Contribution of Fluorescence Spectroscopy to the Determination of Binary Phase Diagrams of Mesomorphic Mixtures. Part II. Investigation of the Induced Smectic A₁ Phase of the System Nonylcyanobiphenyl/ Phenylcyclohexanecarboxylate

Abstract

The fluorescence intensity and the wavelength distribution of binary mesomorphic mixtures composed of 4-nonyl 4′-cyanobiphenyl (9CB) and 4-pentylphenyl 4′-cyclohexane carboxylate (5H5) have been recorded as a function of the temperature in the isotropic, nematic and induced smectic mesophases (S_A1, S_B and S_G).

Two types of fluorescence can be observed in these systems: monomer fluorescence and excimer fluorescence. Their intensity ratio ranges from 0 to ∞ as a function of the composition, of the temperature and of the nature of the phase involved. Monomer fluorescence (I_M) is characteristic of materials of low molecular mobility and high degree of order. It is found in S_B and S_G mesophases. Excimer fluorescence (I_D) on the other hand is predominantly observed in less-ordered and more fluid phases like S_A1, nematic and isotropic phases.

It has been shown in this work that band shape modifications and shifts of the LD max of the fluorescence spectra can be related to changes in the relative importance of radiative and non-radiative deactivation processes of the excited state and that intensity changes allow us to detect phase transitions. A new type of isoemissive point which has the nature of the phase as the variable parameter has also been discovered for these mixtures.

Key Words:

• injected smectic phases
• binary phase diagrams
• fluorescence spectroscopy