Fluorescent NLOphoric Coumarins: A Short Review

Abstract

Franken and co-workers first investigated the second harmonic generation (SHG) in quartz crystal in 1961, and thereafter continuous efforts have been made to develop efficient nonlinear optical (NLO) materials [1]. Compared to electrons, photons can process the information extremely fast (close to the speed of light), and hence $π$-electron-containing NLO materials will be very important in the future photonic technology. The development of NLO materials demands the search of new molecules which can produce a crystal and thereby allow improving the NLO properties. The organic materials are more promising NLO materials over the inorganic ones because their NLO response is of purely electronic origin, and they have properties like high second-order NLO frequency, higher resistance to damage in laser beam, synthetic flexibility and easy incorporation in electronic fabrications [2]. Therefore organic materials with donor-$π$-acceptor (D-$π$-A) type framework are evolved as promising NLO materials [3, 4]. Such D-$π$-A systems perturb the $π$-electron delo-calization of the locally excited (LE) state attained by the absorption of photons, and this can promote intramolecular charge transfer (ICT) [5-8]. Exploiting the synthetic methods, changing the substitution position or substituent groups in such molecular systems leads to modification of their NLO characteristics [9, 10]. The high NLO responses of D-$π$-A type push-pull systems are intrinsically related to ICT excited states [11-14] and hence fluorescence behaviour. In this context the coumarin class of fluorophores has been successfully explored in this area [2, 15-21].