Photophysical properties of new p-phenylene- and benzodithiophene-based fluorophores for luminescent solar concentrators (LSCs)

Article abstract of DOI:10.1016/j.dyepig.2020.108368

In this study, we report on the synthesis of new organic fluorophores containing either the p-phenylene or the benzodithiophene cyclic nucleus connected to thiophene units via triple bonds and carbonyl group, and on their application for the fabrication o

Full text of DOI:10.1016/j.dyepig.2020.108368

Dyes and Pigments 178 (2020) 108368
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Photophysical properties of new p-phenylene- and benzodithiophene-based
fluorophores for luminescent solar concentrators (LSCs)
,
,
,c,*
Gianluigi Albanoa 1, Tony Collia, Tarita Bivera b, Laura Antonella Aronicaa, Andrea Puccia
a
�
Dipartimento di Chimica e Chimica Industriale, Universita di Pisa, Via Giuseppe Moruzzi 13, 56124, Pisa, Italy
b
�
Dipartimento di Farmacia, Universita di Pisa, Via Bonanno 6, 56126, Pisa, Italy
c INSTM, UdR 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:
In this study, we report on the synthesis of new organic fluorophores containing either the p-phenylene or the
benzodithiophene cyclic nucleus connected to thiophene units via triple bonds and carbonyl group, and on their
application for the fabrication of luminescent solar concentrators (LSCs). Their optical properties were evaluated.
Independent of the core, dyes containing the CO-thiophene residues seem to be the most promising for LSCs
applications. In fact, carbonyl groups slightly enhance the quantum yield but significantly increase the red-shift
of the emission so that the superimposition between the absorbance spectrum and the emission one is dimin-
ished. In the case of the benzodithiophene centre, light emission in the yellow-red portion of the spectrum is
achieved. The latter dye is then selected for tests in a poly(methyl methacrylate) (PMMA) matrix. It showed good
compatibility and homogenous distribution, no auto-absorption phenomena, and optical efficiencies of about 8%
at 1 wt. %, i.e. comparable with those PMMA/Lumogen Red films in the same range of concentration.
Luminescent solar concentrator
Organic dye
Optical efficiency
Solvatochromism
Benzodithiophene
Sonogashira reaction
1. Introduction
compensate for these defects luminescent solar concentrators (LSCs)
were developed [7–13].
Recently, high attention has been paid to climate changes due to CO2
emissions generated from carbon-based fuels. As a consequence,
renewable energy sources have become a hot topic in the research
community. One of the most widespread technologies of renewable
energy generation is the use of photovoltaic (PV) systems which convert
sunlight into useable electrical energy [1]. PV technology is unique in its
extreme scalability, ranging from watt-scale individual systems to kilo-
watt- and megawatt-scale distributed domestic and industrial power
systems and to power plants of hundreds of megawatts.
LSCs generally consist of an inert material (often a polymer [14])
containing a suitable dye which, once exposed to sunlight, converts part
of the absorbed radiation into a longer emitting wavelength by fluo-
rescence. The reemitted light can be concentrated via total internal
reflection in the optical waveguide construction. Therefore, solar cells
attached to LSC can generate more electric power than conventionally
used ones. The emitting dyes are the driving force for light concentration
in LSCs cells. There are three main kinds of fluorophores: quantum dots
[15–18], lanthanide-based materials [19,20] and organic dyes. Respect
to the two first classes of compounds, organic dyes are usually less toxic
and their optical properties (Stokes shift, quantum yield, thermal- and
photo-stability) can be optimized by modifying the structure of organic
molecules. Many of them possess one or more heterocyclic nuclei such as
thiazoles, imidazoles, benzothiazoles, benzodithiophenes, oxazines,
lactones, pyrans, pyrroles, thiophenes, pyridines, pyrimidines, and
dithienosiloles [21–34].
Solar cells produce a quantity of electrical energy directly propor-
tional to the total power of the absorbed light. 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 [2–4]. These devices can concentrate the light on small and
highly efficient photovoltaic solar cells, thus lowering the cost of energy
by reducing cell surfaces. Nevertheless, optical concentrators have some
disadvantages such as the need of rotation mechanisms that allow the
concentrator to follow the Sun’s apparent motion, and a cooling system
to disperse the excess heat due to unconverted energy [5,6]. To
Recently we have reported [35] that dimethoxy-1,4-phenylene unit
linked to two (thiophen-2-yl)prop-2-yn-1-one moieties connected to a
PV cell showed optical efficiency of 6.2%, thus supporting the use of this
�
* Corresponding author. Dipartimento di Chimica e Chimica Industriale, Universita di Pisa, Via Giuseppe Moruzzi 13, 56124, Pisa, Italy.
E-mail address: andrea.pucci@unipi.it (A. Pucci).
1
�
Present address: Dipartimento di Chimica, Universita degli Studi di Bari “Aldo Moro”, Via Edoardo Orabona 4, 70,126 Bari, Italy.
https://doi.org/10.1016/j.dyepig.2020.108368
Received 12 February 2020; Received in revised form 13 March 2020; Accepted 13 March 2020
Available online 16 March 2020
0143-7208/© 2020 Elsevier Ltd. All rights reserved.

G. Albano et al.
Dyes and Pigments 178 (2020) 108368
Fig. 1. Chemical structure of the five new fluorophores studied in this work.
compound as a fluorophore for colourless LSC devices.
2.2. Synthesis of p-phenylene-based fluorophores
Prompted by these promising results, here we describe the synthesis
of five new fluorophores containing an electron-rich nucleus linked to
2.2.1. Synthesis of 1,4-bis((S)-3,7-dimethyloctyloxy)-2,5-bis(trimethylsi-
lylethynyl)benzene (9)
–
–
–
–
–
thiophene rings via C C or C C–C O bridges (Fig. 1) and the study of
–
–
–
their optical properties both in solution and in polymer films.
1,4-Bis((S)-3,7-dimethyloctyloxy)-2,5-diiodobenzene (6) (1.00 g,
1.56 mmol), Pd(PPh3)4 (90 mg, 0.078 mmol), CuI (30 mg, 0.157 mmol)
and Et3N (15 mL) were mixed together, then trimethylsilylacetylene (8)
(766 mg, 7.8 mmol) was added dropwise. The resulting mixture was
refluxed under stirring for 7 h, then it was cooled to room temperature,
hydrolyzed with saturated ammonium chloride solution (20 mL) and
extracted with CH2Cl2 (3 � 30 mL). The combined organic phases were
washed with brine (50 mL), dried over anhydrous Na2SO4 and the sol-
vent was removed under vacuum. The crude product was purified by
column chromatography (SiO2, n-pentane) to give 1,4-bis((S)-3,7-
dimethyloctyloxy)-2,5-bis(trimethylsilylethynyl)benzene (9) (900 mg,
yield 99%) as a orangish oil. 1H NMR (400 MHz, CDCl3), δ (ppm): 0.25
(18H, s), 0.87 (12H, d, J ¼ 6.7 Hz), 0.95 (6H, d, J ¼ 6.6 Hz); 1.11–1.20
(6H, m), 1.26–1.37 (6H, m), 1.48–1.62 (4H, m), 1.71–1.79 (2H, m),
1.80–1.88 (2H, m), 3.94–4.02 (4H, m), 6.90 (2H, s). 13C NMR (100 MHz,
CDCl3), δ (ppm): À 0.06 (3C), 19.65, 22.56, 22.68, 24.72, 27.95, 29.76,
36.26, 37.34, 39.29, 67.72, 99.98, 101.08, 113.92, 117.16, 153.99. LC-
MS (APCIþ), m/z: 583.61 [MþH]þ. Anal. calcd for C36H62O2Si2: C,
74.16; H, 10.72; found: C, 74.29; H, 10.59.
2. Experimental section
2.1. Materials and apparatus
Solvents were purified by conventional methods, distilled and stored
over activated molecular sieves under argon. Poly(methyl methacrylate)
(PMMA, Aldrich, MW ¼ 350,000 g/mol, acid number < 1 mg KOH/g)
was 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 ALUGRAM® Xtra SIL G/UV254 (0.2 mm)
purchased from VWR Macherey-Nagel. Column chromatographies were
performed with Fluka silica 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 solution with a Bruker Avance DRX 400 spec-
trometer, operating at a frequency 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.
2.2.2. Synthesis of 1,4-bis((S)-3,7-dimethyloctyloxy)-2,5-diethynylben-
zene (10)
1,4-Bis((S)-3,7-dimethyloctyloxy)-2,5-bis(trimethylsilylethynyl)
benzene (9) (900 mg, 1.54 mmol), 3 M KOH aqueous solution (1.2 mL,
3.6 mmol), THF (68 mL) and methanol (22 mL) were mixed together.
The resulting mixture was left under stirring for 7 h at room tempera-
ture, then it was hydrolyzed with H2O (60 mL) and extracted with
CH2Cl2 (3 � 60 mL). The combined organic phases were washed with
brine (100 mL), dried over anhydrous Na2SO4 and the solvent was
removed under vacuum. The crude product was purified by column
chromatography (SiO2, petroleum ether/CH2Cl2 4:1) to give 1,4-bis((S)-
2

G. Albano et al.
Dyes and Pigments 178 (2020) 108368
3,7-dimethyloctyloxy)-2,5-diethynylbenzene (10) (507 mg, yield 75%)
as a orangish oil. 1H NMR (400 MHz, CDCl3), δ (ppm): 0.86 (12H, d, J ¼
6.7 Hz), 0.94 (6H, d, J ¼ 6.7 Hz), 1.11–1.24 (6H, m), 1.25–1.37 (6H, m),
1.48–1.64 (4H, m), 1.64–1.76 (2H, m), 1.81–1.89 (2H, m), 3.32 (2H, s),
3.96–4.05 (4H, m), 6.96 (2H, s). 13C NMR (100 MHz, CDCl3), δ (ppm):
19.70, 22.59, 22.68, 24.65, 27.95, 29.80, 36.02, 37.23, 39.18, 67.97,
79.79, 82.40, 113.20, 117.60, 153.97. LC-MS (APCIþ), m/z: 439.57
[MþH]þ. Anal. calcd for C30H46O2: C, 82.14; H, 10.57; found: C, 82.19;
H, 10.63.
removed under vacuum. The crude product was purified by column
chromatography (SiO2, petroleum ether/CH2Cl2 4:1) to give 1,4-bis((S)-
3,7-dimethyloctyloxy)-2,5-bis([2,20-bithiophen]-5-ylethynyl)benzene
(3) (330 mg, yield 59%) as yellow solid. 1H NMR (400 MHz, CDCl3), δ
(ppm): 0.88 (12H, d, J ¼ 6.6 Hz), 1.02 (6H, d, J ¼ 6.6 Hz), 1.15–1.47
(12H, m), 1.50–1.59 (2H, m), 1.61–1.73 (2H, m), 1.77–1.88 (2H, m),
1.89–2.00 (2H, m), 4.02–4.18 (4H, m), 7.02 (2H, s), 7.03–7.06 (2H, m),
7.10 (2H, d, J ¼ 4.0 Hz), 7.20 (2H d, J ¼ 4.0 Hz), 7.21–7.23 (2H, m),
7.26–7.28 (2H, m). 13C NMR (100 MHz, CDCl3), δ (ppm): 19.76, 22.61,
22.71, 24.82, 28.00, 29.96, 36.28, 37.41, 39.30, 67.99, 88.24, 90.96,
113.74,116.34, 122.15, 123.61, 124.22, 125.00, 127.95, 132.72,
136.81, 139.08, 153.53. LC-MS (APCIþ), m/z: 767.30 [MþH]þ. Anal.
calcd for C46H54O2S4: C, 72.02; H, 7.09; S, 16.72; found: C, 72.11; H,
7.03; S, 16.72.
2.2.3. Synthesis of 1,4-bis((S)-3,7-dimethyloctyloxy)-2,5-bis(thiophen-2-
ylethynyl)benzene (1)
1,4-Bis((S)-3,7-dimethyloctyloxy)-2,5-diethynylbenzene (10) (150
mg, 0.34 mmol), 2-iodothiophene (164 mg, 0.78 mmol), Pd(PPh3)4 (20
mg, 0.017 mmol), CuI (7 mg, 0.037 mmol) and Et3N (15 mL) were mixed
together. The resulting mixture was refluxed under stirring for 24 h,
then it was cooled to room temperature, hydrolyzed with saturated
ammonium chloride solution (20 mL) and extracted with CH2Cl2 (3 �
30 mL). The combined organic phases were washed with brine (50 mL),
dried over anhydrous Na2SO4 and the solvent was removed under vac-
uum. The crude product was purified by column chromatography (SiO2,
petroleum ether/CH2Cl2 4:1) to give 1,4-bis((S)-3,7-dimethyloctyloxy)-
2,5-bis(thiophen-2-ylethynyl)benzene (1) (189 mg, yield 92%) as a
yellowish oil. 1H NMR (400 MHz, CDCl3), δ (ppm): 0.88 (12H, d, J ¼ 6.6
Hz), 1.01 (6H, d, J ¼ 6.6 Hz), 1.15–1.23 (6H, m), 1.31–1.41 (6H, m),
1.50–1.59 (2H, m), 1.62–1.70 (2H, m), 1.76–1.84 (2H, m), 1.88–1.96
2.3. Synthesis of benzo[1,2-b:4,5-b’]dithiophene-based fluorophores
2.3.1. Synthesis of 2,6-bis(trimethylsilylethynyl)-4,8-bis((S)-3,7-
dimethyloctyloxy)benzo[1,2-b:4,5-b’]dithiophene (12)
2,6-Dibromo-4,8-bis((S)-3,7-dimethyloctyloxy)benzo[1,2-b:4,5-b’]
dithiophene (7) (1.00 g, 1.51 mmol), Pd(PPh3)4 (88 mg, 0.076 mmol),
CuI (29 mg, 0.152 mmol) and Et3N (15 mL) were mixed together, then
trimethylsilylacetylene (8) (445 mg, 4.53 mmol) was added dropwise.
The resulting mixture was refluxed under stirring for 7 h, then it was
cooled to room temperature, hydrolyzed with saturated ammonium
chloride solution (20 mL) and extracted with CH2Cl2 (3 � 30 mL). The
combined organic phases were washed with brine (50 mL), dried over
anhydrous Na2SO4 and the solvent was removed under vacuum. The
crude product was purified by column chromatography (SiO2, n-hex-
ane/CH2Cl2 5:1) to give 2,6-bis(trimethylsilylethynyl)-4,8-bis((S)-3,7-
dimethyloctyloxy) benzo[1,2-b:4,5-b’]dithiophene (12) (976 mg, yield
93%) as a orangish oil. 1H NMR (400 MHz, CDCl3), δ (ppm): 0.27 (18H,
s), 0.87 (12H, d, J ¼ 6.6 Hz), 0.96 (6H, d, J ¼ 6.6 Hz), 1.14–1.22 (6H, m),
1.29–1.41 (6H, m), 1.46–1.59 (2H, m), 1.60–1.69 (2H, m), 1.71–1.81
(2H, m), 1.82–1.92 (2H, m), 4.21–4.29 (4H, m), 7.57 (2H, s). 13C NMR
(100 MHz, CDCl3), δ (ppm): À 0.23 (3C), 19.75, 22.62, 22.73, 24.69,
27.96, 29.73, 37.25, 37.54, 39.23, 72.42, 97.77, 101.65, 122.91,
125.92, 130.09, 131.75, 143.88. LC-MS (APCIþ), m/z: 696.04 [MþH]þ.
Anal. calcd for C40H62O2S2Si2: C, 69.10; H, 8.99; S, 9.22; found: C,
68.97; H, 9.14; S, 9.23.
(2H, m), 4.03–4.12 (4H, m), 7.02–7.04 (4H, m), 7.30–7.32 (4H, m). 13
C
NMR (100 MHz, CDCl3), δ (ppm): 19.70, 22.57, 22.68, 24.74, 27.94,
29.88, 36.23, 37.34, 39.22, 67.90, 88.06, 89.77, 113.71, 116.40,
123.48, 127.06, 127.33, 131.77, 153.46. LC-MS (APCIþ), m/z: 603.45
[MþH]þ. Anal. calcd for C38H50O2S2: C, 75.70; H, 8.36; S, 10.64; found:
C, 75.86; H, 8.31; S, 10.63.
2.2.4. Synthesis of 3,3’-(2,5-bis((S)-3,7-dimethyloctyloxy)-1,4-phenylene)
bis(1-(thiophen-2-yl)prop-2-yn-1-one) (2)
1,4-Bis((S)-3,7-dimethyloctyloxy)-2,5-diethynylbenzene (10) (150
mg, 0.34 mmol), thiophene-2-carbonyl chloride (145 mg, 0.99 mmol),
Pd(PPh3)4 (20 mg, 0.017 mmol) and Et3N (15 mL) were mixed together.
The resulting mixture was left under stirring for 24 h at 50 �C, then it was
cooled to room temperature, hydrolyzed with saturated ammonium
chloride solution (20 mL) and extracted with CH2Cl2 (3 � 30 mL). The
combined organic phases were washed with brine (50 mL), dried over
anhydrous Na2SO4 and the solvent was removed under vacuum. The
crude product was purified by column chromatography (SiO2, n-hex-
ane/CH2Cl2 1:1) to give 3,3’-(2,5-bis((S)-3,7-dimethyloctyloxy)-1,4-
phenylene)bis(1-(thiophen-2-yl)prop-2-yn-1-one) (2) (157 mg, yield
70%) as a yellow solid. 1H NMR (400 MHz, CDCl3), δ (ppm): 0.84 (12H,
d, J ¼ 6.6 Hz), 0.97 (6H, d, J ¼ 6.6 Hz), 1.12–1.25 (6H, m), 1.28–1.38
(6H, m), 1.45–1.55 (2H, m), 1.68–1.78 (4H, m), 1.91–1.99 (2H, m),
4.04–4.13 (4H, m), 7.15–7.18 (4H, m), 7.74 (2H, dd, J ¼ 4.9, 1.3 Hz),
8.15 (2H, dd, J ¼ 3.8, 1.3 Hz). 13C NMR (100 MHz, CDCl3), δ (ppm):
19.51, 22.51, 22.63, 24.57, 27.88, 29.74, 36.22, 37.19, 39.15, 67.84,
87.71, 92.11, 112.93, 117.43, 128.11, 135.36, 136.12, 145.04, 154.91,
169.60. LC-MS (APCIþ), m/z: 659.31[MþH]þ. Anal. calcd for
2.3.2. Synthesis of 2,6-diethynyl-4,8-bis((S)-3,7-dimethyloctyloxy)benzo
[1,2-b:4,5-b’]dithiophene (13)
2,6-Bis(trimethylsilylethynyl)-4,8-bis((S)-3,7-dimethyloctyloxy)
benzo[1,2-b:4,5-b’]dithiophene (12) (976 mg, 1.40 mmol), 3 M KOH
aqueous solution (1.1 mL, 3.3 mmol), THF (68 mL) and methanol (22
mL) were mixed together. The resulting mixture was left under stirring
for 7 h at room temperature, then it was hydrolyzed with H2O (60 mL)
and extracted with CH2Cl2 (3 � 60 mL). The combined organic phases
were washed with brine (100 mL), dried over anhydrous Na2SO4 and the
solvent was removed under vacuum. The crude product was purified by
column chromatography (SiO2, n-hexane/CH2Cl2 5:2) to give 2,6-
diethynyl-4,8-bis((S)-3,7-dimethyloctyloxy)benzo[1,2-b:4,5-b’]dithio-
phene (13) (679 mg, yield 88%) as a brownish solid. 1H NMR (400 MHz,
CDCl3), δ (ppm): 0.89 (12H, d, J ¼ 6.6 Hz), 0.98 (6H, d, J ¼ 6.6 Hz),
1.13–1.23 (6H, m), 1.24–1.44 (6H, m), 1.50–1.60 (2H, m), 1.62–1.72
(2H, m), 1.74–1.82 (2H, m), 1.87–1.95 (2H, m), 3.48 (2H, s), 4.23–4.32
(4H, m), 7.63 (2H, s). 13C NMR (100 MHz, CDCl3), δ (ppm): 19.71,
22.62, 22.72, 24.71, 27.97, 29.72, 37.27, 37.52, 39.24, 72.48, 77.26,
83.44, 121.93, 126.59, 130.13, 131.70, 144.00. LC-MS (APCIþ), m/z:
551.54 [MþH]þ. Anal. calcd for C34H46O2S2: C, 74.13; H, 8.42; S, 11.64;
found: C, 74.26; H, 8.33; S, 11.64.
C
40H50O4S2: C, 72.91; H, 7.65; S, 9.73; found: C, 72.84; H, 7.69; S, 9.72.
2.2.5. Synthesis of 1,4-bis((S)-3,7-dimethyloctyloxy)-2,5-bis([2,20-
bithiophen]-5-ylethynyl)benzene (3)
1,4-Diiodo-2,5-bis[(S)-3,7-dimethyloctyloxy]benzene (6) (470 mg,
0.73 mmol), 5-ethynyl-2,20-bithiophene (11) (250 mg, 1.31 mmol), Pd
(PPh3)4 (43 mg, 0.037 mmol), CuI (14 mg, 0.074 mmol) and Et3N (60
mL) were mixed together. The resulting mixture was left under stirring
at 35 �C for 24 h, then it was cooled to room temperature, hydrolyzed
with a saturated ammonium chloride solution (50 mL) and extracted
with CH2Cl2 (3 � 70 mL). The combined organic phases were washed
with brine (100 mL), dried over anhydrous Na2SO4 and the solvent was
2.3.3. Synthesis of 4,8-bis((S)-3,7-dimethyloctyloxy)-2,6-bis(thiophen-2-
ylethynyl)benzo[1,2-b:4,5-b’]dithiophene (4)
2,6-Diethynyl-4,8-bis((S)-3,7-dimethyloctyloxy)benzo[1,2-b:4,5-b’]
3

G. Albano et al.
Dyes and Pigments 178 (2020) 108368
Scheme 1. Synthesis of 1,4-bis(((S)-3,7-dimethyloctyl)oxy)-2,5-diethynylbenzene (10).
2.4. Characterization
dithiophene (13) (200 mg, 0.36 mmol), 2-iodothiophene (302 mg, 1.44
mmol), Pd(PPh3)2Cl2 (13 mg, 0.019 mmol), CuI (7 mg, 0.037 mmol) and
Et3N (15 mL) were mixed together. The resulting mixture was left under
stirring for 30 h at 50 �C, then it was cooled to room temperature, hy-
drolyzed with saturated ammonium chloride solution (20 mL) and
extracted with CH2Cl2 (3 � 30 mL). The combined organic phases were
washed with brine (50 mL), dried over anhydrous Na2SO4 and the sol-
vent was removed under vacuum. The crude product was purified by
column chromatography (SiO2, petroleum ether/CH2Cl2 9:1) to give
4,8-bis((S)-3,7-dimethyloctyloxy)-2,6-bis(thiophen-2-ylethynyl)benzo
[1,2-b:4,5-b’]dithiophene (4) (193 mg, yield 75%) as a dark yellow
solid. 1H NMR (400 MHz, CDCl3), δ (ppm): 0.88 (12H, d, J ¼ 6.6 Hz),
0.99 (6H, d, J ¼ 6.6 Hz), 1.14–1.27 (6H, m), 1.28–1.43 (6H, m),
1.51–1.60 (2H, m), 1.64–1.72 (2H, m), 1.75–1.83 (2H, m), 1.89–1.97
(2H, m), 4.26–4.35 (4H, m), 7.04–7.06 (2H, m), 7.34–7.37 (4H, m), 7,62
(2H, s). 13C NMR (100 MHz, CDCl3), δ (ppm): 19.77, 22.64, 22.74,
24.73, 28.00, 29.78, 37.32, 37.58, 39.27, 72.50, 86.81, 88.68, 122.41,
122.71, 125.37, 127.31, 128.29, 130.34, 131.96, 132.74, 143.93. LC-MS
(APCIþ), m/z: 716.07 [MþH]þ. Anal. calcd for C42H50O2S4: C, 70.54; H,
7.05; S, 17.94; found: C, 70.61; H, 7.01; S, 17.93.
UV–Vis characterisation of the fluorophores was performed on a
Shimadzu UV-2450 double beam spectrophotometer, with temperature
control to within �0.1 �C. Fluorescence experiments were done either
on a PerkinElmer LS55 spectrofluorometer or on a Horiba Jobin Yvon
FluoroLog®-3 spectrofluorometer, with temperature control to within
�0.1 �C. Stock solutions of each dye in the solvent (chloroform with 1%
EtOH, toluene, acetonitrile, acetone) were typically 10À 2 M, whereas
working solutions were 10À 4 M for absorbance and 10À 5 M in the case of
fluorescence measurements. Temperature was fixed at 23.0 �C. For all
dyes, the stability of absorbance/fluorescence signal over time (several
hours) was checked (not shown). Also, the absence of auto-aggregation
phenomena was confirmed by the constancy of plots of the ratio be-
tween the absorbance values at two distinct wavelengths for different
dye concentrations (Figs. S1–S7 in Supporting Information). Quantum
yields were calculated using Quanta Horiba-Φ integrating sphere con-
nected to a Horiba Jobin Yvon FluoroLog®-3 spectrofluorometer at λexc
¼ λ
.
max abs
PMMA thin films of dye 5 were prepared by drop-casting, i.e. pouring
1.2 mL of a CHCl3 solution containing ~60 mg of the polymer and
different contents (0.2–1.8 wt. %) of 5 on a 50 � 50 � 3 mm optically
pure glass substrate (Edmund Optics Ltd BOROFLOAT window 50 � 50
TS). The glass slides were cleaned with chloroform and immersed 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 per-
formed on a warm hot plate (about 30 �C) and in a closed environment.
The film thickness was measured by a Starrett micrometer to be 25 � 5
2.3.4. Synthesis of 3,3’-(4,8-bis((S)-3,7-dimethyloctyloxy)benzo[1,2-
b:4,5-b’]dithiophene-2,6-diyl)bis(1-(thiophen-2-yl)prop-2-yn-1-one) (5)
2,6-Diethynyl-4,8-bis((S)-3,7-dimethyloctyloxy)benzo[1,2-b:4,5-b’]
dithiophene (13) (200 mg, 0.36 mmol), thiophene-2-carbonyl chloride
(138 mg, 0.94 mmol), Pd(PPh3)4 (21 mg, 0.018 mmol) and Et3N (15 mL)
were mixed together. The resulting mixture was left under stirring for
48 h at 50 �C, then it was cooled to room temperature, hydrolyzed with
saturated ammonium chloride solution (20 mL) and extracted with
CH2Cl2 (3 � 30 mL). The combined organic phases were washed with
brine (50 mL), dried over anhydrous Na2SO4 and the solvent was
removed under vacuum. The crude product was purified by column
chromatography (SiO2, n-hexane/CH2Cl2 1:2) to give 3,3’-(4,8-bis((S)-
3,7-dimethyloctyloxy)benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl)bis(1-
(thiophen-2-yl)prop-2-yn-1-one) (5) (158 mg, yield 57%) as a red-
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 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 when perpendicular to the light
1
orange solid. H NMR (400 MHz, CDCl3), δ (ppm): 0.90 (12H, d, J ¼
6.6 Hz), 1.03 (6H, d, J ¼ 6.6 Hz), 1.16–1.45 (12H, m), 1.53–1.62 (2H,
m), 1.68–1.87 (4H, m), 1.94–2.02 (2H, m), 4.33–4.42 (4H, m), 7.25 (2H,
source (ISC
)
m), 7.79 (2H, d, J ¼ 4.9 Hz), 7.95 (2H, s), 8.05 (2H, d, J ¼ 3.7 Hz). 13
C
ILSC
ISC⋅G
ηopt
¼
NMR (100 MHz, CDCl3), δ (ppm): 19.73, 22.62, 22.71, 24.69, 27.96,
29.78, 37.30, 37.51, 39.23, 72.88, 84.79, 92.07, 120.36, 128.47,
130.45, 131.12, 132.33, 135.36, 135.69, 144.55, 144.70, 168,91. LC-MS
(APCIþ), m/z: 772.05 [MþH]þ. Anal. calcd for C44H50O4S4: C, 68.53; H,
6.54; S, 16.63; found: C, 68.67; H, 6.42; S, 16.64.
where G is the geometrical factor (in our case, G ¼ 16.6), which is the
ratio between the area exposed to the light source and the collecting
area.
4

G. Albano et al.
Dyes and Pigments 178 (2020) 108368
Scheme 2. Synthesis of new p-phenylene-based fluorophores 1 and 2.
Scheme 3. Synthesis of 1,4-bis((S)-3,7-dimethyloctyloxy)-2,5-bis([2,20-bithiophen]-5-ylethynyl)benzene (3).
3. Results and discussion
solubility and also to reduce possible molecules aggregation in the solid
state. Moreover, 3,7-dimethyloctyl moiety was easily available from
citronellol after hydrogenation and subsequent bromination with N-
bromo succinimide (Scheme S1 in Supporting Information). Initially, we
treated diiodo derivative 6 with trimethylsilylacetylene (8) under
Sonogashira reaction conditions and submitted the obtained product 9
to desilylation promoted by KOH in THF/MeOH solution to give 10
(Scheme 1).
3.1. Synthesis of new p-phenylene- and benzodithiophene-based
fluorophores 1-5
Target molecules 1–5 are new compounds and were prepared start-
ing from, respectively, 1,4-bis(((S)-3,7-dimethyloctyl)oxy)-2,5-diiodo-
benzene (6) (Scheme S2 in Supporting Information) and 2,6-dibromo-
4,8-bis(((S)-3,7-dimethyloctyl)oxy)benzo[1,2-b:4,5-b’]dithiophene (7)
(Scheme S4 in Supporting Information). Both nuclei were characterized
by a long branched chain connected to the oxygen atoms to increase the
1,4-Bis(((S)-3,7-dimethyloctyl)oxy)-2,5-diethynylbenzene (10) was
then employed in the synthesis of 1 and 2 via Sonogashira reaction [36,
37] with 2-iodothiophene (Scheme 2, route a) and thiophene-2-carbonyl
Scheme 4. Synthesis of 2,6-diethynyl-4,8-bis((S)-3,7-dimethyloctyloxy)benzo[1,2-b:4,5-b’]dithiophene (13).
5

G. Albano et al.
Dyes and Pigments 178 (2020) 108368
Scheme 5. Synthesis of new benzo[1,2-b:4,5-b’]dithiophene-based fluorophores 4 and 5.
Fig. 2. Appearance of CHCl3 solutions of our new p-phenylene- and benzodithiophene-based fluorophores (from left to right: 10, 1, 3, 2, 13, 4 and 5), illuminated
with daylight (up photo) and irradiated with UV light at 365 nm (down photo).
chloride (Scheme 2, route b) affording the expected products in good to
excellent yields.
derivative 13.
2,6-Diethynyl-4,8-bis((S)-3,7-dimethyloctyloxy)benzo[1,2-b:4,5-b’]
dithiophene (13) was then employed for the synthesis of the final
compounds 4 and 5. Analogously to p-phenylene-based fluorophores 1-
2, a classic Sonogashira reaction and an acyl version of the same process
1,4-Bis((S)-3,7-dimethyloctyloxy)-2,5-bis([2,20-bithiophen]-5-yle-
thynyl)benzene (3) was generated according to a different synthetic
pathway. Indeed, it was prepared by direct Sonogashira reaction of 10
with 1.8 equivalents of 5-ethynyl-2,20-bithiophene (11) (Scheme 3). The
mild experimental conditions employed during the cross-coupling re-
action were due to the thermal instability of 11 which was prepared
immediately before use starting from dithiophene via a three steps
sequence (Scheme S3 in Supporting Information).
–
–
–
–
–
were used for the formation of C C-thiophene and C C–C O-thio-
–
–
–
phene bonds (Scheme 5).
3.2. Spectroscopic characterization in solution
Concerning the synthesis of benzo[1,2-b:4,5-b’]dithiophene (BDT)
derivatives, 7 was treated with an excess of trimethylsilylacetylene (8),
in the presence of Pd(PPh3)4 as the catalyst and Et3N both as the base
and the solvent (Scheme 4). After purification of the crude product, 12
was isolated in almost quantitative yield (93%). Analogously to com-
pound 9, removal of TMS protecting group was easily achieved by means
of aqueous KOH at room temperature, which yielded diethynyl
We started our study with the investigation of the optical properties
of fluorophores 1-5, together with those of the corresponding alkynes 10
and 13, in CHCl3 solutions (Fig. 2). The main results of each compound
(maximum absorbance, extinction coefficient, maximum emission,
Stokes shifts and quantum yield) are collected in Table 1, while full
absorbance and fluorescence emission spectra (Figs. S8–S14) and
absorbance vs. concentration plots (Figs. S15–S25) are depicted in
6

G. Albano et al.
Dyes and Pigments 178 (2020) 108368
Table 1
Optical properties of the p-phenylene- and benzodithiophene-based fluorophores in CHCl3 solution.
[b]
Compound
λmax abs (nm)[a]
ε(λmax) (MÀ 1cmÀ 1
2.8 � 104
)
λmax fl (nm)[c]
Stokes Shift (nm)[d]
68
Quantum yield Φ (%)[e]
403
471
19.2
336
378
0.9 � 104
5.7 � 104
377
409
41
31
17.1
21.1
334
410
4.0 � 104
2.4 � 104
472
454
138
62
26.0
29.7
408
8.5 � 104
46
29.6
330
395
6.4 � 104
2.1 � 104
451
485
581
121
56
44.0
44.4
370
421
6.6 � 104
4.2 � 104
115
64
10.6
13.0
386
459
5.5 � 104
2.4 � 104
195
122
10.5
15.3
a
Maximum of light absorbance.
b
Extinction coefficient.
c
d
e
Maximum of light emission; excitation wavelength is λmax abs
.
Stokes shift is defined as λmax fl - λmax abs
.
Calculated using Quanta Horiba-Φ integrating sphere at λexc ¼ λmax abs
.
Table 2
Optical properties of p-phenylene-based fluorophore 2 in different solvents, T ¼ 23.0 �C.
[b]
Solvent
Polarity
λmax abs (nm)[a]
ε(λmax) (MÀ 1cmÀ 1
)
λmax fl (nm)[c]
Stokes Shift (nm)[d]
Quantum yield Φ (%)[e]
Toluene
2.38
331
404
334
410
405
330
406
3.8 � 104
2.3 � 104
4.0 � 104
2.4 � 104
1.9 � 104
4.3 � 104
2.4 � 104
456
125
52
11.8
18.1
26.0
29.7
42.6
28.5
38.9
Chloroform
4.81
472
138
62
Acetone
21.0
38.8
478
489
73
Acetonitrile
159
83
a
Maximum of light absorbance.
Extinction coefficient.
b
c
d
e
Maximum of light emission; excitation wavelength is λmax abs
.
Stokes shift is defined as λmax fl - λmax abs
.
Calculated using Quanta Horiba-Φ integrating sphere at λexc ¼ λmax abs
.
Supporting Information.
Di-thiophene units further increased Φ up to 30% and enhanced the
bathochromic effect (Fig. S11). Carbonyl groups slightly increased Φ
being supported by the larger red-shift of the emission compared to that
of the absorption, so that the superimposition between absorbance
spectrum and emission one was diminished (Fig. S10); interestingly, the
presence in 2 of long aliphatic chains favoured solubilisation and pro-
duced a quantum yield increase with respect to the methoxy species A
previously described [35]. These features let fluorophore 2 be a better
candidate for application as LSC.
In general, we observed that the extension of conjugation produced
significant bathochromic and auxochromic effects (the latter in the case
of 1–3). As for the species containing the hydroquinone nucleus, by
comparing the data on 10 and 1 (see also Figs. S8–S9), we found that the
insertion of thiophenes on the acetylenic positions produced an increase
in both absorption and emission properties with a quantum yield (Φ)
rising to 20%, even in the presence of a lower Stokes shift. This feature
can be possibly addressed to the increase in molecular rigidity provided
by the enhanced molecular conjugation that, in turn, reduced the
probability of de-excitation processes via non-radiative pathways [38].
The dependence of the optical features of 2 on the solvent was then
tested (Table 2 and Fig. 3). The performances increased by increasing
7

G. Albano et al.
Dyes and Pigments 178 (2020) 108368
Fig. 3. a) Absorbance spectra of 2 in different solvents; concentration ¼ 1.0 � 10À 5 M. b) Emission spectra of 2 in different solvents; concentration ¼ 6.5 � 10À 8 M.
For acetone cut-off at 330 nm.
Table 3
Optical properties of benzodithiophene-based fluorophore 5 in different solvents, T ¼ 23.0 �C.
[b]
Solvent
Polarity
λmax abs (nm)[a]
ε(λmax) (MÀ 1cmÀ 1
)
λmax fl (nm)[c]
Stokes Shift (nm)[d]
Quantum yield Φ (%)[e]
Toluene
2.38
381
459
386
459
379
450
379
445
5.1 � 104
2.1 � 104
5.5 � 104
2.4 � 104
5.6 � 104
2.2 � 104
4.3 � 104
2.4 � 104
552
171
93
60.9
87.0
10.5
15.3
2.7
Chloroform
Acetone
4.81
21.0
38.8
581
574
/
195
122
195
124
/
4.7
Acetonitrile
/
a
Maximum of light absorbance.
Extinction coefficient.
b
c
Maximum of light emission; excitation wavelength is λmax abs
.
d
e
Stokes shift is defined as λmax fl - λmax abs
.
Calculated using Quanta Horiba-Φ integrating sphere at λexc ¼ λmax abs
.
Fig. 4. a) Absorbance spectra of 5 in different solvents; concentration ¼ 1.6 � 10À 5 M. b) Emission spectra of 5 in different solvents; concentration ¼ 1.2 � 10À 6 M.
For acetone cut-off at 330 nm.
8

G. Albano et al.
Dyes and Pigments 178 (2020) 108368
Fig. 5. Absorbance (a) and emission (b) spectra of 5 as thin films in PMMA (thickness 25 � 5
μ
m) at different dye concentrations (0.2–1.8 wt. %). For emission
spectra, excitation wavelength is λmax abs
.
the solvent polarity. In particular, absorption data did not change much
Table 4
while λ
and Φ increased. Fig. S26 in Supporting Information
showedmtahxeflappearance of solutions of 2 in the different solvents,
Optical properties of 5/PMMA thin films at different dye concentrations
(0.2–1.8 wt. %), T ¼ 23.0 �C.
Concentration
(wt. %)
together with
a
Lippert-Mataga plot [39] to enlighten its
λmax abs
λmax fl
Stokes Shift
(nm)[c]
Quantum yield
solvatochromism.
(nm)[a]
(nm)[b]
Φ (%)[d]
Concerning the species with benzodithiophene core (Table 1 and
0.20
0.60
1.00
1.40
1.80
450
450
450
450
450
550
560
568
575
567
100
110
118
125
117
53.3
33.8
27.2
20.1
16.4
Figs. S12–S14), the increase of π-conjugation from 13 to 4 and 5 did not
reflect in a quantum yield increase which, instead, is significantly
decreased. This is likely to be ascribed to the rotation around the alkyne-
thiophene bond. The molar extinction coefficient remained almost un-
changed, but Stokes shift increased and reached 122 nm for compound
5. Therefore, despite a low quantum yield, the very low superimposition
of absorption and emission profiles let 5 be the best candidate for
application in LSCs. Also, light emission of 5 is red-shifted until the re-
gion of maximum traditional silica cells efficiency (λ > 500 nm).
Therefore, similarly to 2, fluorophore 5 was selected for further char-
acterisation in different solvents (Table 3 and Fig. 4).
a
Maximum of light absorbance.
b
c
Maximum of light emission; excitation wavelength is λmax abs
.
Stokes shift is defined as λmax fl - λmax abs
.
d
Calculated using Quanta Horiba-Φ integrating sphere at λexc ¼ λmax abs
.
The obtained results (see also Fig. S27 for the appearance of solutions
of 5 and Lippert-Mataga plot) showed that a polarity increase produced
a marked decrease in fluorophore efficiency. In particular, fluorescence
emission is lost in acetonitrile, and the same sharp drop is observed for Φ
values. On the whole, compound 5 seems a good candidate for LSCs
applications due to light emission in the red, good quantum yield in
toluene (which is close to the PMMA matrix as for polarity), possibly
high dye/matrix compatibility grace to long and branched alkyl chains
that endow high solubility into organic solvents.
Fig. 6. Optical micrographs of 5/PMMA thin films with 1.0 wt. % (left) and 1.4
wt. % (right) dye content, under excitation at 366 nm. Film size ¼ 5 � 5 mm2.
3.3. Spectroscopic characterization of benzodithiophene-based
fluorophore 5 in PMMA films
On the contrary, 5/PMMA films displayed emission features
adversely affected by the fluorophore concentration (Fig. 5b). When
dispersed in PMMA at a concentration of 0.2 wt. %, 5 displayed a bright
fluorescence emission peaked at 550 nm with a Stokes shift of 100 nm
and Φ ¼ 53.3% (Table 4), again in agreement with results collected in
toluene solutions.
Therefore, dye 5 was selected for additional tests as thin films into
the polymeric PMMA matrix, prepared by casting on a 50 � 50 � 3 mm
glass substrate 1.2 mL of a chloroform solution containing 60 mg of
PMMA and different contents (0.2–1.8 wt. %) of 5. The absorption
features of 5/PMMA films (thickness 25 � 5 μm) are displayed in Fig. 5a.
Above this content, the fluorescence band did not display any
evident quenching albeit a significant decrease of Φ up to 20% was
detected at 1.4 wt. % concentration and flanked by a red-shift of its
maximum of 15 nm, which is possibly addressed to auto-absorption
phenomena [40–42]. Actually, the enhanced absorption of light by the
increased amount of fluorophores added in the polymer matrix pro-
gressively contributed in the absorption of part of the emission placed at
Compound 5 in PMMA showed absorption maxima at about 380 and
450 nm, appearing mostly similar to that recorded in toluene solution (i.
e., 381 and 459 nm, respectively) and with absorbance intensities that
increased regularly with fluorophore content. Notably, no evident ab-
sorption bands attributed to the formation of 5 aggregates can be
noticed, most probably thanks to the compatibilizing effect provided by
the alkyl chains linked to the benzodithiophene core.
9

G. Albano et al.
Dyes and Pigments 178 (2020) 108368
investigated organic solvents thanks to the alkyl chains linked to the
chromophoric units. Spectroscopic investigations in solution evidenced
that the fluorophore based on the benzodithiophene core bearing the
carbonyl-thiophene residues appeared as the most promising for LSCs
applications thanks to the good quantum yield (> 60% in toluene), large
Stokes shift (> 90 nm) and emission peaked at 550 nm. PMMA thin films
containing 1 wt. % of the selected fluorophore showed the highest op-
tical efficiency of 8% that is equivalent to that recorded for films con-
taining the red-emitting Lumogen Red and therefore promising for solar
collectors applications.
Declaration of competing interest
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.
CRediT authorship contribution statement
Gianluigi Albano: Data curation. Tony Colli: Data curation. Tarita
Biver: Supervision. Laura Antonella Aronica: Supervision. Andrea
Pucci: Supervision.
Fig. 7. Optical efficiencies (ηopt) of 5/PMMA thin films as a function of fluo-
rophore concentration.
Acknowledgements
high energy (inner-filter effect), i.e. close to the tail of the absorption
spectrum. This phenomenon eventually caused a substantial red-shift of
the emission due to the decreased contribution of the high-energy
portion of the fluorescence band. Nevertheless, the emission of 5 in
PMMA thin films persisted at the highest content (1.8 wt. %), i.e.
showing a broad fluorescence peaked at 567 nm and with a Φ of 16.4%.
Inspections of optical micrographs of 5/PMMA samples taken under the
excitation of a long-range UV lamp at 366 nm (Fig. 6) revealed the
absence of any macroscopic phase segregation of 5 within the polymer
matrix also at high fluorophore content, that is possibly addressed to the
compatibilizing characteristics of the alkyl chains linked to the chro-
mophoric core. Fluorescence quenching of 5 within PMMA can be
therefore addressed to auto-absorption phenomena (i.e., inner-filter ef-
fects), that is possibly neglecting the influence of the aggregation-caused
quenching phenomenon.
Funding from University of Pisa – PRA_2017_28 is gratefully
acknowledged.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2020.108368.
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