Ultraviolet absorption spectra and circular dichroism of divalent metal complexes containing 1,10-phenanthroline and 2,2'-bipyridine

The absorption spectra of the following complexes have been measured at 77°K: Zn bipy32+, Ni bipy32+, Ni en2 bipy2+, Ni phen32+, Ni en2 phen2+, Ni phen2- bipy2+, and Ni bipy2 phen2+. The fluorescence spectrum of Zn bipy32+ has also been determined. The results have been used to sort out the energy levels associated with the π-π* transitions in the region 28000- 40000 cm-1 of the spectra of the complexes. In particular, vibronic coupling theory for trimers has been used to determine the magnitude and sign of the resonance splitting energies (ε) of the tris complexes. An upper limit of 100 cm-1 (of unknown sign) can be placed on ε for bipy and there is evidence to suggest a small positive value of ε for the 37000 cm-1 state of phen in Ni phen32+. The absorption spectra have been used to interpret the circular dichroism reported previously and the vibronic coupling theory has been used successfully to calculate the shape of the circular dichroism curve of l-Ni bipy32+, Current theories of the circular dichroism of these complexes are critically reviewed.