Excited state intramolecular hydrogen atom transfer of phenylethynyl- substituted 2'-hydroxychalcones

Article abstract of DOI:10.1007/s11164-012-0632-8

(E)-1-[2-Hydroxy-4-(phenylethynyl)phenyl]-3-[4-(phenylethynyl)phenyl] prop-2-en-1-one (1), (E)-1-[2-hydroxy-4-(phenylethynyl)phenyl]-3-phenylprop-2- en-1-one (2), and (E)-1-(2-hydroxyphenyl)-3-[4-(phenylethynyl)phenyl]prop-2-en- 1-one (3), which belong to a new class of 2'-hydroxychalcones with phenylethynyl group(s) at the para position of the phenyl ring, were synthesized, and their photochemical properties were investigated. The lowest energy absorption band of 1 peaks at a longer wavelength (383 nm) with a much larger molar extinction coefficient (5.0 × 10⁴ M ^-1 cm^-1) than that of the parent 2'-hydroxychalcone (2'HC) (2.0 × 10⁴ M ^-1 cm^-1 at 318 nm). Upon photoexcitation, all three compounds underwent excited-state intramolecular hydrogen atom transfer (ESIHT) to produce an excited tautomer that emitted fluorescence with a large Stokes shift in the longer wavelength region at 600-700 nm. The quantum yield of the tautomer fluorescence of 1 was not high at 298 K (Φ _f = 9.1 × 10^-5), but was highest among 2'HC and its analogues. The Φ _f values of 1-3 increased 10-30 fold upon reducing the temperature from 298 to 77 K.

Full text of DOI:10.1007/s11164-012-0632-8

Res Chem Intermed (2013) 39:61–72
DOI 10.1007/s11164-012-0632-8
Excited state intramolecular hydrogen atom transfer
of phenylethynyl-substituted 20-hydroxychalcones
•
•
•
Satomi Tasaki Atsuya Momotake Yoko Kanna
Tatsuo Arai
Received: 30 September 2011 / Accepted: 2 December 2011 / Published online: 15 June 2012
Ó Springer Science+Business Media B.V. 2012
Abstract (E)-1-[2-Hydroxy-4-(phenylethynyl)phenyl]-3-[4-(phenylethynyl)phenyl]
prop-2-en-1-one (1), (E)-1-[2-hydroxy-4-(phenylethynyl)phenyl]-3-phenylprop-2-
en-1-one (2), and (E)-1-(2-hydroxyphenyl)-3-[4-(phenylethynyl)phenyl]prop-2-en-
1-one (3), which belong to a new class of 20-hydroxychalcones with phenylethynyl
group(s) at the para position of the phenyl ring, were synthesized, and their pho-
tochemical properties were investigated. The lowest energy absorption band of 1
peaks at a longer wavelength (383 nm) with a much larger molar extinction coef-
ficient (5.0 9 104 M-1 cm-1) than that of the parent 20-hydroxychalcone (20HC)
(2.0 9 104 M-1 cm-1 at 318 nm). Upon photoexcitation, all three compounds
underwent excited-state intramolecular hydrogen atom transfer (ESIHT) to produce
an excited tautomer that emitted fluorescence with a large Stokes shift in the longer
wavelength region at 600–700 nm. The quantum yield of the tautomer fluorescence
of 1 was not high at 298 K (Uf = 9.1 9 10-5), but was highest among 20HC and its
analogues. The Uf values of 1–3 increased 10–30 fold upon reducing the temper-
ature from 298 to 77 K.
Keywords Excited-state hydrogen atom transfer ꢀ Adiabatic reaction ꢀ
20-Hydroxychalcone ꢀ Fluorescence ꢀ Photostability
Introduction
Excited-state intramolecular hydrogen atom transfer (ESIHT) is a fundamental
photochemical process that occurs upon photoexcitation of intramolecularly
S. Tasaki ꢀ A. Momotake ꢀ T. Arai (&)
Graduate School of Pure and Applied Sciences, University of Tsukuba,
Tsukuba, Ibaraki 305-8571, Japan
e-mail: arai@chem.tsukuba.ac.jp
Y. Kanna
Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan
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S. Tasaki et al.
hydrogen bonded compounds in which the hydrogen donating and accepting groups
are in the appropriate geometry to form a pseudo aromatic ring system [1–9]. Upon
photoexcitation, the excited state (N*) of the original normal form (N) immediately
undergoes ESIHT to produce the excited state tautomer form (T*) that decays by
radiative or non-radiative processes, and the resulting ground state tautomer
(T) rapidly reverts back to N by reverse hydrogen atom transfer (Fig. 1). We have
synthesized a variety of ESIHT compounds, for example salicylidene anilines
[10–12], 2-(20hydroxyphenyl)benzazoles (including benzoxazole [13–16], benz-
imidazole [17], and benzothiazole [17]), hemiindigos [18], pyrrole-pyridine [19],
and pyrrole-quinoline [20–22] hydrogen-bonding systems, and have studied their
excited state dynamics.
Among intramolecularly hydrogen bonded molecules, 20-hydroxychalcone
(20HC), an important compound distributed in plants and a precursor of flavonoids
and isoflavonoids [23, 24], is regarded as relatively stable on photoirradiation. One
reason for this low reactivity on photoreaction is believed to be the presence of an
intramolecular hydrogen bond that induces immediate deactivation to the ground
state before undergoing reaction to produce other photoproducts. Chou et al. [25]
studied the excited state reaction of 20HC and discovered tautomer formation in
the excited singlet state by observing the fluorescence emission in the longer
wavelength region of ca 630 nm. This fluorescence emission could not be detected
by conventional fluorescence spectroscopy. When we began our studies on the
dynamic behavior of 20HC in the excited triplet state, there were no reports on either
detection of the triplet state or on detailed reaction processes of the triplet state of
20HC. We found that 20HC underwent ESIHT and intersystem crossing to give the
tautomer in the excited triplet state with an extremely long triplet lifetime of 1 ls,
making observation of the T–T absorption spectra by transient spectroscopy
possible [26]. Furthermore, by synthesizing the cis isomer of 20HC, we revealed that
20HC undergoes only one-way cis-to-trans isomerization by an adiabatic process in
the excited state, with the help of ESIHT, to form a polyene-like structure [27].
Fig. 1 Potential energy surface
of ESIHT of 20HC, where N and
T indicate the normal and
tautomer forms, respectively, as
in the depicted structures. The
energy values are from Ref. [27]
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Excited state intramolecular hydrogen atom transfer
63
Thus, we revealed the triplet state of 20HC undergoes ESIHT and one-way cis-to-
trans isomerization.
Another yet unresolved and important aspect of the excited state behavior of
20HC and related compounds is how to detect fluorescence of the tautomer by
conventional fluorescence spectroscopy. Although the fluorescence efficiency of
20HC is very low (Uf \ 10-6), it can be enhanced by simple chemical modification.
For example, fluorescence of the tautomer of naphthalene-fused 20HC analogues at
approximately 600–700 nm can be detected by use of a commercially available
fluorimeter [28–31]. Thus, extension of the p electron system might be a simple way
of facilitating spectroscopic observation of the tautomer. Recently, we have also
successfully observed fluorescence of the tautomer of 20-hydroxychalcone by
introduction of electron-donating substituents, for example methoxy or dimethyl-
amino groups, at the 4 or 40 positions, which increase the efficiency of fluorescence
of the tautomer [32]. However, the quantum yields of those substituted 20HC
analogues were still low at 298 K (Uf = 2.0 9 10-6–1.1 9 10-5).
In this paper, we focus on a study of the effect of a phenylethynyl substituent on
the photochemical properties of 20HC. The remarkable effects of the phenylethynyl
substituent on the photochemistry of aromatic enediynes [33] and stilbene
(unpublished data) have recently been investigated. A large red shift of the
absorption and fluorescence spectra, dramatic enhancement of the fluorescence
quantum yield, and a decrease of isomerization efficiency from the excited triplet
state, because of the reduced triplet energy of planar conformations, were observed.
The effect of the phenylethynyl unit on the photochemical and photophysical
properties of 20HC should, therefore, also be of interest. Thus, three new
phenylethynyl-substituted 20-hydroxychalcones 1–3 were prepared for this study,
as described below.
Experimental
Materials and general instrumentation
All reagents and solvents of the highest commercial quality were purchased from
Tokyo Chemical Industry, Wako Pure Chemical Industries (Osaka, Japan),
or Aldrich Chemical (Milwaukee, WI, USA) and were used without further
purification. NMR spectra were recorded on a JNM-EX270 (1H NMR at 270 MHz)
or AV-400FT NMR (1H NMR at 400 MHz; 13C at 100 MHz). d values are given in
parts per million relative to tetramethylsilane. Silica gel column chromatography
was performed using silica gel 60N (Kanto Chemical, Tokyo, Japan). IR spectra
were recorded on a Jasco FT-4100 spectrometer.
Measurements
Absorption and fluorescence spectra were measured on Shimadzu UV-1600 and
Hitachi F-4500 fluorescence spectrophotometers, respectively. The concentration
for steady state fluorescence measurements was adjusted so that the absorption
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S. Tasaki et al.
maximum of the excitation wavelength was 0.1 for each sample. Fluorescence
quantum yields were determined relative to anthracene in ethanol (Uf = 0.27) [34]
and were corrected for the solvent refractive index.
Materials
2-Hydroxy-(4-phenylethynyl)acetophenone (4)
A mixture of 4-bromo-2-hydroxyacetophenone (2.73 g, 12.7 mmol), ethynylben-
zene (1.93 g, 18.9 mmol), Pd(PPh3)2Cl2 (461 mg, 0.657 mmol), PPh3 (83 mg,
0.316 mmol), and triethylamine (2.54 g, 25.1 mmol) in 50 mL distilled THF was
stirred for 30 min at room temperature, and CuI (74.9 mg, 0.393 mmol) was then
added. After stirring for 24 h, the reaction was quenched by addition of water and
then extracted with dichloromethane. The organic layer was washed with saturated
sodium bicarbonate solution, dried over MgSO4, filtered, and evaporated. The
residue was purified by silica gel column chromatography (SiO2; chloroform–
1
hexane, 1:2) to give 4 as a yellow solid (1.21 g, 40 %). mp 75–76 °C, H NMR
(270 MHz, CDCl3), d (ppm): 2.63 (s, 3H), 7.04 (d, J = 8.2 Hz, 1H), 7.13 (s, 1H),
7.37 (m, 3H), 7.54 (m, 2H), 7.70 (d, J = 8.2 Hz, 1H), 12.27 (s, 1H). 13C NMR
(100 MHz, CDCl3), d (ppm): 26.6, 88.3, 93.1, 119.2, 121.1, 122.1, 122.4, 128.4
(2C), 129.0, 130.1, 131.3, 131.8 (2C), 162.1, 203.9. Anal. Calcd for C16H12O2: C,
81.34; H, 5.12. Found: C, 81.15; H, 5.15. IR (KBr; 4000–400 cm-1), m: 3444,
3241, 3069, 2922, 2337, 2206, 1763, 1639, 1544, 1367, 1269, 1215, 978, 805, 760,
691 cm-1
.
1-[2-Hydroxy-4-(2-phenylethynyl)phenyl]-3-[4-(2-phenylethynyl)phenyl]prop-2-en-
1-one (1)
To a solution of 4 (234 mg, 0.989 mmol) and (4-phenylethynyl)benzaldehyde
(206 mg, 1.00 mmol) in methanol (15 mL), 2 mol/L KOH in methanol (6 mL) was
added. The mixture was stirred at room temperature for 6 h and then at 40 °C for
18 h. The reaction mixture was poured into iced water, and 2 mol/L HCl was
added until the mixture was acidic (pH 1–2). The mixture was extracted with
dichloromethane and the organic layer was washed with saturated sodium
bicarbonate solution, dried over MgSO4, filtered, and evaporated. The residue
was purified by silica gel column chromatography (SiO2; toluene) to give 1 as an
1
orange solid (135 mg, 32 %). mp 162–163 °C, H NMR (270 MHz, CDCl3), d
(ppm): 7.09 (dd, J = 8.3, 1.5 Hz, 1H), 7.19 (d, J = 1.5 Hz, 1H), 7.37 (m, 6H),
7.59 (m, 9H), 7.90 (d, J = 8.4 Hz, 1H), 7.93 (d, J = 15.3 Hz, 1H), 12.86 (s, 1H).
13C NMR (100 MHz, CDCl3), d (ppm): 88.5, 89.0, 92.2, 93.4, 119.6, 120.4, 121.3,
122.0, 122.5, 122.8, 126.0, 128.4 (2C), 128.5 (2C), 128.7, 128.7 (2C), 129.0, 129.5,
131.3, 131.7 (2C), 131.9 (2C), 132.2 (2C), 134.2, 144.7, 163.4, 192.8. Anal. Calcd
for C31H20O2: C, 87.71; H, 4.75. Found: C, 87.22; H, 4.56. IR (KBr;
4000–400 cm-1), m: 3438, 3050, 2922, 2852, 2335, 2203, 1635, 1569, 1549,
1509, 1354, 1199, 1020, 753, 684 cm-1
.
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Excited state intramolecular hydrogen atom transfer
65
1-[2-Hydroxy-4-(2-phenylethynyl)phenyl]-3-phenylprop-2-en-1-one (2)
To a solution of 4 (238 mg, 1.01 mmol) and benzaldehyde (123 mg, 1.16 mmol) in
methanol (15 mL), 2 mol dm-3 KOH in methanol (5 mL) was added. The mixture
was stirred at room temperature for 9 h and then at 40 °C for 15 h. The reaction
mixture was poured into iced water, and 2 mol dm-3 HCl was added until the
mixture was acidic (pH 1–2). The mixture was extracted with dichloromethane and
the organic layer was washed with saturated sodium bicarbonate solution, dried over
MgSO4, filtered, and evaporated. The residue was purified by silica gel column
chromatography (SiO2; toluene) to give 2 as an orange solid (136 mg, 41 %). mp
135–136 °C, 1H NMR (270 MHz, CDCl3), d (ppm): 7.08 (dd, J = 8.4, 1.7 Hz, 1H),
7.17 (d, J = 1.7 Hz, 1H), 7.37 (m, 3H), 7.44 (m, 3H), 7.55 (m, 2H), 7.65 (m, 3H),
7.89 (d, J = 8.4 Hz, 1H), 7.93 (d, J = 16.2 Hz, 1H), 12.85 (s, 1H). 13C NMR (100
MHz, CDCl3), d (ppm): 88.5, 93.3, 119.6, 119.9, 121.3, 122.0, 122.5, 128.4 (2C),
128.7, 128.9 (2C), 129.0, 129.5 (2C), 131.0, 131.2, 131.8 (2C), 134.5, 145.7, 163.3,
193.0. Anal. Calcd for C23H16O2: C, 85.16; H, 4.97. Found: C, 84.86; H, 5.09. IR
(KBr; 4000–400 cm-1), m: 3442, 3050, 2925, 2340, 2214, 1643, 1575, 1364, 1199,
1151, 979, 868, 755, 693 cm-1
.
1-(2-Hydroxyphenyl)-3-[4-(2-phenylethynyl)phenyl]prop-2-en-1-one (3)
To a solution of 2-hydroxyacetophenone (149 mg, 1.08 mmol) and (4-phenyl-
ethynyl)benzaldehyde (213 mg, 1.03 mmol) in methanol (15 mL), 2 mol dm-3
KOH in methanol (5 mL) was added. The mixture was stirred at room temperature
for 7.5 h and then at 40 °C for 15 h. The reaction mixture was poured into iced
water, and 2 mol dm-3 HCl was added until the mixture was acidic (pH 1–2). The
mixture was extracted with dichloromethane and the organic layer was washed with
saturated sodium bicarbonate solution, dried over MgSO4, filtered, and evaporated.
The residue was purified by silica gel column chromatography (SiO2; toluene) to
1
give 3 as an orange solid (157 mg, 47 %). mp 136–137 °C, H NMR (270 MHz,
CDCl3), d (ppm): 6.96 (dd, J = 8.2, 7.1 Hz, 1H), 7.04 (d, J = 8.2 Hz, 1H), 7.36 (m,
3H), 7.57 (m, 8H), 7.91 (d, J = 15.7 Hz, 1H), 7.93 (d, J = 7.3 Hz, 1H), 12.78 (s,
1H). 13C NMR (100 MHz, CDCl3), d (ppm): 89.0, 92.1, 118.7, 118.9, 120.0, 120.6,
122.8, 125.9, 128.4 (2C), 128.6 (2C), 128.7, 129.6, 131.7 (2C), 132.1 (2C), 134.3,
136.5, 144.4, 163.6, 193.5. Anal. Calcd. for C23H16O2: C, 85.16; H, 4.97. Found: C,
85.07; H, 5.23. IR (KBr; 4000–400 cm-1), m: 3442, 3032, 2922, 2337, 2214, 1639,
1568, 1490, 1445, 1338, 1206, 1163, 1025, 823, 762, 652 cm-1
.
Results and discussion
Molecular design and synthesis
In our previous study, the effect of phenylethynyl substituents at enhancing
fluorescence emission was usually most remarkable when they were introduced at
the para position of the photoresponsive unit [33]. In addition, the introduction of
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S. Tasaki et al.
Fig. 2 Chemical structures of the parent and phenylethynyl-substituted 20-hydroxychalcones (20HC and
1–3)
electron-donating group(s) at the 4 or 40 positions of 20HC was effective for
fluorescence enhancement, as mentioned above. Thus, we designed the three
20-hydroxychalcones having phenylethynyl units at the 4,40 positions, the 40
position, and the 4 position giving compounds 1, 2, and 3, respectively, and their
photochemistry and photophysics were compared with those of the parent 20HC.
The chemical structures of all the compounds are depicted in Fig. 2.
The synthesis of 1 and 2 began with the Sonogashira coupling [35] of
ethynylbenzene and 4-bromo-2-hydroxyacetophenone with Pd(PPh3)2Cl2 as catalyst
to yield 2-hydroxy-(4-phenylethynyl)acetophenone (4), the precursor of both 1 and
2 (Scheme 1). Subsequent aldol condensation of the precursor 4 and 4-(phenyl-
ethynyl)benzaldehyde (for 1) or benzaldehyde (for 2) in KOH–methanol pro-
duced the desired products. Compound 3 was prepared by aldol condensation of
2-hydroxyacetophenone and (4-phenylethynyl)benzaldehyde under the same reac-
tion conditions.
Absorption and fluorescence spectra at 298 K
The phenylethynyl substituent introduced at the 4 and 40 positions has a significant
effect on the spectroscopic properties of the 20-hydroxychalcones (Table 1). The UV
absorption spectrum of the parent 20HC in benzene at 298 K has a peak at 318 nm
with a shoulder at approximately 350 nm, and the molar extinction coefficients are
2.0 9 104 and 1.0 9 104 M-1 cm-1 at 318 and 355 nm, respectively (Fig. 3). The
absorption spectrum of 1, having two phenylethynyl groups at the 4,40 positions,
is the most substantially red-shifted to 383 nm with a large e value (e383 = 5.0 9
104 M-1 cm-1) at the peak wavelength. The shape of the spectrum of 2, in which a
phenylethynyl group is introduced at the 40 position, is somewhat similar to that of
the parent 20HC, but red-shifted to 341 nm with a shoulder at approximately 375
nm, and the e value is twice as high at the peak wavelength (e342 = 3.6 9
104 M-1 cm-1). Compared with the absorption spectrum of 2, the spectrum of 3 is
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Excited state intramolecular hydrogen atom transfer
67
Scheme 1 Synthesis of phenylethynyl-substituted 20-hydroxychalcones 1–3
Table 1 Spectroscopic, photophysical, and photochemical data of 1–3 and 20HC in benzene (298 K) and
MTHF (77 K) solution
Compd. kabs (nm) e (104 M-1 cm-1
)
kfl (nm) Uf (298 K)
Stokes shift (cm-1
)
Uf (77 K)
1
383
342, 372 3.6, 2.8
363 3.9
318, 355 2.0, 1.0
5.0
641
629
630
–
9.1 9 10-5 11,000
6.7 9 10-5 13,000
1.1 9 10-5 12,000
\1.0 9 10-6
1.0 9 10-3
1.5 9 10-3
2.5 9 10-4
2
3
20HC
further red-shifted to 363 nm, despite having only a single phenylethynyl
substituent, and its shape is changed, indicating that the p electrons of the
phenylethynyl unit at the 4 position are delocalized on to the 20HC moiety.
Although the intensity of the fluorescence emission was weak, it could be
observed at room temperature in benzene for 1–3 (Fig. 4). The Stokes shift of
the fluorescence of 1–3 at approximately 600–700 nm is as large as
10,000 cm-1. In addition, the excitation spectra depicted in Fig. 3 are similar
to the corresponding UV absorption spectra. These results indicate that the
observed fluorescence originates from the excited tautomer that is populated
through the ESIHT reaction upon photoexcitation of the normal form. It should
be noted that, even if the quantum yield of the tautomer fluorescence of 1 was
not high (Uf = 9.1 9 10-5), it has the highest value among 20HC and its
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S. Tasaki et al.
Fig. 3 Absorption spectra of 1 (red line), 2 (blue line), 3 (green line), and 20HC (black line) in benzene
at 298 K. (Color figure online)
Fig. 4 Absorption spectra (thin lines), fluorescence spectra (bold lines), and fluorescence excitation
spectra (dotted lines) of 1 (a), 2 (b), and 3 (c) in benzene at 298 K. The concentrations of the compounds
were 2.0 9 10-6, 2.8 9 10-6, and 2.6 9 10-6 mol/L for 1, 2, and 3, respectively. The excitation
wavelengths were 383, 342, and 362 nm for 1, 2, and 3, respectively. The monitoring wavelengths were
640, 625, and 630 nm for 1, 2, and 3, respectively
analogues in solution at 298 K. As already mentioned, detection of the fluo-
rescence of the parent 20HC by use of our fluorimeter was unsuccessful, because
of its very low quantum yield (Uf \ 10-6).
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Excited state intramolecular hydrogen atom transfer
69
Interestingly, despite the marked red-shifts observed in the absorption spectra,
the fluorescence spectra of 1–3 are not very different, or even blue-shifted,
compared with that of the parent 20HC, which had a peak at 635 nm, as reported by
Chou et al. [25], on excitation by an Nd:YAG laser. These similar results, i.e., the
significant absorption red-shift upon introduction of electron-donating groups at the
para position of 20HC, with a blue-shift of the fluorescence spectra, were also
observed in our previous study [32]. These results may indicate that the electron-
donating or p-conjugated substituents affected the ground-state normal form to
shift the absorption spectra of 20HC to a longer wavelength region, whereas the
substituent effect is much less in its tautomer form in the excited state.
Laser flash photolysis
Laser flash photolysis was performed to measure the transient absorption spectra of
1–3 at 298 K. Upon Nd:YAG laser excitation at 355 nm, no transient absorption
spectra for 1–3 were observed, including the excited triplet state that had been
observed for the parent 20HC [27] and for naphthalene-fused analogues of 20HC
[30]. These results indicate that intersystem crossing from the singlet excited state
to the triplet excited state was not significant and the phenylethynyl substituted
20-hydroxychalcones 1–3 were predominantly deactivated from the excited singlet
state non-radiatively at 298 K.
Low-temperature fluorescence spectra
The fluorescence intensity of the tautomers of 1–3 increased in freezing MTHF
glass at 77 K, as shown in Fig. 5. The fluorescence quantum yields of 1–3 at 77 K
were determined to be 1.0 9 10-3, 1.5 9 10-3, and 2.5 9 10-4, for 1, 2, and 3,
respectively; these are 10–30 times larger than those at 298 K. These results
indicate that the non-radiative deactivation of T* was suppressed at low
temperature; however, the Uf values for 1–3 at 77 K are still low. Unlike the
behavior at 298 K, the Uf value of 1 was less than that of 2. One possibility for the
changing order of Uf values from 298 to 77 K is the different efficiency of
intersystem crossing from T* between 1 and 2 at 77 K. In an ab-initio study of the
excited state behavior of diphenylacetylene [36], the excited singlet (Franck–
Condon) state undergoes intersystem crossing via several relaxation steps within the
singlet lifetime. Similarly, 1–3 may undergo intersystem crossing at 77 K and the
efficiency of 1 might be higher than that of 2, resulting in the lower quantum yield
of 1. However, this could not be ascertained by use of our instruments.
Photostability
Although 1–3 have large extinction coefficients in the near-UV to visible region and
undergo ESIHT reactions quite efficiently, the main process that T* undergoes is non-
radiative decay that does not involve a structural change, for example photo-
decomposition by bond cleavage or reaction with surrounding molecules. Further-
more, T, although it cannot be detected, immediately reverts back to N in the ground
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S. Tasaki et al.
Fig. 5 Fluorescence spectra of 1 (a), 2 (b), and 3 (c) in MTHF at 298 K (dotted lines) and at 77 K (solid
lines). The concentrations of the compounds were 2.0 9 10-6, 2.8 9 10-6, and 2.6 9 10-6 M for 1, 2,
and 3, respectively. The excitation wavelengths were 383, 342, and 362 nm for 1, 2, and 3, respectively
state by thermal hydrogen atom transfer. This efficient reaction cycle, as depicted in
Fig. 1, indicates that compounds 1–3 are photo-sensitive but highly photo-stable,
which is in contrast to the parent compound 20HC. Photoirradiation of 20HC resulted
in the formation of 3-hydroxyflavone, which is produced via oxygen-assisted
photocyclization in the excited triplet state, to give 2-phenylchroman-3,4-dione
followed by 1,3-hydrogen migration [25]. In deoxygenated polar solvents, irradiation
of 20HC or methoxy-substituted 20HC causes facile photocyclization to give
4-flavanones [37]. For example, 2-methoxy-substituted 20HC photocyclized with a
quantum yield of 0.3–0.4 in ethyl acetate [38]. This cyclization occurs via the trans-
keto form which is produced by ESIHT to form T* followed by cis–trans
isomerization of the newly produced C=C double bond [29, 38]. A similar reaction
might occur with 1–3, and the above non-radiative deactivation process may produce
the trans-keto form. However, the trans-keto form could not be observed in the
transient absorption measurements of 1–3. Additionally, no detectable change of the
absorption spectra was observed after laser flash photolysis in the diluted solution.
Conclusion
In this paper we report the photochemical properties of a new class of 20-
hydroxychalcones, 1–3, with phenylethynyl group(s) at the para positions of the
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Excited state intramolecular hydrogen atom transfer
71
phenyl rings. The lowest-energy absorption band of 1 peaks at a longer wavelength,
and with a much larger molar extinction coefficient, than that of the parent 20-
hydroxychalcone. Upon photoexcitation, all three compounds underwent ESIHT to
produce an excited tautomer that emitted intense Stokes-shifted fluorescence in the
longer wavelength region at 600–700 nm. The quantum yield of the tautomer
fluorescence of 1 was still low (Uf = 9.1 9 10-5) at 298 K, but was improved, and
the Uf value was the highest among 20HC and its derivatives. The Uf values of 1–3
were increased 10–30 fold on reducing temperature from 298 to 77 K. These
measurements revealed that 1–3 were very photo-stable, even in dilute solution,
because of the highly efficient four-cycle reaction process which involves
photoexcitation, ESIHT reaction, non-radiative decay, and thermal back hydrogen
atom transfer.
Acknowledgments This work was supported by a Grant-in-Aid for Scientific Research in a Priority
Area ‘‘New Frontiers in Photochromism’’ (no. 471) from the Ministry of Education, Culture, Sports,
Science, and Technology (MEXT) and a Grant-in-Aid for Scientific Research (B) (no. 23350075) from
the Japan Society for the Promotion of Science (JSPS).
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