An IR study of poly-1,4-phenylenevinylene (PPV), the 2,5-dimethoxy derivative [(MeO)2-PPV], and their corresponding xanthate precursor polymers and monomers

Spectrochimica Acta Part A 79 (2011) 118–126

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An IR study of poly-1,4-phenylenevinylene (PPV), the 2,5-dimethoxy derivative [(MeO)2 -PPV], and their corresponding xanthate precursor polymers and monomers D. Michael Byler 1 , Yogesh Patel 2 , Georgia A. Arbuckle-Keil ∗ Department of Chemistry, Rutgers, The State University of New Jersey, Camden, NJ 08102, United States

a r t i c l e

i n f o

Article history: Received 11 October 2010 Received in revised form 3 February 2011 Accepted 10 February 2011 Keywords: Infrared spectroscopy Vibrational spectroscopy Conjugated polymers Poly-p-phenylenevinylene (PPV) Xanthate groups

a b s t r a c t A detailed comparison of the infrared (IR) spectra of poly-1,4-phenylenevinylene (PPV), its xanthate precursor polymer, and its bis-xanthate precursor monomer along with the corresponding 2,5-dimethoxy derivatives has provided a clearer basis for characterizing these species with regard to both structure and purity. All the xanthate precursor monomers and polymers exhibit characteristic intense absorptions typical of the xanthate group near 1220, 1110, and 1050 cm−1 . Upon complete conversion of the precursor polymer to the vinylene linked final product, the intense IR peaks of the xanthate group have disappeared and new bands resulting from the vinylene linkages are found. The latter include a moderately strong band near 965 cm−1 due to the out-of-plane –CH CH– deformation of the trans-vinylene conjugated with and linking the phenyl rings into an optoelectronic polymer. Unfortunately, the corresponding C–H stretching vibration of this same group of atoms expected to appear near 3020 cm−1 falls in the same region of the spectrum as the aromatic C–H stretches of the phenyl rings. Similarly, for the 2,5-dimethoxy polymer derivative, [(MeO)2 -PPV], the C–H stretching vibration near 3055 cm−1 contains contributions from both aromatic and vinylene C–H. Density functional theory (DFT) calculations on the monomers were instrumental in assigning the infrared spectra of these materials. This study provides a systemic means for verifying that the precursor monomer has been polymerized into the precursor polymer and that thermal conversion to the conjugated polymer is complete. © 2011 Elsevier B.V. All rights reserved.

1. Introduction 1,4-Phenylenevinylene (PPV) polymers, also named poly-paraphenylenevinylenes, have been the object of numerous studies because of potentially valuable electrically conducting and related optical properties resulting from their conjugated structures [1,2]. In contrast to most inorganic semiconductors, these polymers also have many of the desirable mechanical properties and processing advantages common to ordinary organic polymers. Over the years, investigators have prepared PPV via different synthetic routes starting from a number of different precursors [3,4]. Among the most commonly used are the sulfonium precursor route (SPR) [3,5], the chlorine precursor route (CPR) [6], the xanthate precursor route (XPR) [4,7] and the sulfinyl precursor route [8]. Other methods involve the use of methoxy or acetate leaving groups [9]. For the

∗ Corresponding author. Tel.: +1 856 225 6142; fax: +1 856 225 6506. E-mail addresses: [email protected] (D. Michael Byler), [email protected], [email protected] (G.A. Arbuckle-Keil). 1 Present address: Department of Chemistry, Community College of Philadelphia, Philadelphia, PA 19130, United States. 2 Present address: Cleveland, OH, United States. 1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2011.02.021

preparation of PPV, the xanthate route reportedly has the advantage of producing this polymer in a more amorphous form, with increased photoluminescence efficiency [7]. Compared with PPV itself, the dimethoxy polymer offers the added benefit of having a lower band gap and thus potentially higher conductivity [9]. The lower band gap results from the ␲-electron donating character of the methoxy oxygens [10]. Poly(2,5-dimethoxy-p-phenylene vinylene), referred to herein as (MeO)2 -PPV, has also been prepared via the sulfonium precursor route but care must be taken because this precursor is unstable in aqueous solution [11]. While several workers have reported the principal infrared (IR) bands for PPV [3,12–19], only one has proposed a detailed assignment of its characteristic absorptions throughout the midinfrared region (4000–400 cm−1 ) [3]. By contrast, no such in-depth analysis has been attempted for its dimethoxy analog [12,20,21]. Nor has anyone presented a comprehensive comparison of the spectra of these intriguing conjugated polymers with the corresponding xanthate precursor monomers and polymers from which they are synthesized, beyond giving a brief listing of selected xanthate precursor polymer IR band frequencies [8]. Indeed, this is the first report in the literature of the IR spectra of the bis-xanthate monomers. In the present study, we offer a thorough examina-

D. Michael Byler et al. / Spectrochimica Acta Part A 79 (2011) 118–126

119

Fig. 1. General synthesis scheme for poly-(p-phenylenevinylene) and the 2,5-dimethoxy derivative from the xanthate precursor monomer and polymer, where Y = H for PPV and Y = MeO for the dimethoxy derivative, (MeO)2 -PPV.

tion and comparison of the IR spectra of PPV and its 2,5-dimethoxy derivative [(MeO)2 -PPV or DM-PPV] prepared using the xanthate precursor route [12,19,21,22]. These results should prove invaluable to future researchers as they use IR to verify the purity of the products they prepare for investigation. 2. Experimental 2.1. Materials Potassium O-ethylxanthate, 1,4-bis-(chloromethyl)benzene, methanol, anhydrous tetrahydrofuran (THF), and potassium tbutoxide were purchased from Aldrich Chemical Co., Milwaukee, WI, and were used without further purification. 2,5-Dimethoxy1,4-bis-(chloromethyl)-benzene was kindly supplied by Dr. Bing Hsieh. 2.2. Synthesis of the bis-xanthate PPV precursor monomers The monomer used for the synthesis of the xanthate PPV precursor polymer was prepared from 1,4-bis-(chloromethyl)-benzene by established procedures as outlined in Fig. 1 [12,18,19,22]. The resulting product, 1,4-bis-(S-[O-ethylxanthato]methyl)benzene, is a white crystalline solid. The 1,4-bis-(S-[O-ethylxanthato]methyl)-2,5-(MeO)2 -PPV precursor monomer has a similar physical appearance and was produced in analogous fashion starting with 2,5-dimethoxy-1,4bis-(chloromethyl)benzene [12,22]. 2.3. Synthesis of the xanthate PPV precursor polymers Again, the two corresponding precursor polymers were synthesized as shown in Fig. 1 following published procedures [12,18,19,22]. In contrast to the monomers, both are pale yellow powders. These are soluble in chloroform from which they can be cast into thin films. Typically, 30–60 mg of the precursor polymer was dissolved in 10 mL of chloroform and spread across a smooth surface and the solvent allowed to evaporate under ambient conditions.

2.4. Preparation of the conjugated PPV polymers Heating a freestanding film of the respective xanthate precursor polymer to greater than about 180 ◦ C initiates its conversion into the corresponding conjugated PPV polymer. Continued heating to at least 250 ◦ C, results in complete conversion with concomitant loss of equal numbers of H atoms and xanthate groups (Fig. 1) [12,22]. A freestanding film of conjugated PPV is orange, while the dimethoxy derivative, ((OMe)2 -PPV), is red. 2.5. Infrared spectroscopy All IR spectra were collected on a Varian (formerly Digilab) (Lexington, MA) FTS 6000 FT-IR system. The interferometer has a Ge/KBr beamsplitter, a high-temperature, water-cooled source, a deuterium triglycine sulfate (DTGS) detector with KBr optics, and was operated at 4 cm−1 resolution with an aperture of 2 cm−1 . Before Fourier transformation, the interferogram was multiplied by a triangular apodization function and zero-filled one level. Unless stated otherwise, the IR spectra of the powdered solids were obtained as 7-mm diameter KBr pellets using a hand press and Pike Instruments KBr. For each pellet, approximately 250–500 ␮g of sample was mixed with about 100 mg KBr. Cast precursor polymer films were also examined in situ by transmission infrared spectroscopy while the films were slowly heated to higher temperatures [12,18,21,22]. In general, peak positions and heights were measured by Varian (Digilab) Win-IR Resolutions Pro Peak Pick software without smoothing, base line correction, or other data manipulation. To verify the reproducibility of the parameters of some of the smaller bands, a spectrum of a blank KBr disk was sometimes subtracted from the original spectrum. Many of the spectra were run of 3, 4, or even 5 replicate samples. To facilitate comparison among the spectra for each of the two conjugated polymers and their corresponding precursor polymer and monomers, the relative peak heights of all spectra were roughly normalized as outlined in the following scheme for the four PPV species and p-xylene given in Table 1. The normalized

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D. Michael Byler et al. / Spectrochimica Acta Part A 79 (2011) 118–126

Table 1 Comparison of frequencies (cm−1 ), relative intensities, and assignments for IR spectra of Xa-PPV species (∼D2h ). Pk#

p-Xylene (l) −1

 (cm 1 2 3 4 5 6 7 7b 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63

3135.6 3094.7 3046.4 3019.9 2999.9 2976.0

)

p-(ClCH3 )2 C6 H6 −1

pk ht

 (cm

0.0 0.1 0.4 0.4 0.3 0.0

3131.2 3091.7 3055.0 3032.4 3011.5

0.0 0.0 0.1 0.1 0.1

2970.2

1.7

b

2944.4

0.0

2922.5 2868.6

0.9 0.4

)

pk ht

0.5 0.1 0.1

precursor Xa polymerPPVc

p-(Xa)2 C6 H6 −1

 (cm

−1

pk ht

 (cm

0.0 0.1 0.1 0.3

3111.8 3077.2

0.0 0.1

0.1 0.3

3125.5 3087.7 3048.4 3022.3

3023.0

1.5

2985.2

2.4

2978.3

0.7

2965.8 2935.0 2918.6 2893.4 2866.3

0.2 0.5 0.2 0.4 0.2

2952.7 2930.9 2918.8 2896.0 2852.1

0.1 0.2 0.2 0.2 0.2

2964.5

0.1

2921.6

0.2

2856.0

0.2

2598.3 1906.5 1820.7 1790.0 1680.0 1602.7

0.0 0.2 0.0 0.0 0.0 0.2

1515.6 1472.9

2.4 0.0

1442.4

0.1

1421.9

0.5

1336.6 1301.5

0.3 0.0

1261.6

0.6

1210.8 1180.6

0.1 0.2

1106.3

0.5

1017.8

0.8

)

pk ht

3126.3

0.0

3057.4 3032.1

 (cm

)

2733.0

0.1

2866.8 2828.1 2706.1

1890.5

0.2

1930.2

1.3

1912.0

0.3

1903.0

0.2

1792.8

0.1

1813.1 1701.3

0.6 0.1

1799.9

0.1

1630.4

0.1

1516.5 1495.7 1454.2

2.4 0.0 0.5

1511.7 1478.9

1.8 0.2

1511.9

1.8

1791.3 1698.1 1609.6 1575.9 1511.4

0.1 0.1 0.2 0.1 2.4

0.7 0.5 ∼0.4 2.4 3.2

0.1

4.4

1467.2 1448.2 1440sh 1418.1 1393.9

1468.1

1445.1

1439.7 1418.4 1386.4

0.8 0.8 0.4

1357.0 1305.1 1257.8

1.1 0.1 7.1

1248.1

0.2

1226.3 1202.3

16 4.4

1216.7

1152.0 1109.6

1.5 14

1145.4 1110.2

1417sh 1378.7

∼0 0.2

1323.8

0.0

1272.5 1243.5

0.0 0.0

1219.9

0.1

1213.2

2.5

1119.9

0.4

1143.3

0.7

1103.2

0.2

1105.2 1046.3

1.9 0.4

1042.9 1023.0

0.3 0.2

962.3 935.9

0.0 0.0

1421.1

3.3

1369.5 1304.3 1266.2 1256.0

0.3 0.8 8.0 3.9

1048.4

1.5 4.4

10

2.3

1020.2

0.7

962.5

2.5

972.0

0.5

966.8

0.1

954.6 931.1

0.3 0.1

941.5

0.4

894.2

0.7 6.6

794.9 699.0

5.0 0.0

756.9

10.5

676.8

16.2

653.8

5.6

521.4

5.3

2.7 ∼0

8.0

1020.1

855.9

482.2 ∼410

1046.8

Proposed assignment [Cf. Green [25]]a

Pk#

[2 + 19]a 1616R + 1517ir = 3133 [12 + 19] 1576R + 1517ir = 3093 [23]  CHring b2u [18]  CHring b1u +  CHvinyl a CH2 asym o␾ (a CH3 asym + a CH2 ) i␾ s CH2 sym o␾ + s CH2 sym i␾ (a CH3 asym + a CH2 ) o␾ a Cring -CH2 -S-asym o␾ (s CH2 ) o␾ (of -OEt) s CH3 sym o␾ 2ı CH2 1445 × 2 = 2890 Combination band Combination band [19 + 21] 1516ir + 1023ir = 2539 [16 + 8] 962ia + 935R = 1887 b2u [25 + 29] 1324ir + 482ir = 1806 [16 + 7] 962ia + 836R = 1798 b1u  C C trans vinyl [7 + 28] 836R + 795ir = 1631 b2u Combination band [19] ip ring CC + ıip CHring b1u [3 + 27] 1204R + 292ir = 1496 ıa CH3 asym + ıCH2 ı CH2 ı CH2 (of Cring CH2 S–) i␾ [24] ip ring CC + ıip CHring b2u (ıs CH3 sym + ıs CH2 ) o␾

1 2 3 4 5 6 7 7b 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63

pk ht

0.5

0.0

0.0

20

1362.9 0.8 1289.5 0.6 ∼1261 sh <1

)

1023.0

836.3

643.5

−1

867.4 847.0

2.5 0.4

854.7 827.3

0.3 0.3

866.0 835.5

0.4 2.5

816.2 769.5 693.6

2.3 1.6 2.5

811.3 776.7 719.7

0.2 0.1 0.3

801.0 705.5

1.2 0.0

672.8

0.6

637.9

0.2

557.5 518.0

0.3 1.2

1.4 0.0

2.3

0.7 0.0 0.1

555.0 432.3

429.5

558.2 478.4 439.0

(ıs CH3 sym + ıs CH2 ) i␾ [25]ip ring CC (Kekule) b2u + ␶CH2 ␥ CH2 wag (of –CH2 Cl) or (of Cring CH2 S–) [16 + 27] 962ir + 292ir = 1254 [7 + 17] 835R + ∼410ir = 1245 b1u CXa –O xanthate + ␶CH2 (of EtO–) [20]ip Cring -CXa + ıip CHring b1u [28 + 29] 699ir + 482ir = 1181 [26] (ıip CHring b2u + ␶CH2 ) i␾ ␳CH3 + ıCH2 (of EtO–) + CEtO [26](ıip CHring b2u + ␶CH2 ) o␾ Combination band C–S + C S + C–C + ıa CH3 ␳CH3 [21] ıip ring CCC b1u + ıip CHring ␥op CH trans-vinylene [16] ıop CHring au iad [8] ␥op CHring b2g R or[9 + 27] ␶CH2 [28] ıop CHring b3u + ␳CH3 + ıOCC (of EtO–) [28] ıop CHring b3u + ␳CH3 + ıOCC (of EtO–) + C–S ␳ CH3 + ␶ CH2 [22]ıip CHring b1u or [28]␥op CHring + CXa − CAr [28]␥op CHring + ␥CXa − CAr  C–Cl Complex xanthate mixed mode Combination band ıop OCS2 [29] ıop ring CC + ␥CH [29] ␶op ring CC–CC b3u SCS + ıCOC + a COC

Note: The peak numbers in the first and last columns are given solely to provide a convenient means for comparing peaks that are presumed to correlate among the several species of compounds discussed in this paper. Thus, peaks 28, 34, 45, 52, and 59 are only observed for the DM-PPV species, but not for PPV and its precursors. a The numbers in square brackets under assignments represent the numbering of the fundamental vibrations of the 6 xylene ring C’s and the 6 atoms bonded to them according to Green [25]. b All peak heights (in absorbance units) are normalized as described in Section 2.5, “Infrared spectroscopy”. Peaks with values <0.05 are rounded to 0.0. c PPV synthesized via the xanthate route. d ia and R represent modes for molecules with full D2h symmetry that are inactive in both IR and Raman or that are Raman active only.

D. Michael Byler et al. / Spectrochimica Acta Part A 79 (2011) 118–126

Fig. 2. Infrared spectra of PPV polymers, monomer and model compounds. All spectra are normalized as described in Section 2.5. Then each spectrum was baseline corrected and, except for (a), a constant was added to shift each successively upward along the y axis for ease of visual comparison. (a) p-xylene (liquid); (b) PPV film; (c) xanthate PPV precursor polymer film; (d) bis-xanthate PPV precursor monomer (KBr pellet); (e) 1,4-bis-(chloromethyl)benzene (KBr pellet).

121

Fig. 3. Infrared spectra of DM-PPV polymers, monomer and model compound. All spectra are normalized as described in Section 2.5. Then each spectrum was baseline corrected and, except for (a), a constant was added to shift each successively upward along the y axis for ease of visual comparison. (a) 2,5-dimethoxy-1,4bis-(chloromethyl)benzene (KBr pellet); (b) bis-xanthate DM-PPV monomer (KBr pellet); (c) DM-PPV polymer film; (d) DM-PPV precursor polymer film.

3. Theoretical considerations 3.1. Molecular symmetries, geometries, and fundamental vibrations [23,24]

IR spectra are shown in Fig. 2. First, the peak heights for the xanthate precursor polymer were mathematically adjusted so that the strongest band at 1048 cm−1 , due to the complex xanthate stretching modes, had an absorbance value of 10 (vs). Then the peak intensities of the other spectra were approximately normalized as follows: presuming that the molar absorptivity per xanthate group remains roughly constant, the corresponding extremely strong band of the PPV bis-xanthate monomer was set at twice the above value (20, xs) because this molecule has two xanthate groups per phenyl ring. Many para-disubstituted benzenes exhibit a characteristic ring stretching mode near 1515 cm−1 . For the xanthate monomer, this band has a normalized height of about 1.8 abs. units (w), while for the polymer, the relative absorbance is 2.4 (w). Setting the peak intensity of the 1512 cm−1 band of the bis-dichloromethyl monomer to 1.8 abs. units (w) resulted in the relative height of the most intense band (677 cm−1 ) of this molecule being ∼16 abs. units (xs). By contrast, when the same ring mode of neat 1,4-xylene was set to 2.4 abs. units (w) in height, the strongest band in its spectrum was just 5 abs. units (ms). The ∼1515 cm−1 ring mode for the conjugated PPV polymer was set to 2.4 abs. units resulting in the most intense bands (962 cm−1 and 835 cm−1 ) each being 2.5 abs. units (w) as shown in Table 1 and Fig. 2. Normalization of the (OMe)2 -PPV spectra are a bit more complex as density functional theory calculations based on monomer spectra indicate that most of the xanthate bands have contributions from the methoxy substituent. The ∼1510 cm−1 band has contributions from both the ring and the methoxy substituent, but since there are no xanthate contributions, this band was set to 2.4 for each spectra and all other bands were normalized to this band. For the dichloro monomer, the strongest band is therefore 1040 cm−1 (2.5 abs. units). The xanthate bands are the most intense in the xanthate monomer and precursor polymer as shown in Table 2 and Fig. 3.

The two monomers and four polymers examined in this study all share in common para-disubstituted phenyl rings (Fig. 1). In addition, the dimethoxy-analogs have CH3 O– groups in the 2,5-positions. For both monomers, the 1,4-substituents are S-(Oethyl xanthato)methyl groups, –CH2 Xa, where Xa = –SC( S)OC2 H5 . The two precursor polymers have xanthate-substituted ethanolinkages (–CH2 CHXa–) that concatenate the phenyl rings via the 1,4-positions. Upon thermal conversion of the latter to the respective conjugated polymers, one H and the xanthate moiety have been eliminated from each repeat subunit to transform these links between the phenyls into trans-vinylenes (–CH CH–). Simple para-disubstituted benzenes, p-XC6 H4 X, where X = a single atom such as a halogen, have D2h symmetry. If the x-axis is taken as the one perpendicular to the phenyl ring and the z-axis passes through the two X-atoms on the ring, the 3N − 6 = 30 fundamental vibrations will fall into eight classes [23,25,26]: 6ag + b1g + 3b2g + 5b3g + 2au + 5b1u + 5b2u + 3b3u . Because such molecules have a center of symmetry, i, the mutual exclusion rule prohibits any of these vibrations from being simultaneously Raman and infrared active. For D2h symmetry, then, the first four classes (15 vibrations) are Raman active while the last three are IR active. The two au modes are inactive for either spectroscopy. Of the three classes of IR-active vibrations, the 5b1u + 5b2u involve in-plane vibrations, while the 3b3u modes are all out-ofplane motions. When two Y atoms replace two hydrogens at positions 2 and 5 of a para-disubstituted benzene, the symmetry of the molecule is lowered to C2h . Now the 30 fundamentals are divided among just four classes [23]: 7ag + 8bg + 7au + 8bu .

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D. Michael Byler et al. / Spectrochimica Acta Part A 79 (2011) 118–126

Table 2 Comparison of frequencies (cm−1 ), relative intensities, and assignments for IR spectra of (CH3 O)2 -PPV species (∼C2h ). Pka

1 2 3 4 5 6 7 7b 8 9 9b 10 11 11b 12 13 14 15 16 17 17b 18 19 20 21 22 23 24 25 26 27 28 29 30 31 31b 32 33 34 35 36 37 38 40 39 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63

2,5-(CH3 O)2 – 1,4-(CH2 Cl)2 –C6 H6

(CH3 O)2 Xa monomer

Precursor Xa polymer

(CH3 O)2 -PPV

 (cm−1 )

pk htb

 (cm−1 )

 (cm−1 )

 (cm−1 )

3138.8

0.0

3050.5 3019.1

0.1 0.2

2975sh 2967.2

∼0 0.5

3014.6 2997.0 2979.5 2963.5 2953.5 2933.7

0.0 0.3 0.3 0.2 0.0 0.4 0.0 0.0 0.1 0.0 0.1

0.0 0.0 0.0 0.2

0.3

2908.2

0.0

2859.9

0.1

2837.7

0.2

2913.8 2889.6 2865.9 2847.7 2827.9

1796.6 1737.9

0.0 0.2

1896.9 1846.7 1792.8 1728.8

1632.1

0.0

1512.7

2.4

1463.2 1449.7 1432.1 1409.9

1.5 0.1 0.3 2.0

0.9

1258.4

1.1

1225.0 1194.8 1178.8 1149.6 1136.1

1040.4

1.8 0.1 0.8 0.1 0.5

2.5

pk ht

2962sh

∼0.1

2936.3

0.6

2906sh

0.2

2906.1

0.3

2851.7 2829.0

0.1 0.5

2856.6 2830.3

0.1 0.7

1826.3 1801.6

0.1 0.1

1678.7 1612.3

0.1 0.2

1676.3 1598.9

∼0.3 0.1

1504.7

2.4

1504.0

2.4

1463.6

1.6

1464.5

1.9

1509.0 1484.2 1464.0 1444.1 1421.5 1401.4 1384.7 1355.6

2.4 0.1 1.2 0.1 0.1 1.7 0.1 0.5

1403.1

2.0

1406.8

2.4

1362.9

0.2

1353.3

1.1

1315.8 1288.0

0.6 0.0

1320.7 1289.1

0.3 0.1

1311.5

0.0

1258.9

1.7

1206.6

4.1

1181.0 1150.7 1106.1

0.4 0.0 0.1

[19]ip ring CC + ıip CHring + ␳CH3 O Overtone or combination band ıa CH3 asym, CH3 O– ıs CH2 + ıs CH3 ı CH2 , Cring CH2 S– [24] ip ring CC + ıip CHring ıs CH3 sym, CH3 C– [25]ip ring CC (Kekule) + ␳CH3 O ip ring CC (Kekule) + ␶CH2 + ␳CH3 O ␶ CH2 + ␳ CH3 C– of (EtO–) ␥ CH2 (of –CH2 Cl) ıip CHring + ring CC (Kekule)

2.9

1172.3 1145.4

0.1 0.1

1176.7 1145.0

0.2 0.4

1115.6

2.3

1110.0

1.4

Cring –O– + ıip CHring CXa -O xanthate + ␶CH2 (of EtO–) ␳CH3 O + ıip CHring ıip CHring + ␳ CH3 O + ␶CH2 [26]ıip CHring [26]ıip CHring ıCH2 (of EtO–) + ␳CH3 + CEt O

1051.3

4.0

1045.6

3.6

C–S + C S + C–C + ıa CH3

1037.8 1003.55

3.5 0.1

1003.6

0.3

1.1 0.3

862.5

1.1

863.8

0.5

809.6

0.2

808.0

0.1

764.4

0.2

801.4

0.2

684.1

1.2

685.0

0.3

632.1

0.0

667.0 618.2

0.2 0.1

472.2

0.0

0.2 0.0

Overtone or combination band Overtone or combination band Overtone or combination band Overtone or combination band C C trans vinyl Overtone or combination band

1213.8

877.3 833.0

474.5 451.2

s CH3 sym, CH3 O– s CH3 sym of EtO– s CH3 sym, CH3 O– 2ıs CH3 sym, CH3 C– 2ıs CH3 sym, CH3 O–

3.6 4.7

0.2

0.9

[18 or 23]a  CHring and  CHvinyl [18 or 23] CHring a CH3 asym, CH3 O– a CH2 + a CH3 asym of EtO– s CH2 sym a CH3 asym, CH3 C– a CH2 asym of Cring CH2 S– + a CH3 , CH3 O–

1228.2 1215.9

935.9

609.2

0.4

0.1 0.2

0.5

2.2

2992.5

2951.2 2932.4

901.4

679.2

0.4

0.4

0.5

1.5

3056.5 2989.2

912.8

736.5

pk ht Overtone or combination mode

2938.0

1319.1

pk ht

Proposed assignment

479.2 440.2

0.1 0.3

1045.0 995.2 965.0

3.4 0.1 0.9

[21] ıip ring CCC + CMe –O– (␥CH2 + ␳CH3 ) of EtO– ␥op CH trans-vinylene ␦ip CHring + ␶CH2 (of –CH2 Cl or –CH2 S–)

852.6

0.7

[28] ␦op CHring ␦ip CHring + ␶CH2 ␦ip CHring + ␶ArCH2 S

801.0

0.3

[28]␦op CHring

683.0

0.6

Complex xanthate mixed mode C–Cl Overtone or combination band [22] ıop ring CC + ıop CHring

481.4

0.1

[29] ␶op ring CC–CC ıop CHring + ıSCS + ı CXa OC + ␳CH3 O–

Note: The peak numbers in the first column are given solely to provide a convenient means for comparing peaks that are presumed to correlate among the several species of compounds discussed in this paper. The numbering matches Table 1. Thus peaks 2, 9, 13, 14, 20, 29, 33, 41, 43, 47, 49, 53, 55, 60, and 61 are only observed for the PPV species (Table 1). a The numbers in square backets under assignments represent the numbering of the fundamental vibrations according to Green [25]. b All peak heights (in absorbance units) are normalized as described in Section 2.5, Infrared spectroscopy. Peaks with values <0.05 are rounded to 0.0.

D. Michael Byler et al. / Spectrochimica Acta Part A 79 (2011) 118–126

In this case there are 15 IR active and 15 Raman active modes; none are silent. For each of the eight molecules reported in this paper, discussion of the bands resulting from the vibrations of the phenyl ring and the six atoms bonded to it will focus on the infrared active fundamentals. Of these, two associated with the in-plane (b2u for D2h ; bu for C2h ) and out-of-plane (b3u for D2h ; bu for C2h ) bending motions of the X-groups relative to the plane of the phenyl ring are expected to absorb at wavenumbers below 400 cm−1 [23,25], beyond the range of the interferometer used in this study. For the tetrasubstituted species (where Y = CH3 O–), an additional two modes involving in- and out-of-plane bending of the methoxy oxygens also have frequencies too low to measure. The remaining infrared active modes (11 for D2h ; 13 for C2h ) have frequencies between 3100 and 400 cm−1 . For simple substituted benzenes, some of these vibrations will produce absorptions that represent reasonably good group frequencies. (That is, most of the potential energy of the normal mode is localized in the vibration of a single part of the molecule.) In such cases, especially when these bands also display significant IR intensity, band assignments may be made with reasonable certainty by comparison with detailed reports for model compounds having analogous structure and symmetry [25,26]. Others vibrations, although formally allowed by symmetry, may absorb so weakly that they are difficult to attribute with any certainty. Frequencies of the peaks associated with vibrations of the X- or Y-substituents atoms will, of course, vary depending on the nature and identity of the atoms and the overall symmetry of the system. This is especially true, as in this study, where X = C and Y = OMe, because the masses and force constants of these two atoms are so similar to those of the carbons forming the phenyl ring. Then, kinematic coupling between the motions of the X or Y and the carbons of the ring may result in a normal mode with large contributions from more than one group of atoms so that neither band frequencies nor intensities are easily interpreted solely on the basis of the molecular geometry and structure. Normal modes may also involve significant mixing between vibrations of parts of the molecule that share common symmetry but that have atoms of very different masses. For example, for phenyl rings, CCH deformations often mix with skeletal stretching vibrations of other atoms attached to the ring, like carbon and oxygen, because the vibrational frequencies (energies) that the two groups would have in isolation are sufficiently close in value for this to occur. Furthermore, when X or Y represents a group of atoms with symmetry lower than D2h , such as an O-ethyl xanthate group (∼Cs ), the above symmetry strictures will begin to break down and a few of the otherwise inactive vibrations may appear as additional weak bands in the experimental spectrum [23,27]. For the precursor monomers and polymers, the 11 atoms of each O-ethyl xanthate group will contribute 33 additional fundamental vibrations, although many of these will likely be too weak to observe in the infrared. In the case of the precursor monomers, the presence of two xanthates will lead to 33 in-phase and outof-phase vibrational dyads. Furthermore, the two H’s on each of the two methylene carbons (–CH2 –, where X = C) linking the pair of xanthate groups to the benzene ring of the monomer will add a further 6 vibrational doublets to those enumerated previously. But in most, if not all, cases, the two members of each of these 39 pairs will most likely be nearly degenerate in frequency. Even so, many fewer that 39 + 30 = 69 bands are observed in the IR spectra of the bis-xanthate PPV monomer. For both the xanthate precursor polymers and the corresponding fully conjugated PPV or (MeO)2 -PPV, the two X-substituents (carbons) in the 1,4 positions on the phenyl rings of any one individual subunit are linked to the adjoining subunits by either a single bond or a double bond, respectively. For each subunit of the precur-

123

sor polymer, one of these two carbons (X = C) is an unsubstituted methylene whose two H’s again contribute 6 fundamental modes in addition to the 30 expected for the 12 atoms of the ring. The second methylene carbon has one xanthate group (11 atoms) and one hydrogen. These 11 + 1 = 12 atoms give 33 + 3 = 36 more vibrational modes. The total for the entire polymer repeat unit is 78; again, many fewer than 78 IR bands are found experimentally. For the fully conjugated polymers, the two H’s of the vinylene carbons (–CH CH–, where X = C) that bridge the phenyl rings, contribute six additional normal modes: two CH stretches and four C C–H deformations (two in-plane and two out-of-plane). Because of the presence of a center of symmetry, i, three of these are IR active and three are Raman active. There is also one C C stretching mode for each monomer subunit. In this case, a total of 37 fundamentals is predicted for each polymer subunit. Finally, the two methoxy groups of the three derivatized species potentially will each lead to an additional 15 vibrations; the number of new bands actually observed is likely to be less because local C3v symmetry of the methyls should cause several to be nearly degenerate. 3.2. Calculations Density functional theory (DFT) calculations were used to model the infrared spectra of each of the four monomers. Structure optimization and frequency calculations were performed using Q-Chem 2.0 [28] an electronic structure program. The MOLEKEL [29] graphics package 4.3 [30] was used to visualize the results. The Becke’s three-parameter exchange functional [31] was used in combination with the Lee–Yang–Parr correlation functional [32] (B3LYP) and standard double-zeta (6-31G*) basis set. 4. Results and discussion 4.1. Aromatic and olefinic CH stretching region (3100–3000 cm−1 ) The CH bonds of the phenyl ring characteristically absorb in this part of the infrared spectrum. For example, 1,4-xylene has very weak peaks at 3136 and 3095 cm−1 , presumably combination modes (Table 1), as well as more intense features at 3046, 3020, and 3000 cm−1 [25,33]. The numbers in square brackets in the assignment column in Tables 1 and 2 represent the numbering of the fundamental vibrations for p-xylene numbered according to Green [25]. The peak numbers in the first column are to provide a convenient means for comparison across all monomers and polymers discussed. Slight differences in some wavenumbers have been observed between PPV synthesized via the SPR and XPR [12,13,22]. The values reported in Table 1 refer only to PPV synthesized via the xanthate route. Hydrogens attached to double-bonded carbons (alkenes) have CH stretching frequencies in this region as well. For example, trans-vinylenes (1,1-disubstituted alkenes) give a medium strong IR band between 3050 and 3000 cm−1 [23]. For the conjugated PPV polymer (Table 1), the only band with appreciable intensity in this region is at 3023 cm−1 . Clearly, the aromatic and vinylic CH vibrations both contribute to this band, but the latter is probably responsible for more of the observed intensity than the former. The PPV xanthate precursor monomer and polymer, each have two bands here (Table 1) also due to aromatic CH stretching vibrations. By contrast, for the conjugated polymer, (MeO)2 -PPV, the analogous band is at 3056 cm−1 (Table 2), with contributions from both aromatic and vinylene CH. Neither the monomer nor the corresponding precursor polymer has a band in this region of significant intensity (Table 2) supporting the indication that

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in the conjugated polymer, this band results primarily from the vinylene CH. The calculated spectra of the four monomers were instrumental in proposing assignments in Tables 1 and 2. When a polymer chain forms, it is expected that there will be wavenumber and intensity shifts. Visualization of the vibrational mode of each monomer was applied in conjunction with the calculated wavenumber and relative intensity values to propose each assignment. IR characteristic group frequency tables were also consulted for confirmation that the deduced assignment was reasonable [23,24]. The assignments in Tables 1 and 2 are the best approximations given the calculated vibrational modes for the monomers and show a surprisingly good correlation between the monomers and polymers. 4.2. Aliphatic CH stretching region (3000–2800 cm−1 ) All eight of the monomers and polymers studied have one or more bands in this next region, although the fully conjugated PPV polymer has no alkyl groups, except for one group at each terminus of the polymer chain, and should essentially display little or no absorption due to alkyl CH stretching vibrations here. Weak bands experimentally found (Table 1) for p-xylene at 2944.4, 2922.5 and 2868.6 cm−1 were not labeled by Green [25]. The weak bands reported by Bradley [3] found at 2950, 2920 and 2852 cm−1 in the spectrum of conjugated PPV prepared via the sulfonium precursor route (SPR) may possibly be overtones or combination bands. The aliphatic CH stretches may also arise from small amounts of precursor groups and/or defects on the polymer chain. Analogously, the presence of weak bands at 2965, 2922, and 2856 cm−1 for the conjugated polymer (Table 1) obtained via the xanthate precursor route [4,7] may be combination bands or may suggest that this latter PPV sample has defect units (sp3 rather than sp2 ) or residual precursor. This could be related to the more amorphous nature of the PPV synthesized via the xanthate route [7]. The dimethoxy derivatives all have a characteristic moderately strong band near 2830 cm−1 associated with vibrations of the pair of methoxy groups attached at positions 2 and 5 on the phenyl rings [23]. These dimethoxy derivatives each have two additional CH stretching absorptions due to the methoxy moieties near 2935 and ∼2910 cm−1 . The xanthate groups of both sets of monomers and precursor polymers give other absorptions here that are typical of aliphatic CH stretching (Tables 1 and 2). A general assignment is provided for each line in Tables 1 and 2. For example, in row 8 (Table 1) no wavenumber values are given for p-xylene or the dichloro monomer; the dixanthate monomer and the xanthate precursor polymer have an asymmetric stretch at ∼2930 cm−1 which involves the ring carbon, CH2 and S of xanthate. The corresponding assignment in Table 2 (row 8) also includes a component from the CH3 of the methoxy as vibrations are reported for all four dimethoxy monomers and polymers. Visualization using MOLEKEL [30] confirms the complexity of most of the dimethoxy assignments proposed in Table 2. 4.3. Vinylene stretching region (2000–1620 cm−1 ) Combination bands characteristic of para-substituted phenyls and 1,4-2,5-tetrasubstituted phenyls are found here. For the former, Colthup et al. [23] report that three such summation bands will occur roughly at 1890, 1780, and 1630 cm−1 with a peak height intensity ratio of ∼3:1:2. These weak bands are in rows 15, 17 and 19 of Table 1. The tetrasubstituted phenyl is expected to show a band near 1720 cm−1 with a shoulder of <0.2 its intensity near 1780 cm−1 [23]. For the (MeO)2 -PPV species, a single weak band is found near 1730 cm−1 for the two monomers (Table 2).

The only other fundamentals expected in this region are the –C C– stretch of the cis or trans-vinylenes that conjugate the benzene rings of the converted PPV and (MeO)2 -PPV polymers. Due to their low polarity, such bonds at best give very weak IR absorptions, but should produce measurable Raman scattering. The cis is expected between 1662 and 1631 cm−1 [23]. For unconjugated alkenes with no structural strain, the trans is reported to occur between about 1668 and 1678 cm−1 [23]. Conjugation with the benzene rings could lower the frequency by approximately 40 cm−1 [23]. During conversion of the xanthate precursor polymer, a very weak band increases with heating at 1641 cm−1 consistent with initial formation of cis linkages [12]. This peak diminishes with continued heating and a very weak 1680 cm−1 band for the trans –C C– stretch is found for xanthate PPV (row 18). This band is equally weak in the (MeO)2 -PPV polymer (Table 2). Several investigators report a modest Raman band for PPV near 1626 cm−1 [14,34,35]. 4.4. Aliphatic CH deformation and aromatic ring stretching region (1620–1260 cm−1 ) Vibrational spectroscopic studies of simple substituted benzenes have shown that many of the bands resulting from six stretching vibrations of the phenyl ring interacting with the three in-plane ring deformations and the six in plane CH bending are expected in this part of the spectrum [25,26,36]. For 1,4disubstituted species, seven of these are allowed only in the IR. For PPV and its precursors, one moderately strong peak lies near 1515 cm−1 . The corresponding bands for the (MeO)2 -PPV species appear some 3–12 cm−1 lower in wavenumber. Comparison with the spectra of p-xylene and related model compounds suggest that this mode probably involves a large contribution from the “semicircle” stretch of the phenyl rings coupled with an in-plane aromatic CH deformation of the same symmetry. (For example, b1u for pxylene (D2h ) in Table 1.) A second band with similar intensity at ∼1420 cm−1 for PPV and approximately 1405 cm−1 for (MeO)2 -PPV probably involves the b2u “semicircle” ring stretch. One of the two or three bands found between 1350 and 1260 cm−1 may well be the so-called “Kekule” ring stretch (b2u ). The 1262 cm−1 in PPV falls between two possible combination bands: 962ir + 292ir = 1254 or 962ir + 310R = 1272. It could also be a p-phenylene CH in-plane bend as assigned by Bradley [3] to the 1270 cm−1 band in SPR PPV. All of the ring modes in the dimethoxy monomers and polymer (Table 2) are complex involving the methoxy substituents. Alkyl groups also are expected to exhibit characteristic bands arising from CH bending motions between about 1480 and 1360 cm−1 [23]. Of course, the conjugated PPV has no alkyl functionalities and thus no band of this type. The dimethoxy derivative has an additional peak at 1464 cm−1 which must represent the CH3 deformations of the methoxy groups. The xanthate precursors of both polymers have additional bands here due to the alkyl portions of the xanthate groups and the alkyl groups that links each xanthate to the phenyl ring (Tables 1 and 2). The 1363 band in the dimethoxy precursor decreases in intensity during conversion and a new band increases at 1353 cm−1 . Visualization of the DFT results indicate that these molecular vibrations involve both the methoxy substituent and the ring. Hence, there is no corresponding band in Table 1 (row 28) for the PPV monomers or polymers. 4.5. Aromatic ring bending and C O and C S stretching region (1260–1000 cm−1 ) Generally, disubstituted benzenes have just three IR fundamentals of modest intensity here. The most intense for conjugated PPV at 1106 cm−1 may correspond to an in-plane Cring -H deformation (b2u ). There is an analogous band at 1106 cm−1 in (CH3 O)2 -PPV. A second band must be associated with the Cring -X stretch (X = C

D. Michael Byler et al. / Spectrochimica Acta Part A 79 (2011) 118–126

or O) while the third is an in-plane ring bending vibration (b2u ) (Tables 1 and 2). For the dimethoxy-derivatives, the methyl C–O bond stretching vibration of the methoxy group absorbs with sufficient intensity near 1038–1045 cm−1 in Table 2 (row 44) such that this band probably hides the weak aromatic ring mode. The 1180 cm−1 CH in plane mode for dimethoxy monomers and polymers has a component of the methoxy group. For conjugated PPV, this same mode (1181 cm−1 ) (row 37) could be a combination band or another inplane ring bending vibration [3]. The xanthate precursor polymers have three extremely strong IR bands in this part of the spectrum at ∼1215, 1110, and ∼1045 cm−1 . Comparison with the spectra of other xanthate containing species [37–39] shows that these absorptions are clearly due to the CS and CO stretching vibrations of the xanthate moiety. The frequencies observed for the bis-xanthate monomers are nearly the same, except that the highest energy band apparently splits in two. Perhaps this is a consequence of an interaction between the two identical xanthate groups to give in-phase and out-of-phase vibrations. This additional band overlaps with the ring mode in the ∼1200–1230 cm−1 region. Based on an examination of the effect of organic solvents on the position of the frequencies of the two strongest bands at 1216 and 1046 cm−1 , Little et al. suggest that the former is due to the C–O stretch, while the latter results from the C S stretch [37]. A more recent vibrational analysis of the spectra of xanthate salts, however, suggests that this conclusion is far from certain [38]. Despite differences in structure between these neutral compounds and the negatively charged salt, this region of the IR spectrum is very similar for both these species, as well as for a simple protonated xanthate [40]. The calculated monomer spectra confirm that some of these modes are quite complex, especially in the dimethoxy derivative and cannot be simply labeled as a C S stretch. 4.6. Out-of-plane CH bending region (1000–900 cm−1 ) In this region, trans-vinylenes have a characteristic absorption due to the out-of-plane –CH CH– bending vibration near 965 cm−1 [14,23,34]. Prominent bands are found at this wavenumber for both the converted PPV and (MeO)2 -PPV polymers. Because the precursor polymer has two weak bands at 955 and ∼930 cm−1 , previous workers had suggested that such absorptions indicated that the precursor polymer had partially converted to the final product at room temperature [3]. As these PPV precursor polymer bands occur at distinctly lower frequency than the vinylene out-of-plane –CH CH– bend observed for the converted polymer, the presence of these weak bands seems unlikely to indicate any partial conversion of the precursor to the final product. Indeed, even after several years storage (in a refrigerator at ∼5 ◦ C) these two bands give no evidence of increased intensity as might be expected if either were the result of continued slow conversion to PPV. A future examination of the possible effects of long term storage of samples at room temperature could clarify this point. Furthermore, the PPV xanthate monomer also exhibits a weak absorption at nearly the same frequency (967 cm−1 ), but clearly this molecule has no vinylene group. So we conclude that this 967 cm−1 band in the monomer spectrum must instead arise, of necessity, from the motions of the phenyl ring. Xylene has two extremely weak bands at 962 and 936 cm−1 . Previous vibrational analyses have attributed the former to au outof-plane Cring -H bending mode [25,26,36]. The latter is probably an overtone or combination band. For a 1,4-disubstituted benzene with perfect D2h symmetry, this fundamental will be both IR and Raman inactive. The presence of groups with lower symmetry, such as methyls (C3v ) or xanthate (∼Cs ) obviously weaken the selection rules so that this normally forbidden au mode weakly absorbs in the IR near 965 cm−1 . Given these facts, one may also then conclude

125

with more confidence that the two weak features in the PPV precursor polymer spectrum at 955 and ∼930 cm−1 are associated with the aromatic ring and not with vinylene linkages resulting from partial conversion at ambient conditions of the precursor polymer to the final conjugated product. 4.7. Aromatic Cring H and vinylene –CH CH– bending region (900–400 cm−1 ) Xylene has two strong bands in this region at 795 and 482 cm−1 . On the basis of vibrational analyses, both have been ascribed to b3u modes of the phenyl ring, the former to out-of-plane ring CH bending and the latter to out-of plane ring torsion [25,26,36]. A third fundamental (of b1u symmetry) has been calculated to have a frequency of about 720 cm−1 . The potential energy of the latter mode involves a large contribution from the C-X stretch and thus its frequency and intensity are quite sensitive to the nature of the X substituent. For xylene, it absorbs too weakly to be found in the spectrum. During the thermal elimination process used to convert the SPR PPV to the conjugated polymer, a band increases in intensity at ∼630 cm−1 attributed to cis vinylene out-of-plane –CH CH– bending [41]. With continued heating, isomerization from cis to trans occurs as noted by the decrease in band intensity and the increased intensity of the trans vinylene (965 cm−1 ). In the xanthate PPV precursor, this cis vinylene grows with heating at 639 cm−1 , while for the (MeO)2 -PPV this band shifts to ∼680 cm−1 . Conjugated PPV has an intense peak at ∼835 cm−1 , another about 0.4 its height at ∼800 cm−1 , and a third with but 5% of its intensity at ∼705 cm−1 . For the precursor polymer, the region between 870 and 670 cm−1 has five bands of different intensities. We may reasonably speculate that two of these three PPV bands (along with the analogous two peaks in the xanthate precursor) correspond to the two xylene fundamentals at 795 and 699 cm−1 . Nevertheless, the assignments proposed in this region must be considered tentative. For (MeO)2 -PPV two bands are present: one at 853 cm−1 ; the second at 683 cm−1 . Again their assignment in Table 2 to fundamental vibrations comparable to the above two modes of xylene must be tentative at best. Two of the four bands found for each of the two xanthate (MeO)2 -PPV precursors likely have similar origins. With regards to the 483 cm−1 band of p-xylene, previous studies have labeled this band as due to the out-of-plane b3u ring torsion. All PPV species have a medium to strong band near 555 cm−1 and another near 430 cm−1 (Table 1). The (MeO)2 -PPV molecules, by contrast, each have a lone, but much weaker absorption near 475–480 cm−1 (Table 2). Skeletal stretching and bending vibrations of simple alkyl moieties, such as the ethyl group of the xanthate can also contribute to bands in this part of the spectrum [38,39]. The stretching vibration of the C–S linkage between the ethyl and the dithiocarbonate of the xanthate group is also expected to give a weak absorption between about 730 and 670 cm−1 . Clearly, some of the additional bands of modest strength seen in the spectra of the four xanthate precursor species may be fundamentals that arise from such vibrations. Such bands cannot be assigned without the aid of a more detailed vibrational analysis which is beyond the scope of the present paper. 5. Conclusion A close examination of the IR spectra of conjugated PPV synthesized via the xanthate precursor route, the corresponding dimethoxy derivative, and the six precursor monomer and polymer species are reported. These spectra may be used with confidence to check the purity of these substances when prepared for future

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physical and electrochemical studies. Of critical importance, these results provide a systematic means for verifying that the precursor monomer has been completely polymerized into the precursor polymer and that thermal conversion of the precursor polymer into the conjugated polymer is complete. Comparison of the spectra of the xanthate precursor monomers with those of the respective precursor polymers clearly shows that the intensities of the three xanthate bands between about 1225 and 1045 cm−1 relative to that of the characteristic phenyl ring mode near 1515 cm−1 have decreased by roughly one half. This is consistent with the loss of one out of every two xanthate groups after polymerization of the monomers. The fundamentals reported by Green [25] for p-xylene are noted in Table 1 (numbers in square brackets) and correlate well with the proposed assignments. Density functional theory (DFT) calculations performed on each of the four monomers provided a good correlation with the experimental spectra of these monomers. The molecular visualization of the vibrational modes was used in making the proposed assignments given in Tables 1 and 2. Thermal conversion of the xanthate precursor polymers into the final conjugated form results in the complete loss of the three xanthate bands as expected. Concurrently, a new strong IR band is found in the spectrum of the conjugated polymers at 965 cm−1 . The latter results from the CH deformations of the trans vinylene groups that link one phenyl ring to another to give a ␲-orbital that extends the length of these conjugated polymers. Acknowledgements The authors thank Dr. Bing Hsieh (formerly of Xerox) for providing samples of some of the starting materials. Acknowledgement is made to Dr. Paul Maslen for the DFT calculations and subsequent discussions. Thanks to Ms. Yolanda Liszewski and Mr. Joseph Delpalazzo for experimental IR spectra used to compile data for the analysis given herein. The FTIR spectrophotometer was purchased with funds from an NSF ARI grant (CHE96-01760). This study was partially supported by NSF grant (DMR00-75820). References [1] F. Hide, M.A. Díaz-García, B.J. Schwartz, A.J. Heeger, Accounts Chem. Res. 30 (1997) 430. [2] A. Kraft, A.C. Grimsdale, A.B. Holmes, Angew. Chem. Int. Ed. 37 (1998) 402. [3] D.D.C. Bradley, J. Phys. D 20 (1987) 1389–1410. [4] B.R. Hseih, Polym. Mater. Encyclopedia 9 (1996) 6537.

[5] [6] [7] [8] [9]

[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

[22] [23] [24] [25] [26] [27] [28]

[29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41]

R.A. Wessling, R.G. Zimmerman, U. S. Patent, (1968) 3,401,152. W.J. Swatos, B. Gordon III, Polym. Prep. 31 (1990) 1505. S. Son, A. Dodabalpur, A.J. Lovinger, M.E. Galvin, Science 269 (1995) 376. E. Kesters, S. Gillissen, F. Motmans, L. Lutsen, D. Vanderzande, Macromolecules (2002) 7902. F. Cacialli, in: D.L. Wise, G.E. Wnek, D.J. Trantolo, T.M. Cooper, J.D. Gresser (Eds.), Photonic Polymer Systems: Fundamentals, Methods, and Applications, Marcel Dekker, New York, 1998, p. 103 (Chapter 4). W.J. Mitchell, C. Pena, P.L. Burn, J. Mater. Chem. 12 (2000) 200. P.L. Burn, D.D.C. Bradley, R.H. Friend, D.A. Halliday, A.B. Homes, R.W. Jackson, A. Kraft, J. Chem. Soc. Perkin Trans. 1 (1992) 3225. Y. Liszewski, M.S. Thesis, Rutgers, The State University of New Jersey, Camden, NJ, 2001. H.V. Shah, G.A. Arbuckle, Macromolecules 32 (1999) 1413. A. Sakamoto, Y. Furukawa, M. Tasumi, J. Phys. Chem. 96 (1992) 1490. J.B. Schlenoff, L.-J. Wang, Macromolecules 24 (1991) 6653. J.B. Schlenoff, J. Obrzut, F.E. Karasz, Phys. Rev. B 40 (1989) 11822. J. Obrzut, F.E. Karasz, J. Chem. Phys. 87 (1987) 6178. G.A. Arbuckle-Keil, Y. Liszewski, J. Peng, B. Hsieh, Polym. Prep. 41 (2000) 826. G.A. Arbuckle-Keil, D. Michael Byler, Y. Patel, Polym. Prep. 45 (2004) 240. G.A. Arbuckle-Keil, Y. Liszewski, J. Wilking, B. Hsieh, Polym. Prep. 42 (2001) 306. G.A. Arbuckle-Keil, Y. Liszewski, J. Wilking, B. Hsieh, in: J.E. Puskas, T.E. Long, R.F. Storey, S. Shaikh, C.L. Simmons (Eds.), In Situ Spectroscopy of Monomer and Polymer Synthesis, Kluwer Academic/Plenum Pub., 2003, p. 173. Y. Patel, M.S. Thesis, Rutgers, The State University of New Jersey, Camden, NJ, 2005. N.B. Colthup, L.H. Daly, S.E. Wibberley, Introduction to Infrared and Raman Spectroscopy, 3rd ed., Academic Press, New York, 1990. G. Socrates, Infrared and Raman Characteristic Group Frequencies: Tables and Charts, 3rd ed., Wiley, 2001. J.H.S. Green, Spectrochim. Acta 26A (1970) 1503. J.A. Draeger, Spectrochim. Acta 41A (1985) 607. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed., Wiley–Interscience, New York, 1986, 39. J. Kong, C.A. White, A.I. Krylov, D. Sherrill, R.D. Adamson, T.R. Furlani, M.S. Lee, A.M. Lee, S.R. Gwaltney, T.R. Adams, C. Ochsenfeld, A.T.B. Gilbert, G.S. Kedziora, V.A. Rassolov, D.R. Maurice, N. Nair, Y. Shao, N.A. Besley, P.E. Maslen, J.P. Dombroski, H. Daschel, W. Zhang, P.P. Korambath, J. Baker, E.F.C. Byrd, T. Van Voorhis, M. Oumi, S. Hirata, C.-P. Hsu, N. Ishikawa, J. Florian, A. Warshel, B.G. Johnson, P.M.W. Gill, M. Head-Gordon, J.A. Pople, J. Comp. Chem. 21 (2000) 1532. S. Portmann, H.P. Luthi, Chimia 54 (2000) 766. P. Flukiger, H.P. Luthi, S. Portmann, Weber, MOLEKEL 4.3, Swiss Center for Scientific Computing, Manno, Switzerland, 2000–2002. A.D. Becke, J. Chem. Phys. 98 (1993) 5648. C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. Unpublished mid-IR spectrum of thin film of neat liquid. ´ R. Kostic, ´ L.A. Gribov, I.E. Davidova, Phys. Rev. B 41 (1990) 10744. D. Rakovic, M. Baïtoul, J.P. Buisson, B. Dulieu, J. Wery, O. Chauvet, S. Lefrant, J. Chim. Phys. 95 (1998) 1311. C. La Lau, R.G. Snyder, Spectrochim. Acta 27A (1971) 2073. L.H. Little, G.W. Poling, J. Leja, Can. J. Chem. 39 (1961) 1783. R. Mattes, G. Pauleickhoff, Spectrochim. Acta 29A (1973) 1339. N.B. Colthup, L.P. Powell, Spectrochim. Acta 43A (1987) 317. R. Minkwitz, F. Neikes, Eur. J. Inorg. Chem. (2000) 2283. D.R. Gagnon, J.E. Capistran, F.E. Karasz, R.W. Lenz, S. Antoun, Polymer 28 (1987) 567.