Benzothiazole‐Based Cycloplatinated Chromophores: Synthetic, Optical, and Biological Studies

Cycloplatinated complexes based on 2‐(4‐substituted)benzothiazole ligands of type [Pt(R‐PBT‐κ C , N )Cl(L)] (PBT=2‐phenylbenzothiazole; R=Br ( 1 ), Me 2 N ( 2 ); L=dimethyl sulfoxide (DMSO; a ), 1,3,5‐ triaza‐7‐phosphaadamantane (PTA; b ), triphenylphosphine 3,3′,3′′‐trisulfonate (TPPTS; c )) and [Pt(Br‐PBT‐κ C )Cl(PTA) 2 ] ( 3 ) are presented. On the basis of the photophysical data and time‐dependent (TD)‐DFT calculations ( 1 a and 2 a ), the low‐lying transitions (absorption and emission) were associated with ligand‐center (LC) charge transfer, with minor metal‐to‐ligand charge transfer (MLCT), and intraligand charge transfer (ILCT) [Me 2 N‐PBT→PBT] excited states, respectively. Simultaneous fluorescence/phosphorescence bands were found in fluid solutions (and also in the solid state for 2 a ), which become dominated by triplet emission bands in rigid media at 77 K. The effect of the concentration on emissive behavior of 2 a ,  b indicated the occurrence of aggregation‐induced luminescence properties related to the occurrence of metal–metal and π⋅⋅⋅π interactions, which are more enhanced in 2 a because of the less bulky DMSO ligand. The behavior of 2 a toward para ‐toluenesulfonic acid (PTSA) in aerated acetonitrile and to hydrogen chloride gas in the solid state has been evaluated, thus showing a clear reversible change between the 1 ILCT and 3 LC/ 3 MLCT states due to protonation of the NMe 2 group (theoretical calculations on 2 a‐H + ). Solid 2 a undergoes a surprising oxidation of the Pt II center to Pt IV with concomitant deoxygenation of DMSO, under prolonged reaction with hydrogen chloride gas to afford the Pt IV /dimethyl sulfide complex ( mer ‐[Pt(Me 2 N‐PBT‐ κC , N )Cl 3 (SMe 2 )]; mer ‐4 ), which evolves in solution to fac ‐4 , as confirmed by X‐ray studies. Cytotoxic activity studies on A549 and HeLa cell lines indicated cytotoxic activity of 1 b and 2 a ,  b . In addition, fluorescent cell microscopy revealed cytoplasmic staining, more visible in perinuclear areas. Inhibition of tubulin polymerization by 1 b in both cells is presented as a preliminary mechanism of its cytotoxic action.