Switching of Emissive and NLO Properties in Push-Pull Chromophores with Crescent PPV-like Structures

We report on a series of novel homologous push-pull compounds, in which identical donor (a dimethylamino) and acceptor (a malonate ester) functionalities endcap crescent PPV fragments, bearing respectively 1, 2 and 3 p-phenylenevinylene units in direct linear conjugation (compounds 7-9). The three compounds exhibit striking differences in their linear and nonlinear optical properties. The shorter compound 7 exhibits aggregation-induced emission with a strong luminescence in the solid state (blue emission, photoluminescence quantum yield 38%), and it is nonemissive in solution; the more extended conjugated systems 8 and 9 show classical aggregation-caused quenching in the solid state, while high quantum yield photoluminescence (21 and 93% in toluene) is restored in diluted solutions, through mechanisms involving intramolecular charge transfer in the excited states. EFISH measurements in solutions demonstrate a strong solvent and concentration dependence. As rationalized with the aid of molecular modelling, compound 8 and, more markedly 9, aggregate in stable centrosymmetric dimers in solution.