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

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journal contribution
posted on 05.09.2019, 17:38 by Nicole Benson, Olabisi Suleiman, Samuel O. Odoh, Zachary R. Woydziak
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 energies of all analogues, in both the protonated and deprotonated species, were explored by using quantum-mechanical density functional theory (DFT) and time-dependent DFT calculations. The computed strengths of the intramolecular hydrogen bonds are related to the barriers of rotation about the 5–6 bond and both correlate with the experimentally measured fluorescence quantum yields.