Tetraphenylethene-Based Conjugated Fluoranthene: A Potential Fluorescent Probe for Detection of Nitroaromatic Compounds

Article abstract of DOI:10.1002/chem.201504485

This study reports the synthesis and photophysical properties of a star-shaped, novel, fluoranthene-tetraphenylethene (TFPE) conjugated luminogen, which exhibits aggregation-induced blue-shifted emission (AIBSE). The bulky fluoranthene units at the periph

Full text of DOI:10.1002/chem.201504485

DOI: 10.1002/chem.201504485
Full Paper
&
Structure–Activity Relationships
Tetraphenylethene-Based Conjugated Fluoranthene: A Potential
Fluorescent Probe for Detection of Nitroaromatic Compounds
Yogesh Chandrasekaran, Nutalapati Venkatramaiah, and Satish Patil*[a]
Abstract: This study reports the synthesis and photophysical
properties of a star-shaped, novel, fluoranthene–tetrapheny-
lethene (TFPE) conjugated luminogen, which exhibits aggre-
gation-induced blue-shifted emission (AIBSE). The bulky fluo-
ranthene units at the periphery prevent intramolecular rota-
tion (IMR) of phenyl rings and induces a blueshift with en-
hanced emission. The AIBSE phenomenon was investigated
by solvatochromic and temperature-dependent emission
studies. Nanoaggregates of TFPE, formed by varying the
water/THF ratio, were investigated by SEM and TEM and cor-
related with optical properties. The TFPE conjugate was
found to be a promising fluorescent probe towards the de-
tection of nitroaromatic compounds (NACs), especially for
2,4,6-trinitrophenol (PA) with high sensitivity and a high
Stern–Volmer quenching constant. The study reveals that
nanoaggregates of TFPE formed at 30 and 70% water in
THF showed unprecedented sensitivity with detection limits
of 0.8 and 0.5 ppb, respectively. The nanoaggregates formed
at water fractions of 30 and 70% exhibit high Stern–Volmer
constants (Ksv =79998 and 51120mÀ1, respectively) towards
PA. Fluorescence quenching is ascribed to photoinduced
electron transfer between TFPE and NACs with a static
quenching mechanism. Test strips coated with TFPE lumino-
gen demonstrate fast and ultra-low-level detection of PA for
real-time field analysis.
pyrene,[7] nanoparticles[8] and polymers,[9] were developed as
AIE-active materials for blue and red emission. The bulky
groups on TPE restrict intramolecular bond rotations (=CÀPh
and C=C), thereby inhibiting the non-radiative deactivation
pathway to gain high emission with an enhanced bathochro-
mic shift.[10] Nevertheless, aggregation-induced blue-shifted
emission (AIBSE) is observed in only a few conjugated chromo-
phores. AIBSE was observed in substituted oligophenyleneviny-
lene (OPV) micro-crystals for the first time.[11] Subsequently, Qin
et al. reported the AIBSE effect in amphiphilic oligofluorene,[12]
tentatively attributing the cause to be based on a twisted in-
tramolecular charge transfer (TICT) mechanism. Organic nano-
wires of 9,10-distyrylanthracene (DSA)[13] in THF show orange
emission at l=612 nm (2.03 eV), whereas DSA crystals exhibit
a clearly blue-shifted band at l=518 nm (2.39 eV). The absorp-
tion and emission maxima of a dilute solution of the pyrazine
luminogen, 2,3-dicyano-5,6-diphenylpyrazine (DCDPP),[14] in
THF were l=340 (3.65) and 423 nm (2.93 eV), respectively. At
90% volume fraction of water (fw) in THF, the absorption spec-
trum remains unchanged, whereas the emission spectrum
shows a blue-shifted band at l=413 nm (3.00 eV). A similar
AIBSE effect was also observed with conjugated asymmetric
triazines and 1,2,3,4-tetraphenyl-1,3-butadiene (TPBD).[15] It
has been speculated that the restricted geometry as well as
planarisation of the excited state and reorganisation energy
in the aggregate lead to AIBSE.[16] However, the underlying
mechanism of AIBSE is still unclear due to its microscopic
structural origin and current understanding provides poor in-
sight into how AIBSE can be designed as a feature in molecular
materials. Nevertheless, organic chromophores containing
Introduction
Conjugated organic molecules have attracted a great deal of
attention due to their potential applications in optoelectronic
devices,[1] fluorescent chemosensors and bioprobes.[1] However,
many of them suffer from aggregation-induced quenching
(AIQ), wherein they are highly luminescent in dilute solutions,
but in thin films the luminescence weakens or is even com-
pletely quenched. Tang et al. observed an intriguing phenom-
enon of aggregation-induced emission (AIE) for 1-methyl-
1,2,3,4,5-pentaphenylsilole.[2] The molecule was faintly emissive
in acetonitrile, but became strongly luminescent in the solid
state due to the restriction of intramolecular rotation (IMR) of
the phenyl rings. Over the years, many molecules based on the
AIE phenomenon have been developed and, in particular, tet-
raphenylethene (TPE)-based molecular materials possess inter-
esting properties.[3,4] TPE possesses a simple propeller-type mo-
lecular structure and exhibits AIE: shows non-emissive behav-
iour in solution, but becomes highly luminescent when ag-
glomerated into clusters and in thin films.[2,5] TPE conjugates
with steric bulky chromophores, such as naphthalimides,[6]
[a] Dr. Y. Chandrasekaran, Dr. N. Venkatramaiah, Prof. S. Patil
Solid State and Structural Chemistry Unit
Indian Institute of Science
Bangalore 560012 (India)
Fax: (+91) 80-23601310
E-mail: satish@sscu.iisc.ernet.in
Supporting information for this article and ORCID for one of the authors
are available on the WWW under http://dx.doi.org/10.1002/
chem.201504485.
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bulky units, which exhibit AIE and blue-shifted emission, are
limited.
Herein, we have designed and synthesised a star-shaped
material with TPE as the propeller unit in conjugation with
bulky fluoranthene as an ensemble (TFPE). The photophysical
properties and aggregation behaviour of TFPE in mixtures of
THF/water were systematically investigated. The morphology
of nanoaggregates were studied by SEM and TEM, and their
sizes were determined by means of dynamic light scattering
(DLS) measurements. Remarkably, a blue-shifted emission was
observed in thin films and nanoaggregates of TFPE. The aggre-
gates of TFPE formed in a mixture of THF/water are utilised as
fluorescent probes for the detection of nitroaromatic com-
pounds (NACs). The envisioned material, with highly extended
conjugation endorsed by open cavity structures, allows strong
intermolecular p–p interactions with NACs through intercala-
tion and exhibits Turn OFF fluorescence through a photoin-
duced electron-transfer process.
Figure 1. The optical absorption and fluorescence emission spectra of TFPE
in THF and in thin films on quartz plate.
ened with a bathochromic shift of about 14–20 nm in the thin
film. The emission spectrum of TFPE in THF shows a weak
bluish green emission at lꢀ523 nm with a quantum yield of
12%. This value was low compared with the quantum yield of
other fluoranthene derivatives (ca. 31%).[18] These results sug-
gest that the IMR process of the TPE core has a strong influ-
ence on the excited states of the luminogen, rendering weak
fluorescence. The observed emission in THF predominantly
arises from the fluoranthene units.[19] Thin films of TFPE show
a hypsochromic shift of about 30 nm in comparison with spec-
trum in THF and show a blue emission at lꢀ493 nm with
a quantum yield of 5.4%.
Results and Discussion
The detailed synthetic procedure for the preparation of TFPE is
described in the Experimental Section. In brief, 1,1,2,2-tetra-
kis(4-bromophenyl)ethane (TPE-4Br) was readily prepared from
4,4’-dibromobenzophenone by McMurry coupling. Palladium-
catalysed Sonogashira coupling of TPE-4Br with trimethylsilyl-
acetylene and subsequent desilylation afforded 3 in good yield
(Scheme 1).[17] The star-shaped luminogen, TFPE, was synthes-
ised by a Diels–Alder reaction between 1 and 3 at 2108C for
24 h, resulting in a pale-yellow solid with moderate yields of
about 45% (Scheme 1). The product was isolated and purified
by column chromatography. The NMR, MALDI-TOF MS (m/z
1741.685 [M+H]+) and elemental analysis data further corrob-
orate the final structure of TFPE (Figures S1–S3 in the Support-
ing Information). The thermogravimetric analysis (TGA) and dif-
ferential scanning calorimetry (DSC) results reveal the high
thermal stability of TFPE with a decomposition temperature of
3208C (Figure S4 in the Supporting Information).
The unusual blueshift in the emission spectrum in the solid
state further led us to investigate the photophysical properties
in solvents of different polarity. The absorption bands are
weakly solvatochromic; however, the emission spectra (Fig-
ure 2a) exhibit strong solvatochromism. The emission maxima
shifted from l=537 nm in toluene to l=479 nm in glycerol.
The observed large blueshift of about 58 nm in glycerol pro-
vides evidence for restricted reorientation/IMR of TFPE due to
the high viscosity of the solvent.
To further validate the restricted IMR process, the emission
spectra of TFPE in THF were measured in the temperature
range of 20–300 K (Figure 2b). Above the freezing point of
THF (160 K), the emission intensity gradually increases with in-
creasing temperature. The emission maxima shifted from l=
527 nm at 300 K to l=506 nm at 20 K. The observed blueshift
The absorption spectrum of TFPE in THF shows two distinct
major absorption bands at l=304 and 377 nm, which arise
due to p!p* transitions (Figure 1). These bands were broad-
Scheme 1. Synthetic route for the preparation of TFPE.
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Figure 2. a) Emission spectra of TFPE (lex =370 nm) in different solvents. b) Temperature-dependent emission spectra of TFPE (in THF) in the temperature
range of 300–20 K. c) Emission spectra of TFPE in different mixtures of THF/H2O. d) Change in the quantum yields and non-radiative decay of TFPE in different
mixtures of THF/H2O. e) Photograph showing changes in the emission behaviour of TFPE at different ratios of THF/H2O under the illumination of UV light at
lex =365 nm.
is attributed to restricted rotation of the fluoranthene units
with respect to TPE in the rigid glassy medium below the
freezing point of THF. To understand the AIBSE phenomena,
we have studied the emission behaviour of TFPE in different
mixtures of THF/H2O. As shown in Figure 2c, with increasing
water fractions (fw), the emission intensity of TFPE at l
ꢀ523 nm gradually increases until fw =40%. At fw =50%, the
emission intensity abruptly decreased with a small blueshift in
the emission band. Further increasing the water fraction (fw =
70%) results in a new emission band emerging at lꢀ493 nm
with a 2.3-fold increase in the emission intensity. This emission
spectrum is identical to the spectrum of TFPE in thin films.
Figure 2d shows the change in the quantum yields and
non-radiative decay rate of TFPE in different THF/H2O mixtures.
The quantum yield of TFPE at fw =50% show only 9% while it
is found to be 39.4% for fw =70% in good agreement with
emission spectra. The photograph in Figure 2e reveals the ap-
parent changes in the emission of TFPE in mixtures of THF/H2O
at different ratios under UV light at l=365 nm. The fluores-
cence lifetime study reveals that, upon increasing the water
fraction, the short lifetime of TFPE increases due to an increase
in IMR processes and more planarisation in the excited states
due to aggregation (Table S1 and Figure S6 in the Supporting
Information).
Furthermore, the AIE effect was quantified by the extent of
emission enhancement, aAIE =FF.A/FF.S, in which FF.A and FF.S
are the fluorescence quantum yields in aggregates and solu-
tion, respectively. In solution, TFPE exhibits a weak emission
(FF.S =12%) and a strong emission in aggregates at fw =70%
(FF.A =39.4%), which demonstrates the AIE effect, as evidenced
by the aAIE value (aAIE =3.28). The formation of aggregates and
the self-assembly behaviour of TFPE in different water fractions
was investigated by field-emission (FE) SEM and TEM.
Figure 3 shows SEM images obtained by drop casting a film
on silicon wafer for fw =30, 50, 70 and 90%. The size of the
nanoparticles was further confirmed by DLS measurements
(Figure S8 in the Supporting Information). At lower water frac-
tions (fw =30%), spherical nanostructured aggregates are ob-
served, whereas at fw =50%, the nanoaggregate self-assem-
blies are turned into a rod-shaped morphology, which results
in a decrease in the emission efficiency. At higher water frac-
tions (fw >50%), we have observed the formation of nanoag-
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directly attached to TPE are twisted with a dihedral angle of
1128. As a result, the TPE unit exhibits poor electronic coupling
to the fluoranthene core. The electron density pattern reveals
that the HOMO is localised on TPE, whereas the LUMO is shift-
ed to fluoranthene; this indicates that charge transfer from
TPE to fluoranthene is an energetically favourable process. In
the excited state, the dihedral angle is 1438 and attains more
planarisation of the molecule, which hinders the IMR process
(Figure S9 in the Supporting Information).
Electrochemical studies on TFPE show quasi-reversible oxida-
tion peaks at +1.47 and +1.77 V with reference to the ferro-
cene/ferrocenium (Fc/Fc+) couple. By comparing the oxidation
potentials of TPE and fluoranthene, the oxidation peak at
+1.47 V is assigned to TPE due to a high electron-donating
character (Figure S10 in the Supporting Information). The
LUMO energy level of TFPE is close to the LUMO energy levels
of NACs. This results in a favourable photoinduced electron-
transfer process. This further led us to investigate the ability of
TFPE for the detection of different NACs, such as 2,4,6-trinitro-
phenol (PA), 2,4,6-trinitrotoluene (TNT), 2,4-dinitrophenol
(DNP), 2,4-dinitrotoluene (DNT), 4-nitrophenol (NP), 4-nitroto-
luene (NT), 4-nitrobenzoic acid (NBA) and nitrobenzene (NB).
The nanoaggregates prepared from TFPE in different mixtures
of THF/H2O were treated with different NACs to understand
the effect of aggregation behaviour and morphology of the
nanoparticles on the sensitivity and selectivity. Water fractions
of fw =30 and 70%, at which the fluorescence quantum yields
were higher, exhibited substantial changes in the emission
spectra upon treatment with NACs.
Figure 3. FE-SEM images of TFPE obtained from different mixtures of THF/
H2O: a) fw =30, b) 50, c) 70 and d) 90%.
gregates with similar shapes and sizes ranging from 80 to
200 nm. The formation of nanoaggregates was also verified by
TEM (Figure S7 in the Supporting Information). Upon further
increasing the water fraction to fw >70%, we observed a de-
crease in emission intensity, which was attributed to an in-
crease in the rate of non-radiative decay (Figure 2d).
Computational calculations were performed to understand
the enhancement in emission in TFPE nanoaggregates. We car-
ried out geometry optimisation on the ground and lowest ex-
cited states by using the Gaussian 09 program with the B3LYP/
631g* basis set. The geometry-optimised structure of TFPE in
the ground state shows that the phenyl rings of fluoranthene
Figure 4a and b shows changes in the emission spectra of
TFPE (1”10À5 m) in THF/H2O at fw =30 and 70% upon the addi-
Figure 4. Change in the emission spectra of TFPE (1”10À5 m) in THF/H2O: a) fw =30 and b) 70% upon addition of different concentrations of PA. The inset
shows the Stern–Volmer (SV) plot of TFPE for different concentrations of PA. c) Comparison of the quenching efficiency of TFPE with different NACs.
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tion of different concentrations of PA. The emission maxima at
lꢀ523 nm for fw =30% and l=493 nm for fw =70% gradually
decreases and show unprecedented turn off fluorescence with
about 95 and 92% decreases in the emission intensities upon
the addition of a 83 mm solution of PA. Treating TFPE with TNT
and DNP results in 29 and 75% decreases in emission intensity
for fw =30% and 65 and 67% decreases for fw =70%, respec-
tively. It was found that nitrophenol derivatives showed better
fluorescence quenching behaviour than that of other electron-
deficient compounds and gave high selectivity of TFPE to-
wards DNP and PA. The quenching efficiency of TFPE towards
the detection of PA was 1.5 times higher than that of DNP. The
limit of detection (LOD) for PA was (0.8Æ0.1) and (0.5Æ
0.1) ppb for fw =30 and 70%, respectively (Figure S11 in the
Supporting Information).
Figure 6. Comparison of the SV constants of TFPE for different concentra-
tions of PA at different ratios of THF/H2O.
fluorescence lifetime values of TFPE were invariable for the ad-
dition of different concentrations of PA; this indicated that the
excitons mainly decayed through the static pathway (Fig-
ure S15 in the Supporting Information). The absorption spectra
show gradual increases in the intensity of the p!p* absorp-
tion at lꢀ374 nm with broadening in the band upon the ad-
dition of PA and the appearance of new band at lꢀ418 nm,
which confirms the formation of a charge-transfer complex
(Figure S16 in the Supporting Information). A sharp colour
change from colourless to pale yellow under normal daylight
was observed upon the addition of nitrophenols (NP, DNP and
PA) to the solution of TFPE. In the presence of other NACs, we
did not observe any significant change in the colour and spec-
tral pattern of the solution of TFPE (Figures S17 and 18 in the
Supporting Information). These experimental results clearly in-
dicate that there is a strong interaction between PA and TFPE
compared with other nitro analytes. The binding stoichiometry
of TFPE with PA was determined by means of the modified
Benesi–Hildebrand equation. The fitting data reveals the for-
mation 1:2 stoichiometry between TFPE and PA (Figure S19 in
the Supporting Information). A schematic representation of
possible modes of interaction is given in Figure S20 in the Sup-
porting Information.
The LOD and quenching studies of DNP with TFPE is sum-
marised in Figures S12 and S13 in the Supporting Information.
The addition of PA to TFPE in THF and fw =50% shows
quenching efficiencies of 46 and 83%, respectively (Figure S14
in the Supporting Information). The observed high quenching
efficiency of nanoaggregates formed at fw =30 and 70% for PA
can be attributed to a large surface area and efficient photoin-
duced electron-transfer process. Figure 4c shows the variation
in the quenching efficiency of TFPE at fw =30 and 70% for dif-
ferent NACs. The quenching behaviour was further studied by
using the SV equation, Io/I=1+Ksv[Q], in which Io and I are the
emission intensities of TPFE aggregates in the presence and
absence of quencher (Q). The insets in Figure 4a and b show
the SV plot of TFPE at different concentrations of PA. The SV
plot in Figure 5 shows a linear increase in the Io/I values for dif-
ferent concentrations of PA, which indicates that the quench-
ing process occurs through static quenching phenomena.
1H NMR spectroscopy studies were carried out to gain
a deeper insight into molecular interactions between TFPE and
PA. In a typical 1H NMR spectroscopy titration experiment,
a pale-yellow solution of PA was mixed with TFPE in different
molar ratios (Figure S21 in the Supporting Information). Upon
the addition of PA to TFPE, a multiplet corresponding to pro-
tons of o-Ph of the fluoranthene units is converted into a dou-
blet with a downfield shift of Dd=0.02 ppm. In addition, a mul-
tiplet at d=6.71 ppm corresponding to the protons of m-Ph of
the TPE unit, which shows a downfield shift of Dd=0.03 ppm,
is converted into a well-resolved doublet. These results sug-
gest that PA molecules intercalate with TFPE through efficient
p–p stacking interactions.
Figure 5. Comparison in the fluorescence quenching efficiency of TFPE
(1”10À5 m) in THF/H2O (fw =0, 30, 50 and 70% H2O) treated with different
NACs (100 mm).
The SV quenching constants were quantified and showed
the highest Ksv values of 79998 and 51120mÀ1 for fw =30 and
70%, respectively, towards PA. The Ksv values of TFPE for fw =
50% upon the addition of PA are 17420 and 42974mÀ1
(Figure 6). The Ksv values of TFPE towards NACs follow the
order fw =30%>fw =70%>fw =50%>fw =0%. The obtained
Ksv values at fw =30 and 70% are higher when compared with
those of fluoranthene, TPE and other AIE-active oligo- and
polymers employed for the detection of PA.[16]
To understand the quenching mechanism and variations in
the selectivity of NACs, further investigations were undertaken
by optimising the possible adducts of TFPE and PA. As evi-
denced from the results of electrochemical and DFT studies,
the LUMO energy level of TFPE (À1.96 eV) is closer to the
LUMO energy level of PA (À3.89 eV) than those of other NACs;
this facilitates electron transfer from TFPE to PA and leads to
Static quenching phenomena were further confirmed by
fluorescence lifetime and UV/Vis titration experiments. The
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diode (l=369 nm, 25 ps) and the emission was monitored at l=
470 nm.
high Ksv values. In comparison to TFPE, conjugate TFPE·2PA
shows a decrease in the HOMO–LUMO gap (1.38 eV). The
HOMO of TFPE·2PA is localised on both TPE and fluoranthene
units, whereas the LUMO is localised on PA, and confirms that
photoinduced electron transfer occurs from TFPE to PA. The
binding energy between TFPE and PA is 80 kJmolÀ1 higher
than those of TFPE and DNP adducts, and results in its high
sensitivity. Furthermore, aggregates of TFPE impregnated on
test strips can be used as a portable sensor to detect nitro ex-
plosives. Fast and remarkable turn off fluorescence behaviour
was observed for test strips tainted with different concentra-
tions of PA (Figure S22 in the Supporting Information). Under
UV light irradiation at l=365 nm, the PA mark clearly revealed
a dark pattern and bluish green luminescent for the untainted
area. These tests reveal that visual detection of PA at the fem-
togram level shows its potential for rapid detection and onsite
implementation.
Differential scanning calorimetry (DSC) analysis was carried out on
a Mettler Toledo DSC1 STARe system (chiller cooled) with a N2 flow
of 40 mLminÀ1 with an empty Al pan taken as a standard. All sam-
ples were heated at a rate of 58CminÀ1
.
Redox potentials were determined by cyclic voltammetry experi-
ments by using a CH electrochemical analyser at a scanning rate of
50 mVsÀ1. Solutions of synthesised materials (c=0.2 mg/0.1 mL
CH2Cl2) were drop cast on a working Pt disc electrode. Ag/AgCl
was used as a reference electrode and Pt wire was employed as
a counter electrode. Dry acetonitrile and 0.1m tetrabutylammoni-
um hexafluorophosphate were used as solvent and supporting
electrolyte, respectively. The Fc/Fc+ couple was used as a standard
electrochemical reference. The energies of the HOMO, LUMO and
corresponding band gap were calculated from the oxidation and
reduction potentials. The positions of the HOMO and LUMO were
calculated from Equation (1) and the band gap was determined
from the difference between them.
Conclusion
HOMO ¼ ÀðEox þ 4:5Þ eV
ð1Þ
A star-shaped AIE-active TPE conjugated with fluoranthene
was synthesised. The luminogen exhibited unusual AIBSE in
the solid state and in aggregates. The twisted IMR of the
phenyl rings and excited-state planarisation caused by fluoran-
thene units resulted in blue-shifted emission with enhanced
quantum yields. The nanoaggregates exhibited unique AIBSE
properties with good sensitivity towards the detection of
NACs. The porous nature of TFPE aggregates at fw =30 and
70% showed high sensitivity and selectivity for PA with turn
off fluorescence behaviour. The results demonstrate that the
static quenching phenomenon with a photoinduced electron-
transfer process is dominated by intercalation of NACs with
TFPE.
LUMO ¼ ÀðEred þ 4:5Þ eV
FE-SEM imaging was performed by using a Zeiss scanning electron
microscope at 5 kV after gold coating. Samples were prepared by
drop casting the nanoparticle dispersion on a silicon wafer.
Particle size was determined by DLS measurements of solutions by
using
Corp., Holtsville, NY.
a ZetaPALS potential analyser, Brookhaven Instruments
TEM images were recorded on a Zeol field-emission microscope at
200 kV at room temperature. A dilute solution of nanoparticle dis-
persion was drop cast on a carbon-coated copper grid and dried in
air.
The ground- and excited-state geometry optimisations were car-
ried out by using the Gaussian 09 package. The electron density
profiles were obtained with Gauss View 5.0.8.
Experimental Section
General
1H and 13C NMR spectra were recorded on Bruker 400 and 100 MHz
NMR spectrometers and calibrated by using tetramethylsilane
(TMS) as an internal reference. Chemical shifts are reported in ppm.
Synthesis and characterisation
A solution of 1[18b] (0.070 g, 0.16 mmol) and 3[20] (0.34 g, 0.98 mmol)
in diphenyl ether (8 mL) were added to a hydrothermal bomb and
heated for 24 h in a 2008C oven. Then the reaction mixture was
cooled to room temperature, loaded into a column of silica gel,
and the product was separated by using dichloromethane/hexane
(1:9) as the eluent to obtain the desired product as a pale-yellow
solid (130 mg, 46%). 1H NMR (400 MHz, CDCl3): d=7.79–7.65 (m,
16H), 7.61–7.46 (m, 12H), 7.42–7.15 (m, 38H), 6.88 (d, J=8.3 Hz,
8H), 6.74–6.63 ppm (m, 12H); 13C NMR (100 MHz, CDCl3): d=141.6,
140.9, 140.8, 139.2, 139.0, 138.1, 137.9, 136.7, 136.3, 136.0, 135.7,
133.2, 131.0, 130.7, 130.4, 129.7, 129.2, 128.7, 128.3, 127.8, 127.6,
127.5, 127.4, 126.6, 123.4, 122.9, 77.3, 77.0, 76.7 ppm; MALDI-TOF
MS: m/z: 1741.6 [MÀ1]; elemental analysis calcd (%) for C138H84: C
95.14, H 4.86; found: C 94.93, H 5.18.
The UV/Vis spectra were recorded on a PerkinElmer (Lambda 35)
UV/Vis spectrometer. All absorption spectra in the solution state
were recorded in THF (1”10À5 m) and the spectra in the solid state
were recorded from films of compounds spin coated on quartz
substrate.
Steady-state fluorescence emission studies were carried out with
a Spex FluoroLog-3 spectrofluorometer (Jobin-Yvon Inc.). Tempera-
ture-dependent emission data was recorded by using a Spex 1702/
04 spectrometer equipped with a cryostat and vacuum pump. The
compound was dissolved in THF and excited at l=325 nm with
a HeÀCd laser in the temperature range 300–20 K at temperature
intervals of 20 K.
Fluorescence lifetime measurements were performed in solution
with a Horiba JovinYvon lifetime spectrometer (TCSPC) with a laser
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Acknowledgements
We greatly acknowledge the Centre for Nano Science and En-
gineering, IISc, for extending the SEM and TEM facility. We
thank the Proteomics centre for MALDI and NMR research
centre for NMR studies. Y.C. thanks the University grants com-
mission (UGC), New Delhi for Dr. D. S. Kothari Postdoctoral fel-
lowship. Dr. Deepak Kumar and Shiv Kumar are acknowledged
for their help in electrochemical and NMR experiments and
useful discussions.
Keywords: aggregation · conjugation · fluorescence · sensors ·
structure–activity relationships
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Received: November 9, 2015
Published online on March 1, 2016
Chem. Eur. J. 2016, 22, 5288 – 5294
www.chemeurj.org
5294
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