Drastic Modulation of Stimuli-Responsive Fluorescence by a Subtle Structural Change of Organic Fluorophore and Polymorphism Controlled Mechanofluorochromism

Article abstract of DOI:10.1021/acs.cgd.8b00310

Stimuli-responsive fluorescence modulation of organic fluorophores is closely related to their structural organization, noncovalent interactions, ability to adopt different conformation, and phase change in the solid state. Herein, we have synthesized aggregation enhanced emissive fluorophores, (5-(4-(diphenylamino)benzylidene)-2,2-dimethyl-1,3-dioxane-4,6-dione (1), 5-(4-(diphenylamino)-2-methoxybenzylidene)-2,2-dimethyl-1,3-dioxane-4,6-dione (2), and 5-(4-(diphenylamino)-4-methoxybenzylidene)-2,2-dimethyl-1,3-dioxane-4,6-dione (3)) and demonstrated molecular structure controlled tunable fluorescence (552 to 616 nm, φ_f = 14.6-41.8%) and stimuli responses. One showed thermofluorochromism between 586 and 558 nm at room and liquid N₂. 2 showed tunable fluorescence via polymorphism (2a (550 nm) and 2b (610 nm)). Interestingly, hard crushed -2b polymorph showed two different mechanofluorochromism (MFC) when heated at 80 and 180 °C as well as topochemical conversion from 2b to 2a. In contrast, 2a and 3 displayed usual MFC. Crystal structure, powder X-ray diffraction, and differential scanning calorimetric studies indicated conformational, structural, and phase changes with different stimuli which are responsible for fluorescence switching/tuning. Computational studies revealed that optical band gap modulation depend on the molecular conformation and support the fluorescence modulation.

Full text of DOI:10.1021/acs.cgd.8b00310

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Article
Drastic Modulation of Stimuli-Responsive Fluorescence
by Subtle Structural Change of Organic Fluorophore
and Polymorphism Controlled Mechanofluorochromism
Palamarneri Sivaraman Hariharan, gayathri parthasarathy, Anu Kundu, Subramanian
Karthikeyan, Yoshimitsu Sagara, Dohyun Moon, and Savarimuthu Philip Anthony
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00310 • Publication Date (Web): 24 May 2018
Downloaded from http://pubs.acs.org on May 24, 2018
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Crystal Growth & Design
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Drastic Modulation of Stimuli-Responsive Fluorescence by Subtle Structural Change of Organic
Fluorophore and Polymorphism Controlled Mechanofluorochromism
Palamarneri Sivaraman Hariharan,a Parthasarathy Gayathri,a Anu Kundu,a Subramanian Karthikeyan,b
Yoshimitsu Sagarac Dohyun Moond* Savarimuthu Philip Anthonya*
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a)School of Chemical & Biotechnology, SASTRA Deemed University, Thanjavurꢀ613401, Tamil Nadu, India. Fax:
+914362264120; Tel: +914362264101; Eꢀmail: philip@biotech.sastra.edu
b)PG and Research department of chemistry, Khadir Mohideen College, Adirampattinam, Tamil Nadu, India.
c)Research Institute for Electronic Science, Hokkaido University, N20, W10, KitaꢀKu, Sapporo 001ꢀ0020, Japan
d)Beamline Department, Pohang Accelerator Laboratory, 80 Jigokroꢀ127beongil, Namꢀgu, Pohang, Gyeongbuk, Korea,
Email: dmoon@postech.ac.kr
KEYWORDS. Smart fluorescent materials, fluorescence switching, polymorphism, fluorescence tuning, topochemical con-
version
ABSTRACT: Stimuliꢀresponsive fluorescence modulation of organic fluorophores is closely related to their structural organizaꢀ
tion, nonꢀcovalent interactions, ability to adopt different conformation and phase change in the solid state. Herein, we have syntheꢀ
sized aggregation enhanced emissive (AEE) fluorophores, (5ꢀ(4ꢀ(diphenylamino)benzylidene)ꢀ2,2ꢀdimethylꢀ1,3ꢀdioxaneꢀ4,6ꢀdione
(1), 5ꢀ(4ꢀ(diphenylamino)ꢀ2ꢀmethoxybenzylidene)ꢀ2,2ꢀdimethylꢀ1,3ꢀdioxaneꢀ4,6ꢀdione (2) and 5ꢀ(4ꢀ(diphenylamino)ꢀ4ꢀ
methoxybenzylidene)ꢀ2,2ꢀdimethylꢀ1,3ꢀdioxaneꢀ4,6ꢀdione (3)) and demonstrated molecular structure controlled tunable fluoresꢀ
cence (552 to 616 nm,
Φf = 14.6–41.8 %) and stimuliꢀresponses. 1 showed thermofluorochromism (TFC) between 586 and 558 nm
at room and lig.N2. 2 showed tunable fluorescence via polymorphism (2a (550 nm) and 2b (610 nm)). Interestingly, hard crushed ꢀ
2b polymorph showed two different MFC when heated at 80 and 180 °C as well as topochemical conversion from 2b to 2a. In conꢀ
trast, 2a and 3 displayed usual mechanofluorochromism (MFC). Crystal structure, powder Xꢀray diffraction (PXRD) and differenꢀ
tial scanning calorimetric (DSC) studies indicated conformational, structural and phase change at different stimuli and responsible
for fluorescence switching/tuning. Computational studies revealed optical band gap modulation depend on the molecular conforꢀ
mation and support the fluorescence modulation.
rescence.38 Some of the MFC materials have also showed selfꢀ
reversible gradual change of fluorescence with mechanical
1. Introduction
force.47ꢀ48 Selfꢀreversible and switchable fluorescence has been
The ability of organic fluorophores to exhibit tunable and
reported for TFC materials by controlling rate of heating and
switchable solid state fluorescence have attracted considerable
cooling.49ꢀ51 Mechano and thermoꢀresponsive supramolecular
attention because of their application in optical switches, orꢀ
polymer and unique MFC at higher temperature has also been
ganic lightꢀemitting diodes (OLEDs), data recording, sensors,
recently reported.52,53 Polymorphism provided opportunity to
displays, photo dynamic therapy and bioꢀimaging.1ꢀ12 Particuꢀ
develop tunable organic fluorescent materials without altering
larly, smart fluorescent materials that responds systematic
chemical structure.54ꢀ60 Tunable fluorescence by topochemical
modulation to the external stimuli such as pressure, heat, vaꢀ
conversion, transforming one polymorph to another by thermal
por and pH are highly desirable since that can bring new funcꢀ
treatment and mechanical force, has also been realized.38,54,61
tional adaptive properties. Molecular conformation, packing
However, modulation of stimuliꢀresponse by subtle structural
and nonꢀcovalent interaction strongly influenced on the solid
change, especially polymorphic structure exhibiting different
state fluorescence.13ꢀ22 Fundamental interest and application
MFC depends on the heating temperature and gradual fluoresꢀ
potential in optoelectronic devices lead to the development of
cence change from one polymorph to another remains scarcely
large number of MFC and thermofluorochromic (TFC) derivaꢀ
explored.
tives including imidazoles.23ꢀ42 Most of the reported MFC and
Triphenylamine (TPA) has often been employed for develꢀ
TFC materials showed fluorescence color change between two
oping organic materials for dye sensitized solar cell, organic
emissive or one emissive and one nonꢀemissive state; though
light emitting diode (OLED), field effect transistor, sensor,
multiꢀcolor emitting MFC and TFC materials have also been
reported.43ꢀ46 Mechanical forces convert the fluorescence color
solid state fluorescent and smart fluorescent materials by utiꢀ
lizing nonꢀplanar propeller shape, synthetic tailorability and
via modulating molecular conformation/nonꢀcovalent interacꢀ
good optoelectronic.62ꢀ66 The conformational robustness and
tion/phase of the materials and produce metaꢀstable state
propeller core of TPA hindered close packing of fluorophore
whereas heating or vapor exposure recovered the initial fluoꢀ
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Crystal Growth & Design
in the solid state that produced AEE and MFC.67ꢀ69 Selfꢀ
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(399.44): calcd. C 75.17, H 5.30, N 3.51; found C 75.44, H
5.64, N 3.62. LCMS (ESI) calcd. [M+]: 399.15, found: 399.2.
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reversible and reversible MFC, tunable solid state fluorescence
via polymorphism, selfꢀerasable and rewritable fluorescent
platforms have been demonstrated using TPA by tailoring
acceptor structure, substituent position and halochromic funcꢀ
tionality.28ꢀ32 In this manuscript, we have synthesized three
new fluorophores (1-3) based on TPA donor and meldrum’s
acid acceptor (Scheme 1) that exhibited molecular structure
controlled reversible TFC, polymorphs controlled MFC, topoꢀ
chemical conversion and usual MFC. 1-3 showed tunable
2
Bright orange crystalline powder. Yield: 80%. M. p.: 227
°C, HꢀNMR (300 MHz, CDCl3) δ 8.87 (s, 1H), 8.43 (d, 1H),
1
7.40ꢀ7.35 (m, 4H), 7.25ꢀ7.20 (m, 6H), 6.49 (dd, 1H), 6.34 (d,
1H), 3.68 (s, 3H), 1.76 (s, 6H). 13CꢀNMR (75 MHz, CDCl3) δ
164.9, 163.0, 161.4, 155.7, 151.4, 145.3, 135.4, 129.8, 126.9,
126.0, 113.8, 111.3, 107.1, 103.6, 99.8, 55.6, 27.4. C26H23NO5
(429.46): calcd. C 72.71, H 5.40, N 3.26; found C 72.56, H
5.56, N 3.12. LCMS (ESI) calcd. [M+]: 429.16, found: 429.2.
9
strong fluorescence in the solid state (552 to 616 nm, Φf
=
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14.6–41.8 %). 1 exhibited reversible fluorescence switching
between 586 and 558 nm at room temperature and liq. N2,
respectively. 2 produced polymorphism induced tunable fluoꢀ
rescence (2a (553 nm) and 2b (610 nm). 2a displayed usual
MFC while applying mechanical force and heating. In conꢀ
trast, 2b exhibited two different MFC when heated at 80 and
180 °C as well as topochemical conversion from 2b to 2a.
Hard crushing of 2b showed blue shift of fluorescence (577
nm) with substantial reduction of intensity. Interestingly, heatꢀ
ing at 80 °C exhibited turnꢀon fluorescence at 607 nm (2b)
whereas 180 °C heating converted 2b to 2a and showed fluoꢀ
rescence at 550 nm. 3 showed usual offꢀon fluorescence
switching. Crystal structure analysis of 1 at different temperaꢀ
ture revealed slight molecular conformational change without
packing modulation. PXRD pattern also showed slight variaꢀ
tion in the peak position at different temperature. The polyꢀ
morphs of 2 exhibited distinctly different conformation and
molecular organization in the crystals. PXRD studies of 2b
showed clear change of structure from 2b to 2a with increasꢀ
ing heating temperature. DSC analysis and computational
studies further provided insight on the fluorescence change.
Thus the present studies demonstrated structure controlled
stimuliꢀresponses and rare polymorphism controlled MFC in a
organic fluorophore.
3
Bright red crystalline powder. Yield: 82%. M. p.: 150 °C,
1HꢀNMR (300 MHz, CDCl3) δ 8.29 (s, 1H), 8.09 (d, 2H),
7.39ꢀ7.34 (m, 2H), 7.22ꢀ7.12 (m, 5H), 6.93ꢀ6.86 (m, 4H), 3.83
(s, 3H), 1.77 (s, 6H). 13CꢀNMR (75 MHz, CDCl3) δ 164.8,
161.0, 158.0, 157.6, 153.8, 145.2, 137.9, 137.8, 129.8, 128.6,
126.4, 125.8, 123.1, 117.2, 115.2, 107.6, 103.7, 55.5, 27.4. .
C26H23NO5 (429.46): calcd. C 72.71, H 5.40, N 3.26; found C
72.66, H 5.62, N 3.32. LCMS (ESI) calcd. [M+]: 429.16,
found: 429.2.
Characterization
NMR spectra were measured on a Bruker 300 MHz
AVANCEꢀII. Fluorescence spectra and absolute quantum
yield for all compounds in the solid state were recorded using
Jasco fluorescence spectrometerꢀFPꢀ8300 instruments
equipped with integrating sphere and calibrated light source.
Lifetime measurements were carried out with a Hamamatsu
Photonics QuantaurusꢀTau. DSC scans were recorded on TA
Instruments DSC Q20 differential scanning calorimeter. Each
sample was sealed in a Tzero aluminum pan/lid. Mass spectra
were recorded with a Bruker 320ꢀMS triple quadrupole mass
spectrometer using direct probe insertion method. Powder Xꢀ
ray diffraction (PXRD) patterns were measured using a XRDꢀ
Bruker D8 Advance XRD with Cu Kα radiation (λ = 1.54050
Å) at room temperature. Single crystals were coated with paraꢀ
toneꢀN oil and the diffraction data measured at 100K with
synchrotron radiation (λ = 0.62998 Å) on a ADSC Quantumꢀ
210 detector at 2D SMC with a silicon (111) double crystal
monochromator (DCM) at the Pohang Accelerator Laboratory,
Korea. CCDC Nos. – 1563481ꢀ1563484, 1563486 and
1563487 contain the supplementary crystallographic data for
this paper. The HOMO, LUMO and band gap of all structures
are studied using B3PW91/6ꢀ31+G(d,p) level theory (Gaussiꢀ
an 09 package).
2. Experimental Section
Triphenylamine,
3ꢀmethoxyꢀN,Nꢀdiphenylaniline,
4ꢀ
methoxy triphenylamine, dimethylformamide (DMF, HPLC
grade), phosphorous oxychloride and 2,2’ꢀdimethylꢀ1,3ꢀ
dioxaneꢀ4,6ꢀdione were purchased from SigmaꢀAldrich and
used without further purification. Aldehyde functional group
into triphenylamine, 3ꢀmethoxy triphenylamine and 4ꢀ
methoxy triphenylamine was introduced by following reported
procedure.70
General procedure for synthesizing 1-3 (Scheme S1)
To
the
stirred
solution
of
aldehyde
(4ꢀ
3. Results and Discussion
(diphenylamino)benzaldehyde/4ꢀ(diphenylamino)ꢀ2ꢀmethoxy
benzaldehyde/4ꢀ((4ꢀmethoxyphenyl)(phenyl)amino) benzaldeꢀ
hyde) (1.0 equivalent) in methanol, 1.1 equivalents of 2,2’ꢀ
dimethylꢀ1,3ꢀdioxaneꢀ4,6ꢀdione was added and stirred at room
temperature for 6 hrs. The precipitated product from the reacꢀ
tion mixture was filtered and washed with cold methanol and
dried.
1-3 compounds were easily synthesized via condensing difꢀ
ferent triphenylamine aldehyde with meldrum’s acid (Scheme
1a, S1). 1-3 did not show any measurable fluorescence in the
solution, however, they showed strong fluorescence in the
solid state (Φf = 14.6–41.8 %, Table 1). The weak/nonꢀ
fluorescence in solution and strong solid state fluorescence
indicate the aggregation enhanced emission phenomena
(AEE).24ꢀ26 TPA based donorꢀπꢀacceptor compounds are
known to exhibit AEE phenomena.29ꢀ32,67ꢀ69 Scheme 1b shows
the different stimuliꢀresponsive behavior of 1-3 in the solid
state. Subtle change of 1-3 molecular structure resulted tunaꢀ
ble fluorescence from yellow to red (Fig. 1a). 1 showed orange
fluorescence (λmax = 586 nm) and 3 exhibited red fluorescence
1
1
Orange crystalline powder. Yield: 85%. M. p.: 190 °C, Hꢀ
NMR (300 MHz, CDCl3) δ 8.43 (s, 1H), 8.23 (d, 2H), 7.39ꢀ
7.34 (m, 4H), 7.26ꢀ7.19 (m, 6H), 6.94 (d, 2H), 3.41 (s, 3H),
3.38 (s, 3H).. 13CꢀNMR (75 MHz, CDCl3) δ 163.56, 161.27,
158.37, 153.16, 151.61, 145.43, 138.05, 129.82, 126.73,
125.81, 124.55, 117.92, 112.25, 29.00, 28.31. C25H21NO4
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Crystal Growth & Design
Table 1. Fluorescence data of 1-3.
1
2
1
2a
2b
3
3
4
5
6
Crystals,
max (nm)
586
41.8
553
21.0
610
15.6
616
14.6
λ
Φf (%)
7
Hard
8
9
crushed powꢀ
586
7.6
553
4.6
577
604
5.7
ders,
(nm)
λmax
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23
3.1
Φf (%)
610 (80
°C)
Heated
samples, λmax
(nm)
560
553
590
548
(180 °C)
6.5 (80
°C)
13.5
(180 °C)
29.9
13.5
14.9
Φf (%)
Scheme 1. molecular structure (a) and different stimuli
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responsive fluorescence (b). λexc = 365 nm.
Fig. 2. PXRD pattern of at different temperature and (b) MFC of 1. λexc
=
365 nm (for digital images) and 370 nm (for spectra).
in the solid state (λmax = 616 nm). The asꢀsynthesized powder
of 2 showed red fluorescence and spectra showed peak at 610
nm and a small hump at 548 nm (Fig. S1a). However, crystalꢀ
lization of 2 produced two concomitant polymorphs (2a and
2b) that exhibited yellow (λmax = 553 nm) and red fluoresꢀ
Fig. 1. tunable fluorescence via subtle structural change and
(b) TFC of 1. λexc = 365 nm (for digital images) and 370 nm
(for spectra).
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structural change. Hence MFC of 1 was attributed to the reꢀ
versible switching of crystalline to partial amorphous and vice
versa.
cence (λmax = 610 nm, Fig. S1b). Interestingly, orange fluoresꢀ
cence of asꢀsynthesized powder as well as crystals of ꢀ1
showed blue shift of fluorescence λmax from 586 to 558 nm
upon immersing in liq. N2 (Fig. 1b, S2). The fluorescence was
reversed to initial state upon removing from liq.N2 within few
minutes. PXRD analysis of 1 measured at 298, 193 and 100 K
showed small differences in the peak position upon reducing
to low temperature (Fig. 2a). However, DSC heating and coolꢀ
ing cycle studies did not show any phase transition except
melting at 152 °C (Fig. S3). Due to the instrument limitation,
we were able to record DSC up to ꢀ80 °C whereas 1 showed
color change only when immersed in liq. N2. Hence 1 might be
having phase transition below ꢀ80 °C. Apart from reversible
TFC, 1 also showed usual MFC observed for twisted nonꢀ
planar TPA derivatives. Hard crushing of 1 exhibited strong
reduction of fluorescence intensity without shifting λmax
(Fig.2b, Table 1). The fluorescence was recovered while heatꢀ
1
2
3
4
5
6
7
8
2 crystals grown from CH2Cl2ꢀhexane mixture showed two
different fluorescence color (Fig. S1b), yellow (2a, Table 1)
and red (2b, Table 1). 2a and 2b crystals were separated by
hand. In contrast to 1, both yellow and red crystals of 2 did not
show any fluorescence change while immersing in liq. N2.
However, both 2a and 2b showed MFC (Fig. 3). Hard crushꢀ
ing of 2a showed substantial reduction of fluorescence intensiꢀ
ty without altering fluorescence λmax (Fig. 3a, Table 1). The
fluorescence was recovered by heating the crushed solids at
130 °C (Table 1). Slight breaking of 2b crystals showed enꢀ
hancement of fluorescence intensity with slight blue shift of
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λ
max (600 nm) that might be attributed to defects generation on
the crystals (Fig. 3b).71ꢀ74 Similar to 2a, hard crushing of 2b
also showed strong reduction of fluorescence intensity with
further blue shift of λmax from 600 to 577 nm (Fig. 3b, Table
1). Interestingly, heating of crushed 2b solids at different temꢀ
perature displayed different fluorescence turnꢀon (Fig. 3b,c).
Heating crushed powder at 80 °C exhibited turnꢀon fluoresꢀ
cence at 610 nm (Table 1). However, further increase of heatꢀ
ing temperature (100ꢀ140 °C) leads to slight reduction of fluoꢀ
rescence intensity with gradual blue shift of fluorescence. 2b
crushed powder heated at 160 °C showed two fluorescence
peaks, 540 and 580 nm. The fluorescence of λmax was comꢀ
pletely shifted to 548 nm with good enhancement of intensity
by heating at 180 °C (Table 1). Without crushing, direct heatꢀ
ing of 2b crystals also showed complete conversion of fluoꢀ
rescence from red to yellow at 180 °C (Fig. S5). The
ing at 130 °C (Φf = 29.9 %). Heated solids showed blue shifted
fluorescence λmax that could be attributed to disruption of long
range molecular ordering in the solid state.71ꢀ74 PXRD studies
of 1 indicated that strong crushing converted crystalline mateꢀ
rials to partial amorphous and crystalline phase was recovered
by heating (Fig. S4). The PXRD patterns of heated sample
matched with initial sample and suggest that there is no
Fig. 3. (a) 2a MFC, (b) MFC of 2b with different temperature heating, (c)
digital images of fluorescence change with pressure and temperature and
(d) topochemical conversion of ꢀ2b. λexc = 365 nm (for digital images) and
370 nm (for spectra).
Fig. 4. 2b DSC (a) and PXRD pattern (b) at different temperature.
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Crystal Growth & Design
dimers via CꢀH…π interactions (Fig. 5a, S9). The molecular
topochemical conversion of 2b has been demonstrated in a
1
pattern using hard crushed powder that showed complete conꢀ
version of red fluorescence to yellow upon increasing heating
temperature (Fig. 3d). However, heating of 2a crystals did not
show any significant change of fluorescence. DSC studies of
2a crystals did not show any phase transition before melting
and suggest there is no phase or structural transformation (Fig.
4a). However, 2b crystals exhibited clear phase transition
around 190 °C before melting at 210 °C (Fig. 4a). This supꢀ
ports the topochemical transformation of 2b to 2a. Hard
crushed solids of 2b exhibited two phase transition, one at 80
and another at 140 °C. The first phase transition for converting
amorphous to crystalline form of 2b and second phase transiꢀ
tion is corresponds to topochemical conversion. Hard crushed
80 °C heated 2b powders showed phase transition at 140 C °C
corresponds to topochemical conversion. On the other hand,
hard crushed, higher temperature (140, 160 and 180 °C) heatꢀ
ed powders did not show any phase transition and indicates
completion of structural transformation. PXRD studies reꢀ
vealed different diffraction patterns for 2a and 2b polymorphs
(Fig. 4b). Hard crushing of 2b leads to the decrease of peak
intensity significantly and indicates the formation of partial
amorphous solids. The PXRD peaks obtained for the crushed
solids matched with 2b initial pattern. Heating of crushed solꢀ
ids at 80 °C increased the peak intensity without changing the
pattern that suggests the improvement of crystallinity. In conꢀ
trast, the crushed solids heated at 140 °C revealed complete
change of peaks position which matched with 2a rather than
2b. Higher temperature heating of crushed solids only inꢀ
creased the crystallinity. Thus the first phase transition (80 °C)
of ꢀ2b crushed solids was due to conversion of partial amorꢀ
phous to crystalline whereas second phase transition (140 °C)
could be attributed for structural conversion.
packing in the crystal lattice also did not show significant difꢀ
ferences (Fig. S10). However, comparison of molecular conꢀ
formation revealed slight twisting differences in the triphenylꢀ
amine at different temperature (Fig. 5b, S11, Table S1). The
acceptor (meldrum’s acid) also showed change of twisting
with temperature. The torsional angle (τ) of TPA and melꢀ
drum’s acid decreased from 4.61 at 298K to 3.78 at 100K (Taꢀ
ble S1). The decrease of molecular twist at low temperature
might lead to closer packing of the oppositely oriented moleꢀ
cules and increase of optical band gap. It is noted that CꢀH…O
intermolecular distances of 1 was decreased from 3.219 to
3.195 Å with decreasing temperature from 298 to 100K (Fig.
5a, S9) The molecular conformational and packing change
induced optical band gap modulation and solid state fluoresꢀ
cence tuning of 1 was further supported by DFT calculations
(Fig. 6, Table 2). Single crystal structure of 1 was used for the
calculations. 1 structure revealed gradual increase of optical
band gap with decreasing temperature. 1 at 298 K exhibited
lowest band gap of 3.205 eV whereas it showed 3.24 eV at
100 K. Thus slight conformational change with decreasing
temperature leads to increase of band gap. 2 produced yellow
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3 showed red fluorescence with relatively weak intensity
(Table 1, Fig. S6a) compared to 1 and 2. On the other hand, 3
did not show reversible TFC or polymorphism rather it
showed only usual mechanical shear and heating induced offꢀ
on fluorescence switching. Hard crushing of 3 showed strong
reduction of fluorescence intensity with small blue shift of
fluorescence (Table 1). Heating of hard crushed solids at 130
°C strongly enhanced fluorescence intensity (Table 1). DSC
studies clearly revealed a phase transition between 80 and 90
°C (Fig. S6b). PXRD studies suggested the conversion of crysꢀ
talline to amorphous/partial amorphous state upon hard crushꢀ
ing and amorphous to crystalline state while heating (Fig.
S6c). Thus subtle structural change from simple TPA to methꢀ
oxy substituted TPA significantly influenced on the stimuliꢀ
responsive properties of the resulting materials. The fluoresꢀ
cence lifetime measurements revealed biꢀexponential decay
for 1, 2b and 3 whereas 2a exhibited triꢀexponential decay
(Fig. S7). The short lifetime (4.10 and 9.37 ns (1), 1.28, 7.03,
19.92 ns (2a), 4.02 and 8.46 ns (2b), 2.98 and 5.85 ns (3))
indicates that all compounds shows only fluorescence.
Fig. 5. (a) Dimer and linking of dimers in the crystal lattice of 1 (298 K)
and (b) superimposed molecular conformation of displayed in the crystal
lattice at different temperature. 1ꢀ298K (grey), 193K (yellow) and 100K
(orange). C (grey), N (blue), O (red), H (white); Hꢀbonds (broken line).
d
D…A distances are marked (Å).
To understand the tunable fluorescence and stimuliꢀ
responsive behavior of 1-3, detailed crystal structure analysis
were performed. Single crystal of 1 was grown from slow
evaporation of DCMꢀhexane. 1 structural analysis was perꢀ
formed at 298, 193 and 100 K to gain insight on the reversible
TFC. Single crystal of 1 also exhibited clear color change upꢀ
on cooling from 298 to 100 K (Fig. S8). At all three temperaꢀ
tures, 1 showed similar dimer formation with opposite moꢀ
lecular orientation via CꢀH…O interactions and interlinking of
Figure 6. Molecular orbital plots of the HOMOs and LUMOs of 1 at (I)
298K, (II) 193K and (III) 100K.
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Page 6 of 10
Table 2. Computational HOMO-LUMO band gap value of
1-3.
tational studies of 2 polymorphic structures clearly demonꢀ
1
strated band gap differences (Fig. S15, Table S1). The higher
triphenylamine twisting and lower accepter twisting lead to
higher band gap (3.50 eV) for 2a compared to 2b band gap of
3.182 eV. The stabilization and free energies calculation sugꢀ
gests that 2b structure is more stable than 2a structure by ꢀ3.3
kcal/mol. However, stability of 2a was increased from room
temperature to 453 K by ꢀ26.5 kcal/mol. This might be the
reason for 2b exhibiting topochemical conversion while heatꢀ
ing at 140 °C (Fig. 3b,c,S5). The crystal lattice of 3 (grown
from CH3OH) also revealed πꢀπ interactions induced opposite
molecular oriented dimer and interlinking of dimer by CꢀH…π
interactions (Fig. 8a, S16). Theoretical calculation showed
band gap of 3.187 eV (Fig. S17). Comparison of crystal strucꢀ
tures showed clear conformational changes by OCH3 substituꢀ
tion and polymorphism (Fig. 8b,S18). The modulation of conꢀ
formation and packing lead to tunable fluorescence that was
further supported by theoretical calculation. Further, the strucꢀ
tural and solid state fluorescence comparison indicates that
increase of acceptor twist (meldrum’s acid) lead to red shift of
fluorescence whereas smaller twist showed blue shifted fluoꢀ
rescence.
2
3
4
5
6
7
8
9
Compound
HOMO (eV)
LUMO (eV)
Band gap (eV)
1 (298 K)
1 (193 K)
1 (100 K)
2a
ꢀ5.754
ꢀ5.752
ꢀ5.75
ꢀ2.549
ꢀ2.532
ꢀ2.51
3.205
3.220
3.24
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ꢀ5.721
ꢀ5.633
ꢀ5.635
ꢀ2.221
ꢀ2.451
ꢀ2.448
3.500
3.182
3.187
2b
3
Fig. 7. (a) Superimposed molecular conformation of 2 in polymorphs (2a
(purple) and 2b (pink)) and dimer and linking of dimers in the crystal
lattice of (b) ꢀ2a and (c) ꢀ2b. C (grey), N (blue), O (red), H (white); Hꢀ
bonds (broken line). dD…A distances are marked (Å).
Fig. 8. (a) Dimer and linking of dimers in the crystal lattice of 3 and (b)
comparison of molecular conformation in the crystal lattice of 1-3. C
(grey), N (blue), O (red), H (white); Hꢀbonds (broken line). dD…A distances
are marked (Å).
(2a) and red (2b) polymorphs by slow evaporation of CH2Cl2ꢀ
CH3OH. Structural analysis revealed different molecular conꢀ
formation and crystal packing in the crystal lattice of 2a and
2b (Fig. 7a, S12, Table S1). 2a showed dimer formation with
opposite molecular orientation through πꢀπ interactions
(Fig.7b). The CꢀH…π and CꢀH…O interactions further interꢀ
link the dimers along aꢀaxis. Dimers were also linked along cꢀ
axis via CꢀH…O interactions between methoxy methyl hydroꢀ
gen and meldrum’s acid carbonyl oxygen (Fig. S13). 2b
showed dimer formation via CꢀH…O intermolecular interacꢀ
tions between carbonyl oxygen and phenyl and methoxy meꢀ
thyl hydrogen (Fig. 7c). Further CꢀH…O interactions interlink
the dimers along bꢀaxis (Fig. S14). Triphenylamine in the
crystal lattice of 2a exhibited highest twist compared to the
crystal lattice of 2b. However, meldrum’s acid was strongly
twisted in 2b (τ = 23.56) compared to ꢀ2a (τ = 7.26). Compuꢀ
4. Conclusion
In conclusion, subtle structural change controlled tunable
and stimuli responsive fluorescence including rare polymorph
controlled MFC when heated at different temperature has been
demonstrated. 1-3 showed strong tunable fluorescence in the
solid state (552 to 616 nm, Φf = 14.6–41.8 %). 1 showed reꢀ
versible TFC at liq. N2 and room temperature. Crystal strucꢀ
tural analysis indicated conformational change of TPA upon
reducing temperature and PXRD pattern revealed slight peak
position change. Computational studies supported conforꢀ
mation induced optical band gap modulation. 2b showed rare
two different MFC when heated at 80 and 180 °C and topoꢀ
chemical conversion. PXRD studies confirmed conversion of
partial amorphous to crystalline while heating at 80 °C whereꢀ
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Crystal Growth & Design
tra/Intermolecular Slipped π−π Stackings of Tetraphenylethyleneꢀ
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1
2
3
4
5
6
7
8
studies showed two phase transition (80 and 140 °C) for
strongly crushed 2b. In contrast, 2a and 3 showed usual MFC
via reversible phase transformation. Thus the present studies
suggests that conformationally flexible TPA especially with
OCH3 substituent at ortho position to acceptor could be a poꢀ
tential building unit to develop polymorphic, topochemical as
well as multiꢀstimuli responsive smart fluorescent materials.
Supporting Information: Synthesis scheme, NMR spectra, crystalꢀ
lographic table, fluorescence spectra, crystal structure, HOMOꢀ
LUMO molecular diagram, lifetime, DSC and PXRD studies.
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Corresponding Author
*a)Savarimuthu Philip Anthony, Department of Chemistry, School
of Chemical & Biotechnology, SASTRA Deemed University,
Thanjavurꢀ613401, Tamil Nadu, India.
(philip@biotech.sastra.edu)
*d) Beamline Department, Pohang Accelerator Laboratory,
80 Jigokro-127beongil, Nam-gu, Pohang, Gyeongbuk, Ko-
rea. (dmoon@postech.ac.kr
)
ACKNOWLEDGMENT
Financial support from the Science and Engineering Research
Board (SERB), New Delhi, India (SERB No. EMR/2015/00ꢀ
1891) is acknowledged with gratitude. "Xꢀray crystallography at
the PLSꢀII 2DꢀSMC beamline was supported in part by MSIP and
POSTECH. DSC studies were supported by Basic Science Reꢀ
search Program through the National Research Foundation of
Korea (NRF) funded by the Ministry of Education, Science and
Technology (NRFꢀ2017R1C1B2003111).
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Drastic Modulation of Stimuli-Responsive Fluorescence by Subtle Structural Change of Organic
Fluorophore and Polymorphism Controlled Mechanofluorochromism
Palamarneri Sivaraman Hariharan,a Parthasarathy Gayathri,a Anu Kundu,a Subramanian Karthikeyan,b
Yoshimitsu Sagarac Dohyun Moond* Savarimuthu Philip Anthonya*
Aggregation enhanced emissive fluorophore based on triphenylamine exhibited drastic modulation of
stimuliꢀresponsive behavior and polymorphism and topochemical conversion induced tunable fluoresꢀ
cence.
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