Dual functional and multiple substituted fluorescent star-shaped POSS for a 1 + 1 > 2 explosive vapour detection

Abstract

A dual functional and multiple substituted fluorescent star-shaped POSS has been designed and synthesized. There is effective FRET between the different functional arms. Compared with a single functional probe, the dual functional POSS showed a much better sensing performance to explosive NG and a comparable performance to TNT vapor.

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Polyhedral oligomeric silsesquioxanes (POSS) with rigid cage-like nanostructures comprising an inorganic Si–O–Si core have been commonly used for luminescent material design.^1,2 They possess high thermal and mechanical resistance, together with a rigid steric configuration, and often act as end groups or connecting groups in macromolecules or polymers to increase the incorporated stability and regulate the vapor permeability.^3,4 However, only a few groups have reported the application of POSS as the core structure of a star-shaped molecule. Several groups have introduced pyrene,^5,6 fluorene,^7,8 vinyl benzene,⁹ metal complexes¹⁰ and other donor–acceptor emitters¹¹ on the POSS core to obtain hyperbranched molecules and polymers. These materials could be used in OLEDs, microporous materials, and cellular imaging, and it has been proven that the emission wavelength, charge nature, and diameter of POSS-based fluorescent nanoparticles can be easily tuned via chemical modification of fluorescent arms on the POSS unit. However, to the best of our knowledge, no bifunctional fluorescent branch and multiple substituted POSS (BFA-POSS) have been reported. More functional units on POSS will bring about more functionality and we plan to design and synthesize BFA-POSS for multiple explosives detection, which is difficult to be actualized with one sensing material. Herein, trinitrotoluene (TNT) and nitroglycerin (NG) were chosen as the explosive target as they are two types of very important explosives in terrorist activities.¹²

Recently, we reported that an eight triphenylaminopyrene (PT) substituted polyhedral octavinylsilsesquioxane (POVS) could be applied as high efficient nitrate esters, including an NG probe.¹³ Herein, we also introduce PT as one of the sensing units for nitrate ester detection. Fluorene is known as a violet-blue light emitting material, which has a higher LUMO level relative to TNT. It will emit less short wavelength fluorescence that may experience effective fluorescence resonance energy transfer (FRET) to PT upon photo excitation and therefore magnify the fluorescence of PT and increase the sensing performance.^14–16 Our intention was to excite PT units by dioctylfluorene (DOF) to maximize the efficiency of inside the molecule. We assumed the proportion of DOF is about 4 or 5 fold PT to acquire enough excitation energy of the PT unit (Fig. 1). For comparison their properties, five DOF and PT substituted POSS were also designed and synthesized.

Fig. 1 Chemical structure of P5F1PT.

The synthetic route of the three probes are shown in Scheme 1, taking the reaction conditions reported as a reference.^13,17 DOF-Br and PT-Br were obtained by alkylation and the Suzuki reaction and then coupled relatively with POVS at a feed ratio of 5 : 1 in dried dioxane, using Pd₂(dba)₃ as a catalyst, and Cy₂NMe and P(t-Bu)₃ as ligands. After a column separation, POSS-5DOF and POSS-5PT were both obtained in a yield of 50%. A further reaction of POSS-5DOF with 1 equiv. of PT-Br proceeded with same conditions, provided P5F1PT in a yield of 37%.

Scheme 1 Synthesis of POSS-5DOF, POSS-5PT and P5F1PT.

MALDI-TOF showed the as-synthesized POSS-5PT contains five, six and seven substituted substances. The POSS-5DOF sample consisted of both five and six substituted substances and the P5F1PT sample contained a majority of P5F1PT and a minority of P6F1PT. The ¹H NMR and mass spectra are shown in Fig. S1 and S2.† These results confirmed that by means of precise feed ratio control, diverse functional units could be introduced onto POSS by the simple Heck coupling reaction with decent accuracy and purity for application.

Fig. 2 presented the optical properties of three molecules in the thin film state. The spectra in solution state are shown in Fig. S6.† The surface morphology of the thin films are shown in Fig. S7† by SEM analysis. Fig. 2a shows that the emission spectrum of POSS-5DOF overlaps well with the absorption peak of POSS-5PT, which ensured the FRET between these two units. Fig. 2b shows the emission spectra of P5F1PT excited at 330 nm and 395 nm. Upon excitation at 330 nm, FRET between DOF and PT caused the emission peak of DOF units in 413 nm almost disappeared, and a strong emission band at 498 nm could be observed. While upon excitation at 395 nm, only a much lower emission band of PT was witnessed that proved the efficient intramolecular FRET inside the P5F1PT molecule from the DOF to PT branch.

Fig. 2 (a) Absorption and fluorescence spectra of POSS-5DOF (blue) and POSS-5PT (green) in the thin film state on quartz plates. (b) The absorption spectra (black) and the emission spectra of P5F1PT under UV light of 330 nm (red) and 395 nm (blue) in thin films on quartz plates. Both emission spectra were under same the conditions, and normalized by the max intensity.

We also found that the emission peak of 498 nm was a bit bluer than that of POSS-5PT. It could be easily interpreted that in P5F1PT, DOF units dominate the arms, while the PT branch was highly separated to prevent an intermolecular aggregation or excimer between PT units compared with POSS-5PT.

Calculations based on Dmol³ using Materials Studio software showed that both the LUMO and HOMO orbits of P5F1PT were distributed on the PT unit as presented in Fig. 3. This matched well with the results of the fluorescence spectra that energy would almost entirely transfer from DOF units to the PT unit on the basis of the FRET effect. Moreover, by the calculation, the diameter of the P5F1PT molecule is ∼4 nm, enough for FRET to happen effectively.

Fig. 3 Schematic of HOMO (left) and LUMO (right) orbit distribution of P5F1PT.

The sensing properties of the three probes were investigated, as shown in Fig. 4 and 5. The quenching efficiency of POSS-5DOF to saturated nitroglycerin (NG) vapor was only ∼25%, whereas it reached 72% to TNT vapor under the same conditions, implying it is more suitable as a TNT sensor. An inverse tendency was found for POSS-5PT, the quenching efficiency of which was 83% in NG vapor and 45% in TNT vapor suggesting it is preferable as a NG sensor. Moreover, when it comes to P5F1PT, an efficiency of 69% for TNT and 91% for NG was found, as shown in Fig. 5, which is excellent for both explosive sensing. It is noticeable that compared with POSS-5PT, P5F1PT even showed an 8% enhancement in NG sensing even though POSS-5PT has five PT chains and P5F1PT only has one.

Fig. 4 Quenching efficiency of POSS-5DOF (a) and POSS-5PT (b) in air upon exposure to TNT and NG vapor.

Fig. 5 Quenching efficiencies of P5F1PT in air upon exposure to saturated explosives vapor. The inset images is the original thin film on a quartz plate (left), their fluorescence responses after exposure to TNT (middle) and NG (right) for 300 s relatively. The schematic below shows the comparison of the sensing performance to TNT (red) and NG (blue).

This phenomenon may come from the efficient intramolecular FRET after the photo excitation due to its very small donor–acceptor distance, followed by an efficient photo induced charge transfer from P5F1PT to NG molecules (HOMO and LUMO levels calculated by Materials Studio and cyclic voltammetry are shown in Fig. S3 and S4†). The even better sensing performance of P5F1PT relative to POSS-5PT may lie in two aspects. First, for P5F1PT, the FRET between DOF and PT makes the sensing capability towards NG as a whole. The energy transferred from DOF multiplied the fluorescence efficiency of PT, making the sensing performance comparable with multi-PT-substituted POSS. Second, from Table 1, the 14 nm red shift relative to P5F1PT suggests stronger intermolecular aggregation of POSS-5PT in the solid state, which will decrease the fluorescence efficiency and corresponding sensing performance of the probe. The low aggregation in P5F1PT results from the dilution effect of five DOF units, which makes it exhibit a better sensing performance. Such a dual functionality arm has a 1 + 1 > 2 effect for NG sensing due to the highly efficient intramolecular energy transfer.

Table 1 Optical properties of POSS-5DOF, POSS-5PT and P5F1PT

	Absorption	Emission	Quantum efficiency	Extinction coefficient
POSS-5DOF	329	413	0.82	4.80
POSS-5PT	395	512	0.73	5.26
P5F1PT	330/395	498	0.58	4.75

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To clarify the sensing process, time-resolved fluorescence decay measurements of three probes were carried out to monitor the fluorescence lifetime changes before and after the sensing process.^18,19 As Table S1† shows, the lifetime of POSS-5DOF remained almost unchanged after response to TNT vapor, whereas that of POSS-5PT showed a significant decrease upon contact with NG, which can be proven to be due to static quenching and collisional quenching, respectively. However, P5F1PT demonstrated a slightly decreased lifetime in TNT and an even shorter lifetime in NG than that in TNT. The lifetime change of P5F1PT is less than that of POSS-5PT in NG. This result indicated a combination of two quenching mechanisms occurred for P5F1PT in the explosive detection.

In summary, a dual functional and multiple substituted fluorescent star-shaped POSS has been designed and synthesized. There is effective FRET between the different functional arms. Compared with the POSS probe with single function, the dual functional POSS showed a much better sensing performance to explosive NG and a comparable performance to TNT vapor.

It can be observed that the POSS structure plays a crucial role in the sensing performance towards multiple functional materials. By introducing a different functional arm, the functionality will be enhanced by interchain interaction of the different functional arm. The multi-site skeleton enables multiple types of sensing units to assemble in a single molecule, which may find its use in many fields such sensors, OLEDs and other photoelectronic devices.

Acknowledgements

We thank the research programs from the National Natural Science Foundation of China (Grant No. 61325001, 21273267, 61321492 and 51473182).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra08686a

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