Tricolor Luminescence Switching by Thermal and Mechanical Stimuli in the Crystal Polymorphs of Pyridyl-substituted Fluorene

Article abstract of DOI:10.1002/asia.201801476

Stimuli-responsive organic luminescence-switching materials have attracted much attention for a decade. Most of the reported examples display a reversible two-color luminescence switching, and multicolor-switching materials remain extremely rare. Herein, we report a simple organic molecule, 4,4′-(9,9-dimethyl-9H-fluorene-2,7-diyl)dipyridine (MFDP), which exhibits three different crystal polymorphs (V-MFDP, B-MFDP and G-MFDP) with different luminescent colors. Furthermore, the three crystal polymorphs show a reversible tricolor fluorescent switching from violet to blue and to green upon physical stimuli. The single-crystal structures of the three polymorphs were obtained, and the results indicate that the stimuli-responsive properties of the three polymorphs come from the different stacking modes induced by intermolecular interactions. The competition between weak π–π stacking and weak hydrogen bonding is the main reason for the the phase transformations among the three crystal polymorphs.

Full text of DOI:10.1002/asia.201801476

CHEMISTRY
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Accepted Article
Title: Tricolor luminescence switching by thermal and mechanical
stimuli in the crystal polymorphs of pyridyl substituted fluorene
Authors: Jianping Guan, Fan Xu, Chang Tian, Liang Pu, Maosen Yuan,
and Jinyi Wang
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10.1002/asia.201801476
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Tricolor luminescence switching by thermal and mechanical
stimuli in the crystal polymorphs of pyridyl substituted fluorene
Jianping Guan,+[a] Fan Xu,+[a] Chang Tian,[b] Liang Pu,[a] Mao-Sen Yuan,*[a] and Jinyi Wang*[a]
Abstract: Stimuli-responsive organic luminescence switching
materials have attracted much attention for a decade. Most of the
reported examples display a reversible two-color luminescence
switching, multicolor switched materials remain extremely lacking
due to the un-designability of this kind of smart materials. Herein, we
reported a simple organic molecule, 4,4'-(9,9-dimethyl-9H-fluorene-
2,7-diyl)dipyridine (MFDP), which exhibited three different crystal
polymorphs (V-MFDP, B-MFDP and G-MFDP) with different
luminescent colors. Furthermore, the three crystal polymorphs
showed a reversible tricolor fluorescent switching from violet to blue
and to green upon physical stimuli. The single-crystal structures of
the three polymorphs were obtained, and the results indicate that,
the stimuli response properties of the three polymorphs come from
the different stacking modes induced by intermolecular interactions.
The competition between weak π−π stacking and weak hydrogen
bonding is the main reason for the the phase transformations among
the three crystal polymorphs.
transformations between π-π stacking-directed packing and
hydrogen bond-directed packing. Fluorene, the derivatives of
which generally possess eminent luminescent properties and
have been widely used as optoelectronic materials, has a similar
planar aromatic configuration (π−π stacking factor) as
a
fluorenone fragment but lacks the exposed carbonyl group
(hydrogen bonding factor). We considered that, connecting the
groups of hydrogen bonding factor, e. g., oxygen-containing or
nitrogen-containing groups, to the two sides of fluorene may
achieve the same effect as fluorenone for the construction of
stimuli-responsive SLS molecules. This approach for the design
of functional molecules may likewise result in metastable
solid/crystalline structures because of the competition between
π−π stacking and hydrogen bonding, which is a prerequisite of
the reversible phase transformations of stimuli-responsive SLS
molecules.[42-50] In this paper, we reported a small organic
molecule,
4,4'-(9,9-dimethyl-9H-fluorene-2,7-diyl)dipyridine
(MFDP, Scheme 1), which exhibited three different crystal
polymorphs with different luminescent color. Moreover, all three
polymorphs display high solid luminescent efficiency and
showed a novel tricolor fluorescent switching from violet to blue
and to green upon heating or grinding. The single crystals of the
three crystal polymorphs were obtained, and their phase
Introduction
Luminescent organic solids have attracted much attention owing
to their potential applications as various optical and
optoelectronic materials.[1,2] The luminescence properties of
organic solids depend on not only the molecular chemical
structure, but also on molecular stacking.[3,4] In recent years, a
number of stimuli-responsive organic fluorescent materials have
been reported,[5,6] in which switching to the solid luminescent
color is achieved by altering the mode of molecularly assembled
structures upon external stimulus,[7-25] such as grinding, heating,
and exposure to chemical vapor. This smart approach to
changing the luminescence properties of organic solids is
thought to be more promising and easier than switching
molecularly chemical structures by chemical reactions. To date,
most of the reported examples display a reversible two-color
luminescence switching,[26-40] and multicolor switched materials
remain extremely lacking due to the un-designability of this kind
of smart materials. [41]
transformation
and
photophysical
properties
were
comprehensively investigated. These results provide structural
evidence that reveals the structure−property relationships of
their solid-state fluorescence behaviors and also yields further
insight into the mechanism of luminescence switching.
Scheme 1. The chemical structure and single-crystal structure of MFDP.
Recently, we reported a class of fluorenone compounds that
exhibit stimuli-responsive solid-state luminescence switching
(SLS) properties,[21,22] which derive from the reversible phase
Results and Discussion
Synthesis and characterization
The synthetic approach for compound MFDP is outlined in
Scheme S1 in the Supporting Information (SI). 2,7-Dibromo-9H-
fluorene was used as the starting material. To remove active
hydrogen atoms and to enhance the solubility, full alkylation was
carried out at the 9-position of fluorene with iodomethane
because methyl has an appropriate size for matching the
thickness of the MFDP to facilitate crystallization. The title
compound MFDP was readily synthesized through a Suzuki
[a]
J. Guan, F. Xu, Prof. Dr. M.-S. Yuan, Prof. Dr. J. Wang
College of Chemistry & Pharmacy, Northwest A&F University
Yangling, Shaanxi 712100 (China)
E-mail: yuanms@nwsuaf.edu.cn
jywang@nwsuaf.edu.cn
[b]
[+]
C. Tian
College of Veterinary, Northwest A&F University
Yangling, Shaanxi 712100 (China)
These two authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
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Figure 1. (a) The fluorescence spectra and (b) fluorescence decay profiles of MFDP in THF, crystal V-MFDP, B-MFDP and G-MFDP. Inset: (a) photos of three
polymorphs of MFDP, violet crystal V-MFDP, blue crystal B-MFDP and green crystal G-MFDP under 365 nm UV light.
reaction by reacting 2,7-dibromo-9,9-dimethyl-9H-fluorene and
pyridin-4-ylboronic acid. Details of the synthesis, purification and
single-crystal preparation processes, along with 1H and 13C NMR
spectroscopy data, mass spectrometry results, elemental
analysis characterization and X-ray single crystal structures, can
be found in SI.
compound MFDP packed to form colorless rectangular sheet
crystals (V-MFDP) that exhibited violet emission at 398 nm with
a ΦF value of 0.36 (Figure 1a). However, upon crystallization
through slow diffusion of its THF solution, irregular sheet crystals
(B-MFDP) were obtained that exhibited blue emission at 430 nm
with a ΦF value of 0.52, which is the same as that of the as-
prepared powder precipitated directly from the reaction solution.
Afterwards, the third kind of crystal was grown by the slow
diffusion of n-hexane into the CHCl3 solution of as-prepared
samples to get yellowish block crystals (G-MFDP) with green
emission at 515 nm and a ΦF value of 0.39. The high emission
quantum yield of B-MFDP is derived from the higher rate
constants of radiation decay kf (5.0 × 108 s-1) compared to the
non radiation decay knf (4.6 × 108 s-1). V-MFDP and G-MFDP
exhibit a higher knf (7.6 × 108 s-1 for V-MFDP and 1.4 × 108 s-1 for
G-MFDP) than their respective kf (4.3 × 108 s-1 for V-MFDP and
9.0 × 107 s-1 for G-MFDP).
Figure 1b presents a comparison of the emission spectra of
MFDP in THF, crystal V-MFDP, B-MFDP and G-MFDP. The four
emission spectra exhibited very different profiles and emission
peaks ranging from 377 nm to 515 nm. On the basis of the
reference,[51] we think that the emission of the THF solution
peaked at 377 nm with a vibronic structure should originate from
a single molecule of MFDP. Obviously, the luminescence
properties of the three crystal polymorphs are strictly related to
their intermolecular interactions and molecular packing patterns.
In addition, compared with the three crystal polymorphs, the
THF solution of MFDP shows a short fluorescence decay time
(τave = 0.23 ns), which should be attributed to the free MFDP
single molecule (Figure 1c). The different decay times of the
three crystal polymorphs further prove that they undergo
different relaxation routes or pathways from the molecular
excited state to the ground state in the three different crystal
states, respectively.
Photophysical properties of MFDP in solution
Unlike fluorenone derivatives, which show typical AIE properties,
the pyridyl substituted fluorene compound MFDP exhibited
strong fluorescence both in solution and in solid state. As shown
in Figure S1 (SI) and 1b, the UV-vis absorption and PL spectra
of the 10 μM tetrahydrofuran (THF) solution of MFDP were
recorded, and MFDP exhibited obvious vibronic features due to
the rigid planar framework of fluorene. The absorption maxima
(λabs) and fluorescence maxima (λF) of MFDP in THF are located
at 328 nm and 377 nm respectively. The fluorescence quantum
yield (ΦF) of MFDP in THF is 0.38 (Table S1 in SI), which is
much higher than that of fluorenone compounds. The
solvatochromic properties of MFDP were also explored, and
both the absorption and fluorescence spectra of MFDP were
basically independent of the solvent (Figure S2, SI), which
indicates the insignificant intramolecular charge transfer when
being excited.
Polymorphism and single crystal structure
The original as-prepared solid powders of MFDP emitted strong
blue fluorescence at 432 nm with a ΦF value of 0.47. As
expected, compound MFDP showed polymorphism, and the
three different types of single crystals of MFDP, which exhibited
different morphology and luminescent properties, were obtained
by culturing the single crystals under the different crystallization
conditions. Upon crystallization through slow diffusion of the dry
dichloromethane (DCM) solution of the as-prepared powders,
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Figure 2. (a) X-ray crystallographic packing of violet crystal V-MFDP; (b) illustration of the intermolecular C–H···π and C–H···N interactions; (c) side view and
illustration of the J-aggregation-induced head-to-tail dimers.
In order to obtain more insight into the structure-properties
pyridine rings between the two adjacent molecules in a dimer
relationship and reveal the crystal-state luminescent mechanism, exhibit a short vertical distance of 2.97 Å. In addition, weak
the crystal structures of the three polymorphs were
determined,[52] and selected crystallographic data are listed in
Table S2 in SI. The violet-emission crystal V-MFDP crystallizes
intermolecular C–H···π and C–H···N hydrogen bonds exist
among the molecular pairs (dimers), and hold the dimers
together resulting in the formation of a 3D structure (Figure 2b).
in the orthorhombic space group P212121. As shown in Figure 2a, We think that the violet emission of V-MFDP originates from
the obvious characteristic molecular pairs can be observed in
the packing diagram of V-MFDP. The molecular pairs adopt a
stacking mode of head-to-tail fashion to form dimers. The
molecular frameworks of the two adjacent up-down molecules in
a dimer barely overlap, and the vertical distance of the fluorene
planes between them is 4.58 Å (Figure 2c). However, the
such dimers. The multiple emission peaks of crystal V-MFDP
indicate the different luminescence mechanisms and different
luminescing manners, for instance, the intermolecular charge
transfer through the π (fluorene)···π (fluorene) interactions or π
(pyridine)···π (pyridine) interactions. Further exploitation of the
luminescence mechanism is under investigation.
Figure 3. (a) X-ray crystallographic packing of the blue crystal B-MFDP; (b) side view of the H-aggregation-induced dimers in B-MFDP; (c) illustration of the
intermolecular C–H···π interactions; (d) top view and illustration of the head-to-head dimer in B-MFDP.
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Figure 4. (a) X-ray crystallographic packing and illustration of the “W-shaped” molecular chains in the green crystal G-MFDP; (b) top view of the molecular chain;
(c) illustration of the intermolecular C–H···π interaction; (d) illustration of the molecular chain through intermolecular π···π stacking interactions in G-MFDP.
For blue-emission crystal B-MFDP, the packing mode belongs
to the triclinic space group . The characteristic molecular
a “W” shape. The chains are linked together into a molecular
sheet by weak C–H···π interactions and further stacked into 3D
structures (Figure 4c). The weak C–H···N interactions also exist
in this crystal packing to hold the chains together (Figure S8 in
SI). The green emission of crystal G-MFDP should originate
from the supramolecular system of the molecular chains with the
unstructured luminescence spectral feature (Figure 1b).
P1
pairs can also be found in the packing diagram, as shown in
Figure 3a and b. Different from that of V-MFDP, the molecular
pairs in B-MFDP exhibit a head-to-head stacking fashion.
Intermolecular π···π interactions with a contact distance of 3.54
Å bind every two adjacent molecules to form dimers. The two
molecular planes of dimer behave a certain extent of up-down
overlap (Figure 3d). Meanwhile, these dimers are linked by weak
C–H···π interactions and further stacked into 3D structures
(Figure 3a and c). In addition, weak intermolecular C–H···N
hydrogen bonds are also found to play a role in the stability of
the crystal structures (Figure S7 in SI). Based on the packing
structures, the blue emission of crystal B-MFDP is reasonably
speculated to originate from these dimers. The relatively high
luminous efficiency of crystal B-MFDP may benefit from the
stable stacking configuration of the dimer, which is favorable for
the formation of a static excimer to reduce the nonradiative
decay during the process of optical physics. When the excimer
is excited, they do not undergo any energy-consuming
configurational rearrangement. Then, when the excimer decays
back to the ground state, no repulsive interaction is involved.
Nonradiative decay pathways are blocked, resulting in strong
luminescence.
Stimuli-responsive luminescence switching
The stimuli-responsive luminescence switching properties of the
three crystal polymorphs were thoroughly explored. As shown in
Figure 5, upon heating the violet-emission crystal V-MFDP to
approximately 147 °C using
a microscopic melting-point
apparatus with a heating rate of ~8 K min-1 under ambient
conditions (directly heating the crystal V-MFDP at 147 °C for 20
s), we observed that the luminescence color change of V-MFDP
from violet to blue corresponding to the spectral change from
398 nm to 432 nm. Moreover, grinding the heated V-MFDP (or
crystal B-MFDP) in an agate mortar gave a light white powder,
which exhibited a violet emission (397 nm) similar to the V-
MFDP (Figure 5b). Green-emission crystal G-MFDP can not be
obtained from crystal V-MFDP or B-MFDP through physical
stimuli (grinding, heating, and exposure to chemical vapor),
except for re-crystallizing in n-hexane/CHCl3 solution. The
green-emission crystal G-MFDP can be transformed into the
thermodynamically stable crystal B-MFDP through heating at
190 °C with corresponding emission spectral change (Figure 5c).
In addition, we found that H2O molecules were involved in the
crystal polymorph of B-MFDP. For checking whether H2O
molecules had any effect on the emission property of this
chromophore, the crystalline powders of B-MFDP were heated
until they were completely melted, the emission of B-MFDP was
The green-emission crystal G-MFDP is in an orthorhombic
crystal system and crystallizes in the space group P212121,
which is same as the crystal V-MFDP but with different cell
parameters. In green-emission crystal G-MFDP, the molecules
aggregated into molecular chains based on weak π···π stacking
interactions of intermolecular pyridyl rings with
a contact
distance of 3.57 Å (Figure 4a, 4b and 4d). In a molecular chain,
the G-MFDP molecules are arranged as a hand-in-hand link with
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Figure 5. (a) Photos of luminescence switching of crystal B-MFDP, V-MFDP and G-MFDP upon heating, grinding or recrystal; (b) normalized PL spectra of B-
MFDP, V-MFDP, heated V-MFDP and ground B-MFDP crystalline powders; (c) normalized fluorescence spectra of G-MFDP, B-MFDP and heated G-MFDP
crystalline powders.
powders of both V-MFDP and B-MFDP display sharp
scatteringpeaks that are indicative of highly crystalline order.
After heating of V-MFDP, the characteristic diffraction peaks (2θ
= 12.5 °) of V-MFDP markedly recede to nearly disappear and
the characteristic diffraction peaks (2θ = 8.3 ° and 25.1 °) of B-
MFDP dramatically enhance. That is, the powder pattern of the
heated V-MFDP is almost identical to that of crystal B-MFDP,
which indicates that violet-emission crystal V-MFDP undergoes
a phase transformation to the B-MFDP phase after thermal
treatment at 147 °C. Upon grinding the crystal B-MFDP,
although all of the diffraction peaks obviously decrease, weak
but clear diffraction peaks (2θ = 12.5 °), which are in good
agreement with those of V-MFDP are observed, suggesting the
same molecular arrangement in ground B-MFDP as in the
crystal V-MFDP. Green-emission crystal G-MFDP exhibited an
analogous change of the powder XRD profile after thermal
treatment at 190 °C (Figure 6b), and the characteristic diffraction
peaks of B-MFDP appear. On the other hand, we also noted that
the characteristic diffraction peaks of G-MFDP are reserved to a
certain extent, indicating the incomplete phase transformation
from the G-MFDP state to the B-MFDP state, which is consistent
with the change in the luminescence spectrum with reservation
Figure 6. (a) Powder XRD profiles of B-MFDP, V-MFDP, heated V-MFDP and
ground B-MFDP; (b) powder XRD profiles of B-MFDP, G-MFDP and heated
G-MFDP.
almost not changed during the heating process (Figure S5 in SI),
which is because the presence and leaving of H2O will not affect
the stable stacking configuration of static excimers in the crystal
polymorph of B-MFDP.
To gain a better understanding of the stimuli-responsive
Figure 7. DSC profiles of G-MFDP, V-MFDP and B-MFDP crystals.
luminescence switching of the three crystal polymorphs, we
studied the phase characteristics of the related samples using
powder XRD analysis. As shown in Figure 6a, the pristine crystal
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In this study, we reported a simple organic molecule, 4,4'-(9,9-
dimethyl-9H-fluorene-2,7-diyl)dipyridine (MFDP), which exhibited
three different crystal polymorphs with different luminescent
colors. Furthermore, the three crystal polymorphs showed a
novel tricolor fluorescence switching from violet to blue and to
green upon physical stimuli. Their photophysical properties and
the mechanism of the stimuli-responsive luminescence switching
were thoroughly explored. Their luminescence properties are
strictly related to their intermolecular interactions and molecular
packing patterns in an aggregation state. The single-crystal
structures of the three crystal polymorphs indicate that, the violet
and blue emission of V-MFDP and B-MFDP respectively
originate from the different dimers, and the green emission of G-
MFDP originates from the supramolecular chains based on
weak π···π stacking interactions. The phase transformations
accompanied by luminescence switching among the three
crystal polymorphs are correlated to the three structural
characters of MFDP: appropriate π-conjugated plane of fluorene
fragment, weak hydrogen bonding factor and twist molecular
configuration, which will promote the competition between weak
π−π stacking and weak hydrogen bonding upon external
physical stimuli.
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Keywords: Luminescence Switching • Stimuli-response •
Fluorene •π–π Stacking • Crystal Structure
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Entry for the Table of Contents
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Jianping Guan, Fan Xu, Chang Tian,
Mao-Sen Yuan,* and Jinyi Wang*
Page No. – Page No.
Tricolor luminescence switching by
thermal and mechanical stimuli in the
crystal polymorphs of pyridyl
substituted fluorene
Tricolor neons: This paper described a fluorene-based functional molecule, which
exhibited three different crystal polymorphs with different luminescent colors. The
tricolor luminescence can be reversibly switched by thermal and mechanical stimuli.
The crystal structures revealed the mechanism of the phase transformation and
luminescence switching.
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