Dynamic and static excimer: A versatile platform for single component white-light emission and chelation-enhanced fluorescence

Article abstract of DOI:10.1039/c8tc03035a

The present study demonstrates the tuning of iconic optical features of pyrene through its judicious assembly with a C₃-symmetric triaminoguanidinium ion, leading to the development of a unique multifunctional luminescent material PYTG. By fine tuning the concentration and the external stimuli, such as temperature, the addition of analytes led to a dramatic change in the fluorescence of PYTG, providing an excellent platform for the exploration of the dynamic and static excimer formation. Recently, white-light-emitting molecular probes have received considerable attention from the scientific community due to their great potentials in display devices. Tripodal PYTG was employed as a pure white-light emitter consisting of blue emission from monomers and orange emission from excimers both in solution and thin film with Commission Internationale de l'Eclairage (CIE) coordinates of (0.33, 0.34) and (0.33, 0.38), respectively. The formation of a dynamic excimer along with monomer fluorescence at higher concentrations of PYTG led to tunable emission. Additionally, the rich electronic properties of PYTG allowed efficient detection of trivalent metal ions by turn-on fluorescence and colorimetric sensing of fluoride ions. PYTG selectively detected Fe³⁺ and Al³⁺ with nanomolar sensitivity in a semi-aqueous solution via a chelation-enhanced fluorescence response in the orange-red region (λ_em = 588 nm) by static excimer formation. The 'on-off' fluorescence switching by the consecutive addition of trivalent cations and a chelating ligand EDTA makes PYTG a promising fluorescent molecular switch. A simple and cost-effective strategy, leading to the development of a versatile fluorescent probe and an in-depth exploration of the structure-property relationship as delineated in the present manuscript, will pave the way for the emergence of novel task-specific molecular optical materials.

Full text of DOI:10.1039/c8tc03035a

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Materials Chemistry C
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Dynamic and static excimer: a versatile platform
for single component white-light emission and
chelation-enhanced fluorescence†
Cite this: J. Mater. Chem. C, 2018,
6, 12086
Virendra Kumar,
Bahadur Sk,
Subhankar Kundu
and Abhijit Patra
*
The present study demonstrates the tuning of iconic optical features of pyrene through its judicious
assembly with a C3-symmetric triaminoguanidinium ion, leading to the development of a unique
multifunctional luminescent material PYTG. By fine tuning the concentration and the external stimuli,
such as temperature, the addition of analytes led to a dramatic change in the fluorescence of PYTG,
providing an excellent platform for the exploration of the dynamic and static excimer formation. Recently, white-
light-emitting molecular probes have received considerable attention from the scientific community due to their
great potentials in display devices. Tripodal PYTG was employed as a pure white-light emitter consisting of blue
emission from monomers and orange emission from excimers both in solution and thin film with Commission
Internationale de l’Eclairage (CIE) coordinates of (0.33, 0.34) and (0.33, 0.38), respectively. The formation of a
dynamic excimer along with monomer fluorescence at higher concentrations of PYTG led to tunable emission.
Additionally, the rich electronic properties of PYTG allowed efficient detection of trivalent metal ions by turn-on
fluorescence and colorimetric sensing of fluoride ions. PYTG selectively detected Fe3+ and Al3+ with nanomolar
sensitivity in a semi-aqueous solution via a chelation-enhanced fluorescence response in the orange-red region
(lem = 588 nm) by static excimer formation. The ‘on–off’ fluorescence switching by the consecutive addition of
trivalent cations and a chelating ligand EDTA makes PYTG a promising fluorescent molecular switch. A simple
and cost-effective strategy, leading to the development of a versatile fluorescent probe and an in-depth
exploration of the structure–property relationship as delineated in the present manuscript, will pave the way for
the emergence of novel task-specific molecular optical materials.
Received 20th June 2018,
Accepted 1st October 2018
DOI: 10.1039/c8tc03035a
rsc.li/materials-c
A dynamic excimer is formed due to the collision between the
excited state (lowest singlet) and the ground state molecules.10 On
Introduction
Organic p-conjugated molecules with flexible anchoring groups the other hand, a static excimer is obtained upon photoexcitation
have attracted significant attention owing to their wide range of a ground state dimer formed by weak molecular interactions
of applications from optoelectronics to fluorescence-based such as p–p stacking or hydrophobic interactions.11,12 The classi-
sensing, switching and imaging.1–6 In this context, dual emissive fication of an excimer can also be done based on the nature of the
functional molecular probes are attractive due to ratiometric interaction between its monomeric units, i.e., an intermolecular
signaling and environment-sensitive photophysical properties.7 excimer and an intramolecular excimer.10–17
Among the various strategies demonstrated, tuning the monomer
The formation of dynamic and static excimers and their
and excimer emission is one of the facile ways to develop stimuli- associated emission behaviors can easily be tuned by varying
responsive probes with dual emission.8 Excimer is an excited state the concentration of the probe and in the presence of external
dimer, which is formed due to molecular photoassociation.8,9 stimuli such as temperature, pH, and addition of chemical
Based on the nature of their formation, excimers can be classified species.16,18–20 Hence, a significant effort has been made to
into two different types: dynamic excimers and static excimers.10–12 develop functional materials based on excimer probes for the
detection of different essential elements and bioactive mole-
Department of Chemistry, Indian Institute of Science Education and Research
Bhopal, Bhopal Bypass Road, Bhauri, Bhopal 462066, Madhya Pradesh, India.
E-mail: abhijit@iiserb.ac.in
cules and for the construction of organic light-emitting devices
(OLEDs).21–34 Also, tuning monomer and excimer emissions
across the entire visible region often leads to white-light
emission.35–37 The white light-emitting (WLE) materials have
become increasingly crucial for the low-cost flat-panel display
and lighting devices.38–41 White light emission can be achieved
† Electronic supplementary information (ESI) available: Details of synthesis and
characterization, static and dynamic excimer formation, single component white
light emission, colorimetric sensing of FÀ, chelation-enhanced fluorescence and
computational investigations. See DOI: 10.1039/c8tc03035a
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through the non-covalent assembly of multichromophoric trivalent ions. Systematic investigations revealed PYTG as an
components or covalent linking of multiple emitters in a single efficient chemosensor for the selective detection of the most
molecular platform.42–44 WLE materials have been constructed ubiquitous essential transition metal ion Fe3+ and another
based on inorganic complexes, small organic molecules, important ion in biological systems Al3+. Although, in the past
p-conjugated polymers, porous organic polymers, metal organic few years, many fluorescent sensors have been reported for the
frameworks and nanomaterials.35–59 However, an efficient white- selective detection of Fe3+ or Al3+, a single sensor molecule
light generation maintaining similar intensities of three primary for selectively detecting both trivalent ions with nanomolar
colors, i.e., red (R), green (G) and blue (B) is quite challenging in sensitivity over other divalent and monovalent ions has rarely been
a single molecular platform. Hence, single component WLE demonstrated.29,69 Besides, PYTG can also be employed for the
materials based on small organic molecules are limited.36,53–56
colorimetric detection of FÀ ions. To the best of our knowledge,
Amidst various excimer-based fluorescent probes, a single the present study is the first demonstration of multifunctional
molecular platform allowing the exploration of dynamic and applications including single component white-light emissions
static excimers leading to single component white-light emission both in solution and thin film and selective sensing of Fe3+ and
as well as selective detection of essential elements is still largely Al3+ by a simple C3-symmetric-like tripodal molecular material.
unexplored. In this context, we designed a functional fluorescent
probe (PYTG) comprising pyrene (PY) and triaminoguanidinium
(TG) chloride. Pyrene was chosen due to the ease of function-
Results and discussion
alization and dual emissive nature with intense, well-separated
monomer and excimer bands, which can easily be tuned through
the variation of the local environment as well as external
stimuli.8,18–20,60 The formation of a pyrene excimer is mostly
governed by intermolecular interactions between the excited
state and ground state molecules, indicating the dynamic
nature.10,61 Of late, it has been observed that molecular probes
containing ionophores linked with pyrene are more prone to the
formation of ground state dimers (static excimers) in the
presence of suitable ions.21–23 Herein, we aim to integrate pyrene
units with a flexible organic core so that the simultaneous
probing of both excimers will be convenient.
Triaminoguanidinium chloride was obtained by the reaction
of guanidium chloride and hydrazine (Scheme S2, ESI†).
PYTG was synthesized using a simple Schiff base condensation
reaction between triaminoguanidinium chloride and pyrene-1-
carboxaldehyde (yield: 85%, Scheme 1 and ESI†). The synthetic
protocol is very simple and PYTG can be obtained on the gram-
1
scale. It was characterized by H, 13C NMR and mass spectro-
metry analysis (Fig. S41–S43, ESI†). Thermal stability of PYTG
was monitored with differential scanning calorimetry (DSC)
and thermogravimetric analysis (TGA); it was found to be stable
up to 200 1C (Fig. S1, ESI†).
Photophysical properties of PYTG
We employed the design strategy involving a C3-symmetric
molecular architecture (Scheme S1, ESI†). C3-symmetric mole-
cular materials have been explored in the fields of chiral
catalysis, molecular recognition, optical sensors, organic light
emitting diodes (OLEDs), etc.62–67 However, the utilization of
C3-symmetric molecules for the understanding of the static and
dynamic excimer behavior have not been demonstrated earlier.
The promising scope of tripodal molecular materials inspired
us to choose triaminoguanidinium chloride as an aesthetically
appealing triangular building block.68 The three-terminal
amine units of triaminoguanidinium were utilized for the
incorporation of pyrene units to construct PYTG. The advantages
of triaminoguanidinium ions as the central core are as follows:
(i) the flexibility associated with C3-symmetry allows the for-
mation of a facile excimer both through intra and intermolecular
interactions, which is quite impossible in the C2-symmetric
probes. (ii) The three coordinating sites in the triaminoguanidinium
unit can bind to different external ions, tuning the static and
dynamic excimer behaviors (Scheme S1, ESI†).10,11
The basic understanding of the photophysical properties of
PYTG in a dilute solution laid the foundation of the subsequent
studies on excimer emission. A broad absorption band with two
humps at nearly 390 and 410 nm was observed in chloroform
(CHCl3; Fig. 1a); it significantly red-shifted as compared to the
structured absorption bands of pristine pyrene (230 to 350 nm,
Fig. S2, ESI†). The red-shifted absorption of PYTG can be
ascribed to the p–p* transition with a facile delocalization of
p-electrons from the electron-rich pyrene rings to the electron-
deficient carbon centre of triaminoguanidinium via the imine
bridge. The extensive conjugation in PYTG distorted the rigid
vibrational structure of the pyrene chromophore and caused a
broad absorption band as compared to that of pristine pyrene.
Fig. S3a (ESI†) indicates that there is no significant change in
The exploration of the structure–property relationship facilitated
in-depth understanding of the excimer formation in PYTG by
varying concentration, temperature and presence of guest ions.
The dynamic excimer formation was noticed with varying the
concentration of PYTG as well as the temperature of the
medium. Interestingly, fine-tuning of the concentration of
PYTG in dichloromethane led to the generation of white light.
The static excimer formation was monitored in the presence of
Scheme 1 Synthetic scheme of pyrene-appended Schiff base compound
PYTG with C3-symmetric triaminoguanidinium core.
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fluorescence decay at excimer emission with the major compo-
nent (85%) having a decay time of 23.8 ns (Fig. S7 and Table S2,
ESI†). The triexponential decay profile of PYTG suggested the
presence of multiple emissive species arising due to the mono-
mer, p-conjugated delocalized state and the excimer (intra and/
or intermolecular).11,21 Even though unambiguous assignment
of individual decay times to distinct excited states is difficult,
cardinal photophysical aspects based on the trend in decay
profiles can be deduced. Slower decay with larger contribution
is most likely due to the emission from the stable excimer state.
The fluorescence decay monitored at 470 nm was also best
fitted with a triexponential decay equation. The amplitude
average decay times were nearly 2 ns and 19 ns for monomer
Fig. 1 (a) Normalized (i) absorption, (ii) emission (lexc = 410 nm) and
(iii) excitation (lem = 590 nm) spectra of PYTG (0.5 mM) in CHCl3.
(b) Fluorescence decay profiles (lexc = 408 nm) of PYTG in CHCl3 of
monomer (lem = 470 nm) and excimer (lem = 590 nm) emission bands.
the absorption spectra of PYTG with respect to the polarity of and excimer emissions, respectively. A similar trend of fluores-
solvents. A broad absorption with a hump nearly at 440–460 nm cence decay was also observed in dichloromethane (DCM,
was observed in a highly polar solvent such as dimethyl sulfoxide Fig. S7 and Table S2, ESI†).
(DMSO, Fig. S3a, ESI†); it might be due to the specific interaction
Furthermore, to elucidate the spectroscopic signature of
PYTG, a computational investigation based on density func-
between ionic PYTG and dipolar DMSO.8
Pyrene is known to exhibit monomer emission peaks in the tional theory (DFT) calculations was carried out using the
range of 380–410 nm and excimer emission peaks in the range Gaussian 09 program package. The B3LYP hybrid functional
of 450–500 nm (Fig. S2, ESI†).10,11 In comparison with pyrene, and 6-31G(d,p) basis set were employed for all calculations. The
PYTG exhibits a yellowish-orange fluorescence (fluorescence DFT-optimized ground state HOMO and LUMO distributions of
quantum yield: ff = 26%, 0.5 mM PYTG in CHCl3) with a PYTG showed partial planarity between triaminoguanidinium
peak centred at 590 nm and a shoulder band at 470 nm. The and pyrene moieties (dihedral angles: À311, À281 and À221,
p-electron conjugation involving triaminoguanidinium core Fig. 2). The electron density of HOMO was localized over two
and three pyrene units is responsible for the substantial red- pyrene moieties through imine linkage. However, the LUMO was
shifted emission of PYTG as compared to that of the native predominantly distributed over the central triaminoguanidinium
pyrene (Fig. 1a). Like absorption spectra, the emission spectra core and was also homogeneously distributed over one side of
of PYTG remained nearly unaltered by varying the polarity of three pyrene units through imine linkages. The HOMO–LUMO
the local environments (Fig. S3, ESI†). The two emission bands (along with HOMOÀ2, HOMOÀ1, LUMO+1, LUMO+2) electron
at 430–490 nm and 520–700 nm in different solvents are distributions indicated facile delocalization of p-electrons from
presumably due to monomer emission (from the locally excited the electron-rich pyrene rings to the most electron-deficient
state) and the excimer emission, respectively (Fig. S3b, ESI†). It
was found that the ratio of the intensity of monomer to that of
the excimer emission decreases with increasing polarity of the
solvent (Fig. S4, ESI†).
The fluorescence excitation spectra of PYTG were very simi-
lar to the absorption spectra, indicating that the emission was
governed by a vertical excitation process (Fig. S5, ESI†). The
fluorescence quantum efficiency of PYTG decreased drastically
in the presence of polar solvents (acetonitrile, ethyl acetate,
N,N-dimethylformamide, dimethyl sulphoxide, etc.) containing
heteroatoms (oxygen, nitrogen and sulphur). The possible
reason for such observations is the presence of specific inter-
actions between the electron-rich heteroatoms of the solvents
and the electron-deficient triaminoguanidinium unit of PYTG
(Table S1, ESI†). The fluorescence quantum yields of PYTG
decreased with the increase in concentration (Fig. S6, ESI†);
this observation was ascribed to the formation of the dynamic
excimer as revealed through subsequent studies.
The excited state behaviour of PYTG was monitored using
the time-correlated single photon counting (TCSPC) technique.
The fluorescence decays of PYTG in CHCl3 were measured by
excitation with a 408 nm diode laser, and the decay of the
Fig. 2 The optimized ground-state HOMOÀ2, HOMOÀ1, HOMO, LUMO,
LUMO+1 and LUMO+2 distributions (b spin state) and the respective
monomer (lem = 470 nm) and excimer (lem = 590 nm) emission
peaks was monitored (Fig. 1b). PYTG exhibited triexponential
energy values (eV) of PYTG obtained by the density functional theory
(DFT) calculations at the B3LYP/6-31G(d,p) level.
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carbon centre of triaminoguanidinium through the imine bridge.
Additionally, the intramolecular twisting of PYTG upon photo-
excitation could be predicted. A possible reason for the low
quantum yields of PYTG in polar solvents could be due to non-
radiative deactivation through the stabilization of the twisted excited
state.56 Time-dependent density functional theory (TD-DFT, spin
unrestricted) calculations were also carried out to further explore
orbital contribution (Fig. S39 and Table S6, ESI†). The main transi-
tions occurred from HOMOÀ1 to LUMO and HOMOÀ1 to LUMO+1
(both a and b spin orbitals), and their contributions to the lowest
energy excitations were 49% and 41%, respectively. The HOMO–
LUMO energy levels and the band gap of PYTG obtained from
DFT calculations were further corroborated by cyclic voltammetry
measurements (Fig. 2 and Fig. S40, ESI†).
Fig. 3 Fluorescence decay profiles (lexc = 408 nm) of PYTG in CHCl3 at
(a) monomer (lem = 470 nm) and (b) excimer (lem = 590 nm) emission
bands with varying concentrations (0.5–5 mM).
of the excimer emission in higher time-gated spectra also revealed
the dynamic nature of the excimer. The TRES experiment also
ascertains that the longer decay component in lifetime measure-
ment is due to the PYTG excimer (Fig. 4a, Fig. S12 and Table S3,
ESI†). Additionally, the decrease in the fluorescence quantum yield
with increasing concentration can be ascribed to nonradiative
deactivation due to the formation of the dynamic excimer.
Elucidation of excimer emission
As the photophysical properties of PYTG depend on the excimer
formation, we investigated the sensitivity of the excimer emis-
sion towards the local environment and external stimuli.
Hence, the concentration and temperature of the medium were
varied to tune the excimer emission. The emission spectra of
PYTG with varying concentrations from 0.5 mM to 5.0 mM were
monitored in CHCl3 (Fig. S8, ESI†). The two broad emission
bands at 470 and 590 nm were noticeable at a concentration of
0.5 mM. The excimer peak at such low concentration, which is
not observable for pristine pyrene even at 5 mM (Fig. S2, ESI†),
clearly suggested the formation of intramolecular excimers at a
lower concentration of PYTG. The gradual enhancement of the
emission intensity at 590 nm as compared to that at 470 nm
(I590/I470) might be caused by both intra and intermolecular
excimers at higher concentrations of PYTG (Fig. S9, ESI†).
The CIE chromaticity diagram reveals the tunable emission
behaviour of PYTG with varying concentrations (0.5 mM to
5.0 mM) in comparison with that of pyrene (Fig. S10, ESI†).
The excitation spectra of PYTG were monitored at two distinct
emission bands, i.e., the monomer (470 nm) and the excimer
(590 nm). Interestingly, apart from the minor variation in
relative intensities, the excitation spectra were found to be similar
(Fig. S11, ESI†). This result clearly indicates the dynamic nature of
the excimer.10,61
In addition, the result was further verified with fluorescence
lifetime measurements. Fig. 3a reveals the concentration-
independent decay profiles of PYTG while monitoring the decay
time at 470 nm. A 2-fold increment of amplitude average decay
time of PYTG was noticed by monitoring the decay at the
excimer emission band upon varying the same range of con-
centrations (0.5–5 mM, Fig. 3b and Table S3, ESI†). This result
further suggested that the dynamic excimer can be formed due
to the interaction between the excited and ground state mole-
cules with increasing concentration of PYTG. The dynamic
nature of excimer was further elucidated by time-resolved
emission spectra (TRES) measurements. TRES of PYTG (5 mM,
CHCl3) at room temperature is shown in Fig. 4a. At lower delay
time, an intense peak was observed in the region from 460 to
470 nm, whereas with the increase in delay time, a predominant
peak was noticed in the region from 560 to 570 nm. The emergence
As mentioned earlier, the nature of the excimer emission
can be tuned by external stimuli such as temperature. Hence,
the temperature-dependent emission behaviour of PYTG was
monitored from 263 K to 328 K with the difference of 5 K at two
different concentrations. Fig. 4b presents the temperature-
dependent emission spectra of PYTG at 5 mM concentration
in CHCl3. At a lower temperature (263 K), PYTG exhibited an
intense excimer band at 602 nm and a relatively weaker mono-
mer band at 470 nm. As the temperature was raised from 263 K
to 328 K, the excimer emission showed a significant decrease.
The monomer emission also decreased with gradual increase in
temperature. This result ruled out the possibility of the static
excimer, which mainly shows concomitant increase in the
monomer emission and decrease in the excimer emission with
increasing temperature.70 The decrease in both monomer as well
as excimer emissions with increasing temperature is mostly due
to the collision between PYTG and solvent molecules, resulting
in non-radiative deactivation of the excited state.
The intensity ratio of the excimer emission to that of the
monomer with respect to temperature is depicted in Fig. 4b
(inset). We found that the extent of decrease of the excimer
peak was much greater as compared to that of the monomer
peak with increasing temperature. Evidently, it was noticed that
Fig. 4 (a) Time-resolved emission spectra (TRES) of PYTG in CHCl3 (5 mM)
using picosecond pulsed diode laser (EPL – 408 nm) as an excitation
source depicting the dynamic nature of PYTG excimer and (b) emission
spectra of PYTG in CHCl3 (5 mM) at various temperatures (263–328 K);
inset: the plot of I602/I470 against temperature.
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I602/I470 gradually decreases with increasing temperature. This excimer emission (590 nm). On the other hand, the excimer peak
result clearly indicates the dynamic nature of the excimer. became more intense with increasing concentration of PYTG.
Furthermore, the linear fitting of ln(I602/I470) against the reciprocal Interestingly, at 1.5 mM, two emission bands were found to
of the absolute temperature plot (Stevens–Ban plot) revealed a cover the visible region with almost equal intensity, leading to
point of intersection between the higher and the lower temperature the generation of pure white light (ff = 22%, 1.5 mM in DCM,
regimes, ascertaining the dynamic nature of the excimer (Fig. S13, Fig. S16, ESI†). Furthermore, the increase in concentration
ESI†).71 A similar type of behaviour was observed at a lower caused the enhancement of the excimer emission and shifting
concentration of PYTG (0.5 mM) upon increasing the temperature of the emission colour towards yellowish-orange (Fig. 5b).
(Fig. S14, ESI†). However, the extent of decrease of the excimer peak
The Commission Internationale de l’Eclairage (CIE) chromaticity
intensity was found to be much smaller as compared to that index of the PYTG solution (1.5 mM, DCM) was found to be (0.33,
observed at higher concentration profile (Fig. S15, ESI†).
0.34), which is very close to that of pure white light (0.33, 0.33). It is
also desirable to have white light emission in a solid state or a thin
film for practical applications. We observed strong emission of PYTG
in the viscous solution of poly (methyl methacrylate) (PMMA) in
DCM (Fig. S17, ESI†). Hence, PMMA was chosen to construct the
polymer-supported white-light-emissive thin film of PYTG. A trans-
parent thin film was prepared with PYTG (1.5 mM, DCM) and PMMA
(10 mg mLÀ1, DCM) on a quartz plate via spin coating. The thin film
of PYTG exhibited bright white light emission with CIE coordinates
of (0.33, 0.38) (Fig. 5c and d).
Excimer-assisted white light emission
Single component white-light emitters are superior compared
to multicomponent assemblies due to better stability, color
reproducibility, less phase segregation and negligible colour
aging problems.53–56 The development of a single-molecule
white-light emitter is important albeit challenging. Spectro-
scopic investigations revealed that PYTG showed monomer
emission in the bluish-green (400–525 nm) and excimer emission
in the yellowish-orange regions (550–700 nm, complementary
colour of blue-green). The relative intensities of both the monomer
and excimer emissions could be tuned by varying the concentration
of PYTG (Fig. S8, ESI†). Hence, we assumed that PYTG could
exhibit white light emission at a particular concentration in a
suitable solvent. The emission of PYTG was monitored in dichloro-
methane (DCM) by varying the concentration from 0.1 to 3 mM
(Fig. 5a). At a lower concentration (0.1 mM), the monomer emission
(470 nm) was found to be more prominent as compared to the
Chelation-enhanced fluorescence and conversion of dynamic to
static excimer
The flexible tripodal geometry along with the three coordination
sites of triaminoguanidinium is likely to favour binding with
suitable (sterically and electronically) ions.65,66 Accordingly, the
spectroscopic feature involving excimer emission of PYTG may
alter. We intended to explore the effect on the excimer emission
of PYTG in the presence of various ionic species. The electron-
deficient nature of the central carbon in triaminoguanidinium
prompted us to check the effect of anions on the photophysical
properties of PYTG. The addition of tetrabutylammonium
fluoride salt (1 mM) to the solution of PYTG (3 mM, DMSO)
resulted in a new absorption band around 596 nm, whereas no
such noticeable change was observed in the absorption spectra
with other anions (Fig. S18, ESI†). The limit of detection (LOD) of
FÀ was estimated from the UV–vis titration experiment, and it
was found to be 2.9 mM with the linearity range from 18 to
33 mM (Fig. S19, ESI†). The indigo blue colour in the presence of
FÀ was probably due to deprotonation of –NH groups of the
triaminoguanidinium unit of PYTG, as revealed by the NMR
titration experiment (Fig. S20, ESI†). Furthermore, the change in
absorption spectra upon addition of
a base (tetrabutyl-
ammonium hydroxide) was found to be similar to that observed
with fluoride ions (Fig. S21 and Fig. S18c, ESI†). Thus, the
colorimetric sensing of fluoride ions is due to the deprotonation
of PYTG. PYTG was found to be nonfluorescent in the presence
of FÀ due to the formation of new chemical species, ruling
out further exploration of the excited state behaviour. Hence,
we focused our attention to the chelation-dependent excimer
behaviour of PYTG in the presence of different metal ions.
The effects of aggregation (formation of nanoparticles) and
pH on the emission behaviour of PYTG were also verified prior to the
investigation with metal ions. The spectral features of PYTG (5 mM,
acetonitrile) were monitored in different fractions of acetonitrile–
water mixtures. Blue-shifted absorption (labs = 393 nm) and
Fig. 5 (a) Fluorescence emission spectra (lexc = 410 nm) of PYTG in DCM
recorded at different concentrations (0.1–3 mM); (b) CIE (Commission
Internationale de L’Eclairage) coordinates of PYTG in DCM recorded at
respective concentrations (0.1–3 mM). (c) The emission spectrum of PMMA
[poly(methyl methacrylate)]-coated thin film of PYTG; inset: the digital
photograph of white light-emitting thin film under the illumination at
365 nm and (d) corresponding CIE chromaticity coordinates of the
PYTG-embedded PMMA thin film.
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emission (lem = 460 nm) bands with increasing water fraction successive enhancement in excimer emission at 590 nm was
(more than 40%) were noticed (Fig. S22 and Fig. S23, ESI†). The observed (Fig. 6b). The titration profile with Al3+ was also found
enhanced blue emission was misleading, which was believed to to be similar (Fig. S28, ESI†). The corresponding lowest detec-
be due to the formation of PYTG aggregates; however, it was tion limits were calculated to be 5.4 nM and 14 nM for Fe3+ and
proved to be due to the formation of aggregates of pyrene Al3+ (Fig. S29, S30 and Table S4, ESI†). The Job’s plot analysis
aldehyde generated through hydrolysis of Schiff base (Fig. S24 revealed a maximum inflection point at about 0.7,65 which was
and Fig. S25, ESI†).72 In the pH-dependent study, slight red- in accordance with the 1 : 3 binding stoichiometry between
shifted excimer emission was observed upon addition of an PYTG and both the metal ions (Fig. S31 and Fig. S32, ESI†).
acid (trifluoroacetic acid, Fig. S26, ESI†), whereas a new feature The LCMS-ESI mass spectra of PYTG with Fe3+ and Al3+ showed
and significant decrease in fluorescence intensity were noticed peaks at m/z 1109.6 and 1068.8 corresponding to [PYTG +
in the presence of a base (triethylamine, Fig. S26, ESI†). Hence, 3Fe3+ + 9H2O + MeCN]+ and [PYTG + 3Al3+ + 6ClÀ]+ (Fig. S44
all the photophysical studies of PYTG in the presence of metal and Fig. S45, ESI†). This result further confirmed the 1 : 3
ions were performed in 40% water–acetonitrile (no hydrolysis chelation stoichiometries. The association constants of PYTG
of aldimine linkages) and neutral pH medium. PYTG displayed with metal ions were found to be K11 = 1.2 Â 105, K12 = 4.2 Â 105,
very weak fluorescence in a semi-aqueous solution (3 mM, and K13 = 4.8 Â 105 for Fe3+ and K11 = 1.1 Â 105, K12 = 2.9 Â 105,
acetonitrile : water = 3 : 2, v/v) with a clear signature of mono- and K13 = 8.8 Â 105 for Al3+ (Fig. S33 and Fig. S34, ESI†).14
mer and excimer peaks. Upon addition of various monovalent
The chelation-enhanced fluorescence of PYTG was further
(Na+, K+), divalent (Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, probed for multiple cycles. The reversible chelation behaviour
Zn2+, Cd2+, Hg2+, Pb2+) and trivalent (Al3+, Cr3+, Fe3+) metal ions of PYTG towards both Fe3+ and Al3+ ions was checked in the
(5 eq.) separately, a significant change in the absorption and presence of a strong chelating ligand ethelenediaminetetraace-
emission was only noticed in the case of Fe3+ and Al3+ ions tic acid (EDTA, Fig. 6c). Upon addition of EDTA (10 eq.) to the
(Fig. 6a and Fig. S27, ESI†). The enhancement of the excimer solution of PYTG, which was pre-incubated with 10 eq. of Fe3+
,
emission peak at 590 nm might be due to the strong chelation- an immediate change in the emission intensity was observed
enhanced fluorescence (CHEF) upon addition of trivalent (highly fluorescent [PYTG-Fe3+] to weakly fluorescent PYTG).
Fe3+/Al3+ to PYTG. The chelation of PYTG induces molecular The characteristic fluorescence of the complex was recovered
rigidity and inhibits cis–trans isomerization across the aldimine upon subsequent addition of Fe3+. The fluorescence on–off
bond, reducing nonradiative deactivation.
switching was demonstrated to be facile, reversible and
To investigate the details of the chelation behaviour and the fatigue-resistant even after multiple cycles (Fig. 6c). A similar
sensitivity of PYTG towards Fe3+ and Al3+ ions, we performed a phenomenon was observed in the case of Al3+ (Fig. S35, ESI†).
fluorescence titration experiment in a semi-aqueous solution. The results suggest that the detection of Fe3+ and Al3+ by PYTG
Upon concomitant addition of Fe3+ to PYTG solution, a is highly sensitive and reversible.
Fluorescence decay measurements were obtained using
470 nm and 590 nm emission bands of PYTG in the absence
and presence of Fe3+/Al3+ ions. A slower decay time with a larger
contribution observed for the decay monitored at 590 nm
compared to that at 470 nm with increasing concentration of
metal ions confirmed the formation of an excimer (Fig. 6d). The
amplitude average decay time of metal-chelated PYTG was
found to be constant after addition of 3 eq. of metal ions
(Fig. S36, S37 and Table S5, ESI†). This observation indicated
the formation of a static excimer. The excitation spectra of
PYTG with Fe3+/Al3+ were recorded at both monomer (470 nm)
and excimer (590 nm) emission bands (Fig. S38, ESI†). The
excitation spectra monitored at the excimer emission peak
indicated 15 nm red shift compared to that of the monomer
emission band. The excitation spectra further ascertained the static
nature of the excimer (pre-associated dimer) upon chelation.
Fig. 6 The influence of metal ions on fluorescence of PYTG (3 mM) in
a semi-aqueous solution (MeCN : H2O, 3 : 2, v/v): (a) emission spectra
Conclusion
(lexc = 410 nm) of PYTG with different metal ions (5 eq.), (b) emission
In summary, a multifunctional luminescent molecular material
spectra (lexc = 410 nm) of PYTG with increasing concentration of Fe3+
,
PYTG based on pyrene and a C3-symmetric triaminoguanidinium
core was synthesized via a simple, cost-effective, one-step Schiff
base condensation reaction. A detailed investigation of the dynamic
and static excimer formation was carried out through systematic
(c) fluorescence on–off switching cycles of PYTG using Fe3+ (10 eq.)
and ethylenediaminetetraacetic acid (EDTA, 10 eq.), respectively, and
(d) fluorescence decay profiles (lexc = 408 nm, lem = 590 nm) of PYTG
in the absence and presence of Fe3+ (3 eq.).
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Fig. 7 Schematic illustration of dynamic and static excimer formation of PYTG leading to single component white light emission and chelation-
enhanced fluorescence in the presence of trivalent essential ions (Fe3+ and Al3+).
spectroscopic analysis. The excimer was found to be dynamic in
13C NMR: (126 MHz, DMSO-d6) d (ppm): 149.61, 148.81,
nature upon variation of concentration and temperature; the 132.96, 130.98, 130.31, 129.85, 129.15, 128.88, 127.44, 126.88,
formation of static excimer governed the emission behaviour 126.71, 126.25, 125.80, 125.46, 124.79, 124.07, 123.84, 121.83.
in the presence of trivalent metal ions Fe3+ and Al3+ (Fig. 7).
The fine-tuning of the monomer and excimer fluorescence led to
single component white-light emissions both in solution and
Spectroscopy and computation
polymer-embedded thin film. The lowest detection limits of Fe3+ UV-visible absorption spectra were recorded on a Cary 100
and Al3+ were found to be 5.4 and 14 nM, respectively. Compared to spectrophotometer using 10 mm path length quartz cuvettes.
various fluorescent probes (Table S7, ESI†), PYTG was found to be a Jobin Yvon Horiba Model Fluorolog-3-21 was used for the
truly multifunctional molecular optical material, showing promise steady-state fluorescence measurements. The emission spectra
in the generation of single component white light and selective of white light-emitting thin films were recorded in the front-
detection of trivalent ions. The present study opens up new avenues face geometry. The fluorescence spectra were corrected,
for the development of versatile organic materials through a simple employing the correction files available with Horiba software,
synthetic route for display devices and specific detection of physio- with respect to the excitation light intensity and photomultiplier
logically relevant analytes.
tube (PMT) response. To rule out any possible inner filter effect,
the optical density of the samples was kept low (r0.1). CIE
colour coordinates, colour rendering index (CRI) and correlated
colour temperature (CCT) were obtained using Osram Sylvania
(ColorCalculator, version 6.31) software.
Experimental section
Synthesis and characterization
Time-resolved fluorescence decays were examined by a time-
PYTG was synthesized by a one-step Schiff base condensation correlated single-photon counting (TCSPC) spectrometer (Delta
reaction (Scheme 1). Triaminoguanidinium chloride (0.33 mmol) Flex-01-DD/HORIBA). The excitation source was a Delta diode
was dissolved in 10 mL ethanol/H2O in a 100 mL round- laser (408 nm) having a full width at half maximum of 154 ps
bottomed flask. Then, pyrene-1-carboxaldehyde (1.0 mmol) was and the detector was a picosecond photon detection module
added with vigorous stirring and refluxed for 12 h at 85 1C. The with a photomultiplier tube. An aqueous dispersion of colloidal
resulting precipitate was filtered and washed 3 times with diethyl silica (LUDOX) was used to measure the instrument response
ether. After drying under reduced pressure, PYTG was obtained function. Nonlinear least-squares iteration method was used
as a light-yellow solid (85%). The molecular structure and purity for fitting the decay curves with the help of the decay analysis
of PYTG were established through 1H NMR, 13C NMR, and mass software IBH DAS6 (version 6.8). The fitting parameters (w2) and
spectrometry analysis.
the visual inspection of the residuals were followed to assess
1H NMR: (500 MHz, DMSO-d6) dH (ppm): 12.52 (s, 3H), 10.08 the quality of the data fitting. Time-resolved emission spectra
(s, 3H), 9.19 (d, J = 8.1 Hz, 3H), 8.60 (d, J = 9.4 Hz, 3H), 8.45 (TRES) were recorded (excitation source: 408 nm diode laser)
(d, J = 8.2 Hz, 3H), 8.36 (dd, 9H), 8.27–8.18 (m, 6H), 8.13 (t, J = by collecting the emission at different delay times with the
7.5 Hz, 3H).
wavelength range from 425 to 700 nm at 5 nm intervals.
12092 | J. Mater. Chem. C, 2018, 6, 12086--12094
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Ground state geometry optimizations of PYTG were carried
out using the Gaussian 09 program package. The molecular
structure obtained via AM1 energy minimization in Materials
studio 6.1 was taken as the input geometry and was optimized
at the B3LYP/6-31G(d,p) level. TD-DFT calculations were carried
out using the same functional and basis set and the probable
electronic transitions were analysed.
Journal of Materials Chemistry C
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Conflicts of interest
24 G. K. Bains, S. H. Kim, E. J. Sorin and V. Narayanaswami,
Biochemistry, 2012, 51, 6207.
There is no conflict to declare.
25 Y. Wu, X. Zhang, J. Li, C. Zhang, H. Liang, G. Mao, L. Zhou,
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Acknowledgements
VK gratefully thanks DST-SERB, New Delhi for the National
Post-Doctoral Fellowship (N-PDF) award (PDF/2016/001202). BS
thanks IISERB and SK thanks UGC for the fellowship. Infra-
structural support from IISERB is gratefully acknowledged.
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