chalcones without O–H:O intramolecular hydrogen bonding
undergo mutual cis–trans photoisomerization to give a mixture
of cis and trans isomers. However, we have already reported
that the parent compound 20-hydroxychalcone underwent
one-way cis-to-trans photochemical isomerization around the
double bond controlled by photoinduced hydrogen atom
transfer in the excited state to give 100% trans isomer at the
photostationary state.18,20 As mentioned above, the trans
isomers of substituted HCs with O–H:O intramolecular
hydrogen bonding are also practically stable on photo-
irradiation. However, hydrogen atom transfer in the excited
state did take place as observed spectroscopically. In particular,
ultrafast intramolecular hydrogen atom transfer took place to
produce the emissive tautomer giving fluorescence at a long
wavelength region of red light. Thus, the photochromic
tautomer deactivates by radiative and non-radiative processes
to give the starting compound and this process might occur
in very fast photochromic cycles due to the forward and
backward intramolecular hydrogen atom transfer processes.
absorbance of the sample solution at the excitation wavelength
was adjusted to less than 0.1, and the integration of the
fluorescent spectra over all wavenumbers was plotted against
absorbance at the excitation wavelength. The slope of these
plots gives the relative value of the fluorescence quantum yield,
and the quantum yield of fluorescence emission was then
determined. All the solvents used in this experiments were of
UV and fluorescence spectroscopic grade and were used
without further purification. Only ethanol contains 0.5%
H2O (99.5% purity, Wako).
Synthesis
General procedure for the preparation of 20-hydroxychal-
cones. Substituted 20-hydroxychalcones (Scheme 1) were
prepared by condensation of the appropriate ketones and
aldehydes and the structures were determined by NMR and
elemental analysis.24–26
4,40-Dimethoxy-HC. The mixture of 4-methoxybenzal-
dehyde (1.71 g, 12.5 mmol) and 20-hydroxy-40-methoxyaceto-
phenone (1.75 g, 10.5 mmol) was added in EtOH. 1.5 mol dmꢁ3
KOH in EtOH (35 ml) was added and the mixture stirred
under nitrogen at room temperature for 23 h. This reaction
mixture was neutralized by addition of 1.2 mol dmꢁ3 AcOH in
EtOH. This mixture was extracted with ethyl acetate and the
obtained organic layer was dried with Na2SO4. Then the
solution was evaporated and recrystallized from EtOH to give
20-hydroxy-4,40-dimethoxychalcone as yellow crystals with a
yield of 0.58 g (2.0 mmol, 19.3%). 1H NMR (200 MHz,
CDCl3): d 3.87 (s, 6H, OMe), 6.47–6.52 (m, 2H; Ph–H), 6.95
(d, J = 8.9 Hz, 2H; Ph–H), 7.47 (d, J = 15.3 Hz, 1H;
–HCQCH–), 7.62 (d, J = 8.9 Hz, 2H; Ph–H), 7.87
(d, J = 15.3 Hz, 1H; –HCQCH–), 7.86 (m, 1H; Ph–H),
13.56 (s, 1H; –OH). 13C NMR (100 MHz, CDCl3): 55.38,
55.52, 101.02, 107.55, 114.10, 114.41, 117.74, 127.46, 130.32,
131.08, 144.22, 161.75, 165.98, 166.57, 191.82. Anal. Calc. for
C17H16O4: C, 71.82; H, 5.62. Found: C, 71.83; H, 5.68%.
Conclusions
20-Hydroxychalcones form intramolecular hydrogen bonding.
The parent compound 20-hydroxychalcone undergoes one-way
cis-to-trans photochemical isomerization by way of intra-
molecular hydrogen atom transfer to produce the tautomer
in the excited state followed by cis-to-trans isomerization and
deactivation to the ground state. However, we could not
observe fluorescence emission from the tautomer or any
evidence for occurrence of hydrogen atom transfer in the
excited singlet state in unsubstituted 20-hydroxychalcone. In
this paper, the tautomer fluorescence from 20-hydroxychalcones
was successfully observed by conventional fluorescence
spectrometry. The effect of substituents with different electronic
properties, introduction of dendritic structure, and temperature,
on the efficiency of the tautomer emission produced by
adiabatic intramolecular hydrogen atom transfer was evaluated.
The introduction of the methoxy group as the electron
donating substituent on the benzene ring made it possible to
observe fluorescence emission from the tautomer. The quantum
yield of the red light fluorescence from the transiently
produced tautomer is 1000 times higher at 77 K than that at
room temperature. Even if the effect is small, the fluorescence
quantum yield of the tautomer increased with increasing
generation of the dendrimer. Therefore, one could also
increase the characteristic red fluorescence of the tautomer
by introduction of the dendritic structure surrounding the
20-hydroxychalcone core.
4-Nitro-HC. 1H NMR (270 MHz, CDCl3): d 6.99 (t, J = 8.1
Hz, 1H), 7.06 (d, J = 8.1 Hz, 1H), 7.55 (t, J = 8.1 Hz, 1H),
7.79–7.96 (m, 5H), 8.30 (d, J = 8.1 Hz, 2H), 12.58 (s, 1H).
Anal. Calc. for C15H11NO4: C, 66.91; H, 4.12; N, 5.20. Found:
C, 65.98; H, 4.33; N, 5.20%.
4-Cyano-HC. 1H NMR (270 MHz, CDCl3):
d 6.97
(t, J = 8.1 Hz, 1H), 7.06 (d, J = 8.1 Hz, 1H), 7.54
(t, J = 8.1 Hz, 1H), 7.69–7.74 (m, 5H), 7.85–7.92 (m, 2H),
12.59 (s, 1H). Anal. Calc. for C16H11NO2: C, 77.10; H, 4.45;
N, 5.62. Found: C, 76.61; H, 4.75; N, 5.53%.
4-Cyano-40-methoxy-HC. 1H NMR (270 MHz, CDCl3): d
3.88 (s, 3H), 6.49–6.53 (m, 2H), 7.64 (d, J = 15.4 Hz, 1H), 7.73
(s, 4H), 7.79–7.87 (m, 2H), 13.23 (s, 1H). Anal. Calc. for
C17H13NO3: C, 73.11; H, 4.69; N, 5.02. Found: C, 73.96; H,
4.91; N, 5.95%.
Experimental
Measurements
Absorption and fluorescence spectra were measured on a
Shimadzu UV-1600 and on a Hitachi F-4500 fluorescence
spectrometer, respectively. The quantum yields of fluorescence
emissions were determined by using anthracene (Ff = 0.27)
as a standard. A correction in the difference in refractive
index among the solvents was made for each sample. The
4-Methoxy-HC. 1H NMR (270 MHz, CDCl3): d 3.87
(s, 3H), 6.91–7.04 (m, 4H), 7.46–7.65 (m, 4H), 7.88–7.94
(m, 2H), 12.93 (s, 1H). Anal. Calc. for C16H14O3: C, 75.57;
H, 5.55% Found: C, 75.36; H, 5.62%.
ꢀc
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009
1398 | New J. Chem., 2009, 33, 1393–1401