Ruthenium(iii)-bis(phenolato)bipyridine/TiO2 hybrids: Unprecedented photocatalytic hydrogen evolution

Article abstract of DOI:10.1039/c9dt01506j

In this work, two new bis-(hydroxyphenyl)bipyridine based ruthenium complexes with 4-picoline (coded as MCS-B4M) and isonicotinic acid (coded as MCS-B5M), which act as ancillary ligands, have been synthesized and employed for the first time as photosensit

Full text of DOI:10.1039/c9dt01506j

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Volume 45 Number
1
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January 2016 Pages 1–398
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Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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DOI: 10.1039/C9DT01506J
TOC
Bis-(hydroxyphenyl) bipyridine based ruthenium complexes as photosensitizers for efficient
photocatalytic H evolution.
2

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Ruthenium(III)-bis(phenolato)bipyridine/TiO
2
hybrids:
unprecedented
View Article Online
DOI: 10.1039/C9DT01506J
photocatalytic hydrogen evolution
Received 00th January 20xx,
Accepted 00th January 20xx
Binitendra Naath Mongal, Amritanjali Tiwari, ‡,§ Malapaka Chandrasekharam, †,§ and Ujjwal
Pal
*
†,#
*
‡,§
DOI: 10.1039/x0xx00000x
Abstract
www.rsc.org/
In this work, two new bis-(hydroxyphenyl)bipyridine based ruthenium complexes with 4-picoline (coded as MCS-B4M ) and
isonicotinic acid (coded as MCS-B5M) as ancillary ligands have been synthesized and employed for the first time as
photosensitizers in photocatalytic hydrogen evolution studies. The photocatalyst MCS-B5M/TiO -Pt showed impressive
2
hydrogen generation rate, up to 4.2 mmolh-1 and turn over number (TON) of 84,959 after 5h has been achieved. The
better performance of B5TP over B4TP is due to the higher excited state lifetime of MCS-B5M (~ 2.6 ns) over MCS-B4M
2
(~1.4 ns) leads to higher probability of electron transfer to TiO /Pt composite in case of the former and stronger coupling
of MCS-B5M excited states with the conduction band of the TiO
2
/Pt composite by the –COOH linkers of isonicotinic acid
moiety results in better photosensitization as observed in UV-Vis (DRS mode) absorbance study The comparative study of
the two dyes clearly shows the manifestations of their respective ancillary ligands having contrasting electronic properties.
This work gives
a new class of ruthenium photosensitizers as efficient light harvesting photocatalyst.
Introduction
Cl
COOH Cl
Splitting water into hydrogen and oxygen with sunlight is an
attractive sustainable energy conversion idea compatible with
the concept of energy democracy.1 Efficient photophysical
N
N
processes combined with catalyst driven transformations are
highly warranted to achieve this thermodynamically uphill
2
-4
N
RuIII
reaction. Sensitising a wide band-gap semiconductor with a
redox photosensitizer to produce H has been an effective way
N
RuIII
N
2
N
O
of solar-to-fuel conversion. Widely employed photosensitizers
for this application include polypyridyl complexes of
O
O
O
5
-8
platinum,9,10
iridium,11,12
ruthenium,
organic
N
chromophores,1
3-16
perovskites17 and molecular dyads.
18
N
Recently, our group reported efficient photocatalytic hydrogen
19
evolution using novel thiocyanate based ruthenium dyes. In
continuation of our efforts, we report here, the synthesis of
HOOC
two
complexes (coded as MCS-B4M and MCS-B5M) (Figure 1) and
studied their TiO -Pt composites (coded B4TP and B5TP
respectively) for photocatalytic H production wherein the
reductive half-reaction of H generation from aqueous protons
under visible light irradiation has been discussed. Scheme 1
depicts the synthetic outline for the preparation of both the
complexes. While our work was in progress, a ruthenium
complex similar to MCS-B4M with a hexafluorophosphate
bis-(hydroxyphenyl)bipyridine
based
ruthenium Fig. 1. Structural representation of MCS-B4M (left) and MCS-B5M (right).
counter-anion has been reported by Lau et al. where CAN
driven oxidative degradation of the complex has been
2
2
2
0
studied. To the best of our knowledge, this is the first time
when ruthenium complexes of 6,6’-(bis-phenolato)bypyridine
ligand have been employed as photosensitizers for efficient
2
photocatalytic H
solar spectrum.
2
production under visible light region of the
†Polymer & Functional Materials Division, CSIR-Indian Institute of Chemical
Technology, Hyderabad, India
‡
Centre for Environmental Engineering and Fossil Fuels, CSIR-Indian Institute of
Chemical Technology, Hyderabad, India
§
Academy of Scientific and Innovative Research (AcSIR), New Delhi, India
Electronic Supplementary Information (ESI) available: The supporting
information is available free of charge in the ACS website. Additional
1
information including structural characterization like H-NMR, ESI-MS, HRMS
and DFT optimized parameters.
#
Currently in School of Basic and Applied Sciences, GD Goenka University,
Gurgaon, India
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III
IV
MCS-B4M
N
View Article Online
DOI: 10.1039/C9DT01506J
2B
I
II
(
HO)
Br
N
N
OH
N
HO
HO
MCS-B5M
Br
A
B
A:
B:
6
,6-dibromo-2,2-bipyridine
boronic acid
2
-hydroxybenzene
Reaction conditions: I.
, Na CO , DME:H
II.
Cl , EtOH, 80 °C, reflux,
O(4:1); Ru(DMSO)
4 2
Pd(PPh
3
)
4
2
3
2
2
4 hrs; III. 4-Picoline, EtOH, 80 °C, reflux, 12 hrs IV. Isonicotinic acid, EtOH, 80 °C, reflux, 12 hrs
Scheme 1. Synthetic route of the metal complexes
Experimental
Theoretical Calculation
Materials and Methods
All theoretical calculations were carried out using the density
functional theory (DFT) approach with a suite in Gaussian 09
6
,6’-dibromo-2,2’-bipyridine was purchased from TCI
chemicals. 2-hydroxybenzene boronic acid was purchased
from alfa aesar. Ru(DMSO) Cl was prepared from RuCl
3
22
programs. Ground state geometries are optimized in the gas
phase with the RPBE1PBE functional with def2-SVP with
effective-core potential (ECP) was used for Ru and for the rest
4
2
according to the reported procedure. 4-Picoline and
isonicotinic acid were purchased from sigma Aldrich. Ethanol
and DMSO used were HPLC grade. Water used for the
spectrophotometric and electrochemical studies was purified
by a Milli-Q system. Tetrabutyl ammonium perchlorate was
23
of the elements def2-SVP was used.
Synthesis
Preparation of ligands
synthesized according to literature.21 UV–Vis spectra in 6,6’-bis(2-hydroxyphenyl)-2,2’-bipyridine(LH2):
absorbance and DRS mode were recorded using a Shimadzu dibromo-2,2-bipyridine (0.1 g, 0.32 mmol), 2-hydroxybenzene
UV-3600 spectrophotometer. The pH values were measured in boronic acid (0.88 g, 0.64 mmol) and Na CO (4.48 g, 14 mmol)
Thermos Scientific Orion star pH Benchtop. were dissolved in 20 mL of in DME/H O (4:1, v/v, 20mL). The
Photoluminescence experiments measured in JASCO FP-8300 Pd(PPh (5 mol%) was added as a catalyst. The mixture was
spectrofluorometer. Time-resolved fluorescence stirred at 90°C under N for 24 h. The organic layers, extracted
measurements have been carried out using HORIBA Jobin Yvon with CH Cl and dried with anhydrous Na SO . The crude
spectrofluorometer. The count rates employed were typically product was purified by column chromatography
The
6,6-
2
3
a
4
2
3 4
)
2
2
2
2
4
3
4 -1
1
0 – 10 s . Deconvolution of the data was carried out by the (Hexane:EtOAc 80:20, v/v) to obtain a light yellow coloured
+
1
method of iterative reconvolution of the instrument response solid.(65 mg, 60%). Mass: 341(M+H) H-NMR (CDCl
function and the assumed decay function using DAS-6 14.2(2H,s,NH), 8.15(2H,d,H ), 8.04(4H,m,Hd,h), 7.85(2H,d,H
), 6.98(2H,t,H ).
3
δ):
i
g
),
software. The goodness of the fit of the experimental data to 7.35(2H,t,H
the assumed decay function was judged by the standard
b
), 7.1(2H,d,H
a
c
statistical tests (i.e., random distribution of weighted residuals, Preparation of Complexes
2
the autocorrelation function and the values of reduced χ ). [Ru(III)(L)(4-picoline) ]Cl (MCS-B4M): The ligand LH (100 mg,
2
2
Infrared spectra were recorded as KBr pellets on a Shimadzu 0.29 mmol)) was dissolved in 10 ml EtOH in a round bottom
IR-Prestige21 spectrometer. A Conventional three electrode flask. The solution was slowly heated with drop wise addition
assembly was used under nitrogen to record cyclic of 0.5 mL Et N and then Ru(DMSO) Cl was added and the
3
4
2
voltammograms in CHI6003E potentiostat. The working reaction mixture was refluxed for 24 hours under N2
electrode was glassy carbon. The counter electrode was a atmosphere. Excess 4-picoline was directly added into the
platinum wire and Ag/AgCl was used as the reference reaction mixture and refluxed further for 12 hours. The crude
electrode. The scan rate was 5 mV/s. The 0.10 M anhydrous product was extracted with DCM and purified with
tetrabutylammonium perchlorate (TBAP) solution in the DCM:MeOH (99:1) to give a dark green coloured solid (20 mg).
+
acetonitrile was used as a supporting electrolyte. The 1 mM Mass: 626(M+H) High Resolution Mass Spectrum: 626.12551
solutions of each compound were used for measurement. The (calcd. 626.12503). Elemental analysis calculated for
0
ferrocene/ferrocenium couple was observed at E (ΔEp) = 0.4 V C H N O Ru: % C 65.27, H 4.51, N 8.95; Found %: C 65.38, H
(
34 28 4 2
1
−
1
5 mV) under these experimental conditions. H NMR spectra 4.76, N 8.88. Selected IR bands (cm ): 3304 (νC–H), 1650 (νC=C),
were recorded on a Bruker AVANCE DPX 400 MHz 1386 (νC-O).
spectrometer using Si(CH as internal standard. ESI-MS and [Ru(III)(L)(Isonicotinic acid) ]Cl (MCS-B5M): The synthesis
HRMS of the samples were recorded on Thermofischer procedure is same as that of the above complex with excess
3 4
)
2
orbitrap. EPR spectrum was recorded in JEOL: JES-FA200.
isonicotinic acid in place of 4-picoline. After completion of the
reaction, the solvent is reduced to 1 mL and the product is
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ARTICLE
precipitated with addition of water. The dark green coloured manual. Gas analysis was carried out by regular saVimewpAlritnicgle aOfntliener
DOI: 10.1039/C9DT01506J
crude product is purified with DCM:MeOH (90:10) to give a every hour, and a gas chromatograph (GC) equipped with TCD
+
dark green coloured solid (30 mg). Mass: 686 (M+H) High detector 90 (Agilent 7890) was employed for quantitative
Resolution Mass Spectrum: 686.07715 (calcd. 686.07339). analysis.
24 4 6
Elemental analysis calculated for C34H N O Ru: % C 59.56, H
3
.53, N 8.17; Found %: C 59.63, H3.83, N 8.58. Selected IR
−
1
Results and discussion
Synthetic procedure
bands (cm ): 3290 (νO–H), 2921(νC-H), 1644 (νC=O symm.),
463(νC=C), 1382 (νC=O assym.).
1
The ligand LH2 was prepared by Suzuki coupling of 6,6’-
dibromo-2,2’-bipyridine and 2-hydroxyphenylboronic acid in
DME/H O (4:1) medium. The crude obtained after workup was
2
purified in silica column with 20% EtOAc in hexane solution.
The corresponding metal complexes were prepared via
4 2
reaction of the ligand with Ru(DMSO) Cl and respective
ancillary ligands in EtOH in a one-pot synthesis. The complex
MCS-B4M was eluted with 1% MeOH in DCM whereas MCS-
B5M was eluted with 10% MeOH in DCM from a silica gel
column. The synthetic procedure has been outlined in scheme
Deposition of Pt on TiO
To improve the better photocatalytic activity of H
wt % Pt metal was deposited onto the surface of the
catalysts by using the previously reported method . In 100 ml
glass reaction vessel, 1.0 g of each catalyst was dispersed in 30
2
composite
2
production,
1
15
ml methanol. Then, aqueous solution of H
2
PtCl
6
(0.25 ml, 8 wt
%
aqueous solution) was added into the methanolic
2
suspension of TiO and reaction mixture was irradiated by a
4
50 W Hg lamp for 45 minutes. The resultant Pt-
2
TiO composite of light gray colour was retrieved by
1
.
centrifugation, washed three times with excess methanol and
The complexes MCS-B4M and MCS-B5M sufficiently sensitizes the
TiO as observed from the UV-VIS absorption spectra of the metal
ο
dried under vacuum at 70 C. Pt deposition is confirmed by
2
TEM analysis (see Figure S8 in ESI).
complexes in solution and composites B4TP and B5TP where all the
spectral features of the photosensitizers are well retained in the
Dye adsorption on Pt-TiO
To develop the visible light-driven catalytic systems for water
splitting, Pt nanoparticles were loaded on commercial TiO
nanoparticles by a known method.24 Then, the prepared
sensitizers were adsorbed on TiO -Pt composites by treating
2
complex-TiO
TiO surface is stronger in case of B5TP via the -COOH linker groups
of isonicotinic acid ligand moieties. In the FTIR spectrum of dye
adsorbed TiO -Pt composites, there is an overall shift of the peaks
towards lower frequencies, thus showing adsorption of the
2
composites. The attachment of the complex with the
2
2
2
2
with acetonitrile-ethanol solution (1:1 v/v, 20 mL) of the 0.5
µmol concentration of dyes. After retrieving the sensitizer-
2
5
complexes on TiO
and asymmetric peaks are located at 1420 and 1597 cm , 177 cm
apart, whereas in the free complex, they appear at 1382 and 1644
2
surface. In B5TP, the carboxylate symmetric
-1
-1
TiO
became colourless, indicating that the dyes were completely
adsorbed on TiO -Pt nanoparticles. The FTIR spectra of the
composites have been measured to ascertain the binding
modes of the dyes with TiO (Fig. S10).
2
-Pt composites by centrifugation, the remaining solution
-1
-1
cm , the difference being 262 cm . This is consistent with the
2
2
bidentate coordination of the carboxylate groups with TiO .
EPR Study
2
-1
FTIR: B4TP (cm ): 2971, 2932, 2150, 1843, 1602, 1559, 1459, The EPR spectrum of [Ru(III)(L)(4-picoline)
317, 1261.
rhombic in nature (g ) (Figure 2).The g values have been
B5TP (cm ): 2923, 2858, 1719, 1597, 1555, 1459, 1420, 1308, assigned as g
= 2.184, g = 1.958 and g =1.901 with the order
269.
> g > g . From eqn.1, as the <g> value of 2.018 is in between
2
]Cl (MCS-B4M) is
1
x y z
≠g ≠g
-1
1
2
3
1
g
1
2
3
the <g> = 2.0023 corresponding to free electron and <g> > 2.1
for complete metal based EPR , we can therefore conclude
that the EPR signal originates from mixed contributions of both
Photocatalytic Experiments
Photocatalytic H generation experiments were carried out in a
2
doubly jacketed Pyrex glass reactor with flat optical window
and external cooling jacket. All experiments were carried over
2
1
0 mL aqueous suspension of 15 mg photocatalyst containing
0 vol % of TEOA as sacrificial electron donor (SED). The
solution was adjusted to the desired pH using 1 M hydrochloric
acid. It was then air sealed with a rubber septum. Before light
irradiation dissolved air was removed by 20 min high vacuum
followed by purging of Ar gas. An Oriel Instruments solar
simulator equipped with a 300 W xenon arc lamp system with
cutoff filter (λ ≥ 420 nm) as irradiation source was employed.
Above filter can be easily replaced with any other wavelength
cutoff filter to alter the radiation wavelength regime. Reaction
vessel was kept 20 cm away from the light source to ensure
the one sun condition, as suggested by Oriel solar 85 simulator
Fig. 2 EPR spectrum of MCS-B4M
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Journal Name
metal and ligand.26
ARTICLE
View Article Online
DOI: 10.1039/C9DT01506J
B5TP
B4TP
B
B4TP
B4TP
B
5
5
T
T
P
P
8
6
4
2
0
25
25
MCS-B4M
MCS-B4M
MCS-B5M
MCS-B5M
15000
15000
20
20
15
15
10000
10000
3.09 eV
3.16 eV
10
10
1
2
3
4
5000
5000
hν (eV)
5
5
0
0
0
0
4
4
0
0
0
0
6
6
0
0
0
0
8
8
0
0
0
0
1
1
0
0
0
0
0
0
4
4
0
0
0
0
6
6
0
0
0
0
8
8
0
0
0
0
1000
1000
λ (nm)
λ (nm)
λ (nm)
λ (nm)
Fig.3. UV-Vis absorption spectra of the complexes in acetonitrile. (Left) UV-Vis absorption spectra in DRS mode for composites B4TP and B5TP. Inset Tauc Plot of the B4TP and
B5TP. (right)
[
[
<g> = 261/2 = 2.018] ….(1)
1 2 3
As <g> = {(1/3)(g + g ]
+ g 2)}1/2
2
2
Absorption studies
The synthesized complexes appear green in colour showing a The second oxidation peak appeared at 1.11 V signifying the Ru(III)-
broad absorbance (Figure 3) across the UV-VIS region that may Ru(IV) oxidation whereas in case of MCS-B5M, the anodic peaks for
III/II
be assigned to a combined effect of ligand based transitions, Ru couple appeared at 1.02 V for and Ru(III)-Ru(IV) oxidation at
ligand to metal charge transfer (LMCT) from the phenoxide 1.21 V. The higher anodic potential of MCS-B5M as compared to
ligand to the ruthenium centre and metal-ligand to ligand MCS-B4M is presumably due to the electron withdrawing effect of
3
0
charge transfer (MLL’CT) from ruthenium-phenolate to the isonicotinic acid groups . On the cathodic side, the complexes
bipyridine and isonicotinic acid moieties. A prominent less MCS-B4M and MCS-B5M shows a clear reduction around -0.97 V
intense peak around 700 nm is also observed accounting for and -0.96V respectively which may be based on the bipyridine
the d-d transitions26 as inferred from DFT studies (Figure 7 moiety as inferred from DFT analysis. The second cathodic peak for
3
1
inset). The reflectance mode UV-VIS measurements (DRS) of MCS-B5M at -1.55 V shows the isonicotinic acid based reductions .
the complexes/TiO -Pt composites replicate the spectral
features of the metal complexes in solution. Hence, the TiO -Pt
2
2
Emission studies
composites are properly sensitized with the metal complexes.
In both the solution mode as well as DRS mode, the light
harvesting capabilities of MCS-B5M is higher than that of MCS-
B4M. This is the manifestation of the electron withdrawing
nature of the isonicotinic acids resulting in a large transition
The efficiency of electron injection from the excited state of
the ruthenium complexes to the conduction band of TiO was
2
examined by means of emission spectroscopy (Figure 5).
Although the complexes show high absorption in the UV
region, for the purpose of studying the excited state properties
on visible light irradiation, both the complexes MCS-B4M and
MCS-B5M are excited with light of wavelength 500 nm and 460
nm respectively. In both the cases, the emission was observed
at around 550 nm. The emission was rapidly quenched on
27,28
dipole moment towards the semiconductor interface.
The
band gap values of the composites B5TP and B4TP as
calculated from the Tauc plot using Klubeka-Munk formalism
are 3.09 eV and 3.16 eV respectively. The lowering of band gap
in case of B5TP reflects a better grafting of the MCS-B5M with
2
addition of a miniscule amount of TiO -Pt mixture reflecting
9
TiO
2
via the –COOH anchoring groups.2
clear contribution of the metal complex excited states in
8
electron injection. The excited state lifetime decay signal of
Electrochemical study
MCS-B4M and MCS-B5M in acetonitrile solution has been
The electrochemical properties of both the complexes were studied recorded (see Figure S7 in ESI). MCS-B4M records an average
by means of cyclic voltammetry in acetonitrile (Figure 4). MCS-B4M lifetime of 1.4 ns fitted with a single exponential decay
III/II
showed the first anodic peak at 0.78 V corresponding to Ru
whereas MCS-B5M registers an average lifetime of 4.9 ns fitted
couple and the oxidation of phenolate moiety to a phenoxyl radical. with a multiexponential decay. The higher excited state
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View Article Online
DOI: 10.1039/C9DT01506J
0.000005
0.000005
M
M
C
C
S
S
-
-
B
B
4
4
M
M
0.000030
0.000030
MCS-B4M
MCS-B4M
MCS-B5M
MCS-B5M
MCS-B5M
MCS-B5M
0
0
.
.
0
0
0
0
0
0
0
0
0
0
0
0
0.000025
0.000025
0.000020
0.000020
-0.000005
-0.000005
0.000015
0.000015
-0.000010
-0.000010
0.000010
0.000010
0.000005
0.000005
-0.000015
-0.000015
0.000000
0.000000
-0.000020
-0.000020
-0.000005
-0.000005
-
0
0
.
.
0
0
0
0
0
0
0
0
2
2
5
5
-0.000010
-0.000010
0
0
.
.
0
0
0
0
.
.
2
2
0
0
.
.
4
4
0
0
.
.
6
6
0
0
.
.
8
8
1
1
.
.
0
0
1
1
.
.
2
2
1
1
.
.
4
4
1
1
.
.
6
6
1
1
.
.
8
8
2
2
.
.
0
0
-
-
2
2
.
.
0
0
-
-
1
1
.
.
8
8
-
-
1
1
.
.
6
6
-
-
1
1
.
.
4
4
-
-
1
1
.
.
2
2
-
-
1
1
.
.
0
0
-
-
0
0
.
.
8
8
-
-
0
0
.
.
6
6
-
-
0
0
.
.
4
4
-
-
0
0
.
.
2
2
0.0
0.0
P
P
o
o
t
t
e
e
n
n
t
t
i
i
a
a
l
l
(
(
V
V
)
)
Potential vs. Ag/AgCl (V)
Potential vs. Ag/AgCl (V)
Fig.4. Cyclic Voltammogram of the complexes: anodic wave (left) and cathodic wave (right).
2
lifetime for MCS-B5M may be due to the greater delocalization Mechanistic aspects for H evolution
of the metal-centred electron density over the isonicotinic
moiety as corroborated later in DFT studies.32 There was no From photophysical and electrochemical studies, the two
emission when the complexes were excited at 700 nm.
Photocatalytic hydrogen evolution
pathways that drive the entire photochemical reactions can be
depicted as:
3+
*
3+
[
[
[
Ru ] + hν→[ Ru ]…………………………………………………………..1
3+
4+
-
*Ru ] + TiO
2
-Pt→[Ru ] + TiO
2
-Pt …………………………………...2
The Dye/TiO
for photocatalytic performances in aqueous solution with a
00 W xenon lamp and triethanolamine (TEOA) as sacrificial
electron donor. The data is summarized in Table 1. For
comparison experiments, dye-TiO -Pt composite materials
2
-Pt photocatalysts B4TP and B5TP were tested
3+
2+
*Ru ] + TEOA→[Ru ] + TEOAOX………………………………………3
The oxidative quenching of the complexes with TiO
reaction 2) and subsequent regeneration of the dyes via
reductive quenching by TEOA (reaction 3) is energetically
2
-Pt
3
(
2
12
feasible as shown:
In the case of B4TP photocatalyst
The oxidative quenching represented by reaction numbered
and the reductive quenching by reaction numbered might be
were prepared using commercial N719 via the same
experimental procedure (Figure 6a and b). It is noteworthy to
1
.
4
mention that the performance of MCS-B5M/TiO
photocatalyst (TON 84,600 after 5 h at 0.5 µmol/10 mg
catalyst in 2 mL aqueous TEOA solution: pH-7, details in ESI
2
-Pt
5
~
3+
possible quenching mechanisms of [*Ru ] by electron
transfer:
1
8
Table S1) surpassed the efficiency of the recent report using
more tediously prepared Pt based rhodamine sensitizer (TON
3
+
4+
-Pt-
[
*Ru ] + TiO
2
-Pt → [[Ru ] + TiO
2
……4
..…5
TEOA + [*Ru ] → TEOAox + [Ru2+]
3+
7
1
0700 after 40 hours at 0.025 µmol/20 mg catalyst in 5 mL of
.0 M ascorbic acid solution: pH 4).
The driving force for the oxidative quenching, reaction 4, is
ΔG
ΔG
4
= e E[Ru4+/3+] – E00 – e E(TiO
-Pt )
0/-
4
2
=
+1.11 eV – 3.16 eV – (–0.6 eV) = –1.45 eV.
The driving force for the reductive quenching, reaction
is:
5
5, ΔG ,
ΔG
– (–0.97 eV) – 3.16eV + 0.82 eV = –1.37 eV
= –e E(Ru3+/2+) – E00 + e E([TEOAox/TEOA)
5
=
2
.
In the case of B5TP photocatalyst
The oxidative quenching represented by reaction numbered
and the reductive quenching by reaction numbered might be
6
7
3+
possible quenching mechanisms of [*Ru ] by electron
transfer:
3
+
4+
-Pt-
[
*Ru ] + TiO
2
-Pt → [[Ru ] + TiO
2
…....6
…….7
TEOA + [*Ru ] → TEOAox + [Ru2+]
3+
The driving force for the oxidative quenching, reaction 6, is
ΔG
ΔG
6
= e E[Ru4+/3+] – E00 – e E(TiO
-Pt )
0/-
6
2
2
Fig. 5. Emission quenching of the complexes by TiO -Pt composite.
=
+1.21 eV – 3.09 eV – (–0.6 eV) = –1.28 eV.
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The driving force for the reductive quenching, reaction
is:
7
,
ΔG
7
,
metal complexes (MCS-B4M and MCS-B5M) showed
View Article Online
DOI: 10.1039/C9DT01506J
significantly higher H
sensitized TiO
Similar performance with N719 dye has been reported by
2
evolution than the standard N719 dye
ΔG
– (–0.96 eV) – 3.09eV + 0.82 eV = –1.31 eV
7
= –e E(Ru3+/2+) – E00 + e E([TEOAox/TEOA)
2
-Pt composites in a similar experimental setup.
=
2
4
others. We attribute the observations to the high oxidation
Therefore, for both the complexes, oxidative and reductive potential of 1.21 V for MCS-B5M as compared to 1.12 V for
quenching is feasible. DFT studies2 indicate that the HOMO N719 vs. NHE that results in a faster regeneration of the
2,23
33
3
4
is majorly based on the phenolate moieties and LUMO on the oxidised complexes by sacrificial electron donor (TEOA in this
bipyridine moieties of the chelating ligand for both the case). It is worth noting that the apparent quantum efficiency
complexes (Figure 7 inset). Interestingly, for complex MCS- of the B5TP photocatalyst showed impressively high, ~57% at λ
B4M, the HOMO has considerable contribution from the ≥ 420 nm (details in ESI). Another interesting feature, the
picoline moiety whereas for MCS-B5M, the LUMO is extended remarkable enhancement in the photocatalytic performance
to the isonicotinic acid moieties. The significant tuning of the of MCS-B5M/TiO
photophysical and electrochemical properties of the metal B4M/TiO -Pt is presumably due to the better light harvesting
complexes is achieved by using ancillary ligands with electron capability and higher excited state lifetime coupled with the
2
-Pt composite as compared to that of MCS-
2
35
donating (4-picoline in MCS-B4M) and electron withdrawing favourable electronic directionality
arising from the
(isonicotinic acid in MCS-B5M) groups. It is apparent that proximity of the electron acceptor states to the metal
employing our photocatalysts, we achieved higher TON with complex-semiconductor interface in the former case. Here,
comparatively lower catalyst loading and shorter irradiation Figure 7 shows an energy diagram representative of the
times. The photocatalytic performance of the metal complexes photocatalytic reaction.
was observed to show two distinct features. Firstly, both the
2
Fig.6 a. Time course of Photocatalytic activities and b. Histogram of the MCS-B4M, MCS-B5M and N719 sensitized TiO -Pt (B4TP, B5TP and N719TP) in visible-light-driven
water splitting; reaction conditions: 0.5 µmol/10 mg catalyst in 20 mL 10 vol% aqueous TEOA solution at pH-7.
Table 1. Photocatalytic performance of complexes
aH
bTON
cTOF(h-1)
Photocatalysts
2
Production(μmol)
B4TP
B5TP
N719TP
15467
21239
5336
~61,482
12296
~84,600
~21,152
16920
4230
aH
)/amount of catalyst used. cTOF (turnover frequency) values after 5h.
production after 5h. bTON (turnover number) = (2 X amount of produced
2
H
2
Reaction conditions: 10 mg photocatalyst (0.5 µmol dye) in 20 mL of 10 vol%
neutral aqueous TEOA solution.
Conclusion
Fig.7 Energy diagram depicting the photocatalytic process of the hybrid
systems. The HOMO energy level is determined from the CV of the
complexes in aqueous medium. LUMO energy calculated from the relation
In conclusion, two ruthenium-based complexes of bis-
(phenolato)bipyridine have been synthesized and studied as
ELUMO=EHOMO-E00.
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photosensitizers for photocatalytic hydrogen evolution. 13.
Among the two complexes, MCS-B5M with –COOH linker
R. Abe, K. Shinmei, K. Hara and B. OhtVainewi,ArCtihcleemOniclinael
Communications, 2009, 3577-3579D. OI: 10.1039/C9DT01506J
Q. Li, Y. Che, H. Ji, C. Chen, H. Zhu, W. Ma and J. Zhao,
Physical Chemistry Chemical Physics, 2014, 16, 6550-6554.
A. Tiwari and U. Pal, international journal of hydrogen
energy, 2015, 40, e9079.
1
1
1
1
4.
5.
6.
7.
moieties at the ancillary ligands exhibit efficient capability of
-1
hydrogen evolution of 4.2 mmolh and high TON 84,959 after
h in the presence of sacrificial electron donor under visible
5
light irradiation. The efficient photochemical performance of
the
composite is in conformity with the panchromatic light
A. Kumari, I. Mondal and U. Pal, New Journal of Chemistry,
2
015, 39, 713-720.
K. Maeda, M. Eguchi, W. J. Youngblood and T. E. Mallouk,
Chemistry of Materials, 2009, 21, 3611-3617.
harvesting capability of MCS-B5M throughout the visible light
III/IV
range and a high oxidation potential (Eox=1.21 V for Ru ) in 18.
aqueous media allowing facile regeneration of the oxidised
sensitizer. To the best of our knowledge, this study is the first
G. Li, M. F. Mark, H. Lv, D. W. McCamant and R. Eisenberg,
Journal of the American Chemical Society, 2018, 140,
2575-2586.
G. Koyyada, N. S. Pilli, J. H. Jung, K. K. Mandari, B.
Shanigaram and M. Chandrasekharam, International
Journal of Hydrogen Energy, 2018, 43, 6963-6976.
Y. Liu, G. Chen, S. M. Yiu, C. Y. Wong and T. C. Lau,
ChemCatChem, 2018, 10, 501-504.
1
9.
report of such ruthenium-phenolate complexes working as
photosensitizers for visible-light-driven hydrogen evolution
from water. Moreover, this work also highlights the
importance of ancillary ligands for such photosensitizers in
developing effective photocatalyst systems.
2
0.
1.
2
D. T. Sawyer, A. Sobkowiak and J. L. Roberts Jr.,
Electrochemistry for Chemists, John Wiley & Sons, New
York, second ed. edn., 1995.
Acknowledgements
B.N.M. acknowledges SERB-DST for National Post-Doctoral
Fellowship (Ref. No. PDF/2017/001198). A.T. express thanks to
CSIR for a Senior Research Fellowship and AcSIR for the Ph.D.
enrolment. U.P. and M.C. thanks to DAE/BRNS (GAP-0728) and
DST (File No. DST/TM/SERI/FR/92(G) for their respective
research grants.
2
2.
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M.
A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B.
Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li,
H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L.
Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.
Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H.
Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F.
Ogliaro, M. J. Bearpark, J. Heyd, E. N. Brothers, K. N.
Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K.
Raghavachari, A. P. Rendell, J. C. Burant, S. S. Iyengar, J.
Tomasi, M. Cossi, N. Rega, N. J. Millam, M. Klene, J. E.
Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R.
Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R.
Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K.
Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J.
Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B.
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