Electron rich triphenylamine derivatives (D-π-D) for selective sensing of picric acid in aqueous media

Accepted Manuscript Title: Electron Rich Triphenylamine Derivatives (D-␲-D) for Selective Sensing of Picric acid in Aqueous Media Author: Kumaraguru Duraimurugan Rajendiran Balasaravanan Ayyanar Siva PII: DOI: Reference:

S0925-4005(16)30329-X http://dx.doi.org/doi:10.1016/j.snb.2016.03.035 SNB 19840

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

3-12-2015 7-3-2016 9-3-2016

Please cite this article as: Kumaraguru Duraimurugan, Rajendiran Balasaravanan, Ayyanar Siva, Electron Rich Triphenylamine Derivatives (D-rmpi-D) for Selective Sensing of Picric acid in Aqueous Media, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2016.03.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Highlights A series of dimethyl (linear), tetramethyl (V-shape) and hexamethyl (star like) substituted triphenyl amine derivatives were synthesized and studied their photo physical properties and aggregation induced emission. All the TPA derivatives were sensing for efficient and fast detection of picric acid. From the photophysical studies, all the TPA derivatives were strongly concentration dependent aggregation by π- π interaction and also in the solution state fluorescence study revealed that all the compounds have very binding affinity for picrate anion.

Graphical abstract:

Electron Rich Triphenylamine Derivatives (D--D) for Selective Sensing of Picric acid in Aqueous Media

Kumaraguru Duraimurugan,

Rajendiran Balasaravanan and

Ayyanar Siva* School of Chemistry, Madurai Kamaraj University, Madurai-625 021, Tamilnadu, India.

Electron Rich Triphenylamine Derivatives (D-π-D) for Selective Sensing of Picric acid in Aqueous Media Kumaraguru Duraimurugan, Rajendiran Balasaravanan and Ayyanar Siva* School of Chemistry, Madurai Kamaraj University, Madurai-21, Tamilnadu, India. *Corresponding author: [email protected] KEY WORDS: Triphenylamine, Wittig-Horner reaction, Aggregation Induced Emission, Fluorescent aggregates, Nitro aromatics, Chemo sensors.

Abstract A series of dimethyl substituted triphenylamine derivatives (D-π-D) having linear, V-shaped and star-like molecules have been synthesised by Wittig-Horner reaction. All the molecules exhibit blue light emission and shows fluorescent nano-aggregates in aqueous media due to its aggregation induced emission enhancement property. The three chemosensors undergo highly selective fluorescence quenching with picric acid and also all the fluorescent aggregates serve as potent chemosensors for electron deficient nitro aromatics.

4

1. Introduction Since the finding of the organic molecules with aggregation-induced emission (AIE) they gained considerable attention to their potential applications for optoelectronic devices [1-2], photo memory, fluorescence sensors [3-6], solid-state lighting, and organic lasers. Many research groups are involved in the design and synthesis of new AIE active molecules [7-8] by investigating their aggregate morphologies and manipulating their luminescence. However, a main reason for the AIE effect is the restriction of intramolecular rotation. Unhindered intramolecular rotation of AIE molecules in free-state leads to efficient nonradiative decay and hence quenching of fluorescence emission [9-10]. In the aggregation state, intramolecular rotation is restricted significantly and emission is greatly enhanced. To overcome the aggregation-caused quenching (ACQ) [11-12] effects, bulky groups, branched chain and dendritic species were covalently attached to the fluorophores. A variety of compounds including silole, tetraphenylethene, triphenylethene, distyrylanthracene and triarylamine derivatives have been synthesized and studied for their ACQ [13-14]. In addition to AIE effect, these molecules show detection of nitroaromatics which is another advantage to the flurophore aggregates. Among various nitro derivatives [15-16], the detection of picric acid (PA) is the most important as it is widely used for the manufacture of rocket fuels, fireworks, deadly explosives, and in analytical chemistry of metals and minerals [17-18]. Various analytical methods have been developed for the sensitive detection of nitroaromatic compounds that are based on, chromatography [19], amperometry [20], surface enhanced Raman spectroscopy [21] and energy dispersive X-ray analysis [22]. In recent years, photoluminescence based chemosensors that exploits sensitive fluorescence quenching by nitroaromatic derivatives 5

have also been investigated [23-24]. In comparison with other methods, fluorescence based detection has advantages of high sensitivity and detection for both vapour and solution phases of nitroaromatic explosives at low concentrations [25]. Fluorescence detection is highly tunable, as the reliance on excited-state charge-transfer for detection permits wide latitude for the chemical composition of the indicator and analyte. The detection of trace amount of PA is also very important in combating terrorism, maintaining national security and environmental safety [26]. PA is a strong organic acid, and its vapors are hazardous and cause severe health concerns like headache, weakness, anemia and liver injury [27]. Further, with the electrondeficient nature, the degradation of PA is more difficult in the biological system, which is responsible for many chronic diseases such as sycosis and cancer [28]. Nowadays, major research works are focused on the detection of TNT and less attention has been paid to PA, although its explosive power is superior to that of TNT. So various types of materials have been developed and employed to improve the sensing performance of Nitroexplosives [2940]. Some research groups reported triphenylamine based derivatives (D-A) and its fluorescent aggregates sensing towards electron deficient nitroaromatics in ppm to ppb level [41-42]. In the best of our knowledge, there is no report on sensing behaviour of AIE based on D-π-D molecules towards nitroaromatics. Thus, developing triphenylamine based fluorophores

6

Scheme 1. Synthetic route of DMTPA, TMTPA and HMTPA molecule. having high emission by simple method is a fascinating work for detection of picric acid. Here, we have synthesized triphenylamine flurophores (DMTPA, TMTPA and HMTPA) having donor-π-donor system with the different shapes like linear, V-shaped and a star like molecular structures (Scheme 1). Further, the photophysical properties, aggregation induced emission and sensitivity towards nitroaromatics also studied and reported.

2. Experimental Section 2.1 Materials and Methods All the chemicals were directly used for the synthesis without further purification unless otherwise stated. Triphenylamine, mesitylene and potassium tert butoxide were purchased from Alfa Aesar. N-bromo succinimide and triethylphosphite were purchased from 7

Aldrich. Phosphorous oxychloride was purchased from CDH chemicals. Solvents for chromatography and crystallization were distilled once before use. Technical grade solvents were used for extraction. Dry THF was distilled from sodium/benzophenone and dry DCM was distilled over calcium hydride and stored on KOH pellets. 2.2 Characterization Bruker 300 MHz and Jeol 400 MHz instruments were used to record 1H NMR & 13C NMR spectra. Chemical shift (δ) is reported in parts per million (ppm) relative to residual solvent peaks or tetramethylsilane, and coupling constants are reported in Hertz (Hz). NMR solvents were obtained from Aldrich. The spectra were recorded at room temperature. The multiplicity was denoted s= singlet, d=doublet, t=triplet, q=quartet, m=multiplet. Silica gel-G plates (Merck) were used for TLC analysis with a mixture of n-hexane and ethyl acetate as an eluent. Column chromatography was carried out in silica gel (60-120 mesh) using n-hexane and ethyl acetate as an eluent. The quantum yield of the compounds was calculated by using quinine sulphate as reference. UV absorption was measured in JASCO V-630 and emission was measured in Agilent 8000 spectrofluorometer while FT-IR was recorded in a JASCO FT/IR-410 spectrometer using KBr pellet method. A stock solution of DMTPA, TMTPA and HMTPA in THF with the predetermined concentration of 1x10-3 M was prepared. Aliquots of the samples were transferred into 5mL volumetric flask. After adding the appropriate amount of THF, water was added drop-wise under vigorous stirring to furnish 1x10-5 M solutions with different water fractions. The absorption and emission spectra of these resultant mixtures were measured immediately. Lifetimes were measured in Time Correlated Single Photon Counting (TCSPC). Mass spectra were analysed by Voyager DE PRO Biospectrometry Workstation (Applied Biosystems) matrix assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectroscopy (MS) instrument. Particle size was measured by Malvern instrument (Zetasizer). 8

2.3 Preparation of TPA1 (2) and TPA2 (3) Phosphorus oxy chloride (25 mL) was added drop wise at 0 oC under nitrogen atmosphere to DMF (20mL) and the resulting mixtures was stirred for 1 h. Triphenylamine (5g) in dry THF was added to the above resulting mixture and it was stirred at 110oC for 10h. After cooling to room-temperature, the mixture was poured slowly into ice water, whereby a brown solid in coagulated form obtained. The solid was dissolved in several times of DCM and separated out and washed with brine solution and dried over Na2SO4. The crude product was purified by column chromatography (CH2Cl2/Hexane =1: 3) to get TPA1 (2.5 g, 45%) as a yellow solid. mp: 142-143 oC. 1H NMR (300MHz, CDCl3): δ 9.81 (s, 1H), 7.69-7.67 (d, J = 6Hz, 2H), 7.34-7.32 (d, J = 6Hz, 4H), 7.19-7.17 (t J = 6Hz, 5H), 7.03-7.00(d, J = 9H, 2H). 13

C NMR (75MHz) : 190.4, 153.3, 146.1, 131.3, 129.7, 129.1, 126.3, 125.1, 119.3. TPA2

(2g, 40%) as a bright yellow solid. 1H NMR (300MHz, CDCl3): δ 9.89 (s, 2H), 7.79-7.77 (d, J = 6Hz, 4H), 7.42-7.38 (t, J = 12Hz, 2H), 7.29-7.25(t, J = 12Hz, 1H), 7.20-7.18(d, J = 6Hz, 6H).

13

C NMR (75MHz, CDCl3):190.57, 152.02145.57, 131.34, 130.19, 127.09, 126.54,

122.78.

2.4 Preparation of TPA3 (4) The dialdehyde TPA2 (1 g) was added to an ice-cooled mixture of POCl3 (10 mL) in DMF (10 mL). The resulting mixture was reacting to 110 oC for 4 hrs. After completion of the reaction, the mixture was cooled to room-temperature and poured into ice water, whereby a brown solid precipitated out. The solid was collected by filtration and washed with water. The crude product was purified by column chromatography (CH2Cl2/Hexane = 1:1) to get TPA3 (0.5 g, 38%) as a brown solid; mp: 233-235oC. 1H NMR (400 MHz, CDCl3) δ : 9.95 (s, 3H), 7.86-7.84 (d, J =12Hz, 6H), 7.27-7.25(d, J = 6Hz, 6H).13C NMR (100MHz, CDCl3): δ 190.54, 151.21, 132.89, 131.53 and 124.56.

9

2.5 Preparation of 1-Bromomethyl-3, 5-dimethyl-benzene (6) Mesitylene 5 (5g, 41.6mmol) was taken in dry benzene (50mL) and a catalytic amount of benzyl peroxide was added. Heated the mass up to 50oC and NBS (3.25g, 41.6mmol) was added by delayed addition and further heated to reflux. Then the reaction mass was maintained for 8 hrs at 80 oC. Cooled the mass to RT and filtered through a celite pad and washed with DCM. The organic layer was washed in 1.5N HCl solution (5 x 5 ml) and brine solution (5 x 5 ml). Concentrated and the crude product was purified by column chromatography. Yield is 60% as a colorless liquid. 1H NMR (300 MHz, CDCl3) δ: 7.03(s, 2H), 6.95(s, 1H), 4.46(s, 2H), 2.32(s, 6H).

13

C NMR (75MHz, CDCl3):138.3, 129.8, 126.9,

125.7, 32.4, 20.2 2.6 Preparation of diethyl 3, 5-dimethyl-benzophosphonate (7) Compound 6 (1g, 5.05mmol) was taken in triethylphosphite (10mL) and heated to 140 o

C and maintained for about 6 hrs. Cooled to RT and the crude product was concentrated and

it was absorbed with silica gel (60-120) and eluted with ethyl acetate as an eluent. Isolated product as colorless liquid (1.25g, 98%). 1H NMR (300 MHz, CDCl3) δ: 7.09(s, 2H), 6.88(s, 1H), 4.14-4.10(m, 4H), 3.10-3.03(d, J = 15Hz, 2H), 2.27(s, 6H), 1.34-1.32(t, J = 6Hz, 6H); 13

C NMR(75MHz, CDCl3): 137.8, 137.7, 131.0, 130.9, 128.4, 128.3, 127.9, 127.4, 63.5, 63.4,

62.0, 61.9, 34.2, 32.4, 16.2.

2.7 Preparation of DMTPA (8) TPA1 (100mg) and one equivalent of phosphonate salt was taken in dry THF under nitrogen atmosphere, the KtOBu was added all at once. Then the reaction mixture was stirred for 12 hrs at RT. Quenched the mass with 1.5N HCl and stirred for 30 min. Extracted the mass with ethyl acetate and washed with brine solution and concentrated. The crude product was purified by column chromatography, using 9.5:5 ethyl acetate: pet ether. Yield is 65% 10

(90mg); pale greenish yellow solid. mp 143 0C; FT-IR (KBr): 1583, 1490, 275, 949, 833, 726; 1H NMR (400 MHz, CDCl3) δ :7.38-7.35(d, J = 12Hz, 2H), 7.27-7.23(m, 4H), 7.117.09(d, J = 8Hz, 6H), 7.05-7.01(t, J = 16Hz, 5H), 6.95(s, 1H), 6.91-6.88(d, J = 12Hz, 1H), 2.32(s, 6H); 13C NMR: 147.6, 147.2, 138.0, 137.5, 131.8, 129.2, 129.0, 127.8, 127.3, 124.4, 123.7, 122.9, 21.3; HRMS calculated for C28H25N (m/z):375.1987: found: 376. 2065. 2.8 Preparation of TMTPA (9) (TPA2) (100mg) and two equivalent of phosphonate salt were taken in dry THF under nitrogen atmosphere and four equivalent of KtOBu was added all at once. Then the reaction mixture was stirred for 12 hrs at RT. The mass was quenched with 1.5N HCl solution and stirred for 30 min. Extracted the mass with ethyl acetate and washed with brine solution and concentrated. The crude product was purified by column chromatography using 5% ethyl acetate: pet ether. Yield is 60% (95mg). Yellow solid; mp: 171-1730C. FT-IR (KBr): 1590, 1502, 1274, 957, 836, 726; 1H NMR (400 MHz, CDCl3) δ: 7.40-7.38(d, J = 8Hz, 4H), 7.29-7.25(t, J = 12Hz, 3H), 7.14-7.12(d, J = 8Hz, 6H), 7.08-7.02(m,6H), 6.976.93(d, J = 16Hz, 4H), 6.89(s, 3H), 2.33(s, 12);13C NMR (100MHz): 147.3, 146.9, 138.1, 137.5, 132.0, 129.3, 129.1, 127.7, 127.3, 124.7, 124.2, 124.0, 123.2, 21.2; MALDI (TOF) calculated for C38H35N (m/z):505.2770: found: 506.3659. 2.9 Preparation of HMTPA (10) TPA3 (100 mg) with three equivalent of phosphonate salt was taken in dry THF under nitrogen atmosphere and six equivalent of KtOBu was then added all at once. The reaction mixture was stirred for 12 hrs at RT. Quenched the mass with 1.5N HCl solution and stirred for about 30 min. The mass was extracted with ethyl acetate and washed with brine solution and concentrated it. The crude product was purified by column chromatography; the product was eluted at 5% ethyl acetate: pet ether. Yield is 57 % (110mg). Isolated as a 11

yellow solid; mp: 183 0C; FT-IR (KBr): 1587, 1275, 959, 837, 755. 1H NMR (400 MHz, CDCl3) δ: 7.42-7.40(d, J = 8Hz, 6H), 7.23(s, 2H), 7.13-7.07(m, 12H), 6.98-6.94(d, J = 16Hz, 4H), 6.90(s, 3H), 2.34(s, 8H); 13C NMR (75MHz, CDCl3): 147.03, 138.5, 137.7, 132.7, 129.5, 128.0, 127.9, 127.7, 127.6, 124.6, 21.5; MALDI (TOF) calculated for C48H45N (m/z):635.3552: found: 635.4947. 3. Results and discussion 3.1 Synthesis and characterization of compounds The general synthetic route for the synthesis of the compounds DMTPA, TMTPA and HMTPA are depicted in Scheme 1. All the compounds were synthesized, in moderate yields (70–80%),

by

Wittig-Horner

reactions.

The

precursors,

i.e.

4-(N,N’-

diphenylamino)benzaldehyde (TPA1), 4,4’-diformyltriphenylamine (TPA2) and 4,4’,4’’triformyl-triphenylamine (TPA3) were prepared according to previously reported procedures [43]. Mesitylene was treated with NBS in the presence of benzoyl peroxide as an initiator. It gives 3, 5-dimethyl benzylbromide in very good yield. Then the benzyl bromide was treated with triethylphosphite at 130 oC. It gives phosphonate salt as liquid (Scheme S1, ESI). All the new compounds were identified and characterized by 1H NMR, 13C NMR spectroscopy, high resolution mass spectrometry and FT-IR spectroscopy analysis. 3.2 Optical properties Table 1 Absorption and emission values of DMTPA, TMTPA and HMTPA in THF λemi(nm)

Stoke’s shift(cm-1)

Compounds

λabs(nm)

φfl

DMTPA

300, 362

425

4095

0.4

TMTPA

304, 383

428

2745

0.3

HMTPA

310, 387

431

2638

0.2

12

Fig. 1. Absorption (a) and emission (b) spectra of DMTPA, TMTPA and HMTPA in THF solution (1 × 10−5 M).

The absorption spectra of DMTPA, TMTPA and HMTPA in THF solution exhibited two major bands. The band around 300-330 nm can be attributed to the π-π* transitions and the other one at 360-395 nm can be assigned to intramolecular charge transfer (ICT) (Fig. 1, Table. 1). The detailed absorption and emission spectra of DMTPA, TMTPA and HMTPA in different organic solvents are shown in Fig. S2-S4 . The absorption of DMTPA, TMTPA and HMTPA is bathochromically shifted with the increasing styryl unit in TPA moiety, due to the electron donating ability of styryl unit. The absorption observed for DMTPA at 362 nm in THF is 20nm red shifted for TMTPA (with a shoulder peak characteristic of V-shaped molecules at 355nm) [44-47] and further shifted to 388 nm for HMTPA. A similar trend is also observed in all the cases in different solvents, viz., acetonitrile, DCM DMF and DMSO (Table S1). It indicates that the introduction of styryl groups influenced their photophysical properties. The results revealed that changing the π-conjugated length and/or introducing the methyl group on the electron donating group can influence the electron-donating ability of the triphenylamine moiety. Fig. 2 shows the fluorescence decays of DMTPA, TMTPA and HMTPA in DMF monitored at 457, 473 and 485 nm respectively. DMTPA and TMTPA exhibit a single exponential decay in DMF with a lifetime of 1.89 and 2.02 ns respectively [45]. But, HMTPA exhibit a double exponential decay with a life time of 2.15 ns (81.5%) and 13

1.27 ns (18.5%). This confirms DMTPA and TMTPA derivatives exhibit single conformation in the excited state, but HMTPA exhibit double conformation in the excited state [48].

Fig. 2. Fluorescence decay profile of DMTPA, TMTPA and HMTPA in DMF solution. The emission spectra of DMTPA, TMTPA and HMTPA are very sensitive to the solvent polarity. For the emission of DMTPA, TMTPA and HMTPA, a red shift is observed with the increase in solvent polarity, exhibiting an obvious bathochromic effect. The emission band is found to shift towards longer wavelengths with increasing solvent polarity; this is due to the stabilization of the charge-transfer transition in polar solvents. Further, the higher value of stokes shift on moving from non-polar to polar solvents was observed, indicative of the fact that the intramolecular charge transfer transition is more when compared to the ground state. Further, when increasing the solvent polarity, the emission wavelenth was increasing gradually, due to the charge separation upon excitation, which is confirmed by DimrothReichardt solvent polarity correlations (Fig. 3) [49].

14

6000

Stoke's shift (cm-1)

5500 5000 4500

DMTPA TMTPA HMTPA

4000 3500 3000 2500 34

36

38

40

42

44

46

ET 30

Fig. 3. Dimroth-Reichardt plot for solvent polarity correlations.

3.3 Aggregation Induced emission

Fig. 4. Absorption (a) and emission spectra (b) of DMTPA in various THF-Water mixtures. As all the compounds are insoluble in water, but soluble in organic solvents, we determined the absorption and emission behavior of DMTPA, TMTPA and HMTPA in THF– water mixtures with different water fractions that can enable the fine-tuning of the solvent polarity and the extent of solute aggregation. The UV-vis spectra of DMTPA (1X10-5 M) shown in Fig 4 (a) show an absorption peak at 362 nm in dilute THF solution. As the fraction of water increases, the absorbance intensity also increases gradually up to 50% THF-water mixture with slight red shift. When 60% of water reached, the absorption intensity starts to 15

decrease gradually. On the same time, the spectra of the mixtures with a high fraction of water start to show level-off tails in the longer wavelength region due to the scattering effect of the nanoparticles.

The emission spectra of DMTPA in the THF-water mixtures of different water contents are shown in Fig. 4(b), wherein the emission intensity is weak up to the water fraction of 0% to 20%. Further,

when the water fraction was increased to 30%,

approximately two fold enhancement of emissions is observed. Furthermore, an increase in the percentage of water from 30 to 80% red shifts the emission by 30 nm. This is due to THFwater mixture of a “low” water fraction, the solute molecules steadily assemble in an ordered state to form blue emissive particles, and in the mixture with the “high” water content the solute molecules quickly agglomerate in a random way to form redder amorphous particle [50-51]. These results indicate that chromophore of DMTPA exhibited significant AIEE effect. Similar spectral changes are also observed in TMTPA and HMTPA (Fig. S5, S6) and the spectra of PL intensity vs water fraction are shown in Fig. S7. Further, the particle size of the aggregates of DMTPA, TMTPA and HMTPA were measured by dynamic light scattering (DLS) and the values are 67, 111 and 179 nm respectively (Fig. S8-S10).

3.4 Detection of Picric Acid

16

Fig. 5. Absorption (a) and emission (b) spectra of DMTPA towards addition of PA in THF. (c)Absorption spectra of DMTPA in THF:H2O towards addition of PA. In order to study the picric acid sensor of our newly synthesized TPA derivatives, first we carried out the UV-vis titration study of the DMTPA compound in THF solution (10−5 M) and also THF:H2O (5:5) solution were titrated with addition of PA (10−4 M), and the corresponding responses are shown in Fig. 5 (a, c). When compound DMTPA was titrated with the PA a new intense band observed at 425 nm. At that same time, the colour of the solution gradually changed from blue to pale yellow upon the addition of PA, this is due to the formation of charge-transfer complex of DMTPA with picrate anion [52-53]. A similar trend was also observed in the compounds of TMTPA and HMTPA (Fig. S11, S12). The fluorescence titration was carried out for the compound of DMTPA in water-THF (5:5) mixture. DMTPA showed a fluorescence emission at 430 nm when excited at 362 nm. The addition of PA in to the DMTPA solution, the fluorescence intensity gradually decreased. Further increasing the addition of picric acid (0 to 30 equiv.), fluorescence quenching was observed and the colur change was observed by naked eye and under the illumination of UV light of 365 nm (Fig. 5 b). The quenching efficiency of the compound DMTPA was calculated by using the formula (I0-I)/I0 X 100 where I0 and I are the fluorescence intensities 17

of DMTPA before and after the exposure to the analyte. The quenching efficiency of the compound DMTPA is 97% (Fig. 6a). The observed quenching of the fluorescence is attributed to the energy transfer from photo-excited π-electron rich derivatives of DMTPA to ground state electron deficient containing picric acid. The quenching efficiency can be estimated by Stern-Volmer equation I0/I = 1 + KSV[Q], where I0 is the fluorescence intensity before the addition

of

PA, I is the

fluorescence intensity after the addition of PA and [Q] is the concentration of PA. The SternVolmer plot is found to be a linear at lower concentrations of the PA (Fig. 7b), which indicates that the fluorescence quenching involved a static quenching mechanism. At higher concentrations of PA, the plot is found to be a hyperbolic curve (Fig. 7a), this may be due to the combination of both static and dynamic (collision) quenching [54]. The calculated SternVolmer constant for the DMTPA is 6.37× 105 M-1 which is found to be a higher, when compared to previously reported chemosensors for picric acid [41, 42, 52, 55-57]. Further, the fluorescence titration experiment was carried out by using several analytes in THF for the fluorescence response to evaluate the specific sensing capability of compound DMTPA and the results are shown in Fig. 6 (a). The fluorescence quenching experiment has been performed by the addition of 30 µL (1 × 10-4 M) of different nitroaromatics such as 2,4-dinitrophenol, 4-nitrophenol, 1, 4-dinitrobenzene, dinitrobenzoic acid, nitrotoluene and nitrobenzene taken in THF. All nitroaromatics act as fluorescence quenchers for compound DMTPA (Fig. 6b). From the observed results, DMTPA molecule exclusively brings out the specific fluorescence quenching ability is higher for picric acid than the other nitroaromatics. A similar observation was also found in the TPA derivatives of TMTPA and HMTPA (Fig. S13-S20, Table S2). The aggregates of DMTPA, TMTPA and HMTPA possess detection limits of 30, 30 ppb and 40 ppb, respectively, as fluorescent sensors for PA, which is compared with earlier reported results. [Fig. S21, Table S3]. 18

The stoichiometry of DMTPA-PA complex in THF-water solution was determined using the Benesi-Hildebrand plot of the fluorescence intensity as a function of PA concentration, which is shown in Fig. 8a. The job’s plot clearly indicates the formation of a 1:1 complex between DMTPA and PA (Fig. 8b). The binding affinities were determined by fluorescence quenching experiments and it fits to the Benesi-Hildebrand equation in terms of Ka (association constant) for complexation of DMTPA with PA is 1.67 x 109 M-1 and the values are listed in Table. S2. (Fig. S22). From the observed results, we found the higher the association constant and detection limits for HMTPA when compared to DMTPA and TMTPA. 100

DMTPA

450 400

80

a

Probe NT NB NBA NP DNB DNP PA

b

350

Relative intensity

Quenching efficiency (%)

500

PA

DMTPA

60

DNP 40

300 250 200 150

20

100

NP NB NT

NBA

50

DNB

0

0 1

2

3

4

Analytes

5

6

7

400

450

500

550

Wavelength (nm)

Fig. 6. (a) Selectivity graph of DMTPA towards various nitroaromatic and their corresponding emission spectra (b).

1.30

9 DMTPA

8

a

7

y = a + b*x

Weight

No Weighting

b

7.18333E-4

Pearson's r

0.99449

Adj. R-Square

0.98745 Value

1.20

6

Standard Error

B

Intercept

0.97917

0.00736

B

Slope

0.03283

0.00131

DMTPA 1.15

5

I0/I

I0/I

Equation Residual Sum of Squares

1.25

4

1.10

3 1.05

2 1

1.00

0 0

5

10

15

20

25

30

0

2

Concn(PA)

4

6

8

10

Concn(PA)

Fig. 7. (a) Stern-Volmer plot for quenching of DMTPA upon addition of PA at higher concentration (b) Quenching of DMTPA at lower concentrations. 19

3.0x10

-5

2.5x10

-5

DMTPA

1.5

-5

I0-I

1/[I-I0]

a 2.0x10

b

2.0

DMTPA Linear fit

1.5x10

-5

1.0x10

-5

1.0

0.5

0.0

0.00

0.05

0.10

0.15

0.20

0.25

0.0

0.30

0.2

0.4

0.6

0.8

1.0

Molefraction of PA

1/[PA]

Fig. 8. Benesi-Hildebrand plot of DMTPA with PA (a). Job’s plot shows 1:1 complex formation between DMTPA and PA (b). 0.6

DMTPA

DMTPA

a

b

400

TFA

Absorbance

Relative intensity

0.4

0.2

TFA

300

200

100

0.0

0 300

350

Wavelength (nm)

400

450

400

450

500

550

Wavelength (nm)

Fig. 9. Change in the absorption (a) and fluorescence intensity (b) of DMTPA upon addition of TFA in THF-water mixture. To confirm the ground state complex formation, we carried out the absorbance and fluorescence studies of aggregates of DMTPA with trifluoroacetic acid (TFA), which is strong non-aromatic acid, under the same set of conditions as we used for PA. Which shows a significant change on the absorbance and fluorescence intensity of the DMTPA compared to PA (Fig. 9) [58-59]. Similar spectral changes are also observed in TMTPA and HMTPA (Fig. S23-24). From the observed results our newly synthesized TPA derivatives acts as a very good sensor for aliphatic as well as aromatic acids such as trifluoroacetic acid and picric acid. 20

3.5 NMR Titration Experiment

Fig. 10. 1H NMR spectra of DMTPA and PA with different molar ratio in CDCl3. In addition to the UV and fluorescence titration, we carried out, NMR spectroscopic titrations in CDCl3 for confirming the interaction between the picric acid and DMTPA. During the 1H NMR titration experiments, DMTPA was taken in CDCl3 and PA in CDCl3 from 0 to 40 equivalents and the observed results are given in (Fig. 10). From the NMR spectra revealed that fast deprotonation occurred in PA and that can be interacted with DMTPA. Upon increasing the concentration of PA (0 to 40 equiv.), significantly one new proton signal is formed at 7.30-7.36 ppm. This indicates that the proton involved in hydrogen-bonding between PA and DMTPA and hence the peak intensities are increased, at the same time proton signals are observed extensive very sharp [42]. This is due to the hydrogen bonding is a crucial driving force for the quenching process [52, 63]. A similar trend was also observed in TMTPA as shown in (Fig. S25). 21

3.6 DFT Studies

Fig. 11. Calculated energy level diagrams of PA and DMTPA (a) Energy level diagram of DMTPA after protonation (b). To evaluate the effect of the PA over the other nitroaromatics, the structures of the PA and DMTPA, TMTPA and HMTPA were optimized and the energies of their HOMOs and LUMOs were determined by Gaussian 09 programm and the results are given in Fig. 11. The 22

quantum chemical calculations [61] at the B3LYP/6-31G* basis set reveals that the HOMO level, the electron cloud is largely delocalized on the entire molecule of DMTPA and HMTPA, In the case of TMTPA, electron clouds on stilbene moiety and adjacent phenyl rings. The LUMO energy of DMTPA (- 1.08 eV) lies at much higher energy than the PA (4.13 eV), which favors the electron transfer from the excited state of DMTPA to PA in the fluorescence quenching process [62-63]. However, upon the addition of PA, electron density lies only on stilbene moiety, this is due to high electron mobilation through conjugation in HOMO of DMTPA (-7.92 eV). But in the LUMO, electron density occupy throughout the molecule, this is due the protonation of the triarylamine moiety of DMTPA and hence, PA and DMTPA having almost an equal energy level of LUMO. Further, we calculated the energy gap between the HOMO and LUMO of all the protonated TPA derivatives, the order of the band gap as follows; DMTPA>TMTPA>HMTPA (Table S26). Among the TPA derivatives, HMTPA (star like) has lower the band gap than the others, such as DMTPA (linear) and TMTPA (V-shaped), this is due to the electron clouds (HMTPA) are distributed whole the molecule in the excited state of LUMO level. But the V-shaped molecule of TMTPA electron clouds is delocalised only in the methyl substituted stilbene moiety (Fig. S26). 3.7 Contact Mode

Fig. 12. Photographs of test-strip of DMTPA, TMTPA and HMTPA under naked eye (a) and 365 nm UV-lamp (before and after spotting the solution of PA) (b).

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Detecting the trace amount of picric acid by visually is very essential for security scanning and prompt identification. For this purpose, the test-strip was prepared by dipcoating onto TLC in THF solution of DMTPA, TMTPA and HMTPA followed by drying under a vacuum. Then the strip was spotted one drop of PA solutions and as expected fluorescence quenching was observed by naked eye as well as illuminating under 365 nm UV lamp (Fig. 12). This observation is demonstrated the instantaneous visualization of trace amount of PA. 4. Conclusion In conclusion, a new series of dimethyl (linear), tetramethyl (V-shape) and hexa methyl (star like) substituted triphenylamine derivatives have been synthesized by WittigHorner reaction with very good yield. All the three TPA derivatives were acting as sensor and used for efficient and fast detection of picric acid. Among the three TPA derivatives, DMTPA was a superior sensor for PA detection. From the photophysical studies, it was proved that all the TPA derivatives were strongly concentration dependent aggregation by ππ interaction. Further, the solution state fluorescence study revealed that all the compounds have very binding affinity for picrate anion. Furthermore, we proved that the quenching of fluorescence was observed due to the charge transfer in the ground state complex formation of picrate to triphenylamine derivatives as well as resonance energy transfer between picrate anion and the TPA derivatives.

Acknowledgement K.D & A.S acknowledge to the financial support of the Council of Scientific and Industrial Research, New Delhi, India (Grant No. 01(2540)/11/EMR-II). R.B & A.S acknowledge to the Department of Science and Technology, New Delhi, India (Grant No. 24

SR/F/1584/2012-13) for the financial support. We acknowledge National Centre for Ultrafast Process, University of Madras, Chennai-113 for providing lifetime measurements.

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Biographies

Duraimurugan K received his M.Sc. degree from RKM Vivekananda College from University of Madras in 2004. After that he worked as a Senior Scientist in Syngene International Pvt Ltd, Biocon, Bangalore, India. Currently he is working as a Ph.D. Research Scholar under the guidance of Dr. A. Siva at School of Chemistry, Madurai Kamaraj University. His research interests focus on the synthesis and applications of supramolecules.

Balasaravanan R obtained his M.Sc degree from Madurai Kamaraj University in 2010. Currently he is pursing Ph.D under the guidance of Dr. A. Siva at Madurai Kamaraj University. His research mainly focuses on the synthesis of conjugated and non-conjugated flurophores for sensor applications.

Dr. A. Siva awarded M.Sc and Ph.D. degree from Gandhigram Rural University, Dindigul during the year of 2006. Then he worked as an Associate Scientific Manager at Syngene International Pvt Ltd, A Biocon Company, Bangalore. After that, he moved to South Korea, Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH) for his postdoctoral research under the guidance of Prof. Taiho Park. Currently he is working as an Assistant Professor at Madurai Kamaraj University, Madurai. His research interest includes asymmetric synthesis, organometallic chemistry, supramolecular Chemistry and Dye sensitized solar cells.

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