Single and double intramolecular proton transfers in the electronically excited state of flavone derivatives

Article abstract of DOI:10.1039/c5ra13912k

In an attempt to create a flavone derivative able to take part in Excited State Intramolecular Double Proton Transfer (ESIDPT), we synthesized two carbonyl derivatives of 3,7-dihydroxyflavone, both containing two different proton-transfer sites as well as related carbonyl derivatives of 3-hydroxyflavone and 7-hydroxyflavone. All the examined hydroxyflavones were found to participate in the Excited State Intramolecular Proton Transfer (ESIPT). ESIPT which involves 3-hydroxyl and 4-carbonyl groups was found to have a higher barrier compared to ESIPT involving 7-hydroxyl and 6/8-carbonyl fragments. According to the data presented, 3,7-dihydroxy-2-phenyl-6-(3-phenylpropanoyl)-4H-chromen-4-one undergoes a two-stage ESIDPT with formation of an intermediate tautomer. This kind of ESIDPT leads to a tautomeric form with an abnormally low rate of radiative deactivation of the excited state, which conditions low fluorescence quantum yield. The behavior of 3,7-dihydroxy-4-oxo-2-phenyl-4H-chromene-8-carbaldehyde in the electronically excited state is similar to 3-hydroxyflavone derivatives, thus we conclude the occurrence of a single ESIPT in this compound.

Full text of DOI:10.1039/c5ra13912k

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DOI: 10.1039/C5RA13912K
Journal Name
ARTICLE
Single and double intramolecular proton transfers in the
electronically excited state of flavone derivatives
I. E. Serdiuka,b and A. D. Roshalb
Received 00th January 20xx,
Accepted 00th January 20xx
In an attempt to create a flavone derivative able to take part in the Excited State Intramolecular Double Proton Transfer
(ESIDPT), we synthesized two carbonyl derivatives of 3,7-dihydroxyflavone, both containing two different proton-transfer
sites as well as related carbonyl derivatives of 3-hydroxyflavone and 7-hydroxyflavone. All the examined hydroxyflavones
were found to participate in the Excited State Intramolecular Proton Transfer (ESIPT). ESIPT which involves 3-hydroxyl and
4-carbonyl groups was found to have higher barrier compared to ESIPT involving 7-hydroxyl and 6/8-carbonyl fragments.
According to the data presented, 3,7-dihydroxy-2-phenyl-6-(3-phenylpropanoyl)-4H-chromen-4-one undergoes a two-
stage ESIDPT with formation of an intermediate tautomer. This kind of ESIDPT leads to a tautomeric form with an
abnormally low rate of radiative deactivation of the excited state, which conditions low fluorescence quantum yield.
Behavior of 3,7-dihydroxy-4-oxo-2-phenyl-4H-chromene-8-carbaldehyde in the electronically excited state is similar to the
one of 3-hydroxyflavone derivatives, thus we conclude occurrence of a single ESIPT in this compound.
DOI: 10.1039/x0xx00000x
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acid,4 oxazoles5–7, and chromones8–11, and subsequently inves-
tigated from the point of view of ESIDPT by steady-state and
time-dependent absorption and fluorescent spectroscopies, as
well as ab initio and semiempirical computational techniques.
Generally, it was found that after excitation these compounds
undergo single ESIPT, while occurrence of ESIDPT is much
less favorable. It should be noticed that so far scientists’
attention was mostly paid to compounds with axial symmetry
and the same two PT sites.
Introduction
The Excited State Intramolecular Proton Transfer (ESIPT)
represents one of the fundamental reactions in photochemistry.
The transformation occurs most often in compounds which
contain fragments participating in keto-enol and imine-amine
tautomerization reactions connected by the intramolecular
hydrogen bond (IHB).1–3 Due to its very low energetic barrier
ESIPT is known to be ultrafast at various conditions, which
allows it to successfully compete with other deactivation
pathways of the electronically excited state, such as radiative
deactivation. Followed by a reverse proton transfer in ground
state, ESIPT conditions high photostability of the compounds,
and specific fluorescent properties namely large red-shift of
emission, which is practically insensitive to reabsorption effect
and very sensitive to external hydrogen bonding. ESIPT
fluorophores have various applications in optoelectronic
materials2 and as biological markers and probes.3
Excited State Intramolecular Double Proton Transfer
(ESIDPT) is a very rare process, which involves two proton-
transfer (PT) sites in one molecule. Hypothetically, ESIDPT
can be as useful as ESIPT or even more, if one expects higher
energy losses of the electronically excited molecule before it
emits light. Various bifunctional organic compounds were
designed and synthesized, including derivatives of salicylic
In this article we present synthetic routine and results of
spectroscopy investigations of two novel asymmetric flavone
derivatives:
-chromen-4-one (1a) and 3,7-dihydroxy-4-oxo-2-phenyl-4
chromene-8-carbaldehyde 2a (Chart 1). Both of the
3,7-dihydroxy-2-phenyl-6-(3-phenylpropanoyl)-
4
H
H-
(
)
investigated compounds contain two different PT sites able to
Chart 1 Canonical structures of 1a-d and 2a-d with num-
bering of atoms indicated
O
H
1
O
1
O
8
8
2
7
6
2
R
7
R
3
3
4
4
O
6
R'
R'
5
5
R''
O
O
R, R' = OH, OH (1a);
R, R' = OH, H (1b);
R, R' = OH, OH (2a);
R, R' = OH, H (2b);
R, R' = OH, OCH3 (2c);
R, R' = OCH3, H (2d).
a. Department of Chemistry, University of Gdańsk, Gdańsk, 80-308 Poland. E-mail:
illia.serdiuk@gmail.com
R, R' = OCH3,OH, (1c);
R, R' = OCH3, H (1d);
R'' = (CH2)2Ph
b. Institute of Chemistry, V. N. Karazin Kharkiv National University, Kharkiv, 61022
Ukraine
† Electronic Supplementary Information (ESI) available: for 1H-NMR and mass
spectra of new compounds see DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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participate in keto-enol tautomerization reactions. The first site attempt to synthesize 6-acetyl-3,7-dihydroxy-2-phenyl-4H-
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DOI: 10.1039/C5RA13912K
is represented by a hydroxyl group in position 7 and a carbonyl chromen-4-one starting from the commercially available 1,1’-
fragment in ortho position: 3-phenylpropanoyl in position 6 (4,6-dihydroxy-1,3-phenylene)bisethanone. However, reaction
(1a) and formyl in position 8 (2a). The second site includes of the latter with equimolar quantity of benzaldehyde and other
hydroxyl and carbonyl group in positions 3 and 4, respectively. aldehydes in alkalinic media led to a mixture of mono- and
These fragments are similar to PT sites of ortho
hydroxybenzaldehyde and flavonol (3-hydroxy-2-phenyl-4
-
-
dichalcone derivatives,13 which were difficult to separate. In
order to eliminate formation of the undesired dichalcone
H
chromen-4-one) derivatives, both known to participate in product, 1,1’-(4,6-dihydroxy-1,3-phenylene)bisethanone deri-
ESIPT.12 Compounds 1a and 2a can possibly exist in 4 vatives were designed with subtituent(s) in one of the α
tautomeric forms (Charts 2 and 3), transforming one to another positions (
via intramolecular proton transfer reactions. Forms T6 1a) and dihydroxyphenyl)-2,2-dimethylpropan-1-one (5b) would react
T8 2a) can be produced via PT involving hydroxyl groups in with only one equivalent of benzaldehyde and give
5, Scheme 1). Most probably, 1-(5-acetyl-2,4-
(
(
a
position 7, T4 can appear in the result of PT involving hydroxyl monochalcone derivative. Preparation of 5b was, however,
groups in position 3, and DT can be formed in the result of problematic: the pivaloyl fragment of 1-(2,4-dihydroxyphenyl)-
double PT. The related compounds 7-hydroxy-2-phenyl-6-(3- 2,2-dimethylpropan-1-one
(4b), prepared by reaction of
phenylpropanoyl)-4
methoxy-2-phenyl-6-(3-phenylpropanoyl)-4
1c), and 7-hydroxy-4-oxo-2-phenyl-4
H
-chromen-4-one
(
1b),
3-hydroxy-7- resorcinol with pivaloyl chloride in hot boron trifluoride
H
-chromen-4-one
etherate was unstable in the presence of Lewis acids14 utilized
(
H
-chromene-8- in further acylation step and only 1-(3-acetyl-2,4-
carbaldehyde (2b), 7-hydroxy-3-methoxy-4-oxo-2-phenyl-4
chromene-8-carbaldehyde 2c), which contain only one in a low yield. 1-(2,4-Dihydroxyphenyl)-3-phenylpropan-1-one
hydroxyl group were used as models for investigation of single 4a, Scheme 1) on the other hand, obtained in the same
PT reactions as well as properties of T6 T4, and T8 conditions from resorcinol and 3-phenylpropionic acid, gave
respectively. Compounds 7-methoxy-2-phenyl-6-(3-phenylpro- the desired 1-(5-acetyl-2,4-dihydroxyphenyl)-3-phenylpropan-
panoyl)-4H-chromen-4-one 1d and 7-methoxy-4-oxo-2- 1-one (5a) in the reaction with acetic anhydride in the presence
phenyl-4 -chromene-8-carbaldehyde 2d), which do not of ZnCl2 in a good yield (51%) and minor impurity of 1-(3-
H
-
dihydroxyphenyl)-2,2-dimethylpropan-1-one (6b) was isolated
(
(
,
,
(
)
H
(
contain hydroxyl groups, were used for investigations of the acetyl-2,4-dihydroxyphenyl)-3-phenylpropan-1-one (6a) (8%),
target compounds’ behavior in the absence of PT reactions. which was separated by column chromatography. 5a reacted
Taking into account that 1a-c and 2a-c can possibly exist in with equimolar quantity of benzaldehyde in aqueous N-methyl-
anionic forms besides neutral ones, spectral behavior of these pyrrolidone (NMP) in the presence of KOH leading to the
compounds was investigted in the presence of DBU. This chalcone derivative 1e in 48% yield. When 1e was treated with
helped to define origin of absorbance and fluoresccence of 1a H2O2 in MeOH in the presence of NaOH (Algar-Flynn-
and 2a
.
Oyamada reaction conditions) the desired 1a was not identified
in the reaction mixture. This phenomenon can be explained by
existence of 1e at such conditions in doubly deprotonated form,
RESULTS AND DISSCUSSION
as in the case of (2E)-1-(2,4-dihydroxyphenyl)-3-phenylprop-2-
en-1-one,15 which does not transform to the corresponding 3-
hydroxyflavone derivative. 1e was then converted to the
flavone derivative 1b in hot DMSO in the presence of I2 (yield
53%), which after protection of the hydroxyl group by
Synthesis
In an attempt to obtain a derivative of 3,7-dihydroxyflavone
with a carbonyl fragment in position 6 we firstly made an
Chart 2 Structures of possible tautomeric forms (N, T4, T6,
and DT) of 1a (Chart 1)
Chart 3 Structures of Possible Tautomeric Forms (N, T4, T8,
and DT) of 2a (Chart 1)
N
T4
N
T4
O
H
O
H
H
O
O
R
O
H
O
O
R
O
H
O
H
O
O
+
O
O-
+
O
H
O-
O
H
O
O
H
O
O
H
DT
T6
T8
DT
O
H
O
H
O
R
O
O
R
O
H
O
H
O
H
O
H
O
+
O
+
O
O
O-
O
H
O-
O
O
O
H
H
O
H
2 | J. Name., 2012, 00, 1-3
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dimethylsulfate was oxidized with dimethyldioxirane, genera- alkylated with dimethylsulfate, followed by cleavage of benzyl
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ted in situ by Oxone© and acetone in water-chloroform (1:1, group in HBr solution in acetic acid to yDieOldI: 1102.1.03T9r/eCa5tRmAe13n9t12oKf
v:v) suspension at 0 – 5 °C. The obtained epoxide of 1d was 12 with excess of hexamethylenetetramine in boiling glacial
then hydrolyzed by TFA without isolation, what produced 3- acetic acid (Duff reaction)16 gave 2c in 59% yield. Finally, 2a
hydroxyflavone derivative 1c in 91% yield. Finally, 1a was was obtained by reaction of 2c with boron tribromide. Relative
obtained by reaction of 1c with boron tribromide in CH2Cl2 compound 2b was prepared from 7-hydroxyflavone (14) in the
(yield 87%). The applied synthetic route allowed for obtaining Duff reaction conditions in 67% yield. 14 was prepared starting
not only the target compound 1a, but also the relative ones 1b
,
from 7, using methyl protection for hydroxyl group via stages
1c and 1d, which were used as models in the further described in [17]. 2d was obtained from 2b by methylation with
investigations.
For synthesis of 2a (Scheme 2), 3-hydroxy-7-benzyloxyfla-
vone (10) was chosen as a precursor, synthesized from
commercially available 1-(2,4-dihydroxyphenyl)ethanone ( ),
according to the procedure described earlier:15
was treated
with benzyl chloride, leading to 8a, then converted to chalcone
derivative 9a with benzaldehyde in basic NMP, which was then
oxidatively cyclized in basic MeOH in the presence of H2O2
(Algar-Flynn-Oyamada reaction). 10 was subsequently
dimethylsulfate in CH3CN.
Steady state electronic absorption spectroscopy
7
The compounds investigated exhibit absorption in
methylcyclohexane in the 250 – 400 nm region (Table 1, Fig.
1a and b), which supposedly corresponds to
and 3), usually the most stable in the ground state of flavones.
Spectral properties of anionic forms of the compounds were
7
N form (Chart 2
H3C
HO
O
HO
OH
HO
OH
HO
OH
OH
(CH3CO)2O
RCOOH
O
O
+
O
O
BF3*Et2O,
60 'C, 2 h
CH3COOH,
ZnCl2, 80 'C
1 h
R
CH3
R
R
(51%)
(0%)
5a
5b
6a (8%)
6b (6%)
4a (95%)
4b (97%)
3
1a-e
, 4a, 5a, 6a: R = (CH2)2Ph
20% H2O in NMP,
KOH, RT, 15 h (48%)
1) Oxone,
Acetone, K2CO3,
tert
4b, 5b
,
6b: R =
-butyl
Ph
OH
PhCHO
Ph
CHCl3, H2O, 0 - 5
'C, 50 h
HO
O
H3CO
O
Ph
OH
HO
O
O
H3CO
O
O
Ph
HO
OH
I2
BBr3
(CH3)2SO4
O
O
R
O
O
R
O
O
DMF,
DMSO, 90' C,
4 h (53%)
2) TFA, CHCl3, RT,
0.5 h (89%)
CH2Cl2,
K2CO3, RT,
10 h (92%)
RT, 2h,
(87%)
R
O
O
R
R
1d
1a
1b
1e
1c
Ph
Scheme 1 Synthesis of 1a-d
BnCl or
(CH3)2SO4
BnO
O
Ph
RO
OH Ph
RO
OH
HO
OH
CH3
H2O2
MeOH,
NaOH, RT,
2 h (57%)
PhCHO
CH3
MeOH, KOH,
RT, 10 h
Acetone,
K2CO3, KI,
reflux 6 h
OH
O
O
O
O
9a
9b
8a (87 %)
(81%)
(84 %)
(82%)
7
10
8b
8a 9a
,
: R = CH2Ph;
DMSO, 100' C,
4 h (82%)
CH3CN, K2CO3,
RT, 10 h (87%)
I2
(CH3)2SO4
8b 9b
,
: R = CH3
O
H
BnO
O
Ph
H3CO
O
Ph
HO
O
Ph
HO
O
O
Ph
BBr3
(CH2)6N4
OCH3
CH2Cl2, RT,
2 h (93 %)
CH3CO2H, 80 'C,
6 h (67 %)
O
O
O
11
13
14
2b
CH3CN, K2CO3,
RT, 10 h (81%)
AcOH, RT,
4 h ( 78%)
(CH3)2SO4
HBr
O
H
O
H
O
H
HO
O
Ph
HO
O
Ph
HO
O
Ph
H3CO
O
O
Ph
(CH2)6N4
BBr3
OCH3
CH3CO2H, 80 'C,
6 h (59 %)
CH2Cl2, RT,
2 h (84 %)
OH
OCH3
O
O
O
12
2d
2a
2c
Scheme 2 Synthesis of 2a-d
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Table 1 Spectral parameters of the compounds investigated in methylcyclohexane at 298 and 77 Ka
Journal Name
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DOI: 10.1039/C5RA13912K
Compd cDBU (M) T (K) λabs (nm) λex (nm) λfl (nm) νSt (cm-1) Compd cDBU (M) T (K) λabs (nm) λex (nm) λfl (nm) νSt (cm-1)
1a
298
77
352
348
378
562
563
10940
8690
2a
298
77
339
337
335
338
-
551
415
535
-
11530
5750
10890
-
1×10-4
1×10-3
298
77
430
392
-
-
-
1×10-4
1×10-3
298
77
375c
402
446
-
543
-
4000
-
-
-
-
298
77
298
77
402
404
338
337
390
392
332c
332c
565
541
505
484
518
485
513
481
7180
6270
9780
9010
6320
4900
10630
9330
446
325
325
405
404
336
337
338
415
-
543
546
530
526
489
542
424
540
502
-
4000
12450
11900
5680
4300
11320
6090
11070
4180
-
1b
298
77
324
412
334
2b
298
77
336
392
331c
1×10-4
298
77
1×10-4
298
77
1c
298
77
2c
298
77
1×10-4
298
77
413
295
1×10-4
298
77
381
306
381
382
-
505
475
-
6480
5130
-
1d
298
-
-
-
2d
298
77
496b
77
538b
acDBU – molar concentration of DBU; λabs – position of the maximum of the long-wavelength band in absorption spectra; λex – position of the maximum of
the long-wavelength band in fluorescence excitation spectra; λfl – position of the maximum in the fluorescence emission spectra (for bands with
–1
vibrational modes λfl represents center-weighted position of a band); νSt – Stokes shifts, calculated as a difference between (λex
)
and (λfl)–1
.
bPhosphorescence maxima. c Maxima of the long-wavelength bands obtained by separation.
investigated in DBU solutions. Positions of the long-wave- exhibits complex absorption spectra (Fig. 1b), its long-
length bands of neutral and charged species depend on the wavelength band appears in the 325–350 nm range, similarly to
nature and position of substituents.
2b. Methoxy group at position 3 thus has minimal effect on the
The long-wavelength absorption bands of 1d and 2d are S0-S1 transition energy, which is most probably due to high
positioned at 295 and 306 nm, respectively. The bands of 1b value of the torsion angle between chromone moiety and
and 2b, which contain IHB, are shifted batochromically to 324 methoxy group, as was observed in the case of 3-
and 336 nm, respectively (Table 1, Fig. 1a and b). Compared to methoxyflavone derivatives.20
7-hydroxyflavone, whose absorption band at the same
1a (352 nm) and 2a (339 nm) exhibit the most
conditions is positioned at 291 nm,18 1b and 2b exhibit batochromically shifted absorption among the compounds
batochromic shift, caused by polarization of the hydroxyl group investigated (Table 1, Fig. 1a and b). 3,7-Dihydroxyflavone in
at position 7 due to IHB. Compared to 1b, the long-wavelength the same conditions exhibits absorption at 337 nm (unpublished
absorption band of 1c is batochromically shifted to 334 nm data), close to that of 2a. Thus the carbonyl substituent in
(Table 1, Fig. 1a), which is caused by influence of the hydroxyl position
6 affects the S0-S1 transition energy of 3,7-
group in position 3. Similar spectral effect was observed dihydroxyflavone much more considerably than in position 8.
previously in the case of 3-hydroxyflavone derivatives.19 2c
4 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5RA13912K
(a)
(b)
(d)
(c)
Fig. 1 Steady-state absorption spectra at 298 K: 1a-d (a) and 2a-d (b) in methylcyclohexane; 1a-c (c) and 2a-c (d M
) in 10-4
DBU solution in methylcyclohexane
The long-wavelength absorption bands of the anionic Deprotonation of the second hydroxyl group leads to a 2680
species of the hydroxyflavones investigated, which appear in cm–1 hypsochromic shift. On the other hand, monoanionic
DBU solutions, are considerably shifted batochromically
relative to the neutral ones. Monoanionic species A7 (Chart 4),
formed by deprotonation of the hydroxyl group at position 7 of
1b and 2b, absorb light at 412 and 392 nm, respectively.
Compared to anion of 7-hydroxyflavone18 this corresponds to
the batochromic shift values of 3240 and 2000 cm–1,
respectively, due to electron-withdrawing effect of the carbonyl
substituents. More than 1.5 lower value of the shift in the case
of 2b compared to 1b indicates less effective stabilization of A7
by the carbonyl substituent in position 8 compared to position
6. Absorption maximum of the similar species of 2c is shifted
hypsochromically to 381 nm relative to 2b, which indicates that
due to electron-releasing effect methoxy group at position 3
increases the S0-S1 transition energy in A7. The long-
,
,
1a b
1a c
A3
A7
O
R
O
R
R
O
O
O
O
R
O
O
2a
,b,c
2a
A3
A7
O
H
O
H
H
O
O
O
O
wavelength absorption band of the monoanionic species A3
,
formed by deprotonation of the hydroxyl group at position 3 of
1c, is centered at 413 nm (Chart 4).
A monoanionic form of 1a, which appears in 10-4 M DBU
solution, absorbs light at 430 nm (Table 1, Fig. 1c).
R
O
O
O
Chart 4 Possible monoanionic forms of 1a-c and 2a-c
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species of 2a absorb light at 375 nm, while further broad emission bands centered at 546, and 505, 513 nm,
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deprotonation leads to a 1790 cm–1 batochromic shift (Table 1, respectively (Table 1, Fig. 2c,d,f) with fluorescence quantum
DOI: 10.1039/C5RA13912K
Fig. 1d). The described differences in the spectral behavior of yields less than 1%. In contrast to 1b, vibronic structure of the
anions of 1a and 2a may indicate different order of 2b and 2c bands is visible. When the 1b and 2b
,c solutions are
deprotonation of hydroxyl groups and formation of different cooled to 77 K, their fluorescence intensity increases, maxima
monoanionic forms (Chart 4).
shift to 530 and 484, 481 nm, respectively, and bands become
narrower (Table 1, Fig. 2c,d,f).
Steady state fluorescence emission and excitation
spectroscopy
Fluorescence of anionic forms A7* appear in DBU solutions
of 1b and 2b,c with maxima near 526 and 518, 505 nm,
respectively. At 77 K their intensity increases and maxima shift
to 489 and 485, 475 nm, respectively. In contrast to neutral
solutions, vibronic structure is not observed in the case of the
Since 1a and 2a have two different PT sites which can partici-
pate in ESIPT simultaneously, the experimental investigations
of their electronically excited state behavior are complicated. In
order to understand the features of each PT site, we investigated
the related compounds. 1d and 2d enabled investigation of the
N* forms properties in the absence of IHB and ESIPT. 1b and
2b
,
c
fluorescence in DBU solutions.
As can be concluded from the 3D steady-state fluorescence
spectra and similarity of the long-wavelength bands in
absorption and fluorescence excitation spectra at 298 K and 77
K (Table 1, Fig. S2c,d,f and S3c,d,f), the described emission of
2b,c provided information concerning behavior of hydroxyl
group in position 7 with ortho-carbonyl substituents as well as
spectral features of T6* and T8* forms, respectively. 1c enabled
investigation of ESIPT involving hydroxyl and carbonyl group
in positions 3 and 4, respectively. Fluorescent behavior of 1b,c
and 2b,c in DBU solutions provided information concerning
features of monoanionic forms A7* and A3* as well as
1b and 2b,c in neutral solutions originates from N species in the
ground state. Generally, shape of the emission bands as well as
spectral changes under cooling of neutral solutions differ from
those of DBU ones, which evidences that fluorescence of
anionic forms is absent at such conditions. Most probably, the
T6* and T8* species (Chart 2, 3), respectively, produced via
ESIPT in N* after excitation (Scheme 3), are responsible for
fluorescence of these compounds in methylcyclohexane.
deprotonation order of hydroxyl groups in 1a and 2a
.
The
conclusions on the excited state behavior of 1a and 2a were
thus made based on a comparison of their spectral features with
the related compounds.
Fluorescent properties of 1b,d and 2b,c,d. Fluorescence of 1d
and 2d is observed at neither 298 K nor 77 K. Phosphorescence
of the compounds is registered at 77 K (Table 1, Fig. S1),
which indicates efficient intersystem crossing in their excited
state. Based on previous investigations of 7-hydroxyflavone18
the phenomenon can be due to closeness of the lowest excited
states of π-π* and n-π* nature.
Neither 1b nor 2b
was discussed above, the long-wavelength absorption bands of
1b and 2b are considerably red-shifted compared to 1d and
,c exhibit phosphorescence at 77 K. As
,
c
2d due to IHB. This can emphasize decrease of the π-π* state
energy, which is more sensitive to changes in electronic
structure of chromofores compared to the n-π* one. Increase of
the gap between π-π* and n-π* states prevents intersystem
crossing and appearance of phosphorescence and should
moreover enhance fluorescence of the N* species, as in the case
of 4'-methoxyflavone.21 Absence of the N* fluorescence neither
At 298
K all the hydroxyflavones investigated are
characterized by a single-band fluorescence spectra with the
abnormally large Stokes shifts (Table 1, Fig. 2 and S2)
at 298 K nor 77 K evidences very fast ESIPT in 1b and 2b,c.
Fluorescent properties of 1c. Compound 1c exhibits the most
intensive fluorescence among the investigated series. At room
temperature, its single emission band is observed at 542 nm
(Table 1, Fig. 2e and S2e), fluorescence excitation spectrum is
close to the absorption one (Table 1, Fig. S3e). Under cooling to
77 K the band’s vibronic structure becomes apparent, while its
intensity decreases sharply (Table 1, Fig. 2e). These changes
are accompanied by appearance of a fluorescence emission
band at 424 nm. The excitation spectra, recordered at these
bands maxima are close to absorption spectra at 298 K.
indicating occurrence of efficient ESIPT. 1b and 2b,c exhibit
1b
N*
T6*
O
R
O
O
R
O
ESIPT
H
O
H
O
O
O
Very low intensive fluorescence of anion A3* appear at 502
nm in DBU solutions of 1c at 298 K (Table 1, Fig. 2e). At 77 K
2b (R = H) and 2c (R = OCH3)
T8*
N*
N*
T4*
O
H
O
H
H
H
O
H3CO
O
+
H3CO
O
ESIPT
O
O
O
O
O
ESIPT
O
R
O
R
O-
O
H
R
R
O
O
H
Scheme 4 ESIPT in 1c
Scheme 3 ESIPT in 1b and 2b,c
6 | J. Name., 2012, 00, 1-3
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ARTICLE
no fluorescence of 1c in DBU solutions was detected.
Most probably, the T4* species, produced via ESIPT in N* N*
after excitation (Scheme 4), are responsible for the long- relative compounds.22,23 The substituents’ effect can cause
indicates existence of energetic barrier for the ESIPT reaction
View Article Online
DOI: 10.1039/C5RA13912K
T *, previously observed for 3-hydroxyflavone and

4
wavelength fluorescence emission in neutral solutions of 1c stabilization of the N* form in the case of 1c compared to 3-
both at 298 and 77 K. The observed blue-shifted emission at 77 hydroxyflavone, what can explain increase of the N*
K most probably originates from N* produced by direct reaction barrier. The A3* fluorescence, which was assumed to
excitation. Appearance of the N* fluorescence under cooling appear in solutions of 3-hydroxyflavone in hydrocarbons at 77
T *
4

(a)
(b)
(c)
(d)
(e)
(f)
Fig. 2 Steady-state fluorescence spectra of 1a (a), 2a (b), 1b (c), 2b (d), 1c (e) and 2c (f) in methylcyclohexane at 298 K and 77 K.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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Journal Name
N*
T4*
N*
View Article Online
O
H
O
H
H
O
DOI: 10.1039/C5RA13912K
O
R
O
H
O
H
O
O
+
O
ESIPT
O
H
O-
O
O
H
ESIDPT
DT*
ESIPT
O
H
O
Scheme 6 ESIPT in 2a
T6*
O
H
O
O
R
O
+
H
ESIPT
O
O
O-
mentioned above conclusion on origination of 1a fluorescence
from species in the ground state. Hypothetically, the
O
H
N
R
O
O
H
mentioned above batochromic shift of the excitation spectra of
1a under cooling can be explained by strengthening of both of
its hydrogen bonds, which can decrease relative energy of the
excited Franck-Condon state and thus decrease the S0-S1
Scheme 5 ESIPT and ESIDPT in 1a
K due to the presence of impurities of protic substances,24 can
not have noticeable impact in fluorescence of 1c due to very
low emission intensity.
Fluorescent properties of 1a. The compound fluoresces at 562
nm with a very low quantum yield – 0.11% (Table 2, Fig. 2a).
Based on 3D fluorescence spectra (Fig. S2a) and comparison of
the absorption and fluorescence excitation spectra (Fig. S3a),
transition energy of
N species.
Based on the data presented, one can notice the basic
difference in the spectral behavior of 1a and 1c such as much
lower fluorescence quantum yield (Table 2), absence of the N*
emission at 77 K in the case of 1a and different behavior of
monoanionic forms (Fig 2a,e). This can indicate that 1a does
not exist in T4* form. Compared to 1b, 1a has a tenfold lower
the emission originates from the
N species in the ground state.
fluorescence quantum yield, however, behavior of both
compounds at 77 K is generally similar in both neutral and
DBU methylcyclohexane solutions (Fig 2a,c). These
observations can indicate that hydroxyl group at position 7 has
prior influence on the 1a properties due to its higher acidity
compared to hydroxyl group at position 3. The fluorescence of
When the 1a solution is cooled to 77 K, its fluorescence band
becomes broader with no significant change of the maxima
position (Table 1, Fig. 2a), while its intensity rises 1.5 times.
The fluorescence excitation maxima shifts to 378 nm,
demonstrating a 2280 cm–1 batochromic shift compared to 298
K (Table 1, Fig. S3a). No additional emission in the 400–475
nm region was observed.
1a in the neutral solutions can be, thus, generated by either T6*
DT* or both, but not the T4* form (Scheme 5).
,
1a demonstrates intensive fluorescence at 543 nm in DBU
solutions under cooling to 77 K (Fig 2a). The excitation
spectrum has maxima at 446 nm, which resembles absorption
spectrum of monoanionic form, observed at 298 K (Table 1,
Fig. S3a). The fluorescence behavior of the 1a monoanionic
species at 77 K is similar to 1b, but not 1c, which can indicate
that A7* species are responsible for the 1a fluorescence at 77 K
in the presence of DBU. The excitation spectrum of 1a in
neutral solution at 77 K matches absorption/excitation of
neither its A7* species nor A3* ones of 1c. Dianionic species,
which are present in the 10–3 M DBU solution, are either non-
fluorescent or have low fluorescence intensity relative to A7*
species at 298 and 77 K. Formation of the doubly charged
dianion in neutral methylcyclohexane should be extremely
unfavorable. Based on these data we conclude that
deprotonation is not the reason of the batochromic shift in the
excitation spectrum of 1a at 77 K.
Similarly to 3,7-dihydroxyflavone,19,25 in the ground state of
1a hydroxyl groups and carbonyl group at position 4 should be
of low acidity and basicity, respectively, thus formation of
tautomeric species T3 is unfavorable. As follows from acid-
base properties of acetophenones,26 basicity of the carbonyl
group at position 6 of 1a should be also very low, thus
formation of T6 as well as DT in the ground state is unfavorable
too. Taking into account that concentrations of the investigated
solutions of 1a were below 5×10–5 M, formation of aggregates
is of low probability. These considerations support the
Fluorescent properties of 2a. At 298 K 2a fluoresces at 551
nm with 1.6% quantum yield (Table 2). Based on 3D
fluorescence spectra (Fig. S2b) and comparison of the
absorption and fluorescence excitation spectra (Fig. S3b), the
emission originates from the
N species in the ground state. At
77 K the fluorescence band undergoes a hypsochromic shift to
535 nm, accompanied by rise of a band at 415 nm (Table 1, Fig.
2b), most probably emitted by N* species. The excitation
spectra measured at these bands are similar to that at 298 K
(Table 1, Fig S3b).
Monoanionic forms of 2a, which according to the described
above absorption spectra appear in the 10–4 M DBU solution,
are fluorescent neither at 298 K nor 77 K. This phenomenon, in
contrast to intensive fluorescence of the A7* forms of 2b and
2c, can indicate that hydroxyl group of position 3 of 2a is
deprotonated first, producing A3* form. Different from 1a
deprotonation order of hydroxyl groups in 2a can indicate that
acidity of the hydroxyl group at position 7 is for less extent
affected by the carbonyl substituent at position 8 than at
position 6. This conclusion correlates with the discussed above
influence of the substituents on the A7 spectral properties of 1b
and 2b
.
Broad fluorescence band appear in spectrum of 2a in the 10–
M DBU solution at 565 nm at 298 K and shifts to 541 nm at
3
77 K. According to the excitation spectrum the emission
originates from dianionic form.
8 | J. Name., 2012, 00, 1-3
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kf) and non-radiative deactivation (kd) for their T6* and T8*
forms (Table 2) using the major componeDnOtsI’: 1l0i.f1e0t3im9/Ces5.RAA13c9c1o2rK-
ding to the obtained values, non-radiative processes with a
summary rate of 2.1 – 2.7×109 s–1 are the main deactivation
pathway for both types of forms. T6* form of 1b is
(
View Article Online
(a)
characterized by higher kf value compared to T8* of 2b,c,
which conditions higher quantum fluorescence yield of the
former (Table 2).
In hexane at 550 nm, 1c exhibits a monoexponential decay
of fluorescence with 2.19 ns lifetime (Table 2, Fig. 3a) and no
rise component in the investigated time domain, corresponding
to deactivation of the T4* species. The kf value of T4*
,
calculated based on the assumption that the main deactivation
pathway of the N* form is ESIPT, is the highest among the
compounds investigated, what together with low kd value
conditions the highest quantum yield of fluorescence (Table 2).
Fluorescence decay of 1a at 560 nm differs from the other
compounds investigated: it contains two decay components
with lifetimes of ~ 25 ps and 5.3 ns (Table 2, Fig. 3a). Due to a
drastic difference in the lifetimes, the long-living species have
the overwhelming impact in 1a fluorescence quantum yield
(94%), which enables estimation of their kf and kd values (Table
2). The obtained kf value is extremely low and unique among
the ones of 1b and 1c, which implies that the long-living
species are neither T4* nor T6*. As formation of T4* species in
the excited state of 1a was suggested to be unlikely according
to the steady-state fluorescence investigations, we consider that
the long decay corresponds to DT* and the fast decay
corresponds to T6*. The observed broadening of the 1a
fluorescence emission spectra at 77 K (Fig. 2a) can thus be
explained by co-existence of T6* and DT* forms. Most
probably, that the T6* species are produced by ultrafast ESIPT
with no or very small energetic barrier, similarly to 1b, since no
N* emission is observed even at 77 K. Considering similar
structure and close spectral properties of T6* forms of 1a and
1b, their rates of radiative and non-radiative deactivation
should be of the same ranges. Much faster decay of the T6*
species of 1a can indicate existence of an additional efficient
dark deactivation pathway, which we suppose is ESIPT
resulting in DT*. If one suggests that the main excited state
deactivation route of T6* is ESIPT, the rate constant of the
second stage should be near 4×1010 s–1. The ESIDPT tautomer
is, thus, most probably formed via two single ESIPT reactions:
(b)
Fig. 3 Fluorescence decays of 1a-c
at 298 K recorded at the fluorescence maxima
(a) and 2a-c (b) in hexane
Compared to 2b and 2c, in neutral solutions 2a has a few
times higher fluorescence quantum yield and exhibits N*
fluorescence emission at 77 K, which indicates absence of T8*
form in the excited state of 2a. Even though a model compound
for T4* form of 2a was not available in this study, spectral
properties including positions of the fluorescence emission and
excitation maxima together with behavior at 77 K of 2a and 1c
are very similar. The fluorescence of 2a can be, thus, generated
by either T4* (Scheme 6), DT* forms or both, but not the T8*
as hydroxyl group at position 3 has prior influence on the 2a
,
properties.
N*
T *
DT* (Scheme 5). The second stage should have


6
Time-Resolved Fluorescence Spectroscopy
an energetic barrier, as it is detectable in the sub-nanosecond
time domain. Under cooling the T * DT* transformation
should be suppressed, thus T6* should have higher impact to
fluorescence of 1a
Simultaneous ESIDPT (N*
be accompanied by formation of intermediate species (Scheme
5), which contradicts the observed experimental evidences of
T6* formation at both 298 and 77 K. Simultaneous ESIDPT can
hardly be a barrierless process, because the observed ESIPT
involving hydroxyl group at position 3 proceed with an
energetic barrier. Therefore, at low temperatures, N*
fluorescence of 1a would be registered, which is not the case.

6
Fluorescence decays of 1b and 2b,c in hexane are
biexponential without rise components in the investigated time
domain. The major components have lifetimes of less than 0.5
ns (Table 2, Fig. 3a and b), while the minor ones have very low
intensity and lifetimes near 1.5 – 3.0 ns. Precise determination
of the latter components’ lifetimes is complicated. The major
components most probably characterize decay of T6* and T8*
species, respectively. Appearance of the minor components
may indicate co-existence of T6* and T8* species in two
isomers, different as regards the PT sites conformations. Based
on the assumption that the main deactivation pathway of the N*
.
DT*), if it occurs, should not

forms of 1b and 2b,c is ESIPT, we calculated rates of radiative
This journal is © The Royal Society of Chemistry 20xx
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Journal Name
We thus conclude, that possibility of DT* formation via Elemental analysis was performed on an Elementar Vario El
View Article Online
DOI: 10.1039/C5RA13912K
concerted mechanism, is less likely.
2a in hexane exhibits monoexponential decay of
Cube CHNS analyzer.
a
Absorption and fluorescence, phosphorescence spectra were
fluorescence with lifetime of 1.98 ns with no rise component recorded on
a
Perkin-Elmer Lambda UV/VIS 40
(Fig. 3b). Compared to 2b , deactivation rates of 2a differ a spectrophotometer and Varian Cary Eclipse Fluorescence
,
c
few times (Table 2), which supports the suggestion of absence Spectrophotometer, respectively. Fluorescence and phosphores-
of T8* form in its excited state, based on the steady-state cence emission and excitation spectra were corrected on the
fluorescence investigation. Generally, spectral and kinetic instrumental sensitivity. Fluorescence decay curves were
properties of 2a resemble the T4* form of 1c, a difference measured on
between kf values may be caused by substitution effect. The spectrometer
a
FluoTime 300 fluorescence lifetime
equipped with compact emission
a
DT* species if they appear, should be formed via ESIPT in monochromator, a TimeHarp 300E TCSPC device (minimal
T4*. Taking into account the observed evidence of much lower time resolution 4 ps), a PLS 340 LED-head for sub-nanosecond
energetic barrier of ESIPT taking place in PT site including pulse driven by a PDL 820 device and a MCP-PMT
hydroxyl group in position 7, transformation T * DT* photomultiplier (type R3809U-50) (PicoQuant GmbH,

4
should be too fast to be detected by the techniques used. Germany) controlled by EasyTau system software.
Formation of DT* via concerted ESIDPT seems to us unlikely Investigations of spectral properties were carried out in
due to differences of ESIPT barriers of two sites. The discussed methylcyclohexane solutions with concentrations 1–5×10–5 M.
above different deprotonation order in 1a and 2a can be the Investigations of anionic forms were carried out in the
main reason of different behavior of these compounds. The methylcyclohexane solutions of DBU (1,8-diazabicyclo[5.4.0]
following experimental and theoretical investigations may undec-7-ene). Measurements at 77K were held in FL-1013
probably allow for verification of these suggestions.
liquid nitrogen dewar assembly using quartz 10×10 mm
cuvettes with stopper. Time-resolved fluorescence
a
investigations were held in hexane. The solvents were treated
with LiAlH4 and distilled prior use in order to eliminate trace
amounts of water. The compounds were additionally dried
before measurements at 373 K under reduced pressure.
Table 2 Fluorescence quantum yields and deactivation kinetic
parameters of the excited state of the compounds investigated in
hexanea
Fluorescence quantum yields were determined relative to
quinine sulphate solution in 1 M H2SO4.27 Rates of the excited
state deactivation were calculated according to equations:28
Compd
1a
Form
τ (ns) / f (%)
kf ×10–7 (s–1
)
kd ×10–9 (s–1
)
φ (%)
0.11
DT*
5.89 ± 0.02 / 94
0.02
0.17
,
   k f
T6*
T6*
T4*
T4*
0.025 ± 0.005 / 6
0.473 ± 0.007
2.19 ± 0.03
1.0
1
1b
1c
2a
2.11
5.30
0.81
2.09
0.40
0.50
,
 
k f  kd
11.6
1.6
where
φ – quantum yield of fluorescence, τ – lifetime of the
electronically excited form; kf – rate constant of radiative
deactivation; kd – rate constant of non-radiative deactivation.
1.98 ± 0.04
2b
2c
T8*
T8*
0.377 ± 0.008
0.415 ± 0.004
0.58
0.65
2.65
2.40
0.22
0.27
Synthesis. 7-Hydroxy-2-phenyl-4H-chromen-4-one (14) was
prepared according to [17], its chemical physical properties
correspond to the ones described in literature. Synthetic
procedure and results of analysis for 7-(benzyloxy)-3-hydroxy-
2-phenyl-4H
-chromen-4-one (10) were described previously.15
aτ – lifetime of the electronically excited form; f – fractional contribution,
calculated as  Ai   A  , where Ai are the pre-exponential factors;
f
/
i

i
i
i
kf – rate constant of radiative deactivation; kd – rate constant of non-radiative
deactivation; φ – quantum yield of fluorescence.
General procedure for 1-(2,4-dihydroxyphenyl)-3-phenyl-
propan-1-one (4a) and 1-(2,4-dihydroxyphenyl)-2,2-dime-
thylpropan-1-one (4b). Resorcinol (1.10 g, 10 mmol) and 3-
phenylpropanoic acid (1.50 g, 10 mmol) or pyvaloyl chloride
(1.48 ml, 12 mmol) in boron trifluoride etherate (5.5 ml) were
stirred at 60°C for 2 h, then diluted with sodium acetate
aqueous solution, stirred for 0.5 h at RT and extracted with
ethyl acetate. Crude product after evaporation was purified with
use of flash chromatography (SiO2, 5 % i-PrOH in CHCl3).
4a: White moist solid (2.19 g, 95%), 1H NMR (500 MHz,
EXPERIMENTAL
Reagents of relevant grade for syntheses and spectroscopic
investigations were purchased from Sigma-Aldrich. Identity of
the investigated compounds was confirmed with H1-NMR, C13-
NMR and MALDI TOF MS, their purity was controlled with
TLC and elemental analysis. NMR spectra were recorded on
1
1
500 MHz H (125 MHz 13C) or 200 MHz H spectrometers
with trimethylsilane as reference. Mass spectra were obtained
on MALDI-TOF MS Bruker Daltonics mass spectrometer.
Chromatography was performed on silica gel (230 – 400 mesh)
or Waters HPLC chromatograph, TLC was conducted on
Merck 60 F254 silicagel plates in appropriate eluents.
DMSO-d6
6.23 (d, 1H,
7.17 (m, 1H,
,
δ
): 2.91 (t, 2H,
= 2.2 Hz), 6.34 (dd, 1H,
= 4.3 Hz), 7.26 (d, 4H,
J
= 7.6 Hz), 3.23 (t, 2H,
J
J
= 7.6 Hz),
= 8.9 Hz),
J
J
J
= 2.2 Hz,
J
= 4.3 Hz), 7.80 (d, 1H,
J
m
= 8.9 Hz), 10.60 (broad s, 1H), 12.57 (s, 1H). Mass spectrum,
/z
: 243.3 [M+H]+, 265.1 [M+Na]+, 281.0 [M+K]+.
10 | J. Name., 2012, 00, 1-3
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4b: White moist solid (1.88 g, 97%), 1H NMR (200 MHz, CHCl3). White solid (195 mg, 53%), 1H NMR (500 MHz,
View Article Online
DOI: 10.1039/C5RA13912K
DMSO-d6
,
δ
): 1.30 (s, 9H), 6.25 (d, 1H,
J
J
= 2.4 Hz), 6.32 (dd, CDCl3,
= 8.9 Hz), 10.42 (s, 6.68 (s, 1H), 6.97 (s, 1H), 7.16 (t, 1H,
: 195.2 [M+H]+, 217.2
= 7.6 Hz), 7.25 (t, 2H, = 7.3 Hz), 7.44 – 7.49 (m, 3H), 7.83
(dd, 2H, = 8.0 Hz,
= 1.8 Hz), 8.64 (s, 1H), 12.62 (s, 1H). 13
NMR (125 MHz, CDCl3, ): 205.4, 177.3, 166.2, 163.6, 160.5,
(5a) and 1-(3-acetyl-2,4-dihydroxyphenyl)-3-phenylpropan- 140.2, 131.9, 131.3, 130.0, 129.1, 128.7, 128.4, 126.5, 126.3,
δ): 3.04 (t, 2H, J = 7.6 Hz), 3.42 (t, 2H, J = 7.6 Hz),
1H, = 2.4 Hz,
J
J
= 8.9 Hz), 7.80 (d, 1H,
J = 7.3 Hz), 7.20 (d, 2H,
1H), 12.57 (s, 1H). Mass spectrum,
[M+Na]+.
1-(5-Acetyl-2,4-dihydroxyphenyl)-3-phenylpropan-1-one
m
/
z
J
J
J
J
C
δ
1-one (6a). 1-(2,4-Dihydroxyphenyl)-3-phenylpropan-1-one 4a 118.2, 116.6, 107.2, 105.4, 40.2, 29.7. Mass spectrum,
m/z:
(1.97 mg, 8.1 mmol), zinc chloride (1.66 mg, 12.2 mmol), 371.2 [M+H]+, 393.2 [M+Na]+, 409.2 [M+K]+. Anal. Calcd. for
acetic anhydride (1.54 ml, 16.2 mmol) and glacial acetic acid C24H18O4, %: C 77.91; H 4.90. Found, %: C 77.83; H 4.95.
(10 ml) were stirred at 80°C for 1 h under N2, then poured on 7-Methoxy-2-phenyl-6-(3-phenylpropanoyl)-4
H-chromen-4-
ice and extracted with CHCl3, washed with 0.1 M HCl and then one. 1d): 7-hydroxy-2-phenyl-6-(3-phenylpropanoyl)-4H-
(
water, and dried over MgSO4. The obtained mixture of isomers chromen-4-one 1b (185 mg, 0.5 mmol), dimethylsulfate (69
5a and 6a was separated using column chromatography (SiO2, mg, 0.55 mmol), K2CO3 (76 mg, 0.55 mmol) and DMF (1 ml)
CHCl3).
5a: White moist solid (1.17 g, 51%), 1H NMR (200 MHz, collected precipitate was purified with use of flash
DMSO-d6 ): 2.61 (s, 3H), 2.92 (t, 2H, = 7.6 Hz), 3.43 (t, 2H, chromatography (SiO2, 2% i-PrOH in CHCl3). White solid (176
= 7.6 Hz), 6.36 (s, 1H), 7.18 (m, 1H,
= 4.4 Hz), 8.39 (s, 1H), 12.70 (broad s, 2H). Mass spectrum, Hz), 3.22 (t, 2H,
: 285.2 [M+H]+, 307.1 [M+Na]+.
1H), 7.13 (t, 1H,
= 7.3 Hz), 7.44 – 7.50 (m, 3H), 7.83 (dd, 2H,
= 1.7 Hz), 8.42 (s, 1H). 13C NMR (125 MHz, CDCl3,
= 4.4 Hz), 199.8, 177.3, 163.3, 162.3, 159.4, 141.3, 131.7, 131.5, 129.1,
= 9.0 Hz), 12.89 (broad 128.8, 128.4, 127.5, 126.2, 117.4, 107.8, 99.8, 56.4, 45.1, 30.3.
were stirred at RT for 10 h, and then diluted with water. The
,
δ
J
J
1
J
J
m
= 4.4 Hz), 7.25 (d, 4H, mg, 92%), H NMR (500 MHz, CDCl3,
= 7.6 Hz), 3.92 (s, 3H), 6.71 (s, 1H), 6.94 (s,
= 7.3 Hz), 7.16 (d, 2H, = 7.6 Hz), 7.22 (t,
= 7.9 Hz,
):
δ): 2.97 (t, 2H, J = 7.6
J
J
/
z
J
6a: White moist solid (0.18 g, 8%), 1H NMR (200 MHz, 2H,
J
J
DMSO-d6
= 7.5 Hz), 6.47 (d, 1H,
7.27 (d, 4H, = 4.4 Hz), 8.04 (d, 1H,
,
δ
): 2.58 (s, 3H), 2.92 (t, 2H,
J
= 7.5 Hz), 3.30 (t, 2H,
J
δ
J
J
= 9.0 Hz), 7.18 (m, 1H, J
J
J
s, 1H), 14.24 (broad s, 1H). Mass spectrum,
306.8 [M+Na]+, 322.8 [M+K]+.
m/z m/z
: 284.9 [M+H]+, Mass spectrum, : 385.0 [M+H]+, 407.0 [M+Na]+, 422.9
[M+K]+. Anal. Calcd. for C25H20O4, %: C 78.11; H 5.24.
Found, %: C 78.02; H 5.29.
1-(3-Acetyl-2,4-dihydroxyphenyl)-2,2-dimethylpropan-1-
one
phenyl)-2,2-dimethylpropan-1-one (4b) according to the proce- chromen-4-one.
(
6b). The compound was prepared from 1-(2,4-dihydroxy- 3-Hydroxy-7-methoxy-2-phenyl-6-(3-phenylpropanoyl)-4
1c): 7-Methoxy-2-phenyl-6-(3-phenylpropa-
-chromen-4-one (1d) (0.096 g, 0.25 mmol), acetone
): 1.27 (s, 9H), 2.58 (s, 3H), 6.46 (d, (0.25 ml), CHCl3 (2.5 ml) and water (2.5 ml) were cooled to 0 –
= 9.0 Hz), 7.81 (d, 1H,
= 9.0 Hz), 12.16 (broad s, 1H), 5°С. Oxone© (0.3 g) and K2CO3 (0.2 g, 3.6 mmol) were added
14.12 (broad s, 1H). Mass spectrum, in four portions to the vigorously stirred suspension within 50
: 237.2 [M+H]+.
(2 )-1-[2,4-dihydroxy-5-(3-phenylpropanoyl)phenyl]-3-phe- h, the mixture temperature was maintained at ≤ 5°С. The
H-
(
1
dure described for 5a and 6b. White moist solid (6%). H NMR noyl)-4
(200 MHz, DMSO-d6
1H,
H
,
δ
J
J
m/z
E
nylprop-2-en-1-one. (1e): 1-(5-Acetyl-2,4-dihydroxyphenyl)- organic layer was separated, TFA (0.05 ml) was added, and the
3-phenylpropan-1-one 5a (0.284 mg, 1 mmol), benzaldehyde mixture was stirred for 0.5 h at RT. The residue after
(117 mg, 1.1 mmol), KOH (448 mg, 8 mmol) and 80% aqueous evaporation was reprecipitated from DMSO solution by
NMP (2 ml) were stirred for 15 h at RT, neutralized by 0.1 M dropwise addition of MeOH. Pale yellow powder (89 mg,
1
HCl, extracted with CHCl3 and washed 3 times with water. The 89%), H NMR (500 MHz, CDCl3,
δ
): 2.98 (t, 2H,
J = 7.6 Hz),
crude product was purified with use of flash chromatography 3.23 (t, 2H,
(SiO2, CHCl3). Yellow powder (180 mg, 48 %), H NMR (500 = 7.3 Hz), 7.17 (d, 2H,
J
= 7.6 Hz), 3.93 (s, 3H), 6.93 (s, 1H), 7.13 (t, 1H, J
1
J
= 7.5 Hz), 7.22 (t, 2H,
J
= 7.4 Hz),
MHz, CDCl3,
Hz), 6.51 (s, 1H), 7.25 (t, 1H,
Hz), 7.34 (t, 2H,
7.51 (m, 3H), 7.76 – 7.70 (m, 2H), 7.96 (d, 1H,
8.31 (s, 1H), 13.08 (s, 1H), 13.61 (s, 1H). 13C NMR (125 MHz, 44.1, 29.3. Mass spectrum,
CDCl3,
): 203.7, 191.9, 170.0, 169.0, 146.1, 140.5, 134.4, 439.1 [M+K]+. Anal. Calcd. for C25H20O5, %: C 74.98; H 5.03.
134.3, 131.2, 129.1, 128.8, 128.7, 128.5, 126.6, 119.1, 113.8, Found, %: C 75.05; H 5.10.
113.2, 105.3, 39.7, 30.7. Mass spectrum,
: 373.0 [M+H]+, 3,7-Dihydroxy-2-phenyl-6-(3-phenylpropanoyl)-4
394.9 [M+Na]+, 410.9 [M+K]+. Anal. Calcd. for C24H20O4, %: en-4-one.
1a): To a solution of 3-hydroxy-7-methoxy-2-
phenyl-6-(3-phenylpropanoyl)-4 -chromen-4-one 1c (40 mg,
-chromen-4- 0.1 mmol) in CH2Cl2 (1 ml) under N2, BBr3 (125 mg, 0.5
δ
): 3.15 (t, 2H,
J
J
= 7.7 Hz), 3.36 (t, 2H,
= 7.0 Hz), 7.30 (d, 2H,
J
J
= 7.7 7.40 (t, 1H,
J
= 7.4 Hz), 7.46 (t, 2H,
J
= 7.6 Hz), 8.15 (d, 2H,
J
= 7.6 = 7.9 Hz), 8.45 (s, 1H). 13C NMR (125 MHz, CDCl3,
δ): 198.7,
J
= 7.0 Hz), 7.45 (d, 1H,
J
= 15.5 Hz), 7.47 – 171.7, 161.2, 157.5, 143.5, 140.2, 137.3, 129.8, 129.2, 127.8,
= 15.5 Hz), 127.6, 127.5, 127.4, 126.5, 126.4, 125.1, 113.2, 98.6, 55.4,
: 401.2 [M+H]+, 423.2 [M+Na]+,
J
m/z
δ
m
/
z
H-chrom-
(
C 77.40; H 5.41. Found, %: C 77.03; H 5.49.
H
7-Hydroxy-2-phenyl-6-(3-phenylpropanoyl)-4
H
one.
(1b): (2E)-1-[2,4-dihydroxy-5-(3-phenylpropanoyl)phe- mmol) was added and the mixture was stirred for 2 h. A few
nyl]-3-phenylprop-2-en-1-one 1e (372 mg, 1 mmol), iodine (13 drops of methanol were added, the solution was diluted with
mg, 0.1 mmol) and DMSO (2 ml) were heated at 90°C for 4 h, CH2Cl2 and washed with water. Residue after evaporation was
diluted with thiosulfate aqueous solution, extracted by CHCl3 purified using HPLC with gradient elution (C18, 50 – 80 %
1
and purified by flash chromatography (SiO2, 2% i-PrOH in acetonitrile in water). Pale yellow solid (34 mg, 87%), H NMR
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(500 MHz, CDCl3,
δ
): 3.05 (t, 2H,
J
= 7.6 Hz), 3.43 (t, 2H,
J
=
176.3, 167.0, 162.8, 157.1, 133.4, 132.5, 131.5, 129.8, 127.1,
View Article Online
+
DOI: 10.1039/C5RA13912K
7.6 Hz), 6.82 (broad s, 1H), 6.98 (s, 1H), 7.16 (t, 1H,
Hz), 7.20 (d, 2H, = 7.3 Hz), 7.26 (t, 2H,
1H, = 7.5 Hz), 7.47 (t, 2H, = 7.9 Hz), 8.14 (d, 2H,
Hz), 8.67 (s, 1H), 12.55 (s, 1H). 13C NMR (125 MHz, CDCl3, 2c: Pale yellow powder (59%), H NMR (500 MHz, CDCl3,
): 203.3, 169.1, 164.9, 158.2, 143.9, 140.1, 136.7, 129.4, 3.95 (s, 3H), 7.03 (d, 1H, = 9.0 Hz), 7.54 – 7.60 (m, 3Н), 8.02
129.0, 127.7, 127.4, 127.3, 127.2, 126.6, 126.4, 125.4, 112.5, – 8.07 (m, 2Н); 8.40 (d, 1H, = 9.0 Hz), 10.69 (s, 1Н), 12.49
104.1, 39.1, 28.7. Mass spectrum, ): 192.0, 173.5,
: 386.9 [M+H]+, 408.8 (s, 1Н). 13C NMR (100 MHz, DMSO-d6
[M+Na]+, 424.7 [M+K]+. Anal. Calcd. for C24H18O5, %: C 167.6, 157.0, 154.8, 142.0, 135.0, 131.0, 130.4, 128.8, 128.1,
J
= 7.3 116.8, 116.4, 111.0, 107.8. Mass spectrum,
m
/
z: 267.3 [M+H] .
J
J
= 7.3 Hz), 7.41 (t, Anal. Calcd. for C16H10O4, %: C 72.18; H 3.79. Found, %: C
= 7.9 72.28; H 3.77.
J
J
J
1
δ
):
δ
J
J
m
/
z
, δ
74.60; H 4.70. Found, %: C 74.65; H 4.76.
General procedure for 7-methoxy-4-oxo-2-phenyl-4H-
116.8, 116.2, 108.8, 60.3. Mass spectrum,
319.1 [M+Na]+, 335.0 [M+K]+. Anal. Calcd. for C17H12O5, %:
m/z
: 297.2 [M+H]+,
chromene-8-carbaldehyde (2d) and 7-(benzyloxy)-3- C 68.92; H 4.08. Found, %: C 68.79; H 4.15.
methoxy-2-phenyl-4 -chromen-4-one (11). 7-hydroxy-4- 3,7-Dihydroxy-4-oxo-2-phenyl-4 -chromene-8-carbaldehy-
oxo-2-phenyl-4 -chromene-8-carbaldehyde 2b or 7-(benzyl- de. (2a): the compound was prepared from 7-hydroxy-3-
oxy)-3-hydroxy-2-phenyl-4 -chromen-4-one 10 (1 mmol), methoxy-2-phenyl-4 -chromen-4-one 12 following the
H
H
H
H
H
K2CO3 (276 mg, 2.2 mmol), dimethylsulphate (252 mg, 2.1 procedure described for 1a. Crude product was purified by
mmol) and acetonitrile (10 ml) were stirred at RT for 10 hours, recrystallization from hot MeOH – CHCl3 solution. Pale yellow
1
then diluted with water. The precipitate was filtered off and powder (84%), H NMR (500 MHz, DMSO-d6
,
δ
): 7.09 (d, 1H,
= 6.9 Hz), 7.58 (t, 2H, = 7.5 Hz),
= 7.5 Hz), 9.77 (s, 1Н),
recrystallized from methanol.
2d: White solid (227 mg, 81%), H NMR (200 MHz, DMSO- 8.24 (d, 1H,
d6
3H), 8.20 – 8.32 (m, 3H), 10.54 (s, 1H). 13C NMR (100 MHz, DMSO-d6
DMSO-d6 ): 187.8, 176.5, 166.8, 163.2, 155.6, 133.3, 132.6, 131.7, 130.2, 129.0, 128.0, 115.9, 114.8, 110.5. Mass spectrum,
131.5, 129.8, 127.1, 117.9, 113.3, 111.5, 107.4, 57.9. Mass
: 283.2 [M+H]+. Anal. Calcd. for C16H10O5, %: C 68.09; H
spectrum,
: 281.3 [M+H]+. Anal. Calcd. for C17H12O4, %: C 3.57. Found, %: C 68.01; H 3.61.
J
= 8.9 Hz), 7.50 (t, 1H,
J
J
1
J
= 8.9 Hz), 8.37 (d, 2H, J
, δ
): 4.05 (s, 3H), 7.10 (s, 1H), 7.38 (d, 1H), 7.55 – 7.64 (m, 10.60 (s, 1Н), 12.23 (broad s, 1Н). 13C NMR (125 MHz,
, δ): 190.0, 172.3, 166.9, 155.6, 145.0, 139.8, 133.5,
,
δ
m/z
m/z
72.85; H 4.32. Found, %: C 72.77; H 4.35.
1
11: White solid (308 mg, 86%), H NMR (500 MHz, DMSO-
Conclusions
d6
1.9 Hz), 7.34 – 7.38 (m, 2H), 7.42 (t, 2H,
2H, = 7.6 Hz), 7.56 – 7.61 (m, 3H), 8.00 (d, 1H,
8.01 – 8.05 (m, 2H). Mass spectrum,
,
δ
): 3.82 (s, 3H), 5.27 (s, 2H), 7.14 (dd, 1H,
= 7.0 Hz), 7.49 (d,
= 8.8 Hz),
: 359.1 [M+H]+, 381.2
J = 8.8 Hz, J =
J
Two novel types of carbonyl derivatives of flavones with two
different proton-transfer sites, able to undergo different types
of ESIPT were synthesized. All the hydroxyflavones
investigated undergo ESIPT in their electronically excited
states. ESIPT involving 3-hydroxyl and 4-carbonyl groups was
found to have higher barrier compared to ESIPT involving 7-
hydroxyl and 6/8-carbonyl fragments. 3,7-Dihydroxy-2-phenyl-
J
J
m/z
[M+Na]+, 397.2 [M+K]+. Anal. Calcd. for C23H18O4, %: C
77.08; H 5.06. Found, %: C 76.95; H 5.12.
7-Hydroxy-3-methoxy-2-phenyl-4
H-chromen-4-one. (12): 7-
(Benzyloxy)-3-methoxy-2-phenyl-4
H
-chromen-4-one 11 (358
mg, 1 mmol) was dissolved in 33% HBr solution in acetic acid
(3 ml) and the mixture was stirred at room temperature for 4
hours, then diluted with water. The precipitate was filtered off
and recrystallized from methanol. White solid (209 mg, 78%),
6-(3-phenylpropanoyl)-4H-chromen-4-one
undergoes ESIDPT via formation of a transition tautomer T6*
(
1a
)
probably
.
This kind of ESIDPT leads to a tautomeric form with a very low
rate constant of radiative deactivation of the excited state,
what results in low fluorescence quantum yields. 3,7-
Dihydroxy-4-oxo-2-phenyl-4H-chromene-8-carbaldehyde most
probably undergoes single ESIPT, however, we hope further
investigations will reveal more details on the behavior of this
compound.
1H-NMR (200 MHz, DMSO-d6
(m, 2H), 7.51 – 7.58 (m, 3H), 7.92 (d, 1H,
,
δ
): 3.78 (s, 3H), 6.87 – 6.97
= 9.3 Hz), 7.95 –
: 269.2
J
8.02 (m, 2H), 10.85 (broad s, 1H). Mass spectrum,
m/z
[M+H]+, 291.1 [M+Na]+, 307.0 [M+K]+. Anal. Calcd. for
C16H12O4, %: C 71.64; H 4.51. Found, %: C 71.87; H 4.62.
General procedure for 7-hydroxy-4-oxo-2-phenyl-4
chromene-8-carbaldehyde (2b) and 7-hydroxy-3-methoxy-4-
oxo-2-phenyl-4 -chromene-8-carbaldehyde (2c) (Duff
reaction). 7-Hydroxy-2-phenyl-4 -chromen-4-one 14 or 7-
hydroxy-3-methoxy-2-phenyl-4 -chromen-4-one 12 (1 mmol),
H-
H
Acknowledgements
H
The research was financed by the Polish National Science
Center (NCN) under Grant No. 2014/13/N/ST4/04105. PhD
studies of I. E. Serdiuk were supported by scholarship of the
Polish Bureau for Academic Recognition and International
Exchange (BUWiWM). The authors gratefully acknowledge
Prof. dr hab. Wiesław Wiczk for valuable consultations and
technical support on the low-temperature and time-resolved
fluorescence investigations. A special acknowledgment is
H
hexamethylenetetramine (421 mg, 3 mmol) and acetic acid (4
ml) were refluxed for 2 h, then 1 M HCl was added (2 ml) and
the mixture was stirred for 1 h at 60°С. Precipitate collected
after cooling was recrystallized from MeOH.
1
2b: White powder (67%), H NMR (500 MHz, DMSO-d6
,
δ
):
7.05 (s, 1Н), 7.09 (d, 1H,
J = 8.9 Hz), 7.55 – 7.60 (m, 3Н), 8.14
(d, 1H,
J = 8.9 Hz), 8.16 – 8.21 (m, 2Н); 10.60 (s, 1Н), 12.16
(broad s, 1Н). 13C NMR (100 MHz, DMSO-d6
, δ): 190.5,
12 | J. Name., 2012, 00, 1-3
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addressed to Prof. dr hab. inż. Jerzy Błażejowski for technical
support.
View Article Online
DOI: 10.1039/C5RA13912K
Notes and references
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Graphical and textual abstract for the
Table of contents entry
3,7-Dihydroxyflavone derivatives contain-
ing carbonyl fragments were synthesized.
Results of the fluorescent spectroscopy
investigations indicate that one of them
undergoes Excited State Intramolecular
Double Proton Transfer (ESIDPT).
O
O
hv
H
O
Lifetime - 5.89 ns
Fluorescence QY - 0.11%
O
H
R
O
ESIDPT
ESIPT
O
R
O
O
R
O
+
H
O
H
ESIPT
O-
O
O
H
O
O
H