Water soluble polythiophenes: preparation and applications

Abstract

This review describes the synthesis of different water soluble polythiophenes and their versatile applications. Solubility in water is essential for developing sensors for different bio-molecules and polythiophene derivatives are excellent candidates due to their important optoelectronic properties. A pristine polythiophene chain is hydrophobic and it exhibits aqueous solubility after attachment/grafting of ionic pendent groups or hydrophilic polymer chains on its backbone. A concise account of the different synthetic procedures of preparing water soluble polythiophene is described and all the specific techniques relevant to the synthesis of water soluble polythiophene using cationic, anionic pendent groups and grafting of hydrophilic polymers are discussed. Different grafting processes e.g. “grafting from” and “grafting to” techniques using click chemistry and atom transfer radical polymerization (ATRP) are described in detail. Detections of different bio-molecules such as DNA, RNA, polypeptides, polysaccharides, ATP, UDP and ADP from the excellent opto-electronic properties of aqueous polythiophenes are discussed. The reports on fluorescence based specific sensing of metal ions, and nitro-aromatics using water soluble polythiophenes are also embodied with an up-to-date description of the optoelectronic device applications such as logic gates, molecular thermometers, photovoltaic cells etc. Finally, a summary and outlook is presented discussing the future scope of research on this important subject.

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Sandip Das

Mr Sandip Das received his B.Sc. degree in Chemistry from Midnapore College (Vidyasagar University) and M.Sc. degree in Chemistry from Bengal Engineering and Science University, Shibpur. He joined at Indian Association for the Cultivation of Science as a DST ‘INSPIRE’ research fellow and has been continuing his research under the supervision of Prof. A. K. Nandi. His research interest is synthesis of water soluble polythiophenes by using ATRP technique and its optoelectronic property study.

Dhruba P. Chatterjee

Dr Dhruba P. Chatterjee received his Ph.D. from Jadavpur University in 2006 for his doctoral research at Indian Association for the Cultivation of Science under the supervision of Prof. B. M. Mandal in the field of controlled polymer synthesis and characterization. He then continued in the group of Prof. A. K. Nandi in the field of synthesis, characterization and applications of various graft copolymers. In 2014 he joined as Assistant Professor in the Department of Chemistry, Presidency University, Kolkata and his present research interest includes synthesis of conducting polymer based graft copolymers, membranes and nano particles for sensor applications.

Radhakanta Ghosh

Mr Radhakanta Ghosh received his B.Sc. and M.Sc. degree in Chemistry from Burdwan University, Burdwan and joined at Indian Association for the Cultivation of Science as a ‘CSIR’ research fellow under the supervision of Prof. A. K. Nandi. His research interest is synthesis of polythiophene based graft copolymers by using ATRP technique and preparation of polythiophene based sensor.

Arun K. Nandi

Prof. Arun K. Nandi obtained Ph.D. degree on “Studies on Polymer–Polymer and Polymer–Solvent mixing” and joined Chemistry Department, North Bengal University, Darjeeling. He did post doctoral work at Florida State University with Prof. L. Mandelkern in crystallization of polymers. In 1992 he was appointed at Polymer Science Unit at Indian Association for the Cultivation of Science and he is presently senior professor and head of the unit. His research interests focus on polymer blends, polymer crystallization, polymer and supramolecular gels, polymer nanocomposites, polymer grafting and biomolecular hybrids. He is author of more than 170 papers, and supervised 22 Ph.D. students.

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1. Introduction

In the field of materials science, water soluble, stimuli responsive conjugated polymers showing fluorescence properties are a stunning contender for the fabrication of smart materials.^1–5 Significant research attention is received by the conjugated polymers like polythiophene (PT), polyaniline, polypyrrole, polyacetylene, etc. due to their excellent optical, electrical and electronic properties.^6–9 Due to these excellent properties, conjugated polymers have potential applications in the fields of sensors for biological, inorganic and different organic materials.^10–17 The conjugated polymers exhibit chromic responses tuneable at different physical and chemical conditions^10,18–20 making them a better choice than the small molecular analogs due to their high mechanical strength, versatility in input–output signals (temperature, pH, light, fluorescence, ion etc.)²¹ and high sensitivity because of the co-operative response of the conjugated chains even in the presence of a small perturbation.^10,22,23 Despite these important and useful properties, the solubility of the pristine conjugated polymers is a major problem because of intra and inter chain aggregations causing insolubility in common solvents, especially in water.²⁴ In order to alleviate the problem, attachment of flexible pendent groups onto the conjugated backbone is necessary and in the new sites one can incorporate chemically and biologically active moieties.^24,25 Therefore, the synthesis of water soluble conjugated polymer is very important because of its use in precious biological sensors where water solubility is one of the mandatory criteria.²⁶ Amongst the conjugated polymers polythiophene is the most important for its excellent optoelectronic properties like tuneable fluorescence property, forster resonance energy transfer (FRET), and a tuneable electrical conductivity.^10,22 Generally substituted polythiophenes are two types, one is regioregular and another is regio-irregular, the former is important for photovoltaic and other electronic applications like conductivity, liquid crystallinity, mobility^27,28 and chirality;²⁹ but for sensor applications, where polythiophene acts as a fluorescence probe, regioregularity is not so much important as quenching of fluorescence intensity may take place due to easy inter chain decay of excitons in the aggregated structures of regioregular chains.³⁰ Solvato-chromism is another important property of polythiophenes showing colour changes upon changing from ‘good’ to ‘bad’ solvent³¹ and it occurs due to the changes in chain conformation and/or inter/intra chain aggregation.^32–35

Substituted polythiophenes are semiconducting materials having various technological applications particularly in the fields of field effect transistor (FETs),^27,36,37 polymer light emitting diodes (PLEDs),^38–41 solar cells⁴² and other opto-electronic devices due to its easy fabrication and tunable optoelectronic properties. Fluorescence property of water soluble polythiophenes extend their applications in chemosensors for biological targets (specific detection of deoxyribonucleic acid (DNA), proteins, human thrombin etc.),^4,43,44 explosive materials, cations, anions and for the fabrication of molecular devices like molecular logic gate,^45–47 molecular thermometer⁴⁸ etc. For the sensing process fluorescence signals are suitable as output and hence the water soluble polythiophenes are essential to develop biological sensors and other environment friendly processes.

There are few reviews and book chapters on the synthesis and applications of conjugated polymers where reports on polythiophenes are discussed by McQuade et al.,¹⁰ Schanze et al.,⁴⁹ Leclerc et al.⁵⁰ and McCullough et al.³ Amongst these reviews Schanze et al.⁴⁹ have discussed only about few water soluble polythiophenes e.g. ionomers of polythiophene acetate and of polythiophene ethyl & butyl sulfonate produced by electro-polymerization. Leclerc et al., who is one of the initial leader in developing water soluble polythiophenes, also reported few polythiophene based chemical and biochemical sensors.⁵⁰ So, to the best of our knowledge, there is no exhaustive review on water soluble polythiophenes dealing with different techniques of their synthesis and optoelectronic applications. This review, therefore, covers the synthesis of water soluble polythiophenes using different techniques and sheds light on their optoelectronic properties promising its diverse applications.

2. Water soluble polythiophenes

The first synthesis of water soluble conducting polymers, sodium salt of poly(3-(2-ethanesulfonate)thiophene) (1) and poly(3-(4-butanesulfonate)thiophene) (2), was reported in 1987 by Patil et al. and they introduced a new mechanism of doping, designated as self doping.⁵¹ Water soluble poly(2-(3-thienyloxy)ethanesulfonate) (3) and sodium poly(2-(4-methyl-3-thienyloxy)ethanesulfonate) (4) were reported by Leclerc et al. in 1997.⁵² In 1999, Kim et al. reported water soluble poly(3-thiophene acetic acid) (P3TAA) and potentiometric titration along with pH dependent UV-visible absorption spectra of various modified P3TAA.⁵³ Polyanion and polycation based water-soluble poly(alkoxythiophene) derivatives with high conjugation length, e.g. poly-3-(3′-thienyloxy)propanesulfonate (P3TOPS, 5) and poly-3-(3′-thienyloxy)propyltriethylammonium (P3TOPA, 6) were reported by Lukkari et al. in 2001 (ref. 54) and in subsequent year Ho et al. reported water soluble electroactive and photoactive cationic poly(3-alkoxy-4-methylthiophene) based water soluble polymers (7).⁵⁵ Dore et al. also reported water soluble, fluorescent, cationic polythiophene derivative poly(1H-imidazolium, 1-methyl-3-[2-[(4-methyl-3-thienyl)oxy]ethyl]-chloride) in 2004.⁵⁶ Very recently, a new water soluble cationic polythiophene poly(N,N,N-trimethyl-3-(2-(thiophene-3-yl)acetamido)propan-1-aminium iodide) via two successive post-polymerization functionalization of poly(methyl-2-(thiophene-3-yl)acetate) is reported by Carreon et al.⁵⁷ Thus a series of water soluble polythiophene ionomers are reported in literature and a brief overview of their structure is presented in Scheme 1. Water soluble neutral polythiophenes were produced by grafting of hydrophilic polymer chains like poly(N-isopropyl acrylamide) (PNIPAM),⁵⁸ poly(N,N-dimethyl aminoethyl methacrylate) (PDMAEMA),^24,46 poly(diethylene glycol methylether methacrylate) (PMeO₂MA),⁵⁹ poly(oligo-ethylene glycol methacrylate) (POEGMA),⁵⁹ poly(N,N-diethyl amino ethyl methacrylate) (PDEAEMA)⁶⁰ etc.

Scheme 1 Chemical structures of different water soluble polythiophenes.

Balamurugan et al. produced PNIPAM grafted polythiophene in 2005 by a grafting from technique to prepare water soluble temperature sensitive polythiophene.⁵⁸ Synthesis of a water soluble non ionic polythiophene having oligoethylene glycol side chains and its conformation change in water was reported by Matthews et al. in 2005.⁶¹ Wang et al. grafted PDMAEMA on polythiophene backbone in 2008 to make it water soluble and pH responsive using atom transfer radical polymerization (ATRP) technique.²⁴ From this laboratory, in this decade we have reported a number of water soluble temperature sensitive polythiophene based graft co-polymer polythiophene-g-P(MeO₂MA) (PTD),⁵⁹ polythiophene-g-(MeO₂MA-co-OEGMA) (PTDO)⁵⁹ and water soluble dual responsive (pH and temperature) polythiophene based graft copolymer PT-g-(PMeO₂MA-co-PDMAEMA) (PTDM)⁶² and PT-g-(PMeO₂MA-co-PDEAEMA) (PTDE) produced by ATRP technique.⁶⁰ In Table 1 a classification of water soluble polythiophenes containing different types of pendant groups from the polythiophene backbone is presented.

Table 1 Classification of water soluble polythiophene depending upon its synthetic method

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3. Synthesis of water soluble ionic polythiophenes

The details of synthetic procedures of cationic and anionic polythiophenes are presented below.

3.1. Water soluble cationic polythiophenes

In 2001 Lukkari et al. reported the synthesis of highly water soluble polycationic polymer poly-3-(3′-thienyloxy)propyltriethyl ammonium ion (P3TOPA, 6) from the monomer 3-(3′-thienyloxy)propyltriethylammonium bromide and the polymerization was carried out in anhydrous chloroform (CHCl₃) using ferric chloride (FeCl₃). The monomer 3-(3′-thienyloxy)propyltriethyl ammonium ion was prepared from 3-(3-bromo)-propoxythiophene by using triethylamine (Et₃N) in ethanol. The polymer (P3TOPA, 6) showed high conjugation length both in the solid and in the solution state.⁵⁴ The absorbance maximum of the polymer (P3TOPA) occurs at 582 nm both in water and in dimethyl sulfoxide (DMSO), and the absorbance maxima at lower energy indicate higher conjugation length.⁶³ Ho et al. reported water soluble electroactive and photoactive cationic polythiophene (11) by the oxidative polymerization in presence of FeCl₃ (Scheme 2).⁵⁵ Thiophene monomer (10) was prepared from 3-(2-bromoethoxy)-4-methylthiophene (9) using 1-methyl-imidazole in acetonitrile (CH₃CN) at 70 °C (yield 88%). Pang et al. synthesised thiophene derivative 3-(3-bromo)propoxythiophene (13) from 3-methoxythiophene (12) by treatment with 2-bromo-1-propanol in the presence of NaHSO₄ and the amine-functionalized thiophene monomer (14) was polymerized with FeCl₃ to produce a water soluble cationic polythiophene derivative [poly{3-(3-N,N-diethylaminopropoxy)thiophene} (PDAOT, 16) (Scheme 3)].⁶⁴

Scheme 2 Synthetic procedure of water soluble cationic poly(3-alkoxy-4-methylthiophene)s (reproduced from ref. 55).

Scheme 3 Synthetic procedure of poly[3-(3-N,N-diethylaminopropoxy)thiophene] (PDAOT) (reproduced from ref. 64).

A water soluble cationic polythiophene derivative poly[3-(6-pyridiniumylhexyl)thiophene bromide] (P3PHT⁺ Br⁻) (18) from poly[3-(6-bromohexyl)thiophene] (P3BHT) (17) was synthesised by Rider et al. in 2010 using a simple substitution reaction presented in Scheme 4 and it was prepared by quaternization reaction of pyridine with P3BHT.⁶⁵ In 2012, Li et al. reported a synthetic procedure of another water soluble cationic polythiophene derivative poly[3-(1,1′-dimethyl-4-piperidinemethylene)thiophene-2,5-diyl chloride] (PDPMT-Cl), which was characterized by NMR and gel permeation chromatography (GPC) technique (M̄_n) = 8868 g mol⁻¹, PDI = 2.32. The polymer (PDPMT-Cl, 24) was synthesized from 3-(1,1′-dimethyl-4-piperidinemethylene)thiophene methyl sulphate (23) using FeCl₃ as the oxidizing agent (Scheme 5).

Scheme 4 Quaternization reaction of (P3BHT) with pyridine to produce poly[3-(6-pyridiniumylhexyl)thiophene bromide] (P3PHT⁺ Br⁻) (reproduced from ref. 65).

Scheme 5 Synthetic route of water soluble cationic PDPMT-Cl (reproduced from ref. 66).

The thiophene derivative (23) was prepared from the compound (22) by using Me₂SO₄ in acetonitrile solvent.⁶⁶ Another water soluble cationic polythiophene poly(N,N,N-trimethyl-3-(2-(thiophen-3-yl)acetamido)propan-1-aminium iodide) (29), was synthesized by Carroen et al. from poly(methyl-2-(thiophen-3-yl)acetate) (27) according to Scheme 6. The monomer methyl-3-thiophene methyl acetate (3TMA, 26) was prepared from 3-thiopheneacetic acid (25) using catalytic amount of concentrated H₂SO₄ in dry methanol and the product was received as a pale yellow liquid (yield 82%). Polymer 27 was prepared from monomer 26 by oxidative polymerization using FeCl₃ and the product received as a brown solid (yield 78%). Polymer 29 was synthesized by methylation of polymer 28 with CH₃I in MeOH/Et₂O at room temperature. The product precipitated out as the reaction progressed as a dark brown solid.⁵⁷

Scheme 6 Synthesis of cationic polythiophenes through post-polymerization functionalization of neutral polythiophene (reproduced from ref. 57).

3.2. Water soluble anionic polythiophenes

The first water soluble conducting polymer, sodium salts of poly-3-(2-ethanesulfonate)thiophene and poly-3-(4-butanesulfonate)thiophene was produced by Patil et al.⁵¹ They synthesised the two water soluble anionic polythiophene, sodium poly(3-thiophene-β-ethanesulfonate) (P3-ETSNa, 1) from 3-thiophene-β-ethanesulfonate (31) and sodium poly(3-thiophene-δ-butanesulfonate) (P3-BTSNa, 2) from 3-thiophene-δ-butanesulfonate (32) thiophene derivative by oxidative polymerization. Their corresponding conjugate acids are also water soluble and during oxidation they lose a proton and produce self doped polymers. Formation of thiophene based anionic monomers is presented in Scheme 7.⁶⁷ Water soluble poly(2-(3-thienyloxy)ethanesulfonate) and sodium poly(2-(4-methyl-3-thienyloxy)ethanesulfonate) were reported by Leclerc et al. in 1997 and the monomer sodium-(2-(3-thienyloxy)ethanesulfonate) was synthesized from 3-(2-bromo)ethoxythiophene using Na₂SO₄ (yield 37%). Both the polymers sodium poly(2-(3-thienyloxy)ethanesulfonate) & sodium poly(2-(4-methyl-3-thienyloxy)ethanesulfonate) were synthesized by oxidative polymerization using FeCl₃ and they were isolated from methanol as a dark powder (yield 50–60%).⁵² Kim et al. synthesised water soluble pH responsive polythiophene carboxylic acid (34) by the oxidative polymerization of 3-thiophene methyl acetate (3TMA, 26) using FeCl₃, followed by hydrolysis of ester groups using NaOH and then neutralized by dilute HCl (Scheme 8). The monomer 3TMA was prepared by refluxing 3TAA (25) for 24 h in anhydrous CH₃OH in presence of catalytic amount of H₂SO₄. The solution properties of polymer was studied by potentiometric titration, and UV visible spectroscopy etc.⁵³ The titration curve of P3TAA indicated that its dissociation behaviour was suppressed at the higher pH region (>6) due to the rigid coplanar structure of the main chain stabilizing the ionized carboxylate and un-ionized carboxylic acid, through a hydrogen bonded complex formation.

Scheme 7 Synthetic procedure of 3-ETSNa and 3-BTSNa (reproduced from ref. 67).

Scheme 8 Synthesis procedure of water soluble polythiophene acetic acid by simple oxidative polymerization (reproduced from ref. 53).

McCullough et al. (1997) improved the synthesis of regioregular HT-2,5-poly(thiophene-3-propionic acid) (PTPA, 37) and the polymer was transformed into water soluble ammonium salt (38) by the deprotonation of corresponding polymeric acid in presence of aqueous base (Scheme 9). The oxazoline polythiophene 36 was prepared from thiophene derivative 35 by Pd₂(dba)₃ using CuO customized Stille coupling reaction (yield 84%). Polymer 36 undergoes post polymerization hydrolysis reaction in presence of 3 M hydrochloric acid and produced a dark purple solid product (37). The NMR data indicates regioregularity of the polymer is ∼100%.⁶⁸ Lukkari et al. synthesised the monomer 3-(3′-thienyloxy)propanesulfonate from 3-methoxythiophene using 3-bromopropanol and the synthesized monomer undergoes oxidative polymerization in presence of FeCl₃ to produce water soluble poly-3-(3′-thienyloxy)propanesulfonate (P3TOPS, 5) in 2001.⁵⁴ Richter et al. reported water soluble anionic polythiophenes in 2012 using post polymerization de-protonation of polythiophene end groups by butyllithium followed by subsequent reaction with CO₂ gas. The carboxylic acid functionalized polythiophene (P3T–COOH) was obtained by the acid treatment of the reaction mixture.⁶⁹ Liu et al. in 2013 reported the anionic polythiophene derivative, poly[2-(3-thienyl)ethyloxy-4-butylsulfonate]sodium salt (PTEBS) that has intrinsic peroxidase-like activity which can catalyze the reaction of peroxidise substrate 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of hydrogen peroxide (H₂O₂) to make a blue colouration.⁷⁰

Scheme 9 Synthesis of regioregular, head-to-tail polythiophenes (reproduced from ref. 68).

4. Synthesis of water soluble neutral polythiophenes

The grafting of hydrophilic polymer chains on polythiophene backbone is a fascinating way to produce water soluble neutral polythiophenes and in literature two different types of techniques viz. ‘grafting to’ and ‘grafting from’ are reported. Also there are few reports for the small molecular grafting on polythiophene backbone but in this process synthesized polymer usually aqueous insoluble. A detail of these grafting procedures are embodied here.

4.1. Grafting to process

In case of ‘grafting to’ method at first different hydrophilic polymer chains are synthesized separately and these pre-synthesized polymer chains are then attached on the polythiophene backbone. Recently Cu catalyzed “click” reaction and azide-alkyne cycloaddition are now very popular because of its powerful utility in the field of polymer chemistry. This reaction is one of the most facile, high yield and popular reaction procedures for the “grafting to” process.^71–74 In 2011 Li et al. reported P3HT containing amphiphilic rod-coil diblock copolymer poly(3-hexylthiophene)-block-poly(acrylic acid) (P3HT-b-PAA) (46) (Scheme 10) via click chemistry. The polymer 45 was prepared by using click chemistry between (42) and (44) in presence of N,N,N′,N′,N′′-pentamethyl diethylene triamine (PMDETA)/CuBr as the catalyst system and the polymer (46) was prepared from polymer (45) by acidolysis of polymer (45) with excess trifluoroacetic acid (TFA) in CHCl₃ (yield 97%).

Scheme 10 Synthesis of P3HT-b-PtBA and P3HT-b-PAA (reproduced from ref. 75).

The self-assembly behaviour of the polymer (46) was studied by dynamic light scattering and TEM microscopy.⁷⁵ In the same year the same group also reported the synthesis and self-assembly property of P3HT-block-poly(γ-benzyl-L-glutamate) (51) which is the first report of polythiophene–polypeptide based diblock copolymer (Scheme 11) using Cu catalysed click chemistry (yield 60–72%).⁷⁶ It self assembles into hierarchal structures in solution and in the solid state. Depending upon the aggregation of P3HT chains; the colour of the THF solution of the polymer (51) changes from orange to purple (absorption peak shifts from 442 to 484 nm) by the slow addition of dimethyl formamide. The TEM micrograph also exhibits the spherically aggregated structure of the polymer with average particle size 287 ± 52 nm. This P3HT-block-PBLG, is suggested to exhibit photovoltaic property and is suitable for related applications.⁷⁶ Another water soluble amphiphilic poly(3-hexylthiophene)-graft-poly(ethylene oxide) (P3HT-g-PEO, 56) rod-coil conjugated random copolymers was reported by Mohamed et al. using simple oxidative polymerization with FeCl₃ followed by facile click chemistry {GPC: M_n = 15 000, PDI = 1.69 (Scheme 12)}.⁷⁷ The dynamic light scattering and atomic force microscopy were used to study the self assembled structure of this random copolymer both in solution and in bulk state.

Scheme 11 Synthesis of P3HT–C≡CH (A), PBLG–N₃ (B) and their respective block copolymer P3HT-block-PBLG (reproduced from ref. 76).

Scheme 12 Synthesis of P3HT-g-PEO random copolymers through oxidative polymerization and click reactions (reproduced from ref. 77).

4.2. Grafting from process

The “grafting from” technique is related to the polymerization of monomers from the macroinitiators created at the backbone of the polythiophene. Usually ‘grafting from’ technique is mainly related with the atom transfer radical polymerization (ATRP) of different monomers from the macro-initiator synthesized from thiophene derivative. However ATRP is the most powerful technique for the controlled living radical polymerization. Different types of monomers like acrylate, methacrylate, styrene, acrylamide, etc. are polymerized by ATRP.^78–81 In the ‘grafting from’ process the end bromo functionalized polythiophene is used as a macro-initiator for the formation of second block.^82–85 Different types of ATRP initiator moiety like 2-bromoisobutyryl bromide (2BIB), 2-cloropropyonyl chloride etc. are covalently attached with substituted thiophene monomers to produce thiophene initiator (TI, 57) and then TI undergoes oxidative polymerization producing polythiophene initiator (Scheme 13, PTI, 58) which is essential for grafting from process. The ¹H NMR spectrum of PTI exhibits that the signal corresponding to the methylene proton becomes broader due to polymerization and its splitting into two signals indicates presence of regioirregularity. From the ratio of peak area regioregularity of PTI is calculated to be ∼64%. The number average molecular weight (M̄_n) calculated from GPC using polystyrene as the standard was 38 000.^59,62

Scheme 13 Synthesis of PTI by oxidative polymerization (reproduced from ref. 58).

The major advantage of this technique is that the steric hindrance cannot limit the chain growth of the polymer. However, a disadvantage of this strategy, is the requirement of multiple pre-polymerization modifications as the ATRP initiator must be attached onto the each thiophene unit in the macro-initiator.⁷⁵ Thiophene initiator, 3-[1-ethyl-2-(2-bromoisobutyrate)] thiophene (TI, 57) is prepared from thiophene-3-ethanol (30) by coupling with the ATRP initiator moiety 2-bromoisobutyryl bromide (2BIB) in presence of triethylamine (Et₃N) at 0 °C (yield 80%).^58,60 In the year 2005 Balamurugan et al. reported the synthesis of polythiophene macro-initiator 2,5-poly(3-[1-ethyl-2-(2-bromoisobutyrate)]thiophene) (PTI, 58) from TI (57) via oxidative polymerization with FeCl₃ (yield 83%).⁵⁸ Synthesis of water soluble polythiophene-g-poly(N-isopropyl acrylamide) (PT-g-PNIPAM, 59) by ATRP from PTI was reported by Balamurugan et al.

The monomer NIPAM was polymerized upon PTI macro-initiator using CuBr/1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane catalyst/ligand system (yield 53%) (Scheme 14).⁵⁸ The molecular weight of the polymer measured by GPC was 2 × 10⁶ and the graft copolymer is highly soluble in water (solubility limit 11 g L⁻¹). Light-scattering studies of poly(thiophene-g-PNIPAM) in water showed that the polymer brush changed its molecular conformation at the lower critical solution temperature (LCST) of 32 °C. Water soluble non ionic polythiophene poly(3,4-bis[2-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-(S)-2-methylethoxy]thiophene) was synthesized from the monomer 3,4-bis[2-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-(S)-2-methylethoxy]thiophene by oxidative polymerization in presence of FeCl₃ and chloroform yielding the product as a purple solid. Thiophene derivative 3,4-bis[2-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-(S)-2-methylethoxy]thiophene was synthesized from 3,4-dimethoxythiophene by transeterification with chiral tetra-ethylene glycol at 90 °C (yield 77.7%).⁶¹

Scheme 14 Synthetic procedure of PT-g-PNIPAM (reproduced from ref. 58).

Water soluble pH responsive polythiophene based graft copolymer polythiophene-g-poly(N,N-dimethyl aminoethyl methacrylate), (PD, 60) was synthesized from PTI using ATRP techniques by Wang et al. (Scheme 15). The (M̄_n) calculated from GPC using tetrahydrofuran (THF) as the solvent and polystyrene as the standard was 590 000 (PDI = 3.9). PD exhibited a reversible pH response in aqueous solution as evident from fluorescence spectroscopy.^24,46 We synthesized polythiophene based temperature sensitive water soluble graft copolymer polythiophene-g-poly(diethylene glycol methyl ether methacrylate) (PTD, 61) by ATRP using diethylene glycol methyl ether methacrylate as the monomer, CuCl as the catalyst and 1,1,4,7,10,10-hexamethyl triethylene tetramine (HMTETA) as the ligand at 30 °C. The overall procedure is schematically represented in Scheme 16.⁵⁹

Scheme 15 Synthetic procedure of PD (reproduced from ref. 24).

Scheme 16 Synthetic procedure of PTD & PTDO (reproduced from ref. 59).

Thermo-responsive behavior of the polymer was examined by fluorescence spectroscopy and also by dynamic light scattering (DLS) data. The fluorescence intensity and the particle size of the polymer increased above LCST. Above LCST particle size of the PTD increased due to collapsing of PMeO₂MA chain on the PT core, decreasing non-radiative energy transfer with the water molecules by protecting the PT excitons from quenching with solvent (water) molecules causing an increase of PL intensity.⁵⁹ Another polythiophene based water soluble temperature sensitive graft copolymer polythiophene-g-poly(diethylene glycol methyl ether methacrylate)-co-poly(oligoethylene glycol methyl ether methacrylate) (PTDO, 62) was prepared by using two co-monomers MeO₂MA and OEGMA. ATRP of MeO₂MA and OEGMA from PTI backbone occurs at 30 °C using CuCl as the catalyst, HMTETA as the ligand, and THF as the solvent and the overall procedure is schematically presented in Scheme 16.⁵⁹

The temperature sensitivity of the polymer was checked by fluorescence spectroscopy showing LCST which changes by changing the OEGMA concentration.⁵⁹ The transmission electron microscopy (TEM) micrographs showed that polythiophene chains self-organize as nanospheres, and atomic force microscopy (AFM) indicates that aggregated polythiophene chains are present at the centre surrounded by PMeO₂MA fibers producing core–shell type aggregates. Water soluble polythiophene based stimuli responsive graft copolymer polythiophene-g-poly(diethylene glycol methyl ether methacrylate)-co-poly(N,N-dimethyl aminoethyl methacrylate) (PTDM, 63) was also synthesised from PTI macro-initiator using ‘grafting from’ technique as shown in Scheme 17. In nitrogen atmosphere MeO₂MA and DMAEMA monomers were polymerized on PTI in presence of CuCl/HMTETA as catalyst/ligand system.⁶² Fluorescence intensity of PTDM sharply increases in the temperature range 22–29 °C at pH-9.2 but the fluorescence intensity remain unchanged at low pH-4 & 7.⁶² TEM micrograph exhibits that PTDM also has core shell type of morphology at acidic condition. The pH responsive monomer DMAEMA becomes protonated at low pH originating electrostatic repulsion between two –NMe₂H⁺ groups causing non-collapsing of temperature sensitive PMeO₂MA part even at above the LCST resulting distinct core shell like structure. Synthesis of water soluble polythiophene-g-poly(diethylene glycol methyl ethermethacrylate)-co-poly(N,N-diethyl aminoethyl methacrylate) (PTDE, 64, Scheme 18) polythiophene graft copolymer was performed by ATRP technique. The monomer MeO₂MA and DEAEMA chains had grown from PTI macroinitiator, using CuCl/HMTETA as the catalyst/ligand system (Scheme 18).⁶⁰ PTDE shows good solubility in water showing LCST at ∼42 °C at biological pH (pH = 7).

Scheme 17 Synthetic procedure of PTDM (reproduced from ref. 62).

Scheme 18 Synthetic procedure of polythiophene based graft copolymer PTDE (reproduced from ref. 60).

4.3. Small molecular grafting

Bäuerle et al. reported uracil functionalized bithiophene and tetra-thiophene in 1997. The same group also reported synthesis, characterization and polymerization of nucleobases (uracil, adenine) grafted bithiophene for the discerning recognition of DNA and RNA bases. In the year 1995 Bäuerle et al. also reported the synthesis of novel 15-crown-5 substituted (oligo-)thiophenes in which macrocycle was in the direct conjugation and the properties of the polymer was investigated in presence of alkali metal ions.^86–88

5. Characterization

Usually the polymers are characterized by TEM, AFM and the structures of the polymer are characterized by ¹H NMR and ¹³C NMR spectroscopy techniques. The temperature dependent particle size variations are studied using DLS data. The molecular weight of the polymer is determined by GPC technique.

The TEM micrographs of water soluble dual responsive (temperature, pH) polythiophene based graft copolymer (PTDM, 63) exhibited core–shell (PT at the core and grafted part at the shell, Fig. 1) type morphology at low pH but the distinct core–shell structure was destroyed at high pH due to reduced repulsion between the –NMe₂⁺ ions causing a collapsed aggregate.^59,62 The AFM image of PD (60) exhibited worm-like morphology containing both the backbone (polythiophene) and the grafted PDMAEMA chains in both the height and phase images (Fig. 2). In Fig. 2 the elongated structure of the densely grafted polymer was observed in case of toluene solvent which is good solvent both for polythiophene backbone and grafted chain. The length of the polymer obtained from AFM images varied from 50 to 200 nm. But in case of PD (60) brush copolymer water is a bad solvent for polythiophene and good for PDMAEMA, exhibited the core–shell morphology.²⁴ In case of PTD (61) similar core–shell morphology was observed in AFM when the film was cast from water.⁵⁹

Fig. 1 TEM images of PTDM sample at pH-4, 7 and 9.2 (reproduced from ref. 62).

Fig. 2 AFM images ((A) height; (B) phase) of PT-g-PDMAEMA adsorbed on mica from dilute solution (2 × 10⁻³ g L⁻¹) of toluene (A and B) (reproduced from ref. 24).

The structure and the regioregularity of the PT graft-polymer are characterized by ¹H and ¹³C NMR spectra.⁸⁹ The molecular weight of the water soluble PT graft copolymers were calculated from NMR data.^58,59,62 For the graft copolymers GPC technique is not ideal to determine the accurate molecular weight because of lack of proper standard.^24,59,62 Fig. 3 exhibits that, the elution volume of the PTD (61) decreased with increasing the reaction time signifying an increase of hydrodynamic volume of the graft copolymer with increase of polymerization time confirming the progress of grafting. In case of PTI the calculated molecular weight from GPC is nearly equal with the actual molecular weight^24,90 but for the graft copolymer GPC molecular weight is different from the actual which is usually calculated from ¹H NMR spectrum. For higher molecular weight graft polymers, GPC generally gives lower apparent molecular weights measured from light scattering^91–93 viscosity,^{94–97 1}H NMR etc.⁹⁸

Fig. 3 GPC traces of PTD graft co-polymers with reaction time (1) 2 h, (2) 4 h, (3) 8 h, (4) 18 h (inset: GPC trace of PTI) (reproduced from ref. 59).

The DLS experiment indicated that the water soluble temperature sensitive PT graft-copolymers undergo temperature dependent size change at LCST. In case of poly(thiophene-g-PNIPAM) this temperature is 32 °C.⁵⁸ At low temperature (below LCST) the grafted part of the polymer becomes soluble and the particle size appears mainly due to the insoluble polythiophene chain. But at basic pH and above LCST of the polymers (e.g. for PTD, PTDM, PTDE etc.), the temperature sensitive chain of the polymer becomes insoluble and collapsed upon the polythiophene chain and exhibits large particle size.^58,60,62 The collapsing of the thermoresponsive grafted chains occurs due to the destruction of the favourable H-bonding interaction with the water (solvent) molecules, followed by self-aggregation of the thiophene moieties in the polythiophene chain.

6. Properties and application

In this section we shall discuss the optical properties, electronic properties, stabilization of nanoparticles and photovoltaic properties of the water soluble polythiophenes. A possible application of these properties would also be delineated here.

6.1. Optical properties and their applications

Fluorescence active conjugated polymer based sensors are very much sensitive in presence of very minor perturbation due to the co-operative response of all the chains present in the materials. Polythiophene is one of the most important electron rich conjugated polymers having fluorescence property and it behaves as a donor molecule. Growing applications of polythiophene in the field of biological and chemical sensors, photovoltaic and other optoelectronic applications using the fluorescence property would be discussed here.

6.1.1. Biological sensor. Water-soluble conjugated polymers offer an elite stage of fabricating chemosensors for the biologically relevant targets. Water solubility is the major requirement for the detection of bio-molecules such as DNA, RNA, bovine serum albumin (BSA), natural polyamine, proteins and folic acid etc. The conformational change of the cationic polythiophene derivative in presence of bio-molecules can be perceived by fluorescence spectroscopy. In literature there are some reports upon conjugated polymer based DNA sensor.^{1,5,10,44,99–102} The cationic PT graft co-polymer exhibits yellow colour (λ_max = 397 nm) in buffer solution because of random coil conformation but after addition of ss-DNA the colour changes to red (λ_max = 527 nm) due to the polymer/ss-DNA complex formation causing an extended chain conformation of polythiophene, yielding the red shift.^1,101 The synthesis of water soluble biotin-functionalized polythiophene was reported by Faid et al. and the polymer can perceive the binding of avidin (biotin binding protein) monitored from UV-vis spectroscopy. It is well known that avidin has strong interaction with biotin and the polymer solution exhibits a sharp colour change (violet to yellow) after addition of avidin.^103,104 Water soluble, electroactive, poly(3-alkoxy-4-methylthiophene)s based colorimetric and fluorometric detection of single-strand oligonucleotides or double-stranded nucleic acid depending upon its conformational change was reported by Ho et al. The conformational change of the polymer is due to the electrostatic interaction with single-stranded oligonucleotides and double-stranded nucleic acids.⁵⁵ A fluorometric detection of DNA molecule based on non covalent interaction of DNA molecule and cationic polythiophene derivative poly(3-[(S)-5-amino-5-carboxyl-3-oxapentyl]-2,5-thiophenylene hydrochloride) (8) was reported by Nilsson et al.⁴ In 2004 Nilsson et al. also reported the conformational change of water soluble, zwitterionic and photoactive polythiophene derivative (8) by noncovalent interaction with intracellular signalling small protein Calmodulin (CaM) which changes the conformation of the polymer upon exposure to calcium.¹⁰⁵ In the same year Ho et al. described the use of water soluble cationic polythiophene as a polymeric stain that exclusively transduced the binding of an aptamer (oligonucleic acid or peptide) to its target yielding a clear optical signal. This method can easily and selectively detect the human thrombin in very low concentration.⁴³ In 2005 Nilssion and Inganas et al. developed a negatively charged polythiophene which exhibits optical changes perceived either visually or by absorption and emission spectra. The optical property of the polyelectrolyte changed depending on the negative state or amyloid fibril conformation of the protein with which it formed the complex.¹⁰⁶ Water soluble cationic polythiophene derivatives also can produce interpolymer complex with biomacromolecules like polypeptides,^107,108 and polysaccharides etc.¹⁰⁹ The supramolecular chiral insulated molecular wire was prepared by the self-assembly between achiral water-soluble cationic polythiophene derivatives with a natural polysaccharide, schizophyllan (SPG) reported by Li et al. (Fig. 4).¹⁰⁹ PT-1 is optically inactive, but upon addition of s-SPG, an intense split-type induced CD (ICD) occurs in the π–π* transition region.¹⁰⁹

Fig. 4 (a) Structure of PT-1 and SPG (b) schematic illustration of the chiral insulated wire formation (reproduced from ref. 109).

In the same year same group also reported water soluble cationic polythiophene derivative with its colorimetric and fluorescent responses to ATP by electrostatic and hydrophobic interactions.¹¹⁰ The absorption maximum of the polythiophene derivative is ∼400 nm in water but upon adding increasing amount of ATP the absorption maximum has shifted to 538 nm due to the aggregation with ATP exhibiting a distinct colour change from yellow to pink-red. The absorption maxima of water soluble poly(3-alkoxy-4-methylthiophene)derivative (PMTPA) have shifted from 405 nm to 455 (ADP), 499 (UTP), 540 (GTP), 416 (AMP), 542 (ATP) and 459 nm (CTP), respectively, and this is due to the conformational change of the polythiophene backbone so it is also useful for detection of specific nucleobases containing phosphate groups (Fig. 5).¹¹¹ Fang et al. reported a water-soluble, ferrocene-functionalized polythiophene for label-free electrochemical detection of DNA using HS-modified peptide nucleic acids (PNA) probes on nanogold modifier electrodes.¹¹² In the polyplexes the release of DNA is difficult due to strong ionic interaction in polycation–DNA systems, at the time of gene delivery. On the other hand polyplex formed by nonionic interactions like H-bonding/π–π interaction can facilitate easy detachment of DNA in the process of gene delivery. In our work we have reported the formation of complexes of double stranded (ds)-DNA and poly(3-thiophene acetic acid) at three different compositions produced by non covalent interactions.¹¹³

Fig. 5 Changes in the color of PMTPA solutions (1.0 × 10⁻⁴ M) in 10 mM HEPES buffer (pH = 7.4) induced by the addition of different X-linked nuclear proteins (XNPs) (4.0 × 10⁻⁴ M) (reproduced from ref. 111).

A simple “Turn-on” technique for the rapid detection of protein molecule by water soluble conjugate polymer PDPMT-Cl (24) was reported by Wang et al. and this process was used effectively for the detection of egg and milk proteins.¹¹⁴ Protamines are a class of polycationic proteins for its basic arginine residues. Yao et al. reported a very nice method for colorimetric detection of polycationic protamines by using anionic polythiophene poly(2-(2-(4-methylthiophen-3-yloxy)ethyl)malonateacid) (PMTEMA) system. Interaction of this anionic polythiophene system with polycationic protamines causes a change in polythiophene backbone conformation from random coil to more planer and ordered phase. This change in conformation causes a change in color of the solution along with quenching of fluorescence intensities which are used as outputs for detection.¹¹⁵ The colorimetric and fluorometric recognition of lysophosphatidic acid (LPA) by the water soluble copolythiophenes (CPT9) based on electrostatic and hydrophobic interaction is reported by Lan et al. The UV absorption peak of CPT9 (400 nm) gradually decreases by the addition of LPA and a new peak appears at around 500 nm emerged with an isosbestic point at 442 nm.¹¹⁶ The same group reported 3-phenylthiophene-based water-soluble copolythiophenes (CPT1) for colorimetric and fluorometric recognition of lipopolysaccharide (LPS) which is the major components of the cell membranes of Gram-negative bacteria and Gram-positive bacteria. The absorption peak of the HEPES buffer solution of the CPT1-C at 420 nm gradually decreases by the addition of LPS with a generation of new peak at 540 nm due to LPS induce conformational change of the conjugated polymer backbone. The colour of the polymer solution also changes from yellow to red.¹¹⁷ Exploiting the fluorescent property and low cytotoxicity of the water soluble polythiophene with anticancer tyrosine kinase inhibitor lapatinib as side chain; membrane imaging of living cells was carried out by Wang et al. in 2013.¹¹⁸

6.1.2. Chemical sensor. Water soluble cationic polythiophene which can easily detect the presence of iodide ion over a broad range of other anions (CO₃²⁻, H₂PO₄⁻, F⁻, Cl⁻, Br⁻, CH₃COO⁻, HCO₃⁻, HPO₄²⁻, SO₄²⁻, EDTA⁴⁻, (C₆H₅)₄B⁻) was reported by Ho et al. in 2003. This detection process depends on electrostatic interaction causing conformational changes of water soluble cationic polythiophene. They observed that the yellow colour of the polymer solution quickly changed to red-violet after addition of NaI but the iodine induced optical property is highly dependent on the side chain length of the polymer (Fig. 6).¹¹⁹ Water soluble cationic polythiophene poly(3-(3′-N,N,N-triethylamino-1′-propyloxy)-4-methyl-2,5-thiophene hydrochloride) (7) and a label-free, thymines rich mercury-specific oligonucleotide (MSO) probe was used for specific Hg²⁺ sensor using UV-vis spectroscopy with a detection limit 2.5 μM.¹²⁰ Wang et al. reported colorimetric and fluorometric based selective detection of anionic surfactants with detection limit 10⁻⁹ M by water soluble cationic polythiophene derivative poly[N,N,N-trimethyl-4-(thiophen-3-ylmethylene)-cyclohexanaminium chloride] (PTCA-Cl), and the absorption maximum of water soluble PTCA-Cl had shifted from 489 nm to 434 nm with increasing concentration of sodium dodecylbenzenesulfonate.¹²¹ Detection of anionic surfactants by water soluble cationic polythiophene derivative poly[3-(6-trimethylammoniumhexyl)thiophene] using fluorescence spectroscopy were reported by Evans et al. In presence of anionic surfactants (sodium octyl sulfate and potassium heptadecafluoro-1-octanesulfonate), absorption and fluorescence spectra of the cationic polythiophene exhibited a dramatic change due to ion complex formation.¹²² Recently same cationic polymer, PMNT (7) based Pb²⁺ sensor has been reported by Lu et al. and the colour of the PMNT–TBAA (5′-GGAAGGTGTGGAAGG-3′) system gradually changed from red to yellow with increasing concentration of Pb²⁺ ions.¹²³ Yao et al. reported a new technique for the visual detection of Cu²⁺ ion by using water soluble anionic polythiophene derivative (sodium poly(2-(4-methyl-3-thienyloxy)propanesulfonate, PMTPS)) observing visual colour change. The absorption maxima of PMTPS-kit solution (kit including AD = 1-azidododecane, TAB = N,N,N-trimethylprop-2-yn-1-ammonium bromide) shifted ∼150 nm by the gradual addition of Cu²⁺ ion with a visual colour change from purple to yellow.¹²⁴

Fig. 6 (A) Photographs of solutions of (a) polymer (P) (110 μL of 1.66 × 10⁻⁴ M on a monomer unit basis), (b) P + NaF, (c) P + NaCl, (d) P + NaBr, (e) P + NaI. (B) Assays of photographs A, 4 days after. The quantity of all salts used here is 5 × 10⁻⁸ mol. (C) UV-vis absorption spectra corresponding to the different assays in photograph A (reproduced from ref. 119).

The fluorescence property of water soluble polythiophene (PTDM, 63 & PTDE, 64) is used for the detection of explosive materials such as different nitro-aromatics (picric acid, 2,4-dinitrophenol, paranitrophenol etc.). These nitro compounds are electron deficient compounds and possess low lying π* orbital. On the other hand, polythiophene is electron rich so exitonic transfer occurs from polymer to nitro-aromatic compounds resulting a quenching of fluorescence intensity.⁶²

The quenching is higher in case of picric acid containing maximum number of electron withdrawing nitro groups (minimum sensing concentration 0.5 ppm PA 2.1 μM). The detection of nitro-aromatic is also possible in solid state; the polymer socked filter paper shows black spot by fluorescence quenching in presence of picric acid presented in Fig. 7.⁶² The colorimetric/fluorometric detection of iodine ion (with detection limit 10⁻⁵ M) by the water soluble polythiophene (PDPMT-Cl, 24) was reported by Li et al. in 2012. The absorbance maxima of the polymer shifts from 520 to 450 nm and the colour of the polymer solution changes from reddish violet to yellow, indicating dissociation of polymer aggregates by iodine ions.⁶⁶ Water soluble cationic polythiophene PDAOT supramolecularly interacted with single-walled carbon nanotubes (SWNT) and produced a stable aqueous dispersion of PDAOT–SWNT complexes. The aqueous dispersion of PDAOT–SWNT can be used for the fabrication of amperometric glucose sensor.⁶⁴

Fig. 7 Fluorescence images of polymer samples (PTDM, 63) absorbed in filter paper upon irradiation with 365 nm light: (a) before and (b) after being partially dipped into an aqueous solution of PA (115 μM), (c) with two drops of toluene solution of PA (115 μM) cast upon the filter paper and (d) with the PTDM solution soaked filter paper in contact with solid PA (reproduced from ref. 62).

6.2. Electronic properties and device applications

Water soluble polythiophene based molecular devices such as molecular logic gate and molecular thermometer have been reported from our laboratory. Molecular logic gate exploits a molecule which gives always one output when one or two inputs are applied. Water soluble pH responsive, temperature sensitive PTDM (63) and PTDE (64) systems behave as good molecular logic gates, using temperature and pH as the two inputs and fluorescence intensity as the output. When both the inputs are “ON” (“1”), output is also in “ON” state (“1”) which is the principle of “AND” logic gate.^46,60,62 In case of PTDM (63) when pH is high (pH-9.2) (“1”) and temperature > 29 °C (“1”) the output fluorescence intensity is also high (“1”) but fluorescence intensity keeps a low level (‘0’) when any of the inputs (input 1 or input 2) is kept low, in terms of binary the input conditions are (1, 0) or (0, 1).

Here the polymer follow the principle of “AND” logic gate. Truth table of “AND” logic gate is presented in Table 2.⁶² In case of PTDE at high pH (pH-7, “1”) and high temperature (>40 °C, “1”) the fluorescence intensity is also high (“1”). But when both or any one of the two input is low, in terms of binary (0, 0), (1, 0) and (0, 1) output is also low which is the principle of “AND” logic gate. At pH-7, the fluorescence intensity increases sharply with increasing temperature over ∼40 °C. This is due to the competitive solubilization of insoluble polythiophene core by partly protonated PDEAEMA chain and precipitation of PMeO₂MA chain above its LCST in aqueous solution.⁶⁰

Table 2 Truth table for “AND” logic gate with temperature and pH as inputs and fluorescence intensity as output (reproduced from ref. 62)

Entry	Input1 (temperature/°C)	Input2 (pH)	Output (PL intensity × 10^5)
I	0 (22)	0 (4 or 7)	0 (9.6 or 8.4)
II	1 (29)	0 (4 or 7)	0 (9.3 or 7.9)
III	0 (22)	1 (9.2)	0 (7.7)
IV	1 (29)	1 (9.2)	1 (38.3)

---

6.3. Stabilization of nanoparticles

PD, (60) was also used for the in situ formation of Au nanoparticle (Au NP) by reducing the Au³⁺ ion and the morphology of the Au nano particle changes by the addition of different amounts of RNA concentration. However, the polymer–Au nanocomposite exhibited interesting negative differential resistance (NDR) property in the current–voltage (I–V) curve (Fig. 8)¹²⁵ because, during the preparation of Au NPs a large quantity of charge became adsorbed on Au NPs. This accumulated charge restricts the flow of current, causing a decrease of current flow even with increase of applied voltage and at a higher applied voltage it releases the charges completely, giving very low current. The decrease of current with increase of voltage is contrary to Ohm's law and it is usually known as negative differential resistance (NDR).

Fig. 8 Current–voltage (I–V) characteristic curves of PAu (reproduced from ref. 125).

Water soluble polythiophene–Au nano-composite was used as a pH sensor in aqueous medium using the fluorescence property of polythiophene and here the polymerization of thiophene and formation of Au nanoparticle from HAuCl₄ occurred concomitantly in aqueous medium.¹²⁶ The composite was stable in water and in the pH range of 3.0 to 6.0 the distinguish emissions of the composite changes, providing a solution indicator of pH based on optical properties.¹²⁶

6.4. Photovoltaic application

Recently graphene quantum dots (GQDs) and water soluble polythiophene (PTDM, 63) based PG composite have been used for the fabrication of dye-sensitized solar cells (DSSCs). Here polythiophene acts as a donor and GQDs act as an acceptor. The DSSCs made-up with an indium-tin oxide (ITO)/PG/graphite device using the N719 dye shows a short-circuit current (J_sc) 4.36 mA cm⁻², open-circuit voltage (V_oc) 0.78 V, fill factor 0.52 and a power conversion efficiency (PCE, η) = 1.76% (Fig. 9).¹²⁷

Fig. 9 J–V curve of PG (reproduced from ref. 127).

Water soluble polythiophene derivative HT-poly[3-(6-N,N,N-trimethylammonium)-hexyl thiophene] (P3HTN) was used for bulk heterojunction solar cell with phenyl-C61 butyric acid methyl ester (P3HT–PCBM) which exhibited high stability in air. The PCE of devices improved from 1.8% to 3.3% on annealing.¹²⁸ Sun et al. reported photo-activity of the chemically converted graphene (CCG)/water soluble polythiophene derivative P3TOPS (5) and P3TOPA (6) composite film prepared by layer by layer deposition method.¹²⁹ Due to photoinduced electron transfer from the polythiophenes to CCG it exhibited enhanced photoresponse.

7. Outlook & future scope

Amplification of output signal intensity due to the co-operative response of the polymer chains makes the conjugate polymer systems more attractive in sensor application compared to the small molecules. However, the main bottleneck in their application is insolubility due to rigidity in the backbone structure. Polythiophenes with suitable derivatization in repeating units or grafting of suitable polymer systems may solve this problem to an appreciable extent. Polythiophene is a fluorescent polymer and this property of polythiophene is largely exploited during the development of polythiophene based sensors for biological analytes. These systems promise their applications as sensors for biological molecules, fabrication of molecular logic gates, molecular thermometer etc. with an appreciable degree of precision. Efficiency of these systems may be further improved by introducing even more sensitive functionalities as substituent or polymeric grafts. The optoelectronic property of polythiophenes may be tuned by changing the density of functional groups or by changing the graft density, graft molecular weight etc. This requires tailor made synthesis of the polythiophene based polymeric systems through the development of newer reaction conditions or modification of the existing reactions to develop suitable strategies for synthesis. The modification by organic moieties on polythiophene backbone exhibits significant amount of change in properties for change of external stimuli like pH, temperature, light, ion etc. and it would be quiet useful for developing the optoelectronic devices. Alteration in their morphologies induces a change in conformations of the polythiophene backbone which in effect results in change of optical or electrical response of polythiophene. Therefore, preparation of novel polythiophene based liquid crystalline materials coupled with the electrical or optoelectronic properties of polythiophene systems may open up novel application possibilities. Most importantly, the preparation of polythiophene based bio-conjugates with different bio-molecules (proteins, nucleic acids etc.) on stimuli responsive polythiophenes covalently attached with bio-molecules may find potential applications in biological assay and in the study of detailed structure of bio-molecules making use of solvatochromic or thermo-chromic properties of the polythiophene chains.

Acknowledgements

We gratefully acknowledge DST New Delhi (grant no. SB/SI/OC-11/2013) for financial support. S. Das and R. Ghosh acknowledges DST “INSPIRE” program and CSIR, respectively for providing the fellowship.

References

1. B. Liu and G. C. Bazan, Chem. Mater., 2004, 16, 4467–4476.
2. J. J. Grodzinski, Polym. Adv. Technol., 2002, 13, 615–625.
3. R. D. McCullough, in Handbook of Oligo- and Polythiophenes, 1999, pp. 1–44.
4. K. P. R. Nilsson and O. Inganas, Nat. Mater., 2003, 2, 419–424.
5. L. Chen, D. W. McBranch, H. L. Wang, R. Helgeson, F. Wudl and D. G. Whitten, Proc. Natl. Acad. Sci. U. S. A., 1999, 96, 12287–12292.
6. W. J. Feast, J. Tsibouklis, K. L. Pouwer, L. Groenendaal and E. W. Meijer, Polymer, 1996, 37, 5017–5047.
7. R. D. McCullough, Adv. Mater., 1998, 10, 93–116.
8. M. Leclerc and K. Faïd, Adv. Mater., 1997, 9, 1087–1094.
9. S. Das, D. P. Chatterjee and A. K. Nandi, J. Mater. Chem. A, 2014, 2, 12031–12042.
10. D. T. McQuade, A. E. Pullen and T. M. Swager, Chem. Rev., 2000, 100, 2537–2574.
11. M. D. Disney, J. Zheng, T. M. Swager and P. H. Seeberger, J. Am. Chem. Soc., 2004, 126, 13343–13346.
12. C. H. Fan, K. W. Plaxco and A. J. Heeger, J. Am. Chem. Soc., 2002, 124, 5642–5643.
13. Q. Zhou and T. M. Swager, J. Am. Chem. Soc., 1995, 117, 7017–7018.
14. J. S. Yang and T. M. Swager, J. Am. Chem. Soc., 1998, 120, 11864–11873.
15. M. J. Marsella and T. M. Swager, J. Am. Chem. Soc., 1993, 115, 12214–12215.
16. G. Rimmel and P. Bauerle, Synth. Met., 1999, 102, 1323–1324.
17. B. Wang and M. R. Wasielewski, J. Am. Chem. Soc., 1997, 119, 12–21.
18. J. Liu, E. N. Kadnikova, Y. Liu, M. D. McGehee and J. M. J. Fréchet, J. Am. Chem. Soc., 2004, 126, 9486–9487.
19. G. D. Scholes and K. P. Ghiggino, J. Chem. Phys., 1994, 101, 1251.
20. J. E. Guillet, Polymer Photophysics and Photochemistry, Cambridge University Press, Cambridge, 1985.
21. F. D. Jochum, F. R. Forst and P. Theato, Macromol. Rapid Commun., 2010, 31, 1456–1461.
22. P. S. Heeger and A. J. Heeger, Proc. Natl. Acad. Sci. U. S. A., 1999, 96, 12219–12221.
23. L. Chen, D. W. McBranch, H. L. Wang, R. Hegelson, F. Wudl and D. C. Whitten, Proc. Natl. Acad. Sci. U. S. A., 1999, 96, 12287–12292.
24. M. Wang, S. Zou, G. Guerin, L. Shen, K. Deng, M. Jones, G. C. Walker, G. D. Scholes and M. A. Winnik, Macromolecules, 2008, 41, 6993–7002.
25. F. Garnier, Angew. Chem., Int. Ed. Engl., 1989, 28, 513–517.
26. M. R. Pinto and K. S. Schanze, Synth.Stuttgart, 2002, 9, 1293–1309.
27. B. S. Ong, Y. Wu, P. Liu and S. Gardner, J. Am. Chem. Soc., 2004, 126, 3378–3379.
28. I. McCulloch, M. Heeney, C. Bailey, K. Genevicius, I. MacDonald, M. Shkunov, D. Sparrowe, S. Tierney, R. Wagner, W. Zhang, M. L. Chabinyc, R. J. Kline, M. D. McGehee and M. F. Toney, Nat. Mater., 2006, 5, 328–333.
29. I. Osaka and R. D. Mccullough, Acc. Chem. Res., 2008, 41, 1202–1214.
30. B. Xu and S. Holdcroft, Macromolecules, 1993, 26, 4457–4460.
31. S. D. D. V. Rughooputh, S. Hotta, A. J. Heeger and F. Wudl, J. Polym. Sci., Part B: Polym. Phys., 1987, 25, 1071–1078.
32. J. Kowalik and L. M. Tolbert, Chem. Commun., 2000, 877–878.
33. K. Faied, M. Frechette, M. Ranger, L. Mazerolle, I. Levesque, M. Leclerc, T.-A. Chen and R. D. Rieke, Chem. Mater., 1995, 7, 1390–1396.
34. F. Brustolin, F. Goldoni, E. W. Meijer and N. A. J. M. Sommerdijk, Macromolecules, 2002, 35, 1054–1059.
35. J. J. Apperloo, R. A. J. Janssen, P. R. L. Malenfant and J. M. J. Frcéhet, Macromolecules, 2000, 33, 7038–7043.
36. K. Kaneto, W. Y. Lim, W. Takashima, T. Endo and M. Rikukawa, Jpn. J. Appl. Phys., 2000, 39, L872–L874.
37. H. Sirringhaus, P. J. Brown, R. H. Friend, M. M. Nielsen, K. Bechgaard, B. M. W. Langeveld-Voss, A. J. H. Spiering, R. A. J. Janssen, E. W. Meijer, P. Herwig and D. M. de Leeuw, Nature, 1999, 401, 685–688.
38. M. R. Andersson, O. Inganas, G. Gustafsson, J. C. Gustafsson-Carlberg, D. Selse, T. Hjertberg and O. Wennerstrom, Macromolecules, 1995, 28, 7525–7529.
39. P. Barta, F. Cacialli, R. H. Friend and M. Zagórska, J. Appl. Phys., 1998, 84, 6279–6284.
40. S. H. Ahn, M. Z. Czae, E. R. Kim, H. Lee, S. H. Han, J. Noh and M. Hara, Macromolecules, 2001, 34, 2522–2527.
41. S. Cheylan, H. G. Bolink, A. Fraleoni-Morgera, J. Puigdollers, C. Voz, I. Mencarelli, L. Setti, R. Alcubilla and G. Badenes, Org. Electron., 2007, 8, 641–647.
42. Y. Kim, S. Cook, S. M. Tuladhar, S. A. Choulis, J. Nelson, J. R. Durrant, D. D. C. Bradley, M. Giles, I. Mcculloch, C. Sikha and M. Ree, Nat. Mater., 2006, 5, 197–203.
43. H. A. Ho and M. Leclerc, J. Am. Chem. Soc., 2004, 126, 1384–1387.
44. H. A. Ho, M. Boissinot, M. G. Bergeron, G. Corbeil, K. Dore, D. Boudreau and M. Leclerc, Angew. Chem., 2002, 114, 1618–1621.
45. S. Uchiyama, N. Kawai, A. P. de Silva and K. Iwai, J. Am. Chem. Soc., 2004, 126, 3032–3033.
46. S. Samanta, S. Das, R. K. Layek, D. P. Chatterjee and A. K. Nandi, Soft Matter, 2012, 8, 6066–6072.
47. C. Wei, J. Guo and C. Wang, Macromol. Rapid Commun., 2011, 32, 451–455.
48. S. Uchiyama, Y. Matsumura, A. P. de Silva and K. Iwai, Anal. Chem., 2003, 75, 5926–5935.
49. K. S. Schanze and X. Zhao, Hand book of conducting polymers, ed. T. A. Skotheim and J. Reynolds, 2007, third edn, pp. 1–14.
50. Handbook of Thiophene-Based Materials: Applications in Organic Electronics and Photonics, ed. L. F. Perepichka, D. F. Perepichka, H. A. Ho and M. Leclerc, 2009, ch. 22, pp. 813–831.
51. A. O. Patil, Y. I. N. Basescu, N. Colaneri, J. Chen, F. Wudl and A. J. Heeger, Synth. Met., 1987, 20, 151–159.
52. M. Chayer, K. Faid and M. Leclerc, Chem. Mater., 1997, 9, 2902–2905.
53. B. S. Kim, L. Chen, J. Gong and Y. Osada, Macromolecules, 1999, 32, 3964–3969.
54. J. Lukkari, M. Salomalki, A. Viinikanoja, T. Ääritalo, J. Paukkunen, N. Kocharova and J. Kankare, J. Am. Chem. Soc., 2001, 123, 6083–6091.
55. H. A. Ho, M. Boissinot, M. G. Bergeron, G. Corbeil, K. Dore, D. Boudreau and M. Leclerc, Angew. Chem., Int. Ed., 2002, 41, 1548–1551.
56. K. Dore, S. Dubus, H.-A. Ho, I. Levesque, M. Brunette, G. Corbeil, M. Boissinot, G. Boivin, M. G. Bergeron, D. Boudreau and M. Leclerc, J. Am. Chem. Soc., 2004, 126, 4240–4244.
57. A. C. Carreon, W. L. Santos, J. B. Matson and R. C. So, Polym. Chem., 2014, 5, 314–317.
58. S. S. Balamurugan, G. B. Bantchev, Y. Yang and R. L. McCarley, Angew. Chem., Int. Ed., 2005, 44, 4872–4876.
59. S. Das, S. Samanta, D. P. Chatterjee and A. K. Nandi, J. Polym. Sci., Part A: Polym. Chem., 2013, 51, 1417–1427.
60. S. Das, D. P. Chatterjee and A. K. Nandi, Polym. Int., 2014, 63, 2091–2097.
61. J. R. Matthews, F. Goldoni, A. P. H. J. Schenning and E. W. Meijer, Chem. Commun., 2005, 5503–5505.
62. S. Das, D. P. Chatterjee, S. Samanta and A. K. Nandi, RSC Adv., 2013, 3, 17540–17550.
63. K. Faid, R. Cloutier and M. Leclerc, Macromolecules, 1993, 26, 2501–2507.
64. X. Pang, P. Imin, I. Zhitomirsky and A. Adronov, Macromolecules, 2010, 43, 10376–10381.
65. D. A. Rider, B. J. Worfolk, K. D. Harris, A. Lalany, K. Shahbazi, M. D. Fleischauer, M. J. Brett and J. M. Buriak, Adv. Funct. Mater., 2010, 20, 2404–2415.
66. E. Li, L. Lin, L. Wang, M. Pei, J. Xu and G. Zhang, Macromol. Chem. Phys., 2012, 213, 887–892.
67. A. O. Patil, Y. Ikenoue, F. Wudl and A. J. Heeger, J. Am. Chem. Soc., 1987, 109, 1858–1859.
68. R. D. McCullough, P. C. Ewbank and R. S. Loewe, J. Am. Chem. Soc., 1997, 119, 633–634.
69. T. V. Richter, C. Buhler and S. Ludwigs, J. Am. Chem. Soc., 2012, 134, 43–46.
70. M. Liu, B. Li and X. Cui, Talanta, 2013, 115, 837–841.
71. C. K. Hartmuth, M. G. Finn and K. B. Sharpless, Angew. Chem., Int. Ed., 2001, 40, 2004–2021.
72. J. F. Lutz, Angew. Chem., Int. Ed., 2007, 46, 1018–1025.
73. W. H. Binder and R. Sachsenhofer, Macromol. Rapid Commun., 2007, 28, 15–54.
74. M. Ouchi, T. Terashima and M. Sawamoto, Chem. Rev., 2009, 109, 4963–5050.
75. Z. Li, R. J. Ono, Z. Q. Wu and C. W. Bielawski, Chem. Commun., 2011, 47, 197–199.
76. Z.-Q. Wu, R. J. Ono, Z. Chen, Z. Lia and C. W. Bielawski, Polym. Chem., 2011, 2, 300–302.
77. M. G. Mohamed, C. C. Cheng, Y. C. Lin, C. W. Huang, F. H. Lu, F. C. Chang and S. W. Kuo, RSC Adv., 2014, 4, 21830–21839.
78. K. Matyjaszewski and J. Xia, Chem. Rev., 2001, 101, 2921–2990.
79. W. A. Braunecker and K. Matyjaszewski, Prog. Polym. Sci., 2007, 32, 93–146.
80. J. S. Wang and K. Matyjaszewski, J. Am. Chem. Soc., 1995, 117, 5614–5615.
81. F. D. Lena and K. Matyjaszewskia, Prog. Polym. Sci., 2010, 35, 959–1021.
82. J. Liu, E. Sheina, T. Kowalewski and R. D. McCullough, Angew. Chem., Int. Ed., 2002, 41, 329–332.
83. M. C. Iovu, M. Jeffries-EL, E. E. Sheina, J. R. Cooper and R. D. McCullough, Polymer, 2005, 46, 8582–8586.
84. C. A. Dai, W. C. Yen, Y. H. Lee, C. C. Ho and W. F. Su, J. Am. Chem. Soc., 2007, 129, 11036–11038.
85. G. A. Mussie, G. Srinivas, S. John and C. S. Mihaela, Macromol. Chem. Phys., 2009, 210, 2007–2014.
86. A. Emge and P. Bäuerle, Synth. Met., 1999, 102, 1370–1373.
87. P. Bäuerle and A. Emge, Adv. Mater., 1998, 3, 324–330.
88. P. Bäuerle and St. Scheib, Acta Polym., 1995, 46, 124–129.
89. R. M. Souto Maior, K. Hinkelmann, H. Eckert and F. Wud, Macromolecules, 1990, 23, 1268–1279.
90. G. W. Heffner and D. S. Pearson, Macromolecules, 1991, 24, 6295–6299.
91. T. Norisuye, M. Shibayama and S. Nomura, Polymer, 1998, 39, 2769–2775.
92. S. Zhou, S. Fan, S. C. F. Auyeung and C. Wu, Polymer, 1995, 36, 1341–1346.
93. K. Kubota, S. Fujishige and I. Ando, Polym. J., 1990, 22, 15–20.
94. X. Y. Wu, R. H. Pelton, K. C. Tam, D. R. Woods and A. E. Hamielec, J. Polym. Sci., Part A: Polym. Chem., 1993, 31, 957–962.
95. E. I. Tiktopulo, V. N. Uversky, V. B. Lushchik, S. I. Klenin, V. E. Bychkova and O. B. Ptitsyn, Macromolecules, 1995, 28, 7519–7524.
96. T. Terada, T. Inaba, H. Kitano, Y. Maeda and N. Tsukida, Macromol. Chem. Phys., 1994, 195, 3261–3270.
97. F. M. Winnik, A. R. Davidson, G. K. Hamer and H. Kitano, Macromolecules, 1992, 25, 1876–1880.
98. F. Ganachaud, M. J. Monteiro, R. G. Gilbert, M. A. Dourges, S. H. Thang and E. Rizzardo, Macromolecules, 2000, 33, 6738–6745.
99. H. A. Ho, M. B. Aberem and M. Leclerc, Chem.–Eur. J., 2005, 11, 1718–1724.
100. H. A. Ho, A. Najari and M. Leclerc, Acc. Chem. Res., 2008, 41, 168–178.
101. M. Leclerc, Adv. Mater., 1999, 11, 1491–1498.
102. P. C. Ewbank, G. Nuding, H. Suenaga, R. D. McCullough and S. Shinkai, Tetrahedron Lett., 2001, 42, 155–157.
103. K. Faid and M. Leclerc, Chem. Commun., 1996, 2761–2762.
104. K. Faid and M. Leclerc, J. Am. Chem. Soc., 1998, 120, 5274–5278.
105. K. P. R. Nilsson and O. Inganas, Macromolecules, 2004, 37, 9109–9113.
106. K. P. R. Nilsson, A. Herland, P. Hammarstrom and O. Inganas, Biochemistry, 2005, 44, 3718–3724.
107. K. P. R. Nilsson, J. Rydberg, L. Baltzer and O. Inganas, Proc. Natl. Acad. Sci. U. S. A., 2004, 101, 11197–11202.
108. A. Herland, K. P. R. Nilsson, J. D. M. Olsson, P. Hammarstrom, P. Konradsson and O. Inganas, J. Am. Chem. Soc., 2005, 127, 2317–2323.
109. C. Li, M. Numata, A. H. Bae, K. Sakurai and S. Shinkai, J. Am. Chem. Soc., 2005, 127, 4548–4549.
110. C. Li, M. Numata, M. Takeuchi and S. Shinkai, Angew. Chem., 2005, 117, 6529–6532.
111. Z. Yao, X. Feng, W. Hong, C. Li and G. Shi, Chem. Commun., 2009, 4696–4698.
112. B. Fang, S. Jiao, M. Li, Y. Qua and X. Jiang, Biosens. Bioelectron., 2008, 23, 1175–1179.
113. P. Mukherjee, A. Dawn and A. K. Nandi, Langmuir, 2010, 26, 11025–11034.
114. L. Wang, G. Zhang, M. Pei, L. Hu, E. Li and H. Li, J. Appl. Polym. Sci., 2013, 130, 939–943.
115. Z. Yao, W. Ma, Y. Yang, X. Chen, L. Zhang, C. Lin and H. C. Wu, Org. Biomol. Chem., 2013, 11, 6466–6469.
116. M. Lan, W. Liu, Y. Wang, J. Ge, J. Wu, H. Zhang, J. Chen, W. Zhang and P. Wang, ACS Appl. Mater. Interfaces, 2013, 5, 2283–2288.
117. M. Lan, J. Wu, W. Liu, W. Zhang, J. Ge, H. Zhang, J. Sun, W. Zhao and P. Wang, J. Am. Chem. Soc., 2012, 134, 6685–6694.
118. B. Wang, C. Zhu, L. Liu, F. Lv, Q. Yang and S. Wang, Polym. Chem., 2013, 4, 5212–5215.
119. H. A. Ho and M. Leclerc, J. Am. Chem. Soc., 2003, 125, 4412–4413.
120. X. Liu, Y. Tang, L. Wang, J. Zhang, S. Song, C. Fan and S. Wang, Adv. Mater., 2007, 19, 1471–1474.
121. L. Wang, Q. Feng, X. Wang, M. Pei and G. Zhang, New J. Chem., 2012, 36, 1897–1901.
122. R. C. Evans, M. Knaapila, N. W. Fox, M. Kraft, A. Terry, H. D. Burrows and U. Scherf, Langmuir, 2012, 28, 12348–12356.
123. Y. Lu, X. Li, G. Wang and W. Tang, Biosens. Bioelectron., 2013, 39, 231–235.
124. Z. Yao, Y. Yang, X. Chen, X. Hu, L. Zhang, L. Liu, Y. Zhao and H. C. Wu, Anal. Chem., 2013, 85, 5650–5653.
125. P. Routh, S. Das and A. K. Nandi, RSC Adv., 2012, 2, 11295–11305.
126. B. R. Panda and A. Chattopadhyay, J. Colloid Interface Sci., 2007, 316, 962–967.
127. P. Routh, S. Das, A. Shit, P. Bairi, P. Das and A. K. Nandi, ACS Appl. Mater. Interfaces, 2013, 5, 12672–12680.
128. K. Yao, L. Chen, Y. Chen, F. Li and P. Wang, J. Mater. Chem., 2011, 21, 13780–13784.
129. J. Sun, L. Xiao, D. Meng, J. Geng and Y. Huang, Chem. Commun., 2013, 49, 5538–5540.

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Footnote

† Present Address: Department of chemistry, Presidency University, Kolkata-700 073, India.

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