One-pot direct synthesis of fluorescent polyaniline-porphyrin macrospheres from porphyrin

Article abstract of DOI:10.1002/pola.25840

Porphyrins are very important chromophores as sensitizer in solar cell. Hole and electron transport layers are being used as an important layer to move the electrons and holes away from the sensitizer molecule to electrodes. In this work, a simple process

Full text of DOI:10.1002/pola.25840

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One-Pot Direct Synthesis of Fluorescent Polyaniline-Porphyrin
Macrospheres from Porphyrin
Singh Ajit,1 Srinivasan Palaniappan,1 Putcha Uday Kumar,2 Perikati Madhusudhanachary2
1Polymers & Functional Materials Division, Indian Institute of Chemical Technology, Hyderabad, Andhra Pradesh 500607, India
2Pathology and Electron Microscope Division, National Institute of Nutrition, Hyderabad, Andhra Pradesh 500607, India
Correspondence to: S. Palaniappan (E-mail: palani74@rediffmail.com)
Received 11 October 2011; accepted 1 November 2011; published online 28 November 2011
DOI: 10.1002/pola.25840
ABSTRACT: Porphyrins are very important chromophores as
sensitizer in solar cell. Hole and electron transport layers are
being used as an important layer to move the electrons and
holes away from the sensitizer molecule to electrodes. In this
work, a simple process has been developed for the synthesis
of one layer combination of sensitizer and hole transport, that
is, polyaniline-porphyrin (PANI-TPPS4) system as a bulk hetero-
junction. Macrospheres of fluorescent PANI-TPPS4 materials
were synthesized by one-pot as well as two step processes
directly from porphyrin for the first time. The advantage of this
process is no need to isolate sulfonated porphyrin (TPPS4),
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which is a difficult process.
2011 Wiley Periodicals, Inc.
J Polym Sci Part A: Polym Chem 50: 884–889, 2012
KEYWORDS: conducting polymer; fluorescent polyaniline; mac-
rospheres; fluorescence; synthesis
INTRODUCTION Porphyrins are among the most-studied or-
ganic chromophores, not only because of it’s role as a biolog-
ical light absorber, redox center, and oxygen carrier but also
of it’s attractive chemical properties and potential technologi-
cal applications. Interest in the use of porphyrins as light-
absorbing sensitizers in electrochemical photovoltaic has
intensified over recent years.1–5 The work of porphyrin in so-
lar cell system is to absorb the photolight and then it has to
convert absorbed photolight to intramolecular electrochemical
potential in the form of a charge-separated state as a stored
energy. A next challenge is the movement of electrons and
oxidizing equivalents (holes) away from the molecule to elec-
trodes. One of the ways to achieve it, is by using conducting
polymer with porphyrin that can move electrons or holes
from the site of their generation to a conductor or electrode.6
This has been tried out for with the use of polyaniline, poly-
thiophene, polypyrrole as a separate layer (hole transport)
with dye molecules as a sensitizer in a solar cell devices.7–9
Water soluble meso-tetrakis(4-sulfonatophenyl)porphyrin
(TPPS4) is a very important porphyrin which have all the
characteristics of porphyrin and can be used as a dopant in
PANI synthesis. According to our knowledge, very few works
on PANI-TPPS4 have been reported very recently. Lakshmi-
narayanan et al.13 have synthesized PANI-TPPS4 by doping of
DMF solution of PANI base with DMF solution of TPPS4 for
the study of optical nonlinearity. In the other work, Zhou
et al.14 have synthesized PANI-TPPS4 by electrochemical
polymerization of aniline using Co-TPPS4 for oxygen reduc-
tion. Isolation of sulfonated porphyrin is very difficult one,
very costly and less availability in the market. Hence, synthe-
sis of PANI-TPPS4 using TPPS4 is a difficult process.
In this work, a simple process has been developed for the
synthesis of one layer combination of sensitizer and hole
transport material, that is, polyaniline-sulfonated porphyrin
system as a bulk heterojunction. Macrospheres of fluorescent
polyaniline-sulfonated porphyrin materials were synthesized
by one-pot as well as two step processes directly from por-
phyrin (Fig. 1). The advantage of this process is no need to
isolate sulfonated porphyrin, which is a difficult process and
also less available costly material.
For the combination of porphyrin with conducting polymer, poly-
aniline (PANI) is a very good candidate and from last two decades
it has been widely studied for the electronic and optical applica-
tion.10 PANI has three basic oxidized structures (leucoemeraldine,
emeraldine, and pernigraniline) with different oxidation states,
which are chemically and electrochemically switchable. This
unique property of PANI can be utilized in electron transfer pro-
cess, serving as either electron acceptor (hole acceptance)11 or
electron donor (hole injection)12 depending on the relative posi-
tions of redox potential of excited dye molecule and PANI.
EXPERIMENTAL
Materials
Pyrrole and aniline were purchased from Sigma-Aldrich and
distilled before use. Benzaldehyde, ammonium persulfate,
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FIGURE 1 Synthesis scheme of polyaniline-sulfonated porphyrin directly from TPP by one-pot and two step processes.
sodium hydroxide, and sulfuric acid analytical grade, were
purchased from (S.D. Fine chemicals, India) and used with-
out further purification. Solvents were distilled before use.
of benzaldehyde (0.8 mol) was added in 5 min. duration.
The reaction mixture was further refluxed for 40 min. and
then cooled to room temperature. Then reaction mixture
was filtered, washed with hot distilled water followed by
methanol until colorless filtrate. Purple colored residue was
PANI salt powder was characterized by various techniques.
FTIR spectra of PANI powder was registered on a GC-FTIR
spectrometer 670 Nicolet Nexus (Minnesota) using KBr pel-
let technique. The absorption and fluorescence spectra of the
compounds were recorded on a Jasco 7800 UV–Vis spectro-
photometer (Essex, UK) and FP 777 spectrofluorimeter,
respectively. Morphological studies of PANI powder was per-
formed using Hitachi S3400 scanning electron microscope
(Tokyo, Japan) operating at 15 kV. The sample was mounted
on a double-sided adhesive carbon disk and sputter-coated
with a thin layer of gold (500 Å) to prevent sample from
possible charging. For the determination of the conductivity,
PANI powder was pressed into disk of 13 mm diameter and
about 1.5 mm thickness under a pressure of 400 MPa. The
resistance of the pellet was measured by a four probe
method using constant current source (6220) and nanovolt-
meter (2182A) (Keithley, Cleveland, OH).
ꢀ
obtained and dried in oven at 50 C. Yield - 12.5 g (20%).
Two Step Synthesis of PANI-TPPS4 from TPP
Sulfonation of TPP was performed by using the reported
procedure.16 TPP (0.5 g) was mixed well with concentrated
H2SO4 (50 mL) taken in a 100 mL round-bottomed flask.
The reaction was carried out at 100–110 ꢀC for 4 h under
constant stirring and then cooled to room temperature. The
product was isolated by precipitating the reaction mixture in
acetone, filtered through G-4 sintered glass crucible, thor-
oughly washed with acetone to remove the unreacted TPP
and sulfuric acid. The green product (protonated form of
ꢀ
TPPS4) was dried in oven at 50 C. Yield – 55%.
The isolated protonated form of TPPS4 powder was first con-
verted into sodium salt of TPPS4 using aqueous NaOH solu-
tion. This aqueous sodium salt solution of TPPS4 was used
for the oxidative polymerization of aniline to PANI salt con-
taining TPPS4. In a typical experiment, 0.ꢀ1 g of TPPS4 was
dissolved in 30 mL distilled water at 0–2 C and changed to
sodium salt of TPPS4 by neutralization with dilute sodium
hydroxide solution (pH of the aqueous solution of sodium
salt of TPPS4 ¼ 7). Aniline (0.5 mL) was added in to the
Synthesis of Tetraphenylporphyrin (TPP)
Synthesis of TPP was carried out according to the earlier
reported procedure.15 Propionic acid (1 L) was taken in 2 L
two necked round bottomed flask and refluxed. During
refluxtion of propionic acid, pyrrole (18.6 mL, 0.8 mol) was
added slowly during the period of 5 min and then 26.6 mL
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FIGURE 2 FTIR spectrum of porous PANI-TPPS4 macrospheres
synthesized by one step process.
above aqueous mixture containing sodium salt of TPPS4 (ani-
line:sodium salt of TPPS4 ¼ 1:0.0015 mol ratio) under con-
stant stirring at 0–2 ꢀC, the pH of the reaction mixture was
changed to 7.3. Ammonium persulfate (1.44 g) was dissolved
in 15 mL of distilled water at 0–2 ꢀC (pH – 3.1) and added
to the above mixture slowly over a period of 5 min. On slow
addition of ammonium persulfate solution, the pH of the
reaction mixture changed to 4.8 and simultaneously, the
reaction mixture changed from purple color to green color.
The reaction mixture was constantly stirred at 0–2 ꢀC for
6 h. The final green color solution was filtered, washed with
water and followed by acetone, till the colorless filtrate. The
FIGURE 3 UV–Vis spectra of (a) PANI-TPPS4 and (b) TPPS4 in
DMSO.
RESULTS AND DISCUSSIONS
PANI-TPPS4 was synthesized by in situ chemical oxidation of
aniline in presence of sulfonated porphyrin, which is pre-
pared from porphyrin via two methods. In one-pot synthesis,
porphyrin was sulfonated, converted to sodium salt by neu-
tralizing with aqueous sodium hydroxide solution, then ani-
line was added, which is oxidized by ammonium persulfate to
PANI salt containing TPPS4. In two step synthesis, protonated
TPPS4 was first isolated in powder form by precipitation of
sulfonated porphyrin in acetone and in the second step, TPPS4
was neutralized with aqueous sodium hydroxide solution, then
aniline was added, which is oxidized by ammonium persulfate
to PANI-TPPS4. The conductivity of PANI-TPPS4 salt was found
to be 1.5 ꢁ 10ꢂ2 and 2.2 ꢁ 10ꢂ2 S cmꢂ1 prepared by one and
two step processes, respectively. In chemical oxidation process
of aniline by ammonium persulfate at room temperature,
ammonium persulfate reduces to ammonium sulfate, which
in turn decomposes to ammonia and sulfuric acid. Ammonia
evolves from reaction mixture and due to that acidity of the
ꢀ
powder was dried in oven at 50 C.
Direct Synthesis of PANI-TPPS4 Salt from TPP
TPP (0.5 g) was mixed well with concentrated H2SO4
(50 mL) taken in a 100 mL round-bottomed flask. The reac-
tion was carried out at 100–110 ꢀC for 4 h under constant
stirring and then cooled to room temperature. The reaction
mixture was diluted by slow addition of ice cold water
ꢀ
(50 mL). The green mixture was cooled to 5–10 C and then
carefully neutralized with NaOH up to pH ¼ 7. During this
neutralization process, green color was changed to purple
color, indicating the formation of sodium salt of TPPS4 along
with sodium sulfate in water. This sodium salt of TPPS4 is
used as dopant for in situ preparation of PANI salt contain-
ing TPPS4 by the following procedure. Aniline (1 mL) was
added in to the above aqueous mixture containing sodium
salt of TPPS4 (aniline:sodium salt of TPPS4 ¼ 1:0.0015 mol
ratio) under constant stirring at 0–2 ꢀC, the pH of the reac-
tion mixture was changed to 7.6. Ammonium persulfate
(2.8 g) was dissolved in 15 mL of distilled water at 0–2 ꢀC
(pH–2.7) and added to the above mixture slowly over a
period of 5 min. On slow addition of ammonium persulfate
solution, the pH of the reaction mixture changed to 4.9 and
simultaneously, the reaction mixture changed from purple
color to greenꢀ color. The reaction mixture was constantly
stirred at 0–2 C for 6 h. The final green color solution was
filtered, washed with water and followed by acetone, till the
FIGURE 4 (a) UV–Vis spectrum and (b) fluorescence spectrum
(excited at kmax ¼ 420 nm) of same PANITPPS4 solution in DMSO.
ꢀ
colorless filtrate. The powder was dried in oven at 50 C.
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FIGURE 5 (a) Scanning electron microscopic images at three different magnifications for PANI-TPPS4 synthesized via one-pot direct syn-
thesis. (b) Scanning electron microscopic images at three different magnifications for PANI-TPPS4 synthesized via two step synthesis.
solution increases (H2SO4) and sulfuric acid coordinate to
PANI as dopant. To avoid the decomposition of ammonium
sulfate and participation of H2SO4 as dopꢀant, PANI synthesis
was carried out in low temperature (0–2 C).
gen-quinone bond. Sharp and intense peak at 820 cmꢂ1 is
observed due to 1, 4-disubstituted benzene. These peaks are
very similar to the earlier report for PANI system.17 In addi-
tion to the above peaks, small peak at 3,270 cmꢂ1 is
observed which is due to the NHþ formation, indicating the
presence of doped form of PANI salt. Peaks observed at
1,040 and 1,620 cmꢂ1 are due to mS¼O stretching of TPPS4
present in PANI-TPPS4 salt. This result indicates the presence
of TPPS4 as dopant.
FTIR Study
To confirm the presence of porphyrin on PANI system, FTIR
spectrum of PANI-TPPS4 prepared by one-pot synthesis was
recorded (Fig. 2). Peak at 3,450 cmꢂ1 is due to mNAH str.
vibration, peaks at 3,055, 2,925, 2,850 cmꢂ1 are due to mCAH
str. in PANI. Sharp bands at 1,585 and 1,490 cmꢂ1 are
observed due to mC¼¼C str. in quinoid ring (N¼¼Q¼¼N) and ben-
zenoid rings (N¼¼B¼¼N), respectively. The characteristic mCAN
str. band for QBQ is observed at 1,380 cmꢂ1, for QBC, QBB,
UV–Vis Study
Electronic absorption spectral measurements of TPPS4 as
well as PANI-TPPS4 prepared by one-pot synthesis were car-
ried out in DMSO solution and are shown in Figure 3. The
absorbance spectrum of TPPS4 [purple color solution in
DMSO, Fig. 3(b) dotted line] shows a sharp and intense Soret
BBQ at 1,305 cmꢂ1 and for BBB aromatic at 1,215 cmꢂ1
.
Peak at 1,160 cmꢂ1 is due to the vibrational band of nitro-
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band in the high energy region at 420 nm, accompanied by a
shoulder at around 400 nm. In addition to the Soret band,
four weak low energy Q bands at 515, 551, 590, 646 nm are
also observed. UV–Vis spectrum of TPPS4 are in good agree-
ment with the earlier reports.18,19 UV–Vis spectrum of PANI-
TPPS4 [green color solution in DMSO, Fig. 3(a)] shows all the
characteristics band of TPPS4 as well as one extra band at
350 nm. The presence of extra band at 350 nm is due to the
presence of p!p* transition in PANI salt system.20 This
result indicates the formation of PANI-TPPS4. To confirm the
presence of TPPS4 as dopant on PANI base clearly, PANI-
TPPS4 was dedoped using aqueous sodium hydroxide solu-
tion. Electronic absorption spectrum of the filtrate (purple
color) was recorded and it showed Soret band at 414 and Q
bands at 515, 550, 586, 639 nm, which are similar to that of
the UV–Vis spectrum of TPPS4 [Fig. 3(b)]. This result clearly
indicates the presence of TPPS4 on PANI system.
becomes highly aggregates. According to Tabak and co-
workers19 at pH-4, ꢃ26 TPPS4 molecules aggregates and
form ring like structure. Scolaro and coworkers21 had proved
that aggregation can be fostered by lowering the pH and
increasing the ionic strength of the TPPS4 solution. In our
work reason for the aggregate formation is due to increasing
the acidity of reaction mixture during the addition of ammo-
nium persulfate solution as well as high ionic strength due
to presence of sodium sulfate. Addition of ammonium persul-
fate in the purple solution containing neutral TPPS4 and ani-
line, pH has immediately decreased to ꢃ4.9 and presence of
sodium sulfate increases the ionic strength many fold which
results aggregate formation. Another reason for spherical
morphology is the presence of tetraanionic form of TPPS4 in
PANI-TPPS4.
CONCLUSIONS
Fluorescent PANI-TPPS4 material was successfully prepared
by chemical method for the first time. Moreover, this mate-
rial was synthesized directly from TPP using simple and
cheaper process. The electrical and optical properties of this
polymer suggest that it may be very useful component of
solar energy conversion and LED devices as a bulk hetero-
junction material. This methodology may open up a new
area in materials and electronics field.
Fluorescence Study
Fluorescence and electronic absorption spectrum of PANI-
TPPS4 prepared by direct one-pot synthesis were recorded
for DMSO solution of PANI-TPPS4 and are shown in Figure 4.
Fluorescence spectrum [Fig. 4(b)] was measured for the exci-
tation at kmax 420 nm and observed two bands at 655 nm
and at 718 nm, which are similar to the fluorescence spec-
trum of the tetraanion.13,18 According to Koti and Perias-
amy18 TPPS4 in the tetraanion form shows two emission
bands, whereas dianion emits only single band at 670 nm.
This confirms that TPPS4 is present in tetraanionic form in
PANI-TPPS4 salt.
This research work was carried out in the framework of Council
of Scientific and Industrial Research (CSIR) net work project
(NWP-0025) financed by CSIR, New Delhi, India. One of the
authors (Ajit) is indebted to CSIR, India for Senior Research
Fellowship.
Generally, a similar FTIR, electronic absorption and fluores-
cence spectrum was observed for the PANI-TPPS4 synthe-
sized in two steps when compared to that of the directly
synthesized PANI-TPPS4 sample.
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