Tuning the Extent of Conjugation in Processable Polythiophenes

Chapter 21

Tuning the Extent of Conjugation in Processable Polythiophenes Through Control of Side Chain Density and Regioregularity Seth C . Rasmussen, Bennett D. Straw, and James E. Hutchison

1

Downloaded by IOWA STATE UNIV on February 22, 2017 | http://pubs.acs.org Publication Date: September 16, 1999 | doi: 10.1021/bk-1999-0735.ch021

Department of Chemistry and Materials Science Institute, 1253 University of Oregon, Eugene, O R 97403-1253

Introduction of alkane side chains is a powerful approach to promoting the solubility o f conjugated organic polymers. However, the unintended effects of side chains upon the properties of the polymers can not be ignored. In the case o f polythiophenes, steric interactions involving the side chains can introduce tortional strain between thiophene rings that disrupt π-orbital overlap along the polymer backbone, thus decreasing the degree o f conjugation i n the chain. Eliminating or mimmizing the steric interactions that distort the planarity of the polymer backbone can increase the extent o f conjugation, making these materials more desirable for a number of applications. Here, a short review of the general principles employed to increase conjugation lengths i n n-alkyl side chain substituted polythiophenes is followed by a discussion of specific examples that illustrate these principles. Because the reduction of side chain density is a powerful approach to increasing the conjugation length, particular attention is paid to substituted polythiophenes resulting from polymerization of partially substituted monomers (e.g. monoalkylbithiophenes, mono- and di-alkylterthiophenes and dialkylquaterthiophenes). Regioregular polymers with low side chain density exhibit an extent of conjugation that approaches that predicted from model studies for an ideal infinite polythiophene chain.

Conjugated organic polymers have been studied extensively over the last twenty years, i n part, because o f the fundamental and technological interest i n their optical and electronic properties. Applications that have been demonstrated for these materials include their use as batteries, sensors, electrochromic devices, lightemitting diodes, and field effect transistors.(i-9) A unique advantage of conjugated polymer materials for these applications is that the properties of the polymer can be 1

Corresponding author. E-mail: [email protected].

©1999 American Chemical Society

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

347

348


tuned at the molecular level. Polythiophenes, i n particular, are versatile materials because o f their environmental stability(3,70) and the ease with which they can be synthetically modified.(4-5) A key step i n the development o f polythiophenes as a class o f useful conjugated materials was the preparation of soluble derivatives. The unsubstituted parent polymer is insoluble, however i n 1986 it was found that substitution of long (n > 4) alkyl chains i n the β-position o f the thiophene rings resulted i n polymers that were both soluble and fiisible.(5) Soluble samples can be easily processed and more thoroughly characterized. Through the investigation o f a large number of soluble polythiophene derivatives, the relationships between polymer structure and properties are starting to emerge.(6,5,77) In order to rationally design conjugated polymers with tunable, well-defined electrical and optical properties, the influence of the side chains (number, position, and type) on the properties must be understood.(5-7) For polythiophene, tuning is typically accomplished through variation of side chain number and structure.(47,77,72) Judicious selection o f alkane side chains can result i n enhanced polymer characteristics including: enhanced solubility,(5-7,72-74) solid-state organization through chain self alignment,(74-76) and improved optical and electronic properties.(5,77) In addition to inert hydrocarbon chains, functionalized chains can be used to promote specific interactions with ions or molecules.(5-7,72,76,75,79) Ionizable side chains (e.g. sulfonates or carboxylates) provide internal charge compensation(20) (or self-doping)(45,27-23) as well as impart water solubility, a desirable property for technical applications due to increasing restrictions on the use of organic solvents.(4) A l c o h o l - or ether-containing side chains could potentially control cation transport properties i n solid state batteries, ion-selective electrodes, or membranes.(5-7,12,24-26) Chiral functionalities such as amino acids are o f interest for use i n materials capable o f molecular recognition and asymmetric electrosynthesis.(5-7,72,79) However, the structural and electronic properties of many desirable substituents are often incompatible with conventional synthetic approaches and can adversely affect polymer properties.^-7,11-14)

Side Chain Influence on the Extent of Conjugation in Soluble Polythiophenes A number of studies have addressed the general goal of rationally tuning the properties o f conjugated polymers through synthetic modification. Driven by both fundamental interest and technological applications, numerous studies have focused on extending conjugation lengths. The results of these studies have greatly improved our understanding o f the fundamental structural parameters that strongly influence conjugation. In addition to the primary interest i n understanding the relationship between conjugation and structure, a high degree of conjugation is essential for a number of the applications mentioned above. For example, optimizing the conjugation length is the key to designing l o w band gap materials(77) (including transparent conductors), enhancing electrical conductivity,(5) and maximizing polymer non-linear optical properties.(5) The ability to tune the degree of



349

conjugation is also important i n applications where it is desirable to control the optical absorption and emission such as light emitting diodes(27) and thermochromic devices.( 4 0 ° between rings)(29) the conjugation length w i l l be decreased.(5-7,12-14,28-34) The number, type, and placement of side chains along Figure 1. Steric interactions i n a head-to the backbone can all contribute to increased steric head, head-to-tail triad of an alkyl effects and thus tortional substituted polythiophene. Steric clashes strain. between alkyl side chains and between side chains and the sulfur lone pairs are A s depicted i n Figure 1, indicated. there are two types of steric effects that can influence the planarity of the polymer backbone. Interactions between alkyl chains (chain-chain interactions) are most pronounced for longer alkane chains i n instances where ringring couplings are regiorandom. The second type o f steric interactions involves sulfur lone pair-alkyl side chain steric clashes. These interactions are found i n all substituted polythiophenes, but are most pronounced for polymers containing high side chain densities and/or head-to-head couplings (see Figure 1). Regiorandom poly(3-alkylthiophene)s suffer a loss o f conjugation caused by both o f these types of steric effects. Optical Absorption Spectroscopy of Substituted Polythiophenes. Many investigations have been conducted that probe the effect o f the type and number o f various substituents on the effective conjugation length.(7£) The extent of



350

conjugation i n substituted polythiophenes is typically inferred from U V - v i s spectroscopic data. The wavelength of the π - π * transition (usually recorded i n CHCI3) is taken as an indication o f the conjugation length i n solution. A s the conjugation length increases, the energy of the transition decreases (giving a bathochromic (red) shift). It is important to use the same solvents when comparing solution optical absorption spectra o f different samples because most polythiophenes exhibit solvatochromism.(35) Comparisons between spectra recorded i n good solvents (e.g. CHCI3) and poor solvents (e.g. hexane) must be avoided i f one is interested i n the relative conjugation lengths i n the samples. Little attention has been paid to controlling/determining the extent of aggregation i n solutions of substituted polythiophenes. Aggregation w i l l likely affect the solution state spectra. Investigations aimed at understanding the effects o f aggregation upon these spectra would be very useful. The optical spectra of the polymeric samples are also measured as thin films, usually on glass substrates. In comparison to the solution spectra, the transitions i n the solid state are often red-shifted. This shift is due to an intrachain coil-to-rod conformational change that takes place upon removal o f the solvent and results i n extended chains o f coplanar thiophene rmgs.(36-39) This conformational change coupled with interchain interactions i n the solid state results i n increased electronic derealization and a shift of the U V - v i s transition to lower energy. Care must be taken i n comparing the extent of conjugation i n thin film samples because the absorption spectra depend strongly on sample p r e p a r a t i o n , ^ ) especially film thickness and casting solvent. Approaches to More Highly Conjugated Polythiophenes A number of studies have been undertaken to determine the extent o f conjugation that should be possible i n an ideal polythiophene chain. Numerous theoretical studies on polythiophene predict that conjugation length w i l l increase with increasing number o f coplanar rings i n the chain.(47) In support of this work, alkyl substituted and unsubstituted oligothiophenes have been used as soluble model compounds i n attempts to correlate spectroscopic properties with known conjugation lengths.(42-46) F o r each o f the studies reported, plots o f absorption energy as a function of the reciprocal o f the chain length are linear. Extrapolation o f the data to infinite chain length predicts absorption energies for the ideal extended chains. In the case of end-capped oligothiophenes(43-44) and alkyl substituted oligothiophenes(42,46) these extrapolations yield values i n a narrow range of approximately 2.2-2.3 e V (540-560 nm i n solution).(45-45) Based upon these studies, an ideal, fully conjugated alkyl-substituted polythiophene sample would be expected to have an absorption maximum i n this range. Analogous studies recently reported on fully substituted, regioregular 3-alkylthiophene oligomers yield a value of 2.54 e V (488 nm) upon extrapolation to infinite chain length.(47) The higher energy o f this absorption maximum suggests that high side chain densities can decrease conjugation lengths (vide infra).



351

A number o f approaches have been taken to try to increase the conjugation length i n soluble polythiophenes through the decrease o f side chain induced steric interactions. Generally, the goal is to eliminate clashes between side chains as well as decrease steric repulsion between side chains and the polymer backbone. T o date, three specific approaches have been investigated that eliminate or rmnimize the deleterious steric interactions illustrated i n Figure 1: (i) eliminate steric clashes between side chains, (ii) reduce the magnitude of the steric interactions between the side chains and the polymer backbone, and (iii) reduce the total number o f side chains. First we w i l l review important examples of approaches (i) and (ii). The balance o f this chapter w i l l focus on soluble polythiophenes that have reduced side chain density including recent work on polymers derived from monosubstituted bithiophenes. Throughout the remaining discussion, the influence o f the side chains on the optical absorption spectra i n solution w i l l be emphasized. It is important to keep i n mind that manipulation of alkane side chains w i l l affect polymer solubility. In a fully substituted polymer, there is good solubility, but steric interactions due to the side chains decrease conjugation. O n the other hand, the unsubstituted polymer avoids these steric interactions but is insoluble. The removal of side chains along the polymer has also been shown to decrease solubility. Thus, there w i l l likely be a balance between enhanced conjugation and good solubility.

Elimination of Steric Clashes Between Alkane Side Chains.

There are three main approaches that potentially eliminate the side chain interactions that decrease the degree o f conjugation i n processable polythiophenes: use shorter alkane chains, control the regiochernistry o f the polymer, and increase the distance between side chains along the polymer backbone.

Copolymers of Mixed Alkylthiophenes. The use of shorter side chains potentially eliminates side chain clashes, however, poly(3-alkylthiophene)s with side chains shorter than butyl are not soluble.(75) A n alternative approach is to use a copolymer where long, solubilizing side chains are diluted with short chains such as methyl.(45-54) For a series o f random copolymers, the ratio o f side chain alkyl substituents (R=octyl/R'=methyl) was varied and the optical spectra recorded(45) (Table I). The regioregularity o f these samples was not determined. The effects observed are quite small (< 10 n m red shifts i n the λ™»). Copolymers involving two non-methyl side chains are less conjugated than either o f the analogous homopolymers.(55) A series of regioregular (head-to-tail), random copolymers (R=dodecyl/R'=methyl) has been prepared by M c C u l l o u g h and Jayaraman.(54) Small red shifts i n the optical spectra (450 - » 458 - » 470 « 466 nm) occur upon increasing the fraction o f methyl side chains i n the polymer. In this case, the copolymer formed from two non-methyl monomers has an absorption maximum comparable to the homopolymers.(54f) Unfortunately, the spectra for the regioregular copolymers cannot be directly compared to the those o f the regiorandom forms because the spectra were not measured i n the same solvents (vide supra).


352

Table I. Visible Spectroscopic Data for Random Copolymers of Various 3-Alkylthiophenes Ref R Regio Ratio of λ™* (Solid) λ,,», (Solution) * R' regular R/R' — 48 439 Me 100/0 No c — 48 442 90/10 — 48 447 80/20 — 48 445 70/30 48 — 443 60/40 a

0

1


8

Cl2

Me

1/0 2/1 1/1 1/2

Yes

450 458 470 466

(xylene) (xylene) (xylene) (xylene)

526,561*, 609 526, 565* 520,550* 510,545*, 600

54 54 54 54

C12

c

1/0 1/1

Yes

450 (xylene) 450 (xylene)

526,561*, 609 525,556*, 600

54 54

6

a

b

All side chains are straight chain alkyl where C„ is the number of carbons in the chain. Spectra recorded in CHC1 solution unless otherwise indicated. Where multiple peaks are observed, wavelengths are given for all well-defined peaks. The absorption maximum is indicated by an *. c

3

Regioregular Polyalkylthiophenes. Traditional synthetic methods for polymerization of 3-alkylthiophenes lead to regioisomers with undesirable steric interactions.(73,74,40,56) Three types of coupling (head-to-head ( H H ) , tail-to-tail (TT), or head-to-tail (HT)) lead to the four regioisomer triads shown i n Figure 2. Regiorandom linkages in the polymer backbone lead to steric interactions between alkyl side chains as seen i n the trans configuration of the H T - H H triad. McCullough(743#) and Rieke(75,56) have developed chemical coupling methods i n order to eliminate H H and T T regioisomers. These synthetic methods employ transition metal catalysts to chemically control the coupling of asymmetric monomers thereby producing polymers with only H T couplings.(75,14,40,56) The improvement i n conjugation lengths versus the percent H T coupling can be seen i n representative poly(3-alkylthiophene)s shown i n Table Π. A s the amount of H T coupling i n the polymer increases from the completely random polymer (50% H T couplings) to those made by chemical oxidation with F e C l (-70% H T couplings),(2c?,3i,60,67) the Xmax for the polymers show a 7-13 nm red shift i n CHCI3 solution and a 61-78 nm red shift in the solid state. The regioregular polymers of M c C u l l o u g h and Rieke (>98% H T couplings) show a 9-21 nm red shift in solution and a 50-66 n m red shift i n the solid state compared to the F e C l polymerized polymers. From this data it can be seen that the H T content makes a more drastic impact on the red shift of the visible λ™* for the films than for the polymers i n solution. This has been attributed to nonplanarity o f the polymer chains caused by greater steric strain introduced by H H couplings. 3

3


353

R

R

R

R

R

R TT-HT

HT-HT


R

R

R

R

R

HT-HH

R

HH-TT

Figure 2. Regioisomer triads i n a regiorandom poly(3-alkylthiophene) A n alternative approach to eliminating steric clashes between side chains involves the use of disubstituted 4,4'-dialkyl-2,2 -bithiophenes or 3,3 -dialkyl-2,2'bithiophenes as monomers.(30,ii,57-59) These monomers can only couple i n a head-to-head or tail-to-tail fashion respectively and are thus considered regioregular. Although this approach is effective i n eliminating side chain clashes, die optical absorbance spectra suggest that the large extent of head-to-head couplings greatly distorts the planarity of the π backbone. (30,33) Solution and solid state values for poly(dialkylbithiophene)s range from 390 to 400 nm(30,33,57-59) compared to - 4 3 5 nm for the regiorandom poly(3-alkylthiophene)s i n so\uûon.(33,35,60,61) A final approach to eliminating steric clashes between side chains involves increasing the distance between side chains along the polythiophene backbone. This approach - decreasing the side chain density - w i l l be explored fully, later i n this chapter. ,

,

Reduction of Steric Clashes Between Side Chains and the Polythiophene Backbone. Each side chain has steric contacts with the lone pairs on the sulfur atom in the backbone (Figure 1). These steric interactions induce rotation about the thiophene-thiophene bonds and decrease π orbital overlap along the backbone. Head-to-head couplings involve two side chain-sulfur lone pair interactions and result i n greater steric strain. (30-32,58) There are three main methods for decreasing the steric interactions between the side chains and the polymer backbone. The first involves eliminating head-to-head couplings within the polymer. The second approach is to use side chains that are sterically less bulky than alkyls (e.g. alkoxy).


354

These two approaches w i l l be described i n this section. A final method, decreasing the density of side chains i n the polymer chain, w i l l be discussed i n the next section.

Table II. Comparison of Visible W (nm) for Selected Polyalkylthiophenes in Solution and Films with Respect to Percent H T Coupling. Polymer Random* FeCl McCuIlough Rieke" 50% H T -70% H T >98%HT >98%HT Solution F i l m Solution F i l m Solution F i l m Solution F i l m pC T 428 433 44Ô 494 450 525 449 522 560* 556* 608 605 8

c

3

11

3

11

s


4

pC T

428

438

435

e

505

f

442

525 555* 610

456

526 556* 610

pC T

428

438

441

g

506

g

460

525 559* 610

451

522 556* 608

6

8

a

The percentage HT coupling is the number of HT couplings divided by the sum of HT and HH couplings as determined by NMR. All solution spectra recorded in CHC1 . pC T = poly(3-butylthiophene), pC T = poly(3-hexylthiophene), pC T = poly(3-octylthiophene). Ref. 13. Ref. 14. Ref. 33. Ref. 60. Ref. 61. Ref. 28. Where multiple peaks are observed, wavelengths are given for all well-defined peaks. The absorption maximum is indicated by an *. 3

4

b

6

d

e

c

8

f

8

h

Approaches that Eliminate Head-to-Head Coupling. A s described i n the last section, regioregular poly(3-alkylthiophene)s have more extended conjugation lengths than their regiorandom counterparts. The regular placement o f alkane side chains eliminates both alkane side chain clashes and head-to-head couplings. For a series of poly(3-hexylthiophene)s containing varying amounts o f head-to-tail couplings (Table ΙΠ), the absorption maxima shift to the red with increasing head-totail content.(iJ) In these polymers, the increased conjugation found i n the regioregular forms derives from elimination of both types o f steric effects. In order to isolate the effects of head-to-head couplings, one can compare poly(3,3'-dihexyl2,2'-bithiophene)(57,59) with regioregular poly(3-hexylthiophene).(73) These two polymers have the same side chain length and density and are free o f side chain clashes. The λ™* for the head-to-tail polymer is 60 nm higher than for the head-tohead isomer (both measured as CHCI3 solutions) demonstrating the deleterious effect of the head-to-head couplings upon conjugation length. A n approach that eliminates head-to-head couplings i n regiorandom polymers involves the use of certain "internally" substituted bithiophenes and terthiophenes as monomers (e.g. 3-alkyl-2,2'-bithiophenes and 3 -alkyl-2,2 ;5 ,2"-terthiophenes). Chemical oxidation or coupling results i n regiorandom polymers possessing no head,

,

,


355

to-head couplings. Because these polymers have decreased side chain density, they are considered in the next section.

Table III. Visible Spectroscopic Data for Poly(3-hexylthiophene)s with Varied Percentages of Head-to-Tail Coupling. Film Polymer" Ref % HT Solution Xonaxinm) A™ax(nm) 55, 57 P(C ) BT 0 396 396 13 pC T 50 438 428 13 65 446 428 13 70 451 429 60 80 505 440 13 >98 526,560*, 610 456 0

b

6

2


6

a

b

p(C ) BT= poly^'-dihexyl-^'-bithiophene), pC T = poly(3-hexylthiophene). Spectra recorded in C H C 1 solution unless otherwise indicated. Where multiple peaks are observed, wavelengths are given for all well-defined peaks. The absorption maximum is indicated by an *. 6

2

6

c

3

Alkoxy Substituted Thiophenes and Alkoxy Substituted Bithiophenes. The incorporation of long chain alkoxy substituents has been another approach to increasing the conjugation o f functionalized polythiophenes. Polyalkoxythiophenes often retain the solubility characteristics of alkyl functionalized polythiophenes, but exhibit increased conjugation due to a reduction o f side chain induced steric interactions and the electron donating character of the alkoxy group.(40,64-66) The smaller van der Waals radius of the oxygen atom (1.4 À ) i n comparison to the methylene unit (2.0 À ) results i n less steric repulsion between the side chain oxygen and the thiophene sulfur.(f55) The increased conjugation of these polymers was first demonstrated by poly(3-butoxy-4-methylthiophene) which exhibits a low energy solid state absorbance maxima o f 545 nm.(6?) The ability to reduce steric interactions through the use o f alkoxy side chains has been clearly demonstrated by Cloutier and Leclerc who polymerized 3,3'- and 4,4'-disubsituted-2,2 -bithiophenes containing both mixed alkyl/alkoxy, and dialkoxy substituents.(65) Poly(dialkylbithiophene)s contain all head-to-head couples that result i n low conjugation lengths.(50-32) Stepwise substitution of alkoxy side chains for alkyl side chains extends the conjugation length as demonstrated by dramatic shifts o f the visible transitions to lower energies as shown i n Table IV. Unfortunately, the polythiophenes containing only alkoxy side chains resulted i n only partially soluble materials. In particular, the alkoxy substitutents do not promote the solubilization of the higher molecular weight fractions. The similar solution and solid state absorption maxima suggest that rigid-rod conformational structures prevail i n both states. ,

The red-shifted absorption maxima found for the alkoxy substituted polymers are the result of both the steric and electronic attributes o f the alkoxy side chain. The relative contributions of sterics and electronics have not been unraveled.


356

Table IV. Visible Spectroscopic Data for Alkoxy Functionalized Polythiophenes."

p(3,4-R T)

ρ(4,4·-Κ ΒΤ)


2

Polymer

Ri H H OC Ci

p(3,4-R T) 2

R2

4

c, c,

OC

OC

4

7

4

4

7

8

C10 4

C10

p(%3'-KaBT)

OC OC OC OC OC

oc

C10 C10

p(4,4-R2BT)

pO^'-R^T)

2

4

OC OC OC OC

4

4

4

4

Ref

^.(solution)" 530 590 480 420 408 440

XmuCfilm) 520 460 545 420 545

64 66 63,64 63,64 66 64

390 480 574

390 500 600

31 65 65

460 545

506 582

65 65

—

a

AU side chains are straight chain alkyl where Q, is the number of carbons in the chain. recorded in CHC1 solution.

b

Spectra

3

Reduction of the Number of Side Chains on the Polythiophene Backbone. In polymers o f symmetrical dialkylbithiophenes interactions between alkyl chains are eliminated, but sulfur-alkyl steric repulsions are still present and are pronounced due to the high number o f head-to-head couplings i n these polymers. (5,30-33) Regioregular head-to-tail poly3-alkylthiophenes(73,14,40,56) lack the undesirable head-to-head couplings, but sulfur-alkyl steric interactions are still present. In an effort to increase the conjugation within polyalkylthiophenes, a number o f polymers have been prepared with a reduced number o f alkyl side chains. Decreasing the density o f side chains reduces the number o f sulfur-alkyl interactions, resulting i n more extended conjugation lengths. The most common approaches to the reduction of side chain density i n polyalkylthiophenes is the polymerization o f partially substituted oligothiophenes and the copolymerization o f thiophene/alkylthiophene mixtures. Oligothiophenes investigated include monosubstituted bithiophenes, (28,34,67-69) mono- and di-substituted terthiophenes, (28,70-75) and disubstituted


357

quaterthiophenes.(42,76) Copolymers of thiophene and 3-alkylthiophene also result in polymers with reduced side chain density.(75,77) Partially Substituted Terthiophenes and Higher Oligomers. The initial studies on partially substituted oligothiophenes were reported i n 1991 by Zerbi and co-workers who chemically ( F e C ^ ) polymerized 3,3"-dihexyl-2,2 :5 ,2 -terthiophene.(70,77) This polymer contains one third fewer side chains i n comparison to poly(3-hexylthiophene). The reduction i n side chains results i n low energy solution and solid state U V - v i s absorption maxima (Table V ) indicated by a 20 n m red shift compared to the fully substituted analog.(70,77) The dioctylterthiophene analog, later polymerized by Collard and co-workers,(42) yields a polymer with slightly higher energy maxima possibly due to increased interaction between the longer alkyl side chains.


,

,

,,

Table V . Visible Spectroscopic Data for Polyalkylterthiophenes.

R Polyalkylterthiophene Ri

R2

R3

R4

c c

H

H

H

C18

H H

H H

H H

H H c c

H H c c

H

H

7

c a

8

4

c

6

8

4

3

R4

a

6

8

WCHCI3)

^(film)

Ref

540 510 540 535 517 522

72 28 72 70,71 42 73

468 456 437 499

All side chains are straight chain alkyl where C„ is the number of carbons in the chain.

The success of this early work led others to prepare and polymerize 3 - a l k y l 2,2 :5 ,2"-terthiophenes i n an effort to further decrease the side chain density o f the resulting polymers.(2S, 72) These polyalkylterthiophenes contain side chains on every third thiophene ring and exhibit increased conjugation as evidenced by the reported 468 nm absorption maxima for poly(3 -octyl-2,2 :5 ,2"-terthiophene) i n solution (Table V).(2

Tuning the Extent of Conjugation in Processable Polythiophenes

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