Turn on of room temperature phosphorescence of donor-acceptor-donor type compounds via transformation of excited states by rigid hosts for oxygen sensing

A series of compounds with the different donor moieties (phenothiazine, phenoxazine and acridine) and 1,4-difluorobenzene unit as the acceptor were developed for optical sensors of oxygen. Among three new compounds, oxygen sensing ability of one compound 10,10'-(2,5-difluoro-1,4-phenylene)bis(10 H-phenothiazine) (2PS-2FPh) exhibiting strong room-temperature phosphorescence in rigid matrix Zeonex shows high sensitivity to low concentration of oxygen (<0.1%). To understand the reason of the high oxygen sensitivity of 2PS-2FPh, the effect of a rigid medium on conformational distortion was studied using a set of experimental and theoretical approaches. Doping 2PS-2FPh into Zeonex results in strong room-temperature phosphorescence in a vacuum with singlet-triplet splitting (ΔE_ST) widened to 0.47 eV as compared to 0.17 eV observed for the solution in 2-methyltetrahydrofuran. Molecular dynamic simulation with 2PS-2FPh doped in Zeonex indicates the increased molecular heterogeneity. Concomitantly, the hybrid local and charge transfer triplet state (³HLCT) becomes more local allowing the room temperature phosphorescence to be observed from locally excited triplet states. These results showed that conformational disorder and rigidity of the medium can significantly widen ΔE_ST of donor-acceptor-donor type molecules transforming ³HLCT into ³LE and thus facilitating the room-temperature phosphorescence and high oxygen sensitivity of 2PS-2FPh.