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A Co-assembled Gel of Pyromellitic Dianhydride Derivative and Polyaniline with Optoelectronic and Photovoltaic Properties Partha Bairi, Priyadarshi Chakraborty, Arnab Shit, Sanjoy Mondal, Bappaditya Roy, and Arun K. Nandi Langmuir, Just Accepted Manuscript • Publication Date (Web): 09 Jun 2014 Downloaded from http://pubs.acs.org on June 18, 2014

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A Co-assembled Gel of Pyromellitic Dianhydride Derivative and Polyaniline with Optoelectronic and Photovoltaic Properties Partha Bairi, Priyadarshi Chakraborty, Arnab Shit, Sanjoy Mondal, Bappaditya Roy and Arun K. Nandi* Polymer Science Unit, Indian Association for the cultivation of Science Jadavpur, Kolkata-700 032, INDIA Abstract:

5,5'-(1,3,5,7-tetraoxopyrrolo[3,4-f]isoindole-2,6-diyl)diisophthalic

acid

(PMDIG), is used to produce supramolecular hydrogel via acid-base treatment. The FESEM and AFM micrographs exhibit fibrillar network structure from intermolecular supramolecular interaction, supported from FTIR and UV-vis spectra. The fluorescence intensity of the PMDIG gel is 16 times higher than that of sodium salt of PMDIG with a 42 nm red shift of the emission peak. On adding anilinium chloride solution to PMDIG gel, it transforms into the sol and on spreading solid ammonium persulphate over it a stable hydrogel is produced. The co-assembled PMDIG-PANI gel exhibits fibrillar network morphology and the co-assembly is formed by the supramolecular interaction between the polyaniline (donor) and the PMDIG (acceptor) molecules as evident from FTIR spectra and WAXS results. UV-Vis spectrum of the PMDIG–PANI hydrogel exhibits the characteristic peaks of polaron band transitions of the doped PANI. The PMDIG PANI co-assembled hydrogel has 51 times higher storage modulus; 52 times higher elasticity; 1.4 times increase of stiffness and 5 times increase of fragility than those of PMDIG hydrogel. The PMDIG-PANI xerogel exhibits four orders increase *for correspondence, email: [email protected]

1 ACS Paragon Plus Environment

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of dc-conductivity than that of PMDIG and the I-V characteristics curve exhibits rectification property under white light illumination showing photo-current rectification, a new phenomenon reported here for the supramolecular gel systems. A dye-sensitized solar cell fabricated with an ITO/ PMDIG-PANI /graphite device shows power conversion efficiency (η) of 0.1 %. A discussion on the mechanism of gel formation and sol state of PMDIG-aniline system is included considering the contact angle values of the xerogels. Introduction Low molecular weight gelator (LMWG) is now a widespread term referring to small molecular organic molecules which can gelatinize water or organic solvents at very low concentrations1-4. They have the ability to create an entangled 3D network with high surface area, via self-assembly that entraps the solvent producing the gel of viscoelastic nature; exhibiting a highly complex and hierarchical structure5. The gel properties can be tuned by various external stimuli such as pH, temperature, composition, mechanical force, light etc6-9. In recent times, curiosity has grown in developing soft materials with significant improvement in the material properties, such that they can be useful in some basic applications in diverse fields. So by construction of such an interconnecting 3D micro- or nano-fibrous hybrid network with the requisite organization, new functional materials with desired functionalities can be produced10. These types of hybrid selfassembled soft materials can be prepared by adding polymers, surfactants, additives and nanoparticles to the gelator molecules which promote much enhanced physical and mechanical properties11-15. Conductive hybrid gels have a good scope as a useful


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functional material owing to their applications in super capacitors, fuel cells, rechargeable batteries, solar cells etc16-19. There are very few reports on conducting polymer hydrogels (CPHs). Zhang and co-workers synthesised the hydrogel of

4-(2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl)-

methoxybutane-1-sulfonate by polymerizing the aqueous solution of the monomer with ammonium persulphate (APS), FeCl3 or a mixture of both20. The mixture of FeCl3 and APS exhibits a striking synergistic effect during the synthesis showing much faster gelation rate than those of the components. The gels exhibit a very high conductivity (1100 S m-1) depending on the monomer concentration and catalyst of polymerization. Very recently, Wang and his co-workers showed from rheological study that in a mixture of polyaniline and cellulose solution with a cellulose concentration > 3 wt %, gelation occurs and the gelation temperature of the cellulose solution drops from 60.3 to 30.5 °C with increasing the cellulose concentration21. A strong intermolecular H-bonding between PANI and cellulose has been attributed to the gel formation of the conducting polymer. In a recent report of a conducting polymer hydrogel of polyaniline and phytic acid, the later acts as both the gelator and dopant. The PANI hydrogel with phytic acid exhibits high conductivity (0.11 S·cm−1) and excellent processability through ink-jet printing or spray coating22. Due to its three-dimensional porous nanostructure with high surface area it can be used as supercapacitor electrodes with high capacitance, very good rate capability and cycling stability. This PANI hydrogel can also be used as sensors for glucose oxidase with a fast response time (~ 0.3 s) and superior sensitivity (~16.7 µA·mM−1). Another report of redox-active in-situ hydrogel of hydroquinone (HQ) in presence of chitosan as a template by oxidative polymerization, the resultant polyhydroquinone PHQ forms


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hydrogen bonds with chitosan causing physical gel formation23. From the above reports it is apparent that CPHs exhibit very important material properties, so it is necessary to develop more CPHs as these gels can be used in different optoelectronic, biological, sensing and photovoltaic applications. Due to the progress of civilization energy demand is increasing more and more, so there is a need to search for new renewable and sustainable energy sources. In this context solar energy is the most convenient alternative energy resource and several methods and techniques are devolved for harvesting efficiently the solar energy24-25. Different type of materials are used for the fabrication of solar cell including organic dye, conducting polymers, semiconducting metal nanoparticles, nanodots, nanocarbon materials etc26-31. Molecular gels with electronic properties have also drawn recent attention as it has the ability to transfer information at the molecular level into information at the nanoscopic and macroscopic levels in a stunning way. This is because the π-conjugated gelator molecules have tuneable optoelectronic properties in the sol and gel phase promoting applications in organic field-effect transistors (OFETs) and organic solar cells (OSCs) etc32-37. Wicklein et al have first time prepared a hybrid of n-type semiconducting perylene bisimide organogelator with a p-type conducting polymer poly{N,N=-bis(4-methoxyphenyl)-N-phenyl-N=-4-vinylphenyl-[1,1=-biphenyl]-4,4=diamin}32 and this donor-acceptor interface is suitable for the charge separation and charge transport facilitating the fabrication of a bulk heterojunction solar cell with a power conversion efficiency = 0.041%. Here we have chosen 5,5'-(1,3,5,7tetraoxopyrrolo[3,4-f]isoindole-2,6-diyl)diisophthalic acid (PMDIG) (Scheme-1), a well known electron acceptor molecule38 forming supramolecular gel in water on successive


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base - acid treatment. The supramolecular gel breaks on addition of aniline, but on aging with the ammonium persulphate at the top surface of it for 24 h, a co-assembled supramolecular hydrogel is produced containing the acceptor PMDIG and donor, polyaniline. The mechanical properties of the co-assembled gel have increased dramatically; exhibits excellent optoelectronic, conductivity, photo-rectification and photovoltaic properties, embodied here. A short discussion on the mechanism of gel formation and on the sol state of PMDIG-aniline system in spite of its fibrillar network morphology is also included. O

HOOC

O

N

HOOC

COOH

N

O

COOH

O

PMDIG

NH

NH

n

N

N

m

x

Polyaniline (PANI, EB)

Scheme 1 Molecular structures of the gelator PMDIG and EB form polyaniline

Experimental: Preparation of PMDIG and PMDIG-PANI gels: 10 mg PMDIG, 0.5 ml sodium bicarbonate (NaHCO3, ~15mg/ml), and 1.5 mL water were taken in glass tubes where the final gelator (PMDIG) concentration in the solutions was kept at 0.5% (w/v). The mixture was sealed, sonicated, and was heated to 60°C to make a homogeneous solution. Few drops of dilute HCl were then added into the hot solutions to make the pH neutral (tested by pH paper) which on cooling to room temperature (30 °C) yielded a light yellow gel.


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A stock of anilinium chloride solution was prepared by dissolving 0.4 ml aniline in 20 ml 0.2 N HCl. PMDIG (10 mg) was first dissolved in 0.5 ml NaHCO3 (~15 mg/ml) solution by mild heating and then to this solution 0.5 ml of the above anilinium chloride solution was added. Formation of a yellow colour sol was observed immediately. Then 57 mg (0.25 mmol) powder ammonium persulphate (APS) was spread over the sol and was kept undisturbed at 300C for 24 hours to accomplish the polymerization of aniline. As the polymerization proceeds the respective sol converted into a stable gel with a prominent colour change from yellow to deep green producing the co-assembled PMDIG-PANI gels. The PMDIG-PANI gel was washed repeatedly with water and it was stable for months. The details of experimental procedure and the characterization techniques are presented in supplementary information. Result and Discussion PMDIG hydrogel: In Scheme 1, the structures of the gelator PMDIG and polyaniline (PANI) in emeraldine base form (EB) are presented. Polyaniline in the EB form consists of a mixture of benzonoid and quinonoid structures but in the doped state the radical cations, playing the role of charge carrier along the chain, are produced. PMDIG has four terminal carboxylic acid groups and due to the strong intermolecular H-bonding interaction in the solid state46 the solubility of PMDIG is very poor in water. So it cannot form a hydrogel by the usual heating-cooling technique. To alleviate the problem of solubility in water we have dissolved it by adding a weak base, sodium bicarbonate which converts –COOH groups to -COONa and due to the ionic character of the later solubility of Na-PMDIG in water increases. To this colourless aqueous solution dilute HCl is added and instantly the colourless solution transforms into a light yellow


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semisolid hydrogel at a neutral pH and at 30 °C temperature. The appearance of a light yellow colour in the gel state may be attributed to the π-stacking of PMDIG molecules, producing a lower band gap material than that in the solution state of its sodium salt. The minimum gelation concentration of the hydrogel is 0.3 % (w/v) and the gel melting temperature of the PMDIG hydrogel (0.5 % (w/v)) measured by ‘‘falling steel ball method’’ is 73 0C which is nearly equal to that obtained from DSC study (75 0C; Figure S1).The cooling curve of Fig. S1 also indicates a peak temperature at 58 0C indicating thermoreversible nature of the PMDIG gel. The field emission scanning electron micrograph (FESEM) of PMDIG xerogel (0.5 % (w/v) produced at 30 °C (Figure 1a) exhibits three dimensional fibrillar network structure with a partial tape like morphology. The AFM image in figure 1b confirms the formation of a segmented morphology which may originate from the stepwise overlapping of different self-assembled layers of PMDIG formed due to H-bonding. This result indicates that PMDIG molecules form hydrogel by self organization of PMDIG molecules through intermolecular supramolecular interaction39. a

b

Figure 1(a) SEM images of xerogel of PMDIG hydrogel and (b) AFM image of xerogel of PMDIG hydrogel at 0.5 % (w/v).


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The supramolecular interaction in the gel state can be ascertained from the FTIR spectra (Figure 2) where the FTIR spectra of PMDIG powder, Na-PMDIG and PMDIG xerogel are compared. The carbonyl stretching frequency of carboxylic acid and amide group of PMDIG powder appears at 1724 and 1780 cm-1, respectively. In Na-PMDIG the >C=O vibration peak of -COOH group at ~1724 cm-1 is absent indicating complete transformation of the –COOH group into –COO- ions. In the PMDIG xerogel the >C=O stretching frequency of carboxylic acid has reappeared and is shifted to higher frequency at 1730 cm-1 but that of the amide carbonyl of PMDIG remains unchanged.

This

indicates that >C=O group of amide carbonyl of PMDIG does not take part in the selfassembly processes but the >C=O groups of –COOH groups of PMDIG actively participate in the supramolecular organization causing gelation. The -O-H stretching frequency of carboxylic acid of PMDIG powder (3450 cm-1) shifts to lower frequency (3434 cm-1) for the PMDIG xerogel indicating strong intermolecular hydrogen bond formation with the >C=O groups of –COOH groups during gelation. The increase of the >C=O stretching frequency of the carboxylic acid group may be attributed to the Hbonding interaction between -OH group and >C=O groups of PMDIG hydrogel. Perhaps, there exists an intermolecular H-bonding interaction of stronger magnitude in the powder PMDIG (as evidenced from the sodium bicarbonate treatment required for making it a homogeneous aqueous solution) than that in the PMDIG gel causing a shift of the >C=O peak of PMDIG xerogel to higher frequency40. To investigate any difference in supramolecular packing in the powder and gel state of PMDIG the wide angle X-ray scattering (WAXS) pattern (Figure S2) of the powder and xerogel are compared. Diffraction pattern of the PMDIG xerogels is


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completely different from that of PMDIG powder, indicating that the structures of the gels are different from that of the powder PMDIG. The q value of 9.02 corresponds to

3450 PMDIG powder

% Transmittance

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1780 1724

PMDIG salt

3431

3450

1570 PMDIG gel

1780

1730 1780 PMDIG-PANI gel 3399 3189 1727 1402 1636

4000

3000

2000

1000 -1

Wavenumber (cm ) Figure 2 FTIR spectra of the PMDIG powder, PMDIG-salt and PMDIG, PMDIGPANI xerogels

the π-π stacking distance (0.348 nm) of the PMDIG powder and that for the xerogel it appears at q =8.49 (0.37 nm)41. A probable explanation of the mismatch of d-spacing of PMDIG xerogel with that of the powder is due to the presence of molecular tilt during the π-stacking of the PMDIG molecules in the gel.42 As a result the π-π stacking distance of the PMDIG xerogel is higher by 0.022 nm than that of the PMDIG powder. To get an insight of the supramolecular complex formation in the gel state the UV-Vis spectra of the solution of sodium salt of the PMDIG and that of the PMDIG sol are presented in figure S3. Due to the poor solubility of powder PMDIG in water we have chosen the UV-Vis spectra of sodium salt of PMDIG and the absorption bands observed at 312 nm and 325 nm may be attributed to the π-π* transition and π-π* transition coupled with n-π* transition, respectively.43 The absorption spectra of PMDIG sol (0.1 % (w/v)) exhibits a hump at 290 nm and a board peak around 323 nm. The 22 nm blue shift of the


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π-π* transition peak in the sol state may be attributed to the formation of a stable (lower)

π-band energy state due to the intermolecular π-stacking of the supramolecular complex. This stable π-π complex formation may cause an increase of transition moment integral, thus an increase of absorption intensity of the π-π* transition peak in the sol state. A possible reason for this good π- π stacking is for the planar structure of the H-

bonded PMDIG supramolecule during the gel formation. Pyromellitic diimide (PMDI) shows good luminescence property44 so we expect PMDIG molecules also to exhibit good fluorescence property in the hydrogel state. Fluorescence spectra of the hydrogel and sodium salt of PMDIG are presented in figure 3 and the inset of the figure shows a yellowish green florescence for excitation at 310 nm. Sodium salt of PMDIG shows emission maxima at 427 nm when excited at 310 nm; however, the fluorescence intensity is very low. But the fluorescence intensity of the PMDIG gel is about 16 times higher than that of the sodium salt of PMDIG. The emission peak also shows a red shift to 469 nm. The increase of fluorescence intensity of the PMDIG hydrogel compare to its sodium salt is due to the hydrophobic core formation in the former by H-bonding of the carboxylic groups followed by the self-assembly formation through π- stacking. This hydrophobic core prevents the PMDIG excitons from quenching with the water molecules in the hydrogel. The red shift of 42 nm of the emission maximum of the gel suggests that excitons are stabilised during the gel formation and it is indeed possible for the intermolecular π- stacking. The temperature dependency of the fluorescence emission spectra is presented in figure S4a and 4b for the PMDIG hydrogel and it is apparent that the fluorescence intensity remains unchanged until it reaches its melting temperature (73 0C). From this observation it may be


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concluded that PMDIG gel is stable over a long temperature range (25 to 72 0C) and we have also observed it in naked eyes during ‘falling ball’ method experiments. The decrease of fluorescence intensity at the melting temperature is related to the disaggregation of the hydrogel causing disruption of the hydrophobic cores. As the PMDIG hydrogel is highly stable over a long temperature range (25 to 72 0C) the emission maxima also remains unchanged and it shows a small red shift at the melting point, probably due to the formation of more planar PMDIG through conformational transition in the sol state. PMDIG-Hydrogel 469 nm

6

2

Intensity x 10

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1

427 nm PMDIG - Salt 0 400

500

600

Wavelength (nm)

Figure 3 Fluorescence spectra of PMDIG-salt and PMDIG-hydrogel at concentration 0.5 % (w/v) excited at 310 nm. (Inset: photograph of PMDIG-hydrogel emitting yellowish green light under UV light).

PMDIG-PANI hydrogel We have synthesized co-assembled PMDIG-PANI hydrogel via in-situ polymerization technique. When anilinium chloride solution is added to the PMDIG hydrogel, surprisingly the gel transforms into the sol state. Probably, the anilinium ions enter into the H-bonded channel structure of PMDIG gel and disrupt the strong supramolecular interaction present in the self-organized structure of PMDIG hydrogel. So, we have chosen the solution of sodium salt of PMDIG where on addition of anilinium chloride solution a yellow coloured sol is produced. Here the anilinium ion becomes


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adhered to the PMDIG molecule by double decomposition process eliminating the Na+ and Cl-. On spreading solid ammonium persulphate (APS) over the sol and keeping it undisturbed for 24 hours at 30 0C the above sol transforms into a stable hydrogel. Both falling ball and DSC data indicate that the PMDIG-PANI gel does not melt till 90 0C (Figure S5) and we could not heat it further due to appreciable loss of water. The signature of involvement of the supramolecular interactions between the polyaniline and the PMDIG molecules during the hybrid formation can be clearly understood from the comparison of FTIR spectra of the PMDIG and PMDIG-PANI xerogels with PMDIG powder (Figure 2). The –OH stretching frequency of the carboxylic acid group of the gelator PMDIG is down shifted (3399 cm-1) compared to that of the powder and xerogel of PMDIG at 3450 and 3434 cm-1, respectively and it indicates strong hydrogen bonding interaction between PANI and the gelator. The carbonyl stretching frequency of the carboxylic acid group of the gelator also shifts to higher frequency (1727 cm-1) compared to that of the powder PMDIG (1724 cm-1) and the reason is same as discussed for PMDIG xerogel. The vibration peak at 3189 cm-1 appears in the hybrid xerogel corresponds to the –N-H stretching frequency of PANI. The peaks observed at 1636 cm-1 and 1402 cm-1 are attributed to the stretching in aromatic nuclei and C=N stretching in aromatic compounds, respectively45. The wide angle X-ray scattering (WAXS) pattern (Figure S6) of the hybrid coassembled hydrogel also drastically differs from those of powder & xerogel of the PMDIG and that of pure PANI (Figure S2, Figure S7) indicating co-assembly formation of PANI chain with PMDIG through supramolecular interaction. It is interesting to note that the diffraction peaks in the co-assembled gel is sharper than those of PMDIG and


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PANI and according to Scherrer equation46 the crystallite size of the co-assembled gel is greater than those of the components. No definite reason for this behaviour can be afforded here and one probable reason may be the supramolecular force between the PANI chains of the co-assembly causes the PMDIG supramolecular chains to be more ordered. Also in the diffraction pattern (inset of figure S6) there is a hump at q =1.45-1.8 which may be de-convoluted into two peaks at q =1.49 and 1.66. These peaks correspond to the d values of 2.1 nm and 1.89 nm, respectively which approximately represent the energy minimized lamellar distance (2.26 nm) and molecular distance of the PMDIG molecule (1.79 nm) obtained from molecular mechanics (MMX) program (Figure S8). From these X-ray diffraction results it may be reasonable to get an idea about the structure of the co-assembly by considering the PANI chains to be attached and doped through the four terminal carboxylic acid groups of PMDIG molecule and such PMDIG molecules self assemble by π- stacking with each other. This is in sharp contrast to the pure PMDIG gel where a long supramolecule of PMDIG is produced by H-bonding at the end to end positions between themselves and these long molecules then undergo πstacking to produce the fibrillar structure. For the xerogel the main diffraction peaks (Fig.S6) corresponds to d values of 0.493, 0.462, 0.374, 0.334 and 0.29 nm of which the 0.374 nm value matches very well with the π-stacking distance of PMDIG, discussed earlier. It is evident from the FESEM micrographs (Figure 4a) and transmission electron micrographs (Figure 4b) that the hybrid PMDIG-PANI gel exhibits co-assembled fibrillar network morphology. A comparison of the fibre texture of the PMDIG (Figure S9a) and PMDIG-PANI co-assembled gels indicates that the surface of the later fibres is more


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rough and also its fibrillar diameter (34 ± 11 nm) is lower than those of PMDIG gel (39.0 ±9 nm) (Figure S9a). From a comparison of the above d values of 2.1 nm of PMDIG-PANI and the energy minimized model (Fig.S8) with fibrillar diameter, it might be reasonable to think that ~ 15-16 such PMDIG-PANI units laterally assemble together to produce the fibre. A UV-Vis spectrum of the co-assembled PMDIG–PANI hydrogel dispersed in water (Figure 5) exhibits three characteristic absorption peaks centred at 290, 440, and 791 nm. The absorption peak at 290 nm is attributed to the π-π* transition in the benzenoid rings, whereas the remaining two peaks at 440 and 791 nm are attributed to the polaron band to π* band transition and the π band to polaron band transition of the doped PANI, respectively. The presence of these peaks confirms that PANI chains are in the doped state i.e in the emeraldine salt (ES) state in the co-assembled gel. b

a

Figure 4 (a) FESEM image of co-assembled PMDIG-PANI hydrogel, (inset: photograph of the co-assembled PMDIG–PANI hydrogel) (b) TEM image of the co-assembled hydrogel 2.0

Absorbance

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290 nm 1.5 440 nm

791 nm

1.0

0.5 400

600

800

1000

Wavelength (nm)

Figure 5 UV-Vis spectra of PMDIG–PANI hydrogel dispersed in water


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Rheology As the gels are viscoelastic materials it can store/dissipate energy under oscillatory stress which are characterized by the storage (G') and loss (G") modulii, respectively. In the gel state the rheological parameters should be G'(ω) > G" (ω) and G' (ω) ≈ ω0 for low values of ω where ω is the angular frequency47. Apart from the proof of gel formation, the mechanical strength (G’), fragility (σ*, minimum stress required to rupture the gel), stiffness (G' / G") and elasticity (G' - G")47 of the hydrogels, are calculated from the rheological data (Table S1). A wide invariant linear viscoelastic region (LVR) is observed for PMDIG (figure S10) and the co-assembled PMDIG-PANI (Figure 6a) hydrogels from the dynamic frequency sweep experiments. In addition, the G' value is much higher compared to that of G" in both the cases confirming their gel nature. The PMDIG-PANI co-assembled hydrogel has about 50 times the G' value of the PMDIG hydrogel (TableS1). The elasticity of the PMDIG-PANI gel is 14590 Pa and it is also 52 times higher than that of pure PMDIG hydrogel. The stiffness of the hybrid PMDIG-

10

4

b 10

10

4

G' and G'' (Pa)

G' and G'' (Pa)

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a

10

3

10

2

3

1

10

1

100

Ang. frequency (rad/sec)

10

100

Oss. stress (Pa)

Figure 6 (a) Storage modulus (G') and loss modulus (G") vs. angular frequency plot at a concentration of 1.0 % (w/v) of the co-assembled PMDIG-PANI hydrogel. (b) G' and G" vs. oscillator stress plot at a constant frequency of 1 Hz of the coassembled PMDIG-PANI hydrogel at a concentration of 1.0 % (w/v) at 30 0C


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PANI gel is also much higher (1.4 times) than that of the pure PMDIG hydrogel (Table S1). The higher values of the above mechanical properties of the PMDIG-PANI hydrogel over the pure PMDIG hydrogel may be attributed to the development of a co-assembled nano fibrillar structure. Probably the PANI chains in the lamella can store / dissipate energy through the covalent bond and it is much higher than that through the noncovalent linkages of the PMDIG molecules. The fragility (σ*) of the co-assembled PMDIG-PANI gel is measured from the stress sweep experiment (Figure 6b) and a comparison with that of PMDIG gel at a same concentration indicate that it is 5 times higher than that of PMDIG gel(20 Pa, Table-S1). dc -Conductivity The dc - Conductivity of the xerogels of PMDIG and PMDIG-PANI measured by two probe method using ITO as electrode exhibits the values of 0.6 x 10-8 and 0.3 x 10-4 S/ cm, respectively. The four orders increase of conductivity may be attributed to the co-assembly with the doped conjugated PANI chains which provide an excellent path for the movement of charge carriers in the co-assembled fibrillar network. But in the case of PMDIG xerogel it is probably the π-stacked electrons which contribute to the overall conductivity. Current (I)–voltage (V) characteristics As the pyromellitic diimide (PMDI) is an n-type semiconductor we expect the gelator PMDIG also to exhibit good I-V characteristics which are recorded by scanning from (i) 0 to +5.0 V and then (ii) +5.0 to -5.0 V followed by (iii) a reverse scan from -5 to 0 V. The I–V characteristic curve of the PMDIG xerogel (Figure S11a) clearly indicates its semiconducting nature and it is purely symmetrical in nature.


But the I–V

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characteristic curve of the co-assembled PMDIG-PANI xerogel is not purely symmetrical, showing a very small rectification ratio ≈ 1.2 (Figure S11b)48. The small rectification property of the co-assembled PMDIG-PANI xerogel arises due to the closely spaced p/n junction formation between p-type doped PANI and n-type PMDIG semiconductor. Photo-voltaic Property As the co-assembled PMDIG-PANI hydrogel consists of donor-acceptor component through non-covalent interaction we have further examined the photo current behaviour. The I−V characteristics curves of ITO/ PMDIG-PANI /ITO devices under alternating dark and illumination conditions (Figure 7a) indicates that the photocurrent is higher than dark current in negative bias. The photocurrent of ITO/ PMDIG-PANI /ITO device under alternating dark and illumination conditions at a time interval of 50 s under a bias of +5 V (Figure 7b) indicate that at the initial “on” and “off” cycles there is higher value of photocurrent which decreases at the first two cycles. But at the remaining three cycles the photocurrent is almost stable. One probable way to explain the result is that the trapped space charge is of positive nature and it may be reasonable to think that the protons get adhered to the PANI surface during the co-assembled PMDIG-PANI gel formation (these protons are different from those protons taking part in the doping PANI generating polarons). At the initial 2-3 cycles these adsorbed protons become released on application of positive bias and add to the current flow yielding higher photocurrent. But from the 4th cycles only the contribution of photoelectrons in the current flow exists, showing almost a stable photocurrent in the cycles. An evidence of this assertion may come from almost the same height between the peak and trough current of each cycle49.


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Another important observation from the I-V plot (Figure 7a) is that there is a photo-current rectification in the system with a rectification ratio ≈ 4.450. This is a very new observation in the supramolecular gel systems and a probable reason may be the intimate mixing of donor (PANI) and acceptor (PMDIG) molecules in the co-assembly. It is to be noted here that at the dark condition this device exhibits a rectification ratio of 1.2, (Figure S11b) but on illumination with a light of one sun intensity the rectification ratio increases to 4.4. Actually in PMDIG-PANI, PANI is in the doped state so radical cations (polarons) are produced. On illumination of white light it is quite reasonable to think that electrons and holes of radical cations become well separated and the electrons becomes stabilized by the acceptor PMDIG. Consequently on application of negative bias these electrons move producing higher current than that at positive bias condition. But in the dark state the separation between electrons and holes is not so significant, therefore yielding lower rectification ratio than that under illumination condition. The PMDIGPANI fibers are of 34 nm thickness and few micrometer lengths, so the depletion layer (which usually lies in the range ~1µm) may be produced along the length of the fibre. Here, I-V measurements are made for bulk samples so the usual rectification mechanism of p/n junction formation is likely to be applicable here. Thus, it is confirmed that that there is photo-current rectification under illumination of white light in the PMDIG-PANI co-assembled gel and it is ~4 times higher than that of the dark current. It should be mentioned here that at the first run the I-V characteristics plots under dark and illumination conditions exhibit a high rectification ratio (Figure S12), e.g for the dark condition it is 2.7 and at the illumination condition it is 37. This higher rectification ratio values are due to the space charge stored within the co-


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assembled gel during its preparation, so we have taken the second cycle for rectification measurement. Hence, the above two experiments clearly indicate that there exists space charges of positive nature accumulated during the preparation of the PMDIG-PANI gel, yielding higher rectification ratio and higher photocurrent in the “on” and “off” cycle at the first run. On repeated runs the space charge diminishes significantly. 6

b

a

0

Current (A) x 10-6

-6

5

Current (A) x 10

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Dark -5 -10 Light

-15 -20 -25

4

2

-6

-4

-2

0

2

4

100

6

200

300

400

500

600

Time (Sec)

Voltage (V)

Figure 7 (a) I–V curve of the PMDIG-PANI 2 xerogel under dark and illuminting conditions. (b) Photocurrent cycles showing the turn ‘‘on’’ and ‘‘off’’ by switching the white light illumination on and off for the PMDIG-PANI 2 xerogel.

Dye Sensitized Solar Cell Using the donor / acceptor properties of the components of the PMDIG-PANI coassembled gel we have fabricated a dye-sensitized solar cell (DSSC)25 with an ITO/ PMDIG-PANI /graphite device using 100 mW/cm2 illumination conditions using N-719 dye. For this purpose we have made PMDIG-PANI 1 and PMDIG-PANI 2 samples with two different concentrations of aniline and their subsequent polymerisation. In the PMDIG-PANI1 we have added 10 mmol aniline and polymerised. The resulting PMDIGPANI 1 system exhibits a short-circuit current (Jsc) of 0.29 mA/cm2, an open-circuit voltage (Voc) of 0.56 V, a fill factor (FF) of 0.31 and a power conversion efficiency


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(PCE, η) of 0.05 % (Figure S13 and table S2). In order to increase the short-circuit current we have prepared PMDIG-PANI 2 where we have added 20 mmol aniline. It exhibits a short-circuit current (Jsc) of 0.33 mA/cm2, an open-circuit voltage (Voc) of 0.55 V, a fill factor (FF) of 0.56 and a PCE (η) of 0.1 % (Figure 8). A probable mechanism of the DSSC operation is shown in Scheme-S1. The energy level diagram is drawn taking the LUMO energy of PMDIG39 and that of PANI51 from literature. After absorbing white light by the N-719 dye in the above-fabricated DSSC, an electron of the dye is promoted to its excited state (Scheme S1). The excited electron then enters into the conduction band of PANI and then to that of PMDIG of the co-assembled gel and finally it flows to the external circuit. The process is facilitated by the co-assembly of acceptor PMDIG and the donor conducting polymer. 0.4 2

Cell Current (J mA/cm )

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0.2

0.0 0.0

0.2

0.4

0.6

Cell Voltage (V)

Figure 8 J–V characteristic curve of the PMDIG-PANI 2 xerogel in the ITO/ PMDIG-PANI /graphite device using N719 dye.

Discussion: Here we would like to discuss on the gel and sol formation of PMDIG, PMDIGPANI and PMDIG-aniline (PMDIG-ani) systems. The gel formation of PMDIG is shown in a scheme (Scheme 2) where the carboxylic acid groups of PMDIG produce intermolecular H- bonding causing a longitudinal growth of the fibres and the lateral


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growth of the fibre takes place through the π – stacking of PMDIG cores. But as soon as anilinium hydrochloride solution is added to it, supramolecular interaction between anilinium ions and the carboxylate ions of PMDIG occurs. Here the longitudinal growth occurs through the π – stacking of PMDIG cores and the lateral growth occurs through the π – stacking of aniline molecules supramolecularly attached to the PMDIG molecules. On ploymerization the PANI chains are produced, assisting to supramolecularly interact with the neighbouring units by both the π – stacking and also through ionic interaction of doped imino group of PANI helping to grow in the longitudinal direction. The lateral growth of course occurs through the π – stacking of PMDIG cores as in the PMDIG–ani system. It is now necessary to compare the diameter values of PMDIG, PMDIG-ani and PMDIG-PANI xerogels obtained from TEM microscopy (Fig.4b and FigS.9). It is important to note here that in every system fibrillar network structure is present and the highest average fibre diameter is observed for PMDIG-ani sol (48 ± 7 nm) compared to those of PMDIG gel (39±9 nm) and PMDIG-PANI gel ((34 ± 11 nm). This is because the lateral growth of the PMDIG-ani system is highly favoured for easier π – stacking of aniline monomeric molecules supramolecularly attached to PMDIG than that of

π –

stacking of PANI chains due to the inflexibility present in the PANI chain in PMDIGPANI gel. The lateral growth of the PMDIG-ani system is so highly favoured that it is even easier than the π – stacking of the PMDIG cores in the pure PMDIG gel. So from these results it is apparent that PMDIG-ani system should also produce gel, as the fibre diameters do not differ significantly. To find a suitable explanation the contact angle of the dried systems are measured and it is evident from the figure S14 that PMDIG-PANI has the highest contact angle


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(550); PMDIG has an intermediate value of 300 and PMDIG-ani has a value almost equal to 00 where the water drop spreads immediately after it is cast on it. As the films are cast in a similar way, the contact angle data are indicative of a comparison of the polarity of the fibres of the three systems and polarity order is PMDIG-ani > PMDIG > PMDIGPANI. It is observed that PMDIG-ani fibres do not produce gel but the other two systems produce gel. Due to nano dimension of the fibre diameters there is an appreciable surface force operating amongst the fibres and if it is really large it can entrap the solvent producing the gel. But in case of PMDIG-ani system the force of adhesion of the fibres with water is appreciably large (as contact angle is ~00) and it might be responsible to diminish the force of attraction amongst themselves. This decrease of force operating between the PMDIG-ani fibres makes it inefficient to entrap the water molecules in between them prohibiting the gel formation.

Scheme 2 A schematic models showing the gel formation mechanism of PMDIG, PMDIG-ANI and PMDIG-PANI co-assembled gel.

Conclusion: PMDIG produces supramolecular hydrogel which is characterized by fibrillar network morphology produced from the self organization of PMDIG molecules through


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intermolecular H-bonding interaction and π- stacking process. The fluorescence intensity of the PMDIG gel is 16 times higher than that of sodium salt of PMDIG with a 42 nm red shift of the emission peak arising from the hydrophobic core formation by intermolecular π- stacking of the PMDIG supramolecule. The PMDIG gel transforms into the sol for the addition of anilinium chloride solution and by spreading solid APS over the sol, a stable hydrogel is produced on aging for 24 hours at 30 0C. The co-assembled PMDIG-PANI gel exhibit fibrillar network morphology and the co-assembly is formed by the supramolecular interactions between the polyaniline and the PMDIG molecules and in the PMDIG–PANI hydrogel PANI remains in the doped state. The PMDIG-PANI coassembled hydrogel exhibits a dramatic increase of mechanical property and dcconductivity than that of PMDIG. The I-V curves exhibits good rectification property under illumination of white light showing photo-current rectification for the first time in the supramolecular gel systems. A dye-sensitized solar cell fabricated with an ITO/ PMDIG-PANI /graphite device using 100 mW/cm2 illumination and N-719 dye shows PCE of 0.1 %.

Thus the co-assembled PMDIG-PANI hydrogel exhibit superior

mechanical, optoelectronic and photovoltaic property than that of the PMDIG hydrgel. A discussion on the mechanism of gel formation of PMDIG and PMDIG-PANI and sol state of PMDIG-ani system is included considering the contact angle values of the xerogels. Acknowledgment: We acknowledge the financial support from DST unit of nanoscience at IACS. P.B, P.C, S.M, A.S and B.R acknowledge CSIR, New Delhi for providing the fellowship.


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Graphical Abstract

A Co-assembled Gel of Pyromellitic Dianhydride Derivative and Polyaniline with Optoelectronic and Photovoltaic Properties

Partha Bairi, Priyadarshi Chakraborty, Arnab Shit, Sanjoy Mondal, Bappaditya Roy and Arun K. Nandi*


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A Co-assembled Gel of a Pyromellitic ... - ACS Publications

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