Indian Journal of Pure & Applied Physics
Vol. 42, September 2004, pp 648-652
Fluorescence quenching of anthracene by aniline in different solvents
Annoji Reddy, J Thipperudrappa, D S Biradar, M T Lagare & S M Hanagodimath
Department of Physics, Gulbarga University, Gulbarga 585 106, Karnataka
Received 2 September 2003; revised 9 February 2004; accepted 7 May 2004
Fluorescence quenching of anthracene by aniline in five different solvents has been carried out at room temperature
with a view to understand the quenching mechanisms. Experimental results show positive deviation in the Stern-Volmer plots
in all the solvents studied. In order to interpret these results the ground state complex and sphere of action static quenching
models have been invoked. Using these models various rate parameters have been determined. Ground state complex
formation model is found not to be applicable. Using sphere of action static quenching model various rate parameters have
been determined. The magnitudes of these parameters imply that sphere of action static quenching model agrees well with the
experimental results. Hence, the positive deviation in the S-V plots is attributed to the static and dynamic quenching. Further,
with the use of finite sink approximation model it has been observed that these bimolecular reactions are diffusion limited.
The dependence of Stern-Volmer constant on dielectric constant of the solvents suggests the charge transfer character of the
excited complex.
[Keywords: Fluorescence quenching, Anthracene, Aniline, Stern-Volmer plots]
IPC Code: C 09K 9/00
1 Introduction
Fluorescence quenching of organic molecules in
solution by various quenchers like aniline,
bromobenzene, carbon tetrachloride, ethyltrithiocarbonate, halide ions, metal ions etc. has been
studied by several researchers1-4. The study has been
mainly to understand the nature of bimolecular
reactions taking place both under steady state and
transient conditions. This study has not only been of
importance in physical sciences but also in chemical,
biological and medical sciences5-6. The fluorescence
yield in bimolecular liquid systems is hindered due to
several mechanisms such as static and dynamic
quenching, excimer and exciplex formation, charge
transfer processes, etc. One of the well-known
experimental techniques used to study the role of
fluorescence quenching is to determine the quenching
rate parameter using Stern-Volmer (S-V) plot. If the
quenching mechanism is mainly due to dynamic
process, then it will be largely due to diffusion in
which case diffusion rate parameter, kd, equals the
quenching rate parameter, kq (=Ksv /τ), where Ksv is the
slope of the linear S-V plot and τ is the decay time of
the solute in the absence of quencher. On the other
hand, if the experimental results do not simulate with
the simple linear S-V equation, then it may be due to
one of the above processes other than or along with
diffusion processes. Fluorescence quenching of
anthracene by carbon tetrachloride (CCl4) was studied
by Leiten et al.7 and Lewis et al.8 by steady state
method. In the present study we have used steady
state method to investigate the quenching of
anthracene by aniline in five different organic
solvents namely benzene, toluene, heptane, cyclohexane and ethanenitrile with a view to understand
the nature of quenching mechanism involved in these
systems.
2 Experimental Details
The anthracene was obtained from Koch-Light
Laboratories Ltd., England and is of scintillation
grade. The aniline was obtained from BDH
Laboratory, England and was tested for purity before
use. The solvents were of spectroscopic grade and
were used without any further purification. The
solutions were prepared keeping the concentration of
anthracene fixed (0.25 g/L) and varying the quencher
concentration (0.02 to 0.10 M) in all the solvents.
Fluorescence spectrophotometer F-2000 was used for
fluorescence
intensity
measurements,
with
perpendicular geometry.
3 Results and Discussion
Fluorescence measurements were made by taking
fresh solution each time in a rectangular quartz cell
having an airtight stopper. The solute has maximum
absorption at 366 nm. The solute was excited at 366
REDDY et al.: FLUORESCENCE QUENCHING OF ANTHRACENE
nm and the fluorescence spectrum was recorded. The
maximum fluorescence corresponds to 397 nm. First,
the fluorescence intensity I0 was measured without the
quencher and then the fluorescence intensity I was
measured at different quencher concentrations and at
a fixed solute concentration. The experimental values
are reproducible within 5% of the experimental error.
The S-V plots were obtained using the values of I0 and
I and were found to be non-linear in all the solvents
showing positive deviation and are shown in Fig. 1.
Similar results were observed by others7,8 in case of
anthracene with CCl4 as quencher. Thus, positive
deviation from linearity suggests that quenching is not
purely collisional and this may be attributed either to
the ground state complex formation or to the sphere of
action static quenching model. This quenching
mechanism is shown in the following scheme.
⇌
kdiff
*
F +Q
*
(F . Q)
k-1
Fig. 1 ⎯ Stern-Volmer plots of I0 /I against [Q] in different
solvents
hνa
║
649
|
kf
hνf
kc (Non-emissive)
⇌
KS
F+Q
(F.Q)
where, F is a fluorophore, Q a quencher, F* an excited
fluorophore and kf, kdiff, k-1 and kc are the rate
constants for radiative decay, diffusion, back
diffusion with breakup of exciplex and non-emissive
quenching, respectively.
In order to see whether the ground state complex
formation is partly playing a role, we have used
extended S-V equation9,10
[( I 0 / I ) − 1] /[Q] = ( K sv + k g ) + ( K sv k g )[Q]
…(1)
where Ksv and kg are S-V and ground state association
constants, respectively. In accordance with Eq. (1),
the values of Ksv and kg can easily be determined by
least square fit method. The values of Ksv from this
method were found to be imaginary in all the
solvents. Therefore the role of ground state complex
formation is ruled out in the present case. Also, no
change in emission spectra in the presence of
quencher was observed in all the solvents. These facts
show that Eq. (1) is not applicable for the analysis of
the data corresponding to the observed positive
deviation in the S-V plots. Thus the analysis of the SV plot was made using sphere of action static
quenching model. The instantaneous or static
quenching occurs if the quencher molecule is very
near to, or in contact with the fluorescent molecule at
the exact moment it happens to be excited. This was
explained by the fact that only a certain fraction W of
the excited state is actually quenched by the
collisional mechanism. Some molecules in the excited
state, the fraction of which is (1 - W), are deactivated
almost instantaneously after being formed, because a
quencher molecule happens to be randomly
positioned in the proximity at the time the molecules
are excited and interacts very strongly with them.
Several models were employed (Smoluchowski
model)9,10 to describe this static quenching process, all
leading to the following modified form of the S-V
equation.
(I 0 / I ) =
1 + K sv [Q]
W
…(2)
650
INDIAN J PURE & APPL PHYS, VOL 42, SEPTEMBER 2004
where, Ksv and τ have their usual meanings as
explained earlier and [Q] is the quencher
concentration. The Smoluchowski’s diffusion
controlled equation containing transient term is given
by
K d = 4πN ' RD + 4 R 2 N ' (πD)1 / 2 t −1 / 2
…(3)
where, N' is the Avogadro’s number per millimole, R
is the encounter distance, i.e. the sum of the radii of
the solute and quencher molecules and t is the time.
The retention of the latter term of Eq. (3) leads to an
additional factor, W in Eq. (2), which is given by
W = e −V [Q ]
…(4)
where V is the static quenching constant and
represents an active volume element surrounding the
excited solute molecule.
Instantaneous (static) quenching occurs in a
randomly distributed system when a quencher
happens to reside within a sphere of action with a
volume V/N´, and radius ‘r’ [ V / N' = (4πr3 )/3 ]
surrounding a solute molecule at the time of
excitation.
As W depends on the quencher concentration [Q]
the S-V plots for a quencher with a high quenching
ability generally deviate from linearity. Thus Eq. (2)
is rewritten as
[1−(I / I0 )]/ Q = Ksv (I / I0 ) +(1−W) /[Q]
Fig. 2 ⎯ Plots of [1-(I/I0)]/[Q] against I/I0 in different solvents
Table 1 ⎯ Values of Stern-Volmer constant Ksv, bimolecular
quenching rate parameter kq, static quenching constant V and
kinetic distance r for different solvents
Solvent
From the table we see that the values of k qa and k qb
( k qa is the value of kq determined from lower portion
of the curves of Fig. 1) are approximately equal in all
× 109
(m-1s-1)
…(5)
In order to extract more information from Eq. (5)
it is worth to calculate the value of [1-(I/I0)]/[Q],
because the plot of [1-(I/I0)]/[Q] against I/I0 becomes
linear the slope of which is Ksv and intercept is
(1–W)/[Q]. Fig. 2 shows the plots of [1-(I/I0)]/[Q]
against I/I0 for anthracene with aniline for different
solvents which are linear. The value of Ksv was
obtained using least square fit method by determining
slope in all the solvents and are given in Table 1.
Fluorescence lifetime τ of the solute studied is
obtained from the catalogue of the Koch-Light
laboratories and is given at the bottom of the Table 1.
Bimolecular quenching rate parameter kq was
determined from the experimentally determined
values of Ksv and literature value of τ according to the
relation kq = Ksv /τ (Table 1) and are referred as k qb .
k qa
Ksv
(m-1)
k qb
× 109
(m-1s-1)
V
(m-1dm3)
R
(Å)
Benzene
3.710
16.44
3.355
10.91
16.22
Toluene
4.081
15.18
3.099
10.70
16.11
Heptane
4.636
20.23
4.130
7.13
14.02
Cyclohexane
4.534
19.89
4.060
10.49
16.02
Ethanenitrile
10.204
50.93
10.393
9.83
15.69
RY = 3.47 Å, RQ = 2.84 Å, τ = 4.9 ns
Bimolecular quenching rate parameter determined from linear
portion (low concentration) of the plots I0/I against [Q].
Bimolecular quenching rate parameter determined from sphere of
action static quenching model.
the solvents. This shows that sphere of action model
was able to recover the dynamic bimolecular
quenching constant (kq). Further, no specific
interactions between the solute and the quencher have
been observed. Therefore the positive deviation in the
S-V plots may be attributed to the sphere of action
static quenching model.
REDDY et al.: FLUORESCENCE QUENCHING OF ANTHRACENE
In order to support static and dynamic effects, we
have determined the magnitudes of static quenching
constant V and radii r of sphere of action (or kinetic
distance) using the sphere of action model. With the
use of Eqs (4) and (5) the values of V and r are
determined by least square fit method in all the
solvents and are given in Table 1. Similar results were
also obtained by others11-13.
The radii of the solute (RY) and the quencher (RQ)
molecules were determined by adding the atomic
volumes of all the atoms as suggested by Edward16
and are given at the bottom of the Table 1. From these
values of RY and RQ the sum of the molecular radii of
anthracene and aniline is determined. This sum of the
molecular radii is referred to as encounter distance.
This value of R is then compared with the values of r
to verify whether the reaction is due to sphere of
action model. Since according to Andre et al.17 and
Zeng et al.10, if the distance between the quencher and
the excited molecule lies between the encounter
distance and the kinetic distance, the static effect
takes place especially in the case of steady state
experiments irrespective of ground state complex
formation provided reactions are limited by diffusion.
From Table 1 we see that the values of kinetic
distance r are greater than the encounter distance R
indicating that sphere of action model holds well.
Further, it may also be noted that a positive deviation
in the S-V plot is expected when both static and
dynamic quenching occurs simultaneously9. In order
to find whether the reactions are diffusion limited we
invoked finite sink approximation model. According
to this model18-20 the following modified S-V
relationship is obtained.
K sv−1 = ( K sv0 ) −1 −
where K sv0 =
(2πN ' ) 1 / 3
[Q]1 / 3
4πN ' Dτ
…(6)
4πN ' DRτk a
, N ' is the Avagadro’s
4πN ' DR + k a
number (per millimole), D is the sum of the diffusion
coefficients (mutual diffusion coefficient) of the
reactants and R is the reaction distance at which the
reaction proceeds, ka the activation energy controlled
rate constant. A plot of K sv−1 against [Q]1/3 becomes
linear with negative slope. Mutual diffusion
coefficient D becomes directly accessible from the
slope of the graph exemplified in Eq. (6) and Ksv is
obtained at [Q] = 0 regardless of the relative
651
magnitudes of ka and kd (= 4πN'D R), irrespective of
quenching is diffusion limited or not.
Therefore according to Eq. (6) we need to
determine the values of K sv−1 and [Q]1/3. Where Ksv =
[(I0 /I)-1]/[Q] and [Q] the quencher concentration
from 0.02 to 0.1 M. For efficient quenching processes
(concentration dependent) the value Ksv is often
observed to increase with [Q]. Hence, the values of
Ksv were determined at each quencher concentration
in all the solvents and the values of K sv−1 are also
determined. Fig. 3 shows the plots of K sv−1 against
[Q]1/3. From these figures we see that all the plots in
different solvents are almost linear and small
deviation may be due to experimental uncertainties.
Hence, the linear dependence of K sv−1 on the one-third
power of quencher concentration within the error
limits is confirmed10. Then the least square fit value of
K sv0 (S-V constant at [Q] = 0) was obtained from the
intercept of the plot of K sv−1 against [Q]1/3. Similarly,
mutual diffusion coefficients D were determined from
the slope of the Eq. (6) by least square fit method and
the values of K sv0 and D are presented in Table 2.
Using these values of K sv0 and D, the distance
parameter R' was determined and the values are
presented in the Table 2. In our case R' > R in all the
solvents and hence the values of ka cannot be
determined. According to Joshi et al.21 the
bimolecular reactions are said to be diffusion limited
if the values of kq are greater than 4πN'R'D. Hence the
values of 4πN'R'D are calculated using the
experimentally determined values of R' and D of
Eq. (6) and are presented in the Table 2. In all
solvents the values of kq are greater than 4πN'R'D,
which is an expected result for diffusion limited
reaction21 (Table 2).
Here we made an attempt to compare the results
obtained for anthracene + aniline system with that of
anthracene + CCl4 obtained by others7,8. The values of
Ksv obtained for anthracene + aniline system are high
compared to anthracene + CCl4 system. Further, for
anthracene + CCl4 they have observed an increase in
Ksv with increase in dielectric constant of the solvent.
Similar trend was observed for the present anthracene
+ aniline system (for example for cyclohexane Ksv =
4.534 m-1 and for ethanenitrile Ksv = 10.204 m-1). This
effect of dielectric constant suggests the charge
652
INDIAN J PURE & APPL PHYS, VOL 42, SEPTEMBER 2004
From the ongoing discussion we conclude that
quenching reaction is diffusion limited and both static
and dynamic quenching processes are partly playing a
role in the quenching mechanism. Charge transfer
nature of the excited complex is observed in the
present work.
Acknowledgement
The authors are greateful to Prof. B G Mulimani,
Chairman, Department of Physics, Karnataka
University, Dharwad, for providing facilities to carry
out experiments.
References
1
2
3
4
5
Fig. 3 ⎯ Plots of
Table 2 ⎯ Values of
K sv−1
K
0
sv
versus [Q]1/3 in different solvents
(steady state quenching constant at
[Q] = 0), mutual diffusion coefficient D, distance parameter R',
4πN'DR' and quenching rate parameter kq
Solvent
K
0
sv
(m-1)
D × 10-5
(cm2 s-1)
6
7
8
9
R' (Å) 4πN'R'D × 09 kq × 109
(m-1s-1)
(m-1s-1)
10
11
12
Benzene
14.06
0.325
11.66
2.868
3.355
13
Toluene
12.95
0.295
11.83
2.641
3.099
14
Heptane
16.77
0.466
9.70
3.424
4.130
15
Cyclohexane
16.47
0.388
11.44
3.361
4.060
Ethanenitrile
37.59
0.917
11.05
7.671
10.393
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non-polar solvents (benzene, toluene, heptane and
cyclohexane) can be explained by the greater charge
transfer character of the exciplex in the polar
solvent22.
16
17
18
19
20
21
22
23
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