Hydrogen‐Bonding Effects at Pyrrolic NH Sites: Optical Properties of Benzo[ a ]indolo[2,3‐ c ]carbazole Derivatives

Comparative studies on the photophysical properties of benzo[ a ]indolo[2,3‐ c ]carbazole (BIC) derivatives (i.e., [ 5 ]AH derivatives) and their π‐extended nonplanar analogues (i.e., [ 9 ]AH derivatives) were conducted to elucidate solvent effects associated with hydrogen‐bonding at the NH site. The NH‐free compounds exhibited fluorescence attenuation specifically in acetone, while fluorescence emission was enhanced in polar solvents regardless of N‐alkylation. Theoretical calculation and ultrafast transient absorption spectroscopy were performed to gain insights on the optical responses.