meso-Aryl sapphyrins with heteroatoms; synthesis, characterization, spectral and electrochemical properties

The synthesis, characterization and spectral properties of six new meso-ary l core modified sapphyrins are described. An efficient approach involving a n acid catalyzed condensation of bithiophene diol 1 and modified tripyrrane s 2a-2e allows preparation of the desired meso-aryl sapphyrins in 16-36% yi eld. The product distribution and the isolated yield were found to be depen dent on the nature of the acid catalyst (Lewis acid or protic acid) and its concentration. Protic acid catalyst exclusively gave the expected sapphyri ns while two additional products, an 18 pi tetraphenylporphyrin and a 26 pi modified rubyrin, were isolated under Lewis acid catalysis. An analysis of proton NMR and absorption spectral data suggests that in free base sapphyr ins, the heterocyclic ring opposite to the bithiophene unit is inverted as in N-5 meso-aryl sapphyrin and the degree of inversion is dependent on the nature of the heterocyclic ring. The energy optimized structure calculated from the semi-empirical method substantiates such a conclusion. Protonation of sapphyrins generates respective mono- and dications and the heterocycli c ring retains an inverted structure in contrast to normal N-5 sapphyrins. The triplet excited lifetimes for free base and protonated derivatives are similar both under argon,saturated and air equilibrated conditions, indicat ing that the triplet state quenching by oxygen is minimal. Cyclic voltammet ric studies reveal easier reductions and harder oxidations relative to meso -aryl porphyrins and the Delta(redox) observed for 3d suggests significant reduction of the HOMO-LUMO energy gap consistent with the large red shift o bserved for the Soret band.