Synthesis and characterization of push-pull bithiophene and thieno[3,2-b]thiophene derivatives bearing an ethyne linker as sensitizers for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.orgel.2017.06.048

Five push-pull heterocyclic dyes 4–7, and 9 were synthesized and characterized in order to study the variations in the optical, electronic and photovoltaic properties induced by structural modifications, i.e. different spacer units and anchoring groups. The final push-pull conjugated dyes 6–7 and 9 are formed by a N,N-dimethylaniline donor moiety conjugated with bithiophene, ethynyl-bithiophene or ethynyl-thieno[3,2-b]thiophene spacers and cyanoacetic acid or rhodanine-3-acetic acid as anchoring group. The synthesis of the precursor aldehydes 4–5 was accomplished through a Sonogashira coupling, on the other hand, compound 8 was prepared by Suzuki coupling. Knoevenagel condensation of aldehydes 4, 5 and 8 with cyanoacetic acid or rhodamine-3-acetic acid afforded the final push-pull dyes 6, 7 and 9. Information on conformation, electronic structure and electron distribution was obtained by DFT and TDDFT calculations. This multidisciplinary study regarding the evaluation of the optical, redox and photovoltaic properties of the dyes reveals that compound 7, bearing an ethynyl-bithiophene spacer conjugated with a cyanoacetic acid anchoring group, has the highest conversion efficiency (3.51%) as dye sensitizer in nanocrystalline TiO₂ solar cells. Co-adsorption studies were also performed for dyes 6–7 with N719 as co-adsorbent, and this enhanced dye efficiencies by 33–45%. The best cell performance was obtained by co-adsorbing N719 and dye 7 (75/25 vol%) with 4.66% efficiency.

Full text of DOI:10.1016/j.orgel.2017.06.048

Accepted Manuscript
Synthesis and characterization of push-pull bithiophene and thieno[3,2-b]thiophene
derivatives bearing an ethyne linker as sensitizers for dye-sensitized solar cells
Sara S.M. Fernandes, I. Mesquita, L. Andrade, A. Mendes, Licinia L.G. Justino, Hugh
D. Burrows, M. Manuela M. Raposo
PII:
S1566-1199(17)30305-1
10.1016/j.orgel.2017.06.048
ORGELE 4173
DOI:
Reference:
To appear in: Organic Electronics
Received Date: 23 February 2017
Revised Date: 14 June 2017
Accepted Date: 18 June 2017
Please cite this article as: S.S.M. Fernandes, I. Mesquita, L. Andrade, A. Mendes, L.L.G. Justino,
H.D. Burrows, M.M.M. Raposo, Synthesis and characterization of push-pull bithiophene and
thieno[3,2-b]thiophene derivatives bearing an ethyne linker as sensitizers for dye-sensitized solar cells,
Organic Electronics (2017), doi: 10.1016/j.orgel.2017.06.048.
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ACCEPTED MANUSCRIPT
Graphical abstract
Synthesis and characterization of push-pull bithiophene and thieno[3,2-
b]thiophene derivatives bearing an ethyne linker as sensitizers for dye-
sensitized solar cells
Sara S. M. Fernandes, I. Mesquita, L. Andrade, A. Mendes, Licinia L. G. Justino, Hugh
D. Burrows and M. Manuela M. Raposo*

ACCEPTED MANUSCRIPT
Synthesis and characterization of push-pull bithiophene and thieno[3,2-
b]thiophene derivatives bearing an ethyne linker as sensitizers for dye-sensitized
solar cells
Sara S. M. Fernandes1, I. Mesquita2, L. Andrade2, A. Mendes,2 Licinia L. G. Justino,3 Hugh D.
*
Burrows3 and M. Manuela M. Raposo1
1Centro de Química, Universidade do Minho, Campus de Gualtar, 4710-057, Braga, Portugal
2LEPABE - Faculdade de Engenharia, Universidade do Porto, rua Dr. Roberto Frias, 4200-465
Porto, Portugal
3Centro de Química, Departamento de Química, Universidade de Coimbra, 3004-535. Coimbra,
Portugal
*Corresponding author. Tel: + 351 253 604381; Fax: + 351 253 604382; email:
mfox@quimica.uminho.pt
Abstract: Five push-pull heterocyclic dyes 4-7, and 9 were synthesized and characterized in
order to study the variations in the optical, electronic and photovoltaic properties induced by
structural modifications, i.e. different spacer units and anchoring groups. The final push-pull
conjugated dyes 6-7 and 9 are formed by a N,N-dimethylaniline donor moiety conjugated with
bithiophene, ethynyl-bithiophene or ethynyl-thieno[3,2-b]thiophene spacers and cyanoacetic
acid or rhodanine-3-acetic acid as anchoring group. The synthesis of the precursor aldehydes
4-5 was accomplished through a Sonogashira coupling, on the other hand, compound 8 was
prepared by Suzuki coupling. Knoevenagel condensation of aldehydes 4, 5 and 8 with
cyanoacetic acid or rhodamine-3-acetic acid afforded the final push-pull dyes 6, 7 and 9.
Information on conformation, electronic structure and electron distribution was obtained by DFT
and TDDFTcalculations. This multidisciplinary study regarding the evaluation of the optical,
redox and photovoltaic properties of the dyes reveals that compound 7, bearing an ethynyl-
bithiophene spacer conjugated with a cyanoacetic acid anchoring group, has the highest
conversion efficiency (3.51 %) as dye sensitizer in nanocrystalline TiO2 solar cells. Co-
adsorption studies were also performed for dyes 6-7 with N719 as co-adsorbent, and this
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enhanced dye efficiencies by 33-45 %. The best cell performance was obtained by co-
adsorbing N719 and dye 7 (75/25 vol%) with 4.66 % efficiency.
Keywords: dye sensitized solar cells, metal-free organic dyes, bithiophene, thieno[3,2-
b]thiophene, cyanoacetic acid, rhodanine-3-acetic acid, co-adsorption.
1. Introduction
Metal free donor-acceptor organic dyes have shown interesting features to be used as
sensitizers in dye sensitized solar cells (DSSCs) and thus replacing ruthenium based dyes.
These solar cells based on nanocrystalline TiO2 electrodes are seen as a very promising and
attractive photovoltaic technology due to high incident solar light to electricity conversion
efficiency, good performance in low light conditions and different solar incident angles, low
maintenance, good long-term stability, cheap and sustainable components, multi-colour range
possibilities, semi-transparency and flexibility. Over the last two decades, a wide range of
structural modifications to the donor or acceptor group and π-bridge has been investigated in
order to obtain organic chromophores with high performance for DSSCs [1-17].
In a photovoltaic device, the dye sensitizer plays an important role in the light absorption,
conversion efficiency and the stability of the device. It should possess some essential
characteristics, such as a push-pull structure (donor–π-bridge–acceptor), promoting efficient
charge transfer and separation on going from the ground state to the excited state, broad
spectral response (visible and near-infrared region), photostability, controlled aggregation and
charge recombination, appropriate electronic energies (HOMO, LUMO), good intramolecular
charge transfer, and an anchoring group to strongly bind onto the semiconductor surface [1-18].
Additionally, metal free dyes show advantages such as easy and low cost synthesis and
purification, diversity of molecular structural modifications which offer nearly infinite possibilities
for easily tuning the photophysical and electrochemical properties, and high molar extinction
coefficients. Metal free sensitizers, such as synthetic organic [1-17] and natural dyes [19], have,
therefore, received attention as an alternative to metal complexes for DSSC applications.
However, the lack of absorbance of red and near infra-red light of these compounds is still an
obstacle which needs to be overcome.
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Donor groups commonly used are N,N-dialkylamine, diphenylamine, triphenylamine, carbazole,
indoline, etc [20]. Additionally, electron-rich heterocycles (thiophene, pyrrole, furan) can be used
as auxiliary donor groups in these push-pull systems. Usual spacers or
are ethene, ethyne, benzene or electron-rich heterocycles [1-15, 21-27].
π-conjugated bridges
Previously, dye sensitizers applied in solar cells were mainly ruthenium complexes, consisting
of the central metal ion complexed with organic ligands containing an anchoring group.
Ruthenium is a heavy metal, not widely found in nature, and its toxicity presents a risk for the
environment. Moreover, complicated synthetic processes and difficult purification of the dyes
limit the use of Ru-based dye sensitizers in terms of cost efficiency and environmental
friendliness. However, transition metal complexes still play an important role in DSSC
development, since many of the most efficient and stable dye sensitizers are Ru(II)-based
complexes, due to intense metal-to-ligand charge transfer transitions. The best photovoltaic
efficiency in terms of conversion yield and stability, surpassing 11%, have been achieved by
polypyridyl complexes of Ru and Zn-porphyrin based dyes [28-34].
The first metal free organic dyes used as sensitizers for DSSCs showed poor performances, but
since then several organic compounds have been developed and the corresponding
photovoltaic devices show good performance, with similar efficiencies to the best Ru complexes
(~12%) [35-45]. Recently a high conversion efficiency of over 14% was reported using
collaborative sensitization between two organic dyes bearing two different anchoring groups
[46].
Very frequently, the most efficient DSSCs, contain thiophene groups (e.g. oligothiophenes,
fused thiophenes, etc.) and ethyne units as
π-spacers because of their excellent charge-
transport properties, good -conjugation and high planarity, which allow the best photovoltaic
π
performances [1-15, 35-45]. Following from these earlier achievements, we report the synthesis
and evaluation of optical, redox, and photovoltaic properties of three push-pull organic dyes
bearing a N,N-dimethylaniline donor moiety conjugated with 2,2´-bithiophene, ethynyl-2,2´-
bithiophene or ethynyl-thieno[3,2-b]thiophene spacers and cyanoacetic acid or rhodanine-3-
acetic acid as anchoring group. These new synthesis routes are then coupled with a theoretical
study of the dyes using density functional theory (DFT) and time dependent DFT (TDDFT).
2. Experimental section
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2.1.
Materials and Instruments
4-Ethynyl-N,N-dimethylaniline and cyanoacetic acid were purchased from Aldrich. Rhodanine-3-
acetic acid was purchased from Alfa Aesar. All commercially available reagents and solvents
were used as received. 5-Bromothieno[3,2-b]thiophene-2-carbaldehyde [47], 5’-bromo-2,2’-
bithiophene-5-carbaldehyde
[48],
and
5-formyl-5’-(4-N,N-dimethylaminophenyl)-2,2’-
bithiophene [49] were prepared using the same synthetic methodologies previously reported by
us.
Reaction progress was monitored by thin layer chromatography, 0.25 mm thick precoated silica
plates (Merck Fertigplatten Kieselgel 60 F254), and spots were visualised under UV light.
Purification was achieved by silica gel column chromatography (Merck Kieselgel, 230-400
mesh). NMR spectra were obtained on a BruckerG Avance II 400 at an operating frequency of
1
400 MHz for H and 100.6 MHz for 13C, using the solvent peak as internal reference. The
solvents are indicated in parenthesis before the chemical shifts values (δ relative to TMS). Peak
assignments were made by comparison of chemical shifts, peak multiplicities and J values, and
were supported by spin decoupling-double resonance and bidimensional heteronuclear HMBC
(heteronuclear multiple bond coherence) and HMQC (heteronuclear multiple quantum
coherence) techniques. Melting points were determined on a Gallenkamp apparatus and are
uncorrected. Infrared spectra were recorded on a BOMEM MB 104 spectrophotometer. UV-Vis
absorption spectra were obtained using a Shimadzu UV/2501PC spectrophotometer.
Fluorescence spectra were collected using a FluoroMax-4 spectrofluorometer. The relative
fluorecence quantum yields were determined using fluorescein in an 0.1M aqueous solution of
NaOH (φF=0.79) [50]. Mass spectrometry analysis were performed at the C.A.C.T.I. – Unidad de
Espectrometria de Masas of the University of Vigo, Spain.
2.2.
Synthesis
2.2.1. General procedure for the synthesis of aldehydes 4-5 through Sonogashira coupling
Aldehydes 2 [47] or 3 [48] (0.81 mmol) were coupled with 4-ethynyl-N,N-dimethylaniline 1
(0.114 g, 0.78 mmol), in a mixture of THF (10 mL), trimethylamine (6 mL), Pd(PPh3)2Cl2 (0.023
g, 0.02 mmol) and copper iodide (0.002 g, 0.01 mmol), at reflux under nitrogen. The reaction
was monitored by TLC, which determined the reaction time (16 h). After cooling, 10 mL of water
were added and the reaction mixture was extracted with dichloromethane (3x20 mL). The
combined organic layers were dried over anhydrous MgSO4, filtered, and the solvent
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evaporated under reduced pressure. The crude products were purified by column
chromatography (silica gel, mixtures of chloroform and light petroleum of increasing polarity) to
give the pure compounds 4-5.
2.2.1.1.
5-(2’-(4’’-Dimethylamino)phenyl)ethynyl)thieno[3,2-b]thiophene-2-carbaldehyde
4 Orange solid, η 35 %. Mp. 203–205 ºC. UV (ethanol): λmax nm (ε, M-1cm-1) 409, (34,705). IR
(liquid film) ν 2180, 1664, 1530, 1406, 1226 cm-1. 1H NMR (DMSO-d6, 400 MHz) δ 2.96 (s, 6H,
2xNCH3), 6.72 (d, 2H, H-3’’ and H-5’’, J = 7.0 Hz), 7.39 (d, 2H, H-2’’ and H-6’’, J = 7.0 Hz), 7.74
13
(s, 1H, H-6), 8.34 (s, 1H, H-3), 9.98 (s, 1H, CHO) ppm. C NMR (DMSO-d6, 100 MHz) δ 39.6,
80.9, 98.9, 106.8, 111.8, 124.1, 130.9, 131.1, 132.6, 138.6, 144.3, 145.5, 150.6, 184.8 ppm. MS
(EI) m/z (%) = 312 ([M+H]+, 15), 311 ([M]+, 100), 310 (29), 295 (11). HMRS: m/z (EI) for
C17H13NOS2; calcd: 311.0439; found: 311.0437.
2.2.1.2.
5-(5’-(2’’-(4’’’-(Dimethylamino)phenyl)ethynyl)thiophene-2’-yl)thiophene-2-
1
carbaldehyde 5. Orange solid, η 77 %. Mp. 191–193 ºC (lit. 200–203 ºC [51]). H NMR (DMSO-
d6, 400 MHz) δ 2.96 (s, 6H, NCH3), 6.71 (d, 2H, H-3’’’ and H-5’’’, J = 8.8 Hz), 7.32 (d, 1H, H-3’, J
= 4.0 Hz), 7.35 (d, 2H, H-2’’’ and H-6’’’, J = 9.2 Hz), 7.56 (m, 2H, H-4 and H-4’), 8.00 (d, 1H, H-
3, J = 4.0 Hz), 9.88 (s, 1H, CHO) ppm.
2.2.2. General method of synthesis for compounds 6-7
To a mixture of aldehyde 4 or 5 (0.25 mmol) and 2-cyanoacetic acid (0.30 mmol) in ethanol (15
mL) was added 4 drops of N-triethylamine. The reaction mixture was heated under reflux during
6 h and then cooled to room temperature. The mixture was concentrated by partial evaporation
of the solvent and diethyl ether was added to induce precipitation. The precipitates obtained
were filtered and washed with ethyl ether to give the pure products 6-7.
2.2.2.1.
2-Cyano-3-(2’-(2’’-(4’’’-(dimethylamino)phenyl)ethynyl)thieno[3,2-b]thiophen-5’-
yl)acrylic acid 6. Orange solid, η 53 %. Mp. 215-217 ºC. UV (ethanol): λmax nm (ε, M-1cm-1) 421,
1
(33,786). IR (liquid film) ν 3420, 2361, 2194, 1610, 1531, 1363 cm-1. H NMR (DMSO-d6, 400
MHz) δ 2.96 (s, 6H, 2xNCH3), 6.71 (d, 2H, H-3’’’ and H-5’’’, J = 9.2 Hz), 7.38 (d, 2H, H-2’’’ and
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H-6’’’, J = 8.8 Hz), 7.70 (s, 1H, H-3’), 7.98 (s, 1H, H-6’), 8.15 (s, 1H, =CH) ppm. 13C NMR
(DMSO-d6, 100.6 MHz) δ 43.5, 81.1, 97.8, 107.1, 111.8, 119.1, 124.1, 127.2, 128.0, 132.5,
138.3, 140.4, 141.0, 142.2, 150.5 ppm. MS (ESI) m/z (%) = 379 ([M+H]+, 100), 364 (13). HRMS:
m/z (ESI) found 379.0569; C20H15N2O2S2 requires 379.0497.
2.2.2.2.
2-cyano-3-(5’-(5’’-(2’’’-(4’’’’-(dimethylamino)phenyl)ethynyl)thiophen-2’’-
yl)thiophen-2’-yl)acrylic acid 7 Red solid. η 81 %. Mp. 204-207 ºC. (lit. 235–245 ºC [51]) 1H NMR
(DMSO-d6, 400 MHz) δ 2.96 (s, 6H, NCH3), 6.71 (d, 2H, H-3’’’’ and H-5’’’’, J = 8.8 Hz), 7.28 (d,
1H, H-3’’, J = 3.6 Hz), 7.35 (d, 2H, H-2’’’’ and H-6’’’’, J = 8.8 Hz), 7.41 (d, 1H, H-4’’, J = 3.6 Hz),
7.45 (d, 1H, H-4’, J = 4.0 Hz), 7.66 (d, 1H, H-3’, J = 4.0 Hz), 8.04 (s, 1H, =CH) ppm.
2.2.3. Synthesis of rhodamine dye 9
To a mixture of aldehyde 8 [49] (0.060 g, 0.19 mmol) and rhodanine-3-acetic acid (0.055 g, 0.29
mmol) in toluene (15 mL) was added 2 drops of concentrated acetic acid. The reaction mixture
was heated under reflux during 6 h and then cooled to room temperature. The organic layer was
extracted with chloroform (200 mL), washed with brine, and dried over anhydrous MgSO4.
Evaporation of the solvent under reduced pressure gave the crude product, which was
crystallized from hexane to give the pure product 9. Brown solid, η 31 %. Mp. 199-201 ºC. UV
(ethanol): λmax nm (ε, M-1cm-1) 430, (12,450). IR (liquid film) ν 3360, 2676, 1702, 1585, 1346,
1
1279 cm-1. H NMR (DMSO-d6, 400 MHz) δ 2.95 (s, 6H, 2xNCH3), 4.61 (s, 2H, CH2), 6.74 (d,
2H, H-3'''' and H-5'''', J = 8.4 Hz), 7.34 (d, 1H, H-4''', J = 3.6 Hz), 7.49-7.55 (m, 4H, H-2'''', H-6'''',
H-3''' and H-4''), 7.75 (d, 1H, H-3'', J = 3.6 Hz), 8.08 (s, 1H, =CH) ppm. 13C NMR (DMSO-d6, 100
MHz) δ 43.7, 111.8, 112.3, 118.2, 120.6, 122.5, 125.0, 126.3, 126.43, 127.8, 131.6, 135.1,
138.1, 145.6, 146.4, 150.3, 166.1, 167.2, 191.6 ppm. MS (ESI) m/z (%) = 391 (11), 315 (17),
314 (100), 313 (24), 299 (14). HRMS: m/z (ESI) [M+H]+ found 487.0275; C22H19N2O3S4 requires
487.0279.
2.3.
Cyclic voltammetry
The electrochemical measurements were performed using an AUTOLAB electrochemical
station and a three electrode cell equipped with a vitreous carbon disc working electrode (3
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mm), a platinum wire as counter-electrode and an Ag/AgCl electrode as reference electrode.
The concentration of dyes was 1 mM with 1 mM of [NBu4][BF4] as supporting electrolyte in dry
N,N-dymethylformamide solvent. Cyclic voltammetry was conducted at different scan-rates (20,
50, 100 and 200 mVs-1) and the solutions were deoxygenated bubbling nitrogen before each
measurement. The electrode potentials were measured against the potential of the Fc+/Fc redox
couple, as usual for non-aqueous solvents [52].
2.4.
DSSC preparation
A dye-sensitized solar cell consists of: i) a TiO2 mesoporous layer where dye molecules are
adsorbed, both comprising the working electrode; ii) a platinum counter-electrode; and iii) the
electrolyte containing the iodide/triiodide redox couple. To prepare the working electrodes, FTO
glasses (TCO22-7, 2.2 mm thickness, 7
ꢀ/square, Solaronix, Switzerland) were used, cleaned
in a detergent solution using ultrasonic bath rinsed with water and dried at 60 ºC. After treating
in a UV-O3 system for 15 min, the FTO substrates were immersed into a 40 mM aqueous TiCl4
solution at 70 ºC for 20 min, washed with ethanol and dried with N2. A layer of TiO2 paste (Ti-
Nanoxide T/SP, Solaronix, Switzerland) was coated on the FTO glass by screen-printing, kept
at room temperature for 20 min and then dried for 5 min at 120 ºC. The screen-printing
procedure was repeated 2 more times, in order to achieve 3 layers of TiO2 paste (0.2 cm2 of
circular active area, and 12 µm of thickness). After drying the photoelectrode at 120 ºC, it was
gradually heated (10 ºC min-1) up to 475 ºC for 30 min. The TiO2 electrode was then immersed
into a 0.5 mM dye solution in ethanol and kept at room temperature for 12 h.
To prepare the counter electrodes, two holes were made in the FTO glass using a drilling
machine with a diamond tip. The FTO substrates were then washed as described before. Pt
catalyst (Platisol T/SP, Solaronix, Switzerland) was deposited on the FTO side of the glass by
screen-printing and then heated up to 450 ºC for 10 min.
The dye-covered TiO2 electrode and the Pt counter-electrode were assembled into a sandwich
type cell and sealed with a hot-melt gasket of 25 µm thickness – Surlyn (Meltonix 1170-25,
Solaronix, Switzerland) by hot-pressing. The electrolyte (Iodolyte AN-50, Solaronix, Switzerland)
was injected into the cell through the holes presented in the counter-electrode side. These holes
were then sealed by Surlyn® and a cover glass using a soldering iron.
Two types of co-sensitization of TiO2 films were accomplished: i) by mixing different volume
fractions of the sensitized organic dyes 6-7 with the reference dye N719 at room temperature for
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12 h and ii) sequential co-adsorption of 12 h dipping time of dye N719, followed by 12 h dipping
in the reported dyes 6-7.
2.5.
DSSC characterization measurements
A 150 W xenon light source (Oriel class a solar simulator, Newport, USA) was used to give an
irradiance of 100 mW cm−2 (equivalent of one sun at AM 1.5G) at the surface of solar cells. The
simulator was calibrated using a single crystal Si photodiode (Newport, USA). The current-
voltage characteristics of the cell under these conditions were obtained applying an external
potential bias to the cell and measuring the generated photocurrent with ZENNIUM workstation
(Ref. 2425-C, Zahner Elektrik, Germany). Photovoltaic performance was measured using a
metal mask with an aperture area of 0.2 cm2.
The incident photon-to-current conversion efficiency (IPCE) spectra of the cells were recorded
using a PC-operated setup consisting of a 300 W Xenon lamp as a light source (Newport
66902), a monochromator (Newport Cornerstone 74125), a lock-in amplifier (Newport Merlin
70104), set of optical filters and a light chopper. The photon flux of light incident on the samples
was normalized using a calibrated silicon detector (Newport 70356). The bias light was supplied
by a 100 W halogen lamp, and the light intensity was adjusted to 0.3 sun by the driven current
from a constant current source. Measurements were made at 2 nm wavelength intervals
between 290 and 800 nm.
2.6.
Computational study
The molecular structures of the dyes 6-7, and 9 were optimized without symmetry constraints by
density functional theory (DFT) using the CAM-B3LYP [53] functional with the all-electron 6-
31G(d,p) double-
ζ plus polarization basis sets. The structures were optimized using the
polarizable continuum model (PCM), [54-55] as implemented in GAMESS-US, [56] to simulate
the bulk solvent effects of chloroform. In this model, the solute molecules are placed into a
spherical cavity surrounded by a dielectric continuum. In PCM, default van der Waals radii were
used for all atoms. Additionally, the geometries of the most stable conformers of dyes 6, 7 and 9
were also optimized with the B3LYP [57-58] functional and the same basis set. The Hessian
matrix was calculated for all the equilibrium structures to confirm that they are true minima (no
imaginary frequencies). The vertical excitation energies were calculated for the major
conformers of dyes 6, 7 and 9 by time-dependent DFT (TD-DFT) using the CAM-B3LYP
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functional and the 6-311G(d,p) triple-ζ plus polarization basis sets. The bulk solvent effects of
chloroform were also taken into account in the calculation of the excitation energies.
3. Results and discussion
3.1.
Synthesis and characterization
Push-pull thiophene derivatives 6-7 and 9 were synthesised in order to study the effect of
different -bridges (ethynyl-2,2´-bithiophene or ethynyl-thieno[3,2-b]thiophene) and different
anchoring groups (cyanoacetic acid or rhodanine-3-acetic acid) on their optical, redox and
photovoltaic properties. All these designed -conjugated systems were functionalized with the
N,N-dimethylaniline donor moiety, because of the known high efficiency of this donor group in
DSSCs. Ethynyl-2,2´-bithiophene and ethynyl-thieno[3,2-b]thiophene were used as
bridges/spacers due to the excellent optoelectronic properties of thiophene heterocycle such as
excellent charge transfer properties, efficient -conjugation, low geometric relaxation energy
π
π
π
-
π
upon oxidation, and also the additional planarity introduced into the dye structure by the ethynyl
group which is expected to provide the best photovoltaic performances for the dyes [1-15, 35-
45, 51-52, 59-65]
The aldehydes 4 and 5 were prepared, in fair to good yields (35-77 %), by Sonogashira
coupling of 5-bromothieno[3,2-b]thiophene-2-carbaldehyde 2 or 5-bromo-2,2´-bithiophene 3 with
commercially available 4-ethynyl-N,N-dimethylaniline. The precursors 2 [47], 3 [48] and 8 [49]
were synthesized using the same synthetic methodologies reported by us recently.
The final heterocyclic push-pull conjugated dyes 6-7 were synthesized in good yields (53-81%)
by Knoevenagel condensation of the corresponding aldehyde precursors 4-5 with cyanoacetic
acid in refluxing ethanol, using N-triethylamine as catalyst (Scheme 1, Table 1) and dye 9 was
prepared in 31% yield by refluxing the aldehyde precursor 8 in toluene, with rhodanine-3-acetic
acid in the presence of two drops of acetic acid (Scheme 2, Table 1).
<Schemes 1-2>
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Recently the precursor aldehyde 5 was reported by Cooke and collaborators through the
Sonogoshira coupling in 40% yield using different experimental conditions (Pd/C (10%); CuI,
Ph3P, K2CO3, H2O/DME (1:1) [51]. However, the same compound 5 was prepared by us in a
much higher yield (77%) using different reaction conditions (Pd(PPh3)2Cl2, CuI, TEA, THF).
The same investigators reported also the synthesis of dye 7 in 28% yield by condensation of the
corresponding precursor aldehyde with 2-cyanoacetic acid in acetonitrile at reflux overnight
using piperidine as catalyst. In our experiment the yield of 7 almost triplicated (81%) by reaction
of the precursor aldehyde 5 with 2-cyanoacetic acid in the presence of N-triethylamine as
catalyst in ethanol at reflux during 6 h.
The heterocyclic compounds 4-7 and 9 were completely characterized by standard
spectroscopic techniques.
<Table 1>
3.2.
Optical Properties
The optical properties of push–pull compounds 4-7 and 9 were investigated in ethanol at room
temperature and the experimental data are presented in Table 1.
All dyes exhibit a strong, broad band between 409-472 nm that can be assigned to an internal
charge transfer process (ICT) between the donor and acceptor groups, which depends on the
π
-spacer and on the acceptor/anchoring group.
The substitution of a thienothiophene bridge by a bithiophene spacer, as seen in dye 7, induces
a bathochromic shift in the longest wavelength absorption of 11 nm compared with dye 6
(Figure 1). The same trend was also observed for the corresponding precursor aldehydes 4-5.
Moreover, compound 7 also has the highest molar extinction coefficient, which is advantageous
for the light harvesting process of the sensitizer. As expected, the electronic nature of the
acceptor groups significantly influences the position of this low-energy absorption band. An
increase in the electronic acceptor strength induces a progressive bathochromic shift as can be
observed for aldehyde derivatives 4-5 compared to the corresponding cyanoacetic dyes 6-7
(Table 1). However, the substitution of the ethynyl-bithiophene spacer (7), by the bithiophene
system, and additional substitution of the cyanoacetic acceptor group by the rhodanine-3-acetic
10

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acid moiety (9), leads to a batochromic shift of the absorption band off 40 nm compared to dye
7. This result can be explained considering the loss of the triple bond in the
π-spacer, and the
introduction of the rhodanine-3-acetic acid anchoring group which has an opposite effect due its
higher conjugation compared to the cyanoacetic group.
Push-pull dyes 6-7 have higher molar extinction coefficients (33,786 to 36,080 M-1 cm-1) when
compared to the standard ruthenium ones N3 (13,900 M-1cm-1) [66] and N719 (14,000 M-1cm-1)
[67]. In contrast, the rhodanine dye 9 exhibits a lower ε value of 12,450 M-1cm-1.
The precursor aldehydes 4-5 and push-pull dyes 6-7 and 9 were excited at the wavelength of
maximum absorption, at room temperature, to study their fluorescence properties (Figure 2). All
dyes exhibit weak emissive properties, with relative fluorescence quantum yields ranging from
0.006 to 0.075. The nature of the acceptor end group significantly influences the fluorescence
spectra. As expected, an increase of the strength of the acceptor end group induces a redshift
of the emission, as shown in Table 1 and Figure 2 for the cyanoacetic derivatives 6-7
compared to aldehydes 4-5. All push-pull systems exhibit large Stokes shifts (98-191 nm).
Moreover, compounds 6-7 functionalized with a stronger electron acceptor group exhibit higher
Stokes shifts (170-191 nm) than the corresponding aldehyde derivatives 4-5 (98-120 nm), which
indicates a more pronounced photoinduced ICT.
<Figures 1-2>
3.3.
Electrochemical study
To obtain further insight into the electronic properties of the push-pull dyes, their
electrochemical behavior was studied by cyclic voltammetry in degassed DMF using 0.1 M
tetrabutylammonium tetrafluoroborate as the supporting electrolyte, a glassy carbon working
electrode with a scan rate of 0.1 Vs-1, a Pt counter electrode and Ag/AgCl reference electrode.
The electrochemical studies were performed for compounds 6 and 7 which exhibit higher
efficiency as sensitizers than compound 9, and the relevant electrochemical data are
summarized in Table 2. The electrochemical study of the well know N719 dye was also
performed under the same conditions to compare its electrochemical behavior with dyes 6-7.
It is well known that the electronic nature of the
π-bridge significantly affects the oxidation
potential values. Heterocyclic spacers bearing electron rich heterocycles such as thiophene are
11

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easier to oxidize due to their stronger electron-donating ability, resulting in a higher HOMO
energy level [25, 26].
The analysis of the results showed an oxidation process at +0.67 V for dye 6 and +0.44 V for
dye 7, being both potentials more positive than the iodine redox potential (0.42 V). This is
important, since an oxidation potential greater than that of the redox potential of the iodine
couple is required to reduce the backward electron transfer to electrolyte solution in DSSCs.
Indeed, both devices can show good performance because of their marked difference from the
iodide/triiodide redox couple, leading to a better regeneration of the dye after electron injection
into the conduction band of TiO2 [68]. Moreover, dye 7 exhibits the lowest value for the oxidation
potential, which corresponds to an increase in the HOMO energy (EHOMO = -4.83 V) compared to
compound 6 (EHOMO = -5.06 V) as well as a similar LUMO energy (ELUMO = -2.65 for dye 6 and
ELUMO = -2.68 for dye 7), due to the stronger electron donating ability of the bithiophene when
compared to the thieno[3,2-b]thiophene spacer moiety.
<Table 2>
3.4.
Photovoltaic performance of DSSCs
DSSCs were fabricated using the dyes above described as sensitizers. Details of the device
preparation and characterization were described in the experimental section. The photovoltaic
performances of DSSCs based on dyes 6-7 and 9 characterized under AM 1.5 irradiation (100
mW cm-2) are summarized in Table 3, and the correspondent I-V curves are presented in
Figure 3. The incident photon-to-current conversion efficiency (IPCE) spectra are shown in
Figure 7. The performance of the DSSC fabricated with N719 under the same conditions is also
shown for comparison.
Dyes 6-7 bearing a N,N-dimethylaniline donor moiety conjugated with ethynyl-bithiophene or
ethynyl-thieno[3,2-b]thiophene spacers and cyanoacetic acid as anchoring/acceptor group
exhibit the best efficiencies, 2.63 % and 3.51 %, respectively. The better performance of 7 dye
can be ascribed to a higher photocurrent resulting from having bithiophene as
than the thienothiophene spacer.
π–bridge rather
In contrast, dye 9, functionalized with a N,N-dimethylaminophenyl- group, a bithiophene spacer
and a rhodanine-3-acetic acid as acceptor/anchoring group, shows the poorest performance
12

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compared with dyes 6-7, with an efficiency of only 0.14%. This can be explained as a
consequence of changing the more planar, and more conjugated, ethynyl-bithiophene spacer by
the bithiophene bridge, and also due to substitution of the cyanoacetic acid anchoring moiety
used in dyes 6-7 by the rhodanine-3-acetic acid group. In fact, although using rhodanine-3-
acetic acid as an anchoring group leads to a significant bathochromic shift due to an increase in
the
π-conjugation system, cyanoacrylic is preferable for DSSCs. This happens both due to its
coplanarity with respect to spacer unit and good electron coupling with TiO2, but also because
the LUMO level of rhodanine-3-acetic acid based dyes is centred on the carbonyl and
thiocarbonyl groups, which results in its LUMO being isolated from the –COOH anchoring group
due to the presence of the methylene moiety [69-74].
<Figure 3>
Until now, sensitizers based on bipyridyl complexes of ruthenium have been widely used in
DSSCs due to their very good spectral properties, photostability in operating devices, and high
energy conversion efficiencies [75-78]. An excellent example of a high performance ruthenium
polypyridine complex for this is is-dithiocyanatobis(4, 4’- dicarboxilic acid - 2, 2’-
bipyridine)ruthenium(II), commonly known as the N3 dye [79]. Normally, it is its
tetrabutylammonium salt N719 that has been used as the standard red dye because of its
unmatched performance [80]. This dye is derived from the N3 form by substituting two protons
with two TBA (tetrabutylammonium) cations. Although high efficiency and stability have been
achieved with ruthenium-based dyes, the correspondent DSCs devices yield maximum
efficiencies by using thick TiO2 films due to low molar extinction coefficients (< 20000 M−1cm−1)
ascribed to the metal–ligand charge transfer molecular excitation. In contrast, organic dyes
generally have larger molar extinction coefficients, allowing the preparation of thinner films, and
so minimizing loss during charge transport [81]. On the negative side, they present sharp and
narrow absorptions, seriously weakening the light harvesting capabilities. A route to combine
the advantages of these two families of dyes is co-sensitization. In addition, co-adsorbing
different sensitizers on the TiO2 surface allows investigation of the competition of the dye
adsorption on the TiO2 surface leading to formation of a single monolayer on it, since the use of
multiple dyes gives a lower coverage of each. Thus, using different percentages of Ru-based
13

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dyes mixed with organic ones allows the evaluation of the ease of access that the
iodide/triiodide electrolyte may have to the electrons in the TiO2 conduction band.
Based on these ideas, the best performing dyes 6-7 were co-adsorbed with different volume
fractions of N719. Figures 4 and 5 and Table 3 show the correspondent I-V curves and the
photovoltaic performance parameters of devices containing these mixtures.
<Table 3>
<Figure 4-5>
In the case of dye 6, the best mixture seems to be that with 50 vol% of each dye. From Table 3
we can see that maximum photocurrent and open-circuit voltage are reached for this case (η =
3.81 %, Jsc = 9.49 mA cm-2, Voc = 620 mV). The photocurrent enhancement may be explained
by the band present at 530 nm at the UV-Vis spectra – Figure 6a, while the slight increase in
Voc suggests slower recombination kinetics. The use of N719 co-adsorbed with 6 dye is always
beneficial, presenting power conversion efficiencies always greater than just using the 6 dye.
The broadened UV-Vis absorption spectra of N719 and its TiO2 coverage are responsible for
the enhanced photovoltaic performances observed in the co-sensitization experiments.
<Figure 6>
Different conclusions may be made for the case of dye 7. In this case, the use of N719 only
becomes advantageous for percentages greater than 50 vol%, and even above this value the
photocurrent maintains constant. This means that the use of the bithiophene π-bridge instead of
a thienothiophene spacer for dye 7 strongly enhances its optical properties, which are further
improved only when a large amount of N719 is mixed (responsible for the appearance of the
peak at 530 nm – Figure 6b). Indeed, the optimal conversion efficiency for the system 7/N719 is
found to be approximately the same when using 50 vol% or 75 vol% of N719 dye.
Besides co-adsorption with different percentages of N719 dye, sequential co-adsorption
consisting of 12 h dipping time of dye N719, followed by 12 h in the reported organic dyes was
14

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also performed for assessing differences in the type of dye co-adsorption - Table 3. Based on
IPCE results – Figure 7, it can be concluded that sequential adsorption of N719/new dye when
compared to devices prepared with only each new dye gives a higher quantum efficiency for
wavelengths > 450 nm. This is then evidenced by the power conversion efficiency (PCE); for
example, dye 6 compared to co-adsorption system dye 6/N719 exhibits a PCE increase from
2.63% to 3.52%. Nevertheless, when comparing cells based on sequential adsorption with the
N719 reference device, a significant decrease in the PCE occurs, also illustrated by the IPCE
response. This means that the adsorption of dyes 6 and 7 are competitive with N719, inducing
partial desorption of N719 during the 12 h dipping time of the organic dyes. Besides, comparing
sequential adsorption with co-adsorption with different volume fractions of N719 dye, it can be
concluded that dye 6 gives almost the same PCE when dyes are adsorbed sequentially,
3.52 %, and for the best co-adsorption composition, which corresponds to 50 vol% of each dye
= 3.81 %. In what concerns dye 7, when adsorbed sequentially, it shows a PCE of 3.56 %,
η =
–
η
which is similar to the PCE of dye 7 – 3.51 %. This means that dye 7 is preferably adsorbed in
the TiO2 surface compared with N719. Between 380 nm (limit for TiO2 absorption) and 420 nm,
both N719 and dye 6 show the same IPCE. However, when co-adsorbed the quantum efficiency
is lower. The same is observed for dye 7. This may suggest that dyes 6 and 7 pack less
efficiently when in presence of N719.
Finally, the co-sensitization systems either for dyes 6 and 7 provide evidence for a limited
surface coverage of dye on the TiO2 surface after co-sensitization that favors the approach of
triiodide ions to the TiO2 surface and enhances the recombination of injected electrons, since a
decrease in Voc is observed when compared with N719 dye alone. Since there is also some
photocurrent loss within co-sensitization, in addition to the optical properties of the mixture,
there might also be some aggregation of individual dyes under the co-sensitized conditions that
decreases their injection efficiencies. These two pathways of efficiency loss give important
information for future design of dye synthesis routes.
<Figure 7>
Previously, Cooke et al reported the photovoltaic properties of dye 7 which exhibits an efficiency
of 4.49% under their experimental conditions [51]. A thicker TiO2 layer in-house synthesized
was used, as well as a different electrolyte and Pt counter-electrode deposited by sputtering.
15

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They also performed an additional TiCl4 treatment after TiO2 deposition. These modifications
might be responsible for a higher short-circuit photocurrent. The higher performance value
reached by us for reference N719 dye is attributed to a higher FF value.
3.5.
Computational study
To gain insight into the electronic structure and optical properties of compounds, DFT
calculations were performed for compounds 6-7 and 9.
Compounds 6, 7 and 9 have several possible conformers, differing in the relative arrangement
of some of their units. These structures may have different degrees of conjugation, which will
influence their optical properties. To determine the lowest energy structures for each of these
compounds, a conformational study was carried out at the DFT level. Six conformers of dye 6
were considered. These have different relative arrangements between the carboxylic, the cyano
and the adjacent thienothiophene groups. For the dye 9, in addition to the different possible
orientations of the rhodanine heterocycle ring relative to the neighboring thiophene, the cis or
trans arrangements between the two thiophene moieties also have to be considered. A
computational study was previously published [51] for compound 7, in which two conformers
were considered. One has a trans arrangement of the bithiophene moiety and the second
conformer has a cis arrangement. The cis form was found to account for only 10% of the
population at equilibrium. In our study we will only consider the trans conformation of 7, and we
will also investigate the conformers resulting from different arrangements between the
thiophene, the cyano and the carboxylic groups. A total of six conformers of dye 6, six
conformers of dye 7 and four conformers of dye 9, were optimized at the CAM-B3LYP/DFT
level, taking into account the bulk solvent (chloroform) effects. The optimized structures for the
major conformers are presented in Figures 8a – 8c. For dyes 6 and 9, conformers 6-I and 9-I
are the most stable, accounting for 72.6 and 49.6% of the population at 298 K, respectively
(Table 4). These are followed by conformers 6-II and 9-II with 24.6 and 33.1% of the population,
respectively, for each dye. For dye 7, the major conformer, shown in Figure 8b, accounts for
81.6% of the population (only trans conformers were considered). This structure is different from
that studied in reference [51], and we found our structure to be more stable by 17.0 kJ/mol
(∆G). However, the differences between the two structures are small and do not significantly
affect the main properties calculated for the dye. The optimized geometries of 6-I and 6-II and
the major conformer of dye 7 are, essentially, planar, which favors the efficient electron transfer
16

ACCEPTED MANUSCRIPT
from the donor to the acceptor/anchoring moiety. In contrast, the geometry of conformer 9-I
(and 9-II) is less planar (Figure 8b), having a dihedral angle of 26.1º between the N,N-
dimethylaminophenyl group and the thiophene unit, and an angle of 15.9º between the two
thiophene rings. The rhodanine heterocycle and the adjacent thiophene ring are approximately
coplanar in 9-I and 9-II (torsions in the range 0.3º - 0.7º). The two most stable conformers of dye
9, 9-I and 9-II, have a trans arrangement of the two thiophene rings. The two cis conformers
which are the cis counterparts of 9-I and 9-II were studied and are less stable than 9-I by 4.1
and 4.5 kJ/mol, having populations of 9.2 and 8.1 %.
<Figures 8a-c>
The frontier molecular orbitals of dyes 6, 7 and 9 are shown in Figure 9 (results are shown only
for the dominant conformers I of dyes 6 and 9, but they are similar for conformers II). The
highest occupied molecular orbital (HOMO) in dye 6 is a
donor (N,N-dimethylaniline) and acetylene units and extending to the thienothiophene moiety,
while, the lowest unnocupied molecular orbital (LUMO) is a * orbital localized on the acceptor
π orbital localized essentially on the
π
(cyanoacrylic acid) and on the thienothiophene units (and to a smaller extent on the acetylene
linker). Similarly, in dye 7 the HOMO extends from the donor to the bithiophene moiety and the
LUMO is localized mainly on the cyanoacrylic acid acceptor/anchoring group and extends to the
acetylene bridge. In dye 9, the HOMO and LUMO are localized mainly on the N,N-
dimethylaminophenyl donor and on the rhodanine-3-acetic acid acceptor, respectively.
Therefore, in these dyes, the HOMO-LUMO excitation will result in an overall charge transfer
from the donor to the aceptor groups. However, a significant difference occurs between the
LUMO orbitals of these dyes. The localization of the LUMO on the anchoring –COOH group in
dyes 6 and 7 facilitates the injection of the excited electrons into the TiO2 photoelectrode and
contributes to the good efficiencies observed for the DSSCs based on these dyes. In contrast,
In dye 9, the LUMO is not localized on the –COOH anchoring group, and this hampers an
efficient injection of the excited electrons into the TiO2 electrode, and contributes to the low
efficiency observed for the DSSC.
The vertical excitation energy for the first singlet excitation has been calculated at the TD-CAM-
B3LYP/6-311G(d,p) level for the major conformers of 6, 7 and 9 and the results are given in
Table 4. The calculated maximum absorption wavelengths are 452, 468 and 471 nm for dyes 6
17

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(6-I), 7 and 9 (9-I), (respectively). The calculations overestimate the maximum absorption
wavelengths for dyes 6 and 7 by 31 – 36 nm and predict the correct value for dye 9. The
occurrence of bathochromic shifts on going from dyes 6 to 7, and from 7 to dye 9 are also
correctly predicted by the TD-DFT calculations.
<Table 4>
While the long-range corrected density functionals, such as CAM-B3LYP, seem to perform
better for the TD-DFT prediction of excited states with significant charge-transfer character, [82]
B3LYP is expected to perform better in the calculation of the orbital energies of conjugated
polymers [83]. Therefore, we have chosen the B3LYP functional to calculate the energies of the
frontier molecular orbitals in our systems. The energies of the HOMO and LUMO orbitals in
dyes 6, 7 and 9 have been calculated for the neutral molecules in their ground states, and are
given in Table 4. The energies calculated for dyes 6 and 7 agree very well with the
experimental data obtained from cyclic voltammetry, with absolute errors between 0.00 and 0.05
eV for dye 6 (6-I) and 0.00 and 0.09 eV for dye 7. The HOMO-LUMO gap predicted for dye 9 is
smaller than those predicted for 6 and 7, in accordance with its larger maximum absorption
wavelength. This is caused by the extension of the conjugation length due to the presence of
the rhodanine-3-acetic acid group.
We also note that the calculated energies in the two conformers I and II in dyes 6 and 9 are very
similar. This is probably a consequence of both conformers I and II, in each dye, having
essentially the same degree of conjugation. And this may not be the case for conformers which
have significantly different degrees of conjugation (for example, the cis conformers in these
dyes).
4. Conclusion
Dyes 6-7 and 9 were prepared by Knoevenagel condensation of the respective aldehyde
derivatives 4-5 and 8 with commercially available 2-cyanoacetic acid or with rhodanine-3-acetic
acid.
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The optical, redox and photovoltaic characterization of the compounds reveals that dye 7
bearing an ethynyl-bithiophene spacer shows the best overall conversion efficiency (3.51 %) as
sensitizer in nanocrystalline TiO2 DSSCs.
The significantly lower efficiency of dye 9 is related to the substitution of cyanoacetic acid for
rhodanine-3-acetic acid as an anchoring group.
As expected the co-sensitization systems presents higher power conversion than the individual
synthesized dyes. For dye 6, an improvement of 45 % efficiency was attained with the most
advantageous mixture of 50 % of each dye. In the 7/N719 co-sensitization system, the
introduction of small percentages of N719 is not beneficial at all, but when the addition of more
dye starts to increase efficiency, constant photocurrent is quickly reached at about 50 % of each
dye, improving efficiency by only 33 %. Sequential adsorption was also considered evidencing
that partial desorption of N719 occurs during the 12 h dipping time of the organic dyes. Similar
power conversion efficiencies were obtained for both co-adsorption methods.
DFT and TDDFT calculations have been performed on dyes 6-7 and the results obtained are in
agreement with the experimental data. The calculations also confirm the importance of the
group anchoring the dyes into the TiO2 surface in determining device efficiency as well as the
planarity of the dyes.
Acknowledgments
Thanks are due to: Fundação para a Ciência e Tecnologia (FCT) for PhD grants to S. S. M.
Fernandes (SFRH/BD/87786/2012), and I. Mesquita (PD/BD/105985/2014) and a postdoctoral
grant to L.L.G.Justino. (SFRH/BPD/97026/2013); FEDER-COMPETE for financial support
through the CQ/UM PEst-C/QUI/UI0686/2013 (FCOMP-01-0124-FEDER-037302); European
Research Council (Contract no: 321315) for funding. H. D. Burrows are grateful for funding from
Centro de Química de Coimbra, which is supported by the FC through the programmes
UID/QUI/UI0313/2013 and COMPETE. The NMR spectrometer Bruker Avance III 400 is part of
the National NMR Network and was purchased within the framework of the National Program for
Scientific Re-equipment, contract REDE/1517/RMN/2005 with funds from POCI 2010 (FEDER)
and FCT. The authors also thank the Laboratory for Advanced Computing at the University of
Coimbra for providing computing resources that have contributed to the research results
reported in this paper (URL http://www.lca.uc.pt). The authors would also to acknowledge A. I.
Pereira for sequential co-adsorption devices and Dr. D. Ivanou for IPCE experiments.
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Captions
Figure 1. Normalized absorption spectra of dyes 6-7 and 9 in ethanol (λmax: 6 = 421 nm; 7 =
432 nm; 9 = 430 nm), at room temperature.
Figure 2. Normalized absorption and emission spectra for dyes 4-7 and 9, in ethanol.
Figure 3. I-V curves of DSCs prepared with the synthesized 6-7 and 9.
Figure 4. I-V curves of 6 dye co-adsorbed in different volume fractions with N719 dye.
Figure 5: I-V curves of 7 dye co-adsorbed in different volume fractions with N719 dye.
Figure 6. Normalized absorption spectra in ethanol for co-sensitization systems: a) 6/N719 and
b) 7/N719 dyes.
Figure 7. IPCE spectra of reference dye N719, dyes 6 and 7 and DSSCs with sequential co-
adsorption of dyes 6 and 7.
Figure 8. CAM-B3LYP/6-31G(d,p) optimized geometries of the major conformers of a) dye 6; b)
dye 7; c) dye 9. The bulk solvent effects of chloroform were considered in the calculations.
Figure 9. CAM-B3LYP/6-31G(d,p) highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) of dyes a) 6, b) 7 and c) 9 (the results shown for dyes 6
and 9 concern the two dominant conformers 6-I and 9-I). The bulk solvent effects of chloroform
were considered in the calculations.
28