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Dopant-polymer interaction: MoCl5-doped polyacetylene
Synthetic Metals, 26 (1988) 225 - 236
225
D O P A N T - P O L Y M E R I N T E R A C T I O N : MoCIs-DOPED P O L Y A C E T Y L E N E HSIEN-MING WU and SHOW-AN CHEN* Chemical Engineering Department, National Tsing-Hua University, Hsinch, Taiwan 30043 (Republic of China)
(Received June 1, 1988; accepted July 14, 1988)
Abstract
Po ly acety len e is dope d with MoCls. The doping levels determined using the m e t h o d of n e u t r o n activation analysis indicate that the atom ratio [C1]/[Mo] varies f r o m 5 to 1.6, depending on doping time. The Cls XPS (X-ray p h o t o e l e c t r o n spectroscopy) binding energy (BE) decreases from 283.7 eV for the pristine PA to 282.9 eV, similar to t hat of WC16-doped PA as r e p o r t e d in our previous work, but opposite to that of I2- and AsF 5d o p e d PA. Th e lowering of the BE means that the outer shell electron densities of carbon increase after the doping. The Cl:p XPS of the doped PA shows that there is only one t y p e of chlorine in the adducts. Mo3a XPS shows that there are two states of Mo existing in the doped PA, being state A (MoCI:(C=C)4) and state B (Mo(C=C)n), quite different from those in the WC16-doped PA. T he level created at 1.72 eV related to state B is closer to the HOMO 7r orbital (BE 1.8 eV) of the pristine PA, and is therefore a more shallow level than the state created at 1.24 eV (due to state A Mo). Several olefins f r om more than one PA chain can interact with one Mo atom, allowing electron transfer f r om one PA chain to another.
Introduction Po ly acety len e (PA) film can be doped to become a semiconducting or conducting material and to have various electrical properties, depending on the doping level, t he nature of the d o p a n t and on the interaction between the d o p a n t and PA [1]. Among the dopants used for PA, I: and AsF s are the most studied. XPS studies [2, 3] on these two types of doped PA indicated a charge transfer f r om PA to the dopant, leading to an increase in Cls binding energy (BE). For PA dope d with transition metal halides (WC16 and MoCI5), which was first studied by Rolland and Aldissi [4], electron back-donation f r o m W and Mo in the adduct to olefin and C1- ligand was f o u n d to occur, opposite to t hat of I:- and AsFs-doped PA, as report ed in *Author to whom correspondence should be addressed. 0379-6779/88]$3.50
(~) Elsevier Sequoia/Printed in The Netherlands
226 our previous work [5, 6]. In this work, the d o p a n t / p o l y m e r interaction in the MoCls-doped PA is explored by the use of XPS with detailed analysis, neutron activation analysis and electron spin resonance (e.s.r.). The MoClsdoped PA is found to have two states created at BE 1.24 eV and 1.72 eV, quite different from the WC16-doped PA [6].
Experimental
PA films were prepared using the Shirakawa technique [7] with Ti(OBu)4-Al(Et)3 (mole ratio A1/Ti = 4) as catalyst at --78 °C and using toluene for successive washings. Characterization of the film using i.r. spectroscopy, elemental analysis, density measurements and differential scanning calorimetry has been reported in a previous paper [8]. The PA produced w i t h o u t further treatment has 90% cis and 10% trans content. The PA films were doped at room temperature by first evacuating to remove the trapped gas in the film and then injecting a 4.6 × 10 -4 M MoCls solution in toluene. The doped PA films were subsequently removed from the doping solution and washed with toluene several times. These films were then subject to analysis. The doping level in each PA film after various doping times was determined using neutron activation analysis, by which C1 and Mo contents were determined separately. High purity Mo (99.99%) and NH4C1 (99.8%) were used as standards. The neutron flux used to irradiate the samples, generated from the water pool nuclear reactor of Tsing-Hua University, was 1012 n c m - : s-~. Intensities of the 7-ray irradiation from Mo 99 and C13s in the samples and standards were measured using a multichannel pulse height analyser (Tracor Northern Model TN-1710) with a 50 cm a Ge(Li) 7-energy detector. 7-energies of the isotope C1as are identified at 1642 keV and of isotope Mo 99 at 141 keV. Comparing the counts of the sample with the standards at these two energies, the amounts of Mo and C1 in the samples can be determined. The resolution of the multichannel pulse height analyser used is high, i.e., 3.4 keV for the Co 6° 7-energy 1332.4 keV. X-ray photoelectron spectra of the doped PA films were measured using a Perkin-Elmer model 1905 photoelectron spectrometer equipped with a m o n o c h r o m a t i z e d Mg K a X-ray source (1253.6 eV photons). Calibration of the binding energies was made with reference to a gold target (Au4~7/: = 83.8 eV). In order to assure that there was a good electrical contact between the sample and the spectrometer and that there was no charge effect on the sample, gold powders were sprayed on the sample surface for calibration. In all cases, the BE of Au4~7/2 was 83.8 + 0.1 eV. Electron spin resonance measurements were performed at X-band (9.5 GHz) on a Bruker model 200D 10/12 spectrometer. DPPH (diphenyl picryl hydrazyl) was used as a calibration standard.
227
Results and discussion N e u t r o n activation analysis
By use of the neutron activation analysis method, Mo and C1 contents can be measured in the doped PA. The % weight uptake and atomic ratio [C1]/[Mo] in the doped PA versus doping time are shown in Fig. 1. Before doping, the ratio is 5 for MoCls. The ratio drops to 2.5 after 15 min doping and to 2 and 1.6 after 2 h and 4 h doping respectively. The % weight uptake increases to 6% after 15 min doping and to 9% after 2 h doping, but decreases to 8.4% after 4 h doping. The chemical structure of the doped PA after 2 h doping would be of the d o m i n a n t form, [CH(MoC12)0.00S6]x, which corresponds to one Mo atom to 116 C atoms. E l e c t r o n spin resonance m e a s u r e m e n t s
The spin density of the pristine PA (90% cis content) was determined to be 3.74 X 10 -6 mol spin/g of PA (or 2.251 X 10 is e-/g of PA) at room temperature, which is between those of trans PA (1.44 X 1019 and 1.65 X 1019 e - / g of PA) and c/s PA (an order lower than the value of trans PA) [9]. Each e.s.r, spectrum of the doped PA was found to be a symmetrical Lorentz curve, indicating homogeneous doping. The spin density of the doped PA film versus doping concentration is shown in Fig. 2. The increase of the spin density of MoCls-doped PA is greater than that of AsF s- and WC16-doped PA at the same doping level, indicating the existence of a difference in electronegativity or d o p a n t / p o l y m e r interaction. The g values of the doped PA obtained from the e.s.r, measurement are shown in Table 1, and are close to that of the free electron, 2.0023. The AH value changes only slightly at initial doping levels in comparison to those of AsFs- and WC16-doped PA [9, 6]. Hence, the d o p a n t / p o l y m e r interaction in the MoC1 sdoped PA is expected to be different from that in the WCl6-doped PA.
o
:k
._c
g o~
C~ £]C
~o 2
o
o
l0
1
2
Y:
I 3
Time (hours)
Fig. 1. Atom ratios [C1]/[Mo] and weight uptakes in the MoCls-doped PA adducts at various doping times.
228
o
6I v
o
"a z~
2
® 123
6
.~-
4
& [CH(WCI2)y ] x o [CH ( A s F 5 ) y ] x o [CH (MoCI2)y] x
2 -6 10
I
i
n
2
4
I
I
6
I
I
I
I
8
I
10
Y Doping Concentrotion
I
I
12
I
14
(10-3- )
Fig. 2. Spin density vs. dopant concentration for MoCls-, WC16"and AsFs-doped PA.
TABLE 1 Results of e.p.r, measurements for the adducts Doping time (rain)
/~/pp (Gauss)
g value
0 15 30 60 120 240
7.0 6.5 6.5 6.5 7.0 7.0
2.0025 2.0044 2.0041 2.0041 2.0040 2.0041
X-ray photoelectron spectroscopy (XPS ) (I) Cls core level binding energy Detailed scans of C,s binding energies (BE) of the doped PA at various doping times are given in Fig. 3. T he Cls BE of the pristine PA is 283.7 eV and decreases rapidly to 282.7 eV at 15 min doping time, then gradually increases to a constant value of 282.9 eV. The decrease in C1~ BE indicates an increase of the o u t e r shell electron density of the carbon. The d o p a n t withdraws electrons f r o m the PA, but the carbon of the PA shows increased electron density. This peculiar p h e n o m e n o n is similar to t h a t o f WC16-doped PA [6]. According to the D e w a r - C h a t t - D u n c a n s o n model [10], the elect r o n back-donation exists between olefin segments of PA and Mo metal, and t h e Mo, in principle, can f o r m a double bond with the olefin segment o f PA, as was proposed by us in the case of WC16-doped PA [6].
229
{0) i(1) 12) (3) (4 ] {5)
ormglr',ol PA dopmng15 rain doping 30 mmn doping 1 hour doping 2 hrs doping &hrs.
f~ 15/ i L,I ]
//
/
/
1
i
1
i
'i \
//"
\
\ \
2913
289.3 287.3 285.3 283.3 2~.3
Binding Energy ( e V )
I Li 2793 27?.3 2753
238
236
234
232
230
228
226
22z,
Binding Energy (eV)
Fig. 3. C]s XPS spectra of the MoCls-doped PA adducts at various doping times. Fig. 4. MO3d XPS spectra of the MoCls-doped PA adducts at various doping times.
(3) MO3d core level binding energy The MO3d detailed scans of the MoCls-doped PA at various doping times are shown in Fig. 4. The Moad BE of pure MoCls has a spin-orbital splitting, giving two peaks at 230.8 eV and 234.1 eV. After 30 min doping, these two peaks both shift to lower values at about 229 eV and 232 eV respectively; in addition, there is a shoulder at a b o u t 227 eV. After.doping for more than one hour, these two peaks become shoulders, and two additional peaks at 227 eV and 230 eV are present. Deconvolution of these curves b y assuming a Gaussian shape gives separate peaks, as shown in Figs. 5(a) - (e), whose characteristic values are listed in Table "2. There are two different states of Mo in the d o p e d PA. State A has the MO3ds/2 BE changing from 229.1 eV (15 min doping) to 228.6 eV (4 h doping), and state B has MO3ds/: BE changing from 227.0 eV (15 min doping) to 227.5 eV (2 h doping) and then dropping to 227.3 eV (4 h doping). Since in MoCls, the Mo(V) is of high oxidation state and has a strong electron-withdrawing ability, the [C1]/[Mo] atomic ratio is reduced from 5 to about 1.6 after doping as indicated above. Nordling [11] has established an empirical correlation between measured BE (or chemical shifts) and an atomic charge parameter of the metal atom, qM- By the use of his
230 TABLE 2 Results o f MO3d5/2 a n d MO3d3/2 XPS spectra deconvolution of the adducts at various doping times Doping time (min): Curve No. in Fig. 4: Deconvolution in Fig. 5: MO3d state (BE): State A Peak centre (eV) AE (eV) FWHM (eV) Area (%)
0 0 MOads/2
MO3d3/2
15 1 (a) MO3ds/2
230.8
234.1
229.1
3.3 2.42 61.60
State B Peak centre (eV) AE (eV) FWHM (eV) Area (%)
2.00 38.84
3.1 2,14 46,73
227.0 3.4 1.48 13.62
MO3d3/2
30 2 (b) MO3d5/2
MO3d3/2
232.2
229.0
232.0
2.17 35.43
230.4 1.48 4.22
3.0 2.12 38.55
227.2 2.9 1.58 16.28
2.51 36.38
230.1 1.56 8.78
*FWHM: full width at half maximum.
correlation, the value of qMo as well as the oxidation state of the Mo in the adducts can be determined from the measured Mo3ds/2 BE. A plot of MO3d5/2 BEs of MoCl5, MoC14, MoC13 and Mo (values taken from [12]) v e r s u s their respective qMo yields a straight line, as shown in Fig. 6. From Fig. 6, state A (MO3ds/2 BE from 229.1 eV to 228.6 eV) has qMo ranging from 0.8 to 0.56, which corresponds to the products between MoCI:(C=C)4 (qMo = 0.9744) and MoCI(C=C)s (qMo = 0.6048), according to the prediction in Table 3. The [C1]/[Mo] atom ratio in the adducts obtained from the neutron activation analysis stated above is between 2.5 and 1.6, hence MoC12(C=C)4 complex would be the major end product of state A. State B has t h e MO3ds/2 BE ranging from 227 eV to 227.5 eV, and therefore has a qMo from --0.5 to --0.3 {Fig. 6), which means that there is a higher electron density around the Mo in the adduct than that of Mo(0). This situation often appears in an organometallic c o m p o u n d , such as [(ws-CsHs)Mo(CO)3] 2 having MO3ds/2 BE = 227.2 eV [12]. Since the contribution of one chlorine to qMo is a b o u t +0.4085, as can be obtained from the calculation by use of the Nordling empirical equation, thus chlorine is obviously absent in state B. The M o ( C = C ) , Ir complexes of state B will be the major species for prolonged doping, as indicated in the neutron activation analysis and XPS analysis.
(3) Cl2p core level T h e C12p d e t a i l e d s c a n s o f M o C l s - d o p e d P A a t v a r i o u s d o p i n g t i m e s a r e s h o w n in F i g . 7. T h e C12p B E o f p u r e MoC1 s h a s a s i n g l e p e a k a t 1 9 9 . 6
231
60 3 (c) M°3d5, 2
228.7 3.4 1.16 10.76
227.5 2.8 1.48 39.14
M°3d3, 2
120 4 (d) M°3d5,, 2
232.1
228.8
2.06 17.31
230.3 1.92 32,80
3.5 2,28 27,24
227.5 3.1 1.49 29.76
M°3d3/2
240 5 (e) M°3d5/2
MO3d3/2
232.3
228.6
232.1
2.05 17,34
230.6 1.82 25.66
3.5 1.88 21.00
227.3 3.1 1.47 35.00
Mo metal 6 M°3ds/2
M°3d3,, 2
227.8
230.9
1.77 16.00
230.4 1.48 28.00
3.1 1.15 57.05
1.47 42.94
eV, which can be assigned a s C12p3/2 BE [13]. After doping from 15 min to 4 h, the C12p BE shifts to about 196.4 eV, which is 3.2 eV lower than that of pure MoCls, indicating that the chlorine atom gains electrons after forming the adduct with PA and that there is only one type of chlorine in the adducts. Since C12p3/2 BEs of C1--C1 and C--C1 are about 205 - 207 eV, which are higher than that of metal--C1 (about 1 9 9 - 200 eV) [13], these peaks would not be from C--C1 and C1--C1 species, but due to stronger back-donation of Mo to C1, which increases the outer shell electron density of C1. The decrease in BE of C12p in the adducts in Fig. 7 indicates that there is (d-d)Tr bonding between the Mo and C1. This kind of (d-d) rr-backdonation is similar to the (d-Tr*) 7r-back-donation in the Mo and olefin segm e n t of PA.
(4) Valence levels The valence level spectra of PA, MoC1 s and the doped PA are shown in Fig. 8. The pristine PA valence level spectrum has a peak at 1.8 eV, which should be the highest occupied molecular orbital (HOMO) of the pristine PA [14]. The band gap of the pristine PA is then 1.8 eV here [14]. From the area percentage of these two states of Mo in Table 2, we know that the amounts of MO3d5/2 of state A, MoC12(C=C)4, dominate in the adducts at 15 min (82.16%) and 30 min (74.93%) doping. As doping time increases, the amounts of state B, Mo(C=C), 7r-complex, are larger than those of state A, being 71.94%, 55.43% and 63.0% after 1, 2 and 4 h doping re-
232
1 23/, (a)
1 I I 232 230 228 Binding Energy ( e V )
23Z, (C)
232 230 228 Binding Energy (eV)
t-226
I
236
I
234
I
232
I
230
I
228
~""~
226
(b)
I 23/,
226 (d)
I I I ~ 232 230 228 Binding Energy {eV)
:~--, 226
I / I
234 (e)
I
I
I. . . .
232 230 228 Binding Energy ( e V )
-~--
226
Fig. 5. Deeonvolution of MO3d XPS spectra of the MoCls-doped PA adduets at various doping times: (a) 15 min; (b) 30 min; (c) 1 h; (d) 2 h; (e) 4 h.
spectively. Thus the new peak at 1.24 eV, which is absent in the pristine PA and appears at 15 min and 30 rain doping, can be assigned to state A; the intensity of the latter is weaker than the former. The new peak at about 1.72 eV, which becomes m or e apparent as the doping time increases to
233 > >-
234
Ld
232 -
-
0A
~oC\%
0
0A
0.8
1.2
1.6
2.0
qM0 Fig. 6. Plot of MO3ds/2 binding energy posed by Nordling.
vs.
calculated charge using the correlation pro-
TABLE 3 Estimation of qMo of the Mo state A at several possible chemical structures Chemicala compound
qMo of MoCI n
qMo of Mo(C=C)6_ n
qMo of the adduct b
MoCls(C=C) MoC14(C=C)2 MoC13(C=C)3 MoC12(C=C)4 MoCI(C=C)s Mo(C=C)6 MoCl5 MoC14 MoCl3 MoCl2 Mo
0.4085 0.4085 0.4085 0.4085 0.4085 0 0.4085 0.4085 0.4085 0.4085 0
0.0392 0.0392 0.0392 0.0392 0.0392 0.0392
2.0832 1.7136 1.344 0.9744 0.6048 0.2352 2.044 1.6352
x 5 ×4 ×2 x2 ×1 ×5 ×4 ×3 ×2
×1 X2 ×3 ×4 X5 ×6
1.2264 0.8176 0
aElectronegativity: Mo:1.8; C1:3.25; C=C:2.2 (value taken from Thomas A. Carlson, Photoelectron and Auger Spectroscopy, Plenum Press, New York, 1975). C=C: olefin segment of PA. bqM o of the adduct is obtained by a summation of qMo values of MoC1n and Mo( C = C ) 6 - n ; a positive value of qMo means electrons were removed from Mo at the element state.
1 h a n d 4 h, c a n b e a s s i g n e d t o s t a t e B. T h e p e a k a t 1 . 2 4 eV d i s a p p e a r s after a long doping time. F r o m t h e d i s c u s s i o n o n X P S s p e c t r a o f Cls, MO3d a n d C12p a b o v e , it is k n o w n t h a t t h e 1 . 2 4 eV s t a t e is c r e a t e d b y t h e m o l y b d e n u m 4 d ( t 2 ~ ) o r b i t a l a n d l i g a n d o r b i t a l s , w h i c h i n c l u d e t h e o l e f i n e m p t y 7r* o r b i t a l s a n d c h l o r i n e e m p t y d o r b i t a l s ; a n d t h a t t h e 1 . 7 2 eV level is c r e a t e d b y t h e m o l y b d e n u m 4d(t2g ) o r b i t a l a n d t h e o l e f i n e m p t y n* o r b i t a l o n l y . T h e s e
234
o
"5
C5 UJ
,T, Z
202
200
198 196 19l* 192 Binding Energy (eV)
190
18{}
Fig. 7. C12p XPS spectra o f the M o C l s : d o p e d PA adducts at various times.
1(2) l(3) 1(4) 1(5) 1(6)
doping 15rain. doping 30rain. doping 1hr. doping 2hrs. doping 4hrs. MoCls
SF:OD07 SF:OD06 5F:0£)13 SF:0.012 5F:0.010 SF: 0.0094
~. '~!(~ I
8 6 Binding Energy (eV) Fig. 8. Valence level XPS spectra o f the MoCls-doped P A adducts at various doping times.
ligand orbitals all have fairly low electronegativity compared to the positively charged molybdenum(Mo(V)), so the ligand orbital will lie at a higher energy level than the corresponding m o l y b d e n u m 4d(t2g) orbital. Hence this created bonding ~ molecular orbital will resemble the molybdenum
235 \
elg . . . . . . . .
metol d-orbitols in Oh field
.-'.\ - ] - - i - e ig ~ _ _ -
I I /f empty ligond I^~I"I z'l d, I"I'* or b i t o l s
"~\ I \.------I~ ~.__.L
I
,/ •
///////////////////////////// moleculor
11" bond of PA
orbitols
Fig. 9. Energy level diagram for a ~" s y s t e m w i t h an a c c e p t e r ligand C1
a n d PA in the
MoCls-doped PA. (A 1 = 1.24 eV, A: = 1.72 eV). 4d(t2g) orbital m o r e t h a n the ligand orbital, and conversely the ~* m o l e c u l a r orbital will m o r e closely resemble the ligand orbital, as s h o w n in Fig. 9 (A] = 1.24 eV, A2 = 1.72 eV). T h e level created at 1.72 eV related to state B is closer t o t h e H O M O w orbital (1.8 eV) o f the pristine PA, and is theref o r e a m o r e shallow level state t h a n the created state at 1.24 eV (due to state A Me). Thus state B M e can m a k e the electrical activation energy, Ea, o f t h e d o p e d PA l o w e r t h a n state A Me. This is s u p p o r t e d b y the experi m e n t a l result o f Rolland et al. [4], in which E a shows a sharp decrease at d o p a n t c o n c e n t r a t i o n s , y, f r o m 0 to 0.015 (equivalent to 0 - 4 d o p i n g in o u r case). F r o m the present analysis and the WC16-doped PA e x p e r i m e n t [6], it can be inferred t h a t d i f f e r e n t d o p a n t s can c o o r d i n a t e with PA to yield a d d u c t s o f d i f f e r e n t s t r u c t u r e s and t h u s can create d i f f e r e n t d o p i n g a c c e p t e r level. H e n c e the M o C l s - d o p e d PA should have a m o r e shallow a c c e p t e r level with r e s p e c t to t h e H O M O w orbital (valence level BE 1.8 eV) o f the pristine PA t h a n the WC16-doped PA. This is in a g r e e m e n t with the electrical c o n d u c t i v i t y m e a s u r e m e n t s ; the f o r m e r has lower electrical activation e n e r g y t h a n the latter [4].
Interaction between MoCls and polyacetylene, and the proposed conduction mechanism In t h e MoCls-doped PA adducts, a f t e r doping, the C1 a t o m s in MoC15 can be s u b s t i t u t e d b y the olefins and the chemical s t r u c t u r e f o r m u l a of the a d d u c t s b e c o m e s [MOC12(C=C)4], and [ M o ( C = C ) . ] . Thus, parts o f the C1 a t o m s in t h e MoC1 s are eliminated. In the adducts, the lr e l e c t r o n o f the PA can pass t h r o u g h t h e o - b o n d f r o m the olefin segment of PA to the Me, and t h r o u g h t h e o - b o n d f r o m the Me t o the p w*-orbital o f the olefin. These b a c k - d o n a t e d e l e c t r o n s are located closer to the c a r b o n a t o m . In t h e Me--C1 b o n d i n g , t h e r e is a ( d - d ) n b o n d i n g with e l e c t r o n backd o n a t i o n f r o m the M e to t h e C1 as in the case o f (d-w*) w-back-donation in t h e Me and olefin. Because o f t h e a and w orbital overlap existing b e t w e e n a Me and olefins o f m o r e t h a n o n e PA chain, e l e c t r o n t r a n s f e r m a y be relatively easy t h r o u g h t h e a and w b o n d s t h a t c o n n e c t the d o p a n t and t h e olefins o f PA. So the transverse w-bonding c o n d u c t i o n m e c h a n i s m is p r o p o s e d to exist e x c e p t at light doping, w h e r e the soliton h o p p i n g m e c h a n i s m is fol-
236 l o w e d [1]. T h e transverse ~r-bonding c o n d u c t i o n m e c h a n i s m allows electrons to be t r a n s f e r r e d f r o m o n e PA chain to a n o t h e r t h r o u g h t h e m e t a l Mo bridge, m a i n l y because the m e t a l Mo can c o o r d i n a t e with m u l t i p l e t olefins to f o r m the diolefin and triolefin, etc., c o m p o u n d s with d i f f e r e n t PA chains.
Conclusion In M o C l s < l o p e d p o l y a c e t y l e n e (PA), it is f o u n d t h a t the p 7r-electrons o f t h e olefins can transfer t o the m e t a l d-orbital and t h e n b a c k - d o n a t e t o the p 7r* orbital o f t h e olefins, leading to a decrease in C is binding energy, and t h a t t h e electrons in the d orbital o f the m e t a l a t o m can also backd o n a t e t o t h e C1- ligands, leading t o a decrease in Cl:p binding energy. Several olefins f r o m m o r e t h a n o n e PA chain can interact with o n e m e t a l a t o m , allowing e l e c t r o n t r a n s f e r f r o m one PA chain t o a n o t h e r . In t h e valence level, the o c c u r r e n c e o f t h e state created at 1.24 eV and 1.72 eV is a t t r i b u t e d to t h e partial and t o t a l s u b s t i t u t i o n of chlorine a t o m s b y the olefin, respectively, and the M o C l s - d o p e d PA should have a m o r e shallow a c c e p t o r level with r e s p e c t t o the H O M O lr orbital (valence level BE 1.8 eV) o f t h e pristine PA t h a n the WC16-doped PA.
Acknowledgement T h e a u t h o r s wish to t h a n k the N a t i o n a l Science C o u n c i l for financial aid.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 •14
J. C. W. Chien, Polyacetylene, Academic Press, New York, 1984. W. R. Salaneck and H. R. Thomas, J. Chem. Phys., 72 (1980) 3674. W. R. Salaneck and H. R. Thomas, J. Chem. Phys., 71 (1979) 2044. M. Rolland and M. Aldissi, Polymer 23 (1982) 834. H.-M. Wu and S.-A. Chen, Polym. Commun., 28 (1987) 75. H.-M. Wu and S.-A. Chen, Synth. Met., 20 (1987) 169. H. Shirakawa and S. Ikeda, Synth. Met., 1 (1979/1980) 175. S.-A. Chen and L.-S. Li, Makromol. Chem., Rapid Commun., 4 (1983) 503. I. B. Goldber, H. R. Crowe and P. R. Newan, J. Chem. Phys., 70 (1979) 1132;B. R. Weinberger, E. Ehrenfreund, A. Pron, A. J. Heeger and A. G. MacDiarmid, J. Chem. Phys., 72 (1980) 4749. J. Chatt and L. A. Duncanson, J. Chem. Soc., (1953) 2939. C. Nordling, Angew. Chem. Int. Ed. Engl., 11 (1972)83. S. O. Grim and L. J. Matienzo, Inorg. Chem., 14 (1975) 1014. C. D. Wagner, W. M. Riggs, L. W. Davis, J. F. Mouler and G. E. Muilengerg, Handbook o f X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp., Eden Praire, 1979; D. Briggs and B. Seah, Practical Surface Analysis, John Wiley, New York, 1983. C. R. Fincher, M. Ozaki, M. Tanaka, D. Peebles, L. Lauchlan, A. J. Heeger and A. G. MacDiarmid, Phys. Rev. B, 20 (1979) 15.
225
D O P A N T - P O L Y M E R I N T E R A C T I O N : MoCIs-DOPED P O L Y A C E T Y L E N E HSIEN-MING WU and SHOW-AN CHEN* Chemical Engineering Department, National Tsing-Hua University, Hsinch, Taiwan 30043 (Republic of China)
(Received June 1, 1988; accepted July 14, 1988)
Abstract
Po ly acety len e is dope d with MoCls. The doping levels determined using the m e t h o d of n e u t r o n activation analysis indicate that the atom ratio [C1]/[Mo] varies f r o m 5 to 1.6, depending on doping time. The Cls XPS (X-ray p h o t o e l e c t r o n spectroscopy) binding energy (BE) decreases from 283.7 eV for the pristine PA to 282.9 eV, similar to t hat of WC16-doped PA as r e p o r t e d in our previous work, but opposite to that of I2- and AsF 5d o p e d PA. Th e lowering of the BE means that the outer shell electron densities of carbon increase after the doping. The Cl:p XPS of the doped PA shows that there is only one t y p e of chlorine in the adducts. Mo3a XPS shows that there are two states of Mo existing in the doped PA, being state A (MoCI:(C=C)4) and state B (Mo(C=C)n), quite different from those in the WC16-doped PA. T he level created at 1.72 eV related to state B is closer to the HOMO 7r orbital (BE 1.8 eV) of the pristine PA, and is therefore a more shallow level than the state created at 1.24 eV (due to state A Mo). Several olefins f r om more than one PA chain can interact with one Mo atom, allowing electron transfer f r om one PA chain to another.
Introduction Po ly acety len e (PA) film can be doped to become a semiconducting or conducting material and to have various electrical properties, depending on the doping level, t he nature of the d o p a n t and on the interaction between the d o p a n t and PA [1]. Among the dopants used for PA, I: and AsF s are the most studied. XPS studies [2, 3] on these two types of doped PA indicated a charge transfer f r om PA to the dopant, leading to an increase in Cls binding energy (BE). For PA dope d with transition metal halides (WC16 and MoCI5), which was first studied by Rolland and Aldissi [4], electron back-donation f r o m W and Mo in the adduct to olefin and C1- ligand was f o u n d to occur, opposite to t hat of I:- and AsFs-doped PA, as report ed in *Author to whom correspondence should be addressed. 0379-6779/88]$3.50
(~) Elsevier Sequoia/Printed in The Netherlands
226 our previous work [5, 6]. In this work, the d o p a n t / p o l y m e r interaction in the MoCls-doped PA is explored by the use of XPS with detailed analysis, neutron activation analysis and electron spin resonance (e.s.r.). The MoClsdoped PA is found to have two states created at BE 1.24 eV and 1.72 eV, quite different from the WC16-doped PA [6].
Experimental
PA films were prepared using the Shirakawa technique [7] with Ti(OBu)4-Al(Et)3 (mole ratio A1/Ti = 4) as catalyst at --78 °C and using toluene for successive washings. Characterization of the film using i.r. spectroscopy, elemental analysis, density measurements and differential scanning calorimetry has been reported in a previous paper [8]. The PA produced w i t h o u t further treatment has 90% cis and 10% trans content. The PA films were doped at room temperature by first evacuating to remove the trapped gas in the film and then injecting a 4.6 × 10 -4 M MoCls solution in toluene. The doped PA films were subsequently removed from the doping solution and washed with toluene several times. These films were then subject to analysis. The doping level in each PA film after various doping times was determined using neutron activation analysis, by which C1 and Mo contents were determined separately. High purity Mo (99.99%) and NH4C1 (99.8%) were used as standards. The neutron flux used to irradiate the samples, generated from the water pool nuclear reactor of Tsing-Hua University, was 1012 n c m - : s-~. Intensities of the 7-ray irradiation from Mo 99 and C13s in the samples and standards were measured using a multichannel pulse height analyser (Tracor Northern Model TN-1710) with a 50 cm a Ge(Li) 7-energy detector. 7-energies of the isotope C1as are identified at 1642 keV and of isotope Mo 99 at 141 keV. Comparing the counts of the sample with the standards at these two energies, the amounts of Mo and C1 in the samples can be determined. The resolution of the multichannel pulse height analyser used is high, i.e., 3.4 keV for the Co 6° 7-energy 1332.4 keV. X-ray photoelectron spectra of the doped PA films were measured using a Perkin-Elmer model 1905 photoelectron spectrometer equipped with a m o n o c h r o m a t i z e d Mg K a X-ray source (1253.6 eV photons). Calibration of the binding energies was made with reference to a gold target (Au4~7/: = 83.8 eV). In order to assure that there was a good electrical contact between the sample and the spectrometer and that there was no charge effect on the sample, gold powders were sprayed on the sample surface for calibration. In all cases, the BE of Au4~7/2 was 83.8 + 0.1 eV. Electron spin resonance measurements were performed at X-band (9.5 GHz) on a Bruker model 200D 10/12 spectrometer. DPPH (diphenyl picryl hydrazyl) was used as a calibration standard.
227
Results and discussion N e u t r o n activation analysis
By use of the neutron activation analysis method, Mo and C1 contents can be measured in the doped PA. The % weight uptake and atomic ratio [C1]/[Mo] in the doped PA versus doping time are shown in Fig. 1. Before doping, the ratio is 5 for MoCls. The ratio drops to 2.5 after 15 min doping and to 2 and 1.6 after 2 h and 4 h doping respectively. The % weight uptake increases to 6% after 15 min doping and to 9% after 2 h doping, but decreases to 8.4% after 4 h doping. The chemical structure of the doped PA after 2 h doping would be of the d o m i n a n t form, [CH(MoC12)0.00S6]x, which corresponds to one Mo atom to 116 C atoms. E l e c t r o n spin resonance m e a s u r e m e n t s
The spin density of the pristine PA (90% cis content) was determined to be 3.74 X 10 -6 mol spin/g of PA (or 2.251 X 10 is e-/g of PA) at room temperature, which is between those of trans PA (1.44 X 1019 and 1.65 X 1019 e - / g of PA) and c/s PA (an order lower than the value of trans PA) [9]. Each e.s.r, spectrum of the doped PA was found to be a symmetrical Lorentz curve, indicating homogeneous doping. The spin density of the doped PA film versus doping concentration is shown in Fig. 2. The increase of the spin density of MoCls-doped PA is greater than that of AsF s- and WC16-doped PA at the same doping level, indicating the existence of a difference in electronegativity or d o p a n t / p o l y m e r interaction. The g values of the doped PA obtained from the e.s.r, measurement are shown in Table 1, and are close to that of the free electron, 2.0023. The AH value changes only slightly at initial doping levels in comparison to those of AsFs- and WC16-doped PA [9, 6]. Hence, the d o p a n t / p o l y m e r interaction in the MoC1 sdoped PA is expected to be different from that in the WCl6-doped PA.
o
:k
._c
g o~
C~ £]C
~o 2
o
o
l0
1
2
Y:
I 3
Time (hours)
Fig. 1. Atom ratios [C1]/[Mo] and weight uptakes in the MoCls-doped PA adducts at various doping times.
228
o
6I v
o
"a z~
2
® 123
6
.~-
4
& [CH(WCI2)y ] x o [CH ( A s F 5 ) y ] x o [CH (MoCI2)y] x
2 -6 10
I
i
n
2
4
I
I
6
I
I
I
I
8
I
10
Y Doping Concentrotion
I
I
12
I
14
(10-3- )
Fig. 2. Spin density vs. dopant concentration for MoCls-, WC16"and AsFs-doped PA.
TABLE 1 Results of e.p.r, measurements for the adducts Doping time (rain)
/~/pp (Gauss)
g value
0 15 30 60 120 240
7.0 6.5 6.5 6.5 7.0 7.0
2.0025 2.0044 2.0041 2.0041 2.0040 2.0041
X-ray photoelectron spectroscopy (XPS ) (I) Cls core level binding energy Detailed scans of C,s binding energies (BE) of the doped PA at various doping times are given in Fig. 3. T he Cls BE of the pristine PA is 283.7 eV and decreases rapidly to 282.7 eV at 15 min doping time, then gradually increases to a constant value of 282.9 eV. The decrease in C1~ BE indicates an increase of the o u t e r shell electron density of the carbon. The d o p a n t withdraws electrons f r o m the PA, but the carbon of the PA shows increased electron density. This peculiar p h e n o m e n o n is similar to t h a t o f WC16-doped PA [6]. According to the D e w a r - C h a t t - D u n c a n s o n model [10], the elect r o n back-donation exists between olefin segments of PA and Mo metal, and t h e Mo, in principle, can f o r m a double bond with the olefin segment o f PA, as was proposed by us in the case of WC16-doped PA [6].
229
{0) i(1) 12) (3) (4 ] {5)
ormglr',ol PA dopmng15 rain doping 30 mmn doping 1 hour doping 2 hrs doping &hrs.
f~ 15/ i L,I ]
//
/
/
1
i
1
i
'i \
//"
\
\ \
2913
289.3 287.3 285.3 283.3 2~.3
Binding Energy ( e V )
I Li 2793 27?.3 2753
238
236
234
232
230
228
226
22z,
Binding Energy (eV)
Fig. 3. C]s XPS spectra of the MoCls-doped PA adducts at various doping times. Fig. 4. MO3d XPS spectra of the MoCls-doped PA adducts at various doping times.
(3) MO3d core level binding energy The MO3d detailed scans of the MoCls-doped PA at various doping times are shown in Fig. 4. The Moad BE of pure MoCls has a spin-orbital splitting, giving two peaks at 230.8 eV and 234.1 eV. After 30 min doping, these two peaks both shift to lower values at about 229 eV and 232 eV respectively; in addition, there is a shoulder at a b o u t 227 eV. After.doping for more than one hour, these two peaks become shoulders, and two additional peaks at 227 eV and 230 eV are present. Deconvolution of these curves b y assuming a Gaussian shape gives separate peaks, as shown in Figs. 5(a) - (e), whose characteristic values are listed in Table "2. There are two different states of Mo in the d o p e d PA. State A has the MO3ds/2 BE changing from 229.1 eV (15 min doping) to 228.6 eV (4 h doping), and state B has MO3ds/: BE changing from 227.0 eV (15 min doping) to 227.5 eV (2 h doping) and then dropping to 227.3 eV (4 h doping). Since in MoCls, the Mo(V) is of high oxidation state and has a strong electron-withdrawing ability, the [C1]/[Mo] atomic ratio is reduced from 5 to about 1.6 after doping as indicated above. Nordling [11] has established an empirical correlation between measured BE (or chemical shifts) and an atomic charge parameter of the metal atom, qM- By the use of his
230 TABLE 2 Results o f MO3d5/2 a n d MO3d3/2 XPS spectra deconvolution of the adducts at various doping times Doping time (min): Curve No. in Fig. 4: Deconvolution in Fig. 5: MO3d state (BE): State A Peak centre (eV) AE (eV) FWHM (eV) Area (%)
0 0 MOads/2
MO3d3/2
15 1 (a) MO3ds/2
230.8
234.1
229.1
3.3 2.42 61.60
State B Peak centre (eV) AE (eV) FWHM (eV) Area (%)
2.00 38.84
3.1 2,14 46,73
227.0 3.4 1.48 13.62
MO3d3/2
30 2 (b) MO3d5/2
MO3d3/2
232.2
229.0
232.0
2.17 35.43
230.4 1.48 4.22
3.0 2.12 38.55
227.2 2.9 1.58 16.28
2.51 36.38
230.1 1.56 8.78
*FWHM: full width at half maximum.
correlation, the value of qMo as well as the oxidation state of the Mo in the adducts can be determined from the measured Mo3ds/2 BE. A plot of MO3d5/2 BEs of MoCl5, MoC14, MoC13 and Mo (values taken from [12]) v e r s u s their respective qMo yields a straight line, as shown in Fig. 6. From Fig. 6, state A (MO3ds/2 BE from 229.1 eV to 228.6 eV) has qMo ranging from 0.8 to 0.56, which corresponds to the products between MoCI:(C=C)4 (qMo = 0.9744) and MoCI(C=C)s (qMo = 0.6048), according to the prediction in Table 3. The [C1]/[Mo] atom ratio in the adducts obtained from the neutron activation analysis stated above is between 2.5 and 1.6, hence MoC12(C=C)4 complex would be the major end product of state A. State B has t h e MO3ds/2 BE ranging from 227 eV to 227.5 eV, and therefore has a qMo from --0.5 to --0.3 {Fig. 6), which means that there is a higher electron density around the Mo in the adduct than that of Mo(0). This situation often appears in an organometallic c o m p o u n d , such as [(ws-CsHs)Mo(CO)3] 2 having MO3ds/2 BE = 227.2 eV [12]. Since the contribution of one chlorine to qMo is a b o u t +0.4085, as can be obtained from the calculation by use of the Nordling empirical equation, thus chlorine is obviously absent in state B. The M o ( C = C ) , Ir complexes of state B will be the major species for prolonged doping, as indicated in the neutron activation analysis and XPS analysis.
(3) Cl2p core level T h e C12p d e t a i l e d s c a n s o f M o C l s - d o p e d P A a t v a r i o u s d o p i n g t i m e s a r e s h o w n in F i g . 7. T h e C12p B E o f p u r e MoC1 s h a s a s i n g l e p e a k a t 1 9 9 . 6
231
60 3 (c) M°3d5, 2
228.7 3.4 1.16 10.76
227.5 2.8 1.48 39.14
M°3d3, 2
120 4 (d) M°3d5,, 2
232.1
228.8
2.06 17.31
230.3 1.92 32,80
3.5 2,28 27,24
227.5 3.1 1.49 29.76
M°3d3/2
240 5 (e) M°3d5/2
MO3d3/2
232.3
228.6
232.1
2.05 17,34
230.6 1.82 25.66
3.5 1.88 21.00
227.3 3.1 1.47 35.00
Mo metal 6 M°3ds/2
M°3d3,, 2
227.8
230.9
1.77 16.00
230.4 1.48 28.00
3.1 1.15 57.05
1.47 42.94
eV, which can be assigned a s C12p3/2 BE [13]. After doping from 15 min to 4 h, the C12p BE shifts to about 196.4 eV, which is 3.2 eV lower than that of pure MoCls, indicating that the chlorine atom gains electrons after forming the adduct with PA and that there is only one type of chlorine in the adducts. Since C12p3/2 BEs of C1--C1 and C--C1 are about 205 - 207 eV, which are higher than that of metal--C1 (about 1 9 9 - 200 eV) [13], these peaks would not be from C--C1 and C1--C1 species, but due to stronger back-donation of Mo to C1, which increases the outer shell electron density of C1. The decrease in BE of C12p in the adducts in Fig. 7 indicates that there is (d-d)Tr bonding between the Mo and C1. This kind of (d-d) rr-backdonation is similar to the (d-Tr*) 7r-back-donation in the Mo and olefin segm e n t of PA.
(4) Valence levels The valence level spectra of PA, MoC1 s and the doped PA are shown in Fig. 8. The pristine PA valence level spectrum has a peak at 1.8 eV, which should be the highest occupied molecular orbital (HOMO) of the pristine PA [14]. The band gap of the pristine PA is then 1.8 eV here [14]. From the area percentage of these two states of Mo in Table 2, we know that the amounts of MO3d5/2 of state A, MoC12(C=C)4, dominate in the adducts at 15 min (82.16%) and 30 min (74.93%) doping. As doping time increases, the amounts of state B, Mo(C=C), 7r-complex, are larger than those of state A, being 71.94%, 55.43% and 63.0% after 1, 2 and 4 h doping re-
232
1 23/, (a)
1 I I 232 230 228 Binding Energy ( e V )
23Z, (C)
232 230 228 Binding Energy (eV)
t-226
I
236
I
234
I
232
I
230
I
228
~""~
226
(b)
I 23/,
226 (d)
I I I ~ 232 230 228 Binding Energy {eV)
:~--, 226
I / I
234 (e)
I
I
I. . . .
232 230 228 Binding Energy ( e V )
-~--
226
Fig. 5. Deeonvolution of MO3d XPS spectra of the MoCls-doped PA adduets at various doping times: (a) 15 min; (b) 30 min; (c) 1 h; (d) 2 h; (e) 4 h.
spectively. Thus the new peak at 1.24 eV, which is absent in the pristine PA and appears at 15 min and 30 rain doping, can be assigned to state A; the intensity of the latter is weaker than the former. The new peak at about 1.72 eV, which becomes m or e apparent as the doping time increases to
233 > >-
234
Ld
232 -
-
0A
~oC\%
0
0A
0.8
1.2
1.6
2.0
qM0 Fig. 6. Plot of MO3ds/2 binding energy posed by Nordling.
vs.
calculated charge using the correlation pro-
TABLE 3 Estimation of qMo of the Mo state A at several possible chemical structures Chemicala compound
qMo of MoCI n
qMo of Mo(C=C)6_ n
qMo of the adduct b
MoCls(C=C) MoC14(C=C)2 MoC13(C=C)3 MoC12(C=C)4 MoCI(C=C)s Mo(C=C)6 MoCl5 MoC14 MoCl3 MoCl2 Mo
0.4085 0.4085 0.4085 0.4085 0.4085 0 0.4085 0.4085 0.4085 0.4085 0
0.0392 0.0392 0.0392 0.0392 0.0392 0.0392
2.0832 1.7136 1.344 0.9744 0.6048 0.2352 2.044 1.6352
x 5 ×4 ×2 x2 ×1 ×5 ×4 ×3 ×2
×1 X2 ×3 ×4 X5 ×6
1.2264 0.8176 0
aElectronegativity: Mo:1.8; C1:3.25; C=C:2.2 (value taken from Thomas A. Carlson, Photoelectron and Auger Spectroscopy, Plenum Press, New York, 1975). C=C: olefin segment of PA. bqM o of the adduct is obtained by a summation of qMo values of MoC1n and Mo( C = C ) 6 - n ; a positive value of qMo means electrons were removed from Mo at the element state.
1 h a n d 4 h, c a n b e a s s i g n e d t o s t a t e B. T h e p e a k a t 1 . 2 4 eV d i s a p p e a r s after a long doping time. F r o m t h e d i s c u s s i o n o n X P S s p e c t r a o f Cls, MO3d a n d C12p a b o v e , it is k n o w n t h a t t h e 1 . 2 4 eV s t a t e is c r e a t e d b y t h e m o l y b d e n u m 4 d ( t 2 ~ ) o r b i t a l a n d l i g a n d o r b i t a l s , w h i c h i n c l u d e t h e o l e f i n e m p t y 7r* o r b i t a l s a n d c h l o r i n e e m p t y d o r b i t a l s ; a n d t h a t t h e 1 . 7 2 eV level is c r e a t e d b y t h e m o l y b d e n u m 4d(t2g ) o r b i t a l a n d t h e o l e f i n e m p t y n* o r b i t a l o n l y . T h e s e
234
o
"5
C5 UJ
,T, Z
202
200
198 196 19l* 192 Binding Energy (eV)
190
18{}
Fig. 7. C12p XPS spectra o f the M o C l s : d o p e d PA adducts at various times.
1(2) l(3) 1(4) 1(5) 1(6)
doping 15rain. doping 30rain. doping 1hr. doping 2hrs. doping 4hrs. MoCls
SF:OD07 SF:OD06 5F:0£)13 SF:0.012 5F:0.010 SF: 0.0094
~. '~!(~ I
8 6 Binding Energy (eV) Fig. 8. Valence level XPS spectra o f the MoCls-doped P A adducts at various doping times.
ligand orbitals all have fairly low electronegativity compared to the positively charged molybdenum(Mo(V)), so the ligand orbital will lie at a higher energy level than the corresponding m o l y b d e n u m 4d(t2g) orbital. Hence this created bonding ~ molecular orbital will resemble the molybdenum
235 \
elg . . . . . . . .
metol d-orbitols in Oh field
.-'.\ - ] - - i - e ig ~ _ _ -
I I /f empty ligond I^~I"I z'l d, I"I'* or b i t o l s
"~\ I \.------I~ ~.__.L
I
,/ •
///////////////////////////// moleculor
11" bond of PA
orbitols
Fig. 9. Energy level diagram for a ~" s y s t e m w i t h an a c c e p t e r ligand C1
a n d PA in the
MoCls-doped PA. (A 1 = 1.24 eV, A: = 1.72 eV). 4d(t2g) orbital m o r e t h a n the ligand orbital, and conversely the ~* m o l e c u l a r orbital will m o r e closely resemble the ligand orbital, as s h o w n in Fig. 9 (A] = 1.24 eV, A2 = 1.72 eV). T h e level created at 1.72 eV related to state B is closer t o t h e H O M O w orbital (1.8 eV) o f the pristine PA, and is theref o r e a m o r e shallow level state t h a n the created state at 1.24 eV (due to state A Me). Thus state B M e can m a k e the electrical activation energy, Ea, o f t h e d o p e d PA l o w e r t h a n state A Me. This is s u p p o r t e d b y the experi m e n t a l result o f Rolland et al. [4], in which E a shows a sharp decrease at d o p a n t c o n c e n t r a t i o n s , y, f r o m 0 to 0.015 (equivalent to 0 - 4 d o p i n g in o u r case). F r o m the present analysis and the WC16-doped PA e x p e r i m e n t [6], it can be inferred t h a t d i f f e r e n t d o p a n t s can c o o r d i n a t e with PA to yield a d d u c t s o f d i f f e r e n t s t r u c t u r e s and t h u s can create d i f f e r e n t d o p i n g a c c e p t e r level. H e n c e the M o C l s - d o p e d PA should have a m o r e shallow a c c e p t e r level with r e s p e c t to t h e H O M O w orbital (valence level BE 1.8 eV) o f the pristine PA t h a n the WC16-doped PA. This is in a g r e e m e n t with the electrical c o n d u c t i v i t y m e a s u r e m e n t s ; the f o r m e r has lower electrical activation e n e r g y t h a n the latter [4].
Interaction between MoCls and polyacetylene, and the proposed conduction mechanism In t h e MoCls-doped PA adducts, a f t e r doping, the C1 a t o m s in MoC15 can be s u b s t i t u t e d b y the olefins and the chemical s t r u c t u r e f o r m u l a of the a d d u c t s b e c o m e s [MOC12(C=C)4], and [ M o ( C = C ) . ] . Thus, parts o f the C1 a t o m s in t h e MoC1 s are eliminated. In the adducts, the lr e l e c t r o n o f the PA can pass t h r o u g h t h e o - b o n d f r o m the olefin segment of PA to the Me, and t h r o u g h t h e o - b o n d f r o m the Me t o the p w*-orbital o f the olefin. These b a c k - d o n a t e d e l e c t r o n s are located closer to the c a r b o n a t o m . In t h e Me--C1 b o n d i n g , t h e r e is a ( d - d ) n b o n d i n g with e l e c t r o n backd o n a t i o n f r o m the M e to t h e C1 as in the case o f (d-w*) w-back-donation in t h e Me and olefin. Because o f t h e a and w orbital overlap existing b e t w e e n a Me and olefins o f m o r e t h a n o n e PA chain, e l e c t r o n t r a n s f e r m a y be relatively easy t h r o u g h t h e a and w b o n d s t h a t c o n n e c t the d o p a n t and t h e olefins o f PA. So the transverse w-bonding c o n d u c t i o n m e c h a n i s m is p r o p o s e d to exist e x c e p t at light doping, w h e r e the soliton h o p p i n g m e c h a n i s m is fol-
236 l o w e d [1]. T h e transverse ~r-bonding c o n d u c t i o n m e c h a n i s m allows electrons to be t r a n s f e r r e d f r o m o n e PA chain to a n o t h e r t h r o u g h t h e m e t a l Mo bridge, m a i n l y because the m e t a l Mo can c o o r d i n a t e with m u l t i p l e t olefins to f o r m the diolefin and triolefin, etc., c o m p o u n d s with d i f f e r e n t PA chains.
Conclusion In M o C l s < l o p e d p o l y a c e t y l e n e (PA), it is f o u n d t h a t the p 7r-electrons o f t h e olefins can transfer t o the m e t a l d-orbital and t h e n b a c k - d o n a t e t o the p 7r* orbital o f t h e olefins, leading to a decrease in C is binding energy, and t h a t t h e electrons in the d orbital o f the m e t a l a t o m can also backd o n a t e t o t h e C1- ligands, leading t o a decrease in Cl:p binding energy. Several olefins f r o m m o r e t h a n o n e PA chain can interact with o n e m e t a l a t o m , allowing e l e c t r o n t r a n s f e r f r o m one PA chain t o a n o t h e r . In t h e valence level, the o c c u r r e n c e o f t h e state created at 1.24 eV and 1.72 eV is a t t r i b u t e d to t h e partial and t o t a l s u b s t i t u t i o n of chlorine a t o m s b y the olefin, respectively, and the M o C l s - d o p e d PA should have a m o r e shallow a c c e p t o r level with r e s p e c t t o the H O M O lr orbital (valence level BE 1.8 eV) o f t h e pristine PA t h a n the WC16-doped PA.
Acknowledgement T h e a u t h o r s wish to t h a n k the N a t i o n a l Science C o u n c i l for financial aid.
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