Reversible luminescence switching between single and dual emissions of bipyridinium-type organic crystals

Article abstract of DOI:10.1039/c2cc35685f

An excitation-wavelength-dependent luminescence behavior of a bipyridinium derivative, together with its luminescence switching between single- and dual-emissions upon reversible solid state structural transformation, has been presented.

Full text of DOI:10.1039/c2cc35685f

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Reversible luminescence switching between single and dual
emission of a bipyridinium-type organic crystals
Xu-Hui Jin, Cai-Xia Ren, Jian-Ke Sun, Xue-Jun Zhou, Li-Xuan Cai and Jie Zhang*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
An excitation-wavelength-dependent luminescence behavior
of a bipyridinium derivative, together with its luminescence
switching between single- and dual-emissions upon reversible
solid state structural transformation has been represented.
10 The preparation of multifunctional luminescent materials by
assembly of organic compounds in ordered molecular
arrangements is a subtle but hot topic in organic solid-state
chemistry.1 One of the most important issues in this area is
how to efficiently control solid-state luminescence behaviors
15 and achieve reversible luminescence switching for both
fundamental research and practical applications.2 A promising
approach is to alter molecular conformation and packing mode
through external stimuli, such as temperature, mechanical
pressure and vapor etc.3 However, examples with efficient and
20 reliable switching of solid-state luminescence are limited due
to the difficulty in solid-state structural conversion.3,4
Especially, due to the restriction of molecular species and
crystal engineering, most early established models only
focused on on/off or color switching based on single-color
25 emission,3-5 the precise control of single- and multi-
fluorescence switching of the same molecule in different
homogeneous aggregation states has not been established
despite their great advantages in white light emission
materials and biological sensing applications.6-7
Scheme 1. Molecular structure of the BCbpy zwitterion and
schematic representation of the self-assembly method of Form1.
a
luminescent switching with definite structural variations are
challenging. Although several single molecules with multiple
45 fluorescence emission have been successfully explored in
solutions or films,9 precise crystallographic study on single-
component multiple emissive materials, which may exclude
the possibility of multi-emission origining from various
molecular conformations,10 was not performed.
Recently, we have developed a new bipyridinium molecule
with 2-carboxybiphenyl groups, and observed an interesting
proton-triggered fluorescence switching behavior.11 It appears
that the incorporation of carboxyl groups into the
disubstituted 4,4′-bipyridinium molecular system may greatly
55 improve the luminescence properties of 4,4′-bipyridinium
derivatives, giving rise to a long-wavelength emission in the
visible range based on the intramolecular charge transfer (ICT)
mechanism. Along this line, we extend our study to a mono-
substituted bipyridinium zwitterion BCbpy (HBCbpyCl = 1-
60 (4-carboxybenzyl)-4,4′-bipyridinium chloride, Scheme 1).
Attractively, not only a dual fluorescence emission has been
detected in aqueous solution and the crystalline state for this
single zwitterionic molecule, but also a reversible luminescent
switching between single and dual emissions by the variation
50
30
To achieve multiple photoluminescence, a major challenge
exists in how to judiciously design the molecular system
bearing different emitting states. Different from a few metal
coordination compounds (eg. Ru(II) or Ir(III) polypyridine-
based complexes), which can possess two simultaneously
35 emissive excited states associated with the MLCT transition,8
stable multiple excited states (except for exciplex or excimer
states) commonly means a combination of multicomponent
emitters in a matrix for most organic materials, which
obviously increase the chance of the energy loss by non-
40 radiative pathways via intra- or intermolecular connection in
solid states or concentrated environments. For this reason,
maintaining their emission efficiencies and achieving precise
State Key Laboratory of Structural Chemistry, Fujian Institute of
Research on the Structure of Matter, Chinese Academy of
Sciences, Fuzhou, Fujian 350002, P. R. China.
E-mail: zhangjie@fjirsm.ac.cn; Fax: (+86) 591-83710051
† Electronic Supplementary Information (ESI) available:
Synthesis, characterization and other detailed experimental data.
CCDC 822542-822543. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/b000000x
Fig. 1 Excitation-wavelength-dependent fluorescence emission spectra
of BCbpy molecule in aqueous solution (a) and Form 1 in the solid state
(b). The sharp peaks in (a) are Raman lines.
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in molecular stacking mode has also been established.
structureless broad bands with the excitation maxima at 338
and 370 nm respectively (Fig. S2, †ESI), suggesting that there
are two different emission transitions in the present
luminescence system. The emission peak at 438 nm is similar
The fluorescence emission spectra of the zwitterionic
BCbpy in aqueous solution show an intense emission band at
450 nm with a small shoulder around 540 nm upon excitation
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5
at 315 or 340 nm. With increasing excitation wavelengths, the 45 to that observed for pure 4,4′-bipyridine (Fig. S3, †ESI), and
intensity of the short-wavelength emission gradually
decreases, while the long-wavelength emission gains intensity
and becomes dominant. When the excitation wavelength is
further increased, the intensity of long-wavelength band also
thus is assigned to the intramolecular π···π* transition of the
bipyridinium moiety. The emission band around 537 nm may
correspond to the intramolecular charge transfer (ICT)
emission as we previously reported for the disubstituted
10 begins to decrease (Fig. 1a). To clarify whether this 50 bipyridinium derivative,10 which can be confirmed by solvent-
wavelength-dependent behavior is related to different
conformers of the molecule as common discussion on organic
small molecules,10,12 crystal engineering approach is
considered and column-like single crystals of BCbpy
dependent emission behavior. Upon decreasing solvent
polarity from water to MeCN (Fig. S4, †ESI), this long-
wavelength band becomes invisible, only a dominant emission
band appears at 457 nm. Such a solvent-dependent emission
15 zwitterion (BCbpy·6H2O, Form1) have been sucessfully 55 behavior is typical for ICT fluorescence.13 To achieve multi-
grown from an aqueous solution. X-ray single crystal analysis
reveals that there is only one BCbpy molecule in the
asymmetric unit (Fig. 2a and Fig. S1 in ESI), suggesting that
BCbpy owns homogeneous molecular conformation in Form1.
color fluorescence emission in a single organic molecule, a
typical strategy is to combine two different fluorophores
through covalent bond linkage. However, successful examples
are rare because energy transfer usually quenches one or more
20 Very interesting, the bulk crystals exhibit similar excitation- 60 of the emission pathways.9b,14 The present work suggests that
wavelength-dependent fluorescence behavior to that in
aqueous solution, which can be clearly observed even by
naked eye (Fig. 2e), blue and almost-white emissions can be
observed under a UV lamp with the wavelength of 254 or 365
the appropriate quaternary ammonium modification on nitrogen-
containing organic single-color emitter may tune the electronic
structure of molecular system, achieving the multi-color
luminescence emission through introducing additional ICT
25 nm, respectively. This observation clearly excludes the 65 emission.
possibility that the wavelength-dependent behavior of BCbpy
molecule origins from the multiple molecular conformations,
being distinct from the common red-edge effect of some
organic molecules.10
One interesting structural feature of Form1 is that there
are strong hydrogen-bonding interactions between the lattice
water molecules and BCbpy. The water molecules are linked
together into an infinite 2D water layer composed of 14-
30
The solid-state luminescence spectra provide definite 70 membered water rings (Fig.2b). Adjacent water layers stack in
support for this excitation-wavelength-dependent dual
emission behavior of Form1 (Fig. 1b). When excited at
wavelengths below 340 nm, Form1 exhibits an emission
centered at 438 nm, which is about 12 nm blue shift with
-ABAB- mode along the b-axis and are linked by carboxylate
oxygen atoms through the hydrogen bonding interactions into
a
3D porous supramolecular framework. Each BCbpy
molecule is situated in the center of the 14-memebred water
35 respect to the short-wavelength emission observed in aqueous 75 ring, and offers pyridyl nitrogen and carboxylate oxygen
solution. As the excitation wavelength increased further, the
short-wavelength band shows a decrease in emission intensity,
accompanied by the development of the long-wavelength
emission band centered at 537 nm. The fluorescence
atoms as proton acceptor to form hydrogen bonds with the
molecules within adjacent water layers, resulting in that
BCbpy itself is locked in the cages of the 3D framework (Fig.
2c-d). Such an interesting water molecule-surrounded polar
40 excitation spectra monitored at 438 and 540 nm show 80 environment around BCbpy molecule may provide some
degree of support for the consistent observation on the
luminescence behaviors in aqueous solution and in the solid
state.
Remarkably, as the crystals are ground at room temperature,
85 the blue-color emission excited at 254 nm gradually fades away.
Fig. 2 (a) BCbpy in the structure unit of Form1. (b) 2D water layer
composed of 14-membered water rings. (c) The BCbpy molecule in
the hydrogen-bonded water cages. (d) 3D hydrogen-bonded porous
water framework inside which the BCbpy molecules sit. (e)
Fluorescence images of the original and dehydrated samples of Form1
under a UV lamp with the wavelength of 254 or 365 nm. The blue
emission is weaker as the excitation wavelength of 254 nm is not
Fig. 3 XRPD patterns show the reversible transformation of Form1 and
optimal.
Form2 via dehydration-rehydration.
2
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leads to efficient π–π stacking interactions between bipyridine
units, with the average interplanar distance of 3.49 Å (Fig. 4).
After additional grinding for 12 h at 50 °C, the blue emission
finally disappears, and a new phase Form2 only with single
yellow-color emission can be obtained. Upon keeping Form2 50 Each hydrogen-bonded dimer unit is held together by this π–π
under moisture condition for 48 h, the sample is reverted to the
initial phase with dual fluorescence emission. Elemental analysis
reveals that Form2 is partially dehydrated with the chemical
formula of BCbpy·2H2O. This result indicates that the lattice
waters in the crystal not only play an important role in the
wavelength-dependent luminescence behavior but also can be
stacking interaction into a close-packed layer. It is well
known that the π–π stacking of the planar molecule may
facilitate the formation of detrimental excimeric species and
lead to fluorescence quenching in the solid state.15 Therefore,
55 the disappearance of the blue emission might be due to the
quenching of the intrinsic fluorescence of bipyridinium
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5
10 regarded as a switch between the single- and dual-color emissions
(Fig.2e) The X-ray powder diffraction (XRPD) analysis
demonstrates a reversible structural transformation during the
dehydration and rehydration processes. As shown in Fig. 3, the
phase purity of original samples is supported by a good match
15 between the experimental XRPD pattern and the simulated one
from the single-crystal structure. After grinding the sample at
room temperature for 12 h, some diffraction peaks at the 2θ
values of 16.74, 21.59 and 29.85° disappear, while new peaks at
the 2θ values of 7.52, 11.02, 13.48, 20.99 and 24.65° appear,
20 which indicates that a new crystalline phase emerges. Upon
further grinding at 50 °C for additional 12 h, the diffraction peaks
of the original sample at the 2θ values of 9.64, 11.87, 19.1, 22.87,
23.54 and 31.23 ° completely disappear, suggesting that Form1
has been totally transformed into a new crystalline phase. This
25 new crystalline phase (Form2) is stable at 50 °C. No change in
the positions of diffraction peaks can be observed with increasing
grinding time. After exposure of the dehydrated sample to moist
atmosphere for 48 h, the XRPD pattern becomes identical to that
of the original sample again, meaning that the dehydrated sample
30 can reversibly revert to the original state via rehydration.
chromophore by
luminescence spectrum of Form2 displays a main emission
band at 574 nm, which is about 37 nm red shift from the low-
60 energy emission of Form1 (Fig. S6, †ESI). Such
a
close packed arrangement. The
a
bathochromic effect should be related to the conformational
change of BCbpy molecule in different aggregation states.
In summary, an interesting dual fluorescence emission has
been detected in aqueous solution and the crystalline state for
65 a carboxybenzyl-substituted bipyridinium zwitterion, and a
reversible switching of luminescence between single and dual
emissions by the variation in molecular stacking mode has
also been achieved. The present work develops a kind of
organic small molecule that emits single-color or almost-white
70 light upon controlling excitation wavelength. The merit of
easy synthesis and modification as well as tunable multicolour
photoluminescence of this compound provide the inspiration
for the design of new materials for labelling, sensing, color
display and switching applications
75
This work was supported the NNSF of China (Nos.
20973171/20671090).
By dispersing Form1 in the water-DMSO mixed solution
and allowing slow evaporation, block crystals suitable for X-
ray analysis can be obtained. XRPD analysis reveals that there
is a good match between the simulated pattern from this
35 single-crystal structure data and the experimental pattern of
Form2. Thus, X-ray single crystal structural analysis can be
carried out and clearly reveals that there is one BCbpy
molecule in the asymmetric unit and no water aggregate exists
in Form2. BCbpy molecules are linked by two lattice water
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Fig. 4 Crystal packing mode for Form1 (top) and Form2 (bottom).
Thick yellow lines represent π–π stacking interactions.
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