Dual emission fluorescence/room-temperature phosphorescence of phenothiazine and benzotrifluoride derivatives and its application for optical sensing of oxygen

Abstract Heavy atom-free derivatives of para and ortho isomers of benzotrifluoride and 3,7-di-tert-butyl-phenothiazine were designed, synthesized and studied as active materials for optical sensors of oxygen using both theoretical and experimental approaches. Using these moieties and substitution patterns, we managed to demonstrate room-temperature phosphorescence and to study the effect of the substitution pattern on the triplet emission. Due to the donor-acceptor molecular structure of the synthesized compounds, emission spectra of their solutions have single broad bands resulting from the recombination of intramolecular charge transfer excitons. Dual emission was detected when the derivatives were in solid-state (for pure or doped films) under an inert atmosphere. Having lifetimes of 39 and 40 milliseconds and strong sensitivity to oxygen, the emission was identified as room-temperature phosphorescence. High ratios of intensities (17.44 and 6.88) of room-temperature phosphorescence to fluorescence were observed for the compounds which suggest their potential application as active materials for optical sensors of oxygen. Oxygen sensing abilities of both compounds were tested. Practically the same phosphorescence intensity after several cycles of evacuation and refilling was recorded for one of the designed compounds, thus demonstrating reversibility of oxygen sensitivity and fast oxygen response. The oxygen sensitivity of the ortho isomer was found to be 60.8 at oxygen concentrations lower than 0.2 %.