Perturbing the AIEE activity of pyridine functionalized α-cyanostilbenes with donor substitutions: An experimental and DFT study

Article abstract of DOI:10.1039/c9nj03693h

A series of pyridine functionalized α-cyanostilbenes bearing different amino group donors, carbazole-phenyl (C-1), N-ethyl carbazole (C-3), dimethylamino (DM), julolidine (J-1) and triphenylamine (T-1), were synthesized, and their impact on the aggregatio

Full text of DOI:10.1039/c9nj03693h

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Perturbing the AIEE activity of pyridine
functionalized a-cyanostilbenes with donor
substitutions: an experimental and DFT study†
Cite this: New J. Chem., 2020,
44, 218
Palash Jana,‡ Mahalingavelar Paramasivam, *‡ Shikha Khandelwal, Arnab Dutta
and Sriram Kanvah
*
A series of pyridine functionalized a-cyanostilbenes bearing different amino group donors, carbazole–
phenyl (C-1), N-ethyl carbazole (C-3), dimethylamino (DM), julolidine (J-1) and triphenylamine (T-1), were
synthesized, and their impact on the aggregation propensity of the a-cyanostilbenes was investigated. Distinct
aggregation-induced emission (AIE) features have been obtained in water, where the emission intensity varied
in the following order: C-1 4 DM 4 C-3 4 T-1 4 J-1. The formation of aggregates was substantiated by
fluorescence lifetime decay profiles and their average lifetime correlated with the rigidity of the molecular
framework. The contributing factors behind the AIE phenomena are the bulkiness of the donor segments,
their donating strength, planarity of the molecular framework, the distance between the a-cyano unit and the
donors and its interaction with neighboring units. Based on the experimental observations from this study and
previous research reports, we have used DFT and TDDFT methods to evaluate the donor–AIEE property
relationship. Cyclic voltammetry techniques were used to examine the redox properties of the compounds.
SEM and DLS studies provided us with a detailed picture of the morphology and hydrodynamic radii of the
aggregates, respectively. The results obtained from this study would be helpful to understand the substituent
dependent aggregation propensity of a-cyanostilbenes and provide design guidelines toward the
development of AIE active materials for various optoelectronic and bioimaging applications.
Received 16th July 2019,
Accepted 21st November 2019
DOI: 10.1039/c9nj03693h
rsc.li/njc
radiative channels. The greater the number of free phenyl rings, the
higher the emission intensity, i.e., the AIE effect in HPS is more
Introduction
Aggregation-induced emission (AIE) is a unique phenomenon pronounced than in TPE.10 Like TPE and HPS molecular scaffolds,
that induces emission intensity enhancement or wavelength a-cyanostilbene is a versatile motif that undergoes efficient
shifts upon aggregation.1,2 Various mechanistic findings such intramolecular rotation (IMR) with large torsional distortions and
as restriction of molecular motions (vibration and rotation), is non-emissive in solution. Upon aggregation, the restricted IMR of
planarization, J-aggregate formation, E/Z isomerization, excited- the a-cyano moiety results in planarization, rendering excellent
state intramolecular proton transfer (ESIPT), etc. have been photophysical properties in its aggregated form through effective
reported as contributing factors for the AIE phenomenon.3–5
A
intermolecular interactions.8,11 Unlike HPS or TPE derivatives,
plethora of applications involving the AIE effect of several mole- a-cyanostilbenes influence both emission intensity and wavelength
cular scaffolds and their mechanism have been extensively inves- shifts upon aggregation. It should be noted that the aggregation
tigated to date.4,6–9 Of these, the widely accepted mechanism is type (H or J) is solely dependent on the positional substitution of
the restriction of intramolecular motions typically observed in a-cyanostilbenes.12 For instance, Y. Zhang et al. demonstrated the
tetraphenylethene (TPE) and hexaphenyl silole (HPS) derivatives distinct photophysical features of two isomeric triphenylamine
where only emission intensity enhancement is noted. Hampering based acrylonitriles with respect to the positional substitution of the
the rotation of free phenyl rings in TPE and HPS upon aggregation a-cyano unit to the donor and acceptor groups respectively.13 Isomer
imparts rigidity to the system, dissipating the energy through 1 (TPA-CNa) containing the a-cyano unit proximal to the donor
exhibited a hypsochromic shift with H-type aggregation.14 Similar
Department of Chemistry, Indian Institute of Technology Gandhinagar, Palaj,
observations were noticed by Upamali and coworkers for carba-
zole substituted cyanostilbenes.15 On the other hand, substitution
Gandhinagar-382 355, India. E-mail: sriram@iitgn.ac.in,
paramasivam.org@gmail.com
of the a-cyano unit proximal to the terminal acceptor (spacer) unit
(TPA-CNb) results in a bathochromic shift. Recently, Zhang
† Electronic supplementary information (ESI) available: Additional figures and
1H and 13C NMR of the compounds are provided. See DOI: 10.1039/c9nj03693h
‡ Equal contribution.
et al. also perturbed the aggregation behavior of a-cyanostilbenes
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by proximal substitution of bulkier tetraphenyl imidazole (TPI) formation, and non-linear optical (NLO) applications.26–28 The extent
units. The positional substitution of the bulkier TPI groups hampers of the donor’s influence on the AIEE activity of cyanostilbenes is
the interaction of a-cyanostilbenes with neighboring units to a studied using various experimental techniques and DFT/TDDFT
certain extent. Their aggregation abruptly increases the emission methods. The distance between the donor units and a-cyano moiety
intensity but they show no wavelength shifts despite the presence of distinctly differs for each compound. For instance, the carbazole
imidazole acceptor units. Thus, it can be stated that AIE mainly donor and a-cyano moiety are separated by a phenyl spacer in C-1,
originates from the restriction of intramolecular rotation of tetra- whereas the donor units are directly connected to an a-cyano moiety
phenyl imidazole rings with minimal participation of the for the remaining compounds. The steric repulsion and rigidity of
a-cyanostilbene motif.16 Typically, the incorporation of accep- the donor are expected to have a greater impact in determining
tor units into the cyanostilbene framework dramatically shifts the emission intensity and wavelength shift. Additionally, each
the emission to longer wavelengths with a concomitant large donor unit has its distinguishing characteristics upon aggrega-
Stokes shift. For example, our group developed a series of tion. Our observations are detailed in the following sections.
four coumarin-based AIE active a-cyanostilbenes by varying
Experimental details
the spacers and terminal acceptors without perturbing the
a-cyano moiety. The resultant aggregates shifted the emission
wavelength from the visible to the far red/NIR region with a
large Stokes shift of 158–213 nm.17 Similarly, insertion of an
acrylonitrile group enables emission tunability, yielding fluoro-
phores with a large Stokes shift.18,19 The unique emission
characteristics of a-cyanostilbenes have opened a new platform
towards development of various novel and smart emissive
materials with applications as solid-state light emitters, non-
linear optics, bioprobes, and chemosensors, among others.12,13,20–24
Recently, we also demonstrated the geometrical impact on the
AIE characteristics by varying the structure from linear (mono-
branched) to V-shaped (di-branched) and star-shaped (tri-branched)
molecules.25 We have observed that the AIE effect gradually
decreases upon increasing the number of branching units. This
study reveals that each branch present in the star-shaped molecule
hampers interactions of a-cyano units of other branches.
Scrutiny of these reports shows the paramount importance of
positional substitution of donor/acceptor units to the aggrega-
tion characteristics of a-cyanostilbenes. To study this fact, we
have developed a series of pyridine functionalized acrylonitrile
derivatives with various donor substitutions, carbazole (C-1),
N-ethyl carbazole (C-3), N,N-dimethylamine (DM), julolidine
( J-1) and triphenylamine (T-1) [Fig. 1]. Pyridine acts as an
acceptor moiety and also aids in using these compounds as
stimuli-responsive materials towards use in acid sensing, organogel
All the reagents required for the synthesis of the pyridine
functionalized acrylonitriles were purchased from Sigma-Aldrich,
Alfa Aesar, Acros, and S. D. Fine Chem. The synthesized compounds
were characterized using a Bruker Avance III-500 MHz NMR spectro-
meter and ITMS-probe electron spray ionization (pESI) mass
spectrometer. UV-vis absorption spectra were recorded using an
Analytik Jena Specord 210 Plus, and fluorescence emission studies
were performed using a Horiba-Jobin-Yvon Fluorolog-3 spectrofluori-
meter. The excitation wavelengths were set at the absorption max-
ima (la) of the compounds. All the fluorescence spectra were
recorded in 10 mm path length quartz cuvettes with a slit width
of 2 nm. For the UV-vis absorption & fluorescence measurements,
B10 mM concentrations of chromophores were used prepared from
a stock solution of the compounds in THF or DMSO. The fluores-
cence lifetime measurements were performed using a picosecond
time-correlated single photon counting (TCSPC) setup (Edinburgh
Instruments Ltd, Lifespec II model) employing a picosecond light
emitting diode laser at 405 nm for the excitation of dye solution in
99% water. A reconvolution procedure was used to analyze the decay
by using a proper instrument response function (IRF) obtained by
using LUDOX HS-40 colloidal silica particles in a 40 wt% suspension
in water as the light scatterer. The fluorescence decays [I(t)] were
analyzed in general as a sum of exponentials, eqn (1)
X
Bi expðꢀt=t Þ
(1)
i
IðtÞ ¼
Fig. 1 Structures of the synthesized acrylonitrile derivatives.
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where Bi and ti are the pre-exponential factor and fluorescence aldehyde. Upon completion, the reaction solution was cooled to
lifetime of the ith component. Reduced chi-square (w2) values and room temperature, and the solvent was evaporated to dryness.
the random distribution of the weighted residuals among the data The obtained residue was washed with fresh methanol followed
channels were used to judge the acceptance of the fits. The error by diethyl ether. The precipitate obtained was filtered and dried
associated with the lifetime studies is 0.30–13%. Scanning electron under a vacuum for 1 h to afford the desired products as yellow
microscopy (SEM) analysis was carried out using a field emission to red solid a-acrylonitrile derivatives.
SEM (JSM 7600F JEOL). 10 mL of the sample (B10ꢀ5 M solution) was
C-1. Yield 55% (150 mg): 1H-NMR (500 MHz, DMSO-d6),
drop cast on a Si-wafer. The samples were dried at 35 1C for 16 h to d (ppm): 8.76–8.75 (d, 2H) 8.49 (s, 1H), 8.33–8.28 (dd, 4H), 7.92–
ensure complete removal of any residual water and coated with 7.90 (d, 2H), 7.83–7.82 (d, 2H), 7.56–7.54 (d, 2H), 7.50–7.47
platinum before being analyzed. Electrochemical measurements are (dd, 2H), 7.37–7.33 (dd, 2H) 13C NMR (125 MHz, DMSO-d6): d
performed using a Metrohm Autolab PGSTAT 101 potentiostat at (ppm) 151.06, 145.62, 141.59, 140.12, 139.98, 132.28, 132.03,
room temperature. A three electrode system was used during the 127.29, 126.96, 123.64, 121.10, 120.43, 117.56, 110.37, 108.68.
experiment, a 1 mm glassy carbon disc working electrode, a Pt-wire ITMS (pESI): C26H17N3 (M), [M + H]+ cal. m/z 372.15, found
counter electrode, and Ag/AgCl (in saturated KCl) as a reference m/z 372.17.
electrode. All experiments were performed in dry DMF using
C-3. Yield 55% (150 mg): 1H-NMR (500 MHz, DMSO-d6),
0.1 M tetrabutylammonium tetrafluoroborate (TBAF) as a support- d (ppm): 8.91 (s, 1H), 8.88–8.86 (d, 2H), 8.74 (s, 1H), 8.35–8.33
ing electrolyte. All cyclic voltammetry data reported in this work (d, 1H), 8.19–8.17 (d, 1H), 8.14–8.13 (d, 2H), 7.90–7.89 (d, 1H),
are internally referenced against ferrocene.
7.76–7.75 (d, 1H), 7.60–7.57 (dd, 1H), 7.37–7.34 (dd, 1H), 4.57–
4.56 (q, 2H), 1.40–1.37 (t, 3H) 13C NMR (125 MHz, DMSO-d6)
d (ppm): 142.42, 140.82, 128.22, 127.46, 125.60, 124.14, 123.10,
Computational details
All theoretical calculations have been carried out in this study 122.52, 121.38, 120.91, 120.88, 118.19, 110.65, 79.71, 79.45,
using the Gaussian 09 ab initio software package within the 79.19, 37.94, 14.26. ITMS (pESI): C22H17N3 (M), [M + H]+ cal.
framework of density functional methods.29 The ground state m/z 324.15, found m/z 324.17.
geometry of the compounds was optimized in the gas and
DM. Yield 80% (200 mg): 1H-NMR (500 MHz, DMSO-d6),
solution phase at the DFT/B3LYP/6-311G(d,p) level of theory d (ppm): 8.63–8.61 (d, 2H), 8.11 (s, 1H), 7.96–7.95 (d, 2H), 7.66–
with no symmetry constraints imposed. The optimized geome- 7.65 (2H), 6.87–6.85 (d, 2H), 3.07 (s, 6H) 13C NMR (125 MHz,
tries are subjected to vibrational analysis to ensure global DMSO-d6): d (ppm) 152.9, 150.7, 146.2, 142.8, 132.5, 120.5,
minima on the potential energy surface with no imaginary 119.4, 119.01, 112.1, 99.64. ITMS (pESI): C16H15N3 (M), [M + H]+
frequency. These structural coordinates were used as the input cal. m/z 250.13, found m/z 250.17.
for further single point calculations to examine the electron
J-1. Yield 65% (150 mg): 1H-NMR (500 MHz, DMSO-d6),
density population, TDDFT, and PDOS analysis, etc. Singlet d (ppm): 8.59–8.57 (d, 2H), 7.94 (s, 1H), 7.61–7.60 (d, 2H),
excited state calculations have been employed using time- 7.53 (2H, s), 3.30–3.32 (m, 2H), 2.72–2.70 (m, 4H) 1.91–1.89
dependent DFT (TDDFT) methods. To simulate the experi- (m, 4H) 13C NMR (125 MHz, DMSO-d6): d (ppm) 150.64, 145.97,
mental absorption spectra of the compounds, three different 130.12, 120.86, 119.19, 49.78, 27.65, 21.28. ITMS (pESI):
density functionals, B3LYP, long-range corrected M062X30 and
C
20H19N3 (M), [M + H]+ cal. m/z 302.16, found m/z 302.17.
T-1. Yield 75% (150 mg): 1H-NMR (500 MHz, DMSO-d6),
oB97XD,31 have been used with a common basis set of 6-311G(d,p).
As the absorption of the molecules was recorded in 1,4-dioxane, d (ppm): 8.67–8.66 (d, 2H), 8.20 (s, 1H), 7.93–7.92 (d, 2H), 7.71–
same solvent medium has been used under the conductor-like 7.70 (d, 2H), 7.44–7.41 (dd, 4H), 7.24–7.19 (m, 6H), 6.97–6.95
polarizable continuum model (C-PCM) integral equation formalism (d, 2H) 13C NMR (125 MHz, DMSO-d6) d (ppm): 152.88, 150.72,
within self-consistent field theory was taken into consideration 146.23, 142.82, 132.51, 120.59, 119.45, 119.01, 112.11, 99.62.
to mimic the real environment.32 Density of states (DOS) and ITMS (pESI): C26H19N3 (M), [M + H]+ cal. m/z 374.16, found
projected density of states (PDOS) analysis has been employed m/z 374.33.
to understand the percentage contribution of electron density
from individual segments toward the charge transfer transition
of the compounds against their respective molecular orbital
density coefficients. The simulation of the major portion of the
absorption spectrum, PDOS analysis, and interpretation of the
Results and discussion
Synthesis
nature of the transitions were executed using GaussSum 3.0.2. A series of acrylonitrile derivatives were synthesized with vary-
ing donor substitutions via a condensation reaction of the
corresponding aldehyde derivatives with 4-pyridine acetonitrile
General synthetic procedures
4-Pyridyl acetonitrile hydrochloride [1.6 mmol, 1 eq.] and as depicted in Scheme 1. The corresponding aldehydes of C-1,
0.5 mmol of piperidine were dissolved in 10 mL anhydrous J-1 and T-1 were synthesized using previously published literature
methanol. After 10–15 min of stirring, the desired aldehyde procedures.19,33,34 The commercially obtained aldehydes of C-3
(1 eq., 1.6 mmol) was added to the reaction solution and it was and DM were used without further purification. The structures of
heated to 60 1C for 5 h. The reaction was monitored by thin the cyanostilbenes investigated in this study are depicted in Fig. 1
layer chromatography (TLC) to ensure the consumption of the and the molecules are coded as C-1 (N-phenyl carbazole), C-3
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Scheme 1 General synthetic strategy adopted for the synthesis of acrylonitrile derivatives.
(N-ethyl carbazole), J-1 (julolidine), DM (dimethylamino) and of C-3 is slightly red-shifted due to the enhanced planarity
T-1 (triphenylamine).
achieved through direct conjugation of acrylonitrile with the third
position of carbazole. Compounds DM and T-1 possessing
dimethylamine (DM) and triphenylamine (TPA) donating
segments respectively show similar absorption maxima. The
Absorption and emission studies
The absorption and emission spectra data of the compounds larger absorption maximum (la) for J-1 is ascribed to its increased
recorded in various solvents are shown in Table 1. Among planarity and the electron donating nature of the amino unit of
the compounds studied, C-1 with N-carbazole substitution julolidine. The overall order of absorption is: J-1 4 T-1 B DM 4
possesses the lowest absorption maximum, whereas J-1 with C-3 4 C-1. Upon changing the polarity of the solvents, weaker
julolidine substitution yields the highest absorption maximum solvatochromic absorption behavior was noted except for J-1,
(Fig. 2A). The lower absorption of C-1 is attributed to the poor which exhibits greater solvatochromic shifts (Fig. 2B and Fig. S1,
overlap of orbital interactions due to the large torsional distortion ESI†). The broad absorption profiles noted in water are an
between the carbazole unit and phenyl group.35,36 The absorption indication of aggregate formation. The absorption band of
Table 1 Absorption and emission data of the acrylonitrile derivatives investigated
DM
J-1
C-1
C-3
T-1
Solvents
la
lf
ffa
la
lf
ff
la
lf
ff
la
lf
ff
la
lf
ff
THF
Dioxane
DMF
DMSO
MeOH
H2O
420
418
420
424
425
445
—
546
520
580
591
586
564
609
0.28
0.26
0.20
0.17
0.11
0.35
442
436
451
457
455
462, 513
—
502
499
525
532
526
542
558
0.15
0.17
0.10
0.09
0.06
0.02
384
386
381
383
383
406
—
514
488
552
564
571
531
548
0.65
0.62
0.67
0.66
0.25
0.88
392
389
395
398
398
399
—
450
442
458
459
461
552
574
0.13
0.11
0.19
0.17
0.13
0.27
413
408
422
426
422
421
—
487
484
502
511
500
601
627
0.03
0.03
0.14
0.04
0.01
0.11
Solid
a
Relative quantum yields determined using quinine sulfate (quantum yield: 0.545 0.1 N H2SO4).
Fig. 2 (A) Absorption spectra of the investigated acrylonitrile derivatives in THF (10 mM); (B) absorption spectra of DM recorded in various solvents.
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T-1 is weak with the band at 420 nm and a shoulder at 502 nm substitution at the principal axis leads to intramolecular charge
and J-1 shows two absorption bands at 462 nm and 533 nm transfer, whereas in the current study, the cyano unit on the
(Fig. 2 and Fig. S1, ESI†).
double bond is positioned vertical to the principal axis. Owing
The variation in emission wavelength with changes in to the dynamic intramolecular rotations of the a-cyano unit
solvent polarity is stronger as compared to their spectral with respect to the principal axis of the molecular framework in
absorption changes. The emission profiles exhibit a positive a vertical direction, its solvatochromic behavior distinctly varies
solvatochromic behavior and substantiate the intramolecular for each compound. The reason behind the variations needs
charge transfer (ICT) from the amino donor group to the further investigation.
electron withdrawing cyano and pyridine units.37 Depending
on the strength of the donating group, variable emission shifts
Molecular geometry and frontier energy levels
were noted with DM and C-1 showing a 470 nm shift from To gain deeper insights into the structure and aggregation relation-
dioxane to DMSO [Fig. 3 and Fig. S2, ESI†]. Moderate emission ship of the acrylonitrile derivatives with respect to the donor
shifts have been noted for J-1 (36 nm), C-3 (17 nm) and T-1 substitution, we studied their geometrical, electronic and spectro-
(27 nm). The charge transfer nature of these compounds was scopic properties at the molecular level via DFT/TDDFT methods. In
also qualitatively shown using Lippert–Mataga plots [Fig. S2a, general, the charge transfer propensity is majorly dependent on the
ESI†]. However, unique emission is noted in water where the degree of p-conjugation that is measured via torsional distortion
emission intensity increased from 4.7 to 60 fold depending on between the individual functional building blocks, the donor,
the nature of the substituent. This unique emission in aqueous p-conjugated bridge and acceptor units, present in the molecular
media is attributed to the formation of aggregates. The framework. The higher the planarity, the more efficient the charge
emission maximum observed in the solid state is greater than transfer due to the better overlap of p-orbital interactions. The
the solution phase due to the strong molecular packing in molecular geometries of all the compounds have been optimized
the solids (Fig. S3, ESI†).38,39 It is known that para-cyano using the B3LYP/6-311G(d,p) level of theory.40
Fig. 3 Emission spectra of C-3, DM, and T-1 measured in various solvents.
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Fig. 4 Optimized molecular geometries of the acrylonitrile derivatives with selected torsional angles and bond distances.
Compound C-1 contains three segments (carbazole, phenyl levels, the HOMO and LUMO, calculated from the oxidation
and the acceptor) as shown in Fig. 4 and the remaining compounds potentials of the compounds measured using the CV technique
comprise two parts, i.e., donors linked to the pyridyl unit via an and their comparison with DFT predicted results are displayed
a-cyano moiety. In C-1, the phenyl unit connected to the carbazole is in Fig. 5 and 6, and the corresponding data are summarized in
highly distorted in an out-of-plane fashion with a dihedral angle of Table 2. All the compounds exhibited quasi-reversible oxidation
51.31 on one side and a slight twist of 5.31 with the vinylene part on peaks except J-1 where the oxidation process was found to be
the other side.35 The a-cyano moiety of the C-1 group deviates with a irreversible. The HOMO energy level of the molecules is calculated
distortion of 26.11 from the terminal pyridyl acceptor unit. This from the empirical formula EHOMO = ꢀe[4.8 + Eox] eV. The optical
mainly originates from the steric repulsion induced between the band gap (E0-0) obtained from the absorption onset recorded in
a-cyano unit and ortho-hydrogens of the terminal pyridyl unit. DMF and the LUMO is computed from the following equation:
The other molecules C-3, DM, J-1, and T-1 are two-segmented; the ELUMO = [E0-0 ꢀ EHOMO] eV.
donors maintain co-planarity with the vinylene chain with slight
The pronounced torsional distortion of the carbazole–phenyl
torsional distortions in the range of 3.1–3.81. On the other hand, a segment in C-1 abruptly lowers the HOMO. Connecting the third
significant deviation is seen between the a-cyano unit and pyridyl position of the carbazole in C-3 destabilized the HOMO to a
group for all these compounds in the range of 21.4–23.81. The certain extent due to planarity enhancement. It should be noted
geometrical coordinates of the compounds are provided in Fig. S4 that the conjugation length is remarkably reduced from C-1 to
(ESI†). It can be seen from the molecular geometries that the C-3, which is reflected in their LUMO energy levels i.e., the LUMO
N-atoms of the donor group and the a-cyano moiety are in the of C-1 is lower than that of C-3. Compared to C-3, the HOMO of
same line as the principal axis. The measured bond distances DM is further lowered because of slightly lower delocalization of
between the central styryl carbon and the cyano units are found the amino unit. The strong donating propensity of J-1 leads to
to be around 3.5 Å. The bond distances measured between the pronounced HOMO destabilization and it has the smallest band-
N-atom and the terminal carbon of the donor group are in the gap in the series. The HOMO of T-1 is nearly identical to that of
following order (units in Å): C-3 (7.3) 4 T-1 (5.3) 4 C-1 (4.6) 4 J-1 C-3. The bandgaps of the compounds calculated from the optical
(3.9) 4 DM (2.1). The steric environment caused by the donor units, profiles are in the following order (eV): J-1 (2.61) o C-3 (2.75) o
the distance between the a-cyano unit and donors, and the flexibility T-1 (2.77) o C-1 (2.79) o DM (2.90) [Table 2]. Nevertheless, the
and rigidity imparted by the fused aliphatic chains and aromatic HOMO and LUMO energy levels and the HOMO–LUMO gap
groups may affect the aggregation propensity.
predicted using the DFT/B3LYP/6-311G(d,p) level of theory
To evaluate the redox behavior and energy levels of the undergo little variation, but follow a consistent trend with the
compounds, the cyclic voltammetry (CV) technique was per- experimental energy levels measured [Table 2].
formed in dry dimethylformamide (DMF) containing 0.1 M
Electron density distribution
tetrabutylammonium tetrafluoroborate (TBAF) as a supporting
electrolyte using a 1 mm diameter glassy carbon disc electrode To delve into the type of charge transitions corresponding to
as a working electrode, while a Pt wire and Ag/AgCl in DMF the longer wavelength absorption, whether a p–p* or charge-
solution were used as a counter and reference electrode, transfer transition, we carried out electron density population
respectively. Ferrocene was used as an internal standard during analysis. The electron density of the HOMO in C-1 dominantly
this experiment, and all the potential values were referenced localizes on the carbazole unit, and the extent of delocalization
against the ferrocene couple (FeCp2+/0). The frontier energy was limited to the a-cyano unit without reaching the terminal
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Fig. 5 Schematic diagram illustrating the energy level comparison of the acrylonitrile derivatives estimated from electrochemical-optical profiles and
DFT predicted values along with their computed isodensity (0.02) surfaces of the frontier molecular orbitals involved in S0 - S1 transitions.
pyridyl group. On the other hand, the electron density distribution is didn’t follow the trend of the experimental results and under-
delocalized over the entire molecular framework for the remaining went significant deviation [Fig. 8]. Despite having the same
compounds in the series. In the LUMO, the electron density pattern as the experimental values, the long-range corrected
population is transferred from the donor unit to the a-cyano oB97XD functional provides absorption values with a substantial
and terminal pyridyl groups, but the extent of electron density underestimation of 30–60 nm. In comparison to the other func-
delocalization is majorly governed by the donor units attached. tionals, the results obtained from M06-2X matched well with little
This indicates the presence of ICT transitions in the acrylo- underestimation, and the deviations are in the permissible range
nitrile derivatives. Unfortunately, it is hard to identify the donor’s (Table 3). It is known that the oscillator strength is dependent on
contribution toward the total charge population of the molecules. the wavefunction overlap between the frontier molecular orbitals
Hence, to compute the electron density contribution of the HOMO and LUMO upon photoexcitation. If the overlap is weak,
individual segments toward the charge density population of lower will be the oscillator strength (f).46 In our case, C-1 possesses
the units in terms of molecular orbital coefficients, we have a low f value due to the reduced overlap of orbital interactions
carried out projected density of states (PDOS)41,42 analysis by between the highly twisted carbazole and phenyl groups. As a
grouping the atoms into three segments, donor (D), a-cyano consequence, the major transition involved in the absorption
(CN) and pyridyl (Py) units [Fig. 7]. In the HOMO, we found that maximum is also significantly reduced. C-3 and T-1 come
the pronounced torsional distortion between the carbazole and second in the order [Fig. 8]. The transient dipole moment (me)
phenyl groups is conducive for 94% electron density localization is found to be higher than the ground state. This observation
in C-1. A good electron delocalization characteristic feature has corroborates the positive solvatochromism behavior of the
been found for the other compounds. The electron density a-cyanostilbene derivatives.
distribution of the LUMO distinctly varies with respect to the
donor units. For instance, a marginal increment of 26% electron
AIEE studies
density population is noticed on the a-cyano group in C-1, which To investigate the influence of different amino donor groups on
in turn reduces the electron density of the terminal pyridyl the AIE effect of acrylonitrile derivatives, we measured the
segment. In the two-segment systems (C-3, DM, J-1, and T-1), emission profiles of the compounds by varying the composition
the pyridyl unit prevails to retain the electron density over the of dioxane–water binary mixtures. All the synthesized deriva-
a-cyano group. This analysis would be helpful to understand the tives exhibit the AIE effect in the dioxane–water binary combi-
donor’s influence on the AIEE activity of a-cyano groups in nation, but the emission intensity and wavelength shift of the
the excited state.
aggregates distinctly differ concerning the donor substitution.
To simulate the photophysical features and major transitions In the case of C-1, C-3, and DM, a gradual decrement was
involved corresponding to the maximum spectral region of the observed in the emission intensity along with a bathochromic
compounds, we used the TDDFT approach using different density shift upon the addition of a 70–80% water fraction into
functionals, TDDFT/B3LYP, M06-2X, and oB97XD, with a dioxane. At 100%, a new and broader band was noticed at a
common 6-311G(d,p) basis set.43 As the absorption spectra of longer wavelength with higher intensity (C-1: 44 nm, C-3:
the compounds were recorded in 1,4-dioxane, we have used the 107 nm, DM: 38 nm, J-1: 101 nm; T-1: 122 nm) indicating the
same solvent medium under the CPCM framework to mimic formation of J-aggregates [Fig. S5 (ESI†) and Fig. 9]. The nature
the real environment.44 The simulation of a significant portion of aggregation is not only dependent on the donor units but
of the absorption spectrum and the interpretation of the nature also on the positional substitution of the a-cyano moiety in the
of the transitions were executed using GaussSum 3.0.2.45 molecular framework. For instance, the a-cyano group placed at
Of these functionals, the B3LYP functional simulated values the central position mostly leads to H-type aggregation, and the
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Fig. 6 Cyclic voltammograms of the compounds measured in DMF using a standard three-electrode configuration at a scan rate of 50 mV sꢀ1
.
Table 2 Data obtained from the optical-electrochemical profiles and energy levels obtained from DFT predictions
labs
lmono
(nm)
Emission intensityb
lem shift
Stokes shift
a
a
c
Code
(nm)
laggr (nm)
(ꢁ106 a.u.)
(nm)
(cmꢀ1
)
Eox (V)
HOMOe (eV)
E0-0 (eV)
LUMOg (eV)
b
d
f
C-1
C-3
DM
J-1
386
389
420
437
408
488
554
522
494
474
531
558
560
601
603
7.93
0.79
1.97
0.10
0.68
43
4
38
102
136
7074
7786
4652
6244
7926
0.95
0.56
0.66
0.24
0.54
ꢀ5.75
ꢀ5.36
ꢀ5.46
ꢀ5.04
ꢀ5.34
2.79
2.75
2.90
2.61
2.77
ꢀ2.96
ꢀ2.75
ꢀ2.90
ꢀ2.61
ꢀ2.77
T-1
a
b
c
Absorption and emission spectra of the monomers recorded in dioxane. Emission spectra of the aggregates and their intensity. Emission
d
wavelength shift from monomers to aggregates. Measured in DMF with 0.1 M tetrabutylammonium perchlorate (TBAPC) as a supporting
electrolyte with a scan rate of 50 mV sꢀ1
.
Deduced from the formula HOMO = ꢀe[4.8 + Eox]. Bandgap obtained from the absorption onset of the
e
f
g
compounds. LUMO = [E0-0 ꢀ EHOMO].
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Fig. 7 Percentage molecular orbital density contribution from individual segments, the donor, a-cyano unit, and pyridine, to the charge density
population of the acrylonitrile derivatives.
Fig. 8 TDDFT simulated optical absorption profiles along with their oscillator strength using various functionals, the B3LYP, M06-2X and oB97XD/
6-311G(d,p)/CPCM levels of theory.
Table 3 TDDFT simulated absorption values corresponding to major S0–S1 transitions and ground and transient dipole moment
Code lB3LYP (nm) loB97XD (nm) lM06-2X (nm) Major transitionsc
HOMOb (eV) LUMOb (eV) EH–L (eV) mg (D) me (D) Energy (Hartrees)
a
a
a
b
c
d
C-1 455 (0.50)
C-3 412 (0.72)
343 (1.17)
353 (1.06)
376 (0.97)
378 (1.04)
HOMO - LUMO (72%), ꢀ5.85
Hꢀ2 - LUMO (21%)
ꢀ2.75
ꢀ2.51
3.10
3.07
3.99
7.73
4.93 ꢀ1165.53
9.65 ꢀ1013.08
HOMO - LUMO (87%), ꢀ5.55
Hꢀ2 - LUMO (5%),
Hꢀ1 - LUMO (2%)
DM 411 (1.11)
367 (1.23)
385 (1.22)
379 (1.31)
403 (1.22)
423 (1.21)
401 (1.18)
HOMO - LUMO (94%), ꢀ5.54
Hꢀ1 - LUMO (2%)
ꢀ2.68
ꢀ2.43
ꢀ2.46
2.86
2.88
3.09
8.63
9.28
7.09
10.69
11.79
ꢀ783.15
ꢀ938.04
J-1
430 (1.12)
HOMO - LUMO (94%), ꢀ5.31
Hꢀ2 - LUMO (2%)
T-1 459 (1.04)
HOMO - LUMO (87%), ꢀ5.55
8.20 ꢀ1166.71
Hꢀ1 - LUMO (6%)
a
Values obtained from various density functionals (B3LYP, M06-2X and oB97XD)/6-311G (d, p)/C-PCM (1,4-dioxane) levels of theory, and oscillator
b
c
strength values are given in parenthesis. Values obtained from the DFT/B3LYP/-311G(d,p) level of theory. Ground state dipole moment.
d
Transient dipole moment.
placement at the terminal position renders J-type aggregation.2 the a-cyano moiety must interact freely with the neighboring
It should be remembered that the AIE induced by tetraphenyl- molecules for efficient aggregation. Bulky donor/acceptor
ethene (TPE) derivatives is mainly based on freezing the intra- substitution closer to the a-unit might hamper the desired
molecular motions of four dynamic free phenyl rings, resulting aggregation effect.47,48 Keeping these effects in mind, we
in emission intensity enhancement, but minimal variation examined the impact of the donor’s substitution on the acry-
in their emission wavelength shifts. Unlike the TPE or HPS lonitrile scaffold.
derivatives, the AIE effect in acrylonitrile systems occurs via J or
In C-1, the phenyl p-linker keeps the a-cyano moiety away
H-type aggregation through intermolecular interactions along from the carbazole donor, thereby allowing efficient aggregation.
with restriction of intramolecular rotation and intramolecular The restriction of intramolecular rotation of the carbazole–phenyl
planarization in a synergistic way. As discussed earlier, the donor segment upon aggregation increases the rigidity of the molecular
or acceptor substitutions introduced on the a-cyanostilbene system. As a result, the emission intensity of the C-1 aggregates is
scaffold are of paramount importance for the AIEE effect, i.e., exceptionally high as compared to the other compounds. For C-3,
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Fig. 9 AIE studies of the acrylonitriles performed in a dioxane–water binary solvent system. The inset represents the compounds with weak AIE
characteristics.
the proximity of the a-cyano moiety to the carbazole group and the unit with the neighboring unit yield a bathochromic shift due to
steric effect of N-alkyl substitution lead to minimal AIE effect/ J-type aggregation. The crystal structure reported by Percino et al.
emission wavelength shifts. On the contrary, DM containing the also shows the propensity of the molecule to form J-aggregates.38
smaller dimethylamino group didn’t perturb the a-cyano unit, In T-1, the bulky triphenylamine unit hampers the aggregation of
and a comparable AIE intensity with a wavelength shift of 38 nm a-cyano groups and the same pattern is observed in the case of J-1.
upon aggregation was observed. Among the compounds studied, The more flexible side chains of J-1 reduced the AIE emission
julolidine possesses strong electron-donating propensity and intensity and showed lower aggregation behavior. From these
undergoes efficient J-type aggregation but shows only minor studies, we conclude that the emission in an organic/water mixture
emission intensity enhancement due to its cyclohexyl chain. The gradually shifts from aggregation-induced emission (AIE) charac-
AIE peak of T-1 is higher in emission intensity as compared to J-1 teristics to aggregation caused quenching (ACQ) in the following
and this can be assigned to the restriction of intramolecular order C-1 4 DM 4 C-3 4 T-1 4 J-1. It is solely based on the bulky
rotation of the two phenyl rings that undergo restriction of nature and rigidity of the donor units and their interaction with
molecular motions upon aggregation. J-1 and T-1 showed nearly the a-cyano motif.
the same AIE emission wavelength maxima (of B600 nm). A
In comparison to their emission profiles in organic solvents,
significant shift of 121 nm has been found for T-1 because of its the compounds exhibited a distinct bathochromic shift in
absorption at a shorter wavelength over J-1. We also examined the water, which is a clear indication of molecules undergoing
stability of the compounds based on the energy obtained through aggregation. The possible aggregation sites considered in these
DFT optimized results. The obtained values demonstrated that derivatives are the a-cyano and pyridyl units, which can interact
the energy of T-1 was found to be more stable due to its prominent with the positive sites of the donor parts from neighboring
delocalization over the triphenylamine segment, and for C-1 it can molecules. Hence, the aggregation is mainly determined by the
be assigned its extended conjugation of the molecular system.
negative charge density populated on the N-atom of the cyano
The results obtained from this study reveal that the aggregation and pyridyl units, respectively. For this purpose, we performed
propensity distinctly varies with various donor substitutions such Mulliken population analysis to examine the charge density
as carbazole, triphenylamine, julolidine, and the smaller dimethyl- population on each atom of the title compounds (Fig. S6, ESI†).
amino group. Carbazole and triphenylamine have bulkier and
This analysis is explicitly sensitive to the basis set choice but
rigid donor units, and julolidine is made of fused and flexible can provide the optimal results in a qualitative trend upon
aliphatic chains. In C-1, although the carbazole donor is bulky, the increasing the size of the basis set. For this purpose, we used
cyano unit is distal to the donor group and can interact efficiently. the common but larger 6-311++G(d,p) basis set for all the
However, the sizeable torsional deviation between the carbazole resultant compounds, which produces reliable results. Accord-
and phenyl segments leads to poor overlap of orbital interactions, ing to this study, the charge population on the N-atom of the
enabling hypsochromic emission shifts as compared to the others cyano moiety in all the compounds is given in the following
in the series. It should be noted that the rigidity of the donor unit order: C-1 (ꢀ0.233) o T-1 (ꢀ0.242) E C-3 (ꢀ0.243) o DM
enhanced the AIE intensity but not the wavelength. Likewise, (ꢀ0.247) o J-1 (ꢀ0.252) and the order of the charge population
substitutional changes in C-3 result in dramatic reduction of the on the N-atom of the pyridyl group is: C-1 (ꢀ0.285) o T-1
AIE emission intensity. In DM, bathochromic emission is observed (ꢀ0.289) E C-3 (ꢀ0.289) o DM (ꢀ0.291) E J-1 (ꢀ0.252). In
where the size of the donating group has a weaker influence on the addition, the negative charge density on the N-atom from the
interactions of the cyano unit. Thus, free interactions of the cyano amino donor part indicates its electron donation strength and
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the order obtained is: C-1 (ꢀ0.599) 4 J-1 (ꢀ0.558) 4 C-3 mainly originated from the emission due to the restriction of
(ꢀ0.541) 4 T-1 (ꢀ0.494) 4 DM (ꢀ0.482). Despite the larger intramolecular rotation of the donor segments. For instance,
electron density on the N-atom in C-1, the large distortion the greater emission lifetime of T-1 can be ascribed to freezing
between the carbazole and phenyl segments hampers its donat- the dynamic rotations of the two phenyl rings of the triphenyl-
ing strength. We also performed electrostatic surface potential amine donor segment. This helps to rigidify T-1 upon aggregation
(ESP) analysis of all the compounds and the results are depicted in an efficient way, yielding a longer emission lifetime. For C-1,
in Fig. S7 (ESI†). Alkyl substituted amino group such as dimethyl- the IMR of the carbazole–phenyl segment yields an average
amino units, julolidine, N-ethyl unit of carbazole bears a positive emission lifetime as compared to T-1. Compound C-3 bearing
density population whereas the carbazole of C-1 possesses neutral bulky and aromatic carbazole shows a considerably larger lifetime
charge density.
than DM. The third exponential decay is absent in the case of T-1,
implying that the bulky phenyl rings of the TPA donor perturb the
AIE activities of a-cyanostilbenes to a greater extent. On the
contrary, the carbazole unit in C-1 keeps the cyano moiety away
and allows the molecule to undergo efficient J-type aggregation.
The emission lifetime of C-1 corresponding to the third
exponential is found to be the largest in the series. In comparison
to that of C-1, the emission lifetime of C-3 is marginally
lowered. The smaller dimethylamino donor units’ influence
on the a-cyanostilbenes may be inferior in DM and subsequently
it possesses the lowest emission lifetime (Fig. 10). The second and
third exponential decays can be attributed to the RIR of the donor
and a-cyanostilbene segments, respectively. The observation from
this study demonstrated that the rigidification of the molecular
framework is proportional to the emission lifetime.
Lifetime study
As previously discussed, although various working mechanisms
have been reported behind the AIE phenomena, we found that
two factors mainly determine the AIE characteristics of the
compounds in this study. One is freezing of molecular motions
upon aggregation, helping to open up the radiative channels for
energy dissipation. Second is restriction of intramolecular
rotation of the a-cyano unit and its subsequent planarization,
allowing the molecule to undergo efficient H or J-type aggrega-
tion. Previous research demonstrated that the fluorescence
quantum yield produced by H-type aggregates is remarkably
higher than that of the J-type counterparts, and it is reflected in
their emission lifetimes.2 Before examining the aggregates, it is
important to understand the emission lifetime of the mono-
mers for better comparison. Measurement of the time-resolved
SEM and DLS study
fluorescence lifetime shows monomer emission lifetimes of a The morphology of the compounds was assessed using scanning
few tens of picoseconds, implying their preference for energy electron microscopy and dynamic light scattering measurements.
dissipation through non-radiative channels and due to the The SEM images are shown in Fig. 11 and show disordered
dynamic intramolecular rotation of a-cyanostilbenes. On the aggregates for J-1 and lump like structures for C-1, C-3, and
contrary, the emission lifetime of the aggregates is enhanced DM. A network pattern is noted for T-1. Dynamic light scattering
with a multi-exponential decay. The aggregates of C-1, C-3, and study is used to determine the size of the aggregated particles in
DM exhibited tri-exponential decays, whereas T-1 shows a two- solution [Table S1, ESI†]. All the a-cyanostilbenes have shown a
exponential decay (Fig. 10). In the case of J-1, we were unable to polydispersity index (PdI) of more than 0.2, which is an indication
perform the lifetime decay studies due to its minor emission of the polydisperse nature of the aggregated molecules (Fig. S9,
enhancement. A minimal lifetime of the first exponential decay ESI†). However, the mean diameter of the particles in C-1 is
can be assigned to the possible existence of monomers in the measured to be 172.2 nm, where the aggregation was defined by
solution of aggregates. The second exponential decays of the the a-cyano unit. In the case of T-1, this value is abruptly increased
C-1, C-3 and DM aggregates are in the following order (ns): T-1 to 3940 nm where the aggregation originated from the restriction
(9.23) 4 C-1 (3.44) 4 C-3 (1.84) 4 DM (0.82). These values of intramolecular rotations of phenyl rings from the TPA donor.
Fig. 10 Lifetime decay profiles of C-1, C-3, DM and T-1, and their corresponding data.
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Fig. 11 Drop-cast SEM of J-1 and T-1 [Fig. S8, ESI†].
The PdI value of C-1 illustrated a monodisperse nature. The order Conflicts of interest
of the Z-average value for all a-cyanostilbene molecules is in the
There are no conflicts to declare.
following order (units in nm): T-1 4 C-3 4 J-1 4 DM 4 C-1.
Acknowledgements
Conclusions
The authors acknowledge the Bioinformatics Resources and
Applications Facility (BRAF) C-DAC, Pune for the computational
facilities, Board of Research in Nuclear Sciences (37(2)/14/05/
2016) for the funding and IIT Gandhinagar for infrastructural
facilities.
In summary, we synthesized and characterized a series of
4-pyridyl acrylonitriles bearing different donating groups
[carbazole (C-1 and C-3), dimethylamino (DM), julolidine ( J-1)
and triphenylamine (T-1)]. The restrictions imposed due to the
intramolecular rotation of the aromatic ring structure or donat-
ing groups can explain the changes in the AIE behavior. The
energy levels obtained from the optical-electrochemical profiles
distinctly vary with the nature of the donor substitution.
The bulkiness of the donor substitution has a great impact
on the aggregation propensity of the acrylonitrile derivatives.
The AIE characteristics of the compounds gradually decreased
in the order C-1 4 DM 4 C-3 4 T-1 4 J-1. The rigid carbazole
unit and distal substitution of the cyano unit (C-1) yield a
greater AIE effect due to efficient interactions with neighboring
units. The significant differences found between the AIE inten-
sities in T-1 and J-1 reveals the impact of the rigidity of the
donor unit and its influence on the a-cyano unit, i.e., moving
from AIE to ACQ. The absence of distinct rigidity caused by the
aromatic rings in DM reduced the AIE intensity but yielded a
bathochromic shift. Electron density distribution and projected
DOS analysis of the compounds obtained from DFT and TDDFT
methods reinforced our experimental observations. The AIE effect
and lifetime decay analysis of the compounds are scrutinized to
probe the structure–property relationship of the acrylonitrile
derivatives. Eventually, we found that the geometrical impact of
substitutions, donors, and their substitutions such as alkyl/
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