Aggregation-induced emission active D-π-A binaphthyl luminophore with dual-mode fluorescence

Article abstract of DOI:10.1039/c4ra05871b

A novel aggregation-induced emission active dye consisting of a simple and highly solvatochromic D-π-A type 1,1′-binaphthyl chromophore was synthesised. The new dye exhibited dual-mode fluorescence in response to different types of restriction of intramolecular rotation; aggregation induces green emission, and blue emission is observed in a glassy matrix. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra05871b

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Aggregation-induced emission active D-p-A
binaphthyl luminophore with dual-mode
Cite this: RSC Adv., 2014, 4, 33474
fluorescence†
a
a
b
ac
Received 17th June 2014
Accepted 23rd July 2014
*
Shunsuke Sasaki, Yosuke Niko, Kazunobu Igawa and Gen-ichi Konishi
DOI: 10.1039/c4ra05871b
www.rsc.org/advances
A novel aggregation-induced emission active dye consisting of a However, there are still limited number of D-p-A-type AIE
simple and highly solvatochromic D-p-A type 1,10-binaphthyl chro- luminophores and their molecular structures oen have stil-
mophore was synthesised. The new dye exhibited dual-mode fluo- bene moiety or giant molecules with numerous aryl–aryl
rescence in response to different types of restriction of intramolecular bonds.1,3–8
rotation; aggregation induces green emission, and blue emission is
To extend the scope of D-p-A-type AIE luminophores for
further application, we propose a novel approach to maximise
the restriction of intramolecular rotation (RIR),1 which is
instrumental in the manifestation of AIE, without relying on a
stilbene structure or numerous aryl–aryl bonds. Recently, we
discussed the correlation between excited state polarisation and
the uorescence quantum yield of D-p-A type organic dyes and
pointed out that an organic dye exhibiting strong polarisation
in its excited state tends to be quenched by efficient internal
conversion in polar media.9 According to the discussion, the
excited state of a D-p-A system with strong polarisation should
show AIE without stilbene or multiple aryl–aryl bond structures.
Therefore, we focused on a biaryl D-p-A system, known for its
strong polarisation in the excited state.10,11 By choosing the
appropriate donor–acceptor pair, we could switch the charge
transfer character in the excited state by controlling the angle of
rotation of the biaryl moiety. This strategy enabled us to develop
a novel AIE luminophore with multi-mode luminescence.
In this paper, we report a dual-mode uorescent D-p-A-type
biaryl dye (4-dialkylamino-40-formyl-1,10-binaphthyl) that
exhibits a green emission in its AIE state [charge-transfer (1CT)
emission] and a blue emission in a glassy matrix [localised
emission (LE)]. We succeeded in creating the rst AIE lumino-
phore with a drastic change of its dual-mode emission in
response to RIR, i.e., aggregation and embedding of a lumino-
phore in a glassy matrix.
observed in a glassy matrix.
While a typical organic luminophore shows uorescence
quenching upon aggregation, several classes of organic dyes
exhibit enhanced emission properties with aggregation. Since
this phenomenon was rst discovered by Tang,1 aggregation-
induced emission (AIE)2 has been considered an attractive
topic for applications such as chemosensors, bioimaging
probes, and OLEDs.1,3,4 In particular, AIE luminophores based
on donor and acceptor moieties have been extensively investi-
gated for use in an OLED3 and an in vivo bioimaging probe.4
Very recently, AIE luminophores with multi-mode uorescence
that can respond differently to various external stimuli have
been reported.5 This type of AIE luminophore makes the
multiplex sensing, where different biological substances and
their interaction are simultaneously detected, feasible and there
is an urgent demand to develop multi-mode uorescence from
easily synthesisable, biocompatible, and simple AIE lumino-
phores. D-p-A-type AIE luminophores should be promising
candidates for multiplex sensing applications because they
occasionally exhibit various excited state species, such as the
TICT state, in response to the surrounding environment.6
aDepartment of Organic and Polymeric Materials, Tokyo Institute of Technology,
2-12-1-H-134 O-okayama, Meguro-ku, Tokyo 152-8552, Japan. E-mail: konishi.g.
aa@m.titech.ac.jp; Fax: +81-3-5734-2888; Tel: +81-3-5734-2321
The novel D-p-A-type 1,10-binaphthyl dye 5 was readily syn-
thesised as depicted in Fig. 1. 1,4-Dibromonaphthalene (1) was
converted to 4-bromo-1-piperidylnaphthalene (2) by Buchwald–
Hartwig coupling,12 and the residual bromine was subsequently
borylated with a palladium catalyst to obtain the corresponding
boron pinacol ester 3.13 The synthesized ester 3 underwent
Suzuki–Miyaura coupling14 with 1-bromo-4-formylnaphthalene
bInstitute for Materials Chemistry and Engineering and Department of Molecular and
Material Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan
cPRESTO, Japan Science and Technology Agency (JST), Japan
† Electronic supplementary information (ESI) available: Experimental section:
synthetic procedures, characterizations (1H NMR, 13C NMR, FT-IR spectra),
UV-vis absorption and uorescence spectra, and X-ray crystallographic data.
CCDC 1006100. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4ra05871b
1
(4), obtained from the formylation of 1, to afford 5. H NMR,
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luminescent brightly by suppressing the torsional vibration21 of
its aryl–aryl bond.
The UV-Vis absorption and uorescence spectra of 5 were
measured in a DMF–water mixture with a varying volume frac-
tion of water fw ¼ 0, 0.2, 0.4, 0.6, and 0.8 (Fig. 2). The absorption
peak obtained in fw ¼ 0.8 is signicantly different from those at
other fw values, indicating the formation of an aggregated
state.22 The formation of an aggregate in fw ¼ 0.8 was coincident
with a drastic enhancement in the luminescence; therefore, 5
was conrmed to exhibit aggregation-induced emission (AIE).
Absolute photoluminescence quantum yield (FF) measure-
ments veried the AIE activity of 5. (Table 2) While no lumi-
nescence is observed for fw ¼ 0–0.6, FF reaches a value of 12%
when fw ¼ 0.8 (Table 2). Because the emission maximum of the
aggregated state (lem ¼ 531 nm) is of a much longer wavelength
than those of 1-(N,N-dimethylamino)naphthalene (lem ¼ 385
nm)23 and 1-naphthaldehyde (lem ¼ 420–480 nm),24 it is
reasonable to assign the emission in the aggregated state to a
Fig. 1 Synthesis of D-p-A-type 1,10-binaphthyl dye 5. Reagents and
conditions: (a) piperidine, Pd(OAc)2, BINAP, t-BuONa, toluene, reflux, 3
h, 76%; (b) Pd(PPh3)2Cl2, bis(pinacolato)diboron, AcOK, 1,4-dioxane,
reflux, 12 h, 68%; (c) n-BuLi, anhydrous THF, ꢀ78 ꢁC, 30 min; then
DMF, r.t., 87%; (d) Pd(PPh3)4, Na2CO3, toluene–EtOH–H2O ¼ 5/2/1,
reflux, 12 h, 61%.
13C NMR, FT-IR, melting point measurements, and elemental
analysis were performed to conrm the chemical structure of 5
(for detailed characterization data, see ESI†).
1
transition from the charge transfer state (1CT / S0). Single-
crystal X-ray crystallography reveals that the dihedral angle
between the naphthalene rings of 5 is xed at 63–70ꢁ, where the
naphthalene rings of 1,10-binaphthyl would severely repel each
other.25 Therefore, it can be concluded that the aggregation not
only suppresses the torsional vibration of 5 (RIR effect),1 but
also enhances the planarity of the 1,10-binaphthyl backbone.
Furthermore, since such an AIE activity was not observed from
bare 1,10-binaphthyl (Table S2†), it is reasonable that the AIE
The UV-Vis absorption and uorescence spectra of 5 were
recorded in various solvents (Fig. S6 and S7†). The obtained
absorption maxima labs [nm], uorescence maxima lem [nm],
Stokes shis D~nSt ¼ (~nabs ꢀ ~nem) [cmꢀ1], and absolute photo-
luminescence quantum yields FF are summarized in Table 1
(molar extinction coefficients 3 [Mꢀ1 cmꢀ1] and uorescence
lifetimes s [ns] are listed in Table S1†). The uorescence
maxima exhibit an extraordinary spectral shi, within a narrow
window of solvent polarity, on changing from hexane to
dichloromethane. The uorescence solvatochromism exhibits a
linear dependence on the solvent polarity parameter ET(30)15
(Fig. S8†). The Lippert–Mataga plot16 indicates that 5 exhibits a
prominently strong polarisation (increase in dipole moment
with photoexcitation Dm ¼ 17.7D) compared to other sol-
vatochromic dyes.17,18 As we have previously pointed out,9 such a
highly polarised excited state tends to be controlled by the
energy gap rule19 on internal conversion and is therefore effi-
ciently quenched in polar solvents. Indeed, the uorescence of 5
was strongly quenched in response to an increase in solvent
polarity (Table 1), and furthermore, also weak luminescence in
hexane supports our previous hypothesis20 about emission
property of solvatochromic dyes. Therefore, 5 can be made to be
phenomena of
functionalization.
5
was induced by donor–acceptor
Fig. 2 UV-Vis (a) and fluorescence (b) spectra in DMF–water. (c)
ORTEP drawing of 5.
Table 1 Absorption and fluorescence maxima of 5a
labs
lem
Table 2 Spectroscopic parameters of 5 in DMF–water
Solvent
ET(30)
[nm]
[nm]
D~nSt [cmꢀ1
]
FF
Entry
labs [nm]
lem [nm]
lem [nm] in QYa
FF
Hexane
Toluene
Diethyl ether
THF
Ethyl acetate
Dichloromethane
31
330
332
330
330
329
331
455
510
522
576
577
615
8325
10 513
11 146
12 942
13 064
13 951
0.04
33.9
34.5
37.4
38.1
40.7
0.37 fw ¼ 0
330.0
331.0
329.0
329.0
340.0
No uorescence
No uorescence
No uorescence
470–550
—
—
—
502
531
—
—
—
0.02
0.12
0.34 fw ¼ 0.2
0.11 fw ¼ 0.4
0.06 fw ¼ 0.6
0.04 fw ¼ 0.8
526.4
a
a
The values in the brackets indicate the relative coefficient of each term
of the uorescence lifetime.
These uorescence maxima were obtained from the QY measurements
(Fig. S10†) to identify accurate lem in fw ¼ 0.6.
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Although the suppression of torsional vibration is common applied to a broad range of chromophores in which the pop-
to both states, 5 in a glassy matrix showed different photo- ulation of the 1CT state competes with that of the locally excited
physical behaviour from that observed in the aggregated state. state.
The uorescence spectra of 5 were measured in toluene at 298 K
In conclusion, we have synthesised novel D-p-A-type 1,10-
and 77 K (Fig. 3a). As shown in Fig. 3a, solidifying the toluene binaphthyl dye 5 as an easily synthesisable, small, and simple
solution to a glassy state resulted in a signicant hypsochromic AIE luminophore. The new dye 5 exhibited a strong uorescence
shi in the uorescence maximum of 5. The emission at very solvatochromism in a narrow window of solvent polarity. The
short wavelengths is similar to that observed for the 1Bb / 1S0 aggregation of 5 resulted in an increase in the absolute photo-
transition of 1-naphthaldehyde;24 therefore, it is assignable to luminescence quantum yield. While the aggregated state
the localised emission (LE) from the 1-napthaldehyde moiety of emitted green uorescence, 5 in a glassy matrix showed blue
5. The different modes of emission of 5 in its aggregated state luminescence. The blue emission in glassy matrix implies its
and in solid media can be explained by the planarity of the 1,10- viscosity-responsiveness, like molecular rotors27 used for lipid
binaphthyl backbone. Although the steric repulsion between membrane imaging.28 Such a drastic dual-mode uorescence
the naphthalene rings of 1,10-binaphthyl becomes extremely from an AIE luminophore is unprecedented. In future, we will
severe as the dihedral angle of the aryl–aryl bond gets smaller investigate that the impact of donor–acceptor functionalization
than q z 60–65ꢁ,25 it is known that an even more planar to activate the dual-mode uorescence and AIE.
conformation is needed to populate an excited state polarised
along the long axis, such as the 1CT state (Fig. 3b).26 According
to the discussion on the torsional potential of the 1,10-
Notes and references
binaphthyl backbone, the emission from the 1Bb state in the
glass matrix can be ascribed to the suppression of torsional
motion, which causes the binaphthyl backbone to be unable to
overcome the steric repulsion (Fig. 3b). On the other hand, as
mentioned above, the aggregated state enhances the planar-
isation of the 1,10-binaphthyl backbone (Fig. 2c), and therefore,
the 1CT state, rather than the 1Bb state, would become sterically
advantageous. According to this hypothesis, such a dual-mode
uorescence that depends on the manner of RIR may be
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