Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
media
A. Ghanadzadeh Gilania,∗, E. Moradib, S. Binaya, M. Moghadama
a Department of Chemistry, Faculty of Science, University of Guilan, 41335 Rasht, Iran
b Department of Chemistry, Faculty of Science, Lahijan Branch, Islamic Azad University, Lahijan, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
The absorption spectra of three azoquinolin-8-ol derivatives (o-, m-, p-cyano hydroxy azoquinolin dyes)
were investigated in liquid and liquid crystalline solutions as a function of the solvent polarity. The spec-
tral data of the dyes were compared in both ordinary liquid solvents and liquid crystalline media. Analysis
of the spectral data was used to determine the azo and hydrazone forms in both the environments. The
spectral shifts were correlated by Kamlet–Taft and Katritzky multi-parameter polarity scales. For the
azoquinoline dyes, the azo form is almost entirely dominated in polar anisotropic hosts. In contrast,
the compounds remain dominantly in hydrazone form in some polar solvents such as DMF. The polar-
ized absorption spectra of the compounds in the anisotropic media were measured and their degree of
anisotropies was determined.
Received 14 August 2011
Received in revised form 5 November 2011
Accepted 9 November 2011
Keywords:
Azoquinolin dye
Azo-hydrazone tautomerism
Solvatochromism
Anisotropic media
© 2011 Elsevier B.V. All rights reserved.
Multi-parameter polarity scales
1. Introduction
nematic solvents is important for better understanding their photo-
Azo-hydrazone tautomerism in azo compounds is quite inter-
esting both from theoretical and practical point of view and
has been of particular interest [1–5]. The tautomeric behavior
of a number of hydroxy azo dyes has been identified in var-
transfer between nitrogen and oxygen atoms. The presence of
azo/hydrazone tautomerism in these compounds influence con-
siderably in their unique photo-physical properties, which is in
turn strongly influenced by several factors including temperature,
substituents structure and solvent polarity [9,10].
physical behavior in isotropic and anisotropic surrounding.
In this work, the spectroscopic properties of o-, m-, p-cyano
hydroxy azoquinolin dyes (see Supplementary Fig. 1a) in various
organic solvents and in some anisotropic hosts with different polar-
ities and polarizabilities were studied at room temperature. The
hydroxy azoquinoline compounds investigated in this work pos-
sess an electron-withdrawing group, –C N, attached to the phenyl
ring. One of the principal aims of the investigation was to see how
the location of –C N group affects the spectral and tautomeric
properties of the dyes.
Azo-hydrazone tautomerism has been studied by various optical
techniques, including electronic method. Due to the character-
istic differences between the optical spectra of the dyes in azo
and hydrazone forms, azo/hydrazone tautomeric behavior can be
investigated from the electronic spectra. Therefore, electronic spec-
troscopy is one of the most applicable and successful techniques for
investigating the azo/hydrazone tautomerism. Therefore, the both
tautomers can be detected from the absorption spectra in the visible
region.
Spectroscopic methods, based on absorption spectra, give
valuable information on contribution of different types of
solute–solvent interactions using multi-parameter solvent polar-
ity scales [11–13]. Knowledge of the spectroscopic and solvation
The addition of a dye to a nematic host affects several host
parameters, which is due to the strong mutual interactions between
various kinds of guest–host systems are among the most important
and needed in order to understand and provide further information
about the intermolecular forces in anisotropic matrix. Among the
many interesting properties of dye doped in liquid crystals, absorp-
tion anisotropy is one of the most important [15]. The guest–host
interactions of azo dyes in liquid crystalline (LC) host have been
the subject of much interest in recent years [16–18]. Such inter-
est may arise from their optically switchable properties, due to
photochromic cis–trans isomerization, in LC host.
As far as the authors are aware, no investigation on the
azo/hydrazone tautomerism for the investigated dyes in various
isotropic and anisotropic environments has been attempted. The
most significance of this research is using polarized absorption
method for investigation of the role of anisotropic media on the
∗
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1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.