Functionalization of boron-doped diamond with a push-pull chromophore via Sonogashira and CuAAC chemistry

Article abstract of DOI:10.1039/C8RA07545J

Improving the performance of p-type photoelectrodes represents a key challenge toward significant advancement in the field of tandem dye-sensitized solar cells. Herein, we demonstrate the application of boron-doped nanocrystalline diamond (B:NCD) thin films, covalently functionalized with a dithienopyrrole-benzothiadiazole push-pull chromophore, as alternative photocathodes. First, a primary functional handle is introduced on H-terminated diamond via electrochemical diazonium grafting. Afterwards, Sonogashira cross-coupling and Cu(i) catalyzed azide-alkyne cycloaddition (CuAAC) reactions are employed to attach the chromophore, enabling the comparison of the degree of surface functionalization and the importance of the employed linker at the diamond-dye interface. X-ray photoelectron spectroscopy shows that surface functionalization via CuAAC results in a slightly higher chromophore coverage compared to the Sonogashira cross-coupling. However, photocurrents and photovoltages, obtained by photoelectrochemical and Kelvin probe measurements, are approximately three times larger on photocathodes functionalized via Sonogashira cross-coupling. Surface functionalization via Sonogashira cross-coupling is thus considered the preferential method for the development of diamond-based hybrid photovoltaics.

Full text of DOI:10.1039/C8RA07545J

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Functionalization of boron-doped diamond with
a push–pull chromophore via Sonogashira and
Cite this: RSC Adv., 2018, 8, 33276
CuAAC chemistry†
a
Jorne Raymakers,
Hana Krysova,b Anna Artemenko,c Jan Cˇermak,c
´
Shannon S. Nicley,a Pieter Verstappen,a Sam Gielen,a Alexander Kromka,c
a
a
cd
Ladislav Kavan,b Wouter Maes
and Bohuslav Rezek
* *
Ken Haenen,
Improving the performance of p-type photoelectrodes represents a key challenge toward significant
advancement in the field of tandem dye-sensitized solar cells. Herein, we demonstrate the application of
boron-doped nanocrystalline diamond (B:NCD) thin films, covalently functionalized with
a
dithienopyrrole–benzothiadiazole push–pull chromophore, as alternative photocathodes. First,
a primary functional handle is introduced on H-terminated diamond via electrochemical diazonium
grafting. Afterwards, Sonogashira cross-coupling and Cu(I) catalyzed azide–alkyne cycloaddition (CuAAC)
reactions are employed to attach the chromophore, enabling the comparison of the degree of surface
functionalization and the importance of the employed linker at the diamond-dye interface. X-ray
photoelectron spectroscopy shows that surface functionalization via CuAAC results in a slightly higher
chromophore coverage compared to the Sonogashira cross-coupling. However, photocurrents and
photovoltages, obtained by photoelectrochemical and Kelvin probe measurements, are approximately
three times larger on photocathodes functionalized via Sonogashira cross-coupling. Surface
functionalization via Sonogashira cross-coupling is thus considered the preferential method for the
development of diamond-based hybrid photovoltaics.
Received 10th September 2018
Accepted 18th September 2018
DOI: 10.1039/c8ra07545j
rsc.li/rsc-advances
redox electrolyte.3,4 Further progress can be made by replacing
these cathode materials with photoactive ones toward the
1. Introduction
development of tandem or p,n-DSSCs. Via careful design of the
(organic) chromophores anchored on both electrodes, more
light can be absorbed. Theoretically, this could increase the
maximum efficiency to 43% (in comparison with 33% for single
junction devices).5 Unfortunately, reports on these tandem
devices are very rare and the efficiencies obtained are hardly
comparable with those of classical n-type DSSCs,6–8 which can
mainly be ascribed to the lower performance of the photo-
cathode materials. Until now, the best performing photocath-
odes are based on p-type NiO sensitized with perylene–
thiophene–triphenylamine (PMI-6T-TPA) dyes and tris(acetyla-
cetonato)-iron(III/II) ([Fe(acac)3]0/1–) as redox couple.9 Neverthe-
less, this only resulted in efficiencies of 2.5% and a current
density of 7.65 mA cmꢀ2 at 1 sun illumination. In order to grasp
the full potential of tandem DSSCs, further improvements on
these p-type photoelectrodes are needed.
Dye-sensitized solar cells (DSSCs) are considered as one of the
prominent technologies in the eld of emerging photovoltaics,
next to perovskite, quantum dots and organic solar cells. Aer
three decades of optimization, record efficiencies up to 14.3%
under 1 sun illumination have been achieved.1,2 The most effi-
cient and most intensively studied DSSCs employ n-type elec-
trodes such as TiO2 as the photoanode with platinum or
graphene as counter electrode. These photoelectrochemically
‘silent’ counter electrodes do not contribute to the generation of
photocurrent, but merely serve to transfer the electrons to the
aUHasselt – Hasselt University, Institute for Materials Research (IMO), IMEC vzw,
IMOMEC, Agoralaan 1, 3590 Diepenbeek, Belgium. E-mail: wouter.maes@uhasselt.be
bJ. Heyrovsky Institute of Physical Chemistry, Czech Academy of Sciences, Dolejskova 3,
18223 Prague 8, Czech Republic
c
´
Institute of Physics, Czech Academy of Sciences, Cukrovarnicka 10, 162 00 Prague 6,
Diamond is a promising alternative photocathode material,
especially due to the development of techniques such as
microwave plasma enhanced chemical vapor deposition
(MWPECVD).10 CVD allows the growth of diamond thin lms on
a broad range of substrates over large areas (even m2) and at
a relatively low cost from a mixture of hydrogen and methane.11
By introducing trimethylborane to the gas mixture, diamond
Czech Republic. E-mail: rezek@fzu.cz
d
´
Faculty of Electrical Engineering, Czech Technical University in Prague, Technicka 2,
166 25 Prague 6, Czech Republic
† Electronic supplementary information (ESI) available: NMR spectra of the
alkyne-DTP-BT dye and additional information on the diamond growth, the
Schlenk vials applied for diamond functionalization, AFM lm morphologies,
and
additional
photoelectrochemical
measurements.
See
DOI:
10.1039/c8ra07545j
33276 | RSC Adv., 2018, 8, 33276–33290
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
can be converted from an insulating material to one with either The Sonogashira cross-coupling is a widely employed reaction
semiconducting or even metallic conductivity, depending on for the synthesis of conjugated molecules and polymers due to
the concentration of boron.12 Boron-doped nanocrystalline its broad scope, simplicity and facile monomer synthesis.35 We
diamond (B:NCD) thin lms exhibit a good optical transparency previously optimized this reaction on B:NCD surfaces with
for light with wavelengths from 300 to 1000 nm.13 This means special emphasis on obtaining a high coverage in combination
that by using diamond as a transparent electrode, the attached with minimal surface impurities, both crucial parameters to
(organic) dyes are protected from harmful UV light, as it is obtain efficient photovoltaics.36 In the presented work, a novel
mostly absorbed by the diamond itself. When compared with organic donor–acceptor chromophore, bearing an alkyne func-
NiO, there are several other advantages in utilizing B:NCD as an tional group, was synthesized. The alkyne moiety was intro-
electrode material: (i) the chemical inertness of diamond is duced on the donor part of the chromophore to enable fast hole
benecial when corrosive electrolyte solutions such as I3ꢀ/Iꢀ are injection into the valence band of diamond aer exciton
being employed.14 (ii) B:NCD has a hole diffusion coefficient (2– generation. On the other hand, the acceptor unit will be in close
30 cm2 sꢀ1) that is eight orders of magnitude higher than that contact to the electrolyte solution, ensuring a good electron
for NiO (4 ꢁ 10ꢀ8 cm2 sꢀ1).15 (iii) Diamond surfaces, and H- transfer to the redox couple and thereby minimizing charge
terminated ones in particular, can be readily functionalized recombination. The working principle of p-type DSSCs is illus-
through diazonium chemistry,16–18 photochemical graing,19,20 trated in Fig. 1.
or plasma treatment.21,22 In a subsequent step, light-harvesting
In addition to Sonogashira cross-coupling, alkynylated
molecules can be covalently attached to the surface, leading to chromophores can also be attached onto azidophenyl func-
the formation of carbon–carbon bonds between the dye and the tionalized diamond surfaces via the Cu(I) catalyzed azide–
electrode.23–26 These covalent bonds are stronger than the alkyne cycloaddition (CuAAC).37,38 Comparing these two
coordination bonds formed with NiO and should lead to approaches provides additional insights into the inuence of
improved stability of the diamond–dye interface.
employing different linkers on the structural and electronic
Pioneering research on the application of p-type diamond in properties and the potentially detrimental inuence of metal
photovoltaics was performed in 1999 via the sensitization of residues on the electrode surface.
boron-doped diamond in an electrolyte solution containing
2+
tris(bipyridine)ruthenium(II) (Ru(bpy)3 ).27 In 2008, Zhong
et al. reported the functionalization of diamond with bithio-
phene-C60 (2TC60) via a combination of diazonium graing and
Suzuki cross-coupling.28 This resulted in a current density of
150 nA cmꢀ2, which was later optimized to 4–6 mA cmꢀ2 under 1
sun (100 mW cmꢀ2) illumination.29 In 2014, cis-bis(isothiocya-
nato)bis(2,20-bipyridyl-4,40-dicarboxylato)ruthenium(II) (N3 dye)
was covalently attached to B:NCD via a combination of diazo-
nium, Suzuki, and EDC-NHS chemistry.30 Despite suitable
energy level alignment, photocurrents were limited to 25 nA
cmꢀ2 under 1 sun illumination. Later that year, the Suzuki
cross-coupling efficiency was improved by employing electron-
rich biarylmonodentate phosphine ligands. A surface coverage
of 0.56 monolayers was achieved for an organic molecular wire
(Br-CPDT-Fur) and photocurrents of 150 nA cmꢀ2 under 0.15
sun illumination were generated.31 Translation of this approach
from thin lm to a diamond foam electrode resulted in a 3-fold
increase in photocurrent. Aer illumination for 1 day at 1 sun,
the photocurrent further increased to 15–22 mA cmꢀ2
.
32
However, the ‘aging-activation’ leading to this champion pho-
toelectrochemical performance is not fully understood. More
recently, Krysova et al. compared the non-covalent and covalent
surface modication of B:NCD with 4-(bis(4-[5-(2,2-
dicyanovinyl)thiophene-2-yl]phenyl)amino)benzoic acid (P1
dye).33,34 They concluded that covalent functionalization is
preferred as the incident photon-to-current conversion effi-
Fig. 1 Working principle of a p-type dye DSSC. Under solar illumina-
ciency (IPCE) values were approximately 4 times higher than tion, an electron is excited from the highest occupied molecular
orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of
the dye. This excited electron will result in a reduction of the redox
electrolyte, which will diffuse to the counter electrode, where it is re-
oxidized. Finally, the electron is transferred back to the dye via an
those achieved via the non-covalent approach.
Herein, we further contribute to the development of p-type
diamond as a photocathode material through covalent surface
functionalization with a specic light-harvesting molecule
external circuit, which produces a current and leads to regeneration of
using a combination of diazonium and Sonogashira chemistry. the dye. (VBM ¼ valence band maximum)
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 33276–33290 | 33277

View Article Online
RSC Advances
Paper
the resulting lms with 0.1 M NBu4PF6 in anhydrous aceto-
nitrile as electrolyte solution. To prevent air from entering the
system, the experiments were carried out under a curtain of
argon. Cyclic voltammograms were recorded at a scan rate of
100 mV sꢀ1. For the conversion of V to eV, the onset potentials
of the rst oxidation/reduction peaks were used and refer-
enced to ferrocene/ferrocenium, which has an ionization
potential of ꢀ4.98 eV vs. vacuum.
2. Experimental section
2.1 Materials and instrumentation
All chemicals used were of reagent grade and no further
purication was performed prior to use. Sodium nitrate, 4-
iodoaniline,
4-azidoaniline
hydrochloride,
tris(3-
hydroxypropyltriazolylmethyl)amine (THPTA), N,N,N0,N00,N00-
pentamethyldiethylenetriamine (PMDETA), sodium dieth-
yldithiocarbamate, copper(I) bromide and copper(I) iodide
were purchased from Sigma-Aldrich. Chloroform (CHCl3), dry
tetrahydrofuran (THF), dry N,N-dimethylformamide (DMF),
dry toluene, triethylamine (TEA), diisopropylamine (DIPA), n-
butyl lithium (2.5 M solution in n-hexane; n-BuLi), trimethyltin
chloride, hexylamine, anhydrous copper chloride, sodium tert-
butoxide, tetrabutylammonium uoride (1.0 M solution in
THF; TBAF) and glacial acetic acid were purchased from Acros
Organics. 2,1,3-Benzothiadiazole, 3-bromothiophene, N-iodo-
succinimide, (trimethylsilyl)acetylene and (ꢂ)-2,20-bis(diphe-
nylphosphino)-1,10-dinaphthalene (BINAP) were purchased
from Fluorochem. Tris(dibenzylideneacetone)dipalladium(0)
Pd2(dba)3, tri(ortho-tolyl)phosphine and trans-dichlorobis(-
X-ray photoelectron spectroscopy (XPS) was used to deter-
mine the chemical composition of the diamond surfaces using
an XPS spectrometer (Kratos, AXIS Supra) equipped with
a hemispherical analyzer and a monochromatic Al Ka X-ray
source (1486.6 eV). The XPS spectra were acquired from an
area of 0.7 ꢁ 0.3 mm2 with a takeoff angle of 90ꢃ. The survey XPS
spectra were recorded with a pass energy of 80 eV, whereas the
high-resolution spectrum scans were recorded with a pass
energy of 20 eV. The obtained XPS spectra were calibrated on
283.8 eV binding energy (sp3 carbon phase).39 The CasaXPS
soware with implemented linear baseline and Gaussian line
shapes was used for spectral processing. XPS peak positions
were determined with an accuracy of ꢂ0.2 eV.
Atomic force microscopy (AFM) and Kelvin probe force
microscopy (KPFM) measurements were performed on an
NTEGRA Prima system (NT-MDT) with a Pt/Cr coated probe
(BudgetSensors Multi75E-G) using a value of dz ¼ 0 nm and an
AC voltage of 2 V. The obtained data were line-corrected and
root-mean-square (RMS) values were evaluated from the images
using the NT-MDT soware.
Macroscopic Kelvin probe measurements were performed on
a scanning Kelvin probe system (KP Technologies) using a gold
coated probe (tip of 2 mm in diameter) at a distance dened by
a gradient of 300. Illumination was provided by a cold light
source KL2500 (Schott) with a halogen lamp set to full power
(250 W). The light was introduced by a light guide to the sample
(within 5 cm distance). The photovoltage was calculated as the
difference between the potential values (averaged for 18 s) with
and without illumination. The error bars correspond to the
standard deviation of the averaged potential values.
Photoelectrochemical measurements were performed in
a Ar-saturated 0.1 M Na2SO4 solution containing 5 mM
methyl viologen (MV2+) as an electron carrier in a three-
electrode glass cell equipped with a quartz optical window
for illumination of the B:NCD working electrode. The counter
electrode was platinum and the reference electrode was Ag/
AgCl (sat. KCl). The glass cell was placed in a dark room
and controlled by a potentiostat (Autolab, Ecochemie, B.V.
with NOVA soware). The white light source was an Oriel
xenon lamp, model 6269. The incident light intensity was
focused and calibrated with a standard Si photodiode (PV
Measurements, Inc. USA). The incident white light intensity
was 90 mW cm2 (which is roughly 0.9 sun).
triphenylphosphine)palladium(II)
(Pd(PPh3)2Cl2)
were
purchased from Strem Chemicals. All solvents employed for
rinsing were of HPLC quality.
Preparative (recycling) size exclusion chromatography
(prep-SEC) was performed on a JAI LC-9110 NEXT system
equipped with JAIGEL 1H, 2H, and 3H columns (eluent CHCl3,
ow rate 3.5 mL minꢀ1). Nuclear magnetic resonance (NMR)
chemical shis (d, in ppm) were determined relative to the
residual CHCl3 (7.26 ppm) absorption or the 13C resonance
shi of CDCl3 (77.16 ppm). High resolution electrospray
ionization mass spectrometry (ESI-MS) was performed using
an LTQ Orbitrap Velos Pro mass spectrometer equipped with
an atmospheric pressure ionization source operating in the
nebulizer assisted electrospray mode. The instrument was
calibrated in the m/z range 220–2000 using a standard solution
containing caffeine, MRFA and Ultramark 1621. UV-Vis
absorption spectroscopy measurements were performed on
a VARIAN Cary 5000 UV-Vis spectrophotometer at a scan rate
of 600 nm minꢀ1. The lm for the UV-Vis absorption
measurement was prepared by drop casting a solution of the
chromophore in chloroform on a quartz substrate. The solid-
state UV-Vis absorption spectrum was used to estimate the
optical gap (from the wavelength at the intersection of the
tangent line drawn at the low energy side of the absorption
spectrum with the baseline: Eg (eV) ¼ 1240/(wavelength in
nm)). Electrochemical measurements (cyclic voltammetry)
were performed with an Eco Chemie Autolab PGSTAT 30
potentiostat/galvanostat using a three-electrode microcell with
a platinum working electrode, a platinum counter electrode,
and a Ag/AgNO3 reference electrode (silver wire dipped in
a solution of 0.01 M AgNO3 and 0.1 M NBu4PF6 in anhydrous
acetonitrile). The reference electrode was calibrated against
ferrocene/ferrocenium as an external standard. Samples were
prepared by dip coating the platinum working electrode in the
chromophore solution. The CV measurements were done on
2.2 Dye synthesis
4-Iodobenzenediazonium chloride. 4-Iodoaniline (10.9 mg,
50 mmol) was diazotized 5 min prior to the electrograing
protocol by adding an equimolar amount of NaNO2 (3.45 mg, 50
33278 | RSC Adv., 2018, 8, 33276–33290
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
mmol). 4-Iodoaniline was dissolved in an Ar-purged HCl solu- 50 mL). The combined organic layers were dried over MgSO4,
tion (0.5 M; 9.5 mL) and a NaNO2 solution (0.1 M; 0.5 mL) in ltered, and concentrated in vacuo. The crude material was
HCl (0.5 M) was added, resulting in a nal concentration of puried via preparative size exclusion chromatography (chloro-
5 mM. Aer vortexing the solution, it was used for electro- form), yielding the product as a gray-green oil (1.26 mg, 78%). 1H
chemical graing via cyclic voltammetry.36
NMR (400 MHz, chloroform-d): d ¼ 7.10 (d, J ¼ 5.3 Hz, 1H), 7.02
4-Azidobenzenediazonium chloride. 4-Azidoaniline hydro- (s, 1H), 6.99 (d, J ¼ 5.3 Hz, 1H), 4.19 (t, J ¼ 7.1 Hz, 2H), 1.90–1.81
chloride (8.5 mg, 50 mmol) was diazotized 5 min prior to the (m, 2H), 1.37–1.23 (m, 6H), 0.87 (t, J ¼ 7.1 Hz, 3H), 0.41 (s, 9H).
electrograing protocol by adding an equimolar amount of
4-Bromobenzo[2,1,3]thiadiazole (5). Br2 (8.27 mL, 161.405
NaNO2 (3.45 mg, 50 mmol). 4-Azidoaniline hydrochloride was mmol) was slowly added to a mixture of benzo[2,1,3]thiadiazole
dissolved in an Ar-purged HCl solution (0.5 M; 9.5 mL) and (20.00 g, 146.88 mmol) in 48% HBr (200 mL). Aer reuxing
a NaNO2 solution (0.1 M; 0.5 mL) in HCl (0.5 M) was added, overnight at 150 ꢃC, the reaction was cooled down to room
resulting in a nal concentration of 5 mM. Aer vortexing the temperature and a saturated NaHSO3 solution (100 mL) was
solution, it was used for electrochemical graing via cyclic added at 0 ꢃC. The precipitate was ltered off, washed with
voltammetry.25
water and puried through steam distillation. The distillate was
3,30-Dibromo-2,20-bithiophene (2). Lithium diisopropyl- further puried via recrystallization in methanol, while the
amine (LDA) was freshly prepared by adding n-BuLi (2.5 M residue was puried via column chromatography (silica,
solution in n-hexane, 85.2 mL, 212.9 mmol) to a solution of petroleum ether : ethyl acetate, 95 : 5) and Kugelrohr distilla-
ꢃ
diisopropylamine (28.4 mL, 202.8 mmol) in dry THF (100 mL) at tion (70 C at 2 mm Hg). The combined fractions yielded the
0 C under Ar atmosphere. LDA was added dropwise to a solu- product as a white solid (10.70 g, 34%). 1H NMR (400 MHz,
ꢃ
tion of 3-bromothiophene (19.0 mL, 202.8 mmol) in dry THF chloroform-d): d ¼ 7.98 (dd, J ¼ 8.8, 0.9 Hz, 1H), 7.85 (dd, J ¼
(350 mL) at ꢀ78 ꢃC via a cannula. Aer stirring for 1 h at ꢀ78 ꢃC, 7.2, 0.8 Hz, 1H), 7.49 (dd, J ¼ 8.8, 7.2 Hz, 1H).
ꢃ
CuCl2 (30.0 g, 223.1 mmol) was added at ꢀ78 C. Aer stirring
2-(Benzo[2,1,3]thiadiazol-4-yl)-4-hexyl-4H-dithieno[3,2-b:20,30-
for another hour, the reaction mixture was brought to room d]pyrrole (6). Compounds 4 (1.00 g, 2.35 mmol), 5 (530 mg, 2.46
temperature and allowed to stir overnight. Aer removing the mmol), Pd2(dba)3 (64 mg, 0.070 mmol) and P(o-tolyl)3 (86 mg,
solvent in vacuo, 1 M HCl (400 mL) was added and the crude 0.28 mmol) were loaded in a Schlenk vial and placed under Ar
product was extracted with dichloromethane. The organic atmosphere. Next, dry toluene (10 mL) and dry DMF (4 mL) were
phase was washed with water (2 ꢁ 200 mL) and brine (1 ꢁ 100 added, and the mixture was purged with argon. Aer reuxing for
mL), dried over MgSO4, ltered and concentrated in vacuo. The 1 h, diethyl ether (100 mL) was added and the mixture was
crude product was further puried over a silica plug (hex- washed with water (2 ꢁ 100 mL) and brine (50 mL). The organic
ane : dichloromethane, 90 : 10) and then via Kugelrohr distil- layer was dried over MgSO4, ltered and concentrated in vacuo.
lation (106 ꢃC at 2 mm Hg) and nally recrystallization in EtOH The crude product was puried via column chromatography
to yield the product as white crystals (19.9 g, 60%). 1H NMR (400 (silica, hexane : dichloromethane, 60 : 40), yielding the product
1
MHz, chloroform-d): d ¼ 7.41 (d, J ¼ 5.4 Hz, 2H), 7.08 (d, J ¼ 5.4, as a red solid (930 mg, quantitative). H NMR (400 MHz, chlo-
2H).40
roform-d): d ¼ 8.34 (s, 1H), 7.86 (dd, J ¼ 8.7, 0.9 Hz, 1H) 7.84 (dd, J
4-Hexyl-4H-dithieno[3,2-b:20,30-d]pyrrole (3). A solution of ¼ 7.2, 0.9 Hz, 1H), 7.61 (dd, J ¼ 8.7, 7.2 Hz, 1H), 7.20 (d, J ¼
compound 2 (5.00 g, 15.43 mmol), tBuONa (3.56 g, 37.03 mmol), 5.3 Hz, 1H), 7.02 (d, J ¼ 5.3 Hz, 1H), 4.29 (t, J ¼ 7.0 Hz, 2H), 1.96–
Pd2(dba)3 (0.353 g, 0.39 mmol) and BINAP (0.96 g, 1.54 mmol) in 1.88 (m, 2H), 1.39–1.23 (m, 6H), 0.87 (t, J ¼ 7.0 Hz, 3H).
dry toluene (25 mL) was purged with Ar for 20 min. Hexylamine
2-(Benzo[2,1,3]thiadiazol-4-yl)-4-hexyl-6-iodo-4H-dithieno
(2.0 mL, 12.5 mmol) was added and the mixture was heated to [3,2-b:20,30-d]pyrrole (7). N-Iodosuccinimide (555 mg, 2.47
reux overnight. Aer the reaction was cooled down to room mmol) was added to a solution of compound 6 (930 mg, 2.35
temperature, water (50 mL) was added and the aqueous layer mmol) in acetic acid (40 mL) and chloroform (40 mL). The
was extracted with diethyl ether (3 ꢁ 50 mL). The organic layer reaction mixture was shielded from light and le to stir for 3 h,
was washed with water (2 ꢁ 50 mL) and brine (1 ꢁ 25 mL), dried aer which water (75 mL) and chloroform (75 mL) were added.
over MgSO4, ltered and concentrated in vacuo. The crude The organic layer was washed with a 20% Na2S2O7 solution (3 ꢁ
product was then puried by column chromatography (silica, 75 mL), water (2 ꢁ 75 mL) and brine (75 mL). Next, the organic
petroleum ether : dichloromethane, 95 : 5), yielding a pale solid layer was dried over MgSO4, ltered and concentrated in vacuo.
(3.86 g, 95%). 1H NMR (400 MHz, chloroform-d): d ¼ 7.14 (d, J ¼ The crude product was puried via column chromatography
5.3 Hz, 2H), 7.02 (d, 5.3 Hz, 2H), 4.20 (t, J ¼ 7.1 Hz, 2H), 1.90– (silica, hexane : dichloromethane, 60 : 40), yielding the product
1.80 (m, 2H), 1.37–1.21 (m, 6H), 0.88 (t, J ¼ 7.0 Hz, 3H).40
4-Hexyl-2-(trimethylstannyl)-4H-dithieno[3,2-b:20,30-d]pyrrole
(4). n-BuLi (2.5 M solution in hexane; 1.67 mL, 4.18 mmol) was 0.9 Hz, 1H), 7.60 (dd, J ¼ 8.7, 7.2 Hz, 1H), 7.20 (s, 1H), 4.22 (t, J ¼
added dropwise to a solution of 3 (1.00 g, 3.80 mmol) in THF (10 7.1 Hz, 2H), 1.95–1.85 (m, 2H), 1.40–1.24 (m, 6H), 0.87 (t, J ¼
mL) at ꢀ78 ꢃC. Aer stirring for 1 h at ꢀ78 ꢃC, trimethylstannyl 6.4 Hz, 3H); 13C NMR (100 MHz, chloroform-d): d ¼ 156.3, 152.6,
chloride (1.0 M solution in hexane, 4.75 mL, 4.75 mmol) was 147.7, 145.4, 137.6, 130.3, 129.1, 124.8, 120.9, 120.0, 119.9,
rapidly added and the reaction mixture was gradually brought to 115.8, 113.3, 72.1, 48.1, 32.1, 31.0, 27.3, 23.2, 14.7; calcd for
as a red solid (927 mg, 75%). 1H NMR (400 MHz, chloroform-d):
d ¼ 8.29 (s, 1H), 7.87 (dd, J ¼ 8.7, 0.9 Hz, 1H), 7.82 (dd, J ¼ 7.1,
room temperature. Aer stirring overnight, water (50 mL) was
added and the aqueous layer was extracted with diethyl ether (3 ꢁ
C
20H18IN3S3 [M]+: m/z 522.9708, found: 522.9703.
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 33276–33290 | 33279

View Article Online
RSC Advances
Paper
2-(Benzo[c][2,1,3]thiadiazol-4-yl)-4-hexyl-6-(trimethylsilyle-
2.3 Diamond growth
thynyl)-4H-dithieno[3,2-b:20,30-d]pyrrole (8). Compound
(670 mg, 1.28 mmol), Pd(PPh3)2Cl2 (45 mg, 0.06 mmol) and
CuI (24 mg, 0.13 mmol) were loaded in a Schlenk vial
equipped with a stirring bar under inert atmosphere. Tetra-
hydrofuran (10 mL) and triethylamine (4 mL) were added and
the mixture was purged for 30 min before adding trime-
thylsilylacetylene (0.27 mL, 1.92 mmol). Aer heating over-
night at 50 ꢃC, water (75 mL) was added and the product was
extracted with chloroform (3 ꢁ 75 mL). The organic phase
was dried over MgSO4, ltered and concentrated in vacuo.
The crude product was puried via preparative size exclusion
chromatography, yielding the product as a viscous red oil
(464 mg, 73%). 1H NMR (400 MHz, chloroform-d): d ¼ 8.24 (s,
1H), 7.83 (dd, J ¼ 8.8, 0.9 Hz, 1H), 7.77 (dd, J ¼ 7.2, 0.9 Hz,
1H), 7.55 (dd, J ¼ 8.7, 7.2 Hz, 1H), 7.15 (s, 1H), 4.18 (t, J ¼
7.1 Hz, 2H), 1.87 (p, J ¼ 7.1 Hz, 2H), 1.39–1.21 (m, 7H), 0.89–
0.82 (m, 3H), 0.28 (s, 9H); 13C NMR (100 MHz, chloroform-d):
d ¼ 150.5, 146.7, 141.9, 138.4, 132.7, 124.5, 123.3, 119.1,
115.4, 114.2, 111.0, 110.9, 110.0, 107.3, 94.6, 94.2, 42.2, 26.2,
25.1, 21.5, 17.3, 8.9, ꢀ5.2, ꢀ5.3; HRMS (ESI): calcd for
7
150 nm thick B:NCD lms were grown on (100)-oriented 10 ꢁ
102 mm Si p-type conductive substrates by MWPECVD, with
trimethylborane (TMB) as the boron source. Characterization
performed on samples grown in the same reactor under similar
growth conditions indicated that the B:NCD lms have boron
concentrations of approximately 5 ꢁ 1021 cmꢀ3 and a resistivity
(U) below 5 mU cm.41,42
2.4 Diazonium electrograing
The B:NCD thin lms were functionalized with 4-iodophenyl
and 4-azidophenyl moieties by means of electrochemical dia-
zonium graing of the respective in situ generated benzene-
diazonium chlorides with cyclic voltammetry. The
electrograing was performed with an Autolab PGSTAT30
potentiostat (Eco Chemie B.V.) and was controlled by the GPES
Manager soware. A one-compartment electrochemical cell was
employed in
a three-electrode conguration. Hydrogen-
terminated B:NCD was used as the working electrode, a plat-
inum wire as the counter electrode and a Ag/AgCl/KCl(sat)
electrode as the reference. All potentials are reported versus
this reference electrode at room temperature. Diamond working
electrodes were functionalized in two steps by immersing each
half of the sample in the in situ generated 4-iodo- or 4-azido-
benzenediazonium chloride solution. The electrograing
process was performed by cycling the potential between +500
and ꢀ600 mV vs. Ag/AgCl for 4-iodoaniline and between +300
and ꢀ800 mV vs. Ag/AgCl for 4-azidoaniline at a scan rate of
100 mV sꢀ1 for eight scans.25,36 Aer graing, the B:NCD
samples were sonicated in MilliQ water and subsequently
rinsed with toluene, THF, ethanol, and MilliQ water.
C
25H27N3S3Si [M + Na]+: m/z 516.1034, found: 516.1029.
2-(Benzo[2,1,3]thiadiazol-4-yl)-6-ethynyl-4-hexyl-4H-dithieno
[3,2-b:20,30-d]pyrrole (9). TBAF (1.0 M solution in THF, 1.6 mL,
1.6 mmol) was added to a solution of compound 8 (530 mg,
ꢃ
1.07 mmol) in THF (25 mL) at 0 C. Aer stirring for 40 min,
water (75 mL) was added and the aqueous phase was extracted
with diethyl ether (2 ꢁ 50 mL) and chloroform (2 ꢁ 50 mL).
The combined organic layers were dried over MgSO4, ltered
and concentrated in vacuo. The crude product was puried via
column chromatography (silica, hexane : dichloromethane,
1
60 : 40), yielding the product as a red solid (310 mg, 69%). H
NMR (400 MHz, chloroform-d) d ¼ 8.17 (s, 1H), 7.77 (dd, J ¼
8.7, 0.9 Hz, 1H), 7.67 (dd, J ¼ 7.1, 0.9 Hz, 1H), 7.46 (dd, J ¼ 8.7,
7.1 Hz, 1H), 7.11 (s, 1H), 4.10 (t, J ¼ 7.1 Hz, 2H), 3.48 (s, 1H),
1.83 (p, J ¼ 7.1 Hz, 2H), 1.36–1.19 (m, 6H), 0.89–0.80 (m, 3H);
13C NMR (100 MHz, chloroform-d) d ¼ 155.5, 151.8, 147.1,
143.4, 138.0, 129.6, 128.3, 124.2, 119.4, 119.2, 116.5, 116.1,
114.9, 112.5, 82.1, 78.8, 47.3, 31.4, 30.3, 26.6, 22.5, 14.1; HRMS
(ESI): calcd for C22H19N3S3 [M + Na]+: m/z 444.0639, found:
444.0675.
2.5 Sonogashira cross-coupling
All Sonogashira cross-coupling reactions were prepared in
a glovebox under inert atmosphere. The light-harvesting mole-
cule, palladium catalyst, copper(I) iodide, diamond substrate,
base, and solvent were loaded in a tailor made Schlenk vial
(Fig. S3†) equipped with a magnetic stirring bar. The Schlenk
vial was closed off with a rubber septum, removed from the
glovebox and placed in an oil bath at 55 ꢃC for 16 h. Aer surface
Table 1 Optimization of the Sonogashira cross-coupling conditions for covalent attachment of the alkyne-DTP-BT chromophore through
analysis of the chemical composition on the diamond surface by XPSa
Entry Catalyst system
Dye
Base
TEA
DIPA 4.0
DIPA 3.7
O at% C at% B at% S at% N at% Pd at% Cu at% I at%
1
2
Pd(PPh3)2Cl2 (5 mol%)/CuI (10 mol%)
Pd(PPh3)2Cl2 (5 mol%)/CuI (10 mol%)
(50 mmol)
(50 mmol)
3.6
89.8
87.2
88.0
85.9
87.4
89.5
2.9
3.3
3.7
3.8
2.9
3.4
1.1
1.8
1.6
2.3
2.2
0.2
2.0
3.0
2.6
3.5
3.2
1.7
0.1
0.2
0.1
0.2
0.6
—
—
—
—
—
—
—
0.5
0.5
0.3
0.3
0.3
0.9
3
Pd(PPh3)2Cl2 (10 mol%)/CuI (20 mol%) (50 mmol)
4
Pd(PPh3)2Cl2 (5 mol%)/CuI (10 mol%)
Pd(PPh3)2Cl2 (5 mol%)/CuI (10 mol%)
—
(100 mmol) DIPA 4.0
(100 mmol) DIPA 3.4
(50 mmol)
5b
6
DIPA 4.3
a
General reaction conditions: dry THF (6 mL), base (3 mL), 60 ꢃC, 18 h; TEA ¼ triethylamine, DIPA ¼ diisopropylamine. b Reaction outside of the
glovebox.
33280 | RSC Adv., 2018, 8, 33276–33290
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
Table 2 Optimization of the CuAAC conditions for covalent attachment of the alkyne-DTP-BT chromophore through analysis of the chemical
composition on the diamond surface by XPSa
Entry
Catalyst/ligand
O at%
C at%
B at%
S at%
N at%
Cu at%
1
2
3
4
CuI (100 mol%)/PMDETA (200 mol%)
CuI (10 mol%)/THPTA (20 mol%)
CuBr (100 mol%)/PMDETA (200 mol%)
—
4.1
3.4
3.1
4.3
84.6
85.9
85.2
89.0
3.0
1.9
2.1
2.2
1.9
2.2
2.5
0.1
6.4
6.6
7.1
4.4
—
—
—
—
a
General reaction conditions: dye (50 mmol), dry THF (6 mL), dry DMF (3 mL), 50 ꢃC, 18 h; THPTA ¼ tris(3-hydroxypropyltriazolylmethyl)amine,
PMDETA ¼ N,N,N0,N00,N00-pentamethyldiethylenetriamine.
functionalization, all B:NCD samples were sonicated in toluene illustrated in Scheme 1. For the functionalization via Sonoga-
(p.a.), a 10% sodium diethyldithiocarbamate solution, acetone, shira cross-coupling, a iodophenyl layer was rst graed onto
ethanol, and MilliQ water, before drying under a strong stream the diamond surface via diazonium chemistry. When the
of nitrogen. More detailed information on the exact reaction CuAAC reaction was employed, an azidophenyl layer was
conditions applied can be found in Table 1.
primarily graed on the surface instead.
2.6 Cu(I) catalyzed azide–alkyne cycloaddition
3.1 Chromophore synthesis
All Cu(I) catalyzed azide–alkyne cycloaddition reactions were
prepared in a glovebox under inert atmosphere. The light-
harvesting molecule, copper source, ligand, diamond
substrate and solvents were loaded in a tailor made Schlenk vial
(Fig. S3†) equipped with a magnetic stirring bar. The Schlenk
vial was closed off with a rubber septum, removed from the
glovebox and placed in an oil bath at 55 ꢃC for 16 h. Aer surface
functionalization, all B:NCD samples were sonicated in toluene
(p.a.), a 10% sodium diethyldithiocarbamate solution, acetone,
ethanol and MilliQ water, before drying under a strong stream
of nitrogen. More detailed information on the exact reaction
conditions applied can be found in Table 2.
A light-harvesting donor–acceptor type dye was synthesized to
facilitate charge separation of the generated hole and electron
aer excitation. As we target a high surface coverage, the dye
itself should preferentially be rather small, without too long
side chains, as this might inuence the coupling reactions.
Additionally, to obtain a low optical gap chromophore, strong
donor and acceptor units have to be combined in a push–pull
approach. In this case, the electron rich dithienopyrrole unit
was selected as the donor part and combined with a benzo-
thiadiazole acceptor building block.43,44 The synthesis of the
alkyne
functionalized
dithieno[3,2-b:20,30-d]pyrrole-2,1,3-
benzothiadiazole (alkyne-DTP-BT) chromophore is shown in
Scheme 2. Dithienopyrrole 3 was synthesized in two steps via
ꢃ
regiospecic lithiation of 3-bromothiophene (1) at ꢀ78 C and
3. Results and discussion
subsequent Ullmann coupling, resulting in 3,30-dibromo-2,20-
The covalent functionalization of B:NCD electrodes with alky- bithiophene (2). Aerwards, the bithiophene was fused using
nylated chromophores was performed in two steps, as a Buchwald–Hartwig amination with hexylamine to generate
Scheme 1 General functionalization strategy of boron-doped nanocrystalline diamond for the design of p-type photoelectrodes.a a(i) In situ
diazotization of 4-iodoaniline or 4-azidoaniline; (ii) electrochemical reduction and subsequent grafting of the corresponding diazonium salt; (iii)
Sonogashira cross-coupling (a) and Cu(I) catalyzed azide–alkyne cycloaddition (CuAAC) (b) using an alkyne functionalized dithieno[3,2-b:20,30-d]
pyrrole-2,1,3-benzothiadiazole (alkyne-DTP-BT) chromophore.
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 33276–33290 | 33281

View Article Online
RSC Advances
Paper
Scheme 2 Synthetic route toward the alkyne functionalized dithieno[3,2-b:20,30-d]pyrrole-2,1,3-benzothiadiazole (alkyne-DTP-BT) chromo-
phore.a a(i) LDA, CuCl2, THF; (ii) t-BuONa, hexylamine, Pd2(dba)3, BINAP, toluene; (iii) n-BuLi, Me3SnCl, THF; (iv) Pd2(dba)3, P(o-tolyl)3, toluene,
DMF; (v) NIS, CHCl3, AcOH; (vi) TMS-acetylene, Pd(PPh3)2Cl2, CuI, THF, TEA; (vii) TBAF, THF.
the donor building block 3, which was then stannylated and resolution I 3d3/2 and I 3d5/2 peaks at 632 and 622 eV, respec-
coupled to 4-bromo-2,1,3-benzothiadiazole (5) via a Stille cross- tively (Fig. 3, top). XPS analysis of the high resolution iodine
coupling. Finally, an alkyne functionality was introduced on the peaks revealed that the relative atomic concentration of iodine
DTP-BT chromophore 6 in 3 steps through iodination with N- on the sample surface is about 0.8 at%, which is in agreement
iodosuccinimide (NIS),40 Sonogashira cross-coupling with tri- with previous results.36
methylsilylacetylene and deprotection with tetrabutylammo-
For the functionalization of diamond via CuAAC chemistry,
nium uoride (TBAF). UV-Vis analysis of the newly synthesized an azide functional handle is required on the surface. The azide
dye (9) revealed two strong absorption bands peaking at wave- groups were also introduced via electrochemical diazonium
lengths of 335 and 474 nm. As expected, this results in a red dye. graing, employing 4-azidoaniline hydrochloride. The reduc-
The molar extinction coefficient (3 ¼ 18 600 Mꢀ1 cmꢀ1 at lmax
¼
tive potential scan was slightly adjusted to ensure optimal
474) is slightly lower compared to chromophores previously functionalization according to a literature protocol,25 by cycling
employed for the sensitization of diamond lms.31,34 HOMO the potential between +300 and ꢀ800 mV. Complete passivation
(ꢀ5.21 eV) and LUMO (ꢀ3.29 eV) energy levels were estimated of the diamond surface was again obtained aer just one scan,
from the oxidation and reduction onsets as measured by cyclic as shown in the cyclic voltammogram (Fig. 2, bottom). Graing
voltammetry. Alternatively, the HOMO energy level can also be of the azidophenyl layer was also conrmed by XPS through the
estimated by subtracting the optical gap (2.30 eV) from the presence of the N 1s signal. Deconvolution of the high
LUMO energy level, resulting in EHOMO ¼ ꢀ5.59 eV.
resolution N 1s peak (Fig. 3, bottom) revealed three distinct
nitrogen species at 399.0, 400.1 and 404.1 eV.45,46 The two
signals at the lowest binding energy can be assigned to the more
electron rich (outer) azide nitrogen atoms, whereas the signal at
higher binding energy comes from the more electron decient
(central) azide nitrogen atom.47 The surface nitrogen content
calculated from the high resolution N 1s peak was about 4.8
at%. It should be mentioned that this nitrogen concentration is
quite high for nanocrystalline diamond and could be related to
multilayer formation (via a radical mechanism and/or azo
coupling) during the graing procedure.17
3.2 Primary surface functionalization: diazonium graing
To immobilize the alkyne-DTP-BT chromophore onto a B:NCD
electrode via Sonogashira cross-coupling, an iodine moiety was
introduced via electrochemical diazonium graing of 4-iodoa-
niline, as reported previously.36 The diazonium salt was gener-
ated in situ via diazotization of 4-iodoaniline with an equimolar
amount of NaNO2 in HCl (0.5 M). Subsequently, a freshly grown
H-terminated diamond thin lm was employed as working
electrode in a cyclic voltammetry experiment and submerged in
the aforementioned solution. The potential was then cycled
eight times between +500 and ꢀ600 mV (versus a Ag/AgCl
reference electrode). Hereby, an electron is ejected from the
3.3 Secondary surface functionalization: Sonogashira cross-
coupling and CuAAC chemistry
diamond electrode into the solution, resulting in a reduction of For the development of efficient photocathodes, two parameters
the diazonium salt to the corresponding radical, which can gra are very important, i.e. the amount of dye immobilized on the
onto the surface. Going to more negative potentials can lead to surface and the contamination thereof, as this can lead to
further reduction of the radical to an anion, which is detri- charge trapping of generated holes and electrons. Recently, we
mental for the graing process. The cyclic voltammogram demonstrated that the iodine functionalized diamond surface
(Fig. 2, top) shows the efficiency of the functionalization is a suitable platform for covalent attachment of organic
protocol as complete passivation of the diamond surface is molecules through Sonogashira cross-coupling.36 A broad range
obtained aer one cycle. Actual graing of the iodophenyl of reaction conditions were evaluated for the coupling of the 2-
moieties was conrmed by XPS through the detection of high ethynylthiophene model system. In all cases, a decent surface
33282 | RSC Adv., 2018, 8, 33276–33290
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
Fig. 3 High resolution I 3d and N 1s XPS spectra for iodophenyl (top)
and azidophenyl (bottom) functionalized B:NCD electrodes, respec-
tively. The N 1s peak was deconvoluted into three peaks at 399.0,
400.1 and 404.1 eV.45,46
Fig. 2 Cyclic voltammograms showing the electrochemical grafting
of in situ generated 4-iodobenzenediazonium chloride (top) and 4-
azidobenzenediazonium chloride (bottom) on H-terminated B:NCD
electrodes at a scan rate of 100 mV sꢀ1 for 8 scans.
transition metals from organic and aqueous reaction media.48
Additionally, the signal of the B 1s peak was still detected by
XPS, which indicates that a relatively thin layer of molecules on
the diamond surface is obtained, since the attenuation length
of the employed X-rays (1486.6 eV) is approximately 3.5 nm for
carbon materials.49 By employing DIPA as a base (Table 1, entry
2), the degree of surface functionalization improved (the S
content increased to 1.8 at%), while keeping the amount of
metal impurities at a minimum. Doubling the concentration of
the catalyst and co-catalyst (Table 1, entry 3) did not signi-
cantly inuence the outcome of the surface functionalization.
On the other hand, doubling the concentration of the dye to 100
mmol (Table 1, entry 4) did increase the relative amount of S on
the surface to 2.3 at%, without drastically changing the amount
of metal residues. When performing the reaction outside of the
glovebox (Table 1, entry 5), a similar surface coverage of sulfur
(2.2 at%) was achieved, but the Pd concentration noticeably
increased (from 0.2 to 0.6 at%). This could tentatively be
attributed to the fact that oxygen can lead to the formation of
palladium black, which sticks to the surface, even aer
coverage was obtained with a relatively low amount of surface
contamination by catalyst residues. Accordingly, the optimal
catalyst system, Pd(PPh3)2Cl2, was selected for the functionali-
zation of B:NCD with the alkyne-DTP-BT chromophore (Scheme
3). For this specic dye molecule, the reaction conditions were
ne-tuned by varying the employed base, catalyst concentration,
and chromophore concentration. The degree of surface func-
tionalization and contamination were estimated by the relative
atomic concentrations of sulfur, palladium, and copper, as
obtained from the XPS spectra (Table 1).
Initially (Table 1, entry 1), 50 mmol of the alkyne-DTP-BT dye
was dissolved in a mixture of THF and TEA (2 : 1), with 5 mol%
Pd(PPh3)2Cl2 as a catalyst and 10 mol% CuI as co-catalyst. This
lead to a surface coverage of 1.1 at% S, while the relative surface
concentration of Pd was as low as 0.1 at% and no Cu was
detected by XPS. These low amounts of metal residues were
obtained by adding an additional step to the rinsing procedure,
i.e. sonication in a 10 m/v% solution of sodium dieth-
yldithiocarbamate in MilliQ water. These dithiocarbamates
have proven to be especially efficient for the removal of
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 33276–33290 | 33283

View Article Online
RSC Advances
Paper
Scheme 3 (i) Oxidative addition, (ii) transmetallation, and (iii) reductive elimination for the Sonogashira cross-coupling on iodophenyl func-
tionalized B:NCD surfaces.
thorough rinsing. Finally, a control experiment was performed directly provides the required copper(I) and reduces the amount
(Table 1, entry 6) without addition of the catalytic system to of reagents that have to be added, which is benecial to mini-
conrm actual cross-coupling. A small amount of dye (0.2 at% mize surface fouling. The highest surface coverage based on the
of sulfur) appears to physisorb on the surface, but this is an relative atomic concentration of sulfur was obtained by
order of magnitude lower than in the case of coupling via the employing 50 mmol of the dye in combination with 100 mol% of
Sonogashira reaction.
CuBr and PMDETA as a stabilizing ligand (Table 2, entry 3). This
Apart from the Sonogashira cross-coupling, alkynylated surface coverage of 2.5 at% S is slightly higher than the 2.3%
chromophores can also be attached to diamond surfaces by obtained for the Sonogashira cross-coupling. Additionally, no
employing CuAAC reactions, oen referred to as a ‘click’ copper residues were detected on the surface by XPS. Based on
chemistry approach (Scheme 4).50 The success of this protocol these results, we can summarize that, in terms of surface
for diamond functionalization was demonstrated by Szunerits functionalization, the CuAAC reaction is slightly more effective
and coworkers by successfully ‘clicking’ both ferrocene and than the Sonogashira cross-coupling, and, importantly, no
thiophenes onto azide functionalized diamond surfaces with surface contamination was observed at all. Conrmation of the
a conversion efficiency of approximately 50%.38,51
actual CuAAC reaction was again provided by performing
A small optimization of the reaction conditions for the a control experiment (Table 2, entry 4), which shows only
CuAAC functionalization was performed. Since the alkyne-DTP- a small amount of physisorbed dye (0.1 at% S).
BT dye is not soluble in aqueous media, the traditional catalyst
The changes on the diamond surface during the two-step
system, containing CuSO4 and sodium ascorbate to reduce functionalization approach for both the Sonogashira cross-
copper(II) to copper(I), was replaced with CuI or CuBr. This coupling and the CuAAC reaction are visualized in Fig. 4. In
Scheme 4 Simplified mechanism for the Cu(I) catalyzed azide–alkyne cycloaddition (CuAAC) on azidophenyl functionalized B:NCD surfaces.52
33284 | RSC Adv., 2018, 8, 33276–33290
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
Fig. 4 Normalized XPS survey spectra for the surface functionalization of H-terminated B:NCD via (a) Sonogashira cross-coupling (Table 1, entry
4) or (b) CuAAC chemistry (Table 2, entry 2). Deconvoluted high resolution S 2p (c and e) and N 1s (d and f) signals after attachment of the dye via
Sonogashira or CuAAC are provided below the XPS survey spectra.
Fig. 4a, the appearance of the iodine signal aer diazonium energy of 399.9 eV was detected by XPS. This peak can be
graing of 4-iodoaniline can be seen, as well as the appearance assigned to the sp2 hybridized nitrogen atoms that are present
of the nitrogen and sulfur signals from the dye aer Sonoga- in the push–pull chromophore. However, when CuAAC chem-
shira cross-coupling. Based on the ratio of the relative iodine istry was employed for surface functionalization, an additional
surface concentration before (0.8 at%) and aer (0.3 at%) the peak was detected at 401.1 eV. This peak is typically observed for
Sonogashira cross-coupling, the conversion efficiency of the 1,2,3-triazoles formed aer the azide–alkyne cycloaddition
secondary surface functionalization step was calculated to be reaction.45,46
approximately 63%. Similarly, Fig. 4b shows the graing of 4-
azidoaniline on H-terminated B:NCD lms and subsequent
3.4 Surface morphology and Kelvin probe measurements
‘clicking’ of the dye. Unfortunately, in this case no approxima-
tion of the reaction efficiency could be made since we are not
able to distinguish between the nitrogen species in the 1,2,3-
triazole linker and the dye. The high resolution S 2p peaks
(Fig. 4c and e) look similar for the two coupling strategies and
are composed of three main peaks at binding energies of 166.2,
165.4 and 163.9 eV. The two peaks of the deconvoluted high
resolution S 2p signal with lowest binding energy are related to
the S 2p1/3 (163.9 eV) and S 2p2/3 (165.4 eV) peaks of a S–C bond.
These signals arise from the two thiophenes in the dithieno-
pyrrole donor unit.36,38,53 As a result, the peak at 166.2 eV should
be attributed to the N–S–N bond in the benzothiadiazole
acceptor unit. The very weak peak located at 168.5 eV can
possibly be attributed to oxidized sulfur groups (SOx),36 while
the peak at 162.0 eV can be referenced to disuldes,54 which
could be formed during XPS irradiation of the samples.
Contrary to the S 2p signal, XPS analysis did reveal signicant
differences in the deconvoluted high resolution N 1s signal for
substrates functionalized via Sonogashira (Fig. 4d) and CuAAC
(Fig. 4f) chemistry. In both cases, a large N 1s peak at a binding
Kelvin probe measurements have been proven to be highly
useful to assess the photovoltage generation on various dia-
mond based opto-electronic devices, from Schottky photodi-
odes on boron doped diamonds to hybrid systems composed of
diamond and organic dyes.55,56 Kelvin probe force microscopy
(KPFM) images of H-terminated B:NCD and electrodes func-
tionalized with DTP-BT chromophores via Sonogashira cross-
coupling and CuAAC chemistry are shown in Fig. 5. The
surface potential of the bare H-B:NCD substrate correlates very
well with the surface morphology obtained by AFM (see
Fig. S4†). Crystalline grains are well resolvable as their facets
and edges have slightly different potentials. Yet the overall
potential variations remain within 5 mV RMS. Noticeably, the
potential variations remain about the same aer the chromo-
phore is graed to the diamond via CuAAC or Sonogashira
chemistry, as shown in the graph in Fig. 5d. However, the
surface potential landscape changes signicantly: the diamond
grains are not resolvable anymore (in spite of some potential
uctuations). This means that the surface potential is
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 33276–33290 | 33285

View Article Online
RSC Advances
Paper
Fig. 5 KPFM images of (a) a bare H-terminated B:NCD thin film, (b) a B:NCD film with the chromophore coupled via CuAAC chemistry, and (c)
a B:NCD film with the chromophore coupled via Sonogashira. The Z scale is set to 20 mV in all images to visualize subtle potential differences. (d)
Surface potential RMS roughness from the KPFM images. (e) Macroscopically measured surface photovoltages and (f) surface potential profiles in
time in dark (shaded area) and under illumination (white area) for the three sample types. The potential profiles are stacked along the y-axis for
better visibility. For Cu(I)AAC, CuI was used as the catalyst, while for Cu(Br)AAC, CuBr was used instead (according to Table 2).
homogenized by the chromophore graing (within 5 mV RMS). layer is very thin and does not noticeably affect the surface
It also shows that the surface is well covered by the graed morphology (as evidenced by Fig. S4†).
molecules that create a uniform layer. At the same time, the
33286 | RSC Adv., 2018, 8, 33276–33290
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
The photovoltages measured by scanning Kelvin probe (SKP)
RSC Advances
microscopy on the studied samples are shown in Fig. 5e. One
can see that the largest photovoltage was obtained for Sonoga-
shira coupling of the chromophore. It is also noticeable that the
lowest photovoltage is obtained for CuAAC (CuBr catalyst)
chemistry, which provided the best graing efficiency in terms
of XPS analysis. The photovoltage values were obtained from
multiple experiments, and the error bars correspond to the
standard deviations of the averaged values. Fig. 5f illustrates the
SKP proles taken in dark and under halogen light illumination
on each type of sample. From such proles, the photovoltage
data was extracted. The potential proles suggest that Sonoga-
shira cross-coupling would provide the most pronounced pho-
toresponse from the given chromophore dye.
3.5 Photoelectrochemical measurements
To complement the photovoltage measurements of the func-
tionalized electrodes, photoelectrochemical measurements
were performed using H-terminated, iodophenyl, azidophenyl
and DTP-BT functionalized B:NCD thin lms. The diamond
electrodes were employed as the working electrode in a three-
electrode voltammetry setup and they were illuminated with
chopped white light in an aqueous electrolyte solution of
methyl viologen (MV2+), which functioned as a redox mediator.
When performing chronoamperometric measurements (Fig. 6),
the electrodes were equilibrated in the dark for a short period
before switching the light source on and off for intervals of
approximately 10 seconds. Turning on the light source resulted
in the generation of a photocurrent, which was fully reversible.
Applying a negative bias was shown to positively inuence the
cathodic photocurrents as a net vectorial electron ow from the
cathode to the counter electrode via the redox electrolyte is
being generated. Hence, a negative bias of ꢀ0.2 V was applied
during chronoamperometry.57,58
Fig. 6 (top) clearly shows a good correlation between the
relative atomic contractions of S on the surface and the gener-
ated photocurrents under white light illumination of 0.9 sun.
The diamond electrode that revealed the lowest surface
coverage (1.1 at% S) aer Sonogashira cross-coupling had the
weakest photoresponse of ca. 200 nA cmꢀ2. Upon obtaining
a higher surface coverage of 1.8 at% S, the photocurrent
increased to 300 nA cmꢀ2, and the highest photocurrent
densities (450 nA cmꢀ2) were obtained for the diamond elec-
trode that had the highest amount of dye attached on the
surface (2.3 at% S). It should be mentioned that the initial
photocurrent decreases over a rather short period of time.
However, when illuminating for a few hours, one can see the
gradual return of the photocurrent to its original value
(Fig. S5†). The higher photocurrents correspond with an
increase of the absorption between 350 and 400 nm in the IPCE
spectrum (Fig. S6†). This ‘aging-activation’ phenomenon is re-
ported in all long-term studies on diamond-based photocath-
odes so far, but remains to be fully understood.32,34
Photocurrents were also measured for the sample that con-
tained a higher amount of surface contamination (0.6 at% Pd)
(Table 1, entry 5). Despite having a similar surface coverage (2.2
Fig. 6 Photocurrent densities (nA cmꢀ2) of H-terminated (B:NCD-H),
iodophenyl (B:NCD-I), azidophenyl (B:NCD-N3) and DTP-BT func-
tionalized (B:NCD-DTP-BT) diamond electrodes in a 5 mM methyl
viologen solution (in 0.1 M Na2SO4) under white light illumination (90
mW cmꢀ2; simulated AM 1.5G solar spectrum, 10 s 1 : 1 dark/light
interval) at a bias of ꢀ0.2 V. Surface functionalization was performed
via Sonogashira cross-coupling (top) and CuAAC (bottom).
at% S) to the electrode with the highest initial photocurrents
(Table 1, entry 4), the generated currents were about three times
lower. This gives a rst indication that the presence of palla-
dium has a negative effect on the photocurrent. In a control
experiment, no photocurrent was obtained upon solar illumi-
nation of H-terminated B:NCD, whereas the current for a iodo-
phenyl functionalized electrode was extremely small.
For the functionalization of the diamond electrodes via
CuAAC chemistry (Fig. 6, bottom), a similar relation between
surface coverage and photocurrent generation was observed.
Surprisingly, the observed currents are signicantly lower
compared to functionalization via Sonogashira, even though
the surface coverage is slightly higher and the system is
completely devoid of metal contamination (based on XPS).
When we compare the functionalization strategies, two major
differences are noted. The Sonogashira cross-coupling employs
an alkyne linker to attach the dye to the surface and aer the
reaction residual iodine groups remain on the surface. The
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 33276–33290 | 33287

View Article Online
RSC Advances
Paper
CuAAC reaction on the other hand employs a 1,2,3-triazole the photocurrent was observed from the best performing
linker and has remnant azide groups. Since the photocurrents samples, up to 600 and 240 nA cmꢀ2, respectively. However, for
generated by both iodophenyl and azidophenyl functionalized the Sonogashira cross-coupling, the difference in photocurrent
surfaces were negligible, we can assume that the type of residual between the electrodes with 1.1% S (Fig. 7, top, green curve) and
groups on the surface has a limited inuence on the photo- 1.8% S (Fig. 7, top, blue curve) becomes less pronounced. This
current. Therefore, the main difference in photocurrent gener- effect is related to the increasing dark currents at a lower bias as
ation is likely related to the employed linker. For now, the exact illustrated in the chopped-light polarization curve in Fig. S7.†
inuence of the alkyne and triazole linkers on the generated Additionally, the photocurrent decrease of the electrode with
photocurrents remains unknown. Possibly, the rigidity (advan- the highest initial photocurrent density (Fig. 7, top, cyan curve)
tageous for orbital overlap) and higher electron density on the is more pronounced.
alkyne could be more favorable for transporting the generated
To assess the potential of the Sonogashira cross-coupling for
hole (upon excitation of an electron from the HOMO to the the development of ‘all-carbon’ photovoltaics, we nally compared
LUMO of the dye) from the chromophore to the hole conducting our results with the functionalization of B:NCD thin lms via
B:NCD electrode, leading to lower interfacial charge combina- Suzuki cross-coupling.31 This reaction was previously employed to
tion and higher currents. Yet, further experiments and covalently link Br-BT-Rho and Br-CPDT-Fur dyes to a phenyl-
computational studies should be performed to conrm this.
boronate functionalized diamond surface. Photoelectrochemical
To further improve photocurrent generation, a more nega- measurements resulted in currents of 70 and 150 nA cmꢀ2 under
tive bias of ꢀ0.3 V was applied, as shown in Fig. 7. For both the white light illumination with an intensity of 0.15 sun (AM 1.5; 15
Sonogashira cross-coupling and CuAAC reaction, an increase of mW cmꢀ2) at a bias of ꢀ0.2 V, respectively. To facilitate compar-
ison with our results, we assumed a proportional relation between
the solar illumination intensity and photocurrent generation for
Br-BT-Rho and Br-CPDT-Fur. Doing so, these chromophores
should give currents of 420 and 900 nA cmꢀ2 under 0.9 sun illu-
mination (AM 1.5; 90 mW cmꢀ2). Compared to the Br-BT-Rho
functionalized electrodes, our system gives similar currents.
However, it should be mentioned that the Br-BT-Rho chromophore
is more suitable for diamond sensitization due to a better posi-
tioning of the HOMO of the dye with respect to the valence band
maximum (VBM) of the B:NCD photocathode and its higher molar
extinction coefficient (3 ¼ 43 200 Mꢀ1 cmꢀ1). The Br-CPDT-Rho
functionalized electrodes perform even better, as there is a larger
driving force for hole injection into the B:NCD electrode and since
this dye has an even higher molar extinction coefficient (3 ¼
73 000 Mꢀ1 cmꢀ1).31 Despite the less favorable optical properties of
the alkyne-DTP-BT dye, photocurrents within the same order of
magnitude were obtained for the Sonogashira cross-coupling. This
proves the strength of this approach for surface functionalization
and indicates that higher photocurrents are within reach.
4. Conclusions
A
mono-alkynylated donor–acceptor chromophore was
successfully synthesized and covalently anchored onto boron-
doped nanocrystalline diamond thin lms. The functionaliza-
tion was achieved through two different two-step functionali-
zation approaches, i.e. electrochemical diazonium graing and
subsequent Sonogashira cross-coupling or CuAAC chemistry. In
terms of surface functionalization, the CuAAC reaction was
superior, affording a slightly higher chromophore coverage in
the absence of surface contamination. However, in terms of
photoresponse, the Sonogashira cross-coupling resulted in
current densities (450 nA cmꢀ2 at ꢀ0.2 V) that are approximately
three times higher than those for the CuAAC reaction (140 nA
cmꢀ2 at ꢀ0.2 V). For the rst time we also showed photovoltages
generated in such hybrid systems, where again Sonogashira
cross-coupling of the chromophore provided the most
pronounced effect. Current work focuses on the development of
Fig. 7 Photocurrent densities (nA cmꢀ2) of DTP-BT functionalized
(B:NCD-DTP-BT) diamond electrodes in a 5 mM methyl viologen
solution (in 0.1 M Na2SO4) under white light illumination (90 mW cmꢀ2
;
simulated AM 1.5G solar spectrum, 10 s 1 : 1 dark/light interval) at a bias
of ꢀ0.3 V. Surface functionalization was performed via Sonogashira
cross-coupling (top) and CuAAC (bottom).
33288 | RSC Adv., 2018, 8, 33276–33290
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
alkynylated chromophores that have higher molar extinction 18 J. Wang, M. A. Firestone, O. Auciello and J. A. Carlisle,
coefficients and more optimal energy level alignment with
Langmuir, 2004, 20, 11450–11456.
regards to the diamond photocathode.
19 X. Wang, P. E. Colavita, J. A. Streifer, J. E. Butler and
R. J. Hamers, J. Phys. Chem. C, 2010, 114, 4067–4074.
20 X. Wang, E. C. Landis, R. Franking and R. J. Hamers, Acc.
Chem. Res., 2010, 43, 1205–1215.
Conflicts of interest
There are no conicts of interest.
21 R. Bogdanowicz, M. Sawczak, P. Niedzialkowski, P. Zieba,
B. Finke, J. Ryl, J. Karczewski and T. Ossowski, J. Phys.
Chem. C, 2014, 118, 8014–8025.
22 T. Nakamura and T. Ohana, Jpn. J. Appl. Phys., 2012, 51,
085201.
Acknowledgements
The authors thank UHasselt, the Research Foundation – Flan-
ders (FWO Vlaanderen), the Czech Science Foundation projects 23 Y. L. Zhong and K. P. Loh, Chem.–Asian J., 2010, 5, 1532–
13-31783S and 15-01809S, and the European Regional Devel- 1540.
opment Fund project CZ.02.1.01/0.0/0.0/15 003/0000464 for 24 D. Meziane, A. Barras, A. Kromka, J. Houdkova,
nancial support. J. Raymakers and S. Gielen thank the FWO for
their PhD fellowships. P. Verstappen is a postdoctoral fellow of
the FWO.
R. Boukherroub and S. Szunerits, Anal. Chem., 2012, 84,
194–200.
25 W. S. Yeap, M. S. Murib, W. Cuypers, X. Liu, B. van Grinsven,
M. Ameloot, M. Fahlman, P. Wagner, W. Maes and
K. Haenen, ChemElectroChem, 2014, 1, 1145–1154.
26 K. Natsui, T. Yamamoto, M. Akahori and Y. Einaga, ACS Appl.
Mater. Interfaces, 2015, 7, 887–894.
References
¨
1 B. O'Regan and M. Gratzel, Nature, 1991, 353, 737–740.
2 K. Kakiage, Y. Aoyama, T. Yano, K. Oya, J. Fujisawa and 27 S. Nakabayashi, N. Ohta and A. Fujishima, Phys. Chem.
M. Hanaya, Chem. Commun., 2015, 51, 15894–15897. Chem. Phys., 1999, 1, 3993–3997.
3 S. Mathew, A. Yella, P. Gao, R. Humphry-Baker, 28 Y. L. Zhong, K. P. Loh, A. Midya and Z.-K. Chen, Chem.
B. F. Curchod, N. Ashari-Astani, I. Tavernelli, Mater., 2008, 20, 3137–3144.
U. Rothlisberger, M. K. Nazeeruddin and M. Gratzel, Nat. 29 Y. L. Zhong, A. Midya, Z. Ng, Z. K. Chen, M. Daenen,
Chem., 2014, 6, 242–247.
4 L. Kavan, Curr. Opin. Electrochem., 2017, 2, 88–96.
M. Nesladek and K. P. Loh, J. Am. Chem. Soc., 2008, 130,
17218–17219.
5 C. J. Wood, G. H. Summers and E. A. Gibson, Chem. 30 W. S. Yeap, X. Liu, D. Bevk, A. Pasquarelli, L. Lutsen,
Commun., 2015, 51, 3915–3918.
6 A. Nattestad, A. J. Mozer, M. K. Fischer, Y. B. Cheng,
M. Fahlman, W. Maes and K. Haenen, ACS Appl. Mater.
Interfaces, 2014, 6, 10322–10329.
A. Mishra, P. Bauerle and U. Bach, Nat. Mater., 2010, 9, 31– 31 W. S. Yeap, D. Bevk, X. Liu, H. Krysova, A. Pasquarelli,
35.
D. Vanderzande, L. Lutsen, L. Kavan, M. Fahlman,
W. Maes and K. Haenen, RSC Adv., 2014, 4, 42044–42053.
32 H. Krysova, L. Kavan, Z. V. Zivcova, W. S. Yeap,
P. Verstappen, W. Maes, K. Haenen, F. Gao and
C. E. Nebel, RSC Adv., 2015, 5, 81069–81077.
33 H. Krysova, Z. Vlckova-Zivcova, J. Barton, V. Petrak,
M. Nesladek, P. Cigler and L. Kavan, Phys. Chem. Chem.
Phys., 2015, 17, 1165–1172.
7 A. Nattestad, I. Perera and L. Spiccia, J. Photochem. Photobiol.,
C, 2016, 28, 44–71.
8 Y. Farre, M. Raissi, A. Fihey, Y. Pellegrin, E. Blart,
D. Jacquemin and F. Odobel, ChemSusChem, 2017, 10,
2618–2625.
9 I. R. Perera, T. Daeneke, S. Makuta, Z. Yu, Y. Tachibana,
A. Mishra, P. Bauerle, C. A. Ohlin, U. Bach and L. Spiccia,
Angew. Chem., Int. Ed. Engl., 2015, 54, 3758–3762.
10 K. Kobashi, K. Nishimura, Y. Kawate and T. Horiuchi, Phys.
Rev. B: Condens. Matter Mater. Phys., 1988, 38, 4067–4084.
34 H. Krysova, J. Barton, V. Petrak, R. Jurok, M. Kuchar,
P. Cigler and L. Kavan, Phys. Chem. Chem. Phys., 2016, 18,
16444–16450.
11 R. J. Nemanich, J. A. Carlisle, A. Hirata and K. Haenen, MRS 35 R. Chinchilla and C. Najera, Chem. Soc. Rev., 2011, 40, 5084–
Bull., 2014, 39, 490–494. 5121.
12 J. V. Macpherson, Phys. Chem. Chem. Phys., 2015, 17, 2935– 36 J. Raymakers, A. Artemenko, S. S. Nicley, P. Stenclova,
ˇ
´
2949.
A. Kromka, K. Haenen, W. Maes and B. Rezek, J. Phys.
Chem. C, 2017, 121, 23446–23454.
37 M. Meldal and C. W. Tornoe, Chem. Rev., 2008, 108, 2952–
3015.
38 M. Wang, M. R. Das, M. Li, R. Boukherroub and S. Szunerits,
J. Phys. Chem. C, 2009, 113, 17082–17086.
13 C. H. Y. X. Lim, Y. L. Zhong, S. Janssens, M. Nesladek and
K. P. Loh, Adv. Funct. Mater., 2010, 20, 1313–1318.
14 J. Stotter, Y. Show, S. Wang and G. Swain, Chem. Mater.,
2005, 17, 4880–4888.
`
15 P.-N. Volpe, J. Pernot, P. Muret and F. Omnes, Appl. Phys.
`
`
Lett., 2009, 94, 092102.
39 S. Ghodbane, D. Ballutaud, F. Omnes and C. Agnes, Diamond
Relat. Mater., 2010, 19, 630–636.
16 Y. R. Leroux, H. Fei, J. M. Noel, C. Roux and P. Hapiot, J. Am.
Chem. Soc., 2010, 132, 14039–14041.
17 J. Pinson and F. Podvorica, Chem. Soc. Rev., 2005, 34, 429–
439.
40 A. Gupta, M. M. A. Kelson, V. Armel, A. Bilic and
S. V. Bhosale, Tetrahedron, 2014, 70, 2141–2150.
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 33276–33290 | 33289

View Article Online
RSC Advances
Paper
´
´
41 S. D. Janssens, P. Pobedinskas, J. Vacik, V. Petrakova, 49 P. J. Cumpson and M. P. Seah, Surf. Interface Anal., 1997, 25,
´
B. Ruttens, J. D'Haen, M. Nesladek, K. Haenen and
430–446.
P. Wagner, New J. Phys., 2011, 13, 083008.
50 T. Meinhardt, D. Lang, H. Dill and A. Krueger, Adv. Funct.
Mater., 2011, 21, 494–500.
51 M. R. Das, M. Wang, S. Szunerits, L. Gengembre and
R. Boukherroub, Chem. Commun., 2009, 25, 2753–2755.
42 S. Drijkoningen, S. D. Janssens, P. Pobedinskas, S. Koizumi,
M. K. Van Bael and K. Haenen, Sci. Rep., 2016, 6, 35667.
43 L. E. Polander, L. Pandey, S. Barlow, S. P. Tiwari, C. Risko,
´
´
´
B. Kippelen, J.-L. Bredas and S. R. Marder, J. Phys. Chem. 52 S. Dıez-Gonzalez, Catal. Sci. Technol., 2011, 1, 166–178.
C, 2011, 115, 23149–23163.
44 H. Zhou, L. Yang and W. You, Macromolecules, 2012, 45, 607–
632.
53 E. O. Sako, H. Kondoh, I. Nakai, A. Nambu, T. Nakamura and
T. Ohta, Chem. Phys. Lett., 2005, 413, 267–271.
54 X. Chen, Y. Peng and D. Bradshaw, Int. J. Miner. Process.,
45 R. D. Rohde, H. D. Agnew, W. S. Yeo, R. C. Bailey and
J. R. Heath, J. Am. Chem. Soc., 2006, 128, 9518–9525.
46 A. C. Gouget-Laemmel, J. Yang, M. A. Lodhi, A. Siriwardena,
2013, 125, 129–136.
ˇ
´
55 J. Cermak, Y. Koide, D. Takeuchi and B. Rezek, J. Appl. Phys.,
2014, 115, 053105.
ˇ
´
ˇ
´
D. Aureau, R. Boukherroub, J. N. Chazalviel, F. Ozanam and 56 B. Rezek, J. Cermak, A. Kromka, M. Ledinsky and J. Kocka,
S. Szunerits, J. Phys. Chem. C, 2012, 117, 368–375.
Diamond Relat. Mater., 2009, 18, 249–252.
47 T. Heinrich, C. H. H. Traulsen, E. Darlatt, S. Richter, 57 D. Hirayama, K. Takimiya, Y. Aso, T. Otsubo, T. Hasobe,
J. Poppenberg, N. L. Traulsen, I. Linder, A. Lippitz,
P. M. Dietrich, B. Dib, W. E. S. Unger and C. A. Schalley,
RSC Adv., 2014, 4, 17694–17702.
H. Yamada, H. Imahori, S. Fukuzumi and Y. Sakata, J. Am.
Chem. Soc., 2002, 124, 532–533.
58 T. Otsubo, Y. Aso and K. Takimiya, J. Mater. Chem., 2002, 12,
2565–2575.
48 W. P. Gallagher and A. Vo, Org. Process Res. Dev., 2014, 19,
1369–1373.
33290 | RSC Adv., 2018, 8, 33276–33290
This journal is © The Royal Society of Chemistry 2018