Theoretical investigation of polymer chain stability in the metal coordinated azorubine and cyclam complex

- Ni(II)- and Zn(II)-coordinated cyclam-azorubine complexes are described using DFT.
- Ni(II) coordinated system shows a reasonably small singlet-triplet energy gap.
- Intersystem crossing gives high barrier between singlet and triplet geometries.
- For the Zn(II) case only the singlet state is energetically favorable.
- The stability of a longer polymer (oligomer) chain was described using DFTB.

Theoretical investigations have been performed for unit systems with Ni(II) and Zn(II) coordination between azorubine and 1,4,8,11-tetraazacyclotetradecane (cyclam) complexes using the conventional DFT and the DFT-based tight binding (DFTB) methods. Two different geometries (short and long) and spin states (singlet and triplet) of the model system built by two mesylate groups and the cyclam ring together with Ni(II) and Zn(II) ions were energetically characterized. For the Ni(II) coordination complex the triplet geometry is preferred, but one could not exclude also the presence of the singlet spin configuration due to the huge energy barrier defined by the intersystem crossing. The intersystem crossing geometry of the singlet-triplet transition was studied in details and the corresponding spin-orbit couplings were discussed. For the Zn(II) coordination complex only the singlet state was found. Polymer chain build up from four unit systems presents irregular forms with strong coordination bonds between units.

67