Fluorescence quenching of organic molecule by insulator

Article abstract of DOI:10.1016/j.saa.2013.04.093

A new kind of fluorophore 2-(4-fluorophenyl)-1-phenyl-1H-benzo[d]imidazole (FPPBI) has been synthesized and characterized by 1H NMR, 13C NMR, mass spectral studies and single crystal XRD. The energy transfer from FPPBI to Al ₂O₃ nanocrystals has been studied by absorption, fluorescence and lifetime spectroscopic methods. The association between nanoparticles and FPPBI is explained from both absorption and fluorescence quenching data. The distance between FPPBI and Al₂O₃ as well as the critical energy transfer distance has been deduced.

Full text of DOI:10.1016/j.saa.2013.04.093

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 112 (2013) 417–421
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Fluorescence quenching of organic molecule by insulator
⇑
C. Karunakaran, J. Jayabharathi , K. Jayamoorthy
Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamilnadu, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
Electron transfer from excited state of
benzimidazole to alumina is not
possible.
ꢀ
ꢀ
Energy transfer from the organic
molecule to the alumina lattice.
Benzimidazole is adsorbed on the
surface of alumina nanoparticles.
a r t i c l e i n f o
a b s t r a c t
Article history:
A new kind of fluorophore 2-(4-fluorophenyl)-1-phenyl-1H-benzo[d]imidazole (FPPBI) has been synthe-
sized and characterized by 1H NMR, 13C NMR, mass spectral studies and single crystal XRD. The energy
transfer from FPPBI to Al2O3 nanocrystals has been studied by absorption, fluorescence and lifetime spec-
troscopic methods. The association between nanoparticles and FPPBI is explained from both absorption
and fluorescence quenching data. The distance between FPPBI and Al2O3 as well as the critical energy
transfer distance has been deduced.
Received 15 November 2012
Received in revised form 12 April 2013
Accepted 24 April 2013
Available online 3 May 2013
Keywords:
Ó 2013 Elsevier B.V. All rights reserved.
Benzimidazole
Nanocrystal
Quencher
Single crystal XRD
Introduction
dots makes them quite attractive, showing some unique optical
properties such as high quantum yield, symmetrical emission
Nanotechnology, inextricably a multidisciplinary field, has an
explosive growth in the past decade due to its extremely more
applications in various fields of nanoscale electronics, optics, mag-
netics, energy, catalysis, nanomedicines, clothing, cosmetics, etc.
[1]. Nanoparticle probes compared with organic dyes acting as bio-
sensors in chemical and biochemical fields have been researched
recently and their applications are becoming more extensive.
These probes have been applied to the ultrasensitive detection of
proteins, DNA sequencing, clinical diagnostics, etc. [2]. Compared
to conventional dyes, confinement of electronic states of quantum
spectra, broad-band excitation, photostability, and readily tunable
spectra [3–7]. The combination of nanoparticles and biological
molecules has attracted tremendous attention, where nanoparti-
cles could present a versatile nanoscale interface for biomolecular
recognition [8–10]. The electron-transfer process at the
semiconductor–dye interface has been successfully utilized in the
development of solar cells, electronic devices, heterogeneous pho-
tocatalysis and waste water treatment [11,12].
Imidazoles play important role in materials science due to their
optoelectronic properties [13–16]. They are used as ligands for the
synthesis of metal complexes of ruthenium(II), copper(II), cobal-
t(II), nickel(II), manganese(II), iridium(III) and several lanthanides
for nonlinear optical (NLO) applications. Some researchers have
⇑
Corresponding author. Tel.: +91 9443940735.
E-mail address: jtchalam2005@yahoo.co.in (J. Jayabharathi).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.04.093

418
C. Karunakaran et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 112 (2013) 417–421
reported the specific interaction between nanoparticles and pro-
tein as well as other biomolecules [17–20]. There are many reports
on the photoinduced electron transfer from organic molecule to
nanoparticle semiconductors [21–23]. However there are no re-
ports on the photophysical studies of fluorescent molecules with
insulator nanoparticles. In this article we attempt to find the inter-
action between insulator and organic molecule.
fluxed at 80 °C in ethanol. The reaction was monitored by TLC and
purified by column chromatography using petroleum ether (60–80°)
as the eluent. Yield: 50%. mp = 96 °C, Anal. calcd. for C19H13FN2: 1H
NMR (400 MHz, CDCl3): d6.98–7.02 (T, 2H), 7.26–7.36 (m, 4H), 7.50–
7.55 (m, 6H), 7.87–7.88 (d, 1H). 13C NMR (100 MHz, CDCl3): d110.50,
115.44, 115.64, 119.83, 123.13, 123.47, 126.14, 126.18, 127.43,
128.75, 130.01, 131.41, 131.49, 136.84, 137.19, 142.90, 151.47,
162.16, 164.65. MS: m/e 289.17, calcd 289.11 [M+1].
Materials and methods
Results and discussion
Materials
Crystal structure
4-Fluoro benzaldehyde, N-Phenyl-o-phenylenediamine and all
other reagents have been purchased from Sigma–Aldrich chemicals
and used without further purification. The Al2O3 nanoparticles
used was characterized by Karunakaran et al. [24]; the average
FPPBI [25] is a monoclinic crystal with space group P21/n. The
cell dimensions are a = 8.7527 Å, b = 10.1342 Å, c = 17.0211 Å. OR-
TEP diagram of FPPBI presented in Fig. 1a, shows that the benz-
imidazole unit is almost planar [maximum deviation = 0.0342 (9)
A for C6]. Fig. 1b displays the crystal packing diagram of FPPBI.
The dihedral angles between the planes of the benzimidazole and
the phenyl and the fluorobenzene groups are 58.94 (3) and 51.43
(3)°, respectively. The dihedral angle between the planes of the
phenyl and the fluorobenzene rings is 60.17 (6)°. Intermolecular
C4AH4ꢃ ꢃ ꢃF4, C7AH7ꢃ ꢃ ꢃF4 and C26AH26ꢃ ꢃ ꢃF4 hydrogen bonds and
crystal size and surface area are 11 nm and 148 S (m2 gꢁ1
respectively.
)
Measurements
The 1H and 13C NMR spectra of the FPPBI have been recorded on a
Bruker 400 MHz NMR instrument. Mass spectra of the sample have
been recorded on Thermo Fischer LC–Mass spectrometer in FAB
mode. The fluorescence quenching measurements have been carried
out using a Perkin Elmer LS55 spectrofluorimeter. The excitation and
weak C16AH16ꢃ ꢃ ꢃ
p
and C22AH22ꢃ ꢃ ꢃ
p interactions involving the
fused benzene ring are found in the crystal structure. Optimization
of FPPBI have been performed by DFT at B3LYP/6-31G(d,p) using
Gaussian-03. All these XRD data are in good agreement with the
theoretical values listed Table 1. However, from the theoretical val-
ues it can be found that most of the optimized bond lengths, bond
angles and dihedral angles are slightly higher than that of XRD val-
ues. These deviations can be attributed to the fact that the theoret-
ical calculations were aimed at the isolated molecule in the
gaseous phase where as the XRD results are about the molecule
in the solid state. The 1H and 13C NMR spectral data along with
the mass spectral results support the structure of the synthesized
molecule.
emission slit widths (each 10 nm) and scan rate (600 nm minꢁ1
)
were kept constant for all the measurements. The absorption spec-
tral measurements were recorded by using a Perkin Elmer Lambda
35 spectrophotometer. An ethanolic solution of the FPPBI of required
concentration (1 ꢂ 10ꢁ8 M) was mixed with nanoparticles dis-
persed in ethanol at different loading and the absorbance and emis-
sion spectra were recorded. The lifetime spectra have been recorded
on Fluorocube-01-NL lifetime system provided with emission
monochromator and diode excitation. Single crystal XRD has been
recorded in Agilent Xcalibur Ruby Gemini diffractometer. The Radi-
ation source is Enhance (Mo) X-ray Source. Graphite monochroma-
tor used is 10.5081 pixels mmꢁ1 for detector resolution.
Absorption and fluorescence quenching characteristics of FPPBI –
nanoparticles
General procedure for the synthesis of 2-(4-fluorophenyl)-1-phenyl-
1H-benzo[d]imidazole
Fig. 2 displays the absorption spectra of FPPBI in the presence of
Al2O3 nanocrystals dispersed at different loading and also in their
absence. It enhances the absorbance of FPPBI remarkably without
shifting its absorption maximum at 260 nm. This indicates that
A mixture of 4-Fluoro benzaldehyde (1 mmol), N-phenyl-o-phenyl-
enediamine (1 mmol) and ammonium acetate (2.5 mmol) was re-
Fig. 1. (a) ORTEP diagram of FPPBI with displacement ellipsoids drawn at the 50% probability level and (b) packing diagram of FPPBI.

C. Karunakaran et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 112 (2013) 417–421
419
Table 1
Selected bond lengths (Å), bond angles (°) and torsional angles (°) of FPPBI.
Bond lengths (Å)
Experimental XRD (Å)
Bond angles (°)
Experimental XRD (°)
Torsional angles (°)
Experimental XRD (°)
F4AC24
N1AC2
N1AC8
N1AC11
N3AC2
N3AC9
C2AC21
C7AC8
C8AC9
C14AC15
C15AC16
C21AC22
C24AC25
C25AC26
1.3633(1.3500)
1.3890(1.4947)
1.3833(1.4586)
1.4298(1.4700)
1.3186(1.3671)
1.3908(1.4604)
1.4735(1.5400)
1.3921(1.3863)
1.3961(1.4769)
1.3891(1.4014)
1.3952(1.4014)
1.3787(1.4014)
1.3787(1.4014)
1.3893(1.4014)
C2AN1AC8
C2AN1AC11
C8AN1AC11
C2AN3AC9
N1AC2AN3
N3AC2AC21
N1AC2AC21
N1AC8AC7
N1AC8AC9
N3AC9AC4
N3AC9AC8
N1AC11AC12
N1AC11AC16
F4AC24AC23
106.15(101.64)
128.03(113.68)
125.75(113.30)
104.91(105.49)
113.10(113.56)
123.76(123.21)
123.13(123.23)
131.97(130.71)
105.45(108.28)
129.50(130.69)
110.38(108.25)
119.29(120.00)
119.75(120.00)
118.21(120.00)
C8AN1AC2AN3
C8AN1AC2AC21
C11AN1AC2AN3
C11AN1AC2AC21
C2AN1AC8AC7
ꢁ0.88(17.57)
179.88(ꢁ162.03)
ꢁ178.02(139.68)
2.74(ꢁ39.92)
ꢁ175.66(166.13)
1.24(ꢁ13.73)
C2AN1AC8AC9
C11AN1AC8AC7
C11AN1AC8AC9
C2AN1AC11AC12
C2AN1AC11AC16
N1AC2AC21AC22
N1AC2AC21AC26
N3AC2AC21AC22
N3AC2AC21AC26
F4AC24AC25AC26
1.56(43.76)
178.46(ꢁ136.10)
ꢁ123.95(152.30)
56.07(ꢁ27.70)
50.81(ꢁ0.22)
ꢁ132.26(179.78)
ꢁ128.35(ꢁ179.78)
48.56(0.22)
178.48(180.00)
Values within the parentheses corresponds to theoretical values.
Al2O3 nanocrystals to the solution of FPPBI resulted in the quench-
ing of its fluorescence. Fig. 3 displays the effect of increasing con-
centration of Al2O3 nanocrystals on the fluorescence spectrum of
FPPBI. This quenching behavior is similar to the studies reported
earlier [26]. The apparent association constants (Kapp) have been
obtained from the fluorescence quenching data using the following
equation:
1=ðF0 ꢁ FÞ ¼ 1=ðF0 ꢁ FÞ þ 1=KappðF0 ꢁ FÞ ½nanoparticlesꢄ
ð1Þ
where Kapp is the apparent association constant, F0 is the initial fluo-
rescence intensity of FPPBI, F is the fluorescence intensity of FPPBI
adsorbed on nanoparticles. A good linear relationship between 1/
(F0 ꢁ F) and the reciprocal concentration of nanoparticles is seen.
From the slope, the values of Kapp have been assessed for FPPBI –
nanoparticles.
Fig. 2. Absorption spectra of FPPBI in presence and absence of Al2O3 ((a) Bare FPPBI
The fluorescence quenching behavior is usually described by
Stern–Volmer relation I0/I = 1 + KSV [Q]. Here, I0 and I are the fluo-
rescence intensities in the absence and presence of quencher, KSV is
the Stern–Volmer constant related to the bimolecular quenching
rate constant and Q is the quencher. Fig. 4 presents the Stern–Vol-
mer plots. The ability of the excited state FPPBI to inject its elec-
trons into the conduction band of nanoparticles is determined
from the energy difference between the conduction band of nano-
particles and excited state oxidation potential of FPPBI.
1 ꢂ 10-8 M, (b) to (g) FPPBI 1 ꢂ 10-8 M and Al2O3 1 ꢂ 10-5 M to 4.5 ꢂ 10-5
M
respectively).
the insulator nanocrystals do not modify the excitation process of
the FPPBI. The enhanced absorption at 260 nm observed with the
dispersed Al2O3 nanocrystals are due to adsorption of the FPPBI
on surface of nanocrystals. This is because of effective energy
transfer from the excited state of FPPBI to the alumina lattice.
The fluorescence quenching technique is applied to study the
interaction between Al2O3 nanocrystals and FPPBI. Addition of
Binding constant and number of binding sites
The FTIR spectrum of FPPBI shows the ˜C@N stretching vibra-
tion at 1602 cmꢁ1. This band is shifted when FPPBI is adsorbed
on the Al2O3 nanocrystals; a new band appears at 1624 cmꢁ1
.
These observations show that the FPPBI is bound to the surface
of nanoparticles. Static quenching arises from the formation of
complex between organic molecule and the quencher and the
binding constants (K) have been calculated by employing the
equation:
log½ðF0 ꢁ FÞ=Fꢄ ¼ log K þ n log ½Qꢄ
ð2Þ
where K is the binding constant of Al2O3 nanoparticles with FPPBI
and the calculated binding constant is 2.22 ꢂ 104. The value of ‘‘n’’
is close to unity for insulator Al2O3 which indicates single binding
of the organic molecule to insulator Al2O3 surface. The reported
binding constant of the FPPBI with insulator Al2O3 nanoparticle is
far less than those with semiconductor nanoparticles such as
WO3, CuO, and Fe2O3: the respective binding constants are
1.96 ꢂ 109, 1.86 ꢂ 108, and 5.69 ꢂ 106 [27].
Fig. 3. Fluorescence spectra of FPPBI in the presence and absence of Al2O3 ((a) Bare
FPPBI 1 ꢂ 10-8 M, (b) to (g) FPPBI 1 ꢂ 10-8 M and Al2O3 1 ꢂ 10-5 M to 4.5 ꢂ 10-5
M
respectively).

420
C. Karunakaran et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 112 (2013) 417–421
Fig. 4. Stern–Volmer plot of FPPBI with various concentrations of Al2O3
nanocrystals.
Fig. 5. Overlapping spectra of (i) acceptor fluorescence and (ii) donor absorption.
to the nanoparticles. The energy transfer efficiencies (E) is calcu-
lated using the equation E = 1 ꢁ (I/I0) is 0.12. Here, I is the emission
intensity of donor in the presence of acceptor and I0 is the emission
intensity of the donor alone. From the above results it is clear that,
in presence of Al2O3 nanoparticles, the fluorescence intensity of
FPPBI is reduced (from I0 to I) by energy transfer to nanoparticles.
Energy transfer between Al2O3 nanocrystals and FPPBI
According to Forster’s non-radiative energy transfer theory [28],
the energy transfer efficiency is related not only to the distance be-
tween the acceptor and donor (r0), but also to the critical energy
transfer distance (R0). That is
E ¼ R60=ðR60 þ r60Þ
ð3Þ
Fluorescence lifetime measurements
where R0 is the critical distance when the transfer efficiency is 50%.
An alternative way to rationalize the binding behavior in the
present study is by considering the fluorescence lifetime of FPPBI
with nanoparticles. The experimental decay curves were first fitted
to a biexponential,
R60 ¼ 8:8 ꢂ 10ꢁ25K2Nꢁ4u
where K2 is the spatial orientation factor of the dipole, N is the
refractive index of the medium, is the fluorescence quantum yield
J
ð4Þ
u
of the donor and J is the overlap integral of the fluorescence emis-
sion spectrum of the donor and the absorption spectrum of the
acceptor; Fig. 5 presents the overlap. The value of J can be calculated
by using the following equation:
fðtÞ ¼
a
1 expðꢁt=s1Þ þ
a
2 expðꢁt=s2
Þ
ð6Þ
where a1 and s1 are respectively, the pre-exponential factor and
lifetime of the various excited states involved. This model is based
on the assumption that one, two or three fluorescent states are
present in the solution. If Ni molecules are excited at zero time,
the fluorescence quantum yield of the ith component ai is propor-
tional to the ratio aisi/N and the ai factors are related to the absor-
bances of the various substances at the excitation wavelength. The
fluorescence decay curves of all nanoparticles with FPPBI were re-
corded in ethanol. Laser excitation was set at 290 nm and the fluo-
rescence signal was measured at emission wavelength of individual
compound. The fluorescence decay was fitted with a biexponential
function and the decay time, radiative (kr), non-radiative (knr) con-
stants and energy transfer rate constants (ket) are presented in Ta-
ble 2. Al2O3 nanoparticles bound to FPPBI change the fluorescence
lifetime. The time decay results are shown in Fig. 6. The half-life
Z
J ¼ FðkÞ
e
ðkÞk4dk=FðkÞdk
ð5Þ
where F(k) is the fluorescence intensity of the donor,
e(k) is molar
absorptivity of the acceptor. The parameter J can be evaluated by
integrating the spectral parameters in Eq. (5) as 4.99 ꢂ 10ꢁ12 cm3 -
L molꢁ1. Under these experimental conditions, the value of R0 calcu-
lated from Eq. (3) is found to be about 0.75 nm for insulator
nanocrystals; the values of K2 (=2/3) and N (=1.3467) used are from
the literature [29] and the
u value is from the present study. Obvi-
ously, the calculated value of R0 is in the range of maximal critical
distance. This is in accordance with the conditions of Forster’s
non-radiative energy transfer theory [30,28], indicating the static
quenching interaction between nanoparticles and FPPBI. The value
of r0 obtained using Eq. (2) is less than 8 nm [Al2O3: 1.04 nm].
The fact that the value of r0 is larger than that of R0 in the present
study also reveals the operation of static-type of quenching mecha-
nism [29]. The decrease in fluorescence intensity is attributed to en-
ergy transfer between FPPBI and the Al2O3 nanoparticles. This
makes possible the energy transfer from the excited state of FPPBI
of excited state I (s1) and II (s2) are of the order of 1.3 ns and
8.8 ns. Excited state II is more predominant in the case of isolated
molecule as seen from Table 2. Examination of the lifetime of the
two states shown that they do not very significantly due to binding.
The energy transfer rate constant (ket) is 0.03. The radiative and
non-radiative rate constants do not change when the nanoparticles
are bound to the FPPBI.
Table 2
Biexponential fitting parameter of fluorescence decay.
Compound
s1 (ns)
s2 (ns)
a1
a2
x2
saverage
kr
knr
ket
FPPBI
FPPBI + Al2O3
1.27
1.29
8.94
8.84
4.4731 ꢂ 10ꢁ2 (90.00)
7.0769 ꢂ 10ꢁ4 (10.00)
1.19
1.28
1.39
1.46
0.15
0.14
0.57
0.54
–
0.03
4.3048 ꢂ 10ꢁ2 (86.21)
1.0024 ꢂ 10ꢁ3 (13.79)
Values within the parentheses corresponds to relative amplitude.

C. Karunakaran et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 112 (2013) 417–421
421
Fig. 6. Fluorescence lifetime spectra of FPPBI in presence and absence of Al2O3 nanocrystals.
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Fluorophore 2-(4-fluorophenyl)-1-phenyl-1H-benzo[d]imidaz-
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through azomethine nitrogen. The distance between the FPPBI
and Al2O3 nanocrystals, deduced on the basis of Forester’s non-
radiation energy transfer theory are close to 3.12 nm and the crit-
ical energy transfer distances are found to be about 0.75 nm.
Quenching of fluorescence by nanoparticulate alumina is most
likely due to energy transfer. Increase in the non-radiative path-
ways, occasioned by the coupling between the molecule leads to
change the fluorescence lifetime with Al2O3 nanocrystals.
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Acknowledgments
One of the authors Prof. J. Jayabharathi is thankful to DST [No.
SR/S1/IC-73/2010], UGC (F.No. 36-21/2008 (SR)) and DRDO
(NRB-213/MAT/10-11) for providing funds to this research study.
Mr. K. Jayamoorthy is thankful to DST [No. SR/S1/IC-73/2010] for
providing fellowship.
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