Comparative Raman spectroscopy studies of photosensitive polymers

~° ~ ~'~ ~

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ELSEVIER

MATERIALS CHEMISTRYAND PHYSICS Materials Chemistry and Physics 55 (1998) 202-208

Comparative Raman spectroscopy studies of photosensitive polymers F. Barbet a, D. Bormann a,,, M. Warenghem a, B. Khelifa a, Y. Kurios b, y. Reznikov b, F. Simoni c Laboratoire de Physicachhnie des lnte~fiwes et Applications, Universitd d'Artois, Facu#~ Jean Perrin, SP 18, rue Jean Souvraz, F-62307 Lens Cedex. France Institute of Physics of the Academy of Sciences of Ukraine, prospect Nauki 46, Kyiv 252022. Ukraine ~"Dipartimento di Scienze dei Materiali e della Terra, Universita di Ancona, via Brecce Bianche. 1-60131 Ancona, Italy

Received 1 August 1997: accepted 16 January 1998

Abstract

We present Raman studies on various substituted polyvinylcinnamates, [-CH,CH(QCCH=CHC6Hs)-],,, which are well "known for undergoing cycloaddition when exposed to UV radiation. Our comparative studies on these unphototransformed polymers show that the substitutions on the aryl cycle play a fundamental role in the vibrational dynamics of these molecules. We have been able to identify the characteristic modes of each type of fluoro-substitution (mona, para and ortho), and as a result, it is possible to determine whether the two isomers and two rotating conformers coexist in the samples. In order to proceed to further investigations on the phototransformation process, we have determined how the C=C and C=O double-bond stretching modes behave versus the type of substitution. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Polymers; Isomers and conformers; Raman spectroscopy

2 3

1. Introduction

The first experiments on phototransformation in the soIid state were performed at the end of the last century [ 1 ], but this chemico-physical science topic came back to interest in the 1960s, as great technical improvements were realized to probe the deep-seated structure of matter. Since that period, both theoretical and experimental work has been done in order to understand the various mechanisms that occur in the phototransformation process [ 1-3 ]. Photosensitive polymers are of great interest, namely, in the processing of microelectronic devices as resins for photolithography [ 4 - 6 ] or in nonlinear optics as photorefractive materials [7 ]. In another field of applied physics, some authors have recently used the dichroic properties of these polymers to realize tunable oftentants for liquid crystals [ 8 - 1 0 ] . Poly (vinylcinnamates), the rough formula of which is [-CH,_CH(O2CCH=CHC6Hs)-]~, are composed of a cinnamoyl side group attached to a vinyl chain (Fig. 1). Several experiments have been performed over the last 25 years on the polyvinylcinnamate and cinnamoyl groups, like fluorescence, UV and infrared spectroscopy, N M R and dielectric studies [1-3,1 1-15]. However, * Corresponding author. TeL: + 33-321-79-17-24; Fax: + 33-321-79-1717; E-marl: [email protected]

~n O~S~ (a) O

1~4 (b) 6 5

Fig. 1. Nomenclature t~r our various samples: (a) poly(vinylcinnamate); (b) aryl substitutions, t-4 substituted (para), 1-2 or 1-6 substituted tortho). 1-3 or 1-5 substituted (meta). no systematic study by means of Raman spectroscopy has been reported on these polymers. In this paper, we focus our attention on the cinnamoyl side chains of the polymers.

2. E x p e r i m e n t a l The measurements have been performed on ttu'ee photopolymers: poly (vinylcinnamate), poly (vinyl-para-fluorocinnamate) and a 'mixture' of two polymers composed mainly of poly(vinylcinnamate) and a little poly(vinylortho-fluorocinnamate) (Fig. 1 (a) and ( b ) ) . For the sake of simplicity we will use the following acronyms: PVCN, PVCN-F and M-PVCN (which stands for 'mixed' PVCN).

0254-0584/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PH S0254-0584 (98) 00037-6

203

F. Bat'bet et aL /Materials Chemist~, and Physics 55 (1998) 202-208

PVCN is a commercial polymer ( S i g m a - A l d r i c h - H u k a ) , in contrast to the PVCN-F and the M-PVCN which were synthesized at the Institute of Bioorganic Chemistry and Petrochemistry of the Academy of Sciences of Ukraine. Each polymer was studied without any treatment after synthesis, at room temperature. Our investigations have been performed using a DILOR XY 800 Raman spectrophotometer in a microprobe configuration. An argon laser pump beam ( 1 mW) has been focused onto the sample through a microscope. The power density is a few m W / c m 2 onto a 1 txm2 area of the sample. The working wavelengths were carefully chosen as 514.5 and 488 nm, in order to avoid the phototransformation of the polymers that occurs at wavelengths < 3 3 0 nm [11,15]. In our experiments, we explored the spectral range between 200 and 3500 c m - ~ with a spectral resolution of 2 c m - 1

3. Results and discussion The spectrum of each polymer in the total spectral range 200-3500 c m - ~ is shown the Fig. 2. We first notice a common feature of the three spectra: the PVCN, PVCN-F and M-PVCN spectra are characterized by the existence of photoluminescence. We can suggest two main reasons to explain this effect. First, we can expect the presence of synthesis residues in our samples. Secondly, chromophore groups like carbon-carbon double bonds ( C = C ) , carbon-oxygen double L

i

I

bonds ( C = O ) or aryl cycles are known to be able to produce such photoluminescence [ 16]. The chemical groups contained in the cinnamoyl sidechain have vibration modes which are characteristic and well defined in Raman spectroscopy. The first is the carbonyl group ( C = O double bond), which has stretching modes close to 1700 c m - ~. The second group is the aryI cycle, the vibration frequencies of which are sensitive to the position and chemical nature of the substituents [ 17,18 ]. This allows our samples to be distinguished from each other without any ambiguity. The third and most interesting group is the C = C double bond which links the ester group and the aryl cycle. Indeed the C = C double bond plays the prominent role in the photocycloaddition process [ 1-3,12-15]. The carbon-carbon double bonds present Raman transitions close to 1630 c m - ~ as they are adjacent to an ester group [ 17,18 ]. Finally, these considerations lead us to focus our attention onto two spectral domains: the first one below 1250 cm -~, where the aryl cycle produces many characteristic modes, and the second one around 1650 c m - ~. In the following, we present in detail these spectral domains, and we assign the characteristic peaks for each sample. The spectra in the range below 1250 c m - ~ are shown in Fig. 3. In the three left-most columns of Table 1 are listed a few aryI-cycle vibration frequencies extracted from our spectra, restricted only to the characteristic modes of our samples. I

I

I

I

?.

I

-'. ..

~,. :,.

I

{

A

b)

i,'. °.

"

'1

.,1 v

:

_=

.=

!i .3

•

[

.' -.. !,,,,

..... .~.L~ ...... ::-

i1! I

1000 500

1000

1500

2000

2500

3000

3500

:, T

P

1050

i

I

ll00

r

I

f

1150

I

1200

1250

Raman shift (cm "l)

Fig. 2. Raman spectra of PVCN (a), M-PVCN (b) and PVCN-F (c), The

gig. 3. Raman spectra of PVCN (a), M-PVCN (b) and PVCN-F (c) in the aryl stretching and C-O single-bond stretching range. The arrows are markers for weak modes as explained in the text. The excitation wavelength is

excitation wavelength is 514.5 rim.

514.5 nm.

Raman shift (era "I)

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F. Barber et al. / Materials Chemistry and Physics 55 (I998) 202-208

Table 1 Characteristic aryl vibration modes of our various PVCNs. The **-labelled values correspond to the FT-IR data; the values marked * are the calculated frequencies; every piece of data has an uncertainty of + 1 cm- ~, unless other,vise specified. X denotes an unobser~,edmode that was predicted and - a nonactive mode for the considered substitution Measured frequencies (cm -~ ) PVCN

PVCN-F

406 621

-

557 ~* 714 769

_ -

Nature of the vibrations M-PVCN

403

403 62l

639 _ 7t4 768**

-

X

833

1003 -

997 1016

1031 1075"* -

1184 -

-

1579±2 1601 -

1030 1077"* -

1164 1421

1451 1498

116l 1167 1185

1515

ring semi-circie stretch mixed with in-plane C-H bend and in-plane C-H bend

-

1602 1604

'ring breathing'

-

1149

1161

I003

out-of-plane ring bend by quadrant in-plane ring bend by quadrant out-of-plane ring bend by sextant 5 (mono)- or 4 (ortho)-adjacent C-H wag (754+9*) 4 ( ortho)-adjacent C-H wag (939 + 5* ) 2 (para)-adjacent C-H wag (842*) 3 ( meta)-adjacent C-H wag (783 +_6* ) 1 (meta) lone C-H wag (868+8*)

t452 I498

ring semicircle stretch strongly mixed with in-plane C-H bend

I578 i601 -

ring quadrant stretch mixed with in-plane C-H bend

-

The assignations of these lines are given in the right-most c o l u m n on the following basis, In the considered range, the aryl cycles present vibration frequencies which are sensitive to the position and chemical nature of the substituents. The literature reports 30 vibrations modes for a monosubstitued aromatic ring; however, most of them are quite weak or absent in R a m a n spectroscopy [17,18]. A m o n g the important modes, the in-plane C - H w a g g i n g modes are affected by electronic withdrawing or donating effects caused by the substituent [ 18], which explains the shifts observed for these lines between our different polymers (Table 1). In the case of m o n o s u b s d t u e d aryls, this electronic influence mainly affects the C - H w a g g i n g modes with frequencies close to 900 c m - ~ , whereas the modes with frequencies near 750 c m - ~ are more sensitive to mechanical influence. One can estimate the mode frequency for each substitution: the obtained values are listed in the fourth c o l u m n of Table 1 ( m a r k e d with an asterisk), where we indicate the standard deviation expected for each mode [ 17,18]. To do this, we have used the following procedure. Since the disubstitued aryl n-adjacent C - H wagging mode frequency can be considered as the c o m b i n a t i o n between the 5-adj acent C - H wagging mode of two monosubstitued aryls, the resulting frequency u of the n - C - H disubstitued aryl wagging mode can be calcu-

lated to the first order of approximation by the following linear relation, established by Colthup [ 19] : v--oe(ul + u2)+/3 (in c m - 1)

( 1)

where v~ and z,2 stand respectively for the frequency of the 5-adjacent C - H wagging mode of each m o n o s u b s t i t u e d aryl. The values for both the parameters ( oe,/3), which are different for each type of substitution, are tabulated in Table 2 [ 19]. W e used for u~ and u2 the reported vibration frequencies [1%18]: v I ( - C H = C H 2 ) = 9 0 9 cm - I and ~ 2 ( - F ) = 8 9 6 cm-L, except for the case n = 2 (aryl p a r a - s u b s t i t u t i o n ) where it is necessary to consider the 5-adjacent C - H wagging mode with a frequency near 750 c m - 1 because this mode correlates better with the electronic effect [ t7,18]. Thus for Table 2 Values for the pair of parameters ( a./3) for various substituted aryl cycles from Ref. [19] Substitution

Values of ( o< 13) in cm-

Mono/orrho near 770 cm-~ Ortho near 940 cm- ~ Para Meta t lone C-H wagging Meta 3 adjacent C-H wagging

(0.5, --149) (0.5, + 36) ( 0.6, - 75 ) (0.9, -757) (0.5, --120)

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F. Barbet et al. / Materials Chemist O, and Physics 55 (J998) 2 0 2 - 2 0 8

the case n = 2 (para-substituted aryls), we performed the calculations with the frequencies v ~ ( - C H = C H a ) = 7 7 5 cm -~ and z ~ ( - F ) = 7 5 3 cm - t . We assumed that a C = C double bond is a satisfactory approximation for the whole side chain, with respect to this vibration mode. Our calculations are found to be in good agreement with our measurements. Indeed, we observed only Raman lines corresponding to calculated adjacent C - H wagging-mode frequencies of mono-substituted aryl for PVCN, and para-substituted aryl for PVCN-F. The concentration of para-substituted aryI in M-PVCN is too low and prevents its spectrum from exhibiting the characteristic line of the 4-adjacent C - H wagging mode calculated at 939 +_5 cm - ~. Moreover in the case of a mono- or meta-substituted aryl cycle, the two modes near 990 and 1010 c m - ~, which belong to the same symmetry species, are supposed to merge together and to give rise to two other modes, one at 1000-+ i0 c m and in the spectral range 1060-1260 cm-~ for the second [20]. This mixing does not occur in the case of ortho- or para-substituted aryl cycles. Actually the PVCN and MPVCN (which is composed mainly ofmono-substituted aryl) spectra exhibit a line at 1003 _+3 c m - 1 as expected, in contrast to the PVCN-F spectrum (which is composed ofparasubstituted aryl), which shows two modes, one at 997_+ 1 cm-~ and the second at 1016+ 1 cm -1 (Table 1 and Fig. 3). In the spectral range 1150-1205 c m - i there are several well-resolved peaks (Fig. 3). Except for the line at 1204 + 1 cm - 1, which can be attributed to the C - O stretching mode in esters [21], these modes are assigned to ring semicircle stretching modes mixed with in-plane C - H bending modes and @ C - H bending modes (Table 1), where ~ is the aryl cycle. In the spectral range 1205-1250 cm -~, our spectra of MPVCN and PVCN-F show two lines, at 1233_+ 1 and at 1242+ 1 cm-~ for the M-PVCN sample, andat 1233 _+ 1 and 1239-+ 1 cm -x for the PVCN-F one. These modes appear more weakly on the M-PVCN spectrum than on the PVCNF one. Disregarding the fact that the literature reports the presence of only one mode at 1230 c m - 1, assigned to a ~-F stretching mode [ 17,20,22-24], the existence of a second line can be due to the composition of the samples: M-PVCN is a mixture with a few ortho-substituted aryl cycles, whereas PVCN-F is a pure material. Actually, we cannot yet explain the presence of this additional mode with certainty. These two lines are naturally absent from the spectrum of PVCN. We have complemented the Raman analysis with some experiments in transmission mode on a FT-IR Brucker IFS55 spectrophotometer. The polymer powders have been mixed with dried KBr at a ratio of 1% by weight, then pressed under 10 tons in order to obtain thin pellets. In the IR studies, we obtained a spectral resolution of 4 c m - ~. The IR spectra are presented in Fig. 4, and give further information in the spectral range 400-1000 cm-~, where the Raman lines are very weak or absent (data marked ** in Table 1 ). The substituent influence on the aryI-cycle vibration modes also appears in the spectral range 1590-1610 cm-~. In this

spectral range four modes are found and their frequencies are independent of the chemical nature of the substituents for a given position of the substitution. Three out of these four modes are very sensitive to the substitution position, each type of substitution leading to a characteristic frequency shift [20,22-24]. It is therefore possible by spotting these lines to determine the position of the substitution on the aryl cycle. Indeed, the para-substituted aryls exhibit their spectral lines at 1421_+1, 1515+_1, 1602_+1 and 1604+1 cm -1, which cannot be confused with those of mono- or ortho-substituted aryl which stand at 1452 -+ 1, 1498 -+ i, 1578 -+ 2 and 160I + 1 c m - 1. As shown in Table 1, two frequencies correspond to semicircle stretching modes of the aryl cycle, and the two others correspond to ring quadrant stretching modes; both pairs of modes are variously mixed with C - H bending modes [18]. Thus, our studies led us to determine the most characteristic modes of the aryI cycle in the cinnamoyl side-chain of our polymers with respect to the type of substitution. After having shown the interest of the aryl-cycle vibrationmode study, we now focus on the C = C and the C = O doublebond vibrations. A comparison of the spectra of our three polymers in the spectral range close to 1600 cm-~ is shown in Fig. 5. We notice first in the range 1620-1640 c m - ~ a strong band for our three polymers, which is well known to be assigned to the stretching mode of the C = C double bond [ 17,18]. The different trials we have performed on this band indicate I

i (a)

(b) l .=. [,.

I 500

,

I 1000

,

I 1500

,

I 2000

Frequency (cm "I) Fig. 4. IR transmittance spectra of PVCN (a), M-PVCN (b) and PVCN-F

(c) in the range 400-2200 cm- ~.

K Barber et al./ Materials Chemist9' and Physics 55 (1998) 202-208

206

clearly that to be fitted it takes at least two Lorentzian curves in the case of PVCN and M-PVCN, and only one in the case of PVCN-F. Moreover, in the case of the PVCN and MPVCN spectra, the previous band has a pronounced shoulder, contrary to the PVCN-F spectrum which exhibits two tight lines at 1648 _+5 and 1684 + i cm - 1. Considering these three spectra, we are led to the conjecture that this spectral range contains at least three components, the frequencies of which are listed in Table 3. This hypothesis is consistent with the existence of both different trans- and cis-isomers that each molecule can have, and also with the existence of two rotating conformers S-trans and S-cis for each isomer [1,15], as shown in Fig. 6. Thus we would expect to observe four C = C stretching modes in our spectra; only three of them can be definitively evidenced. Different reasons can explain this: one possibility is that our experimental conditions were inadequate for observing the fourth mode; another is that the type of molecule associated with the fourth mode is present in too small an amount in our samples and is screened by a stronger line of another mode. So we cannot state definitely that this mode is absent in our spectra. We expect a shift of these modes depending on the substituent position. Our observations are in good agreement with this prediction: the C = C double-bond vibration is indeed responsible in our spectra respectively for three lines at frequencies of 1622 + 1, 1634 _+ 1 and 1640 +_2 cm - ~for PVCN I

I

I

I

a)

.=

I

I

/ ~ :i\

b)

A

,'~

._v"

e)

I

I-.

!

k,..._j"xN..__j\

,

I

___,'j L___Y' :

,

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I

1450

,

I

1500

,

l

1550

,

I

1600

1650

,

I

1700

,

I

,

1750

Raman shift (era "l) Fig. 5. Raman spectra of PVCN (a), M-PVCN (b) and PVCN-F (c) in the C = O and C = C double-bond stretching range, The arrows are markers for weak modes as explained in the text. The excitation wavelength is 514.5 rim.

Table 3 Measured values tbr the C = C and C = O double-bond stretching modes Involved

Measured frequencies ( c m - ~)

bond

PVCN

PVCN-F

M-PVCN

C=C

i622+ 1 I634+ ! 1640 +_2

t63t+ t

i621 +_ 1 I633+1 i641 + 1

1648 5:3 1684 +- 1 1702+- 1

C=O 1707 _+3 1714_+2

I699+_ I 1709 +_ 1

1753+_ I I763 + 1

(mono-substituted aryl cycle) and M-PVCN (containing a little ortho-fluorocinnamate), and for three lines at 1631 + 1, 1648+_5 and 1684_+ I cm -~ for PVCN-F (constituted by para-fluorocinnamate), as shown in Table 3. Moreover, we observe a shift of the lines corresponding to the C = C stretching mode between PVCN and PVCN-F. This effect is predicted to be the consequence of the modification of the C = C bond strength under the additional conjugation caused by the fluor in the case of PVCN-F. This shift is expected to be of a magnitude of more or less 60 c m - ~ [ 17], which is fully in agreement with the maximum 62 cm - ~shift measured in our spectra when considering the lowest frequency of PVCN and the highest one of PVCN-F. Regarding the C = O double bond, we expect a line close to 1710 cm -* observed in experiments performed on poly(vinylacetate) [25] which differs only from our polymers by the absence of the aryl cycle. The spectrum of PVCN shown in Fig. 5 presents a peak with an asymmetric shape that is best fitted using two lines at the frequencies t707 +_ and 1714 + 2 cm - 1, although the PVCN-F spectrum exhibits two well-separated lines at frequencies of 1702+_1 and 1753+_1 cm -~. Finally, as M-PVCN is a mixture of poly(vinylcinnamate) with poly(vinyl-ortho-flurocinnamate), its spectrum is the result of the superposition of their own spectra: we have been abte to fit this spectrum with at least three lines at 1699+1, 1709+_1 and 1763+_1 cm - ~ (Table 3). According to the C = O double bond, in first approximation we can compare the cinnamoyl group of PVCN to an o~unsaturated ketone. Thus the spectral separation of 7 c m - 1 between the two lines corresponding to the C = O bond in PVCN is compatible with that of 10 cm-~ announced for unsaturated ketones [26-29]. This undoubling of the C = O stretching mode is due to the presence of the two conformers that each molecule can have. There is no report in the literature of any enlargement of the splitting of the C = O stretching mode due to an isomeric effect, whereas the importance of the mesomeric effect is shown in esters [20,30]. Since we remember the large electronegativity of fluor, we can invoke this effect to explain the increasing spectral deviationbetween

207

F. Barber et al. / Materials Chemistr)" and Physics 55 (1998) 202-208

Cis P V C N

Trans PVCN

o

\ //

S- Trans

//

/5

o

o

O

\

O S-CIs

Fig. 6. Schematicrepresentationsof the different isomers and their related conformersof poly(vinyicinnamate)[ 1,i5].

the two components of the C = O vibration between PVCN and PVCN-F.

4. C o n c l u s i o n s Our investigations tackle a little-exploited field in the case of poly(vinylcinnamate): vibrational studies by Raman spectroscopy. Through the spectral analysis we have performed, we have been able to determine the behaviour of some awlcycle modes and of the C = O and C = C double-bond stretching modes for poly(vinylcinnamate) under the influence of various aryl-fluoro substitutions. We have shown without any ambiguity the existence of various isomers and rotatomers in our samples. This preliminary work will be of great interest in further Raman spectroscopy studies about the phototransformation processes of the poly(vinylcinnamate) family.

Acknowledgements

We would like to thank the 'R6gion Nord - Pas-de-Calais' and the 'Conseil G6n6ral du D6partement du Pas-de-Calais' for financial support, contract number 96 26 01 58.

References

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F. Barber et a l . / Materials Chemisz~3, and Physics 55 (t998) 202-208

[ 17] D. Lin-Vien, N.B. Colthup, W.G. Fateley, J.G. Graselli, in: Handbook or Infrared and Raman Characteristic Frequencies of Organic Molecules, Academic Press, Boston, t99l. [ 18] N.B. Colthup, L.H. Daly, S.E. Wiberley, in: Introduction to Infrared and Raman Spectroscopy, 3rd edn., Academic Press, Boston, 1990. [ t9] N.B. Colthup, Appl. Spectrosc. 30 (1976) 589. [201 F.R. Dollish, W.G. Fateley, F.F. Bentley, in: Characteristic Raman Frequencies of Organic Compounds, John Wiley, New York, 1974. [21] R.G. Fowler, R.M. Smith, J. Opt. Soc. Am. 43 (1953) 1054. [22] J.R. Scherer. in: Planar Vibrations of Chorinated Benzenes, Dow Chemical Company, Midland, MI, 1963. [23] J.R. Scherer, Spectrochim. Acta 21 (1965) 321.

[24] K.W.F. Kohlraush, in: Ramanspectren, Akad, Vedagsges, Leipzig, 1943. [25] F. Barbet, D. Bormann, B. Khelifa, unpublished. [26] A.J. Bowles, W.O. Georges, W.F. Maddams, J. Chem. Soc. B (1969) 810. [27] F.H. Cottee, B.P. Straughan, C.J, Timmons, W.F. Forbes, R. ShiltofiT, J. Chem. Soc. B (1967) 1I46. [28] M. Mecke, K. Noack, Spectrochim. Acta 12 (1958) 391. [29] R. Barlet, M. Montague, P. Arnaud, Spectrochim. Acta 25 (19695 1081. [30] LJ. Bellamy, Spectrochim. Acta 13 { 1958) 60.