Paramagnetism and Fluorescence of Zinc(II) Tripyrrindione: A Luminescent Radical Based on a Redox-Active Biopyrrin

Ritika Gautam, Steven J. Petritis, Andrei V. Astashkin, Elisa Tomat

Research output: Contribution to journalArticlepeer-review

21 Scopus citations


The ability of bilins and other biopyrrins to form fluorescent zinc complexes has been known for more than a century; however, the exact identity of the emissive species remains uncertain in many cases. Herein, we characterize the hitherto elusive zinc complex of tripyrrin-1,14-dione, an analogue of several orange urinary pigments. As previously observed for its Pd(II), Cu(II), and Ni(II) complexes, tripyrrindione binds Zn(II) as a dianionic radical and forms a paramagnetic complex carrying an unpaired electron on the ligand π-system. This species is stable at room temperature and undergoes quasi-reversible ligand-based redox chemistry. Although the complex is isolated as a coordination dimer in the solid state, optical absorption and electron paramagnetic resonance spectroscopic studies indicate that the monomer is prevalent in a tetrahydrofuran solution. The paramagnetic Zn(II) tripyrrindione complex is brightly fluorescent (λ abs = 599 nm, λ em = 644 nm, φ F = 0.23 in THF), and its study provides a molecular basis for the observation, made over several decades since the 1930s, of fluorescent behavior of tripyrrindione pigments in the presence of zinc salts. The zinc-bound tripyrrindione radical is thus a new addition to the limited number of stable radicals that are fluorescent at room temperature.

Original languageEnglish (US)
Pages (from-to)15240-15246
Number of pages7
JournalInorganic Chemistry
Issue number24
StatePublished - Dec 17 2018
Externally publishedYes

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry


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