A simple D-π-A system of phenanthroimidazole-π-fluorenone for highly efficient non-doped bipolar AIE luminogens: Synthesis, and molecular optical, thermal and electrochemical properties

Article abstract of DOI:10.1039/c9nj05226g

A series of hybrid organic bipolar fluorescent aggregation-induced emission (AIE) luminogens have been synthesized with D-π-A architecture using phenanthroimidazole as a conjugate by incorporating a fluorenone moiety at the C2 position of a rigid skeleton

Full text of DOI:10.1039/c9nj05226g

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DOI: 10.1039/C9NJ05226G
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A simple D–π–A system of phenanthroimidazole-π-fluorenone for
highly efficient non-doped bipolar AIE luminogens: Synthesis,
molecular optical, thermal and electrochemical properties
Received 00th January 20xx,
Accepted 00th January 20xx
Kathirvelan Devarajan,a Mayakrishnan Sivakalai,b Uma Maheswari Narayanan,b Chinmoy Biswas,c
Sai Santosh Kumar Raavi,c Tarun K. Panda* a
DOI: 10.1039/x0xx00000x
A series of hybrid organic bipolar fluorescent aggregation-induced emission (AIE) luminogens have been synthesized with
D–π–A architecture using phenanthroimidazole as a conjugate by incorporating a fluorenone moiety at the C2 position of a
rigid skeleton of 1,2-diphenyl-1H phenanthro[9,10-d]imidazole. The synthesis was achieved by the Suzuki coupling reaction
between 2-(4-bromophenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole and 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)-9H-fluoren-9-one. All derivatives of the product were found to be thermally stable, with high melting and high
decomposition temperatures. The phenanthroimidazoles P1–P5 displayed the natural properties of AIE. The wide colour
contrast between the green and yellow regions were demonstrated by these luminogens. Among them, P1 displayed higher
quantum yield and long lifetime properties. Thus, the obtained results clearly demonstrate all the luminogens are promising
materials for research and use in optoelectronic devices, sensing applications, biological probes and intercellular imaging.
platform for luminophore research.8–9 In fact, this concept is exactly
opposite to ACQ and has been getting considerable attention from
many researchers around the world because these AIE luminogens
have tuneable colour and high quantum yields and can be used in
various applications from optics and electronics to energy and
bioscience materials.
Introduction
Organic fluorescent materials are finding various applications.
For example, in optoelectronic devices, data storage, memory
devices,1 fluorescent probe for bio applications,2–3 and thin-film
transistors. When compared with inorganic emitters, organic
materials demonstrate many advantages, including high brightness,4
large Stokes shift,5 quantum efficiency, good photostability,6 and
biocompatibility. Over the past two decades, many scientists
worldwide have been attracted by organic conjugate molecules.
These luminophores emit bright light in solution when dissolved in
various solvents. However, a problem arises when utilising these
classic luminophores in a solid-state or in an aggregated state, a
quenching effect called ACQ occurs, which involves a drastic
deterioration in luminescence efficiency, during the transformation
from solution to solid or aggregated state.7 This is mainly because of
slow non-radiative decay and intermolecular π–π stacking
interactions. Studies undertaken to find ways to overcome
quenching, as published in the literature, had witnessed no solution
to this phenomenon.
Tremendous research has been devoted to applying AIE
luminogens in the solid-state form to a variety of technologies,
especially as luminescent sensors and light-emitting devices.10 For
example, these organic AIE dyes are used as chemosensors for
detecting explosives, toxic metals and volatile organic compounds.11
Also, it has huge potential for use in imaging intracellular organelles,
evaluating enzymatic activity, long-term cell tracking and in vivo
imaging of cancer cell lines.12 Moreover, light-emitting devices are
predominantly used in the solid-state, which plays a vital role in the
emission efficiency of the AIE luminogens.
To achieve efficient fluorescent materials for light-emitting
applications, the molecule must be designed in such a way that
a bipolar donor-acceptor system is incorporated together in the
designed single entity for lowering the charge injection barrier
and to broaden the carrier combination zone in the emissive
layer.13 Hence, an excellent blue-green emitter can be obtained
by incorporating donor and acceptor in a single molecule with
different arrangements of substituents. Phenanthroimidazole is
a promising material that is constructed from a variety of
materials with different donors and acceptors, as it is ambipolar
in nature.14
In 2001, the concept of aggregation-induced emission (AIE) was
devised by Tang, where aggregate formation enhances light emission
instead of a reducing it, and this discovery made a new and vast
a. Department of Chemistry, Indian Institute of Technology Hyderabad, 502285,
India. E-mail; (tpanda@iith.ac.in)
b. Organic & Bioorganic Chemistry Laboratory, CSIR-Central Leather Research
Institute, Chennai, 600020, India.
Phenanthroimidazole has received substantial attention due to
its exceptional photophysical properties and photostability. It emits
near-ultraviolet region and the luminogen PI is rigid and non-planar.
When the moiety is incorporated with an appropriate combination
c. Department of Physics, Indian Institute of Technology Hyderabad, 502 285,
India.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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O
of donor and acceptor, it leads to products which display high
thermal stability, good charge-transporting property, wide bandgap,
highest film-forming capacity and high quantum yield.15 Several
research groups have utilised phenanthroimidazole as a promising
bipolar emitter of blue-green light in various ways, such as in live-cell
imaging, toxic metal sensing and organic light-emitting diodes
(OLEDs). OLEDs16, in particular, have received much attention, since
they have numerous advantages high power conversion efficiency,
fast response time, low-cost fabrication, and planner light source and
have thus replaced traditional light sources to a great extent in
emerging economies.
O
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DOI: 10.1039/C9NJ05226G
N
O
Pd(PPh3)
4
N
B
+
N
K2CO3, THF/H2
85 oC, N2, 24 h
O
O
Br
N
R
R
S1,S2,S3,S5
P1, P2, P3, P5
R = H, OMe, CN, butyl
O
O
O
Pd(PPh3)
4
N
N
N
B
+
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K2CO3, THF/H2
85 oC, N2, 24 h
O
O
N
Br
P4
S4
In order to gain deeper insight into the fascinating nature of
phenanthroimidazole, as well as to explore its prospective
technological applications, our research group began by synthesising
five phenanthroimidazole emitters by incorporating various groups
of donor and acceptor, with an ambient length of the spacer at the
C2 position. This position was chosen since excessive length of spacer
causes a red and broad-shifted emission and different substituents
(donating or withdrawing) at the para position of the phenyl ring at
the N1 position of imidazole ring of PI which inhibits π-π stacking and
thus restricts molecular interaction.17 Until now, the majority of
fluorescent phenanthroimidazole incorporated various moieties and
the materials display red-shifted emission behaviour in solution or
solid-state upon conjugation with acceptor groups. Consequently,
the correct mechanistic understanding of the incorporation process
must be achieved to develop a fine-tuneable donor and acceptor
push−pull systems. It is believed that charge transfers, termed
twisted intramolecular charge transfer (TICT),18 between
phenanthroimidazole and fluorenone will improve the quantity of
singlet exciton yield, and that the phenyl π bridge between
phenanthroimidazole and fluorenone promotes the formation of the
LE state and thereby improves fluorescence efficiency.19
Scheme 1. Synthetic route for compounds P1–P5 by Suzuki Coupling.
Fig. 1. Molecular structures of Phenanthroimidazole-core molecules.
Molecular structure and crystal packing
A single crystal of compound P3 was obtained through slow
evaporation of the THF solution. The molecular structure and solid-
state packing features of the synthesised compound P3 were
confirmed by single-crystal X-ray analysis (Fig. 2). The obtained data
has been submitted to the Cambridge Crystallographic Data Centre
(CCDC) and the deposition number is 1947005.
Hence, in this combination, phenanthroimidazole acts as a weak
donor and fluorenone is used as the moderate acceptor, where both
moieties are linked by a phenyl or naphthyl π bridge and this strategy
has been adopted for obtaining good thermal properties and high
morphological stabilities when PI unit is imparted with fluorenone.
Moreover, these five non-doped emitters are expected to exhibit
very high quantum yields, high lifetimes, low turn-on voltages and
stable emission colours.
Results and discussion
Synthesis and identification
Fig. 2. Molecular structure of the luminogen P3.
To synthesise compounds P1–P5, the Suzuki coupling reaction was
used. S1–5 (1.00 equ.) were treated with fluoren-9-one moiety (1.00
equ.) using Pd(PPh3)4 (10.0 mol %) and K2CO3 (2.0 equ.) in THF/H2O
(7:3). After the reaction was completed, the crude residue was
purified through a silica gel column using hexane and DCM as eluents
and a yellow solid was isolated, with yields of 62% to 85%. The
Table 1 Sec. 8 ESI† contains important and relevant data. Fig. 3
provides details of packing order, which are useful to understand the
possible intermolecular interactions of P3. The benzene ring linked
with the –CN group at the N1 position of imidazole found to be
virtually perpendicular to the adjacent fluorenone plane. The
intermolecular interaction between two molecules in the unit cell is
linked by offset face-to-face π – π stacking between the fluorenone
and the phenanthroimidazole planes and the value is 3.552 Å with
the dihedral angle of 5.23ᵒ, which implies that the interaction is
significantly weak.
1
synthesised compounds were analysed using H NMR, 13C NMR, IR
and mass spectrometry techniques.
The designed compounds were synthesised according to the path
illustrated in Scheme 1.
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Two neighbouring parallel phenanthroimidazole PI moiety
planes overlap to a certain degree and the distance between the two
molecules is 3.978 Å. Hence, the possibility of π–π stacking
interaction can be ignored due to the normally accepted concept
that an effective intermolecular interaction frequently occurs within
an effective distance when it is less than 3.7 Å.
The packing diagram of P3 discloses tight packingV. iPew3AprtoicsleseOsnslienes
two molecules in the unit cell, with two mutual hydrogen bonding
DOI: 10.1039/C9NJ05226G
interactions between the –CN group. Hydrogen bonding at the N2
position of the imidazole ring results in improvement20 in the
photoluminescence (PL) intensity of P3 and can be easily ascribed to
the constrained packing of molecules in the aggregated state at fw-
50%. As shown in the crystal, the dye material P3 exhibits a highly
twisted molecular conformation with large dihedral angles, which
measure 72.89ᵒ between the PI plane and the adjacent benzene ring,
38.06ᵒ between fluorenone plane and the side benzene ring 58.36ᵒ
between the phenanthroimidazole plane and the bridging benzene
ring respectively. Single-crystal X-ray analysis of compound P3 shows
that it adopts a large twisted intramolecular configuration, which
leads to aggregation in the solid-state. Hence, this phenomenon is
valuable to be used as a non-doped emitter in OLEDs.
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Fig. 3. Single crystal X-ray packaging diagram of P-3.
Fig. 4. (a) Thermogravimetric analysis (TGA) curves of P1–P5. (b) Glass transition temperature by differential scanning calorimetry (DSC) of P1–P5.
Thermal properties
Theoretical studies
Calculations under the density functional theory (DFT) were carried
Good thermal stability of a luminogen plays a significant role
in obtaining high performance with a long lifetime of
optoelectronic devices. In particular, the glass transition
temperature of an organic luminophore is very important – it
influences device stability and lifetime, since irretrievable
damage may be possible to the fabricated device when heated
above glass temperature. Hence, thermal property
measurements were recorded for the synthesised compounds
using differential scanning calorimetry (DSC), as shown in Fig 4a.
out using the Gaussian 09 software program. Quantum chemical
calculations were performed to obtain a complete picture of the
electronic properties of bipolar luminogens.22 As shown in Fig. S32–
36 (Sec.9 ESI†), the highest occupied molecular orbital (HOMO) of all
luminogens (P1–P5) are located entirely on phenanthroimidazole
backbone, whereas the lowest occupied molecular orbital (LUMO) is
located completely on the fluorenone moiety due to their strong
electronic behaviour. This reveals the separation of the HOMO and
LUMO states. There is no electron density distribution observed in
donating or withdrawing groups on the central N-phenyl group of
imidazole ring. However, HOMO and LUMO located
phenanthroimidazole and fluorenone moiety, respectively. The
energy level of HOMO and LUMO are given in Table 1. The HOMO–
LUMO energy levels were practically estimated by cyclic
voltammetry (CV) measurements.
o
The decomposition temperature (Td) started at 423 C for 7%
weight loss of P3 and P5 and the high Td can be attributed to the
large rigid plane of phenanthroimidazole-incorporated
fluorenone.20
However, Td for P1, P2 and P5 were noted as approximately
253 oC. Glass transition temperature and melting point are also
vital and observed values for compounds P1–P4 are set out in
Fig. 4b. However, a very low glass transition temperature was
observed for compound P5. This can be attributed to its
amorphous nature, as it has a long alkyl chain in the PI
backbone. This clearly indicates that all the molecules possess
The electrochemical properties of luminogens (P1–P5) were
examined, based on DFT studies, by cyclic voltammetry using
tetrabutylammonium perchlorate (0.1 M) as a supporting electrolyte
in HPLC grade dichloromethane (DCM) solution at a scan rate of 100
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mV s−1. The calculated HOMO and LUMO energy levels of the mechanisms in electroluminescent devices. By subtracting the band
View Article Online
luminogens are given in Fig. 5. The outcome, reinforced with DFT, gaps from the HOMO energy levels, the LUMO energy levels were
DOI: 10.1039/C9NJ05226G
allows for general inferences to be made about the charge-transport calculated and the details of calculation are provided (Sec. 9,10 ESI†).
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Table 1. Theoretical HOMO and LUMO readings of compounds P1-P5
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HOMO
LUMO
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P1
P2
P3
P4
P5
backbone with a phenyl or naphthalene ring used as a spacer in
We assessed the HOMO and LUMO levels of the system based between. It is believed that the threshold potential of oxidation
on the onset oxidative and reductive potentials luminogens.23
increases with decreasing electron-donating strength. The
combined DFT and CV data are given in Table 2. Thus, the
Phenanthroimidazole luminogens P1–P5 are bipolar in bandgap of methoxy substituted P2 has a lesser band gap than
nature. They can both donate and receive electrons in push-pull that of the luminogens. Interestingly, the results revealed that
systems. However, we have incorporated a strong electron luminogens were constructed in a Donor-π-Acceptor sequence,
acceptor –fluorenone moiety – in the phenanthroimidazole with potentially bipolar transporting properties.
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Table 2. Electrochemical properties of P1–P5 materials
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DOI: 10.1039/C9NJ05226G
Comp
ound
Theoretical study from
DFT
Experimental study
from CV
LUMO
(eV)
HOMO
(eV)
LUMO
(eV)
Eg
(eV)
HOMO
(eV)
E g
(eV)
P1
P2
P3
P4
P5
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-5.22
-5.49
-5.22
-5.23
-2.40
-2.38
-2.48
-2.41
-2.39
2.86
2.84
3.00
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2.83
-5.07
-4.84
-5.44
-5.12
-5.16
-2.21
-2.36
-2.59
-2.30
-2.32
2.86
2.48
2.84
2.81
2.83
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Fig. 6. Solvatochromic PL spectra of P1
Consequently, DMSO is the best solvent to realise the AIE
effect on the luminophore, with water being added
incrementally.
The organic luminophores (P1–P5) offer the best
opportunity to understand the TICT effect, which consists of the
D-π-A system with various structural combinations of moderate
electron-donating phenanthroimidazole and electron-deficient
fluorenone. AIE behaviour was explored by UV/vis absorption
spectra of P1 in DMSO–water combinations, by a gradual
increase in the percentage of water, as given in Fig. 7a. The
luminogen P1 displayed two typical absorption bands
positioned at shorter (313–367 nm) and longer wavelength
regions (416–468 nm) in DMSO. The first transition could be
assigned to the π-π* transition of phenanthroimidazole,
whereas the second band may originate from TICT between the
donor and acceptor. Interestingly, the absorption band at fw-
50% of DMSO–water solvent showed a slight red shift in both
the transition and CT transition compared to the water fraction
at fw-0% with minor spectral broadening. In the solution state,
the rotational and vibrational motions of the
phenanthroimidazole core and the fluorenone units work
together to disperse the excited-state energy.
Fig. 5. Cyclic voltammetry curves of P1–P5 materials.
Aggregation-induced emission
The photophysical properties of fluorophores P1–P5 were
explored in solution by varying solvent polarity, as the AIE
phenomenon is strongly dependent on solvent polarity. The
synthesised compounds P1–P5 are highly soluble in many
organic solvents such as tetrahydrofuran, acetone,
dichloromethane, DMSO, etc.; however, they are sparingly
soluble in water, ethanol and methanol owing to the
hydrophobic nature of their aromatic rings. For better insight
and to choose the appropriate solvent for realising the AIE
effect compounds P1–P5, the solvent effects on
ultraviolet/visible (UV/vis) absorption as well as PL of P1 were
initially studied, using various solvents ranging from the
nonpolar solvent DCM to the high polar solvent DMSO (10 µM).
As shown in Fig. 6, the PL spectrum in non-polar solvent toluene
expressed maximum intensity, while in DMSO, the spectrum
expressed minimal intensity.
The PL spectra of the luminogen P1 is shown in Fig. 7b. As
we expected, the emission is completely quenched at fw-0%.
With gradual incremental addition of water to
DMSO/luminophores from fw-10% to fw-40%, the emission
intensity slowly increases, with a slight red shift owing to the
enhancement of solvent polarity.27 However, the emission
drastically increases to maximum emission band at 550 nm in
DMSO/water mixture fw-50% which can be attributed to π-π*.
This can be attributed to the deteriorated solvating ability of the
DMSO/water; in other words, the luminogen molecule begins
to aggregate at this point.
The solvent effects can be correlated to the fact that faint or
non-fluorescence in more polar solvents may be due to rapid
interconversion from the emissive local excited (LE) state to the
low emissive TICT state, while robust emission in nonpolar
solvents or in the thin film state may be owing to the restricted
intramolecular motions (RIM).24, 25
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contrast with those of their solutions or aggregated states, as shown
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in Fig. 8c. and the intensity is in the order P1>P5>P4>P2>P3.
DOI: 10.1039/C9NJ05226G
This order provides noteworthy information on the mode of
energy transfer. Moreover, we conducted a 3D study for
fluorescence spectrum of P1 in the steady-state mode to
understand the nature of emission maxima at various excitation
wavelengths, from 300 to 450 nm, and emissions in the window
between 400 nm and 800 nm in a thin film state, as shown in
Fig.9. The intensity was plotted as a function of excitation
wavelength and emission wavelength. We can conclude from
the figure that the emission property is independent of
excitation wavelength. The 3D of fluorescence spectrum of P4
and P5 has been given in Figure S41-42 (Sec. 11.2 ESI†).
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Fig. 7. (a) Absorption (b) PL spectrum of P1 in DMSO/Water mixtures
with varying water fraction (fw); Con-10µM; Excitation wavelength-
380 nm.
Surprisingly, increasing in the water fraction results in the
max
gradual decrease in intensity of the emission λ
fw-90% with consistent emission maxima λ
from fw-50% to
max
at 550 nm. When
water content increases beyond the saturation point (fw-50%), the
solvating power of the DMSO–water mixture decreases, which is
attributed to the fact the packing mode of the molecule’s changes
from orderly to casual. The decrement of the intensity of the
luminogen from fw-60% to fw-90% can be attributed to this
phenomenon. 27
Fig. 9. The 3D fluorescence spectrum of P1 in thin film state.
Fig. 10. Time Resolved (TCSPC) life time measurement of (a) P1-P5 in
Thin film, (b) fw 0% (c) fw 50%
Fig. 8. (a) Steady State PL Measurement of P1–P5 at 380 nm (b) DMSO:
water fw 0% (b) DMSO: water fw 50%. (c)PL emission intensity of P1–P5 in
the thin-film state.
Besides, the steady-state and time-dependent fluorescence
lifetime decay experiments of luminophores P1–P5 were
carried out in the solution and thin-film state to identify the
significance of aggregation in the changing aspects of decay of
the excited state. The results for thin film and solution state are
depicted in Fig. 10 and in Table 3. Complete data of lifetime
decay may be found in S59–72 (Sec.11.6). These results
afforded additional information on the excited state of the
fluorophores. With emission at 561nm, the mono-exponential
lifetime studies of P1–P5 in thin-film shows that P1 has a
maximum lifetime of 12.34 ns, while P2 has a shorter lifetime of
2.67 ns. Bi-exponential lifetime data has also been provided,
A similar pattern was observed for all the remaining fluorophores
(P2–P5). The fluorescence intensities of solutions of dyes (P1–P5) in
DMSO/water increased remarkably with the addition of water and
these fluorophores displayed high emission colours, which varied
from green to yellow to red in the following manner at fw 50%
P1>P5>P3>P4>P2, depending on the substituent effect on the phenyl
group in para position with different substitution on N1 position of
phenanthroimidazole. The intensity of plot b is 300% stronger than
plot a, as shown in Fig. 8. The UV and PL spectra of luminophores P2–
P5, when the water fraction was increased from 0 to 90%, are given
in (11.1 ESI†). The PL spectra of all the luminogens in thin-film are in
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and these decays are typical of donor-acceptor systems with a
ARTICLE
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View Article Online
DOI: 10.1039/C9NJ05226G
π-π* transition and TICT state.
5
6
Table 3. Lifetime of Thin Film Samples
7
8
9
Sample
P1
τ1
τ2
12.34
2.67
5.76
4.94
8.93
17.53
6.32
9.70
9.25
18.00
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60
P2
P3
P4
Fig. 11. The CIE chromaticity diagram for thin film photoluminescence.
P5
Conclusions
The quantum yield (ΦF) was measured using the integrated
sphere method using thin-film state and DMSO–water mixtures
in various water fractions (fw). The solution state quantum yield
was measured using a direct excitation method at 380 nm. The
ΦF of P1 shows a maximum of 19.92% and a minimum of 3.69%,
as shown by P2 in the solution state. However, the quantum
yield of Solid-state reveals that ΦF of P1, P4, P5 is twice the
value of the solution state. (S52–56- Sec.11.5 ESI†). The
Commission Internationale de l'Eclairage (CIE) chromaticity
diagram28 can be plotted for reckonable estimation of PL, and
the plot is in Fig. 11.
We have successfully designed and synthesised a series of five
phenanthroimidazole materials by integrating phenanthroimidazole,
which acts as a weak donor, and fluorenone, which is a moderate
acceptor, using the D-π-A model. This integration was carried out by
different substitutions in the phenyl ring of imidazole at N1 and two
different spacers at C2, which were subsequently characterised.
Among all these materials, luminogen P1 showed good quantum
yield and longer lifetime. Moreover, all the synthesised materials
displayed good thermal properties with a high glass transition
temperature. This improved thermal stability makes these materials
more attractive as potential non-doped emitters, which may be
applied variously, from optics and electronics to energy and
bioscience materials. To make use of these luminescent materials,
further investigations are underway in our laboratory.
Table 4. Quantum Yield Measurement
Sample
Quantum yield
/solid state
40.39%
Quantum yield
/solution state
19.92%
Experimental Section
General Information
P1
P2
P3
P4
P5
All chemicals were purchased from commercial sources – in
particular, Sigma Aldrich – and used as such without any further
purification. The starting materials, S1–S5, were prepared by
condensation of phenantrolene-9, 10-dione core, amine
derivatives and aldehyde derivatives in the presence of
ammonium acetate and glacial acetic acid. The synthetic
strategy to obtain the targeted phenanthroimidazole
fluorophores was carried out by reacting to the condensed
product with relevant boronic acid using the Suzuki coupling
reaction. The synthesised compounds (P1–P5) were analysed –
1 H NMR, 13C NMR and DEPT spectroscopy were performed on
400 MHz and 100 MHz Bruker spectrometer. The chemical shift
values in parts per million (ppm) with TMS (0 ppm) and CDCl3-
5.66%
3.69%
6.35%
6.56%
9.59%
4.06%
24.94%
8.17%
The chromaticity measurement is to estimate the possibility
of application of samples P1–P5 in OLED and other photonic
domains. The excitation wavelength used is 380 nm to excite
the samples for PL emission. We observed that different
substitutions in the phenanthroimidazole backbone make the
shift from red emitters to blue emitters in the following order
of samples: P3–P2–P5–P4–P1. Based on this chromaticity plot,
all the luminogens, P1–P5, can be used for OLED applications
covering a range of wavelengths.
1
DMSO-d6 were used as the standard for H NMR and 13C NMR
spectra. TLC analysis was carried out using Merck silica gel 60
F254 plates. The UV/vis absorption spectra of all compounds
were recorded in DMSO and water on a Bruker UV/vis
spectrophotometer. FT–IR spectra were measured using Bruker
FT–IR. Emission spectra were recorded in a Bruker fluorescence
spectrophotometer. The excitation and emission slits are 3 nm
wide and all measurements were taken at 298 K. The product
was purified using the column chromatographic technique on
Merck silica gel (100–200 mesh).
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To an oven-dried 50 mL, double-neck round-bottomed flask 143.6, 141.5, 139.99, 137.5, 135.1, 135.0, 134.6, V1ie3w3A.2rt,icl1e3O1nl.in4e,
fitted with a reflux condenser, the starting materials (which 130.3, 130.2, 129.9, 129.5, 129.3, 128.6, 128.5, 127.4, 127.4,
were prepared using the methodology adopted in previous 126.7, 126.5, 125.7, 125.1, 124.6, 124.3, 123.3, 122.9, 122.9,
literature in Sec. 2. ESI†), 2-(4-bromophenyl)-1-phenyl-1H- 121.1, 120.9, 120.6, 115.5, 55.8 ppm; ES-MS: (ESI) m/z =
phenanthro[9,10-d]imidazole (S1–S5), were added, along with Calculated for C41H24N3O [M + H] + 574.66, Found 574.29.
DOI: 10.1039/C9NJ05226G
0.6
mmol
(1.0
equiv),
2-(4,4,5,5-tetramethyl-1,3,2- 2-(6-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2
dioxaborolan-2-yl)-9H-fluoren-9 one 0.6 mmol (1.0 equiv), yl)naphthalen-2-yl)-9H-fluoren-9-one (P4)
Pd(PPh3)4 (10.0 mol %) and K2CO3 (2.0 equiv) in THF/H2O (7:3).
Yellow solid; Isolated Yield (276 mg) 77 %; FT-IR (cm-1) Neat;
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
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60
The reaction mixture was refluxed at 85oC for 24 hours. After 3061, 2918, 2850, 1710, 1597, 1524, 1489, 1451, 1376, 1291,
cooling to ambient temperature, the reaction mixture was 1233, 1181, 1037, 999, 909, 882, 811, 724; 1H NMR (400 MHz,
diluted with DCM, filtered through a celite pad and washed with CDCl3) δ 8.93 (d, J = 7.8 Hz, 1H), 8.79 (d, J = 8.4 Hz, 1H), 8.73 (d,
20–40 mL of DCM, and then dried over anhydrous Na2SO4. The J = 8.3 Hz, 1H), 8.02 (d, J = 6.2 Hz, 3H), 7.84 – 7.73 (m, 5H), 7.70
filtrate was concentrated under reduced pressure. The crude – 7.52 (m, 10H), 7.29 (d, J = 23.0 Hz, 5H) ppm; 13C NMR (100
residue was purified through a silica gel (100–200 mesh) column MHz, CDCl3) δ 193.9, 142.0, 139.0, 138.21, 137.8, 135.2, 135.0,
using Hexane: DCM as eluent to obtain the desired products, 134.7, 134.3, 133.5, 133.4, 132.5, 130.4, 130.0, 129.5, 129.4,
P1–P5 (Fig.1).
129.3, 129.0, 128.5, 128.3, 127.5, 127.4, 126.5, 125.8, 125.5,
125.5, 125.1, 124.6, 124.3, 124.3, 123.3, 123.2, 122.9, 122.9,
121.1, 120.9, 120.6 ppm; ES-MS: (ESI) m/z = Calculated for
C44H27N2O 599.17, Found 599.25.
Spectral Data
2-(4-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)-
9H-fluoren-9-one (P1)
2-(4-(1-(4-butylphenyl)-1H-phenanthro[9,10-d]imidazol-2-
yl)phenyl)-9H-fluoren-9-one (P5)
Yellow solid; Isolated Yield (229 mg) 70 %; FT-IR (cm-1) Neat;
1701, 1596, 1495, 1446, 1379, 1289, 1179, 1148, 931, 825, 760,
729, 70; 1H NMR (400 MHz, CDCl3) δ 8.87 (d, J = 7.5 Hz, 1H), 8.78
(d, J = 7.5 Hz, 1H), 8.72 (d, J = 7.6 Hz, 1H), 7.87 (s, 1H), 7.70 (dd,
J = 29.0, 6.0 Hz, 8H), 7.54 (dd, J = 20.9, 6.6 Hz, 7H), 7.38 – 7.26
(m, 4H), 7.21 (d, J = 7.1 Hz, 1H) ppm; 13C NMR (100 MHz, CDCl3)
δ 193.5, 144.1, 143.4, 141.1, 139.8, 134.8, 134.7, 133.1, 130.2,
129.9, 129.8, 129.7, 129.1, 129.1, 129.1, 128.9, 128.2, 128.2,
127.2, 127.2, 127.1, 127.1, 126.4, 126.3, 125.6, 124.9, 124.3,
123.9, 123.1, 122.6, 122.6, 120.8, 120.7, 120.5 ppm; ES-MS: m/z
Yellow solid; Isolated Yield (307 mg) 85 %; FT-IR (cm-1) Neat;
2920, 2853, 1700, 1600, 1507, 1449, 1415, 1380, 1291, 1236,
1
1182, 1149, 1109, 938, 825, 730; H NMR (400 MHz, CDCl3) δ
8.89 (dd, J = 8.0, 1.4 Hz, 1H), 8.76 (d, J = 8.4 Hz, 1H), 8.70 (d, J =
8.3 Hz, 1H), 7.87 (d, J = 1.7 Hz, 1H), 7.75 (ddd, J = 8.0, 7.0, 1.1
Hz, 1H), 7.71 – 7.63 (m, 5H), 7.52 (ddt, J = 10.2, 8.4, 4.0 Hz, 6H),
7.43 (s, 4H), 7.31 – 7.20 (m, 4H), 2.85 – 2.79 (m, 2H), 1.80 – 1.72
(m, 2H), 1.46 (dq, J = 14.6, 7.4 Hz, 2H), 1.02 (t, J = 7.4 Hz, 3H)
ppm; 13C NMR (100 MHz, CDCl3) δ 193.8, 150.4, 145.1, 144.3,
143.6, 141.5, 139.9,137.6, 136.3, 135.1, 134.9, 134.6, 133.2,
130.3, 130.3, 129.9, 129.5, 129.3, 128.9, 128.6, 128.4, 127.4,
127.4, 126.6, 126.4, 125.8, 125.0, 124.8, 124.2, 123.3, 122.9,
122.9, 121.1, 120.9, 120.6, 35.6, 33.5, 22.4, 14.1 ppm; ES-MS:
(ESI) m/z = Calculated for C44H33N2O [M + H] + 605.76, Found
605.30
+
(ESI) = calculated for C40H25N2O [M + H] 549.6530, Found
549.25.
4-(2-(4-(9-oxo-9H-fluoren-2-yl)phenyl)-1H-phenanthro[9,10-
d]imidazol-1-yl)benzonitrile (P2)
Yellow solid;Isolated Yield (268 mg)78 %; FT-IR (cm-1) Neat;
3053, 2917, 2847, 1703, 1602, 1509, 1450, 1246, 1177, 1029,
940, 831, 751, 722, 679; 1H NMR (400 MHz, CDCl3) δ 8.86 (d, J =
7.9 Hz, 1H), 8.78 (d, J = 8.4 Hz, 1H), 8.71 (d, J = 8.3 Hz, 1H), 7.93
(d, J = 8.2 Hz, 2H), 7.87 (s, 1H), 7.77 (t, J = 7.4 Hz, 1H), 7.68 (q, J
Thermal studies
Thermogravimetric analysis (TGA) was carried out using a TA
= 5.5, 5.0 Hz, 5H), 7.60 – 7.48 (m, 8H), 7.31 (t, J = 7.4 Hz, 2H), SDTQ-600 instrument. The findings of the TGA of all five
7.10 (d, J = 8.2 Hz, 1H) ppm; 13C NMR (100 MHz, CDCl3) δ 193.8, luminogens were studied under a nitrogen environment at a
150.3, 144.2, 143.8, 142.9, 141.1, 140.6, 138.1, 135.1, 135.0, heating rate of 5 ℃ min-1 from 30 to 1000 ℃. Differential
134.6, 134.2, 133.2, 130.4, 130.1, 129.6, 129.4, 129.4, 128.5, scanning calorimetry (DSC) was performed on a TA SDTQ – 200
127.9, 127.7, 127.0, 126.9, 126.7, 126.2, 125.5, 124.6, 124.6, instrument and was carried out at a heating rate of 10 ℃ from
123.3, 122.9, 122.5, 120.9, 120.7, 120.6, 117.8, 114.1 ppm; ES- 30 to 500 ℃ under a nitrogen environment.
+
MS: m/z (ESI) = Calculated for C41H27N2O2 [M + H] 579.67,
Found 579.25.
Photophysical studies
2-(4-(1-(4-methoxyphenyl)-1H-phenanthro[9,10-d]imidazol-2-
yl)phenyl)-9H-fluoren-9-one (P3)
The steady-state and time-resolved PL properties were
investigated using a fluorescence spectrometer from Edinburgh
Instruments (FLS 1000). To measure steady-state luminescence,
the sample was excited using 380 and 420 nm collimated beam
from the excitation monochromator of the spectrometer, which
is pumped using a 450 W Xe2 continuous xenon lamp. After
passing through an emission monochromator, the emission
spectrum is scanned and detected in the high-gain red-sensitive
Yellow solid; Isolated Yield (212 mg) 62 %; FT-IR (cm-1) Neat;
3050, 2918, 2851, 2227, 1698, 1599, 1506, 1447, 1374, 1289,
1
1147, 1110, 1017, 938, 838, 720; H NMR (400 MHz, CDCl3) δ
8.88 (d, J = 7.9 Hz, 1H), 8.77 (d, J = 8.4 Hz, 1H), 8.71 (d, J = 8.3
Hz, 1H), 7.89 (d, J = 1.3 Hz, 1H), 7.76 – 7.65 (m, 6H), 7.58 – 7.44
(m, 8H), 7.33 – 7.28 (m, 3H), 7.12 (d, J = 8.8 Hz, 2H), 3.97 (s, 3H)
ppm; 13C NMR (100 MHz, CDCl3) δ 193.9, 160.6, 150.7, 144.4,
8 | J. Name., 2012, 00, 1-3
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photomultiplier (PMT) detector with spectral coverage from 2. (a) H. Kobayashi, M. Ogawa, R. Alford, P. L. Choyke and Y.
400 nm to ~ 800 nm.
View Article Online
Urano, Chem. Rev., 2010, 110, 2620; (b)DDO.I:O1u0.s1h03ik9i/,CH9N. JK0o5j2im26aG,
The luminescence lifetime was measured using the standard T. Terai, M. Arita, K. Hanaoka, Y. Urano and T. Nagano, J. Am.
time-correlated single-photon counting (TCSPC) technique Chem. Soc., 2010, 132, 2795; (c) X. Chen, S. Nam, G. Kim, N.
using a picosecond pulsed diode laser of wavelength 405 nm Song, Y. Jeong, I. Shin, S. K. Kim, J. Kim, S. Park and J. Yoon,
from Edinburgh Instruments (EPL-405) with a repetition rate of Chem. Commun., 2010, 46, 8953.
200 kHz. The decay is fitted using deconvolution fit analysis 3. (a) Q. Zhao, C. Huang and F. Li, Chem. Soc. Rev., 2011, 40,
using the IRF to extract the lifetime parameters from the whole 2508; (b) R. Guliyev, A. Coskun and E. U. Akkaya, J. Am. Chem.
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time-resolved measurement.
Soc., 2009, 131, 9007; (c) J. Shinar and R. Shinar, J. Phys. D: Appl.
Phys., 2008, 41, 133001; (d) J. Liang, B. Z. Tang and B. Liu, Chem.
Soc. Rev., 2015, 44, 2798; (e) J. Mei, Y. Hong, J. W. Y. Lam, A.
Theoretical calculations
Density functional theory (DFT) calculation of compounds Qin, Y. Tang and B. Z. Tang, Adv. Mater., 2014, 26, 5429.
P1–P5 was carried out using Gaussian 09 program package.29 4. (a) S. V. Eliseeva and J. C. G. Bunzli, Chem. Soc. Rev., 2010, 39,
The ground state geometries of these luminogens were 189; (b) Z. Q. Chen, Z. Q. Bian and C. H. Huang, Adv. Mater.,
optimised using DFT-B3LYP-6-311G basis set.30, 31 To obtain the 2010, 22, 1534; (c) Y. Tao, C. Yang and J. Qin, Chem. Soc. Rev.,
information on absorption properties, time-dependent density 2011, 40, 2943.
functional theory (TDDFT) calculations were performed using 5. D. Janczewski, Y. Zhang, G. Das, D. Yi, P. Padmanabhan, K.
optimised geometries.
Bhako, T. Tan and S. Selvan, Microsc. Res. Tech., 2011, 74, 563.
6. Z. Li, Y. Zhang and S. Jiang, Adv. Mater., 2008, 20, 4765.
7. (a) X. Zhang, D. Gorl, V. Stepanenko, F. Wurthner, Angew.
Chem. Int. Ed., 2014, 53 , 1270; (b) S. W. Thomas III , G. D. Joly,
T. M. Swager, Chem. Rev., 2007, 107, 1339; (c) F. J. M. Hoeben,
P. Jonkheijm, E. W. Meijer, A. P. H. J. Schenning, Chem. Rev.,
2005, 105, 1491; (d) U. H. F. Bunz, Chem. Rev., 2000, 100, 1605;
(e) D. Wang, J. Qian, S. He, J. S. Park, K.-S. Lee, S. Han and Y. Mu,
Biomaterials, 2011, 32, 5880; (f) F. Ito, R. Ohta, Y. Yokota, K.
Ueno, H. Misawa and T. Nagamura, Opt.Photonics J., 2013, 3,
27; (g) S. M. Borisov, O. S. Wolfbeis, Chem. Rev., 2008, 108, 423;
(h) H. Saigusa, E. C. Lim, Acc. Chem. Res., 1996, 29, 171; (i) L.
Chen, S. Xu, D. Mcbranch, D. Whitten, J. Am. Chem. Soc., 2000,
122, 9302.
8. (a) J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu, H. S.
Kwok, X. Zhan, Y. Liu, D. Zhu, B. Z. Tang, Chem. Commun., 2001,
1740–1741; (b) J. Mei, N. L. C. Leung, R. T. K. Kwok, J. W. Y. Lam
and B. Z. Tang, Chem. Rev., 2015, 115, 11718; (c) Z. Zhao, B. He
and B. Z. Tang, Chem. Sci., 2015, 6, 5347; (d) J. Mei, Y. Hong, J.
W. Y. Lam, A. Qin, Y. Tang and B. Z. Tang, Adv. Mater., 2014, 26,
5429.
Cyclic voltammetry studies
The oxidation and reduction properties can be used to find
HOMO and LUMO energy levels. These are important parameters for
the configuration of the materials. The oxidation and reduction
potential of all the dyes were carried out using a cyclic voltammetry
instrument. 0.1M tetra-n-butylammonium hexafluorophosphate
(TBAPF6) was dissolved in deoxygenated DCM using nitrogen
bubbling, which was used as
a supporting electrolyte. The
electrochemical cell having three electrodes was equipped to carry
out the measurement. Among these three cells, a glassy carbon
electrode and platinum electrode were utilised as a working and
counter electrode respectively. For the reference electrode, the
Ag/AgCl electrode was utilised.
Conflicts of interest
There are no conflicts to declare.
Acknowledgments
We thankfully acknowledge the financial support from SERB-
NPDF(PDF/2016/001195) and IITH for providing facilities. RSSK
acknowledges the financial support for the following projects nos.
9. (a) R. Hu, N. L. C. Leung and B. Z. Tang, Chem. Soc. Rev., 2014,
43, 4494; (b) Y. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Soc.
DST/YSS/2015/000008, BRICS/PilotCall2/IEEE-OSC/2018 (G) and Rev., 2011, 40, 5361; (c) J. Mei, N. L. C. Leung, R. T. K. Kwok, J.
SPARC/2018-2019/P301/SL.
W. Y. Lam and B. Z. Tang Chem. Rev.,2015, 115, 21, 11718; (d)
Y. Chen, J. W. Y. Lam, R. T. K. Kwok, B. Liu, and B. Z. Tang. Mater.
Horiz., 2019, 6, 428.
10.(a) T. Wang, X. Su, X. Zhang, X. Nie, L. Huang, X. Zhang, X. Sun, Y.
Luo, and G. Zhang, Adv. Mater. 2019, 1904273; (b) Yuanhui Sun,
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New Journal of Chemistry
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DOI: 10.1039/C9NJ05226G
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A New Class of Bipolar AIE luminogens: Synthesis of Phenanthrimidazole-π-
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Flurenone system with Molecular Optical, Thermal and Electrochemical Properties
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Kathirvelan Devarajana, Mayakrishnan Sivakalaib, Uma Maheswari Narayananb, Chinmoy
Biswasc, Sai Santosh Kumar Raavi c, Tarun K Panda*a
We report synthesis of phenanthroimidazole based dyes incorporating fluorenone using
Suzuki coupling and their aggregation induced emission characterisations.