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Photoinduced orientational transformations in polar Langmuir-Blodgett films
Thin Solid Films, 179 (1989) 493-496
493
PHOTOINDUCED ORIENTATIONAL TRANSFORMATIONS IN POLAR L A N G M U I R - B L O D G E T T FILMS M. I. BARNIK, S. P. PALTO, V. A. KHAVRICHEV, N. M. SHTYKOV AND S. G. YUDIN Organic Intermediates and Dyes Institute, Moscow 103787 (U.S.S.R.) (Received April 25, 1989; accepted May 16, 1989)
Multimolecular Langmuir-Blodgett films exhibiting the photoinduced optical anisotropy effect have been obtained: Illumination of the films in the dye absorption spectrum leads to changes in their optical and polar characteristics.
1. INTRODUCTION
PreviouslyL 2 we reported the photoinduced optical anisotropy (POA) effect in Langmuir-Blodgett (LB) films of surface-active dyes. The essence of the effect is as follows. The initially isotropic LB film becomes optically anisotropic when illuminated by linearly polarized light whose spectral composition coincides with that of the dye absorption band. If the light is directed along the normal to the LB film, the latter acquires properties of a uniaxial crystal with an optical axis lying in the film plane and displays absorbance dichroism. Under illumination the dye molecules are oriented in such a way that their absorption oscillators become perpendicular to the electric vector of the polarized light wave. The optical axis is oriented in the same direction. The same effect is observed in films of these dyes obtained by vacuum evaporation 3. Here, in addition to the above effect, the dye optical axis and absorption oscillators of the film affected by natural (non-polarized) light become parallel to the wavevector of the inducing radiation. In refs. 1 and 2 the POA effect in LB films of dyes exhibiting no polar structures was studied. In this paper we report the preparation of polar LB films exhibiting the POA effect, as well as an investigation of their structural transformations under inducing radiation. 2. EXPERIMENTAL DETAILS
The polar LB films were obtained using the azo dye of the following structural formula: C4HgNH I ~ ~ N = N - - ~ S O 2 N H 2 0040-6090/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
494
M . I . BARNIK et
al.
Multimolecular X-type LB films were deposited onto quartz plates by successive transfers of monolayers from the water surface by the horizontal lifting method at a surface pressure n = 10 dyn c m - i, pH 6.0 and 22 °C. The POA effect was induced by illumination of the LB films by natural or polarized light using a microscope (the spectral band of the illumination is 450-550 nm~. The film structure variations due to illumination were examined by measuring dichroic absorption spectra and using the modulation Stark spectroscopy technique. The spectral study was carried out with a Pye Unicum 8800 spectrophotometer. The Stark spectra were investigated according to the procedure described in ref. 4. The essence of the Stark effect is as follows. The energetic levels of molecules vary in an electric field. For a molecule with a dipole moment, the Stark effect is generally due to a difference in the values of the molecular dipole moment for the excited (#e) and ground (#o) states. The spectral absorption shift Av for a fixed dipole molecule in the field E is defined by the relationship Av=
-~(Pe-Po)" E = -~E.A#
(1)
and depends on mutual orientation of the vectors A# and E(h is Planck's constant). In the case of an ordered molecular ensemble, averaging by the possible molecular orientations results in a shift in the electronic absorption band to either the longwave or the shortwave regions of the spectrum depending on the direction of the electric field relative to the predominant orientation of the dipoles. We measured the modulation spectra of the Stark effect. An a.c. electric field offrequencyf = 1000 Hz was applied to the sample and the relative change -AT ~ - (2) = TET T(2 ) in the sample transmittance was measured (here Te and T are the transmittances of the sample in the presence and the absence respectively of the applied electric field) as a function of the wavelength 2. The LB films for the Stark spectra measurement were prepared on quartz plates with SnOz electrodes. A semitransparent aluminium layer obtained by vacuum evaporation at a pressure of 10- 5 Torr was used as the upper electrode. The LB films for absorption spectra measurement were directly deposited onto quartz substrates without SnOz electrodes. In all cases the number of monolayers in the LB films was 48. 3.
RESULTS AND DISCUSSION
The characteristic spectra of the optical density D(2) and the Stark effect LB films of the dye 1 are shown in Fig. 1. The spectra were measured for normal incidence of light onto the film. Non-illuminated films do not possess any absorbance dichroism. However, the presence of absorbance shows that the chromophore group of the dye molecules is oriented at an angle to the substrate. The films exhibit a linear Stark effect. The Stark spectrum (AT/T)(2) correlates with the shape of the first derivative of the optical density spectrum D(2) and is
(AT/T)(2) of the initial
PHOTOINDUCED TRANSFORMATIONSIN LB FILMS
495
observed at the modulating field frequency. Such a behaviour of the Stark spectrum is indicative of polarity of the LB structure. From the relationship A T ( 1 ) = (A# cos O)E dD 12 In 10
d2
hc
(2)
one can calculate the value of the p a r a m e t e r A # c o s 0 = (A/a), which is a characteristic of the polarity of a m e d i u m consisting of an ordered ensemble of molecular dipoles. Here A# is the difference between the molecular dipole m o m e n t s in the excited a n d ground states, and 0 is the angle between the vector A/a and the direction of the external electric field E. Estimation using formula (2) gives (A/a) = 1.8 D (1 D = 10-18 c.g.s, units). After illumination by natural (non-polarized) light the optical density in the shortwave absorption b a n d remains practically the same. Consequently, activating illumination does not affect the total a m o u n t of the film substance (no sublim a t i o n takes place). The LB film a b s o r b a n c e in the longwave spectral region (2ms, = 450 rim) contributed by the molecular c h r o m o p h o r e of the dye remains isotropic. The optical density, however, decreases perceptibly (Fig. 2, spectrum 1). This means that the longwave oscillators of the dye molecules align predominantly along the lightwave p r o p a g a t i o n direction. The amplitude of the linear Stark effect of the film illuminated by natural light decreases almost 20 times (Fig. 2, spectrum 2). Considering that the optical density in the spectral region from 450 to 500 nm decreases by a factor of approximately 5, we obtain a decrease in (A/a) by a factor of a b o u t 4. Consequently, the LB film polarity decreases noticeably.
?
5
5
I
3~'6 ~oo ~ t l l
t
I I, 6o0 ~t, nm
,
t 1
I /I I V
[ -,
I
i 300
\
-',/,
, ',\
t
,
I
9,,nm
500 '
,/
-,9.
Fig. 1. Optical density (spectrum 1) and linear Stark effect (the modulating voltage amplitude is 3 V) (spectrum 2) in the initial LB film comprising 48 monolayers. The scale factor for spectrum 1 is 0.2 and for spectrum 2 - 5 x 10- 3. Fig. 2. Optical density (spectrum 1) and linear Stark effect (the modulating voltage amplitude is 3 V) (spectrum 2) in the LB film comprising 48 monolayers after illumination by non-polarized light (P ~ 90 J cm- 2 ). The scale factor for spectrum 1 is 0.1, and for spectrum 2 - 2 x 10- 4.
496
M.I. BARNIK e t al.
Figure 3 shows polarization absorption spectra after plane-polarized light activation of the LB film. Spectra are shown for polarizations of the probing illumination parallel (DiI) and perpendicular (D±) to the activating polarization. The absorption spectra dichroism indicates that under activating light the molecular oscillators of the dye align perpendicular to the lightwave electric vector. The order parameter obtained from the formula S=
D±-DIr
D± + 2DII is found to be about 0.7. The value of (Ap), defined from the Stark effect spectra and absorption spectra measured with non-polarized probing illumination, for the film activated by planepolarized light (Fig. 4) is 1.60, which within the experimental error coincides with the value of (Ap> for non-illuminated films. Thus, with the POA effect, orientational order can be implemented in the LB film plane almost without disturbance of its polar characteristics. ~), L ~ T / T 0,8 0.6
!
0.4
0
i
~t I. ~#,
',J
I
400
\a
|
,
0,?. I
0
I
¢
300
,
!
,
I 500
i
1
i
[ 700
~nm
I
i
"',%""l 600 ~,.m / I / I
V
Fig. 3. Dichroic absorption spectra of the LB filmilluminated by plane-polarizedlight. Fig. 4. Optical density (spectrum 1) and linear Stark effect(the modulating voltage amplitude is 3 V) (spectrum 2) in the LB film illuminated by plane-polarizedlight. The scalefactor for spectrum I is 0.2 and for spectrum 2 - 2 × 10-3. ACKNOWLEDGMENTS The authors are thankful to Dr. V. T. Lazareva for making available the dye and to Dr. V. I. Muratov for measuring the optical absorption spectra. REFERENCES 1 V.M. Kozenkov, S. G. Yudin, E. G. Katyshev, S, P. Palto, V. T. Lazareva and V. A. Barachevsky, Pis'ma Zh. Tekh. Fiz., 12 (1986) 1267. 2 M. 1. Barnik, V. M. Kozenkov, N. M. Shtykov, S. P. Palto and S. 13. Yudin, J. Mol. Electron., 5 (1988) 54. 3 V, M. Kozenkov, V. S. Doroshenko, E. G. Katyshev, N. E. Minchenko, S. G. Yudin, V. T. Lazareva, M, I. Barnik and V. A. Baraehevsky,Proc. 3rd U.S.S.R. Conf. on Computational Optoelectronics, Yerevan. November 1987, Abstracts, Part 2, Academyof Sciencesof Armenia, p. 61. 4 L.M. Blinov,S. P. Palto and S. G. Yudin, J. Mol. Electron., 5 (1988) 42.
493
PHOTOINDUCED ORIENTATIONAL TRANSFORMATIONS IN POLAR L A N G M U I R - B L O D G E T T FILMS M. I. BARNIK, S. P. PALTO, V. A. KHAVRICHEV, N. M. SHTYKOV AND S. G. YUDIN Organic Intermediates and Dyes Institute, Moscow 103787 (U.S.S.R.) (Received April 25, 1989; accepted May 16, 1989)
Multimolecular Langmuir-Blodgett films exhibiting the photoinduced optical anisotropy effect have been obtained: Illumination of the films in the dye absorption spectrum leads to changes in their optical and polar characteristics.
1. INTRODUCTION
PreviouslyL 2 we reported the photoinduced optical anisotropy (POA) effect in Langmuir-Blodgett (LB) films of surface-active dyes. The essence of the effect is as follows. The initially isotropic LB film becomes optically anisotropic when illuminated by linearly polarized light whose spectral composition coincides with that of the dye absorption band. If the light is directed along the normal to the LB film, the latter acquires properties of a uniaxial crystal with an optical axis lying in the film plane and displays absorbance dichroism. Under illumination the dye molecules are oriented in such a way that their absorption oscillators become perpendicular to the electric vector of the polarized light wave. The optical axis is oriented in the same direction. The same effect is observed in films of these dyes obtained by vacuum evaporation 3. Here, in addition to the above effect, the dye optical axis and absorption oscillators of the film affected by natural (non-polarized) light become parallel to the wavevector of the inducing radiation. In refs. 1 and 2 the POA effect in LB films of dyes exhibiting no polar structures was studied. In this paper we report the preparation of polar LB films exhibiting the POA effect, as well as an investigation of their structural transformations under inducing radiation. 2. EXPERIMENTAL DETAILS
The polar LB films were obtained using the azo dye of the following structural formula: C4HgNH I ~ ~ N = N - - ~ S O 2 N H 2 0040-6090/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
494
M . I . BARNIK et
al.
Multimolecular X-type LB films were deposited onto quartz plates by successive transfers of monolayers from the water surface by the horizontal lifting method at a surface pressure n = 10 dyn c m - i, pH 6.0 and 22 °C. The POA effect was induced by illumination of the LB films by natural or polarized light using a microscope (the spectral band of the illumination is 450-550 nm~. The film structure variations due to illumination were examined by measuring dichroic absorption spectra and using the modulation Stark spectroscopy technique. The spectral study was carried out with a Pye Unicum 8800 spectrophotometer. The Stark spectra were investigated according to the procedure described in ref. 4. The essence of the Stark effect is as follows. The energetic levels of molecules vary in an electric field. For a molecule with a dipole moment, the Stark effect is generally due to a difference in the values of the molecular dipole moment for the excited (#e) and ground (#o) states. The spectral absorption shift Av for a fixed dipole molecule in the field E is defined by the relationship Av=
-~(Pe-Po)" E = -~E.A#
(1)
and depends on mutual orientation of the vectors A# and E(h is Planck's constant). In the case of an ordered molecular ensemble, averaging by the possible molecular orientations results in a shift in the electronic absorption band to either the longwave or the shortwave regions of the spectrum depending on the direction of the electric field relative to the predominant orientation of the dipoles. We measured the modulation spectra of the Stark effect. An a.c. electric field offrequencyf = 1000 Hz was applied to the sample and the relative change -AT ~ - (2) = TET T(2 ) in the sample transmittance was measured (here Te and T are the transmittances of the sample in the presence and the absence respectively of the applied electric field) as a function of the wavelength 2. The LB films for the Stark spectra measurement were prepared on quartz plates with SnOz electrodes. A semitransparent aluminium layer obtained by vacuum evaporation at a pressure of 10- 5 Torr was used as the upper electrode. The LB films for absorption spectra measurement were directly deposited onto quartz substrates without SnOz electrodes. In all cases the number of monolayers in the LB films was 48. 3.
RESULTS AND DISCUSSION
The characteristic spectra of the optical density D(2) and the Stark effect LB films of the dye 1 are shown in Fig. 1. The spectra were measured for normal incidence of light onto the film. Non-illuminated films do not possess any absorbance dichroism. However, the presence of absorbance shows that the chromophore group of the dye molecules is oriented at an angle to the substrate. The films exhibit a linear Stark effect. The Stark spectrum (AT/T)(2) correlates with the shape of the first derivative of the optical density spectrum D(2) and is
(AT/T)(2) of the initial
PHOTOINDUCED TRANSFORMATIONSIN LB FILMS
495
observed at the modulating field frequency. Such a behaviour of the Stark spectrum is indicative of polarity of the LB structure. From the relationship A T ( 1 ) = (A# cos O)E dD 12 In 10
d2
hc
(2)
one can calculate the value of the p a r a m e t e r A # c o s 0 = (A/a), which is a characteristic of the polarity of a m e d i u m consisting of an ordered ensemble of molecular dipoles. Here A# is the difference between the molecular dipole m o m e n t s in the excited a n d ground states, and 0 is the angle between the vector A/a and the direction of the external electric field E. Estimation using formula (2) gives (A/a) = 1.8 D (1 D = 10-18 c.g.s, units). After illumination by natural (non-polarized) light the optical density in the shortwave absorption b a n d remains practically the same. Consequently, activating illumination does not affect the total a m o u n t of the film substance (no sublim a t i o n takes place). The LB film a b s o r b a n c e in the longwave spectral region (2ms, = 450 rim) contributed by the molecular c h r o m o p h o r e of the dye remains isotropic. The optical density, however, decreases perceptibly (Fig. 2, spectrum 1). This means that the longwave oscillators of the dye molecules align predominantly along the lightwave p r o p a g a t i o n direction. The amplitude of the linear Stark effect of the film illuminated by natural light decreases almost 20 times (Fig. 2, spectrum 2). Considering that the optical density in the spectral region from 450 to 500 nm decreases by a factor of approximately 5, we obtain a decrease in (A/a) by a factor of a b o u t 4. Consequently, the LB film polarity decreases noticeably.
?
5
5
I
3~'6 ~oo ~ t l l
t
I I, 6o0 ~t, nm
,
t 1
I /I I V
[ -,
I
i 300
\
-',/,
, ',\
t
,
I
9,,nm
500 '
,/
-,9.
Fig. 1. Optical density (spectrum 1) and linear Stark effect (the modulating voltage amplitude is 3 V) (spectrum 2) in the initial LB film comprising 48 monolayers. The scale factor for spectrum 1 is 0.2 and for spectrum 2 - 5 x 10- 3. Fig. 2. Optical density (spectrum 1) and linear Stark effect (the modulating voltage amplitude is 3 V) (spectrum 2) in the LB film comprising 48 monolayers after illumination by non-polarized light (P ~ 90 J cm- 2 ). The scale factor for spectrum 1 is 0.1, and for spectrum 2 - 2 x 10- 4.
496
M.I. BARNIK e t al.
Figure 3 shows polarization absorption spectra after plane-polarized light activation of the LB film. Spectra are shown for polarizations of the probing illumination parallel (DiI) and perpendicular (D±) to the activating polarization. The absorption spectra dichroism indicates that under activating light the molecular oscillators of the dye align perpendicular to the lightwave electric vector. The order parameter obtained from the formula S=
D±-DIr
D± + 2DII is found to be about 0.7. The value of (Ap), defined from the Stark effect spectra and absorption spectra measured with non-polarized probing illumination, for the film activated by planepolarized light (Fig. 4) is 1.60, which within the experimental error coincides with the value of (Ap> for non-illuminated films. Thus, with the POA effect, orientational order can be implemented in the LB film plane almost without disturbance of its polar characteristics. ~), L ~ T / T 0,8 0.6
!
0.4
0
i
~t I. ~#,
',J
I
400
\a
|
,
0,?. I
0
I
¢
300
,
!
,
I 500
i
1
i
[ 700
~nm
I
i
"',%""l 600 ~,.m / I / I
V
Fig. 3. Dichroic absorption spectra of the LB filmilluminated by plane-polarizedlight. Fig. 4. Optical density (spectrum 1) and linear Stark effect(the modulating voltage amplitude is 3 V) (spectrum 2) in the LB film illuminated by plane-polarizedlight. The scalefactor for spectrum I is 0.2 and for spectrum 2 - 2 × 10-3. ACKNOWLEDGMENTS The authors are thankful to Dr. V. T. Lazareva for making available the dye and to Dr. V. I. Muratov for measuring the optical absorption spectra. REFERENCES 1 V.M. Kozenkov, S. G. Yudin, E. G. Katyshev, S, P. Palto, V. T. Lazareva and V. A. Barachevsky, Pis'ma Zh. Tekh. Fiz., 12 (1986) 1267. 2 M. 1. Barnik, V. M. Kozenkov, N. M. Shtykov, S. P. Palto and S. 13. Yudin, J. Mol. Electron., 5 (1988) 54. 3 V, M. Kozenkov, V. S. Doroshenko, E. G. Katyshev, N. E. Minchenko, S. G. Yudin, V. T. Lazareva, M, I. Barnik and V. A. Baraehevsky,Proc. 3rd U.S.S.R. Conf. on Computational Optoelectronics, Yerevan. November 1987, Abstracts, Part 2, Academyof Sciencesof Armenia, p. 61. 4 L.M. Blinov,S. P. Palto and S. G. Yudin, J. Mol. Electron., 5 (1988) 42.