White-light emission from simple AIE-ESIPT-excimer tripled single molecular system

Article abstract of DOI:10.1039/c6nj03070j

A simple organic molecule (L₃) has been systematically designed and developed to generate white-light emission from a single-component system. It could also tune several emission colors owing to its diverse spectral nature with varying water fractions in methanol-water and acetonitrile-water mixtures. Essentially, the introduction of an aggregation-induced emission (AIE) phenomenon and excimer-forming ability in the molecular system provided the scope for a dual emission, whereas ESIPT (excited state intramolecular proton transfer) coupled-AIE phenomenon acted as an additional means of adjusting the emission wavelength of the corresponding emission peak by varying the solvent polarity. Detailed AFM (atomic force microscopy), DLS (dynamic light scattering) and X-ray crystallographic studies were carried out to validate the mechanism of generation of white-light emission.

Full text of DOI:10.1039/c6nj03070j

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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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White light emission from simple AIE-ESIPT-Excimer tripled single
molecular system
Received 00th January 20xx,
Accepted 00th January 20xx
Soham Samanta, Utsab Manna and Gopal Das*
A simple organic molecule (L3) has been systematically designed and developed to generate white light emission from a
single component system. It also could tune several emission colors due to its diverse spectral nature with varying water
fractions in methanol-water mixture and acetonitrile-water mixture. Essentially introducing aggregation-induced emission
(AIE) phenomenon and Excimer formation ability in the molecular system provided the scope for dual emission whereas
ESIPT (excited state intramolecular proton transfer) coupled-AIE phenomenon worked as an additional handle to adjust
emission wavelength of the corresponding emission peak by varying solvent polarity. Detailed AFM (atomic force
microscope), DLS (dynamic light scattering) and X-ray crystallographic studies were carried out to validate the mechanism
of white light emission output.
DOI: 10.1039/x0xx00000x
www.rsc.org/
molecules display weak emission in dilute solutions, which is
enhanced dramatically due to aggregation of the fluorophore
species in solution or in the solid state. Usually AIE or AIEE is
Introduction
In recent time, white-light-emitting organic materials and
devices have grabbed the limelight due to their various
important lighting applications. To achieve an ideal white-light
emission a system requires either simultaneous emission of
three primary RGB (red, green and blue) colors or at least two
complementary colors having almost similar distribution of
emission intensities covering the entire visible wavelength
(400 to 700 nm) region.1 Hence, most of the reported white-
light emitters deal with combination of several components
having different emission output colors covering the entire
visible spectrum.2-7 However, generating white-light emission
from the combination of multiple components is a relatively
complicated process with much less stability. Compared to
these multicomponent white-light emitters, a single white-
light-emitting organic molecule could offer advantages in
terms of its improved stability and reproducibility, as well as a
simpler fabrication process.8-11 However, examples of
acquiring white-light emission from a single organic molecule
are very handful.
explained by restriction of intramolecular rotation (RIR),
intramolecular charge-transfer (ICT), excimer formation, and
excited-state proton-transfer (ESPT) processes.16-18 Although
the RIR and ICT processes has already been studied exclusively
in explaining the AIE phenomenon, ESPT theory discussed
seldom in this regard. Even though a few articles referred to
ESIPT theory, they failed to elucidate AIE behavior in the light
of excited-state proton-transfer (ESIPT) process in depth by
investigating crystal structures. More often, multiple color
emission can be achieved from AIE or AIEE responsive single
molecules due to different conformation and/or packing
modes in the aggregated state, or due to a change in the
degree of aggregation which may lead to white light
emission.19-20
Most recently, organic molecules having aggregation induced
emission (AIE) or aggregation induced emission enhancement
(AIEE)12-13 and excited state intramolecular proton transfer
(ESIPT),14-15 properties are being extensively used for
generating tunable luminescence responses. Fluorescent
organic molecules that can display aggregation-induced
emission (AIE) properties have come to the limelight as these
Scheme 1: Designing of L1, L2 and L3
Moreover, as pyrene more often indulges in self-assembly to
assimilate excimer emission, introducing pyrene moiety in a
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AIE-ESIPT coupled system might provide an additional Calculated yield: 83%. 1H NMR [600 MHz, CDCl3, TMS, J (Hz), δ
emission peak in aggregated form.21-22 Hence, designing AIE- (ppm)]: 13.45 (1H, s), 9.83 (1H, s), 9.32 (1H, s), 8.96 (1H, d,
ESIPT coupled simple organic molecules with suitable J=8.4), 8.22 (1H, d, J=8.4), 8.10 (1H, d, J=6.6), 8.00 (1H, d,
combination of fluorophore could open up the scope for J=7.8), 7.94 (1H, d, J=7.8), 7.88 (1H, d, J=9.0), 7.81 (1H, d,
achieving tunable white light emission. However thorough J=8.4), 7.69 (1H, t, J=7.8), 7.60-7.58 (3H, m), 7.41 (1H, t, J=7.2),
literature study revealed that the possibility of designing 7.27 (1H, d, J=10.2). 13C NMR [150 MHz, CDCl3, TMS, δ (ppm)]:
simple organic molecule with the abilities of showing AIE, 161.86, 161.59, 161.46, 134.81, 134.15, 133.03, 132.43,
ESIPT and excimer formation together rarely explored till date. 131.54, 129.91, 129.49, 129.38, 129.11, 128.39, 128.10,
Thus, in our continuous pursuit to develop various fluorogenic 127.80, 126.61, 125.57, 124.79, 123.90, 120.36, 119.46,
systems23-30 with several optical applications in the present 108.54. ESI-MS (positive mode, m/z) Calculated for C22H16N2O:
manuscript we would like to discuss systematic development 325.1340. Found: 325.1300 [(M+H+)].
of AIE active bis-imine molecules (L1, L2 and L3) with interesting
ESIPT properties to achieve pure white light emission from
single organic molecule system by suitable variation of
fluorophoric units (Scheme 1).
Experimental
General Information and Materials
All the materials for synthesis were purchased from
commercial suppliers and used without further purification.
The absorption spectra were recorded on a Perkin-Elmer
Lamda-25 UV−vis spectrophotometer using 10 mm quartz
cuvettes. Fluorescence measurements were performed on a
Horiba Fluoromax-4 using 10 mm path length cuvettes with a
slit width of 3 nm at 298 K. The mass spectra were obtained
using Waters Q-ToF Premier mass spectrometer. NMR spectra
were recorded on a Bruker Advance 600 MHz instrument. The
chemical shifts were recorded in parts per million (ppm) scale.
The following abbreviations are used to describe spin
multiplicities in 1H NMR spectra: s = singlet; d = doublet; t =
triplet; m = multiplet.
Scheme 2: Synthesis of L1, L2 and L3
Synthesis of L3
L3 was synthesized using a similar procedure like L1, wherein
only 1-Pyrenecarboxaldehyde was used in place of
benzaldehyde (Scheme 2). Needle shaped orange crystal was
obtained from DCM at room temperature.
Synthesis of L1
Following reported procedure 1-(hydrazonomethyl)naphthale-
ne-2-ol was synthesized (Step 1, scheme 2).23 In Step 2, 1.0
mmol of this product was mixed with 1.1 mmol of
benzaldehyde and stirred for 12 h to obtain a pale yellow
colored solid product. The product was then filtered, washed
thoroughly with methanol and eventually dried in a desiccator.
Calculated yield: 76%. 1H NMR [600 MHz, CDCl3, TMS, J (Hz), δ
(ppm)]: 13.531 (1H, s) or 13.00 (1H*, s), 9.86 (1H, s) or 9.63
(1H*, s), 9.67 (1H, s), 8.91 (1H, d, J=9.0), 8.72 (1H, d, J=7.8),
8.26-8.23 (5H, m), 8.17 (1H, d, J=9.6), 8.10 (1H, d, J=7.8), 8.06
(1H, t, J=7.2), 7.88 (1H, d, J=8.4), 7.81 (1H, d, J=7.8), 7.60 (1H, t,
J=7.2), 7.42 (1H, t, J=7.2), 7.29 (1H, d, J=9.0). 13C NMR [150
MHz, CDCl3, TMS, δ (ppm)]: 161.63, 161.51, 161.40, 135.15,
134.78, 133.84, 133.05, 131.46, 130.81, 130.77, 129.39,
128.41, 128.10, 127.64, 126.59, 126.52, 126.33, 125.29,
125.19, 124.76, 123.90, 122.85, 120.39, 119.47, 119.35. ESI-MS
(positive mode, m/z) Calculated for C28H18N2O: 399.1497.
Found: 399.1528 [(M+H+)].
1
Calculated yield: 86%. H NMR [600 MHz, CDCl3, TMS, J (Hz), δ
(ppm)]: 13.37 (1H, s), 9.73 (1H, s), 8.69 (1H, s), 8.19 (1H, d,
J=8.4), 7.86-7.90 (3H, m), 7.81 (1H, d, J=8.4), 7.58 (1H, t, J=7.2),
7.51-7.49 (3H, m), 7.41 (1H, t, J=7.2), 7.27 (1H, d, J=15.0). 13C
NMR [150 MHz, CDCl3, TMS, δ (ppm)]: 161.69, 161.59, 161.27,
134.65, 133.80, 132.87, 131.58, 129.22, 128.98, 128.72,
128.22, 127.94, 123.74, 120.15, 120.23, 119.28, 108.32. ESI-MS
(positive mode, m/z) Calculated for C18H14N2O: 275.1184.
Found: 275.1154 [(M+H+)].
Crystallographic Refinement Details
Intensity data for the all crystals were collected Mo-Kα
radiation (λ = 0.71073Å) at 298(2) K, with increasing ω (width
of 0.3° per frame) at a scan speed of 6 s/ frame on a Bruker
SMART APEX diffractometer equipped with CCD area detector.
The data integration and reduction were processed with
SAINT31 software. An empirical absorption correction was
Synthesis of L2
L2 was synthesized using a similar procedure like L1, wherein
only 1-naphthaldehyde was used in place of benzaldehyde
(Scheme 2).
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applied to the collected reflections with SADABS.32 The
structures were solved by direct methods using SHELXTL33 and
were refined on F2 by the full-matrix least-squares technique
using the SHELXL-97 program package.34 Graphics are
generated using MERCURY 3.0.35 Non-hydrogen atoms are
treated anisotropically. The hydrogen atoms are geometrically
fixed.
Results and discussion
Rationale behind the design of L1, L2 and L3:
Schiff base compounds L1, L2 and L3 were rationally designed
and synthesized in order to achieve tunable white light
emission (Scheme 1&2). Essentially the idea to keep bis-imine
core intact along with naphthol moiety in all L1, L2 and L3 was
fundamentally based on the theory that it would inherently
introduce AIE behavior in these types of compounds likewise
our previously reported AIE active systems.23, 25 Eventually this
AIE active behavior would allow the system to show high
fluorescence. However, it was necessary to link AIE
phenomenon with dual emission behavior (comprised of
complementary emission colors) to achieve white light. Hence
we systematically introduced the additional phenyl (in L1) or
naphthyl (L2) or pyrene (in L3) moiety to acquire extra emission
peak. These variation of the phenyl (in L1) or naphthyl (L2) or
pyrene (in L3) moiety was based on the fundamental notion
CCDC No. 1434540, Formula - C28H18N2O, Fw - 398.44, Crystal
system – monoclinic, Space group P 21, a = 9.6949(8) Å, b =
16.3365(19) Å, c = 12.5230(1) Å, α = 90°, β = 96.097°(8), γ =
90°, V = 2056 Å3, Z = 4, Dc = 1.342 g cm-3, μ (MoKα) = 10.082
mm-1, F(000) = 832.0, T = 298(2) K, θmax. = 26.99, Total no. of
reflections = 8784, Independent reflections = 7079, Observed
reflections = 3947, Parameters refined = 561, Flack parameters
= -3.9(10), R1, I > 2σ(I) = 0.1277, wR2, I > 2σ(I) = 0.3054, GOF
(F2) = 1.189.
UV−Vis and fluorescence spectroscopic studies
that unlike the phenyl (in L1
) or naphthyl (L2) moiety
Stock solutions of L1 and L2 (5 × 10−3 mol. L−1) were prepared in
DMSO. The solution of L1 or L2 was then diluted to 10 × 10−6
mol. L−1 for fluorescence spectral studies with CH3OH/aqueous
mixed solvents having different water fractions (varied from 0-
100%). However, for UV-Visible experiments the final
concentration of the experimental solutions of L1 or L2 were
adjusted at 25 µM by diluting the respective stock solutions.
On the other hand, a stock solution of L3 was also prepared in
DMSO having concentration 1.25 × 10−3 mol. L−1. The solution
introducing pyrene (in L3) moiety would generate a distinct
extra emission peak due to its tendency to form excimer.
Moreover, we also envisaged that imine-naphthol excited
state intramolecular proton transfer could be instrumental in
acquiring solvent polarity dependent emission maxima
(generated due to AIE) shift.
UV-Visible spectroscopic study of L1, L2 and L3:
of L
3 was then diluted to 10 × 10−6 mol. L−1 for spectral studies
The UV-Vis spectra of L1, L2 and L3 (25 µM, 25 µM and 10 µM
respectively) in pure methanol revealed sharp absorbance
maxima at 379, 387 and 416 nm respectively which may be
attributed to the π-π* charge transfer transitions of the
concerned molecules (Figure S11, ESI). It was interesting to
note that gradual addition of water into its methanol solution
(both UV-Visible and fluorescence) by taking only 16 µL stock
solution of the molecule and making the final volume 2 mL
adding CH3OH/aqueous or THF/aqueous or CH3CN/aqueous
mixed solvents with different water fractions (varied from 0-
100%). Fluorescence and UV-visible experiments were carried
out in quartz optical cells of 1 cm path length.
led to regular decrease of the absorption bands for L1, L2 and
L3 likely due to the formation of molecular aggregates (Figure
S11, ESI). Absorption spectra taken with varying water fraction
in a methanol-water mixture too revealed that gradual
increase in the water fraction could result in the systematic
reduction of the absorbance peak accompanied by ~10-12 nm
Dynamic light scattering studies
Dynamic light scattering (DLS) experiments were performed on
a Malvern Zetasizer Nano ZS instrument equipped with a 4.0
mW He–Ne laser operating at a wavelength of 633 nm at room
temperature. It was carried out with optically clear solutions of
red shifts for every molecular system (L1
, L2 and L3). Eventually
UV-Vis spectra of L1, L2 and L3 in the methanol-water mixed
L1, L2 and L3 (10 μM) in 5:5 MeOH-H2O and 1:9 MeOH-H2O
solvents with higher water fractions (>60%) also witnessed
increase in the base lines which also reiterated the formation
of aggregates (Figure S11, ESI). Hence initially it was
mixed solvents to observe the change in particle size upon
increasing water fraction. The solutions were equilibrated for
30 minutes before taking the measurements.
anticipated that each of the designed molecules L1, L2 and L3
might exhibit aggregation induced emission property.36
Atomic Force Microscope (AFM) Studies
The morphology of the aggregates were investigated from NT-
MDT micro-40 AFM instrument using a semi-contact mode at a
Studies of AIE (aggregation-induced emission) properties of L1& L2:
All the three designed organic molecules (L1
found to be very weakly emissive in pure methanol. However,
it was interesting to note that these weakly emissive L1 L2 and
, L2 and L3) were
scan rate of 1 Hz. Solutions of L1, L2 and L3 (10 µM) in 1:9
MeOH-H2O mixed solvent were respectively drop-casted on
three different cover slips and left open to atmosphere for 12
h, followed by desiccation prior to acquiring AFM images. For
L3 (10 µM) another AFM image was taken wherein a solution
of L3 (10 µM) in 1:9 THF-H2O mixed solvent was drop-casted on
a cover slip.
,
L3 in methanol became highly emissive in a methanol–water
mixture with higher water contents (Figure 1 & 2).
In pure methanol, L1 displayed a very weak emission band with
an obscure maximum at around 485 nm when excited at 380
nm (Figure 1A). Emission spectra of L1 recorded with varying
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water fraction in a methanol-water mixed solvent revealed spectral studies strongly recommended that both L1 and L2
that though increase in the water fraction up to 70% did not could demonstrate aggregation induced emission behavior.
induce any noticeable change in the spectral signature of L1
,
Dynamic light scattering (DLS) studies of L1 and L2 in a mixed
further increase in the water fraction (80% or above) could aqueous medium revealed that the average particle size
lead to dramatic enhancement in its emission intensity. increased to 232 nm and 370 nm from 136 nm and 259 nm
Meanwhile this huge TURN-ON fluorescence of L1 in the respectively (Figure S12 & S13, ESI) for L1 and L2 upon
methanol-water mixture having water content more than 80% increasing water content of the medium from 50% to 90%.
or equal also witnessed about 25 nm red shift in the emission Moreover, evidence for the aggregation phenomena in
maxima to introduce a new emission maximum at 510 nm. L2 methanol water mixture with higher water content (9:1
followed similar trends in the fluorescence spectral behavior.
methanol-water; v/v) was strengthened by obtaining
corresponding atomic force microscope images of L1 and L2
(Figure S14, ESI). Solid state/film state fluorescence spectra of
L1 and L2 also authenticated the AIE active nature of these
molecules by showing strong fluorescence with emission
maxima at 525 nm and 548 nm respectively, when excited at
380 nm (Figure S15, ESI). Thus, beyond any doubt L1 and L2 are
classic AIE active compounds which can display dramatic
enhancement in its fluorescence as
a consequence of
restricted intra-molecular rotation (RIR) due to aggregation in
solution or in the solid state. It also should be paid attention
that due to aggregation induced emission; L1 exhibited strong
green fluorescence whereas L2 illuminated a strong greenish-
yellow fluorescence (Figure 1A & B INSET). However, as our
aim was to develop a system capable of displaying white light
emission, it was necessary to link AIE phenomenon with dual
emission behavior (comprised of complementary emission
colors). Hence we designed the molecule L3, where we kept
the bis-imine core intact with naphthol moiety which is
actually responsible for the AIE behavior of these types of
compounds.23,
25
However, in place of phenyl (in L1) or
naphthyl (L2) moiety we introduced another fluorophore
(pyrene moiety) which itself can emit bluish-green
fluorescence in aqueous methanol (beacause of excimer
formation) to achieve effective dual emission.
Fluorescence spectroscopic studies of L3:
In pure methanol medium, L3 displayed two clear but very low
intensity emission maxima at ~460 nm (peak 1) and ~516 nm
(
peak 2) upon excitation at 380 nm; which can be attributed to
the presence of both pyrene and hydroxy-naphthalene moiety
in the molecule (Figure 2A). It was observed that increasing
water fraction of a methanol-water mixed solvent (from 0%)
up to 20% led to gradual increase in the intensity of L3
,
Figure 1: Fluorescence spectra of L1 (10 µM) (A) and L2 (10 µM) (B) upon changing the
water fraction of methanol-water mixed solvent; λex = 380 nm. INSET: change in the
intensity and visual changes in fluorescence (under UV light) with different water
fractions.
wherein the emission peak around 460 nm increased more
steadily compared to the emission peak at 516 nm (Figure 2A).
Interestingly, increasing water fraction from 20% to 30% not
only induced certain increase in the overall emission intensity
but also tempted sudden significant red shift in the emission
peak 2 from 516 nm to 556 nm whereas emission peak 1
experienced negligible shift. Increasing the water fraction from
30% to 40% also resulted in substantial enhancement of the
Upon excitation at 380 nm, L2 also rendered a broad weak
emission band at around 494 nm (Figure 1B) in methanol
medium. Increase in water fraction up to 50 % in a methanol-
water mixture showed very insignificant changes in the
fluorescence spectra of L2. However, further increase in the
water fraction (60% or above) resulted in the unprecedented
enhancement in its emission intensity accompanied by 21 nm
red shift in the emission maxima and thus new emission
maxima arose at 525 nm (Figure 1B). Hence, these detailed
intensity in both of its emission peaks (peak1 and peak 2
)
besides causing 10 nm further red shift of the emission peak 2
from 546 nm to 556 nm. Similarly, changing the water fraction
to 50% led to further red shift of its maxima (peak 2) from 556
nm to 570 nm along with slight growth of its intensity (peak 2).
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Figure 3: (A) CIE chromacity diagram of the spectra of L3 in different water fractions [(I)
0-60% water fractions & (II) 70-100% water fractions] in methanol-water mixture; (B)
AFM image of the aggregates obtained from a drop casted solution of L3 in 9:1
methanol water solvent.
Figure 2: (A) Fluorescence spectra of L3 (10 µM); upon changing the water fraction of
methanol-water mixed solvent; λex = 380 nm; (B) Visual changes in fluorescence (under
UV light) with different water fractions.
White light emission and plausible mechanism:
However, it was important to note that emission peak 1
,
Hence, L3 not only can display dual emission in methanol-
water mixture but also apparently could tune several emission
colors due to its diverse spectral natures with varying water
fractions. Photograph of the resultant experimental solution of
L3 under UV-light in methanol-water mixture with varying
water content (0-100%) indeed showed that L3 could emit
various fluorescence colors with various intensities based on
variation of the water percentage. As evident from the figure
2B, in 70, 80 and 90% water content L3 emitted white
fluorescence in methanol-water mixture. CIE chromacity
diagram too reiterated that fluorescence spectra of L3 with 70,
80 and 90% water content represented the white light
emission (Figure 3A) with CIE-coordinates (0.35, 0.35), (0.36,
0.35) and (0.33, 0.37) respectively. However, among them,
emission spectra of L3 in 9:1 methanol-water mixture has the
closest coordinates (0.33, 0.37) compared with pure white
light (0.33, 0.33). Hence a white light emission was achieved
from a single AIE active molecular system, developed through
a step by step systematic progress in molecular design.
neither showed any change in it’s intensity nor experienced
any shift upon changing the water fraction from 40% to 50%.
Compared to 50% water fraction, in 60% water fraction L3
witnessed slight decrease in the intensity of both of its
emission maxima. Meanwhile, emission peak 1 witnessed ~ 5
nm red shift whereas emission peak 2 displayed ~10 nm blue
shift to exhibit two emission maxima at 465 nm and 560 nm
respectively upon altering the water fraction from 50% to
60%.Thereafter increasing water fraction to 70% did not show
any change in the spectral nature of L3 except causing slight
enhancement in the emission intensity (Figure 2A). Increasing
water fraction from 70% to 80% induced almost no change
except producing very small red shift in the emission peak 1
from 465 nm to 468 nm.
However, upon changing the water fraction to 90%, there was
reasonable reduction in the overall emission intensity of L3
,
accompanied by further slight (~4 nm) red shift in its emission
maximum at 468 nm (peak 1) to facilitate emergence of a new
emission maximum at 472 nm whereas the other emission
peak at 560 nm (peak 2) remained unaltered (Figure 2A). As a
result of this L3 displayed dual emission with two distinct
emission peaks at 472 nm (peak 1) and 560 nm (peak 2) with
almost same intensities in methanol-water mixed solvent
having 90% water content (Figure 2A). Altering water fraction
from 90% to almost 100% water medium (99.99%) revealed no
significant shifts in the emission maxima but the emission
maximum at 472 nm reduced slightly with simultaneous
substantial enhancement of the emission peak intensity at 560
nm.
It is worth mentioning that when non-protic solvent
acetonitrile was used instead of methanol and fluorescence
spectra of L3 were recorded in the acetonitrile-water mixture
having different water fractions; it also resulted in similar
spectral outcome to indicate generation of white light at 40,
50 and 60% water content (Figure 4, main manuscript and
Figure S16, ESI). So, essentially white light can be obtained
from the molecule L3 in variety of solvents; which further
enhances the scope of its utilization.
Interestingly, likewise the case of L1 and L2; DLS studies of L3 in
a mixed aqueous medium too revealed that the average
particle size increased from 298 nm to 393 nm upon increasing
water fraction of the medium from 50% to 90% in a methanol-
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37-38
.
water mixture (Figure 5). Thus undoubtedly L3 also formed L3
When the emission spectra of compound L3 were
aggregates in a methanol-water mixture with higher water recorded in various solvents with different solvent polarity,
contents. Alongside aggregation phenomena in methanol there was clear red shift in the emission peak 2 along with
water mixture with higher water content (9:1 methanol-water; increase in intensity upon increasing the solvent polarity
v/v) was further corroborated by atomic force microscope (Figure S17, ESI). This observation indicated towards the
images of L3 (Figure 3B) aggregates. It was also noted that L3 involvement of ESIPT process in the system.
can exhibit strong fluorescence in solid state/ film state (Figure
S15, ESI); which further qualified the molecule as a classic AIE-
gen.
Figure 5: DLS-based particle size analysis of L3 (10μM) upon changing the solvent from
5:5 methanol-water to 1:9 methanol-water.
Moreover, when solvent polarity was increased by gradually
increasing the water fraction (Figure 6A) of a THF-water mixed
solvent (water content varied from 0% to 100%); it was
interesting to note that the emission intensities of L3 at 510
nm increased and red shifted with the increase of the polarity
of the solution (Figure 6A). This implied that a significant
intramolecular electronic push–pull phenomenon exists in the
bonds of N···H–O. As L3 is very poorly soluble in water,
increasing water fraction in a methanol-water or a THF-water
mixture would help aggregation (Figure 3B & 6B) of L3
molecules. This aggregation might restrict the intramolecular
torsional motions in L3 to attain more planar structure which
would essentially be favorable for the proton transfer from –
OH to the Nimine atom upon photo-excitation.37 This ESIPT
process was expected to fully restrict the intramolecular
rotation and help molecules of L3 to attain a ‘‘lock’’ structure
(Scheme 3), which accounted for the gradual red shift in the
band maxima (peak 2) from 516 to 570 nm (Figure 2A) upon
increasing water fraction (up to 80%).38
On the other hand, it is squarely acknowledged that pyrene
excimer emission λem is much less variable and generally
located at 410–480 nm. Hence, peak 1 of L3 (with λem~460 nm)
can be attributed to the excimer formation due to the
presence of pyrene moiety in it. The appearance of the
excimer emission peaks in the fluorescence spectra of L3 may
be due to close proximity of two L3 units at aggregated state.
This assumption is also verified by taking fluorescence spectra
of L3 at three different concentrations (1 µM, 3 µM, 5μM and
10 µM) which revealed that increase in the concentrations of
L3 resulted in the systematic increase in the excimer peak
(Figure S18, ESI). Moreover, fluorescence spectra of L3 taken in
non-protic solvents such as hexane (poor solvent for L3) and
acetonitrile (moderately good solvent) revealed that even
though in acetonitrile emission peak 1 of L3 is absent, in
hexane it can display a characteristic triple humped peak
corresponding to typical pyrene moiety (Figure S19, ESI).
Figure 4: (A) Fluorescence spectra of L3 (10 µM); upon changing the water fraction of
acetonitrile-water mixed solvent; λex = 365 nm; (B) CIE chromacity diagram of the
spectra of L3 in different water fractions in acetonitrile-water mixture.
However, even though the involvement of AIE phenomenon in
the spectral outcome of L3 was unquestionable, the diverse
spectral characters for different water fractions prompt us to
suspect that some other factor(s) too might have some role in
the spectral output. A close look at the structure of L3
suggested that it could potentially undergo ESIPT process.
However it was important to understand the extent of
involvement of ESIPT in influencing AIE behavior of L3. The
fluorescence spectra of L3 were recorded in solvents with
varying polarity in order to verify whether proton-transfer
really exists or not; and how it impacts on the excited state of
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Hence, emission peak 1 can be attributed to pyrene excimer X-Ray Crystal structure of L3:
formation. Also the absence of any prominent peaks (Figure
The crystal structure of L3 also supported the premise that AIE,
ESIPT and excimer-formation are simultaneously operational in
L3 (Figure 7). We obtained the block shaped orange crystal of
L3 from DCM solvent by slow evaporation which found to be
highly fluorescent in nature in the solid state itself. X-ray single
crystal structure of L3 revealed that it adopted a non-
centrosymmetric molecular arrangement with a polar space
group P(2)1 (experimental methods). The crystal packing of L3
suggested that two different types of orientations are present
in a unit cell of L3, which basically backed the probability of
forming dimer (excimer) in solution upon aggregation (Figure
7).
S20, ESI) in the region of 410–480 nm for L1 and L2 further
reiterated that introducing pyrene moiety in L3 actually
embedded the extra emission peak to harvest dual emission.
Hence, it would be fair to assess that introducing AIE
phenomenon and excimer formation ability in the molecular
system provided the scope for dual emission whereas ESIPT
coupled-AIE phenomenon offered an additional handle to
adjust emission wavelength of the corresponding emission
peak suitably by varying solvent polarity to yield effective
white light emission (Scheme 3).
Figure 6: (A) Fluorescence spectra of L3 (10 µM) upon changing the water fraction of
THF-water mixed solvent; (B) AFM images of the aggregates obtained from the drop
casted solution of L3 (10 µM) in 1:9 THF-water mixed solvent.
Figure 7: (A) Intra-molecular and intermolecular interactions; (B) ORTEP molecular
structure and (C) molecular packing in single crystals of L3.
One type of orientation of L3 subjected to four intermolecular
short interactions (two C–H···π & two C–H···O) whereas other
type of orientation led to six intermolecular short interactions
(four C–H···π & two C–H···O). It may be mentioned here that all
the existed intermolecular C–H···O interactions have the
distances in the range of 2.60–2.66 Å whereas intermolecular
C–H···π interactions have distances in the range of 2.85–2.86
Å; which underlines the strong nature of these short
interactions (Figure 7A). These types of strong interactions
could be instrumental in reducing the non-radiative
deactivation of excitons by locking the molecular motion in the
crystal lattice.39-40 In other words presence of the strong C–
H···π
& C–H···O intermolecular interactions (not π–π
Scheme 3: Scheme for AIE-ESIPT-Excimer tripled white-light emission.
interactions) in L3 helped to enhance the molecular rigidity and
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9
Q. Y. Yang and J. M. Lehn, Angew. Chem. Int. Ed., 2014, 53
4572-4577; Angew. Chem. 2014, 126, 4660-4665.
,
initiated its turn-on emission significantly due to the restriction
of intramolecular rotation. Hence, crystal structure directly
attested the AIE active nature of L3. Besides the intermolecular
interactions the single crystal of L3 showed that there is a
strong intramolecular interaction between –OH and Nimine
which opened up the scope for a proton transfer through
hexa-cyclic ring from O–H to Nimine. Hence more likely these
interactions (both intra and intermolecular) led to the large
red shift of the emission peak 2 accompanied by gradual
increase in the emission intensity upon increasing water
fraction due to the formation of aggregates.
10 M. Han, Y. Tian, Z. Yuan, L. Zhu and B. Ma, Angew. Chem. Int.
Ed., 2014, 53, 1090810912.
11 S. H. Kim, S. Park, J. E. Kwon and S. Y. Park, Adv. Funct.
Mater., 2011, 21, 644-651.
12 J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu, H. S.
Kwok, X. Zhan, Y. Liu, D. Zhuc and B. Z. Tang, Chem.
Commun., 2001, 1740-1741.
13 Y. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Soc. Rev., 2011,
40, 5361-5388 and references therein.
14 J. E. Kwon and S. Y. Park, Adv. Mater., 2011, 23, 3615-3642.
15 J. Zhao, S. Ji, Y. Chen, H. Guo and P. Yang, Phys. Chem. Chem.
Phys., 2012, 14, 8803-8817.
16 Z. Yang, W. Qin, J. W. Y. Lam, S. Chen, H. H. Y. Sung, I. D.
Williams and B. Z. Tang, Chem. Sci., 2013,4, 3725-3730.
17 V. S. Padalkar, D. Sakamaki, K. Kuwada, N. Tohnai, T.
Akutagawa, K. Sakai and S. Seki, RSC Adv., 2016, 6, 26941-
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18 J. Mei, N. L. C. Leung, R. T. K. Kwok, J. W. Y. Lam and B. Z.
Tang, Chem. Rev., 2015, 115, 11718-11940.
19 X. Luo, J. Li, C. Li, L. Heng, Y. Q. Dong, Z. Liu, Z. Bo and B. Z.
Tang, Adv. Mater., 2011, 23, 3261-3265.
20 H. Tong, Y. Hong, Y. Dong, Y. Ren, M. Haussler, J. W. Y. Lam,
K. S. Wong and B. Z. Tang, J. Phys. Chem. B, 2007, 111, 2000-
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21 F. M. Winnick, Chem. Rev., 1993, 93, 587-614.
22 D. Sahoo, V. Narayanaswami, C. M. Kay, R. O. Ryan,
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9008
Conclusions
In summary we have systematically designed and developed of
a simple organic molecule (L3) to generate white light emission
from single component. It also has the potential to tune
several emission colors by varying water fractions in methanol-
water mixture, due to its diverse spectral nature. Introducing
aggregation-induced emission (AIE) phenomenon (peak 2) and
excimer formation
(peak 1) helped in securing steady
enhancement in the fluorescence intensities of the respective
dual emission peaks upon aggregation whereas insertion of
ESIPT (excited state intramolecular proton transfer)
phenomenon provided additional tool to handle the shift in
emission maxima through changing solvent polarity. DLS
(dynamic light scattering), AFM (atomic force microscope)
Studies and crystal structure analysis directly validated the
proposed lighting scheme. We hope this work would open a
new window in engendering various tunable lighting
applications from low cost simple single molecular devices.
25 S. Samanta, U. Manna, T. Ray and G. Das, Dalton Trans.,
2015, 44, 18902-18910
26 S. Samanta, S. Goswami, A. Ramesh and G. Das, J.
Photochem. Photobiol. A, 2015, 310, 45-51.
27 S. Samanta, T. Ray, F. Haque and G. Das, J. Lumin., 2016, 171
13-18.
,
28 S. Samanta, S. Goswami, A. Ramesh and G. Das, Sens.
Actuators B., 2014, 194, 120-126.
Acknowledgements
29 S. Samanta, B. K. Datta, M. Boral, A. Nandan and G. Das,
Analyst, 2016,141, 4388-4393.
30 S. Samanta, P. Dey, A. Ramesh and G. Das, Chem. Commun,
2016, 52, 10381-10384.
31 SAINT and XPREP, 5.1 ed.; Siemens Industrial Automation
Inc.: Madison, WI, 1995. G. M.Sheldrick.
G.D. acknowledges CSIR (01/2727/13/EMR-II) and Science &
Engineering Research Board (SR/S1/OC-62/2011), India for
financial support, CIF IITG for providing instrument facilities.
SS, MB and UM acknowledge IIT Guwahati for fellowship.
32 SADABS, empirical absorption Correction Program;
University of Göttingen: Göttingen, Germany, 1997.
33 G. M. Sheldrick, SHELXTL Reference Manual: Version 5.1;
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Table of Contents
Rationally designed simple organic molecule exhibits white light emission in methanol/water
and acetonitrile/water mixed solvents.