Phenothiazine derivatives - synthesis, characterization, and theoretical studies with an emphasis on the solvatochromic properties

Abstract A series of ethynylated N-octylphenothiazine aldehydes (PTZ1–PTZ4) with terminal substituents of different electronic ability (p-methoxyphenyl, p-fluorophenyl, p-trifluoromethylphenyl and 2,5-bis(trifluoromethyl)phenyl) acting as A/D-π-D-A system were synthesized and fully characterized. The influence of these terminal groups on their photophysical, thermal, and biological properties was thoroughly examined and confronted with theoretical calculations. The novel compounds exhibit high thermal stability up to 238–318 °C. Compounds showed an intramolecular charge transfer (ICT) absorption band at about 399 nm and emission band at 535 nm with quantum yields between 40 and 63% in chloroform solution. All compounds exhibit positive solvatochromism in the full range of solvent polarities from hexane to DMSO, whereas in the methanol solution they demonstrate the hypsochromic shift of emission maximum compared to DMSO with significant fluorescence quenching. PTZ1–PTZ4 compounds possess a significant emission from blue to yellow region 488–542 nm, high quantum yields 24–91%, and lifetimes ranging from 6.03 to 8.56 ns in solvents of different polarity. The marked positive solvatochromism indicates the considerable ICT in PTZ1–PTZ4 compounds. A noticeable increase in dipole moment in the excited state calculated in various solvent causes o significant red shift in the emission spectrum of PTZ1–PTZ4. This work demonstrates the strong variation of photophysical properties with a simple alteration of the chemical structure of the phenothiazine framework by terminal groups. The toxicity of the compounds that were obtained was evaluated on cancer as well as normal fibroblast cell lines. The applicatory potential of those phenothiazines as cellular probes have been also evaluated.