Synthesis and optoelectronic properties of chemically modified bi-fluorenylidenes

Article abstract of DOI:10.1039/c5tc03501e

The development of new light harvesting materials is a key issue for the progress of the research on organic & hybrid photovoltaics. Here, we report a new class of organic sensitizers based on the bi-fluorenylidene moiety as π-linker within the donor-π-linker-acceptor (D-π-A) scheme. The new dyes are endowed with electron donor and electron acceptor units at strategic positions in order to improve their electronic and light-harvesting properties. The comprehensive study of these compounds through the use of different experimental and theoretical techniques, provides an in-depth understanding of their electronic and photophysical properties, and reveal their interest as photovoltaic materials.

Full text of DOI:10.1039/c5tc03501e

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Synthesis and Optoelectronic Properties of Chemically Modified Bi-
fluorenylidenes.
5
Mateusz Wielopolski,a Magdalena Marszalek,b Fulvio G. Brunetti,c Damien Joly,c Joaquín Calbo,d Juan Aragó,d Jacques-E.
Moser,a Robin Humphry-Baker,b Shaik M. Zakeeruddin,b Juan Luis Delgado,*,e,f Michael Grätzel,*,b Enrique Ortí,*,d and
Nazario Martín,*,c,g
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
First published on the web Xth XXXXXXXXX 200X
10 DOI: 10.1039/b000000x
The development of new light harvesting materials is a key issue for the progress of the research on organic & hybrid photovoltaics. Here, we report
a new class of organic sensitizers based on the bi-fluorenylidene moiety as π-linker within the donor–π-linker–acceptor (D–π–A) scheme. The new dyes
are endowed with electron donor and electron acceptor units at strategic positions in order to improve their electronic and light-harvesting properties.
The comprehensive study of these compounds through the use of different experimental and theoretical techniques, provides an in-depth
15 understanding of their electronic and photophysical properties, and reveal their interest as photovoltaic materials.
acyd or rhodanine as the accepting fragment.7 This structural
versatility illustrates the scientific interest and progress in the
Introduction
Within the field of renewable energies, solar energy is expected
to play a prominent role. The search for new materials able to
20 efficiently convert solar energy into electrical power is a current
challenge for materials scientists.1 The utilization of light-
harvesting materials would bring new advantages such as the
possibility of processing directly from solution, affording lighter
and cheaper flexible solar devices.
quest for new molecular architectures to be used as sensitizers
in DSSCs.
60
According to the above mentioned reports on organic dyes,
requirements such as the presence of an anchoring group,
efficient coupling to the conduction band of TiO2, and high molar
absorption coefficients can be fulfilled by chemical modification
of the dye structure through the introduction of electron-
25
Dye-sensitized solar cells (DSSCs) based on organic dyes
adsorbed on TiO2 semiconductor electrodes emerged as a new
generation of sustainable photovoltaic devices.2 Their attraction
to chemists, physicists and engineers originates not only from
the already established high incident-photon-to-current
65 donating and -accepting groups and by expansion of π-
conjugation. Assuring directional transport of electrons and
chemical stability, and preventing dye aggregation, on the other
hand, is achieved through the introduction of sterically hindering
substituents such as hydrophobic long alkyl chains and aromatic
70 units onto the chromophore skeleton.5d In summary, to obtain
new and efficient organic dye sensitizers for DSSCs, novel
molecular designs capable of controlling not only the
photophysical and electrochemical properties of the dyes
themselves but also their molecular orientation and
75 arrangement on the TiO2 surface are necessary.8
Herein, we present a new class of organic sensitizers based
on fluorene and bi-fluorenylidene as π-linkers within the donor–
π-linker–acceptor (D–π–A) scheme. DSSCs based on 2-donor, 7-
acceptor, 9-alkyl chain fluorenes have been extensively studied.9
80 However, bi-fluorenylidenes have been comparatively less
studied. Bi-fluorenylidenes and their thiophene-based analogues
have been used as electron acceptors10 or as monomers to
prepare low bandgap polymers.11 In this work, we have carried
out a systematic study on the synthesis of a variety of fluorene
85 derivatives, such as 2,7-donor, 9-acceptor and 3,6-donor, 9-
acceptor fluorenes. Furthermore, we have also prepared a series
of suitably functionalized bi-fluorenylidenes endowed with
electron-donor and electron-acceptor units at strategic
positions. In particular, this work aims to establish the
90 relationship between precise and controlled changes of the
molecular architecture and their impact on the electronic
structure of the dyes. Photophysical and theoretical studies
provide in-depth insights into the effects of electronic structure
modification on the charge transfer between the dyes and TiO2.
95 All investigated molecules are shown in Chart 1. F2a, F3a and
30 conversion efficiencies and low-cost production but also from
the scientific interest in their operational principles.3 The
fabrication of high-performance DSSCs requires the
development of efficient organic dyes, whose molecular
structures are optimized to provide sufficient light-harvesting
35 features, good electronic communication between the dye and
the conduction or valence band of the semiconductor, and a
controlled molecular orientation on the semiconductor surface.4
Due to the almost infinite synthetic versatility and the high
potential in molecular design, precise control of the
40 photophysical and electrochemical properties may be achieved
by the modification of the chromophore skeleton or the
introduction of substituents.5 Hence,
a whole arsenal of
molecular structures of organic dye sensitizers for DSSCs is
present in the literature,and the information about the
45 relationship between the chemical structur of the dyes and their
photovoltaic performances in DSSCs is steadily increasing. In
fact, families of organic dyes, which exhibit high DSSC
performances, such as polyenes, hemicyanines, thiophene-based
dyes, coumarins, indolines, heteropolycyclic dyes, boron
50 dipyrromethenes
(BODIPYs),
merocyanines,
xanthenes,
perylenes, carbazoles, porphyrins, catechols, polymeric dyes,
squaraines, cyanines, and phthalocyanines (Pcs) have been
reported.6 In our group we have also designed exTTF- and
hemiexTTF-based materials, where exTTF stands for 9,10-di(1,3-
55 dithiol-2-ylidene)-9,10-dihydroanthracene, bearing cyanoacrylic
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F3b are mono-fluorene dyes with two diphenylamine (DPA)
donor units linked to the phenyl rings of the fluorene moiety and
Absorption spectra (Cary 5, Varian), fluorescence spectra
(Fluorolog 3, Horiba) and NMR spectra (AVANCE-400, Bruker)
a
cyanoacrylic acid (CA) group as the acceptor. For the 30 were recorded with the indicated instruments.
connection of the DPAs, different linkage patterns have been
applied. Taking F1 as the mono-fluorene reference structure, the
donor units in F2a and F3a are linked in meta and para positions,
respectively. Compared with F3a, F3b presents a conjugated
phenylenevinylene spacer connecting the DPA units to the
fluorene core. On the other hand, BF1, BF3a, BF3b and BF3c
10 consist of a bi-fluorenylidene core and a carboxylic acid (COOH)
accepting unit. BF3a and BF3b contain DPA donors attached via
the same π-conjugated linkers as in F3a and F3b, respectively,
and BF1 and BF3c lack of an explicit donor part and constitute
reference compounds. The investigation focuses on the charge
15 transfer properties and their dependence on parameters such as
i) the variation of the anchoring moieties: CA vs COOH, ii) the
change in the distance and the π-conjugated linker between the
donor and the anchoring/acceptor groups, and iii) the difference
in the conjugation pattern between meta and para substitution.
20 Thus, we explore the effects of specific structural modifications
on the electronic, photophysical, and photovoltaic features of
the dyes.
5
Differential pulse voltammetry (DPV) measurements
The measurements were conducted using an Autolab system
(PGSTAT30, Metrohm, Switzerland). A typical, three-electrode
35 setup consisting of
electrode), a Pt plate (auxiliary electrode) and a Pt wire (quasi-
reference electrode) was used along with the
a glassy carbon electrode (working
ferrocene/ferrocenium redox couple as an internal standard. The
concentration of the dye in dichloromethane solvent was 0.1
40 mM and 0.1 M TBAPF6 was added to improve the conductivity.
Differential pulse voltammetry was performed in order to enable
precise determination of the oxidation potentials
Computational Methods
45 The molecular geometry of the dyes was fully optimized in vacuo
and in dichloromethane solution using density functional theory
(DFT) calculations with the B3LYP12 functional and the cc-pVDZ13
basis set as implemented in the Gaussian 09 (version C.01)14
program package. The radical cation species were obtained by
50 fully relaxing the optimized geometries of the neutral dyes with
a charge of +1 assuming a doublet spin configuration. Ionization
potentials were calculated as the adiabatic energy difference
between the fully-relaxed cationic and neutral species in their
respective doublet and singlet electronic ground state. Spin
55 densities were computed to predict the localization of the
unpaired electron in the cationic species. Time-dependent DFT
(TD-DFT)15 calculations of the lowest-lying singlet/doublet states
were performed for all neutral/cation compounds using
dimethylformamide (DMF) as solvent. Solvent effects were taken
60 into account using the polarizable continuum model (PCM).16
Molecular orbitals and spin densities were plotted using
Chemcraft 1.6.17
Chart 1. Chemical structure of the dye molecules.
Ph2N
NPh2
Ph2N
NPh2
NC
COOH
Ph2N
NC
F2a
COOH
NC
F3a
COOH
F1
NPh2
NC
COOH
F3b
NPh2
Ph2N
Photophysical Studies
65 Time-resolved transient absorption measurements were
performed on dye-sensitized, 3 μm-thick, transparent TiO2
mesoporous films screen-printed on non-conducting microscope
slides immersed in dye solutions. The pump-probe technique
uses
a compact CPA-2001, 1 kHz, Ti:Sapphire-amplified
H3CO
OCH3
70 femtosecond laser (Clark-MXR), with a pulse width of about 120
fs and a pulse energy of 1 mJ at a central wavelength of 775 nm.
The output beam was split into two parts for pumping a double-
stage noncollinear optical parametric amplifier (NOPA) and to
produce a white light continuum in a sapphire plate or 387 nm
75 UV light by second harmonic generation of the CPA output in a
thin BBO crystal. The NOPA was pumped by 200 μJ pulses at a
central wavelength of 775 nm and the excitation wavelength
was tuned to 480 nm to generate pulses of approximately 10 μJ.
The output pulses of the NOPA were compressed in an SF10-
80 glass prism pair compressor down to duration of less than 60 fs
(fwhm). Iris diaphragms were used to decrease the pulse energy
down to a few micro joules for the pump and to less than 1 μJ
for the probe beam. Transient spectra were measured using a
white light continuum (WLC) for probing.
HOOC
BF3b
Ph2N
NPh2
HOOC
BF3c
HOOC
BF1
HOOC
BF3a
25
85
The nanosecond laser flash photolysis employed 7 ns pulses
to excite the sample at λ = 480 nm and using a 30 Hz repetition
Experimental Section
2
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rate. A Powerlite 7030 frequency-doubled Q-switched Nd:YAG
laser (Continuum, Santa Clara, California, USA) served as a light
source. The laser beam output was expanded by a plano-
concave lens to irradiate a large cross-section of the sample,
whose surface was kept at a 40° angle to the excitation beam.
The laser fluence on the sample was kept at a low level (35 μJ
cm–2 per pulse) to ensure that, on average, less than one
electron is injected per nanocrystalline TiO2 particle on pulsed
irradiation. The probe light, produced by a continuous wave
5
10 xenon arc lamp, was first passed through a monochromator
tuned at different wavelengths to probe a broad spectral range,
various optical elements, the sample, and then through a second
monochromator, before being detected by
photomultiplier tube (Hamamatsu, R9110).
a
fast
15
Device fabrication
State-of-the-art double-layer mesoporous TiO2 layer (8 μm of 20
nm particle (DSL 18NR-T, DYESOL) plus 5 μm of 400 nm light
scattering particles (HPW-400NRD, CCIC)) was employed on FTO
Scheme 1. Synthesis of F2a, F3a and F3b.
20 conducting glass (Solar-4 mm, Nippon Sheet Glass Co, Ltd.). The
double-layer TiO2 film was sensitized by immersing it into a
THF/EtOH (1:4) solution of the respective dye (0.3 mM) for 15 h
at room temperature. The composition of the electrolyte was 1.0
M 1,3-di-methylimidazolium iodide, 30 mM I2, 0.5 M tert-
25 butylpyridine, 0.1 M lithium iodide and 0.1 M guanidinium
thiocyanate in a mixture of acetonitrile and valeronitrile (85/15,
v/v). A platinized FTO conducting glass (LOF TECH 7, Pilkington)
was used as counter electrode. The spectral distribution of the
light source simulates the AM 1.5G solar irradiation
30 characteristics (Xe 450W / K113, filter) with a spectral mismatch
of less than 4%. The exposed area of the devices was 0.16 cm2, a
black tape mask being used to exclude any stray light. An
antireflection film (ARCTOP, Mihama Co.) was attached on the
photoanode side.
55
In the bi-fluorenylidene (BF) series, esters were obtained
using Barton’s two-fold coupling between fluorenones 3, 4 and 9
which were prior converted to fluorenthiones, and 4-
carboxymethylester-fluoren-9-tosylhydrazone
(8)
in
the
presence of iodobenzene diacetate (Scheme 2). The esters were
60 then hydrolyzed to obtain BF1, BF3a, BF3b and BF3c.
TsHN
N
O
NH2-NHTs
CO2Me
CO2Me
8
R
R
R
R
Lawesson's
reagent
35
Results and discussion
S
O
3
4
9
R=NPh2
R=Ph2NPV
R=OMe
10
11
12
13
R=H
R=NPh2
R=Ph2NPV
R=OMe
Synthesis
New dyes based on different fluorene substitution patterns, such
as 2,7-donor, 9-acceptor and 3,6 donor, 9-acceptor fluorenes or
40 even bi-fluorenylidene units, have been carefully designed.
Compounds F2a, F3a and F3b were prepared through a
multi-step synthetic route starting with the preparation of
fluorenone precursors 1, 3 and 4, respectively (see the
Supporting Information, SI). The synthesis of 1, 3 and 4 was
45 achieved through the Buchwald reaction on the corresponding
bromofluorenone precursors. Knoevenagel condensation
followed by hydrolysis in the presence of LiOH afforded
compounds F2a, F3a and F3b (Scheme 1). The reference
compound F1 was prepared following a previously reported
50 procedure.18
iodobenzene
diacetate
R
R
R
R
1) LiOH
2) HCl
HO2C
MeO2C
BF1
R=H
14
15
16
17
R=H
BF3a
BF3b
BF3c
R=NPh2
R=Ph2NPV
R=OMe
R=NPh2
R=Ph2NPV
R=OMe
Scheme 2. Synthesis of bi-fluorenylidene derivatives BF1, BF3a,
BF3b, and BF3c. Compound 10 was synthesized according to an
65 already described procedure.19
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