EXCITON-TYPE SPLITTING OF ELECTRONIC STATES IN HYDROGEN-BONDED MOLECULAR DIMERS OF N-HETEROCYCLICS

Abstract

The excition theory of Frenkel has been greatly extended by $Davydov^{1,2,3}$ to molecular crystal interactions and to intramolecular interactions in composite molecules (biphenyl and p-polyphenyls). Molecular dimers of two main types have been studied recently and found to exhibit a very large electronic term splitting. London-force dimers of dye molecules have been studies experimentally by numerous authors. The monomers are arranged essentially in parallel planes in the dimer, and invariably, mainly a blue-shift of the strong dipole-allowed transition of the dye is observed, amounting to $1000-5000 cm^{-1}$. Simpson $et al^{4}$ applied the excition type of treatment to this problem. A general extension to London-force polymers of dye molecules has also been $made,^{5}$ and general selection selection rules presented. Hydrogen-bonded dimers of N-heterocyclic molecules present unusual interest from several points of view. The intermolecular hydrogen bonds cause the monomers to form a co-planar dimer in some cases. Depending on whether the transition moments of the monomers are perpendicular or parallel to the dimer axis, a blue shift or red shift, respectively, is expected to be observed; or a splitting into two components is expected if the monomer transition moments are oblique to the dimer axis. Dimers of imidosole, bensimidozole , $N, N^{1}$-diphenylformamidine , and 3, 6-diominoacridine have been studied in solution at room temperature. The results are in general agreement with an excition-type of interaction. For example, for the last molecule, the two lowest transitions of the monomer are mutually perpendicularly polarized, and the blue shift of $2600 cm^{-1}$ and a red shift of $1400 cm^{-1}$ for the two respective bands upon going from the monomer to the dimer are qualitatively in accord with theory. Dimers involving purines, and certain mixed dimers, are also under investigation. Such hydrogen-bonded dimers of N-heterocyclic molecules are of great biological importance, e.g., they occur as the fundamental core of structure of nucleic acids. The role of the dimers as energy transfer agents is strongly suggested by the spectroscopic results.