Synthesis and characterization of a new series of dibenzofulvene based organic dyes for DSSCs

Article abstract of DOI:10.1016/j.dyepig.2016.02.030

Three new 2D-π-A dyes (TK4, TK5 and TK6), composed of diarylamine donor groups, a dibenzofulvene-thiophene core as the π-bridge, and a cyanoacrylic acid anchoring group as the acceptor, have been successfully designed, synthesized, and characterized both experimentally and computationally. The performance in DSSC solar cells has been also studied. Octyloxy chains were introduced on the backbone of the dye, in order to increase donor capability, avoid aggregation side effects and increase physical insulation between electrolyte system and the TiO₂ layer. The dye containing the octyloxy chains on the donor group and two thiophene ring as an extension of π-bridge showed the best photovoltaic performance with a maximum of solar energy-to-electricity conversion yield of 7.8% under AM 1.5 irradiation (100 mW/cm²).

Full text of DOI:10.1016/j.dyepig.2016.02.030

Accepted Manuscript
Synthesis and characterization of a new series of dibenzofulvene based organic dyes
for DSSCs
Agostina L. Capodilupo, Luisa De Marco, Giuseppina A. Corrente, Roberto Giannuzzi,
Eduardo Fabiano, Antonio Cardone, Giuseppe Gigli, Giuseppe Ciccarella
PII:
S0143-7208(16)30048-1
10.1016/j.dyepig.2016.02.030
DYPI 5120
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 5 November 2015
Revised Date: 12 February 2016
Accepted Date: 26 February 2016
Please cite this article as: Capodilupo AL, De Marco L, Corrente GA, Giannuzzi R, Fabiano E, Cardone
A, Gigli G, Ciccarella G, Synthesis and characterization of a new series of dibenzofulvene based organic
dyes for DSSCs, Dyes and Pigments (2016), doi: 10.1016/j.dyepig.2016.02.030.
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ACCEPTED MANUSCRIPT
Synthesis and characterization of a new series of
dibenzofulvene based organic dyes for DSSCs.
Agostina L. Capodilupo,a* Luisa De Marco,b Giuseppina A. Corrente,a,b,c Roberto Giannuzzi,b
Eduardo Fabiano,b,d Antonio Cardone,e Giuseppe Giglia,f and Giuseppe Ciccarella.a,g*
Agostina Lina Capodilupo a*
a
CNR NANOTEC - Institute of Nanotechnology c/o Campus Ecotekne, University of Salento,
Via Monteroni - 73100 Lecce Italy. E-mail: agostina.capodilupo@nanotec.cnr.it
Luisa De Marco b
b
Center for Biomolecular Nanotechnologies (CBN) Fondazione Istituto Italiano di Tecnologia
(IIT), Via Barsanti 1, Arnesano, 73010, Italy. E-mail: luisa.demarco@iit.it
Giuseppina Anna Corrente a,b,c
a
CNR NANOTEC - Institute of Nanotechnology c/o Campus Ecotekne, University of Salento,
Via Monteroni - 73100 Lecce Italy.
b
Center for Biomolecular Nanotechnologies (CBN) Fondazione Istituto Italiano di Tecnologia
(IIT), Via Barsanti 1, Arnesano, 73010, Italy.
c
Dipartimento di Ingegneria dell'Innovazione, Università del Salento, via Monteroni, 73100,
Lecce, Italy. E-mail: giuseppinaanna.corrente@unisalento.it
1

ACCEPTED MANUSCRIPT
Roberto Giannuzzi b
b
Center for Biomolecular Nanotechnologies (CBN) Fondazione Istituto Italiano di Tecnologia
(IIT), Via Barsanti 1, Arnesano, 73010, Italy. E-mail: roberto.giannuzzi@iit.it
Eduardo Fabiano b,d
b
Center for Biomolecular Nanotechnologies (CBN) Fondazione Istituto Italiano di Tecnologia
(IIT), Via Barsanti 1, Arnesano, 73010, Italy.
d
Istituto Nanoscienze–CNR, Euromediterranean Center for Nanomaterial Modelling and
Technology
(ECMT),
Via
per
Arnesano,
73100
Lecce,
Italy.
E-mail:
eduardo.fabiano@nano.cnr.it
Antonio Cardone e
e
Istituto di Chimica dei Composti OrganoMetallici (ICCOM) – Consiglio Nazionale delle
Ricerche CNR, via Orabona, 4 – 720125 Bari, Italy. E-mail: cardone@ba.iccom.cnr.it.
Giuseppe Gigli a,f
a
CNR NANOTEC - Institute of Nanotechnology c/o Campus Ecotekne, University of Salento,
Via Monteroni - 73100 Lecce Italy.
f Dipartimento di Matematica e Fisica ”Ennio De Giorgi”, Università del Salento, via Monteroni,
73100, Lecce, Italy. E-mail: giuseppe.gigli@unisalento.it
Giuseppe Ciccarella a,g*
2

ACCEPTED MANUSCRIPT
a
CNR NANOTEC - Institute of Nanotechnology c/o Campus Ecotekne, University of Salento,
Via Monteroni - 73100 Lecce Italy.
g
Dipartimento di Scienze e Tecnologie Biologiche e Ambientali Universita' del Salento c/o
Edificio
"La
Stecca"
via
Monteroni,
73100
Lecce,
Italy.
E-mail:
giuseppe.ciccarella@unisalento.it
Corresponding author at CNR NANOTEC - Institute of Nanotechnology c/o Campus
Ecotekne, University of Salento, Via Monteroni
agostina.capodilupo@nanotec.cnr.it.
-
73100 Lecce, Italy. E-mail:
Corresponding author at Dipartimento di Scienze e Tecnologie Biologiche e Ambientali,
Università del Salento, Via Monteroni, 73100, Lecce, Italy. E-mail:
giuseppe.ciccarella@unisalento.it.
3

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ABSTRACT
Three new 2D-π-A dyes (TK4, TK5 and TK6), composed of diarylamine donor groups, a
dibenzofulvene-thiophene core as the π-bridge, and a cyanoacrylic acid anchoring group as the
acceptor, have been successfully designed, synthesized, and characterized both experimentally
and computationally. The performance in DSSC solar cells has been also studied. Octyloxy
chains were introduced on the backbone of the dye, in order to increase donor capability, avoid
aggregation side effects and increase physical insulation between electrolyte system and the TiO2
layer. The dye containing the octyloxy chains on the donor group and two thiophene ring as an
extension of π-bridge showed the best photovoltaic performance with a maximum of solar
energy-to-electricity conversion yield of 7.8% under AM 1.5 irradiation (100 mW/cm²).
4

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KEYWORDS:
Dibenzofulvene, Dyes, Dye-sensitized solar cells, energy conversion, Diarylamine.
5

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1 INTRODUCTION
Dye-sensitized solar cells (DSSCs) have attracted sustained attention over the past decades
because of their potential in low-cost solar-to-electricity conversion.[1, 2] Sensitizers play a
critical role in light harvesting and electron injection and thereby affect the power conversion
efficiency (PCE) of the DSSCs.[3] Compared to expensive ruthenium complexes, metal-free
organic dyes promise modest fabrication costs and grant flexibility in molecular tailoring.[4-6]
Moreover, the metal free organic dyes have much stronger light-harvesting ability than metal
complexes thanks to higher molar extinction coefficients. The common structure of solar dyes
(D-π-A)[7-10] involves a donor (D) connected to an electron acceptor (A) by a π-bridge. This
structure allows a fine-tuning of molecular absorption to attain panchromatic light-harvesting,
taking advantage of the efficient intramolecular charge transfer (ICT) and the large control of
electrochemical properties. Up to now, the cyanoacrylic acid unit, which is a strong electron-
deficient anchoring group, is the most widely used acceptor moiety.[11, 12] On the other hand,
electron-rich units such as diphenylamine (DPA),[13-18] carbazole,[19-22] and
phenothiazine[23-25] are mostly adopted as donors. In addition to these, many different
conjugated building blocks have been introduced as bridges between the donor and the
acceptor.[26, 27] Despite its many advantages, the D-π-A sensitizer structure also shows several
limitations. In particular, it has a rod-like configuration, which may promote undesirable dye
aggregation and charge recombination.[28] Indeed, a close π-π aggregation can lead not only to
self-quenching and to a reduction of electron injection into TiO2 but also to the instability of the
organic dyes due to the formation of excited triplet states and unstable radicals under light
irradiation.[29] Several strategies have been adopted to overcome this problem. For example, the
introduction of long chains into the dye[30] and the employment of starburst dyes with a bulky
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multi-donor arrangement.[27, 31, 32] The choice of a suitable donor group together with the
optimization of its structure has, in fact, not only a positive effect on the absorption properties of
the dye, but can also modify the charge recombination rate in the device, as a consequence of the
redefinition of the energy levels, avoiding dye aggregation and enhancing the stability of the
sensitizers.[33] With the above considerations in mind, we have designed and synthesized three
new metal-free organic dyes with the starburst 2D-π-A structure (Figure 1), labelled TK4, TK5
and TK6. This set, which extends our previous work on dibenzofulvene-containing dyes,[17]
consists of dyes having para-alkoxy substituted diphenylamines as donor groups, the
cyanoacrylic acid as the acceptor/anchoring group, and a dibenzofulvene core as the π-linker.
Our strategy in the choice of the branched 2D-π-A structure of the dyes, was to pursue
characteristics as high electron-donating ability, reduced dye-aggregation, and the charge
recombination, with the aim to achieve enhanced photovoltaic performance. Theoretical
calculations using time-dependent density functional theory (TD-DFT) have also been employed
in order to optimize the geometry and to visualize location of the HOMOs and LUMOs. The
dyes have been used as sensitizers in liquid DSSC fabrication, correlating the molecular structure
with the device performance. As a result, a maximum efficiency of 7.8% has been recorded for
the TK6 dye.
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Figure 1. Structures of the sensitizers TK4-TK6
2 EXPERIMENTAL SECTION
2.1 Materials and instrumentation.
The synthetic routes of TK4-TK6 are detailed in Scheme 1. The solvents were freshly distilled
prior to use, according to standard procedures. Commercial reagents were used as received. FT-
1
IR spectra were measured on a Perkin-Elmer 1710 spectrophotometer using dry KBr pellets. H
13
and C NMR spectra were recorded on a Bruker 400 MHz spectrometer. LC-MS spectra were
acquired with an Agilent 6300 Series Ion Trap interfaced to an Agilent 1200 HPLC, in the
following general conditions: atmospheric pressure chemical ionization, positive ions, eluent
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chloroform, flow rate 0.200 mL min1, drying gas flow 5.0 L min1, nebulizer pressure 60 psi,
drying gas temperature 350 °C, vaporizer temperature 325 °C, mass range 100–2200 m/z. UV-
Vis absorption spectra were recorded on a Varian-Cary 500 spectrophotometer. Elemental
analyses were done by a Carlo Erba CHNS-O EA1108-Elemental Analyzer. Electrochemical
characterization of dyes was carried out by cyclic voltammetry (CV) using a AutoLab
potentiostat (Methrom). A typical three-electrode cell was assembled with a glassy carbon disk-
working electrode, a Pt-wire auxiliary electrode, and an Ag/AgCl non-aqueous reference
electrode. Cyclic voltammograms were acquired at 0.1 V s 1 scan rate on 1mM dye solutions
prepared in the electrolyte solution, which consisted of 0.1M tetrabutylammonium
hexafluorophosphate (TBAPF6) in dichloromethane (CH2Cl2). All the solutions were previously
degassed with N2 and kept under a N2 atmosphere. Ferrocene was added as an internal reference
for calibration of the potential scale versus Fc+/Fc couple. Potentials referred to ferrocene were
then converted to Normal Hydrogen Electrode (NHE) by addition of 0.63 V.
2.2 Computational details.
All calculations were carried out with the TURBOMOLE[34] program package using the
COSMO solvation model.[35, 36] Ground-state calculations and geometry optimizations were
performed using the BLOC functional.[37, 38] Time-dependent calculations were carried out
with the BHLYP functional.[39] In all calculations a def2-TZVP basis set[40] was employed. To
quantify the charge transfer character of each excited state two indicators were considered: the
overlap indicator
unoccupied orbitals involved in the excitation; the electron displacement indicator
measures the distance between the ground-state and the excited-state charge distributions. We
Λ,[41] which is computed as the spatial overlap between the occupied and
ꢀr,[42] which
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recall that small values of
Λ
(i.e. a small overlap between the occupied and unoccupied orbitals)
indicate a greater charge-transfer character (when
Λ = 0 an electron is completely transferred). In
contrast, larger r values indicate a more pronounced charge-transfer character. To study the
ꢀ
intermolecular interactions in the dimers of the new dyes calculations were performed using the
APBE-D3 functional[43, 44] with a def2-TZVP basis set[45] and the resolution of identity
approximation. [46-48]
2.3 Fabrication of DSSC devices and photovoltaic measurements.
Fluorine-doped tin oxide (FTO, 15
cleaned in a detergent solution using an ultrasonic bath for 15 min, and then rinsed with water
and ethanol. Double-layer electrodes (overall thickness 20 m) were prepared as follows: two
layers of commercial colloidal titania paste (Dyesol 18NR-T) were deposited onto the FTO glass
using the doctor blade technique and gradually heated in air to obtain a ~15 m transparent
nanocrystalline film. The temperature gradient was programmed as follows: 170°C (40 min),
350°C (15 min) and 430°C (30 min). This procedure was repeated for the scattering layer (5 m),
ꢁ/sq, provided by Solaronix S.A.) glass plates were first
ꢂ
ꢂ
ꢂ
prepared by a Solaronix D/SP colloidal paste. The double-layer electrode was sintered at 450°C
for 30 min. The thickness of the sintered photo-anodes were measured using a profilometer
(Veeco Dektak 150 Surface Profiler). The dye loading was performed by keeping the electrodes
for 14 h and under the dark in 0.2 mM (CH3CN:CHCl3 = 1:0.01 v/v) (dye TK4) or 0.2 mM THF
solutions (dyes TK5 and TK6), containing when needed, a known amount of chenodeoxycholic
acid (CDCA). The counter-electrodes were prepared by sputtering a 50 nm Pt layer on a hole-
drilled cleaned FTO plate. In a typical device construction procedure, the photo-anode and the
counter-electrode were faced and assembled using a suitably cut 50 µm thick Surlyn® hot-melt
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gasket for sealing. The redox electrolyte (0.1 M LiI, 0.03 M I2, 0.6 M 1-methyl-3-
propylimidazolium iodide, and 0.5 M tert-butylpyridine in dry acetonitrile) was injected into the
vacuum spaces between the electrodes through pre-drilled holes on the back of the counter
electrode. The holes were sealed using Surlyn® hot melt film and a cover glass. Photocurrent-
voltage measurements were performed using a Keithley unit (Model 2400 Source Meter). A
Newport AM 1.5 Solar Simulator (Model 91160A equipped with a 1000 W xenon arc lamp)
serving as a light source. The light intensity (or radiant power) was calibrated to 100 mW cm–2
using as reference a Si solar cell. The incident photon-to-current conversion efficiency (IPCE)
was measured by the DC method, using a computer-controlled xenon arc lamp (Newport, 140 W,
67005) coupled with a monochromator (Newport Cornerstore 260 Oriel 74125). The light
intensity was measured by a calibrated silicon UV-photodetector (Oriel 71675), and the short
circuit currents of DSSCs were measured by using a dual channel optical power/energy meter,
(Newport 2936-C). The surface concentrations (dye loading) of the dyes were assessed by
spectrophotometric determination as follows: double layered photoanodes (15 + 5 ꢂ
m, 1 cm2)
were sensitized with the same solutions used for devices; then the dyes were completely
desorbed from the TiO2 surface by immersing the substrates in a 0.01 M tetrabutylammonium
hydroxide in DMF solution. The evaluation of the dye concentration in the solvent, obtained by
UV-Vis measurements, allowed an estimate of the amount of the adsorbed molecules, expressed
in terms of moles of dye anchored per unit area of the photoelectrode to be calculated.
Electrochemical impedance spectroscopy (EIS) was per-formed by an AUTOLAB PGSTAT
302N (Eco Chemie B.V.) in a frequency range between 100 kHz and 10 mHz. The impedance
measurements were carried out at different voltage biases, under 1.0 sun illumination. The
resulting impedance spectra were fitted by using ZView (Scribner Associates) software.
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2.4 Synthetic procedures
The synthetic routes of these dyes were shown in Scheme 1, and the detailed synthetic
procedures are as follows.
2.4.1. 3,6-dibromo-9H-fluoren-9-one, 1. Was synthesized following the literature procedure.[49]
2.4.2. 3,6-dibromo-9H-fluorene, 2.
To a suspension of aluminium chloride (1.177 g, 8.85 mmol) and borane-tert-butylamine (1.539
g, 17.7 mmol) in anhydrous CH2Cl2 (40 mL) at 0°C, compound 1 (1 g, 2.95 mmol) was added in
15 min. The resulting mixture was allowed to warm at r. t, and stirred for 4 h before quenching
with a 0.1 M hydrochloric acid aq. solution. The crude residue was extracted with CH2Cl2 and
the combined organic layers were dried over anhydrous sodium sulfate. The solvent was
evaporated under a reduced pressure, and the residue was purified by flash chromatography
(silica gel), using 98/2 hexane/dichloromethane as the eluent. The pure product 2 was obtained as
a white solid (80% yield). 1H-NMR (400 MHz, CDCl3, ppm):
J1 = 8.02, J2 = 1.78, 2H) 7.41 (d, J = 8.02, 2H) 7.39 (s, 2H). 13C (100 MHz, CDCl3, ppm)
δ 7.66 (d, J = 1.67, 2H) 7.45 (dd,
δ
142.62, 142.20, 130.23, 126.54, 123.38, 121.01, 36.27. MS (APCI): calcd for C13H8Br2, 324.01;
found: m/z = 325.01 [M+H]+.
2.4.3. 2-(5-((3,6-dibromo-9H-fluoren-9-ylidene)methyl)thiophen-2-yl)-5,5-dimethyl-1,3-dioxane,
3a
.
A mixture of potassium tert-butoxide (0.08 g, 0.64 mmol), compound 2 (0.2 g, 0.64 mmol) and
5-(5,5-dimethyl-1,3-dioxan-2-yl)thiophene-2-carbaldehyde (i) (0.16 g, 0.7 mmol) in absolute
ethanol (4 mL) was stirred in an ultrasonic bath for 30 min. The reaction mixture was cooled at rt
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and the resulting precipitate was filtered and washed with absolute ethanol. The pure product 3a
was obtained as a yellow solid (84% yield). 1H-NMR (400 MHz, CDCl3)
δ 8.036 (d, J = 8.36 Hz,
1H) 7.805 (dd, J1 = 1.54 Hz, J2 = 4.53 Hz, 1H) 7.675 (d, J = 1.09 Hz, 1H) 7.599 (d, J = 7.99 Hz,
1H) 7.552 (d, J = 9.88 Hz, 1H) 7.498 (dd, J1 = 1.59 Hz, J2 = 7.86 Hz, 1H) 7.456 (dd, J1 = 1.79
Hz, J2 = 8.21 Hz, 1H) 7.328 (m, 1H) 7.176 (d, J = 3.80 Hz, 1H) 5.687 (s, 1H) 3.805 (d, J = 11.28
Hz, 2H) 3.686 (d, J = 10.63 Hz, 2H) 1.314 (s, 3H) 0.832 (s, 3H) 13C-NMR (100 MHz, CDCl3)
δ
141.5, 139.2, 138.6, 138.3, 132.6, 130.2, 130.1, 129.6, 129.4, 125.5, 125.4, 124.0, 123.0, 122.9,
122.7, 122.1, 121.5, 120.3, 99.4, 97.9, 77.4, 30.1, 22.8, 21.7. MS (APCI): calcd for
C24H20Br2O2S, 531.05; found: m/z = 532.05 [M+H]+. FT-IR (KBr):
ν 2954, 2849, 1707, 1605,
1592, 1573, 1470, 1418, 1378, 1306, 1205, 1099, 1054, 1026, 989, 966, 922, 869, 821, 774 cm-1.
Anal. Calcd for C24H20Br2O2S: C, 54.16; H, 3.79; S, 6.02. Found: C, 54.19; H, 3.75; S, 6.03.
2.4.4.
2-(5'-((3,6-dibromo-9H-fluoren-9-ylidene)methyl)-[2,2'-bithiophen]-5-yl)-5,5-dimethyl-
1,3-dioxane, 3b
.
A mixture of potassium tert-butoxide (0.065 g, 0.58 mmol), compound 2 (0.186 g, 0.58 mmol)
and 5'-(5,5-dimethyl-1,3-dioxan-2-yl)-[2,2'-bithiophene]-5-carbaldehyde (3’) (0.178 g, 0.58
mmol) in absolute ethanol (5 mL) was stirred in an ultrasonic bath for 30 min. The reaction
mixture was cooled at r. t. and the resulting precipitate was filtered and washed with absolute
ethanol. The pure product 3b was obtained as an orange solid (77% yield). 1H-NMR (400 MHz,
CDCl3)
δ 8.197 (d, J = 8.40 Hz, 1H) 7.809 (dd, J1 = 1.75 Hz, J2 = 9.54 Hz, 2H) 7.585 (d, J =
8.23 Hz, 1H) 7.549 (s, 1H) 7.452 (dd, J1 = 1.74 Hz, J2 = 8.20 Hz, 1H) 7.388 (dd, J1 = 1.85 Hz, J2
= 8.37 Hz, 1H) 7.353 (d, J = 3.79 Hz, 1H) 7.196 (d, J = 3.79 Hz, 1H) 7.149 (d, J = 3.70 Hz, 1H)
7.079 (d, J = 3.54 Hz, 1H) 5.644 (s, 1H) 3.789 (d, J = 11.21 Hz, 2H) 3.667 (d, J = 10.91 Hz, 2H)
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1.310 (s, 3H) 0.824 (s, 3H) 13C-NMR (100 MHz, CDCl3)
δ 141.5, 141.2, 140.1, 139.0, 138.5,
137.3, 136.7, 134.6, 133.5, 132.5, 131.5, 130.2, 130.1, 125.8, 125.5, 124.1, 123.6, 123.1, 122.9,
122.6, 122.0, 121.3, 120.0, 97.9, 77.4, 30.0, 22.8, 21.6. MS (APCI): calcd for C28H22Br2O2S2,
613.94; found: m/z = 614.97 [M+H]+. FT-IR (KBr):
ν 2952, 2851, 1609, 1589, 1469, 1420,
1376, 1366, 1322, 1307, 1212, 1185, 1087, 1067, 1015, 988, 965, 863, 816, 792 cm-1. Anal.
Calcd for C28H22Br2O2S2: C, 54.74; H, 3.61; S, 10.44. Found: C, 54.71; H, 3.62; S, 10.46.
2.4.5.
9-((5-(5,5-dimethyl-1,3-dioxan-2-yl)thiophen-2-yl)methylene)-N3,N3,N6,N6-tetrakis(4-
methoxyphenyl)-9H-fluorene-3,6-diamine, 4a
.
To a suspension of Pd(dba)2 (0.008 g, 0.014 mmol) and PtBu3 (0.16 mL, 0.16 mmol, 1M in
toluene) in anhydrous toluene (5 mL), were added 3a (0.2 g, 0.38 mmol), bis(4-
methoxyphenyl)amine (0.19 g, 0.84 mmol) and sodium tert-butoxide (0.08 g, 0.84 mmol). The
resulting mixture was reacted under a microwave irradiation at a constant temperature of 110°C
for 50 min. The solvent was removed under a reduced pressure, and the residue was purified by
column chromatography (silica gel, 50/50 hexane/CH2Cl2) to give the pure product as a red solid
1
(67% yield). H-NMR (400 MHz, DMSO)
δ 7.879 (d, J = 8.65 Hz, 1H) 7.691 (d, J = 8.46 Hz,
1H) 7.422 (s, 1H) 7.267 (d, J = 3.64 Hz, 1H) 7.114 (d, J = 3.78 Hz, 1H) 6.984 (m, 8H) 6.880 (m,
10H) 6.696 (dd, J1 = 2.15 Hz, J2 = 8.43 Hz, 1H) 6.563 (dd, J1 = 2.28, J2 = 8.61, 1H) 5.688 (s,
1H) 3.716 (s, 12H) 3.638 (d, 4H) 1.147 (s, 3H) 0.731 (s, 3H) 13C-NMR (100 MHz, DMSO)
δ
156.2, 155.8, 149.5, 149.1, 142.7, 141.8, 140.5, 139.3, 139.0, 134.7, 133.1, 129.2, 128.4, 127.0,
126.5, 126.1, 124.9, 121.8, 120.1, 118.4, 115.2, 115.18, 115.1. MS (APCI): calcd for
C52H48N2O6S, 828.3; found: m/z = 829.4 [M+H]+. FT-IR (KBr):
ν 2956, 2833, 1687, 1596, 1505,
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1484, 1451, 1361, 1329, 1276, 1246, 1177, 1105, 1032, 911, 827, 783, 677 cm-1. Anal. Calcd for
C52H48N2O6S: C, 75.34; H, 5.84; S, 3.87. Found: C, 75.34; H, 5.86; S, 3.88.
2.4.6.
9-((5'-(5,5-dimethyl-1,3-dioxan-2-yl)-[2,2'-bithiophen]-5-yl)methylene)-N3,N3,N6,N6-
tetrakis(4-methoxyphenyl)-9H-fluorene-3,6-diamine, 4b
.
To a suspension of Pd(dba)2 (0.015 g, 0.024 mmol) and PtBu3 (0.096 mL, 0.096 mmol, 1M in
toluene) in anhydrous toluene (5 mL), were added 3b (0.18 g, 0.3 mmol), bis(4-
methoxyphenyl)amine (0.18 g, 1.17 mmol) and sodium tert-butoxide (0.131 g, 1.17 mmol). The
resulting mixture was reacted under a microwave irradiation at a constant temperature of 110°C
for 50 min. The solvent was removed under a reduced pressure, and the residue was purified by
column chromatography, (silica gel, 50/50 hexane/CH2Cl2) to give the pure product 4b as a
powder purple (70% yield) 1H-NMR (400 MHz, CDCl3)
δ 8.109 (d, J = 8.58 Hz, 1H) 7.51 (d, J
= 8.43 Hz, 1H) 7.259 (d, J = 3.93 Hz, 1H) 7.237 (s, 1H) 7.186 (s, 2H) 7.136 (d, J = 3.72 Hz, 1H)
7.066 (m, 10H) 6.845 (m, 9H) 6.754 (d, J = 8.61 Hz, 1H) 5.625 (s, 1H) 3.801 (s, 14H) 3.653 (d, J
= 10.97 Hz, 2H) 1.315 (s, 3H) 0.820 (s, 3H) 13C-NMR (100 MHz, CDCl3)
δ 155.7, 155.4, 149.2,
148.9, 142.1, 141.2, 140.7, 140.3, 139.5, 139.1, 138.0, 137.4, 135.3, 133.8, 129.9, 129.4, 126.2,
125.7, 125.6, 125.0, 123.9, 123.0, 12.2, 120.6, 119.7, 114.6, 114.5, 113.9, 113.0, 112.0, 98.0,
55.3, 55.29, 30.0, 22.8, 21.7. MS (APCI): calcd for C56H50N2O6S2, 910.31; found: m/z = 911.4
[M+H]+. FT-IR (KBr):
ν 2948, 2830, 1600, 1502, 1452, 1377, 1322, 1272, 1238, 1178, 1095,
1034, 983, 825 cm-1. Anal. Calcd for C56H50N2O6S2: C, 73.82; H, 5.53; S, 7.04. Found: C, 73.83;
H, 5.55; S, 7.07.
15

ACCEPTED MANUSCRIPT
2.4.7.
9-((5'-(5,5-dimethyl-1,3-dioxan-2-yl)-[2,2'-bithiophen]-5-yl)methylene)-N3,N3,N6,N6-
tetrakis(4-(octyloxy)phenyl)-9H-fluorene-3,6-diamine, 4c
.
To a suspension of Pd(dba)2 (0.012 g, 0.02 mmol) and PtBu3 (0.08 mL, 0.08 mmol, 1M in
toluene) in anhydrous toluene (5 mL), were added 3b (0.128 g, 0.2 mmol), bis(4-
(octyloxy)phenyl)amine (0.22 g, 0.5 mmol) and sodium tert-butoxide (0.058 g, 0.52 mmol). The
resulting mixture was reacted under a microwave irradiation at a constant temperature of 110°C
for 50 min. The solvent was removed under a reduced pressure, and the residue was purified by
column chromatography (silica gel, 7/3 hexane/CH2Cl2) to give the pure product 4c as a red solid
(60% yield). 1H-NMR (400 MHz, CDCl3)
δ 8.07 (d, J = 8.61 Hz, 1H) 7.50 (d, J = 8.40 Hz, 1H)
7.25 (d, J = 3.81, 1H) 7.22 (t, J = 4.31 Hz, 2H) 7.18 (s, 2H) 7.12 (d, J = 3.74 Hz, 1H) 7.08 (d, J =
3.69 Hz, 1H) 7.04-7.00 (m, 9H) 6.85 (d, J = 2.11 Hz, 1H) 6.81 (dd, J1 = 8.98, J2 = 1.61 Hz, 4H)
6.72 (dd, J1 = 8.62, J2 = 2.33 Hz, 1H) 5.62 (s, 1H) 3.92 (t, J1 =5.06 Hz, 8H) 3.77 (d, J = 11.27
Hz, 2H) 3.65 (d, J = 10.61 Hz, 2H) 1.81-1.73 (m, 8H) 1.35-1.26 (m, 40H) 0.93-0.88 (m, 15H)
0.81 (s, 3H). 13C-NMR (100 MHz, CDCl3)
δ 155.2, 154.9, 141.0, 140.6, 139.2, 137.9, 137.4,
129.7, 129.3, 126.1, 125.6, 123.9, 122.9, 115.1, 115.0, 99.4, 98.4, 98.0, 68.1, 31.6, 30.0, 29.5,
29.2, 29.1, 25.9, 22.8, 22.5, 21.6, 13.9. MS (APCI): calcd for C84H106N2O6S2, 1302.7; found: m/z
= 1303.8 [M+H]+. FT-IR (KBr):
ν 2951, 2855, 1600, 1504, 1480, 1469, 1452, 1384, 1240, 1171,
1099, 1027, 825, 722 cm-1. Anal. Calcd for C84H106N2O6S2: C, 77.38; H, 8.19; S, 4.92. Found: C,
77.37; H, 8.21; S, 4.90.
2.4.8. 5-(5,5-dimethyl-1,3-dioxan-2-yl)thiophene-2-carbaldehyde, 1’
.
2,5-thiophene-dicarbaldehyde (2 g, 14.28 mmol), neopentylglycol (1.56 g, 15 mmol) and p-
toluenesulfonic acid (0.142 g, 0.75 mmol) were dissolved in dry toluene (50 mL). The reaction
16

ACCEPTED MANUSCRIPT
mixture was refluxed for 5 h and then cooled to rt. The solvent was removed under a reduced
pressure and the residue was dissolved in ethyl acetate and washed with brine. The organic layer
was dried over anhydrous sodium sulfate and the solvent evaporated under a reduced pressure.
The crude redidue was purified by flash chromatography on SiO2, using a mixture 9/1
hexane/ethyl acetate as the eluent. The pure product I was obtained as a white solid (85% yield).
1H-NMR (400 MHz, CDCl3):
5.64 (s, 1H) 3.78 (d, J = 11.30 Hz, 2H) 3.65 (d, J = 11.24 Hz, 2H) 1.27 (s, 3H) 0.82 (s, 3H). 13C-
δ 9.90 (s, 1H) 7.67 (d, J = 3.84 Hz, 1H) 7.23 (d, J = 3.84 Hz, 1H)
NMR (100 MHz, CDCl3),
δ 183.0, 151.2, 143.3, 135.5, 125.7, 97.3, 77.3, 30.1, 22.7, 21.6. MS
(APCI): calcd for C11H14O3S, 226,07; found: m/z = 227.07 [M+H]+. FT-IR (KBr):
ν
2965, 2855,
2821, 1668, 1539, 1476, 1375, 1300, 1228, 1218, 1189, 1098, 1056, 1033, 1019, 988, 961, 813
cm-1. Anal. Calcd for C11H14O3S: C, 58.39; H, 6.24; S, 14.17. Found: C, 58.40; H, 6.22; S, 14.20.
2.4.9. 2-(5-bromothiophen-2-yl)-5,5-dimethyl-1,3-dioxane, 2’
.
5-bromothiophene-2-carbaldehyde (5 g, 26 mmol), neopentylglycol (4.16 g, 40 mmol) and p-
toluenesulfonic acid (0.38 g, 2 mmol) were dissolved in dry toluene (100 mL). The reaction
mixture was refluxed for 5 h and then cooled at r. t. The solvent was removed under a reduced
pressure and the residue was dissolved in ethyl acetate and washed with brine. The organic layer
was dried over anhydrous sodium sulfate and the solvent evaporated under a reduced pressure.
The crude residue was purified by flash chromatography on SiO2 using a mixture 9/1
hexane/ethyl acetate as the eluent. The pure product 2’ was obtained as a white solid (95%
1
yield). H-NMR (400 MHz, CDCl3)
δ 6.94 (d, J = 3.78 Hz, 1 H), 6.88 (dd, J1 = 3.78 Hz, J2 =
0.73 Hz, 1 H), 5.54 (s, 1 H), 3.75 (d, J = 11.30 Hz, 2 H) 3.62 (d, J = 11.30 Hz, 2H), 1.26 (s, 3H),
0.79 (s, 3H). 13C-NMR (100 MHz, CDCl3)
δ
143.17, 129.17, 125.07, 113.22, 97.93, 77.12, 30.0,
17

ACCEPTED MANUSCRIPT
22.71, 21.60. MS (APCI): calcd for C10H13BrO2S, 277.98; found: m/z = 278.00 [M+H]+. FT-IR
(KBr):
ν 2956, 2850, 1554, 1471, 1448, 1394, 1375, 1368, 1298, 1231, 1210, 1190, 1097, 1021,
986, 972, 930, 922, 796, 777 cm-1. Anal. Calcd for C10H13BrO2S: C, 43.33; H, 4.73; S, 11.57.
Found: C, 43.33; H, 4.72; S, 11.55.
2.4.10.
5'-(5,5-dimethyl-1,3-dioxan-2-yl)-[2,2'-bithiophene]-5-carbaldehyde,
3’.
5-
formylthiophen-2-ylboronic acid (1.24 g, 7.94 mmol), 2’ (2,0 g, 7.22 mmol) Na2CO3 (4.59 g,
43.32 mmol), toluene (35 mL), ethanol (8.75 mL) and water (8.75 mL) were purged with argon
for 1.5-2 h. Pd(PPh3)4 (0.083g, 0.072 mmol) was added and the reaction mixture was stirred at
reflux for 20 h, Then, the reaction mixture was cooled at rt, poured into water and extracted with
dichloromethane. The organic layer was washed with water, saturated aqueous NaHCO3 solution
and brine, then dried over anhydrous sodium sulfate. The solvent was evaporated under a
reduced pressure and the crude residue was purified by column chromatography on silica gel
1
(eluent: hexane/dichloromethane 60/40), to give the pure product 3’ (60% yield). H-NMR (400
MHz, CDCl3)
δ 9.84 (s, 1H), 7.65 (d, J = 3.84 Hz, 1H), 7.23 (d, J = 3.84 Hz, 1H), 7.08 (d, J =
3.76 Hz, 1H), 7.01 (d, J = 3.72 Hz, 1H), 5.62 (s, 1H), 3.75 (d, J = 11.24 Hz, 2H) 3.66 (d, J =
13
11.23 Hz, 2H), 1.28 (s, 3H), 0.80 (s, 3H). C-NMR (100 MHz, CDCl3)
δ 182.3, 146.9, 143.0,
141.6, 137.1, 135.9, 126.0, 125.4, 124.1, 123.0, 97.9, 97.6, 30.0, 22.8, 21.6. MS (APCI): calcd
for C15H16O3S, 308.05; found: m/z = 309.05 [M+H]+. FT-IR (KBr):
ν
2947, 2861, 1650, 1474,
1455, 1433, 1378, 1309, 1298, 1224, 1214, 1199, 1159, 1096, 1042, 1027, 1014, 987, 965, 800,
776 cm-1. Anal. Calcd for C15H16O3S2: C, 58.42; H, 5.23; S, 20.79. Found: C, 58.44; H, 5.23; S,
20.78.
18

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2.4.11. bis(4-(octyloxy)phenyl)amine, 4’
.
To a 50 mL round-bottomed flask were added, under a nitrogen atmosphere, anhydrous toluene
10 mL, 4-(octyloxy)aniline (1.8 g, 8.5 mmol) and 1-bromo-4-(octyloxy)benzene (2.13 g, 7.32
mmol). Then Pd2dba3 (0.076 g, 0.073 mmol), 1,1’-bis(diphenylphosphino)ferrocene (0.081 g,
0.146 mmol) and sodium tert-butoxide (1.0 g, 10.46 mmol) were added. The reaction mixture
was stirred at reflux for 4h, then cooled at rt and filtered through a short pad of celite. The
solvent was removed at a reduced pressure and the crude product was purified by flash
chromatography, using a mixture 8/2 hexane/dichloromethane as the eluent, to give the pure
1
product 4’ (70% yield). H-NMR (400 MHz, CDCl3):
δ 6.93 (m, 4H) 6.82 (m,4H), 5.27 (s, 1H)
3.94 (m, 4H) 1.77 (m, 4H), 1.45 (m, 4H) 1.33 (m, 16H), 0.87 (m, 6H) 13C-NMR (100 MHz,
CDCl3) 129.2, 120.2, 114.3, 68.4, 31.6, 29.2, 29.1, 25.9, 22.5, 13.9. MS (APCI): calcd for
δ
C28H43NO2, 425.33; found: m/z = 426.33 [M+H]+.
2.4.12. 3-(5-((3,6-bis(bis(4-methoxyphenyl)amino)-9H-fluoren-9-ylidene)methyl)thiophen-2-yl)-
2-cyanoacrylic acid, TK4
.
A mixture of 4a (0.083 g, 0.1 mmol), cyanoacetic acid (0.17 g, 2.0 mmol), acetic acid (20 mL)
and ammonium acetate (0.095 g, 1.2 mmol) was heated at 120°C for 12 h. The resulting solution
was poured into ice-cold water to produce a precipitate. Then, the precipitate was dissolved in
dichloromethane and washed several times with water. The organic layer was dried over
anhydrous sodium sulfate, and the solvent removed by rotary evaporation. The crude product
was purified by flash chromatography on SiO2, using a mixture 10/1 dichloromethane/methanol
1
as the eluent. The pure product TK4 was obtained as a purple solid (75% yield). H-NMR (400
MHz, DMSO-d8):
δ
8.36 (s, 1H), 8.01 (d, J = 8.69 Hz, 1 H), 7.90 (d, J = 3.93 Hz, 1H) 7.73 (d, J
19

ACCEPTED MANUSCRIPT
= 8.52 Hz, 7.57, (d, J = 4.26 Hz, 1 H) 7.52 (s, 1H), 7.04-6.98 (m, 8H), 6.91-6.84 (m, 10H), 6.66
(dd, J1 = 5.47, J2 = 2.04 Hz, 1H), 6.54 (dd, J1 = 8.62, J2 = 2.24 Hz, 1H) 3.73 (s, 12H). 13C-NMR
(100 MHz, DMSO-d8) δ 156.43, 156.07, 150.07, 150.03, 149.71, 142.40, 140.25, 139.71, 139.34,
132.73, 127.54, 127.36, 126.82, 115.28, 115.21, 55.51. MS (APCI): calcd for C50H39N3O6S,
809.26; found: m/z = 810.26 [M+H]+. ]+. FT-IR (KBr):
ν
3428, 2927, 2830, 1600, 1568, 1506,
1450, 1346, 1323, 1272, 1240, 1179, 1126, 1104, 1033, 825, 715 cm-1. Anal. Calcd for
C50H39N3O6S: C, 74.15; H, 4.85; N, 5.19; S, 3.96. Found: C, 74.14; H, 4.85; N, 5.18; S, 3.93.
2.4.13.
3-(5'-((3,6-bis(bis(4-methoxyphenyl)amino)-9H-fluoren-9-ylidene)methyl)-[2,2'-
bithiophen]-5-yl)-2-cyanoacrylic acid, TK5
.
A mixture of 4b (0.1 g, 0.126 mmol), cyanoacetic acid (0.214 g, 2.52 mmol), acetic acid (40 mL)
and ammonium acetate (0.117 g, 1.81 mmol) was heated at 120°C for 12 h. The resulting
solution was poured into ice-cold water to produce a precipitate. Then, the precipitate was
dissolved in dichloromethane and washed several times with water. The organic layer was dried
over anhydrous sodium sulfate, and the solvent removed by rotary evaporation. The pure product
1
TK5 was obtained as a red solid (85% yield). H-NMR (400 MHz, DMSO-d8)
δ 8.15 (s, 1H)
8.10 (d, J = 8.64 Hz, 1H) 7.73 (d, J = 3.03 Hz, 1H), 7.70 (d, J = 8.55 Hz, 1H), 7.60 ( d, J = 8.62
Hz, 1H), 7.53-7.50 (m, 2H), 7.47 (d, J =11.40, 1H), 7.37 (d, J = 3.83 Hz, 1H), 7.05-6.95 (m, 8H),
6.91-6.84 (m, 8H), 6.71-6.67 (m, 2H), 6.62 (dd, J1 = 8.65, J2 = 2.16 Hz, 1H), 6.42 (dd, J1 = 8.68,
J2 = 2.21 Hz, 1H), 3.73 (s, 12 H). 13C-NMR (100 MHz, DMSO-d8)
δ 169.6, 156.3, 156.29,
156.2, 155.9, 149.7, 149.6, 149.5, 149.2, 142.0, 140.9, 140.5, 140.3, 140.0, 139.9, 139.0 138.5
136.5, 135.2, 134.8, 133.2, 132.1, 128.5, 128.1, 128.0, 127.9, 127.2, 127.19, 127.0, 126.6, 125.3,
115.3, 115.2, 55.5. MS (APCI): calcd for C54H41N3O6S2, 891.24; found: m/z = 892.26 [M+H]+.
20

ACCEPTED MANUSCRIPT
FT-IR (KBr):
ν
3434, 2929, 2832, 1602, 1504, 1454, 1368, 1324, 1275, 1239, 1180, 1106, 1035,
827 cm-1. Anal. Calcd for C54H41N3O6S2: C, 72.71; H, 4.63; N, 4.71; S, 7.19. Found: C, 72.69; H,
4.65; N, 4.72; S, 7.18.
2.4.14.
3-(5'-((3,6-bis(bis(4-(octyloxy)phenyl)amino)-9H-fluoren-9-ylidene)methyl)-2,2'-
bithiophen-5-yl)-2-cyanoacrylic acid, TK6
.
A mixture of 4c (0.2 g, 0.15 mmol), cyanoacetic acid (0.214 g, 3.06 mmol), acetic acid (40 mL)
and ammonium acetate (0.141 g, 1.84 mmol) was heated at 120°C for 12 h. The resulting
solution was poured into ice-cold water to produce a precipitate. Then, the precipitate was
dissolved in dichloromethane and washed several times with water. The organic layer was dried
over anhydrous sodium sulfate, and the solvent removed by rotary evaporation. The pure product
1
TK6 was obtained as a red solid (70% yield). H-NMR (400 MHz, CDCl3)
δ 8.40 (d, J = 6.26
Hz, 1H) 8.29 (d, J = 3.05 Hz, 1H) 7.98 (d, J = 8.70 Hz, 1H), 7.80 (d, J = 4.28 Hz, 1H), 7.71 (d, J
= 4.28 Hz, 1H), 7.60 (d, J = 8.54 Hz, 1H), 7.52 (s, 1H), 7.50 (d, J = 8.34 Hz, 1H), 7.16 (s, 1H),
7.12 (m, 1H), 7.04-6.96 (m, 8H) 6.80 (m, 8H) 6.70 (dd, J1 = 1.96 Hz, J2 = 8.34 Hz, 1H) 6.54 (m,
1H) 3.92 (m, 8H) 1.77-1.30 (m, 48H) 0.89 (m, 12H). MS (APCI): calcd for C82H97N3O6S2,
1283.68; found: m/z = 1284.70 [M+H]+. FT-IR (KBr):
ν 3434, 2923, 2853, 1602, 1504, 1486,
1470, 1455, 1386, 1238, 1168, 1108, 1028, 826, 721 cm-1. Anal. Calcd for C82H97N3O6S2: C,
76.66; H, 7.61; N, 3.27; S, 4.99. Found: C, 76.66; H, 7.60; N, 3.25; S, 4.98.
3 RESULTS AND DISCUSSION
3.1. Synthesis of organic dyes.
21

ACCEPTED MANUSCRIPT
The synthesized organic dyes (TK) are shown in Figure 1, while the synthetic pathways are
depicted in Scheme 1. Compound 2 was synthesized by a reduction with aluminum chloride and
borane tert-butylamine complex[50] of compound 1. Compounds 3a and 3b were prepared by an
ultrasound-assisted reaction of compound 2 with 5-(5,5-dimethyl-1,3-dioxan-2-yl)thiophene-2-
carbaldehyde (1’) and 5'-(5,5-dimethyl-1,3-dioxan-2-yl)-2,2'-bithiophene-5-carbaldehyde (3’),
respectively. The reactions were carried out in short times with good yields, and the products
were readily purified. Compound 1’ was obtained by reacting thiophene-2,5-dicarbaldehyde
(commercial) with 2,2-dimethylpropane-1,3-diol. Compound 2’ was obtained by treating 5-
bromothiophene-2-carbaldehyde with 2,2-dimethylpropane-1,3-diol, and then the resulting
product was reacted with (5-formylthiophen-2-yl)boronic acid under typical Suzuki reaction
conditions to prepare compound 3’. Compound 4a, 4b and 4c were obtained starting from 3a and
3b by a Buchwald-Hartwig cross coupling reaction (C-N amination) with 2.2 equivalents of
diarylamine in the presence of Pd(dba)2 (dba = dibenzylideneacetone) and P(tBu)3,[51] in a
microwave reactor. Finally, TK4, TK5 and TK6 dyes were prepared via a Knoevenagel
condensation, reacting cyanoacetic acid with compunds 4a, 4b and 4c, respectively, in the
presence of ammonium acetate.
22

ACCEPTED MANUSCRIPT
Scheme 1. Synthesis of organic dyes TK4, TK5 and TK6.
3.2. Photophysical properties.
23

ACCEPTED MANUSCRIPT
UV-vis absorption spectra of dyes in CH2Cl2 solution (ca. 10-5 M) are shown in Figure 2, (the
fluorescence spectra have been reported in the Supp. Info.) Absorption data are summarized in
Table 1. As shown in Figure 2, all spectra show strong bands in the ranges 260 − 350 nm and
350 − 600 nm. The higher energy bands are attributed to local π−π* transitions, while the lower
energy bands are attributed to charge transfer (CT) transitions. TK4 shows two maxima
= 13200 and 10100 M-1cm-1,
absorption peaked at 360 and 548 nm, with extinction coefficients
ε
respectively. The sensitizer TK5 can be considered as obtained by incorporating two thiophene
rings, instead of one, as spacer between the DBF-core and the acceptor unit in the basic structure
of TK4. TK5 shows two maxima absorption peaked at 413 nm (ε
= 24800 M-1cm-1) and 513 nm
(17800 M-1cm-1), almost twice TK4. TK6 can be considered as obtained by substituting the
methoxy groups with longer octyloxy chains in the basic structure of TK5. TK6 show two
maxima absorption peaked at 420 nm (
ε
= 34500 M-1cm-1) and 516 nm ( = 25100 M-1cm-1).
ε
70
60
50
40
30
20
10
TK4
TK5
TK6
0
300
400
500
600
700
800
Wavelenght (nm)
Figure 2. Absorption spectra of TK dyes in CH2Cl2.
24

ACCEPTED MANUSCRIPT
3.3. Electrochemical properties.
Cyclic voltammetry (CV) was carried out to investigate the possibility of electron injection
from the excited state of the organic dyes to the conduction band (CB) of TiO2. The curves are
collected in the Figure 3, and the corresponding data are depicted in Table 1.
From the onset of the first oxidation potential were calculated HOMO energies, while the
LUMO energies were extimated from ꢀꢁꢂꢃꢂ − ꢀꢄꢅꢄ,[52, 53] and are ultimately based on the
absolute value for the normal hydrogen electrode (NHE), which was critically assessed in a
fundamental review paper. [54] The optical band gap E0-0 was derived from the absorption edge.
The LUMO values (-1.16, -1.22 and -1.15 V, for TK4, TK5 and TK6, respectively) are
sufficiently more negative than the conduction band edge of TiO2 (-0.5 vs. NHE),[55] to forecast
an efficient electron injection from excited dye into the TiO2 electrode. Dye regeneration is
essential to achieve a high stability and operational lifetime for the DSSCs. Efficient dye
regeneration is possible if the ground-state redox potential of the dye is more positive than that of
the electrolyte redox potential. The HOMO levels estimated (0.64, 0.68 and 0.73 V for TK4,
TK5 and TK6, respectively) are more positive than that of I¯ /I3¯ redox couple (0.4 V vs.
NHE),[1] which is necessary to ensure that the neutral dye is effectively regenerated after being
oxidized.
25

ACCEPTED MANUSCRIPT
Figure 3. Cycle voltammograms of dyes in CH2Cl2 with 0.1 M Bu4NPF6 supporting electrolyte.
Table 1. Absorption data, oxidation potentials, HOMO and LUMO levels, optical energy gaps.
Dye
λabs
(ε
× 103M-1cm-1)a HOMO (V E0-0c(eV)
LUMO d(V
(nm)
vs. NHE)
vs. NHE)
548 (10.1)
513 (17.8)
516 (25.1)
0,64
0.68
0.73
1.82
1.90
1.88
-1.18
-1.22
-1.15
TK4
TK5
TK6
aAbsorption maximum in dichloromethane (~10-5 M) solution. b HOMO (vs. NHE) of the dyes
by cyclic voltammetry in CH2Cl2 solution containing 0.1 M of tetrabutylammonium
c
hexafluorophosphate with a scan rate of 100 mVs-1 and calibrated against ferrocene. The
bandgap, E0-0 was determined from onset of absorption spectrum. d LUMO = HOMO - E0-0.
3.4. Theoretical approach.
26

ACCEPTED MANUSCRIPT
To understand better the electronic and optical properties of the three new dyes, we performed
a density functional theory (DFT) and time-dependent (TD) DFT investigations. The TD-DFT
results, reported in Table 2, are in reasonably good agreement with the experimental spectra and
allow the identification of several singlet excitations underlying the broad absorption bands
observed experimentally (see Figure S15). All the dyes exhibit rather similar photophysical
properties for the lowest-lying optically active excitations. Thus, we find that the lowest
excitation lay in all cases at 2.12-2.5 eV, they are well described by a single particle transition
from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital
(LUMO), and they display a significant charge-transfer character (Λ=0.6 and ꢀr=5-6Å) (see
Table 2). Iso-density plots of the HOMO and LUMO orbitals are reported in Figure 4. The
highest excitations are found in an energy interval between S1 and about 3.5 eV. They show
similar features as the S1 excited state, but generally an even larger charge-transfer character (as
suggested by the values of the
Λ and ꢀr indicators).
A comparison of the TK4 and TK5 results shows that the effect of the insertion of a thiophene
unit into the spacer is to slightly increase the effective conjugation length. This effect is visible in
the plots of the orbitals and can be also inferred by an inspection of the HOMO and LUMO
energies of the two dyes as well as of the many-body ionization potentials (IP) and electron
affinities (EA) reported in Table 3. Consequently, a little lowering of the S1 excitation energy is
found.
27

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Table 2 Excitation energies and absorption spectra
Dye
Excitation
Energy
(eV)
Single-particle
transition
Λ
ꢀr (Å)
S1
S2
S3
S1
S2
S3
S4
S1
S2
S3
S4
2.33
2.40
3.35
2.17
2.48
3.15
3.31
2.56
2.86
3.52
3.70
H
→
L (86%)
0.61
0.39
0.70
0.58
0.29
0.65
0.67
0.61
0.32
0.68
0.71
5.05
6.19
3.22
6.66
16.51
9.48
9.25
6.54
16.47
9.43
9.21
TK4
H-1
H-2
→
L(82%)
L (88%)
→
H→L (84%)
TK5
TK6
H-1
→
→
L (79%)
L (88%)
H-2
H
→
→
L+1(86%)
L (83%)
H
H-1
→
→
L (79%)
L (79%)
H-2
H→L+1 (85%)
TD-DFT excitation energies (eV), oscillator strengths, main single-particle transitions, overlap
indicator , and electron displacement indicator r, for the lowest-lying singlet excitations of TK4,
TK5, and TK6.
Λ
ꢀ
At the same time, the additional thiophene unit favors the charge separation in the excited
states, as indicated by the values of r, having a potentially positive impact on the photovoltaic
ꢀ
properties of the material, as already found in our previous work.[17] On the contrary, the
inclusion of branched substituents on the phenyl rings, which marks the difference between TK6
and TK5, is found to have a minor impact on the electronic properties of the dye, leaving frontier
orbitals as well as IP and EA almost unchanged.
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Figure 4. Isodensity plots of the most relevant frontier orbitals of the dyes.
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