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 (λem) of about 600 nm and quantum yields (ΦF) 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 (λem =
693 nm, ΦF = 73% for 4; λem = 654 nm, ΦF = 93% for 5).
The largely red-shifted emissions of 4 and 5 as well as the significantly improved ΦF are very
unusual. Furthermore, the ΦF 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.