Novel highly emissive H-aggregates with aggregate fluorescence change in a phenylbenzoxazole-based system

Article abstract of DOI:10.1039/c4cc02564d

Fibrous nanoaggregates of a new benzoxazole-based derivative have been reported. This derivative exhibits not only H-aggregates but also strong yellow fluorescence, which is different from the traditional understanding of H-aggregates.

Full text of DOI:10.1039/c4cc02564d

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Novel highly emissive H-aggregates with
aggregate fluorescence change in a
phenylbenzoxazole-based system†
Cite this: Chem. Commun., 2014,
50, 8723
Received 8th April 2014,
Accepted 19th May 2014
Lianke Wang,a Yanfang Shen,a Mingdi Yang,ab Xiuzhen Zhang,c Weinan Xu,a
Qiuju Zhu,a Jieying Wu,a Yupeng Tiana and Hongping Zhou*a
DOI: 10.1039/c4cc02564d
www.rsc.org/chemcomm
Fibrous nanoaggregates of a new benzoxazole-based derivative have
been reported. This derivative exhibits not only H-aggregates but also
strong yellow fluorescence, which is different from the traditional
understanding of H-aggregates.
Scheme 1 (a) The molecular structures of 2a, 2b and 2c, (b) photographs
of powder and crystals of 2b under UV light.
Designing novel chromophores with excellent optical properties in
the condensed state has been a popular research area because of
their potential applications in organic light emitting diodes
(OLEDs),1 fluorescent sensors,2 etc. The restriction of intramolecular scanning calorimetry (DSC) results indicate that all 3 prepared
rotation (RIR) or planarization and J-aggregation reported by compounds possess high melting points with Tm at 139.9, 235.8
Tang3 and Park4 favor enhanced emission. The frequently and 201.9 1C (Table 1), respectively, and high thermal stability
reported aggregation-caused quenching fluorescence (ACQ)5 with Td at 294, 273 and 294 1C (Table 1), respectively.
in H-aggregates has greatly limited their potential applications.
We initially investigated the absorption and fluorescence
At present, various excellent chromophores with aggregation- spectra of 2a–2c (10 mM) in various polarity solvents (Table S1
induced emission3 or enhanced emission4 (AIE/AIEE) properties and Fig. S1, ESI†). The absorption and fluorescence spectra of 2a–2c
have been developed by many research groups, such as tetra- demonstrate quite similar maxima in the different solvents,
phenylethene,6 cyanostilbene7 and 9,10-distyrylanthracene indicating a slight solvent dependence. The target molecules
derivatives,8 which all avoid H-aggregation.
show strong absorption bands at 388, 327 and 414 nm, respectively,
Considering the interesting fluorescence properties of benzox- and emission maxima (lem) at 499, 389 and 495 nm, respectively.
azole, we utilized benzoxazole as building blocks and changed The weak absorption peaks at 300–320 nm are mainly attributed to
the terminal substituents to build new phenylbenzoxazole-based p–p* electronic transitions derived from the benzoxazolyl-benzene
chromophores. The structures of the target molecules are shown in unit.9 The molar extinction coefficient values also are in agreement
Scheme 1 (see also ESI† Scheme S1). In this communication, we with p–p* transitions.10 Theoretical calculations are a reliable
reported the highly emissive H-aggregate 2b and the unique AIEE method for qualitative indication of the absorption properties.11
phenomenon with large red shifts and remarkable color We calculated the frontier molecular orbitals based on the molecular
change. Thermogravimetric analysis (TGA) and differential
Table 1 Optical properties of the target molecules
a College of Chemistry and Chemical Engineering, Key Laboratory of Functional
l
em/nm
Inorganic Materials Chemistry of Anhui Province, Anhui University, Hefei 230601,
P. R. China. E-mail: youthmd98@163.com, zhpzhp@263.net;
Fax: +86-551-63861279; Tel: +86-551-63861279
Pure Mixed Powder Crystal
Compd
labs/nm
Tm (1C) Td (1C)
b Key Laboratory of Opto-Electronic Information Acquisition and Manipulation of
Ministry of Education, Anhui University, Hefei 230601, P. R. China
c Center of Modern Experimental Technology, Anhui University, Hefei 230039,
P. R. China. E-mail: ahzhangxz@ahu.edu.cn
2a
2b
2c
388
327
414
499
389
495
518
542
564
552
581
564
593
588
564
139.9
235.8
201.9
294
273
294
l
abs = the maxima absorption peak, lem = the maxima emission peak, in
dilute ethanol solution (10À5 mol LÀ1), pure = pure ethanol solution,
mixed = a mixed water–ethanol solution with fw = 0.90, Td = temperature
for 5% weight loss, Tm = melting temperature recorded using a
differential scanning calorimeter.
† Electronic supplementary information (ESI) available: Synthesis, characteriza-
tion, spectra, SEM images and theoretical calculations. CCDC 994632–994634.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4cc02564d
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Fig. 1 The fluorescence spectra of 2b and 2c in water–ethanol mixtures
with different water fractions (fw). The inset depicts the changes of PL peak
intensity with different fw. Photos of 2b and 2c in water–ethanol mixtures
with different fw, taken under UV light. Concentration: 50 mM.
Fig. 2 (a) SEM image of the nanoaggregates of 2b prepared in an ethanol/
water (2 : 8) mixture at a concentration of 50 mM. (b) The PL spectrum of 2b
in different concentrations with fw = 80%.
immediately in the mixture of ethanol–water with fw = 80% (Fig. S3a,
conformations in the crystal structures. Further details are ESI†). It is noteworthy that 2b molecules aggregate to form entangled
given in the ESI† and the results are in good agreement with three-dimensional networks consisting of interconnected
the experimental absorption spectra. All data indicate that the fibrous nanoaggregates, which has so far never been reported
introduction of diethylamino groups can change the p-conjugation in benzoxazole-based compounds (Fig. 2a).12 2c, aggregates into
degree of benzoxazolyl compounds.
rice-shaped nanoparticles in ethanol–water mixtures (Fig. S3c,
2a–2c exhibit weak green, blue and cyan emissions in ESI†). These morphologies must be responsible for the observed
diluted ethanol solution under UV light (inset in Fig. S2, ESI† enhanced emission and the impressive color changes.
and Fig. 1), respectively. Firstly, the fluorescence intensity of 2a
We also comparatively investigated their emissions in different
is almost linearly increasing step-by-step with the increasing of states. Their normalized fluorescence spectra are shown in Fig. S4
water fraction, accompanied with a red-shift in the lem values. (ESI†) and relative emission wavelength are shown in Table 1. All
The light emission reaches its maximum value when fw = 70%, compounds exhibit the minimum emission wavelength in pure
which is 6.4-fold higher than that in pure ethanol solution. 2b ethanol solution and the maximum emission wavelength in
exhibits different fluorescent behavior in ethanol–water solutions the crystalline state. 2a and 2b show the same trend: lem/Pure
with different fw values. Upon addition of water, the emission lem/Mixed o lem/Powder o lem/Crystal, while 2c displays lem/Pure
o
o
intensity and wavelength show a slight decrease and a red shift,
lem/Mixed E lem/Powder E lem/Crystal.
respectively. When fw Z 60% (Fig. 1), the yellow emission band
According to some other AIE/AIEE-active systems, the reasons
centred at 542 nm rapidly turns on. The inset photos (Fig. 1) for these unique fluorescent behaviors can be tentatively ascribed
taken under UV light show the color change and we can clearly to the different molecular conformations and packing modes in the
see that the blue solution vividly changes to yellow. In addition, different states.13 In pure ethanol solution, these compounds
2c also exhibits emission changes from cyan to orange with a molecules are molecularly isolated without any interactions
red-shift of 70 nm after addition of the different amounts of between the adjacent molecules, and the molecules have highly
water. The fluorescence intensity remain unchanged until fw is twisted conformations due to the intramolecular steric hindrance,
increased to 60%, the solutions emit bright yellow light with which leads to the minimum lem. The molecules of 2a–2c in the
lem at 528 nm. The emission intensity reaches its maximum crystal state are arranged so regularly that the rotatable single
value at fw = 70% and the lem red-shifts to 564 nm when fw = 90%. bonds are locked due to the multiple physical intermolecular
Such ratiometric fluorescence changes with large red-shifts of 150 interactions (see the following crystal discussion). These increase
and 70 nm for 2b and 2c in benzoxazole-based compounds have so the molecular planarity and strengthen the p-conjugated systems,
far never been reported, not to mention the impressive color which contribute to the maximum lem. The powder states have less
changes. These unique AIEE behaviors are very significant in the regularity compared with that in the crystal states. In condensed
field of fluorescent probes. The absorption spectra of 2a and 2c show solution, 2a shows common features just like some other AIE/AIEE-
a slight bathochromic shift and a level-off tail in the long-wavelength active systems, while 2b may form multimers in a mixture with
regions. The tail is caused by the Mie scattering effect,6 which high water content. The emission wavelength of 2c in the mixed
suggests the formation of nanoaggregates. The absorption spectrum solution is close to that in the powder and the crystal states,
of 2b exhibits a slight blue-shift and a few new absorption peaks suggesting that they originate from the same emitting species with
arise between 370 and 420 nm when fw Z 60%, which indicate the similar molecular conformation and interactions.
formation of new species.
To obtain a better understanding of the enhanced emission
To gain further insight into the unique morphology of the and impressive color change of 2a–2c, we investigated their
nanoaggregates of these benzoxazole-based compounds in the crystal structures. The single crystals were obtained by slow
mixture system, the mixture solutions were subjected to scanning evaporation from ethanol, CH2Cl2–ethanol and CH2Cl2–CH3CN
electron microscopy (SEM) observation. As can be seen from the solution, respectively and their crystallographic data are sum-
photos shown in Fig. 2a and Fig. S3 (ESI†), the three benzoxazole- marized in Table S2 (ESI†).
based compounds aggregate to form three different shapes in the
As shown in Fig. S5 (ESI†), due to the existence of intra-
mixture system. A large number of block nanoparticles of 2a formed molecular hydrogen bonds, they all have better planarity. The
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ORTEP diagram with atom numbering and high-dimensional these existing multiple interactions. Under the synergetic effect of
structures of 2a is shown in Fig. S6 (ESI†). The adjacent three intra- and intermolecular interactions, intramolecular motions are
molecules are cross-linked into a one-dimensional band in a locked and the molecular conformations are more rigid and planar.
face-to-edge way via multiple C–HÁ Á Áp interactions (d = 2.743, Thus, the physical restriction of intramolecular motions and
2.823, 2.964 and 2.905 Å) and C–HÁÁÁN hydrogen bonds (d = 2.780 aggregation-induced planarization induced by the molecular aggre-
and 2.747 Å). Furthermore, more molecules are interconnected by gation can be considered as the mechanisms of the enhanced
the above-mentioned intermolecular interactions to form two- emission phenomena of 2a and 2c. For 2b, it is obvious that 2b
dimensional layer structures. The molecular structure of 2b in molecules have better planarity and aggregate to form multimers in
the crystal state is nearly coplanar (Fig. S7, ESI†). We can clearly see face-to-face manner according to its crystal structure. In general, the
from Fig. S7c (ESI†) that the two molecules are initially cross formation of H-aggregates where molecules are aligned parallel to
stacked in edge-to-edge manner to form an L-shaped dimer each other with strong intermolecular interactions is characterized
through C–HÁÁÁO hydrogen bonds between the hydroxystyryl hydro- by blue-shifted absorption bands, which tend to induce the non-
gen atoms and the hydroxy oxygen atom, with the distance radiative deactivation process.14 The absorption bands and the
measured at 2.716 and 2.680 Å, respectively. As shown in Fig. S7b crystal structures all prove that 2b molecules aggregate to form
(ESI†), two 2b molecules are slip-stacking together in face-to-face H-aggregates. The formation of multimers shows an obvious concen-
type to form H-aggregates through pÁÁÁp stacking interactions with tration dependence. When the concentration of 2b is lower than 1
the distance measured at 3.477 Å. The two dimers are then mM in the ethanol/water (2 : 8) mixture, the yellow emission at
connected with each other in a middle-to-middle way to form 542 nm disappeared (Fig. 2b and Fig. S13, ESI†), which indicates
a #-shaped tetramer through C–HÁÁÁN (d = 2.564 Å) and C–HÁÁÁO the absence of the special emissive species that show yellow emis-
(d = 2.668 Å) hydrogen bonds between the hydrogen atoms of the sion. Thus, we can speculate that the emission in the yellow region is
benzoxazolyl and phenyl groups of one molecule and the nitrogen attributed to the emission of H-aggregates, which is quite different
and oxygen atoms of the benzoxazolyl group of the adjacent from the traditional understanding of H-aggregates.
molecule. The #-aggregates are further expanded to form a two-
Generally, high radiative rates can efficiently enhance emis-
dimensional multimer crossing layer structure by the above- sion, while the active exciton migration is detrimental to
mentioned type of hydrogen bonds and pÁÁÁp stacking interactions. emission.15 Whether the emission quenches or strengthens is
The formation scheme of the two-dimensional expanded cross determined by the competition between the two opposite
packing layer structure is shown in Fig. 3. For 2c, its crystal factors. Obviously, the positive effect is dominant in the emis-
structure is similar to that of 2a. Fig. S9b (ESI†) shows all sion process of 2b. It is very likely that the specific slip-stacking
intermolecular interactions on one molecule, including C–HÁÁÁp and the dimer formed by multiple intermolecular hydrogen
(d = 2.735, 2.844 and 2.852 Å), C–OÁÁÁp (d = 3.010 Å) and C–HÁÁÁN bonds play essential roles in its high emission, which effectively
(d = 2.847 and 2.699 Å) interactions. 2c molecules are linked into slow exciton motion, turn on radiative decay channel and avoid
two-dimensional bands via the above intermolecular interactions strong H-type coupling to some extent, thus H-aggregates of 2b
(Fig. S9c, ESI†). Furthermore, these band structures are connected exhibit high emission.
in a head-to-head way to form a three-dimensional W-shaped
Considering the color changes in solution and nanoaggre-
stacking structure by another type of hydrogen bond between the gates, the application of 2b nanoaggregates to detect highly
nitrogen atoms on the oxazolyl rings and the hydrogen atoms of volatile organic solvents was investigated on thin-layer chro-
phenyl rings, with the distance measured at 2.796 Å.
matography plates (Fig. S12, ESI†). Under 365 nm UV light at
According to the crystal structures, the obvious enhanced room temperature, 2b nanoaggregates show bright yellow
emission and unique color change can be well explained by fluorescence which turns blue in an atmosphere of dichloro-
methane vapour. The color can be recovered after the vapour is
removed. Such fluorescence switching indicates that 2b exhi-
bits different states in different conditions. A similar fluores-
cence change of 2b can also be observed in other solvents, such
as tetrahydrofuran and chloroform. Thus, the fluorescence
color changes can be used in detecting highly volatile organic
solvents because they can be distinguished easily by eye.
In conclusion, we have revealed enhanced emission with large
red shifts and color change of a novel benzoxazole-based derivative.
In contrast to the general observation that H-aggregate formation
quenches the light emission of chromophores, 2b exhibit H-type
aggregates and efficient emission in the aggregated state. The
specific slip-stacking and the dimer formed by multiple inter-
molecular interactions avoid strong H-type coupling and are
responsible for highly emissive H-type aggregates. The response
Fig. 3 Formation scheme of two-dimensional expanded cross packing
to volatile organic solvents with blue/yellow fluorescent switching
layer structure of the 2b molecules. All the hydrogen atoms are omitted for
clarity.
was also demonstrated. 2a and 2c show common AIEE features.
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We acknowledge the financial support from New Century
Excellent Talents in University (China), the Doctoral Program
Foundation of the Ministry of Education of China
(20113401110004), the National Natural Science Foundation
of China (21271003, 21271004), the Natural Science Foundation
of Education Committee of Anhui Province (KJ2012A024), the
Natural Science Foundation of Anhui Province (1208085MB22),
the 211 Project of Anhui University, the Ministry of Education
Funded Projects Focus on Returned Overseas Scholar, Higher
Education Revitalization Plan Talent Project (2013) and the
College Students Innovative Entrepreneurial Training Program
of Anhui University (201310357025, 201310357155).
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