Synthesis and Combined Experimental and Theoretical Characterization of Dihydro-tetraaza-acenes.

We present a combined experimental and theoretical study of electronic and optical properties of dihydro-tetraaza-acenes (DHTA n ). Using solvent-free condensation, we are able to synthesize not only DHTA5 but also the longer DHTA6 and DHTA7 molecules. We then investigate their gas-phase electronic structures by means of ab initio density functional calculations employing an optimally tuned range-separated hybrid functional. By comparing with the parent linear oligoacenes ( n A) and based on computed ionization potentials and electron affinities, we predict DHTA n molecules to be more stable than acenes of the same length, where we expect DHTA n molecules to be persistent at least up to n = 7 rings. We further exploit the analogy with n A by analyzing the entire intramolecular π-band structure of the DHTA n molecules. This clearly reveals that the additional two electrons donated by the dihydropyrazine group are delocalized over the entire molecule and contribute to its π-electron system. As a consequence, the symmetry of the frontier orbitals of DHTA n differs from that of the parent n A molecule. This also affects the UV-vis absorption spectra which have been measured for DHTA5, 6, and 7 dissolved in dimethyl sulfoxide and analyzed by means of excited state calculations within a time-dependent density functional theory framework.