Detection of Nitroaromatic Explosives Based on Fluorescence

Article pubs.acs.org/JPCC

Detection of Nitroaromatic Explosives Based on Fluorescence Quenching of Silafluorene- and Silole-Containing Polymers: A TimeDependent Density Functional Theory Study Burcu Dedeoglu,† Antonio Monari,*,‡,§ Thibaud Etienne,‡,§,∥ Viktorya Aviyente,†,⊥ and Alimet Sema Ö zen*,# †

Department of Chemistry, Bogazici University, Bebek, Istanbul 34342, Turkey Théorie-Modélisation-Simulation, Université de Lorraine − Nancy, SRSMC, Boulevard des Aiguillettes, BP 70239, 54506 Vandoeuvre-lés-Nancy Cedex, France § Théorie-Modélisation-Simulation, CNRS, SRSMC, Boulevard des Aiguillettes, BP 70239, 54506 Vandoeuvre-lés-Nancy Cedex, France ∥ Unité de Chimie Physique Théorique et Structurale, Université de Namur, Rue de Bruxelles 61, 5000 Namur, Belgium ⊥ The Science Academy, Beşiktaş, Istanbul 34330, Turkey # Faculty of Science and Letters, Piri Reis University, Tuzla, Istanbul 34940, Turkey ‡

S Supporting Information *

ABSTRACT: Poly(silafluorene-phenylenedivinylene)s and poly((tetraphenyl)-silole- phenylenedivinylene)s are promising materials to be used as chemical sensors for explosives detection. The optoelectronic properties of these polymers as well as their constituent units have been investigated by modeling the properties of their excited states. Natural Transition Orbital analysis and topological ϕS descriptor assessment have been used to qualitatively and quantitatively characterize the physical nature of the transitions constituting the absorption spectra. The main transitions observed in all oligomers are associated to be a π−π* transition of the bridging moiety. Lower energy transitions of charge transfer character are further considered to understand the fluorescence quenching mechanism upon the complexation of these polymers with the analytes. Indeed the charge-transfer character of the first excited state leads to the emergence of thermal deactivation channels and hence to luminescence quenching.

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effect.2 This will lead to a general bathochromic shift of both absorption and emission spectra that will easily cover the visible wavelength region. Silafluorenes and siloles are the two mostly studied silicon-containing π-conjugated systems to be used as chemical sensors for detection of explosives, because of their improved optoelectronic properties.3−10 Delocalization is further enhanced when silafluorenes and siloles are copolymerized with conjugated organic systems. Indeed, silafluorene and silole copolymers (5 and 6, Scheme 1) have been synthesized for use in the detection of explosives by catalytic hydrosilylation of 1,4-diethylnylbenzene. The fluorescence quenching of these polymers by explosives containing nitro groups has been reported, and it has been proposed that Lewis acid−base interactions between the lone pairs of the nitro groups of the explosives and the silicon center promote the binding of the analytes to the polymers.8 Photoluminescence data for monomers, dimers, and polymers

INTRODUCTION The detection of explosive materials to be used in various security and military issues continues to attract the attention of many researchers. Besides physical detection methods, such as X-ray imaging, thermal neutron analysis, or gas chromatography, which offer high sensitivity and specificity, but some of which are expensive or not easy for field deployment, fluorescence methods are promising because they can be incorporated into portable microelectronic devices and are lower in cost. Detection based on fluorescence quenching of conjugated photoluminescent polymers in detection of explosives is a promising technology to be developed.1 The use of fluorescent polymers in detection of explosives has been adapted for both instrumental and visual imaging approaches. These conjugated polymers allow the detection of explosives by means of signal gain or loss in response to interactions with analytes. Hetero-π-conjugated molecules are powerful building blocks for optoelectronic materials. The inclusion of silicon as the heteroatom in these systems can decrease virtual orbital energy levels substantially due to a strong σ*−π* hyperconjugation © 2014 American Chemical Society

Received: May 22, 2014 Revised: August 12, 2014 Published: September 16, 2014 23946

dx.doi.org/10.1021/jp505025t | J. Phys. Chem. C 2014, 118, 23946−23953

The Journal of Physical Chemistry C

Article

Scheme 1. Structures of Silafluorene- and Silole-Containing Monomers (H2SF and H2silole), Model Dimers (1 and 2), Dimers (3 and 4), and Polymers (5 and 6)

(Table 1) as well as the solid-state detection limits (ng cm−2) for various explosives by fluorescence quenching of polymers 5

as compared to H2SF and 3. This could make it a better candidate for applications involving improved exciton mobility. Quenching of photoluminescence of silafluorene- and silolecontaining polymers by the analyte were measured, and the Stern−Volmer equation was used to quantify the quenching efficiencies for several analytes containing nitroaromatic, nitrate ester, and nitramine functionalities.5,7−9 A linear Stern−Volmer relationship was observed in all cases suggesting that the photoluminescence quenching might occur by either a static or dynamic process. No change of the fluorescence lifetimes as a function of TNT concentration pointed out that the photoluminescence quenching occurs dominantly by a static mechanism where the emitting species binds the quencher; thus, a polymer−quencher complex is formed.9 In that study, the observed linear dependence of the Stern−Volmer constant, KSV, on the reduction potential of the analytes indicated that the quenching mechanism is dominated by a photoinduced charge transfer from the polymer to the analyte. We have already shown that the resulting energy levels of the complex that is formed between the polymers and the analytes change dramatically to allow probable charge transfer to occur (Table S1 in ref 2). The aim of this study is to investigate the fluorescence quenching mechanism to detect various explosives containing nitro groups (Scheme 2), with the use of silafluorene- and

Table 1. Summary of Photoluminescence Data for Monomers, Dimers, and Polymers8 H2SF 1 5 H2silole 2 6

λabsa (nm)

solution λflua (nm)

thin-film λflub (nm)

288 290 294 305 306 322

343 348 359 482 482 487

365 380 447 485 482 478

a UV−vis and fluorescence taken in toluene. bEmission maximum for thin layer of fluorophore absorbed onto TLC plate.

and 6 have been reported.8 In our previous study, we have shown that two types of interactions, π−π stacking and Lewis acid−base, favor the binding of analytes to polymers.2 This was done by modeling, at the quantum mechanics level, the interactions between different explosives and dimers representative of silafluorene- and silole-containing polymers (5 and 6). For the silole-containing materials, the fluorescence emission maxima of 2 was observed to be almost the same with its monomer H2silole, and accordingly, the corresponding polymer 6 showed a slight bathochromic shift of 5 nm in toluene. These results were attributed to the orthogonal alignment of the bridging organic π system and the Si−C σ* orbitals in dimer 2.8 The fluorescence emission maximum of the silafluorene dimer 3 and polymer 5 are only very slightly redshifted 5 and 16 nm, respectively, from the one of the monomer H2SF due to a better alignment for conjugation of the bridging organic π system and the Si−C σ* orbitals.8 Interestingly, one observes a significant broadening of the emission spectrum of polymer 5

Scheme 2. Structures of Explosives (TNT, DNT, PA, RDX) and Nonexplosive Nitrobenzene (NB)

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due to the fact that only one couple of occupied and virtual NTOs has a significant weight and totally describes the electronic transition.18 Basically, the transition can be entirely described as electron removal from the occupied NTO (“hole” density) and electron accumulation into the virtual NTO (“electron” density). NTOs have been calculated using a postprocessing code developed in our laboratory and freely available under GPL license19 while they are also attainable within Gaussian 09.16 Moreover, using our public available code, we also computed the φS descriptor to quantitatively assess for the charge-transfer nature of the different transitions. For the reader’s convenience, let us remind that φS is an index based on the spatial overlap between attachment and detachment densities and assuming values comprised from zero to one by construction.20,21 φS values close to zero are representative of a charge-transfer transition while local excitation will give values close to 1.0.

silole-containing oligomers (Scheme 1). In order to perform such a task, we will need to model the behavior of excited states and in particular to follow their relaxation in the case of the dimers and of the dimer−explosive complexes. To achieve a good compromise between computational cost and accuracy, we will therefore use time-dependent density functional theory (TD-DFT) techniques to assess the properties of the excited states. Computational Methodology and Benchmark Calculations. Geometry optimizations are performed using DFT11−13 at the M06-2X/6-31G(d)14 level of theory. All stationary points have been characterized by a frequency analysis. Charge analysis has been carried out by using full natural population analysis with M06-2X/6-31G(d).15 All calculations have been carried out using the Gaussian 09 program package.16 TD-DFT has been utilized to calculate the excitation energies and the oscillator strengths and to simulate the electronic absorption and fluorescence spectra. Absorption spectra have been calculated as vertical excitation energies from the groundstate equilibrium geometry, while fluorescence emission has again been obtained considering vertical transitions taking place from the geometry corresponding to the minimum of the excited-state potential energy surface. In order to simulate the band shape, vertical transitions have been convoluted using Gaussian functions of constant fixed-width at half-length of 0.3 eV. Single point TD-DFT calculations have also been performed with M06-2X/6-31+G(d) even if M06-2X/631G(d) is shown to reproduce the experimental absorption wavelength of 1 quite well (Table S1, Supporting Information).2 Nevertheless, the influence of the basis set has been carefully analyzed by systematically adding diffuse and polarization functions (Table S2, Supporting Information). Since the effect of the basis set size is rather negligible (not exceeding 10 nm), excited-state geometry optimizations were performed at the M06-2X/6-31G(d) level of theory. Yet, the absorption and emission spectra have been simulated both in the gas phase and in toluene. Throughout, solvent effects have been modeled by using the linear response polarizable continuum method (PCM) for absorption as implemented in Gaussian 09 code.17 In the case of emission, we used the state-specific equilibrium version of the PCM methodology to take into account the effects of the solvent reorganization on emission wavelengths. The absorption bands in toluene are only slightly redshifted with higher intensities than those of the ones in the gas phase, but they have the same characteristics (Figure S1, Supporting Information) with differences not exceeding 15−20 nm. Also, in the case of emission, no substantial difference in the behavior of the fluorophore was evidenced when solvent was taken into account. At the TD-DFT level, excited states are represented as a linear combination of singly excited determinants, that is, as a linear combination of excitations from occupied to virtual Kohn−Sham orbitals. This expression in terms of molecular orbitals can be cumbersome, since many occupied/virtual orbital couples can enter the expansion with similar weight. A possible improvement to get a deeper understanding of the physical nature of the electronic transitions leans on the representation of an excited state in terms of the natural transition orbitals (NTOs).18 Indeed, by performing a singular value decomposition of the transition density matrix, one may express the excited state as a transition from occupied to virtual NTOs. The advantage over the Kohn−Sham representation is

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RESULTS AND DISCUSSION Optical Properties. Monomers and Dimers. The computed absorption spectra of silafluorene-containing monomer (H2SF), model dimer 1, and dimer 3 convoluted using Gaussian functions are reported in Figure 1, together with the

Figure 1. TD-DFT (M06-2X/6-31+G(d)) predicted absorption spectra for H2SF, 1, 3, and 1,4-divinylbenzene in toluene.

calculated spectrum of the phenylenedivinylene bridging moiety, 1,4-divinylbenzene. The dimer and model dimer both give an intense absorption at around 300 nm that compares very well with experimental results (Table 1). Note also that, in the case of the dimer 3 and especially in the gas phase (Figure S2, Supporting Information), one can notice an evident shoulder at around 270 nm that is actually absent in the case of the model dimer 1. This shoulder is due to a transition centered on the second phenylenedivinylene moiety present in dimer 3. For comparison, we also report the spectra of the constituent units H2SF and 1,4-divinylbenzene of the dimer. First, we can note that the oscillator strengths of the two constituents are much lower than the ones of the dimer and the model dimer. More in detail, we can observe that the lowest energy transition of H2SF happens at about 265−270 nm with a low intense and very large band. This is once again in good agreement with the experimental values (Table 1). The phenylenedivinylene bridge on the contrary is characterized by a relatively stronger absorption maximum appearing at 280 23948



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Figure 2. First transition NTO couples (M06-2X/6-31+G(d)) of H2SF, 1,4-divinylbenzene, and 1 in toluene. The left NTO is the occupied one, and the right NTO is the virtual one.

nm. It has to be noted the relative good agreement with the experimental results;8 indeed, the main transition was modeled with a relative error of only about 10−15 nm. Moreover, the position of the shoulder due to the absorption of the second unit was correctly previewed as compared to the experiment. The nature of the different excited states can be easily determined by analyzing the corresponding NTOs (Figure 2). Indeed, and as expected, both H2SF and 1,4-divinylbenzene give local transitions that are mainly of π−π* nature. It is also interesting to note the important participation of the Si atom in the case of H2SF. Obviously, the case of the dimers can be much more complex. The occupied and virtual NTOs for the first transition responsible for the ∼300 nm absorption band show that the transition is mainly ascribed to be a π−π* localized in the bridging moiety (phenylenedivinylene). The silafluorene rings are not only innocent spectators. Indeed, some density delocalization over the Si atoms is noticeable both in the occupied and virtual NTOs for the first transition of 1 (Figure 2) and 3 (Figure S3, Supporting Information). The coupling between the bridging and the silafluorene units may explain the redshift of the transition (∼20 nm), compared to the pure π−π* one obtained for 1,4-divinylbenzene alone. The reasons for the important difference in intensities between monomers and oligomers can be related to the difference in the local environment of the chromophoric units that result in an important increase of the value of the transition dipole moments and hence of absorption intensity. The same tendencies in the absorption spectra as before can be observed in the case of the silole-containing monomer (H2silole), model dimer 2, and dimer 4 (Figure 3). Once again, the main absorption peak (∼300 nm) of the dimer and the model dimer is significantly redshifted compared to the 1,4divinylbenzene equivalent one (280 nm). As well as in the previous case, the oscillator strengths are much higher upon dimerization. Interestingly enough, for this class of compounds, we can also notice a lower energy and a much less intense band appearing for the monomer as well as the dimer and the model dimer at around 360 nm. Once again, the analysis of the NTOs can elucidate the nature of the transitions; in particular, we can again ascribe the 300 nm transition of 2 (Figure 4) and 4 (Figure S3, Supporting Information) to a mainly π−π* excitation centered on the bridging unit, and again, some

Figure 3. TD-DFT (M06-2X/6-31+G(d)) predicted absorption spectra for H2silole, 2, and 4 in toluene.

participation of the silole ring is evidenced. The lowest energy transition (∼360 nm) on the contrary appears totally localized on the silole ring (Figure 4), an occurrence that perfectly explains the fact that this transition is almost unaltered upon dimerization. Complexes. Absorption spectra of different analytes are given in Figure S4 (Supporting Information). In particular and coherently with previous work, we considered four aromatic analytes (TNT, PA, DNT, and NB), and one nonaromatic analyte (RDX). In all the cases, they show intense absorption in the UV region, with a weak but not negligible tail at around 320 nm (Figure S4, Supporting Information). Binding of silafluorene- and silole-containing polymers to explosives was investigated by modeling the complexes between nitrocontaining explosives and dimers of these polymers.2 As an illustrative example, the spectra of individual dimer 3, TNT, and their complex, 3−TNT are shown in Figure 5. The main absorption of TNT is centered in the UV region. On the contrary, upon complexation, one can observe an evident modification of the ∼300 nm absorption band characteristic of dimer 3. In particular, the intensities are lowered, and the absorption maxima are very slightly redshifted with a double 23949



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Figure 4. First and main transition NTO couples (M06-2X/6-31+G(d)) of H2silole, 2, and 4 in toluene. The left NTO is the occupied one, and the right NTO is the virtual one.

Figure 5. TD-DFT (M06-2X/6-31+G(d)) predicted absorption spectra for 3, TNT, and 3−TNT in toluene.

Figure 6. TD-DFT (M06-2X/6-31+G(d)) predicted absorption spectra for the complexes between 3 and analytes in toluene.

maxima structure. Interestingly enough, one can now see a lowintense tail centered around 375 nm that was not present neither in the spectrum of TNT alone nor in the one of the dimer. Spectra of the complexes with other explosives (Figure 6) show the same tendency as the spectrum of 3−TNT: strong absorption around 300 nm and a low-absorption tail at longer wavelengths (∼370 nm). As far as the NTO analysis (Figure S5, Supporting Information) is concerned, we may first recognize that the most intense transition (∼300 nm) is invariantly characterized as π−π* and is always localized on the dimer and in particular on the phenylenedivinylene bridge. The participation of the aromatic analyte to this transition is always extremely weak. An important exception is the case of RDX for whom chargetransfer nature can be evidenced going from the bridge of the dimer to the acceptor nitramine group. The fact that, in almost all the cases, the most intense transition is keeping a local nature explains the small change on the absorption spectra maxima upon complexation. It is however interesting to analyze

the lowest energy transition that gives the small intense absorption tail. Indeed, since this tail is due to the first excited state, it will be extremely important to modulate the emission properties of the aggregate, since following Kasha’s rule, emission can be modeled as taking place from the lowest energy excited state, only. In Figure 7 and Figure S6 (Supporting Information), we report NTOs for the first excited state; contrary to the previous transition, we may here observe an impressive charge transfer from the phenylenedivinylene bridge of the dimer to the explosive. Charge transfer is present for all the analytes concerned, especially when they present an aromatic core. Note also that the geometry of the complex is ideal to favor an important charge transfer, since it allows a good overlap between the π systems of the bridge and of the analyte in the case of the aromatic explosives or between the bridge and the acceptor nitramine group in the case of 3−RDX. For a comparison between molecular orbital and NTO analyses, the 23950



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formation of a ground-state complex. There is a significant change in the spectrum of free dimer 3 (red spectrum in Figure 5) when it is complexed with TNT (blue spectrum in Figure 5) indicating a static mechanism. Thus, the complexes formed by the fluorophores have been considered to understand the nature of the excited states. According to Kasha’s rule,22,23 the emission from the excited fluorophore is exclusively happening from the lowest singlet excited state (S1). Any excitation to a higher electronic state will lead to a fast relaxation to the S1 state before the possible emission to the ground state (S0). This can be basically interpreted by the fact that the energy differences between excited states are much lower than the one between the S1 and the S0 states. As an example, in Figure 8, we report the excited-

Figure 7. NTOs (M06-2X/6-31+G(d)) of complexes in the first excited state in toluene.

weights of the principal transitions that contribute more than 10% are given in Table S3 (Supporting Information). From a more quantitative point of view, charge transfer is evidenced by calculating the charges of the analytes in the complexes for both first and main excited states, and the charge-transfer character of these transitions is also evaluated with a descriptor, φS, that allows quantitative representation of the charge-separation nature of a chromophore upon light absorption (Table 2).20,21 All aromatic explosives follow the

Figure 8. Vertical excitation energies (eV) for 3 and 3−DNT.

state energies of the individual dimer 3 and its complex with DNT, 3−DNT. Since the energy difference between S1 and S0 is of the order of 3.5−4.0 eV while the energy difference between the other excited states is at most of the order of some fraction of electronvolts, the validity of Kasha’s rule can be safely assumed. In Table 3, we report the emission wavelengths for the dimer 3 and different complexes with explosives obtained upon the

Table 2. Charges of the Analytes in the Complexes for the Ground State, the First Excited State (ES1), and the Main Excited State (ESmain) and the Overlap Descriptor, φS, of the First and Main Excited States φS

q 3−TNT 3−PA 3−DNT 3−RDX 3−NB a

GS

ES1

ESmain

ES1

ESmain

−0.001 0.0036 0.015 0.029 0.007

−0.869 −0.922 −0.847 −0.414 −0.728

−0.124 −0.084 −0.210

0.357 0.260 0.416 0.804 0.493

0.856 0.867 0.733

a

−0.052

Table 3. Fluorescence Wavelengths (nm) of 3, 3−DNT, and 3−RDX with the Oscillator Strengths in Parenthesesa

a

3 3−DNT 3−RDX

0.869

The first excited state of 3−RDX corresponds to the main transition.

λflu in the gas-phase (nm)

λflu in toluene (nm)

342 (1.697) 622 (0.110) 739 (0.047)

341 (1.778) 718 (0.009) 807 (0.004)

The fluorescence wavelengths of 3−TNT, 3−PA, and 3−NB show the same tendency (shift to ∼600−700 nm and an intense decrease of the oscillator strength) as 3−DNT and 3−RDX. However, these numerical values are not reported; because of the extremely flat potential energy surface of these compounds, only loose convergence criteria were met. a

main trend with the most intense transition showing almost no charge transfer while already at the Franck−Condon region almost one full electron is transferred to the analyte for the first excited state. In the case of 3−RDX, the transition was charge transfer from a chemical point of view, since 0.5 electrons were transferred from different groups. However, as proved by the high φS value, from a spatial point of view, the charge transfer happens at quite a short range. The less amount of electronic charge transferred for this case can be due to the fact that, in the case of aromatic explosives, the whole π system of the analyte was participating to the charge transfer that is now only restricted to the acceptor group. Detection of Explosives by Fluorescence Quenching. The fluorescence quenching process of silafluorene- and silolecontaining polymers occurs due to the formation of complexes in the ground state mentioned earlier. The perturbation of the absorption spectrum of the fluorophore also indicates

first excited-state geometry optimization using TD-DFT, both in toluene and the gas phase. The fluorescence wavelength of 3 alone is calculated at about 340 nm (Table 3) that is close to the experimental λflu taken in toluene (Table 1). Moreover, the emission oscillator strength is quite high suggesting a fully allowed fluorescence, again in agreement with experimental evidence. This is due to π−π* character of the transition. On the contrary, when we consider the case of explosive−dimer complexes, we invariably see that emission wavelength is shifted in the red or infrared region of the spectrum. Moreover, the oscillator strengths become extremely close to zero that implies 23951



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(DPT-2009K120520) and Bogazici University Polymer Research Center (PRC). A.M. thanks CNRS for the funding of the “chaire d’excellence” programm. T.E. thanks the ANR “Balance Supra” for financial support.

that, upon complexation with explosives (whether aromatic or not) and because of the presence of a charge transfer for the first excited state, the fluorescence should be quenched. Anyway, one should be aware of the fact that fluorescence intensities not only depend on the radiative rate (proportional to the oscillator strengths) but also on the nonradiative rate. In the present case, especially for the charge-transfer states characterized by very low-energy emission, the latter can be competitive with the radiative pathways, hence giving rise to even more important quenching phenomena. The photophysical and analytical properties of the complexes can therefore be easily interpreted on that basis. Indeed, as soon as one molecule of explosive complexes with the detecting polymer, it totally quenches emission from one of the emitting units of the polymer (the phenylenedivinylene bridges). This can also explain the linear law relating quenching to the explosive concentration. In addition, since the quenching happens to be total for every explosive tested here, the sensitivity of the polymer will mostly depend on the complexation affinity with different explosives: the higher the affinity, the better the sensitivity.

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(1) Meaney, M. S.; McGuffin, V. L. Investigation of Common Fluorophores for the Detection of Nitrated Explosives by Fluorescence Quenching. Anal. Chim. Acta 2008, 610, 57−67. (2) Dedeoğlu, B.; Aviyente, V.; Ö zen, A. S. Computational Insight into the Explosive Detection Mechanisms in Silafluorene- and SiloleContaining Photoluminescent Polymers. J. Phys. Chem. C 2014, 118, 6385−6397. (3) Sohn, H.; Calhoun, R. M.; Sailor, M. J.; Trogler, W. C. Detection of TNT and Picric Acid on Surfaces and in Seawater by Using Photoluminescent Polysiloles. Angew. Chem., Int. Ed. 2001, 40, 2104− 2105. (4) Risko, C.; Kushto, G. P.; Kafafi, Z. H.; Bredas, J. L. Electronic Properties of Silole-Based Organic Semiconductors. J. Chem. Phys. 2005, 121, 9031−9038. (5) Sanchez, J. C.; DiPasquale, A. G.; Rheingold, A. L.; Trogler, W. C. Synthesis, Luminescence Properties and Explosives Sensing with 1,1Tetraphenylsilole- and 1,1-Silafluorene-Vinylene Polymers. Chem. Mater. 2007, 19, 6459−6470. (6) Sanchez, J. C.; Trogler, W. C. Hydrosilylation of Diynes as a Route to Functional Polymers Delocalized through Silicon. Macromol. Chem. Phys. 2008, 209, 1528−1540. (7) Sanchez, J. C.; Trogler, W. C. Efficient Blue-Emitting Silafluorene-Fluorene-Conjugated Copolymers: Selective Turn-Off/ Turn-On Detection of Explosives. J. Mater. Chem. 2008, 18, 3143− 3156. (8) Sanchez, J. C.; Urbas, S. A.; Toal, S. J.; DiPasquale, A. G.; Rheingold, A. L.; Trogler, W. C. Catalytic Hydrosilylation Routes to Divinylbenzene Bridged Silole and Silafluorene Polymers. Applications to Surface Imaging of Explosive Particulates. Macromolecules 2008, 41, 1237−1245. (9) Sohn, H.; Sailor, M. J.; Magde, D.; Trogler, W. C. Detection of Nitroaromatic Explosives Based on Photoluminescent Polymers Containing Metalloles. J. Am. Chem. Soc. 2003, 125, 3821−3830. (10) Yamaguchi, Y. Design of Novel a σ*-π* Conjugated Polysilanes. Synth. Met. 1996, 82, 149−153. (11) Hohenberg, P.; Kohn, W. Inhomogeneous Electron Gas. Phys. Rev. 1964, 136, B864−B871. (12) Kohn, W.; Sham, L. J. Self-Consistent Equations Including Exchange and Correlation Effects. Phys. Rev. 1965, 140, A1133− A1138. (13) Parr, R. G.; Weitao, Y. Density-Functional Theory of Atoms and Molecules; Oxford University Press: Oxford, U.K., 1989. (14) Zhao, Y.; Truhlar, D. G. The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: Two New Functionals and Systematic Testing of Four M06-Class Functionals and 12 Other Functionals. Theor. Chem. Acc. 2008, 120, 215−241. (15) Reed, A. E.; Weinstock, R. B.; Weinhold, F. Natural-Population Analysis. J. Chem. Phys. 1985, 83, 735−746. (16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, B. M. V.; Petersson, G. A.; Nakatsuji, H.; et al. Gaussian 09; Gaussian Inc.: Wallingford, CT, 2010. (17) Tomasi, J.; Mennucci, B.; Cances, E. The IEF Version of the PCM Solvation Method: An Overview of a New Method Addressed to Study Molecular Solutes at the QM ab Initio Level. J. Mol. Struct.: THEOCHEM 1999, 464, 211−226. (18) Martin, R. L. Natural Transition Orbitals. J. Chem. Phys. 2003, 118, 4775−4777. (19) NAncy_EX Project homepage. http://nancyex.sourceforge.net/.

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CONCLUSIONS The improved optoelectronic properties of silafluorene- and silole-containing polymers have been shown to arise from the increased delocalization of the conjugated bridge through the silicon centers in the silafluorene and silole moieties. The complexation of silafluorene-containing polymers with the analytes produces a tail in the absorption spectrum of the complexes. This tail has been shown to be responsible for the charge transfer from the polymer to the analyte as it should be the case for the static quenching mechanism. The shift of the fluorescence signal of the complexes out of the vis range and the extreme lowering of the oscillator strengths prove the suitability of the silafluorene-containing polymers as chemical sensors for the analytes based on the fluorescence quenching technique.

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ASSOCIATED CONTENT

S Supporting Information *

Tables listing the benchmark study for λabs (nm) of 1 and the percent contributions of molecular orbitals to the related transition; TD-DFT predicted absorption spectra for H2SF, 1, and 3 in the gas phase and in toluene and for the analytes TNT, PA, DNT, RDX, and NB in toluene; NTO couples of 3, 4, and the complexes; and complete ref 16. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*(A.M.) Phone: +33 383684380; e-mail: antonio.monari@ univ-lorraine.fr. *(A.S.Ö .) Phone: +90 2165810050/1362; e-mail: asozen@ pirireis.edu.tr. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS ̇ AK under grant number This project is supported by TÜ BIT 111T174. Computing resources used in this work were provided by the TUBITAK ULAKBIM High Performance and Grid Computing Center, the State Planning Organization 23952



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(20) Etienne, T.; Assfeld, X.; Monari, A. Towards a Quantitative Assessment of Electronic Transitions’ Charge-Transfer Character. J. Chem. Theory Comput. 2014, 10, 3896−3905. (21) Etienne, T.; Assfeld, X.; Monari, A. A New Insight into the Topology of Excited States through Detachment/Attachment Density Matrices-Based Centroids of Charge. J. Chem. Theory Comput. 2014, 10, 3906−3914. (22) Kasha, M. Characterization of Electronic Transitions in Complex Molecules. Discuss. Faraday Soc. 1950, 14−19. (23) McGlynn, S. P.; Azumi, T.; Kinoshito, M. Molecular Spectroscopy of the Triplet State; Prentice-Hall: Englewood Cliffs, NJ, 1969.

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Detection of Nitroaromatic Explosives Based on Fluorescence

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