A terthiophene (3T) derivative of 5-(1-pyrenyl)-2,2 0 :5 0 ,2 00 -terthiophene (Py-3T) was synthesized and chemically immobilized onto a glass wafer surface via a flexible spacer by employing a single-layer chemistry technique. Unlike the film fabricated in the same way but with 3T as the fluorophore, the film fabricated in the present study possesses unprecedented photochemical stability at ambient conditions. Fluorescence studies revealed that the emission of the film as fabricated is significantly and selectively quenched by the presence of nitroaromatic compounds (NACs), a group of typical explosives, both in the vapor phase and in aqueous solution. Experimental and theoretical studies demonstrated that the quenching may be a result of electron transfer from the electron-rich Py-3T to the electron-deficient NACs. It was found that for vapor phase sensing, the response time and the quenching efficiency of the systems are dominantly determined by the vapor pressures of the NACs tested. The sensing performances of the film to NACs in aqueous phase were also investigated. In this case, however, the specific binding of the film to picric acid (PA), a typical NAC, makes the compound show a superior quenching efficiency than other NACs. Moreover, the response is fast and reaches equilibrium within 90 s. Furthermore, acids, bases, apple juice, perfume, and commonly found organic solvents etc. show little effect upon the sensing in aqueous phase. Both the vapor phase sensing and the aqueous solution sensing are reversible. Furthermore, the film is stable for at least 6 months provided it is properly preserved. The basic contribution of the present work is not only creating a new fluorescent film of superior sensing properties to NACs in the vapor phase, in particular to PA in the aqueous phase, but also providing a new photochemically stable fluorophore, which may combine the advantages of small molecular fluorophores and those of conjugated polymers/oligomers, for developing new fluorescent sensing films.
A new fluorescent derivative of cholesterol, N,N'-(N-(2-(3β-cholest-5-en-3yl-formamido)ethyl) pyrene-1-sulfonamido)ethyl perylene-3,4:9,10-tetracarboxylic acid bisimide (CPPBI), was designed and synthesized. In the design, pyrene (Py) and perylene bisimide (PBI) were specially chosen as the energy donor and the acceptor, respectively. Fluorescence studies revealed that (1) CPPBI shows a strong tendency to form supra-molecular assemblies, (2) the assemblies possess a high efficiency of fluorescence resonance energy transfer (FRET) via intermolecular interactions, and (3) the profile and position of its fluorescence emission are highly dependent upon the nature of its medium, but the medium shows little effect on the efficiency of the energy transfer, suggesting that the chromophores including both Py and PBI units enjoy some rotational and/or translational mobility in the aggregated state of the compound. Temperature- and concentration-dependent (1)H NMR spectroscopy studies revealed that both hydrogen-bonding and π-π stacking play a great role in stabilizing the assemblies of the compound, and confirmed the existence of π-π stacking between the Py moieties and between the PBI residues of the compound, of which the donor and the acceptor may have arranged in an appropriate orientation and at a suitable distance which are the key factors to determine the FRET efficiency. Moreover, the CPPBI-based film possesses unusual photochemical stability, and its emission is sensitive to the presence of some organic vapors, in particular aniline.
A butterfly-shaped pyrene derivative of cholesterol, namely, N,N'-(ethane-1,2-diyl)-bis(N-(2-(chol-amino)ethyl)pyrene-1-sulfonamide) (ECPS), has been designed and synthesized. Solvent effect studies revealed that in good solvents such as n-hexane, benzene, and 1,4-dioxane, the profile of the fluorescence emission of the compound is characterized by pyrene monomer emission, but in poor solvent such as water, the emission is dominated by pyrene excimer emission. Quantitatively speaking, the ratio of the excimer emission to monomer emission changes from 50 to 0 when ECPS is dissolved in water and n-hexane, respectively. In contrast, for a commonly used polarity probe pyrene, the ratio of I3/I1 varies only from ~0.6 to ~1.7, where I3 and I1 stand for the intensities of the fluorescence emission at peak 3 and peak 1, respectively. This value suggests that a more powerful discriminating ability of the new compound in polarity sensing. Furthermore, unlike the main components of the compound, pyrene and cholesterol, its main chain is composed of multiple hydrophilic structures, and it is this structure that makes the emission of the compound in organic solvents sensitive to the presence of water. Accordingly, the applicability of the compound in determination of the trace amount of water in some organic solvents was evaluated. As expected, the detection limit of the compound toward water in acetonitrile reaches 7 ppm, a result never reached before. Furthermore, the fluorescence emission of the compound is also sensitive to viscosity variation. Therefore, it is assumed that ECPS may be used both as a polarity probe and a viscosity probe. On the bases of a series of steady-state and time-resolved fluorescence, as well as dynamic light scattering studies, a structural model was proposed to rationalize the fluorescence behavior of the compound in different solvents and its polarity and viscosity probing performances.
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