Solvent effects on the photophysics and photoreactivity of 3-hydroxyflavone: A combined spectroscopic and kinetic study

Article abstract of DOI:10.1016/j.molliq.2014.12.001

The photophysical and photochemical properties of 3-hydroxyflavone (3HF) have been investigated in six solvents with different chemical and physical properties: methanol, 2,2,2-trifluoroethanol (TFE), tetrahydrofuran (THF), chloroform (CHCl₃), acetonitrile (MeCN), and dimethylsulfoxide (DMSO). The polarity of the solvents and their ability to interact with 3HF by donating and accepting hydrogen bonds were found to strongly influence the UV-Vis absorption spectra and the emission spectra, as well as other photophysical parameters such as fluorescence quantum yield and fluorescence lifetime. In addition, the nature of the solvent was found to influence the kinetics of 3HF photorearrangement to 3-hydroxy-3-phenyl-1,2 indiandione, the reaction being the fastest in TFE, and the slowest in DMSO. The results were interpreted in terms of specific solute-solvent interactions.

Full text of DOI:10.1016/j.molliq.2014.12.001

Journal of Molecular Liquids 205 (2015) 110–114
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Solvent effects on the photophysics and photoreactivity of
3-hydroxyflavone: A combined spectroscopic and kinetic study
Stefano Protti a,b, Alberto Mezzetti b,c,
⁎
a
PhotoGreen Lab, Department of Chemistry, University of Pavia, V. Le Taramelli 12, 27100 Pavia, Italy
Laboratoire de Spectrochimie Infrarouge et Raman LASIR, Unité Mixte de Recherche 8516, Université Lille 1, Bat. C5, Cité Scientifique, 59655 Villeneuve d'Ascq, France
Service de Bioénergétique, Biologie Structurale et Mécanismes SB2SM, Unité Mixte de Recherche 8221, IBITeC-S, Bat. 532, CEA-Saclay, 91191 Gif-sur-Yvette, France
b
c
a r t i c l e i n f o
a b s t r a c t
Available online 13 December 2014
The photophysical and photochemical properties of 3-hydroxyflavone (3HF) have been investigated in six sol-
vents with different chemical and physical properties: methanol, 2,2,2-trifluoroethanol (TFE), tetrahydrofuran
(THF), chloroform (CHCl3), acetonitrile (MeCN), and dimethylsulfoxide (DMSO). The polarity of the solvents
and their ability to interact with 3HF by donating and accepting hydrogen bonds were found to strongly influence
the UV–Vis absorption spectra and the emission spectra, as well as other photophysical parameters such as fluo-
rescence quantum yield and fluorescence lifetime. In addition, the nature of the solvent was found to influence
the kinetics of 3HF photorearrangement to 3-hydroxy-3-phenyl-1,2 indiandione, the reaction being the fastest
in TFE, and the slowest in DMSO. The results were interpreted in terms of specific solute–solvent interactions.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
chemical properties of the surrounding molecular microenvironment
[9]. Thus, the relative intensity of the emission deriving from 1N and
Flavonols are a class of flavonoids characterized by a 3-
hydroxyflavone (IUPAC name 3-hydroxy-2-phenylchromen-4-one)
moiety. They are largely present in the vegetal kingdom [1] and
they can act as anti-oxidants [2].
1T, the position of the maxima of the emission peaks as well as the kinet-
ics of ESIPT process strongly depend upon the molecular environment
around 3HF (reviewed in [7–9,12]; for more details, see [12–15] and ref-
erences therein). Interestingly, the tunable properties of 3HF suggested
its use as a laser dye [16] and for other technological applications (re-
cently reviewed in [9]), including its use as a matrix for matrix-
assisted laser desorption/ionization (MALDI) tissue imaging ([17]).
Apart from the influence on the emission properties of 3HF, the mo-
lecular environment can also induce a partial ground-state deproton-
ation of 3HF to give the corresponding anion A ([14,18–20] and refs.
therein) whose photophysical properties (including its emission hνA)
significantly differs from those of neutral 3HF. It should be noticed
that solvent-induced (or environment-induced) deprotonation of OH
groups is a quite general and recently-discovered feature of flavonols
(see [21] and refs. therein) with important consequences in biochemis-
try and analytical chemistry.
The extreme sensitivity of 3HF photophysics to the surrounding envi-
ronment has suggested its use as a fluorescent molecular probe in many
different fields, such as biophysics, material science, supercritical fluids
and physical organic chemistry (see [9] for a recent review). Concerning
the photochemistry, 3HF can undergo two different photoreactions,
namely a unimolecular photo-rearrangement [22, Fig. 2] or a photo-
oxygenation reaction [23], the nature of the surrounding environment as
well as the presence of oxygen favoring one of the two pathways. Both
3HF photoreactivity and the reaction rate of the photo-rearrangement re-
action were also found to be strongly solvent-dependent, even though
contradicting results have been reported [14,22–26].
The simplest molecule belonging to this class, synthetic derivative 3-
hydroxyflavone (3HF, 1) has attracted great interest due to its peculiar
photophysical and photochemical properties. These are both intrinsical-
ly related to the Excited State Intramolecular Proton Transfer (ESIPT in
Fig. 1) that occurs from the excited state (1N) of 3HF, to produce the sin-
glet tautomer 1T, characterized by an emission centered at 520–540 nm
(hνT). The resulting ground-state tautomer T undergoes a non-radiative
back proton transfer (BPT) to give 3HF in its fundamental state (N). Fur-
thermore, when ESIPT is hampered, fluorescence can also occur from
the singlet excited state (1N), with a broad emission centered at ca.
400 nm (hνΝ) [3,4].
Because of the presence of two easily observable emission bands,
3HF has become a model system to investigate intramolecular excited-
state proton transfer reactions (for reviews see [5–9]). Currently, several
aspects of 3HF photophysics are still unclear ([8,9] and refs. therein; for
a recent debate, see [10,11]). An interesting aspect is the dependence of
photophysical properties of 3HF on the physical, physicochemical and
Abbreviations: 3HF, 3-hydroxyflavone; DMSO, dimethylsulfoxide; HPLC, high-
performance liquid chromatography; MeCN, acetonitrile; MeOH, methanol; NMR, Nuclear
Magnetic Resonance; THF, tetrahydrofuran; UV–Vis, ultraviolet–visible
⁎
Corresponding author at: Laboratoire de Spectrochimie Infrarouge et Raman LASIR,
Unité Mixte de Recherche 8516, Université Lille 1, Bat. C5, Cité Scientifique, 59655
Villeneuve d'Ascq, France.
http://dx.doi.org/10.1016/j.molliq.2014.12.001
0167-7322/© 2014 Elsevier B.V. All rights reserved.

S. Protti, A. Mezzetti / Journal of Molecular Liquids 205 (2015) 110–114
111
Fig. 1. Excited State Intramolecular Proton Transfer (ESIPT) and ground state proton transfer occurring in 3HF. N indicates the normal state of 3HF in its fundamental state, 1N indicates the
excited singlet state of 3HF, T indicates the fundamental state of the tautomer, 1T the excited singlet state of the tautomer, A the fundamental state to deprotonated 3HF, and 1A the excited
singlet state of the deprotonated 3HF.
It is important to underline that the interest in fully understanding
the factors governing the photoreactivity of 3HF comes from different
research areas. The possibility of exploiting 3HF derivatives for photo-
stimulated organic synthesis has been reported [9,27]. Another intrigu-
ing aspect is the fact that the photo-oxygenation reaction mimics the
oxygenation mechanism on 3HF and other flavonols catalyzed by the
enzyme quercetin 2,3-dioxygenase [28]. Finally, understanding how
3HF photochemistry is influenced by its molecular environment is a
fundamental point in order to improve the photostability of 3HF-
based technological devices [9,29,30].
In view of these premises, we have carried out a multidisciplinary
approach combining organic photochemistry and spectroscopy analy-
sis, to clarify how the environment influences the photophysics and
the photoreactivity of 3HF in a series of solvents of different physical,
physicochemical, and chemical parameters. The results have been com-
pared to those reported in the literature for other solvents and rational-
ized in terms of specific solute–solvent interactions.
trifluoroethanol (TFE, 99% purity) were commercially available
(Sigma-Aldrich) and used as received. Tetrahydrofuran (THF, Sigma-
Aldrich) was filtered on alumina (to avoid the presence of hydroqui-
nones as stabilizers) and distilled over sodium before use.
Disappearance quantum yield (Φ−1) values as well as kinetic analy-
ses were measured in an optical bench equipped with a high pressure
Hg lamp (maximum of emission at 317 nm), and the reaction course
followed by UV–Vis analyses.
Preparative irradiations were performed in a multilamp reactor
fitted with six 15 W Hg phosphor coated lamps (maximum of emission
310 nm).
UV–Vis absorption spectra were recorded on a V-550 Jasco spectro-
photometer with cells of 1 cm path length.
Fluorescence spectra were performed by using a LS-55 Perkin Elmer
spectrofluorimeter. Fluorescence lifetime measurements were mea-
sured on a Single Photon Counting instrument (Edinburgh).
High-performance liquid chromatography (HPLC) analyses were car-
ried out on a HPLC apparatus equipped with two pump-980 (Jasco), and
a spectrophotometer UV-975 (Jasco) as detector (column: AQUASIL C-18
(250 × 4.6 mm); eluent: MeCN/water, flux 1.0 mL min−1).
1H and 13C Nuclear Magnetic Resonance (NMR) spectra were
recorded on a Bruker 300 MHz spectrometer.
2. Experimental
3HF (1) was purchased from Sigma-Aldrich and recrystallized from
cyclohexane before use. Methanol (MeOH, HPLC purity grade), acetoni-
trile (MeCN, HPLC purity grade), dimethylsulfoxide (DMSO, dry, N99.9%
purity), chloroform (CHCl3, ACS reagent purity grade) and 2,2,2-
HPLC–MS analysis was performed on a Thermo Scientific LTQ Linear
Ion Trap.
3. Results and discussion
3.1. Photophysical characterization
The UV–Vis spectra of 3HF recorded in the six examined solvents are
shown in Fig. 3. The position of the absorption bands is indicated in
Table 1. In DMSO (and, to a lesser degree, in MeOH) a red-shifted
band is also present for λ N 390 nm. According to literature [14,18,20]
this was assigned to the deprotonated 3HF (produced by solvent-
Fig. 2. Photorearrangement of 3HF to give 3-hydroxy-3-phenyl-1,2 indiandione.

112
S. Protti, A. Mezzetti / Journal of Molecular Liquids 205 (2015) 110–114
Fig. 3. UV–Vis spectra of 3HF (4.2 × 10−5 M) in the examined solvents. Position of the
band is indicated in Table 1.
Fig. 4. Emission spectra of 3HF in the examined media. λexc was set in correspondence of
the absorption maximum of the corresponding UV–Vis spectrum, around 345 nm.
induced ground state deprotonation of the 3-OH group). As shown in
Fig. 3 and Table 1, the effect of the solvent on the position of the absorp-
tion bands for neutral 3HF is generally limited. The only exception is the
case of TFE, where the absorption band, that in the case of the other five
solvents is centered around 305 nm (and assigned to the HOMO-
1 → LUMO transition [31]), is red-shifted at 314 nm. A similar behavior
was observed by our research group for MeOH–H2O 1:1 mixture [14].
A much more pronounced effect can be observed in the fluorescence
spectra shown in Fig. 4. Whereas 3HF in CHCl3 shows exclusively emis-
sion from the tautomer state 1T, in more polar and protic solvents fluo-
rescence relative to the 1N state is also observed. The effect is stronger in
protic solvents (the strongest case being the most acidic TFE), and can
be easily interpreted as a perturbing effect on the ESIPT reaction given
by the formation of a hydrogen bond between the hydroxyl group of
the solvent (donor) and the C_O group of 3HF (acceptor) [4]. This hy-
drogen bond hampers the formation of the tautomer and as a conse-
quence favors direct de-excitation (radiative and non-radiative) from
the 1N state. Interestingly, a significant fluorescence from the 1N state
is also observed in THF and, to a slightly lesser degree, in DMSO. Fluores-
cence from 1N state is conversely very weak in MeCN. The presence of
fluorescence band 1N state can be again rationalized as a perturbation
on the ESIPT by the solvent, which in this case most probably involves
a hydrogen bond between the OH group of 3HF (3-OH, donor) and the
oxygen atom of the solvent (DMSO, THF), which in this case acts as an
acceptor (see Fig. 5). Conversely, a similar interaction is negligible
between the 3-OH group and the nitrogen atom of MeCN (whose β
value of hydrogen bond accepting basicity is much lower than those of
THF and DMSO, see Table 1).
In addition, contributions arising from 1A fluorescence are also
present in solvents that favor deprotonation of 3HF [18,20].
Solvents have also a great influence on the position of the emission
maxima, in particular on the band of 1T fluorescence, which shifts
from 507 nm (TFE) to 534 nm (DMSO). A similar effect had already
been reported before ([4,13] and refs. therein). The simplest explana-
tion for this phenomenon is that the hydrogen bonding effect of the sol-
vent (acting as a donor), when present, influences the energy of both
the ground and the excited states of the tautomer [4]; (for the value of
hydrogen bonding donor acidity of the examined solvents see Table 1).
It is beyond the scope of this paper to report a detailed characteriza-
tion of 3HF fluorescence in terms of classical parameters such as inten-
sity ratio between 1N and 1T emission bands, a fundamental parameter
to exploit 3HF and its derivatives as fluorescent probes [13]. Here, the
photophysical characterization is a prerequisite to understand the effect
of solvation on 3HF photochemistry.
Another interesting photophysical parameter for 3HF is the fluores-
cence quantum yield ΦF, especially of the 1T state: the highest ΦF (1T) is
observed in the case of CHCl3, the lowest being the one reported in
DMSO. It should be emphasized that in DMSO a probably strong – and
possibly quite stable – hydrogen bond interaction between the 3-OH
group of 3HF and the S_O group has been postulated [18], which
most probably favors deactivation of the excitation energy through in-
ternal conversion.
Table 1
A key parameter to understand 3HF–solvent interactions is also the
fluorescence lifetime (see Table 2): in this case, the largest value is
found for the apolar solvent CHCl3 (0.50 ns), whereas a lower value is
found for aprotic solvents (i.e. THF, τ = 0.21 ns, and MeCN, τ =
0.22 ns) which are not supposed to strongly interact with 3HF. In the
protic solvent MeOH the shortest measured fluorescence lifetime is ob-
served (0.11 ns), whereas in the more acidic protic solvent TFE and in
DMSO (the latter aprotic but with a strong hydrogen bond acceptor ca-
pability β, favoring specific solute–solvent interaction) the values are
very small and below the time resolution of our instrument. In this
case, most probably the strength and possible presence of more than
one hydrogen bond interaction between the solvent and 3HF is respon-
sible for this behavior.
Photophysical and photochemical parameters measured for 3HF in the examined solvents.
ΦF: fluorescence quantum yield; Φ−1: disappearance quantum yield. α: hydrogen bond
donor acidity according to the Kamlet–Taft scale [33]. β: hydrogen bonding acceptor basic-
ity according to the Kamlet–Taft scale [33]. π*: solvent dipolarity/polarizability according
to the Kamlet–Taft scale. εr = dielectric constant. Values in parentheses are less certain.
a
Solvent Physical and
chemical
λ
abs (nm), ε ∗ 10−3 λem
ΦF
Φ−1
(M−1 cm−1
)
(nm)
parameters
TFE
α = 1.51; β = 0
345, 13.2
407, 507 0.024,
0.046
404, 528 0.0008, 0.011
0.06
400, 534 0.0034, 0.007
0.0125
0.014
π* = 0.73; εr = 8.55 314, 9.5
α = 0.19; β = 0.31 341, 19.8
π* = 0.75; εr = 37.5 305, 15.2
DMSO α = 0; β = 0.76 348, 17.8
π* = 1.00; εr = 46.7 308, 11.0
MeCN
THF
α = 0; β = 0.55
π* = 0.58; εr = 7.6
(α = 0.44); β = 0
343, 18.8
305, 13.2
345, 18.0
395, 532 0.005,
0.030
0.00105
3.2. Photoreactivity of 3HF in the examined solvents
CHCl3
520
0.19
0.0086
0.0089
π* = 0.58; εr = 4.81 309, 12.8
MeOH α = (0.62); β = 0.93 345, 18.3
π* = 0.60; εr = 32.7 306, 13.2
406, 531 0.009,
0.020
The investigation of 3HF photoreactivity is particularly interesting as
it has been reported that the solvent can modify the reaction mechanism
[14,22–26]. In fact, contrasting reports on the followed reaction pathway
a
ΦF was measured by using the ΦF (0.069) of 3HF in ethyl acetate as reference [32].

S. Protti, A. Mezzetti / Journal of Molecular Liquids 205 (2015) 110–114
113
Fig. 5. Scheme of 3HF–solvent interactions for TFE solution (b) and DMSO solution (c). The intramolecular hydrogen bond in isolated 3HF is also shown in (a).
in different solvents are found in the literature. Whereas Ficarra and co-
workers reported 3HF photorearrangement in CH2Cl2, cyclohexane,
acetonitrile, and photooxygenation in air saturated MeOH [24,25] (sug-
gesting therefore that protic solvents favor photooxygenation and aprotic
solvents favor photorearrangement); other groups (including ours) re-
port different results: photorearrangement in MeOH [14,26], in MeOH/
H2O [14], in benzene/isopropyl alcohol (1:1) [22] and DMSO [14] and
photo-oxygenation in non-polar solvents [23]. All the latter data suggest
a scenario (photooxygenation reaction taking place only in non-polar sol-
vents, photorearrangement reaction taking place in all other cases)
which is opposite to the one proposed by Ficarra and coworkers [24,
25]. In addition, a solvent effect on the reaction kinetics has been report-
ed: the photorearrangement reaction is faster in MeOH than in a 1:1
MeOH/H2O mixture [14].
A detailed investigation of the influence of all the possible parameters
(the presence or absence of oxygen, temperature, wavelength of irradia-
tion, etc.) on the reaction pathways is beyond the scope of this paper.
Our aim is to rationalize, in terms of specific solute–solvent interactions,
how the environment surrounding 3HF modifies its photochemistry and
its photophysics. For this reason, we focused our attention on de-aerated
solutions, in order to make the photooxygenation reaction impossible.
We found that in all the six studied solvents photo-rearrangement
takes place, yielding – as a primary product – the indiandione 2. In the
case of CHCl3 the situation looks complicated by the presence – along
with indiandione – of other products, whose detailed characterization
is in progress in our labs. Similarly, analogous byproducts are also
found in MeOH.
In agreement with Yokoe and coworkers [26], we also found that the
indiandione 2 can also undergo photoinduced decarbonylative degrada-
tion to form the lactone 3 (3-phenylisobenzofuran-1(3H)-one) in all
solvents with the exception of DMSO. Compound 3 was characterized
by NMR analysis and HPLC–MS (1H NMR (δ, CDCl3): 6.45 (s, 1H),
7.25–7.50 (m, 6H), 7.60–7.70 (m, 2H), 7.95–8.00 (d, 1H, J = 8 Hz); 1H
NMR (δ, CDCl3): 82.6 (CH), 122.7 (CH), 125.6 (CH), 126.9 (CH), 128.5,
128.9 (CH), 129.2 (CH), 129.3 (CH), 134.2 (CH), 136.3, 149.6, 170.1.
HPLC–MS (m/z); 210 (15), 209 (100), 152 (25), 77 (15)). The photore-
action of 2 is blocked in DMSO most probably because this solvent ab-
sorbs in the UV region where 2 has its most red-shifted absorption
band (~295 nm [14], see Fig. 6).
Two key pieces of information to understand the role played by the
solvent are the quantum yield of 3HF photodecomposition (Table 1)
and the kinetics under λirr = 317 nm (see Fig. 7). As it can be noticed,
the kinetics follows the order: TFE N MeOH ~ THF ~ MeCN N CHCl3 N
DMSO. The low reactivity of 3HF in DMSO can be explained only in
part as an effect of solvent filter (DMSO weakly absorbs at 310 nm).
We suggest that the quite strong (and probably quite stable) hydrogen
bond interaction between the 3-OH moiety of 3HF and the S_O of
DMSO [18] plays a key role in hampering the photorearrangement
mechanism. Interestingly, THF, which also can accept a hydrogen
bond from the 3-OH group of 3HF, but has a lower hydrogen bonding
acceptor basicity compared to DMSO (see Table 1), has a kinetics very
similar to MeCN and MeOH. This suggests that the strength and the sta-
bility of the hydrogen bonding interaction 3-OH\O_S rather than its
existence have a slowing effect on the photoreaction kinetics.
It can also be easily deduced from Fig. 7 that the polarity of the sol-
vent (this is the case of MeCN; DMSO, as explained above, is a very pe-
culiar case) and the proticity of the solvent (measured through the
hydrogen bond donor acidity α; this is the case of TFE and MeOH) are
both factors which accelerate the photoreaction. It is interesting to
point out that the reaction is faster in TFE (α = 1.51) than in MeOH
(α = 0.62), but in the presence of water (in MeOH/H2O 1:1 mixture,
for example see Ref. [14]) photorearrangement rate resulted lower, de-
spite the fact that H2O has a stronger hydrogen bond donor acidity (α =
1.17) than MeOH and is more polar. Therefore the hydrogen-bond
donor acidity of the solvent is not the only parameter to be taken into
account when investigating the influence of the solvent on 3HF
photorearrangement. Most probably, in the case of the MeOH/H2O
(1:1) mixture, specific water–3HF interaction plays a key role (for in-
stance, the possible simultaneous presence of hydrogen bonds between
a water molecule – acting in both cases as the donor – and the solute,
acting as an acceptor through the oxygen atoms of both the C_O and
the 3-OH moieties).
The present results are in rough agreement with a previous kinetic
study of the photochemistry of 3HF carried out in cyclohexane, MeCN
and MeOH [25]. However, in [25] the authors suggest the existence of
different reaction mechanisms (see above), and do not report data in
the absence of oxygen (as in our study) so that a direct comparison is
not feasible.
4. Conclusions
The photophysical and photochemical properties of 3HF depend
strongly on its interaction with the solvent. Apart from the role played
by hydrogen-bonding donor solvents (MeOH, TFE), some interesting as-
pects emerge, as for instance the peculiar interaction between 3HF and
DMSO, most probably through a 3OH\O_S intermolecular hydrogen
bond. Such an interaction could explain both the mechanism of 3HF
Table 2
Fluorescence lifetime (τ) measured for 3HF in different solvents. λem indicates the
emission wavelength used.
Solvent
λem (nm)
τ (ns)
CHCl3
THF
MeCN
MeOH
524
537
528
531
0.50
0.21
0.22
0.11
Fig. 6. Photodecomposition of 3-hydroxy-3-phenyl-1,2 indiandione to yield 3-
phenylisobenzofuran-1(3H)-one.

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S. Protti, A. Mezzetti / Journal of Molecular Liquids 205 (2015) 110–114
More detailed experiments, combining vibrational spectroscopy
(which can probe directly solute–solvent interactions) and dynamical
quantum mechanics calculations, are in progress in our laboratories to
better clarify the interactions between 3HF and organic solvents. This
will make it possible to rationalize the environment effect not only on
photochemical and photophysical properties, but also on other aspects
such as the solvent-dependent reactivity towards other species.
Acknowledgments
S.P. acknowledges partial financial support by the University Lille 1,
France through the “Invited Professors 2014 program” which made it
possible his stay in Lille. We thank B. Mannucci (Centro Grandi
Strumenti, Pavia, Italy) for precious help in the measurements. This
work was partially supported by the French Infrastructure for Integrat-
ed Structural Biology (FRISBI) ANR-10-INSB-05-01.
References
Fig. 7. Kinetics of 3HF photodecomposition in the examined solvents. The reaction has
been monitored via UV–Vis spectroscopy analyses (see experimental part for further de-
tails). Included is a list of factors which characterize 3HF–solvent interactions in each so-
lution and possibly influencing the kinetics of 3HF photodecomposition. DMSO: high
hydrogen bond acceptor basicity (β = 0.76), leading to a strong and possibly stable hydro-
gen bond interaction between the 3-OH moiety of 3HF (donor) and the oxygen atom of a
DMSO molecule (acceptor). DMSO has also a very high polarity (εr = 46.7). CHCl3: no sig-
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gen bond donor acidity (α = 1.51) and zero hydrogen bond basicity. This means that TFE
can interact with 3HF acting only as a hydrogen bond donor.
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