A combined theoretical and experimental study was performed on
diarylethenes and diarylethene-capped sexithiophenes aiming at an
improved understanding of the electrochemical and photochemical ring-opening
and ring-closing mechanisms. Theoretical calculations, based on DFT
and TDDFT, suggested that the spatial distribution and the occupancy
of the frontier orbitals determine and control the diarylethenes’
ring-opening and ring-closing upon photoirradiation and electrochemical
oxidation. Optimized geometries, potential energy surfaces, and activation
energies between the open-ring and closed-ring forms were calculated
for diarylethenes in the neutral ground state, excited states, and
mono- and dicationic states. Analysis of the frontier orbitals was
employed to understand the cyclization and cycloreversion of diarylethenes
and to predict and explain the switching properties of diarylethene-capped
sexithiophene molecular wires. The TDDFT data were verified with experimentally
measured UV/vis spectra. The DFT calculations estimated open-shell
ground states of diarylethene-capped sexithiophene dications, which
were verified with EPR spectroscopy, and the broadening of the peaks
in the EPR spectra were explained with the calculated singlet−triplet
splitting. The good agreement of experiment and theory allows for
the understanding of switching behavior of diarylethenes in solutions,
in metal break junctions, in monolayers on metal surfaces, and as
a part of complex organic molecular wires.