Dissymmetric Bis(dipyrrinato)zinc(II) Complexes: Rich Variety and Bright Red to Near-Infrared Luminescence with a Large Pseudo-Stokes Shift

Bis­(dipyrrinato)­metal­(II)
and tris­(dipyrrinato)­metal­(III) complexes
have been regarded as much less useful luminophores than their boron
difluoride counterparts (4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes, BODIPYs), especially in polar solvent. We proposed previously
that dissymmetry in such metal complexes (i.e., two different dipyrrinato
ligands in one molecule) improves their fluorescence quantum efficiencies.
In this work, we demonstrate the universality and utility of our methodology
by synthesizing eight new dissymmetric bis­(dipyrrinato)­zinc­(II) complexes
and comparing them with corresponding symmetric complexes. Single-crystal
X-ray diffraction analysis, 1H and 13C NMR spectroscopy,
and high-resolution mass spectrometry confirm the retention of dissymmetry
in both solution and solid states. The dissymmetric complexes all
show greater photoluminescence (PL) quantum yields (ϕPL) than the corresponding symmetric complexes, allowing red to near-infrared
emissions with large pseudo-Stokes shifts. The best performance achieves
a maximum PL wavelength of 671 nm, a pseudo-Stokes shift of 5400 cm–1, and ϕPL of 0.62–0.72 in
toluene (dielectric constant εs = 2.4), dichloromethane
(εs = 9.1), acetone (εs = 21.4),
and ethanol (εs = 24.3). The large pseudo-Stokes
shift is distinctive considering BODIPYs with small Stokes shifts
(∼500 cm–1), and the ϕPL values are higher than or comparable to those of BODIPYs fluorescing
at similar wavelengths. Electrochemistry and density functional theory
calculations illustrate that frontier orbital ordering in the dissymmetric
complexes meets the condition for efficient PL proposed in our theory.