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Title: An experimental and theoretical study of the solvatochromism of merocyanine dyes
Author: Morley, R. M.
Awarding Body: University of Wales Swansea
Current Institution: Swansea University
Date of Award: 1998
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The spectroscopic properties of solvent soluble derivatives of Brooker's merocyanine 4-[(1-methyl-4-(1H)-pyridinylidene)-ethylidene]-2,5-cyclohexadien-1-one, that can in principle exist in two distinct canonical forms, have been assessed both experimentally and theoretically using molecular orbital methods. 1H and 13C NMR evidence in a range of solvents suggests that the merocyanine exists as a resonance hybrid which is weighted towards the zwitterion even in solvents of low dielectric constants. In protic solvents, the large hypsochromic shift observed for the merocyanine 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 merocyanine in aprotic solvents. The large shifts observed arise because solvents with large dielectric constants have a much greater stabilising effect on the more polar ground state of the merocyanine than they do on the first excited state. While the same method predicts stable hydrogen bonded structures for a dihydrate and hexahydrate, it is unable to reproduce the known hypsochromic shift for these solvated species. In contrast, a version of the CNDO/S method does predict the correct trends on hydration though the magnitude of the effect is less than that found experimentally. The origin of the two main absorption bands found in the visible region of the spectrum of Brooker's merocyanine in solvents such as chloroform and tetrahydrofuran has been explored experimentally using 1H and 13C 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 non-planar conformations of the merocyanine which are predicted to absorb at a longer wavelength than the favoured planar conformer. Spectroscopic analysis at variable temperature is supportive with the intensity of the longer wavelength absorption reducing with decreasing temperature.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available