Theoretical investigation on the spectroscopic properties of Zn porphyrin and Zn tetrapyrrin

• The ring-opened Zn tetrapyrrin has a helical geometry.
• The Q-band of Zn tetrapyrrin is red-shifted and intense compared to zn porphyrin.
• Fluorescence intensity is higher for Zn tetraphyrin than Zn porphyrin.

Exploring the relationship between the geometrical structures and spectral properties has great significance to design some desirable materials. For important pyrrole contained macromolecules, there is a new complex of Zn tetrapyrrin with opened chain ligand derived from breaking one methine bridged of Zn porphyrin. The frontier molecular orbitals, absorption and emission properties of such Zn tetrapyrrin complexes are investigated by density functional theory (DFT) and its time-dependent density functional theory (TD-DFT) methods. Compared to Zn porphyrin, the HOMOs and LUMOs are no longer degenerate, the lowest lying absorption and emission of Zn tetrapyrrin start to increase in intensity with the shifting of charge transfer transition band to near-infrared region. We hope these theoretical studies will assist the design of novel molecular materials.

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