Steady-state and time-resolved fluorescence behavior of C153 and PRODAN in room-temperature ionic liquids

Article abstract of DOI:10.1021/jp0143591

Room temperature ionic liquids have emerged as nature-friendly media suitable for various applications. In this work, steady-state and time-resolved fluorescence behavior of coumarin 153 (C153) and 6-propionyl-2-dimethylaminonaphthalene (PRODAN) have been reported in two room-temperature sonic liquids based on substituted imidazolium salts. The steady-state fluorescence behavior of the two probe molecules suggests that these liquids are more polar than acetonitrile. The polarity of these media is found to be comparable to that of 2-propanol. No significant difference in the polarity of the two media could be observed although the cationic components of the two liquids contain alkyl substitutents of different chain length. Both the probe molecules display a wavelength-dependent fluorescence decay behavior. The decay profile at the red edge of the spectrum consists of an initial rise followed by a slow decay, while no rise could be observed at the blue edge of the spectrum. The solvation dynamics of the two probe molecules in the two room-temperature ionic liquids have been studied from the time-dependence of the spectral shift function constructed from the wavelength-dependent fluorescence decay profiles. The solvation of the probe molecules occurs on two different time scales with the absolute values strongly dependent on the cationic component of the ionic liquids. The average solvation time is found to be fairly long and dependent on the probe molecule used.

Full text of DOI:10.1021/jp0143591

6670
J. Phys. Chem. A 2002, 106, 6670-6675
Steady-State and Time-Resolved Fluorescence Behavior of C153 and PRODAN in
Room-Temperature Ionic Liquids
Rana Karmakar and Anunay Samanta*
School of Chemistry, UniVersity of Hyderabad, Hyderabad 500 046, India
ReceiVed: NoVember 29, 2001; In Final Form: May 10, 2002
Room temperature ionic liquids have emerged as nature-friendly media suitable for various applications. In
this work, steady-state and time-resolved fluorescence behavior of coumarin 153 (C153) and 6-propionyl-
2-dimethylaminonaphthalene (PRODAN) have been reported in two room-temperature ionic liquids based
on substituted imidazolium salts. The steady-state fluorescence behavior of the two probe molecules suggests
that these liquids are more polar than acetonitrile. The polarity of these media is found to be comparable to
that of 2-propanol. No significant difference in the polarity of the two media could be observed although the
cationic components of the two liquids contain alkyl substitutents of different chain length. Both the probe
molecules display a wavelength-dependent fluorescence decay behavior. The decay profile at the red edge of
the spectrum consists of an initial rise followed by a slow decay, while no rise could be observed at the blue
edge of the spectrum. The solvation dynamics of the two probe molecules in the two room-temperature ionic
liquids have been studied from the time-dependence of the spectral shift function constructed from the
wavelength-dependent fluorescence decay profiles. The solvation of the probe molecules occurs on two different
time scales with the absolute values strongly dependent on the cationic component of the ionic liquids. The
average solvation time is found to be fairly long and dependent on the probe molecule used.
1. Introduction
polarity,19-23 nitriles,24 N-methylamides,25 alkyl cyanides,26
water and organized assembly in water,1,2,27-30 inorganic molten
salt at high temperature,31-33 and ionic salt solutions.34
Study of the time-dependent response of the solvent molecules
to a newly created charge distribution in a dissolved solute has
been an attractive topic of investigation in recent years.1-7 From
the experimental point of view, the time-dependent shift of the
fluorescence spectrum of a probe molecule in polar media is
quantitatively measured following excitation by an ultra-short
laser pulse to obtain information on the solvation dynamics.
The theoretical treatments to understanding the solvent response
to the newly created charge distribution of the fluorophore
immersed in a polar solvent can be broadly categorized into
two approaches. The first approach, commonly referred to as
“simple continuum” models, is essentially based on the initial
treatments of Bakshiev8 and Mazurenko,9 in which the micro-
scopic structure of the solvent is disregarded and instead, the
solvent is considered as a continuous, homogeneous fluid and
is characterized by bulk, frequency-dependent dielectric response
function.10-14 The continuum models predict exponential sol-
vation kinetics with a lifetime equal to the longitudinal relaxation
time, τL of the solvent. This bulk dielectric property is related
to much slower Debye relaxation time, τD according to the
equation τL ) (ꢀ∞/ꢀ0)τD, where ꢀ∞ and ꢀ0 are the optical and
static dielectric constants of the solvent. The second theoretical
approach, initially proposed by Wolynes15 and later developed
by Rips,16,17 and Nichols and Calef,18 recognizes the molecular
nature of the solvent. In this mean spherical approximation
(MSA) model, the solvent molecules are considered as dipolar
hard spheres. The dynamical MSA model predicts a more
complex dynamics and a range of relaxation times between τL
and τD instead of a single relaxation time τL predicted by the
continuum theory. Some of the media in which the solvation
dynamics has been extensively studied are alcohols of different
Room-temperature ionic liquids have emerged as a new kind
of media alternative to the conventionally used organic solvents,
which are generally volatile, flammable, and hazardous
chemicals.35-49 Even though ionic liquids such as [EtNH3][NO3],
which has a melting point of 12° C, have been known since
1914,36 it is only very recently that the usefulness of these
systems as a new kind of media for synthetic, electrochemical,
and catalytic applications is being explored.35-52 Some of the
properties that make the room-temperature ionic liquids attrac-
tive media for various applications are the wide liquid range,
nonvolatility (negligible vapor pressure), nonflammable nature,
less reactivity, and the ability to dissolve a large variety of
organic and inorganic substances including even the polymer
materials in high concentration. Many of these properties have
made the room-temperature ionic liquids a nature-friendly
“Green Solvent”.35,53-55 The room-temperature ionic liquids that
are currently the focus of extensive investigation are generally
a substituted imidazolium or a pyridinium salt. Even though
the solvent properties of different halogenoaluminate(III) ionic
liquids were explored as early as in 1986,56 very little is known
about the various properties of the contemporary ionic liquids
despite their extensive usage in synthetic applications. Studies
on the viscosity and density of the ionic liquids by Seddon and
his group suggest that these are moderate to highly viscous
liquids (depending on the length of the alkyl chain attached to
the imidazolium moiety) at room temperature.57 Very few
photophysical studies have been carried out so far in these
media. A few UV-vis absorption and fluorescence studies have
been carried out with a view to measuring the polarity of these
media. These studies suggest that imidazolium and pyridinium
ionic liquids are sufficiently polar media. Gordon and co-
* Corresponding author. E-mail: assc@uohyd.ernet.in.
10.1021/jp0143591 CCC: $22.00 © 2002 American Chemical Society
Published on Web 06/20/2002

C153 and PRODAN in Room-Temperature Ionic Liquids
J. Phys. Chem. A, Vol. 106, No. 28, 2002 6671
CHART 1
PRODAN was thought to be due to the twisted intramolecular
charge transfer (TICT) nature of the emitting state, a recent study
suggests that the change in the dipole moment on excitation of
the systems is rather low (around 4.4-5.0 D) and the molecule
emits from a locally excited state. Hydrogen bonding interaction
seems to play a major role in dictating the location of the
fluorescence state in alcoholic or aqueous media.64
2. Experimental Section
2.1. Materials. Laser grade C153 from Eastman Kodak and
6-propionyl-2-dimethylaminonaphthalene (PRODAN) from Mo-
lecular Probes were used without any further purification. NaBF4
(97%) and 1-methylimidazole (99%), required for the synthesis
of the room-temperature ionic liquids, were procured from
Lancaster and the later was distilled in vacuo before using, while
1-chlorobutane (HPLC grade), bromoethane (98%), and 1,1,1-
trichloroethane (99%) were obtained from Aldrich.
2.1.1. Preparation of 1-Butyl-3-methylimidazolium Tetrafluo-
roborate, [BMIM][BF4]. Air- and water-stable [BMIM][BF4]
was prepared by the metathesis of 1-butyl-3-methylimidazolium
chloride [BMIM]Cl with NaBF4 in water at room temperature
following a standard procedure.47 [BMIM]Cl was first prepared
from 1-methylimidazole and 1-chlorobutane in N2 atmosphere
on heating at 70 °C for more than 24 h and the highly viscous
oily molten salt so obtained was purified according to the
literature procedure.65
workers have shown recently that the polarity of an ionic liquid
is largely determined by the cationic component of the solvent.48
In an earlier paper, we reported the solvation dynamics in a
room-temperature ionic liquid, 1-butyl-3-methylimidazolium
tetrafluoroborate [BMIM][BF4]. Using C153 as the probe
molecule, we observed a biphasic solvation dynamics with an
average solvation time of 2.13 ns. To obtain a better under-
standing of the dynamics in these media it is necessary to
systematically vary the constituting ions of the ionic liquids and
also to examine the dynamical behavior with more than one
probe molecule. This paper presents the results of a study dealing
with these aspects.
In addition to [BMIM][BF4], we have chosen 1-ethyl-3-
methylimidazolium tetrafluoroborate [EMIM][BF4] (Chart 1) as
the second medium for the study of solvation dynamics because
of the air and water stability of these two solvents and the ease
of their preparation.58 While the constituting anionic component
is identical in the two liquids, the latter possesses an imidazolium
cation with a relatively shorter alkyl chain and is miscible in
all proportion with water even at low temperature. The miscibil-
ity of [BMIM][BF4] is comparatively low with ice-cold water.
Even though the tetrafluoroborate ionic liquids are relatively
more hygroscopic than those containing [PF6]- or [Tf2N]-
anion,41,59 all of them are far more stable, low-melting, less
moisture sensitive, and less corrosive than the conventional
halogenoaluminate ionic liquids. Moreover, the present systems,
which possess a fluorinated anion, are less viscous than those
containing chloride ions. This is due to the fact that the viscosity
of the room-temperature ionic liquids is largely controlled by
the strength of the hydrogen bonding interaction between the
C2-hydrogen of the imidazolium ring and the halogen atoms in
the corresponding anion.41
We have chosen coumarin 153 (C153) and PRODAN as the
probe molecules to estimate the time-dependent Stokes shift of
the emission spectrum and to determine the solvation dynamics.
C153 has been extensively used as a probe for the study of the
solvation dynamics in a variety of media60,61 in view of the fact
that the intramolecular charge transfer fluorescence band of
C153 is highly sensitive to the polarity of the medium and also,
due to the fact that the S0 T S1 transition of C153 is
uncomplicated by other nearby transition or interfering reactions.
A recent time-resolved dielectric loss measurement indicated
that the change in the dipole moment of the molecule on
electronic excitation lies between 4.9 and 5.4 D.62 PRODAN is
also extensively used as a probe molecule to measure the
microscopic polarity of various chemical and biological media.
That the position of the fluorescence maximum of PRODAN
is extremely sensitive to the polarity of the medium is evident
from the fact that the molecule shows a red shift of ∼130 nm
when the medium is changed from cyclohexane to water.63
Though the hypersensitivity of the fluorescence maximum of
2.1.2. Preparation of 1-Ethyl-3-methylimidazolium Tetrafluo-
roborate, [EMIM][BF4]. [EMIM][BF4] was prepared by the
metathesis between 1-ethyl-3-methylimidazolium bromide,
[EMIM]Br, with NaBF4 in acetone at room temperature. This
method is similar to the procedure adopted for the preparation
of [BMIM][BF4] by Suarez et al.66 [EMIM]Br was synthesized
by refluxing 1-methylimidazole with freshly distilled bromo-
ethane in 1,1,1-trichloroethane at 80 °C temperature for 2 h
according to the modified procedure described by Bonhote and
co-workers.41 It is to be noted that we avoided preparing
[EMIM][BF4] using conventional silver method47 as we found
that even a trace quantity of silver was enough to destroy the
fluorescence probe molecules within a short time. The water
content and purity of the liquid salt were checked by both IR
and NMR. No precipitation of silver halide on addition of
aqueous silver nitrate solution in the final tetrafluoroborate liquid
salts suggests the absence of any halide impurity above the
detection limit.
2.2. Instrumentation. The absorption and steady-state fluo-
rescence spectra were recorded on Shimadzu UV-Vis-NIR
spectrophotometer (UV-3101PC) and Spex spectrofluorimeter
(Fluoromax 3), respectively. The time-resolved fluorescence
decay profiles at various emission wavelengths were measured
using a picosecond single photon counting spectrophotometer
(Model IBH 5000U). In this setup, a diode-pumped Millena
(5W) CW laser output (Spectra Physics) at 532 nm was used
to pump a Ti-Sapphire rod in Tsunnami picosecond Mode
Locked Laser system (Spectra Physics Model 4960 M3S). The
frequency-doubled 375 nm output was used for excitation of
the samples. The emission was detected at right angle to the
excitation beam using a Hamamatsu 323P MCP photomultiplier.
The instrument response time was 50 ps (fwhm).
2.3. Methods.The ionic liquids were kept in high vacuum at
70-80 °C for at least 7 h to remove any moisture present in
these media. Such treated liquids were slowly allowed to cool
to room temperature under vacuum prior to the sample prepara-
tion. A 2.5 mL sample of the liquid salt was used in each case
to prepare the solution of the fluorescence probe at room

6672 J. Phys. Chem. A, Vol. 106, No. 28, 2002
Karmakar and Samanta
temperature. Proper precaution was taken to avoid moisture
absorption by the ionic liquid while transferring it to the quartz
cuvette. The cuvettes were flushed with nitrogen prior to the
transfer of the solution and were sealed by septum and Parafilm
immediately after transfer to prevent moisture from contaminat-
ing the solution. The sealed solutions were found to be stable
for several weeks.
The time-resolved fluorescence was measured by TCSPC
technique monitored at 16-20 different wavelengths across the
entire emission spectrum. The detailed procedure was described
elsewhere.67 Briefly, each decay curve was fit to a sum-of-
exponentials function using an iterative reconvolution program
(IBH). This procedure increased the effective time-resolution
of the experiments to ∼25 ps. The quality of the fit was assessed
by the ø2 values and the residuals. The impulse response
functions at different wavelengths were calculated from those
best-fitted curves. The time-resolved emission spectra at various
times were calculated from the appropriately normalized
intensity decay function from steady-state intensity for different
wavelengths at different times. As it is quite difficult to
determine the peak frequency from the emission spectra, the
time-resolved spectra were first fit to a log-normal line shape
function, known for its excellent representation of the fluores-
cence band shape68 and at each time the peak frequency νj(t)
was calculated. The solvation dynamics described by the
normalized Stokes shift correlation function, C(t), which is
defined as,10,13
Figure 1. Normalized steady-state fluorescence spectra of PRODAN
(i) and C153 (ii) in [EMIM][BF4] at 25 °C; λexc ) 375 nm. The
fluorescence spectra have been corrected for the instrumental response.
TABLE 1: Steady-State Fluorescencea Data of C153 and
PRODAN and the Estimated Polarity Values of the Two
Room-Temperature Ionic Liquids in the ET(30) Scale
ET(30) value of the medium as
max
flu
max
νj
estimated from the νj of
flu
medium
C153 PRODAN
C153
PRODAN
νj(t) - νj(∞)
νj(0) - νj(∞)
[EMIM][BF4] 18587
[BMIM][BF4] 18622b
20964
21053
49.1
47.5
47.1
C(t) )
(1)
48.9b
a Excitation wavelength was 375 nm. b From ref 67.
was calculated using the peak frequency of the time-resolved
emission spectra. The quantities, νj(0), νj(t), and νj(∞) are the
peak frequencies immediately after excitation, at an intermediate
time and at infinite time after excitation. The correlation function
in each case was found to be biexponential in nature. The decay
1
alkyl chain of the imidazolium cation in [BMIM][BF4]. It is to
be noted here that these estimates of the polarity of those two
ionic liquids are in fairly good agreement with those obtained
by Muldoon et al. from the absorption maximum of the betaine
dye.48
was fit to a biexponential decay function, C(t) ) a1e-t/τ
+
a2e-t/τ where τ1 and τ2 are the two relaxation times having
amplitudes of a1 and a2 respectively. The method of determi-
nation of νj(0) from the time-resolved data is dependent on the
finite time-resolution of the instrumental response. Hence, the
initial response of the solvent that occurs within the first ∼25
ps (time-resolution of the present setup) would not be included
in the so obtained C(t) function.
2
The time-resolved decay profiles of the probe molecules are
found to be dependent on the monitoring wavelengths. When
monitored at longer wavelength (toward the red edge of the
fluorescence spectrum) the decay profile consisted of an initial
rise followed by a slow decay. However, the rise component
could not be observed when the fluorescence is monitored at
the shorter wavelength region. This wavelength-dependent decay
behavior of the two fluorescence probes is the signature of
solvation dynamics that is occurring on the time scale compa-
rable to that used in the measurement. Figure 2 illustrates the
wavelength-dependent decay behavior of the two molecules in
[EMIM][BF4]. The quality of the fit to the decay profiles has
been shown in Figure 3.
The time-dependent fluorescence spectra of the molecules,
constructed from the decay profiles by following the procedure
stated in Section 2.3, are shown in Figure 4 as dotted lines.
The values of the correlation function, C(t) at various times
were estimated from the time-resolved emission spectra using
eq 1. The time-dependence of C(t) is shown in Figure 5 along
with the best fit to the data. Research groups of Barbara22 and
Huppert31-33,70 have used a stretched factor â in order to
improve their biexponential fitting parameters.However, in our
case, we did not find any significant improvement to the quality
of the fit by varying the â value from 0.4 to 0.9. The relaxation
times obtained on fitting the data are given in Table 2. The
salient features of the solvation dynamics can be summarized
as follows.
3. Results and Discussion
The fluorescence spectra of the two probe molecules in
[EMIM][BF4] are shown in Figure 1. The wavenumbers
corresponding to the steady-state fluorescence maximum
max
flu
(νj ) of C153 and PRODAN in the room-temperature ionic
liquids are shown in Table 1. The polarity of the two media (in
terms of Reichardt’s solvent polarity parameter, ET(30)69 is
max
estimated from the linear relationship between νj and ET(30)
flu
max
in several conventional solvents and the measured νj values
flu
in the two ionic liquids. The estimated polarity values of the
two media are shown in Table 1. Even though C153 gives a
relatively higher estimate of the polarity of the media compared
to that obtained with PRODAN, the data suggest that both
[EMIM][BF4] and [BMIM][BF4] are more polar than acetonitrile
(ET(30) of 45.6), but much less polar than methanol (ET(30) of
55.4). The polarity of the two media resembles more that of
2-propanol (ET(30) of 48.4). Both the probe molecules indicate
that [BMIM][BF4] is marginally less polar than [EMIM][BF4].
This is in agreement with the presence of a relatively longer

C153 and PRODAN in Room-Temperature Ionic Liquids
J. Phys. Chem. A, Vol. 106, No. 28, 2002 6673
Figure 2. Time-resolved fluorescence behavior of PRODAN (a) and
C153 (b) in [EMIM][BF4]. The monitoring wavelengths are (i) 560,
(ii) 430, and (iii) 390 nm in (a) and (i) 650, (ii) 500, and (iii) 450 nm
in (b). λexc ) 375 nm. The excitation lamp profile is shown in (iv) in
both cases.
TABLE 2: Relaxation Parameters of the Probe Molecules in
the Room Temperature Ionic Liquidsa
Figure 3. Fluorescence decay profile of C153 in [EMIM][BF4] at 450
nm (i) and PRODAN in [EMIM][BF4] at 390 nm (ii) along with the
best fit (solid line). The data have been fitted to a triexponential
function.
relaxation time/ps amplitude
viscosityb/
cP
<τ>c/
ps
medium
probe
τ1
τ2
a1 a2
[EMIM][BF4]
66.5 C153
125
1290 0.73 0.27 440
2130 0.74 0.26 680
3980 0.50 0.50 2130
3330 0.62 0.38 1440
results in terms of the model suggested by Huppert et al. rather
than using the continuum models or the MSA model, which
are applicable to dynamics in molecular solvents. According to
Huppert and co-workers, the biphasic nature of the dynamics
is due to the difference in the transport properties of the cations
and the anions, which differ in their sizes. If Huppert’s model
is followed, then the short-component of the dynamics has to
be assigned to the motion of the smaller species, i.e., the anions,
while the long-component is due to the motion of the relatively
larger species, the cations. As the ionic solvation involves a
cooperative movement of the ions, the absolute values of the
two components, as pointed out by Huppert et al., are dependent
on the size of the larger species, the cation. This is why the τ
values obtained for the short and the long components in the
two liquids are different. However, if the slow component were
exclusively due to the cations, as proposed by Huppert, then
one would have expected a smaller amplitude of this component
with an increase in the size of the cations. The present results
(vide Table 2) show exactly opposite behavior. Interestingly,
even though Huppert also observed a similar behavior, no
explanation was offered to account for the change of the
amplitude of the two components. The amplitude of the two
components can only be accounted for if it is assumed that the
fast component is solely due to the initial response of the smaller
species, i.e., the anion, while the long component arises due to
PRODAN 175
[BMIM][BF4] 154.0 C153d
278
PRODAN 280
a At 20 °C. b From ref 57. c Average relaxation time <τ> is defined
as, <τ> ) a1τ1 + a2τ2, where (a1 + a2 ) 1). d From ref 67.
The solvation dynamics in both cases is found to be clearly
biphasic consisting of a relatively fast component with a τ value
of 125-175 ps in the case of [EMIM][BF4] and 278-280 ps
in the case of [BMIM][BF4]. The long component, on the other
hand, has a τ value of 1.3-2.1 ns in the case of [EMIM][BF4]
and 3.3-4.0 ns in the case of [BMIM][BF4]. The average
solvation time is calculated to be 440-680 ps in [EMIM][BF4]
and 1.4-2.1 ns in [BMIM][BF4]. The solvation dynamics is
found to be dependent on the probe molecule employed in the
study. In the case of [EMIM][BF4], an average solvation time
of 440 ps is obtained using C153, while the relaxation time is
found to be 680 ps with PRODAN. Similarly, in the case of
[BMIM][BF4], τ value varied between 1.4 and 2.1 ns depend-
ing on the probe molecule employed.
Essentially, the main features of the solvation dynamics in
the room-temperature ionic liquids, such as the biphasic nature
and the probe dependence of the dynamics, are similar to what
was observed in the case of the molten salts by Huppert and
co-workers.31-33 Hence, we attempt to interpret the present

6674 J. Phys. Chem. A, Vol. 106, No. 28, 2002
Karmakar and Samanta
Figure 5. Decay profile of the response function, C(t) of PRODAN
in [EMIM][BF4] (b), PRODAN in [BMIM][BF4] (2), and C153 in
[EMIM][BF4] (9). The points denote the actual values of C(t), while
the solid line represents the biexponential fit.
nanosecond time scale. On the basis of the present results, the
fast initial response of the biphasic dynamics is attributed to
the motion of the relatively smaller species, the anions, while
the slow component originates from the collective motion of
the cations and the anions.
Acknowledgment. The authors thank the Council of Sci-
entific and Industrial Research (CSIR) and Department of
Science and Technology (DST), Government of India, for
supporting this work and Dr. P. Ramamurthy for his help during
the measurement of the picosecond time-resolved decay profiles
at the National Centre for Ultrafast Processes, Chennai. The
research fellowship to R.K. was provided by CSIR. We also
thank Prof. K. R. Seddon for providing us the viscosity data of
the ionic liquids prior to publication.
References and Notes
Figure 4. Time-resolved emission spectra (normalized at the peak
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