An interpretation of the structure of Langmuir-Blodgett films of octa-substituted metal-free phthalocyanine molecules from optical spectra

Ultraviolet-visible absorption spectra of Langmuir-Blodgett (LB) films of octa-substituted metal-free phthalocyanine molecules are decomposed into Gaussian-Lorentzian components in order to compare the in-plane molecular arrangements of the two groups of the compounds with short and long alkyl chains. LB films of long-chain molecules represent the Davydov splitting of the Q band, which corresponds to the 'herringbone' molecular structure and the angle between the molecules is estimated to be 75 degrees. For short-chain molecules, the largely broadened 'blue' component of the Davydov doublets is suppressed and this can be explained by the domination of an anisotropic stack-like molecular arrangement in LB films.