Iso-Oriented Fluorescent Colloidal Nanocrystals of Bis-Cyanostyryl Thiophenes: Crucial Secondary Halogen Interactions toward Stability and Transport

Abstract

Water-dispersible, single-crystalline, and fluorescent colloidal crystals with curved edges were fabricated from π-conjugated chromophores 2,5-bis(α-cyano-4-bromostyryl)thiophene (CN-BST) and 2,5-bis(α-cyanostyryl)thiophene (CN-ST) via reprecipitation. The colloidal crystal growth occurred in a nonclassical pathway through alignment of seed particles. The effects of torsion and planarity were examined through DFT calculations and photoluminescence spectroscopy. The bromine end groups were shown not to have a significant role in bringing absorption or fluorescent spectral changes, however, had endowed the molecules to interact in an angular geometry, conferring additional stability and rigidity to the CN-BST colloidal crystals. Most importantly, secondary Br–Br interactions in the solid state shielded and stabilized the associative dimeric interactions of the type, (i) face-to-face π-stack, (ii) slip-stack, (iii) C–H···N, and (iv) C–H···π with different strengths. Both classes of colloidal crystals exhibit...