Pre-resonance Raman studies of some mesogens - TB4A, TB7A and TB10A

Pre-resonance Raman spectra of certain liquid crystalline compounds, TB4A, TB7A and TB10A, are reported. The anomaly observed in the pre-resonance Ram an spectra in the three compounds was initially explained by the Albrecht-H utley Model, but its failure leads us to explain the anomalous intensity en hancement mechanism by invoking an interference effect between a weak dipol e-forbidden excited state and a nearby strong electronic level via vibratio nal modes. The presence of a 2(1)A(g) dipole-forbidden excited state in TB4 A and TB10A is deduced after an elaborate discussion of their centro-symmet ric structure. The structural difference of TB7A in comparison with TB4A an d TB10A is also discussed by taking their X-ray data into consideration.