This paper is devoted to the study of the photodegradation of pi-conjugated fluorescent materials used as chemical sensors for explosives detection. The selected pi-conjugated fluorescent material was a phenylene-ethynylene diimine with di-ether substituents (PH-DE-DI). A rapid disappearance of the fluorescence emission occurred when the material was exposed to solar light in ambient air. To understand this degradation, modifications to the chemical structure of thin films exposed to UV-visible irradiation were characterised using various techniques. HS-SPME-GC-MS experiments showed that sample irradiation induced the release of numerous volatile products in a relatively high abundance. IR spectroscopy coupled with derivatisation treatments allowed the identification of the photoproducts trapped in the irradiated thin films. To improve the understanding of the degradation mechanism of PH-DE-DI, the present paper also focused on the photo-ageing of phenylene-ethynylene materials: (i) without the end diimine function and the di-ether groups and (ii) the PH-DE-DI polymer. Based on the identification of several photoproducts, the primary pathways of photodegradation involving a chain radical oxidation process were discussed.