π‐Extended Push‐Pull‐Type Bicyclic Fluorophores Based on Quinoline and Naphthyridine Frameworks with an Iminophosphorane Fragment

Emissive quinoline and naphthyridine derivatives, bearing an iminophosphorane (IPP) fragment as an electron‐donating group and two trifluoromethyl groups as electron‐withdrawing groups, were prepared as electron push‐pull‐type bicyclic fluorophores: quinoline‐IPP ( 1 ) and naphthyridine‐IPP ( 2 ). Compared to those of the precursor bicyclic amines without the IPP moiety, the fluorescence quantum yields (Φ f ) of 1 and 2 were considerably higher, not only in low polarity solvents but also polar solvents. Moreover, the molar absorption coefficients (ϵ) were enhanced; thus, 1 and 2 are fluorophores with relatively high brightness (Φ f ×ϵ). Using density functional theory calculations, the t indices were calculated, and these reveal that locally excited character is dominant in both molecules. Because of the sterically bulky IPP fragments, 1 and 2 emitted with moderate Φ f values, even in the solid state. Further, by exploiting the low crystallinity of 1 and 2 , emissive amorphous thin films were prepared.