Influence of trimethoxy-substituted positions on fluorescence of heteroaryl chalcone derivatives

Article abstract of DOI:10.2478/s11696-011-0084-4

Three series of heteroaryl chalcones, (E)-1-(2-pyridyl)-3-(X)prop-2-en-1- one (Ia-Ic), (E)-1-(2- thienyl)-3-(X)prop-2-en-1-one (IIa-IIc), and (E)-1-(2-furyl)-3-(X)prop-2-en-1-one (IIIa-IIIc), where X = 2,4,5- trimethoxyphenyl (for series a), X = 2,4,6-tri

Full text of DOI:10.2478/s11696-011-0084-4

Chemical Papers 65 (6) 890–897 (2011)
DOI: 10.2478/s11696-011-0084-4
ORIGINAL PAPER
Influence of trimethoxy-substituted positions on fluorescence
of heteroaryl chalcone derivatives
a
b
aThitipone Suwunwong, Suchada Chantrapromma*, Hoong-Kun Fun
aDepartment of Chemistry and Centre of Excellence for Innovation in Chemistry, Faculty of Science,
Prince of Songkla University, Hat-Yai, Songkhla 90112, Thailand
bX-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia
Received 21 April 2011; Revised 6 June 2011; Accepted 15 June 2011
Three series of heteroaryl chalcones, (E)-1-(2-pyridyl)-3-(X)prop-2-en-1-one (Ia–Ic), (E)-1-(2-
thienyl)-3-(X)prop-2-en-1-one (IIa–IIc), and (E)-1-(2-furyl)-3-(X)prop-2-en-1-one (IIIa–IIIc), where
X = 2,4,5-trimethoxyphenyl (for series a), X = 2,4,6-trimethoxyphenyl (for series b), and X =
3,4,5-trimethoxyphenyl (for series c) were synthesised using basic catalysed aldol condensation and
characterised using 1H NMR and FT-IR spectroscopies. Compound IIa was also characterised by
single crystal X-ray analysis. The absorption and fluorescence emission spectra of these compounds
revealed that the absorption and fluorescence depended on the heterocycle rings and trimethoxy-
substituted phenyl rings linked to the enone system. The position of methoxy groups substantially
affected the fluorescent properties. Compounds Ia–IIIa containing the 2,4,5-trimethoxyphenyl moi-
ety exhibited the red-shift phenomenon and strong emission fluorescence.
c
ꢀ 2011 Institute of Chemistry, Slovak Academy of Sciences
Keywords: heteroaryl chalcones, fluorescence, aldol condensation, X-ray crystallography, trimethoxyphenyl
Introduction
the fluorescent property of the compounds and there
have been many reports on their fluorescent properties
(Knyazhansky et al., 1998; Sun et al., 2005). Our study
was focused on the influence of the different positions
of substituents on some heteroaryl chalcones, specifi-
cally pyridyl, thienyl, and furyl chalcones. In addition,
it has been found that electron-donating substituted
groups in the π-conjugate system can enhance the flu-
orescent properties of heteroaryl chalcones (Musil et
al., 2007). It is known that the OCH3 functional group
is mostly found in natural products and that these
compounds exhibited interesting applications not only
as far as their bioactivities (Salem & Werbovetz, 2005;
Sivakumar et al., 2010; Zangade et al., 2010) but also
where their fluorescent properties (Hirano et al., 2004;
Maiti et al., 1999) are concerned. Numerous applica-
tions of fluorescent materials and the fluorescence ca-
pability of the above compounds led us to synthesise
nine small molecules of heteroaryl chalcones (Ia–Ic,
IIa–IIc, and IIIa–IIIc) containing trimethoxyphenyl
Chalcones (1,3-diphenyl-2-propen-1-one derivati-
ves) (Schlangen et al., 2009; Sivakumar et al., 2009;
Sun & Cui, 2008) were synthesised using the base-
catalysed Claisen–Schmidt condensation reaction or
the aldol condensation reaction (Navarini et al., 2009).
They have many applications, involving biological ac-
tivities such as antibacterial (Nielsen et al., 2004), an-
tifungal (Batovska et al., 2007; López et al., 2001),
anti-inflammatory (Won et al., 2005), and anti-tumour
(Romagnoli et al., 2008) properties, as well as non-
linear optical (NLO) (Crasta et al., 2004) and elec-
troactive fluorescent properties (Jung et al., 2008).
Chalcones with the latter properties were used as flu-
orescent dyes (Jung et al., 2008), light-emitting diodes
(LEDs) (Fayed & Awad, 2004), fluorescent probes
(Xu et al., 2005), and fluorescent sensors (Niu et al.,
2006). The previous studies revealed that the hetero-
cyclic part in the fluorophore consistently increased
*Corresponding author, e-mail: suchada.c@psu.ac.th

T. Suwunwong et al./Chemical Papers 65 (6) 890–897 (2011)
891
2-acetylthiophene (0.35 mL, 2 mmol), a solution
of the respective trimethoxybenzaldehyde (2,4,5-tri-
methoxyphenyl for IIa, 2,4,6-trimethoxyphenyl for IIb,
and 3,4,5-trimethoxyphenyl for IIc) (0.40 g, 2 mmol)
in ethanol (20 mL), and 30 % aqueous NaOH (5 mL)
at 5◦C for about 3 h. A second recrystallisation from
hot acetone gave 0.45 g (1.48 mmol, 74 %) of IIa, 0.54
g (1.77 mmol, 89 %) of IIb, and 0.44 g (1.44 mmol, 72
%) of IIc.
Analogously, furyl chalcones (IIIa–IIIc) were pre-
pared from ethanol (15 mL), 2-acetylfuran (0.22 g,
2 mmol), a solution of the respective trimethoxyben-
zaldehyde (2,4,5-trimethoxyphenyl for the synthesis
of IIIa, 2,4,6-trimethoxyphenyl for IIIb, and 3,4,5-
trimethoxyphenyl for IIIc) (0.40 g, 2 mmol) in ethanol
(15 mL), and 20 % aqueous NaOH (5 mL). The mix-
ture was allowed to react at 5◦C for about 5 h. A
second recrystallisation from hot acetone gave 0.37 g
(1.27 mmol, 64 %) of IIIa, 0.44 g (1.51 mmol, 76 %)
of IIIb, and 0.44 g (1.51 mmol, 76 %) of IIIc.
O
O
O
NaOH
EtOH
R
CH3 + H
R
R'
R'
I–III
Fig. 1. Synthesis of heteroaryl chalcones I–III.
moiety located in various positions of the benzene
rings (Fig. 1) with both potential bioactivity and flu-
orescent properties. Attempts have been made to syn-
thesise and elucidate the effects of structure and the
positions substituted on the absorption and fluores-
cence properties of this class of compounds.
Experimental
Ethanol, acetone, and chloroform were purchased
from Merck Ltd. (Thailand) and were used as received.
Sodium hydroxide was purchased from Lab-Scan An-
alytical Sciences, Lab-Scan Asia. Co. (Thailand).
2-Acetylpyridine, 2-acethylthiophene, 2-acetylfuran,
and all trimethoxybenzaldehydes were obtained from
Fluka (Sigma–Aldrich Chemical Co., Thailand) and
were used without purification. IR spectra (KBr pel-
lets) were recorded in the range of 4000–400 cm−1
using a Perkin–Elmer FT-IR System Spectrum BX
spectrophotometer. 1H NMR spectra (300 MHz) were
recorded on a Bruker Ultra ShieldTM, using CDCl3 as
solvent and tetramethylsilane as the internal standard.
UV-VIS spectra were recorded on a SPECORD S100
(Analytik Jena AG, Germany). Fluorescence emission
spectra were recorded on a Perkin–Elmer LS 55 lumi-
nescence spectrometer at the ambient temperature.
Results and discussion
The heteroaryl chalcones were synthesised in high
yields (more than 60 %) by aldol condensation
(Fig. 1). The reaction time and the substituent R
(pyridyl, thienyl, and furyl) in the starting methyl ke-
tone play important roles in the product formation.
The yields and some physico-chemical properties of
all nine heteroaryl chalcones (photochemically stable
compounds) are given in Table 1.
The IR spectra of compounds I–III revealed the
stretching vibrations of aromatic C—H bonds at
about 3100–3000 cm−1 and C—H bonds of OCH3 at
∼ 2900 cm−1. The strong peak of stretching vib−ra1tions
—
of C O bonds was observed at 1700–1600 cm and
—
Synthesis of pyridyl chalcones (Ia–Ic), thienyl
chalcones (IIa–IIc), and furyl chalcones (IIIa–
IIIc)
—
stretching vibrations of aromatic C C bonds were
—
observed at 1515–1610 cm−1 (Table 2). In the 1H
NMR spectra, two signals of trans-arranged hydro-
gen atoms at δ 7.32–8.42 with coupling constant J
= 15.6–15.9 Hz, were observed. The spectra of 2,4,5-
trimethoxyphenyl derivatives showed three singlet sig-
nals of three OCH3 in ortho, para, and meta positions
at δ 3.95–3.91. As for 2,4,6-trimethoxyphenyl deriva-
tives, the spectra showed singlet peak of two equiva-
lent OCH3 in ortho positions at δ 3.94–3.91 and one
peak of OCH3 in para position at δ 3.88–3.86. Finally,
the spectra of 3,4,5-trimethoxyphenyl derivatives ex-
hibited singlet peak of two equivalent OCH3 in meta
positions at δ 3.92–3.91 and one peak of OCH3 in para
position at δ 3.95–3.93 (Table 2).
A mixture of ethanol (10 mL), 2-acetylpyridine
(0.20 mL, 2 mmol), the solution of the respec-
tive trimethoxybenzaldehyde (2,4,5-trimethoxyphenyl
for Ia, 2,4,6-trimethoxyphenyl for Ib, and 3,4,5-
trimethoxyphenyl for Ic) (0.40 g, 2 mmol) in ethanol
(20 mL), and 30 % aqueous NaOH (5 mL) was stirred
in an ice bath at 5◦C for about 4 h. The resulting
solid was collected by filtration, washed with distilled
water and dried to give an amorphous yellow-coloured
solid. This solid was dissolved in 80 mL of hot acetone.
The hot solution was filtered and allowed to cool to
room temperature. The yellow-coloured microcrystals
were collected by filtration. A second recrystallisation
from hot acetone gave 0.37 g (1.20 mmol, 61 %) of Ia,
0.42 g (1.40 mmol, 70 %) of Ib, and 0.40 g (1.32 mmol,
66 %) of Ic. Pyridyl chalcones (Ia–Ic) were kept under
N2 atmosphere in order to prevent their oxidation.
Thienyl chalcones (IIa–IIc) were synthesised in a
similar manner, using a mixture of ethanol (10 mL),
The UV-VIS spectra of the heteroaryl chalcones
exhibit characteristic K and R bands. The absorbance
bands in the UV region of 200–300 nm are consid-
ered to be the allowed π→π* transitions of the aro-
matic system and the forbidden n→π* transitions of
carbonyl and heterocyclic part (K band) and another
band in the region of 300–470 nm is the allowed π→π*

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T. Suwunwong et al./Chemical Papers 65 (6) 890–897 (2011)
Table 1. Characteristics of the compounds prepareda
Yield
%
M.p.
◦C
Compound
R
Rꢀ
Appearance
Ia
Ib
Ic
2,4,5-OCH3
2,4,6-OCH3
3,4,5-OCH3
61
70
66
155–156
119–120
161–162
Yellow
Pale yellow
Pale yellow
2-Pyridyl
2-Thienyl
2-Furyl
IIa
IIb
IIc
2,4,5-OCH3
2,4,6-OCH3
3,4,5-OCH3
74
89
72
128–129
108–109
147–148
Yellow
Pale yellow
Pale yellow
IIIa
IIIb
IIIc
2,4,5-OCH3
2,4,6-OCH3
3,4,5-OCH3
64
76
76
83–84
117–118
117–118
Yellow
Pale yellow
Pale yellow
a) Apart from Ia, all the compounds have already been described in the literature. For Ib, see Cordaro et al. (2002), Suzuki et
al. (1995a, 1995b); for Ic, see Prasad et al. (2007a, 2008); for IIa, see Fun et al. (2008); for IIb, see Lavrushin et al. (1961, 1962),
Tsukerman et al. (1967, 1969); for IIc, see Dudeja et al. (1993), Katiyar et al. (1974), Lawrence et al. (2000), Roman (2004), Prasad
et al. (2007b), Lin et al. (2007), Lin and Chuang (2007), Suwunwong et al. (2009), Ramesh et al. (2009a, 2009b, 2010), Dulawat et
al. (2010), Ramesh and Rao (2010); for IIIa, see Nepali et al. (2011); for IIIb, see Fun et al. (2010a), Werts et al. (2005); for IIIc,
see Fun et al. (2010b), Nepali et al. (2011), Singhal and Mishra (1985), Singhal and Misra (1986).
Table 2. Spectral characterisation of heteroaryl chalcones I–III
Compound
Spectral dataa
IR, ν˜/cm−1: 3081 (m, CHar), 2937–2960 (s, CH3), 1638 (s, C O), 1576–1591 (s, C C), 1272–1317 (s, C—N),
—
—
—
—
Ia
1194–1208 (s, C—O)
1H NMR (CDCl3), δ: 8.76 (dd, 1H, J = 0.6 Hz, J = 4.8 Hz, Hpyr), 8.26 (d, 1H, J = 15.9 Hz, Htrans), 8.14 (dd, 1H,
J = 0.9 Hz, J = 6.6 Hz, Hpyr), 8.11 (d, 1H, J = 15.9 Hz, Htrans), 7.92 (dt, 1H, J = 1.8 Hz, J = 7.8 Hz, Hpyr), 7.54
(dt, 2H, J = 0.9 Hz, J = 4.8 Hz, Hpyr), 7.28–6.59 (s, 2H, Har), 3.95–3.91 (s, 9H, OCH3)
—
—
Ib
Ic
IR, ν˜/cm−1: 2972 (m, CHar), 1669 (s, C O), 1576 (s, C C), 1200 (s, C—N), 1123 (s, C—O), 810 (w, C—H)
—
—
1H NMR (CDCl3), δ: 8.73 (d, 1H, J = 4.5 Hz, Hpyr), 8.42–8.31 (d, 2H, J = 16.2 Hz, Htrans), 8.10 (d, 1H, J = 7.8
Hz, Hpyr), 7.89–7.49 (dt, 2H, J = 1.5 Hz, J = 7.8 Hz, Hpyr), 6.18 (s, 2H, Har), 3.95–3.88 (9H, s, OCH3)
IR, ν˜/cm−1: 2998–2976 (m, CHar), 2943 (s, CH3), 1668 (s, C O), 1610–1579 (s, C C), 1322–1290 (s, C—N), 1125
—
—
—
—
(s, C—O)
1H NMR (CDCl3), δ: 8.76 (dd, 1H, J = 0.9 Hz, J = 4.8 Hz, Hpyr), 8.20 (dd, 1H, J = 0.9 Hz, J = 6.9 Hz, Hpyr),
8.18 (d, 1H, J = 15.9 Hz, Htrans), 7.90 (dt, 1H, J = 1.8 Hz, J = 7.5 Hz, Hpyr), 7.88 (d, 1H, J = 15.9 Hz, Htrans),
7.50 (dt, 1H, J = 0.9 Hz, J = 4.8 Hz, Hpyr), 6.96 (s, 2H, Har), 3.94–3.91 (9H, s, OCH3)
IR, ν˜/cm−1: 3107 (m, CHar), 2932 (s, CH3) 1638–1611 (s, C O), 1583–1509 (s, C C), 1206 (s, C—O)
—
—
—
—
IIa
1H NMR (CDCl3), δ: 8.12 (d, 1H, J = 15.6 Hz, Htrans), 7.86 (dd, 1H, J = 0.9 Hz, J = 3.9 Hz, Hth), 7.65 (dd, 1H,
J = 0.9 Hz, J = 4.8 Hz, Hth), 7.39 (d, 1H, J = 15.6 Hz, Htrans), 7.17 (t, 1H, J = 3.9 Hz, Hth), 7.12–6.53 (2H, s,
Har), 3.95–3.91 (9H, s, OCH3)
—
—
IIb
IIc
IR, ν˜/cm−1: 2937 (m, CHar), 1638 (s, C O), 1610–1570 (s, C C), 1204 (s, C—O), 1124 (s, C—S)
—
—
1H NMR (CDCl3), δ: 8.28 (d, 1H, J = 15.6, Htrans), 7.81 (d, 1H, J = 5.1 Hz, Hth), 7.79 (d, 1H, J = 15.6, Htrans),
7.60 (d, 1H, J = 5.1 Hz, Hth), 7.15 (t, 1H, J = 5.1 Hz, Hth), 6.14 (s, 2H, Har), 3.92–3.86 (9H, s, OCH3).
IR, ν˜/cm−1: 3086–2975 (m, CHar), 2943 (s, CH3), 1646 (s, C O), 1598–1579 (s, C C), 1125 (s, C—O), 1004–979
—
—
—
—
(s, C—H)
1H NMR (CDCl3), δ: 7.90 (dd, 1H, J = 0.9 Hz, J = 3.9 Hz, Hth), 7.79 (d, 1H, J = 15.6 Hz, Htrans), 7.70 (dd, 1H,
J = 0.9 Hz, J = 3.9 Hz, Hth), 7.32 (d, 1H, J = 15.6 Hz, Htrans), 7.21 (t, 1H, J = 3.9 Hz, Hth), 6.88 (s, 2H, Har),
3.95–3.92 (9H, s, OCH3)
IR, ν˜/cm−1: 3404 (m, CHar), 2985 (s, CH3), 1642 (s, C O), 1585–1515 (s, C C), 1217 (s, C—O), 1040–1026 (s,
—
—
—
—
IIIa
C—H)
1H NMR (CDCl3), δ: 8.19 (d, 1H, J = 15.6 Hz, Htrans), 7.64 (d, 1H, J = 3.9 Hz, Hfur), 7.39 (d, 1H, J = 15.6 Hz,
Htrans), 7.30 (d, 1H, J = 3.9 Hz, Hfur), 7.14–6.53 (s, 2H, Har), 6.58 (t, 1H, J = 3.9 Hz, Hfur), 3.95–3.92 (s, 9H,
OCH3)
—
—
IIIb
IIIc
IR, ν˜/cm−1: 3001–2977 (m, CHar), 2946 (s, CH3), 1654 (s, C O), 1582 (s, C C), 1123 (s, C—O), 1064–1011 (s,
—
—
C—H)
1H NMR (CDCl3), δ: 8.24 (d, 1H, J = 15.6 Hz, Htrans), 7.74 (d, 1H, J = 15.9 Hz, Htrans), 7.67 (d, 1H, J = 3.9 Hz,
Hfur), 7.25 (d, 1H, J = 3.9 Hz, Hfur), 6.60 (t, 1H, J = 1.8Hz, Hfur), 6.17 (s, 2H, Har), 3.94–3.88 (s, 9H, OCH3)
IR, ν˜/cm−1: 3001–2977 (m, CHar), 2946 (s, CH3), 1654 (s, C O), 1582 (s, C C), 1123 (s, C—O), 1064–1011 (s,
—
—
—
—
C—H)
1H NMR (CDCl3), δ: 8.24 (d, 1H, J = 15.6 Hz, Htrans), 7.74 (d, 1H, J = 15.9 Hz, Htrans), 7.67 (d, 1H, J = 3.9 Hz,
Hfur), 7.25 (d, 1H, J = 3.9 Hz, Hfur), 6.60 (t, 1H, J = 1.8Hz, Hfur), 6.17 (s, 2H, Har), 3.94–3.88 (s, 9H, OCH3)
a) The IR bands are denoted as m – medium; s – strong; aromatic moieties are denoted as ar – aryl, fur – furyl, th – thienyl.

T. Suwunwong et al./Chemical Papers 65 (6) 890–897 (2011)
893
Table 3. Absorption maxima (λmax) and molar absorptivity (ε) of chalcones I–III (5 µM in chloroform)
K band
R band
Compound
λmax/nm
ε/(m2 mol−1
)
λmax/nm
ε/(m2 mol−1
)
Ia
Ib
276
248
263
248
273
267
264
245
312
260
244
5.8508
1.3540
1.3520
1.9600
3.8800
1.4320
1.1240
0.6096
0.5740
1.6900
1.7000
403
372
2.4828
1.2800
Ic
IIa
IIb
IIc
IIIa
355
395
368
350
396
2.3420
2.3600
0.6580
1.5960
0.7984
IIIb
IIIc
365
352
1.5343
1.3020
0.3
0.25
0.2
Table 4. Fluorescence spectral data (excitation (λex), absorp-
tion (λabs), and emission (λem) maxima, fluorescent
quantum yield (ΦF))a,b and Stokes shift of heteroaryl
chalcones I–III (5 µM in chloroform)
1
5
2
0.15
0.1
Compound λex/nm λabs/nm λem/nm
ΦF Stokes shift/nm
8
3
4
Ia
Ib
Ic
IIa
IIb
IIc
IIIa
IIIb
IIIc
401
350
377
394
354
374
394
352
372
403
372
355
395
368
350
396
365
352
530
453
502
504
433
485
501
431
481
0.18
< 0.01
0.05
0.28
< 0.01
0.06
0.20
< 0.01
0.08
127
81
147
109
65
135
105
66
0.05
7
9
6
0
200
250
300
350
400
450
500
Wavelength/nm
Fig. 2. UV-VIS absorption spectra of heteroaryl chalcones I–
III (5 µM in chloroform): Ia (1), IIa (2), Ic (3), IIIc
(4), IIIb (5), Ib (6), IIb (7), IIc (8), IIIa (9).
129
a) The emission wavelength was set at 485 nm for excitation
spectra study and the excitation wavelength was set at 380 nm
for emission spectra study; b) coumarin 1 (in ethanol, ΦF
0.73) was used as the standard for ΦF measurements.
=
transition (R band) with intra-molecular charge trans-
fer (ICT) to the enone system (Zhang et al., 2008;
Gaber et al., 2008) as shown in Fig. 2. The molar
absorptivity (ε) given in Table 3 was used to com-
pare the spectra of different compounds because it was
used to consider an intrinsic property of species (Per-
cino et al., 2011). It was found that the K band of
compound Ia displayed a hyperchromic shift. More-
over, the strongest absorbance of the ICT band (R
band) was also observed in Ia. The absorption maxima
(λmax) of the ICT band of pyridyl chalcones (Ia–Ic)
present small red-shift phenomena around 4–10 nm
when compared with those of thienyl (IIa–IIc) and
furyl chalcones (IIIa–IIIc) (Table 3, Fig. 2). These are
attributed to the capacity of the pyridine moiety to be
more aromatic than that of furan and thiophene. Het-
eroaryl chalcones Ia–IIIa with 2,4,5-trimethoxyphenyl
chromophore (OCH3 groups located in ortho, meta,
and para positions) exhibit λmax of the ICT transition
which is shifted to the longer wavelength (red-shift) by
ca 30 nm when compared with those containing 2,4,6-
trimethoxyphenyl (OCH3 groups located in ortho, or-
tho, and para positions) and by ca 50 nm when com-
pared with those containing 3,4,5-trimethoxyphenyl
(OCH3 groups located in meta, meta, and para po-
sitions), as shown in Table 3. These shifts can be in-
terpreted by the capability of electrons from the aux-
ochrome OCH3 group in the ortho and para positions
of the enone system to yield a more efficient resonance
effect than the electrons in the meta position. With
2,4,6-trimethoxyphenyl moiety, π-conjugated delocal-
isation was obstructed by the steric effect of two OCH3
groups in ortho positions.
The absorption spectra of the ICT band (Fig. 2) of
all heteroaryl chalcones exhibited λmax in the range of
300–470 nm. The maximum absorbance was observed
for compound Ia. As a consequence, the excitation
wavelengths were selected and set at 380 nm in or-
der to study their emission spectra. Fig. 3 presents
the fluorescence emission spectra of heteroaryl chal-
cones I–III in chloroform solution (5 µM). The max-
imum emission was observed for compound IIa. The
comparison of the emission of these compounds can
be interpreted as that the emission maximum wave-
length (λem) of pyridyl chalcones exhibits red-shift

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T. Suwunwong et al./Chemical Papers 65 (6) 890–897 (2011)
50
B
700
600
500
400
300
200
100
0
A
1
40
7
8
30
2
20
3
9
10
6
5
4
0
400
500
600
700
800
400
500
600
700
Wavelength/nm
Wavelength/nm
Fig. 3. Fluorescence emission spectra of heteroaryl chalcones (5 µM in chloroform) excited at 380 nm at room temperature:
compounds IIa (1), Ia (2), IIIa (3), IIc (4), IIIc (5), Ic (6) using slit width 4.5 (A) and compounds IIb (7), Ib (8), IIIb (9)
using slit width 10 (B).
around 15–30 nm when compared with that of thienyl
and furyl chalcones (Table 4). This corresponds to the
red-shift in absorption spectra caused by the pyri-
dine moiety with the most distinctive aromaticity.
However, the λem of thienyl and furyl chalcones hav-
ing the same Rꢀ group are almost comparable. The
electron-donating methoxy groups in different posi-
tions also play an important role on the fluorescent
properties of these heteroaryl chalcones. Compounds
Ia–IIIa which contain the 2,4,5-trimethoxyphenyl moi-
ety exhibit strong fluorescence emissions in compari-
son with other compounds which have methoxy sub-
stituents in different positions. It can be seen that the
λem of Ia–IIIa were red-shifted by 70 nm and 20 nm as
compared with the λem of Ib–IIIb and Ic–IIIc, respec-
tively (Fig. 3, Table 4). The shifts were caused by the
electron-donating OCH3 in ortho and meta positions
which increased their fluorescence emission intensity
and λem. By contrast, heteroaryl chalcones containing
2,4,6-trimethoxyphenyl exhibited the lowest emission
intensity (in the decreasing order IIIb, Ib, IIb), and λem
which were caused by the steric effect of the two OCH3
groups in ortho position which decreased the elec-
tron delocalisation of heteroaryl chalcones. A previous
work (Fahrni et al., 2003) concludes that Stokes shift
is usually associated with the attached substituent
groups and it is enhanced in compounds with a strong
electron-donating group, as was also indicated in this
work. It was found that the compounds containing
3,4,5-trimethoxyphenyl (Ic–IIIc) showed the highest
Stokes shift values (Table 4). The fluorescent quan-
tum yields of heteroaryl chalcones I–III in chloroform
using coumarin 1 (ΦF = 0.73 in EtOH) as the stan-
dard sample are presented in Table 4. The following
equation was used to calculate the fluorescent quan-
tum yield (Φ) of each compound:
where subscripts ST and X denote the standard and
test respectively, S is the integrated emission band
area, A is the absorbance at the excited wavelength
and n is the solvent refractive index (Rhys Williams et
al., 1983). It was found that thienyl chalcones showed
higher values of ΦF in comparison with those of furyl
and pyridyl chalcones (Table 4). However, the ΦF val-
ues of these trimethoxy-substituted heteroaryl chal-
cones are lower than those of the dimethylamino-
substituted heteroaryl chalcones (Gaber et al., 2008).
It is important to note that thiophene moiety is a bet-
ter electron-donor than furan and pyridine and it is
also a good fluorophore in the molecule, exhibiting flu-
orescence even in diluted solutions (Sun & Cui, 2009).
Clearly, compounds Ia–IIIa showed higher values of
ΦF compared with Ib–IIIb and Ic–IIIc.
The crystal structures of heteroaryl chalcones IIc
(Suwunwong et al., 2009), IIIb (Fun et al., 2010a),
and IIIc (Fun et al., 2010b) have already been re-
ported. Crystallographic data of IIa were collected us-
ing the Oxford Cryosystem Cobra low-temperature at-
tached to a Bruker Apex2 CCD diffractometer with a
graphite monochromated Mo Kα radiation at a detec-
tor distance of 5 cm using APEX2 software (Bruker
AXS, 2005). The data were reduced using the Bruker
SAINT program (Siemens AXS, 1998) and the empir-
ical absorption corrections were performed using the
SADABS program (Sheldrick, 2003). The structures
were solved by direct methods and refined by least-
squares using the Bruker SHELXTL software package
(Bruker AXS, 2001). All non-hydrogen atoms were re-
fined anisotropically. The hydroxyl H atoms were lo-
cated from the difference maps and were isotropically
refined. The remaining H atoms were placed in calcu-
lated positions C—H distances in the range of 0.93–
˚
0.98 A after examining their positions in the difference
map. The Uiso values were constrained as 1.5Ueq of
the carrier atoms for methyl H atoms and 1.2Ueq for
hydroxyl and the other remaining H atoms. The final
ꢀ
ꢁ ꢀ
ꢁ ꢀ
ꢁ
SX
AST
AX
nX2
n2ST
ΦX = ΦST
(1)
SST

T. Suwunwong et al./Chemical Papers 65 (6) 890–897 (2011)
895
Table 5. Crystallographic dataa and structure refinement for
compound IIa
bridge Crystallographic Data Centre, CCDC No.
739791. Copies of this information may be ob-
tained free of charge from The Director, CCDC,
12 Union Road, Cambridge CB2 1EZ, UK (fax:
0044-1223-336-033; e-mail: deposit@ccdc.cam.ac.uk or
http://www.ccdc.cam.ac.uk).
Empirical formula
Formula mass
C20H34O2
306.47
Temperature, T (K)
Crystal system, space group
100(1)
Monoclinic, P21/c
7.4877(2)
7.9781(2)
24.3557(6)
90
˚
a (A)
˚
b (A)
Conclusions
˚
c (A)
α (◦)
β (◦)
γ (◦)
The series of heteroaryl chalcones containing three
methoxy groups (2,4,5-, 2,4,6-, and 3,4,5-trimethoxy
derivatives) were synthesised by the basic catalysed
aldol condensation. Their structures were determined
by IR and 1H NMR spectra. By way of representa-
tion, the crystal structure of compound IIa was de-
termined by X-ray diffraction analysis. The results re-
vealed that heterocyclic groups had a significant effect
on the intensities of the emission spectra. In this re-
spect, thienyl is a better fluorophore than the furyl
and pyridyl moieties. It was also found that 2,4,5-
trimethoxy derivatives exhibited the most intense flu-
orescence spectra in chloroform solution. Compounds
Ia, IIa, and IIIa showed intense fluorescence spectra
and emitted green-yellow light at 530 nm, 504 nm,
and 501 nm, respectively. Heteroaryl chalcones with
trimethoxyphenyl moiety in ortho, meta, and para po-
sitions can exhibit strong fluorescent properties as in-
dicated by the fluorescent quantum yield values of Ia,
IIa, and IIIa (0.18, 0.28, and 0.20, respectively) which
are higher than those of the other compounds in the
study.
96.838(2)
90
1444.60(6)
4
1.399
0.237
3
˚
Unit-cell volume, V (A )
Formula per unit cell, Z
Density, Dcalcd (g cm−3
)
Absorption coefficient, µ (mm−1
Crystal size (mm)
)
0.56 × 0.34 × 0.25
Reflections collected/unique
Final R indices
17551/4502
R1 = 0.0677, wR2 = 0.1454
a) Standard deviations in parentheses.
Acknowledgements. The authors thank the Centre of Excel-
lence for Innovation in Chemistry (PERCH-CIC), Commis-
sion on Higher Education, Ministry of Education, Thailand
for financial support. The authors also thank the Thailand Re-
search Fund (Grant No. RSA 5280033), Prince of Songkla Uni-
versity and Universiti Sains Malaysia for the Research Univer-
sity Grant No. 1001/PFIZIK/811160. TS wishes to thank Dr.
Nawong Boonnak for his useful discussions.
Fig. 4. ORTEP plot of compound IIa.
refinement converged well. Materials for publication
were prepared using SHELXTL (Bruker AXS, 2001)
and PLATON (Spek, 2003) software. The molecular
structure of compound IIa is shown in Fig. 4. The ba-
sic crystallographic data are summarised in Table 5.
The molecule is slightly twisted and the thio-
phene ring is disordered over two positions. The di-
hedral angles between the thiophene rings, (S1/C10—
C13) and (S1X/C10/C11X—C13X), with the 2,4,5-
trimethoxyphenyl ring are 13.7(2)◦ and 10.0(17)◦, re-
spectively. The three methoxy groups have two differ-
ent conformations, two are co-planar (torsion angles
C14—O1—C2—C1 = −1.4(3)◦ and C15—O2—C3—
C4 = 2.8(3)◦) and one is slightly twisted (torsion angle
C16—O3—C5—C4 = −16.7(3)◦) with the attached
benzene ring.
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