THEORETICAL DETERMINATION OF THE MOLECULAR AND SOLID-STATE ELECTRONIC-STRUCTURES OF PHTHALOCYANINE AND LARGELY EXTENDED PHTHALOCYANINE MACROCYCLES

The molecular and solid-state electronic structures of metal-free phth alocyanine and a series of linearly benzoannulated phthalocyanines hav e been investigated using the valence effective Hamiltonian (VEH) quan tum-chemical method. Geometry optimizations show that, from the molecu lar structure standpoint, phthalocyanine-based macrocycles are the res ult of joining four polyacenic units to the C8N8 central ring. The ele ctronic structure calculated for the parent phthalocyanine is compared with that of porphyrin, and the consequences of benzoannulation and m eso-tetraaza substitution on the optical properties of phthalocyanines are discussed. The VEH results obtained for extended phthalocyanines are in agreement with available photoemission, cyclic voltammetry and optical absorption data and help to rationalize the evolution of the e lectronic properties. The first ionization energy is predicted to decr ease with linear benzoannulation and asymptotically converges to an ex trapolated value of -5.7 eV. Strikingly, a non-convergent behaviour is obtained for the HOMO-LUMO energy gap and very low excitation energie s are predicted for extended phthalocyanines. Band-structure calculati ons have been performed for one-dimensional stacks of the molecules in vestigated. The variation of the bandwidth with the staggering angle a nd the intermolecular separation provides a coherent picture of the el ectrical conductivities observed experimentally in crystals and polyme rs. Very small bandgaps lower than 0.5 eV are predicted for extended p hthalocyanines.