Heterogeneously catalyzed phosphine-free heck cross-coupling reaction of aryl halides with reusable palladium(II) schiff base complex

Article abstract of DOI:10.1590/S0103-50532011000200018

The coupling between aryl halides with terminal alkenes (Heck reaction) has been investigated in the presence of a polystyrene-supported Schiff base palladium(II) catalyst. The present catalyst is air and moisture-stable and has significant catalytic activity in Heck cross-coupling reactions under operating conditions. Various aryl halides and terminal alkenes were coupled smoothly under air, to afford the corresponding cross-coupled products in excellent yields. Furthermore, the catalyst can be easily recovered quantitatively by simple filtration and reused up to seven cycles without losing its activity.

Full text of DOI:10.1590/S0103-50532011000200018

J. Braz. Chem. Soc., Vol. 22, No. 2, 319-326, 2011.
Printed in Brazil - ©2011 Sociedade Brasileira de Química
0103 - 5053 $6.00+0.00
Article
A
Heterogeneously Catalyzed Phosphine-Free Heck Cross-Coupling Reaction of Aryl
Halides with Reusable Palladium(II) Schiff Base Complex
Manirul Islam,*,a Paramita Mondal,a Kazi Tuhinab and Anupam S. Roya
aDepartment of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B. India
bDepartment of Chemistry, B.S. College, S-24 P.G.S., 743329, W.B. India
O acoplamento entre haletos de arila e terminais alceno (reação de Heck) foi investigado
na presença de catalisadores de paládio (II) suportados em poliestirenos contendo bases de
Schiff. O catalisador é estável frente ao ar e umidade, e tem atividade catalítica significativa
nas reações de acoplamento cruzado de Heck sob condições normais. Vários haletos de arila e
alcenos terminais foram facilmente acoplados na presença de ar, fornecendo os correspondentes
produtos de acoplamaneto cruzado, em ótimos rendimentos. O catalisador é facilmente recuperado
quantitativamente por filtração simples e reutilizado por até sete ciclos sem perda de atividade.
The coupling between aryl halides with terminal alkenes (Heck reaction) has been investigated
in the presence of a polystyrene-supported Schiff base palladium(II) catalyst. The present catalyst
is air and moisture-stable and has significant catalytic activity in Heck cross-coupling reactions
under operating conditions. Various aryl halides and terminal alkenes were coupled smoothly
under air, to afford the corresponding cross-coupled products in excellent yields. Furthermore, the
catalyst can be easily recovered quantitatively by simple filtration and reused up to seven cycles
without losing its activity.
Keywords: polystyrene, schiff base, palladium(II) complex, heterogeneous catalysis, Heck
reaction
Introduction
of homogeneous phosphine catalysts is the contamination
of products by palladium-phosphine and separation of
products from the catalytic media. From an industrial as
well as an economic standpoint, a phosphine-free process
would provide much needed impetus to the development
of improved catalyst systems for Heck couplings.21-25
Further, such a process would be advantageous for
synthetic chemists who would generally prefer not to use
expensive and air-sensitive phosphine ligands. In addition,
the elimination of phosphines would be welcome because
industrial wastes containing them would require treatment
for environmental purposes.
Theheterogenizationofhomogeneouscatalyticreactions
has attracted great attention from a wide range of organic
chemists over the last few decades.26-30 Immobilizations of
homogeneous catalysts into insoluble polymeric supports
by covalent bonding or complexation enables the easy
recovery and reuse of catalysts. Polystyrene is one of the
most widely employed macromolecular supports.31-34 This
supported material offers several advantageous features in
heterogeneous catalysis such as thermal stability, selectivity
Transition metal-catalyzed carbon-carbon bond forming
reactions are widely employed methods in modern organic
chemistry.1-3 The use of such catalytic procedures has found
applications in the preparation of many natural products,
as well as in materials science and the agrochemical
industry.4-7 With various metals being employed in coupling
reactions, palladium probably is the most frequently chosen
transition metal due to its excellent catalytic efficiency in
this type of reactions.8-12 The palladium catalyzed Heck
cross-coupling reaction of aryl halides with terminal
alkenes, which provides a powerful tool for the formation
of aryl-alkenes,13-15 has been widely applied to such diverse
areas as natural product syntheses and material sciences.
The most common catalytic system used for this reaction
is the homogeneous palladium-phosphine complexes16-20
as PdCl2(PPh3)2 or Pd(PPh3)4 in presence of base. One key
challenge for the commercial development and practical use
*e-mail: manir65@rediffmail.com

320
Heterogeneously Catalyzed Phosphine-Free Heck Cross-Coupling Reaction
J. Braz. Chem. Soc.
Pd(OAc)2
PhCHO
NH2
N=CH
N
OAc
OAc
HC
(1)
Pd
Pd
CH
(2)
N
CH CH2
(3)
Polystyrene (PS)
n
Scheme 1. Synthesis of polymer-anchored Pd(II) Schiff base complex.
and recyclability to homogeneous systems and ease of
their separation from the reaction products also leading to
operational flexibility.
Our continuing interest in palladium-catalyzed organic
transformations35-38 prompted us to extend our attention
to the palladium catalyzed Heck reaction. Herein we
have reported an efficient procedure for a heterogeneous
phosphine-free Heck cross-coupling reaction of aryl halides
with terminal alkenes using PS-[(C6H4CH=N)Pd(OAc)]2 as
the catalyst and K2CO3 as the base.
(Figure 1B) is shown. The SEM images of polymer-
anchored ligand and metal complex clearly shows the
morphological change which occurred on the surface of
the polymer matrix after loading of the metal (Figure 1).
Flake like particles are observed in case of polymer-
anchored ligand. After metal loading spherical shape of
the particles are quite visible throughout the specimen.
Further EDX analysis of polymer-anchored metal complex
shows palladium content along with C which suggested the
formation of metal complex with the anchored ligand. The
EDX profile of catalyst is presented in Figure 2.
Results and Discussion
The outline for the preparation of the polymer-anchored
Schiff base complex is presented in Scheme 1. It was
readily prepared through a two-step procedure.36 Firstly, the
polystyrene amine (1) (1 g) was reacted with benzaldehyde
(3 mL) in dry toluene (10 mL) under reflux condition for
72 h to afford polymer Schiff base ligand (2). This polymer-
anchored Schiff base ligand (1 g) subsequently reacted with
palladium acetate (0.5 g) in acetic acid (10 mL) at 80 oC for
10 h to generate the corresponding polystyrene-supported
palladium(II) complex (3).
Figure 1. FE SEM image of polymer-anchored Schiff base ligand (A) and
polymer-anchored palladium(II) Schiff base complex (B).
Due to insolubility of the polymer-anchored Pd(II)
complex in all common organic solvents, its structural
investigations was limited only to its physico-chemical
properties, scanning electron microscope-microprobe
analyses (SEM-EDX), thermogravimetric-differential
thermal (TGA-DTA), infrared (IR) and ultraviolet-visible
(UV-Vis) spectral data. The complete incorporation of
the organic substructure in the material was confirmed
by elemental analysis. The metal content of polymer-
anchored Pd(II) complex determined by atomic absorption
spectrophotometer (AAS) suggested 2.23 wt% metal
loading in the immobilized palladium complex. In Figure 1
the FE SEM image of polystyrene anchored Schiff base
ligand (Figure 1A) and anchored palladium complex
Figure 2. EDX spectrum of PS[(C6H4CH=N)Pd(OAc)]2 complex.

Vol. 22, No. 2, 2011
Islam et al.
321
o
The catalyst is thermally stable up to 250 C. The IR
spectrum of polymer-anchored Schiff base ligand showed
characteristic IR peak at 1635 cm-1 may be assigned to the
-C=N stretching vibration of imine which on complexation
shifted towards lower frequency to 1615 cm-1 suggested
bond formation between Pd and ligand. Other characteristic
peaks at 1585, 1430 cm-1 (νCOO bridged),39 720 cm-1
(orthometallation),40 and 455 cm-1 (νPd-N)41 also support
the formation of palladium(II) complex. The UV-Vis spectra
provided further evidence for the presence of palladium on
polymer support. The absorption maxima at 295 nm may
be attributed to the π→π* transition in polymer and phenyl
moiety and the absorption at higher range (370-440 nm)
may be due to the π→π* and n→π* transitions of imine
system in conjugation with the aromatic nuclei.
Figure 3. The influence of reaction temperature on Heck cross-coupling
reaction.
In order to evaluate the catalytic activity of the
polystyrene-supported palladium(II) Schiff base complex,
the Heck cross coupling reaction of aryl halides with
terminal alkenes was studied (Scheme 2). Initially, to
determine the optimum reaction conditions, the influences
of metal precursors, temperatures, solvents, bases,
base-substrate ratio and amounts of the catalyst on the
catalytic property of the present palladium complex were
investigated taking the reaction of 4-bromonitrobenzene
with styrene as model reaction. Since no air-sensitive
phosphine was used, all the coupling reactions were
conducted under air atmosphere.
PdCl2 did not show competitive result in this case (Table 1,
entry 5). Using metal salts without any ligand gave partial
conversion of the substrate (Table 1, entries 6, 7).A control
experiment indicated that the coupling reaction did not
occur in the absence of catalyst (Table 1, entry 18).
Another essential factor which affects the performance
of a catalytic reaction is the amount of catalyst used.
In order to optimize the catalyst amount, different
amounts of catalyst were used in the coupling reaction
of 4-bromonitrobenzene with styrene. The results are
presented in Table 1 (entries 1-4). The best results were
obtained with 0.5 mol% of Pd catalyst (Table 1, entry 3).
The Heck cross coupling reaction was found to be highly
dependent on the nature of solvent.As reported in literature,
polar aprotic solvents tend to give the best results for the
Heck coupling reaction.42,43 We employed several solvents in
the Heck model reaction. Among evaluated polar and non-
polar solvents, dimethylformamide (DMF) was the most
productive solvent (Table 1, entry 3) which is consistent
with the previous report. Other polar aprotic solvents, such
as dimethylsulfoxide (DMSO) and acetonitrile (MeCN),
gave moderate conversion (Table 1, entries 9, 10). Non
polar solvent, such as toluene, affords very poor conversion
(Table 1, entry 11). Extensive application of eco-friendly
solvents such as water in chemical reaction is a touching
appeal of green chemistry.44-46 We also performed the
model coupling reaction in neat water but yield was not
satisfactory (Table 1, entry 12). A mixed solvent system,
however, did show better result (Table 1, entry 13).
R2
Catalyst
+
R2
X
R1
R1
base, solvent
X = l, Br, Cl
R1 = H, COMe, NO2,CH3, OCH3
R2 = C6H5, COOMe
Scheme 2. Heck cross-coupling reaction.
The reaction temperature has a very strong influence on
the Heck reaction. No product formation was observed at
room temperature. As the temperature increased gradually
o
from 30-90 C, the conversion of 4-bromonitrobenzene
increased. We found that the reaction performed at 90 oC
appeared to provide the best result and highest yield of
coupled product was obtained (Figure 3).
The catalytic performance of a coupling reaction is
also greatly affected by the particular metal precursors
employed. Pd(OAc)2 and PdCl2 as metal precursors
attached with polystyrene anchored Schiff base ligand were
studied for the coupling reaction of 4-bromonitrobenzene
and styrene. As shown in Table 1, an excellent yield
was obtained with Pd(OAc)2 as palladium source
(Table 1, entry 3). Unexpectedly, the commonly used
The effect of base on the catalytic performance of
this system was investigated by taking the model reaction
in different bases. Among several organic and inorganic
bases, potassium carbonate (K2CO3) was found to be the

322
Heterogeneously Catalyzed Phosphine-Free Heck Cross-Coupling Reaction
J. Braz. Chem. Soc.
Table 1. Optimal reaction conditions for Heck cross-coupling reaction of 4-bromo nitrobenzene and styrenea
Catalyst
O2N
+
O2N
Br
Cat. Conc.
mol% of Pd
entry Catalyst
Solvent
Base
Base:Substrate
time (h)
Yieldb (%)
1
PS[(C6H4CH=N)Pd(OAc)]2
0.1
DMF
DMF
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
NaOH
K3PO4
Et3N
2:1
2:1
2:1
2:1
2:1
2:1
2:1
1:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
-
8
8
23
2
PS[(C6H4CH=N)Pd(OAc)]2
PS[(C6H4CH=N)Pd(OAc)]2
PS[(C6H4CH=N)Pd(OAc)]2
PS[(C6H4CH=N)Pd(Cl)]2
Pd(OAc)2
0.25
0.5
70
3
DMF
8
97
4
0.75
0.5
DMF
8
97
5
DMF
8
69
6
0.5
DMF
8
63
7
PdCl2
0.5
DMF
8
48
8
PS[(C6H4CH=N)Pd(OAc)]2
PS[(C6H4CH=N)Pd(OAc)]2
PS[(C6H4CH=N)Pd(OAc)]2
PS[(C6H4CH=N)Pd(OAc)]2
PS[(C6H4CH=N)Pd(OAc)]2
PS[(C6H4CH=N)Pd(OAc)]2
PS[(C6H4CH=N)Pd(OAc)]2
PS[(C6H4CH=N)Pd(OAc)]2
PS[(C6H4CH=N)Pd(OAc)]2
PS[(C6H4CH=N)Pd(OAc)]2
No catalyst
0.5
DMF
8
52
9
0.5
DMSO
MeCN
Toluene
H2O
8
85
10
11
12
13
14
15
16
17
18
19
0.5
8
76
0.5
8
18
0.5
8
11
0.5
H2O-DMF
DMF
8
45
0.5
8
83
0.5
DMF
8
70
73
0.5
DMF
8
0.5
DMF
8
39
0.5
DMF
K2CO3
No base
24
24
No reaction
No reaction
PS[(C6H4CH=N)Pd(OAc)]2
0.5
DMF
aReaction conditions: 4-bromonitrobenzene (1.0 mmol), styrene (2.0 mmol), solvent (6 mL); all the reactions were carried out in air. bConversion of reactant
was determined by GC and GC MS analysis using dodecane as internal standard.
most effective base as highest conversion was obtained
and thus chosen as the preferred base for the reactions
(Table 1, entry 3). Other inorganic bases were less effective
and affords moderate yield of coupled product (Table 1,
entries 14-16). Low conversion was achieved when organic
base, triethylamine (Et3N), was used (Table 1, entry 17).
This may be due in part to blocking of free coordination
sites on the palladium center.47,48 The base substrate mole
ratio also has a significant influence in the Heck coupling
reaction. Two different mole-ratios were tested (Table 1)
and among them, 2:1 mole ratio of base: substrate
(Table 1, entry 3) was found to be best. It was observed
that rate of the reaction drops significantly with decrease
the ratio to 1: 1 (Table 1, entry 8).
Under these optimized reaction conditions, a variety of
electronically and structurally diverse aryl halides could be
coupled very efficiently with terminal alkenes, affording
the desired products in excellent yields. Styrene gave the
corresponding coupled product in higher yields than methyl
acrylate. Non-substituted aryl halides, iodobenzene and
bromobenzene, gave excellent yield of coupled product
(Table 2, entries 1, 2, 11, 12). Aryl bromides bearing an
electron-withdrawing group can be employed to generate
the desired product by coupling with styrene and methyl
acrylate in excellent yield (Table 2, entries 3, 4, 13, 14).Aryl
bromides containing electron-donating groups required
longer reaction time to obtain high yields (Table 2, entries
5, 6, 15, 16). 3-Bromopyridine (Table 2, entries 7, 17) and
sterically hindered 1-bromo-naphthalene (Table 2, entries
8, 18) could provide the corresponding products in good
yields. In all coupled product, E-stereochemistry was
obtained. For the electron-neutral aryl chloride, the catalyst
was inefficient and the conversion of chlorobenzene was
very low (Table 2, entries 9, 19). However, the coupling
of electron-deficient aryl chloride, 4-chloroacetophenone,
provided a moderate yield (Table 2, entries 10, 20).
For comparison, the catalytic activity of the
present polymer-anchored catalyst PS[(C6H4CH=N)
Pd(OAc)]2 and corresponding homogeneous catalyst
[(C6H4CH=N) Pd(OAc)]2 were examined for the reaction of
4-bromonitrobenzene with styrene.As revealed in Table 3,
compatible performances were observed for both systems

Vol. 22, No. 2, 2011
Islam et al.
323
(Table 3, entries 1 and 2). Polymer-anchored catalyst gets
advantages due to its recycling and reusable properties.
Thermal stability of the present heterogeneous catalyst
is higher than its homogeneous analogue and the catalyst
can withstand in more drastic reaction than homogeneous
catalyst. We also compared the catalytic activity of the
present catalyst with other reported system. It is seen that
present catalyst exhibited higher conversions and yields
compared to the other reported system49,50 (Table 3, entries
3 and 4). In addition, the necessary reaction temperature is
lower and lower reaction time is required.
Hot-filtration test was performed in the Heck cross-
coupling reaction of 4-bromonitrobenzene with styrene
to investigate whether the reaction proceeded in a
heterogeneous or a homogeneous fashion.After continuing
the reaction for 4 h, the catalyst was removed by filtration
and the determined conversion was 46%. The resulting
filtrate was subjected to heating for further 4 h. From gas
chromatography (GC) result it reveals that after separation
of the catalyst no conversion takes place in the filtrate
part. This confirms that the reaction did not proceed upon
the removal of the solid catalyst. Further, no evidence for
leaching of palladium or decomposition of the complex
catalyst was observed during the catalytic reaction and
no palladium could be detected by atomic absorption
spectroscopic measurement of the filtrate after removal of
catalyst. These studies clearly demonstrated that palladium
was intact to a considerable extent with the heterogeneous
support, and no significant amount of leaching occurred
during reaction.
The advantage of the heterogeneous catalysis is
the possibility of recovering and reusing the catalyst
during the reaction. We studied the reusability of
the present heterogeneous palladium catalyst in the
Heck cross-coupling reaction taking the reaction of
4-bromonitrobenzene and styrene. After completion
Table 2. Heck reaction of aryl halides with terminal alkenes using PS[(C6H4CH=N P)Pd(OAc)]2 complex catalysta
entry
1
Alkene
Arylhalide
Productb,d
time (h)
6
Yieldc (%)
100 (95)
2
3
4
5
6
7
Br
8
8
92 (87)
97 (93)
95 (93)
89 (82)
91 (87)
O2N
Br
O2N
H3COC
H3CO
H3C
H3COC
H3CO
H3C
Br
8
Br
12
12
Br
Br
12
10
88
N
N
8
92 (88)
Br
9
Cl
24
24
15
53
10
H3COC
Cl
H3COC

324
Heterogeneously Catalyzed Phosphine-Free Heck Cross-Coupling Reaction
J. Braz. Chem. Soc.
Table 2. Continuation
entry
11
Alkene
Arylhalide
Productb,d
time (h)
6
Yieldc (%)
97 (94)
O
I
OMe
12
13
14
15
16
17
O
Br
8
8
89 (83)
94 (90)
91 (85)
83 (79)
86
OMe
O
O2N
Br
OMe
O2N
H3COC
H3CO
H3C
O
H3COC
H3CO
H3C
Br
8
OMe
OMe
O
Br
12
12
12
O
O
OMe
Br
OMe
O
Br
82
OMe
N
N
18
O
12
87 (82)
OMe
Br
19
20
O
Cl
24
24
7
OMe
O
H3COC
Cl
37
OMe
H3COC
aReaction conditions: arylhalide (1.0 mmol), alkene (2.0 mmol), K2CO3 (2.0 mmol), DMF (6 mL), catalyst (0.5 mol% of Pd);All the reactions were carried
out in air. bProducts were identified by comparison of their GC MS and 1H NMRspectral data those reported in the literature (whenever possible). cYield
of the reactant was determined by GC and GC MS analysis using dodecane as internal standard. The isolated yields are given in parentheses. dIn all the
coupling reactions, only E-stereochemistry was observed.
Table 3. Comparison of catalytic activity of the present catalyst in the Heck cross-coupling reaction with its homogeneous analogue and other related
reported systems
Reaction
Catalyst
Reaction conditions
Yield (%)
Ref.
This work
This work
49
4-bromo nitrobenzene + styrene
PS[(C6H4CH=N)Pd(OAc)]2
[(C6H4CH=N)Pd(OAc)]2
Pd-PMMA
K2CO3, DMF, 0.5 mol% of Pd, 90 oC, 8 h
K2CO3, DMF, 0.5 mol% of Pd, 90 oC, 8 h
Et3N, DMF, 1 wt% of Pd, 100 oC, 8 h
Bu3N, NMP, 0.015 mmol Pd, 130 oC, 12 h
97
92
84
90
MCM-41-SH-Pd(0)
50
of the reaction, the catalyst was recovered by simple
filtration and washed with ethyl acetate followed by
acetone then dried in reduced pressure at 40 C. The
recovered catalyst was employed in the next run with
further addition of substrates in appropriate amount
under optimum reaction conditions, and no substantial
loss of activity was observed up to seven cycles. It was
also observed from the IR spectral studies that there
was no change in the nature of the catalyst even after
the seventh cycle.
o

Vol. 22, No. 2, 2011
Islam et al.
325
Conclusions
General experimental procedure for Heck reaction
In summary, we have synthesized a simple
heterogeneous palladium complex which is successfully
employed as an excellent catalyst for Heck cross-
coupling reactions. The catalyst is more efficient with
respect to quantity of catalyst required, the yields
obtained and a much wider range of substrates can be
utilized including electron-rich, electron-deficient and
electronically neutral aryl halides. The main advantages
of the present catalyst are easy synthesis of the catalyst,
phosphine free conditions as well as convenient handling
due to insensitivity to air and moisture which allows
the reactions to be conducted in the undried solvent
under air. The catalyst is highly reusable and shows
seven time reusability without appreciable decrease in
its initial activity.
A mixture of aryl halide (1.0 mmol), alkene (2.0 mmol),
K2CO3 (2.0 mmol), DMF (6 mL) and 1.0 mol% of catalyst
was stirred at 90 C under air. To study the progress of
o
the reaction, the mixtures were collected at different time
interval and quantified by GC analysis. At the end of the
reaction, the catalyst was separated by simple filtration.
Filtrate was dried over sodium sulfate (Na2SO4), filtered,
concentrated and the residue was purified by flash column
chromatography on silica gel. The product was analyzed
by GC MS. All the prepared compounds are known and
compared with authentic sample.
Recovery and reuse of the catalyst
The Heck reaction was carried out with 4-bromo-
nitrobenzene (1.0 mmol), styrene (2.0 mmol), K2CO3
(2.0 mmol) and catalyst (0.5 mol% of Pd) in DMF
(6 mL). Each reaction mixture was stirred at 90 oC in air
atmosphere. After completion of the reaction, the mixture
was cooled and the catalyst was separated from the liquid
mixture by filtration. The filtrate was analyzed by CG to
determine the yield of the product. The recovered catalyst
was washed thoroughly with ethyl acetate and acetone and
Experimental
Materials
All the reagents were analytical grade and used as
such without further purification. Solvents were purified
and dried according to standard procedures. Macroporous
polystyrene beads cross linked with divinylbenzene,
was purchased from Aldrich Chemical Company, USA.
Palladium acetate and palladium chloride were procured
fromArora Matthey and used as such. Other reagents were
purchased from Merck.
o
dried under reduced pressure at 40 C. The catalyst was
totally recovered. From AAS analysis it is seen that the
metal content of the recovered catalyst remained almost
identical to that of fresh catalyst. After that, the recovered
catalyst was reused for the next reaction under the same
reaction conditions as mentioned above.
A Perkin-Elmer, USA, 2400C elemental analyzer
was used to collect microanalytical data (C, H and N).
The metal content in the polymer-anchored complex was
analyzed using a Varian, USA, AA240 atomic absorption
spectrophotometer (AAS). The reaction products were
analyzed by Varian, USA, 3400 gas chromatograph
equipped with a 30m CP-SIL8CB capillary column and
a flame ionization detector. All reaction products were
identified by using an Agilent, USA, GC MS (QP-5050).
Surface morphology of functionalized polystyrene ligand
and metal complex were analyzed using a scanning
electron microscope (SEM) (ZEISS EVO40, England)
equipped with EDX facility. The thermal stability of the
immobilized catalyst was determined using a Mettler
Toledo, Switzerland, TGA/DTA 851. The FTIR spectra
of the immobilized catalyst were recorded on a Perkin-
Elmer, USA, FTIR 783 spectrophotometer using KBr
pellets. Diffuse reflectance UV-Vis spectra was taken
using a Shimadzu, Japan, UV-2401PC double beam
spectrophotometer having an integrating sphere attachment
for solid samples.
Acknowledgments
We acknowledge the Department of Science and
Technology (DST), Council of Scientific and Industrial
Research (CSIR) and the University Grant Commission
(UGC), New Delhi, India for funding. One of the authors,
K.T., is thankful to the University Grants Commission
(Eastern Region), India, for financial support. We also
thank the DST and UGC, New Delhi, India for providing
instrumental support under the FIST and SAP program.
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Submitted: June 17, 2010
Published online: October 5, 2010