The effect of heavy atoms on photoinduced electron injection from nonthermalized and thermalized donor states of MII-polypyridyl (M=Ru/Os) complexes to nanoparticulate TiO2 surfaces: An ultrafast time-resolved absorption study

Article abstract of DOI:10.1002/chem.200901937

We have synthesized ruthenium(II)- and osmium(II)-polypyridyl complexes ([M(bpy)₂L]²⁺, in which M = Os^IIor Ru ^II bpy = 2,2-bipyridyl, and L = 4-(2,2-bipyridinyl-4- yl)benzene-l,2-diol) and studied the interfacia

Full text of DOI:10.1002/chem.200901937

FULL PAPER
DOI: 10.1002/chem.200901937
The Effect of Heavy Atoms on Photoinduced Electron Injection from
Nonthermalized and Thermalized Donor States of MII–Polypyridyl
ACHTUNGTRENNUNG
Sandeep Verma,[a] Prasenjit Kar,[b] Amitava Das,*[b] Dipak K. Palit,[a] and
Hirendra N. Ghosh*[a]
Abstract: We have synthesized ruthe-
nium(II)– and osmium(II)–polypyridyl
complexes ([M in which
(bpy)2L]2+
M=OsII or RuII, bpy=2,2’-bipyridyl,
and L=4-(2,2’-bipyridinyl-4-yl)ben-
zene-1,2-diol) and studied the interfa-
cial electron-transfer process on a TiO2
nanoparticle surface using femtosecond
system, single-exponential electron in-
jection takes place from photoexcited
nonthermalized metal-to-ligand charge
transfer (MLCT) states. However, in
the case of the OsII/TiO2 system, elec-
tron injection takes place biexponen-
tially from both nonthermalized and
thermalized MLCT states (mainly
3MLCT states). Larger spin–orbit cou-
pling for the heavier transition-metal
osmium, relative to that of ruthenium,
accounts for the more efficient popula-
3
G
,
tion of the MLCT states in the OsII-
based dye during the electron-injection
process that yields biexponential dy-
namics. Our results tend to suggest that
appropriately
designed
OsII–poly-
pyridyl dye can be a better sensitizer
molecule relative to its RuII analogue
not only due to much broader absorp-
tion in the visible region of the solar-
emission spectrum, but also on account
of slower charge recombination.
transient-absorption
spectroscopy.
Ruthenium(II)- and osmium(II)-based
dyes have a similar molecular struc-
ture; nevertheless, we have observed
quite different interfacial electron-
transfer dynamics (both forward and
backward). In the case of the RuII/TiO2
Keywords: charge transfer · elec-
tron transfer · nanoparticles · pho-
tochemistry · spin–orbit coupling
Introduction
these solar cells is an area of intense interest as this provides
the insight for understanding the dynamics of the photoin-
duced electron-injection process(es), which have been car-
ried out in both experimental[1–14] and theoretical[15] investi-
gations. These studies were expected to help in developing
solar cells with better photon-to-current conversion efficien-
cy (IPCE). In this context, RuII–[2–6,12] and OsII–polypyridine
complexes[7–11,13] have been used extensively by various re-
searchers as the prospective photosensitizer molecule for
the development of dye-sensitized solar cells (DSSCs).[2]
The osmium complexes may have certain advantages when
compared with the analogous ruthenium complexes, because
the ruthenium polypyridyl-based photosensitizer molecules
do not absorb solar photons below approximately 2.1 eV,[13]
and as a result around 0.8 V is expected to be wasted in the
Sensitization of nanocrystalline TiO2 electrodes with molec-
ular chromophores forms the basis of efficient solar energy
conversion in regenerative photoelectrochemical cells.[1] The
study of interfacial electron-transfer dynamics involved in
[a] S. Verma, Dr. D. K. Palit, Dr. H. N. Ghosh
Radiation and Photochemistry Division
Bhabha Atomic Research Centre, Mumbai 400 085 (India)
Fax : (+91)22-25505151
E-mail: hnghosh@barc.gov.in
[b] P. Kar, Dr. A. Das
Central Salt and Marine Chemicals Research Institute
Bhavnagar 364002, Gujarat (India)
Fax : (+91)278-567-562
ACTHNUTRGNEUNG
reaction of [RuIII(bpy)3]3+ (bpy=2,2’-bipyridyl) and Iꢀ to
E-mail: amitava@csmcri.org
sustain faradic current flow in the cell.[16] OsII–polypyridyl
complexes[2d,7–11,13,16–20], on account of their lower energy-ab-
sorption band in the visible region of the spectrum, may be
more suitable for wider absorption of the solar spectrum
Supporting information for this article, including details of ESMS
spectra, transient spectra for the cation radicals, and additional fig-
ures, is available on the WWW under http://dx.doi.org/10.1002/
chem.200901937.
Chem. Eur. J. 2010, 16, 611 – 619
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
611

and thus may act as a better sensitizer molecule in photo-
electrochemical energy conversion.
to see the effect of lowering the energy of the excited singlet
or triplet MLCT band of the donor orbital[6] and the ISC
process on electron-injection dynamics.
In most of the studies reported so far—including very
recent ones[2d]—the anchoring groups used to attach the
metal–dye complexes to the nanoparticles are carboxy-
lates.[1–11] However, use of carboxylates as an anchoring
group has some disadvantages. The ground-state pKa of the
carboxylates is too low (pKa ꢁ3.5)[2b] to ensure strong bind-
ing, and in the presence of water, slow desorption of the
photosensitizers may occur, which can limit the long-term
stability of the cell. The pKa value for the catecholate
system is considerably higher (pKa >9.6), which ensures
more efficient binding with TiO2 nanoparticles, even at
higher pH values.[5,12] It would therefore be interesting to
synthesize catecholate-based osmium complexes and study
the electron-transfer dynamics onto a TiO2 nanoparticle sur-
face.
Experimental Section
Materials: Titanium(IV) tetraisopropoxide (TiACHTNUGTRENN[UG OCHACHTUNGTRNE(NGUN CH3)2]4; Aldrich,
97%), RuCl3·xH2O, 2,2’-bipyridyl, 3,4-dimethoxybenzaldehyde, and acet-
aldehyde were procured from Sigma–Aldrich and were used as received.
Pyridine and ethanol, used as solvent, were dried and distilled prior to
use. Nanopure water (Barnsted System, USA) was used for making aque-
ous solutions. All other reagents were of analytical reagent grade and
procured from S.D. Fine Chemicals (India). HPLC-grade acetonitrile (E.
Mark, Bombay, India) was used for all spectrophotometric studies. Sol-
vents were degassed thoroughly with Indian oxygen-limited analytical re-
agent (IOLAR)-grade dinitrogen gas before use in the preparation of all
standard solutions. 3-(3,4-Dimethoxyphenyl)propanal (A), pyridacyl pyri-
dinium iodide salt (B), and 4-(2,2’-bipyridinyl-4-yl)benzene-1,2-diol (L)
were synthesized following previously reported procedures and are pro-
vided in the Supporting Information.
In our earlier report,[13] we discussed the interfacial elec-
tron-transfer (IET) dynamics in a longer time domain
(nanosecond) by monitoring charge-transfer emission in-
volving OsII-based sensitizer molecules bound to nanoparti-
culate TiO2 through catecholate functionality using time-re-
solved emission spectroscopy. However, to understand the
ET mechanism in this system in detail and the involvement
of different MLCT states during the ET process, it is very
important to study IET dynamics on an ultrafast timescale.
It has been reported in the literature[3,4] that for related
RuII–polypyridyl complexes the major component of the
electron-transfer process(es) is extremely fast from the ini-
tially populated, vibronically nonthermalized excited singlet
metal-to-ligand charge-transfer (1MLCT) states prior to
electronic and nuclear relaxation of the molecule. Simulta-
Analytical methods: Microanalyses (C, H, N) were performed using a
Perkin–Elmer 4100 elemental analyzer. FTIR spectra were recorded
either as KBr pellets or as solutions in acetonitrile in a cell fitted with a
KBr window, using a Perkin–Elmer Spectra GX 2000 spectrometer.
1H NMR spectra were recorded using a Bruker 200 MHz FT NMR spec-
trometer (model: Avance DPX 200) using CD3CN as the solvent and tet-
ramethylsilane (TMS) as an internal standard; ESIMS measurements
were carried out using a Waters QTof-Micro instrument. Electronic spec-
tra were recorded using a Shimadzu UV-3101 PC spectrophotometer;
steady-state luminescence spectra were recorded using a Perkin–Elmer
LS 50B luminescence spectrofluorimeter outfitted with a red-sensitive
photomultiplier. Electrochemical experiments were performed using a
CH-660A electrochemical instrument with a conventional three-electrode
cell assembly. A saturated Ag/AgCl reference and a platinum working
electrode were used for all measurements. Ferrocene was added at the
end of each measurement of the cyclic voltammetric experiment as an in-
ternal standard and all potentials are quoted versus the ferrocene/ferro-
cenium (Fc/Fc+) couple.
3
neous electron injection from the thermalized MLCT states
has also been observed and contributes to the multiexpo-
nential electron injection. In our earlier investigations[5,6] in
which we used ruthenium–polypyridyl photosensitizers,
Nanoparticle preparation: Nanometer-sized TiO2 was prepared by con-
trolled hydrolysis of titanium(IV) tetraisopropoxide.[21–23] A solution of
TiACHTUNTRGENNUG[OCHACHTUNGTRNEN(GUN CH3)2]4 (5 mL) dissolved in isopropyl alcohol (95 mL; Aldrich)
namely, TiO2 nanoparticles sensitized by [RuII
ACHTUNGTRENNUNG
was added dropwise (1 mLminꢀ1) to nanopure water (900 mL; 28C) at
pH 1.5 (adjusted with HNO3). The solution was continuously stirred for
10–12 h until a transparent colloid was formed. The colloidal solution
was concentrated at 35–408C with a rotary evaporator and then dried
with a nitrogen stream to yield a white powder. In the present work all
colloidal samples were prepared after dispersing the dry TiO2 nanoparti-
cles in water (15 gLꢀ1).
CH=CH-catechol)][5] and [RuII
ACTHNUGRTEN(NUGN SCN)2ACHTUNGTRENNUNG
echol)][6] (bpy=2,2’-bipyridine), we observed only a single-
exponential electron injection in TiO2 CB (CB = conduc-
tion band) from photoexcited MLCT states. We also ob-
served that the strong electronic coupling between dye–TiO2
systems facilitates electron injection from nonthermalized
MLCT states. The question that automatically arises is
whether strong coupling should always facilitate single-expo-
nential electron-injection kinetics or whether the energy of
the photoexcited donor orbital(s) together with the rate of
the intersystem-crossing (ISC) process influences the elec-
tron-transfer dynamics. To address this issue, we have syn-
thesized ruthenium(II)– and osmium(II)–polypyridyl com-
Synthesis of complex 1:
A solution of [RuACHTUNGTRENUGN(bpy)2Cl2]·2H2O (0.07 g,
0.072 mmol) and L (0.02 g, 0.07 mmol) dissolved in an ethanol/water mix-
ture (1:1, v/v, 50 mL) was heated to reflux for 8 h with constant stirring
under an inert atmosphere. Then ethanol was removed under vacuum
and the desired crude complex was precipitated as a red-orange solid by
adding an excess amount of aqueous KPF6 solution. This was filtered off,
washed with cold water, and air-dried. This crude product was further pu-
rified by gravity chromatography by using silica as the stationary phase
and CH3CN-saturated aqueous KPF6 solution (98:2, v/v) as the eluent.
Acetonitrile was then removed under vacuum, and the desired pure com-
plex was extracted into the dichloromethane layer by solvent extraction.
The dichloromethane was removed under reduced pressure to isolate the
pure compound (0.070 g, 52%). 1H NMR (CD3CN): d=8.70 (d, J=
9.0 Hz, 2H; H6,6’ (L)), 8.51 (d, J=8.2 Hz, 4H; 2H6,6’ (bpy)), 8.06 (t, J=
7.6 Hz, 4H; 2H4,4’ (bpy)), 7.82–7.58 (m, 7H; 2H5,5’ (bpy), H3,3’,4’ (L)),
7.42–7.29 (m, 8H; 2H3,3’ (bpy), 2H5,5’ (L), H5,6 (phenyl)), 6.99 ppm (d, J=
8.2 Hz, 1H; H3 (phenyl)); IR (KBr): n˜ =3450 (-OH), 1604 (C=C, C=N),
842 cmꢀ1 (PF6); E1/2 (vs. Ag/AgCl) {DE}=1.32 {102} (RuII/III), ꢀ1.38 {80}
plexes ([MACHTUNGTRENNUNG
(bpy)2L]2+ (in which M=OsII or RuII, bpy=2,2’-
bipyridyl, and L=4-(2,2’-bipyridinyl-4-yl)benzene-1,2-diol)
and studied IET processes between nanoparticulate TiO2
using femtosecond transient-absorption spectroscopy. In
general, the ISC process is known to be much higher for the
heavier metal ions like osmium due to higher spin–orbit
coupling, so the choice of OsII complex basically allows us
612
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Chem. Eur. J. 2010, 16, 611 – 619

Heavy Atoms
FULL PAPER
(L/L·ꢀ), ꢀ1.62 {100} (bpy/bpy·ꢀ), ꢀ1.9 V {85 mV} (bpy/bpy·ꢀ); ESMS: m/z
(%): 823 (29) [M+ꢀPF6]; elemental analysis calcd (%) for 1: C 44.68, H
2.92, N 8.69; found: C 44.7, H 2.8, N 8.5.
Results and Discussion
Synthesis of complexes 1 and 2: 3-(3,4-Dimethoxyphenyl)-
propanal (A) and pyridacyl pyridinium iodide salt (B+Iꢀ)
were allowed to react for the synthesis of Me2L by following
the basic synthetic methodology generally adopted for the
synthesis of pyridine derivatives (Scheme 1). Then the de-
Synthesis of complex 2: A solution of [OsACHTNURGTNE(UNG bpy)2Cl2]·2H2O (0.070 g,
0.114 mmol) and L (0.030 g, 0.114 mmol) dissolved in an ethanol/water
mixture (1:1, v/v, 50 mL) was heated to reflux for 16 h with constant stir-
ring under an inert atmosphere. Ethanol was then removed under
vacuum, and the desired crude complex was precipitated as a dark green
solid by adding an excess amount of
aqueous KPF6 solution. This was fil-
tered off, washed with cold water, and
air-dried. This crude product was fur-
ther purified by gravity chromatogra-
phy by using silica as the stationary
phase and CH3CN-saturated aqueous
KPF6 solution (98:2, v/v) as the eluent.
Acetonitrile was then removed under
vacuum, and the desired pure complex
was extracted into the dichlorome-
thane layer by solvent extraction. The
dichloromethane was removed under
reduced pressure to isolate the pure
compound (0.062 g, 52%). 1H NMR
(CD3CN): d=8.63 (d, J=8.4 Hz, 2H;
H6,6’ (L)), 8.48 (d, J=8.2 Hz, 4H;
2H6,6’ (bpy)), 7.86 (t, J=7.8 Hz, 4H;
2H4,4’ (bpy)), 7.72–7.48 (m, 7H; 2H5,5’
(bpy), H3,3’,4’ (L)), 7.37–7.27 (m, 8H;
2H3,3’ (bpy), 2H5,5’ (L), H5,6 (phenyl)),
6.99 ppm (d, J=8.2 Hz, 1H; H3
Scheme 1. Reaction scheme for the synthesis of 4-(2,2’-bipyridinyl-4-yl)benzene-1,2-diol (L) and complexes 1
and 2. i) CH3CHO, EtOH, 28% NaOH, 08C, 30 min; ii) pyridine, I2, reflux, 90 min, N2 atmosphere;
iii) CH3COOH, CH3COOꢀNH4+; iv) pyridine–HCl, 1908C, 180 min.
(phenyl)); IR (KBr): n˜ =3450 (-OH),
1603 (C=C, C=N), 847 cmꢀ1 (PF6); E1/2
(vs. Ag/AgCl) {DE}=1.14 {105} (OsII/
III), ꢀ1.24 {80} (L/L·ꢀ),ꢀ1.48 {100}
(bpy/bpy
*
ꢀ), ꢀ1.73 V {110 mV} (bpy/
sired ligand L was obtained by reacting Me2L with molten
bpy
*
ꢀ); ESMS: m/z (%): 911 (85) [M+ꢀPF6], 766 (60) [M+ꢀ2PF6]; ele-
pyridinium hydrochloride salt at 1908C. This ligand L was
used for the reaction with [Ru(bpy)2Cl2] and [Os(bpy)2Cl2]
for the synthesis of the desired complexes 1 and 2, respec-
tively. For the synthesis of complex 1, [Ru(bpy)2Cl2]·2H2O
mental analysis calcd (%) for 2: C 40.90, H 2.67, N 7.95; found: C 40.8,
A
ACHTUNGTRENNUNG
H 2.6, N 7.7.
Femtosecond visible spectrometer: The femtosecond tunable visible spec-
trometer was developed based on a multipass amplified femtosecond Ti:-
sapphire laser system from Avesta, Russia (1 kHz repetition rate at
800 nm, 50 fs, 800 mJpulseꢀ1), and has been described earlier.[24,25] 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 both 800 nm (fundamental) and its frequency-doubled 400 nm as ex-
citation sources. To generate pump pulses at 400 nm, one part of 800 nm
with 200 mJpulseꢀ1 was frequency-doubled in b-BaB2O4 (BBO) crystals.
To generate visible probe pulses, about 3 mJ of the 800 nm beam was fo-
cused onto a 1.5 mm-thick sapphire window. The intensity of the 800 nm
beam was adjusted by iris size and neutral density filters to obtain a
stable white light continuum in the 400 to over 1000 nm region. The
probe pulses were split into the signal and reference beams and were de-
tected by two matched photodiodes with variable gain. We kept the spot
sizes of the pump beam and probe beam at the crossing point, which was
around 500 and 300 mm, respectively. The excitation-energy density (at
both 800 and 400 nm) was adjusted to approximately 2500 mJcmꢀ2. The
noise level of the white light was about 0.5% with occasional spikes due
to oscillator fluctuation. We have noticed that most laser noise is low-fre-
quency 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 <0.3%. The in-
strument response function (IRF) for 400 nm excitation was obtained by
fitting the rise time of the bleaching of the sodium salt of meso-tetra-
kis(4-sulfonatophenyl)porphyrin (TPPS) at 710 nm and was found to be
120 fs.
AHCTUNGTRENNUNG
was allowed to react at reflux temperature with one molar
equivalent of L in ethanol as solvent, which was expected to
yield the complex 1 in optimum yield. This was further puri-
fied by gravity chromatography. Analytical data matched
well with the proposed formulation for this complex. Com-
plex 2 was synthesized using the same procedure as adopted
for complex 1. Purity of the isolated complexes after column
chromatography was checked by various analytical and spec-
troscopic methods. Analytical and spectroscopic data ob-
tained for these two complexes agreed well with the pro-
posed structure for the respective complexes.
Cyclic voltammetry: Electrochemical studies revealed that
MII/III redox potentials (with respect to the normal hydrogen
electrode (NHE)) for complexes 1 and 2 are +1.32 (RuII/III
)
and +1.14 V (OsII/III), respectively, and this difference in
MII/III (M=Ru/Os) redox potential was reflected in the ob-
served redshift of the MLCT band for the Os complex rela-
tive to the Ru complex.
UV/Vis absorption spectra: Prior to ultrafast transient ab-
sorption studies, we carried out steady-state absorption
Chem. Eur. J. 2010, 16, 611 – 619
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613

A. Das, H. N. Ghosh et al.
measurements of both the complexes in different media.
Figure 1 shows the optical absorption spectra of complexes
1 and 2 and the respective complexes bound to nanoparticu-
Excited-state dynamics of free complexes 1 and 2 in acetoni-
trile: To understand photoinduced interfacial electron-injec-
tion dynamics involving TiO2 nanoparticles sensitized by
complex 1 and 2, it is essential to understand the excited-
state dynamics of the sensitizer molecules in nonreactive
media like acetonitrile. We have carried out femtosecond
transient absorption studies in the visible and near-IR
region to monitor the excited-state dynamics of the sensitiz-
er molecules 1 and 2. Figure 2 shows the transient absorp-
Figure 1. Optical absorption spectra of a) complex 1 in water, b) complex
2 in water, c) 1 on the TiO2 nanoparticle surface, and d) 2 on the TiO2
nanoparticle surface dispersed in water (the concentration of 1 was kept
to around 38 mm, and that of 2 was kept to around 30 mm).
late TiO2 dispersed in water. Let us begin by discussing the
optical absorption behavior of complex 1. Figure 1a shows a
low-energy band at 426 and 456 nm. The band at 426 nm
*
can be attributed to a dRuII !pbpy-based metal-to-ligand
charge-transfer transition. The other band at 456 nm is at-
*
tributed to dRuII !pL-based MLCT transition. Since the exci-
tation wavelength is 400 nm in the transient absorption
measurements, we have not given much attention to charac-
terizing the higher energy bands of RuII complexes. Now let
us discuss the optical absorption behavior of complex 2,
which is shown in Figure 1b. Broad absorption bands with
maxima at 443 and 486 nm (Figure 1b) are assigned to two
*
*
dOsII !pbpy- (at ꢁ443 nm) and dOsII !pL-based (at ꢁ486 nm)
MLCT transitions. The broad absorption feature exhibited
by the OsII complex (complex 2) in the 550–750 nm region is
attributed to spin-forbidden MLCT transitions, which gain
intensity in the osmium complexes because of enhanced
spin–orbit coupling, a common feature of heavy metal cen-
ters.[13,14] The absorption bands at 400–550 nm are attributed
to the MLCT singlet state for complexes 1 and 2.[5,13] After
the addition of TiO2 nanoparticles, the absorption spectrum
(Figure 1c) of 1 becomes broad and redshifted with an ap-
preciable increase in absorbance of the band at 460 nm.
Once again, after the addition of TiO2 nanoparticles, the ab-
sorption spectrum of complex 2 (Figure 1d) becomes broad
and shifted to a longer wavelength with an appreciable in-
Figure 2. Transient absorption spectra of a solution of complex 1 in aceto-
*
nitrile following excitation with a 400 nm laser pulse at 200 ( ) and
~
^
500 fs (N) and 1 ( ), 5 (&), and 10 ps ( ) time delays (top). Kinetic
trace of the excited triplet state (3MLCT) of 1 at 710 nm (bottom).
tion spectra of 1 at different time delays in acetonitrile fol-
lowing 400 nm laser-light excitation. The transients show a
major absorption band in the 500–1000 nm wavelength
region centered on 750 nm with simultaneous bleaching
below the 500 nm region. The transient absorption bands
can be ascribed to the excited-state absorption (ESA) of 1.
It has been reported in recent literature that an ISC process
(singlet to triplet) for RuII–polypyridyl complexes is very
fast (<50 fs).[26] Consequently, in the present investigation,
we have attributed this ESA to the excited triplet-state
(3MLCT) absorption, as it is believed that the singlet-to-trip-
let conversion might have occurred within the pulse width
of the instrument. It is interesting to observe that the ampli-
*
*
crease in absorbance for both (dpOs!pbpy and dpOs!pL ) of
the MLCT bands. Redshift and broadening of the visible
band for both complexes 1 and 2 in the presence of TiO2
could be attributed to the strong interaction between the
sensitizer and nanoparticles.
614
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Chem. Eur. J. 2010, 16, 611 – 619

Heavy Atoms
FULL PAPER
tude of the transient absorption increases with time
(Figure 2), and after a certain time delay it does not decay
for 1 ns. Figure 2 (bottom) shows the transient absorption
decay kinetics at 710 nm for 1 in acetonitrile, and this could
be best fit with time constants of <100 fs (94.3%) and 7 ps
(6.7%). The first growth component is pulse-width limited
(<100 fs), which can be attributed to the formation of the
singlet and/or triplet MLCT state. However, the second
component grows with a time constant of 7 ps and can be at-
tributed to vibrational relaxation processes. As we can ob-
serve in Figure 1a, the absorption band of complex 1 ex-
tends up to 550 nm and on excitation at 400 nm it is quite
natural that vibrationally hot (nonthermalized-state) mole-
cules will be formed. This nonthermalized molecule will
take some time to relax and reach the thermalized state,
which can be attributed to vibrational relaxation. It is re-
ported in the literature that the lifetimes of the triplet state
(3MLCT) of the analogous RuII–polypyridyl complexes are
>100 ns or so.[27] Thus, the longer lifetime of the triplet state
is actually reflected in the kinetic traces shown in Figure 2
(bottom), which does not decay for 1 ns.
Figure 3 shows the transient absorption spectra of a solu-
tion of complex 2 in acetonitrile following excitation with a
400 nm laser pulse with an approximately 100 fs pulse
width. It shows bleaching in the 470–690 nm region and a
broad positive absorption band in the 710–1000 nm region.
The bleaching region consists of two peaks: one in the 470–
490 nm region and another in the 530–690 nm region. Both
the bleaching peak positions match well with absorbance
peaks in the optical absorption spectra of complex 2 (see
Figure 1b). Bleaching around the 470–510 nm region is due
Figure 3. The transient absorption spectra of a solution of complex 2 in
1
to the MLCT absorption band and that in the 530–690 nm
*
acetonitrile following excitation with a 400 nm laser pulse at 200 ( ) and
3
~
!
region is due to the spin-forbidden MLCT absorption band.
^
500 fs ( ) and 1 (N), 5 (&), 10 ( ), and 30 ps ( ) time delays (top). Ki-
netic trace of the excited triplet state (3MLCT) of 2 at 710 nm (bottom).
The spin–orbit coupling in the osmium complex is more
prominent relative to that in the ruthenium complex. This
!
3
allows a direct MLCT S0 transition, which results in ex-
tended optical absorption in the 530–690 nm region by the
OsII–polypyridyl complex. Therefore the bleaching in the
530–690 nm region can be attributed to the spin-forbidden
sorption spectra of TiO2 nanoparticles sensitized by complex
1 at different time delays. The spectrum at each time delay
consists of bleaching below the 500 nm wavelength region,
an absorption band at 510–630 nm that peaks at around
550 nm, and another broad positive absorption band in the
650–1000 nm region. The absorption peak at 550 nm can be
attributed to 1·+ (cation radical; see the Supporting Infor-
mation). The broad absorption band in the 650–1000 nm
region has been attributed to the conduction-band electrons
in the nanoparticles.[22,28,29] Previously, it has been reported
by us and many other research groups that conduction-band
electrons can be detected by characteristic visible,[22,28,29]
near-IR,[30] and mid-IR[3,31,32] absorption. Injection time in
the present investigation was determined by monitoring the
appearance time of the transient absorption signal of the in-
3
S0! MLCT transition. The positive absorption band in the
710–1000 nm region is ascribed to mixed 1MLCT and
3MLCT excited-state absorption. It is well documented in
the literature that the ISC process is very fast (<50 fs) in a
heavy transition metal like osmium, so a pulse-width-limited
3
1MLCT state to MLCT state conversion is expected. There-
fore, in the present investigation, the broad positive transi-
ent absorption band is dominated by the excited 3MLCT
state absorption. The transient decay profile monitored at
710 nm can be fit with biexponential time constants of t1 =
150 ps (5.3%) and t2 >1 ns (94.75%). Long-lived species (>
1 ns) can be attributed to MLCT triplet states of complex 2.
jected electron (eꢀTiO ) at 1000 nm. Electron-injection dy-
2
Transient absorption measurements of 1/TiO2 and 2/TiO2:
We have carried out transient absorption experiments for 1/
TiO2 and 2/TiO2 systems following excitation with a laser
source of 400 nm to explore the interfacial ET dynamics on
the semiconductor surface. Figure 4 shows the transient ab-
namics (growth kinetics of the transient absorption signal)
were found to be fit by a single exponential with a time con-
stant of <100 fs. We discuss the mechanism of single-expo-
nential electron-injection dynamics in the present system in
detail in the next section. We have also monitored the
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615

A. Das, H. N. Ghosh et al.
Figure 5. Transient absorption spectra of TiO2 nanoparticles sensitized by
2 in water at 200, 400, 600, and 800 fs and 5 and 20 ps time delays after
excitation at 400 nm. Bottom: The kinetic trace at 490 nm.
Figure 4. Transient absorption spectra of TiO2 nanoparticles sensitized by
~
!
*
1 in water at 200 ( ) and 500 fs ( ) and 1 (N), 5 (&), 10 ( ), and 50 ps
^
(
) time delays after excitation at 400 nm (top). The kinetic trace at
acetonitrile+95% water) and is shown in the Supporting In-
formation. Transient absorption spectra show two negative
absorption bands peaking at 490 and 600 nm; these can be
attributed to the bleaching due to ground-state absorption
(Figure 1d). The injection time in the present investigation
was determined by monitoring the appearance signal of the
490 nm (bottom).
bleaching at 490 nm, which primarily tells us the charge-re-
combination dynamics (back electron transfer, or BET) be-
tween the electron (eꢀTiO ) and the parent cation (RuIII
2
center) to determine back-electron-transfer dynamics
(shown in Figure 4, bottom). The bleaching recovery kinet-
ics at 490 nm and transient decay kinetics at 1000 nm for the
1/TiO2 system can be fit with multiexponential time con-
stants of t1 =0.8 ps (29%), t2 =20 ps (21%), and t3 >300 ps
(50%).
injected electron (eꢀTiO ) at 1000 nm. The electron-injection
2
dynamics (growth kinetics in the transient absorption signal)
in the 2/TiO2 system can be fit with biexponential time con-
stants of t1inj <100 fs (42%) and ti2nj =1.7 ps (58%). The
mechanism of biexponential electron injection in strongly
coupled dye–nanoparticle systems is discussed in the next
section. We have also monitored the bleaching at 490 nm,
which primarily gives the time constants of the BET be-
tween the injected electron and the parent cation (OsIII).
The bleaching recovery kinetics at 490 nm (Figure 5,
bottom) and transient decay kinetics at 1000 nm (Figure 6B)
for the 2/TiO2 system can be fit with multiexponential time
constants of t1 =2.0 ps (12%), t2 =35 ps (17%), and t3 >
300 ps (71%).
We have also carried out transient absorption studies with
the 2/TiO2 system following excitation with a laser light at
400 nm. Figure 5 shows the transient absorption spectra of
TiO2 nanoparticles sensitized by 2 at different time delays.
Here the absorption band at 550 nm has also been attribut-
ed to the cation radical (2·+; see the Supporting Informa-
tion) and the broad band in the wavelength region 700–
1000 nm can be attributed to the injected electron in the
conduction band in TiO2 nanoparticles (eꢀTiO ). We have as-
2
signed the cation radical of 2 on the basis of the results ob-
tained in a complementary pulse radiolysis experiment, in
which 2·+ was generated selectively by the reaction of the
N3 radical with 2 in N2O saturated aqueous solution (5%
Interfacial electron-transfer dynamics: Interfacial electron-
transfer dynamics in ruthenium–polypyridyl complexes—in
particular [Ru
ACHTUNGTRNENUG(dcbpy)2ACHTUNGTERN(NUGN NCS)2], widely known as the ruthe-
nium–N3 complex (dcbpy=4,4’-(dicarboxylic acid)-2,2’-bi-
616
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Chem. Eur. J. 2010, 16, 611 – 619

Heavy Atoms
FULL PAPER
in which electronic coupling is considered to be moderate
with TiO2 nanoparticulate. However, in our earlier investi-
gation,[5,6] we reported only a single exponential and pulse-
width-limited (<100 fs) electron injection from photoexcit-
ed MLCT states into the conduction band of TiO2 nanopar-
ticles. In those we used [Ru
and [Ru(bpy)
(SCN)2(bp-CH=CH-catechol)][6] as the photo-
(bpy)2(bp-CH=CH-catechol)][5]
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
sensitiser, in which the sensitizers are coupled strongly with
the nanoparticles through catecholate binding. Therein we
have demonstrated that the electron-injection process com-
petes with the thermalization process in the photoexcited
state of strong coupling dye molecules. Our earlier results
indicated that electron injection took place predominantly
from the nonthermalized singlet state (1MLCT) and/or the
nonthermalized triplet state (3MLCT) or a combination of
both. Strong coupling in dye–nanoparticle systems facilitates
ultrafast single-exponential electron injection, which com-
petes with the thermalization process of the excited states.
To find out whether strong coupling always gives single-
exponential injection and how the presence of heavy atoms
affects electron injection in strongly coupled dye–nanoparti-
cle systems, we have carried out transient absorption studies
in structurally similar ruthenium– and osmium–polypyridyl
complexes with a catechol moiety on the TiO2 nanoparticle
surface. In an earlier section, we discussed transient absorp-
tion measurements in both 1/TiO2 and 2/TiO2 systems. To
compare the electron-transfer dynamics (both electron injec-
tion and BET), the transient absorption signals at 1000 nm
following excitation with 400 nm laser pulses for complexes
1 and 2 on the TiO2 nanoparticle surface are shown in
Figure 6. Traces on a shorter timescale are shown as in
Figure 6, bottom. It is evident from curve A (shown in the
bottom of Figure 6) that the electron-injection kinetics in 1/
TiO2 are single exponential with a pulse-width-limited (<
100 fs) time constant. This single-exponential electron injec-
tion for 1/TiO2 could be attributed to the injection from
nonthermalized MLCT states. Here, due to strong coupling
through the catecholate moiety, electron injection competes
Figure 6. Kinetic decay trace of the injected electron in the conduction
band at 1000 nm in 1/TiO2 (A) and 2/TiO2 (B) following excitation with a
400 nm laser pulse. Bottom: The kinetic trace on a shorter timescale.
pyridine)-sensitized TiO2 nanomaterials—has been a subject
of intense research interest for more than a decade. It has
been shown by femtosecond transient IR and near-IR spec-
troscopy that in most of the cases[3] injection happens in the
ultrafast time domain and occurs prior to vibrational relaxa-
tion of the thermally equilibrated MLCT manifold (i.e.,
from “hot” excited states or the Frank–Condon state itself).
However, most authors have argued in favor of the presence
of a sub-100 fs component and considered it to be the domi-
nant channel with additional contributions of slower nonex-
ponential electron injection with a distribution of character-
istic times from 1 ps to tens of picoseconds. Durrant et al.[33]
reported biexponential electron injection (EI) with <150 fs
(50%) and approximately 1.2 ps (50%) time constants
3
with the ISC rate, which leads to thermalized MLCT states.
On the other hand, in the 2/TiO2 system (shown in curve B),
electron-injection dynamics are found to be biexponential
with time constants of t1inj <100 fs (42%) and t2inj =1.7 ps
(58%). Here the shorter component could be attributed to
the electron injection from nonthermalized MLCT states
and the longer component could be attributed to electron
for a carboxylate-functionalized [RuII
ACHTNUGTRENNUG(dcbpy)2CAHTUNGTRNE(NUGN NCS)2]/TiO2
3
system, in which the slower component was attributed to
thermalized MLCT states. It has been clearly demonstrated
by Benko et al.[4] that electron injection in the Ru–N3/TiO2
injection predominantly from thermalized MLCT states. In
Scheme 2, we have shown the mechanistic pathways of elec-
tron transfer and the relaxation dynamics of both dyes (1
and 2) bound to the TiO2 surface. The ground- and excited-
state energy levels of these complexes were determined by
cyclic voltammetry and the optical absorption spectrum of
MLCT-based singlet and triplet states. It is interesting to see
that the ET pathways are different for the two complexes,
even though the energy levels of the complexes do not
differ appreciably. As Os is the heavier transition metal, the
spin–orbit coupling for OsII complexes is larger relative to
analogous RuII complexes and for complex 2 the ISC pro-
1
system occurs mainly from the nonthermalized MLCT state
with a smaller contribution from the 3MLCT state. Later,
Piotrowiak and co-workers[34] reported multiexponential
electron-injection kinetics in
a [RuACHTNUTGREG(NNUN bpy)2CAHTUNGRTENUN(GN dcbpy)]/TiO2
system. Lewis and McCusker[10] also reported multiexponen-
tial EI in structurally identical RuII– and OsII–polypyridyl
complexes with carboxylate anchoring on the TiO2 surface.
Interestingly, in all such cases multiexponential electron-in-
jection dynamics were observed for carboxylate sensitizers
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617

A. Das, H. N. Ghosh et al.
plexes 1 and 2 to couple strongly with the nanoparticles
with the formation of a charge-transfer complex. Femtosec-
ond transient absorption measurements have been carried
out to study IET dynamics in both [RuII
ACHTUNGTRENNUNG
TiO2 (namely, 1/TiO2) and [OsII
ACHTUNGTRENNUNG
(namely, 2/TiO2) systems. Upon excitation with a 400 nm
laser pulse, bleaching of the adsorbed dye, transient absorp-
tion for the dye cations (M·+), and a broad absorption band
for the conduction-band electron are observed. Electron in-
jection is found to be single exponential and pulse-width
limited (<100 fs) in the 1/TiO2 system, thereby indicating
injection from the nonthermalized singlet state (1MLCT),
and/or nonthermalized triplet state (3MLCT), or a combina-
tion of both. On the other hand, in the 2/TiO2 system elec-
tron-injection dynamics are found to be biexponential with
time constants of ti1nj <100 fs (42%) and ti2nj =1.7 ps (58%).
Here the shorter component could be attributed to the elec-
tron injection from nonthermalized MLCT states and the
longer component could be attributed to electron injection
from thermalized 3MLCT states. Back-electron-transfer
(BET) dynamics were studied by monitoring the decay ki-
netics of the cation radical, the injected electron in the con-
duction band of TiO2, and also from recovery kinetics of the
bleaching of the adsorbed dye. BET kinetics for the 1/TiO2
system can be fit with multiexponential time constants of
t1 =0.8 ps (29%), t2 =20 ps (21%), and t3 >300 ps (50%),
and for the 2/TiO2 system the kinetics can be best fit with
time constants of t1 =2.0 ps (12%), t2 =35 ps (17%), and
t3 >300 ps (71%). It is interesting to see that BET dynamics
are slower for the 2/TiO2 system than the 1/TiO2 system.
Scheme 2. Mechanistic scheme showing a three-level model that consists
of the ground state (S0), excited triplet state (3MLCT), and excited singlet
(1MLCT) state of 1 and 2 adsorbed on TiO2 nanoparticles. CB=Conduc-
tion band; VB=valence band.
cess is therefore expected to be much faster and more effi-
cient. Thus upon photoexcitation the 3MLCT state gets
populated much faster through an efficient ISC process
1
from initially populated MLCT states relative to the analo-
gous RuII complex 1 and electron injection takes place from
both the states with different time constants (Scheme 2).
We have also monitored back-electron-transfer (BET)
time constants for both the systems by monitoring kinetic
decay traces at 1000 nm as well as bleaching recovery kinet-
ics at 490 nm for both the systems. BET kinetics for the 1/
TiO2 system can be fit with multiexponential time constants
of t1 =0.8 ps (29%), t2 =20 ps (21%), and t3 > 300 ps
(50%), and for the 2/TiO2 system the kinetics can be best fit
with time constants of t1 =2.0 ps (12%), t2 =35 ps (17%),
and t3 >300 ps (71%). Interestingly, the BET dynamics
have been found to be slower in the 2/TiO2 system relative
to the 1/TiO2 system. Furthermore, we have also confirmed
that in a longer time domain (up to 500 ms) the BET reac-
tion is slower in the 2/TiO2 system relative to the 1/TiO2
system (see the Supporting Information).
Acknowledgements
We thank Dr. T. Mukherjee and Dr. S. K. Sarkar of BARC, and Dr. P. K.
Ghosh of CSMCRI for their constant encouragement. H.N.G. thanks
BRNS, and A.D. thanks DST and BRNS for financial assistance.
[1] B. OꢁRegan, M. Grꢂtzel, Nature 1991, 353, 737.
[2] a) S. Ardo, G. J. Meyer, Chem. Soc. Rev. 2009, 38, 115; b) M. K. Na-
zeeruddin, P. Pechy, T. Renouard, S. M. Zakeeruddin, R. Humphry-
Baker, P. Comte, P. Liska, L. Cevey, E. Costa, V. Shklover, L. Spic-
cia, G. B. Deacon, C. A. Bignozzi, M. Gratzel, J. Am. Chem. Soc.
2001, 123, 1613; c) M. K. Nazeeruddin, S. M. Zakeeruddin, R.
Humphry-Baker, M. Jirousek, P. Liska, N. Vlachopoulus, V. Skhlov-
er, C. H. Fischer, M. Gratzel, Inorg. Chem. 1999, 38, 6298; d) D. Ku-
ciauskas, M. S. Freund, H. B. Gray, J. R. Winkler, N. S. Lewis, J.
Phys. Chem. B 2001, 105, 392; e) S. Altobello, R. Argazzi, S. Cara-
mori, C. Contado, S. Da Fre, P. Rubino, C. Chonꢃ, G. Larramona,
C. A. Bignozzi, J. Am. Chem. Soc. 2005, 127, 15342; f) F. Gao, Y.
Wang, J. Zhang, D. Shi, M. Wang, R. Humphry-Baker, P. Wang,
S. M. Zakeeruddin, M. Grꢂtzel, Chem. Commun. 2008, 2635; g) B.
Dietzek, W. Kiefer, J. Blumhoff, L. BÅttcher, S. Rau, D. Walther, U.
Uhlemann, M. Schmitt, J. Popp, Chem. Eur. J. 2006, 12, 5105.
[3] J. B. Asbury, E. Hao, Q. Y. Wang, H. N. Ghosh, T. Lian, J. Phys.
Chem. B 2001, 105, 4545.
Conclusion
We have synthesized a ruthenium(II)– and an osmium(II)–
polypyridyl complex ([MACHTNUTGRNEUNG
(bpy)2L]2+, in which M=RuII (1)
or OsII (2), bpy=2,2’-bipyridyl, and L=4-(2,2’-bipyridinyl-4-
yl)benzene-1,2-diol), and characterized them by means of
various analytical and spectroscopic techniques. Optical ab-
sorption studies show that complex 1 has a strong MLCT ab-
sorption band at 400–500 nm, which is attributed to a
1MLCT (spin-allowed) transition. Complex 2, however, fea-
tured two strong MLCT absorption bands in the 400–550
and 550–750 nm regions, which are attributed to 1MLCT
[4] G. Benkç, J. Kallioinen, J. E. I. Korppi-Tommola, A. P. Yartsev, V.
Sundstrom, J. Am. Chem. Soc. 2002, 124, 489.
[5] G. Ramakrishna, D. A. Jose, D. KrishnaKumar, A. Das D. K. Palit,
H. N. Ghosh, J. Phys. Chem. B 2005, 109, 15445.
3
(spin-allowed) and MLCT (spin-forbidden) transitions. The
pendant catecholate moiety of the complexes allows com-
618
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Heavy Atoms
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[6] P. Kar, S. Verma, A. Das, H. N. Ghosh, J. Phys. Chem. C 2009, 113,
7970.
[19] F. Barigelletti, L. De Cola, V. Balzani, R. Hage, J. G. Haasnoot, J.
Reedijk, J. G. Vos, Inorg. Chem. 1991, 30, 641.
[7] G. Sauvꢃ, M. E. Cass, S. J. Doig, I. Lauermann, K. Pomykal, N. S.
Lewis, J. Phys. Chem. B 2000, 104, 3488.
[20] R. Argazzi, G. Larramona, C. Contado, C. A. Bignozzi, J. Photo-
chem. Photobiol. A 2004, 164, 15.
[8] C. Chiorboli, M. A. J. Rodgers, F. Scandola, J. Am. Chem. Soc. 2003,
125, 483.
[21] D. Bahnemann, A. Henglein, J. Lilie, L. Spanhel, J. Phys. Chem.
1984, 88, 709.
[9] T. A. Heimer, E. J. Heilweil, C. A. Bignozzi, G. J. Meyer, J. Phys.
Chem. A 2000, 104, 4256.
[10] D. Kuciauskas, J. E. Monat, R. Villahermosa, H. B. Gray, N. S.
Lewis, J. K. McCusker, J. Phys. Chem. B 2002, 106, 9347.
[11] P. G. Hoertz, D. W. Thompson, L. A. Friedman, G. J. Meyer, J. Am.
Chem. Soc. 2002, 124, 9690.
[22] H. N. Ghosh, J. Phys. Chem. B 1999, 103, 10382.
[23] G. Ramakrishna, H. N. Ghosh, Langmuir 2003, 19, 505.
[24] D. Kuciauskas, M. S. Freund, H. B. Gray, J. R. Winkler, N. S. Lewis,
J. Phys. Chem. B 2001, 105, 392.
[25] S. Kaniyankandy, S. Verma, J. A. Mondal, D. K. Palit, H. N. Ghosh,
J. Phys. Chem. C 2009, 113, 3593.
[12] C. R. Rice, M. D. Ward, M. K. Nazeeruddin, M. Gratzel, New J.
Chem. 2000, 24, 651.
[26] A. C. Bhasikuttan, M. Suzuki, S. Nakashima, T. Okada, J. Am.
Chem. Soc. 2002, 124, 8398.
[13] S. Verma, P. Kar, A. Das, D. K. Palit, H. N. Ghosh, J. Phys. Chem. C
2008, 112, 2918.
[27] A. Vlꢄek, Jr., Coord. Chem. Rev. 2000, 200, 933.
[28] J. Z Zhang, Acc. Chem. Res. 1997, 30, 423.
[14] E. M. Kober, J. V. Caspar, B. P. Sullivan, T. J Meyer, Inorg. Chem.
1988, 27, 4587.
[29] D. P. Colombo, Jr., R. M. Bowman, J. Phys. Chem. 1996, 100, 18445.
[30] T. Hannappel, B. Burfeindt, W. Storck, F. Willig, J. Phys. Chem. B
1997, 101, 6799.
[31] H. N. Ghosh, J. B. Asbury, T. Lian, J. Phys. Chem. B 1998, 102, 6482.
[32] J. B. Asbury, R. J. Ellingson, H. N. Ghosh, S. Ferrere, A. Nozik, T.
Lian, J. Phys. Chem. B 1999, 103, 3110.
[15] a) L. G. C. Rego, V. S. Batista, J. Am. Chem. Soc. 2003, 125, 7989;
b) W. R. Duncan, O. V. Prezhdo, Annu. Rev. Phys. Chem. 2007, 58,
143, and references therein. c) A. Petersson, M. Ratner, H. O. Karls-
son, J. Phys. Chem. B 2000, 104, 8498; d) D. A. Gaal, J. E. McGar-
rah, F. Liu, J. E. Cook, J. T. Hupp, Phys. Chem. Chem. Phys. 2007, 9,
853.
[33] Y. Tachibana, J. E. Moser, M. Grꢂtzel, D. R. Klug, J R. Durrant, J.
Phys. Chem. 1996, 100, 20056.
[16] A. Hagfeldt, M. Gratzel, Chem. Rev. 1995, 95, 49.
[17] D. Xu, J. Z. Zhang, B. Hong, J. Phys. Chem. A 2001, 105, 7979.
[18] H. B. Baudin, J. Davidsson, S. Serroni, A. Juris, V. Balzani, S. Cam-
pagna, L. Hammarstrom, J. Phys. Chem. A 2002, 106, 4312.
[34] T. D. M. Bell, C. Pagba, M. Myahkostupov, J. Hofkens, P. Piotro-
wiak, J. Phys. Chem. B 2006, 110, 25314.
Received: July 13, 2009
Published online: December 8, 2009
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619