# Light-Induced Tyrosine Radical Formation  from Ruthenium-Tyrosine Complex Anchored to  SnO2 Semiconductor

Authors: Raed Ghanem
Journal: International Journal of Photoenergy
 (2008)

---

## Abstract

Steady state spectroscopy and flash photolysis measurements were used to study the multistep electron transfer in a supramolecular complex adsorbed on the surface of nanocrystallineSnO2 in order to mimic the function of the tyrosineZ and chlorophyll unit P680 in natural photosystem II (PSII). A ruthenium(II)-tris(bipyridyl) complex covalently linked to an L-tyrosine ethyl ester through an amide bond was anchored to the surface of nanocrystalline SnO2 via four carboxylic acid groups linked to the bpy ligands. The apparent association constant for the association between ruthenium(II)-tris(bipyridyl) complex and SnO2 is 3.2×106M-1  and a degree of association up to 99% was determined. 450 nm excitation of the complex promotes an electron to a metal-to-ligand charge transfer (MLCT) excited state, from which the electron is injected into SnO2. The photogeneration of Ru(III) is followed by an intramolecular electron transfer from tyrosine to Ru(III), regenerating the photosensitizer Ru(II) and forming the tyrosyl radical. The tyrosyl radical is formed in less than 3 microseconds with a yield of 32%.


---

## Body

## 1. Introduction

Production of  tyrosine radical (TyrZ) is the primary step
on the photosynthetic energy conversion in plants due to the crucial role of
the TyrZ in photosynthetic energy conversion through the oxidation
of water into molecular oxygen.  This process (regardless of the exact mechanism of photosynthetic water oxidation) [1–5] is initiated when
the primary donor chlorophyll P680 absorbs light, which results in
transferring an electron from the excited state chlorophyll, via several steps,
to quinone on the acceptor side of PSII, leading to the oxidation of P680, in which P680+ is produced. Moreover, photogenerated P680+ recaptures an electron by oxidizing a nearby
tyrosine (TyrZ) into a neutral tyrosyl radical (TyrZ•) [3, 4]. Once
the tyrosyl radical is formed, it oxidizes the water molecule by one of two mechanisms.
(I) The radical in its turn oxidizes a tetranuclear Mn-cluster bound to PSII,
and after four consecutive turnovers, two water molecules are oxidized into
molecular oxygen; (II) tyrosine radical (TyrZ) catalyzes water
oxidation by abstracting a hydrogen atom from a water molecule coordinated to
the manganese cluster Mn4 [1, 2].Efforts
to design supramolecular complexes to mimic the important part  of light-driven
process in PSII in order to construct an artificial photosynthetic system for
fuel production are continuous. Ruthenium-tris(bipyridine) complexes have been
used as a model compound to mimic the PSII photochemistry in which ruthenium acts
as a photosensitizer in analogy of P680 in PSII. In previous studies,
model complexes (Ru(II)(bpy(COOH)2)2(4-Me-4′CONH-L-tyrosine
ethyl ester-2,2′-bpy) 2PF6 (1) with its analogue (1a)
and (Ru(II)(bpy)2(4-Me-4′-CONH-L-tyrosine
ethyl ester-2,2′-bpy))
2PF6 (2) were studied to mimic the TyrZ-P680 functional unit of PSII by generating a tyrosyl radical by means of intramolecular
electron transfer from the tyrosine moiety to the photogenerated Ru(III) (i.e., upon
excitation of  Ru(II)(byp)3 moiety with visible light, electrons are transferred from the excited state of
the Ru(II) to an electron acceptor producing Ru(III), then an electron is
transferred to the oxidized ruthenium from tyrosine moiety leading to recovery
of Ru(II) and tyrosyl radical formation [6–9]). Also it was found that in the
presence of
an external electron acceptor (i.e., methyl viologen or cobalt
pentaaminechloride), the photogenerated tyrosyl radical can oxidize a binuclear
manganese cluster from its Mn(III/III) state to the Mn(III/IV) state [8, 9]. Despite
the proven functionality of the artificial system, the use of an external
electron acceptor has the disadvantage of diffusion-limited electron transfer
from the excited state of the ruthenium complex to the external electron
acceptor [6–9]. However, it was
demonstrated that the use of a nanocrystalline semiconductor (i.e., TiO2) as an electron acceptor with Ru-tyrosine complex overcomes the diffusion-controlled
nature of the electron transfer since the electron injection was reported to
occur on femtosecond to picosecond time scale [10]. Also, the possibility of
multistep electron transfer in complex (1) adsorbed on the surface of
TiO2 via four carboxylic acid groups linked to the bpy ligands was
demonstrated. Excitation of complex (1) promotes an electron to a
metal-to-ligand charge transfer (MLCT) excited state from which the electron is
injected into TiO2 resulting in the formation of Ru(III). The photogeneration
of Ru(III) is followed by an intramolecular electron transfer from tyrosine to
Ru(III) regenerating the photosensitizer Ru(II) and forming the tyrosyl radical
with a yield of 15%. The low yield of tyrosine production is a result of a
fast back electron transfer reaction from the nanocrystalline TiO2 to the photogenerated Ru(III) [10]. Most of ultrafast interfacial electron transfer studies are focused on
TiO2 where the rate of electron injection of dye adsorbed onto TiO2 was reported to be multiexponential and mainly in the femtosecond regime [11–14]. Recently, it was
demonstrated that other wide bandgap semiconductors like SnO2 and ZnO are promising [15]. In this paper, we will demonstrate
the possibility of obtaining a multistep electron transfer in a supramolecular
complex (1)  anchored to nanocrystalline Tin(IV) dioxide (SnO2)
with a target to improve the efficiency of tyrosine radical production (see Figure 1).Chemical structure of complexes (1) and (1a).


## 2. Experimental Section

### 2.1. Materials

Synthesis  and characterization of Ru-(II)(bpy(COOH)2)2(4-Me-4′CONH-L-tyrosine
ethyl ester-2,2′-bpy) 2PF6 (1)
and its analogue Ru(II)-tris(bipyridyl) (1a) have been described
elsewhere [10]. A stable suspension (15%) of SnO2 was purchased directly from Alfa Chemicals.The stock solutions of (1) and (1a) were prepared in methanol and stored in the dark. The sample
solutions were prepared in water by adding a few microliters of SnO2 solution to freshly prepared solutions of (1) or (1a) to obtain the desired
concentration. Sample degradation was checked out by measuring the absorption
spectra before and after flash photolysis measurements—no sample
degradation was observed during the flash photolysis experiment. The pH of the solution
was adjusted with HCl or NaOH and measured with a Metrohm pH meter.


### 2.2. Spectroscopic Measurements

Luminescence spectra were recorded using a SPEX Fluorolog fluorimeter by
exciting the sample at 450 nm. Absorption spectra were recorded on a
Jasco-V-530 spectrophotometer in a 1 cm  quartz cell. Transient absorption
experiments were conducted with a nanosecond laser flash photolysis setup. The
excitation pulse at 450 nm was obtained from a Quanta-Ray master optical parametric
oscillator (MOPO) pumped by a Quanta-Ray 230 Nd:YAG laser, having a pulse
energy of 1-2 mJ and duration of approximately 7 nanoseconds. The probe light
from a xenon arc lamp (75 W) was focused to a 1 mm diameter spot overlapping
with the unfocused pump beam of 2.5 mm diameter. After the 1 cm quartz cell
containing the sample, the probe light was passed through two single grating
monochromators and detected by a photomultiplier tube (Hamamatsu R928).The transient spectra at different delay times were constructed
from average kinetic traces measured at different wavelengths. All kinetic
traces and spectral profiles were fitted using the data-analysis package
Microcal Origin.


## 2.1. Materials

Synthesis  and characterization of Ru-(II)(bpy(COOH)2)2(4-Me-4′CONH-L-tyrosine
ethyl ester-2,2′-bpy) 2PF6 (1)
and its analogue Ru(II)-tris(bipyridyl) (1a) have been described
elsewhere [10]. A stable suspension (15%) of SnO2 was purchased directly from Alfa Chemicals.The stock solutions of (1) and (1a) were prepared in methanol and stored in the dark. The sample
solutions were prepared in water by adding a few microliters of SnO2 solution to freshly prepared solutions of (1) or (1a) to obtain the desired
concentration. Sample degradation was checked out by measuring the absorption
spectra before and after flash photolysis measurements—no sample
degradation was observed during the flash photolysis experiment. The pH of the solution
was adjusted with HCl or NaOH and measured with a Metrohm pH meter.


## 2.2. Spectroscopic Measurements

Luminescence spectra were recorded using a SPEX Fluorolog fluorimeter by
exciting the sample at 450 nm. Absorption spectra were recorded on a
Jasco-V-530 spectrophotometer in a 1 cm  quartz cell. Transient absorption
experiments were conducted with a nanosecond laser flash photolysis setup. The
excitation pulse at 450 nm was obtained from a Quanta-Ray master optical parametric
oscillator (MOPO) pumped by a Quanta-Ray 230 Nd:YAG laser, having a pulse
energy of 1-2 mJ and duration of approximately 7 nanoseconds. The probe light
from a xenon arc lamp (75 W) was focused to a 1 mm diameter spot overlapping
with the unfocused pump beam of 2.5 mm diameter. After the 1 cm quartz cell
containing the sample, the probe light was passed through two single grating
monochromators and detected by a photomultiplier tube (Hamamatsu R928).The transient spectra at different delay times were constructed
from average kinetic traces measured at different wavelengths. All kinetic
traces and spectral profiles were fitted using the data-analysis package
Microcal Origin.


## 3. Results and Discussion

The absorption and emission spectra of (1) and (1a) (Figure 2) were similar to those of Ru(bpy)3+ complexes [16, 17]; the intense band at ∼300 nm is attributed to the ligand-centered
(LC) band which arises from a π to π* transition. The band at 475 nm which was assigned
to a metal-to-ligand charge transfer (MLCT) as a result of a d to π* transition
is red-shifted by ∼25 nm (depending on the pH) compared to Ru(bpy)3+ complexes [16, 17] due to the carboxylate  substituents.
Deprotonation of the carboxylic acid group affects the LUMO energy of the
ligand. The MLCT band is blue-shifted to higher energy by 10 nm upon increasing
the pH of the solution from 1 to 7 [18, 19]. The shift of the MLCT
absorption band at 470 nm of (1), as a function of pH, was used to
determine the pKa value of the ground state [20]. The analysis of the pH
profile of (1) resulted in two inflection points at pH = 0.4 and pH = 2
which were attributed to the loss of two carboxylate protons at each inflection
point.Figure 2
Normalized absorption and emission spectra of complex (1). The emission spectra were excited at 450 nm.The emission intensity also shows a pH dependence; the lowest MLCT
excited state exhibits an intense luminescence emission band at∼625 nm, at pH
7, and  it is red-shifted by ∼25 nm at pH 1 (∼650 nm). The pH dependence can be
explained by the differences in electron accepting/donating  properties of the
protonated/deprotonated forms
of the lig- ands [10].SnO2 has an isoelectric point at pH ∼3.5 [15];
that is, at
pH<3.5, SnO2 is positively charged [SnOH+]; thus, a
condi- tion of  pH<3.5 represents an optimal condition for the
elec- trostatic attraction for the adsorption of (1) and (1a) onto
the surface of the semiconductor (i.e., pKa values of (1) are
2 and 0.4). The quenching of the emission spectra of com- plexes (1) and (1a) with the addition of SnO2 was used to  study the efficiency of the
attachment of both complexes (1)  and (1a) onto the surface of the
SnO2. The luminescence in- tensity of (1) is decreased with the increase of SnO2 concen- tration
until it reaches a constant value at 90 μM [SnO2](see   Figure 3). Moreover, the
adsorption of (1) onto the surface of  SnO2 causes a small
blue shift (from 430 nm to 420 nm); in  fact, this observation is completely
opposite to the previously  reported system of 1-TiO2 where a red shift
was observed  and attributed for the delocalization of π* orbital [10] of the
bipyridine ring with conduction band. The origin of the blue  shift observed for 1-SnO2 could be attributed to (I) aggrega- tion behavior of (1) on the surface of
SnO2, (II) fully depro- tonation of (1), and (III) a weak
electronic coupling between  the dye molecule and the semiconductor surface. In
fact, sim- ilar blue shift has been reported for other ruthenium dye,  N719, adsorbed
onto the surface of nanocrystalline
SnO2  [15], and it is also observed for other molecular dyes such as  perylene adsorbed onto nanocrystalline SnO2 [21]. On the  other hand, the
similarity of the quenching pattern observed  for both complexes shows that the
presence of tyrosine in (1)   does not
affect the functionality of the carboxylic acid group;  therefore, the steady
state analysis is valid for (1) and (1a).Figure 3
Luminescence emission spectra of complex (1) in the presence of
various concentrations of SnO2.The observed quenching of the MLCT was
attributed to  an electron injection from the excited state of the dye  into  the
conduction band of SnO2 [13, 22–26]. Energy differences
between the conduction band of the semiconductor and the  redox potential of the
excited ruthenium, ERu(III/II), are the ma- jor driving
forces for the electron injection. It is not pos- sible to electrochemically
determine the Ru(III/II) oxidation  potential in aqueous media
because of the background wa- ter oxidation current; however, the Ru(III/II) redox potential  could be estimated from the value of Ru(dcb)32+ in acetoni- trile [27, 28]; the Ru(III/II) redox potential is
estimated versus  NHE electrode to be 1.37 eV and 1.2 eV for the fully
proto- nated and deprotonated forms of complex (1), respectively.  Taking
into account the excited state energy of (1) (c.a 2 eV,  λmax⁡∼470 nm) and the energy
band-gap of SnO2 (approx- imately 3.8 eV), we can conclude that the electron
injection  from the excited state of the dye into the conduction band  of
semiconductor is thermodynamically allowed. Kamat [23]  relates the quenching of
the excited state by colloidal semi- conductor to the adsorption of the dye onto
the surface of  semiconductor (i.e., SnO2). The extent of the
quenching of  fluorescence intensity by the SnO2 colloidal particle
reflects  the amount of chemisorbed dye onto the surface of the SnO2  particle. In order to obtain an association constant (Ka) for  the
dye-SnO2 interaction, quenching  of the luminescence intensity of (1) and (1a) by SnO2 was analyzed by considering an equilibrium
between the adsorbed and unadsorbed dye molecules:
(1)Dye–COO−+SnOH+⇄Ka[Dye–COO–Sn≡].
The observed
emission quantum yield ϕf of the dye in the presence of colloid is related
to the emission quantum yield of the unadsorbed dye ϕfo and the emission quantum yield of the adsorbed molecule, ϕf′, as follows:
(2)ϕf=(1−α)ϕfo+αϕf′,where α is the
degree of association between the SnO2 particles and complex (1). Under pseudo-first-order reaction condition
([SnO2] ≫ [dye]), α can be expressed as(3)α=Ka[SnO2]1+Ka[SnO2].
By substituting (3) into (2), the
quenching of luminescence emission of (1) or (1a) by SnO2 can
be represented as(4)1(ϕfo−ϕf)=1(ϕfo−ϕf′)+1Ka(ϕfo−ϕf′)[SnO2].
The emission
quantum yields (ϕ) were measured by
comparison of the emission spectrum of the sample with the emission spectrum of
a reference compound (tryptophan) using the following equation:(5)ϕx=DxDrAr(λr)Ax(λx)ϕr,
where D is
the area under the luminescence spectrum, A(λ) is the corresponding absorbance
at the excitation wavelength, and the subscripts r and x  refer to reference and sample solutions [24].
Plot of the emission quantum yield of the dye complex (1) in the
presence of SnO2 colloidal versus the concentration of SnO2 according to (4) results in straight line (linear fitting results with
correlation coefficient R of 0.998) in which the apparent association constant
and the emission quantum yield of the adsorbed molecule could be estimated from
the slope and intercept, respectively. The apparent association constant of 2.3×106M−1 and the emission quantum yield of the adsorbed
molecule (ϕf′∼0.002)
were obtained with a degree of association equal to 99% (i.e., 99% of complex (1) adsorbed onto the surface of SnO2).
Again, a similar pattern was observed for the reference compound (1a).Figure4 shows the transient
absorption spectra of complexes
(1) and (1a) (20 μM of (1)/(1a) and 90 μM of SnO2 at pH
∼ 2). The photoinduced electron transfer processes were initiated with the
excitation of MLCT (through absorption of photon at λ=450 nm) which promotes the electron from a Ru d-orbital
to π* orbital of the ligand, from which an electron can be injected into the
conduction band of SnO2. The above mechanism leads to the formation
of Ru(III) cation and the bleaching of Ru(II) MLCT ground state [18, 26].
Analogously to other ruthenium complexes and taking into account the energetic
point of view, we can assume that the electron transfer from the dye to SnO2 occurs in an ultrafast subpicosecond to femtosecond time scale [15, 22].
For ruthenium-tyrosine complexes (like complex (1)) anchored to the nanocrystalline semiconductors, the second electron transfer
from the tyrosine moiety to the dye cation Ru(III) is the rate-limiting step. The
photogenerated Ru(III) can return to the Ru(II) ground state either by charge
recombination (back electron transfer from
the nanocrystalline SnO2) or by a second intramolecular
electron transfer process from the linked tyrosine moiety [7, 9]. Distinguishing
between these two ways needs a careful investigation of the transient
absorption spectra.Transient absorption spectra of adsorbed complexes (1) (a) and (1a) (b) at pH 2 with 90 μM
of SnO2 at different delay times after excitation at 450 nm.
(a)(b)The time
evolution of the transient absorption spectra of the (1)-SnO2 system, Figure 4(a), shows that the absorption
recovery of Ru(II) is accompanied by the formation of a tyrosyl radical which
is known  to have an absorption band at
410 nm (i.e., the positive signal at 410 nm) [29]. Tyrosyl radical is indeed
formed as a result of the intramolecular electron transfer from the tyrosine
moiety to the photogenerated Ru(III). The confirmation of the above conclusion
was achieved in a separate experiment on the analogue complex (1a) which
lacks the tyrosine moiety; therefore, no absorbance feature due to the
generation of tyrosyl radical should be observed [10]. The transient absorption
spectra of complex (1a) anchored to SnO2. Figure 4(b) shows that the strong bleaching of the
MLCT band (as a result of the electron injection into the SnO2) was followed
by an absorption recovery at 470 nm (as a result of recombination between
electrons in the nanocrystalline SnO2 and Ru(III)). As it was previously
illustrated for (1) or (1a)-TiO2 systems, the
absorption recovery of the reference complex (1a) is apparently slower
than the absorption recovery recorded for complex (1) (the bleaching
signal of complex (1) is completely recovered within, approximately, 80 microseconds,
whereas it was not recover for complex (1a)); in fact, the recovery
of the bleaching signal in the (1)-SnO2 system is indeed faster
than the (1)-TiO2 system.The yield of Ru(III) to tyrosyl radical
can be estimated from the relative absorption magnitude of Ru(II) ground state
bleaching at early times and from
the 410 nm band corresponding to the tyrosyl radical; the absorption coefficient of the
tyrosyl radical isε=3000M−1cm−1, and for complex (1) it is ε=15000M−1cm−1.
Despite the fact that the electron injection rate for the Ru-SnO2 system is slower than the Ru-TiO2 system [10], it is interesting to
observe that for (1)-SnO2 system the yield of Ru(III) to tyrosyl radical conversion was about 32% compared
to 15% for (1)-TiO2 system. The relatively low yield of tyrosine-Ru(III) electron transfer into TiO2 was explained by a fast  back electron
recombination process from TiO2 to Ru(III). Indeed, in the case of
electron transfer from Ru(III) to SnO2, the charge recombination seems to be slower than
the case of TiO2 (recombination rate constants are 8.0×104s−1 and 1.1×105s−1 for (1)-SnO2 and (1)-TiO2 systems,
resp.) which could be attributed to the efficiency of attachment of complex (1) onto the surface of SnO2 (∼99% adsorption); the above analysis was
confirmed through the comparison of the kinetics of Ru(II) absorption recovery
at 470 nm for complexes (1) and (1a) (Figure 5). The absorption recovery
of complex (1a) reflects only the dynamic of the charge recombination of
(1a) and SnO2 since it was the only pathway of the recovery
of the excited Ru(II); thus, a direct comparison of the kinetics shown in
Figure 5 enables us to obtain the time constants of tyrosine-Ru(III) electron
transfer and the back electron transfer recombination.
A multiexponential
fit of the kinetics data for complex (1a) at 470 nm results in time
constants of τ1≈0.5 (60%) microsecond
and τ2=30.6±0.5 (40%) microseconds. The
fastest component,  ≈0.5 microsecond, is partly attributed to the unadsorbed
dye molecule (i.e., luminescence lifetime of the dye was 0.72 microsecond) [10];
moreover, it is known that the charge recombination is also a multiexponential
process which occurs in nanosecond time scale; thus, the  ≈0.5-microsecond component could also be due to
recombination between Ru(III) and electrons injected into SnO2. The
second microsecond time constant (τ2=30.6±0.5 (40%) microseconds) reflects the
recombination between Ru(III) and the electrons injected into SnO2; from
both time constants, effective average rate constant of 8×104s−1 could be calculated. On the other hand, a multiexponential fitting of the 470 nm kinetic for
complex (1), which contains also a
component corresponding to the tyrosine-to-Ru(III) electron transfer, results
in three time constants τ1≈0.5 (60%) microsecond,
τ2=30.6±0.5 (4%) microseconds, and
τ3=1.3±0.2 (36%) microseconds with an effective average rate
constant of 5.1×105s−1 for tyrosine-to-Ru(III) and
recombination between Ru(III) and electrons injected into SnO2.
Moreover, the analysis of the kinetics for complex (1) data measured at 410 nm, which
corresponds to the formation of the tyrosyl radical, gave the same time
constant (within the experimental error) as at 470 nm. On the other hand, in
case of complex (1a), which has no tyrosine moiety, kinetics measured at 410
nm reflect only the
absorption recovery due to the recombination between Ru(III) and SnO2 electrons and gave the same time constant as at 470 nm.Kinetics of the adsorbed complexes (1) and (1a) measured after 450 nm excitation at pH 2 with 90 μM of SnO2; the kinetics were recorded at 470 nm (a) and at 410 nm (b).
(a)(b)The rate constant for the intramolecular electron transfer
from tyrosine to Ru(III) in the presence of SnO2 (k=5.1×105s−1) was faster than the one obtained for the same complex in the
presence of TiO2 (k=3.8×105s−1) and indeed
faster than the rate obtained in the presence of an external electron acceptor
MV2+ in solution [9]. In fact, analysis of kinetic traces is a little
bit complicated and it is not easy to resolve the rate constants of the two
steps—the rate constant
for the tyrosine-to-Ru(III) and the rate constant for the recombination between
Ru(III) and electrons injected into SnO2 due to the competition
between them. On the other hand, even with the competition between the two
processes, the average effective rate constants still could be estimated for
both processes because  the analysis of
the kinetics data at 470 nm or 410 nm for complex (1) contains component
which corresponds
to the tyrosine-to-Ru(III) electron transfer, while the kinetics data at the
same wavelength for the analogue reference complex (1a) reflect only the
charge recombination process; moreover, even with the poorly defined kinetic,
it is clear that the tyrosyl radical formation is improved by a factor of two
compared to the (1)-TiO2 system. From the above results, it
seems that using SnO2 as an electron acceptor increases the
efficiency of the attachment of complex (1) on the surface of the
semiconductor up to unity which slows the back electron recombination, slightly
increases the electron transfer rate constant, and indeed improves the
efficiency of the tyrosyl radical formation. Moreover, the use of SnO2 seems to improve the separation of the recombination process from the
tyrosine-Ru(III) electron transfer, but still suffers from a competitive back
electron recombination which, in fact, is responsible for the relatively low
yield of the tyrosine-Ru(III) electron transfer.


## 4. Conclusion

We have demonstrated that the anchoring of Ru(bpy)3 complex (1) with four carboxylic
acid groups linked to the bpy ligands with tin (IV) dioxide, SnO2, improves
the attachment of the dye onto the surface of the semiconductors (Ka=2.3×106M−1 with a degree of association up to 99%). Anchoring
of complex (1) to SnO2 not only enables ultrafast injection
of electrons from the excited MLCT state into the conduction band of SnO2,
but also improves the second intramolecular electron transfer from the tyrosine
moiety to the photogenerated Ru(III) (KET∼5×105s−1).
The second intramolecular electron transfer step (i.e., the rate-limiting step)
leads to the formation of tyrosyl radical which can be observed at 410 nm. The
yield of Ru(III)-tyrosyl radical conversion is indeed higher (∼32%) than the
case of TiO2, but still limited as a result of a competing charge
recombination between Ru(III) and the photoinjected electrons in the SnO2 transfer. We can conclude that using the nanocrystalline SnO2 slows the
charge recombination, thus enhancing the Ru(III)-to-tyrosyl radical conversion, and
therefore improves the functionality of the system. It is important to emphasize
on the needs for the design of a more efficient complex to overcome the charge
recombination problem through increasing the space between the positive charge
density of the Ru and the semiconductor, which could slow down the charge
recombination and thus enhance the tyrosyl radical formation. Recently, faiz et
al. [13] demonstrated that the incorporation of α-cyclodextrin (α-CD) into ruthenium dye improves the power
conversion efficiency in solid-state dye sensitized solar cell (i.e., the
efficiency of the cell sensitized with Ru-dye incorporated with α-CD is increased by 40% compared
to the Ru-dye without α-CD). We think that using
a similar idea of incorporating α-CD with our complex (1) could improve the
functionality of the system through slowing down the charge recombination rate
and, therefore, increase the tyrosyl
radical formation.



---

*Source: 524142-2008-02-14.xml*