Quantum-Chemical Calculations of the Geometric Structures and Electronic Spectra of Phthalocyanines MgPc and H2Pc and Their β-Octaphenyl Derivatives

Geometric structures of phthalocyanines (MgPc, H 2 Pc) and their β-octaphenyl derivatives (MgPcPh 8 , H 2 PcPh 8 ) were calculated by the DFT PBE/TZVP method; excited electronic states, by the modifi ed INDO/Sm method. An examination of the bond lengths taking into account data for porphyrazines MgTAP and H2TAP showed that the weight of the internal 16-atom macroheterocycle in the electronic structures of MgPc and MgPcPh8 increased as compared to MgTAP whereas the contribution of the 18-atom azacyclopolyene of the free bases H2Pc and H2PcPh8 decreased as compared to H 2 TAP. The two lowest unoccupied MOs and highest occupied MO of the examined phthalocyanines were 70% localized on the internal 16-atom macrocycle (INDO/Sm data). Filled MOs of the next lowest energy had strong mixing of the π-AOs of the 16-atom macrocycle with π-MOs of annelated benzene rings (MgPc and H 2 Pc) and also with π-MOs of the phenyl rings (MgPcPh 8 and H 2 PcPh 8 ). The Q-state energies calculated by INDO/Sm agreed with the experimental data within 200–400 cm –1 . Several ππ * -transitions characterized by local excitation of the 16-atom macrocycle and electron transfer between the 16-atom ring and the benzene rings were mainly responsible for the observed broad absorption spectrum of the phthalocyanines in the range 27,000–37,000 cm–1 (Soret band). The two strongest calculated ππ * -transitions in this range agreed qualitatively with the experimental ones. However, the calculated energy was overestimated by an average of ~3000 cm –1 . The calculated energy of the strongest transition for MgPcPh8 corresponded well with the experimental Soret band maximum (overestimated by only 900 cm –1 ).