Aggregation induced emission (AIE) of trifluoromethyl substituted distyrylbenzenes

Article abstract of DOI:10.1039/c2cc32380j

The AIE properties of two trifluoromethyl substituted distyrylbenzene model compounds were compared. The fluorescence quantum efficiency of these molecules can be modulated by tuning their subtle solid-state intermolecular interactions.

Full text of DOI:10.1039/c2cc32380j

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COMMUNICATION
Aggregation induced emission (AIE) of trifluoromethyl substituted
distyrylbenzenesw
Zhengwei Shi,a Joshua Davies,a Sei-Hum Jang,a Werner Kaminskyb and Alex K.-Y. Jen*ab
Received 2nd April 2012, Accepted 11th June 2012
DOI: 10.1039/c2cc32380j
The AIE properties of two trifluoromethyl substituted distyryl-
benzene model compounds were compared. The fluorescence quantum
efficiency of these molecules can be modulated by tuning their subtle
solid-state intermolecular interactions.
Fluorescent molecular materials have been intensively studied
as active media for organic light-emitting diodes (OLEDs),
lasers, and sensor applications.1,2 Many of the fluorescent
molecules have efficient emission in solution, however, the
emission tends to vanish in solid-state due to aggregation-
Fig. 1 Chemical structures of MeO-CF3DSB (a) and CF3DSB (b), and
caused quenching (ACQ). Although ACQ is quite detrimental
to material performance in devices,3 recent reports from
single crystal X-ray structures of MeO-CF3DSB (c) and CF3DSB (d).
Tang et al. and Park et al., showed that several molecular
supplementary information, ESI, for the detailed synthesis
systems can display strong fluorescence upon concentration or
solidification, while their dilute solutions show weak emission.4–6
The aggregation-induced emission (AIE) phenomenon is generally
ascribed to changes of molecular geometry in solid-state, which
result in suppressed non-radiative relaxation pathways of excited
species to enhance fluorescence efficiency. More recently, the result
from Dinca et al. suggested that spatial isolation of the rigid
p-conjugation framework of a molecule with bulky substituents is
effective in enhancing molecular fluorescence.7
and characterizationw).
CF3 is known to be a strong electron-withdrawing group
and is able to modulate material properties.9 Unlike the linear
cyano group (CN),10 the CF3 group imposes a significant
spatial constraint on the molecular skeleton due to its steric
hindrance. To our surprise, the study of the single crystal
structure of CF3DSB reveals that CF3 substituents impede the
molecule from adopting the expected anti-conformation.
Moreover, severe torsion between lateral phenyl groups and
the phenylenevinylene core is observed. As a result, the
effective p conjugation is interrupted by the syn-defect.11
The highly electronegative nature (EN = 4.0 according to
the Pauling scale) of the F atoms on the CF3 group also tends
to induce hydrogen bonding with surrounding hydrogen
atoms. As a result, it not only significantly affects the mole-
cular geometry, but also creates a pronounced influence on the
molecular packing modes in solid-state.
In this communication, we report the design, synthesis,
crystal structure, and optical properties of a novel AIE
molecule – a methoxy-substituted trifluoromethyldistyrylbenzene
(MeO-CF3DSB) (Fig. 1a). Especially, the emphasis is placed on
the role of intermolecular interactions in enhancing solid-state
fluorescence. For comparison, the unsubstituted trifluoromethyl-
distyrylbenzene (CF3DSB) (Fig. 1b) was also synthesized. The
CF3DSB was first reported by Holmes et al.8 which contains a
bulky trifluoromethyl (CF3) group on its vinylenic linkage.
The target compound, MeO-CF3DSB, was synthesized
in 79% yield using the Wittig–Horner reaction to condense
2,5-bis(diethoxyphosphorylmethylene)-1,4-dimethoxybenzene
and 40-methoxy-2,2,2-trifluoroacetophenone (see the electronic
The CF3 group also inductively affects the peripheral phenyl
rings even though it has a considerable twisted angle to the
p electron core. This creates subtle changes on packing of
MeO-CF3DSB, which has electron-donating methoxy groups
at the para positions of its peripheral phenyl rings. The
methoxy groups add additional resonance structures at the
peripheral phenyl rings. Two additional methoxy groups were
also introduced onto the central phenyl group in order to
increase the resonance conjugation path of p electrons along
the phenylenevinylene core.
a Department of Materials Science and Engineering, University of
Washington, Seattle, Washington 98195, USA.
E-mail: ajen@u.washington.edu
b Department of Chemistry, University of Washington, Seattle,
Washington 98195, USA
Fig. 2 shows the 1H NMR (A) and 19F NMR (B) spectra of
MeO-CF3DSB and CF3DSB. The analysis of the 1H NMR
spectra reveals that both molecules are centrosymmetric, and
w Electronic supplementary information (ESI) available. CCDC
884441 and 874372. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c2cc32380j
c
7880 Chem. Commun., 2012, 48, 7880–7882
This journal is The Royal Society of Chemistry 2012

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absorption spectrum of MeO-CF3DSB shows a noticeable
shoulder at 274 nm, which is corresponding to the localized
transitions of the terminal ends of the molecule. Both MeO-
CF3DSB and CF3DSB (see inset of Fig. 3) show blue fluores-
cence in their solid form, but only show very weak fluorescence
in dilute THF solution. The fluorescence quantum efficiencies
(Ffl) of MeO-CF3DSB and CF3DSB powders are 56.3% and
11.2%, respectively (Ffl in solid-state is obtained in a calibrated
integrating sphere, using 9,10-diphenylanthracene as the standard),
which clearly indicates the AIE characteristics of MeO-CF3DSB
and CF3DSB. However, there is a pronounced Ffl difference
between CF3DSB and MeO-CF3DSB. In addition, the non-
emissive dilute solutions of MeO-CF3DSB and CF3DSB
(10 mM in THF) become strongly fluorescent when a significant
amount of water was added to induce dispersed aggregates as
is commonly used to study AIE molecules; (See inset of Fig. 3
and Fig. S3w).6
To investigate the effect of spatial arrangement of MeO-
CF3DSB on its fluorescence in solid-state, single crystals of
MeO-CF3DSB and CF3DSB were grown by slow evaporation
of chloroform solvent under ambient condition, and the X-ray
diffraction results of single crystals were refined and analyzed.
MeO-CF3DSB forms monoclinic crystals having the P21/c
space group and there are two molecules present in a unit
cell, as shown in Fig. 1c. A full crystal analysis of collected
data is provided in Table S1 in the ESI,w where the crystallographic
information of CF3DSB (Fig. 1d) is also provided for comparison.
As shown in Fig. 1c, the X-ray crystal structure of MeO-CF3DSB
unequivocally confirms that the molecule is in syn-conformation
and possesses a high symmetry with an inversion center, which
is consistent with the NMR results. An important feature of
the MeO-CF3DSB crystal, akin to CF3DSB, is that the
through-space distance of the intramolecular Fꢀ ꢀ ꢀH bonds
(C12–F1ꢀ ꢀ ꢀH4–C4: 2.26 A) are much shorter than the
sum of the van der Waals radii for H and F (H = 1.20 A,
F = 1.47 A). Even though it is known in the literature that
the interactions between donor(D)–Hꢀ ꢀ ꢀacceptor(A) angle
below 120 1C are less significant,16 it can be regarded as a
beneficial contact on the vinylenic segments for the molecular
syn-conformation.
Fig. 2 1H NMR (A) and 19F NMR (B) spectra of MeO-CF3DSB
(top) and CF3DSB (bottom) in chloroform-d3. The spectral peaks are
not on the same vertical scales.
the protons on the vinylenic segments appear to be a clear
doublet with a coupling constant of 1.5 Hz. Consequently, the
19F NMR spectra of MeO-CF3DSB and CF3DSB show a
distinct doublet peak with the similar corresponding coupling
constant. The coupling constant indicates the existence of
intramolecular C–Fꢀ ꢀ ꢀH–C hydrogen bonding on the vinylenic
segments of the molecule.12 Moreover, the clear difference of
chemical shift between the aromatic proton b (d, 6.90 ppm) and
c (d, 7.21 ppm) on the lateral phenyl ring of MeO-CF3DSB,
compared to those of CF3DSB (proton b0, 7.23 ppm; proton c0,
7.34 ppm), strongly suggests the anisotropic electron distribu-
tion within the para-substituted peripheral phenyl rings of
MeO-CF3DSB with the substituent of methoxy groups. The
natural bond orbital (NBO) study clearly corroborates that the
substitution of methoxy groups poses a considerable influence
on the p electron delocalization in both the phenylenevinylene
core and the peripheral phenyl moieties of MeO-CF3DSB as
shown in Fig. S2.w13
Both MeO-CF3DSB and CF3DSB molecules are soluble in
common organic solvents such as THF, chloroform, and
acetone, but insoluble in water. The absorption spectra of
MeO-CF3DSB and CF3DSB in dilute THF solution are shown in
Fig. 3, with lmax at 370 nm and 297 nm, respectively, corresponding
to the main electronic transition of the phenylenevinylene core
(Fig. S1w).14 It is worthy to note that MeO-CF3DSB and CF3DSB
show significant hypsochromic shift of the lowest energy electronic
transitions compared to the non-CF3 substituted DSB analogues.15
The effective conjugation is confined between the CF3
groups on the phenylenevinylene core owing to highly twisted
terminal phenyl groups. By comparing with CF3DSB, the
In addition, the severe torsion between the lateral phenyl
group and the central phenylenevinylene framework is a
Fig. 3 Absorption (in THF solution) and emission (in solid powders)
spectra of MeO-CF3DSB (red) and CF3DSB (black). Inserted photo-
graphs of MeO-CF3DSB in powder form (a), 10 mM THF solution (b),
10 mM THF-water mixture (fw = 90%) (c) taken under UV illumination
with the excitation wavelength of 365 nm.
Fig. 4 Crystal structure and short contact in MeO-CF3DSB single
crystal; (a) unit cell packing diagram viewed down the b axis;
(b) packing of terminal phenyl rings viewed down the b axis.
c
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Chem. Commun., 2012, 48, 7880–7882 7881

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Table 1 Physical properties of MeO-CF3DSB and CF3DSB
lab (nm)a
lem (nm)b
Solutionc
Aggregated
Solution (Ffl)c
Solid (Ffl)e
DHfusion (kcal molꢁ1
)
f
f
Dye
Tm (1C)
MeO-CF3DSB
CF3DSB
370 (275)
297
369 (275)
297
487 (o0.010)
409 (o0.010)
490 (0.563)
409 (0.112)
183
149
11.24
6.71
a
b
Absorption maximu, ꢂ1 nm. Emission maximum, ꢂ1 nm; emission spectrum taken at a lex of 380 nm for MeO-CF3DSB and 310 nm for
c
d
e
f
CF3DSB. In THF (10 mM). In a THF/water mixture (10 : 90 by volume). In solid powder form, ꢂ0.002. Analytic results of differential
scanning calorimetry (DSC) at a heating rate of 10 1C minꢁ1 on thermo-equilibrated samples: Tm, melting temperature, ꢂ1 1C; DHfusion energetics
for melting transition, ꢂ0.1 kcal molꢁ1
.
distinct feature. Consistent with the large torsion angle for
CF3DSB (84.31), the torsion angle for MeO-CF3DSB is 78.11.
These large dihedral angles hamper the effective charge transfer
between the lateral phenyl group and the phenylenevinylene
core. As a result, their effective conjugation lengths are much
shorter than all anti-linkage in un-substituted DSB analogues.
Moreover, the methoxy substituents on MeO-CF3DSB pose a
significant influence on molecular packing in solid-state. As
shown in Fig. 4a and Fig. S4,w an interlayer network was
formed along the b axis. In addition to the close contacts along
the columns, a network of close contacts within each column is
also observed.
its emission efficiency can be modulated by fine-tuning the
subtle intermolecular interactions in solid-state. This thorough
investigation provides useful insights for rational design of
highly efficient AIE dyes for optoelectronics and sensing
applications.
The authors thank the support from the National Science
Foundation (DMR-0120967). A. K.-Y. J. thanks the Boeing
Foundation. The authors thank A. Holmes for providing
X-ray data of CF3DSB.
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In conclusion, we have demonstrated that a highly fluorescent
AIE molecule can be made with high quantum yield (56.3%) and
c
7882 Chem. Commun., 2012, 48, 7880–7882
This journal is The Royal Society of Chemistry 2012