Characterization of in-plane and out-of-plane molecular orientation in Langmuir-Blodgett films of merocyanine dyes using electron spin resonance

Thin Solid Films, 159 (1988) 285-291

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CHARACTERIZATION OF IN-PLANE AND OUT-OF-PLANE M O L E C U L A R O R I E N T A T I O N IN L A N G M U I R - B L O D G E T T F I L M S O F M E R O C Y A N I N E DYES U S I N G E L E C T R O N SPIN R E S O N A N C E * SHIN-ICHI KURODA, KEIICHI IKEGAMI, KAZUHIRO SAITO, MITSUYOSHI SAITO AND MICHIO SUGI Electrotechnical Laboratory, Sakuramura, Ibaraki 305 (Japan} (Received July 27, 1987; accepted August 10, 1987)

Electron spin resonance measurements are performed on Langrnuir-Blodgett films of a merocyanine dye diluted with arachidic acid. Stable signals are detected and spectra show a clear anisotropy, where in-plane structures are found to exhibit hyperfine splitting owing to a nitrogen nucleus of the dye molecule, from the frequency dependence of the spectral line shape at the X and Q bands. Line shape analysis using a computer simulation determines the distribution of molecular orientations, where the pn orbital axis of the molecule is nearly parallel to the film plane and makes an angle of about 60 ° with the dipping direction. This angle shows a correlation with the structure of J aggregate proposed by the optical analysis and the obtained in-plane distribution function explains the observed dichroic ratio of the J band well, suggesting that the radical is associated with the J aggregate. For the out-of-plane distribution, the molecules are found to be highly organized along the plane normal.

1. INTRODUCTION

The orientation of molecules is one of the fundamental problems in the microscopic characterization of Langmuir-Blodgett (LB) films. Electron spin resonance (ESR) spectroscopy has been employed to study the molecular orientation in copper-containing LB films by studying the out-of-plane anisotropy of the spectra 1-3. In those works the tilting of the principal axis of O tensor with respect to the plane normal was determined. In addition, the existence of the in-plane preferential orientation of molecules has been already pointed out in refs. 1 and 2; however, the spectra were measured for only one direction in the film plane and no detailed measurements and analysis of the in-plane spectra were presented. Recently4, we have reported the first direct measurements and analysis of the in-plane ESR spectra in the LB system using LB films of a merocyanine dye (DS) (Fig. 1) diluted with arachidic acid (C20). This system contains high-concentration stable n electron radicals in the dark condition, which possibly arise from an * Paper presented at the Third International Conference on Langmuir-Blodgett Films, Grttingen, F.R.G., July 26-31, 1987. 0040-6090/87/$3.50

© ElsevierSequoia/Printedin The Netherlands

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x

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C18H37 O CH2COOH

Fig. 1. Chemica•structure•fasurfac••activ•mer•cyanine.Thede•niti•n•fthem•••cu•arc••rdinat•sis also shown.

intermolecular charge transfer in dye aggregates 5 7. A clear hyperfine structure due to a nitrogen nucleus in the dye molecule is resolved for the in-plane spectra, permitting the detailed analysis of the in-plane molecular orientation using an ESR spectrum simulation method 4. In this paper, we report a more complete discussion of the molecular orientation using a spectrum simulation incorporating not only in-plane but also out-of-plane distribution of the molecular orientation. The results confirmed our previous determination of the in-plane distribution function and also showed that the molecules are highly organized along the plane normal. 2.

EXPERIMENTAL DETAILS

The films were deposited using a standard vertical dipping method as described previously 6. Substrates were sheets of polyethylene terephthalate (Lumilar, manufactured by the Toray Co.) of 0.1 mm thickness, coated with five layers of cadmium arachidate. ESR measurements were performed with Varian spectrometers. 3.

RESULTS AND DISCUSSION

The full curves in Fig. 2 show the orientation dependence of the first derivative ESR spectra at the X band (9.47 GHz) in a DS film at room temperature with a microwave power of 2 mW. The direction of the external magnetic field is normal to the film plane in Fig. 2(a) whereas it lies in the film plane and makes an angle of 0 °, 45 ° and 90 ° with the dipping direction in Figs. 2(b)-2(d) respectively. Figures 2(b)-2(d) are the first ESR measurements of in-plane anisotropy in LB system. The numbers in the figure show the g values of the field positions as indicated. F r o m the anisotropy in the plane, the structures at higher field side of the inplane spectra in Fig. 2 were suggested to result from a triplet hyperfine splitting due to a nitrogen nucleus in the molecule 6. This fact has been confirmed from the frequency dependence of the spectral line shape at the X and Q bands, using the fact that the hyperfine splitting should be independent of the frequency of measurement 4. The observed spectrum is characterized by the preferentially oriented radical species with a single nitrogen hyperfine coupling and thus provides the basis for the analysis of the distribution of the molecular axis. One complication, however, arises from the fact that the spectra contain the signals from two radical species having different spin lattice relaxation times 5' 6. The species responsible for the nitrogen coupling (species A) has been identified as the one having the longer spin lattice relaxation times from the saturation behaviour as well as electron nuclear double resonance-induced ESR spectra 4'6' 7. The line shape

ESR CHARACTERIZATION OF MEROCYANINE LB FILMS

287

(a)

(b) - -

1---

(c) ,U2.009

5G m

I

I

I

Fig. 2. Orientation dependence offirst-derivative E S R s p e c t r a o f a 1000-layer DS film with a m o l a r ratio of [DS]:[C2o] = 1:2 at r o o m temperature ( ): (a) the external magnetic field is normal to the film

plane; (b)-(d) the external magnetic field lies in the plane and makes an angle of 0°, 45° and 90° respectively with the dipping direction of the substrate. The numbers show the g values of the fields as indicated. - . . . . , the simulated curves of the primary radical species responsible for the observed nitrogen hyperfine structure. The difference between the observed and calculated curves is due to the overlapping signal of the second radical species4' 7. Resonance fields corresponding to the principal components of the g and the hyperfine tensors are shown at the bottom of the figure. analysis was c a r r i e d o u t for species A after the o b s e r v e d s p e c t r a were a p p r o x i m a t e l y d e c o n v o l u t e d into two c o m p o n e n t s . E S R s p e c t r a of a n electron radical are well d e s c r i b e d by the following spin h a m i l t o n i a n 8: )eto = # B S . g . H + S . A . I

(1)

T h e first term is the electronic Z e e m a n energy a n d the second t e r m is the hyperfine c o u p l i n g with a nucleus, a n i t r o g e n nucleus in this case. S, I a n d H d e n o t e the electron spin, n u c l e a r spin a n d the external m a g n e t i c field respectively, g a n d A are the g t e n s o r a n d the hyperfine t e n s o r respectively. In Fig. 1, the definition of the c o o r d i n a t e axes in the m o l e c u l e is also shown, w h e r e the x, y a n d z axes are parallel to the h y d r o p h o b i c chain b o n d e d to a n i t r o g e n a t o m , the long axis of the c h r o m o p h o r e a n d the prt o r b i t a l axis, respectively. T h e n i t r o g e n hyperfine splitting shows a clear triplet splitting due to nuclear spin 1 a l o n g the z axis a n d a m u c h w e a k e r splitting a l o n g the d i r e c t i o n p e r p e n d i c u l a r to z axis. T h e lack of a triplet structure in the s p e c t r u m a l o n g the p l a n e n o r m a l in Fig. 2(a) shows t h a t the z axis is n e a r l y confined in the film plane. F i g u r e 3 shows the Euler angles (0, q~, ~b), in the usual n o t a t i o n , t h a t define the m o l e c u l a r orientation. X, Y a n d Z r e p r e s e n t the c o o r d i n a t e s fixed to the substrate, w h e r e the X axis a n d the Z axis are p a r a l l e l to the d i p p i n g d i r e c t i o n a n d the p l a n e n o r m a l respectively, x, y a n d z are the c o o r d i n a t e s fixed to the m o l e c u l e defined in Fig. 1.0 a n d ~b define the o r i e n t a t i o n of the z axis (the pn o r b i t a l axis) for the out-of-

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dippin.g~

~

direction

/ J

V

~

~t~ubstrat

e

Fig. 3. Euler angles to define the molecular orientation in the substrate. The X, Y and Z axes are the coordinates fixed to the substrate, where the X and Z axes are parallel to the dipping direction and the plane normal respectively.The x, y and z axes show the coordinate fixed to the molecule, as defined in Fig. 1. plane and the in-plane orientations of the substrate. ~b shows the rotation of the y axis (the long axis of the chromophore) around the z axis, thus representing the outof-plane deviation of the y axis when 0 ~ 90 °. The direction cosines (l~,m,nl) with i = x, y or z of those molecular axes with respect to the substrate coordinate can be expressed using the Euler angles. When the direction cosines of the external field to the substrate are given by (L, M, N), the direction cosines (l, m, n) of the external field with respect to the molecular coordinate are expressed as 1 = 121.Yc= Ll:, + M m x + Nn:,

(2)

and so on. Then the molecule shows triplet lines with the g and hyperfine coupling values given as follows 8:

g2 = gx212 +gy2m2 + g z Z n 2 A 2 = Ax2l 2 +

(3)

Ay2m2 + A 2 n 2

(4)

The ESR spectrum I ( H ) for the given orientation of the external magnetic field is obtained by summing the contributions from all orientations of molecules considering the orientation distribution as G { H - H, es(O, c~, ~k)}P(0, q~,~b)sin 0 dO d~b d@ (5) 0 Here the function G is the line shape function, which is assumed to be gaussian with isotropic linewidth, for simplicity. Hre~ represents the resonance fields of the triplet lines determined by g and A in eqns. (3) and (4). The function P is the distribution function for the molecular orientation. In our previous paper 4 we have shown that the z and y axes are nearly confined in the plane: 0 ~ 90 °, ~b ~ 0 °. Considering this point, we assume that the function P can be decoupled into the product of the distribution functions of each angle, the form previously adopted being retained. The in-plane distribution function for q~, for example becomes I(H) =

P(q~) ~ exp~ +exp{

sinZ(qS-- ~bo)~

2-q n g 3 sin2(q9+ ~b°)~

2

J

(6)

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~bo denotes the most probable orientation of the z axis in the plane and by symmetry of the system - ~bo is considered with equal weight. 6 is the width of the distribution. For the calcuation of the spectra, the following parameter values were deduced from the observed g values and hyperfine splittings: gx = 2.0068, gy = 2.0093, gz = 2.0023 and A= = 5.5 G. Ax = Ay = 1.1 G and the peak-to-peak width of 1.8 G of the gaussian function in eqn. (5) were estimated from the width of the spectrum along the plane normal in Fig. 2(a) and the widths of structures in Figs. 2(b)-2(d). Using those values, good agreement between the calculated and observed spectra was obtained for the distribution functions, of which the parameter values for eqn. (6) are as listed below. The calculated spectra are shown by broken lines in Fig. 2. The difference between the observed and calculated curves is due to the overlapping signal from the second radical species as mentioned above 4' 7. The in-plane distribution parameters are ~b0 = 60°-65 ° with 6 - 25°-30 °, which are consistent with the previous estimates. These values are mainly determined from the in-plane line shape. The out-of-plane distribution parameters are 0 0 - 9 0 ° ~ 7° with 6 = 6°-8 ° and ~k0 ~ 5 ° with 6 ,~ 10 °. These parameters directly characterize the deviation of the prt orbital axis (z axis) or the long axis of the chromophore (y axis) out of the plane, which leads to the slight asymmetry of the nearly gaussian line shape observed along the plane normal in Fig. 2(a) due to the g value distribution. Figure 4(a) shows the in-plane distribution function. The most probable orientation of the z axis is at an angle of about 60 ° from the dipping direction. Figure 4(b) shows a schematic picture of the most probable orientation of the molecules. In as-grown DS films, the majority of molecules form the so-called J aggregate 9'1 o. The angle ~ of about 30 ° between the y axis (long axis of the chromophore) and the dipping direction shows a good coincidence with the structure of the J aggregate expected from the analysis of the J band peak position using Kuhn's extended dipole model 1~, if we assume that the one-dimensional aggregate is aligned along the

"~

~) ~

0o 30° 60° 90° 120o150° 180°

(a)

dipping direction

p~ orbital

(b)

Fig. 4. (a) The distribution function P(~) of the p~ orbital axis in the plane, determined by the simulation study. The parameter values are ~o = 60° and 6 = 25 ° in eqn. (6): , P(~b); . . . . . , behaviours oftbe two terms on the right-hand side of eqn. (6). (b) Schematic view of the average orientation of molecules in the film plane. The long axis of the chromophore is expressed by a thick line. ~ is the angle between the direction connecting the centres of adjacent molecules and the y axis of the molecule in dye aggregates. (P0 denotes the average of (p.

et al.

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dipping direction. This coincidence suggests that the radical molecules are associated with the J aggregate 4. Furthermore, the optical dichroic ratio of the J band has been found to be explained well by the above-obtained in-plane distribution function. The dichroic ratio R between the peak heights of optical absorbance for the electric vector parallel and perpendicular to the dipping direction is easily obtained as

R=(cosZ(2-cb)~l(sinZ(2-qb>>

(7,

Here ( ) means the average obtained using the above-determined distribution function. The calculated value of R was about 1.8, which is in good agreement with the observed 12 dichroic ratio of 1.6-2.0. This fact further suggests that the radical is associated with the J aggregate. Moreover, a good correlation has been found between the changes in the dichroic ratio obtained from ESR line shape analysis and those optically observed when the dipping speed was varied lz. As for the out-ofplane distribution, the molecules show a fairly good organization of orientation (5 < 10°). This is reasonably expected from the lamellar structure of LB films. The above discussions strongly suggest that ESR spectroscopy of the stable radical can sense the orientation of the majority of molecules forming the J aggregate in the DS system. By probing the nitrogen hyperfine structure, further structural studies may be performed, such as the study of conformational change due to heat treatment ~3, which causes the dissociation of J aggregate formed in asgrown state 14. Finally, the spin density on the nitrogen atom of 0.17, estimated from the observed nitrogen hyperfine coupling, is obviously the important microscopic quantity. This directly probes the wavefunction of the dye molecules in the aggregated state. Theoretical study of the electronic states of J aggregates may be of interest in examining the observed spin density. ACKNOWLEDGMENTS

The authors are grateful to Dr. J. Kondo, Dr. Y. Kawabata, Professor K. Fukuda, Dr. H. Nakahara and Dr. N. Minari for valuable discussions. Dr. K. Someno and Dr. M. Kaise are thanked for permission to use the Q band spectrometer. REFERENCES 1 J. Messier and G. Marc, J. Phys. (Paris), 32 (1971) 799. 2 P.A. Chollet, J. Phys. C, 7(1974)4127. 3 M. Vandevyver, A. Barraud, A. Ruaudel-Teixier, P, Maillard and C. Gianotti, J. Colloidlnterface Sci., 85 (1982) 571. 4 S. Kuroda, K. Ikegami, M. Sugi and S. Iizima, Solid State Commun., 58 (1986) 493. 5 S. Kuroda, M. Sugi and S. Iizima, Thin Solid Films, 99 (1983) 21. 6 S. Kuroda, M. Sugi and S. Iizima, Thin Solid Films, 133 (1985) 189. 7 S. Kuroda, K. Ikegami, K. Saito, and M. Sugi, J. Phys. Soc. Jpn., 56 (1987) 3319. 8 J.E. Wertz and J. R. Bolton, Electron Spin Resonance, McGraw-Hill, New York, 1972. 9 M. Sugi and S. lizima, Thin Solid Films, 68 (1980) 199.

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10 M. Sugi, T. Fukui, S. Iizima and K. Iriyama, Mol. Cryst. Liq. Cryst., 62 (1980) 165. I 1 H. Nakahara, K. Fukuda, D. M6bius and H. Kuhn, J. Phys. Chem., 90 (1986) 6144. 12 N. Minari, K. Ikegami, S. Kuroda, K. Saito, M. Saito and M. Sugi, paper presented at the 48th Autumn Meet. of the Japan Society of Applied Physics, Nagoya, 1987. 13 H. Oyanagi, M. Sugi, S. Kuroda, S. Iizima, T. lshiguro and T. Matsushita, Thin Solid Films, 133 (1985) 181. 14 M. Sugi, M. Saito, T. Fukui and S. Iizima, Thin Solid Films, 129 (1983) 15.