Through space charge transfer and quadratic nonlinear optical (NLO) properties in alternated stacks of 2-amino-1,3-benzothiazole-ethylcoumarin-3- carboxylate charge transfer complexes: From the molecular to the bulk NLO response

The title compound is a charge transfer complex with an experimental low lying excitation located around 420 nm. It is used as a model compound for the investigation of a possible strategy towards NLO materials obtained by "through space" charge transfer. Its solid state structure (Pc space group) reveals non-centrosymmetric alternating chains of donor-acceptor entities, built from the two independent 2-amino-1,3-benzothiazole entities hydrogen-bonded in the crystal unit cell. Each independent chain gives rise to a one-dimensional charge transfer, and therefore a non-vanishing NLO response along the chain. The angle between the glide mirror and the charge transfer direction is close to the optimized value (61°) and leads to two extended lattices of high NLO capability over the whole extent of the crystal. Unfortunately, an angle of 175° is observed between the two lattices, because of the unexpected effect of the hydrogen bonding, thus cancelling most of the NLO response. The efficiency of the material is therefore limited to 0.15 times that of urea in second harmonic generation (SHG). Nevertheless, this investigation indicates that charge transfer complexes could be envisioned for an alternative approach to molecular materials with SHG capabilities.