COLLOIDS AND A

Colloids and Surfaces SURFACES A: Physicochemical and Engineering Aspects 126 (1997) 181-188 ELSEVIER

Photochemistry in stilbene-containing monolayers. Part II: effect of cation binding on photoisomerization

O. Karthaus, M. Hioki, M. Shimomura * Research Institute for Electronic Science, Hokkaido University, Sapporo, Japan

Received 16 August 1996; accepted 6 November 1996

Abstract

A new stilbene-containing amphiphile with the chromophore in the hydrophilic headgroup was synthesized, and the photoisomerization of the stilbene sulfonate was investigated at the air-water interface. It could be shown that the isomerization leads to a decrease in area per molecule. The binding of inorganic and organic cations from the subphase occurs in the nanomolar concentration range. Binding was verified by UV and fluorescence spectroscopy and by electron spectroscopy for chemical analysis. Upon complexation with alkaline earth metals, the stilbene photoisomerization proceeds apparently faster, whereas organic cations of the viologen type quench the photoreaction. This is the first example of a control of photoisomerization in monolayers by cation complexation. © 1997 Elsevier Science B.V.

Keywords: Control of photoisomerization; Cation binding; Monolayer; Stilbene sulfonate; Trans-cis photo- isomerization

1. Introduction

Photochemical reactions are very important in both natural and artificial organized systems. In materials science, photoisomerizations in orga- nized media such as Langrnuir monolayers or Langmuir-Blodgett (LB) films can be used, for example, for thea l te ra t ion of the structure of Langmuir monolayers [1,2] and LB films [3,4], or the switching of electrical conductivity [5]. In these cases, the photoreaction can be partially controlled by parameters such as irradiation wavelength [6], irradiation time, or temperature [7]. Despite the fact that binding of inorganic cations to negatively charged surface monolayers of amphiphiles has been studied since 1935 [8], up until now no

* Corresponding author.

0927-7757/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PH S0927-7757 (96)03965-9

reports about the control of photoreactions by cation complexation have been made.

Stilbenes are well-known photochromic com- pounds exhibiting trans-cis photoisomerization. They can be easily functionalized and incorporated into amphiphilic compounds. In contrast to previ- ous reports in which the stilbene is incorporated in the hydrophobic part of an amphiphile [7,9], we report the synthesis and monolayer formation of a new type of amphiphile derived from diami- nostilbene disulfonate [10], which resembles the previously reported 'gemini-surfactants' [ 11 ] with the basic feature of a stiff, aromatic headgroup with directly attached charged groups.

At the air-water interface, the hydrophilic stil- bene chromophore should orient parallel to the interface, as schematically shown in Fig. 1. In this orientation both sulfonate groups are in the favor-

182 O. Karthaus et aZ / Colloids Surfaces A: Physicochem. Eng. Aspects 126 (1997) 181 188

air

water 0---q

Fig. 1. Schematic drawing of the stilbene orientation at the air- water interface and the change in structure on isomerization. The shaded ovals depict the area per molecule at the air- water interface.

able contact with water. Trans-c& photoisomeriza- tion of the stilbene leads, then, to a change in the structure of the headgroup, and the footprint of the cis isomer is smaller than that of the trans

form. The influence of photoisomerization of 1 on the n-A isotherms and the monolayer morphology was described recently [12]. Here, we wish to focus on the influence of organic and inorganic cations on the photoisomerization of monolayers of 1.

2. Synthesis and experimental conditions:

N,N'-bis-(hexadecanoyl)-4,4'-diaminostilbene- 2,2'-disulfonate 1 was synthesized by dissolv- ing 4,4'-Diaminostilbene-2,2'-disulfonate (TCI, Tokyo) (18.5g) and NaOH (4g) in 100ml of water. During rapid stirring, a solution of 25 g hexadecanoylchloride in 50 ml CC14 was added dropwise. The pH of the water phase was kept between 7 and 8 by adding aqueous NaOH. After 1 h of stirring, the insoluble material was filtered

and dried. The crude material contains much hexa- decanoic acid as a side product and was recrystal- lized from tetrahydrofuran. The yield was 420 mg (0.9%). Purity and chemical structure were con- firmed by nuclear magnetic resonance (NMR) spectra and elemental analysis [12].

N,N'-bis-(2-hydroxyethyl )-4,4'-bipyridinium bro- mide 2 was synthesized by dissolving 4,4'-bipyridyl (TCI, Tokyo) (20 g) and 2-bromoethanol (TCI, Tokyo) (32 g) in 150 ml dimethylformamide and stirring at 70°C for 48 h. The residue was filtered and recrystallized from ethylacetate. The yield was quantitative. The purity and chemical structure were checked by NMR, thin layer chromatography and elemental analysis. N,N'bis-(3-sulfonato- propyl)-4,4'-bipyridinium 3 was synthesized according to the literature [13].

Monolayer experiments were carried out with a computer controlled film balance [14] (FSD series, USI system; area, 150 ×458 mm 2, compression speed 0.1 nm 2 molecule-l). The water subphase, which was purified by a Milli-Q system (Millipore Corp.), had a resistivity of > 18 MI) cm, was kept at 20 + 0.5°C. For the spreading solutions, 3-5 mg of 1 were dissolved in 600 ~tl of dimethylsulfoxide (DMSO), and diluted to 10ml by CHC13 (both solvents spectroscopic grade, Merck UVasole). For irradiation, a multiband UV (254/366 nm) lamp (UVGL-58, UVP Inc.) was fixed at a 20 cm dis- tance over the film balance so the whole film was irradiated. The light intensities at the air-water interface were estimated by using a photodiode (Hamamatsu Photonics, Japan) and had the following values: 240 ~tWcm -2 at 254nm; and 560 luW cm -2 at 366 nm. Calculation of the molec-

R R

R 386n sCL s o I I

R = H31015 --C-N-- I

H

H O ~ / = ~ / = k (~ N ~ N ~ . / ~ O H

2

3S" v " N ~ _ _ ~ N ~ S O 3

3

o. Karthaus et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 126 (1997) 181-188 183

ular dimensions were based on computer-assisted models (Chem 3D Plus, Cambridge Scientific Computing, Inc.). All monolayers were transferred at 2 0 m N m -1 to solid supports. Glass slides (24 × 60 mm 2, Matsunami Co., Japan) for electron spectroscopy for chemical analysis (ESCA) meas- urements and quartz slides for the UV and fluo- rescence measurements were cleaned with a detergent solution (dcn 90, Decon Laboratories, England) followed by immersion in alkaline water- ethanol with sonication. The slides were kept under water until use. ESCA measurements were carried out with a Shimadsu ESCA-850 spectrome- ter. Fluorescence spectra were measured with a Jasco FP-770 spectrofluorometer; excitation and emission slit widths were 3 nm. Fluorescence life- times were measured with a setup described in the literature [15]. A cavity-dumped dye laser pumped by a continuous wave mode-locked Nd 3+ YAG laser with a pulse width of 6 ps was used. Then the frequency was doubled (giving a wavelength of 362 nm) for excitation.

3. Results and discussion

When spread from a CHC13-DMSO solution on a clean water surface of a Langmuir film balance, 1 forms a monomolecular layer at the air-water interface. From the r~-A isotherm in Fig. 2 one can deduce that a stable monolayer with a solid analogous character on a pure water subphase is formed [16].

The area per molecule of more than 1 nm 2 at the collapse point (deflection point of the curve on compression) is very large compared with the expected area derived from the cross-sections of the two alkyl chains ( 2 x 0 . 2 n m 2) [16]. On the other hand, the measured area is in good agreement with the area per molecule of the stil- bene sulfonate chromophore in the side-on orienta- tion at the air-water interface. Calculations of the cross-sectional area of 1 reveal an area per mole- cule of 1.2 nm 2 for the stilbene chromophore. In this orientation both sulfonate groups are in favor- able contact with water, and, in addition, are available for the binding of cations from the subphase. Two kinds of cations were used in this

4° t _.onw ,er 35 ~ _ ~ ~ 1 ~ , ~ , , , , / on MgCI 2

~" 30 BaCI on

I I I I I 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

area / molecule (nm 2)

Fig. 2. ~-A isotherms of 1 on water and on 10-5 M metal salt subphases.

study: organic dications derived from bipyridines and inorganic cations.

Some of the 7t-A isotherms on subphases con- taining various divalent metal cations differ sig- nificantly from that on pure water. Some of the metal chlorides of group II elements (Ca, Sr and Ba) cause a clear and distinct decrease of the collapse pressure of 1, whereas MgCI2 shows a small effect (Fig. 2). The collapse area remains nearly constant, which indicates that the binding occurs under the monolayer of 1. The decrease in collapse pressure is due to a binding of the cations to the monolayer and the subsequent destabiliza- tion of the latter. Ca 2+, Sr 2 + and Ba 2 ÷ are known to bind to sulfonates and sulfate more strongly than Mg z+. This is, for example, reflected in the solubility of their sulfates in water (MgSO 4 250 g 1 - 1, CaSO 4 2 g 1 - 1, SrSO 4 0.1 g 1 - 1, BaSO4 2 .4mgl -x at 20°C). The more strongly binding cations Ca 2+ , Sr 2 + and Ba 2+ might then be able to distort the conformation of the stilbene chromo- phore owing to the strong cation-sulfonate inter- action. The altered structure then results in a different, lower collapse pressure. Fig. 3 shows the concentration dependence of the collapse pressure. In the case of the group II elements [Fig. 3(a)], a clear decrease in the collapse pressure can be seen at a concentration of 500 nM for CaCI2, SrCI2 and BaC12. The new, lower value for the collapse pressure is constant over a wide concentration

184 O. Karthaus et al. / Colloids Surfaces A." Physicochem. Eng. Aspects 126 (1997) 181-188

Z

Q. t~ = 0 0

a) 3s

8

30'

25 1 . 7

Mg2 + \ ,

Ca 2+ / ill [] [] [] . Ba 2+

• • A ~ []

• • • o \ Sr2+

I I I I 1 0 -6 10 s 10 4 10 -3

salt concentration {M) I 0 -z

b)

35 ---~

Z E

,~ 30 a

o,) r~ t~

0

zs \\1 0 10 .7

o

[] • \ Na +

• [] • ~ Co2+

[] ~ Eu3+

O & AI3+ • A~

o • O ~ pb2+

O

I I I I I 0 6 I 0 s 10 .4 I 0 3 I 0 -z

salt concentration (M)

Fig. 3. Collapse pressure of Langmuir monolayers of 1 for different concentrations o f group II metal chlorides in the subphase: Mg 2 ÷, • ; Ca 2 +, []; Sr 2 ÷, • ; Ba 2 + • ; pure water, ×. (b) Collapse pressure of Langmuir monolayers o f 2 for different concentrations of metal chlorides in the subphase: Pb 2÷ , ©; Co 2÷ , • ; AI 3+, A; Na ÷ , V; Eu 3+ , IS]; x , pure water.

range indicating that the binding sites are already saturated at low concentrations (500 nM). As well as the group II metals, other metal cations have a similar effect on the monolayer of 1 [Fig. 3(b)]. A13+ and Pb 2+ have the same effect as the group II metals with the plateau of the collapse pressure of 1 between 1 and 100 ~tM. Other cations, such as Co z + and Eu 3 +, induce a monotonous decrease of the collapse pressure with increasing concen- tration, indicating a much weaker binding.

In order to confirm the binding of the cations, monolayers from Ca 2 +-containing subphases were transferred to glass substrates and subjected to ESCA analysis. Table 1 shows the dependence of the ratio between the integrated intensities of the peaks in the X-ray photoelectron spectroscopy spectra. The intensities are corrected by the sensi-

Table 1 Integrated atomic ratios from the electron spectroscopy for chemical analysis spectra of monolayers of 1 to transferred solid supports at various concentrations of CaCI2 in the subphase

Ca z+ concentration (M) 0 10 7 10-6 10-s

Ca:S ratio - - 0.328 0.637 0.518 N:S ratio 0.775 1.115 0.864 0.925 Ca:stilbene ratio a - - 0.656 1.274 1.036

"Calculated from the Ca:S atomic ratio.

tivities of each element to ESCA. One can see that at above 1 ~tM of Ca 2 + in the subphase the binding ratio of Ca 2 + to stilbene is around 1. In order to show the limits of the quantitative evaluation of the signals, the nitrogen:sulfur (N:S) ratio is given, with a calculated value of unity for 1.

This binding process is not limited to inorganic cations; organic cations, such as alkylated ammo- nium salts or pyridinium and bipyridinium deriva- tives, can also interact with the stilbene sulfonate monolayer. In these cases as well, even though the effect is not as significant as in the case of inorganic cations, a change in the n-A isotherms can be observed, and the collapse area is only marginally influenced by the complexation at low concen- trations of cation (Fig. 4). Hence, it can be con- cluded that in this case also the binding occurs under the monolayer.

The binding of the bipyridinium salts to the monolayer was confirmed by UV and fluorescence spectroscopy of monolayers transferred to glass plates. The incorporation of 2 into the transferred films can be seen as an increase in absorbance at 260 nm, the absorption maximum of 2. Electron acceptors such as 2 can act as quenchers of the excited state of stilbenes, and thus the incorpora- tion of 2 into the monolayer can also be followed by fluorescence spectroscopy. Fig. 5 shows the

o. Karthaus et aL / Colloids Surfaces A." Physicochem. Eng. Aspects 126 (1997) 181-188 185

fluorescence of a transferred stilbene monolayer upon excitation with 320 nm light. The fluores- cence decreases with increasing concentration of 2 in the water subphase. Hence, it can be concluded that at concentrations higher than 500 nM, all

40

35 z 30 g ~ 25

20- Q .

~ 15- 0

~= 10-

5 -

o I 0.4 0.6

- - 5 x 1 0 ~

1 x 106 M

I I 0.8 1 1.2

area / molecule (rim 2)

on water

1 X 10 .7 M ~,2x 107M

I 1.4 1.6

Fig. 4. n-A isotherms of 1 on subphases containing 2. The con- centration of 2 is given for each isotherm.

0.10

0 .08 ~ / 1 xl O-7M .,~,,~,

t -

O.06 (~ / \ . 2xl 0"7M o

J X (.-

0.04 O

LL

0.02

0 350 400 450 500 550

w a v e l e n g t h / n m

Fig. 5. Fluorescence spectra of monolayers transferred from subphases containing 2 to quartz slides. 2=x = 320 nm. The con- centration of 2 in the subphase is given for each spectra.

stilbene sites are bound with 2, and thus completely quenched. The measurement of the fluorescence lifetime shows that the stilbene fluorescence life- time (95 and 410 ps) is not changed by increased incorporation of bipyridinium compounds, and thus static quenching takes place.

A second feature of the stilbene monolayers is their response to irradiation. Stilbenes are known to exhibit trans-cis isomerization on irradiation [17], and this behavior could be confirmed by irradiating a bilayer solution of 1 in water [12]. Upon irradiation at the a i r -water interface, the z-A isotherms change drastically [12]. After 15 min of irradiation at 366 nm of a monolayer of 1 and subsequent compression, the solid-analogous monolayer of the trans-stilbene is converted into a liquid-analogous monolayer with a much smaller collapse area, but a larger collapse pressure (Fig. 6). Irradiation times between 0 and 15 min produce rc-A isotherms of intermediate states of the photoreaction. Prolonged irradiation times of up to 1 h do not change the isotherms significantly and it can be assumed that the stilbene moieties in the monolayer are in the trans-cis photostation- ary equilibrium after 15 min of irradiation.

Even though metal salts, e.g. CaC12, have an effect on the trans-1 monolayer, the irradiated monolayers of 1 seem to be totally insensitive towards cations in the subphase. The n-A iso-

60

g 40 j O min

1 min

ao--2o_, s mi,

10-

o I I I I - - 7 L 0.4 0.6 0.8 1 1.2 1.4

a r e a / molecule (nm 2) 6

Fig. 6. n-A isotherms of 1 after different times of irradiation with 366 nm light. The monolayers were irradiated before compression.

186 O. Karthaus et aL / Colloids Surfaces A: Physicochem. Eng. Aspects 126 (1997) 181-188

therms on pure water and o n C a 2+- , Sr 2+-, B a 2 +- and P b 2 + - c o n t a i n i n g subphases do not differ from each other. However, the cations bind to the monolayer, as shown by ESCA experiments. A ratio of 1:1 for calcium to 1 was found when a monolayer of 1 was transferred after irradiation from a 1 gM solution of CaC12.

The kinetics of this photoreaction can be moni- tored in isobar experiments in which the pressure of the monolayer is kept constant and the area-vs.- time (A-t) curves are recorded. The photoreaction can be clearly seen as a decrease of the area per molecule on irradiation of a compressed mono- layer at a surface pressure of 20 mN m-1 (Fig. 7).

On complexation of these monolayers with inor- ganic or organic cations, the kinetics of the isomer- ization reaction changes drastically. Irradiation of the monolayer on a subphase including the inor- ganic cations which cause a decrease in collapse pressure, namely Ca 2+, Sr 2+ and Ba 2+, leads to an apparent increase in photoreactivity (Fig. 7). Also, M g 2+ has no effect in this case. The threshold concentration of this effect is the same as in the previous rc-A experiments, 500 nM. Further kinetic experiments shed light on a possible reason for the enhancement of the trans cis photoreaction. It was described earlier that the trans-cis photoisomeriza- tion at the air-water interface is reversible under certain conditions. When cis-1 is irradiated with

254 nm light, trans-1 can be recovered. This recov- ery on pure water subphase is completely reversible at pressures below 5 mN m - 1 and partially revers- ible at higher pressures [12]. Fig. 8 shows the A-t isobars of 1 at 2 0 m N m -1 on subphases con- taining 1 gM salt. The A-t isobar of 1 on MgCI2 subphase exhibits the same response to irradiation as the monolayer on water, and the photoreaction is partially reversible as can be seen by the increase in area per molecule on irradiation with 254 nm light. In contrast, the isobars on CaC12, SrC12 and BaC12 subphases exhibit a much smaller response to irradiation with 254 nm. As discussed before, the alkaline earth metal cations are known to bind more strongly to sulfonates than Mg 2+. The results of the kinetic experiments thus indicate that the complexes of cis-1 with alkaline earth metal cations are thermodynamically somewhat stabilized. Even though the rc-A isotherms of 1 after irradiation with 366 nm light on Ca 2÷ containing subphases do not differ from the isotherms measured on pure water subphase, ESCA measurements could show that indeed calcium is also incorporated in the monolayer of cis-1. Thus, the stabilizing effect of the alkaline earth metals is effective on the molecu- lar scale, as indicated by the influence on the photoreaction, but has no macroscopic expression in the 7r-A isotherms.

Other explanations for the enhanced trans-cis photoreaction do not explain these results suffi-

1.05

1 i l l l I • I I

0.95 /Ik~ water

366 n

o 0 . 8 5 -

o.6 2 +

075

0.7

0.65 0 100 200 300 400 500 600 700

time (sec)

Fig. 7. Isobars at 20 m N m - 1 for monolayers of 1 on 10 -5 M metal salt subphases. The arrow indicates the start of the irradi- ation with 366 nm light.

1 ~ ~ 366nm v , ~ 254nm

il\ i I

-4 II s[c, c.c, o7! 06 I [ " ~ BaCl2

0 500 1000 1500 2000 time (see)

Fig. 8. Isobars at 20 mN m -1 for monolayers of 1 on 10 -5 M metal salt subphases on consecutive irradiation with 366 and 254 nm light.

O. Karthaus et al. / Colloids Surfaces A." Physicochem. Eng. Aspects 126 (1997) 181-188 187

ciently. If the activation barrier during the photore- action was lowered by the cation complexation, then the back photoreaction from cis-1 to trans-1 by irradiation at 254 nm should also proceed more rapidly. Although sensitized photoisomerization of stilbene has been reported, it is unlikely in this case of inorganic guests [ 18]. In addition, the effect of increased free volume in the monolayer, which was reported previously [19], does not apply for our case because the area per molecule does not increase upon complexation.

On the contrary, organic cations such as pyridin- ium or bipyridinium cations suppress the stilbene photoreaction. The organic cations bind to the stilbene monolayer, and act as a quencher of the stilbene exited state (see Fig. 5). Hence, the photo- reaction is apparently slower. As an example of the retardation of the photoisomerization, the effect of 2 in the subphase on the photoreaction of 1 is demonstrated (Fig. 9). The quenching of the photoreaction occurs in the same concentration range as the fluorescence quenching (see Fig. 5), indicating that both effects have the same origin. In order to show that the binding of the organic cation 2 to monolayers of I is due to electrostatic interaction, control experiments with a zwitterionic viologen derivative 3 were carried out. It can be seen in Fig. 9 that the quenching ability of 3 is much smaller than that of 2. At 1 gM concen- tration, 3 exhibits no effect on the photoisomeriza- tion of 1, whereas 2 induces a complete quenching

1.05

1

"~" 0.95- t~

o~ 0.9-

o.85-

0.8- t~

075 -

0.7

366 ~l ' ~ "~"

I I I I I I I

- - 1 x 1 0 6 M

- - 5 x 10-7M

j l x l O ' S M

2x 10"7M

--1x10-TM

j l x l O ' S M

pure water

0 100 200 300 400 500 600 700 lime (sec)

Fig. 9. Isobars at 20 mN m - 1 for monolayers of 1 on subphases containing 2 ( - - ) and 3 (- - -). The concentration of 2 and 3 is given for each curve. The arrow indicates the start of the irradiation with 366 nm light.

of the photoreaction. The slight quenching of the photoreaction at 10 ~tM of 3 can be explained by a dynamic quenching due to diffusion to the inter- face and collision with 1.

4. C o n c l u s i o n

We described the synthesis and monolayer prop- erties of a new type of photoactive amphiphile. It could be shown that monolayers of the stilbene sulfonate amphiphile are sensitive towards inor- ganic and organic cations in the subphase. Binding of these cations to the monolayers was confirmed by ESCA and spectroscopy. Furthermore, this work presents the first example of the control of a photoreaction in oriented media on complexation of the chromophore with counterions. This com- plexation may lead to an increase or a decrease in the apparent rate constant for photoisomerization depending on the nature of the counterion.

A c k n o w l e d g m e n t

We thank Dr Y. Ebara and Prof. Y. Okahata, Tokyo Institute of Technology, for help in the ESCA measurements, and Dr G. Nishimura, Research Institute for Electronic Science, Hokkaido University, for the fluorescence lifetime measurements.

This work was partially funded by a grant- in-aid on priority-area-research 'Photoreaction Dynamics' from the Ministry of Education, Science and Culture, Japan.

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