Synthesis and piezochromic luminescence study of a coumarin hydrozone compound

Article abstract of DOI:10.1039/c6cc02937j

A novel coumarin hydrozone compound which exhibits piezochromic luminescence upon grinding was prepared. The piezofluorochromic properties were reversible upon fuming or heating. The intermolecular hydrogen bonds have been observed by single-crystal X-ray structural analysis, which are believed to make a major contribution to the piezofluorochromic properties.

Full text of DOI:10.1039/c6cc02937j

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Synthesis and piezochromic luminescence study
of a coumarin hydrozone compound†
Cite this:DOI: 10.1039/c6cc02937j
Haiyan Yu, Wang Ren, Hongguang Lu, Yanan Liang and Qiusheng Wang*
Received 8th April 2016,
Accepted 11th May 2016
DOI: 10.1039/c6cc02937j
www.rsc.org/chemcomm
A novel coumarin hydrozone compound which exhibits piezochromic can be readily prepared by a single step condensation and shows
luminescence upon grinding was prepared. The piezofluorochromic concentration-dependent emission spectra and color change. In
properties were reversible upon fuming or heating. The intermolecular addition, a change in solid-state luminescence can be induced
hydrogen bonds have been observed by single-crystal X-ray structural by mechanical grinding and luminescent reversion by fuming
analysis, which are believed to make a major contribution to the in dichloromethane (DCM). We believe that this mechano-
piezofluorochromic properties.
responsive luminescent compound will have practical applica-
tion in the development of pressure sensors and security inks.
7-Diethylamino-3-(40-methoxylbenzoylhydrazone)methylcoumarin
Piezochromic luminescent materials (PLMs), a class of ‘‘smart’’
materials, have been extensively studied over the last decade (DBHC) was synthesized via a simple method according to Scheme S1
due to their promising potential as rewritable optical media, (see details in the ESI†). In order to verify the lowest energy spatial
pressure sensors, security inks and memory chips.1 Araki and conformation of DBHC, density functional theory (DFT) calculations
co-workers synthesized tetraphenylpyrene (TPPy) compounds were performed (Scheme 1). Geometry optimizations were performed
bearing four hexyl amide groups at the para-position of phenyl using the Gaussian 09 program. The B3LYP exchange–correlation
units which showed an emission color change upon grinding.2 functional8 and the 6-31G* basis-set9 were employed. The SMD
Tang et al. and Kato et al. prepared various types of tetraphenyl- solvation model10 was used to represent a tetrahydrofuran (THF)
ethylene (TPE)-based dyes independently, which were used as solvent environment. The optimized conformation structure of
fluorescent sensors for the detection of bioactive substances.3 compound DBHC can be seen in Fig. 1. The calculations also
Chi and co-workers reported a series of multifunctional 9,10-distyryl- revealed an intramolecular charge transfer (ICT) trend from the
anthracene (DSA) derivatives, which exhibit multifunctional N,N-diethylamino moiety to the p-methoxyl phenyl group in DBHC.
properties, including aggregation-induced emission (AIE), mechano- The changes in the calculated electron density that can be seen
chromic luminescence, vapochromism, and thermochromism.4 from the HOMO and LUMO of DBHC are shown in Fig. 1.
In addition, other types of organic dyes exhibiting piezochromic
luminescence properties have also been developed.5
The optical properties of DBHC were investigated using its
photoluminescence (PL) spectra in THF solution and in the solid
Based on Kitaigorodoskii’s close-packing principle6 and Etter’s state. The THF solution of DBHC (1 mM) showed the lumines-
hydrogen-bond rule,7 p–p interaction and hydrogen bonding cence maxima (lem) at 488 nm. Interestingly, the fluorescence
interaction are the two critical factors in designing tunable-state emission wavelength red shifted from 488 nm to 572 nm when
luminescent materials. For example, in the studies of Araki and the concentration of DBHC was increased to 5 mM (Fig. 2). This
co-workers, they concluded that hydrogen bond-directed inter- concentration-dependent bathochromic luminescence shift of
action is the dominant factor controlling molecular packing.2 In DBHC in THF could be attributed to enhanced p–p interaction
seeking novel versatile piezofluorochromic compounds, we design and hydrogen bonding interaction.
a special coumarin hydrozone derivative, which can form inter-
molecular hydrogen bonding in the solid state. The compound
Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion,
School of Chemistry & Chemical Engineering, Tianjin University of Technology,
Tianjin 300384, China. E-mail: wangqsh@tjut.edu.cn
† Electronic supplementary information (ESI) available: Experimental details,
DSC measurement, and NMR spectra. CCDC 1472934. For ESI and crystallo-
Scheme 1 Chemical structure of DBHC.
graphic data in CIF or other electronic format see DOI: 10.1039/c6cc02937j
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Fig. 1 Calculated spatial electron distribution of LUMO and HOMO and
the optimized conformation structure of DBHC.
Fig. 3 (a) Images of DBHC taken under 365 nm UV light using different
processes. The fumed sample was obtained by fuming the ground sample
in DCM vapour for 5 min; the reground sample was obtained by grinding
the fumed sample; the heated sample was obtained by heating the
reground sample. (b) Photoluminescence spectra of DBHC under different
treatments.
states are 0.14 (pristine), 0.47 (ground), 0.10 (fumed), 0.36
(reground) and 0.08 (heated), respectively.
To study the mechanism of the piezofluorochromism of
DBHC, wide-angle X-ray diffraction (XRD) and differential
scanning calorimetry (DSC) measurements were carried out.
As shown in Fig. 4, the XRD curve of the pristine sample
exhibited sharp diffraction peaks that indicate a well-ordered
microcrystalline structure. In contrast, the diffraction peaks
disappeared or decreased after grinding, showing that the
structural orders of the pristine sample were disrupted. Either
the intensity or the number of diffraction peaks was recovered by
fuming or heating the ground sample, indicating the recovery
of the microcrystalline structure. Additionally, the ground solid
powder of DBHC showed a cold-crystallization transition peak
Fig. 2 (a) Normalized PL spectra and (b) color change of DBHC in THF
with increasing concentrations of DBHC (1 mM–5 mM), lex = 432 nm.
In the solid state, DBHC displayed red-shifted luminescence
upon being ground using a pestle. Images of DBHC obtained
under UV light, including grinding, fuming, regrinding, and
heating, are shown in Fig. 3a. The pristine sample showed a
luminescence color change from bright yellow to orange-red
after grinding. In addition, the initial state could be recovered
by fuming the ground powder with DCM vapor, which indicates
that the piezofluorochromic properties of DBHC are reversible.
Thermal annealing of DBHC at 170 1C also resulted in a similar
luminescent reversion. Regrinding in a mortar caused a change
in the luminescence color of the fumed sample from bright
yellow to orange-red, and the bright yellow luminescence was
recovered upon heating. These reversible switches can also be
monitored by PL spectra (Fig. 3b) and diffuse reflectance
absorption spectra (Fig. S2, ESI†). It was found that the PL
spectra of the pristine sample exhibited a red-shift after grinding.
Blue-shifts of the PL spectra were observed upon fuming and
heating. The fluorescence quantum yields (Ff) of the corresponding
Fig. 4 XRD curves of DBHC as a powder with different treatments.
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electron-donating group (N,N-diethylamino) to the hydrazone
moiety, resulting in the luminescence bathochromic shift.5b In
the solid powder state, hydrogen-bonded dimers converted to a
slightly disordered state induced by grinding, and this structural
change is accompanied by a luminescence color change from
bright yellow to orange-red.2b The ordered hydrogen bond-
directed packing can be reversed by thermal treatment or DCM
fuming of the ground solid powder. All these indicated that
DBHC was a piezochromic compound.
In summary, a new coumarin benzoyl hydrazone compound has
been synthesized, which showed interesting piezochromic lumines-
cence properties. The spectroscopic properties and luminescence
color change in the solid state were reversible upon grinding and
fuming or heating. The hydrogen bond-directed structure showed
slight disorder that was caused by mechanical grinding, which could
be restored by fuming or heating. These results provide a design
method to use new hydrogen-bonding sites as dominant factors for
designing piezochromic luminescent materials. We believe that
these studies can help researchers obtain a deep insight into the
piezofluorochromic mechanism and develop rewritable media,
pressure sensors and security inks in the future.
We are grateful for funding from the National Natural Science
Foundation of Tianjin (No. 14JCTPJC00549). We thank Dr Q. Wang
(University of South Carolina, UA) for the single crystal and PL
spectra measurements. We would also like to acknowledge the
support of the National Training Program of Innovation and
Entrepreneurship for Undergraduates.
Fig. 5 (a) Displacement ellipsoid plot of the molecular structure. (b)
Molecules associate into dimers by NHꢀ ꢀ ꢀO hydrogen bonding (red dotted
bond) across an inversion center. (c) Molecular packing pattern of
hydrogen-bonded dimers.
at 112 1C prior to melting at 268 1C, indicating an exothermal
recrystallization process of the ground powder in a metastable
amorphous phase converting to a stable crystalline phase. The
DSC curves of DBHC are shown in Fig. S1 (see details in the ESI†).
To obtain further insights into the piezochromic mechanism of
DBHC, a single-crystal X-ray structural analysis was carried out. As
shown in Fig. 5, DBHC packed in a head-to-tail orientation to form
J-aggregates. Intermolecular hydrogen bonds N3–HꢀꢀꢀO1* (2.93 Å)
and N3*–Hꢀ ꢀ ꢀO1 (2.93 Å) were found. Compared with the lowest
energy spatial conformation of DBHC in THF solution, N3–H
inverted to the same orientation as C1QO1 in a single crystal
and formed intermolecular hydrogen bonds. Upon the bonding
of intermolecular hydrogen bonds, two molecules constructed
the slipped packing pattern. The free rotation of N2–N3 and
N2*–N3* bonds was locked, and stable hydrogen-bonded dimers
were obtained.
Based on these results, we postulate that the red-shifts of
photoluminescence spectra in THF solution and in the solid
state were caused by the non-covalent interaction (Fig. 6). Upon
increasing the concentration of DBHC in THF from 1 mM to 5 mM,
more hydrogen-bonded dimers formed. The larger p-conjugated
electron system induced the red-shift of emission spectra and color
change of solution. In addition, a stable hydrogen bond-directed
structure promoted intramolecular charge transfer (ICT) from the
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Fig. 6 Proposed mechanisms of piezofluorochromic properties of DBHC.
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