A Br-substituted phenanthroimidazole derivative with aggregation induced emission from intermolecular halogen-hydrogen interactions

Article abstract of DOI:10.1039/c5cc00490j

A unique aggregation induced emission (AIE) active emitter based on a Br-substituted phenanthroimidazole derivative was reported. The yellowish green emission with high quantum yield and a large Stokes shift (～150 nm) in the aggregated state is proposed t

Full text of DOI:10.1039/c5cc00490j

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A Br-substituted phenanthroimidazole derivative
with aggregation induced emission from
Cite this:DOI: 10.1039/c5cc00490j
intermolecular halogen–hydrogen interactions†
Ying Zhang,abc Jun-Hao Wang,b Ji Zhengb and Dan Li*b
Received 18th January 2015,
Accepted 3rd March 2015
DOI: 10.1039/c5cc00490j
www.rsc.org/chemcomm
A unique aggregation induced emission (AIE) active emitter based on
Considering the unique effect of halogen in many interesting
a Br-substituted phenanthroimidazole derivative was reported. The luminescence phenomena and the high quantum yield, good
yellowish green emission with high quantum yield and a large Stokes stability and diverse availability of phenanthroimidazole deriva-
shift (B150 nm) in the aggregated state is proposed to be obtained tives, herein, we present an AIE active compound based on a
from weak interactions especially C–HÁ Á ÁBr interactions.
phenanthroimidazole derivative, 2-(5-bromothiophen-2-yl)-1-(4-
(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazole,11 denoted as
Luminogenic materials with aggregation induced emission t-PhIm-Thi-Br (Scheme 1 and Scheme S1, ESI†). This compound
(AIE) performance have attracted much attention for their wide exhibits a very unique AIE phenomenon with the emission intensity
range of applications in organic electronics and biosensors.1 significantly enhanced and a new emission band emerged with a
In the past decade, many AIE materials have been reported,2–10 very large Stokes shift in both the nano-aggregation state and the
and most of them have been ascribed to an intramolecular solid state. In DMF, t-PhIm-Thi-Br is almost non-emissive with a
restricted rotation (RIR) mechanism proposed by Tang et al.2–7b
however, this may not be a full story. Other AIE active complexes emission of quantum yield up to 0.19 in pure water.
;
quantum yield of 0.007, whereas it turns on with green-yellow
obtained from J aggregation,6 excimer formation,7a,c activated
The emission spectra of t-PhIm-Thi-Br in DMF–H2O (water
phosphorescence,8,9 aurophilic interactions10 and so on have also content fw = 0–100%) solutions show different emission max-
been documented. AIE active molecules are highly luminescent in ima and curve profiles (Fig. 1a) reflecting a process of dynamic
the solid state, which is a prerequisite for most display applications. aggregation with three different stages.
In spite of many reports on AIE active materials with various
In Stage I ( fw o 50%), the blue emissions (lmax = 410 nm) of
mechanisms, it is still highly significant to find new AIE systems t-PhIm-Thi-Br in DMF–H2O with the water content less than
as candidates for developing new functional luminescent materials. 50% are very weak. Since t-PhIm-Thi-Br can be well dissolved in
Recently, Kim et al. reported an interesting AIE emitter.9 DMF, it is reasonable to believe that solute molecules disperse
Unlike most AIE active molecules, Br substituted benzaldehyde in solutions with such low water portions. The weak emission
shows not only enhanced intensity but also an evidently large of t-PhIm-Thi-Br might be due to the intramolecular rotation of
Stokes shift due to the directed heavy atom phosphorescence. the thiophene and t-Bu-benzene rings4–7b or the heavy atom
The large Stokes shift upon aggregation gives this material potential effect of Br. To determine this, a similar molecule t-PhIm-Thi-H
significant role in bio-imaging or bio-sensing. However, this material without a Br substitute was synthesized for comparison.
can only exhibit phosphorescence emission in bulk crystallization or In good solvents, t-PhIm-Thi-H shows an emission mainly at
the co-crystal state, which may hamper its practical application in
organic light emitting diodes (OLED) or bio-sensing.
a Department of Chemistry, School of Life Science and Technology, Jinan University,
Guangzhou 510632, China
b Department of Chemistry and Key Laboratory for Preparation and Application of
Ordered Structural Materials of Guangdong Province, Shantou University,
Guangdong 515063, China. E-mail: dli@stu.edu.cn
c Institute of Molecular Science, Shanxi University, Taiyuan 030006, China
Scheme 1 Structure of t-PhIm-Thi-Br and its photographs in the mixtures
of DMF and water (water contents 0–100%) taken under 365 nm hand-lamp
irradiation.
† Electronic supplementary information (ESI) available: Experimental details,
crystal data, physical measurements. CCDC 1043767 and 1043768. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c5cc00490j
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experiment giving a size distribution of B196 nm. Note that the
aggregation nano-particles well dispersed in water and the
solution is clear (Fig. S5, ESI†).
The emission profile of t-PhIm-Thi-Br in the solid state (Fig. S2,
ESI†) is similar to those in DMF–H2O with fw 4 50% and shows a
significant red-shift from those of fw r 50% (Fig. 1a) peaked
at B440, 500 and 540 nm. A reasonable deduction of the longer
wavelength emissions maybe due to stronger molecular inter-
actions in the solid state and in the nano-aggregation state.14
Single crystals of t-PhIm-Thi-Br and t-PhIm-Thi-H were
obtained by slow vaporization of dichloromethane solutions
Fig. 1 Emission (a) and UV-Vis absorption (b) spectra of t-PhIm-Thi-Br in a
mixture of DMF and water with the water content from 0 to 100% (10 mM).
B410 nm similar to that of t-PhIm-Thi-Br (Fig. S1, ESI†). The of the corresponding compounds. The results showed that both
difference is that the emission quantum yield of t-PhIm-Thi-H structures (Fig. 3a and b) have a very similar geometry and that
is much higher (0.99 in DMF). Thus the heavy atom effect of Br the thiophene ring is co-planar to the phenanthroimidazole
should be the key factor that makes the emission weaker.
conjugate perpendicular to t-Bu-benzene with a degree of
When the water content reaches 50% (Fig. 1a, Stage II, fw = 50%), nearly 901. In the structure of t-PhIm-Thi-Br, two adjacent
the emission intensity of t-PhIm-Thi-Br decreased slightly molecules are dense packed as a dimer by p–p interactions
and the peak red-shifted from 410 nm to 440 nm. A long tail (3.59 Å) between the thiophene ring from one molecule and the
after 400 nm in the UV-Vis absorption curve appears (Fig. 1b). phenanthrene ring from the other in a head-to-tail fashion.
Similar luminescence spectral maxima can also be found both The dimers further extend in the b direction by C–HÁ Á Áp
in the solid state and pure water of t-PhIm-Thi-H (Fig. S2, ESI†). interactions (2.95 Å) forming a packing row (Fig. 3a, Fig. S6a
We propose that t-PhIm-Thi-Br molecules began to aggregate and b, ESI†). A similar packing row is also found in the
showing an excimer emission in this stage.12
structure of t-PhIm-Thi-H with p–p interactions of 3.62 Å and
Increasing the water content to more than 50% (Stage III, C–HÁ Á Áp interactions of 2.86 Å (Fig. 3b, Fig. S6c and d, ESI†). One
fw 4 50%), another new emission band emerged mainly at 500 difference in the structures of t-PhIm-Thi-Br and t-PhIm-Thi-H is
and 540 nm with two shoulders at 440 nm and 585 nm, with the their packing between two adjacent rows: t-PhIm-Thi-Br in the
colour changing from faint deep blue to enhanced yellow-green same direction vs. t-PhIm-Thi-H in the opposite (Fig. 3a and b).
which can be discerned by the naked eye. Further, the emission Another difference is the existence of C–HÁ Á ÁBr interactions
lifetime of t-PhIm-Thi-Br was determined to be B4.7 ms mon- (3.08 Å) between two adjacent rows in t-PhIm-Thi-Br. The PXRD
itored at 450–585 nm in H2O (Fig. S3a–e, ESI†). The lifetime is patterns of nano-aggregates of t-PhIm-Thi-Br in Stage III
relatively long for an organic molecule. However, this cannot be matched well with the simulated ones from the single crystal
attributed to phosphorescence by suppression of molecule rotation structure (Fig S7, ESI†). The structural differences should respond
and vibration reported by others13 because of the absence of the to the different emissions of yellow green for t-PhIm-Thi-Br and
longer wavelength emission from the spectra of t-PhIm-Thi-Br in a blue for t-PhIm-Thi-H.
glassy polymer of PMMA (5 mg/400 mg). When the concentration
was increased up to 10 mg/50 mg, the new emission around
540 nm emerged, indicating that the newly observed emission
was caused by intermolecular interaction formation (Fig. S4, ESI†).
We speculate that the gradually enhanced intensity of the obvious
red-shift emission might be caused by J-aggregate formation, which
typically shows a distinct absorption red-shift6c,12 (Fig. 1b).
To prove the existing form of the aggregation state, SEM and
DLS experiments of t-PhIm-Thi-Br in pure water at 10 mM were
carried out (Fig. 2). SEM clearly gave an image of uniform nano-
particles (B200 nm), which was further proved by the DLS
Fig. 3 Crystal packing of t-PhIm-Thi-Br (a) and t-PhIm-Thi-H (b), color
codes for elements: S, yellow ball; Br, red ball. Color codes for interactions:
p–p, red dotted lines; C–HÁ Á Áp, blue dotted lines; C–HÁ Á ÁBr, green dotted
lines. Illustration of head-to-tail packing of t-PhIm-Thi-Br (c) and t-PhIm-
Fig. 2 Morphology (a) and size distribution (b) of t-PhIm-Thi-Br in the
aggregation state in pure water (10 mM).
Thi-H (d), blue and red bars representing the hydrogen-rich t-butyl group
and –Br, respectively.
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To find out the origin of the longer wavelength emission of exhibit low luminescence efficiency in pure H2O (see pictures
t-PhIm-Thi-Br, we further synthesized some other similar mole- at the bottom of Fig. S11, ESI†), likely due to the different
cules (Scheme S1, ESI†) under the following considerations: (1) aggregation types affected by different moieties.6a,c,16 As shown
replacing Br with benzene (t-PhIm-Thi-Ph) to further verify that in Fig. S11e (ESI†), absorption maximum peaks of COOH-PhIm-
the Br substituent is crucial; (2) replacing Br with Cl (t-PhIm- Thi-Br are blue-shifted with the opposite shift direction towards
Thi-Cl) to find the main factor from the electron withdrawing AIE-active t-PhIm-Thi-Br and PhIm-Thi-Br, which can be attri-
effect, heavy atom effect and C–HÁ Á ÁX (X = halogen) interaction; buted to be the H-aggregate formation.6c
(3) replacing the thiophene ring with benzene (t-PhIm-Ph-Br)
Thus, the N-aromatic moiety and the halogen atom and
to test the role of thiophene; (4) replacing t-Bu with H (PhIm- thiophene ring discussed above are some of the main factors
Thi-Br) or COOH (COOH-PhIm-Thi-Br) and (5) opening the that affect the emission behaviour significantly. More concre-
rigid phenanthrene (t-BIm-Thi-Br) to see if the resulting mole- tely, supramolecular interactions of C–HÁ Á Áp, pÁ Á Áp especially
cule has any influence on the emission.
C–HÁ Á ÁX (X = halogen) are responsible for the unique emission
The photoluminescence spectra of the above mentioned behaviour of t-PhIm-Thi-Br. And the enhanced emission should
molecules for comparison in DMF–H2O (fw = 0–100%) solutions be caused by the strengthening of C–HÁ Á ÁBr interactions for
are shown in Fig. S8, S9 and S11 (ESI†).
J aggregate formation.17 The essence of this AIE luminescence is
The photoluminescence behaviors of t-PhIm-Thi-H and t-PhIm- still under investigation and may be documented in the future.
Thi-Ph (Fig. S8, ESI†) are very different from that of t-PhIm-Thi-Br. In conclusion, we report a unique AIE active emitter based on a
They show strong emission in DMF with much higher quantum phenanthroimidazole derivative showing high quantum yield and a
yield (t-PhIm-Thi-H: 0.99, t-PhIm-Thi-Ph: 0.75). Upon addition of large Stokes shift (B150 nm). By careful study of emission proper-
more than 70% of water for t-PhIm-Thi-H (60% for t-PhIm-Thi-Ph), ties (in good solvents, in the nano-aggregation state and solid state)
the emission intensity decreases with the maxima slightly red- and comparison of structures, spectroscopy of a series of designed
shifted of about 15 nm (35 nm for t-PhIm-Thi-Ph), ascribed to a derivatives, we found that the combination effect of J-aggregate
common ACQ (aggregation caused quenching) phenomenon. This formation and C–HÁÁÁBr interaction is crucial for the unique
result further implied that C–HÁÁÁBr interactions in t-PhIm-Thi-Br emission behaviour, which may provide an opportunity to develop
play a crucial role in the strong yellowish green emission.
OLED and bio-sensing/imaging materials.
t-PhIm-Thi-Cl shows strong blue emission in DMF (Ff = 0.39,
This work was financially supported by the National Basic
lmax = 412 nm), suggesting that the heavy atom effect does not Research Program of China (No. 2013CB834803), the National
exist anymore when Br is replaced by Cl. The replacement of Natural Science Foundation of China (No. 91222202 and
H in t-PhIm-Thi-H with Br or Cl only causes a little change in 21171114) and Scientific Research Start-up Funds of Shanxi
the UV-Vis absorption peak (Fig. 1b, Fig. S8c and S9c, ESI†), University (020354010).
indicating that electron-withdrawal does not significantly affect
the orbital levels (see also HOMO and LUMO orbitals obtained
from DFT calculations in Fig. S10, ESI†).
Notes and references
1 (a) Y. N. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Commun., 2009,
With increasing water contents, t-PhIm-Thi-Cl exhibits a similar
4332; (b) Y. N. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Soc. Rev.,
emission behavior to t-PhIm-Thi-Br (Fig. S9a, ESI†), peaking at 501
and 538 nm with two shoulders at 459 and 582 nm. However, the
intensity ratio of emissions in the blue region (B450 nm) and
yellowish green region (500–600 nm) is higher than that of t-PhIm-
Thi-Br, and thus the emission color in pure water is cyan rather
than yellowish green. Thus the halogen atoms (Br/Cl) responsible
for C–HÁ Á ÁX (X = Br, Cl) interactions in the crystal structures
should be an important factor affecting the emission energies.
The thiophene ring plays a crucial role which can be proved
directly by the emission spectrum of t-PhIm-Ph-Br (Fig. S9b, ESI†).
The emission is weak in DMF with a maximum wavelength at
391 nm, and its intensity decreases with a small red-shift with
increasing water contents. The absorption locates at 362 nm with a
larger energy band than t-PhIm-Thi-Br, due to which thiophene can
raise the HOMO energy effectively according to our previous
report.11,15 It is very probable that the electronic structure and
chemical structure of the thiophene ring are important for the
unique emission properties in this system.
2011, 40, 5361; (c) Z. G. Chi, X. Q. Zhang, B. J. Xu, X. Zhou, C. P. Ma,
Y. Zhang, S. W. Liu and J. R. Xu, Chem. Soc. Rev., 2012, 41, 3878.
2 (a) J. Huang, N. Sun, J. Yang, R. Tang, Q. Li, D. Ma, J. Qin and Z. Li,
J. Mater. Chem., 2012, 22, 12001; (b) R. Misra, T. Jadhav, B. Dhokale
and S. M. Mobin, Chem. Commun., 2014, 50, 9076.
3 (a) J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu, H. S. Kwok,
X. Zhan, Y. Liu, D. Zhu and B. Z. Tang, Chem. Commun., 2001, 1740;
(b) B. Z. Tang, X. Zhan, G. Yu, P. P. S. Lee, Y. Liu and D. Zhu, J. Mater.
Chem., 2001, 11, 2974.
4 (a) K. Itami, Y. Ohashi and J. I. Yoshida, J. Org. Chem., 2005,
70, 2778; (b) Q. Zeng, Z. Li, Y. Dong, C. Di, A. Qin, Y. Hong, L. Ji,
Z. Zhu, C. K. W. Jim, G. Yu, Q. Li, Z. Li, Y. Liu, J. Qin and B. Z. Tang,
Chem. Commun., 2007, 70.
5 (a) B. K. An, S. K. Kwon, S. D. Jung and S. Y. Park, J. Am. Chem. Soc.,
2002, 124, 14410; (b) M. Han and M. Hara, J. Am. Chem. Soc., 2005,
127, 10951; (c) Y. Li, F. Li, H. Zhang, Z. Xie, W. Xie, H. Xu, B. Li,
F. Shen, L. Ye, M. Hanif, D. Ma and Y. Ma, Chem. Commun., 2007,
231; (d) S. J. Yoon, J. W. Chung, J. Gierschner, K. S. Kim, M. G. Choi,
D. Kim and S. Y. Park, J. Am. Chem. Soc., 2010, 132, 13675.
6 (a) Y. Qian, S. Li, G. Zhang, Q. Wang, S. Wang, H. Xu, C. Li, Y. Li and
G. Q. Yang, J. Phys. Chem. B, 2007, 111, 5861; (b) Y. Zhang,
J. H. Wang, W. Zheng, T. Chen, Q. X. Tong and D. Li, J. Mater.
Chem. B, 2014, 2, 4159; (c) B. K. An, S. K. Kwon, S. D. Jung and
S. Y. Park, J. Am. Chem. Soc., 2002, 124, 14410.
PhIm-Thi-Br (also COOH-PhIm-Thi-Br, t-BIm-Thi-Br) shows
deep blue emission in DMF and a new emission band with a large
Stokes shift in H2O (Fig. S11, ESI†), similar to that of t-PhIm-Thi-Br.
The difference is that COOH-PhIm-Thi-Br and t-BIm-Thi-Br
7 (a) Q. Zhao, L. Li, F. Li, M. Yu, Z. Liu, T. Yi and C. Huang,
Chem. Commun., 2008, 685; (b) Y. You, H. S. Huh, K. S. Kim,
S. W. Lee, D. Kim and S. Y. Park, Chem. Commun., 2008, 3998;
(c) K. Huang, H. Wu, M. Shi, F. Li, T. Yi and C. Huang, Chem.
Commun., 2009, 1243.
This journal is ©The Royal Society of Chemistry 2015
Chem. Commun.

View Article Online
Communication
ChemComm
8 Z. Zhang, B. Xu, J. Su, L. Shen, Y. Xie and H. Tian, Angew. Chem., 12 Y. Qian, S. Li, Q. Wang, X. Sheng, S. Wu, S. Wang, J. Li and G. Yang,
2011, 123, 11858. Soft Matter, 2012, 8, 757.
9 (a) W. Z. Yuan, X. Y. Shen, H. Zhao, J. W. Y. Lam, L. Tang, P. Lu, 13 D. Lee, O. Bolton, B. C. Kim, J. H. Youk, S. Takayama and J. Kim,
C. Wang, Y. Liu, Z. Wang, Q. Zheng, J. Z. Sun, Y. Ma and B. Z. Tang, J. Am. Chem. Soc., 2013, 135, 6325.
J. Phys. Chem. C, 2010, 114, 6090; (b) O. Bolton, K. Lee, H. J. Kim, 14 L. L. Zhu, X. Li, Q. Zhang, X. Ma, M. H. Li, H. C. Zhang, Z. Luo,
K. Y. Lin and J. Kim, Nat. Chem., 2011, 3, 205. H. Ågren and Y. L. Zhao, J. Am. Chem. Soc., 2013, 135, 5175.
10 (a) W. X. Ni, M. Li, J. Zheng, S. Z. Zhan, Y. M. Qiu, S. W. Ng and D. Li, 15 Y. Zhang, S. L. Lai, Q. X. Tong, M. F. Lo, T. W. Ng, M. Y. Chan,
Angew. Chem., Int. Ed., 2013, 52, 13472; (b) W. X. Ni, Y. M. Qiu, M. Li,
J. Zheng, R. W. Y. Sun, S. Z. Zhan, S. W. Ng and D. Li, J. Am. Chem.
Z. C. Wen, J. He, K. S. Jeff, X. L. Tang, W. M. Liu, C. C. Ko, P. F. Wang
and C. S. Lee, Chem. Mater., 2012, 24, 61.
Soc., 2014, 136, 9532; (c) Z. Luo, X. Yuan, Y. Yu, Q. Zhang, D. T. Leong, 16 (a) H. Zhang, Z. Zhang, K. Ye, J. Zhang and Y. Wang, Adv. Mater.,
¨
J. Y. Lee and J. Xie, J. Am. Chem. Soc., 2012, 134, 16662; (d) F. K.-W.
Hau, T. K.-M. Lee, E. C.-C. Cheng, V. K.-M. Au and V. W.-W. Yam,
Proc. Natl. Acad. Sci. U. S. A., 2014, 111, 15900.
2006, 18, 2369; (b) F. Wu¨rthner, T. E. Kaiser and C. R. S. Moller,
Angew. Chem., Int. Ed., 2011, 50, 3376.
17 (a) Y. Sagara and T. Kato, Nat. Chem., 2009, 1, 1605; (b) Y. Sagara and
T. Kato, Angew. Chem., 2011, 123, 9294; (c) K. Nagura, S. Saito,
H. Yusa, H. Yamawaki, H. Fujihisa, H. Sato, Y. Shimoikeda and
S. Yamaguchi, J. Am. Chem. Soc., 2013, 135, 10322.
11 Y. Zhang, S. L. Lai, Q. X. Tong, M. Y. Chan, T. W. Ng, Z. C. Wen,
G. Q. Zhang, S. T. Lee, H. L. Kwong and C. S. Lee, J. Mater. Chem.,
2011, 21, 8206.
Chem. Commun.
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