Blue/red linear dichroic emission from a highly anisotropic crystal of triarylmethane dye conjugated with phenoxo-zinc complexes

Article abstract of DOI:10.1002/chem.201002950

We have developed a novel triphenylmethane-based hexanuclear zinc complex that exhibits peculiar photochemical and photophysical properties. Upon UV irradiation, the compound turned from colorless to reddish purple, while the color of emission turned from

Full text of DOI:10.1002/chem.201002950

DOI: 10.1002/chem.201002950
Blue/Red Linear Dichroic Emission from a Highly Anisotropic Crystal of
Triarylmethane Dye Conjugated with Phenoxo-Zinc Complexes
Hirohiko Houjou,*[a] Shunpei Takezawa,[b] Issei Oyamada,[b] Kazunari Matsumura,[b]
Hidetake Seino,[a] Isao Yoshikawa,[a] Yasushi Mizobe†,[a] and Koji Araki[a]
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FULL PAPER
Abstract: We have developed a novel
triphenylmethane-based hexanuclear
of the ligand part. It was suggested that
an intramolecular energy-transfer
mechanism operates to give rise to the
red emission. The UV treatment of a
single crystal results in simultaneous
emission of orthogonally polarized blue
and red light. This color switching,
namely linear dichroic emission was so
distinct that one can recognize with by
sight through optical microscope. The
columnar arrangement of molecules in
the crystal clearly accounts for the ob-
served polarization of the emission.
zinc complex that exhibits peculiar
photochemical and photophysical prop-
erties. Upon UV irradiation, the com-
pound turned from colorless to reddish
purple, while the color of emission
turned from blue to red. The color
change was attributed to an oxidation
Keywords: crystal engineering · en-
ergy transfer · luminescence · polar-
ized emission · transition metals
Introduction
oped[10] and applied to a variety of functional dyes, including
nonlinear optical materials and near-infrared dyes.[11–14]
Also, the combinations with various inorganic materials lead
to new functions utilizing the intrinsic electronic properties
of TAM and the related dye compounds.[15] However, there
have been few studies on well-defined metal complexes as-
sembled with a TAM core. In this paper, we will present a
novel triangular triphenylmethane derivative that has m-phe-
noxo-bridged dinuclear zinc moieties at each apex. This C3-
symmetric hexanuclear Zn complex was crystallized in hex-
agonal rods and exhibited interesting photoproperties, in-
cluding UV-induced color change and blue/red bimodal
emissions with orthogonal polarization planes. We describe
herein the relationship between the molecular structure,
crystal packing, and spectroscopic properties of this materi-
al.
Organic molecules with multiple emission modes in the visi-
ble region have recently attracted a great deal attention,
largely because they are promising candidates as chromo-
phores in white-light emitting devices, displays, computer
memories, switches in wave optics, chemical sensing probes,
and so forth.[1] Thus far, several attempts have been made to
create such molecules with efficient emission over multiple
modes. Although solid-state luminescence is more interest-
ing and useful for industrial applications, various intra- and
intermolecular interactions in the condensed phase often
result in diminished emission efficiency. Crystal packing or
aggregation pattern can play a decisive role in controlling
solid-state luminescence,[2] which is probably related to the
mutual arrangement of the transition moment vectors of the
chromophores.
A highly axle-symmetric molecule with a rigid core can
be a useful building unit that forces functional moieties to
have a preferred arrangement in solid state.[3] Triarylme-
thane (TAM) derivatives meeting this prerequisite have
been extensively studied and applied to supramolecular
complexes,[4] nano-scale fibers,[5] self-assembled monolay-
ers,[6] and liquid crystals.[7] Furthermore, it is interesting that
the “leuco” form of TAM suffers oxidative dissociation by
photoirradiation, releasing hydride, hydroxide, or cyanide
from the central carbon atom.[8] As a result, a virtually
planar methylium p-system is responsible for the vivid
colors of synthetic dyes such as crystal violet.[9] By virtue of
this interesting property, a variety of TAMs with different
aryl groups, including heteroaromatic rings, were devel-
Results and Discussion
Compound 1H3 was synthesized from 4,4’,4’’-trihydroxytri-
phenylmethane by converting the phenol groups into 2,6-bis-
(N,N-bis(2-pyridylmethyl)aminomethyl)phenol
units. Subsequently, 1H3 was complexed with zinc acetate
G
(bpmpH)
(Scheme 1, further details are available in the Supporting In-
ꢀ
ꢀ
formation). Addition of PF6 or BF4 ions to the solution of
[Zn61
AHCTUNGTRENNUNG
tion of a hexagonal crystalline solid precipitate. The crystal-
line precipitate was colorless to slightly pink in color. Al-
though the H NMR spectrum showed a series of broadened
peaks in the methylene region, comparison with the spec-
trum of a model compound shows that every methylene
proton couples with its geminal counterpart (Figure S1 in
the Supporting Information). This observation suggests a
suppressed intramolecular motion due to the rigidity and
steric hindrance of the molecular framework.
1
[a] Prof. H. Houjou, Dr. H. Seino, I. Yoshikawa, Prof. Y. Mizobe,
Prof. K. Araki
Institute of Industrial Science, University of Tokyo
4-6-1 Komaba, Meguro-ku, Tokyo 153-8505 (Japan)
Fax : (+81)3-5452-6367
We obtained a single crystal of [Zn61
ble for X-ray crystal-structure analysis (Figure 1). The cen-
tral methine carbon of [Zn61
(OAc)6]3+ unit lies on the crys-
ACHTUTGNNERNUG(OAc)6]CAHTUNGTRNEN[UGN BF4]3 suita-
E-mail: houjou@iis.u-tokyo.ac.jp
[b] S. Takezawa, I. Oyamada, Prof. K. Matsumura
Department of Material Science
AHCTUNGTRENNUNG
tallographic c axis, and one third of the unit is crystallo-
Shibaura Institute of Technology, 3-7-5 Toyosu
Tokyo135-8548 (Japan)
graphically independent. There are two more independent
[Zn61
of these three are essentially same. The structure of each
[Zn2A
(bpmp)]+ unit was similar to those reported for analo-
(OAc)6]3+ units in the crystal, although the structure
ACHTUNGTRENNUNG
[†] Deceased.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002950.
CTHUNGTRENNUNG
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H. Houjou et al.
Scheme 1. Synthesis and structure of the triphenylmethane-based hexanu-
clear complex. i) dimethylamine, paraformaldehyde, reflux in MeOH,
30 min; ii) bis(2-pyridylmethyl)amine, liquid paraffin, 1308C, 30 h; iii)
zinc acetate dihydrate, MeOH.
Figure 1. Crystal structure of [Zn61ACHTUNRGTENNUG(OAc)6]AHCTUNGTRNEN[UGN BF4]3. Top: A representative
hexanuclear unit. Bottom: the crystal packing viewed along the c axis. In
bottom view the methine carbon atom and its surroundings are shown in
pink. The zinc atoms and its surroundings are shown as gray polyhedra.
gous dinuclear complexes.[16] The central methine carbon
adopts a considerably flattened pyramidal structure (angles
of H-CACHTUNGTRENNUNG(methine)-CACHTUNGTRENNUNG(phenyl) are 103.3, 105.0, and 105.38),
probably due to steric repulsion among the [Zn
units.
(bpmp)]+
2ACHTUNGTRENNUNG
A single crystal of [Zn61
100 mm3 in size) was set in an optical microscope and irradi-
ated with high-pressure Hg lamp (100 W, 380>l>
ACHTUNGTRENNUG(OAc)6]ACHTUNGTREN(NGUN BF4)3 (ca. 10ꢁ10ꢁ
a
330 nm). At the beginning of the irradiation, the crystal ex-
hibited a blue emission, but as time elapsed, the emission
turned a reddish orange through reddish purple. Moreover,
we found that the emission was a mixture of blue and red
rays that were orthogonally polarized (Figure 2). Simultane-
ous with the emission change, the crystal turned reddish
purple.
The emission spectra (l>420 nm) of the single crystal
were measured by varying the transmission axis of the ana-
lyzer relative to the crystallographic c axis (Figure S2 in the
Supporting Information). The spectra measured under con-
ditions in which the transmission is parallel to the c axis (c-
parallel) showed an emission band with the highest peak at
490 nm, which weakened as time elapsed. The spectra mea-
sured under conditions in which the transmission is perpen-
dicular to the c axis (c-perpendicular) showed a bimodal
band with emission maxima at 460 and 620 nm. A sharp de-
pression around 550 nm is attributable to re-absorption due
Figure 2. Optical micrographs of a single crystal of [Zn61ACTHNURGTENN(UG OAc)6]ACHTUNGTRENNUNG[BF4]
after UV irradiation for about 5 min. The emission images shown in a)
and b) were taken using parallel and perpendicular positions, respective-
ly, of the analyzer transmission axis. The transmission images shown in c)
and d) are of the same sample observed using parallel and perpendicular
polarizer positions, respectively.
to a photoproduct (vide infra). The emission at 460 nm de-
creased more rapidly as time elapsed and corresponded to a
relative increase of emission at 620 nm. Namely, the photo-
product has an emission around 620 nm, which compensates
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Dichroic Emission
FULL PAPER
for the decrease of the original emission. The spectral
changes seemed saturated within an irradiation time of
32 min (Figure S2 in the Supporting Information), and we
confirmed by microscopic observation that the crystal
hardly collapsed during this period. The spectra of the satu-
rated state strongly showed linear dichroism in the emission
(Figure 3). No such distinct blue/red switching has been re-
ported so far in studies of dichroic emission or polarized
emission.[17]
We observed similar photoluminescence behavior for
[Zn61ACHTNUTRGENN(GU OAc)6]CAHTUNGTRNEN[UGN PF6]3 (Figure S4 in the Supporting Informa-
tion), though the crystal thickness was insufficient for mea-
surement of single-crystal emission spectra. To elucidate the
origin of the photoinduced color change, we allowed the PF6
salt to turn reddish orange by prolonged UV irradiation.
The absorption spectra of the photoproduct in solution
(CH2Cl2, 1ꢁ10ꢀ4 m) showed a peak around 540 nm, which
was not observed for the sample before irradiation
(Figure 4, top). This band has a shapeless profile, typical of
Figure 4. Top: Absorption spectra of [Zn61ACHTUNGTRNNEUG(OAc)6]ACHTUGNTREN(NUGN PF6)3 before radiation
(dashed line) and after UV irradiation (solid line). Absorption spectra of
aurin under basic conditions (MeOH, 1ꢁ10ꢀ5 m; dotted line). Bottom:
&
Fluorescence spectra (solid lines: (&) lex =350 nm, ( ) lex =500 nm) and
&
excitation spectra (dashed lines: (&) lem =448 nm, ( ) lem =578 nm) of
the UV-irradiated sample of [Zn61ACTHUNTRGENNUG(OAc)6]AHCTUNGTREN(NUGN PF6)3.
Figure 3. Top: Emission spectra of a single crystal of [Zn61ACHTUNRTGENN(UG OAc)6]ACHUTGNTREN[NGUN BF4]3
after 32 min of UV irradiation measured with the analyzer at c-parallel
(solid line) and at c-perpendicular (dashed line) positions. Bottom: Opti-
cal micrographs of the single crystal corresponding to the above condi-
tions.
TAM-based dyes.[9] It is instructive to compare the observed
absorption spectra (Figure 4, top) with that of aurin (1,4-
benzoquinone-bis(4-hydroxyphenyl)methide) under basic
conditions (Scheme S1 in the Supporting Information). The
1
intense absorption around 540 nm is attributed to the A1!
As can be seen from Figure 2c and d, the degree of ab-
sorption is also strikingly dependent on the polarization
plane of the incident light. We measured absorption spectra
1E transition, which has two transition dipoles in the p-con-
jugated plane.[9] Apparent similarity between these spectra
suggests that the photoproduct involves an oxidized quinoi-
of a single crystal of [Zn61
G
N
dal form, such as [Zn62ACTHNGUTERNNUG
(OAc)6]4+ (Scheme 2). Based on the
the transmission axis coincides with the crystallographic c
axis (Figure S3 in the Supporting Information). By compari-
son of the above results with the crystal structure, it is evi-
dent that the observed linear dichroic emission and absorp-
tion are related to the TAM framework stacked along the c
axis. We assume that the chromophore of the photoproduct
has transition dipoles parallel to the a and b axes, implying
that an extended p-conjugated system was formed in the
plane perpendicular to the c axis.
ratio of absorbance (1:7), we estimate the gross ratio of
photo-conversion to be 10–20%. Since the extremely flat-
tened pyramidal structure around the central methine
carbon atom (Figure 1, top) suggests a predisposition for ar-
omatization, this photoreaction does not cause serious strain
in the crystal lattice.
Fluorescence and excitation spectra were also measured
for the UV-irradiated sample of [Zn61
ACHTUNGTRENNUG(OAc)6]AHCTUNGTRENN(GUN PF6)3 in solu-
tion (CH2Cl2, 1ꢁ10ꢀ4 m). Upon excitation at 350 nm, we ob-
Chem. Eur. J. 2011, 17, 1122 – 1127
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H. Houjou et al.
[Zn61
ACHTUNGTREN(NUG OAc)6]ACHTUNGTREN(NGUN BF4)3 crystal under c-perpendicular conditions
(Figure 3, bottom). The aurin-like core then emits at 600 nm
1
1
corresponding to E! A1 transition, for which the transition
dipoles are perpendicular to the c axis. This proposed mech-
anism accounts for the linear dichroic emission observed for
the hexagonal crystals of [Zn61
(OAc)6][PF6]3 (Figure 5).
ACHTUGTNRNENUG(OAc)6]AHCTUNGTRENN[GUN BF4]3 and [Zn61-
N
ACHTUNGTRENNUNG
Scheme 2. Structure of the oxidized form of the TAM-based complex.
Figure 5. Schematic representation of the linear dichroic emission of the
[Zn62ACHTUNTRGENNU(G OAc)6]4 in the [Zn61ACHUTNGTERN(NUGN OAc)6]X3 crystal.
served an intense emission band with a peak at 448 nm, ac-
companied by a weak band around 570 nm (Figure 4,
bottom). Note that the weak band around 570 nm was com-
pletely absent in the spectrum of the non-irradiated sample
(Figure S5 in the Supporting Information). An excitation
spectrum for the emission at 448 nm shows two peaks at 320
and 370 nm, which are roughly in agreement with the ab-
+
Conclusion
In summary, we have developed a novel conjugated dye
molecule composed of a TAM unit and m-phenoxo-bridged
dinuclear zinc complex that shows a clear linear dichroic
emission. The symmetric structure and rigidity of the molec-
ular framework allows for crystallization of the compound
in a hexagonal columnar fashion with a highly anisotropic
molecular arrangement. The molecule in the “leuco” form
undergoes oxidation to the “dye” form by UV irradiation
and then shows blue/red dual emissions that have perpendic-
ular polarization planes. By changing the direction of the di-
chroic filter, the blue/red switching is so distinct that it is
recognizable with the naked-eye. We attribute the red emis-
sion to the central aurin-like moiety and intra- and intermo-
lecular energy-transfer mechanisms that effectively distrib-
ute the polarized light with different wavelength. In this
system, the photophysical property of aurin, which is nor-
mally non-fluorescent, was modified by embedding it in
more favorable molecular surroundings. This study clearly
demonstrates that appropriate molecular design can lead to
the development of new materials with unique properties
that originate from the molecular arrangement.
sorption maxima of [Zn61ACHTUNRTGENN(UG OAc)6]AHCTUNGTRENN[UNG PF6]3, while the intensity
at 370 nm is prominently higher in the excitation spectrum.
On the other hand, the excitation spectrum for the emission
at 578 nm shows an intense peak at 548 nm, while the afore-
mentioned peaks around 320 and 370 nm are relatively
small. The profile around 548 nm is quite similar to the ab-
sorption band of aurin, suggesting that the newly appeared
emission around 570 nm is attributable to the oxidized qui-
noidal form of [Zn61ACTHNUTRGNEUNG
(OAc)6]3+. As expected, altering the ex-
citation wavelength to 500 nm resulted in the observation of
an intense emission band with a peak at 578 nm. Note that
aurin itself does not fluoresce either in solution or in the
solid state due to vibrational energy loss from fluctuation of
the phenyl rings. The weak but observable emission from
the aurin core in the complex may have resulted from the
bulkiness of the chelating moiety that suppresses the intra-
molecular motion. From the excitation spectra described
above, we conclude that irradiation at 330 nm can cause
emission at 578 nm through a partial energy transfer from
the peripheral zinc complex units to the central aurin-like
moiety.
It is possible that a similar process occurs in the crystal-
line state. Namely, irradiation of UV light causes electronic
Experimental Section
excitation in the [Zn2ACTHNUTRGNEUNG
(bpmp)]+ units which emit at 490 nm.
Spectroscopic measurements: The absorption and luminescent spectra of
a single crystal was measured by using a conventional polarized optical
microscope (Olympus, BX-51) equipped with an optical fiber that was
connected to a spectrometer (Ocean Optics, USB4000). For absorption
spectra, a halogen lamp was used as a light source, and the transmission
with wavelength of 400–800 nm was detected. For luminescent spectra,
high pressure Hg lamp (output 100 W) was employed with a band-pass
Simultaneously, partial energy transfer occurs between the
complex units to the aurin-like moiety in the photo-oxidized
species. In addition, re-absorption of the emitted light by
the aurin-like moiety is implied from the sharp depression
around 540 nm in the emission spectra observed for the
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Dichroic Emission
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Received: October 13, 2010
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