A blue-green-emitting 3D supramolecular compound: Synthesis, crystal structure and effect of solvents and temperature on the luminescent properties

Article abstract of DOI:10.1080/10610278.2013.792340

A blue-green-emitting three-dimensional supramolecular compound (C ₁₀O₂N₂H₈)(C₉O ₇H₆) (1) was synthesised under hydrothermal conditions and structurally characterised by elemental analysis, IR spectrum, ¹H NMR and single-crystal X-ray diffraction. The crystal belongs to triclinic system with P 1 space group. The crystal structure is stabilised by O-H.O, O-H.N hydrogen bonds and π-π interactions (π-π stacking distance is 3.282 A). Compound 1 exhibits intense green luminescence in solid state at 298 K (λ_em = 546 nm). In addition, absorption and fluorescence characteristics of compound 1 have been investigated in different solvents (DMSO, CH₃CN and CH₃OH). The results show that compound 1 exhibits a large red shift in both absorption and emission spectra as solvent polarity increases (polarity: DMSO>CH₃CN>CH ₃OH), indicating a change in dipole moment of compound 1 upon excitation. Although the emission spectra of compound 1 in CH₃OH are close to it in dimethyl sulfoxide (DMSO), it is revealed that the luminescence behaviour of compound 1 depends not only on the polarity of environment but also on the hydrogen bonding properties of the solvent. Meanwhile, temperature strongly affects the emission spectra of compound 1. Emission peaks of compound 1 were blue shift at 77 K than those at 298 K in both solid state (ca. 142 nm) and solution (ca. 6-23 nm), which was due to the non-radiative transition decreases at low temperature. Moreover, the quantum yield and fluorescence lifetime of compound 1 were also measured, which increased with increasing polarity of solvent, lifetime in DMSO at 298 K (τ₁ = 0.92 μs, τ₂ = 8.71 μs) was the longest one in solvents (298 K: τ₁ = 0.87-0.92 μs, τ₂ = 7.50-8.71 μs; 77 K: τ₁ = 0.72-0.90 μs, τ₂ = 6.88-7.45 μs), which was also shorter than that in solid state (298 K: τ₁ = 1.13 μs, τ₂ = 7.50 μs; 77 K: τ₁ = 0.97 μs, τ₂ = 8.97 μs). This was probably because of the weak polarity environment of compound 1 in solid state.

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A blue-green-emitting 3D supramolecular compound:
synthesis, crystal structure and effect of solvents and
temperature on the luminescent properties
Xin-Ming Wang a , Liang-Sheng Qiang a , Rui-Qing Fan a , Ping Wang a & Yu-Lin Yang a
a Department of Chemistry , Harbin Institute of Technology , Harbin , 150001 , P.R. China
Published online: 07 Jun 2013.
To cite this article: Xin-Ming Wang , Liang-Sheng Qiang , Rui-Qing Fan , Ping Wang & Yu-Lin Yang (2013) A blue-green-emitting
3D supramolecular compound: synthesis, crystal structure and effect of solvents and temperature on the luminescent
properties, Supramolecular Chemistry, 25:7, 416-423, DOI: 10.1080/10610278.2013.792340
To link to this article: http://dx.doi.org/10.1080/10610278.2013.792340
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Supramolecular Chemistry, 2013
Vol. 25, No. 7, 416–423, http://dx.doi.org/10.1080/10610278.2013.792340
A blue-green-emitting 3D supramolecular compound: synthesis, crystal structure and effect
of solvents and temperature on the luminescent properties
Xin-Ming Wang, Liang-Sheng Qiang, Rui-Qing Fan*, Ping Wang and Yu-Lin Yang
Department of Chemistry, Harbin Institute of Technology, Harbin 150001, P.R. China
(Received 11 January 2013; final version received 29 March 2013)
A blue-green-emitting three-dimensional supramolecular compound (C10O2N2H8)(C9O7H6) (1) was synthesised under
1
hydrothermal conditions and structurally characterised by elemental analysis, IR spectrum, H NMR and single-crystal
¯
X-ray diffraction. The crystal belongs to triclinic system with P 1 space group. The crystal structure is stabilised by
˚
OZH· · ·O, OZH· · ·N hydrogen bonds and p–p interactions (p–p stacking distance is 3.282 A). Compound 1 exhibits
intense green luminescence in solid state at 298 K (lem ¼ 546 nm). In addition, absorption and fluorescence characteristics
of compound 1 have been investigated in different solvents (DMSO, CH3CN and CH3OH). The results show that compound
1 exhibits a large red shift in both absorption and emission spectra as solvent polarity increases (polarity:
DMSO . CH3CN . CH3OH), indicating a change in dipole moment of compound 1 upon excitation. Although the
emission spectra of compound 1 in CH3OH are close to it in dimethyl sulfoxide (DMSO), it is revealed that the luminescence
behaviour of compound 1 depends not only on the polarity of environment but also on the hydrogen bonding properties of
the solvent. Meanwhile, temperature strongly affects the emission spectra of compound 1. Emission peaks of compound 1
were blue shift at 77 K than those at 298 K in both solid state (ca. 142 nm) and solution (ca. 6–23 nm), which was due to the
non-radiative transition decreases at low temperature. Moreover, the quantum yield and fluorescence lifetime of compound 1
were also measured, which increased with increasing polarity of solvent, lifetime in DMSO at 298 K (t1 ¼ 0.92 ms,
t2 ¼ 8.71 ms) was the longest one in solvents (298 K: t1 ¼ 0.87–0.92 ms, t2 ¼ 7.50–8.71 ms; 77 K: t1 ¼ 0.72–0.90 ms,
t2 ¼ 6.88–7.45 ms), which was also shorter than that in solid state (298 K: t1 ¼ 1.13 ms, t2 ¼ 7.50 ms; 77 K: t1 ¼ 0.97 ms,
t2 ¼ 8.97 ms). This was probably because of the weak polarity environment of compound 1 in solid state.
Keywords: supramolecular compound; hydrogen bonds; fluorescence characteristics in different solvents; quantum yield
1. Introduction
attention due to their potential application in materials
with luminescent and nonlinear optical properties (12–16).
In this study, we designed and synthesised the
supramolecular compound 1 which possesses green
emission in solid state. Compound 1 consists of a 2-
hydroxyl-1,3,5-tricarboxylic acid (H4OTBA) and a 4-(1H-
Imidazol-1-yl) benzoic acid (HIBA), which are connected
by hydrogen bonds. As other organic acid–base
compounds, HIBA is an aromatic carboxylic acid and N-
containing compound. What is a fascinating and unique
feature of HIBA is that its structure contains imidazole
ring and benzene ring which are linked by a CZN single
bond. Imidazole and its derivatives exhibit a broad
spectrum of biological and biochemical activity, processes
occurring in living systems have been attributed to them
(17). Imidazoles have been applied as a chromophore with
tunable absorption wavelength (18). They also attract
special attention of the researchers who work on metal–
organic frameworks and organic molecular crystals in
recent years (19–21). The aromatic rings of HIBA could
make p–p stacking possible, as well as hydrogen bonds,
Various non-covalent weak interactions play significant
roles in supramolecular architecture. In crystal engineering
(1) and supramolecular chemistry (2), hydrogen bonding
system (3), as well as various non-bonding weak
interactions (4) such as p–p stacking interactions, is the
main key for the organisation of self-assembling
supramolecule. In molecular recognition, hydrogen bond
(OZH· · ·O, OZH· · ·N and NZH· · ·O) is the most
important interaction because of its strength and
directional properties (5). Aromatic p–p stacking
interactions are all pervasive in nature in molecular level;
they play important roles in the stacking of supramolecule
(6). In addition, some crystals of organic small molecules
not only have potential advantages in lower synthetic costs
but also possess organic light emitting properties (7, 8).
The compounds with p–p stacking interactions mostly
have aromatic structures, and could take place p–p (9,
*
10) or n–p (11) transitions which are responsible for
*
most of the optical processes. Many novel complexes with
non-covalent interactions have attracted considerable
*Corresponding author. Email: fanruiqing@hit.edu.cn
q 2013 Taylor & Francis

Supramolecular Chemistry
417
and build a steady supramolecular structure (22, 23).
In addition, in order to study the effects of surrounding
environment on different fluorescence properties (e.g.
maximum emission wavelength, fluorescence quantum
yield and fluorescence lifetime) of compound 1, we
recorded the fluorescence spectra and fluorescence decay
curves of compound 1 in solid state and solutions (DMSO,
CH3CN and CH3OH) at 298 and 77 K, the relevance of
environment to luminescent properties of compound 1 is
discussed in detail.
riding atoms with isotropic displacement parameters. All
non-hydrogen atoms were refined anisotropically. Crystal-
lographic data for compound 1 are given in Table 1, and
selected bond lengths and bond angles are listed in Table
S1 of the Supplementary Information, available online.
Cambridge Crystallographic Data Centre as supplemen-
tary publication No. CCDC–918773. Copies of the data
can be obtained free of charge on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (fax: þ44
1223/336 033; deposit@ccdc.cam.ac.uk).
2. Experimental
3. Results and discussion
2.1. General characterisation
3.1. Structure description
All solvents and reagents were of analytical reagent grade.
Elemental analyses (C, H and N) were performed on a
Perkin-Elmer 240c elemental analyser. IR spectra were
recorded by Nicolet Impact 410 FTIR spectrometer (KBr
The result of single-crystal X-ray diffraction analysis
revealed that compound 1 is triclinic and crystallises in a
space group. The asymmetric unit contains one H4OTBA
and one HIBA molecule, which is shown in Figure 1. The
supramolecular structure of compound 1 was established
by asymmetric unit through non-covalent intermolecular
and intramolecular interactions including hydrogen bond-
ing and p–p stacking interactions (the parameters of
hydrogen bonds are given in Table 2), the crystal packing
structure of compound 1 is shown in Figure 2.
1
pellets, range in 4000–400 cm21). H NMR (400 MHz)
spectra were recorded on a Bruker ACF 400 MHz at room
temperature (rt). (UV–vis spectra were obtained on a
Perkin-Elmer Lambda 20 spectrometer. Photolumines-
cence analysis and fluorescence decay curves were
recorded on Edinburgh FLS920 fluorescence spectrometer
at 298 and 77 K.
This conformation is stabilised by intramolecular/inter-
molecular hydrogen bonding and p–p stackinginteractions.
In compound 1, following ones can be found: OZH·· ·O,
OZH· · ·N, and donor· · ·acceptor distance is in the range of
2.2. Synthesis of compound (C10O2N2H8)(C9O7H6) (1)
˚
˚
3.055–2.639 A. The shortest distance 2.639 A refers to the
intramolecular O9ZH9· · ·O8 (/OHO ¼ 166.638) hydro-
genbond. ForHIBAmolecule, theconjugatedimidazolering
and benzene ring are not co-planar but with a dihedral angle
about 408. The angle that carboxyl deviated from benzene
ring of HIBA molecule is 6.798. For H4OTBA molecule,
O1ZH1· · ·O2 (/OHO ¼ 139.488) and O2ZH2· · ·O1
(/OHO ¼ 141.808) intramolecular hydrogen bonds lead to
deviations of the carboxyls from benzene ring average plane:
1-carboxylate group deviates by 15.968, 3-carboxylate group
by 59.798 and 5-carboxylate group by 2.068. The hydroxyl
group and its corresponding benzene ring are almost co-
A
mixture of 2-hydroxyl-1,3,5-tricarboxylic acid
(H4OTBA) (0.0226 g), HIBA (0.0188 g), with a molar
ratio of 1:1 was mixed in the distilled water (5 mL). After
stirring for 1 h in air, it was transferred into a 15 mL Teflon-
lined stainless steel autoclave and heated at 1608C for
4 days. After cooling to the rt, the mixture was washed with
distilled water, filtered off and dried, then yellow block
crystals were obtained in a 54% yield, based on H4OTBA.
The resultant crystal was stable in air and insoluble in
water or common organic solvents such as dichloro-
methane, toluene and tetrahydrofuran (THF). IR bands
(KBr pellet, cm21): 3448 s, 3137 w, 1608 m, 1525 m, 1413
s, 1305 m. Anal. calcd for compound 1 (%): C, 55.08; H,
3.40; N, 6.76. Found: C, 55.24; H, 3.57; N, 6.60.
˚
planar with the deviation is only about 0.093 A. There is a
significant intermolecular hydrogen bond interaction
O2ZH2· · ·N2 in solid state of compound 1, its bond length
˚
is 3.055 A (/OHN ¼ 139.588). The p–p stacking distance
˚
between imidazole ring and benzene ring is 3.282 A. These
2.3. X-ray diffraction analysis
two ring planes are not parallel-displaced geometry with the
dihedral angle is 10.148.
Suitable crystals of compound 1 were mounted on glass
fibres for single-crystal X-ray measurement. Reflection
data were collected on a Siemens SMART 1000 CCD
diffractometer equipped with graphite-monochromated
3.2. IR spectra of 1
˚
Mo Ka radiation (l ¼ 0.71073 A), operating at
298 ^ 2 K. The structure was solved by direct method
and refined by full-matrix least squares based on F 2 using
the SHELXTL 5.1 software package (24). The hydrogen
atoms were placed at calculated positions and refined as
The IR spectrum of compound 1 is shown in Figure S1 of
the Supplementary Information, available online. In the IR
spectrum, the absorption bands centred at 3448 cm21 may
be assigned to the stretching vibrations of phenolic

418
X.-M. Wang et al.
Table 1. Crystal data and structure refinement parameters of compound 1.
Empirical formula
C19H14N2O9
Formula weight
T (K)
414.32
293(2)
Triclinic
Crystal system
Space group
¯
P 1
˚
a (A)
8.0830(16)
9.6180(19)
11.538(2)
˚
b (A)
˚
c (A)
a (8)
b (8)
g (8)
91.14(3)
101.74(3)
98.74(3)
866.8(3)
2
1.587
0.129
428
3
˚
V (A )
Z
D (Mg m23
)
m (mm21
F(000)
)
Crystal size (mm)
u Ranges (8)
Max. and min. transmission
Limiting indices
0.50 £ 0.30 £ 0.19
3.10–25.00
0.9758 and 0.9383
29 # h # 9
211 # k # 11
213 # l # 13
6865/3043 (R(int) ¼ 0.0241)
3043/2/271
Full-matrix least squares on F 2
1.101
Reflections collected/unique
Data/restraints/parameters
Refinement method
GOF
Final R indices (I . 2 s(I))
R indices (all data)
Largest diff. peak and hole (e · A
a
b
R1a ¼ 0.0779, wR2b ¼ 0.2384
R1 ¼ 0.0914, wR2 ¼ 0.2506
0.736 and 20.637
23
)
˚
a R1 ¼ SkFoj 2 jFck/SjFoj.
b wR2 ¼ (S(w (Fo2 2 Fc2)2)/S(w(F2o)2))1/2
.
hydroxyl. The band at 3137 cm21 is associated with the
benzene ring stretching vibrations. The splitting absorption
bands at 1608(s) cm21, 1525(s) cm21, 1413(s) cm21 and
1305(s) cm21 are attributed to carboxylate group
asymmetric stretching vibrations (nas) and symmetric
stretching vibrations (ns), respectively (25). Characteristic
bands of carboxyl unit appear at 1720–1690 cm21 (26),
whereas the stretching bands of carboxyl groups in
compound 1 shift towards lower frequencies and their
intensity are slightly reduced, which indicate that there exit
Figure 1. Crystal structure of compound 1.

Supramolecular Chemistry
419
7.80 (s, 1H, HIBA–H8), 8.17 (m, 2H, HIBA–H2,6), 8.63–
8.64 (d, 2H, H4OTBA–H4,6), 8.93 (s, 1H, HIBA–COOH),
1
10.49 (m, 3H, H4OTBA–COOH). H NMR (CH3OH-d4,
Table 2. Important hydrogen bond interactions in compound 1.
DZH
˚
(A)
H· · ·A
˚
(A)
D· · ·A
˚
(A)
/DZH· · ·A
(8)
DZH· · ·A
400 MHz) d (ppm) data of compound 1 are 4.93 (s, 1H,
H4OTBA–H2), 7.74 (s, 1H, HIBA–H10), 7.84–7.87 (t,
3H, HIBA–H3,5,11), 8.08 (s, 1H, HIBA–H8), 8.25–8.26 (t,
2H, HIBA–H2,6), 8.75–8.76 (t, 2H, H4OTBA–H4,6), 8.94
(s, 1H, HIBA–COOH), 10.95 (m, 3H, H4OTBA–COOH).
O1ZH1· · ·O2 0.820(1) 1.989(1) 2.666(1)
O2ZH2· · ·O1 0.820(1) 1.973(1) 2.666(1)
O2ZH2· · ·N2 0.820(1) 2.384(1) 3.055(1)
O9ZH9· · ·O8 0.820(1) 1.835(1) 2.639(1)
139.48(1)
141.80(1)
139.58(1)
166.63(1)
1
In the three H NMR spectra of compound 1, signals of
compound 1 are uniform on the whole. Especially the
characteristic peak positions of H4OTBA–COOH in
DMSO-d6, CH3CN-d3 and CH3OH-d4 are almost the
intermolecular or intramolecular hydrogen bond inter-
actions in solid state. This kind of non-covalent bond could
inhibit the molecular vibration (27), this was further
confirmed by single-crystal X-ray diffraction analysis.
1
same. H NMR data indicate that compound 1 keeps the
same structure in DMSO, CH3CN and CH3OH solution.
3.3. 1H NMR spectra
3.4. Spectral behaviour of 1 in different solvents
In order to explain the structure of compound 1 in three
1
polar solvents (DMSO, CH3CN and CH3OH), H NMR
3.4.1. UV–vis absorption spectra
experiments of compound 1 were carried out in DMSO-d6,
CH3CN-d3 and CH3OH-d4, respectively (Figures S2–S4
In UV–vis spectra, the bands in the wavelength range
230–270 nm could be assigned to the excitation of p-
electrons of aromatic system. These bands are sensitive to
substitution at aromatic rings and their positions are
influenced by changing the solvent polarity (28).
1
of the Supplementary Information, available online). H
NMR (DMSO-d6, 400 MHz) d (ppm) data of compound 1
are 5.00 (s, 1H, H4OTBA–H2), 7.23 (s, 1H, HIBA–H10),
7.83–7.85 (t, 3H, HIBA–H3,5,11), 7.93 (s, 1H, HIBA–H8),
8.05–8.08 (t, 2H, HIBA–H2,6), 8.53 (t, 2H, H4OTBA–
Effects of solvent polarity on the electronic absorption
of compound 1 were studied in three organic polar solvents
(polarity: DMSO . CH3CN . CH3OH) at 298 K because
compound 1 is insoluble in water or common organic
solvents. The absorption spectral characteristics of
compound 1 are shown in Figure 3. The absorption
maximum of compound 1 is observed to shift from 229 to
269 nm with an increase in the solvent polarity from
H
4,6), 8.60 (s, 1H, HIBA–COOH), 10.70 (m, 3H,
H4OTBA–COOH). 1H NMR (CH3CN-d3, 400 MHz) d
(ppm) data of compound 1 are 4.76 (s, 1H, H4OTBA–H2),
7.58 (s, 1H, HIBA–H10), 7.69 (m, 3H, HIBA–H3,5,11),
CH CN to DMSO, corresponding to p ! p electronic
*
3
transition of the aromatic rings (29). The red shift implies
that the value of dipole moment is higher in the excited-
state structure than it in ground state (30), so the excited
state is more sensitive than ground state. With the
2.8
DMSO
CH3OH
2.4
CH3CN
2.0
1.6
1.2
0.8
0.4
0.0
200
250
300
350
400
Wavelength/nm
Figure 2. (a) Supramolecular structure of compound 1, (b)
hydrogen bonding interactions between the molecules and (c)
packing structure for compound 1.
Figure 3. The UV–vis spectra of compound 1 in DMSO,
CH3OH and CH3CN solutions at 298 K.

420
X.-M. Wang et al.
increasing of solvents polarity, excited state p* is to be
more stable than ground state p. Thus on going to more
polar solvents, the excited state is to be lower energy level,
which reduced the energy difference between ground state
and excited state, so the absorption spectra are red shift
with the increase of solvent polarity.
(a)
DMSO 298 K
CH3OH 298 K
CH3CN 298 K
1.0
0.8
0.6
0.4
0.2
0.0
However, it is important to note that although CH3CN
possesses a more strong polarity than CH3OH, the UV
absorption spectra in CH3OH (l ¼ 243 nm) are red shift
than that in CH3CN (l ¼ 229 nm). It may be due to that in
polar solvent, the donor ability of hydrogen bond is
responsible for a contributing factor for higher extent of
stabilisation of excited state (31). CH3OH is polar protic
solvent, which is a hydrogen bond donor with strong
polarity; it has very strong interaction with solute through
hydrogen bond. CH3CN is polar aprotic solvent; this kind
of solvent cannot be used as a hydrogen bond donor (32).
As a result, there has been a red shift to longer wavelength
in CH3OH than in CH3CN.
400
450
500
550
600
Wavelength/nm
(b)
DMSO 77 K
CH3OH 77 K
1.0
0.8
0.6
0.4
0.2
0.0
CH3CN 77 K
3.4.2. Fluorescence spectra
To investigate interaction between the compound
molecule and solvent, the emission spectra of compound
1 in solutions (CH3OH, CH3CN and DMSO) with the same
concentration (1.0 £ 1025 mol/L) were examined by
fluorescence spectrophotometry at 298 and 77 K, the
results are shown in Figure 4. It is shown that compound 1
displays blue-violet fluorescence emission in these polar
aprotic or polar protic solvents in the 400–600 nm region.
The emission peaks positions are sensitive to the polarity
of medium. Ongoing from CH3CN to DMSO, low-energy
bands of the emission bands of compound 1 are at ca.
476–483 nm, and high-energy emission bands are at ca.
400
450
500
550
600
Wavelength/nm
Figure 4. Normalised emission spectra of compound 1 in
DMSO, CH3OH and CH3CN solutions (concentration:
(M) < 1025 M) at (a) 298 K and (b) 77 K.
428–447 nm, which can be attributed to p ! p
*
the low-energy emission bands appear as shoulders. We
consider the blue shift on the change of the emitting state is
closely related to vibrational coupling. At low tempera-
ture, the vibrations are hardly restricted by the interaction
of energy level (35), in that case, energy of the transition
from excited to ground state increase.
fluorescence emission (33). It undergoes a red shift (ca.
19 nm) with increasing the solvent polarity from CH3CN
to DMSO, which is attributed to that the ground-state
energy distribution is not affected to a greater extent
possibly due to the less polar nature than that in excited
state. Although DMSO possesses a more strong polarity
than CH3OH, the fluorescence spectra in these two
solvents are almost the same. It is due to the donor ability
of CH3OH as polar protic solvent (34). The hydrogen bond
donor nature of CH3OH seems to stabilise the excited state
of the electronic transitions through solute–solvent
interactions. DMSO is polar aprotic solvent, which cannot
be used as a hydrogen bond donor (32). As a result, the
fluorescence spectra in DMSO and CH3OH solvents are
almost the same.
3.4.3. Fluorescence quantum yield and time-resolved
fluorescence decay
The fluorescence quantum yields (Fƒ) of 1 were
determined in DMSO, CH3CN and CH3OH using diluted
solution (1.0 £ 1025 mol/L). Quinine sulphate with known
quantum yield acts as reference fluorophore. The
expression Fu=Fs ¼ Fu=FsAs=Aun2u=ns2 was used for
calculating the quantum yields, in which the indices u
and s refer to the sample of the compound (with unknown
quantum yield) and the reference fluorophore, F is the area
under the fluorescent emission curves, A denotes the
absorbance at the wavelength of excitation and n is
The change of temperature from 298 to 77 K caused a
blue shift of emission peaks (ca. 6–23 nm) in the three
solutions of compound 1 (Figure 5). Low- and high-energy
emission bands of compound 1 are at ca. 469–477 and
421–444 nm, from CH3CN to DMSO, respectively, and

Supramolecular Chemistry
421
298 and 77 K, it showed that the lifetimes of compound 1
in solution not depend strongly on the temperature but on
solution (Figure 6), corresponding data are given in
Table 3. The fluorescence lifetime in DMSO at 298 K is
the longest one, t1 ¼ 0.92 ms (81.28%), t2 ¼ 8.71 ms
(18.72%). Although in CH3OH, the fluorescence lifetime
is substantially shortened comparison with that in aprotic
solvents (DMSO and CH3CN). This can be attributed to
hydrogen bonding interaction in excited state of compound
1, which enhances the non-radiative deactivation of the
excited state (37).
(a)
DMSO 298 K
DMSO 77 K
1.0
0.8
0.6
0.4
0.2
0.0
400
450
500
550
600
650
Wavelength/nm
3.5. Fluorescence spectra of 1 in solid state
(b)
The emission band for compound 1 is located at 546 nm,
exhibiting strong green fluorescence in solid state at 298 K
(lex ¼ 338 nm) (Figure 7). In the same condition, discrete
H4OTBA molecule displays a broad peak at 481 nm
(lex ¼ 320 nm) (Figure 7). In contrast to H4OTBA and in
solutions, the emission band of compound 1 in solid state
is red shift, which may be due to the intermolecular
hydrogen bonds. The hydrogen bonds can effectively
decrease the HOMO–LUMO energy gap, so as to
CH3CN 298 K
CH3CN 77 K
1.0
0.8
0.6
0.4
0.2
0.0
(a)
400
450
500
550
600
650
DMSO 298 K
Wavelength/nm
CH3CN 298 K
CH3OH 298 K
1000
100
10
(c)
CH3OH 298 K
CH3OH 77 K
1.0
0.8
0.6
0.4
0.2
0.0
20
30
40
Time/µs
50
60
400
450
500
550
600
650
(b)
1000
100
10
Wavelength/nm
DMSO 77 K
CH3CN 77 K
CH3OH 77 K
Figure 5. Normalised emission spectra of compound 1 at 298 K
compared with it at 77 K in DMSO, CH3OH and CH3CN
solutions (concentration: (M) < 1025 M).
the refractive index of the solvent (36). The value of the
quantum yields for compound 1 in DMSO, CH3CN and
CH3OH is 0.079, 0.012 and 0.166, respectively, which
depends strongly on solvent properties. The remarkable
increase in quantum yield (Fƒ) in polar protic solvent
(CH3OH) is observed than that in polar aprotic medium
(DMSO, CH3CN). This may be due to differential
contribution of hydrogen bonding interactions.
20
30
40
50
60
Time/µs
The fluorescence lifetimes in solution (DMSO,
CH3CN and CH3OH) of compound 1 were measured at
Figure 6. The decay curves of compound 1 measured in DMSO,
CH3OH and CH3CN solutions at (a) 298 K and (b) 77 K.

422
X.-M. Wang et al.
Table 3. Photoluminescent data for compound 1 and H4OTBA.
Absorption (nm)
Compound 1/dm3 cm21 mol21
Excitation
(l, nm)
Emission
(lmax., nm)
Lifetime
(ms)
Quantum
yields (F)a
Conditionsb
1
269
–
243
–
229
–
–
291
289
285
324
281
283
338
316
320
444, 483
t1 ¼ 0.92 (81.28%) t2 ¼ 8.71 (18.72%)
0.079
–
0.166
–
0.012
–
–
–
–
DMSO, 298 K
DMSO, 77 K
CH3OH, 298 K
CH3OH, 77 K
CH3CN, 298 K
CH3CN, 77 K
Solid state, 298 K
Solid state, 77 K
Solid state, 298 K
444, 477sh t1 ¼ 0.90 (68.89%) t2 ¼ 7.45 (31.11%)
447, 485
t1 ¼ 0.79 (78.64%) t2 ¼ 6.34 (21.36%)
424, 473sh t1 ¼ 0.72 (72.17%) t2 ¼ 6.88 (27.83%)
428, 476sh t1 ¼ 0.87 (75.57%) t2 ¼ 7.50 (24.43%)
421, 469sh t1 ¼ 0.73 (52.80%) t2 ¼ 7.03 (47.20%)
546
404
487
t1 ¼ 1.13 (44.03%) t2 ¼ 7.50 (55.97%)
t1 ¼ 0.97 (65.05%) t2 ¼ 8.97 (34.95%)
t1 ¼ 1.05 (54.18%) t2 ¼ 5.56 (45.82%)
–
–
H4OTBA
a Determined using quinine sulphate in 0.1 M sulphuric acid ((M) ¼ 1 £ 1024 M, Fs ¼ 0.546).
b Concentration in DMSO, CH3CN and CH3OH solution: (M) ¼ 1 £ 1025 M.
influence the p ! p transitions (38). The luminescent
we can see the strong blue shift in emission spectra of
compound 1 (ca. 142 nm) (Figure 7), and the lifetime of
compound 1 decreases at 77 K [t1 ¼ 0.97 ms (65.05%),
t2 ¼ 8.97 ms (34.95%)]. It is due to the radiative rate
increases and the non-radiative rate decreases for
compound 1 in solid state at low temperature (40).
*
lifetime for compound 1 in solid state is determined to be:
t1 ¼ 1.13 ms (44.03%), t2 ¼ 7.50 ms (55.97%), which is
longer than that in DMSO. It probably because of the less
polar nature of the environment in solid state (39). At 77 K,
(a)
4. Conclusion
H4OTBA
298 K
1.0
The supramolecular compound 1 was successfully
synthesised under hydrothermal methods and its single
crystal structure was confirmed by X-ray diffraction
analysis. In crystal structure, significant non-covalent
interactions are noted, involving OZH· · ·O, OZH· · ·N and
77 K
0.8
0.6
0.4
0.2
0.0
˚
p–p stacking (p–p stacking distance is 3.282 A), on
account of which a three-dimensional feature was formed.
At 298 K, compound 1 exhibits an intense green
fluorescent emission at 546 nm (lex ¼ 338 nm) in solid
state. Good fluorescent properties of compound 1 reveal
that it is a good fluorescence material candidate and has
potential applications in optical devices. In addition, we
found that the peak position of compound 1 is sensitive to
the polarity of the medium as well as specific solute–
solvent interaction such as hydrogen bonding. It exhibits
blue-purple fluorescent emissions from DMSO, CH3OH to
CH3CN solution at 298 K, and the UV–vis spectra show a
large red shift, as the solvent polarity increases, the
maximum shift was 40 nm. Moreover, the fluorescence
quantum yield and lifetime of compound 1 are dependent
on the polarity of the solvent. On changing temperature
from 298 to 77 K, certain extent of changes occur in
fluorescence spectra of compound 1 in both solution and
solid state, the maximum shift is 23 and 142 nm towards
blue, respectively.
400
450
500
550
600
650
700
Wavelength/nm
(b)
1000
H4OTBA
298 K
77 K
100
10
20
30
40
50
60
Time/µs
Acknowledgements
Figure 7. Normalised emission spectra (a) and decay curves (b)
of H4OTBA at 298 K and those of compound 1 at 298 and 77 K in
solid state.
This work was supported by the National Natural Science
Foundation of China (Grant Nos 21071035 and 21171044),
National Key Basic Research Program of China (973 Program,

Supramolecular Chemistry
423
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