Direct validation of the restriction of intramolecular rotation hypothesis via the synthesis of novel ortho-methyl substituted tetraphenylethenes and their application in cell imaging

Article abstract of DOI:10.1039/c4cc04241g

We demonstrate a novel synthetic approach to ortho-methyl substituted tetraphenylethene materials, which can be utilised for directly validating the restriction of intramolecular rotation hypothesis as the basic mechanism of aggregation-induced emission phenomenon and cell imaging. This journal is

Full text of DOI:10.1039/c4cc04241g

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Direct validation of the restriction of intramolecular
rotation hypothesis via the synthesis of novel
ortho-methyl substituted tetraphenylethenes and
their application in cell imaging†
Cite this:DOI: 10.1039/c4cc04241g
Received 3rd June 2014,
Accepted 9th July 2014
a
a
a
a
a
Guo-Feng Zhang, Ze-Qiang Chen, Matthew P. Aldred,* Zhe Hu, Tao Chen,
DOI: 10.1039/c4cc04241g
a
b
a
Zhenli Huang, Xianggao Meng* and Ming-Qiang Zhu*
www.rsc.org/chemcomm
We demonstrate a novel synthetic approach to ortho-methyl According to the molecular architecture designed by Tang and
6
substituted tetraphenylethene materials, which can be utilised for co-workers, the intramolecular rotation in hexaphenylsilole
directly validating the restriction of intramolecular rotation hypoth- (HPS) can be greatly impeded via the introduction of the bulky
esis as the basic mechanism of aggregation-induced emission isopropyl groups on phenyl rings with ortho-positions in silole
phenomenon and cell imaging.
regioisomers. With respect to TPEs, intensive endeavours, such
as the construction of conjugated microporous polymers, coor-
During the recent decades, a lot of attention has been paid to design dinative immobilization, locking the phenyl rings of TPE and
and synthesize novel organic fluorescent materials for the potential host–guest inclusion, have been devoted to probing the intri-
applications in sensors, optoelectronic devices and bioimaging, guing AIE-active characteristics and the possible mechanism of
7
which is probably due to the variability and flexibility of chemical the AIE phenomenon. However, the strategies for straight-
1
modifications. Unfortunately, most fluorophores encounter a forward validation of the RIR hypothesis for TPE analogues at
common problem called ‘‘aggregation caused quenching (ACQ)’’ molecular level are still seldom reported, which is probably due
in the solid state because of the intrinsic intermolecular p–p stacking to the difficulties in chemical syntheses and modifications.
2
and/or self-absorption related to a small Stokes shift. In 2001, an
Our aim was to directly verify the RIR hypothesis for the TPE
opposite and abnormal phenomenon named ‘‘aggregation-induced derivatives at molecular level via facile chemical modifications,
3
emission (AIE)’’ was observed by Tang’s group. A facile strategy and investigate the influence of steric congestion between the
to overcome the ACQ in fluorophores is by bonding them with contiguous phenyl rings in the TPE unit. However, the well-
8
AIE-active counterparts. Since then, various fluorophores with AIE developed approaches (Scheme S1, ESI†): McMurry reaction
4
9
characteristic have been reported and investigated. In the previous and Rathore’s procedures for producing TPE derivatives can
experimental observations and theoretical calculations, the proposed hardly meet the increasing demand for diverse structural varia-
hypotheses for the underlying mechanism of this new AIE pheno- tions, which is due to the impractical to access the requisite diaryl
menon are still ambiguous such as the restriction of intramolecular ketones. Therefore, it is imperative to introduce an alternative way
5
rotation (RIR), E/Z isomerisation and J-aggregation formation.
to afford the ortho-substituted TPEs to directly validate the RIR
Therefore, it is urgently desirable to acquire novel molecules via hypothesis for AIE phenomenon by chemical modifications.
simple modifications to directly validate the proposed hypothesises. Herein, we report a versatile approach to the synthesis of ortho-
Most materials possessing AIE properties studied in literature methyl substituted TPE via Corey–Fuchs reaction and subsequent
2b,4
10
are associated with silole or tetraphenylethene (TPE) motifs.
Suzuki coupling reaction. The synthetic route is outlined in
Scheme 1, and the final products were characterized by NMR, MS,
a
Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic
Information, Huazhong University of Science and Technology, Wuhan, Hubei,
4
30074, China. E-mail: mqzhu@hust.edu.cn, mpaldred@hotmail.com;
Fax: +862787793419; Tel: +862787793419
b
Key Laboratory of Pesticide and Chemical Biology of Ministry of Education,
College of Chemistry, Central China Normal University, Wuhan, Hubei 430079,
China. E-mail: mengxianggao@mail.ccnu.edu.cn
†
Electronic supplementary information (ESI) available: Experimental proce-
dures/characterisation, optical spectra, molecular simulation and crystallo-
graphic information files. CCDC 1006150–1006152. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c4cc04241g
Scheme 1 Synthetic route to novel ortho-methyl substituted TPEs.
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THF–water mixtures are probably due to the effect of agglomera-
1
1
tion and morphology transition.
As we can envisage, the steric hindrance between the relevantly
geminal phenyl rings is gradually enhanced with increasing
number of ortho-methyl groups. With respect to TMTPE, due to
the introduction of tetra(ortho-methyl) groups in TPE, the motion
or vibration of the phenyl rings is extremely suppressed, which
imparts the enhanced fluorescence in diluted solutions. In com-
parison with DMTPE, the rigidity of TMTPE molecule is probably
improved as well, on account of the steric repulsion of tetra(ortho-
methyl) groups in TPE, giving rising to fluorescence QYs boosted
from 0.32% (DMTPE) to 64.3% (TMTPE) in THF solution
(Table 1). Namely, by internal control through the introduction
of ortho-methyl groups, the PL properties of the TPE derivatives
can be modulated, indicating that steric congestion of the relevant
phenyl rings can fix the molecules and significantly inhibit the
non-radiance annihilation process, causing TMTPE to exhibit
high luminescence even in dilute solution. Based on steric
Fig. 1 (a) Normalised UV-vis absorption of TPE, DMTPE and TMTPE in
THF solution and PL spectra of TPE, DMTPE and TMTPE in solid state. (b) PL
ꢀ5
spectra of TPE, DMTPE and TMTPE in THF solution (10 M) under same
conditions (inset: their related fluorescence picture under 365 nm irradia-
tion). (c) Emission spectra of DMTPE in THF–water mixtures (inset:
fluorescence picture of DMTPE in 10% and 95% water under 365 nm hindrance triggered by the introduction of ortho-methyl groups
0
irradiation). (d) AIE effect (aAIE = I/I ) of TPE, DMTPE and TMTPE.
in the TPE unit, RIR as the fundamental mechanism of the
intriguing AIE phenomenon, is directly verified through simple
and X-ray crystallography. The resultant TPE, di(ortho-methyl)- chemical modifications. Probably, the investigations on AIE
substituted TPE (DMTPE) and tetra(ortho-methyl)-substituted TPE phenomenon will be adequately emerging due to a definite
(
TMTPE) were afforded with excellent yields, and they could be understanding of the mechanism of AIE characteristics, which
dissolved in common solvents with excellent solubility. The will be also beneficial for the novel molecular design.
detailed procedures are depicted in ESI.† Verified by powder X-ray diffraction (PXRD) showing several
The maximum UV-vis absorption peaks (labs) of TPE, DMTPE sharp peaks (Fig. S3, ESI†), all the associated pristine states are
and TMTPE in THF solutions are located at 308 nm, 300 nm and indicative of crystallinity. The fluorescence lifetime of these
314 nm, respectively (Fig. 1a and Table 1). The individual maxi- analogues are gradually increased from 1.37 ns (TPE), 2.74 ns
mum PL emission peaks (lem) of TPE, DMTPE and TMTPE in (DMTPE) to 4.63 ns (TMTPE) (Fig. S4, ESI†). Density functional
solid state are present at 448 nm, 417 nm and 429 nm, respec- theory (DFT) calculations were performed using Gaussian 09
tively. Measured under the same conditions, the PL intensity of program using B3LYP/6-31G(d) basis set, demonstrating that
TMTPE in THF solution, with the QY up to 64.3%, is much higher the optimized structures of DMTPE and TMTPE are quite
than DMTPE and TPE (Fig. 1b). As shown in Fig. 1b (inset contorted. Accordingly, the HOMO and LUMO of these analo-
ꢀ
5
picture), the THF solution of TMTPE (10 M) emits bright gues are located on the whole molecules (Fig. S5, ESI†). The
cyan-fluorescence under 365 nm irradiation, while vague fluores- energy level of TPE, DMTPE and TMTPE obtained from calcula-
cence can be observed by naked eye for both TPE and DMTPE. tions are 4.12, 4.29 and 4.20 eV, respectively.
Similar to TPE (Fig. S1, ESI†), DMTPE exhibits archetypal AIE
characteristic. With the water faction increased from 10% to the adjacent phenyl rings in these TPE derivatives, single crystals
0%, the PL intensity displays unnoticeable variations (Fig. 1c). were obtained via slow solution evaporation process. Their
To further investigate intermolecular interactions between
8
The PL intensity is dramatically enhanced as the water faction individual ORTEP structures and detailed crystallographic data
promoted from 80% to 95% (Fig. 1c, inset picture). On the are shown in Fig. 2 and Fig. S6–S8 (ESI†), respectively. Calculated
contrary, TMTPE shows little AIE-active attribute (Fig. 1d and by PLATON, no classical pꢁ ꢁ ꢁp interactions can be observed in all
Fig. S2, ESI†), and the profile alterations of PL spectra in different of them. In the case of TPE, C–Hꢁ ꢁ ꢁp interaction (Fig. 2d) with
the distance of 2.95 Å (H23ꢁ ꢁ ꢁCg
, Cg is a centroid defined
C9–C14
by some specified atoms) can be observed. For DMTPE, the
phenyl group C22–C27 and methyl C28 atoms are disordered
over two sites with the site occupancies of 0.70(1) : 0.30(1) for the
Table 1 Optical properties
a
lem/nm
F
F /%
major and minor components, respectively. Only one kind of
b
c
d
l
abs/nm Solution Solid Solution Aggregate
a
AIE
E
g
t/ns
0
C–Hꢁꢁꢁp interaction with the distance of 2.73 Å (H26 ꢁꢁꢁCgC15–C20
)
TPE
308
DMTPE 300
TMTPE 314
n.d.
n.d.
481
448 0.20
417 0.32
429 64.3
13.8
20.5
96.4
69
64
3.48 1.37 is seen. The two DMTPE molecules form a dimer in the molecular
3.61 2.74
packing associated with C–Hꢁꢁꢁp interactions (Fig. 2e). With
1.5 3.50 4.63
respect to TMTPE, the asymmetric unit composed of one fourth
of the molecule with all the atoms are symmetry-disordered by the
four-fold axis. No apparent pꢁꢁꢁp or C–Hꢁꢁꢁp can be observed in
bulky crystal, instead the whole molecules pack via weak van der
a
9
F
,10-Diphenylanthracene (F = 0.9 in cyclohexane) was used as the
b
standard. Aggregates formed in 95% water-THF solvent mixture.
c
d
a
AIE = FF,aggre/FF,sol
wavelength (lonset) of the absorption spectra.
.
g
Optical band gap (E ) estimated from the onset
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and subsequent Suzuki coupling reactions. The introduction of
ortho-methyl groups in DMTPE and TMTPE has a significant
influence on the AIE effect, which is due to the enhancement of
steric hindrance in the contiguous phenyl rings with ortho-
groups. Through internal control via the introduction of ortho-
groups, the RIR hypothesis as the basic mechanism for the
AIE-active TPE derivatives is directly validated. Moreover, with
further modifications, TPE derivatives with the introduction of
bulky groups at the ortho-positions may be promising candi-
dates for the fabrication of OLEDs and luminescent visualizers
for cell imaging.
This work was supported by the NSFC (21174045), 973
Program of China (2013CB922104). We also thank Analytical
and Testing Center of Huazhong University of Science and
Technology.
Fig. 2 ORTEP molecular structures of (a) TPE; (b) DMTPE; (c) TMTPE
shown as 30% thermal ellipsoid probability. The respective molecular
packing mode of single crystal: (d) TPE; (e) DMTPE; (f) TMTPE, symmetry Notes and references
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1
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