TD-DFT calculations and MCD spectroscopy of porphyrin and phthalocyanine analogues: rational design of photosensitizers for PDT and NIR region sensor applications

Geometry optimizations and TD-DFT calculations have been carried out on series of fused-ring-expanded phthalonitriles, phthalocyanines, and aza-dipyrromethene boron difluoride (aza-BODIPY) dyes and trends in their optical and redox properties have been analyzed. The potential utility of fused-ring-expanded phthalocyanine and aza-BODIPY analogues for photodynamic therapy and near infrared region sensor applications is assessed on this basis. Recent attempts to prepare fused-ring-expanded aza-BODIPY analogues with benzene, pyrazine, and naphthalene rings have demonstrated that the properties of aza-BODIPYs vary markedly when different fused ring systems are added to the β-carbons of the pyrrole rings. A comparison of the TD-DFT calculations demonstrates that, as has previously been postulated, trends in the optical spectra, redox properties, and electronic structures of aza-BODIPYs follow those observed for the phthalonitrile precursors and the analogous phthalocyanines despite the absence of a fully conjugated macrocyclic perimeter that obeys Huckel’s rule.