M. Watanabe et al. / Tetrahedron Letters 56 (2015) 1548–1551
1549
charge transfer (MLCT) character is reduced and therefore leads to
the reduction of electron donating ability (Figs. 1–3).
Dyads Fc0–Fc3 were synthesized from ferrocene-2-
carboxyaldehyde (1).11 The aldehyde moiety of 1 was reduced by
NaBH4 to give alcohol (2),12 which was immediately converted to
a
Wittig salt with triphenylphosphine hydrobromide (3).13
Donor–acceptor dyad (Fc0) was synthesized by Knoevenagel con-
densation condition with cyanoacetic acid. The oligothiophene
spacer groups were introduced via Wittig reactions. The oligothio-
phene moieties, that is, thiophene-2,5-dicarboxaldehyde, [2,20-
bithiophene]-5,50-dicarbaldehyde, and [2,20:50,200-terthiophene]-
5,500-dicarbaldehyde reacted with 3 to afford cis/trans isomers,
which were readily converted to the pure trans form of aldehydes
(4–6) upon the catalysis of I2 in CHCl3 (31–78%). In 1H NMR spec-
tra, the characteristic aldehyde peaks were confirmed at d 10 ppm.
Knoevenagel condensation of 4–6 with cyanoacetic acid gave the
oligothiophene dyads Fc1–3 in 10–30%. In 1H NMR spectra, the
aldehyde peaks at around 10 ppm disappeared, while in 13C NMR
spectra, the characteristic peaks of carboxylic acid were appeared
at 163–168 ppm. In infrared spectra, the absorption at 2214–
2222 cmꢁ1 was assigned to the stretching of cyanide and that at
1575–1584 cmꢁ1 was assigned to the vibration of carbonyl moiety
of carboxylic acids. The molecular weight of dyads Fc0–Fc3 was
confirmed by using MALDI/MS spectrometry (Scheme 1).
Figure 2. UV/vis spectra of dyads Fc0–Fc3 in CH2Cl2 solution (1.0 ꢂ 10ꢁ4 M).
The absorption spectra of ferrocene dyads were measured in
CH2Cl2 solutions (10ꢁ4 M). In the spectrum of compound Fc, there
are three characteristic transitions, that is, two MLCT bands at 456
(
e
= 2700) and 329 (
(ICT) band at 245 (
Fc0, three transition bands are observed with slight shifts: the low-
est one appeared as a broad peak at 525 nm ( = 2640) and the
other two at 383 (1710) and 323 ( = 15,780) nm with bathochro-
e
= 700) nm and an internal charge-transfer
e
= 88,800) nm (Fig S1a).14 In the spectrum of
Figure 3. Cyclic voltammogrum of dyads Fc0–Fc3 in 0.1 M nBu4NPF6 of CH2Cl2
solution. Scan rate is 100 mV/s, dyad of concentration is 1.0 ꢂ 10ꢁ3 M.
e
e
mic shifts. When a thiophene group was inserted between the
donor and the acceptor, the lowest energy transition of Fc1 showed
HOMO levels of Fc0–Fc3 were estimated to be 0.39–0.59 V (vs
NHE), which were too close to the oxidation potential of Iꢁ/I3ꢁ
(0.4 V vs NHE). The charge flow cannot be conducted effectively,
so that they cannot be used as workable sensitizers for DSSCs.15
To further investigate the nature of radical cations after oxidation,
electrochemical absorption spectra were recorded. It is reported
that the photo-induced radical cation of ferrocene displays the
characteristic lowest energy band at 617 nm with very weak inten-
sity in CHCl3 or CCl4 solutions.16 The electrochemically generated
radical cation of ferrocene (1.0 ꢂ 10ꢁ3 M in 0.1 M nBu4NPF6 in CH2-
Cl2 solution) shows a similar position at 616 nm (Fig. S1b). All mea-
surements of dyads were done under a similar condition to that of
ferrocene. The characteristic absorption peak of Fc0 was found at
853 nm with low intensity. In the ground state, the lowest energy
bands of dyads Fc2 and Fc3 were blue-shifted compared with Fc1.
In contrast to the ground state, the radical cation of the dyads
showed a red-shift with the increased number of thiophene unit,
that is, for Fc1 at 942 nm, for Fc2 at 1051 nm, and Fc3 at
1183 nm, respectively.
To confirm the electron coupling in the neutral and radical
cation states, time dependent density functional theoretical (TD-
DFT) computation was performed at B3LYP/def-TZVPP level by
the TURBOMOLE program.17 The 20 excited states for neutral state
and 10 excited states for radical cation state were calculated and
the trend agrees well with experimental results (Figs. S2 and S3).
Typical examples of selected molecular orbitals for Fc0 and Fc3
are illustrated in Figure 4, and those for Fc2 and Fc3 in Figure S2.
In the absorption spectra, a blue shift was found at the lowest
energy band when the thiophenylene moieties were inserted
between the donor and acceptor. TD-DFT computation results indi-
cated that the 1st excitation of Fc0 mainly dominates from HOMO-
1 to the second lowest unoccupied molecular orbital (LUMO+1)
and LUMO+2, and HOMO to LUMO+1 transition, with HOMO-1 to
LUMO, LUMO+2, and LUMO+3 transition, respectively, (Table 1).
a further bathochromic effect at 560 nm (
that of Fc0. The other two peaks of Fc1 appeared at 439
= 22,690) and 290 ( = 10,980) nm. When two and three thio-
phene groups are inserted, like Fc2 and Fc3, the spectra showed
slight blue shifts, with respect to that of Fc2, at 556 ( = 7100),
466 ( = 14,110), and 347 ( = 6300) nm, respectively. The corre-
sponding ones of Fc3 appeared at 552 (sh, = 12,090), 494
= 16,930), and 382 ( = 8390) nm.
To evaluate the redox property of dyads Fc0–Fc3, cyclic voltam-
e = 8170) comparing with
(e
e
e
e
e
e
(e
e
metry was measured in CH2Cl2 with 0.1 M of nBu4NPF6 as support-
ing electrolyte. For all dyads the oxidation potential showed quasi-
ox
1=2
reversible waves. The oxidation potential of Fc0 is E ðIÞ = ꢁ73 mV
(vs Fc/Fc+), suggesting a higher oxidation ability along with the
elongation of
p-conjugation. However, when thiophene spacers
are inserted between ferrocene and 2-cyanobut-3-enoic acid moi-
ox
1=2
ety, oxidation abilities were decreased, that is, E ðIÞ = ꢁ49 mV for
ox
1=2
ox
1=2
Fc1, E ðIÞ = +88 mV for Fc2, E ðIÞ = +128 mV for Fc3, respective-
ly, (vs Fc/Fc+). The energy potentials are summarized in the
Figure S3.
The result of electrochemical measurement of dyads indicated
redox property varied with the change of spacer groups. The
Figure 1. Ferrocene-containing donor–bridge-acceptor dyads of structures Fc0–Fc3
in this study.
Watanabe, Motonori
