Theoretical Study On The Spectral Properties Of Zinc Porphyrin Derivatives

Zinc porphyrins and their derivatives have been widely applied in organic light emitting diodes,dye-sensitized solar cells,catalysis and photodynamic therapy due to their special structures,excellent photophysical properties and photochemical properties.Moreover,the photoelectrochemical properties of zinc porphyrin derivatives can be effectively adjusted by modifying their molecular structures with various chemical methods.At present,the modification of zinc porphyrin complexes is mainly at their meso and ? sites.Generally,such modification strategies can increase the conjugation degree and maintain the planarity of zinc porphyrin.However,compared with planar zinc porphyrin derivatives,the structures and properties of non-planar zinc porphyrin derivatives have rarely been reported.In this paper,a series of nonplanar zinc porphyrin complexes with different ?-conjugated structures and different aromaticity were theoretical investigated by using quantum chemical method.The ground state and excited state properties related to photoelectric functions were calculated.Meantime,the relationship between molecular structures and molecular properties of these nonplanar zinc porphyrin complexes were studied,which could provide theoretical basis for the design and synthesis of novel photoelectric functional materials.This work mainly includes the following three aspects.1.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,named as open-ring Zn porphyrin.Density functional theory(DFT)and time-dependent density functional theory(TD-DFT)are used to analyze the geometrical structure,frontier molecular orbital properties,absorption spectra and emission spectra for the open-ring zinc porphyrin complexes.Comparing to the closed-ring zinc porphyrin complexes,the four pyrrole groups in the open-ring zinc porphyrin are no longer coplanar,the HOMOs and LUMOs are no longer degenerate,the lowest lying absorption and emission start to increase in intensity with the shifting of charge transfer transition band to near-infrared region.Moreover,compared with the closed-ring zinc porphyrin,the open-ring zinc porphyrin complex has a relatively large Stokes shift due to its greater flexibility.We hope these theoretical studies will assist the design of novel molecular materials.2.The special molecular structures and spectra properties of a series of different ?-conjugation tetrap yrrole zinc complexes are systematically studied based on TD-DFT calculations and four-orbital model theory.The ?-conjugation effects on the geometrical structures and absorption spectra properties are predicted.The calculated results show that breaking the conjugate structure of zinc porphyrin,the molecular structures of these tetra pyrrole zinc complexes can be changed from planar structure to distorted tetrahedral structure.At the same time,the frontier molecular orbitals(HOMO-1,HOMO,LUMO and LUMO+1)of these tetra pyrrole zinc complexes are no longer separated from other molecular orbitals in energy,the orbital with metal distribution is approaching to lowest unoccupied molecular orbital(LUMO),and the absorption spectra is no longer weak at Q-like band and no longer intense at B-like band with the breakdown of the ?-conjugation of zinc porphyrins.Studying the relationship between the molecular structure and spectral properties of these tetra pyrrole zinc complexes will help people better understand Gouterman's four-orbital theory.Moreover,we hope this study can provide theoretical basis and clues for the experimental synthesis of effective and stable zinc pyrrole complexes.3.The purpose of this work described herein is to design highly efficient photosensitizers(PS)for photodynamic therapy(PDT)in theory.A series of expanded Zn porphyrins have been studied as light activated PS.Their main photophysical properties are systematically calculated by using density functional theory and its time-dependent extension.The mechanisms of PDT are discussed.All the considered candidates exhibit intense absorption in the therapeutic window,efficient intersystem crossing,and sufficient energy for singlet molecular oxygen production.Accordingly,the designed Zn pentaphyrins and sapphyrins would be proposed as potential PS for PDT.Moreover,the therapeutic effects of Zn pentaphyrins and sapphyrins are better than those of the referenced Zn iso-pentaphyrin.It is expected that the results could provide a new way to design and develop PS for PDT application.