- Email: [email protected]
New synthetic routes to polyacenes
ELSEVIER
Synthetic
Metals
101 (1999)
100-101
New Synthetic Routes to Polyacenes J-P. N. Schmit, K. Davidson and A. Iraqi* School of Physics & Chemistry, University of Lancaster, Lancaster LA1 4YA UK
Abstract A new synthetic method toward the preparation of polyacene-type polymers is presented The new route involves condensation polymerisalion of 2,5-fUnctionalised-l,~bi~~o~e~yl)~~~ via acid catalysed reactions. Ladder-type polymers with bromosubstituents have been obtained and charactetised by gel permeation chromatography as well as spectrosccpic and thermogravimetric methods. Prehminary studies revealed a certain degree of aromatisation in the precnrsor polymers prior to oxidation reactions Keywords: Low-Bandgap conjugated polymers, polyacene.
1. Introduction
prqmsity for formation of six-membered rings especially if these reactions are can&I out at low concentrations.
Over the last few years, there has been a great deal of research interest in developing organic conjugated polymers with narrow energy band gaps. Theoretical studies have shown that polyaceues which could be considered as single strands of graphite, could satisfy these requirements with estimated energy band gaps of 0.4 eV[l]. However, the synthesis of such materials has proved challenging since it requires the fwmation of structurally well defined precursor ladder polymers prior to their aromatisation. A number of synthetic approaches have been adopted towards prcducing ladder-conjugated polymers such as the use of a succession of Diels-Alder-type cycloadditions starting t?om bifimctional d&es and dienophiies[2] or the synthesis of single stranded polymers followed by intramolecular ring closure to produce double stranded ladder-type stmclures[3]. Most recently, based on thermogravimetric and infimed analyses, Baker et al.[4] have shown that the acidcatalysed condensation of 1,2-bis(methoxymethyl)benzene led to network polymers which could have a polyacene structure while in the same time, similar reactions involving 1,3- or 1,4bis(methoxymethyl)benzenes led to single-stranded network polymerS* In this work we present our findings on a new route to poiyacene premrsors via the acid-catalysed condensation polymerisation of 2,5-functional&d-1,4-bis(allroxymethyl) benzenes. With such intermediates, side reactions involving electrophilic attack at the 2,5-positions of the benzene ring that could lead to branching or nonlinear ladder structures during the polperhation reaction are eliminated. Conceptually, the driving force behind intramolecular electrophilic reactions once an intermolecular electrophilic reaction between two monomers or a monomer and a macromolecule has taken place is the * E-mail: [email protected]
0379-6779/99/$ - see front matter 0 1999 Elsevier PII: so379-6779(98)01247-S
2. Experimental General synthetic and iastmnentai techniques are described elsewhere [5]. 2,5dibromo-pxylene was available commercially and used without further puritication. 1,4-di@romomethy~-2Jdibromobcnzene: To a solution of 2,5-dibromo-pxylene (10 g, 37.8 mmol) in CC4 (200 cm3) was added N-bromosu ccinimide (14.84 g, 83.2 mmol) and berrzoyl peroxide (few crystals). The solution was refluxed overnight. Upon cooling to room temperature, it was filtered to remove the suc&imide produced and the solvent evaporated to dryness. The product was separated upon repeated recrystalhsations from petroleum ether (4060 “C) as a white solid. (yield: 4.6J g; 29%).‘H NMR (CDCl3, TMS) 8~: 4.48 (s, 6H), 7.65 (s, 2HJ 2,5dibromo-1,4di(etho.zyn~~be3?ze11e: A solution of sodium ethoxide (4.5 g; 66.2 mmol) in ethanol (40 cm3) was added dropwise at 0 “C to a solution of 1,4di~romomethyl)2,5dibromobenzene (10 g; 23.3 mmol) in THF (100 cm3). The mixture was left to rise to room temperature overnight and was poured over water (250 cm3) and the product extracted witi diethyl ether. The ether extracts were combined and dried over magnesium su&te and the solvent evaporated to dryness to yield the product as a light yellow powder. (yield: 6.1 g, 73%). ‘H NMR (CDc13, TMS) &-I: 1.28 (t, 6H), 3.59 (q 4K), 4.50 (s, 4H), 7.63 (s, 2H). Polymer Jynfhesjs: To a solution of 2,5dibromo-1,4di(ethoxymethyl> (1 g; 2.36 mmol) in methylene chloride (15 cm3) was added suIfuric acid (30 cm3) at room temperature. A dark coloration developed and the solution was left stirring overnight The solution was poured over water (300 cm3) and the product extracted with methylene chloride (300 cm3). The organic extracts were dried over magnesium sulfate and the solvent evapcrated to dryness to produce a gray solid. (Yield: 0.28g, 38%).
Science S.A. All rights reserved.
J-P.N.
Schtnit
ct al. / Synthetic
3. Results and discussion Polymer 4 was prepared according to scheme 1. The polymer was obtained as a gray powder that was readily soluble in THF and partially soluble in methylene chloride and chloroform. Br
Br
Br 1
Br
Metals
101 (1999)
101
100-101
Fig 2 shows the thermogravimetric analysis of polymer 4. The analysis was conducted under a nitrogen atmosphere. The scan shows the go& thermal stability which is associated with ladder type structures. An initial weight loss of - 5% is observed up to a temperature of 350 “C, followed by a greater rate of weight loss between 350 and 570 “C. The weight remaining after 570 Y! and up to 800 “C remains unchanged and corresponds to 32 % of the initial weight of the polymer. The main weight loss is due to the elimination of bromine atoms. These constitute 62% of the total weight of the polymer.
2
I
NaOEt
f$-J
Br
H+
EtKY$& Br
n
2
4
Scheme 1 Preparation of substituted polyacene precursors
101
Gel permeation chromatography analysis on polymer 4 in THF using polystyrene standards revealed a bi-nodal peak with an average molecular number of MN = 2397 and polydispersity of 3.65, corresponding to a degree of polymeristion of DP = 9 (i.e. up to 18 rings). It must also be noted that the first node showed the presence of a high molecular weight material (MN = 10143 and a polydispersity of 1.78 (i.e. Dp = 39 and up to 78 rings). The UV-Vis spectrum of 4 in X-IF is shown in Fig I. It reveals that a certain degree of aromatisation has already taken place since distinct absorption peaks at 338,386 and 310 nm are clearly observed. These bands should correspond to some anthracene and hexacene units formed along the ladder polymer chains. 2.0 ,
F s 15 0 ;I a
l.O-
/
/
/
I
1
/
100 200 300 400 500 600 700 800 Temperature o C
Fig 2 Thermogravimetric analysis of 4. 4. Conclusions We have shown that the acid catalysed condensation polymerisation of 2,5-dibromo-l,4-di(ethoxymethyl)benzene could yield to ladder-type polymers with linear structures. With such intermediates, side reactions involving electrophihc attack at the 2,5-positions of the benzene ring that could lead to during the branching or nonlinear ladder structures polymer&&on reaction are eliminated. Preliminary experiments have shown that the polymers produced are partly dehydrogenated with a certain number of anthracene and hexacene units already formed along the ladder structure of the polymer backbone. Experiments are underway to reduce the bromine substituents on these polymers and lead to purely unsubstituted polyacenes. Other 2,5-fimctionalised-1,4-bis(alkoxymethyl)benzene and 2,5functionalisfzd-l,4-bis(chloromethyl)benzene intermediates with alkyl- or alkoxy-substituents that could lead to tractable polyacenes are also being investigated.
n
“,
5. References
r
OS-
0.0 !
200
.
300
nm400
Fig 1 UV-Vis spectrum of 4 in THF.
500
4
fjO0
[l] M. Kertesz,A4acrumolecule.s, 1995,28, 1475. [2] U. Scherf and K. Mullen, Synthesis, 1992,23. [3] U. ScherfandK. Mullen Makromol.Chem.&pidCommun., 1991, 12,489. [4] C. J. Ruud, C. Wang and G. L. Baker, Synth. Met., 1997,84,
363. [S] A. Iraqi and G. W. Barker, J. Mater. Chem., 1998,825.
Synthetic
Metals
101 (1999)
100-101
New Synthetic Routes to Polyacenes J-P. N. Schmit, K. Davidson and A. Iraqi* School of Physics & Chemistry, University of Lancaster, Lancaster LA1 4YA UK
Abstract A new synthetic method toward the preparation of polyacene-type polymers is presented The new route involves condensation polymerisalion of 2,5-fUnctionalised-l,~bi~~o~e~yl)~~~ via acid catalysed reactions. Ladder-type polymers with bromosubstituents have been obtained and charactetised by gel permeation chromatography as well as spectrosccpic and thermogravimetric methods. Prehminary studies revealed a certain degree of aromatisation in the precnrsor polymers prior to oxidation reactions Keywords: Low-Bandgap conjugated polymers, polyacene.
1. Introduction
prqmsity for formation of six-membered rings especially if these reactions are can&I out at low concentrations.
Over the last few years, there has been a great deal of research interest in developing organic conjugated polymers with narrow energy band gaps. Theoretical studies have shown that polyaceues which could be considered as single strands of graphite, could satisfy these requirements with estimated energy band gaps of 0.4 eV[l]. However, the synthesis of such materials has proved challenging since it requires the fwmation of structurally well defined precursor ladder polymers prior to their aromatisation. A number of synthetic approaches have been adopted towards prcducing ladder-conjugated polymers such as the use of a succession of Diels-Alder-type cycloadditions starting t?om bifimctional d&es and dienophiies[2] or the synthesis of single stranded polymers followed by intramolecular ring closure to produce double stranded ladder-type stmclures[3]. Most recently, based on thermogravimetric and infimed analyses, Baker et al.[4] have shown that the acidcatalysed condensation of 1,2-bis(methoxymethyl)benzene led to network polymers which could have a polyacene structure while in the same time, similar reactions involving 1,3- or 1,4bis(methoxymethyl)benzenes led to single-stranded network polymerS* In this work we present our findings on a new route to poiyacene premrsors via the acid-catalysed condensation polymerisation of 2,5-functional&d-1,4-bis(allroxymethyl) benzenes. With such intermediates, side reactions involving electrophilic attack at the 2,5-positions of the benzene ring that could lead to branching or nonlinear ladder structures during the polperhation reaction are eliminated. Conceptually, the driving force behind intramolecular electrophilic reactions once an intermolecular electrophilic reaction between two monomers or a monomer and a macromolecule has taken place is the * E-mail: [email protected]
0379-6779/99/$ - see front matter 0 1999 Elsevier PII: so379-6779(98)01247-S
2. Experimental General synthetic and iastmnentai techniques are described elsewhere [5]. 2,5dibromo-pxylene was available commercially and used without further puritication. 1,4-di@romomethy~-2Jdibromobcnzene: To a solution of 2,5-dibromo-pxylene (10 g, 37.8 mmol) in CC4 (200 cm3) was added N-bromosu ccinimide (14.84 g, 83.2 mmol) and berrzoyl peroxide (few crystals). The solution was refluxed overnight. Upon cooling to room temperature, it was filtered to remove the suc&imide produced and the solvent evaporated to dryness. The product was separated upon repeated recrystalhsations from petroleum ether (4060 “C) as a white solid. (yield: 4.6J g; 29%).‘H NMR (CDCl3, TMS) 8~: 4.48 (s, 6H), 7.65 (s, 2HJ 2,5dibromo-1,4di(etho.zyn~~be3?ze11e: A solution of sodium ethoxide (4.5 g; 66.2 mmol) in ethanol (40 cm3) was added dropwise at 0 “C to a solution of 1,4di~romomethyl)2,5dibromobenzene (10 g; 23.3 mmol) in THF (100 cm3). The mixture was left to rise to room temperature overnight and was poured over water (250 cm3) and the product extracted witi diethyl ether. The ether extracts were combined and dried over magnesium su&te and the solvent evaporated to dryness to yield the product as a light yellow powder. (yield: 6.1 g, 73%). ‘H NMR (CDc13, TMS) &-I: 1.28 (t, 6H), 3.59 (q 4K), 4.50 (s, 4H), 7.63 (s, 2H). Polymer Jynfhesjs: To a solution of 2,5dibromo-1,4di(ethoxymethyl> (1 g; 2.36 mmol) in methylene chloride (15 cm3) was added suIfuric acid (30 cm3) at room temperature. A dark coloration developed and the solution was left stirring overnight The solution was poured over water (300 cm3) and the product extracted with methylene chloride (300 cm3). The organic extracts were dried over magnesium sulfate and the solvent evapcrated to dryness to produce a gray solid. (Yield: 0.28g, 38%).
Science S.A. All rights reserved.
J-P.N.
Schtnit
ct al. / Synthetic
3. Results and discussion Polymer 4 was prepared according to scheme 1. The polymer was obtained as a gray powder that was readily soluble in THF and partially soluble in methylene chloride and chloroform. Br
Br
Br 1
Br
Metals
101 (1999)
101
100-101
Fig 2 shows the thermogravimetric analysis of polymer 4. The analysis was conducted under a nitrogen atmosphere. The scan shows the go& thermal stability which is associated with ladder type structures. An initial weight loss of - 5% is observed up to a temperature of 350 “C, followed by a greater rate of weight loss between 350 and 570 “C. The weight remaining after 570 Y! and up to 800 “C remains unchanged and corresponds to 32 % of the initial weight of the polymer. The main weight loss is due to the elimination of bromine atoms. These constitute 62% of the total weight of the polymer.
2
I
NaOEt
f$-J
Br
H+
EtKY$& Br
n
2
4
Scheme 1 Preparation of substituted polyacene precursors
101
Gel permeation chromatography analysis on polymer 4 in THF using polystyrene standards revealed a bi-nodal peak with an average molecular number of MN = 2397 and polydispersity of 3.65, corresponding to a degree of polymeristion of DP = 9 (i.e. up to 18 rings). It must also be noted that the first node showed the presence of a high molecular weight material (MN = 10143 and a polydispersity of 1.78 (i.e. Dp = 39 and up to 78 rings). The UV-Vis spectrum of 4 in X-IF is shown in Fig I. It reveals that a certain degree of aromatisation has already taken place since distinct absorption peaks at 338,386 and 310 nm are clearly observed. These bands should correspond to some anthracene and hexacene units formed along the ladder polymer chains. 2.0 ,
F s 15 0 ;I a
l.O-
/
/
/
I
1
/
100 200 300 400 500 600 700 800 Temperature o C
Fig 2 Thermogravimetric analysis of 4. 4. Conclusions We have shown that the acid catalysed condensation polymerisation of 2,5-dibromo-l,4-di(ethoxymethyl)benzene could yield to ladder-type polymers with linear structures. With such intermediates, side reactions involving electrophihc attack at the 2,5-positions of the benzene ring that could lead to during the branching or nonlinear ladder structures polymer&&on reaction are eliminated. Preliminary experiments have shown that the polymers produced are partly dehydrogenated with a certain number of anthracene and hexacene units already formed along the ladder structure of the polymer backbone. Experiments are underway to reduce the bromine substituents on these polymers and lead to purely unsubstituted polyacenes. Other 2,5-fimctionalised-1,4-bis(alkoxymethyl)benzene and 2,5functionalisfzd-l,4-bis(chloromethyl)benzene intermediates with alkyl- or alkoxy-substituents that could lead to tractable polyacenes are also being investigated.
n
“,
5. References
r
OS-
0.0 !
200
.
300
nm400
Fig 1 UV-Vis spectrum of 4 in THF.
500
4
fjO0
[l] M. Kertesz,A4acrumolecule.s, 1995,28, 1475. [2] U. Scherf and K. Mullen, Synthesis, 1992,23. [3] U. ScherfandK. Mullen Makromol.Chem.&pidCommun., 1991, 12,489. [4] C. J. Ruud, C. Wang and G. L. Baker, Synth. Met., 1997,84,
363. [S] A. Iraqi and G. W. Barker, J. Mater. Chem., 1998,825.