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Pt(II)C^N^N-Based Luminophore/Micellar Adducts for Sensing Nitroaromatic Explosives Prasenjit Maity, Aarti Bhatt, Bhavesh Agrawal, and Atanu Jana Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00869 • Publication Date (Web): 10 Apr 2017 Downloaded from http://pubs.acs.org on April 11, 2017

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Pt(II)C^N^N-Based Luminophore/Micellar Sensing Nitroaromatic Explosives

Adducts

for

Prasenjit Maity,*,# Aarti Bhatt,# Bhavesh Agrawal# and Atanu Jana*,†,‡ #

Institute of Research and Development, Gujarat Forensic Sciences University, Gandhinagar – 382007, India † Department of Chemistry, University of Sheffield, Sheffield, S3 7HF, UK ‡ Present Address: Institute for Supramolecular Chemistry and Catalysis, Shanghai University, Shanghai, 200444, China E-mails: [email protected]; [email protected]

ABSTRACT: Two luminescent cyclometalated Pt(II)-complexes, 1•Pt and 2•Pt, respectively, were synthesized by using unsymmetrical C^N^N ligands having different alkyl substituents. These π-electron rich complexes are used for sensing various electron deficient nitroaromatic explosives, e.g., 4-nitrotoluene (NT), 2,4-dinitrotoluene (DNT), 2,4,6-trinitrotoluene (TNT) and 2,4,6-trinitrophenol (TNP), in aqueous, non-aqueous, as well as in solid state as paper strip with maximum detection limit of ca. 10−9 M. It was demonstrated that the sparingly soluble 2•Pt complex becomes water soluble in presence of all kinds of surfactants viz. cationic (e.g., cetyl trimethylammonium bromide, CTAB), anionic (e.g., sodium dodecyl sulfate, SDS) and neutral (e.g., Triton X-100). This may be due to the incorporation of its long lyophilic tail group (−C12H25) inside the micellar core, exposing planar Pt(II)C^N^N head group to the aqueous bulk phase. It was also observed that, the extent of solubility of these Pt(II)-complexes in micellar media, strongly depends on the length of existing alkyl chain. For instance, the presence of longer dodecyl chain makes 2•Pt complex ca. 1000 fold more soluble than the complex 1•Pt, which contains a shorter propyl chain. Their sensing behavior essentially arises by the quenching of Pt(II)based intense luminescence due to supramolecular charge transfer (CT) process originated from Pt(II)C^N^N-antenna to the electron deficient nitroaromatic explosives. Our present work exhibits that the micellar adducts formed by highly luminophoric material and surfactant molecules, could effectively detect such kind of explosives in aqueous medium with better sensitivity compare to what were observed in other media.

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INTRODUCTION Polynitroaromatic molecules e.g., DNT, TNP, and most importantly TNT are widely used as explosives by several terrorist groups and separatist organizations with sole perseverance of mass killing. They are also extensively used in crude oil extraction, mining industries, by road and transport authorities for making tunnels, railroads and highways through mountainous regions. The catastrophic effect of detonations and the contaminated soil and ground water are of serious threats for mankind in many parts of our planet and thus high-end detection of these molecules are now research priorities1,2 in present era. Therefore, development of a simple and cost-effective methodology for the detection of nitroaromatic explosives with high degree of precision is necessary. There are several techniques corroborated with the detection of explosive traces e.g., surface enhanced Raman spectroscopy (SERS),3 surface plasmon resonance (SPR),4,5 gas and liquid chromatographies,6 mass spectrometry,7 ion-mobility spectroscopy (IMS),8 molecular imprinting,9 electrochemical detection10 and many others. A popular practice is the use of trained dog, which can detect down to parts-perquadrillion concentration (10−15 mg mL−1) of TNT. However, the constant training and maintenance of canines are expensive and often their performance is distracted by neighborhood external stimuli like sound, smell and weather conditions.11 Luminescence based technique12−33 is another widely used method due to its extremely high sensitivity (works even at parts-per-billion level), easy portability, quick response and inexpensive processing cost. Several chemical scaffolds have been designed and synthesized for this purpose with most dominant mechanism relied on luminescence quenching.12−32 However, an exceptional case where sensing behavior depends on luminescence enhancement has also been demonstrated.33 Various groups reported a number of functional materials like conjugated organic/inorganic polymers,12−15 porous metal– organic frameworks (MOFs),16−19 covalent organic frameworks (COFs),20 dendrimers,21 quantum dots22,23, hybrid metal nanoparticles,24,25 lanthanide based nanocomposites26,27 polymer nanocomposite28 and π-electron rich small organic molecules29 for sensing applications. Among the aforesaid chemical scaffolds, conjugated organic polymer based sensors12 have been proven most sensitive e.g., pentiptycene containing poly(phenyleneethynylene) derivative was marketed as Fido by Nomadics Inc with detection limit up to


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10−15 mg mL−1 with TNT, whereas, they are synthetically challenging, and have poor solubility in aqueous phase detection. Porous MOFs and COFs are also highly sensitive but have same synthetic challenges and solubility issues. There are also quite a few tetrathiavulvalene (TTF) derived flexible calix[4]pyrrole macrocycles34−36 reported by Sessler and coworkers for the naked eye detection of nitroaromatic explosives, however, to make those compounds highly skilled synthetic hand is necessary. Compared to the abovementioned materials, π-electron rich small organic molecule-based sensors29 are easy to synthesize with desired solubility and well-defined photophysical properties. The fundamental mechanism associated with the luminescence quenching behavior of these molecular sensors is photoinduced electron transfer (PET) from electron rich fluorophore to electron deficient nitroaromatic explosives. On the other hand, luminescent transition metal complexes are another category of small molecules but rarely explored for explosive detection despite their many advantages to offer. Synthetic simplicity, easy tailoring of functional groups, flexibility of luminescence property by structural modifications and unique photophysical properties are their attractive features. Reddy et al. first demonstrated the application of an aggregationinduced phosphorescent emissive Ir(III)-complex30 for solution phase (in acetone:water (30:70 v/v), as solvent mixture) and vapor phase detection of TNT. A phosphorescent Ir(III)-complex31 was also reported by Zhang et al. for detection of nitroaromatics in dichloromethane (DCM). Recently, Mosca and co-workers reported Ru(II)- and Ir(III)based luminescent complexes32 for the detection of DNT and TNT in polar organic solvents (acetonitrile, ethanol and DCM). However, there are only few such examples are reported so far in recent literature. Therefore, the scarcity of such kind of luminescent small transition metal complexes encouraged us to explore the prospect of newer class of molecules for detection of explosives. As discussed in this section, all these reported compounds in recent literature performed exclusively in organic solvents.30−32 Nevertheless, detection of nitroaromatics contaminated with aquatic environment in trace level is more important due to its adverse effect on marine life and human health.37 Subsequently, it would be appealing if a single molecule can effectively sense trace amount of nitroaromatic explosives in both organic solvents as well as in aqueous medium.


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It was also observed that the use of micellar media have been proven superior for some selective chemosensors for detection of metal ions38−40 in aqueous medium. In recent literature, few micelle-based sensors were reported41−47 for the detection of explosives materials. For example, Bu et al. demonstrated41−44 the use of micellar domain in a heterogeneous surface for the extraction of hydrophobic nitroaromatics, pesticides, drugs and anti oxidants by using electrochemical method. A micelle-based pyrene encapsulated silica nanocomposite was also reported for the detection of nitroaromatics.45 It was also established that pyrenes move inside the micellar core in aqueous media and behave as sensor for nitroaromatic detection through quenching of excimer emission.46,47 We report herein an ideal technique where suitable surfactants are used to make the hydrophobic transition metal-based luminophore nicely soluble in water over its critical micellar concentration (CMC) to sense nitroaromatic explosives. In our present work, we used hydrophobic organometallic Pt(II)C^N^N complex as the luminophoric material which is grafted onto the surface of the micelle instead of locating inside hydrophobic core. Therefore, the donor−acceptor induced CT complexation resulted in presence of electron deficient nitroaromatic explosives in surface. In this context, the luminescence characteristics of the selected luminophore in bulk solution and in microenvironments (i.e., in micellar media) were meticulously performed. The chemical structures of the key chelate ligands, 1 and 2, along with their corresponding cyclometallated Pt(II)-complexes used under present studies are given in Chart 1.

N

1

N N C3H 7

N

N

Pt

N N

Pt

N N C12H25

C3H 7 N

2

N N

Cl

Cl

1••Pt

2••Pt

C12H25

Chart 1: The chemical structures of the ligands (1 and 2) used to prepare luminophores 1•Pt and 2•Pt, respectively under present studies.

As we discussed earlier, the major focus of our present work is the use of micellar


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media to solubilize the lipophilic organometallic luminophores in aqueous media and demonstrate their improved sensitivity compare to other testing media. The schematic representations

of

the

proposed

luminophore−micelle

adduct

formation

and

supramolecular host-guest complexation upon addition of nitroaromatic explosives are given in Scheme 1. Detailed photophysical investigation on these luminophore−micelle adducts revealed that upon addition of each analyte, quenching of emission were observed. This might be due to the donor−acceptor kind of supramolecular interactions operating between Pt(II)C^N^N head groups (donor) grafted to the micellar surface and the added nitroaromatics (acceptor), sandwiched between two neighboring Pt(II)C^N^N units.

Scheme 1: Proposed model of luminophore−micelle adduct and formation of supramolecular complex upon guest addition (e.g., various nitroaromatic explosives) responsible for luminescence quenching.

EXPERIMENTAL SECTION Materials and Methods. All the starting materials and solvents were purchased from commercial sources (Sigma Aldrich Chemical Company and Finer Chemicals) and used as received unless stated otherwise. Milli-Q water and spectroscopic grade solvents were used for all measurements. FTIR-spectra were recorded on a JASCO FT-4600 spectrophotometer by using KBr pellet of samples. UV−vis spectra were measured on a JASCO V-670


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spectrophotometer. Emission spectra were recorded using a JASCO FP700 spectrometer. Low temperature (77 K) emission spectra were recorded with a quartz tube placed into a liquid nitrogen Dewar. 1H (400 and 500 MHz) and 13C (100 and 125 MHz) NMR spectra were recorded using the Bruker NMR spectrometers. Electrospray (ESI) mass spectra were measured using micromass LCT instrument. Sample and standard solutions were degassed with at least three freeze-pump-thaw cycles. Emission quantum yields were measured by the method of Demas and Crosby48 with [Ru(bpy)3](PF6)2 in degassed acetonitrile as the standard (Φref = 0.062). Dynamic light scattering (DLS) experiments were performed using a Malvern DLS instrument (Zetasizer Nano ZSP model). Photoluminescence (PL) lifetime measurements of the complexes in various conditions were carried out by using a picosecond time-correlated single photon counting (TCSPC) instrument (Edinburgh Instruments Ltd, Lifespec II model). A picosecond light emitting diode laser (Nano LED, λex = 405 nm) was used as excitation source. The emission polarizer at a magic angle of 54.7° by a photomultiplier tube (TBX-07C) was used to collect the PL decays. The instrument response function (IRF, fwhm TNT (97%) > DNT (61%) and NT (43%) in DCM.

Figure 2. PL titration spectra of 2•Pt in DCM at 298 K with various amount of (a) TNP, (b) TNT, (c) DNT and (d) NT. (e) Stern-Volmer plots obtained from the PL spectra of Figures (a)−(d) associated with the complex 2•Pt; and (f) Stern-Volmer plots obtained from spectra S15 to S18 in ESI associated with 1•Pt resulted from PL titration using these four analytes (TNP, TNT, DNT and NT) in DCM under identical experimental conditions.

Explosive Detection in Water Using Micellar Host. It was ascribed that, the attachment of long hydrophobic aliphatic chain to the luminophore facilitate the formation of luminophore−micelle adduct in water. This may be due to the incorporation of aliphatic chain inside the hydrophobic micellar core, keeping the luminophoric Pt(II)C^N^N head groups exposed to the surface of the micelle which creates a platform to interact with planar nitroaromatic molecules (cf. Scheme 1).


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Thus, we have chosen two Pt(II)-complexes with same core structures but varied in alkyl chain lengths, to investigate its role in aqueous micellar media. As depicted in Scheme 1, we proposed that 2•Pt easily incorporated inside the micelle core due to its longer (−C12H25) hydrophobic aliphatic chain, which is not favorable in case of 1•Pt, containing a shorter (−C3H7) aliphatic chain. As a test run, we mixed both the Pt(II)-complexes in various micellar media and stirred the mixture for overnight at room temperature. During experiments, the aqueous surfactant solutions were taken two times higher (i.e., 16 mM SDS, 0.5 mM Trion X-100 and 2 mM CTAB, respectively) than their respective critical micellar concentrations (CMC) and 0.01 mmol of each complex (4.92 mg 1•Pt and 6.2 mg 2•Pt) were added to each surfactant with a total volume of 1L. The solutions were filtered to remove, if any, undissolved residue of Pt(II)-complexes. Interestingly, it was found that 2•Pt was completely dissolved in all three surfactant solutions, whereas, 1•Pt was mostly insoluble as evidenced by its presence in filter paper (Figure S22 in ESI) as residue. This experiment unequivocally supports that longer aliphatic chain favors the formation of luminophore−micelle adduct in aqueous micellar host. The emission spectra of each three sets of 2•Pt/surfactant and 1•Pt/surfactant solutions were measured, which revealed strong emission profiles of 2•Pt solutions and very negligible emission outputs from 1•Pt solutions (Figures S23). The electron poor planar explosive molecules are thought to be sandwiched between two planar Pt(II)C^N^N head groups which are exposed to the micellar surface, form donor−acceptor charge transfer complexes. The relative emission intensities and peak profiles were scrutinized thoroughly which revealed that the emission intensities follow the order: 2•Pt in alcoholic glass at 77 K > 2•Pt/Triton X-100 at RT ≈ 2•Pt/SDS at RT ≈ 2•Pt/CTAB at RT > 2•Pt in DCM at RT. Compare to the emission profile of Pt(II)-complex in DCM, the 2•Pt/micellar adducts show slightly blue shifted peaks at 508 nm and 545 nm and a weak band at 580 nm along with 4−8 nm narrower FWHM values (Figures 1b and Table 1). This significant observation depicts that the vibronic coupling based broadening and non-radiative excited electronic transition is slightly suppressed in micellar environment, which was also observed at glass temperature. Solution based PL titrations were performed with various concentrations of TNP against 2•Pt/Triton X-100, 2•Pt/SDS, 2•Pt/CTAB and the corresponding results are


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shown in Figures 3a−3c along with their Stern-Volmer plots given in Figure 3d. The calculated Stern-Volmer constants are 8.8 × 103 (for SDS), 1.06 × 104 (for Triton X-100) and 9.06 × 103 M−1 (for CTAB), respectively, which predicts that among these surfactants, the neutral surfactant (Triton X-100) showed better efficacy compared to cationic (CTAB) and anionic (SDS) surfactant. The superiority of neutral surfactant over ionic surfactants is thought to be due to the better supramolecular complexation of TNP onto the neutral surface of 2•Pt/Triton X-100. Such interaction may be slightly disturbed in ionic surfactants, mainly due to interference of charged species with neutral TNP, although the luminophore−micelle adduct formation is identical in all three micelles as evidenced from their PL profiles and DLS studies (vide infra). A similar type of PL titrations were also performed with TNT against 2•Pt/Triton X-100, 2•Pt/SDS, 2•Pt/CTAB and the corresponding results are given in Figure S24 along with their Stern-Volmer plots. It is worth noting that, a methanolic solution of TNT was prepared first as TNT is sparingly soluble in water. This alcoholic solution was diluted with measured quantity of water and used during titration experiments. The calculated Stern-Volmer constants for TNT predicted by titration results are 1.02 × 104 (for SDS), 1.19 × 104 (for Triton X-100) and 1.07 × 103 M−1 (for CTAB), respectively, in aqueous medium. These values again support that neutral surfactant (Triton X-100) is most efficient compared to cationic (CTAB) and anionic (SDS) surfactants. Interestingly, it was observed that the Stern-Volmer constants for sensing TNT are marginally higher than TNP in all these three surfactants used for experiments. This observation could be ascribed due to the more hydrophobic nature of TNT that makes it advantageous over hydrophilic TNP during supramolecular complexation in the medium. It is to be mentioned that in our systems, hydrophobic extraction of micellar core (cf. Scheme 1) might be operative upon addition of nitroaromatics, but in a lesser extent with compare to what were observed in other micellar systems reported.41−44 This might be due to the trace amount of added nitroaromatic molecules (TNT, TNP) present in solution which were immediately seized by the micellar surface, resulting strong supramolecular complexation with Pt(II)C^N^N-antenna. Therefore, the controlled addition of titrant allowed only very trace amount of hydrophobic explosives move inside the hydrophobic micellar core. This phenomenon is clearly observed from all the related


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photophysical

experiments,

which

also

indicate that

direct

contact

between

donor−acceptor moieties is operative in major extent that promotes supramolecular complexation onto the micellar surface, resulting a significant luminescence quenching. Moreover, we maintained higher concentration of luminophore-micellar adduct with compared to the concentration of added explosives, assuming most of them were engaged in supramolecular CT complexation in surface. To understand the robustness and versatility of the system we carefully examined the effects of temperature and pH of the medium against luminescence property of 2•Pt/micellar adduct in presence and absence of nitroaromatic analytes. We select TNT (hydrophobic in nature) and TNP (hydrophilic in nature) as the representatives of each kind for these experiments. It was observed that the luminescence intensity of 2•Pt/micellar adduct in water remains unchanged from 10 ºC to 40 ºC and after that shows ≈ 10% decrease (Figure S25). The quenching of PL intensity after addition of TNT and TNP remains almost same throughout the temperature range with standard error of ± 5% (Figure S25). The robustness of adduct was also verified within a wide pH range of 4 to 13 and the PL intensity of 2•Pt/micellar adducts were examined with and without added analytes. It was observed that PL intensity of 2•Pt/micellar adduct remains same for all pH ranges in case of Trioton X-100 and CTAB, whereas, SDS micelle shows drastic drop of PL value after pH 9 (Figure S26). This might be due to the precipitation of 2•Pt from SDS micellar solution beyond pH 9 and we did not use this system for analysis of nitroaromatics. The PL quenching effect upon addition of TNT and TNP is very similar for all three surfactants in the pH range of 4 to 9. At higher pH (for example, at pH = 9 to 13), TNP shows 30% lower quenching efficiency than TNT for CTAB and Triton X-100 micelle. The formation of phenolic anionic species perhaps increases its interaction with surfactant head groups rather than supramolecular interaction with 2•Pt. However, these studies validates the robustness of our 2•Pt/micellar system over a wide temperature range (10 ºC to 60 ºC) and shows that for TNP the optimum pH range is 4 to 9 and for TNT we can even measure it in the pH ranges 4 to 13.


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Figure 3. PL titration spectra of 2•Pt/micelle adducts in water with incremental addition of TNP (a) Triton X-100 micelle, (b) SDS micelle, (c) CTAB micelle and (d) Stern-Volmer plots obtained from the above three titration results.

To understand the effect of surfactant concentration for detection efficiency of TNP, a series of titrations were performed with various concentrations of Triton X-100. For each experiment the Stern-Volmer constant was calculated (see Figure S27 in ESI). The calculated Stern-Volmer constants show a minimum value below the CMC (0.1 and 0.2 mM of Triton X-100), after which over a long range of concentration (ca. from 0.3 mM to 0.7 mM) it is almost steady and then decreased on increasing concentration. By comparing the Stern-Volmer constants for TNP sensing in organic solvent (DCM) and in aqueous micellar media, we established that micelle could boost the detection efficiency by 1.5 times. Thus, the combination of hydrophobic fluorophore in a micellar environment has the advantage of the remarkable solubility of the fluorophore in water,


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substantially can detect trace level nitroaromatics with better efficacy than what was observed in organic solvent.

Dynamic Light Scattering Measurement: To determine the change, if any, of the hydrodynamic diameters (Dh) of three surfactant solutions and their corresponding 2•Pt inclusion adducts, DLS measurements were performed in aqueous medium. It was observed that the measured hydrodynamic diameters for 16 mM SDS, 0.5 mM Triton X-100 and 2 mM CTAB are 6.5, 5.9 and 5.9 nm respectively, which are similar to the reported literature values.50 The hydrodynamic diameters of the above solutions after adduct formation with 2•Pt were found to be 6.8, 5.5 and 6.4 nm for SDS, Triton X-100 and CTAB respectively (see Figure 4). This observation clearly indicates that the micellar structures remain intact after incorporation of 2•Pt without any significant deformity.

Figure 4: Size distribution histograms of the assemblies determined by DLS measurements carried out in aqueous solutions of (a) 0.5 mM of Triton X-100, (b) 0.5 mM of Triton X-100 + 0.01mM 2•Pt, (c) 16 mM of SDS, (d) 16 mM of SDS + 0.01mM 2•Pt, (e) 2 mM of CTAB and (f) 2 mM of CTAB + 0.01mM 2•Pt. All the experiments were carried out at 298 K.


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Luminescence Lifetime Measurements: To understand the effect of nitroaromatics to the excited state photophysical property of 2•Pt, the excited state lifetime measurements were performed in solution state at ambient condition. In carrying out this experiment, the lifetime studies of 2•Pt with and without addition of TNT and TNP were performed in DCM as well as in Triton X100 in water at 298 K. The corresponding lifetime decay traces are given in Figure 5. It was noticed that the excited state lifetime of 2•Pt (45 ns in DCM) is reduced to 33 ns after addition of 2 equivalent of TNT. Similarly, the excited state lifetime of 2•Pt in Triton X-100 is 49 ns, which further reduced to 31 ns after addition of 2 equivalent of TNP. These results further support the basis of PL quenching caused by excited state electron transfer from donor (2•Pt) to acceptor (nitro aromatics).

Figure 5. Lifetime decay traces for the complex 2•Pt and in presence of TNT and TNP, as recorded in DCM and Triton X-100 micellar medium at RT.

Vapor and Solid Phase Detection of TNT by Using a Paper Based Strip. For direct application of these organometallic luminophores, vapor and solid phase detection of TNT were also demonstrated. For this purpose, paper based strips


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were prepared by immersing the Whatman filter papers (2 cm × 2 cm) in a DCM solution of the luminophores (1 × 10−3 M) for 5 minutes. Subsequently, the strip was air dried in dark and then exposed to TNT vapor by placing it over a closed vial containing 25 mg of solid TNT for a specific period of time. The solid-state emission spectra of the strips were measured at different time intervals (Figures 6a−6b) and gradual attenuation of emission intensities were observed with increasing exposure times. It was observed that the emission profiles of the test strips exactly matched with the emission profile of DCM solution of 2•Pt and 1•Pt. This indicates that intermolecular π−π stacking and excimer formation processes are non-operative here. The visual effect of these test strips in visible and UV (365 nm) lights are shown (Figure 6c−g). The presence of solid TNT particles on test strips was also detected through quenched emission of the selected area (Figure 6f). Trace level detection of explosive molecules present in fingerprint residue is another challenging task in forensic investigative research.51 We also performed preliminary studies to identify TNT contaminated human thump impression on paper strip. For doing this experiment, a finger was dipped into TNT powder and then solid particles were removed from the thump before pressing it on the test strip. The optical image of tested human thump on the test strip is shown in Figure 6g, where the quenched emission area represents the presence of TNT. To quantify the sensing efficacy of test strips, 20 µL of TNT solution in DCM with varying concentrations were applied on each strips and their PL intensities were compared (Figure S28). The result demonstrates the efficiency of 2•Pt impregnated test strips with detection limit of 10−9 molar.


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Figure 6. The time monitored solid-state PL spectra of impregnated test strips of 1•Pt (a), and 2•Pt (b), respectively, in contact with TNT vapor in a closed vial. The visual change of 2•Pt impregnated test strip under visible light (c), under 365 nm UV light (d), under UV light after TNT vapor treatment (e), under UV light after treating with solid TNT crystals (f) and under UV light after pressing TNT contaminated thump impression (g).

SUMMARY AND CONCLUSION In summary, we described herein the design and synthesis of two luminescent Pt(II)C^N^N complexes for detection of nitroaromatic explosives in aqueous, organic, vapor and solid phase. Both the complexes showed comparable sensing performances in organic solvent (e.g., in DCM) with the detection limit of 10−6 M and in vapor phase as a test strip. However, due to the presence of longer hydrophobic aliphatic chain, 2•Pt becomes more water soluble in presence of neutral, anionic or cationic micellar hosts with enhanced sensitivity towards explosive detection. We assumed that in our present case, the donor−acceptor-based supramolecular CT interaction is the prime factor for sensing the nitroaromatics, which involves a prominent luminescence quenching. The complex 2•Pt in micellar media showed higher detection efficiency with KSV value of 1.06 × 104 with compared to what was observed in DCM (KSV of 7.5 × 103). We also


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successfully demonstrated that, the presence of long hydrophobic chain ensures its insertion to the micellar core leaving Pt(II)C^N^N head group exposed towards the surface of the micelle. This was attributed by the partial ionic character and adequate hydrophilicity of Pt(II)C^N^N head group. The lower vibrational degrees and suppressed non-radiative relaxation pathways of 2•Pt complex in micellar host results enhanced emission and at the same time, its easy accessibility to aqueous nitroaromatic molecules improves the detection limit compared to free fluorophore in organic solvent. We believe that our proposed methodology could be generalized, where introducing sufficient hydrophobicity to a fluorophore will make it usable in both organic solvent and in water using micellar host with high detection efficiency.

ASSOCIATED CONTENT Supporting Information Synthesis of all these material, characterization data along with additional photophysical data are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (P.M.). *E-mail: [email protected] (A.J.).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS P.M. gratefully acknowledges Science and Engineering Research Board (SERB), Government of India, (Grant number SERB/FT/007-2014) for financial assistance. A.J. thanks the European Commission for a Marie Curie International Incoming Fellowship (MC-IIF-FP7).


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For TOC

Pt(II)C^N^N-Based Luminophore/Micellar Sensing Nitroaromatic Explosives

Adducts

for

Prasenjit Maity,*,# Aarti Bhatt,# Bhavesh Agrawal# and Atanu Jana*,†,‡ #

Institute of Research and Development, Gujarat Forensic Sciences University, Gandhinagar – 382007, India † Department of Chemistry, University of Sheffield, Sheffield, S3 7HF, UK ‡ Present Address: Institute for Supramolecular Chemistry and Catalysis, Shanghai University, Shanghai, 200444, China E-mails: [email protected]; [email protected]

Two luminous cyclometalated Pt(II)C^N^N-complexes, 1•Pt and 2•Pt, were used for sensing nitroaromatic explosives, e.g., 4-nitrotoluene (NT), 2,4-dinitrotoluene (DNT), 2,4,6-trinitrotoluene (TNT) and 2,4,6-trinitrophenol (TNP). Their competence in aqueous, non-aqueous, as well as in solid state was demonstrated. However, better sensitivity towards explosive detection was witnessed in various micellar adducts of complex 2•Pt, with compare to other commonly used media.


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Pt(II) - ACS Publications - American Chemical Society

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