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Molecular cis-trans switching in amphiphilic monolayers containing azobenzene moieties
7Tun .S'Mid t"ih.~. _4_" "~ I 1994) 122
122
126
Molecular cis-trans switching in amphiphilic monolayers containing azobenzene moieties Jtirgen Maack, Ramesh C. Ahuja and Dietmar M6bius .~lav-I)lum'k-h?stitut /fir Bi.phy.~ikali.~che
('hemw. qm I'}tl~hcrg. I)-37077 (Hittmk, e. ((;erma.y)
Hiroaki Tachibana and Mutsuyoshi Matsumoto ..\"ational In.stitutc ol Mulo'ial.s und ('lwmica/ Re.scar4lt. I-I Iligashi. li~ukuha, lharaki ~05 L/apa.)
Abstract A novel principle of molecular architecture For monolaycrs containing azobcnzcnc moieties is presented which allows the chromophores to bc organized with a high density and with sufficient mobility l\)r photochemical change. Tiffs monolayer represents a new medium for a data storage system each single azobenzene group is m principle capable of storing the information of I bit. The physical properties of this monolayer have been investigated at the air water interface. I)ata slorage in the micrometer range is demonstrated with a single monolayer transferred on glass.
I. Introduction In oxygen-free media no side reactions of the photochemical cis lrans isomerization of azobenzene are known [1]. This reversible photochemical reaction as shown schematically below was found to be of first order in both directions [2]:
The system under investigation is a mixture of the amphiphilic azobenzenc A820Py and dimyristoylphosphalidic acid ( D M P A ) (molar ratio, 1:1 ). We follow the photochemical isomerization by recording the change in non-resonant properties. Further we demonstrate the technical potentiality of this system.
2. Experimental details
""=; 0"-'0 TRANS
CIS
Cis and trans isomers difl'cr in their absorption spectra and other physical observables. The cis isomer has a dipole moment of 3 D, whereas the trans isomer has no dipole moment at all [3]. A variety of amphiphilic molecules containing azobenzene moieties have been synthesized and monolayer studies have been carried out [4, 5]. Photochemical isomerization leads to changes in most of the monolayer properties including the surface pressure at a constant area or the area at a constant surlace pressure [6]. the absorption spectra [7] and the dielectric properties [8]. An important aim of this work is to develop an intk~rmation storage system, in which the single azobenzenc group is the elementary storage entity. Towards this end, we present a molecular organization principle, which allows high density organization of azobenzene moieties all provided with sufficient mobility for photochemical change. Isomerization proceeds without disturbing the structural organization of the monolaycr.
0040-6090.,'94/S 7.00 .S'£'DI 0040-6090( 93 )(14146-.1
D M P A was obtained from the Sigma Chemical Con> pany. N-[p-(p-Octylphenylazo)-phenyloxy] eicosylpyridiniurn bromide (A820Py, for the structure see Fig. I) was synthesized by the method described earlier [7]. Filtered light of a 200 W IIBO mercury lamp was used
50-
c
......
..........- 0 ' ,
40
z
E P_. 30
. . . ............. .
20 100% TRANS
I ~
• ".
85% CIS
10tJ3 0---
00
. -+-----
0.2
0.4
016
0.8
i0
1.
i
12
.......... ,,,i
1.4
1.6
A r e a / M o l e c u l e [ n m 2] Fig. 1. Sulf~lce pressure area isolhernln of a nlollolayer o f / \ g 2 0 P ' , al the air vea(er interface. The isomer content o1" lhc differen( i s o t h e r m s is indicated in the figure (cis content = 10(1" , tram, content: subphase, water: 7+= 18 C'I.
1994
I~lnexicr Sequoia. All rights rc,,cr\ed
J. Maack et al./ Moh'cular ci.~-Irans .switching in monohoers containing azohettzetw moietie.s
to illuminate the monolayer. The trans isomer was obtained by keeping the spreading solution (1 mM in chloroform) for several days at room temperature in the dark. The cis isomer was prepared by illumination of the spreading solution with 365 nm radiation. The concentration of cis isomer in the solution was determined from the absorption spectra. The monolayers at the a i r - w a t e r interface were prepared on a rectangular Teflon trough. The surface pressure was measured by the Wilhelmy method; the surface potentials at the a i r - w a t e r interface were measured with the vibrating-plate method [9]. Illumination was performed through a quartz window at the bottom of thc trough just under the vibrating plate. Approximately 10% of the monolayer area was exposed to light. The application of Brewster angle reflectometry to monolayers has been introduced a few years ago [10]. The light beam of a laser diode (2 = 685 nm) passes through a G l a n - T h o m p s o n polarizer that is set for p-polarization. The reflected beam was detected with a photomultiplier. For the cis trans isomerization, illumination was done using a quartz fiber bundle located above the trough and the light was directed onto the region of laser spot for Brewster reflectometry. In this experimental configuration, 5% ot" the monolayer area has been exposed to actinic radiation. Monolayers have transferrcd by the L a n g m u i r - B l o d gett (LB) technique on float-glass substrates. Light exposure of the monolayer film was performed through a micrometer scale on a glass substrate, which was put onto the transferred monolayer. Image reading was done with a Brewster angle microscope (Nanofilm Technologic G m b H , G6ttingen, Germany) [11].
3. Results and discussion The azobenzene amphiphile A820Py is made up of four parts: the pyridinium head group carried a positive charge, a spacer of 20 methylene units connected to the pyridinium nitrogen, the azobenzene moiety and finally a hydrophobic tail eight carbon atoms long (Fig. 1). In a monolayer, the molecules are packed in a very compact way, as can be seen from the surface pressure isotherm profile in Fig. 1, full curve. The film collapses at 0.31 nm 2 molecule ~ ( 3 5 m N m-~), an area that is determined by the azobenzcne moiety. The chromophorcs form H aggregates at surface pressure above 1 0 m N m ), as seen in the reflection spectra (a bluc shift occurs) [12]. The cis isomer of A820Py has a strong dipole mometat and adopts a kinked conformation imposed by the cis azobenzenc. Obviously, the molecular packing ot" the cis isomer of A820Py at the interface is completely
123
50.
40. Z
E 30.
20-
10-
0 0.0
tO~~°~ 0'1 --. contof, enttrans isomer 0.2
0.4
0.6
0.8
1.0
Area per h y d r o c a r b o n chain [nm 2] Fig. 2. Surface pressure area isotherms of mixed I)MPA A820Py monolaycrs (molar ratio, I:1) at the air water interface with different trans isomer fractions. The trans content of the different isotherms is indicated in the figure (cis content = 100"/,,- trans content; subphase, water: T = 18 C). In the compressed state at 3 0 m N m ), all isotherms show the same area per hydrocarbon chain of 0.21 nm-', independent of their isomer composition.
different: the surface pressure isotherm is much more expanded and shows fluid-like behavior (Fig. 1, dotted line). The area per molecule at 20 mN m ~ is 0.76 nm 2 in contrast with 0.35 nm 2 for the trans isomer. The cis trans isomerization of the pure A820Py monolayer therefore leads to large changes in the molecular organization, making it unsuitable for information storage purpose. To overcome the disadvantages ot" the pure A820Py film, we have used a mixed monolayer of D M P A and A820Py (molar ratio, 1:1). The surface pressure area isotherms of this mixed monolayer at various amounts of the trans isomer A820Py are shown in Fig. 2. Although at large areas per hydrocarbon chain the surface pressure shows different behaviors depending on the content of cis isomer, all isotherms m~rge in the compressed state at an area of 0.21 nm 2 per hydrocarbon chain. This surprising experimental result is interpreted in the scheme of monolayer organization shown in Fig. 3. The essential features of the molecular arrangement of the mixed D M P A - A 8 2 0 P y monolayer, (molar ratio, 1:1), are as follows. (1) Homogeneous mixing of the two components at the molecular level is assured owing to the oppositely charged head groups. (2) In the compressed state the D M P A and the spacer chain of the A820Py are tightly packed and form a rigid matrix with a mean area of 0.20 nm 2 per hydrocarbon chain. (3) The D M P A amphiphiles act as spacer molecules which keep the azobenzene moieties apart, thereby allowing tbr unhindered c i s - t r a n s isomerization. Reversible photochemical isomerization can be carried out very efficiently by illuminating the film with
.I. ,%l¢lo(k ('t (11. :' .~lolecular ci.~ I,'an.~ .~'llit(lhitl,t~ in m o r n , l a y e r s (OIlta#l#t.l~ azoh('llEot(" moietie.~
124
AIR (.~111 .
(.,}1 :
(,ll c
( 0 2~:~ [
t El:.
I ~Clb;,',
WATER Fig 3. Schema, tic illustration of the molecular packing arrangement of the mixed DMPA Ag20P>. monolay.cr in the condensed state.
UV radiation (330 nm < 2 < 390 nm), with radiation of 365 nm or with radiation of 436 nm [7]. After a few minutes the photostationary states are reached, in which the cis isomer content is about 90% (UV illumination and 365 nm illumination) and 10% (436 nm illumination) [ 13]. The surface potential and surface pressure were measured during the isomerization process of the mixed monolayer at constant area ( Fig. 4). 10% of the monolayer area was exposed to radiation. The surface potentim changes by 100mV, while the surface pressure changes only slightly. The constancy of the surface pressure of 26 m N m-~ is another piece of very strong evidence for the proposed molecular packing illustrated
E
I
25 4-
.
~
J-650 vv
t
X
2o-~
in Fig. 3. Thc azobenzenc moieties have sufficient space to isomerize without hindrance. We think that the azobenzene moieties in this mixed monolayer arc separated from each other and that isomerization is reversible. The most important contribution to the change in surfacc potential can be attributed to the change in dipole moment going from trans to cis isomer. However. reorganization of the m-CH3 end groups and of the hydrophilic head groups and charge displacement in the diffuse double layer may also contribute to this change. Therefore it is not possible to calculate the orientation of the azobenzene moieties from the surl:ace potential data. Nevertheless, the extent of surface potential change has to be emphasized in comparison with changes in A V of for example 10 mV in a photochemical reaction of monolayers containing a spiropyran [14], or similar changes for an anthocyanidine system (not m photostationary states) [15]. Further investigations of the mixed monolayer have been carried out by reflectometry at the Brewster angle [10]. The non-resonant method detects the dieletric properties as well as the organization of the molecules within the monolayer [16]. With the set-up used. the pure air water interface reflects a fraction of 0.15 × 10 " while a monolayer of arachidic acid at the a i r - w a t e r interface reflects a fraction of 1.0 × 10 " of the incident light. Owing to the long aliphatic chains and the aromatic systems of the A820Py, the reflectivity of the mixed D M P A - A 8 2 0 P y film is stronger by a factor of 4 {Fig. 5). Illumination of the monolayer with 365 or 436 nm light causes large changes in reflectivity (Fig. 5). The time dependence of the reflectivity contains information on the kinetics of the isomerization processes of the azobenzenc in the monolaycr. The reflcclivity changes could be fitted by single-exponential functions. Semilogarithmic plots arc shown in Fig. 6. where R , is
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.~ is
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.
.
.
.
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~
~
~
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~
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~
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400
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o
.
".U.,
-~
oL
.
am '0
% -
.
~. i-
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Time [min] Fig. 4. Surface potential and surtacc pressure kinetics of the isomcrization m the mixed DMPA Ag20Py monolayer. (molar ratio 1:1: 7 = 18 (7"). The area per hydrocarbon chain is 0.21 nm z. The illumination cycle is as follows: UV radiation, dark. 436 nm light. IJV radiation, dark etc. (3 rain duration each).
0
,'o
2'o
I
3o
4;
o'o
7o
Time train] Fig. 5. Brewstcr angle rcflcctivity kinetics o f the cls trans isornerizalion o f the I ) M P A A820Py monolayers, (molar ratio. I:I) at the a i r water interface (illumination with 365 or 436 nm light, subphasc, water" T = 2 0 (').
J. Maack el al. / Molecular ci,s trans .switching in momdavers containing uzohenccne moietie.s 6.r
5~..... ~ ~
43
C I S
• TRANS
I
2
f" ....
•°
I
I
o • °
°
,
I
0 .... i 42.0
i
i
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42.4
42.6
42.8
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(a)
6 TRANS
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5 4.
125
patterns in such systems by surface plasmon microscopy [8]. We have transferred one single m o n o l a y e r o f A820Py on float glass and exposed it through a mask to UV radiation in order to create cis isomers in the unprotected regions. After removing the mask, a picture of the surface was recorded with the Brewster angle microscope [11]. There is a strong contrast between cis and trans isomers: the numbers 6 7 8, where the m o n o l a y c r has been protected from exposure, can be seen dark against a bright b a c k g r o u n d (Fig. 7(a)). Thc same experiment has been pcrl\~rmed with the mixed D M P A - A 8 2 0 P y m o n o l a y e r (molar ratio, 1:1) (Fig. 7(b)) and again dark numbers appear. Because of thc reduccd c h r o m o p h o r e density the contrast is weakcr. The regions with high cis isomer content reflect morc ligth than the regions with low cis isomer content. This shows that the contrast at the a i r - g l a s s interthce is inverse to the air water interface ((;,¢i Fig. 5).
\
.fi
51.0
i
51'2
....
T
51.4
(b)
Time [min] Fig. 6. Semilogarithmic plot of the Brewster refleclivity signal (data from I"ig. 5) rs. lime: (a) 436 nm illumination: (b) 365 nm illuminalion. R , is the reflection signal of the corresponding photostationar~
state.
the reflection signal of the corresponding photostationary state. The different slopes o f the straight lines are due to differcnt q u a n t u m yields o f the photoisomerization rcactions, different absorption behavior of the isomers and different intensities o f the m o n o c h r o m a t i c illumination at 365 and 436 nm. We assume a linear relation between the measured signal R - R, and the trans isomer content. With this assumption we find that the photochemical azobenzene isomerization o f A820Py in the mixed m o n o l a y e r is a rcaction o f first order, as photochcmical isomerization of azobenzene is in solution. The very strong reflectivity signal o f the trans m o n o l a y e r before illumination ( Fig. 5) is surprising and not yet understood. A few years ago, a different, very intersting strategy for separating azobenzene c h r o m o p h o r e s in a m o n o layer fi'om each other was investigated. In that case a non-amphiphilic azobenzene was incorporated into an amphiphilic 13-cyclodextrin [ 17]. Unfortunately, the c i s trans isomerization was no longer o f first order, indicating some u n k n o w n proccsscs. Knoll and co-workers [8] prepared I,B multilayers o f azobenzene containing films on metal-coated glass substrates. They demonstrated the ability to write and read
(a)
(b) Fig. 7. (u) Pattern in one monolayer of A820P+,, at the air glass interface measured by Brewster angle microscop}. The numbers 6 7 8 were writen b+,, masked ilhmlination of 365 nm radiation through a metal standard. Dark a r e a s c o n t a i n trans isomcrs, and bright areas contain cis isomers. (b) A mixed I)MPA A820Py monola.xer (molar ratio, I:1)+ was used instead of the pure lilm The contrast is less because of the redttced dcnsit+,, of azoben:,'enc moieties in the fihn.
126
,l. Alaa(k ('! al..' .~]olelltdar ci~ Iralt~ ,~ll'ilchilt[~ ill mo#tolal'o'~ conlaitlilt£, a2ol)ell_-elw moietl~'~
Thc origin of the scattering points in the picture is unclear. Either it is due to the non-planar substratc or it arises from non-ideal I.B transfer. This experiment demonstrates the potentiality of the presented system for data storage devices. It is reasonable to expect that high resolution scanning techniques arc able Io discriminate between the single cis and Irans isomer molecules.
4. Conclusion W e h a v e a s s e n | b l c d all organized m o n o l a y e r at the air water interl, tce, in which azobcnzene moieties are separated from each other and can be photoisomerized without mechanical strain of the monolayer. The principle of this arrangement is applicable for separating a large variety' of amphiphilic modified chromophores. The strong effect of isomcrization on the surface potential may find applications in microelectronics. The most interesting potential application of this systein is a new kind of data storage medium with molecular density.
Acknowledgment Financial support by the Bundesministeriuin fib" Forschung und Technologie (BMFT-03M4008D) is gratefully acknowledged.
References I I|. Rau, m II. l)i.irr and H. Bouas-Laurcnt Icds.). l'lu>toclmnmwn, I!]scvicr. Amsterdam. 1990. p. 165. 2 G. Zimmcrman, I,.-Y. ( ' h o w and I ._J. Paik. ,I. Am. (Twin. A , , , 80 (1958) 352s. 3 (i. S. Ilartlc.', arid J. ~,". [..c I:dvrc../. ('hem..c;m.. (19391 531. 4 J. l Iccsemann. ,I..-Ira, ('hem..S'm .. 102 (198(I) 2167. 5 I I. Nakahara and K. FLlktlda..1. (;dl,,id. Inl('rl~lcc Nil., '7311983) 53o. 6 1. Scki and K. lchimura, 77m~ X.lid I-'ihn~, 179(1989)77. 7 1t. Tachib;ula, J. (JOlO. 1". Nakiullura. M. MatsumolO. I{. Manda, II. Nimo. A. Yabc lind V. K;.lWabata, 7hm 5o1,1 /-7hm. 179 1198q) 207. 8 M. Sa~odny, A. Schmidt. M. Stature, W. Knoll. ('. t'rban and II. Ringsdorf. l'Mvm. ,Idc. Techmd., 2 I1991) 127. 9 V. \"ogcl aild D. M/.~bius..I. ('olkml Inlerlu~c Sci.. 126 (19881 408. 10 D. 116nig and I). M6bius. Thin Sulid 1"7hn~. 210 211 11992) 64. I D. 116nig and D. M(~,hius, ('he,hi. I>hl'.v.I.cH.. /c35(19921 51). 2 J. Maack. R. ('. Ahuja. It. Tachibana and [). M 6 b i u s . . I . Phl~ (7win.. ,;ubmhled for publication. 3 R. ( . Ahuja. J. Maack. t[. Tachibana and D. M{:,biu~,..I. I'hl'.v. ('kun,'., submitted for publication. 14 1{. I{. Pol',nlcropoulos :.lnd 1). M(:,bius. Her. Bun,~un.ec.~. t'hl'~. ('hem., ,S'.l (1979) 1215. 15 1). M6bius. 11. B{ichcr, K. K u h n and .I Sondermann. Bet. Bun~cnj,~e~. Phis. (Tw,.n.. E;' (1969) 845. 16 (i. (),,crbcck, I). tl6nig and D. M6bius, 771m .',kdkl kThn.+. 242 (19q4) 213. 17 ,.%. Yabc. Y. Kav. abata. II. Niino. M. T;.iluika, A. ()uchi. tl+ Takahashi. S. l'amura. D.,'. Tagaki. II. Nakahara and K. I'ukuda, (7win. l.cll.. 11988) I.
122
126
Molecular cis-trans switching in amphiphilic monolayers containing azobenzene moieties Jtirgen Maack, Ramesh C. Ahuja and Dietmar M6bius .~lav-I)lum'k-h?stitut /fir Bi.phy.~ikali.~che
('hemw. qm I'}tl~hcrg. I)-37077 (Hittmk, e. ((;erma.y)
Hiroaki Tachibana and Mutsuyoshi Matsumoto ..\"ational In.stitutc ol Mulo'ial.s und ('lwmica/ Re.scar4lt. I-I Iligashi. li~ukuha, lharaki ~05 L/apa.)
Abstract A novel principle of molecular architecture For monolaycrs containing azobcnzcnc moieties is presented which allows the chromophores to bc organized with a high density and with sufficient mobility l\)r photochemical change. Tiffs monolayer represents a new medium for a data storage system each single azobenzene group is m principle capable of storing the information of I bit. The physical properties of this monolayer have been investigated at the air water interface. I)ata slorage in the micrometer range is demonstrated with a single monolayer transferred on glass.
I. Introduction In oxygen-free media no side reactions of the photochemical cis lrans isomerization of azobenzene are known [1]. This reversible photochemical reaction as shown schematically below was found to be of first order in both directions [2]:
The system under investigation is a mixture of the amphiphilic azobenzenc A820Py and dimyristoylphosphalidic acid ( D M P A ) (molar ratio, 1:1 ). We follow the photochemical isomerization by recording the change in non-resonant properties. Further we demonstrate the technical potentiality of this system.
2. Experimental details
""=; 0"-'0 TRANS
CIS
Cis and trans isomers difl'cr in their absorption spectra and other physical observables. The cis isomer has a dipole moment of 3 D, whereas the trans isomer has no dipole moment at all [3]. A variety of amphiphilic molecules containing azobenzene moieties have been synthesized and monolayer studies have been carried out [4, 5]. Photochemical isomerization leads to changes in most of the monolayer properties including the surface pressure at a constant area or the area at a constant surlace pressure [6]. the absorption spectra [7] and the dielectric properties [8]. An important aim of this work is to develop an intk~rmation storage system, in which the single azobenzenc group is the elementary storage entity. Towards this end, we present a molecular organization principle, which allows high density organization of azobenzene moieties all provided with sufficient mobility for photochemical change. Isomerization proceeds without disturbing the structural organization of the monolaycr.
0040-6090.,'94/S 7.00 .S'£'DI 0040-6090( 93 )(14146-.1
D M P A was obtained from the Sigma Chemical Con> pany. N-[p-(p-Octylphenylazo)-phenyloxy] eicosylpyridiniurn bromide (A820Py, for the structure see Fig. I) was synthesized by the method described earlier [7]. Filtered light of a 200 W IIBO mercury lamp was used
50-
c
......
..........- 0 ' ,
40
z
E P_. 30
. . . ............. .
20 100% TRANS
I ~
• ".
85% CIS
10tJ3 0---
00
. -+-----
0.2
0.4
016
0.8
i0
1.
i
12
.......... ,,,i
1.4
1.6
A r e a / M o l e c u l e [ n m 2] Fig. 1. Sulf~lce pressure area isolhernln of a nlollolayer o f / \ g 2 0 P ' , al the air vea(er interface. The isomer content o1" lhc differen( i s o t h e r m s is indicated in the figure (cis content = 10(1" , tram, content: subphase, water: 7+= 18 C'I.
1994
I~lnexicr Sequoia. All rights rc,,cr\ed
J. Maack et al./ Moh'cular ci.~-Irans .switching in monohoers containing azohettzetw moietie.s
to illuminate the monolayer. The trans isomer was obtained by keeping the spreading solution (1 mM in chloroform) for several days at room temperature in the dark. The cis isomer was prepared by illumination of the spreading solution with 365 nm radiation. The concentration of cis isomer in the solution was determined from the absorption spectra. The monolayers at the a i r - w a t e r interface were prepared on a rectangular Teflon trough. The surface pressure was measured by the Wilhelmy method; the surface potentials at the a i r - w a t e r interface were measured with the vibrating-plate method [9]. Illumination was performed through a quartz window at the bottom of thc trough just under the vibrating plate. Approximately 10% of the monolayer area was exposed to light. The application of Brewster angle reflectometry to monolayers has been introduced a few years ago [10]. The light beam of a laser diode (2 = 685 nm) passes through a G l a n - T h o m p s o n polarizer that is set for p-polarization. The reflected beam was detected with a photomultiplier. For the cis trans isomerization, illumination was done using a quartz fiber bundle located above the trough and the light was directed onto the region of laser spot for Brewster reflectometry. In this experimental configuration, 5% ot" the monolayer area has been exposed to actinic radiation. Monolayers have transferrcd by the L a n g m u i r - B l o d gett (LB) technique on float-glass substrates. Light exposure of the monolayer film was performed through a micrometer scale on a glass substrate, which was put onto the transferred monolayer. Image reading was done with a Brewster angle microscope (Nanofilm Technologic G m b H , G6ttingen, Germany) [11].
3. Results and discussion The azobenzene amphiphile A820Py is made up of four parts: the pyridinium head group carried a positive charge, a spacer of 20 methylene units connected to the pyridinium nitrogen, the azobenzene moiety and finally a hydrophobic tail eight carbon atoms long (Fig. 1). In a monolayer, the molecules are packed in a very compact way, as can be seen from the surface pressure isotherm profile in Fig. 1, full curve. The film collapses at 0.31 nm 2 molecule ~ ( 3 5 m N m-~), an area that is determined by the azobenzcne moiety. The chromophorcs form H aggregates at surface pressure above 1 0 m N m ), as seen in the reflection spectra (a bluc shift occurs) [12]. The cis isomer of A820Py has a strong dipole mometat and adopts a kinked conformation imposed by the cis azobenzenc. Obviously, the molecular packing ot" the cis isomer of A820Py at the interface is completely
123
50.
40. Z
E 30.
20-
10-
0 0.0
tO~~°~ 0'1 --. contof, enttrans isomer 0.2
0.4
0.6
0.8
1.0
Area per h y d r o c a r b o n chain [nm 2] Fig. 2. Surface pressure area isotherms of mixed I)MPA A820Py monolaycrs (molar ratio, I:1) at the air water interface with different trans isomer fractions. The trans content of the different isotherms is indicated in the figure (cis content = 100"/,,- trans content; subphase, water: T = 18 C). In the compressed state at 3 0 m N m ), all isotherms show the same area per hydrocarbon chain of 0.21 nm-', independent of their isomer composition.
different: the surface pressure isotherm is much more expanded and shows fluid-like behavior (Fig. 1, dotted line). The area per molecule at 20 mN m ~ is 0.76 nm 2 in contrast with 0.35 nm 2 for the trans isomer. The cis trans isomerization of the pure A820Py monolayer therefore leads to large changes in the molecular organization, making it unsuitable for information storage purpose. To overcome the disadvantages ot" the pure A820Py film, we have used a mixed monolayer of D M P A and A820Py (molar ratio, 1:1). The surface pressure area isotherms of this mixed monolayer at various amounts of the trans isomer A820Py are shown in Fig. 2. Although at large areas per hydrocarbon chain the surface pressure shows different behaviors depending on the content of cis isomer, all isotherms m~rge in the compressed state at an area of 0.21 nm 2 per hydrocarbon chain. This surprising experimental result is interpreted in the scheme of monolayer organization shown in Fig. 3. The essential features of the molecular arrangement of the mixed D M P A - A 8 2 0 P y monolayer, (molar ratio, 1:1), are as follows. (1) Homogeneous mixing of the two components at the molecular level is assured owing to the oppositely charged head groups. (2) In the compressed state the D M P A and the spacer chain of the A820Py are tightly packed and form a rigid matrix with a mean area of 0.20 nm 2 per hydrocarbon chain. (3) The D M P A amphiphiles act as spacer molecules which keep the azobenzene moieties apart, thereby allowing tbr unhindered c i s - t r a n s isomerization. Reversible photochemical isomerization can be carried out very efficiently by illuminating the film with
.I. ,%l¢lo(k ('t (11. :' .~lolecular ci.~ I,'an.~ .~'llit(lhitl,t~ in m o r n , l a y e r s (OIlta#l#t.l~ azoh('llEot(" moietie.~
124
AIR (.~111 .
(.,}1 :
(,ll c
( 0 2~:~ [
t El:.
I ~Clb;,',
WATER Fig 3. Schema, tic illustration of the molecular packing arrangement of the mixed DMPA Ag20P>. monolay.cr in the condensed state.
UV radiation (330 nm < 2 < 390 nm), with radiation of 365 nm or with radiation of 436 nm [7]. After a few minutes the photostationary states are reached, in which the cis isomer content is about 90% (UV illumination and 365 nm illumination) and 10% (436 nm illumination) [ 13]. The surface potential and surface pressure were measured during the isomerization process of the mixed monolayer at constant area ( Fig. 4). 10% of the monolayer area was exposed to radiation. The surface potentim changes by 100mV, while the surface pressure changes only slightly. The constancy of the surface pressure of 26 m N m-~ is another piece of very strong evidence for the proposed molecular packing illustrated
E
I
25 4-
.
~
J-650 vv
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in Fig. 3. Thc azobenzenc moieties have sufficient space to isomerize without hindrance. We think that the azobenzene moieties in this mixed monolayer arc separated from each other and that isomerization is reversible. The most important contribution to the change in surfacc potential can be attributed to the change in dipole moment going from trans to cis isomer. However. reorganization of the m-CH3 end groups and of the hydrophilic head groups and charge displacement in the diffuse double layer may also contribute to this change. Therefore it is not possible to calculate the orientation of the azobenzene moieties from the surl:ace potential data. Nevertheless, the extent of surface potential change has to be emphasized in comparison with changes in A V of for example 10 mV in a photochemical reaction of monolayers containing a spiropyran [14], or similar changes for an anthocyanidine system (not m photostationary states) [15]. Further investigations of the mixed monolayer have been carried out by reflectometry at the Brewster angle [10]. The non-resonant method detects the dieletric properties as well as the organization of the molecules within the monolayer [16]. With the set-up used. the pure air water interface reflects a fraction of 0.15 × 10 " while a monolayer of arachidic acid at the a i r - w a t e r interface reflects a fraction of 1.0 × 10 " of the incident light. Owing to the long aliphatic chains and the aromatic systems of the A820Py, the reflectivity of the mixed D M P A - A 8 2 0 P y film is stronger by a factor of 4 {Fig. 5). Illumination of the monolayer with 365 or 436 nm light causes large changes in reflectivity (Fig. 5). The time dependence of the reflectivity contains information on the kinetics of the isomerization processes of the azobenzenc in the monolaycr. The reflcclivity changes could be fitted by single-exponential functions. Semilogarithmic plots arc shown in Fig. 6. where R , is
AV
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o
.
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-~
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.
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.
~. i-
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Time [min] Fig. 4. Surface potential and surtacc pressure kinetics of the isomcrization m the mixed DMPA Ag20Py monolayer. (molar ratio 1:1: 7 = 18 (7"). The area per hydrocarbon chain is 0.21 nm z. The illumination cycle is as follows: UV radiation, dark. 436 nm light. IJV radiation, dark etc. (3 rain duration each).
0
,'o
2'o
I
3o
4;
o'o
7o
Time train] Fig. 5. Brewstcr angle rcflcctivity kinetics o f the cls trans isornerizalion o f the I ) M P A A820Py monolayers, (molar ratio. I:I) at the a i r water interface (illumination with 365 or 436 nm light, subphasc, water" T = 2 0 (').
J. Maack el al. / Molecular ci,s trans .switching in momdavers containing uzohenccne moietie.s 6.r
5~..... ~ ~
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(a)
6 TRANS
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125
patterns in such systems by surface plasmon microscopy [8]. We have transferred one single m o n o l a y e r o f A820Py on float glass and exposed it through a mask to UV radiation in order to create cis isomers in the unprotected regions. After removing the mask, a picture of the surface was recorded with the Brewster angle microscope [11]. There is a strong contrast between cis and trans isomers: the numbers 6 7 8, where the m o n o l a y c r has been protected from exposure, can be seen dark against a bright b a c k g r o u n d (Fig. 7(a)). Thc same experiment has been pcrl\~rmed with the mixed D M P A - A 8 2 0 P y m o n o l a y e r (molar ratio, 1:1) (Fig. 7(b)) and again dark numbers appear. Because of thc reduccd c h r o m o p h o r e density the contrast is weakcr. The regions with high cis isomer content reflect morc ligth than the regions with low cis isomer content. This shows that the contrast at the a i r - g l a s s interthce is inverse to the air water interface ((;,¢i Fig. 5).
\
.fi
51.0
i
51'2
....
T
51.4
(b)
Time [min] Fig. 6. Semilogarithmic plot of the Brewster refleclivity signal (data from I"ig. 5) rs. lime: (a) 436 nm illumination: (b) 365 nm illuminalion. R , is the reflection signal of the corresponding photostationar~
state.
the reflection signal of the corresponding photostationary state. The different slopes o f the straight lines are due to differcnt q u a n t u m yields o f the photoisomerization rcactions, different absorption behavior of the isomers and different intensities o f the m o n o c h r o m a t i c illumination at 365 and 436 nm. We assume a linear relation between the measured signal R - R, and the trans isomer content. With this assumption we find that the photochemical azobenzene isomerization o f A820Py in the mixed m o n o l a y e r is a rcaction o f first order, as photochcmical isomerization of azobenzene is in solution. The very strong reflectivity signal o f the trans m o n o l a y e r before illumination ( Fig. 5) is surprising and not yet understood. A few years ago, a different, very intersting strategy for separating azobenzene c h r o m o p h o r e s in a m o n o layer fi'om each other was investigated. In that case a non-amphiphilic azobenzene was incorporated into an amphiphilic 13-cyclodextrin [ 17]. Unfortunately, the c i s trans isomerization was no longer o f first order, indicating some u n k n o w n proccsscs. Knoll and co-workers [8] prepared I,B multilayers o f azobenzene containing films on metal-coated glass substrates. They demonstrated the ability to write and read
(a)
(b) Fig. 7. (u) Pattern in one monolayer of A820P+,, at the air glass interface measured by Brewster angle microscop}. The numbers 6 7 8 were writen b+,, masked ilhmlination of 365 nm radiation through a metal standard. Dark a r e a s c o n t a i n trans isomcrs, and bright areas contain cis isomers. (b) A mixed I)MPA A820Py monola.xer (molar ratio, I:1)+ was used instead of the pure lilm The contrast is less because of the redttced dcnsit+,, of azoben:,'enc moieties in the fihn.
126
,l. Alaa(k ('! al..' .~]olelltdar ci~ Iralt~ ,~ll'ilchilt[~ ill mo#tolal'o'~ conlaitlilt£, a2ol)ell_-elw moietl~'~
Thc origin of the scattering points in the picture is unclear. Either it is due to the non-planar substratc or it arises from non-ideal I.B transfer. This experiment demonstrates the potentiality of the presented system for data storage devices. It is reasonable to expect that high resolution scanning techniques arc able Io discriminate between the single cis and Irans isomer molecules.
4. Conclusion W e h a v e a s s e n | b l c d all organized m o n o l a y e r at the air water interl, tce, in which azobcnzene moieties are separated from each other and can be photoisomerized without mechanical strain of the monolayer. The principle of this arrangement is applicable for separating a large variety' of amphiphilic modified chromophores. The strong effect of isomcrization on the surface potential may find applications in microelectronics. The most interesting potential application of this systein is a new kind of data storage medium with molecular density.
Acknowledgment Financial support by the Bundesministeriuin fib" Forschung und Technologie (BMFT-03M4008D) is gratefully acknowledged.
References I I|. Rau, m II. l)i.irr and H. Bouas-Laurcnt Icds.). l'lu>toclmnmwn, I!]scvicr. Amsterdam. 1990. p. 165. 2 G. Zimmcrman, I,.-Y. ( ' h o w and I ._J. Paik. ,I. Am. (Twin. A , , , 80 (1958) 352s. 3 (i. S. Ilartlc.', arid J. ~,". [..c I:dvrc../. ('hem..c;m.. (19391 531. 4 J. l Iccsemann. ,I..-Ira, ('hem..S'm .. 102 (198(I) 2167. 5 I I. Nakahara and K. FLlktlda..1. (;dl,,id. Inl('rl~lcc Nil., '7311983) 53o. 6 1. Scki and K. lchimura, 77m~ X.lid I-'ihn~, 179(1989)77. 7 1t. Tachib;ula, J. (JOlO. 1". Nakiullura. M. MatsumolO. I{. Manda, II. Nimo. A. Yabc lind V. K;.lWabata, 7hm 5o1,1 /-7hm. 179 1198q) 207. 8 M. Sa~odny, A. Schmidt. M. Stature, W. Knoll. ('. t'rban and II. Ringsdorf. l'Mvm. ,Idc. Techmd., 2 I1991) 127. 9 V. \"ogcl aild D. M/.~bius..I. ('olkml Inlerlu~c Sci.. 126 (19881 408. 10 D. 116nig and I). M6bius. Thin Sulid 1"7hn~. 210 211 11992) 64. I D. 116nig and D. M(~,hius, ('he,hi. I>hl'.v.I.cH.. /c35(19921 51). 2 J. Maack. R. ('. Ahuja. It. Tachibana and [). M 6 b i u s . . I . Phl~ (7win.. ,;ubmhled for publication. 3 R. ( . Ahuja. J. Maack. t[. Tachibana and D. M{:,biu~,..I. I'hl'.v. ('kun,'., submitted for publication. 14 1{. I{. Pol',nlcropoulos :.lnd 1). M(:,bius. Her. Bun,~un.ec.~. t'hl'~. ('hem., ,S'.l (1979) 1215. 15 1). M6bius. 11. B{ichcr, K. K u h n and .I Sondermann. Bet. Bun~cnj,~e~. Phis. (Tw,.n.. E;' (1969) 845. 16 (i. (),,crbcck, I). tl6nig and D. M6bius, 771m .',kdkl kThn.+. 242 (19q4) 213. 17 ,.%. Yabc. Y. Kav. abata. II. Niino. M. T;.iluika, A. ()uchi. tl+ Takahashi. S. l'amura. D.,'. Tagaki. II. Nakahara and K. I'ukuda, (7win. l.cll.. 11988) I.