Photoreactions of 2-(furan-2-yl)-3-hydroxy-4H-chromen-4-one and 3-hydroxy-2-(thiophene-2-yl)-4 H -chromen-4-one using cyclohexane and acetonitrile as solvents

Article abstract of DOI:10.1039/c7pp00106a

Photolysis of the titled chromenones was carried out at their longest absorption band (～360 nm) using cyclohexane (CH) and acetonitrile (ACN) as solvents, in both aerated and de-aerated solutions. Different dimeric photoproducts were formed with both chromenones in aerated solutions. On photolysing 2-(furan-2-yl)-3-hydroxy-4H-chromen-4-one (FHC) in aerated cyclohexane, 2-(furan-2-yl)-2-{[2-(furan-2yl)-4-oxo-4H-chromen-3-yl]oxy}-2H-chromene-3,4-dione (a dehydrodimer) was formed, and on photolysing 3-hydroxy-2-(thiophene-2-yl)-4H-chromen-4-one (THC) in aerated ACN, a different dimeric product was isolated and identified. The corresponding 3-aryl-3-hydroxy-1,2-indandiones were also detected with FHC in ACN and with THC in CH, in addition to the dimeric products in both cases. On the other hand, in the de-aerated solutions, only the corresponding 1,2-indandiones were detected. 3-(Furan-2-yl)isobenzofuran-1(3H)-one as a secondary product was also detected with FHC in both solvents. An attempt was made to isolate the spectra of the photoproducts in situ. Excited State Intramolecular Proton Transfer (ESIPT) and Excited State Intramolecular Charge Transfer (ESICT) processes complicate the photodynamics of the reaction, making it difficult to predict the mechanisms of the photoreactions. However, tentative mechanisms have been proposed for the formation of the photoproducts.

Full text of DOI:10.1039/c7pp00106a

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Photoreactions of 2-(furan-2-yl)-3-hydroxy-4H-
chromen-4-one and 3-hydroxy-2-(thiophene-
Cite this: DOI: 10.1039/c7pp00106a
2
-yl)-4H-chromen-4-one using cyclohexane
and acetonitrile as solvents†
Kulvir Kaur, Ranbir Kaur, Jyoti Tomar and Manisha Bansal*
Photolysis of the titled chromenones was carried out at their longest absorption band (∼360 nm) using
cyclohexane (CH) and acetonitrile (ACN) as solvents, in both aerated and de-aerated solutions. Different
dimeric photoproducts were formed with both chromenones in aerated solutions. On photolysing
2
-(furan-2-yl)-3-hydroxy-4H-chromen-4-one (FHC) in aerated cyclohexane, 2-(furan-2-yl)-2-{[2-(furan-
yl)-4-oxo-4H-chromen-3-yl]oxy}-2H-chromene-3,4-dione (a dehydrodimer) was formed, and on
2
photolysing 3-hydroxy-2-(thiophene-2-yl)-4H-chromen-4-one (THC) in aerated ACN, a different dimeric
product was isolated and identified. The corresponding 3-aryl-3-hydroxy-1,2-indandiones were also
detected with FHC in ACN and with THC in CH, in addition to the dimeric products in both cases. On the
other hand, in the de-aerated solutions, only the corresponding 1,2-indandiones were detected.
3-(Furan-2-yl)isobenzofuran-1(3H)-one as a secondary product was also detected with FHC in both
solvents. An attempt was made to isolate the spectra of the photoproducts in situ. Excited State
Intramolecular Proton Transfer (ESIPT) and Excited State Intramolecular Charge Transfer (ESICT)
processes complicate the photodynamics of the reaction, making it difficult to predict the mechanisms of
the photoreactions. However, tentative mechanisms have been proposed for the formation of the
photoproducts.
Received 22nd March 2017,
Accepted 12th May 2017
DOI: 10.1039/c7pp00106a
rsc.li/pps
flavone (3-HF) has not drawn the interest of researchers up to
the desired extent. The photoreactions of 3-HF have been
Both absorption and emission spectroscopic studies of 2-aryl- studied for a long time but are not yet fully understood.
Introduction
1
7
3
-hydroxy-4H-chromenones (3HCs) have attracted scientists in Westhuizen et al. have exhaustively reviewed the photo-
1
–16
18
the last 30–40 years.
This is mainly because of the Excited chemistry of different classes of flavonoids. Matsuura and
State Intramolecular Proton Transfer (ESIPT) phenomenon co-workers irradiated 3-hydroxyflavone (3-HF) in isopropyl
occurring in these chromenones. The effects of different para- alcohol-benzene with a high pressure mercury lamp through
meters, especially the substituents and solvents in the 3-HCs, pyrex and obtained 3-aryl-3-hydroxy-1,2-indandione as the
4
–8
19
have been widely studied.
Dual emission observation (one photoproduct in an almost quantitative yield. They reported
2
2
emission from the normal excited state (N*) and one emission that the photorearrangement of 3-HF is due to (σ + π ) cyclo-
from the excited state of the tautomer (T*) formed by ESIPT in addition of the substrate in its triplet state, leading to the
3
HCs) has been used to provide information about the formation of 1,2-indandione via a 2,3-epoxy-2-hydroxy-1-indan-
physico-chemical properties of the microenvironment, both by
the position and the relative intensities of the two emission
1
2
bands. On the other hand, the photochemistry of 3-hydroxy-
Department of Chemistry, Punjabi University, Patiala-147002, Punjab, India.
E-mail: jindal_manisha@yahoo.co.in; Fax: +91 01752286682; Tel: +91 9417812003
1
†
Electronic supplementary information (ESI) available: H-NMR, IR and LC-MS
1
data for the dehydrodimer of FHC (Fig. S1–S3); H-NMR, IR and LC-MS data for
the anti-dimer of THC (Fig. S4–S6); IR data for the 1,2-indandione of THC
(Fig. S7); and GC-MS data for the 1,2-indandione, phthalide derivative and reac-
tant of FHC (Fig. S8–S10). See DOI: 10.1039/c7pp00106a
Scheme 1 Photorearrangement of 3-hydroxyflavone.
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dione intermediate (Scheme 1). Later on, they expanded the
range of photolysed flavonols. Yokoe and co-workers ir- chromen-4-one (FHC). For the preparation of FHC, 4.08 g
radiated flavonol in methanol and obtained the corresponding (0.03 moles) of 2′-hydroxyacetophenone was condensed with
Preparation and purification of 2-(furan-2-yl)-3-hydroxy-4H-
2
0
3
1
-(phenyl-2-yl)isobenzofuran-1(3H)-one product, in addition to 2.88 g (0.03 moles) of furan-2-carbaldehyde in the presence of
,2-indandione, which is the primary product. Thereafter, sodium hydroxide (2.5 g) in ethanol at 5 ± 1 °C. The reaction
2
1
Ficarra and co-workers reported the photolysis of 3-HF, which mixture which became deep red in color after 30 minutes was
gave the same product, i.e. 1,2-indandione, using aerated or stirred for a further 7–8 hours. Thereafter, it was poured over
2
2
oxygen free acetonitrile and dichloromethane as the solvent.
ice and neutralized with dilute HCl to obtain the chalcone,
However, Yokoe and co-workers reported that the oxygenated which was crystallized from methanol to afford orange-yellow
2
1
products dominated using aerated methanol as the solvent.
needles. The 2.4 g (out of 6.2 g) of chalcone thus obtained was
The conclusion of Ficarra et al. was that molecular oxygen subjected to AFO (Algar–Flynn–Oyamada) reaction conditions
affects the photochemical reactions of flavones in alcoholic in sodium hydroxide (3.5 g) and ethanol (25 ml) stirred at
(
protic-polar) and hydrocarbon (non-polar) solutions, and 5 ± 1 °C with dropwise addition of 30% (w/v) H
does not participate in the reactions in acetonitrile and over an hour. This mixture was stirred for a further 5–6 hours,
dichloromethane, which is under doubt. and the resulting reaction mixture on neutralization with
2 2
O (7.0 ml)
Protti et al. investigated the photostability of 3-hydroxy- dilute HCl gave a yellow mass of FHC. It was crystallized twice
flavone (3HF) and its complexes with Al(III), Zn(II) and Pb(II) in from MeOH : CHCl (1 : 1 v/v) to give fine pale yellow needles
3
2
3
methanol. Upon irradiating the solution of 3HF in deaerated (85% yield) of FHC, with an m.p. of 177–178 °C, which agrees
DMSO, they obtained the same photorearranged product, i.e. with previous reports.
Preparation and purification of 3-hydroxy-2-(thiophene-2-yl)-
not that of Ficarra. In their recent studies on the photo- 4H-chromen-4-one (THC). THC was prepared by a similar
reactivity of 3-HF, they again contradicted the observations of method to that described above, except that in the conden-
Ficarra et al. Zoltan Szakacs et al. studied the photooxygen- sation step, thiophene-2-carbaldehyde was used in place of
1
3,25,26
1
9–21
1
,2-indandione. This also supports the earlier studies,
but
2
2
1
7
ation of 4′-diethylamino-3-hydroxyflavones in acetonitrile to furan-2-carbaldehyde. It was crystallized from MeOH : CHCl
3
give O-4-diethylaminobenzoyl salicylic acid, a product similar (1 : 1 v/v) to give dense yellow needles of THC, with an m.p. of
2
4
27
to that of 3-HF in methanol.
160–162 °C and a yield of 63%. The structures of FHC and
1
On replacing the 2-phenyl ring with a 2-furanyl ring, there THC were also confirmed by their IR and H NMR spectra. The
are lots of changes in the ESIPT process, both quantitatively sharp m.p., single spots on the TLC plates and sharp peaks in
1
4
and qualitatively. Therefore the two titled chromenones, the UV/Vis spectra confirm the purity of the compounds.
FHC and THC, were chosen in the present work. Their photo-
chemistry and the results of their photochemical reactions are
interesting, as predicted from their spectroscopic studies.
Photolysis assembly
A medium pressure mercury lamp (125 W) with a built-in
filter, emitting at 366 nm, was used for photolysis. Some low
intensity emission lines at wavelengths lower than 340 nm
were eliminated by incorporating a pyrex glass cut off filter.
A small fan was provided near the photolysis cell in order to
avoid excessive heating due to the lamp. A 50 ml pyrex glass
Experimental
Materials and methods
All solvents used, such as acetonitrile (ACN) and cyclohexane tube with a side tube to flush nitrogen/oxygen gases and a
CH), were of spectroscopic grade and were purchased from water condenser to avoid evaporation of the solvent was used
S.D. Fine Chemical and Loba Chemie. The reagents used for to carry out bulk photolysis.
the preparation of the 2-aryl-3-hydroxychromones were
double beam UV/Vis spectrophotometer (UV-1800
purchased from Sigma Aldrich, USA. For deoxygenation, nitrogen Shimadzu) equipped with UV-probe software was employed for
gas (99.5%) was flushed from the solutions to be photolysed.
recording UV/Vis spectra. Fluorescence spectra were recorded
-(Furan-2-yl)-3-hydroxy-4H-chromen-4-one (FHC) and using
Shimadzu spectroflurophotometer (RF-5301PC).
(
A
2
a
3
-hydroxy-2-(thiophene-2-yl)-4H-chromen-4-one (THC) used in IR spectra were recorded using a FTIR spectrophotometer
1
the present studies (Fig. 1) were synthesized by preparing their (PerkinElmer, RZX). H NMR spectra were recorded using a
corresponding chalcones.
Bruker Avancell 400NMR spectrophotometer with TMS as an
internal standard. Mass spectra were recorded using LC-MS
(
Waters Micromass Q-TOF) and GC-MS (Shimadzu QP 2010
Plus) spectrometers.
Results
Fig. 2 and 3 show the absorption and emission spectra of FHC
and THC in CH (I) and ACN (II). By carefully examining the
Fig. 1 Structures of the 2-aryl-3-hydroxychromenone derivatives.
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Fig. 5 Spectra of the photolysed solutions of THC (5.9 × 10− M) in
CH (I) and in ACN (II).
5
Fig. 2 Absorption and emission spectra of FHC (5.2 × 10− M) in CH (I)
5
and ACN (II).
It is clear from the decreasing absorbance values from 300
to 365 nm that both 3-HCs are photodegraded. There appears
to be a different pattern in the spectra from 230 to 290 nm in
both Fig. 4 and 5. Obviously, this is because of the formation
of product(s) which have different molar absorption coefficient
(
s) than that of the reactant(s).
Identification and characterization of the photoproducts of
FHC in different solvent systems
The photoproduct formed using CH as the solvent. For the
purpose of identifying the photoproduct, bulk photolysis in
several separate 50 ml solutions was carried out for 3–4 h using
aerated cyclohexane as the solvent. Because of the low solubility
of the reactant in cyclohexane (about 100 ppm), only low con-
centrations (∼5.2 × 10− M) could be used for photolysis.
In addition to the inherent low yield of product(s) from the
photochemical reactions, here we encountered two other
major problems; firstly, the reactant has very low solubility in
cyclohexane; secondly, the product deposits on the walls of the
container after a brief time of photolysis. Also, it takes away
the reactant from the solution along with it and interrupts the
light path. Because of this, the product is completely insoluble
in cyclohexane. Therefore, it was possible only after a long
struggle to isolate a small amount of the photoproduct as a
brown mass, which was crystallized from methanol :
chloroform (1 : 1) to yield shining flakes of brown product. The
appearance of only a single spot showing light blue fluo-
rescence on a TLC plate confirms its purity.
Fig. 3 Absorption and emission spectra of THC (5.9 × 10− M) in CH (I)
and ACN (II).
5
4
finer details of the spectra, it is clear that there are two sets of
main absorption bands, similar to the spectra of general
flavones and their hydroxyl substituted derivatives. Band I in
the region of 270–380 nm is due to the cynnamoilic part and
band II around 250 nm is due to the benzenic part of the
molecule. Both FHC and THC have a high molar absorption
coefficient (2 × 10 L mol cm ) at around 365 nm in both
CH and ACN solvents. Therefore in this study, the longest
wavelength region of the absorption band I, i.e. around
4
−1
−1
3
60 nm, was chosen as the irradiation wavelength.
Fig. 4 and 5 show the spectral changes with time on photo-
−
5
lysing a 3 ml solution of the 3-HCs (∼5.0 × 10 M) taken in an
3
airtight teflon covered quartz cuvette (1 × 1 × 4.5 cm ).
This product decomposes on melting at 225–230 °C. The
mass spectrum of this product shows molecular ion peaks
+
+
+
(
m/z) at 454 [M ] along with peaks at 453 [M − 1], 455 [M + 1]
+
and 475 [M + Na]. The IR spectrum shows two intense bands
at 1725 and 1613 cm− and one shoulder around 1640 cm
1
−1
which indicates the presence of the 1,2-dicarbonyl group. Also,
−1
no peak is observed around 3300 cm (there is an intense
band at 3258 cm− in the IR spectra of the reactant), which
1
shows the absence of the –OH group in the product. The
1
3 3
H NMR (CD COCD ) spectrum shows the presence of
14 protons, all in the aromatic region from δ 6.6–8.0 ppm.
The UV/Vis absorption spectrum shows one main absorption
band at 252 nm and a low intensity peak around 320 nm. The
product appears to be a dehydrodimer of FHC (MW = 454),
having the structure shown in Fig. 6, as all the data (m.p.,
Fig. 4 Spectra of the photolysed solutions of FHC (5.2 × 10−5 M) in
CH (I) and in ACN (II).
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Scheme 2 Product formation.
Fig. 6 The structure of 2-(furan-2-yl)-2-{[2-(furan-2yl)-4-oxo-4H-
chromen-3-yl]oxy}-2H-chromene-3,4-dione (the dehydrodimer, DDH).
3
-(furan-2-yl)isobenzofuran-1(3H)-one (3b) (Scheme 2). It was
further observed that 2b was unstable even in solution and
IR and UV) are similar to that of the dehydrodimer of was probably converting to 3b (as noticed from the changes in
2
8,29
3
-hydroxyflavone (3-HF), as reported earlier.
its UV spectrum).
1
The finer details of the H NMR spectrum are also in agree-
ment with this structure of the product.
δ 7.8–7.9 ppm [dd, 2H, H-5, H-5″, J = 7.8, J = 1.7];
0 m
δ 7.78–7.8 ppm [distorted m, 2H, H-3′, H-5′]; δ 7.61–7.64 ppm
Identification and characterization of the photoproducts of
THC
[
dd, 1H, H-4′, J = 7.2, J = 6.4]; δ 7.5–7.54 ppm [dd, 3H, H-8,
The photoproduct formed using ACN as the solvent. As
0
0
H-8″, H-5′′′, J
0
= 7.2, J
m
= 1.7]; δ 7.21 ppm [dd, 1H, H-3′′′]; described above for FHC, several separate 50 ml solutions of
δ 6.92–6.96 ppm [m, 4H, H-6, H-7, H-6″, H-7″ Ar-H]; THC (∼6.0 × 10− M) were photolysed for the pre-determined
5
δ 6.62–6.64 ppm [dd, 1H, H-4′′′, J = 7.5, J = 5.5].
optimized time of 35 hours. TLC analysis, using n-hexane :
0
0
One furan ring is in conjugation with the cynnamoilic part ethyl acetate (50 : 50) as the solvent, shows only one photopro-
of the molecule, due to which the protons H-3′′′, H-4′′′ and duct with more than a 90% yield. Thereafter, the solvent was
H-5′′′ are displayed separately in the spectrum, whereas due to distilled out from the photolysed solution and the remaining
rotation of the other furan ring around a single bond, the solid was crystallized from methanol : chloroform to obtain
protons H-3′ and H-5′ merge with each other, which supports white flakes, which decompose on melting from 250 to 252 °C.
this structure. Furthermore, this product reformed the reactant The IR spectrum (neat) of this product shows a sharp peak at
2
9
−1
upon treatment with dilute HCl in methanol.
1640 cm
indicating the presence of the –CvO group.
−
1
The photoproduct formed using ACN as the solvent. The A broad intense band around 3432 cm shows the presence of
aerated solution of FHC (∼5.2 × 10 M) was photolysed for 2 h the –OH group in the product.
using acetonitrile as the solvent. TLC analysis showed two pro-
−
4
1
The H-NMR (400 MHz, CD COCD ) spectrum displayed 14
3 3
ducts: one of which was the same as in cyclohexane as the aromatic protons in the region of δ 6.9–7.91 ppm. The mass
solvent, and the other which had properties corresponding to spectrum of this product shows the molecular ion peak at 527
+
those of 1,2-indandione. However, on photolysing the deoxyge- [M + K] indicating that the product is a dimer (MW 488) of
nated solutions, two products were detected by GC-MS (Fig. 7). the reactant molecule (MW 244). The absence of the 1,2-diketo
The product peak with a retention time (RT) of 11.10 minutes group and presence of the –OH group in the product shows
had a MW of 200, the peak at 13.15 minutes was due to the that this product is an anti-dimeric product, having the
reactant (MW of 228), and the product peak with a RT of structure shown in Fig. 8. A similar product has also been
1
2
5.7 minutes had a MW of 228. It appears that the reported from the photoreactions of chromen-2-carboxylic
28 MW compound is 2b, and the 200 MW compound is ester.
3
0
Fig. 7 Chromatogram from GC-MS of the photolysed (deoxygenated) solution of FHC in ACN.
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identified by comparing its melting point (152–154 °C) and UV
and IR spectra with reported data for the similar product of
3
1
Cl-THC. UV/Visible spectra of all the products are given in
Fig. 9. Data for the photoproducts of FHC and THC under
different environments are summarized in Table 1.
UV/Vis absorption spectra of photoproducts in situ
An attempt has been made to separate and analyze the UV
spectra of the photoproduct(s) in situ. Fortunately, all photo-
products (both dimers, both 1,2-indandiones (2b and 2c) and
both phthalide derivatives (3b and 3c)) do not absorb at
λ > 330 nm, and both reactants have two major peaks beyond
this wavelength. Therefore, it is possible to separate the
spectra of the photoproduct(s) from the total spectra of the
photolysed mixture by subtracting the remaining weighed
spectra of the reactant(s).
Fig. 8 Structure of the dimer of THC.
1
The finer details of the H-NMR spectrum clearly support
this structure. A multiplet at δ 7.54–7.64 ppm is equivalent to
the aromatic protons H-6, H-7, H-6″ and H-7″ of the two ben-
zenic rings. The protons H-5 and H-5″, which are alpha to the
carbonyl group, show a multiplet at δ 7.84–7.88 ppm, whereas
the two protons H-8 and H-8″ are upfield at δ 7.11–7.24 ppm.
Two protons of the 5-membered heterocyclic rings, H-5′ and
H-5″, appear as doublets of doublets at δ 7.93 and 7.91 ppm
respectively. However, H-3′ (δ 7.44 ppm) and H-3′′′ (δ 7.37 ppm)
show as quartets. The most upfield signals at δ 7.0 and
Fig. 10 shows the spectra of the photoproducts of FHC
obtained by subtracting the spectra of the unreacted FHC in
CH and ACN. The regular growth of the peaks at 254, 282 and
320 nm and the isobestic points in the original spectra (Fig. 3)
indicate the growth of the primary products. The separation of
the photoproduct formed in CH interrupts the transparency of
the solution, due to which the subtracted spectra are less
reliable here.
Similar information can be expected from the subtracted
spectra for THC (Fig. 11). Here, the subtracted spectra in both
solvents at lower times of photolysis, when multiplied by a
suitable factor, overlap with the spectra at higher times of
photolysis. This clearly shows that no secondary reaction is
taking place.
6
.9 ppm are due to H-4′ and H-4′′′.
The photoproduct formed using CH as the solvent. TLC ana-
lysis of the photolysed (aerated) solution of THC using cyclo-
hexane as the solvent shows the formation of two products:
one (anti-dimer) with a ∼20% yield, and another with more
1
9
than 60% yield, identified as 1,2-indandione (2c). 2c was
Fig. 10 Spectra of the photoproducts obtained by subtracting the
unreacted reactant from the photolysed mixtures of FHC in CH (I) and
ACN (II) at different times of photolysis (cf. Fig. 3).
Fig. 9 UV/Visible spectra of 2c (I), DDH (II), anti-dimer (III) and 3b (IV)
using ACN as the solvent.
Table 1 Photoproducts formed from the photolysis of FHC and THC in CH and ACN
Cyclohexane
Acetonitrile
Aerated
Compound
Aerated
De-oxygenated
De-oxygenated
FHC
THC
DDH
2c + anti-dimer
2b + 3b
2c
DDH + 2b
Anti-dimer
2b + 3b
No reaction
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Fig. 11 Spectra of the product obtained by subtracting the unreacted reactant from the photolysed mixtures of THC in CH (I) and ACN (II) at
different times of photolysis (cf. Fig. 4) and the spectra of the photoproduct of THC in ACN under different conditions (III).
An important observation was also made by comparing a is non-planar with respect to the 3-HC moiety, while the
set of four spectra in ACN (Fig. 11 (III)): (I) a freshly photolysed furanyl and thiophenyl groups have similar sized rings and are
2
6
solution, (II) when this solution was kept overnight, (III) after planar with respect to the 3-HC moiety of the flavonol. The
evaporating (I) then adding the solvent, and (IV) the spectra of planarity of the 5-membered ring supports conjugation.
a separated sample of the anti-dimer of THC. It appears that During electronic excitation of these flavonols, in addition to
the spectra of the product(s) in a freshly photolysed solution ESIPT, the Excited State Intramolecular Charge Transfer
get closer (shift in the peak at 232 nm and splitting of the (ESICT) process also participates. The contribution of the
peak at 275 nm) to that of the anti-dimer of THC with time phenyl ring at the 2-position to the ESICT effect is small
and on evaporation.
because of its non-planarity, and thus it does not support con-
jugation. In FHC and THC, furanyl and thiophenyl rings
become major contributors as electron donors, unlike in 3-HF,
where one oxygen atom is the only contributor as an electron
donor. Furthermore, furanyl and thiophenyl rings are also
Discussion
2
6,33
It is well known that in most photochemical reactions, the sub- electron rich compared to the phenyl ring.
strate initially goes to the excited singlet state and then to the
The behavior of FHC is exclusively different for ESIPT
triplet state through ISC, from which the subsequent photo- reactions. It is not solely dependent on its electron donating
chemical reactions are initiated. In the case of 3-hydroxy- capacity in its ground state, but it has been suggested that the
chromenones, there is an additional and major process, ESIPT behavior of FHC could be due to its strong excited state
ESIPT, which contributes to the mechanism of their photo- electron donor ability, as compared to that for the phenyl group
32
reactions. Since ESIPT is highly sensitive to the environment, or even better electron-donating substituents. It has also been
the structure of the chromenones and other parameters will shown that the behavior of FHC is different during studies on
affect their photochemistry. In the present work, it is clear that the variation of IN/IT* with the polarity of the solvent for
the photoreactions of FHC and THC are not only different to different electron donating substituents at the 2-position.
those of 3-HF but also to each other (cf. introduction). In the
Taking all these facts together, it is not an easy task to give
photoreactions of FHC, only one product dehydrodimer was the mechanism(s) for the formation of the products in the
detected using CH as the solvent, and two more products present studies. However, the following steps for the formation
(2b and 3b) were formed in ACN, as shown in Table 1. In the of the photoproducts from FHC and THC in aerated solutions
case of THC, only one product anti-dimer of THC was formed have been proposed.
using ACN as the solvent, and one additional product (2c) was
formed in CH. In both cases, the presence of oxygen is formed.
required for the formation of the corresponding dimers. It is
worth mentioning that no dimer has been reported in the
photoreactions of 3-HF, neither in the presence nor in the
absence of oxygen. It appears that replacing phenyl with
thiophenyl and furanyl at the 2-position of 3-HF changes the
photodynamics and the resulting photoreactions.
In the first step, the peroxide intermediate (Scheme 3) is
It has been reported that electron donating groups at the
2
-position result in slower ESIPT reactions, thus increasing the
3
2
I /I ratio. We have also observed, compared and reported
N
T
the comparative data on the emission spectra of 3-HF, FHC
2
6,33
and THC.
Among these three flavonols, 3-HF is different
Scheme 3 Peroxide formation by the addition of oxygen to the reac-
from the other two. In 3-HF, the phenyl group at the 2-position tant molecule in tautomeric form.
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As it was suggested for 3-HF that the peroxide intermediate
was formed by combination of the ground state of the tauto-
mer and the ground state of oxygen, it appeared that the for-
mation of the peroxide intermediate (A) and the intermediate
(
B) takes place by the combination of some different states of
oxygen and/or chromenones in the case of FHC and THC. The
formation of (C) has been proposed because the final product
appears to be formed during the process of separating the
product. It is indicated by Fig. 11 (III) that the final dimeric
product appears to be formed on heating. Such peroxides (C)
are well known to decompose on heating. Also, this type of
dimer has been shown to be formed upon irradiation of
chromone-2-carboxylic esters using ACN as the solvent, and it
has been proposed that this type of dimer is formed from the
Scheme 4 Proposed mechanism for the formation of DDH from the
peroxide.
3
0
triplet excited states.
The formation of 1,2-indandione (1b) in the photolysis of
3-HF has been shown to take place from the excited triplet
In the photooxygenation of 3-HF, it has been suggested that state of the normal form by Matsuura et al.18,19 As in the
such an intermediate peroxide is formed by the combination excited states of 3-hydroxychromenones, fluorescence emission
of the ground states of both the tautomer of 3-HF and from the tautomeric form dominates (Fig. 2 and 3), especially
3
4
oxygen. Different mechanisms appear to operate for the sub- in non-polar solvents. It is deduced that the photochemical
sequent formation of the dimeric products from FHC and reactions should initiate from the tautomer form. The type of
THC. In the case of FHC, the hydrogen abstraction reaction photoreaction for the formation of 1,2-indandione (1b)
should take place before the formation of the final products suggested by Matsuura et al. is unlikely to proceed from the
3
5
(Scheme 4).
tautomer triplet state. Pi-Tai Chou et al.
studied the
A similar mechanism has been suggested for the reactions dynamics of the decay of the excited state of 3-HF by transient
2
8
of singlet oxygen with 3-HF. On the other hand, a similar absorption, two-step laser excitation and steady state photo-
peroxide intermediate formed from THC appears to follow a lysis. They proposed that the triplet state of the normal form is
different course of reaction (Scheme 5).
regenerated in the proton transfer cycle which gives 1,2-indan-
Scheme 5 Proposed mechanism for the formation of the anti-dimer of THC from the peroxide.
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dione. Here, in our reactions in the absence of oxygen, 1,2-
indandione (2b) and the corresponding phthalide (3b) are
formed in the case of FHC in both CH and ACN solvents
the new principle in design of fluorescence sensors, J. Am.
Chem. Soc., 2002, 124, 12372–12379.
7 S. Ercelen, A. S. Klymchenko and A. P. Demchenko,
Ultrasensitive fluorescent probe for the hydrophobic range
of solvent polarities, Anal. Chim. Acta, 2002, 464, 273–287.
8 A. P. Demchenko, S. Ercelen, A. D. Roshal and
A. S. Klymchenko, Excited state proton transfer reaction in
a new benzofuryl 3-hydroxychromone derivative: the in-
fluence of low-polar solvents, Pol. J. Chem., 2002, 76, 1287–
1299.
(Table 1). The 1,2-indandione (2b) of FHC appears to be
unstable and decomposes to the corresponding phthalide
derivative (3b). The fact that there is no reaction of THC in
deoxygenated ACN solvent is unexplainable. The assignment
of the excited states, either of the normal or tautomer forms
involved in the formation of the different products, is beyond
the scope of this study.
9
A. S. Klymchenko, V. G. Pivovarenko and A. P. Demchenko,
Elimination of the hydrogen bonding effect on the solvato-
chromism of 3-hydroxyflavones, J. Phys. Chem. A, 2003, 107,
Conclusion
4211–4216.
The subsequent reactions after photooxygenation with the two
seemingly similar chromenones studied here are different not
only to those from 3-hydroxyflavone but also to each other.
Solvents play a crucial role in the mechanism and thus affect
the formation of the final products. However, in the de-aerated
solutions, the corresponding 1,2-indandione is the product in
all cases. It is difficult to elucidate the photodynamics and
mechanism of these photoreactions because of the complexity
created by the ESIPT and ESICT processes.
1
1
0 A. S. Klymchenko, A. P. Demchenko, V. G. Pivovarenko,
G. Duportail and Y. Mely, Multiparametric color-changing
fluorescence probes, J. Fluoresc., 2003, 13, 291–295.
1 A. S. Klymchenko, V. G. Pivovarenko and A. P. Demchenko,
Perturbation of planarity as the possible mechanism of
solvent dependent variations of fluorescence quantum
yield in 2-aryl-3-hydroxychromones, Spectrochim. Acta, Part
A, 2003, 59, 787–792.
2 A. S. Klymchenko and A. P. Demchenko, Multiparametric
probing of intermolecular interactions with fluorescent dye
exhibiting excited state intramolecular proton transfer,
Phys. Chem. Chem. Phys., 2003, 5, 461–468.
1
Acknowledgements
1
3 A. N. Bader, V. G. Pivovarenko, A. P. Demchenko, F. Ariese
and C. Gooijer, Solvent influence on excited-state intra-
molecular proton transfer in 3-hydroxychromone deri-
vatives studied by cryogenic high-resolution fluorescence
spectroscopy, Spectrochim. Acta, Part A, 2003, 59, 1593–
One of the authors is grateful to UGC, New Delhi for awarding
the UGC-BSR fellowship. We also acknowledge the regional
Sophisticated Instrumentation Centre, Chandigarh, for record-
ing the IR, NMR and mass spectra. Our heartiest thanks are
due to Prof. W. R. Bansal (Retd., Chemistry Department,
Punjabi University, Patiala) for helpful discussion.
1603.
1
4 A. S. Klymchenko and A. P. Demchenko, 3-
Hydroxychromone dyes exhibiting excited-state intra-
molecular proton transfer in water with efficient two-band
fluorescence, New J. Chem., 2004, 28, 687–692.
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