Highly Efficient Crystallization Induced Emissive Materials Based on a Simple Push-Pull Molecular Structure

Solid state luminescent materials are the subject of ever growing interest both from a scientific and a technological point of view. Aggregation caused quenching (ACQ) processes however represent an obstacle to the development of most luminogens in the condensed phase. This is why particularly fascinating are those materials showing higher emission intensity in the solid state than in solution. Here we report on 4-dialkylamino-2-benzylidene malonic acid dialkyl esters, very simple push-pull molecules, which are weakly emissive in solution but becomes highly emissive in the aggregated phase according to what has been indicated as aggregation induced emission (AIE).Thanks to combined emission and NMR spectroscopies at different temperatures on the prototype compound 4-dimethylamino-2-benzylidene malonic acid dimethyl ester both in solution and in the solid state we give full evidence that a restricted intramolecular rotation (RIR) phemomenon, in particular the rigidification of the rotation around the aryl main axis of the compound, is at the origin of this behavior. Moreover, we show that this compound possesses interesting acidochromic properties opening to sensor and optoelectronic applications.