J. Phys. Chem. 1984, 88, 2893-2898 shape as the ones formed by SDS alone at high concentrations but they are larger in size. Further investigation should be directed at establishing the generality of these results. In particular, mixtures of other similar ionic surfactants should be studied.
Acknowledgment. We acknowledge useful discussions with H.
2893
T. Davis, D. F. Evans, B. W. Ninham, and L. E. Scriven and are grateful to S. Mukherjee for technical assistance. This research was supported by the Department of Energy and the National Science Foundation. Registry No. SDS, 151-21-3;SHBS, 67267-95-2.
Synthesis, Structure, and Excimer Formation of Aromatic Cholesteric Liquid Crystals Masahiko Sisido,*t Kazuhiko Takeuchi,* and Yukio Imanishif Research Center for Medical Polymers and Biomaterials and Department of Polymer Chemistry, Kyoto Japan (Received: November 16, 1983) University, Kyoto 606,~
Cholesteryl w-arylalkanoates having phenyl, 1-naphthyl, 2-naphthyl, and 1-pyrenylgroups as aromatic groups were synthesized, and some of those were found to form the cholesteric liquid crystal (CLC) phase. The pitch and the screw sense of the helix were determined from the selective reflection wavelength and the circular dichroism. The pitch was around 800 nm for cholesteryl 5 4 1-naphthy1)pentanoate and about 970 nm for cholesteryl34 1-naphthyl)propionate. The helix sense was left-handed for all cholesteryl esters which form the CLC phase. The orientation of naphthyl groups within each quasi-nematic layer was qualitatively discussed from the sign of circular dichroism at the IL, absorption band. The short axis of the 1-naphthyl group was parallel to the optical or the molecular axis of the quasi-nematic layer in the CLC of cholesteryl 1-naphthylpropionate and -pentanoate, whereas the short axis of 2-naphthyl group was perpendicular to the optical axis of the CLC of cholesteryl 2-naphthylpropionate. The orientation of the excimer transition moment was investigated by circularly polarized fluorescence spectroscopy. The transition moments for all chromophoric CLC’s were found to be nearly perpendicular to the optical axis. From these spectroscopic data, an intralayer excimer configuration for 1- and 2-naphthyl groups was proposed. In the mixture of cholesteryl 3 4 1-pyrenyl)propionate (60 mol %) and 3-phenylpropionate (40 mol %), the excimer formation was more efficient in the CLC phase than in the crystalline phase.
Introduction Molecular assemblies in which chromophores are regularly arranged may show some characteristic optical properties (birefringence, dichroism, and others including nonlinear phenomena) and electric or electronic properties (energy transfer, electron transfer, electric conductivity, and photoconductivity). These properties are often different from those observed in amorphous assemblies on a quantitative or even a qualitative level. Molecular crystals of chromophoric substances have been extensively studied with respect to their optical and electronic properties.’ Although their properties are unique and useful for some practical purposes, the application of organic crystals is limited because of their mechanical instability and their low processibility. In search for ordered assemblies of chromophores which are applicable to practice, we have undertaken synthesis of chromophoric substances which form cholesteric liquid crystals (CLC) phases. In the CLC phase, molecules are oriented along a particular direction within a quasi-nematic layer, and the layers pile up spirally, forming a macrohelical texture.2 The particular arrangement may affect the electronic interactions between chromophores and will lead to some characteristic optical and electronic properties of the CLC. The purpose of this study is to clarify the arrangement of chromophores in the CLC phase and to investigate their interactions by spectroscopic methods. Studies on optical and spectroscopic properties of guest chromophores embedded in CLC have been r e p ~ r t e d . ~Chirooptical ,~ properties such as circular dichroism (CD)5,6and circularly polarized fluorescence (CPF)’+ are especially noteworthy. However, in the host-guest systems, the arrangement of the guest chromophores may be more or less disturbed as compared with the ordering of the host CLC, and the host CLC structure may be somewhat disordered by the presence of the guest molecules. Furthermore, since the concentration of the guest chromophores I
Research Center for Medical Polymers and Biomaterials. Department of Polymer Chemistry.
0022-3654/84/2088-2893$01.50/0
TABLE I: Elemental Analysis of Cholesteryl w-Arylalkanoates found, % calcd, % compd C H C H Ph(CHz),COOCho 83.52 10.75 83.34 10.49 Ph(CHJqCOOCh0 83.35 10.82 83.61 10.53 1-Nap(CH2),COOCho 84.66 10.18 84.45 9.92 l-Nap(CH,),COOCho 84.44 10.11 84.65 9.98 2-NapCH2COOCho 84.25 9.75 84.42 9.81 2-Nap(CH2),COOCho 84.71 9.94 84.45 9.92 2-Nap(CH2),COOCho 84.28 10.11 84.65 9.98 l-Pyr(CH,),COOCho 85.73 9.20 85.93 9.09 1-Pyr(CH,)$OOCho 85.93 9.30 85.92 9.21 1-Pyr(CH2),COOCho 85.46 9.32 86.05 9.18 must be kept low enough to maintain the CLC structure, studies on interchromophoric interaction are difficult. In this paper we will describe synthesis and photochemical properties of CLC’s of several cholesteryl w-arylalkanoates (I). Hereafter, compound I will be referred to as Ar-n, where n indicates the number of methylene groups connecting the aromatic group to the ester group. Some of the cholesteryl esters were found ~
~
~~~~
(1) For example, see: Davydov, A. S. “Theory of Molecular Excitons”;
Plenum Press: New York, 1971. (2) Chandrasekhar, S. “Liquid Crystals”; Cambridge University Press: London, 1977. (3) Anderson, V. C.; Craig, B. B.; Weiss, R. G. J. Am. Chem. SOC.1981, 103, 7169. (4) Anderson, V. C.; Craig, B. B.; Weiss, R. G. J. Am. Chem. SOC.1982, 104, 2972. ( 5 ) Sackmann, E.; Voss, J. Chem. Phys. Lett. 1972, 14, 528. (6) Saeva, F. D.; O h , G. R. J . Am. Chem. SOC.1973, 95, 7882. (7) Mainusch, K.-J.; Stegemeyer, H. Ber. Bunsenges. Phys. Chem. 1974,
78, 927. ( 8 ) Pollmann, P.; Mainusch, K.-J.; Stegemeyer, H. Z . Phys. Chem. (Wiesbaden) 1976, 103, 295. (9) Stegemeyer, H.; Stille, W.; Pollmann, P. Isr. J . Chem. 1979, 18, 312.
0 1984 American Chemical Society
2894 The Journal of Physical Chemistry, Vol. 88, No. 13, 1984
Sisido et al. TABLE II: Phase Transition of Cholesteryl w-Arylalkanoates 'Cha - Is0 isotropic liquid cholesteric LC
--
A h o - C r y
crystal
I
to form the CLC phase by themselves, and their interchromophoric interactions such as excimer formation were studied. The orientation of chromophores in the CLC phase was determined by the sign of the CD spectrum, and the orientation of the excimer transition moment was qualitatively inferred from CPF spectroscopy. Experimental Section
Materials. Cholesteryl w-arylalkanoates were prepared by the esterification of cholesterol (analytical grade) with the corresponding w-arylalkanoyl chlorides. w-Arylalkanoic acids were synthesized from the corresponding aryl aldehyde through the Knovenagel condensation followed by the catalytic hydrogenation for the n = 2 series1h12and from unsubstituted aromatic hydrocarbons through the Friedel-Crafts acylation with glutaric anhydride followed by the reduction with hydrazine for n = 4 The acids were converted to acid chlorides with oxalyl chloride or phosphorus pentachloride, and then the acid chlorides were mixed with an equal molar amount of cholesterol in benzene containing a small amount of pyridine. The cholesteryl esters were recrystallized from tert-amyl alcohol or from an ether/methanol mixture. Results of the elemental analysis are collected in Table I. Spectroscopic Measurements. The chromophoric CLC's were placed between a pair of quartz plates as a thin capillary film of 1-10-pm thickness, and the plates were fixed in a thermostated jacket. It has been known that in the thin film of CLC's, the helix axis is oriented perpendicularly to the plates. The perpendicular orientation was confirmed in the present study by the observation of a sharp selective reflection. Surface fluorescence spectra and selective reflections were recorded on a Hitachi MPF-4 instrument. For the fluorescence spectra the excitation light was introduced with an incident angle of 56O, and the surface fluorescence was detected from the perpendicular direction to the surface. The samples were not deoxygenated throughout the present study. The surface reflection was measured with an incident light perpendicular to the plates, and the reflection was observed from the same direction as the incident beam. The above measurement was possible by inserting a half-mirror at 45' to the incident light beam. CD spectra were measured with a JASCO 5-20 instrument. In this case the plates containing the CLC were inserted between two thermostated metal blocks with holes of 5-mm diameter, through which the incident light was introduced and the transmitted light was taken out. The -CPF instrument was a JASCO FCD-1F type, which has been described previously.15 The same sample cell as used in the CD measurement was used, and therefore the fluorescence was taken from the opposite side of the incident light beam. The refractive index was measured by an Abbe-type refractometer (Shimadzu Seisakusho, Kyoto, Japan). Refractive indices for light propagating parallel and perpendicular to the helix axis of the CLC were obtained.I6 In some cases a monochromatic light was (10) Muller, E. Angew. Chem. 1949, 61, 179. (11) Chatterjea, J. N.; Prasad, K. J. J. Indian Chem. SOC.1960, 37, 357. (12) Backmann, W.E.; Carmack, M. J . Am. Chem. SOC.1941.63, 2494. (13) Regros, R.; Cagniant, R. C. R.Hebd. Seances Acad. Sci. 1960,251, 553. (14) Perin-Rousel, 0.;Buu-Hoi, N. P.; Jacquignon, P.; Phuoc-Hien, Do. Bull. SOC.Chim. Fr. 1971, 2160. (15) Sisido, M.; Egusa, S.; Okamoto, A,; Imanishi, Y .J . Am. Chem. Soc. 1983, 105, 3351.
TChrrCry,
compd Ph(CH2)zCOOCho ..
mp, "C 109-112 (1 11.5) Ph(CH,),COOCho 98-99 (98.3) 1-NapCH2COOCho 94 1-Nap(CH2)2COOCho 104 1-Nap(CH2),COOCho 87 2-NapCH2COOCho 157 2-Nap(CH2)2COOCho 149 2-Nap(CH2),COOCho 127 9-Ant(CH2),COOCho 124-1 26 1-Pyr(CH2),COOCho 164-1 66 1-Pyr(CH,),COOCho 111 1-Pyr(CH2)4COOCho 163
OC OC 109-112 (114.1) 66 (79)' . , TChc-Iso,
91 (91.3)
59 (59)"
b 49 48 b 149 113 b 122 b b
b
b 107 75 b 89 b b
aEnnulat, R. D.; Brown, A. J. Mol. Cryst. Liq. Cryst. 1971, 12, 367. bNo cholesteric phase.
"A-
0-0" Pyr-212-Nap-2
012 ' 0;
0:60 0:8 1.b 4 0 0.2 0.4 0.6 0.8 1.0
2 N a p -2 / Ph-2
Pyr-2/Ph -2
v
W
Cho
Cho
60
Pyr-4/2-Nap-4
2-Nap-4/Ph-4
Pyr-41Ph-4
Figure 1. Phase diagrams for two-component mixtures of cholesteryl w-arylalkanoates. The abscissa indicates molar fraction of the first component. Notations for phases are as follows: Is0 = isotropic liquid, Cho = cholesteric liquid crystal, Sc = supercooling liquid, and C = crystal.
introduced to the refractometer to obtain the dispersion of the refractive indices. Results and Discussion Phase Diagramsfor Cholesteryl o-Arylalkanoates. Some of the cholesteryl esters synthesized formed the CLC phase and showed a monotropic transition; Le., the CLC phase was observed only when the temperature was decreased from the melting point. Table I1 lists melting points, critical temperatures TCho,for the reversible change between isotropic liquid and CLC, and transition temperatures TW, for the irreversible change from CLC to crystal. Only cholesteryl esters having even numbers of methylene groups ( n = 2 or 4) form the CLC phase. This behavior has been found for the series of cholesterylw-phenylalkanoates." No CLC phase was attained, however, for 1-Pyr(CH2),COOCho (Pyr-4) and (16) Miiller, W. U.; Stegemeyer, H. Ber. Bunsenges. Phys. Chem. 1973,
77, 20. (17) Ennulat, R. D.; Brown, A. J Mol. Crysr. Liq. Cryst. 1971, 12, 367.
The Journal of Physical Chemistry, Vol. 88, No. 13, 1984 2895
Aromatic Cholesteric Liquid Crystals I
I
I
I
I
I
I
$000
a
600
a
l-Nap(CH2)4COOCho
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2-Nap(CHz)4COOCho
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CH$4C00 ChO
I
I
I
I
100
60
20
I
Temp
1
1
500
I
300
700
140
("CI
Figure 2. Selective reflection wavelengths (0)and helix pitches ( 0 )of some single-componentliquid crystals plotted as a function of tempera-
400 500 WAVELENGTH (nm)
600
700
Figure 4. Circular dichroism of 1-Nap2 ((-) 6.2-pm thickness, (---) 1.3-pm thickness at 40 "C), 1-Nap4 11.4-pmthickness at 40 "C), and 2-Nap4 ((-*-) 1O.pm thickness at 90 "C). ((-e.)
ture.
8oon 'La
T r a n s i t i o n Moment
Molecular Axis
Figure 5. Probable molecular conformations of 1-Nap-4 (-) Nap-4 (---) in the CLC phase.
700
I
I
I
60
I
I
100 Temp ("0
Figure 3. Selective reflection wavelengths of mixed liquid crystals of
Pyr-2/Ph-2 having different compositions. 9-Ant(CH2)2COOCho(Ant-2). No phase other than the CLC, isotropic liquid, and crystalline phases was detected by a polarized microscope observation of the cholesteryl esters. The transition temperatures from the CLC to the crystal phase in Table I1 are much higher than room temperature, except for the cases of 1-Nap-2 and 1-Nap-4. The temperature range for the CLC phase can be lowered by mixing different cholesteryl esters. The phase diagrams for several two-component mixtures are shown in Figure 1. For example, an equimolar mixture of Pyr-2 with 2-Nap-2 forms the CLC phase at room temperature. Pyr-4, which does not exhibit the CLC phase by itself, forms the homogeneous CLC when mixed with other CLC-forming substances. The homogeneity of the mixed CLC was confirmed by the observation of the selective reflection at an intermediate wavelength between those of the two components (see below). Helix Pitch and Helix Sense of the Chromophoric Cholesteric Liquid Crystals and Orientation of the Chromophores within a Layer. It is well-known that a CLC having a helix pitch P and average refractive index n selectively reflects a light of wavelength A, = nP.18 The reflection occurs only for a circularly polarized light of the same screw sense as the CLC; i.e., a right-handed CLC reflects only right-handed circularly polarized light.'* Hence, measurement of the reflection wavelength and the circular dichroism provides information on the helix pitch and the helix sense. Figure 2 shows the reflection wavelengths of some chromophoric (18) De Vries, H1. Acta Crystallogr. 1951,4, 219.
and 2-
CLC's as a function of temperature. The helix pitch was calculated from the reflection wavelengths by using the average refractive index for a light propagating along the helix axis, which was measured with a refractometer. The pitches for 1-Nap-2 and 1-Nap-4 are plotted also in Figure 2 (solid circles). It is clear that the lower the temperature, the longer the helix pitch. The reflection wavelengths for mixed CLC's of Pyr-2 with Ph-2 are plotted in Figure 3 for different compositions of the mixture. Each mixed CLC shows a single A,, at an intermediate wavelength between those of the two components, although A,, of Pyr-2 is longer than 800 nm and could not be observed. The presence of single A,, indicates the homogeneity of the mixed CLC's. CD spectra of 1-Nap-2, 1-Nap-4, and 2-Nap-4 in the CLC phase are shown in Figure 4. Positive peaks at the selective reflection region indicate that left-handed circularly polarized light is selectively reflected, and therefore the helix sense of the CLC's is left-handed. It was found that all CLC's listed in Table I1 form left-handed helices. This result is in harmony with the fact that between those of the all mixed CLC's showed intermediate A, components. Contrary to the reflection region, the sign of CD spectrum at the 'La absorption band (around 290 nm) depends on the link position to the naphthyl group. According to the optical theory of CLC,5 on the average, a left-handed CLC exhibits negative CD peaks when the absorption transition moment is nearly parallel to the optical axis or the molecular axis of the quasi-nematic layer. Positive CD is expected when the transition rnoment is nearly perpendicular to the optical axis. The CD spectra in Figure 4 indicate that the transition moment of the naphthyl 'La band, which is directed along the short axis of the naphthyl ring,19 is parallel to the molecular axis of 1-Nap-2 and 1-Nap-4, but it is perpendicular to the molecular axis of 2-Nap-4. The orientation suggested from the CD spectra is consistent with the molecular structure shown in Figure 5, in which an extended conformation is assumed for -(CH,),-CO-0sequence. In the figure the Ar-CH2 bond takes a gauche state so that the aromatic group lies on the quasi-nematic layer. An empirical conformation energy calculation for 1-Nap-CH2-CH,-CH, supports the stability of (19) Mnurrel, J. N. "The Theory of Electronic Spectra of Organic Molecules ; Wiley: New York, 1963.
2896
The Journal of Physical Chemistry, Vol. 88, No. 13, I984
Sisido et al. I
I
I
I
0.
0.'
d; 0.C
5
c.
VI
0
1
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i I
I
400
500
400
500
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Figure 6. Fluorescence (bottom) and CPF (top) spectra of some cholesteryl w-arylalkanoatesin the CLC phase: (A) 1-Nap-2 at 40 O C , (B) 1-Nap4 at 40 OC (dashed line indicates that the CPF spectrum is distorted by the selective reflection), (C) 2-Nap-2 at 80 O C (---) and 2-Nap-4 at 80 O C (-), (D) Pyr-2/Ph-2 (60/40) mixed CLC at 80 OC.
the gauche state as compared to the trans state.*" Fluorescence Spectra and Circularly Polarized Fluorescence of Chromophoric Cholesteric Liquid Crystals. Figure 6 shows fluorescence (bottom) and CPF (top) spectra of the chromophoric CLC's. Excimer formation is evident for 1-naphthyl and pyrenyl esters, but little excimer fluorescence was detected in the 2naphthyl derivatives. The presence of excimer in 2-Nap-4 is, however, evident from an intense C P F signal at the excimer fluorescence region, as will be discussed later. (20) Sisido, M.; Shimada, K. J . Am. Chem. SOC.1977, 99, 7785.
In general, fluorescence emitted from chiral chromophores is circularly polarized, and the C P F spectroscopy provides information on the geometrical or the electronic structure of the chromophore in the excited state.*] Fluorescence emitted from chromophores embedded in the CLC phase has been found to be strongly circularly and its sign and intensity are theoretically related with the helix sense, the helix pitch, refractive index, and the averaged orientation of the fluorescence transition moment in the quasi-nematic layer.* Stegemeyer and co-w~rkers'~ (21) Richardson, F. S.; Riehl, J. P. Chem. Reu. 1977, 77, 773.
Aromatic Cholesteric Liquid Crystals
I
The Journal of Physical Chemistry, Vol. 88, No. 13, 1984 2897 I
I
\I
Cholesteric Layer Plane
( a ) l n t r a l a y e r Excimer 400
600
500 WAVELENGTH (nrn)
\
\
Figure 8. Steady-statefluorescence spectra of a mixture of Pyr-2/Ph-2 (60/40) in three different types of phases: (-), CLC phase at 40, 60, 80, and 100 OC from the top; (---), crystal phase at 40, 60, and 80 O C from the top; (-.-), isotropic liquid phase at 120, 140, and 160 OC from the top. I
-1 (b)
l
l
TCho
I
I
I
I
1
l n t e r l a y e r Excirner
Figure 7. Probable configurations of excimers of cholesteryl 1- and 2-naphthylalkanoatesin the CLC phase. Excimer fluorescence transition moments are indicated for 1-naphthyl (e) and for 2-naphthyl (+--*) derivatives. In configuration (a), two naphthyl groups lying on the same layer twist to form a sandwich-like configuration. The molecules are viewed from the top of the layer. In configurtion (b), two naphthyl groups lying on the adjacent layers form the excimer configuration. In this case a side view of the CLC is shown.
reported the C P F of chromophores dispersed in the CLC phase at a particular wavelength. However, no CPF spectrum, Le., CPF as a function of fluorescence wavelength, has been reported for chromophoric CLC. The top curves of Figure 6 are C P F spectra plotted in terms of Kuhn's luminescence dissymmetry factor g,, as an ordinate
I
0'5
t
0.2'
i
/
Y
I
i.2
I
2.6
I
I
3.0
I
I
I
3.4
l / T ( l / K ) x103
where ILand I , are the intensities of left and right circularly polarized fluorescence, respectively. All chromophoric CLC's exhibited positive C P F at the excimer fluorescence region. According to Pollmann et a1.,* a positive C P F signal is expected for a fluorescence transition polarized perpendicularly to the optical axis in a left-handed CLC. Therefore, the results in Figure 6 suggest that the excimer fluorescence transition moment is nearly perpendicular to the optical axis for all chromophoric CLC's investigated. A perpendicular orientation has been reported for the transition moment of pyrene excimers in the nematic LC22 and in the CLC.9 A modified PPP-CI molecular orbital calculation has been reported for naphthalene excimers having various geometrical configuration^.^^ The result predicted that the fluorescence transition is forbidden for a sandwich-type dimer having Dlh symmetry, but small deviations from the symmetrical configuration allow two excimer transitions, one perpendicular to the aromatic ring and the other parallel to the long axis of naphthalene. It has not been established yet which transition contributes more significantly, although perpendicular transition have been observed in some aromatic e x c i m e r ~ . ~ ~ In view of the above theoretical prediction and the C P F results, two possible geometrical configurations which lead to the perpendicular excimer transition moment with respect to the optical axis are proposed and shown in Figure 7 . In configuration (a), two adjacent naphthyl groups twist by about 90° from the plane of the quasi-nematic layer to attain a sandwich-like excimer configuration. In this case the excimer transition moment which (22) Sackmann, E.; Rehm, D. Chem. Phys. Lett. 1970, 4, 537. (23) Hoshi, T.; Kobayashi, M.; Yoshino, J.; Komura, M.; Tanizaki, Y .Eer. Eunsenges. Phys. Chem. 1979, 83, 821. (24) Suzuki, S . ; Fujii, T.; Yoshiike, N. Chem. Phys. Lett. 1979, 62, 287.
Figure 9. Fluorescence intensity ratios of excimer (480 nm) to monomer (379 nm) of Pyr-2/Ph-2 (60/40) mixture in the CLC (O), crystal ( o ) , and isotropic liquid (A)phases.
is perpendicular to the aromatic rings makes a major contribution to the positive C P F observed. In configuration (b), the two naphthyl groups in the adjacent layers approach to within about 3.5 %I to achieve an excimer configuration. In this case the excimer transition moment which is parallel to the long axis of naphthyl groups shoudl contribute to the positive CPF signal. Configuration (a) seems more likely, because it predicts positive excimer CPF signals for both 1-naphthyl and 2-naphthyl cholesteryl esters. An explanation for the positive C P F signals observed in both the 1and 2-naphthyl esters in terms of configuration (b) is difficult, since different orientations for the two naphthyl groups have been suggested from CD spectra (Figure 5). In Figure 6, no significant C P F signal was observed in the monomer fluorescence region. The absence of a C P F signal suggests no preferred orientation of the transition moment of the monomer fluorescence in the chromophoric CLC. However, monomer fluorescence from a small amount of Pyr-2 (less than 1 mol %) in a mixed CLC of cholesteryl nonanoate (35 mol %) and cholesteryl chloride (65 mol %) showed intense CPF spectra which oscillate frequently between positive and negative values, at the temperatures above and below the nematic point (43 "C) of the mixture.25 It is supposed in the case of Figure 6, the high chromophore concentration may have caused interchromophoric interactions which mixed up differently polarized vibronic states and led to the absence of polarization of the monomer fluorescence. This kind of phenomenon has been known as a concentration depolarization in the linear polarization of fluorescence.26 (25) Sisido, M.; Takeuchi, K.; Imanishi, Y. Chem. Lett. 1983, 961. (26) Dorr, F.Angew. Chem. 1966, 78, 457.
2898
J. Phys. Chem. 1984, 88, 2898-2902
Excimer Formation in the Cholesteric Liquid Crystal, Isotropic Liquid, and Crystalline Phases. Figure 8 shows fluorescence spectra of the mixture of Pyr-2 and Ph-2 (60/40) in the three types of phases at different temperature^.^^ The excimer/monomer ratio is largest in the CLC phase. The same situation was also observed in the 1-Nap-2 case. Ratios of fluorescence intensities for excimer (480 nm) and monomer (390 nm) peaks were calculated from the fluorescence spectra and are shown in Figure 9. It is evident from the figure that the ratios as well as their temperature dependences for the three phases differ substantially with each other. The excimer formation of a relatively small amount of pyrene molecules dispersed in CLC has been reported by Anderson et al.4 Their results indicate only a small difference between the excimer/monomer ratios and their temperature dependences in (27) As has been noted by Novak et al. (Mol. Crysr. Liq. Cryst. 1973,20, 213), the fluorescence intensity changes erratically with temperature and phase changes. This may be ascribed to the change in the path length of the exciting light due to light scattering by the molecular assemblies. Therefore, any discussion on the absolute intensity of fluorescencewas avoided in this study.
the CLC and isotropic liquid phase, when the data for the CLC phase were extrapolated into the temperature range for the isotropic liquid phase. The small difference may be expected, since the pyrene molecule can diffuse in the CLC phase as well as in the isotropic liquid phase, although some kind of motions may be restricted in the former phase. In the present system, since the chromophoric groups are covalently bound to CLC-forming components,the excimer formation should be affected substantially by the phase changes. The difference in the efficiencies of the excimer formation may be interpreted by the differences in the densities of the excimer-forming sites and/or in the frequencies of the excitation energy migration in the three phases. A detailed kinetic study on the excimer formation processes will clarify this problem, and it is now under way. Registry No. Ph(CH2),COOCho, 14914-99-9;Ph(CH2)4COOCho, 14978-35-9; 1-Nap(CH2),COOCho, 89922-21-4; I-Nap(CH2)4COOCho,89922-22-5; 2-NapCH2COOCho, 89922-23-6; 2Nap(CH2)2COOCho,89922-24-7;2-Nap(CH2)4COOCho, 89922-25-8; 1-Pyr(CH2)2COOCho,86851-21-0;1-Pyr(CH2)3COOCho,7608 1-97-5; 1-Pyr(CH,),COOCho, 89922-26-9; 1-NapCH,COOCho, 89922-27-0; 9-Ant(CH2)2COOCho,8685 1-22-1.
Ab Initio Self-Consistent Field Calculations of Lithium Atom Insertion into a Carbon-Hydrogen Bond of Methane John G. McCaffrey, Raymond A. Poirier,+ Geoffrey A. Ozin,* and Imre G. Csizmadia Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S IAl (Received: August 12, 1983)
The energy and geometries of the lowest 2AIand ,E states of the product resulting from lithium atom insertion into a C-H bond of methane in C,,symmetry were obtained by ab initio molecular orbital calculations. Geometries were optimized with three different basis sets, STO-3G, 3-21G, and 6-31G**. The possible pathways to insertion of the lithium atom into a CH bond of methane were also considered. The geometry of the more stable 2Al state suggested that the inserted molecule is best described as a methyl radical interacting with lithium hydride, where the C-Li bond can be taken as a single electron bond. The 2E state showed a different behavior in that a stronger interaction exists between the methyl and lithium hydride, with the C-Li bond considerably shorter and correspondingly longer C-H bonds. In this case the C-Li bond can be considered as a two-electron bond. The barrier for the formation of methyllithium hydride appears to be associated with the abstraction of the hydride. A brief comparison of the bonding in CH3CuH with that in CH3LiH showed considerable similarities.
Introduction Insertion of single metal atoms into the C-H bonds of saturated hydrocarbons has been observed recently.'-4 Matrix isolation techniques have been used extensively in this area. Copper and iron are the two metals best documented with regard to insertion. In the case of copper, insertion into a C-H bond of methane occurs when an isolated copper atom in a frozen CH4 or CH4 rare gas matrix is excited from its 2Selectronic ground state to its P excited state. Experimental evidence for metal atom photoinsertion into a C-H bond of CH4 comes mostly from UV-vis, infrared, and electron spin resonance spectroscopy in conjunction with detailed, isotopic substitution and kinetic measurements. In the case of copper, infrared studies show, besides CH,CuH, bands ascribed to CuH and CH, as major components and CH3Cu and H as minor ones. Photoinsertion of iron atoms into CH4 has been shown to occur by Margrave et ale2using broad-band irradiation and by Ozin et aL3 using narrow-band excitation at the 300-nm atomic
I
Present address: Scarborough College, Physical Sciences Division (Chemistry), University of Toronto, Toronto, Ontario, Canada MIC 1A4. 0022-3654/84/2088-2898$01.50/0
iron resonance lines, the product being exclusively CH3FeH. Lithium is found in conjunction with many organic molecules and finds use as a synthetic reagent in organic and organometallic chemistry. As a result, the nature of the lithium-carbon bond in these molecules has been of considerable theoretical i n t e r e ~ t . ~ - ~ Synthetically, organolithium reagents are very useful as a source (1) G. A. Ozin, D.F. McIntosh, S . A. Mitchell, and J. Garcia Prieto, J . Am. Chem. SOC.,103, 1574 (1981). (2) W. E. Billups, M. M. Konarski, R. H. Hague, and J. L. Margrave, J . Am. Chem. SOC.,102, 7393 (1980). (3) G. A. Ozin and J. G. McCaffrey, J. Am. Chem. SOC., 104, 7351 (1982). (4) K. J. Klabunde and Y.Tanaka, J . Am. Chem. Soc., 105,3544(1983). (5) A. Streitwieser, J. E. Williams, S. Alexandrato, and J. M. McKelvey, J . Am. Chem. Soc., 102,4572 (1980). (6) G. D.Graham, D. S. Marynick, and Lipscomb, J . Am. Chem. Soc., 102, 4572 (1980). (7) J. B. Collins, J. D. Dill, E. D. Jemmis, Y . Apeldig, P. von R. Schleyer, R. Seeger, and J. A. Pople, J . Am. Chem. SOC.,98, 5419 (1976). (8) N. C. Baird, R. Barr, and R. F. Datta, J . Orgunomet. Chem., 59, 65 (1973). (9) M. F. Guest, I. H. Hillier, and V. R. Saunders, J . Orgunomet. Chem., 44, 59 (1972).
0 1984 American Chemical Society
