FUNDAMENTAL-STUDIES ON BROOKERS MEROCYANINE

The spectroscopic properties of solvent soluble derivatives of Brooker 's simple merocyanine yridinylidene)ethylidene]-2,5-cyclohexadien-1-on e, which can in principle exist in two distinct canonical forms, have been assessed both experimentally and theoretically using molecular or bital methods. H-1 and C-13 NMR evidence in a range of solvents sugges ts that the merocyanine exists as a resonance hybrid which is weighted toward the zwitterion even in solvents of low dielectric constants. I n protic solvents, the large hypsochromic shift observed for the meroc yanine in the visible region arises from both a dielectric effect and a hydrogen bonding effect. Theoretically, the PM3/COSMO method gives a reasonable account of the structure and spectroscopic shifts of the m erocyanine in aprotic solvents, ?The large shifts observed arise becau se solvents with large dielectric constants have a much greater stabil izing effect on the more polar ground state of the merocyanine than th ey do on the first excited state. While the same method predicts stabl e hydrogen-bonded structures for a dihydrate and hexahydrate, it is un able to reproduce the known hypsochromic shift for these solvated spec ies, In contrast, a version of the CNDO/S method does predict the corr ect trends on hydration though the magnitude of the effect is less tha n that found experimentally.