Aggregation induced emission of 9-Anthraldehyde microstructures and its selective sensing behavior towards picric acid

Accepted Manuscript Aggregation induced emission of 9-Anthraldehyde microstructures and its selective sensing behavior towards picric acid

Debasish Das, Ashim Maity, Milan Shyamal, Samir Maity, Naren Mudi, Ajay Misra PII: DOI: Reference:

S0167-7322(17)34932-2 doi:10.1016/j.molliq.2018.03.129 MOLLIQ 8903

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

16 October 2017 14 March 2018 31 March 2018

Please cite this article as: Debasish Das, Ashim Maity, Milan Shyamal, Samir Maity, Naren Mudi, Ajay Misra , Aggregation induced emission of 9-Anthraldehyde microstructures and its selective sensing behavior towards picric acid. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Molliq(2017), doi:10.1016/j.molliq.2018.03.129

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ACCEPTED MANUSCRIPT

Aggregation Induced Emission of 9-Anthraldehyde Microstructures and Its Selective Sensing Behavior towards Picric Acid Debasish Das, Ashim Maity, Milan Shyamal, Samir Maity, Naren Mudi and Ajay Misra

Department of Chemistry and Chemical Technology, Vidyasagar University, Midnapore

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721102, W.B, India

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Abstract

A novel material showing aggregation induced emission (AIE) is developed by reprecipitation

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method using 9-Anthraldehyde (9-AC), where Sodium dodecyl sulfate (SDS) was used as

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morphology directing agent. 1-dimension rod and 2-dimension elongated hexagon shaped morphology of 9-AC aggregates have been synthesized. Morphology of the materials was

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characterized using optical microscopy. Photophysical properties of the hydrosol were studied

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using UV-Vis, steady state and time resolved fluorescence emission techniques. Computation of second order Fukui parameter as local reactivity descriptor on each atomic center of the titled

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compound also substantiate that the neighboring 9-AC molecules are arranged in trans

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conformation in its aggregated structures and this is in conformity with the crystal structure of 9AC. The ‘turn off’ fluorescence property of aggregated 9-AC has been utilized for selective detection of picric acid and the fluorescence quench ing has been explained due to ground state complexation between 9-AC and picric acid. The observed detection limit of picric acid was found as low as 8.07µM. Keywords: 9-Anthraldehyde, Aggregates, Morphology, aggregation induced emission (AIE), Fukui parameter, DFT. 1

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1. Introduction [1]

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nanometer and micrometer-sized crystals of

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functional organic molecules. Presence of van der-Waals intermolecular interactions offers large

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variability in their composition and physical properties. The orientation of building units in

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inorganic crystals is identical since atoms can be regarded as hard spheres; in contrast, the stacking mode of organic molecules acting a significant role in the properties of low dimensional

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organic material. Most of the organic chromospheres which are highly fluorescent in solution at low concentration show a drastic decrease of their emission efficiency in the solid state. This

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behavior is generally attributed to interactions that provide non-radiative decay routes,

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intermolecular π–π* stacking interactions of fluorophore etc. This well known phenomenon is

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known as the aggregation caused quenching (ACQ) effect [2]. This ACQ effect is responsible for unsatisfactory photoluminescence (PL) efficiency of organic luminescent materials in the solid

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state and is a great obstacle towards the development of efficient optoelectronic devices using

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organic luminescent materials. It has driven researchers to synthesize anti-ACQ type materials to improve the emission efficiency of organic luminescent materials in their aggregated or solid state. This major problem was solved by Tang [3] and Park [4] et al. in 2001 by developing new organic luminescent materials that exhibit stronger emission properties in the solid state than in their solution phase. These molecules are classified into two different groups. In the first group, the molecules are non-emissive in a good solvent but become highly luminescent in their aggregated form, thus behaving exactly opposite to the conventional ACQ effect. This unusual 2

ACCEPTED MANUSCRIPT fluorescence phenomenon was referred as to ‘‘aggregation-induced emission’’ (AIE). Restriction of intramolecular torsional/rotational motions is identified as the main cause for the AIE effect. In the second group, the molecules are weakly luminescent in solution but their efficiency increases upon aggregation. This is known as aggregation induced emission enhancement

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(AIEE) [5-7].

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The strong effect of electron confinement on electron-hole pairs in all three directions

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result in the size-tunable optoelectronic properties of semiconducting quantum dots [8]. But this is not expected in organic molecular crystals (OMCs), because of small radius of the Frenkel

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exciton [9]. The primary differences between inorganic and organic semi-conductors are in the

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band width, or the degree of orbital overlap. In the case of OMCs, the electronic [10] and optical properties such as phototransistors [11,12], memory devices [13] are fundamentally different

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from those of inorganic semi-conductors, because of weak van der Waals intermolecular forces

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[14,15]. The controlling of size, shape and hence the properties of OMCs is still a challenge and an important aspect in the development of material science. Much effort has been devoted to

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synthesize organic nano/micro particles having various size and shapes. These include zero

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dimensional (0-D) spherical or tetrahedral quantum dots [16, 17], one-dimensional (1-D) nano rods and wires from small organic compound [18-20] and two-dimensional (2-D) nanoribbons

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and nanotubes [21], nanoplates [22], nanowires [23] microcapsule [24], sub-microtube [25] organic nano flower [26], etc. Various techniques were developed to prepare organic nano/micro particle, such as reprecipitation [27,28], physical vapor deposition [29], ultra-sonication [30], microemultion [31], template method [32], self-organization [33], postchemistry [34-37] etc. Among the above methods, reprecipitation is one of the most preferential routes towards the cost-effective large-scale production of nano/micro building blocks. Reprecipitation is the 3

ACCEPTED MANUSCRIPT quickly injection of micro amounts of the solution of the material in a good solvent, into macro amounts of poor solvent. In this process, the sudden change of environment for organic molecules induces precipitation. Herein, we report a self-inducing template growth to produce microstructure of 9-AC in

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the presence of SDS as morphology directing agent. The morphology of the as prepared

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microstructures was studied by optical microscope and scanning electron microscope (SEM).

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Photo physical properties of the aqueous dispersion of 9-Anthraldehyde microstructures were investigated using UV-Vis absorption and steady state as well as time resolved fluorescence

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emission techniques. Nitroaromatic compounds are electron deficient and they prefer to form

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aromatic π-interactions complexes with the electron-rich aromatic host. The presence of nitro groups can further enhance the complexation through hydrogen bonds with suitable donor groups

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on the host molecules. Among the different nitroaromatic compounds, picric acid (PA) is one of

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the powerful explosive. The AIE property of aggregated 9-AC hydrosol is utilized for selective fluorescence ‘turn-off’ sensing of picric acid [38]. It can selectively sense PA by forming

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ground state complex with PA in its aggregated state.

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2.1. Materials

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2. Materials and methods

9-Anthraldehyde (9-AC) and sodium dodecyl sulfate (SDS) were purchased from SigmaAldrich Chemical Corp. Ethanol was obtained from E-Mark India Ltd. All of the reagents were of analytical grade and were used without further treatment. SDS was recrystallized from 1:1 water methanol mixtures. Triply distilled deionized water was used throughout the experiments. 2.2. Synthesis of 9-Anthraldehyde Microparticles 4

ACCEPTED MANUSCRIPT 9-Anthraldehyde microstructures were synthesized by reprecipitation method where SDS was used as soft template. In a typical preparation, small volume of 9-Anthraldehyde (50mM) in ethanol was injected into 5 mL of continuously stirred aqueous SDS (10mM)/water at room temperature (25°C). Volume of 9-Anthraldehyde and concentration of SDS were varied to

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synthesize different shaped 9-Anthraldehyde microstructures.

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Sample-a, b & c were prepared by injecting 0.05mL 0.1mL and 0.2mL 9-Anthraldehyde

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(50mM) into 5ml 10mM aqueous SDS solution respectively. On the other hand sample- d, e & f were prepared by injecting 0.05mL, 0.1mL and 0.2mL 9-Anthraldehyde (50mM) into 5ml water

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respectively with vigorous stirring. After 5 min of vigorous stirring, each solution was kept

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undisturbed for 30 min at room temperature before characterization and subsequent analysis. 2.8. Characterization

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Optical microscopy images were taken using a NIKON ECLIPSE LV100POL upright

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microscope equipped with a 12V-50W halogen lamp. The samples for optical microscopic study were prepared by placing a drop of colloidal solution onto a clean glass slide. The UV–Vis

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spectroscopic measurements were done in a 1cm quartz cuvette with a Shimadzu UV-1800

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spectrophotometer. Steady state fluorescence spectra were recorded using Hitachi F-7000 Fluorescence Spectrophotometer. Fluorescence lifetime of samples were measured using TCSPC

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set up from PTI, U.S, equipped with sub-nanosecond pulsed LED source (370 nm) having pulse width 600 ps (FWHM) operating at high repetition rate of 10 MHz driven by PDL 800-B driver, PicoQuant, Germany. Instrumental resolution of the setup is 100 ps. Lamp profiles were measured with a band-pass of 3 nm using Ludox as the scatterer. The decay parameters were recovered using a non-linear iterative fitting procedure based on the Marquardt algorithm [39].

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ACCEPTED MANUSCRIPT The quality of fit was assessed over the entire decay, and tested with a plot of weighted residuals and other statistical parameters e.g. the reduced χ2 ratio [40].

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2.4. Computational study:

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   r  f r        N  vr   vr  N

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ρ

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[41].

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ρ

 r  f r     N  

 r  f r     N  

 V r 

 V r 

 N 1 r   N r  ……………………….. (2)

 N r    N 1 r  ………………………….. (3)

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ρ

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ρ

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Δ

≡ Δ

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η

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   f r  f 2 r        N  vr   vr  N ………………………………. (4)

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ACCEPTED MANUSCRIPT As mentioned above, DD allows one to obtain simultaneously the preferable sites for nucleophilic attacks (f(2)(r) > 0) and the preferable sites for electrophilic attacks (f(2)(r) < 0) into

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the system at point ‘r’.

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9-

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[55].

3. Results and discussion

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3.1. Optical microscopic study

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Optical microscopic study was carried out to get an idea about the morphology of the

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aggregated structures. The aggregated hydrosol with increasing concentration of 9-AC for a fixed concentration of SDS (10mM) are shown as sample-a, b & c in fig. 1. Morphology of the

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aggregated particle is rod like (sample- a) at low concentration of 9-AC. On the other hand, elongated hexagonal plate as well as bar shaped mixed morphology are obtained for sample- b

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and uniform elongated hexagonal plate shaped morphology of 9-AC microparticles are obtained for sample-c. Distinct morphologies for sample-a to c are observed using fluorescence microscopy (fig. 2). Upon excitation with blue light Sample-a shows yellowish green luminescence, sample-b shows green luminescence and sample-c exhibits rectangular plate like microcrystals having clear edges with orange luminescence. Dark field’s view of the sample-a, b & c using polarizer and analyzer assembly show different colors depending on the direction of 8

ACCEPTED MANUSCRIPT incident radiation and it reveals the anisotropic nature of the as prepared microcrystal (fig. 3). Our optical microscopy study on aggregated hydrosol of 9-AC in the absence of SDS ( sampled, e & f) shows no regular morphologies and rather larger uneven aggregates are formed. 3.3. Role of SDS

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Concentration of SDS (10mM) used for each sample is higher than the critical micelle

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concentration of SDS. The micellar core acts as good solvent to solubilize 9-AC within the

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microcavity of SDS micelle. Increasing concentration of 9-AC in SDS solution, reaches its super saturation value within the miceller cavity and upon standing without any disturbances; 9-AC is

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precipitated out as microcrystals within the micro cavity. A similar experiment without SDS

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results precipitation of the samples from water and the resulting aggregated structures of 9-AC

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shows no regular shapes.

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3.4. UV-Vis Study

The UV-Vis absorption spectra of 9-AC in EtOH shows two distinct absorption band. The

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two sharp absorption band (234nm and 263 nm) below 300nm are due to of π–π* transition of

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the aromatic ring of 9-AC and the broad absorption band in the region 350nm-475nm which shifted to the red with increasing solvent polarity is due to charge transfer transition between the

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aromatic anthracene moiety to the aldehyde group of 9-AC. Fig.4 also illustrates that the intensity of the CT band increases within the aggregated hydrosol of 9-AC. It seems the aldehyde group of 9-AC within aggregated hydrosol can take favorable orientation (planar) for charge transfer leading to enhanced intensity of CT band. UV-Vis spectra of 9-AC hydrosol in SDS (10mM) also show similar CT transition (fig. 4). 3.5. Emission study 9

ACCEPTED MANUSCRIPT When 9-AC (10-5M) is dissolved in its good solvent EtOH, no photoluminescence spectra were observed upon photoexcitation. The disappearance of fluorescence is caused by the ICT effect in which intramolecular charge transfer occurs from the electron-donating anthracene moiety to the electron-accepting -CHO group. The aggregated hydrosol of 9-AC shows intense

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broad emission (fig. 5) corresponding to the excimer emission of the fluorophores [56]. In case

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of sample - a, b, c it is observed that the PL intensity increases and red shifted from 515 to

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522nm with increasing concentration of 9-AC and reaches its maximum value when the concentration of 9-AC reaches to 2 mM. It is also interesting to note that the emission maximum

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is shifted to the red with the increased size of microcrystals. The extent of staking i.e. overlap of

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π orbitals become more and more until it reaches the thermodynamically stable larger crystalline aggregates and it is responsible for red shift of emission band with increasing size of the

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aggregated structures within the hydrosol.

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In order to understand the environment of aggregates in 9-AC microcrystals, we have carried out time resolved fluorescence study of the 9-AC hydrosols with 370 nm excitation, and

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the emission was monitored deliberately at 420 nm for every samples. Decay profile of the

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hydrosols is shown in fig. 6. Fluorescence lifetime of the samples (table 1) was evaluated by deconvoluting the response function from the decay curves. Fluorescence decay profiles of

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aggregated 9-AC hydrosols are fitted with single exponential decay. The observed fluorescence lifetime of aggregated 9-AC hydrosols i.e. samples- a, b and c are ~0.932 ns, 0.935 ns, 1.29 ns respectively. On the other hand, the observed fluorescence lifetime for sample d, e and f are

1.679 ns, 1.743 ns and 2.45 ns respectively. Morphology study of both set of hydrosol reveal that 9-AC forms crystalline aggregates in presence of SDS and amorphous with no regular structure in the absence of SDS. Though SDS encapsulated 9-AC hydrosol shows intense 10

ACCEPTED MANUSCRIPT emission, the strong interaction among the neighbouring 9-AC induces the excited 9-AC to decay faster compare to 9-AC hydrosol in water where 9-AC molecules are oriented rather disordered manner. Thus the faster emission decay of sample-a, b, c compare to sample-d, e, f has been explained due to strong interaction among the neighbouring 9-AC molecules in sample-

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a, b, c compare to sample-d, e, f.

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3.6. Mechanism of AIE effect

The non-emissive property of excited 9-AC in ethanol is due to the ICT effect in which

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intramolecular charge transfer occurs from the electron-donating anthracene moiety to the electron-accepting -CHO group. In order to understand the distribution of electron density within

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the aromatic rings of 9-AC, we optimized the ground state geometry of 9-AC using density

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functional theory (DFT) with B3LYP hybrid functional and 6-311G (d,p) basis function. Our computed HOMO electron density (Fig. 7) illustrates that electron densities are localized within

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Anthracene group. On the other hand, LUMO electron densities are localized to the entire

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geometry of 9-AC. This delocalization of excited electronic energy to the freely rotating -CHO groups is responsible for opening up the nonradiative deactivation channels of excited 9-AC in

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its isolated form in solution.

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We also observe that the non emissive 9-AC in ethanol becomes highly emissive when aggregation takes place in SDS miceller solution. This observation suggests that 9-AC in hydrosol is an AIE active compound. 3.7. Theoretical study An earlier report by M. Ehrenberg [57] suggests that the single crystal of 9-AC has trans configuration of the –CHO groups with respect to the anthracene skeleton and the neighboring 911

ACCEPTED MANUSCRIPT AC molecules are arranged in head to tell parallel geometry. In order to understand the possible driving force for attaining such a slipped head to tell conformation, we have computed second order Fukui parameter as local reactivity indexes for each atomic centre of 9-AC. It has been discussed in section 2.4 that a positive value of f(2)(r) at a particular atomic centre is a measure of

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its electrophilicity and the negative value indicates its nucleophilicity. Our computed f(2)(r) for

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each atomic center of 9-AC is shown in table-2. Crystal structure of 9-AC shows that the carbon

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atom no 1C, 3C, 7C, 10C, 13C, 14C, 9C, 8C, 4C and 6C of one 9-AC molecule are projected towards the carbon atom no. 6C, 4C, 8C, 9C, 14C, 13C, 10C, 7C, 3C and 1C of its neighboring

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9-AC molecule. Atoms of one 9-AC unit are involved in nonbonded interaction with another 9-

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AC can be understood form the f(2)(r) values listed in table-3. Computed f(2)(r) values as shown in table-3 illustrates that except atom pairs 2C-5C and 11C-15C, all other atom pairs are involved

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strong nonbonded interaction to attain stable structure of 9-AC dimer as suggested from its single

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crystal data. Thus our computed results are in conformity with the crystal structure of 9-AC where two immediate neighbor of 9-AC lie on centres of symmetry and hence the CHO groups

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must be in the trans configuration with respect to the anthracene skeleton (scheme-1).

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Scheme-1: Staked conformation of 9-AC dimer.

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4. Picric Acid Sensor:

The strong emission of 9-AC aggregates prompted us to explore their potential application as

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chemo sensors for the trace detection of nitroaromatics such as 1,4-dinitrobenzoic acid (DNBA),

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2,4-dinitrophenol (DNP), 2,4-dinitrotoluene (DNT), 4-nitrophenol (4-NP), picric acid (PA) etc. Fluorescence titration between hydrosol of sample-f and aqueous PA was carried out at room

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temperature, as shown in Fig. 8. The fluorescence spectrum of 9-AC hydrosol exhibits an intense emission peak at 521 nm. Fluorescence is effectively quenched upon addition of PA. Intriguingly, the remarkable fluorescence intensity of hydrosol was steadily quenched manifold upon gradual addition of PA (0–50 μM). Fig. 8 shows the changes in the fluorescence spectrum upon titration of the hydrosol against picric acid solution. The emission intensity quenched

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ACCEPTED MANUSCRIPT manifold upon gradual addition of PA (0–50 μM) to 9-AC hydrosol and at the same time the PL spectral profile is shifted to the blue region. To establish the better selectivity with fluorescence quenching rate of hydrosol toward PA, we determined the Stern−Volmer quenching constant using the expression I0/I = 1 + KSV[Q],

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where I0 and I are the fluorescence intensities of 9-AC hydrosol in the absence and presence of

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quencher PA and [Q] is the concentration of PA. The plot of I0/I versus [PA] shows an upward curvature instead of linear relationship (inset of fig. 9), indicating that quenching efficiency

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increases with increasing concentration of quencher and may be termed as super amplified quenching effect. At lower concentration of PA, a linear Stern−Volmer plot (I0/I vs [PA]) was

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obtained (inset of Fig. 9) and it furnishes a quenching constant (KSV) of 1.89 × 105 M–1.

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UV–Vis absorption titration was performed to get a quantitative idea of interaction

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between PA and 9-AC hydrosol at room temperature, as depicted in Fig. 10. The successive addition of PA in the range 0–50 μM to the hydrosol of 9-AC exhibits blue shift and increase of

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absorbance at wavelength region 415 nm with the regular increase of picric acid concentration.

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The gradual increase of absorbance suggests the formation of ground state complexation between electron deficient picric acid and hydrosol of 9-AC. In order to check the inner filter effect of

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picric acid, we have rechecked the absorption spectra of hydrosol in presence of picric acid by subtracting the absorbance spectra of picric acid in water. As the absorption band of PA lies within the region of absorption maxima of aggregated hydrosol of 9-AC, the quenching in fluorescence intensities can be accredited to the absorption of the excitation energy by PA rather than by hydrosol. Therefore, the fluorescence titration of aggregated hydrosol was performed (Fig. 11) by the addition of equivalent amount of PA at various excitation wavelengths (300, 14

ACCEPTED MANUSCRIPT 310, 320, 330, 340, 350, 360, 370, 380, 390 and 400 nm). However, at different excitation wavelengths, the emission intensities of aggregated hydrosol were different but the quenching percentages by PA were almost same at all excitation wavelengths. These consequences overlook the possibility of reducing the fluorescence intensities of aggregated hydrosol because

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of masking by PA. To establish the specific selectivity toward PA, we performed the fluorescence quenching

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study of hydrosol in the presence of different aromatic nitro compounds (100 μM) like 2,4-

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dinitrophenol (2,4-DNP), nitrophenol (NP), 2,4-dinitrotoluene (2,4-DNT), 2,4-dinitrobenzoic acid (2,4-DNBA), nitrobenzene (fig.12) and it is observed that the quenching efficiency of PA is

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much higher than other nitro aromatics. The lowest detection limit of PA was determined using

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3σ method and the corresponding value is 8.07µM (fig.13)

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4.1. Sensing Mechanism

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A nonlinear Stern-Volmer plot at high concentration of PA indicates that more than one type of quenching mechanisms take place simultaneously. Fluorescence lifetime measurement of

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the hydrosol in presence of picric acid may through some light on the possible mechanism of interaction between the fluorophore and the quencher. Static quenching does not affect the

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fluorescence lifetime of the fluorophore due to the formation of a non-fluorescent fluorophorequencher complex and dynamic quenching will quench the exited state lifetime of the fluorophore due to the collisional energy transfer from excited fluorophore to the quencher, PA. Therefore, we measured fluorescence lifetime of hydrosol in the absence and presence of PA as depicted in Fig. 14 and found to be invariant for both the cases, which specifies that quenching is static in nature and ground state complex which facilitates the charge transfer from the donor 15

ACCEPTED MANUSCRIPT (electron rich hydrosol) to the acceptor (electron deficient PA) is formed. These results confirmed the possibility of quenching of fluorescence intensity of hydrosol via ground state complexation with PA. 5. Conclusion

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In summary, morphologically interesting structures of 9-AC aggregates were prepared

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via a simple self-assembly method using sodium dodecyl sulfate (SDS) miceller core as soft template. 9-AC has almost no emission in its diluted solution. But the aggregated hydrosol of 9-

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AC shows strong emission and it is termed as aggregation induced emission (AIE). Another excellent utility of this AIE active molecule is its selective sensitivity towards picric acid (PA)

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with quenching constant 1.89 × 105 M–1. It is further explained with the help of both steady state

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and time resolved emission study that the fluorescence quenching of 9-AC hydrosol in presence of picric acid takes place through static quenching mechanism. In order to understand the mode

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of interactions and the orientation of 9-AC molecules in their aggregated structure, we computed

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condensed Fukui function and second order Fukui function f(2)(r) as local reactivity descriptor for each atomic center of 9-AC. Our computational study reveals that the neighboring 9-AC

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molecules are arranged in tarns geometry in the aggregated structures and this is also in

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agreement with the orientation of 9-AC within its single crystal structures as reported earlier in the literature.

Corresponding Author: E-mail: [email protected] Fax: +91 3222 275329 Acknowledgment 16

ACCEPTED MANUSCRIPT S.M and N.M thank CSIR, New Delhi and M.S thanks RGNF, UGC for their individual fellowship. We gratefully acknowledge the help render by USIC, Vidyasagar University for doing fluorescence measurements. We are also thankful to Presidency University, Kolkata for

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their help in doing fluorescence lifetime measurement study.

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[56] H. Xie, X. Jiang, F. Zeng, C. Yu, S. Wu, A novel ratiometric fluorescent probe through aggregation-inducedemission and analyte-induced excimer dissociation, Sensors and Actuators B 203 (2014) 504–510. [57] M. Ehrehberg, The Crystal Structure of the Dimer of 9-Anthraldhyde, Acta Cryst B24 (1968) 1123-1125.of 9-Anthraldehydehe Crystal Structure of NE EHRENBERGMAMARIANNE EHRENBERGRIANNE EHRENBERG

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Legend to the figures: Fig. 1. Optical microscopic images of 9-AC microcrystals, (a) sample-a, (b) sample-b and (c) sample-c Fig. 2. Fluorescence microscopic images of 9-AC microcrystals upon blue light (a) sample-a, (b) sample-b, (c) sample-c, 23

ACCEPTED MANUSCRIPT Fig. 3. Polarizing microscopic images of 9-AC microcrystals, (a) sample-a, (b) sample-b, (c) sample-c, Fig. 4. (a) UV–visible absorption spectra of (i) 9-AC monomer in EtOH and aqueous suspensions of its microcrystal; (ii) sample-a, (iii) sample-b, and (iv) sample-c; (b) UV–visible absorption spectra of (i) 9-AC monomer in EtOH, (ii) sample- d, (iii) sample-e, and (iv) sample-

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Fig. 5. (a) Fluorescence emission spectra of aqueous suspensions of its microcrystals; (i) sample-

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a, (ii) sample-b, (iii) sample-c; (b) Fluorescence emission spectra of (i) sample-d, (ii) sample-e, (iii) sample-f.

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Fig. 6. (a) Fluorescence decay profiles of (i) lamp (ii) sample-a, (iii) sample-b (iv) sample-c; (b) Fluorescence decay profiles of (i) lamp (ii) sample-d, (iii) sample-e (iv) sample-f. All samples

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were excited at 370 nm.

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Fig. 7. HOMO and LUMO electron densities of 9-AC calculated using DFT -B3LYP/6311G(d,p) level of theory.

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Fig. 8. Change in fluorescence spectra of aggregated hydrosol of 9-AC in presence of varying

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concentration of PA.

Fig. 9. Stern-Volmer plot of corresponding fluorescence quenching of 9-AC hydrosol in

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presence of different amount of PA. (The inset shows the linear part of the plot of I0/I versus

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[PA] for 9-AC hydrosol.)

Fig. 10. UV–Vis absorption spectra of (i) 50mM Picric acid in water, (ii) 9-AC hydrosol (sample-f) (iii) 9-AC hydrosol in presence of 50mM Picric acid in water. Fig. 11. The percentages quenching of fluorescence emission of aggregated hydrosol of 9-AC upon addition of PA (20 μM) at different excitation wavelengths (300-400 nm). Fig. 12. Fluorescence quenching efficiency diagram of 9-AC hydrosol in presence PA (100 µM) and with same concentration of other aromatic nitro compounds separately. 24

ACCEPTED MANUSCRIPT Fig. 13. a) Fluorescence intensity versus 9-AC (µM) concentration plot for measuring standard deviation; b) IO/I versus PA/[M] plot for measuring slope. Fig. 14. Fluorescence decay profile of (i) lamp (ii) aggregated hydrosol and (iii) hydrosol with

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Table 1: Fluorescence life time of aggregated 9-AC

τav (ns)

χ2

0.932

1.260

Sample -b

0.935

0.915

Sample -c

1.29

Sample -d

1.679

Sample -e

1.743

Sample -f

2.45

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0.879

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0.862 1.011

ACCEPTED MANUSCRIPT Table 2: Electrophilic f+ and nucleophilic f− condensed Fukui functions and second order Fukui

f+ × 10−3

f− × 10−3

f (2)(r) × 10−3

1C

-64

-49

-15

2C

-32

-72

40

3C

-12

22

4C

14

7

5C

-54

-68

14

6C

-33

-61

22

7C

-81

-138

57

8C

-143

-106

-37

9C

15

14

1

10C

-19

25

-44

11C

-31

-83

52

12H

-12

-15

3

13C

-67

-54

-13

14C

-36

-53

17

15C

-55

-80

25

-28

-29

1

-18

-19

1

-24

-23

-1

-29

-28

-1

-29

-23

-6

-29

-28

-1

-30

-29

-1

23H

-24

-23

-1

24C

-53

11

-64

25O

-73

-77

-16

26H

-27

-22

-5

20H 21H 22H

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19H

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17H 18H

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16H

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parameter ( f (2)(r)) of all the atomic centers of 9-Anthraldehyde calculated using DFT B3LYP 6-31G (d,p) level of theory.

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ACCEPTED MANUSCRIPT Table – 3: Computed value of f(2)(r) of possible interacting atomic centers in 9-AC dimer. 9-AC(upper)

9-AC(lower)

f(2)(r)  103

Atom No.

f(2)(r)  103

1

-15

6

22

3

-34

4

7

4

7

3

6

22

1

7

57

8

-37

9

1

10

-44

13

-13

14

17

-15

8

-37

7

57

10

-44

9

1

14

17

13

-13

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Atom No.

Graphical Abstract

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Highlights

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 Morphologically interesting 9-Anthraldehyde microcrystals are synthesized.

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 1-dimension rod, 2-dimension elongated hexagon shaped microstructures are obtained.

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 Local reactivity descriptors are computed to explain the possible staking sites.

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 Aggregated 9- Anthraldehyde hydrosol is utilized for selective sensing of picric acid.

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