuvet. The samples in this study were then sealed under vacuum of less than torr. The sealed cuvets were thermostated at approximately 10 deg above their melting points for 3-4 h and then slowly cooled during the next 24 h. The resultant materials were transparent crystals which filled the cell as if it were a single crystal. Samples prepared in this manner were used for the temperature studies reported herein. Instrumentation. The single beam polarized transmission spectrometer (PTS) consisted of a 450-W xenon lamp and shutter, a Heath EU-700 monochromator, two Nicol polarizers, a Barnes Engineering temperature controlled sample chamber, a Heath EU-701-30 photomultiplier unit, a Hewlett-Packard 3465A digital voltmeter, and/or a Hewlett-Packard 7128A x-t recorder. A Versa/matic V, 0-5000-V dc power supply and switching arrangement were added for the electric field part of the study. Procedure for Temperature Study. The cuvet containing the crystal was placed in the PTS and allowed to equilibrate at the maximum temperature, 348 K, used here for a period of 48 h before data were collected. When a fresh crystal was placed in the spectrometer, it would anneal and the transmittance would increase from 20 to 300% before the transmittance became stable. The sample annealing was probably due to the “flowing” of the orientationally disordered crystal which removed microscopic fractures. The temperature was slowly reduced at the rate of 5 K per day. Data were obtained at each new temperature until four readings were reproducible to within A10 mV of photomultiplier output. The sensitivity of the PTS was determined with a distilled water standard. The temperature of the sample cavity of the spectrometer was constant to f0.5 K and the first polarizer was at 45O with respect to the second polarizer. All data were collected at 520 nm. The 520-nm wavelength was selected
0022-365417912083-1876$01 .OO/O 0 1979 American Chemical Society
The Journal of Physical Chemistry, Vol. 83, No. 14, 1979
Orientationally Disordered Crystals
TABLE I : Orientationally Disordered Mesophases” CY phase 4 phase MP -~ compd T AS T AS T AS ~
1877
1.4-
~~~
1-bromoadamantane 389 10.17 308 24.02 2 8 1 3.68 1-vinyl-o-carborane 352 8.45 284 23.01 208 13.93 a
Tin Kelvin; AS, in joules per mole-degree.
1.3-
1.2r
:.1
I 1.6
1.5
I
~
R
T
~
I
~
~
Figure 2. The log of the polarized transmitted light intensity (mV) as a function of 1IRTX lo3 for 1-bromoadamantane in the 01 mesophase at 520 nm.
15
“6
8
’7
I / R T x103 Figure 1. The log of the polarized transmitted light intensity (mV) as a function of 1IRT X l o 3 for 1-vinyl-o-carborane in the 01 mesophase at 520 nm.
x
750-
e m K
because the transmittance spectrum of the single beam instrument has a plateau at this wavelength, whereas xenon radiation peaks appeared in the spectrum at shorter wavelengths. The orientationally disordered crystals investigated in this study do not possess any chromophoric groups in the visible spectra, and scanning the spectra between 370 and 700 nm showed no measurable differences as a function of temperature and applied electric field. Procedure for Electric Field Study. The l-bromoadamantane and 1-vinyl-o-carborane were examined in a Kerr-type cell with quartz windows and gold-plated electrodes. The Kerr cell was filled in the manner described above to produce a transparent “single” crystal, except that the Kerr cell was not vacuum sealed. The Kerr cell had an electrode gap of 0.33 cm and an optical path length of 0.30 cm. The operation of the cell was standardized with nitrobenzene. As in the temperature study discussed above, the first polarizer was at 45” with respect to the second polarizer, while the electric field vector was a t 90” to the transmitted light beam. The measurements were carried out on the crystals of 1-bromoadamantane in both the cy (50 “C) and the p (24 “ C ) phases and in the cy (24 “C) phase only of 1-vinyl-o-carborane. The results of the electric field portion of this study were found to be reproducible within f l mV of photomultiplier output.
Results The pure samples of 1-bromoadamantane and 1vinyl-o-carborane were studied as a function of temperature in the PTS. The mesophase transition temperatures and entropies of these compounds are shown in Table I. At each temperature recorded, the intensity of the transmitted light was allowed to reach an equilibrium value, which was obtained in 2-4 h. The results of the PTS work on these two systems is shown in Figures 1 and 2, as a plot of the log I vs. 1 / R T X lo3. Linear regression analysis of the data points gave the straight lines shown
-
+J c)
c
700-
1 A p p l i e d F i e l d x 13-3 V i c r r
Flgure 3. The polarized transmitted light intensity (mV) as a function of the applied field for I-vinyl-o-carborane in the a mesophase at 520 nm.
in these figures. In the case of 1-vinyl-o-carborane, the data were continuous over the temperature range which encompassed the cy phase, Figure 1. However, in the case of 1-bromoadamantane, Figure 2, there was a discontinuity in the data for the a phase. This “knee” in the curve occurred approximately 15 deg above the phase transition temperature from the 01 to the /3 phase. However, within the a mesophase the log I vs. 1/RT X lo3 plot yielded a linear relationship with essentially the same slope on both sides of the discontinuity. These results were found in both cases to be indpendent of wavelength in the range 370-700 nm. In the electric field portion of the study, it was found that both 1-bromoadamantane and 1-vinyl-o-carborane gave very similar results in the a phase, as that shown for 1-vinyl-o-carborane in Figure 3. In the p phase of 1bromoadamantane, the results were very different as shown in Figure 4. The differences between the a and p phase can be enumerated as follows: (1)in the 01 phase the applied field for onset of a change in the light intensity is much lower than in the phase; (2) the change in the polarized transmission spectrum intensity with applied field is much larger in the cy phase as compared to the fl phase; (3) the potential to completely order the 01 phase is higher than that of the f? phase; and (4) the polarized
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The Journal of Physical Chemistry, Vol. 83,No. 14, 1979
T. J. Klingen and
L. R. Sherman
TABLE 11: Rotational Energy Barriers
1
t
i
-= 450t
E
NMR 23.8: 27.2b 13.3c
Reference 11. Reference 13. Reference 1 2 Error in E estimated at 5%. All E’s in kJ/mol.
a
+-
4001
0
temp range, compd K PTSd 1-bromoadamantane 308-348 26.4 1-vinyl-o-carborane 298-348 17.2
o
3
d
9
6
Applied Field x
V/cm
Figure 4. The polarized transmitted light intensity (mV) as a function of the applied field for 1-bromoadamantane in the mesophase at 520 nm.
0
transmitted light intensity in the @ phase is much less than that in the a phase under the same conditions. This last observation occurs even though the material appears to be transparent and has the same transmitted light intensity in both phases when the polarizers are removed from the PTS.
Discussion The compounds 1-bromoadamantane and l-vinyl-ocarborane have been shown to exhibit a “Kerr effect” in both the a and @ phases. Since the Kerr effect occurs through the alignment of the dipole moments of the molecules, a t its maximum applied field, the high field plateau region of Figures 3 and 4,the molecules of the material are aligned as completely as possible by the electric field. As this alignment begins, low applied field plateau of Figures 3 and 4, the change from maximum polarized transmitted light intensity to the minimum transmission of light, the material within the Kerr cell must be acting as a polarizer itself to rotate the light beam to greater than 45O, Le., to cause a decrease in the polarized transmitted light intensity. It was noted, Figures 3 and 4,that the electric field required to begin the alignment of molecules occurred at a much lower applied field in the a phase than in the @ phase. This is what would be expected, since there is greater rotational freedom in the a phase than in the @ Also, it was noted that the change in the polarized transmitted light intensity with applied field was much smaller in the P phase than in the a phase. This is most probably due to the partial alignment of the molecules in the @ phase compared to the a phase. This reasoning is supported by the fact that, in the absence of the first and second polarizers, the transmitted light intensity through the sample was the same in both phases. If the material is partially aligned before the field is applied, one would expect the applied field for onset of the Kerr effect to occur at a higher applied potential, which is observed for the @ phase compared to the a phase. As a result of these considerations, changes in the polarized transmitted light intensity can be qualitatively related to the order of the material, i.e., to the relative amount of rotational freedom of the particular phase. Since the electrical effects on the polarized transmission spectra of orientationally disordered crystals clearly relates the intensity of the transmitted light to rotational order in the mesophase, the introduction of energy in the form of heat should within a given mesosphase cause increased rotation of the molecules. An increase of rotation of the molecules should then in turn cause the partial alignment of the molecules inherent in the mesophase to decrease,
resulting in a decreased polarizing action of the media on the PTS light beam. This phenomenon would result in an increase of transmitted light intensity with increasing temperature, which was observed in both systems investigated. In view of the Arrhenius type dependence on temperature exhibited by both systems within the a phase and the relationship between polarized transmitted light intensity and rotational freedom of the system demonstrated by the electric field studies, the data in Figures 1 and 2 may be interpreted as a measure of the rotational energy barrier in the a mesophase. The Arrhenius energies of rotation in the a phase are shown in Table 11, compared with the literature ~a1uesll-l~ of the energy barrier for rotation determined by nuclear magnetic resonance methods. In both cases there is reasonably good agreement between the PTS method and the NMR method for determination of the energy barrier to rotation. The discontinuity observed in the a phase of 1bromoadamantane between 315 and 325 K was found to be reproducible. Heimer13 also observed a discontinuity in this same temperature range in determining the rotational energy barrier of 1-bromoadamantane from NMR TI, experiments. Since this discontinuity begins 17 deg above the phase transition and the Arrhenius energies calculated on both sides of the discontinuity are essentially the same, this effect is not related to the a-@ phase transition. Since an increase in temperature increases the fraction of the molecules with sufficient energy to rotate, thereby increasing the polarized transmission light intensity by decreasing the alignment of the molecules in the mesophase, this discontinuity must be associated with the heat energy going into the release of some other degree of freedom than the rotational one. One might speculate that this discontinuity is due to a strong liberational mode becoming allowed at the expense of other modes in the phonon spectrum, Le., with no overall activation energy.14 The results reported here suggest that the PTS can be used as a direct method for determining rotational energy barriers in at least the a mesophase of orientationally disordered (plastic) crystals. However, additional systems should be investigated in order to establish the general validity and limitations of the PTS method for this purpose.
Acknowledgment. The authors express their appreciation to Professor N. E. Heimer for making preprints of his NMR papers available to us prior to publication. One of us (L.R.S.) thanks the U S . Department of Energy for providing post-doctoral support during the course of this investigation. This research was supported by the U.S. Department of Energy under Research Contract NO. EY-76-S-05-3781. References and Notes (1) T. J. Klingen and J. R. Wright, Mol. Cyst. Liq. Cyst., 13, 173 (1971). (2) T. J. Klingen and J. R. Wrlght, Mol. Cyst. Liq. Cyst., 16,283 (1972). (3) T. J. Klingen and J. H. Kindsvater, Mol. Cyst. Liq. Cyst., 26,365 (1974). (4) T. J. Kiingen and D. R. Hepburn, Jr., J. Inorg. Nucl. Chem., 37,1343 (1975).
Thermal Modulation Spectroscopy (5) T. J. Klingen and J. R. Wright, US. Patent No. 3711 180, 1973. (6) T. J. Klingen, US. Patent No. 3 949 224, 1976. (7) J. G. Aston, “Physics and Chemistry of the Organic Solid State”, Vol. 1, D. Fox, M. M. Labes, and A. Weissberger, Ed., Wiley, New York, 1963. (8) G. M. Hood and J. N. Sherwood, Mol. C y s t . , 1, 97 (1966). (9) H. M. Hawthorne and J . N. Sherwood, Trans. Fara&y SOC.,66, 1783 (1970).
The Journal of Physical Chemistty, Vol. 83, No. 74, 7979
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(10) R. S.Porter, E. M. Barrall, 11, and J. F. Johnson, Acc. Chem. Res., 2, 53 (1969). (11) I. P. Aleksandrova, V. S. Repeta, and M. V. Zobtovin, Kristallograflya, 22, 194 (1977). (12) N. E. Heimer, Mol. C y s t . Liq. Cyst., in press. (13) N. E. Heimer, unpublished data. (14) C. Kettel, “Introduction to Solid State Physics”, 5th ed, Wiley, New York, 1976.
Thermal Modulation Spectroscopy Applied to Inorganic Compounds with Near Degenerate Excited States K. W. Hipps” Department of Chemistry and Chemical Physics Program, Washington State University, Pullman, Washington 99 164
and A. H. Francls Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48109 (Received January 8, 1979) Publication costs assisted by the Petroleum Research Fund
The technique of thermal modulation as applied to emission spectroscopy is discussed. It is demonstrated that the thermal modulation method is a useful technique for studying inorganic molecules and ions with (a) non-Boltzmann excited state populations, (b) multiple levels responsible for the observed emission even if these levels are in thermal equilibrium, (c) vibronically allowed transitions. Emission and thermally modulated emission spectra are presented for the PO2-, tri~[2,2’-bipyridine]ruthenium(II)~+, and tris[4,7-diphenyl-o-phenanthroline]r~thenium(II)~+ ions, and for ruthenocene. The generality of this method is emphasized by presenting data on samples which take the form of single crystals, powders, and plastics. Thermally modulated emission spectroscopy is found to be analogous to optically detected EPR with broad band microwave radiation. The energy range available to thermal modulation, however, exceeds that of conventional microwave techniques by several orders of magnitude.
Introduction Thermal modulation techniques were first employed in the study of semiconductors and metals.’ Changes in band gap properties with temperature were correlated with the observed thermal modulation (TM) reflectance spectra of these materials. Since this early use of TM, other applications of the technique have been reported. Loh has reported the T M of absorption spectra of several minerals2i3in which the relative enhancement of weak, but temperature dependent, vibronically allowed electronic transitions afforded by the T M method allowed detailed assignments to be made of bands which appeared only as shoulders in the normal absorption spectrum. Loh et aL4 also utilized thermally modulated absorption spectroscopy to investigate f d transitions of Ce3+in CaF,. TM was introduced to molecular crystal spectroscopy by Francis and c o - ~ o r k e r s .In ~ their work, the equivalence of T M of emission and optically detected magnetic resonance, performed with a broad band microwave source, was demonstrated for tetrachlorobenzene doped durene. Hunter et aL5 also provided an example of the use of T M for studying and identifying trap-to-trap energy migration. In later papers6J the utility of TM techniques for studying electron-phonon interactions in molecular crystals was explored. The integral thermal modulation signal was shown to be a convenient experimental quantity which relates directly to the extent of phonon participation in the electronic transition mechanism. Theoretical expressions for the zero phonon T M line shape were developed and were in satisfactory agreement with experi-
-
ment. Recently, Baker et a1.8 have commented on the difficulty of distinguishing thermal modulation and optically detected infrared double resonance spectra. In this paper we will present those aspects of emission T M spectroscopy which are especially pertinent to inorganic spectroscopy. We will concentrate on those systems which have “zero field splitting” of the excited term from whence luminescence originates. In the examples to be presented the energy separation of the excited levels varies from less than a wavenumber to tens of wavenumbers. Since variation in temperature leads to a variation in population for the emitting levels (whether or not they are in thermal equilibrium), the TM spectrum will be characteristic of the difference in emission band shapes for those levels. We find that the band shapes of the thermal modulation of emission spectra are good indicators of the existence of multiple level emission and of the transition mechanism for the respective levels.
Experimental Section General. The experimental configuration was as depicted in Figure l. Samples were bonded to the face of a thin film heater and cooled to liquid helium temperatures in a variable temperature liquid helium cryostat. Typically, the ambient temperature was about 5 K and the sample and heater were in thermal contact with a helium vapor bath. Sample luminescence was excited by a properly filtered UV light source, dispersed by a 1-m monochromater, and detected by an EMI9558QB photomultiplier. When recording the normal emission
0022-365417912083-1879$01.00100 1979 American Chemlcal Society
