Pyrazole, Imidazole,and Isoindolone Dipyrrinone Analogues:pH-Dependent Fluorophores That Red-Shift Emission Frequencies in aBasic Solution

Dipyrrinones are nonfluorescent yellow-pigmented constituents of bilirubin that undergo Z to E isomerization when excited with UV/blue light. Mechanical restriction of the E/Z isomerization process results in highly fluorescent compounds such as N,N-methylene-bridged dipyrrinones and xanthoglows. This manuscript describes the first examples of dipyrrinone analogues, which exhibit fluorescence without covalently linking the pyrole–pyrrolidine nitrogen atoms. Instead these analogues restrict E/Z isomerization through intramolecular hydrogen bonding, resulting in mild to moderately fluorescent compounds (ΦF = 0.01–0.30). Further, in basic solutions (pH > 12), the dipyrrinone analogues readily deprotonate and absorption/emission profiles of the fluorophores red-shifts by 10–49 nm. Directly from commercial materials, 10 analogues were prepared in 41–96% yields in one step. To estimate the capacity of which intramolecular hydrogen bonding has upon restricting the E/Z isomerization process, conformational energi...