Phenothiazinyl merocyanine dyes with variable substitution patterns on the peripheral benzene ring were synthesized in good yields by Knoevenagel condensation of the corresponding phenothiazinyl aldehydes and N-methylrhodanine or indan-1,3-dione. The electronic properties were investigated by cyclic voltammetry, absorption, electro-optical absorption, and emission spectroscopy. All these merocyanines reveal reversible redox behavior that stems from the phenothiazinyl-centered oxidation to give stable radical cations. The redox potentials strongly correlate with Hammett σ(p) parameters. All merocyanines reveal large Stokes shifts. They also display a pronounced emissive solvatochromism, which is caused by large dipole moment changes upon excitation from the ground to the excited state. These findings are supported by solvatochromism studies and time-dependent DFT computations.

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.

Dumbbell-shaped diphenothiazines bridged by conjugatively linked (hetero)aromatic moieties were synthesized in a modular fashion by Suzuki-Miyaura coupling in good yields. The electronic structure was studied by DFT computations, determining the geometry optimized lowest energy conformers and scrutinizing the Kohn-Sham FMOs. The torsional deviation from coplanarity is predominantly influencing the electronic structure, i.e., by deviation from ideal overlap and maximal electron transmission. The reversible oxidation potentials assigned to the phenothiazinyl electrophores in most cases can thereby be qualitatively rationalized. All dumbbell-shaped diphenothiazines are strongly luminescent, which can be attributed to extended π-electron conjugation with considerable excited state electronic coupling as a consequence of large structural and electronic distributional changes upon photoexcitation.