Low quantum yields of relaxed electron transfer products of moderately coupled ruthenium(II)-cobalt(III) compounds on the subpicosecond laser excitation

Article abstract of DOI:10.1021/jp037259z

Photoinduced electron-transfer reactions of [(tpy)Ru^II(tpy-tpy) Co^III(tpy)]5+ (tpy = 2,2′:6′,2″-terpyridine and tpy-tpy = 6′,6″-bis(2-pyridyl)-2,2′:4′,4″: 2″,2″′-quarterpyridyne), [(tpy)Ru^II(tpy-ph-tpy) Co^III(tpy)]5+ (tpy-ptitpy = 1,4-bis[2,2′:6′,2″- terpyridine-4′-yl]benzene), and [(bpy)₂Ru^II(tpphz) Co^III(bpy)₂]⁵⁺ (bpy = 2,2′-bipyridine, and tpphz = tetrapyrido[3,2-a:2′,3′-c:3″,2″-h:: 2″′,3″′-j]phenazine) were studied in the range 140-298 K by means of subpicosecond transient absorption spectroscopy. ³MLCT(Ru) of [(tpy)Ru^II(L-L)Co^III(tpy)] ⁵+ (L-L: tpy-tpy and tpy-ph-tpy) underwent intramolecular electron-transfer (EET) reaction to form [²Ru^III(tpy)(L-L) ²Co^II(tpy)]⁵⁺ in a shorter time than 10 ps. Low quantum yields of the EET products (0.53 for tpy-tpy and 0.41 for tpy-ph-tpy in butyronitrile at 298 K) measured before the fast return electron-transfer were found as in the intermolecular electron-transfer quenching of ³MLCT(Ru) by [Co(tpy)₂]³⁺. The quantum yield of [(tpy)-Ru^III(tpy-tpy)Co^II(tpy)]⁵⁺ (Φ) decreases to 0.38 in a slowly reorganizing solvent of propylene carbonate and that of [(tpy)Ru^II(tpy-ph-tpy)Co^III(tpy)]⁵⁺ to 0.21 at 180 K. The reductions in the EET yields, 1 - Φ, can be ascribed to a fast transition of the nonrelaxed EET product to the lowest triplet d-d* state of [Co^III(tpy)₂] moiety during the solvent reorganization. A tunneling transition of the nonrelaxed EET products to the lowest lying d-d excited-state of [Co^III(tpy)₂] moiety takes place as a hole transfer, HT, from the dπ-orbital of Ru(III) to that of Co(II) with a configuration of dπ⁶da*. An electronic coupling of dπ(Ru)-dπ(Co) estimated from the intensity of inter-valence transition of [(tpy)Ru^III(L-L)Ru^II(tpy)]⁵⁺ (Collin, J.-P.; Laine, P.; Launay, J.-P.; Sauvage, J.-P.; Sour, A. J. Chem. Soc., Chem. Commun. 1993, 434) is large enough to carry out the HT. The weak coupling of dπ(Ru)-dπ(Co) in the case of [(bpy)₂Ru(tpphz) Co(bpy)₂]⁵⁺ attenuates the HT rate to bring about a high yield of EET (0.8). The possibility that energy-transfer of ³MLCT(Ru) to the Co(III) moiety via an intermediate super-exchange coupling between dπ(Ru) and dπ(Co) occurs in competition with the EET of [(tpy)Ru ^III(tpy-ph-tpy)Co^II(tpy)]⁵⁺ is also pursued.

Full text of DOI:10.1021/jp037259z

© Copyright 2004 by the American Chemical Society
VOLUME 108, NUMBER 22, JUNE 3, 2004
ARTICLES
Low Quantum Yields of Relaxed Electron Transfer Products of Moderately Coupled
Ruthenium(II)-Cobalt(III) Compounds on the Subpicosecond Laser Excitation
Hiroaki Torieda, Koichi Nozaki, Akio Yoshimura, and Takeshi Ohno*
Department of Chemistry, Graduate School of Science, Osaka UniVersity, 1-16 Machikaneyama, Toyonaka,
Osaka 560-0043, Japan
ReceiVed: October 29, 2003; In Final Form: March 16, 2004
Photoinduced electron-transfer reactions of [(tpy)RuII(tpy-tpy)CoIII(tpy)]5+ (tpy ) 2,2′:6′,2′′-terpyridine and
tpy-tpy ) 6′,6′′-bis(2-pyridyl)-2,2′:4′,4′′:2′′,2′′′-quarterpyridyne), [(tpy)RuII(tpy-ph-tpy)CoIII(tpy)]5+ (tpy-ph-
tpy ) 1,4-bis[2,2′:6′,2′′-terpyridine-4′-yl]benzene), and [(bpy)2RuII(tpphz)CoIII(bpy)2]5+ (bpy ) 2,2′-bipyridine,
and tpphz ) tetrapyrido[3,2-a:2′,3′-c:3′′,2′′-h::2′′′,3′′′-j]phenazine) were studied in the range 140-298 K by
means of subpicosecond transient absorption spectroscopy. 3MLCT(Ru) of [(tpy)RuII(L-L)CoIII(tpy)]5+ (L-
L: tpy-tpy and tpy-ph-tpy) underwent intramolecular electron-transfer (EET) reaction to form [2RuIII(tpy)(L-
L)2CoII(tpy)]5+ in a shorter time than 10 ps. Low quantum yields of the EET products (0.53 for tpy-tpy and
0.41 for tpy-ph-tpy in butyronitrile at 298 K) measured before the fast return electron-transfer were found as
in the intermolecular electron-transfer quenching of 3MLCT(Ru) by [Co(tpy)2]3+. The quantum yield of [(tpy)-
RuIII(tpy-tpy)CoII(tpy)]5+ (Φ) decreases to 0.38 in a slowly reorganizing solvent of propylene carbonate and
that of [(tpy)RuII(tpy-ph-tpy)CoIII(tpy)]5+ to 0.21 at 180 K. The reductions in the EET yields, 1 - Φ, can be
ascribed to a fast transition of the nonrelaxed EET product to the lowest triplet d-d* state of [CoIII(tpy)2]
moiety during the solvent reorganization. A tunneling transition of the nonrelaxed EET products to the lowest
lying d-d excited-state of [CoIII(tpy)2] moiety takes place as a hole transfer, HT, from the dπ-orbital of
Ru(III) to that of Co(II) with a configuration of dπ6dσ*. An electronic coupling of dπ(Ru)-dπ(Co) estimated
from the intensity of inter-valence transition of [(tpy)RuIII(L-L)RuII(tpy)]5+ (Collin, J.-P.; Laine, P.; Launay,
J.-P.; Sauvage, J.-P.; Sour, A. J. Chem. Soc., Chem. Commun. 1993, 434) is large enough to carry out the
HT. The weak coupling of dπ(Ru)-dπ(Co) in the case of [(bpy)2Ru(tpphz)Co(bpy)2]5+ attenuates the HT
3
rate to bring about a high yield of EET (0.8). The possibility that energy-transfer of MLCT(Ru) to the
Co(III) moiety via an intermediate super-exchange coupling between dπ(Ru) and dπ(Co) occurs in competition
with the EET of [(tpy)RuIII(tpy-ph-tpy)CoII(tpy)]5+ is also pursued.
Introduction
donor-acceptor compound takes place faster than 1012 s-1
,
electronic coupling between an oxidized donor and a reduced
acceptor is expected to be large enough to carry out a fast RET.
Furthermore, the Gibbs free energy surface of EET product
could have an intersection with that of the reactant not only at
the ground state but also at the electronic excited state.7
According to theories of tunneling transition between potential-
energy surfaces, a fraction of the tunneling transition depends
on the strength of electronic coupling between diabatic states,
the intersection angle between diabatic states, and the excess
Reaction products of excited-state electron-transfer (EET) are
likely to undergo return electron-transfer (RET) reducing
apparent quantum yield of EET product formation. A transient
absorption spectroscopy has revealed that fast RET occurs within
geminate radical pairs formed in bimolecular quenching of
excited molecule1-4 and in photoexcitation of charge-transfer
compounds.5,6 Provided that EET within a chemically linked
* Corrresponding author. E-mail: ohno@ch.wani.osaka-u.ac.jp.
10.1021/jp037259z CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/07/2004

4820 J. Phys. Chem. A, Vol. 108, No. 22, 2004
Torieda et al.
thermal energy of the nonrelaxed state.8,9 EETs of donor-
acceptor compounds in solution are followed by reorganization
of atoms in the product and of surrounding solvent molecules
to redistribute the change of standard Gibbs free energy to many
vibrational modes of the product and kinetic or solvation mode
of surrounding solvent molecules. Some extent of entropy
change is involved in the reorganization. It takes less than a
couple of picoseconds to redistribute the thermal energy among
low-frequency modes of vibration and solvation.10-12 Further-
more, the nonrelaxed product of EET could undergo a tunneling
transition to lower lying states in competition with the thermal
relaxation.
5,6-diamino-1,10-phenanthroline, which gave rise to 1,10-
phenanthroline-5,6-dione.18,19 [Ru(bpy)2(1,10-phenanthroline-
5,6-dione)](PF6)2‚2H2O was prepared from cis-[Ru(bpy)2Cl2]‚
3H2O and 1,10-phenanthroline-5,6-dione by refluxing for 3 h.
[Ru(bpy)2(tpphz)](PF6)2‚2H2O was prepared from [Ru(bpy)2-
(1,10-phenanthroline-5,6-dion)]2+ and 5,6-diamino-1,10-phenan-
throline by refluxing for 2 h.19 cis-[Co(bpy)2Cl2]Cl‚2H2O was
prepared according to the literature.20
[Ru(bpy)2(tpphz)Co(bpy)2](PF6)5 was prepared from a solu-
tion of [Ru(bpy)2(tpphz)](PF6)2‚2H2O (26 mg) and [Co(bpy)2-
Cl2]Cl ‚2H2O (16 mg) in 1:1 ethanol-acetonitrile (40 mL) by
refluxing for 2 h, and then the solvent was evaporated. The
residue was dissolved in water and then excess amount of NH4-
PF6 was added. The solid product was filtered out and purified
through CM Sephadex C-25 column and Sephadex LH-20
column. Yield: 7 mg (16%). [Ru(bpy)2(tpphz)Ru(bpy)2](PF6)4
was prepared according to the literature.20
Measurement. Preparation of the Sample Solutions. Ac-
etonitrile (AN) and propylene carbonate (PC) were used as a
solvent for the measurement at 298 K. Butyronitrile (BN) was
used as a solvent for the measurement in the region 170-298
K. The solvents were purified by distillation. The sample
solutions were prepared just before the measurements.
Cyclic Voltammetry. Redox potentials of the metal com-
plexes were measured by means of differencial-pulse voltam-
metry using a dc pulse polarograph (Huso, HECS-312B).
Voltamograms were recorded by using a platinum disk electrode
(diameter 0.5 mm) in a solution of the complex containing 0.05
M of tetraethylammonium perchlorate as a supporting electro-
lyte.
Small yields of the relaxed ET product demonstrate the
occurrence of either RET on the way to the relaxed product or
competing processes of the excited-state such as excitation
energy transfer. Quantum yields of intramolecular reactions in
the fast quenching of excited state ΦEET have, however, been
rarely determined,13 because the lifetimes of excited states are
too short to determine the formation of excited-state undergoing
EET even on using a subpicosecond transient photometry. In
this paper, the number of photon absorbed by a chromophore
is counted for by employing a transient actinometer of [Ru-
(bpy)3]2+, which is converted to the countable lowest excited
state, 3MLCT(Ru), in 100 fs.14,15 Low quantum yields of
electron-transfer products in a bimolecular electron-transfer
3
quenching of MLCT(Ru) of [RuL3]2+ (L:2,2′-bipyridine and
2,2′-bipyridine-4,4′-dicarboxylate) with [Co(tpy)2]3+ (tpy:2,2′:
6′,2′′-terpyridine) have been ascribed to either RET of geminate
radical-pair formed in the electron-transfer quenching or a rapid
radiationless transition of exciplex formed.16 A previous paper
revealed that both EET and RET with a temperature-independent
rate of 1010-1012 s-1 occurred in [(tpy)Ru(tpy-ph-tpy)Co(tpy)]5+
and [(tpy)Ru(tpy-tpy)Co(tpy)]5+ containing a [Co(tpy)2]2+ moi-
ety.17 The fast processes of EET and RET have been accounted
for in terms of through-ligand electronic coupling between
localized d-orbitals of metal centers and small structural change
of the Co(II) moiety. Quantum yields of [RuIII(L-L)CoII]5+
formation, however, have not been determined for these
Determination of the Quantum Yields of Electron-
Transfer Product. The quantum yield of excited-state formation
or excited-state reactions on the laser excitation was determined
in the following procedures, provided that the conversion took
place during the excitation pulse. Rates of reactant disappearance
are proportional to the laser flux absorbed by the reactant, Iabs
,
and the quantum yield of excited-state formation or reaction,
Φ. The ET-reaction rate of a reactant in a cell with path length
of d cm is written in terms of laser flux irradiated to the
reactant,I(t), and the absorbance of the sample, RC(t)d,
3
compounds because EET quenching of MLCT(Ru) was too
3
rapid to determine the production of MLCT(Ru).
In this study, quantum yields of excited-state ET reactions,
EET, for rigid donor-acceptor compounds, [Ru(bpy)2(tpphz)-
dC(t)
dt
Φ
103
d
103
d
-
) Φ Iabs
) Φ I(t)(1 - exp(- RC(t)d)) ×
Co(bpy)2](PF6)5 (bpy ) 2,2′-bipyridine, tpphz ) tetrapyrido
[3,2-a:2′,3′-c:3′′,2′′-h::2′′′,3′′′-j]phenazine), [Ru(tpy)(tpy-ph-tpy)-
Co(tpy)](PF6)5(tpy ) 2,2′:6′,2′′-terpyridine, tpy-ph-tpy ) 1,4-
bis[2,2′:6′,2′′-terpyridine-4′-yl]benzene) and [Ru(tpy)(tpy-tpy)-
Co(tpy)] (PF6)5 (tpy-tpy ) 6′,6′′-bis (2-pyridyl)-2,2′:4′,4′′:2′′,2′′′-
quarterpyridyne) were determined by means of transient
actinometry. Time-resolved absorption spectroscopy after the
laser excitation in the region 10-13-10-7s will reveal a different
reaction sequence for [RuII(bpy)2(tpphz)CoIII(bpy)2]5+ from
those for [RuII(tpy)(tpy-ph-tpy)CoIII(tpy)]5+ and [RuII(tpy)(tpy-
tpy)CoIII(tpy)]5+. Solvent and temperature dependence of the
ET reaction yield were also investigated.
(1)
Provided that the formation of either the reacting excited state
or the ET product occurs during the laser pulse to decrease the
concentration of reactant at the ground-state, integrations of the
left-hand term over the concentration and of the right-hand term
over time during the laser pulse give the following equation
ln(eRc d - 1) ) ln(eRc d - 1) - 103 ΦRI
(2)
a
b
where ca, cb, R, d, and I are the concentration after and before
the laser excitation, the molar absorption coefficient of a reactant
at the wavelength of the excitation laser, the optical length, and
the integrated laser flux, respectively. By employing [Ru-
(bpy)3]2+ as a chemical actinometer, the integrated laser flux,
I, can be determined using, d, Φ1( ) 1), R1, c1a, and c1b, the
last of which was calculated from the formation of 3MLCT on
the basis of ∆Abs450 nm by using the difference molar absorption
coefficient ∆ꢀ ) -9800 M-1 cm-1 at 450 nm.21 Provided that
the width of the laser pulse is long enough to produce the ET
Experimental Section
Materials. [Ru(tpy)(tpy-ph-tpy)Co(tpy)](PF6)5‚6H2O and
[(tpy)Ru(tpy-tpy)Co(tpy)](PF6)5. The preparation of these
compounds was described in the previous paper.17
[Ru(bpy)2(tpphz)Co(bpy)2](PF6)5 and [Ru(bpy)2(tpphz)-
Ru(bpy)2](PF6)4. The ligand 1,10-phenanthroline-5,6-dione was
prepared as follows. 5-Nitro-6-amino-1,10-phenanthroline was
prepared by refluxing 5-nitro-1,10-phenanthroline with hydroxy-
lamine hydrochrolide in ethanol and then was converted to
product, the quantum yield of ET product, [RuIII(L-L)CoII]5+
,
Φ2, can be estimated by plotting ln(eR c d - 1) against 103R2I.
2
2a

Quantum Yield of Excited-State Electron Transfer
J. Phys. Chem. A, Vol. 108, No. 22, 2004 4821
TABLE 1: Phosphoresence Peaks at 77 K and the Decay
Provided that EET takes place after a laser pulse, the formation
of 3MLCT(Ru), c2b - c2a, can be calculated from R2, c2a and I.
Formations of the ET product,c2b - c2a, were obtained from
a difference absorbance at the peak wavelength of MLCT
absorption right after the disappearance of 3MLCT. The molar
absorption coefficient of the ET product was determined as the
sum of those of [RuIII(L-L)]3+ and [CoII(L-L)]2+, of which
the former was prepared by means of electrochemical oxidation
of [RuII(L-L)]2+. Differences in the molar absorption coefficient
between the reactant and the product are the following, ∆ꢀ490
3
Rate Constants of CT(Ru), k1, and [RuIII(L-L)CoII], kRET
,
at 298 K
phos
max
compounds
ν˜ /cm-1 k1/109 s-1 kRET/109 s-1
[Ru(tpy)(tpy-ph-tpy)Ru(tpy)]5+ 15 500a
[Ru(tpy)(tpy-ph-tpy)Co(tpy)]5+
0.30b
320b
0.20a
1000b
0.0009*
0.013*
1400
81, 4.9b
520, 28b
[Ru(tpy)(tpy-tpy)Ru(tpy)]5+
[Ru(tpy)(tpy-tpy)Co(tpy)]5+
[Ru(bpy)2(tpphz)]2+
14 900a
16 730
16 680
[Ru(bpy)2(tpphz)Ru(bpy)2]4+
[Ru(bpy)2(tpphz)Co(bpy)2]5+
0.02
) -34400 M-1 cm-1 for [RuII(tpy)(tpy-ph-tpy)CoIII(tpy)]5+
∆ꢀ515 ) -24400 M-1 cm-1 for [RuII(tpy)(tpy-tpy)CoIII(tpy)]5+
and ∆ꢀ450 ) -30 000 M-1 cm-1 for [RuIII(bpy)2(tpphz) CoII-
(bpy)2]5+
,
,
a Reference 16. b Reference 17.
TABLE 2: Redox Potentials of the Component Compounds
and Gibbs Free Energy Change of the Return
Electron-Transfer Process in BN at 298 K
.
Time-Resolved Difference Absorption Spectra. Time-
resolved difference absorption spectra were obtained by using
the second harmonic (SHG) of Q-switched Nd3+:YAG laser
(Continuum Surelite I-10, λex ) 532 nm, fwhm 4 ns) and the
SHG of a mode-locked Nd3+:YAG laser (λex ) 532 nm, fwhm
17 ps) for the excitation.
White light from a Xe-arc lamp was used for acquisition of
absorption spectra of longer-living species than 10 ns.21 A white
light pulse with a delay of 20-6000 ps for the excitation was
produced by focusing the fundamental oscillation laser light into
a flowing H2O/D2O (1:1 by volume) solution.21 The experi-
mental setup and the characteristics of a subpicosencond laser
was shown in ref 22. The temperature of the sample solutions
(77-300 K) was controlled by the use of a cryostat (Oxford
DN1704) and a controller (Oxford ITC4).
a
a
b
compounds
Eo(Ru3+/2+
)
Eo(Co3+/2+
)
∆G°
RET
[Co(bpy)3]3+
0.28
0.25b
0.25b
(0.25)c
[Co(tpy)2]3+
[(tpy)Ru(tpy-ph-tpy)Co(tpy)]5+
[(tpy)Ru(tpy-tpy)Co(tpy)]5+
[Ru(bpy)2(tpphz)]2+
1.24b
1.26b
1.24
1.3
-0.99b
-0.97b
[Ru(bpy)2(tpphz)Ru(bpy)2]4+
[Ru(bpy)2(tpphz)Co(bpy)2]5+
1.3
(0.25)c
-1.02b
a Vs SCN in AN containing 0.1 M tetraethylammonium perchlorate.
b Reference 17. c Eo(Co3+/2+) is assumed to be either that of [Co(bpy)3]3+
or [Co(tpy)2]3+
.
TABLE 3: Temperature Dependence of the Decay Rate
3
Constant (k1) of CT(Ru), the Quantum Yield of ET Product
Formation (ΦEET) and the Rate Constants (kRET) of the
Recovery Processes of the Ground State Absorption of
[(tpy)Ru(tpy-ph-tpy)Co(tpy)]5+ in BN
Results
T/K
∆GR°ET/eV
k1s/1012a
ΦEET
kRET/1010 s-1 a
[RuIII(tpy)(L)L)CoII(tpy)]5+ (L)L: tpyp-py-tpy and tpy-
tpy). 1. Transient Absorption Changes and Sequential
Reactions. A subpicosecond laser excitation of a N2-bubbled
BN solution of [RuIII(tpy)(tpy-ph-tpy)CoII(tpy)]5+ produced a
negative absorbance below 540 nm due to the smaller molar
extinction coefficient of 3MLCT of Ru(II) moiety than the
ground state and a broad band of 3MLCT with the maximum at
650 nm in a red and near infrared region. The broad band due
to 3MLCT of Ru(II) moiety decayed in 2 ps with a rate constant
of (3.2 ( 0.5) × 1011s-1. [(tpy)2RuIII(tpy-ph-tpy)2CoII (tpy)]5+
in eqs 4 and 5.17 The resultant difference absorption consisted
of a negative band below 570 nm and a weak and broad band
in a red region, both of which were ascribed to the bleaching
of the MLCT band and the formation of the LMCT band,
respectively. The recovery of the MLCT absorption band during
297
260
220
180
0.99
1.03
1.05
1.07
0.32
0.32
0.28
0.25
0.41
0.31
0.23
0.21
3.9
4.4
4.5
4.6
a Reference 17.
As for [(tpy)RuII(tpy-tpy)CoIII(tpy)]5+ in BN, the subpico-
second laser excitation produced a difference absorption spec-
trum characteristic of MLCT, of which a broad band at 620
3
nm was monitored to decay with a rate constant of 1.0 × 1012
s-1. The MLCT absorption band bleached was multiexponen-
tially recovered in 100 ps. The rate constant of the slow recovery
was 2.8 × 1010 s-1. That of the intermediate recovery was
determined to be 1.5 × 1011 s-1 on the basis of the fastest
recovery rate (1.0 × 1012 s-1) of MLCT band from MLCT
3
3
the disappearance of MLCT (eq 5) demonstrates a low yield
and the slow one (2.8 × 1010 s-1) from EET product. The
production of [Ru(III)-Co(II)]5+ was determined from the
absorbance-bleached MLCT band.
of [(tpy)2RuIII(tpy-ph-tpy)2CoII(tpy)]5+. The subsequent biex-
ponential recovery of the MLCT band was a result of RET of
[CoII(tpy)2] moiety with rate constants of 8.1 × 1010 and 0.49
× 1010 s-1 17
.
The production of the [RuIII(tpy)2] moiety was
2. Formation Quantum Yield of the Electron-Transfer
Product. The formation of the EET product was determined
by extrapolating the recovery of the MLCT band from EET
product. The formation of the excited state (4.5 × 10-5 M) of
[(tpy)Ru(tpy-ph-tpy)Co(tpy)]5+ was evaluated by using eq 2
from the absorbances before and after the excitation (ꢀCbd and
ꢀCad) and the integrated laser flux (0.90 × 10-7 mol/cm2), the
latter of which was obtained by using [Ru(bpy)3]2+ (6.4 × 10-5
M) as a chemical actinometer. The quantum yields of [(tpy)-
RuIII(tpy-ph-tpy)CoII(tpy)]5+ were independent of the solvents,
0.39 in AN, 0.41 in BN, and 0.39 in PC. Lowering the
temperature to 180 K reduced the yield to 0.21 in BN as is
shown in Table 3.
determined from the absorbance recovered of MLCT band from
the EET product.
[(tpy)Ru(tpy-ph-tpy)Co(tpy)]5+ + hν f 3MLCT(Ru) (3)
3MLCT(Ru) f [(tpy) 2RuIII(tpy-ph-tpy)2CoII(tpy)]5+ (4)
3MLCT(Ru) f [(tpy) RuII(tpy-ph-tpy)CoIII(tpy)]5+ (5)
[(tpy) 2RuIII(tpy-ph-tpy)2CoII(tpy)]5+
[(tpy) 2RuIII(tpy-ph-tpy)4CoII(tpy)]5+ (6)
f
[(tpy) 2RuIII(tpy-ph-tpy)2CoII(tpy)]5+
f
The quantum yield of [(tpy)RuIII(tpy-tpy)CoII(tpy)]5+ on the
subpicosecond excitation was determined in a similar way. The
[(tpy) RuII(tpy-ph-tpy)CoIII(tpy)]5+ (7)

4822 J. Phys. Chem. A, Vol. 108, No. 22, 2004
Torieda et al.
Figure 2. Absorption spectra of [(bpy)2Ru(tpphz)Co(bpy)2]5+ and its
components in AN. Solid line: [(bpy)2Ru(tpphz)Co(bpy)2]5+. Dashed
line: [(bpy)2Ru(tpphz)]2+. Dotted line: [Co(bpy)2Cl2]+.
The coordination of tpphz to [CoIII(bpy)2] moiety enhances the
molar absorption coefficients (ꢀ) of the π-π transition of tpphz
at 350-400 nm compared with [Ru(tpy)(tpphz)]2+
.
2. Transient Absorption Change and Sequential Reaction.
A N2-bubbled AN solution of [(bpy)2Ru(tpphz)Co(bpy)2]5+
under the laser excitation displayed the bleaching of the MLCT
band below 520 nm and the formation of a positive band with
the maximum at 610 nm in the red and nearinfrared regions as
Figure 3 shows. Each of the spectral changes was obtained as
a difference absorption spectrum referred to the spectrum before
the excitation. The transient difference spectrum observed
immediately after the subpicosecond laser excitation of [(bpy)2Ru-
(tpphz)Co(bpy)2]5+ was similar to the primary excited-state
Figure 1. Molecular structures.
3
formation of the CT (2.0 × 10-5 M) was evaluated from the
(3CT(Ru)) absorption spectrum of [(bpy)2Ru(tpphz)Ru(bpy)2]4+
,
absorbance of the reactant before and after the laser excitation
at 400 nm, molar absorption coefficient (R) and the integrated
laser flux (0.90 × 10-7 mo/lcm2). The formation of [(tpy)RuIII-
(tpy-tpy)CoII(tpy)]5+, 1.1 × 10-5 M, was determined by
extrapolating the recovery of MLCT band from the EET
product.. The quantum yield of [(tpy)RuIII(tpy-tpy)CoII(tpy)]5+
was 0.65 in AN, 0.53 in BN, and 0.38 in PC.
which was very rapidly converted to an absorption spectrum of
differently solvated charge-transfer state in polar solvent.24,25
A partial bleaching of the MLCT band and the formation of a
broad band in a longer wave wavelength than 530 nm are
3
characteristic of CT(Ru) excited states, though the intensity
and width of a broad band are sensitive to the polarity of solvents
for [(bpy)2M(tpphz)M′(bpy)2]4+ (M ) Ru(II) and M′ ) Ru(II)
or Os(II)). In 2 ps, the initial difference absorbance of -0.08 at
450 nm became more negative (-0.175) at 450 nm and that of
0.04 at 600 nm was reduced to 0.02 at 600 nm, respectively,
with a rate constant of 1.4 × 1012 s-1. Moreover, the difference
absorbance at 450 nm became slightly more negative (-0.185)
in 20 ps with a rate constant of 1.3 × 1011 s-1. The slight
absorption change monitored at 450 nm may be somewhat of a
relaxation process of molecular vibration, spin-multiplicity, or
solvent reorganization that produces a stable form of the
electron-transfer product. The reaction product exhibited an
absorption spectrum similar to the sum of the absorption spectra
of [(bpy)2RuIII(tpphz)]3+ electronically prepared and [CoII(bpy)2-
Cl2], indicating that the reaction is the following
The initial difference absorbance at 490 or 515 nm at the
peak time of the laser pulse can be ascribed to a difference in
the absorbance between the ground state and MLCT(Ru). By
3
3
employing the production of MLCT determined above, one
calculates the difference in the molar extinction coefficient, ∆ꢀ,
to be 17 100 M-1 cm-1 at 490 nm and 19 300 M-1 cm-1 at
515 nm for [(tpy)RuIII(tpy-ph-tpy)CoII(tpy)]5+ and [(tpy)RuIII-
(tpy-tpy)CoII(tpy)]5+, respectively.
[(bpy)2RuII(tpphz)CoIII(bpy)2]5+. 1. Absorption Spectra
and Redox Potentials. The peak potentials of differencial pulse
voltamograms (DPV) vs Fc+/Fc for [Ru(bpy)2(tpphz)]2+, [Co-
(bpy)2(tpphz)]3+, and [(bpy)2Ru(tpphz)Co(bpy)2]5+ were ob-
tained at 298 K and are shown as the redox potential in Table
2. For the binuclear compounds, the peaks in the higher and
the lower potential region of DPV correspond to E°(Co3+/2+
and E°(Ru3+/2+), respectively.
)
2
3[(bpy)2RuII(tpphz)CoIII(bpy)2]5+ f 3[(bpy)2 RuIII
(tpphz)2CoII(bpy)2]5+ (8)
An absorption spectrum of [RuII(bpy)2(tpphz)]2+ consists of
three bands, a broad MLCT band with the peak at 445 nm, a
structured π-π band of tpphz with two peaks at 380 and 360
nm and a sharp π-π band of bpy coordinating to a divalent
cation at 290 nm as is shown in Figure 2. [Co(bpy)2Cl2]+
displays a structured π-π band around 310 nm, which is
characteristic of bpy coordinating to a trivalent cation. The
absorption spectrum of [RuII(bpy)2(tpphz)CoIII(bpy)2]5+ is nearly
equal to the sum of those of the components mentioned above.
The recovery of the MLCT band of the original reactant took
place with a rate constant of 2.1 × 107 s-1 at 298 K. The rate
constant decreased as the temperature decreased down to 170
K as is shown in Figure 4.
3. Formation Yield of ET Product. The formation quantum
yield of ET product, [(bpy)2RuIII(tpphz)CoII(bpy)2]5+, on the
nanosecond laser excitation was evaluated by plotting the left-

Quantum Yield of Excited-State Electron Transfer
J. Phys. Chem. A, Vol. 108, No. 22, 2004 4823
Figure 5. Plot of ln(exp(RdCa) - 1), against 103RI for a solution of
[(bpy)2RuI(tpphz)CoIII(bpy)2]5+
.
The formation of CT(Ru) of [Ru(bpy)2(tpphz)Co(bpy)2]5+
on the subpicosecond laser excitation was also estimated by
using eq 2. The integrated laser flux (0.9 × 10-7 mol/cm2) was
obtained by using [Ru(bpy)3]2+ as an actinometer. The product
formation of [(bpy)2RuIII(tpphz)CoII(bpy)2]5+, Cb - Ca, was
determined from the difference absorbance at 450 nm and at
100 ps. The quantum yield of [(bpy)2RuIII(tpphz)CoII(bpy)2]5+
formation in BN ΦEET at 298 K was 0.84, which slightly
decreased to 0.77 at 173 K.
3
Figure 3. (a) Subpicosecond transient absorption spectra of [(bpy)2Ru-
(tpphz)Co(bpy)2]5+ at 0.25 (solid line), 1 (dot), and 10 ps (dash-dot-
dash) and [(bpy)2Ru(tpphz)Ru(bpy)2]4+ at 0.3 ps (fine solid line). (b)
Difference absorption spectra of [(bpy)2Ru(tpphz)Co(bpy)2]5+ in AN
on the nanosecond laser excitation. Solid line: at 20 ns. Dashed line:
at 50 ns. Dash-dot-dashed line: at 100 ns. Dotted line: at 200 ns.
Discussion
1. Formation and decay of [RuIII(bpy)2(tpphz)CoII-
(bpy)2]5+. The primary difference absorption spectrum on
the subpicosecond laser excitation of [(bpy)2Ru(tpphz)Co-
(bpy)2]5+ is ascribed to the formation of 3CT(Ru) by comparing
the primary excited-state absorption of [Ru(bpy)2(tpphz)Ru-
(bpy)2]4+ 24,25 The 3MLCT(Ru) decayed with a rate constant of
.
1.4 × 1012 s-1 to produce the second transient species, which
is assigned to the EET product, 3[(bpy)2 RuIII(tpphz)2CoII-
2
(bpy)2]5+, from the difference absorption spectrum. The absorp-
2
tion spectrum of the stable EET product, [(bpy)2 RuIII(tpphz)4-
CoII(bpy)2]5+, with a lifetime of 480 ns on the 532 nm
nanosecond laser excitation was indifferent from the nascent
EET product on the subpicosecond excitation. The quantum
yield of the EET product, [2RuIII(bpy)2(tpphz)4CoII(bpy)2]5+, on
the 4 ns laser excitation (0.77) is in agreement with that on the
400 nm subpicosecond laser excitation (0.84). The high yield
is consistent with the reported quantum yields for intermolecular
Figure 4. Temperature dependence of the recovery rate of [(bpy)2Ru-
(tpphz)Co(bpy)2]5+ in BN. A solid line is the least-squares fit to the
equation,
4
EET product of Co(II) moiety.4,26 High yields of geminate-
radical pairs (ΦEET ) 0.94-1.0) were obtained in the intermo-
3
lecular quenching of MLCT(Ru) of [Ru(bpy)3]2+ by either
2πH2
(∆HR°ET - T∆SR°ET + λ)2
4λkBT
RET
[Co(phen)3]3+ or [Co(bpy)3]2+, where the high quantum yields
were calculated from the yields of ET product formation in the
ln kRET ) ln
-
,
p
4πk Tλ
x
B
∆HR°ET, ∆S° , HRET, and λ are -1.4 eV, 1.3 meV/K, 0.51 meV, and
2.05 eV, reRsEpTectively.
bulk (ΦEETfCE 0.93) and the cage-escape fractions (fCE
)
)
0.99).4 A unity yield for the intramolecular redox reaction of
[(bpy)2Ru(bpy-(CH2)2-bpy)Co(bpy)2]5+ was also observed.26
3
hand term of eq 2, ln(eR c d - 1), against 103R2I as is shown in
Figure 5. The integrated laser flux (I ) 4.77 × 10-7 mol/cm2)
The MLCT(Ru) of [RuII(bpy)2(tpphz)CoIII(bpy)2]5+ in BN
2
2a
undergoes EET with a much faster rate of 1.4 × 1012 s-1 than
3
was obtained by inserting the unity formation-yield of CT-
[(bpy)2RuII(tpphz)OsIII(bpy)2]5+ with a rate of 0.13 × 1010
(Ru) of [Ru(bpy)3]2+ and the absorbances before and after the
laser excitation into eq 2. The quantum yield of the long-living
ET product, [(bpy)2RuIII(tpphz)CoII(bpy)2]5+, is estimated to be
0.77.
s-1 24,25 The nuclear factor κn for EET to the [CoIII(bpy)2] moiety
.
is much larger than that for EET to the [OsIII(bpy)2] moiety,
since the ergonicity of the former (-1 eV) is much favorable
compared with the latter (-1.7 eV).

4824 J. Phys. Chem. A, Vol. 108, No. 22, 2004
Torieda et al.
TABLE 4: Dielectric Constants and Longitudinal Relaxation
Time of Solvents, Solvent Reorganization Energy of RET,
structure with a weak ligand field on average and splitting of
2+
dπ and dσ*, while Co(bpy)3 has an octahedral structure. (2)
3
and Decay Rate Constants (k1) of CT(Ru) and the
2+
The lowest state of Co(tpy)2 has a doublet configuration of
Formation Yields (ΦEET) of [(tpy)RuIII(L-L)CoII(tpy)]5+
(L-L: tpy-ph-tpy and tpy-tpy) at 298 K
dπ6dσ*, while Co(bpy)32+ a quartet d-electron configuration of
dπ5dσ*2. (3) The reorganization energy of ET is 0.28 eV for
[Co(tpy)2]3+/2+ and 0.9 eV for [Co(bpy)3]3+/2+, respectively.17
L-L
tpy‚tpy‚tpy
tpy‚tpy
3. Gibbs Free Energy Surfaces of the Excited State and
the Ground State of Reactant and EET Product. The 3MLCT
of [(tpy)RuII(tpy-tpy)CoIII(tpy)]5+ was subjected to rapid in-
tramolecular quenching in 1 ps, followed by frequency changes
of the molecular vibration modes and the solvation modes and
displacement of atoms in the product and of surrounding solvent
molecules in a couple of picoseconds. The intermolecular flow
of thermal energy achieves the entropy change of EET product.
solvent
τl/psa
ꢀs
AN
0.9
36
BN
3.6
24
PC
4.9
65
AN
0.9
36
BN
3.6
24
PC
4.9
65
ꢀop
1.78
1.91
2.02
1.78
1.91
2.02
λo/eVb
1.1
32
0.39
1.0
32
0.41
0.95
76
0.39
0.88
>200
0.65
0.81
100
0.53
0.80
190
0.38
k1/1010 s-1
ΦEET
a Reference 27. b Calculated by using the two-sphere model and
presuming the distances of electron transfer, 1.3 and 0.94 nm, for the
tpy-ph-tpy compound and the tpy-tpy compound, respectively. λo values
in acetonitrile are larger than the transition energy of the intervalence
band by 0.1 e.29
The vertical energy gap of EET, ∆G° + λEET, between
EET
3MLCT and EET product before the nonvertical reorganization
is slightly positive on the basis of ∆G° (-1.0 eV), intramo-
EET
lecular reorganization energy around the ruthenium (0.2 eV)
and the cobalt (0.28 eV17), and solvent reorganization (0.72 eV).
Then, the Gibbs free energy surface of 3MLCT at 298 K has an
intersection with that of EET product, [(tpy)RuIII(tpy-tpy)2CoII-
(tpy)]5+, around the bottom of the surfaces along with both of
the intramolecular coordinate and solvent coordinate, as shown
in Figure 6. In Figure 6, a Gibbs free energy surface of the
EET product, [2RuIII-2CoII]5+, is shown in black dash-dot-
dash line as a function of one coordinate with the other
coordinate fixed and a Gibbs energy surface of the EET product
fully relaxed is shown in gray dash-dot-dash line. The solvent
reorganization energy was estimated on the basis of the two-
sphere model27 with a shorter electron-transfer distance of 0.84
nm than the metal-metal distance by 0.3 nm because of the
TABLE 5: Temperature Dependence of the Decay Rate
3
Constant (k1) of CT(Ru), the Quantum Yield of ET Product
Formation (ΦET), and the Rate Constants (kRET) of the
Recovery Processes of the Ground State Absorption of
[(bpy)2Ru(tpphz)Co(bpy)2]5+ in BN
T/K
-∆GR°ET/eV
k1/1012 s-1
ΦET
kRET/109 s-1
295
275
247
221
194
173
0.97
1.03
1.07
1.13
1.4
0.84
0.82
0.78
0.78
0.021
0.020
0.0175
0.014
0.011
0.0097
1.17
0.75
2. Quantum Yields of EET Products of [(tpy)RuII(L)L)-
3
far location of the excited electron from the Ru(III) of CT-
3
CoIII(tpy)]5+ (L)L: tpy-ph-tpy and tpy-tpy). MLCT(Ru)
(Ru) state. As for [(tpy)RuII(tpy-ph-tpy)CoIII(tpy)]5+, the Gibbs
of [(tpy)RuII(L-L)CoIII(tpy)]5+ (L-L:tpy-ph-tpy and tpy-tpy)
underwent EET from the π* orbital of L-L to one of the dσ*
orbitals of [CoIII(tpy)2] moiety. The thermally relaxed products
of the EET reaction, [(tpy)RuIII(L-L)2CoII(tpy)]5+, have life-
times of 25 and 3 ps, respectively,17 for which the production
yields were determined from the absorbance-bleached MLCT
band by means of transient photometry. The numbers of photon
absorbed by the solute were determined by means of transient
actinometry. The quantum yields of an electron-transfer reaction
product, [(tpy)2RuIII(tpy-tpy)2CoII(tpy)]5+, 0.65 in AN, 0.53 in
BN, and 0.40 in PC look to be in correlation with not the
dielectric constants of solvents (ꢀ ) 36, 24, and 65 for AN,
BN, and PN, respectively) but the dielectric relaxation times of
solvent (τl ) 0.38, 2, and 10 ps, for AN, BN, and PN,
respectively) as is shown in Table 4. The transition of the
nonrelaxed EET product to the original reactant is suggested to
occur during the solvent relaxation. The product yield of [(tpy)-
RuIII(tpy-ph-tpy)CoII(tpy)]5+ decreased from 0.41 at 300 K to
0.21 at 180 K, while it was independent of the solvents. Not
only the figures but also the temperature dependence of the
quantum yields of [(tpy)2RuIII(L-L)2CoII(tpy)]5+ (L-L:tpy-ph-
tpy and tpy-tpy) are in contrast to those of [L2RuIII(L-L)-
CoL2]5+ (L-L: tpphz, and bpy-(CH2)2-bpy26). Nonunity yields
of geminate-radical pairs of Ru3+-Co2+ (Φ ) 0.60-0.80) were
only observed in the intermolecular quenching of 3MLCT(Ru)
of [RuL3]2+ (L ) 2,2′-bipyridine-4,4′-dicarboxylato or bpy) by
3
free energy surface of MLCT has an intersection with the
displaced free energy surface of EET product because of the
larger solvent reorganization energy (0.85 eV) than that of
[(tpy)RuII(tpy-tpy)CoIII(tpy)]5+
.
Nascent EET products of [(tpy)2RuIII(L-L)2CoII(tpy)]5+ just
formed in the quenching of 3MLCT have a chance of transition
to the reactant not only at the ground state but also at the excited
states lying intermediately between 3MLCT(Ru) and the ground
state during the thermal relaxation, where the nitrogen atoms
of pyridyl groups coordinating to Co(II) get far from the cobalt,
from 19630 to 202 pm31 on the average. The Gibbs free energy
surface of relaxed EET product is horizontally and vertically
displayed with a reorganization energy of 0.28 eV along with
the intramolecular coordinate and of 0.81 eV along with the
solvent coordinate referring to the ground state of reactant (open
circles in Figure 7).16,17,22 The Gibbs free energy surface of the
EET product with a relaxed solvation has an intersection with
that of reactant around the bottom as is shown in Figure 7,
because the Gibbs free energy change (-1.0 eV) is close to the
net reorganization energy (1.1 eV). The lowest energy surface
in gray is drawn for the reactant fully relaxed.
As for [(bpy)2RuII(tpphz)CoIII(bpy)2]5+, the Gibbs free energy
surface of EET product is horizontally and vertically displaced
along with both the solvent coordinate and the intramolecular
coordinate as referring to that of reactant as shown in Figure 8.
After a transition to the Gibbs free energy surface of reactant,
the nitrogen atoms of pyridyl groups get closer to the cobalt
during its oxidation from 213 pm32 to 193 pm33 and the
surrounding solvent molecules are reorganized to the inverted
electric dipole of Ru2+-Co3+ to accomplish the entropy change
of 1.3 meV/K. The reorganization energy of RET with an
[Co(tpy)2]3+ 4
.
It should be noticed that the low quantum yields were met in
the EET quenching of 3MLCT(Ru) by [CoIII(tpy)2] moiety.
3+/2+
Several different features of Co(tpy)2
from those of
Co(bpy)33+/2+ might be related to the low yields of EET product
formation. (1) Co(tpy)23+/2+ has a distorted square-bipyramidal

Quantum Yield of Excited-State Electron Transfer
J. Phys. Chem. A, Vol. 108, No. 22, 2004 4825
3
Figure 6. Schematic Gibbs free energy surfaces of CT(Ru), EET product and excitation energy-transfer product of the triplet (d-d) state of
[(tpy)RuII(tpy-tpy)CoIII(bpy)]5+. A Gibbs free energy surface of EET product is vertically relaxed to the surface of fully relaxed product (gray line)
within a couple of picoseconds. Transitions EET, EnT, and HT are shown as gray, fine gray, and white arrows, respectively.
Figure 7. Schematic Gibbs free energy surfaces of EET product ([(tpy)2RuIII(tpy-tpy)2CoII(tpy)]5+) thermally relaxed, triplet excited hole-transfer
product ([(tpy)RuII(tpy-tpy)3CoIII(tpy)]5+) with electronic configurations of 1(dπ6) for Ru(II) and 3(dπ5dσ*) of Co(III) and RET product. The Gibbs
free energy surface of the RET product is vertically relaxed to that of fully relaxed product (gray line) within a couple of picoseconds. Transitions
are shown as in Figure 6.
electron-transfer distance of 1.1 nm in BN is 0.92 eV along
with the solvent coordinate and 0.9 eV along with the intramo-
lecular coordinate, respectively.
[Co(CN)6]3- 37 Since the weaker ligand-field of [Co(tpy)2]3+
.
than [Co(CN)6]3- is assumed to reduce the Franck-Condon
energy of the optical singlet-triplet d-d transition to 0.4 eV,
the enthalpy of the formation of lowest triplet state is close to
1 eV. The Gibbs free energy change of the formation of 3[CoIII-
There are triplet d-d excited-states of [CoIII(tpy)2] moiety
3
below MLCT(Ru). An optical transition to the lowest triplet
d-d excited state of [CoIII(tpy)2] with a configuration of dπ5dσ*
is expected to emerge at lower energy than the corresponding
one of [Co(bpy)3]3+ (1.8 eV34), because the equatorial ligand
(tpy)2], ∆G° , is estimated to be ≈ 0.82 eV, because the
3(d-d)
entropy change, ∆S3 , of 3[CoIII(tpy)2] with highly dense
(d-d)
vibrational states is as large as the entropy change of [CoII-
2
field of [Co(tpy)2]3+ is much weaker than that of [Co(bpy)3]3+
.
(tpy)2] formation (0.6 meV/K22)) with a similar configuration
The peak energy of the lowest d-d singlet-triplet band of [Co-
(tpy)2]3+ is inferred to be lower (1.35 eV) than the singlet-
triplet absorption-band peak observed of [Co(bpy)3]3+ on the
basis of the difference (0.7 eV) in the energy of the lowest triplet
d-d excited state between [Ru(tpy)2]2+ and [Ru(bpy)3]2+, which
was estimated to be 2.1 and 2.8 eV, respectively, from the
of dπ6dσ*. The Gibbs free energy surface of (dπ5dσ*) has a
3
similar displacement to that of the EET product along with the
intramolecular coordinate, while Franck-Condon energy of
optical transition (0.4 eV) is similar to the reorganization energy
of RET.
It is noteworthy that the triplet d-d excited states of [CoIII-
(tpy)2] moiety are considerably displaced from the EET products
along with the solvent coordinate as is shown in Figures 6 and
7. The transition to 3(dπ5dσ*) occurs as a HT from 2[RuIII(tpy)2]
with an electronic configuration of dπ5 to 2[CoII(tpy)2] with an
electronic configuration of dπ6dσ*. The solvent reorganization
3
temperature dependence of radiation-less transition rate of -
CT(Ru).35,36 The peak energy of the triplet excited d-d transition
of [Co(tpy)2]3+ is shifted from the 0-0 energy by the Franck-
Condon energy of optical transition, which is estimated to be
0.75 eV from the Stokes shift (1.5 eV ) 3.2 eV (the absorption
peak) - 1.7 eV (the phosphorescence)) in the case of

4826 J. Phys. Chem. A, Vol. 108, No. 22, 2004
Torieda et al.
3
2
Figure 8. Schematic Gibbs free energy surface of CT(Ru), EET product of [(bpy)2 RuII(tpphz)4CoII(bpy)2]5+, excitation energy-transfer product
of triplet (d-d) state with electronic configurations of 1(dπ6) for Ru(II) and 5(dπ4dσ*2) of Co(III), and RET product. The Gibbs free energy surface
of the RET product is vertically relaxed to that fully relaxed (gray line). Transitions are shown as in Figure 6.
3
energy of the HT between the EET product and (dπ5dσ*) is
the same as the RET (0.9-1.0 eV) as is shown in Figure 7.
The solvents may vary both the reorganization energy of HT
and the rate of solvent relaxation, which considerably affect
Marcus normal region. Provided that the product of rate-
controlled process is the reactant at the ground-state, the
ergonicity ∆G° is -0.97 eV and the reorganization energy
RET
λ
RET is the difference in the Gibbs free energy of [RuII(tpphz)-
the transition rate of EET product to (dπ5dσ*).
CoIII]5+ between the minimum energy point and the point
corresponding to the minimum energy of [RuIII(tpphz)CoII]5+
3
In the case of [(bpy)2Ru(tpphz)Co(bpy)2]5+, the Gibbs free
energy change of lowest d-d triplet excited state formations
of 3[CoIII(bpy)2] moiety is inferred to be 1.0 eV by taking into
account the absorption peak of optical transition to triplet d-d
excited-state (1.8 eV), the entropy change of 1.3 meV/K, and
the Franck-Condon energy (0.4-0.5 eV). A HT from 2[RuIII-
(bpy)2] to 4[CoII(bpy)2] produces the quintet d-d excited state
.
The less negative values of ∆G°RET at high temperatures due to
negative entropy change (∆S° ) -1.3 meV) of the RET
RET
process attenuates the temperature effect on the RET rate,
reducing the apparent energy of activation (0.028 eV). By means
of simulation, the optimized value of λRET (2.05 eV) and HRET
(0.51 mV) were obtained by assuming the same magnitude of
∆S° as that (∆S° ) -1.3 meV K-1) of the redox pair,
with an electronic configuration of (dπ4dσ*2), whose energy
5
RET
RET
level may be close to the lowest triplet state. The Gibbs free
energy surface of the quintet lowest excited state is largely
displaced from the EET product along with the solvent
coordinate as shown in Figure 8.
[Ru(bpy)2(2,6-bis(2-pyridyl)benzodiimidazole)Co(bpy)2]5+.22 The
large value of λRET can be the sum of the reorganization energies
of the products and the surrounding solvent in the RET. Since
the solvent reorganization energy in BN is estimated to be 0.91
eV by using two-sphere model with a donor-acceptor distance
of 1.1 nm, the rest of the reorganization energy (1.14 eV) is
assigned as the Gibbs energy difference of [RuII(tpphz)CoIII]5+
3
A quantum chemical calculation of the energies of MLCT,
EET product, and 3,5(d-d) of the [CoIIIL2] moiety and the
tunneling transition among them is in progress to give insight
into the phosphorescence quenching, RET, and HT in the case
of [(tpy)RuIII(tpy-tpy)CoII(tpy)]5+, and [(bpy)2Ru(tpphz)Co-
III
between the minimum energy point (rCo - N ) 0.193 pm)
II
and the point (rCo - N ) 0.213 pm) corresponding to the
(bpy)2]5+
.
minimum energy of [RuIII(tpphz)CoII]5+. The intramolecular
reorganization energy for the oxidation of CoII is in agreement
with the quantum chemically calculated one (0.87-0.90 eV)
for [Co(bpy)3]3+/2+ using a density functional theory (DFT).17
4. RET of the EET Product To Form the Ground State
of [RuII(L)L)CoIII]. The EET products are thermally relaxed
to the bottom of the Gibbs free energy surface within a couple
of picoseconds to accomplish the entropy change. The recovery
of the ground-state-absorption of [RuII(bpy)2(tpphz)CoIII-
(bpy)2]5+ occurred with a rate constant of 2.1 × 107 s-1 at 298
K as follows, [RuIII(bpy)2(tpphz)4CoII(bpy)2]5+ f [RuII(bpy)2-
(tpphz)CoIII(bpy)2]5+. The slow recovery of [RuII(bpy)2(tpphz)-
CoIII(bpy)2]5+ is written using an effective frequency, νn, a
nuclear coefficient,øn, and an electronic transmission coefficient,
øel,
It is instructive to compare the RET rate of [2RuIII(tpphz)4-
CoII]5+ with [2RuIII(tpy-ph-tpy)2CoII]5+ and [2RuIII(tpphz)1-
OsII]5+. The small magnitude of the nuclear factor (4.6 × 10-3
)
is ascribed to the large intramolecular reorganization energy (1.1
eV) compared with [(tpy)2RuIII(tpy-ph-tpy)2CoII(tpy)]5+ (0.28
2
eV). If the lifetime of [CoII(bpy)2] moiety were long enough,
the RET of the doublet cobalt(II) moiety in [(bpy)2RuIII(tpphz)2-
CoII(bpy)2]5+ could occur with a rate of 4.6 × 109 s-1, which
is calculated by assuming the same electronic coupling strength
(0.51 meV) as that of [(bpy)2RuIII(tpphz)4CoII(bpy)2]5+ and the
zero intramolecular reorganization energy. The extent of the
electronic coupling (0.51 meV) for RET of [2RuIII(tpphz)4CoII]5+
is not so small compared with [2RuIII(tpphz)1OsII]5+ (1.6 meV),24
while through-ligand superexchange coupling between dπ(RuIII)
and dσ*(CoII) is thought to be much smaller than dπ(RuIII)-dπ-
(OsII).
kRET(T) ) νnønøel )
2
2
HRET
4πk Tλ
(∆GR°ET + λRET
2λRETkBT
)
2π
p
exp -
(9)
[
]
x
B
RET
the second of which is expressed in terms of vertical energy
difference, ∆G° + λRET, of electron-transfer process in the
RET

Quantum Yield of Excited-State Electron Transfer
J. Phys. Chem. A, Vol. 108, No. 22, 2004 4827
The relaxed EET products of [(tpy)RuIII(tpy-tpy)CoII(tpy)]5+
and [(tpy)RuIII(tpy-ph-tpy)CoII(tpy)]5+ disappear to recover the
reactant at the ground state with 3.2 × 1011s-1 and 4 × 1010
s-1, respectively, at 298 K.17 The absence of temperature
dependence of the recovery rate demonstrates that the relaxed
product carries out a transition to either the ground or the excited
state of the original reactant near to the bottom of free energy
surface. The slow rate constant, 1010-1011 s-1, of an activation-
less process suggests a low transmission coefficient of the
intersurface transition. Provided that the final state of RET is
the reactant at the ground state, the magnitudes of HRETs are
evaluated to be 1.9 and 4.6 meV, respectively, from the
temperature dependence of the recovery rate of the original
low-frequency vibrations of the product molecule and of the
surrounding solvent to accomplish the entropy change. Conse-
quently, nonequilibrium free energy of the reorganization41
consists of entropic term, too. The nonrelaxed EET product in
PC with a long relaxation time conceivably has many chances
3
of intersurface tunneling with that of (dπ5dσ*) of the [CoIII-
(tpy)2] moiety. The temperature dependence of ΦEET in BN is
understood in terms of longer relaxation time at lower temper-
atures.
Intramolecular HT needs the large strength of electronic
coupling between Ru(III) and Co(II). Both dπ-dπ interaction
of dyz(RuIII)-dyz(CoII) and dxy(RuIII)-dxy(CoII) are thought to
be strong enough to cause the HT of the EET product, [2RuIII-
(dπ5)-2CoII(dπ6dσ*)]5+, due to a superexchange interaction via
the py electrons of bridging ligand of tpy-tpy and tpy-ph-tpy.
The strong dπ-dπ interaction can be seen in the intensity of
the MMCT transition. The intensive MMCT bands between Ru-
(II) and Ru(III) in [(tpy)RuII(L-L)RuIII(tpy)]5+ (HII-III ) 30
meV for tpy-ph-tpy and 47 meV for tpy-tpy)37 demonstrate the
dπ-dπ interaction is intermediate. Though the replacement of
1RuII by 2CoII might reduce dπ-dπ interaction to some extent,
a HT from 2RuIII to 2CoII forming the triplet d-d excited states
of [CoIII(tpy)2] moiety with an electronic configuration of
dπ5dσ* much more efficiently occurs via strong dπ-dπ
interaction than RET of [(tpy)RuIII(tpy-tpy)CoII(tpy)]5+ via weak
dπ-dσ* interaction. The triplet d-d excited state of the [CoIII-
(tpy)2] moiety, 3(dπ5dσ*), might decay via nonradiative transi-
tion immediately after the formation of the triplet d-d excited
state.
reactant, ∆G° (- 0.99 eV)and λRET (1.1 eV for the tpy-ph-
RET
tpy compound and 1.0 eV for the tpy-tpy compound). The
difference in the rate constants (3.2 × 1011s-1, 3.9 × 1010 s-1
,
and 4.6 × 109s-1) among [2Ru(L-L)2Co]5+ (L-L: tpy-tpy,
tpy-ph-tpy and tpphz) originates from those in the square of
II
dyz(xy)(RuIII) - d(x -y )(Co ) coupling (4.6, 1.9, and 0.51 meV).
2
2
The electronic coupling between dyz(xy)(RuIII) and d(x -y )(Co )
II
2
2
is thought to be smaller than dyz(xy)(RuIII)-dyz(xy)(CoII) coupling,
because d(x -y )(Co ) are unable to overlap with dyz(xy)(RuIII) via
delocalized π-orbitals of the bridging ligand without any
vibronic coupling. Actually, the extents of electronic coupling
between dyz(xy)(RuIII) and dyz(xy)(RuII) in [(tpy)RuII(L-L)RuIII-
(tpy)]5+ or dyz(xy)(RuIII) and dyz(xy)(OsII) in [(bpy)2RuIII(L-L)-
OsII(bpy)2]5+ were evaluated to be 47 meV37 for the tpy-tpy
compound, 30 meV37 for the tpy-ph-tpy compound, and 1.6
meV23 for the tpphz compound, respectively. The involving of
II
2
2
II
yz(xy)(RuIII)-dyz(xy)(CoII) coupling in dyz(xy)(RuIII)-d(x -y )(Co )
2
2
d
Intersurface tunneling transition between adiabatic states
depends on the intersecting angle between adiabatic states and
strength of electronic coupling between diabatic states as well
as thermal relaxation rate. According to Nakamura and Zhu,7,8
the probability of tunneling transition between adiabatic states
increases as the thermal energy of the product increases.
Moreover, the greater coupling between the energy surfaces in
a strong coupling region makes the energy surfaces more distant
and the intersurface tunneling difficult where the energy surfaces
intersect with the same sign of slopes (Landau-Zener cross-
ing),7,8 while the more coupling between the energy surfaces
in a weak coupling region increases the tunneling rate. Such a
strong dπ-dπ interaction in [(tpy)RuIII(tpy-tpy)CoII(tpy)]5+
could make the adiabatic energy surface more distant from that
of 3(d-d) (Co(III)) to prevent the tunneling transition. This effect
on the fraction of tunneling transition might account for the
higher yield of [(tpy)RuIII(tpy-tpy)CoII(tpy)]5+ than [(tpy)RuIII-
coupling via vibronic coupling might enhances the rate of RET.
The extents of dyz(xy)(RuIII)-d(x -y )(Co ) coupling (4.6, 1.9, and
0.51 meV) are too small for the RET of nonrelaxed EET product
to compete with thermal relaxation.
II
2
2
5. Hole-Transfer of RuIII(L)L)2CoII To Form Excited
2
States of [1RuII(L)L)3CoIII]. Another possible process for the
reactant recovery consists of the formation of a triplet d-d
excited state 3(dπ5dσ*) of the Co(III) moiety followed by a rapid
radiationless transition of the triplet excited state. The transition
to the triplet state, 3(dπ5dσ*), occurs as a HT between 2Ru(III)
with dπ5 and Co(II) with dπ6dσ*. By employing ∆G° of
2
HT
-0.15 eV and λHT,s of +0.82 eV for [(tpy)Ru(tpy-tpy)Co-
(tpy)]5+, the extent of HHT is estimated to be large (57 meV)
from the recovery rate of the relaxed product at 298 K. The
activation-less recovery from the relaxed product is inconsistent
with the activation energy of HT (0.16 eV) calculated on the
(tpy-ph-tpy)CoII(tpy)]5+
.
based of eq 9, ∆G° of -0.15 eV, and λHT,s of 1 eV. Since a
HT
HT
In the case of [(bpy)2RuIII(tpphz)CoII(bpy)2]5+, a transition
to the reactant at the ground state is retarded by the large
intramolecular reorganization energy (0.9 eV) of ET in addition
to the solvent reorganization energy (0.92 eV) as is shown in
Figure 8. The nascent doublet state of EET product is able to
undergo HT before the thermal relaxation, only if super-
exchange interaction of dπ-dπ between RuIII and CoII is strong
enough. The extent of dπ-dπ interaction between RuIII and OsII
was estimated to be as small (1.6 meV) for [RuIII(tpphz)OsII]5+
from the rate of RET based on eq 9. The absence of an
intervalence optical transition of [RuIII(tpphz)RuII]5+ 24 indicates
that the dπ-dπ coupling between RuIII and RuII via tpphz is
much smaller. It turns out that weak dπ-dπ superexchange
interaction through tpphz between RuIII and CoII may be unable
to open the channel of HT during the thermal relaxation of
nonrelaxed EET product reducing the quantum yield of EET
more negative value of ∆G° only allows the activationless
transition, the HT is able to develop from the nonrelaxed product
of EET.
The thermal relaxation of the nascent EET product is
controlled by the rates of solvent relaxation, intramolecular
vibration redistribution IVR, and intermolecular energy transfer
IET between a solute molecule and surrounding solvent
molecules. Since most of solvents are relaxed in not a single-
exponential mode but multiexponential one,39 the effective
relaxation time of viscous solvent for the formation of a charged
solute is longer than the average one depending on the coupling
between the modes of intramolecular vibration and solvation.
Time-resolved spectroscopy of electronic transition10b,38 and anti-
Stokes resonance Raman10a,12 revealed that the thermal relax-
ation of vibrationally excited molecule in the condensed phase
occur in a shorter time than a couple of picoseconds. Thermal
energy released during the relaxation is redistributed into the
product in the case of [(bpy)2Ru(tpphz)Co(bpy)2]5+
.

4828 J. Phys. Chem. A, Vol. 108, No. 22, 2004
Torieda et al.
6. Competing Process with Electron Transfer. Provided
that the excitation energy transfer (EnT) of 3MLCT(Ru) occurs
in competition with EET, the yield of EET product is reduced.
Yields Smaller than 0.5 were given for the formation of
[(dmb)2RuIII(L-(CH2)n-L)OsII(bpy)2]5+ (dmb, 4,7-dimethyl-2,2′-
bipyridine; L, 2-(2-pyridyl)-1-benzimidazolyl; n ) 3, 4, 5) in
the intramolecular phosphorescence quenching of [(dmb)2RuII-
through-ligand electronic coupling of dπ(Ru)-dπ(Co), which
was estimated from the strong intensity of inter-valence transi-
tion of [(tpy)RuIII(L-L)RuII(tpy)]5+ 38
, allows the hole-transfer
between [RuIII(tpy)2] and [CoII(tpy)2]. The possibility that the
lower lying d-d excited-states are produced in competition with
the ET of [(tpy)RuIII(tpy-ph-tpy)CoII(tpy)]5+ is also pursued. In
the case of [(bpy)2RuIII(tpphz)-CoII(bpy)2]5+, both RET and HT
are retarded very much by a small superexchange electronic
interaction between dπ(RuIII) and dπ(CoII).
(L-(CH2)n-L)OsIII(bpy)2]5+ 13b
where the energy accepting level
,
of [OsIII(bpy)2] moiety was the triplet ligand-to-metal charge
3
transfer state. Excitation energy transfer of MLCT(Ru) to the
References and Notes
cobalt(III) moiety is able to be in competition with EET, since
the triplet d-d excited-states of the [CoIII(tpy)2] moiety of [(tpy)-
RuII(tpy-ph-tpy)CoIII(tpy)]5+ and [(tpy)RuII(tpy-tpy)CoIII(tpy)]5+
are low enough (-1.04 and -0.94 eV, respectively). Since the
optical transitions to the triplet excited-states of [CoIII(tpy)2]
moiety are spin-forbidden, only exchange interaction is able to
cause the EnT. The interactions not only between π*(L-) and
dσ*(CoIII)but also between dπ(RuIII) and dπ(CoII) are required
for the EnT, of which the former and the latter are necessary
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3
for the EET of MLCT(Ru) to [CoIII(tpy)2] moiety and for the
HT of RuIII to [CoII(tpy)2] moiety, respectively. Consequently,
the interaction between π*(L-) and dσ*(CoIII) necessary for the
3
EET of MLCT(Ru) is more favorably satisfied than both the
interactions of π*(L-)-dσ*(CoIII) and dπ(RuIII)-dπ(CoII) for
3
the EnT of MLCT(Ru).
The rate of the EnT can be comparable with that of the EET,
only when the Franck-Condon factor of the EET process is
less favorable than that of the EnT. Following energy-gap law
of nonradiative transitions, Franck-Condon factors of EET and
EnT decrease as the vertical free-energy gaps of EET
(- ∆G° - λEET,i - λEET,s) and EnT (- ∆G° - λEnT,i
-
EET
EnT
λ
EnT,s), respectively. As the magnitudes of ∆G° , λEnT,i, and
EnT
λ
EnT,s are -1.15, +0.45,42 and 0 eV, respectively, and those of
EET
∆G° , λEET,i and λEET,s are -0.93, 0.38,42 and 0.85 eV,17
respectively, for [(tpy)RuII(tpy-ph-tpy)CoIII(tpy)]5+, the EnT with
a vertical free-energy gap of 0.7 eV is more preferable than
EET with the negative energy gap (-0.30 eV). It is probable
that the second lowest triplet state of [CoIII(tpy)2] moiety as an
energy acceptor improves the Franck-Condon factor of EnT
more. The small solvent reorganization (0.72 eV) of EET for
[(tpy)Ru(tpy-tpy)Co(tpy)]5+ due to a small Ru-Co distance is
expected to shift the vertical free-energy gap of EET to the less
negative (-0.22 eV). This might lead the more contribution of
3
EET to the decay of MLCT resulting in the larger EET yield
of [(tpy)RuII(tpy-tpy)CoIII(tpy)]5+ than that of [(tpy)RuII(tpy-
ph-tpy)CoIII(tpy)]5+
.
Conclusion
EET reaction products of [RuII(L-L)CoIII]5+ (L-L: tpy-ph-
tpy, tpy-tpy, and tpphz) were formed in the intramolecular
electron-transfer quenching of 3MLCT(Ru) in a time shorter than
10 ps for subpicosecond laser excitation. A transient actinometry
using [Ru(bpy)3]2+ was applied to evaluate the quantum yield
of EET product. The yields of the EET reaction products in
BN, ΦEET, were 0.41 and 0.53 for L-L ) tpy-ph-tpy and for
L-L ) tpy-tpy of [2RuIII(tpy)(L-L)2CoII(tpy)]5+, respectively.
The ET yield of [(bpy)2 RuIII(tpphz)4CoII(bpy)2]5+ was 0.77-
2
0.84. The smaller yields of EET product in PC with a slow
relaxation time and at lower temperatures suggest that the triplet
d-d excited state of [CoIII(tpy)2] moiety is formed during the
solvent relaxation. A tunneling transition of the nonrelaxed EET
products to the lowest d-d excited-states of [CoIII(tpy)2] moiety
takes place as a hole-transfer from [RuIII(tpy)2] to [CoII(tpy)2]
generating an electronic configuration of dπ6dσ*. A strong
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263, 209.

Quantum Yield of Excited-State Electron Transfer
J. Phys. Chem. A, Vol. 108, No. 22, 2004 4829
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and [Co(tpy)2]3+ are estimated to be 0.28,17 0.12,43 and 0.35 eV, respectively.
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