Excitonic Luminescence of Hemiporphyrazines

Metal-free hemiporphyrazine (HpH2) is a notoriously
insoluble material possessing interesting photophysical properties.
Here we report the synthesis, structure, and photophysical properties
of an octahedral zinc trans-ditriflate hemiporphyrazine
complex “HpH2Zn­(OTf)2” that contains
a neutral hemiporphyrazine ligand. The photophysical properties of
hemiporphyrazine are largely unaffected by introduction of zinc­(II)
triflate, but a dramatic increase in solubility is observed. HpH2Zn­(OTf)2 therefore provides a convenient model
system to evaluate the impact of aggregation on the photophysical
properties of hemiporphyrazine. Soluble aggregates and crystalline
materials containing planar hemiporphyrazines exhibit relatively strong
absorbance of visible light (450–600 nm) and red luminescence
(600–700 nm). Hemiporphyrazine monohydrate (HpH2·H2O), in contrast, has a nonplanar “saddle-shaped”
conformation that exhibits very little absorbance of visible light
in solution or in the solid state. Upon photoexcitation at 380 nm,
HpH2Zn­(OTf)2 and HpH2 exhibit multiwavelength
emissions centered at 450 and 650 nm. Emissions at 450 nm are highly
anisotropic, while emissions at 650 nm are fully depolarized with
respect to a plane-polarized excitation source. Taken together, our
results suggest that excitonic coupling of aggregated and crystalline
hemiporphyrazines results in increased absorbance and emission of
visible light from S0 ↔ S1 transitions
that are usually symmetry forbidden in isolated species. In contrast
to previously proposed theories involving excited-state intramolecular
proton transfer, we propose that the multiple-wavelength luminescent
emissions of HpH2Zn­(OTf)2 and HpH2 are due to emissive S1 and S2 states in aggregated
and crystalline hemiporphyrazines. These results may provide a better
understanding of the nonlinear optical properties of these materials
in solution and in the solid state.