Stereomutation and chiroptical bias in the kinetically controlled supramolecular polymerization of cyano-luminogens

Abstract

The synthesis of two pairs of enantiomeric cyano-luminogens 1 and 2, in which the central chromophore is a p-phenylene or a 2,5-dithienylbenzene moiety, respectively, is described and their supramolecular polymerization under kinetic and thermodynamic control investigated. Compounds 1 and 2 form supramolecular polymers by quadruple H-bonding arrays between the amide groups and the π-stacking of the central aromatic moieties. In addition, the peripheral benzamide units are able to form intramolecularly H-bonded pseudocycles that behave as metastable monomer M* thus affording kinetically and thermodynamically controlled aggregated species AggI and AggII. The chiroptical and emissive features of compounds 1 and 2 strongly depend on the aggregation state and the nature of the central aromatic unit. Compounds 1 exhibit a bisignated dichroic response of different intensity but with similar sign for both AggI₁ and AggII₁ species, which suggests the formation of helical aggregates. In fact, these helical supramolecular polymers can be visualized by AFM imaging. Furthermore, both AggI and AggII species formed by the self-assembly of compounds 1 show CPL (circularly polarized light) activity of opposite sign depending on the aggregation state. Thienyl-derivatives 2 display dissimilar chiroptical, morphological and emissive characteristics for the corresponding kinetically and thermodynamically controlled aggregated species AggI and AggII in comparison to those registered for compounds 1. Thus, a stereomutation phenomenon is observed in the AggI₂ → AggII₂ conversion. In addition, AggI₂ is arranged into nanoparticles that evolve to helical aggregates to afford AggII₂. The dissimilar chiroptical and morphological features of AggI₂ and AggII₂ are also appreciated in the emissive properties. Thus, whilst AggI₂ experiences a clear AIE (aggregation induced emission) process and CPL activity, the thermodynamically controlled AggII₂ undergoes an ACQ (aggregation caused quenching) process in which the CPL activity is cancelled.