Visible-light photochromism of triarylamine- Or ferrocene-bound diethynylethenes that switches electronic communication between redox sites and luminescence

Article abstract of DOI:10.1002/chem.200800732

Redox-active ferrocene- and triarylamine-terminated diethynylethene derivatives have been synthesized and their photochromic properties and switching behavior based on through-bond electronic communication between the two redox sites, as well as their emi

Full text of DOI:10.1002/chem.200800732

DOI: 10.1002/chem.200800732
Visible-Light Photochromism of Triarylamine- or Ferrocene-Bound
Diethynylethenes that Switches Electronic Communication between
Redox Sites and Luminescence
Ryota Sakamoto, Shoko Kume, and Hiroshi Nishihara*[a]
Abstract: Redox-active ferrocene- and
triarylamine-terminated diethynyleth-
ene derivatives have been synthesized
and their photochromic properties and
switching behavior based on through-
bond electronic communication be-
tween the two redox sites, as well as
their emissions, have been examined.
higher efficiency in 2 may be attributed
a switch in the strength of the electron-
ic communication between the two
redox sites in their mixed-valence
states (DE0’ values are 70 and 48 mV
for (E)-1 and (Z)-1, and 74 and 63mV
for (E)-2 and (Z)-2). In the case of 2,
further evaluations were carried out
through intervalence charge-transfer
(IVCT) band analyses and DFT calcu-
lations. We have also demonstrated
that steric repulsion between the
methyl ester moieties in the Z form is
implicated in the reduction in the
through-bond electronic communica-
tion. Compound 2 exhibits photolumi-
nescence, which is more efficient in the
E form than in the Z form, whereas 1
and 3 show no photoluminescence.
to the absence of the heavy atom effect
3
and of a low-lying LF state, which are
characteristic of ferrocenyl compounds.
This proposition is further supported
by the fact that bisACHTREU(NG ferrocenylbuta-1,3-
diynyl)ethene 3, which, unlike 1, is free
from steric interference between the
two ferrocenyl groups in the Z form,
does not show a significant improve-
Both bis(ferrocenylethynyl)
and bis(triarylaminoethynyl)
A
G
1
2
G
ACHTREUNG
show visible-light photochromism in-
duced by donor–acceptor charge-trans-
fer (CT) transitions from the ferrocene
ment in its photoACHTREUiNG somerization quan-
tum yields (FE!Z =6.2”10À5, FZ!E
=
3.4”10À5). The visible-light photo-
or triarylamine to the diethynyl
A
chromism of 1 and 2 is accompanied by
moieties. The reversibility and quan-
tum yields of the photochromism of 2
Keywords: electrochemistry. · elec-
(FE!Z =6.1”10À2,
FZ!E =1.4”10À2)
tronic communication
· lumines-
are far higher than those of 1 (FE!Z
=
cence · oxidation · photochromism
8.6”10À6, FZ!E =2.5”10À6).
The
Introduction
can constitute molecular devices in themselves,[5] as well as
models for switchable molecule-based electronic circuits,[6]
in which the p-bridges may be regarded as conductive nano-
wires. Among the potential systems that may be controlled
by external stimuli, photocontrollable ones are the most fa-
vorable in terms of their practicable reversibility. Ethynyl-
The exploration of electronic communication through organ-
ic p-bridges in mixed-valence compounds is of great impor-
tance in clarifying electron-transfer and -transport issues.[1]
Above all, control of this electronic communication by
means of external stimuli (photons,[2] protons,[3] ions,[4] etc.)
has attracted much attention because these kinds of systems
ACHRTEeUNG thACHRTEeUGN ne, which can be regarded as the fundamental building
block of polydiacetylene,[7] is suitable for investigations of
the type described above because it has a highly extended
p-system, reversible and efficient Z/E photochromism, both
isomers are thermally stable, and it is amenable to synthetic
diversity.[8] We have found a new class of diethynylethene
[a] Dr. R. Sakamoto,+ Dr. S. Kume, Prof. Dr. H. Nishihara
Department of Chemistry, Graduate 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
frameworks, namely dimethyl 2,3-bisACHTRE(UNG ethynyl)fumarate/mal-
eate, to represent excellent photochromic and acceptor frag-
ments.[9] In combination with ferrocene, which is a good
donor and shows rather intense electronic communication
through p-bridges,[10] the unique system (E)/(Z)-1, dimethyl
2,3-bis(ferrocenylethynyl)fumarate/maleate (Figure 1), was
established to produce an optical switch in the electronic
[+] Current address: Department of Chemistry
Faculty of Science, Tokyo University of Science
1-3, Kagurazaka, Shinjuku-ku, Tokyo, 162-8601 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800732.
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FULL PAPER
have been published on triarylamine-based mixed-valence
species[4,17] because a wide range of p-bridges may be intro-
duced, giving rise to intense, well-isolated intervalence
charge-transfer (IVCT) bands; however, no system has yet
been produced with an optical switch in the electronic com-
munication between triarylamine sites.
Against the background described above, we report
herein on (E)/(Z)-2, or dimethyl 2,3-bis(N,N-di-4-methoxy-
phenyl-4-aminophenylethynyl)fumarate/maleate (Figure 1),
which displays efficient and reversible visible light Z/E pho-
tochromism that is accompanied by changes in the strengths
of the fluorescence and electronic communication between
the two triarylamine sites. Electrochemical, IVCT, and DFT
calculation analyses have been employed to evaluate this
electronic communication. We also discuss steric repulsion
between the methyl ester moieties in the Z form, which
plays a role in reducing the electronic communication. Possi-
ble obstruction between the two ferrocenyl groups in the Z
form (Figure S1) may be invoked in order to account for the
low photoisomerization quantum yields of 1. Therefore, to
further justify the contribution of the electronic structure of
the triarylamine to the drastic increase in the photoisomeri-
zation efficiency of 2, we examined the photochromic be-
havior of (E)/(Z)-3, or dimethyl 2,3-bis(ferrocenylbuta-1,3-
diynyl)fumarate/maleate (Figure 1), which has longer diyne
bonds to circumvent this kind of steric hindrance (Fig-
ure S1).
Figure 1. Diethynylethenes 1–4.
communication with photoisomerization triggered by the ex-
citation of a donor–acceptor charge-transfer (CT) band in
the visible region, as reported previously in a short commu-
nication.[9]
Results and Discussion
Characterization: The E/Z configurations of 1–4 were deter-
mined as follows. For (E)-2, a single-crystal X-ray structure
analysis was carried out (Figure 2), as was also the case for
Thus far, a considerable number of metal (organometal-
lics and transition metal complexes,[11,12] nanoparticles,[13]
surfaces[14])–organic photochromic ensembles have been re-
ported, in which the photochromic abilities are suppressed
through electron- or energy-transfer quenching processes.
Compound 1 also suffers from this kind of drawback, with
quite low quantum yields for the Z/E photoisomerization
(FE!Z =8.6”10À6, FZ!E =2.5”10À6),[9] which we ascribe to
the quenching ability of the ferrocenyl moieties based on
3
the heavy atom effect of the FeII ion[15a] and a low-lying LF
Figure 2. ORTEP drawing of (E)-2 with thermal ellipsoids drawn at the
50% probability level. Hydrogen atoms have been omitted for clarity.
state.[15] Moreover, 1 lacks an E-rich photostationary state
(PSS), although we have confirmed the existence of both
E!Z and Z!E photoisomerization pathways.[9]
(E)-1 and (Z)-1.[9] For (Z)-2, (E)-3, and (E)-4, the E/Z con-
figurations were verified by checking the C=O vibrations in
their IR spectra (Figure S2). The two C=O bonds give rise
to symmetric and antisymmetric harmonic vibrations
through the central C=C bond. In (E)-1 and (E)-2, both of
which have Ci symmetry in the crystal phase, only the anti-
symmetric vibration is allowed, whereas in (Z)-1, in which
the inversion symmetry is lost, two signals are seen as a
result of both vibrations. (Z)-2, (E)-3, and (E)-4 also con-
form to these trends. (Z)-3 and (Z)-4 were generated in
photoirradiation experiments (see later) and were identified
Triarylamine derivatives display redox[16] and electronic
communication[4,17] behavior quite similar to that of ferro-
cene derivatives, which is based on an n orbital on the cen-
tral N atom. It is the absence/presence of the heavy-atom
3
effect and of the low-lying quenching LF excited state that
define the critical difference between triarylamine and fer-
rocene. This difference is strongly reflected in their photo-
chemical behavior. For example, triarylamine-based dyes
tend to be highly emissive from singlet excited states,[18]
whereas ferrocenyl compounds seldom show lumines-
cence.[10] In this decade, a substantial number of reports
1
from their H NMR spectra without isolation.
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H. Nishihara et al.
Electronic spectra: Figure 3a shows the electronic spectra of
the E forms of 1–4 in dichloromethane. In the UV region,
each of the compounds shows an intense p–p* band charac-
teristic of ethynylethene derivatives,[8] whereas unique visi-
ble bands are observed for all but (E)-4, which does not
contain a donor moiety. Incidentally, tris(4-methoxyphenyl)-
amine only shows absorptions below 400 nm,[19] while ferro-
cene absorbs faintly in the visible region.[15] By means of
time-dependent density functional theory (TD-DFT) calcu-
lations, these visible bands could be assigned to CT transi-
tions from the HOMO, the out-of-phase combination of the
which may be attributed to the extension of the p-conjuga-
tion.
Figure 4 shows overlays of the electronic spectra of the E
and Z forms of 1–4 in toluene. (Z)-2 shows increased molar
absorptivity on the short-wavelength side of the CT bands
compared with (E)-2, whereas 1 and 3 do not show such re-
versals. In the Z form, the higher CT transition should be al-
lowed due to the loss of inversion symmetry. According to
the TD-DFT calculations, the E-to-Z conformational change
in triarylamine-containing 2 is accompanied by a substantial
increase in the oscillator strength of the higher CT transi-
tion, with a significant decrease in that of the lowest CT
transition (Figure S4). In contrast, in ferrocene-containing 1
and 3, the rise of the higher CT band is not so pronounced
as to cause the reversal of the absorption in the visible
region (Figures S6 and S8).
All of the compounds exhibited slight blueshifts in their
p–p* and CT bands in the Z forms (Figure 4). It has been
reported that ethynylethene derivatives without steric hin-
drance do not display such blueshifts upon E-to-Z isomeri-
zation.[8c,d] Figure 5 shows the optimized structures of (E)-4
and (Z)-4. The E form has a coplanar structure, whereas in
the Z form the two methyl ester moieties are substantially
tilted because of their mutual steric repulsion. This series of
results indicates that the structural hindrance of the methyl
ester moieties in the Z form, which is common to all of the
ferrocene
the diethynylethene(p), to the LUMO, the diethynyleth-
ACHTREUNGene(p*) (Figure 3b–d; Figures S3, S5, and S7 in the Support-
ACHTREUNG(dx2Ày2) or triarylamine(n) with a contribution from
ing Information). These results do not contradict previous
data for ferrocene-[20] and triarylamine-based[18] p-conjugat-
ed donor–acceptor pigments. It may be noted that there
should exist another higher-lying CT band, from HOMO-1
or HOMO-2, the in-phase combination of the two redox
sites, to the LUMO in (E)-1, (E)-2, and (E)-3 (Figure 3b–d;
Figures S3, S5, and S7); however, the higher CT transition
is, in principle, Laporte-forbidden in Ci-symmetric E forms.
The lower CT band in (E)-2 is far more intense than that in
(E)-1, reaching emax ꢀ5”104 molÀ1 dm3 cmÀ1. (E)-3 showed
enhanced molar absorptivities and redshifts for both its p–
p* and lower CT bands compared with those of (E)-1,
Figure 3. a) Electronic spectra of (E)-1 to (E)-4 in dichloromethane. b)–d) Main transitions in the lower and higher CT bands: b) (E)-1; c) (E)-2; d) (E)-
3.
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Photochromism in Substituted Diethynylethenes
FULL PAPER
Figure 4. Overlays of the electronic spectra of the E and Z forms of 1–4 in toluene. Those of the Z forms were measured directly for 1, and calculated
mathematically from photoirradiation experiments described in a later section.
tegrals of the 1H NMR spectra. Time-course UV/Vis spectral
changes (Figure S10) revealed the quantum yields for the
photoisomerization of 2 to be larger than those of 1 (Fig-
AHCTREuUNG res S12 and S13) by almost four orders of magnitude
(Table 1). These values for 2 are also superior or similar to
those for the organic analogue 4 upon excitation of the p–
p* bands (Table 1, Figures S15 and S16).
Figure 5. Optimized structures of (E)-4 and (Z)-4.
compounds considered here, is sufficiently pronounced to
disturb the p-conjugation.
Table 1. Photoisomerization quantum yields of 1–4 in toluene.
102FE!Z
102FZ!E
Irradiation [nm][a]
1
2
0.00086
4.4
6.1
0.0062
1.2
0.00025
0.89
1.4
0.0034
1.2
546
546
405
Photoisomerization: In toluene, excitation of the CT bands
of (E)-2 with light at 578 nm caused a UV/Vis spectral
change with isosbestic points (Figure 6a), which was reflect-
ed in a drastic color change (Figure 6c). This reaction was
3
4
578
436[b]
1
further tracked by means of H NMR spectroscopy, which
[a] Excitation of the CT bands. [b] Excitation of the p–p* bands.
revealed one-step E-to-Z photoisomerization (Figure 6b and
Figure S9). The proportions of the Z isomer reached >99%
(irradiation with light at 578 nm) and 97% (irradiation with
light at 546 nm), as calculated on the basis of the relative in-
It is noteworthy that upon irradiation with light at
405 nm, at which the absorption of (Z)-2 increased com-
Figure 6. a) UV/Vis and b) 1H NMR spectral changes of (E)-2 in toluene and [D8]toluene, respectively, upon irradiation with light at 578 and 405 nm.
The percentages given in a) indicate the proportion of the Z isomer in each state. The asterisk in b) indicates the presence of signals derived from the
solvent. The scale for the methoxy protons is reduced to one-third of that for the aromatic protons. c) Photograph of (E)-2 before and after irradiation
with light at 578 or 405 nm.
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H. Nishihara et al.
pared with that of (E)-2 as shown in Figure 4, the PSS
moved in the E-richer direction compared with what was
found upon irradiation with light at 578 nm, again with high
efficiency (Table 1, Figure 6, and Figure S11). Such switching
behavior has never been observed for 1, for which there is
no reversal of the absorption in the visible region in the Z
form (Figure 4).
Compound 3 is free from the steric congestion between
its ferrocenyl groups in the Z form (Figure S1). When (E)-3
in toluene was irradiated with light at 578 nm, one-step UV/
1
Vis and H NMR spectral changes similar to those noted for
(E)-1 (Figure S12) were observed, indicating E-to-Z photo-
ACHTREUNGisomerization (Figure 7). The proportion of the Z isomer in
the PSS was 79% (Figure 7), and the quantum yields of the
photoisomerization were only slightly higher than those in
the case of 1 (Table 1 and Figure S14). It is also noteworthy
that, in contrast to 1, compound 3 did not show an E-richer
PSS.
Figure 8. Absorption and fluorescence spectra of (E)-2 and (Z)-2 in tolu-
ene at ambient temperature upon excitation of the CT bands. At the ex-
citation wavelength (525 nm), both samples have identical optical densi-
ties.
Fluorescence spectra: At ambient temperature, both (E)-2
and (Z)-2 show emissions with wavelength maxima at
639 nm and 637 nm, respectively, upon excitation at the CT
bands (Figure 8). Relatively large Stokes shifts are observed
This series of results indicates that the far higher photo-
ACHTREUNGisomerization quantum yields observed for 2 as compared to
1
those found for 1 is not attributable to the steric issue (Fig-
ure S1), but to the difference in the electronic structures be-
tween triarylamine and ferrocene, that is, the absence or ex-
istence of the heavy atom effect[15a] and a low-lying 3LF
state.[15] This proposition is further supported by the fact
that stilbene derivatives modified with organic substituents
such as nitro and dialkylamino groups show relatively high
photoisomerization quantum yields from both their singlet
and triplet excited states,[21] whereas styrylferrocene under-
goes significantly less efficient photoisomerization, which is
ascribed to fast internal conversion and intersystem crossing
from singlet excited states to photoisomerizable triplet excit-
in both isomers, which are typical of fluorescence from CT
excited states in donor–acceptor-type triarylamine-based
dyes.[18] The efficiency of the fluorescence is higher in (E)-2
than in (Z)-2. In contrast, both 1 and 3, containing ferrocen-
yl units, show no emission.
Electrochemistry: The electronic communication between
the two triarylamine sites in the mixed-valence states in (E)-
2 and (Z)-2 was evaluated by means of cyclic voltammetry.
Figure 9 and Figure S17 show cyclic voltammograms and
their simulations for (E)-2 and (Z)-2 in 0.1m nBu4NBF4 in
dichloromethane. The voltammograms of both isomers were
simulated with two one-electron reversible processes, attrib-
utable to the stepwise redox changes of the two triarylamine
sites.[4,17] The separation between the two redox potentials,
DE0’, was larger in the E form (74 mV for (E)-2 and 63mV
3
ed states, and subsequent effective deactivation via the LF
channel.[11]
Figure 7. a) UV/Vis and b) 1H NMR spectral changes of (E)-3 in toluene and [D8]toluene, respectively, upon irradiation with light at 578 nm. The percen-
tages given in a) indicate the proportion of the Z isomer in each state.
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Photochromism in Substituted Diethynylethenes
FULL PAPER
on triarylamines tend to show intense and well-resolved
IVCT bands compared with those based on organometallics
and transition metal complexes,[17] we also subjected (E)-2
and (Z)-2 to IVCT analyses with chemical oxidation. Tris(4-
bromophenyl)ammonium
hexachloroantimonate
(E0’=
0.70 V vs ferrocenium/ferrocene in dichloromethane[23]) was
used as an oxidant, considering the oxidation potentials of
the triarylamine moieties in (E)-2 and (Z)-2. The oxidizing
reagent (0.1 equiv) was added to neutral (E)-2 and (Z)-2 in
dichloromethane, such that the ratios between the monocat-
ion and dication exceeded 100 according to Kcomp calculated
from the electrochemical data given above.
Figure 10a shows the electronic spectra of monocationic
(E)-2 and (Z)-2. As in previous reports, each spectrum can
be deconvoluted with three Gaussians (Figure 10b, c; Fig-
AHCTREUNG
a ACHTREUNG
band from the diethynylethene(p) to the triarylammo-
nium(n),[17h] and an IVCT band,[17] respectively. The sym-
metrical IVCT bands indicate that both (E)-2+ and (Z)-2+
conform to Robin and Day class II,[24] rather than class III,
or lie at the border of classes II and III.[1c,d,17a,25] It is worth
noting that there is a substantial difference in the strengths
of the IVCT bands between (E)-2+ and (Z)-2+.
Figure 9. a) Cyclic voltammograms of (E)-2 and (Z)-2 (1.1 mm) in 0.1m
nBu4NBF4 in dichloromethane at a sweep rate of 100 mVsÀ1. b),c) Exper-
imental and simulated cyclic voltammograms: b) (E)-2; c) (Z)-2. See Fig-
ures S17 and S18 for details of the parameters used for the simulations.
When a Gaussian-shaped curve is employed for the IVCT
band, the off-diagonal matrix coupling element Hab can be
for (Z)-2), as was also the case for 1 (70 mV for (E)-1 and
48 mV for (Z)-1; Figure S19).[9] Since, in the present cases,
the through-space distances between the two redox sites are
obviously far shorter in the Z forms (11.9 Š for (E)-1 and
7.5 Š for (Z)-1; 17.4 Š for (E)-2 and 10.8 Š for (Z)-2, the
Fe–Fe or N–N distances of the neutral species based on
DFT calculations), DE0’ should be dominated by the
through-bond communication, rather than the through-
space coulombic repulsion and direct contact between the
redox sites.[22] The weaker through-bond communication in
the Z forms is attributable to the disruption of the p-conju-
gation by the steric repulsion between the methyl ester moi-
eties, which clearly arises in both 1 and 2 (Figures 4 and 5).
obtained from Hushꢁs equation [Eq. (1)][26]
:
1=2
Hab ¼ 2:06 Â 10À2RÀ1ðemaxnmaxn1=2
Þ
ꢀ
ꢀ
ð1Þ
where emax, n¯max, n¯1/2, and R are the molar extinction coeffi-
cient and wavenumber at the absorption maximum, the full-
width at half-maximum of the IVCT band, and the distance
between the two redox sites, respectively. Application of
Equation (1) with the emax, n¯max, and n¯1/2 values derived from
the curve fittings (Figures S21 and S22), and R from the N–
N distances of the neutral species from the DFT calcula-
tions, gave a slightly larger Hab for the E form (534 cmÀ1 for
(E)-2+ and 529 cmÀ1 for (Z)-2+). It should be noted that
there is a high possibility of a larger underestimation of Hab,
which results from a greater overestimation of R in (E)-2+
Electronic spectra of monocationic (E)-2 and (Z)-2: Taking
advantage of the fact that mixed-valence compounds based
Figure 10. a) Overlay of the electronic spectra of (E)-2+ and (Z)-2+ in dichloromethane. b),c) Curve fitting with three Gaussians: b) (E)-2+; c) (Z)-2+.
See Figures S21 and S22 for details of the parameters used for the fittings.
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H. Nishihara et al.
than in (Z)-2+. The determination of R is one of the most
difficult problems in the application of Hushꢁs equation, be-
cause stronger delocalization of the surplus charge over the
We have also demonstrated that both isomers of 2 show
fluorescence in fluid solution, the efficiency of which is
higher in the E form.
redox sites and p-bridge produces
a
shorter effective
The strength of the electronic communication between
the redox sites in 1 and 2 has been quantitatively evaluated
from cyclic voltammograms and their simulations, with
weaker through-bond interactions being identified in both Z
forms. This phenomenon stems mainly from steric hindrance
between the methyl ester moieties and the resulting distor-
tion of the p-system, which is reflected in blueshifts in the
electronic spectra and in the DFT calculations. Further eval-
uations for 2 by IVCT analyses and DFT calculations are
consistent with the electrochemical data.
The results reported herein have been obtained through
sophisticated considerations of both electronic and structur-
al aspects of functional molecular fragments, as well as how
these act in unison. This work and its concept constitute a
paradigm for the construction of molecular devices, as well
as providing models for photoswitchable molecular electron-
ic circuits.
charge-transfer distance.[27] The electrochemical data indi-
cate that the delocalization over the two triarylamine sites is
larger in the E form (Figure 9). The disruption of the p-con-
jugation in the Z form by the steric repulsion of the methyl
ester moieties (Figures 4 and 5) suggests that the delocaliza-
tion over the diethynylethene p-bridge is also greater in the
E form. In addition, the vector connecting the two redox
centers is almost directed along the diethynylethene p-
bridge in the E form, whereas in the Z form there is a large
discrepancy. This topological difference also leads to a more
pronounced reduction of R in the E form by delocalization
over the diethynylethene p-bridge.
Coropceanu et al. have suggested[17c,h,28] that the extent of
the electronic communication can be evaluated from MO
calculations on neutral species according to Koopmanꢁs the-
orem:
Hab ¼ 1=2ðeHOMOÀeHOMOÀ1
Þ
ð2Þ
where eHOMO and eHOMOÀ1 denote the energy levels of the
HOMO and HOMO-1, respectively, of the neutral species.
Assignment of the results of the DFT calculations afforded
a larger value for the E form (1524 cmÀ1 for (E)-2+ and
1286 cmÀ1 for (Z)-2+) (Figure S23). This trend is consistent
with our experimental results and discussion.
Experimental Section
Dimethyl 2,3-dibromofumarate,[29] (E)-1,[9] (E)-4,[9] N,N-di-4-methoxy-
phenyl-4-ethynylphenylamine,[30] and ethynylferrocene[10d] were prepared
according to previous reports. Et3N was distilled and stored over KOH
pellets. THF was distilled from metallic Na and benzophenone. Other
chemicals were used as purchased unless otherwise stated.
Dimethyl 2,3-bis(N,N-di-4-methoxyphenyl-4-aminophenylethynyl)fuma-
rate [(E)-2]: Et3N (40 mL) was added to a mixture of dimethyl 2,3-dibro-
mofumarate (400 mg, 1.3mmol), CuI (27 mg), [PdCl 2ACHTRE(UNG PPh3)2] (100 mg),
Conclusions
and N,N-di-4-methoxyphenyl-4-ethynylphenylamine (940 mg, 2.8 mmol)
under a nitrogen atmosphere. The pale-yellow suspension was heated to
1008C, whereupon it turned deep-red in color. After the mixture had
been heated under reflux for 2 h, it was allowed to cool to ambient tem-
perature, whereupon dichloromethane was added to quench the reaction.
The deep-red suspension was filtered through Celite, and the filtrate was
concentrated to dryness in vacuo. The deep-red residue was purified by
column chromatography on basic alumina (activity grade II–III), eluting
with hexane/dichloromethane 1:1. The deep-red fraction was collected
and concentrated to furnish (E)-2 as a deep-red solid (770 mg, 73%). Re-
crystallization from dichloromethane/hexane with protection from light
produced deep-red crystals of (E)-2. 1H NMR (500 MHz, [D8]toluene):
d=7.42 (d, J=9.2 Hz, 4H), 6.96–6.93(m, 8H), 6.83(d, J=9.1 Hz, 4H),
6.64 (d, J=9.6 Hz, 8H), 3.48 (s, 6H), 3.30 ppm (s, 12H); elemental analy-
sis calcd (%) for C50H42O8N2: C 75.17, H 5.30, N 3.51; found: C 74.94, H
5.37, N 3.22.
We have created a new visible-light photochromic system, 2,
that can reversibly and efficiently undergo switching of its
through-bond electronic communication between the two
triarylACHTREUNGamine sites and fluorescence intensity. Compounds 1–
3 have two characteristic bands in the visible region, ascri-
bed to CT transitions from the in-phase and out-of-phase
combinations of the two triarylamine(n) or ferrocene(d) to
the diethynylethene(p*). These compounds undergo Z/E
photochromism upon excitation of their CT bands. The effi-
ciency of the photochromism in fully organic 2 (FE!Z =6.1”
10À2, FZ!E =1.4”10À2) is far higher than that in organome-
tallic 1 (FE!Z =8.6”10À6, FZ!E =2.5”10À6) or in 3 (FE!Z
=
6.2”10À5, FZ!E =3.4”10À5). This series of results indicates
that the difference in the electronic structures of triaryla-
mine and ferrocene, rather than steric repulsion between
the two ferrocenyl units in (Z)-1, is mainly responsible for
the drastic difference in efficiency between 1 and 2.
Dimethyl 2,3-bis(N,N-di-4-methoxyphenyl-4-aminophenylethynyl)maleate
[(Z)-2]: A solution of (E)-2 (200 mg, 0.25 mmol) in toluene (120 mL) was
irradiated for 24 h at 546 and 578 nm by means of a high-pressure Hg
lamp equipped with a cut-off filter. After completion of the reaction was
confirmed by TLC, the solution was concentrated and the residue ob-
tained was purified by column chromatography on basic alumina (activity
grade II–III) eluting with hexane/dichloromethane (1:1, v/v). The second
red fraction was collected and concentrated to give (Z)-2 (185 mg, 92%)
as a red solid. Recrystallization from dichloromethane/hexane with pro-
tection from light produced exclusively (Z)-2 as a red powder. 1H NMR
(500 MHz, [D8]toluene): d=7.40 (d, J=9.2 Hz, 4H), 6.90 (d, J=9.5 Hz,
8H), 6.79 (d, J=9.2 Hz, 4H), 6.61 (d, J=9.6 Hz, 8H), 3.47 (s, 6H),
3.30 ppm (s, 12H); elemental analysis calcd (%) for C50H42O8N2: C 75.17,
H 5.30, N 3.51; found: C 75.03, H 5.55, N 3.35.
Compound 2 shows reversible Z/E switching behavior in
response to irradiation at 578 nm and 405 nm, which is unat-
tainable with ferrocene-containing compounds 1 and 3. This
phenomenon stems from the dramatic changes in the
strengths of the two CT bands between (E)-2 and (Z)-2.
6984
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 6978 – 6986

Photochromism in Substituted Diethynylethenes
FULL PAPER
1-Ferrocenyl-4-triisopropylsilylbuta-1,3-diyne: Triisopropylsilylacetylene
(22 mL, 0.11 mol), [PdCl2ACHTRE(UNG PPh3)2] (838 mg, 1.2 mmol), and CuI (460 mg,
Electrochemistry: Electrochemical data were acquired with an ALS-
650B voltammetric analyzer (BAS Inc.). A series of measurements was
carried out in a standard one-compartment cell, using 3mm f glassy
carbon (Tokai Carbon Co., Ltd.) as a working electrode, platinum wire
(The Nilaco Corporation) as a counter electrode, and an Ag/Ag+ refer-
ence electrode. As an internal standard, decamethylferrocene (E0’=
À551 mV vs Fc+/Fc under our measurement conditions) was added after
each measurement, and its redox wave was used to determine the solu-
tion resistance and double-layer capacitance (Figures S18 and S20). Digi-
Sim 3.03b (BAS Inc.) was used to simulate the voltammograms.
nBu4NBF4 (Sigma–Aldrich Co.) was recrystallized from EtOH. Dichloro-
methane (HPLC grade, Kanto Chemicals Co., Inc.) was used as received.
Ferrocene (Kanto Chemicals Co., Inc.) was recrystallized from dichloro-
methane/hexane. Decamethylferrocene was synthesized according to a
previous report.[36]
2.4 mmol) were suspended in a mixture of THF (100 mL) and Et3N
(100 mL) under an aerobic atmosphere. A solution of ethynylferrocene
(5.0 g, 0.024 mol) in THF (120 mL) was added dropwise to this suspen-
sion over 5 min. After stirring the brown suspension for 1 h, diethyl ether
(300 mL) was added. The resulting mixture was then filtered through
Celite. After evaporation of the volatiles in vacuo, the dark-brown resi-
due obtained was purified by column chromatography on alumina (activ-
ity grade II–III) eluting with hexane. The first orange fraction was col-
lected and concentrated to give 1-ferrocenyl-4-triisopropylsilylbuta-1,3-
diyne as a dark-brown oil, which subsequently crystallized (2.2 g, 24%).
1H NMR (500 MHz, [D1]chloroform): d=4.50 (dd, J=1.8, 1.8 Hz, 2H),
4.26 (s, 5H), 4.24 (dd, J=1.8, 1.8 Hz, 2H), 1.12–1.10 ppm (m, 21H); ele-
mental analysis calcd (%) for C23H30FeSi: C 70.76, H 7.75; found: C
70.61, H 7.82.
Apparatus: UV/Vis spectra were measured with Jasco V-570 and Hew-
lett–Packard 8453UV/Vis spectrometers, IR spectra with a Jasco FT/IR-
620v spectrometer, H NMR spectra with a Bruker DRX 500 (500 MHz)
spectrometer, and fluorescence spectra with a Hitachi F-4500 spectro-
fluorimeter.
Ferrocenylbuta-1,3-diyne: Under a nitrogen atmosphere, 1-ferrocenyl-4-
triisopropylsilylbuta-1,3-diyne (404 mg, 1.0 mmol) was dissolved in THF
(90 mL). A solution of Bu4NF in THF (1.1 mL, 1.1 mmol) and a few
drops of water were added to the solution. After stirring the mixture for
5 min, the reaction was quenched by the addition of water and diethyl
ether. The organic phase was separated, and washed with water and
brine. The brown ethereal solution was dried over Na2SO4, diluted with
1,4-dioxane (100 mL), and then concentrated in vacuo until the diethyl
ether and THF were removed. The resulting solution of 1-ferrocenylbu-
ta-1,3-diyne in 1,4-dioxane was used directly in the next reaction because
of the low stability of this compound in concentrated solution or in the
solid state.
1
Acknowledgements
The authors thank Prof. Dr. Takeshi Yamamura (Tokyo University of Sci-
ence) for his kind consideration. This work was supported by Grants-in-
Aid for Scientific Research (nos. 16047204 [area 434] and 17205007) and
a grant from the 21st Century COE Program for Frontiers in Fundamen-
tal Chemistry from MEXT, Japan.
Dimethyl 2,3-bis(ferrocenylbuta-1,3-diynyl)fumarate [(E)-3]: Under a ni-
trogen atmosphere, [PdCl2ACHTREU(NG PPh3)2] (16 mg), CuI (16 mg), and 2,3-dibro-
mofumarate (150 mg, 0.49 mmol) were suspended in a solution of 1-ferro-
cenylbuta-1,3-diyne (1.0 mmol) in 1,4-dioxane (100 mL) and Et3N
(30 mL). The suspension was refluxed for 1 h, whereupon the color
changed from orange to purple. The mixture was then cooled to ambient
temperature, whereupon dichloromethane was added to quench the reac-
tion. The suspension was filtered through Celite, and the filtrate was con-
centrated in vacuo. The black residue was then purified by column chro-
matography on alumina (activity grade II–III) eluting with hexane/di-
chloromethane 3:2. Recrystallization from hexane/dichloromethane pro-
duced tiny deep-purple crystals of (E)-3 (54 mg, 18%). 1H NMR
(500 MHz, [D8]toluene): d=4.23(dd, J=1.8, 1.8 Hz, 4H), 3.91 (s, 10H),
3.86 (dd, J=1.8, 1.8 Hz, 4H), 3.31 ppm (s, 6H); elemental analysis calcd
(%) for C34H24O4Fe2: C 67.14, H 3.98; found: C 66.98, H 4.20.
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CCCDC 661918 contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
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TD-DFT calculations were executed under the same conditions as listed
above. Solvent effects were not considered in any of the processes. This
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man et al.[35] None of the compounds showed any significant difference in
their photochemical behavior between aerobic and anaerobic conditions.
Chem. Eur. J. 2008, 14, 6978 – 6986
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Received: April 16, 2008
Published online: June 23, 2008
6986
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