Wirelike dinuclear ruthenium(II)polyterpyridine complexes based on D–P–A architecture: Experimental and theoretical investigation

Article abstract of DOI:10.1016/j.jphotochem.2017.03.009

To accomplish the goal of prolonged excited state, efficient for interfacial electron injection and low electron-hole recombination (LUMO?→?HOMO of complex) we have synthesized a heteroleptic complexes 1 and 2 based on D–P–A architecture (where P?=?Photosensitizer, A?=?Acceptor and D?=?Donor). The complexes 1 and 2 show the average excited state lifetimes (τ_avg) of 25?ns and 12.67?ns respectively compared to 0.25?ns for [Ru(tpy)₂]2PF₆. The τ_avg of such an order is sufficient for performing the interfacial electron transfer into the conduction band of TiO₂ and redox chemistry of excited state. The electrochemical studies exhibit the oxidation potential (E_ox) of 1.23?V and 1.27?V vs SCE with associated excited-state redox potentials (E_ox^*=?0.95?V and ?0.85?V and E_red^*=1.0?V and 0.88?V for complexes 1 and 2 respectively) have evidence for stronger reductant and oxidant behaviour in ¹MLCT excited state. Subsequent interfacial study with TiO₂ nanocrystal is in good agreement with photo induced electron injection from LUMO of complex to conduction band (CB) of TiO₂ as LUMO level (?3.22?eV and 3.16?eV for complexes 1 and 2 respectively) of complexes is above the CB of TiO₂ (?3.65?eV). The experimental photophysical results of both the complexes are well supported by time dependent density functional theory.

Full text of DOI:10.1016/j.jphotochem.2017.03.009

Journal of Photochemistry and Photobiology A: Chemistry 340 (2017) 170–180
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Wirelike dinuclear ruthenium(II)polyterpyridine complexes based on
D–P–A architecture: Experimental and theoretical investigation
*
Pallavi Singh, Prem Jyoti Singh Rana, Prasenjit Kar
Department of Chemistry, Indian Institute of Technology, Roorkee 247667, India
A R T I C L E I N F O
A B S T R A C T
Article history:
To accomplish the goal of prolonged excited state, efficient for interfacial electron injection and low
electron-hole recombination (LUMO ! HOMO of complex) we have synthesized heteroleptic
complexes 1 and 2 based on D–P–A architecture (where P = Photosensitizer, A = Acceptor and D = Donor).
The complexes 1 and 2 show the average excited state lifetimes ( ) of 25 ns and 12.67 ns respectively
Received 30 November 2016
Received in revised form 4 March 2017
Accepted 7 March 2017
a
Available online 14 March 2017
t
avg
compared to 0.25 ns for [Ru(tpy)2]2PF6. The
interfacial electron transfer into the conduction band of TiO2 and redox chemistry of excited state. The
electrochemical studies exhibit the oxidation potential (E ) of 1.23 V and 1.27 V vs SCE with associated
t
avg
of such an order is sufficient for performing the
Keywords:
Dinuclear ruthenium terpyridyl
Donor–acceptor complex
Interfacial electron transfer
Excited state lifetime
ox
ꢀ
ꢀ
excited-state redox potentials (Eox ¼ ꢁ0:95 V and ꢁ0.85 V and Ered ¼ 1:0 V and 0.88 V for complexes 1
and 2 respectively) have evidence for stronger reductant and oxidant behaviour in 1MLCT excited state.
Subsequent interfacial study with TiO2 nanocrystal is in good agreement with photo induced electron
injection from LUMO of complex to conduction band (CB) of TiO2 as LUMO level (ꢁ3.22 eV and 3.16 eV for
complexes 1 and 2 respectively) of complexes is above the CB of TiO2 (ꢁ3.65 eV). The experimental
photophysical results of both the complexes are well supported by time dependent density functional
theory.
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
in 3MLCT and 3MC energy level [24]. As a result major relaxation
occurs via non-emissive deactivation pathway through 3MC
From the last couple of decade ruthenium(II)polypyridine
complexes found their extensive application in field of energy
conversion systems like electroluminescent devices [1–6] and dye
sensitized solar cell [7–11]. These applications are based on the
extraordinary photophysical, photochemical and electrochemical
properties of ruthenium(II)polypyridine complexes [12–14].
Among them the polynuclear complexes are more promising
one [15]. We have preferred binuclear ruthenium(II)polyterpyr-
idine complex with highly conjugated terpyridine linker based on
wire like architecture over achiral mononuclear [Ru(tpy)2]2PF6.
This selection is based on the fact that the binuclear ruthenium(II)
polyterpyridine complex involves long range charge separation
that increase the lifetime of binuclear complex which is essential
for efficient interfacial electron transfer [16–21]. However, the
mononuclear [Ru(tpy)2]2PF6 have a disadvantage of short lifetime
of ꢂ250 ps and lower reduction potential of excited state [22,23].
This is due to unfavourable bite angle of tpy that weaken ligand
field thereby smaller d–d splitting occur which allow equilibration
excited state which diminishes RT luminescence [23]. Literature
reveal various ways to accomplish prolonged excited state by
cyclometallation [25–28], incorporation of strong
-acceptor ligands [29,30] (effectively shift 3MC to higher energy)
and widening of NꢁꢁRuꢁꢁN bite angle [31,32].
s-donor and
p
Taking the concern of prolonged lifetime for efficient electron
transfer to semiconducting material TiO2 in photovoltaic material
we have designed and synthesized two D–P–A complexes [(L2)Ru
(L1)Ru(L4)]4PF6 and [(L3)Ru(L1)Ru(L4)]4PF6 named complexes 1
and 2 respectively shown in Schemes 1 and 2. The design of such
complexes basically incorporates end capping of spacer (L1) with
terpyridyl ligand bearing donor and acceptor redox active groups
(Schemes 1 and 2). Here either complex involves incorporation of
strong donor (L2 and L3 in case of complexes 1 and 2 respectively)
and strong
p
-acceptor (L4) at 40 position of both end capping
terpyridine unit. Literature reviews show that strong
s-donor
moiety stabilize 3MLCT transition by destabilizing the metal
centred t2g orbital (HOMO) more effectively compared to the lifting
of ligand centred LUMO simultaneously it also destabilize 3MC
excited state. This HOMO destabilization is due to transfer of
electron density from the
p orbital of donor which is not involved
* Corresponding author.
E-mail addresses: prkarfcy@iitr.ac.in, kar.prasen@gmail.com (P. Kar).
http://dx.doi.org/10.1016/j.jphotochem.2017.03.009
1010-6030/© 2017 Elsevier B.V. All rights reserved.

P. Singh et al. / Journal of Photochemistry and Photobiology A: Chemistry 340 (2017) 170–180
171
in 3MLCT. The acceptor group stabilize 3MLCT by lowering the
ligand centred LUMO more prominently compared to lowering of
the metal centred t2g orbital HOMO, with simultaneous destabili-
zation of 3MC state. Here ꢁꢁCOOH group functions both as good
acceptor and as anchoring group for dye sensitized solar cell (DSSC)
[33–36]. These two effects balance each other thereby the decrease
of the 3MLCT transition. This energy gap engineering between
3MLCT and 3MC level inhibited surface crossing to some extent to
obtained RT luminescence [37]. In addition to this, selection of
spacer having rigid conformation provides better electronic
communication between metal centres through bond coupling
[38]. They provide extended conjugation which increases the light
harvesting of molecule. Other than this, spacer control complex
architecture by tuning intercomponent distance and angles.
The basic requirement of long excited state lifetime is achieved
to some extent by the selection of above ligand moieties as both the
complexes 1 and 2 show the average excited state lifetimes (
t
)
avg
of 25 ns and 12.67 ns respectively obtained from TCSPC measure-
ment compared to 0.25 ns for [Ru(tpy)2]2PF6. The of such an
t
avg
order is sufficient for performing the interfacial electron transfer
with TiO2 and redox chemistry of excited state. The electrochemi-
cal study exhibit the oxidation potential E of 1.23 V and 1.27 V vs
ox
ꢀ
SCE with associated excited-state redox potentials (Eox ¼ ꢁ0:95 V
ꢀ
and ꢁ0.85 V and Ered ¼ 1:0 V and 0.88 V for complexes 1 and 2
respectively) have evidence for stronger reductant and oxidant
behaviour in 1MLCT excited state. Subsequent interfacial study
with TiO2 nanocrystal is in good agreement with electron injection
from LUMO of complex to CB of TiO2. The experimental
Scheme 1. Synthesis of ligands.
Scheme 2. Synthesis of complexes 1 and 2.

172
P. Singh et al. / Journal of Photochemistry and Photobiology A: Chemistry 340 (2017) 170–180
Table 1
photophysical results in both the complexes are well supported by
theoretical TDDFT studies.
Spectroscopic and photophysical data of complexes in acetonitrile.
lmax (1MLCT)
lmax
(p
!
p
*)
lem (lexc)
at RT
(e
ꢃ 104)Mꢁ1 cmꢁ1
(e
ꢃ 104)Mꢁ1 cmꢁ1
2. Result and discussion
Complex 1
495 nm (1.24)
498 nm (1.85)
474 nm (0.17)
286 nm (3.24),
309 nm (3.33)
286 nm (4.02),
306 nm (4.36)
270 nm (0.48),
307 nm (0.78)
661 nm
(495 nm)
670 nm
(498 nm)
629 nm
We have synthesized the two complex [(L2)Ru(L1)Ru(L4)]4PF6
and [(L3)Ru(L1)Ru(L4)]4PF6 designated as complexes 1 and 2
respectively based on D–P–A architecture according to Schemes 1
and 2. The detailed synthesis procedure is provided in Section 4.
Complex 2
[Ru(tpy)2](PF6)2 [39]
2.1. UV–vis absorption
complexes 1 and 2 is indicative of the fact that both metal centres
The experimental UV–vis absorption spectra of both complexes
1 and 2 are shown in Fig.1 and their relevant spectroscopic data are
summarized in Table 1. Both complexes show very similar
absorption profile but the molar extinction coefficient for complex
2 is almost 1.5 fold greater than complex 1. This point up the extent
to which the auxochrome ꢁꢁOMe alters the light harvesting
characteristic of complex 2 over phenyl group in complex 1. The
absorption pattern for both complexes in the UV region at
are oxidized at same oxidation potential E of 1.23 V and 1.27 V vs
ox
SCE respectively. The oxidation of both ruthenium centre at the
same potential were further supported by single peak in
differential pulse voltammetry (DPV), which further diminishes
the possibility of electronic coupling between two metal centres,
essential for long range fast electron transfer (Figure. SI 2) [41].
However, measurement of potential difference considered as a
poor indicator of electronic coupling [42–44]. Moreover, the E
ox
ꢂ285 nm and 305 nm were assigned for
p–p* transition in the
seems to be shifted cathodically by 70 mV and 30 mV for
nearly identical ligand backbone. The absorption spectrum in the
visible region is characterized by the presence of single strong
absorption band at 495 nm in complex 1 and at 498 nm in complex
2 and found to be red shifted from lmax 475 nm for [Ru(tpy)2](PF6)2
[39,40]. This is solely described for 1MLCT transition involving
transfer of electron density from Ru(II) ! L1 spacer ligand, Ru
(II) ! L4 acceptor ligand and Ru(II) ! tpy of L2/L3 donor ligand, this
is further supported by TDDFT studies.
complexes 1 and 2 respectively with respect to E of 1.30 V for
ox
[Ru(tpy)2]2PF6. This positive metal centred oxidation potential
directly corresponds to electron donating nature of respective
ligand L2 and L3 in complexes 1 and 2 respectively [45]. Other than
this, there is no more oxidation waves present in the anodic region.
As like previous reports, the reduction is ligand localized
(Fig. 2b) [46–48]. The first single reversible redox wave in the
cathodic region at ꢁ1.18 V and ꢁ1.24 V for complexes 1and 2, this is
positively shifted compared to first reduction potential of [Ru
(tpy)2]2PF6 (ꢁ1.24 V vs SCE). The second reversible redox potential
at ꢁ1.42 V and ꢁ1.44 V for complexes 1 and 2 shifted towards
positive axis with respect to 1.46 V for [Ru(tpy)2]2PF6. The third
reduction potential for both complexes is observed at the potential
1.98 V.
2.2. Photoluminescence spectra
The photoluminescence behaviour of both complexes was
investigated upon excitation at their respective lmax in acetonitrile
at room temperature (Fig. 1). Both of them show a broad emission
extending from 540 nm to 800 nm with an emission maximum at
661 nm and 670 nm for complexes 1 and 2 respectively. This
emission mainly corresponds to 3MLCT ! S0 transition that
involves relaxation from 3MLCT states which was populated
directly from photoexcited 1MLCT states via an ultrafast Inter
System Crossing (ISC) process.
Further, Rehm–Weller equation [49] was used to determine the
ꢀ
ꢀ
excited-state oxidation ðEox Þ and reduction ðEred Þ potentials of
complexes in 1MLCT excited state using ground state redox
potentials
ꢀ
Eox ¼ Eox ꢁ E0ꢁ0
and
2.3. Cyclic voltammetry
ꢀ
Ered ¼ Ered þ E0ꢁ0
where E and E
ox
are the oxidation and reduction potentials in
The cyclic voltammogram (CV) of complexes in the anodic
region show single reversible redox wave for two electron
oxidation process (Fig. 2a ). This electrode reaction corresponds
to one electron oxidation of each (RuII/RuIII) centre present in
either type of complexes [41]. The peak profile analysis of
red
ground state obtained from CV, E0–0 is the energy difference
between the lowest vibrational levels of ground electronic state
potential energy curve and first excited electronic state potential
energy curve and correspond to intersection point of absorption
Fig. 1. (a) Absorption spectra and (b) photoluminescence spectra of complexes 1 and 2 in acetonitrile.

P. Singh et al. / Journal of Photochemistry and Photobiology A: Chemistry 340 (2017) 170–180
173
Fig. 2. Cyclic voltammogram of (a) metal based oxidation and (b) ligand based reduction in complexes 1 and 2 (1 mM complex in acetonitrile using 0.1 M (Bu)4NPF6
supporting electrolyte and [Fc]+/[Fc]0 as internal standard with scan rate of 200 mV sꢁ1).
Table 2
Ground and excited state electrochemical properties of complexes in acetonitrile.
ꢀ
ꢀ
E
ox
(Ru II/Ru III) SCE (V)
E
(V)
E
(V)
E (V)
red 3
HOMO (eV)
LUMO (eV)
E0–0 (eV)
Eox (V)
Ered (V)
vs
red 1
red 2
Complex 1
Complex 2
[Ru(tpy)2] (PF6)2 [40]
1.23
1.27
1.30
ꢁ1.18
ꢁ1.24
ꢁ1.24
ꢁ1.42
ꢁ1.44
ꢁ1.46
ꢁ1.98
ꢁ1.98
ꢁ5.63
ꢁ5.67
ꢁ5.70
ꢁ3.22
ꢁ3.16
ꢁ3.16
2.18
2.12
ꢁ0.95
ꢁ0.85
1.00
0.88
and emission spectra where the emission spectra is normalized to
broad with an observable absorbance increment as compared to
their bare complex solution (Fig. 3a) [51–56]. This increment in
absorption spectra attributed to the increased absorption derived
through TiO2 binding simultaneously the characteristic shoulder
appeared at 580 nm after TiO2 binding is assigned for the charge
transfer complex formation through strong electronic coupling
between the TiO2 nanoparticles and ꢁꢁCOOH anchoring group of
complexes [57,58]. The emission of each complex is quenched
significantly under the influence of colloidal dispersion of TiO2
nanoparticle (Fig. 3b). It arises due to efficient photo induced
electron transfer from excited state (LUMO) of complex to CB of
TiO2 followed by depopulation of excited state that lead to
quenched photoluminescence [59].
lowest energy 1MLCT transition. The Eox and Ered values were
summarized in Table 2 showing stronger reductants and oxidants
behaviour of complexes in 1MLCT excited state.
ꢀ
ꢀ
2.4. Interfacial electron transfer studies
The interfacial electron transfer dynamics involving electron
injection from LUMO of sensitizer to CB of TiO2 depends upon the
binding interaction of sensitizer with TiO2 nanoparticle and is
regarded as the primary step towards charge separation in DSSCs
[50]. This phenomenon of electron transfer is well illustrated by
comparing the absorption and photoluminescence behaviour of
bare complex solution and complex solution with colloidal
dispersion of semiconducting TiO2 nanoparticles shown in
Fig. 3. The resulting complex–TiO2 system consists of a complex
anchored to co-ordinatively unsaturated Ti(IV) hard Lewis centres
present in TiO2 nanocrystal through the anchoring group (ꢁꢁCOOꢁ)
of the complex molecule. These binding influence the excited state
properties and interfacial behaviour. On binding with TiO2
nanoparticles, the absorption spectra for each complex became
2.5. Time correlated single photon counting (TCSPC)
In addition to steady state absorption and emission studies, the
Time Correlated Single Photon Counting (TCSPC) technique can
also be the efficient tool for monitoring the interfacial electron
transfer. Both of the complexes show tri-exponentially fitted decay
kinetics with their respective decay time and related amplitude
Fig. 3. (a) Optical absorption spectra and (b) photoluminescence spectra of complexes 1 and 2 in acetonitrile using complex concentration 20 mM and TiO2 nanoparticle 20 g/
L.

174
P. Singh et al. / Journal of Photochemistry and Photobiology A: Chemistry 340 (2017) 170–180
Table 3
Decay parameter obtained from the decay kinetic fitting of complexes 1 and 2 in different environments.
Average life time (tavg) in acetonitrile
Average life time (tavg) on TiO2 nanoparticle
Complex 1
tavg = 25 ns
tavg = 10.39 ns
t1 = 9.82 ns (14.5%)
t2 = 2.88 ns (80.22%)
t3 = 52.23 ns (5.28%)
t1 = 2.99 ns (28.55%)
t2 = 0.27 ns (64.58%)
t3 = 17.14 ns (6.87%)
x
2 = 1.31
x
2 = 1.36
Complex 2
tavg = 12.67 ns
tavg = 6.8 ns
t1 = 0.80 ns (8.98%)
t2 = 3.51 ns (79.51%)
t3 = 22.69 ns (11.61%)
t1 = 0.04 ns (75.65%)
t2 = 3.34 ns (21.62%)
t3 = 13.95 ns(2.73%)
x
–
x
2 = 1.11
0.25 ns
2 = 1.10
[Ru(tpy)2](PF6)2 [39]
given in Table 3, in aerated acetonitrile solution under 460 nm laser
pulse photoexcitation (Fig. 4). We have calculated the average
lifetimes of the excited state obtained from decay parameter using
following equation
respective complexes therefore, we have obtained comparative
quenched decay pattern for complex 1/2–TiO2 system (Fig. 4). This
was further supported by decrease in decay time of component in
case of complex 1/2–TiO2 system with respect to their respective
complexes. In case of complex 1 the decay time of first component
n
2
X
aiti
aiti
(t1) decrease from 9.82 ns to 2.99 ns, second component (t2)
tavg
¼
decrease from 2.88 ns to 0.27 ns and third component (t3) decrease
from 52.23 ns to 17.14 ns. Likewise, the complex 2 shows decrease
in the decay time of all three components in its complex 2–TiO2
system. The quenched decay pattern along with decrease in decay
time of component in complex 1/2–TiO2 system indicates
interfacial charge transfer from LUMO of complex to CB of TiO2.
i¼1
where
t
= average lifetime, a = amplitude of the components
avg i
represented in percentage,
t = decay times of the component,
i
n = no. of decay times.
The complexes 1 and 2 show the average excited state lifetimes
t ) of 25 ns and 12.67 ns respectively compared to 0.25 ns for
avg
(
[Ru(tpy)2]2PF6 (Fig. 4 and Table 3). Therefore, substitution of
2.6. Computational studies
p
-acceptor substituent and
s
-donor substituent at 40-position of
the tpy in order to tune the luminescence lifetime was justified to
some extent. Previous reports show multiphasic photoinduced
charge transfer kinetics with an ultrafast component in the 10–
100 fs time range and another slower component in the 10–100 ps
time range [60]. This imply that obtained average lifetime of such
an order is sufficient for the study of interfacial charge dynamics
with semiconducting TiO2 and other redox chemistry associated
with excited state [61,62]. We have also carried out the TCSPC
study of complex–TiO2 system in order to ensure the interfacial
electron transfer. Further, both complexes display tri-exponential-
ly fitted decay kinetics for complex–TiO2 system under same laser
pulse photoexcitation and their respective decay parameter are
given in Table 3 (Fig. 4). It is noteworthy that the phosphorescence
decay of complex 1/2–TiO2 system were more rapid than their
The geometry optimization of complexes was performed at
B3LYP/LANL2DZ level of theory using Gaussian 09 programs with
default convergence criteria [63–68]. The self-consistent reaction
field (SCRF) calculation was performed using Tomasi's Conductor-
like polarizable continuum model (CPCM) to observe the solvent
effect on complexes due to acetonitrile (Fig. 5). Using the ground
state optimized geometry, the theoretical absorption spectra of the
complexes were calculated at B3LYP/LANL2DZ level of theory
within the framework of the time dependent density functional
theory (TDDFT) method (Fig. 6). The geometric parameters of each
complex in the singlet state reasonably reproduce the correspond-
ing experimental values.
The theoretical spectra include excitations from 392 nm to
527 nm because first 30 excitations are calculated mainly to focus
Fig. 4. Time correlated single photon counting studies of complexes in different environments: (a) complex 1 and complex 1–TiO2, and (b) complex 2 and complex 2–TiO2 in
aerated acetonitrile under 460 nm using complex concentration 20
the decay pattern.
mM and TiO2 nanoparticle 20 g/L. Here solid circle represent the fitting result and hollow circle represent

P. Singh et al. / Journal of Photochemistry and Photobiology A: Chemistry 340 (2017) 170–180
175
Fig. 5. Optimized geometry of complexes 1 and 2 at B3LYP/LANL2DZ in acetonitrile.
on visible region. Fig. 6 shows absorption peak at 491 nm for
complex 1 (exp. lmax 495 nm) and 489 nm for complex 2 (exp. lmax
498 nm) involving electronic transition from (S0 ! S9) with
oscillator strength 1.3801 and 1.2610 respectively. In this electronic
transition (S0 ! S9), there is contribution from following molecular
orbital H–5 ! L + 3 (8.82%), H–4 ! L + 1 (12.47%), H–1 ! L (55%),
H ! L + 2 (18.23%) in case of complex 1 and H–5 ! L + 3 (8.94%),
H–4 ! L + 1 (12.76%), H–1 ! L (51.37%), H ! L + 2 (20.40%) in case of
complex 2 (Table 4). Both complexes show nearly similar electron
density distribution in various molecular orbital involved in this
particular range of interest (Figs. 7 and 8). In HOMO, the electron
density is mainly localized on the Ru(II) centre (belonging to donor
ligand), central pyridine ring of donor tpy, attached phenylene and
donor substituents (ꢁPh in case of L2 and ꢁOCH3 in case of L3 in
complexes 1 and 2 respectively). In LUMO, the electron density is
mainly localized on the spacer and slightly on both Ru(II) centres.
In the transition H-5 ! L + 3, the electron density which was
present mainly on both Ru(II) centres and slightly on central
pyridine ring of spacer were shifted towards tpy part of spacer and
to some extent on the both Ru(II) centres with contribution 8.82%
and 8.94% for complexes 1 and 2 respectively. The transition
H–4 ! L + 1 having contribution of 12.47% and 12.76% for respec-
tive complexes 1 and 2, involve migration of electron density from
the Ru(II) centre (attached to L4 ligand) to entire L4 ligand with a
small electron cloud over the Ru(II) adjacent to acceptor L4 ligand.
In H–1 ! L transition having major contribution of 55% and
51.37% for respective complexes 1 and 2, involve transfer of
electron density from complex HOMO-1 (where electron cloud is
mainly localized on both the Ru(II) centres and on both central
pyridine and phenylene of spacer) to LUMO.
The remaining transition H ! L + 2 having contribution of
18.23% and 20.40% for respective complexes 1 and 2, involve the
migration of electron cloud from HOMO to Ru(II) and attached
entire donor ligand (L2 in case of complex 1 and L3 in case of 2). All
these transitions correspond to 1MLCT band occurred in visible
region of absorption spectrum in both the complexes. This
computational study show the band gap of 3.09 eV and 3.04 eV
for complexes 1 and 2 respectively which is near to the bandgap
2.41 eV and 2.51 eV for respective complexes 1 and 2 obtained from
CV measurement.
Fig. 6. The experimental electronic absorption spectra and TDDFTcalculated energy transitions and oscillator strengths shown as vertical blue lines. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of this article.)

176
P. Singh et al. / Journal of Photochemistry and Photobiology A: Chemistry 340 (2017) 170–180
Table 4
Theoretical absorption wavelength (labs), oscillator strength (f) and orbital transitions of complexes in acetonitrile.
Complex
Electronic transitions
labs
f (a.u.)
Transition assignment
(nm)
(eV)
2.52
Complex 1
S0 ! S9
491.05 (495)
1.3801
H–5 ! L + 3 (8.82%)
H–4 ! L + 1 (12.47%)
H–1 ! L (55%)
H ! L + 2 (18.23%)
H–5 ! L + 3 (8.94%)
H–4 ! L + 1 (12.76%)
H–1 ! L (51.37%)
H ! L + 2 (20.40%)
Complex 2
S0 ! S9
489.45 (498)
2.53
1.2610
Fig. 7. Frontier orbitals of complex 1 involved in various electronic transitions.
1
3. Conclusion
H NMR – 1H NMR spectra were recorded on Jeol 400 MHz and
Bruker 500 MHz spectrometer.
The above findings suggest complexes 1 and 2 are appropriate
as photosensitizer for dye sensitized solar cell having prolonged
excited state life time with good oxidation and reduction potentials
values. In addition, the excited state redox potential values are also
evident for easier transfer of electron from LUMO of sensitizer to
CB of TiO2. This was further support by interfacial electron transfer
from LUMO of sensitizer to CB of TiO2 through study of complexes
in the colloidal dispersion of nanocrystalline TiO2. The above
findings suggest that the synthesized complexes efficiently behave
as sensitizer in Dye sensitized solar Cell application.
CHNS analysis – Elementar vario Micro instrument was used
for the CHNS analysis of samples.
Matrix-Assisted Laser Desorption/Ionization mass spectrom-
etry (MALDI) – We have recorded MALDI in combination with time
of flight mass spectrometer (TOF MS) for detection of molecular
mass of complex using Bruker deltonics ultrafleXtremeTM. The
MALDI matrix HABA is prepared by dissolving 2 mg mLꢁ1 in
appropriate solvent like THF or acetonitrile followed by gentle heat
to dissolve solid completely. Then diluted solution of complex was
mixed with matrix solution in suitable proportion, placed 5–10 mL
of this solution on MALDI sample plate, and kept this sample under
ion source after complete drying.
4. Experimental
HRMS – High resolution mass spectrometry (HRMS) was
performed using Bruker mass spectrometer.
4.1. Materials and instruments
UV–vis
– UV–vis absorption spectra were recorded by
Shimadzu UV–vis 2450 spectrophotometer using 10ꢁ5 M solution
of complex. The sensitization of complex takes place by adding
colloidal solution of TiO2 and kept for 24 h for complete
All reactions were carried out under N2 atmosphere (Scheme 1).
All reagents were purchased from Sigma–Aldrich.

P. Singh et al. / Journal of Photochemistry and Photobiology A: Chemistry 340 (2017) 170–180
177
Fig. 8. Frontier orbitals of complex 2 involved in various electronic transitions.
sensitization. This was followed by filtration using microfilter of
22 M.
Photoluminescence
a mixture of solvent ethanol and water (4/1, v/v) and dried in
vacuum. This was followed by recrystallization in the mixed
m
–
Horiba scientific Fluoromax-4C
solvent CHCl3:Et2O. Light yellow solid. 1H NMR (400 MHz, CDCl3)
d
spectrophotometer was used to record photoluminescence spec-
tra. The fluorescence of complex was measured in air-equilibrated
solution of acetonitrile.
TCSPC – TCSPC experiment for lifetime measurement was
recorded using HORIBA Jobin Yvon FluoroCube Spectrofluorome-
ter.
8.80 (s, 4H), 8.77–8.74 (m, 4H), 8.71–8.67 (4H), 8.07 (s, 4H),
7.92–7.86 (m, 4H), 7.39–7.35 (m, 4H). HRMS (ESI, m/z) [M+Na]+
calcd. for C36H24N6: 563.1960; found 563.1895.Cal C 79.98, H 4.47,
N 15.55, Observed C 79.94, N 15.36, H 3.86.
4.2.2. 40-([1,10-Biphenyl]-4-yl)-2,20:60,200-terpyridine (L2)
Infra-red spectroscopy – IR spectrum was recorded using
Thermo scientific Nicolet 6700 and film was prepared by grinding
the sample with previously dried KBr.
2-Acetyl pyridine 1.0 g (8.25 mmol) and 4-phenyl benzaldehyde
0.782 mg (4.3 mmol) was dissolved in 40 mL of ethanol, KOH
0.630 mg (11 mmol) was dissolved in 25 mL NH3 (aq) 30% and was
further added to the previous solution. This mixture was stirred for
1 day at 60 ꢄC. The precipitate was filtered, washed with a mixture
of solvent ethanol and water (4/1, v/v) and dried in vacuum. White
Cyclic Voltammetry
– Electrochemical experiments were
performed in a standard electrochemical cell under nitrogen
atmosphere using a CH instruments model 440 potentiostat. This
three electrode system comprises of glassy carbon electrode as the
working electrode, platinum wire as the counter electrode and
saturated calomel electrode as a reference electrode. Polishing of
glassy carbon electrode was performed prior to experiment All
measurements were performed in 1 mM solution of complex in
ACN using 0.1 M (Bu)4NPF6 supporting electrolyte to ensure
sufficient electrical conductivity and [Fc]+/[Fc]0 as internal
solid m.p. 198–199 ꢄC, 1H NMR (CDCl3, 400 MHz):
d ppm 7.35–7.39
(m, 3H), 7.48 (t, J = 8.0 Hz, 2H), 7.68 (d, J = 8.0 Hz, 2H), 7.74 (d,
J = 8.0 Hz, 2H), 7.9–7.86 (m, 2H), 8.00 (d, J = 8.0 Hz, 2H), 8.69 (d,
J = 8.0 Hz, 2H), 8.76–8.73 (m, 2H), 8.80 (s, 2H). Cal C, 84.13; H, 4.97;
N, 10.90, found: C, 84.16; H, 5.24; N, 10.58.
standard with scan rate of 200 mV sꢁ1
.
4.2.3. 40-(3,4-Dimethoxyphenyl) 2,20:60,200-terpyridine (L3)
2-Acetyl pyridine 1.0 g (8.25 mmol) and 3,4-dimethoxy benzal-
dehyde 0.714 mg (4.3 mmol) was dissolved in 40 mL of ethanol,
KOH 0.630 mg (11 mmol) was dissolved in 25 mL NH3 (aq) 30% and
was further added to the previous solution. This mixture was
stirred for 1 day at 60 ꢄC. The precipitate was filtered, washed with
a mixture of solvent ethanol and water (4/1, v/v) and dried in
vacuum. Light green solid. 1H NMR, CDC13.d (ppm): 3.97 (s, 3 H),
4.03 (s, 3H), 7.00 (d, J = 8.0 Hz, 1H), 7.36 (ddd, J = 7.6, 4.8, 1.2 Hz, 2H),
7.40 (d, J = 2.0 Hz, 1H), 7.51 (dd, J = 8.4, 2.0, 1H), 7.89 (td, J = 7.8,
1.8 Hz, 2H), 8.67 (m, 4H), 8.73 (m, 2H). HRMS (ESI, m/z) [M+Na]+
4.2. Synthesis of ligands and complexes
4.2.1. 2-(4-(4-(2,6-Di(pyridin-2-yl)pyridin-4-yl)phenyl)-6-(pyridin-
2-yl)pyridin-2-yl)pyridine (L1)
2-Acetyl pyridine 2.0 g (16.5 mmol) and terephthalaldehyde
0.576 g (4.3 mmol) was dissolved in 40 mL of ethanol, KOH
0.630 mg (11 mmol) was dissolved in 25 mL NH3 (aq) 30% and
was further added to the previous solution. This mixture was
stirred for 1 day at 60 ꢄC. The precipitate was filtered, washed with

178
P. Singh et al. / Journal of Photochemistry and Photobiology A: Chemistry 340 (2017) 170–180
calcd. for C23H19N3O2: 392.1375; found 392.1362. Cal C 74.78, H
5.18, N 11.37, Observed C 74.57, H 5.22, N 11.15.
4.3. Synthesis of nanoparticle TiO2
Nanoparticle TiO2 was synthesized by controlled hydrolysis of
titanium(IV)tetraisopropoxide.[54–59] A solution of 5 mL of Ti
[OCH(CH3)2]4 dissolved with 95 mL of isopropyl alcohol was added
dropwise (1 mL/min) to 900 mL of milli q water (2 ꢄC) at pH 1.5
(adjusted with HNO3). The solution was incessantly stirred for 10–
12 h until a translucent colloidal solution was formed. This solution
was concentrated at 35–40 ꢄC with a rotary evaporator and then
dried out with a nitrogen stream to yield a white powder. In the
present work all colloidal samples were prepared after dispersing
the dry TiO2 nanoparticles in water (15 g/L).
4.2.4. 40-(4 Carboxyphenyl)-2,2060:200-terpyridine (L4)
4-Carboxybenzaldehyde (360 mg, 2.4 mmol) and 2-Acetylpyr-
idine (580 mg, 4.8 mmol) and were dissolved in CH3OH (16 mL) by
stirring for 5 min, followed by addition of 15% KOH (14.2 mL) and
concentrated NH4OH (1.6 mL). The mixture was allowed to stand at
ambient temperature for 24 h. The emulsion formed was filtered
off and washed with CHCl3 (8 mL) and cold CH3OH/H2O (1:1)
(8 mL). The crude product was suspended in CH3OH/H2O (80:20),
and the mixture was stirred and sonicated at 35 ꢄC until a clear
solution was obtained. This was then acidified to pH = 2 by addition
of 1 M HCl, resulting in the formation of a white precipitate that
was collected by vacuum filtration and rinsed with cold water. The
solid was dried by means of vacuum filtration to give the pure
Acknowledgements
PK gratefully acknowledges the Council of Scientific & Industrial
Research CSIR (01(2796)/14/EMR-II), New Delhi, India for financial
support. PJSR acknowledges MHRD, India, for his senior research
fellowship. PS acknowledges MHRD, India, for her junior research
fellowship.
product as a white solid. IR (KBr) 1699 cmꢁ1
(
nC O); 1H NMR
¼
(400 MHz, DMSO-D6) 9.04 (d, J = 8 Hz, 2H), 8.95 (s, 2H), 8.93 (d,
d
J = 8 Hz, 2H), 8.42 (t, J = 8 Hz, 2H), 8.20–8.13 (m, 4H), 7.86 (t, J = 8 Hz,
2H). HRMS (ESI, m/z) [M+H]+ calcd. for C25H15N3O2: 354.1243;
found 354.1204. Cal C 74.78, H 4.28, N 11.89, O 9.06, Observed C
74.67, H 4.39, N 11.64, O 9.15.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in
4.2.5. [(L2)Ru(L1)Ru(L4)]4PF6 (complex 1)
the
online
version,
at
http://dx.doi.org/10.1016/j.
RuCl3 3H2O (5.52 mmol, 1.145 g) and L1 (1.657 mmol, 0.896 g)
ꢅ
jphotochem.2017.03.009.
were dissolved in 70 mL of a miscellaneous solvent of chloroform
and C2H5OH (1/3, v/v). This mixture was refluxed for 5 h and then
condensed to 12 mL. The dark brown precipitate was filtered,
washed with C2H5OH, and dried in vacuo. The solid collected was
dissolved in acetonitrile and purified by column chromatography
using alumina. Yield 16.3%. 1 equivalent of [Ru(L1)Ru]2Cl3 was
reacted with 1.0 equivalent of L2 were dissolved in 60 mL of a
miscellaneous solvent of chloroform and C2H5OH (1/3, v/v). This
mixture was refluxed for 5 h and then condensed to 15 mL. The
dark brown precipitate was filtered, washed with C2H5OH, and
dried in vacuo followed by purification using column chromatog-
raphy in alumina adsorbent. Further 1 equivalent of [Ru(L1)Ru(L2)]
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