Two polymorphs and cocrystal of styryl-pyridine derivatives with tuned emission induced by Co2+ and Zn(phen)32+

Article abstract of DOI:10.1039/c3ce42069h

A styryl-pyridine derivative, 2,5-dibutoxy-1,4-bis[2-(4-pyridyl)ethenyl] benzene (L), has been designed and synthesised. Two polymorphs of L and crystals of [Zn(phen)₃]₂L(ClO₄)₄ were obtained by different crystallisation conditions. Polymorph-α was prepared by a solvent evaporation method. However, polymorph-β and a cocrystal of [Zn(phen)₃]₂L(ClO₄)₄ were obtained by different inducing reagents. It has been found that there are, in total, four independent conformations of the molecule L in the three kinds of crystals. We have calculated the standard molar enthalpy of formation (ΔH_m) of each conformation and demonstrated that the stabilisation energy of each conformation significantly relies on both the planarity of the molecules (L) and bond length of conjugated bridges. Furthermore, the photophysical properties of the three kinds of crystals also exhibit differences in the vibration and absorption spectroscopies, solid-state photoluminescence and time-resolved fluorescence, which may arise from the chromophore's stacking modes and the intermolecular electronic interactions in the crystals. These results revealed that the optical properties of functional organic materials could be tuned by controlling their crystallisation environment for a specific molecule. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3ce42069h

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Two polymorphs and cocrystal of styryl-pyridine
derivatives with tuned emission induced by Co2+
and Zn(phen)3 †
Cite this: CrystEngComm, 2014, 16,
2+
2039
a
a
a
a
a
*
Ruilong Zhang, Qiong Zhang, Zhaodi Liu, Longmei Yang, Jieying Wu,
a
a
ab
*
Hongping Zhou, Jiaxiang Yang and Yupeng Tian
A styryl-pyridine derivative, 2,5-dibutoxy-1,4-bis[2-(4-pyridyl)ethenyl]benzene (L), has been designed and
synthesised. Two polymorphs of L and crystals of [Zn(phen)3]2L(ClO4)4 were obtained by different
crystallisation conditions. Polymorph-α was prepared by a solvent evaporation method. However,
polymorph-β and a cocrystal of [Zn(phen)3]2L(ClO4)4 were obtained by different inducing reagents. It has
been found that there are, in total, four independent conformations of the molecule L in the three kinds
of crystals. We have calculated the standard molar enthalpy of formation (ΔHm) of each conformation
and demonstrated that the stabilisation energy of each conformation significantly relies on both the
planarity of the molecules (L) and bond length of conjugated bridges. Furthermore, the photophysical
properties of the three kinds of crystals also exhibit differences in the vibration and absorption spectroscopies,
solid-state photoluminescence and time-resolved fluorescence, which may arise from the chromophore's
stacking modes and the intermolecular electronic interactions in the crystals. These results revealed
that the optical properties of functional organic materials could be tuned by controlling their
crystallisation environment for a specific molecule.
Received 13th October 2013,
Accepted 3rd December 2013
DOI: 10.1039/c3ce42069h
www.rsc.org/crystengcomm
solids composed of at least two types of neutral chemical
species.10,15 As a large family of non-bonding interactions,
Introduction
Nowadays, solid state lighting (SSL) of organic chromophores
has attracted a great deal of attention due to their potential
applications in devices, such as light-emitting diodes,1–3
photovoltaic devices,4–6 and sensors.7–9 Tuning and controlling
the luminescent color of an organic material is crucial to meet
the light-emission with an appropriate wavelength.10,11 It is
well known that the optical properties of chromophores in the
solid state strongly depend on the molecular structure and
intermolecular interactions.12–14 Recent advances in organic
fluorescent crystals have focused on material polymorphism
and cocrystallisation, which are, generally, used to modulate
the properties of organic molecules in crystals. Polymorphs
can be defined as the existence of a particular compound in
more than one crystal structure and cocrystals are molecular
hydrogen-bonding and π–π interactions play an important
role in crystal formation as a driving force to stabilize the
crystals.15–18 Adding appropriate inducing reagents is an available
approach for polymorphs and cocrystals. Also, various effective
additives have been reported in previous work, including poly-
mers, organic compounds, metal ions and so on.15,19,20 For
example, Wang reported that polymorphs of N,N-di(n-butyl)
quinacridone (DBQA) can be obtained from a mixed solution
of DBQA and 7,7,8,8-tetracyanoquinodimethane or perylene-
3,4,9,10-tetracarboxydiimide. The most crucial reason is that the
added reagents have π-conjugated systems similar to DBQA.15
In our previous work, two morphologies of 4′-(4-{2-[4-(N,N-
diphenylamino)-phenyl]ethenyl}phenyl)-2,2′:6′,2″-terpyridine
appear as Eu3+/Ag+ induced features.20
Herein, we report two polymorphs of 2,5-dibutoxy-1,4-bis[2-(4-
pyridyl) ethenyl]benzene (L) and a cocrystal, [Zn(phen)3]2L(ClO4)4
(phen = 1,10-phenanthroline), between L and Zn(phen)3(ClO4)2
which were obtained by different inducing reagents. It is very
interesting to find that the one chromophore molecule L
could exist as four kinds of structures in different crystallisation
environments and emit discriminate colors. The correlations
between the conformation and standard molar enthalpy of forma-
tion (ΔHm) have been interpreted, relying on both experimental
data and theoretical calculations. Finally, the relationships
a Department of Chemistry, Anhui University and Key Laboratory of Functional
Inorganic Materials of Chemistry of Anhui Province, Hefei 230039, PR China.
E-mail: yptian@ahu.edu.cn
b State Key Laboratory of Coordination Chemistry, Nanjing University, 210093,
Nanjing, PR China
† Electronic supplementary information (ESI) available: HRMS and NMR
spectra, X-ray crystallography, powder XRD and IR characterisation, packing
modes of the crystals and theoretical calculation results are described. CCDC
729228, 771894, and 737794. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c3ce42069h
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between their structures and photoluminescence properties
have been investigated.
were added to a stirred solution of 1 (5.56 g, 0.025 mol), with
three 50 mL portions of 40% formalin at 20 min intervals.
During the period of addition, hydrogen chloride gas was
passed through. Stirring and the introduction of hydrogen
chloride gas were continued for a further 3 h. 150 mL con-
centrated hydrogen chloride was then added. After cooling,
the solid material was filtered off, and recrystallised from
ethanol to give compound 2 as white crystals in a 75% yield.
1H NMR (400 MHz, d6-DMSO, δ) 6.935 (s, 4H, ArH), 4.653
(s, 4H, CH3Cl), 4.029–3.997 (m, 4H, –CH2–), 1.843–1.773
(m, 4H, –CH2–), 1.595–1.513 (m, 4H, –CH2–), 1.026–0.989
(m, 6H, –CH3); IR (KBr, cm−1) 2960, 1650, 1514, 1415, 1259,
1223, 1044, 867, 755, 608.
A solution of compound 2 (5.23 g, 0.016 mol) and
triphenylphosphine (8.92 g, 0.034 mol) in 100 mL of anhy-
drous ethanol was stirred and refluxed for 8 h. After distilling
the ethanol and washing with benzene three times, the white
solid 3 was obtained in a 93% yield. The obtained product
was used in the next step without characterisation.
Experimental section
Materials and physical measurements
The reagents and solvents employed were commercially avail-
able and used as received without further purification. IR
spectra were recorded with a Nicolet FT-IR Nexus 870 instru-
ment (KBr disks) in the 400–4000 cm−1 region. 1H NMR
spectra were performed on a Bruker 400 MHz Ultrashield
spectrometer and are reported as parts per million (ppm)
from TMS (δ). Mass spectra (MS) were obtained using an
Agilent Q-TOF 6540 mass spectrometer. Elemental analyses
were performed with a Perkin-Elmer 240C elemental analyzer.
Absorption spectra were recorded on a UV-3600 spectropho-
tometer. The fluorescence spectra were determined with an
F-4500 Spectro-fluorophotome. The powder X-ray diffraction
(XRD) patterns were recorded using a 18 KW advance X-ray
diffractometer with Cu Kα radiation (λ = 1.54056 Å). For time-
resolved fluorescence measurements, the fluorescence signals
were collimated and focused onto the entrance slit of a
monochromator with the output plane equipped with a
photomultiplier tube (HORIBA HuoroMax-4P). The decays
were analysed by “least-squares”. The quality of the exponen-
tial fits was evaluated by the goodness of fit (χ2). Fluores-
cence imaging of the crystals was taken on a Leica DMI6000
fluorescence microscope.
Subsequently, t-BuOK (8.98 g, 0.080 mol) was placed into
a dry mortar and milled. Then, 3 (8.45 g, 0.010 mol) and iso-
nicotinaldehyde (4.28 g, 0.040 mol) were added and mixed.
The mixture was milled vigorously for about 10 min. After
the reaction was completed (monitored by TLC), the mixture
was dispersed in 50 mL H2O. The solution was extracted
three times with 50 mL CH2Cl2. The dichloromethane solu-
tion was dried for 12 h and concentrated. The product was
purified through a silica gel column eluted with petroleum
ether–ethyl acetate (V : V = 8 : 1). A yellow solid was obtained
Synthesis of 2,5-dibutoxy-1,4-bis[2-(4-pyridyl)ethenyl] benzene (L)
1
in a 73% yield. H NMR (400 MHz, d6-DMSO, δ): 8.570–8.556
The general synthetic scheme for the preparation of 2,5-
dibutoxy-1,4-bis[2-(4-pyridyl)ethenyl]benzene is outlined in
Scheme 1.
(d, J = 5.6 Hz, 4H, –pyridine–H ), 7.685–7.644 (d, J = 16.4 Hz,
2H, –CHCH–), 7.510–7.496 (d, J = 5.6 Hz, 4H, –pyridine–H),
7.417–7.375 (d, J = 16.8 Hz, 2H, –CHCH–), 7.394 (s, 2H,
–ArH), 4.133–4.101 (t, 4H, –CH2–), 1.853–1.784 (m, 4H, –CH2–),
1.579–1.487 (m, 4H, –CH2–), 1.013–0.977 (t, 6H, –CH3); 13C
NMR (400 MHz, CDCl3, δC): 150.12, 131.99, 131.49, 131.40,
128.78, 128.67, 127.05, 120.64, 68.52, 30.85, 18.93, 13.76.
Anal. Calc. for C28H32N2O2: C, 78.47; H, 7.53; N, 6.54 (%).
Found: C, 78.50; H, 7.49; N, 6.58 (%). HRMS (m/z, ES) Calcu-
lated for C28H33N2O2 m/z = 429.2542 [M +H]. Found m/z =
429.2514.
Hydroquinone (11.13 g, 0.10 mol) and sodium hydroxide
(8.35 g, 0.21 mol) were dissolved in ethanol. 1-Bromobutane
(27.97 g, 0.20 mol) was added dropwise to the solution and
refluxed for 8 h. The solution was then poured into water (1 L),
and the precipitate was filtered off and recrystallised from
ethanol to give compound 1 as a white solid in an 85%
yield. 1H NMR (400 MHz, d6-DMSO, δ) 6.624 (s, 4H, –ArH),
3.896–3.863 (m, 4H, –CH2–), 1.692–1.622 (m, 4H, –CH2–),
1.466–1.373 (m, 4H, –CH2–), 0.942–0.905 (m, 6H, –CH3);
IR (KBr, cm−1), 2958, 1869, 1639, 1510, 1237, 1043, 831,
767, 535.
Crystallisation of polymorph-α.
L, as a powder, was dissolved in a CHCl3 solution at room
temperature. Yellow needle crystals of polymorph-α suitable
for single crystal X-ray diffraction analysis were obtained after
five days.
Compound 2 was synthesised by the reported method.21
20 mL dioxane and 50 mL concentrated hydrogen chloride
Crystallisation of polymorph-β
First, the CoL(NO3)2 complex was prepared by mixing L
and Co(NO3)2 (molar ratio = 1 : 1) in ethanol. Anal. Calc.
for CoC28H32N4O8: C, 55.00; H, 5.27; N, 9.16 (%). Found:
C, 55.12; H, 5.24; N, 9.22 (%).
Then, CoL(NO3)2 as a powder was dissolved in DMF at
room temperature. Yellow block crystals of polymorph-β
Scheme 1 Synthetic route for target compound L.
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suitable for single crystal X-ray analysis were obtained after
two weeks.
(Van der Waals forces). However, polymorph-α was obtained
instead of polymorph-β without inducing reagents. There-
fore, Co2+ should decrease the activation energy for the
formation of polymorph-β effectively. The third crystal,
Preparation of [Zn(phen)3]2L(ClO4)4
[Zn(phen)3]2L(ClO4)4, was obtained from
a mixture of
Zn(ClO4)2·6H2O (0.37 g, 1.0 mmol), phen (0.18 g, 2.0 mmol)
and L (0.42 g, 1.0 mmol) were dissolved in methanol (20 mL).
The mixture was refluxed for 2 h, cooled to room temperature
and filtered. Orange crystals were obtained in a 42% yield.
IR (KBr, cm−1), 3489, 3064, 2928, 1589, 1516, 1312, 1226,
1189, 724, 621. Anal. Calc. for C100H80Cl4N14O18Zn2: C, 58.92;
H, 3.96; N, 9.62 (%). Found: C, 59.01; H, 3.91; N, 9.65 (%).
Zn(ClO4)2, phen and L due to strong interactions between
L and the Zn(phen)3(ClO4)2 complex. Detailed information
on the differences will be further confirmed by single
crystal X-ray diffraction analysis.
Crystal structures
All the chromophore (L) molecules with different conforma-
tions consist of two pyridine rings, a styryl unit and two
butoxy chains in the three kinds of crystals.
X-ray crystallography
Single-crystal X-ray diffraction measurements were carried
out on a Bruker Smart 1000 CCD diffractometer equipped
with a graphite crystal monochromator situated in the inci-
dent beam for data collection at room temperature. The
determination of the unit cell parameters and data collec-
tions were performed with Mo Kα radiation (λ = 0.71073 Å).
The unit cell dimensions were obtained with least-squares
refinements, and all the structures were solved by direct
methods using SHELXL-97. The other non-hydrogen atoms
were located in successive difference Fourier syntheses. The
final refinement was performed by full-matrix least-squares
methods with anisotropic thermal parameters for non-
hydrogen atoms on F2. The hydrogen atoms were added theo-
retically and riding on the concerned atoms. The crystal-
lographic data for the three crystals are shown in Table S1
in the ESI.† Additional crystallographic details and com-
plete listings of the compounds have been deposited with
the Cambridge Crystallographic Data Center as supple-
mentary publications with reference number CCDC 729228
for polymorph-α, 771894 for polymorph-β, and 737794 for
[Zn(phen)3]2L(ClO4)4.
The molecular structure of L in polymorph-α is displayed
in Fig. 1a. In polymorph-α, the pyridine ring and the benzene
ring are connected with C5–C6: 1.467 Å, C6C7: 1.321 Å,
C7–C8: 1.469 Å and the dihedral angle between the pyridine
ring and the central benzene unit is 44.05°. CC exists in a
trans-isomer in molecule L. It is obvious that no π–π inter-
actions exist in the polymorph. Meanwhile, there are C–H⋯N
and C–H⋯π interactions between the neighboring molecules
(see Fig. 2a). The distances of the C–H⋯N and C–H⋯π inter-
actions are 2.733 Å and 2.832 Å, respectively.
As shown in Fig. 1b, there are two other conformations in
the polymorph-β crystal of the chromophore molecule L. In
conformation 2, the linkage connects the pyridine ring with
the central benzene unit with bond lengths of C19–C20:
1.488 Å, C20C21: 1.252 Å, C21–C22: 1.511 Å. In conforma-
tion 3, the linkage connects the pyridine ring with the central
benzene unit with bond lengths of C5–C6: 1.500 Å, C6C7:
1.154 Å, C7–C8: 1.480 Å. The dihedral angles between the
pyridine ring and the central benzene unit are 8.00° in con-
formation 2 and 9.53° in conformation 3. These results indicate
that the π-conjugated degree of the chromophore moiety in
Results and discussion
Preparation of the polymorphs and cocrystal
The two kinds of polymorph crystals were obtained from
different crystallisation procedures. Polymorph-α was pre-
pared by a solvent evaporation method and polymorph-β
was generated from CoL(NO3)2. Infrared spectra (IR) provide
unambiguous proof for the polymorphism. As shown in
Fig. S3 in the ESI,† polymorph-α and polymorph-β exhibit
different bands in the range from 1600 cm−1 and 1000 cm−1,
which can be assigned to the vibrations of the benzene
and pyridine moieties. XRD patterns provide further experi-
mental evidence for the existence of polymorph-α and
polymorph-β. Fig. S4 in the ESI† shows the experimental
and simulated XRD patterns of polymorph-α and polymorph-β.
Furthermore, the melting points of the two polymorphs were
investigated. The melting point of polymorph-α (196.6 °C)
is lower than that of polymorph-β (201.7 °C). Obviously,
polymorph-β is more stable than polymorph-α because the
melting point relies on the interactions between molecules
Fig. 1 Crystal structures of polymorph-α (a), polymorph-β (b) and
[Zn(phen)3]2L(ClO4)4 (c) with the H atoms omitted for clarity. (d) The
comparative planarities of the four configurations of the chromophore
molecule L with H atoms and butoxy chains omitted for clarity.
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conformation 1. Therefore, it can be inferred that the planarity
of π-conjugated chromophore molecules is a crucial factor
for their stabilisation as crystals.
From analysis of the crystals, conformations 2, 3 and 4
possess similar planarity and different linkage lengths (CC
bond lengths in the linking molecules are 1.252 Å, 1.154 Å
and 1.320 Å in conformation 2, 3 and 4, respectively). How-
ever, the calculation results indicate that the ΔHm value of
conformation 3 is the highest one and the ΔHm values of con-
formations 2 and 4 are almost the same. The CC bond
length is so short (1.154 Å) that the π-electron delocalisation
has been handicapped to a certain extent in conformation 3.
Accordingly, the energy of conformation 3 is the highest one,
although it possesses good planarity. It can be explained that
the bond length of the linkage connecting the pyridine ring
with the central benzene unit is another crucial factor for
their stabilisation as crystals.
Fig.
2 The intermolecular interactions of polymorph-α (a),
polymorph-β (b) and [Zn(phen)3]2L(ClO4)4 (c).
conformation 2 is higher than that in conformation 3. Fig. 2b
illustrates that there are two types of C–H⋯N interactions
between N2 and C–H in adjacent molecules. However, there
are no interactions between N1 and the neighboring molecules.
The structure of the metal complex [Zn(phen)3]2L(ClO4)4 is
shown in Fig. 1c. In molecule L, the pyridine rings and the
benzene ring are linked with C5–C6: 1.452 Å, C6C7: 1.320 Å
and C7–C8: 1.452 Å, and the dihedral angle between the
central benzene ring and the pyridine ring is 10.24°. These
results suggest that there is a more highly conjugated system
in molecule L existing in the crystal of the metal complex
[Zn(phen)3]2L(ClO4)4. Therefore, there are strong π–π inter-
actions between phen and L in this crystal (Fig. 2c). The
shortest distance between the benzene ring in chromophore
Absorption and fluorescence spectra
The absorption and fluorescence properties of the three
crystals were investigated at different determination conditions.
The results indicate that the absorption and emission spec-
tra of the three crystals exhibit almost the same behavior in
dilute solution (1 × 10−5 mol L−1). Therefore, we focused on
their optical properties in the solid state.
Fig. 3 presents a comparison of the absorption spectra of
polymorph-α, polymorph-β, [Zn(phen)3]2L(ClO4)4 and L in a
dilute DMF solution (1 × 10−5 mol L−1). Polymorph-α,
polymorph-β and [Zn(phen)3]2L(ClO4)4 exhibit an absorption
band at around 392 nm, 403 nm, and 412 nm, respectively,
in the solid state. The π–π interactions between L and
2+
L and phen in Zn(phen)3 is 3.288 Å, and between the
pyridine and phen is 3.303 Å. On comparing the two polymorphs,
it is interesting to note that there are no interactions bet-
ween the chromophores. The chromophore L is isolated by
Zn(phen)3 cations. As counter-ions, the ClO4 anions not
only keep the charge balance, but connect the adjacent
Zn(phen)3 cations through C–H⋯O interactions. However,
there are no interactions between the ClO4 anions and L.
Therefore, the cocrystal should be induced by Zn(phen)3
cations.
2+
−
2+
Zn(phen)3 can format oligomers, resulting in the lowest
excitation energy of [Zn(phen)3]2L(ClO4)4.15,23 In addition, the
absorption spectra of solid-state polymorph-β show a remark-
able red shift compared to that of polymorph-α, which is
caused by the different packing modes in the two crystals.
2+
−
2+
Correlations between the conformation and energy
To further understand the correlations between conformation
and energy, DFT theoretical calculation was performed using
the Gaussian 09 software package22 at the B3LYP, BLYP,
B3P86/6-31G(d, p) level with the ground state geometries of
the four conformations obtained from their crystal structures.
Standard molar enthalpy of formation (ΔHm) values for the
four conformations are listed in Table S2 in the ESI.† For
molecule L in each conformation, the one with the lower
ΔHm value is relatively stable, while that with a higher ΔHm
value is unstable.
As described above, the conformations 1 and 4 have
similar linkage lengths but the dihedral angles between the
pyridine moiety and central benzene unit are 44.05° and 10.24°,
respectively. The different dihedral angles cause the ΔHm
value of conformation 4 to be much lower than that of
Fig. 3 Normalized absorption spectra of polymorph-α, polymorph-β
and [Zn(phen)3]2L(ClO4)4 in the solid state.
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Fig. 4 Normalized fluorescence spectra (λex = 400 nm) of polymorph-α,
polymorph-β and [Zn(phen)3]2L(ClO4)4 in the solid state.
Fig. 5 Fluorescence decay curves for the three crystals. (Fluorescence
lifetimes were monitored at 505 nm for polymorph-α, 525 nm for
polymorph-β and 488 nm for [Zn(phen)3]2L(ClO4)4.)
The packing diagrams (Fig. S5 and S6, ESI†) show that the
packing mode of polymorph-β is beneficial to the overlap
between the chromophore molecules compared to that of
polymorph-α. Enhancing the overlap degree of the chromo-
phores can lead to an extended π-electronic system and
enable the molecules to be excited more easily.24
similar to in dilute solution, due to there being no interactions
between adjacent chromophore molecules. The fluorescence
lifetime of polymorph-β is the longest, suggesting that the over-
lap degree within the chromophore molecule of polymorph-β
is higher than that of polymorph-α.
Fig. 4 shows the emission spectra of polymorph-α, poly-
morph-β, [Zn(phen)3]2L(ClO4)4 and L in dilute solution. The
emission maximum band of the crystals of polymorph-α is
located at a wavelength of 505 nm, which undergoes a red
shift compared with that of L in dilute solution, due to the
much stronger intermolecular interactions between the chro-
mophore molecules.25–27 In contrast to the spectrum of poly-
morph-α, the emission band at 525 nm of polymorph-β is
red-shifted by 20 nm, which is consistant with the absorption
spectra above.
Conclusions
In conclusion, in the present work, we found that the same
molecule, L, exists in three kinds of single crystals, poly-
morph-α, polymorph-β and complex [Zn(phen)3]2L(ClO4)4, on
tuning the crystallisation conditions. In addition, there are,
in total, four independent conformations of molecule L in
the three crystals. Relying on both experiments and theoreti-
cal calculations, it can be concluded that the CC double
bond length and the coplanarity between the pyridine and
central benzene units can effectively reduce the stabilisation
energy of the chromophore molecules. When the CC double
bond length is too short, the coplanarity becomes less
important. Furthermore, an investigation of their optical
properties shows that the three conformations display different
colors (see Fig. 6) due to their different molecular configura-
tions and interactions. These findings may have potential
Interestingly, the emission band of [Zn(phen)3]2L(ClO4)4
shows an obvious blue-shift compared to those of poly-
morphs-α/β, which is similar to that in dilute solution. The
spectral features can be explained by the fact that the chro-
2+
mophore molecules are isolated by Zn(phen)3 in the crys-
tal of [Zn(phen)3]2L(ClO4)4 (Fig. 2c). The distance between
them is as long as about 6.9 Å. The interaction between
the chromophore molecules is so weak that it can be
ignored. Therefore, [Zn(phen)3]2L(ClO4)4 exhibits monomer
emission.
Solid-state fluorescence lifetimes
To obtain further insight into the photophysical properties,
the fluorescence lifetimes (τ) were monitored at the maxi-
mum emission wavelength of the three crystals and the corre-
sponding fluorescence decay curves are shown in Fig. 5.
Zn(phen)3]2L(ClO4)4 has the shortest fluorescence lifetime
and polymorph-β has the longest fluorescence lifetime (see
Table S3, ESI†). The results provide further evidence that the
fluorophore in [Zn(phen)3]2L(ClO4)4 favors monomer emission,
Fig. 6 Photographs of the crystals of polymorph-α, polymorph-β and
[Zn(phen)3]2L(ClO4)4 under UV excitation (λex = 365 nm).
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applications for crystal engineering to design different
crystals and color-tuned optical materials.
10 D. Yan, A. Delori, G. O. Lloyd, T. Friščić, G. M. Day,
W. Jones, J. Lu, M. Wei, D. G. Evans and X. Duan,
Angew. Chem., Int. Ed., 2011, 50, 12483.
11 B. Dong, M. Wang, C. Xu, Q. Feng and Y. Wang,
Cryst. Growth Des., 2012, 12, 5986.
Acknowledgements
12 C. W. Lehmann, Angew. Chem., Int. Ed., 2011, 50, 5616.
13 G. Zhang, J. Lu, M. Sabat and C. L. Fraser, J. Am. Chem. Soc.,
2010, 132, 2160.
14 H. Y. Zhang, Z. L. Zhang, K. Q. Ye, J. Y. Zhang and Y. Wang,
Adv. Mater., 2006, 18, 2369.
15 Y. Fan, Y. Zhao, L. Ye, B. Li, G. Yang and Y. Wang,
Cryst. Growth Des., 2009, 9, 1421.
16 Z. Zhang, Y. Zhang, D. Yao, H. Bi, I. Javed, Y. Fan, H. Zhang
and Y. Wang, Cryst. Growth Des., 2009, 9, 5069.
17 D. Cinčić, T. Friščić and W. Jones, J. Am. Chem. Soc.,
2008, 130, 7524.
This work was supported by the National Natural Science
Foundation of China (21271004, 21071001, 21271003,
51142011), the Natural Science Foundation of Anhui Province
(1208085MB22), the Ministry of Education Funded Projects
Focus on returned overseas scholar, the Program for New
Century Excellent Talents in University (China), and the
Doctoral Program Foundation of Ministry of Education of
China (20113401110004).
Notes and references
18 Z. Xie, L. Liu, B. Yang, G. Yang, L. Ye, M. Li and Y. Ma,
Cryst. Growth Des., 2005, 5, 1959.
19 C. P. Price, A. L. Grzesiak and A. J. Matzger, J. Am. Chem. Soc.,
2005, 127, 5512.
1 J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks,
K. Mackay, R. H. Friend, P. L. Burns and A. B. Holmes,
Nature, 1990, 347, 539.
2 Y. Yamaguchi, T. Ochi, S. Miyamura, T. Tanaka,
S. Kobayashi, T. Wakamiya, Y. Matsubara and Z. I. Yoshida,
J. Am. Chem. Soc., 2006, 128, 4504.
3 C. A. Strassert, C. H. Chien, M. D. Galvez Lopez,
D. Kourkoulos, D. Hertel, K. Meerholz and L. De Cola,
Angew. Chem., Int. Ed., 2011, 50, 946.
4 M. O'Neill and S. M. Kelly, Adv. Mater., 2003, 15, 1135.
5 M. K. Fischer, S. Wenger, M. Wang, A. Mishra,
S. M. Zakeeruddin, M. Grätzel and P. Bäuerle, Chem. Mater.,
2010, 22, 1836.
6 N. Cai, S. J. Moon, L. Cevey-Ha, T. Moehl, R. Humphry-Baker,
P. Wang, S. M. Zakeeruddin and M. Grätzel, Nano Lett.,
2011, 11, 1452.
20 Y. H. Gao, W. Zhou, L. Li, J. Y. Wu, H. P. Zhou, J. X. Yang
and Y. P. Tian, Cryst. Growth Des., 2008, 9, 253.
21 J. H. Wood and R. E. Gibson, J. Am. Chem. Soc., 1949,
71, 393.
22 T. Schwich, M. P. Cifuentes, P. A. Gugger, M. Samoc and
M. G. Humphrey, Adv. Mater., 2011, 23, 1433.
23 K. Ye, J. Wang, H. Sun, Y. Liu, Z. Mu, F. S. M. Jiang,
J. Y. Zhang, H. X. Zhang, Y. Wang and C. M. Che, J. Phys.
Chem. B, 2005, 109, 8008.
24 D. Xiao, L. Xi, W. Yang, H. Fu, Z. Shuai, Y. Fang and J. Yao,
J. Am. Chem. Soc., 2003, 125, 6740.
25 S. Li, L. He, F. Xiong, Y. Li and G. Yang, J. Phys. Chem. B,
2004, 108, 10887.
7 K. L. Wong, G. L. Law, Y. Y. Yang and W. T. Wong,
Adv. Mater., 2011, 18, 1051.
26 Z. R. Grabowski, K. Rotkiewicz and W. Rettig, Chem. Rev.,
2003, 103, 3899.
8 H. Liu, J. Mack, Q. Guo, H. Lu, N. Kobayashi and Z. Shen,
Chem. Commun., 2011, 47, 12092.
9 D. Yan, J. Lu, J. Ma, M. Wei, D. G. Evans and X. Duan,
Angew. Chem., Int. Ed., 2011, 50, 720.
27 L. Li, Y. P. Tian, J. X. Yang, P. P. Sun, J. Y. Wu, H. P. Zhou,
S. Y. Zhang, B. K. Jin, X. J. Xing, C. K. Wang, M. Li,
G. H. Cheng, H. H. Tang, W. H. Huang, X. T. Tao and
M. H. Jiang, Chem.–Asian J., 2009, 4, 668.
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