Hyun-Sook Jang

Unique effects of the chain lengths and anions of tetra-alkylammonium salts on quenching pyrene excimer.

Pyrene (Py) excimer, through its unique fluorescence quenching, exhibits high sensitivity and high selectivity in detecting specific electron-deficient molecules, providing a potential platform for sensing technology, optical switch, and probing hydrophobicity of molecular environment. In solution state, its quenching mechanism has been well-studied. However, there remain many unknown properties regarding the quenching mechanism of the solid-state Py excimer. In this paper, the effects of a series of tetra-alkylammonium salts (with a variety of chain lengths and anions) on Py excimer quenching are investigated to identify the controlling parameters of the fluorescence quenching in the binary system. Several experimental approaches including steady-state fluorescence spectroscopy, UV absorption, (13)C-nuclear magnetic resonance (NMR) spectra, X-ray diffraction, scanning electron microscopy, and time-dependent fluorescence decay are employed to seek for the fundamental understanding of the quenching mechanism. The result indicates a unique quenching effect of tetrabutylammonium cation on the pyrene excimer, and which is not observed in the other cations with different chain lengths (the same associated hexafluorophosphate anions). Meanwhile, hexafluorophosphate anion (in the presence of tetrabutylammonium) is able to effectively retain Py excimer fluorescence when the system is prepared by evaporating solvent at high temperature. It is also confirmed that dynamic quenching is involved in the process. Hydrophobic environment around Py molecules shows strong correlation with the formation of Py excimer. The knowledge obtained in this study provides the insights to how the interaction between salt and Py molecule affects the excimer fluorescence.

Insight into the interactions between pyrene and polystyrene for efficient quenching nitroaromatic explosives

A pyrene (Py) in polystyrene (PS) matrix shows rapid fluorescence quenching in the presence of 2,4-dinitrotoluene (2,4-DNT). The fluorescence quenching does not occur in the absence of the PS matrix, implying that the molecular architecture of the PS chain is critical. However, the function of the PS matrix is not well understood. Here, we investigate various Py/PS binary thin films containing PS of diverse molecular architecture (i.e., linear, centipede and 4-arm star) and molecular weight (i.e., 2.5, 35, 192, 350 and 900 kDa) to understand the effects of these molecular descriptors on the fluorescence quenching. The findings suggest that the electron-rich nature of Py/PS facilitates the photoinduced electron transfer from Py/PS to the electron-deficient 2,4-DNT, resulting in effective quenching of Py excimers. Moreover, cyclic voltammetry (CV) and UV-vis absorption verified that PS reduces the lowest unoccupied molecular orbital level of Py, promoting the Py excimer quenching efficiency in the presence of nitroaromatic molecules. The quenching process is found to be independent of molecular architecture and molecular weight, suggesting that energy migration along the PS backbone may not be the key mechanism. This simple and concise concept provide the insight into the selection for highly-efficient sensing materials.