DOI: 10.2478/s11532-007-0042-8Research article

CEJC 5(4) 2007 981–995

Synthesis and photochromic behavior of someazo-polysiloxanes modified with nucleobases or

donor-acceptor groups

Ramona Enea1, Ana-Maria Resmerita1, Laura Petraru2,

Cristian Grigoras3, Dan Scutaru1, Cristofor I. Simionescu1,Nicolae Hurduc1∗

1 Department of Natural and Synthetic Polymers, Faculty of Chemical Engineering,Technical University of Iasi, Bd. Mangeron 71, 700050-Iasi, Romania

2 Institute of Applied Synthetic Chemistry 163 Division of Macromolecular Chemistry,Vienna University of Technology, Getreidemarkt 9, A-1060 Wien, Austria

3 “P.Poni” Insitute of Macromolecular Chemistry of Iasi,Aleea Gr. Ghica Voda 41A, 700487-Iasi, Romania

Received 01 May 2007; accepted 21 June 2007

Abstract: This paper presents the syntheses, characterization and photochromic behavior of some newazo-polysiloxanes modified with uracil, cytosine or nitro-phenolic groups. Taking into consideration thepossibility of generating H-bonds or donor/acceptor interactions, this class of materials present a potentialapplicability in the immobilization of biomolecules and their nano-manipulations. Also, such compoundsare capable of producing a fluid phase, with directional flowing capacity. For all these polymers, themolecular modeling simulations have shown disordered structures, which would generate amorphousphases, a very important aspect for molecules’ nano-manipulation. The photochromic behavior in thepresence of UV irradiation was also evaluated.c© Versita Warsaw and Springer-Verlag Berlin Heidelberg. All rights reserved.

Keywords: azobenzene functionalized polymers, photochromic behavior

1 Introduction

Azobenzene functionalized polymers are interesting and potentially useful materials due

to the reorganization process at nanometric scale, under UV/VIS irradiation, which,have

∗ E-mail: nhurduc@ch.tuiasi.ro

982 R. Enea et al. / Central European Journal of Chemistry 5(4) 2007 981–995

applications in optoelectronics and biomolecules nano-manipulation [1–4]. The trans-cis

isomerization of the azo-groups, connected in the polymeric main- or side-chain is respon-

sible for this behavior. This process is accompanied by molecule reorganization at the

supramolecular level. Light induced conformational changes also form the primary step

in the mass transport phenomena, observed in the azobenzenic polymeric films. Two

independent research groups reported the possibility of obtaining a surface relief grat-

ing (SRG) by UV irradiation of a polymer containing azobenzene [5, 6]. Until now the

mechanisms responsible for the surface material structuring have not been completely

elucidated despite the fact that several models have been proposed to account for the

photo-induced mass transport [7–15]. Different proposed mechanisms assume the driv-

ing forces to be the isomerization pressure, the gradient electric force, the asymmetric

diffusion, the mean-field forces, or the permittivity gradients [6, 11–15]. Recent stud-

ies developed in our research group, allow us to propose a mechanism that can explain

the azo-polymers flowing even at temperatures in the range of 30 − 40◦C under Tg [16].

The mechanism is based on the concept of conformational instability, a special state of

matter induced by the UV irradiation of the azo-polymers. Additionally, Karageorgiev

et al. demonstrated the possibility of generating a directional flow at the surface of an

azo-polymeric film using a polarized UV laser source [3]. These results open a new inter-

esting research direction, related to the molecules’ nano-manipulation, with special focus

on biomolecules.

The purpose of this paper is to report the syntheses and photochromic behavior of

some new azo-polysiloxanes modified with uracil, cytosine or nitro-phenolic groups, ma-

terials susceptible to be used for molecules immobilization and nano-manipulation. The

present paper is a part of a large study (developed) based on many azo-materials. This

study tries to identify the best candidates for the immobilization and nano-manipulation

processes and, at the same time, to elucidate the azo-materials surface relief grating

mechanism. For the nano-manipulation processes, a crucial aspect is to generate a photo-

induced fluid phase and to realize, at the same time, the molecules immobilization at the

azo-polymeric surface film. To generate a photo-induced phase, a flexible main chain is

recommended and this is the reason why we decided to use a polysiloxanic structure.

From the point of view of the photochromic behavior, for generating a photo-induced

fluid phase, similar rates are necessary for both trans/cis and cis/trans isomerization

processes. The operational conditions (the presence or absence of the natural visible

light) can influence in a crucial manner the ratio between these two opposite processes.

This is the reason why special attention has been accorded to the photochromic behavior

of the synthesized materials. The chemical structure of the photochromic unit (azoben-

zene group) is very important for generating a photo-induced fluid phase. To realize this

desiderate, it is necessary that the trans-cis isomerization rate have similar values on

the time scale domain compared to the cis-trans relaxation rate. This desiderate can be

realized in different ways:

(1) by using a p-nitro-p’-amino-azobenzene group type; for this azo-group, both trans-

cis and cis-trans isomerization process take place at the same excitation wavelength;

R. Enea et al. / Central European Journal of Chemistry 5(4) 2007 981–995 983

as a consequence, during UV irradiation, a continuous trans-cis-trans isomerization

takes place, inducing the conformational instability state (even in the absence of

visible light); this solution was used by the majority research groups;

(2) another possibility, reported only by us, is to use specific experimental conditions,

in our case the presence of the visible light simultaneous with UV irradiation, that

can increase the relaxation cis-trans rate, the result being the continuously trans-

cis-trans isomerization process, that induce the conformational instability state.

2 Experimental part

All the chemical products were purchased from Aldrich and used without supplementary

purification.

In a typical polymer modification reaction, 0.3 g of polysiloxane containing chloroben-

zyl groups in the side-chain was dissolved in 6 mL DMSO and mixed with the cor-

responding amount (based on the chloromethyl groups content) of sodium salt of 4-

(phenylazo)phenol and 0.5−0.7 g tetrabutylammonium hydrogen-sulphate. The reaction

mixture was introduced in a flask and heated for 5 h at 90◦C. The polymer was precip-

itated in methanol and washed 5 − 6 times (with methanol) to eliminate the unreacted

products. The polymer was dried under vacuum. In the second step, 0.5 g of azo-

polysiloxane was dissolved in 7 ml DMSO, under stirring; an equivalent quantity of uracil

(or nitro-phenols) and 0.1 g K2CO3 were added and then the reaction mixture was heated

9 h at 55◦C. The polymer was precipitated in methanol and washed 3 − 4 times (with

methanol), filtered and dried under vacuum.

The polymers were characterized by 1H-NMR, GPC, DSC and UV spectroscopy.

The 1H-NMR spectra were recorded on a Brucker 300 MHz device (solvents DMSO or

CDCl3). GPC experiments were carried out in THF solution at 30◦C, at a flow rate

of 1 cm3/min using a Spectra Physics 8800 gel permeation chromatograph (polystyrene

standards). DSC thermograms were recorded on a Perkin Elmer DSC calorimeter with

10◦C/min heating rate. The photochromic behavior (azobenzene trans-cis isomerization

and cis-trans relaxation phenomena) was investigated in solution and in solid state by UV

spectroscopy (BOECO S1 UV spectrophotometer); in the last case, thin films deposed

on the surface of a quartz cell were used. To induce a trans-cis photoisomerization of

the azo-groups, the solutions or films were irradiated using a UV lamp (50 W) equipped

with 350 nm filter. The azobenzene photoisomerization kinetic could be investigated

due to the fact that the trans and cis isomer absorptions are situated at different wave

length values: a strong absorption at 350 nm corresponds to the trans isomer and a weak

absorption at 450 nm is characteristic to the cis one. The kinetic curves were obtained

by monitoring the signal situated at 350 nm, corresponding to the trans-isomer.

The molecular simulations were effectuated by using a Materials Studio (4.0) soft-

ware [17]. The polymer chains’ geometry optimization was effectuated by using a Molec-

ular Mechanic method, Forcite module (Dreiding and PCFF force fields, alternatively

with molecular dynamics, in order to identify the global minimum energy value).

984 R. Enea et al. / Central European Journal of Chemistry 5(4) 2007 981–995

3 Results and discussion

The polysiloxane containing chlorobenzyl groups in the side-chain were obtained starting

from dichloro(4-chloromethylphenylethyl)methylsilane. The details concerning the syn-

thesis procedure have been reported previously [18]. The modified polysiloxanes were ob-

tained in two-steps reactions, according to Figure 1. To obtain modified azo-polysiloxanes

containing uracil, cytosine or nitro-phenolic groups in the side-chain, the polysiloxane was

modified in the first step with 4-hydroxyazobenzene (60− 80% substitution degree) and,

in the second one, the unreacted chlorobenzyl groups were modified with uracil, cytosine

or nitro-phenols, according to the procedures found in literature [19].

Fig. 1 Synthesis route to the azo-polysiloxanes modified with uracil or nitro-phenolic

groups.

In the case of the nucleobase modified azo-polysiloxanes, the polymer substitution

degree was monitored by using the signals corresponding to the -CH2Cl groups (4.5 ppm),

that shift to 5.0 ppm after the connection of the azobenzene, and 4.85 ppm or 4.8 ppm,

after the connection of the uracil or cytosine, respectively. Moreover, there are additional

characteristic signals for the uracil group (at 5.5 and 11.2 ppm) and for cytosine (at

5.85 ppm). Typical 1H-NMR spectra of an unmodified polysiloxane (A), a polysiloxane

modified with uracil (B) and a polysiloxane modified with azobenzene and uracil (C) are

presented in Figure 3. A supplementary comment must be effectuated concerning the 1H-

NMR signals situated in 0.8− 3.0 ppm domain. These signals are due to the presence of

two siloxane structural units types: α and β (Figure 2). Details concerning the 1H-NMR

spectra interpretation of the unmodified polysiloxane were previously reported [17].

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(a) β-isomer (b) α-isomer

Fig. 2 α- and β-type structural units, corresponding to the polysiloxane containing

chlorobenzyl groups.

In the case of the azo-polysiloxane modified with 2,4-dinitrophenol, the substitution

degree was calculated by using the signals corresponding to the aromatic domain (protons

f, k and i - Figure 4).

Table 1 Some characteristics of the synthesized polymers.

Sample Substituents Substitution Mnnumber degree (%)

1a - - 43002 azophenol 85 72503 uracil 14.1 7900

azophenol 74.54 cytosine 17.3 7450

azophenol 75.25 p-nitrophenol 23.0 7200

azophenol 72.56 dinitrophenol 21.8 8000

azophenol 73.0

The molecular weights (Mn) of the modified polymers have values in the range of

7,000 to 8,000. Previous studies developed on the polysiloxanes containing chlorobenzyl

groups have shown a good agreement concerning the Mn values between the H-NMR and

GPC methods [18]. The polymers Tg values were situated between 27 and 43◦C, as a

function of the substituent structure.

986 R. Enea et al. / Central European Journal of Chemistry 5(4) 2007 981–995

(a)

(b)

(c)

Fig. 3 1H-NMR spectra of un-substituted polysiloxane in CDCl3 (A), substituted with

uracil in DMSO (B) and substituted with azobenzene and uracil in CDCl3(C)

R. Enea et al. / Central European Journal of Chemistry 5(4) 2007 981–995 987

Fig. 4 1H-NMR spectra corresponding to the Sample 5 (CDCl3).

The molecular simulations showed a very important aspect about the geometry of

the polymers: the azo-polysiloxanes modified with nucleobases or donor groups present

disordered structures, susceptible to generate amorphous phases (Figure 5 and 6). The

azo-groups and the uracil or nitro-phenolic groups are randomly distributed around the

main-chain direction that can be expected to induce only short range ordered systems

(amorphous). From the conformational point of view, the polymeric chains have the

tendency to generate amorphous phases, but taking into consideration the main-chain

flexibility, the azobenzenic groups can be capable of generating small ordered domains.

This is the conclusion from the DSC analyses, which reflect slow exothermal signals during

the heating scan (Figures 7 and 8). The crystallization processes demonstrated by DSC

analyses takes place above Tg (around 50◦C) and are accompanied by low exothermal

processes (< 10 mJ) that reflect only a small crystallization degree (that do not disturb

the photo-isomerization process). For the applications in the nano-manipulation field,

this aspect is very important because the polymer film surface must be transformed into

a fluid state. If the polymer supramolecular organization is in a crystalline system, the

photo-fluidization process of the film surface may become very difficult.

988 R. Enea et al. / Central European Journal of Chemistry 5(4) 2007 981–995

Fig. 5 Minimum energy geometry corresponding to an azo-polysiloxane modified with

uracil groups.

Fig. 6 Minimum energy geometry corresponding to an azo-polysiloxane modified with

nitrophenolic groups.

Supplementary comments are necessary concerning the DSC curves profile. Figure 7

presents the DSC heating/cooling curves corresponding to the Sample 2 (Table 1). One

R. Enea et al. / Central European Journal of Chemistry 5(4) 2007 981–995 989

can observe a slow exothermal signal that appears immediately after the glass transition,

which could be due to an ordering process generated by the azobenzenic groups’ interac-

tions. This exothermal process is succeeded by an endothermic one (at 61◦C), that can be

attributed to a melting process of the ordering phase that appear at 54◦C. Aggregation

and crystallization processes above 50◦C, due to azobenzene association dipoles, were

reported recently by Yager et al. [20]. Probably, a similar association process takes place

for our azo-polysiloxanes, accentuated by the presence of the nucleobases. It must be

taken into consideration that the exothermal effect corresponding to the crystallization

process is most important in the case of Sample 3 as compared to Sample 2.

Fig. 7 Second DSC heating/cooling cycle, corresponding to the Sample 3 (10 K/min)

Fig. 8 Second DSC heating/cooling cycle, corresponding to the Sample 2 (10 K/min)

A similar behavior, but much less evident, is present in the polysiloxane substituted

only with azobenzene 85% (Figure 8). Apparently, the presence of the nucleobase favours

the ordering process that takes place immediately after the glass transition. But taking

into consideration the low intensity of signals, the ordering degree is not that high enough

to disturb the photo-fluidization process.

990 R. Enea et al. / Central European Journal of Chemistry 5(4) 2007 981–995

From the point of view of practical application, one of the most interesting azo-

polysiloxanes’ properties is the possibility to use these materials in the field of biomolecules

immobilization and nano-manipulation [19]. For this reason, it is important to know the

photochromic behavior of the synthesized azo-polymers following UV irradiation, espe-

cially in the solid state. It is important that the response rate under UV irradiation in the

solid state be high, comparable to that corresponding to the solution. Figure 10 presents

the kinetic curves of the trans-cis photo-isomerization process (obtained by spectroscopic

UV measurements) corresponding to Sample 3 (Table 1). In the case of solution mea-

surements, 5 mg/100 ml solutions in chloroform were used.

Fig. 9 The kinetic curves corresponding to the trans-cis isomerization in solution and in

solid state and cis-trans relaxation processes in solid state of Sample 3.

As it can be observed, there are not substantial differences between the isomerization

rates in solution or in solid state. The maximum conversion degree, which is around 80%

is similar in both cases. The high response speed of the azobenzenic groups in the solid

state may be explained by the flexibility of the main chain with polysiloxanic structure

and by the amorphous structure that assures a high free volume. But a supplementary

discussion is necessary concerning the relaxation process. One of the main problems

concerning the photoinduced flowing state is the necessity to generate a continuously

trans-cis-trans isomerization during the film UV irradiation process that can induce the

conformational instability state [16]. As a consequence, in the case of the azobenzenic

substituted polymers, the relaxation cis-trans process (stimulated by the visible light)

may have a similar rate value reported to the trans-cis isomerization one (stimulated by

UV light). From the point of view of the relaxation rate, one can observe that there is a

significant difference between these two processes, only 20% of the cis-form content being

recuperated after 100 s. As a comparison, in Figure 10 is presented the photochromic

behavior of the polysiloxane substituted only with azobenzene (Sample 2), where one can

observe a good concordance between the rate values of the two opposite processes, the

relaxation being complete after 350 s.

This difference concerning the photochromic behavior can be explained only by the

re-organization processes that take place in the solid state after the UV irradiation, that

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diminish in an important manner the free-volume and hinder the relaxation cis-trans

process. These re-organization phenomena are favored by the H-bond formation by the

nucleobase with itself (between the nucleobase moieties). The possibility to form such

H-bonds is a well known process, especially in the case of thymine that has a similar

structure with uracil [21, 22].

The idea that H-bonds formation can diminish the relaxation cis-trans rate is in agree-

ment with the photochromic behavior of the azo-polysiloxane substituted with cytosine.

As one can see in Figure 11, the relaxation rate is lower compared to the Sample 2, but

higher when compared with Sample 3. From this point of view, cytosine is recommended

mostly for nano-manipulation applications. This conclusion is in agreement with previous

studies that recommend adenine and not thymine for nano-manipulation [16].

Fig. 10 The kinetic curves corresponding to the trans-cis isomerization and cis-trans

relaxation processes of Sample 2 (in solid state).

Fig. 11 The kinetic curves corresponding to the trans-cis isomerization in solution and

in solid state and cis-trans relaxation processes in solid state of Sample 4.

As may be seen in Figures 12 and 13, the situation is different in the case of the azo-

polysiloxanes modified with nitro-phenolic groups (Samples 5 and 6). The nitro-phenolic

moiety cannot form H-bond and, as a consequence, the relaxation process stimulated by

992 R. Enea et al. / Central European Journal of Chemistry 5(4) 2007 981–995

visible light is complete after 500 s. But in these cases, the behavior in the solution and

in the solid state is quite different. There is a difference of 15 − 20% concerning the

maximum conversion degree of the azo-groups in the cis-form. Even the nitro-phenolic

groups cannot generate H-bonds, but they can generate dipole-dipole interactions and,

as a consequence a free volume diminishing in the solid state can take place. More of the

azo-benzenic groups may interact with nitro-phenolic moiety, especially in the cis-form,

when the dipole moment increases from 0.1 to 3.5 D. No major differences concerning the

photochromic behavior between nitro- and dinitro-phenol substituted azo-polysiloxanes

are observed.

Fig. 12 The kinetic curves corresponding to the trans-cis isomerization process in solution

and in solid state and cis-trans relaxation processes in solid state of Sample 5.

Fig. 13 The kinetic curves corresponding to the trans-cis isomerization process in solution

and in solid state and cis-trans relaxation processes in solid state of Sample 6.

R. Enea et al. / Central European Journal of Chemistry 5(4) 2007 981–995 993

4 Conclusions

Azo-polysiloxanes modified with uracil, cytosine, 4-nitrophenolic and 2,4-dinitro-phenolic

groups were synthesized and characterized.

The molecular modeling simulation demonstrated disordered structures, which will

generate amorphous phases. This an important aspect for the molecules and biomolecules

nano-manipulation, because the polymer film surface may be transformed in a fluid state

under UV-VIS light stimuli.

Photochromic behavior in the presence of UV irradiation was evaluated for all the

synthesized azo-polymers. In the case of the uracil or cytosine substituent, the maximum

conversion degree in azo-cis groups is similar in solution and in solid state. The situation

is different if one uses 4-nitrophenol or 2,4-dinitrophenol as substituents; the maximum

conversion degree is lower if compared with nucleobases systems and different in the

solution and solid states. These differences can be explained by the stronger physical

interactions that take place in the case of nitro-phenols, which diminish the free volume

necessary for azo-groups trans-cis isomerization.

Concerning the cis-trans relaxation process, one can underline that all the nucle-

obase substituted azo-polysiloxanes present lower rate values, when comparing with the

polysiloxane substituted only with azobenzene groups. This behavior can be due to a

re-organization process at supramolecular level (more compact) after the UV irradia-

tion, supplementary stabilized by the H-bonds. The nucleobase influences the cis-trans

relaxation rate. From this point of view, cytosine is mostly recommended for the nano-

manipulation applications as compared to uracil.

This class of materials present a potential interest in the field of molecules or biomolecules’

immobilization and nano-manipulations, taking into consideration the fact that the poly-

mers contain substituents capable of generating H-bonds or donor/acceptor interactions,

and at the same time also generate a directional photo-induced fluid phase (due to the

presence of the azobenzenic groups).

5 Acknowledgements

The authors would like to thank the Ministry of Education of Romania for financial

support. The research activity concerning the nucleobase substituted polysiloxanes were

effectuated in the frame of the Matnantech Program (CEEX 107/2006).

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