Synthesis of tunable, red fluorescent aggregation-enhanced emissive organic fluorophores: Stimuli-responsive high contrast off-on fluorescence switching

Article abstract of DOI:10.1039/c7ce01867c

Triphenylamine (TPA) and N-methylbarbituric acid/indanedione-based donor-acceptor derivatives were synthesized and demonstrated molecular conformation and packing dependent tunable fluorescence from yellow to red in the solid state. TPA with N-methylbarbi

Full text of DOI:10.1039/c7ce01867c

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Tiddo J. Mooibroek, Antonio Frontera et al.
Towards design strategies for anion–π interactions in crystal engineering
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DOI: 10.1039/C7CE01867C
Journal Name
ARTICLE
Synthesis of tunable, red fluorescent aggregation enhanced
emissive organic fluorophores: Stimuli responsive high contrast
off-on fluorescence switching
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
DOI: 10.1039/x0xx00000x
Palamarneri Sivaraman Hariharan, Parthasarathy Gayathri, Anu Kundu, Subramanian
b c a
Karthikeyan, Dohyun Moon* and Savarimuthu Philip Anthony*
www.rsc.org/
Triphenylamine (TPA) and N-methylbarbituric acid/indanedione based donor-acceptor derivatives were synthesized and
demonstrated molecular conformation and packing dependent tunable fluorescence from yellow to red in the solid state.
TPA with N-methylbarbituric acid (BA-1) showed bright yellow fluorescence (λmax = 550 nm,
substitution at phenyl rings of TPA, (BA-2 and BA-3) produced strong red (λmax = 602 nm,
fluorescent solids (λmax = 582 nm, = 19.1 %). Indanedione acceptor dyes exhibited red to deep red fluorescence. ID-1
showed red fluorescence at 604 nm ( = 17.7 %) whereas ID-2 and ID-3 showed fluorescence at 611 ( = 19.4 %) and 636
nm ( = 14.1 %) in the solid state. Solid state structural analysis revealed alteration of molecular conformation and
packing by OCH substitution and lead to tunable fluorescence. BA and ID compounds showed turn-off/substantially
reduced fluorescence intensity upon hard crushing ( = 1.2 to 3.1 %). Interestingly, heating of BA and ID crushed powders
lead to turn-on/significant enhancement of fluorescence intensity ( = 5.6 to 22.5 %). Powder X-ray diffraction (PXRD)
Φ
f
= 22.8 %) whereas OCH
3
Φ
f
= 41.1 %) and orange
Φ
f
Φ
f
Φ
f
Φ
f
3
Φ
f
Φ
f
studies indicated the conversion of crystalline to amorphous and amorphous to crystalline phase by hard crushing and
heating. The reversible conversion of crystalline to amorphous phase was responsible for fluorescence switching of BA and
ID. Computational studies have been performed to get the insight on the energy level modulation upon changing of the
molecular conformation. Thus we have presented facile preparation of strong red fluorescent dyes with high contrast
stimuli responsive reversible dark and bright fluorescence switching in the solid state.
intermolecular interactions via substituent or by applying
external stimuli such as mechanical force, heat and solvent
exposure lead to tunable and switchable solid state
Introduction
Switching and tuning of organic solid state fluorescence
have received strong interest in recent years because of their
application potential in various optoelectronic devices
including sensors, displays, data storage, optical switches and
3
,5-8
fluorescence.
Often,
conformationally
twisted
tetraphenylpyrene, pyrene- and anthracene-based liquid
crystals, cyanostilbene and triphenylamine derivatives have
1
,2
been utilized to develop reversible fluorescence switching with
9
security inks. Organic fluorophore conformation, packing
and intermolecular interactions plays important role to make
the molecules to emit in the solid state as well as switching
external
force.
Fluorescence-phosphorescence
dual
mechanoluminescence at room temperature has been
1
0
3
reported
for
terphenyl-dioxaborolane
stacking mode
derivative.
dependent
and tuning of fluorescence. For example, strongly fluorescent
Substitution
controlled
π
-conjugated fluorophores become non-fluorescent in the
mechanofluorochromism was observed in 3-aryl-2-cyano
solid state due to the aggregation caused fluorescence
quenching (ACQ). In contrast, non-planar fluorophore that
prevents close packing in the solid state produced aggregation
enhanced emission (AEE) in the solid state via rigidification of
1
1
acrylamide derivatives in the solid state. Temperature
induced reversible phase changes in aromatic amine,
terpyridine and excited state intramolecular proton transfer
(ESIPT) compounds produced reversible fluorescence
4
fluorophore by weak intermolecular interactions. Further,
1
2
switching. Cyano-substituted oligo(p-phenylene vinylene)
derivative exhibited crystalline to crystalline phase transition
when grinding at high temperature and exhibited fluorescence
subtle alteration of molecular conformation, packing and
1
3
switching. However, compared to external stimuli induced
mechanofluorochromism, relatively less number of organic
a.
Department of Chemistry, School of Chemical & Biotechnology, SASTRA
University, Thanjavur-613401, Tamil Nadu, India. E-mail:
philip@biotech.sastra.edu
Department of Chemistry, Kalasalingam University, Krishnan Kovil–626126, India
Beamline Department, Pohang Accelerator Laboratory, 80 Jigokro-127beongil,
Nam-gu, Pohang, Gyeongbuk, Korea. E-mail: dmoon@postech.ac.kr
materials showed high-contrast on-off fluorescence
2a, 12b,14
b.
c.
switching.
For example, acceptor-donor-acceptor triad
based on dicyanodistyrylbenzene exhibited high contrast
15
reversible on-off fluorescence by applying external force.
d.
Electronic Supplementary Information (ESI) available: Synthetic scheme, NMR
data, fluorescence spectra, crystal structures, PXRD pattern.. See
DOI: 10.1039/x0xx00000x
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Diphenylquinoxaline core with anthracene showed heat methylbarbituric acid was added and stirred at room
1
6
responsive on-off fluorescence switching in the solid state.
A
temperature for 6 hrs. The precipitated product from the
unique piezochromic fluorescent organic crystals with pressure reaction mixture was filtered and washed with cold methanol
dependent reversible off-on fluorescence switching has been and dried.
1
7
reported recently.
5-(4-(diphenylamino)benzylidene)-1,3-dimethylpyrimidine-
2
2
The non-planar propeller shape, synthetic tailorability and 2,4,6(1H,3H,5H)-trione (BA-1)
good optoelectronic properties of triphenylamine (TPA) has
1
Bright yellow powder. Yield: 80%. M. p.: 190
MHz, CDCl ) δ 8.43 (s, 1H), 8.23 (d, 2H), 7.39-7.34 (m, 4H),
.26-7.19 (m, 6H), 6.94 (d, 2H), 3.41 (s, 3H), 3.38 (s, 3H). C-
NMR (75 MHz, CDCl ) δ 163.56, 161.27, 158.37, 153.16,
51.61, 145.43, 138.05, 129.82, 126.73, 125.81, 124.55,
17.92, 112.25, 29.00, 28.31. LCMS (ESI) calcd. [M+]: 412.16,
°C, H-NMR (300
been exploited for fabricating organic materials for dye
sensitized solar cell, organic light emitting diode (OLED), field
effect transistor, sensor, solid state fluorescent and smart
3
13
7
3
1
8
fluorescent materials. The conformational robustness and
twisted propeller core of TPA prevented close packing of
fluorophore in the solid state and often produced AEE and
1
1
found: 412.2.
-(4-(diphenylamino)-2-methoxybenzylidene)-1,3-
dimethylpyrimidine-2,4,6(1H,3H,5H)-trione (BA-2).
1
9
mechanofluorochromism.
Self-reversible and reversible
5
mechanofluoro-chromism, tunable solid state fluorescence via
polymorphism, self-erasable and rewritable fluorescent
platforms have been demonstrated using TPA by tailoring
acceptor structure, substituent position and halochromic
1
Bright orange powder. Yield: 78%. M. p.: 227
MHz, CDCl ) δ 8.98 (s, 1H), 8.58 (d, 1H), 7.39-7.34 (m, 4H),
7.23-7.19 (m, 6H), 6.51 (dd, 1H), 6.35 (d, 1H), 3.68 (s, 3H), 3.40
°C, H-NMR (300
3
2
0
functionality. In this manuscript, we report the facile
synthesis of red emissive, high contrast off-on fluorescence
switching triphenylamine derivatives and molecular
conformation and packing dependent tunable fluorescence
from yellow to red in the solid state. BA-1 showed yellow
fluorescence whereas methoxy substituted BA-2 and BA-3
produced red and orange fluorescent solids (Table 1). ID
1
3
(
s 3H), 3.36 (s, 3H). C-NMR (75 MHz, CDCl
3
) δ 163.80, 163.15,
1
1
2
61.42, 155.49, 152.74, 151.83, 145.53, 135.87, 129.74,
26.86, 125.82, 114.72, 111.43, 110.87, 99.83, 55.68, 28.85,
8.26. LCMS (ESI) calcd. [M+]: 442.17, found: 442.2.
5
-(4-((4-methoxyphenyl)(phenyl)amino)benzylidene)-1,3-
dimethylpyrimidine-2,4,6(1H,3H,5H)-trione (BA-3).
1
derivatives (ID-1 to 3) showed red to deep red fluorescence in Bright orange red powder. Yield: 75%. M. p.: 150
the solid state (Table 1). Absolute quantum yield measurement (300 MHz, CDCl ) δ 8.42 (s, 1H), 8.23 (d, 2H), 7.38-7.33 (m, 2H),
confirmed strong fluorescence of these compounds in the solid 7.22-7.13 (m, 5H), 6.94-6.86 (m, 4H), 3.83 (s, 3H), 3.41 (s, 3H),
°C, H-NMR
3
1
3
3
state (Table 1). Interestingly, all derivatives showed turn-off 3.38 (s, 3H). C-NMR (75 MHz, CDCl ) δ 163.66, 161.32,
fluorescence (Table 1) upon strong mechanical crushing and 158.38, 157.89, 153.35, 151.65, 145.37, 138.27, 138.05,
heating induced turn-on fluorescence (Table 1). Single crystal 129.74, 128.58, 126.32, 125.64, 124.03, 116.99, 115.17,
X-ray analysis revealed molecular conformational and packing 111.74, 55.54, 28.98, 28.29. LCMS (ESI) calcd. [M+]: 442.17,
change depending on the substituent and position that lead to found: 442.2.
tunable fluorescence. PXRD studies suggested the conversion General procedure for synthesizing indanedione derivatives
of crystalline to amorphous and vice versa upon crushing and (Scheme S2)
heating was responsible for fluorescence switching.
To the stirred solution of aldehyde (4-(diphenylamino)
Computational studies have been performed to get the insight
benzaldehyde/4-(diphenylamino)-2-methoxy-benzaldehyde/4-
on the energy level modulation upon changing of the
((4-methoxyphenyl)(phenyl)amino)benzaldehyde)
(1.0
molecular conformation.
equivalent) in methanol, 1.1 equivalents of indanedione was
added and stirred at room temperature for 6 hrs. The
precipitated product from the reaction mixture was filtered
and washed with cold methanol and dried.
Experimental Section
Triphenylamine, 3-methoxy-N,N-diphenylaniline, 4-methoxy 2-(4-(diphenylamino)benzylidene)-1H-indene-1,3(2H)-dione (ID-1)
triphenylamine, dimethylformamide (DMF, HPLC grade),
phosphorous oxychloride, N-Methyl barbituric acid and
indanedione were purchased from Sigma-Aldrich and used
without further purification. Aldehyde functional group into
triphenylamine, 3-methoxy triphenylamine and 4-methoxy
triphenylamine was introduced by following reported
1
C, H-NMR
Bright orange red powder. Yield: 75%. M. p.: 221
°
(
300 MHz, CDCl
m, 3H), 7.39-7.34 (m, 4H), 7.23-7.19 (m, 6H), 7.01 (d, 2H). C-
) δ 191.29, 189.66, 152.72, 146.60,
45.75, 142.36, 139.97, 136.82, 134.79, 134.55, 129.77,
26.57, 125.74, 125.52, 125.31, 122.85, 122.82, 118.87. LCMS
3
) δ 8.42 (d, 2H), 7.97-7.92 (m, 2H), 7.78-7.74
13
(
NMR (75 MHz, CDCl
3
1
1
2
1
procedure.
(ESI) calcd. [M+]: 402.14, found: 402.2.
General procedure for synthesizing N-Methylbarbituric acid
derivatives (Scheme S1)
2
-(4-(diphenylamino)-2-methoxybenzylidene)-1H-indene-1,3(2H)-
dione (ID-2)
To
the
stirred
solution
of
aldehyde
(4-
1
Bright orange red powder. Yield: 80%. M. p.: 243 °C, H-NMR
(diphenylamino)benzaldehyde/4-(diphenylamino)-2-methoxy
(
300 MHz, CDCl
3
) δ 9.15 (d, 1H), 8.44 (s, 1H), 7.94-7.90 (m, 2H),
benzaldehyde/4-((4-methoxyphenyl)(phenyl)amino) benzalde-
hyde) (1.0 equivalent) in methanol, 1.1 equivalents of N-
7
.75-7.70 (m, 2H), 7.40-7.35 (m, 4H), 7.24-7.19 (m, 6H), 6.60
13
dd, 1H), 6.40 (d, 1H), 3.71 (s, 3H). C-NMR (75 MHz, CDCl ) δ
3
(
2
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1
1
1
91.71, 189.94, 162.77, 155.18, 145.65, 142.24, 140.58, donor-
π
39.93, 136.03, 134.46, 134.25, 129.71, 126.75, 125.63, phenomena.
-acceptor compounds are known to exhibit AEE
0
2
Interestingly, BA-1-3 showed tunable
24.03, 122.59, 115.56, 111.76, 100.52, 55.58. LCMS (ESI) fluorescence from yellow to red depend on the substitution
calcd. [M+]: 432.16, found: 432.2.
position. The unsubstituted BA-1 showed strong yellow
fluorescence whereas methoxy substitution at the phenyl ring
2
1
-(4-((4-methoxyphenyl)(phenyl)amino)benzylidene)-1H-indene-
,3(2H)-dione (ID-3)
Bright orange red powder. Yield: 82%. Melting point: 176
1
°
C,
H-NMR (300 MHz, CDCl ) δ 8.40 (d, 2H), 7.97-7.92 (m, 2H),
3
7
.77-7.73 (m, 3H), 7.38-7.33 (m, 2H), 7.22-7.13 (m, 5H), 6.96-
1
.89 (m, 4H), 3.84 (s, 3H). C-NMR (75 MHz, CDCl ) δ 191.40,
3
6
1
1
1
3
89.70, 157.72, 153.08, 146.69, 145.70, 142.34, 139.95,
38.40, 136.97, 134.71, 134.45, 129.69, 128.51, 126.13,
25.30, 125.19, 124.87, 122.78, 122.75, 117.86, 115.15, 55.54.
LCMS (ESI) calcd. [M+]: 432.16, found: 432.2.
Characterization
Fluorescence spectra and absolute quantum yield for all
compounds in the solid state were recorded using Jasco
fluorescence spectrometer-FP-8300 instruments equipped
with integrating sphere and calibrated light source (D2 and
W1). Powder X-ray diffraction (PXRD) patterns were measured
using a XRD- Bruker D8 Advance XRD with Cu Kα radiation (
.54050 Å). Single crystals were coated with paratone-N oil
and the diffraction data measured at 100K with synchrotron
radiation ( = 0.62998 Å) on a ADSC Quantum-210 detector at
D SMC with a silicon (111) double crystal monochromator
DCM) at the Pohang Accelerator Laboratory, Korea. CCDC Nos.
1579626-1579630 contain the supplementary
λ =
1
λ
2
(
–
crystallographic data for this paper. The HOMO, LUMO and
band gap of all structures are studied using B3PW91/6-
31+G(d,p) level theory (Gaussian 09 package).
3 3
Fig. 1. BA and ID (a) digital fluorescence images in (I) CH CN and (II) water-CH CN
mixture (95:5) and (b) solid state fluorescence spectra. The inset in (b) shows the digital
fluorescence images of corresponding crystals. (λexc = 370 nm (spectra), 365 nm (digital
images)).
Table 1. Absolute quantum yields of BA and ID derivatives.
λ
abs, in
Hard
crushed
Crystals
%,
λmax
(nm)
λmax
(nm) (%,
Heated
λmax
(nm)
CH
3
CN
(
Φ
f
)
Φ
f
)
(
nm)
(%,
Φ
f
)
BA-1
BA-2
BA-3
ID-1
ID-2
ID-3
458
462
468
477
482
488
22.8
41.1
19.1
17.7
19. 4
14.1
550
602
582
604
611
636
3.1
530
572
557
626
593
650
14.5
22.5
7.7
530
572
557
602
595
606
1.3
1.3
3.1
1.4
1.2
14.1
10.1
10.0
Chart 1. Molecular structure of BA and ID molecules.
Results and Discussion
in which acceptor is attached (BA-2) produced red
-acceptor fluorescence solids (Table 1). BA-3, in which methoxy group
Chart 1 shows the structures of TPA donor-
π
derivatives based on N-methylbarbituric acid and indanedione. substituted at different phenyl ring of TPA, exhibited orange
In solution, both BA-1-3 and ID-1-3 compounds showed only fluorescence (Table 1) in the solid state. BA-2 exhibited higher
weak fluorescence (Fig. 1a, Table S1, Fig. S1). In contrast, both fluorescence intensity compared to BA-1 and 3 (Table 1). ID
derivatives exhibited enhanced fluorescence in the solid state compounds showed tunable strong red fluorescence between
(
Fig. 1b, Table 1, Table S1). The weak/non-fluorescence in 604 and 636 nm depend on the methoxy substitution position
solution and strong solid state fluorescence indicate the (Table 1). ID-2 exhibited higher fluorescence efficiency
aggregation enhanced emission phenomena (AEE). TPA based compared to ID-1 and 3 (Table 1). Slight breaking of BA crystals
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2
4
showed blue shifting of fluorescence (Fig. 2, S1). However, ID-1 has already been reported. ID-2 and ID-3 showed only
both BA and ID compounds did not show significant change in reduction of fluorescence intensity without showing significant
3
the absorption in CH CN (Fig. S2). The crystals of BA-1 showed change of λmax. The heating of hard crushed BA and ID solids
fluorescence at 550 nm that blue shifted to 541 nm upon slight exhibited turn-on fluorescence; though relatively low
breaking (Fig. S3a). Similarly slight breaking of BA-2 and BA-3 compared to initial fluorescence intensity (Fig. 2, 3, S3, S4,
crystals also showed blue shifting of fluorescence from 602 to Table 1). After heating, both BA-1-3 and ID-1-3 crushed solids
573 nm (BA-2, Fig. 2a) and 582 to 570 nm (BA-3, Fig. S3b). showed enhanced fluorescence intensity (Table 1). It is noted
Organic molecular materials are known to exhibit blue shift as that crushed followed by heated BA solids showed blue shifted
well as strong enhancement of emission when the long range emission maximum compared to crystals/slightly broken
5
a,20b,23
molecular ordering in the crystals was disturbed.
However, ID-1-3 did not show significant shift in the molecular ordering in the crystalline phase.
fluorescence max upon slight breaking of crystals (Fig. 2b, S4). subsequent crushing and heating cycle did not show significant
crystals. This could be due to the disruption of the long range
5
a,20b,d,23
However,
λ
Importantly, the strong fluorescence of BA and ID compounds modulation in the emission maximum (Fig. S5). We have also
were completely turn-off by hard crushing (Fig. 2, 3, S3, S4, prepared TPA based donor-acceptor dyes with barbituric acid
Table 1). BA-1 showed absolute quantum yield of (Φf) 22.8 % without N-methyl to understand the role acceptor structure on
before crushing whereas after crushing it exhibited only 3.1%. the reversible fluorescence switching. Interestingly, the as
BA-2 that showed highest fluorescence efficiency in the crystal synthesized TPA (DPBT-1) and 3-Methoxy TPA (DPBT-2)
and BA-3 also becomes weakly emissive solids after crushing derivatives with barbituric acid showed red fluorescence at
(Table 1). Similarly, strongly fluorescent ID-1, ID-2 and BA-2 602 nm (Φf = 12.3 and 13.6 %) and hard crushing completely
crystals also converted to weakly fluorescent solids by quenched the fluorescence (Fig. S6). However, heating did not
crushing. (Fig. 3, Table 1). Hard crushing of ID-1 showed red
exhibit reversible turn-on fluorescence. This might be due to
the strong intermolecular H-bonding capability of the
2
5
barbituric acid via N-H...O interactions. Thus N-methylation
of nitrogen that prevents strong H-bonding network structure
formation played important role on the reversible
fluorescence switching.
Fig. 3. Digital images of mechanofluorochromism of (a) BA and (b) ID compounds by
hard crushing followed by heating. (λexc = 365 nm).
Single crystals of BA-1-3 and ID-1-3 were grown to get the
insight on the molecular conformation and packing to
understand the solid state fluorescence tuning and switching.
Except ID-2, we were able to obtain good quality crystals of all
other compounds by slow evaporation. In all the molecules
Fig. 2. Solid state fluorescence switching of (a) BA-2 and (b) ID-1 by external stimuli. TPA exhibited twisted molecular conformation. BA-1 showed
(λexc = 370 nm).
H-bonding interaction between N-methylbarbituric acid
carbonyl oxygen and phenyl hydrogen atoms of TPA and N-
methylbarbituric acid carbon and nitrogen (Fig. 4). One of the
carbonyl oxygen atom of N-methylbarbituric acid exhibited H-
shift of fluorescence (λmax = 626 nm) with substantial
reduction of intensity (Fig. 3). The mechanofluorochromism of
4
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bonding interaction with phenyl hydrogen atoms (Fig. 4a,b). bonding interactions between methoxy oxygen and phenyl
Another carbonyl oxygen of N-methylbarbituric acid showed hydrogen and C-H… interactions between phenyl hydrogen
π
C=O…C and C=O…N interactions and these interactions lead to and indanedione (Fig. 5b, S9). The combined weak
the formation of herringbone stacking structure in the crystal intermolecular interactions produced square structure in the
lattice (Fig. 4c). In contrast, BA-2 showed only H-bonding crystal lattice. Figure 6 shows the comparison of molecular
interaction between para carbonyl oxygen of N- conformation of BA and ID molecules in the crystal lattice. The
methylbarbituric acid and phenyl hydrogen (Fig. 4d). The donor TPA phenyl group and acceptor N-methylbarbituric acid
intermolecular interaction produced dimer that was further adopts coplanar conformation in BA-1 and BA-3 (Fig. 6a).
interconnected by H-bonding. However, BA-3 exhibited H- Similarly, ID-1 also showed coplanar conformation between
bonding and C=O…C and C=O…N interactions similar to BA-1 in donor TPA phenyl group and acceptor indanedione. The
the crystal lattice and produced similar herringbone structural coplanar conformation in BA-1, BA-3 and ID-1 produced strong
arrangement (Fig. S7). It is noted that barbituric acid without intermolecular interactions with extended antiparallel dipole
methyl group on the nitrogen showed strong H-bonding with
arrangement (H-aggregation) in the crystal lattice (Fig.
Fig. 5. (a) Dimer formation in ID-1 and (b) square structure formation in ID-2 via H-
bonding interaction in the crystal lattice. C (grey), N (blue), O (red), H (white). H-
bonding and C-O…π interactions (broken line) distances are marked in Å.
Fig. 4. (a)Dimer, (b) H-bonding interaction of other oxygen of N-methyl barbituric acid,
(
c) extended H-bonding interaction along b-axis in the crystal lattice of BA-1 and (d) H- 4,5a,S5). Whereas BA-2 and ID-3 exhibited twisted
bonding interaction in the crystal lattice of BA-2. C (grey), N (blue), O (red), H (white).
conformation between TPA phenyl donor and acceptor (N-
methylbarbituric acid (BA-2) and indanedione (ID-3)). The
twisted donor-acceptor conformation in BA-2 and ID-3 could
increase the proportion of charge transfer states that lead to
H-bonding, C-O…π interactions (broken line) distances are marked in Å.
2
5
2
D
hexagonal network structure.
formation through H-bonding interactions between
indanedione oxygen and phenyl hydrogen and interactions
ID-1 showed dimer
red shifting of fluorescence
with strong interactions might lead to blue shift of
fluorescence max. The superimposing of BA and ID structures
λmax. The coplanar conformation
π-π
between carbonyl and alkene further connects the dimer in
the crystal lattice along a-axis (Fig. 5a). The H-bonding
between oxygen and phenyl hydrogen of indanedione
connects the network along b-axis (Fig. S8). ID-3 showed H-
λ
revealed that TPA also adopted different conformation depend
on the substitution and position of substitution (Fig. 6b,c).
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However, the acceptors (N-methylbarbituric acid and and strong diffraction peaks and confirmed the crystallinity of
indanedione) in both the derivatives did not show significant the sample. After hard crushing, the sharp diffraction peaks
variation. The modulation of TPA conformation could also were disappeared or the intensity was substantially reduced
influence on the solid state fluorescence of TPA based donor- (Fig. 8. S11,12). Hence hard crushing converted crystalline
2
0
acceptor derivatives.
materials to amorphous phase or partially amorphous.
The molecular conformational change induced optical band However, heating of the hard crushed compounds showed
gap modulation and solid state fluorescence tuning was
further supported by DFT calculations that were performed
using single crystal structure of BA and ID (Fig. 7, S10, Table 2).
The HOMO, LUMO and band gap of all structures are studied
using B3PW91/6-31+G(d,p) level of theory. The isodensity
surface plot (isodensity contour = 0.02) of the highest occupied
molecular orbitals (HOMOs) of BA and ID indicated that
electron density was predominantly localized on the TPA
donor phenyl group and slightly over the acceptor. The
electron density in the lowest unoccupied molecular orbitals
(LUMOs) was mainly localized on the N-methylbarbituric acid
and indanedione acceptor. The calculated band gap indicates
that BA-2 showed lowest band gap of 3.03 eV compared to BA-
1
and BA-3 (Table 2). ID-3 showed band gap of 2.78 eV
Fig. 7. Molecular orbital plots of the HOMOs and LUMOs of BA compounds.
Table 2.HOMO and LUMO energy levels of BA and ID derivatives.
HOMO
eV)
LUMO
(eV)
Band gap
(eV)
Compound
(
BA-1
BA-2
BA-3
ID-1
-5.75
-5.57
-5.65
-5.62
-5.39
-2.52
-2.55
-2.41
-2.62
-2.61
-3.23
-3.03
-3.24
-3.00
ID-3
-2.78
clear strong diffraction peaks that suggest the regeneration of
crystalline phase from amorphous phase. DSC studies of
crushed BA and ID compounds were also exhibited clear phase
transition between 70 and 100 °C (Fig. S13,14). The matching
of regenerated diffraction peaks after heating with the initial
peaks position suggest that the compound did not undergo
any structural change after crushing and heating. It is noted
that hard crushed and heated BA-1 compounds showed some
additional PXRD peaks compared to the initial crystals.
However, the PXRD pattern of heated samples perfectly
matched with the pattern generated using single crystal data
Fig.6. Molecular conformation of (a) BA and ID crystal lattice and superimposed images
of (b) BA-1-3 and (c) ID-1 and 3. C (grey), N (blue), O (red), H (white). (BA-1 (pink), BA-2
(
green), BA-3 (blue), ID-1 (orange) and ID-3 (grey).
compared to 3.00 eV of ID-1. The lower optical band gap leads (Fig. S15) and indicate that there is no structural change. Thus
to red fluorescence in BA-2 and ID compounds. Thus DFT the off-on fluorescence switching of BA and ID was due to the
calculation also supported the change of optical band gap due reversible transformation of crystalline to amorphous and vice
to the modulation of molecular conformation. To understand versa.
the mechanism of reversible fluorescence change of BA and ID
compounds towards external stimuli, PXRD studies were
Conclusion
performed before and after hard crushing as well as after
heating (Fig. 8, S11,12). The initial compounds showed clear
6
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In conclusion, we have synthesized triphenylamine (TPA) studies revealed reversible conversion of crystalline to
and N-methylbarbituric acid/indanedione based donor- amorphous and amorphous to crystalline was responsible for
acceptor dye that exhibited molecular conformation and fluorescence switching. Thus attachment of π-conjugated flat
packing dependent tunable fluorescence from yellow to red in N-methylbarbituric acid and indanedione acceptor with TPA
the solid state. The coplanar conformation between donor and donor produced tunable, red fluorescent organic dyes that
acceptor in BA-1 and BA-3 showed bright yellow and orange could exhibit external stimuli responsive high contrast
fluorescence. The increasing electronic conjugation in BA-3 red reversible off-on fluorescence switching.
Acknowledgement
Financial support from the Science and Engineering Research
Board (SERB), New Delhi, India (SERB No. EMR/2015/00-1891)
is acknowledged with gratitude. "X-ray crystallography at the
PLS-II 2D-SMC beamline was supported in part by MSIP and
POSTECH. DSC studies were supported by Basic Science
Research Program through the National Research Foundation
of Korea (NRF) funded by the Ministry of Education, Science
and Technology (NRF-2017R1C1B2003111).
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Graphical Abstract
Synthesis of tunable, red fluorescent aggregation enhanced emissive organic fluorophores:
Stimuli responsive high contrast off-on fluorescence switching
Molecular conformation controlled tunable fluorescence and stimuli responsive high contrast off-
on high contrast off-on mechanofluorochromism has been demonstrated.