Dipyrrins serve as monovalent bidentate ligand molecules that coordinate to various cations. Their BF 2 complexes, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene and its derivatives (BODIPYs), exhibit excellent photostability, strong light absorption, and high fluorescence quantum yield, thereby encouraging their application in various fields, e.g., as biological and biomedical fluorescent markers. Dipyrrin may also accept a wide variety of metal ions spontaneously. However, dipyrrin metal complexes have been disregarded from materials science research. This review article summarizes recent progress in bis(dipyrrinato)metal(II) and tris(dipyrrinato)metal(III) complexes from the viewpoint of materials chemistry. Section 2 describes a series of efforts aimed to realize intense luminescence superior to or comparable with that of BODIPYs. The spontaneous coordination of these complexes enables them to construct self-assembled nanoarchitectures, such as supramolecules and coordination polymers that form one-dimensional nanowires, two-dimensional nanosheets, and metal-organic frameworks. Section 3 describes such alluring molecular superstructures. Section 4 discusses potential applications based on these nanoarchitectures, such as thermoelectric and photoelectric conversion. aimed to realize intense luminescence superior to or comparable with that of BODIPYs. Section 3 discusses the selfassembly of bis and tris(dipyrrinato)metal complexes, which results in the formation of alluring nanoarchitectures, such as supramolecules and coordination polymers giving rise to onedimensional nanowires, two-dimensional nanosheets, and metal-organic frameworks (MOFs) and porous coordination polymers (PCPs). Section 4 describes potential applications based on these nanoarchitectures. Pursuit of bright luminescenceA plain BODIPY shows intense absorption and bright fluorescence at approximately 500 nm, which are derived from the 1 π-π * transition of the dipyrrinato ligand. 5-16 The absorption and fluorescence may be redshifted upon the introduction of substituents on the dipyrrinato ligand, covering the region 500-900 nm. BODIPYs are good fluorophores even in polar solvents such as water. In sharp contrast, dipyrrin metal complexes have long been believed to be non-luminescent or weakly luminescent. This drawback seriously reduces the value of dipyrrinato-metal complexes in applications in which they serve, for example, as photosensitizers. In this section, the authors concentrate on efforts to improve the luminescent ability of bis and tris(bisdipyrrinato)metal complexes. For comprehensive knowledge on the whole types of luminescent dipyrrinato-metal complexes (i.e. mono(dipyrrinato)metal complexes with ancillary ligands other than dipyrrins), please see a comprehensive review article contributed by Baudron. 2

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, (1)H and (13)C 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.

Heteroleptic zinc(II) complexes synthesized using achiral dipyrrinato and chiral bis(oxazoline) ligands show bright fluorescence with quantum efficiencies of up to 0.70. The fluorescence originates from the (1)π-π* photoexcited state localized exclusively on the dipyrrinato ligand. Furthermore, the luminescence is circularly polarized despite the achirality of the dipyrrinato ligand. Single-crystal X-ray structure analysis discloses that the chiral bis(oxazoline) ligand undergoes intramolecular π-π stacking with the dipyrrinato ligand, inducing axial chirality in the dipyrrinato moiety.

Heteroleptic zinc(II) complexes synthesized using achiral dipyrrinato and chiral bis(oxazoline) ligands show bright fluorescence with quantum efficiencies of up to 0.70. The fluorescence originates from the 1 p-p * photoexcited state localized exclusively on the dipyrrinato ligand. Furthermore, the luminescence is circularly polarized despite the achirality of the dipyrrinato ligand. Single-crystal X-ray structure analysis discloses that the chiral bis(oxazoline) ligand undergoes intramolecular p-p stacking with the dipyrrinato ligand, inducing axial chirality in the dipyrrinato moiety.

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