= Dipyrromethene
{title2=$\mathrm{C}_9\mathrm{H}_8\mathrm{N}_2$}
= Dipyrrin
{synonym}
\b[Dipyrromethene], also called <dipyrrin>, is a conjugated nitrogen-containing framework built from two pyrrole-derived rings joined through a methine-containing connection. The conventional dipyrrin name refers to the 2,2′-linked system. IUPAC retains dipyrromethane as a distinct name and introduces dipyrrin for the corresponding unsaturated framework.<Nomenclature of Tetrapyrroles by IUPAC-IUB Joint Commission on Biochemical Nomenclature>[TP-6 and TP-7, retained and introduced trivial names]{ref}
The unsubstituted neutral parent has formula $\mathrm{C_9H_8N_2}$; its attachment pattern and protonation state matter when discussing derivatives.<Dipyrrin in PubChem>[CID 3083424, Molecular Formula and description]{ref} A dipyrrin-derived ligand and its boron complex are chemically different species, even when they share a recognizable skeleton.
An original atom-accounting example starts with neutral $\mathrm{C_9H_8N_2}$. Removing one proton gives a ligand composition $\mathrm{C_9H_7N_2^-}$. Combining that formal ligand with a boron-difluoride center gives the neutral simplest BODIPY composition $\mathrm{C_9H_7BF_2N_2}$. This is a charge and composition check, not a claim that free isolated fragments are the actual synthesis intermediates. Substituted dyes have additional atoms and their own molecular formulas.
The extended <conjugated system> makes the framework useful for optical dyes, while coordination and substitution alter the electronic structure. <BODIPY> dyes are therefore a family of complexes rather than another name for free dipyrromethene. The primary ChEBI dye-class definition identifies that coordinated boron-containing family.<BODIPY dyes in ChEBI>[CHEBI:51123, Definition]{ref}
A proposed analytical comparison would record absorption spectra before and after a defined coordination or protonation change, with concentration and solvent held known. A changed band can support a change in electronic structure, but its amplitude alone cannot distinguish conversion, concentration loss and altered <molar absorption coefficient>. A <Beer-Lambert law> calibration needs the identity of the absorbing species.
<Dipyrrin protonation and optical response> is consequently a separate question from merely recognizing the skeleton. Specify the free compound, ligand or complex, its substituents and the measured optical observable before transferring a property between members of the family.
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