Synthesis, electrical and optical properties of asymmetrically monoalkoxy-substituted PPV derivatives

Synthetic Metals, 55-57 (1993) 908-913

908

SYNTHESIS, ELECTRICAL AND OPTICAL PROPERTIES OF ASYMMETRICALLY MONOALKOXY-SUBSHTUTED PPV DERIVATIVES

H.K. SHIM*, D.H. HWANG and J.I. LEE Department of Chemistry, Korea Advanced Institute of Science and Technology, Taejon (Korea) K.-S. LEE Department of Macromolecular Science, Han Nam University, Taejon 300-791 (Korea)

ABSTRACT Poly(2-methoxy-1,4-phenylenevinylene),

PMPV,

and

its

copolymers

containing

both

unsubstituted and 2-methoxy-l,4-phenylenevinylene (MPV) units were prepared in thin films from their water-soluble proecursor polymers, and monobutoxy- and monododecyloxy-substitutcd PPV derivatives were also prepared via the water-soluble precursor method. Drawn films (L/L o = 6) of PMPV

could be

doped with I2 vapor and

FeC13 to give conductivities of 3.2 and 4.0

Scmq, respectively. Conductivity values of long alkoxy-subsituted PPVs were decreased by increasing the carbon number of mono substituted alkoxy side chain. Conductivity of FeCI 3 doped copolymer films ranged from 1-100 Scm-1 depending on the composition of the copolymcrs. The third-order nonlinear optical susceptibility, Z(3)(3to ; to, to, to) for PMPV thin film has been investigated by degenerate four-wave mixing technique using 400 femtosecond pulses at 602nm. A relatively large 3rd-order susceptibility value (Z(3)

=

7.9 x 10"10 esp,) with a subpicosecond response was

observed. INTRODUCTION High molecular weight poly(1,4-phenylenevinylene), PPV, and its derivatives can be obtained in film form via water-soluble sulfonium salt precursor polymers. 1-3 The PPV derivatives are attracting much interest as conductive materials after doping with various dopants as well as materials for non-linear optical applications because their delocalized polyconjugated structures arc expected to possess fast response time and large third-order optical st,sccptibility. 4'5 A PPV derivative such as poly(2,5-dimethoxy-l,4-phenylenevinylene), PDMPV, shows easy dopability to 12 vapor or FeCI 3 leading to high conductivities. 6 The presence of strong electron-donors leads to a reduction in band gap and ionization potential of the conjugated polymers, to facilitate oxidation with weak oxidants such as 12 and FeCI 3. Conjugated polymers have received much recent attention due to their exciting nonlinear optical properties, and their advantages as materials for nonlinear optical applications. 7 In addition, they have mechanical strength, environmental stability, high optical damage threshold, and low optical losses. 037%6779/93/$6.00

© 1 9 9 3 - Elsevier Sequoia. All rights reserved

909

In this paper, we describe the syntheses of poly(2-methoxy-l,4-phenylcnevinylene), PMPV, and copolymers containing both unsubstituted (PV) and 2-methoxy-l,4-phenylenevinylene(MPV) units. We also prepared poly (2-n-butoxy-l,4-phenylenevinylene), PBPV, and poly(2-n-dodecyloxy1,4-phenylenevinylene), PDPV. Especially, we observed relatively large value of the third-order susceptibility, Z(s)(3w ; ~o, 0~,w) for PMPV thin film, which are closely tied to the degree of w-electron delocalization.

The

synthetic route and structures of monoalkoxy-subsitutcd PPV

derivatives and PV-MPV paired copolymers are shown below.

OCH3 (R)

OCH3 (R)

NBS, BPO

CH3/~--"~~}~CH3

CCI 4 / reflux

BrCH2~

CH2Br

MeOH, 50°C

1

OCH3 (R) ~SCH2/_...~CH2S+~ Br"

Br

/ OCH3 (R) 1.OH', 0°C 2. dialysis, 3days

film -

-

casting

-E-O-c.2--:

Br S

I9

3

2

zX / VAC

E _ ~ OCH3(R)

CH=O4--~n

210 °C, 20 h

PMPV

r PBPV: R = oc41--19 G L PDPV : R = OC12H2s-I

OCH3 Poly (PV-co-MPV)

EXPERIMENTAL Monomer synthesis The bisbromomethyl compounds, 1, were prepared by reacting the corresponding 2-alkoxy-pxylene with N-bromosuccinimide (NBS) in CC14. The sulfonium salt monomers, 2-alkoxy-l,4phenylenedimethylene

bis(tetrahydrotbiophenium

bromide),

2,

were

obtained

by

reacting

bisbromomethyl compound with excess tetrahydrothiophene for 20hrs. at 50°C in methanol. Pure salt monomers were obtained by concentration of the reaction solutions, precipitation in cold acetone, filtration, and vacuum drying. The products were white crystalline powder, and were very hygroscopic. Polymerization The sulfonium salt monomers, 2, were polymerized in a NaOH solution (1.0M) at 0°C for lhr under nitrogen atmosphere to obtain precursor polymers. A homogeneous viscous solutions were obtained. For the copolymers, the total moles of the two monomer salts were kept constant,

910

but their mole ratio was varied. The solutions, after neutralized with 1.0N-HCI, were dialyzed using a dialysis tube (Sigma) with a molecular cut-off at 12,000. Films were cast from these solutions. The precursor

polymer

films were

subjected to

thermal treatment in vacuum at

210°C for 20hrs. to transform into the final polyconjugated polymer films. The thin films for measuring third-order susceptibility, Z(3)(3to ; to, to, to) were coated onto fused quartz by spin coating technique. Analysis and measurement 1H-NMR spectra of the monomers were recorded on a Brucker AM 300 spectrometer. FFIR

spectra of the polymers were recorded on Bomen Michelson series FI'-IR sepetromctcr.

Differential scanning calorimetry (DSC) and thermogravimetry (TGA) were perfomed at a heating rate of 10°C/min with DuPont 9900 analyzer. UV-visible spectra were obtained with a PerkinElmer spectrophotometer. Conductivities were measured on a rectangular piece of film on a fourin-line probe unit on which resistance measurements were performed using a Keithley 197 digital multimeter. The third-harmonic generation measurement in PMPV thin film using a degenerate four-wave mixing (DFWM) technique was performed on a continuous wave Nd:YAG laser and tunable dye laser. The dye pulses were amplified by frequency-doubled pulses from a 30Hz O-switched pulsed Nd:YAG laser to generate around 400fs nearly transform-limited pulses with the encrgy of 0.4 mJ at 602nm. The peak power was 400MW/cm 2. RESULTS AND D I S C U S S I O N Thermal analyses of the elimination reaction of the precursor polymers indicated that major weight losses occurred at about 100°C and 190°C. The lower temperature loss corresponds to the

loss

of

water

and

the

higher

temperature

one

to

the

elimination

of

HCI

and

tetrahydrothiophene. The IR spectra of the precursor polymers showed sharp absorption at 3023 and 96(/cm -1, which correspond to stretching mode and the out-of-plane bending mode of trans vinylene = C H groups. These absorption bands must result from partial elimination of HCI and thiophene during film casting.

The films

of the

final copolymers

exhibited

sharp and

strong IR absorption

at 960cm 1 indicating the vinylene C = C bonds were entirely of the trans configuration. This means that polymer main chains are transformed to polyconjugated structures. Fig. 1 shows UV-visible spectra the fully eliminated films of PMPV, PBPV, PDPV and PVMPV copolymers. The broad, long wavelength absorptions with maxima around 370-460 nm are due to 7t-'n* transitions of the polyconjugated systems. The bathochromic shift by the methoxy groups on phenylene rings in PV-MPV copolymers is significant and this can be explained by the electron-donating effect of the substituents. The profiles of the absorption spectra of PBPV and PDPV are similar to that for PMPV. We observed that there is no trend of maximum absorption position according to the length of alkoxy side chains. It is explained that maximum absorption position can be attributed to the differences in molecular weights of the polymers and also of the

degree of elimination.

(602nm) used in DFWM experiment.

The

PMPV

sample is nonabsorbing at the wavelength

911

I

C

a

R B A

E

a

I

300

I

I

400

I

I

500

i

I

600

nm

Fig. 1. UV-VIS spectra of polyconjugated polymers of : a) PMPV, b) 75-poly(PV-co-MPV), c) 42-poly(PV-co-MPV), d) PBPV, and e) PDPV. The degree of monomer conversions (55-98%) and the polymer yields (20-63 Wt.%) after dialysis are comparable to or slightly higher than those reported for other similar polymerization systems. 2'8 All of the precursor polymer films cast from aqueous precursor solutions appeared homogeneous and transparent. Table 1 summarizes the data for compositions of the final COlX)lymers, draw ratios and the maximum conductivity values. The compositions of the final copolymers were calculated from oxygen contents. The content of MPV units incorpnratcd in the copolymers was significantly higher than that in feed, suggesting that MPV-monomer, 2, is more reactive than unsubstituted PV-monomcr. On doping with FeCI 3 and 12 vapor, the polymer films became black and highly electroconductive. Conductivity values of asymmetrically monoalkoxysubstituted PMPV, PBPV and PDPV are quite lower than that of PDMPV. Monoalkoxy-substitutcd PPVs lower molecular packing ability owing to their asymmetric polymer structures. Ahhough these polymers contain one electron-donating alkoxy substituents, their conductivity values arc not high because of their low packing ability. This means that conductivity is significantly influenced by the chain contact because of the hopping mechanism. However, these monoalkoxy-subsitutcd PPVs have a great advantage of high drawability. They can be drawn up to 6-10 times their initial length at the temperature range of 110-120°C using a zone-heating apparatus. Whereas, PDMPV could not be drawn to any extent during or prior to the elimination reaction. Conductivity of FeCl3-doped copolymer films ranged from 10°-102 ~ m -1 depending on composition. As the content of MPV unit in the copolymer increased, the electrical conductivity steadly dccrc~lscd

912 T A B L E 1. Conductivities of the Doped Polymers

PV : MPV

PV : MPV

In Feed

Actual

Draw Ratio L/Lo

Conductivity, Scm -1 FeCl 3, I2

Polymers a

Doping Ratio, FcCI4-/RU b

92-poly(PV-co-MPV)

17:83

8.2 : 91.8

1 6.5

8.3x10 -1 7.0

0.72 0.99

75-poly(PV-co-MPV)

50:50

25.0 : 75.0

1 6.5

3.6 23.1

0.57 0.71

42-poly(PV-co-MPV)

83:17

58.3 : 41.7

1 7.5

10.1 31.2

0.95 1.38

27-poly(PV-co-MPV)

91:9

73.2 : 26.8

1 8.0

2.1 97.0

0.89 1.05

PMPV

1 6.0

2.8x10 -1 4.0

1.3 3.2

PBPV

1 6.0

3.2x10 -2 1.8

1.0 6.6

PDPV

1 10.0

8.6x10 -3 1.2x10 -2

2.7x10 -4 2.2x10 -3

aThe numerical values stand for mole % of MPV units in final copolymers. bRU stands for average repeating unit.

as shown in Table 1. The drawn copolymer film containing 27 mole % of MPV unit, 27-poly (PV-co-MPV), when doped with FeCI3, showed m a x i m u m value of conductivity of - 1 0 2 Scm -1. Fig. 2 shows the subpicosecond degenerate four-wave mixing (DFWM) response of PMPV thin film. The Z(3)(3to ; t0, to, t0) value of PMPV thin film was evaluated by comparing the strength of the conjugated D F W M signal at low incident photon flux with that of CS 2 according to the following relationship; Zs (3)

¢(3)

=(

ns

I¢ )2___

n¢

gS

( _I_s ) ~ I.¢

a t:

exp(-, e/2)il-exp(-,, 60]

where n is refractive index, 1 is the incident length, a is the linear absorption coefficient and I is the intensity of D F W M signal. The subscript c and s refer to CS2 and the sample, respectively. The refractive index was determined by a Rudolph Ellipsometry (Auto El-IV). The value of Z (3) =

6.8 x 10 -13 esp. was used as the reference value for CS29. The measured Z (3) value

for PMPV was 7.9 x 10 -l° esp..

913 0.100. 0.080-

o*~*J,#

tO

"~ 0.060-

• •g,

,"

~ 0.040 T CD

0"020T 0.000/ -2

I

-1 0 Delay Time (ps)

I

1

2

Fig. 2. DFWM signal absorbed for PMPV as a function of the forward beam delay (wavelength 602 nm, 400 fs pulses).

Wc conclude that the PPV derivatives are attractive as nonlinear materials, duc to thcir relatively large nonlinearity, durability, stability, ease of synthesis and processing, and the possibility of tailoring the band gap. ACKNOWLEDG EMENTS It is gratefully acknowledged that this rescarch was supported by thc Korea ~icncc amt Engineering Foundation. REFERENCES 1. I. Murase, J.D. Capistran,

D,R. Gagnon and R,W. Lenz, Mol. Cryst., Lig. Cryst.,

118

(1985) 327. 2. H.K. Shim, R.W. Lenz and J.I. Jin, Makromnl.

Chem, 190 (1989) 389.

3. J.I. Jin, C.K. Park and H.K. Shim, J. polym: Sci., Polym Chem., 29 (1991) 93. 4. T. Kamo, K. Kubodera, S. Tomaru, T. Kurihara, S. Saito, T. isutsui and S. Tokito, Electron. Lett., 23 (1987) 1095. 5. C J . Wung, K.S. Lee, P.N. Prasad, C.K. Park, H.K. Shim and J.l. Jin, unpublished rcsuhs. 6. C.C. Han, R.W. Lenz and F.E. Karasz, P01ym. Commun., 28 (1987), 261. 7. D. McBranch, M. Sinclair, A.J. Heeger, A.O, Patil. S. Shi, S. Askari and F. Wudl, Synth. Met., 29 (1989) E85-E90. 8. J.I. Jin, H.J. Kang and H.K. Shim, Bull. Korean Chem. S(x:., 11(5) (1990) 415. 9. N.P. Xuan, J.L. Ferrier, J. Gazengel, and G. Riw)rie, Opt. Commun., 51 (1984) 433.