Molecular Conformation Engineering To Achieve Longer and Brighter Deep Red/Near-Infrared Emission in Crystalline State.

A series of molecules 1 - 5 containing the same fluorophore and different alkyl chains are synthesized to reveal the significant effect of molecular conformations on the emission properties. In crystalline state, molecules 1 - 3 exhibit strong orange emissions with maxima (λ em ) of about 600 nm and quantum yields (Φ F ) of around 60%, while molecules 4 and 5 display much longer emissions to the deep red/near-infrared (NIR) region as well as even higher efficiencies (λ em = 693 nm, Φ F = 73% for 4 ; λ em = 654 nm, Φ F = 93% for 5 ). The largely red-shifted emissions of 4 and 5 as well as the significantly improved Φ F are very unusual. Furthermore, the Φ F of 4 and 5 represent the highest values among organic solids with similar deep red/NIR emission wavelengths. On the basis of the experimental measurements and theoretical calculations, the new molecular design of conformation engineering, the impressive emission properties, and the potential NIR fluorescence sensing and lasing applications are comprehensively investigated.