Molecular engineering of triphenylamine based aggregation enhanced emissive fluorophore: Structure-dependent mechanochromism and self-reversible fluorescence switching

Article abstract of DOI:10.1021/acs.cgd.6b01363

Triphenylamine (TPA), a propeller-shaped optoelectronic molecule, has been used to generate stimuli-responsive smart fluorescent organic materials and correlate the effect of subtle structural changes on the molecular packing and mechanochromic fluorescen

Full text of DOI:10.1021/acs.cgd.6b01363

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Article
Molecular Engineering of Triphenylamine based Aggregation
Enhanced Emissive Fluorophore: Structure Dependent
Mechanochromism and Self-Reversible Fluorescence Switching
Palamarneri Sivaraman Hariharan, Viki Kumar Prasad, Surajit Nandi,
Anakuthil Anoop, Dohyun Moon, and Savarimuthu Philip Anthony
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01363 • Publication Date (Web): 21 Nov 2016
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Molecular Engineering of Triphenylamine based Aggregation Enhanced Emissive
Fluorophore: Structure Dependent Mechanochromism and Self-Reversible Fluores-
cence Switching
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Palamarneri Sivaraman Hariharan,a) Viki Kumar Prasad,b) Surajit Nandi,b) Anakuthil Anoop,b) Dohyun
Moon*c) and Savarimuthu Philip Anthony*a)
a)Department of Chemistry, School of Chemical & Biotechnology, SASTRA University, Thanjavur – 613401, Tamil
Nadu, INDIA. E-mail: philip@biotech.sastra.edu
b)Department of Chemistry, Indian Institute of Technology Kharagpur, West Bengal 721302 INDIA
c)Beamline Department, Pohang Accelerator Laboratory, 80 Jigokro-127beongil, Nam-gu, Pohang, Gyeongbuk, Ko-
rea. E-mail: dmoon@postech.ac.kr
KEYWORDS: stimuli-responsive organic fluorescent materials, smart fluorescent materials, triphenylamine, aggrega-
tion enhanced emission
ABSTRACT: Triphenylamine (TPA), a propeller shaped optoelectronic molecule, has been used to generꢀ
ate stimuliꢀresponsive smart fluorescent organic materials and correlate the effect of subtle structural
change on the molecular packing and mechanochromic fluorescence (MCF). The substituent (OCH3)
position in the TPA phenyl ring and acceptors (malononitrile, cyanoacetamide, cyanoacetic acid, ethyl
cyanoacetate and diethylmalonate) influenced strongly on the solid state and mechanochromic fluoresꢀ
cence as well as molecular packing. The structureꢀproperty studies revealed that (i) TPA derivatives
without OCH3 substituent exhibit strong fluorescence (Φf = 85 % (TCAADꢀ1, 55 % (TDEM)), (ii) highꢀ
er dihedral angle (τ) between donor (aminophenyl) and acceptor lead to weak/non fluorescent material,
(iii) substituent at ortho position to acceptor increased the dihedral angle (τ = 26.49 (TCAADꢀ2), τ =
27.14 (TDMM)) and (iv) the increase of alkyl groups produced selfꢀreversible high contrast offꢀon fluoꢀ
rescence switching materials (TDEM). PXRD studies indicate that stimuli induced reversible phase
transformation from crystalline to amorphous and viceꢀversa was responsible for fluorescence switchꢀ
ing. The computational studies also supported that OCH3 substitution at ortho to acceptor increased the
dihedral angle and optical band gap. Thus, the present studies provide a structural insight for designing
TPA based organic molecules for developing new smart organic materials.
actions substantially modulates the excitonic interactions
and solid state fluorescence.21-25 Derivatives of tetra-
phenylpyrene, tetraphenylethenes, cyanostilbene, phe-
nanthroimidazole, boron-diketonate and dendritic mole-
cules are some of the molecular building blocks extensive-
ly employed for mechano and vapochromism.26-30 In most
of the MCF materials, reversible fluorescence switching
was achieved by applying secondary force such as heat-
ing/vapor exposing. Only few classes of materials exhibit-
ed self-reversible fluorescence without the need of sec-
ondary force.31-34 Nevertheless, it is still a great challenge
to predict structural design for organic molecules to ex-
hibit MCF especially self-reversible and high contrast flu-
orescence switching. Hence, there is a strong interest in
exploring MCF molecules with subtle structural change to
Introduction
Stimuli-active smart fluorescent materials have been
received significant interest because of their fundamental
importance and application potential in sensors, security
inks, data storage and optoelectronic devices.1-8 Particu-
larly, developing organic mechanochromic fluorescence
(MCF) materials that exhibit reversible fluorescence
changes without undergoing any structural change have
made rapid progress in recent years.9-13 For example,
fluorophores with non-planar twisted molecular confor-
mation whose structures adopt more planar conformation
and exhibit phase transition from crystalline to amor-
phous phase by mechanical grinding have often been re-
ported for MCF.14-20 Moreover, subtle alternation of mo-
lecular packing by disrupting weak intermolecular inter-
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understand the relationship between molecular structures
perature dependent positive fluorescence enhancement
and resulting MCF properties at the molecular level.35-37
by introducing aldehyde functionality and polymorphism,
mechanochromism and nanofabrication induced fluores-
cence switching by attaching malononitrile with methoxy
substituted TPA.33,50 However, systematic studies of struc-
tural modification such as acceptor groups or substituent
position in the phenyl ring of TPA on their fluorescence
properties have never been explored in detail and such
studies are expected to provide structural insight for de-
signing new MCF molecules with self-reversible or stimu-
li-induced reversible fluorescence. Herein, we have at-
tempted to correlate the acceptor structure and position
of methoxy group in the phenyl ring of TPA on the mo-
lecular packing and solid state fluorescence especially
MCF properties by synthesizing TPA-based donor-π-
acceptor (D-π-A) molecules with different acceptors
(malanonitrile, cyanoacetamide, cyanoacetic acid, ethyl
cyanoacetate and diethylmalonate, Scheme-1). Interest-
ingly, acceptor structure and substituent position exhibit-
ed strong influence on the molecular conformation and
packing that significantly modulated their solid state and
mechanochromic fluorescence. TCAAD-2 and TDMM
that showed higher dihedral angle between donor TPA
and acceptor did not show any solid state fluorescence.
The comparison of solid state structure and fluorescence
properties revealed that TPA derivatives without methoxy
substituent in the phenyl ring showed strong fluores-
cence. The increase of alkyl groups produced self-
reversible high contrast off-on fluorescence switching
materials. Theoretical studies also confirmed the increase
of dihedral angle and band gap when OCH3 is substituted
at ortho to acceptor in TPA. PXRD studies indicate that
the reversible phase change from crystalline to amor-
phous was responsible for fluorescence switching. Thus
the present studies could provide a structural insight for
designing new TPA based MCF organic molecules.
Triphenylamine (TPA), a propeller shaped nonplanar
optoelectronic molecule, has been employed as electron
donor (D) moiety in fabricating dyes for dye-sensitized
solar cells (DSSC), organic field effect transistor and solid
state fluorescent materials for OLED device applica-
tions.38-40 The nonplanar twisted structure that prevented
close stacking often produced aggregation induced emis-
sive materials whereas synthetic tailorability allowed to
engineer the molecular structure with different substitu-
ent or acceptor (A) groups.41-45 The nonplanar molecular
structure with D-π-A configuration provided the basis for
developing solid state fluorescent materials with MCF
properties. TPA substituted with dicyanodistyrylbenzene
π-conjugated molecule reported for high contrast fluores-
cence switching.46 TPA based D-A-D molecular structure
revealed temperature dependent fluorescence in solution
and phase transition dependent mechanochromism.47
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Experimental Section
Chemicals: Acetonitrile (CH3CN, HPLC grade), tetra-
hydrofuran (THF, HPLC grade), dimethylformamide
(DMF, HPLC grade), phosphorous oxychloride, triphenyl-
amine, 3-methoxy-N,N-diphenylaniline, 4-methoxy tri-
phenylamine, cyanoacetic acid, cyanoacetamide, ethylcy-
anoacetate, malononitrile and diethylmalonoate were
purchased from Sigma-Aldrich and used without further
purification.
General procedure for synthesizing 1-3:
Aldehyde was introduced into TPA by following a re-
ported Vilsmeier–Haack formylation procedure.51 Typical-
ly, phosphorous oxychloride (1.1 mL, 12 mmol) was added
drop-wise to DMF (2 mL, 24 mmol) at 0 °C (cooled in ice-
salt mixture) and the reaction mixture was stirred for 45
minutes. Then, TPA compound (triphenylamine/3-
Scheme 1. Structures of TPA based D- π-A molecules.
Highly twisted benzoyl group attached TPA showed me-
chanical force induced off-on fluorescence switching.48
Self-reversible fluorescence switching has been observed
with TPA substituted with aldehyde and methyl groups in
the phenyl ring.31 Malononitrile attached TPA showed
phase transition induced reversible fluorescence switch-
ing.49 Recently, we have reported self-reversible and tem-
methoxy-N,N-diphenylaniline/4-methoxy
-N,N-
diphenylaniline, (0.98 g, 4 mmol)) was added slowly.
Then the reaction mixture was brought to room tempera-
ture and stirred at 60°C for 2 h. After cooling, the reaction
mixture was poured into ice-water (200 mL). The solution
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was neutralized with 1 M NaOH solution and extracted
117.30, 101.38. C22H17N3O (339.14): calcd. C 77.86, H 5.05, N
with CH2Cl2. The combined extract was washed with wa-
ter and brine solution and finally dried over Na2SO4. The
solvent was evaporated and purified using column chro-
matography (ethyl acetate:hexane (1:10)).
12.38; found C 77.52, H 5.40, N 12.16. LCMS (ESI) calcd.
[M+H]+: 340.14, found: 340.2.
TCAAD-2: Light orange colored powder and recrystal-
lized from DMSO and obtained as an orange colored crys-
1
4-(diphenylamino)benzaldehyde (1): (yield 90%). Mp
132-136 °C. 1H NMR (300 MHz, CDCl3) δ 9.81 (s, 1H), 7.66-
7.70 (d, 2H), 7.31-7.38 (m, 4H), 7.14-7.19 (m, 6H), 6.99-7.03
(d, 2H). 13C NMR (125 MHz, CDCl3) δ 190.46, 153.37, 146.16,
131.32, 129.74, 129.10, 126.33, 125.12, 119.35.
tal. Yield: 75%. H NMR (300 MHz, DMSO-d6) δ 8.33 (s,
1H), 8.00-8.04 (m, 1H), 7.62-7.66 (b,2H), 7.40-7.45 (m,4H),
7.21-7.26 (m, 6H), 6.43-6.45 (m, 2H), 3.64 (s, 3H). 13C
NMR (125 MHz, DMSO-d6) δ 163.23, 159.91, 152.98, 145.33,
144.14, 129.92, 129.28, 126.34, 125.48, 117.69, 112.54, 111.29,
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100.99, 100.28, 55.51. C23H19N3O2 (369.15): calcd. C 74.78,
4-(diphenylamino)-2-methoxybenzaldehyde (2): (yield
93%). Mp 132-137 °C. 1H NMR (300 MHz, CDCl3) δ 10.23 (s,
1H), 7.64-7.67 (d, 1H), 7.31-7.37 (m, 4H), 7.14-7.20 (m, 6H),
6.51-6.54 (dd, 1H), 6.44-6.45 (d, 1H), 3.69 (s, 3H). 13C NMR
(125 MHz, CDCl3) δ 187.85, 163.12, 154.87, 146.10, 129.91,
129.67, 126.47, 125.17, 118.21, 112.23, 101.73, 55.41.
H 5.18, N 11.37; found C 74.52, H 5.31, N 11.24. LCMS
(ESI) calcd. [M+H]+: 370.15, found: 370.2
TCAAD-3: Light orange colored powder and recrystal-
lized from acetonitrile and obtained as an orange colored
1
crystal. Yield: 82%. H NMR (500 MHz, DMSO-d6) δ 8.00
(s, 1H), 7.79-7.82 (d, 2H), 7.79- (b,1H), 7.72 (b, 1H), 7.37-
7.43 (m, 2H), 7.17-7.24 (m, 5H), 6.99-7.02 (d, 2H), 6.77-
6.82 (d, 2H), 3.77 (s, 3H). 13C NMR (125 MHz, DMSO-d6) δ
163.38, 157.26, 151.51, 149.81, 145.44, 137.93, 132.12, 129.85,
128.56, 125.75, 125.08, 122.62, 117.45, 117.33, 115.35, 100.63,
4-(N-(4-methoxyphenyl)-N-phenylamino)
benzalde-
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hyde (3): (yield 85%). Mp 67-68 °C. H NMR (300 MHz,
CDCl3) δ 9.78 (s, 1H), 7.63-7.67 (d, 2H), 7.31-7.34 (m, 2H),
7.11-7.18 (m, 5H), 6.88-6.95 (m, 4H), 3.82 (s, 3H). 13C NMR
(125 MHz, CDCl3) δ 190.4, 157.6, 153.7, 146.2, 138.9, 131.4,
129.7, 128.5, 125.9, 124.9, 118.2, 115.2, 55.6.
55.29. C23H19N3O2 (369.15): calcd. C 74.78, H 5.18, N
11.37; found C 74.48, H 5.06, N 11.14. LCMS (ESI) calcd.
[M+H]+: 370.15, found: 370.2
Synthesis of TCMN-3
3 (1.0mmol) and malononitrile (1.2mmol) were dis-
solved in ethanol and refluxed for 4 hrs. Upon cooling to
room temperature, it produced reddish orange colored
powder that was filtered, washed by cold ethanol and
dried under vacuum. The product was further purified by
recrystallization from acetonitrile.
TCAA-2: Bright orange colored powder and recrystal-
lized from acetonitrile. Yield: 82%. 1H NMR (300 MHz,
DMSO d6) δ 13.52 (b, 1H), 8.44 (s, 1H), 8.11-8.44 (d, 1H),
7.42-7.47 (m,4 H), 7.24-7.29 (m, 6H), 6.39-6.40 (d, 1H),
6.42-6.45 (dd, 1H), 3.65 (s, 3H). 13C NMR (125 MHz,
CDCl3) δ. 163.07, 159.12, 152.52, 145.03, 143.62, 128.61, 128.21,
125.26, 124.52, 115.95, 110.44, 109.59, 98.80, 95.00, 54.22.
TCMN-3: Red colored powder and recrystallized from
acetonitrile. Yield: 80%. H1 NMR (300 MHz, DMSO-d6) δ
8.24 (s, 1H), 7.85-7.88 (d, 2H), 7.31-7.53 (m, 2H), 7.27-7.29
(m, 6H), 7.06-7.12 (m, 2H), 6.82-6.87 (m, 2H), 3.84 (s, 3H).
C13 NMR (125 MHz, DMSO-d6) δ 159.20, 157.71, 153.34,
144.67, 137.18, 133.13, 130.03, 128.74, 126.46, 126.08, 121.93,
C23H18N2O3 (370.13): calcd. C 74.58, H 4.90, N 7.56; found
C 74.72, H 5.06, N 7.42. LCMS (ESI) calcd. [M+H]+: 371.13,
found: 371.2
TCAA-3: Obtained as red colored gel. Yield: 85%. 1H
NMR (300 MHz, CDCl3) δ 13.50 (b, 1H), 8.11 (s, 1H), 7.39-
7.45 (m, 2H), 7.18-7.23 (m, 5H), 6.99-7.04 (d, 2H), 6.77-
6.80 (d, 2H), 3.78 (s, 3H). 13C NMR (125 MHz, CDCl3) δ
164.19, 157.42, 153.24, 152.21, 145.16, 137.66, 129.91, 128.64,
126.04, 125.45, 122.11, 117.13, 116.90, 115.37, 97.23, 55.30.
116.36, 115.49, 115.44, 114.57, 72.88, 55.33. C23H17N3O
(
351.14): calcd. C 78.61, H 4.88, N 11.96; found C 78.80, H
4.71, N 11.40. LCMS (ESI) calcd. [M+H]+: 352.14, found:
352.2.
General synthesis of TCAAD, TCAA and TCAEA
C23H18N2O3 (370.13): calcd. C 74.58, H 4.90, N 7.56; found
C 74.68, H 4.86, N 7.44. LCMS (ESI) calcd. [M+H]+: 371.13,
These compounds were synthesized by following a re-
ported procedure.56 Typically, triphenylamine aldehyde
(1/2/3, 1mmol), cyanoacetamide/cyanoacetic acid/ethyl
cyanoacetate (1.5 mmol) and ammonium acetate (150 mg)
were added into glacial acetic acid (5 mL). The reaction
mixture was heated at 80 °C for 4 hrs. After cooling to
room temperature, it was poured into ice-water. The pre-
cipitate was filtered, washed by distilled water, cold ace-
tonitrile and dried under vacuum. The product was fur-
ther purified by recrystallization.
found: 371.3
TCAEA-1: Bright yellow colored powder and recrystal-
lized from DMSO. Yield: 75%. 1H NMR (300 MHz, DMSO-
d6) δ 8.09 (s, 1H), 7.83-7.86 d, 2H), 7.33-7.38 (m, 4H), 7.17–
7. 21 (m, 6H), 6.96–6.99 (d, 2H), 4.31-4.39 (q, 2H), 1.36-1.40
(t, 3H). 13C NMR (125 MHz, DMSO-d6) δ 163.61, 153.97,
152.43, 145.66, 135.13, 129.76, 126.42, 125.46, 123.35, 119.09,
116.76, 97.26, 62.18, 14.21. C24H20N2O2 (368.15): calcd. C
78.24, H 5.47, N 7.60; found C 78.12, H 5.34, N 7.52. LCMS
(ESI) calcd. [M+H]+: 369.15, found: 369.3
TCAAD-1: Bright yellow colored powder and recrystal-
1
lized from acetonitrile. Yield: 82%. H NMR (300 MHz,
TCAEA-2: Light yellow colored powder and recrystal-
lized from DMSO. Yield: 78%. 1H NMR (300 MHz, DMSO-
d6) δ 8.64 (s, 1H), 8.26-8.29 (d, 1H), 7.33-7.88 (m, 4H),
7.17–7. 21 (m, 6H), 6.53–6.57 (dd, 1H), 6.40-6.41 (d, 1H),
DMSO-d6) δ 8.03 (s, 1H), 7.82-7.85 (d, 2H), 7.76 (s,1H),
7.63 (s,1H), 7.39-7.45 (m, 4H),7.18-7.25 (m, 6H), 6.88-6.91
(d, 2H). 13C NMR (125 MHz, DMSO-d6) δ 163.27, 151.05,
149.72, 145.46, 132.07, 129.98, 126.18, 125.35, 123.48, 118.69,
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4.30-4.37 (q, 2H), 3.66 (s, 3H), 1.35-1.39 (t, 3H). C NMR
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eter with integrating sphere. Mass spectra were recorded with
(125 MHz, DMSO-d6) δ 164.11, 161.04, 154.35, 148.09, 145.67,
130.48, 129.73, 126.60, 125.55, 117.47, 113.18, 112.55, 100.98,
95.98, 61.97, 55.54, 14.28. C25H22N2O3 (398.16): calcd. C
a Bruker 320ꢀMS triple quadrupole mass spectrometer using
direct probe insertion method. The powder Xꢀray diffraction
(PXRD) patterns were measured using a XRDꢀ Bruker D8
75.36, H 5.57, N 7.03; found C 75.40, H 5.42, N 7.12.
LCMS (ESI) calcd. [M+H]+: 399.16, found: 399.3
Advance XRD with Cu Kα radiation (
ed in the 2 range from 10 to 50
coated with paratoneꢀN oil and the diffraction data measured
at 100K with synchrotron radiation ( = 0.62998 Å) on a
λ
= 1.54050 Å) operatꢀ
θ
ο
ο. Single crystals were
TCAEA-3: Orange red colored powder and recrystal-
1
lized from methanol at low temperature. Yield: 80%. H
λ
NMR (300 MHz, DMSO-d6) δ 8.08 (s, 1H), 7.81-7.84 (d,
2H), 7.32-7.37 (m, 2H), 7.12–7. 19 (m, 5H), 6.89–6.92 (d,
4H), 4.31-4.38 (q, 2H), 3.827 (s, 3H), 1.35-1.39 (t, 3H). 13C
NMR (125 MHz, DMSO-d6) δ 163.78, 157.66, 154.04, 152.78,
145.62, 138.30, 133.23, 129.67, 128.42, 125.96, 125.23, 122.72,
ADSC Quantumꢀ210 detector at 2D SMC with a silicon
(111) double crystal monochromator (DCM) at the Pohang
Accelerator Laboratory, Korea. CCDC Nos. – 1448374
(TCAEA-1), 1448375 (TCAEA-2), 1448377 (TCAEA-3),
1448378 (TCAA-2), 1448484 (TCAAD-3, 1448485 (TCAAD-
2), 1448486 (TDEM), 1448488 (TDMM) and 1448490
(TCMN-3) contains the supplementary crystallographic
data for this paper.
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118.02, 116.91, 115.15, 96.58, 62.14, 55.49, 14.21. C25H22N2O3
(
398.16): calcd. C 75.36, H 5.57, N 7.03; found C 75.43, H
5.68, N 6.91. LCMS (ESI) calcd. [M+H]+: 399.16, found:
399.4
Computational Studies
Synthesis of TDEM and TDMM
All the density functional theory calculations were per-
formed with the Turbomole program package.52 All the
structures were optimized fully without any symmetry
constraints. For the molecules where the crystal data were
available, the geometry from the crystal data were used as
initial guess and for others the geometry were modeled
keeping similar conformation to the available data. We
used the hybrid PBE0 functional53 and def2-TZVP54 basis
set for all calculations. To confirm the proper conver-
gence to minima, the vibrational frequencies were com-
puted at the same level of theory and the absence of im-
aginary frequency was confirmed. For the optical part, the
calculation on the properties has been carried out with
time dependent-density functional theory (TD-DFT) for-
malism with PBE0/def2-TZVP level of theory. The geome-
tries optimized at the ground state were used for TD-DFT
calculations. To include the solvent effect, conductor-like
screening model (COSMO)55 with acetonitrile as solvent
(ε=35.688) was used in the calculation of vertical excita-
tion with TD-DFT. In the TD-DFT calculations, the use of
PBE0 functional has provided accurate results for medium
and large size molecules.
1 (1.0 mmol), diethylmalonate (1.2 mmol) and piperidine
(1.0 mmol) were added into ethanol (10 mL). The reaction
mixture was stirred at room temperature for 10 days. Then
ethanol was removed using rotary evaporator and extract-
ed 5 times using dichloromethane and the combined ex-
tract was washed with brine solution and dried over
Na2SO4. The solvent was removed using rotary evaporator
to produce the product as yellow colored gel. Recrystalli-
zation from hexane gave bright greenish yellow crystals of
TDEM. However, diethylmalonate did not react with In a
same synthetic procedure was followed for using 2 in
methanol. The diethylmalonate ester exchanged with
methanol solvent and produced methyl derivative. Unex-
pectedly, the reaction did not progress in pure ethanol
solvent. The product was obtained as yellow powder and
recrystallized from acetonitrile.
TDEM: Yield: 65%. 1H NMR (300 MHz, CDCl3) δ 7.62 (s,
1H), 7.28-7.33 (m, 6H), 7.09-7.14 (m, 6H), 6.94–6.97 (m,
2H), 4.25-4.38 (m, 4H), 1.30-1.35 (t, 6H). 13C NMR (125
MHz, CDCl3) δ 167.40, 164.68, 150.18, 146.61, 141.69, 131.20,
129.56, 125.72, 125.18, 124.37, 122.68, 120.81, 61.58, 61.39,
14.21, 14.00. C26H25NO4 (415.18): calcd. C 75.16, H 6.06, N
Result and Discussion
3.37; found C 75.26, H 5.92, N 3.24. LCMS (ESI) calcd.
[M+H]+: 416.18, found: 416.3
Scheme 1 shows the TPA derivatives of conjugated D-π-A
molecules explored for MCF properties. The synthesized
TPA derivatives did not show any measurable fluores-
cence in solution state. However, they exhibited strong to
weak fluorescence in the solid state except TCAAD-2 and
TDMM-2 (Table 1). The non-fluorescence in solution and
strong solid state fluorescence indicate the aggregation
enhanced emission phenomena (AEE). It is noted that
TPA based D-π-A compounds are known to exhibit AEE
phenomena.49 The mechanochromism of TCMN-1 has
already been reported by Y. Cao et al. that showed re-
versible fluorescence switching between 581 to 602 nm
upon strong grinding and heating.49 Recently, we have
reported TCMN-2, a methoxy group substituted at ortho
to malononitrile, that showed tunable fluorescence via
polymorphism and mechanochromism.50 The strong
grinding/heating of TCMN-2 switched solid state fluores-
1
TDMM: Yield: 68%. H NMR (300 MHz, CDCl3) δ 8.070
(s, 1H), 7.26-7.33 (m, 4H), 7.09-7.19 (m, 6H), 6.47–6.49 (d,
2H), 3.81-3.82 (s, 6H), 3.64 (s, 3H). 13C NMR (125 MHz,
CDCl3) δ 168.06, 165.37, 159.46, 151.95, 146.52, 138.02,
129.93, 129.50, 125.87, 124.45, 121.43, 114.69, 113.29, 103.24,
55.41, 52.41, 52.37. C25H23NO5 (417.16): calcd. C 71.93, H
5.55, N 3.36; found C 71.82, H 5.38, N 3.42. LCMS (ESI)
calcd. [M+H]+: 418.16, found: 418.3.
Spectroscopy and structural characterization
Absorption and fluorescence spectra were recorded us-
ing Perking Elmer Lambda 1050 and Jasco fluorescence
spectrometer-FP-8200 instruments. Solid state fluores-
cence spectra were recorded by exciting at 370 nm with 5
nm slit width in the detection window. Fluorescence quanꢀ
tum yields (
Φf) of solid samples were measured using a
Horiba Jobin Yvon model FL3ꢀ22 Fluorolog spectrofluorimꢀ
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cence between 562 and 545 nm. However, TCMN-3,
TCAAD-3 crystals showed orange-yellow fluorescence
methoxy group at para position of phenyl ring, showed
solid state fluorescence at 581 nm that was blue shifted to
565 nm by slight breaking (Fig. 1a). The blue shift of fluo-
rescence by slight breaking might be due to disrupting
long range order of molecular packing in the single crys-
tals. The PXRD pattern of slightly broken crystals perfect-
ly matched with simulated PXRD pattern generated from
single crystal data and confirmed that there was no struc-
tural change (Fig. S1a). The strong grinding lead to weak
fluorescence (turn-off) and heating turn-on the fluores-
cence without altering λmax. The repeated grind-
ing/heating of TCMN-3 showed reversible off-on fluores-
cence switching for several cycles without significant
change (Fig. S2). Thus, malononitrile with simple TPA
(TCMN-1) showed fluorescence switching whereas meth-
oxy group at ortho to malononitrile (TCMN-1) exhibited
polymorphism and fluorescence switching. But TCMN-3,
methoxy group at para position, showed off-on fluores-
cence switching with mechanical pressure and heating.
PXRD studies of strongly grounded TCMN-3 powder sug-
gested conversion of crystalline to amorphous phase (Fig.
S1b). However, heating converted the amorphous phase to
crystalline phase. Thus, the reversible off-on fluorescence
switching of TCMN-3 was due to reversible change of ma-
terial phase with pressure/heating.
with λmax at 560 nm (Fig. 1b). The slight breaking of crys-
tals blue shifted the fluorescence from 560 to 548 nm
whereas strong grinding red shifted to 560 nm with re-
duced intensity. Heating of grounded powder blue shifted
the fluorescence to strong yellowish-green. The fluores-
cence can be reversibly switched between 560 and 548 nm
by grinding/heating (Fig. S3). PXRD studies indicated the
formation of amorphous phase upon strong grinding that
was converted to crystalline phase by heating (Fig. S4).
The perfect matching of PXRD pattern of crystals and
heated powder with simulated pattern from single crystal
data confirmed the same structure.
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Table 1. Absolute quantum yield, λmax and dihedral
angle displayed by the molecule in the crystals.
Quantum yields
f)
Dihedral
angle (τ)
Compound
TCMNꢀ1
λ
max (nm)
(
Φ
54%
6.55
588a
535b
TCMNꢀ2
26%,a 48%b
0.36,a 0.29b
TCMNꢀ3
TCAADꢀ1
TCAADꢀ2
TCAADꢀ3
TCAAꢀ1
TCAAꢀ2
TCAEAꢀ1
TCAEAꢀ2
TCAEAꢀ3
TDEM
47%
85%
581
520
5.49
5.88
26.49, 4.12c
No fluorescence
52%
562
565
617
506
542
547
504
6.04
30%
4.52
24%
3.11
45%
8.20
23%
18.65
15.81
3.09
28%
55%
TDMM
No fluorescence
27.14
TPA with cyanoacetamide (TCAAD-1), a strong H-
bonding acceptor, showed intense green fluorescence at
520 nm (Table 1).57 It exhibited red shifting of fluores-
cence from 520 to 560 nm upon strong grinding and heat-
ing switched the fluorescence to initial state. Surprisingly,
TCAAD-2 that has methoxy at ortho position to acceptor
did not show any solid state fluorescence. However,
Fig. 1. Mechanochromic fluorescent responses of (a) TCMN-3
and (b) TCAAD-3. Digital fluorescent images are shown in
the bottom of the spectra. λexc = 370 (for spectra) and 365 nm
(for digital images).
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Ethyl cyanoacetate substituted TPA compounds, TCAEA-1,
Cyanoacetic acid acceptor substituted TPA derivatives,
TCAA-1 and TCAA-2, showed weak fluorescence at 565
and 617 nm, respectively (Fig. S5). Slight breaking of
TCAA-1 single crystals blue shifted the fluorescence λmax
to 550 nm with increased intensity and strong grinding
reduced the intensity significantly with red shift of λmax to
565 nm (Fig. S5a). Although TCAA-1 showed blue shift of
fluorescence to 550 nm by heating, it showed only slight
increase of intensity. PXRD studies revealed that heating
did not convert TCAA-1 amorphous phase (obtained by
strong grinding) to crystalline significantly (Fig. S6). The
solid state fluorescence of TCAA-1 has also been tuned by
nanofabrication.58 In contrast, hard crushing blue shifted
the fluorescence of TCCA-2 from 617 to 560 nm and heat-
ing strongly increased the fluorescence intensity without
changing fluorescence λmax (Fig. S5b). Further, off-on flu-
orescence switching by repeated hard crushing/heating
showed good recovery of fluorescence without significant
change in the intensity or λmax (Fig. S7). PXRD studies
indicated clear conversion of amorphous phase to crystal-
line after heating (Fig. S8). Since TCAA-3 was obtained as
semi-solid, we could not perform any mechanochromic
fluorescence studies.
TCAEA-2 and TCAEA-3, showed strong solid state
fluorescence at 506, 533 and 547 nm, respectively (Fig. 2, S9).
The strong green fluorescence of TCAEA-1 was red shifted
from 506 to 548 nm (yellow) with reduced intensity by hard
crushing (Fig. 2a). Strong enhancement of fluorescence
intensity with blue shift was observed by heating (487 nm).
The fluorescence switching of TCAEA-1 from green to yellow
and vice-versa have also been performed for several cycles to
confirm the high reversibility (Fig. S10). PXRD studies
indicate that TCAEA-1 crystalline phase was converted to
amorphous phase by strong crushing and heating
transformed again to crystalline (Fig. 3a). It is noted that
experimental PXRD patterns perfectly matched with
simulated pattern from single crystal data (Fig. S11). TCAEA-2
showed strong greenish-yellow fluorescence at 533 nm. The
hard crushing lead to quenching of fluorescence intensity
without changing λmax and heating turn-on the fluorescence
with blue shift (533 to 504 nm (Fig. 2b). PXRD studies
revealed reversible change of crystalline to amorphous and
vice-versa by mechanical crushing/heating (Fig. 3b, S12).
Similar to TCAEA-1, the fluorescence of TCAEA-2 can be
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Fig. 3. PXRD pattern of (a) TCAEA-1 and (b) TCAEA-2.
reversibly switched by repeated crushing and heating (Fig.
S13). TCAEA-3 exhibited yellow fluorescence (Fig. 4, S9).
Similar to TCAEA-1 and 2, hard crushing of TCAEA-3 showed
red shifts of fluorescence (547 to 570 nm). But, the red
shifted fluorescence was slowly self-recovered to initial state
without applying secondary force. The self-recovered
fluorescence spectra showed an additional hump at 490 nm.
Fig. 2. Mechanochromic fluorescent responses of (a) TCAEA-
1 and (b) TCAEA-2. Digital fluorescent images are shown in
the bottom of the spectra. λexc = 370 (for spectra) and 365 nm
(for digital images).
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gen.49 TCMN-2 showed polymorphism and crystals ob-
However, PXRD showed similar pattern for both initial and
self-recovered TCAEA-3 sample and ruled-out any structural
change (Fig. S14).
tained from methanol showed similar dimer and 1D struc-
ture formation via π…π and H-bonding interaction be-
tween cyano nitrogen and phenyl hydrogen in the crystal
lattice.50 TCMN-2 crystals grown from acetonitrile did not
show any π…π interactions or extended networks in the
crystal lattice, though the molecules adopt opposite di-
pole arrangement. In TCMN-3, both π…π (3.353 Å) and H-
bonding interactions between phenyl hydrogen and cyano
nitrogen atoms (dD…A =3.484 Å and θDꢀH…A =141.84°) re-
sulted in the formation of dimer structure with opposite
dipole arrangement (Fig. 6a). The dimer was further
linked to four different dimers via C-H…π (dD…A =3.347 Å)
and H-bonding interactions (dD…A =3.383 Å and θDꢀH…A
=154.35°) (Fig. 6b).
Interestingly, diethylmalonate attached TPA (TDEM)
showed high contrast self-reversible off-on fluorescence
switching. TDEM showed strong bluish-green fluores-
cence at 498 nm (Table 1, Fig. 5a, S15). The fluorescence
was completely quenched by strong crushing. However,
the fluorescence intensity was self-recovered to initial
state within 20 min without applying secondary force.
TDEM fluorescence can be reversibly switched off-on for
many cycles by strong crushing (Fig. S16). The complete
quenching of fluorescence by strong crushing could be
due to the conversion of crystalline to amorphous
phase.20,33 The amorphous phase self-reversed to crystal-
line state with time after removing the mechanical force
that turn-on the fluorescence. PXRD studies confirmed
the self-recovery of crystalline phase with time that per-
fectly matched with the initial structure (Fig. 5b, S17).
Surprisingly, TDMM, a dimethylmalonate derivative, did
not show any solid state fluorescence. It is noted that di-
ethylmalonate substituted TPA (TDEM) was easily ob-
tained using ethanol as solvent in the reaction (TDEM).
But methoxy substituted aldehyde, 2, and diethylmalo-
nate did not produce any clear product in ethanol. Hence
methanol was used as solvent that produced dimethylma-
lonate substituted compound (TDMM) rather diethylma-
lonate derivative. The reaction between aldehyde- 3, and
diethylmalonate produced only viscous liquid product.
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Fig. 5. TDEM (a) self-reversible mechanochromic fluorescent
Fig. 4. Mechanochromic fluorescent responses of TCAEA-3
with digital fluorescent images. λexc = 370 (for spectra) and
365 nm (for digital images).
responses with digital images and (b) PXRD pattern. λexc
=
370 (for spectra) and 365 nm (for digital images).
The amide functionality of TCAAD compounds was ex-
pected to form strong intermolecular H-bonding interac-
tions in the crystal lattice. TCAAD-1 formed 1D network
structure via N-H…O (dD…A =3.042 Å and θDꢀH…A =148.70°)
and C-H…O (dD…A =3.194 Å and θDꢀH…A =118.36°) interac-
tions (Fig. S18). It did not show commonly observed am-
Single crystal X-ray analysis was performed to get the
insight on the solid state fluorescence and MCF proper-
ties of TPA derivatives. TPA adopted propeller shape in
the crystal lattice. TCMN-1 formed dimer via π…π interac-
tion that was further linked to 1D structure by H-bonding
interaction between cyano nitrogen and phenyl hydro-
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Diethylmalonate acceptor substituted TDEA showed dimer
formation via π…π interactions (3.320 Å and 3.383 Å, Fig. 9a).
ide-amide complementary
interactions.
However,
TCAAD-2 and TCAAD-3 exhibited usual amide-amide
complementary H-bonding interaction (dD…A =2.761, 3.036
Å and θDꢀH…A =161.36°, 153.26° (TCAADꢀ2); dD…A =2.883 Å
and θDꢀH…A =164.08° (TCAADꢀ3)) and N-H…NC (dD…A
=3.337 Å and θDꢀH…A =152.67° (TCAADꢀ1); dD…A =3.120 Å
and θDꢀH…A =145.89° (TCAADꢀ3)) intermolecular interaction
lead to network structure formation in the crystal lattice (Fig.
6c,d). The two molecules present in the unit cell of TCAAD-2
showed different dihedral angle (τ = 26.49 and 4.12, Fig. S19,
Table 1). Further, both the molecules are linked by comple-
mentary amide H-bonding interaction. TCAA-1, a cyanoace-
tic acid attached TPA, did not exhibit commonly observed
acid-acid dimer in the crystal lattice.25 It showed helical net-
work structure via O-H…NC intermolecular H-bonding in-
teractions (dD…A =2.752 Å and θDꢀH…A =158.43°, Fig. S20).
However, methoxy group substituted TCAA-2 exhibited typi-
cal carboxylic acid-acid dimer (dD…A =2.598 Å and θDꢀH…A
The dimers are further linked by C-H…O intermolecular
interactions between carbonyl oxygen and methylene
hydrogen atoms (dD…A = 3.329 Å and θDꢀH…A = 142.35°, Fig.
9b). TDMM, dimethylmalonate based derivative produced
dimer structure via C-H…O intermolecular interactions
between carbonyl oxygen and phenyl hydrogen atoms (dD…A
= 3.311 Å and θDꢀH…A = 149.45°; Fig. 9c). The dimer was further
linked into extended network structure by another C-H…O
intermolecular interactions between carbonyl oxygen and
methyl hydrogen atoms of methoxy group (dD…A = 3.364 Å
and θDꢀH…A = 151.24°).
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=175.07°) that was further linked by π…π (3.274
) interactions in the crystal lattice (Fig. 7).
Å and 3.257
Å
Fig. 7. Dimer and interlinking of dimer in the crystal lattice
of TCAA-2. C (grey), N (blue), O (red), H (white); H-bonds
(broken line). dD…A distances are marked (Å).
Fig. 6. Dimer and linking of dimers in the crystal lattice of
(a,b) TCMN-3, (c) TCAAD-2 and (d) TCAAD-3. C (grey), N
(blue), O (red), H (white); H-bonds (broken line). dD…A
distances are marked (Å).
TCAEA-1 showed a square like structure through π…π
(3.366-3.398 Å) and C-H…O (dD…A =3.135 Å and θDꢀH…A
114.49°) intermolecular interactions between four
different molecules (Fig. 8a). Whereas C-H…π (3.375 Å)
=
Fig. 8. Molecular arrangement of (a) TCAEA-1 (b) TCAEA-2
and (c) TCAEA-3 by weak intermolecular interactions in the
crystal lattice. C (grey), N (blue), O (red), H (white); H-
bonds (broken line). dD…A distances are marked (Å).
and C-H…O
(dD…A =3.423 Å and θDꢀH…A = 161.13°)
intermolecular interactions in the crystal lattice of
TCAEA-2 resulted in dimer structure with antiparallel
arrangement of molecules that was further interlinked by
weak C-H…π interactions (3.458 Å, Fig. 8b). However,
TCAEA-3 showed only weak C-H…O intermolecular
interactions between carbonyl oxygen and methylene and
phenyl hydrogen atoms in the crystal lattice (dD…A = 3.404
Å, 3.432 Å and θDꢀH…A = 145.77° and 156.64°, Fig. 8c).
The comparison of solid state fluorescence with molecular
structures of TPA based D-π-A derivatives revealed that TPA
without methoxy substituent in the phenyl ring showed
strong fluorescence (Table-1). The increased dihedral angle
between donor phenyl and acceptor in TCAAD-2 and TDMM
lead to non-fluorescence. The methoxy group at ortho
position to alkene acceptors tends to increase the dihedral
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angle. This was further confirmed by computational studies
due to weak intermolecular interactions and relaxation of
(Table-S1, S2 and Fig. S21). In all the molecules, the
substitution at the acceptor phenyl ring (R3) blue shifted the
excitation wavelength and increased the dihedral angle (Φ1).
As Φ1 represents the planarity of the alkene group with the
phenyl ring, an increment in the value can causes lose of π-
electron conjugation that increased the excitation energy.
The increase of Φ1 leads to a higher blue shift in the
absorbance spectra from TCMN-1 to TCMN-2. It is noted that
TDMM (with as well as without OCH3) unusually exhibited
higher twist between aminophenyl and alkene acceptor,
though ortho substitution showed slightly higher dihedral
angle.
alkyl groups. It is noted that TCAAD-2 and TCAAD-3 ex-
hibit similar amide-amide complementary interactions in
the crystal lattice, however, TCAAD-3 showed strong fluo-
rescence whereas TCAAD-2 is non-fluorescence. Similar-
ly, TDEM and TDMM exhibited similar extended network
structure by weak interactions but TDEM alone showed
strong solid state fluorescence. This was due to the higher
twist of alkene acceptor group in both TCAAD-2 and
TDMM.
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Conclusion
In conclusion, we have synthesized TPA based D-π-A
derivatives and systematically explored effect of subtle
structural change on the molecular conformation and
packing in the solid state and their influence on the stim-
uli-responsive fluorescence. TPA derivatives except
TCAAD-2 and TDMM exhibited aggregation enhanced
emission in the solid state. Importantly, all fluorescent
solids showed stimuli-induced reversible fluorescence
changes. The comparison of solid state structure and fluo-
rescence indicated that position of methoxy group and
dihedral angle between donor TPA and alkene acceptor
played significant role on the MCF properties. TPA deriv-
atives without methoxy substituent showed strong fluo-
rescence (Φf = 85 % (TCAAD-1, 55 % (TDEM)) that could
be due to the better packing and rigidification of fluoro-
phore in the solid state. Higher dihedral angle leads to
weak/non-fluorescence. TCAAD-2 and TDMM that
showed high dihedral angle (26.49, 27.14) did not exhibit
any fluorescence in the solid state. The methoxy group at
ortho position to alkene acceptor increased the dihedral
angle between donor and acceptor. The increase of alkyl
groups at TPA structure lead to self-reversible high con-
trast off-on fluorescence switching. Subtle intermolecular
H-bonding change in the crystal lattice of TCAA-1 dis-
played poor fluorescence recovery after strong crushing
whereas typical off-on fluorescence switching was ob-
served with TCAA-2. The theoretical studies further sub-
stantiated the role of substituent position on the solid
state fluorescence. PXRD studies suggest that the fluores-
cence switching was due to the reversible phase conver-
sion of materials from crystalline to amorphous and vice-
versa. Thus, the present studies provide better structural
insight for designing new smart organic fluorescent mate-
rials.
Fig. 9. The dimer and network structure formation by weak
interactions in the crystal lattice of (a, b) TDEM and (c, d)
TDMM. C (grey), N (blue), O (red), H (white); H-bonds
(broken line). dD…A distances are marked (Å).
In contrast, substitution at the donor phenyl ring did
not show significant influence on the dihedral angle of
alkene group. However, it does influence on the excita-
tion energy of TPA derivatives. The substitution at the
donor phenyl ring of TCMN, TCAAD, and TCAA series
showed red shift of excitation energy. From the MO dia-
grams, it is clear that the substituted methoxy group acts
as donor in all the cases when it is in the donor phenyl
group. Therefore, it decreased the HOMO-LUMO separa-
tion. Hence bulky group substitution closer to acceptor
group in the TPA molecules might be unfavorable for de-
veloping strong solid state fluorescent material. Further,
TCAA-2 crushed sample showed good recovery of fluores-
cence after heating compared to TCAA-1. TCAA-1 showed
helical structure via OH…NC intermolecular H-bonding
interactions whereas TCAA-2 exhibit commonly observed
acid-acid dimer formation (Fig. 7, S20). The strong crush-
ing of TCAA-1 might have forced the molecules to adopt
acid-acid dimer structure along with phase change that
could not be completely reversed by heating. Nanofabri-
cated TCAA-1 that showed acid-acid dimer interactions
showed blue shifted fluorescence compared to crystals.54
In contrast, TCAA-2 that showed strong acid-acid dimer
in the crystal lattice undergoes complete reversible phase
transition by crushing/heating and switched the fluores-
cence. The self-recovery of fluorescence with alkyl groups
substituted TPA molecules after strong crushing could be
Acknowledgement
Financial support from the Science and Engineering Re-
search Board (SERB), New Delhi, India (SERB No.
EMR/2015/00-1891, SB/FT/CS-182/2011) is acknowledged with
gratitude. The CRF facility of SASTRA University is also
acknowledged for absorption spectroscopy. "X-ray crystallog-
raphy at the PLS-II 2D-SMC beamline was supported in part
by MSIP and POSTECH. We thank Department of Chemis-
try, IIT Kharagpur for computational facility and LCMS". The
mass spectra were supported by the Basic Science Research
Program of the National Research Foundation of Korea
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(15) Wang, Y.; Liu, W.; Bu, L.; Li, J.; Zheng, M.; Zhang, D.; Sun,
(NRF) funded by the Ministry of Education, Science and
Technology (NRF-2014R1A1A2058815).
M.; Tao, Y.; Xue, S.; Yang, W. Reversible piezochromic lumines-
cence of 9,10-bis[(N-alkylcarbazol-3-yl)vinyl]anthracenes and the
dependence on N-alkyl chain length. J. Mater. Chem. C, 2013, 1,
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mation from H- to J-Aggregated Perylene Bisimide Dyes by
Complexation with Cyanurates†. Angew. Chem. Int. Ed., 2008, 47,
3367-3371.
SUPPORTING INFORMATION
Fluorescence spectra, fluorescence switching cycle, PXRD
patter and crystal structures.
AUTHOR INFORMATION
Corresponding Author
9
(17) Varghese, S.; Das, S. Role of Molecular Packing in Deter-
mining Solid-State Optical Properties of π-Conjugated Materials.
J. Phys. Chem. Lett. 2011, 2, 863–873.
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* Savarimuthu Philip Anthony, philip@biotech.sastra.edu.
* Dohyun Moon, dmoon@postech.ac.kr.
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Molecular Engineering of Triphenylamine based Aggregation Enhanced Emissive Fluorophore: Structure
Dependent Mechanochromism and Self-Reversible Fluorescence Switching
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Palamarneri Sivaraman Hariharan, Viki Kumar Prasad, Surajit Nandi, Anakuthil Anoop, Dohyun Moon
and Savarimuthu Philip Anthony
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Stimuli responsive smart fluorescent triphenylamine derivatives have been synthesized that exhibited
reversible and selfꢀreversible mechanochromism as well as non fluorescence solids depend on the posiꢀ
tion of substitution and acceptor structure.
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