Aggregation properties of nitroporphyrins: comparisons between solid-state and solution structures

The aggregation properties of a series of 5-nitro derivatives of octaethylporphyrine were investigated. Results showed that strong pi-pi dimeric or linear aggregates of nitro-substituted metalloporphyrins were observed in the solid state. Analysis of several porphyrin molecules showed a requirement for an asymmetric electron distribution in the system. Weak aggregation was observed in the free-base porphyrin while intermediate to strong dimerization was shown by corresponding zinc complexes.