Influence of UV irradiation on the blue and red light photoinduced

processes in azobenzene polyesters

F.J. Rodrıgueza, C. Sancheza, B. Villacampaa, R. Alcalaa,*, R. Casesa, M.V. Colladosb,S. Hvilstedc, M. Stranged

aDepartamento de Fısica de la Materia Condensada, Instituto de Ciencia de Materiales de Aragon, Universidad de Zaragoza, 50009 Zaragoza, SpainbDepartamento de Fısica Aplicada, Universidad de Zaragoza, 50009 Zaragoza, Spain

cDanish Polymer Centre, Department of Chemical Engineering, Technical University of Denmark, DK-2800 Kgs Lyngby, DenmarkdRisø National Laboratory, DK-4000 Roskilde, Denmark

Received 7 January 2004; received in revised form 2 June 2004; accepted 15 June 2004

Abstract

Birefringence induced in a series of liquid crystalline side-chain azobenzene polyesters with different substituent groups was investigated

under irradiation with 488 and 633 nm linearly polarized lights. Two different initial conditions have been used: the effect of a previous

irradiation with UV light that yields the films into the isotropic state at room temperature (RT) was compared with the quenching from

temperatures above the isotropic transition temperature Ti. UV–visible spectra of the thermally quenched films show the presence of

aggregates when measured at RT. We have found that UV light irradiation creates a high concentration of cis isomers and breaks the

aggregates, but they are formed again after a few days in dark at RT. Orientation of the chromophores perpendicular to the polarization of the

488 nm light and parallel to the polarization of the 633 nm light was confirmed by dichroism measurements.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Azobenzene; Polyesters; Photoinduced optical anisotropy

1. Introduction

Photoinduced anisotropy in polymer films containing

azobenzene units has been extensively studied [1–16].

Anisotropy is usually induced by illumination with linearly

polarised blue or green light (488 or 514 nm) from an ArC

laser. Under this illumination, the orientation of the azo

units changes through trans–cis–trans isomerization pro-

cesses and a preferential orientation of the trans moieties,

with its axis perpendicular to the polarisation direction of

the exciting light, can be achieved. High and stable values of

the optical anisotropy, which can be checked by dichroism

and birefringence measurements, have been obtained,

mainly in films of liquid crystalline polymers (LCP) with

the azo units in the side chain. The strong molecular

interactions among the azo moieties in LCP seems to

0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymer.2004.06.036

* Corresponding author. Tel.: C34-976761333; fax: C34-976761229

E-mail address: ralcala@unizar.es (R. Alcala).

enhance the photoinduced anisotropy and to improve its

stability.

Optical anisotropy has also been induced in some azo

polymer films under illumination with linearly polarised red

light from a He–Ne laser (633 nm), after pre-irradiation with

either blue or ultraviolet (UV) light [17–19]. In this case,

and depending on the irradiation conditions, the induced

orientation of the azo moieties can either be parallel or

perpendicular to the polarization direction of the red light. If

the films are first irradiated with unpolarised UV light, a

subsequent red light irradiation induces, in a first step, a

preferential orientation of the trans moieties parallel to the

red light polarization [18,20,21]. However, if the red light

irradiation goes on for long times, a change in the

preferential orientation direction from parallel to perpen-

dicular has been found in some polymers [18]. That

perpendicular orientation can also be induced in some

azo-polymers under red light irradiation without a previous

pre-irradiation with UV light [18,22].

Polymer 45 (2004) 6003–6012

www.elsevier.com/locate/polymer

Fig. 1. (a) Chemical structure of the P6x4 polymers. (b) trans and cis

configurations of the azo moieties.

F.J. Rodrıguez et al. / Polymer 45 (2004) 6003–60126004

Pre-irradiation with UV light produces a high concen-

tration of the cis isomer. Subsequent red light irradiation

induces the preferential back isomerization of the cis

moeities with its axis parallel to the red light polarisation

direction, to the trans form. But the presence of high cis

content in the films seems to produce some other changes in

the properties of the films. Thus, it has been reported that

UV irradiation can induce a liquid crystal (LC) to isotropic

phase transition at temperatures well below Ti (LC to

isotropic transition temperature without irradiation) [23].

This transition is usually accompanied by a strong decrease

in the light scattered by the film [24]. UV irradiation is also

able to break the aggregates of azobenzene moieties that

appear in many polymers and this can play an important role

in the photoinduced processes [25–27].

From all these results it is clear that the presence of the

cis isomer, induced by UV irradiation, strongly influences

the optical properties and the response of azobenzene

polymer films to light irradiation. In this paper we present a

study of the influence of UV pre-irradiation on the optical

properties of a series of side chain LC azobenzene

polyesters. The different polymers in this series were

chosen to have different terminal groups. In this way the

thermal properties as well as the aggregation trend are

modified along the series and the influence of those

properties on the photoinduced processes can be explored.

Optical absorption, dichroism and birefringence have been

studied using different irradiation conditions with either

blue light (488 nm) or red light (633 nm). The influence of

UV pre-irradiation on the photoinduced processes has been

analysed.

2. Experimental

The liquid crystalline polyesters used in this work were

prepared in a K2CO3 catalyzed transesterification of

equimolar amounts of diphenyl adipate [28] and the

appropriately 4-substituted (X) 2-[6-[4-[(4-Xphenyl)azo]-

phenoxy]hexyl]1,3-propanediol [29] in the melt under

vacuum at elevated temperature [30]. Further details are

provided elsewhere [31,32]. The chemical structure of the

investigated polyesters P6x4 is given in Fig. 1(a), while the

trans and cis configurations of the azo chromophore are

given in Fig. 1(b). The five different polyester substituents

Table 1

Transition temperatures and enthalpies and molecular mass of polyesters P6x4 m

x R Tg (8C) Ti (8C) D

c OCH3 22 63 1

e CF3 44 73 5

fa CH3 29 61 7

65 5

j Cl 9 54 2

k Br 24 81 3

a This polymer shows three phase transitions.

are shown in Table 1. Their glass transition temperatures

(Tg) and their isotropic transition temperatures (Ti) as well

as their molecular masses and transition enthalpies are also

collected in Table 1.

Films were produced by casting a solution of the

polymers in chloroform (0.5 mg in 200 ml) onto clean

glass substrates. Thickness was measured using a DEKTAK

profilometer. Thickness value ranged from 200 to 400 nm.

Before performing any experiment, all the films were heated

up to 20 8C above Ti for 10 min. Films were then fast

quenched to RT by putting them on a metallic plate.

A high pressure Hg lamp followed by a band pass UV

filter (maximum transmission at 350 nm) was used as the

UV light source. The measured light intensity was about

0.5 mW/cm2 in the film. The polymer films have been

easured heating up the polymers (third run) at 3 8C/min

H (cal/g) Mn Mw Mw/Mn

0.53 10,000 16,000 1.6

.81 27,000 42,000 1.6

.37 34,000 55,000 1.6

.36

.32 31,000 61,000 2.0

.99 35,000 62,000 1.8

F.J. Rodrıguez et al. / Polymer 45 (2004) 6003–6012 6005

examined in the polarization microscope before and after

irradiation with UV light. Fast-quenched films show a

random distribution of micro-domains. However, we were

not able to identify the type of phase of those domains. In

films irradiated with UV light, the domain structure is barely

observed and the light scattering produced by the film is

drastically reduced. Vertically polarized light beams at 488

and 633 nm were provided by ArC and He–Ne lasers,

respectively. The intensity used was 75 mW/cm2 for the

488 nm light and 1 W/cm2 for the 633 nm light. Optical

absorption measurements have been performed in a Cary

500 Scan UV–Vis-NIR spectrophotometer. Linear (verti-

cal/horizontal) polarization of the measuring beams was

achieved using a Glan-Thomson prism. Birefringence

measurements were performed using the setup shown in

Fig. 2. The sample was placed between crossed polarizers

with their polarization directions at G458 with the vertical

axis. The light from a 780 nm diode laser transmitted

through the polarizer-sample-polarizer system was

measured with a Si detector as a probe of the photoinduced

anisotropy. The transmitted intensity I is given by the

equation:

I Z I0 sin2ðpjDnjd=lÞ

where I0 is the intensity transmitted by the as quenched films

between parallel polarizers, d the film thickness, Dn the

Fig. 2. Experimental set-up for bi

birefringence of the sample and l the wavelength of the

measuring light (780 nm). In isotropic samples (DnZ0), no

transmitted light is expected and thus, birefringence

measurements also enable us to detect the transition from

an anisotropic (Is0) to an isotropic (IZ0) state. An optical

pyrometer was used to check that the temperature of the

films did not increase during irradiation.

3. Experimental results and discussion

As we said above, the main goal of the present work is to

study the influence of UV light pre-irradiation on both the

optical properties and the photoinduced optical effects,

under several irradiation conditions, in films of side chain

liquid crystalline azo-polyesters with different end substi-

tutions. The polyesters generally have relatively high

molecular masses. As seen from Table 1 most of the

polyesters have Mn>27,000 stemming from DP’s >50 with

corresponding polydispersities (Mw/Mn) from 1.6 to 2.0.

P6c4 has a somehow lowerMn, however, this has previously

not caused significant changes in physical properties for this

type of polyesters [30]. Azobenzene units can appear in two

isomeric forms: trans and cis. Before any treatment, the azo

moieties are in the trans form that is the most stable one.

However, the azo moieties show a tendency to form

refringence measurements.

F.J. Rodrıguez et al. / Polymer 45 (2004) 6003–60126006

different types of aggregates and the photoinduced effects

depend on the aggregation state. To check for the presence

of these aggregates in our films we have compared the

optical absorption spectrum of the different P6x4 polymer

measured at RT in a chloroform solution, with the spectra of

the polymer films measured at TZTiC10 8C and at RT

(after quenching the films from the isotropic phase to RT).

The results are given in Fig. 3. It can be seen that

independently of the end substitution, all the spectra

measured in solution show a main absorption band at

about 360 nm and a shoulder at about 450 nm that are

associated with the p–p* and n–p* transitions of the

azobenzene moieties, respectively. The spectra of the films

measured at TZTiC10 8C are similar (although slightly

broader) to those in solution but the ones at RT show new

bands that are due to the formation of different types of

azobenzene aggregates [16,26]. Besides, the strong decrease

Fig. 3. Optical absorption spectra of the different polyester measured:.. at RT in

from the isotropic phase to RT.

in the optical absorption when measured at RT can be

related with a tendency of the azobenzene units to be

oriented out of the plane of the film. This type of behaviour

has been previously reported by several authors [27,33,34].

In conclusion, the RT spectra of the different polymer films

show remarkable differences, indicating that the formation

of aggregates and their optical absorptions are strongly

influenced by the end substitution.

In a first step we have studied the time evolution of the

birefringence (Dn) induced with 350 nm linearly polarized

light in the polymer films quenched from the isotropic

phase. The evolution is similar in all the polymers. As an

example we give in Fig. 4 the evolution for P6c4. It can be

seen that birefringence increases with irradiation time,

reaches a maximum and then goes back to zero. The initial

growth is associated with the photoinduced trans–cis–trans

isomerization. Under irradiation with UV light in the

CHCl3 solution;.. at TZTiC10 8C in films; — at RT in films quenched

Fig. 4. Evolution of birefringence with time in a P6c4 film under irradiation

at RT with 350 nm linearly polarized light.

F.J. Rodrıguez et al. / Polymer 45 (2004) 6003–6012 6007

absorption band of the trans isomer this can be transformed

to the cis form, which can go back to the trans state either by

thermal or by photoinduced processes. If the UV light is

linearly polarized the photoexcitation of trans moieties is

proportional to cos2 q, q being the angle between the

molecular axis and the light polarization direction. Thus, the

isomerization rate to the cis state is bigger for the trans

moieties parallel to the light polarization and decreases

when q increases. Through this photoselection (as well as

some rotational diffusion) process a preferential orientation

of the trans moieties in the plane perpendicular to the

polarization direction of the exciting light is induced. Since

the trans moieties have a high anisotropy (while that of the

cis moieties is much smaller) the trans preferential

orientation gives place to the measured birefringence.

As the irradiation goes on, the concentration of trans

isomers decreases. This can induce a decrease in birefrin-

gence since Dn is mainly associated with the transmoieties.

On the other hand, while the rod like shape of the trans

moieties favours the formation of the liquid crystalline

phase, the bent shape of the cis ones tends to destroy that

phase and thus the Ti of the LC polymer decreases [23]. If Tigoes below the irradiation temperature, the polymer

converts to the isotropic phase. These two effects can

account for the decrease in birefringence for long irradiation

time. However, since the conversion from trans to cis is not

complete, some residual anisotropy should remain in the

films, unless that the polymer becomes isotropic. Thus, we

conclude that UV irradiation induces a transition from the

liquid crystalline phase to the isotropic one. This transition

is also accompanied by a decrease in the light scattered by

the film. Similar results have been previously reported

[23–35].

The optical absorption spectrum of the films has also

been measured after irradiation with the unpolarized 350 nm

light. The results are given in Fig. 5. A strong decrease is

observed in the bands associated with the trans moieties

while new bands (mainly two bands at about 310 and

450 nm) appear, that are due to the cis isomer. The spectrum

has been measured after several time intervals in dark at RT.

From these measurements it has been found that the lifetime

of the cis isomers is of several hours in all our polymers.

After 1 day, the samples show a recovery of the trans

absorption but the 360 nm band is narrower than in the ‘as

quenched’ films (see Fig. 3). This indicates that irradiation

with 350 nm light produces a breakdown of the azo

aggregates. After some days at RT aggregation can be

clearly observed in some of the films.

The time evolution of birefringence under irradiation

with linearly polarized 488 nm light has also been measured

in films quenched from the isotropic phase, before and after

irradiation with 350 nm light. The results are shown in Fig.

6. It can be seen that the qualitative evolution of Dn with theirradiation time in the P6c4, P6e4 and P6k4 is similar with

and without pre-irradiation with 350 nm light. In all the

cases birefringence grows up and reaches a saturation value.

When the 488 nm light is switched off, a slight decrease in

Dn (not shown in the figure) is observed, and then Dnremains stable if the films are kept in dark. However, the

saturation values achieved after 350 nm light pre-irradiation

are much bigger than those obtained without that pre-

irradiation. The increase is of an order of magnitude in some

of the films.

In P6f4 and P6j4 samples even the qualitative evolution

of Dn under 488 nm light irradiation is different before and

after pre-irradiation with 350 nm light. Without pre-

irradiation the time evolution of Dn in these two polymers

is similar to that observed in the P6c4, P6e4 and P6k4 films.

However, Dn evolution under 488 nm irradiation in pre-

irradiated P6j4 samples shows a faster initial increase, goes

through a maximum an then slowly decreases when

irradiation time goes on. Besides, the Dn values achieved

after pre-irradiation are smaller than those obtained without

pre-irradiation, in contrast with the results in P6c4, P6e4 and

P6k4 samples. This behaviour could be associated with the

low value of Tg for P6j4 and the mutual influence of photo-

orientation and self-organisation processes [25–27,33,34].

In P6f4 a strong qualitative change is observed in the time

evolution of Dn under 488 nm light irradiation after pre-

irradiation with 350 nm light. In this case, the Dn vs. time

evolution is much slower in pre-irradiated films, but reaches

a saturation value bigger than the one achieved without pre-

irradiation.

Besides, another difference in the Dn vs. time evolution

under 488 nm light irradiation, before and after pre-

irradiation with 350 nm light, has been observed in all the

films. Without pre-irradiation, the growth of photoinduced

Dn begins as soon as the 488 nm light is switched on.

However, a delay of a few seconds (different delays for

different polymers) between the switching on of the 488 nm

light irradiation and the increase in Dn has been observed inpre-irradiated films.

The induction of birefringence with linearly polarized

488 nm light in samples quenched from the isotropic phase

has been extensively reported [5,16]. It is due to

photoselection and rotational diffusion of azo moieties

Fig. 5. Optical absorption spectra of the films taken immediately after long time irradiation at RT with unpolarized 350 nm light and after some periods in dark

at RT. The spectra before irradiation are also shown.

F.J. Rodrıguez et al. / Polymer 45 (2004) 6003–60126008

through the angle dependent trans–cis–trans isomerization

processes. As in the case of irradiation with 350 nm light,

trans moieties excited with 488 nm light can experience an

isomerization to the cis state, which can be followed by a

molecular reorientation before going back to the trans state.

Since the excitation probability decreases when the angle

between the trans molecular axis and the polarization

direction of the exciting light increases, a preferential

orientation of the trans moieties in a plane perpendicular to

the 488 nm light polarization is achieved. This has been

checked in our films by dichroism measurements in the

360 nm band due to the trans moieties.

The preferential orientation of the trans moieties gives

place to the observed birefringence. However, in contrast

with the behaviour obtained with polarized 350 nm light, Dndoes not go back to zero when the irradiation with polarized

488 nm light goes on. This can be associated with the lower

concentration of cis moieties induced with 488 nm light, as

compared with that induced with 350 nm light (as checked

by optical absorption measurements in the 450 nm region).

Because of this, the decrease in Ti under blue light

irradiation is not enough to reach RT and thus, no transition

to the isotropic state has been induced with 488 nm light in

our experimental conditions.

Concerning the changes induced by pre-irradiation with

350 nm light on the evolution of Dn vs. 488 nm light

irradiation time, we can say the following. After UV pre-

irradiation the films have a high concentration of cis isomers

and likely are in an isotropic state (Ti!RT). Since both

trans and cis isomers absorb in the 488 nm region,

irradiation with light of this wavelength produces both

trans–cis and cis–trans photoinduced isomerization.

Through these processes the concentration of cis isomers

is reduced in such a way that, after a few seconds, the Ti of

the irradiated polymers becomes higher than RT. This could

explain the delay of the increase in Dn growth with respect

Fig. 6. Birefringence of the different films as a function of time under RT irradiation with linearly polarized 488 nm light. // after quenching from the

isotropic phase to RT; — after pre-irradiation with unpolarized 350 nm light.

F.J. Rodrıguez et al. / Polymer 45 (2004) 6003–6012 6009

to the switching on of the 488 nm light irradiation. Then,

photo-orientation of trans moieties begins with a process

similar to that in the films that have not been pre-irradiated.

However, since the initial cis concentration is higher in the

pre-irradiated samples Ti is lower. Besides, the azo

aggregates that are present before UV irradiation are broken

after irradiation. These two facts, the decrease in Ti and the

breaking of aggregates, can produce and increase in the

mobility of the transmoieties similar to the one achieved by

increasing the temperature of the film. It has been reported

that the evolution of Dn with irradiation time is strongly

dependent on temperature [10,36]. The production effi-

ciency of Dn increases at low temperatures, goes through a

maximum and then decreases again to zero at high

temperatures. Thus, the effect of UV pre-irradiation could

be understood as equivalent to a heating up of the films and

this could account for the results shown in Fig. 6. A detailed

study of the temperature and light power dependence of all

these processes is under way in our laboratory.

Birefringence induced with linearly polarized 633 nm

light has also been measured before and after pre-irradiation

with UV light. It is known that red light can induce a

preferential orientation of the trans moieties that can either

be parallel or perpendicular to the polarization of the red

light, depending on the polymer and the irradiation

conditions. In our case, no birefringence has been induced

with 633 nm light in films quenched from the isotropic state.

After pre-irradiation with UV light some birefringence has

been observed. The results are given in Fig. 7. It can be seen

that in all the films, a photoinduced birefringence begins to

grow after a delay time. The growth of Dn continues, in

some of the films, for several hours, and then reaches a

saturation value. When the 633 nm light is switched off Dnremains stable if the samples are kept in dark. The saturation

Fig. 7. Birefringence of the different films as a function of time under RT irradiation with linearly polarized 633 nm light. The films were pre-irradiated with

unpolarized 350 nm light.

F.J. Rodrıguez et al. / Polymer 45 (2004) 6003–60126010

values of Dn are, in all the cases, smaller than those obtained

with 488 nm light in pre-irradiated films.

To check for the kind of molecular orientation that has

been achieved under red light irradiation, dichroism

measurements have been performed. The sign of the

photoinduced dichroism is the same for all the polymers.

As an example we give in Fig. 8 the results corresponding to

P6c4 and P6j4, which show the highest dichroism values. It

can be seen that, after 633 nm light irradiation, the

dichroism that appears in the 360 nm band shows that

the trans moieties are preferentially oriented parallel to the

polarization of the exciting light. As we said above this type

of orientation has been observed in several polymers after

pre-irradiation [18,20,21]. It has been associated with a

photoinduced angle-selective cis–trans isomerization and

thus, an initial concentration of cis moieties is needed.

Because of this, the orientation parallel to the polarization of

the red light, has only been observed in pre-irradiated films.

It has been recently reported that, in some polymers, when

the 633 nm light irradiation goes on for long times the

orientation slowly changes from parallel to perpendicular

with respect to the red light polarization [18]. This second

step is associated with the same type of processes that give

place to the photoinduced orientation with 488 nm light.

However, although we have kept the red light irradiation for

several days, no change of the molecular orientation has

been observed in our polymers. This can be due to the small

absorption of the trans molecules at 633 nm and to the

aggregation of azobenzene moieties.

Finally we want to comment on the observed delay

between the growth of Dn and the beginning of red light

irradiation. It has been found that the delay decreases if the

Fig. 8. Dichroism induced at RT with 633 nm light in P6c4 and P6j4

polymer films. Films were pre-irradiated at RT with unpolarized 350 nm

light. The polarization of the measuring light is parallel (v) or perpendicular

(h) to that of the exciting 633 nm light.

F.J. Rodrıguez et al. / Polymer 45 (2004) 6003–6012 6011

sample is kept in dark for some time after UV pre-

irradiation. As in the case of 488 nm irradiation, we propose

that after UV irradiation the film is in the isotropic phase,

because of the decrease in Ti induced by the presence of cis

isomers. When red light irradiation begins the cis concen-

tration decreases, due to the cis–trans photoinduced

isomerization. This gives place to an increase in Ti and,

when it becomes higher than RT, the polymer goes back to

the liquid crystalline phase. Then, since the photoinduced

cis–trans isomerization is angle dependent, the preferential

orientation is induced. The decrease in the delay when the

sample is kept in dark before red light irradiation can be

understood as due to a decrease in the initial cis

concentration associated with thermal cis–trans

isomerization.

4. Conclusion

No aggregation of the azobenzenes was found above Ti in

the polyesters studied in this work, but they aggregate in a

few seconds after a fast quenching to RT. Although the

absorption spectra of the different polymers are quite similar

in solution and when the films are measured at T>Tinoticeable differences are observed when the same films are

measured at RT. Molecular interactions among the

chromophores modify their absorption spectra. These

interactions as well as the transition temperatures depend

on the end substitutions. Thus, different polymers show

different behaviour. A photoinduced phase transition to the

isotropic state can be induced in all the polymer films at RT

under irradiation with UV light. This irradiation also breaks

the azo aggregates present in the ‘as quenched’ films. When

cis population decreases while keeping the films in dark at

RT, aggregation appears again at least in some of the films.

The kinetic and the saturation values of photoinduced

birefringence with 488 nm light are very different if the

films are previously irradiated with UV light or fast

quenched from the isotropic state. In both cases the

orientation of the azobenzene is perpendicular to light

polarization.

Irradiation with 633 nm light after previous UV

irradiation orients the azobenzene parallel to light polariz-

ation although no orientation is detected without UV pre-

irradiation.

Acknowledgements

The financial support from the CICYT, Spain, under

project No. MAT2002-04118-02 is gratefully acknowl-

edged. This work has been performed within the COST P8

Action.

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