Effects of the introduction of tertiary amino linkages on the properties of phenothiazine derivatives as delayed emission fluorophores

Two phenothiazine-based electroactive compounds were synthesized by Buchwald-Hartwig cross-coupling reaction. The compounds consist of pyridine- or benzonitrile accepting moiety and phenothiazine donor fragments linked through tertiary amino linkage. The resulting electroactive compounds showed high thermal stability with 10% weight loss temperatures of 270 and 308 oC. Both the compounds showed reversible oxidation after repeated scans of cyclic voltammetry. Experimental results in combination with the outcomes of TD-DFT calculations, were employed to examine the emission nature of the target compounds. By demonstrating the correct choice of acceptor moiety and linking topology, the synthesized compounds were found to be emitters exhibiting efficient RTP. With emission lifetime reaching 26.04 ms and spin-orbit coupling value of the triplet states with the singlet ground state of 18.72 cm-1, the radiative recombination of the triplet state is ensured. Having lifetimes of 26 ms and strong sensitivity to oxygen, the benzonitrile-containing phenothiazine derivative demonstrated fast oxygen response with the quenching constant of 4.97*10-4 ppm-1 at low oxygen concentrations that is comparable to the corresponding characteristics of reported RTP-based sensors.