1418
Table 1. The electrochemical data of 1 and 2a
E
1/2(ox)
/V
E1/2(red)
/V
¦E1/2
/V
Compound
1
2
1.29
1.31
1.25
1.26
¹1.06
¹1.02
¹1.37
¹1.36
2.35
2.33
2.62
2.62
[Ru(bpy)2(phen)]
[Ru(bpy)3]
aCyclic voltammograms for all compounds were measured in
1.0 mM acetonitrile solutions containing 0.1 M Bu4NPF6,
using a Ag/AgNO3/CH3CN reference electrode (+0.37 V vs.
SCE; calibrated with Fc0/+) and Pt working electrode.
complexes near +1.3 V are typical for the Ru3+/2+ couple of
ruthenium(II) polypyridyl complexes with bipyridine and/or
phenanthroline derivatives, while the first reduction potentials
(E1/2(red)) are assigned to the first reduction process of the
phenanthroline or bipyridine ligands. An interest finding is that
the E1/2(red) values of two complexes (1: ¹1.06 V, 2: ¹1.02 V)
have a higher potential than those of [Ru(bpy)3](PF6)2 (¹1.36 V)
or [Ru(bpy)2(phen)](PF6)2 (¹1.37 V) under the same measure-
ment conditions. These first reduction potentials of compounds
might indicate that the first reduction occurred at the phen-
anthroline derivative ligands, AcL1 and AcL2, in these
complexes; in addition, it means that the diethynylphenanthro-
line in the complex accepts the electron from a ruthenium center
under the photoexcited MLCT state. The validity of this
explanation is supported by our previously reported results.6,11
Furthermore, the relation between the emission energy of
maximum intensity and potential difference, ¦E1/2, listed in
Table 1 was consistent with the relation in many ruthenium(II)
polypyridyl complexes reported in our previous article.6 The
second reduction potentials of novel ruthenium(II) complexes
were observed at near ¹1.3 V and these reduction might be
assigned the reduction of one of bpy ligands in these complexes.
However, the reversibility on the multiple sweeping in present
complexes was very low, and the EC reaction must be occurred
after the second reduction of these complexes.
For the electrochemistry of SAM with ruthenium complexes
on a gold electrode, a precleaned highly flat gold thin film sheet
on mica was used to form a thiol-bonded monolayer by
immersion of this gold sheet into an acetonitrile solution of
1.0 mM 1 or 2. An acetate-protecting group on the thiophenyl
substituent in the ligand was removed in situ by adding NH3(aq)
to obtain the thiol complexes 3 or 4. After 1 day of immersion,
the SAM of the ruthenium complex on the gold surface was
firmly developed and the sheet was thoroughly rinsed with
acetonitrile. Details regarding preparation of the self-assembled
ruthenium complex monolayer on the gold electrode are
described in the Supporting Information.10 The cyclic voltam-
mograms of SAM with 3 and 4 on gold sheet electrodes were
measured with different scan rates and referenced against SCE
with a silver wire as a quasi-reference electrode under similar
conditions to those used for the acetyl-protected ruthenium
complexes 1 and 2 in the solution described above. Reversible
oxidation waves of 3 and 4 for the Ru3+/2+ redox process
occurred near +1.3 V, very similar to the values obtained in
solutions of 1 and 2. Self-assembled complex 3 on the gold
surface exhibited the characteristic electrochemical behavior of
surface-confined species with small peak separation (example
Figure 2. UV-vis absorption (left) and emission (right)
spectra upon excitation at 425 nm of 1 (solid line) and 2 (dash
line) at room temperature in acetonitrile.
line ring were detected in AcL1 and 1, respectively, while the
observed phenanthroline proton signals for AcL2 and 2 were
three sets of signals corresponding to the protons of the
symmetric diethynylphenanthroline skeleton.10
Figure 2 shows the absorption spectra of Ru(II) complexes
1 and 2 in CH3CN at room temperature. The absorption bands in
the 400-550 nm region of these complexes were assigned to
typical MLCT transitions observed for ruthenium(II) polypyr-
idyl complexes with bipyridine and/or phenanthroline deriva-
tives. The absorption bands in the 300-400 nm region were
primarily assigned to ³-³*(diethynylphenanthroline) transitions,
and this assignment was supported by UV-vis absorption spectra
of novel ligands, AcL1 and AcL2, as shown in Figure S1.10 The
spectrum of 2 shows the lowest energy ³-³*(AcL2) transition
band near 370 nm. A similar absorption band for 1 was detected
at 340 nm, and the molar extinction coefficient (¾) for the lowest
³-³*(AcL1) absorption of 1 was found to be smaller than that
for 2. This red shift and high ¾ of the lowest energy ³-³*
absorption in 2 is likely due to greater electron delocalization by
the two acetylthiophenyl substitutions on the diethynylphenan-
throline skeleton in the AcL2 ligand compared with AcL1.5,11
The emission spectra of 1 and 2 in Figure 2 show similar
phosphorescent bands assignable to the triplet MLCT transition
in the long wavelength region (1: 638 nm, 2: 671 nm) at room
temperature. These complexes have a higher emission quantum
yield (º) (1: º = 0.156, 2: º = 0.113) than standard ruthe-
nium(II) complexes, [Ru(bpy)3](PF6)2 (º = 0.095).9 Further-
more, these emissions at 77 K display the phosphorescent bands
with weak vibronic progressions assignable to the triplet MLCT
transition in the short wavelength region (1: 594 nm, 2: 631 nm)
(Figure S2).10
The electrochemical properties of ruthenium(II) complexes
1 and 2 were investigated by cyclic voltammetry in acetonitrile.
The resulting electrochemical data are collected in Table 1.
These complexes exhibited reversible oxidation waves and
reversible first reduction waves in the potential range +1.50 to
¹1.50 V versus SCE. The oxidation potentials (E1/2(ox)) of two
Chem. Lett. 2012, 41, 1417-1419
© 2012 The Chemical Society of Japan
Shiotsuka, Michito
