Synthesis of water-soluble Ni(II) complexes and their role in photo-induced electron transfer with MPA-CdTe quantum dots

Article abstract of DOI:10.1007/s11120-019-00668-z

Abstract: Photocatalytic water splitting using solar energy for hydrogen production offers a promising alternative form of storable and clean energy for the future. To design an artificial photosynthesis system that is cost-effective and scalable, earth a

Full text of DOI:10.1007/s11120-019-00668-z

Photosynthesis Research
https://doi.org/10.1007/s11120-019-00668-z
ORIGINAL ARTICLE
Synthesis of water‑soluble Ni(II) complexes and their role
in photo‑induced electron transfer with MPA‑CdTe quantum dots
1
2
2
1
Niharika Krishna Botcha · Rithvik R. Gutha · Seyed M. Sadeghi · Anusree Mukherjee
Received: 31 May 2019 / Accepted: 13 August 2019
©
Springer Nature B.V. 2019
Abstract
Photocatalytic water splitting using solar energy for hydrogen production oꢀers a promising alternative form of storable and
clean energy for the future. To design an artiꢁcial photosynthesis system that is cost-eꢀective and scalable, earth abundant
elements must be used to develop each of the components of the assembly. To develop artiꢁcial photosynthetic systems, we
need to couple a catalyst for proton reduction to a photosensitizer and understand the mechanism of photo-induced electron
transfer from the photosensitizer to the catalyst that serves as the fundamental step for photocatalysis. Therefore, our work
is focused on the study of light driven electron transfer kinetics from the quantum dot systems made with inorganic chalco-
genides in the presence of Ni-based reduction catalysts. Herein, we report the synthesis and characterization of four Ni(II)
complexes of tetradentate ligands with amine and pyridine functionalities (N2/Py2) and their interactions with CdTe quantum
dots stabilized by 3-mercaptopropionic acid. The lifetime of the quantum dots was investigated in the presence of the Ni
complexes and absorbance, emission and electrochemical measurements were performed to gain a deeper understanding of
the photo-induced electron transfer process.
Graphic abstract
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11120-019-00668-z) contains
supplementary material, which is available to authorized users.
Extended author information available on the last page of the article
Vol.:(0
1
123456789)
3

Photosynthesis Research
Keywords Nickel complexes · Quantum dots · Photo-induced electron transfer · Biomimetic systems · Artiꢁcial
photosynthesis
Abbreviations
absorption of light (ii) charge transfer from the photosensi-
tizer to a catalyst; and (iii) chemical reactions driven by the
redox potential stored in the catalyst to couple that electron
transfer to chemical bond formation (Wang et al. 2015). To
develop highly eꢂcient artiꢁcial photocatalytic systems,
the key issue to address is the right and eꢀective combi-
nation of the photosensitizer and the catalyst (Wang et al.
2015). The photosensitizers must have good stability, broad
spectral absorptions in the visible light region to maximize
the use of solar energy, and long excited state lifetimes for
eꢀective charge separation. Early eꢀorts on ꢁnding photo-
sensitizers relied heavily on molecular light absorbers such
AP
Artiꢁcial photosynthesis
QDs
MPA
Quantum dots
3-Mercaptopropionic acid
UV–Vis Ultraviolet-Visible
K
k
Stern–Volmer quenching constant
Bimolecular quenching rate constant
sv
q
TCSPC Time-correlated single-photon counting
E
E
E
Bandgap
g
Valence band energy level
Conduction band energy level
Normal hydrogen electrode
Nuclear magnetic resonance
vb
cb
NHE
2
+
NMR
as [Ru(bpy) ] and other organic dyes such as ꢃuorescein
3
ESI–MS Electrospray ionization mass spectrometry
and eosin Y, however, in the past two decades quantum dots
as potential light harvesters have garnered signiꢁcant atten-
tion (Li et al. 2018). The advantages oꢀered by quantum
dots (QDs) include their unique size-dependent absorption
properties, large absorption cross-sections over a broad spec-
tral range, long exciton lifetimes and superior photostability
(Gross et al. 2014; Liu et al. 2015; Sadeghi et al. 2018, 2019;
Wu et al. 2014).
RT
Room temperature
Introduction
The increasing global demand for energy has compelled the
need to look for a clean alternative source that is safe and
sustainable like solar energy (Gust et al. 2001). The energy
provided by the sun in an hour is more than that consumed
by the world in a year (Wu et al. 2014). Solar energy has the
unique potential to sustain the ever-growing energy needs
of humanity in the most environmentally benign manner
One of the strategies for sustainable artiꢁcial photosyn-
thetic systems are homogeneous molecular catalytic systems
with earth-abundant elements for the reduction of water
because of their ease of synthesis, tunable redox properties
and their cost-eꢀectiveness (Artero and Fontecave 2013;
Du and Eisenberg 2012; Fukuzumi et al. 2013; Wang et al.
2015). Over the past two decades, molecular catalysts, espe-
cially biomimetic complexes composed of earth abundant
elements such as Fe, Co and Ni, have been developed and
(
Vullev 2011). However, the intermittent and diꢀuse nature
of the sunlight mandates the need to store solar energy in a
cost-eꢀective way (Mulfort and Utschig 2016). Photosynthe-
sis provides an excellent paradigm to store solar energy in
the form of chemical bonds. This has inspired scientists to
design artiꢁcial photosynthesis (AP) systems that will cap-
ture and convert solar energy to chemical energy by under-
standing the processes in natural photosynthesis and mim-
icking their solar energy conversion mechanisms in simple
man-made models and systems, but with better eꢂciencies
for future scale-up applications (Kaerkaes et al. 2014; Kalya-
nasundaram and Graetzel 2010; Kim et al. 2015; Mao and
Shen 2013; Zhou et al. 2016). Over the past, several decades,
several AP systems have been designed and developed for
the utilization of sunlight and eꢂcient production of solar
fuels by mimicking the key structural elements and functions
of natural photosynthesis (Utschig et al. 2015; Xu and Zhang
studied for the catalytic proton reduction for H generation
2
(Berardi et al. 2014; Wen and Li 2013; Xu and Zhang 2015).
Particularly Ni-based molecular catalytic system supported
by N2P2 ligands has shown excellent proton reduction capa-
bilities under electrocatalytic conditions (Helm et al. 2011;
Kilgore et al. 2011; Wilson et al. 2006, 2007). These suc-
cessful DuBois catalysts have led to the design of several
AP systems where Ni or its complexes were coupled to dif-
ferent light-harvesting materials (Gross et al. 2014; Silver
et al. 2013).
In the last decade, there has been growing interest to
design AP systems using QDs as photosensitizers and
hydrogenases or their functional mimics as the catalysts
in water (Brown et al. 2010, 2012; Gimbert-Surinach et al.
2014; Greene et al. 2012; Li et al. 2013a, b; Nann et al.
2010; Wang et al. 2011) Water-soluble CdTe QDs serve
as excellent photosensitizers because of their high extinc-
tion coeꢂcients as well as the intrinsic advantages they
oꢀer such as the high-photoluminescence quantum yields
2
015). The high speciꢁc enthalpy of combustion as well as
water being the benign combustion product make H an ideal
2
solar fuel (Armaroli and Balzani 2007; Esswein and Nocera
2
007; Wang and Li 2017; Wu et al. 2014).
There are three fundamental steps in an eꢂcient AP sys-
tems: (i) charge separation in a photosensitizer due to the
1
3

Photosynthesis Research
and the quantum conꢁnement eꢀects (Gaponik et al. 2002;
Gimbert-Surinach et al. 2014; Shi et al. 2006; Zhang et al.
(Kankanamalage et al. 2019). In the current work water-sol-
uble CdTe quantum dots stabilized by 3-mercaptopropionic
acid (MPA-CdTe QDs) were selected as the photosensitizer
due to their aqueous dispersion and economical advantage
(Li et al. 2013a; Wang et al., 2011). This work focuses on
understanding the kinetics of photoinduced electron transfer
of these QDs in the presence of complexes 1–4 to pave the
way to develop a better multicomponent AP system compris-
ing inorganic chalcogenide-based QDs and earth-abundant
transition metal complexes.
2
003). Speciꢁcally, the assemblies of MPA-CdTe QDs (or
MPA-CdSe QDs) along with molecular catalysts in aque-
ous solutions are able to photocatalytically produce H with
2
exceptional activity and impressive durability under visible
light irradiation (Huang et al. 2012; Wang et al. 2014; 2015;
Wu et al. 2014; Zhou et al. 2017). However, most of these
reports focus on performance of the photocatalytic system
toward hydrogen production very little is known about the
kinetics of photo-induced electron transfer, crucial for eꢀec-
tive photocatalysis.
To improve eꢂciency of an existing AP system and to
design the next-generation system, it is essential to under-
stand the mechanism of photoinduced electron transfer from
the photosensitizer to the catalyst. This understanding will
ensure a proper design of an AP system that is a water-
soluble, scalable and sustainable alternative. In the present
work we report synthesis and characterization of four water-
soluble mononuclear nickel complexes (Fig. 1) supported
by tetradentate ligands with amine and pyridine function-
alities (N2/Py2). Complexes 1–4 were carefully designed
to tune their electronic and steric properties. In a separate
study, we have shown that complexes 1 and 2 both can pro-
Results and discussion
The syntheses of the ligands were performed by follow-
ing the published procedures (Pella et al. 2018; Singh et al.
2017). Complexes 1–4 were synthesized by mixing equimo-
lar amounts of the nickel salt with the respective ligands. In
water, complexes 1–4 all gave faintly green-colored solutions
with optical bands in the visible region with weak intensities
(Fig. 2). All absorption data including the peak position and
molar absorptivity are summarized in the Table 1.
The optical spectrum of complex 1 matches with previ-
ously reported spectral data (Singh et al. 2017). The spectral
features of the complexes 1–4 are consistent with six-coordi-
nate octahedral geometry for Ni(II) (Guadalupe Hernandez
et al. 2006; Liu et al. 2012; Mautner et al. 2009; Singh et al.,
duce H from proton reduction under electrocatalytic and
2
photocatalytic conditions using organic and inorganic dyes
2
017). The band in the near-UV region is characteristic of
an intra-ligand charge transfer transition (Anacona et al.
009). In the visible region, all the Ni(II) complexes exhibit
two d–d bands with low intensities around 564–568 nm and
20–934 nm which can be adequately represented by the
2
9
Fig. 2 Optical spectra of complexes 1 (black solid line), 2 (red-
dashed line), 3 (blue dash-dotted line) and 4 (green-dotted line) in
water using a quartz cuvette with 1 cm path length. All complexes are
dissolved in water to give an 8 mM concentration
Fig. 1 Synthesized nickel complexes 1–4 with bispicen (N2/Py2)
ligand system
1
3

Photosynthesis Research
Table 1 Optical and
electrochemical parameters for
complexes 1–4
Complex Peak posi- Molar absorp- Peak posi- Molar absorp- Peak
Epc versus NHE (V)
−1
−1
tion λ
tivity (M
tion λ
(nm)
tivity (M
shoulder λ
max
max
−1
−1
(
nm)
cm
)
cm
)
(nm)
1
2
3
4
566
569
564
568
5
7
14
6
922
934
922
920
10
13
16
11
400, 790
400, 790
450, 790
390, 790
−0.221
−0.259
−0.275
−0.313
3
3
3
A
A
to T (F) transition in the 550–800 nm range and the
the addition of the Ni complex 1 (Fig. S4a) showed no
remarkable changes suggesting that the structure and prop-
erties of the MPA-CdTe QDs were not impacted upon the
addition of the complex. However, the emission intensity
at 638 nm was dramatically quenched upon the addition
of the complexes (Fig. 3).
2
g
g
1g
3
to T (F) transition in the 850–1100 nm range (Guru-
2
2g
moorthy et al. 2016; Liu et al. 2012; Mautner et al. 2009;
Sadhu et al. 2015; Singh et al. 2017). The optical spectra
of the complexes 1–4 match reasonably well with the other
8
reported complexes of high spin d Ni(II) in an octahedral
geometry (Singh et al. 2017).
Furthermore, the luminescence quenching could be
directly correlated to the concentration of the complex 1
with maximum quenching to be observed with 500 equiva-
lents. The concentration dependent quenching was fur-
ther investigated using Stern–Volmer equation for 1, that
The reduction potentials of the Ni complexes 1–4 were
measured using cyclic voltammetry (Fig. S1a–d). The com-
plexes were scanned from +1 V to −1 V in water with 0.1 M
KCl as the supporting electrolyte with a scan rate varied
−
1
10
−1 −1
from 100 to 500 mV s . All four Ni complexes exhibited
cathodic waves corresponding to the Ni(II)/Ni(I) reduction
yielded a quenching rate constant of 5.4 × 10
M
s .
Similar eꢀects were observed for the other three nickel
complexes 2, 3 and 4. In all the cases, optical bands of
the QDs remain unchanged upon addition of the complex
solution (Fig. S4b–d) but emission spectra showed steady
decline in emission intensity (Fig. S5a–c). Table 2 lists the
(
Kankanamalage et al. 2019; Zilbermann et al. 2005) around
−
0.509 V versus Ag/AgCl for 1, −0.547 V for 2, −0.563 V
for 3, and −0.601 V for 4, respectively, well within the range
of other known Ni complexes. The potentials with respect
to the normal hydrogen electrode (NHE) for the complexes
Stern–Volmer quenching constant K and the bimolecu-
sv
1
–4 are listed in Table 1.
lar quenching rate constant k for all the complexes 1–4
q
The MPA-CdTe QDs were characterized by measuring
(Fig. S6a–d). Stern–Volmer quenching constant for all the
complexes are reasonably similar. Similarly, bimolecular
rate constants for 1, 2, 3 and 4 are all in the same order
further corroborating their similar potential as eꢀective
quenchers.
their UV–Vis absorption spectra and the luminescent spec-
tra as shown in the Figs. S2 and S3, respectively (see SI).
The concentration of the MPA-CdTe QDs was determined as
−
6
−1
2
.8×10 M (1 mg mL ) using the Beer–Lambert law (Fig.
S2). On the basis of the absorption band centered at 620 nm,
the particle size of the MPA-CdTe QDs was determined to
be 3.9 nm using the Eqs. S1 and S2 (see SI) (Yu et al. 2003).
The bandgap of CdTe is narrow (around 1.44 eV), and so
all wavelengths of visible light provide suꢂcient energy for
the excitation of the electrons (Amelia et al. 2012; Haram
et al. 2011).
Luminescence quenching
The MPA-CdTe QDs exhibit absorption bands in the range
of 550–650 nm (Fig. S2). The photoluminescence of the
MPA-CdTe QDs were measured in water at room tem-
perature following excitation at 514 nm. These QDs are
luminescent at room temperature with emission maximum
at 638 nm (Fig. S3). To probe impact of luminescent inten-
sity upon addition of nickel complexes, solution of com-
plexes 1–4 in water was added to the solutions of MPA-
CdTe QDs. Optical spectra of the MPA-CdTe QDs upon
−
6
Fig. 3 Luminescence spectra of MPA-CdTe QDs (1.4×10 M,
−1
0
.5 mg mL ) with increasing concentration of complex 1 (0 to
−3
0.75×10 M) in water (excitation wavelength: 514 nm)
1
3

Photosynthesis Research
Table 2 The Stern–Volmer quenching constant Ksv and the Bimolec-
ular quenching rate constant k for complexes 1–4
q
Complexes
Stern–Volmer quenching Bimolecular quenching
−1
−1
constant K (M
)
rate constant k (M
q
−1
sv
s
)
3
10
10
10
10
1
2
3
4
1.3×10
5.4×10
8.0×10
7.8×10
3.7×10
3
1.9×10
3
1.9×10
2
8.5×10
Luminescence decay
A number of processes could lead to the quenching, such
as energy transfer, electron transfer or complex formation.
Because of the minimal spectroscopic overlap of the absorp-
tion of the complexes 1–4 (Fig. 1) and the emission of the
MPA-CdTe QDs (Figure S3), the quenching could not be
attributed to the energy transfer process from the excited
MPA-CdTe QDs to the complexes 1–4. Therefore, the
luminescence quenching most likely arises due to electron
transfer from the excited MPA-CdTe QDs to the complexes
−
6
Fig. 4 The luminescence decay of MPA-CdTe QDs (1.4×10 M,
−1
0
.5 mg mL ) with increasing concentrations of complex 1 (0 to
.75×10− M) in water (excitation wavelength: 450 nm)
3
0
of the original lifetime. All other complexes showed similar
concentration dependence on the decay rate (Fig. S7a–c) and
signiꢁcant faster decay as summarized in Table 3. Therefore,
in all the four cases the decay of the MPA-CdTe QDs in the
presence of the nickel complexes 1–4 were almost twice as
fast.
1–4. To further corroborate the above data, we measured the
luminescence lifetime of the MPA-CdTe QDs using time-
correlated single-photon counting (TCSPC) system and
monitored lifetime changes in the presence of Ni complexes.
Analysis of the decay revealed that the luminescence lifetime
of the MPA-CdTe QDs was shortened considerably with the
progressive addition of the complexes 1–4 (Table 3). Since
we observe a decrease in the lifetime of the MPA-CdTe QDs
in the presence of the complexes 1–4, we can eliminate the
possibility of complex formation between them.
To establish that the photoluminescence quenching and
luminescence decay rate are not a result of the concentration
change of the QDs solution, experiments were performed
where MPA-CdTe QDs solutions were diluted with the addi-
tion of water achieving the concentrations of 0.15 mM to
0.75 mM (Fig. S8a, b). These control experiments showed
no appreciable change in the luminescence intensity of the
MPA-CdTe QDs in the presence of added water. No sig-
niꢁcant quenching was observed upon addition of water,
moreover no changes in lifetime decay in the absence of
the complexes 1–4 (Table S2). These control experiments
demonstrate that electron transfer from the QDs to the Ni
complexes is responsible for the change in excited state
dynamics as measured with the luminescence quenching
and decay experiments.
The decay lifetime was best ꢁt by a biexponential func-
tion (Table S1a–d). The average lifetime of MPA-CdTe QDs
in water at room temperature was 24 ns. It was observed that
on the addition of the complex 1 to the solution of the QDs
resulted in faster decay with concentration dependence sug-
gesting electron transfer in the multicomponent molecular
system with freely diꢀusing donors and acceptors (Fig. 4).
It is observed that the lifetime of the QDs with the addition
of 1 falls from 24 to 13 ns which is a decrease to almost half
Table 3 Mean lifetimes of the
Mean lifetime for
(ns)
Mean lifetime for
2 (ns)
Mean lifetime for
3 (ns)
Mean
quantum dot in the presence
1
lifetime for
of 1–4
4
(ns)
QD
24
16
14
14
13
13
24
15
13
12
12
11
24
16
14
13
12
12
24
19
17
16
15
15
QD+0.15 mM
QD+0.30 mM
QD+0.45 mM
QD+0.60 mM
QD+0.75 mM
1
3

Photosynthesis Research
were able to reduce lifetime of QDs suggesting that they can
accept electrons from the quantum dots. Surprisingly, chang-
ing the ligand architecture by varying the donation property
of the pyridine group (1 vs. 2) or having a rigid backbone (1
compared to 3 and 4) did not lead to signiꢁcant change in
the decay lifetimes (Fig. 5, Table 1). Future work includes
the addition of the sacriꢁcial electron donor to complete the
three-component system and investigate whether the ligands
have an impact on the amount of hydrogen produced, despite
having similar eꢀects in the photo-induced electron transfer
rates.
Experimental section
Fig. 5 Potentials (vs. NHE) of the excited MPA-CdTe QDs and the
cathodic peak potentials (vs. NHE) of the complexes 1–4
Materials and methods
All reagents and solvents were purchased from Millipore-
Sigma and Fisher Scientiꢁc and directly used for synthesis
without further puriꢁcation unless otherwise mentioned.
1,2-dimethylcyclohexane-1,2-diamine was obtained from
Alfa Aesar. Water-soluble Cadmium telluride quantum dots
stabilized by 3-mercaptopropionic acid (MPA-CdTe QDs)
were purchased from NNCrystal US Corp. Deuterated sol-
vents used for Nuclear magnetic resonance (NMR) spectros-
copy were purchased from Cambridge Isotope Laboratories,
Inc.
Therefore, the luminescence quenching and decay of
MPA-CdTe QDs can be attributed to electron transfer from
the excited MPA-CdTe QDs to the Ni complexes 1–4. To
further conꢁrm this hypothesis, we used a combination of
spectroscopic and electrochemical studies. The bandgap (E )
g
calculated from the absorption spectra of the MPA-CdTe
QDs is 2 eV (Fig. S2) (Bae et al. 2004; Gimbert-Surinach
et al. 2014; Poznyak et al. 2005). The anodic peak potential
of the MPA-CdTe QDs (Fig. S9) is +0.907 V versus Ag/
AgCl, which gives an estimated value for the valence band
1
H NMR experiments were performed in deuterated
energy level (E ) as +1.2 V versus NHE (Gimbert-Surinach
solvents on a Varian Inova-500 MHz instrument at 25 °C.
Electrospray ionization mass spectrometry (ESI–MS)
experiments were performed on a Thermo Scientiꢁc LTQ
Orbitrap XL Hybrid FT Mass Spectrometer in positive ioni-
zation mode. Elemental analyses were performed by Atlan-
tic Microlab, Inc. (Norcross, GA). Electronic absorption
spectra were measured on an Agilent Cary 8454 UV–Vis
spectrometer in a quartz cuvette with a 5 mm path length.
Electrochemical measurements were carried out on a BASi-
C3 Epsilon electrochemical instrument using a glassy car-
bon working electrode, a platinum wire auxiliary electrode,
and Ag/AgCl reference electrode. All cyclic voltammetry
experiments were performed under nitrogen atmosphere
using 0.1 M KCl as the supporting electrolyte. Graphical
representation and data analysis were performed using Ori-
gin program.
vb
et al. 2014; Rajh et al. 1993; Wang et al. 2011). Utilizing
the values for E and E , the calculated value for the con-
vb
g
duction band energy level (E ) for the MPA-CdTe QDs is
cb
−
0.8 V versus NHE (Gimbert-Surinach et al. 2014; Li et al.
2
013a; Rajh et al. 1993; Wang et al. 2011). The cathodic
peak potentials of the complexes 1–4 versus NHE are sum-
marized in Table 1. Based on our ꢁndings, since the energy
of the conduction band of the MPA-CdTe QDs is more nega-
tive than the cathodic peak potentials of the complexes 1–4,
electron transfer from the excited MPA-CdTe QDs to the Ni
complexes 1–4 is energetically favorable (Fig. 5) (Li et al.
2
014; Rajh et al. 1993).
Conclusion
The Emission spectra were recorded using a TE-cooled
QEPRO spectrometer from Ocean Optics. A continuous
wave laser diode with 514 nm wavelength was used as an
excitation source. A combination of dichroic mirrors and
ꢁlters were used to separate the emission of the quantum
dots from the laser beam before sending it to the spectrom-
eter. To measure the lifetime of the quantum dots, we used
time-correlated single-photon counting (TCSPC) system
(Picoquant Time harp 260) combined with 30-ps pulsed
The synthesis and characterization of a series of water-sol-
uble mononuclear Ni(II) complexes supported by the N2/
Py2 tetradentate ligand family were reported. Photolumi-
nescence properties of this MPA-CdTe QDs system were
investigated in detail in the presence of nickel complexes,
to gain fundamental understanding about their prospects
for potential photocatalytic systems. All Ni complexes 1–4
investigated in this paper showed promising results, they
1
3

Photosynthesis Research
laser with a 450 nm wavelength. All the above emission and
lifetime measurements were performed with special optical
glass cells with a pathlength of 10 mm.
light brown oil was dissolved in dichloromethane (10 mL).
The above solution was added dropwise to a solution of
N,N′-dimethylethylenediamine 0.493 mL (4.58 mmol) in
dichloromethane (15 mL). Aqueous solution of 1 M sodium
hydroxide (10 mL) was slowly added and solution was
stirred for additional 60 h at room temperature. After 60 h
of stirring was the rapid addition of a second fraction of
aqueous 1 M sodium hydroxide (10 mL, 10 mmol), the prod-
uct was extracted with dichloromethane (3× 25 mL). The
combined organic layers were dried over anhydrous sodium
sulfate and ꢁltered. Subsequently, the excess solvent was
Synthesis of N,N′‑dimethyl‑N,N′‑bis‑(pyridine‑2‑ylm
ethyl)‑1,2‑diaminoethane (BPMEN)
The ligand BPMEN was synthesized via a previously
reported procedure (Singh et al. 2017). A solution of potas-
sium carbonate (5.1 g, 37 mmol) in 15 mL water was drop-
wise added to the aqueous solution of 2-(chloromethyl)-
pyridine hydrochloride (3 g, 18.3 mmol in 10 mL). After
about 30 min of stirring at room temperature, the reaction
mixture was extracted with dichloromethane (3× 20 mL).
The combined organic extracts were dried over anhydrous
sodium sulfate. The solution was ꢁltered, and the solvent
was removed under vacuum. The resulted residue was then
dissolved in dichloromethane (10 mL). The above solu-
tion was added dropwise to a solution of N,N′-dimethyl-
ethylenediamine (0.942 mL, 8.75 mmol) in dichlorometh-
ane (25 mL). After this addition, 20 mL of aqueous sodium
hydroxide (1 M) was added slowly and the reaction mixture
was stirred for next 60 h at room temperature. After stirring
was ꢁnished, another fraction of sodium hydroxide (20 mL,
evaporated by vacuum to aꢀord brown color viscous oil
1
(1.71 g, Yield 97%). H NMR (500 MHz, Methanol-d ) δ
4
8.08 (s, 2H, pyridine ring), 3.76 (s, 6H, –O–CH –Py), 3.57
3
(s, 4H, –CH –CH –Py), 2.56 (s, 4H, –CH –CH –), 2.28 (d,
2
2
2
2
6H, CH –Py), 2.24 (d, 6H, CH –Py), 2.16 (s, 6H, –N–CH ).
3
3
3
+
+
+
ESI–MS : [345BPMEN+H] =387.32 m/z (experimental)
+
387.27 m/z (theoretical).
Synthesis of N1,N2‑dimethyl‑N1,N2‑bis(pyridin‑2‑yl
methyl)‑cyclohexane‑1,2‑diamine (BPMCN)
The ligand BPMCN was synthesized via a previously
reported procedure (Pella et al. 2018). Brieꢃy, 1,2-dimethyl-
cyclohexane-1,2-diamine (162 mg, 1.14 mmol) was dis-
solved in acetonitrile (15 mL) in a round-bottom ꢃask. Tri-
ethylamine (0.78 mL, 5.6 mmol) was charged to the solution,
and 2-(chloromethyl)-pyridine (374 mg, 2.27 mmol) was
added. The solution as brought to reꢃux in air overnight,
then cooled to room temperature and condensed under
reduced pressure. The resulting crude solid was dissolved
in 30 mL of dichloromethane and washed with 30 mL of
saturated sodium bicarbonate (aq). The organic layer was
extracted, and the aqueous layer was washed 2×30 mL with
dichloromethane. The organic layers were collected, dried
over sodium sulfate, ꢁltered, and condensed under reduced
pressure to yield a brown oil. The crude product was puri-
ꢁed using silica gel chromatography with 86% Ethyl ace-
tate/10% Methanol/4% Ammonium hydroxide to yield the
1
M) was added rapidly. The reaction mixture was extracted
with dichloromethane (3×50 mL) and the combined organic
portion was dried over anhydrous sodium sulfate. Evapora-
tion of solvent led to isolation of the ligand BPMEN as a
1
dark orange oil. (2.1 g, Yield −89%) H NMR (500 MHz,
Methanol-d ) δ 8.45 (d, 2H, pyridine ring), 7.76 (m, 2H, pyr-
4
idine ring), 7.52 (d, 2H, pyridine ring), 7.30 (m, 2H, pyridine
ring), 3.67 (s, 4H, –N–CH –Py), 2.63 (s, 4H, –CH –CH –),
2
2
2
+
+
2
.26 (s, 6H, N–CH ). ESI–MS : [BPMEN+H] =271.15 m/
3
+
+
z (experimental) 271.19 m/z (theoretical).
Synthesis of [N,N′‑dimethyl‑N,N′‑bis‑(4‑methoxy‑3,5
‑
dimethylpyridine‑2‑ylmethyl)‑1,2 diaminoethane.
345BPMEN)
(
The ligand 345BPMEN was synthesized by modifying the
previously reported procedure (Singh et al. 2017). To a solu-
tion of 2-chloromethyl-4-methoxy-3,5-dimethylpyridine
hydrochloride 2.032 g (9.15 mmol) in 10 mL of water, a
solution of potassium carbonate (2.55 g, (18.45 mmol) in
water (10 mL) was added dropwise. After potassium carbon-
ate addition, very thick white ppts were formed and solution
solidiꢁed. Additional amount of water (50 mL) was added
into the mixture. After water addition, the reaction mixture
was stirred at room temperature for next 30 min followed by
solvent extraction with dichloromethane (3×20 mL). The
combined dichloromethane layer was treated with anhy-
drous sodium sulfate. The solution was ꢁltered, and the sol-
vent was removed by rotatory evaporation. The collected
desired product as a light brown oil (0.15 g, Yield 41%).
1
H NMR (500 MHz, CDCl ) δ 8.47 (d, 2H), 7.59 (d, 4H),
3
7.12 (m, 2H), 3.87 (q, 4H), 2.67 (d, 2H), 2.29 (s, 6H), 2.02
+
(2H), 1.73 (d, 2H), 1.29 (m, 2H), 1.17 (t, 2H). ESI–MS :
+
+
+
[BPMCN+H] =325.31 m/z (experimental) 325.23 m/z
(theoretical).
Synthesis (‑)‑2‑(((S)‑2‑((S)‑1‑(pyridin‑2‑ylmethyl)
pyrrolidin‑2‑yl)pyrrolidin‑1‑yl)methyl)pyridine
(PDP)
The ligand PDP was synthesized via a previously reported
procedure (Pella et al. 2018). A 100 mL round-bottom ꢃask
was charged with a stir bar, (S,S)-2,2′-bispyrrolidine tartrate
1
3

Photosynthesis Research
(
S2) (0.25 equiv, 1.0 g, 3.45 mmol) and water (7.5 mL),
Synthesis of Ni(BPMCN)Cl (3)
2
and dichloromethane (7.5 mL). Solid sodium hydroxide
pellets (1.6 equiv, 0.883 g, 22.05 mmol) were added, fol-
lowed by 2-(chloromethyl)-pyridine (0.55 equiv, 1.243 g,
Addition of the ligand BPMCN (0.033 g, 0.1 mmol)
dissolved in methanol (2 mL) to a stirring solution of
NiCl ·6H O (0.024 g, 0.1 mmol) in methanol (2 mL)
7
.575 mmol). After 18 h stirring at room temperature, the
2
2
reaction mixture was diluted with 1 M sodium hydroxide.
The aqueous layer was extracted with dichloromethane
resulted in the formation of a green solution. Evapora-
tion of solvent gave a green color powder (0.039 g, Yield
+
(3×12.5 mL), and the organic extracts were combined, dried
87%). ESI–MS (in CH OH) for [Ni(BPMCN)Cl] (z = 1)
3
over magnesium sulfate, and concentrated in vacuo. The
crude ligand thus obtained was puriꢁed by silica gel chroma-
tography (5% Methanol/2% Ammonium hydroxide/Dichlo-
romethane) and the collected fractions were combined,
washed with 1 M sodium hydroxide, dried over magnesium
m/z 417.19 (experimental) m/z 417.14 (theoretical) and
2
+
for [Ni(BPMCN)] (z=2) m/z 191.25 (experimental) m/z
191.11 (theoretical). Elemental analysis Calculated (%): C
47.28, H 6.74, N 11.03, Cl 13.95 Found (%): C 47.45, H
6.51, N 11.51, Cl 14.78. UV–Vis λ in nm (ε in M− cm )
1
−1
max
sulfate, and concentrated in vacuo to provide a brown oil
in MeOH at RT: 588 (13), 994 (13).
1
(
(
0.515 g, Yield 46%). H NMR (500 MHz, CDCl ) δ 8.50
3
dd, 2H), 7.60 (dt, 2H), 7.40 (d, 2H), 7.11 (dd, 2H), 4.20 (d,
Synthesis of Ni(PDP)Cl (4)
2
2
2
H), 3.51 (d, 2H), 3.00 (p, 2H), 2.80 (m, 2H), 2.24 (appq,
+
+
H), 1.77-1.64 (m, 8H). ESI–MS : [PDP+H] =323.32 m/
Similarly, when the ligand PDP (0.1 g, 3 mmol) dissolved
in methanol (2 mL) was added dropwise to a stirring solu-
tion of NiCl ·6H O (0.72 g, 3 mmol) in methanol (2 mL),
+
+
z (experimental) 323.22 m/z (theoretical).
2
2
a green solution was obtained. Evaporation of solvent gave
Synthesis of Ni(BPMEN)Cl (1)
2
a green color powder (0.271 g, Yield 83%). ESI-MS (in
+
CH OH) for [Ni(BPMCN)Cl] (z=1) m/z 415.19 (experi-
3
The complex 1 was prepared by following a published pro-
2
+
mental) m/z 415.12 (theoretical) and for [Ni(BPMCN)]
cedure (Singh et al. 2017).The ligand BPMEN (0.238 g,
(
z=2) m/z 190.24 (experimental) m/z 190.01 (theoretical).
1
mmol) dissolved in methanol (4 mL) was added dropwise
Elemental analysis Calculated (%): C 51.1, H 6, N 11.92,
Cl 15.08 Found (%): C 51.38, H 5.84, N 12.03, Cl 15.71.
to a stirring solution of NiCl .6H O (0.27 g, 1 mmol) in
2
2
methanol (4 mL). Blue green solution was obtained immedi-
−
1
−1
UV–Vis λ
in nm (ε in M cm ) in MeOH at RT: 581
max
ately upon addition. Evaporation of solvent gave a blue green
(
9), 976 (11).
color powder (0.396 g, Yield 99%). ESI–MS (in CH OH)
3
+
for [Ni(BPMEN)Cl] (z=1) m/z 363.13 (experimental) m/z
Sample preparation and measurements
2
+
3
1
63.09 (theoretical) and for [Ni(BPMEN)] (z = 2) m/z
64.21 (experimental) m/z 164.06 (theoretical). Elemental
The UV–Vis absorption, emission and lifetime measure-
ments were performed under anaerobic conditions at room
temperature. A 1 mL solution of the MPA-CdTe QDs of
analysis Calculated (%): C 45.37, H 6.27, N 12.45 Found
−
1
(
%): C 45.18, H 5.82, N 13.3. UV–Vis λ in nm (ε in M
max
−
1
cm ) in MeOH at RT: 603 (15), 986 (26).
−
6
−1
concentration 1.4 × 10 M (0.5 mg mL ) in water was
prepared. 30 mM stock solutions of the complexes 1–4 in
water were also prepared. The MPA-CdTe QDs solutions
were purged with nitrogen gas for 30 s in a micro cuvette
(10 mm pathlength) whereas the Ni complexes 1–4 stock
solutions were purged with nitrogen gas for 90 s.
Synthesis of Ni(345BPMEN)Cl (2)
2
The complex 2 was prepared by following similar proce-
dure as reported for 1. The ligand 345BPMEN (0.198 g,
0
.512 mmol) dissolved in methanol (3 mL) was added
To the MPA-CdTe QDs solution in the cuvette, an aliquot
of 10 μL of the stock solution of 1 was added with the help
of a gas tight syringe. Similar procedure was followed for the
remaining complexes and all the measurements.
dropwise to a stirring solution of NiCl ·6H O (0.122 g,
2
2
0
.512 mmol) in methanol (3 mL). The color of the solution
turned immediately bluish green upon addition. Evaporation
of solvent gave a blue green color powder (0.262 g, Yield
The cyclic voltammetry experiments were performed
under a nitrogen atmosphere with Ag/AgCl as the reference
electrode and 0.1 M KCl as the supporting electrolyte. To
measure the potential of the MPA-CdTe QDs samples were
scanned from − 1.5 to + 1.5 V in water with a scan rate
+
9
9%). ESI–MS (in CH OH) for [Ni(345BPMEN)Cl] (z=1)
3
m/z 479.25 (experimental) m/z 479.17 (theoretical) and for
2
+
[
Ni(345BPMEN)] (z=2) m/z 222.26 (experimental) m/z
2
22.10 (theoretical). Elemental analysis Calculated (%): C
7.29, H 7.25, N 9.59 Found (%): C 47.56, H 6.69, N 10.42.
−
1
4
varied from 100 to 500 mV s . Similarly, the complexes
were measured in water using a potential window of −1 V
in nm (ε in M− cm ) in MeOH at RT: 610
1
−1
UV–Vis λ
max
−
1
(
16), 974 (28).
to +1 V with a scan rate varied from 100 to 500 mV s .
1
3

Photosynthesis Research
Acknowledgements This work was funded by the University of Ala-
hydrogenase assemblies. J Am Chem Soc 134(27):11108–11111.
https://doi.org/10.1021/ja3042367
bama in Huntsville, Individual Investigator Distinguished Research
(
IIDR) awards to Anusree Mukherjee and Seyed M. Sadeghi.
Gross MA, Reynal A, Durrant JR, Reisner E (2014) Versatile photo-
catalytic systems for H2 generation in water based on an eꢂcient
DuBois-type nickel catalyst. J Am Chem Soc 136(1):356–366.
https://doi.org/10.1021/ja410592d
Compliance with ethical standards
Guadalupe Hernandez HJ, Pandiyan T, Bernes S (2006) Mercaptoeth-
anesulfonic acid (CoM imitator) interaction studies with nickel(II)
complexes of pyridyl groups containing tetradentate ligands: syn-
thesis, structure, spectra and redox properties. Inorg Chim Acta
Conflict of interest The authors declare no competing ꢁnancial inter-
est.
3
59(1):1–12. https://doi.org/10.1016/j.ica.2005.09.036
Gurumoorthy P, Mahendiran D, Kalilur Rahiman A (2016) Theo-
retical calculations, DNA interaction, topoisomerase I and phos-
phatidylinositol-3-kinase studies of water soluble mixed-ligand
nickel(II) complexes. Chem. Biol. Interact. 248:21–35. https://
doi.org/10.1016/j.cbi.2016.02.002
References
Amelia M, Lincheneau C, Silvi S, Credi A (2012) Electrochemical
properties of CdSe and CdTe quantum dots. Chem Soc Rev
Gust D, Moore TA, Moore AL (2001) Mimicking photosynthetic solar
energy transduction. Acc Chem Res 34(1):40–48. https://doi.
org/10.1021/ar9801301
4
1(17):5728–5743. https://doi.org/10.1039/c2cs35117j
Anacona JR, Marquez VE, Jimenez Y (2009) Synthesis and charac-
terization of manganese(II), nickel(II), copper(II) and zinc(II)
Schiff-base complexes derived from 1,10-phenanthroline-
Haram SK, Kshirsagar A, Gujarathi YD, Ingole PP, Nene OA, Markad
GB, Nanavati SP (2011) Quantum conꢁnement in CdTe quantum
dots: investigation through cyclic voltammetry supported by den-
sity functional theory (DFT). J Phys Chem C 115(14):6243–6249.
https://doi.org/10.1021/jp111463f
2
6
,9-dicarboxaldehyde and 2-mercaptoethylamine. J Coord Chem
2(7):1172–1179. https://doi.org/10.1080/00958970802382768
Armaroli N, Balzani V (2007) The future of energy supply: challenges
and opportunities. Angew Chem Int Ed 46(1+2):52–66. https://
doi.org/10.1002/anie.200602373
Helm ML, Stewart MP, Bullock RM, DuBois MR, DuBois DL (2011)
A synthetic nickel electrocatalyst with a turnover frequency above
−1
Artero V, Fontecave M (2013) Solar fuels generation and molecular
systems: is it homogeneous or heterogeneous catalysis? Chem
Soc Rev 42(6):2338–2356. https://doi.org/10.1039/C2CS35334B
Bae Y, Myung N, Bard AJ (2004) Electrochemistry and electrogen-
erated chemiluminescence of CdTe nanoparticles. Nano Lett
100,000 s for H2 production. Science 333(6044):863–866. https
://doi.org/10.1126/science.1205864
Huang J, Mulfort KL, Du P, Chen LX (2012) Photodriven charge
separation dynamics in CdSe/zns core/shell quantum dot/coba-
loxime hybrid for eꢂcient hydrogen production. J Am Chem Soc
134(40):16472–16475. https://doi.org/10.1021/ja3062584
Kaerkaes MD, Verho O, Johnston EV, Aakermark B (2014) Artiꢁcial
photosynthesis: molecular systems for catalytic water oxidation.
Chem Rev 114(24):11863–12001. https://doi.org/10.1021/cr400
572f
4
(6):1153–1161. https://doi.org/10.1021/nl049516x
Berardi S, Drouet S, Francas L, Gimbert-Surinach C, Guttentag M,
Richmond C, Llobet A (2014) Molecular artiꢁcial photosynthe-
sis. Chem Soc Rev 43(22):7501–7519. https://doi.org/10.1039/
C3CS60405E
Brown KA, Dayal S, Ai X, Rumbles G, King PW (2010) Controlled
assembly of hydrogenase-CdTe nanocrystal hybrids for solar
hydrogen production. J Am Chem Soc 132(28):9672–9680. https
Kalyanasundaram K, Graetzel M (2010) Artiꢁcial photosynthesis:
biomimetic approaches to solar energy conversion and storage.
Curr Opin Biotechnol 21(3):298–310. https://doi.org/10.1016/j.
copbio.2010.03.021
:
//doi.org/10.1021/ja101031r
Brown KA, Wilker MB, Boehm M, Dukovic G, King PW (2012)
Characterization of photochemical processes for H2 production
by CdS nanorod-[FeFe] hydrogenase complexes. J Am Chem Soc
Kankanamalage PHA, Ekanayake DM, Singh N, de Morais ACP,
Mazumder S, Verani CN, Mukherjee A, Lanznaster M (2019)
Eꢀect of ligand substituents on nickel and copper [N4] complexes:
electronic and redox behavior, and reactivity towards protons.
New J Chem. https://doi.org/10.1039/C9NJ01283D
1
34(12):5627–5636. https://doi.org/10.1021/ja2116348
Du P, Eisenberg R (2012) Catalysts made of earth-abundant ele-
ments (Co, Ni, Fe) for water splitting: recent progress and future
challenges. Energy Environ Sci 5(3):6012–6021. https://doi.
org/10.1039/c2ee03250c
Kilgore UJ, Roberts JAS, Pool DH, Appel AM, Stewart MP, DuBois
MR et al (2011) [Ni(PPh2NC6H4X2)2]2+complexes as electro-
catalysts for H2 production: eꢀect of substituents, acids, and water
on catalytic rates. J Am Chem Soc 133(15):5861–5872. https://
doi.org/10.1021/ja109755f
Esswein AJ, Nocera DG (2007) Hydrogen production by molecu-
lar photocatalysis. Chem Rev 107(10):4022–4047. https://doi.
org/10.1021/cr050193e
Kim D, Sakimoto KK, Hong D, Yang P (2015) Artiꢁcial photosynthe-
sis for sustainable fuel and chemical production. Angew Chem Int
Ed 54(11):3259–3266. https://doi.org/10.1002/anie.201409116
Li C-B, Li Z-J, Yu S, Wang G-X, Wang F, Meng Q-Y, Wu L-Z (2013a)
Interface-directed assembly of a simple precursor of [FeFe]-
H2ase mimics on CdSe QDs for photosynthetic hydrogen evolu-
tion in water. Energy Environ Sci 6(9):2597–2602. https://doi.
org/10.1039/c3ee40992a
Fukuzumi S, Hong D, Yamada Y (2013) Bioinspired photocatalytic
water reduction and oxidation with earth-abundant metal cata-
lysts. J Phys Chem Lett 4(20):3458–3467. https://doi.org/10.1021/
jz401560x
Gaponik N, Talapin DV, Rogach AL, Hoppe K, Shevchenko EV, Kor-
nowski A et al (2002) Thiol-capping of CdTe nanocrystals: an
alternative to organometallic synthetic routes. J Phys Chem B
1
06(29):7177–7185. https://doi.org/10.1021/jp025541k
Li Z-J, Li X-B, Wang J-J, Yu S, Li C-B, Tung C-H, Wu L-Z (2013b) A
robust “artiꢁcial catalyst” in situ formed from CdTe QDs and inor-
ganic cobalt salts for photocatalytic hydrogen evolution. Energy
Environ Sci 6(2):465–469. https://doi.org/10.1039/C2EE23898E
Li Z-J, Fan X-B, Li X-B, Li J-X, Ye C, Wang J-J, Wu L-Z (2014)
Visible light catalysis-assisted assembly of Nih-QD hollow
nanospheres in situ via hydrogen bubbles. J Am Chem Soc
136(23):8261–8268. https://doi.org/10.1021/ja5047236
Gimbert-Surinach C, Albero J, Stoll T, Fortage J, Collomb M-N,
Deronzier A et al (2014) Eꢂcient and limiting reactions in aque-
ous light-induced hydrogen evolution systems using molecular
catalysts and quantum dots. J Am Chem Soc 136(21):7655–7661.
https://doi.org/10.1021/ja501489h
Greene BL, Joseph CA, Maroney MJ, Dyer RB (2012) Direct evidence
of active-site reduction and photodriven catalysis in sensitized
1
3

Photosynthesis Research
Li X-B, Tung C-H, Wu L-Z (2018) Semiconducting quantum dots for
artiꢁcial photosynthesis. Nat Rev Chem 2(8):160–173. https://doi.
org/10.1038/s41570-018-0024-8
Vullev VI (2011) From biomimesis to bioinspiration: what’s the ben-
eꢁt for solar energy conversion applications? J Phys Chem Lett
2(5):503–508. https://doi.org/10.1021/jz1016069
Liu W, Migdisov A, Williams-Jones A (2012) The stability of aqueous
nickel(II) chloride complexes in hydrothermal solutions: results of
UV–Visible spectroscopic experiments. Geochim Cosmochim Acta
Wang X, Li C (2017) Interfacial charge transfer in semiconductor-
molecular photocatalyst systems for proton reduction. J Photochem
Photobiol C 33:165–179. https://doi.org/10.1016/j.jphotochem
rev.2017.10.003
9
4:276–290. https://doi.org/10.1016/j.gca.2012.04.055
Liu C, Qiu F, Peterson JJ, Krauss TD (2015) Aqueous photogenera-
tion of H2 with CdSe nanocrystals and nickel catalysts: electron
transfer dynamics. J Phys Chem B 119(24):7349–7357. https://doi.
org/10.1021/jp510935w
Wang F, Wang W-G, Wang X-J, Wang H-Y, Tung C-H, Wu L-Z (2011)
A highly eꢂcient photocatalytic system for hydrogen production by
a robust hydrogenase mimic in an aqueous solution. Angew Chem
Int Ed 50(14):3193–3197. https://doi.org/10.1002/anie.201006352
(S3193/3191–S3193/3116)
Mao SS, Shen S (2013) Hydrogen production catalysing artiꢁcial pho-
tosynthesis. Nat Photonics 7(12):944–946. https://doi.org/10.1038/
nphoton.2013.326
Wang D, Zhang Y, Chen W (2014) A novel nickel-thiourea-triethylamine
complex adsorbed on graphitic C3N4 for low-cost solar hydro-
gen production. Chem Commun 50(14):1754–1756. https://doi.
org/10.1039/c3cc48141g
Mautner FA, Mikuriya M, Ishida H, Sakiyama H, Louka FR, Humphrey
JW, Massoud SS (2009) Dicyanamido-metal(II) complexes. Part
4
: synthesis, structure and magnetic characterization of polynu-
Wang M, Han K, Zhang S, Sun L (2015) Integration of organometallic
complexes with semiconductors and other nanomaterials for pho-
tocatalytic H2 production. Coord Chem Rev 287:1–14. https://doi.
org/10.1016/j.ccr.2014.12.005
clear Cu(II) and Ni(II) complexes bridged by μ-1,5-dicyanamide.
Inorg Chim Acta 362(11):4073–4080. https://doi.org/10.1016/j.
ica.2009.05.066
Mulfort KL, Utschig LM (2016) Modular homogeneous chromophore-
catalyst assemblies. Acc Chem Res 49(5):835–843. https://doi.
org/10.1021/acs.accounts.5b00539
Wen F, Li C (2013) Hybrid artiꢁcial photosynthetic systems comprising
semiconductors as light harvesters and biomimetic complexes as
molecular cocatalysts. Acc Chem Res 46(11):2355–2364. https://
doi.org/10.1021/ar300224u
Nann T, Ibrahim SK, Woi P-M, Xu S, Ziegler J, Pickett CJ (2010) Water
splitting by visible light: a nanophotocathode for hydrogen produc-
tion. Angew Chem Int Ed 49(9):1574–1577. https://doi.org/10.1002/
anie.200906262
Wilson AD, Newell RH, McNevin MJ, Muckerman JT, Rakowski DuBois
M, DuBois DL (2006) Hydrogen oxidation and production using
nickel-based molecular catalysts with positioned proton relays. J
Am Chem Soc 128(1):358–366. https://doi.org/10.1021/ja056442y
Wilson AD, Shoemaker RK, Miedaner A, Muckerman JT, DuBois DL,
Dubois MR (2007) Nature of hydrogen interaction with Ni(II) com-
plexes containing cyclic phosphine ligands with pendant nitrogen
bases. Proc Natl Acad Sci USA 104(17):6951–6956. https://doi.
org/10.1073/pnas.0608928104
Pella BJ, Niklas J, Poluektov OG, Mukherjee A (2018) Eꢀects of den-
ticity and ligand rigidity on reactivity of copper complexes with
cumyl hydroperoxide. Inorg Chim Acta 483:71–78. https://doi.
org/10.1016/j.ica.2018.07.035
Poznyak SK, Osipovich NP, Shavel A, Talapin DV, Gao M, Eychmueller
A, Gaponik N (2005) Size-dependent electrochemical behavior of
thiol-capped CdTe nanocrystals in aqueous solution. J Phys Chem
B 109(3):1094–1100. https://doi.org/10.1021/jp0460801
Rajh T, Micic OI, Nozik AJ (1993) Synthesis and characterization of
surface-modiꢁed colloidal cadmium telluride quantum dots. J Phys
Chem 97(46):11999–12003. https://doi.org/10.1021/j100148a026
Sadeghi SM, Wing WJ, Gutha RR, Wilt JS, Wu JZ (2018) Balancing
silicon/aluminum oxide junctions for super-plasmonic emission
enhancement of quantum dots via plasmonic metaꢁlms. Nanoscale
Wu L-Z, Chen B, Li Z-J, Tung C-H (2014) Enhancement of the eꢂciency
of photocatalytic reduction of protons to hydrogen via molecular
assembly. Acc Chem Res 47(7):2177–2185. https://doi.org/10.1021/
ar500140r
Xu Y, Zhang B (2015) Hydrogen photogeneration from water on the bio-
mimetic hybrid artiꢁcial photocatalytic systems of semiconductors
and earth-abundant metal complexes: progress and challenges. Catal
Sci Technol 5(6):3084–3096. https://doi.org/10.1039/C5CY00365B
Yu WW, Qu L, Guo W, Peng X (2003) Experimental determination of the
extinction coeꢂcient of CdTe, CdSe, and CdS nanocrystals. Chem
Mater 15(14):2854–2860. https://doi.org/10.1021/cm034081k
Zhang H, Zhou Z, Yang B, Gao M (2003) The inꢃuence of carboxyl
groups on the photoluminescence of mercaptocarboxylic acid-sta-
bilized CdTe nanoparticles. J Phys Chem B 107(1):8–13. https://doi.
org/10.1021/jp025910c
10(10):4825–4832. https://doi.org/10.1039/C7NR09396A
Sadeghi SM, Wing WJ, Gutha RR, Goul RW, Wu JZ (2019) Functional
metal-oxide plasmonic metastructures: ultrabright semiconductor
quantum dots with polarized spontaneous emission and suppressed
auger recombination. Phys Rev Appl 11(2):024045. https://doi.
org/10.1103/PhysRevApplied.11.024045
Sadhu MH, Solanki A, Kumar SB (2015) Mixed ligand complexes of
copper(II), cobalt(II), nickel(II) and zinc(II) with thiocyanate and
pyrazole based tetradentate ligand: syntheses, characterizations
and structures. Polyhedron 100:206–214. https://doi.org/10.1016/j.
poly.2015.07.067
Zhou H, Yan R, Zhang D, Fan T (2016) Challenges and perspec-
tives in designing artiꢁcial photosynthetic systems. Chem Eur J
22(29):9870–9885. https://doi.org/10.1002/chem.201600289
Zhou Y, Yang S, Huang J (2017) Light-driven hydrogen production from
aqueous solutions based on a new Dubois-type nickel catalyst. Phys
Chem Chem Phys 19(11):7471–7475. https://doi.org/10.1039/
C7CP00247E
Shi L, Hernandez B, Selke M (2006) Singlet oxygen generation from
water-soluble quantum dot-organic dye nanocomposites. J Am
Chem Soc 128(19):6278–6279. https://doi.org/10.1021/ja057959c
Silver SC, Niklas J, Du P, Poluektov OG, Tiede DM, Utschig LM (2013)
Protein delivery of a Ni catalyst to photosystem I for light-driven
hydrogen production. J Am Chem Soc 135(36):13246–13249. https
Zilbermann I, Maimon E, Cohen H, Meyerstein D (2005) Redox
chemistry of nickel complexes in aqueous solutions. Chem Rev
105(6):2609–2626. https://doi.org/10.1021/cr030717f
://doi.org/10.1021/ja405277g
Singh N, Niklas J, Poluektov O, Van Heuvelen KM, Mukherjee A (2017)
Mononuclear nickel(II) and copper(II) coordination complexes sup-
ported by bispicen ligand derivatives: experimental and computa-
tional studies. Inorg Chim Acta 455(Part_1):221–230. https://doi.
org/10.1016/j.ica.2016.09.001
Publisher’s Note Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional aꢂliations.
Utschig LM, Soltau SR, Tiede DM (2015) Light-driven hydrogen pro-
duction from photosystem I-catalyst hybrids. Curr Opin Chem Biol
25:1–8. https://doi.org/10.1016/j.cbpa.2014.11.019
1
3

Photosynthesis Research
Aꢀliations
1
2
2
1
Niharika Krishna Botcha · Rithvik R. Gutha · Seyed M. Sadeghi · Anusree Mukherjee
2
*
Anusree Mukherjee
Department of Physics and Astronomy, The University
of Alabama in Huntsville, 301 Sparkman Drive, Huntsville,
AL 35899, USA
anusree.mukherjee@uah.edu
1
Department of Chemistry, The University of Alabama
in Huntsville, 301 Sparkman Drive, Huntsville, AL 35899,
USA
1
3