Synthesis, spectroscopic characterisation, electron microscopic study and thermogravimetric analysis of a phosphorus-containing dendrimer with phloroglucinol as a core unit

Article abstract of DOI:10.3184/030823410X12887232916505

A phosphorus-containing dendrimer of up to four generations with phloroglucinol as a core unit was synthesised using a divergent growth method. A repetitive synthetic sequence of several types of reactions was performed by using P(O)Cl₃, 3-hydr

Full text of DOI:10.3184/030823410X12887232916505

JOURNAL OF CHEMICAL RESEARCH 2010
RESEARCH PAPER 643
NOVEMBER, 643–647
Synthesis, spectroscopic characterisation, electron microscopic study
and thermogravimetric analysis of a phosphorus-containing dendrimer
with phloroglucinol as a core unit
Echchukattula Dadapeer and Chamarthi Naga Raju*
Department of Chemistry, Sri Venkateswara University, Tirupati-517 502, India
A phosphorus-containing dendrimer of up to four generations with phloroglucinol as a core unit was synthesised
using a divergent growth method. A repetitive synthetic sequence of several types of reactions was performed by
using P(O)Cl3, 3-hydroxybenzaldehyde, 4-hydroxybenzaldehyde and p-phenylenediamine for the synthesis of this
dendritic molecule G4 involving simple condensation reactions. This Schiff’s base macromolecule possesses 12 imine
bonds and 6-OH groups on the periphery. The structures of the intermediate generations G1, G2, G3 were confirmed
by IR, 1H, 13C, 31P NMR and LC/EI MS. The final dendrimer G4 was characterised by IR, 1H, 13C, 31P NMR, Maldi-tof mass
spectrometer and C, H, N analysis. The scanning electronic microscopic study and thermogravimetric analysis were
also performed for the final dendrimer (G4).
Keywords: phosphorus-containing dendrimer, divergent growth method, phloroglucinol, Schiff’s base, scanning electron
microscopy, thermogravimetric analysis
Well-defined, monodispersed macromolecules with uniform
branched structures are called dendritic macromolecules. They
constitute a ubiquitous type of precisely defined polymers,1
potentially usable in numerous applications. Recently, there
has been a growing interest in the synthesis of dendrimeric
systems which offers the opportunity to generate monodis-
perse, structure-controlled macromolecular architectures
similar to those observed in biological systems.2 The phospho-
rus-containing dendrimers are useful for biological purposes.3
The presence of phosphorus on the surface or within the cas-
cade structure of dendrimers, or at the focal point of dendrons
confers on phosphorus dendrimers fascinating properties.4
Among other useful applications of phosphorus dendrimers,
they can be used as hydrogels, hybrid organic–inorganic
materials, nanolatex, nanotubes, microcapsules and fibres.5 As
much to the extent that a dendrimer possesses its own structure
and molecular properties, identification of new monomers
for dendrimer synthesis continues to be attractive. Thermo gra-
vimetric analysis (TGA) is generally employed to determine
characteristics of materials such as polymers and dendrimers,
to determine degradation temperature, absorbed moisture con-
tent, etc. Simultaneous TGA-DTA/DSC measures both heat
flow and weight changes (TGA) in a material as a function of
temperature or time in a controlled atmosphere. The comple-
mentary information obtained allows differentiation between
endothermic and exothermic events which have no associated
weight loss (e.g., melting point) and those which involve a
weight loss (e.g., degradation). So thermal stability appears as
an important point for the most applications of the dendrimers,
and particularly in the field of materials science.6 Scanning
electron microscopy (SEM) gives the information about the
sample’s surface topography, composition, electrical con-
ductivity, etc. SEM micrographs have a characteristic three-
dimensional appearance useful for understanding the surface
structure of the sample.
dendrimer. We have adopted a divergent growth method for
the preparation of the dendritic molecule involving simple
condensation reactions.
Results and discussion
Synthesis of the phosphorus-containing dendrimer (G4) having
phloroglucinol as a core unit is presented in Scheme 1. The
first step of the synthesis was a condensation reaction of
phloroglucinol (G0) with POCl3 in presence of triethylamine
at –15 to 40 °C in dry THF with stirring for 2–3 h to afford the
corresponding G1. After filtration, G1 was then treated with
3-hydroxy benzaldehyde in dry THF in presence of triethyl-
amine at 40–45 °C with stirring for 4–5 h to form G2. Then
compound G2 was reacted with p-phenylenediamine in reflux-
ing dry EtOH to afford G3. Finally the compound G3 was
treated with 4-hydroxybenzaldehyde in dry EtOH at reflux
temperature for 5–6 h to afford the dendritic macromolecule
G4. As the size of the dendron increases the reaction becomes
sluggish and the yield is low. To improve the yield of G4, alco-
hol is taken in excess and the temperature is maintained at
reflux with continued stirring for long periods. Scheme 1 sum-
marises the preparation of the dendrimer G4 and the yield of
the final dendrimer was a moderate 60%. The intermediates
1
G1, G2, G3 were characterised by IR, H, 13C, 31P NMR and
LC/EI mass spectrometry. The synthetic and analytical data of
the dendrimer G4 are given in the experimental section. The
dendrimer G4 exhibited absorption bands for –OH and P=O,
in the regions 3368 and 1256 cm−1 respectively. P–O–C(aromatic)
gave two absorptions in the regions 980 and 1192 cm−1.8,9 An
absorption band for CH=N is observed at 1601 cm−1. In the 1H
NMR spectra (500 MHz) of G4, the aromatic protons gave
multiplet in the region δ 6.57–7.79. The aromatic –OH protons
resonated at δ 9.78 as a singlet. The CH=N protons gave chem-
ical shifts in the region 8.45 and 8.51 in two environments.
The 13C NMR spectrum was recorded for G4 and the data are
given in the experimental part. The aromatic carbons resonated
in the region 114.8–160.5 ppm and the CH=N Carbon signal
was observed at δ 163.8. G4 gave a 31P NMR signal10 at δ –1.86.
The dendrons G1, G2 and G3 showed phosphorus –31 resonance
signals in the expected regions and their data are given in the
experimental section. The mass spectroscopic data of the final
In our endeavour to synthesise a new type of dendrimer, we
report here the synthesis of a phosphorus-containing dendrimer
of four generations, with phloroglucinol 1,3,5-trihydroxyben-
zene as a core unit. Phloroglucinol was used previously by
Chow and co-workers7 as the branching unit of dendron deriv-
atives. We have chosen alsothe symmetrically functionalised
phloroglucinol as the core and have chosen P(O)Cl3, 3-hydroxy
benzaldehyde, 4-hydroxybenzaldehyde and p-phenylenedi-
amine as branching components in the synthesis of the
Table 1 Mass data of G4 obtained from MALDI
Dendrimer
G4
Calculated mass
2156.08
MALDI mass
2156.00
* Correspondent. E-mail: rajuchamarthi10@gmail.com

644 JOURNAL OF CHEMICAL RESEARCH 2010
Scheme 1

JOURNAL OF CHEMICAL RESEARCH 2010 645
TGA-DTA
dendrimer G4 are determined by matrix assisted laser desorp-
tion /ionisation (MALDI) mass spectrometry. As expected, the
mass obtained from the MALDI measurements corresponds
closely to the calculated value.
The surface topography of the molecule was studied by
SEM. The molecular decomposition of the dendrimer G4 was
investigated by TGA and DTA.
TGA was carried out on G4 to determine changes in weight in
relation to change in temperature. Similarly, DTA, a thermo-
analytic technique was also carried out on G4 to detect the
changes in the sample, either exothermic or endothermic.
Simultaneous TGA-DTA, which measures both heat flow and
weight changes in a material as a function of temperature in
a controlled air atmosphere was recorded. The TGA-DTA
analysis of the compound was recorded up to 400 °C. The
TGA graph shows that the compound is stable up to 85 °C and
that minor decomposition starts at around 90 °C with a cor-
responding weight loss of approximately 5% water and con-
tinues up to 100 °C. The calculated water loss is 5.3%. Then
the compound is stable up to 220 °C. Then a sudden decompo-
sition starts at 220 °C with an observed weight loss of approx-
imately 58% and this continues up to around 400 °C. A stable
product of the compound is indicated by the constant weight
in the plateau of the thermogram (100–200 °C). An overlay of
TGA and DTA plots for the test compound (G4) up to 400 °C
is shown in Fig. 2. From the DTA curve, the heat of reaction
was calculated. The DTA profile (Fig. 3) shows an endotherm
peak at 95 °C corresponding to the loss of water and an endo-
therm at 45 °C, corresponding perhaps to the loss of a small
branch. The exotherm at 220 °C seems to be due to a major
degradation of the dendrimer structure.
SEM
SEM images depict the surface structure of the dendritic mate-
rial (G4). In order to get a deeper insight into the properties of
the surface of the C6H4–OH terminated dendritic macromo-
lecular material, we have also carried out an SEM analysis.
With increasing size it was clear that the overall surface mor-
phology of the material tended to assume a nanoscale size
gravel shape at a line width of 2 µm. Figure 1 shows the micro-
graph of the material with a line width of 10 µm and Fig. 2
shows the particle structure of the material with a line width of
2 µm.
Experimental
All the reagents used in this study were purchased from Sigma-Aldrich
Chemical Company and used without further purification. THF and
EtOH were dried by standard methods. TLC was performed on pre-
coated plates with silica gel 60F254 (Merk).Column chromatography
was performed on silica gel (0.040–0.063 mm, Macherey Nagel).The
melting points were determined on a Buchi R-535 (Flawil, Switzer-
land) melting point apparatus and are uncorrected. IR Spectra were
recorded on JASCO Japan FT IR-5300 spectrometer at the University
of Hyderabad using KBr optics.1H and 13C NMR spectra of G4 were
recorded on a Bruker A VIII 500 MHz NMR spectrometer at IIT,
1
Chennai, operating at 500.13 MHz for H, and 125.75 MHz for 13C
NMR; data were recorded in the solvent DMSO-d6 and chemical shifts
were referenced to TMS (1H and 13C). 1H and 13C NMR spectra of G1,
G2 and G3 were recorded on a Bruker A VIII 400 MHz NMR spec-
trometer at Laila Impex, Vizayawada, operating at 400.13 MHz for 1H
and 100.61 MHz for 13C NMR; data were recorded in the solvent
DMSO-d6 and chemical shifts were referenced to TMS (1H and 13C).
31P NMR were recorded on a Bruker ACF Supercon 200 spectrometer
operating at 81 MHz at the University of Hyderabad, Hyderabad. 31
P
Fig. 1 SEM picture of the dendrimer G4.
NMR data were recorded in the solvent CDCl3 or DMSO-d6 and
chemical shifts were referenced to 85% H3PO4. EI mass spectra of
intermediate dendrons G1, G2 and G3 were recorded on a JEOL
GCmate at IIT, Chennai and for mass chromatogram, LC Mass
spectra of intermediate dendrons G1, G2 and G3 were recorded on
LCMS-2010A Shimadzu, spectrometer at University of Hyderabad,
Hyderabad. The MALDI mass spectrum of the final dendrimer G4
was recorded using a Applied Biosystems MALDI-TOF Voyger depro
spectrometer. The sample was run using Sinapic acid as the matrix
with DMSO as the solvent in the dried-droplet preparation method,
performed at IIT, Madras, Chennai. TGA-DTA measurement was
taken using a SDT Q600 V8.2 built instrument, performed at IISc,
Bangalore. SEM images were taken with a Carl Zeiss, EVO MA15
Instrument. SEM operated at 20 kV, performed at Department of
Physics, S. V. University, Tirupati, India. Elemental analyses were
performed using EA 1112 Thermo Finnigan instrument, France, at
University of Hyderabad, Hyderabad, India.
Preparation of G1: A solution of POCl3 (1. 39 mL, 0.015 mol) (1) in
20 mL of dry THF was added dropwise over a period of 20 min to a
mixture of stirred solution of phloroglucinol (G0) (0.63g, 0.005 mol)
in 25 mL of dry THF and triethylamine (2.08 mL, 0.015 mol) at
–15 °C. After stirring for 3 h at 40 °C, formation of G1 was ascer-
tained by TLC analysis run in a 2:8 mixture of ethyl acetate and
hexane and the average Rf value observed was 0.75. Triethylamine
hydrochloride was removed by filtration. The filtrate (G1) was used
for the next reaction step without further purification. The compound
Fig. 2 Particle structure of G4.

646 JOURNAL OF CHEMICAL RESEARCH 2010
Fig. 3 TGA-DTA of the dendrimer G4: top curve, DTA; lower curve, TGA.
(G1) thus obtained was characterised by IR, 1H, 13C, 31P NMR , LC/EI
mass and C, H, N analysis. Yield 80%, υmax (KBr, cm−1): 1272 (P=O).
1H (400 MHz, DMSO-d6), 6.84(3H, s, ArH). 13C (100 MHz, DMSO-
d6), 154.0 (C01, C03 and C05), 96.5 (C02, C04 and C06). 31P NMR (81
MHz, CDCl3). –0.35 ppm. EI MS: m/z 473.10 [M]+., 475.00 [M+2]+.,
477.00 [M+4]+.. LC MS: m/z 473.00 [M]+ 475.00 [M+2]+., 477.00
[M+4]+.. Anal. Calcd for C6H3Cl6O6P3: C, 15.12; H, 0.63. Found: C,
15.21; H, 0.68%.
13C (100 MHz, DMSO-d6), 153.3 (C01, C03 and C05), 96.3 (C02, C04 and
C06), 151.1 (C11),114.1 (C12),136.6 (C13), 117.6 (C14 ),129.3(C15), 119.5
(C16), 142. 6 (C21),122.4 (C22/ C26),113.7 (C23/ C25), 148.0 (C24 ) and
157.7 (Ar-CH=N). 31P (81 MHz, CDCl3), –19.22 ppm. EI MS: m/z
+
1532.03 (M+H)+ , LC MS: m/z 1532.15 (M+H) . Anal. Calcd for
C84H69N12O12P3: C, 65.88; H, 4.54; N, 10.98%. Found: C, 65.71; H,
4.58; N, 10.86%.
Preparation of G4: To a stirred solution of G3 in 30 mL of dry EtOH,
a solution of 4-hydroxybenzaldehyde (4) (3.66 g, 0.03 mol) in 25 mL
of dry EtOH, was added at room temperature. After stirring for 5h at
reflux temperature, formation of G4 was ascertained by TLC analysis
run in a 4:6 mixture of ethyl acetate and hexane and the average Rf
value observed was 0.35. The solvent was evaporated under reduced
pressure to afford a crude product. This was purified by washing with
ethyl acetate and hexane. Then G4 was purified by column chromatog-
raphy on silica gel (eluent: ethyl acetate) to afford a brown coloured
powder in 60% yield. The imine thus obtained was characterised
by IR, 1H, 13C, 31P NMR, MALDI-TOF mass spectrometry and C, H,
N analysis. Yield 60%, υmax (KBr,cm−1): 3368 (Ar-OH), 1256 (P=O),
Praparation of G2: To a mixture of stirred solution of the filtrate
G1 and triethylamine (4.5 mL, 0.03 mol), a solution of 3-hydroxy-
benzaldehyde (2) (3.66 g, 0.03 mol) in 25 mL of dry THF was added
dropwise over a period of 20 min at 0–5 °C. After stirring for 4 h at
40–45 °C, formation of G2 was ascertained by TLC analysis run in a
4:6 mixture of ethyl acetate and hexane and the average Rf value
observed was 0.65.Triethylamine hydrochloride was removed by fil-
tration. The solvent was evaporated under reduced pressure to obtain
a crude product. This was purified by column chromatography eluting
with ethyl acetate:hexane (2:8). The compound thus obtained was
characterised by IR, 1H, 13C, 31P NMR, LC/EI mass spectrometry and
C, H, N analysis and used for the next reaction step. Yield 75%, υmax
(KBr, cm−1): 1255 (P=O), 941, 1167 (P–O–Caromatic), 1693 (Ar-CHO).
1H (400 MHz, DMSO-d6), 7.17–7.40 (27H, m, ArH), 9.91(6H, s,
Ar-CHO). 13C (100 MHz, DMSO-d6), 153.9 (C01, C03 and C05), 96.5
(C02, C04 and C06), 149.01 (C11),115.0 (C12),137.6 (C13), 120.6
(C14),130.0 (C15) 121.9 (C16) and 193.0 (Ar-CHO). 31P (81 MHz,
CDCl3), –13.05 ppm. EI MS: m/z 991.15 [M+H]+. , LC MS: m/z 991.30
1
980, 1192 (P–O–Caromatic), 1601 (CH=N). H (500 MHz, DMSO-d6),
9.78 (6H, s, Ar-OH), 6.84–7.79 (75 H, m, ArH), 8.44, 8.51 (12H, s,
–CH=N). 13C (125 MHz, DMSO-d6), 154.9 (C01/ C03/ C05), 96.4 (C02/
C04/ C06, 149.7 (C11), 114.9 (C12, 138.6(C13, 120.1 (C14), 130.3 (C15),
122.2 (C16, 147.4 (C21/ C24), 122.9 (C22/ C23/ C25/ C26, 128.8 (C31), 131.1
(C32/ C36, 116.3 (C33/ C35), 160.5 (C34), 163.8 (CH=N). 31P (81 MHz,
CDCl3), –1.86 ppm. [MALDI-TOF]: m/z 2156.00 [M+H]+. Anal.
Calcd for C126H93N12O18P3: C, 70.19; H, 4.35; N, 7.80%. Found:
C, 70.10; H, 4.41; N, 7.85%.
+
(M+H) . Anal. Calcd for C48H33O18P3: C, 58.19; H, 3.36. Found:
C, 58.25; H, 3.31%.
Preparation of G3: To a stirred solution of G2 in 25 mL of dry EtOH,
a solution of p-phenylenediamine (3) (3.24 g, 0.03 mol) in 25 mL of
dry EtOH, was added at room temperature. After stirring for 5h at
reflux temperature, formation of G3 was ascertained by TLC analysis
run in a 3:7 mixture of ethyl acetate and hexane and the average Rf
value observed was 0.65. The solvent was evaporated under reduced
pressure to get a crude product. This was purified by column chroma-
tography eluting with ethyl acetate and hexane (3:7). The imine thus
obtained was characterised by IR, 1H, 13C, 31P NMR, LC/EI Mass and
C, H, N analysis and used for the next reaction step. Yield 70%, υmax
(KBr, cm−1): 1248 (P=O), 956, 1176 ((P–O–Caromatic), 3217 str, 1450
Conclusion
The synthesis of a novel dendritic macromolecule G4 has been
accomplished. The condensation reactions were performed
in dry tetrahydrofuran in the presence of triethylamine and the
Schiff’s base preparations were perfomed in dry ethanol. The
deeper surface topography of the dendritic molecule G4 was
observed by SEM. This reveals a nano-sized gravel particle
with a diameter of 2 µm. The thermal stability and changes in
weight in relation to change in temperature and the heat of the
final dendritic molecule G4 were studied by TGA-DTA analy-
sis. This reveals that the compound G4 starts decomposition at
1
bending (Ar-NH2), 1622 (CH=N). H (400 MHz, DMSO-d6), 6.60–
7.33 (51H, m, ArH), 8.51(6H, s, Ar-CH=N), 5.26 (12H, s, Ar-NH2).

JOURNAL OF CHEMICAL RESEARCH 2010 647
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Received 17 June 2010; accepted 27 September 2010
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