Synthesis of new bis[1-(thiophenyl)propynones] as potential organic dyes for colorless luminescent solar concentrators (LSCs)

Article abstract of DOI:10.1016/j.dyepig.2019.108100

New luminophores having different aryl nuclei and propynones moieties have been obtained via Sonogashira reactions. Their optical properties were evaluated and indicated that carbonyl groups are responsible for significant bathochromic effects and high St

Full text of DOI:10.1016/j.dyepig.2019.108100

Dyes and Pigments xxx (xxxx) xxx
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Synthesis of new bis[1-(thiophenyl)propynones] as potential organic dyes
for colorless luminescent solar concentrators (LSCs)
Gianluigi Albano, Tony Colli, Luigi Nucci, Rima Charaf, Tarita Biver, Andrea Pucci,
Laura Antonella Aronica*
�
Dipartimento di Chimica e Chimica Industriale, Universita di Pisa, Via Giuseppe Moruzzi 13, 56124, Pisa, Italy
A R T I C L E I N F O
A B S T R A C T
Keywords:
New luminophores having different aryl nuclei and propynones moieties have been obtained via Sonogashira
reactions. Their optical properties were evaluated and indicated that carbonyl groups are responsible for sig-
nificant bathochromic effects and high Stokes shifts. The insertion of -OMe groups on the central benzene unit
gives to the fluorophore high optical efficiency (7.7%) when homogeneously dispersed in a poly(cyclohexyl
methacrylate) (PCMA) film and connected to a PV cell.
Luminescent solar concentrator
Organic dye
Solvatochromism
Sonogashira reaction
1. Introduction
on the edge of the collector thus increasing the efficiency of the system.
Fluorophores are the driving force for light concentration in LSCs
Climate changes due to pollution, together with the continuous in-
crease in energy demand, have pushed research towards the use of
renewable sources such as solar energy which is free and ubiquitous, and
which utilization does not imply carbon dioxide emissions in the at-
mosphere. The exploitation of solar energy can take place by means of
photo-thermal, photochemical or photovoltaic technology. The field of
greatest interest concerns the photovoltaic (PV) sector, which regards
with direct conversion of electromagnetic radiation into electrical en-
ergy. Solar cells produce a quantity of electrical energy, proportional to
the total power of the absorbed light [1]. Hence, if the intensity of the
incident light is increased, a linear response in energy production will be
observed: this is the principle on which geometric solar concentrators
are based [2]. However, this technology has some disadvantages such as
a low efficiency in diffused light conditions and the need of a good
system of dispersion of excess heat due to unconverted energy [3].
To compensate for the defects of optical concentrators, luminescent
solar concentrators (LSCs) were developed [4]. Since 1982 when Her-
mann used for the first time this term [5], the literature on LSCs has been
rapidly enriched with publications [6–11] and industrial patents
[12–16]. LSCs are optical devices consisting of a polymeric or glass
matrix and a suitable dye dispersed within the matrix itself. The dye has
the function of absorbing the incident solar radiation and re-emitting
light radiation at longer wavelengths by fluorescence. The emitted
light is trapped within the material and concentrated towards a solar cell
cells. An effective luminophore must meet different requirements: broad
spectral absorption, good matching between the dye emitted spectrum
and the external quantum efficiency of the PV-cell (generally in the red
zone), large Stokes shift (no or low overlap in absorption and emission
spectra), high luminescent efficiency (quantum yield, QY) and thermal-
and photo-stability. Different luminophores have been investigated:
quantum dots [17], lanthanide-based materials [18] and organic dyes
[19], i.e. organic molecules featured by an extended planar π–system.
Respect to the two first classes of compounds, organic dyes are usually
less toxic and their optical properties can be modulated by changing
portions of the carbon backbone or introducing specific functional
groups. Dyes proposed for LSCs applications are coumarins, rhodamines,
pyrromethane 580, naphthalimides, perylenebisimides, bipyridines,
dicarbocyanins, dicyanomethylenes, oxazines, and phthalocyanines
[20–33]. Among all these classes, only a very limited number of dyes
have resulted in being suitable for LSCs. Therefore, a continuous
investigation towards really efficient luminophores is in progress. In this
context, the development of a simple synthetic methodology for
obtaining π-conjugated variously functionalized systems could pave the
way to new luminophores. Moreover, the application of LSC as archi-
tectural windows has also triggered interest in visible transparent fluo-
rophores, which were found to provide acceptable optical efficiencies
with a negligible degree of colored tinting [34]. Notwithstanding this
exciting approach to harvesting solar energy for the building integrated
* Corresponding author.
E-mail address: laura.antonella.aronica@unipi.it (L.A. Aronica).
https://doi.org/10.1016/j.dyepig.2019.108100
Received 25 October 2019; Received in revised form 22 November 2019; Accepted 2 December 2019
Available online 5 December 2019
0143-7208/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Gianluigi Albano, Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2019.108100

G. Albano et al.
Dyes and Pigments xxx (xxxx) xxx
photovoltaics, challenges are still open to enhance the ultimate LSC
optical efficiency [10].
mg, 1.0 mmol), 3-methylthiophene-2-carbonyl chloride (4b) (402 mg,
2.5 mmol), PdCl2(PPh3)2 (14 mg, 0.02 mmol) and Et3N (20 mL) were
mixed together. The crude product was purified through column chro-
matography (SiO2, n-hexane/CH2Cl2 1:5), giving 277 mg (yield 74%) of
3,3’-(1,4-phenylene)bis(1-(3-methylthiophen-2-yl)prop-2-yn-1-one)
(1b) as yellow solid. Mp: 195–198 �C. 1H NMR (400 MHz, CDCl3), δ
(ppm): 2.66 (6H, s); 6.98 (2H, d, J ¼ 4.4 Hz); 7.56 (2H, d, J ¼ 4.4 Hz);
7.66 (4H, s). 13C NMR (100 MHz, CDCl3), δ (ppm): 16.67; 89.51; 90.06;
122.35; 132.86 (2C); 132.97; 133.37; 137.62; 146.56; 169.41. LC-MS
APCI (þ): calcd for C22H14O2S2: 374.04; found m/z [MþH]þ: 375.1.
Anal. calcd for C22H14O2S2: C, 70.56; H, 3.77; S, 17.13; found: C, 70.49;
H, 3.82; S, 17.13.
Here we report our principal results regarding Sonogashira based
synthesis of new bis[1-(thiophenyl)propynones] derivatives and the
evaluation of their optical properties also in terms of transparent solar
collectors for PV.
2. Experimental section
2.1. Materials and apparatus
Solvents were purified by conventional methods, distilled and stored
over activated molecular sieves under argon. Starting substrates
thiophene-2-carbonyl chloride (4a), 3-methylthiophene-2-carbonyl
chloride (4b) and 3-chlorothiophene-2-carbonyl chloride (4c) were
purchased from Sigma Aldrich and used as received. All the other
chemicals were purchased from commercial sources and used as
received without purification. All the operations under inert atmosphere
were carried out using standard Schlenk techniques and employing
dried nitrogen. For all reactions, conversion was monitored by thin-layer
chromatography (TLC) analysis on pre-coated silica gel plates ALU-
GRAM® Xtra SIL G/UV254 (0.2 mm) purchased from VWR Macherey-
Nagel. Column chromatographies were performed with Fluka silica
2.2.3. 3,3’-(1,4-Phenylene)bis(1-(3-chlorothiophen-2-yl)prop-2-yn-1-one)
(1c)
According to the general procedure, 1,4-diethynylbenzene (3) (126
mg, 1.0 mmol), 3-chlorothiophene-2-carbonyl chloride (4c) (453 mg,
2.5 mmol), PdCl2(PPh3)2 (14 mg, 0.02 mmol) and Et3N (20 mL) were
mixed together. The crude product was purified through column chro-
matography (SiO2, n-hexane/CH2Cl2 1:5), giving 208 mg (yield 50%) of
3,3’-(1,4-phenylene)bis(1-(3-chlorothiophen-2-yl)prop-2-yn-1-one)
(1c) as light yellow solid. Mp: 215–218 �C.1H NMR (400 MHz,
DMSO‑d6), δ (ppm): 7.39 (2H, d, J ¼ 5.2 Hz); 7.89 (4H, s); 8.25 (2H, d, J
¼ 5.2 Hz). 13C NMR (100 MHz, DMSO‑d6), δ (ppm): 89.38; 91.90;
122.05; 130.90; 131.94; 133.87 (2C); 135.99; 136.76; 167.09. LC-MS
APCI (þ): calcd for C20H8Cl2O2S2: 413.93; found m/z [MþH]þ: 415.9.
Anal. calcd for C20H8Cl2O2S2: C, 57.84; H, 1.94; S, 15.44; found: C,
58.02; H, 1.89; S, 15.43.
gel, pore size 60 Å, 70–230 mesh, 63–200
μ
m. 1H NMR and 13C NMR
spectra were recorded at room temperature in CDCl3 or DMSO‑d6 so-
lution with a Bruker Avance DRX 400 spectrometer, operating at a fre-
quency of 400 MHz for 1H and 100 MHz for 13C, using the residual
solvent peak as internal reference; chemical shifts (δ) values are given in
parts per million (ppm) and coupling constants (J) in Hertz. Mass spectra
were obtained with an Applied Biosystems-MDS Sciex API 4000 triple
quadrupole mass spectrometer (Concord, Ont., Canada), equipped with
a Turbo-V ion-spray (TIS) source. Elemental analyses were performed on
a Elementar Vario Micro Cube CHN-analyzer.
2.2.4. 3,3’-(1,4-Phenylene)bis(1-(4-phenylthiophen-2-yl)prop-2-yn-1-
one) (1d)
According to the general procedure, 1,4-diethynylbenzene (3) (126
mg, 1.0 mmol), 4-phenylthiophene-2-carbonyl chloride (4d) (557 mg,
2.5 mmol), PdCl2(PPh3)2 (14 mg, 0.02 mmol) and Et3N (20 mL) were
mixed together. The crude product was purified through column chro-
matography (SiO2, n-hexane/CH2Cl2 1:5), giving 409 mg (yield 82%) of
3,3’-(1,4-phenylene)bis(1-(4-phenylthiophen-2-yl)prop-2-yn-1-one)
(1d) as light yellow solid. Mp: 182–185 �C.1H NMR (400 MHz, CDCl3), δ
(ppm): 7.33–7.36 (2H, m); 7.41–7.45 (4H, m); 7.58–7.60 (4H, m); 7.71
(4H, s); 7.83 (2H, d, J ¼ 1.6 Hz); 8.21 (2H, d, J ¼ 1.6 Hz). 13C NMR (100
MHz, CDCl3), δ (ppm): 88.53; 90.08; 122.29; 126.40 (2C); 128.09;
129.08 (2C); 130.19; 133.08 (2C); 133.51; 134.44; 143.82; 144.97;
169.26. LC-MS APCI (þ): calcd for C32H18O2S2: 498.07; found m/z
[MþH]þ: 499.1. Anal. calcd for C32H18O2S2: C, 77.08; H, 3.64; S, 12.86;
found: C, 77.15; H, 3.59; S, 12.86.
2.2. Synthesis of bis[1-(thiophenyl)propynone] dyes
General procedure: in a typical run, diethynylarene (1.0 mmol),
thiophene acid chloride (2.5 mmol), PdCl2(PPh3)2 (2 mol%) and Et3N
(20 mL) were mixed together in a 50 mL two-necked round bottom flask.
The resulting mixture was left under stirring for 28 h at 50 �C, then it was
cooled to room temperature, hydrolyzed with saturated ammonium
chloride solution (20 mL) and extracted with CH2Cl2 (3x30 mL). The
combined organic phases were washed with brine, dried over anhydrous
Na2SO4 and the solvent was removed under vacuum. All the crude
products were purified through column chromatography on silica gel
and characterized with 1H NMR, 13C NMR, LC-MS and elemental anal-
ysis techniques.
2.2.5. 3,3’-(1,4-Phenylene)bis(1-(3-ethoxythiophen-2-yl)prop-2-yn-1-
one) (1e)
According to the general procedure, 1,4-diethynylbenzene (3) (126
mg, 1.0 mmol), 3-ethoxythiophene-2-carbonyl chloride (4e) (477 mg,
2.5 mmol), PdCl2(PPh3)2 (14 mg, 0.02 mmol) and Et3N (20 mL) were
mixed together. The crude product was purified through column chro-
matography (SiO2, n-hexane/CH2Cl2 1:4), giving 252 mg (yield 58%) of
3,3’-(1,4-phenylene)bis(1-(3-ethoxythiophen-2-yl)prop-2-yn-1-one)
(1e) as yellow-orange solid. Mp: 216–218 �C.1H NMR (400 MHz,
CDCl3), δ (ppm): 1.45 (6H, t, J ¼ 6.8 Hz); 4.24 (4H, q, J ¼ 6.8 Hz); 6.85
(2H, d, J ¼ 5.2 Hz); 7.57 (2H, d, J ¼ 5.2 Hz); 7.61 (4H, s). 13C NMR (100
MHz, CDCl3), δ (ppm): 15.05; 67.80; 89.73; 89.98; 116.88 (2C); 122.67;
132.64 (2C); 134.67; 161.82; 167.59. LC-MS APCI (þ): calcd for
2.2.1. 3,3’-(1,4-Phenylene)bis(1-(thiophen-2-yl)prop-2-yn-1-one) (1a)
According to the general procedure, 1,4-diethynylbenzene (3) (126
mg, 1.0 mmol), thiophene-2-carbonyl chloride (4a) (367 mg, 2.5 mmol),
PdCl2(PPh3)2 (14 mg, 0.02 mmol) and Et3N (20 mL) were mixed
together. The crude product was purified through column chromatog-
raphy (SiO2, n-hexane/CHCl3 1:1), giving 288 mg (yield 83%) of 3,3’-
(1,4-phenylene)bis(1-(thiophen-2-yl)prop-2-yn-1-one) (1a) as yellow
solid. Mp: 208–210 �C. 1H NMR (400 MHz, CDCl3), δ (ppm): 7.19 (2H,
dd, J ¼ 4.8, 3.8 Hz); 7.68 (4H, s); 7.75 (2H, dd, J ¼ 4.8, 1.2 Hz); 8.00
(2H, dd, J ¼ 3.8, 1.2 Hz). 13C NMR (100 MHz, CDCl3), δ (ppm): 88.53;
89.74; 122.30; 128.46; 133.05 (2C); 135.34; 135.70; 144.66; 169.35.
LC-MS APCI (þ): calcd for C20H10O2S2: 346.01; found m/z [MþH]þ:
347.1. Anal. calcd for C20H10O2S2: C, 69.34; H, 2.91; S, 18.51; found: C,
69.55; H, 2.86; S, 18.52.
C
C
24H18O4S2: 434.06; found m/z [MþH]þ: 435.4. Anal. calcd for
24H18O4S2: C, 66.34; H, 4.18; S, 14.76; found: C, 66.09; H, 4.21; S,
14.77.
2.2.6. 3,3’-(1,4-Phenylene)bis(1-(benzo[b]thiophen-2-yl)prop-2-yn-1-
one) (1f)
2.2.2. 3,3’-(1,4-Phenylene)bis(1-(3-methylthiophen-2-yl)prop-2-yn-1-
one) (1b)
According to the general procedure, 1,4-diethynylbenzene (3) (126
According to the general procedure, 1,4-diethynylbenzene (3) (126
2

G. Albano et al.
Dyes and Pigments xxx (xxxx) xxx
Scheme 1. Synthetic approach to (1,4-phenylene)bis(thiophenylpropynones) dyes 1.
mg, 1.0 mmol), benzo[b]thiophene-2-carbonyl chloride (4f) (492 mg,
2.5 mmol), PdCl2(PPh3)2 (14 mg, 0.02 mmol) and Et3N (20 mL) were
mixed together. The crude product was purified through column chro-
matography (SiO2, n-hexane/CH2Cl2 1:3), giving 201 mg (yield 45%) of
3,3’-(1,4-phenylene)bis(1-(benzo[b]thiophen-2-yl)prop-2-yn-1-one)
(1f) as orange solid. Mp: 206–208 �C.1H NMR (400 MHz, CDCl3), δ
(ppm): 7.42–7.46 (2H, m); 7.49–7.53 (2H, m); 7.76 (4H, s); 7.89 (2H, d,
J ¼ 8.0 Hz); 7.95 (2H, d, J ¼ 8.0 Hz); 8.27 (2H, s). 13C NMR (100 MHz,
CDCl3), δ (ppm): 88.49; 90.41; 122.37; 123.16; 125.39; 126.46; 128.26;
132.97; 133.15 (2C); 138.73; 143.34; 144.07; 170.82. LC-MS APCI (þ):
calcd for C28H14O2S2: 446.04; found m/z [MþH]þ: 447.2. Anal. calcd for
C28H14O2S2: C, 75.31; H, 3.16; S, 14.36; found: C, 75.55; H, 3.14; S,
14.35.
132.25; 133.38 (2C); 147.41; 157.18; 168.70. LC-MS APCI (þ): calcd for
C20H8N2O6S2: 435.98; found m/z [MþH]þ: 437.0. Anal. calcd for
C20H8N2O6S2: C, 55.04; H, 1.85; N, 6.42; S, 14.69; found: C, 54.91; H,
1.92; N, 6.43; S, 14.70.
2.2.9. 3,3’-([1,10-Biphenyl]-4,40-diyl)bis(1-(thiophen-2-yl)prop-2-yn-1-
one) (10a)
According to the general procedure, 4,40-diethynyl-1,10-biphenyl
(9a) (202 mg, 1.0 mmol), thiophene-2-carbonyl chloride (4a) (367 mg,
2.5 mmol), PdCl2(PPh3)2 (14 mg, 0.02 mmol) and Et3N (20 mL) were
mixed together. The crude product was purified through column chro-
matography (SiO2, n-hexane/CH2Cl2 1:4), giving 321 mg (yield 76%) of
3,3’-([1,10-biphenyl]-4,40-diyl)bis(1-(thiophen-2-yl)prop-2-yn-1-one)
(10a) as light yellow solid. Mp: 206–208 �C.1H NMR (400 MHz, CDCl3),
δ (ppm): 7.19–7.21 (2H, m); 7.66 (4H, d, J ¼ 8.4 Hz); 7.73–7.76 (6H, m);
8.02 (2H, d, J ¼ 4.4 Hz). 13C NMR (100 MHz, CDCl3), δ (ppm): 87.50;
91.15; 119.73; 127.37 (2C); 128.36; 133.67 (2C); 135.05; 135.32;
142.07; 144.93; 169,62. LC-MS APCI (þ): calcd for C26H14O2S2: 422.04;
found m/z [MþH]þ: 423.2. Anal. calcd for C26H14O2S2: C, 73.91; H,
3.34; S, 15.18; found: C, 73.68; H, 3.35; S, 15.18.
2.2.7. 3,3’-(1,4-Phenylene)bis(1-(5-chlorothiophen-2-yl)prop-2-yn-1-one)
(1g)
According to the general procedure, 1,4-diethynylbenzene (3) (126
mg, 1.0 mmol), 5-chlorothiophene-2-carbonyl chloride (4g) (453 mg,
2.5 mmol), PdCl2(PPh3)2 (14 mg, 0.02 mmol) and Et3N (20 mL) were
mixed together. The crude product was purified through column chro-
matography (SiO2, n-hexane/CH2Cl2 1:5), giving 324 mg (yield 78%) of
3,3’-(1,4-phenylene)bis(1-(5-chlorothiophen-2-yl)prop-2-yn-1-one)
(1g) as light pink solid. Mp: 255–257 �C.1H NMR (400 MHz, CDCl3), δ
(ppm): 7.05 (1H, d, J ¼ 4.1 Hz); 7.69 (2H, s); 7.80 (1H, d, J ¼ 4.1 Hz).
13C NMR (100 MHz, CDCl3), δ (ppm): 87.83; 90.32; 122.21; 128.01;
133.09 (2C); 134.77; 141.69; 142.88; 168.25. LC-MS APCI (þ): calcd for
2.2.10. 3,3’-(Naphthalene-2,6-diyl)bis(1-(thiophen-2-yl)prop-2-yn-1-one)
(10b)
According to the general procedure, 2,6-diethynylnaphthalene (9b)
(176 mg, 1.0 mmol), thiophene-2-carbonyl chloride (4a) (367 mg, 2.5
mmol), PdCl2(PPh3)2 (14 mg, 0.02 mmol) and Et3N (20 mL) were mixed
together. The crude product was purified through column chromatog-
raphy (SiO2, n-hexane/CH2Cl2 1:4 → CH2Cl2), giving 202 mg (yield
51%) of 3,3’-(naphthalene-2,6-diyl)bis(1-(thiophen-2-yl)prop-2-yn-1-
one) (10b) as light yellow solid. Mp: 210–213 �C.1H NMR (400 MHz,
CDCl3), δ (ppm): 7.19–7.22 (2H, m); 7.71 (2H, d, J ¼ 8.4 Hz); 7.75 (2H,
d, J ¼ 4.4 Hz); 7.89 (2H, d, J ¼ 8.4 Hz); 8.05 (2H, d, J ¼ 4.4 Hz); 8.22
(2H, s). 13C NMR (100 MHz, CDCl3), δ (ppm): 87.60; 91.01; 119.43;
128.41; 128.74; 129.57; 133.14; 133.89; 135.15; 135.46, 144.89;
169.51. LC-MS APCI (þ): calcd for C24H12O2S2: 396.03; found m/z
[MþH]þ: 397.2. Anal. calcd for C24H12O2S2: C, 72.70; H, 3.05; S, 16.17;
found: C, 72.77; H, 3.09; S, 16.16.
C
C
20H8Cl2O2S2: 413.93; found m/z [MþH]þ: 415.9. Anal. calcd for
20H8Cl2O2S2: C, 57.84; H, 1.94; S, 15.44; found: C, 58.08; H, 1.86; S,
15.44.
2.2.8. 3,3’-(1,4-Phenylene)bis(1-(5-nitrothiophen-2-yl)prop-2-yn-1-one)
(1h)
According to the general procedure, 1,4-diethynylbenzene (3) (126
mg, 1.0 mmol), 5-nitrothiophene-2-carbonyl chloride (4h) (479 mg, 2.5
mmol), PdCl2(PPh3)2 (14 mg, 0.02 mmol) and Et3N (20 mL) were mixed
together. The crude product was purified through column chromatog-
raphy (SiO2, n-hexane/AcOEt 9:1 → 7:3), giving 231 mg (yield 53%) of
3,3’-(1,4-phenylene)bis(1-(5-nitrothiophen-2-yl)prop-2-yn-1-one) (1h)
as yellow-orange solid. Mp: 219–221 �C.1H NMR (400 MHz, CDCl3), δ
(ppm): 7.77 (4H, s); 7.90 (2H, d, J ¼ 4.2 Hz); 7.97 (2H, d, J ¼ 4.2 Hz).
13C NMR (100 MHz, CDCl3), δ (ppm): 87.62; 92.41; 122.03; 128.20;
Table 1
Photophysical properties of (1,4-phenylene)bis(thiophenylpropynones) dyes 1a-
h.
Abs b
Fluoc
Compound
εa
λ
λ
SSd
Φe
max
max
1a
1b
1c
1d
1e
1f
3.8
3.8
3.8
3.4
2.5
4.5
5.8
4.6
335
338
339
328
323
343
340
352
443
450
/
108
112
/
0.2
f
/
g
/
467
507
451
/
139
184
108
/
0.6
0.2
0.2
g
1g
/
g
1h
/
/
/
a
Extinction coefficient (104 MÀ 1 cmÀ 1).
Maximum of light absorbance (nm).
Maximum of light emission (nm).
b
c
d
e
f
Fluo
max
Abs
SS ¼ Stokes shift (nm) ¼ λ
Quantum yield (%).
Less than 0.1%.
- λ
.
max
Fig. 1. Chemical structure of acid chlorides chosen as coupling partner of 3 for
g
the synthesis of (1,4-phenylene)bis(thiophenylpropynones) dyes.
Not determined.
3

G. Albano et al.
Dyes and Pigments xxx (xxxx) xxx
2.2.12. 3,3’-(2,5-Dimethoxy-1,4-phenylene)bis(1-(thiophen-2-yl)prop-2-
yn-1-one) (15)
Table 2
Photophysical properties of arylbis(thiophenylpropinones) 10a-c.
Compound
εa
λ
SSd
Φe
Abs b
Fluoc
λ
max
According to the general procedure, 1,4-diethynyl-2,5-dimethoxy-
benzene (13) (186 mg, 1.0 mmol), thiophene-2-carbonyl chloride (4a)
(367 mg, 2.5 mmol), PdCl2(PPh3)2 (14 mg, 0.02 mmol) and Et3N (20
mL) were mixed together. The crude product was purified through
column chromatography (SiO2, n-hexane/CH2Cl2 1:5), giving 346 mg
(yield 85%) of 3,3’-(2,5-dimethoxy-1,4-phenylene)bis(1-(thiophen-2-yl)
prop-2-yn-1-one) (15) as yellow-green solid. Mp: 264–267 (dec.) �C.1H
NMR (400 MHz, CDCl3), δ (ppm): 3.98 (6H, s); 7.19 (2H, s); 7.23–7.25
(2H, m); 7.78 (2H, dd, J ¼ 4.9, 1.2 Hz); 8.15 (2H, dd, J ¼ 3.8, 1.2 Hz).
13C NMR (100 MHz, CDCl3), δ (ppm): 56.89; 87.60; 92.59; 113.19;
116.88; 128.66; 135.78; 136.20; 145.33; 155.85; 169.08. LC-MS APCI
(þ): calcd for C22H14O4S2: 406.03; found m/z [MþH]þ: 407.2. Anal.
calcd for C22H14O4S2: C, 65.01; H, 3.47; S, 15.78; found: C, 64.83; H,
3.41; S, 15.78.
max
f
10a
10b
10c
3.8
5.3
3,4
346
346
397
420
428
456
74
82
59
/
f
/
5.8
a
Extinction coefficient (104 MÀ 1 cmÀ 1).
Maximum of light absorbance (nm).
Maximum of light emission (nm).
b
c
d
e
f
Fluo
max
Abs
SS ¼ Stokes shift (nm) ¼ λ
Quantum yield (%).
Less than 0.1%.
- λ
.
max
2.2.11. 3,3’-(Naphthalene-1,4-diyl)bis(1-(thiophen-2-yl)prop-2-yn-1-one)
(10c)
According to the general procedure, 1,4-diethynylnaphthalene (9c)
(176 mg, 1.0 mmol), thiophene-2-carbonyl chloride (4a) (367 mg, 2.5
mmol), PdCl2(PPh3)2 (14 mg, 0.02 mmol) and Et3N (20 mL) were mixed
together. The crude product was purified through column chromatog-
raphy (SiO2, n-hexane/CH2Cl2 1:4 → CH2Cl2), giving 266 mg (yield
67%) of 3,3’-(naphthalene-1,4-diyl)bis(1-(thiophen-2-yl)prop-2-yn-1-
one) (10c) as yellow solid. Mp: 212–214 �C.1H NMR (400 MHz, CDCl3),
δ (ppm): 7.22–7.23 (2H, m); 7.73–7.76 (2H, m); 7.78 (2H, dd, J ¼ 4.4,
1.2 Hz); 7.92 (2H, s); 8.09 (2H, dd, J ¼ 4.4, 1.2 Hz); 8.44–8.48 (2H, m).
13C NMR (100 MHz, CDCl3), δ (ppm): 88.39; 92.99; 120.78; 126.50;
128.56; 128.70; 131.87; 133.27; 135.13; 135.65; 144.82; 169.30. LC-MS
APCI (þ): calcd for C24H12O2S2: 396.03; found m/z [MþH]þ: 397.2.
Anal. calcd for C24H12O2S2: C, 72.70; H, 3.05; S, 16.17; found: C, 72.83;
H, 3.03; S, 16.17.
2.3. Characterization
UV–Vis absorption measurements in solution were done using a
PerkinElmer Lambda 650 spectrophotometer, with temperature control
to within �0.1 �C. Fluorescence measurements in solution were per-
formed using a Horiba Jobin Yvon FluoroLog®-3 spectrofluorometer,
with temperature control to within �0.1 �C.
Quantum yields (Φ) in solution are calculated according to a
comparative method, involving the use of a standard with known
quantum yield, ΦST (perylene in cyclohexane, ΦST ¼ 0.94; fluorescein in
0.1 M aqueous NaOH solution, ΦST ¼ 0.95; quinine sulfate in 0.1 M
aqueous H2SO4 solution, ΦST ¼ 0.54) [35]; the equation used is
Scheme 2. Acyl Sonogashira reactions between 1,4-diethynylbenzene (3) and thiophene carbonyl chlorides 4a-h.
Table 3
Optical properties of dimethoxy-substituted dyes 13–15.
Compounds
Abs c
max
Fluod
max
Yield (%)a
εb
λ
λ
SSe
28
Φf
85
60
85
1.0
345
377
403
373
409
471
12.3
3.8
1.9
32
68
23.3
19.2
a
b
Yield of pure product.
Extinction coefficient (104 MÀ 1 cmÀ 1).
c
Maximum of light absorbance (nm).
d
Maximum of light emission (nm).
e
f
Fluo
max
Abs
SS ¼ Stokes shift (nm) ¼ λ
- λ
.
max
Quantum yield (%).
4

G. Albano et al.
Dyes and Pigments xxx (xxxx) xxx
Table 4
Optical properties of 3,3’-(2,5-dimethoxy-1,4-phenylene)bis(1-(thiophen-2-yl)
prop-2-yn-1-one) (15) recorded in different solvents.
ηa
ϵb
λ
λ
SSe
Φf
Abs c
Fluod
Solvent
max
max
Toluene
CHCl3
1.496
1.445
1.359
1.344
2.4
401
403
402
403
453
471
471
484
52
68
69
81
16.1
19.2
38.4
40.8
4.8
Acetone
21.0
38.8
Acetonitrile
a
Refractive index.
b
c
d
e
f
Dielectric constant.
Maximum of light absorbance (nm).
Maximum of light emission.
Fluo
max
Abs
SS ¼ Stokes shift (nm) ¼ λ
- λ
.
max
Quantum yield (%).
rx
ð
ηxÞ2
Φx
¼
ΦST
⋅
⋅
ηSTÞ2
rST ð
where rx and rST are the slopes of a fluorescence area vs. absorbance
plot for the dye and standard, respectively, while ηx and ηST are the
refractive index of the solvents used for dye and standard solutions,
respectively (ηchloroform ¼ 1.445; ηcyclohexane ¼ 1.426; ηwater ¼ 1.33).
15/PCMA thin films were prepared by drop casting, i.e. pouring 0.8
mL chloroform solution containing 120.0 mg of the polymer and the
Scheme 3. Synthesis and optical properties of 1,4-bis(thiophen-2-ylethynyl)
benzene (6).
Fig. 2. UV–Vis absorption (a) and emission (b) normalized spectra of 3,3’-(2,5-dimethoxy-1,4-phenylene)bis(1-(thiophen-2-yl)prop-2-yn-1-one) (15) in different
solvents. Sample concentration: 1.7 � 10À 5 M for absorption spectra, 1.0 � 10À 6 M for emission spectra; cell length: 1 cm; excitation wavelength: 335 nm.
Scheme 4. Synthetic approach to arylbis(thiophenylpropinones) 10a-c.
5

G. Albano et al.
Dyes and Pigments xxx (xxxx) xxx
when perpendicular to the light source (ISC
)
ILSC
ISC⋅G
ηopt
¼
where G is the geometrical factor (in our case, G ¼ 13.3), which is the
ratio between the area exposed to the light source and the collecting
area.
3. Results and discussion
We started our study with the synthesis of (1,4-phenylene)bis(thio-
phenylpropynones) 1 (Scheme 1) characterized by the presence of a
–
benzene ring linked to thiophenyl moieties through a C�C–C ¼O
–
spacer. The synthetic strategy chosen for the preparation of these com-
pounds consists of two main steps: the synthesis of the central diethynyl
functionalized nucleus and the subsequent coupling with thiophene
carbonyl chlorides (Scheme 1). Both steps are based on Sonogashira
cross-coupling reactions: the first is the “classic” version of the reaction
[36], the second consists of the acyl copper-free Sonogashira coupling
[37,38].
In details, 1,4-diethynylbenzene (3) was obtained from the reaction
between commercial available 1,4-dibromobenzene (2) and an excess of
ethynyltrimethylsilane, followed by quantitative desilylation of product
5 performed with aqueous solution of NaOH (see Scheme S1 in Sup-
porting Information). As coupling partner of 1,4-diethynylbenzene (3),
eight different acid chlorides were chosen (Fig. 1). While compounds 4a-
c were commercially available, acid chlorides 4d-4f were easily pre-
pared starting from the methyl esters by hydrolysis with aqueous NaOH.
The obtained carboxylic acids were converted into the acid chlorides by
treatment with oxalyl chloride. Analogously, chlorides 4g-h were ob-
tained treating the commercial corresponding acids with (COCl)2
(Scheme S2).
Fig. 3. 1.0 � 10À 6 M solutions of 3,3’-(2,5-dimethoxy-1,4-phenylene)bis(1-
(thiophen-2-yl)prop-2-yn-1-one) (15) dye in different solvents (from left to
right: toluene, chloroform, acetone, acetonitrile), illuminated with daylight (up
photo) and irradiated with UV light at 335 nm (down photo).
Since it is known that decarbonyation and C�C–C�C side reactions
[39,40] may occur if acid Sonogashira reactions are performed at high
temperature, a preliminary cross coupling between 1,4-diethynylben-
zene (3) and thiophene-2-carbonyl chloride (4a) was performed at
room temperature for 28 h. The analysis of crude product indicated an
almost complete conversion of the reagents but the formation of a
mixture of both mono- and di-thiophene carbonyl derivatives (Scheme
S3 in Supporting Information).
In order to increase the chemoselectivity of the reaction, a second
run was carried out at 50 �C. In this case the total consumption of the
reagents occurred and the exclusive formation of expected product 1a
was observed (yield 83%). As a consequence, the improved experi-
mental reaction conditions (50 �C, 28 h, 1 mmol of 1,4-diethynylben-
zene (3), 2.5 mmol of thiophene carbonyl chloride, 20 mL of Et3N,
and 2 mol% of PdCl2(PPh3)2) were applied to the acyl Sonogashira re-
action between 3 and thiophene carbonyl chlorides 4 (Scheme 2). The
reactions afforded exclusively di-carbonyl compounds 1a-h and proved
amenable to acid chlorides containing both electron-donating and
electron-withdrawing functionality, delivering products in good yields,
regardless of the position of the functional group on the thiophene ring.
Study of the optical properties of compounds 1a-h was undertaken in
CHCl3 solutions. The main results obtained are collected in Table 1,
while full absorbance and fluorescence emission spectra are depicted in
Figs. S1–S8 in Supporting Information.
Fig. 4. UV–Vis absorption spectra of 3,3’-(2,5-dimethoxy-1,4-phenylene)bis(1-
(thiophen-2-yl)prop-2-yn-1-one) (15) in CHCl3 recorded each 15 min for 6 h.
Sample concentration: 1.7 � 10À 5 M; cell length: 1 cm.
proper amount of fluorophore to obtain concentrations in the range
0.7–1 wt% on 50 � 50 � 3 mm optically pure glass substrate (Edmund
Optics Ltd BOROFLOAT window 50 � 50 TS). The glass slides were
cleaned with chloroform and immerged in 6 M HCl for at least 12 h;
then, they were rinsed with water, acetone and isopropanol and dried
for 8 h at 120 �C. Solvent evaporation was performed on a warm hot
plate (about 30 �C) and in a closed environment. The film thickness was
Products 1c, 1g and 1h, featured by electron-withdrawing moieties
did not show fluorescence when excited at 335 nm wavelength [41-43].
On the contrary, compounds 1a,b,d,e,f showed emission spectra with
high Stokes shifts (108–168 nm), indicating low auto-absorption in so-
lution and good potentiality as fluorophores in LSC devices. Unfortu-
nately their quantum yields were almost negligible (less than 1%).
In order to evaluate the effect of carbonyl group on the optical
properties of (1,4-phenylene)bis(thiophenylpropynones) 1, 1,4-diethy-
nylbenzene 3 was reacted with 2-iodothiophene 5 affording 1,4-bis
measured by a Starrett micrometer to be 200 � 10 μm.
The optical efficiency of the LSC was measured by using a solar
simulating lamp (ORIEL® LCS-100 solar simulator 94011A S/N: 322,
AM1.5G std filter: 69 mW/cmÀ 2 at 254 mm). The PV module (IXYS
SLMD121H08L mono solar cell 86 � 14 mm) was connected to Keysight
Technologies B2900 Series Precision Source/Measure Unit. The optical
efficiency ηopt was evaluated from the concentration factor C, which is
the ratio between the short circuit current measured in the case of the
cell over the LSC edge (ILSC) and short circuit current of the bare cell
6

G. Albano et al.
Dyes and Pigments xxx (xxxx) xxx
Fig. 5. Absorption and emission of 3,3’-(2,5-dimethoxy-1,4-phenylene)bis(1-(thiophen-2-yl)prop-2-yn-1-one) (15) in PCMA film at a concentration of 0.7 wt%.
(thiophen-2-ylethynyl)benzene 6 in good yield (Scheme 3).
compounds 14 and then to 15 the extension of conjugation determines a
red-shift of the spectrum, but with an absorption maximum still peaked
close to the near-UV region.
A comparison between optical properties of 6 (Scheme 3 and
Figure S9) and 1a (Table 1 and Fig. S1) clearly indicated that the
presence of CO determines an increase of the Stokes shift, a significant
bathochromic effect in emission spectra, but also a dramatic reduction of
quantum yield (0.2 vs. 9.9%). A similar trend had been observed [32]
when we studied the photophysical features of two triphenyl amine
derivatives containing ethynyl linkers to thiophene or carbon-
ylthiophene moieties. The presence of CO involved higher Stokes shifts
and increase in both absorption and emission maximum wavelengths.
Thus, with the aim to improve the optical properties, bis(thio-
phenylpropynones) containing nucleus with extended conjugation were
synthesized (Scheme 4). Analogously to the synthetic sequence
employed for the synthesis of derivatives 1a-h, central units were pre-
pared starting from the corresponding dibromo derivatives 7a-c, which
were coupled with trimethylsilylacetylene generating 8a-c; subsequent
desilylation step (KOH) afforded diynes 9a-c in good yields (Scheme S4).
The optical properties of dyes 10a-c in CHCl3 solution are collected
in Table 2 and Figs. S10–S12. While biphenyl (10a) and 2,6-naphtyl
(10b) derivatives showed optical features very similar to 3,3’-(1,4-
phenylene)bis(1-(thiophen-2-yl)prop-2-yn-1-one) (1a) in terms of
maximum absorbance and emission wavelength, 3,3’-(naphthalene-1,4-
Moreover, the presence of CO causes an increasing of the Stokes shift
from 28 to 68 nm. If we compare the optical properties of 15 with
compound 1a it is evident that the presence of -OMe group involves a
clear improvement of the quantum yield, from 0.2 for 1a to about 20%
for 14 and 15, possibly due to a restricted molecular mobility.
Furthermore, since 15 appeared as a promising fluorophore for colorless
solar collectors, further investigations were carried out. Notably sol-
vatochromism analyses were performed in solvents with different po-
larities but refractive index (η) close to that of acrylate polymers, i.e. the
most common matrix for LSC devices (Table 4 and Figs. 2 and 3).
As it is evident, the choice of the solvent has a great influence on the
photophysical properties of 15. In particular, as polarity increases from
toluene to acetonitrile, a significant enhancement of quantum yield (i.e.
16.1 in toluene respect to 40.8% in acetonitrile) and Stokes shift (from
Abs
52 to 81 nm) were detected. In the different solvents, λ
is constant
max
Fluo
and λ
increases as polarity increases. This shift of the emission to
max
lower energy is due to the stabilization of the (polar) excited state by the
polar solvent molecules. The increase in the quantum yield with solvent
polarity is related to a decrease of non-radiative decays, in agreement
with the increased values of Stokes shift [45].
diyl)bis(1-(thiophen-2-yl)prop-2-yn-1-one) (10c) showed a significant
Abs
bathochromic effect (λ
¼ 397 nm for 10c, 335 for 1a) together with
All solutions were also checked for stability in time: absorption
spectra collected over all the range were totally super imposable over 6 h
of continuous irradiation at the excitation wavelength (Fig. 4).
A preliminarily investigation of 15 as a potential fluorophore for
colorless LSC was carried out by dispersing 0.7 wt% of the dye in a
polymer matrix of poly(cyclohexylmethacrylate) (PCMA). PCMA is
100% amorphous, transparent and commercially available, features that
make this matrix suitable for LSC applications (Fig. 5). In the PCMA film
max
an increase of quantum yield (5.8% for 10c vs. 0.2% for 1a). Such results
could be related to both the increasing of π-conjugation and to the
different geometry of 10c respect to 1a [44].
Considering that the presence of an electron-donating group such as
-OEt had determined a remarkable bathochromic effect and a very high
SS value (Table 1, 1e) the syntheses of 2,2’-((2,5-dimethoxy-1,4-phe-
nylene)bis(ethyne-2,1-diyl))dithiophene (14) and 3,3’-(2,5-dimethoxy-
1,4-phenylene)bis(1-(thiophen-2-yl)prop-2-yn-1-one) (15) were carried
out (Scheme S5). Central nucleus 1,4-diethynyl-2,5-dimethoxybenzene
(13) was easily prepared starting from commercially available 1,4-
dibromo-2,5-dimethoxybenzene (11) which was coupled with ethynyl-
trimethylsilane affording 12 (Scheme S5, step a). After desilylation with
KOH/MeOH (Scheme S5, step b), product 13 was generated in high
yield. and was then successfully employed in the synthesis of 14 and 15
via Sonogashira reactions (Scheme S5, steps c-d). The optical properties
in CHCl3 solution of both compounds are described in Table 3 and
compared with those of precursor 13 (full absorbance and fluorescence
emission spectra of 13–15 are reported in Figs. S13–S15 in Supporting
Information). As is evident from data collected in Table 3, wavelengths
of absorption and emission maxima of the three compounds vary
significantly with their structure. Indeed, going from diyne 13 to
(thickness of about 200 μm), 15 showed absorption in the near-UV re-
gion with a negligible contribution in the visible range of the light
spectrum (at 450 nm, about 80% of transmitted light, Fig. 5). 15 dis-
played an emission maximum at about 470–480 nm, in agreement with
data collected in solution (Table 4). The performances of the PCMA/15
film as solar collector were determined on optically pure 50 � 50 � 3
mm glass by using a Si-based PV cell attached to one edge of the LSC. The
data acquired followed a peculiar trend, i.e. optical efficiency increasing
with 15 content and levelling off for concentration higher than 0.7 wt%
possibly due to adverse dissipative phenomena. Notably, maximum
optical efficiencies of 7.7% was obtained and resulted higher than those
of previously investigated bis-azido fluorophores for colorless LSC and
determined with the same laboratory setup [34].
7

G. Albano et al.
Dyes and Pigments xxx (xxxx) xxx
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In summary, an easy methodology for the preparation of new bis
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has been developed. All compounds were generated in good to high
yields and were tested as organic dyes for LSCs. The presence of carbonyl
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emission spectra. Moreover, increasing
π-conjugation of thiophenyl
lateral unit determined an increase in Stokes shifts up to 180 nm, thus
indicating low auto-absorption in solution. Replacement of benzene ring
with naphthalene nucleus involved an increment of quantum yield but
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solvents with different polarity and refractive index (η), with an increase
of quantum yield up to ~ 41% in acetonitrile. Finally a PCMA film of 15
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Declaration of competing interest
[27] Zhou W, Wang M-C, Zhao X. Poly(methyl methacrylate) (PMMA) doped with
DCJTB for luminescent solar concentrator applications. Sol Energy 2015;115:
569–76.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
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Appendix A. Supplementary data
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