The chromoionophore trans-2,4-dinitrostilbene-4′-monoaza-18-crown-6 (DMC) in CH3OH is characterized by a long-wavelength absorption (λmax = 476 nm) which is attributed to a charge transfer (CT) transition from the amine function to the nitro substituents. On protonation or complexation with Ba2+ or Pb2+ the amine is blocked and the CT transition is replaced by a ππ* transition of the stilbene moiety at higher energies. The photochemistry of all species is in accord with these assignment. While the photolysis of DMC leads to the formation of an isatogen, DMCH+, [Ba(DMC)]2+ and [Pb(DMC)]2+ undergo trans-cis photoisomerization of the stilbene moiety.