Pulse radiolysis on radical anions of sigma-conjugated polymers, organopolysilane and organopolygermane

0146-5724/89 $3.00+ 0.00 Copyright © 1989PergamonPress plc

Radiat. Phys. Chem. Vol. 34, No. 4, pp. 587-590, 1989 Int. J. Radiat. AppL Instrum., Part C Printed in Great Britain. All rights reserved

PULSE RADIOLYSIS ON RADICAL ANIONS OF SIGMA-CONJUGATED POLYMERS, ORGANOPOLYSILANE A N D ORGANOPOLYGERMANE HIROSHI BANt, AKINOBUTANAKAl, NARIYUKIHAYASHI2, SEIICHITAGAWA2 and YONEHOTABATA2 ~NTT LSI Laboratories, Morinosato, Atsugi, Kanagawa 243-01 and 2The University of Tokyo, Tokai, Ibaraki 319-11, Japan Abstraet--Organopolysilane and organopolygermane solutions in tetrahydrofuran are investigated by pulse radiolysis. These a-conjugated polymers become stable radical anions by an electron transfer reaction involving solvated electrons. The extra charge of the polymer radical anions is thought to be delocalized on the polymer skeleton.

INTRODUCTION Recently, chemistry and physics concerning organopolysilanes have received much attention. The organopolysilane backbone is a linear chain of silicon atoms, while ordinal polymers have a chain of carbon atoms. Ultraviolet irradiation has easily scissored Si-Si bonds in the main chain (Trefonas et aL, 1983a). This photochemical reaction has received much investigation, because it is potentially important for use as a photoresist in microlithography (Miller et aL, 1986; Zeigler et al., 1985) and a photoinitiator in radical polymerization of vinyl monomers (West, 1986). Organopolysilanes become strong SiC ceramics by thermal pyrolysis (Yajima et al., 1976). Organopolysilanes exhibit long absorption bands at 300-360 nm (West, 1986). These absorption bands are attributed to the transition from a silicon a-orbital to an anti-bonding a-orbital in alkyl-substituted polysilanes (Takeda et al., 1986). In aryl-substituted polysilanes, n-character of aryl groups are mixed with the silicon a-orbital (Harrah and Zeigler, 1987; Loubriel and Zeigler, 1986; Takeda et al., 1986). As the chain length increases, polysilanes have shown a bathochromic shift (Trefonas et al., 1983a) which is usually observed .in a n-conjugated system. When they have been doped with strong acceptors such as AsFs, polysilanes become p-type semiconductors with conductance up to 1 S cm -~ (West et al., 1981). They have exhibited photoconductivity, with holes being carriers (Fujino, 1987; Kepler et al., 1987). Poly(methylphenylsilane) exhibits third harmonic generation, a nonlinear optical effect (Kajzar et al., 1986). These electronic properties mainly arise from the conjugation of a-bonds in the silicon skelton, and -character of aryl pendant groups contributes to the enhancement or the stability of these properties. a-Bonds in the usual carbon polymers are not con587

jugated, however, molecular orbital (MO) calculation has indicated that a-orbitals for the polysilane silicon skeleton are conjugated (Takeda et al., 1986). Thus, they can be called a-conjugated polymers. Organopolygermanes (Trefonas and West, 1985) are also a-conjugated polymers whose main chains consist of Ge-Ge bonds. They exhibit almost similar physical and chemical properties to polysilanes. The study on polygermanes may clarify the fundamental characteristics of a-conjugated polymers. The excited or charged states of the a-conjugated system are interesting in connection to the electronic properties such as conductivity. These states are also important as the intermediates in pattern fabrication by electron beam microlithography. We have already reported the observation of polysilane radical anions (Ban et aL, 1987) and the effect (Ban et al., 1988) of side chains using pulse radiolysis. This paper deals with the summary on polysilane radical anions and newly observed polygermane radical anion.

EXPERIMENTAL

Organosilane and organogermane polymers were synthesized according to the conventional method (Trefonas et aL, 1983b; Wesson and Williams, 1980), the dechlorination of dichloride monomers with molten sodium metal in refluxed toluene. Linear permethylsilane oligomers, CH3-(Si(CH3)2)n-CH 3 (n = 4, 5, 6; Kumada and Ishikawa, 1963) were also synthesized using N a - K alloy, and distilled under low pressure. Molecular structures of organopolysilanes and organopolygermanes are drawn in Fig. 1. Pulse radiolysis measurement was performed using the linear accelerator system (LINAC) in the University of Tokyo (Kobayashi et al., 1981, 1983; Tabata et al., 1986). Polymer solutions in tetrahydrofuran (THF) prepared on the vacuum line were irradiated

588

Hmosm BAN et al. R

R

R'

R'

Fig. 1. Molecular structures of organopolysilane and organopolygermane. R and R' are alkyl or aryl groups. with a 35 MeV electron pulse at room temperature, and transient ultraviolet (UV) absorption spectra were recorded• The pulse width was 2 ns and the data was recorded for 200 or 500 ns in ordinal investigation in nanosecond experimental regime. A new pulse radiolysis system (Tabata et al., 1986) was also employed to observe the primary process in the picosecond regime. RESULTS

AND

DISCUSSION

Tetrahydrofuran solutions of polysilanes was irradiated with electron pulses (Ban et al., 1987). Figure 2 shows typical transient absorption spectra observed for 5 mM poly(methylpropylsilane) (PMPrS) solution. A strong absorption band of PMPrS radical anion with a peak at 363 nm gradually grew after an electron pulse, and no other intense absorption bands were observed in the wavelength region between 300 and 700 nm. This assignment of the PMPrS radical anion was confirmed by electron transfer from the radical anion to chloroform and pyrene. PMPrS radical anion was produced by the reaction of PMPrS with the solvated electron. The apparent rate constant of this reaction, k was (3.4+0.5) x 109M -[ s -l in the following reaction: P + e~l

,

P'-

(1)

where P and e~o] denote PMPrS and the solvated

0.6

electron, respectively• Each charged polymer chain possesses only one excess electron, because the concentration of PMPrS radical anion was quite low compared with that of PMPrS (Ban et al., 1988). Various polysilanes similarly became radical anions, for example, poly(methylphenylsilane) (PMPS), dimethylsilane-methylphenylsilanecopolymer and so on. Polysilane radical anions scarcely decayed in 500 ns. Irie et al. (1988) reported that the half-life of a polysilane radical anion in methyltetrahydrofuran is ca 30/as. The primary generation process of PMPrS radical anion was observed using a twin LINAC system with 18 ps time resolution (Tabata et al., 1986). Figure 3 shows that the optical density at 370 nm gradually increased after an electron pulse. The rate constant of this growth was estimated to be 3.8 x 10 9 M -~ s 1. This rate constant coincides with that obtained for the nanosecond experimental regime. The solid curve in Fig. 3 represents the simulation using the rate constant of 3.8 x 10 9 M ~s -1 in equation (1). The solid curve fits the data points well, confirming that this rising curve represents the primary stage of the electron transfer reaction from the solvated electron to PMPrS. The effect of silicon chain length on the transition bands of the radical anions was investigated. T H F solutions of linear permethylsilane oligomers, ca3-(Si(Ca3 )2)~---CH3(n = 4, 5, 6) were irradiated by 2 ns electron pulses. Transient spectra of their radical anions were shown in Fig. 4. As the chain length increased from tetramer to hexamer, the absorption peaks shifted to longer wavelength, from 310 to 315 and 320 nm, and the absorption intensities increased. These absorption maxima and intensities tend to approach those of high molecular weight PMPrS radical anion, which is a typical example of aklylsubstituted polysilane radical anions. The delocalization of the extra charge probably cause this bathocromic shift. Neutral polysilane absorption bands of tr-a* transition exhibit a bathocromic shift as the length of silicon chain increases (Trefonas et al., 1983a). This shift saturates at the silicon chain

>,

,~¢n r..

0.4

0.4

• ."......, •.;.~

._~

=

0 ~D

-t;e-.,

y

,

0.2 ,..

m

.r'

~ " "i. .':" 4-

=~ 0

340

360

300

wavelength /

400 nm

Fig. 2. Transient absorption spectra observed for 5 mM poly(methylpropylsilane) solution in tetrahydrofuran at 5 (O), 30 (O), 55 (A), 300 (A) ns after an electron pulse. Cell length is 1cm.

0

I

I

i

I

0

0.5

1

1.5

time

/ns

Fig. 3. Primary process of the generation of poly(methylpropylsilane) radical anion observed with the twin LINAC system.

Radical anions of a-conjugated polymers

0.8

Table 1. Absorptionmaxima(Am.)and extinctioncoefficients(~)° of organopolysilanesand their radical anions Polymers Radical anions ~.,,ffi, ~ ( x 103 ).~,~ ¢ ( x 105 (nm) M-Icm -I ) (rim) M-' cm-') PMPrSb 306 5.9 363 1.6 PPnMS 305 6.6 357 2.1 DMS-MHxS 305 6.5 356 1.8 DMS-MPrS 302 5.2 356 1.6 DMS-PnMS 303 6.8 355 1.5 DMS-MPS 331 7.0 370 1.6 PMPS 340 11.0 372 1.6 DPS-DMS 340 5.0 ca 380 1.4 DPS-MPrS 352 6.0 ca 385 1.4 DPS-PnMS 356 8.4 390 1.4 °Extinction coefficients are defined as per average Si unit for polymers and per added electronfor radicalanions. bAbbreviationsare referred in the text.

0.7 0.6 .-

o.s

"o

0.4

f

I 0

0.2

0.1 0

300

350

589

400

wavelength

450 /

nm

Fig. 4. The effect of silicon chain length on the absorption spectra of the radical anions. (©) poly(methylpropylsilane) with degree of polymerization equal to 2400; (0) permethylsilane hexamer; (A) permethylsilane pentamer; (A) permethylsilane tetramer.

MPrS), and poly(diphenylsilane-co-fl-phenethylmethylsilane) (DPS-PnMS) showed their radical anion absorption bands at 380-390 nm. Absorption maxima of polysilane radical anions containing phenethyl groups such as poly(fl-phenethylmethylsilane) (PPnMS) and poly(dimethylsilane-co-flphenethylmethylsilane) (DMS-PnMS) lay around 360 nm, the region for the alkylpolysilane radical anions. PPnMS and DMS-PnMS radical anions behaved like alkylpolysilane radical anions with regard to the absorption maximum, although they possess phenyl groups in side chains. This indicates that the phenyl groups indirectly attached to the main chain does not influence the absorption bands. The effect of phenyl groups on the absorption bands was clearly observed. The extinction coefficients were also determined to be 1.4-2.1 x 105 M -l cm -j on the basis of pyrene radical anion, with an experimental error of 10-20% (Ban e t a l . , 1988). Poly(dihutylgermane) (PDBG), a typical organopolygermane, was investigated in a similar way. A strong absorption band was observed after an electron pulse as shown in Fig. 5. This absorption band

length equal to c a 30 Si-Si bonds. In Fig. 4, there was considerable difference in absorption maximum and insensity between PMPrS and hexamer radical anions. Saturation of the bathocromic shift for polysilane radical anions seems to require a much longer silicon chain than the hexamer. This suggests that the extra charge of the polysilane radical anions can be delocalized on the relatively long silicon chain. Hexamethyldisilane (dimer) and dodecamethylcyclohexasilane (cyclic hexamer) were also examined. However, their radical anions could not be observed by our pulse radiolysis measurement at the room temperature, although Carberry e t al. (1969) reported that cyclic organosilane oligomers such as (Si(CH3)2) . (/1 = 4--6) can be chemically reduced to radical anions by using alkali metal at the low temperature. The extra charge of polysilane radical anions is thought to be essentially on the silicon chain, however, it interacts with aryl side chains. Absorption maxima and extinction coefficients of radical anions of organosilane homopolymers and copolymers are summarized in Table 1. Polysilanes with alkyl sidechains such as PMPrS, poly(dimethylsilane._.q methylcyclobexylsilane) (DMS-MHxS), and poly(dimethylsilane-co-methylpropylsilane) (DMS-MPrS), o 0.1 showed their radical anion absorption bands at 355-363 nm. Polysilanes containing methylphenylsilylene units such as PMPS and poly(dimethylo I I I I 350 400 450 500 silane-co-methylphenylsilane) (DMS-MPS) showed their radical anion absorption bands at c a 370 nm. wave length / nm Polysilanes with dipbenylsilylene units such as Fig. 5. Transient UV absorption spectra of poly(dibutylpoly(diphenylsilane-co-dimethylsilane)(DPS-DMS), germane radical anion immediately (©) and 50 ns (Q) after poly(diphenylsilane-co-methylpropylsilane) (DPSan electron pulse.

0.4

0.3

e-

"o 0.2 m 0~

590

HIROSH]BAN et al. (a)

(b)

==

0= e~ ¢o

lOns time

2""

Fig. 6. (a) Growth of the absorption of poly(dibutylgermane) radical anion in tetrahydrofuran monitored at 400 nm. (b) Decay of the absorption of poly(dibutylgermane) radical anion due to being scavenged by 29 mM tetrachloromethane in tetrahydrofuran.

lasted a long time in THF, and decayed on addition of a small amount of carbontetrachloride as shown in Fig. 6. This absorption band was also scavenged by pyrene, and pyrene radical anion was produced. From these observations, this absorption band was assigned to the PDBG radical anion. Based on the similarity of polysilane and polygermane, we believe that the extra charge on PDBG radical anion is delocalized on the main chain of germanium atoms. The absorption peak of PDBG radical anion lying at 405 nm was 78 nm longer than that of PDBG in neutral state, 327 nm. This difference in absorption wavelength between the radical anion and the neutral state was relatively large compared with polysilanes. PDBG radical anion was characterized as having a longer absorption maximum and a broader spectrum shape than polysilane radical anions. Absorption bands of alkyl and aryl polysilane radical anions had full width at half maximum of ca 30 and ca 40 nm, respectively. However, the figure for PDBG was ca 50 nm. In the case of polysilane radical anions, the absorption maximum became longer and the spectrum shape became broader due to the content of phenyl pendant groups. In the case of PDBG radical anion, it has only alkyi group and exhibits a longer and broader absorption band. Thus, the extra charge on PDBG radical anion is more delocalized than that on polysilane radical anions. This may arise from the difference in the property of the a-conjugation, that is composed of Si3p orbital in polysilanes and Ge4p orbital in polygermanes. CONCLUSION a-Conjugated polymers, organopolysilanes and the organopolygermane were investigated by pulse radiolysis. Both became polymer radical anions by the electron transfer reaction involving a solvated electron. Absorption bands of polysilane radical anions exhibited a bathocromic shift as the length of the silicon chain increased. Saturation of this bathocromic shift seems to require much longer silicon chain than the hexamer, compared

with the absorption band of high molecular weight poly(methylpropylsilane) radical anion. This suggests that the extra charge can be delocalized on relatively long silicon chain. Absorption maxima also shifted to longer wavelength due to the electronic interaction between phenyl pendant groups and the silicon main chain. Poly(dibutylgermane) radical anion had an absorption maximum at 405 nm. It was characterized as having a longer absorption maximum and a broader spectrum shape compared with polysilane radical anions. This probably arose from the difference in the property of tx-conjugation, that is composed of Si3p orbital in polysilanes and Geap orbital in polygermanes. Acknowledgement--The authors wish to express their

appreciate to Ken Sukegawa for his useful discussions and advice. REFERENCES Ban H., Sukegawa K. and Tagawa S. (1987) Macromolecules 20, 1775, Ban H., Sukegawa K. and Tagawa S. (1988) Macromolecules 21, 45. Carberry E., West R. and Glass G. E. (1969) J. Am. Chem. Soc. 91, 5446. Fujino M. (1987) Chem. Phys. Left. 138, 451. Harrah L. A. and Zeigler J. M. (1987) Macromolecules 20, 601. Irie S., Oka K. and Irie M. (1988) Macromolecules 21, ll0. Kajzar F., Messier J. and Rosilio C. (1986) J. AppL Phys. 60, 3040. Kepler, R. G., Zeigler J. M., Harrah L. A. and Kurtz S. R. (1987) Phys. Rev. B35, 2818. Kobayashi H., Ueda T. Kobayashi T., Tagawa S. and Tabata Y. (1981) Nuel. Instrum. Meth. 179, 223. Kobayashi H., Ueda T., Kobayashi T., Washio M., Tagawa S. and Tabata Y. (1983) Radiat. Phys. Chem. 21, 13. Kumada M. and Ishikawa M. (1963) J. Organometall. Chem. 1, 153. Loubriel G. and Zeigler J. M. (1986) Phys. Rev. 1333,4203. Miller R. D., Hofer D., Fickes O. N., Wilson C. G., Marinero E., Trefonas P. III and West R. (1986) Polym. Eng. Sci. 26, 1129. Tabata Y., Kobayashi H., Washio M., Yoshida Y., Hayashi N. and Tagawa S. (1986) J. RadioanaL Nucl. Chem. 101, 163. Takeda K., Teramae H. and Matsumoto N. (1986) J. Am. Chem. Soc. 108, 8186. Trefonas P., West R., Miller R. D. and Hofer D. (1983a) J. Polym. Sci. Polym. Lett. Ed. 21, 823. Trefonas P. III, West R., Miller R. D. and Hofer D. (1983a) J. Polym. Sci. Polym. Lett. Ed. 21, 823. Trefonas P. III, Djurovich P. I., Zhang X.-H., West R., Miller R. D. and Hofer D. (1983b) J. Polym. Sci. Polym. Lett. Ed. 21, 819. Wesson J. P. and Williams T. C. (1980) J. Polym. Sci. Polym. Chem. Ed. 18, 959. West R. (1986) J. Organometall. Chem. 300, 327. West R., David L. D., Djurovich P. I., Stearley K. L., Srinivasan K. S. V. and Yu H. (1981) J. Am. Chem. Soc. 103, 7352. Yajima S., Omori M., Hayashi J., Okamura K., Matsuzawa T. and Liau L. (1976) Chem. Lett. 551. Zeigler J. M., Harrah L. A. and Johnson A. W. (1985) SPIE Ad. Resist Technol. Process. I1 539, 166.