Covalent functionalization and solubilization of multi-walled carbon nanotubes by using zinc and copper complexes of meso-tetra(4-aminophenyl) porphyrin

Article abstract of DOI:10.1016/j.jorganchem.2018.02.031

Zinc (II) and copper (II) complexes of mesotetra(4-aminophenyl)porphyrin (Zn-TAP and Cu-TAP) were covalently connected to multi-walled carbon nanotube (MWCNT) through an amide linkage and characterized by using many spectroscopic techniques. Significant m

Full text of DOI:10.1016/j.jorganchem.2018.02.031

Accepted Manuscript
Covalent functionalization and solubilization of multi-walled carbon nanotubes by
using zinc and copper complexes of meso-tetra(4-aminophenyl) porphyrin
G. Prabhavathi, R. Yamuna, Alifia C. Jafer
PII:
S0022-328X(18)30130-X
10.1016/j.jorganchem.2018.02.031
JOM 20330
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 16 September 2017
Revised Date: 6 February 2018
Accepted Date: 21 February 2018
Please cite this article as: G. Prabhavathi, R. Yamuna, A.C. Jafer, Covalent functionalization
and solubilization of multi-walled carbon nanotubes by using zinc and copper complexes of
meso-tetra(4-aminophenyl) porphyrin, Journal of Organometallic Chemistry (2018), doi: 10.1016/
j.jorganchem.2018.02.031.
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ACCEPTED MANUSCRIPT
Covalent Functionalization and Solubilization of Multi-walled Carbon Nanotubes by Using
Zinc and Copper Complexes of Meso-tetra(4-aminophenyl) porphyrin
G. Prabhavathia, R. Yamuna*b and Alifia C. Jaferb
a Research and Development Centre, Bharathiar University, Coimbatore – 641 046, India. bCenter of Excellence in
Advanced Materials and Green Technologies (CoE-AMGT), Dept. of Sciences, Amrita School of Engineering,
Coimbatore, Amrita Vishwa Vidyapeetham, India. ryamuna1@gmail.com.
1. Introduction
Carbon nanotubes (CNTs) stand out as distinctive materials owing to their variety of
physical and chemical properties such as high surface area, high electrical and thermal
conductivity, structural integrity, significant mechanical strength and chemical inertness [1-3].
CNTs are used as electrode materials in electrochemical devices in order to promote electron
transfer reactions [4,5]. Literature studies show that cobalt corrole adsorbed on the surface of
multiwalled carbon nanotubes (MWCNTs) demonstrated an exceptional electro-catalytic
property towards the reduction of oxygen [6]. Furthermore, cobalt-porphyrin complex that
attached to multiwalled carbon nanotubes (CNTs) have shown higher catalytic performance for
oxygen-reduction reaction (ORR) in acidic medium, indicating the advantages of supramolecular
complex formed by electro-catalytic metalloporphyrin covalently linked to CNTs [7]. Recent
studies on heterogeneous nano catalysts containing iron (III) porphyrin covalently attached onto
the surface of MWCNTs showed to be an efficient catalyst in the epoxidation of alkene under
mild conditions [8]. Zhao et. al. constructed a sensor for the detection of nitrite in water sample
by immobilizing hemoglobin on MWCNTs modified glass carbon electrode [9]. Recent studies
have also shown that MWCNT-based nanohybrid systems in which MWCNTs are covalently
functionalized with dye molecules as electron donor-acceptor ensembles (D-A) have attracted
intense scientific interest due to their enhanced photo-electronic properties [10,11]. MWCNTs
are preferred to other carbon nanomaterials as components of electron (D-A) ensembles due to
the presence of large number of concentric graphitic tubes. Nevertheless, the use of MWCNTs in
building various photovoltaic and non-linear optical (NLO) materials have been impeded due to
their poor solubility in most of the organic and aqueous solvents and also difficulties associated
with processing. Therefore, significant effort has been devoted towards the covalent
functionalization of MWCNTs with chromophoric molecules in order to simultaneously promote
the solubility and also to improve the photo-electronic properties of the nano-hybrid [12,13].

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Porphyrins have fascinated significant attention for applications in solar cells and
molecular electronics in owing to their interesting biological, optical, electronic and structural
properties [14-16]. These molecules possess unique optoelectronic properties due to the small
reorganization energies and this, together with their large π-conjugated systems [17]. Fine tuning
of these properties can be achieved through varying the substituents at the macrocyclic periphery
or central metal ion [18]. Literature study also shows that covalently functionalized
metalloporphyrin (Zn or Cu) with carbon nanomaterials exhibited enhanced NLO and optical
limiting properties [19-21].
In
this
paper,
covalent
functionalization
of
5,10,15,20-meso-tetra(4-
aminophenyl)porphyrinato Zn (II) complex (Zn-TAP, Chart 1) and 5,10,15,20-meso-tetra(4-
aminophenyl)porphyrinato Cu (II) complex (Cu-TAP, Chart 1) with MWCNTs (nano hybrid (I)
and (II), respectively) through an amide linkage has been reported. The chemical structure of
synthesized nano hybrids I and II have been testified by Raman, Fourier transform infrared (FT-
IR) and X-ray photoelectron (XPS) spectroscopy. Thermal stability of these nano hybrids were
studied by thermogravimetric analysis (TGA). Morphology of the nano hybrids were analysed by
Transmission electron microscopy (TEM). Linear optical studies of these nano hybrids are
reported.
NH2
NH2
N
N
N
N
N
N
Zn
Cu
H2N
H2N
NH2
NH2
N
N
NH2
NH2
(a)
(b)
Chart 1. Chemical structure of a) Zn-TAP and b) Cu-TAP

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2.
Materials and Methods
2.1. Materials
Pyrrole, Zinc (II) acetate dihydrate and copper (II) acetate monohydrate were purchased
from Merck. Other reagents such as chloroform (CHCl3), dry tetrahydrofuran (THF, Finar) and
N,N-dimethylformamide (DMF) were purified by using standard procedures. MWCNTs were
purchased from Yunnan Great (Group) Co., Ltd. (China). Carboxyl functionalized MWCNTs
(MWCNT–COOH) and TAP were synthesized by using the literature procedure [22].
2.2. Characterization and Instrumentation
Fourier transform infrared spectra (FT-IR) were measured on a Thermo Nicolet, iS10 FT-
IR spectrophotometer using KBr pellet method. Thermogravimetric analysis (TGA) was carried
o
out using SDT Q600 simultaneous DSC-TGA with a heating rate of 5 C/min under N2 purge
(100 ml/min). Absorption studies were carried out with a SHIMADZU, UV-1800 UV
spectrophotometer. Nuclear magnetic resonance (NMR) spectra were measured on a 500 MHz
Bruker NMR spectroscopy using tetramethylsilane (TMS) as the internal standard. The
chemicals shifts were expressed in parts per million (ppm). Raman spectra were obtained from
the micro Raman spectroscopy (The CRAIC Technologies™). Photo emission spectra were
obtained by using a SHIMADZU RF-6000 spectrofluorophotometer. JEOL JEM 2100 high
resolution transmission electron microscopy (HR-TEM) instrument was used to record the TEM
images of nano-hybrid. X-ray photoelectron spectroscopy (XPS) analysis was performed in
KRATOS Axis Ultra (Kratos Analytical, Manchester, United Kingdom). Electron spin resonance
(EPR) spectrum was recorded in Varian E112 spectrometer.
2.3. Synthesis of 5,10,15,20-meso-tetra(4-aminophenyl) porphyrinato Zn (II) complex:
Zinc (II) acetate dihydrate (0.227 g, 1.034 mmol) and 5, 10, 15, 20-mesotetra(4-
aminophenyl) porphyrin (0.100 g, 0.148 mmol) were refluxed in DMF solvent (15 mL) for 3 h at
80 oC. Thin layer chromatography (TLC) was used to check the completion of reaction. Rota-
evaporator was used to remove the excess DMF solvent. Further the residue was purified by
washing five times with water (15 mL) through sintered crucible in order to remove excess zinc
acetate and other impurities from the product. Finally, trituration of the crude product with ethyl
acetate (5 mL) and followed by drying the product under vacuum afforded the title compound as
purple solid. Yield: 0.072 g, 80 %. 1H nuclear magnetic resonance spectroscopy (1H NMR) in d6-

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dimethylsulfoxide (d6-DMSO): 5.42 (s, 8H), 6.95 (d, 8H), 7.79 (d, 8H), 8.81 (m, 8H) (see
supporting information Figure.S1).
2.4. Synthesis of 5,10,15,20-meso-tetra(4-aminophenyl) porphyrinato Cu (II) complex:
5, 10, 15, 20-mesotetra-(4-aminophenyl) porphyrin (0.050 g, 0.074 mmol) was refluxed
with copper (II) acetate monohydrate (0.207 g, 1.036 mmol) in dimethylsulfoxide (15 mL) at
0
100 C for 4 h. After completion of the reaction as indicated by TLC, the DMSO solvent was
removed by using rota-evaporator. Further the residue was purified by washing five times with
water (15 mL) through sintered crucible in order to remove excess copper (II) acetate and other
impurities from the product. Finally, trituration of the crude product with ethyl acetate (5 mL)
and followed by drying the product under vacuum afforded title compound. Yield: 75%. The
formation of the complex has been confirmed by the electron spin resonance (EPR) spectrum
recorded in DMF solvent at room temperature. The gII value < 2.3 indicates a co-ordinate
covalent character of the Cu-N bonds in the copper-porphyrinic complexes (see supporting
information Figure S2).
2.5. Synthesis of MWCNT-CONH-ZnTAP nano hybrid
o
MWCNT–COOH (100 mg) was refluxed at 70 C for 24 h with SOCl2 (75 mL) under
argon atmosphere in the presence of DMF (1.8 mL) to form acyl chloride functionalized
MWCNT (MWCNT–COCl). Excess SOCl2 was removed by vacuum distillation and the
remaining residue was washed with dry THF. MWCNT–COCl (100 mg) and Zn-TAP (100 mg)
o
were dissolved in DMF (30 mL). Further this mixture was refluxed at 130 C for 72 h in
presence of 1 mL triethylamine (Et3N) under argon atmosphere. The resultant solution was
cooled to room temperature. Excess DMF were removed by using rotary evaporator. 100 ml of
ether was added into the reaction mixture in order to precipitate the product. The residue was
separated through ultracentrifugation. Finally, the precipitate was purified by washing with THF
(3 mL) and further washed five times with CHCl3 which is followed by filtration in order to
remove excess Zn-TAP and other impurities from the residue. TLC was used to check the filtrate
to confirm the absence of Zn-TAP in the final washing. The resultant product was washed with
water in order to remove Et3N.HCl from the nano hybrid and it was dried under vacuum.
2.6. Synthesis of MWCNT-CONH-CuTAP nano hybrid
MWCNT-CuTAP nano hybrid (2a) was synthesized by following the above procedure by
using Cu-TAP instead of Zn-TAP.

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3.
Result and discussion
3.1. Synthesis of materials
Zn-TAP and Cu-TAP were synthesized by refluxing their corresponding acetate salts with
TAP in DMF and DMSO, respectively. Zn-TAP and Cu-TAP were covalently linked to
MWCNT through an amide bond by refluxing the porphyrin complex with MWCNT–COCl at
130 oC for 72 h in DMF solvent under argon in presence of triethylamine (Et3N) as shown in the
scheme 1.
Scheme1. Reaction scheme for the covalent functionalization of MWCNTs with Zn-TAP
and Cu-TAP
The chemical structure of nano-hybrids I and II were testified by FT-IR spectra as shown in
Figure 1. In FT-IR spectra, MWCNT–COOH reveal characteristic peak at 3414 cm-1 which is
attributed to the O–H vibration frequency. The peak at 1709 cm-1 in MWCNT-COOH is
attributed to the carbonyl (C=O) stretching frequency of carboxyl moieties (COOH). This peak is
shifted to 1688 cm-1 and 1692 cm-1 in the nano-hybrids I and II, respectively due to the formation
of amide bond (–CO-NH-). Whereas this (C=O) peak in nano-hybrid I and II are slightly shifted
to higher frequency compared to that of MWCNT-TAP (1638 cm-1) [22]. The N-H stretching
and bending vibration frequencies of the porphyrinic core for TAP appeared at ~ 3330 cm-1 and
963 cm-1, respectively [22]. Whereas these two peaks are nearly disappeared in Zn-TAP and Cu-
TAP which is owing to the replacement of hydrogen atom in the N-H bond by metal ions [23].
Whereas a strong peak observed near 1000 cm-1 which corresponds to the skeletal ring vibration

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of metal substituted porphyrin [24]. The peak at ~ 3380 cm-1 in Zn-TAP and Cu-TAP is owing to
the N-H stretching frequency of amino (–NH2) group. Furthermore nano hybrids I and II also
show single broad band at 3415 and 3433 cm-1, respectively due to the N-H stretching mode of
(–CO–NH, -NH2). Two more new striking bands appeared in the nano-hybrids I and II at 1256,
1586 and 1237, 1599 cm-1, respectively which is attributed to the stretching vibration of C-N of
amide bond and porphyrin C=C, respectively [25,26]. These noticeable modifications in the FT-
IR spectra reveal the formation of covalently connected nano hybrids I and II (separate FT-IR
spectra are given in the supporting information Figures S3 and S4 for clarity). Furthermore FT-
IR spectra of Cu-TAP and nano hybrid II show higher frequency shift in their peaks compared to
Zn-TAP and its nano hybrid, indicating different electronic interaction of metal ions.
Figure.1. FT-IR spectra of MWCNT, MWCNTCOOH, Zn-TAP, Cu-TAP, MWCNT-
ZnTAP (I) and MWCNT-CuTAP (II)
Figure.2. depicts the Raman spectra of MWCNT, MWCNT-COOH, nano hybrids I and II.
All these materials show two Raman bands in the first order region. In general, the band
appeared at ~1300 cm-1 is often called as the D-band and another band centered at ~1500 cm-1 is

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known as the tangential G-band. The D-band is attributed to the structural defects or disorder
present in the graphitic sp2-hybridized carbon systems. However G-band is related to the planar
stretching vibrational mode of graphitic carbon atoms [18]. The second order - or 2D-band
which appear around 2600 cm-1 is corresponds to the overtone of the D-band. The ratio of D
band intensity relative to that of the G band (ID/IG) is utilized as a method to assess the quality of
nanotubes. MWCNT, MWCNT-COOH, nano hybrids I and II exhibit ID/IG ratio of 0.93, 1.06,
0.96 and 1.01, respectively. After covalent functionalization with Zn-TAP or Cu-TAP, the
intensity of D band of nano hybrids I and II was reduced compared with MWCNT-COOH.
Furthermore, there is a slight increase in the ID/IG ratio of MWCNT-ZnTAP or MWCNT-CuTAP
as compared to that of MWCNT and MWCNT-TAP [22]. Similarly, D and G bands position of
MWCNT-COOH, MWCNT-ZnTAP, and MWCNT-CuTAP exhibit higher frequency shift
(stiffening) compared with MWCNT. This higher frequency shift in nano hybrids I and II is
possibly due to the metal which acts as Lewis acid owing to its co-ordination with TAP molecule
that acts as Lewis base. These observations are clear evidence for the chemical functionalization
of MWCNTs, subsequently Raman spectroscopy is very sensitive to the electronic structure.
Figure.2. Raman spectra of MWCNT, MWCNT-COOH, MWCNT-ZnTAP and MWCNT-
CuTAP

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TGA has been widely employed to study the upper temperature limit of a material and also
for the determination of covalent functionalization of MWCNTs. TGA results of MWCNT,
MWCNT-COOH, ZnTAP, CuTAP, nano hybrids I and II are shown in the Figure.3. TGA plot
reveals that MWCNT, Cu-TAP and Zn-TAP are thermally more stable and shows less than 20 %
weight loss in the temperature range 50-800 oC. TGA plot of MWCNT-COOH reveals that 27 %
o
weight loss in the temperature range of 50–500 C which is attributed to the elimination of
physisorbed water, decarboxylation, removal of –OH and other metal impurities on the surface
of MWCNT–COOH. Nano hybrids I and II show approximately 40 and 23 % weight loss in the
temperature range of 180-800 oC, respectively. This weight loss relates to the loss of Zn-TAP or
Cu-TAP from the nano hybrids which were covalently linked to the MWCNTs. Nano hybrid II
shows more thermal stability compared to nano hybrid I and also compared to the reported
similar nano hybrids [18,22].
Figure.3. TGA curves of MWCNTCOOH, MWCNT, Cu-TAP, Zn-TAP, MWCNT-ZnTAP
(I) and MWCNT-CuTAP (II)
Morphology study of two nano hybrids I and II have been carried out by using HR-TEM
analysis. Images showing the morphology of MWCNT, MWCNT-COOH, nano hybrids I and II
are presented in the Figures 4 and 5. As shown in Figure 4a, the surface of the pure MWCNTs is

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relatively smooth and tubular structure can be clearly distinguished. However, the observed dark
spot in the magnified image of MWCNT (Figure 4b) indicates the presence of amorphous carbon
[27]. Tem images of MWCNT-COOH (Figure 4c and 4d) indicate the decrease in the quantity of
amorphous carbon. Whereas the Figure 4c shows the presence of defect sites along the MWCNT
side walls due to oxidation. Nevertheless, after functionalization with Zn-TAP or Cu-TAP as
shown in the Figure 5, the MWCNTs surfaces became rougher and also the observed dark spots
on the surface of carbon nanotubes clearly reveal the metalloporphyrin functionalization (see
supporting information Figure S5). Tubular morphology was retained even after
functionalization.
a
b
d
c
Figure.4. TEM images of (a & b) MWCNT (c & d) MWCNT-COOH

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a
b
c
d
Figure.5. TEM images of (a & b) MWCNT-ZnTAP (I) and (c & d) MWCNT-CuTAP (II)

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a
b
Figure.6. XPS spectra of a) MWCNT & b) MWCNT-COOH
a

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b
Figure.7. XPS spectra of a) MWCNT-ZnTAP and b) MWCNT-CuTAP
Figure.8. Deconvolutions of C 1s and N 1s for MWCNT-ZnTAP

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Figure.9. Deconvolutions of C 1s and N 1s for MWCNT-CuTAP
XPS has become an essential tool for the confirmation of elemental composition and
further establish the electronic information of carbon nanomaterials. Figures 6 & 7 reveal the XP
spectra of MWCNTs, MWCNT-COOH, MWCNT-ZnTAP and MWCNT-CuTAP. All samples
demonstrate C 1s and O 1s peak. The presence of N 1s band and higher intensity O 1s band in
the spectra of nano hybrids I and II denote the functionalization of organic molecules on the
surface of MWCNTs. MWCNTs mainly contain carbon (94.51 %) and with low content of 5.49
% of oxygen (Figure 6a, Table 1). The presence of oxygen in MWCNTs is attributed to the
atmospheric moisture that bound to the MWCNTs [19]. The C 1s spectrum of MWCNTs (see
supporting information Figure S6a) exhibited four distinct peaks at 284.7, 284.9, 285.6 and 291.3
eV and these four peaks corresponds to sp2 carbon atoms, defects on the nanotube structure,
carbonyl (-C=O) groups and π–π* transition loss, respectively [20]. XP spectrum of MWCNT-
COOH shows higher intensity O 1s band due to the oxidation of MWCNTs (Figure 6b, Table 1).
We have find out the % of oxidization in MWCNTs using the peak position of oxygen
containing functional groups from the literature [(C-OH (285.8 eV), O=C-O (288.9 eV), C=O
(286.8 eV)] [27]. The % of oxidation (C-OH = 12.61 %, O=C-OH = 5.19 %, C=O = 4.31 %) was
obtained by analyzing the C 1s region of MWCNT-COOH (see supporting information Figure

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S6b). Some more new peaks were appeared in the C 1s XPS spectra of MWCNT-ZnTAP and
MWCNT-CuTAP, after linking porphyrin to MWCNTs, indicating the covalent functionalization
of MWCNTs with metallo porphyrins as shown in the Figures. 8 and 9. Furthermore covalent
linking of metallo porphyrin with MWCNTs results in a considerable decrease in the intensity of
C 1s peak of nano hybrids I and II relative to that of MWCNTs. The elemental composition
obtained from XPS analysis were tabulated in the Table 1. Deconvolution of the N 1s spectrum
of nano hybrids I and II exhibited the following contributions with the binding energies of 398.5,
400 and 398.1, 399.1 eV (sp2 and sp3 N atom bonded to Zn and Cu respectively) [28], and other
peaks which corresponds to the C-N, N-H, C=N and N-C=O bonds as shown in the Figures.8 and
9. We estimated the % of metallo porphyrins linked with MWCNTs using the peak position of
C-N (285.7 eV) and pyrolidinic nitrogen (400.2 eV)] [29]. The amount of Zn-TAP or Cu-TAP
functionalized with MWCNTs are found to be 5.18 and 5.00 % using the C-N binding energy of
nano hybrid I and II, respectively. Similarly, 2.14 and 3.26 % from the pyrolidinic nitrogen
present in the nano hybrid I and II, respectively. Further XP spectra of nano hybrid I show new
peaks related to Zn 2p1/2 and Zn 2p3/2 with the binding energies of 1044 and 1019 eV as shown in
the inlet of Figure 7a. Whereas inlet of Figure 7b shows 4 peaks with binding energies of 935,
943, 954 and 962 eV for nano hybrid II which correspond to Cu 2p3/2, 2p3/2 satellite line, 2p1/2
and 2p1/2 satellite line, respectively [30]. Moreover these four peaks are slightly broad as
compared to that of nano hybrid I which may be due to the paramagnetic nature of Cu. Thus XP
spectra clearly indicates the covalent functionalization of Zn-TAP and Cu-TAP with MWCNTs.
Table. 1. Elemental composition [wt %] of pure MWCNTs, nano hybrid I and II obtained from
XPS analysis (using areas of the respective peaks)
Sample
At wt %
C1s
O1s
N1s
-
MWCNT
94.51
79.14
75.68
64.75
5.49
20.86
17.78
27.3
MWCNT-COOH
MWCNT-ZnTAP
MWCNT-CuTAP
-
6.54
7.94

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Figure.10. UV-vis absorption spectra with the 1cm path length for MWCNT-COOH (1
mg/100 mL), Zn-TAP (7.1 x 10-6 M), MWCNT-ZnTAP (0.7 mg/100mL), Cu-TAP (3.39 x 10-
5 M), MWCNT-CuTAP (3.5 mg/100 mL) and controlled samples in DMF solvent
Figure 10 depicts UV-vis absorption spectra of MWCNT-COOH, Zn-TAP, Cu-TAP,
physically blended sample of MWCNT-COOH with Zn-TAP or Cu-TAP (as a controlled
sample, after sonicating the mixture for several hours), nano hybrids I and II in DMF solvent.
Zn-TAP and Cu-TAP reveal a strong Soret band at 437 nm and 425 nm, respectively. Both the
metal complexes show fewer Q peaks (two peaks) in the range 500 – 700 nm compared to
porphyrin free base (TAP, 4 peaks). This is due to improved symmetry on metalation relative to
the free base porphyrin from D2h (rectangular) to D4h (square) symmetry [24]. Generally,
simpler spectrum results from more symmetrical molecule. Soret band of Cu-TAP and nano
hybrid II show hypsochromic shift (blue shift) and also intensity of the peak is reduced
(hypochromic shift) as compared to TAP, Zn-TAP and their nano hybrids. This is owing to the
pronounced back bonding of paramagnetic Cu metal orbitals dπ (dxz and dyz) to porphyrin π*
orbital [24]. However, zinc metal coordination to TAP affect the π-π* energy gap (slightly

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reduced) causing a slight red shift in both Zn-TAP and nano hybrid I (see table 2) compared to
that of TAP and MWCNT-TAP. These findings clearly indicate that varying electronic energy
level due to different metal ion substitution. Soret band of nano hybrids I and II reveal a blue
shift as compared to pure Zn-TAP and Cu-TAP, respectively. This is evidence for the covalent
linking of Zn-TAP or Cu-TAP with MWCNT [26]. Whereas there is no shift in the Soret band of
physically blended samples as compared to metalloporphyrins. These nano hybrids also show
improved solubility similar to MWCNT-TAP [22]. Detailed absorption studies in different
solvents for example acetone, methanol, DMF, DMSO and THF have been carried out to
indicate the charge transfer between Zn-TAP or Cu-TAP with MWCNTs. Nano hybrid I exhibits
good solubility in all these solvents except THF (disperse after sonication). In order to determine
the solubility, we plotted Soret band absorption value of nano-hybrid I in these solvents against
concentrations to get a standard curve as given in the supporting information Figures S7 and S8.
However, nano hybrid II is disperse well after sonication. Nano hybrids I and II, Zn-TAP and
Cu-TAP reveal a red shift in the absorption spectra with increasing solvent polarity as shown in
Table 2. However, the red shift is more pronounced in the case of nano hybrid I and Zn-TAP as
compared to nano hybrid II and Cu-TAP. Consequently these nano hybrids also exhibit positive
solvatochromism similar to the reported MWCNT-TAP [22]. Due to the formation hydrogen
bond with the methanol, all these compounds exhibit blue shift in methanol.
Photoluminescence (PL) spectra of Zn-TAP, and nano hybrid I in DMF solvent are
presented in the Figure 11. Upon excitation of Zn-TAP and nano hybrid I at 430 nm, the
solution of MWCNT-ZnTAP exhibits 62 % fluorescent quenching of emission peak at 622 nm
as compared to that of pure Zn-TAP. This significant quenching is attributed to photo induced
electron/energy transfer between Zn-TAP and MWCNTs of nano hybrid I [22]. Nano hybrid I
displays two well resolved peaks at 622 nm and another at 676 nm with almost same intensity,
whereas n-TA shows broad pea at n with slight hu p n . Moreover, the peak of
MWCNT-ZnTAP (622 nm) is blue shifted as compared to Zn-TAP (633 nm). These finding
substantiates the covalent linking of Zn-TAP to MWCNT. The peaks of MWCNT-ZnTAP
exhibit red shift as compared to the reported nano hybrid MWCNT-SnTPP [18]. This is due to
the presence of electron donating amino group in the TAP moiety of nano hybrid I. Upon
excitation at different wavelength the nano hybrid I exhibits similar quenching (82 %) for the
peak at 633 nm as shown in Figure S9 (see supporting information λex = 450 nm). Nevertheless,

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the intensity of peak at 675 nm is increased by increasing the excitation wavelength.
Fluorescence emission studies were performed in different solvents such as DMSO, DMF,
methanol and THF at excitation wavelengths of 430 nm as shown in the Figure 12. Similar
quenching was observed for MWCNT-ZnTAP in all these solvents. However, the intensity of the
peak at 675 nm is varies with the polarity of solvent. This clearly indicates the effect of metal
substitution which alter the electronic energy level of nano hybrid. We observed bathochromic
shift (positive solvatochromism) by increasing the solvent polarity except in methanol which
shows a blue shift due to hydrogen bonding [22]. Quantum yield (ɸF) for the Zn-TAP and
MWCNT-ZnTAP were determined using TPP as reference at 20 oC (the measurement details are
given in the supporting information). We found that the ɸF of Zn-TAP is 0.12 whereas the ɸF of
MWCNT-ZnTAP is 0.08. The decrease in the ɸF is due to the charge transfer between Zn-TAP
and MWCNT in the hybrid. However, no peaks were observed in the fluorescent spectra of Cu-
TAP and MWCNT-CuTAP, due to paramagnetic effect of copper.
Table 2. Soret band peak position of Cu-TAP, MWCNT-CuTAP, Zn-TAP, and MWCNT-
ZnTAP in different solvents
Solvent with
decreasing
polarity
DMSO
Zn-TAP MWCNT-
Cu-TAP
(in nm)
MWCNT-
CuTAP
(in nm)
425
ZnTAP
(in nm)
(in nm)
440
437
428
433
434
432
425
429
425
425
415
420
DMF
423
Methanol
THF
413
416

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Figure.11. PL spectra of Zn-TAP and MWCNT-ZnTAP in DMF solvent (λex = 430 nm)
with the same absorbance value at the Soret band (Abs = 0.24)
Figure.12. PL spectra of MWCNT-ZnTAP and Zn-TAP in different solvents (λex = 430
nm) with the same absorbance value at the Soret band (Abs = 0.24)
4. Conclusion

ACCEPTED MANUSCRIPT
Zn and Cu tetraaminophenylporphyrin complexes were covalently connected with MWCNTs
through an amide linkage. Significant modification in the FT-IR and XP spectra confirms the
covalent functionalization of Zn-TAP and Cu-TAP with MWCNT. Raman spectra of MWCNT-
ZnTAP and MWCNT-CuTAP exhibited higher frequency shift compared with MWCNT. This is
possibly due to the metal co-ordination with TAP molecule. However, these nano hybrids
exhibited slightly lower frequency shift compared with MWCNT-COOH. MWCNT-CuTAP
shows more thermal stability compared to MWCNT-ZnTAP and also compared to the reported
similar nano hybrids. Absorption spectra of Cu-TAP and its nano hybrid show blue shift due to
the paramagnetic nature of Cu metal ion. Moreover, PL spectra of MWCNT-ZnTAP show, there
is a fluorescence quenching of Zn-TAP Soret band by MWCNTs in various solvents. This
quenching is owing to the covalent functionalization of Zn-TAP with MWCNT molecules in the
nano hybrid. This further substantiates the energy or electron transfer between Zn-TAP and the
MWCNTs.
Acknowledgements
We Thank COE-AMGT (MHRD, New Delhi) for their funding. We also thank Mr. S.
Sarath, ACNSMM, AIMS Kochi for recoding XPS and Mr. Shibu, DST-SAIF, Kochi for
recording TEM images.
Supporting Information available
NMR spectrum of Zn-TAP, EPR spectrum of Cu-TAP, IR spectra of nano hybrids I and II,
Tem images, deconvolution peak of MWCNT & MWCNT-COOH C 1s, solubility studies, PL
spectra of Zn-TAP and nano hybrid I in λex = 4 0 n , ɸF measurement details.
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Highlights
•
•
Zn and Cu tetraaminophenylporphyrin complexes were covalently connected with
MWCNTs.
Significant modification in the FT-IR and XP spectra confirms the covalent
functionalization of Zn-TAP and Cu-TAP with MWCNT.
These nano-hybrids exhibit positive solvatochromism
•
•
PL spectra of MWCNT-ZnTAP exhibited fluorescence quenching of Zn-TAP Soret band
by MWCNTs in various solvents.