Stabilization through p-dimethylaminobenzaldehyde of a new NLO-active phase of [E-4-(4-dimethylaminostyryl)-1-methylpyridinium] iodide: Synthesis, structural characterization and theoretical investigation of its electronic properties

Co-crystallization of p-N(CH3)2C6H4CHO and E-4-(4-dimethylaminostyryl)-1-methylpyridinium iodide ([DAMS]I) gives a new solid state form (A), characterized by high non-linear-optical (NLO) activity and quite different from the well known [DAMS]I salt (centrosymmetric and therefore inactive). The X-ray structural characterization, although affected by the extended disorder of p-N(CH3)2C6H4CHO, addresses a new kind of aggregation for the [DAMS+] chromophore molecules. In fact, together with the often encountered J-type aggregation, an unprecedented "fishbone" coupling is observed. Calculations using time-dependent density functional theory (TDDFT) prove that the absence of a J-aggregation band in the electronic absorption spectrum is due to the additional intermolecular interaction that quenches the expected J-type signal. Correlation between supramolecular arrangements of the chromophores and NLO properties is also discussed. © 2010 The Royal Society of Chemistry.