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Analysis of the electronic structure of polyacetylene based on its optical absorption spectra
S q~thetic Metals, 3P (1991) 367-378
367
Analysis of the electronic structure of polyacetylene based on its optical absorption spectra V. M. K o b r y a n s k y
and E. A. T e r e s h k o
Ir~,titute of Che,rdccd Physics, U.S.S.R. Academy of Sciences, Moscow (U.S.S.R.)
(Received September 11, 1990; accepted September 17, 1990)
Abstract A comparison of the characteristics of the electronic absorption spectra for polyacetylene (PA) prepared by different methods has been carried out. The comparison shows that the concentration of PA molecules (or their fragments) with a bandgap of different widths depends mainly on the crystalline structure and morphology of PA as well as on the concentration of conformational and chemical defects in the macromolecules. The dependence on chain length is evident only for low-molecular weight samples.
1. I n t r o d u c t i o n It is k n o w n t h a t p o l y a c e t y l e n e (PA) is a s e m i c o n d u c t o r with a c o n d u c t i v i t y of 10 -9 a n d 10 -~ o h m -1 c m -1 for c/s- a n d t r a n s - f o r m s , respectively. After d o p i n g the PA with e l e c t r o n - d o n o r or e l e c t r o n - a c c e p t o r c o m p o u n d s its c o n d u c t i v i t y i n c r e a s e s to 10'~-10 s o h m 1 c m - 1 and it c h a n g e s into a metallic state [1-3]. The w i d t h of the b a n d g a p ( 2 - 1 . 4 eV) in n e u t r a l t r a n s - P A as d e t e r m i n e d f r o m the e l e c t r o n i c a b s o r p t i o n s p e c t r u m d e p e n d s on h o w the PA w a s obtained. The p o s i t i o n a n d s h a p e of the a b s o r p t i o n b a n d in the e l e c t r o n i c s p e c t r a of different PA m o d i f i c a t i o n s c a n b e divided into five types. The first t y p e includes s t a n d a r d PA ( s e v e r a l m i c r o n s to s e v e r a l m i l l i m e t r e s thick) p r e p a r e d in the p r e s e n c e of S h i r a k a w a [4] and L u t t i n g e r [5] catalytic s y s t e m s . T h e s p e c t r a of c/s-PA are c h a r a c t e r i z e d b y a wide b a n d at 25 0 0 0 - 1 6 0 0 0 c m - 1 with t w o v i b r a t i o n a l m a x i m a at 18 0 0 0 a n d 16 4 0 0 e r a a n d a s h a r p s h o r t - w a v e drop, while t h o s e of t r a n s - P A c o n t a i n a wide b a n d at 1 6 0 0 0 - 1 2 0 0 0 c m -1 [6]. The s e c o n d t y p e includes thin PA films ( 1 0 0 0 - 2 0 0 0 / ~ ) . A g e n e r a l v i e w of the a b s o r p t i o n s p e c t r a of thin films is similar to t h a t of the a b s o r p t i o n s p e c t r a of s t a n d a r d film e x c e p t t h a t in the latter c a s e all the a b s o r p t i o n b a n d s are shifted in the s h o r t - w a v e d i r e c t i o n [7]. T h e third t y p e includes PA p r e p a r e d f r o m s a t u r a t e d p o l y m e r s . T h e e l e c t r o n i c a b s o r p t i o n s p e c t r a of t h e s e p o l y m e r s are c h a r a c t e r i z e d b y the c o m p l e t e a b s e n c e of a v i b r a t i o n a l s t r u c t u r e of b a n d s at a high d e g r e e of c o n v e r s i o n a n d a c o n s i d e r a b l e ( 0 . 5 - 1 . 0 eV) shift o f the a b s o r p t i o n b a n d
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368
maximum of t r a n s - P A into the short-wave region as compared with that observed for standard films [8]. The fourth type includes block and grafted PA copolymers with saturated macromolecules. A Shirakawa catalyst has been used in graft and some of these copolymers are soluble. The absorption spectra of these copolymers resemble that of the third-type PA. As a rule the position of the enveloping curve is shifted into the short-wave region as compared with the absorption bands in standard films [9, 10]. The fifth type calls for special consideration. It includes soluble PA compositions with saturated polymers the electronic absorption spectra of which are characterized by fine structure. Such compositions can sometimes be obtained by polymerizing acetylene in solutions of saturated polymers. The polymerization of acetylene by using the Shirakawa catalytic system in a polystyrene solution yielded a product which is thought to be a suspension of PA in a polystyrene matrix [ 11 ]. The electronic absorption spectra of this composition is characterized by narrow absorption bands with a vibrational structure. A similar absorption spectrum has been described for a PA composition with polyvinylbutyral (PVB) which has been synthesized by polymerizing acetylene in a PVB solution [12]. In the present work we have investigated the relationship between the electronic absorption spectra and the structure and morphology of PA. Samples of PA and PA-PVB compositions were prepared with the use of a Luttinger catalyst.
2. Experimental PA and PA-PVB compositions were obtained by polymerizing acetylene in ethanol containing the required amount of Co(NO3)-NaBH4 catalyst and PVB according to a known method [12]. The Co(NO3)2:NaBH4 ratio was kept constant in all experiments at 4:1. For PA-PVB compositions the concentration of PVB was varied from 0.2 to 2.5 wt.%. The absorption spectra in the range 5 0 0 0 0 - 1 2 600 cm -1 were obtained with the aid of a Specord UV-Vis instrument; in the 1 4 0 0 0 - 3 5 0 0 cm -1 region a Specord NIR-61 instrument was used. PA and PA-PVB samples needed for spectral studies were made by dispersing the reaction mixture immediately after the synthesis. The gels were diluted to 0.001 wt.% PA and then dispersed by ultrasound or high-speed stirring for 30 min. The viscosity of the suspensions was increased by adding PVB solution up to 3 wt.% to increase their stability. Films of PA and PA-PVB compositions were prepared by suspension casting on horizontal supports. Electron microscopic investigations were carried out with the aid of a YEOL-YCM-35C scanning electron microscope. Before measurement the films were coated with a layer of gold (100-200 /~ thick) by the ionic sputtering method.
369
3. R e s u l t s
3.1. Physicochemical characteristics and morphology of PA and PA-PVB compositions The polymerization of acetylene in the presence of Co(NO3)2-NaBH4 in pure ethanol and in PVB solutions at concentrations of < 1.2 wt.%. PVB leads to the formation of thixotropic gels. Suspensions made from these gels are unstable and their consistency is gradually restored. The rate of structuralization and the physicochemical properties of the suspensions and gels depend on the composition of the polymerization solution. With an increase in the concentration of PVB in the polymerization solution the stability of the gels decreases and the time required for the suspension to return to the form of gel increases. The polymerization of acetylene in a solution containing 1.5 wt.% PVB results in the formation of a low-viscosity gel which dissolves on stirring. After stirring is s t oppe d no structuralization occurs for a long time. This means that the structuralization ability and thixotropic properties of PA-PVB compositions in the liquid phase decreases with an increase in the concentration of PVB in the polymerization solution, and that at a concentration of 1.5 wt.% PVB it virtually disappears. The polymerization of acetylene in a PVB solution at > 1.5 wt.% PVB results in the formation of coloured solutions which are not thixotropic. We shall refer to compositions which are obtained at a higher concentration of PVB and which are not thixotropic in the liquid as soluble compositions, and PA obtained under these conditions as the soluble form of PA. Morphological investigations of PA-PVB compositions with the aid of a scanning electron microscope showed that changes in their physicomechanical properties in the liquid phase are related to their morphology. Polymerization in solutions containing up to 0.6 wt.% PVB results in the formation of fibrils 3 0 0 - 5 0 0 /~ in diameter (Fig. 1). At 0 . 6 - 1 . 2 wt.% PVB thicker and shorter fibrils are formed which gradually begin to resemble
Fig. 1. Surface of PA film obtained in C2HsOH solution ( × 4 0 0 0 0 , reduced in reproduction 379/0). Fig. 2. Surface of PA-PVB film obtained in 1.5% solution of PVB ( × 4 0 0 0 0 , reproduction 37%.)
reduced in
370 ellipsoids. At 1.5 wt.% of the r e a c t i o n solution globules 1 5 0 - 5 0 0 / ~ in d i a m e t e r are f o r m e d (Fig. 2). On a f u r t h e r i n c r e a s e in c o n c e n t r a t i o n of PVB, soluble c o m p o s i t i o n s c o n t i n u e to be p r o d u c e d w i t h o u t any s u b - m o l e c u l a r f o r m a t i o n s w h e n e x a m i n e d u n d e r × 105 magnification. If we a s s u m e that the physicom e c h a n i c a l p r o p e r t i e s of the c o m p o s i t i o n s in the liquid p h a s e are directly related to the d i m e n s i o n and the f o r m o f the particles, t h e n o n a n a l o g y with data [9] for soluble P A - p o l y b u t a d i e n e c o m p o s i t i o n s we can s p e a k h e r e of particles 50 ,~ in diameter. Thus, the p o l y m e r i z a t i o n o f a c e t y l e n e in PVB solutions in the p r e s e n c e of a Luttinger catalyst yields c o m p o s i t i o n s the m o r p h o l o g y o f which varies f r o m fibrillar to globular and t h e n c h a n g e s into soluble forms.
3.2. Electronic absorption spectra of P A - P V B compositions Figure 3 shows the a b s o r p t i o n s p e c t r u m of a fibrillar PA s u s p e n s i o n o b t a i n e d by p o l y m e r i z i n g a c e t y l e n e in ethanol. The s p e c t r u m contains the b a n d of c/s-PA with two vibrational m a x i m a at 18 000 and 1 6 4 0 0 c m -1 and a s h o u l d e r of the trans-PA in the 1 6 0 0 0 - 1 2 0 0 0 c m -1 r e g i o n w i t h o u t a vibrational structure. It can also be that the short-wave d r o p o f the main a b s o r p t i o n b a n d gradually d e c r e a s e s with an increase in the f r e q u e n c y , and the a b s o r p t i o n e x t e n d s into the ultraviolet region. The p o l y m e r i z a t i o n o f a c e t y l e n e in ethanol with different c o n c e n t r a t i o n s o f the catalyst and different r e a c t i o n times r e s u l t e d in the f o r m a t i o n of PA fibrils 2 0 0 - 9 0 0 / ~ in diameter. Their a b s o r p t i o n s p e c t r a s h o w e d a t e n d e n c y t o w a r d s a slight long-wave shift of a b s o r p t i o n b a n d s with an i n c r e a s e in the d i a m e t e r of the fibrils. The shift of the positions of the m a x i m a a m o u n t s to 3 0 0 - 4 0 0 c m - ~ as the d i a m e t e r o f the fibrils i n c r e a s e s f r o m 200 to 9 0 0 / ~ . A c o m p a r i s o n of t h e s e data with data r e p o r t e d in the literature indicates that the position of the m a x i m a in the optical a b s o r p t i o n s p e c t r a nearly coincides for any s a m p l e s o f similar m o r p h o l o g y r e g a r d l e s s o f h o w the PA s a m p l e s w e r e p r e p a r e d or h o w their
/.0
o.s o,s
o.~
q~
o,l
0
,
~ cM " l
0 ~o
26
22
18
/~ -•/O'J
Fig. 3. Absorption spectrum of PA suspension obtained in C2H6OH solution. Fig. 4. Absorption spectrum of PA-PVB soluble compositions obtained in 1.5% solution of PVB.
371 a b s o r p t i o n s p e c t r a w e r e r e c o r d e d . A b s o r p t i o n s p e c t r a a n a l o g o u s to t h a t s h o w n in Fig. 3 are c h a r a c t e r i s t i c of s t a n d a r d PA films o b t a i n e d b y the S h i r a k a w a m e t h o d , d e s p i t e the fact t h a t the films w e r e d e t e r m i n e d b y reflection s p e c t r a a n d w e r e p r e p a r e d in the p r e s e n c e of a different catalytic s y s t e m . T h e a b s o r p t i o n s p e c t r a of fibrillar P A - P V B c o m p o s i t i o n s are similar to t h o s e of p u r e PA. H o w e v e r , a c h a n g e in c o m p o s i t i o n s with a g l o b u l a r m o r p h o l o g y a n d in soluble c o m p o s i t i o n s is a c c o m p a n i e d b y c o n s i d e r a b l e c h a n g e s in all the p a r a m e t e r s of their a b s o r p t i o n s p e c t r a . Figure 4 s h o w s the a b s o r p t i o n s p e c t r u m of a soluble P A - P V B c o m p o s i t i o n o b t a i n e d at 1.8 wt.% PVB c o n c e n t r a t i o n in the p o l y m e r i z a t i o n solution. In spite of a g e n e r a l similarity with the a b s o r p t i o n s p e c t r a of fibrillar PA, this a b s o r p t i o n s p e c t r u m is shifted by 2 0 0 0 c m - 1 t o w a r d s h o r t e r w a v e l e n g t h s ; it r e s e m b l e s m o r e the s p e c t r a of individual p o l y e n e s in solution. In the 22 0 0 0 - 1 7 0 0 0 c m - 1 r e g i o n lies an a b s o r p t i o n b a n d of cis-PA. Against the b a c k g r o u n d of t h r e e a b s o r p t i o n m a x i m a at 20 180, 1 8 9 3 0 a n d 1 7 7 4 0 c m -1 lies the s h o u l d e r due to t r a n s PA on w h i c h t h e r e are t h r e e p o o r l y r e s o l v e d v i b r a t i o n a l m a x i m a at 1 5 4 8 0 , 14 9 6 0 a n d 13 9 6 0 c m -1. In the s h o r t - w a v e r e g i o n of the s a g of the a b s o r p t i o n b a n d at 40 0 0 0 - 2 2 0 0 0 c m -1 t h e r e are eleven p o o r l y r e s o l v e d m a x i m a a n d inflections. The a b s o r p t i o n s p e c t r a of soluble P A - P V B c o m p o s i t i o n s o b t a i n e d at > 1.8 wt.% PVB in the r e a c t i o n solution r e s e m b l e t h a t s h o w n in Fig. 4. H e r e , with an i n c r e a s e in the c o n c e n t r a t i o n of PVB a f u r t h e r shift is o b s e r v e d of all the a b s o r p t i o n b a n d s t o w a r d s h o r t e r w a v e l e n g t h s . The m a g n i t u d e of the shift is p r o p o r t i o n a l to the c o n c e n t r a t i o n o f PVB in the r e a c t i o n solution. T a b l e 1 s u m m a r i z e s b o t h o u r d a t a a n d k n o w n r e s u l t s on the f r e q u e n c i e s of l o n g - w a v e v i b r a t i o n a l m a x i m a in the a b s o r p t i o n of different t y p e s of c/sa n d t r a n s - P A in the a b s o r p t i o n s p e c t r a o f w h i c h a v i b r a t i o n a l s t r u c t u r e is p r e s e n t at least for o n e of the PA forms. F o r s o m e t y p e s of PA a Vibrational s t r u c t u r e is o b s e r v e d for c/s- a n d t r a n s - f o r m s . A s c a n be s e e n f r o m t h e s e data, the shift in the p o s i t i o n of the m a x i m a on g o i n g o v e r f r o m the c/sto the t r a n s - f o r m in this c a s e a m o u n t s to 3 8 0 0 - 4 2 0 0 c m - 1, a n d the m a g n i t u d e of the shift is Virtually i n d e p e n d e n t o f the PA type. By a s s u m i n g t h a t A v c i s - t r a n s d e p e n d s only slightly on the t y p e of PA w e c a n e s t i m a t e the w i d t h of the b a n d g a p for the t r a n s - f o r m of PA t y p e s the a b s o r p t i o n s p e c t r a of w h i c h s h o w a v i b r a t i o n a l s t r u c t u r e for the c / s - f o r m alone. T h e s e e s t i m a t i o n s are g i v e n in a c o l u m n ( t r a n s - P A c a l c u l a t e d m a x i m a ) of T a b l e 1. The p a r a m e t e r s of the a b s o r p t i o n s p e c t r a of t h o s e PA t y p e s in the a b s o r p t i o n s p e c t r a of w h i c h v i b r a t i o n a l s t r u c t u r e s are a b s e n t for at least o n e of the f o r m s are n o t s h o w n in T a b l e 1 a n d will not b e d i s c u s s e d in the p r e s e n t w o r k . As can b e s e e n f r o m d a t a in T a b l e 1, the difference in the p o s i t i o n of the a b s o r p t i o n m a x i m a for PA of different m o r p h o l o g i e s is quantitatively r e l a t e d to the d i m e n s i o n of the p a r t i c l e s of c o r r e s p o n d i n g s a m p l e s . A m a x i m a l shift ( ~ 2 3 0 0 c m - 1 ) into the s h o r t - w a v e r e g i o n o c c u r s during t h e t r a n s i t i o n f r o m fibrillar PA ( 9 0 0 ~ in d i a m e t e r ) to soluble c o m p o s i t i o n s . An analysis of t h e o b t a i n e d results m a k e s it p o s s i b l e to identify t h r e e i m p o r t a n t c h a r a c t e r i s t i c s of the e l e c t r o n i c a b s o r p t i o n s p e c t r a o f PA:
372 TABLE 1 Spectral characteristics of different PA compositions PA type (catalyst)
Morphology (dimension, /~)
Maxima ( c m - 1) c/s-Form
trans-Form
Exp.
Exp. 12000
Source
Calc.
Standard PA (Luttinger; suspension)
fibrils (300)
16500
Standard PA (Luttinger; suspension)
fibrils (900)
16200
Composition PA-PVB
globules (150-200)
17700
13960
present work
Composition PA-PVB
soluble form
18100
14300
present work
Standard PA Shirakawa film
fibrils (300-500)
16400
12OOO
[6]
Thin PA film
fibrils (30-50)
17100
13000
[71
Composition PA-polystyrene
soluble film
17150
13800
[11]
present work 12000
present work
(i) The transition from fibrillar PA to soluble forms is accom pani ed by a shift in the absorption maxima of the c/s- and t r a n s - P A into the shortwave region. For soluble PA the position of the maxima in the absorption spectra is shifted into the short-wave region with an increase in the PVB concentration in the polymerization solution. (ii) For soluble PA-PVB compositions the vibrational structure of c/sand t r a n s - f o r m s is present in the short-range spectral region. (iii) The transition from fibrillar PA to soluble compositions is a c c o m p a n i e d by narrowing of the absorption bands because of a decrease in the absorption intensity in the short-wave region. In this case the absorption of soluble compositions begins to resemble the absorption bands of individual polyenes in the solution, but first is shifted into the long-wave region. We shall discuss only the position of the maxima of the absorption bands.
4. D i s c u s s i o n
The analysis of the typical spectra of PA is based on the theory of electronic structure of the polyene chain and s p e c t r o s c o p y of individual
373 polyenes. According to existing concepts there should be a bandgap in a polyene chain of infinite length owing to the alternating bond lengths and interelectronic correctional interaction [ 13, 14]. Calculations for short polyenes as a rule yield results that coincide with the experimental data when the correlation of electrons is taken into account. Both theoretical analysis and experimental results indicate that a lower excited state in short polyenes has A~ symmetry. Direct electron transitions into this state are of low intensity and have been observed only in the case of a twop h o t on excitation for N from 3 to 6 [15, 16]. Long-wave optical absorption is due to the transition into the Bu state. It has been shown that the energy of transition depends on the num ber of conjugated double bonds, the conformations of the chain and the dielectric characteristics of the medium. Fluorescence in short polyenes is regarded and proceeds from the 2Ag excited state or the mixed 2Ag+/Bu state, formed with a change in the conformation of the polyene chain [15, 16]. In PA the position of the lower excited state, 2Ag, has not been established. By using the method of two-photon s p e c t r o s c o p y the energy of the 2Ag state was found to be ~ 1.93 eV [17], i.e., greater than the lower edge of the bandgap ( ~ 1.4 eV). However, it was assumed [15] that the 2Ag state in long polyenes b e c o m e s degenerate with the level of the neutral soliton. The optical absorption band of PA in the 2 - 1 . 4 eV region is related to the dipole transition into the exciton band or into the conductivity band [13]. From photoconductivity data for t r a n s - P A it follows that the conductivity band extends even below the edge of the bandgap [18]. It can be supposed that an inter-band optical transition in PA is of the same nature as the long-wave dipole transition in short-chain polyenes. This proposition is s u p por t ed by gradual change in the characteristics of the optical transition from fibrillar PA to soluble compositions (shown in the present work) and by similar values of the intensities of the r e d u c e d bands for optical absorption of PA and short polyenes. For short polyenes the intensity of the absorption band A g - - ~ B u increases linearly with the number of double bonds in the macromolecular chain [19]. The extinction calculated per double bond equals ~ 104 cm -1 1 tool- 1. The intensity of the absorption band for a film (density 0.4 g cm -3) of t r a n s - P A (prepared by the Shirakawa method) is 1.5 105 cm -1 [20]. This value also co r r es ponds to extinction if recalculated per double bond, i.e. ~ 104 cm ~ 1 tool 1. In other words, the magnitude of the dipole m o m e n t of optical transition and consequently the degree of charge separation in the initially formed e l e c t r o n - h o l e pair are similar for PA and short-chain polyenes. It can be assumed that a possible discrepancy between the order position of the electron levels 2A~ and Bu in short-chain polyenes and the same states in PA is related to an increase in the polarizability of polyene with an increase in the length of the polyene chain in the solid state. Most investigations of the electronic spectra of short-chain polyenes have been carried out on solutions. At the same time PA is insoluble in any solvent and all its spectral
374 characteristics belong to the solid aggregate state. For molecules with a high degree of electronic polarizability and larger dipole m om ent s of electronic transitions, strong long-wave shifts of absorption bands were observed during the transition from solutions to the solid phase. Such shifts have been found for many types of molecules with a developed conjugated system, for example, for tetraseleniumtetracene [21], cyanine dyes [22] and fl-carotene [23]. The order of location of the bands of the excited state in crystalline PA can vary relative to their location in the individual polyene chain owing to the electronic polarization of molecules in the region of dipole transition and to the intermolecular exchange interactions in the ground and excited states. In the simplest case we can consider that the positions of the bands shift only to the polarizability of the surroundings of dipole transitions. Hence, the energy of the exciton band and the conductivity band should decrease, these bands becoming populated with the bound and free elect r o n - h o l e pairs, respectively. The conductivity band undergoes a larger shift so that can be below the exciton band. Qualitatively, the shift of the absorption band in inter-zonal transition in PA relative to its position in a hypothetical unfolded polyene chain i n v a c u o can be estimated on the assumption that the mechanism of the interaction, the dipole m o m e n t of electronic transition and the medium when changing from short polyenes in a solution to crystalline PA remain unchanged. For short-chain polyene in solution the relationship between the position of the optical absorption band and the polarizability of the medium can be determined experimentally. The relationship between the shift in the frequency of optical transition and the electronic part of the dielectric permeability, x = n 2, can be defined as follows:
where Avi_j is the shift of the frequency of the long-wave vibrational m axi m um of the absorption band when going from solvent i to solvent j; K is the constant which takes into a c e ount the energy of the dipole form ed i n v a c u o and the efficiency of its interaction with the medium; )(i=n~ is the refractive index of solvent i raised to the s econd power; and ,~ =n~ is the refractive index of solvent j raised to the second power. Measurements of Avi_j for a series of individual polyenes showed that in the range of N from 3 to 6 the magnitude of the shift is independent of N. Deviations are observed only for N > 6 and are probably due to the transition to non-true solutions. As an example, Table 2 summarizes the values of K calculated on the basis of experimental data for a series of dimethylpolyenes and unsubstituted polycene in different solvents. Data in Table 2 show that the magnitude of K depends only slightly on the choice of the solvent pair and chain length despite some differences in the end groups. By averaging all values for K in Table 2 we h a v e / ~ = 3 1 877 cm -1. This value was used in estimating Av when going over to the PA crystal.
375 TABLE 2 Values of K calculated on the basis of experimental data for a series of dimethylpolyenes and unsubstituted polycene in different solvents N
3 4 5 6
CH~-CH=CH-}NCH chloroform (a)
H {-CH ~ CH:)-NH isooctane (b)
CH-(-CH=CH~NCH dimethyl ether (c)
r~= 1.4467 (cm l), [24]
n = 1.391 45 (cm 1), [19]
n = 1.3526 (cm 1), [25]
35842 31596 28612 26316
37313 32895 29940 27473
38023 33445 30675 28409
Ka__e
Ka-b
32073 27191 30338 30779
37717 33307 34051 29667
The effective d e g r e e of dielectric p e r m e a b i l i t y of different t y p e s of PA h a s b e e n d e t e r m i n e d [ 2 6 - 2 8 ] . The o b t a i n e d v a l u e s lie in the r a n g e 3 < X < 7. A c c o r d i n g to Fig. 1 the t r a n s i t i o n f r o m i n v a c u o to the m e d i u m with X = 3 gives A v = 2 1 2 5 7 c m - 1 ; with X = 7 , A v = 2 7 3 2 3 c m -1. The d e p e n d e n c e of the width of the b a n d g a p on the chain length a n d the m a g n i t u d e of h v for N o oo h a s b e e n a n a l y s e d [29] b y u s i n g different t h e o r e t i c a l m o d e l s a n d b y e x t r a p o l a t i n g e x p e r i m e n t a l r e s u l t s for a series of s h o r t - c h a i n p o l y e n e s . It w a s s h o w n t h a t with a c c o u n t t a k e n of the e x c i t e d c o n f i g u r a t i o n s o n e - e l e c t r o n m o d e l s give the PN~N-1 d e p e n d e n c e in the c a s e of the Ag ---,Bu transition, while e x c i t o n m o d e l s w h i c h d e s c r i b e the chain with the aid of localized m o l e c u l a r orbitals of e t h y l e n e give the VN ~ N - 2 d e p e n d e n c e . Analysis of e x p e r i m e n t a l results for a H-(-CH=CH-)-NH series [19] g a v e a d e p e n d e n c e w h i c h is similar t o PN~N -1. H o w e v e r , for a series of dimet h y l p o l y e n e s [ 24, 25 ] this d e p e n d e n c e w a s n o t quite fulfilled. It w a s i m p o s s i b l e , on the b a s i s of results, to d r a w definite c o n c l u s i o n s c o n c e r n i n g the f o r m of the v . ~ f ( N ) function. In r e c e n t years, with the d e v e l o p m e n t of the S u - S c h r e i f f e r - H e e g e r [30] m o d e l a n d in an analysis of the p h o t o i n d u c e d a b s o r p t i o n s p e c t r a of P A [ 31 ] it h a s b e e n s h o w n t h a t a m o r e c o r r e c t t h e o r e t i c a l d e s c r i p t i o n of the PA e l e c t r o n s t r u c t u r e r e q u i r e s c o m p e n s a t i o n for the c o r r e l a t i o n of e l e c t r o n s [ 3 2 - 3 4 ] . F o r the p r e s e n t w o r k w e h a v e p r o c e s s e d e x p e r i m e n t a l v a l u e s of b,N for a series of s h o r t p o l y e n e s b y the l e a s t - s q u a r e s m e t h o d a n d b y u s i n g two t y p e s of functions: vN = v~ + A e x p ( - B N )
(2)
+D/(N+
(3)
VN =
Lr~
C)
w h e r e 72y is the f r e q u e n c y of the l o n g - w a v e v i b r a t i o n a l m a x i m u m in the o p t i c a l s p e c t r u m of a p o l y e n e m o l e c u l e haxSng length N; vo¢ is the f r e q u e n c y of the l o n g - w a v e v i b r a t i o n a l m a x i m u m in the o p t i c a l s p e c t r u m of a p o l y e n e m o l e c u l e of infinite length; a n d A, B, C a n d D are c o n s t a n t s , C v a l u e s b e i n g in the r a n g e f r o m 0 to 1.
376 An analysis shows that for the entire series of polyene the exponential function describes the experimental results with a smaller standard deviation than the ~ N - 1 function. As an example, Table 3 summarizes the parameters of eqns. (2) and (3) and the standard deviation
2~1/2
for several series of polyenes. The parameters of the exponential functions given in Table 3 correspond to the fast overreaching of the limits of ~N as a function of N, and for N = 30 the magnitude of VN-- ~ lies in the range of 8 . 7 - 0 . 3 7 cm -1, which is much less than the width of the vibrational maxima in the optical spectra. From this it follows that the difference in the frequency of long-wave vibrational maxima in the optical absorption spectra of different types of PA shown in Table 1 cannot be explained solely by changes in the length of the conjugated chain. Most of the experimental results on the structure of crystalline PA and model c o m p o u n d s obtained by X-ray, electronic and neut ron diffraction methods have been reviewed [35]. A comparison of the parameters of the crystalline structure for different types of PA shows that for c/s-PA practically all diffraction methods and all PA types give similar parameters for the lattice cell. In the case of t r a n s - P A identical parameters for the lattice cell have been determined only for samples which have not been subjected to strong mechanical action during synthesis or subsequent treatment. Since, in the present work, all types of PA were p r e p a r e d with the same catalyst and without the use of strong mechanical fields, we can assume that transition from fibrillar PA to soluble forms is not accom pani ed by changes in the parameters of the crystalline structure e xc e pt for the size of the crystallites. The analysis made above permits us to relate the position of the band in the optical absorption of different types of PA to their morphology. This relationship is fulfilled through changes in the effective dielectric permeability according to the mechanism of the dimensional effect. In other words, the shift of the absorption bands into the short-wave region during the transition from fibrillar PA to soluble compositions is due to the transition from conditions of absorption in the solid phase to absorption in solutions. Crystalline PA is a strongly anisotropic substance. The magnitudes of dielectric permeability in the direction perpendicular and parallel to the axis of the polymeric chain were found [28] to be X± = 1.8 and Xrl = 7.1, respectively. Proceeding from this we can ex p ect that the dimensional effects would be strongly anisotropic in nature. An attempt has been made to investigate the dimensional effect for an anisotropic substance by taking as an example needle-shaped monocrystals of radical-ion salts dispersed in a polymeric matrix [36]. The dimensional effect for PA compositions in a polymeric matrix will be described in our next paper.
diethyl e t h e r
CHa{- CH = CH -)-NCHj
chloroform
CH:~{- CH = CH -)-NCH 3
isooctane
H {- CH = CH -)-NH
Polycne and solvent
')
21810
22224
20032
(cm
v~
I I N = lit.
T h e p a r a m e t e r s of tile f u n c t i o n s
TABLE 3
41189
44091
38991
+Aexp(-BN)
0.31
0.39
0.28
B
m
146
132
87
')
16063
15990
13933
v~
(cm
')
Av
(cm
1.0
0.4
1.0
C
VN = v~ + D / ( N + C )
85643
67765
94329
D
385
200
152
Av (cm
')
-q -q
378
References 1 C. K. Chiang, M. A. Druy, S. K. Gau, A. J. Heeger, H. Shirakawa, E. Y. Louis and A. G. MacDiarmid, J. Am. Chem. Soc., 100 (1978) 1013. 2 T. Y. Skotheim, Handbook of Conducting Polymers, Marcel Dekker, New York, 1986. 3 N. Basescu, Z. K. Liu, D. Moses, A. J. Heeger, H. Naarmann and N. Theophilou, Nature (London), 6121 (1987) 403. 4 T. Ito, H. Shirakawa and S. Ikeda, J. Polym. Sci., Polym. Chem. Ed., 12 (1974) 11. 5 V. Enkelmann, G. Lieser, W. Mueller, G. Wegner, Angew. Makromol. Chem., 94 (1981) 105. 6 H. Eckhardt, J. Chem. Phys., 79 (1983) 2085. 7 M. Tanaka, A. W a t a n a b e and I. Tanaka, Bull. Chem. Soc. Jpn., 53 (1980) 3430. 8 R. H. Friend, Physics and Chemistry of Electronics and Ions in Condensed Matter, Reidel, Dordrecht, 1984, p. 625. 9 G. L. Baker and F. S. Bates, Macromolecules, 17 (1984) 2619. 10 M. Aldissi, J. Chem. Soc., Chem. Commun., 20 (1984) 1347. 11 F. S. Bates and G. L. Baker, Macromolecules, 16 (1983) 1013. 12 V. M. Kobi:cansky, N. Zh. Zurabyan, T. O. Nagapetyan and V. K. Skachkova, Vysokomol. Soedin. Ser. B, 29 (1987) 625. 13 J. L. Br6das, R. Q. Chance, R. H. B a u g h m a n and R. Sibley, J. Chem. Phys., 76 (1982) 3673. 14 R. Peierls, Quantum Chemistry of Solids, Inostrannaia Literatura, Moscow, 1956. 15 B. S. Hadson and B. E. Kohler, Synth. Met., 9 (1984) 241. 16 B. S. Hadson, B. E. Kohler and K. Schulten, in E. C. Lim (ed.), Excited States, Vol. 6, Academic Press, New York, 1982, p. 1. 17 F. Kajzar, S. Etemad, G. L. Baker and J. Messier, Synth. Met., 17 (1987) 563. 18 L. Lauchlan, S. Etemad, T. C. Chung, A. J. Heeger and A. G. MacDiarmid, Phys. Rev. B., 24 (1981) 3701. 19 F. Sondheimer, D. A. Ben-Wphraim and R. Wolovsky, J. Am. Chem. Soc., 87 (1961) 1675. 20 M. Fujimoto, K. Kamiya, Y. Tanaka and M. Tanaka, Synth. Met., 10 (1985) 367. 21 I. Shirotani, Y. Kamura, H. Inokuchi and T. Hirooka, Chem. Phys. Lett., 40 (1976) 257. 22 T. H. James, The Theory of Photographic Processes, Macmillan, New York, 4th edn., 1977. 23 E. U. Finkelshtein, E. V. Alekscev and E. L. Kozlov, Zh. Org. Khim., 10 (1974) 1027. 24 P. Nayler and M. C. Whiting, J. Chem. Soc., 9 (1955) 3037. 25 F. Bohlmann and H. I. Mannhardt, Chem. Bet., 89 (1956) 1307. 26 F. Devreux, I. Doerg, L. Mihaly, S. Pekker, A. Yanovssy and M. Kertesz, J. Polym. Sci., Polym. Phys. Ed., 19 (1981) 743. 27 A. Feldbaum, Y. W. Park, A. J. Heeger, A. G. MacDiarmid, G. Wnek, F. Karasz and Y. C. W. Chien, J. Polym. Sci., Polym. Phys. Lett., 19 (1981) 173. 28 H. Kahlert and G. Leising, Mol. Cryst. Liq. Cryst., 117 (1985) 1. 29 A. Szabo, I. Langlet and I. P. Malrien, Chem. Phys., 13 (1976) 173. 30 W. P. Su, I. R. Schrieffer and A. J. Heeger, Phys. Rev. B, 22 (1980) 2099. 31 Z. Vardeny and J. Tauc, Phys. Rev. Lett., 54 (1985) 1844. 32 D. Baeriswyl, D. K. Campbell and S. Mazumdar, Phys. Rev. Lett., 56 (1986) 1509. 33 Z. Vardeny and J. Tauc, Phys. Rev. Lett., 56 (1986) 1510. 34 I. C. Hicks and Tinka Gammel, Phys. Rev. B, 37 (1988) 6315. 35 R. H. Baughman, B. E. Kohler, I. J. Levy and C. Spangler, Synth. Met., 11 (1985) 37. 36 G. C. Papavassilion and S. S. Spanou, J. Chem. Soc., Faraday Trans. 2, 73 (1977) 1425.
367
Analysis of the electronic structure of polyacetylene based on its optical absorption spectra V. M. K o b r y a n s k y
and E. A. T e r e s h k o
Ir~,titute of Che,rdccd Physics, U.S.S.R. Academy of Sciences, Moscow (U.S.S.R.)
(Received September 11, 1990; accepted September 17, 1990)
Abstract A comparison of the characteristics of the electronic absorption spectra for polyacetylene (PA) prepared by different methods has been carried out. The comparison shows that the concentration of PA molecules (or their fragments) with a bandgap of different widths depends mainly on the crystalline structure and morphology of PA as well as on the concentration of conformational and chemical defects in the macromolecules. The dependence on chain length is evident only for low-molecular weight samples.
1. I n t r o d u c t i o n It is k n o w n t h a t p o l y a c e t y l e n e (PA) is a s e m i c o n d u c t o r with a c o n d u c t i v i t y of 10 -9 a n d 10 -~ o h m -1 c m -1 for c/s- a n d t r a n s - f o r m s , respectively. After d o p i n g the PA with e l e c t r o n - d o n o r or e l e c t r o n - a c c e p t o r c o m p o u n d s its c o n d u c t i v i t y i n c r e a s e s to 10'~-10 s o h m 1 c m - 1 and it c h a n g e s into a metallic state [1-3]. The w i d t h of the b a n d g a p ( 2 - 1 . 4 eV) in n e u t r a l t r a n s - P A as d e t e r m i n e d f r o m the e l e c t r o n i c a b s o r p t i o n s p e c t r u m d e p e n d s on h o w the PA w a s obtained. The p o s i t i o n a n d s h a p e of the a b s o r p t i o n b a n d in the e l e c t r o n i c s p e c t r a of different PA m o d i f i c a t i o n s c a n b e divided into five types. The first t y p e includes s t a n d a r d PA ( s e v e r a l m i c r o n s to s e v e r a l m i l l i m e t r e s thick) p r e p a r e d in the p r e s e n c e of S h i r a k a w a [4] and L u t t i n g e r [5] catalytic s y s t e m s . T h e s p e c t r a of c/s-PA are c h a r a c t e r i z e d b y a wide b a n d at 25 0 0 0 - 1 6 0 0 0 c m - 1 with t w o v i b r a t i o n a l m a x i m a at 18 0 0 0 a n d 16 4 0 0 e r a a n d a s h a r p s h o r t - w a v e drop, while t h o s e of t r a n s - P A c o n t a i n a wide b a n d at 1 6 0 0 0 - 1 2 0 0 0 c m -1 [6]. The s e c o n d t y p e includes thin PA films ( 1 0 0 0 - 2 0 0 0 / ~ ) . A g e n e r a l v i e w of the a b s o r p t i o n s p e c t r a of thin films is similar to t h a t of the a b s o r p t i o n s p e c t r a of s t a n d a r d film e x c e p t t h a t in the latter c a s e all the a b s o r p t i o n b a n d s are shifted in the s h o r t - w a v e d i r e c t i o n [7]. T h e third t y p e includes PA p r e p a r e d f r o m s a t u r a t e d p o l y m e r s . T h e e l e c t r o n i c a b s o r p t i o n s p e c t r a of t h e s e p o l y m e r s are c h a r a c t e r i z e d b y the c o m p l e t e a b s e n c e of a v i b r a t i o n a l s t r u c t u r e of b a n d s at a high d e g r e e of c o n v e r s i o n a n d a c o n s i d e r a b l e ( 0 . 5 - 1 . 0 eV) shift o f the a b s o r p t i o n b a n d
0379-6779/91/$3.50
© Elsevier Sequoia'Printed in The Netherlands
368
maximum of t r a n s - P A into the short-wave region as compared with that observed for standard films [8]. The fourth type includes block and grafted PA copolymers with saturated macromolecules. A Shirakawa catalyst has been used in graft and some of these copolymers are soluble. The absorption spectra of these copolymers resemble that of the third-type PA. As a rule the position of the enveloping curve is shifted into the short-wave region as compared with the absorption bands in standard films [9, 10]. The fifth type calls for special consideration. It includes soluble PA compositions with saturated polymers the electronic absorption spectra of which are characterized by fine structure. Such compositions can sometimes be obtained by polymerizing acetylene in solutions of saturated polymers. The polymerization of acetylene by using the Shirakawa catalytic system in a polystyrene solution yielded a product which is thought to be a suspension of PA in a polystyrene matrix [ 11 ]. The electronic absorption spectra of this composition is characterized by narrow absorption bands with a vibrational structure. A similar absorption spectrum has been described for a PA composition with polyvinylbutyral (PVB) which has been synthesized by polymerizing acetylene in a PVB solution [12]. In the present work we have investigated the relationship between the electronic absorption spectra and the structure and morphology of PA. Samples of PA and PA-PVB compositions were prepared with the use of a Luttinger catalyst.
2. Experimental PA and PA-PVB compositions were obtained by polymerizing acetylene in ethanol containing the required amount of Co(NO3)-NaBH4 catalyst and PVB according to a known method [12]. The Co(NO3)2:NaBH4 ratio was kept constant in all experiments at 4:1. For PA-PVB compositions the concentration of PVB was varied from 0.2 to 2.5 wt.%. The absorption spectra in the range 5 0 0 0 0 - 1 2 600 cm -1 were obtained with the aid of a Specord UV-Vis instrument; in the 1 4 0 0 0 - 3 5 0 0 cm -1 region a Specord NIR-61 instrument was used. PA and PA-PVB samples needed for spectral studies were made by dispersing the reaction mixture immediately after the synthesis. The gels were diluted to 0.001 wt.% PA and then dispersed by ultrasound or high-speed stirring for 30 min. The viscosity of the suspensions was increased by adding PVB solution up to 3 wt.% to increase their stability. Films of PA and PA-PVB compositions were prepared by suspension casting on horizontal supports. Electron microscopic investigations were carried out with the aid of a YEOL-YCM-35C scanning electron microscope. Before measurement the films were coated with a layer of gold (100-200 /~ thick) by the ionic sputtering method.
369
3. R e s u l t s
3.1. Physicochemical characteristics and morphology of PA and PA-PVB compositions The polymerization of acetylene in the presence of Co(NO3)2-NaBH4 in pure ethanol and in PVB solutions at concentrations of < 1.2 wt.%. PVB leads to the formation of thixotropic gels. Suspensions made from these gels are unstable and their consistency is gradually restored. The rate of structuralization and the physicochemical properties of the suspensions and gels depend on the composition of the polymerization solution. With an increase in the concentration of PVB in the polymerization solution the stability of the gels decreases and the time required for the suspension to return to the form of gel increases. The polymerization of acetylene in a solution containing 1.5 wt.% PVB results in the formation of a low-viscosity gel which dissolves on stirring. After stirring is s t oppe d no structuralization occurs for a long time. This means that the structuralization ability and thixotropic properties of PA-PVB compositions in the liquid phase decreases with an increase in the concentration of PVB in the polymerization solution, and that at a concentration of 1.5 wt.% PVB it virtually disappears. The polymerization of acetylene in a PVB solution at > 1.5 wt.% PVB results in the formation of coloured solutions which are not thixotropic. We shall refer to compositions which are obtained at a higher concentration of PVB and which are not thixotropic in the liquid as soluble compositions, and PA obtained under these conditions as the soluble form of PA. Morphological investigations of PA-PVB compositions with the aid of a scanning electron microscope showed that changes in their physicomechanical properties in the liquid phase are related to their morphology. Polymerization in solutions containing up to 0.6 wt.% PVB results in the formation of fibrils 3 0 0 - 5 0 0 /~ in diameter (Fig. 1). At 0 . 6 - 1 . 2 wt.% PVB thicker and shorter fibrils are formed which gradually begin to resemble
Fig. 1. Surface of PA film obtained in C2HsOH solution ( × 4 0 0 0 0 , reduced in reproduction 379/0). Fig. 2. Surface of PA-PVB film obtained in 1.5% solution of PVB ( × 4 0 0 0 0 , reproduction 37%.)
reduced in
370 ellipsoids. At 1.5 wt.% of the r e a c t i o n solution globules 1 5 0 - 5 0 0 / ~ in d i a m e t e r are f o r m e d (Fig. 2). On a f u r t h e r i n c r e a s e in c o n c e n t r a t i o n of PVB, soluble c o m p o s i t i o n s c o n t i n u e to be p r o d u c e d w i t h o u t any s u b - m o l e c u l a r f o r m a t i o n s w h e n e x a m i n e d u n d e r × 105 magnification. If we a s s u m e that the physicom e c h a n i c a l p r o p e r t i e s of the c o m p o s i t i o n s in the liquid p h a s e are directly related to the d i m e n s i o n and the f o r m o f the particles, t h e n o n a n a l o g y with data [9] for soluble P A - p o l y b u t a d i e n e c o m p o s i t i o n s we can s p e a k h e r e of particles 50 ,~ in diameter. Thus, the p o l y m e r i z a t i o n o f a c e t y l e n e in PVB solutions in the p r e s e n c e of a Luttinger catalyst yields c o m p o s i t i o n s the m o r p h o l o g y o f which varies f r o m fibrillar to globular and t h e n c h a n g e s into soluble forms.
3.2. Electronic absorption spectra of P A - P V B compositions Figure 3 shows the a b s o r p t i o n s p e c t r u m of a fibrillar PA s u s p e n s i o n o b t a i n e d by p o l y m e r i z i n g a c e t y l e n e in ethanol. The s p e c t r u m contains the b a n d of c/s-PA with two vibrational m a x i m a at 18 000 and 1 6 4 0 0 c m -1 and a s h o u l d e r of the trans-PA in the 1 6 0 0 0 - 1 2 0 0 0 c m -1 r e g i o n w i t h o u t a vibrational structure. It can also be that the short-wave d r o p o f the main a b s o r p t i o n b a n d gradually d e c r e a s e s with an increase in the f r e q u e n c y , and the a b s o r p t i o n e x t e n d s into the ultraviolet region. The p o l y m e r i z a t i o n o f a c e t y l e n e in ethanol with different c o n c e n t r a t i o n s o f the catalyst and different r e a c t i o n times r e s u l t e d in the f o r m a t i o n of PA fibrils 2 0 0 - 9 0 0 / ~ in diameter. Their a b s o r p t i o n s p e c t r a s h o w e d a t e n d e n c y t o w a r d s a slight long-wave shift of a b s o r p t i o n b a n d s with an i n c r e a s e in the d i a m e t e r of the fibrils. The shift of the positions of the m a x i m a a m o u n t s to 3 0 0 - 4 0 0 c m - ~ as the d i a m e t e r o f the fibrils i n c r e a s e s f r o m 200 to 9 0 0 / ~ . A c o m p a r i s o n of t h e s e data with data r e p o r t e d in the literature indicates that the position of the m a x i m a in the optical a b s o r p t i o n s p e c t r a nearly coincides for any s a m p l e s o f similar m o r p h o l o g y r e g a r d l e s s o f h o w the PA s a m p l e s w e r e p r e p a r e d or h o w their
/.0
o.s o,s
o.~
q~
o,l
0
,
~ cM " l
0 ~o
26
22
18
/~ -•/O'J
Fig. 3. Absorption spectrum of PA suspension obtained in C2H6OH solution. Fig. 4. Absorption spectrum of PA-PVB soluble compositions obtained in 1.5% solution of PVB.
371 a b s o r p t i o n s p e c t r a w e r e r e c o r d e d . A b s o r p t i o n s p e c t r a a n a l o g o u s to t h a t s h o w n in Fig. 3 are c h a r a c t e r i s t i c of s t a n d a r d PA films o b t a i n e d b y the S h i r a k a w a m e t h o d , d e s p i t e the fact t h a t the films w e r e d e t e r m i n e d b y reflection s p e c t r a a n d w e r e p r e p a r e d in the p r e s e n c e of a different catalytic s y s t e m . T h e a b s o r p t i o n s p e c t r a of fibrillar P A - P V B c o m p o s i t i o n s are similar to t h o s e of p u r e PA. H o w e v e r , a c h a n g e in c o m p o s i t i o n s with a g l o b u l a r m o r p h o l o g y a n d in soluble c o m p o s i t i o n s is a c c o m p a n i e d b y c o n s i d e r a b l e c h a n g e s in all the p a r a m e t e r s of their a b s o r p t i o n s p e c t r a . Figure 4 s h o w s the a b s o r p t i o n s p e c t r u m of a soluble P A - P V B c o m p o s i t i o n o b t a i n e d at 1.8 wt.% PVB c o n c e n t r a t i o n in the p o l y m e r i z a t i o n solution. In spite of a g e n e r a l similarity with the a b s o r p t i o n s p e c t r a of fibrillar PA, this a b s o r p t i o n s p e c t r u m is shifted by 2 0 0 0 c m - 1 t o w a r d s h o r t e r w a v e l e n g t h s ; it r e s e m b l e s m o r e the s p e c t r a of individual p o l y e n e s in solution. In the 22 0 0 0 - 1 7 0 0 0 c m - 1 r e g i o n lies an a b s o r p t i o n b a n d of cis-PA. Against the b a c k g r o u n d of t h r e e a b s o r p t i o n m a x i m a at 20 180, 1 8 9 3 0 a n d 1 7 7 4 0 c m -1 lies the s h o u l d e r due to t r a n s PA on w h i c h t h e r e are t h r e e p o o r l y r e s o l v e d v i b r a t i o n a l m a x i m a at 1 5 4 8 0 , 14 9 6 0 a n d 13 9 6 0 c m -1. In the s h o r t - w a v e r e g i o n of the s a g of the a b s o r p t i o n b a n d at 40 0 0 0 - 2 2 0 0 0 c m -1 t h e r e are eleven p o o r l y r e s o l v e d m a x i m a a n d inflections. The a b s o r p t i o n s p e c t r a of soluble P A - P V B c o m p o s i t i o n s o b t a i n e d at > 1.8 wt.% PVB in the r e a c t i o n solution r e s e m b l e t h a t s h o w n in Fig. 4. H e r e , with an i n c r e a s e in the c o n c e n t r a t i o n of PVB a f u r t h e r shift is o b s e r v e d of all the a b s o r p t i o n b a n d s t o w a r d s h o r t e r w a v e l e n g t h s . The m a g n i t u d e of the shift is p r o p o r t i o n a l to the c o n c e n t r a t i o n o f PVB in the r e a c t i o n solution. T a b l e 1 s u m m a r i z e s b o t h o u r d a t a a n d k n o w n r e s u l t s on the f r e q u e n c i e s of l o n g - w a v e v i b r a t i o n a l m a x i m a in the a b s o r p t i o n of different t y p e s of c/sa n d t r a n s - P A in the a b s o r p t i o n s p e c t r a o f w h i c h a v i b r a t i o n a l s t r u c t u r e is p r e s e n t at least for o n e of the PA forms. F o r s o m e t y p e s of PA a Vibrational s t r u c t u r e is o b s e r v e d for c/s- a n d t r a n s - f o r m s . A s c a n be s e e n f r o m t h e s e data, the shift in the p o s i t i o n of the m a x i m a on g o i n g o v e r f r o m the c/sto the t r a n s - f o r m in this c a s e a m o u n t s to 3 8 0 0 - 4 2 0 0 c m - 1, a n d the m a g n i t u d e of the shift is Virtually i n d e p e n d e n t o f the PA type. By a s s u m i n g t h a t A v c i s - t r a n s d e p e n d s only slightly on the t y p e of PA w e c a n e s t i m a t e the w i d t h of the b a n d g a p for the t r a n s - f o r m of PA t y p e s the a b s o r p t i o n s p e c t r a of w h i c h s h o w a v i b r a t i o n a l s t r u c t u r e for the c / s - f o r m alone. T h e s e e s t i m a t i o n s are g i v e n in a c o l u m n ( t r a n s - P A c a l c u l a t e d m a x i m a ) of T a b l e 1. The p a r a m e t e r s of the a b s o r p t i o n s p e c t r a of t h o s e PA t y p e s in the a b s o r p t i o n s p e c t r a of w h i c h v i b r a t i o n a l s t r u c t u r e s are a b s e n t for at least o n e of the f o r m s are n o t s h o w n in T a b l e 1 a n d will not b e d i s c u s s e d in the p r e s e n t w o r k . As can b e s e e n f r o m d a t a in T a b l e 1, the difference in the p o s i t i o n of the a b s o r p t i o n m a x i m a for PA of different m o r p h o l o g i e s is quantitatively r e l a t e d to the d i m e n s i o n of the p a r t i c l e s of c o r r e s p o n d i n g s a m p l e s . A m a x i m a l shift ( ~ 2 3 0 0 c m - 1 ) into the s h o r t - w a v e r e g i o n o c c u r s during t h e t r a n s i t i o n f r o m fibrillar PA ( 9 0 0 ~ in d i a m e t e r ) to soluble c o m p o s i t i o n s . An analysis of t h e o b t a i n e d results m a k e s it p o s s i b l e to identify t h r e e i m p o r t a n t c h a r a c t e r i s t i c s of the e l e c t r o n i c a b s o r p t i o n s p e c t r a o f PA:
372 TABLE 1 Spectral characteristics of different PA compositions PA type (catalyst)
Morphology (dimension, /~)
Maxima ( c m - 1) c/s-Form
trans-Form
Exp.
Exp. 12000
Source
Calc.
Standard PA (Luttinger; suspension)
fibrils (300)
16500
Standard PA (Luttinger; suspension)
fibrils (900)
16200
Composition PA-PVB
globules (150-200)
17700
13960
present work
Composition PA-PVB
soluble form
18100
14300
present work
Standard PA Shirakawa film
fibrils (300-500)
16400
12OOO
[6]
Thin PA film
fibrils (30-50)
17100
13000
[71
Composition PA-polystyrene
soluble film
17150
13800
[11]
present work 12000
present work
(i) The transition from fibrillar PA to soluble forms is accom pani ed by a shift in the absorption maxima of the c/s- and t r a n s - P A into the shortwave region. For soluble PA the position of the maxima in the absorption spectra is shifted into the short-wave region with an increase in the PVB concentration in the polymerization solution. (ii) For soluble PA-PVB compositions the vibrational structure of c/sand t r a n s - f o r m s is present in the short-range spectral region. (iii) The transition from fibrillar PA to soluble compositions is a c c o m p a n i e d by narrowing of the absorption bands because of a decrease in the absorption intensity in the short-wave region. In this case the absorption of soluble compositions begins to resemble the absorption bands of individual polyenes in the solution, but first is shifted into the long-wave region. We shall discuss only the position of the maxima of the absorption bands.
4. D i s c u s s i o n
The analysis of the typical spectra of PA is based on the theory of electronic structure of the polyene chain and s p e c t r o s c o p y of individual
373 polyenes. According to existing concepts there should be a bandgap in a polyene chain of infinite length owing to the alternating bond lengths and interelectronic correctional interaction [ 13, 14]. Calculations for short polyenes as a rule yield results that coincide with the experimental data when the correlation of electrons is taken into account. Both theoretical analysis and experimental results indicate that a lower excited state in short polyenes has A~ symmetry. Direct electron transitions into this state are of low intensity and have been observed only in the case of a twop h o t on excitation for N from 3 to 6 [15, 16]. Long-wave optical absorption is due to the transition into the Bu state. It has been shown that the energy of transition depends on the num ber of conjugated double bonds, the conformations of the chain and the dielectric characteristics of the medium. Fluorescence in short polyenes is regarded and proceeds from the 2Ag excited state or the mixed 2Ag+/Bu state, formed with a change in the conformation of the polyene chain [15, 16]. In PA the position of the lower excited state, 2Ag, has not been established. By using the method of two-photon s p e c t r o s c o p y the energy of the 2Ag state was found to be ~ 1.93 eV [17], i.e., greater than the lower edge of the bandgap ( ~ 1.4 eV). However, it was assumed [15] that the 2Ag state in long polyenes b e c o m e s degenerate with the level of the neutral soliton. The optical absorption band of PA in the 2 - 1 . 4 eV region is related to the dipole transition into the exciton band or into the conductivity band [13]. From photoconductivity data for t r a n s - P A it follows that the conductivity band extends even below the edge of the bandgap [18]. It can be supposed that an inter-band optical transition in PA is of the same nature as the long-wave dipole transition in short-chain polyenes. This proposition is s u p por t ed by gradual change in the characteristics of the optical transition from fibrillar PA to soluble compositions (shown in the present work) and by similar values of the intensities of the r e d u c e d bands for optical absorption of PA and short polyenes. For short polyenes the intensity of the absorption band A g - - ~ B u increases linearly with the number of double bonds in the macromolecular chain [19]. The extinction calculated per double bond equals ~ 104 cm -1 1 tool- 1. The intensity of the absorption band for a film (density 0.4 g cm -3) of t r a n s - P A (prepared by the Shirakawa method) is 1.5 105 cm -1 [20]. This value also co r r es ponds to extinction if recalculated per double bond, i.e. ~ 104 cm ~ 1 tool 1. In other words, the magnitude of the dipole m o m e n t of optical transition and consequently the degree of charge separation in the initially formed e l e c t r o n - h o l e pair are similar for PA and short-chain polyenes. It can be assumed that a possible discrepancy between the order position of the electron levels 2A~ and Bu in short-chain polyenes and the same states in PA is related to an increase in the polarizability of polyene with an increase in the length of the polyene chain in the solid state. Most investigations of the electronic spectra of short-chain polyenes have been carried out on solutions. At the same time PA is insoluble in any solvent and all its spectral
374 characteristics belong to the solid aggregate state. For molecules with a high degree of electronic polarizability and larger dipole m om ent s of electronic transitions, strong long-wave shifts of absorption bands were observed during the transition from solutions to the solid phase. Such shifts have been found for many types of molecules with a developed conjugated system, for example, for tetraseleniumtetracene [21], cyanine dyes [22] and fl-carotene [23]. The order of location of the bands of the excited state in crystalline PA can vary relative to their location in the individual polyene chain owing to the electronic polarization of molecules in the region of dipole transition and to the intermolecular exchange interactions in the ground and excited states. In the simplest case we can consider that the positions of the bands shift only to the polarizability of the surroundings of dipole transitions. Hence, the energy of the exciton band and the conductivity band should decrease, these bands becoming populated with the bound and free elect r o n - h o l e pairs, respectively. The conductivity band undergoes a larger shift so that can be below the exciton band. Qualitatively, the shift of the absorption band in inter-zonal transition in PA relative to its position in a hypothetical unfolded polyene chain i n v a c u o can be estimated on the assumption that the mechanism of the interaction, the dipole m o m e n t of electronic transition and the medium when changing from short polyenes in a solution to crystalline PA remain unchanged. For short-chain polyene in solution the relationship between the position of the optical absorption band and the polarizability of the medium can be determined experimentally. The relationship between the shift in the frequency of optical transition and the electronic part of the dielectric permeability, x = n 2, can be defined as follows:
where Avi_j is the shift of the frequency of the long-wave vibrational m axi m um of the absorption band when going from solvent i to solvent j; K is the constant which takes into a c e ount the energy of the dipole form ed i n v a c u o and the efficiency of its interaction with the medium; )(i=n~ is the refractive index of solvent i raised to the s econd power; and ,~ =n~ is the refractive index of solvent j raised to the second power. Measurements of Avi_j for a series of individual polyenes showed that in the range of N from 3 to 6 the magnitude of the shift is independent of N. Deviations are observed only for N > 6 and are probably due to the transition to non-true solutions. As an example, Table 2 summarizes the values of K calculated on the basis of experimental data for a series of dimethylpolyenes and unsubstituted polycene in different solvents. Data in Table 2 show that the magnitude of K depends only slightly on the choice of the solvent pair and chain length despite some differences in the end groups. By averaging all values for K in Table 2 we h a v e / ~ = 3 1 877 cm -1. This value was used in estimating Av when going over to the PA crystal.
375 TABLE 2 Values of K calculated on the basis of experimental data for a series of dimethylpolyenes and unsubstituted polycene in different solvents N
3 4 5 6
CH~-CH=CH-}NCH chloroform (a)
H {-CH ~ CH:)-NH isooctane (b)
CH-(-CH=CH~NCH dimethyl ether (c)
r~= 1.4467 (cm l), [24]
n = 1.391 45 (cm 1), [19]
n = 1.3526 (cm 1), [25]
35842 31596 28612 26316
37313 32895 29940 27473
38023 33445 30675 28409
Ka__e
Ka-b
32073 27191 30338 30779
37717 33307 34051 29667
The effective d e g r e e of dielectric p e r m e a b i l i t y of different t y p e s of PA h a s b e e n d e t e r m i n e d [ 2 6 - 2 8 ] . The o b t a i n e d v a l u e s lie in the r a n g e 3 < X < 7. A c c o r d i n g to Fig. 1 the t r a n s i t i o n f r o m i n v a c u o to the m e d i u m with X = 3 gives A v = 2 1 2 5 7 c m - 1 ; with X = 7 , A v = 2 7 3 2 3 c m -1. The d e p e n d e n c e of the width of the b a n d g a p on the chain length a n d the m a g n i t u d e of h v for N o oo h a s b e e n a n a l y s e d [29] b y u s i n g different t h e o r e t i c a l m o d e l s a n d b y e x t r a p o l a t i n g e x p e r i m e n t a l r e s u l t s for a series of s h o r t - c h a i n p o l y e n e s . It w a s s h o w n t h a t with a c c o u n t t a k e n of the e x c i t e d c o n f i g u r a t i o n s o n e - e l e c t r o n m o d e l s give the PN~N-1 d e p e n d e n c e in the c a s e of the Ag ---,Bu transition, while e x c i t o n m o d e l s w h i c h d e s c r i b e the chain with the aid of localized m o l e c u l a r orbitals of e t h y l e n e give the VN ~ N - 2 d e p e n d e n c e . Analysis of e x p e r i m e n t a l results for a H-(-CH=CH-)-NH series [19] g a v e a d e p e n d e n c e w h i c h is similar t o PN~N -1. H o w e v e r , for a series of dimet h y l p o l y e n e s [ 24, 25 ] this d e p e n d e n c e w a s n o t quite fulfilled. It w a s i m p o s s i b l e , on the b a s i s of results, to d r a w definite c o n c l u s i o n s c o n c e r n i n g the f o r m of the v . ~ f ( N ) function. In r e c e n t years, with the d e v e l o p m e n t of the S u - S c h r e i f f e r - H e e g e r [30] m o d e l a n d in an analysis of the p h o t o i n d u c e d a b s o r p t i o n s p e c t r a of P A [ 31 ] it h a s b e e n s h o w n t h a t a m o r e c o r r e c t t h e o r e t i c a l d e s c r i p t i o n of the PA e l e c t r o n s t r u c t u r e r e q u i r e s c o m p e n s a t i o n for the c o r r e l a t i o n of e l e c t r o n s [ 3 2 - 3 4 ] . F o r the p r e s e n t w o r k w e h a v e p r o c e s s e d e x p e r i m e n t a l v a l u e s of b,N for a series of s h o r t p o l y e n e s b y the l e a s t - s q u a r e s m e t h o d a n d b y u s i n g two t y p e s of functions: vN = v~ + A e x p ( - B N )
(2)
+D/(N+
(3)
VN =
Lr~
C)
w h e r e 72y is the f r e q u e n c y of the l o n g - w a v e v i b r a t i o n a l m a x i m u m in the o p t i c a l s p e c t r u m of a p o l y e n e m o l e c u l e haxSng length N; vo¢ is the f r e q u e n c y of the l o n g - w a v e v i b r a t i o n a l m a x i m u m in the o p t i c a l s p e c t r u m of a p o l y e n e m o l e c u l e of infinite length; a n d A, B, C a n d D are c o n s t a n t s , C v a l u e s b e i n g in the r a n g e f r o m 0 to 1.
376 An analysis shows that for the entire series of polyene the exponential function describes the experimental results with a smaller standard deviation than the ~ N - 1 function. As an example, Table 3 summarizes the parameters of eqns. (2) and (3) and the standard deviation
2~1/2
for several series of polyenes. The parameters of the exponential functions given in Table 3 correspond to the fast overreaching of the limits of ~N as a function of N, and for N = 30 the magnitude of VN-- ~ lies in the range of 8 . 7 - 0 . 3 7 cm -1, which is much less than the width of the vibrational maxima in the optical spectra. From this it follows that the difference in the frequency of long-wave vibrational maxima in the optical absorption spectra of different types of PA shown in Table 1 cannot be explained solely by changes in the length of the conjugated chain. Most of the experimental results on the structure of crystalline PA and model c o m p o u n d s obtained by X-ray, electronic and neut ron diffraction methods have been reviewed [35]. A comparison of the parameters of the crystalline structure for different types of PA shows that for c/s-PA practically all diffraction methods and all PA types give similar parameters for the lattice cell. In the case of t r a n s - P A identical parameters for the lattice cell have been determined only for samples which have not been subjected to strong mechanical action during synthesis or subsequent treatment. Since, in the present work, all types of PA were p r e p a r e d with the same catalyst and without the use of strong mechanical fields, we can assume that transition from fibrillar PA to soluble forms is not accom pani ed by changes in the parameters of the crystalline structure e xc e pt for the size of the crystallites. The analysis made above permits us to relate the position of the band in the optical absorption of different types of PA to their morphology. This relationship is fulfilled through changes in the effective dielectric permeability according to the mechanism of the dimensional effect. In other words, the shift of the absorption bands into the short-wave region during the transition from fibrillar PA to soluble compositions is due to the transition from conditions of absorption in the solid phase to absorption in solutions. Crystalline PA is a strongly anisotropic substance. The magnitudes of dielectric permeability in the direction perpendicular and parallel to the axis of the polymeric chain were found [28] to be X± = 1.8 and Xrl = 7.1, respectively. Proceeding from this we can ex p ect that the dimensional effects would be strongly anisotropic in nature. An attempt has been made to investigate the dimensional effect for an anisotropic substance by taking as an example needle-shaped monocrystals of radical-ion salts dispersed in a polymeric matrix [36]. The dimensional effect for PA compositions in a polymeric matrix will be described in our next paper.
diethyl e t h e r
CHa{- CH = CH -)-NCHj
chloroform
CH:~{- CH = CH -)-NCH 3
isooctane
H {- CH = CH -)-NH
Polycne and solvent
')
21810
22224
20032
(cm
v~
I I N = lit.
T h e p a r a m e t e r s of tile f u n c t i o n s
TABLE 3
41189
44091
38991
+Aexp(-BN)
0.31
0.39
0.28
B
m
146
132
87
')
16063
15990
13933
v~
(cm
')
Av
(cm
1.0
0.4
1.0
C
VN = v~ + D / ( N + C )
85643
67765
94329
D
385
200
152
Av (cm
')
-q -q
378
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