Styrylbenzene organogels and how the cyano groups tune the aggregation-induced emission

Article abstract of DOI:10.1016/j.dyepig.2021.109427

Fluorescence switching of π-gelators has emerged as a field of great interest because of their promising applications. In this work we present two families of luminogens based on bis(styrylbenzene) and bis(α-cyanostyryl)benzene that incorporate acetamido groups and hydrocarbon chains to induce gelation. The presence of cyano groups in the molecular skeleton produces Aggregation Induced Emission (AIE) from solution to the gel while their absence gives the Aggregation Caused Quenching (ACQ) effect. Density Functional Theory calculations were performed in order to shed light on the origin of these opposite phenomena. The bis(styrylbenzene) compound showed a high quantum yield in solution and nearly planar structure was theoretically predicted. A significant drop in the quantum yield was observed after gelation, probably due to the π-π intermolecular interactions favoured by the planar scaffold. In contrast, the more twisted structure predicted for the cyano compounds could prevent such π-π intermolecular interactions and this would favour fluorescence emission. In addition, the twisted molecular structure predicted for the cyano derivative could induce Z/E photosiomerisation that suppresses the emission in solution. However, the framework of intermolecular contacts that arises during the gelation process could restrict torsional motions to the detriment of photoisomerization, thus resulting in AIE-active compounds. Furthermore, the reorganisation energy and Huang–Rhys factors associated with the non-radiative relaxation were calculated. The highest values were found for some vibrational modes in the low energy region in the case of the cyano compound and these could contribute to the non-radiative relaxation in solution. These intramolecular motions could be blocked in the gel, thus increasing the fluorescence emission. The introduction of cyano groups in the carbon scaffold could therefore provide a strategy for the rational design of new active AIE organogels.

Full text of DOI:10.1016/j.dyepig.2021.109427

Dyes and Pigments 192 (2021) 109427
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Styrylbenzene organogels and how the cyano groups tune the
aggregation-induced emission
,
,b,*
Rocío Domíngueza b, Amparo Navarroc, Joaquín C. García-Martíneza
a
´
´
´
´
´˜
Universidad de Castilla-La Mancha, Departamento de Química Inorganica, Organica y Bioquímica, Facultad de Farmacia, C/ Jose María Sanchez Ibanez s/n, 02008,
Albacete, Spain
b Universidad de Castilla-La Mancha, Regional Center for Biomedical Research (CRIB), C/ Almansa 13, 02008, Albacete, Spain
c
´
´
Department of Physical and Analytical Chemistry, Faculty of Experimental Sciences, Universidad de Jaen, Campus Las Lagunillas, 23071, Jaen, Spain
A R T I C L E I N F O
A B S T R A C T
Keywords:
Fluorescence switching of
π
-gelators has emerged as a field of great interest because of their promising appli-
Organogel
cations. In this work we present two families of luminogens based on bis(styrylbenzene) and bis(
α
-cyanostyryl)
Aggregation induced emission
benzene that incorporate acetamido groups and hydrocarbon chains to induce gelation. The presence of cyano
groups in the molecular skeleton produces Aggregation Induced Emission (AIE) from solution to the gel while
their absence gives the Aggregation Caused Quenching (ACQ) effect. Density Functional Theory calculations
were performed in order to shed light on the origin of these opposite phenomena. The bis(styrylbenzene)
compound showed a high quantum yield in solution and nearly planar structure was theoretically predicted. A
α
-cyanostyrylbenzenes
Fluorescence
DFT
significant drop in the quantum yield was observed after gelation, probably due to the
teractions favoured by the planar scaffold. In contrast, the more twisted structure predicted for the cyano
compounds could prevent such intermolecular interactions and this would favour fluorescence emission. In
π-π intermolecular in-
π-π
addition, the twisted molecular structure predicted for the cyano derivative could induce Z/E photosiomerisation
that suppresses the emission in solution. However, the framework of intermolecular contacts that arises during
the gelation process could restrict torsional motions to the detriment of photoisomerization, thus resulting in
AIE-active compounds. Furthermore, the reorganisation energy and Huang–Rhys factors associated with the non-
radiative relaxation were calculated. The highest values were found for some vibrational modes in the low en-
ergy region in the case of the cyano compound and these could contribute to the non-radiative relaxation in
solution. These intramolecular motions could be blocked in the gel, thus increasing the fluorescence emission.
The introduction of cyano groups in the carbon scaffold could therefore provide a strategy for the rational design
of new active AIE organogels.
1. Introduction
exploration of new gels was originally a random process, but nowadays
it is known that gelators must have certain structural characteristics.
Organogels are semi-solid materials in which an organic solvent is
immobilized by a solute, the so-called gelator, to give a three-
dimensional network composed of self-assembled gelator fibres. In
recent years, low molecular weight organogels (LMOGs) have received
considerable attention in supramolecular chemistry and materials sci-
ence due to their potential applications in cosmetic formulations [1],
drug delivery [2,3], sensors [4–6], template synthesis [7], or optoelec-
tronics [4,8–10], amongst others [11]. In organogels the gelator
self-assembles usually through weak intermolecular interactions such as
One of these is the promotion of supramolecular interactions through
functional groups, e.g., hydrogen bond formation with urea or amide
groups. The presence of lipophilic groups such as hydrocarbon chains or
cholesterol is another important structural requirement to maintain the
subtle balance between solubility and precipitation of the gelator
molecule in a given solvent [3,11,12]. In any case, the presence of these
groups does not guarantee that a compound will form a gel since the
spatial arrangement, directionality, and rigidity of both the molecule
and the functional groups are critical for the gelation process.
electrostatic, dipole-dipole, hydrogen bonding and
π-
π
interactions. The
Among the different LMOGs, gelators based on
π
-conjugated organic
´
´
´
* Corresponding author. Universidad de Castilla-La Mancha, Departamento de Química Inorganica, Organica y Bioquímica, Facultad de Farmacia, C/ Jose María
´
´˜
Sanchez Ibanez s/n, 02008, Albacete, Spain.
E-mail address: JoaquinC.Garcia@uclm.es (J.C. García-Martínez).
https://doi.org/10.1016/j.dyepig.2021.109427
Received 8 March 2021; Received in revised form 22 April 2021; Accepted 22 April 2021
Available online 13 May 2021
0143-7208/© 2021 Elsevier Ltd. All rights reserved.

R. Domínguez et al.
Dyes and Pigments 192 (2021) 109427
Scheme 1. Molecular structures of compounds 1–5.
compounds, also known as
π
-gelators [12], have attracted significant
in order to establish structure-property relationships that could aid the
rational design of new materials.
attention as photo-responsive materials due to promising properties and
applications of the so-called ‘π-gels’ [11,13]. The intermolecular in-
teractions established in most luminescent organic molecules induce the
so-called aggregation-caused quenching (ACQ) of emission in the gel
phase, which could limit the performance of optical devices. The con-
2. Materials and methods
2.1. Measurements and characterisation
trary
effects,
i.e.,
aggregation-induced
emission
(AIE),
aggregation-induced enhanced emission (AIEE) and solid-state lumi-
nescence enhancement (SLE), could offer a potential solution to this
challenge since the materials are non-emissive when they are dissolved
in good solvents but highly emissive in the gel or solid state as aggre-
gates [14–22]. Thus, it is desirable to design and synthesise new gelators
with an AIE or AIEE effect since organogels are thermoreversible and
thermal fluorescence modulation could be achieved. Although the syn-
1H NMR and 13C NMR spectra were acquired at room temperature.
The chemical shifts (δ) are given in ppm and are referenced to the re-
sidual protons of the deuterated solvent or carbon nuclei of chloroform
(1H, δ = 7.27 ppm; 13C, δ = 77.0 ppm) or DMSO (1H, δ = 2.50 ppm; 13C,
δ = 39.5 ppm). Acidic impurities in CDCl3 were removed by treatment
with anhydrous K2CO3. IR spectra were recorded on an FT-IR spectro-
photometer equipped with an ATR accessory and the main peaks are
given in cmꢀ 1. MALDI-TOF mass spectrometry was carried out on an
Autoflex maX (Bruker). Quartz cuvettes (Hellma Analytics) of 10 mm
were employed for all the absorption and emission measurements in
liquid samples. UV–Vis absorption spectra were acquired in a V-650
(Jasco) spectrophotometer at a scan rate of 600 nm minꢀ 1. A Peltier
accessory was employed to control the temperature of the spectropho-
tometer measuring cell. Fluorescence emission spectra were recorded on
an FLS920 spectrofluorometer (Edinburgh Instruments) equipped with a
time correlated single photon counting (TCSPC) detector. A TLC 50
temperature-controlled cuvette holder (Quantum Northwest) was used
for the measurements (the temperature was kept constant at 298 K
except for the temperature-dependence experiments). Quantum yields
were measured on an FS5 spectrofluorometer (Edinburgh Instruments)
equipped with an integrating sphere, a 150 W Xe lamp as the light source
and a TCSPC detector. Quantum yield calculations were carried out
using F980 Software from Edinburgh Instruments.
thesis of α-cyanostilbenes has been known since 2000 [23,24], their
incorporation into organogel structures is relatively recent [15,25–31].
These compounds have shown remarkable optical features in addition to
interesting electrical properties, and hence they are recognised as very
suitable and versatile options for the development of functional mate-
rials [32]. Most cyanostilbene-based compounds produce AIEE in the
aggregated state, solid state and organogels. To our knowledge, com-
parison of the physical and optical properties of two families of π-gelators
that differ only in the presence of a nitrile in the alpha positions of the
stilbene has rarely been considered [33]. The work described here
concerned the synthesis and study of two families of small molecules,
based on bis(styrylbenzene) and bis(α-cyanostyryl)benzene, that incor-
porate two strategic groups for gel induction, namely an acetamido
group and a hydrocarbon chain (Scheme 1). The appropriate arrange-
ment of these polar and non-polar functional groups in the chemical
structure is critical for organogel formation. In order to determine its
role in the gel formation, the related molecules were also prepared but
with some parameters modified, such as the length or the absence of an
alkoxy chain and the absence of the acetamido groups (Scheme 1). Thus,
molecules 1–3 could form organogels and self-assemble through inter-
molecular hydrogen bonding through neighbouring acetamido groups.
Furthermore, the important role that the cyano group plays in tuning
the photophysical properties of the organogel was studied. The intro-
duction of this electron-withdrawing group in a series of styrylbenzene
derivatives was extensively studied by Hanack et al., who concluded
that the cyano group produces a red shift in the optoelectronic proper-
ties and reduces the optical band gap [34,35]. The thermal and optical
properties were investigated and, from a theoretical perspective, the
molecular geometry and photophysical properties of the new com-
pounds were also studied by means of Density Functional Theory (DFT)
2.2. Electrochemical experiments
Cyclic voltammetry (CV) and Osteryoung square wave voltammetry
(OSWV) experiments were carried out in ODCB/acetonitrile 4:1 solu-
tions. Tetrabutylammonium perchlorate (0.1 M as supporting electro-
lyte) was purchased from Merck and was used without purification.
Solutions were deoxygenated by nitrogen bubbling prior to each
experiment, which were run under a nitrogen atmosphere. Experiments
were carried out in a one-compartment cell equipped with a glassy
carbon working electrode (ø = 2 mm), a platinum wire counter electrode
and an Ag/AgNO3 (0.01 M in the supporting electrolyte) electrode as
reference. The scan rate was 100 mV sꢀ 1. The ferrocene/ferrocenium
redox couple (Fc/Fc+) was used as an internal reference in order to
2

R. Domínguez et al.
Dyes and Pigments 192 (2021) 109427
Scheme 2. Synthetic route to 1–4.
determine the oxidation.
by Reimers [42] according to:
∑
∑
λ =
λi =
ℏωiSi
2.3. Scanning Electron Microscopy (SEM)
i
i
The microstructure was evaluated by Scanning Electron Microscopy
(SEM) using a Jeol 6490LV electron microscope equipped with SE and
BSE detectors and an Oxford Link EDS Probe. The samples were pre-
pared by placing a spatula tip of the organogel on top of a sampler stub
and allowing them to dry for 6 h at room temperature. Specimens for
SEM observation required coating (Au–Pt, Emitech) to avoid problems
related to the surface charging-up.
where ωi is the wavenumber associated with the vibrational mode i and
Si is the Huang–Rhys (HR) factor. The geometry optimizations for
clusters were performed in the gas phase at the PBE0/6-31G* level of
theory considering Grimme’s D3 dispersion correction with the original
D3 damping function as implemented in Gaussian D.01 [43].
2.5. Gelation test
A weighed sample of compounds 1–5 in 0.5 mL of the different sol-
vents (CH2Cl2, CHCl3, ethyl acetate, THF, ethanol, toluene, DMSO,
hexane, acetonitrile and DMF) were heated in screw-capped glass vials
in a water bath until the solid had dissolved. The sample was allowed to
cool down to room temperature, kept for a certain time and the aggre-
gation state was assessed. Organogel formation was tested by the ‘stable
to inversion of the test tube’ method. If flow was not observed on
inverting the vial, a stable gel had formed (G). If the solution was
retained, it was marked as soluble (S), and if a precipitate had appeared,
it was noted as precipitate (P). A sample was denoted Insoluble (I) when
the compound did not dissolve. Repeated heating and cooling confirmed
the thermoreversibility of the gelation process. The minimum gelator
concentration (MGC) was determined from the minimum amount of
compound required to achieve gel formation at room temperature.
2.4. Computational details
Full geometry optimisations for the ground and first excited states
were performed using the density functional theory (DFT) method
implemented in the Gaussian 09 programme package (version D.01)
[36]. PBE0 [37] and CAM-B3LYP [38] functionals were used along with
the 6-31+G** basis set. Both functionals have proven to be suitable to
estimate the singlet excited state of organic molecules in the compre-
hensive TD-DFT benchmark conducted by Jacquemin et al. [39] The
nature of the optimized geometries in the ground and excited states was
evaluated by computing vibrational frequencies to check the absence of
imaginary frequencies. The long alkoxy chains were replaced by
methoxy groups to reduce the computational cost. The effect of the
solvent was included by the polarisable continuum model (PCM) as
implemented in the Gaussian package [40,41]. Time-dependent DFT
calculations at the TD-PBE0/6-31+G** and TD-CAM-B3LYP/6-31+G**
levels of theory were performed in tetrahydrofuran (THF), toluene and
dichloromethane (CH2Cl2). The reorganisation energy upon excitation,
λ, was calculated using the programme DUSHIN, which was developed
3

R. Domínguez et al.
Dyes and Pigments 192 (2021) 109427
Scheme 3. Synthetic route to 5.
Scheme 4. Molecular structures and selected dihedral angles (in degrees) for the ground state calculated at the PBE0/6-31+G** level of theory for compound 1 in
CH2Cl2 solution and 3 in toluene solution.
3. Results and discussion
singlet at 10.3 ppm (1, 2 and 5, DMSO‑d6) or 7.3 ppm (3, CDCl3), which
was assigned to the proton of the amide group. Additionally, the singlet
associated with the proton of the vinylene group appeared at around 8
ppm in 1, 2 and 5, and at 7.59 ppm in 4. The E configuration of the
double bond in 3 was confirmed by coupling constants of around 16 Hz.
The stereochemistry of compound 1 was determined by performing NOE
experiments (Fig. S11). Irradiation of the vinyl proton signals at 8.02
ppm resulted in a response in both protons of the central aromatic ring
and vice versa. This suggests that, in solution, the molecule mainly has a
Z configuration for both double bonds, as shown in Scheme 4. Confor-
mational analysis was performed at the PBE0/6-31+G** and CAM-
B3LYP/6-31+G** levels of theory and the results were consistent with
the results obtained in the NMR experiments (see Scheme 4, Table S1
and Figs. S12–S15 in SI). The calculations allowed us to analyse the
structures in greater detail. Non-planar structures were predicted for
compound 1, with dihedral angles of ~30◦ between the central phenyl
ring and the two vinylene arms. However, more planar structures were
found for compound 3, with dihedral angles of ~10◦ (see
3.1. Synthesis and thermal properties
The synthetic strategy for the preparation of the new phenyl-
enevinylene derivatives is shown in Schemes 2 and 3. Firstly, alkylation
of 3,5-dimethylphenol with the corresponding halide (twelve or six
carbons in length) led to compounds 6 and 7, which were transformed
into the corresponding dibromides 8 and 9 by treatment with NBS in
CCl4 and irradiation with a 200 W IR heat lamp. Diacetonitriles 10, 11
and 13 were synthesised by nucleophilic substitution using sodium cy-
anide and tetrabutylammonium bromide in dichloromethane/water
under reflux. The final products 1, 2, 4 and 5 were obtained by a
Knoevenagel reaction in NaOMe/MeOH from the corresponding diac-
etonitrile and 4-acetamidobenzaldehyde (in the case of 1, 2 and 5) or 4-
tert-butylbenzaldehyde (in the case of 4). Derivative 3 was synthesised
by Horner–Wadsworth–Emmons reaction using potassium tert-butoxide
as base, 4-acetamidobenzaldehyde and diphosphonate 12, which was
prepared from dibromide 8 by an Arbuzov reaction.
Figs. S12–S15). All compounds were characterised by 1H NMR and 13
C
The 1H NMR spectrum of each diamide (Figs. S6–10) contained a
NMR spectroscopy, EI mass spectrometry and FT-IR spectroscopy to
4

R. Domínguez et al.
Dyes and Pigments 192 (2021) 109427
Fig. 1. TGA (up) and DSC (down) curves of novel compounds 1, 2, 3 and 5 at a
scan rate of 10 ◦C minꢀ 1
.
confirm the structures (see ESI).
Thermogravimetric analysis (TGA) and Differential Scanning Calo-
rimetry (DSC) were carried out in order to investigate the stability and
thermal behaviour of all new solids over a wide temperature range. TGA
was performed from 40 ◦C to 650 ◦C under a N2 atmosphere and all
molecules displayed good thermal stability up to 350 ◦C in the cases of 1,
2 and 3, and to 280 ◦C for 5 (Fig. 1, top).
DSC analysis showed an endothermic peak associated with the
melting point of the molecules at 210 ◦C (1 and 2), 185 ◦C (3) and 242 ◦C
(5) when the samples were heated under a N2 atmosphere (Fig. 1, bot-
tom). Compound 1 also showed an exothermic peak upon cooling to
around 110 ◦C and this is associated with a crystallisation process. The
other new compounds did not show exothermic peaks under the mea-
surement conditions. The variations in the melting points provide an
initial view of the intermolecular interactions in the solid state. Com-
pound 4 is a yellow oil at room temperature, and this suggests that
replacement of terminal amide groups by sterically hindered groups
such tert-butyl precludes a good interaction between the molecules and
thus solidification. Compound 3 has the lowest melting point of the
compounds that are solids at room temperature. Comparison of
Fig. 2. (a) UV–Vis absorption spectra in THF solution, (b) fluorescence spectra
in THF solution and (c) solid state of compounds 1–5. Concentrations of all
solutions range 2.5–5.5 × 10ꢀ 6 M such that the absorbance was between 0.1
and 0.3.
compound 3 with 1, 2 and 5 indicates that the presence of cyano groups
would allow the formation of hydrogen bonds between neighbouring
molecules. These kinds of interactions have been described in crystals of
a similar cyanostilbene compound [44]. Finally, compound 5 has the
5

R. Domínguez et al.
Dyes and Pigments 192 (2021) 109427
Table 1
Table 3
exp
Experimental maximum absorption wavelength (λ ), molar absorption coef-
Organo gelation abilitiesa of compounds 1–5 in different organic solvents and
their dielectric constants (κ) and dipolar moment (D) [49].
ab
max
ficient (ε), maximum emission wavelength (λ ), fluorescence quantum yield
em
(ΦF) and Stokes shifts of derivatives 1–5 in solution and as powders.
Solvent
κ
D
Compounds
exp
max
Comp.
Solvent
λ
ε
λ
ΦF (%)b
Stokes shift
abs
em
1
2
3
4
5
(nm)a
(Lmolꢀ 1cmꢀ 1
)
(nm)a
(cmꢀ 1
)
Hexane
Toluene
CHCl3
1.9
2.4
4.8
6
0
I
I
I
S
S
S
S
S
S
S
P
S
P
I
0.4
1.0
1.8
1.6
1.6
1.7
3.5
3.8
3.9
I
I
G
S
S
S
S
P
P
S
S
I
1
2
3
CH2Cl2
THF
348
357
–
348
354
–
329
333
332
–
325
324
346
349
–
56717
55435
–
54909
77397
–
70968
99585
80749
–
67416
50816
74721
82331
–
435
430
556
430
423
543
397
383
381
412
417
417
424
418
591
0
5747
4755
–
5480
4606
–
5206
3920
3873
–
6788
6883
5316
4730
–
G
P
S
G
P
P
S
S
G
P
S
G
P
P
S
S
I
0
Ethyl Acetate
THF
I
solid
7.2
0
7.5
9.1
24.6
36.6
38.2
47
I
CH2Cl2
THF
CH2Cl2
Ethanol
Acetonitrile
DMF
I
0
I
solid
7.1
56
58
81
3.8
0
I
CH2Cl2
THF
S
S
DMSO
Toluene
solid
a
G: gel; S: soluble; I: insoluble; P: precipitate.
4
5
CH2Cl2
THF
0
CH2Cl2
THF
0
incorporation of the acetamido group also results in a 25 nm red shift
(comparison between compounds 1 and 4). It is worth noting that
although compound 5 has the same molecular structure as compounds 1
and 2, the introduction of a hydrophobic alkoxy chain did not produce a
relevant shift in the UV–vis maxima. The emission spectra of derivatives
1–5 in THF solution are depicted in Fig. 2 (bottom) and the data are
summarised in Table 1. In a similar way to the absorption spectra, the
emission spectra did not exhibit a vibronic fine structure except in the
case of compound 3. This finding suggests a more planar geometry of the
excited state compared to the cyano compounds. All molecules dis-
played similar maximum emission wavelengths (420–430 nm) except
for 3, the molecule without the cyano groups, which exhibited the
lowest maximum emission and the lowest Stokes shift.
0
solid
3.0
a
UV–Vis absorption and emission spectra were recorded in dilute CH2Cl2 and
THF solutions (2.5–5.5 × 10ꢀ 6 M) such that the absorbance was between 0.1 and
0.3.
b
Fluorescence quantum yield was recorded in dilute CH2Cl2 and THF solu-
tions (0.8–1.5 × 10ꢀ 6 M) such that the absorbance was below 0.1.
highest melting point of all of the compounds. Comparison of this
compound with 1 and 2, which have aliphatic chains, shows that the
chains must interfere sterically with the intermolecular interactions. In
any case – and with the aim of forming gels – not only must intermo-
lecular interactions take place between molecules but there must also be
a hydrophobic/hydrophilic balance and an appropriate distribution of
the functional groups around the structure to allow a suitable orienta-
tion and arrangement for the formation of the three-dimensional
network of the gel.
The fluorescence spectra were also recorded in solid state for mole-
cules 1, 2, 3 and 5 (Fig. 2, bottom, and Table 1). The derivatives with the
cyano groups (1, 2 and 5) showed large bathochromic shifts with respect
to the spectra recorded in solution. Fluorescence quantum yields were
also determined in solution and in solid state. The molecules with cyano
groups (1, 2, 4 and 5) exhibited a fluorescence quantum yield of 0% in
solution and this increased to 3% in case of compound 5 and 7.2% for
compounds 1 and 2 in solid state. However, derivative 3 had values
56.2%, 58.2% and 80.5% in CH2Cl2, THF and toluene solution, respec-
tively, and this decreased markedly in the solid state. These data predict
different photophysical behaviour depending on the presence of the
cyano groups in the molecular structure, with aggregation-induced
emission (AIE) predicted in 1, 2 and 5, and aggregation-caused
quenching (ACQ) in 3 from solution to solid state.
3.2. Photophysical and electrochemical properties in solution and solid
state
The UV–Vis absorption and emission spectra of 1–5 were recorded in
different solvents, and the corresponding photophysical data are shown
in Fig. 2, Fig. S16 and are summarised in Table 1.
The UV–Vis absorption spectra in THF showed a broad absorption
band without a vibronic fine structure and this was located at around
354 nm for 1, 2 and 5, 333 nm for 3, and 324 nm for 4 (Fig. 2, top). These
absorption maxima in the UV spectra are similar to those of other
styrylbenzenes described with branches in a meta arrangement [18,19,
45,46]. The functional groups attached to the styryl units tune the op-
tical properties more strongly than functionalisation on the central
benzene. Thus, despite the fact that compounds 1 and 3 have the same
carbon scaffold, the presence of cyano groups in compound 1 induces a
red shift of ~20 nm compared to compound 3 (Table 1). The
Electrochemical measurements were also carried out in order to
determine the HOMO energy level of each new molecule. Reduction
waves were not observed because they appear beyond the experimental
potential window. The data are presented in Fig. S17 and are summar-
ised in Table 2. The new phenylenevinylene derivatives 1–5 showed an
irreversible oxidation process within the accessible electrochemical
window. This process is assigned to the formation of the radical cation of
the central bis(styrylbenzene) unit. The absence of the electron-
Table 2
Theoretical and experimental energy values for the HOMO and LUMO levels and bandgap (in eV).
Compound
PBE0/6-31+G**
CAM-B3LYP/6-31+G**
Electrochemical data
Eox1 (V)a
b
EHOMO
ELUMO
ΔEH-L
EHOMO
ELUMO
ΔEH-L
EHOMO
1
2
3
4
5
ꢀ 6.276c
ꢀ 2.389c
3.887
ꢀ 7.355c
ꢀ 1.314c
6.041
1.09
1.11
0.73
1.05
1.14
ꢀ 6.19
ꢀ 6.21
ꢀ 5.83
ꢀ 6.15
ꢀ 6.24
ꢀ 5.731d
ꢀ 1.842d
3.889
ꢀ 6.809d
ꢀ 0.816d
5.993
a
OSWV (Osteryoung square wave voltammetry) value.
b
c
d
HOMO energy levels were estimated from OSWV oxidation curves, assuming the absolute energy level of ferrocene/ferrocenium to be 5.1 eV below vacuum.
Theoretical calculation performed in CH2Cl2.
Theoretical calculation performed in toluene.
6

R. Domínguez et al.
Dyes and Pigments 192 (2021) 109427
Fig. 3. Thermoreversible sol-gel phase transition of 1 (in chloroform, (a) solution; (b) organogel) and 3 (in toluene, (c) solution; (d) organogel).
accepting cyano groups in 3 leads to a lower oxidation potential (around
0.4 V) compared to the other molecules in the series. The length of the
alkoxy chain has very little effect on the oxidation properties (1 and 2)
and the absence of this chain in 5 increases the oxidation potential to the
highest value within this family. These results are consistent with the
trend previously observed in the absorption spectra. In conclusion, the
presence of cyano groups induces stabilisation of the HOMO molecular
orbital.
Table 4
exp
Experimental maximum absorption wavelength (λ ), fluorescence emission
ab
wavelength maxima (λeemxp), and quantum yield (ΦF).
(nm)a
λ
(nm)a
ΦF (%)
exp
exp
em
Compound
State
λ
ab
1
Gel CH2Cl2
352
349
–
556
546
559
552
554
540
540
523
410
409
3.8
2.0
5.0
3.1
6.2
2.7
4.0
1.6
11.5
8.6
Gel CHCl3
Xerogel CH2Cl2
Xerogel CHCl3
Gel CH2Cl2
–
2
3
345
345
–
–
326
–
Gel CHCl3
3.3. Gelation abilities
Xerogel CH2Cl2
Xerogel CHCl3
Gel Toluene
Xerogel Toluene
The gelation behaviours of derivatives 1–5 were investigated in
various solvents using the ‘stable to inversion of a test tube’ method [47,
48], and the solutions were heated until boiling point of the solvent and
cold down until room temperature. The results are summarised in
Table 3. It was found that compounds 1 and 2, which contain the alkoxy,
acetamido and cyano groups, were able to form stable and transparent
gels in dichloromethane and chloroform (Fig. 3 and Fig. S18). These
compounds were soluble in THF, DMSO and DMF, and precipitated in
ethyl acetate, ethanol and acetonitrile when the temperature decreased.
The compounds were insoluble in nonpolar solvents such as toluene and
hexane. Compound 3 was also able to form an opaque gel in toluene
(Fig. 3), whereas in dichloromethane and chloroform solutions were
obtained. The minimum gelator concentration (MGC) was 18 mg mLꢀ 1
in all cases. Derivatives 4 and 5 were unable to form gels in any of the
tested solvents; compound 4 was soluble and 5 was insoluble in most of
the solvents.
gels could be recovered after the hot solutions were cooled naturally,
thus demonstrating the thermoreversibility of the gelation process.
Representative photographs of the organogels are shown in Fig. 3 and
Fig. S18.
3.4. Photophysical properties in the gel state and xerogels
UV–Vis absorption and fluorescence spectra were also measured in
the gel state (Fig. 4 and Fig. S19, Table 4). The maximum absorption
wavelength barely varied with respect to solution values for the three
compounds (1–3) and gelation solvents, but the absorption band is
broader in all cases due to aggregation of the molecules and scattering
during the measurement. In contrast, the maximum emission wave-
lengths were red-shifted with respect to the solution spectra (both THF
and dichloromethane), particularly for molecules 1 and 2 (≈110–120
nm) (Fig. 4). The nature of the self-assembly in an organogel structure is
difficult to establish unambiguously [50–52].
These results suggest the importance of the amide groups to promote
gel formation, probably due to the formation of hydrogen-bonding in-
teractions between neighbours’ molecules. The role of the amide groups
in the formation of hydrogen bonds in a supramolecular gel of cyanos-
tilbenes was proposed by Park et al.28 Compound 5, despite the presence
of acetamido groups, was insoluble in common organic solvents even at
high temperature and this is due to the absence of a hydrocarbon chain.
The length of the hydrocarbon chain does not play a key role in gel
formation. Cyano groups modify the photophysical properties and the
gelation abilities of this family of molecules, with the result that 1 and 2
gelate in polar solvents such as dichloromethane and chloroform, and 3
gelates in apolar solvents such as toluene (Fig. 3).
In order to obtain information about the organisation of the mole-
cules during the gel formation, the cooling process from the hot solution
and the formation of the organogel was monitored by fluorescence
spectroscopy (Fig. 5 and Figs. S20 and S21). It was found for molecules 1
and 2 that the fluorescence intensity increased significantly relative to
the solution state during the gelation process, thus showing an AIE effect
in the gel state. Although organogel formation occurs within four mi-
nutes after the cooling process, as determined with the naked-eye, the
reorganisation of the molecules continues and the fluorescence intensity
The organogels were obtained within four minutes after cooling the
hot solutions and they were destroyed on reheating the samples. The
7

R. Domínguez et al.
Dyes and Pigments 192 (2021) 109427
Fig. 5. Fluorescence spectra monitoring the formation of the organogel in (top)
1 (organogel in dichloromethane) and (bottom) 3 (organogel in toluene) during
the cooling process.
Fig. 4. (a) Emission spectra of compounds 1–3 in THF solution (1 – red, 2 –
black, 3 - green) and (b) gel state (dichloromethane gel of 1 – red; chloroform
gel of 1 – wine; dichloromethane gel of 2 – black; chloroform gel of 2 – dark
grey; toluene gel of 3 – green).
transition is S0→S1 in CH2Cl2 and this was calculated at 337 nm with
CAM-B3LYP, which is consistent with the experimental absorption (348
nm). The molecular orbitals involved in this transition are HOMO-
1→LUMO and HOMO→LUMO+1. In the case of compound 3, the most
intense transition is S0→S1 and was calculated in toluene at 330 nm,
again in very good agreement with the experimental value (332 nm).
The molecular orbitals involved in this transition are HOMO-1→LUMO.
The molecular orbitals and energy levels calculated at the CAM-B3LYP/
6-31+G** level of theory are represented in Fig. 6 and it can be seen that
the molecular orbitals are distributed over the whole molecular struc-
ture (similar results were obtained from PBE0/6-31+G**, see Fig. S22).
The relative theoretical values of the HOMO energy levels are in
agreement with the CV experimental measurements and show a higher
stabilisation of the HOMO in the case of compound 1 (see Table 2).
The first excited state S1 was optimized for compounds 1 and 3 in
CH2Cl2 and toluene, respectively. The geometry changes after photo-
excitation calculated at the PBE0/6-31+G** level of theory are shown in
Fig. 7. One dihedral angle of compound 1 connecting the central phenyl
ring with the vinylene arm undergoes a significant change from ~30◦ to
2◦ on going from S0 to S1. However, CAM-B3LYP/6-31+G** produces a
strong distortion of the first excited state, with one of the arms almost
perpendicular (~90◦) to the central phenyl ring (see Fig. S23). This
could correspond to the transition state (the so-called ‘phantom’ state)
across which the cis-trans isomerisation could occur upon excitation. The
photoisomerization of stilbenes has attracted significant attention for
several decades from both the theoretical and experimental point of
increases for up to nine hours. These results are consistent with the
fluorescence quantum yields found in solution and the gel state
(Table 1). A red-shift in the emission bands was also observed from 528
nm in the hot solution to 557 nm in the gel state. In contrast to de-
rivatives 1 and 2, compound 3 exhibited the opposite behaviour during
the cooling process (Fig. 5, bottom). The fluorescence spectra registered
during gel formation for compound 3 shows a decrease of the intensity
being the quantum yield upon gel formation of ΦgFel = 11.5% while the
fluorescence quantum yield calculated for molecule 3 in toluene was
ΦtFoluene solution = 81% (see Table 1). This result reveals an aggregation-
caused quenching (ACQ) effect from solution to the gel state for com-
pound 3.
Density Functional Theory calculations were performed with the aim
of shedding light on the AIE and ACQ effects observed on changing from
solution to the solid/gel state. The theoretical calculations were only
performed on compounds 1 and 3 because they are the only ones that
formed a gel. As previously mentioned, the initial conformational
analysis was performed at the PBE0/6-31+G** and CAM-B3LYP/6-
31+G** levels of theory (see Table S1 and Figs. S12–S15 in SI). Vertical
electronic transitions were calculated in different solvents for the most
stable conformers of compounds 1 and 3 (see Table 5) and a reasonable
agreement was obtained with the experimental absorption measure-
ments in Table 1. CAM-B3LYP gave the best match between experi-
mental and theoretical observations. For compound 1, the most intense
8

R. Domínguez et al.
Dyes and Pigments 192 (2021) 109427
Table 5
Experimental absorption and calculated electronic transitions, oscillator
strength (f), main components of the S0→S1 transition (% contribution) at the
TD-PBE0/6-31+G** and TD-CAM-B3LYP/6-31+G** levels of theory for com-
pounds 1 and 3.
exp
calc
Comp.
Solvent
λ
(eV
λ
(eV
f
Transition
% Contribution
abs
abs
[nm])
[nm])
PBE0/6-31þG**
1
CH2Cl2
3.56
3.30
1.58
0.29
1.55
S0→S1
S0→S2
S0→S1
H-1→L(67),
H→L(11)
[348]
[376.1]
3.37
H→L(28),
[367.6]
3.30
H→L+1(63)
H-1→L(68),
H→L(12),
1
THF
3.47
[357]
[375.7]
H→L+1(19)
H→L(11)
3.37
0.31
1.43
0.48
1.52
0.26
S0→S2
S0→S1
S0→S2
S0→S1
S0→S2
H→L(27),
[367.5]
3.41
H→L+1(64)
H-1→L(95)
3
3
Toluene
THF
3.73
[332]
[364.2]
3.53
H→L(98)
H-1→L(90)
H→L(93)
[351.0]
3.33
3.72
[333]
[372.5]
3.41
[363.6]
CAM-B3LYP/6-31þG**
1
1
3
3
CH2Cl2
3.56
3.68
1.95
0.53
1.93
0.54
2.24
0.38
2.25
0.36
S0→S1
S0→S2
S0→S1
S0→S2
S0→S1
S0→S2
S0→S1
S0→S2
H-1→L(48),
H→L+1(46)
H-1→L+1(33),
H→L(59)
[348]
[337.3]
3.81
Fig. 6. Molecular orbitals and energy levels (in eV) for compounds 1 in CH2Cl2
solution and 3 in toluene solution calculated at the CAM-B3LYP/6-31+G**
level of theory (isocontour plots 0.02 a.u.).
[325.2]
3.68
THF
3.47
H-1→L(46),
H→L+1(45)
H-1→L+1(33),
H→L(57)
[357]
[336.8]
3.82
and vibrations (RIV) is a widely accepted mechanism to explain the AIE
and ACQ effects observed in a variety of luminogens [20]. In addition,
other mechanisms have also been proposed for different AIE systems
including E/Z isomerisation [16] and restriction access to conical
intersection (RACI) between ground and excited states [55,56], amongst
others. In the context of the RIV mechanism, computation of the reor-
ganisation energy and the dimensionless Huang–Rhys factors allows
quantification of the contribution of vibrational modes to the
non-radiative relaxation channels by internal conversion. The reorgan-
isation energy and Huang–Rhys factors calculated for compounds 1 and
3 at the PBE0/6-31+G** level of theory (the results for CAM-B3LYP are
shown in Figs. S25 and S26) are presented in Fig. 8.
[324.5]
3.76
Toluene
THF
3.73
H-1→L(70),
H→L+1(17)
H-1→L+1(12),
H→L(76)
[332]
[329.9]
3.85
[322.4]
3.76
3.72
H-1→L(70),
H→L+1(17)
H-1→L+1(12),
H→L(77)
[333]
[329.4]
3.86
[321.5]
view (see ref. 50, and references therein). A thorough description of the
excited states and potential energy surface involved in the cis-trans
isomerisation is beyond the scope of this work and could become com-
plex and highly demanding from a computational point of view.
For compound 3, the changes in the molecular geometry from S0 to
It can be seen in Fig. 8 that there is a large contribution to the
reorganisation energy in compound 1 from two vibrational modes in the
low energy region, for which large Huang–Rhys factors of around 24 and
18, respectively, were calculated (see Table S3). The atomic displace-
ments of these vibrational modes are represented in Fig. S27 and these
are associated with twisting motions of the molecule as a whole in which
the acetamido and cyano groups are strongly involved. These vibra-
tional modes could help to dissipate the absorbed energy in solution.
However, the framework of intermolecular interactions in the solid/gel
state through acetamido and cyano groups (vide infra) could block these
vibrational motions to yield bright states – in agreement with the AIE
effect for this compound.
S
1 are less pronounced (see Fig. 7) and PBE0 and CAM-B3LYP behave
similarly. Thus, the three dihedral angles are reduced from 6◦, 10◦ and
3◦ to 1–0◦ and this results in an almost planar structure for the first
excited state at the PBE0/6-31+G** level of theory. Similar results were
obtained on using CAM-B3LYP/6-31+G** (see Fig. S24 in SI).
The almost planar geometry predicted for compound 3 in the first
excited state is in agreement with the vibronic structure observed in the
emission spectrum in solution (see Fig. 2). In addition, the larger change
predicted in the geometry of compound 1 on going from the ground to
the excited state is in agreement with the larger Stokes shift observed for
this compound when compared with compound 3. In the latter case, the
modifications of the dihedral angles are smaller and, as a consequence, a
smaller Stokes shift was observed experimentally.
Besides restriction to the vibrational motions, cyanostilbenes usually
show cis-trans photoisomerization in solution – a process that quenches
the emission [29,30,57]. Thus, the predicted twisted molecular structure
of compound 1 could favour intramolecular motions that facilitate the
isomerisation in solution upon excitation. These intramolecular motions
could be blocked in the solid/gel state due to the intermolecular contacts
among the acetamido and cyano groups responsible of the gel formation
and this would hinder the photoisomerization and enhance the emission
upon aggregation [33,58].
The S1→S0 emission wavelength for compound 3 was calculated with
PBE0 in toluene at 527 nm and the result was far from the experimental
exp
value (λ
= 381 nm) with ΔE = 0.9 eV. Unlike PBE0, this difference
em
calc
becomes negligible (ΔE = 0.04 eV) on using CAM-B3LYP (λ
= 386
em
nm). For compound 1, PBE0 predicts reasonable agreement between
theoretical (λceamlc = 505 nm) and experimental (λeemxp = 435 nm) emission
in CH2Cl2 with ΔE = 0.4 eV, i.e., within the range of expected deviations
[53,54].
In the case of compound 3, the largest contributions to the reor-
ganisation energy come from the vibrational modes in the high energy
region for which very small Huang–Rhys factors were predicted (see
Fig. 8 and Table S3). This non-radiative deactivation pathway would not
Restriction of Intramolecular Motions (RIM) through rotations (RIR)
9

R. Domínguez et al.
Dyes and Pigments 192 (2021) 109427
Fig. 7. Selected dihedral angles (in degrees) for the ground S0 and first excited state S1 calculated at the PBE0/6-31+G** level of theory for compound 1 (right) in
CH2Cl2 solution and 3 (left) in toluene solution.
Fig. 8. Reorganisation energy (in meV) and Huang-Rhys factors for the ground state calculated at the PBE0/6-31+G** level of theory for compound 1 (bottom) and 3
(top) in CH2Cl2 and toluene solution, respectively.
be determinant in dissipating the absorbed energy in solution. In addi-
tion, the nearly planar structure predicted for the compound 3 would not
favour the photoisomerization process in solution [59,60].
The cyano groups also lead to strong polarisation of the electronics of the
molecule, as suggested by the high Stokes shifts. In the excited state,
geometric optimisation suggests that these compounds may undergo cis-
trans photoisomerization, which would also imply non-radiative deac-
tivation of the excited state. When the compounds gel, the mechanisms
of vibrational non-radiative deactivation and cis-trans photo-
isomerization could be blocked by steric hindrance, and the excited state
is radiatively deactivated, thus causing an increase in fluorescence. In
contrast, compound 3 has a flat geometry both in the ground and excited
states, which allows the observation of the vibronic structure in the
emission spectrum in solution. The non-radiative deactivation of the
excited state of compound 3 is produced, fundamentally, by contribu-
tions of vibrational modes in the high-energy region. The flat structure
predicted for this compound would not favour the photoisomerization
process in solution in which a high quantum yield is observed. In
The origin of the ACQ observed in compound 3 could be that the
planar structure would favour π-π intermolecular interactions in the
solid/gel state. As discussed in previous publications [14], this kind of
intermolecular interaction quenches the fluorescence emission, a situ-
ation in agreement with the experimental observations. In contrast, the
more twisted structure predicted for the cyano compounds could pre-
vent such intermolecular interactions in the solid/gel state and this
would favour fluorescence emission.
In summary, the incorporation of cyano groups in the bis(styryl)
benzene structure therefore produces changes in both the geometry and
the electronics of the molecule when compared to compound 3, which
does not contain cyano groups. In its ground state, the styrene branches
in compound 1 are twisted by 30◦ and this is flattened upon excitation.
In addition, this arrangement causes vibrational modes associated with
twisting motions that deactivate the excited state in a non-radiative way.
contrast, it should lead to better stacking and favour the π-π interactions
in the solid or gel state, which in turn leads to quenching of the
fluorescence.
10

R. Domínguez et al.
Dyes and Pigments 192 (2021) 109427
and xerogel states (Fig. S30) and nor were the fluorescence quantum
yields (Table 4). These results indicate similar interactions in these two
phases. Representation of the maximum emission wavelength in a
Commission International de l’Eclairage (CIE) chromaticity diagram for
compounds 1–3 in the various states (solution, solid, gel and xerogel)
provided an insight into the differences in the optical properties of the
molecules with (1 and 2) and without (3) cyano groups (Fig. 9 and
Fig. S31). The results corroborate those discussed above. Dichloro-
methane and THF solutions of 1 exhibit CIE coordinates of (0.18, 0.17)
in the blue region, whereas when the molecules are aggregated (solid,
gel and xerogel) the coordinates are (0.22, 0.55) in the yellowish green
region. In contrast, significant differences were not observed between
the different states for compound 3 (blue region).
SEM micrographs of compounds 1 and 3 are shown in Fig. 10.
Samples were prepared by allowing a small amount of the corresponding
organogels to dry on a SEM sample stub. Although the molecular
structures of the two compounds only differ in the presence of two cyano
groups, their supramolecular self-assembly in the gel is completely
different. The xerogel of compound 1 (Fig. 10, top) shows a layered or
laminar structure, as evidenced by the cracks produced by rapid drying
and contraction of the gel. In contrast, the xerogel of compound 3
(Fig. 10, bottom) forms fibre bundles that interlock with each other to
form a net. In order to determine the size of the bundles and fibres, the
images were processed with DigitalMicrograph 3.7.4. and at least 100
elements were measured in different pictures to estimate the size dis-
tribution (Fig. S32). The bundles were 5.49 ± 0.2 μm width and the fi-
bres that make up these bundles have a thickness of 343 ± 20 nm. These
images suggest that compound 1 has a predominant bidimensional
interaction between molecules whereas compound 3 has more tridi-
mensional self-assembly.
Since only those compounds containing acetamido groups form the
gels (1, 2 and 3) and the morphology of the gels is different depending
on whether the molecule has a cyano group, the infrared spectra of
compounds 1, 2, 3 and 4 were compared (Fig. S35) in order to identify
the functional groups involved in the intermolecular interactions
responsible for the supramolecular organisation in the xerogel. Being
structurally very similar, the position of the characteristic bands can
help us to define the structural role of each functional groups in the
xerogels. The experimental observations were supported calculating the
vibrational frequencies of similar compounds to 1 and 3 (see Fig. S34
and Table S4) at the PBE0-D3/6-31G* level of theory (scaled by 0.9519
according to Ref. [61]).
The theoretical calculations predict the stretching vibrational mode
at 2242 cmꢀ 1 for the cyano groups unbounded and at 2237 cmꢀ 1 for the
cyano groups bonded by hydrogen (Table S4). This slight difference of
only 5 cmꢀ 1 indicates that the stretching vibration of the CN will be
almost unaffected if the cyano group participates in hydrogen bonding,
probably due to the strength of the triple bond. This is confirmed
experimentally when comparing the vibrational frequencies of the
cyano group in solid in compound 4 (2216 cmꢀ 1), which has no amido
groups, and the vibrational frequencies of the cyano group in the
xerogels of compounds 1 and 2 (2214 cmꢀ 1 and 2216 cmꢀ 1) which do
have amido groups and no band shifting is observed (Fig. S33). For this
reason, the principal source of information is found when studying the
acetamido group, in the carbonyl and NH stretching vibration bands.
The stretching vibration of the carbonyl group prediction shown a
shift towards lower wavenumbers when the carbonyl group is involved
in hydrogen bonding (Table S4). Experimental infrared spectrum of
compound 3 shows a band centered at 1661 cmꢀ 1 meanwhile com-
pounds 1 and 2 show bands less shifted at 1683 cmꢀ 1 and 1675 cmꢀ 1
(Fig. S33). This indicates that the hydrogen bonds formed in compound
3 are stronger and with a higher carbonyl participation than in the case
of compounds 1 and 2. Additionally, in compounds 1 and 2, two
shoulders are observed at higher wavenumbers (1695 cmꢀ 1 and 1694
cmꢀ 1, Fig. S33) which may indicate the presence of carbonyl groups that
do not participate in the hydrogen bonds and that are unbounded. These
Fig. 9. CIE1931 chromaticity diagram of 3 (top) and 1 (bottom) in solution,
solid, gel and xerogel state.
3.5. Xerogel characterisation
TGA, DSC, fluorescence emission spectra, SEM images and ATR-FTIR
were registered for the xerogels obtained from the corresponding
organogels by slow evaporation of the solvent at room temperature (in
the case of dichloromethane and chloroform) or by removing the solvent
under vacuum (in the case of toluene).
In a similar way to the solids, TGA experiments performed from 40 ◦C
to 800 ◦C under a N2 atmosphere showed that all xerogels are thermally
stable up to 350 ◦C (Fig. S28). DSC curves of xerogels of compounds 1
and 2 displayed the same endothermic peak as in the solid and this is
associated with the melting point of the molecule. Other endothermic
peaks were also observed at lower temperatures relative to the solid-
solid phase transitions between different crystalline structures in the
xerogel (Fig. S29). An exothermic peak was also assigned to a crystal-
lisation process. In the case of compound 3, only the endothermic peak
at 185 ◦C was observed, and this is due to melting of the solid.
The fluorescence spectra were not significantly different for the gel
11

R. Domínguez et al.
Dyes and Pigments 192 (2021) 109427
Fig. 10. SEM images at different magnitude of amplification of compound 1 (top) and 3 (bottom) from their corresponding xerogel.
Fig. 11. Schematic representation of the proposed molecular arrangement in the gel for compound 1 along with the optimized structure of a tetramer at the PBE0-
D3/6-31G* level of theory.
results point out that the carbonyl group would be involved in hydrogen
bonds in compounds 3 and different carbonyl groups are present in
compounds 1 and 2.
in the infrared spectrum and agrees with the calculated for a bonded NH
at 3302 cmꢀ 1 (average value) instead of the unbounded at 3423 cmꢀ 1
(average value) confirming that the acetamido group plays the critical
role in the formation of the gel in compound 3 (Table S4).
Finally, the NH stretching of compound 3 was assigned at 3290 cmꢀ 1
12

R. Domínguez et al.
Dyes and Pigments 192 (2021) 109427
Fig. 12. Schematic representations of the proposed molecular arrangements in the gel for compound 3 along with the optimized structures of the cyclic (a) and
stacked (b) trimer at the PBE0-D3/6-31G* level of theory.
For compounds 1 and 2, NH stretching was theoretically predicted at
3464 cmꢀ 1 (average value) and at 3358 cmꢀ 1 (average value) for un-
bounded and bonded, respectively (Table S4). Experimentally, com-
pound 1 and 2 present main peaks at 3366 cmꢀ 1 and 3328 cmꢀ 1 that
agrees with the theoretically predicted for bonded NH (Fig. S33).
Additionally, other peaks were observed in the region corresponding to
the NH stretching that is not observed for compound 3. This suggests
that different kind of NH participate in hydrogen bonds involving
different functional groups in the formation of the gels for compounds 1
and 2.
of 2.086 Å. In the case the hydrogen bonds through acetamido groups,
the mean value for N–H⋯O bond angles is 156.3◦ and for bond distances
1.990 Å (see Fig. S35).
For compound 3, to be consistent with the size of the fibre and fully
connected through the amido groups, we propose two kind of tridi-
mensional self-assembly shown in Fig. 12: a) three molecules would be
interconnected through hydrogen bonds forming a cycle which would be
stacked one inside the other to form the fibre; b) molecules would be
stacked on top of each other resulting in a helical structure. These mo-
lecular arrangements were explored through DFT calculations. In both
cases, our theoretical calculations predict diameters around 28 Å and 25
Å for the arrangements a and b, respectively, which are consistent with
the size of the fibre estimated by SEM (see Figs. S36 and S37). For the
N–H⋯O hydrogen bonds, the mean values predicted for bond angles
were 162.1◦ and 166.2◦, and for bond distances 1.907 Å and 1.950 Å, for
arrangements a and b, respectively (see Figs. S36 and S37). In both
molecular organizations, the predicted C⋯C intermolecular distances
Taking into account all these results, we propose a two-dimensional
self-assembly for compound 1 in gel state in which molecules are net-
worked through two kind of hydrogen bonds: those established between
cyano and acetamido groups, and those in which only acetamido groups
are involved (see Fig. 11). This molecular arrangement would hinder
intramolecular vibrations and rotations as well as photoisomerization
promoting the radiative electronic relaxation upon gelation. DFT cal-
culations were performed in order to get some insight about the strength
of the hydrogen bonds and hence the stability the bidimensional struc-
ture. The molecular geometry of a model tetramer was optimized at the
PBE0-D3/6-31G* level of theory. Fig. 11 shows the optimized structure
including bond lengths and angles of the hydrogen bonds. According to
that, each cyano group is bonded to acetamido moiety being the mean
value for N–H⋯N bond angles of 169.4◦ and for N–H⋯N bond distances
between stacked molecules are close to the typical
π stacking distances
(3.5 Å) facilitating intermolecular interactions which would be
π-π
responsible for the observed decrease of the fluorescence emission in the
gel. As a conclusion, our theoretical calculations predict that the
hydrogen bonds formed in the proposed self-assemblies could be clas-
sified as moderate according to Jeffrey’s criteria which would provide
stability to the supramolecular structures formed during the gelation
13

R. Domínguez et al.
Dyes and Pigments 192 (2021) 109427
´
process [62].
Economía y Conocimiento. Junta de Andalucía’ (FQM-337) and ‘Accion
´
1’ (Universidad de Jaen, Spain). R. D. thanks the Junta de Comunidades
4. Conclusions
de Castilla-La Mancha, Fondo Social Europeo (FSE) and Iniciativa de
Empleo Juvenil (EIJ) for the postdoctoral grant.
Two families of small molecules based on bis(styrylbenzene) and bis
(
α
-cyanostyryl)benzene have been synthesised and characterised by
Appendix A. Supplementary data
spectroscopic and modelling methods. These compounds were used as
organic gelators. The appropriate incorporation of polar and non-polar
functional groups on the structures, such as acetamido and hydrocar-
bon chains, are critical for gel induction. It was found that amide groups
are essential for gelation due to the formation of hydrogen bonds in the
supramolecular structure. Optical data revealed different photophysical
behaviour depending on the presence or absence of the cyano groups: in
the first case (molecules 1 and 2) an AIE effect was observed from the
solution to the gel, whereas an ACQ effect was recorded in the second
case (molecule 3). Both effects could be easily followed by monitoring
the fluorescence spectra during gel formation. In the same way, different
behaviours of the solvents involved in the gelation process were found,
where 1 and 2 gelated in dichloromethane and chloroform (polar sol-
vents) and 3 gelated in toluene (apolar solvent). Density Functional
Theory calculations were carried out to gain an insight into these
opposite effects. For the cyano derivative 1, the predicted twisted
structure could favour intramolecular motions that promote cis-trans
photoisomerization in solution upon excitation and quench the emis-
sion, whereas these intramolecular motions would be blocked in the
solid and gel states, thus preventing the photoisomerization and
enhancing the emission upon aggregation. In addition, the RIV mecha-
nism could also contribute to the AIE effect. In the case of compound 3,
almost planar structures were predicted both in the ground and excited
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2021.109427.
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