Self-reversible thermofluorochromism of D-A-D triphenylamine derivatives and the effect of molecular conformation and packing

Article abstract of DOI:10.1039/c7ce01538k

Triphenylamine (TPA) based donor-acceptor-donor (D-A-D) compounds were synthesized showing aggregation enhanced emission (AEE) and molecular conformation and packing dependent rare self-reversible thermofluorochromism in the solid state. 1, with TPA subst

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Self-reversible thermofluorochromism of D-A-D triphenylamine
derivatives and effect of molecular conformation and packing
Anu Kundu,a Subramanian Karthikeyan,b Dohyun Moon*c and Savarimuthu Philip Anthony*a
Received 00th January 20xx,
Accepted 00th January 20xx
Triphenylamine (TPA) based donor-acceptor-donor (D-A-D) compounds were synthesized that showed aggregation
enhanced emission (AEE) and molecular conformation and packing dependent rare self-reversible thermofluorochromism
in the solid state. 1, TPA substituted with phenylenediacetonitrile at both end displayed twisted molecular conformation
whereas OCH3 substitution at ortho position of TPA (3-MTPA, 2) resulted in twisted conformation at one end and coplanar
conformation at other end. 4-methoxytriphenylamine (4-MTPA) attached acceptor (3) displayed coplanar conformation on
both end of the acceptor. Highly twisted conformation in 1 lead to strong fluorescence (16.8 %) and coplanar
conformation in 3 resulted in weak fluorescence (1.6 %) in the solid state. 2 showed moderate fluorescence (8.4 %) in the
solid state. Importantly, 1 and 2 exhibited self-reversible thermofluorochromism with heating and cooling whereas 3 did
not show any fluorescence switching with temperature. Computational studies indicate that the molecular conformation
and OCH3substitution influences on the HOMO-LUMO energy level of TPA derivatives.
DOI: 10.1039/x0xx00000x
www.rsc.org/
tools for constructing solid state fluorescent materials.4
Numerous organic solid state fluorescent materials have been
emerged based on the AEE strategy using diverse core
Introduction
Molecular conformation, packing and supramolecular
interactions of -conjugated organic molecule in the solid
structures such as including tetraphenylethenes, siloles,
fulvenes, boron derivatives and conjugated polymers.5,6 Some
of the AEE materials exhibited reversible fluorescence
switching upon external stimuli mostly applying mechanical
pressure followed by heating or solvent vapor exposure.7 Ping
Lu group reported a unique piezochromic fluorescent organic
crystals that showed pressure dependent reversible off-on
fluorescence switching.8 Fluorescence-phosphorescence dual
mechanoluminescence at room temperature was observed
with terphenyl-dioxaborolane derivative.9 3-aryl-2-cyano
acrylamide derivatives showed stacking mode dependent
mechanofluorochromism in the solid state.10 However, AEE
materials that exhibit temperature induced reversible
fluorescence are rarely reported.11
π
state plays significant role on their electrical as well as optical
properties.1 In particular, smart fluorescent organic materials
that exhibit reversible fluorescence switching to the different
external stimuli such as light, heat and pressure depends on
the ability of the molecule to display varied conformation,
packing and intermolecular interactions in the solid state.2 For
example, strongly fluorescent π-conjugated organic molecule
in solution often converted weak/non-fluorescent in the solid
state due to aggregation caused fluorescent quenching (ACQ).
To overcome the ACQ, different strategies such as utilization of
non-planar fluorophore, supramolecular fluorescent host-
guest systems, excited state intramolecular proton transfer
systems, substituting with bulky groups and controlling
intermolecular interactions have been explored for developing
solid state fluorescent materials.3 Particularly, the discovery of
aggregation enhanced emission (AEE) phenomenon in the last
decade, an opposite effect of ACQ in which weak or non-
fluorescent molecules in solution exhibited strong
fluorescence in the solid state via intermolecular interactions
induced fluorophore rigidification and restriction of
intramolecular rotation (RIR), has offered new methods and
Triphenylamine (TPA),
a propeller shaped nonplanar
optoelectronic molecule, has been extensively used for
developing organic solar cell dyes, field effect transistor,
sensors, solid state fluorescent and smart fluorescent
materials.12 The conformationally robust, twisted propeller
core aryl structure that prevent close stacking produced AEE in
the solid state with mechanofluorochromism.13 The structure
of the acceptor, substituent position and halochromic
functionality has been exploited to demonstrate self-
reversible, reversible mechanofluorochromism, polymorphism
induced tunable fluorescence and self-erasable and rewritable
platoforms.14 In this manuscript, we report the synthesis of
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, Kalasalingam University, Krishnan Kovil–626126, India
c. Beamline Department, Pohang Accelerator Laboratory, 80 Jigokro-127beongil,
Nam-gu, Pohang, Gyeongbuk, Korea. E-mail: dmoon@postech.ac.kr
d.Electronic Supplementary Information (ESI) available: [Synthesis, NMR data,
absorption spectra, crystallographic table for PyBAP and PyBAP-PA, crystal
structures, PXRD pattern, excitation spectra. CCDC: 1534730 and 1534731]. See
DOI: 10.1039/x0xx00000x
triphenylamine
based
donor-acceptor-donor
(D-A-D)
molecules and investigation of methoxy substituent effect on
the molecular conformation and packing as well as self-
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reversible thermofluorochromism in the solid state. Absorption and fluorescence spectra were measured using
Triphenylamine attached phenylenediacetonitrile ( ) showed Perking Elmer Lambda 1050 and Jasco fluorescence
1
highly twisted molecular conformation whereas coplanar spectrometer-FP-8300 instruments. Solid state fluorescence
conformation was observed with 4-methoxytriphenylamine spectra were recorded by exciting at 370 nm with 5 nm slit
attached acceptor
phenylenediacetonitrile (
well as coplanar conformation. In
conformation whereas trans conformation was observed in
and . All three molecules showed AEE in the solid state. The B3PW91/6ꢀ31+G(d,p) level theory (Gaussian 09 package). The
formation of highly twisted conformation in resulted in powder X-ray diffraction (PXRD) patterns were measured using
strong fluorescence (quantum yield ( f) = 16.8) whereas that a XRD- Bruker D8 Advance XRD with Cu Kα radiation (
has both twisted as well as planar conformation showed 1.54050 Å) operated in the 2
range from 10ο to 50ο. Single
showed only weak crystals were coated with paratone-N oil and the diffraction
and exhibited self- data measured at 100K with synchrotron radiation (
(
3
). 3-methoxytriphenylamine attached width in the detection window. Fluorescence quantum yields
) displayed twisted conformation as f) of solid samples were measured using a Jasco (FP-8300)
, CN groups showed cis spectrofluorimeter with integrating sphere. The HOMO,
LUMO and band gap of all structures are studied using
2
(Φ
1
2
3
1
Φ
2
λ =
θ
moderate fluorescence (Φf = 8.4).
fluorescence (Φf = 1.6). Importantly,
3
1
2
λ =
reversible fluorescence switching with heating and cooling. But 0.62998 Å) on a ADSC Quantum-210 detector at 2D SMC with a
did not show any fluorescence change with temperature. silicon (111) double crystal monochromator (DCM) at the
Computational studies have also been performed to get the Pohang Accelerator Laboratory, Korea. CCDC Nos. – 1450595
3
insight on the energy level modulation upon changing of the
molecular conformation.
(1), 1570578 (2) and 1570577 (3) contains the supplementary
crystallographic data for this paper.
Experimental Section
Triphenylamine (TPA), 4-methoxy triphenylamine (4-MTPA), 3-
methoxy triphenylamine (3MTPA), dimethylformamide (DMF,
HPLC grade), phosphorous oxychloride and 1, 4-
phenylenediacetonitrile was purchased from Sigma-Aldrich
and used without further purification. The solvents and NaOH
were obtained from Merck India. Aldehyde functional group
was introduced into TPA/4-MTPA/3-MTPA by following a
reported procedure.14a,b
Scheme 1. Molecular structure of TPA based D-A-D molecules.
General procedure for synthesizing 1, 2 and 3
1,4-phenylenediacetonitrile and aldehyde functionalized
triphenylamine compound (1:2 molar ratio) was dissolved in
ethanol (10 mL) and stirred at room temperature. To this
reaction mixture, ethanol solution (5 mL) of NaOH (0.5eq.) was
added drop-wise and stirred for another 2 h at room
temperature. The formed precipitate was filtered, washed
Results and Discussion
Aldehyde functionality into TPA, 3-MTPA and 4-MTPA was
introduced using Vilsmeier–Haack formylation reaction using
POCl3 and DMF. Triphenylamine based donor-acceptor-donor
π-conjugated organic molecules (1-3) were synthesized by
condensing aldehyde functionalized TPA donor molecules with
1,4-phenylenediacetonitrile acceptor (Scheme 1). All three
products were formed as orange-red precipitates. The
absorption spectra of 1-3 showed λmax around 450 nm and
OCH3 substitution did not exhibit significant influence on the
absorption in CH3CN (Fig. S1). All three donor-acceptor-donor
compounds (1-3) did not show any fluorescence in solution
state.
with cold ethanol and dried under vacuum.
1
: Yield = 85%. M.p = 245°C. H NMR (300 MHz, CDCl3, ppm) δ
1
7.80 (d, J = 8.4 Hz, 1H), 7.69 (s, 1H), 7.48-7.45 (m, 1H), 7.35-
7.28 (m, 3H), 7.13-7.07 (m, 1H), 7.06-7.03 (m, 2H). 13C NMR
(CDCl3, ppm) δ 150.2, 146.5, 141.8, 135.1, 131.2, 130.8, 129.6,
126.1, 126.1, 125.8, 124.5, 120.7, 118.5, 106.6.
1
: Yield = 86%. M.p = 267°C. H NMR (300 MHz, CDCl3, ppm) δ
2
8.15 (d, J = 8.4 Hz, 1H), 7.95 (s, 1H), 7.69 (s, 1H), 7.35-7.24 (m,
5H), 7.16-7.09 (m, 6H), 6.66-6.61 (m, 1H), 6.54-6.49 (m, 1H),
3.67 (s, 3H). 13C NMR (CDCl3, ppm) δ 159.1, 151.7, 146.6, 136.4,
135.3, 129.5, 128.9, 126.1, 125.9, 124.5, 118.9, 115.9, 113.6,
106.4, 103.1, 55.6.
3: Yield = 85%. 1H NMR (300 MHz, CDCl3, ppm) δ 7.77 (d, J =
8.4 Hz, 2H), 7.68 (s, 2H), 7.45 (s, 1H), 7.34-7.24 (m, 2H), 7.17-
7.10 (m, 5H), 7.05-6.94 (m, 2H), 6.90-6.84 (m, 2H), 3.83 (s, 3H).
13C NMR (CDCl3, ppm) δ 157.2, 150.5, 146.5, 142.0, 139.2,
135.1, 130.9, 129.5, 128.2, 126.0, 125.4, 125.2, 124.1, 119.4,
118.7, 115.1, 106.0, 55.5.
Interestingly,
1
and
2 showed strong fluorescence in the
solid state whereas
3
showed weak but enhanced fluorescence
compare to solution state (Fig. 1a). This phenomenon suggests
the aggregation enhanced emissive (AEE) behaviour of these
compounds.4-7 Further, the AEE phenomenon was confirmed
by slow evaporation of
formation of orange-red emitting solids with slow evaporation
of CH3CN solvent (Fig. 1b). showed broad fluorescence
between 545 nm to 590 nm with λmax at 563 nm. and
showed slightly red shifted fluorescence λmax at 576 nm. It is
noted that the fluorescence spectra of was comparatively
1 and 2 on a filter paper that exhibited
1
2
3
Spectroscopy and structural characterization
3
2 | J. Name., 2012, 00, 1-3
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To get the insights on the solid state fluorescence change,
single crystals of were attempted to grow from different
1-3
solvents (ethyl acetate, toluene, CH3Cl, CH2Cl2, CH3CN, CH3OH,
Fig. 1. (a) Fluorescence spectra of 1-3 and (b) AEE behaviour of 1 and 2 in CH3CN drops
(λexc = 370 nm (spectra), 365 nm (digital images)).
broad than
measurement also confirmed strong fluorescence of
and 2 (8.4 %). showed very weak fluorescence (1.6 %).
2
but sharper than
1. Absolute quantum yield
1
(16.8 %)
3
TPA based donor-acceptor molecules are known to exhibit
mechanofluorochromism.13,14 In contrast, all three compounds
did not show any fluorescence switching or intensity
modulation upon strong grounding of the solids. However,
Fig. 2. Thermofluorochromism of 1 and 2 (a) fluorescence spectra and (b) digital images
(λexc = 370 nm (spectra), 365 nm (digital images)).
DMF and DMSO). We were able to grow quality single crystals
compound
induced fluorescence switching (Fig. 2a).
fluorescence with a max at 563 nm and solids exhibited orange
fluorescence. Interestingly, orange fluorescence of was
1
and
2
exhibited rare self-reversible temperature
of
only and
suitable single crystals for structural analysis. The single
crystals of obtained from ethyl acetate and DCM-diethyl
1
from ethyl acetate and DCM-diethyl ether,
2 from toluene
1
showed broad
3
from DMSO. The other solvents did not produce
λ
1
1
ether showed a similar structure with the reported literature.15
Triphenylamine in all compounds displayed propeller
conformation (Fig. 3, Table 1). Interestingly, both the CN
converted to yellow fluorescent solids with increasing heat
(Fig. 2b). The yellow fluorescence was converted to orange
upon cooling. Fluorescence spectra of
blue shift in the λmax (560 nm) and clear reduction of
broadening. showed quenching of fluorescence with increase
of heating and self-recovered the fluorescence upon cooling.
As observed with , fluorescence spectra of also exhibited
only slight change after heating. In contrast to the spectra,
both and showed visible color change of solids (orange to
yellow ( ) and orange to non-fluorescent ( )) in the solids
1 showed only small
groups of acceptor in
1 displayed cis conformation whereas 2
2
and 3, in which methoxy donor attached at different position
of phenyl in TPA, exhibited trans conformation (Fig. 3a). TPA
phenyl groups attached to phenylenediacetonitrile adopted
twisted conformation (Fig. 3b, torsion angle (τ1 (C1-C2-C3-C4)) =
1
2
1
2
24.90
°
and τ2
attached to ortho position of TPA, displayed highly twisted
= -21.38°). 2, methoxy (OCH3) group
(C5-C6-C7-C8)
1
2
upon heating and cooling. This could be due to the change of
temperature in the sample before performing the spectral
measurement. But the digital images of solids were taken
while samples were on the hot surface. Heating and cooling
cycle measurements demonstrate clear fluorescence change
conformation on one end and coplanar conformation on the
other end of the phenylenediacetonitrile (τ1 = 33.06
1.90 ). In contrast, showed coplanar conformation on both
sides of the TPA phenyl groups (Fig. 3c), one side of the phenyl
twisted up (τ1 = 16.17 ) and other side of the phenyl twisted
down ( 2 = 16.17 ).
The twisted molecular conformation prevented any close
packing of molecules in the crystal lattice. showed only C-
N… intermolecular interaction (3.240 Å) that leads to the
° and τ2 =
°
3
°
for both
1
and
2
up to five cycles (Fig. S2). The excitation of
τ
°
spectra of
1
and
2 at room temperature and hot condition also
exhibited small change (Fig. S3). In contrast,
3 did not show
1
either fluorescent switching or fluorescence quenching upon
heating.
π
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Table 1. Crystallographic data of 2 and 3.
Compound
chemical formula
formula mass
crystal system
space group
2
(1570578)
3 (1570577)
C107H84N8O4
1545.82
C54H50N4O4S2
883.10
monoclinic
P21/c
Triclinic
Pꢀ1
a
b
c
(Å)
(Å)
(Å)
9.7430(19)
8.5910(17)
48.988(10)
6.9670(14)
10.922(2)
15.965(3)
89.68(3), 87.94(3),
78.03(3)
α
, β,
γ
(deg)
90, 93.39(3), 90
4093.2(14)
unit cell
volume(Å3)
1187.6(4)
temp (K)
100
2
243
1
Z
abs coeff (ꢁ/mm−1)
no.reflns measured
no. independent
reflns
0.060
38285
0.111
12323
10950
0.0425
0.0482
6197
Rint
0.0373
0.0550
final R1 values (I >
2σ(I))
final R2(F2) values
0.1334
0.0602
0.1352
0.1209
(I > 2σ(I))
final R1 values (all
data)
final R2(F2) values
0.1415
1.073
0.1586
0.854
(all data)
GOF on F2
formation of dimer with slipped face to face arrangement (Fig.
4a). The assembly of next dimer produced a ladder like
structure in the crystal lattice. The molecular packing
suggested well separation of molecules along a and c axis (Fig.
4b).
2
showed dimer formation through multiple weak
((3.321 Å), (between
intermolecular interactions, C-H…
π
π…π
cyano carbon and phenyl (3.370 Å) and C-H…O hydrogen
bonding between methyl C-H and methoxy oxygen (3.362 Å).
These dimers are further linked along b axis through cyano-
Fig.3. Molecular conformation of (a) 1, (b) 2 and (c) 3 in the crystal lattice. C (grey), N
(blue), O (red), H (white).
phenyl
along b axis showed ABAB pattern (Fig. S4). The molecular
packing of further revealed the inclusion of toluene solvent
π…π interactions (Fig. 5a). The molecular arrangement
The different weak intermolecular interactions observed in
the crystal lattice of
1
-
3
rigidified the fluorophore in the solid
solid state fluorescence
2
state and lead to the AEE.4-7
1-3
molecule in the crystal lattice (Fig. 5b). The toluene was
stabilized in the crystal lattice by hydrogen bonding between
cyano nitrogen and phenyl hydrogen, methoxy oxygen and
intensity differences and thermofluorochromism could be
explained based on the molecular conformation in the crystal
structure. Highly twisted molecular conformation of
ends lead to the strong enhancement of fluorescence (16.8 %)
whereas one end twisted conformation in exhibited
moderate fluorescence (8.4 %) and coplanar conformation in
resulted in weak fluorescence (1.6 %). The inclusion of DMSO
solvent in the crystal lattice of might have also contributed
for weak fluorescence. The slight red shift of fluorescence λmax
for and compared to might be due to the coplanar
1 at both
methyl hydrogen of methyl and C-H…π interactions (Fig. S5). 3
also exhibited dimer arrangement in the crystal lattice without
any intermolecular interactions (Fig. 6a). It included DMSO
solvent molecules in the crystal lattice. These DMSO molecules
2
3
forms intermolecular hydrogen bonding with
3 and connect
3
the molecules along a axis (Fig. 6b). PXRD patterns of 1-3
perfectly matched with the simulated pattern using single
crystal structure that confirmed the phase purity of the sample
(Fig. S6).
2
3
1
4 | J. Name., 2012, 00, 1-3
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Fig. 4. (a) Ladder like arrangement and (b) molecular packing in the crystal lattice of 1.
C (grey), N (blue), O (red), H (white). C-N…π interactions (broken line) distances are
marked in Å.
Fig. 6. (a) Dimer structure and (b) H-bonding interactions in the crystal lattice of 3. C
(grey), N (blue), O (red), S (yellow) H (white). H-bonding interactions (broken line)
distances are marked in Å.
arrangement of TPA phenyl groups. Further, to get the insight
of the molecular conformation and substitution on the HOMO-
LUMO energy level change, DFT calculations were performed
using single crystal structure of 1, 2 and
The isodensity surface plot (isodensity contour = 0.02) of the
highest occupied molecular orbitals (HOMOs) of indicated
3 (Fig. 7, Table S1)).
1
that electron density was predominantly localized on the TPA
and slightly over the dicyanophenyl acceptor. The electron
density in the lowest occupied molecular orbitals (LUMOs) was
mainly localized on the dicyanophenyl acceptor (Fig. 7). In
contrast, HOMO of
2 was mainly localized on the OCH3
substituted phenyl and dicyanophenyl acceptor and LUMO was
mainly localized on the dicyanophenyl acceptor and coplanar
phenyl group. In 3, HOMO was localized on the dicyanophenyl
acceptor and adjacent phenyl groups and LUMO was occupied
on the acceptor and slightly on the adjacent phenyl groups
with small change of electron density. Comparison of electron
density change indicates that
density shift in the HOMO-LUMO compared to
calculated HOMO level of is -5.336 eV and LUMO level is -
2.498 eV. HOMO and LUMO level of and are -5.173 (2), -
5.172 (3) and -2.302 (2) and -2.340 eV, respectively. The
1
and
2
showed higher electron
3.
The
1
Fig. 5. (a) Different intermolecular interactions and (b) molecular packing in the crystal
2
3
lattice of 2.
C (grey), N (blue), O (red), H (white). H-bonding, C-H…π and π…π
interactions (broken line) distances are marked in Å.
calculated band gap of
1,
2
and
3
is 2.838, 2.871 and 2.832 eV,
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Acknowledgement
Financial support from the Science and Engineering Research
Board (SERB), New Delhi, India (SERB No. EMR/2015/00-1891)
is acknowledged with gratitude. The CRF facility of SASTRA
University is also acknowledged for absorption spectroscopy.
"X-ray crystallography at the PLS-II 2D-SMC beamline was
supported in part by MSIP and POSTECH.
Notes and references
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support the solid state fluorescence change of
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3
conformation might not undergo any conformational change
with heat. DSC studies of both 1 and 2 exhibited clear phase
transition at 116 and 95 °C, respectively (Fig. S7). DSC of
exhibited another phase transition at 121
2 also
°
C that might be due
to the loss of toluene from the crystal lattice. PXRD studies
confirmed that both 1 and 2 did not undergo any structural
change after heating (Fig. S8).
3
4
Conclusion
In conclusion, we have synthesized TPA based D-A-D AEE
compounds and demonstrated substitution dependent
molecular conformation, packing and rare self-reversible
thermofluorochromism in the solid state. Highly twisted
molecular conformation and same side orientation of cyano
group was observed with
orientation of cyano group and one twisted and one coplanar
conformation. In contrast, revealed symmetrical coplanar
conformation with opposite side orientation of cyano groups
in the crystal lattice. Interestingly, and showed self-
reversible fluorescence switching with heating and cooling but
did not show any change of fluorescence. PXRD studies
confirmed that there is no structural change in and after
1 whereas 2 exhibited opposite side
3
5
1
2
3
1
2
heating. Computational studies provided insight on the energy
level modulation upon altering the molecular conformation by
changing the substituent. Thus the present studies indicate
that molecule with twisted molecular conformation without
strong intermolecular interactions in the crystal lattice could
6
be
interesting
for
generating
self-reversible
thermofluorochromism.
6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C7CE01538K
Self-reversible thermofluorochromism of D-A-D triphenylamine derivatives and effect of
molecular conformation and packing
Triphenylamine based donor-acceptor-donor compounds exhibited conformation and packing
dependent solid state fluorescence and self-reversible thermofluorochromism.