Red/Near-Infrared Luminescence Tuning of Group-14 Element Complexes of Dipyrrins Based on a Central Atom

A dipyrrin complex
has been one of the most utilized fluorescent dyes, and a variety
of dipyrrin complexes show intriguing functions based on the various
coordination structures of the central element. We now report the
synthesis, structure, and photophysical properties of germanium and
stannane complexes of the N2O2-type tetradentate dipyrrin, L·Ge and L·Sn, which are heavier analogues of the previously
reported dipyrrin silicon complex, L·Si. The central
group-14 atoms of the monomeric complexes have geometries close to
trigonal bipyramidal (TBP), in which the contribution of the square-pyramidal
(SP) character becomes higher as the central atom is heavier. Interestingly, L·Sn formed a dimeric structure in the crystal. All
complexes L·Si, L·Ge, and L·Sn showed a fluorescence in the red/NIR region. Fluorescence
quantum yields of L·Ge and L·Sn are higher than that of L·Si. These results indicated
that the central atom on the dipyrrin complexes contributes not only
to the geometry difference but also to tuning the fluorescence properties.