Aggregation induced emission of excited-state intramolecular proton transfer compounds: Nanofabrication mediated white light emitting nanoparticles

Article abstract of DOI:10.1021/acs.cgd.6b00396

Excited-state intramolecular proton transfer (ESIPT) compounds, 2-(2-((2-hydroxybenzylidine)amino)phenoxy)benzonitrile (1) and 2-(4-((2-hydroxy-4-methoxybenzylidine)amino)phenoxy)benzonitrile (2) exhibited aggregation induced enhanced emission (AIEE) in t

Full text of DOI:10.1021/acs.cgd.6b00396

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Article
Aggregation induced emission of excited state intramolecular proton transfer
compounds: Nanofabrication mediated white light emitting nanoparticles
Anu Kundu, Palamarneri Sivaraman Hariharan, Kaliyan Prabakaran
Prabakaran, Dohyun Moon, and Savarimuthu Philip Anthony
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00396 • Publication Date (Web): 21 Apr 2016
Downloaded from http://pubs.acs.org on April 23, 2016
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Aggregation induced emission of excited state intramolecular proton transfer
compounds: Nanofabrication mediated white light emitting nanoparticles
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Anu Kundu,a P. S. Hariharan,a K. Prabakaran,a Dohyun Moonb* and Savarimuthu Philip
Anthonya*
a)School of Chemical & Biotechnology, SASTRA University, Thanjavurꢀ613401, Tamil Nadu,
India. Fax: +914362264120; Tel: +914362264101; Eꢀmail: philip@biotech.sastra.edu
b)Beamline Department, Pohang Accelerator Laboratory, 80 Jigokroꢀ127beongil, Namꢀgu,
Pohang, Gyeongbuk, Korea, Email: dmoon@postech.ac.kr
ABSTRACT: Excited state intramolecular proton transfer (ESIPT) compounds, 2ꢀ(2ꢀ((2ꢀ
hydroxybenzylidine)amino) phenoxy) benzonitrile (1) and 2ꢀ(4ꢀ((2ꢀhydroxyꢀ4ꢀmethoxy
benzylidine)amino) phenoxy) benzonitrile (2) exhibited aggregation induced enhanced emission
(AIEE) in the solid state and nanofabrication mediated white light emission. The strong
intermolecular interactions (Hꢀbonding, CꢀH…π and π… π) restrict the free rotation and rigidify
the fluorophores, 1 and 2, in the crystal lattice that lead to enhanced fluorescence in the solid
state of 1 (yellow, λmax = 535 nm, Φf = 38 %) and 2 (greenishꢀyellow, λmax = 525 nm, Φf = 21
%). Surprisingly, nanofabrication of 1 and 2 produced metaꢀstable white light emitting
nanoparticles (Φf = 24 % (1), 19 % (2)) that slowly converted into stable yellow (1) or greenishꢀ
yellow (2) fluorescent nanoparticles with increasing time. The time dependent fluorescence,
morphological and metal ion interacting studies indicate the formation of soft nanoparticles in
which both blue fluorescent cyanophenoxy and yellow/greenishꢀyellow fluorescent salicylidene
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group behave independently. 1 and 2 nanoparticles undergo morphological change from
spherical to plates with increasing time. Thus simple ESIPT molecules showed solid state
fluorescent and nanofabrication induced tuneable fluorescence particularly white fluorescent
metaꢀstable nanoparticles.
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Introduction
Organic solid state fluorescent materials have played significant role in developing modern
optoelectronic devices such as light emitting diodes, flexible display, organic lasers, photovoltaic
cells and sensors.1ꢀ7 Particularly, small organic materials and nanostructures with tuneable
fluorescence are important for developing multiple color emitting devices. However, aggregation
caused quenching (ACQ) remains a big challenge for solid state fluorescence since that
promoted nonꢀradiative decay process caused by strong intermolecular vibronic interactions and
excimer formation.8 Hence various approaches including use of twisted molecular conformation
and supramolecular complexation have been explored for generating organic solid state
fluorescence systems.9ꢀ12
In recent years, a new class of organic functional materials that exhibit unusual
aggregationꢀinduced enhanced emission (AIEE) in the solid state has emerged as an important
topic both in the field of fundamental research and device applications.13ꢀ14 The AIEE molecules
are nonꢀfluorescence or weakly fluorescent in the solution but show strongly enhanced
fluorescence in the aggregated/solid state by rigidifying fluorophore and restricting
intramolecular rotation. Tetraphenylethylene, cyanostilbene, triphenylethylene and triarylamine
derivatives are some of the organic molecules employed for generating AIEE materials.15ꢀ20
Organic fluorescent compounds that exhibit excited state intramolecular proton transfer (ESIPT)
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mechanism constitute another interesting class of functional materials because of their unique
fourꢀlevel cyclic protonꢀtransfer reactions.21 The ESIPT compounds exhibit large Stokes shifts
and unique fluorescence that made them as potential candidates for wide application in
molecular switches, laser dyes, sensors and UV photostabilizers.22ꢀ24
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Tuneable organic solid state fluorescence often realized via systematic modulation of
structures, polymorphism, nanofabrication, controlling supramolecular interactions and inclusion
of guest molecules.25ꢀ33 AIEE and ESIPT mechanism has also been utilized for fabricating
organic materials with tuneable emission.34ꢀ36 AIEE responsive single molecule with twisted
molecular conformation could show multiꢀcolor emission due to external stimuli induced
conformational change or different molecular packing in the aggregated state.37ꢀ41 The tuneable
multiꢀcolor fluorescence of some AIEE and ESIPT materials have also lead to whiteꢀlight
emission.42ꢀ43 The simultaneous generation of three primary color (red, green and blue) or at least
any two color with equal intensity covering wide visible spectrum would be ideal for whiteꢀlight
emitter. White light emitting organic molecular assemblies in nanostructures, solution and solid
state have often been realized using multiple components via Forster resonance energyꢀtransfer
(FRET) between donor and acceptor. However, white light emission from single small organic
fluorescent molecules are rarely reported.44ꢀ46 Recently, Maity et al. reported unique ESIPT
couple AIEE in a single organic molecule for generating multiple emission including white light
emission.47
In this manuscript, we report AIEE from simple ESIPT compounds, 1 and 2, and
nanofabrication induced tuneable fluorescence including white light emission and fluorescence
sensing of Cu2+ and Co2+ metal ions. Single crystal analysis revealed strong intermolecular
interactions (Hꢀbonding, CꢀH…π, π… π) in the crystal lattice of 1 and 2 that restrict free rotation
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and rigidify the fluorophores. This leads to strong enhancement of fluorescence in the solid state.
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1 and 2 showed yellow (λmax = 535 nm,
Φf = 38 %) and greenishꢀyellow (λmax = 525 nm, Φf =
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21 %) emission, respectively. Nanofabrication of 1 and 2 in water produced rare metaꢀstable
white light emitting nanoparticles that slowly transform into yellow/greenish yellow emitting
nanoparticles. SEM studies revealed morphological change of 1 and 2 nanoparticles from
spherical to plates with increasing time and support the fluorescence change. Further, selective
fluorescence quenching of salicylidene group by Cu2+ and Co2+ metal ions demonstrate the
formation of soft nanoparticles in which cyanophenoxy and salicylidene groups are behaving
independently. The tuneable fluorescence particularly white light emission of simple ESIPT
molecules could be potential interest in optoelectronic applications.
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Materials and methods
2ꢀfluoroꢀnitrobenzene, 4ꢀfluoroꢀnitrobenzene, 2ꢀcyanophenol, NaBH4, Pd/C, salicylaldehyde
and 2ꢀhydroxyꢀ4ꢀmethoxy benzaldehyde was obtained from sigmaꢀAldrich and used as received.
The solvents were obtained from Merck India. All chemicals are used as received. The metal ion
solutions used for the colorimetric sensor experiments were prepared in MillꢀQ water. Ultrapure
water produced using a MilliꢀQ apparatus (Millipore), was used in all experiments.
Synthesis of 1 and 2:
Schiff base ESIPT molecules, 1 and 2 was synthesized following our reported procedure
(Scheme S1).48 Typically, salicylaldehyde (1.1mmol) dissolved in ethanol (20 ml) was added
dropꢀwise into ethanol (5 ml) solution of 2ꢀ(2ꢀcyanophenoxy)aminobenzene (1mmol) under
stirring at room temperature. The resultant reaction mixture was refluxed for 5 h. The product
was precipitated out upon cooling the reaction mixture to room temperature. The precipitate was
filtered, washed with ethanol and dried under vacuum. 2 has also been synthesized in a similar
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procedure
but
by
using
4ꢀmethoxysalicylaldehyde
(1.1mmol)
and
4ꢀ(2ꢀ
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cyanophenoxy)aminobenzene in ethanol solution.
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1: Yield = 85 %. m.p. 171ꢀ173 °C. H NMR (CDCl3) δ 12.419 (s, 1H (OH)), 8.674 (s, 1H
(imine H)), 7.617ꢀ7.640 (d, 1H), 7.448ꢀ7.453 (d, 1H), 7.427ꢀ7.434 (t, 1H), 7.408ꢀ7.413 (d, 1H),
7.309ꢀ7.332 (m, 4H), 7.175ꢀ7.192 (m, 1H), 7.072ꢀ7.110 (t, 1H), 6.885ꢀ6.922 (m, 2H), 6.754ꢀ
6.777 (d, 1H). 13C NMR (CDCl3) δ 164.77, 161.10, 159.49, 147.03, 141.14, 134.19, 133.92,
133.57, 132.64, 127.98, 126.58, 122.68, 122.21, 121.27, 119.06, 119.01, 117.28, 115.93, 115. 60,
102.81.
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1
2: Yield = 90 %. m.p. 116ꢀ118 °C. H NMR (CDCl3) δ 13.601 (s, 1H (OH)), 8.674 (s, 1H
(imine H)), 7.661ꢀ7.676 (d, 1H), 7.512ꢀ7.516 (t, 1H), 7.472ꢀ7.497 (m, 3H), 7.262ꢀ7.306 (m, 3H),
6.892ꢀ6.913 (d, 1H), 6.492ꢀ6.516 (t, 2H), 3.85 (s, 3H). 13C NMR (CDCl3) δ 164.19, 163.82,
161.71, 159.86, 153.55, 145.67, 134.40, 134.09, 133.73, 123.11, 122.73, 121.05, 117.08, 116.06,
113.18, 107.07, 103.90, 101.22, 55.61.
Nanoparticles fabrication of 1 and 2: The stock CH3CN solution (10 ml) of 1 and 2 (10ꢀ3 M)
was prepared and stored. 100 µl of this solution was rapidly injected into 20 ml of water under
vigorous stirring at room temperature. The stirring was allowed to continue for another 30
minutes. The samples were subjected absorption, fluorescence and microscopy studies
immediately as well as after 24 h. The nanoparticles of 1 and 2 were collected on the aluminium
foil surface and dried under vacuum for microscopy analysis. Spectroscopic analysis of the
nanoparticles was carried out while keeping the sample directly in the solution. The effect of
temperature, pH, presence of different ions (cations/anions) in the water and stirring speed has
been explored on the formation of nanoparticles and fluorescence change.
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Microscopy characterization: The morphologies and sizes of the nanoparticles were examined
using field emission scanning electron microscopy ((FEꢀSEM) (JSMꢀ6701F, JEOL Japan INC)
with an accelerating voltage 30 kV and filament current of 20 mA for 45 seconds. The samples
are stuck onto a doubleꢀface conducting carbon tape mounted on a brass stub. Prior to analysis,
the samples were coated with a thin layer of gold.
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Structural analysis: Single crystal of 1 and 2 was coated with paratoneꢀN oil and the
diffraction data measured at 100K with synchrotron radiation (
λ = 0.62998 Å) on a ADSC
Quantumꢀ210 detector at 2D SMC with a silicon (111) double crystal monochromator (DCM) at
the Pohang Accelerator Laboratory, Korea. The ADSC Q210 ADX program49 was used for data
collection (detector distance is 63mm, omega scan; ∆ω = 3º, exposure time is 2 sec per frame)
and HKL3000sm (Ver. 703r)50 was used for cell refinement, reduction and absorption correction.
The crystal structure of 1 and 2 was solved by the direct method with SHELXꢀXT (2014/4)
program and refined by fullꢀmatrix leastꢀsquares calculations with the SHELXꢀXL (2014/7)
program package.51 All nonꢀhydrogen atoms except some atoms with high thermal anisotropies
and all the solvent atoms were refined anisotropically. The hydrogen atoms were assigned
isotropic displacement coefficients U(H) = 1.2U (C,N) or 1.5U(Cmethyl), and their coordinates
were allowed to ride on their respective atoms. CCDCꢀ1414929 (1) and 1414930 (2) contains the
supplementary crystallographic data for this paper. The data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre,
12, Union Road, Cambridge CB2 1EX, UK.
Spectroscopy and characterization: Absorption and fluorescence spectra were recorded using
Perking Elmer Lambda 1050 and Jasco fluorescence spectrometerꢀFPꢀ8200 instruments.
Fluorescence quantum yields (Φf) of solid samples were measured using a Horiba JobinꢀYvon
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model FL3ꢀ22 Fluorolog spectrofluorimeter with integrating sphere. 1 and 2 solids and
nanoparticles were excited at 370 nm for measuring quantum yields. IR spectra were recorded
using KBr disks on a Shimadzu affinity spectrophotometer over the range 4000 ꢀ 400 cmꢀ1 using
KBr as the reference.
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Result and Discussion
1 and 2 Schiff base compound was synthesized by condensing corresponding aromatic
aldehyde and amine precursors in ethanol as shown in Scheme S1.48 The amine precursors were
prepared by reaction between 2ꢀcyanophenol and 2ꢀfluoro or 4ꢀfluoro nitrobenzene in presence
of K2CO3 in DMSO at 110 °C and subsequent reduction of nitro group using NaBH4 in presence
of catalytic amount of Pd/C in methanol. Absorption studies of 1 and 2 showed πꢀ π* transition
between 340ꢀ345 nm in different solvents (Fig. S1). The absorption maxima of 1 and 2 did not
shift significantly in different solvents that ruledꢀout the solvatochromism. Further, 1 and 2 did
not show fluorescence in any solvent that could be due to the free rotation of CꢀC bond and C=N
isomerization.
Interestingly, 1 and 2 showed strong AIEE in the solid state (Fig. 1a). 1 showed yellow
fluorescence (λmax = 535 nm,
Φf = 38 %) whereas greenishꢀyellow fluorescence (λmax = 525 nm,
Φf = 21 %) was observed for 2 (Fig. 1b). Further, the fluorescence spectra of 1 and 2 showed a
small hump at 558 and 500 nm, respectively. The excitation spectra of 1 and 2 in the solid state
showed πꢀ π* transition at 370 nm which is 25 nm red shifted relative to absorption maxima in
solution. The red shift of absorption peak suggests the Jꢀaggregation of 1 and 2 fluorophores in
the solid state. The long stoke shift of 1 (165 nm) and 2 (155 nm) could be due to the ESIPT
phenomena of Schiff base unit. To get the insight on the molecular organization, single crystals
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of 1 and 2 were grown from methanol and toluene, respectively. The structural analysis of 1 and
2 revealed strong intramolecular hydrogen bond interaction between phenolic hydroxyl and
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Fig. 1. (a) Molecular structure with digital fluorescence images and (b) excitation and emission
spectra of 1 and 2. λexc = 365 nm.
Fig. 2. (a) Molecular conformation of 1 (a) and 2 (b) in the crystal lattice. C (grey), N (blue), O
(red), H (white); Hꢀbonds (broken line). Intramolecular Hꢀbond distances are marked in Å.
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imine nitrogen. The Schiff base molecular unit of 1 and 2 adopts perfectly planar conformation
in the solid state whereas cyanophenoxy adopts nearly perpendicular orientation (Fig. 2).
Further, the weak intermolecular interactions, Hꢀbonding (phenoxy hydroxyl and cyanophenyl
hydrogen) and CH...π, in the crystal lattice of 1 lead to the formation of zig-zag structure along
cꢀaxis (Fig. 3a). However 1 did not form any intermolecular interactions in other two directions
and are well separated (Fig. S2ꢀ3). The intermolecular Hꢀbonding between methoxy oxygen and
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Fig. 3. Zig-Zag chain of 1 (a), dimer formation (b) and extended Hꢀbond interaction in the crystal
lattice (c) of 2. N (blue), O (red), H (white); Hꢀbonds (broken line). Hꢀbond and CꢀH...π
distances are marked in Å.
cyanophenyl hydrogen and π...π interactions lead to the formation of dimer with opposite
molecular orientation in the crystal lattice of 2 (Fig. 3b). The dimer is further connected in all
three directions via Hꢀbonding and π...π interactions (Fig. 3c, S4). These weak intermolecular
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interactions rigidify the 1 and 2 fluorophore in the crystal lattice and exhibit AIEE properties.
The offꢀset stack structural arrangement confirms the Jꢀaggregation of 1 and 2 fluorophores in
the solid state.
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Further to confirm the AIEE, nanoparticles of 1 and 2 were fabricated using fast precipitation
method by swiftly injecting CH3CN solution of 1/2 (good solvent) into water (poor solvent). 1 in
CH3CN (100 %) and up to 80 % of water solution (20:80 CH3CN and water) did not show any
florescence. However, injecting 1 (25 µL) into water (100 %) showed the formation of strongly
enhanced fluorescent nanoparticles (Fig. 4a). Similarly, 2 also produced strong fluorescent
nanoparticles when injecting into water (90% as well as 100 %, Fig. 4b). Surprisingly, the
fluorescence spectra of 1 and 2 exhibited dual emission peak, blue and yellow/greenish yellow
fluorescence with nearly equal intensity. Both 1 and 2 showed a new peak at 410 nm along with
molecular crystals emission at 532 nm (1)/523 nm (2). The appearance of blue and
yellow/greenish yellow emission in the nanoparticles of 1 and 2 (100 % water) lead to white light
emission (Φf = 24 % (1), 19 % (2)), a phenomenon rarely observed in a simple single organic
fluorescent compound. It is noted that equal intensity of blue, green and red fluorescence or at
least either two is required for observing white emission.52 The CIE color coordinates of white
emission were calculated from fluorescence spectra (0.275, 0.306 (1); 0.232, 0.365 (2)). The
ideal CIE coordinates for pure white light is (0.33, 0.33). Single organic fluorophores emit dual
fluorescence when the structure contains different color emitting group or by halochromic effect
that produced several emitting species.47,53 For example, different degree of protonation in
dithienophospholeꢀbased donor–acceptor conjugated fluorophore53 and disturbing the
conjugation by protonation in aminoꢀtrimethylammonium oligophenylene vinylene lead to white
light emission.54 Similarly, the evolution of weak supramolecular interaction in the aggregates
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Fig. 4. Nanoparticles evolution of study of 1 (a) and 2 (b) by measuring fluorescence spectra.
Digital images of nanoparticles fluorescence is shown in the inset. λexc = 365 nm.
of fluoreneꢀbased gold(I) complex resulted in single component white light emitting materials.55
The molecular structure of 1 and 2 contain both blue emitting cyanophenoxy and green/greenish
yellow emitting salicylidene Schiff base group (Fig. 1a). The blue emission from cyanophenoxy
group was confirmed by synthesizing 2ꢀphenoxycyanobenzonitrile that showed blue
fluorescence at 432 nm in the solid state (Fig. S5). The greenishꢀyellow/yellow fluorescence
could be due to salicylidene Schiff base group. In solid state of 1 and 2, both blue and
yellow/greenishꢀyellow emitting groups are held together by supramolecular interactions that
produced only single fluorescence peak and no blue fluorescence was observed. However, it
appears that blue and yellow/greenishꢀyellow emitting units are independent in the initial
aggregates and showed dual fluorescence. The nanoparticles morphological studies, time
dependent fluorescent and excitation spectra and metal ions sensing that are discussed in the
following paragraphs have also support the above hypothesis. The nanoparticles evolution has
also been monitored using absorption spectroscopy. It is noted that 1 and 2 in CH3CN showed
absorption λmax at 345 nm. The absorption peak of 1 did not undergo any significant change as
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well as no new peak has been observed up to 90 % water solution (Fig. 5a). However,
introducing 1 into 100 % water exhibited a clear red shifted absorption peak at 378 nm (Fig. 5a).
The absorption peak of 2 was steadily decreased with appearance of new absorption at 402 nm as
increasing water percentage (Fig. 5b). 2 in 90 and 100 % water showed a clear red shifted new
absorption at 402 nm and the peak at 345 nm has been completely disappeared. The red shifted
absorption suggests the Jꢀaggregation of molecules in the nanoparticles that was evidenced from
single crystal Xꢀray structure (Fig. 3).
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Fig. 5. Nanoparticles evolution of study of 1 (a) and 2 (b) by measuring absorption spectra.
The white light emitting nanoparticles of both 1 and 2 are stable for 1 h. After that they
slowly changed into yellow (1) or greenishꢀyellow (2) emitting nanoparticles with increasing
time. Hence 1 and 2 white light emitting nanoparticles are metaꢀstable. Time dependent
fluorescence studies were performed for of 1 and 2 nanoparticles to gain the insight on the
appearance of dual fluorescence and color change (Fig. 6). The asꢀfabricated nanoparticles of 1
and 2 (100 % water sample) showed two strong emission with nearly equal intensity; one at 410
nm (blue emission) another at 532 nm (1, yellow) or 522 nm (2, greenish yellow) initially. The
nanoparticles solution exhibited clear white emission when excited by UV lamp (λexc = 365 nm).
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The intensity of blue emission peak was slowly decreased and yellow or greenishꢀyellow
emission peak has been intensified with increasing time. After 16 h, the blue emission peak has
been completely disappeared in both 1 and 2 nanoparticles. The excitations spectra of 1 and 2
nanoparticles have also showed clear shift of peak position with increasing time (Fig. S6). The
initial excitation spectrum of 1 nanoparticles showed peak at 378 nm that slowly blue shifted to
370 nm with increasing time (Fig. S6a). Similarly, excitation spectra of 2 showed a peak at 385
nm initially and slowly blue shifted to 370 nm with time (Fig. S6b). It is noted that excitation
spectra of 1 and 2 molecular crystals have also showed peak at similar position (370 nm, Fig.
1b). Further, the time dependent excitation spectra showed a small hump at longer wavelength
(435 nm) for both 1 and 2 nanoparticles initially along with strong peak at 378 nm (1) and 385
nm (2). The longer wavelength peak might be due to the salicylidene unit. With increasing time,
this peak slowly disappeared and merged with main strong peak that clearly broadened with blue
shift. The λcutꢀoff for 1 and 2 nanoparticles after 24h is 440 nm and 480 nm, respectively. The
excitation spectra of 2ꢀphenoxycyanobenzonitrile, a model compound prepared to study the
cyanophenoxy fluorescence, showed a peak at 378 nm (Fig. S5). Thus initially both blue
emitting cyanophenoxy and yellow/greenishꢀyellow emitting salicylidene groups might not be
having strong interactions and emit separately. As increasing time, the nanoparticles slowly
evolved and produced one single peak of longer wavelength that is matched with bulk materials
emission. These studies indicate that 1 and 2 fluorophores adopt different molecular arrangement
initially in the nanoparticles that reꢀorganize with time and adopt molecular arrangement similar
in the molecular crystals of 1/2. This was further confirmed by PXRD studies that exhibited
perfect matching of 1 and 2 nanoparticles (after 24 h) pattern with simulated PXRD pattern from
single crystal data (Fig. S7). However, we could not perform PXRD measurement for white light
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emitting nanoparticles since it was metaꢀstable and centrifugation converted into
yellow/greenishꢀyellow emitting particles. FTIR studies of 1 and 2 crystals, powders and
nanoparticles are essentially same (Fig. S8). Thus the modulation of fluorescence in the
nanoparticles with time is due to the change of molecular arrangement in the nanoparticles
evolution. 1 and 2 nanoparticles fabricated at different temperatures (10, 25 and 60 °C) have also
showed dual fluorescence with small variations in the intensity (Fig. S9). However, it was
observed that nanoparticles fabricated at higher temperature (60 °C) were converted into stable
nanoparticles with single emission fast compared to low and room temperature fabrication.
Further, it was observed that freezing white light emitting nanoparticles solution has also
converted to yellow/greenishꢀyellow emitting nanoparticles immediately. It is noted that room
temperature fabricated 1 and 2 nanoparticles exhibited dual emission with equal intensity
whereas longer wavelength emission dominates in the nanoparticles fabricated at lower
temperature (S9). Thus applying any external stimuli appear to help the conversion of metaꢀ
stable to stable nanoparticles.
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Fig. 6. Change of nanoparticles fluorescence intensity and λmax of 1 (a) and 2 (b) with time.
Digital images of nanoparticles fluorescence with time is shown in the inset. λexc = 365 nm.
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Further SEM studies of 1 and 2 nanoparticles with time revealed interesting morphological
changes and support the fluorescence modulation. The white light emitting 1 and 2 nanoparticles
fabricated initially (100 % water) showed clear spherical morphology with size range between
20ꢀ30 nm (Fig. 7a,c). The spherical nanoparticles further assembled into micron sized big balls
in 1 whereas stripꢀlike arrangements observed in 2. The spherical nanoparticles of 1 and 2
undergoes morphological change from spherical nanoparticles to nano plates with increasing
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Fig. 7. SEM images of nanoparticles at initial stage (a (1) and c (2)) and after 24 h (b (1) and d
(2)). The scale bar in the SEM image is 100 nm.
Scheme 1. Schematic diagram of fluorescent and morphological changes of 1 and 2 nanoparticles
with increasing time.
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time (after 24 h) in both sample (Fig. 7b,d). 1 exhibited featureless nanoplates with thickness
between 40ꢀ50 nm and diameter range 100ꢀ200 nm. However, 2 showed clear hexagonal
nanoplates formation with diameter between 200ꢀ300 nm. PXRD confirms the formation of
crystalline nanoplates with similar structure to their single crystals (Fig. S7). Although the exact
structural arrangement in the white light emitting 1 and 2 spherical nanoparticles is difficult to
predict, the above studies clearly suggest that fluorophores adopt different molecular orientation
in the soft spherical nanoparticles initially in which both blue and yellow/greenishꢀyellow
emitting groups could emit separately. It is noted that morphology and soft surface of organic
functional nanoparticles can modulate the fluorescence.56ꢀ58 Initially, when the nanoparticles are
soft, there might be only the strong intramolecular hydrogen bonding in the aggregates and the
weak intermolecular hydrogen boding (CH…π, CꢀH…O) might not have been formed (Fig. 3).
Hence it appears that there is no energy transfer between blue emitting cyanophenoxy group and
yellow or greenishꢀyellow emitting salicylidene unit (Scheme 1). However, the evolution of
weak intermolecular hydrogen bonding converts the soft spherical nanoparticles to hard
nanoplates and the rigidified fluorophores exhibited either yellow (1) or greenishꢀyellow (2)
emission. The evolution of weak intermolecular interaction has already been shown to influence
on the nanoparticles hardness and optical properties.58
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The molecular structure of 1 and 2 consist of metal ions interacting phenolic chelating ligand.
It is noted that 1 and 2 in solution exhibited selective colorimetric sensing of Cu2+ and Fe3+ ions
via coordination.48 The asꢀfabricated 1 and 2 nanoparticles that exhibited white fluorescence
initially due to dual fluorescence (blue and yellow/greenishꢀyellow) from phenolic chelating
salicylidene unit and cyanophenoxy group. The time dependent fluorescence and SEM studies
further indicates that both salicylidene and cyanophenoxy group behave independently at initial
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stage of nanoparticles evolution. The independent salicylidene unit might interact with metal
ions that could be used to further demonstrate the dynamic nature of the nanoparticles. Hence, 1
and 2 spherical white fluorescent nanoparticles were explored for selective fluorescence sensing
of metal ions in aqueous solution. Interestingly, addition of Cu2+ and Co2+ ions into white
fluoresccent nanoparticles completely quenched the salicylidene unit fluorescence (532 nm (1) or
522 nm (2), Fig. 8). The blue fluorescence peak has not been affected significantly. It is noted
that Cu2+ addition showed partial reduction in the blue fluorescence intensity in nanoparticles of
1. Addition of other metal ions did not show significant change in the fluorescence. The selective
quenching of fluorescence could be due to the interaction of metal ions with nanoparticles via
metal ions interacting phenolic chelating functional groups (Scheme S1). However, addition of
metal ions into 1 and 2 nanoparticles after 24 h did not show fluorescence quenching for any
metal ions. This might be due to the formation of hard nanoplates. These studies further support
the formation of soft spherical nanoparticles initially in which cyanophenoxy and salicylidene
unit behave independently and emit both blue and yellow/greenishꢀyellow emission. The
formation of soft spherical nanoparticles might facilitate the metal ions to interact with phenolic
chelating functionality. The hard nanoplates completely block the metal ions interaction with
chelating functionality. Thus the morphological change of nanoparticles (spherical to plates)
accompanies with complete reꢀorganization of fluorophores arrangement and support the
fluorescence tuning from white to yellow/greenish yellow. The fluorescence intensity of 1 and 2
nanoparticles decreased gradually with increasing the concentration of Cu2+/Co2+ metal ions
(Fig. 9, S10ꢀ11). However, in contrast to molecular fluorescent probes that often exhibit
complete fluorescent quenching at 1:2 or 1:3 metal:ligand ratio depending on the coordination
geometry of metal ions,59 1 and 2 nanoparticles required higher concentration of metal ions
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(Cu2+/Co2+) for complete fluorescent quenching. This could be due to the difficulty in the
diffusion of metal ions with aggregated nanoparticles and evolution of nanoparticles with
increasing time. To get the insight on the complex formation, we have tried to single crystals of
Cu2+/Co2+ with 1 and 2 by dissolving in DMSO. Although, Co2+ did not produce quality single
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Fig. 8. Fluorescence spectra of white light emitting nanoparticles of 1 (a) and 2 (b) with different
metal ions. Digital images of nanoparticles with different metal ions is shown for 1 (a).
crystals to collect the data, Cu2+ with 1 and 2 gave good crystals that clearly demonstrate the
formation of 1:2 metal:ligand ratio coordination complex (Fig. S12). Further it has also
confirmed involvement of salicylidene unit alone in the coordination with metal ions. Based on
this result, we propose the formation of 1:2 coordination complexes for Co2+ also with 1 and 2
(Scheme S1). The interference studies indicate that Cu2+/Co2+ could quench the fluorescence 1
and 2 nanoparticles in presence of other metal cations (Fig. S13). However, the reason for
selective coordination of Cu2+/Co2+ with 1 and 2 ligands are not clear at present. Combining
single crystal structure of 1 and 2, 1 and 2 with Cu2+, selective interaction of white fluorescent
nanoparticles with metal ions, morphological and fluorescence change, it can proposed that
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initial nonꢀplanar salicylidene unit in the white fluorescent nanoparticles slowly evolve into
planar conformation along with strong intramolecular Hꢀbonding and produced dominant
yellow/greenishꢀyellow emitting nanoparticles. Thus these studies have also clearly demonstrate
the dynamic nature of 1 and 2 nanoparticles that produced metaꢀstable nanoparticles initially and
converted to stable nanoparticles with increasing time.
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Fig. 9. Change of fluorescence intensity of 1 (a) and 2 (b) nanoparticles (fabricated by injecting
10 µL of 1 and 2 (10ꢀ3 M) into 2 mL water) with concentration of Co2+ and Cu2+ ions.
Conclusion
In conclusion, simple ESIPT compounds, 1 and 2, exhibited AIEE in the solid state and
interestingly nanofabrication induced rare white light emitting metaꢀstable nanoparticles. Single
crystal analysis confirmed the restriction of free rotation and rigidification of fluorophores in the
solid state via strong intermolecular interactions (Hꢀbonding, CꢀH…π, π… π). The rigidification
of fluorophores in the solid state lead to enhanced fluorescence for 1 (yellow, λmax = 535 nm)
and 2 (greenishꢀyellow, λmax = 525 nm). Interestingly, 1 and 2 produced metaꢀstable white light
emitting nanoparticles in water that showed blue and yellow or greenishꢀyellow dual
fluorescence from phenoxybenzonitrile and salicylidene unit separately. The white fluorescent
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metaꢀstable nanoparticles were slowly converted into corresponding bulk materials fluorescence
with increasing time. Time dependent fluorescence, morphological and metal ions interaction
studies indicate the formation of soft nanoparticles in which blue fluorescent cyanophenoxy and
yellow/greenishꢀyellow fluorescent salicylidene group produce fluorescence independently. The
soft spherical nanoparticles converted into hard nanoplates with increasing time that matched
with the bulk materials through evolving supramolecular interactions in the nanocrystals. The
selective quenching of salicylidene fluorescence in the dual fluorescent metaꢀstable nanoparticles
by Cu2+/Co2+ metal ions support the separation of both groups at initial nanoparticles. Hard
nanoplates did not show any fluorescence quenching. Thus, simple ESIPT molecules showed
solid state florescence and nanofabrication mediated white fluorescent metaꢀstable nanoparticles.
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Acknowledgements
Financial supports from DST, New Delhi, India (DST Fast Track Scheme No. SR/FT/CSꢀ
03/2011 (G) and SR/FST/ETIꢀ284/2011(c)) and CRF facility, SASTRA University are
acknowledged with gratitude. AK and PSH thank SASTRA University for research fellowship.
"Xꢀray crystallography at the PLSꢀII 2DꢀSMC beamline was supported in part by MSIP and
POSTECH. We thank Dr. C. R. Ramanathan for NMR spectra.
Supporting Information
Absorption spectra, crystallographic table for 1 (CCDC No. 1414929) and 2 (CCDC. No.
1414930) and fluorescence spectra are available in the supporting information.
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Aggregation induced emission of excited state intramolecular proton transfer
compounds: Nanofabrication mediated white light emitting nanoparticles
Anu Kundu,a P. S. Hariharan,a K. Prabakaran,a Dohyun Moonb* and Savarimuthu Philip
Anthonya*
ESIPT compounds exhibited aggregation induced enhanced emission and unusual
nanofabrication induced metaꢀstable white light emitting that exhibited morphological,
fluorescence and metal ion interaction changes with increasing time.
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