ESIPT-active multi-color aggregation-induced emission features of triphenylamine-salicylaldehyde-based unsymmetrical azine family

Article abstract of DOI:10.1039/c9ce01490j

The development of multi-color aggregation-induced emission (AIE)-featured azine molecules with excited-state intramolecular proton transfer (ESIPT) characteristics has drawn significant interest in recent years. In this study, we report the construction of donor-acceptor-structured triphenylamine-functionalized unsymmetrical azine molecules (L₁-L₄) prepared from 4-(hydrazonomethyl)-N,N-diphenylaniline with different salicylaldehyde derivatives. By changing the electron donating ability at the ESIPT moiety, we could tune the optical properties of the newly synthesized molecules. All the compounds exhibited the AIE behavior in a THF/water mixture and the excited-state intramolecular proton transfer phenomenon. Further, the tuning of a peripheral substituent in the salicylaldehyde moiety resulted in different emission colors in the aggregated state. The crystal structure of all the compounds (L₁-L₄) revealed that the multiple weak interactions present in the solid-state structure lead to various supramolecular networks. In addition, these molecules showed a prominent positive solvatochromic effect, which was confirmed by their solvent polarity-dependent emission behavior. The appearance of dual emission bands in the solid as well as in the solution state, lifetime values and HOMO, LUMO energy gap of the keto and enol forms strongly support the occurrence of ESIPT in all the compounds (L₁-L₄). Furthermore, the presence of two different electron donating groups in L₃ (triphenylamine and diethylamino) induced pH-dependent emissive features in solution.

Full text of DOI:10.1039/c9ce01490j

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PAPER
ESIPT-active multi-color aggregation-induced
emission features of triphenylamine–salicyl-
aldehyde-based unsymmetrical azine family†
Cite this: CrystEngComm, 2020, 22,
213
b
Moorthy Mathivanan, a Balamurugan Tharmalingam, a Chia-Her Lin,
c
Baskaran Vijaya Pandiyan,
Viruthachalam Thiagarajan d and
a
*
Balasubramanian Murugesapandian
The development of multi-color aggregation-induced emission (AIE)-featured azine molecules with
excited-state intramolecular proton transfer (ESIPT) characteristics has drawn significant interest in recent
years. In this study, we report the construction of donor-acceptor-structured triphenylamine-
functionalized unsymmetrical azine molecules (L1–L4) prepared from 4-(hydrazonomethyl)-N,N-
diphenylaniline with different salicylaldehyde derivatives. By changing the electron donating ability at the
ESIPT moiety, we could tune the optical properties of the newly synthesized molecules. All the compounds
exhibited the AIE behavior in a THF/water mixture and the excited-state intramolecular proton transfer
phenomenon. Further, the tuning of a peripheral substituent in the salicylaldehyde moiety resulted in differ-
ent emission colors in the aggregated state. The crystal structure of all the compounds (L1–L4) revealed that
the multiple weak interactions present in the solid-state structure lead to various supramolecular networks.
In addition, these molecules showed a prominent positive solvatochromic effect, which was confirmed by
their solvent polarity-dependent emission behavior. The appearance of dual emission bands in the solid as
well as in the solution state, lifetime values and HOMO, LUMO energy gap of the keto and enol forms
strongly support the occurrence of ESIPT in all the compounds (L1–L4). Furthermore, the presence of two
different electron donating groups in L3 (triphenylamine and diethylamino) induced pH-dependent emissive
features in solution.
Received 21st September 2019,
Accepted 22nd November 2019
DOI: 10.1039/c9ce01490j
rsc.li/crystengcomm
dependent fluorescent switches and temperature-dependent
fluorescence enhancement behaviors.6–9
1. Introduction
The design and construction of substituent-dependent
multicolored aggregation-induced emissive materials have
attracted considerable attention by virtue of their growing
applications in physics, chemistry, materials science, and
biology.1–4 Among the various organic fluorescent materials,
triphenylamine-based compounds have attracted intense
attention for the construction of multifunctional materials.
Since triphenylamine is a non-planar propeller-shaped, easily
tunable molecule,5 it has been widely used for the develop-
ment of compounds with AIE features, polymorphism-
The applications of traditional fluorophores in the solid
state are hindered by their unique characteristic of
aggregation-caused quenching, and thus these molecules are
highly emissive in the solution state and non-emissive in the
solid state. Tang et al. developed a new molecule that showed
interesting behavior called aggregation-induced emission
(AIE), which is contrary to the emission exhibited by tradi-
tional fluorophores.10 These molecules exhibit a weak emis-
sion in the solution state and the rigidification of their mo-
tion in the aggregated state or solid state enhances their
emission behavior. Accordingly, the AIE molecules have
attracted substantial research attention because of their
broad range of applications in optoelectronic devices, light
emitting diodes, photomemory, photovoltaic cells, photo-
emitters, fluorescence sensors, organic lasers and solid-state
emission.11–16 Recently, AIE-active molecules have also been
employed for theranostic applications.17–20 In the last decade,
a variety of AIE-active molecules have been reported based
on different mechanisms, which include restricted intra-
molecular rotation (RIR), cis–trans isomerisation, twisted
a Department of Chemistry, Bharathiar University, Coimbatore, 641046, Tamil
Nadu, India. E-mail: mpandian@gmail.com; Fax: +91 422 2422387;
Tel: +91 422 2428312
b Department of Chemistry, National Taiwan Normal University, Taipei, Taiwan
c Department of Biotechnology, Indian Institute of Technology, Madras, India
d School of Chemistry, Bharathidasan University, Tiruchirappalli, 620024, Tamil
Nadu, India
† Electronic supplementary information (ESI) available. CCDC 1892317 (L1),
1892316 (L2), 1892315 (L3) and 1892318 (L4). For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c9ce01490j
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intramolecular charge transfer (TICT) and excited-state intra-
molecular proton transfer (ESIPT).21–27 ESIPT-featured or-
ganic fluorophores have received considerable attention due
to their inherent applications in laser dyes, optical sensors
and optoelectronic devices.28–32 Moreover, these types of mol-
ecules are usually characterized by spectral sensitivity to the
surrounding medium, dual emission band formation (enol
form and keto form) with large Stoke shifts and photo-
stability. The formation of H-bonding from the hydroxyl or
amine group (donor) to the nitrogen of the imine group or
oxygen of the carbonyl group (acceptor) in the molecule is a
prerequisite for the ESIPT behavior. In this context, the con-
struction of AIE-active molecules bearing ESIPT core unsym-
metrical azine derivatives has not attracted much attention
in the research community.
Azine-based compounds are Schiff bases synthesized from
the reaction of two molecules of identical (symmetrical
azines), or different (unsymmetrical azines) carbonyl com-
pounds with hydrazine.33 Particularly, unsymmetrical azines
are very interesting owing to the presence of two dissimilar
moieties with unique features, which are connected by a
freely rotatable N–N bond, resulting in a combination of two
inherent features in the final compound. To date, reports re-
garding the development of ESIPT-active unsymmetrical
azines with AIE features have been scarce.34–38 Recently, our
group started to study the multi-functional behavior of spe-
cially designed molecules.39,40
in aqueous THF. The solid-state structure of the molecules
clearly confirmed the presence of strong intramolecular
hydrogen bonding between the imine nitrogen and pheno-
lic OH group (CHN—HO), which facilitates ESIPT behav-
ior in the excited state. Further occurrence of substituent-
dependent multiple weak interactions in the solid-state
structure of all the compounds results in a unique supra-
molecular network for the different molecules. Further-
more, tuning the electronic and steric features in the pe-
riphery of the salicylaldehyde moiety resulted in multi-
color emission in the aggregated state with different wave-
lengths (L1: yellow, L2: greenish yellow, L3: green, and L4:
orange) in THF/water mixture.
2. Experimental methods
2.1. Materials and measurements
Triphenylamine, hydrazine hydrate, salicylaldehyde, 2-hy-
droxy-1-naphthaldehyde, 3-methoxy salicylaldehyde and
4-(N,N-diethylamino)salicylaldehyde were purchased from
Sigma-Aldrich. All the analytical grade solvents and regents
used for the synthesis were purchased from commercial
sources and used after purification via standard methods.
1H NMR and 13C NMR were performed on
a Bruker
Avance 400 MHz instrument and chemical shifts values
are reported in ppm (parts per million) using TMS as the
internal reference. Coupling constant values are expressed
in hertz (Hz) and the following abbreviations are used to
express multiplicities: singlet (s), doublet (d), triplet (t)
quartet (q) and multiplet (m). Mass spectra of the com-
pounds were measured using a TOF-MS analyzer for accu-
rate HRMS. FT-IR spectra of the compounds were
recorded on a Bruker Tensor 27 with KBr pellets. Absorp-
tion spectral data was collected using a JASCO V-630 UV-
visible spectrometer in the wavelength range of 250–800
nm. The emission spectral data was recorded on a JASCO
FP-8300 spectrofluorometer in the wavelength range of
400–800 nm. A quartz cuvette with a path length of 1.0
cm was used to record absorption and emission spectral
Herein, we report the design and synthesis of a donor-
acceptor architecture triphenylamine-appended unsymmet-
rical azine family (Scheme 1) from simple precursors. All
the molecules exhibit positive solvatochromism, AIE behav-
ior in THF/water mixture and ESIPT phenomenon. Inter-
estingly, compound L3 shows pH-dependent fluorescence
data. Solid-state emission data was recorded using
a
HORIBA JOBIN YVON Fluoromax-4 spectrofluorometer. Op-
timization of all geometries was performed in the Gauss-
ian 09W program package using DFT. The B3LYP hybrid
density functional and 6-31G* basis set were used for all
atoms. DLS (dynamic light scattering) measurements were
performed on a Malvern Zetasizer Nano ZS instrument
equipped with a 4.0 mW helium–neon laser at a wave-
length of 633 nm. FE-SEM images of the compounds were
taken on
energy-dispersive X-ray spectrometer. Fluorescence lifetime
was measured with Horiba DeltaFlex TCSPC system
a JEOL JSM6480LV system coupled with an
a
equipped with a NanoLED laser (371 nm, Pulse width
<200 ps) excitation source and a PPD-850 photon detec-
tion module. The solid-state quantum yield of the com-
pounds was measured using a CARY-Eclipse with an inte-
grating sphere fluorescence spectrophotometer.
Scheme 1 Synthetic routes for compounds L1–L4.
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2.2. General procedure for UV-vis and fluorescence
measurement
129.56, 126.26, 125.66, 124.29, 121.23, 119.42, 117.96, 116.94.
HRMS: m/z (M + H+) (ESI,† Fig. S3) calcd. 392.1763, found
392.1756 (ESI,† Fig. S4). FT-IR (KBr) ν (cm−1): 3434 (–OH),
1596 (–CHN).
1 × 10−5 M stock solutions of L1–L4 were prepared for solvent
effect studies. 1 × 10−4 M stock solutions of L1–L4 were pre-
pared in tetrahydrofuran (THF) solvent for aggregation study.
Double distilled ultrapure water and THF solvents were used
for the aggregation studies. pH study of the compound L3 (1
× 10−4 M) was performed in the pH range of 3.0 to 10.0 using
aqueous THF (double distilled water buffered with Tris HCl
at various pH).
2.6. Synthesis of ligand L2
L2 was synthesized used the same procedure as L1, in which
2-hydroxy-1-naphthaldehyde (0.206 g) was used instead of
salicylaldehyde (Scheme 1). Mp: 172–176 °C. Yield: 0.421 g
(91%). Characterization of L2: 1H NMR (400 MHz, CDCl3)
(ESI,† Fig. S5) δ 13.43 (s, 1H), 9.65 (s, 1H), 8.57 (s, 1H), 8.14
(d, J = 4.0 Hz, 1H), 7.81 (d, J = 4.0 Hz, 1H), 7.76 (d, J = 4.0 Hz,
1H), 7.68 (d, J = 8.0 Hz, 1H), 7.51–7.54 (t, J = 8.0 Hz, 1H),
7.34–7.37 (t, J = 8.0 Hz, 1H), 7.27–7.32 (q, 2H), 7.20–7.24 (t,
J = 8.0 Hz, 2H), 7.15 (d, J = 8.0 Hz, 4H), 7.09–7.12 (t, J = 8.0
Hz, 2H), 7.06 (d, J = 8.0 Hz, 4H). 13C NMR (100 MHz, CDCl3)
(ESI,† Fig. S6) δ 165.93, 165.81, 165.45, 165.02, 164.85, 155.64,
151.53, 151.36, 139.00, 137.34, 134.69, 134.44, 134.37, 133.92,
132.79, 132.75, 130.98, 130.43, 130.18, 129.23, 128.52, 125.67,
123.86, 113.07. HRMS: m/z (M + H+) (ESI† Fig. S7) calcd
442.1919, found 442.1760 (ESI,† Fig. S8). FT-IR (KBr) ν (cm−1):
3421 (–OH), 1590 (–CHN).
2.3. Quantum yield calculation
The fluorescence quantum yield experiment was performed
using following formula, and perylene (quantum yield is 0.94
in cyclohexane) and quinine sulfate dyes (quantum yield is
0.546 in 0.5 M H2SO4) were used as the standard for the cal-
culation of the quantum yield of L1–L4 (at 411 nm for L1, L2
and L3 and 355 nm for L4).41
ꢀ
ꢁꢀ ꢁꢀ
ꢁ
As
Ar
f s
f r
ηr2
ηr2
ϕs ¼
ϕr
where ϕs and ϕr are the fluorescence quantum yield of the
sample and reference, respectively. As and Ar are the area un-
der the curve of sample and reference emission spectrum, re-
spectively. The terms fs and fr represent the absorption opti-
cal density value of the sample and reference respectively,
and η is the refractive index of the solvent.
2.7. Synthesis of ligand L3
L3 was synthesized using the same procedure as L1, in which
4-(N,N-diethylamino)salicylaldehyde (0.231 g) was used instead
of salicylaldehyde (Scheme 1). Mp: 168–173 °C. Yield: 0.311 g
(93%). Characterization of L3: 1H NMR (400 MHz, CDCl3)
(ESI,† Fig. S9) δ 12.05 (s, 1H), 8.59 (s, 1H), 8.47 (s, 1H), 7.63
(d, J = 4.0 Hz, 2H), 7.28–7.32 (q, 2H), 7.16 (d, J = 4.0 Hz, 2H),
7.14 (d, J = 4.0 Hz, 2H), 7.11 (d, J = 4.0 Hz, 2H), 7.10 (d, J =
4.0 Hz, 2H), 7.05 (s, 1H), 7.08, (d, J = 4.0 Hz, 1H), 6.27 (d, J =
4.0 Hz, 1H), 6.25 (d, J = 4.0 Hz, 1H), 6.22 (d, J = 4.0 Hz, 1H),
3.37–3.42 (q, 4H), 1.19–1.23 (t, J = 8.0 Hz, 6H). 13C NMR (100
MHz, CDCl3) (ESI,† Fig. S10) δ 163.34, 161.81, 158.75, 151.37,
150.26, 147.01, 133.56, 129.46, 129.27, 127.38, 125.37, 123.92,
121.76, 107.01, 103.94, 97.82, 44.57, 12.71. HRMS: m/z (M +
Na+) (ESI,† Fig. S11) calcd 463.2498, Found 463.2502 (ESI,†
Fig. S12). FT-IR (KBr) υ (cm−1): 3468 (–OH), 1593 (–CHN).
2.4. Synthesis of 4-(hydrazinomethyl)-N,N-diphenylaniline
(HADA)
The starting material 4-(diphenylamino)benzaldehyde was
synthesized according to the following literature method.42
In a 100 mL round-bottom flask, 10 mmol (2.73 g) of
4-(diphenylamino)benzaldehyde was added and it was
dissolved in 50 mL of ethanol with constant stirring. To this
solution, excess hydrazine hydrate (5.0 mL, 100 mmol) was
added. The mixture was allowed to stir for 8 h and then
refluxed for 2 h to obtain a pale yellow-colored solid product,
which was filtered and washed thoroughly with ethanol.
2.8. Synthesis of ligand L4
2.5. Synthesis of ligand L1
L4 was synthesized using the same procedure as L1, in which
3-methoxy salicylaldehyde (0.182 g) was used instead of
salicylaldehyde (Scheme 1). Mp: 174–178 °C. Yield: 0.51 g
(86%). Characterization of L4: 1H NMR, (ESI,† Fig. S13) δ
12.10 (s, 1H), 8.73 (s, 1H), 8.52 (s, 1H), 7.67 (d, J = 8.0 Hz,
2H), 7.30–7.33 (t, J = 8.0 Hz, 2H), 7.12–7.17 (t, J = 8.0 Hz, 4H),
7.05 (d, J = 4.0 Hz, 4H), 6.96 (d, J = 4.0 Hz, 4H), 6.87–6.91 (t,
J = 8.0 Hz, 1H), 3.94 (s, 3H).13C NMR (100 MHz, CDCl3) (ESI,†
Fig. S14) δ 163.86, 161.93, 151.05, 149.68, 148.31, 146.78,
129.90, 129.56, 126.22, 125.66, 124.29, 123.78, 121.20, 119.07,
118.04, 114.49, 56.24. HRMS: m/z (M + Na+) (ESI,† Fig. S15)
calcd 444.1688, Found 444.1685 (ESI,† Fig. S16). FT-IR (KBr) ν
(cm−1): 3446 (–OH), 2928 (OCH3), 1589 (–CHN–).
1.0 mmol (0.287 g) of 4-(hydrazinomethyl)-N,N-diphenylaniline
was mixed with 1.2 mmol (0.127 mL) of salicylaldehyde in ab-
solute ethanol and stirred for 12 h to obtain a pale yellow
solid product. The product was then filtered, washed thor-
oughly with ethanol and finally dried in a vacuum desiccator.
Mp: 161–165 °C. Yield: 0.295 g (72%). Characterization of L1:
1H NMR (400 MHz, CDCl3) (ESI,† Fig. S1) δ 11.85 (s, 1H), 8.73
(s, 1H), 8.54 (s, 1H), 7.66 (d, J = 4.0 Hz, 2H), 7.30–7.36 (m,
2H), 7.15 (d, J = 8.0 Hz, 4H) 7.10–7.13 (t, J = 8.0 Hz, 4H), 7.05
(d, J = 4.0 Hz, 2H), 7.01 (d, J = 4.0 Hz, 2H), 6.92–6.95 (t, J =
8.0 Hz, 2H). 13C NMR (100 MHz, CDCl3) (ESI,† Fig. S2) δ
163.83, 161.81, 159.76, 151.04, 146.78, 132.57, 132.16, 129.86,
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2.9. X-ray single-crystal structure analysis
in the excited state, which in turn leads to the keto form
emission in the longer wavelength region. Furthermore, the
substituents present in periphery can alter the ICT and ESIPT
features through steric as well as electronic contribution,
which results in tunable optical responses. The structures of
compounds L1–L4 were unambiguously characterized by dif-
ferent spectroscopic techniques such as 1H NMR, 13C NMR,
mass spectroscopy, FT-IR and single crystal X-ray diffraction
techniques (Fig. S1–S16†).
X-ray diffraction measurements were performed using a
Bruker AXS SMART APEX II diffractometer (Bruker AXS, Mad-
ison, WI, USA) (Mo-Kα radiation, graphite monochromator, λ
= 0.71073 Å). Raw data was collected with a scan by combina-
tion of both ω and ϕ angles. Data analysis was first corrected
for Lorentz and polarization methods, and the SADABS pro-
gram in APEX2 (ref. 43) was applied to run the absorption
correction. Further calculations and refinements were
performed using the APEX2 programs. CCDC data (1892317
(L1), 1892316 (L2), 1892315 (L3) and 1892318 (L4)) (Table S3†).
3.1. Photophysical behaviors of L1–L4
The optical properties of L1–L4 were thoroughly investigated
in various organic solvents with different polarities. As shown
in Fig. S17,† all the compounds exhibited two absorption
bands. The shorter wavelength absorption band at around
290 nm was assigned to the π–π* transition, whereas the lon-
ger wavelength absorption band at around 410 nm corre-
sponds to the intramolecular charge transfer (ICT) transition
within the azine moiety owing to the presence of donor-
acceptor groups (∼290, 395 nm (L1), ∼300, 411 nm (L2),
∼296, 411 nm (L3) and ∼306, 360 nm (L4)). The absorption
and emission data of all four compounds in various solvents
are presented in Table S1.† Triphenylamine is a well-known
π-electron-donating group with high donor potency, and
salicylaldimine is a common acceptor as well as ESIPT-active
unit, and all the compounds possess donor acceptor units
with additional substituents in their periphery. The absorp-
tion spectral data of all the compounds (L1–L4) in various sol-
vents illustrate that the peak position is independent of the
solvent polarity. From nonpolar to polar solvent, only a slight
absorption band shift was observed for all the compounds
studied (Fig. S17†).
Unlike absorption studies, the emission spectral data showed
interesting results based on the molecule and solvent polarity.
Upon excitation at the longer wavelength absorption maximum,
L1, L2 and L3 exhibited dual emission in various aprotic solvents
(Fig. 2). Two different emission trends were observed depending
upon the solvent polarity. In polar protic and aprotic solvents
such as methanol, DMSO and acetonitrile, charge transfer emis-
sion was dominant, whereas in the case of toluene, THF and
chloroform, dual fluorescence was observed due to both CT and
ESIPT emission. The shorter wavelength emission at around
450 nm is due to the CT emission from the enol form (E*),
3. Result and discussion
The construction of donor-acceptor based organic fluoro-
phore molecules has attracted increasing attention in recent
owing to their various potential applications in the field of
sensors, optoelectronic devices, up-conversion fluorescence,
organic field-effect transistors (OFETs), solid-state lighting
and light-emitting diodes.44–53 Furthermore, tuning the opti-
cal characteristics of molecules by engineering the electronic
and steric nature of substituents is of current interest.
Triphenylamine is a propeller-shaped non-planar fluorophore
and its periphery can be easily tuned by simple reactions.
Herein, to construct the multifunctional organic materials,
the electron-rich triphenylamine was utilized as the electron
donor and AIE-active core and coupled with an electron ac-
ceptor imine nitrogen, which was further linked to ESIPT-
active salicylaldimine units via an azine linkage (Fig. 1). Four
different triphenylamine-appended unsymmetrical azines
containing various salicylaldehyde derivatives were synthe-
sized via a stepwise route from simple precursors such as
4-(diphenylamino) benzaldehyde, hydrazine hydrate and
different substituted salicylaldehydes such as simple
salicylaldehyde (L1), 2-hydroxy-1-naphthaldehyde (L2), 4-(N,N-
diethylamino)
salicylaldehyde
(L3)
and
3-methoxy-
salicylaldehyde (L4) (Scheme 1).
All the synthesized compounds L1–L4 contain an electron-
rich triphenylamine unit as the donor moiety and
salicylaldimine unit as the acceptor moiety, which are
connected by a rotatable N–N bond, and as a whole, unsym-
metrical azine family can function as donor-acceptor (D–A)
push–pull conjugates. Further, these molecules undergo
excited-state intramolecular proton transfer (ESIPT) reaction
Fig. 1 Design principle of compounds L1–L4.
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Fig. 2 Normalized emission spectra of compounds (A) L1, (B) L2, (C) L3 and (D) L4 in different solvents with varying polarities. The emission peaks
showed a bathochromic shift with an increase in the polarity of the solvent.
Fig. 3 A) Fluorescence spectra of L1 in a THF-water mixture (100 μM; λex = 395 nm). B) Plots of fluorescence intensity at 548 nm versus THF-
water fraction. C) Fluorescence photograph of L1 with increasing fw (0–99%) under a 365 nm UV lamp.
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Fig. 4 A) Fluorescence spectra of L2 in THF-water mixture (100 μM; λex = 411 nm). B) Plots of fluorescence intensity at 530 nm versus THF-water
fraction. C) Fluorescence photograph of L2 with increasing fw (0–99%) under a 365 nm UV lamp.
Fig. 5 A) Fluorescence spectra of L3 in THF-water mixture (100 μM; λex = 411 nm). B) Plots of fluorescence intensity at 520 nm values versus THF-
water fractions. C) Fluorescence photograph of L3 with increasing fw (0–99%) under a 365 nm UV lamp.
whereas the longer emission at around 550 nm is due to the
keto form (K*) formed via ESIPT reaction. The occurrence of
dual emission in all three compounds confirmed the presence
of the ESIPT phenomenon in the molecules. In the case of L3,
the longer wavelength keto emission was largely blue shifted
compared to L1 and L2, which may be due to the destabilization
of the ESIPT pathway with the increase in the electron density
at the oxygen atom by the strong electron donating diethyl-
amino group in the salicylaldehyde moiety. In contrast to L1–L3,
L4 showed a single emission peak due to ICT in all the solvents
studied. This may be owing to the presence of the strong
electron-donating methoxy group present at ortho position to
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Fig. 6 A) Fluorescence spectra of L4 in THF-water mixture (100 μM; λex = 370 nm). B) Plots of fluorescence intensity at 613 nm versus THF-water
fraction. C) Fluorescence photograph of L4 with increasing fw (0–99%) under a 365 nm UV lamp.
the –OH group in the compound, which in turn led to the sup-
pression of the ESIPT reaction. The solid-state emission for all
the compounds was recorded and results indicate the presence
of both keto and ICT emission states (Fig. S19†). All the mole-
cules show a very low quantum yield in all the solvents studied
(<0.1). This may be due to the increase in non-radiative path-
ways during the relaxation of the excited state via intramolecular
rotation around the N–N, and –CHN– bonds and C–N
bond of the phenyl rotors in the triphenylamine moiety.
able bonds and easily isomerisable –CH=N units. The ab-
sorption spectra of all the compounds were recorded in a
THF-water mixture, where level-off tails appeared at a lon-
ger wavelength when fw exceeded 60%, which confirmed
the formation of aggregates (Fig. S21†). To check the AIE
behavior of the synthesized compounds, fluorescence exper-
iments were performed for all the compounds in THF-water
mixtures with various water contents. In pure THF, L1, L2
and L3 showed a very weak dual emission when excited at
the longer wavelength absorption maximum, which is due
to the occurrence of free rotation around the N–N sin-
gle bond, –CHN– bonds and C–N bond of the phenyl ro-
tors in the triphenylamine moiety, which destroyed the ex-
cited state by a non-radiative path. Upon the incremental
addition of water fraction to the L1–L3 solutions, no signifi-
cant increase in the emission intensity was observed up to
30% water fraction. After 30% water fraction, a gradual in-
crease in emission intensity was observed up to 99% water
content for L1 and L2, and 70% water content for L3. L1, L2
and L3 showed a fluorescence enhancement of 45-, 12- and
25-fold with a significant bathochromic shift of 6 nm, 10
nm and 58 nm (Fig. 3–5), respectively. As shown in Fig. 5,
the emission intensity of L3 increased up to 70% water
fraction due to the aggregation of its molecules, which
resulted in aggregation-induced emission behavior. How-
ever, a further increase in water fraction led to decrease in
emission intensity due to precipitation of the aggregated
molecules, which was confirmed by the decrease in absor-
bance together with the formation of a red-edged absorp-
tion due to scattering of light by the aggregated particles.
3.2. AIE behavior of L1–L4
The AIE characteristics of compounds L1–L4 were explored
due to the presence of the D–A unit, various freely rotat-
Fig. 7 Photographic images of compounds L1–L4 in different forms
under a UV lamp (365 nm). (A) Crystal, (B) pristine, and (C) aggregate
states generated by centrifuging suspensions of THF-water mixtures
(fw – 90%).
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Fig. 8 FE-SEM images of the compounds in the aggregates state: A) L1, B) L2, C) L3 and D) L4.
On the other hand, L4 showed a gradual enhancement in
emission intensity with the incremental addition of water
fraction up to 50% water content in the THF-water mixture
(Fig. 6). After 50% water content, the formation of a new
red-shifted intense emission together with a blue-shifted less
intense emission was noticed. The observed enhanced emis-
sion at 553 nm up to 50% water content was assigned to
the ICT band of aggregated species. A further increase in
the water fraction resulted in the movement of L4 from a hy-
drophilic to hydrophobic environment in the newly formed
aggregated state, which in turn led to the formation of a
keto emission at 613 nm via the ESIPT reaction together
with an increase in emission intensity. The formation of a
new emission peak at 613 nm together with an increase in
emission intensity on increasing the water fraction is consis-
tent with the color change from green to orange, which indi-
cates that the emission occurred from the keto state formed
by ESIPT.
The observed fluorescence enhancement phenomenon
was also visible to the naked eye under a UV lamp (365 nm)
and different colors were observed for compounds with dif-
ferent substituents such as yellow (L1), greenish yellow (L2),
green (L3) and orange (L4) in the aggregated state (Fig. 3C–6-
C). This result indicates that the tuning of AIE color of L1–L4
can be tailored by tweaking the substituents in the salicyl-
aldehyde part of azine family.
All four compounds displayed weak emissions in highly
soluble solvents due to the occurrence of free rotation around
the N–N single bond, which destroyed the excited state
by non-radiative pathways. When fw increased to 50–99%, the
compounds started to form aggregates, which restricted the
intramolecular motions (restriction of intramolecular rota-
tion) and led to an intense emission through radiative relaxa-
tion of the excited state. Moreover, fluorescence measure-
ments in the THF-water and THF-glycerol mixtures
(Fig. S22†) confirmed the presence of the restriction of
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Fig. 9 Molecular structures of compounds L1–L4.
Fig. 10 A) Dimer structure of L1 through C–H—O interaction in a head-to-tail manner. B) Intermolecular interaction of an L1 molecule with neigh-
boring molecules. C) One dimensional polymeric chain-like structure of compound L1.
intramolecular rotation (RIR) mechanism in all the com-
pounds. Further, to validate the formation of aggregates, dy-
namic light scattering (DLS) measurements were carried out
for compounds L1–L4 in THF and in the water fraction with
the maximum fluorescence enhancement (for aggregates).
The result shows that the size of the particle increased from
negligible (<5 nm) to the well-defined size of 1271 nm (L1),
2712 nm (L2), 571 nm (L3) and 825 nm (L4) upon increasing
the water fraction (Fig. S23†), which confirms the formation
of aggregates.
The solid-state fluorescence images of crystals, pristine
and aggregates (centrifuged aggregates obtained at 90% water
content) of L1–L4 are shown in Fig. 7 (under UV lamp (365
nm)), which clearly indicate that presence of substituent-
dependent solid-state fluorescence in L1–L4. The above all re-
sults confirmed the enhanced emission behavior of the com-
pounds due to the formation of aggregates. In addition, the
solid-state quantum yield for all the compounds was mea-
sured using an integrating sphere, which was found to be
0.28 (L1), 0.36 (L2), 0.13 (L3) and 0.31 (L4). These values are
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Fig. 11 A) Dimer structure of L2 through C–H—O interaction in a head-to-tail manner. B) Intermolecular interaction of an L2 molecule with neigh-
boring molecules. C) One-dimensional polymeric chain-like structure of compound L2. D) Front view of the molecule L2 showing the C–H—N
intermolecular interaction.
higher than the quantum yields observed in the solution
state and it clearly indicate that the RIR mechanism is re-
sponsible for the enhanced emission in the solid state of
L1–L4 as well as for aggregation-induced emission. Further-
more, field-emission scanning electron microscopy (FE-SEM)
measurements of the corresponding aggregate showed the
clear morphology of the aggregated states, as shown in Fig. 8.
The fluorescence lifetimes were measured to understand
the behavior of the azine molecules in the excited state. Ex-
cept L4, the other three molecules showed a single exponen-
tial decay and their lifetime values in different solvents are
presented in Table S1.† The shorter ps component observed
in the emission maxima is due to the intramolecular hydro-
gen bonded keto form. In contrast, the L4 decay curves were
fitted with a biexponential decay. The biexponential decay
containing a major contribution from a faster ps component
is similar to other three-azine molecules, and a longer ns life-
time component with minor contribution was observed. The
longer component may be due to the charge transfer from
the donor to the acceptor moieties within L3. Detailed femto
up conversion studies are needed to understand the clear pic-
ture of the excited state reactions. In the aggregated state
(90% water : 10% THF mixture), all four azine molecules
showed a biexponential decay with one major shorter compo-
nent with lifetimes in the range of ps and another minor
component with a longer ns lifetime (Table S2†). The longer
ns component may be due to the emission from the aggre-
gated state.
3.3. Single crystal structure
Single crystals of all the compounds were grown using aceto-
nitrile solvent via the slow evaporation method and they crys-
¯
tallized in the triclinic, P1 (L1), monoclinic, P21/c (L2), mono-
¯
clinic, P21/n (L3) and triclinic, P1 (L4) space groups. The
molecular structures of all the compounds are shown in
Fig. 9. In all the compounds, the salicylaldimine and diphe-
nylamine moieties adopt a trans orientation around the N2–
N3 bond. The crystal structure of each compound authenti-
cated the existence of strong intra-molecular hydrogen bond
(O–H—N) interactions (1.897 Å (L1), 1.851 Å (L2), 1.919 Å (L3)
and 1.916 Å (L4)) between the –OH groups of the salicyl-
aldehyde moieties with the imine nitrogen (N3) of
4-(hydrazonomethyl)-N,N-diphenylaniline groups and led to
the planarity in the salicylaldimine unit, which is responsible
for the proton transfer process in the excited state (Fig. 9).
Further, the phenyl ring of triphenylamine, which is
connected to the salicylaldimine unit via an N–N bond, is not
present in the single plane with salicylaldimine moiety and
their respective dihedral angles are 11.085(5) for L1, 27.739(8)
for L2, for 16.796(9) L3 and 34.023(5) for L4 (Fig. S24†). Collec-
tively all the molecules adopted a propeller-shaped molecular
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Fig. 12 A) Dimer structure of L3 through C–H—O interaction in a head-to-tail manner. B) Front view of the L3 molecule showing the C–H—N inter-
molecular interaction. C) One-dimensional polymeric chain-like structure of compound L3. D) Intermolecular interaction of an L3 molecule with
neighboring molecules.
structure. The presence of a freely rotatable N–N single bond
and strong intra-molecular hydrogen bonding interaction fa-
cilitate the ESIPT and AIE behavior in all the molecules. In
the solution state, the compounds displayed weak emission
as a consequence of the free N–N bond rotation, but in the
crystalline state the rotation was restricted, thus promoting
the fluorescence and thereby revealing the AIE behavior of
the molecules. The neighboring molecules are connected by
various weak interactions such as C–H—O, C–H—N and C–
H—π interactions.
molecules via weak intermolecular interactions (Fig. 10B).
Further, it forms a one-dimensional polymeric chain-like
structure through C–H—O and C–H—π interactions
(Fig. 10C).
Similarly to L1, compound L2 also forms head-to-tail di-
mers via the strong C–H—O interaction between the phenolic
O and ortho C–H proton from the free phenyl group in the
triphenylamine moiety (2.693 Å), resulting in the formation
of J aggregates (Fig. 11A). A single molecule is connected to 6
neighboring molecules via weak intermolecular interactions
(Fig. 11B). Further, through C–H—O (2.693 Å) and C–H—N
(2.738 Å) interaction, the molecule forms a one-dimensional
chain-like structure (Fig. 11C).
Compound L1 forms head-to-tail dimers by strong C–H—
O interactions (2.678 Å), leading to J aggregate structures
(Fig. 10A). Each molecule is connected to three neighboring
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Fig. 13 A) Dimer structure of L4 through C—C interaction in slipped head-to-tail manner. B) One-dimensional polymeric ladder-like structure of
compound L4. C) Front view of the L4 molecule showing the C–H—N intermolecular interaction. D) Intermolecular interaction of an L4 molecule
with neighboring molecules.
Contradictory to the dimeric structure of L1 and L2, L3
exhibited a different type of dimer, i.e. in-plane dimeric struc-
ture via strong C–H—O interaction between the oxygen atom
of the salicylaldehyde moieties and C–H (ortho to imine nitro-
gen) of the triphenylamine moieties (2.718 Å) (Fig. 12A). Fur-
ther, a C–H—N interaction (2.615 Å) was also observed be-
tween imine nitrogen atom (N2) and ortho C–H of the free
phenyl group of the triphenylamine moiety (Fig. 12B), which
is similar to L1. The dimer molecules were connected in a
head-to-tail manner, which clearly confirms the formation of
J aggregates. Collectively, it forms a supramolecular dimeric
one-dimensional polymeric chain, as presented in Fig. 12C
by multiple weak interactions. Further one molecule is
connected with 7 neighboring molecules by CH—N and
CH—O interactions (Fig. 12D).
(Fig. 13A). Each dimer is attached to the adjacent dimers via C–
H—C and C–H—H-C interactions between the o-methoxy salicyl-
aldehyde in the adjacent molecules and C–H—H-C interactions
between the phenyl rings of the triphenylamine unit and collec-
tively lead to a one-dimensional ladder-like polymeric chain
(Fig. 13B). Similarly to compound L2 and L3, L4 also forms a
one-dimensional structure via the C–H—N interaction between
the meta C–H hydrogen of the free phenyl group of the
triphenylamine moiety with the N3 imine nitrogen (2.652 Å)
(Fig. 13C), which is similar to L2. Each molecule is connected to
5 neighboring molecules through C–H—O, C–H—N, and C–H—
H–C interactions (Fig. 13D).
Collectively, molecules L1–L4 displayed various interesting
supra-molecular networks via weak intermolecular interac-
tions such as C–H—O, C–H—N and C—C (Fig. 14), which are
tuned by the different substituents in the salicylaldehyde
moiety. The weak intermolecular interaction present in mole-
cules L1–L4 plays a dominant role in the AIE characteristic by
Unlike compounds L1–L3, compound L4 forms a different
type of dimer, such as slipped head-to-tail dimer with a centroid
to centroid distance of 4.603 Å and C—C distance of 3.285 Å
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Fig. 14 Supramolecular network structures of compounds A) L1 B) L2 C) L3 and D) L4.
hindering the deactivation of the excited state via non-
radiative dissipation and improves the radiative pathway in
the aggregated state or in the solid state. Further, these dif-
ferent supramolecular interactions in L1 to L4 lead to various
interesting morphologies in the molecular self-assembled ag-
gregated states, which can be clearly seen in the SEM images
of the compounds (Fig. 8). For example, compounds L1 and
L3 have a similar type of C–H—N interaction in the solid
state, resulting in a similar morphology in their SEM images.
However, compound L4 possess a different type of weak inter-
action compared to the other three compounds, resulting in
a different morphology in the aggregated state.
gregates of L1–L4 with varying pH in the range of 3.0–10.0
in aqueous THF solution (fw L1: 99%, L2: 99%, L3: 70%
and L4: 99%) buffered by Tris-HCl (Fig. S25†). Surpris-
ingly, only L3 exhibited pH-dependent emission features
upon varying the pH from 3 to 10. It showed high fluo-
rescence emission in basic medium, whereas in highly
acidic pH, it was non-emissive with a red shift in its
emission maxima, and while with a decrease in the acidic
conditions, its emission behavior gradually increased
(Fig. 15). In general, the salicylaldehyde Schiff base un-
dergoes hydrolysis in strong acidic medium, which is due
to the electrophilic addition reactions of protons from the
acid source to the CHN bond in hydrazones. Further,
only compound L3 has two different electron donating
groups (triphenylamine and diethylamino) in both sides of
the azine functionality, which collectively increase the
electron density on the CHN groups, making it least
3.4. pH response of L3
To study the pH-dependent photophysical properties of
the probes, we performed fluorescence experiments for ag-
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stable in high acidic medium with a titration jump at 4.5
pH. At low pH such as lower than 4.5 pH, the acid pro-
tons can easily attack the CHN double bond through
electrophilic addition, resulting in the formation of
4-(hydrazonomethyl)-N,N-diphenylaniline (HADA) and corre-
sponding 4-(N,N-diethylamino)salicylaldehyde (DESAL).1,4,54,55
To gain deep understanding of the hydrolysis mecha-
nism, we measured the proton NMR spectra for bare L3 and
with the addition of 50 μL TFA–D2O mixture (Fig. 16). The
1H NMR spectrum of the resultant species displayed a shift
in the OH proton peak from δ 12.05 to 11.65 ppm. Mean-
while, the formation of new peaks was observed at δ 9.49
and 5.43 ppm, corresponding to the formyl group of 4-(N,N-
diethylamino)salicylaldehyde (DESAL) and hydrazine group of
the 4-(hydrazonomethyl)-N,N-diphenylaniline (HADA) moiety,
respectively. This result clearly confirms the occurrence of
cleavage at the imine linkage in the acidic medium,
whereas, the compound is stable in basic medium. Further,
the hydrolysis process of L3 was confirmed by ESI-MS analy-
sis in diluted TFA solution. In the presence of TFA, L3
showed two different peaks at m/z 288.1477 and 194.1177,
which correspond to 4-(hydrazonomethyl)-N,N-diphenylaniline
(HADA) and 4-(N,N-diethylamino)salicylaldehyde (DESAL), re-
spectively, together with the original molecular ion peak at
463.2488 (Fig. S26†). Furthermore, the comparison of the ab-
sorption spectra of 4-(N,N-diethylamino)salicylaldehyde with
the results from the pH-dependent UV-vis spectral measure-
ments of L3 clearly indicates that the absorption behavior
of the cleaved species is similar to that in 4-(N,N-
diethylamino)salicylaldehyde (Fig. S27†). Thus, the above cu-
mulative results suggest that L3 undergoes hydrolysis
reaction in acidic medium, which results in fluorescence
quenching.
Fig. 15 A) Emission peak intensities of L3 as a function of pH value. B)
Fluorescence photograph of L3 at different pH values.
3.5. Theoretical calculations
The miscellaneous emissive properties of all the
triphenylamine derivatives may be due to the different
electronic structure of the molecules.31,56 To validate clear in-
sight into the geometry and electronic structures of L1–L4 we
performed DFT calculations using the Gaussian 09W pro-
Fig. 16 1H NMR spectra of compound L3 before and after the addition
of TFA (50 μL) in CDCl3 solvent.
Fig. 17 Enol form HOMO, LUMO frontier molecular orbitals and energy levels of compounds L1–L4.
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Fig. 18 Keto form HOMO, LUMO frontier molecular orbitals and energy levels of compounds L1–L4.
gram. The B3LYP hybrid density functional and 6-31G* basis Conflicts of interest
set were used to optimize the geometries. Fig. 17 and 18
show the frontier molecular orbital amplitude plots and en-
There are no conflicts to declare.
ergy levels of the highest occupied molecular orbital
(HOMOs) and lowest unoccupied molecular orbital (LUMOs)
of L1–L4. The molecular orbital amplitudes indicate that the
Acknowledgements
B. M. thanks SERB, New Delhi, India (YSS/2015/000037), for
financial support and UGC, New Delhi, for a start-up grant
and UGC FRP faculty award [F.4-5IJ94-FRP)/2014IJBSR)]. M. M.
acknowledges the Department of Science and Technology
(DST), New Delhi for fellowship [SR/PURSE Phase 2/17IJG)].
V. T. thank SERB, DST (YSS/2014/00026), DST Nano Mission
(DST/NM/NB-2018/10(G)) and UGC-FRP [F.4-5(24-FRP)/2013(BSR)]
for financial support to this research work.
HOMOs are localized on both the triphenylamine donor
phenyl groups and some extend over the imine nitrogen.
Meanwhile, the LUMOs are slightly localized on the
triphenylamine phenyl ring connected with the imine group
and localized over the salicylaldehyde, hydroxyl naphth-
aldehyde, o-vanillin and 4IJN,N-diethylamino) salicylaldehyde
groups. Moreover, all the compounds exhibit intramolecular
hydrogen bonding and the band gap was powerfully
influenced by the molecular packing and conformation. Fur-
ther, the energy gaps for the enol and keto isomers of all the
triphenylamine derivatives are well correlated with the enol
and keto emission wavelength range observed in the emis-
sion measurements. This result clearly indicates the occur-
rence of ESIPT behavior in all the compounds.
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