A density functional theory study on the porphyrin isomers: effect of meso-bridge length, relative stabilities, cis-trans isomerism

Abstract

B3LYP/6-311+G** calculations are employed to study the structures, stabilities of 1,2 (syn) and the 1,3 (anti) tautomeric forms of porphyrin isomers with varying meso-bridge lengths. A total of 46 structures obtained by considering the geometrical isomers of 1,2 (syn) and the 1,3 (anti) tautomeric forms, 44 were characterized as distinct minima on potential energy surface. Among the anti and syn tautomeric form the porphycene isomer 4a and 4a′ is computed to be the most stable with a strong intramolecular hydrogen bonding. A –(CH)₂– linker in Z form with significant π-delocalization and the E form isomers are somewhat distorted into bowl-like structure in order to avoid the steric repulsion between the inner protons. In the isomers with trimethine interpyrrole linker, the E forms 7b and 7b′ are having lower energy than the Z form due to the unstrained bond angles in these isomers and the Z form is destabilized by severe bond angle. In isomer [4.0.0.0], which has connectivity with the maximum possible bipyrrolic, linkage Z isomer is having high energy this is due to possibly strained bonds that are directly connected to the pyrrole rings.