A molecular insight on the supramolecular assembly of thiophene polymers

Article abstract of DOI:10.1039/c2jm30550j

Herein, the relationship between supramolecularly self-assembled microstructures and the chemical structures of photoluminescent molecules is discussed. A series of non-amphiphilic molecules with different length alkyl chains (4-14), central mesogenic groups, and chemical linkers were designed and synthesized. Intricate analysis shows that the supramolecular assembly varies with change in torsion angle of the thiophene linking group on elongation of the alkyl chain. Small angle X-ray scattering measurement shows long range molecular interaction for lower alkyl chain length, AFM reveals circular domains for octyl and decyl alkyl group polymers. UV-Visible and photoluminescence studies in solution and thin-film show that the J-aggregate patterns for the circular domain of the polymer backbone has a major effect on the polymer absorbance, and produces a larger shift for the photoluminescence signals at higher wavelengths. Excitation polarisation studies show a change in molecular ordering in various colour regions with change in aggregation by means of maximum dichroic ratio for the circular domain assembly. Charge carrier mobility and cyclic voltammetry studies show higher mobility and reduced band gap for these circular domain polymers. Dynamic light scattering studies of octyl alkyl chain polymers show a bimodal distribution, which shows the apparent nature of anisotropy in solution. The Royal Society of Chemistry.

Full text of DOI:10.1039/c2jm30550j

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PAPER
A molecular insight on the supramolecular assembly of thiophene polymers†
Chandrasekaran Suryanarayanan, Ezhakudiyan Ravindran, Soundaram Jeevarathinam Ananthakrishnan,
*
Narayanasastri Somanathan and Asit Baran Mandal
Received 29th January 2012, Accepted 27th July 2012
DOI: 10.1039/c2jm30550j
Herein, the relationship between supramolecularly self-assembled microstructures and the chemical
structures of photoluminescent molecules is discussed. A series of non-amphiphilic molecules with
different length alkyl chains (4–14), central mesogenic groups, and chemical linkers were designed and
synthesized. Intricate analysis shows that the supramolecular assembly varies with change in torsion
angle of the thiophene linking group on elongation of the alkyl chain. Small angle X-ray scattering
measurement shows long range molecular interaction for lower alkyl chain length, AFM reveals
circular domains for octyl and decyl alkyl group polymers. UV-Visible and photoluminescence studies
in solution and thin-film show that the J-aggregate patterns for the circular domain of the polymer
backbone has a major effect on the polymer absorbance, and produces a larger shift for the
photoluminescence signals at higher wavelengths. Excitation polarisation studies show a change in
molecular ordering in various colour regions with change in aggregation by means of maximum
dichroic ratio for the circular domain assembly. Charge carrier mobility and cyclic voltammetry studies
show higher mobility and reduced band gap for these circular domain polymers. Dynamic light
scattering studies of octyl alkyl chain polymers show a bimodal distribution, which shows the apparent
nature of anisotropy in solution.
by Langmuir Blodgett,7 hydrogel,8 liquid crystal9 and amphilic
1. Introduction
molecules10 etc. However, a report on the innate self assembly
behaviour of the molecule is indeed to reduce the additional
processing method. Aggregation induced molecular assembly
using hydrophobic–hydrophilic pendant groups makes the
conjugated polymers exhibit some unique and interesting
properties.11 The solvation of functional groups dissolved in
hydrophilic solvents increases hydrophobic groups interaction
increases intra-chain and/or inter-chain energy transfer.12
Juan Torras and co-workers13 did a theoretical study on the
conformational properties of poly[(R)-3-(4-(4-ethyl-2-oxazolin-
2-yl) phenyl) thiophene] on aggregation, and found that
the syn–gauche conformation between continuous backbone
thiophene rings favours a self-assembled helical aggregate based
on stacked octamers and a bent double helix aggregate in
large oligomers. Although conceptually interesting, reports on
solvent-induced self-assembled systems based on highly
conjugated molecules that possess significant hole or
electron mobilities remain rare in the literature.14 An insight
into molecular ordering by means of an experimental study
is necessary. In order to investigate the dependence of the
self-assembling behaviour on the proportion of the side chain,
we designed APTC polymers (Scheme 1) of various alkyl
chains with same main chain of thiophene and azomethine
chromophoric group. Accordingly, the variations in
chromophore segment depend only on the alignment of the
alkyl chain.
Organic semiconductors are carbon-rich compounds with a
tailored structure to optimize a particular function, such as
charge mobility or luminescent properties etc. Thiophene
enclosed polymers have been the specific goal of interest for high-
end electrical and optical materials.1 The apparent fragility of
organic materials has also opened the door to a suite of inno-
vative molecules in optical switching,2 biosensing3 and data
storage.4 Supramolecular functional materials have various
structures, which contribute to diverse functionality.5 The
formation of supramolecular ordering is due to the cooperative
effect of noncovalent forces, such as hydrogen bonding, p
stacking, dipolar and van der Waals interactions. This is the
driving force behind molecular self-assembly, leading to a variety
of novel supramolecular architectures.6
The change in supramolecular organisation amplifies indi-
vidual molecule photochemical or electrochemical properties,
with the properties of the chromophoric units being at the
supramolecular levels. The molecular organisation is improved
Polymer Division, Central Leather Research Institute, Network Institute of
Solar Energy, Adayar, Chennai, India. E-mail: nsomanathan@rediffmail.
com; Fax: +91-044-24911589; Tel: +91-044-24437189
† Electronic supplementary information (ESI) available: 1H NMR and
ESI-MS of all samples. Excitation polarisation of all samples, SEM
images, PL anisotropy and linear dichroism. See DOI:
10.1039/c2jm30550j
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Scheme 1 Synthesis route of 4-{(E)-[(4-alkoxy phenyl imino] methyl} phenyl thiophene-3-carboxylate (APTC) monomer. (i) Dicyclocarbodiimide/
dimethyl amino pyridine RT, 24 h. (ii) HCl/ethanol, reflux, 5 h (iii) K2CO3/dimethylformamide, 75 ꢁC, 4 h (iv) Microwave 40 W, ethanol, 10 min.
All the monomers were synthesised according to Scheme 1.
NMR, mass spectra and elemental analysis confirmed the
structures of the synthesized monomers. The monomers of
various alkoxy chain lengths were electro polymerised using
cyclic voltammetry. The photochromic property and self-
assembling behaviour of thiophene polymer on varying alkyl
chain length was studied in detail.
use application. Small angle X-ray scattering measurements of
PAPTC were carried out by using BL17A (powder X-ray scattering
end station) X-ray diffractometer of the National Synchrotron
Radiation Research Center (NSRRC), Taiwan. The incident beam
is focused on a toroidal mirror and monochromated (l ¼ 1.333
ꢀ
621A for BL17A) by a Ge double-crystal monochromator. The
beam is mainly collimated by a 20 cm collimator of 1 mm diameter
channel. The sample-to-detector distance was kept at 300 mm. X-
Ray diffraction data collected by using a mar345 imaging plate
detector and corrected for background scattering. Charge carrier
mobility were measured by using space charge limited current
(SCLC) method using Keithley 2400 source meter for ITO/poly-
mer/aluminium layered device.
2. Experimental
2.1. Material and characterisation
All starting materials were brought from Sigma Aldrich and used
as received. The solvents used were of analytical grade. All reac-
tions were monitored by thin-layer chromatography carried out on
0.2 mm Merck silica gel plates (60F-254). Column chromatography
2.2. Synthesis
1
was performed on silica gel (Merck, 60–100 mesh). H and 13C
NMR spectra were recorded on a JEOL ECA 500 MHz instru-
ment. Samples were dissolved in CDCl3 and tetramethylsilane was
used as an internal standard. ESI mass spectra were recorded on
Thermo Finnigan LCQ Advantage MAX ESI-Mass Spectrometer,
elemental analysis of monomers were done on Euro Vector S.P.A,
Euro EA 3000 CHNS elemental analyzer. UV-Visible absorption
spectra were recorded on Varian Carey 50 Bio UV-Visible spec-
trophotometer. Photoluminescence spectra were measured on
Varian Carey eclipse fluorescence spectrophotometer (with
concentration of 1 mg in 100 ml of CH2Cl2 for PL, UV-Visible and
life time measurements). Excitation polarisation experiment was
done according to the similar procedure reported elsewhere.15
Cyclic voltammetry measurements were taken on a CH instru-
ments, CHI600D electrochemical work station. Dynamic light
scattering experiments were done on a dynamic light scattering
instrument from Malvern instruments, UK. Fluorescence life time
experiments were done on a IBH Fluorescence Lifetime System
(picosecond resolution), while the exponential fitting were done
using Horiba Jobin Yuon decay analysis software. Atomic force
micrographs were obtained on Nova 1.0.26 RC1 Atomic force
microscope with NT-MDT solver software for analysis, silicon
cantilever (SII) with average frequency of 260–630 kHz with force
constant of 28–91 N mꢀ1 were used in semi contact mode. Samples
for AFM were prepared by spin coating the polymer solution
(CH2Cl2) over Indium tin oxide (ITO) substrate for simulated end
2.2.1. Synthesis of 4-formyl phenyl thiophene 3-carboxylate
(1). Thiophene 3-carboxylic acid (0.078 mol), 4-hydroxy benz-
aldehyde (0.078 mol), dicyclo hexyl carbodiimide (0.078 mol)
dissolved in 25 mL of dichloromethane (DCM) and dimethyl
amino pyridine (0.0078 mol) was finally added and stirred at
room temperature for 24 h. The contents were filtered and the
organic solution was washed with dilute HCl and 2% KOH
solution. The compound was recrystallized in isopropanol (yield
1
¼ 62%.). H NMR (CDCl3, TMS, d): 10.01(s, 1H), 8.34(q, 1H),
7.96 (d, 2H), 7.66(d, 1H), 7.4(m, 3H). 13C NMR (CDCl3, TMS,
d): 191.0, 160.4, 155.5, 134.7, 134.1, 132.2, 131.4, 128.3, 126.8,
122.6. ESI-MS Experimental (predicted): 232.72 (232.02),
elemental analysis: C12H8O3S C: 62.56, H: 3.57, O: 20.87, S:
13.81% (cal. C:62.07,H:3.45, O:20.69, S: 13.79%).
2.2.2. Synthesis of N-(4- alkoxy phenyl) acetamide (2). N-(4-
Hydroxy phenyl) acetamide (0.02 mol) dissolved in 100 mL of
dimethyl formamide (DMF) and potassium carbonate (0.02 mol)
was added and the contents were stirred and heated to 80 ꢁC,
alkyl bromide (0.02 mol) was added slowly to ꢁthe contents and
reaction mixture was stirred for 3 h at 70–80 C. The reaction
mixture was poured into cold water and neutralized using dilute
HCl. The product was filtered and recrystallized using petroleum
ether (yield ¼ 80%). 1H NMR of HPTC (CDCl3, TMS, d):
7.53(d, 1H), 7.36(d, 2H), 6.82(d, 2H), 3.91(t, 2H), 2.11 (s, 3H),
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1.75(m, 2H), 1.42(m, 2H), 1.29(m, 8H), 0.87(t, 3H). 13C NMR of
HPTC (CDCl3, TMS, d): 168.5, 130.93, 122, 114.8, 68.4, 31.9,
29.4, 29.36, 29.33, 29.1, 24.3, 22.7 and 14.1. ESI-MS Experi-
mental (predicted): 235.76 (235.16). Elemental analysis:
C14H21NO2 C: 71.89, H: 9.22, N: 6.22, O: 13.88%.(Cal.C: 71.49,
H: 8.94, N: 5.96, O: 13.61%)
voltammograms (CV) of repeated oxidative cycles is presented in
the ESI.† The plotting of current response versus number of
cycles show that the radical cations produced on oxidative scans
couple to produce oxidized oligomers which electro precipitate
onto the electrode surface through diffusion controlled
processes.16
The redox properties of this series of materials were investi-
gated by CV (Fig. 1). Table 1 shows oxidation potential (Eox) and
reduction potential (Ered) of polymers and their respective elec-
trochemical band gap (Egec). The oxidative scan (positive scan) of
respective polymer CV shows redox potential behaviour for all
polymers and very low onset oxidation potential for PDPTC of
1.24 V with very low current response. In reductive scan (nega-
tive scan) the polymers undergo a reversible multielectron (two
or three) reduction arising from the aromatic core and reduction
potential decreases on increasing chain length up to PDPTC.17
The HOMO (EHOMO) and LUMO (ELUMO) as well as the band
gaps (Egec) of the polymers were calculated from the values of
oxidation and reduction potential. Band gaps of the polymers
decreases up to PDPTC and increases for PDDPTC and
PTDPTC.
2.2.3. Synthesis of 4-alkoxy aniline (3). N-(4-Alkoxy phenyl)
acetamide (3.17 mmol) was treated with 3 N HCl in 50 ml of
C2H5OH and refluxed for 6 h. The reaction mixture was poured
into water and neutralized using 2% KOH solution the amine
obtained was extracted using ether. 1H NMR of HPTC (CDCl3,
TMS, d): 7.0(d, 2H), 6.79(d, 2H), 3.8(t, 2H), 2.02(m, 2H), 1.42(m,
2H), 1.3(m, 4H), 0.87(t, 3H). 13C NMR of HPTC (CDCl3, TMS,
d): 152.5, 139.8, 116.5, 115.7, 68.8, 32, 29.7, 29.5 and 29.45. ESI-
MS Experimental (predicted): 193.90 (193.29). Elemental anal-
ysis: C12H19NO C: 74.85, H: 9.98, N: 7.86, O: 8.64% (Cal. C:
74.61, H: 9.84, N: 7.25, O:8.29%).
2.2.4. Synthesis of 4-{(E)-[(4-alkoxy phenyl) imino] methyl}
phenyl thiophene 3- carboxylate}. 4-Alkoxy aniline (4.3 mmol) and
4-formyl phenyl thiophene 3-carboxylate (4.31 mmol) were reacted
in ethanol medium using microwave reactor. The obtained reaction
mixture was diluted with methanol and recrystallized using iso-
propanol (yield ¼ 62%). The representative NMR data obtained
for HPTC is presented below. 1H NMR of HPTC (CDCl3, TMS,
d): 8.48(s, 1H), 8.3(d, 1H), 7.95(d, 2H), 6.9(d, 2H), 3.97(d, 2H),
1.8(m, 2H), 1.46(m, 2H), 1.3(m, 4H), 0.88(t, 3H). 13C NMR of
HPTC (CDCl3, TMS, d): 160.7, 158, 157, 152.8, 144.5, 134.4, 134.3,
132.6, 129, 128.3, 126.6, 122.3, 122.1, 115.08, 68.08, 31.4, 19.4 and
13.98. ESI-MS Experimental (predicted): 408.13 (407.53).
Elemental analysis: C24 H25 N O3 S C: 71.1, H: 6.78, N: 3.84, O:
11.86, S: 7.9% (Cal.C: 70.76, H: 6.14,N: 3.44, O: 11.79, S:7.86%)
3.2. UV/Vis and photoluminescence (PL) study of solution and
thin film
The absorption and photoluminescence (PL) spectrum (exc. 320–
340 nm range) of monomers and polymers are presented in
Fig. 2. The absorption edge extends around 380 nm in the
monomers, which get extended and red shifted above 400 nm in
polymers. In the case of monomers, the lmax is around 325 nm for
all the monomers, while the emission maximum is around 385
nm. For PHPTC, POPTC and PDPTC, the absorption max. in
solution is around 323 nm, which is red shifted for PDDPTC,
PTDPTC and PBPTC. The PL maximum is obtained around
400–405 nm, which get red shifted by about 75–80 nm for
POPTC and PDPTC. Comparison of the PL of monomer to
polymer18,19 clearly indicate the red shifting in POPTC and
PDPTC which suggest the formation of ‘J’ aggregates in the
above polymers. Fluorescence anisotropy (solution) and linear
dichroism (in thin films) were studied for different monomers
and polymers and the obtained results are presented in Table
S1(a) and (b).† The fluorescence anisotropy was found to be
maximum for OPTC. In the case of polymers, POPTC and
PDPTC exclusively show higher anisotropy when compared to
other polymers. The linear dichroism data also shows that
2.2.5. Polymerization. Electrochemical polymerisation was
done using the same procedure reported in ref. 15. Electrochemical
polymerizations were carried out in a conventional three electrode
cell using Pt working electrode of 2 mm diameter, a 1 cm2 platinum
flag counter electrode, and a nonaqueous Ag/0.01 M Ag+ (silver
wire in 0.1 M tetra butyl ammonium perchlorate (TBAP) in
acetonitrile) reference electrode. Electrochemical polymerizations
were carried out using cyclic voltammetry. Solutions used for the
electrochemical studies were prepared from freshly distilled ACN
and contained 10 mM monomer and 0.1 M electrolyte. The bulk
polymers of APTC were prepared by electrochemical polymeri-
zation over stainless steel plates (3 in diameter) at a current density
of 2 mA cmꢀ2 for 30 mL of stock solution containing acetonitrile,
0.1 M electrolyte (TBAPF6) and 10 mM APTC monomer. The
polymer thus obtained on the stainless steel plate was further
reduced at ꢀ2 V and washed with acetonitrile. The polymer
removed from the electrode and dried under vacuum at 50 ꢁC (Mn
¼ 7475, Mw ¼ 20 780 and PDI ¼ 2.78).
3. Results and discussion
3.1. Cyclic voltammetry
All the synthesized APTC monomers were polymerised using
oxidative electrochemical polymerization. The typical cyclic
Fig. 1 Cyclic voltammograms of the six APTC polymers over Pt elec-
trode in 0.1 mol Lꢀ1 Bu4N+ClO4ꢀ in acetonitrile solution.
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Table 1 Absorption and emission peaks in solution and thin film, fluorescence life time (ss), CV data oxidation reduction peak and its respective band
gap. Charge carrier mobility (m)
lmax (nm)a
lemi (nm)b
m
Eoxi Ered
(V)c (V)c
EHOMO ELUMO Eg
(eV)d (eV)e
ꢂ 10ꢀ4
Sample
PBPTC
Solution Thin film Solution
Thin film
ss ꢂ 10ꢀ9 s (%)
(eV)f (cm2 Vꢀ1 sꢀ1
)
260.4,
338
212.1,
234.6,
292.3
211.9,
234.4,
289.2
213.1,
239.4,
291.1
213.4,
241.2,
301.2,
212.6,
242.7,
295.4
212.5,
241.7,
289.2
302.5, 386.3,
465.9, 587.9,
744.1
260.2, 491.3, 0.8496(76.71), 4.4602(22.79) 1.42 ꢀ1.06 ꢀ6.13 ꢀ3.65 2.48
7.19
8.2
529.5, 566.9,
722.7
PHPTC
POPTC
PDPTC
269.5,
336.9
302, 379, 463.9, 264.8, 490.3, 0.7962(69.12), 3.4281(30.88) 1.45 ꢀ1.05 ꢀ6.16 ꢀ3.66 2.5
586.6726.4 533.2, 566.8,
721.9
269.7,
340.4,
303.3, 378.7,
480.2, 588.8,
724.6
301.7, 368.1,
480.9, 586.3,
751.9
302.3, 389.1,
484.8, 586.6,
722.3
302.8, 367.9,
485.9, 579.6,
692.7
264.6, 490.2, 0.8755(73.73), 4.0565(26.27) 1.46 ꢀ0.91 ꢀ6.17 ꢀ3.8
2.37
7.02
12.46
525.8, 567.9,
727.8
262.8,
340.1
261.8, 492.1, 0.8582(83.12), 5.1079(16.88) 1.24 ꢀ1.06 ꢀ5.95 ꢀ3.65 2.3
528.4, 564.9,
711.2
PDDPTC 266.9,
339.8
267.6, 487.8, 0.8436(67.09), 3.8630(32.91) 1.6 ꢀ0.97 ꢀ6.31 ꢀ3.74 2.57 27.61
520.3, 566.3,
716.1
PTDPTC 270.3,
338.4
270.2, 489.3, 0.8291(68.69), 3.8735(31.31) 1.55 ꢀ0.98 ꢀ6.26 ꢀ3.73 2.53 63.8
533.2, 566.8,
723.5
a
b
c
ꢀ
Absorption peak. Emission peak. Measured in 0.1 mol Lꢀ1 solution of Bu4N+ClO4 in CH3CN with a Pt electrode and Ag/AgCl reference
electrode. d EHOMO ¼ ꢀe(Eox + 4.82) (eV). e ELUMO ¼ ꢀe(Ered + 4.82) (eV). f Eg ¼ e(Eoxi ꢀ Ered) (eV).
ec
Thin film UV spectra (Table S1a†) exhibit blue shift when
compared with solution spectra, with 212 nm of maximum
absorption. PL signals in cast films are blue-shifted by about
55 nm with respect to solution. However, similar trend in peak
absorption and emission intensity observed in both solution and
thin film in which PHPTC shows maximum absorption intensity
and POPTC of maximum emission intensity. The emission peak
intensity shows unequal variation in solution, but not in thin film
PL emission (ESI†). The PL signals of APTC in cast films did not
exhibit a remarkable change with respect to the solution, sug-
gesting that the emission signal is generated by more than one
type of aggregate.
3.3. Morphology studies correlation with excitation
polarisation
Fig. 3 shows the AFM morphologies characterised over ITO
substrate, so it might be a reasonable model for the growth from
a technological view. The morphology results show the depen-
dence of alkyl chain length on supramolecular assembly. The
PHPTC polymer AFM micrograph (Fig. 3a) shows uniform
coarse grain surface domains of different orientation. The inset
image shows local equalized graph and line profile analysis of the
same AFM micrograph shows uniform symmetry and molecular
orientation adopted by the molecules. Local equalization enables
the contrast of fine structure of surface objects in case if the
topography has a number of specific scales, for example a fine
structure against the background of significant differences of
topography height with large characteristic length.
Fig. 2 Solution absorption and emission spectra of all (a) monomers (b)
polymer.
POPTC thin films show higher anisotropy when compared to
other polymers in different excitations. At 212 nm, POPTC and
PDPTC show higher linear dichroism when compared to other
polymers.19 Formation of H aggregates are commonly observed
for stacking of conjugated oligomers. Reports of J aggregate are
highly limited and usually require alignment or thin film
formation.20 The red shift found to be higher in the case of
POPTC and PTDPTC hints about the effect of aggregation in
solution. Solution spectra of PTDPTC exhibit higher red emis-
sion at 720 nm.
The POPTC polymer AFM micrograph (Fig. 3b) shows
circular domain and line profile analysis shows uniform micro-
graph scale of 890 nm thickness wall with varied radius of
circular domain. The intricate line profile analysis of supramo-
lecular domain show no uniform curvature of two profile height,
and hence it is not completely circular. The aggregates in the self
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likely to define the p stacking direction as the principal axis of
self-assembly. Additional change in the domain area reflects the
variation of the angle between the planes of the thiophene rings
and ester group with alkyl chain. The PDPTC (Fig. 3c) polymer
too shows a similar pattern as POPTC but the domain is not
continuous because of the mismatch (circle in graphics and AFM
image Fig. 3) in aggregate pattern with change in torsional angle
in main chain. The PTDPTC polymer shows higher aggregation
with worm like pattern and inset local equalisation graph shows
visible molecular orientation with maze like pattern in local
equalized graph.21
Small angle X-ray diffraction (Fig. 3e) shows sharp peaks
responsible for close construction of molecules and strong
intermolecular p–p stacking by aggregation-induced planariza-
tion of the molecule.22 The sharp reflection at the low angle (2q)
ꢁ
ꢀ
1.934 corresponds to a layer spacing of 35.0 A for PDDPTC and
this may be the reason for change in trend in electrochemical
band gap (Eg) obtained from CV due to the close overlap of
molecules. The POPTC SAXS spectrum shows second low angle
reflection and four additional reflection explicitly validate the
circular domains pattern in AFM. Increase in interlayer packing
in molecule increases the d spacing value and the number of
internal reflections among lattices increases. Interestingly the
peak pattern observed by POPTC looks similar to PHPTC SAXS
pattern; however, the 2q values are low for POPTC with one
additional reflection at 2q value 4.441ꢁ with d spacing value of
ꢀ
17.22 A due to short range molecular interaction. PTDPTC
shows broad reflection with one sharp peak at 4.438ꢁ with d
ꢀ
spacing 17.44 A.
Excitation polarisation experiment was used to detect the
strength of the excitonic coupling depending on the different
molecular branches p–p localized or delocalized excited states
with important consequences for the material properties and
behaviour whether the resulting emission can be characterized in
one of two ways; either the emission is isotropic, or it is aniso-
tropic. Fig. 4 shows excitation polarisation of PHPTC and
POPTC and the bottom bar graph shows the normalized pho-
toluminescence (PL) intensity at various emission regions versus
effect of alkoxy chain length. Also, the dichroic ratio values
calculated and presented in Fig. 4. PHPTC polymer shows
highest PL emission (Fig. 4c) compared to all other polymers.
The reduced PL intensity in higher aggregates is due to the
precipitation of aromatic residues with formation of amorphous
aggregates, in most cases, the excimer and exciplex formation
caused by the intermolecular p–p stacking induces collisional
interactions between molecules in the ground and excited states
and results in subsequent fluorescence quenching. PL polar-
isation experiment23 shows polariser angle 180ꢁ of maximum PL
output; it might be the molecular axis direction of optical tran-
sition dipole of the polymer. It is noteworthy that the maxima/
minima of PL output varies with the change in structural
morphology, PHPTC polymer (Fig. 4a) PL polarisation emission
spectra shows three maxima and two minima of emission,
whereas in the case of POPTC polymer (Fig. 4b) only one
maxima of emission is observed. This may be attributed to the
intense bisignate excitonic space coupling of two or more chro-
mophores. To clarify the optical anisotropy induced by the
internal molecular orientation of polymer, the dichroic ratio was
calculated by taking ratio of maxima and minima of emission in
Fig. 3 AFM scanned images of (a) PHPTC (b) POPTC, (c) PDPTC, (d)
PTDPTC inset graph shows local equalized graph of respective polymer
the below figure is their respective line profile analysis. (e) SAXS of all
ꢀ
polymers the inside value shows the respective ‘d’ spacing value in A. (f)
Schematic representation of ‘J aggregate’ leading to circular domain
formation.
assembling system studied here leads us to hypothesize that J
aggregates of the thiophene moieties are formed through bundle
interactions and forms hierarchical structures as shown in
Fig. 3f. Because hydrogen bonding is typically stronger than the
p–p interactions of oligothiophene, the orientation of hydrogen
bonds among azomethine with stacking of thiophene rings is
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Fig. 4 Excitation polarisation data of (a) PHPTC and (b) POPTC measured by varying the polariser angle with constant excitation wavelength. (c) PL
intensity values at various colour region, (d) polarisation dichroic ratio values of all samples at various colour region.
various regions. The results obtained from PL dichroic ratio
replicates the AFM morphology output of circular domain
pattern that PDPTC polymer shows higher dichroic ratio denote
maximum anisotropy in it. PTDPTC shows higher dichroic ratio
for 719 emissions of all polymers.
polymers, as supported by AFM analysis. All APTC polymers
other than POPTC, show single hydrodynamic radius distribu-
tion indicates the supramolecular assembly exists in solution
itself for POPTC polymer.25 The first peak in (Fig. 5b) represents
the radius of gyration and the second peak corresponds to the
length of the cylinder shape. That is to say that we expected the
first polymer binding event to enhance the affinity of the second
(and, for 2, the third) polymer binding event to form aligned
supramolecular assemblies; such a process is necessary if we are
to avoid the formation of random assemblies.
The major change in dichroic ratio of all polymers is due to the
tilted stacking of the p-conjugated backbone of molecules, which
gives rise to an angle between the transition dipole moment and
the molecular axis. This internal molecular alignment results in
the anisotropy of refractive index and produces strong birefrin-
gence and polarized PL emission. This result also symbolizes that
the PL emission sensitivity differentiates the variation in self-
assembly which is usually driven by weak intermolecular forces
such as hydrogen bonding, p–p stacking, or van der Waals
forces, p-conjugated segments can be rationally designed to have
their strongest p–p interactions along the principal axis.
Fluorescence life time measurement was done to prioritize the
effect of aggregation and fluorescence quenching. Table 1
summarizes fluorescence life times estimated on the basis of
fitting curves for fluorescence decays measured in DCM (at
25 ꢁC). All of the polymers exhibit bi-exponential anisotropy
decays in DCM solution. However, the relative time of decay of
two decays varies with the aggregation of polymers.26 In all the
cases, the higher percentage of species shows lower FL life time
scale. Note that highly aggregated polymers show long lived first
populated FL lifetime than second one. PDPTC polymer shows
higher life time of about 5.1079 ns (16.88%) and POPTC
87.545 ns (73.73%).
3.4. Dynamic light scattering (DLS) and fluorescence life time
measurements
The physical–chemical properties of scattering objects present in
the dilute solutions were probed using dynamic light scattering
(DLS). The hydrodynamic radius (Rh) of the particles, was
calculated from the Stokes–Einstein relation.24 The size distri-
bution of the nanostructures depends on the proportion of alkyl
side chains and variation in torsional angle in thiophene with
respect to it. PHPTC polymer exhibits very low Rh of all
3.5. Charge carrier mobility
Charge transport of semiconducting polymers varies with
process parameters such as the solvent boiling point, film
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Chuanjun Xia and co-workers30 reported the same type of
micro domain for electro polymerised, surface grafted self
assembled monolayer of carbazole and the formation of micro
domain. The cause for this micro domain is not due to air bubble
formation over electrode surface and the property is only thin
film dependent. However, the above paper has failed to find the
cause for it. The cause for the formation is studied in detail for
our system and interesting results of anisotropy in solution from
DLS experiments were obtained. Since the above paper is on a
self assembled monolayer of carbazole, it can be believed that the
molecular ordering in our system is also better.
4. Conclusions
The self-assembling behaviour of these polymers has been
systematically investigated to reveal the relationship between the
supramolecular self-assembly and their chemical structures.
Intricate analysis of polymer self assembly is studied in detail by
changing the end alkyl group. The AFM and XRD analyses
show the circular domain formation for POPTC and PDPTC.
UV-visible and fluorescence analysis supports J-aggregate
formation. The change in alkyl chain length brings a change in
the central unit and thiophene torsional angle with formation of
various supramolecular assemblies.
Acknowledgements
The authors would like to thank CSIR-NWP55 for funding and
the authors also thank Dr A. Ajayaghosh and Mr R. Thirumalai,
NIIST, Trivandrum for their help in AFM studies. Authors
would also like to thank A. Prasannan and Po-Da Hong
National Taiwan University of Science and Technology, Taipei-
10607, Taiwan and W. T. Chung of National Synchrotron
Radiation Research Center (NSRRC), Taiwan for Synchrotron
XRD measurement.
Fig. 5 Hydrodynamic radius (Rh) of the polymer solution (ꢂ10ꢀ6 M)
using dynamic light scattering (a) PHPTC, (b)POPTC, (c) PDPTC,
PTDPTC.
Notes and references
1 O. Bubnova, Z. Ullah Khan, A. Malti, S. Braun, M. Fahlman,
M. Berggren and X. Crispin, Nat. Mater., 2011, 10, 429; Y. Yuan,
T. J. Reece, P. Sharma, S. Poddar, S. Ducharme, A. Gruverman,
Y. Yang and J. Huang, Nat. Mater., 2011, 10, 296; G. Gunbas and
L. Toppare, Chem. Commun., 2012, 48, 1083.
2 P. M. Beaujuge, S. Ellinger and J. R. Reynolds, Nat. Mater., 2008, 8,
795.
3 Y. Song, W. Wei and X. Qu, Adv. Mater., 2011, 23, 4215.
4 C. Ye, M. Li, J. Luo, L. Chen, Z. Tang, J. Pei, L. Jiang, Y. Song and
D. Zhu, J. Mater. Chem., 2012, 22, 4299.
5 A. Ciesielski, C. A. Palma, M. Bonini and P. Samorı, Adv. Mater.,
2010, 22, 3506; J. Liang Li and X. Yang Liu, Adv. Funct. Mater.,
2010, 20, 3196.
6 K. Helttunen, K. Salorinne, T. Barboza, H. C. Barbosa, A. Suhonen
and M. Nissinen, New J. Chem., 2012, 36, 789; J. Choi, S. H. Kim,
W. Lee, C. Yoon and J. P. Kim, New J. Chem., 2012, 36,
812.
7 J. Matsui, S. Yoshida, T. Mikayama, A. Aoki and T. Miyashita,
Langmuir, 2005, 21, 5343.
8 K. Venkata Rao, K. Jayaramulu, T. Kumar Maji and S. J. George,
Angew. Chem., Int. Ed., 2010, 49, 4218.
9 B. P. Roderick, J. L. F. Mary, E. L. Foster and R. C. Advincula, ACS
Appl. Mater. Interfaces, 2011, 3, 817.
10 R. R. Ramireddy, K. R. Raghupathi, D. A. Torres and
S. Thayumanavan, New J. Chem., 2012, 36, 340.
11 M. R. Pinto and K. S. Schanze, Synthesis, 2002, 9, 1293; C. V. Hoven,
A. Garcia, G. C. Bazan and T. Q. Nguyen, Adv. Mater., 2008, 20,
deposition method (drop casting versus spin coating), and
modifying the semiconductor–dielectric interface,27 all of which
directly impact the self organization of polymer and supramo-
lecular assembly. Results of charge carrier mobility actuate
the same effect of molecular assembly with lower mobility for
POPTC and then attain a maximum mobility of 63.8 ꢂ 10ꢀ4
cm2 V sꢀ1 for PTDPTC. Zen et al. illustrated decrease in
mobility is primarily due to the larger distortion of the
conjugated backbone and it has significant impact on interchain
interactions, thus lowering the mobility.28 Thus, the coexistence
of ordered and disordered domains implies that charge trans-
port would be strongly dependant on the nature of both regions
reliant on molecular packing structure. The ramification of
alkyl chain brings in change of phenyl rings aggregate, creates
large overlap of p orbital and generates a large bandwidth for
charge transfer. This may also be due to large-scale phase
separation, decreased diffusional escape probability for
mobile charge carriers, and hence increased recombination of
charges.29
ꢁ
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 18975–18982 | 18981

View Article Online
3793; H. Jiang, P. Taranekar, J. R. Reynolds and K. S. Schanze,
Angew. Chem., Int. Ed., 2009, 48, 4300.
12 A. Duarte, K. Y. Pu, B. Liuand and G. C. Bazan, Chem. Mater., 2011,
23, 501.
ꢂ
13 J. Torras, C. Sanchez-Navas, O. Bertranb and C. Aleman, Phys.
Chem. Chem. Phys., 2012, 14, 1881.
14 B. W. Messmore, J. F. Hulvat, E. D. Sone and S. I. Stupp, J. Am.
Chem. Soc., 2004, 126, 14452; F. S. Schoonbeek, J. H. van Esch,
B. Wegewijs, D. B. A. Rep, M. P. de Haas, T. M. Klapwijk,
R. M. Kellogg and B. L. Feringa, Angew. Chem., Int. Ed., 1999, 38,
1393; S. Yagai, T. Kinoshita, Y. Kikkawa, T. Karatsu,
A. Kitamura, Y. Honsho and S. Seki, Chem.–Eur. J., 2009, 15, 9320.
15 S. Radhkrishnan, N. Somanathan and T. Narasimhaswamy, J.
Polym.Sci., Part B: Polym. Phys., 2008, 46, 1463.
21 M. C. Quiles, T. Ferenczi, T. Agostinelli, P. G. Etchegoin, Y. Kim,
T. D. Anthopoulos, P. N. Stavrinou, D. D. C. Bradley and
J. Nelson, Nat. Mater., 2007, 7, 158.
22 H. Peterlik and P. Fratzl, Monatsh. Chem., 2006, 137,
529.
23 Q. Bao, B. Min Goh, B. Yan, T. Yu, Z. Shen and K. Ping Loh, Adv.
Mater., 2010, 22, 3661.
24 V. Petta, J. Moradian-Oldak, S. N. Yannopoulos and
N. Bouropoulos, Eur. J. Oral Sci., 2006, 114, 308.
25 F. Qiu, C. Tu, Y. Chen, Y. Shi, L. Song, R. Wang, X. Zhu, B. Zhu,
D. Yan and T. Han, Chem.–Eur. J., 2010, 16, 12710; K. Aissou,
€
A. Pfaff, C. Giacomelli, C. Travelet, A. H. E. Muller and
R. Borsali, Macromol. Rapid Commun., 2011, 32, 912.
26 K. Viertler, A. Wewerka, F. Stelzer, G. Fytas and D. Vlassopoulos,
Macromol. Chem. Phys., 2001, 202, 3174.
27 Y. Z. Wu, C. R. Zhang and D. J. Zhang, Appl. Phys. Lett., 2009, 95,
033508.
16 V. Seshadri, L. Wu and G. A. Sotzing, Langmuir, 2003, 19, 9479;
S. Radhakrishnan and N. Somanathan, J. Mater. Chem., 2006, 16, 2990.
17 G. A. Sotzing and K. Lee, Macromolecules, 2002, 35, 7281.
€
€
18 F. Wurthner, T. E. Kaiser and C. R. Saha-Muller, Angew. Chem., Int.
Ed., 2011, 50, 3376; R. H. Chien, C. T. Lai and J. L. Hong, J. Phys.
Chem. C, 2011, 115, 5958.
28 A. Zen, J. Pflaum, S. Hirschmann, W. Zhuang, F. Jaiser,
U. Asawapirom, J. P. Rabe, U. Scherf and D. Neher, Adv. Funct.
Mater., 2004, 14, 757.
19 D. M. Eisele, J. Knoester, S. Kirstein, J. P. Rabe and D. A. Vanden
Bout, Nat. Nanotechnol., 2009, 4, 658; E. T. Niles, J. D. Roehling,
H. Yamagata, A. J. Wise, F. C. Spano, A. J. Moule and J. K. Grey,
J. Phys. Chem. Lett., 2012, 3, 259.
20 E. Da Como, M. A. Loi, M. Murgia, R. Zamboni and M. Muccini,
J. Am. Chem. Soc., 2006, 128, 4277.
29 G. Adam, A. Pivrikas, A. M. Ramil, S. Tadesse, Tl. Yohannes,
N. S. Sariciftcia and D. A. M. Egbe, J. Mater. Chem., 2011, 21,
2594; W. Zhang, K. Ochi, M. Fujiki, M. Naito, M. Ishikawa,
K. Kaneto, W. Takashima, A. Saeki and S. Seki, Adv. Funct.
Mater., 2010, 20, 3941.
30 C. Xia and R. C. Advincula, Chem. Mater., 2001, 13, 1682.
18982 | J. Mater. Chem., 2012, 22, 18975–18982
This journal is ª The Royal Society of Chemistry 2012