LIGHT REFLECTION BY COMPLEX DYE MONOLAYERS

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LIGHT REFLECTION BY COMPLEX DYE MONOLAYERS

D. Möbius

To cite this version:

D. Möbius. LIGHT REFLECTION BY COMPLEX DYE MONOLAYERS. Journal de Physique

Colloques, 1983, 44 (C10), pp.C10-441-C10-450. �10.1051/jphyscol:19831087�. �jpa-00223545�

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JOURNAL DE PHYSIQUE

Colloque CIO, suppliment a u n012, Tome 44, dgcembre 1983 page C 10-44 1

L I G H T REFLECTION BY COMPLEX DYE MONOLAYERS

Max-PZanck-fistitut fiir BiophysikaZische Ckemie (Karl-Friedrich-Bonhoeffer- Institut), AbteiZung MoZekuZarer Systemaufbau, D-3400 G~ttingen-NikoZausberg, Am Fassberg, F.R. G.

Resume

-

La reflexion de la lumiPre par une monocouche de colorant

a

une interface est amplifiee en raison de la relation de phase entre lronde rgflgchie 2 l'interface et les ondes diffu- sees de fayon coh6rente par les chromophores individuels.

L'augmentation de la reflexion dans le domaine spectral de la bande d'absorption du colorant depend lineairement de la densite des chromophores aux faibles densites. Dans les cas oii on observe la contribution dgpendant du carre de la densite que l'on attend, la constante d'amortissement de la diffusion cohe- rente de la lumiPre peut Etre determinee Zi partir de mesures drabsorption et de reflexion. La mgthode par reflexion est un outil extrEmement utile pour l1&tude des ph6nomPnes interfa- ciaux tels que l'adsorption, la formation de complexe ou la photoisom6risation, et pour le contrdle de l'organisation de monocouches.

Abstract-The reflection of light by dye monolayers at interfaces 1s enhanced due to the phase relation of the wave reflected at the interface and the coherently scattered waves from the individual chromophores. The enhancement of the reflection in the spectral range of the dye absorption band depends linearly on the density of chromophores at small densities. In cases where the expected contribution that depends on the square of the density is observed, the damping constant of coherent light scattering can be determined from absorption and reflection measurement. The reflection method is an extremely valuable tool for the investigation of interfacial processes like adsorption, complex formation or photoisomerization and for control of monolayer organization.

Amphiphilic dye molecules can be organized in monomolecular layers at the air-water interface by dropping a solution of the dye on the water surface and packing the molecules after evaporation of the solvent. Ex- ternal control of the molecular organization is not only provided by the variation of the surface pressure exerted on the monolayer but a' so by the possibility of formation of complex monolayers, which con- sist of various components of well defined molar fractions, and by t!

interactions with the underlying aqueous substrate. Therefore, these organized monolayers are far more complex than the single component m nolayers, and molecular interactions can be studied systematically by variation of the appropriate parameter / I / .

With these monolayers as subunits, organized systems on solid sub- strates can be constructed by using the Langmuir-Blodgett technique of stepwise transfer of monolayers from the air-water interface to solid surfaces / 2 / . The optical and electrical properties of a great varietl of such systems have been investiqated / 3 / .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831087

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C 10-442 JOURNAL DE PHYSIQUE

1

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LIGHT REFLECTION AND ABSORPTION OF UNSUPPORTED MONOLAYERS

In the classical approach dye chromophores are represented by oscillators of resonance frequency

vo.

Due to coherent scattering of the incident wave a monolayer of such oscillators has the absorption AD

and the reflection

if the amplitude of the incident wave is only negligibly attenuated by the monolayer /4/. In these equations, f is the oscillator strength, eo the charge of the electron, mo the mass of the electron, c the speed of light,

v

the frequency of the incident light,

vo

the resonance frequency of the oscillators and v' the damping constant of coherent scattering. N is the density of oscillators, and the factor 3/2 in Equs. (1) and (2) is due to the assumption that the oscillators are statistically oriented in the monolayer plane. This situation is usually observed in the case of monolayers containing amphiphilic cyanine dyes / 5 / . Whereas the absorption depends linearly on the density of oscillators, N, the reflection is a function of ~ 2 .

2 - LIGHT REFLECTION AND ABSORPTION OF MONOLAYERS AT INTERFACES In all practical systems, monolayers of oscillators, i. e. dye molecules, are located at interfaces or are embedded in a medium with a refractive index n > 1. Since the incident light wave has to cross the interface between the air (or vacuum) and the medium, part of the incident wave is reflected af this interface. The incident light wave then reaches the monolayer of oscillators. Each oscillator emits a spherical wave, and since the oscillators are located in a plane, all the coherently emitted spherical waves form a plane wave scattered backward and another wave scattered forward. In order to obtain the to- tal reflection caused by the system of interface and oscillator monolayer, the phase relation between the contributions has to be taken into account. If the dye monolayer is located at the interface, the reflection R of the system dye monolayer/substrate is /6/

D,S

In this equation, RS is the reflection of the substrate in the absence of the dye. The increase of reflection due to the presence of a dye monolayer at the interface is therefore

The first term on the right side of Equ. (4) depends linearly on the dye density N at the interface since RS is constant, and the second term is a function of N~ (see Equs. (1 ) and (2) )

.

The relative contributions of the terms will be discussed in Sections 4 and 5.

The absorption of a dye monolayer is usually measured in a transmission experiment. This includes the reflection of the system in the spectral

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range of dye absorption, and the absorption AD of the dye monolayer is obtained by

The expression (I-T) is measured with respect to a corresponding monolayer system without dye monolayer. If the dye density N is very small the reflection term is'determined by the first term on the right side of Equ. (4) which contains the phase relation between the plane wave reflected from the dye monolayer and the wave reflected from the interface. This phase relation changes if the monolayer is placed in the medium at systematically varied distances from the interface. If this distance is increased by deposition of more and more fatty acid monolayers on top of the dye monolayer the observed transmission changes periodically / 7 / . The experimentally accessible (I-T) reaches maxima and minima according to the phase shift P between the two plane waves.

From these extreme values of (I-T) the absorption AD is obtained /5/

as

The complex monolayers may be formed by controlled adsorption on top of a solid surface or by surface chemical reactions. Optimal control is provided when the monolayers are formed at the air-water interface.

The water surface comes closer to the ideal of a geometrically flat interface than solid surfaces. Further, the reflection RS can easily be calculated from the well known refractive index of water. Thus, the clean water surface is an excellent reference for the absolute reflection measurement.

The reflection method is a very elegant way to obtain information on the molecular organization of dye monolayers at the interface since the measurement is not obscured by molecules dissolved in the underlying substrate. However, for quantitative investigations, the relative contributions of the two terms on the right side of Equ. (4), the "linear term" and the "squared term" must be evaluated.

3

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THE LINEAR TERM IN LIGHT REFLECTION

The reflection ( R D , ~ - RS) of complex dye monolayers at the air-water interface has been measured with a spectrometer using quartz fiber bundles as light guides and a clean water surface as reference and standard / 6 / . In Fig. 1 the reflection spectrum of a monolayer containing the amphiphilic porphyrin P I in a matrix of arachidic acid ( A A ) and methylarachidate (MA). The Soret band with the reflection maximum at 430 nm and the Q-bands at 520 nm and 590 nm can be identified / 8 / . The porphyrin dye has been selected for the study of the dependence of the reflection on the dye density in the monolayer since this dye does not show any aggregation in the density range investigated.

The density N of the porphyrin can be varied by changing the ratio of chromophore to hydrophobic chain, which is given by the number m of matrix molecules ( A A and MA) plus 4 chains (attached to the chromophore)

.

Since each chain occupies 0.195 nm2 in a densely packed monolayer the density N = ( (m+4)

-

0.195 nm2)-'. From the reflection spectra of various mixed monolayers the reflection (RD

-

RS) (430 nm) at the maximum was obtained, and the values are plott&8 in Fig. 2 against the density N.

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JOURNAL DE PHYSIQUE

Wavelength /nm

Fig. 1

-

Reflection spectrum (RD

-

RS) of a mixed monolayer of amphi- philic porphyrin PI (structure s&e inset), arachidic acid (AA) and methylarachidate (MA), molar ratios P I : AA : MA = 1:10:10 at a surface pressure of 20 dyn/cm on bidistilled water.

Fig. 2

-

Reflection (RD

s -

RS) (430 nm) at the maximum of the Soret band of mixed monolayerg of the porphyrin PI (see Fig. I ) , arachidic acid and methylarachidate plotted against the density N of the porphyrin. A linear relationship is observed.

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Clearly, a linear dependence of the reflection (RD,c+

-

RS! of the dye density N is observed. The linear term in Equ. (4) IS dominating and the squared term can be neglected in these monolayers.

4

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THE SOUARED TERM R, IN LIGHT REFLECTION

In the case of homogeneous absorption and reflection bands, the band shape is given by the frequency dependence of Eqs. ( 1 ) and ( 2 )

,

and v' is obtained from the full width half maximum (Av). In the case of inhomogeneous bands, however,

Av

is larger than v' and additional information is required for the determination of v'. The observation of the squared term (second term on the right side of Equ. (4)) allows to eva- luate the damping constant v' of the coherent light scattering in the case of inhomogeneous bands.

A contribution of the squared term to the reflection ( R

-

^{RS) can}

only be observed if the chromophore density is very hi$: In monolayers of the J-aggregates of cyanine dyes /9/ the chromophore packing is very regular and provides an extremely high dye density. Fig. 3 shows the absorption spectrum (I-T) and the reflection spectrum (RD,s

-

^RS)

(dashed line) of a monolayer of the cyanine dye CY (structure s/ee inset of Fig. 3) organized in J-aggregates. The spectra are char4cZer- ized by the narrow and high aggregate band and a small band at s%orter waves attributed to monomeric dye. The maximum of the aggregate band

Wavelength Inm

Fig. 3

-

Absorption spectrum (I-T) and reflection spectrum (RDIS

-

RS) (dashed line) of a monolayer of the cyanine CY and hexadecane (HD) molar ratio CY : HD = 1:l. The absorption was obtained from a monolayer transferred to a glass plate, the reflection spectrum was measured from the surface of bidistilled water at a surface pressure of 20 dyn/cm.

The strong and narrow band is attributed to J-aggregates (thin line at the short wavelength side), the small band at shorter waves to monomeric dye.

is slightly shifted from 585 nm (reflection from the water surface) to

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ClO-446 JOURNAL DE PHYSIQUE

577 nm ( a b s o r p t i o n o f a monolayer t r a n s f e r r e d t o a g l a s s p l a t e ) due t o t h e d i f f e r e n t environment o f t h e chromophores.

The a b s o r p t i o n AD a t t h e maximum i s determined from (I-T) = 0.227 and ( R D , S

-

RS) = 0.070 by a l l o w i n g f o r t h e d i f f e r e n t r e f r a c t i v e i n d i c e s of w a t e r and g l a s s , and AD = 0.148 i s o b t a i n e d . Then, t h e r e f l e c t i o n i s c a l c u l a t e d a c c o r d i n g t o Equ. ( 4 ) w i t h RS = 0.02 ( w a t e r ) , RD = 2 0 4 9 . Comparison o f t h i s RD which i s t h e s q u a r e d c o n t r i b u t i o n t o t h e t o t a l r e f l e c t i o n , w i t h t h e l i n e a r t e r m , AD

6

⁼0.021 shows, t h a t i n - deed t h e s q u a r e d t e r m dominat6s i n l i g h t r e f l e c t i o n by J - a g g r e g a t e dye monolayers.

T h i s o f f e r s t h e p o s s i b i l i t y o f d e t e r m i n i n g t h e damping c o n s t a n t v ' . However, t h e i n t e r p r e t a t i o n o f t h e q u a n t i t i e s AD and RD i s n o t a s s i m p l e a s i n t h e c a s e o f t h e p o r p h y r i n monolayers. The J - a g g r e g a t e mo- n o l a y e r s c o n s i s t o f domains l a r g e compared w i t h t h e wavelength of t h e i n c i d e n t l i g h t . The d e n s i t y N o f chromophores i n a l l domains i s t h e same i n which t h e chromophores a r e a l l a l i g n e d p a r a l l e l t o each o t h e r . The a b s o r p t i o n A and r e f l e c t i o n R of monolayer w i t h inhomogeneous band a r e i n t h i s c a s e

I n t h e s e e q u a t i o n s + ( v o ) i s t h e d i s t r i b u t i o n f u n c t i o n o f t h e o s c i l l a - t o r s w i t h d i f f e r e n t r e s o n a n c e f r e q u e n c i e s vo. F u r t h e r , it i s assumed, t h a t a l l o s c i l l a t o r s have t h e same damping c o n s t a n t v ' . From t h e s e e q u a t i o n s

The o s c i l l a t o r s t r e n g t h f i s o b t a i n e d from Equ. ( 7 )

The damping c o n s t a n t v ' can t h e n b e d e t e r m i n e d a c c o r d i n g t o

The a b s o r p t i o n of t h e s e domains i s s t r o n g l y p o l a r i z e d , and t h e r e f o r e o n l y h a l f o f t h e i n c i d e n t u n p o l a r i z e d l i g h t i s a c t i v e . F u r t h e r , t h e a g g r e g a t e o c c u p i e s o n l y a c e r t a i n f r a c t i o n a J o f t h e t o t a l monolayer a r e a , t h e remaining p a r t b e i n g f i l l e d by monomeric dye. The p o l a r i z e d a b s o r p t i o n AJ and r e f l e c t i o n RJ a r e t h e r e f o r e

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Accordingly w i t h a = 0.6 t h e p o l a r i z e d a b s o r p t i o n and r e f l e c t i o n

AJ

= 0.493 and RJ = O . l i 3 have t o be i n t r o d u c e d i n Equ. ( 1 1 ) f o r t h e d e t e r - m i n a t i o n o f v ' . With t h e i n t e g r a t e d a b s o r p t i o n o f t h e J - a g g r e g a t e ,

J J A J d ( v ) = 8.59 x 1012 s-I t h e damping c o n s t a n t v ' = 4.1 x 1012 s-I i s o b t a i n e d . T h i s i s a b o u t 3 t i m e s s m a l l e r t h a n Av = 1 . 3 x 1013 s - I

( s e e F i g . 31, and c o n s e q u e n t l y t h e J - a g g r e g a t e band i s inhomogeneous by a f a c t o r o f 3.

T h i s d i s c u s s i o n i s n o t c o m p l e t e l y j u s t i f i e d by t h e used t h e o r e t i c a l approach. Obviously, t h e assumption made i n t h e d e r i v a t i o n o f Eqs. ( I ) ,

( 2 1 , and ( 4 ) t h a t t h e i n c i d e n t wave i s o n l y n e g l i g i b l y a t t e n u a t e d by t h e c o h e r e n t l y s c a t t e r i n g monolayer, i s no l o n g e r t r u e . However, t h e o b s e r v a t i o n of such l a r g e e f f e c t s i n a b s o r p t i o n and r e f l e c t i o n a s i n t h e c a s e of J - a g g r e g a t e monolayers and t h e p o s s i b i l i t y o f e v a l u a t i n g t h e damping c o n s t a n t o f t h e c o h e r e n t p r o c e s s may s t i m u l a t e t h e deve- lopment and a p p l i c a t i o n of a more a d e q u a t e t h e o r y .

6

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INTERFACIAL REACTIONS INVESTIGATED BY THE REFLECTION METHOD 6.1 Adsorption o f dye molecules from t h e aqueous s u b s t r a t e t o a mono-

l a y e r a t t h e i n t e r f a c e .

A w a t e r s o l u b l e p o r p h y r i n , ~ 2 , (tetrasodium-mesotetra-(4-sulfonatophe- n y 1 ) - p o r p h y r i n ) w i t h 4 n e g a t i v e c h a r g e s p e r chromophore i s r e j e c t e d from t h e w a t e r s u r f a c e i f a condensed monolayer o f a r a c h i d i c a c i d i s p r e s e n t a t t h e i n t e r f a c e . Even a t c o n c e n t r a t i o n s of P2 of M i n t h e aqueous subphase no dye is d e t e c t a b l e a t t h e i n t e r f a c e , s e e F i g . 4 , c u r v e 1 / 6 / . C l e a r l y , t h e n e g a t i v e l y charged chromophores c a n n o t

LOO

Wavelength /nrn

F i g . 4

-

R e f l e c t i o n ( R D , s

-

R S ) of monolayers a t t h e s u r f a c e o f aqueous s o l u t i o n s o f t h e p o r p h y r i n P2, c u r v e 1: Monolayer o f a r a c h i d i c a c i d , c o n c e n t r a t i o n of P2 10-3 M; no p o r p h y r i n a d s o r p t i o n . Curve 2 : Mixed monolayer o f e i c o s y l a m i n (EA) and m e t h y l a r a c h i d a t e ( M A ) , molar r a t i o EA:MA = 1 : 4 on 2 x l o - ? M aqueous P 2 s o l u t i o n ; s t r o n g a d s o r p t i o n o f t h e p o r p h y r i n t o t h i s monolayer a s shown by t h e dye r e f l e c t i o n .

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CIO-448 JOURNAL DE PHYSIQUE

approach t h e i n t e r f a c e a g a i n s t t h e i n t e r f a c i a l p o t e n t i a l o f t h e c a r b o - x y l a t e groups.

When t h e monolayer a t t h e i n t e r f a c e i s p r o v i d e d w i t h p o s i t i v e l y c h a r g e d head g r o u p s a s i n t h e c a s e o f mixed monolayers of e i c o s y l a m i n e , EA, and m e t h y l a r a c h i d a t e , MA, t h e p o r p h y r i n i s s t r o n g l y a d s o r b e d , and t h e r e f l e c t i o n spectrum shows t h e t y p i c a l S o r e t band a t 430 nm, s e e F i g . 4 , c u r v e 2 / 6 / .

T h i s a d s o r p t i o n t e c h n i q u e c o u f d b e a n e x t r e m e l y u s e f u l way of o r g a n i - z i n g monolayers a t t h e a i r - w a t e r i n t e r f a c e s i n c e no s p e c i a l l y d e s i g n e d a m p h i p h i l i c d y e s a s i n t r o d u c e d i n t h e p r e v i o u s s e c t i o n s would be r e - q u i r e d . Indeed, t h e o r g a n i z a t i o n by a d s o r p t i o n c a n be c o n t r o l l e d u s i n g an a p p r o p r i a t e m a t r i x monolayer. I n t h e c a s e o f t h e p o r p h y r i n P2 one might want t o form a monolayer o f chromophores d e n s e l y packed i n a f l a t p o s i i o n . S i n c e t h e p o r p h y r i n chromophore o c c u p i e s an a r e a o f a b o u t

5

4 nm

,

t h e m a t r i x monolayer s h o u l d p r o v i d e 4 p o s i t i v e c h a r g e s , i. e . 4 m o l e c u l e s o f EA p e r 4 nm2, and t h e a r e a n o t occupied by t h e s e must be f i l l e d w i t h n e u t r a l MA m o l e c u l e s . T h i s s i m p l e c o n s i d e r a t i o n l e a d s t o t h e molar r a t i o of amine t o e s t e r i n t h e m a t r i x o f EA : MA = 1:4 f o r o p t i m a l complex dye / m a t r i x monolayer o r g a n i z a t i o n . Optimal mono- l a y e r b e h a v i o r i s indeed observed i n t h i s c a s e w i t h r e s p e c t t o mecha- n i c a l p r o p e r t i e s ( s u r f a c e p r e s s u r e

-

a r e a i s o t h e r m ) and t h e l i g h t r e - f l e c t i o n (D. Mobius and H. G r u n i g e r , u n p u b l i s h e d ) .

6 . 2 Complex f o r m a t i o n a t t h e a i r - w a t e r i n t e r f a c e

The f o r m a t i o n o f complexes by t h e r e a c t i o n o f a f r e e b a s e p o r p h y r i n w i t h d i v a l e n t c a t i o n s i n aqueous s o l u t i o n s may proceed v i a a t r a n s i - t i o n s t a t e i n which one c a t i o n i s s i t t i n g a t o p (SAT complex) t h e f r e e b a s e whereas t h e c a t i o n f i n a l l y b e i n g i n c o r p o r a t e d a p p r o a c h e s t h e por- p h y r i n r i n g from t h e o t h e r s i d e / l o / . Such a mechanism would n o t be p o s s i b l e i n an i n t e r f a c i a l r e a c t i o n , and t h e i n v e s t i g a t i o n o f t h e me- t a l l a t i o n o f t h e p o r p h y r i n PI i n monolayers i s t h e r e f o r e p a r t i c u l a r l y i n t e r e s t i n g .

On aqueous s o l u t i o n s of cadmium c h l o r i d e t h e p o r p h y r i n PI ( s t r u c t u r e , s e e F i g . 1 ) i n mixed monolayers w i t h a r a c h i d i c a c i d (AA) and methyl- a r a c h i d a t e ( M A ) , e . g . PI : AA : MA = 1 : 10 : 10, i s m e t a l l a t e d . T h i s i s deduced from t h e change i n t h e r e f l e c t i o n s p e c t r u m , s i n c e t h e S o r e t band o f t h e cadmium c c x p l e x h a s i t s maximum a t 455 nm ( f r e e b a s e : 430 nm) / I ? / . The molar f r a c t i o n o f t h e complex i n t h e p o r p h y r i n mono- l a y e r depends on t h e cadmium c o n c e n t r a t i o n i n t h e aqueous subphase and on t h e composition o f t h e monolayer m a t r i x . S i n c e t h e p o r p h y r i n P I c a r r i e s 4 p o s i t i v e c h a r g e s p e r chromophore, t h e l o c a l p o t e n t i a l r e - j e c t s t h e d i v a l e n t c a t i o n s . Only when t h i s l o c a l p o t e n t i a l i s compen- s a t e d by t h e i n t e r f a c i a l p o t e n t i a l o f t h e monolayer m a t r i x , t h e approach and i n c o r p o r a t i o n o f t h e cadmium i o n i s o b s e r v e d . From a de- t a i l e d s t u d y of t h e s e i n f l u e n c e s t h e e q u i l i b r i u m c o n s t a n t o f complex f o r m a t i o n c a n b e e v a l u a t e d (D. Mijbius and H . G r u n i g e r , u n p u b l i s h e d ) . 6 . 3 P h o t o i s o m e r i z a t i o n of an a m p h i p h i l i c photochromic system

The r e v e r s i b l e p h o t o i s o m e r i z a t i o n o f a s p i r o p y r a n w i t h an a b s o r p t i o n i n t h e n e a r UV l e a d s t o t h e f o r m a t i o n of a merocyanine w i t h an a b s o r p - t i o n band i n t h e v i s i b l e r a n g e o f t h e spectrum. Mixed monolayers of t h e a m p h i p h i l i c s p i r o p y r a n (SP) ( s e e F i g . 5 ) and o c t a d e c a n o l ( O D ) , mo- l a r r a t i o SP : OD = 1:5, expand d u r i n g t h e p h o t o r e a c t i o n a t c o n s t a n t s u r f a c e p r e s s u r e s i n c e t h e merocyanine r e q u i r e s a l a r g e r a r e a p e r mole- c u l e t h a n t h e s p i r o p y r a n . The e x p e c t e d change i n t h e r e f l e c t i o n spectrum i s o b s e r v e d / 1 1 / , s e e F i g . 5.

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Wavelength I nm

F i g . 5 - R e f l e c t i o n ( R D

-

RS) of a mixed monolayer o f t h e amphiphi- l i c s p i r o p y r a n (SP) a n d ' o c t a d e c a n o l ( O D ) , molar r a t i o SP : OD = 1:5, b e f o r e ( l o w e r t r a c e ) and a f t e r p h o t o i s o m e r i z a t i o n t o t h e merocyanine,

(MC, upper t r a c e ) ; s u r f a c e p r e s s u r e : 10 dyn/cm; s u b s t r a t e : b i d i s t i l l e d w a t e r .

The s p i r o p y r a n shows no a d d i t i o n a l l i g h t r e f l e c t i o n i n t h e r a n g e between 500 nm and 700 nm, whereas t h e merocyanine formed by UV i r r a d i - a t i o n h a s a r e f l e c t i o n band w i t h maximum a t 620 nm. The merocyanine can be i s o m e r i z e d t o t h e s p i r o p y r a n by i r r a d i a t i o n w i t h g r e e n l i g h t . A t c o n s t a n t monolayer a r e a , t h e s u r f a c e p r e s s u r e r i s e s r a p i d l y on

f l a s h i l l u m i n a t i o n of t h e s p i r o p y r a n w i t h r a d i a t i o n . T h i s phenomenon o f f e r s t h e p o s s i b i l i t y o f g e n e r a t i n g a s u r f a c e p r e s s u r e jump. T h i s c o u l d be u s e d t o s t u d y r e l a x a t i o n phenomena i n a d i f f e r e n t monolayer i n c o n t a c t w i t h t h e photochromic monolayer. P r e l i m i n a r y o b s e r v a t i o n s o f t h e f o r m a t i o n of J - a g g r e g a t e s induced by r a i s i n g t h e s u r f a c e p r e s - s u r e o v e r t h e v a l u e r e q u i r e d f o r t h e t r a n s i t i o n from monomeric t o ag- g r e g a t e dye by t h i s method have d e m o n s t r a t e d t h e f e a s i b i l i t y of t h i s t e c h n i q u e . However, t h e a n a l y s i s of t h e r e s u l t s i s c o m p l i c a t e d by t h e t r a n s m i s s i o n of t h e s u r f a c e p r e s s u r e jump from t h e s p o t o f g e n e r a t i o n a c r o s s t h e unexposed monolayer t o t h e monolayer undergoing t h e t r a n - s i t i o n .

REFERENCES

/ I / MOBIUS D . , Acc. Chem. Res.

Ifl

(1981) 63.

/ 2 / BLODGETT K . B . , and LANGMUIR I . , Phys. Rev.

51

(1937) 964.

/ 3 / s e e e . g. t h e S p e c i a l I s s u e on Langmuir-Blodgett F i l m s , BARLOW W . A. ( G u e s t - E d . ) , Thin S o l i d F i l m s

68

(1980) 1-288;

P r o c e e d i n g s of t h e F i r s t I n t e r n a t i o n a l Conference on Langmuir- B l o d g e t t F i l m s , Thin S o l i d F i l m s

99

( 1 9 8 3 ) .

/ 4 / KUHN W. i n Stereochemie, Ed. H . Freudenberg, F. D e u t i c k e , L e i p z i g , Wien, 1933, p . 340 f f .

/ 5 / KUHN H . , MGBIUS D . , and BUCHER H . , i n P h y s i c a l Methods of Che- m i s t r y , Eds. A . Weissberger and B. R o s s i t e r , Vol. I , P t . 3B, John Wiley and Sons, I n c . , New York 1972.

(11)

C 10-450 JOURNAL

DE

PHYSIQUE

/ 6 / G R W I G E R H . , M i i B I U S D . , a n d MEYER H . , J. C h e m . P h y s . i n p r i n t . / 7 / DREXHAGE K . H . , i n P r o g r e s s i n O p t i c s X I I , E d . E . W o l f , N o r t h -

H o l l a n d A m s t e r d a m 1 9 7 4 , p. 1 6 5 .

/ 8 / GOUTERMANN M . , i n T h e P o r p h y r i n s , E d . D . D o l p h i n , A c a d e m i c P r e s s , N e w Y o r k 1 9 7 8 , V o l . 111, p. 1 .

/ 9 / BuCHER H . , and KUHN H . , C h e m . P h y s . L e t t . 6 ( 1 9 7 0 ) 1 8 3 .

/ l o /

LONG0 F. R . , BROWN E . M., RAU W. G . , . a n d A ~ L E R A . D . , i n T h e P o r p h y r i n , E d . D. D o l p h i n , A c a d e m i c P r e s s , N e w Y o r k 1 9 7 8 , V 0 l .

v,

p. 4 5 9

/ 1 1 / MGBIUS D . , M o l . C r y s t . L i q . C r y s t . i n p r i n t .