ORIGINAL PAPER Synthesis of novel diphenylamine-based fluorescent styryl colorants and study of their thermal, photophysical, and electrochemical properties Yogesh A. Sonawane • Rajkumar N. Rajule • Ganapati S. Shankarling Received: 17 November 2009 / Accepted: 29 June 2010 / Published online: 5 August 2010 Ó Springer-Verlag 2010 Abstract Three novel ‘‘Y’’-shaped acceptor–p-donor– p-acceptor-type compounds were synthesized from 4,4 0 -(hexylimino)bis(benzaldehyde) as a donor and 2-methylthiazolo[4,5-b]quinoxaline derivatives as strong electron acceptors condensed by classical Knoevenagel condensation. Their absorption, emission, and thermal properties and electrochemical stability were investigated. It was found that the strong electron acceptor–donor chromophoric system of these compounds showed high Stokes shift, excellent thermal stability, and electrochem- ical reversibility. The solvatochromic behavior of these colorants was studied by using various solvents such as toluene, chloroform, ethyl acetate, tetrahydrofuran, meth- anol, and N,N-dimethylformamide in increasing order of polarity. The dyes were characterized by means of ele- mental analysis, 1 H NMR, and mass spectrometry. Keywords Dyes Á Absorption spectra Á Fluorescent colorants Á Cyclic voltammetry Á Knoevenagel condensation Introduction Electron transfer (ET) is one of the most fundamental phenomena in chemistry and biology [1]. Organic fluo- rescent heterocyclic chromophores have a wide range of applications in biochemistry and electronics such as molecular probes [2], organic light-emitting diodes [3], photovoltaic cells [4], and in traditional textile and polymer fields [5]. Electron donors like triphenylamine and diphenylamine with high electron mobility and thermal and photochemical stability are commonly used as hole-transporting materials or light-emitting materials for balanced charge injection. Recently triphenylamine derivatives have been widely investigated [6]. N-Alkyldiphenylamines have not been exploited much in the past, but attract increasing interest as they exhibit better solubility in common organic solvents, excellent thermal and electrochemical stability, electron- donating ability, and optoelectronic properties [7]. The rigidity and non-planar structure of N-alkyldiphenylamines offer a sterically well-hindered feature. The presence of two electron acceptor moieties increases the electron affinity of the compounds [8]. From the viewpoint of molecular design, structural features such as the long p conjugation style, the molecular planarity, and the length of the conjugated bridge play important roles in functional chromophores, which are highly promising for electronic applications like photonics materials [9, 10], holographic optical data storage [11], and organic photorefractive materials. It has been demonstrated that diarylamine- and triarylamine-based chromophores have extremely high electro-optic properties [12]. Exploitation of electron acceptor fused quinoxaline systems as fluorescent styryl dyes is of interest because of their strong fluorescence [13, 14] and their versatile applications [15, 16]. Quinox- alines are in general easy to prepare [17–19]. In this research work, our interest was to design and develop novel heterocycle-based chromophores with excellent photophysical and thermal properties. We report the synthesis and study of new chromophores based on 4,4 0 -(hexylimino)bis(benzaldehyde) (11). We have espe- cially focused on the condensation and synthesis of two Y. A. Sonawane Á R. N. Rajule Á G. S. Shankarling (&) Department of Dyestuff Technology, Institute of Chemical Technology, N. P. Marg, Matunga, Mumbai 400019, India e-mail: gss@udct.org; gsshankarling@gmail.com 123 Monatsh Chem (2010) 141:1145–1151 DOI 10.1007/s00706-010-0365-6