Open AccessPyrazole, Imidazole, and Isoindolone Dipyrrinone Analogues: pH-Dependent Fluorophores That Red-Shift Emission Frequencies in a Basic Solution
Author(s) -
Nicole Benson,
Olabisi Suleiman,
Samuel O. Odoh,
Zachary R. Woydziak
Publication year - 2019
Publication title -
journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.2
H-Index - 228
eISSN - 1520-6904
pISSN - 0022-3263
DOI - 10.1021/acs.joc.9b01708
Subject(s) - intramolecular force , isomerization , chemistry , density functional theory , hydrogen bond , fluorescence , photochemistry , deprotonation , protonation , pyrrolidine , imidazole , computational chemistry , molecule , stereochemistry , organic chemistry , catalysis , ion , physics , quantum mechanics
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 (Φ F = 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.