Y. Shiina et al. / Tetrahedron Letters 53 (2012) 1249–1252
1251
Figure 1. Absorption (blue) and emission (red) spectra of Ac–Ru2+–F (solid line) and
Figure 2. Transient absorption spectra of V2+–Ru2+–Y in deaerated acetonitrile;
kex = 355 nm.
V2+–Ru2+–Y (dashed line) at 298 K in deaerated acetonitrile.
Table 1
In conclusion, novel ruthenium tris(2,20-bipyridine)-type com-
plexes tethered to peptides containing the unnatural amino acid
5Bpy were synthesized. This molecular design allows synthesis of
the ruthenium tris(2,20-bipyridine) derivatives bearing two differ-
ent functional groups. In order to demonstrate this strategy, a novel
ruthenium tris(bipyridine) derivatives having viologen and tyrosine
were synthesized. Attaching the viologen to the ruthenium complex
afforded the short lifetime of the excited states compared with the
reference complex without both viologen and tyrosine, indicating
that the electron transfer occurred from the excited states to the
viologen. Preliminary photophysical results showed that the life-
time of the charge separation state is within a couple of nanosec-
onds. Further study on pico-second time-resolved spectroscopy
will be planned for these ruthenium complexes.
Photophysical properties of the ruthenium–peptide complexes in deaerated
acetonitrile
Complex
kabs/nm
kem/nm
U
s/ns
V2+–Ru2+–Y
454
649
0.007
35.1 (79.8%)
664 (20.2%)
667
Ac–Ru2+–F
454
649
0.056
versus SCE in acetonitrile, is known to become a good quencher for
oxidative quenching of [Ru(bpy)3]2+⁄.1 On the other hand, the oxida-
tion potential of tyrosine in water is 0.93 V versus NHE at pH 7,23,24
which is more positive than the excited state oxidation potential
(E⁄(Ru2+⁄/Ru+) = 0.84 V vs NHE) but more negative than the ground
state oxidation potential (E(Ru2+/Ru3+) = 1.26 V vs NHE) of [Ru(b-
py)3]2+ 1
. This indicates that the reductive quenching of tyrosine is
negligible and electron transfer occurs from tyrosine to the oxidized
ruthenium complex, which generates by oxidative quenching with
the viologen unit. Therefore, the major shorter-lived component
as well as the smaller quantum yield than Ac–Ru2+–F arises from
the electron transfer from the excited states of the ruthenium com-
plex to the viologen.25 From the lifetime of the major short-lived
component of V2+–Ru2+–Y, the intramolecular electron transfer rate
Acknowledgments
We thank Dr. Shohei Tashiro and Prof. Mitsuhiko Shionoya at
School of Science, the University of Tokyo for the ESI-TOF-MS mea-
surements. This research was supported by the PRESTO Program of
JST and a Grant-in-Aid for Scientific Research (C) from the Ministry
of Education, Culture, Sports, Science, and Technology (21550163).
(kET) in V2+–Ru2+–Y is estimated to be ꢂ2.70 ꢀ 107 sꢁ1 26
.
Covalently linked [Ru(bpy)3]2+–viologen dyads have been syn-
thesized as physical models for the photosynthetic reaction cen-
ter.27–29 The lifetimes for the charge separation (CS) states have
been observed in the range of 300 ps–3 ns. On the other hand, a
[Ru(bpy)3]2+–tyrosine conjugate has been synthesized as a model
for the photosystem II, where a tyrosine group (TyrZ) donates an
electron to the primary electron donor (P680+).24 In the work, the
electron transfer from the tyrosine group to [Ru(bpy)3]3+, which
is generated with oxidation quenching by viologen, is researched.
However, to the best of our knowledge, there are few studies for
covalently linked viologen–[Ru(bpy)3]2+–tyrosine molecules. To
estimate the lifetime for the CS state, nanosecond transition
absorption spectra of V2+–Ru2+–Y in acetonitrile were measured
(Fig. 2). The spectra showed a unique broad band at 500–600 nm,
which had never been observed in the ones for ruthenium
tris(bipyridyl) derivatives. However, it was also observed for Ac–
Ru2+–F (compare Fig. 2 with Fig. S5 in Supplementary data). This
indicates that the broad band at 500–600 nm comes from the ex-
cited triplet states of the ruthenium complex and suggests that
the lifetime for the CS state is within a couple of nanoseconds.
Supplementary data
Supplementary data associated with this article can be found, in
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