A molecular orbital study on a tetra-aza macrocycle containing 2,2′-bipyridines and its lithium complex

Abstract

Singlet transition energies and oscillator strengths of two configurational isomers of a dicyano dibutyl tetra-aza macrocycle containing four pyridine rings and their lithium complexes were calculated using the CNDO/S CI method at ab initio RHF/3-21G optimized geometries. The results were in good agreement with those derived from the UV-Visible absorption spectra. The drastic spectral changes observed upon complexation in the trans-isomer were found to be due to the significant conformational change in the bipyridine moiety and the resulting energy separation of the frontier π orbitals. The close proximity between the triplet states and the singlet excited state suggested that the intersystem crossing might occur in the trans form of the lithium free macrocycle. This is one of the possible explanations of the large discrepancy in fluorescence intensities between the trans-isomer of the macrocycle and its lithium complex.