Ab initio calculation of the electronic spectrum of azobenzene dyes and its impact on the design of optical data storage materials

Electronic excitation energies of 16 azobenzene dyes have been calculated by ab initio methods within the second-order polarization propagator approximation (SOPPA). Good agreement with expriment is found for the lowest singlet and triplet states for both the trans- and cis-azobenzene molecules. The differences are in the range of +/-0.3 eV, with the exception of the lowest n --> pi* transition in trans-azobenzene, where a deviation of -0.64 eV is found. The lowest,pi --> pi* transition in trans-azobenzene, on the other hand, is particularly well reprensented with a deviation of only -0.15 eV. Furthermore, the experimental singlet pi --> pi* transitions are reproduced for a set of azobenzene dyes with different electron donor and acceptor groups and the correct shifts in excitation energy are obtained for the different substituents. It has also hen demonstrated that ab initio methods can be used to determine suitable candidates for azo components used in materials for data storage.