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Electric field induced changes in the optical absorption of a merocyanine dye
Volume
3, number
7
CHEMICAL
ELECTRIC IN
THE
OPTICAL
H. BijCHER,
PHYSICS
FIELD ABSORPTION
J. WIEGAND,
LETTERS
INDUCED OF
CHANGES
A MEROCYANINE
B. B. SNAVELY*“,
Received
July 1969
DYE
*
K. H. BECK and H. KUHN
23 May 1969
A field induced linear and quadratic change in the optical absorption of a merocyanine dye at 295%. 630K and 60K is reported. The dye was incorporated in a fatty acid monolayer capacitor, which allows the application of external fieId strength up to 5 X lo6 V/cm at low values of applied voltages.
It is well mown
that solvatochromic
shifts
of
due to changes in the Onsager reaction field of the solvent which are of the order of lo7 V/cm. By analogy it is to be expected that a strong externally applied electric field should also produce a change in the absorption spectrum, an effect which has been termed “electrochromism”. The theory of electrochromism has been considered by Ceekalla and Liptay [lJ, Platt [2], Labhart [3] and observation of the effect has been reported in refs. [I] and [3] and by Malyey, Feher and Manzerall [4], Sauter and Albrecht [5], and Powers et aL [5]. In this paper we report the observation of electrochromic effects in a merocyanine dye at temperatures from 6OK to 295OK and at a field strength of 2.5 x lo6 V/cm. The dye was incorporated in a fatty-acid molecular monolayer capacitor which made possible the use of electric fields up to 5 x 106 V/cm with low values of applied voltage. Changes h optical absorption resulting from the application of an electric field were detected using a lock-in technique. The form of the samples studied is shown schematically in fig. 1. A transparent aluminium film was formed on a microscope slide by evaporation and coated with the monolayer assembly using the monolayer-assembly-technique redyes
are
* This work was supported by the Deutsche Forschungsgemeinschaft. We are grateful to Dozent Dr. Ch. Reichardt for a sample of the merocyanine dye. ** On leave of absence from Eastman Kodak Co., Research Laboratories, Rochester, N.Y.
508
ported
in refs.
layers
were
The
eighth
[6,7].
built film
Seven
up on this
contained
Cd-arachidate
mono-
substrate-electrode.
the surface
active
merocyanine dye mixed with arachidate in the molecular ratio of 1 : 5, giving a dye concentration of 8.3 X IO18 molecules per cm2. Seven additional arachidate monolayers were added giving a total of fitJeen layers. The thiclmess of each layer is 26.6A yielding a total thickness of the dielectric of 400-4. Sample separation was completed with the evaporation of a second transparent electrode. Sample quality was tested by measuring the capacitance and conductance. The capacitance was found to be inversely proportional to the number of monolayers [?‘I and for a fifteen layer capacitor had a value of 0.056 PF per cm2. The conductance depended upon the applied voltage and frequency. Properly made samples had a conductance of approximately 1.5 x 10-5 ohm/cm2 at 0s3 Vrms and 500 Hz. Application of voltages greater than 13 V,,, destroyed the samples. Properly made samples burnt out homogeneously over the total area. The samples were mounted in an optical dewar to allow low temperature measurements. Electrochromism was observed as the change in light intensity produced by a 500 Hz sinusoidal voltage applied to the sample electrodes. A monochromatic light which passed through the sample in a direction parallel to the electric field was used in the measurement. The light transmitted by the sample was detected by a photomultiplier and lock-in amplifier (PAR model HR8). Measurements of the signal at the first
Volume 3, number 7
CHEMICAL
PHYSICS
LETTERS
July
1969
NDIUM CONTbCT
s
I-DISTANCE
--12mm-
Fig.
1.
Schematic
representation
of
the
= 15 = 26.6 a=400
cw~-(cn~)l~-c //O
Q
ARACHIDIC -ACID
‘3
SURFACE ACTIVE
sarcple
and highs2 harmonics of the frequency of the applied field were made. The absorption spectrum of the dye within the monolayer assembly was measured in the dewar using a sample-in sample-out technique. For this purpose a sample without electrodes was prepared, which contained the dye only in one half of the sample area. The other half served as the reference for the absorption measurement. Optical absorption and electrochromism for a particular dye at applied field with a peak amplitude of 2.5 x lo6 V/cm, corresponding to an applied voltage of 2.1 Vrms over a dielectric thickness of 400A, are shown in fig. 2. The dye [8], for which the structural formula is given in the inset, exhibited a linear (first harmonic) and a quadratic (second harmonic) electrochromic effect. Harmonics higher than the second order were not detectable. The results are given for the electric field directed from layer 15 to layer 1. The results obtained at 295OK are presented in fig. 2a and those obtained at 83’K and 6OK are given in fig. 2b. No changes in the electrochromism spectra were observed between 83O and 6OK. The optical absorption spectrum was measured only at 295OK and 83OK. The data presented are taken from samples with mean performance. With different samples the magnitude of both, the linear and quadratic effect may change independently by a factor as great as 1.5; the shapes and proportions of the spectra, however, did not change. No bleaching or electrolysis of the dye occurred during the measuring procedure.
used
for
mezzxarin
ii -I ‘OH
DYE cHo*%l,-
CHROHOPHW
g electrothromism
-10
of
I ! \
b) WK.6’K
organic
dyes_
II-
-.__. /-’
Fig. 2. Absorption spectrum (-), linear electrochromism spectrum (--- ) and quadratic electrochromism spectrum (0-. ) of the merocyanine dye l-stearyl-2(3’ nitro - 3’ formylallyliden)-1.2-di-hydrochinoliu at room temperature (a) and low temper,ture @I_ 509
Volume 3, number 7
CHEMICAL PHY=CS
At the applied low temperatures, the absorption and electrochromism spectra are shifted by approximately 0.026 eV towards higher energy, leaving the relative positions of the spectral features unchanged Roth, the linear and quadratic electrochromism increased by a factor of 2, only the broad negative peak of the quadratic effect at 2.65 eV decreases. The spectrum of the linear electrochromic effect is consistent with a spectral shift of the low energy absorption peak of 2 x lo-10 eV per V/cm, caused by a difference in the dipole moments of the ground state and the first excited state. The main qualitative deviations from the spectrum of a pure bathochromic shift of the low energy absorption band are the zero at 2.305 eV (at 295OK) and the low energy negative peak. It is also noted that the Icw energy positive peak of th? electrochromism spectrum is not exactly at the maximum slope of the absorption spectrum. These facts are explained by assuming a band with a maximum at 2.27 eV (at 295OK) in the back ground of the low energy tail of the absorption band. In the electric field of fig. 2 this band is shifted by 10-9 eV per V/cm to the higher energy. This shift corresponds to a change of the component of the dipole moment in the direction of the applied electric field obtained by -citing the dye, Ap = -10-9 eV per V/cm = -10-9 X x 4.8 x IO-lOj10-18 Debye = -$ Debye. The fact that the zero at 2.42 eV of the linear electrochromism spectrum is at the main peak of the absorption spectrum excludes the contribution of a molecular reorientation effect in this region since a change of absorption caused by a reorientation of the dye molecules in the electric field would be proportional to the absorption. Furthermore a pronounced orientation effect is not to be expected particularly at low temperatures since the molecules are incorporated within a rigid structure. The spectrum of the quadratic electrochromism corresponds to a decrease of both absorption maxima and an increase of the band assumed in the background of the low energy tail of the absorption band with a maximum at 2.27 eV (at 295OK). The decrease of the maximum at 2.58 eV suggests a torsional rearrangement of dimers and this assumption is support& by the pronounced decrease of the effect at low temperature. This point willbe considered in greater detail in a later publication. The electric field produces a stress in the dielectric of the capacitor proportional to the square of the applied field and the quadratic electrochromism may contain a pressure induced ab510
LETTERS
July 1969
sorption change. Samara, Riggleman and Drickamer [9] studied the effect of high pressure on the optical absorption of cyanine dyes and found a shift of the absorption edge of 10-6 eV/bar for a symmetrical trimethin cyanine. A shift of the same order of magnitude would be expected for the merocynnine dye as well. The applied field strength of 2.5 x 106 V/cm produces a pressure of 7 bar. Using the results of ref. [9] a shift of less than 10-5 eV is expected. This value is much smaller than the observed effect produced by the electric field and it is concluded that pressure effects are negligible_ A difficulty in a quantitative analysis of the electrochromism spectra arises from the fact, that the exact orientation of the dye molecule in the monolayer is not known. Comparison of the absorption spectra measured using parallel and perpendicular polarized light at an incidence angle of 45O indicates that the main optical transition moment which is assumed to be parallel to the long axis of the molecule, lies in a plane parallel to the electrodes being randomly orientated in this plane. The orientation of the short axis of the molecules is unknown. However, the component of the short axis in the direction of the electric field should have the same sign for all molecules, since the molecules are fixed in the monolayer by their stearyl substituent [S]. Consequently, the components of the permanent and induced dipole moments in the direction of the electric field should have the same sign and this assumption is consistent with the fact that the linear electrochromic effect changed its sign if the dye was incorporated in monolayer no. 9 instead of no. 8, which obviously changed the orientation of the short axis. Careful investigation of the absorption bands with moments parallel to the short axis of the dye molecule using polarized light should make it possible to determine the exact molecular orientation and facilitate the quantitative interpretation of the results. The monolayer assembly technique has been shown to be useful in the hvestigation of electrochromism of a merocyanine dye. This technique should be a useful tool for studying the electrooptical properties of many molecules in particular those of biological significance such as chlorophyll, which can be easily incorporated into the monolayer capacitor.
Volume
3, number
7
CHEMICAL
PHYSICS
REFEREPJCES [l]
W. Liptay and J. Czekalla. Z. Naturforsch. 15a (1960) 1072; J. Czekalls. Chimin 15 (1961) 26: W. Liptay and J. Czekalla. Z. Elektrochem.. Ber. Bunsenges. Physik. Chem. 65 (1961) 721; W. Liptay, Angew. Chem. 81 (l969) 195. [Z] J. R. Platt, J. Chem. Phys. 34 (1961) 862. [3] H. Labhart. Chimia 15 (1961) 20; H. Labhart, Tetrahedron 19 (1963) suppl. 2. [4] H. Malley, G. Feher and M. Mauzerall. J. Mol. Spectry 25 (1968) 544.
LETTERS
July I_969
[5] H. Sauter and A. C.Albrecht. Chem. Phys. Letters 2 (1968) 8: J. C. Pcwers et al., J. Chem. Phys. 36 (I962) 2893. [6] H. Bticher, K. H. Drexbage, hf. Fleck. H-Kuhn. G. hIttbius. F. P. Sch;ifer. J. Sondermann. W_Sperling. P. Tillmann and J. Wiegand. Mol. Cry&. 2 (1967) 199. [7] K. H. Drexbage and H. Kuhn, in: Basic problems in thin film physics (Gottingen. 1966). [8] C. Reichardt. Tetrahedron Letters 8 (I965} 429. 191S. A. Samara, B. M. Riggleman and H.G. Drickamer. J. Chem. Phys. 37 (1962) 1482.
51i
3, number
7
CHEMICAL
ELECTRIC IN
THE
OPTICAL
H. BijCHER,
PHYSICS
FIELD ABSORPTION
J. WIEGAND,
LETTERS
INDUCED OF
CHANGES
A MEROCYANINE
B. B. SNAVELY*“,
Received
July 1969
DYE
*
K. H. BECK and H. KUHN
23 May 1969
A field induced linear and quadratic change in the optical absorption of a merocyanine dye at 295%. 630K and 60K is reported. The dye was incorporated in a fatty acid monolayer capacitor, which allows the application of external fieId strength up to 5 X lo6 V/cm at low values of applied voltages.
It is well mown
that solvatochromic
shifts
of
due to changes in the Onsager reaction field of the solvent which are of the order of lo7 V/cm. By analogy it is to be expected that a strong externally applied electric field should also produce a change in the absorption spectrum, an effect which has been termed “electrochromism”. The theory of electrochromism has been considered by Ceekalla and Liptay [lJ, Platt [2], Labhart [3] and observation of the effect has been reported in refs. [I] and [3] and by Malyey, Feher and Manzerall [4], Sauter and Albrecht [5], and Powers et aL [5]. In this paper we report the observation of electrochromic effects in a merocyanine dye at temperatures from 6OK to 295OK and at a field strength of 2.5 x lo6 V/cm. The dye was incorporated in a fatty-acid molecular monolayer capacitor which made possible the use of electric fields up to 5 x 106 V/cm with low values of applied voltage. Changes h optical absorption resulting from the application of an electric field were detected using a lock-in technique. The form of the samples studied is shown schematically in fig. 1. A transparent aluminium film was formed on a microscope slide by evaporation and coated with the monolayer assembly using the monolayer-assembly-technique redyes
are
* This work was supported by the Deutsche Forschungsgemeinschaft. We are grateful to Dozent Dr. Ch. Reichardt for a sample of the merocyanine dye. ** On leave of absence from Eastman Kodak Co., Research Laboratories, Rochester, N.Y.
508
ported
in refs.
layers
were
The
eighth
[6,7].
built film
Seven
up on this
contained
Cd-arachidate
mono-
substrate-electrode.
the surface
active
merocyanine dye mixed with arachidate in the molecular ratio of 1 : 5, giving a dye concentration of 8.3 X IO18 molecules per cm2. Seven additional arachidate monolayers were added giving a total of fitJeen layers. The thiclmess of each layer is 26.6A yielding a total thickness of the dielectric of 400-4. Sample separation was completed with the evaporation of a second transparent electrode. Sample quality was tested by measuring the capacitance and conductance. The capacitance was found to be inversely proportional to the number of monolayers [?‘I and for a fifteen layer capacitor had a value of 0.056 PF per cm2. The conductance depended upon the applied voltage and frequency. Properly made samples had a conductance of approximately 1.5 x 10-5 ohm/cm2 at 0s3 Vrms and 500 Hz. Application of voltages greater than 13 V,,, destroyed the samples. Properly made samples burnt out homogeneously over the total area. The samples were mounted in an optical dewar to allow low temperature measurements. Electrochromism was observed as the change in light intensity produced by a 500 Hz sinusoidal voltage applied to the sample electrodes. A monochromatic light which passed through the sample in a direction parallel to the electric field was used in the measurement. The light transmitted by the sample was detected by a photomultiplier and lock-in amplifier (PAR model HR8). Measurements of the signal at the first
Volume 3, number 7
CHEMICAL
PHYSICS
LETTERS
July
1969
NDIUM CONTbCT
s
I-DISTANCE
--12mm-
Fig.
1.
Schematic
representation
of
the
= 15 = 26.6 a=400
cw~-(cn~)l~-c //O
Q
ARACHIDIC -ACID
‘3
SURFACE ACTIVE
sarcple
and highs2 harmonics of the frequency of the applied field were made. The absorption spectrum of the dye within the monolayer assembly was measured in the dewar using a sample-in sample-out technique. For this purpose a sample without electrodes was prepared, which contained the dye only in one half of the sample area. The other half served as the reference for the absorption measurement. Optical absorption and electrochromism for a particular dye at applied field with a peak amplitude of 2.5 x lo6 V/cm, corresponding to an applied voltage of 2.1 Vrms over a dielectric thickness of 400A, are shown in fig. 2. The dye [8], for which the structural formula is given in the inset, exhibited a linear (first harmonic) and a quadratic (second harmonic) electrochromic effect. Harmonics higher than the second order were not detectable. The results are given for the electric field directed from layer 15 to layer 1. The results obtained at 295OK are presented in fig. 2a and those obtained at 83’K and 6OK are given in fig. 2b. No changes in the electrochromism spectra were observed between 83O and 6OK. The optical absorption spectrum was measured only at 295OK and 83OK. The data presented are taken from samples with mean performance. With different samples the magnitude of both, the linear and quadratic effect may change independently by a factor as great as 1.5; the shapes and proportions of the spectra, however, did not change. No bleaching or electrolysis of the dye occurred during the measuring procedure.
used
for
mezzxarin
ii -I ‘OH
DYE cHo*%l,-
CHROHOPHW
g electrothromism
-10
of
I ! \
b) WK.6’K
organic
dyes_
II-
-.__. /-’
Fig. 2. Absorption spectrum (-), linear electrochromism spectrum (--- ) and quadratic electrochromism spectrum (0-. ) of the merocyanine dye l-stearyl-2(3’ nitro - 3’ formylallyliden)-1.2-di-hydrochinoliu at room temperature (a) and low temper,ture @I_ 509
Volume 3, number 7
CHEMICAL PHY=CS
At the applied low temperatures, the absorption and electrochromism spectra are shifted by approximately 0.026 eV towards higher energy, leaving the relative positions of the spectral features unchanged Roth, the linear and quadratic electrochromism increased by a factor of 2, only the broad negative peak of the quadratic effect at 2.65 eV decreases. The spectrum of the linear electrochromic effect is consistent with a spectral shift of the low energy absorption peak of 2 x lo-10 eV per V/cm, caused by a difference in the dipole moments of the ground state and the first excited state. The main qualitative deviations from the spectrum of a pure bathochromic shift of the low energy absorption band are the zero at 2.305 eV (at 295OK) and the low energy negative peak. It is also noted that the Icw energy positive peak of th? electrochromism spectrum is not exactly at the maximum slope of the absorption spectrum. These facts are explained by assuming a band with a maximum at 2.27 eV (at 295OK) in the back ground of the low energy tail of the absorption band. In the electric field of fig. 2 this band is shifted by 10-9 eV per V/cm to the higher energy. This shift corresponds to a change of the component of the dipole moment in the direction of the applied electric field obtained by -citing the dye, Ap = -10-9 eV per V/cm = -10-9 X x 4.8 x IO-lOj10-18 Debye = -$ Debye. The fact that the zero at 2.42 eV of the linear electrochromism spectrum is at the main peak of the absorption spectrum excludes the contribution of a molecular reorientation effect in this region since a change of absorption caused by a reorientation of the dye molecules in the electric field would be proportional to the absorption. Furthermore a pronounced orientation effect is not to be expected particularly at low temperatures since the molecules are incorporated within a rigid structure. The spectrum of the quadratic electrochromism corresponds to a decrease of both absorption maxima and an increase of the band assumed in the background of the low energy tail of the absorption band with a maximum at 2.27 eV (at 295OK). The decrease of the maximum at 2.58 eV suggests a torsional rearrangement of dimers and this assumption is support& by the pronounced decrease of the effect at low temperature. This point willbe considered in greater detail in a later publication. The electric field produces a stress in the dielectric of the capacitor proportional to the square of the applied field and the quadratic electrochromism may contain a pressure induced ab510
LETTERS
July 1969
sorption change. Samara, Riggleman and Drickamer [9] studied the effect of high pressure on the optical absorption of cyanine dyes and found a shift of the absorption edge of 10-6 eV/bar for a symmetrical trimethin cyanine. A shift of the same order of magnitude would be expected for the merocynnine dye as well. The applied field strength of 2.5 x 106 V/cm produces a pressure of 7 bar. Using the results of ref. [9] a shift of less than 10-5 eV is expected. This value is much smaller than the observed effect produced by the electric field and it is concluded that pressure effects are negligible_ A difficulty in a quantitative analysis of the electrochromism spectra arises from the fact, that the exact orientation of the dye molecule in the monolayer is not known. Comparison of the absorption spectra measured using parallel and perpendicular polarized light at an incidence angle of 45O indicates that the main optical transition moment which is assumed to be parallel to the long axis of the molecule, lies in a plane parallel to the electrodes being randomly orientated in this plane. The orientation of the short axis of the molecules is unknown. However, the component of the short axis in the direction of the electric field should have the same sign for all molecules, since the molecules are fixed in the monolayer by their stearyl substituent [S]. Consequently, the components of the permanent and induced dipole moments in the direction of the electric field should have the same sign and this assumption is consistent with the fact that the linear electrochromic effect changed its sign if the dye was incorporated in monolayer no. 9 instead of no. 8, which obviously changed the orientation of the short axis. Careful investigation of the absorption bands with moments parallel to the short axis of the dye molecule using polarized light should make it possible to determine the exact molecular orientation and facilitate the quantitative interpretation of the results. The monolayer assembly technique has been shown to be useful in the hvestigation of electrochromism of a merocyanine dye. This technique should be a useful tool for studying the electrooptical properties of many molecules in particular those of biological significance such as chlorophyll, which can be easily incorporated into the monolayer capacitor.
Volume
3, number
7
CHEMICAL
PHYSICS
REFEREPJCES [l]
W. Liptay and J. Czekalla. Z. Naturforsch. 15a (1960) 1072; J. Czekalls. Chimin 15 (1961) 26: W. Liptay and J. Czekalla. Z. Elektrochem.. Ber. Bunsenges. Physik. Chem. 65 (1961) 721; W. Liptay, Angew. Chem. 81 (l969) 195. [Z] J. R. Platt, J. Chem. Phys. 34 (1961) 862. [3] H. Labhart. Chimia 15 (1961) 20; H. Labhart, Tetrahedron 19 (1963) suppl. 2. [4] H. Malley, G. Feher and M. Mauzerall. J. Mol. Spectry 25 (1968) 544.
LETTERS
July I_969
[5] H. Sauter and A. C.Albrecht. Chem. Phys. Letters 2 (1968) 8: J. C. Pcwers et al., J. Chem. Phys. 36 (I962) 2893. [6] H. Bticher, K. H. Drexbage, hf. Fleck. H-Kuhn. G. hIttbius. F. P. Sch;ifer. J. Sondermann. W_Sperling. P. Tillmann and J. Wiegand. Mol. Cry&. 2 (1967) 199. [7] K. H. Drexbage and H. Kuhn, in: Basic problems in thin film physics (Gottingen. 1966). [8] C. Reichardt. Tetrahedron Letters 8 (I965} 429. 191S. A. Samara, B. M. Riggleman and H.G. Drickamer. J. Chem. Phys. 37 (1962) 1482.
51i