Abstract: The structure of free base phthalocyanine (H2Pc) has been predicted using density functional theory (DFT) at the B3LYP/6-31G level and time-dependent density functional theory (TDDFT) has been applied to the excited states of H2Pc.The effects of tetraaza substitutions and tetrabenzo annulations on the molecular orbitals and excited states of H2Pc were examined by the comparison with H2P, H2Pz and H2TBP.These substitutions lead to the inversion of HOMO-1 (132 b1u) and HOMO-3 (130 b1u) and as a result the spectra of H2Pc could not be interpreted by using the Gouterman four-orbital model. The effects of tetraaza subsitution are especially obvious which cause not only the B band but also the Q bands of H2Pz and H2Pc unable to be interpreted by using the Gouterman four-orbital model. Both the tetraaza substitutions and tetrabenzo annulations cause the oscillator strengths of Q band stronger, so the oscillator strengths of H2Pc are the largest among these compounds. The TDDFT energies are in good agreement with the experimental spectra.

Key words: Free base phthalocyanine, Density fanctional theory(DFT), Time-dependent density functional theory(TDDFT), Ground state, Excited state