Lowering the Band Gap of Ethylenedioxythiophene Polymers

Chapter 22

Lowering the Band Gap of Ethylenedioxythiophene Polymers: Cyanovinylene-Linked Biheterocycles

Downloaded by CHINESE UNIV OF HONG KONG on June 24, 2016 | http://pubs.acs.org Publication Date: September 16, 1999 | doi: 10.1021/bk-1999-0735.ch022

Christopher A. Thomas and John R. Reynolds Department of Chemistry, Center for Macromolecular Science and Engineering, University of Florida, Gainesville, FL 32611-7200

Abstract: L o w band gap organic electrochromic polymers show promise for devices that can switch absorptive/transmissive states i n both the infrared and visible spectral regions. Here we describe a rationale for lowering the band gaps of conducting polymers by incorporating donoracceptor units along a polymer backbone. In this instance, we have e m p l o y e d 3,4-ethylenedioxythiophene ( E D O T ) and cyanovinylene linkages as electron rich and electron poor components respectively. The substitution pattern around the v i n y l group has been varied resulting i n polymers with bandgaps ranging from 1.1-1.6 e V when prepared v i a o x i d a t i v e e l e c t r o p o l y m e r i z a t i o n f o l l o w e d b y subsequent charge neutralization. The polymers have been characterized by c y c l i c voltammetry and spectroelectrochemistry with coloration efficiencies and chronoabsorptometry results reported.

V a r y i n g the electron density along the main chain of conjugated polymers allows a high degree of control of numerous properties including redox switching potential, electrochromic transitions, luminescence energies and the ability to store charge i n electrochemical capacitors and rechargeable batteries (7). Recently, E D O T based polymers have grown i n importance based on their use as highly transmissive antistatic coatings with high conductivity and stability due to their l o w oxidation potentials (2). The high transmissivity of the conducting form is due to the electron rich nature of the polymer backbone yielding a relatively low electronic bandgap o f 1.6 e V . Our research group has been especially interested i n the electrochromic properties of electropolymerized EDOT-based polymers (3). Due to its low bandgap, P E D O T is one of relatively few conjugated polymers that are cathodically coloring. A s such, it is colored a deep-blue i n the reduced state and switches to a highly transmissive neutral gray or sky blue in its conducting oxidized state. W e find that varying the e l e c t r o n i c character o f the l i n k i n g groups between thiophene rings i n bis(EDOT)arylene monomers allows us to tune the band gap over a broad spectral range. F o r example poly(l,2-(2-(3,4-ethylenedioxythienyl))vinylene) ( P B E D O T - V ) exhibits a band gap o f 1.4 e V (3a), w h i l e poly(3,6-(2-(3,4-ethylenedioxythienyl))carbazole) has a band gap of 2.5 e V (3e). B y utilizing a combination of complementary anodically and cathodically c o l o r i n g polymers, dual-polymer

©1999 American Chemical Society

Hsieh and Wei; Semiconducting Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

367

368 electrochromic devices were prepared that switch between a highly transmissive color-neutral state and a highly absorbing blue state (4). A benefit of using the l o w oxidation potential polymers in these devices is that they are highly stable to tens of thousands of deep double-potential switches.


F i v e factors have been identified which affect the band gap i n conjugated polymers (5). These factors are interrelated and affect the overall band gap to varying degrees by summation of the individual contributions. B e l o w , these factors are listed along with the specific requirements necessary in order to elicit band gap reduction. 1)

2)

3)

4)

5)

Bond length alternation: The difference in length between single and double bonds in a conjugated polymer is related to the Peierls distortion that is responsible for the opening of a band gap at the Fermi level. L o w bond length alternation leads to a smaller distortion which in turn lowers the band gap. Deviation from planarity: Orbital overlap varies with the cosine o f the twist angle and the band gap is directly related to the π overlap. Polythiophenes are known for rotational disorder, which must be minimized in order to ensure efficient orbital overlap. Resonance contributions: In polymers derived from heterocyclic aromatic polymers there is a competition between electron density localized on the ring and delocalized over some larger region of the backbone. Donor-Acceptor substituent effects: One approach to band gap reduction relies on alternating donor-acceptor units in a π conjugated polymer. There is an ideal strength for each of the donor and acceptor units (6). Interchain effects: Organization of polymer chains into a solid and the transport differences that result play an important role in the properties of C P s (7).

In this work we explore the effect of varying the electron rich nature, and thus the donor strength, of a set of four monomer subunits. In this family we use E D O T and thiophene as the donor variants and cyanovinylene as the acceptor to prepare a family of polymers which have band gaps ranging from 1.1 to 1.6 e V . Figure 1 schematically shows the expected relationship between the donor and acceptor frontier orbital energies of the monomer subunits and the energies o f the polymer. W e observe the H O M O energy to be related to the electron rich character of the heterocycle donor and the L U M O energy to be coupled to the acceptor unit. Consequently, the L U M O level is expected to remain constant over a series o f polymers where the acceptor unit is identical.

Synthetic Approach The cyanovinylene moiety is constructed by the Knoevenagel condensation o f a suitable aldehyde with an aryl acetonitrile derivative using potassium r-butoxide. E D O T carboxaldehyde was prepared by the V i l s m e i e r formylation o f E D O T with P O C l and D M F while E D O T acetonitrile was prepared by the Ni(acac) catalyzed coupling of E D O T - Z n C l with bromoacetonitrile as shown below (8). ?

2

1 ) 1 equiv DMF, POCI CH CICH CI . 2) H 0 Ο 2

2

2

Ο

3

/ ° \

1 ) BuLi, -78 °C, THF

/ — \ 2) ZnCI , -78 to 0 °C η Ο ) BrCH CN, PPh Cy \ / Ni(acac) , THF 60 °C 2

3

2

2

2

Q

' •


369

-D-D-D-D-

-A-D-A-D-

-A-A-A-A-

LUMO Eg,D

E? Φ c

Eg, U-A


LU

HOMO

High HOMO Donor Unit

Polymer

Low LUMO Acceptor Unit

F i g u r e 1: A donor is combined with an acceptor to yield a monomer with a hybrid electronic structure.

s

NC

-o

t

s.

Th-CNV-EDOT

-S

BEDOT-CNV

NC KOfiu S - ^ EtOH, reflux

BTh-CNV

KO/Bu EtOH, reflux

EDOT-CNV-Th

Scheme 1: Monomers were prepared by Knoevengel condensation of an aldehyde with an acetonitrile derivative.


370

2-Thiophenecarboxaldehyde and 2-thiopheneacetonitrile were o b t a i n e d commercially from A l d r i c h . The Knoevenagel route is illustrated in Scheme 1. Yields range from 62-93% and all monomers gave satisfactory * H - N M R , C - N M R and F A B - H R M S spectra. It should be noted that T h - C N V - E D O T was initially communicated by Roncali et al (9).


1 3

E l e c t r o p o l y m e r i z a t i o n . These monomers were anodically polymerized v i a repeated potential scanning on Pt button electrodes or deposited potentiostatically on ITO/glass at low potential to yield electrode confined films which are light blue. Table 1 lists some relevant properties of these materials. The onset is defined as the potential at which current starts to rise during the first scan of polymerization and low values are indicative of clean polymerization. A s illustrated in Figure 2, this oxidation potential is ca. 250 m V 2

T a b l e 1: Electrochemical properties of cyanovinylenes on a 0.73 c m Pt button electrode vs A g / A g reference electrode +

Monomer Designation BTh-CNV BEDOT-CNV Th-CNV-EDOT EDOT-CNV-Th

Monomer Properties E /V E ^ / V ÔT8 LIO 0.58 0.70 0.78 0.90 0.75 >1.20 nn!Wt

Polymer Properties E anodic E cathodic (+0S) (-0.1) -1.67 (+0.15) -1.52 (+0.4) -1.6 1 / 7

m

i

0.0

0.2

0.4

0.6

0.8

Π

E /eV Π6 1.1 1.3 1.2 f f

1

1.0

+

Potential /V(vs Ag/Ag )

F i g u r e 2: Repeated potential scanning electropolymerization of 10 m M T h C N V - E D O T in 0.1 M B u N C 1 0 - ( T B A P ) at Pt. Scan rate was 100 m V / s . +

4

4


371


l o w e r than the parent E D O T and is i n the range for very effective electropolymerization/deposition. The peak potential for T h - C N V - E D O T is noted along with the peak potentials for several other commonly used thiophene based monomers. Repeated scanning shows evolution of a polymer redox process at l o w potential coupled with an increased monomer peak current on sequential scans due to increasing surface area of the polymer modified electrode. A blue film appears on the electrode and the scan rate dependence of the polymer cyclic voltammetry (50-250 mV/s) indicates that the f i l m is electrode confined, that is, the peak current for the oxidative doping process scales linearly with the scan rate.

Electronic and Electrochemical Properties Polymer E l e c t r o c h e m i s t r y . Cyclic voltammetry in monomer free tetrabutylammonium perchlorate ( T B A P ) / a c e t o n i t r i l e ( A C N ) indicates a broad oxidative p-doping process characteristic o f this type of conducting polymer. Spectroelectrochemistry on ITO/glass in the U V - V i s - N I R region confirms the pdoping process. These spectra indicate a single peak for the neutral form o f the polymer which gradually decreases in intensity with increased oxidative doping to a state which is highly absorbing at lower energies with a gradual tail into the visible region. The onset for the oxidative doping process is taken to be the upper edge of the H O M O and increases systematically as the electron rich nature of the flanking heterocycle increases. The onset for the polymer reduction process is taken to be the L U M O and remains essentially the same as expected for a constant acceptor strength. A s is characteristic of other reducible thiophene containing systems, this family o f polymers displays a set of pre-peaks, small peaks at less extreme potentials than the main polymer redox processes. Disagreement persists over the origin o f these peaks but in this system, the oxidative pre-peak grows in only after a polymer reduction and the reductive pre-peak grows in after polymer oxidation. Additionally, the pre-peaks are electrochemically coupled to one another, ruling out charge trapping as a possible explanation for them. Zotti has proposed that the pre-peaks arise from quinoid moieties formed from trace water at the 3 or 4 positions of a thiophene ring (70). In the case of B E D O T - C N V , a system with blocked 3 and 4 positions, the pre-peaks presumably arise from reaction at the vinyl group although we note pre-peaks in other compounds prepared i n our laboratories with blocked 3 and 4 positions and no v i n y l group. Polymer reduction of T h - C N V - E D O T i n A C N shows remarkably symmetrical peak currents when tetrabutylammonium is used as the electrolyte cation i n an inert, water free atmosphere. The peak currents do not decay over ca. 50 scans and the shape of the redox process remains constant. In L i C 1 0 / A C N , the shape of the reduction process is not symmetrical and is highly broadened compared to the corresponding T B A P electrochemistry. Additionally, the peak currents decay rapidly over a period of only ten scans leaving little electroactivity in the resulting polymer. The difference in electrochemistry between the two supporting electrolytes is attributed to the small size of L i pinning the negative charge on the backbone forming a tight ion pair with limited d e r e a l i z a t i o n (#). In a general sense, the stability of reduced polymers has been examined and it is expected that with typical overpotentials, the E for the reduction couple must be more positive than about ca. -500 m V for an air stable device to be formed (9). 4

+

1 / 2


372

E l e c t r o c h r o m i s m . The E values for the polymers as determined by C V were confirmed using in situ spectroelectrochemical analysis. A t an applied potential of - 7 5 0 m V all of the polymers are in their neutral form allowing a band gap to be estimated as the onset of the π to π * transition (e.g. the smallest energy necessary to populate the π * orbital). This method agrees well (± 0.1 e V ) with the bandgaps taken from C V . A s the polymer film is oxidized to sequentially higher potentials, a lower energy valence to bipolaron transition is observed consistent with charge carrier formation. In the oxidized form of these polymers, there is an absorptive tail which extends into the visible region limiting the amount of optical contrast available between the neutral and o x i d i z e d forms at the absorbance m a x i m u m . This absorbance, which extends through the near IR and into the mid-IR, makes these materials useful as I R electrochromics.


g

IR and visible light electrochromic characterization of T h - C N V - E D O T was performed by employing an experiment where the potential was stepped rapidly between the completely oxidized or neutral (labeled " R e d " in the figure) states. The percent transmittance at the λ (610 nm), 1064 or 1550 n m was measured simultaneously. The switching results for 610 and 1064 n m are shown in Figure. 3. The relative position of the oxidized and neutral forms of the polymer are inverted in Π13Χ

100

1064 nm

610 nm Red

80 H

Ox

60 s

0*

Ox 20

Red

_L 20

30

40

0

40

10

Time /Seconds

F i g u r e 3. Chronoabsorptometry of T h - C N V - E D O T on ITO/glass in A C N / T B A P . Potentials were stepped between +1.0 V (Ox) and -750 m V (Red) at a frequency of 1 Hz. the figure because the two wavelengths studied are on opposite sides of the isobestic point. Because of the absorptive tail of the oxidized form of the polymer at λ ^ , the electrochromic contrast changed on the order A % T / A % T : λ < 1064 n m < 1550 nm suggesting they w i l l have further enhanced contrast at longer wavelengths. Coloration efficiency (4) is defined as the change in optical density per unit charge 0 X

n e u t r a l

π13Χ


373

density and is a parameter used to compare electrochromic devices. F o r a 150 n m thick f i l m o f T h - C N V - E D O T on ITO/glass, the electrochromic contrast was 330 c m / C at 1064 n m and 400 c m / C at 1550 n m . F o r comparative purposes, electrochromic metal oxides (e.g. W 0 , I r 0 ) have coloration efficiencies between 20-200 c m / C (10). A s such, organic electrochromic materials based on E D O T have considerable promise i n devices mat operate over this regime. 2

2

3

2


2

A c k n o w l e d g e m e n t s . W e acknowledge early contributions to this work by D r . Gregory A . Sotzing. Funding from the National Science Foundation ( C H E 96-29854) and the A F O S R (F49620-96-1-0067) is greatly appreciated.

1 See for an extensive review the "Handbook of Conducting Polymers", Skotheim, T.A.; Elsenbaumer, R.L.; Reynolds, J.R., Marcel Dekker Inc., New York, NY, 1998. 2 a) Jonas, F.; Heywang, G.; Schidtberg, W. Ger. Offen. DE 3,813,589,1989.b) Jonas, F.; Heywang, G.; Schidtberg, W.; Heinze, J.; Dietrich, M. Eur. Pat. App. EP 339,340, 1989. c) Jonas, F.; Heywang, G.; Schidtberg, W.; Heinze, J.; Dietrich, M . U.S. Patent No. 5,035,926, 1991. d) Heywang, G.; Jonas, F. Adv. Mater. 1992, 4, 116. 3 a) Sotzing, G. Α.; Reynolds, J. R. J. Chem. Soc., Chem. Commun. 1995, 703. b) Sotzing, G. Α.; Reynolds, J. R.; Steel, P. J. Chem. Mater. 1996, 8, 882. c) Sankaran, B.; Reynolds, J. R. Macromolecules 1997, 30, 2582. d) Reddinger, J. L.; Sotzing, G. Α.; Reynolds, J. R.; J. Chem. Soc., Chem. Commun. 1996, 1777. e) Sotzing, G. Α.; Reddinger, J. L.; Katritzky, A. R.; Soloducho, J.; Musgrave, R.; Steel, P. J.; Reynolds, J. R. Chem. Mater. 1997, 9 , 1578. f) Sotzing, G. Α.; Thomas, C. Α.; Reynolds, J. R.; Steel, P. J.; Macromolecules, In Press. 4 Sapp, S. Α.; Sotzing, G. Α.; Reddinger, J. L.; Reynolds, J. R. Adv. Mater. 1996, 8, 808. Sapp, S. Α.; Sotzing, G. Α., Reynolds, J. R., Chem. Mater. 1998, 10, 2101. 5 Roncali, J. Chem. Rev. 1997, 97, 173-205. 6 Pagani, G.; Berlin, Α.; Canavesi, A; Schiavon, G.; Zecchin, S; Zotti, G. Adv. Mater. 1996, 819. 7 Smith, R. C.; Fischer, W. M. ; Gin, D. L. J. Am Chem. Soc. 1997, 119, 4092-4093. PPV emission intensity is increased when PPV is formed as a single isolated chain in a polymerized LC matrix. 8 Torbjörn, F.; Klingstedt, T. Synthesis 1987, 1, 40. 9 Ho, Η. Α.; Brisset, H.; Frere, P.; Roncali, J. J. Chem. Soc. Chem. Commun. 1995, 2309. 10 Zotti, G.; Schiavon, G.; Zecchin, S. Synth. Met., 1995, 72, 275-281. 9 de Leeuw, D.M.; Simenon, M. M. J.; Brown, A. R.; Einerhand, R. E. F. Synth. Met., 1997, 87, 53-59. 10 a) Faugnan, B.W.; Crandall, R. S.; Heyman, P. M. RCA Rev. 1975, 36, 177. b) Hitchman, M. J. J. Electroanal. Chem. 1977, 85, 135. c) Dautremont-Smith, W. C. DisplaysI1982, 3.


Lowering the Band Gap of Ethylenedioxythiophene Polymers

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