E/Z isomerism in isoporphycene and its nickel complex. Strong influence of peripheral substituents on the molecular structure

We investigated the influence of peripheral ethyl substituents on the relative stability of the Z and E isomers of octaethylisoporphycene 1a using calculations at DFT level. Compared to the parent compound isoporphycene (1), for which the Z isomer is found to have a lower energy than the E isomer, the order of stability is reversed, if the ethyl groups are taken into consideration. The predicted preference of the E configuration in 1a agrees with experimental observations. An analysis of the molecular geometries reveals that the different behaviour of 1 and 1a results from steric interactions between the ethyl substituents and between these substituents and H atoms of the (CH)3 bridge. For the nickel complex of isoporphycene (2) the calculated energy difference between Z and E isomer is larger and the effect of ethyl substitution is less pronounced than in the case of the free base. As a result, the Z-configured form remains the more stable isomer in the octaethyl derivative 2a.