β-Diketones bearing electron-donating chromophores and a novel β-triketone: Synthesis and reversible fluorescence behavior

Article abstract of DOI:10.1016/j.tetlet.2006.02.019

Saturated and unsaturated β-diketones bearing 4-N,N- dimethylaminophenyl substituent and a novel β-triketone were synthesized. These β-diketones exist in both cis-enol and keto forms in solution, and their relative contents were determined by ¹

Full text of DOI:10.1016/j.tetlet.2006.02.019

Tetrahedron Letters 47 (2006) 2623–2626
b-Diketones bearing electron-donating chromophores and a novel
b-triketone: synthesis and reversible fluorescence behavior
Xin Zhang, Zi-Chen Li,* Ning Xu, Kai-Bo Li, Song Lin, Feng-Zhu Lu,
Fu-Sheng Du and Fu-Mian Li
Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and Molecular Engineering,
Peking University, Beijing 100871, China
Received 29 September 2005; revised 2 February 2006; accepted 6 February 2006
Available online 24 February 2006
Abstract—Saturated and unsaturated b-diketones bearing 4-N,N-dimethylaminophenyl substituent and a novel b-triketone were
synthesized. These b-diketones exist in both cis-enol and keto forms in solution, and their relative contents were determined by
1H NMR measurements. In contrast, for the b-triketone, only enol form was observed in a solution. A strong fluorescence quench-
ing was observed for unsaturated diketones as compared with the saturated ones. These b-diketones displayed reversible photo-
induced ketonization as revealed by UV–vis spectroscopy. The fluorescence emission of the saturated b-diketones faded gradually
during UV irradiation. The fluorescence emission of these saturated b-diketones can thus be reversibly switched between the enol
form ‘ON’ and the keto form ‘OFF’ based on keto-enol tautomerization.
ꢀ 2006 Elsevier Ltd. All rights reserved.
b-Diketones are well known to have two forms, that is
enol and keto forms. One of the most interesting prop-
erties is the conversion between the two forms, which
was termed as keto-enol tautomerism.1 The enol form
is more stable than the keto form in a solution, and
the tautomeric equilibrium can be affected by various
factors such as solvent polarity,2 substitution groups,3
pH values, and UV light irradiation.4 The photoinduced
ketonizations of b-diketones generally occur after UV
irradiation.5 The photoinduced ketonization process is
reversible. In the darkness the keto form can be con-
verted to the cis-enol form.6,17 Although a wide range
of b-diketones and their keto-enol tautomerism have
been studied extensively, little attention was paid to their
fluorescence behavior because of the general fluores-
cence quenching of the n–p* triplet states of b-diketones.
The corresponding unsaturated b-diketone 3 was also
synthesized, which was symbolized as A(=)–D,8 where
A(=) is the electron-accepting C@C bond attached with
b-diketone groups, and D is the N,N-dimethylaniline
donor. In addition, as an extension of b-diketones, a
novel b-triketone 4 was synthesized. Their keto-enol
tautomerism and fluorescence behaviors were investi-
gated. To the best of our knowledge, this is the first
report on the reversible fluorescence behavior based on
keto-enol tautomerism.
For the synthesis of b-diketones, several synthetic routes
have been developed: (1) Claisen condensation of corre-
sponding esters with esters, or ketones with esters in the
presence of sodium hydride (NaH), tetramethyl orthosili-
cate/cesium fluoride, sodium ethoxide, or n-butylli-
thium;9–11 (2) acylations of b-alkylketones with acid
chlorides in the presence of pyridine or triethylamine;12
and (3) aldol condensation of aldehydes or b-alkyl-
ketones to yield the 3-hydroxyl ketones, followed by an
oxidation.13 b-Diketones 1 and 2 were synthesized using
the route (1) by Claisen condensation of 1-(4-(dimethyl-
amino)phenyl) ethanone with ethyl acetate and methyl
pivalate, respectively, using dibenzo-18-crown-6 and
ethyl ether/diethylene glycol dimethyl ether (5/1, v/v)
as solvents in the presence of sodium hydride (NaH)
as shown in Scheme 1. The synthesis of b-diketones 3
For the past decade, we had studied the synthesis, poly-
merization, and fluorescence behavior of vinyl and
maleimide monomers bearing electron-donating or elec-
tron-accepting chromophores.7,8 Here, we designed and
synthesized new b-diketones 1 and 2, which contain elec-
tron-donating chromophores as shown in Scheme 1.
*
Corresponding author. Tel.: +86 10 6275 1708; fax: +86 10 6275
7155; e-mail: zcli@pku.edu.cn
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.02.019

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X. Zhang et al. / Tetrahedron Letters 47 (2006) 2623–2626
H
O
O
O
O
O
1
(i)
O
+
+
N
N
1.0
O
:
:
:
8.1
H
O
O
O
O
O
(ii)
2
3
O
N
N
1.0
O
8.4
N
H
O
O
O
O
(iii)
+
Cl
N
N
1.0
5.1
Keto Form
Enol Form
O
O
O
O
O
O
Cl
O
(iv)
O
+
4
H
O
Enol Form
Scheme 1. Synthetic routes to the saturated b-diketones 1 and 2, the unsaturated b-diketone 3, and b-triketone 4. Reagents and conditions: (i) NaH,
dibenzo-18-crown-6, rt; (ii) NaH, Et2O/diglyme (5/1, v/v), rt; (iii) NaNH2, Et2O/diglyme (4/1, v/v); (iv) Mg (OC2H5)2, Et2O/EtOH (4/1, v/v), 0–5 ꢁC.
and 4 is based on an improved acylation employing
diethyl ether/diethylene glycol dimethyl ether (4/1, v/v)
as solvent under strong basic conditions, that is sodium
amide (NaNH2) for 3 and magnesium ethanolate
(Mg(OC2H5)2) for 4. There are few reports on the acyl-
ation reaction under such strong basic conditions. The
synthetic details and spectroscopic characterizations
were provided in Supplementary materials.
For b-triketone 4, the NMR signal of the ketonic methen-
yl proton (CH(COR)3) was not observed. The integral
ratio (9.05/1.00) of the resonance peaks of the methyl
protons (2(CH3CO)– and CH3C(R)@) and enolic hydr-
oxyl proton (OH) indicates that the b-triketone 4 only
exists in one form, that is enol form in a solution as
illustrated in Scheme 1.
These b-diketones are thermally unstable as revealed by
thermal gravimetric analysis (TGA)/differential thermal
analysis (DTA) measurements (Figs. S-4 and S-5, and
Scheme S-1 in Supplementary materials).
Four compounds were characterized with H NMR spec-
troscopy. The singlets at d 4.00 of 1, d 4.04 of 2, d 4.23 of
3 were assigned to the methylene protons (–CO–CH2–
CO–) of the corresponding keto tautomers. The singlets
at d 6.08 of 1, d 6.17 of 2, d 6.34 of 3 were assigned to the
vinyl protons (–C(–OH)@CH–) of the corresponding
enol tautomers. The singlet at d 16.45 of 1, d 16.82 of
2, d 16.40 of 3, d 16.64 of 4 were assigned to the hydro-
xyl protons (OH) of the corresponding enol tautomers.
The chemical shifts of the enolic hydroxyl protons
became sharp and shifted downfield as compared with
the hydroxyl protons in general, suggesting the existence
of strong H-bonding. These results indicate that the b-
diketones 1, 2, and 3 exist in two forms, that is keto
and enol forms in a solution.
Compared with the saturated b-diketones 1 and 2, the
absorption spectra of unsaturated A(=)–D 3 show a
bathochromic shift of 22 nm as shown in Figure 1. This
2.0
Absorption Emission
80
Saturated
Saturated
Unsaturated
1
2
1.5
1.0
0.5
0.0
A
(=)-D 3
60
40
20
0
1
According to the assignments of H NMR peaks, the
relative ratios of enol versus keto tautomers are 8.1/
1.0 for 1, 8.4/1.0 for 2, 5.1/1.0 for 3 in CDCl3, which
were calculated from the integral ratios of the enolic
vinyl and ketonic methylene (1/2) protons. The enol form
predominates. The enol/keto ratios of the b-diketones 1,
2, and 3 in acetonitrile-d3 are 3.9/1.0, 4.1/1.0, and 3.2/
1.0, respectively. This may be due to the fact that
intramolecular H-bonding interaction was weakened in
acetonitrile therefore the less contents of the enol
forms.
200
300
400
500
600
Wavelength /nm
Figure 1. UV–vis absorption and fluorescence spectra of the saturated
b-diketones 1 and 2, the unsaturated b-diketone 3 in acetonitrile.
Concentration: 2.0 · 10ꢀ5 M.

X. Zhang et al. / Tetrahedron Letters 47 (2006) 2623–2626
2625
may be attributed to the more extended and conjugated
structure of the enol form of unsaturated 3. In contrast,
their fluorescence spectra are distinct. The saturated b-
diketones 1 and 2 display a strong fluorescence in aceto-
nitrile with the quantum yields (Q)14 of 2.41% and
2.35%, respectively. However, very weak fluorescence
(Q < 0.02%) was observed for the unsaturated A(=)–D
3 as shown in Figure 1.
tically decrease from 2.41% before UV irradiation to a
very low value (<0.05%) after UV irradiation. The fluo-
rescence behavior can be rationalized by the enol-keto
tautomerism. In the cis-enol form, the hydroxyl and car-
bonyl groups form a six-member ring structure through
H-bonding interaction. The energy minimum conforma-
tion of the enol form was calculated by a semiempirical
(AM1) method using a Gaussian98 program to be a pla-
nar conjugated and rigid structure of the whole molecule
(Scheme S-2 in Supplementary materials). One hydroxyl
group is adjacent to the N,N-dimethylaniline donor
(Scheme 2). These lead to a strong fluorescence emis-
sion. In other words, the fluorescence was switched to
‘ON’ state in cis-enol form. In the keto form, two car-
bonyl groups are relatively free due to the rotation of
carbon–carbon bonds. The energy minimum conforma-
tion of the keto form displays a dihedral angle of 53.3ꢁ
between the two carbonyl groups (Scheme S-2), and the
whole molecule no longer has rigid and conjugated
structure. One carbonyl group is adjacent to the N,N-
dimethylaniline donor. These lead to the fluorescence
quenching through the intramolecular free rotation
and the n–p* excited state of carbonyl groups. The fluo-
rescence was switched to ‘OFF’ state in keto form. With
UV irradiation, the photoinduced ketonization oc-
curred, and the enol form was converted into the keto
form. The fluorescence emission was switched from
‘ON’ to ‘OFF’.
For b-diketone 1, the absorption intensity at the long
wavelength of 365 nm (p–p* transition of the enol struc-
ture)15,16 was decreased gradually during UV irradiation
as shown in Figure 2. A new absorption peak at the
short wavelength of 337 nm (attributed to the keto
structure)16 appeared and increased gradually. An isos-
bestic point was observed at 348 nm, implying that the
keto tautomer increased and the enol tautomer
decreased during UV irradiation. This process is called
as photoinduced ketonization or photoketonization.1
The photoinduced ketonization was also observed for
2 and 3.
Interestingly, the fluorescence emission of b-diketone 1
was found to be fading during UV irradiation, as shown
in Figure 3A. The fluorescence quantum yields drama-
According to the UV absorption and emission wave-
lengths of b-diketones and AM1 semiempirical calcula-
tion, the energy level diagrams of enol and keto forms
were established for the b-diketone 1. The energy transi-
tion pathways of keto and enol forms are illustrated in
Supplementary materials (Scheme S-3). In the enol
form, the lowest excited state (1p–p*, 365 nm, 3.08 eV)
is localized at aromatic donors, strong emission was
observed at 443 nm. In ketone form, the lowest excited
state (3n–p*, 352 nm, 3.20 eV) is localized at the two car-
bonyl groups, which is lower than the excited singlet
state (1p–p*, 337 nm, 3.34 eV), thus, no fluorescence
was observed due to the intersystem crossing (excited
singlet state ! excited triplet state) according to
Lakowicz.22
Figure 2. UV–vis absorption spectra of b-diketone 1 in acetonitrile
during UV irradiation. Irradiation time: 0, 15, 40, 60, 90, 120, 150, 190,
220 min. Concentration: 2.0 · 10ꢀ5 M.
This photoinduced ketonization process is reversible.
The keto form can be converted back into the cis-enol
form in darkness.6,17 The fluorescence was restored in
darkness as shown in Figure 3B. Thus, the fluorescence
was switched reversibly from ‘OFF’ to ‘ON’ state. The
same result was also observed for b-diketone 2.
A
0 min
15 min
40 min
60 min
90 min
120 min
150 min
190 min
220 min
360 min
B
300 min
15 min
0 min
400
500
400
500
Wavelength / nm
Wavelength / nm
Figure 3. Fluorescence spectra of b-diketone 1 in acetonitrile during
UV irradiation (A) from top: 0, 15, 40, 60, 90, 120, 150, 190, 220,
360 min; and in darkness (B) from bottom: 0, 15, 300 min.
Scheme 2. Schematic representation for the reversible fluorescence
ON/OFF of b-diketone 1 based on the keto-enol tautomerization.
k
ex = 360 nm. Concentration: 2.0 · 10ꢀ5 M.

2626
X. Zhang et al. / Tetrahedron Letters 47 (2006) 2623–2626
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donors were found to have both keto and cis-enol forms
in a solution. The enol is the predominant form in a
solution as revealed by NMR spectroscopy. In contrast,
the novel b-triketone 4 exists in only one enol form in
the solution. The saturated b-diketones 1 and 2 display
a strong fluorescence, however, almost no fluorescence
was observed for the unsaturated b-diketone A(=)–D 3.
Reversible photoinduced ketonization of these b-di-
ketones was observed. The fluorescence intensities de-
creased gradually during photoinduced ketonization.
Based on the reversible keto-enol tautomerization, the
fluorescence emission was reversibly switched between
the ‘ON’ state in cis-enol form, and the ‘OFF’ state in
keto form. Such reversible photochemical and photo-
physical properties at a molecular level may provide
an opportunity in designing a novel class of molecular
switches and devices.
14. (a) Fluorescence quantum yields (Q) were calculated from
the integrated intensity under the emission band (I) using
the following equation:
Acknowledgments
n2
nr2
ODr
Ir OD
I
Q ¼ Qr
Financial support from the National Natural Science
Foundation of China (No. 50003001) is gratefully
acknowledged. Authors thank Dr. Liu-He Wei and
Ms. Xiao-Nan Huang for kind help.
where OD is the optical density of the solution at the
excitation wavelength, and n is the refractive index. The
optical density of the solution for the calculation of
quantum yields was less than 0.1 at the excitation
wavelength. 9,10-Diphenylanthrene in cyclohexane was
used as reference (Qr = 0.90); (b) Eaton, D. F. Pure. Appl.
Chem. 1988, 60, 1107.
Supplementary data
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1
Synthesis and spectroscopic data of 1, 2, 3, and 4; H
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NMR spectra of 2 and 3 in CDCl3; H NMR spectrum
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mal behavior of 3; absorption spectrum of 4 in aceto-
nitrile; schematic representation for the energy minimum
conformations of the enol and keto forms of 1 and 3;
energy level diagrams of enol and keto forms of 1. Supple-
mentary data associated with this article can be found,
in the online version, at doi:10.1016/j.tetlet.2006.02.019.
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