Dicyanovinylcoumarin as a turn-on fluorescent sensor for cyanide ion

Article abstract of DOI:10.1016/j.jphotochem.2017.10.021

Dicyanovinylcoumarin chemosensors (4a, 4b and 4c) were designed and synthesized by the Knoevenagel condensation reaction. The electronic absorption and emission spectra of the dicyanovinylcoumarin isomers exhibited red shifted absorption and less emission as compared to their precursor's coumarin aldehydes. The sensing behavior of dicyanovinylcoumarin isomers were studied for different anions. In these dicyanovinylcoumarin isomers, the 7-substituted isomer shows high selectivity towards CN^? in the presence of other F^?, Cl^?, Br^?, I^?, ClO₄^?, HSO₄^? and NO₂^? ions. The effect of CN^? on the structure of dicyanovinylcoumarin isomer was studied by performing DFT calculations. The theoretical calculations show strong agreement with the experimental results. The detection limit for CN^? were found to be 1.14 × 10^?8 M for 7-substitued dicyanovinylcoumarin isomer 4b.

Full text of DOI:10.1016/j.jphotochem.2017.10.021

Accepted Manuscript
Title: Dicyanovinylcoumarin as a turn-on fluorescent sensor
for cyanide ion
Authors: T. Sheshashena Reddy, Myung-Seok Choi
PII:
S1010-6030(17)31286-8
DOI:
Reference:
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11-10-2017
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a
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Dicyanovinylcoumarin as a turn-on fluorescent sensor for cyanide ion
T. Sheshashena Reddy, Myung-Seok Choi*
Department of Materials Chemistry and Engineering,
Konkuk University, 120 neungdong-ro, Gwangjin-gu, Seoul, Korea.
Corresponding author.
*
E-mail address: mchoi@konkuk.ac.kr (Myung-Seok Choi)
Graphical Abstract
Highlights
➢
Synthesis of dicyanovinylcoumarins (4a, 4b and 4c) by Knoevenagel condensation
reaction.
➢
The dicyanovinylcoumarin (4b) shows turn-on fluorescence towards cyanide anion.
−
−8
➢
The dicyanovinylcoumarin 4b detection limit for CN was found to be 1.14 ×10 M.
1

Abstract: Dicyanovinylcoumarin chemosensors (4a, 4b and 4c) were designed and
synthesized by the Knoevenagel condensation reaction. The electronic absorption and emission
spectra of the dicyanovinylcoumarin isomers exhibited red shifted absorption and less emission
as compared to their precursor’s coumarin aldehydes. The sensing behavior of
dicyanovinylcoumarin isomers were studied for different anions. In these
−
dicyanovinylcoumarin isomers, the 7-substituted isomer shows high selectivity towards CN
−
−
−
−
−
−
−
−
in the presence of other F , Cl , Br , I , ClO
4
, HSO
4
and NO
2
ions. The effect of CN on
the structure of dicyanovinylcoumarin isomer was studied by performing DFT calculations.
The theoretical calculations show strong agreement with the experimental results. The
−
−8
detection limit for CN were found to be 1.14×10 M for 7-substitued dicyanovinylcoumarin
isomer 4b.
Keywords: Dicyanovinylcoumarin, fluorescence, sensor, Sonogashira cross–coupling reaction
and cyanide ion.
1
.
Introduction
The design and development of new chemosensors for the detection of anions is an area of
growing interest in supramolecular chemistry due to key roles of anions in chemical and
biological events [1-4]. The cyanide anion is extremely toxic to living organisms and is used
in various applications, such as gold mining, dyes and textiles, electroplating, synthetic
fertilizers, pesticides, automobiles, metallurgy and pharmaceuticals [5-8]. Hence, several
fluorescent chemosensors have been reported for the detection of cyanide anions during recent
years [9-12]. Coumarins are a group of natural compounds found in a variety of plant sources,
as well as widely studied functional materials [13]. Coumarin derivatives are widely used as
laser dyes, and they are attractive fluorescent molecules due to their extended spectral range,
strong fluorescence, large Stokes-shift, and high thermal and chemical stability [14-23].
Coumarin derivatives are widely used in various applications, including fluorescent probes,
laser dyes, organic light emitting diodes, solar cells, nonlinear optical chromophores and
2

biomedicines [24-30]. In recent years, chemosensors containing a dicyanovinyl group have
attracted considerable attention due to their applications in biology and anion sensing [31-38].
Jalal Isaad et al., reported water-soluble polymer containing coumarin-dicyanovinyl motif [39].
In water-soluble polymer containing coumarin-dicyanovinyl motif, dicyanovinyl group is
attached at C-3 carbon of coumarin ring. Water-soluble polymer fluorescence emission
-
maximum is at 581 nm. Upon addition of CN ion to the water-soluble polymer, its fluorescence
was decreased and blue shifted to 500 nm.
Generally, non-substituted parent coumarin (2-oxo-2H-chromene) is not fluorescent,
and optical properties of substituted coumarins depend highly on the type and position of the
substituent on the coumarin ring [40]. Coumarin derivatives substituted at the 7-position with
an electron donating group are highly fluorescent compared to 6 and 8-position substituted
coumarins [41]. Coumarin derivatives substituted at the 7-position with an electron donating
group (amino or hydroxyl groups) are highly fluorescent in the blue or green region [42]. In
this context, herein, we report the design and synthesis of three dicyanovinylcoumarin isomers
(4a, 4b and 4c) and their anion sensing ability.
2
2
. Experimental
.1. Materials
All the chemicals were purchased from commercial sources and used without further
1
13
purification. H NMR and C NMR spectra were performed on 600 MHz and 150 MHz Bruker
1
Ultra shield (Avance–III) Nano Bay spectrometer. All the spectra were recorded at 298K. H
NMR data are reported as follows: s: singlet, d: doublet, t: triplet, bs: broad singlet and coupling
1
13
constants, J, are given in Hz. Chemical shifts in H NMR and C NMR spectra were reported
in parts per million (ppm) with TMS (0 ppm) and CDCl (77.00) as standards. Mass
spectroscopy was performed on gas chromatography high resolution mass spectrometer, JMS-
00 (JEOL, Japan), 6890 series (Agilent, USA) (EI-MS). TLC analysis was carried out using
3
7
silica gel 60 F254 plates. UV–vis absorption spectra of all compounds were recorded in THF on
a Jasco V-670 UV–visible Spectrophotometer. Emission spectra were taken in a PerkinElmer
LS 55 fluorescence spectrometer. The excitation and emission slits were 2.5/2.5 nm for the
emission measurements. All the measurements were done at 298K. For fluorescence titrations,
−
−
−
−
−
−
−
−
the anion (F , Cl , Br , I , ClO
4
, HSO
4
, NO
2
and CN ) stock solutions were prepared in
−
3
THF in the order of 10 M. The dicyanovinylcoumarins (4a, 4b, 4c) stock solution was
3

prepared (C=1mM) in THF. Working solutions of dicyanovinylcoumarins (4a, 4b, 4c) and
anions were freshly prepared from stock solutions. Column chromatography was performed on
Merck silica gel (230-400 mesh). The anions were used in their tetrabutylammonium salts.
2
.2. Synthesis
General synthetic procedure for 3a/3b/3c: 6-Bromocoumarin (0.5 g, 2.23 mmol), (4–
ethynyl)benzaldehyde (0.29 g, 2.23mmol) were dissolved in THF-triethylamine (1:1, v/v, 120
mL) and the mixture was deaerated for 10 min with nitrogen bubbling and then Pd(PPh Cl
31 mg, 2mol%), PPh (23 mg, 4 mol%)and CuI (8 mg, 2mol%) were added. The solution was
3
)
2
2
(
3
deaerated for an additional 5 min; after that, reaction was left under nitrogen at 60 °C for 12h.
After completion of the reaction, the reaction mixture was cooled at room temperature and the
solvent was evaporated. The crude product was dissolved in CH
2
Cl and purified by a column
2
chromatography on a silica gel using (chloroform) as an eluent.
1
Synthesis of 3a. Yield, 61 % (0.37 g), H NMR (600 MHz, CDCl
3
) δ 10.04 (s, 1H, -CHO), 7.89
(d, J =8.25, 2H, aromatic), 7.70-7.68 (m, 5H, aromatic), 7.35 (d, J =9.17, 1H, aromatic), 6.48
1
3
(
d, J =9.55, 1H, aromatic). C (150 MHz, CDCl
34.89, 132.15, 131.81, 129.82, 127.88, 119.28, 117.80, 117.35, 117.30, 91.58, 88.79. MS
m/z) 275 (M+H).
3
) δ 192.67, 159.58, 153.76, 143.58, 135.70,
1
(
General synthetic procedure for dicyanovinylcoumarins 4a/4b/4c. 3a (0.2 g, 0.73 mmol) and
malononitrile (0.057 g, 0.875 mmol) were dissolved in 20 mL of ethanol with two drops of
piperidine. The reaction mixture refluxed for 4 h, and the solvent was removed under reduced
pressure. The crude product was dissolved in CH
2
Cl and purified by column chromatography
2
on a silica gel using (chloroform) as an eluent.
1
Synthesis of 4a. Yield 68 % (0.16 g), H NMR (600 MHz, CDCl
3
) δ 8.56 (s, 1H, - C=CH),
8
7
.08 (d, J =9.35 Hz, 1H, aromatic), 8.02-7.99 (m, 3H, aromatic), 7.82-7.80 (m, 3H, aromatic),
1
3
.48 (d, J =8.5 Hz, 1H, aromatic), 6.59 (d, J =10.2 Hz, 1H, aromatic). C (150 MHz, CDCl )
3
δ 160.194, 159.55, 153.84, 143.57, 134.93, 132.32, 131.88, 131.34, 130.89, 127.60, 119.30,
+
1
17.67, 117.37, 117.35, 114.29, 113.27, 92.42, 88.87, 82.16. HRMS (m/z): [M ] calculated for
C
21
H
10
N
2
O
2
: 322.0742; Found: 322.0746.
1
Synthesis of 3b. Yield 63 % (0.387 g), H NMR (600 MHz, CDCl
3
) δ 10.05(s, 1H, -CHO),
7
.91-7.89 (m, 2H, aromatic), 7.73-7.69 (m, 3H, aromatic), 7.50-7.43 (m, 3H, aromatic), 6.46
1
3
(
d, J =9.34, 1H, aromatic). C (150 MHz, CDCl
3
) δ 191.33, 160.22, 153.77, 142.65, 135.87,
4

1
32.32, 129.62, 128.53, 127.87, 127.75, 126.06, 119.77, 119.17, 117.32, 91.64, 91.61. HRMS
+
(
m/z): [M ] calculated for C18
H
10
O
3
: 274.0630; Found: 274.0629.
1
Synthesis of 4b. Yield 76 % (0.18 g), H NMR (600 MHz, CDCl
3
) δ 7.94-7.89(m, 2H,
aromatic), 7.77 (s, 1H, C=CH), 7.73-7.69 (m, 3H, aromatic), 7.52-7.44 (m, 3H, aromatic), 6.54
1
3
(
d, J =9.6, 1H, aromatic). C (150 MHz, CDCl
3
) δ 160.12, 158.36, 153.76, 142.60, 132.67,
1
9
30.77, 130.66, 128.83, 127.93, 127.80, 125.69, 119.86, 119.39, 117.50, 113.53, 112.44, 93.04,
1.31, 83.39. HRMS (m/z): [M+H] calculated for C21
H
11
N
2
O
2
: 323.0821; Found: 322.0825.
) δ 10.06(s, 1H, -CHO), 8.22
1
Synthesis of 3c. Yield 69 % (0.42 g), H NMR (600 MHz, CDCl
3
(d, J =9.60, 1H, aromatic), 7.93 (d, J =8.59,2H, aromatic), 7.75 (d, J =8.08,2H, aromatic), 7.56-
1
3
7
.50 (m, 2H, aromatic), 7.37-7.35(m, 1H, aromatic), 6.54 (d, J =9.60, 1H, aromatic). C (150
) δ 192.74, 159.55, 153.82, 141.68, 136.05, 132.50, 132.12, 129.77, 128.47,
27.34, 120.41, 119.55, 117.92, 117.73, 94.53, 88.35. MS (m/z) 275 (M+H).
MHz, CDCl
3
1
1
Synthesis of 4c. Yield 85 % (0.2 g), H NMR (600 MHz, DMSO-d
.381 (d, J =9.60, 1H, aromatic), 8.03 (d, J =8.4,2H, aromatic, phenyl), 7.94 (d, J =8.4,2H,
aromatic, phenyl), 7.708-7.646 (m, 2H, aromatic), 7.52 (d, J =7.6, 1H, aromatic), 6.62 (d, J
6
) δ 8.586(s, 1H, =CH),
8
1
3
=
10, 1H, aromatic). C (150 MHz, CDCl ) δ 160.202, 159.540, 153.837, 141.691, 132.657,
3
1
1
32.146, 131.778, 130.787, 128.52, 127.01, 120.292, 119.614, 118.062, 117.787, 114.263,
13.263, 94.559, 89.127, 82.566. MS (m/z) 322 (M+H).
3
3
. Results and Discussion
.1. Synthesis
The synthetic route of dicyanovinylcoumarins (4a, 4b and 4c) are shown in Scheme 1
and the structures are outlined in Chart 1. Bromophenols were reacted with malic acid in the
presence of sulfuric acid to give bromo coumarins (2a, 2b and 2c) [43-45]. Coumarin aldehydes
(3a, 3b and 3c) were synthesized by the Pd–catalyzed Sonogashira cross–coupling reaction of
bromocoumarins (2a, 2b and 2c) with 4–ethynylbenzaldehyde respectively [46]. Coumarin
1
13
aldehydes were characterized by H, C NMR (Figure S1-S12) spectra, and High-resolution
mass spectrometry (HRMS) techniques. Coumarin aldehydes 3b was synthesized by the
Sonogashira cross–coupling reaction of 7-bromocoumarin (2b) with 4–ethynylbenzaldehyde
using the catalyst Pd(PPh
3
)2, CuI, PPh in tetrahydrofuran (THF) solvent, triethylamine as base
3
at 60 °C for 12h, which resulted 3b in 63% yield. In the proton NMR spectrum of 3b, an
aldehyde proton show a singlet signal at 10.05 ppm and coumarin characteristic C-3 proton
5

show a doublet signal at 6.46 ppm indicates the formation 3b. Further conformation was
obtained from the mass spectroscopy. In the mass spectrum, the molecular ion peak observed
at 274 [M+] confirms the formation of 3b. Dicyanovinylcoumarins (4a, 4b and 4c) were
synthesized from its precursor aldehydes (3a, 3b and 3c) reacting with malononitrile in EtOH
1
13
solvent [47]. Dicyanovinylcoumarins were characterized by H, C NMR spectra (Figure S1-
S12), and High-resolution mass spectrometry (HRMS) techniques. In the proton NMR
spectrum of 4b, disappearance of an aldehyde proton at 10.05 ppm and the appearance of a
new singlet signal at 7.77 ppm confirms the formation of 4b. Further conformation was
1
3
obtained from C NMR and mass spectroscopy. In the mass spectrum, the molecular ion peak
observed at 322 [M+] confirms the formation of 4b.
3
.2. Photophysical properties
The electronic absorption and emission spectra of the dicyanovinylcoumarins (4a, 4b,
c) and its precursors the coumarinaldehydes (3a, 3b and 3c) were recorded in THF at room
4
temperature (Figure 1-3 and Table 1). The coumarinaldehyde 3b absorbs at 345 nm with a
shoulder at 360 nm attributed to the π–π* electronic transition, whereas 3a and 3c absorbs at
3
15 nm and 322 nm attributed to the π–π* electronic transition. The coumarinaldehyde 3b show
red shifted absorption compared to coumarinaldehydes 3a and 3c. The dicyanovinylcoumarin
b absorbs at 365 nm attributed to the π–π* electronic transition with a shoulder at 385 nm
4
may be attributed to the charge transfer. The dicyanovinylcoumarins 4a and 4c absorbs at 360
and 356 nm attributed to the π–π* electronic transition respectively. The dicyanovinylcoumarin
4
b show red shifted absorption compared to dicyanovinylcoumarins 4a and 4c. The
dicyanovinylcoumarins 4a, 4b and 4c exhibited weak florescence and an emission maximum
are located at 482, 447 and 436 nm respectively.
3
.3. Anion binding studies
The anion sensing ability of the dicyanovinylcoumarins 4a, 4b and 4c were investigated
−
−
−
−
−
−
by the fluorescence emission spectra, using various anions (F , Cl , Br , I , NO
2
, HSO
4
,
−
−
ClO
4
and CN ) in THF solvent. The anions were used in their tetrabutylammonium salts.
Among the dicyanovinylcoumarins (4a, 4b and 4c), a dicyanovinyl group substituted at the 7-
−
position of coumarin i.e., 4b shows turn-on fluorescence behavior in the presence of (F ) and
−
−
−
−
−
−
cyanide (CN ) ions, and no significant change in the presence of Cl , Br , I , NO
2
, HSO ,
4
6

−
and ClO
4
anions (Figures 4). However, dicyanovinyl group substituted at the 6 and 8-position
of coumarin, 4a and 4c, are not showing significant changes in the presence of anions (Figure
5
). This may be due to that the substituent at positions 6, 7 or 8 is a strong electron withdrawing
−
group. After gradual addition of the CN ions to the solution of 4b in THF, the emission
intensity of 4b at 450 nm gradually decreased and a new emission maximum observed at 386
−
nm (Figure 4b). After addition of CN ion, one isosbestic point was observed at 443 nm. The
−
addition of CN ion caused blue shift in emission maximum at 386 nm and fluorescence
−
increased (Figure 4b). The increase in the fluorescence intensity of 4b with the added CN ion
−
7
−6
2
follows a linear relationship between 2.30 ×10 M to 2.26 ×10 M (R =0.994). The detection
−
limit of dicyanovinylcoumarin 4b for CN ion was up to 11.4 nM, which is far lower than the
maximum level for cyanide in drinking water according to WHO guidelines [44].
The dicyanovinylcoumarin 4a show a weak turn-off fluorescence behavior in the presence of
F− and CN− anions (Figure 5a). After addition of F− and CN− anions to the
dicyanovinylcoumarin 4a, its emission maximum was blue shifted to 434 nm and 422 nm
−
−
−
respectively. However, no significant changes were observed in the presence of Cl , Br , I ,
NO anions (Figures 5a). The dicyanovinylcoumarin 4c show weak turn-
−, HSO −, and ClO −
on fluorescence behavior in the presence of F and CN anions. Whereas, no significant
2
4
4
−
−
−
−
−
−, HSO −
, and ClO −
anions (Figures 5b).
changes observed in the presence of Cl , Br , I , NO
2
4
4
−
−
After addition of F and CN anions to the dicyanovinylcoumarin 4c, its emission maximum
was blue shifted to 398 nm and 398 nm respectively. The sensing ability of the
−
dicyanovinylcoumarins towards CN anion is in the order of 4b>4c>4a.
1
3
.4. Sensing mechanism and H-NMR study
1
To gain further insight into the cyanide binding, H-NMR spectrum was recorded after the
1
addition of cyanide ion to the 4b. The comparison of H–NMR spectra of 4b before and after
addition of cyanide ion is shown in Figure 6, and it can be observed that the addition of cyanide
1
ion to the 4b results in a upfield shift of the Ha proton form 7.77 ppm to 4.38 ppm. The H–
−
NMR results clearly confirm that the Michael addition of CN on the C=C double bond yields
−
a new C-C single bond between CN and the α-carbon of the dicyanovinyl group. Michael
−
addition of CN caused the break of intramolecular charge transfer in the 4b and resulted in
“turn-on” fluorescence behavior.
3
.5. Theoretical Study
7

To understand the structure and electronic properties of dicyanovinylcoumarins 4a, 4b
and 4c, time-dependent density functional theory (TD–DFT) calculations were performed
using Gaussian 09 program. The TD–DFT calculations were performed by using Gaussian 09
program 6–31G** for C, N, and H at B3LYP level for the dicyanovinylcoumarins 4a, 4b and
4
c [48-52]. The frontier molecular orbitals of the 4a, 4b, 4c and 4b-CN are shown in figure
S13 and S14. In 4a, HOMO and LUMO are located on the coumarin ring and dicyanovinyl
group. In 4b, HOMO is primarily located on the coumarin ring and LUMO is majorly located
on the dicyanovinyl group. In 4b, slight charge transfer occurs from the coumarin ring oxygen
atom to dicyanovinyl group. In 4b-CN, frontier molecular orbitals HOMO and LUMO are
majorly localized on coumarin ring and it indicates that the breaking of conjugation and
intramolecular charge transfer between coumarin ring and dicyanovinyl group. In 4c, HOMO
and LUMO are located on the coumarin ring and dicyanovinyl group. The theoretical band gap
values of 3a-3c and 4a-4c calculated at B3LYP level are 3.50, 3.22, 3.61, 3.16, 3.09 and 3.24
eV respectively (Table 1). The optical band gap values of of 3a-3c and 4a-4c are 3.36, 3.31,
3
.39, 2.94, 2.91 and 2.98 eV respectively. The optical band gap values agree with theoretical
band gap values.
4
. Conclusions
Dicyanovinylcoumarins 4a, 4b and 4c were synthesized by the Knoevenagel
condensation reaction and characterized by NMR and mass spectra. The dicyanovinylcoumarin
4
b shows “turn-on” fluorescence behavior upon addition of fluoride and cyanide ions. The
−
sensing ability of the dicyanovinylcoumarins towards CN is in the order of 4b>4c>4a. The
−
−8
dicyanovinylcoumarin 4b detection limit for CN was found to be 1.14 ×10 M.
5
. Acknowledgements
This work was supported by the Technology Innovation Program (10047756,
Development tetra-pyrrole type for color, light-emitting, detecting devices) funded by the
Ministry of Trade, Industry and Energy.
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Figure Legends
Figure 1. Normalized absorption (solid lines) and fluorescence spectra (dotted lines) of 3a (λex
=328 nm) and 4a (λex =360 nm) in THF solvent at 25 °C.
Figure 2. Normalized absorption (solid lines) and fluorescence spectra (dotted lines) of 3b (λex
360 nm) and 4b (λex =365 nm) in THF solvent at 25 °C.
=
1
3

Figure 3. Normalized absorption (solid lines) and fluorescence spectra (dotted lines) of 3c (λex
340 nm) and 4c (λex =356 nm) in THF solvent at 25 °C.
=
Figure 4. a) Effect of various anions on fluorescence emission spectra of 4b in THF (λex =365
-6
nm) and b) fluorescence titration spectra of 4b (7.76 x 10 M) with TBACN in THF solvent at
5 °C.
2
-6
Figure 5. Effect of various anions on fluorescence emission spectra of a) 4a (7.76 x 10 M)
-6
(λ
ex =360 nm) and b) 4c (λex =356 nm) (7.76 x 10 M) in THF solvent at 25 °C.
1
Figure 6. H–NMR spectra of dicyanovinylcoumarin 4b (top) and 4b with TBACN (bottom)
in CDCl .
3
Chart 1. Structure of dicyanovinylcoumarins 4a, 4b and 4c.
Scheme 1. Synthesis of dicyanovinylcoumarins (4a, 4b and 4c).
1
4

Figure 1. Normalized absorption (solid lines) and fluorescence spectra (dotted lines) of 3a (λex
328 nm) and 4a (λex =360 nm) in THF solvent at 25 °C.
=
Figure 2. Normalized absorption (solid lines) and fluorescence spectra (dotted lines) of 3b (λex
360 nm) and 4b (λex =365 nm) in THF solvent at 25 °C.
=
1
5

Figure 3. Normalized absorption (solid lines) and fluorescence spectra (dotted lines) of 3c (λex
=340 nm) and 4c (λex =356 nm) in THF solvent at 25 °C.
a
b
Figure 4. a) Effect of various anions on fluorescence emission spectra of 4b in THF (λex =365
-6
nm) and b) fluorescence titration spectra of 4b (7.76 x 10 M) with TBACN in THF solvent at
5 °C.
2
1
6

a
b
-6
Figure 5. Effect of various anions on fluorescence emission spectra of a) 4a (7.76 x 10 M)
-6
(λ
ex =360 nm) and b) 4c (λex =356 nm) (7.76 x 10 M) in THF solvent at 25 °C.
δ (ppm)
1
Figure 6. H–NMR spectra of dicyanovinylcoumarin 4b (top) and 4b with TBACN (bottom)
in CDCl .
3
1
7

4
a
4b
4c
Chart 1. Structure of dicyanovinylcoumarins 4a, 4b and 4c.
i) Malic acid, H
2
SO
4
, 120°C, 6h; ii) 4-Ethynylbenzaldehyde, Pd(PPh
h, Reflux; iii) Malononitrile, Ethanol, Piperidine, reflux, 4h.
Scheme 1. Synthesis of dicyanovinylcoumarins (4a, 4b and 4c).
3
)
2
Cl
2
, CuI, THF, N atm,
2
8
Table 1. Photophysical properties of 3a–3c and 4a–4c.
1
8

Compounds
number
λ
max(nm)
λ
em(nm)
Stokes-shift
Δν (cm )
Optical
band gap
Theoretical band gap (eV)
-
1
HOMO (eV)
LUMO (eV)
g
E (eV)
(
eV)
3
3
a
315, 328
345, 360
321, 340
360
411
383
386
482
447
436
6077
1667
3504
7023
5025
5154
3.36
3.31
3.39
2.94
2.91
2.98
6.22
6.09
6.24
6.32
6.20
6.32
2.72
2.87
2.62
3.16
3.10
3.07
3.50
3.22
3.61
3.16
3.09
3.24
b
3
4
c
a
4
b
365
4
c
356
1
9