Extremely efficient and reversible visible-light photochromism and accompanying switch of electronic communication in N-phenylcarbazole-appended diethynylethene

Article abstract of DOI:10.1002/chem.201200794

Photoisomerization: N-phenylcarbazole-conjugated dimethyl 2,3-diethynylfumarate/maleate, E/Z-4, shows highly efficient and reversible visible-light E/Z photoisomerization (φ_total=0.40) upon excitation with an intramolecular charge-transfer band. This photochromism induces switching of electronic communication between the two N-phenylcarbazole moieties in the one-electron oxidized mixed-valent state (see scheme). Copyright

Full text of DOI:10.1002/chem.201200794

COMMUNICATION
DOI: 10.1002/chem.201200794
Extremely Efficient and Reversible Visible-Light Photochromism and
Accompanying Switch of Electronic Communication in N-Phenylcarbazole-
Appended Diethynylethene
Mikihiro Hayashi, Ryota Sakamoto,* and Hiroshi Nishihara*[a]
Top-down methods, long used for the creation of materi-
als and devices, are to be replaced by bottom-up methods,
the ultimate goal of which is the establishment of unimolec-
ular devices.[1] In exploiting these bottom-up methods, the
integration of various functionalities (e.g., electro-, photo-,
and magnetoproperties) in a single molecule is the key to
the fabrication of high-end materials and devices. Structural
changes induced by photonic stimuli are of importance as
a component of multifunctionality, because they are accom-
Figure 1. Molecular structures of the E and Z forms of 1–4.
panied by electronic structural changes, which can control
other functionalities of the molecules.[2] To realize molecular
systems with multifunctionality, various kinds of conjugates
among organic photochromics and functional molecular
fragments have been synthesized.[2] In this decade, our
group has contributed to this research field,[3,4] and among
these pieces of research, we have revealed that conjugates
comprising dimethyl 2,3-diethynylfumarate as an electron-
withdrawing photochromic moiety (Scheme 1),[5] and ferro-
munication between the two ferrocene or triarylamine sites
in the one-electron oxidized mixed-valent (MV) state: E-1+
and E-2+ featured greater electronic communication than
Z-1+ and Z-2+. The remaining problem in this series of mo-
lecular systems is low-photoisomerization-quantum yields:
Ftotal(=FE!Z+FZ!E), in which no less than 1.1ꢀ10ꢀ5 and
0.075 for E/Z-1 and E/Z-2, respectively.
N-phenylcarbazole (NPC), which is a fused analogue of
triarylamine, is utilized as hole-transport materials in organ-
ic electronics. Many of these materials feature dimer, trimer,
and higher oligomer-molecular structures,[6] in which intra-
molecular through-bond hole hopping among NPC moieties
is also expected to be involved in the hole transportation, as
well as an intermolecular through-space one. MV com-
pounds with electronic communication experience intramo-
lecular charge migration among their redox sites;[7] hence,
studies on NPC-based MV compounds can contribute to
fundamental chemistry of organic electronics. However,
there is no research report that treats NPC oligomers from
the viewpoint of MV chemistry.
Scheme 1. Photochromic 2,3-diethynylfumarate and 2,3-diethynylmaleate
frameworks.
cene and triarylamine as electron-donating redox sites, E-
1 and E-2 (Figure 1), presented synergetic effects.[4] Both
compounds underwent visible-light photochromism between
their Z forms upon excitation with donor–acceptor intramo-
lecular charge-transfer (ICT) transitions from the d orbital
of ferrocene and the n orbital of triarylamine, respectively,
to the p* orbital of dimethyl 2,3-diethynylfumarate. This
photochromism was followed by a change in electronic com-
Herein, we synthesized NPC-appended dimethyl 2,3-di-
ethynylfumarate E-3 and E-4 (Figure 1) and disclosed the
synergetic photochemical and electrochemical properties of
E-4. It showed reversible visible-light photochromism be-
tween Z-4 with quantum efficiency much higher than those
of previously reported E-1 and E-2. We also clarified the
modulation of electronic communication between two NPC
moieties in MV E/Z-4+ upon the photoisomerization.
[a] M. Hayashi, Dr. R. Sakamoto, Prof. Dr. H. Nishihara
Department of Chemistry
1
Graduate School of Science, The University of Tokyo
Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Fax : (+81)3-5841-8063
E-mail: sakamoto@chem.s.u-tokyo.ac.jp
nisihara@chem.s.u-tokyo.ac.jp
All compounds were identified with H NMR spectrosco-
py and high-resolution fast-atom bombardment (FAB) MS.
The E and Z configurations were determined by single-crys-
tal X-ray structure analysis for E-4 and Z-4 (Figure 2).[8] We
also disclosed the crystal structure of E-3 (Figure S1 in the
Supporting Information).[8] Noteworthy is the difference in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200794.
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Figure 3. Top: UV/Vis (solid) and fluorescence (dotted) spectra of E-3
(orange) and E-4 (red) in toluene. Bottom: Chief transition in the ICT
band of E-4 estimated by TDDFT calculation.
Table 1. Photochemical parameters of E/Z-3 and E/Z-4 in toluene.
Compound
Absorption
emax [mꢀ1 cmꢀ1
Fluorescence
lmax [nm]
]
lmax [nm]
FF
E-3
Z-3
E-4
Z-4
420
398
450
434
31000
18000
28000
19000
541
548
618
637
0.04
0.07
0.13
0.08
Figure 2. ORTEP drawings of E-4·CHCl3 and Z-4 with 50% probability
of ellipsoids. Gray: carbon; red: oxygen; blue: nitrogen. Hydrogen atoms
and chloroform are omitted for clarity.
lmax =22 nm (0.16 eV) for E/Z-3 and lmax =16 nm (0.10 eV)
for E/Z-4. This stems from the steric repulsion between the
methyl ester moieties suggested in single-crystal X-ray struc-
ture analysis and DFT calculation (Figures 1, 2, S4, and S5
in the Supporting Information), and resultant disturbance of
the p conjugation of the dimethyl 2,3-diethynylmaleate
moiety. We note that ethynylethenes without steric hin-
drance as such did not display the blueshift of absorptions
upon E-to-Z isomerization.[5a] Figures 3a and S6 in the Sup-
porting Information also show the fluorescence spectra
upon excitation with the ICT band, and their florescence ef-
ficiencies (FF) are tabulated in Table 1.
molecular planarity in the E and Z forms: E-4 features
a highly planar p-conjugated framework; on the other hand,
significant distortion with respect to the methyl ester moiet-
ies was found in Z-4 (Figures 2 and S4 in the Supporting In-
formation). This distortion stems from the steric repulsion
of the methyl ester moieties (Figure 1). The distortion in Z-
4 was also demonstrated in DFT calculation (Figure S5 in
the Supporting Information), thereby being valid in the so-
lution phase.
Figure 3a shows the UV/Vis spectra of E-3 and E-4 in tol-
uene, which feature intense and broad absorptions in the
visible region. By means of time-dependent density func-
tional theory (TDDFT) calculations (Figures 3b, S2, and S3,
Tables S1 and S2 in the Supporting Information), we as-
signed these visible bands to ICT transitions from the
HOMO (chiefly distributed on the n orbitals of the NPC
moieties), to the LUMO (p* orbital of the dimethyl 2,3-di-
ethynylfumarate moiety). The absorption maximum (lmax) of
the ICT band of E-4 is redshifted by 30 nm (0.20 eV) com-
pared with that of E-3 (Table 1), as a result of the strong
electron-donating ability of the methoxy group on the NPCs
and resultant elevation of the HOMO level. Figure S6 in the
Supporting Information shows the absorption spectra of Z-3
and Z-4, and their numerical data are collected in Table 1.
From the comparison between the E and Z forms, one can
find that lmax of the ICT band is blueshifted in the Z forms:
Compound E-4 underwent photoisomerization to Z-4
upon irradiation with visible light (l=500 nm), which corre-
sponded to the excitation of the ICT band (Scheme 2). This
Scheme 2. Reversible E/Z photoisomerization in E/Z-4.
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Visible-Light Photochromism in Diethynylethenes
COMMUNICATION
Figure 4. Time-course UV/Vis spectral change of E-4 in toluene upon ir-
radiation with l=500 nm light. The inset shows a time-course 1H NMR
spectral change in [D8]toluene under the same condition.
reaction was traced by time-course UV/Vis (Figure 4) and
1H NMR spectral changes (inset of Figure 4). The former
possessed an isosbestic point, which ensured the absence of
side reactions. Irradiation at l=500 nm to E-4 gave rise to
a nearly quantitative conversion to Z-4: the proportion of
the Z-isomer in the photostationary state (PS) reached
1
>99%, judging from the integral ratio in the H NMR spec-
Figure 5. Cyclic voltammograms (black solid line) of a) E-4 and b) Z-4 in
nBu4NBF4-dichloromethane (0.1m) and its simulation (red circles). E0’(1),
E0’(2), and DE0’ are extracted from the simulation.
trum. Both E and Z isomers were thermally stable, because
the spectra did not change at all when the solution samples
were stored in the dark. We note that the E/Z ratio at the
PS could be tuned reversibly by changing the excitation
wavelength (Scheme 2). For example, upon irradiation at
l=400 nm, the proportion of the Z isomer in the PS was re-
duced to 71% (Figures S7 and S8 in the Supporting Infor-
mation).
The E/Z photoisomerization-quantum yields in 4 upon ir-
radiation with l=400 nm light was calculated by the proce-
dure reported by Zimmerman et al.[9] Figures S9 and S10 in
the Supporting Information display the experimental plot
and equations, respectively. This calculation gave FE!Z and
FZ!E of 0.30 and 0.10, thereby affording Ftotal of 0.40.
Therefore, the Ftotal of E/Z-4 was far greater than that of
E/Z-1 (Ftotal =1.1ꢀ10ꢀ5) and 5.3 times greater than that of
E/Z-2 (Ftotal =0.075).
the Supporting Information). According to the simulation,
E0’(1) and E0’(2) are 496 and 541 mV for E-4, and 501 and
532 mV for Z-4 (Figure 5). The separation of the formal po-
tentials between the two chemically equivalent redox sites,
DE0’ =E0’(2)ꢀE0’(1), can be a good indicator for electronic
communication.[7] Value DE0’ is greater in E-4 (45 mV) than
in Z-4 (31 mV); therefore, E-4 should possess more intense
electronic communication in the monocationic MV state.
We note DE0’ is modulated by other factors, such as Cou-
lomb repulsion, statistical term, inductive effect, and so
on.[10] However, these sidebar factors should be nearly iden-
tical in E-4 and Z-4, except for the Coulomb factor. This
through-space interaction should be more intense in the Z
ꢀ
Pursuit of solid-state photoisomerization in E/Z-3 and
E/Z-4 was unsuccessful. Instead, both E-3 and E-4 exhibited
strong fluorescence even in the solid state (Figure S11 in the
Supporting Information), which might be appreciated in
solid-phase applications, such as electroluminescence devi-
ces.
To quantify the modulation of electronic communication
in the one-electron oxidized MV state upon the photoisome-
rization, cyclic voltammetry was conducted for both E-4 and
Z-4 (Figure 5). Both isomers underwent two reversible and
close one-electron oxidations with respect to the two NPC
moieties. Therefore, simulation of the voltammograms was
conducted to acquire the formal potentials of the two oxida-
tions of the two NPCs, E0’(1) and E0’(2). The result of the
simulations is also shown in Figure 5 as red circles (for the
detail parameters of the simulation, see Tables S3 and S4 in
form (8.4 ꢂ for the N N bond length, from the single-crys-
tal X-ray structure of Z-4) than in the E form (17.2 ꢂ, from
that of E-4). Nevertheless, the greater DE0’ was observed in
E-4; hence, one can conclude that the electronic communi-
cation is truly more intense in E-4 than in Z-4. The weaker
electronic communication in Z-4 should originate from the
steric repulsion between the methyl ester moieties (Fig-
ures 1, 2, S4, and S5 in the Supporting Information), which
disturbs the p conjugation between the two NPCs,[4b] as evi-
denced by the blueshift of the ICT band in the Z form (Fig-
ures 3, S6 in the Supporting Information, and Table 1).
We note that the methoxy group at the 3- and 3’-positions
in E/Z-4 is essential for the reversible electrochemical be-
havior: without methoxy substituents, E-3 underwent irre-
versible oxidations (data not shown). Instead, oxidative elec-
tropolymerization was observed.[11] Thus, when an ITO plate
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M. Hayashi, R. Sakamoto, and H. Nishihara
Bull. Chem. Soc. Jpn. 2004, 77, 407; e) H. Nishihara, Coord. Chem.
Rev. 2005, 249, 1468.
was employed as a working electrode, E-3 was electrochemi-
cally deposited onto the ITO plate to form insoluble but
electrochemically active films (Figure S12 in the Supporting
Information).
[3] a) T. Yutaka, M. Kurihara, K. Kubo, H. Nishihara, Inorg. Chem.
2000, 39, 3438; b) M. Kurihara, A. Hirooka, S. Kume, M. Sugimoto,
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Murata, T. Ozeki, H. Nishihara, J. Am. Chem. Soc. 2005, 127, 490;
d) R. Sakamoto, M. Murata, S. Kume, H. Sampei, M. Sugimoto, H.
Nishihara, Chem. Commun. 2005, 1215; e) S. Nagashima, M.
Murata, H. Nishihara, Angew. Chem. 2006, 118, 4404; Angew.
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[5] Photoisomerization of diethynylethene and tetraethynylethene has
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Chem. Int. Ed. 1999, 38, 674.
In conclusion, we synthesized a new photochromic mole-
cule E-4 comprising two NPCs and dimethyl 2,3-diethynylfu-
marate. Compound E-4 underwent reversible E/Z photoiso-
merization upon irradiation with visible light, corresponding
to excitation with the ICT band. This photochromism was
highly efficient (Ftotal =0.40), superior to analogue
E-1 (Ftotal =1.1ꢀ10ꢀ5) and E-2 (0.075) with ferrocene and
triarylamine instead of NPC. This photochromism is accom-
panied by the modulation of electronic communication in
the one-electron oxidized MV state: E-4 featured more in-
tense interaction than Z-4. MV chemistry with NPCs is
quite rare, and E/Z-4 is the first molecular system that real-
izes a photonic switch of the intramolecular hole delocaliza-
tion between the two NPC sites.
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Acknowledgements
The authors acknowledge Grants-in-Aid from MEXT of Japan (Nos.
24750054 and 21108002, area 2107, Coordination Programming), the
Global COE Program for Chemistry Innovation, and the Sasakawa Sci-
entific Research Grant from The Japan Science Society for financial sup-
port.
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[8] CCDC 858938 (E-3), 865251, (E-4·CHCl3), and 858937 (Z-4).
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Keywords: carbazole · electrochemistry · fluorescence ·
photochemistry · photochromism
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Received: March 9, 2012
Published online: && &&, 0000
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These are not the final page numbers!

Visible-Light Photochromism in Diethynylethenes
COMMUNICATION
Photoisomerization: N-phenylcarba-
zole-conjugated dimethyl 2,3-diethy-
nylfumarate/maleate, E/Z-4, shows
highly efficient and reversible visible-
light E/Z photoisomerization
(Ftotal =0.40) upon excitation with an
intramolecular charge-transfer band.
This photochromism induces switching
of electronic communication between
the two N-phenylcarbazole moieties in
the one-electron oxidized mixed-valent
state (see scheme).
Photochemistry
M. Hayashi, R. Sakamoto,*
H. Nishihara* ................. &&&& — &&&&
Extremely Efficient and Reversible
Visible-Light Photochromism and
Accompanying Switch of Electronic
Communication in N-Phenylcarbazole-
Appended Diethynylethene
Chem. Eur. J. 2012, 00, 0 – 0
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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