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K. Wang et al. / Tetrahedron xxx (xxxx) xxx
because of their intense fluorescence after the deprotonation of
lactam bond and rich modification sites [13e16]. They also can be
used as optoelectronic device applications including organic field
effect transistors (OFETs), organic solar cells (OSCs), organic light-
emitting diodes (OLEDs) owing to the excellent properties of the
thermal and photostability [17e24], planarity of the chromophore
and ambipolar charge carrier transport. The planarity and electron-
deficient pyrrole site of DPP dyes are exactly what the leveling
agent needs, the planar electron-deficient region may exert selec-
tive adsorption on high current density cathode surface and
leveling the deposition behavior. Unfortunately, the strong inter-
heterocyclic units onto DPP to alleviate this twisted geometry [29],
so the DPP derivatives with furan and thiophene units were syn-
thesized and at the same time the effect of selenium in lowering the
lowest unoccupied molecular orbitals (LUMOs) energy level was
also explored [30,31]. Moreover, the decrease in the bandgap also
can be attributed to the decrease in aromaticity and an increase in
quinoidal character in the selenophene derivative [31]. TDPP was
synthesized following the reported literature [12], selenophene-2-
carbonitrile was prepared as described by Martin Heeney [30]. The
synthetic route of selenophene-2-carbonitrile reaction with diethyl
succinate in the presence of sodium is the same with TDPP. The
obtained deep red DPP derivatives had low solubility because of the
strong intermolecular interactions between the NeH and C]O
groups. The alkyl chains were connected to the nitrogen atoms of
the DPP unit in the presence of CH3CN with 1,6-dibromohexane and
potassium tert-butoxide at 80 ꢀC. After the N-alkylation, the ob-
tained four DPP derivatives were done further reacted with the
quaternary ammonium salts at the temperature of 80 ꢀC, with
CH3CN as solvent, NMe3$HCl as nucleophilic reagent and NaHCO3
as base. After that, the column chromatogram was carried out to
purify the crude product then the target products (p-ClDPP-QAS,
FDPP-QAS, TDPP-QAS and SeDPP-QAS) were given and the main
synthetic routes are shown in Scheme 1. The detailed synthetic
routes and characterization are provided in the supporting
information.
molecular hydrogen bond and p-p stacking properties lead to poor
water solubility that impedes the usage of DPP pigments as leveler
in copper electroplating. In our previous work, the strategy of
quaternary ammonium salt was introduced to significantly
enhance the water solubility of Cl-DPP (Chlorine substituted DPP)
via grafted on amide site and successfully applied into printed
circuit boards (PCBs) electroplating [25]. Afterwards, the aryl-
modification strategy was proposed to enhance the adsorption
performance of DPP-based levelers [26]. However, the structure-
performance correlation is not clear and the mentioned aryl-
modification strategy is not very satisfactory because the most
effective leveler CF3-DPP (trifluoromethyl substituted DPP) still
shows relative larger bandgap.
Herein, the bandgap engineering strategy was proposed by
substituting the conventional aryl groups of DPP with varies of
heterocycles of to obtain the more efficient leveler with smaller
bandgap. To be specific, four DPP-based quaternary ammonium
salts (compounds p-ClDPP-QAS, FDPP-QAS, TDPP-QAS and SeDPP-
QAS) were designed and synthesized with different aromatic
rings of DPP skeletons as levelers in copper electroplating [27]. The
electrochemical behaviors of these DPP derivatives were also
characterized by a series of electrochemical tests containing Gal-
vanostatic measurements (GMs), Cyclic voltammetry (CV) and
potentiodynamic polarization curves to analyze the effects of ad-
ditives on cathodic polarization. Besides, the wetting performance
of four DPP derivatives were evaluated by contact angle. The results
show that the inhibiting abilities of FDPP-QAS, TDPP-QAS and
SeDPP-QAS on copper electrodeposition increasingly improve and
these three novel levelers are all better than the previously re-
ported p-ClDPP-QAS [19]. To explain why there are different
inhibiting abilities of each derivative, contact angle, XPS and
quantum chemical calculation were carried out. The electronic
properties and molecule orbital information well explained the
inhibiting ability. According to these measurements, SeDPP-QAS
was chosen as representative leveler for through-holes electro-
plating process to evaluate the practical leveling performance. In
addition, its influence on the surface morphology was also inves-
tigated by field-emission scanning electron microscopy (FE-SEM)
and X-ray diffraction (XRD).
2.2. Characterization
1H NMR and 13C NMR spectra were recorded with a Bruker
Avance 400 spectrometer at 400 MHz and 100 MHz respectively.
Tetramethylsilane was used as an internal reference and CDCl3 or
CD3OD as a solvent. The chemical shifts (d) and coupling constants
(J) were expressed in parts per million and hertz, respectively. A
Micromass GCTTM mass spectrometer was used to record the mass
(see the Supporting Information) spectra. Chemicals were
commercially available and used without purification. Chroma-
tography: column chromatography was performed with silica gel or
aluminum oxide, AR, neutral (200e300 mesh ASTM). Contact angle
was tested by Shanghai Zhong Chen Contact Angle Measuring In-
strument (JC2000D). A PGSTAT302N (Auto-Lab) was used for all
electrochemical tests. Plating performances were evaluated by the
cross-section images detected by field-emission scanning electron
microscopy (FE-SEM, Hitachi S3400-N). Photoluminescence spectra
were examined using a Varian Cary Eclipse spectrophotometer.
Absorption spectra were measured on a Thermo UVeVis spectro-
photometer. XPS were tested by Rotating Anode X-ray Powder
Diffractometer(18KW/D/max2550VB/PC).
2.3. Electrochemical measurements
We used a 3 mm diameter platinum rotating disk electrode (Pt-
RDE) as a base for the working electrode (WE). A Pt stick with a
diameter of 2 mm was used as counter electrode (CE), and a silver/
silver chloride electrode (Ag/AgCl) was served as the reference
electrode (RE). The composition of the base electrolyte was 60 mg/L
NaCl, 60 g/L CuSO4$5H2O and 200 g/L H2SO4 for all electrochemical
tests. Polyethylene glycol (PEG, MW ¼ 10000, SIGMA), NaCl (Fisher,
Certified ACS), and bis(3-sulfopropyl) disulfide (SPS; Jiangsu
Mengde New Materials Technology Co., Ltd.) were additives. The
four DPP derivatives used as levelers in this work were p-ClDPP-
QAS, FDPP-QAS, TDPP-QAS, and SeDPP-QAS, respectively.
2. Experimental
2.1. Synthesis of the additives
The DPP derivatives with different aromatic ring units at 3 and 6
positions of the pyrrolopyrrole core were synthesized from an aryl
or heteroaryl nitrile (Ar-CN) and succinate. The synthesis routes are
shown in Scheme 1. p-ClDPP was prepared according to the syn-
thetic methodology reported in the previous literature [20]. The
presence of space steric caused by benzene ring at the 3 and 6
positions of the DPP core enhances the repulsive interaction with
the adjacent lactam ring resulting in a distorted molecular geom-
etry [28]. In order to attain a highly co-planar molecular backbone
A constant scan rate of 50 mV/s was used for cyclic voltammetry
experiments. The domain of study ranged from 1.57 to 0.20 V. A
thin copper layer with a thickness of 500 nm was pre-deposited
for efficient
p
-p
stacking, we introduced five membered
onto the Pt-RDE in
a
pre-deposition bath before each
Please cite this article as: K. Wang et al., Engineering aromatic heterocycle strategy: Improving copper electrodeposition performance via tuning