Comparison of Z⇄E isomerization in Langmuir–Blodgett layers and in solution

Materials Science and Engineering C 22 (2002) 91 – 98 www.elsevier.com/locate/msec

Comparison of Z V E isomerization in Langmuir–Blodgett layers and in solution Izabella Zawisza a, Renata Bilewicz a,*, Krzysztof Janus b, Juliusz Sworakowski b, Elz˙bieta Luboch c, Jan F. Biernat c a

Department of Chemistry, University of Warsaw, ul. Pasteura 1, 02093 Warsaw, Poland b Institute of Physical and Theoretical Chemistry, Technical University of WroclBaw, Wybrzez˙e Wyspian´skiego 27, 50370 WroclBaw, Poland c Department of Chemical Technology, Technical University of Gdan´sk, ul. Narutowicza 11, 80952 Gdan´sk, Poland Received 21 August 2001; received in revised form 12 December 2001; accepted 20 December 2001

Abstract ZVE isomerization processes in the Langmuir – Blodgett (LB) monolayer and in the bulk of the solution were compared. The molecules studied are crown ethers with the electro- and photoactive azobenzene moiety as part of the macrocyclic ring. The rates of the photoinduced E ! Z isomerization and of the spontaneous thermal Z ! E reaction were calculated on the basis of the electrochemical and spectroscopic measurements. Isomerization was found to be faster in the monolayer assembly than in the bulk of the solution. On the other hand, the activation energies of the thermal Z ! E isomerization in the film and in the heptane solution were comparable. Entropy of the isomerization reaction is larger in the film indicating the lower stability of the Z form upon immobilization. The higher rate of isomerization in monolayers compared to what was observed in solutions was explained as due to shorter intermolecular distances and uniform orientation of molecules in the monolayer. These features, unique for well-organized monolayers, allow for the cooperative ‘‘domino type’’ behavior of molecules leading to a faster isomerization process than observed in the bulk of the solution. D 2002 Elsevier Science B.V. All rights reserved. Keywords: ZVE isomerization; Langmuir – Blodgett; Heptane solution

1. Introduction Molecules containing chromophores under the illumination with light of the appropriate wavelength undergo various kinds of photoreactions: isomerization [1,2], ionization [3], dimerization [4] or cleavage [5,6]. In preparing the supramolecular devices based on the photoactive compounds, it is important to know what the changes are in the isomerization processes that may take place upon immobilizing the molecules on the solid substrates. Wellorganized molecular assemblies such as the Langmuir – Blodgett (LB) monolayers of the amphiphilic compounds containing chromophore moieties are often used for the studies of various kinds of photoreactions, which may proceed under the irradiation of the film. *

Corresponding author. Tel.: +48-22-822-0211x345; fax: +48-22-8224889. E-mail address: [email protected] (R. Bilewicz).

Our goal was to compare the Z V E isomerization processes of the two azocrown ethers (Scheme 1) in the Langmuir – Blodgett monolayer and in the bulk of the solution. The amphiphilic derivatives of the simple azobenzene in the liposome [1,7,8], bilayers [9,10], polymer films [11,12] and in the Langmuir –Blodgett mono- and multilayers [13 – 15] were reported to undergo the reversible cisVtrans photoisomerization. In micelles containing liquid and flexible phospholipids, the trans azobenzene moiety has enough space to reorient itself and to form a larger cis isomer [2], whereas in the closely packed Langmuir– Blodgett or in the self-assembled monolayers, the lack of free volume slowed down or inhibited the rate of isomerization [15 –17]. The use of two component films [16,18], introduction of the bulky alkyl substituents into the azobenzenes [19,20] or incorporation of the azobenzene moiety into the bulky molecules, e.g. crown ethers [21] or calixresorcinarenes [22], were some of the solutions proposed in order to obtain molecular assemblies providing

0928-4931/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 8 - 4 9 3 1 ( 0 2 ) 0 0 0 0 2 - 4

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Scheme 1. Structures of azocrown ethers in the Z and E forms.

enough space for the rearrangements needed during isomerization. In our group, attention is paid to the studies of the amphiphilic crown ethers with the azobenzene moiety as part of the macroring [23 – 33]. Structures of many of these compounds [23,34a] as well as their complexes [34b] are known. These compounds were separated into Z (cis) and E (trans) isomers of different complexing abilities. At the air – water interface, both isomers form stable monolayers [25,26], which can be transferred onto the solid substrates, e.g. electrodes. The azo group introduces the electroactive properties to the molecule, hence the monolayer-covered electrode can be studied using the electroanalytical methods [27 –30]. In aqueous solutions, the azo group is reduced to the hydrazo compound, and in alkaline solutions, Z and E isomers can be recognized by their different reduction potentials [29,30]. In the present work, we studied the kinetics of E ! Z isomerization induced by the UV irradiation and of the reverse spontaneous thermal isomerization from the Z to the E form. The kinetics of both reactions is usually monitored by spectrophotometry. We showed that due to the different reduction potentials of the isomers, the Z V E process in the monolayer could be conveniently monitored by voltammetry.

2. Experimental All materials were of analytical grade. The synthesis of azocrown ethers and separation into the Z- and E-isomers is described elsewhere [25,34a]. 2.1. Formation of Langmuir – Blodgett monolayers The solutions of molecules were prepared daily. Distilled water, used as the subphase, was passed through a Milli-Q water purification system. Chloroform (Aldrich) was employed as the spreading solvent. Surface pressure and sur-

face potential vs. area per molecule isotherms were recorded using the KSV LB Trough 5000 equipped with two hydrophobic barriers and a Wilhelmy balance as a surfacepressure sensor. Software version KSV-5000 was used to control the experiments. To protect the experimental setup from dust, it was placed in the laminar flow hood in which the temperature was kept constant at 20 F 1
C. Surface pressure was recorded simultaneously as a function of the molecular area. The accuracy of the measurements was F 2 ˚ 2 molecule1 for the area per molecule and F 0.1 mN m1 A for the surface pressure. The monolayers were transferred onto a thin mercury film on a silver wire or hydrophobic quartz substrates. Monolayers were transferred onto the thin mercury film electrode at a surface pressure of 20 mN m1 by withdrawing the substrate at a speed of 3 mm min1. The transfer ratio measurement is provided with the software and defined as the ratio of the decrease of the surface area of the monolayer on water to the surface area of the electrode substrate onto which the monolayer is transferred. The transfer ratio is close to 1 point of the quantitative transfer onto the electrode. Quartz substrate was hydrophobized by exposing it to the vapors of 1,1,1,3,3,3-hexamethyldisilazane (Aldrich) for 30 min. Quartz plate dimensions were 3.5  1 cm. Mono- and multilayers were transferred onto the hydrophobic quartz substrate at a surface pressure of 15 mN m1 by withdrawing and immersing (Y-type deposition) at a rate of 10 mm min1. Up to 16 layers could be deposited on the quartz substrate with a mean transfer ratio of 0.96 F 0.12. 2.2. Electrochemistry For the electrochemical studies of azocrown in the solution, 1 mM solution of azocrown ether in methylene chloride and 0.2 M tetrabutylammonium perchlorate as the supporting electrolyte were used. Voltammetry experiments were done in three-electrode arrangement with the calomel reference electrode, platinum foil and thin mercury film

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electrode (TMFE). The silver wire, precleaned in concentrated perchloric acid, was touched to a drop of mercury and cathodically polarized in 0.1 M KOH to obtain a shiny and uniform layer of mercury about 1 mm in thickness. In the studies of the monolayer-modified electrodes, an aqueous 0.1 M solution of LiOH was used as the supporting electrolyte solution. Eco Chemie Autolab system was used as the potentiostat. UV irradiation of the azocrown ether monolayer on the TMFE surface and in the solution was done using the mercury lamp (OSRAM36/73, 36 W cm2 power and 365 F 10 nm wavelength). 2.3. Spectroscopy Azocrown ether E-L16 was dissolved in the n-heptane solution, the concentration of the chromophore was 3.35  105 mol dm3. Spectroscopic experiments were done using the Perkin Elmer Lambda 20 and Shimadzu UV2101PC spectrophotometers supplied with thermostated chambers capable of stabilizing the temperature of the samples between 280 and 400 K to within 0.1 K. The photoisomerization of the samples was performed at 298 K, using a 200-W mercury lamp and appropriate combinations of filters transmitting at 313, 365 and 465 nm.

3. Results and discussion 3.1. Langmuir monolayers of Z and E isomers of azocrown ethers Z and E isomers of azocrown L13 and L16 form stable monolayers at the air – water interface [29]. The larger 16membered (L16) macrocycle possesses two bulky t-octyl substituents, whereas the smaller 13-membered (L13) azocrown ether contains two n-octyl chains (see Scheme 1). The Z-isomers of both azocrown ethers occupy smaller areas per molecule than the E form (Fig. 1). The larger the macrocyle cavity is, the larger the area per molecule occupied by an azocrown ether molecule in the monolayer. The area per molecule obtained by extrapolation of the isotherm to zero pressure for the t-octyl E-L13 was reported to be 0.96 nm2 [26,35], while the t-octyl E-L16 is 1.14 nm2. When the alkyl chains are n-octyl, the area per molecule for the E-L13 is 0.97 nm2 but the collapse pressure is significantly higher. It means that the packing of the molecules in the monolayer is more efficient when the substituents are normal alkyl chains, which may therefore affect the isomerization rate. When two crown ethers of identical substituents (t-octyl) and different sizes are compared, the area per molecule and the collapse pressure are higher for the larger crown. Since t-octyl E-L16 and n-octyl E-L13 could be separated into isomers, the Langmuir monolayers of these L13 and L16 azocrowns were transferred onto the solid substrates using the Langmuir– Blodgett method, and the E V Z isomerization was investigated.

Fig. 1. Langmuir isotherms of the Z (—) and E (- - -) isomers of azocrown ethers (A) L13 and (B) L16 at the air – water interface.

3.2. Electrochemical monitoring of EVZ isomerization reaction under UV irradiation Z and E isomers of azocrown ethers in the monolayer assembly are characterized by different reduction potentials [29,30]. In the monolayer on the electrode surface, the reduction of the Z form to the hydrazo compound and its reoxidation gives exclusively E isomer in the next reduction half-cycle [29,30]. In order to reverse the isomerization process, to transform the isomer E back to Z, the monolayer has to be UV-irradiated. Since both isomers of azocrowns in highly alkaline solutions have different reduction potentials, the voltammetric method is a convenient tool for monitoring the progress of isomerization. Fig. 2 presents the voltammograms of L13 and L16 azocrowns recorded following the different times of the UV irradiation. When the time of irradiation increases, the voltammetric signal corresponding to the electroreduction of the Z form increases. The E form is transformed into the Z form (Fig. 2A). The electroreduction of the larger azocrown, L16, in the alkaline medium is more complicated. After the reduc-

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The changes in the surface concentration of the E isomer with time are described by the equation: 

dCE ¼ kEZ CE  kZE CZ dt

ð1Þ

where CE and CZ are the surface concentrations of the E and Z isomers, respectively, kEZ is the E ! Z isomerization rate constant, whereas kZE is the rate constant of the Z ! E reaction, and t is the time. Since the isomerization reaction attains the photostationary state, the rates of the isomerization reaction can be calculated from the equations given below:

ðkEZ þ kZE Þt ¼ ln

Ceq Z C0

Ceq CZ Z  C0 C0

ð2Þ

and 0

kZE kEZ

Fig. 2. Cyclic voltammograms of azocrown ethers in LB monolayer on TMFE after different times of UV light irradiation at 365 nm: (A) L13, (a) 0, (b) 30 and (c) 60 min; (B) L16 (a) 0, (b) 10, (c) 15 and (d) 30 min in 0.1 M LiOH solution, v = 0.1 V s1. Insets: The dependence of the changes in the surface concentration of the Z form: ln[CZ,eq/C0/(CZ,eq/C0  CZ/C0)] on the time of irradiation of the monolayer of (A) L13 and (B) L16.

tion of the Z or E isomer of azocrown in the second voltammetric cycle, the reduction peak is shifted towards a more negative potential (see peak c2V in Fig. 2B). The changes in the voltammogram are due to the change in the mechanism of the azocompound reduction. During the reduction of the azocompound in this highly alkaline solution, the Li+ cations present in the electrolyte solution probably bind to the azo moiety instead of the protons [36]. This reaction takes place in the ground state of the azobenzene moiety. Changes in the surface concentration of the Z isomer upon different times of the UV irradiation (insets of Fig. 2A and B) reflect the progress of the isomerization reaction until the photostationary state (PSS) of the two isomers is attained. In the photostationary state, 77% of the initial E isomers of L13 present on the electrode surface is converted into the Z isomer. Under PSS conditions for the larger azocrown L16, only 41% of the molecules is in the Z form.

1 Ceq E B C0 C C ¼K¼B @ Ceq A Z C0

ð3Þ

where CZeq and CEeq are the surface concentrations of the Z and E forms at PSS, respectively (see insets of Fig. 2). The rate constants (kZE and kEZ), under the UV irradiation with the lamp power of 36 W cm2 are: kEZ = (1.6 F 0.2)  104 s1 for L13 and (2.2 F 0.1)  104 s1 for L16, and kZE = (0.5 F 0.2)  104 s1 for L13 and (3.2 F 0.1)  104 s1 for L16. In the smaller macrocycle, L13, the strain in the molecule during the E V Z isomerization process is higher than the more flexible azocrown, L16. This may explain the higher rate constant of isomerization compared to the stiffer L13 azocrown. Also, in the monolayer at the air – water interface, the azocrown L16 forms a more expanded film than L13 (Fig. 1). Thus, the isomerization reactions in the monolayers of the larger crown were faster. The rate of azocrown ether t-octyl L16 isomerization in the Langmuir– Blodgett monolayer was compared with the rate of isomerization of the same compound but dissolved in methylene chloride. In the solution phase, the Z isomer is reduced to a more negative potential than the E isomer. This result, opposite to the sequence observed for monolayers, is in agreement with the data reported by Laviron and Mugnier [37] for azobenzene. UV irradiation of the solution containing the E isomer (the intensity of the incident light being approximately the same as in the experiments with the LB monolayers) induces the isomerization to the Z form, and after attaining the photostationary state, the content of the Z isomer in the mixture is 60%. The higher content of the Z isomer of L16 in the photostationary state in the solution than in the Langmuir– Blodgett monolayer may be due to the different environments of the molecules in the solution and in the monolayer. Similar behaviour was reported for other types of molecules undergoing the isomerization reactions — amphiphilic derivatives of stilbene and azobenzene [7 –9].

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Interestingly, the rate constants calculated for the Z V E photoisomerization of the L16 azocrown on the basis of the electrochemical measurements are: kEZ = (1.6 F 0.2)  104 s1 and kZE = (1.1 F 0.2)  104 s1, hence the process is slower than in the monolayer assemblies. This result, although unexpected, can be explained assuming that the cooperative behavior of the molecules in the monolayer enhances the rate of isomerization. Alternatively, it may also be proposed that the absorption coefficient is higher in the monolayer due to the uniform orientation of molecules with the azo bond along the substrate surface in contrary to the situation in the solution. 3.3. UV –vis spectroscopy of Z and E isomers of azocrown ether in Langmuir –Blodgett films and in solution Fig. 3 presents the UV – vis spectra of Langmuir – Blodgett layers of azocrown ether E-L16 transferred on the quartz substrate. The absorbance of a single monolayer of azocrown is very low (0.006 at 370 nm) and is not well resolved from the background noise. Therefore, Y-type multilayers were prepared on the quartz substrate for the spectroscopic measurements. The absorbance increases linearly with the increase of the number of the Langmuir– Blodgett transferred layers (inset Fig. 3). For up to 16 layers, this dependence is a straight line with a correlation of 0.997. The azocrown ether is a convenient compound to buildup wellorganized materials on the solid substrates. Ten-monolayer thick Langmuir – Blodgett films of azocrown L16 were deposited on hydrophobic quartz and used further in the isomerization studies. UV –vis spectra of the Langmuir –Blodgett (LB) film of the azocrown ether L16, on a hydrophobic quartz substrate, were compared to those of the diluted solution of

Fig. 4. UV – vis spectra of azocrown ether L16 (A) in LB film, (B) in 3.35  105 M solution in n-heptane recorded for (a) pure E isomer, (b) PSS at 313 nm irradiation, (c) PSS at 365 nm irradiation and (d) -calculated for the pure Z isomer.

L16 in n-heptane. Fig. 4 presents the spectra of the pure E and Z isomers and the mixtures of isomers at the photostationary states obtained upon irradiation at 313 and 365 nm. The spectrum of the Z form was calculated using the Fischer [38] method from the spectrum of the E form and the two spectra at two different photostationary states. After the irradiation of the sample by light at two different wavelengths (k1 and k2), the contents of the Z and E isomers of azocrown at two stationary states were calculated. The absorbance of the metastable cis (Z ) isomer is described by the equation [38]: AZ ¼ AE  ðAk  AE Þ=a

Fig. 3. UV – vis spectra of azocrown ether, E-L16 in the Langmuir – Blodgett film on the quartz substrate transferred at P = 15 mN m1 for the different numbers of bilayers transferred onto the quartz substrate: (a)  1, (b)  4, (c)  6, (d)  10, (e)  12 and (f ) 16. Inset: The dependence of absorbance at 370 nm on the number of LB bilayers of azocrown transferred onto the substrate.

ð4Þ

where AZ, AE and Ak are the values of absorbance of the pure Z and E isomers and of the mixture of isomers at PSS, respectively; a is the molar fraction of the cis isomer at PSS. It should be noted at this point that Eq. (4) has been derived assuming that the quantum yields at the two wavelengths used in the experiment, UE(k1) and UE(k2), are equal, as are UZ(k1) and UZ(k2). In general, the assumption does not

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have to be fulfilled. In the experiment reported in this paper, the absorption at both wavelengths is due to the same type of transition, thus a possible wavelength dependence should not be too strong. For the parent azobenzene dissolved in isooctane, Zimmerman et al. [39] found UEZ313 = 0.10, UEZ365 = 0.12, UZE313 = 0.42 and UZE365 = 0.48. There is no reason to expect a much stronger dependence in our compounds, thus our assumption would result in an error in the order of a few percent. Furthermore, the calculated spectrum of the E form of our crown ethers reasonably compares with those determined experimentally for the E azobenzene [39,40]. The intensive band associated with the p ! p* transition in the E form of the azocrown ether is split into two maxima peaking at 368 and 302 nm in the solution, and at 370 and 305 nm in the LB film (Fig. 4). The splitting is probably due to an asymmetry of the azobenzene moiety incorporated into the crown ether ring. The molar absorption coefficients for azocrown E-L16 in the LB film was calculated according to the procedure described in Ref. [15]. The molar absorption coefficient of E-L16 for the p ! p* transition are equal to 8490 (at 365 nm) and 8815 dm3 mol  1 cm  1 (at 370 nm) in the solution and in the Langmuir –Blodgett film, respectively. In the monolayer assembly, the molar absorption coefficient is hence higher than in the solution. UV irradiation (365 nm) induces a transformation of the E isomer into the Z form, resulting in a decrease in the intensity of the band and a shift in the maxima which in the extrapolated spectrum of the Z form appear at 325 and around 280 nm (the latter one partly hidden under a much stronger short-wavelength band). The E ! Z conversion in the solution is more efficient than in the Langmuir – Blodgett films: at PSS attained under 365 nm irradiation, 82% of the E azocrown is converted into the Z form when the azocrown remains in the solution phase, whereas in the monolayer assembly, the conversion reaches only 30% (Fig. 4). In a two-dimensional phase, the space requiring the rearrangement in the LB film decreases the efficiency of isomerization. The band corresponding to the n ! p* transition for the E form is twice weaker and red shifted compared to that of the Z form. The maxima appeared at 460 and 445 nm for the E and Z forms, respectively. Table 1 The values of the rate constants of thermal Z ! E isomerization reaction of azocrown ether in Langmuir – Blodgett film and in n-heptane solution T (K)

kdark in LB film ( 105 s1)

313 318 323 328 333 338 343 353

1.4 2.4 3.5 6.0 12.8

kdark in heptane solution ( 105 s1)

1.4 2.8 5.5 7.8 13.9 37.6

Fig. 5. The Arrhenius plot of the rate constants vs. the reciprocal temperature for azocrown ether L16 (n) in LB film and (.) in the n-heptane solution.

Small differences in the positions and relative intensities of the absorption bands for azocrown ether molecules in the bulk of the solution and in the LB multilayers confirm the homogeneity of the film, low tendency to form aggregates and utility of these molecules in the fabrication of the novel organized materials via the layer by layer deposition. 3.4. Monitoring of thermal Z!E isomerization of azocrown ether Metastable Z form, generated under the UV irradiation in the dark, undergoes a spontaneous isomerization to the E form. The process can be conveniently monitored by following the changes of absorbance at a wavelength characteristic of the E form (365 nm in the solution and 370 nm in the LB film). The changes of absorbance are described by the equation: ln

Ainf  A ¼ kdark t Ainf  A0

ð5Þ

where A0 is the initial absorbance, Ainf is the absorbance after attaining a constant value, and A is the absorbance at the time of measurement. The values of the rate constants were determined at temperatures ranging between 313 and 333 K for the Langmuir –Blodgett film, and between 323 and 353 K for the n-heptane solution. The narrow temperature range covered by our experiment was due to the instability of the LB films of L16 at temperatures exceeding about 340 K. Table 1 contains the values of the rate constants of the thermal Z ! E isomerization in the Langmuir– Blodgett film and in the bulk of the solution. The rate of the thermal Z ! E isomerization in the Langmuir – Blodgett multilayer is also higher than in the solution confirming that the rate of isomerization may be accelerated by the intermolecular interactions in the well-packed film. The steric changes in the single azocrown molecule would then induce the

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changes in the neighbouring molecules giving the effect of ‘‘domino’’ which leads to the observed increase in the rate of isomerization in the monolayer. Within an experimental error, the temperature dependencies of the rate constants were found to follow the Arrhenius equation (Fig. 5):   Ea kdark ¼ m exp  ð6Þ RT where Ea is the activation energy, R is the gas constant, T is the temperature in K, and m is the frequency factor. The values of the activation enthalpy and activation entropy were obtained from the relationships [41,42]: DH # ¼ Ea  RT

ð7Þ

and

  kT  1 DS # ¼ R ln h

97

 28.8 J mol1 K1) than in the solution (DS # =  35.8 J mol1 K1). These results indicate lower stability of the azocrown in the Z form in the film than in the solution. Moreover, the isomerization process is connected with conformational changes in the molecule. In the solution, intuitively, the azocrown ether molecules have more space to undergo steric changes required in the isomerization process. We observed a higher rate of the thermal and photoinduced isomerization processes in the Langmuir – Blodgett films than in the solution, which suggest that the intermolecular interactions in well-organized assembly are responsible for the increase in the rate of the isomerization process. The instability of the Z isomer and the molecular interactions in the monolayer assembly lead to the enhancement in the rate of isomerization observed in the Langmuir– Blodgett film compared to the solution phase.

ð8Þ 4. Conclusions

#

#

where DH is the activation enthalpy, DS is the activation entropy of the thermal isomerization reaction, k is the Boltzmann’s constant, and h is the Planck’s constant. The results are given in Table 2. A higher frequency factor (Table 2) is found for the azocrown in the monolayer assembly compared to the value calculated for the azocrown in the solution phase. This resembles the situation of the azobenzene derivatives in liposomes where the frequency factor is also higher than in the acetone solution [7,8]. Activation energies (Table 2) for the spontaneous thermal Z ! E isomerization are comparable in the Langmuir– Blodgett monolayer and in the heptane solution. It is interesting to compare the value obtained with the results reported for the simple (unsubstituted) azobenzene and for the differently substituted azobenzene derivatives. The activation energy determined for the simple azobenzene in the solution [43,44] amounts to 97 kJ mol1, whereas in crystals of the simple azobenzene, it is equal to 223 kJ mol1. The values reported for the substituted azobenzenes in the solution are in the range 70– 110 kJ mol1, depending on the solvent, and the size and polarity of the substituent [45 – 47]. In the Langmuir– Blodgett film, the value of the activation energy is close to the values obtained in liquid state, thus confirming the fluidity of the film. The entropy of the activation of the Z ! E isomerization process in the monolayer has a more positive value (DS # =

Table 2 The activation parameters of thermal Z ! E isomerization reaction of the azocrown ether L16 in LB film on quartz and in n-heptane solution Parameter

LB monolayer

Heptane solution

Ea (kJ mol1) m (s1) DS # (J mol1 K1) DH # (kJ mol1)

100.0 5.7  1011  28.8 97.3

100.2 2.0  1011  35.8 97.5

DS # and DH # were calculated using Eqs. (7) and (8) at 323 K.

The azobenzene introduced into the crown ether macrocycle is able to undergo the reversible Z V E photoinduced isomerization both in the solution and in the monolayer assembly. The azocrown ethers under study provided enough free space and flexibility for the isomerization process to take place. Z and E isomers can be recognized electrochemically by the differences in the values of the reduction potentials. In the monolayer on the electrode surface, the reduction of the Z form followed by reoxidation gives exclusively the E isomer in the following voltammetric scan. The reverse process: E ! Z isomerization was investigated during the UV irradiation of the film. In the monolayer of the smaller L13 azocrown, 77% of the initial E form was converted into the Z isomer, whereas only 41% was converted in the case of the larger L16 azocrown. The values of the rate constants in the Langmuir – Blodgett film were compared with the value in the bulk of the solution and the rate of isomerization was found to be faster in the LB film than in the solution. The spectroscopic measurements in the rates of the reverse Z ! E thermal isomerization in the Langmuir – Blodgett film and in the bulk solution confirmed that the rate of the process is higher in the film than in the solution. The molar absorption coefficients of the E-L16 for the p ! p* transition are equal to 8490 (at 365 nm) and 8815 dm3 mol1 cm1 (at 370 nm) in the solution and in the Langmuir – Blodgett film, respectively. In the monolayer assembly, the molar absorption coefficient is hence higher than in the solution, but not sufficiently higher to explain the observed differences in the isomerization rates. Small differences in the positions and relative intensities of the absorption bands for azocrown ether molecules in the bulk of the solution and in the LB multilayers confirm the homogeneity of the film, and the low tendency to form aggregates. On the other hand, the values of the activation energies of the thermal Z ! E isomerization in the LB film and in the

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solution are comparable and typical for the values obtained in the solution confirming the fluidity of the film. The monolayer of azocrown ether is known to be in a liquidlike state at the air – water interface [25] and also on the surface of the mercury electrode [33]. In view of these results, it is proposed that the higher rate of isomerization reaction found in the monolayers compared to the behavior in the solution are due to the small intermolecular distances allowing the cooperative interactions, which facilitate the molecular rearrangements during the isomerization process. These well-organized Langmuir – Blodgett monolayers of azocrown ether molecules may be considered interesting for the fabrication of the novel light-controlled materials via the layer by layer deposition on the solid substrates.

Acknowledgements This work was supported by the KBN Grant No. 3T09A 115 18, DS Grant No. 014169/003 from the Technical University of Gdan´sk and in part by the Technical University of WroclBaw.

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