Synthesis and photophysical properties of tetraphenylethylene derivatives as luminescent downshifting materials for organic photovoltaic applications

Article abstract of DOI:10.1016/j.dyepig.2021.109724

Luminescent Down-Shifting (LDS) is an optical approach applied in several photovoltaic (PV) technologies in which high energy solar radiation is converted to a wavelength region where the response of the photovoltaic devices is better. The use of LDS laye

Full text of DOI:10.1016/j.dyepig.2021.109724

Dyes and Pigments 195 (2021) 109724
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and photophysical properties of tetraphenylethylene derivatives
as luminescent downshifting materials for organic
photovoltaic applications
*
´
˜
Helio Lopes Barros, Maria Alexandra Esteves, Maria Joao Brites
National Laboratory of Energy and Geology (LNEG), Renewable Energies and Energy Efficiency Unit. (UEREE), Estrada do Paço do Lumiar, 22, 1649-038, Lisboa,
Portugal
A R T I C L E I N F O
A B S T R A C T
Keywords:
Luminescent Down-Shifting (LDS) is an optical approach applied in several photovoltaic (PV) technologies in
which high energy solar radiation is converted to a wavelength region where the response of the photovoltaic
devices is better. The use of LDS layers on organic photovoltaics (OPV) could serve two purposes: to prevent cell
degradation by filtering the incident ultraviolet (UV) radiation and to improve the spectral response of PV cells at
short-wavelength. This work reports, the design and synthesis of a series of tetraphenylethylene (TPE) derivatives
Organic solar cells
Luminescent dyes
UV filters
Luminescent down-shifting
Intramolecular charge-transfer
based on TPE-A or A-π-TPE-π-A molecular structure featuring various electron-acceptor (A) groups. The photo-
physical properties of the new LDS compounds were systematically studied in 1,4-dioxane solution and film
(Zeonex) by UV–visible absorption and fluorescence spectroscopy, and electrochemical properties studied by
cyclic voltammetry. Thermal stability of the new LDS compounds was evaluated by Thermogravimetric analysis
(TGA). Theoretical computational studies provided evidence of existence of intramolecular charge-transfer (ICT)
between frontier orbitals of donor and acceptor moieties. The good photophysical and thermal properties of the
synthesized TPE derivatives, associated with high molar absorption coefficients in UV spectrum and emission
maximum in the range of 476–531 nm, make them promising candidates for LDS layers in OPV application.
1. Introduction
efficiency and stability enhancements via LDS layer on these PV mate-
rials and devices were investigated only in a few number of published
Luminescent Down-Shifting (LDS) is an optical approach applied in
several photovoltaic (PV) technologies to improve the spectral match
between solar cells and the distribution of sunlight (AM 1.5 G). The LDS
layer consists of a polymer sheet (e.g poly (methyl methacrylate PMMA,
ethylene vinyl acetate EVA polymer) with luminescent molecules
embedded in it, which can absorb the short wavelength (λ) photons,
where the PV cell responds poorly (λ < 400 nm) and re-emit photons in
the visible spectrum, where its spectral response is much greater.
The use of LDS layers positioned on top of the PV cell (Scheme 1)
could serve two purposes: to prevent cell degradation by filtering the
incident ultraviolet (UV) radiation and to improve the spectral response
of PV cells at short-wavelength [1]. So far, the majority of studies have
focused on the effects of LDS in the external quantum efficiency (EQE) of
solar cells [2–5]. Although the stability issue is a major obstacle for the
commercialization of the emerging PV technologies, such as organic
photovoltaic (OPV) and perovskite (PSC) cells, the potential for
studies [1,6,7]. It’s known that some components of OPV, namely the
active layer, are susceptible to photodegradation, due to the combined
effect of the UV component of solar radiation and oxygen from the
ambient air [8–10]. Kettle at al [1]. and Fernandes et al. [7,11] have
reported promising results demonstrating the proof of concept of
improvement of both the lifetime and efficiency of OPV cells by applying
LDS layers based on a lanthanide-based metal complex [1] or a mixing of
two luminescent materials forming multi-dye blends, respectively [7,
11].
Over the years, a large number of luminescent materials, such as
organic dyes [7], quantum dots [12,13] and rare–earth ion complexes
[14,15] have been investigated for their use as LDS layers. In particular,
organic dyes have been highlighted for presenting high absorption co-
efficient, good Luminescent Quantum Yield (LQY), large Stokes shift i. e,
good shift between absorption and emission bands, and good solubility
in polymeric matrix [11,16]. However, most of the organic dyes already
* Corresponding author.
E-mail addresses: helio.barros@lneg.pt (H.L. Barros), alexandra.esteves@lneg.pt (M.A. Esteves), mjoao.brites@lneg.pt (M.J. Brites).
https://doi.org/10.1016/j.dyepig.2021.109724
Received 5 March 2021; Received in revised form 21 July 2021; Accepted 14 August 2021
Available online 15 August 2021
0143-7208/© 2021 Elsevier Ltd. All rights reserved.

H.L. Barros et al.
Dyes and Pigments 195 (2021) 109724
purchased from TCI and were used without further purification, except
for tetrahydrofuran (THF) and acetonitrile which were previously dried
by standard procedures [23]. Zeonex and PMMA (polymethyl methac-
rylate) polymers were purchased from Aldrich. Hexyl-2-cyanoacetate
and Pd(PPh3)2Cl2 were prepared by previously described procedures
[24,25]. Spectroscopic solvents (Eurisotop® and Cambridge isotope
Laboratories, Inc®) were used to prepare samples for spectroscopic
characterization. 1H and 13C nuclear magnetic resonance (NMR) spectra
were obtained with a BRUKER, AVANCE 400 MHz Ultra Shield spec-
trometer at the Faculty of Sciences of Lisbon University. The chemical
shifts are reported in δ (ppm) and coupling constants in Hertz (Hz).
Fourier-transform infrared spectroscopy (FTIR) spectra were obtained in
KBr pellet and NaCl cells using an Elmer Spectrum BX v5.3.1 spec-
trometer. High resolution mass spectra (HR-MS) with electron spray
ionization (ESI) were performed on a microTOF-Focus mass spectrom-
eter at CACTI of Vigo University. Melting points were determined in a
Reichert Thermovar apparatus and were uncorrected. Thin layer chro-
matography (TLC) was performed on silica gel 60 F254 plates with 0.2
mm layer thickness from Merck, and column chromatography (CC) was
carried out with Merck Si gel 60 (230–400 mesh). The synthesized
products were observed under UV light at 254, 365 or 395 nm. Ab-
sorption spectra were recorded in quartz cuvettes using a Shimadzu
UV-3101PC UV–Vis–NIR spectrophotometer. The fluorescence mea-
surements were obtained using a Horiba Jobin Yvon Fluorolog 3 spec-
trometer. The excitation wavelength beam was adjusted to the
maximum absorbance of the derivatives in films and the maximum
wavelength of the reference standard for measurement in solution of 1,
4-dioxane. The photophysical experiments were all performed at room
temperature and in a concentration range from 10ꢀ 4 to 10ꢀ 5 M. The
Zeonex-based films were prepared by mixing 1 mg of synthesized dye
and 100 mg of polymer (Zeonex) in 1 mL of toluene. The mixture was
stirred at room temperature overnight or until the polymer was
completely dissolved. The dye-polymer mixture was poured into a
quartz piece (4 × 1 cm) by drop-casting using a Pasteur pipette. The
solvent was evaporated under room temperature and further dried in a
desiccator under vacuum for 48 h. The PMMA-based films were pre-
pared by solution of 25 mg of PMMA in 1 mL of toluene with different
concentration of dye D8 and D11 (0.03, 0.05, 0.11, 0.34 and 0.57% wt).
The mixture solution was sonicated in an ultrasound bath for 20 min and
deposited into a quartz piece (4 × 1 cm) by spin-coating at 100 rpm for
Scheme 1. Schematic of the down-shifting mechanism of a PV cell with a LDS
layer on top.
tested for LDS [7,17] were designed for other applications (e.g BASF
Lumogen dyes [18], showing narrow absorption bands or spectral
mismatch between their emission light and the spectral response of solar
cells.
To improve the efficiency and stability of OPV, an ideal LDS material
should exhibit optical properties, such as wide absorption of UV
component of solar radiation, high absorption coefficient, high Lumi-
nescence Quantum Yield (LQY), high transmittance, large Stokes Shift
and long-term photo and thermal stability [7].
The tetraphenylethene (TPE) molecule, exhibiting Aggregation-
induced Enhanced Emission (AEE) characteristics, is a versatile build-
ing block, for the development of luminescent materials with efficient
solid-state emissions [19]. TPE derivatives have been reported as sen-
sitizers for dye-sensitized solar cells [20,21], however up to date, their
use as LDS materials are scarce and limited to Cadmium Tellurium
(CdTe) PV. Li et al. [5] reported the applications of some TPE dyes for
CdTe solar cell, with an increase in the output short circuit current
density (Jsc) of 5.69% for a small cell and 8.88% for a large panel. These
fluorophores proved to exhibit superior capabilities of LDS when
compared with BASF Lumogen dyes, violet (V570) and yellow (Y083)
commercially available [22].
60s using
a
LAURELL®
–
technologies corporation, model
WS-650HZ-23NPPB/OND. The solvent was evaporated under room
temperature and further dried in a desiccator under vacuum for 24 h.
One way to fine-tuned optical properties of organic dyes is achieved
by manipulating the charge densities of the donor and/or the acceptor,
allowing to reach various Intramolecular Charge transfer (ICT) rates,
from the donor to the acceptor upon molecular excitation. A high ICT
rate will result in a large change in the transition dipole moment of the
molecule, and consequently induces a large Stokes shift.
2.2. Synthetic procedures
2.2.1. 2,6-Difluoro-4-(1,2,2-triphenylvinyl)benzaldehyde (6)
A mixture of 2-bromo-1,1,2-triphenylethylene 5 (168 mg, 0.5 mmol),
Pd(PPh3)2Cl2 (30 mg, 0.05 mmol) and K2CO3 (690 mg, 5 mmol) in 20
mL of THF/H2O (4:1) was stirred in reflux at 85 ◦C under N2. The
coupling reagent, 3,5-difluoro-4-formylphenyl boronic acid (111 mg,
0.6 mmol), was dissolved in THF and added to the reaction mixture after
30 min using a syringe. The reaction was followed by thin layer chro-
matography using a mixture of hexane/ethyl acetate (5:1) as eluent. As
soon as the reaction was completed (24 h), the reaction mixture was
filtered in vacuum over Celite to remove metal catalyst. After that, 30
mL of water was added and extracted twice with 15 mL of dichloro-
methane. The organic phase was dried over anhydrous magnesium
sulfate and the solvent evaporated under reduced pressure. The product
was purified by column chromatography on silica gel, using dichloro-
methane/hexane (1/2) as eluent. The product was obtained as a light
In this study, we report the synthesis of luminescent down shifter
tetraphenylethene (TPE) derivatives designed for enhancing efficiency
and stability of OPV cells. Donor-Acceptor TPE derivatives were syn-
thesized using TPE core as donor and as an AEE functional group, linked
to one or two acceptors (A), with different electron withdrawing
strength, with the aim to establish a structure-properties relationship of
emission behavior. The electron-acceptor groups selected were nitro,
cyano, di-cyano, cyanoacetic acid, or hexylcyanoacetate. The photo-
physical and electrochemical properties of TPE-A and A-π-TPE-π-A
luminescent dyes were investigated, and the obtained results make them
promising LDS materials for OPV and PSC applications.
2. Experimental Section
yellow solid (163 mg,
η
= 82%). m. p.: 121–123 ◦C; FTIR (KBr): 3053,
2.1. Materials and methods
–
–
3020 (C–H aromatic), 2865, 2774 (C–H aldehyde) 1700, 1625 (C
O
–
carbonyl), 1560 (C C aromatic ring), 1443, 1420, 1409, 1208 (C–F),
–
All reagents and solvents used in the synthesis, purification, photo-
physical and electrochemical studies of the TPE derivatives were
870, 761 (C–H aromatic), 698 cmꢀ 1 (C C alkene). H NMR (400 MHz,
1
–
–
CD3COCD3): δ 6.76 (2H, d, J = 8 Hz), 7.03–7.23 (15H, m), 10.18 (1H, s)
2

H.L. Barros et al.
Dyes and Pigments 195 (2021) 109724
ppm. 13C NMR (100 MHz, CD3COCD3): δ 113.0, 115.7, 115.9, 128.0,
128.1, 128.4, 128.6, 129 (2C), 131.7 (2C), 131.9, 138.9, 142.7, 143.3,
143.4, 145.4, 153.9, 161.9, 164.5, 184.3 ppm. HRMS (ESI) [M+H]+: m/
z calcd. (C27H18F2O) 397.1405; found 397.1391.
2.2.5. 1,2-bis(4-Bromophenyl)-1,2-diphenylethene (8)
TiCl4 1 M in toluene (4.21 mL, 4.21 mmol) was added to a mixture of
4-bromobenzophenone (7) (1 g, 3.83 mmol) and zinc dust (0.50 g, 7.66
mmol) in dry THF (10 mL) in a round bottom flask under nitrogen at-
mosphere. The mixture was stirred at reflux temperature until complete
consumption of 7 (20 h) as judged by TLC (dichloromethane/hexane, 1/
3). The mixture was quenched with 10% K2CO3 solution and filtered.
The filtrate was extracted three times with THF and the resulting organic
layer was washed with brine and dried over anhydrous magnesium
sulfate. After solvent evaporation, the crude product was purified by
silica column chromatography using dichloromethane/hexane (1/3) as
eluent. A mixture (1/1) of the cis and trans isomers of the product was
obtained as a white solid after recrystallization from dichloromethane/
2.2.2. Hexyl 2-cyano-3-(2,6-difluoro-4-(1,2,2-triphenylvinyl)phenyl)acry-
late (D6)
A mixture of 2,6-difluoro-4-(1,2,2-triphenylvinyl)benzaldehyde (6)
(120 mg, 0.302 mmol), hexyl-2-cyanoacetate (77 mg, 0.453 mmol) and
ammonium acetate (9.4 mg, 0.121 mmol) in 10 mL of glacial acetic acid
was stirred at 120 ◦C in reflux under N2 for 3 h. The reaction was fol-
lowed by thin layer chromatography using a mixture of dichloro-
methane/hexane (1/2) as eluent. After cooling to room temperature, a
saturated sodium bicarbonate solution was added and the resulting
mixture was extracted by ethyl acetate. The combined organic phases
were dried with anhydrous magnesium sulfate and concentrated in
vacuo. Purification was done by column chromatography on silica gel,
using dichloromethane/hexane (1/2 then 1/1) as eluent. The product
methanol (0.636 g,
η
= 68%). m. p.: 183–185 ◦C; FTIR (KBr): 3074,
–
3055, 3020 (C–H aromatic), 1487, 1442, 1409, 1391 (C C aromatic),
–
1071, 1010, 805, 758, 696 cmꢀ 1 (C C alkene, C–H aromatic, C–Br). H
1
–
–
NMR (400 MHz, CD3COCD3): δ 6.98 (4H, t, J = 8 Hz), 7.03–7.07 (4H,
m), 7.13–7.18 (6H, m), 7.31 (2H, d, J = 8 Hz), 7.36 (2H, d, J = 8 Hz)
ppm. 13C NMR (100 MHz, CD3COCD3): δ 121.0, 121.2, 127.7, 127.8,
128.7, 128.9, 131.7, 131.8, 131.9, 133.8, 133.9, 141.4 (2C), 143.5,
143.6, 143.7, 143.8 ppm. HRMS (ESI) [M+H]+: m/z calcd. (C26H18Br2)
490.9834; found 490.9852.
was obtained as a thick yellow oil (137 mg,
η = 83%). FTIR (NaCl):
3079, 3056, 3025 (C–H aromatic), 2957, 2930, 2859 (C–H alkyl) 1734,
–
–
–
1701, 1626 (C O), 1560, 1492, 1444 (C C aromatic ring), 1258, 1215,
–
1200, 1098, 1027 (C–F, C–O), 762, 699 cmꢀ 1 (C C alkene, C–H aro-
–
–
matic); 1H NMR (400 MHz, CD3COCD3): δ 0.89 (3H, s), 1.28–1.44 (6H,
m), 1.74 (2H, t, J = 4 Hz), 4.32 (2H, brs, 6.83 (2H, d, J = 12 Hz),
7.06–7.23 (15H, m), 8.18 (1H, s) ppm. 13C NMR (100 MHz, CD3COCD3):
δ 14.2, 23.2, 26.1, 29.1, 32.1, 67.6, 109.3, 112.0, 114.3, 115.3, 115.6,
128.0, 128.1, 128.4, 128.6 (2C), 129.0 (2C), 129.1 (2C), 131.7, (2C),
131.9 (2C), 138.7 (2C), 139.0, 143.0, 143.3, 143.4, 145.3, 148.8, 151.9,
159.4 (2C), 161.9 ppm. HRMS (ESI) [M+H]+: m/z calcd. (C36H31F2NO2)
548.2356; found 548.2401.
2.2.6. 4′,4′′′-(1,2-diphenylethene-1,2-diyl)bis(3,5-difluoro-[1,1′-
biphenyl]-4-carbaldehyde) (9)
Compound 9 was synthesized by the procedure used for compound 6.
The method was used with 1,2-bis(4-bromophenyl)-1,2-diphenylethene
(8) (147 mg, 0.3 mmol), 3,5-difluoro-4-formylphenyl boronic acid (204
mg, 1.1 mmol), Pd(PPh3)2Cl2 (42 mg, 0.06 mmol) and K2CO3 (0.83 g,
6.0 mmol). Purification was performed by silica column chromatog-
raphy using ethyl acetate/hexane (1/3) as eluent, to give a mixture
(1:1.2) of the cis and trans isomers of the product as a yellow solid (121
2.2.3. 2-(2,6-Difluoro-4-(1,2,2 triphenylvinyl)benzylidene)malononitrile
(D7)
mg,
η
= 66%). m. p.: 92–94 ◦C; FTIR (KBr): 3024 (C–H aromatic), 2867
–
Compound D7 was synthesized by the procedure used for compound
D6. The method was used with 2,6-difluoro-4-(1,2,2-triphenylvinyl)
(C–H aldehyde) 1701, 1626 (C O), 1570, 1550, 1432, 1410, 1396
–
ꢀ 1
–
(C C aromatic), 1200, 1183, 1039 (C–F), 894, 701 cm (C–H aro-
–
benzaldehyde (6) (150 mg, 0.378 mmol) and malonitrile (36
μ
L,
matic, C–H alkene); 1H NMR (400 MHz, CD3COCD3): δ 7.10–7.20 (12H,
m), 7.24 (2H, d, J = 8 Hz), 7.45 (2H, d, J = 8 Hz), 7.46 (2H, d, J = 8 Hz),
7.63 (4H, t, J = 8 Hz), 10.28 (1H, s), 10.30 (1H, s) ppm. 13C NMR (100
MHz, CD3COCD3): δ 110.9, 111.2, 113.4, 127.3, 127.5, 127.7, 127.9,
128.8, 128.9, 131.9, 132.0, 132.8, 132.9, 135.9, 136.0, 142.0, 144.0,
146.1, 149.4, 162.8, 162.9, 165.5, 184.4 ppm. HRMS (ESI) [M+H]+: m/
z calcd. (C40H24F4O2) 613.1791; found 613.1780.
0.567 mmol). Purification was performed by silica column chromatog-
raphy using dichloromethane/hexane (1/1) as eluent, and the product
was obtained as a yellow foam (140 mg,
η
= 83%). m. p.: 155–157 ◦C;
FTIR (KBr): 3056, 3019 (C–H aromatic), 2922 (C–H alkyl), 1626, 1598,
ꢀ 1
–
–
–
1560, 1421 (C C aromatic ring), 1026, 1272 (C–F), 697 cm (C
C
–
alkene, C–H aromatic) 1H NMR (400 MHz, CD3COCD3): δ 6.87 (2H, d, J
= 12 Hz), 7.06–7.24 (15H, m), 8.21 (1H, s) ppm. 13C NMR (100 MHz,
CD3COCD3): δ 90.4, 108.8, 112.4, 114.1, 115.5, 115.8, 128.1, 128.2,
128.5, 128.6 (2C), 129.0 (2C), 129.1 (2C), 131.7 (4C), 131.9 (2C),
138.8, 142.7, 143.3, 143.4, 145.7, 149.5, 153.4, 159.3, 161.8. Ppm.
HRMS (ESI) [M]+: m/z calcd. (C30H18F2N2) 444.1438; found 444.1463.
2.2.7. Di-hexyl 3,3’-((1,2-diphenylethene-1,2-diyl)bis(3,5-difluoro-[1,1′-
biphenyl]-4′,4-diyl)) (2E,2′E)-bis(2-cyanoacrylate) (D9)
Compound D9 was synthesized by the procedure used for compound
D6. The method was used with 4′,4’’-(1,2-diphenylethene-1,2-diyl)bis
(3,5-difluorobiphenyl-4-carbaldehyde) (9) (107 mg, 0.175 mmol) and
hexyl-2-cyanoacetate (89 mg, 0.525 mmol). Purification was performed
by silica column chromatography using dichloromethane/hexane (1/1)
as eluent. A mixture of the cis and trans isomers of the product was
2.2.4. 2-Cyano-3-(2,6-difluoro-4-(1,2,2-triphenylvinyl)phenyl)acrylic
acid (D8)
Compound D8 was synthesized by the procedure used for compound
D6. The method was used with 2,6-difluoro-4-(1,2,2-triphenylvinyl)
benzaldehyde (6) (150 mg, 0.378 mmol) and 2-cyanoacetic acid (48
mg, 0.567 mmol). Purification was performed by silica column chro-
matography, using ethyl acetate/methanol (9/1) as eluent and the
obtained as a thick yellow oil (120 mg,
η = 75%). FTIR (NaCl): 3056
(C–H aromatic), 2957, 2932, 2860 (C–H alkyl), 1732, 1629 (C = 0),
ꢀ 1
–
1433, 1261 (C C aromatic), 1222, 1100, 1028 (C–O, C–F), 701 cm
–
(C–H aromatic, C C alkene); 1H NMR (400 MHz, CD3COCD3): δ 0.89
–
–
product was obtained as an orange foam (136 mg,
η
= 77%). m. p.:
(6H, s), 1.26–1.46 (12H, m), 1.77 (4H, brs), 4.34 (4H, s), 7.11–7.26
224–226 ◦C; FTIR (KBr): 3421 (O–H carboxylic acid), 3056, 3023 (C–H
(14H, m), 7.54 (4H, d, J = 8 Hz), 7.66 (4H, t, J 8), 8.30 (2H, s) ppm. 13
C
–
–
–
aromatic), 1628 (C O), 1597, 1560, 1491, 1419, 1389 (C C aromatic
NMR (100 MHz, CD3COCD3): δ 14.3, 23.3, 26.2, 29.2, 32.2, 67.7,
110.0110.7, 111.0, 112.4, 114.5, 127.3, 127.4, 127.8, 128.0, 128.8,
129.0, 132.0, 132.1, 132.9 (2C), 136.2, 142.1, 144.1, 144.2143.2,
146.0, 146.1, 148.0, 160.4, 162.0, 162.9 ppm. HRMS (ESI) [M+H]+: m/
z calcd. (C58H50F4N2O4) 915.3740; found 915.3778.
–
ring), 1026 (C–O, C–F), 698 cmꢀ 1 (C C alkene, C–H aromatic); H NMR
1
–
–
(400 MHz, CD3COCD3): δ 6.65 (2H, d, J = 12 Hz), 7.05–7.16 (15H, m),
8.04 (1H, s) ppm. 13C NMR (100 MHz, CD3COCD3): δ 65.1, 109.6, 114.0,
114.2, 115.5, 126.9, 127.0, 127.3, 127.6 (2C), 127.9 (2C), 128.0 (2C),
130.7 (2C), 130.8 (2C), 130.9 (2C), 138.1138.5, 142.0, 142.4, 142.7,
143.7, 148.7, 158.2, 160.7, 166.0 ppm. HRMS (ESI) [M+H]+: m/z calcd.
(C30H19F2NO2) 464.1462; found 464.1469.
2.2.8. (2-(4-Nitrophenyl)ethene-1,1,2-triyl)tribenzene (D10)
Compound D10 was synthesized by the procedure used for com-
pound 6. The method was used with 2-bromo-1,1,2-triphenylethylene
3

H.L. Barros et al.
Dyes and Pigments 195 (2021) 109724
(250 mg, 0.746 mmol), 4-nitrophenyl boronic acid (149 mg, 0.895
mmol), Pd(PPh3)2Cl2 (52.0 mg, 0.0746 mmol) and K2CO3 (1.03 mg,
7.46 mmol). Purification was performed by silica column chromatog-
raphy using ethyl acetate/hexane (1/5) as eluent, and the product was
in dry acetonitrile, with concentration of dyes fixed at 2.5 × 10ꢀ 3 M. The
potential was calibrated against ferrocene/ferrocenium couple (Fc/Fc+).
The HOMO energy level was calculated from the equation: E (HOMO) =
onset
onset
-(Eox
+ 4.8) eV, where Eox
is the onset oxidation potential
obtained as a yellow solid (211 mg,
η
= 75%). m. p.: 148–149 ◦C; FTIR
versus Ag/Ag+ [26,27].
–
(KBr): 3053 (C–H aromatic), 1590 1512 (N–O), 1491, 1345 (C C aro-
–
1
matic), 748, 699 cmꢀ 1 (C–H aromatic, C C alkene); H NMR (400 MHz,
–
–
2.5. Quantum chemical calculations
CD3COCD3) δ 8.02 (2H, d, J = 7.8 Hz), 7.31 (2H, d, J = 7.5 Hz),
7.24–6.94 (15H, m) ppm. 13C NMR (100 MHz, CD3COCD3) δ 151.0,
146.1, 143.7, 142.9, 142.8, 142.6, 139.1, 132.1, 131.0, 130.9, 128.0,
128.0, 127.8, 127.2, 127.0, 122.8 ppm. HRMS (ESI) [M+H]+: m/z calcd.
(C26H19NO2) 378.1494; found 387.1493.
Theoretical calculations were performed with Density Functional
Theory (DFT) using the Gaussian 03 Package [28]. Geometry optimi-
zation was carried out using the B3LYP/6-31G (d) functional and basis
set. All calculations were performed in the gas phase. Single point cal-
culations for HOMO (Highest Occupied Molecular Orbital) and LUMO
(Lowest Unoccupied Molecular Orbital) were determined at ground
state (S0) and first excited state (S1) from optimized geometries. The
electron density and spatial orientation of the HOMO and LUMO, and
electrostatic potential maps (ESP) [29] were obtained using the same
function. Electronic transitions were calculated using ZINDO method
and STO-6G basis set in ArgusLab software from optimized geometry by
B3LYP/6-31G (d) in Gaussian 03. To visualize the results, the Gauss view
5 software was used.
2.2.9. 4-(1,2,2-Triphenylvinyl)benzonitrile (D11)
Compound D11 was synthesized by the procedure used for com-
pound 6. The method was used with 2-bromo-1,1,2-triphenylethylene
(200 mg, 0.597 mmol), 4-cyanophenyl boronic acid (149 mg, 0.895
mmol), Pd(PPh3)2Cl2 (42 mg, 0.060 mmol) and K2CO3 (825 mg, 5.970
mmol). Purification was performed by silica column chromatography
using ethyl acetate/hexane (1/5) as eluent, and obtaining a white
product (151 mg,
η
= 71%). m. p.: 171–173 ◦C; FTIR (KBr): 3051 (C–H
–
aromatic), 2228 (CN nitrile), 1602, 1489, 1442, 1074 (C C aromatic),
–
826, 744, 630, 596 cmꢀ 1 (C C alkene, C–H aromatic); 1H NMR (400
–
–
3. Results and discussion
MHz, CD3COCD3) δ 7.54 (2H, d, J = 8.1 Hz), 7.24 (2H, d, J = 8.1 Hz),
7.17 (9H, td, J = 6.5, 3.1 Hz), 7.06 (6H, dd, J = 6.6, 2.8 Hz) ppm. 13C
NMR (100 MHz, CD3COCD3) δ 148.8, 143.2, 143.0, 142.9, 142.7, 139.5,
132.0, 131.5, 131.0, 130.9, 128.0, 128.0, 127.7, 127.0, 126.9, 118.4,
109.8 ppm. HRMS (ESI) [M+H]+: m/z calcd. (C27H19N) 358.1596;
found 358.1598.
3.1. Synthesis of TPE derivatives
Here we report the design, synthesis and characterization of new
luminescent TPE derivatives (Scheme 1) comprising TPE moiety as a
donating group linked to (1) one of the following electron acceptors:
nitro, cyano, dicyano, cyanoacetic acid, hexyl cyanoacetate, (TPE-A) or
(2) linked to two electron acceptors: nitro or hexyl cyanoacetate,
2.2.10. 1,2-bis(4′-Nitro-[1,1′-biphenyl]-4-yl)-1,2-diphenylethylene (D12)
Compound D12 was synthesized by the procedure used for com-
pound 6. The method used 1,2-bis(4-bromophenyl)-1,2-diphenylethene
(8) (155 mg, 0.316 mmol), nitrophenyl boronic acid (127 mg, 0.759
mmol), Pd(PPh3)2Cl2 (45 mg, 0.064 mmol) and K2CO3 (873 mg, 6.32
mmol). Purification was performed by silica column chromatography
using ethyl acetate/hexane (1/5) as eluent, to give a mixture (1:3) of the
through a phenyl or di-fluorophenyl bridge (A-
The new luminescent materials with a TPE-A (D6-8, 10–11) and A-
-TPE- -A (D9, D12) molecular structure were synthesized in moderate
π-TPE-π-A), respectively.
π
π
to high yields (45–83%) using the Suzuki coupling reaction and Knoe-
venagel condensation. The synthetic routes are presented in Scheme 2
and the detailed synthetic procedures are described in the Experimental
Section. All synthesized derivatives were structurally characterized by
1H and 13C NMR spectroscopy, FTIR, and mass spectrometry (see the
Experimental Section for more details. The spectral data are consistent
with the proposed structures.
cis and trans isomers of the product as a yellow solid (82 mg,
η = 45%).
m. p.: 254–256 ◦C; FTIR (KBr): 3024 (C–H aromatic), 1594, 1509 (N–O),
ꢀ 1
–
–
–
1342, 1108 (C C aromatic), 805, 698 cm
(C–H aromatic, C C
–
alkene); 1H NMR (400 MHz, CDCl3) δ 8.58 (1H, d, J = 6.8 Hz), 8.47 (3H,
d), 8.28 (1H, d, J = 7.2 Hz), 8.10 (3H, d, J = 7.5 Hz), 7.81 (3H, t, J = 8.6
Hz), 7.38 (10H, dt, J = 28.6, 13.3 Hz) ppm. 13C NMR (100 MHz,
CD3COCD3) δ 141.8 (3C), 141.7, 139.8, 139.2138.0, 135.7, 131.5,
131.4, 127.0 (2C) 126.2, 126.1, 123.1, 122.8, 122.7, 122.3, 121.8,
121.7, 121.6, 121.4, 119.2, 118.9 ppm. HRMS (ESI) [M+H]+: m/z calcd.
(C38H26N2O4) 575.1971; found 575.1953.
3.2. Photophysical properties
The photophysical properties of TPE derivatives D6-D12 were stud-
ied in 1,4-dioxane solution and in Zeonex film. Fig. 1 shows the
normalized absorption and emission spectra and the relevant spectro-
scopic data are summarized in Table 1. The absorption spectra of the
TPE derivatives in 1,4-dioxane (Fig. 1A) are quite different, with a
strong absorption band at around 250–300 nm, and a low to high in-
tensity band around 325–400 nm depending on the acceptor group
linked to the TPE core. The band at the higher wavelength is attributed
to ICT from electron donor to electron acceptor group, whilst the lowest
2.3. Thermal analysis
Thermogravimetric analysis (TGA) and Differential Scanning Calo-
rimetry (DSC) were carried on a SETSYS Evolution 1750/Simultaneous
TGA DSC Thermal Analyzer (Setaram) under nitrogen atmosphere with
gas flow of 20 mL minꢀ 1. The sample with initial mass ranged from
3.369 to 12.281 mg was analyzed in a platinum crucible with temper-
wavelength band is due to
π–π* transition from TPE core and depends on
the conjugated -system of the molecule [30]. The derivatives D11
π
ature ranged from 20 to 600 ◦C and heating rate of 10 ◦C.minꢀ 1
.
(cyano) and D12 (di-NO2) show intensive ICT absorption bands with the
maximum wavelength (λmax) at 355 nm nm and 323 nm, respectively.
The molar absorption coefficients (Ɛmax) for all derivatives ranged be-
tween 8394 and 37,809 Mꢀ 1 cmꢀ 1 at the absorption maximum (Table 1).
Derivatives D9 and D12 with two acceptor groups in their molecular
2.4. Electrochemical properties
Cyclic voltammetry (CV) was performed on a potentiostat/galvano-
stat Gamry Instrument reference 600. The electrochemical cell was
configured to work with three electrodes, a platinum disk, platinum wire
and Ag/AgCl (3 M KCl solution) as working, counter and reference
electrodes, respectively. The experiment was carried out at room tem-
perature and scan rate of 50 mV/s. The supporting electrolyte was a
solution of 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6)
structure (A-
π
-TPE-
π
-A), show the higher Ɛ
values, 37,809 and 22,
498 Mꢀ 1cmꢀ 1, respectively, which could bemdauxe to their greater conju-
gated π-system. Specifically, for the di-substituted TPE D9, Ɛmax value is
3 times greater than the correspondent mono-substituted derivative D6
and a slight redshift of 9 nm is observed. It is also noteworthy that the
introduction of a hexyl chain in cyanoacetic acid group of D8, reduce the
4

H.L. Barros et al.
Dyes and Pigments 195 (2021) 109724
Scheme 2. Synthetic routes and molecular structures of TPE derivatives D6-D12.
Ɛmax value from 12,237 to 8394 M-1cm-1 causing a redshift of 14 nm.
The solution photoluminescence (PL) spectra of D6-D12 derivatives
(Fig. 1B) show a maximum emission band (λem) between 401 and 547
nm. In the case of D6, D7, D8, D10, and D11, a shoulder is also observed
around 502, 425, 497, 415, 483 nm, respectively, that disappears in the
Zeonex film PL emission spectra (Fig. 1C). We attribute this observation
to an excited-state rotamer with weak ICT formed in solution which
disappears in solid state, due to the suppression of excited state intra-
molecular rotation of bulky groups in a rigid medium [31]. In the case of
the di-substituted TPE derivatives D9 and D12, a unique emission band
is observed, at 362 nm and 345 nm, respectively, since the existence of
rotamers in solution is restricted due to their large chemical structures.
The Stokes shifts of the compounds in solution, range between 68 and
184 nm and reflect the feature of ICT that is intrinsically associated with
the fluorophores [40]. Fluorescence quantum yields (ɸ ) of D6-D12 in 1,
F
4-dioxane solution, were determined by comparative method using
coumarin 153 in ethanol as fluorescence standard (ɸ 32%), and the
F
obtained values range from 0.05 to 0.73%. These results are consistent
5

H.L. Barros et al.
Dyes and Pigments 195 (2021) 109724
Fig. 1. Normalized absorption (solid line) (A), fluorescence emission (dashed line) (B) spectra of D6-D12 in 1,4-dioxane and Zeonex film (C). The normalized
External Quantum Efficiency (EQE) curve of P3HT:PCBM solar cell [17] is shown as black dotted line (C). Images of Zeonex films based on D6-D12 derivatives under
white and ultraviolet light of 365 nm (D), and TPE derivatives powders under ultraviolet light of 365 nm (E).
Table 1
Photophysical properties of TPE derivatives in 1,4-dioxane (experimental and theoretical) and Zeonex films.
UV–Vis absorption λmax (nm)
Fluorescence emissionb λmax (nm)
Stokes shift λ (nm)
1,4-dioxane
Film
Theoretical
1,4-dioxane
Film
1,4-dioxane
Film
D6
353 (8394)a
371 (11,473)
339 (12,267)
362 (37,809)
355 (11,633)
323 (12,928)
345 (22,498)
364
388
344
372
359
323
357
378
392
379
353
413
331
359
435 (0.36)c
547 (0.14)
407 (0.05)
538 (0.73)
532 (0.07)
401 (<LOD)d
529 (0.13)
494
515
531
495
512
476
505
82
130
127
187
123
153
153
148
D7
176
68
D8
D9
176
177
78
D10
D11
D12
184
a
Experimental molar absorption coefficient in Mꢀ 1 cmꢀ 1 in parenthesis.
b
c
The λmax abs of each derivatives was used as excitation wavelength.
Photoluminescence Quantum yield (%) in parenthesis. Coumarin 153 (38%) in ethanol with excitation at 420 nm was used as fluorescence standard.
d
LOD – Limit Of Detection.
with the visually observed low fluorescent of D6-D12 in solutions. In the
out to the AEE characteristic already observed in other TPE derivatives
[32,33], which is caused by the inhibition of intramolecular rotation in
the single molecule state.
case of D11 it was not possible to measured ɸ , because the fluorescence
F
signal intensity is below the Limit of detection (LOD). Fig. 1 (D and E)
shows the images of synthesized TPE derivatives in solid state under
ultraviolet light of 365 nm, where an intense fluorescence emission of
The photophysical properties of D6-D12 were also studied in films,
using Zeonex polymer, which has good optical transparency in the near
UV range [34]. Fig. 1C depicts the absorption and emission spectra of
blue, green and yellow colors can be seen. The low ɸ of D6-D12 in
F
solution and the high fluorescence visually observed in solid state point
D6-D12 in films. As shown, there is no significant difference in the λ
max
6

H.L. Barros et al.
Dyes and Pigments 195 (2021) 109724
of TPE derivatives in both solution and films spectra. In fact only a
redshift of approximately 10 nm is observed between their 1,4-dioxane
solution and Zeonex solid samples. It is well known by the
Franck-Condon principle [35] that the electron transition speed is
extremely faster (femtoseconds) than the process of molecular relaxa-
tion. Consequently, the absorption process is less environmentally
dependent which can explain the little difference between the λmax of
D6-D12 in solution and films. On the other hand, the emission process is
more environmentally dependent and the shape and the maximum
wavelength of the emission bands (λem) changed considerably in film
spectra.
redshift of 25 nm observed in the λem may be due to the presence of the
carboxylic group that facilitate the intermolecular interaction and
hydrogen bonds, leading to the formation of J-aggregates, which are
side-by-side molecular arrangements that cause bathochromic shift in
the absorption and emission spectra [36–38]. On the other hand, the
absorbance of films (Fig. S3) increased by increasing the concentration
of the dye over the entire concentration range tested. The absorption and
emission spectra of D8-and D11-LDS films obtained with five different
dye concentrations (0.03, 0.06, 0.11, 0.34 and 0.57 wt%), showed that
the higher absorption and fluorescence intensity were achieved for films
containing 0.34 wt% of D8 and D11.
In this case, only a single emission band in the range 476–531 nm is
observed for all derivatives. As shown in Fig. S1, the aforementioned
shoulder observed in PL solution spectra of D6, D8 and D11, attributed
to the low energy ICT emitting conformer, becomes dominant in film
spectra with an emission maximum at 494, 531 and 476 nm, respec-
tively. As expected the observed redshift in the λem leads to an increase
of Stokes shift. In the case of D7, D9, D10 and D12 films a blue shift
between 20 and 43 nm is observed in the single emission band, leading
to a decrease in Stokes shift values. Nevertheless, in Zeonex films all
derivatives show large Stokes shift between 123 and 187 nm, which will
allow to minimize self-absorption losses in LDS application.
3.3. Thermal analysis
The melting points of the D7, D10 and D11 range from 148 to 173 ◦C.
D6 and D9 with the hexylcyanoacetate acceptor group in the molecular
structure, are viscous oils. The derivative D8 and D12 show higher
melting point, which are 224–226 ◦C and 254–256 ◦C, respectively.
Thermal stability of the TPE derivatives was evaluated by TGA
analysis and TGA (A) and dTGA (B) thermograms are shown in Fig. 3. All
derivatives exhibited initial mass loss at temperature above 225 ◦C. For
derivatives D9, a mass loss of 18% occurred between 125 and 250 ◦C
(Fig. S5), possibly due to the evaporation of the solvent trapped inside.
In the case of D7, D10 and D11 the mass loss occur between 225 and
400 ◦C and the estimated temperature at which the degradation rate is
maximum (Tp, the peak temperature of the DTGA curve- Fig. 3b), is Tp
343, 351 and 347 ◦C, respectively.
Luminescent polymethylmethacrylate (PMMA) films were prepared
by spin coating technique from polymer solution doped with different
concentrations of the TPE derivatives D8 and D11. These were chosen
because they present its maximum absorption in the UVA part of the
spectrum (310–410 nm range) having also the highest Stokes shift
values, 187 and 153 nm, respectively (Fig. S2). The polymer solution
was doped by different dye concentrations ranging from 0.03 to 0.57 wt
%, and PL emission spectra of LDS films are shown in Fig. 2 and Fig. S4,
in the wavelength range of 400–700 nm. Excitation wavelength of 370
and 380 nm were used for D8 and D11, respectively. A plot of the
maximum fluorescence intensity as a function of the dye concentration
is shown in Fig. 2 (inset).
The derivatives D6 and D12 are thermally more stable than the other
ones, with the mass loss occurring between 250 and 550 ◦C, and Tp of
391 and 410 ◦C, respectively. The TGA results suggest that the type of
acceptor substituents affects the thermal properties of synthesized TPE
derivatives. Knowing that operating temperature of PV panels can ge as
hot as 65 ◦C, we conclude that all compounds (D6-D12) have good
thermal stability for use in LDS layer.
The spectra show a remarkable dependence on the dye concentration
since the fluorescence intensity increases with increasing concentration
up to 0.34 wt %. For higher concentrations, the emission intensity starts
to decrease, and in the case of D8 a redshift of 25 nm in λem is also
observed. This may be explained by the fact that in a higher concen-
tration range, the non-planar molecules, D8 and D11, become more
3.4. Electrochemical properties
The electrochemical properties of D6-D12 were determined by cyclic
voltammetry in freshly distilled acetonitrile solution using 0.1 M of
tetrabutylammonium hexafluorophosphate (TBAPF6) as supporting
electrolyte (Fig. 4). The potential was externally calibrated by ferro-
cene/ferrocenium couple (Fc/Fc+) and then was calculated versus NHE
electrode. Cyclic voltammograms of D6, D9, D10 and D11 show quasi-
reversible waves with two oxidation peaks between 1 and 1.5 eV
whereas in derivatives D7, D8 and D12 the oxidation process is irre-
versible. The HOMO levels were estimated from the first oxidation onset
closely packed and consequently the intermolecular πꢀ π stacking in-
teractions are enhanced, which promotes the formation of aggregates
with ordered or random structures. With the increase of this competing
force in the luminogen system, the AEE gives rise to fluorescence
Aggregation-Caused Quenching (ACQ), which is responsible for the
observed decreasing of fluorescence emission. In the case of D8, the
Fig. 2. Effect of concentration of the dyes D8 and D11 on the fluorescence emission of the LDS film.
7

H.L. Barros et al.
Dyes and Pigments 195 (2021) 109724
Fig. 3. Thermogravimetric (A) and derivative thermogravimetric (B) curves of synthesized TPE derivatives under nitrogen atmosphere at a heating rate of
10 ◦C minꢀ 1
.
Table 2
Electrochemical properties of TPE derivatives determined by cyclic voltammetry
and DFT method.
TPE
Eox
λ inta
E-
Experimental (CV)
Theoretical (DFT)
onset
(nm)
E0b
(eV)
E
E
E
E
vs Fc/
Fc+
HOMOc
(eV)
LUMOd
(eV)
HOMOe
(eV)
LUMOe
(eV)
D6
0.95
1.08
0.99
1.07
1.04
1.02
0.88
429
450
459
440
429
394
419
2.89
2.76
2.7
ꢀ 5.75
ꢀ 5.88
ꢀ 5.79
ꢀ 5.87
ꢀ 5.84
ꢀ 5.82
ꢀ 5.68
ꢀ 2.86
ꢀ 3.12
ꢀ 3.09
ꢀ 3.06
ꢀ 2.95
ꢀ 2.67
ꢀ 2.72
ꢀ 5.74
ꢀ 5.9
ꢀ 2.45
ꢀ 2.86
ꢀ 2.56
ꢀ 2,64
ꢀ 2.56
ꢀ 2.01
ꢀ 2.56
D7
D8
ꢀ 5.80
ꢀ 5.71
ꢀ 5.61
ꢀ 5.9
D9
2.81
2.89
3.15
2.96
D10
D11
D12
ꢀ 5.71
a
The intersect of the normalized absorption and the emission spectra in solid
films.
b
E0-0 values were estimated from the intercept of the normalized absorption
and emission spectra.
c
Deduced from equation E HOMO = - (Eoxonset+4.8).
d
e
Deduced from equation E LUMO = E HOMO + E0-0
.
Calculated with B3LYP/6-31G(d) in gaussian 03.
Theory (DFT) using Gaussian 03 software package [28] in order to better
understand the photophysical properties of synthesized TPE derivatives.
The calculated transitions associated with their oscillator strengths,
excitation energies and configuration interaction (CI) coefficients, as
well as experimental wavelengths for comparison purpose are shown in
Table S1. The calculation results allowed to observe that the transition of
greater wavelength occurred between HOMO and LUMO. A good cor-
relation is observed between the experimental and theoretical absorp-
tion maxima in 1,4-dioxane with small to moderate variation of 24, 21,
40, 9, 58, 6 and 14 nm for D6, D7, D8, D9, D10, D11 and D12, respec-
tively. If compared with experimental absorption spectra data in Zeonex
films, the difference is smaller (14, 4, 35, 19, 54, 8 and 2 nm for D6, D7,
D8, D9, D10, D11 and D12, respectively).
Fig. 4. Cyclic voltammograms of TPE derivatives in acetonitrile with TBAPF6
(0.1 M) as the electrolyte; scan rate: 50 mV/s. All voltammograms were
referenced to internal ferrocene (Fc/Fc+).
The geometry structures of the TPE derivatives were optimized at the
B3LYP/6-31G (d) level [40,41] and the electron density distribution for
the HOMO and LUMO orbitals and its corresponding energy levels were
subsequently calculated (Fig. 5). For all TPE derivatives, the HOMO
orbital is located predominantly on the TPE core and has a greater
contribution from the pz orbital. On the other hand, the LUMO orbital is
located on the substituted acceptor moiety, except for D11, where it is
uniformly distributed throughout the molecular structure due to the
weak acceptor character of the cyano group. In general, HOMO is
located orthogonal to the plane of the molecule and it is a bonding
wave (Eoxonset) by using the empirical equation (E HOMO = - (Eoxonset
+ 4.8)) [39]. The energy gap values (E0-0) were estimated from the
intercept of the normalized absorption and emission spectra in film.
The LUMO levels were determined by equation (E LUMO = E HOMO
+ E0-0). Electrochemical properties are present in Table 2. The energy
gap determined by cyclic voltammetry measurement for these new de-
rivatives (2.76–3.15 eV) is similar to the energy gap reported in the
literature (2.75–3.11 eV) for other TPE derivatives [27]. However, it is
noticeable that the substituents on the TPE units affect the HOMO and
LUMO energy levels of the TPE derivatives.
molecular orbital (π), while LUMO is also orthogonal to the plane of the
molecule, but as a non-bonding molecular orbital (π*).
The results of theoretical studies also evidenced the presence of both
π–π* and ICT transition for D6-D12, although, the contribution of one
3.5. Quantum chemical calculations
transition can be greater depending on the nature of the electrons donor
Theoretical calculations were performed with Density Functional
8

H.L. Barros et al.
Dyes and Pigments 195 (2021) 109724
Fig. 5. Electron density distributions of HOMO and LUMO orbitals and corresponding energy levels (isovalue = 0.02 au).
and acceptor [42,43]. For example, as shown in Fig. 5, the well sepa-
rated electronic distributions in the HOMO and LUMO orbitals at two
different ends featuring donor and acceptor characters for all de-
rivatives, except for D11, is expected to produce a charge-transfer be-
tween HOMO and LUMO [44]. The weak charge separation observed
between the TPE core and the cyano group in D11 was also reflected in
its experimental absorption spectrum, where a considerable difference
in wavelength was observed in relation to other derivatives due to the
electron-withdrawing strength of the acceptor group. The electrostatic
potential maps (ESP) were also determined by TDF method and are
shown in Fig. S6.
4. Conclusion
In this paper, we have successfully synthesized a series of TPE LDS
compounds, presenting in their TPE-A or A- -TPE- -A molecular struc-
π
π
ture, groups (A) with different electron withdrawing strength. The re-
sults of the quantum chemical studies are consistent with experimental
results, evidencing the existence of ICT between donor and acceptor
moieties through the analysis of the electron density of frontier orbitals.
All compounds present high absorption in the UV region (from 323 nm
to 388 nm) and fluorescence emission band close to the region of higher
external quantum yield of organic solar cells (476–531 nm). The large
Stokes shift found in all compounds (from 123 up to 187 nm), will allow
to minimize self-absorption losses in LDS layer application. All de-
rivatives exhibited high thermal stability with the decomposition tem-
perature above 225 ◦C. In compounds D6 and D9, the presence of the
hexyl chain in the cyanoacetate group has impact on their physical state,
i.e. these are highly viscous oil instead of solids. The preliminary studies
on D8-and D11-based LDS layers (TPE derivatives with larger stoke shift,
187 and 153 nm, respectively) prepared at different dye concentrations,
points out to 0.34 wt% of dye-doped PMMA films, in which high
The estimated energy levels of HOMO and LUMO orbitals as well as
the energies gap determined by theoretical calculations are consistent
with those determined by cyclic voltammetry (CV) measurement
(Table 2). The calculated energies gap between frontiers orbitals by DFT
ranged from 3.04 to 3.89 eV, and are also in accordance with the gap
energies reported in the literature (2.94–3.48 eV) [27] for other TPE
derivatives, using the same functional and basis set (B3LYP/6-31G).
9

H.L. Barros et al.
Dyes and Pigments 195 (2021) 109724
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Helio Lopes Barros: Investigation, Writing – original draft, Visual-
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Writing – review & editing, Data curation. Maria Joao Brites:
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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.
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Acknowledgments
This work was supported by Horizon H2020/European Commission
under the Project IDEAS (Novel building Integration Designs for
increased Efficiencies in Advanced climatically tunable renewable en-
ergy Systems) Grant agreement ID: 815271. H. Barros was supported by
a postdoctoral grant from the same project.
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