Theoretical investigation of the molecular, electronic structures and vibrational spectra of a series of first transition metal phthalocyanines

A theoretical investigation of the fully optimized geometries and electronic structures of metallophthalocyanines FePc, CoPc, NiPc, CuPc and ZnPc has been conducted with the density functional theory (DFT) method. A comparison between the different molecules for the geometry, molecular orbital, and atomic charge is made. The simulated order of the sizes of the central hole is FePc > CoPc > NiPc < CuPc < ZnPc, which is in complete accord with the experiment. Moreover, the HOMO-LUMO gaps vary in the order of FePc < CoPc > NiPc > CuPc > ZnPc, and the atomic charges of the central metal (M = Fe, Co, Ni, Cu, Zn) ions vary in the same order, FePc > CoPc > NiPc < CuPc < ZnPc, as the length of N-M bond. The vibrational spectra for these five compounds have also been calculated at the density functional B3LYP or UB3LYP level using the 6-31G(d) basis set. Detailed assignments of the vibrational bands in the IR and Raman spectra have been made based on assistance of animated pictures. The simulated IR and Ra spectra for the five derivatives are compared with the experimental absorption spectra, and very good consistency has been obtained. The experimental medium intensity bands associated with the metal-ligand vibrations which appear as singlet peaks at 909, 911, 915, 898 and 888 cm^-1 for FePc, CoPc, NiPc, CuPc and ZnPc in IR spectra show the order of NiPc > CoPc > FePc > CuPc > ZnPc, and the corresponding peaks predicted at 894, 896, 898, 882 and 871 cm^-1, respectively, also exhibit the same order as above-mentioned. Moreover, the lines of fit through plots of the experimental IR and Ra frequencies versus the calculated ones show very good correlations.