Electron Transfer in Biology and the Solid State - American Chemical

23 Stabilization of Conducting Heteroaromatic Polymers

Downloaded by UNIV OF LEEDS on June 18, 2016 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1990-0226.ch023

in Large-Pore Zeolite Channels Thomas B e i n1, Patricia Enzel , Francois Beuneu , and Libero Z u p p i r o l i 1

1 2

2

2

Department of Chemistry, University of New Mexico, Albuquerque, NM 87131 Laboratoire des Solides Irradiés, S. Ε. S. I., École Polytechnique, F-91128 Palaiseau Cedex, France

Different strategies to encapsulate polymeric chains in ordered hosts are discussed. Pyrrole was polymerized as a model within the crystalline channel system of faujasite (three-dimensional) and mordenite (one-dimensional) zeolite molecular sieves. Polymerization required the presence of an intrazeolite oxidant such as ferric or cupric ions. No reaction was observed with the Νa and Fe(II) forms of the zeolites. The yield of the reaction was highest with vapor-phase pyrrole, and it was low in aqueous solvents. The systems were characterized with a combination of electronic, infrared, and Raman spectroscopic data. Electron spin resonance measurements were used to explore the trans port properties, and preliminary bulk powder conductivity meas urementswere performed to evaluate potential conduction paths on the outside of the zeolite crystals. The intrazeolite heteroaromatic polymer chains represent the first example of host-stabilized one -dimensionalmolecular conductors, or molecular wires.

L· H E D I S C O V E R Y O F D O P E D C O N J U G A T E D P O L Y M E R S has g e n e r a t e d

sub

stantial research interest, b o t h at t h e f u n d a m e n t a l l e v e l a n d i n v i e w o f possible applications (1-3). T h e s e p o l y m e r s i n c l u d e d o p e d p o l y a c e t y l e n e , p o l y a n i l i n e , p o l y p y r r o l e , a n d other polyheterocycles. P o t e n t i a l applications based o n t h e c o n d u c t i n g properties o f these systems range from l i g h t - w e i g h t batteries, antistatic e q u i p m e n t , a n d m i c r o e l e c t r o n i c s to speculative concepts such as " m o l e c u l a r e l e c t r o n i c " devices (4, 5). 0065-2393/90/0226-0433$06.00/0 © 1990 American Chemical Society

Johnson et al.; Electron Transfer in Biology and the Solid State Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

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E L E C T R O N TRANSFER IN BIOLOGY A N D T H E SOLID STATE

P o l y p y r r o l e (PPy), for example, has b e e n extensively s t u d i e d i n the form of t h i n films deposited o n electrode surfaces (6). E l e c t r o c h e m i c a l oxidative p o l y m e r i z a t i o n of p y r r o l e w i t h anions (e.g., C 1 0 ~ , H S 0 ~ ) present i n so l u t i o n results i n r e l a t i v e l y air-stable, h i g h l y c o n d u c t i n g films. P o l y p y r r o l e can also be p r e p a r e d v i a c h e m i c a l oxidation of p y r r o l e w i t h Cu(II) or Fe(III) salts i n solution (7-10). T h e p r o p o s e d reaction p a t h (6) v i a cation radicals is s h o w n i n S c h e m e I. 4


H

4

H

Scheme I. Oxidative polymerization of pyrrole. I n contrast to inorganic semiconductors that are c h a r a c t e r i z e d b y t h r e e d i m e n s i o n a l covalent b o n d i n g a n d h i g h c a r r i e r m o b i l i t i e s a n d are adequately d e s c r i b e d b y rigid-band models, interactions i n organic p o l y m e r s are h i g h l y anisotropic (11). A t o m s are covalently l i n k e d a l o n g the chains, whereas i n t e r c h a i n interactions are m u c h w e a k e r ; these structural features can cause collective instabilities such as P e i e r l s distortions. D o p i n g occurs b y charge transfer b e t w e e n the intercalated dopant molecules or atoms a n d the organic chains, a n d it can result i n substantial local relaxations of the c h a i n geometry. N e w l o c a l i z e d electronic states i n the gap are i n t r o d u c e d b y these chargetransfer-induced local geometric modifications of the p o l y m e r . I n heteroaromatic p o l y m e r s such as P P y , the g r o u n d state is n o n d e -


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generate; the ground-state geometry w i t h aromatic structure w i t h i n rings a n d single bonds b e t w e e n rings is m o r e stable than the c o r r e s p o n d i n g q u i n o i d resonance structure. Because the q u i n o i d structure has a smaller b a n d gap (lower i o n i z a t i o n p o t e n t i a l a n d larger e l e c t r o n affinity), the i n t r o d u c t i o n of a charge o n the c h a i n can result i n relaxation from the aromatic to the q u i n o i d structure. T h e r e s u l t i n g electronic structure of P P y as a f u n c t i o n of


d o p i n g is d e p i c t e d schematically i n F i g u r e 1.

Figure 1. Band structure of polypyrrole as a function of doping level (schematic). Key: A, neutral polymer; B, polaron ( + ), spin = Vfe, three new transitions; C, bipolaron ( + +), spin = 0, two transitions; D, heavily doped, bipolaron bands. R e c e n t studies (12, 13) conclude that at l o w d o p i n g levels, the chains are i o n i z e d to p r o d u c e a radical cation (polaron) that is " p i n n e d " to the c o u n t e r i o n a n d does not c o n t r i b u t e significantly to the c o n d u c t i v i t y . A t h i g h e r d o p i n g levels, polarons can c o m b i n e or i o n i z e to f o r m spinless d i cations (bipolarons) that are associated w i t h a q u i n o i d segment e x t e n d i n g o v e r four to five rings. T h e bipolarons are assumed to transfer charge v i a i n t e r c h a i n h o p p i n g that corresponds to the o b s e r v e d spinless c o n d u c t i v i t y .


436


T h i s m o d e l is consistent w i t h the absence of P a u l i s u s c e p t i b i l i t y i n the h i g h l y c o n d u c t i n g f o r m of P P y (13). T h e study of the c o n d u c t i o n m e c h a n i s m of these p o l y m e r s has b e e n i m p e d e d b y the l o w l e v e l of structural d e f i n i t i o n of most samples. T h e amorphous products of e l e c t r o c h e m i c a l a n d c h e m i c a l oxidative p o l y m e r i zation reactions present a w i d e range of possible i n t e r c h a i n interactions (e. g.,


islands w i t h m i c r o c r y s t a l l i n e o r d e r separated b y amorphous regions, u n k n o w n degrees o f c r o s s - l i n k i n g , or b u n d l e s o f fibers c o m b i n e d i n d i s o r d e r e d arrays). F u n d a m e n t a l studies o f the e l e c t r o n i c structure a n d c o n d u c t i o n m e c h a n i s m o f c o n d u c t i n g p o l y m e r s w o u l d benefit substantially i f the l o w d i m e n s i o n a l structures w e r e available as isolated, s t r u c t u r a l l y w e l l d e f i n e d , c h e m i c a l l y accessible entities. T h e goal o f o u r research p r o g r a m i n this area is to d e s i g n c o r r e s p o n d i n g m o d e l systems v i a encapsulation o f p o l y m e r i c c h a i n conductors i n l o w - d i m e n s i o n a l , crystalline host lattices, p a r t i c u l a r l y i n zeolites. E m b e d d i n g a single c h a i n i n a zeolite m a t r i x is also o f interest, because the e l e c t r o n i c properties of single chains i n t h e s o l i d state are not easily available. C h a i n segments, e v e n i f short, are i n t e r e s t i n g b e cause o f p o t e n t i a l q u a n t u m - s i z e effects o n the e l e c t r o n i c s t r u c t u r e that have b e e n o b s e r v e d i n c o l l o i d a l s e m i c o n d u c t o r systems, b o t h i n suspension a n d stabilized i n zeolite host systems (14-17). T h i s c h a p t e r reports o n o u r i n i t i a l progress (18) i n the encapsulation of heteroaromatic conjugated p o l y m e r s i n large-pore zeolites, p a r t i c u l a r l y p o l y p y r r o l e . W e r e c e n t l y s u c c e e d e d i n s y n t h e s i z i n g p o l y a n i l i n e (19, 20) a n d p o l y t h i o p h e n e (21) i n zeolite Y a n d m o r d e n i t e . P r e v i o u s l y r e p o r t e d strategies for the d e s i g n o f l o w - d i m e n s i o n a l structures are discussed i n the f o l l o w i n g sections.

Stabilization of Low-Dimensional Polymer Structures U r e a a n d o t h e r organic hosts have b e e n e x p l o r e d for the r a d i a t i o n - i n d u c e d i n c l u s i o n p o l y m e r i z a t i o n o f clathrated m o n o m e r s s u c h as b u t a d i e n e (22). W e l l - s t u d i e d examples i n c l u d e v i n y l c h l o r i d e , a c r y l o n i t r i l e a n d b u t a d i e n e i n u r e a , b u t a d i e n e , p e n t a d i e n e i n deoxycholic a c i d , a n d e t h y l e n e a n d p r o p y l e n e i n p e r h y d r o t r i p h e n y l e n e . P o l y m e r i z a t i o n i n the clathrates is u s u a l l y i n d u c e d b y exposure to h i g h - e n e r g y r a d i a t i o n that produces radicals d e r i v e d from host a n d guest m o l e c u l e s , a n d proceeds v i a a l i v i n g radical m e c h a n i s m . I n c l u s i o n p o l y m e r i z a t i o n can result i n a h i g h degree o f steric c o n t r o l of the r e s u l t i n g i n c l u d e d p o l y m e r chains. T o o u r k n o w l e d g e , no conjugated, c o n d u c t i n g clathrate systems have yet b e e n s y n t h e s i z e d . I f the d i m e n s i o n o f c h a n n e l - s h a p e d hosts is e x t e n d e d s u c h that u n h i n d e r e d diffusion o f m o n o m e r s can occur, c o n d u c t i n g p o l y m e r s can b e f o r m e d o n an electrode surface. P o l y p y r r o l e a n d poly(3-methylthiophene) fibrils w i t h diameters b e t w e e n 0.03 a n d 1 μπι at 10-μηι l e n g t h have b e e n s y n t h e s i z e d


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i n m i c r o f i l t e r m e m b r a n e s (Nuclepore) (23, 24). S t r e t c h a l i g n m e n t has often b e e n e m p l o y e d to create d i r e c t i o n a l anisotropy i n p r e f o r m e d p o l y m e r s , such as i n polyacetylene (25-27). L i q u i d crystal p o l y m e r i z a t i o n u n d e r a m a g n e t i c field is an alternative t e c h n i q u e to achieve a l i g n m e n t (28, 29). Inorganic layer c o m p o u n d s have b e e n e x p l o r e d for the p o l y m e r i z a t i o n of organic c o m p o u n d s i n t w o d i m e n s i o n s (30). P r o m i n e n t examples are based o n layer perovskite h a l i d e salts w i t h the general f o r m u l a ( R C H N H 3 ) M X , w h e r e M is a d i v a l e n t m e t a l such as M n , F e , C u , a n d C d , a n d X is a h a l i d e i o n (31). T h e organic cations separate the layers f o r m e d b y the m e t a l halides. I f the organic cations contain 2,4-diene units, the c o r r e s p o n d i n g l a y e r e d c o m p o u n d s can be p o l y m e r i z e d w i t h g a m m a i r r a d i a t i o n to f o r m 1,4-disubstituted frans-polybutadienes (32, 33). H o w e v e r , i f r e l a t e d d i a c e t y l e n e - c o n t a i n i n g layer perovsldtes are p o l y m e r i z e d , the o r i g i n a l lattice is gradually b r o k e n u p (30). R e c e n t studies e x p l o r e d the intercalative p o l y m e r i z a t i o n of p y r r o l e , t h i o p h e n e , a n d a n i l i n e i n l a y e r e d F e O C l (and V O ) (34-36); this p o l y m e r i z a t i o n exploits the w e l l - e s t a b l i s h e d oxidative intercalation of organic molecules (37, 38) w i t h concomitant r e d u c t i o n of F e O C l . It was f o u n d that p y r r o l e intercalated i n t o F e O C l a n d p o l y m e r i z e d to result i n a n i n o r g a n i c c o n d u c t i n g p o l y m e r h y b r i d structure w i t h a n i n t e r l a y e r spacing i n c r e a s e d b y 5.23 Â.


2

2

2

4

s

O t h e r t w o - d i m e n s i o n a l systems that w i l l not b e discussed h e r e i n c l u d e the e l e c t r o p o l y m e r i z a t i o n o f a n i l i n e intercalated i n m o n t m o r i l l o n i t e clay (39) a n d p o l y m e r i z a t i o n s i n L a n g m u i r - B l o d g e t t films o r b i l a y e r m e m b r a n e s (40). Efforts to f o r m composites b e t w e e n p o l y p y r r o l e a n d a v a r i e t y of porous materials such as p a p e r , c l o t h , or w o o d have b e e n based o n an approach comparable to that u s e d i n the present study. T y p i c a l l y , the respective m a t e r i a l was i m p r e g n a t e d w i t h an oxidant s u c h as F e C l (41, 42) a n d s u b 3

sequently contacted w i t h p y r r o l e vapor or solution. P o l y p y r r o l e (and p o l y aniline) have b e e n i n c l u d e d i n perfluorosulfonated i o n o m e r m e m b r a n e s (Nafion) b y stepwise treatment w i t h aqueous f e r r i c c h l o r i d e a n d the m o n omers (43). T h i s b r i e f survey indicates that no single chains of conjugated systems w i t h long-range o r d e r have yet b e e n s t a b i l i z e d i n s o l i d matrices. W e w i l l show that o u r approach, based u p o n intrazeolite p o l y m e r i z a t i o n reactions, succeeds i n the formation of such systems.

Zeolites as Microporous Host Structures Zeolites (44-47) are crystalline o p e n framework m e t a l oxide structures (clas sically aluminosilicates w i t h h y d r o p h i l i c surfaces) w i t h p o r e sizes b e t w e e n 3 a n d 12 Â a n d exchangeable cations c o m p e n s a t i n g for the negative charge of the framework. T h e topologies of these systems i n c l u d e o n e - d i m e n s i o n a l channels, i n t e r s e c t i n g t w o - d i m e n s i o n a l channels, a n d t h r e e - d i m e n s i o n a l o p e n frameworks. R e c e n t d e v e l o p m e n t s i n c l u d e h y d r o p h o b i c structures


438


w i t h compositions close to S i 0 (48), i n c o r p o r a t i o n o f transition metals i n t o the framework (49), a n d t h e discovery o f m e t a l a l u m i n o p h o s p h a t e sieves (50). A l k a l i m e t a l cations, possibly coordinated to o x y g e n - m e t a l rings w i t h C o r C s y m m e t r y i n zeolite Y ( F i g u r e 2), c a n b e exchanged for t r a n s i t i o n m e t a l ions a n d the system c a n b e heat-treated to i n d u c e cation m i g r a t i o n and r e m o v a l of water. T a b l e I presents a b r i e f d e s c r i p t i o n o f zeolite structure types u s e d i n this study. 2

3 v

2 v

A

Β


100

010 Figure 2. Structure of faujasite (A) and mordenite (B). T h e s e features make zeolites e x t r e m e l y attractive candidates as hosts for p o l y m e r i c conductors. T h e y offer w e l l - d e f i n e d , stable crystalline c h a n n e l structures w i t h d i m e n s i o n s at t h e m o l e c u l a r l e v e l . T h e nature o f the i n t e r n a l surface, as d e t e r m i n e d b y cation type a n d other factors, w i l l affect the d i p o l a r and redox surface interactions a n d diffusion rates of polar versus n o n p o l a r m o n o m e r s . P r e v i o u s w o r k related to t h e present study i n c l u d e s t h e catalytic formation o f polyacetylene from acetylene o n t h e external surface o f C o Y and N i Y zeolites (51) a n d o n K X zeolite (52). T h i s chapter presents recent results o n t h e successful encapsulation o f p o l y p y r r o l e chains i n large-pore t h r e e - d i m e n s i o n a l a n d o n e - d i m e n s i o n a l z e o lite hosts. T h e host structures e x a m i n e d i n this study i n c l u d e zeolite Y (threed i m e n s i o n a l c h a n n e l system, 7.5-Â pores), m o r d e n i t e (one-dimensional channels, 7 - A pores), a n d zeolite A (three-dimensional channels, 4 . 1 - A pores). P o l y p y r r o l e was oxidatively p o l y m e r i z e d i n C u ( I I ) - o r Fe(III)-cont a i n i n g zeolite pores, such as NaY + FeS0 (aq) 4

NaFe(II)Y

^

° > C

pyrrole

NaFe(III)Y

(vapor/solvent)

> PPy/NaFe(II)Y


(1)


8

12

2

2

8

12

2

2

12

2

2

2

8

[Na [(Al0 ) (Si0 ) ] 27 H 0 ] N a g s K A l O ^ S i O ^ a J 240 H 0 Na [(Al0 ) (Si0 )4o] 24 H 0

y

Unit Cell Composition α, β β, 26-hedron(II) complex 5-1

Cage Type

a

Main Channels, A 4,1 ***

η ^ *** 6.7 χ 7.0*** 2.9 χ 5.7*

"The number of stars (*) at the channel description indicates the dimensionality of channel connections.

LTA, Linde A F AU, Faujasite MOR, Mordenite

Name

Table I. Representative Zeolite Structure Types


440


T h e intrazeolite oxidation o f Fe(II) w i t h oxygen p r o b a b l y i n v o l v e s t h e for m a t i o n o f F e ( I I I ) - 0 - F e ( I I I ) bridges i n appropriate c o o r d i n a t i o n sites (53), although generation o f some amorphous i r o n oxide cannot b e e x c l u d e d (54). T h e systems w e r e characterized w i t h a c o m b i n a t i o n o f e l e c t r o n i c , i n frared, a n d R a m a n spectroscopic data. E l e c t r o n s p i n resonance ( E S R ) a n d c o n d u c t i v i t y measurements w e r e u s e d to explore t h e transport p r o p e r t i e s .


Experimental Details Sample Preparation. Zeolite Precursor Materials. Fe(II)Y zeolite was pre paredfromthe sodium form of zeolite Y (Linde LZ-Y52, Alfa) by conventional aqueous ion exchange with ferrous sulfate under nitrogen atmosphere and dried at 295 Κ under nitrogen (54). The resulting ferrous Y zeolite [unit cell compositions, based upon ion exchange, Fe6Na44(Al0 )56(Si0 )i36 for solution loadings and Fei Na36(AlO )56(SiO )i36 for vapor-phase loadings] was heated under a flow of oxygen (40 mL/min) in a quartz tube reactor at a rate of 1 K/min to 370 Κ (10 h), then to 620 Κ (3 h), and subsequently evacuated for 2 h at that temperature to give Fe(III)Y. The color of the zeolite changed from white to light brown during this treatment. Ferrous mordenite (Fe(II)M) was similarly prepared from Na mordenite (LZM-5, Union Carbide) by ion exchange to give white Fe3Na (AlO )8(SiO )40. This material was oxidized in a scheme similar to that used with ferrous Y and generated light brown Fe(III)M. Cu(II) forms of these zeolites and zeolite A were obtained by ion exchange with Cu(N0 ) , resulting in samples CuY, C u M , and CuA with 15, 2.5, and 8 Cu ions per unit cell (u.c.), respectively. Dehydrated sodium zeolites (heated at 1 K/min up to 720 Κ under vacuum, kept at 720 Κ for 10 h) and the metalcontaining dry samples were stored under nitrogen i n a glove box prior to use. 2

0

2

2

2

2

3

2

2

2

Bulk Polypyrrole. Bulk polypyrrole was prepared by chemical oxidation of pyrrole (Aldrich) with F e C l 6 H 0 (Aldrich) according to published procedures (7, 8, 55). Oxidation with an oxidant:pyrrole ratio of 2.4 in aqueous solution at 292 K, carried out under nitrogen, represents the optimal reaction conditions for highest yield and conductivity (7). 3

e

2

Intrazeolite Polymerization of Pyrrole. Pyrrole was diflused into the pore sys tem of the dry zeolites in a variety of solvents and via gas-phase adsorption (Table II). In the solvent reactions, 0.500 g of Fe(III)Y was suspended in a solution of 5.16 mg of pyrrole (0.077 mmol) in 50 mL of the respective solvent and stirred at 295 Κ for 15 h under nitrogen. Similarly, 0.500 g of Fe(III)M was reacted with 12.5 mg (0.187 mmol) of pyrrole. For the vapor-phase reactions, ~0.5 g of dry zeolite was weighed into a small quartz reactor, evacuated at a vacuum line at 1.33 mPa (10 torr), and equilibrated with 270 Pa (2 torr) of degassed pyrrole at 295 Κ for 1 h. -5

Characterization. Fourier transform infrared (FTIR) spectra were taken at 4- cm resolution (Mattson Polaris instrument) and were analyzed with the ICON software. Electronic absorption spectra of the samples dispersed in glycerin were obtained with a spectrophotometer (PE 356) at 2-nm resolution. ESR spectra between 40 and 300 Κ were obtained with a Varian Ε-109 instrument operating at X-band frequencies. Conductivity measurements at 295 Κ were carried out with pressed wafers of the bulk polymers and of the zeolite powders by using the four-point technique (56). Resonance Raman spectra were generated by irradiating the sample with 5 mW at 457.9 nm. Scanning electron micrographs were taken with an Hitachi 5- 800 microscope. 1


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Table II. Zeolite-Pyrrole Samples Sample

Loading with Pyrrole

0

h


NaY-V NaM-V Fe(II)Y-V Fe(II)M-V Fe(III)Y-V Fe(III)Y-W Fe(III)Y-T Cu(II)Y-V Fe(III)M-V Fe(III)M-W Fe(III)M-T Cu(II)M-V Cu(II)A-V

Color

Surface Capacity Reaction Time, h 0

41 2 40 2 39 2 2 50 1.0 1.2 1.2 0.7 0.3

1 1 1 1 1 15 15 1 1 15 15 1 1

0.2 0.06 0.2 0.06 0.2 0.2 0.2 0.2 0.06 0.06 0.06 0.06 0.3

white white white white black/turquoise light green grey/ turquoise black/turquoise turquoise light green light green turquoise very light green

"Abbreviations: Y, zeolite Y; M , mordenite; A, zeolite A; V, vapor phase; W, water; T, toluene. ^Molecules per unit cell. T h e surface capacities for l-μπι crystals are based on 0.6-nm diameter for pyrrole; normalized per zeolite unit cell.

Results and Discussion I f p y r r o l e is a l l o w e d to diffuse into large-pore zeolites that contain ferric ions, t h e color o f the r e s u l t i n g adduct changes slowly f r o m light b r o w n to different shades o f turquoise green a n d black. T h i s s t r i k i n g reaction appears to b e c o m p l e t e d w i t h i n a few minutes i f t h e p y r r o l e is a d m i t t e d as vapor into the d r y zeolite, a n d w i t h i n a few hours i f diffused from solution (Table II). I n contrast, p y r r o l e i n zeolites c o n t a i n i n g o n l y s o d i u m ions o r Fe(II) ions does not react to form c o l o r e d products. Irrespective o f p y r r o l e c o n centration a n d m e d i u m (different solvents o r vapor phase), n o n e o f these b l a n k experiments r e s u l t e d i n a color change o f the sample (Table II). T h i s b e h a v i o r strongly suggests that p y r r o l e p o l y m e r i z e s to p o l y p y r r o l e i n a redox reaction that involves the intrazeolite ferric o r c u p r i c ions. B u l k p o l y p y r r o l e has b e e n synthesized c h e m i c a l l y v i a oxidative c o u p l i n g o f p y r r o l e i n various solutions a n d suspensions o f ferric c h l o r i d e . A n o v e r a l l reaction s c h e m e i n H 0 / C H O H solution has b e e n proposed (57). 2

2

5

4C H N + 9 F e C l * n H 0 - ^ (C H N) C14

5

3

2

4

3

4

+

+ 8HC1 + 9 F e C l * n H 0 2

2

(2)

T h e f o l l o w i n g discussion o f the sample characteristics confirms the f o r m a t i o n of intrazeolite p o l y p y r r o l e .

Sample Characteristics. IR Spectra. B u l k p o l y p y r r o l e is charac t e r i z e d b y a b r o a d I R absorption that extends from a n e l e c t r o n i c absorption b a n d at ~ 1 e V (8000 c m " ) d o w n to about 1700 c m " a n d obscures t h e C - H a n d N - H vibrations o f the p o l y m e r (6). T h e o x i d i z e d a n d t h e n e u t r a l forms 1

1


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of the p o l y p y r r o l e have v e r y s i m i l a r C - C a n d C - H p y r r o l e r i n g v i b r a t i o n s i n the 1 6 0 0 - 8 0 0 - c m " r e g i o n (58, 59). I n contrast to p y r r o l e , the r i n g v i b r a tions o f the p o l y m e r are relatively b r o a d a n d can b e d i s t i n g u i s h e d from r e m a i n i n g u n r e a c t e d m o n o m e r . T h e intrazeolite p o l y p y r r o l e shows I R bands q u i t e s i m i l a r to the t y p i c a l set o f bands characteristic for b u l k p o l y p y r r o l e . H o w e v e r , c e r t a i n shifts that v a r y w i t h host a n d p r e p a r a t i o n conditions are 1

o b s e r v e d ( F i g u r e 3). A c c o r d i n g to the e l e c t r i c a l p o t e n t i a l o f t h e zeolite, t h e p o l y m e r chains s h o u l d not exactly d u p l i c a t e the b u l k s p e c t r u m . F o r instance, bands o f sample F e ( I I I ) Y - V p r e p a r e d from v a p o r phase o c c u r at 1572 (1540), 1473 (1452), 1309 (1280-1300), a n d 790 (791) c m " . Positions of b u l k p o l y p y r r o l e are g i v e n i n parentheses (58, 59). Downloaded by UNIV OF LEEDS on June 18, 2016 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1990-0226.ch023

1

T h e I R spectra of zeolite Y samples show a h i g h e r c o n c e n t r a t i o n of p o l y p y r r o l e t h a n the m o r d e n i t e samples, p r o b a b l y because o f the h i g h e r p o r e v o l u m e fraction available i n the f o r m e r host. Intrazeolite P P y r e c o v e r e d after dissolution of the host i n H F shows features v e r y s i m i l a r to those o f the b u l k p o l y m e r ( F i g u r e 3). F o r m a t i o n of p o l y p y r r o l e a c c o r d i n g to the oxidative reaction m e c h a n i s m is expected to p r o d u c e two protons p e r m o n o m e r . T h e s e protons m u s t b e accommodated i n the zeolite, b u t the I R data of these samples d o not s h o w clear h y d r o x y l features. T h i s result is not s u r p r i s i n g i n v i e w of the strong e l e c t r o n i c absorption e x t e n d i n g b e y o n d the h i g h - e n e r g y part of the spec t r u m . S i m i l a r observations are n o t e d i n the p o l y p y r r o l e - F e O C l system (60). W i t h its p r o n o u n c e d a c i d - b a s e a n d ion-exchange p r o p e r t i e s , the zeolite framework (or intrazeolite F e species) is e x p e c t e d to a c c o m m o d a t e the a d d i t i o n a l p r o t o n concentration. Raman Spectra of Intrazeolite Polypyrrole. B u l k p o l y p y r r o l e is k n o w n to e x h i b i t weak R a m a n spectra, b u t r e s o l v e d bands i n the 8 0 0 - 1 6 0 0 c m " range have r e c e n t l y b e e n o b t a i n e d w i t h s m o o t h films f o r m e d o n electrode surfaces (61). Resonance R a m a n spectra o f p o l y p y r r o l e - z e o l i t e samples (e.g., sample F e ( I I I ) Y - V ) show weak b u t r e s o l v e d bands at 1598 a n d 1418 c m " that c o r r e s p o n d to the vibrations at 1591 a n d 1418 c m r e p o r t e d for p o l y p y r r o l e film g r o w n o n a P t electrode (62). 1

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Location of the Polymer Chains. It is clear from t h e c o m b i n e d spec troscopic e v i d e n c e discussed a n d from the e l e c t r o n i c spectra that p o l y p y r r o l e forms w h e n p y r r o l e reacts w i t h zeolites c o n t a i n i n g ferric a n d c u p r i c ions. I n the context of synthetic strategies for the d e s i g n of m o l e c u l a r " w i r e s " , it is i m p o r t a n t to d e t e r m i n e the location of the p o l y p y r r o l e w i t h respect to the host. A film o f p o l y p y r r o l e at the outside o f the zeolite crystals w o u l d not constitute a system o f isolated m o l e c u l a r chains. Because of the i n s o l u b l e nature of the p o l y m e r , extraction experiments do not p r o v i d e m u c h i n f o r m a t i o n . H o w e v e r , significant i n d i r e c t e v i d e n c e confirms the p i c t u r e of z e o lite-encapsulated p o l y p y r r o l e .



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Electron Transfer in Biology and the Solid State - American Chemical

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