Visible-light photochromism of bis(ferrocenylethynyl)ethenes switches electronic communication between ferrocene sites

Article abstract of DOI:10.1002/anie.200601067

(Figure Presented) Communicating better over a long distance: The bis(ferrocenylethynyl)ethene 1 undergoes E→Z photoisomerization upon excitation of a charge-transfer band with visible light (546 nm). This structural change leads to a decrease in the "thr

Full text of DOI:10.1002/anie.200601067

Angewandte
Chemie
or the p* orbital of the bridge[6] is key to the excellent
nonlinear optical properties of ferrocene(donor)–acceptor
combinations[5] and, for example, the green-light-induced
trans-to-cis isomerization of the azobenzene moiety in ferro-
cenylazobenzenes.[6]
To our knowledge, light-triggered electronic communica-
tion between ferrocenes has never been reported before. In
the present study, we adopted an ethynylethene p frame-
work[8] as a bridge fragment, which undergoes Z–E photoiso-
ꢀ
merization on the central C C double bond. The greatest
benefit of introducing this class of photochromic compounds
is the thermal stability of the Z and E isomers,[8] which is
important in terms of their applications as, for example,
molecular switching devices. Additionally, the possibility of
ꢀ
Ferrocene Derivatives
modifying substituents on the central C C double bond is also
likely to be important in terms of tuning the photoisomeriza-
tion behavior and mixed-valence communication by the
electronic perturbation of the p system. These properties of
ethynylethene are superior to those of other photochromic
species such as the azo group. Our attempts at quantitative
analysis of the mixed-valence interaction in cis-azoferrocene
have not been successful because of the low stability of the
cis isomer.[9,10]
DOI: 10.1002/anie.200601067
Visible-Light Photochromism of
Bis(ferrocenylethynyl)ethenes Switches
Electronic Communication between Ferrocene
Sites**
Herein, we report two types of ferrocene-conjugated
ethynylethenes, (E)-1 and (E)-2 (Scheme 1), which differ in
the presence of a p or s substituent on the central double
bond. Their photophysical properties also contrast with each
other: (E)-1 showed visible-light photochromism, whereas
Ryota Sakamoto, Masaki Murata, and
Hiroshi Nishihara*
Ferrocene has many attractive features, such as excellent
reversible redox properties, high solubility in organic
media, and high modifiability with organic chemical
methods,[1] and it remains a promising molecular
fragment for molecular devices. Related to the redox
properties, intramolecular mixed-valence interactions
between multiple ferrocene sites have been regarded
as a key to the realization of molecular electronics,
such as molecular quantum cellular automata
(QCA),[2] and the fundamental evaluation of their
ability as molecular electronic wires.[3]
Ferrocene exhibits an excellent affinity for organic
p-bridge systems, with strong electronic coupling
between the Fe d orbitals and the p orbitals of the
cyclopentadienyl ring and p bridges. This mutual
interaction can yield an intramolecular mixed-valence
interaction[4] and a charge-transfer (CT) band in the
Scheme 1. Syntheses of (E)-1, (Z)-1, and (E)-2. EDC-HCl=1-ethyl-3-(dimethylaminopropyl)-
carbodiimide hydrochloride; DIBAL-H=diisobutylaluminumlithium hydride; DMAP=4-
(N,N-dimethyl)aminopyridine.
visible region.[5–7] This CT transition from ferrocene
d orbitals to the LUMO of either the acceptor group[5]
(E)-2 did not show any photochromic behavior. Also
discussed is the decrease in the strength of the mixed-valence
interaction between the two ferrocene moities accompanying
E!Z isomerization in 1. Compounds (E)-1 and (E)-2 were
synthesized according to the procedure outlined in Scheme 1.
The Z isomer of 1 was obtained as a minor by-product during
the synthesis of (E)-1.[11] The absolute configurations of (E)-1
and (Z)-1 were confirmed by single-crystal X-ray analysis
(Figure 1).[12] The configuration of 2 (E) was assigned with the
aid of IR spectroscopy, by comparison of the absorptions
[*] R. Sakamoto, Dr. M. Murata, Prof. Dr. H. Nishihara
Department of Chemistry
School of Science
The University of Tokyo
7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-0033 (Japan)
Fax: (+81)3-5841-8063
E-mail: nisihara@chem.s.u-tokyo.ac.jp
[**] This work was supported by Grants-in-Aid for Scientific Research
(Nos. 16047204 (area 434) and 17205007) and by a grant from The
21st Century COE Program for Frontiers in Fundamental Chemistry
from MEXT, Japan.
ꢀ
derived from the carbonyl and C C triple bonds of 2 with
those of (E)-1 and (Z)-1.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
stepwise decrease observed in the p–p* band, as is typical in
the E-to-Z transformation,[8] as well as in the CT band with
time (Figure 3). In addition, one isosbestic point was observed
which confirmed the absence of side reactions such as
decomposition. A one-step reaction from the E to Z form
was observed by 1H NMR spectroscopy (Figure 3). The
Figure 1. ORTEP drawings of (E)-1 and (Z)-1 with thermal ellipsoids
set at the 50% probability level. Hydrogen atoms are omitted.
Both (E)-1 and (E)-2 displayed electronic absorption
bands in the visible region, together with the p–p* band in the
UV region (Figure 2). On the other hand, the analogues (E)-3
ꢁ
=
ꢁ
(p-tol-C C-(COOMe)C C(COOMe)-C C-p-tol) and (E)-4
Figure 3. Time-course UV/Vis (upper) and 1H NMR (lower) spectral
changes of (E)-1 upon irradiation with light at 546 nm and 578 nm in
dichloromethane and [D2]dichloromethane. PSS=photostationary
state.
proportion of Z isomer in the photostationary state was
calculated to be 89% based on the ratio of the integrals. This
value is much higher than the percentage found for azo
compounds[6,14] upon excitation of the CT transition from the
metal moiety to the p* orbital of the azo moiety. The quantum
yields in toluene upon irradiation with green light at
Figure 2. Electronic spectra of (E)-1, (Z)-1, (E)-2, and (E)-3 in dichloro-
^
methane, the main transition in the CT band of (E)-1 (marked with ),
and photos of each compound in dichloromethane.
546 nm[15] were found to be FE!Z = 1.7 ” 10ꢀ5 and FZ!E
=
0.47 ” 10ꢀ5 [16,17]
.
Additionally, p–p* excitation with UV light
ꢁ
=
ꢁ
(p-tol-C C-(CH2OAc)C C(CH2OAc)-C C-p-tol),
which
at 365 nm also afforded E!Z isomerization, which is an
lack the ferrocene moieties, showed only the p–p* band.
Such dual absorptions were previously seen in ferrocene-
containing pigments, which are excellent nonlinear optical
materials,[5b] and in our azo precursors.[6,9,10] Time-dependent
density functional theory (TD-DFT) calculations indicate
that these bands in the visible region are assignable to CT-like
transitions from the ferrocene d to the ethynylethene p*
orbitals (Figure 2), with some contribution by the enyne
bridge, which again showed the same features as in two
previous studies.[5b,6] Lower extinction coefficients (e) were
observed for both the p–p* and CT bands in (Z)-1 compared
to those in (E)-1. With regards to (E)-2, a decrease in e and a
blue shift in the CT band were found (Figure 2).
intrinsic property in ethynylethenes.[8]
In contrast, (E)-2 showed no photochemical response to
excitation of the CT band in the visible region or to excitation
of the p–p* band in the UV region, as confirmed also by
1H NMR spectroscopy. If we consider the photoisomerization
behavior together with the differences in the electronic
spectra, it can be concluded that the substituents attached
directly to the central double bond contribute greatly to the
photochemical properties in our ferrocene-conjugated ethy-
nylethene system. This speculation is well supported by the
photochemical responses of the analogues that lack the
ferrocene moieties. For example, (E)-3 showed E!Z photo-
isomerization upon excitation of the p–p* band, just as (E)-1.
On the contrary, (E)-4 revealed no photochromic behavior
just as (E)-2, but instead it underwent photochemical
degradation.
Irradiation of (E)-1 in dichloromethane with a mixture of
green (546 nm) and yellow (578 nm) light from a superhigh-
pressure Hg lamp[13] led to excitation of the CT band, with a
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Inorg. Chem. 1996, 35, 3735 – 3740; c) M. Rosenblum, N. Brown,
We evaluated the mixed-valence interaction by studying
the splitting between the two redox potentials, DE0’, as
determined from cyclic voltammograms of (E)-1 and (Z)-1
measured in dichloromethane containing nBu4NBF4
(0.1 moldmꢀ3). The result showed a larger DE0’ value in
(E)-1 (70 mV) than in (Z)-1 (48 mV) (Figure 4).[18] This is
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Figure 4. Cyclic voltammograms of (E)-1 and (Z)-1
(1.2ꢀ10ꢀ3 moldmꢀ3) in dichloromethane containing nBu4NBF4
(0.1 moldmꢀ3) at a sweep rate of 100 mVsꢀ1. The potential was
measured relative to the ferrocenium/ferrocene couple.
[11] This may be attributable to the fact that the central double bond
rotated when the reactant was on a Pd atom. However, the
possibility that stray light triggered the E!Z photoisomeriza-
tion in the reaction pot or during purification cannot be
excluded.
consistent with the fact that p conjugation, namely through-
bond interaction,[4] is stronger in the E form and spatial
interaction[19] is negligible because the two redox sites are well
separated even in (Z)-1 (Fe–Fe 6.17 Š), as indicated by the
single-crystal X-ray structural analysis (Figure 2).
In summary, the perturbation by the transition-metal
complex (ferrocene) by the organic photochromic framework
(ethynylethene) and vice versa in the system reported here
were both sophisticatedly utilized to attain visible-light
photochromism that causes a change in electronic communi-
cation between the ferrocene sites. This result is expected to
contribute to the development of well-defined supramolec-
ular devices.
[12] Intensity data were collected at 113(2) K on a RIGAKU
Mercury CCD using monochromated MoKa radiation (l =
0.71073 Š). (E)-1: C30H24O4Fe2, Mr = 560.19, P21/c, a =
15.739(8) Š, b = 7.404(4) Š, c = 10.196(5) Š, b = 95.294(3)8,
V= 1183(1) Š3, Z = 2, m = 1.263 mmꢀ1
, unique reflections =
2696 [R(int) = 0.0176], R1 = 0.0269 [I > 2.00s(I)], wR2 = 0.0646
¯
[I > 2.00s(I)]. (Z)-1: C30H24O4Fe2, Mr = 560.19, P1, a =
9.968(5) Š, b = 11.115(6) Š, c = 12.022(6) Š, a = 111.838(7)8,
b = 92.640(6)8, g = 96.769(6)8, V= 1222(1) Š3, Z = 2, m =
1.223 mmꢀ1, unique reflections = 5353 [R(int) = 0.0176], R1 =
0.0337
[I > 2.00s(I)],
wR2 = 0.0935
[I > 2.00s(I)].
CCDC 600138 ((E)-1) and 600139 ((Z)-1) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[13] Cut-off filter Y-47 and IR cut-off filter IRA-25S by Asahi
Techno Glass Inc. were used to extract light with wavelengths of
546 nm and 578 nm.
[14] R. Sakamoto, M. Murata, S. Kume, H. Sampei, M. Sugimoto, H.
Nishihara, Chem. Commun. 2005, 1215 – 1217.
[15] Light with a wavelength of 546 nm was separated with a Jasco
CT-10T monochromator.
Received: March 17, 2006
Revised: May 1, 2006
Published online: June 26, 2006
Keywords: electrochemistry · metallocenes · mixed-valent
.
compounds · molecular devices · photochromism
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