Spectroscopic studies on Brooker's merocyanine

The origin of the multiple absorption bands of Brooker's merocyanine, 1- (4-aza-4-methylphenyl)-2-trans-(4-oxyphenyl)ethene, found in the visible region of the spectrum in solvents such as chloroform and tetrahydrofuran, has been explored experimentally using H-1 and C-13 NMR spectroscopy and theoretically using molecular orbital methods. While NMR analysis rules out the presence of a cis conformer, AM1/COSMO calculations in various solvents strongly suggest that there are a number of stable nonplanar conformations of the merocyanine as well as a dimer which are predicted to absorb at longer and shorter wavelengths, respectively, than the favored planar conformer. Spectroscopic analysis at variable temperature is supportive with the intensity of the longer wavelength absorption initially reducing with decreasing temperature, followed by an increase in the concentration of dimers and possibly aggregates. Further support is provided by synthesis of the corresponding nonplanar merocyanine containing methyl groups at the double bond and at the ortho positions of the oxyphenyl ring, which absorbs at longer wavelength than its related planar analogue.