Mechanofluorochromism, polymorphism and thermochromism of novel D-π-A piperidin-1-yl-substitued isoquinoline derivatives

Article abstract of DOI:10.1039/c9tc03942b

Two novel D-π-A isoquinoline derivatives IQ-TPA and IQ-PC using triphenylamine and 9-phenyl-9H-carbazole as an electron donor, respectively, were synthesized to investigate their applications in mechanofluorochromic (MFC) materials. The design was inspire

Full text of DOI:10.1039/c9tc03942b

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Mechanofluorochromism, polymorphism and
thermochromism of novel D–p–A piperidin-1-yl-
substitued isoquinoline derivatives†
Cite this: J. Mater. Chem. C, 2019,
7, 12580
Yating Chen, Xinyu Zhang, Mengzhu Wang, Jingqing Peng, Yunbing Zhou,
Xiaobo Huang, * Wenxia Gao,
Miaochang Liu
and Huayue Wu
*
Two novel D–p–A isoquinoline derivatives IQ-TPA and IQ-PC using triphenylamine and 9-phenyl-9H-carbazole
as an electron donor, respectively, were synthesized to investigate their applications in mechanofluorochromic
(MFC) materials. The design was inspired by an accidentally obtained piperidine-1-substituted isoquinoline
intermediate emitting strong blue solid-state fluorescence, which was obtained from the reaction of a 4H-pyran
derivative with piperidine by a complex ring-opening/closing mechanism. These two compounds with highly
twisted conformations both had two crystalline polymorphs in which the piperidin-1-yl group was located on
different sides of the isoquinoline core. The different emissions of the polymorphs were confirmed to originate
from their different molecular conformations. For IQ-TPA, yellow-emitting IQ-TPA-y could be converted
into green-emitting IQ-TPA-g and exhibited hypsochromic MFC and thermochromic properties by the
transformation between different crystalline states, respectively. Regarding IQ-PC, the two polymorphs did
not exhibit MFC activities owing to the strong intermolecular interactions in the crystalline state, while the
transition from green-emitting IQ-PC-g to yellow-emitting IQ-PC-y could be achieved by an annealing
treatment. This indicated that different electron-donating groups led to different solid-state fluorescence
stimulus-responsive properties. This work provides some useful information for the development of
traditional fluorescent molecules with MFC, polymorphic, and thermochromic properties.
Received 20th July 2019,
Accepted 18th September 2019
DOI: 10.1039/c9tc03942b
rsc.li/materials-c
in 2007,3 an amide-substituted 1,3,6,8-tetraphenylpyrene derivative,
some organic compounds with MFC properties have been devel-
Introduction
Stimulus-responsive organic fluorescent materials in the solid oped in recent years; but up to now, most of them have been
state can undergo emission color changes under external stimuli concentrated on tetraphenylethylene,4 distyrylanthracene,5 and
such as pressure, acid vapor, temperature, or organic solvent cyanoethylene6 as basic structural units, and the structural types
vapor, showing mechanofluorochromic (MFC), acidochromic, are not abundant enough. Obviously, as a vast treasure trove of
thermochromic, and vapochromic properties, respectively, and fluorescent molecules, the conventional organic fluorescent
they have attracted great attention from researchers in recent years molecular skeletons have not been fully utilized.2c A very impor-
because of their important application prospects in the fields tant reason is that traditional organic fluorescent molecules are
of fluorescent sensors, information storage, safety inks, anti- mostly rigid planar molecules with large p-conjugated systems,
counterfeiting commodities, and organic light-emitting devices.1 which means they easily form excimers and exciplexes in the
Among these fluorescent materials, MFC-active materials, whose aggregated state by the strong p–p stacking interactions, and
fluorescence color changes often originate from morphological thus result in weakened or quenched solid-state fluorescence.7
changes, are of particular importance because they often also have This kind of aggregation-induced quenching effect greatly limits
thermochromic and vapochromic properties.2 Since Sagara and their applications in organic solid-state fluorescent materials.
Araki et al. reported the first pure organic MFC-active compound Therefore, the proper structural modification of traditional
fluorophores to obtain novel MFC-active molecules and the
College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou,
325035, P. R. China. E-mail: xiaobhuang@wzu.edu.cn, huayuewu@wzu.edu.cn
† Electronic supplementary information (ESI) available: Experimental, char-
acterization, crystal structures, fluorescence spectra, absorption spectra, photo-
physical data, and other additional information. CCDC 1940020–1940023. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c9tc03942b
elucidation of their structure–activity relationships are still a
subject worthy of study.
Recently, our group’s effort has been devoted to the develop-
ment of stimulus-responsive solid-state fluorescent materials based
on two traditional fluorescent molecular skeletons: 4H-pyran unit8
and its N-substituted derivative (1,4-dihydropyridine unit),9
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and synthesize two novel D–p–A piperidin-1-yl-substitued iso-
quinoline derivatives IQ-TPA and IQ-PC using triphenylamine
and 9-phenyl-9H-carbazole as an electron donor, respectively
(Scheme 1). These two compounds were expected to exhibit
twisted structures and could be used as candidates for MFC-
active compounds based on the following reasons. Firstly, the
introduction of electron-donating triphenylamine/9-phenyl-9H-
carbazole into PIQ endowed the target molecules with a D–p–A
structure, which could be used to regulate emissions by intra-
molecular charge transfer (ICT) and was also conducive to
realizing the fluorescence color changes under external stimu-
lation according to the previous reports.12 Secondly, the large
steric hindrance of the triphenylamine/9-phenyl-9H-carbazole
group could further enhance the distortion of molecular con-
formations, which was beneficial to obtaining loose stacking
arrangements and appropriate crystallinity, and thus might
endow the target compounds with MFC properties.13 Finally,
the existence of the cyano group, acetylene bond, and aromatic
rings in the molecular structures also helps to increase the
possibility of the formation of polymorphs by adjusting mole-
cular stacking arrangements through C–Hꢀ ꢀ ꢀp bonds, C–Hꢀ ꢀ ꢀN
hydrogen bonds, and p–p stacking interactions. The generation
of the polymorphs for a particular molecule is favorable for
enriching the fluorescence color changes in MFC materials.14
On this basis, the photophysical properties in solvent and solid
state, the solid-state fluorescent stimulus-responsive proper-
ties, and the relationship between structures and properties of
IQ-TPA and IQ-PC were investigated in detail.
Scheme 1 Chemical structures of compounds 1 and PIQ, and the target
compounds IQ-TPA and IQ-PC.
whose derivatives have the advantages of simple structures, easy
modification, convenient synthesis, and excellent photophysical
properties.10 In the course of investigating the synthesis of these
kinds of compounds, unexpectedly, a novel isoquinoline derivative,
8-hydroxy-3,6-dimethyl-1-(piperidin-1-yl)isoquinoline-7-carbonitrile
(PIQ), was obtained from an unreported reaction of a 4H-pyran
derivative 2-(2,6-dimethyl-4H-pyran-4-ylidene)-3-oxopentanedinitrile
(1) with piperidine (Scheme 1). This compound had a twisted
conformation due to the existence of piperidin-1-yl group at the
1-position of the isoquinoline core and emitted strong blue
solid-state fluorescence with a fluorescence quantum yield (FF)
of 44%. It is worth mentioning that although isoquinoline is an
excellent traditional structural unit for constructing various
fluorescent materials,11 to the best of our knowledge,
isoquinoline-based pure organic compounds with MFC proper-
ties have not been reported. Although the solid sample of PIQ
Results and discussion
Synthesis and characterization
did not show MFC activity owing to its strong crystallization Scheme 2 displays the synthetic routes of IQ-TPA and IQ-PC.
ability and thus its unchanged morphology before and after 2-(2,6-Dimethyl-4H-pyran-4-ylidene)-3-oxopentanedinitrile (1) could
grinding (Fig. S1, ESI†), the finding of PIQ inspired us to design be obtained from the reaction of compound 2 with cyanoacetic acid
Scheme 2 Synthetic routes of IQ-TPA and IQ-PC.
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Table 1 UV-vis absorption and emission data of IQ-TPA and IQ-PC in
different solvents
using acetic anhydride as the solvent according to previous
literature.15 The key intermediate 8-hydroxy-3,6-dimethyl-1-
(piperidin-1-yl)isoquinoline-7-carbonitrile (PIQ) was prepared
by the reaction of compound 1 with piperidine in acetonitrile
as solvent in the yield of 85.0%, which has not been reported in
previous literature. The synthesis of PIQ might undergo a
complex ring-opening/closing mechanism, which is shown in
Scheme S1 (ESI†). The reaction of PIQ with trifluoromethane-
sulfonic anhydride gave 7-cyano-3,6-dimethyl-1-(piperidin-1-
yl)isoquinolin-8-yl trifluoromethanesulfonate (3) in 80.5% yield.16
IQ-TPA and IQ-PC were obtained from the palladium-catalyzed
Sonogashira–Hagihara reaction of compound 3 with compound 8
and compound 9 in the yields of 47.5% and 67.8%, respectively,
using a procedure similar to previous literature.17 The experimental
results of 1H and 13C NMR spectroscopy, HRMS-ESI spectrometry,
and single-crystal X-ray diffraction analysis confirmed the chemical
structures of IQ-TPA and IQ-PC.
Compound
Hexane CHCl3 THF
CH3OH DMSO
max
em
IQ-TPA
l
l
l
l
(nm) 482
(nm) 406
(nm) 499
(nm) 366
0
509
415
528
370
511
409
527
367
535
409
540
360
556
412
549
366
max
abs
max
em
IQ-PC
max
abs
Df
0.155
0.210 0.308
0.263
naked eye under a 365 nm UV lamp, indicating that these com-
pounds exhibit solvent-dependent activities in their emissions. The
solvatochromic properties of these compounds should be attributed
to the fact that the increase of the polarity is advantageous to
stabilizing the excited states of the molecules and enhance the
ICT effect, and thus results in the red shifts of the emissions.19
Polymorphism and crystal structures
IQ-TPA and IQ-PC both exhibited polymorphic properties.
For IQ-TPA (Fig. 2a), two kinds of single crystals IQ-TPA-g and
IQ-TPA-y were cultured from the slow diffusion of chloroform
and methanol mixtures (v/v = 2 : 1 for the former and v/v = 1 : 2
for the latter) and they exhibited green and yellow fluorescence
centered at 528 and 552 nm with FF values of 30 and 12%,
respectively. Regarding IQ-PC (Fig. 2b), green-emitting IQ-PC-g
Photophysical properties in solutions
The target compounds are soluble in hexane, chloroform, dichloro-
methane, tetrahydrofuran (THF), dimethyl sulfoxide, and methanol.
Fig. 1 displays the UV-vis absorption and fluorescence spectra
of IQ-TPA and IQ-PC in THF solution. The maximum absorp-
tion wavelength (lmabasx) of IQ-TPA and IQ-PC is 409 and 367 nm
in the long wavelength 300–500 nm region, respectively, which
should be ascribed to the delocalized p-conjugation, indicating
that IQ-TPA has stronger electron-donating ability than IQ-PC
in THF solution. However, IQ-PC has a larger maximum emis-
(lem = 527 nm, FF = 43%) and yellow-emitting IQ-PC-y (lem
=
542 nm, FF = 54%) were cultured by the slow evaporation of
chloroform/ethyl acetate (v/v = 1 : 2) and chloroform/petroleum
ether (v/v = 1 : 1), respectively. IQ-TPA-g has a larger radiative
rate constant (kf = 3.26 ꢁ 107 sꢂ1) and smaller non-radiative rate
constant (knr = 7.61 ꢁ 107 sꢂ1) than IQ-TPA-y (kf = 1.19 ꢁ 107 sꢂ1 and
knr = 8.76 ꢁ 107 sꢂ1), and thus it shows a higher FF value (Table 2).
max
em
max
sion wavelength (l
= 527 nm) compared with IQ-TPA (l
=
em
511 nm), suggesting that the THF solvent is more conducive to
stabilizing the excited states of IQ-PC molecules. Considering
that solvent polarity often affects the photophysical properties
of D–p–A-type fluorescent molecules,12d,18 the UV-vis absorption
and fluorescence spectra of IQ-TPA (Fig. S2, ESI†) and IQ-PC
(Fig. S3, ESI†) in various solvents were further investigated and
the corresponding data are shown in Table 1. The absorption
spectra of these compounds only show slight changes in
different polar solvents, which indicates that the solvent polarity
has no obvious influence on the ground states of the molecules.
However, their emission spectra exhibit an obvious red shift with
the increase of the orientation polarizability (Df ) of the solvent
from hexane (Df = 0) to MeOH (Df = 0.308), and the fluorescence
color changes from cyan to yellow can be observed with the
Fig. 2 The normalized fluorescence spectra of the polymorphs of IQ-TPA (a)
and IQ-PC (b). Insets: Emission pictures of the polymorphs of IQ-TPA and
IQ-PC under UV light (lex = 365 nm) obtained on a Leica DMI3000B
inverted optical microscope.
Fig. 1 The normalized fluorescence and absorption spectra of IQ-TPA
and IQ-PC in THF solvent. Concentration: 1 ꢁ 10ꢂ5 M.
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Similarly, IQ-PC-y has stronger luminescence ability compared
with IQ-PC-g.
Furthermore, the single crystal structures of the IQ-TPA and
IQ-PC polymorphs are investigated. Fig. 3 shows the molecular
conformations of their polymorphs and the corresponding
crystallographic data are collected in Table S1 (ESI†). For the
polymorphs of IQ-TPA and IQ-PC, the different photophysical
properties can be discussed based on the single crystal struc-
tures because their X-ray diffraction (XRD) curves match well
with the simulated XRD curves obtained from the corres-
ponding single crystals (Fig. S4 and S5, ESI†). As shown in
Fig. 3a and b, although both the polymorphs of IQ-TPA have
twisted conformations, one obvious difference is that the
piperidin-1-yl group in IQ-TPA-g is located below the isoquino-
line core, while that in IQ-TPA-y is located above the isoquino-
Fig. 3 Single-crystal structures of IQ-TPA and IQ-PC: (a) IQ-TPA-g;
(b) IQ-TPA-y; (c) IQ-PC-g; (d) IQ-PC-y. Hydrogen atoms are omitted
and the piperidin-1-yl units are marked in red.
line core. Even so, IQ-TPA-g (Fig. 4b) and IQ-TPA-y (Fig. S6b, their solid-state absorption spectra (Fig. S8a, ESI†). Therefore,
ESI†) adopt a similar zigzag packing arrangement containing IQ-TPA-g exhibits a smaller emission wavelength than IQ-TPA-y.
the characteristic molecular pairs. For IQ-TPA-g, the molecules
Similar to those of IQ-TPA, the piperidin-1-yl groups in the
within one pair are stabilized by the C13–H13Aꢀ ꢀ ꢀp (Ph) bond two polymorphs of IQ-PC are located on different sides of the
with a distance of 3.180 Å, and different pairs are linked by the isoquinoline core and the highly twisted molecular conforma-
C22–H22ꢀ ꢀ ꢀp (Ph) bond and C37–H37ꢀ ꢀ ꢀp (Ph) bond with a tions result in the absence of pꢀ ꢀ ꢀp stacking interactions (Fig. 3c
distance of 3.246 and 3.033 Å, respectively (Fig. 4a). Analogously, and d). Moreover, IQ-PC-g (Fig. 5c) and IQ-PC-y (Fig. S9c, ESI†)
the molecules in IQ-TPA-y (Fig. S6a, ESI†) are also stabilized by adopt similar zigzag packing arrangements. Different from
the C–Hꢀ ꢀ ꢀp bonds and the corresponding distances in these IQ-TPA, the molecules in the polymorphs of IQ-PC are stabilized
bonds are very close to those in IQ-TPA-g. The C–Hꢀ ꢀ ꢀp bonds in by both C–Hꢀ ꢀ ꢀp bonds (2.693–3.756 Å) and C–Hꢀ ꢀ ꢀN hydrogen
the polymorphs can fix the molecular conformations and thus bonds (2.715–2.734 Å), which suggests that the IQ-PC poly-
prevent the internal motions in the molecules, which results morphs have stronger intermolecular interactions (Fig. 5 and
in their solid-state emissions. The distances of two molecules in Fig. S9, ESI†). Although the corresponding distances of the non-
IQ-TPA-g (Fig. 4a) and IQ-TPA-y (Fig. S6a, ESI†) within one pair covalent interactions in IQ-PC-g and IQ-PC-y are very close, the
are measured to be 4.052 and 4.059, respectively, and the larger dihedral angle (between ring A and plane C) and torsion
aromatic rings hardly overlap; as a result, there are no pꢀ ꢀ ꢀp angle (between ring A and ring B) in IQ-PC-g are larger than
stacking interactions in the crystal structures of these two those in IQ-PC-y, which can be used to explain the more twisted
polymorphs. Therefore, the emissions of the polymorphs should conformation and blueshifted emission of the former (Fig. S7c
depend on their molecular conformations.13f,h,20 For IQ-TPA-g and d, ESI†).
and IQ-TPA-y, the isoquinoline rings are not in the same plane
due to the presence of piperidin-1-yl with steric hindrance and
MFC and thermochromic properties
the dihedral angles between benzene ring (A) and pyridine ring We first investigated the solid-state fluorescence stimulus-
(referred to as plane C) are 15.641 and 15.161, and the torsion responsive behaviors of IQ-TPA-g and IQ-TPA-y. Upon grinding,
angles between ring A and the benzene ring (B) in the triphenyl- IQ-TPA-g showed no obvious change in the fluorescence color
amine unit are 30.111 and 29.821, respectively (Fig. S7a and b, (Fig. 6A) and fluorescence spectrum (Fig. 7a). Unlike IQ-TPA-g,
ESI†). The results indicate IQ-TPA-g has a more twisted mole- IQ-TPA-y exhibited a 24 nm blue shift in the fluorescence
cular conformation compared with IQ-TPA-y. Further proof is spectra and transformed into green-emitting ground samples
that IQ-TPA-g has a smaller bandgap than IQ-TPA-y based on (lem = 528 nm), revealing that IQ-TPA-y exhibited MFC activity.
Table 2 The emission properties and decay parameters of the polymorphs of IQ-TPA and IQ-PC
Emission decay
Sample
lem (nm)
FF (%)
A1/A2 (%)
t1 (ns)
t2 (ns)
hti (ns)
kfc (sꢂ1
)
knr (sꢂ1
)
c
a
a
a
b
IQ-TPA-g
IQ-TPA-y
IQ-PC-g
IQ-PC-y
528
552
527
542
30
12
43
54
63/37
46/54
66/34
64/36
1.29
1.30
7.03
7.86
22.66
17.51
75.89
66.13
9.20
10.05
30.44
28.84
3.26 ꢁ 107
1.19 ꢁ 107
1.41 ꢁ 107
1.87 ꢁ 107
7.61 ꢁ 107
8.76 ꢁ 107
1.87 ꢁ 107
1.60 ꢁ 107
a
Obtained from the following equation: I = A1 exp(ꢂt/t1) + A2 exp(ꢂt/t2); herein, t1 and A1 represent the lifetime and amplitude of the shorter-lived
b
species, t2 and A1 represent the lifetime and amplitude of the shorter-lived species, respectively. The weighted mean lifetime hti was obtained
c
from the following equation: hti = (A1t1 + A2t2)/(A1 + A2). kf and knr are the radiative rate constant and non-radiative rate constant obtained from
the following equations: kf = FF/hti and knr = (1 ꢂ FF)/hti, respectively.
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Fig. 4 Single crystal IQ-TPA-g: (a) intermolecular interactions; (b) zigzag stacking arrangement viewed along the a-axis; (c) stacking arrangement along
the b-axis.
Fig. 5 Single crystal IQ-PC-g: (a and b) intermolecular interactions; (c) zigzag stacking arrangement viewed along the c-axis.
When IQ-TPA-y was annealed at the melting point (100 1C) for
To understand the reasons for the solid-state fluorescence
5 h, the resultant annealed sample exhibited green fluorescence stimulus-responsive behaviors of the IQ-TPA and IQ-PC poly-
and had almost identical emission to that of IQ-TPA-g (Fig. S10, morphs, the XRD and solid-state UV-vis absorption experi-
ESI†). This indicated that IQ-TPA-y also exhibited thermo- ments were conducted. The diffraction peaks of IQ-TPA-g,
chromic activity and could be transformed into IQ-TPA-g by IQ-PC-g, and IQ-PC-y in the XRD patterns showed no obvious
the annealing treatment. For the polymorphs of IQ-PC, grinding changes before and after grinding (Fig. 8 and Fig. S13, ESI†).
could not result in the changes of the emission wavelengths and This result indicated that their crystalline structures remained
fluorescence colors of IQ-PC-g and IQ-PC-y (Fig. 6B and Fig. S11, unchanged and no morphological changes occurred, which
ESI†), suggesting non-MFC characteristics. After being annealed could account for their non-MFC characteristics. For IQ-TPA-y,
at the melting point (208 1C) for 2 h, green-emitting IQ-PC-g was after being ground, the resultant diffraction peaks in the XRD
converted to yellow-emitting IQ-PC-y (Fig. S12, ESI†), suggesting curve displayed obvious changes and agreed well with those of
its thermochromic property.
IQ-TPA-g (Fig. 8). Moreover, the solid-state absorption spectrum
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Fig. 6 (A) Photographs of the solid samples of IQ-TPA recorded under 365 nm UV irradiation: (a) IQ-TPA-g; (b) IQ-TPA-y; (c) ground sample.
(B) Photographs of the solid samples of IQ-PC recorded under 365 nm UV illumination: (a) IQ-PC-g; (b) the ground sample of IQ-PC-g; (c) IQ-PC-y;
(d) the ground sample of IQ-PC-y.
Fig. 8 XRD patterns of the polymorphs of IQ-TPA before and after
grinding.
were carried out (Fig. S16, ESI†). For IQ-TPA, as shown in
Fig. S16a (ESI†), the DSC analysis showed that IQ-TPA-g dis-
played one endothermic peak (230.0 1C) and IQ-TPA-y displayed
two endothermic peaks (95.1 and 229.9 1C). The annealed
sample of IQ-TPA-y exhibited one endothermic peak (230.2 1C),
which was similar to IQ-TPA-g. The endothermic peaks at
approximately 230 1C revealed that these samples have almost
the same melting points. The evident endothermic peak at
95.1 1C for IQ-TPA-y indicated that it was metastable and could
be transformed into relatively stable IQ-TPA-g upon an annealing
treatment at 100 1C, which was ascribed to a crystal-to-crystal
transition.21 Regarding IQ-PC, as shown in Fig. S16b (ESI†),
IQ-PC-g, IQ-PC-y, and the annealed sample of IQ-PC-g all exhibited
Fig. 7 Normalized fluorescence spectra of IQ-TPA-g (a) and IQ-TPA-y (b)
before and after grinding.
of the IQ-TPA-y ground sample was in good agreement with sharp endothermic peaks at 283.4, 284.2, and 285.4 1C, respec-
those of IQ-TPA-g and its ground sample (Fig. S14, ESI†). These tively, revealing that they had similar melting points. During the
results indicated that IQ-TPA-y could be converted to IQ-TPA-g process of heating, IQ-PC-g showed a distinct exothermic peak at
upon grinding and its MFC properties were attributed to 203.2 1C before the melting point, whereas no peak was observed
the transformation from one crystalline state to another. After for IQ-PC-y. Upon annealing at 208 1C, green-emitting IQ-PC-g
IQ-TPA-y and IQ-PC-g were annealed, the XRD patterns of the could be converted into yellow-emitting IQ-PC-y, indicating that
annealed samples were similar to those of IQ-TPA-g and IQ-PC-y the exothermic peak at 203.2 1C should correspond to the
(Fig. S15, ESI†), respectively, revealing that their thermochromic crystal-to-crystal transition from IQ-PC-g to the thermodynami-
properties originated from the transition between different cally stable IQ-PC-y.22
crystalline states.
Based on the above experimental results, it can be concluded
To understand the nature of the phase transition, differential that the introduced electron-donating group plays a very impor-
scanning calorimetry (DSC) experiments on IQ-TPA and IQ-PC tant role in the MFC activities of the isoquinoline derivatives.
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The polymorphs of IQ-TPA are only stabilized by C–Hꢀ ꢀ ꢀp Notes and references
bonds, while those of IQ-PC are stabilized by both C–Hꢀ ꢀ ꢀp
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bonds and C–Hꢀ ꢀ ꢀN hydrogen bonds, suggesting that the IQ-PC
polymorphs have stronger intermolecular interactions and
therefore their crystalline structures were hardly destroyed
by grinding. This can be used to explain why the introduction
of 9-phenyl-9H-carbazole is not conducive to the generation of
the MFC properties. For the IQ-TPA polymorphs, IQ-TPA-y can
be transformed into IQ-TPA-g by an annealing treatment,
indicating that IQ-TPA-g is a more stable crystalline state
compared with IQ-TPA-y. Upon grinding, the molecular con-
formations of IQ-TPA-y in the crystalline structure are changed
and transformed into stable IQ-TPA-g, which results in hypso-
chromic MFC activity.
Conclusions
Two novel D–p–A isoquinoline derivatives IQ-TPA and IQ-PC
using triphenylamine and 9-phenyl-9H-carbazole as an electron
donor, respectively, were designed and synthesized to investi-
gate their MFC activities, which was inspired by the accidentally
obtained isoquinoline derivative PIQ emitting strong blue
solid-state fluorescence. PIQ was prepared from the reaction
of a 4H-pyran derivative with piperidine by a complex ring-
opening/closing mechanism. Each of them exhibited a poly-
morphic property and had two crystalline polymorphs obtained
from different crystallization conditions. Crystal structural
analyses revealed that the different emissions of the poly-
morphs mainly originated from the different molecular con-
formations, in which the piperidin-1-yl group was located on
different sides of the isoquinoline core. For IQ-TPA, yellow-
emitting IQ-TPA-y exhibited hypsochromic MFC and thermo-
chromic properties and converted to green-emitting IQ-TPA-g
upon grinding/annealing, which was attributed to the transition
between different crystalline states. The strong intermolecular
interactions in IQ-PC polymorphs made it difficult to destroy
their crystal structures, thus they exhibited non-MFC properties.
Green-emitting IQ-PC-g could be changed to yellow-emitting
IQ-PC-y, revealing bathochromic thermochromic activity. This
work provides some useful information for the synthesis and
development of novel fluorescent materials with MFC, poly-
morphic, and thermochromic properties based on traditional
isoquinoline fluorophore.
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There are no conflicts to declare.
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Acknowledgements
The authors gratefully acknowledge the financial support from
the National Natural Science Foundation of China (No. 21572165
and 21472140) and the Graduate Innovation Foundation of
Wenzhou University (No. 3162018035).
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