The role of the chromophore size and shape on the SHG stability of PMMA films with embebbed NLO active macrocyclic chromophores based on a cyclotetrasiloxane scaffold

In this work, the second harmonic generation (SHG) and the nonlinear optical (NLO) coefficients d33 of PMMA films, containing 4 wt % NLO active macrocyclic chromophores based on a cyclotetrasiloxane scaffold and of their monomeric structurally homologous NLO active chromophores, were obtained by an electrical poling process and measured at room temperature during a period of many months, after switching off of the orienting electric field. The composite films based on the macrocyclic NLO chromophores have shown much higher d33 values and stability in the time if compared to films based on the monomeric structurally homologous NLO chromophores. Such higher stability in the time is related to a higher decay time t2 and to a lower rotational diffusional coefficient DR, as a consequence of the increased size of the macrocyclic NLO chromophores. We have observed that also the shape of the NLO chromophores may play a role. A good stability in the time of the d33 values is maintained if the PMMA films, containing the macrocyclic NLO chromophores, are heated up to about 50 °C.