The excitation of 3-hydroxyflavone (3HF) to its second excited singlet state (S2) gives rise to dual fluorescence in supercritical carbon dioxide. The ultraviolet fluorescence originated from the S2 state of 3HF is well separated from the green emission emanating from the tautomeric form, produced via the excited state intramolecular proton transfer. The relative intensity of the S2 to the tautomer fluorescence (S2/T) has been studied as a function of pressure and temperature. It is shown that this ratio reflects the microheterogeneity of the supercritical CO2, and confirms the value of fluorometric probes in disclosing the microscopic properties of supercritical fluids.http://www.sciencedirect.com/science/article/B6TFN-4BVP7G9-3/1/02dd61c567fe3e9c8d6ac86a01f79ce

Chattopadhyay, Nitin

Barroso, Mónica

Serpa, Carlos

Isilda Silva, M.

Arnaut, Luís G.

Formosinho, Sebastião J.

Black and white portrait photograph of Helen VanDeventer, part-time Instructor in Latin, Eastern State High School, 1947-1949; Assistant Professor in Foreign Languages, 1960-1972.https://thekeep.eiu.edu/archives_faculty_sz/1243/thumbnail.jp

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The Keep

Helen VanDeventer

Eastern Illinois University

Estudo Geral

Chemical Physics Letters 387 (2004) 263–266
www.elsevier.com/locate/cplettPhotophysics of 3-hydroxyﬂavone in supercritical CO2:
a probe to study the microenvironment of SCF
Nitin Chattopadhyay a,b,*, Monica Barroso a, Carlos Serpa a, M. Isilda Silva a,
Luis G. Arnaut a,*, Sebasti~ao J. Formosinho a
a Department of Chemistry, Coimbra University, P-3004-535, Portugal
b Department of Chemistry, Jadavpur University, Calcutta 700 032, India
Received 12 January 2004
Published online: 4 March 2004Abstract
The excitation of 3-hydroxyﬂavone (3HF) to its second excited singlet state (S2) gives rise to dual ﬂuorescence in supercritical
carbon dioxide. The ultraviolet ﬂuorescence originated from the S2 state of 3HF is well separated from the green emission emanating
from the tautomeric form, produced via the excited state intramolecular proton transfer. The relative intensity of the S2 to the
tautomer ﬂuorescence (S2/T) has been studied as a function of pressure and temperature. It is shown that this ratio reﬂects the
microheterogeneity of the supercritical CO2, and conﬁrms the value of ﬂuorometric probes in disclosing the microscopic properties
of supercritical ﬂuids.
 2004 Elsevier B.V. All rights reserved.1. Introduction
Supercritical ﬂuids (SCF) are getting enhanced at-
tention in diverse ﬁelds, in view of their potential tech-
nological applications [1–5]. One of the most important
properties of SCF is the local density eﬀect on the solute
molecules [6–8]. The density of a SCF is between that of
a gas and a normal liquid, and can be easily tuned with a
change in pressure under isothermal conditions. Con-
sequently, medium properties can be continuously var-
ied without a change in the molecular nature of the
solvent. Because of its high sensitivity, ﬂuorometric
technique is gradually establishing its importance in
characterising the SCF properties along with other
techniques [9–13].
3-Hydroxyﬂavone (3HF) is known to exhibit excited
state intramolecular proton transfer (ESIPT) reaction in
solutions giving rise to dual ﬂuorescence [14–21]. The
ESIPT reaction of this ﬂuorophore has been extensively
studied since its discovery by Kasha and co-workers.* Corresponding authors.
E-mail addresses: pcnitin@yahoo.com (N. Chattopadhyay),
lgarnaut@ci.uc.pt (L.G. Arnaut).
0009-2614/$ - see front matter  2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2004.01.122A recent work of Uchiyama et al. [22] where they have
determined the solubility of the ﬂavone and 3HF from
the mass of solute trapped by decompressing and the
volume of CO2, has shown that 3HF is soluble in su-
percritical CO2. In the preceding Letter [23], we reported
that upon excitation of 3HF in sc-CO2 to its second
excited singlet state (S2), a signiﬁcant ﬂuorescence em-
anating from the S2 state is observed. In this Letter, we
report the relative intensities of the S2 to the tautomer
ﬂuorescence (S2/T) as a function of the pressure and
temperature of sc-CO2. The present report establishes
that 3HF can be used as a probe to monitor the mi-
croenvironment of sc-CO2.2. Experimental
3HF (Aldrich) was puriﬁed by repeated recrystallisa-
tion from ethanol. Trace of water was removed from the
puriﬁed sample by putting it into an evacuator with oc-
casional heating. Purity of the sample was checked
through thin layer chromatography (with a 3:1 mixture
of dichloromethane n-hexane as elluent) and spectro-
scopic measures. For preparation of the mother solution,
300 350 400 450 500 550
T  = 314K
N
or
m
al
is
ed
 fl
uo
re
sc
en
ce
 in
te
ns
ity
Wavelength  (nm)
 91 bar
 96 bar
 102 bar
 112 bar
 130 bar
Fig. 1. Fluorescence spectra of 3HF in sc-CO2 at diﬀerent pressures at
314 K. The spectra are normalised at the maximum of the tautomer
ﬂuorescence band.
264 N. Chattopadhyay et al. / Chemical Physics Letters 387 (2004) 263–266spectroscopic grade diethylether (Aldrich) was used after
drying it further by distilling over metallic sodium. The
dry ether was then stored over molecular sieves.
A Shimadzu UV-2001 spectrophotometer and a Spex
Fluorolog 3 spectroﬂuorometer were used for the ab-
sorption and ﬂuorescence measurements, respectively.
The supercritical cell with its accessories has been
described in our previous article [9]. For spectroscopic
measurements, the sample was prepared in the following
way. A solution of 3HF (1 105 M) was made in di-
ethylether and was loaded in the clean SCF cell. The
solvent ether was then completely evaporated. The cell
was evacuated and ﬁlled with CO2 after thermostating at
the experimental temperature. The pressure inside the
supercritical cell was measured using a OMEGA DP20
digital pressure indicator (0.25%) and temperature
within the cell was controlled and measured using
a OMEGA CN77000 series temperature controller
(0.5 C).80 90 100 110 120 130 140 150
2
4
6
8
10
12
14
16
18
20
 308 K
 311 K
 317 K
 320 K
S 
 / 
T
2
Pressure (bar)
Fig. 2. Plot of S2/T ﬂuorescence ratio of 3HF in sc-CO2 as a function
of pressure at diﬀerent temperatures. The temperatures are given in the
inset. Plot of the data at 314 K has been dropped for the clarity of the
ﬁgure.3. Results and discussion
It is known that 3HF undergoes ESIPT in solution
phase as well as in jet cooled isolated condition. In hy-
drogen-bonding solvents, ESIPT gives rise to a dual
ﬂuorescence, the higher energy one corresponding to the
molecular species (prior to ESIPT) and the lower energy
one corresponding to the tautomer. To the best of our
knowledge, there is no report where the photophysical
properties of 3HF have been exploited to monitor the
microenvironment of supercritical ﬂuids.
We showed in the preceding Letter that excitation of
3HF in sc-CO2 at 291 nm gives a S2 ﬂuorescence band in
the region 300–350 nm together with the Stokes’ shifted
ﬂuorescence from the tautomeric species peaking at
520 nm (Fig. 1 of [23]). The diﬀerent excitation spectra
of these bands suggested that two ground-state precur-
sors should be present in sc-CO2 and it was proposed
that they diﬀer in their solvation environment. Thus, it is
expected that a change in the local molecular density in
the supercritical media should lead to a modiﬁcation in
the relative S2 to the tautomer ﬂuorescence (S2/T).
Fig. 1 shows that the relative amplitude of S2/T in
Sc-CO2 has a characteristic pressure dependence. Since
the two emission bands are widely separated, the ab-
solute heights of the maxima of the individual emissions
corresponding to the two (S2 and T) bands were used
for the calculation of the relative intensity (S2/T). The
ﬂuorescence spectra represented in Fig. 1 were norma-
lised at the tautomer emission maximum of 3HF, and
reﬂect the variation of the emission behaviour with
pressure at 314 K. Fig. 2 plots S2/T against pressure at
diﬀerent temperatures. At any particular temperature
over the range studied, an increase in pressure leads to a
decrease in S2/T until a certain pressure (Pmin) isreached. For pressures higher than Pmin, we did not
observe any further decrease of S2/T. This gives rise to a
break point (Pmin) indicating a change in the organisa-
tion of the environment around the ﬂuorophore. With
an increase in the temperature, Pmin shifts towards
higher pressures with a concomitant reduction in the
sharpness of the break point. These observations are
very similar to those of our previous experimental study
with benzil [9] and to the theoretical work of Bartle et
al. [24], that were correlated with the kinematic behav-
iour of sc-CO2. Thus, the S2/T ﬂuorescence ratio of 3HF
can also be used to monitor the kinematic behaviour of
sc-CO2.
The similarity between the representation of the S2/T
ratio as a function of pressure and temperature with the
308 310 312 314 316 318 320
80
90
100
110
120
130
P 
   
(ba
r)
m
in
Temperature (K)
Fig. 3. Variation of Pmin against temperature.
N. Chattopadhyay et al. / Chemical Physics Letters 387 (2004) 263–266 265corresponding plot in our previous study [9], corrobo-
rates our earlier study and invites a similar interpreta-
tion for all the observations. They can be interpreted on
the basis of a microscopic view of sc-CO2, considering
the microenvironment around the ﬂuorophore at vari-
ous pressures.
An increase in pressure induces the CO2 molecules to
rearrange around the ﬂuorophore and form a solvent
shell around the probe [9]. Once a ﬁrst solvation shell is
formed, further increase in pressure is expected to have a
rather modest eﬀect on the probe. Thus, a saturation
eﬀect is expected at higher pressures. The break point
(Pmin) in the plot indicates a saturation of the inner
solvent shell. The inner shell solvation can approxi-
mately be described by Langmuir equilibrium model
[25]. With a further increase in pressure, the solvent
molecules assemble in the secondary solvation shell. We
argued in the preceding article [23] that the precursors of
the S2 normal emission and tautomer emission diﬀer in
the size of their solvation shell. At lower pressures, CO2
clusters of diﬀerent sizes may be formed around the
solute molecules and co-exist in rapid equilibrium. At
higher pressures, such clusters merge into a more ho-
mogeneous microscopic structure. The decrease of the
S2/T ratio with the pressure indicates that the normal S2
emission is favoured by incomplete solvation shells.
Considering that the tautomer is formed by ESIPT and
that it is perturbed by external hydrogen bonding to the
solvent molecules, the high intensity of the normal S2
emission at lower pressures suggests that CO2 forms
stronger hydrogen bonds in incomplete solvation shells.
In fact, we expect to ﬁnd a larger energy stabilisation in
the association of an isolated CO2 molecule to a hy-
droxyl group, than when such association has to be
made at the cost of the interaction of that CO2 molecule
with others of the same kind. Clearly, sc-CO2 makes a
microheterogeneous environment rather than a homo-
geneous one around the ﬂuorophore. The concept of
microheterogeneity of the supercritical environments
has already been adopted for diﬀerent SCFs including
sc-CO2 [4,9,26–29].
With an increase in the temperature, the kinetic en-
ergy of the solvent molecules increases, thereby requir-
ing a higher pressure to form large clusters. This is
consistent with the observed shift of the Pmin towards
higher pressure at elevated temperature. Moreover, the
increased molecular velocities at high temperatures
make onset of the saturation less precise, leading to a
reduction of the sharpness of the change of S2/T value
near the break point, as shown in Fig. 2. Fig. 3 shows
that Pmin is a linear function of the temperature. The Pmin
values at diﬀerent temperatures determined in this work
are in good agreement with those reported in our earlier
work [9], although the two systems diﬀer in their
photophysics. The similarity between these results
suggests that, at least for ﬂuorophores of comparabledimensions (the molecular lengths of 3HF and trans-
benzil are 11.25 and 11.65 A, respectively), Pmin is
independent of the ﬂuorophore and reﬂects the micro-
scopic properties of sc-CO2.4. Conclusion
The present work, in conjunction with our earlier
work [9], establishes the value of spectroscopic/ﬂuoro-
metric probes in disclosing the microscopic properties of
supercritical ﬂuids. With an increase in pressure, sc-CO2
evolves from an ensemble of clusters of diﬀerent sizes
surrounding the probe molecule, to a solvent shell that
encloses the probe molecule. This change modiﬁes the
ﬂuorometric behaviour of the probe, and provides in-
formation on the microheterogeneous environment
created by sc-CO2.Acknowledgements
Financial support from GRICES, Government of
Portugal, and DST, Government of India, through the
Indo-Portuguese project is gratefully acknowledged. We
also thank Fundac~ao para a Cie^ncia e a Tecnologia for a
Grant (BD/1332/2000).References
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