A density functional theory study on photophysical properties of red light-emitting materials: Meso-substituted porphyrins

The density functional theory (DFT) has been applied to investigate the photophysical properties of meso-substituted porphyrins: 5,15-difluorenylporphyrin (DFP), 5,10,15,20-tetrafluorenylporphyrin (TFP), Zn-5,10,15,20-tetrafluorenylporphyrin (FPZ) and Zn-5,10,15,20-tetrafluorenoneporphyrin (OPZ). The geometry structures, frontier molecular orbitals, ionization potential (IP), electron affinity (EA) and reorganization energy (λ) together with the absorption and emission wavelengths of these compounds were investigated. Theoretical calculations indicated that the introduction of meso-substitutions brought about significant effect on the photophysical properties. The TFP molecule has lowest hole injection energy barrier of all the studied compounds, and OPZ owns lowest electron injection energy barrier of them. The equilibrium of the hole and electron transport of FPZ is better than those of porphyrin (FBP), DPZ, TFP, and OPZ. The electron reorganization energies (λelectron) of FBP, DFP, TFP, FPZ and OPZ are 0.22 eV, 0.20 eV, 0.23 eV, 0.26 eV and 0.13 eV, respectively, which are smaller than tris(8-hydroxyquinolinato) aluminum(III), thus these molecules are potential electron transport materials. Their visible absorption wavelengths are calculated by TD-B3LYP and semi-empirical ZINDO methods, and these results agree with corresponding experimental values. The fluorescence spectra of these complexes predicted by TD-HF method exhibit more consistent results with experimental data than by TD-B3LYP method.