Spectra of porphyrins

The review opens by presenting the absorption spectra for three series of porphyrins derived from the basic skeleton: (a) compounds obtained by simple substitution; (b) compounds obtained by reduction of one or more pyrrole rings; and (c) compounds obtained from fusion of aromatic rings onto the basic skeleton. The spectra are discussed in terms of a four orbital model—that is intensity changes and energy shifts are related to the properties of two top filled and two lowest empty pi orbitals. Emission spectra of metal porphyrins are then discussed, three metal series being distinguished: (1) In closed shell metals, the continuous enhancement of phosphorescence at the expense of fluorescence is attributed to spin-orbit coupling. (2) In paramagnetic metals, observed effects are attributed to the existence of a state at the same energy as the usual triplet but with multiplicity the same as the ground state; its intensity is ascribed to exchange interactions. (3) In diamagnetic metals with unfilled d shells, peculiar emission properties are attributed to enhanced spin orbit coupling due to low lying metal triplets. The review closes by discussing n-π transitions and triplet-triplet spectra.