The effects of positional isomers, protonation and solvent on the morphologies and photophysical properties of boron difluoride complex microcrystals

Article abstract of DOI:10.1039/c6ce00202a

Positional isomers, protonation and solvent provide useful strategies for morphology-controllable fabrication of organic microcrystals. Among them, the solvent unprecedentedly induces a globate polyhedron morphology with unusual orange-red phosphorescence. The changes in morphology also induce interesting changes in the photophysical properties of boron difluoride organic microcrystals.

Full text of DOI:10.1039/c6ce00202a

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The effects of positional isomers, protonation and
solvent on the morphologies and photophysical
properties of boron difluoride complex
microcrystals†
Cite this: DOI: 10.1039/c6ce00202a
Received 25th January 2016,
Accepted 22nd February 2016
DOI: 10.1039/c6ce00202a
*
Cong Zhang, Chen Shen and Guoping Yong
www.rsc.org/crystengcomm
Positional isomers, protonation and solvent provide useful
strategies for morphology-controllable fabrication of organic
microcrystals. Among them, the solvent unprecedentedly induces
a globate polyhedron morphology with unusual orange-red phos-
phorescence. The changes in morphology also induce interesting
changes in the photophysical properties of boron difluoride or-
ganic microcrystals.
teristics or changing solvent quality. Varughese and Draper
reported the influence of isomerism on the morphology of or-
ganic micro/nanocrystals.4b Very recently, we have discovered
that the protonation and/or anion induced morphological
changes in boron difluoride (BF2) complex microcrystals2n
and acid–base vapor can trigger a reversible morphological
transformation with interesting “on/off” switching behav-
iors.6 Herein, two novel methyl-substituted BF2 complexes
were synthesized, which reveal the effects of the positional
isomers of methyl groups on the morphologies. It is interest-
ingly found that protonation (pH change) and solvent also
have important influences on the morphologies. Especially,
the solvent can even induce an unusual globate polyhedron
morphology. We also demonstrate that morphologies exhibit
obvious effects on the photophysical properties. The major
purpose of this work is to unveil the fact that the morpholog-
ical properties of organic microcrystals are tunable by appro-
priate factors such as positional isomers, protonation and
solvent.
Organic micro/nanomaterials have drawn more and more
attention due to their interesting properties and potential
technological applications in many fields.1 Organic micro/
nanostructures with various morphologies such as particles,
rods, wires, sheets, cubes and fibers have been fabricated by
using different approaches.2 Among them, the reprecipitation
method is considered as an effective and facile process to
prepare
organic
micro/nanomaterials
with
unique
morphology. However, the morphology/size-controllable
fabrication of organic micro/nanomaterials is still a challenge
when using this strategy. Therefore, there is increasing
interest in developing preparation methods to control the
morphology/size of organic micro/nanostructural materials,
and further exploring the effects of morphology on their
properties.
Organic microcrystals with various morphologies are easily
self-assembled through supramolecular interactions.3 Other
factors such as the substituted group, positional isomer, pH
change, solvent, heat, light, and/or concentration also have
important effects on the morphology/size of organic micro/
nanostructural materials.3,4 Tian et al. found that substituted
groups on molecules are responsible for the final shape of
the organic micro/nanostructures,5 and the corresponding
nanoparticles and nanorods have been fabricated by either
taking advantage of the different molecular structure charac-
A 7,7′ (or 8,8′)-dimethyl-2,3′-biimidazoij1,2-a]pyridin-2′-one
radical (Hdmbipo−•, Scheme 1) was synthesized by a similar
procedure to that of a 2,3′-biimidazoij1,2-a]pyridin-2′-one radi-
cal.7 The BF2 complexes (BF2-7,7′-dmbipo and BF2-8,8′-
dmbipo) were synthesized from the corresponding
Department of Chemistry, University of Science and Technology of China, Hefei
230026, PR China. E-mail: gpyong@ustc.edu.cn; Fax: +86 551 6360 1592;
Tel: +86 551 6360 1584
† Electronic supplementary information (ESI) available: Experimental procedures
and measurements; characterization data. See DOI: 10.1039/c6ce00202a
Scheme
corresponding hydrochlorides.
1 Synthesis of boron difluoride complexes and the
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Hdmbipo−• with BF3·OEt2 in DMF solution (Scheme 1). The
protonated BF2 complexes (BF2-7,7′-dmbipoHCl and BF2-8,8′-
dmbipoHCl) were prepared from the corresponding BF2-
dmbipo by a dissolution–reprecipitation method in DMF (or
DMSO)–HCl solution. The scanning electron microscopy
(SEM) images reveal that BF2-7,7′-dmbipo (termed microcrys-
tal A) and BF2-8,8′-dmbipo (termed microcrystal B) microcrys-
tals present a one-dimensional (1D) rod-like morphology with
an average diameter of 2.0 μm (Fig. 1a) and a three-
dimensional (3D) brick-like morphology with an average size
of 24.3 μm × 10.0 μm × 5.7 μm (Fig. 1b), respectively. The
sharp peaks in the powder X-ray diffraction (PXRD) patterns
confirm the microstructures of microcrystals A and B to be
highly crystalline (Fig. S1, ESI†). The PXRD patterns indicate
that microcrystals A and B possess various microstructures.
Because microcrystals A and B were fabricated under the
same conditions, the factors such as solvent, pH, and temper-
ature, etc. are not responsible for the formation of different
morphologies. Instead, the positional isomeric methyl groups
have a profound effect on the morphologies of microcrystals
A and B. The morphology variations of micro/nanocrystals as
a function of steric hindrance, π–π stacking, and hydrogen
bonding are known in the literature, although the manipula-
tion of morphology by altering the molecular geometry only
has a limited success.8,4b Thus, a rational and simple ap-
proach to fabricate organic microcrystal materials is highly
desirable for understanding the various factors that influence
the morphologies. Two isomers exhibit different steric hin-
drance and molecular geometries (Scheme 1), and these in
turn are reflected in the morphologies. BF2-7,7′-dmbipo
should possess a longer molecular size compared to BF2-8,8′-
dmbipo (Scheme 1). Thus, BF2-7,7′-dmbipo molecules with a
“linear” geometry were self-assembled into 1D rod-like micro-
crystal A, whereas BF2-8,8′-dmbipo molecules with a relatively
“rectangular” geometry were constructed into 3D brick-like
microcrystal B. This change in morphology can therefore be
attributed to the different interactions at the supramolecular
level brought about by the variation of the methyl group posi-
tions in the molecules. Moreover, compared to BF2-7,7′-
dmbipo, the larger steric hindrance of the methyl groups in
BF2-8,8′-dmbipo is also an important factor in the formation
of the 3D brick-like morphology. The observed steric effects
could hinder the formation of 1D nanostructures, and thus
the formation of thin platelets and nanoparticles have been
reported instead.9,4b Therefore, the results reveal that the
morphology of the organic small molecular microcrystals can
be facilely tuned by varying the isomer or the steric
hindrance.
Interestingly, when microcrystals A and B were dissolved
in DMF–HCl solution by the dissolution–reprecipitation
method, the resulting microcrystals C (BF2-7,7′-dmbipoHCl)
and D (BF2-8,8′-dmbipoHCl) disclose a flower-like morphology
composed of two-dimensional (2D) microsheets (Fig. 2a) and
a 1D microstrip morphology (Fig. 2b), respectively. The sharp
peaks in the PXRD patterns also confirm the microstructures
of microcrystals C and D to be highly crystalline (Fig. S1,
ESI†). From the SEM images, one can see that the thickness
(about 100 nm) of the microsheets (Fig. 2a) is thinner than
that of the microstrips (thickness of about 160 nm, Fig. 2b).
The above results reveal that the protonation (pH change)
and/or Cl− anion can also trigger morphological changes.2n
Fig. 1 SEM images of 1D microrods of microcrystal A (a) and 3D
Fig. 2 SEM images of flower-like microcrystal C composed of 2D
microbricks of microcrystal B (b).
microsheets (a) and 1D microstrips of microcrystal D (b).
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The exact mechanism for the protonation and/or Cl− anion
induced morphology is still not clear, but we can hypothesize
that the Cl− anions were selectively adsorbed on a specific
crystal facet due to the electrostatic attraction, resulting in
the anisotropic growth which gives the microsheets and the
microstrips of microcrystals C and D, respectively.
with one strong emission peak at 506 nm and one shoulder
at ca. 436 nm (Fig. 4), indicating that it might emit green-
white light (Fig. 5a). However, microcrystal B only emits
green light (Fig. 5b), as indicated by its PL spectrum with an
emission peak at about 511 nm (Fig. 4). Moreover, the pro-
tonated microcrystals C and D reveal obviously blue-shifted
PL spectra, with microcrystal C having an emission peak at
447 nm (Fig. 4), meaning that it emits blue light (Fig. 5c),
and microcrystal D having a strong emission peak at 455 nm,
as well as two shoulders at 483 and 575 nm (Fig. 4), implying
that it emits blue-white light (Fig. 5d). Unprecedentedly,
microcrystal E exhibits an obviously red-shifted PL spectrum
with an emission peak at about 622 nm (Fig. 4), meaning
that it emits orange-red light (Fig. 5e). Moreover, microcrystal
E also emits orange light when illuminated with ambient
light (Fig. 5f), due to its maximum excitation wavelength at
about 455 nm (Fig. S2, ESI†). The detailed emission mecha-
nism for microcrystals A–E is not clear at present; however,
as aggregation-induced emission, their emission mechanism
can be ascribed to morphology-dependent luminescence.11
Take microcrystals C and D for example, the phosphorescent
nature of microcrystals A–E is confirmed by the microsecond-
order decay lifetime in the solid state (Fig. S3, ESI†). The dif-
ferent phosphorescent colors of microcrystals A–E should be
ascribed to their various morphologies owing to the
morphology-dependent luminescence.11 Especially, the
globate polyhedron induced orange-red phosphorescence is
very unusual, probably attributable to the 0D morphology
and/or the lower crystalline state of microcrystal E. Both Ito
et al.12 and Sagara et al.13 have reported that red-shifted
emission in luminescent materials is related to the amor-
phous state and that the crystalline state is responsible for
the blue-shifted emission. The red-shifted emission of micro-
crystal E would be supported by its PXRD pattern with a
lower crystalline state, compared to microcrystals A–D (Fig.
S1, ESI†).
Moreover, when microcrystal B was dissolved in DMSO–
HCl solution, the resulting microcrystal
E
(BF2-8,8′-
dmbipoHCl) reveals an unusual zero-dimensional (0D)
globate polyhedron morphology with an average diameter of
22.8 μm (Fig. 3), which is also crystalline as confirmed by the
PXRD pattern (Fig. S1, ESI†). To our knowledge, the globate
polyhedron morphology has not been reported in organic
microcrystals. Obviously, when DMSO was used as a solvent
under the same conditions for the fabrication of microcrystal
D, the resulting microcrystal E displayed a quite different
morphology from microcrystal D. It is noteworthy that the
solvent is the only difference in the reaction conditions for
the preparation of such two kinds of morphologies by the
reprecipitation method. As a result, the different morphol-
ogies of microcrystals D and E are ascribed to the solvent-
induced morphological changes.4 It is well known that polar
solvents have a stronger interaction with polar molecules.
The use of DMF as a solvent led to the formation of 1D
microstrips of microcrystal D (Fig. 2b), owing to the relatively
low polarity of the DMF, while the reaction in DMSO which
has a higher polarity10 resulted in the unique globate polyhe-
dron morphology of microcrystal E (Fig. 3), attributable to
the stronger interaction between the “rectangular” BF2-8,8′-
dmbipoH+ cation and DMSO which both have a higher polar-
ity. However, when DMSO was used as a solvent for the prep-
aration of the BF2-7,7′-dmbipoHCl microcrystal, a flower-like
morphology similar to that of microcrystal C was also
obtained, possibly because DMSO which has a higher polarity
did not exhibit an effective interaction with the “linear” BF2-
7,7′-dmbipoH+ cation which has a lower polarity, unlike in
the case of the preparation of the BF2-8,8′-dmbipoHCl
microcrystal.
The photoluminescence (PL) spectrum of microcrystal A
displays a broad emission band in the range of 400–675 nm
Fig. 4 Normalized PL spectra of microcrystals A (a), B (b), C (c), D (d)
Fig. 3 SEM image of globate polyhedra of microcrystal E.
and E (e).
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orange-red phosphorescence and lower NIR absorption,
due to its “isotropical” 0D globate polyhedron morphology.
The importance of this work evidences the fact that the
positional isomers, protonation and solvent can tune mor-
phological properties. Our results could also be helpful for
searching novel organic micro/nanomaterials with poten-
tial applications.
Fig. 5 Photographs of microcrystals A (a), B (b), C (c), D (d) and E (e)
under 365 nm UV lamp irradiation, and microcrystal E under ambient
light irradiation (f).
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (NSFC; grant 21172210).
The phosphorescence quantum yield (PHQY) of microcrys-
tals A–E is 8.7 0.9%, 5.6 0.6%, 22.4 2%, 13.8 1% and
2.1 0.2%, respectively, upon excitation at 365 nm.14,2n Inter-
estingly, microcrystal C shows a higher PHQY, which should
be ascribed to the local field enhancement effect induced by
the anisotropic microsheet structure2n,15 in the flower-like
morphology (Fig. 2a) and/or the thinner sheet thickness
(about 100 nm). Moreover, the lower PHQY of microcrystal E
could be due to its 0D morphology and larger diameter
(about 22.8 μm, Fig. 3). The various PHQYs of microcrystals
A–E should be attributed to their different morphology/size
effects.2n On the other hand, microcrystals A–E display inter-
esting near-infrared (NIR) absorption characteristics with
multiple absorption bands from 1180 nm to 2500 nm
(Fig. S4–S6, ESI†), which can be looked as aggregation-
induced NIR absorption.16 Interestingly, compared to micro-
crystals A and B (Fig. S4, ESI†), microcrystals C and D reveal
an enhanced NIR absorption (Fig. S5, ESI†). Especially,
microcrystal C shows an absorption intensity in the NIR re-
gion (above 2000 nm) that is even higher than that in the
UV-vis region (∼408 nm). Such an enhanced NIR absorption
is also ascribed to the morphological anisotropy of the 2D
microsheets and 1D microstrips. It should be noted that
compared to microcrystals A–D, microcrystal E exhibits not
only fewer NIR absorption bands, but also a lower absorp-
tion intensity in the NIR region (Fig. S6, ESI†). This pheno-
menon should be attributed to its “isotropical” 0D globate
polyhedron morphology and larger diameter.
In conclusion, we have achieved the in situ syntheses of
1D microrods and 3D microbricks of BF2-dmbipo micro-
crystals tuned by the positional isomeric methyl groups,
from which morphology-controllable flower-like microcrys-
tals composed of 2D microsheets and 1D microstrips were
respectively fabricated through protonation by the dissolu-
tion–reprecipitation method. Unprecedentedly, DMSO as
solvent can even induce a globate polyhedron morphology
which is the first reported in organic microcrystals. The
changes in morphology induce distinctive changes in the
photophysical properties. Significantly, the 2D microsheets
in the flower-like microcrystal exhibit higher PHQY (22.4
2%) and enhanced NIR absorption, which are attributed to
the morphological anisotropy effects of the 2D microsheets.
Interestingly, the globate polyhedron microcrystal reveals
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