Improving the Electropolymerization Properties of Fluorene-Bridged Dicarbazole Monomers through Polyfluoroalkyl Side Chains

Article abstract of DOI:10.1021/acs.langmuir.9b01141

The facile functionalization of the fluorene scaffold at the 2,7-positions was utilized to provide access to two soluble carbazole-π-carbazole derivatives CFC-H1 and CFC-F1 featuring fully hydrogenated and polyfluorinated alkyl chains at the 9-position of

Full text of DOI:10.1021/acs.langmuir.9b01141

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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge
Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage
Improving the Electropolymerization Properties of Fluorene-
Bridged Dicarbazole Monomers through Polyfluoroalkyl Side Chains
Ilir Pecnikaj, Simonetta Orlandi, Gianluca Pozzi, Maria Victoria Cappellari, Gabriela Marzari,
Luciana Fernandez, Maximiliano Andrés Zensich, Laura Hernandez, and Fernando Fungo
Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b01141 • Publication Date (Web): 04 Jun 2019
Downloaded from http://pubs.acs.org on June 7, 2019
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Improving the Electropolymerization Properties of Fluorene-Bridged Dicarbazole
Monomers through Polyfluoroalkyl Side Chains
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Ilir Pecnikaj, Simonetta Orlandi and Gianluca Pozzi *
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Istituto di Scienze e Tecnologie Molecolari del Consiglio Nazionale delle
Ricerche, ISTM-CNR, Via Golgi 19, 20133 Milano, Italy
María Victoria Cappellari, Gabriela Marzari, Luciana Fernández, Maximiliano Andrés
Zensich, Laura Hernandez, and Fernando Fungo*
Departamento de Química, Universidad Nacional de Río Cuarto, Agencia Postal 3
(X5804BYA), Río Cuarto, Argentina.
ABSTRACT: The facile functionalization of the fluorene scaffold at the 2,7-positions
was utilized to provide access to two soluble carbazole-π-carbazole derivatives CFC-H1
and CFC-F1 featuring fully hydrogenated- and polyfluorinated alkyl chains at the 9-
position of the fluorene π-bridging unit, respectively. The optical and electrochemical
properties of the new dicarbazoles were investigated. Their electrochemical
polymerization over Pt and indium tin oxide electrodes allowed the generation of electro-
active polymeric films, which physicochemical characteristics were strongly dependent
on the kind of alkyl chain present on the fluorene bridge. In particular, the
electropolymerization of the polyfluorinated monomer allowed the fabrication of thin
films with good electrical conductivity, reversible electrochemical processes,
electrochromic properties and enhanced water repellency with respect to its non-
fluorinated analog.
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Keywords: Polymers, Electrodeposition, Optoelectronic materials, Electrochromism
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INTRODUCTION
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Considerable research efforts are currently devoted to the development of organic
macromolecules and polymeric compounds for application in the production of electronic
and optoelectronic devices.1–4 Actually, a wealth of multifunctional materials with
tailored physical characteristics can be obtained from a relatively restricted pool of
unsaturated building blocks, thanks to ingenious organic synthesis techniques.5,6 It is thus
possible to create organic conducting materials with adequate optical and electronic
properties, which utilization can make a great contribution to the advancement of
optoelectronic devices such as electrochromic windows,7,8 solar cells,9–12 and lighting
systems.13 Inexpensive fluorene and carbazole derivatives are fit to this purpose. Indeed,
conjugated materials featuring carbazole units either in the π-conjugated backbone or as
pendant groups display excellent thermal and photochemical stabilities and possess high
charge carrier mobility, making them attractive materials for a variety of applications.14–
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In an analogous way, fluorene and 9,9’-spirobifluorene derivatives showing high
photoluminescence (PL) quantum yields and electroluminescence (EL) efficiencies, as
well as high thermal stabilities, have drawn much attention.3,11,17–22 Finally, fluorene and
carbazole units can be combined in the same molecular structure leading to a variety of
compounds with interesting hole transporting and electroluminescence properties.23–27
The materials used in the fabrication of optoelectronic devices must provide
certain essential characteristics, such as: charge transport, strong interaction with light
(absorption and/or emission), chemical stability that avoids material degradation when
the device is operating, and a right relative position in the energy scale of the valence
bands (VB)-conduction band (CB) regarding the devices contacts work function, which
ensures the flow of charges in the correct directions. However, for a material to be useful,
it must be apt to the formation and deposition of thin films (thicknesses in the order of
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nanometers), which is a central aspect in the construction of optoelectronic devices. In
this context, the electrodeposition of electroactive compounds is an attractive and
promising technique for the production of organic material films, since it has the
advantage to produce the film in one-step at room temperature, which requires that the
monomer is soluble, polymerizable, and that the solubility of the generated polymer is
low enough to induce its precipitation on the working electrode. It also has other
advantages, such as permitting a fine control of the film thickness, variations of its
morphology (e.g. porosity or roughness), an excellent adhesion of the electrodeposited
film to the electrode, and the formation of patterns on the film.28 In addition, the
electropolymerization approach offers unique opportunities to tune the wettability
properties of the obtained film by just using judicious chemical modifications of the
monomer, although this strategy has been so far applied to a restricted number of
monomer cores, such as thiophene and pyrrole derivatives.29–40 All these features are
important parameters in the manufacture of optoelectronic devices.
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In this context, the development of fluorene/carbazole hybrid monomers that can
give rise to electrodeposited coatings is particularly appealing.41–45 In fact, carbazole and
N-substituted carbazoles undergo electrochemical oxidative coupling under relatively
low oxidation potential to give nitrogen- and ring-coupled dimers, respectively.46 These
characteristics enable the deposition of polymeric films over conductive substrates by
electrochemical oxidation of monomers bearing at least two carbazole substituents.47–49
Thus, electropolymerizable fluorene-based monomers were designed in which the sp3
carbon atom of fluorene (9-position) is connected to the N atoms of two electroactive
carbazole units through flexible alkyl linkers.44,45 In this case, efficient polymerization
processes could be observed only for monomers containing at least two functionalized
fluorene units.44 The use of simple D-π-D conjugated monomers where two carbazoles
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donor groups D are directly linked to the phenyl rings of the π-bridging fluorene
represents another viable option,43 as also shown by early examples of
electropolymerizable bipolar 9,9’-spirobifluorene derivatives.41,42 Besides ease of
synthesis, this approach offers the opportunity to introduce tailored substituents at the 9-
position of fluorene, in particular long alkyl- or polyfluoroalkyl chains, which could be
used to facilitate the electropolymerization process and to tune the final properties of the
electrodeposited material, as previously shown in the case of simple thiophene or pyrrole
derivatives.29-40
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It is well known that the introduction of highly fluorinated moieties in polymeric
structures induces valuable features such as improved thermal stability, chemical and
oxidative resistance, low dielectric constants and low refractive index, and oil and water
repellency.50 This last property is crucial for the long-term durability of optoelectronic
devices, which is seriously challenged by the deleterious effects of moisture on the
charged organic materials produced when the devices are operating.51 The optoelectronic
characteristics of these fluorinated polymers are also very interesting. As an example, the
replacement of hydrogen atoms with fluorine results in lower LUMO and HOMO energy
states, producing enhanced EL efficiency, while the lower HOMO energies are
responsible for increased oxidation potentials and enhanced stability against photo-
oxidation.52 A limited number of conjugated poly- and oligofluorenes bearing poly- or
perfluoroalkyl chains at the 9-position have been thus studied, focusing on their PL, EL
and charge transport properties.53–58 The surface properties of these compounds, obtained
by transition metal-catalyzed cross-coupling reactions preceded by cumbersome
multistep syntheses of properly functionalized monomers, were not investigated. It should
be also noted that the attempted electropolymerization of fluorene derivatives bearing a
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single semifluorinated chain at the 9-position failed to give the corresponding
polyfluorenes.56
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Based on these premises, we here report a facile electrochemical polymerization
approach to the fabrication of conductive films starting from simple D-π-D conjugated
carbazole-fluorene-carbazole (CFC) monomers bearing at the 9-position either alkyl- or
polyfluoroalkyl chains of similar length (Figure 1). It will be shown that the nature of
these substituents significantly improves the film formation and affects the morphology,
water repellency and electrochromic behavior of the resulting materials.
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CFC-H1
CFC-F1
R = n-C8H17
R = n-C8F17
N
N
R
R
Figure 1. Monomers structure.
EXPERIMENTAL SECTION
Synthesis procedures.
General remarks. All available reagents were purchased from commercial sources
and were used without any further purification. Solvents were purified by standard
methods and dried if necessary. 2,7-Diiodo-9H-fluorene 1,59 and 9,9-bis(1H,1H,2H,2H-
perfluorodecyl)-9H-fluorene 357 were prepared as described in the literature. Reactions
were monitored by thin layer chromatography (TLC) that was conducted on plates
precoated with silica gel Si 60-F254 (Merck, Germany). Column chromatography was
carried out on silica gel SI 60 (Merck, Germany), mesh size 0.063 – 0.200 mm
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(gravimetric) or 0.040 – 0.063 mm (flash). H NMR and C NMR were recorded on a
Bruker Avance 400 spectrometer (400 and 100.6 MHz, respectively); 19F NMR spectra
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were recorded on a Bruker AC 300 spectrometer (282 MHz). Elemental analyses were
carried out by the Departmental Service of Microanalysis (University of Milano).
9,9-Didecyl-2,7-diiodo-9H-fluorene (2). Powder KOH (1.00 g, 17.9 mmol) was
added to a stirred solution of 2,7-diiodo-9H-fluorene 1 (1.67 g, 4.0 mmol) in DMSO (15
mL) under nitrogen atmosphere. After 5 min, KI (0.12 g, 0.7 mmol) was added, followed
by 1-bromodecane (2.65 g, 12.0 mmol. The reaction mixture was stirred under nitrogen
for 15 h at RT, then it was poured into H2O (100 mL). The aqueous phase was extracted
with Et2O (3 x 30 mL). The combined organic phases were dried over MgSO4 and
evaporated under reduced pressure. The residue was purified by column chromatography
(silica gel, petroleum ether) to give the bis alkylated fluorene 2 as a pale yellow oil (1.90
g, 68%.). 1H NMR (400 MHz, CDCl3) δ 7.67 – 7.60 (m, 4H), 7.40 (d, J = 8.4 Hz, 2H),
1.91-1.86 (m, 4H), 1.29-1.05 (m, 28H), 0.86 (t, J = 6.6 Hz, 6H), 0.63-0.52 (m, 4H); 13C
NMR (101 MHz, CDCl3) δ 152.51, 139.77, 136.03, 132.03, 121.52, 93.16, 55.56, 40.10,
31.91, 29.88, 29.55 (2CH2), 29.32, 29.21, 23.65, 22.71, 14.17. Anal. calcd for C33H48I2:
C, 56.74; H, 6.93; found: C, 56.81; H, 7.03.
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9,9-Bis(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluorodecyl)-2,7-diiodo-
9H-fluorene (4). A 25 mL round-bottom flask equipped with a magnetic stir bar and a
reflux condenser was charged with fluorene derivative 3 (1.06 g, 1.0 mmol), iodine (0.19
g, 0.75 mmol), iodic acid (0.09 g, 0.5 mmol), water (0.3 mL), acetic acid (7 mL) and 98%
H2SO4 (0.06 mL). The mixture was heated under reflux for 4h under vigorous stirring,
then it was cooled to RT. The white precipitate formed was filtered off, washed with water
and dried under vacuum. It was then dissolved in the minimal amount of CH2Cl2 and
reprecipitated by adding dry ethanol. The crude compound was further purified by column
chromatography (silica gel, hexane) to give the title compound as a white solid (1.02 g,
78% yield). mp 117–118 °C; 1H NMR (400 MHz, CDCl3) δ 7.77 (d, J = 8.0 Hz, 2H), 7.69
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(s, 2H), 7.47 (d, J = 8.0 Hz, 2H), 2.41 – 2.17 (m, 4H), 1.36 – 1.18 (m, 4H); 13C NMR (101
MHz, CDCl3) δ 148.01, 139.76, 137.12, 131.73, 122.05, 120.57 – 107.19 (m, C8F17),
93.52, 54.93, 30.15, 26.14 (t, JCF = 22 Hz); 19F NMR (377 MHz, CDCl3) δ -81.73 (t, J =
9.6 Hz, 6F), -115.36 (s, 4F), -122.60 – -123.10 (m, 12F), -123.71 (s, 4F), -124.76 (s, 4F),
-127.10 (s, 4F). Anal. calcd for C33H14F34I2: C, 30.25; H, 1.08; found: C, 30.13; H, 1.12.
9,9'-(9,9-didecyl-9H-fluorene-2,7-diyl)bis(9H-carbazole) (CFC-H1) A flame-
dried Schlenk tube equipped with a stir bar was charged with fluorene 2 (1.40 g, 2.0
mmol), carbazole (1.00 g, 6.0 mmol), dry K2CO3 (1.24 g, 9.0 mmol), copper bronze (0.77
g, 12.0 mmol), 18-crown-6 (0.16 g, 0.6 mmol) and dry 1,2-dichlorobenzene (15 mL)
under nitrogen atmosphere. The reaction mixture was deaerated by freeze-pump-thaw
cycles (3 times), purged with nitrogen and heated at 180 °C under stirring for 48h. The
mixture was cooled to RT, diluted with DCM (30 mL) and filtered through a short pad of
Celite. The organic solvents were removed by evaporation under vacuum and the residue
was purified by flash column chromatography (silica gel, petroleum ether/DCM 10/1).
The title compound was obtained as an off-white glassy solid (1.46 g, 94% yield). 1H
NMR (400 MHz, CDCl3) δ 8.19 (d, J = 7.8 Hz, 4H), 7.99 (d, J = 7.8 Hz, 2H), 7.65 – 7.57
(m, 4H), 7.51 – 7.42 (m, 8H), 7.33 (ddd, J = 7.9, 6.5, 1.7 Hz, 4H), 2.08 – 1.99 (m, 4H),
1.36 – 1.05 (m, 28H), 0.91– 0.96 (m, 4H), 0.83 (t, J = 6.9 Hz, 6H); 13C NMR (101 MHz,
CDCl3) δ 152.89, 140.99, 139.55, 136.70, 125.96 (2 CH), 123.41, 121.86, 121.03, 120.42,
119.95, 109.77, 55.72, 40.23, 31.88, 30.03, 29.61, 29.56, 29.42, 29.28, 24.18, 22.65,
14.11. Anal. calcd for C57H64N2: C, 88.09; H, 8.30; N, 3..60; found: C, 88.15; H, 8.46; N,
3.49.
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9,9'-(9,9-bis(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluorodecyl)-9H-
fluorene-2,7-diyl)bis(9H-carbazole) (CFC-F1). A flame-dried Schlenk tube equipped
with a magnetic stir bar was charged with fluorene 4 (1.31 g, 1.0 mmol), carbazole
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(0.50 g, 3.0 mmol), dry K2CO3 (1.07 g, 5.0 mmol), copper bronze (0.38 g, 6.0 mmol), 18-
crown-6 (79 mg, 0.3 mmol) and dry 1,2-dichlorobenzene (10 mL) under nitrogen
atmosphere. The reaction mixture was deaerated by freeze-pump-thaw cycles (3 times),
purged with nitrogen and heated at 180 °C under stirring for 60h. The mixture was cooled
to RT, diluted with DCM (30 mL) and filtered through a short pad of Celite. The organic
solvents were removed by evaporation under vacuum and the crude product was purified
by flash column chromatography (silica gel, petroleum ether/DCM 6/1). The fractions
containing the products were collected, evaporated to dryness and the residue was
recrystallized from hexane to give the title compound as a white solid (0.87 g, 63%
yield). mp 89–90 °C; 1H NMR (400 MHz, CD2Cl2) δ 8.20 (d, J = 7.8 Hz, 4H), 8.11 (d, J
= 8.0 Hz, 2H), 7.76 (dd, J = 8.0, 1.8 Hz, 2H), 7.72 (d, J = 1.7 Hz, 2H), 7.48 – 7.43 (m,
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8H), 7.37 – 7.30 (m, 4H), 2.57 – 2.36 (m, 4H), 1.77 – 1.54 (m, 4H). C NMR (101 MHz,
CD2Cl2) δ 148.98, 141.37, 140.14, 138.45, 128.17, 126.64, 124.07, 122.71, 122.01,
120.94, 120.82, 110.11, 120.91 – 107.51 (m, C8F17), 55.86, 30.85, 26.80 (d, JCF = 22 Hz);
19F NMR (377 MHz, CDCl3) δ -81.71 (t, J = 9.6 Hz, 6F), -115.42 (s, 4F), -122.60 – -
123.04 (m, 12F), -123.69 (s, 4F), -124.60 (s, 4F), -127.08 (s, 4F). Anal. calcd for
C57H30F34N2: C, 49.29; H, 2.18, N, 2.02; found: C, 49.18; H, 2.22, N, 1.96.
Optical and electrochemical characterization of monomers and polymers films.
For the optical characterization a diode array HP 8452 spectrometer and a Spex
FluoroMax fluorometer were used to measure the absorption and fluorescence spectra,
respectively. Spectra in 1,2-dichloroethane (DCE) solution were acquired at room
temperature using 1 cm path length quartz cells.
The voltammetric characterization of the redox processes and
electropolymerization of the monomers was performed with a CH Instrument 700 E
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pontentiostat using a silver wire quasi-reference electrode and a loop Pt as counter
electrode in a conventional three-electrode cell. Pt disk, (2 mm in diameter) polished on
a felt pad with 0.3 m alumina and sequentially sonicated in water and absolute ethanol,
was used as working electrode for the voltammetric characterization of the redox
processes and indium tin oxide (ITO) electrodes (Delta Technologies) were used for the
electropolymerization.
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Electrochemical studies of monomers were carried out in DCE with 0.10 M tetra-
n-butylammonium hexafluorophosphate (TBAPF6) as supporting electrolyte. All the
electrochemical responses of the electropolymerized films were obtained in DCE solution
with 0.10 M TBAPF6 as supporting electrolyte. A silver wire quasi-reference electrode
was calibrated using ferrocenium/ferrocene (Fc+/Fc) couple with potential Fc+/Fc taken
as 0.4 V vs. saturated calomel electrode (SCE).60
Spectroelectrochemical experiments were carried out in a homemade cell built
from a commercial UV–visible cuvette. The ITO-coated glass was used as working
electrode; a Pt wire was used as counter electrode, and an Ag wire was used as the
reference electrode. These experiments were carried out in DCE with 0.10 M TBAPF6
supporting electrolyte. The cell was placed in the optical path of the sample light beam.
The background correction was obtained by taking an UV–vis spectrum of a blank cell
(an electrochemical cell with an ITO working electrode without the polymer film) with
conditions and parameters identical to those of the polymer experiment.
SEM micrographs were collected using Carl Zeiss EVO MA 10 SEM, with
electron beam energy of 18 KeV.
AFM characterization of the polymer films on ITO electrodes was performed
using an Agilent 5500 SPM microscope (Agilent Technologies, Inc.) working in Acoustic
AC Mode. Commercial silicon cantilever probes, with aluminum backside coating and
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nominal tip radius of 10 nm (MikroMasch, NSC15/Al BS/15, spring constant ranging 20–
75 N/m), were employed just under their fundamental resonance frequencies of about 325
kHz. The experiments were performed in stationary dry-air atmosphere. Images were
treated and analyzed using Gwyddion, an open source software for the visualization and
analysis of scanning probe microscopy data.
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Contact angle measurements were performed using an optical contact angle meter
(Attension Theta Optical Tensiometer) at room temperature and humidity. A milli Q
water droplet (droplet size: 5 l) was placed with a Hamilton syringe (25 l) on the
surface of the electrodeposited films. The contact angles reported values are the averages
of at least three consecutive measurements (3 water droplets). Samples for contact angle
measurements were prepared using the same procedure as those used for
spectroelectrochemistry onto an ITO and they were washed with solvent to eliminate the
electrolyte that could remain on the electrode.
RESULTS AND DISCUSSION
Synthesis of CFC monomers.
The polyfluorinated monomer CFC-F1 and its fully hydrogenated analog CFC-
H1 were synthesized starting from 2,7-diodofluorene 1 and 9,9-bis(1H,1H,2H,2H-
perfluorodecyl)-9H-fluorene 3, respectively, according to the straightforward two steps
sequences outlined in Scheme 1.
The introduction of two n-decyl chains at the 9-
position of 1 to give the CFC-H1 precursor 2 was easily achieved by nucleophilic
substitution reaction with 1-bromodecane in the presence of a substoichiometric amount
of KI. An excess of KOH was used to generate the nucleophilic 9-fluorenyl anions. A
slightly different route had to be employed in order to get good yields of the desired
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polyfluorinated intermediate 4. In that case, the fluorene derivative 3 was first
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synthesized, followed by iodination of the 2,7-positions of the fluorene scaffold.
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N
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H
n-C10H21Br
KOH/KI
I
I
I
I
CFC-H1
CFC-F1
Cu/K2CO3/18-C-6
C10H21 C10H21
1
2
N
H
I2/HIO3
I
I
Cu/K2CO3/18-C-6
AcOH/H2SO4
C8F17
C8F17
C8F17
C8F17
3
4
Scheme 1. Synthesis of CFC monomers.
Indeed, attempts at introducing two C8F17(CH2)2- chains at the 9-position of 1
were frustrated by the massive occurrence of side reactions favored by the strong electron
withdrawing character of the -C8F17 group.57 Bis N-arylation of intermediates 2 and 3
with carbazole was then performed under mild Ullmann-type conditions, affording CFC-
H1 and CFC-F1, respectively, in excellent yields.
Optical Properties of CFC Monomers:
The electronic transitions of CFC-H1 and CFC-F1 were investigated by
measuring their absorption and emission spectra in diluted DCE solutions. The obtained
results are shown in Figure 2 and the main photophysical parameters are summarized in
Table I. A comparison of both spectra shows almost identical optical transition features,
which indicates that the insertion of fluorinated alkyl chains at the 9 position of fluorene
does not affect the conjugated core of the chromophore system. CFC monomers exhibit
two clear absorptions bands at ~290 nm and ~340 nm. The light absorption at 290 nm is
due to π–π* electron transition of carbazole moiety, while the lower energy transition is
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assigned to electron transfer from nitrogen lone pairs to the π*orbital of fluorene.20,24,43,61–
64 As reported in Table I, the π–π* electron transition occurs at the same wavelength for
both dyes, as the torsion that allows the -bonding between the carbazole and fluorene
moieties minimizes the extension of the conjugation length between the two aromatic
centers. On the other hand, the band assigned to the electron transfer processes between
carbazole and fluorene is 4 nm shifted towards lower energy for CFC-F1 with respect to
CFC-H1, which indicates the existence of a residual inductive effect exerted by the
polyfluorinated alkyl chains.
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a
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b
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 (nm)
Figure 2. Absorption (solid line) and emission (dot line) spectra of CFC-H1 (red) and
CFC-F1 (blue), recorded in DCE solution. The emissions were taken with excitation at
338 nm for CFC-H1 and 342 nm for CFC-F1.
The photoluminescence properties of CFC-H1 and CFC-F1 were studied in DCE
solution by exciting the monomers at the wavelength maximum of the lower energy light
absorption band. As shown in the dyes emission spectra reported in Figure 2, CFC-H1
and CFC-F1 emit light at 363 and 370 nm with quantum yields 0.77 and 0.69,
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respectively. On the other hand, it was observed that the CFCs dyes fluorescence
excitation spectra at different excitation energies were superimposable to their absorption
spectra and in addition, a unique fluorescence emission spectrum was obtained
irrespective of the excitation wavelength. These results support the absence of light
emitting impurities in the CFCs dyes solution.
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Table 1. Photophysical and electrochemical parameters of the compounds.
Absorption
Emission
Oxidation
Potentials
Energy Levelsf
푂푛푠푒푡
c
휆푀표푛표
휆푀표푛표
퐸푀표푛표 퐸퐹푖푙푚
퐸푔
Compd
HOMO LUMO
휆
휙퐸푚
퐹푖푙푚
(nm)b
(nm)a
(nm)a
(V)d
(V)e
(eV)
(eV)
(eV)
CFC-H1
292
338
397
363
0.77
0.69
0.81
0.96
1.06
0.43
-5.53
-2.41 3.12
CFC-F1
292
342
404
370
0.84
0.95
1.02
0.44
-5.54
-2.47 3.07
aSpectra measured in DCE. The emission spectra were measured by excitation at the
maximum wavelength of the lower energy absorption bands. bLow energy optical onset
of the electrodeposited films, as determined from the intersection between the
projection of the corresponding spectrum in the reduced state and the base line.
cEmission quantum yield obtained in DCE. Standard used were 9,10-
diphenylanthracene.
(9,10-diphenylanthracene) = 1 in cyclohexane.65 dPeak
휙퐹퐿
potential. eOnset potentials of the electrodeposited films determined from intersection
between the baseline and the current signal. The potentials values are expressed in volt
vs. Fc/Fc+. fCalculated HOMO and LUMO energies of the films in the absolute scale
(eV) as obtained using the following equations:
,
퐻푂푀푂 = ― (퐸퐹푖푙푚 +5.1) 퐿푈푀푂 =
푂푛푠푒푡
60
and
.
퐸푔 = 1240/휆퐹푖푙푚
― (퐸푔 ― 퐻푂푀푂)
Electrochemical Properties of CFC Monomers: Electropolymerization and Film
Formation
Cyclic voltammetry experiments were carried out to obtain information about the
electrochemical processes of the monomers and their capability to form electropolymers.
The results obtained are shown in Figure 3. It is interesting to note that CFC molecular
structures are similar, the only difference being the nature of the saturated chains (C10H21-
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or C8F17(CH2)2-) in the 9-position of the fluorene moiety, (Figure 1). Since the fluorinated
chain in CFC-F1 is linked to the fluorene conjugated center of the system by a non-
conjugated bridge, therefore, a small influence by inductive effect on the electrochemical
behavior of the dye was expected, similarly to what was observed in photophysical
studies. However, a general inspection of the CFCs cyclic voltamperograms shapes
presented in Figures 3aI and 3bI shows remarkable differences in the behavior of the
two monomers, which indicates that the introduction of fluorinated chains notably affects
the CFCs redox properties. The cyclic voltammograms of CFCs dyes measured on
freshly polished Pt electrode show three oxidation processes in the first scan to positive
potentials (red line) around 0.85 V, 0.95 V and 1.00 V. Where, in the case of CFC-H1,
the detected first and second oxidation processes are manifested as a shoulder of a wave
with a maximum at 1.06 V (see arrows in Figure 3aI), while for CFC-F1, those redox
processes are not so well defined as in the case of CFC-H1 and they overlap in a wave
with a maximum at 0.88 V (see arrows in Figure 3bI). Then, when the potential scan
direction is inverted, complementary reduction waves to the previous oxidation processes
are detected: at 0.78 V and 0.91 V for CFC-H1, at 0.81 V for CFC-F1), and another at
0.54 V for both CFCs dyes (see Figures 3aI and 3bI). During the second cycle a new
oxidation peak at 0.62 V is identified regarding the first potential swept, which is
complementary to the reduction process observed at 0.54 V in the first cathodic sweep
(see black line in Figures 3aI and 3bI). Finally, when the electrode potential is
continually cycled (see the black line in Figures 3aI and 3bI) in both molecular systems
all the voltamperogram current waves show a progressive growth, but with a different
rate. Clearly, the CFC-F1 current growth rate is bigger than CFC-H1. This behavior can
be ascribed to the development of a conductive and electroactive film on the working
electrode surface. In concordance, when the electrodes were removed from the
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electrochemical cell, washed with fresh solvent to eliminate any rest of monomer, and
then transferred to a monomer-free supporting electrolyte solution, electrochemical
signals characteristic of redox active modified electrodes were observed (Error!
Reference source not found.aII and 3bII).
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a I)
a II)
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Figures 3b II
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+
+
E (Volts) vs. Fc/Fc
E (Volts) vs. Fc/Fc
Figure 3. First (red) and 2nd-10th (black) cyclic voltammograms of CFC-H1 (aI) and
CFC-F1 (bI). Electrochemical response of the CFC-H1 (aII) and CFC-F1 (bII)
electrodeposited films. Conditions: Pt working electrode, electrolyte TBAPF6 0.1 M
solution in DCE, v=0.1 V/s.
The cyclic voltammograms of the CFC-H1 and CFC-F1 electrodeposited films
show two oxidation waves with peak potentials at 0.57 V / 0.93 V and 0.62 V / 0.85 V,
respectively. However, as previously pointed out, CFC-F1 is much more efficient than
CFC-H1 in terms of film electrodeposition rate. This is clearly shown in Figure 3, where
applying ten potential scan cycles in equal experimental conditions, CFC-F1 presents a
bigger current increment between successive voltamperometric cycles producing four
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times more film oxidation current than CFC-H1, which can be associated with a greater
amount of electrodeposited material on the electrode surface.
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For both CFC monomers, the first oxidation signal observed in the first
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potential scan (see the red line and inserted arrows in Figure 3aI and 3bI) can be assigned
to the oxidation of the fluorene ring; while the second and third electrochemical signals
are the oxidation of each carbazole units. The oxidation of the second carbazole moiety
occurs at a more anodic potential, because a higher coulombic attraction must be
overcome in order to remove a third electron from the dicationic charged species formed
in the previous oxidation steps.20 The reactive 2,7 positions of the fluorene core in the
CFCs dyes are blocked by two nitrogen atoms, which stabilize the cation radical. In fact,
when the potential is swept until the beginning of the first oxidation wave, where the
carbazole units are not oxidized, the current growth with the number of potential sweep
cycles is not observed. In addition, it is observed that the CFC-F1 first oxidation process
is 30 mV shifted to more anodic potentials with respect to CFC-H1 (See Table I)
evidencing a fluorinated chain effect on the fluorene ring. On the other hand, as it is shown
in Figures 3aI and 3bI, when the scan potential reaches the second oxidation process, the
typical electrodeposition process stimulated by coupling of two carbazole cation radicals
starts.41,42,48,49 Indeed, the oxidation of one carbazole center leads to the formation of a
radical cation that undergoes a dimerization reaction to produce less soluble dicarbazole,
which precipitates on the electrode surface forming a polymeric film.46,47
It is known that the carbazole dimer structural motif allows the extension of the
conjugation length compared to the single carbazole unit, which makes the dimer obtained
in the first step of the electropolymerization process more easily oxidizable than the
starting monomer.24,41,42,46,48,49 Thus, the carbazole dimer produces new voltamperometric
signals in the second potential scan cycle around 0.62 V and 0.90 V corresponding to its
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radical cation and dication, respectively (See Figure 3). Consequently, the oxidation
wave of the dimer dication overlaps with the ones corresponding to the starting CFC
monomers, masking their electrochemical signals. For CFC-F1, which shows a higher
electrodeposition rate, this effect is manifest, producing changes in the current magnitude
of the monomer oxidation peaks, and therefore, changes in the voltammograms shape in
the first potential scan regarding CFC-H1 (see red line in Figure 3aI and 3bI).
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The CFC films showed high stability when they were subjected to several
oxidation-reduction cycles, and their redox behavior is comparable to that observed with
other carbazole derivatives.41,42,48 However, the comparative analysis of the
electrochemical data clearly demonstrates the unexpected role played by the pendant
polyfluoroalkyl chains, which improves enormously the film electro-formation in case of
CFC-F1. This effect cannot be ascribed to any electronic effect, but rather to the well-
known deviation of the solubility properties of compounds featuring polyfluoroalkyl
chains from those of their non-fluorinated analogs.66,67 In general, the higher the number
of polyfluoroalkyl substituents, the lower the affinity of the compound for standard
organic solvents. So, the presence of four C8F17(CH2)2- chains in the dimeric form of
CFC-F1 produced in the first scan favors the separation of this product from the organic
solvent and the precipitation on the electrode, which is not the case for the fully soluble
dimeric form of CFC-H1. The same happens for the oligomeric forms of CFC-F1
generated in the subsequent scans, with the result of an increased rate of formation of the
polymeric film on the electrode. Rigidity of the terminal C8F17 groups might also play a
role, by enhancing the microstructural ordering of the fluorinated polymer.68
Morphological and Surface Properties of Electrodeposited Films
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The morphology characteristics of the film surface are a significant parameter
associated to the possible use of organic materials as active layers in optoelectronic and
electronic devices.25,67 Figure 4 shows SEM images of the surfaces of CFC-H1 and
CFC-F1 electropolymer films obtained under the same experimental conditions used in
the electrochemical studies described above. Despite the identical deposition method
used, the different surface characteristics of the two films are blatant.
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Figure 4. From up to down, scanning electron and atomic force microscopy images and
digital camera photographs of a water droplet on the film surface obtained by applied 10
growth cycles. Left CFC-H1 and right CFC-F1.
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The CFC-H1 film is uneven and it does not cover the ITO electrode surface
showing unfilled spaces. On the contrary, the CFC-F1 polymeric structure shows a
granulate aspect and homogeneously full coats the outside of the ITO electrode. The
morphology of the CFC-F1 film surface shows that its formation goes through a
nucleation and growth process that ends up in the formation of a continuous film with
globular-shaped features. Thus, the observed films surfaces differences between the two
modified electrodes are in agreement with the highest CFC-F1 electrodeposition rate
regarding CFC-H1, which allows to electrodeposit, in homogenous way, a greater
amount of material on the electrode surface. Moreover, as seen in Figure 4, the aspect of
the CFCs electrodes surfaces morphologies images obtained by AFM corroborates their
surface characteristic found in the SEM study. The AFM imagens exhibit that CFC-H1
modified ITO electrode has a non-uniform and crannied surface, while CFC-F1 shows
smooth surface formed by globular-shaped mosaics. From the AFM data were calculated
the films thickness of 196 nm for CFC-F1 and 72 nm for CFC-H1, which also is in
agreement with the better film formation capability of CFC-F1 molecular system.
The creation of active layers endowed with intrinsic water repellency, which
relieves the negative effects of moisture on the charged species produced under
exercise conditions,51 has been recognized as an effective strategy for the
enhancement of the long-term durability of optoelectronic devices.69 The static
contact angle formed by a water drop standing on the film surface provides a direct
estimate of the hydrophilic/hydrophobic properties of the surface, with water
repellency increasing with the measured contact angle.70 Thus, a surface is
commonly defined as hydrophilic if the value of the contact angle is less than 90º,
as hydrophobic if the value of the contact angle is greater than 90º.71 Comparison
of the water contact angle values for the two CFC electropolymer films (Figure 4
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and Table 1) clearly shows that the introduction of polyfluoroalkyl chain on the
fluorene ring leads to a dramatic increase, from 82° for CFC-H1 to 127° for CFC-
F1. The contact angle measured in the case of the CFC-H1 film is surprisingly low
for an organic material that holds long hydrophobic alkyl chains in its chemical
composition. This is due to an incomplete coverage of the ITO electrode, as
evidenced in Figure 4. Thus, the electrodeposited CFC-H1 film does not
efficiently shelter the substrate and the observed water contact angle is closer to
that of the clean ITO surface, which corresponds to 75°. On the other hand, the
superficial hydrophobic properties of the defect-free CFC-F1 film are typical of a
polymeric material characterized by the presence of polyfluoroalkyl residues.50,72,73
These results show that the introduction of polyfluoroalkyl side chains in
fluorene/carbazole hybrid monomers allows to form by electrochemical techniques
thin film materials with good morphological and water repellency characteristics,
which are highly desirable for the fabrication of optoelectronic devices.
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Spectrolectrochemical Properties of Electrodeposited Films.
To get information about the optoelectronic proprieties of the CFC
electrodeposited films, spectroelectrochemical experiments were carried out on polymer
films electrochemically generated over ITO electrodes applying the same experimental
conditions used in the electrochemical studies described above. Nevertheless, for these
spectroelectrochemical experiments the films growth was adjusted by the applied number
of potential cycles in order to optimize the film light absorption. The absorption spectra
of the CFC films at different applied potentials are shown Figure 5. The potential applied
to each spectrum corresponds to the one identified with a dot of the same color in the
corresponding films voltamperograms showed in Figure 3aII and 3bII. Both films
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present three redox states being these neutral (partly transparent in the visible region of
the electromagnetic spectrum), semioxidized (yellow), and fully oxidized (green). The
pictures inserted in Figure 5 showed the CFC polymer films colorations at different
oxidation states.
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0.35 V
a)
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400
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900 1000
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900 1000
 (nm)
 (nm)
Figure 5. Spectroelectrochemical measurements of a) CFC-H1 film (20 growth cycles)
and b) CFC-F1 film (5 growth cycles), at different applied potentials. The potentials
applied to obtain the different traces were the same for both monomers. The inset
photographs show the electrodes coloration at different redox states and the color frame
indicates the corresponding spectrum.
The electro-optical properties of the CFC film have origin mainly in the carbazole
dimer. The light absorption onset of the CFC-H1 and CFC-F1 films in their neutral state
(light absorption spectra in black in Figure 5) is located at ~404 nm, as determined by
the intersection between the projection of the light absorption band tail and the baseline.
This UV-band is due to the π–π* transition in the conjugated system and it has a maximum
in the UV-region at ~340 nm close to the ITO optical cut and therefore, it is not shown
in Figure 5. However, both films in their neutral state shown a light absorption in the
zone of 430 nm, which can be assigned to the bicarbazole radicals cation. This would
indicate that after to the electrochemical films formation, some positively charged traps
remain. When the applied potential to the modified electrode reaches higher values than
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the films oxidation onset (See Table I), two new electronic transitions are observed, one
with peak maximum at 422 nm and another broader at ~950 nm (see the green, blue and
magenta traces). In addition, when the potential advances through the second oxidation
process of CFC films (see cyan and red dots and traces in Figure 3 and Figure 5,
respectively) the higher energy absorption band starts to decrease in absorbance intensity;
while a new broad band at 800 nm is developed. This behavior is typical of
electrodeposited carbazole derivative materials and these bands can be assigned to the
formation of bicarbazole radicals cation and dications by oxidation of the polymeric
film.23,48,61
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SUMMARY AND CONCLUSIONS
We have developed two easily accessible electropolymerizable molecules that
feature fluorene, bearing either alkyl or polyfluoroalkyl chains in the 9-position, as a
bridge between two carbazole moieties. Adequate conditions for the growth of conductive
polymers over platinum electrodes and over semitransparent ITO have been established.
The proposed electropolymerization mechanism involves the reaction between two
carbazole radical cations to form dicarbazole linkages between fluorene units. The
morphology characteristics and the rate of deposition of the electropolymerized films
generated in this process are heavily dependent on the nature of the pendant saturated
chains on the fluorene bridge. In the case of polyfluoroalkyl chains (monomer CFC-F1)
the film is homogeneous and very stable, with good adherence to the electrode surface.
The nature of the pendant chains has also a deep impact on the water affinity of the films,
which exhibit clear hydrophobic characteristics in the case of CFC-F1, as proved by static
contact angle measurements. In addition, the materials obtained show electrochromic
properties, with the coloration of the films switching from uncolored to green upon
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oxidation due to the applied potential. These findings open perspectives for application
in the field of damp- or waterproofing optical coatings and the manufacturing of
intelligent windows.
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ACKNOWLEDGMENTS
We thank the CNR/Regione Lombardia project “Integrated Zero Emission Buildings”,
Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET-Argentina),
Agencia Nacional de Promoción Cientıfica y Tecnológica (ANPCYT Argentina),
Universidad Nacional de Río Cuarto. M.V.C., G.M, M.Z, L.F, and F.F. are scientific
members of CONICET.
AUTHOR INFORMATION
Corresponding Authors
* ffungo@exa.unrc.edu.ar; gianluca.pozzi@istm.cnr.it
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