Phenothiazine Cruciforms:  Synthesis and Metallochromic Properties

We report the synthesis and characterization of five novel phenothiazine-containing cruciforms (5−9). The targets were prepared by a NaH-promoted Horner reaction of tetraethyl(2,5-diiodo-1,4-phenylene)bis(methylene)diphosphonate with 10-hexyl-10H-phenothiazine-3-carbaldehyde. The formed intermediary 3,3‘-(1E,1‘E)-2,2‘-(2,5-diiodo-1,4-phenylene)bis(ethene-2,1-diyl)bis(10-hexyl-10H-phenothiazine) was reacted with several different aromatic alkynes (1-tert-butyl-4-ethynylbenzene, N,N-dibutyl-4-ethynylaniline, 1-ethynyl-3-(trifluoromethyl)benzene, and 1-ethynyl-3,5-bis(trifluoromethyl)benzene) to give the corresponding cruciform fluororphores (XF). The XFs were fully characterized by NMR and IR spectroscopy and then exposed to trifluoroacetic acid as well as to several metal triflates. The XFs show dramatic shifts in emission and to a lesser extent in absorption when exposed to magnesium triflate or zinc triflate. In the case of magnesium triflate, a blue shift in emission was observed; in contrast, addition of zinc triflate results in either quenching or a red-shifted emission. Due to the electronic situation, these XFs display spatially separated frontier molecular orbitals, allowing the HOMO or the LUMO of the XFs to be addressed independently by addition of zinc or magnesium ions. Phenothiazine XFs may have potential in array-type sensory applications for metal cations.