Light induced fluorescence enhancement in MEH-PPV thin films

Influence of Air Flow on Luminescence Quenching in Polymer Films towards Explosives Detection Using Drones

Explosive detection has become an increased priority in recent years for homeland security and counter-terrorism applications. Although drones may not be able to pinpoint the exact location of the landmines and explosives, the identification of the explosive vapor present in the surrounding air provides significant information and comfort to the personnel and explosives removal equipment operators. Several optical methods, such as the luminescence quenching of fluorescent polymers, have been used for explosive detection. In order to utilize sensing technique via unmanned vehicles or drones, it is very important to study how the air flow affects the luminescence quenching. We investigated the effects of air flow on the quenching efficiency of Poly(2,5-di(2′-ethylhexyl)-1,4-ethynylene) (PEE) by TNT molecules. We treated the TNT molecules incorporated into the polymer film as non-radiative recombination centers, and found that the time derivative of the non-radiative recombination rates was greater with faster air flows. Our investigations show that relatively high air flow into an optical sensing part is crucial to achieving fast PL quenching. We also found that a “continuous light excitation” condition during the exposure of TNT vapor greatly influences the PL quenching.

Continuous Illumination of a Conjugated Polymer Causes Strong Enhancement of Photoluminescence

We present measurements of absorbance and photoluminescence (PL) for films of poly(3-(2,5-dioctylphenyl)thiophene) (PDOPT) as a function of temperature (T) and time (t) of illumination. While having no detectable influence on absorbance of this conjugated polymer, our experiments clearly revealed that illumination of PDOPT caused a significant increase in the PL intensity (I_PL(T,t)), that is, the emission probability of PDOPT. Without illumination, we always observed a decrease in I_PL with time. An increase in I_PL was only detectable when the sample was illuminated. Interestingly, while absorption and emission of photons occur on a time scale of nanoseconds, the here-reported changes in the emission probability were slow and occurred on a time scale of minutes to hours. The influence of illumination on changes in I_PL(T,t) was qualitatively similar for slowly and rapidly crystallized PDOPT, that is, the degree of crystallinity was not decisive for the observation. The rate of the increase in I_PL depended clearly on the power of the illumination light source. As a function of the illumination time, the change in I_PL(T,t) was nonmonotonic and depended on sample temperature. We speculate that changes in polymer interactions caused by excited electronic states might have induced slow changes in polymer conformations.