Second harmonic generation in nonsymmetrical squaraines: tuning of the directional charge transfer character in highly delocalized dyes

The interest in squaraine compounds, a relatively old class of cyanine-like dyes, has been recently renewed due to their potential usefulness in a large number of technologically relevant fields such as two photon absorbing materials, field effect transistors, solar cells, NIR emitting fluorescent probes and sensitizers for photo dynamic therapy. In the case of symmetric compounds, the squaraine electronic structure is reminiscent of that of symmetric cyanines. In the case of nonsymmetric squaraines the HOMO-LUMO transition is characterized by a charge transfer component responsible for a sometimes sizeable band broadening, also reflected in a relevant second order nonlinear optical behaviour. The aim of the present paper is the design, synthesis and multidisciplinary characterization of a series of nonsymmetrical heterocycle-based squaraines, a study directed towards a general understanding of the deviation from the cyanine limit in this class of dyes. We exploit the electric field induced second harmonic generation technique as a tool for the charge transfer character evaluation and, together with other hints coming from UV-Vis, NMR and cyclic voltammetry, we show how the directionality of the HOMO-LUMO excitation can be characterized and accounted for in terms of the most relevant contribution of canonical structures for the squaraine ground state description. DFT/TDDFT calculations provide further insight to the electronic structure of representative compounds.