7450
J. Phys. Chem. 1992,96,1450-7454
Resonant Nonlinear Optical Properties of Spin-Cast Rims of Soluble Otlgomeric Bridged (Phthalocyaninato)ruthenium(I I ) Complexes A. Crund, A. Kaltbeitzel? A. Matby, R. Schwarz, C. Bubeck,* Max-Planck-Institut fiir Polymerforschung, Postfach 3148, W-6500 Mainz, Germany
P. Vermehren? and M. Hanack Institut fiir Organische Chemie der Universitiit Tiibingen, Auf der Morgenstelle 18, W-7400 Tiibingen, Germany (Received: April 7,1992)
Spin-cast filmsof the oligomeric bridged (phthalocyaninato)ruthe~um(II)complex [t-Bu,PcRu(dib)], which is axially linked by a p-diisocyanobenzene (dib) bridging ligand, were investigated by degenerate four wave mixing with laser wavelengths varied over the complete range of the Qband absorption and by third harmonic generation at 1064 nm. The magnitudes ) ~(~)(-u;w,w,-w) and characteristic dependence on the absorption coefficient U(W) were determined. of ~ ( ~ ) ( - 3 w ; o , o , wand By comparison with other lower-dimensionalorganic materials with a conjugated *-electron system, we conclude that the optical properties of this [t-Bu4PcRu(dib)], oligomer are mainly determined by the two-dimensional character of the phthalocyanine units, which behave as saturable absorbers with a relaxation time of 70 A 5 ps.
1. Introduction The third-order nonlinear optical properties of phthalocyanines (Pc) have gained considerable interest recently,’” because of their twdmensional(2D) conjugated *-electron system and their high thermal and chemical stability. The latter is a prerequisite for any application of their resonantly enhanced optical nonlinearities such as the saturable absorption of the large-oscillator strength The great Qband, which is located in the near-infrared potential of organic materials for nonlinear optics is given by the possibility to design molecular systems with the required ahsorption properties and the facility to prepare thin solid films from solutions. Bridged Pc-transition metal complexes have been synthesized and investigated recently as models for onedimensional (1D) organic conductors.* A large variety of macrocycles, bridging ligands, and metal complexes can be combined. Here we shall present the first description of the third-order nonlinear optical properties of a typical representative of this class of materials. It is a phthalocyaninederivative with a ruthenium central metal which is bridged by p-diisocyanobenzene (dib) and a phthalocyanine ring being substituted by four terr-butyl groups.9 The chemical structure of this oligomer, abbreviated [t-Bu4PcRu(dib)], is shown in Figure 1. The thirdsrder nonlinear optical susceptibilities ~ ( ~ ) ( - 3 w ; w , o , w ) and xQ)(-o;w,w,-w) of organic materials can be well characterized by third harmonic generation (THG) and degenerate four wave mixing (DFWM) experiments.1° These methods have been used to characterize and classify various conjugated polymers and dye systems with respect to their dimensionality and kind of electronic coupling effects between the chromophore^.^*' For example the class of polymers with a onedimensional (1D) conjugated ?r-electronsystem has very large ~ ( ~ ) ( - 3 w ; w , w , ovalues. ) ) increase with a power law Eod It was found that their x ( ~values of their optical gap Eoll-12in accordance with the scaling law of Flyt~anis.l~9’~ By means of resonant DFWM experiments, it was found that ~ ( ~ ) ( - w ; w , w , - w )scales linearly with the absorption coefficient a(@) if the laser frequency w is tuned into the main absorption band of 1D conjugated po1~mers.I~ This behavior can be attributed to phase-space filling effects by 1D excitons.” Dye systems with a 2D conjugated ?r-electron system, which are electronically isolated from each other, behave very different. Their ~ ( ~ ) ( - 3 w ; w , o , wvalues ) are more than 1 order of magnitude With smaller as compared to a 1D system with similar E0.12918*19
resonant DFWM experiments it was found that ~ ( ~ ) ( w ; w , w , - w ) of rhodamine 6G increases with aZ(o)as expected for saturable absorption in electronically isolated two-level systems.14 Therefore THG and DFWM experiments will be used here also as spectroscopic and analytical tools. We present quantitative data on the spectral dependence of ~ ( ~ ) ( - w ; w , w , - w )of [tBu4PcRu(dib)], solid films and their relaxation times and addreas the question whether the [r-Bu$cRu(dib)], oligomer behaves like a - l D electronic system or if the coupling of the bridging ligand is too small, that the 2D character of the Pc *-electron system prevails. 2. ExperimeatalSection
2.1. Material and F h Preparation. The synthesis of the oligomeric bridged tetra-tert-butyl(phtha1ocyaninato)ruthenium(11) complex, [t-Bu4PcRu(dib)],, in which the bridging ligand is p-diisocyanobenzene(dib), and the determination of the chain length by NMR spectroscopy were described in detail recently.9,2032rWe studied an oligomer with an average degree of polymerization of approximately 12. Solid films of [t-Bu4PcRu(dib)], were prepared by spin-casting from chloroform solutions on hydrophilic microscope slides or quartz substrates. The thickness of the films was measured by means of a Tencor 200 alpha step profiler. 2.2. Nonlinear Optical a d Spectroscopic Methods. The DFWM experiments were performed using a folded BOXCARS configuration. The output of a ultrafast dye laser system (Coherent Antares and 700 series dye laser with saturable absorber jet and cavity dumper) was split into three beams with variable delay and focusad on the sample with a lens (focal length 16 mm). The intensity at the sample position could be varied in the range of 0.1-15 GW/cm2. The time resolution is given by the laser pulse width of approximately 1.5 ps. Improvements in the detection sensitivity by means of a time modulation method enabled DFWM investigations over the full tuning range of several laser dyes without the need of an amplifier system. The time-dependence of the transient grating was measured by varying the delay time AT, of one of the three beams with respect to the other two beams which generate the grating. In the case of an exponential decay, the relaxation time T can be obtained from the diffracted intensity Z according to22923
Author to whom correspondence should be addressed. ‘Present address: Hoechst AG, Postfach 800320,W-6230Frankfurt 80, Germany.
0022-3654/92/2096-1450$03.oo/o
- ?)
2AT,
I
exp(
In a semilogarithmicplot of I against ATp the relaxation time T 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 18, 1992 7451
Spin-Cast Films of [t-Bu4PcRu(dib)],
A
M =Ru
Figure 1. Schematic drawing of the oligomeric bridged (tetra-rert-butylphthalocyaninato)ruthenium(II)complex, [r-Bu,PcRu(dib)],.
40
36
[nml Figure 2. Absorption spectrum of a solution of [r-Bu4PcRu(dib)],in CHCI,with a concentration of e = 8 X lod mol/L.
can be derived. The effective evaluated using the equation
x(') value of
1[nml Figure 3. Absorption spectrum and dispersion of thin film of [t-Bu4PcRu(dib)],.
x(~)(-w;w,w,-w)
of a
the sample was
' 2adexp(ad) 1 - exp(-2ad)
(2)
where a,n, d, and I are the absorption coefilcient, refractive index, thickness of the sample, and the intensity of the generated beam, respectively. The subscript 'ref refers to the CS2reference with a value of xg) = 6.8 X emz4 The experimental setup for the THG measurements was based on the Maker fringe method as described r e ~ e n t l y . *Infrared ~~~~ light pulses (A, = 1064 nm) with 35-ps pulse duration, generated by an actively/pssively modelocked Nd:YAG laser were focused on the sample with the focal region in vacuum. For reference we used ~(~)(-3w;o,w,w) = 3.1 1 X esu for fused silica at 1064 nm.26 The harmonic intensity was analyzed as a function of the incidence angle via a formalism including all free and bound waves.25 The film thickness, the refractive indices, and the absorption coefficient at o and 3w were taken into account. For the determination of both the absolute value and the phase of ~(~)(-3o;o,o,w) we first investigated a sample with the film on the front side of the substrate. Finally the Maker fringe pattern of the substrate alone was measured after removal of the film without displacement of the sample. Reflection and absorption spectra were measured with a Perkin-Elmer Model Lambda 9 spectrometer in the visible range. For
x Inml Figure 4. Dispersion of the refractive index for a film of [r-Bu4PcRu(dibll,.
reflection measurements the quartz substrate was used with a rough and black backside to eliminate backscattered light. The incident light was s-polarized and aligned at an angle of 1 5 O to the substrate normal. The refractive index n was evaluated by solving the Fresnel equations without any fit parameter.27-28 3. Experimeat.lResulQ The absorption spectrum of a solution of [t-Bu4PcRu(dib)In in CHC13 with a concentration of c = 8 X 10" mol/L is shown in Figure 2. The absorption spectrum and the dispersion of the resonant x(')(-u;o,o,-u) values of a film of [r-Bu,PcRu(dib)], with a thickness of d = 160 nm are shown in Figure 3. In comparison to the solution spectrum in Figure 2 the absorption
Grund et al.
7452 The Journal of Physical Chemistry, Vol. 96, No. 18, 1992 in
0
-2 -
-1
I
B
EL v
c -2
-3 -10
10
30
50
70
90
110
ATP [PSI
Figure 5. Time dependence of a transient grating in a thin film of [r-Bu,PcRu(dib)], (semilogarithmic plot, XL = 600 nm).
+
-30
.
.
-10
.
.
10
.
I
30
Angle of Incidence [DEG]
Figure 7. Third harmonic intensity as a function of the incidence angle for a 160 nm thick film of [t-Bu,PcRu(dib)], (bottom) and of the fused silica substrate alone (top). The solid lines represent the calculated Maker fringe pattern including all free and bound waves.
The Maker fringe pattern measured by THG of a film of [r-Bu,PcRu(dib)], and the fused silica substrate are depicted in Figure 7. The fringes have been calculated including all free and bound waves as described recently?* The best fit gave an absolute value of ~ ( ~ ) ( - 3 w ; w , w , w=) (3.7 f 1.5) X lo-'* esu with a phase of 155 f loo. This value is about 4 orders of magnitude smaller than the largest ~ ( ~ ) ( - w ; w , w , - wdata ) obtained by DFWM. This results from the different resonance contributions to x ( ~in) the DFWM and THG experiments. I' 20
I
100
1 200
[a.u.]
F
i 6. Doublelogarithmic plot of the diffracted intensity I versus the average laser power ( l o for ) a film of [t-Bu&Ru(dib)], at a wavelength of 595 nm.
spectrum of the film is considerably broadened. The dispersion of the refractive index was obtained from reflectometry measurements. The result is depicted in Figure 4. These values were taken into account in the evaluation of x ( ~ ) If . investigated near the absorption maximum of the Q-band, the film showed strong degradation effects although the repetition rate of the exciting laser pulses was 40 kHz and the intensity was reduced to 100 MW/cmZ. This sample decomposition may be due to either photochemical or thermal reactions or due to laser-induced desorption processes. For these x ( ~values ) a larger error must be taken into account (seeFigure 3). The experimental error of x ( ~ ) in the range of low absorption coefficients corresponds to the size of the measuring points depicted in Figure 3. The relaxation behavior of a transient grating of a [tBu4PcRu(dib)], film is shown in Figure 5 . The relaxation time T of the transient grating can be evaluated from the slope of a semilogarithmic plot of the diffracted intensity as a function of the delay time AT, using relation 1. A major relaxation time T of 70 f 5 p at a laser wavelength XL = 600 nm was found. An influence of XL on T could not be observed in the Wavelength range between 560 and 750 nm. The dependence of the diffracted intensity I on the excitation intensity Io at the sample position is shown in Figure 6. In the low intensity limit we obtain I a Io3, as expected due to the interaction of the three pump fields.29 At average laser powers larger than Io = 100 mW, corresponding to an intensity at the sample of approximately 11 GW/cmz, deviations from this dependence m r due to saturation effectdo or due to self-action effects derived from the intensity dependence of the complex refractive index.
4. Discussion The comparison of the absorption spectra of the solution (Figure 2) and films of [t-Bu4PcRu(dib)], (Figure 3) show that an inhomogeneous broadening of the main absorption band occurs in the film similar to earlier investigations! Thii broadening effect is due to a dipoledipole i n t e r a c t i ~ n ~ between ~ s ~ ~ phthalocyanine units of adjacent oligomers which are arranged in many different geometrical positions. The fast relaxation time of 70 ps in the solid film of [tBu4PcRu(dib)], probably has an electronic origin. The following photophysical mechanisms may be responsible for this: (a) The dense packing of the [t-Bu4PcRu(dib)], units in the solid film leads to a shortening of the lifetime of excited states. Reduction of the fluorescence lifetime of densely packed films as compared to solution is known as concentration quenching.3335 (b) The inhomogenously broadened absorption spectrum (Figure 3) of the film of [t-Bu4PcRu(dib)], is due to different sites with different resonance frequencies. In the DFWM experiment one selected absorption site corresponding to the laser frequency w is excited. From this site a very fast energy transfer of the excitation energy to neighboring sites can follow. Fast energy transfer processes to trap states are well-known in hostguest systems in organic moIecular cry~tals.9~7~~ The characteristic time of this energy transfer process is measured by DFWM. The relaxation of the trap states occurs either radiationless or via delayed fluorescence. (c) Energy migration, for example exciton diffusion to the dark areas of the optical grating or recombination of mobile exciton states, has been d i s c ~ s s e d . An ~ ~ *energy ~ ~ migration process of this type requires exciton diffusion lengths in the order of the grating constant A. In our experiment A was 4 bm at a wavelength of 700 nm. In a disordered system like a spin-cast film of [tBu4PcRu(dib)],, the diffusion length of the incoherent excitons is expected to be much shorter than A; therefore this process is not probable. The dependence of ~ ( ~ ) ( - w ; w , w , - o )on a for a film of [?Bu4PcRu(dib)], is depicted in Figure 8. The x ( ~values ) scale
The Journal of Physical Chemistry, Vol. 96, No. 18, 1992 1453
Spin-Cast Films of [t-Bu4PcRu(dib)],
-7
.
I
5
.
3- ,o’9
3:
-
4 .
0 X
10-10 .
,oil 103
/i io4
105
or a series of rylene molecule^.'^ Therefore we assume that the difference between the large ~(~)(-3w;w,o,w) data of 1D conjugated polymers and the smaller ~(~)(-3w;w,w,w) values of planar dye systems is a characteristic manifestation of dimensionality effects in the optical properties of organic compounds. This is in line with considerations by Wu et ale3
106
a [cm”]
Figure 8. xO)(-O;w,w,-w) as a function of the absorption coefficient for a thin film of [r-Bu,PcRu(dib)], if the laser wavelength XL is varied over the whole range of the Q absorption band.
with the square of a. This scaling was also measured with a thin film of rhodamine 6G.I4-I8 This indicates saturable absorption in two-level systems which is the simplest approach to describe resonant DFWM. If the laser frequency w coincides with the frequency of the optical transition 52,f between the ground and final states, the resonant nonlinear optical susceptibility for parallel polarizations of the beams can be written as40g41 I ~ ( ~ ) ( - w ; w , w , - ~= ) l aoTl Tdr8d4 (3) In eq 3 p,f is the transition dipole moment between g a n d 3 T I and T2 are the population relaxation and dephasing times, respectively. The factor 4contains several physical constants which depend on the system of units and the molecular arrangement. The absorption coefficient at the center of a homogeneously broadened line is a. Ip8#. As outlined above, the absorption spectrum of films of [t-Bu,PcRu(dib)], is inhomogeneously broadened. Therefore the measured absorption coefficient a(@) is built up by a statistical distribution of a multitude of absorbing units and we obtain I ~ ( ~ ) ( - w ; w , w , - w ) l= aTIT2/a2(w) (4) N
Under selective laser excitation we describe our DFWM results in the case of solid films of [t-Bu,PcRu(dib)], with transient spectral hole burning in an inhomogeneously broadened line of saturable two-level systems.41The ratio of the homogeneous to inhomogeneous broadening is not known in this specific example. The quadratic dependence of I~(~)(-w;w,w,-w)lon a(@)displayed in Figure 8 is in very good agreement with eq 4. Sites which are not resonantly excited by the laser frequency wL have their major influence presumably only on the relaxation times TI’and T i . The observed scaling X(~)(-W;W,W,-W) a2(w) further indicates that the Pc units in this [t-Bu4PcRu(dib)], oligomer are not strongly coupled by the dib bridge. If strong excitonic coupling effects between the repeat units of [t-Bu,PcRu(dib)], would occur, we would expect a similar linear relationship between x ( ~ ) ( - w ; w,o,-w) and a ( w ) as observed in polymers with an extended 1D conjugated r-electron system.I4 Obviously these extended excitation states do not exist in [t-Bu4PcRu(dib)], oligomers. Another characteristic distinction between [t-Bu,PcRu(dib)], oligomers and the class of 1D conjugated polymers can be seen in the magnitude of ~ ( ~ ) ( - 3 w ; w , w , wobtained ) from THG mea~ u r e m e n t s . l ~Although -~~ the measured phase of = 155 10’ indicates contributions from two and three photon resonances, the modulus of ~(~)(-3w;w,o,w) is more than an order of magnitude smaller as compared to representative data of 1D conjugated polymers extrapolated to the same spectral position of their low energy absorption maximum. However the ~(~)(-3w;w,w,w) value of [t-Bu,PcRu(dib)],, favorably compares with the data of other dye systems which have a planar conjugated *-electron distribution like rhodamine 6G N
*
5. Conclusion We conclude that the optical properties of [t-Bu4PcRu(dib)], oligomers are mainly determined by the individual phthalocyanine units. The diisocyanobenzenebridging ligand does not lead to a one-dimensional character of the optical properties of these [t-Bu,PcRu(dib)], oligomers. The magnitude of ~(~)(-3o;w,w,o) is similar to other dye systems with a planar distribution of their conjugated *-electron system, but it is considerably smaller as compared to polymers which have a conjugated one-dimensional *-electron system. In resonant DFWM experiments with solid films, the Pc units behave as saturable absorbers. Small intra- and intermolecular dipole coupling effects exist in solid films of [t-Bu4PcRu(dib)],. These effects are responsible for the spectral broadening of the absorption band and the short relaxation time of 70 ps found by DFWM. Acknowledgment. We thank Professor G. Wegner for helpful and stimulating interest in this work. We also thank H. Menges for considerable technical support. This work is supported by the BMFT under the project number 03M4008E9. Registry No. [t-Bu,PcRu(dib)],, 126894-94-8. References and Notes (1) Ho, Z. Z.; Ju, C. Y.; Hetherington, W. M. J . Appl. Phys. 1987, 62, 716. (2) Ho, Z. Z.; Peyghambariam, N. Chem. Phys. Lett. 1988, 148, 107. (3).Wu, J. W.; Heflin, J. R.; Norwood, R. A.; Wong, K. Y.; ZamaniKhamiri, 0.;Garito, A. F.; Kalyanaraman, P.;Sounik, J. J . Opt. SOC.Am. E 1989, 6 , 707. (4) Kaltbeitzel, A.; Neher, D.; Bubeck, C.; Sauer, T.; Wegner, G.; Caseri, W. In Electronic Properties of Conjugated Polymers III; Kuzmany, H., Mehring, M., Roth, S., Eds.; Springer Series in Solid State Science; Springer: Berlin, 1989; Vol. 91, p 220. ( 5 ) Shirk, J. S.; Lindle, J. R.; Bartoli, F. J.; Hoffman, C. A.; Kafafi, Z. H.; Snow, A. W. Appl. Phys. Lett. 1989, 55, 13. (6) Wang, N. Q.;Cai, Y. M.; Heflin, J. R.; Wu, J. W.; Rodenberger, D. C.; Garito, A. F. Polymer 1991, 10, 1752. (7) Kosonocky, W. F.; Harrison, S. E. J . Appl. Phys. 1966, 37, 13. (8) (a) Hanack, M.;Datz, A.; Fay, R. In Handbook of Conducting Polymers; Skotheim, T. A., Ed.; Marcel Dekker: New York, 1986; Vol. 1, p 133. (b) Schultz, H.; Lehmann, H.; Rein, M.; Hanack, M. Struct. Bonding (Berlin) 1990, 74, 41. (9) Hanack, M.; Vermehren, P. Synth. Met. 1989, 32, 257. (IO) Chemla, D. S., Zyss, J., Eds. Nonlinear Optical Properties of Organic Molecules and Crystals; Academic Press: New York, 1987. (11) Neher, D.; Kaltbeitzel, A.; Wolf, A,; Bubeck, C.; Wegner, G. In Conjugated Polymeric Materials: Opportunities in Electronics, Opto-Electronics and Molecular Electronics; Bredas, J. L., Chance, R. R., Eds.;NATO AS1 Series E 182, Kluwer Acad. Publ.: Dordrecht, 1990; p 387. (12) Bubeck, C.; Grund, A.; Kaltbeitzel, A.; Neher, D.; Mathy, A.; Wegner, G. In Organic Molecules for Nonlinear Optics and Photonics; Messier, J., Kajzar, F., Prasad, P. N., Eds.; NATO AS1 Series E 194, Kluwer Acad. Publ.: Dordrecht, 1991; p 335. (13) Neher, D.; Kaltbeitzel, A.; Wolf, A,; Bubeck, C.; Wegner, G. J. Phys. D, Appl. Phys. 1991, 24, 1193. (14) Bubeck, C.; Kaltbeitzel, A,; Grund, A,; Leclerc, M. Chem. Phys. 1991, 154, 343. (15) Agrawal, G. P.; Cojan, C.; Flytzanis, C. Phys. Rev. B 1978, 17, 776. (1 6) Flytzanis, C. In Nonlinear Optical Properties of Organic Molecules and Crystals;Chemla, D. S., Zyss, J., Eds.; Academic Press: New York, 1987; Vol. 11, p 121. (17) Greene, B. I.; Orenstein, S.;Millard, R. R. Phys. Reu. Lett. 1987.58, 2750. (18) Grund, A.; Mathy, A.; Kaltbeitzel, A,; Neher, D.; Bubeck, C.; Wegner, G. In Organic Materials for Nonlinear Optics II; Hann, R. A., Bloor, D., Eds.; Royal Society of Chemistry: London, 1991; p 288. (19) Schrader, S.; Koch, K. H.; Mathy, A,; Bubeck, C.; Miillen, K.; Wegner, G. Synth. Met. 1991, 43/1-2, 3223. (20) Markovitsi, D.; Hanack, M.; Vermehren, P. J . Chem. SOC.,Faraday Trans. 1991,87,455. (21) Vermehren, P. Ph.D. Thesis, University of Tiibingen, Germany, 1989. (22) Carter, G. M.; Hryniewicz, J. V.; Thakur, M. K.; Chen, Y. J.; Meyer, S. E. Appl. Phys. Lett. 1986, 49, 16. (23) Carter, G. M. J. Opt. Soc. Am. E 1987, 4 , 1018.
J . Phys. Chem. 1992, 96, 7454-1451
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(24) Xuan, N. P.; Ferrier, J. L.; Gazengel, J.; Rivoire, G. Opt. Commun. 1984,51, 433. (25) Neher, D.; Wolf, A.; Bubeck, C.; Wegner, G. Chem. Phys. Lett. 1989, 163, 116. (26) Kajzar, F.; Messier, J. Phys. Reu. A 1985,32, 2352. (27) Schwarz, R.; Goedel, W. A.; Somanathan, N.; Bubeck, C.; Scheunemann, U.; Hickel, W.; Wegner, G. In Electronic Properties of Conjugated Polymers; Kuzmany, H.; Mehring, M., Roth, S., Eds.; Springer Series in Solid-state Sciences; Springer: Berlin, 1992; Vol. 107, p 337. (28) Schwarz, R. Doctoral thesis, Mainz, 1992. (29) Shen, Y. R. The Principles of Nonlinear Optics; J. Wiley: New York, 1984. (30) Fisher, R. A. Optical Phase Conjugation; Academic Press: New York, 1983. (31) Kasha, M.; Rawls, H. R.; Ashraf El-Bayouni Pure Appl. Chem. 1965, 11, 371. (32) Kasha, M. In Spectroscopy of the Excited Stare; DiBartolo, B., Ed.; Plenum Press: New York, 1976; p 337.
(33) Wehry, E. L.; Ed. Modern Fluorescence Spectroscopy; .. Heyden: . London, 1976 (34) Berlman, I. B. J . Phys. Chem. 1970,74, 3085. (35) Boulu. L. G.: Patterson. L. K.: Chauvet. J. P.: Kozak. J. J. J. Chem. Phys. 1987,86, 503. (36) Pope, M., Swenberg, C. E., Eds. Electronic Processes in Organic Crystals; Clarendon Press: Oxford, 1982. (37) Wolf, H. C. In Festkdrperprobleme 4; Sauter, F., Ed.; Vieweg: Braunschweig, 1965. (38) Salcedo. J. R.:Sieaman, - A. E.: Dlott. D. D.; Fayer, M. D. Phys. Rev. &ti. 1'978, 41, 131. (39) Fayer, M. D. In Spectroscopy and Excitation Dynamics of Condensed Molecular Systems; Agranovich, V. M., Hochstrasser, R. M., Eds.; North Holland: Amsterdam, 1983; p 233. (40) Levenson, M. D.; Kano, S. S. Introduction to Nonlinear Laser Spectroscopy, revised ed.; Academic Press: New York, 1988. (41) Butcher, P. N.; Cotter, D. The Elements of Nonlinear Optics; Cambridge University Press: Cambridge, 1990. ~~
Chemlcal Shift Changes on Mlcelllzatlon of Linear Alkyl Benzenesulfonate and Oleate S . Das,* R. G. Bhirud, N. Nayyar, K. S. Narayan, and V. V. Kumar Hindustan Lever Research Centre, ICT Link Road, Andheri East, Bombay 400 099, India (Received: April 27, 1992)
The proton NMR of linear alkyl benzenesulfonate (LABS) has been studied as a function of concentration. On micellization, aromatic protons meta to the sulfonate group show a large upfield shift (0.25 ppm) whereas the aromatic protons in ortho position remain unshifted. When water is solubilized in polar core of the LABS reverse micelles in CDC13,it is aromatic protons ortho to the sulfonate group that show a large upfield shift whereas the aromatic protons in meta position remain unshifted. This difference in behavior of meta/ortho protons in aqueous and nonaqueous environments has been utilized to determine the extent of water penetration in LABS micelle. These rsults show that ortho protons exist in a polar environment while the meta protons are in a hydrocarbon environment. On comicellizationof LABS and oleate, a-CH2 protons of oleate show an upfield shift due to the ring current of aromatic group of LABS. There is no change in the LABS spectrum due to comicellization.
Introduction Linear alkyl benzenesulfonate (LABS) is one of the commercially important surfactants finding a wide range of industrial applications.' It is the largest tonnage anionic after soap. In addition, the presence of a number of isomers and the aromatic ring in the hydrophobic part make the study of this molecule extremely interesting. In recent years, the micelle structure of LABS has been investigated using a variety of techniques such as S A N S Zfluorescence spectroscopy? and NMR.4 Some of the important structural features of organization of LABS in mic e l l e ~ and ~ - ~comicellizatiod have been well studied. However, the transition from the monomeric to the micellar state particularly with respect to chemical shifts in NMR have not been investigated.6 In an early study,' Inoue and Nakagawa have reported some interesting chain length dependent changes in chemical shifts of w-(phenylalky1)trimethylammonium bromides and ascribed these as arising out of the terminal aromatic ring. Recent work in our laboratory has also indicated conformation-dependent changes on micellization in the case of sodium ricinoleate.' In view of this, a detailed NMR study of LABS was undertaken. It was also felt desirable to examine whether NMR could be used to establish comicellization. For this purpose, sodium oleate which also has a special molecular feature (a double bond in 9-10position) was selected as a cosurfactant. Several interesting features have been obtained with respect to the chemical shifts of the ortho and meta protons of LABS on micellization and a-methylene protons of oleate on comicellization, and they are summarized in this report.
Experimental Section Two types of LABS, both greater than 99% purity, have been used in these studies. C11 LABS was a mixture of all isomers 0022-3654/92/2096-7454%03.00/0
TABLE I: 'H Chemical Shift Assignments (in ppm) of 6-PK12 LABS 1 2 diff (below cmc) (above cmc) between (0.1 mM) (100 mM) 2 and 1 ortho 7.8 7.75 +0.05 meta
PhCH (C6) a-methylenes (C5, C7)
7.5 2.7 1.67-1.6
other methylenes (C2-C4, C8-Cll)
1.22
terminal methyls (Cl,C l 2 )
0.8
7.18 2.47 1.5-1.54 1.19
+0.32 +0.23
1.195 0.84
+0.025 -0.04
0.77
+0.03
-0.03
and C12 LABS was a pure 6-phenyl isomer (6-PhLABS). Both the materials were obtained as gifts from Unilever Research, Port Sunlight, U.K. Oleic acid was from Aldrich Gold label (purity 99%). Sodium oleate was prepared by neutralization in DzOwith sodium carbonate to minimize the water of neutralization. The NMR spectra were taken on Varian X L 300 and Bruker W H 270 at 37 OC. No internal standard was used, and the chemical shifts were determined with respect to HDO at 4.78 ppm. The water peak was suppressed. The NMR chemical shifts are quite reproducible since the same results have been obtained with two different samples on two spectrometers of different frequencies.
Results A schematic of 6-PhLABS molecule is given in Figure 1 indicating the numbering of the various carbons. The NMR spectra for 6-PhLABS as a function of concentration are given in Figure 2. The concentration range spans 0.1-100 mM (0.05-50 X cmc). 0 1992 American Chemical Society
