A theoretical investigation on the π-conjugation effect on the structures and spectral properties of tetra pyrrole zinc complexes

• Tetra pyrrole Zn complexes with different π-conjugation in skeleton are studied.
• Spectral bands variation related to the different geometries.
• Quasi Gouterman's four-orbital model is not for all tetra pyrrole Zn complexes.
• The higher electronic excited states are bright except Zn2−(Pyr)4.

The structures and properties of a family of different π-conjugation tetra pyrrole Zn complexes are reported. The tetra pyrrole moiety can be divided as porphyrin, one tripyrrin and one pyrrole, two dipyrrines, one dipyrrin and two pyrroles, and four pyrroles. The Zn complexes possess different geometries and tunable spectral bands depending on the mode of the different metal-coordination. For Zn porphyrin, the frontier molecular orbitals (FMOs) are the major contribution to the intense B-like band and weak Q-like band absorptions, and this is in agreement with the Gouterman’s four-orbital model. But because of the breakdown of the π-conjugation from complex 1 to 5, the FMOs are no longer separated from the other MOs in energy, the orbital with metal distribution is approaching to lowest unoccupied molecular orbital (LUMO), the absorption band is no longer intense at B-like bond and weak at Q-like bond as Gouterman's four-orbital model mentioned. The calculated fluorescence spectra in toluene solution show that fluorescence can be observed in the visible region of complexes 1–4 because of the bright higher electronic excited states.

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