Electronic spectra of porphyrins in the solid state: Newly observed transitions, collective and structural effects, and protein-mimicking environments

Solid-state porphyrins allow detailed electronic spectroscopy, evaluation of structural effects, and new catalytic environments not possible in solution. A difficulty with studying porphyrins and other highly absorbing species with UV-vis spectroscopy in the solid state is the opacity of the samples. Fortunately, this is in fact a major plus with specular reflectance spectroscopy. This paper reviews several solid-state porphyrin systems having notable delocalized electrons, structural effects on spectra and environments that allow heterogeneous catalysis and protein-mimicking environments, all studied with polarized specular reflectance spectroscopy. The porphyrins include free-base and metalloporphyrins with simple (octaethyl) through complex (tetraaza-tetrabenzo) substituents and ring insertions. Applications include new electronic transition assignments, determination of ring vs. metal conductivity, correlation of ring puckering to red-shifting of Q and Soret transitions, and identifying the presence of and environmental effects on porphyrins in network solids.