Synthesis, steady-state, and femtosecond transient absorption studies of resorcinol bound ruthenium(II)- and osmium(II)-polypyridyl Complexes on nano-TiO2 surface in water

Article abstract of DOI:10.1021/ic4003548

The synthesis of two new ruthenium(II)- and osmium(II)-polypyridyl complexes 3 and 4, respectively, with resorcinol as the enediol anchoring moiety, is described. Steady-state photochemical and electrochemical studies of the two sensitizer dyes confirm strong binding of the dyes to TiO₂ in water. Femtosecond transient absorption studies have been carried out on the dye-TiO₂ systems in water to reveal <120 fs and 1.5 ps electron injection times along with 30% slower back electron transfer time for the ruthenium complex 3. However, the corresponding osmium complex 4 shows strikingly different behavior for which only a <120 fs ultrafast injection is observed. Most remarkably, the back electron transfer is faster as compared to the corresponding catechol analogue of the dye. The origin and the consequences of such profound effects on the ultrafast interfacial dynamics are discussed. This Article on the electron transfer dynamics of the aforesaid systems reinforces the possibility of resorcinol being explored and developed as an extremely efficient binding moiety for use in dye-sensitized solar cells.

Full text of DOI:10.1021/ic4003548

Article
pubs.acs.org/IC
Synthesis, Steady-State, and Femtosecond Transient Absorption
Studies of Resorcinol Bound Ruthenium(II)- and Osmium(II)-
polypyridyl Complexes on Nano-TiO2 Surface in Water
Tanmay Banerjee,† Sreejith Kaniyankandy,‡ Amitava Das,*,† and Hirendra Nath Ghosh*,‡
†Central Salt and Marine Chemicals Research Institute, Bhavnagar 364002, India
‡Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India
S
* Supporting Information
ABSTRACT: The synthesis of two new ruthenium(II)- and osmium(II)-
polypyridyl complexes 3 and 4, respectively, with resorcinol as the enediol
anchoring moiety, is described. Steady-state photochemical and electrochemical
studies of the two sensitizer dyes confirm strong binding of the dyes to TiO2 in
water. Femtosecond transient absorption studies have been carried out on the
dye−TiO2 systems in water to reveal <120 fs and 1.5 ps electron injection times
along with 30% slower back electron transfer time for the ruthenium complex 3.
However, the corresponding osmium complex 4 shows strikingly different
behavior for which only a <120 fs ultrafast injection is observed. Most
remarkably, the back electron transfer is faster as compared to the
corresponding catechol analogue of the dye. The origin and the consequences
of such profound effects on the ultrafast interfacial dynamics are discussed. This
Article on the electron transfer dynamics of the aforesaid systems reinforces the
possibility of resorcinol being explored and developed as an extremely efficient binding moiety for use in dye-sensitized solar
cells.
Despite the above fact, sensitizer dyes with catecholate
anchoring groups have failed to compete successfully with those
with carboxylate and phosphonate anchors, in terms of their
energy conversion efficiencies.6a This is because the interfacial
electron transfer reaction in such ene-diol bound TiO2 systems
falls in the nonergodic regime because of a very strong coupling
of the dye with the semiconductor. In a nonergodic system, the
reaction proceeds before electronic, vibrational, and rotational
energy equilibration over all of the degrees of freedom.7 If the
electron injection process at the dye−semiconductor interface
is really faster than the intramolecular energy relaxation
processes such as vibrational relaxation, internal conversion,
and intersystem crossing, then the surplus vibronic energy of
the photo-excited sensitizer molecule is dispelled quickly into
the semiconductor particle, thus preventing parallel destructive
side reactions that might influence the long-term stability of the
system. Thus, a nonergodic electron injection pathway in a dye-
sensitized TiO2 nanoparticle system implies the prospect of
complete utilization of the energy of the photon.8 Indeed, solar
energy conversion in dye-sensitized devices is most efficient
when operating in the nonergodic limit.2j However, this
nonergodicity in the electron transfer for catecholate bound
TiO2 systems can seldom be harvested. This is because of the
very strong coupling of the dye with the semiconductor, which
1. INTRODUCTION
Contemporary literature reinforces the importance of opti-
mization of the interfacial electron transfer parameters in the
design of a resourceful dye-sensitized solar cell.1 This
elementary yet complex process has therefore attracted a
large number of research groups who are involved in an effort
to understand and control the sensitization events.2
Sensitization of a semiconductor is achieved by chemical
anchoring of dye molecules to the semiconductor nanosurface.
The choice of anchoring groups, for example, carboxyl,
phosphonate (the ones most explored), acetylacetonate, has
been shown to significantly influence the interfacial electron
transfer dynamics, and extensive studies have consequently
been done in this regard.3
However, in water the stability of the carboxyl and the
phosphonate ester linkages is limited to a small range of pH
values due to the related protolytic equilibrium.4 In aqueous
solutions of higher pH, hydrolysis and subsequent dechelation
of the anchoring group from the Ti(IV) groups in TiO2 can
occur on the surface. Most often, equilibration of the surface
titanol groups takes place quite slowly so that dechelation
shows only in a few long-term stability tests. The pH range
where the devices can be used in aqueous media is rather
limited for this reason.5 This led to the development of
acetylacetonates and catecholates as anchoring groups, most
importantly, catecholates, which have considerably higher pKa
values, and hence chances of dechelation are greatly reduced.3j,6
Received: February 5, 2013
Published: April 5, 2013
© 2013 American Chemical Society
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Scheme 1. Molecular Structures of 3 and 4
manifests itself not only in an ultrafast forward electron transfer
but also in the very fast backward electron transfer (BET) rates,
rates that are much faster than dyes with carboxylic
anchors.3f−h,6b,9
Several approaches have been adopted by our research group
in this regard to slow this BET rate appreciably. Secondary
electron-donating groups have been appended to the primary
Ru(II) redox center9e following numerous earlier works by
effect of the alteration in the anchoring moiety as discussed
above on the interfacial electron transfer dynamics of the
ruthenium(II)/osmium(II)-polypyridyl complexes, 3 and 4,
respectively (Scheme 1), in aqueous environment, as probed by
femtosecond transient absorption spectroscopic study. This
study is therefore the first report on the synthesis of
ruthenium(II)/osmium(II)-polypyridyl-resorcinol dyes and is
the only report on the time-resolved femtosecond transient
absorption spectroscopy of osmium(II)-polypyridyl-resorcinol
dye on the TiO2 surface. In addition, we believe that the
present work would add significantly to the ongoing research
efforts toward the development of aqueous dye-sensitized
photoelectrochemical fuel cells.1g−i
Meyer and colleagues and also Gratzel and colleagues involving
̈
carboxylate and phosphonate bound dyes.10 The effects of
these groups are however superseded by the extremely fast
charge recombination rates of catecholate binding. A related
study suggested that Ru(II)−polypyridyl complexes comprising
of ligand-localized charge transfer (LLCT) states can be a
better photosensitizer in terms of improved electron injection
yield and slow BET processes than complexes comprised of
metal to ligand charge transfer (MLCT) states.11 However, the
complicated synthetic procedure of these complexes always
places a question mark on practical applicability. Another of our
studies employed sequential energy and electron transfer in a
catecholate bound Ru−Os−Ru hybrid sensitizer molecule, and
a slower BET rate could be obtained.12 This slow BET rate was
argued to be associated with the delocalization of the hole in
the trinuclear complex radical cation. However, such big
molecules again present serious impediments during syntheses
and also lead to an obvious lower surface coverage and might
consequently lead to lower photocurrent yields. The develop-
ment of an ene-diol tagged viable sensitizer system therefore
still remains a busy plinth of modern research in this area.
This problem of fast BET because of the very strong binding
in such catecholate bound systems is presumably because of the
hydroxyl groups binding to TiO2 in a very localized manner,
that is, to a single Ti4+ atom forming a five-membered chelate
ring.13 If the binding of the enediol dye is made to involve
multiple Ti4+ centers, then the overlap of the dye LUMO with
the Ti 3d orbital network would not be that localized, and one
can expect the effective binding to become weaker. Binding of
the dye to multiple Ti4+ centers is also expected to enhance the
electron delocalization in the TiO2 nanocrystal after initial
electron injection, which in turn is expected to decrease the
BET rate.
2. EXPERIMENTAL SECTION
a. Materials. Titanium(IV) tetraisopropoxide (97%), isopropyl
alcohol, 4,4′-dimethyl-2,2′-bipyridine, n-butyl lithium, 3,5-dimethox-
ybenzaldehyde, 2,2′-bipyridine, ruthenium trichloride hydrate, ammo-
nium hexachloro osmate and Dowex MR-3 mixed bed ion-exchange
resin have been obtained from Sigma-Aldrich and used as received.
Solvents like THF, pyridine and isopropyl alcohol have been dried and
distilled prior to use. Nanopure water (Barnsted System, U.S.) has
been used for making all aqueous solutions. All other reagents (AR
grade) have been procured from S. D. Fine Chemicals (India).
Solvents have been degassed thoroughly with IOLAR grade dinitrogen
gas before use in the preparation of standard solutions. Ru(bpy)2Cl2,15
Os(bpy)2Cl2,16 56b and 617 have been prepared following previously
reported procedures.
b. Analytical Methods. FTIR spectra have been recorded as KBr
pellets in a cell fitted with a KBr window, using a Perkin-Elmer Spectra
1
GX 2000 spectrometer. H NMR spectra have been recorded on a
Bruker 200 MHz FT NMR (model: Avance-DPX 200) using
tetramethylsilane (TMS) as an internal standard. ESI−MS measure-
ments have been carried out on a Waters QTof-Micro instrument.
Microanalyses (C, H, N) have been performed using a Perkin-Elmer
4100 elemental analyzer. Electronic spectra have been recorded with a
Varian Cary 500 UV−vis−NIR spectrophotometer; room-temperature
luminescence spectra have been recorded with an Edinburgh Xe 900
luminescence spectrofluorimeter, fitted with a red-sensitive photo-
multiplier tube. Electrochemical experiments have been performed in
acetonitrile using a bipotentiostat (AFCPBI, PINE Instrument Co.)
electrochemical instrument with a conventional three-electrode cell
assembly. An Ag/AgCl, saturated KCl reference and a platinum
working electrode have been used for all measurements. Ferrocene has
been used as an internal standard. It is more conventional to report the
potential values vs NHE while reporting and comparing interfacial
electron transfer dynamics. All potentials have therefore been quoted
with respect to the normal hydrgen electrode in water.18
To explore this hypothesis, an innovative ruthenium(II)-,
osmium(II)-, rhenium(I)-polypyridyl-based sensitizer system
containing resorcinol instead of catechol as the enediol anchor
has been developed. Our very recent work on the ultrafast
interfacial electron transfer dynamics of the ruthenium(II)- and
rhenium(I)-polypyridyl complexes on oleic acid capped TiO2 in
chloroform has indeed corroborated our hypothesis.14 As a part
of our continuing research on the ultrafast dynamics of such
resorcinol bound dye−TiO2 systems, we report herein the
c. Femtosecond Visible Spectrometer. The femtosecond
tunable visible spectrometer has been developed based on a multipass
amplified femtosecond Ti:sapphire laser system supplied by Thales,
France.19 The pulses of 20 fs duration and 4 nJ energy per pulse at 800
nm, obtained from a self-mode-locked Ti:sapphire laser oscillator
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(Synergy 20, Femtolaser, Austria), are amplified in a regenerative and
two-pass amplifier pumped by a 20 W DPSS laser (JADE) to generate
40 fs laser pulses of about 1.2 mJ energy at a repetition rate of 1 kHz.
The 800 nm output pulse from the multipass amplifier is split into two
parts to generate pump and probe pulses. In the present investigation,
we have used frequency doubled 400 nm as the excitation sources. To
generate pump pulses at 400 nm, one part of the 800 nm output with
200 μJ/pulse is frequency doubled in BBO crystals. To generate visible
probe pulses, about 3 μJ of the 800 nm beam is focused onto a 1.5 mm
thick sapphire window. The intensity of the 800 nm beam is adjusted
by iris size and ND filters to obtain a stable white light continuum in
the 400−1000 nm region. The probe pulses are split into the signal
and reference beams and are detected by two matched photodiodes
with variable gain. We have kept the spot sizes of the pump beam and
probe beam at the crossing point around 500 and 300 μm,
respectively. The noise level of the white light is about ∼0.5% with
occasional spikes due to oscillator fluctuation. We have noticed that
most laser noise is low-frequency noise and can be eliminated by
comparing the adjacent probe laser pulses (pump blocked vs
unblocked using a mechanical chopper). The typical noise in the
measured absorbance change is about <0.3%. The instrument response
function (IRF) for 400 nm excitation has been obtained by fitting the
rise time of the bleach of sodium salt of meso-tetrakis(4-
sulfonatophenyl)porphyrin (TPPS) at 710 nm and has been found
to be 120 fs. The data analysis and fitting at individual wavelengths
have been done with the LabView program.
d. Synthesis of TiO2 Nanoparticles. TiO2 nanoparticles have
been prepared by controlled hydrolysis of titanium(IV) tetraisoprop-
oxide.20 A solution of 5 mL of Ti[OCH(CH3)2]4 (Aldrich, 97%) in 95
mL of isopropyl alcohol is added dropwise (1 mL/min) to 900 mL of
nanopure water (2 °C) at pH 1.5 (adjusted with HNO3). The solution
is continuously stirred for 10−12 h until the formation of a transparent
colloid. The colloidal solution is concentrated at 35−40 °C with a
rotary evaporator and then dried with nitrogen stream to yield a white
powder. The transmission electron microscopy (TEM) images show
the particle size to be ∼3 nm, and specific area electron diffraction
(SAED) pattern confirms the particles to be crystalline anatase (Figure
S1). In the present work, all colloidal samples (15 g/L) have been
prepared following an already reported procedure.21
e. Preparation of Sample Solution. The complexes discussed
are insoluble in water, and so sensitization has been done by dissolving
them in the least possible volume of acetonitrile (this volume is less
than 1% of the total volume) and then by adding the dissolved dye
into an aqueous colloidal solution of the nanoparticles. The resulting
solutions are stirred for one-half an hour and then kept in dark
overnight for the dye to covalently bind to TiO2. For all of the
measurements, the sample solutions have been deoxygenated by
continuously bubbling high-purity nitrogen (99.95 IOLAR grade from
Indian Oxygen Co. Ltd., India) through the solutions.
f. Synthesis. i. 4-(3,5-Dimethoxystyryl)-4′-methyl-2,2′-bipyridine
(1). To a solution of 4,4′-dimethyl-2,2′-bipyridine (1.84 g, 10 mmol) in
ice-cold THF (30 mL) under N2 is added a solution of lithium
diisopropylamide (10 mmol; freshly prepared by mixing 6.65 mL of
1.6 M n-BuLi in hexanes and 1.325 mL of dry diisopropylamine at
room temperature under N2) dropwise over 15 min. The deep
chocolate brown solution is stirred at 0 °C for 1 h, after which a
solution of 3,5-dimethoxybenzaldehyde (1.66 g, 10 mmol) in THF (20
mL) is added dropwise while maintaining the temperature at −30 °C.
The mixture is allowed to warm to room temperature and stirred
overnight. After quenching the reaction with water and evaporation to
dryness in vacuum, an oily mass is obtained. This is dissolved in
chloroform and solvent extraction is done to remove the ionic
impurities. The organic layer is dried with anhydrous sodium sulfate
and evaporated to give a crude oily residue (the crude intermediate
alcohol) which is used without purification for the subsequent
dehydration. The oil is dissolved in dry pyridine (50 mL) and a
solution of POCl3 (1.16 mL, 12.5 mmol) in dry pyridine (50 mL) is
added dropwise under N2 at room temperature with vigorous stirring.
After 1 h, the pyridine is evaporated in a vacuum and crushed ice is
added; the mixture is left for 30 min to ensure that all residual POCl3
is destroyed. The pH of the aqueous solution is adjusted to between 3
and 4 and unwanted organic materials are extracted with CH2Cl2. The
aqueous solution is then made just alkaline (pH ≈ 8) and the crude
products are extracted into CH2Cl2, which is subsequently dried with
anhydrous sodium sulfate and evaporated to give a yellowish brown
solid. This is purified by column chromatography over silica using
chloroform/methanol as the eluent to give the desired product in pure
form. Yield: 1.66 g, 50%. ESI−MS (m/z): calculated for C21H20N2O2,
1
332.15; observed, 333.27 [M + 1]+. H NMR (200 MHz, CDCl3): δ
(ppm) 8.39 (1H, d, J = 5 Hz, H6′ (bpy)); 8.35 (1H, d, J = 5 Hz, H5′
(bpy)); 8.30 (1H, s, H3′ (bpy)); 8.06 (1H, s, H3 (bpy)); 7.09 (1H, d, J
= 13.2 Hz, Hethenyl); 7.05−7.02 (1H, m, H6 (bpy)); 6.87 (1H, d, J = 5
Hz, H5 (bpy)); 6.80 (1H, d, J = 16.4 Hz, Hethenyl); 6.47 (2H, d, J = 2
Hz, H2(phenyl)); 6.24−6.21 (1H, m, H4(phenyl)); 3.57 (6H, s,
−OCH3); 2.17 (3H, s, −CH3). IR (KBr pellet, cm−1): 1591 (ν(C
C)), 1056 (ν(C−O)). Anal. Calcd for C21H20N2O2: C, 75.88; H, 6.06;
N, 8.43. Found: C, 75.5; H, 6.26; N, 8.47.
ii. 5-(2-(4′-Methyl-2,2′-bipyridin-4-yl)vinyl)benzene-1,3-diol (2).
32 mL of analytical grade pyridine is taken in a round bottomed
flask equipped for distillation, and 35.2 mL of concentrated
hydrochloric acid is added to it with continuous stirring. This mixture
is then heated for 2 h at 220 °C to distill off the water from the
mixture. After the mixture was cooled to 140 °C, 1 (1 g, 3.01 mmol) is
added as a solid, and the reaction mixture is stirred and heated at 200
°C for 4 h. The reaction mixture is then cooled, and 30 mL of water is
added to dissolve the crude mixture. The pH of the resulting solution
is then raised to 6−6.5 by slowly adding a dilute solution of caustic
soda to precipitate the desired compound. This precipitate is then
filtered through a grade 4 sintered glass crucible, washed with large
volumes of water, and dried overnight in a vacuum desiccator to give
compound 2 in pure form. Yield: 870 mg, 95%. ESI−MS (m/z):
1
calculated for C19H16N2O2, 304.12; observed, 305.4 [M + 1]+. H
NMR (200 MHz, CD3CN): δ (ppm) 8.58 (1H, d, J = 5.2 Hz, H6′
(bpy)); 8.53 (1H, d, J = 5 Hz, H5′ (bpy)); 8.38 (1H, s, H3′ (bpy));
8.16 (1H, s, H3 (bpy)); 7.57 (1H, d, J = 5 Hz, H6 (bpy)); 7.42 (1H, d,
J = 16.2 Hz, Hethenyl); 7.33 (1H, d, J = 4.6 Hz, H5 (bpy)); 7.13 (1H, d,
J = 16.4 Hz, Hethenyl); 6.59−6.58 (2H, m, H2(phenyl)); 6.28−6.26
(1H, m, H4(phenyl)); 2.49 (3H, s, −CH3). IR (KBr pellet, cm−1):
3430 (ν(OH)), 1589 (ν(CC)). Anal. Calcd for C19H16N2O2: C,
74.98; H, 5.3; N, 9.2. Found: C, 75.2; H, 5.15; N, 9.1.
iii. {Bis-(2,2′-bpy)-(5-[2-(4′-methyl-2,2′-bipyridin-4-yl)vinyl]-
benzene-1,3-diol)} Ruthenium(II) Hexafluorophosphate (3). Ru-
(bpy)2Cl2·2H2O (86 mg, 0.165 mmol) and 2 (50.2 mg, 0.165
mmol) are refluxed in ethanol for 8 h with continuous stirring. The
solvent is then evaporated and the product is made soluble in a
minimum volume of water. Saturated aqueous NH4PF6 (10 mol
equivalents) is added to the resulting solution to precipitate the
desired Ru(II)-polypyridyl complex as the hexafluorophosphate salt.
This is kept as such for 4−5 h in a refrigerator to ensure complete
precipitation after which it is filtered, washed with large volumes of
cold water and dried in a vacuum desiccator. The crude compound so
obtained is purified by column chromatography over silica using
acetonitrile/water/saturated aqueous KPF6 as the eluent. The second
fraction is collected and the solvent is removed to isolate a red solid,
which is redissolved in dichloromethane and two drops of acetonitrile,
and solvent extraction is done to remove the excess KPF6 used in the
eluent. The organic phase is dried over anhydrous sodium sulfate and
is evaporated to dryness to give the desired product in pure form.
Yield: 58.3 mg (35%). ESI-MS (m/z): calculated for
−
C39H32N6O2PF6Ru, 863.09 [M − PF6 ]+’ observed, 863.41 [M −
−
1
PF6 ]+. H NMR (200 MHz, CD3CN): δ (ppm) 8.62−8.58 (1H, m,
H6′ (bpy-res)); 8.53−8.50 (2H, m, H6 (bpy) and/or H6′ (bpy); 8.49−
8.46 (2H, m, H6 (bpy) and/or H6′ (bpy)); 8.09−8.01 (4H, m, H3
(bpy) and H3′ (bpy)); 7.81 (1H, d, J = 5.6 Hz, H4 (bpy) or H4′
(bpy)); 7.75−7.68 (3H, m, H4 (bpy) and H4′ (bpy)); 7.60 (1H, d, J =
4.6 Hz, H5′ (bpy-res)); 7.55−7.49 (2H, m, H3′ (bpy-res) and Hethenyl);
7.43−7.36 (5H, m, H5 (bpy) and H5′ (bpy), H3 (bpy-res)); 7.29−7.22
(3H, m, H5 (bpy-res) and H6 (bpy-res), Hethenyl); 6.62 (2H, d, J = 2
Hz, H2(phenyl)); 6.34 (1H, t, J = 2 Hz, H4(phenyl)); 2.56 (3H, s,
−CH3(bpy-res)). IR (KBr pellet, cm−1): 3432 (ν(OH)), 1606 (ν(C
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a
Scheme 2. Reaction Scheme for the Synthesis of 3 and 4
a
a = LDA, THF, 0 °C; b = 3,5-dimethoxybenzaldehyde, THF, −30 °C; c = POCl3, pyridine, room temperature; d = pyridine, HCl, 200 °C; e =
Ru(bpy)2Cl2·2H2O, EtOH, reflux; f = saturated aqueous NH4PF6; g = OS(bpy)2Cl2, EtOH, reflux; h = saturated aqueous NH4PF6.
C)), 840 (ν(PF6)). Anal. Calcd for C39H32N6O2P2F12Ru: C, 46.48; H,
3. RESULTS AND DISCUSSION
3.20; N, 8.34; Found: C, 46.47; H, 3.18; N, 8.38.
The synthetic methodology adopted for the synthesis of 3 and
iv. {Bis-(2,2′-bpy)-(5-[2-(4′-methyl-2,2′-bipyridin-4-yl)vinyl]-
4 is shown in Scheme 2. The synthesis of 1 is done via a lithium
diisopropyl amide-mediated C−C bond formation reaction to
give the precursor alcohol, which is dehydrated to 1 using
POCl3 in pyridine solvent that itself acts as the base. Removal
of the methyl group in 1 is done with molten pyridinium
hydrochloride to give 2.
benzene-1,3-diol)} Osmium(II) Hexafluorophosphate (4). This is
prepared by a similar procedure as adopted for the synthesis of 3.
Os(bpy)2Cl2·2H2O (100 mg, 0.165 mmol) and 2 (50.2 mg, 0.165
mmol) are refluxed in ethanol for 8 h with continuous stirring. The
solvent then is evaporated and the product is made soluble in a
minimum volume of water. Saturated aqueous KPF6 (10 mol equiv) is
added to the resulting solution to precipitate the desired Os(II)-
polypyridyl complex as the hexafluorophosphate salt. This is kept as
such for 4−5 h in a refrigerator to ensure complete precipitation, after
which it is filtered, washed with cold water and dried in a vacuum
desiccator. The crude compound so obtained is purified by column
chromatography over alumina using acetonitrile/water/saturated
aqueous KPF6 as the eluent. The last fraction is collected and the
solvent is removed to isolate a greenish-black solid, which is
redissolved in dichloromethane and two drops of acetonitrile and
solvent extraction is done to remove the excess KPF6 used in the
eluent. The organic phase is dried over anhydrous sodium sulfate and
is evaporated to dryness to give the desired product. Yield: 57.6 mg
(32%). ESI−MS (m/z): calculated for C39H32N6O2PF6Os, 953.15 [M
The final ruthenium- and osmium-complexes, 3 and 4, are
prepared by reactions of 2, respectively, with Ru(bpy)2Cl2 and
Os(bpy)2Cl2 in ethanol. The purity of the complexes is checked
by different analytical and spectroscopic methods and the
analytical data match well with that of the expected
formulation. 2,2′-Bipyridine (bpy) ligands are employed to
make sure that the MLCT excited state has electrons localized
on the surface-bound ligand for all of the sensitizers. This is
because the surface-bound ligand, that is, the bipyridine ligand
with appended resorcinol moiety, because of its extended
conjugated structure, is energetically low lying as compared to
the unsubstituted bipyridine ligands (vide infra). As a result of
the low lying nature of the surface-bound bipyridine-resorcinol
ligand in 3 and 4, upon excitation, population of the
corresponding MLCT state is expected to dominate over that
constituted by the other unsubstituted ligands. Moreover, the
unsubstituted 2,2′-bpy ligands being at higher energies, electron
density from these ligands is expected to get transferred to the
surface-bound bipyridine-resorcinol ligand with time.
−
−
−
1
− PF6 ]+; observed, 953.34 [M − PF6 ]+, 404.18 [M − 2PF6 ]2+. H
NMR (200 MHz, CD3CN): δ (ppm) 8.57−8.49 (1H, m, H6′ (bpy-
res)); 8.51−8.47 (2H, m, H6 (bpy) and/or H6′ (bpy); 8.47−8.43 (2H,
m, H6 (bpy) and/or H6′ (bpy)); 7.89−7.81 (4H, m, H3 (bpy) and H3′
(bpy)); 7.71 (1H, d, J = 5.6 Hz, H4 (bpy) or H4′ (bpy)); 7.66−7.62
(3H, m, H4 (bpy) and H4′ (bpy)); 7.54 (1H, d, J = 4.6 Hz, H5′ (bpy-
res)); 7.50−7.42 (2H, m, H3′ (bpy-res) and Hethenyl); 7.34−7.25 (5H,
m, H5 (bpy) and H5′ (bpy), H3 (bpy-res)); 7.21−7.13 (3H, m, H5
(bpy-res) and H6 (bpy-res), Hethenyl); 6.61 (2H, d, J = 2 Hz,
H2(phenyl)); 6.32 (1H, t, J = 2 Hz, H4(phenyl)); 2.64 (3H, s,
−CH3(bpy-res)). IR (KBr pellet, cm−1): 3433 (ν(OH)), 1608 (ν(C
C)), 838 (ν(PF6)). Anal. Calcd for C39H32N6O2P2F12Os: C, 42.7; H,
2.94; N, 7.66. Found: C, 42.6; H, 2.9; N, 7.8.
Absorption and emission spectra recorded for 3 and 4 in
water:acetonitrile 99:1 (v/v) medium are shown in Figure 1.
The absorption spectrum is characterized by strong ligand
centered π−π* transitions at 288 and 296 nm for 3 and 4,
respectively; while the band at 330 nm can be assigned either to
delocalized interligand charge transfer or to metal centered
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and only two bipyridyl centered reductions can be observed at
−1.39 and −1.52 V in the +2 to −2 V potential window
available. For 4, the Os2+/Os3+ reversible redox couple appears
at 0.8 V (ΔE = 58 mV) (Figure 2) and bipyridyl centered
reductions can be seen at −1.37 and −1.58 V. An important
point to note is that the metal centered redox potential values
in these resorcinol dyes are significantly more negative than the
respective catechol dyes, 5 and 6 (Scheme 3).
For 5, the Ru2+/Ru3+ redox couple appears at 1.32 V, while
the Os2+/Os3+ redox couple appears at 1.14 V in 6.6b,17 Intra-
and/or intermolecular hydrogen bonding is expected to be
more prominent for the catechol moiety in 5 and 6 than in the
resorcinol counterpart in 3 and 4. Probably a greater
participation of the catechol moiety in this aforesaid hydrogen
bonding curtails the electron density from active involvement in
+R resonance effects toward the sensitizer core in 5 and 6.23
This effect, being most likely less dominant for the resorcinol
moiety in 3 and 4, is presumably being reflected in lower values
of the metal centered redox potentials. Also, this effect is most
likely more prominent in case of the Os(II)-based dyes, and
hence a greater difference in the potential values is seen.
Knowledge about the position of the excited state of the dye
with respect to the conduction band of TiO2 is essential to
comprehend the feasibility of electron injection from the dye to
the semiconductor nanoparticles.
Figure 1. Absorption and emission spectra of 3 (red) and 4 (blue) in
water:acetonitrile 99:1 (v/v) medium. The concentrations of both 3
and 4 are 1 × 10−5 M. The dotted lines show the normalized excitation
spectra.
(MC) transitions.22 The familiar metal to ligand charge transfer
1
comprised of overlapping MLCT based dM(II)→π*bpy [M =
Ru(II)/Os(II)] manifests in the low energy broad absorption
band peaking at 460 nm for 3 (ε = 1.58 × 104 M−1 cm−1) and
at 488 nm for 4 (ε = 1.44 × 104 M−1 cm−1).22 The Os(II)
center, as compared to the Ru(II) center, is characterized by a
significantly higher spin orbit coupling, and this is responsible
for the allowedness of the spin forbidden 3MLCT based
dOs(II)→π*bpy transition,2h,22b resulting in another broad
absorption between 550 and 710 nm for 4 (ε500 = 1.437 ×
104 M−1 cm−1, ε630 = 3730 M−1 cm−1). The emission spectrum
E0−0 transition energies for the 3MLCT states of 3 and 4 are
calculated to be 2.16 and 1.78 eV, respectively, from the
respective excitation and emission spectra. The excited-state
potentials E(S+/S*) are thus −0.91 and −0.98 V for 3 and 4,
respectively, following the equation [E(S+/S*)] = [E(S+/S)] −
3
9e,24
The E0−0 values for the 1MLCT states are
of 3 consists of a broad MLCT-based emission with λmax at
E0−0
.
640 nm when excited at 460 nm. For 4, this emission peaks at
λmax = 750 nm when excited at either 488 or 630 nm.
Excitation spectra of 3 and 4 (Figure 1) agree well with their
respective absorption and absorptance spectra (Figure S2),
indicating that the observed photoluminescence is from the
sensitizers.3h
approximated to be 2.41 and 2.21 eV for 3 and 4, respectively,
from the onset of optical absorption (unlike the estimation of
E0−0 for 3MLCT states, which have been done as stated above).
The corresponding excited-state potentials are thus −1.16 and
−1.41 V for 3 and 4, respectively. The excited-state potentials
being above the conduction band level, electron injection from
the excited state of the sensitizers into the conduction band of
TiO2 becomes thermodynamically feasible.25
Electrochemical studies on 3 (Figure 2) reveal a reversible
Ru2+/Ru3+ redox couple at 1.25 V (ΔE = 69 mV) (vide supra),
The absorption spectrum becomes broad with simultaneous
increase in absorbance on steady addition of an aqueous
solution of TiO2 nanoparticles to the aforesaid complexes
(Figure 3a and b), which indicates significant interaction of the
dye molecules with the TiO2 nanoparticles. Unlike many dyes
with pendant catechol groups for binding to TiO2, no new
charge transfer band appears.9a,d,26 This suggests weaker
electronic coupling of the resorcinol dyes, 3 and 4, to TiO2
than the catechol dyes 5 and 6. However, the fact that the
MLCT absorption does become broad and the OD increases
only means that charge transfer interaction is there, but the
binding is probably not strong enough to manifest this CT
interaction as a new band.6b,9b,c,e,10e,11,12,17 This conclusion is
further supported by the fact that the emission spectra of 3 and
4 in solution are identical to that on TiO2.
However, the emission intensity is radically diminished on
TiO2 surface (Figure 4) (TiO2 concentration is only 5 g/L for
the fluorescence spectra in Figure 4 as compared to 15g/L for
the transient absorption studies), suggesting substantial
quenching of the excited state by electron injection3f,9c,e,10e,17
and hence negligible contribution of excited-state features in
the transient absorption data on TiO2 (vide infra).
Figure 2. Cyclic voltammogram of 3 (pink, right-hand side layer in
figure) and 4 (navy blue, left-hand side layer in figure) in acetonitrile.
Ag/AgCl (saturated KCl) has been used as the reference electrode.
Scan rate for both of the scans is 100 mV/s. Ligand-based reductions
are omitted because only the metal-based redox potentials have been
used in the interpretation of data throughout this Article. The peak on
the left side in both of the voltammograms is due to the ferrocene/
ferrocenium couple, which has been added as an internal standard. A
difference in the concentrations of potassium chloride solutions in the
Fc/Fc+
For a comprehensive understanding of the electron transfer
reference electrodes presumably causes the difference in the E1/2
values in the two voltammograms.
dynamics of 3 and 4 on TiO2 nanoparticle surface, a detailed
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Scheme 3. Molecular Structures of 5 and 6
Figure 5. Transient absorption spectrum of 3 at different delay times
in water:acetonitrile 99:1 (v/v) medium measured after 400 nm
(fwhm <120 fs) excitation. Concentration of 3 is ∼200 μM. Shown in
the inset is the kinetic trace for 3 monitored at 900 nm.
Figure 3. Changes in absorbance of (a) 3 (1.034 × 10−5 M) on TiO2
(0−4.06 g/L) and (b) 4 (0.9266 × 10−5 M) on TiO2 (0−3.52 g/L) in
water:acetonitrile 99:1 (v/v) medium. Shown as a dashed line is the
spectral profile of TiO2 (15 g/L) in water:acetonitrile 99:1 (v/v)
medium.
negative absorption due to the ground-state bleach below 500
nm.6b,9a,b,11
The kinetic trace of 3 at 900 nm (Figure 5 inset) shows a 1
ps (19.2%) and a 25 ps (23.2%) increase in oscillator strength
after the initial excited-state population occurring in pulse width
limited time (<120 fs, +57.6%). McCusker et al. studied the
ultrafast dynamics of the excited-state evolution of a series of
ruthenium polypyridyl complexes pumping at 400 nm and
could observe a 5 ps component in the pump−probe difference
absorption kinetics monitored at 532 nm that they assigned as
the vibrational cooling dynamics in the 3MLCT excited state.28
Their assignment was corroborated by the fact that similar
dynamics could again be observed for a comparable complex
when excited with a 400 nm pump and probed at 510 nm;
while with 480 nm pump at the same probe wavelength the
complex did not show any dynamics beyond 200 fs.29
Interestingly, in the complex with appended phenyl rings, the
component due to vibrational cooling decreased to 2 ps
because of the phenyl ring rotation that acted as an easy
pathway for dispelling the excess vibrational energy. Therefore,
considering the non rigid structure and also the appended
hydroxyl groups that might arbitrate additional relaxation
pathways, more so in solvent water, it would not be injudicious
to assign the 1 ps component to vibrational relaxation in the
3MLCT manifolds. In addition, the average solvation relaxation
time, ⟨τs⟩, in water and in acetonitrile is reported to be ∼500−
600 fs.30 This and the fact that on TiO2 surface we have a
similar 1.5 ps injection component whose amplitude is probe
wavelength dependent (vide infra) rule out the possibility of
this component in the free dye arising due to solvation. This
assignment is further corroborated by the appearance of a
similar 0.7 ps in the dynamics of 3 in methanol:chloroform 99:1
Figure 4. Fluorescence quenching of 3 (1 × 10−5 M) (left layer, red)
and 4 (1 × 10−5 M) (right layer, blue) on TiO2 (5 g/L)
water:acetonitrile 99:1 (v/v) medium.
study of the excited-state dynamics of the free sensitizer
molecules is essential. Transient absorption spectrum has been
recorded both in acetonitrile and in water:acetonitrile 99:1 (v/
v), and the kinetic data are very similar in the two solvents.
Figure 5 shows the transient absorption spectrum of 3 in
water:acetonitrile 99:1 (v/v) medium. The transient absorption
spectrum of 3 shows a broad absorption ranging from 500 to
1000 nm. Recent work by Chergui et al. employing broadband
fluorescence spectroscopy has revealed that intersystem
crossing in ruthenium polypyridyl complexes occurs in 15
10 fs.27 The absorption feature, therefore, in the transient
absorption spectrum can be assigned to excited triplet state
absorption.6b,9a−c,11 This is accompanied by a simultaneous
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(v/v) solution and on oleic acid capped TiO2 in the same
solvent.14
ably is the reason for which the aforesaid interligand electron
redistribution occurs at a faster rate in 4 than in 3.
Transient absorption spectrum of 3 on TiO2 nanoparticle
surface is shown in Figure 7a. The spectrum shows two major
As told before, the average solvation relaxation time, ⟨τs⟩, in
water and in acetonitrile is reported to be ∼500−600 fs. This
eliminates the possibility of the 25 ps component arising due to
solvation. The transient studies have been carried out by
excitation at the blue edge of the MLCT band. This necessitates
simultaneous electron localization in all three bipyridine ligands
after initial photoexcitation. The 25 ps component is therefore
tentatively assigned to interligand electron redistribution along
31
3
the MLCT potential energy surface. The lifetimes of the
MLCT state of Ru(II)-polypyridyl complexes are known to be
>100 ns,32 so we can safely assume that it is the long lifetime of
this state that is mirrored in the decay trace for 3 that does not
decay until 400 ps.
The transient absorption spectrum of 4 (Figure 6) shows two
bands at 540−720 and 760−1000 nm in addition to a negative
Figure 7. Transient absorption spectrum of (a) 3 and (b) 4 on TiO2
(15 g/L) in water:acetonitrile 99:1 (v/v) medium at different delay
times measured after 400 nm (fwhm <120 fs) excitation.
Concentrations of 3 and 4 are ∼100 μM.
absorption bands in the regions 500−700 and 760−900 nm.
The former band can be irrefutably assigned to the formation of
the cation radical (3·+). Assignment of this band has been made
on the basis of the results obtained in a complementary pulse
radiolysis experiment where 3·+ has been generated selectively
by the reaction of N3 radical with 3 in N2O-saturated aqueous
solution (5% acetonitrile + 95% water) (Figure S3a, Supporting
Information). This assignment is also corroborated by the
previously reported transient absorption spectrum of 5 where
the cation radical appears in the 500−670 nm region with the
absorption peak at 590 nm.6b Numerous other studies on
similar ruthenium polypyridyl complexes corroborate this
assignment.9a−c,e
The lower energy absorption band in the 760−900 nm
region can be assigned to electrons in the conduction band of
TiO2 nanoparticles. Again, this assignment is corroborated by
previous literature reports where it has been shown that
electrons in the conduction band can be detected by visible,
near-IR, and mid-IR absorption.3a,e,35 In addition to these
bands, a negative absorption can be seen below 500 nm due to
the ground-state bleach.6b,9a,b,11 The transient absorption
spectrum of 4 (Figure 7b) again presents two absorption
bands between 560−680 and 720−900 nm. The first
absorption band can again be assigned to the formation of
cation radical (4·+) based on our complementary pulse
radiolysis experiment to generate 4·+ selectively as described
above and also by previous results from our group on a similar
osmium complex (Figure S3b, Supporting Information).9b The
cation radical appears at 520−660 nm in this reported complex
where there is no double bond between bipyridine and
catechol. Apparently, the enhanced conjugation because of the
presence of the double bond in 4 shifts the absorption band of
the cation radical to higher wavelengths.
Figure 6. Transient absorption spectrum of 4 at different delay times
in water:acetonitrile 99:1 (v/v) medium measured after 400 nm
(fwhm <120 fs) excitation. Concentration of 4 is ∼200 μM. Shown in
the inset is the kinetic trace for 4 monitored at 900 nm.
absorption below 520 nm due to ground-state bleach.
Following an argument similar to that in the case of 3, these
bands are assigned to excited triplet state absorption.9b An
important point to note in the transient absorption spectrum is
the absence of any bleach band in the 560−750 nm region,
3
corresponding to the ground-state MLCT absorption. This is
probably due to the higher molar extinction coefficient of the
excited state relative to the aforesaid ground-state absorption so
that the former dominates the transient absorption spectrum.
The kinetic trace monitored for 4 at 900 nm (Figure 6 inset)
shows a 13 ps rise (10.2%) after initial population of the excited
state (<120 fs, 89.8%). This 13 ps component is again assigned
tentatively to interligand electron redistribution along the
3MLCT potential energy surface.33
The redox potential values could be obtained only for two
bipyridine ligands in 3 and 4 from the electrochemical studies
(vide supra). The most positive of these, that is, −1.39 V for 3
and −1.37 V for 4, indisputably corresponds to the reduction of
the coordinated ligand 2.6b The other redox potential value is
−1.52 V for 3, that is, 0.13 V more negative. On the other hand,
this difference is 0.21 V for 4 for which the second reduction
appears at −1.58 V. This implies the fact that, as compared to 3,
for 4, the 3MLCT potential energy surface corresponding to the
ligand 2 is energetically more distantly placed than the other
2,2′-bipyridine ligands. Considering Marcus normal region
behavior of interligand electron redistribution,34 this presum-
Before discussing the individual kinetics of 3 and 4 on TiO2,
a brief review of the kinetic data of the 5−TiO2 system is
necessary. One of the biggest endeavors of the ultrafast
spectroscopists working in the area of dye-sensitized solar cells
worldwide is perhaps the maintenance of a steady and high
electron concentration in the conduction band of TiO2 after the
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elementary charge injection process. Studies done on the metal
polypyridyl dyes with catecholate anchoring groups reveal two
very important findings, the first being the fact that electron
injection occurs single exponentially in less than 100 fs (this
temporal resolution does not allow an accurate determination
of the rate of electron injection process).6b,9a−d,11 This
corresponds to an electron injection rate of >1013 s−1, which
has been explained in light of a very strong coupling of the dye
and a large density of acceptor states in TiO2 such that the
electron transfer process falls in the so-called nonergodic
limit.7,8 Electron injection in the nonergodic regime is valuable
in the sense that it ensures exploitation of the full energy of the
incident photons by taking place before thermal electron
relaxation and reorganization of the inner-sphere ligand
environment. This nonergodicity, however, cannot be harvested
in such systems because of the very deleterious BET reaction.
The BET of the electrons to the oxidized ruthenium center
involves a d orbital localized on the ruthenium metal whose
electronic overlap with the TiO2 conduction band is little and is
further lessened by a spatial contraction of the wave function
upon oxidation of Ru(II) to Ru(III).36 As a result, the
electronic coupling element for the BET reaction is 1 or 2
orders of magnitude smaller as compared to that for electron
injection, reducing the back reaction rate by at least the same
factor.36 For 5 bound to TiO2, even with this expected slowing,
the BET rate is much faster as compared to the recovery of the
ground state of the classical N3 dye, which occurs in
microsecond−millisecond time scales.3f,36 This makes this
catechol dye incongruous for any sensible applications.
for the ground-state bleach recovery of the 3−TiO2 and 5−
TiO2 systems both monitored at 480 nm.
Figure 8. Comparison of the kinetic traces of 3 and 5 adsorbed on
TiO2 at 480 nm (lower panel) and comparison of the electron
injection dynamics of the 3−TiO2 system with that of the 5−TiO2
system with the kinetic traces normalized with respect to the ultrafast
growth component (upper panel).
The kinetic trace of 3−TiO2 at 480 nm can be fitted
multiexponentially with τ1 < 120 fs (−100%), τ2 = 1.2 ps
(+29%), τ3 = 33 ps (+1.3%), and τ4 > 400 ps (+69.7%) in
contrast to the τ1 < 120 fs (−100%), τ2 = 1.5 ps (+37.6%), τ3 =
70 ps (+19.4%), and τ4 > 400 ps (+43%) fit of the kinetic trace
for 5−TiO2 (Table 1). The ground-state bleach recovery
Table 1. Lifetimes of the Transients for 3− and 5−TiO2
Systems at Different Wavelengths
To slow this inherently fast BET, several approaches have
been adopted by our research group, unfortunately not to much
effect. The effects of secondary electron donating groups
appended to the primary Ru(II) redox center are outcasted by
the fast geminate charge recombination rates.9e A related study
suggested that Ru(II)−polypyridyl complexes comprising of
LLCT states can be a better photosensitizer in terms of
improved electron injection yield and slow BET processes than
complexes comprised of MLCT states.11 However, the
complicated synthetic procedure of these complexes places a
question mark on their practical applicability. Another study
from our group employed sequential energy and electron
transfer in a catecholate bound Ru−Os−Ru cluster, and a
slower BET rate could be obtained.12 This slow BET rate was
argued to be associated with the delocalization of the hole in
the trinuclear complex radical cation. Nevertheless, for practical
applications, such bulky molecules might present serious
shortcomings in terms of not only synthetic viability but also
photocurrent yields because of their lower surface coverage and
probable tendency to aggregate. On the basis of this knowledge,
we set out on the task of lowering the BET rates by a
modification of the molecular skeleton itself, thus avoiding any
cumbersome synthesis and exclusive of making the molecule
any bulkier, and have developed a ruthenium(II)/osmium(II)/
rhenium(I)-polypyridyl-based sensitizer system containing
resorcinol instead of catechol as the enediol anchor. Our very
recent work on the ultrafast interfacial electron transfer
dynamics of the ruthenium(II)- and rhenium(I)-polypyridyl
complexes on oleic acid capped TiO2 in chloroform has indeed
corroborated our hypothesis.14
monitoring
system
wavelength (nm)
lifetimes
3-TiO2
480
<120 fs (−100%), 1.2 ps (+29%), 33 ps
(+1.3%), >400 ps (+69.7%)
640
900
480
900
<120 fs (+89.5%), 1.5 ps (+10.5%), 33 ps
(−3.1%), >400 ps (−96%)
<120 fs (+92.5%), 1.5 ps (+7.5%), 33 ps
(−65%), >400 ps (−92%)
5-T1O2
<120 fs (−100%), 1.5 ps (+37.6%), 70 ps
(+19.4%), >400 ps (+43%)
<120 fs (+100%), 1.5 ps (−34.8%), 70 ps
(−26.8%), >400 ps (−38.4%)
represents the true geminate recombination dynamics and a
look at the residuals at 400 ps suggests a not less than 30%
slowing in this primary charge recombination rate, such a
phenomenal effect having been brought about just by changing
the position of the hydroxyl groups with respect to each other.
The electron injection time in the 3−TiO2 system is
monitored by the time of appearance of the electron signal at
900 nm. Similar to that on oleic acid capped TiO2 in
chloroform,14 electron injection is found to be fitting with τ1
< 120 fs (+92.5%) and τ2 = 1.5 ps (+7.5%) at 900 nm followed
by a decay with time constants τ3 = 33 ps (−6.5%) and τ4 > 400
ps (−92%) (Figure 8, upper panel). This is in stark contrast to
a < 120 fs only electron injection from the hot 1MLCT and/or
3MLCT states observed in case of the 5−TiO2 system which
decays with τ1 = 1.5 ps (−34.8%), τ2 = 70 ps (−26.8%), and τ3
> 400 ps (−38.4%) time constants (Table 1). It may be noted
that injection component in addition to the pulse width limited
<120 fs component is a very rare observation for ruthenium
polypyridyl enediol complexes as sensitizers.
An analysis of the kinetic traces of 3−TiO2 at various
wavelengths presents the most striking results. The lower panel
of Figure 8 shows a comparison of the normalized kinetic traces
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To comprehend the true origin of the 1.5 ps injection
component in the kinetic trace of the electron signal, we
recorded the kinetic trace for the decay of the cation radical
signal at 640 nm (Figure 8, upper panel) and it is found to be
fitting with τ1 < 120 fs (+89.5%), τ2 = 1.5 ps (+10.5%), τ3 = 33
ps (−3.1%), and τ4 > 400 ps (−96%). Again, two injection
times are observed; however, the amplitude of the 1.5 ps
component is greater than that observed in case of electron
signal. The concerned difference, although small, represents a
genuine difference in amplitudes, and, keeping in mind the
extremely small noise (<0.1%) at these wavelengths, any fitting
errors can be safely ruled out. While discussing the kinetic data
for the free dye, we came across a 1 ps component that we
assigned to a thermal electron relaxation to the thexi state.
Thus, we can see that the 1.5 ps component observed in the
kinetic traces of the conduction band electron signal and the
cation radical signal corresponds to the component due to
vibrational relaxation in the free dye dynamics and is therefore
attributed to electron injection from the thermally relaxed
3MLCT states. That this component is indeed due to electron
injection from the thermally relaxed states is proved by the fact
that the relative amplitude of the component is greater in case
of the cation radical signal where the probe pulse (at 640 nm) is
of higher energy than that when the electron signal is being
monitored (at 900 nm). The pulse width limited component,
on the other hand, is attributed to electron injection from hot
singlet/triplet Franck−Condon states as in the case for 5−
TiO2. An interesting point to note is that the residuals at 400 ps
for the cation radical in the 3−TiO2 system at 640 nm are no
less than 96% as compared to a mere 38% for the 5−TiO2
system at the same time.
The electron injection rate from the dye to the semi-
conductor is determined by many factors, an important one of
which is the potential energy difference between the dye excited
state and conduction band edge.35c,37 A higher excited state of
the dye results in injection in a higher density of states, thus
resulting in a very fast injection. For 5, the 3MLCT state lies at
−0.68 V, while for 3, the same state lies at −0.91 V. With the
TiO2 conduction band edge at −0.5 V,25 it is evident that in the
3−TiO2 system the electron injection rate should be faster.
Instead, we observe a slower component. This clearly suggests
that some other factor is being operational.
One of the other factors affecting the electron injection rate
is the electronic coupling between the electron-donating orbital
of the adsorbate and the electron-accepting orbital of the
semiconductor.3e,35c Apparently, in both 3 and 5, the π* orbital
of the almost similarly substituted bipyridine is the electron-
donating ligand overlapping with the π symmetry t2g d orbital
of Ti4+ in TiO2 However, the situation is somewhat more
intricate.
orbital network of TiO2 as compared to that in case of catechol.
Formation constants have also been calculated to show that
binding of resorcinol on TiO2 is relatively weaker as compared
to the binding of catechol.38 We therefore believe that this
comparatively weaker resorcinolate binding serves as the cause
for the slower injection component in case of 3−TiO2 as
compared to that in the 5−TiO2 system similar to that on oleic
acid capped TiO2 in chloroform.14
Furthermore, transient absorption studies carried out on the
aforesaid resorcinol−TiO2 system showed long time offset,
which persisted with significant amplitudes up to >200 ps as
compared to almost complete recombination for the molecule
catechol binding to TiO2.38
These observations tend to suggest that, as a result of
increasing the number of Ti atoms and the Ti−Ti distance
involved in the binding, not only is the electronic coupling
(HAB) reduced, but there is also a significant change in the
electronic structure of the excited state involving the π* orbital
of the bipyridine ligand and the π symmetry t2g d orbital of Ti4+
in TiO2 as one moves from catechol to resorcinol. This change
in the electronic structure of the excited state implies a much
greater degree of charge delocalization as we move from
catechol to resorcinol. In other words, as we move from
catechol to resorcinol, because of the reduced electronic
coupling and because of this enhanced charge diffusion along
different directions into the bulk TiO2, a significantly slower
charge recombination rate in the kinetic traces of 3 on TiO2 is
observed. A similar observation on oleic acid capped TiO2 in
chloroform further corroborates this hypothesis.14
Following the Marcus semiclassical electron transfer theory,
the BET rate can be expressed as a function of the driving force,
reorganization energy, and the coupling element in the
following way (eq 1):39
⎛
⎜
⎞
⎟
2π
2
kBET
=
[HAB]
⎝
⎠
ℏ
(1)
(ΔG0 + Λ)2
4ΛkT
⎧
⎫
⎬
⎭
1
⎨
exp −
⎩
4πΛkT
Now, the charge recombination dynamics in a ruthenium
polypyridyl dye−TiO2 system falls in the Marcus inverted
regime where with an increase in the thermodynamic driving
force (−ΔG0), the BET rate decreases provided the coupling
element remains the same (eq 1).3i,9a,11,26,40 For the sake of
argument, let us suppose that the coupling elements for 3−
TiO2 and 5−TiO2 systems are indeed the same. Reorganization
energy values are generally very small as compared to the
driving force in the inverted region (−ΔG0 ≫ λ, respective
values for the classical N-719 dye being 1.5 and 0.3
eV);36,38,40,41 the changes in the reorganization energy values,
if any, will therefore be insignificant with respect to the changes
Catechol is long known to interact with TiO2 with the
formation of a stable five-membered ring involving only one Ti
atom of the whole nanocluster.13 This is certainly much
improbable of an interaction to occur in case of resorcinol
where the hydroxyl groups are disposed further away from each
other. Any reasonable interaction is therefore expected to
involve at least more than one Ti atom. This proposition is
proved by one of the recent works from our group where
theoretical calculations have proved that the energy minimized
structure for the molecule resorcinol binding to TiO2 on the
{101} anatase plane of a titanium dioxide cluster [Ti8O32]
involves multiple nonadjacent Ti atoms.38 This suggests a
probably weaker overlap of the bipyridine π* orbital with the d
in the driving force values. Now, ΔG0 is EC − ES/S+ 9a,e,42 where
,
EC is the potential of the conduction band edge (−0.5 V)25 and
ES/S+ is the ground-state redox potential of the dye, which is
1.25 V for 3 and 1.32 V for 5. Using the above equation, the
(−ΔG0) values for 3−TiO2 and 5−TiO2 systems are,
respectively, calculated to be 1.75 and 1.82 eV. The
recombination should therefore be faster for the 3−TiO2
system, which is contrary to what we observe. Thus, we see
that the electron recombination rate cannot be explained on the
basis of changes in free energy alone and that coupling plays a
significant role.
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Logically, a similar reasoning, as thought of for explaining the
injection dynamics of 3 on TiO2, should apply in case of the 4−
TiO2 system as well. To our surprise, this is not the case.
Electron injection in both 4−TiO2 and 6−TiO2 systems is
pulse width limited and is found to be fitting to <120 fs only, as
monitored by the time of appearance of the conduction band
electron signal. The kinetic trace for the decay of the signal in
the 4−TiO2 system recorded at 620 nm can be fitted with 0.9
ps (−29.4%), 22 ps (−9.4%), and >400 ps (−61.2%) time
constants (Figure S4, Supporting Information), while the decay
of the conduction band electron signal at 900 nm can be fitted
with 0.9 ps (−20.9%), 22 ps (−10.4%), and >400 ps (−68.7%)
time constants (Figure 9 and Table 2). On the other hand, the
This, therefore, might be the reason for the absence of any
slow component in the electron injection parameters, which
supersedes the effects of weaker coupling of resorcinolate
binding. Ultrafast <120 fs electron injection in the already
reported rhenium(I)-polypyridyl resorcinol complex further
corroborates this hypothesis.14 The lower panel of Figure 9
compares the BET dynamics of 4−TiO2 and 6−TiO2 systems
based on the recovery of the ground-state bleach monitored at
480 nm. The recovery in the 4−TiO2 system can be fitted with
0.9 ps (+28.5%), 22 ps (+10.3%), and >400 ps (+61.2%), while
that in the 6−TiO2 system can be fitted with 1.2 ps (+16.5%),
10 ps (+3.5%), and >400 ps (+80%) (Table 2). The
recombination dynamics in the 4−TiO2 system is therefore
∼20% faster than that in the 6−TiO2 system. This also is
wholly contrary to that observed in case of the corresponding
ruthenium complex 3 on TiO2. The aforesaid trend is visible in
the electron signal at 900 nm as well but is not prominent
because of possible electron trapping at various trap sites of
different trap energies and distances from the adsorbed dye.44
This faster recombination observed in case of the 4−TiO2
system can be explained again on the basis of Marcus theory.
We have mentioned before that the charge recombination
dynamics in such systems falls in the Marcus inverted regime
where with an increase in the thermodynamic driving force
(−ΔG0), the BET rate (given by eq 1) decreases provided
coupling parameters are the same. Electron recombination in
the case of the 3−TiO2 system is seen to be governed by the
nature of electronic coupling of the dye to TiO2 (vide supra).
This factor therefore must apply in case of the 4−TiO2 system
as well, and this would have governed the recombination rate
provided the other factors were maintained the same, and this is
not the case for the osmium complexes. The (−ΔG0) values for
the 4−TiO2 and 6−TiO2 systems are calculated to be 1.3 and
1.64 eV, respectively. A significantly lower (−ΔG0) value (0.34
eV) in the 4−TiO2 system, which again presides over the
electronic coupling effects, presumably results in the faster
recombination observed. In case of the ruthenium complexes,
this difference in the (−ΔG0) values between the catecholate
and resorcinolate binding analogues is only 0.07 eV, which, we
believe, is not enough to outdo the effects of resorcinolate
binding.
Figure 9. Comparison of the kinetic traces of 4 and 6 adsorbed on
TiO2 at 480 nm (lower panel) and 900 nm (upper panel).
Table 2. Lifetimes of the Transients at Different
Wavelengths for 4− and 6−TiO2 Systems
monitoring
wavelength
(nm)
system
lifetimes
4−TiO2
480
<120 fs (−100%), 0.9 ps (+28.5%), 22 ps
(+10.3%), >400 ps (+61.2%)
620
900
480
900
<120 fs (+100%), 0.9 ps (−29.4%), 22 ps
(−9.4%), >400 ps (−61.2%)
<120 fs (+100%), 0.9 ps (−20.9%), 22 ps
(−10.4%), >400 ps (−68.7%)
6−TiO2
<120 fs (−100%), 1.2 ps (+16.5%), 10 ps
(+3.5%), >400 ps (+80%)
<120 fs (+100%), 1.2 ps (−22.5%), 10 ps
(−3.5%), >400 ps (−74%)
4. CONCLUSION
Femtosecond transient absorption studies with our newly
synthesized sensitizer dyes reinforce the fact that by replacing
catechol-based enediol anchoring groups with those based on
resorcinol, that is, by merely changing the position of the
hydroxyl groups, the exceedingly deleterious geminate charge
recombination in ruthenium polypyridyl enediol-based sensi-
tizers can be slowed outstandingly. This substitute anchoring
group, as a result of its weak binding with TiO2 and similar to
that on oleic acid capped TiO2 in chloroform, elicits electron
injection simultaneously from the thermalized 3MLCT states in
addition to the sub 100 fs electron injection from the hot
singlet/triplet MLCT states. This study therefore strengthens
our assertion of resorcinol to be a very promising anchoring
group toward the development of efficient sensitizers for use in
dye-sensitized solar cells. Studies on the efficiency and the
current voltage characteristics of such dyes are currently
underway in our laboratory. A point, however, to be taken
care of while designing the dyes is that the ground-state redox
level of the dye must not be too cathodic to prevent possible
inverted region effects presiding over effects of weak binding, as
decay of the conduction band electron signal for the 6−TiO2
system at 900 nm can be fitted with 1.2 ps (−22.5%), 10 ps
(−3.5%), and >400 ps (−74%) time constants.
Lian and co-workers studied a series of ruthenium
polypyridyl complexes, and they proved that in the non-
adiabatic limit, provided coupling and reorganization energy
parameters are the same, with an increase in the excited-state
energies the electron injection rates become faster.35c,43 They
explained their observation by considering electron injection in
successively higher density of states with successive increase in
the excited-state energies of the sensitizer dyes. In our case, we
3
observe that the MLCT excited state of 4 lies higher at −0.98
V as compared to that of 3 at −0.91 V (Scheme 4). Following
an analogy similar to that of Lian et al., this means that for the
4−TiO2 system electron injection takes place in a relatively
higher density of states as compared to that in the case of the
3−TiO2 system.
5375
dx.doi.org/10.1021/ic4003548 | Inorg. Chem. 2013, 52, 5366−5377

Inorganic Chemistry
Article
Scheme 4. Mechanistic Scheme Showing the Different Electron Transfer Processes in 3, 4, 5, and 6 Adsorbed on TiO2
Concepcion, J. J.; Jurss, J. W.; Hoertz, P. G.; Luo, H.; Chen, C.;
Kenneth, H. G.; Meyer, T. J. J. Phys. Chem. C 2011, 115, 7081. (i)
More references of the works from the Solar Fuels EFRC centered at
UNC, Chapel Hill, on the use of dye-sensitized architectures for
aqueous photoelectrochemical fuel-forming applications can be found
at http://www.efrc.unc.edu/.
(2) (a) Ardo, S.; Meyer, G. J. Chem. Soc. Rev. 2009, 38, 115.
(b) Abrahamsson, M.; Johansson, P. G.; Ardo, S.; Kopecky, A.;
Galoppini, E.; Meyer, G. J. J. Phys. Chem. Lett. 2010, 1, 1725. (c) Guo,
J.; She, C.; Lian, T. J. Phys. Chem. C 2007, 111, 8979. (d) Clifford, J.
is evident from a study of the BET rate of the corresponding
Os(II)-polypyridyl dye.
ASSOCIATED CONTENT
* Supporting Information
■
S
Transient absorption spectrum of the cation radical of 3 and 4
obtained from one electron pulse radiolysis, kinetic trace for the
decay of the cation radical in the 4−TiO2 system, SAED
pattern of TiO2 nanoparticles, and comparison of the
absorptance and the excitation spectra of 3 and 4. This
material is available free of charge via the Internet at http://
pubs.acs.org.
N.; Palomares, E.; Nazeeruddin, M. K.; Gratzel, M.; Durrant, J. R. J.
̈
Phys. Chem. C 2007, 111, 6561. (e) Maggio, E.; Martsinovich, N.;
Troisi, A. J. Phys. Chem. C 2012, 116, 7638. (f) Barzykin, A. V.;
Tachiya, M. J. Phys. Chem. B 2002, 106, 4356. (g) Kallioinen, J.;
Benko, G.; Sundstrom, V.; Korppi-Tommola, J. E. I.; Yartsev, A. P. J.
Phys. Chem. B 2002, 106, 4396. (h) Kuciauskas, D.; Monat, J. E.;
Villahermosa, R.; Gray, H. B.; Lewis, N. S.; McCusker, J. K. J. Phys.
Chem. B 2002, 106, 9347. (i) Palomares, E.; Martinez-Diaz, M. V.;
Haque, S. A.; Torres, T.; Durrant, J. R. Chem. Commun. 2004, 2112.
(j) McFarland, S. A.; Lee, F. S.; Cheng, K. H. W. Y.; Cozens, F. L.;
Schepp, N. P. J. Am. Chem. Soc. 2005, 127, 7065.
AUTHOR INFORMATION
Corresponding Author
■
*Fax: (+) 91-22-25505331/25505151 (H.N.G.); 00-91-278-
2567562 (A.D.). E-mail: hnghosh@barc.gov.in (H.N.G.);
amitava@csmcri.org (A.D.).
Notes
The authors declare no competing financial interest.
(3) (a) Hannappel, T.; Burfeindt, B.; Storck, W.; Willig, F. J. Phys.
Chem. B 1997, 101, 6799. (b) She, C.; Guo, J.; Irle, S.; Morokuma, K.;
Mohler, D. L.; Zabri, H.; Odobel, F.; Youm, K.-T.; Liu, F.; Hupp, J. T.;
Lian, T. J. Phys. Chem. A 2007, 111, 6832. (c) Chen, Y.-S.; Li, C.;
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ACKNOWLEDGMENTS
■
We thank Dr. D. K. Palit, Dr. S. K. Sarkar, and Dr. T.
Mukherjee of Bhabha Atomic Research Centre of Mumbai and
Dr. P. K. Ghosh of Central Salt & Marine Chemicals Research
Institute (CSIR), Bhavnagar, for their encouragement and
continuous support. A.D. and H.N.G. acknowledge BRNS for
financial support. A.D. also thanks MNRE/CSIR for funds
through CSIR TAP-SUN Programme. T.B. acknowledges
BRNS & CSIR for research fellowship. We also thank
Analytical Discipline and Centralized Instrument Facilities,
CSMCRI, for assistance with different analytical and spectro-
scopic measurements.
J. E.; Gratzel, M.; Klug, D. R.; Durrant, J. R. J. Phys. Chem. 1996, 100,
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