Electronic Structure and UV Absorption Spectra of 2-Pyridylacetylenes: An Intramolecular Orbital Interaction between the n Orbital on the Nitrogen Atom of the Pyridine Ring and the In-plane π Orbital of the Ethynylene Group

Article abstract of DOI:10.1246/bcsj.71.329

The UV absorption spectra of some pyridylacetylenes were measured in various media at room temperature and 77 K, and the origin of the electronic absorption bands of these compounds was clarified compared with the results of MO calculations. For instance, it has been found that the 304.6, 271.7, 221, and 216 nm bands of l-(2-pyridyl)-2-phenylethyne in cyclohexane can be assigned to the calculated S₁← S₀, S₉←S₀, S₉←S₀, and S₁₀←S₀ transitions, respectively, in which the observed 271.7 nm band (S₅←S₀) is ascribed as a new type transition from the n orbital on the aza-nitrogen atom of the pyridine ring to the in-plane π^* orbital on the sp-carbon atoms of the ethynylene group. Furthermore, it is revealed that the observed 259.6 nm band of l,2-di(2-pyridyl)ethyne (D2PE) can also be assigned to a new type of transition caused by weak interactions between the n orbitals and the in-plane π orbital of the ethynylene group. This result indicates that two n orbitals of the terminal pyridine rings of D2PE may interact with each other via the central in-plane π orbital.