PHOTOCHROMIC CROWN-ETHER COMPLEXES - A RAMAN-SPECTROSCOPIC STUDY

Resonance and preresonance Raman spectra have been obtained in the region 400-1700 cm-1 for some benzothiazolium and indolinium steryl dyes containing a crown ether ring. Spectra arising from the trans isomers are observed selectively due to the resonance effect, and the principal features can be attributed to modes of the central conjugated Ph-N+ = C-C = C-Ph unit present in each of these molecules. Complex formation between the dye molecules and Mg2+ in acetonitrile solution results in intramolecular electron transfer. This is observed in the Raman spectra as a downshift of a band assigned to Ph-O vibration in the crown ether unit, and upshifts of several bands associated with the Ph-N+ = C-C = C-Ph unit, including the phenyl ring, C = C and +N = C stretches. The results demonstrate the sensitivity of the Raman spectra to changes in the structure and bonding within these photochromic complexing agents on binding to metal ions, and indicate that they may serve as a useful probe for the complicated photoisomerization and complexation reactions of these interesting systems.