Photoinduced electron-transfer from benzimidazole to nanocrystals

Article abstract of DOI:10.1016/j.molliq.2012.10.032

The dynamics of photoinduced electron injection and energy transfer from benzimidazole to CuO, Fe₂O₃, WO₃ and Al ₂O₃ nanoparticles has been studied by FT-IR, absorption and fluorescence spectroscopic

Full text of DOI:10.1016/j.molliq.2012.10.032

Journal of Molecular Liquids 177 (2013) 295–300
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Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Photoinduced electron-transfer from benzimidazole to nanocrystals
C. Karunakaran, J. Jayabharathi ⁎, K. Jayamoorthy, P. Vinayagamoorthy
Department of Chemistry, Annamalai University, Annamalainagar 608002, Tamilnadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 April 2012
Received in revised form 24 October 2012
Accepted 27 October 2012
Available online 13 November 2012
The dynamics of photoinduced electron injection and energy transfer from benzimidazole to CuO, Fe O , WO
3
and Al O nanoparticles has been studied by FT-IR, absorption and fluorescence spectroscopic methods. The
2 3
2
3
association between nanoparticles and the benzimidazole derivative was explained from both absorption
and fluorescence quenching data [Kapp=2.11×106
−1
(CuO); 4.11×106
−1
6
−1
M
M
2 3
(Fe O ); 2.91×10 M
−1
(WO
3
) and 3.85×106
M
2 3
(Al O )]. There is good agreement between these values of Kapp obtained from
the data of fluorescence quenching with those determined from the absorption spectral changes which
highlighted the validity of the association between benzimidazole and nanoparticles. The distance between
Keywords:
Benzimidazole
Nanocrystals
Electron transfer
Energy transfer
the benzimidazole derivative and nanoparticles (r
(R ~1.70 nm) has been calculated. By applying the Rehm–Weller equation, the free energy change (ΔGet) for
electron injection has also been calculated.
0
~3.12 nm) as well as the critical energy transfer distance
0
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
in supramolecular chemistry [14]. They are important building blocks
for the synthesis of proton, anion and cation sensors. As far as our
knowledge is concerned, detailed study on the photoinduced interac-
tion of nanoparticles with bioactive benzimidazole derivatives [15] has
not been reported till now. The fluorescence quenching technique is
applied to study the interaction between nanomaterials and the benz-
imidazole derivative to infer the association and also the energy transfer
Nanotechnology, inextricably a multidisciplinary field, has an ex-
plosive growth in the past decade due to its extremely hefty applica-
tions in various fields of nanoscale electronics, optics, magnetics,
energy, catalysis, nano medicines, clothing, cosmetics, etc. [1]. Nano-
crystalline semiconductors with unique properties such as large surface
area, pore structure, embedded effect and size effect are recognized to
have potential applications in genomics, proteomics and bioanalytical
fields. The combination of nanoparticles and biological molecules has
attracted tremendous attention, where nanoparticles could present a
versatile nanoscale interface for biomolecular recognition [2–4]. Some
researchers have reported the specific interaction between nanoparticles
and protein as well as other biomolecules [5–9]. The electron-transfer
process at the semiconductor–dye interface has been successfully
utilized in the development of solar cells, electronic devices, heteroge-
neous photocatalysis and waste water treatment [10,11]. The efficiency
of these processes depends on the properties of the sensitizers, semi-
conductor and their interaction under photo excitation. Recently, semi-
conductor composite nanostructures have become an attractive topic
because of their potential applications in different fields [12].
between them (Scheme 1). According to the plot of log [(F −F)/F] ver-
0
sus log [nanoparticles], the binding constants have been determined.
Further, the mechanism of electron transfer process on the basis of
energy level diagram has also been proposed in this paper. Distances
between the benzimidazole derivative and nanosurfaces (r
0
~3.12 nm)
as well as the critical energy transfer distance (R
been calculated.
0
~2.30 nm) have also
2. Materials and methods
2.1. Materials
Benzaldehyde, o-phenylenediamine and all other reagents have
been purchased from Himedia chemicals and used without further
purification. The nanoparticles used were those supplied by Sigma-
Aldrich and characterized by Karunakaran et al. [16]; the average
crystal size (D) and surface area (S) are listed in Table 1.
Imidazoles display a broad spectrum of biological activities such as
antiviral, antiulcer, antihypertension and anticancer properties [13].
They have also found application as a chromophore with high extinction
coefficient, readily tunable absorption wavelength, and fluorophoric
properties and are desirable as a large planar synthetic building block
2.2. Measurements
The H and 13C NMR spectra of the ligand have been recorded on a
Bruker 400 MHz NMR instrument. Mass spectrum was recorded
using Agilant 1100 mass spectrometer. The fluorescence quenching
measurements have been carried out using a Perkin Elmer LS55
1
⁎
Corresponding author at: Department of Chemistry, Annamalai University,
Annamalainagar 608 002, Tamilnadu, India. Tel.: +91 9443940735.
E-mail address: jtchalam2005@yahoo.co.in (J. Jayabharathi).
0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molliq.2012.10.032

296
C. Karunakaran et al. / Journal of Molecular Liquids 177 (2013) 295–300
applicable to alumina; alumina is an insulator and the possible explana-
tion is energy transfer from the excited state of the ligand to the alumina
lattice.
3.2. FT-IR characteristics of benzimidazole derivative-nanoparticles
Fig. 2 shows the FT-IR spectra of the benzimidazole derivative
(broken line) and it is bound to the nanoparticle (solid line). The spec-
trum of the benzimidazole derivative shows the >C_N stretching vibra-
−
1
tion at 1604 cm . This band is shifted in the benzimidazole derivative
−
1
bound to nanoparticle; a new band appears at 1633 cm . These obser-
vations show that the benzimidazole derivative is adsorbed on the sur-
face of nanoparticles.
Scheme 1. Photoinduced charge injection and charge separation.
spectrofluorometer. The excitation wavelength was 271 nm and the
3
.3. Fluorescence quenching characteristics
emission was monitored at 361 nm. The excitation and emission slit
−
1
widths (each 5 nm) and scan rate (600 nm min ) were kept constant
for all the measurements. The absorption spectral measurements were
recorded by using a Perkin Elmer Lambda 35 spectrophotometer. An
ethanolic solution of the benzimidazole derivative of required concen-
Addition of nanoparticles to the solution of the benzimidazole de-
rivative resulted in the quenching of its fluorescence emission and the
values are tabulated in Table 3. Fig. 3 displays the effect of increasing
concentration of nanoparticles on the fluorescence emission spectrum of
benzimidazole derivative [17]. The apparent association constants (Kapp
have been obtained from the fluorescence quenching data using the
−
8
tration (1×10
M) was mixed with nanoparticles dispersed in
)
ethanol at different loadings and the absorbance and emission spectra
were recorded.
following equation
2
.3. General procedure for the synthesis of ligand
1
=ðF –FÞ ¼ 1=ðF –FÞ þ 1=K ðF −FÞ½nanoparticlesꢀ
ð1Þ
0
0
app
0
A mixture of benzaldehyde (2 mmol), o-phenylenediamine (1 mmol)
where Kapp is the apparent association constant, F
0
is the initial fluores-
and ammonium acetate (2.5 mmol) was refluxed at 80 °C in ethanol. The
reaction was monitored by TLC and purified by column chromatography
using petroleum ether:ethyl acetate (9:1) as the eluent.
cence intensity of the benzimidazole derivative, F is the fluorescence in-
tensity of the benzimidazole derivative adsorbed on nanoparticles and F
is the observed fluorescence intensity at its maximum. A good linear
relationship between 1/(F −F) and the reciprocal concentration of
0
2
.3.1. 1-benzyl-2-phenyl-1H-benzo[d]imidazole
Yield: 50%. mp=124 °C, anal. calcd. for C20H N : C, 84.48; H,
16 2
nanoparticles is seen. From the slope, the values of apparent association
constants (Kapp) have been assessed for benzimidazole derivative-
nanoparticles.
1
5
.67; N, 9.85. Found: C, 84.12; H, 5.79; N, 10.09. H NMR (400 MHz,
): δ5.43 (S, 2H), 7.125–7.140 (d, 2H) (J=6 Hz), 7.23–7.28
m, 2H), 7.32–7.37 (m, 4H), 7.45–7.50 (m, 3H), 7.71–7.72 (t, 2H)
J=5.4 Hz), 7.90–7.91 (d, 1H) (J=6.8 Hz). 13C NMR (100 MHz, CDCl
):
δ 48.39 (\CH carbon), 110.53, 120.02, 122.68, 123.04, 125.99, 127.78,
28.76, 129.06, 129.28, 129.91, 130.14, 136.10, 136.43, 143.24, 154.19
aromatic carbons). MS: m/e 284.3, calcd. 285.13 [M+1].
CDCl
(
(
3
The fluorescence quenching behavior is usually described by the
Stern–Volmer relation:
3
I0=I ¼ 1 þ KSV ½Qꢀ
where, I and I are the fluorescence intensities in the absence and
ð2Þ
2
1
(
0
presence of quencher, KSV is the Stern–Volmer constant related to the
bimolecular quenching rate constant and Q is the quencher (Fig. 4).
The ability of the excited state benzimidazole derivative to inject
its electrons into the conduction band of nanoparticles is determined
from the energy difference between the conduction band of nanoparticles
and excited state oxidation potential of the benzimidazole derivative.
According to the equation Es*/s+ =Es/s+−E , the oxidation potential of
s
excited singlet state benzimidazole derivative is −1.97 V (vs. NHE),
where, Es/s+ is the oxidation potential of the benzimidazole derivative,
3
. Results and discussion
3
.1. Absorption of benzimidazole derivative-nanoparticles
The absorption spectra of the benzimidazole derivative in the pres-
2 3 3 2 3
ence of CuO, Fe O , WO and Al O nanoparticles dispersed at different
loadings and also in their absence are displayed in Fig. 1 and the values
are tabulated in Table 2. The nanoparticles enhance the absorbance of
the benzimidazole derivative remarkably without shifting its absorp-
tion maximum (361 nm). This indicates that the nanocrystals do not
modify the excitation process of the ligand. The enhanced absorption
at 271 nm observed with the dispersed semiconductor nanoparticles
are due to adsorption of the benzimidazole derivative on the semicon-
ductor surface. This is because of effective transfer of electron from the
excited state of the benzimidazole derivative to the conduction band
of the semiconductor nanoparticles. However, this inference is not
s
0.21 V (vs. NHE) and E is the excited state energy, 2.18 eV; the excited
state energy of the benzimidazole derivative is calculated from the fluo-
rescence maximum based on the reported method [18]. The energy
level of the conduction band of semiconductor nanoparticles is shown
in Scheme 2 [19]. It suggests that the electron transfer from excited
state benzimidazole derivative to the conduction band of nanoparticulate
semiconductors is energetically favorable.
3.4. Binding constant and number of binding sites
Static quenching arises from the formation of complex between
Table 1
Crystallite size (D), surface area (S) and apparent association constants (Kapp).
fluorophore and the quencher and the binding constants (K) have been
calculated by using the equation
Nanocrystal Crystal structure
D (nm) S (m2 g−1
)
10−6
K
app
δ,γ-Al Cubic:tetragonal :: 0.65:0.35 11
2
O
3
148
39
33
3.85
2.91
4.11
2.11
log½ðF –FÞ=Fꢀ ¼ logK þ n log½Qꢀ
ð3Þ
0
WO
3
Primitive monoclinic
Cubic
23
28
32
δ-Fe
2
O
3
where K is the binding constant of nanoparticles with benzimidazole de-
rivative and the calculated binding constant values are [2.11×10 (CuO),
CuO
End centered monoclinic
36
6

C. Karunakaran et al. / Journal of Molecular Liquids 177 (2013) 295–300
297
2 3 3 2 3
Fig. 1. Absorption spectra of benzimidazole derivative in the presence and absence of CuO, Fe O , WO and Al O nanoparticles.
2
.91×106 (WO ), 3.85×106 (Al ), 4.11×106 (Fe
3 2 3 2 3
O O )]. The order of
binding constant K is
CuO > WO3 > Al O > Fe O :
2
3
2
3
The value of “n” is close to unity which indicates single binding of
the organic molecule to the nanoparticulate surface. Quenching of
fluorophore of the benzimidazole derivative by nanoparticle may
also be due to the binding of the fluorophore of the organic molecule
with the nanoparticulate semiconductors (Scheme 3).
Fig. 2. FT-IR spectra of benzimidazole (broken line) and benzimidazole bound with
nanoparticulate copper oxide (solid line).
Table 2
Table 3
Absorption intensity of benzimidazole derivative in presence and absence of CuO,
Emission intensity of the benzimidazole derivative in the presence and absence of CuO,
Fe
2
O
3
, WO
3
and Al
2
O
3
nanoparticles.
2 3 3 2 3
Fe O , WO and Al O nanoparticles.
Materials and concentration Absorption Absorption Absorption Absorption
Materials and concentration
Emission
intensity
of CuO
Emission
intensity
Emission
intensity
Emission
intensity
of Al O
2 3
intensity
of CuO
intensity
of Fe
intensity
intensity
of Al
2
O
3
of WO
3
2
O
3
of Fe
2
O
3
of WO
3
Bare ligand 1∗10−
8
M
0.010
0.015
0.023
0.010
0.017
0.023
0.010
0.018
0.028
0.010
0.015
0.021
Bare ligand 1∗10
−8
M
563
528
563
515
563
524
563
505
−
5
−8
Bare nanoparticle 1∗10
M
Ligand 1∗10
M+
Ligand 1∗10− M+
8
nanoparticle 1∗10
−5
M
M
M
−
−
−
5
−8
nanoparticle 1∗10
Ligand 1∗10− M+
nanoparticle 2∗10
Ligand 1∗10− M+
nanoparticle 3∗10
Ligand 1∗10− M+
M
M
Ligand 1∗10
M+
510
473
441
406
381
78
459
416
376
344
312
79
489
457
426
388
363
78
463
411
376
347
321
79
8
−5
−5
0.032
0.041
0.048
0.058
0.061
0.033
0.043
0.052
0.060
0.068
0.035
0.042
0.051
0.061
0.070
0.024
0.034
0.041
0.048
0.052
nanoparticle 2∗10
5
−8
Ligand 1∗10
M+
8
nanoparticle 3∗10
5
−8
M
Ligand 1∗10
M+
8
−5
nanoparticle 3.5∗10
M
−
5
5
−8
nanoparticle 3.5∗10
M
Ligand 1∗10
M+
Ligand 1∗10− M+
8
nanoparticle 4∗10
−5
M
−
−8
nanoparticle 4∗10
M
Ligand 1∗10
M+
Ligand 1∗10− M+
8
nanoparticle 4.5∗10
Bare nanoparticle 1∗10
−5
M
nanoparticle 4.5∗10−
5
M
−5
M

298
C. Karunakaran et al. / Journal of Molecular Liquids 177 (2013) 295–300
2 3 3 2 3
Fig. 3. Fluorescence spectra of benzimidazole derivative in the presence and absence of various concentrations of CuO, Fe O , WO and Al O nanoparticles.
3
.5. Energy transfer between nanoparticles and the benzimidazole derivative
2 3
in the case of insulators (Al O ). The excited state energy of the benz-
imidazole derivative, as shown in Scheme 2, is larger than the conduc-
tance band energy levels of nanosemiconductors [20]. This makes
possible the energy transfer from the excited state of the benzimidazole
derivative to the nanoparticles.
The decrease in fluorescence intensity is attributed to electron
transfer between benzimidazole derivative and the nanoparticles in
the case of semiconductors (CuO, WO and Fe ) and energy transfer
3
2 3
O
2 3 3 2 3
Fig. 4. Stern–Volmer plot of benzimidazole with various concentrations of CuO, Fe O , WO and Al O nanoparticles.

C. Karunakaran et al. / Journal of Molecular Liquids 177 (2013) 295–300
299
Fig. 5. Overlapping of (i) absorption and (ii) fluorescence spectra of donor and acceptor.
0
where, R is the critical distance when the transfer efficiency is 50%.
6
−25
2
−4
R
¼ 8:8 ꢁ 10
K N φJ
ð6Þ
0
2
where, K is the spatial orientation factor of the dipole, N is the refrac-
tive index of the medium, φ is the fluorescence quantum yield of the
donor and J is the overlap integral of the fluorescence emission spectrum
of the donor and the absorption spectrum of the acceptor (Fig. 5). The
value of J can be calculated by using Eq. (7),
4
J ¼ ∫FðλÞεðλÞλ dλ=FðλÞdλ
ð7Þ
where, F(λ) is the fluorescence intensity of the donor, and ε(λ) is
the molar absorptivity of the acceptor. The parameter J can be
evaluated by integrating the spectral parameters in Eq. (7) as
−
14
1
.85×10
0
. Under these experimental conditions, the value of R cal-
culated from Eq. (6) is found to be about 2.30 nm in all cases; the values
2
of K (=2/3) and N (=1.3467) used are from the literature [22] and the
Scheme 2. Schematic diagram describing the electron-donating energy level of
benzimidazole.
0
φ value is from the present study. Obviously, the calculated value of R is
in the range of maximal critical distance. This is in accordance with the
conditions of Forster's non-radiative energy transfer theory [23,24], in-
dicating the static quenching interaction between nanoparticles and the
Energy transfer efficiency (E) is calculated from the following equa-
tion as 0.12
benzimidazole derivative. The value of r
Eq. (4) is less than 8 nm. The fact that the value of r
that of R in the present study also reveals the operation of static-type
of quenching mechanism [25].
0
(3.12 nm) obtained using
is larger than
E ¼ 1–ðI=I0Þ
where, I is the emission intensity of the donor in the presence of accep-
ð4Þ
0
0
tor and I is the emission intensity of the donor alone. From the above
results it is clear that, in the presence of nanoparticles, the fluorescence
0
3.6. Calculation of free energy change (ΔGet) for electron transfer processes
intensity of the benzimidazole derivative is reduced (from I
energy transfer to nanoparticles.
According to Forster's non-radiative energy transfer theory [21],
the energy transfer efficiency is related not only to the distance between
0
to I) by
The thermodynamic feasibility of excited state electron transfer
reaction has been confirmed by the calculation of free energy change
by employing the well known Rehm–Weller expression [26].
the acceptor and donor (r
0
), but also to the critical energy transfer dis-
1=2
1=2
ΔGet ¼ E ðoxÞ−E
−E þ C
ð8Þ
tance (R
0
). That is
ðredÞ
s
ꢀ
ꢁ
where, E1/2(ox) is the oxidation potential of the benzimidazole deriva-
tive (0.21 V), E (red) is the reduction potential of nanoparticles, that
6
6
6
E ¼ R 0=
R
þ r
ð5Þ
0
0
1/2
Scheme 3. Electrostatic interaction of the benzimidazole derivative with the nanoparticle surface.

300
C. Karunakaran et al. / Journal of Molecular Liquids 177 (2013) 295–300
[
[
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. Conclusions
-benzyl-2-phenyl-1H-benzo[d]imidazole is adsorbed on the surface
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