Stimuli responsive reversible high contrast off-on fluorescence switching of simple aryl-ether amine based aggregation-induced enhanced emission materials

Article abstract of DOI:10.1039/c5ra17570d

Aryl-ether amine based simple Schiff base molecules (1-5) showed aggregation induced enhanced emission (AIEE) in the solid state and rare stimuli responsive fluorescence off-on switching. The hard grounding of 1 resulted in irreversible fluorescence tuning from greenish-yellow to green. Heating or solvent exposure did not result in any fluorescence reversibility. Interestingly, the hard grounding of 2-5 led to the quenching of the solid state fluorescence and heating/solvent exposure produced clear bright fluorescence. Importantly, 2-5 exhibited reversible off-on fluorescence switching for several cycles without significant change in fluorescence intensity. It is noted that the turn-on fluorescence of 2-5 was slightly blue shifted compared to the initial solids. PXRD studies suggest that the switching of dark to bright fluorescence and vice versa of 2-5 is due to the reversible change of crystalline to amorphous phase and more planarization of the twisted structure. Single crystal analysis of 1 and 5 confirmed the twisted molecular conformation and strong intermolecular interactions in the crystal lattice, which led to AIEE by restricting free rotation and rigidifying the fluorophores in the solid state. The high contrast dark and bright fluorescence switching of 2-5 could be of potential interest in optoelectronic applications.

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Stimuli responsive reversible high contrast off-on fluorescence switching of simple aryl-
ether amine based aggregation induced enhanced emission materials
Aryl-ether amine based simple Schiff base molecules showed rare stimuli responsive off-on
fluorescence switching with high contrast.

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DOI: 10.1039/C5RA17570D
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Stimuli responsive reversible high contrast off-on fluorescence switching
of simple aryl-ether amine based aggregation induced enhanced
emission materials
Anu Kundu,a P. S. Hariharan,a K. Prabakaran,a Dohyun Moonb* and Savarimuthu Philip Anthonya*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Arylꢀether amine based simple Schiff base molecules (1ꢀ5) showed aggregation induced enhanced
emission (AIEE) in the solid state and rare stimuli responsive fluorescence offꢀon switching. Hard
grounding of 1 showed irreversible fluorescence tuning from greenishꢀyellow to green. Heating or solvent
10 exposure did not show any reversibility of the fluorescence. Interestingly, hard grounding of 2ꢀ5 lead to
the quenching of solid state fluorescence and heating/solvent exposure produced clear bright
fluorescence. Importantly, 2ꢀ5 exhibited reversible offꢀon fluorescence switching for several cycles
without significant change of fluorescence intensity. It is noted that the turnꢀon fluorescence of 2ꢀ5 was
slightly blue shifted compared to the initial solids. PXRD studies suggest that the switching of dark to
15 bright fluorescence and vice versa of 2ꢀ5 is due to the reversible change of crystalline to amorphous phase
and more planarization of twisted structure. Single crystal analysis of 1 and 5 confirmed the twisted
molecular conformation and strong intermolecular interactions in the crystal lattice that lead to AIEE by
restricting the free rotation and rigidifying the fluorophores in the solid state. The high contrast dark and
bright fluorescence switching of 2ꢀ5 could be of potential interest in optoelectronic applications.
45 switching due to the change of fluorophore conformation or
phase of the materials.8,9 For example, conformationally twisted
tetraphenylpyrene, pyreneꢀ and anthraceneꢀbased liquid crystals,
cyanostilbene and triphenylamine derivatives exhibited reversible
fluorescence switching with external force.10 Most of these
50 materials showed only the color transformation with external
force rather than the change of fluorescence efficiency that
hamper the materials to be used for highꢀcontrast fluorescence
recording.
Park’s et al. reported the unique example of high contrast
55 reversible fluorescence onꢀoff by external force using
dicyanodistyrylbenzene based donorꢀacceptorꢀdonor triad.11
Solvent inclusion/removal in coordination complexes showed
dramatic fluorescence changes in intensity.12 However, the
examples of fluorescent material that exhibit external stimuli such
60 as heat, pressure and vapor responsive fluorescence onꢀoff
switching with high contrast are rarely reported compared to
change of color.6a,13,14 The integration of anthracene unit with
AEE active diphenylquinoxaline core lead to heat responsive
fluorescence onꢀoff in the solid state.15 Zhang et al recently
65 reported heating and mechanical force induced fluorescence onꢀ
off in a triphenylamine based fluorophore.16 In our effort to
develop new organic solid state fluorescence switching
materials,17 we report here the synthesis of arylꢀether amine based
Schiff base molecules that exhibit AIEE in the solid state and
70 stimuli responsive reversible fluorescence offꢀon switching
(Schemeꢀ1). Schiff bases were chosen in view of their facile
20 Introduction
Developing stimuli responsive organic fluorescent materials
particularly with repeated fluorescence switching has received
considerable interest because of their potential applications in
sensors, displays, switches, security inks, optoelectronic devices,
25 and data storage.1,2 In general, switching of organic solid state
fluorescence have often been achieved either by altering the
fluorophore packing and conformation or controlling the
intermolecular interactions in the solid state structures.3ꢀ5 The
external stimuli such as mechanical force, heat and solvent
30 exposure often changes the molecular conformation or phase of
the materials (crystalline to amorphous) and lead to switching of
solid state fluorescence. Organic molecules that exhibit
temperature dependent molecular packing in crystals showed
highly reversible fluorescence switching.6 The inclusion and
35 removal of guest molecules in the crystal lattices of
supramolecular organic fluorescent host materials lead to
fluorescence switching.5 In recent years, aggregation induced
enhanced emission (AIEE) phenomenon, weakly or nonꢀemissive
fluorophores in solution but strong emission in the aggregated
40 state, has been successfully used for developing efficient solid
state fluorescent materials.7 The rigidification of fluorophores in
the aggregated state restrict the intramolecular rotation and
activate the radiative decays. Some of the strongly fluorescent
AEE materials exhibited external stimuli responsive fluorescence
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synthesis and good photophysical properties that found
applications in many research fields such as chemicals molecular
optoelectronics, photochromism and medicine.18 The greenishꢀ
4H), 6.96 (d, J = 9.2 Hz, 1H). 13C NMR (100 MHz, Chloroformꢀ
d) δ 173.1, 157.0, 153.1, 148.8, 137.4, 135.8, 133.5, 129.9, 129.5,
128.2, 127.2, 127.0, 124.5, 123.6, 123.6, 123.4, 120.1, 119.0,
yellow fluorescence of 1 in the solid state was tuned to green by 60 118.8, 118.6, 108.9. m/z Calcd. for C23H17NO2 (M+H): 339.13,
5
hard grounding. 1 did not show any fluorescence switching.
However, 2ꢀ5 exhibited clear offꢀon fluorescence switching and
showed reversible dark and bright fluorescence with external
force. The greenishꢀyellow or yellow fluorescence of 2ꢀ5 was
found: 340.1.
3: Yield = 82 %. m.p. 132.6 °C. 1H NMR (400 MHz,
Chloroformꢀd) δ 15.53 (s, 1H), 9.33 (s, 1H), 8.09 (dd, J = 4 Hz,
1H), 7.78 (dd, J = 4.4 Hz, 1H), 7.71 (d, J = 7.6 Hz, 1H), 7.51 (t, J
quenched by hard grounding and heating or solvent exposure 65 = 7.6 Hz, 1H), 7.39 ꢀ 7.32 (m, 5H), 7.16 ꢀ 7.04 (m, 6H). 13C NMR
10 turnꢀon the fluorescence. The turnꢀon fluorescence of 2ꢀ5 after
the external stimuli (grounding/heat/solvent vapor) was slightly
blue shifted compared to initial solids. Importantly, the
fluorescence offꢀon switching of 2ꢀ5 could be reproduced for
several cycles by repeated grounding and heating. PXRD studies
15 suggest that fluorescence onꢀoff switching could be attributed to
the reversible change of crystalline to amorphous phase. Single
(100 MHz, Chloroformꢀd) δ 168.8, 157.0, 156.0, 154.8, 141.1,
136.2, 133.1, 129.9, 129.3, 128.0, 127.4, 123.5, 123.5, 121.8,
119.8, 118.9, 108.9. m/z Calcd. for C23H17NO2 (M+H): 339.13,
found: 340.1.
70 Synthesis of 4 and 5
Ethanol solution (30 ml) of aldehyde (1.1mmol, 2ꢀhydroxyꢀ4ꢀ
methoxy benzaldehyde (4), 2ꢀhydroxy naphthaldehyde (5)) was
added dropꢀwise into 2ꢀ(2'ꢀaminophenoxy)benzene carboxylic
acid (1mmol) dissolved in ethanol (10 ml) under stirring at room
75 temperature. After the addition, the reaction mixture was refluxed
for 10 h. The cooling of reaction mixture produced precipitate
that was filtered and washed with ethanol and dried under
vacuum.
crystal analysis of
1 and 5 showed twisted molecular
conformation and strong intermolecular interactions in the crystal
lattice that rigidify the fluorophores and induced AIEE. Thus
20 simple Schiff base organic molecules showed rare high contrast
stimuli responsive fluorescence offꢀon switching in the solid state
that could be of interest in optoelectronic device studies.
4: Yield = 85 %. m.p. 158.8 °C. 1H NMR (300 MHz, d6ꢀDMSO)
80 δ 13.50 (s, 1H), 12.84 (s, 1H), 8.91 (s, 1H), 7.83 (dd, J = 1.8 Hz,
1H), 7.57 ꢀ 7.54 (m, 2H), 7.46 (d, J = 8.7 Hz, 1H), 7.28 ꢀ 7.25 (m,
3H), 6.97 ꢀ 6.94 (m, 1H), 6.81 (d, J = 8.1 Hz, 1H), 6.49 (dd, J =
2.7, 2.4 Hz, 1H), 6.37 (d, J = 2.4 Hz, 1H), 3.77 (s, 3H). 13C NMR
(75 MHz, d6ꢀDMSO) δ 166.2, 163.6, 163.4, 162.6, 155.8, 149.0,
85 138.9, 134.0, 133.4, 131.5, 124.7, 122.9, 120.0, 119.7, 118.3,
112.8, 106.6, 100.6, 55.3. m/z Calcd. for C21H17NO5 (M+H):
363.11, found: 364.1.
Materials and methods
2ꢀfluoroꢀnitrobenzene,
4ꢀfluoroꢀnitrobenzene,
phenol,
2ꢀ
25 cyanophenol, NaBH4, Pd/C, 2ꢀhydroxy naphthaldehyde and 2ꢀ
hydroxyꢀ4ꢀmethoxy benzaldehyde was obtained from sigmaꢀ
Aldrich and used as received. The solvents were obtained from
Merck India. All chemicals are used as received. The arylꢀether
amine precursors and Schiff base molecules (1ꢀ5) were
30 synthesized by following our recently reported procedure
(Scheme S1).19 The melting points of the compounds were
obtained from DSC spectra (Fig. S1)
5: Yield = 86 %. m.p. 224 °C. 1H NMR (300 MHz, d6ꢀDMSO) δ
15.79 (s, 1H), 12.98 (s, 1H), 9.68 (s, 1H), 8.45 (d, J = 8.4 Hz,
90 1H), 8.08 ꢀ 8.05 (dd, J = 3.1 Hz, 1H), 7.92 (d, J = 7.8 Hz, 1H),
7.84 (d, J = 9.3 Hz, 1H), 7.72 (d, J = 7.5 Hz, 1H), 7.61 – 7.53 (m,
2H), 7.34 ꢀ 7.22 (m, 4H), 7.08 (d, J = 8.1 Hz, 1H), 6.86 ꢀ 6.78 (m,
2H). 13C NMR (300 MHz, d6ꢀDMSO) δ 173.4, 166.0, 154.7,
153.1, 148.9, 137.4, 133.7, 133.3, 131.7, 128.9, 128.0, 127.0,
95 126.2, 124.2, 123.8, 123.7, 123.3, 123.2, 120.9, 120.0, 119.0,
117.3, 108.3. m/z Calcd. for C24H17NO4 (M+H): 383.12, found:
384.1.
Synthesis of 1-3
1 was synthesized by adding ethanol (20 ml) dissolved 2ꢀ
35 hydroxyꢀ4ꢀmethoxy benzaldehyde (1.1mmol) into stirring
solution of 2ꢀaminophenoxybenzene (1mmol) in ethanol (5 ml)
dropꢀwise under ambient condition. The resultant reaction
mixture was refluxed for 4 h. The product was precipitated out
upon cooling the reaction mixture to room temperature. The
40 precipitate was filtered, washed with ethanol and dried under
vacuum. 2 has been synthesized in a similar procedure but by
using 2ꢀhydroxy naphthaldehyde (1.1mmol) in ethanol whereas 3
was prepared using4ꢀaminophenoxybenzene (1mmol)and 2ꢀ
hydroxy naphthaldehyde (1.1mmol)in ethanol.
Spectroscopy and structural characterization:
Absorption and fluorescence spectra were recorded using Perking
100 Elmer Lambda 1050 and Jasco fluorescence spectrometerꢀFPꢀ
8200 instruments. Fluorescence quantum yields (Φf) of solid
samples were measured using a Horiba Jobin Yvon model FL3ꢀ
22 Fluorolog spectrofluorimeter with integrating sphere. The
powder Xꢀray diffraction (PXRD) patterns were measured using a
105 XRDꢀ Bruker D8 Advance XRD with Cu Kα radiation (λ =
1.54050 Å) operated in the 2θ range from 10ο to 50ο. Single
crystal of 1 and 5 was coated with paratoneꢀN oil and the
45 1: Yield = 85 %. m.p. 117.2 °C. 1H NMR (400 MHz,
Chloroformꢀd) δ 13.54 (s, 1H), 8.57 (s, 1H), 7.32 ꢀ 7.27 (m, 3H),
7.23 ꢀ 7.19 (m, 3H), 7.07 ꢀ 7.05 (m, 2H), 6.98 ꢀ 6.95 (m, 2H),
6.45 ꢀ 6.42 (dd, J = 1.2, 2.0 Hz, 2H), 3.80 (s, 3H). 13C NMR (100
MHz, Chloroformꢀd) δ 164.5, 164.2, 162.0, 157.8, 149.3, 140.4,
50 133.6, 129.8, 127.4, 124.9, 122.9, 121.3, 120.4, 117.6, 113.3,
107.2, 101.2, 55.5. m/z Calcd. for C20H17NO3 (M+H): 319.12,
found: 320.1.
diffraction data measured at 100K with synchrotron radiation (
λ
= 0.62998 Å) on a ADSC Quantumꢀ210 detector at 2D SMC with
110 a silicon (111) double crystal monochromator (DCM) at the
Pohang Accelerator Laboratory, Korea. CCDCꢀ1418264
(1),1417738 (5) contains the supplementary crystallographic data
for this paper.
2: Yield = 90 %. m.p. 163.2 °C. 1H NMR (400 MHz,
Chloroformꢀd) δ 15.40 (s, 1H), 9.32 (s, 1H), 7.97 (d, J = 8.4 Hz,
55 1H), 7.70 (d, J = 9.2 Hz, 1H), 7.63 (d, J = 8 Hz, 1H), 7.49 ꢀ 7.45
(m, 2H), 7.36 ꢀ 7.25 (m, 3H), 7.23 ꢀ 7.19 (m, 2H), 7.12 ꢀ 7.02 (m,
2
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Spectroscopy and characterization: Absorption and fluorescence
spectra were recorded using Perking Elmer Lambda 1050 and
Jasco fluorescence spectrometerꢀFPꢀ8200 instruments. IR spectra
were recorded using KBr disks on
a Shimadzu affinity
5
spectrophotometer over the range 4000 ꢀ 400 cmꢀ1 using KBr as
the reference.
Result and Discussion
The synthesis of amine precursors and the corresponding Schiff
base molecules (1ꢀ5) are shown in scheme S1. The reaction
10 between phenol and 2ꢀfluoroꢀnitrobenzene or 4ꢀfluoroꢀ
nitrobenzene in presence of K2CO3 in DMSO at 110 °C yielded
the corresponding arylꢀether with nitro group at ortho or para
position. The subsequent nitro group reduction produced amine
precursors for 1ꢀ3. The reaction between 2ꢀcyanophenol and 2ꢀ
15 fluoroꢀnitrobenzene in presence of K2CO3 in DMSO at 110 °C
resulted in the formation of 2ꢀ(2ꢀnitrophenoxy)benzonitrile. The
nitro group reduction followed by oxidation of CN to COOH
produced amine precursor for 4 and 5. The Schiff base molecules
(1ꢀ5) were synthesized using a simple condensation reaction
20 between the amine and aromatic aldehydes in ethanol. The
synthesized Schiff base compounds (1ꢀ5) showed greenishꢀ
yellow to yellow fluorescence in the solid state. However, 1ꢀ5 did
not show any fluorescence in solution state that could be due to
the free rotation of single bond and isomerism of imine (C=N).20
25 The observation of fluorescence only in the solid state suggest the
AIEE phenomena of 1ꢀ5.
30 Fig. 1. Fluorescence tuning of 1 (digital images and fluorescence spectra)
by hard grounding. λexc = 365 nm.
Scheme 1. Molecular structures of 1ꢀ5.
Fig. 2. Digital images and fluorescence spectra of offꢀon fluorescence
switching of (a) 2 and (b) 3. λexc = 365 nm.
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%). However, strong grounding of 4 completely quenched the
fluorescence (Φf = 10 %). Heating/solvent exposure switched the
35 fluorescence from dark to bright greenish yellow. The
fluorescence spectra showed a enahnced fluorescence at 504 nm.
The excitation spectra of 4 showed clear peaks at 370 and 492 nm
(Fig. S4). The strong grounding showed the disappearance of
peak at 370 nm and blue shift of 480 nm peak to 465 nm.
40 Although heating did not show clear peak at 370 nm, the intensity
has been clearly enhanced with slight red shift of 465 nm peak to
478 nm. This result support the fluorescence change of 4 by hard
grounding and heating. The fluorescence of 4 can also be
repeatedly switched offꢀon for several cycles by hard grounding
45 and heating (Fig. 4b). The asꢀsynthesized powder of 5 showed
strong yellow fluorescence at 536 nm (Fig. 5a, Φf = 48 %).
Surprisingly, the crystals of
5 showed only very weak
fluorescence (Φf = 6 %). However, slight breaking of crystals
Fig. 3. External stimuli induced reversible offꢀon fluorescence switching
between dark and bright state of (a) 2 and (b) 3.
1 showed greenishꢀyellow fluorescence with emission peak at
520 nm (Fig. 1, Φf = 40 %). The strong grounding of 1 blue
shifted the emission to 508 nm and showed green fluorescence
(Φf = 43 %). Heating or solvent vapor exposure did not show any
fluorescence switching. The methoxyꢀsalicylaldehyde group was
replaced with naphthaldehyde in 2. The isomeric compound of 2
5
10 and 3 also showed greenishꢀyellow solid state fluorescence (Fig.
2). Fluorescence spectra of 2 and 3 showed emission at 532 and
527 nm, respectively (Φf = 46 % (2), 53 % (3). In contrast to 1,
strong grounding of 2 and 3 exhibited quenching of fluorescence
(Φf = 5 % (2), 7 % (3). The grounded powder showed only very
15 weak fluorescence. Interestingly, the quenched fluorescence of 2
and
3 grounded powder was switched to bright green
fluorescence upon heating or solvent vapor exposure. The digital
images and fluorescence spectra confirmed the bright green
fluorescence with enhanced intensity. It is noted that grounding
20 and heating of 2 and 3 blue shifted fluorescence. 2 showed
fluorescence at 525 nm along with a small hump at 500 nm after
Fig. 4. (a) Digital images and fluorescence spectra of 4 and (b) external
50
stimuli induced reversible fluorescence offꢀon switching. λexc = 365
nm.
grounding and heating. Whereas
3 exhibited two clear
fluorescence peaks at 503 and 522 nm after grounding and
heating. The excitation of spectra of 2 and 3 showed a small
25 change after grounding and heating (Fig. S2 and S3). Importantly,
2 and 3 showed reversible fluorescence offꢀon switching (dark to
bright fluorescence) without significant loss of intensity for more
than five cycles by repeated grounding and heating (Fig. 3).
The carboxylic acid group was introduced at ortho position of
30 1 and 2 to further rigidifying the fluorophore in the solid state via
intermolecular Hꢀbonding interactions (4 and 5). 4 exhibited
strong greenishꢀyellow fluorescence at 517 nm (Fig. 4a, Φf = 58
lead to strong enhancement of fluorescence intensity (Φf = 34
%)Similar to 2ꢀ4, hard grounding quenched the fluorescence
intensity of 5 (Φf = 5 %) and heating/solvent exposure produced
55 bright greenishꢀyellow fluorescence. The strongly grounded
powder showed very weak fluorescence and heating showed
strong enhancement of intensity. The fluorescence λmax was
slightly blue shifted from 536 to 532 nm after grounding and
heating. The excitation spectra of crystals and strongly grounded
60 powder did not show any clear peak however, slightly broken
4
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crystals and heated samples exhibited strong peaks with perfect 25 conformationally flexible arylꢀether is known to adopt different
matching (Fig. S5). The solid state fluorescence of 5 also
exhibited repeated fluorescence offꢀon switching from dark to
bright state for several cycles without significant loss of intensity
by grounding and heating (Fig. 5b). Thus simple arylꢀether based
conformation in the crystals including four different
conformation in a single supramolecular structure.21 The imine
and methoxyꢀsalicylaldehyde are in same plane in 1 whereas
naphthaldehyde in 5 is slighted twisted (Fig. 6b,d). The hydroxy
5
Schiff base compounds (2ꢀ5) exhibited rare stimuli responsive 30 group of 1 and 5 formed strong intramolecular Hꢀbonding
fluorescence offꢀon switching and 1 showed fluorescence tuning.
Importantly, 2ꢀ5 showed reversible dark and bright fluorescence
switching with good contrast for several cycles.
interactions with imine nitrogen in the crystal lattice (Fig. 7).
Further, the strong intermolecular πꢀπ interactions between imine
phenyl group and methoxy phenyl group produced stairꢀlike
network structure in the crystal lattice (Fig. 7a). In 5,
35 intermolecular Hꢀbonding between hydroxyl groups lead to the
formation of 1ꢀD chains (Fig. 7b). Strong CꢀH…O (imine
hydrogen and acid carbonyl oxygen) and πꢀπ intermolecular
interactions between the imine naphthyl groups lead to
interdigitation of chains with faceꢀtoꢀface molecular arrangement
40 of imine naphthyl groups (Fig. 7c). The faceꢀtoꢀface arrangement
of imine naphthyl group might be the reason for fluorescence
quenching in the crystals of 5 (Fig. 5a). The strong face to face πꢀ
π interactions are known to quench the solid state fluorescence of
πꢀconjugated aromatic molecules.22 The slight breaking might
45 have disturbed the faceꢀface arrangement that turned on the
fluorescence. Although, 1 also showed πꢀπ interactions in the
crystal lattice but it showed slipꢀstacking molecular arrangment
between two different aromatic molecules. The structural studies
of 1 and 5 clearly indicate the strong intra and intermolecular
50 interactions in the crystal lattices. Thus, it is expected that 2ꢀ4
also could be having strong intermolecular interactions in the
solid state due to structural similarity. These strong
intermolecular interactions could restrict the free rotation and
imine isomerism that induces AIEE in the solid state. The
55 restriction of free rotation in Schiff base molecules by molecular
design showed AIEE.23
10 Fig. 5. (a) Digital images and fluorescence spectra of 5 and (b) external
stimuli induced reversible fluorescence offꢀon switching. λexc = 365
nm.
In an effort to understand the molecular arrangement and AIEE
phenomena of 1ꢀ5 in the solid state, single crystals were
15 attempted to grow from different solvents that includes methanol,
ethanol, ethyl acetate, chloroform, dichloromethane, acetonitrile
and acetone. 2 and 3 gave only thin plates from all solvents which
did not show any Xꢀray diffraction. 4 formed more of amorphous
powder with less crystalline phase. However, single crystals of 1
20 and 5 was obtained from methanol and dimethylsulfoxide,
respectively (Table S1 and S2). 1 and 5 showed twisted
molecular conformation in the crystal lattice (Fig. 6). The phenyl
group of aryl ether adopted perpendicular orientation with respect
to imine attached phenyl group in both 1 and 5. It is noted that the
Fig. 6. Molecular conformation with intramolecular Hꢀbonding
60
interactions in the crystals lattice of (a, b) 1 and (c, d) 5. N (blue), O
(red), H (white); Hꢀbonds (broken line). Hꢀbond distances are marked
in Å.
The external stimuli induced switching or tuning of organic solid
65 state fluorescence is generally due to the change of molecular
conformation or phase (crystalline to amorphous).3,4 To gain the
insight on the switching and tuning of 1ꢀ5 solid state
fluorescence, PXRD measurements were performed before and
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face to face molecular organization in the solid state that
30 quenches the fluorescence. Hence it is speculated that strong
grounding promotes face to face assembly in naphthaldehyde
substituted compounds (2, 3 and 5) and might quenched the
fluorescence. More structural studies are required to clearly
understand the offꢀon fluorescence switching of Schiff base
35 compounds which is currently underprogress in our lab.
Fig. 7. (a) Stairꢀlike network structure formation via πꢀπ interactions in 1
and (b, c) linear chain and interdigitation chain via Hꢀbonding and πꢀ
π interactions in 5. N (blue), O (red), H (white); Hꢀbonds (broken
line). Hꢀbond and πꢀπ interaction distances are marked in Å.
5
after applying the external force. The perfect matching of
simulated and experimental diffraction pattern of 1 confirmed the
phase purity of the sample (Fig. S6). The strong grounding of 1
modified the strong diffraction to weak diffraction peaks and
suggests the formation of partial amorphous phase. The change of
Fig. 8. PXRD pattern of (a) 2 and (b) 5.
Conclusion
In conclusion, we have demonstrated external stimuli dependent
rare high contrast fluorescence offꢀon switching using simple
40 arylꢀether amine based Schiff base molecules (1ꢀ5) that showed
aggregation induced enhanced emission (AIEE) in the solid state.
1 showed irreversible fluorescence tuning from greenishꢀyellow
to green upon hard grounding of solids. Heating or solvent
exposure did not switch the fluorescence of 1. In contrast, strong
45 grounding of 2ꢀ5 solids exhibited quenching of solid state
fluorescence. Interestingly, heating/solvent exposure of strongly
grounded powder (2ꢀ5) induced clear bright fluorescence.
Importantly, reversible fluorescence offꢀon switching in 2ꢀ5 has
been demonstrated for more than five cycles by repeated
50 grounding and heating. PXRD studies suggest that the switching
dark and bright fluorescence of 2ꢀ5 is due to the reversible
change of crystalline to amorphous phase. Single crystal analysis
of 1 and 5 confirmed the twisted molecular conformation and
strong intermolecular interactions in the crystal lattice that would
55 restrict the free rotation and rigidifies the fluorophores. Thus rare
high contrast dark and bright fluorescence switching have been
10 crystalline to amorphous phase of 1 blue shifted the fluorescence
λmax from 520 to 508 nm. 2 and 3 that formed thin needles
showed clear diffraction peaks (Fig. 8a, S7). Strong grounding
lead to the formation of amorphous phase, however, heating
converted the amorphous to crystalline phase. PXRD studies of 4
15 that showed only weak diffraction peaks indicate the formation of
more amorphous phase (Fig. S8). The weak diffraction peaks also
disappear or reduced the intensity upon strong grounding and
heating showed slight recovery of peaks. Similar to 1, the perfect
matching of simulated and experimental pattern confirmed the
20 phase purity of 5 (Fig. S9). The slight breaking of 5 produced
more peaks and strong grounding lead to the formation of
amorphous phase (Fig. 8b). Heating or solvent exposure
converted the amorphous phase to crystalline phase. Thus PXRD
studies indicate that switching offꢀon fluorescence of 2ꢀ5 and
25 fluorescence tuning in 1 is due to the conversion of crystalline to
amorphous phase with external stimuli. However, the reason for
the fluorescence tuning in 1 and switching offꢀon in 2ꢀ5 by
external stimuli is not clear. Single crystal studies of 5 showed
6
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Acknowledgements
70
Financial supports from DST, New Delhi, India (DST Fast Track
Scheme No. SR/FT/CSꢀ03/2011 (G) and SR/FST/ETIꢀ
284/2011(c)) and CRF facility, SASTRA University are
acknowledged with gratitude. AK and PSH thanks SASTRA
University for research fellowship. "Xꢀray crystallography at the
PLSꢀII 2DꢀSMC beamline was supported in part by MSIP and
5
75
10 POSTECH. We thank Dr. C. R. Ramanathan for helping in NMR
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Notes and references
a)School of Chemical & Biotechnology, SASTRA University, Thanjavur-
613401, Tamil Nadu, India. Fax: +914362264120; Tel: +914362264101;
15 E-mail: philip@biotech.sastra.edu
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b)Beamline Department, Pohang Accelerator Laboratory, 80 Jigokro-
127beongil, Nam-gu, Pohang, Gyeongbuk, Korea, Email:
dmoon@postech.ac.kr
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