Correlation of Electronic Properties and Raman Spectra Within a Series of Polymethine Dyes

Abstract

The vibrational spectra obtained for a series of polymethine dyes show clear effects of π-electron derealization. The polymcthines have a structure of the form

$$ {\text{ > }}{{{\text{N}}}^{{\text{ + }}}}{\text{ = CH - (CII = CH}}{{{\text{)}}}_{{{\text{n - 1}}}}}{\text{ - N < }} $$

((1))

and the electronic absorption of their π-system is well described by simple Hückel MO — theory [1], π-electron derealization expresses itself in the vibrational spectra by an essential contribution of interaction forces between different bond stretching coordinates, which must be taken into consideration to fit the observed frequency distribution. Furthermore the Raman intensity ratio between the different vibrational bands can be explained quantitatively by considering the alterations of the overall molecular polarizability for the delocalized electrons resulting from the distortion of single bonds in the molecule. Vice versa from an analysis of the vibrational spectra bond specific informations about π-electron derealization can be obtained.