A photoluminescent molecular host with aggregation-induced emission enhancement, multi-stimuli responsive properties and tunable photoluminescence host-guest interaction in the solid state

Article abstract of DOI:10.1016/j.jphotochem.2019.112106

The development of controllable multimodal luminescent materials that are responsive to external stimuli has been a challenge. In this study, a photoluminescent (PL) molecular host with aggregation-induced emission enhancement (AIEE) characteristic and mu

Full text of DOI:10.1016/j.jphotochem.2019.112106

Journal of Photochemistry & Photobiology A: Chemistry 386 (2020) 112106
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
A photoluminescent molecular host with aggregation-induced emission
enhancement, multi-stimuli responsive properties and tunable
photoluminescence host-guest interaction in the solid state
⁎
Hamideh Nikookar, Parviz Rashidi-Ranjbar
School Of Chemistry, College of Science, University of Tehran, Tehran, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords:
Photoluminescence
Aggregation-induced emission enhancement
Stimuli-responsive
Host-guest interactions
Pi-Pi interactions
The development of controllable multimodal luminescent materials that are responsive to external stimuli has
been a challenge. In this study, a photoluminescent (PL) molecular host with aggregation-induced emission
enhancement (AIEE) characteristic and multi-stimuli responsive properties has been designed, synthesized and
its fluorescence response to mechanical, thermal, anion and guest stimuli have been studied in the solid state.
Host 1 displays reversible tricolor-changing PL triggered by mechanical and thermal stimulations. Furthermore,
anion exchange has demonstrated that the emission is affected by the anion size and the affinity between cation
and anion moieties, resulting in enhancement or quenching effects. Additionally, grinding was used for inclusion
of guests in the host molecular packing to tune the PL properties, most probably due to π-π interactions between
molecular host and aromatic guests.
1. Introduction
and drug delivery [19]. π-π interactions not only lessen solubility of
organic compounds but also induce ACQ effect in luminogenic mole-
The stimuli-responsive materials are highly interested because of
cules resulting in significant fluorescence intensity and quantum yield
decrease in solution or solid state that limits their applications. This
phenomenon occurs when the aromatic sheets accumulate in stack
form, and the excited states of aggregates often relax via non-radiative
pathways [4d]. Various approaches have been developed to prevail
with the ACQ effect in π-conjugated systems. For example, branched
chains [20], bulky cyclics [21], spiro kinks [22], and dendritic wedges
[23] have been attached to aromatic rings. This strategy led to the
discovery of aggregation-induced emission (AIE) or aggregation-in-
duced emission enhancement (AIEE) materials that are non-emissive or
weak emissive in dilute solution and strong emissive in aggregated
forms [24]. The engineering of π–π interaction systems pursue good
solvation, high quantum yield, good charge transfer (CT), and other
optoelectronic properties.
their applications in memory, sensors, bio-imaging, and displays [1]. A
wide variety of external stimuli which induce changes in chemical and
physical properties of materials have been reported, such as electric
current [2], irradiation,[3] aggregation [4], solvent polarity [5], tem-
perature [6], mechanical force [7], pH[8] and organic solvent vapor
[
9].
Although the dichromic fluorescent systems have been successfully
developed in the solid state [10], there are few examples of materials
that display multicolor PL with stimuli.[11] Such an extent of various
applications in diverse fields enforces researchers to design new com-
pounds as single molecules [12], polymers [13], metal-organic frame-
works [14], quantum dots [15] and nanoparticles [16] which provide
tunable multicolor emission in solution or solid state.
On the other hands, noncovalent interactions in host-guest com-
plexes display excellent strategy to modify chemo-physical properties of
materials and provide color spectra through adjustment of emission
light [17]. Hydrogen bonding, metal-ligand coordination, and π-π
stacking interactions have been reported in PL tunable systems [18],
π–π interactions are facile, non-destructive and particularly promising
for applications from material science to molecular biology such as
nucleic acids sequencing, protein folding, biosensors, crystal packing,
The development of supramolecular systems with host-guest inter-
actions to design color-tunable luminogens based on the modification of
core fluorophores is challenging [25]. However, there have been a few
reports [26] of the host-guest systems such as cyclodextrin or cucurbit
[8] uril that display multicolor PL due to interactions with a fluorescent
guest molecule. Traditionally, solution-phase crystallization has been
used to form inclusion compounds in supramolecular chemistry. How-
ever, it is difficult to find an appropriate solvent for co-crystallization of
⁎
Corresponding author.
E-mail address: parvizrashidi2@ut.ac.ir (P. Rashidi-Ranjbar).
https://doi.org/10.1016/j.jphotochem.2019.112106
Received 9 May 2019; Received in revised form 22 September 2019; Accepted 25 September 2019
Available online 26 September 2019
1010-6030/ © 2019 Elsevier B.V. All rights reserved.

H. Nikookar and P. Rashidi-Ranjbar
Journal of Photochemistry & Photobiology A: Chemistry 386 (2020) 112106
connected to a Fluoromax 4 fluorometer. Excitation performed at
380 nm.Powder X-ray Diffraction (PXRD) was collected on PANalytical-
X'PERT PRO MPD diffractometer using CuK irradiation. Scanning
α
Electron Microscopy (SEM) micrographs obtained by a MIRA3, TESCAN
instrument equipped with Energy Dispersive X-ray (EDX) and elemental
mapping. Thermogravimetric Analysis (TGA) conducted on a TA In-
struments Q50 Thermogravimetric System and samples were heated at
a rate of 20 °C/min. The balance and purge flows were 40 ml/min and
60 ml/min respectively. Electrostatic Potential Surface (EPS) calcula-
tions for guests were performed using SPARTAN 10 program. Optimi-
zation of geometry and Time-Dependent Density Functional Theory
(
TD-DFT) were performed using Gaussian 16 program. The structures
were optimized initially by semi-empirical PM6 method and the opti-
mized structures were used as input files for the DFT calculations at
B3LYP/6-31 G(d) level of theory.
2.2. Synthesis of 1,1′-(2,5-bis(dodecyloxy)-1,4-phenylene)bis(methylene)
bis(4-(1H-phenanthro[9,10-d]imidazol-2-yl)pyridinium)
hexafluorophosphate (1)
Scheme 1. The structure of molecular host 1 and reference molecule 2.
many of the host-guest systems. Mechanical methods including solid
state grinding and solvent assistant grinding could be used to prepare
molecular co-crystals in simple and effective ways [27], and sometimes
the resulting crystals are different from the ones from solution or
melting crystallization path [28]. The interactions between the guest
and the host are enhanced when the solvent is omitted [29].
Herein, we report a new type of fluorogenic material based on
phenanthroimidazole moiety showing a mechanical and thermal re-
sponse, as well as the response due to anion type and guest interactions.
Phenanthroimizole fluorophore has been used previously to design
mechanoresponsive materials [30]. The molecular design of a host is
essential for suitable guest-recognition by π-π interaction to have a
sensitive fluorescence response. For this purpose, we have designed and
A sealed tube was charged with 2,5-bis(bromomethyl)-1,4-bis(do-
decyloxy)benzene (4((0.5 mmol, 0.32 g), 2-(4-Pyridinyl)-1H-phenan-
thro [9,10-d]imidazole (5) (1.05 mmol, 0.31 g) and DMF (5 mL) and the
mixture was heated to 100 °C for 6 h. After cooling, the mixture was
filtered off, washed with ethyl acetate and chloroform respectively, and
dried at room temperature to afford yellow powder (yield: 70%).
−1
mp > 230 °C. ATR-FTIR (cm ) υ 3551-3296, 3039, 2935, 2847, 1635,
421. 1H-NMR (500 MHz, DMSO-d
) δ 9.30 (d, J =6.5 Hz, py-ring,
1
4
4
8
6
H), 9.06 – 8.94 (m, phenan-ring, 4 H), 8.89 (d, J =4.5 Hz, py-ring,
H), 8.56 – 8.42 (m, phenan-ring, 4 H), 7.91-7.81 (m, phenan-ring,
H), 7.57 (s, OR-phen-ring, 2 H), 5.91 (s, methylene, 4 H), 4.06 (s, O-
methylene, 4 H), 1.70 (s, CH
2
, 4 H), 1.28 – 0.81 (m, CH
2
, 36 H), 0.77–
13
0
.70 (m, CH
3
, 6 H). C NMR could not be obtained because of low
synthesized the molecular host
1
using fluorescent phenan-
solubility.
throimidazole pincers -with both π-donor and π-acceptor fragments-
and benzylic spacer (Scheme 1).This dual nature of 1 causes the
emission band to shift to longer wavelengths which are very important
for biological applications and allows more effective interactions be-
tween host and different guests through π-π interaction. Subsequently,
we have studied the interactions between 1 as a host molecule with
aromatic guests by grinding to form tunable color. Spectroscopy tech-
niques were employed to investigate the effectiveness of grinding pro-
cedure to study the host-guest interactions and to compare it with the
host-guest co-precipitates. The model compound 2 was synthesized as a
reference for host-guest interaction, if any, to be compared with the
molecular host 1.
Ammonium hexafluorophosphate (0.8 mmol, 0.13 g) was added to
the suspension of 1-Br (0.3 mmol, 0.37 g) in methanol (30 mL). The
mixture was stirred at room temperature for one hour. Then it was
filtered off, washed with methanol and water respectively and dried at
ambient temperature. Host 1 was obtained in quantitative yield as a
1
yellow powder. mp > 230 °C. H NMR (500 MHz, DMSO-d
6
) δ 9.27 (d,
J =6.3 Hz, py-ring, 4 H), 9.00 (d, J =8.5 Hz, phenan-ring, 2 H), 8.97
d, J =8.5 Hz, phenan-ring, 2 H), 8.87 (d, J =6.3 Hz, py-ring, 4 H),
(
8
2
.51 (d, J =8.0 Hz, phenan-ring, 2 H), 8.45 (d, J =8.5 Hz, phenan-ring,
H), 7.86 (m, phenan-ring, 8 H), 7.55 (s, OR-phen-ring, 2 H), 5.89 (s,
methylene, 4 H), 4.06 (t, J =5.8 Hz, O-methylene, 4 H), 1.70 (p, J
=
1
7.3 Hz, CH
6 H), 0.95 – 0.79 (m, CH
NMR (126 MHz, DMSO-d
35.51, 129.96, 128.78, 128.61, 128.25, 128.17, 127.48, 124.54,
124.36, 124.19, 123.83, 123.74, 122.18, 121.32, 119.38, 116.19,
2
, 4 H), 1.21 (q, J =7.3 Hz, CH
2
, 4 H), 1.17 – 0.99 (m, CH
2
,
C
1
3
2
, 16 H), 0.73 (t, J =7.3 Hz, CH
3
, 6 H).
2. Experimental section
6
) δ 156.30, 150.75, 146.28, 145.91, 139.92,
1
2.1. Materials and methods
6
8.61, 60.09, 31.15, 29.32, 29.28, 29.13, 29.11, 28.93, 28.55, 28.48,
2
+
2+
All chemicals were purchased from Merck and used without further
purification. All solvents were lab grade. The compounds 3 [31], 4 [32]
and 5 [33] used in this study were synthesized according to the pre-
25.80, 21.98, 13.86. HRMS (ESI) calcd for C72
531.3244; found: 531.3266.
H
82
N
6
O
2
[M]
:
1
13
viously reported procedures. H NMR and C NMR spectra were re-
corded at room temperature by a Varian 500 MHz spectrometer. A
Bruker TENSOR27 instrument measured FTIR-ATR spectra. Solution
phase UV/Vis spectroscopy has been studied using a PerkinElmer
Lambda 850 UV/Vis spectrophotometer. Diffuse Reflectance Spectra
3. Results and discussion
3.1. Molecular design
Recently, in addition to 1H-NMR spectroscopy or single crystal
formation, fluorescence spectrophotometry as an appropriate technique
is used to study host-guest complexation because of easy detection and
high sensitivity to low concentration of the complex [34]. Therefore,
host 1 with fluorescent pincers and benzylic spacer was designed to
study the interaction with aromatic guests. The pincer segment has a
donor-acceptor characteristic causing strong CT from phenanthrene
moiety to the pyridinium ring. The DFT method was used to optimize
(
DRS) were obtained using an Avantes Avaspec-2048-TEC spectrometer
with AvaLamp DH-S setup using BaSO as the reflectance sample. Solid
4
state and solution photoluminescence spectroscopy analyses were car-
ried out by an Agilent G9800A fluorescence spectrophotometer. Solid
sample holder with 10 × 10 mm window used for powder compounds.
The absolute solid-state fluorescence quantum yield measurements
conducted on a Horiba Scientific Quanta-φ integrated sphere setup,
2

H. Nikookar and P. Rashidi-Ranjbar
Journal of Photochemistry & Photobiology A: Chemistry 386 (2020) 112106
the geometry and the optimized structure was utilized to calculate the
electronic properties of host 1 by TD-DFT method at the level of B3LYP/
6-31 G(d). The optimized structure of host 1 in global minimum con-
formation shows that the pincer fragments adopt a planar structure and
anti positions around the spacer with a torsional angle of 160°. The
highest occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) of host 1 are presented in Fig. S1 which
shows that electron density distribution of HOMO is localized on the
phenanthrene core, whereas LUMO is located mainly on pyridinium
ring.The calculated results revealed the existence of the intramolecular
charge transfer (ICT) process from phenanthrene as donor and pyr-
idinium ring as acceptor through imidazole π-bridge in 1. The large
dipole moment in excited state (ES) compared to ground state (GS) is
another evidence for the ICT.[35] The calculated dipole moment of host
1
in the GS and ES are 3.47 D and 14.08 D, respectively. The calculated
π→π* transition (from the calculated UV–vis spectrum) is 1.54 eV
which could be related to phenanthrene π orbitals (HOMO) to pyr-
idinium ring π* orbitals (LUMO). (Fig. S2)
Fig. 1. Photographs (under daylight (top) and irradiation at 254 nm (down))
and emission spectra of host 1 in acetonitrile/water mixtures (λex =420 nm, c
=0.4 μM) with different water fractions (fw).
3.2. Synthesis
The host 1 was prepared through the reaction between 2,5-bis
bromomethyl)-1,4-bis(dodecyloxy)benzene (4) and 2-(4-Pyridinyl)-
H-phenanthro [9,10-d]imidazole (5) in DMF at 100 °C to form 1-Br,
different fractions of water to study the AIEE behavior of 1. The solu-
tion of 1 in acetonitrile emits a greenish yellow light which does not
change remarkably with increasing the water fraction up to 30%. Along
with the formation of aggregates by addition of water from 40% to 70%
as the solution turns cloudy, the PL intensity gradually increases and
the emission shifts to yellow light (Fig. 1a). However, the PL intensity
decreases when the water content reaches to 80% probably due to in-
crease in the aggregate size or decrease in the crystallinity of particles.
(
1
and then the anion was exchanged by treating with ammonium hexa-
fluorophosphate (Scheme 2). O-alkylation of hydroquinone formed
compound 4 by 1-bromododecane in DMF followed by addition of
methylene bromide using para-formaldehyde and hydrobromic acid in
acetic acid. Simultaneously, compound 5 was synthesized from pyr-
idine-4-carbaldehyde, 9,10-phenanthrenedione and an excess amount
of ammonium acetate in acetic acid. Simple reactions, good yields, easy
and inexpensive purifications (without usual chromatography proce-
dures) are the advantages of synthetic procedure. Alkylation takes place
at position 1 of the pyridine ring rather than imidazole. This expression
is supported by the more significant changes in chemical shift of pyr-
idine protons of compound 2 compared to compound 5 in the 1HNMR
spectrum, i.e., a downfield shift of about 0.7 ppm for the ortho protons
in pyridinium ring compared to the non-alkylated pyridine ring (Fig.
S3). Hexafluorophosphate salt of 1 has been used for further in-
vestigation because the bromide salt is not emissive enough in the solid
state. Unfortunately, despite all efforts, crystallization of host 1 was not
successful, probably because of the existence of long alkyl chains.
[
36] At the same time the emission wavelength is shifted hypsochro-
mically from 528 nm to 545 nm by increasing the solvent polarity by
addition of water. These results imply that 1 could act as an AIEE active
fluorescent molecule. Compared to host 1, reference molecule 2 shows
neither ACQ nor AIE effect with increasing water to THF ratio and no
aggregation is observed in solution. (Fig. S4)
3.4. Mechano- and thermoresponsive behavior of 1
The host 1 illustrates reversible mechanochromic and thermo-
chromic behavior. The optical properties of host 1 before and after
grinding was evaluated by diffuse-reflectance spectra (DRS) measure-
ments (Fig. S5a). The host 1 shows a broad absorption peak between
250–500 nm. Grinding causes the absorption edge to shift to higher
wavelengths (about 20 nm). This shift was observed as a change in color
from yellow to orange by naked eyes. It has been found that the band
gap is related to crystallinity [37] and particle size [38], mechanical
grinding crashes the microcrystalline structure and might enforce
3
.3. Aggregation-induced emission enhancement (AIEE)
Host 1 is not soluble in most organic solvents. It only dissolves in a
few polar solvents, in which, it did not show any solvato-chromic ef-
fects. The fluorescence analyses were performed in acetonitrile with
Scheme 2. Synthesis of molecular host 1, i) 1-Bromododecane, K
DMF, reflux, v) NH PF , acetone, r.t.
2
CO
3 2 3 4
, DMF, reflux ii) CH O, HBr/CH COOH, iii) Pyridine-4-carbaldehyde, NH OAC, acetic acid, iv)
4
6
3

H. Nikookar and P. Rashidi-Ranjbar
Journal of Photochemistry & Photobiology A: Chemistry 386 (2020) 112106
its high decomposition temperatures in the range of 255 − 280 °C. The
TGA profile of host 1 reveals a 3% weight loss at around 90 °C probably
due to the release of the trapped acetonitrile (Fig. S6). In addition, a
change in color to deep-orange was observed in daylight when the as-
prepared or ground powders were heated up to 120 °C, whereas the
fluorescence intensity quenched utterly. After cooling to room tem-
perature, the color turned immediately back to orange and the fluor-
escence emission intensity increased again. Interestingly, λmax of
emission of the cooled powder was observed to be identical to the λmax
of the ground compound, but not the original powder before heating
(λmax =556 nm).With this findings in hand, we propose that grinding
in addition to force sticking small aggregates to the bigger one, might
also causes the release of the trapped solvent molecules resulting to
color change [11a,41].
Also, the mechano and thermoresponsive behaviour were observed
in reference molecule 2. Mechanical force shifts the emission peak to
higher wavelength (about 30 nm). Similarly heating 2 causes too little
color change although the spectrum of cooled powder was identical to
initial sample. (Fig. S7)
Fig. 2. PXRD patterns and SEM images of host 1 before and after grinding.
adhesion of small particles of the amorphous phase to each other
leading to decrease the band gap from 2.30 eV for as-prepared host to
3.5. Guest-responsive behavior
2.22 eV for the ground one (Fig. S5b). This issue was supported by SEM
images and PXRD patterns as shown in Fig. 2. Before grinding, the host
exhibits irregularly block-like aggregated microstructures with size less
than 1 μm. The mechanical grinding leads to a more amorphous and
bulky structure. The broader and shorter signals for the ground com-
pound compared to the sharp and intense signals of the host 1 in the
PXRD patterns is a sign of the crashed microcrystalline structure and
disordered molecular packing due to grinding. This comminuted par-
ticles sticks together to form an amorphous phase with larger particle
size.
To consider the influence of different guests on the optical proper-
ties of host 1, first, the optical properties of host 1 and carbazole (CAB)
as a guest was studied in the solution and then in the solid state. As
shown in Fig. 4a, the absorbance spectrum of host 1 in acetonitrile
shows notable broadband at 420 nm which could be attributed either to
π-π* transitions or formation of ICT between donor-acceptor moieties
of the host itself. Addition of CAB as a guest to the solution did not have
any distinctive effect on the absorption probably due to the weakness of
π-π interactions compared with the interactions between the polar
pincer and the solvent molecules, hindering the efficient interaction
with guest. Also, for the solution of equimolar of host 1 and CAB, a
weak CT band was observed at 650 nm. This result indicates that the CT
complex was formed between host 1 and CAB. Likewise, a very little
quenching of the broad emission band located at 527 nm in PL spectrum
was observed via the addition of an equimolar amount of CAB to the
solution of 1 (Fig. 4b).
Moreover, the fluorescence emission changes from yellow (λmax
=
548 nm) to orange (λmax =556 nm) by grinding. On the other hand,
f
the host 1 has a strong emission with an absolute quantum yield (φ ) as
much as 31% in the solid state and when it is ground, the absolute
quantum yield decreases to 24%. Although the contrast is low between
the two states but there are different methods to increase contrast in
mechanochromic materials [39]. The optical properties of the ground
compound returns to its initial state (as-prepared host) in contact with
acetonitrile vapors (Fig. 3). It seems that the solvent molecules are able
to reorganize damaged molecular packing. The very same effect has
been reported in the literature [40]. According to thermogravimetric
analysis (TGA), the excellent thermal stability of host 1 is indicated by
For inclusion of guests into the host packing in the solid state, a
mixture of pure materials was ground physically by agate mortar and
pestle. A red shift occurred in the DRS spectrum of the ground mixture
of 1 with CAB; comparing this spectrum with the one for the ground
compound, the shift is more when the equivalent amount of CAB
Fig. 3. a) Reversible diagram of host 1 photographs with different stimuli under UV light (irradiation at 254 nm) (right) and daylight (left); b) Emission spectra
excited at 330 nm) of host 1 under different stimuli.
(
4

H. Nikookar and P. Rashidi-Ranjbar
Journal of Photochemistry & Photobiology A: Chemistry 386 (2020) 112106
Fig. 4. a) The absorption spectra of host 1 in acetonitrile (2 μM) with a different equivalent of carbazole; b) The emission spectra (λex: 420 nm) of host 1 in
acetonitrile (2 μM) in the presence of an equivalent of carbazole.
grounds with host 1. The band gap also decreases more with the ad-
dition of CAB (Fig. S5). It should be noted from PXRD and SEM results
that the crystallinity and particle size are almost the same for the
ground host and the inclusion complex. Guest inclusion could reduce
electronic band gap through CT in donor-acceptor systems which leads
to an improved electrical conductivity [42].The optical band gap of the
different series of electron-rich aromatic compounds developed a color
range from army green to red-orange and caused significant fluores-
cence quenching. Because the host was dicationic, we chose electron-
rich aromatic guests which expected to have maximum interaction. An
extended π-π-stacking interaction seems to be the dominant interaction
since some of these aromatic guests could not have any other type of
intermolecular interactions. The broad emission band of host 1 at
556 nm shifted within 40 nm (in the range of 542–582 nm) with dif-
ferent guests such as 1,5-naphthalene diol (NDO), phenanthrene
(Phen), anthracene (AN), pyrene (Py), 1,4-phenylene diamine (PDA),
1
⊂ CAB was calculated to be 2.15 eV.
During the grinding process, the color of the mixture gradually
changed in day light, as well as under UV irradiation (Fig. 5a). The
formation of CT complex as a consequence of grinding host 1 with
Fig. 5. a) Photographs of host 1 and its complex with guest under daylight and UV lamp (irradiation at 254 nm); b) The solid state emission spectra (excited at
30 nm) of host 1 with different guests (1:1); c) PXRD pattern of host 1 as-prepared (cyan line), 1 in the ground state (yellow line), 1 ground with carbazole (green
3
line), 1 and carbazole co-precipitated (pink line) and theoretical powder pattern from a CIF of carbazole crystal (blue line).
5

H. Nikookar and P. Rashidi-Ranjbar
Journal of Photochemistry & Photobiology A: Chemistry 386 (2020) 112106
fluoren-9-one (Flu), carbazole (CAB) and phenenthroline (PhN)
(CIEE) effect [45] was observed in sample which was produced by co-
precipitation method which is supported by PXRD results. Also, the red
shift in co-precipitation method (λmax = 579 nm) is more than grinding
one (λmax = 567 nm). Because the molecules in the solution have more
chance to make suitable arrangement for effective interactions. In the
PXRD pattern of the co-precipitated complex, new peaks appeared
while the crystalline structure of the host preserved. The similarity of
this pattern with the one from pure host 1 as-prepared, could be used as
evidence to prove that the observed red shift is due to the interaction of
1 with CAB. The new peaks are similar to CAB pattern obtained from its
CIF file. The peaks in the PXRD pattern of the co-precipitation proce-
dure are sharp while in the ground material presence of more amor-
phous phase causes peak broadening.
There remains a question, whether the structure of host 1 is ne-
cessary to interact with guests or the pincer alone could be enough? To
answer this question and to have more insight about guest-responsive
behavior, compound 2, as a reference molecule, has been synthesized
and its interaction with CAB as a guest was compared with the host 1.
As can be seen from Fig. 6b, the ground mixture of CAB with compound
2 does not show any difference with the ground pure 2. In conclusion,
the whole structure of 1 is responsible for the observed phenomena.
(
5
Fig. 5b). None of the guests have emission band in the range of
00–700 nm [43]. Therefore, they should have no contribution in the
observed fluorescence emission. Engineering the effective interactions
and prediction of π-donor strengths of guest molecules have been de-
scribed previously by employing the calculated molecular electrostatic
potential surface and Hammet substituent constants [44]. We used
electrostatic potential maps to align guest interaction trends in order to
interpret the acquired results in this study (Fig. S8). The calculations
were carried out by DFT method using B3LYP/6-31 G(d) level of theory.
PDA and NDO showed the most quench in PL diagram. PDA is com-
pletely quenched but the absolute quantum yield for NDO is 1.8%.
According to calculated electrostatic potential maps, these guest mo-
lecules provide high electron density surfaces and are expected to show
the most efficient interactions with the electron-deficient host 1. Py,
PhN, Phen and CAB are known to be suitable electron donors, showing
more quenching effect compared with fluoren-9-one and anthracene.
The absolute quantum yield for 1 ⊂ Py, CAB, PhN and Phen complexes
are 4.6, 4.8, 4.9 and 5.1% respectively, which are about five times
lower than the ground form. The absolute quantum yield for Flu and An
are 14 and 8.1% respectively, due to weak interaction with host 1.
Furthermore, Py and CAB illustrate bathochromic (red) shift. These
results indicate the strong CT interaction between host and guest that
leads to quenching and red shift in the spectrum.
4. Counterion responsive behavior
Besides, SEM-EDX elemental mapping was used to confirm if the
inclusion occurs uniformly during the grinding of host 1/guest or only a
physical mixture is formed. For this purpose, the mixture of host 1 and
Organic salts play an important role in construction of supramole-
cular assemblies [46] and the arrangement in self-assembled structures
could be simply modified by changing the counter ion [47]. Moreover,
the ionic materials are often conductive and therefore can find potential
applications in solar cells or photovoltaic devices [48] and some of
them have also been used in imaging [49]. The electrostatic attraction
between the cationic chromophore and anionic moiety could be used to
tune the luminescence properties of the emissive compounds [50]. It is
proved that anion exchange can affect the emission because of the
coulombic attraction and steric effects which appear in molecular
packing of crystals or aggregates [51]. The effect of counterion on light
emission was studied by measuring the fluorescence spectra of host 1
with different anions. The absolute quantum yield and the λmax changes
drastically when anion exchange takes place. The amount of quantum
3,6-dibromocarbazole as a guest, grounded together and elemental
distribution was studied. The Br atom merely was presented in the guest
molecule while P and F atoms existed just in the host molecule. Fig. S9,
shows that all of the elements (C, O, Br, P, and F) are distributed
homogeneously, indicating that grinding was able to put the host and
the guest uniformly together. Also, it can be concluded from the
homogeneity that the product is an inclusion compound rather than a
physically mixture. Because, obtaining a completely homogenous
blends from physically mixing microcrystalline structures is a very rare
case.
The difference between the grinding procedure and co-precipitation
to form inclusion compounds was studied by co-precipitation of CAB
and the host 1 from the acetonitrile-chloroform mixture through sol-
vent evaporation. Then, the PL spectrum of this micro-crystalline
−
−
−
yield for F , SCN and Br− anions are low (about 1.54, 1.26 and
−
0
.88% respectively) while I and B(Ph) anions/host 1 are not emis-
sive. However, PF , ClO and Cl anions/host 1 are highly emissive
and quantum yield are 31.5, 11.9 and 7.2% respectively. The shift due
to anion exchange occurs in the range of 522–578 nm (Fig.7). Tetra-
phenylborate is a very big anion. Thus, the steric repulsion between the
4
−
−
−
powder compared with the ground one; grinding of 1 with CAB (φ
f
=
6
4
4
.8%) shows more quenching effect than co-precipitation procedure (φ
f
=
8.9%) (Fig. 6a). The crystalline-induced emission enhancement
Fig. 6. a) The solid state PL spectra of compound 1 and its carbazole complex prepared by grinding and evaporation procedure (excited at 330 nm) and photograph
under irradiation at 254 nm) of host 1-carbazole with different method preparation; b) The solid state emission spectra of compound 2 and its grinding mixture with
carbazole (1:1).
(
6

H. Nikookar and P. Rashidi-Ranjbar
Journal of Photochemistry & Photobiology A: Chemistry 386 (2020) 112106
Acknowledgements
The financial support by the Research Council of the University of
Tehran is acknowledged. The authors are grateful to Ehsan Hamzepoor
for useful help with DFT calculations.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.jphotochem.2019.
112106.
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